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Protein Kinase C Activation: Isozyme-Specific Effects on Metabolism and Cardiovascular Complications in Diabetes

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Protein Kinase C Activation: Isozyme-Specific Effects on Metabolism and Cardiovascular Complications in Diabetes Diabetologia 2001) 44: 659±673 Ó Springer-Verlag 2001 Reviews Protein kinase C activation: isozyme-specific effects on metabolism and cardiovascular complications in diabetes I.Idris, S.Gray, R.Donnelly School of Medical and Surgical Sciences, University of Nottingham, & Jenny O'Neil Diabetes Centre, Derbyshire Royal Infirmary, Derby, UK Abstract Protein kinase C PKC) is a family of multi- tivation especially but not exclusively PKC-b) has functional isoenzymes, activated by diacylglycerols been strongly implicated in the pathogenesis of dia- DAGs), which play a central role in signal transduc- betic microangiopathy and macroangiopathy through tion and intracellular crosstalk by phosphorylating at a host of undesirable effects on endothelial function, serine/threonine residues an array of substrates, in- VSM contractility and growth, angiogenesis, gene cluding cell-surface receptors, enzymes, contractile transcription in part by MAP-kinase activation) and proteins, transcription factors and other kinases. In- vascular permeability. Interventions that increase dividual isozymes vary in their pattern of tissue and DAG metabolism e.g. vitamin E) and/or inhibit subcellular distribution, function and Ca2+/phospho- PKC isozymes e.g. the b-selective inhibitor lipid cofactor requirements, and in diabetes there is LY333531) ameliorate the biochemical and function- widespread activation of the DAG-PKC pathway in al consequences of DAG-PKC activation in experi- metabolic, cardiovascular and renal tissues. In liver, mental diabetes, for example improving retinal blood muscle and adipose tissue, PKC isozymes have been flow and albuminuria in parallel with reductionsin implicated both asmediatorsand inhibitorsof insulin membrane-associated PKC isozyme activities. Thus, action. Activation of DAG-sensitive PKC isoforms, a greater understanding of the functional diversity such as PKC-v and PKC-e, down-regulatesinsulinre- and pathophysiological regulation of PKC isozymes ceptor signalling and could be an important biochem- islikely to have important clinical and therapeutic ical mechanism linking dysregulated lipid metabo- benefits. [Diabetologia 2001) 44: 659±673] lism and insulin resistance in muscle. On the other hand, atypical PKC isozymes, such as PKC-z and Keywords Protein kinase C, PKC isoenzymes, insulin PKC-l, have been identified asdownstreamtargets resistance, diacylglycerol, vascular complications, en- of PI-3-kinase involved in insulin-stimulated glucose dothelial dysfunction, vascular permeability, PKC-b uptake, especially in adipocytes. PKC Glucose-induced de novo synthesis of palmitate- rich) DAG and sustained isozyme-selective PKC ac- Received: 8 January 2001 and in revised form: 12 February thase kinase 3-beta; IRS-1, insulin receptor substrate-1; MAP- 2001 kinase, mitogen-activated protein kinase; NO, nitric oxide; cNOS & iNOS, constitutive & inducible nitric oxide synthase; Corresponding author: Prof. R. Donnelly, Division of Vascular PA, phosphatidic acid; PC, phosphatidylcholine; PI-3-kinase, Medicine, University of Nottingham, Derbyshire Royal Infir- phosphoinositide-3-kinase; PKC, protein kinase C c, conven- mary, Derby, DE1 2QY, UK tional; n, novel; a, atypical groups); PLA2, phospholipase A2; Abbreviations: Akt, also called protein kinase B; bFGF, basic PS, phosphatidylserine; TG, triglyceride; TGFb, transforming fibroblast growth factor; cAMP, cyclic adenosine monophos- growth factor-beta; TNF-a, tumour necrosis factor-alpha; phate; DAG, diacylglycerol; ET-1, endothelin-1; ECM, extra- VPF or VEGF), vascular permeability factor. cellular matrix; GS, glycogen synthase; GSK-3b, glycogen syn- 660 I.Idriset al.: Protein kinaseC and diabetes Fig.1. The catalytic and regulatory domain structures of con- in diacylglycerols DAG) or exposureto exogenous ventional, novel and atypical PKC isozymes with conserved re- tumour-promoting agents known as phorbol esters. gions C1±C4) and variable regions V1±V5), and binding sites The primary structure of PKC, a single polypeptide for Calcium, phosphatidylserine PS), DAG and ATP chain, can be divided into 4 conserved domains C1±C4) separated by 5 variable regions V1±V5) Fig.1). The NH2-terminal half of the polypeptide i.e. C1, C2, V1, V2 and part of V3) constitutes the Protein kinase C PKC): a family of multifunctional regulatory domain, while the CO2H-terminal half isoenzymes i.e. C3, C4, V4 and V5) formsthe catalytic domain [4]. The functional domainsare located in the con- Adding and removing phosphate groups is an impor- served regions. Thus, the C1-region, which contains tant physiological mechanism for regulating intracel- two cysteine-rich zinc finger-like regions, is responsi- lular proteins, including enzymes, receptors and sec- ble for DAG or phorbol ester binding. The C2-region ond messengers. Thus, a variety of receptor-mediated is responsible for Ca2+-binding while the C3-region responses and metabolic pathways are activated and containsthe ATP binding siteand the C4-region con- deactivated by intracellular kinases enzymes that tainsthe main catalytic site Fig.1). add phosphate groups) and phosphatases enzymes A total of 12 isozymes of PKC have so far been that remove phosphate groups) which in turn are cloned and characterised [5]. Brain and liver contain themselves regulated by extrinsic biochemical signals most PKCs but each isoform shows a different pat- such as hormones and growth factors. Cellular kinases tern of tissue distribution, substrate specificity and are broadly divided into two types: those that phos- cofactor requirements Table 1). The Group A clas- phorylate proteins at tyrosine residues tyrosine kinas- sical) PKC isoforms cPKC-a,-bI,-bII and -g) are cal- es) and those that phosphorylate serine and threonine cium- and phospholipid-dependent and possess the sites serine/threonine kinases). There are three major full primary amino acid sequence C1±C4 and serine/threonine kinases widely distributed in all tis- V1±V5). They are activated by phosphatidylserine sues: cyclic AMP-dependant protein kinase also PS), Ca2+ and DAG or phorbol ester). Group B known as protein kinase A, PKA), Akt also known novel) PKCs nPKC- d,-e,-h,-v and -m) lack the as protein kinase B, PKB) and the calcium-phospho- C2-region and are therefore calcium-independent lipid activated kinase, protein kinase C PKC). but phospholipid-dependent; they require PS and PKC was first identified over 20 years ago as a sin- DAG for activation. The Group C atypical) PKC gle proteolytically-activated kinase in rat brain [1] isoforms aPKC-i,-l and -z) also lack a C2-region, but in fact PKC isa family of structurallyand func- aswell asone of the zinc finger-like cysteine-richmo- tionally related proteinsderived from multiple genes tifsin the C1 region, and are calcium and phospholip- at least 3) and from alternative splicing of single id independent. Both PKC-l and PKC-i are species mRNA transcripts Fig.1) [2, 3]. Activation and homologues. Although the atypical PKCs are depen- translocation of PKC from the cytosol to the plasma dent on the presence of PS for their catalytic activity, membrane occurs in response to a transient increase these isoforms are not activated by Ca2+, DAG or I.Idriset al.: Protein kinaseC and diabetes 661 Table 1. Protein Kinase C isoforms Isoform Tissue Distribution Co-factor requirements Ca2+ DAG PS Conventional cPKCs a widespread 333 bI widespread 333 bII widespread 333 g brain 333 Novel nPKCs d widespread ± 33 e brain, heart ± 33 h heart, skin, lung ± 33 v muscle, brain, blood cells ± 33 m lung epithelial cells± 33 Atypical aPKCs z widespread ± ± 3 i/l * kidney, brain, pancreas± ± 3 phorbol esters. In addition to the allosteric effectors, ed hydrolysis of phosphatidylcholine and de novo post-translational phosphorylation of PKC also ap- synthesis of DAG from phosphatidic acid [20]. pears to constitute an important mechanism for regu- In addition, DAG-mediated activation of PKC is lating PKC translocation and isozyme activity [5, 6]. augmented by specific NEFAs of varying chain Activation of one or more isoforms of PKC leads lengths. For example, non-esterified fatty acids and to a variety of biological responses, including changes their CoA esters especially arachidonic, oleic, linole- in cell proliferation and differentiation, transmem- ic and linolenic acids) appear to activate PKC syner- brane ion transport, glucose and lipid metabolism, gistically with DAG [21, 22], and it has been suggest- smooth muscle contraction and gene expression ed that cis-unsaturated fatty acids act as enhancer [7±9]. Very little isknown about the functional spe- molecules [23]. Thus, in diabetes increased NEFA cificity of different PKC isoforms and experimental concentrationscould enhance hyperglycaemia-in- work has been hampered by the lack of isoform-spe- duced PKC activation, independently of de novo syn- cific substrates for use in radioenzymatic assays. thesis of DAG [24]. Pathophysiological studies have implicated PKC-b and PKC-d in hyperglycaemia-induced vascular dys- function [10]. They have implicated PKC-z in insulin signalling [11±13] and abnormalities of cell growth and differentiation, including hyperplasia and hyper- trophy [14], and PKC-e and PKC-v in muscle insulin resistance [15±17]. DAG-mediated PKC activation in diabetes. Intracel- lular release of DAG is the primary step leading to activation and translocation of PKC and various spe- ciesof DAG varying in fatty acid composition)are generated from 4 principal sources Fig.2). These sources are, firstly, classical receptor-mediated,
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