THE CARDIAC FATTY ACID METABOLIC PATHWAY
IN HEART FAILURE
by
Eric E. Morgan
Submitted in partial fulfillment of the requirements
For the degree of Doctor of Philosophy
Thesis Advisor: Dr. William C. Stanley
Department of Physiology and Biophysics
CASE WESTERN RESERVE UNIVERSITY
May, 2006
CASE WESTERN RESERVE UNIVERSITY
SCHOOL OF GRADUATE STUDIES
We hereby approve the thesis/dissertation of
Eric E. Morgan
candidate for the Ph.D degree *.
(signed) Richard Eckert (chair of the committee)
William C. Stanley
Brian D. Hoit
Steve Fisher
Matthais Buck
(date) 01/20/06
*We also certify that written approval has been obtained for any proprietary material contained therein.
ii
DEDICATION
This work is dedicated to Drs. William Stanley, Margaret Chandler and Brian Hoit, whose unique perspectives on research collectively afforded me a very rich and well- rounded educational experience.
iii TABLE OF CONTENTS
Dedication...... ii
Table of Contents……………………………………………………………………..…iii
List of Tables…………………………………………………………...……………….vii
List of Figures……………………………………………………………………………ix
Acknowledgments…………………………………...... ………………………………...xi
List of Abbreviations………………………………………………………………...…xii
Abstract…………………………………………………………..………...…………....xv
Chapter 1: Energy Metabolism in the Heart……………………………….……...... 1
1.1. Introduction……………………………………………………………………...... 1
1.2. Myocardial Carbohydrate and Fatty Acid Metabolism…………………....……...2
1.2.1. Overview…………………………………………………………...……...2
1.2.2. Carbohydrate Metabolism………………………………………………....3
1.2.3. Fatty Acid Metabolism……………………………………………………5
1.3. Regulation of Expression of Fatty Acid Oxidation Enzymes by Nuclear
Receptors…………………………………………………………………….……8
1.4. Overview of Heart Failure……………………………………………………….11
1.5. Myocardial Lipid Accumulation and Heart Failure……………………………...13
1.6. Metabolic Phenotype of the Failing Heart……………………………………….14
1.7. Substrate Selection and Contractile Function in Heart Failure……………...... 17
1.8. Rationale and Hypothesis……………………………………………………..…18
iv Chapter 2: Validation of Echocardiographic Methods for Assessing Left
Ventricular Dysfunction in Rats with Myocardial Infarction…...... ….24
2.1. Introduction………………………………………………………………………24
2.2. Materials and Methods…………………………………………………………...25
2.2.1. Study Design and Induction of Myocardial Infarction…………………..25
2.2.2. Echocardiography………………………………………………………..26
2.2.3. Hemodynamic Measurements……………………………………………30
2.2.4. Statistical Analysis……………………………………………………….30
2.3. Results……………………………………………………………………………31
2.3.1. Hemodynamics…………………………………………………………..31
2.3.2. Echocardiography………………………………………………………..31
2.3.3. Intra- and Inter-observer error………………………………………...…31
2.3.4. WMSI and MPI Correlations…………………………………………….35
2.4. Discussion………………………………………………………………………..35
2.5. Limitations……………………………………………………………………… 40
2.6. Conclusions………………………………………………………………………40
Chapter 3: Effects of Coronary Artery Ligation Induced Heart Failure
on Cardiac Metabolic Enzyme Gene and Protein Expression…...... 42
3.1. Introduction…………………………………………………………….………...42
3.2. Methods………………...……………………………………………….………..44
3.2.1. Study Design and Induction of Myocardial Infarction…………………..44
2.3.2. Echocardiography………………………………………………………..44
3.2.3. Hemodynamic Measurements……………………………………………45
v 3.2.4. RNA Extraction and Quantitative RT-PCR……………………………...46
3.2.5. Western Immunoblot Analysis…………………………………………..46
3.2.6. Metabolites and Enzyme Activities…………………………………...... 47
3.2.7. Statistical Analysis……………………………………………………….47
3.3. Results………………….……………………………….………………………..47
3.3.1. Body and Heart Mass……………………...………………….………….47
3.3.2. Cardiac Function……………………..….…………………….…………48
3.3.3. mRNA Expression…………………………...……………..……………52
3.3.4. Enzyme Activity and Protein Expression…………..….….……………..52
3.3.5. Metabolite Levels……………………………………………...………...52
3.4. Discussion………………………………………………………………………..60
Chapter 4: Effects of Chronic Activation of Peroxisome Proliferator Activated
Receptor Alpha or High Fat Feeding in a Rat Infarct Model of Heart
Failure…………………………………………………………………..….63
4.1. Introduction……...……………………………………………………...…….....63
4.2. Methods………………………………………………………………………….64
4.2.1. Study Design and Induction of Myocardial Infarction…………………..64
4.2.2. Echocardiography………………………………………………………..65
4.2.3. Hemodynamic Measurements……………………………………………66
4.2.4. Metabolic Products and Enzyme Activity……………………………….67
4.2.5. RNA Extraction and Quantitative RT-PCR……………………………...67
4.2.6. Western Immunoblot Analysis…………………………………………..68
4.2.7. Statistical Analysis……………………………………………………….68
vi 4.3. Results………………………………..…………………………………………...69
4.3.1. Body and Heart Mass………………………………………………….…69
4.3.2. Cardiac Function…………………………………………………………69
4.3.3. Triglyceride and Ceramide………………………………………………69
4.3.4. mRNA Expression……………………………………………………….75
4.3.5. Protein Expression and Enzyme Activity………………………………..75
4.4. Discussion………………………………………………………...…………..….78
Chapter 5: Discussion and Future Directions…………………………………...... 82
5.1. Thesis Summary………………………………………………………………....82
5.2. Discussion and Future Directions………………………………………………83
5.2.1. Time Course of Fatty Acid Metabolic Down-regulation in Heart
Failure……………………………………………….…………………...84
5.2.2. Role of Myocardial Triglyceride and Ceramide Accumulation in the
Progression of Heart Failure……………………………………………..85
5.2.3. Role of Nuclear Receptor Activation in Heart Failure…………………..86
5.2.4. Pharmacologic Manipulations of Myocardial Metabolism for the
Preventions of Heart Failure Following Myocardial Infarction…………90
5.3. Conclusion………………………………………………………………………91
Reference List…………………………………………………………………………...92
vii LIST OF TABLES
Table 2-1: Hemodynamic variables in normal and infarcted rats……………….…...... 32
Table 2-2: Echocardiographic variables in normal and infarcted rats………………….33
Table 2-3: Results of linear regression analysis comparing intra- and Inter-
investigator measurements of MPI and WMSI…………...…………...…….34
Table 2-4: Correlation coefficients between WMSI and MPI and left ventricular
pressure measurements and echocardiographic parameters in normal
and infarcted rats…………………………………………………………....37
Table 2-5: Results of forward stepwise linear regression analysis for WMSI
And MPI using peak +dP/dt, cardiac index, peak end diastolic
pressure, area of fractional shortening, end diastolic area and tau
as independent variables…………………………………………………….38
Table 3-1: Body and heart mass in normal and infarcted rats………………………….49
Table 3-2: Left ventricular pressures and heart rates and echocardiographic data
in normal and infarcted rats……….……….…...…………..………….……51
Table 3-3: mRNA expression in normal and infarcted rats…………………………….53
Table 3-4: Protein expression in normal and infarcted rats at 20 weeks post
infarction or sham surgery………………………………………………….56
Table 3-5: Medium chain acyl-CoA dehydrogenase and citrate synthase
activity in normal and infarcted rats………………………………………..56
Table 3-6: Plasma fatty acid and triglyceride concentrations in normal and
infarcted rats………………………………………………………………...57
viii Table 4-1: Body and heart masses in infarcted, infarcted and high fat fed,
and infarcted and fenofibrate treated rats…………………………………...70
Table 4-2: Hemodynamic and echocardiographic measurements in infarcted,
infarcted and high fat fed, and infarcted and fenofibrate treated rats………72
Table 4-3: Plasma free fatty acids and triglycerides in infarcted, infarcted and high
fat fed, and infarcted and fenofibrate treated rats…………………………...73
Table 4-4: Protein expression in infarcted, infarcted and high fat fed, and infarcted
and fenofibrate treated rats………………………………………………….73
Table 4-5: Medium chain acyl-CoA dehydrogenase and citrate synthase activity in
infarcted, infarcted and high fat fed, and infarcted and fenofibrate treated
rats…………………………………………………………………………..77
ix LIST OF FIGURES
Figure 1-1: Schematic representation of the carbohydrate and fatty acid
metabolic pathways………………………………………………………….4
Figure 1-2: Schematic representation of the regulation of metabolic genes
in cardiomyocytes by stimulation of the peroxisome proliferator-
activated receptor α (PPARα) and retinoic X receptor (RXRα)…………….9
Figure 1-3: Major end points determined in this thesis………………………………...20
Figure 2-1: Parasternal long and short axis views of rat left ventricle…………………27
Figure 2-2: Doppler color directed pulse-wave recording of mitral and aortic flow
for the determination of MPI………………………………………………29
Figure 2-3: Left ventricular peak +/-dP/dt, area fractional shortening and
cardiac index plotted as a function of WMSI and MPI……………………36
Figure 3-1: Fractional area shortening and ANP expression in normal and
infarcted rats at 8 and 20 weeks post infarction or sham surgery………….50
Figure 3-2: mRNA expression of non-PPARα regulated genes in normal and
infarcted rats at 8 and 20 weeks post infarction or sham surgery………….54
Figure 3-3: mRNA expression of PPARα regulated genes in normal and infarcted
rats at 8 and 20 weeks post infarction or sham surgery……………………55
Figure 3-4: Myocardial C16 ceramide content in normal and infarcted rats at 8
and 20 weeks post infarction or sham surgery……………………………..58
Figure 3-5: Protein expression in normal and infarcted rats at 20 weeks post
infarction or sham surgery………………………………………………….59
x Figure 4-1: Left ventricular mass to body mass ratio in infarcted, infarcted and
high fat fed, and infarcted and fenofibrate treated rats…………………….71
Figure 4-2: Myocardial tissue triglyceride and C16 ceramide content in
infarcted, infarcted and high fat fed, and infarcted and
fenofibrate treated rats……………………………………………………..74
Figure 4-3: mRNA expression of non-PPARα and PPARα regulated genes in
infarcted, infarcted and high fat fed, and infarcted and fenofibrate
treated rats………………………………………………………………….76
Figure 5-1: Late stage heart failure associated decreases in expression of PPARα
and/or RXRα may precipitate the myocardial substrate switch by
reducing the amount of fatty acid oxidation enzyme mRNA transcribed…88
Figure 5-2: In heart failure, phosphorylation of PPARα by the ERK pathway may
prevent transcription of fatty acid oxidation enzymes……………………..89
xi
ACKNOLEDGEMENTS
I thank my Advisor, Dr. William Stanley and my committee members, Drs. Brian Hoit,
Steven Fisher, Matthais Buck, and Richard Eckert for their guidance and support; Dr
Margaret Chandler for her encouragement, patience and understanding; Dr. Martin
Young for affording me the opportunity to enhance my technical abilities abroad and for his generosity and kindness; Tracy McElfresh for her excellent technical assistance and camaraderie, and Dr. Isidore Okere, Naveen Sharma, Kristen King, and Julie Rennison, for their devoted friendship.
xii LIST OF ABREVIATIONS
3-KAT 3-Ketoacyl-CoA Thiolase
ACC Acetyl-CoA Carboxylase
AMPK 5’AMP activated protein kinase
ANP Atrial Natriuretic Peptide
BNP Brain Natriuretic Peptide
CAT Carnitine Acyl-Transferase
CAC Citric Acid Cylce
CI Cardiac Index
CO Cardiac Output
CPT-I Carnitine Palmitoyltransferase I
CPT-II Carnitine Palmitoyltransferase II
CS Citrate Synthase dP/dt First derivative of left ventricular pressure
EDA End Diastolic Area
EDD End Diastolic Dimension
EDP End Diastolic Pressure
ERK Extracellular-Signal-Regulated Kinase
ESA End Systolic Area
ESD End Systolic Dimension
FABP Fatty Acid Binding Protein
xiii FACS Fatty Acyl-CoA Synthase
FAO Fatty Acid Oxidation
FENO Fenofibrate
FA- Free Fatty Acid anion
FFA Free Fatty Acid
FS Fractional Shortening
FSa Area Fractional Shortening
HF Heart Failure hLpL Human Lipoprotein Lipase
HR Heart Rate
INF Infarcted
LDH Lactate Dehydrogenase
LV Left Ventricle
LVDa Left Ventricular Diastolic area
LVDd Left Ventricular Diastolic dimension
LVP Left Ventricular Pressure
LVSd Left Ventricular Systolic dimension
LVSa Left Ventricular Systolic area
MCD Malonyl-CoA Decarboxylase
MCAD Medium Chain Acyl-CoA Dehydrogenase
MPI Myocardial Performance Index
MTE-1 Mitochondrial Thioesterase I
xiv PDK Pyruvate Dehydrogease Kinase
PPARα Peroxisome Proliferator Activated Receptor α
PPRE Peroxisome Proliferator Response Element
RAS Renin-Angiotension System
RV Right Ventricle
RXRα Retinoid X Receptor α
TG Triglyceride
TNFα Tumor Necrosis Factor α
UCP Uncoupling Protein
VTI Velocity Time Integral
WMSI Wall Motion Score Index
xv The Fatty Acid Metabolic Pathway in Heart Failure
Abstract
By
ERIC E. MORGAN
Studies in advanced heart failure (HF) show down-regulation of fatty acid oxidation (FAO) genes, possibly due to decreased expression of the nuclear transcription factors peroxisome proliferator activated receptor α (PPARα) and/or retinoid X receptor α
(RXRα). The time course of this down-regulation, however, is unclear. Additionally, myocardial accumulation of lipids is associated with cardiac dysfunction and has been proposed to be a causative factor in the progression of HF. The goal of this dissertation was to test the hypotheses that (i) mRNA and protein levels of PPARα, RXRα and the mRNA and protein expression of PPARα/RXRα regulated FAO enzymes would progressively decrease in HF, and (ii) activation of the fatty acid metabolic pathway with either a direct PPARα agonist or high fat feeding would increase myocardial triglyceride and/or ceramide and exacerbate left ventricular (LV) dysfunction and dilation in established HF.
Studies were conducted in the rat infarct model of HF. Rats underwent left coronary artery ligation or sham surgery and 8 weeks later were either untreated, fed a high fat diet (45% kcal from fat), or fed the PPARα agonist fenofibrate (150 mg·kg-1·day-
1). At either 8 or 20 weeks post ligation (12 weeks after initiation of treatment), LV
xvi function was assessed by echocardiography and LV catheterization. Non-infarcted LV
tissue was freeze-clamped for later mRNA, protein and biochemical analysis.
The results show that mRNA down-regulation of PPARα regulated metabolic
genes in HF occurs despite no decrease in the protein expression of PPARα or RXRα and
that the down-regulation of mRNA occurs relatively early in the progression of HF
whereas protein down-regulation is a late-stage phenomenon. In addition, these studies
indicate that neither up-regulation of the fatty acid metabolic pathway with a PPARα
agonist nor accumulation of myocardial triglyceride exacerbates LV dysfunction or dilation in a model of established HF.
In conclusion, reduced mRNA expression of PPARα/RXRα regulated genes is not dependent on reduced protein expression of PPARα and/or RXRα, and there is a
disconnect between metabolic mRNA expression and protein levels in HF. Additionally,
PPARα activation and myocardial triglyceride accumulation do not affect the progression
of LV dysfunction and dilation in established HF.
xvii Chapter 1
Energy Metabolism in the Heart
1.1. Introduction
Recent studies suggest that changes in cardiac substrate metabolism affect left
ventricular (LV) function and remodeling in heart failure (HF). While the healthy adult
heart uses fatty acids as the primary substrate for the generation of ATP, in advanced
heart failure, the myocardium down-regulates mRNA and protein expression of fatty acid
oxidation enzymes and switches to greater carbohydrate oxidation. At present, it is unclear as to how or when in the development and progression of HF this metabolic switch occurs, and it is not known if this metabolic shift affects LV function or dilation in
HF. Additional studies suggest that myocardial accumulation of ceramides and triglycerides lead to cardiac “lipotoxicity” and result in myocardial dysfunction and dilation, though the role of these lipid intermediates in the development and progression of HF has not been examined. This section reviews heart failure, and myocardial metabolism in the healthy and failed heart. Specifically, this chapter addresses: (i) myocardial carbohydrate and fatty acid oxidation, (ii) the role of nuclear receptors in the regulation of the fatty acid metabolic pathway, (iii) myocardial lipotoxicity, (iv) the metabolic profile of the failing heart, and (v) the effect of substrate selection on cardiac efficiency.
1 1.2. Myocardial Carbohydrate and Fatty Acid Metabolism
1.2.1. Overview
The myocardium has a perpetually high energy demand, and it relies on a constant supply of oxygen and metabolic fuels to maintain ATP-dependent cellular processes including ion transport, intracellular Ca2+ homeostasis, and contractile function. Under
normoxic conditions, almost all myocardial ATP formation results from oxidative
phosphorylation in the mitochondria, although a small percentage of ATP (<5%) is
derived from glycolysis and GTP formation in the citric acid cycle (CAC).
Mitochondrial oxidative phosphorylation is driven by electrons donated from the
reducing equivalents NADH and FADH2 to the electron transport chain (ETC) located in
the inner mitochondrial membrane. A small amount of NADH is generated via the decarboxylation of pyruvate to acetyl-CoA by pyruvate dehydrogenase (PDH) and from the action of the dehydrogenases of glycolysis and of lactate dehydrogenase. The
majority of NADH and FADH2, however, is produced from the conversion of fatty acids
to acetyl-CoA moieties in the fatty acid β-oxidation pathway and from the oxidation of
acetyl-CoA in the CAC.
In the normal adult myocardium, the CAC is fueled by a continuous supply of
acetyl-CoA from the oxidation of carbohydrates and fatty acids. In the well perfused
state, 60-90% of myocardial acetyl CoA is derived from fatty acid β-oxidation, and 10-
40% comes from the oxidation of pyruvate, generated in approximately equal amounts
from glycolysis and lactate oxidation (44; 89; 107; 159; 160). The degree to which the healthy heart uses fatty acids and carbohydrates is variable and dependent on pathway regulation (allosteric enzyme control and substrate/product concentrations) (107; 118).
2 An additional level of regulation stems from changes in the expression and activity of key
metabolic proteins involved in the uptake and oxidation of fatty acids and carbohydrates.
1.2.2. Carbohydrate Metabolism
Glucose and lactate metabolism provide 10-40% of the myocardial acetyl-CoA which feeds into the CAC. Glycolysis is the multi-step pathway in which glucose 6- phosphate and NAD+ are converted to pyruvate and NADH. In anaerobic glycolysis, two
ATP per molecule of glucose are generated. Additionally, lactate can be metabolized in
the cytosol to produce pyruvate in a reversible reaction catalyzed by lactate
dehydrogenase (LDH). The pyruvate and NADH formed from these two processes can
be converted back to lactate and NAD+ in the cytosol by LDH, carboxylated to form
oxaloacetate or malate (by pyruvate carboxylase and malic enzyme respectively) or
transported into the mitochondria, where the pyruvate is decarboxylated by PDH to form
acetyl-CoA, which is further oxidized to CO2 in the CAC to complete the process of
aerobic oxidative glycolysis.
The committed, irreversible step of carbohydrate oxidation is catalyzed by PDH, a
multi-enzyme complex located in the mitochondrial matrix (75; 118; 141; 158). The flux
through PDH is tightly regulated, both by allosteric enzyme control and substrate/product
concentrations. PDH is allosterically inactivated by phosphorylation of the E1 subunit of
the enzyme complex by PDH kinase (PDK) and is reactivated by dephosphorlyation by
PDH phosphatase (75; 118; 119; 158) (Figure 1-1). The activity of the kinase is dependent on the ratios of acetyl-CoA/free CoA and NADH/NAD+ and pyruvate
3 Lactate Glucose Nonesterified Fatty Acids
Cytosol GLYCOLYSIS Lactate - Pyruvate FA Fatty Acyl-CoA Outer Membrane CPT-I Fatty Acyl-Carnitine Inner Membrane UCP3 CAT CPT-II PDK-4 PDH FA- Fatty Acyl-CoA MTE-1 MCAD Mitochondrial Matrix Acetyl-CoA Fatty Acid β-Oxidation
CO CitricAcid 2 Cycle
Figure 1-1: Schematic representation of the carbohydrate and fatty acid metabolic pathways. Abbreviations: PDH, pyruvate dehydrogenase; PDK-4, pyruvate dehydrogenase kinase-4; CPT-I, carnitine palmitoyltransferase I; CPT-II, carnitine palmitoyltransferase II; CAT, carnitine acyl-transferase; ; MTE-1, mitochondrial thioesterase I and UCP-3, uncoupling protein 3; MCAD, medium chain acyl-CoA dehydrogenase; FA-, fatty acid anion. Key PPARα regulated proteins are highlighted in yellow.
4 concentrations (75; 119; 157). When reactants (i.e. NAD+, CoA, and pyruvate) are at
high concentrations, PDK is inhibited and PDH activity increases. On the other hand, if product concentrations are high (ie NADH and Acetyl-CoA), PDK is stimulated to phosphorylate PDH, thus inactivating it. PDH phosphatase, which dephosphorylates
PDH and thus increases the amount of active PDH, is stimulated by the metal cations
Ca2+ and Mg2+ (96).
The majority of the substrate for myocardial glycolysis is derived from an
exogenous supply of glucose. Glucose transport into the myocytes is controlled by the
transmembrane glucose gradient and is further regulated by the glucose transporters
GLUT-4 and GLUT-1 in the sarcolemma. In response to increased work demand,
ischemia or insulin release, glucose transporters (primarily GLUT-4) are translocated
from intracellular vesicles to the sarcolemmal membrane, thus enhancing transmembrane
glucose transport and increasing myocardial glucose uptake (143; 166; 167). Additional
substrate for glycolysis comes from the glycogen pool within the heart. Glycogenolysis
is stimulated by adrenergic stimulation, decreased tissue ATP content and increased
organic phosphate, as is seen in intense exercise or ischemia (47; 60; 143), and the
myocardial concentration of glycogen is increased when exogenous levels of glucose or
insulin are elevated (81; 84; 143).
1.2.3. Fatty Acid Metabolism
The rate of myocardial fatty acid uptake is primarily determined by circulating
levels of fatty acids, which can range from approximately 0.2 to 0.8 mM in healthy
humans over the course of a day (6; 94; 160). Plasma fatty acids are primarily supplied
5 by diet, but during fasting or starvation, the activation of hormone-sensitive lipase results in triglyceride breakdown and the release of fatty acids into the circulation (93; 134;
152). Hormone-sensitive lipase is activated by phosphorylation resulting from stimulation by glucagon or catecholamines. On the other hand, insulin serves to dephosphorylate and thus inhibit hormone-sensitive lipase, reducing lipolysis in the post- prandial state.
Fatty acid movement into the cardiomyocyte occurs by either passive diffusion or by protein mediated transport (153). Once inside the cell, the fatty acid moiety is bound by a fatty acid binding protein (FABP) and are esterified to fatty acyl-CoA by fatty acyl-
CoA synthase (FACS)(107). The fatty acyl-CoA can then be further esterified to triglyceride by glycerolphosphate acyl-transferase (24; 94; 153) or can enter the mitochondria to undergo β-oxidation. Uptake of long chain fatty acids into the
mitochondrion occurs via the carnitine palmitoyltransferase system, a key regulatory step
of fatty acid oxidation (76; 94; 97; 133; 134). In this process, long-chain fatty acyl-CoA moieties are first converted to long-chain fatty acylcarnitine by carnitine palmitoyltransferase I (CPT-I) in the compartment between the inner and outer mitochondrial membranes. The newly formed long-chain fatty acylcarnitine is then transported across the inner mitochondrial membrane in exchange for free carnitine by carnitine acyl-transferase (CAT). Finally the long-chain fatty acylcarnitine is converted back to long-chain acyl-CoA in the mitochondrial matrix by carnitine palmitoyltransferase II (CPT-II, Figure 1-1)
The rate of fatty acid uptake by the mitochondria is regulated by the activity of
CPT-I (76; 94; 133; 134). In the heart, CPT-I activity is strongly inhibited by malonyl-
6 CoA, which binds to CPT-I on the cytosolic side of the enzyme (76; 99; 172), thus a fall
in malonyl-CoA increases fatty acid oxidation (51; 82), and an increase reduces fatty acid
oxidation (128; 142). Malonyl-CoA is formed in the cytosol by the carboxylation of
acetyl-CoA by acetyl-CoA carboxylase (ACC) and is degraded by the activity of
malonyl-CoA decarboxylase (MCD), which converts malonyl-CoA back to acetyl-CoA
and CO2 (30; 48; 51-53; 77; 82; 128; 129). The activity of ACC is inhibited by
phosphorylation by 5’AMP activated protein kinase (AMPK)(31; 82; 129). Thus, when
AMPK is activated, ACC activity is diminished and there is a decrease in malonyl-CoA
formation, which, in turn, leads to reduced inhibition of CPT-I and increased fatty acid
oxidation (31; 32).
Once taken up into the mitochondria, fatty acyl-CoA molecules are cleaved into
two carbon units of acetyl-CoA by the enzymes of the β-oxidation pathway, generating
NADH and FADH2 in the process. Fatty acid β-oxidation occurs in the mitochondrial matrix and consists of four reactions, with specific enzymes for each step. The enzymes include acyl-CoA dehydrogenase, enoyl-CoA hydratase, 3-hydroxyacyl-CoA dehydrogenase, and 3-ketoacyl-CoA thiolase (3-KAT), each with specific enzymes for short, medium and long chain length substrates (5; 6; 133). The last step of the cycle, catalyzed by 3-KAT, regenerates acyl-CoA to enter another cycle of β-oxidation. The
activity of Acyl-CoA dehydrogenase and 3-hydroxyacyl-CoA dehydrogenase produce
FADH2 and NADH, respectively, and the acetyl-CoA formed from the oxidation of fatty
acids enters the CAC in order to generate more NADH and FADH2.
7 1.3. Regulation of Expression of Fatty Acid Oxidation Enzymes by Nuclear
Receptors
Fatty acids up-regulate fatty acid metabolic proteins via activation of gene
transcription (4; 63). Although the regulation of the expression of these enzymes and
proteins is not well understood, recent studies have focused on a class of ligand activated
transcription factors called peroxisome proliferator-activated receptors (PPARs) (4; 63).
These transcription factors regulate gene expression by forming heterodimers with retinoid X receptor α (RXRα) and recruiting several coactivators including PGC-1α (4;
63; 64; 154). When formed, the PPAR/RXR/PGC-1 complex binds to PPAR response
elements (PPRE) located within promoter regions and thus up-regulates the transcription
of many of the genes that control many metabolic enzymes (Figure 1-2).
There are three PPAR isoforms: α, β/δ, and γ. Of these, PPARα and PPARβ/δ are
the primary regulators of fatty acid metabolism in the heart, and recent evidence suggests
that PPARα and PPARβ/δ may play equally important roles in the regulation of cardiac fatty acid metabolism (19; 63). PPARγ is expressed at very low levels in cardiomyocytes and does not appear to play a direct role in the regulation of fatty acid oxidation in the heart (46; 73). PPARγ activation, however, may indirectly regulate fatty acid oxidation by decreasing the fatty acid concentration to which the heart is exposed. PPARγ activators have been shown to sequester fatty acids in adipocytes and lower circulating fatty acids and triglycerides, therefore reducing the exposure of the myocardium to fatty acids (63) . Thus, in vivo, PPARγ ligands can reduce myocardial fatty acid uptake and oxidation by decreasing plasma fatty acid content.
8 9-cis retinoic acid
RXRα PPARα
Endogenous ligands (long chain fatty acid) PGC-1
PPRE FAO enzyme genes CPT-1 MCAD PDK4 MTE-1 nucleus UCP3 Others
sarcolemma
Figure 1-2: Schematic representation of the regulation of metabolic genes in cardiomyocytes by stimulation of the peroxisome proliferator-activated receptor α
(PPARα) and retinoid X receptor α (RXRα). Abbreviations: PPRE, peroxisome proliferator activated response element; CPT-I, carnitine palmitoyltransferase I; MCAD, medium acyl-CoA dehydrogenase; PDK-4, pyruvate dehydrogenase kinase-4; MTE-1, mitochondrial thioesterase I; and UCP-3, uncoupling protein 3. (Adapted from Stanley et al. 2005).
9 In order to form dimers and subsequently bind to the PPRE, PPARα and RXRα must first be activated by the binding of a ligand. The activating ligand for RXRα is 9- cis-retinoic acid (4), whereas PPARα is stimulated by a broad array of ligands, including long chain polyunsaturated fatty acids and eicosanoids and, to a lesser extent, long chain saturated fatty acids (40). Studies in primary cardiac and neonatal myocytes have shown that administration of long chain saturated and monounsaturated fatty acids increase the mRNA expression of several PPARα regulated genes (9; 46). Similarly, increasing the long chain, but not medium chain fatty acid supply, up-regulates uncoupling protein 3
(UCP3, a PPARα regulated gene) mRNA and protein levels in both skeletal and cardiac muscles, with the greatest effect being seen in cardiac muscle (57). Therefore, PPARα act as lipid sensors in the cell, increasing capacity for fatty acid oxidation in response to elevated levels of intracellular lipid (4; 63). PPARα can also be activated by pharmacological ligands including fenofibrate or WY-14,643 (46; 63; 168). Once activated and bound to the PPRE, the PPARα/RXRα complex increases the rate of transcription of fatty acid oxidation genes including CPT-I, medium acyl-CoA dehydrogenase (MCAD) and PDK-4 (the inhibitory kinase of PDH) (9; 46; 54; 86; 147).
It has also been recently demonstrated that the expression UCP3 and mitochondrial thioesterase I (MTE-I), key enzymes involved in the extrusion of fatty acyl-CoA from the mitochondria, are regulated by PPARα (56; 146; 169). Figure 1-1 illustrates some of the key proteins regulated by PPARα and their position in the fatty acid metabolic pathway.
PPARα over-expressing and PPARα null mice have provided evidence that
PPARα serves to regulate fatty acid metabolic gene expression and function as a as lipid sensor in the cell. Transgenic mice with cardiac specific over-expression of PPARα
10 show up-regulation of the proteins involved in fatty acid metabolism, increased
myocardial fatty acid uptake and triglyceride accumulation, and they develop of LV
dysfunction and dilation (37; 38). This cardiomyopathic phenotype is exacerbated when
these mice are fed a high fat diet, resulting in an enhanced triglyceride accumulation, and
a decrease in fractional shortening (37; 38). On the other hand, PPARα null mice show
reduced cardiac expression of genes involved in the cellular uptake, mitochondrial transport and oxidation of fatty acids (28; 85; 156). Furthermore, myocardial fatty acid
uptake and oxidation is decreased while glucose oxidation is increased in these mice (15)
and when subjected to fasting or pharmacologic inhibition of fatty acid oxidation, PPARα
null mice develop cardiac myocyte lipid accumulation and hepatic steatosis (78; 88).
Collectively, these results have defined PPARα as an important regulator of myocardial
fatty acid metabolism.
1.4. Overview of Heart Failure
Over the last 30 years, there has been a steady rise in mortality attributable to
heart failure (HF) (72). HF has recently been defined as “a complex clinical syndrome
that can result from any structural or functional cardiac disorder that impairs the ability of
the ventricle to fill with or eject blood” (62). Thus, HF is not a specific disease but a
syndrome that is dependent upon many factors including etiology, duration, coronary
artery disease and ischemia, diabetes, hypertension and obesity (72). Approximately two-
thirds of all HF patients, however, have a history of ischemic heart disease (72) . The
remainder have no history of coronary artery disease, and are considered to have
“idiopathic cardiomyopathy” (72).
11 Classically, HF is marked by enlarged ventricular chambers and increased heart mass, although numerous additional structural and biochemical abnormalities are associated with HF, including defects in mitochondria and adrenergic signal transduction
(72). In the early stages of HF, patients are often asymptomatic at rest or become symptomatic when performing more vigorous activities such as exercise or climbing stairs (10). The early stages of HF often go unnoticed because other compensatory physiologic mechanisms are activated and thus disguise the underlying problem. For example, activation of neurohormonal systems, especially the release of norepinephrine by adrenergic cardiac nerves, increases myocardial contractility and activates the renin- angiotension system (RAS) (10). The RAS is also triggered by low cardiac output states, where reduced renal blood flow activates renal baroreceptors to cause release of renin
(10). When activated, the RAS acts to retain sodium and water in order to maintain arterial pressure and increase preload to sustain cardiac performance via the Frank-
Starling mechanism. In addition, myocyte hypertrophy results in increased contractile tissue mass, which unloads individual muscle fibers and redistributes the work load (10).
The capacity of natural physiologic mechanisms to sustain cardiac performance in
HF is finite, and when chronically maintained, these mechanisms become maladaptive and lead to the progression of HF. Augmented salt and water retention, for example, can lead to pulmonary congestion and peripheral edema. Chronic activation of the sympathetic nervous system leads to increased myocardial energy expenditure without necessarily increasing the supply of metabolic substrates and oxygen (10; 107).
Additionally, myocyte hypertrophy leads to cardiac cell deterioration and death. This, in turn, contributes to the progression of HF by precipitating ventricular remodeling, where
12 the LV chamber volume is increased, ventricular and septal walls thin, and myocardial
fibrosis develops (10). Ultimately, this remodeling leads to the enlarged ventricular
chambers and increased heart mass that is the classical anatomic finding in HF.
In the late stages of HF, the myocardium has a reduced content of ATP resulting from an inability to generate ATP by oxidative metabolism, thus reducing the heart’s ability to do contractile work (1; 33; 71; 111). Although the consequences of metabolic dysfunction in HF are not well understood, recent studies suggest that changes in cardiac
substrate metabolism may affect left ventricular (LV) function and remodeling in heart failure (87; 108; 127).
1.5. Myocardial Lipid Accumulation and Heart Failure
Epidemiological studies have demonstrated that obese people have a decreased life expectancy, a greater mortality from cardiovascular disease and a greater risk for developing HF (39; 59; 74). However, the mechanism by which an increased lipid load may lead to HF is unclear. Recent evidence from small animal studies suggests that obesity and elevated plasma fatty acid and triglycerides may lead to myocardial
“lipotoxicity,” a condition characterized by lipid accumulation, ventricular dilation and
contractile dysfunction (37; 98; 131; 151; 164; 165; 173). In PPARα over-expressing
mice, there is an up-regulation of the proteins involved in fatty acid metabolism,
accumulation of triglyceride and ceramide in the myocardium, and development of LV
dysfunction and dilation (37; 38). In other transgenic mouse models where proteins
regulating fatty acid transport or esterification are over-expressed, there is lipid
accumulation, cardiac hypertrophy, development of LV dysfunction and premature death
13 (21; 22). Likewise, in hLpLGPI transgenic mice that over-express human lipoprotein lipase (hLpL), there is increased lipid stores and decreased cardiac function (164).
Although the cellular mechanisms whereby lipid accumulation reduces LV function and increases cardiac dilation are unclear (131), one possibility is that increased myocardial triglyceride accumulation increases susceptibility to arrhythmia and reduced contractile function (122). Another possibility is that ceramide, a lipid second messenger involved in the apoptotic response, accumulates in the myocardium and results in the initiation of apoptosis and inhibition of mitochondrial respiration, thus leading to LV dysfunction and dilation (27; 50; 79; 138; 173). Obese Zucker diabetic rats, for example, develop cardiac dilation and LV dysfunction that correspond with increased myocardial triglyceride and ceramide content and DNA laddering, an index of apoptosis (173).
The downstream signaling pathways through which ceramides initiate apoptosis is unclear; however, direct targets of ceramide include the pro-apoptotic enzymes ceramide- activated protein kinase, protein kinase Cζ, and ceramide activated phosphatase (95) .
Ceramide can also affect the mitogen-activated protein kinase and c-Jun N-terminal kinase signaling cascades or the activation of NF-KB, thus leading to growth arrest and apoptosis (95; 106). Although the toxic effects of lipid accumulation in the heart have been clearly demonstrated, the effect of increased myocardial triglyceride and ceramide accumulation on the development and progression of HF has been largely uninvestigated.
1.6. Metabolic Phenotype of the Failing Heart
Although it is well known that the healthy adult heart uses fatty acids as the primary substrate for the generation of ATP (107), less is known about myocardial substrate metabolism in HF. In general, the data indicate that in the early stages of HF,
14 there is a normal or elevated rate of fatty acid oxidation, and that in advanced or end- stage HF, fatty acid oxidation is down-regulated. Early studies indirectly assessed myocardial metabolism in HF and suggested that during the development of HF, the heart switches its metabolism away from fatty acids to carbohydrate metabolism for energy production (5; 7; 36; 161). Sack et al. established that mRNA and protein levels of both
MCAD and long chain acyl-CoA dehydrogenase are reduced in explanted failed hearts obtained from NYHA functional class III and IV transplant recipients compared with unused donor hearts (127). While these findings suggest that the reduction in fatty acid oxidation observed in HF may result from the coordinately repressed expression of fatty acid metabolic pathway, the time course and role of this metabolic switch in the development and progression of HF remains unknown.
There are no direct measurements of myocardial substrate oxidation in HF patients, and indirect measurements provide somewhat conflicting data. A study conducted in class II and III HF patients revealed an increase in myocardial uptake and oxidation of fatty acids and a decrease in glucose uptake and oxidation compared to age- matched healthy individuals (110). Similar results were obtained in a PET analysis of class III HF patients that revealed an increase in labeled myocardial fatty acid uptake and a decrease in deoxyglucose uptake compared to healthy subjects (148). Another PET study, however, conducted in patients with idiopathic dilated cardiomyopathy showed a greater myocardial glucose uptake and less fatty acid uptake compared to normal individuals (26).
Animal models of HF have provided additional insight into the changes in cardiac metabolism that accompany heart failure. Studies in the canine rapid pacing model of HF
15 using isotopic tracers to directly measure substrate oxidation showed that myocardial substrate metabolism is relatively normal during the early and middle stages of heart failure, but suddenly switches away from fatty acid toward glucose oxidation in end-stage failure (87; 108). In a canine model of less severe HF, there were no differences in myocardial glucose, lactate or fatty acid metabolism compared to normal dogs, suggesting that down-regulation of fatty acid oxidation occurs only in more severe or late-stage HF (16). Data from the rat infarct model of heart failure suggest that there is down-regulation of the mRNA for fatty acid oxidation enzymes as early as 8 weeks after infarction in rats with severe heart failure (121; 124), and when perfused, the isolated hearts demonstrate a significant increase in glucose oxidation with no change in palmitate oxidation (121). These results suggest that in the early stages of HF there is increased glucose metabolism but not a reduction in exogenous fatty acid metabolism.
Although the mechanisms responsible for the down-regulation of the fatty acid oxidation pathway in advanced heart failure are not clear, recent studies suggest that decreased expression of PPARα and/or RXRα may play a key role in the myocardial substrate switch observed under these conditions (63; 65; 69; 108; 156). A recent human study found that PPARα protein levels were decreased in failed hearts (69), and studies in dogs with pacing induced heart failure have shown that myocardial protein levels of
RXRα (but not PPARα) are reduced in advanced heart failure (108). Species variation and/or differences in heart failure etiology may explain, in part, the discrepancies in myocardial PPARα and RXRα protein expression observed in these studies. It remains unclear, however, if the down-regulation of PPARα and/or RXRα and fatty acid
16 oxidation enzymes occurs early in the development of heart failure, or if it is observed only in late-stage heart failure.
1.7. Substrate Selection and Contractile Function in Heart Failure
Clinical and animal studies show that acutely switching substrate oxidation in HF can increase LV power and efficiency (3; 17; 25; 41; 123; 125; 126; 143). The efficiency of the heart is dependent upon the relative contribution of fatty acid and carbohydrate oxidation to ATP production. For a given uptake of oxygen, the contractile performance of the heart is greater when the heart is oxidizing more glucose and lactate and less fatty acids (12; 80; 100; 101; 135). In isolated rat hearts, for example, the mechanical power of the left ventricle was shown to be greater at a given rate of oxygen consumption when glucose rather than fatty acids were the sole exogenous substrate (12). In closed-chest dogs, an increased rate of myocardial fatty acid uptake resulted in a 26% increase in oxygen consumption without a change in LV power output (100; 101), and elevations in plasma free fatty acids resulted in similar decreases in cardiac efficiency in healthy humans and pigs (80; 135). Furthermore, administration of ranolazine, a partial fatty acid oxidation inhibitor, to dogs with heart failure improved mechanical efficiency of the left ventricle, suggesting that pharmacologic manipulation of myocardial metabolism may be a viable option for the treatment of heart failure (17).
The mechanisms for reduced cardiac efficiency with high fatty acid oxidation are unclear; however, fatty acid oxidation requires more oxygen for a given rate of ATP synthesis than do carbohydrates (140). Additionally, fatty acid moieties are capable of uncoupling oxidative phosphorylation, resulting in the “wasting” of oxygen by the
17 mitochondria (8; 116). Thus, a greater oxygen uptake for a given rate of ATP formation
would be required when fatty acids are the substrate for oxidative phosphorylation (8;
116). Furthermore, long-chain fatty acids have been shown to activate sarcolemmal Ca2+
channels of cultured myocytes and thus increase the uptake of Ca2+ into the cytosol (58).
This resulting in “calcium overload” may result in an increase in the rate of ATP
hydrolysis required to maintain normal cytosolic Ca2+ cycling (58).
It has also been suggested that fatty acids reduce cardiac efficiency by wasting
ATP (and, thus, oxygen) through the shuttling of long chain fatty acids out of the
mitochondria by UCP3 (56; 132). Once inside the mitochondria, excess long-chain fatty acyl-CoA can be converted to a free fatty acid anion (FFA-) and a free CoA by the action
of MTE-1 (49) (Figure 1-1). The FFA- is then transported out of the mitochondria into
the cytosol by UCP3, where it can be reesterified by FACS to long-chain fatty acyl-CoA
in a process that requires 2 ATP (56; 61). The reesterified long-chain fatty acid may then
move back into the mitochondria, resulting in a futile cycle and the wasting of ATP (42;
67). Furthermore, once in the intramembraneous space, the FFA- can associate with a
proton and move back into the mitochondrial matrix and relinquish the proton (42; 67).
The net result of this process is a leak of protons, as is seen in classic uncoupling, but
with no net flux of fatty acids and a reduction of the P:O ratio (42; 67).
1.8. Rationale and Hypotheses
In the late stages of heart failure, there is a metabolic switch away from fatty acid
oxidation to carbohydrate oxidation in the heart, and this switch occurs concomitantly
with a down-regulation of the fatty acid metabolic pathway. As summarized in the
sections above, expression of the enzymes and proteins of the fatty acid oxidation
18 metabolic pathway are under the transcriptional control of the nuclear receptors PPARα and RXRα. It remains unclear, however, when in the progression of HF the down- regulation of the fatty acid metabolic pathway occurs, and how this change is precipitated. Furthermore, it is unknown if this metabolic switch has functional consequences on the failing myocardium. In addition, the role of myocardial ceramide and triglyceride accumulation on LV dysfunction and dilation in HF has not been established. Accordingly, the goal of the research described in this dissertation was to examine 1) the time course of fatty acid metabolic pathway down-regulation in HF and 2) determine the effects of fatty acid metabolic pathway up-regulation and the role of ceramide and triglyceride accumulation on LV dysfunction and dilation in HF. The major end points of this work are summarized in Figure 1-3.
The following questions were addressed:
1. What is the time course of fatty acid metabolic pathway down-regulation in HF?
Are decreases in mRNA matched by decreased protein expression, and are the
decreases in mRNA expression related to PPARα and RXRα protein content?
In advanced heart failure, the myocardium down-regulates mRNA and protein expression of fatty acid oxidation enzymes and switches to greater carbohydrate oxidation (120; 127). Although the mechanisms responsible for the down-regulation of the fatty acid oxidation pathway in advanced heart failure are not clear, recent studies
19
END POINTS Ligation R x “Lipotoxicity”
?
Healthy Heart Failure Cardiac triglyercide & ceramide content
LV Systolic PPAR/RXR
Function Dilation/Hypertrophy Activity FAO Pathway
-EDP -End Systolic/Diastolic -PPAR/RXR -FAO protein -+dP/dt Dimensions protein levels expression -Fractional -Right ventricular -FAO mRNA -MCAD shortening mass/ Body mass ratio -MCAD -CPT-1 -Cardiac Index -CPT-I -PDK-4 (cardiac output) -PDK-4 -MTE-1 MCAD Activity -UCP3
Figure 1-3: Major end points determined in this thesis. Abbreviations: Rx, experimental treatment; EDP, end diastolic pressure; PPAR, peroxisome proliferatior- activated receptor; RXR, retinoid X receptor; FAO, fatty acid oxidation enzyme; MCAD, medium acyl-CoA dehydrogenase; CPT-I, carnitine palmitoyltransferase I; PDK-4, pyruvate dehydrogenase kinase-4; MTE-1, mitochondrial thioesterase I; and UCP-3, uncoupling protein 3.
20 suggest that decreased expression of the nuclear transcription factors peroxisome proliferator activated receptor α (PPARα) and the retinoid X receptor α (RXRα) may play a key role in the myocardial substrate switch observed under these conditions (63). It remains unclear, however, if the down-regulation of PPARα and/or RXRα and fatty acid oxidation enzymes occurs early in the development of HF, or if it is observed only as a late stage phenomenon. Thus, my goal was to test the hypothesis that mRNA and protein levels of PPARα, RXRα and the mRNA and protein expression of key fatty acid oxidation enzymes would progressively decrease in the failing heart.
2. Does up-regulation of the fatty acid metabolic pathway result in increased
progression of cardiac dysfunction and dilation in HF?
PPARα over-expressing mice show up-regulation of the proteins involved in fatty acid metabolism, increased myocardial fatty acid uptake, and development of LV dysfunction and dilation (37; 38). Additional work has shown that administration of a
PPARα agonist during the initial 10 days of aortic constriction-induced pressure overload up-regulates the fatty acid metabolic pathway and worsens LV dysfunction (168).
Similarly, PPARα activation by fenofibrate leads to increased heart failure markers
(ANF, BNP and TNFα) and decreased cardiac function in hLpLGPI transgenic mice that over-express human lipoprotein lipase (155). The effect of up-regulation of the fatty acid metabolic pathway on the progression of LV dysfunction and dilation in HF, however, has not been explored. Thus, my goal was to determine the consequence of long term up-
21 regulation of the myocardial fatty acid metabolic pathway through PPARα activation or feeding a lipid enriched diet on cardiac function and dilation in HF.
3. Does up-regulation of the fatty acid metabolic pathway result in increased
myocardial triglyceride and/or ceramide content in established HF?
Myocardial accumulation of ceramide and triglyceride is associated with cardiac dilatation and reduced contractility and has been proposed to be a potential causative factor in the progression of heart failure (21; 38; 173). Furthermore, over-expression of
PPARα in transgenic mice increases myocardial ceramide content and causes left ventricular remodeling and dysfunction. Thus, it was my goal to test the hypothesis that activation of the fatty acid metabolic pathway with either a direct PPARα agonist or high fat feeding would increase myocardial triglyceride and/or ceramide, and exacerbate LV dysfunction and dilation in established HF.
The work conducted in this thesis was done using the well established rat infarct model of heart failure. The rat infarct model was chosen because it recapitulates much of the pathophysiology of LV dysfunction after myocardial infarction in humans (113; 114) and is useful in the evaluation of experimental therapies for heart failure (112; 115).
While it is possible to measure systolic and diastolic LV function ex vivo in this model, this approach does not allow for the serial measurements required by this study. Thus, there was a need for a noninvasive method to reliably and repeatedly assess in vivo LV dysfunction in the infarcted rat. To meet these needs, we developed a novel 13-segment
22 wall motion score index (WMSI) and validated this index as well as the Doppler myocardial performance index (MPI) in the post-infarction rat model of heart failure.
23 Chapter 2
Validation of Echocardiographic Methods for Assessing Left
Ventricular Dysfunction in Rats with Myocardial Infarction
2.1. Introduction
The rat infarct model has been used extensively because it recapitulates much of
the pathophysiology of left ventricular (LV) dysfunction after myocardial infarction in humans (113; 114) and is useful in the evaluation of experimental therapies for heart failure (112; 115). While coronary artery ligation is generally an effective means for inducing LV dysfunction, infarcted rat hearts demonstrate a wide range of LV dilation and contractile dysfunction (124). Because of the high variability in LV remodeling and contractile abnormalities following coronary artery ligation, it is important to have a reliable method to assess LV dysfunction in this model. Although systolic and diastolic
LV function can be accurately assessed ex vivo, this approach does not allow for serial measurements. Thus, there is a need for diagnostic tools that can reliably, repeatedly, and non-invasively assess in vivo LV dysfunction in the infarcted rat.
In humans, M-mode and 2-dimensional (2-D) echocardiography accurately describe LV dimensions, geometry and systolic and diastolic function, and aortic Doppler waveform analysis can estimate LV stroke volume and assess LV diastolic performance
(35). While many clinical echocardiographic measures have proven useful in detecting
LV dysfunction in rats (2; 13; 92; 117; 130; 136; 137), two important echocardiographic
24 markers of LV function, the Doppler myocardial performance index (MPI) and the wall
motion score index (WMSI) have not been validated for the quantification of LV
dysfunction in the rat infarct model. The objective of this study was to validate a novel
13-segment WMSI and the MPI in the post-infarction rat model of heart failure.
2.2. Materials and Methods
2.2.1. Study Design and Induction of Myocardial Infarction
This study was conducted in accordance with the Guide for the Care and Use of
Laboratory Animals (NIH publication Number 85-23) and the Institutional Animal Care
and Use Committee at Case Western Reserve University. All echocardiographic studies,
left ventricular pressure studies and data interpretation were conducted in a blinded
fashion.
Twenty-nine 8 week old male Wistar-Kyoto rats weighing ~350 g were
anesthetized with 1.5 – 2.0% isoflurane, intubated and ventilated on a Harvard ventilator.
In 22 rats an infarct was induced by ligation of the left coronary artery as previously
described (2). The ribs were then approximated, the lungs inflated and the chest closed.
Sham animals (n=7) were subjected to the same surgical procedure without coronary
artery ligation. Echocardiography was performed 8 weeks later under anesthesia (1.5 –
2.0% isoflurane by mask) as described below. Two days later, the rats were anesthetized, intubated and ventilated for recording of LV pressures as described below.
25 2.2.2. Echocardiography
LV function was evaluated by echocardiography using a Sequoia C256 System
(Siemens Medical) with a 15 MHz linear array transducer. The rats were anesthetized
with 1.5-2.0% isoflurane by mask, the chest was shaved, the animal was situated in the
supine position on a warming pad (Deltaphase), and ECG limb electrodes were placed.
2-D guided M-mode, 2-dimensional, and Doppler echocardiographic studies of aortic and
transmitral flows were performed from parasternal and foreshortened apical windows.
Study duration was typically 15-20 minutes per animal.
All data were analyzed offline with software resident on the ultrasound system at
the end of the study. LV wall motion was analyzed with a 13 segment model. The wall
segments were visualized from 2-D images taken from the parasternal long axis and from
the basal and mid-papillary short axes (Figure 2-1). Regional wall motion was graded in each segment according to the scheme adopted by the American Society of
Echocardiography where 1= normal, 2= hypokinetic, 3= akinetic, 4= dyskinetic and 5= aneurysmal (5). Motion in the anteroseptal and posterior wall segments was scored from the clearer of either the parasternal long or short axis, but not both. WMSI was defined as the total of the wall motion scores divided by the number of segments scored.
LV end diastolic (LVDa) and end systolic (LVSa) areas were planimetered from the parasternal long-axis, and area fractional shortening (FSa) was calculated according
to the following formula:
FSa = [(LVDa – LVSa)/ LVDa] X 100%
26
Figure 2-1. A. Parasternal long axis view of LV. The LV wall is divided into 5 segments as shown: BAS: Basal Anteroseptal; MAS: Mid Anteroseptal; APX: apex; MP:
Mid Posterior; BP: Basal Posterior. B. Mid-papillary parasternal short axis view of LV.
The LV wall is divided into 6 segments as shown: MAS: Mid Anteroseptal; MA: Mid
Anterior; ML: Mid Lateral; MP: Mid Posterior; MI: Mid Inferior; MS: Mid Septal. RV denotes the right ventricular space. The basal parasternal short axis view is similarly divided (not shown).
27 LV end diastolic (LVDd) and end systolic (LVSd) diameters were determined from the short axis view at the mid-papillary level, and LV fractional shortening (FS) was calculated according to the following equation:
FS = [(LVDd – LVSd)/ LVDd] X 100%
MPI and cardiac output (CO) were calculated using color-flow directed Doppler pulsed-wave tracings of mitral and aortic flow measured at the level of the LV outflow tract from the apical four-chamber view. Aortic outflow and mitral inflow waveforms were recorded when the mitral and aortic flows were distinct and both aortic and mitral valve clicks were clearly visible. Ejection time and the isovolumic contraction and relaxation times were calculated from three consecutive beats and averaged to calculate
MPI. MPI was defined as the sum of the isovolumic contraction and relaxation times divided by the ejection time (Figure 2-2).
Cardiac output (CO) was estimated using the equation:
CO= VTI x π x (aortic diameter/2)2 x HR where VTI is the velocity time integral of aortic flow, HR is heart rate, and the aortic diameter was measured from the parasternal long axis 2-D view. Cardiac Index (CI) was calculated by dividing the CO by the body weight.
To assess intra- and inter-observer variability of WMSI and MPI, echocardiographic data from a randomly selected subset of 10 animals were analyzed by two independent observers under the same conditions. One scorer analyzed the data
28
Figure 2-2. Doppler color directed pulsed-wave recording of mitral and aortic flow for the determination of the MPI index. MPI is calculated as (b-a)/a.
29 twice on separate days to evaluate intra-observer differences. Intra- and inter-observer
variability was calculated as the differences between the two observations divided by the average of the two observations, and by linear regression analysis.
2.2.3. Hemodynamic Measurements
Two days after echocardiography, rats were intubated, ventilated, and
anesthetized (1.5-2.0% isoflurane). A 3.5 Fr microtip pressure transducer catheter
(Millar Instruments) was introduced into the LV via the right carotid artery. Heart rate
(HR), maximum LV pressure (LVP), end diastolic pressure (EDP), peak ± dP/dt, and the
time constant of isovolumic LV relaxation (Tau) were recorded using a Digi-Med® Heart
Performance Analyzer-τ ™ at 0.5 second intervals and subsequently averaged over a 30 second period.
2.2.4. Statistical Analysis
Data are expressed as group means ± S.E. Groups were compared with Student’s t-test. WMSI and MPI were correlated with measurements of LV function using the
Pearson product moment correlation as data were normally distributed. Differences were
considered significant when P< 0.05. Forward stepwise regression analyses were
performed for WMSI and MPI (dependent variables) with EDP, tau, + dP/dt, EDA, FSa,
and CI as independent variables.
30 2.3. Results
2.3.1. Hemodynamics
Coronary artery ligation resulted in a significant increase in EDP and tau, and
significant decreases in peak LVP, peak ± dP/dt, and developed pressure (Table 2.1).
There was no significant difference in HR.
2.3.2. Echocardiography
2-dimensional and 2-D guided M-mode images taken at the mid-papillary level
showed that myocardial infarction produced significant increases in end diastolic and end
systolic areas and dimensions and significant reductions in their respective percent
fractional shortening (Table 2.2). Doppler analysis of aortic outflow showed that the
velocity time integral of aortic flow and cardiac index was significantly reduced, and the
MPI was significantly increased in the infarcted animals. There was no difference in HR
between the two groups.
2.3.3. Intra- and inter-observer error
Intra- and inter-observer differences for WMSI were 10 ± 4% and 10 ± 3%, and
for MPI were 9 ± 3% and 9 ± 2%, respectively. The linear regression analyses demonstrated a high statistically significant relationship between readings (Table 2.3).
31 Table 2.1. Hemodynamic variables in sham and infarcted rats. Values are mean ± SEM. dP/dt is the first derivative of pressure.
Sham (n=7) Infarcted (n=22) P value
Heart Rate 338 ± 15 298 ± 11 0.085 (beats•min-1) Peak LVP 106 ± 7 91 ± 3 <0.05 (mmHg) Peak LVEDP 7 ± 1 13 ± 2 <0.05 (mmHg) Developed LV Pressure 99 ± 6 78 ± 3 <0.05 (mmHg) Peak LV +dP/dt 6723 ± 486 4514 ± 268 <0.001 (mmHg•sec-1) Peak LV -dP/dt -1) -4657 ± 511 -3556 ± 192 <0.05 (mmHg•sec Tau 15 ± 1 19 ± 1 <0.05
32 Table 2.2. Echocardiographic variables in sham and infarcted rats. Values are mean ±
SEM.
Sham (n=7) Infarcted (n=22) P Value
Heart Rate 226 ± 9 237 ± 10 0.557 (beats•min-1) End Diastolic Dimension 0.76 ± 0.03 0.95 ± 0.04 <0.05 (cm) End Systolic Dimension 0.37 ± 0.02 0.69 ± 0.06 <0.05 (cm) Fractional Shortening 51 ± 3 29 ± 3 < 0.001 (%) End Diastolic Area 0.72 ± 0.07 0.96 ± 0.04 <0.05 (cm2) End Systolic Area 0.27 ± 0.05 0.64 ± 0.06 <0.05 (cm2) Area Fractional 62 ± 2 36 ± 4 < 0.001 Shortening (%) Aortic Velocity Time 2.7 ± 0.2 1.7 ± 0.1 < 0.001 Integral (cm) Ejection Time 68 ± 3 69 ± 2 0.681 (msec) Isovolumic Contraction + Isovolumic Relaxation 21 ± 2 40 ± 3 <0.05 Time (msec) MPI 0.30 ± 0.02 0.58 ± 0.05 <0.05 Cardiac Index 121 ± 15 75 ± 10 <0.05 (ml·min-1·kg-1)
33 Table 2.3. Results of linear regression analysis comparing intra- and inter-investigator measurements of MPI and WMSI (n=10).
Slope Y-Intercept r r P value MPI Intra- 0.94 0.02 0.893 <0.001 investigator Inter- 1.02 0.00 0.905 <0.001 investigator WMSI Intra- 1.42 -0.45 0.913 <0.001 investigator Inter- 1.16 -0.14 0.833 <0.003 investigator
34 2.3.4. WMSI and MPI Correlations
Individual wall segment scores ranged from 1 (normal) to 3 (akinetic); no dyskinetic or aneurysmal segments were observed. WMSI ranged from 1.0 to 2.8, and was significantly and positively correlated with end diastolic pressure, tau, left ventricular dimensions and areas, and MPI (Figure 2.3, Table 2.4). WMSI was negatively correlated with peak ±dP/dt, developed pressure, fractional shortening, aortic velocity time integral, and cardiac index. There was no significant correlation between
WMSI and LV peak systolic pressure (Table 2.4).
MPI ranged from 0.24 to 0.94, and was significantly and positively correlated with end diastolic pressure, tau, and left ventricular dimensions and areas. MPI was significantly inversely correlated with peak ±dP/dt, developed pressure, fractional shortening, aortic velocity time integral, and cardiac index (Figure 2.3, Table 2.4). There was no significant correlation between MPI and LV peak systolic pressure (Table 2.4).
Forward stepwise linear regression analyses revealed that of the 6 variables tested,
EDP, FSa, +dP/dt and CI are independent determinants of WMSI (r = .994) and CI and
+dP/dt are independent determinants of MPI (r= .781) (Table 2.5).
2.4. Discussion
Wall motion scoring provides an accurate semiquantitative index of regional and global LV function and prognosis after myocardial infarction (14; 68; 70; 102), but is used infrequently in animal models. Although the American Society of Echocardiography recommends dividing the human heart into 16 segments (35), the 16 segment model cannot be applied directly to the rat heart because the small size of the heart limits the
35
Figure 2.3. Left ventricular peak +dP/dt, area fractional shortening, and cardiac index plotted as a function of the WMSI (left panels) and the MPI (right panels).
36 Table 2.4. Correlation coefficients between WMSI and MPI and left ventricular pressure measurements and echocardiographic parameters in sham and infracted rats. (n=29).
Values are mean ± SEM. dP/dt is the first derivative of pressure.
WMSI MPI
Correlation Correlation Coefficient P value Coefficient P value
Peak LVP -0.332 0.072 -0.280 0.134
Peak EDP 0.719 <0.001 0.420 <0.05
Developed Pressure -0.579 <0.001 -0.420 <0.05
Peak LV +dP/dt -0.684 <0.001 -0.488 <0.05
Peak LV -dP/dt -0.566 <0.05 -0.436 <0.05
Tau 0.680 <0.001 0.482 <0.05
End Diastolic Dimension 0.804 <0.001 0.444 <0.05
End Systolic Dimension 0.877 <0.001 0.466 <0.05
Fractional Shortening -0.791 <0.001 -0.662 <0.001
End Diastolic Area 0.675 <0.001 0.444 <0.05
End Systolic Area 0.798 <0.001 0.432 <0.05
Area of Fractional Shortening -0.850 <0.001 -0.526 <0.05
Aortic Velocity Time Integral -0.802 <0.001 -0.631 <0.001
Cardiac Index -0.741 <0.001 -0.736 <0.001
MPI 0.707 <0.001
37 Table 2.5. Results of forward stepwise linear regression analyses for WMSI and MPI
using peak +dP/dt, CI, peak EDP, area of fractional shortening, EDA and tau as independent variables. Values are listed for significant independent variables only.
dP/dt is the first derivative of pressure.
WMSI (r= 0.944) MPI (r = 0.781)
Independent Variable F-to Remove P value F-to Remove P value
Peak LV +dP/dt 13.26 0.001 4.45 < 0.05
Cardiac Index 8.71 < 0.01 24.24 < 0.001
Peak EDP 4.94 < 0.05
Area of Fractional 21.00 < 0.001 Shortening
38 number of echocardiographic windows and wall segments that can be visualized. Thus,
in order to determine WMSI in the rat, it was necessary to develop a model that depends on fewer echocardiographic windows and wall segments. In the present scoring system, the long axis is divided into 5 segments, while the short axis, like the 16 segment model, is divided into 6 segments. Multivariate analysis indicates that the 13 segment derived
WMSI is a strong predictor of pressure- and volume-derived indices of LV function.
Moreover, the close association of WMSI with numerous invasive and noninvasive indices of systolic and diastolic LV function shows it to be a powerful and important global index of cardiac function.
The Doppler derived MPI correlates with invasive measurements of LV systolic and diastolic function in patients with ischemic heart disease and idiopathic dilated cardiomyopathy (149) and has been shown to reliably indicate global LV dysfunction in patients with acute myocardial infarction (70) and aortic stenosis (105). The applicability of the MPI to small research animals was recently demonstrated by Broberg et. al. who showed that MPI strongly correlates with peak +dP/dt over a range of hemodynamic conditions produced by pharmacological and load alterations in mice (11). In addition,
MPI has been shown recently to be valuable in evaluating LV function in normal and spontaneously hypertensive rats (137), and in rats with hypertrophy induced by aortic banding (130). In the present study, we confirmed that MPI is significantly correlated with +dP/dt and other invasive and noninvasive indices of LV function and extended these findings to the chronic rat infarction model. A major advantage of MPI is that its derivation is independent of geometry, making it particularly well-suited for studying rats throughout the course of a disease associated with chamber remodeling.
39 2.5. Limitations
Several potential limitations of this study merit mention. First, unlike the 16
segment model, the 13 segment model assesses apical wall movement from the
parasternal long-axis rather than parasternal short- and apical 4-chamber views. As a result, apical wall movement is assessed in anteroseptal and posterior segments only and
therefore, portions of the septal, inferior, anterior and lateral regions of the apex are not
viewed and scored. Despite the omission of these regions, the 13 segment model clearly
functions to accurately assess LV function in the rat. Second, in one ligated rat (excluded
from the study), an irregular rhythm prevented the acquisition of mitral and aortic flow
tracings suitable for the calculation of MPI. Therefore, it may not be possible to utilize
MPI as a measure of LV function in rats with cardiac arrhythmias. Third, rats were
studied under isoflurane anesthesia; however, both the echocardiographic and
hemodynamic studies were performed with the same type and depth of anesthesia,
thereby facilitating comparisons in a similar, albeit depressed, state. Finally,
echocardiographic and hemodynamic measurements were not simultaneously acquired.
However, despite this delay, the correlations between the noninvasive and invasive
indices were strong.
2.6. Conclusion
The present study demonstrates for the first time that in the rat infarction model, a
13 segment model for WMSI and an index of myocardial performance provides accurate,
noninvasive and repeatable measurements of in vivo LV systolic and diastolic function.
These findings are of considerable interest insofar as longitudinal studies are often critical
40 in the experimental design of studies that use the rat coronary artery ligation model of heart failure.
41 Chapter 3
Effects of Coronary Artery Ligation Induced Heart Failure on
Cardiac Metabolic Enzyme Gene and Protein Expression
3.1. Introduction
Recent studies suggest that changes in cardiac substrate metabolism affect left ventricular (LV) function and remodeling in heart failure (144). The healthy adult heart uses fatty acids as the primary substrate for the generation of ATP, and during the early stages of heart failure, the myocardium continues to derive most of its energy from fatty acids in animal models and humans (110; 148). On the other hand, in advanced heart failure there is down-regulation of the mRNA and protein expression of fatty acid oxidation enzymes, and the myocardium switches to greater carbohydrate oxidation (120;
127). The time course and mechanisms responsible for the down-regulation of the fatty acid oxidation pathway in advanced heart failure are not clear.
Recent studies suggest that decreased expression of the nuclear transcription factors peroxisome proliferator activated receptor α (PPARα) and the retinoid X receptor
α (RXRα) may play a key role in the myocardial substrate switch observed under these conditions (63) . These proteins form a heterodimer and increase transcription of genes encoding key enzymes involved in fatty acid metabolism, such as medium chain acyl-
CoA dehydrogenase (MCAD), carnitine palmitoyl transferase I (CPT-I), uncoupling protein-3 (UCP3), and mitochondrial thioesterase-1 (MTE-1) and the inhibitory kinase of
42 pyruvate dehydrogenase, pyruvate dehydrogenase kinase-4 (PDK-4) (63; 146; 163; 169).
Studies in patients (69) and animals (65; 108) with advanced end stage heart failure show that there is a significant reduction of PPARα and/or RXRα mRNA and/or protein levels compared to non-failing hearts. It remains unclear, however, if the down-regulation of
PPARα and/or RXRα and fatty acid oxidation enzymes occurs early in the development of heart failure, or if it is observed only in the late-stage.
The primary goal of the present study was to test the hypothesis that the mRNA
and protein levels of PPARα, RXRα and the mRNA and protein expression of key fatty
acid oxidation enzymes would progressively decrease in the failing heart. Studies were
performed using the well established rat infarct model of heart failure (114), and
measurements of LV function, and the mRNA and protein levels of PPARα, RXRα and
enzymes involved in fatty acid metabolism were made at 8 and 20 weeks following
induction of heart failure by coronary artery ligation or sham surgery.
Elevated myocardial ceramide levels are associated with cardiac dilatation and
reduced contractility, (22; 37; 173), however it is unclear if ceramide contributes to
cardiac dysfunction in heart failure. Ceramide is a toxic lipid intermediate that can
impair mitochondrial function and trigger apoptosis, and is associated with cardiac
dysfunction and LV chamber dilation rat and mice models of increased myocardial fatty
acid metabolism (22; 37; 173). Thus, we hypothesized that heart failure would result in a
progressive increase in myocardial ceramide content compared to sham animals.
43 3.2.1. Methods
3.2.1. Study Design and Induction of Myocardial Infarction
This study was conducted in accordance with the Guide for the Care and Use of
Laboratory Animals (NIH publication Number 85-23) and the Institutional Animal Care and Use Committee at Case Western Reserve University. All cardiovascular and biochemical measurements were conducted in an investigator blinded fashion. Animals were maintained on a reverse 12h:12h light:dark cycle (i.e. lights off at 7AM), and all procedures and tissue harvests were performed in the fed state between 3 and 6 hours into the dark phase of the cycle. It has been established that the myocardial content of mRNA for metabolic genes plateau during this time period (147).
Heart failure was induced by myocardial infarction as previously described (104).
Briefly, eight week old male Wistar-Kyoto rats weighing ~250 g were anesthetized with
1.5 – 2.0% isoflurane, intubated and ventilated. An infarct was induced by ligation of the left coronary artery, and sham animals were subjected to the same surgical procedure without coronary artery ligation. Sham and ligated rats were randomly assigned to either an 8 week (11 sham and 11 ligated) or 20 week post-infarction (10 sham and 10 ligated) study group, at which times echocardiography was performed and LV pressures were measured as described below.
3.2.2. Echocardiography
LV function was evaluated by echocardiography using a Sequoia C256 System
(Siemens Medical) with a 15 MHz linear array transducer as previously described (104).
Briefly, at either 8 or 20 weeks post surgery, rats were anesthetized with 1.5-2.0%
44 isoflurane by mask, the chest was shaved, the animal was situated in the supine position on a warming pad, and ECG limb electrodes were placed. Two-dimensional (2D), 2D- guided M-mode, and Doppler echocardiographic studies of aortic and transmitral flows were performed from parasternal and foreshortened apical windows. End diastolic and end systolic areas (EDA and ESA) were measured using software resident on the ultrasonograph, and the relative wall motion score index, fractional area shortening, myocardial performance index (MPI), and cardiac index (CI) were calculated as previously described (104).
3.2.3. Hemodynamic Measurements
At either 8 or 20 weeks post surgery, rats were intubated, ventilated, and anesthetized (1.5-2.0% isoflurane) as previously described (104). A 3.5 Fr microtip pressure transducer catheter (Millar Instruments) was introduced into the LV via the right carotid artery. Heart rate (HR), maximum LV pressure, peak end diastolic pressure
(EDP), peak ± dP/dt, and the time constant of isovolumic LV relaxation (Tau, the time constant in milliseconds of the best-fit monoexponential pressure decay from the pressure at maximum –dP/dt to a positive pressure above the previous LV end diastolic pressure by 10 mmHg or a duration of 20 ms, whichever happens first) were recorded using a
Digi-Med® Heart Performance Analyzer-τ™ over a 30 s period. Following the measurement of LV pressure, the heart was excised, the scar tissue was visually identified and removed, and the non-infarcted LV tissue was frozen in liquid nitrogen and stored at -80 C for later biochemical analysis.
45 3.2.4. RNA Extraction and Quantitative RT-PCR
RNA extraction and quantitative RT-PCR were performed on frozen powdered
LV tissue (isolated either 8 or 20 weeks post surgery) using previously described
methods (23; 45; 55). Specific quantitative assays were designed from rat sequences
available in GenBank for expression of PPARα, RXRα, citrate synthase (CS), uncoupling
protein 2 (UCP2), atrial natriuretic peptide (ANP) and genes that are known to be
regulated by PPARα: MCAD, CPT-I, PDK-4, UCP3, and MTE-1 (9; 146; 147; 162; 169-
171). Standard RNA was made for all assays by the T7 polymerase method (Ambion),
using total RNA isolated from rat hearts. The correlation between the Ct (the number of
PCR cycles required for the fluorescent signal to reach a detection threshold) and the
amount of standard was linear over at least a 5-log range of RNA for all assays. To control for sample-to-sample differences in RNA concentration, the mRNA level for cyclophilin was quantitatively measured in each sample. Expression of cyclophilin was not different among the experimental groups; therefore, the PCR data are reported as the number of transcripts per number of cyclophilin molecules.
3.2.5. Western Immunoblot Analysis
Protein was extracted from frozen powdered LV tissue as previously described
(108). Either 50 or 75 micrograms of total protein was separated by electrophoresis in
10% SDS-PAGE gels and transferred onto a PVDF membrane. Membranes were incubated with specific antibodies to PPARα, RXRα (1:100; Santa Cruz Biotechnology,
Inc.), or MCAD (1:2000; Caymen Chemical). After conjugation with the secondary
antibody, the membranes were developed in a chemiluminescence substrate solution
46 (Santa Cruz Biotechnology, Inc.), and bands were quantified using commercially
available software. All samples were run in duplicate and normalized to a standard sample (normal male Wistar-Kyoto rat) that was loaded in duplicate on each gel.
3.2.6. Metabolites and Enzyme Activities
Plasma free fatty acids and triglycerides were determined using spectrophotometric assays, and the maximal activities of MCAD and CS were measured in frozen powdered LV tissue (isolated either 8 or 20 weeks post surgery), as previously described (87; 108; 139). C16-ceramide content in non-infarcted LV tissue was measured by a capillary gas chromatographic procedure with a flame ionization detector using C17-ceramide as an internal standard, as previously described (150).
3.2.7. Statistical Analysis
Data are expressed as group means ± S.E. Groups were compared by two-way
ANOVA followed by Bonferroni t-test for multiple comparisons. For all statistical analysis, significance was accepted at P<0.05.
3.3. Results
3.3.1. Body and Heart Mass
Body mass increased significantly from 8 to 20 weeks in both groups, with no differences between sham and heart failure groups (Table 3-1). Total LV mass (LV mass
+ scar tissue mass) was significantly higher in the 20 week groups compared to the 8 week groups, and there were no differences between sham and heart failure groups. The
47 mean scar tissue mass was the same for the 8 and 20 week heart failure groups. The LV
mass/body mass ratio was lower in the 20 week sham group compared to the 8 week
shams and was higher in the 20 week heart failure group compared to the 20 week shams.
There was no difference in the LV mass/body mass ratio between the 8 and 20 week heart
failure groups. RV mass and the RV mass/body mass ratio were both significantly higher
in the heart failure animals compared to shams at both 8 and 20 weeks (Table 3-1).
3.3.2. Cardiac Function
Rats with heart failure had reduced LV fractional area of shortening and greater
LV end diastolic and end systolic cross sectional areas than sham animals at both 8 and
20 weeks (Figure 3-1, Table 3-2). There was a significant increase in LV end systolic
area and a decrease in fractional area of shortening from 8 to 20 weeks, suggesting a
progressive deterioration in LV systolic function (Figure 3-1, Table 3-2). The wall
motion score index, myocardial performance index and cardiac index were significantly
reduced in the heart failure animals compared to sham at 8 and 20 weeks, with no further
change from 8 to 20 weeks (Table 3-2).
The LV dysfunction that resulted from coronary ligation was not accompanied by changes in heart rate or peak LV pressure at 8 or 20 weeks (Table 3-2). LV EDP and Tau were elevated in the heart failure groups at both 8 and 20 week. Peak +dP/dt was significantly reduced in the heart failure rats, and -dP/dt tended to be reduced but the difference did not reach statistical significance (P=0.064). There were no differences in
LV pressure data between the 8 and 20 week heart failure groups.
48 Table 3-1. Body and heart mass in sham and heart failure rats.
SHAM Heart Failure 8 Week 20 Week 8 Week 20 Week
Body mass 501 ± 21 643 ± 24*g 519 ± 15 591 ± 42* (g) Total LV mass 1.12 ± 0.04 1.32 ± 0.04*g 1.14 ± 0.04 1.33± 0.08*‡ (g) Scar mass - - 0.21 ± 0.02 0.21 ± 0.03 (g) LV mass/body 2.29 ± 0.10 2.06 ± 0.06g 2.16 ± 0.07 2.29 ± 0.10** mass (mg/g) RV mass (g) 0.30 ± 0.02 0.32 ± 01 0.41 ± 0.03₤g 0.48 ± 0.06₤** RV mass/body 0.60 ± 0.04 0.50 ± 0.02 0.82 ± 0.06 g 0.83 ± 0.12 ** mass (mg/g) ₤ ₤
Values are mean ± SEM. *P<0.001 compared to 8 week group; gP<0.05 compared to 8 week Sham; ‡P<0.05 compared to 8 week heart failure; **P<0.05 compared to 20 week
Sham; ₤ P< 0.001 compared to Sham group.
49 80 Sham Heart Failure P<0.05
60 P<0.05
40 *
*
20
Fractional Area of Shortening (%)
0 8wk 20wk 8wk 20wk
40 Sham Heart Failure
P<0.05 * 30
20 *
10 ANP mRNA
0 (molecules/molecule cyclophilin mRNA) 8wk 20wk 8wk 20wk
Figure 3-1. Upper Panel: Percent fractional area of shortening for 8 and 20 week sham and heart failure rats. Lower Panel: ANP mRNA expression in 8 and 20 week sham and heart failure rats normalized to cyclophilin. *P<0.05 compared to respective 8 and 20 week shams.
50 Table 3-2. Left ventricular pressures and heart rates and echocardiographic data in sham and heart failure rats.
SHAM Heart Failure 8 Week 20 Week 8 Week 20 Week
Heart Rate (beats•min-1) 337 ± 13 319 ± 18 330 ± 13 318 ± 17 Peak LV Pressure (mmHg) 106 ± 5 112 ± 7 97 ± 6 107 ± 6 Peak LV End Diastolic Pressure 8 ± 1 9 ± 1 10 ± 1* 15 ± 3*g (mmHg) Peak LV +dP/dt (mmHg•sec-1) 6510 ± 380 6850 ± 620 5150 ± 440* 5710 ±640* Peak LV -dP/dt (mmHg•sec-1) -4800 ± 370 -5170 ± 760 -3890 ± 390 -4230 ± 460
Tau (mSec) 15 ± 1 14 ± 1 18 ± 1* 19 ± 2*g LV End Diastolic Area (cm2) 0.72 ± 0.06 0.83 ± 0.08 0.98 ± 0.08* 1.21 ± 0.17*g LV End Systolic Area (cm2) 0.24 ± 0.04 0.35 ± 0.05g 0.63 ± 0.06*‡ 0.90 ± 0.16*g₤ Wall Motion Score Index 1.0 ± 0.0 1.0 ± 0.0 2.1 ± 0.1*‡ 2.3 ± 0.1*g Myocardial Performance 0.31 ± 0.02 0.33 ± 0.03 0.66 ± 0.06*‡ 0.60 ± 0.05*g Index Cardiac Index 111 ± 13 118 ± 11 63 ± 9*‡ 64 ± 8*g (ml·min-1·kg-1)
Values are mean ± SEM. *P<0.05 compared to Sham group (main effect for treatment); gP<0.05 compared to 20 week Sham; ‡P<0.05 compared to 8 week Sham; ₤P< 0.05 compared to 8 week heart failure.
51 3.3.3. mRNA Expression
mRNA expression was normalized to the gene cyclophilin, which did not differ between groups (data not shown). ANP was elevated approximately 200% and 580% in the 8 and 20 week heart failure animals, respectively, compared to shams (Figure 3-1).
There was a significant increase in ANP expression from 8 to 20 weeks in the heart failure groups (Figure 3-1).
Expression of PPARα, RXRα, CS and genes that are known to be regulated by
PPARα (MCAD, CPT-I, PDK-4, UCP3, and MTE-1) were significantly reduced by heart failure, but were not different between the 8 and 20 week time points (Table 3-3, Figures
3-2 and 3-3). UCP2 gene expression was not altered following infarction.
3.3.4. Enzyme Activity and Protein Expression
Protein expression for PPAR-α, RXR-α, and MCAD was not different among groups (Table 3-4, Figure 3-5). CS activity was significantly reduced in the heart failure animals compared to shams. In contrast, MCAD activity (either normalized to g wet weight tissue or normalized to CS activity) was not different between the groups (Table
3-5). The ~15% reduction in CS activity and the lack of a change in the ratio of
MCAD/CS suggests a modest reduction in mitochondrial content with heart failure.
3.3.5. Metabolite Levels
Plasma free fatty acids were increased in the 20 week animals compared to the 8 week groups, but there were no differences between sham and heart failure groups (Table
3-6). Plasma triglyceride content did not differ among the groups (Table 3-6). Tissue
52 Table 3-3. mRNA expression in sham and heart failure rats normalized to cyclophilin
(mole mRNA/mole cyclophilin mRNA).
SHAM Heart Failure
8 Week 20 Week 8 Week 20 Week (n=9) (n=9) (n=9) (n=9) PPAR-α 0.234 ± 0.063g 0.205 ± 0.021 0.137 ± 0.014* 0.159 ± 0.010* RXR-α 0.293 ± 0.044 0.291 ± 0.031 0.212 ± 0.016* 0.206 ± 0.035*‡ CS 1.93 ± 0.17 2.21 ± 0.23 1.72 ± 0.17* 1.60 ± 0.13*‡ UCP2 6.57 ± 0.95 7.32 ± 1.07 6.75 ± 1.24 6.50 ± 1.09 MCAD 8.26 ± 0.69 9.65 ± 1.52 5.98 ± 0.77* 5.48± 0.53*‡ CPT I 1.83 ± 0.08 1.90 ± 0.14 1.63 ± 0.09* 1.47 ± 0.15*‡ PDK4 2.46 ± 0.44 2.51 ± 0.23 2.07 ± 0.47* 1.40 ± 0.19*‡ UCP3 0.029 ± 0.004 0.034 ± 0.005 0.024 ± 0.003* 0.018 ± 0.004*‡ MTE1 0.792 ± 0.114 0.847 ± 0.161 0.634 ± 0.111* 0.450 ± 0.064*‡
Values are mean ± SEM. *P< 0.05 compared to Sham group; gP<0.05 compared to 8 week heart failure; ‡P<0.001 compared to 20 week Sham.
53
160 8 WK Sham 8 WK HF 20 WK Sham 20 WK HF
120
* * * * 80 * *
40 % of% 20 Week Sham
0 PPAR RXR CS UCP2
Figure 3-2. mRNA expression of non-PPARα regulated genes in 8 and 20 week sham and heart failure (HF) rats expressed as percent of 20 week sham. *P<0.05 compared to
Sham group (main effect for treatment).
54
8 WK Sham 20 WK Sham 8 WK HF 20 WK HF 120
100 * * * * 80 * * * * * 60 *
40
% of 20 Week% of Sham 20
0 MCAD CPT-I PDK4 UCP3 MTE-1
Figure 3-3. mRNA expression of PPARα regulated genes in 8 and 20 week sham and heart failure (HF) rats expressed as percent of 20 week sham. *P<0.05 compared to
Sham group (main effect for treatment).
55
Table 3-4. Protein expression in sham and heart failure rats at 20 weeks.
Sham Heart Failure
PPAR-α 70 ± 15 76 ± 20
RXR-α 103 ± 14 113 ± 14
MCAD 94 ± 11 110 ± 15
Values are percent of standard, expressed as mean ± SEM.
Table 3-5. Medium chain acyl-CoA dehydrogenase (MCAD) and citrate synthase (CS) activities in sham and heart failure rats.
SHAM Heart Failure 8 Week 20 Week 8 Week 20 Week
MCAD 9.26 ± 1.04 8.20 ± 0.48 7.44 ± 0.49 7.78 ± 0.68 (µmols•gww-1•min-1) CS 135 ± 9 130 ± 8 117 ± 4* 116 ± 6* (µmols•gww-1•min-1) MCAD/CS 0.067±0.004 0.063±0.002 0.063±0.003 0.066±0.003
Values are mean ± SEM. *P< 0.05 compared to Sham.
56 Table 3-6. Plasma fatty acid and triglyceride concentrations in sham and heart failure rats.
SHAM Heart Failure 8 Week 20 Week 8 Week 20 Week
Plasma Free Fatty Acids (µmol/mL) 0.30 ± 0.04 0.41 ± 0.07* 0.20 ± 0.02 0.33 ± 0.05*
Plasma Triglycerides 0.74 ± 0.05 0.93 ± 0.06 0.85 ± 0.07 0.90 ± 0.08 (mg/mL)
Values are mean ± SEM. *P=0.004 compared to 8 week group.
57
Sham Heart Failure
P<0.05 2.5 P<0.05
2.0
1.5
1.0
(nmol/gww) 0.5
Tissue Ceramide 0
8wk 20wk 8wk 20wk
Figure 3-4. Myocardial C16-ceramide content (nmol/gww) in 8 and 20 week sham and heart failure rats.
58 Sham Heart Failure
PPARα
RXRα
MCAD
1.5 20 week Sham 20 week Heart Failure
1.0
0.5 Fraction of 20 week Sham
0.0 PPARα RXRα MCAD
Figure 3-5. Top: Western blots from representative gel of a sham and heart failure rat, each run in duplicate. Bottom: Mean data for blot intensity expressed as a fraction of the mean of the sham group. All samples were run in duplicate and gels were normalized to a standard sample of normal rat myocardium run in duplicate on each gel.
59 levels of C16-ceramide were not elevated in the 8 week heart failure group compared to sham, but were significantly higher in the 20 week heart failure group compared to the 8 week heart failure animals and the 20 week shams (Figure 3-4).
3.4. Discussion
The present study showed that there were no alterations in PPARα, RXRα or
MCAD protein levels in rats with infarct induced heart failure. There was a clear down- regulation of the mRNA for the nuclear receptors PPARα and RXRα, and for PPARα regulated genes; howeve,r this was not translated into reduced protein expression.
Despite a progressive deterioration in fractional area of shortening and increases in LV end systolic area and ANP mRNA from 8 to 20 weeks post-infarction, there was not a corresponding fall in the expression of genes encoding proteins involved in fatty acid metabolism. A modest but significant ~15% decrease in CS activity at both the 8 and 20 week post-infarct time points suggest that there was a reduction in mitochondrial content with heart failure without a selective down-regulation of fatty acid oxidation enzymes.
These results are consistent with the concept that down-regulation of fatty acid oxidation enzymes is a late stage occurrence in heart failure.
Previous work in patients (69; 120; 127) and animals (65; 87; 124) found reduced mRNA expression for fatty acid oxidation enzymes, PPARα and/or RXRα in severe late- stage heart failure; however, little is know about the time course of this effect over the progression of failure. In studies utilizing the rat coronary artery ligation model of heart failure, mRNA levels of PPARα, CPT-I and of medium chain, long chain and very long chain acyl-CoA dehydrogenase were shown to be reduced at 20 or 24 weeks post ligation
60 (65; 124). The findings of the present study agree well with the previously observed
changes in mRNA in late stage heart failure. Furthermore, we find that repression of these genes occurs eight weeks following the induction of heart failure and is not associated with reduced PPARα or RXRα protein expression.
In contrast with previous studies in patients and animals with late stage heart failure, the results of this investigation demonstrate that protein levels and activity of
MCAD were not reduced despite down-regulation of the mRNA. Sack et al. showed that protein levels of both MCAD and long chain acyl-CoA dehydrogenase are reduced in patients with end stage heart failure (127). Myocardial protein levels of MCAD, RXRα
(but not PPARα) and MCAD activity are reduced in end stage pacing induced heart
failure in dogs (108). Rosenblatt-Velin et al. reported a significant decrease in MCAD protein content in a subset of coronary artery ligated rat hearts isolated 20 weeks after ligation (124). For inclusion in this group, however, the rats had to exhibit 5 signs of severe heart failure (assessed by the Feldman criteria for heart failure); of the sixteen rats ligated, only six met this requirement (124). In the present study, MCAD protein content and activity were determined from LV tissue taken from all infarcted rat hearts, and this difference in inclusion criteria may explain the disparity between the MCAD protein results of this study and those reported previously.
Elevated myocardial ceramide levels are associated with cardiac dilatation and reduced contractility, (22; 37; 173), however it is unclear if ceramide contributes to cardiac dysfunction in heart failure. In this study there was no increase in myocardial
C16-ceramide levels 8 weeks after coronary artery ligation, despite significantly reduced
LV function and chamber dilation. At 20 weeks post ligation, however, tissue levels of
61 ceramide increased by 40% in heart failure rats relative to shams (Figure 3-4), which corresponded with a significant increase in LV end systolic area and a decrease in fractional area of shortening from 8 to 20 weeks post infarction (Table 3-2). While it is intriguing to speculate that greater ceramide accumulation may contribute to progressive cardiac dysfunction in heart failure, a causal link remains to be demonstrated.
In summary, there is mRNA down-regulation of PPARα regulated genes in rats with infarct-induced heart failure despite no decrease in the protein expression of PPARα or RXRα. Although there was a progressive increase in LV end systolic area and ANP mRNA expression and deterioration in fractional area of shortening, there were no differences in mRNA levels for metabolic proteins between 8 and 20 weeks postinfarction. In addition, MCAD protein levels and activity were not altered despite downregulation of MCAD mRNA. Thus, reduced mRNA expression of PPARα/RXRα regulated fatty acid oxidation enzymes is not dependent on reduced protein expression of
PPARα and/or RXRα.
62 Chapter 4
Effects of Chronic Activation of Peroxisome Proliferator
Activated Receptor Alpha of High Fat Feeding in a Rat Infarct
Model of Heart Failure
4.1. Introduction
Accumulation of lipids and their derivatives in the heart have been associated with cardiac dysfunction, and may be a potential causative factor in the progression of heart failure (21; 22; 37; 38; 164; 173). Elevated cardiac triglyceride (TG) content has been observed in models of hyperlipidemia, diabetes and obesity (173), in transgenic mouse models in which proteins promoting fatty acid uptake are over-expressed (21; 22;
164), and in mice with down-regulated fatty acid oxidation enzymes (34; 83). The fatty acid-activated nuclear transcription factor peroxisome proliferator activated receptor α
(PPARα) is a key modulator of the expression of genes encoding proteins controlling fatty acid uptake and metabolism (40). Transgenic mice with cardiac over-expression of
PPARα have increased myocardial fatty acid uptake, triglyceride and ceramide accumulation, cardiac hypertrophy and left ventricular (LV) dilation that is exacerbated by high fat feeding (37; 38). Up-regulation of myocardial fatty acid uptake and esterification in transgenic mice has clear adverse effects on cardiac function and
63 structure (21; 22; 38; 131), however it is unclear if these finding can be translated to
clinical conditions like heart failure (145).
There is clear evidence that up-regulation of myocardial fatty acid uptake through genetic manipulations can result in “cardiac lipotoxity” (21; 22; 37; 38; 164). From a clinical perspective, however, it is important to know if relevant dietary and pharmacology manipulations (e.g. high fat feeding or activation of PPARα) have adverse effects on LV function and remodeling in established heart failure. The present study tested the hypothesis that activation of fatty acid metabolism with either a direct PPARα agonist or high fat feeding would increase myocardial triglyceride and/or ceramide content, and exacerbate LV dysfunction and remodeling in established heart failure.
Studies were conducted in the rat infarct model of heart failure (113), and treatment was initiated 8 weeks after myocardial infarction and continued for 12 additional weeks.
Since cardiac dysfunction has been associated with myocardial ceramide accumulation
(22; 173), we also assessed cardiac ceramide content.
4.2. Methods
4.2.1. Study Design and Induction of Myocardial Infarction
This study was conducted in accordance with the Guide for the Care and Use of
Laboratory Animals (NIH publication Number 85-23) and the Institutional Animal Care and Use Committee at Case Western Reserve University. All cardiovascular and biochemical measurements were conducted in an investigator blinded fashion. Data from sham operated and infarcted animals has been presented in a separate report (103).
Animals were maintained on a reverse 12h:12h light:dark cycle (i.e. lights off at 7AM),
64 and all procedures and tissue harvests were performed in the fed state between 3 and 6
hours into the dark phase of the cycle. It has been established that the myocardial content
of mRNA for metabolic genes plateau during this time period (147).
Heart failure was induced by coronary artery ligation as previously described in
detail (2). Eight week old male Wistar-Kyoto rats (~250 g) were anesthetized with
isoflurane (1.5–2.0%), intubated and ventilated. A myocardial infarction was induced by
left coronary artery ligation and sham animals underwent a similar surgical procedure
without arterial ligation. Eight weeks after surgery, LV function was evaluated by
echocardiography (described below), and the infarcted rats were assigned to either no-
treatment (INF, n=10), treatment with a high-fat diet (Research Diets, Inc., 45% of
calories from fat [37% saturated 46% mono- and 19% polyunsaturated fatty acids]; INF +
Fat, n=15) or treatment with the PPARα agonist fenofibrate (150 mg·kg-1·day-1, , INF +
Feno, n=13, milled into the food based on previously recorded average daily food intake).
Animals were assigned so that the initial level of LV dysfunction (determined by the wall motion score index as previously described (104) was the same in each group. After 12 weeks of treatment with either high fat diet or fenofibrate (20 week post surgery), echocardiography was again performed and LV pressures were measured as described below.
4.2.2. Echocardiography
LV function was evaluated by echocardiography using a Sequoia C256 System
(Siemens Medical) with a 15 MHz linear array transducer as previously described (104).
Briefly, rats were anesthetized with 1.5-2.0% isoflurane by mask, the chest was shaved,
65 the animal was situated in the supine position on a warming pad, and ECG limb electrodes were placed. Two-dimensional (2D), 2D-guided M-mode, and Doppler echocardiographic studies of aortic and transmitral flows were performed from parasternal and foreshortened apical windows. End-diastolic and end systolic dimensions
(EDD and ESD) were measured using software resident on the ultrasonograph, and the relative wall motion score index, fractional shortening (FS), myocardial performance index (MPI), and cardiac index (CI) were calculated as previously described (104).
4.2.3. Hemodynamic Measurements
After 12 weeks of treatment (20 weeks post surgery), rats were anesthetized (1.5-
2.0% isoflurane), intubated, and ventilated as previously described (104). A microtip pressure transducer catheter (3.5 Fr, Millar Instruments) was introduced via the right carotid artery into the LV. Measurements of heart rate (HR), maximum LV pressure,
peak end diastolic pressure (EDP), peak positive and negative dP/dt, and tau, the time
constant of isovolumic LV relaxation (previously described (103)) were recorded using a
Digi-Med® Heart Performance Analyzer-τ ™ over a 30 s period.
66 4.2.4. Metabolic Products and Enzyme Activity
Plasma free fatty acids (FFA) and triglycerides (TG) were measured using a
commercially available enzymatic spectrophotometric kit (Wako Chemicals, USA,
Richmond, VA). Myocardial activities of medium chain acyl-CoA dehydrogenase
(MCAD) and citrate synthase (CS) and tissue TG content were measured from homogenate extracts using enzymatic spectrophotometer methods as previously described
(87; 108; 109; 139). C16-ceramide content in the LV was measured by a capillary gas chromatographic procedure with a flame ionization detector using C17-ceramide as an internal standard, as previously described by Tserng & Griffin (150).
4.2.5. RNA Extraction and Quantitative RT-PCR
RNA extraction and quantitative RT-PCR were performed on frozen powdered
LV tissue using previously described methods (23; 45; 55). Specific quantitative assays were designed from rat sequences available in GenBank for expression of PPARα,
RXRα, CS, uncoupling protein 2 (UCP2), atrial natriuretic peptide (ANP) and genes that are known to be regulated by PPARα: MCAD, carnitine palmitoyltransferase-I (CPT-I), pyruvate dehydrogenase kinase-4 (PDK-4), uncoupling protein-3 (UCP3), and mitochondrial thioesterase-1 (MTE-1) (9; 55; 146; 147; 162; 169-171). Standard RNA was made for all assays by the T7 polymerase method (Ambion), using total RNA isolated from rat hearts. The correlation between the Ct (the number of PCR cycles
required for the fluorescent signal to reach a detection threshold) and the amount of
standard was linear over at least a 5-log range of RNA for all assays. To control for
sample-to-sample differences in RNA concentration and to adjust for possible changes in
67 overall expression with heart failure, the mRNA level for 18S was quantitatively measured in each sample. The number of 18S molecules was not different among the experimental groups; (data not shown); therefore, the PCR data are normalized to 18S and expressed as a percent of sham.
4.2.6. Western Immunoblot Analysis
Protein was extracted from frozen powdered LV tissue as previously described
(108). Either 50 or 75 micrograms of total protein was separated by electrophoresis in
10% SDS-PAGE gels and transferred onto a PVDF membrane. Membranes were incubated with specific antibodies to PPARα, RXRα, (1:100; Santa Cruz Biotechnology,
Inc.) or MCAD (1:2000; Caymen Chemical). After conjugation with the secondary antibody, the membranes were developed in a chemiluminescence substrate solution
(Santa Cruz Biotechnology, Inc.), and bands were quantified using commercially available software. All samples were run in duplicate and normalized to a standard sample loaded on each gel.
4.2.7. Statistical Analysis
Differences among the INF, INF + Fat and INF + Feno groups were determined using Kruskal Wallis one-way analysis of variance (ANOVA) with a Dunn’s pair-wise multiple comparison. Data are expressed as group means ± standard error. For all statistical analysis, significance was accepted at P<0.05.
68 4.3. Results
4.3.1. Body and Heart Mass
Administration of a high fat diet (INF + Fat group) significantly increased body mass compared to the INF and INF + Feno groups. Total LV mass (LV+scar tissue mass) and the LV mass/body mass ratio were significantly higher in the INF + Feno group compared to INF and INF + Fat groups (Table 4-1, Figure 4-1), however, the mean scar tissue mass was the same in all infarcted groups. RV mass, the RV mass/body mass ratio, and the RV/LV ratio did not differ among the infarcted groups (Table 4-1).
4.3.2. Cardiac Function
Myocardial infarction produced LV enlargement and decreased systolic and diastolic LV function, as previously noted (103). There were no significant differences in any echocardiographic or LV pressure measurement following 12 weeks of treatment with either a high fat diet or fenofibrate compared to the untreated INF group (Table 4-2).
4.3.3. Triglyceride and Ceramide
Fenofibrate administration lowered plasma FFA and TG compared to the INF +
Fat group (Table 4-3), and high-fat feeding increased tissue TG levels compared to the untreated INF and INF + Feno groups (Figure 4-2, upper panel). Compared to Sham, the
INF group had significantly higher tissue ceramide content (Figure 4-2, lower panel), but there were no differences in plasma FFA or TG, or in tissue TG. Neither fenofibrate administration nor high fat feeding had an effect on tissue ceramide content when compared to the untreated INF group (Figure 4-2).
69
Table 4-1. Body and heart masses in infarcted (INF), high-fat treated (INF+Fat) and
fenofibrate treated (INF+Feno) rats.
INF INF+Fat INF+Feno
Body mass (g) 591 ± 42 707 ± 21* 562 ± 23
Total LV mass (g) 1.33 ± 0.08 1.36 ± 0.04 1.56 ± 0.04 †
Scar mass (g) 0.21 ± 0.03 0.21 ± 0.02 0.22 ± 0.03
RV mass (g) 0.50 ± 0.08 0.56 ± 0.06 0.54 ± 0.05 RV mass/body mass 0.83 ± 0.12 0.81 ± 0.09 1.00 ± 0.14 (mg/g) RV/LV 0.37 ± 0.05 0.42 ± 0.04 0.34 ± 0.03
Values are mean ± SEM. * P<0.05 INF+Fat compared to INF and INF+Feno; † P<0.05
INF+Feno compared to INF and INF+Fat.
70
† 3.5 †
(mg/kg) 2.5 * mass
1.5
0.5 LV mass/ body mass/ LV
Sham INF INF+ INF+
Fat Feno
Figure 4-1. Left ventricular mass to body mass ratio in Sham, INF, INF + Fat and INF +
Feno groups. * P<0.05 INF compared to sham; † P<0.05 INF + Feno compared to INF
and INF + Fat.
71
Table 4-2. Hemodynamic and echocardiographic measurements in infarcted (INF), high-fat treated (INF+Fat) and fenofibrate treated (INF+Feno) rats. Values are mean ± SEM.
INF INF+Fat INF+Feno
HEMODYNAMIC VARIABLES -1 Heart Rate (beats•min ) 318 ± 17 326 ± 8 350 ± 21
Peak LV Pressure (mmHg) 107 ± 6 102 ± 4 106 ± 4 Peak LV End Diastolic Pressure 15 ± 3 13 ± 2 15 ± 2 (mmHg)
-1 Peak LV +dP/dt (mmHg•sec ) 5710 ±640 5500 ± 320 5620 ±340
-1 Peak LV -dP/dt (mmHg•sec ) -4230 ± 460 -4110± 310 -4100 ± 230
Tau (mSec) 19 ± 2 17 ± 1 19 ± 1
ECHOCARDIOGRAPHIC
VARIABLES
LV End Diastolic Dimension (cm) 1.11 ± 0.09 1.03 ± 0.05 1.08 ± 0.04
LV End Systolic Dimension (cm) 0.90 ± 0.10 0.81 ± 0.07 0.85 ± 0.06
Fractional Shortening (%) 21 ± 3 23 ± 3 22 ± 4
Wall Motion Score Index 2.3 ± 0.1 2.2 ± 0.1 2.3 ± 0.1
Myocardial Performance Index 0.60 ± 0.05 0.50 ± 0.03 0.61 ± 0.03
-1 -1 Cardiac Index (ml•min •kg ) 64 ± 8 52 ± 6 58 ± 6
72
Table 4-3. Plasma free fatty acids and triglycerides in infarcted (INF), high-fat treated
(INF+Fat) and fenofibrate treated (INF+Feno) rats.
INF INF+Fat INF+Feno
Plasma Free Fatty Acids (µmol/ml ) 0.33 ± 0.05 0.50 ± 0.05 0.18 ± 0.05* Plasma Triglycerides (mg/ml) 0.90 ± 0.08 1.28 ± 0.15 0.66 ± 0.04*
Values are mean ± SEM. * P<0.05 INF+Feno compared to INF+Fat.
Table 4-4. Protein expression in infarcted (INF), high-fat treated (INF+Fat) and fenofibrate treated (INF+Feno) rats.
INF INF + Fat INF + Feno
PPARα 108 ± 27 108 ± 17 97 ± 15
RXRα 109 ± 13 112 ± 7 98 ± 12
MCAD 117 ± 16 117 ± 8 179 ± 24 *†
Values are percent of Sham ± SEM. * P<0.05 INF+Feno compared to INF+Fat; †
P=0.055 INF+Feno compared to INF.
73 † †
6.0
4.0 gww )
mol/ 2.0 µ ( Tissue Triglyceride Tissue Triglyceride 0
N.S. 2.5 * ) 1.5 nmol/gww ( 0.5 Tissue Ceramides Sham INF INF INF
+Fat +Feno
Figure 4-2. Myocardial tissue triglyceride and C16 ceramide content in Sham, INF, INF
+ Fat and INF + Feno groups. * P<0.05 compared to Sham; † P<0.05 compared to INF
+ Fat; N.S. indicates no statistical differences among INF, INF + Fat and INF + Feno groups.
74 4.3.4. mRNA Expression
Atrial natriuretic peptide (ANP) was significantly elevated in the INF group compared to sham, but was not different among the treatment groups (Figure 4-3, upper
panel). mRNA expression of the genes known to be regulated by PPARα (MCAD, PDK-
4, MTE-1 and UCP3) were significantly decreased in the INF group compared to shams
(Figure 4-3, lower panel). There were no differences in any of the mRNA levels between
the INF and the INF + Fat groups. Treatment with fenofibrate increased the expression
of MCAD, CPT-I, PDK-4, and MTE-1 compared to the INF and INF+Fat groups (Figure
4-3, lower panel). UCP3 expression, was significantly increased in the INF+Feno group
compared to the INF group and tended to be higher than that of the high-fat treated
group, but this difference did not reach significance (P=0.068, Figure 4-3).
4.3.5. Protein Expression and Enzyme Activity
Protein expression for PPARα and RXRα, was not different among groups;
however, fenofibrate administration significantly increased MCAD protein expression
relative to the INF+Fat group and tended to increase expression relative to untreated INF
group (P=0.055, Table 4-4). CS activity was not different between groups, but MCAD
activity (either normalized to g wet weight tissue or CS activity) was significantly
increased by fenofibrate treatment compared to INF and INF+Fat (Table 4-5).
75 1200 Sham INF INF + Fat 800 * INF + Feno
400
mRNA expression ( % of Sham) % ( expression mRNA 0 ANF PPARα RXRα CS UCP2
600 †
† 400 † ‡ P=0.06
† † 200
* * * *
0 mRNA expression ( % of Sham) MCAD mCPT-I PDK4 MTE-1 UCP3
Figure 4-3. Upper panel: mRNA expression of non-PPARα regulated genes. Lower panel: mRNA expression of PPARα regulated genes. All expression values are normalized to 18S and expressed as a percent of sham. * P<0.05 compared to INF and
INF+Fat; † P<0.05 compared to INF only.
76 Table 4-5. Medium chain acyl-CoA dehydrogenase (MCAD) and citrate synthase (CS) activities and MCAD/CS ratio in infarcted (INF), high-fat treated (INF+Fat) and fenofibrate treated (INF+Feno) rats.
INF INF+Fat INF+Feno
-1 -1 MCAD (µmols•gww •min ) 7.78 ± 0.65 7.77 ± 0.57 15.0 ± 1.14 *
-1 -1 CS (µmols•gww •min ) 116 ± 6 120 ± 5 133 ± 12
MCAD/CS 0.066±0.003 0.064±0.003 0.11±0.004 *
Values are mean ± SEM. * P<0.05 INF+Feno compared to INF and Inf+Fat.
77 4.4. Discussion
The present study demonstrates that prolonged administration of a PPARα agonist increases fatty acid oxidation capacity and causes LV hypertrophy in rats with heart failure, but does not affect myocardial triglyceride or ceramide levels, nor exacerbate LV dysfunction and remodeling. In contrast, high fat feeding significantly increases cardiac triglyceride stores, but again does not exacerbate LV dysfunction. Thus, in a model of established LV dysfunction and dilation, neither increased fatty acid oxidation capacity nor accumulation of myocardial triglyceride alone exacerbates heart failure.
The lack of a functional consequence of PPARα activation on LV contractile function contrasts with previous work that showed impaired cardiac function in cardiac specific PPARα over-expressing mice, and in rats fed a PPARα agonist during the initial development of cardiac hypertrophy induced by aortic banding (37; 38; 168). In the current investigation, fenofibrate treatment increased MCAD mRNA and protein expression and enzyme activity in rats with established heart failure, but did not worsen
LV dysfunction or dilation. It is important to note in this study that PPARα activation did not result in an increase in myocardial triglyceride or ceramide content. On the other hand, we made the unique observation that treatment with PPARα agonist caused LV hypertrophy (24% increase in LV/body mass ratio) without any evidence of further contractile dysfunction or LV remodeling. The mechanism for this effect is unclear, but perhaps activation of PPARα stimulates hypertrophy through signaling mechanisms that are not maladaptive (29). Jamshidi et al found that A G/C polymorphism in intron 7 of the PPARα gene in young men predicted greater LV growth in response to exercise training and hypertension (66). It is not yet known if this mutation activates or inhibits
78 PPARα activation of gene expression, but it nevertheless demonstrates a link between
PPARα and LV hypertrophy (65) .
In contrast to the fenofibrate-induced up-regulation of the fatty acid metabolic pathway, administration of a high-fat diet did not significantly increase the transcription of PPARα regulated genes in the rat infarct model of heart failure. Although PPARα can recognize a broad array of ligands, including long chain polyunsaturated fatty acids and eicosanoids, the response to long chain saturated and monounsaturated fatty acids is markedly reduced (40). Administration of long chain saturated and monounsaturated fatty acids did result in moderate increases in the mRNA expression of several PPARα regulated genes, however this was seen only in isolated cardiomyocytes (46). Similarly, increasing the long chain, but not medium chain fatty acid supply, up-regulates UCP3 (a
PPARα regulated gene) mRNA and protein levels in both skeletal and cardiac muscles, with the greatest effect being seen in cardiac muscle (57). The lack of activation we observed may result from the fatty acid composition of our diet. The high-fat diet used in this study contained primarily saturated and monounsaturated fatty acids of varying chain lengths, which may have been inadequate for optimal activation of PPARα.
Previous work has shown that elevated myocardial ceramide and triglyceride levels are associated with cardiac dilatation and reduced contractility, and that the
reduction of these lipid intermediates is associated with improved cardiac function (21;
38; 173). In this study, we hypothesized that increased delivery of fatty acids to the myocardium would result in a mismatch between fatty acid import and utilization, resulting in an accumulation of myocardial ceramide and triglyceride and an exacerbation of LV dysfunction and dilation in heart failure. Although we found ceramide levels to be
79 increased in the heart failure animals compared to sham (103), increasing fatty acid delivery to the heart via a high-fat diet did not further increase cardiac ceramide content or exacerbate LV dysfunction or dilation observed in this study. On the other hand, high- fat fed animals had significantly higher myocardial triglyceride content compared to the infarcted rats fed normal chow with no increase in LV dysfunction or dilation. These results are consistent with previous studies using isolated cells or transgenic mice where the ability to synthesize cardiac triglycerides has been suggested to play a critical role in protection from “lipotoxicity” by diverting excess fatty acids from cytotoxic pathways
(90; 91). The data further suggests that myocardial ceramide accumulation is not dependent on the quantity of fatty acids delivered to the myocardium, and that myocardial ceramide content is not affected by cardiac triglyceride accumulation in heart failure.
There are several limitations of the present study that need to be addressed. First, although fenofibrate administration up-regulated the fatty acid metabolic pathway, we did not measure myocardial fatty acid uptake or oxidation directly. These measurements require ex vivo perfusion, which would have precluded the biochemical measurements made in the present study. Secondly, the lack of effect of high fat feeding on the progression of heart failure and activation of PPARα regulated genes may be due to the lipid composition and duration of treatment with the high-fat diet. Finally, it is possible that despite elevations in myocardial triglycerides, the duration of treatment was insufficient to up-regulate the fatty acid metabolic pathway or accelerate the progression of heart failure. Future studies should consider increasing both the content of the long chain fatty acid moieties and the length of treatment with high-fat feeding.
80 In summary, prolonged administration of a PPARα agonist up-regulated the fatty acid metabolic pathway and caused LV hypertrophy, but did not affect LV dysfunction and remodeling in a rat model of infarct-induced heart failure. In addition, high fat feeding significantly increased cardiac triglyceride stores, but also did not exacerbate LV dysfunction or remodeling. Thus, LV dysfunction and dilation are not worsened despite up-regulation of the fatty acid metabolic pathway or accumulation of myocardial triglyceride in the rat infarct model of heart failure.
81 Chapter 5
Discussion and Future Directions
5.1. Thesis Summary
The novel findings of this study are:
1) In the rat infarct model of HF, a novel 13 segment model for determining the wall motion score index (WMSI) and the myocardial performance index (MPI) provide accurate, noninvasive and repeatable measurements of in vivo LV systolic and diastolic function. These findings are of considerable interest insofar as longitudinal studies are often critical in the experimental design of studies that use the rat coronary artery ligation model of heart failure.
2) These studies demonstrate that there is mRNA down-regulation of PPARα,
RXRα, as well as PPARα regulated metabolic genes in rats with infarct-induced HF despite no decrease in the protein expression of PPARα or RXRα. Additionally, the down-regulation of PPAR/RXR regulated mRNA was shown to occur within 8 weeks following myocardial occlusion without decreasing any further despite continued decreases in LV function and increases in LV dilation, C-16 ceramide content and ANP expression between 8 and 20 weeks post coronary artery occlusion. The results of this
82 work illustrate that the decreases in PPAR/RXR regulated mRNA observed in HF do not necessarily translate into decreased protein content or activity.
3) These studies indicate that prolonged administration of a PPARα agonist up- regulates the fatty acid metabolic pathway and causes LV hypertrophy, but does not affect tissue triglyceride or ceramide accumulation or LV dysfunction and remodeling in the rat model of infarct-induced heart failure.
4) This work shows that myocardial ceramide content is not affected by a high fat diet. On the other hand, high fat feeding significantly increases cardiac triglyceride stores, but does not exacerbate LV dysfunction or remodeling. Thus, in a model of established LV dysfunction and dilation, the up-regulation of the fatty acid metabolic pathway or accumulation of myocardial triglyceride does not exacerbate heart failure.
5.2. Discussion and Future Directions
The studies described in the previous chapters shed light on the role of myocardial lipid accumulation and nuclear receptor activation on cardiac function and remodeling in
HF, and they have helped to define the time course for the down-regulation of the fatty acid oxidation pathway in advanced heart failure. There still remain, however, numerous unanswered questions.
83 5.2.1. Time Course of Fatty Acid Metabolic Pathway Down-regulation in Heart
Failure
Although down-regulation of the fatty acid metabolic pathway is observed in late-
stage HF, it is unclear when in the progression of HF this down-regulation occurs. In
these studies, mRNA, and protein levels and activity of fatty acid metabolic proteins were
measured at 8 and 20 weeks post coronary artery occlusion. Down-regulation of mRNA
was seen as at 8 weeks, and there was no further decrease observed at 20 weeks. Fatty
acid metabolic protein levels and MCAD activity, however, were not decreased at either
time point, despite the reductions in corresponding mRNA. Thus, it appears that mRNA down-regulation begins before 8 weeks post ligation, whereas protein down-regulation occurs later in the progression of HF (ie. more than 20 weeks post ligation). Additional studies are needed to confirm these findings, and should include measurements of mRNA expression at time points immediately following coronary artery ligation in order to determine when in the progression of HF mRNA down-regulation occurs. On the other hand, longer range studies (ie greater than 20 weeks) are needed to determine when protein levels and activities become reduced in HF. It is likely that there is a range of time in which mRNA and protein down-regulation will be observed. For example,
Rosenblatt-Velin et al. reported a significant decrease in MCAD protein content in rat hearts at 20 weeks after coronary artery ligation, but for inclusion in this group, the rats had to exhibit 5 signs of severe heart failure (assessed by the Feldman criteria for heart failure) (124). Of the sixteen rats ligated, only six had achieved this criteria by 20 weeks
(124). Thus, the data suggest that protein down-regulation is associated with severe,
84 symptomatic end-stage HF, but longer studies are needed to determine the range of time over which protein down-regulation is observed in this model of HF.
5.2.2. Role of Myocardial Triglyceride and Ceramide Accumulation in the
Progression of Heart Failure
Accumulation of lipids and related intermediates in the heart is frequently associated with cardiac dysfunction and has been proposed to be a potential causative factor in the progression of heart failure (21; 22; 37; 38; 164; 173). In these studies, I sought to determine if feeding a high fat diet affects cardiac dysfunction and dilation in
HF and alters myocardial triglyceride and ceramide levels. Although high fat feeding increased myocardial triglyceride levels, it did not exacerbate LV dysfunction or dilation in established HF. It remains unknown, however, if increased triglyceride levels could affect the development of HF (i.e. do elevated myocardial triglyceride levels increase the likely-hood or rate of development of HF after infarct?). To address this question, serial measurements of LV function and dilation could be made early after coronary artery occlusion in rats that are fed a high fat diet prior to infarction. When compared to rats fed normal chow, this data may provide insight into the role of myocardial triglyceride load in the development of HF after infarct.
The role of myocardial triglyceride load in the later stages of HF is also unknown.
It is possible that increased myocardial triglycerides may exacerbate LV dilation and dysfunction in the end stages of HF (i.e beyond 20 weeks post ligation). On the other hand, increased myocardial triglyceride stores may reduce the rate of progression of HF.
Previous studies using isolated cells or transgenic mice have suggested that the ability to
85 synthesize cardiac triglycerides may play a critical role in protection from “lipotoxicity” by diverting excess fatty acids from cytotoxic pathways (90; 91). Thus, additional long- term studies are required to investigate the role of a high-fat diet and increased myocardial triglyceride levels on the progression of HF.
The studies described in this work demonstrate that high-fat feeding does not increase myocardial C16-ceramide levels in HF. Additionally, there was no increase in myocardial C16-ceramide levels 8 weeks after coronary artery ligation, despite significantly reduced LV function and chamber dilation. At 20 weeks post ligation, however, tissue levels of ceramide increased by 40% in heart failure rats, which corresponded with a significant increase in LV end systolic area and a decrease in fractional area of shortening from 8 to 20 weeks post infarction. These results are similar to those observed by Finck et al, who reported a similar increase in ceramide and reduction in LV fractional shortening in cardiac specific PPARα over-expressing mice fed a high fat diet (37). Although it appears that ceramide accumulation may contribute to progression of cardiac dysfunction in heart failure, additional studies are necessary to clearly demonstrate a causal link.
5.2.3. Role of Nuclear Receptor Activation in Heart Failure
Recent studies suggest that decreased expression of the nuclear transcription factors peroxisome proliferator activated receptor α (PPARα) and the retinoid X receptor
α (RXRα) may play a key role in the myocardial substrate switch observed in advanced heart failure (63). Studies in patients (69) and animals (65; 108) with advanced end stage heart failure show that there is a significant reduction of PPARα and/or RXRα mRNA
86 and/or protein levels compared to non-failing hearts, thus suggesting that a decrease in mRNA expression of PPARα and/or RXRα may precipitate the myocardial substrate switch by reducing the amount of fatty acid oxidation enzyme mRNA transcribed (Figure
5-1). The results of these investigations, however, suggest that the level of PPARα mRNA expression may have a limited role in the progression of HF. For example, downregulation of PPARα regulated metabolic genes in rats with infarct-induced HF was observed despite no decrease in the protein expression of PPARα or RXRα. There is growing evidence that the activity of PPARα is inhibited by phosophorylation via the
ERK pathway during hypertrophic processes (43). It is possible, therefore, that in the early stages of heart failure (i.e. before observable downregulation of PPARα and/or
RXRα), phosphorylation of PPARα prevents transcription of fatty acid oxidation enzymes (Figure 5-2). Additionally, these studies show that pharmacological upregulation and activation of the fatty acid metabolic pathway does not affect the progression of HF from 8 to 20 weeks post coronary artery ligation. However, it is unknown if continued activation of the fatty acid metabolic pathway affects LV dysfunction and dilation in the late-stages of HF, or how early PPARα activation might affect the development of HF immediately after infarct. Additional studies measuring
LV function and dilation at time points immediately and long after (beyond 20 weeks) coronary artery ligation in rats receiving a PPARα agonist prior to infarction may shed light on the role of PPARα on the early development and late-stage progression of HF.
87
RXRá PPARα Healthy Heart
nucleus CPT-1 MCAD PDK4 PPRE FAO enzyme genes MTE-1 UCP3 Others Heart Failure
nucleus
Reduced PPRE FAO enzyme genes Transcription
Figure 5-1. Late stage heart failure associated decreases in expression of PPARα and/or
RXRα may precipitate the myocardial substrate switch by reducing the amount of fatty acid oxidation enzyme mRNA transcribed.
88
PPARα
Phosphorylation Via ERK Pathway
RXRá P PPARα Heart Failure
X nucleus CPT-1 MCAD PDK4 PPRE FAO enzyme genes MTE-1 UCP3 Others
Figure 5-2. In heart failure, phosphorylation of PPARα by the ERK pathway may prevent transcription of fatty acid oxidation enzymes (43).
89 In contrast to PPARα, the physiologic role of the nuclear transcription factor
PPARβ/δ in the heart has not been extensively studied. Recent evidence, however,
suggests that cardiac PPARβ/δ target genes overlap with PPARα, and that PPARβ/δ may
in fact play an equally important, if not a greater role in the regulation of cardiac fatty
acid metabolism (18; 64). Administration of PPARβ/δ ligands have been shown to induce fatty acid metabolic gene expression and increase palmitate oxidation rates in both wild type and PPARα null mice (20; 46). Additionally, cardiac specific deletion of the
PPARβ/δ gene has been shown to result in reduced expression of fatty acid metabolic genes and diminished palitate oxidation rates (18). In contrast to PPARα null mice,
however, PPARβ/δ null mice do not develop cardiomyopathy under fasting conditions,
but rather, under basal conditions, suggesting that PPARβ/δ may serve to regulate basal
metabolism, whereas PPARα may be more important for the regulation of fatty acid
uptake (18; 64). Thus, in order to better understand the role of nuclear receptor activation
in HF, future studies must address the affects of PPARβ/δ activation on the development
and progression of LV dysfunction and dilation in HF.
5.2.4. Pharmacologic Manipulations of Myocardial Metabolism for the
Prevention of Heart Failure Following Myocardial Infarction
Numerous clinical and animal studies have shown that treatment with partial fatty
acid oxidation inhibitors improves LV function in and decreases the progression of HF
(3; 17; 25; 41; 123; 125; 126; 143). In humans, administration of partial fatty acid
oxidation inhibitors also reduces the symptoms of chronic myocardial ischemic
conditions such as chronic stable angina (41; 123; 125; 143). It is unknown, however, if
90 administration of a partial fatty acid oxidation inhibitor might prevent or slow the development of HF that often develops as a result of myocardial ischemic injury. To address this question, serial measurements of LV function and dilation could be made beginning shortly after coronary artery ligation in rats that are treated with a partial fatty acid oxidation inhibitor immediately following infarction. When compared to untreated rats, this data may provide insight into the role of myocardial fatty acid oxidation in the development of HF after ischemic injury.
5.3. Conclusion
The studies contained in this dissertation show that mRNA down-regulation of
PPARα regulated metabolic genes in rats with infarct-induced HF occurs despite no decrease in the protein expression of PPARα or RXRα, and that the down-regulation of mRNA occurs relatively early in the progression of HF. In addition, these studies indicate that up-regulation of the fatty acid metabolic pathway with a PPARα agonist or accumulation of myocardial triglyceride does not exacerbate LV dysfunction or dilation in a model of established HF. Assessment of mRNA expression at time points immediately following coronary artery ligation and determination of protein content at time points long after infarction will enhance our understanding of the natural time course of the metabolic changes in HF. Manipulating the expression of fatty acid metabolic proteins in the rat model of HF with PPARβ/δ agonists will help to further define the role of nuclear transcription factors in the development and progression of HF.
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