Alterations in Plasma and Tissue Lipids Associated with Obesity and Metabolic Syndrome

Clinical Science (2008) 114, 183–193 (Printed in Great Britain) doi:10.1042/CS20070115 183

REVIEW Alterations in plasma and tissue lipids associated with obesity and metabolic syndrome

Concepcion´ M. AGUILERA∗, Mercedes GIL-CAMPOS†, Ramon´ CANETE˜ † and Angel´ GIL∗ ∗Department of Biochemistry and Molecular Biology II, Institute of Nutrition and Food Technology, School of Pharmacy, University of Granada, Campus de Cartuja 18071 Granada, Spain, and †Unit of Pediatric Endocrinology, Reina Soﬁa University Hospital, Avda Men´endez Pidal s/n, Cordoba,´ Spain

ABSTRACT

The MS (metabolic syndrome) is a cluster of clinical and biochemical abnormalities characterized by central obesity, dyslipidaemia [hypertriglyceridaemia and decreased HDL-C (high- density lipoprotein cholesterol)], glucose intolerance and hypertension. Insulin resistance, hyperleptinaemia and low plasma levels of adiponectin are also widely related to features of the MS. This review focuses on lipid metabolism alterations associated with the MS, paying special attention to changes in plasma lipids and cellular fatty acid oxidation. Lipid metabolism alterations in liver and peripheral tissues are addressed, with particular reference to adipose and muscle tissues, and the mechanisms by which some adipokines, namely leptin and adiponectin, mediate the regulation of fatty acid oxidation in those tissues. Activation of the AMPK (AMP- dependent kinase) pathway, together with a subsequent increase in fatty acid oxidation, appear to constitute the main mechanism of action of these hormones in the regulation of lipid metabolism. Decreased activation of AMPK appears to have a role in the development of features of the MS. In addition, alteration of AMPK signalling in the hypothalamus, which may function as a sensor of nutrient availability, integrating multiple nutritional and hormonal signals, may have a key role in the appearance of the MS.

INTRODUCTION released through the action of LPL (lipoprotein lipase), an insulin-stimulated enzyme. In addition, glucose provides White adipose tissue, the body’s major energy store, is the glycerol backbone for TAG. In humans, FAs can composed of TAGs (triacylglycerols), produced mainly also be synthesized from glucose, although the rate of by FAs (fatty acids) derived from chylomicrons and cir- synthesis is lower than in rodents. Adipose tissue is also culating VLDLs [very-LDL (low density lipoproteins)], able to release NEFAs [non-esteriﬁed FAs (‘free FAs’)];

Key words: AMP-dependent kinase (AMPK), fatty acid oxidation, hypothalamus, lipid, metabolic syndrome, non-esteriﬁed fatty acid (NEFA), obesity. Abbreviations: ACC, acetyl-CoA carboxylase; AgRP, agouti protein; AMPK, AMP-activated protein kinase; apo, apolipoprotein; CE, cholesterol ester; CETP,CE transfer protein; CPT-1, carnitine-palmitoyl transferase-1; D6D, 6 desaturase; FA, fatty acid; FAS, FA synthase; HDL, high-density lipoprotein; HDL-C, HDL cholesterol; HL, hepatic lipase; HSL, hormone-sensitive lipase; IR, insulin resistance; LC-CoA, long-chain acyl-CoA; LCFA, long-chain FA; LDL, low-density lipoprotein; LPL, lipoprotein lipase; MS , metabolic syndrome; MUFA, monounsaturated FA; NEFA, non-esteriﬁed FA; NPY, neuropeptide Y; PI3K, phosphoinositide 3-kinase; PL, phospholipid; PPAR, peroxisome-proliferator-activated receptor; PUFA, polyunsaturated FA; SFA, saturated FA; TAG, triacylglycerol; Tri-C, triacsin C; VLDL, very-LDL. Correspondence: Professor Angel´ Gil (email [email protected]).

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during lipolysis, TAGs are hydrolysed in a reaction liver, adipose tissue and muscle tissue. Attention is also catalysed by HSL (hormone-sensitive lipase), which is, paid to the LCFA (long-chain FA)-mediated mechani- in turn, regulated by numerous factors and hormones. sms by which the hypothalamus controls energy homo- Catecholamines, by binding to β-adrenergic receptors, eostasis, and to alterations linked to obesity and the MS. stimulate lipolysis, whereas insulin inhibits this process [1]. TAG accumulation is enhanced by the preferential PLASMA LIPOPROTEINS AND THE MS channelling of nutrients into adipose tissue, rather than into muscle or other tissues for fairly immediate The MS can best be explained by viewing abdominal oxidation. Overall, it would seem reasonable to assume adipose tissue as an endocrine organ that releases that alterations which limit lipolysis and oxidation of FAs, excess NEFAs and adipokines into the circulation. First, and those which stimulate lipogenesis (the two processes increased blood NEFAs inhibit the uptake of glucose are often linked), are the cause of, or at least are related to, by muscle [19]. Although the pancreas manufactures obesity [2–4]. A number of published findings point to a extra insulin, there is not enough to counter the hyper- link between human obesity and genetic defects affecting glycaemia, thus explaining the paradox of fasting the lipolytic pathway [5]. Oxidation of FAs depends hyperglycaemia despite increased plasma insulin levels, on the availability of substrates and on co-adjuvant which is known as IR [20]. Hyperglycaemia and increased mechanisms for their transport into mitochondria. In circulating NEFAs provide the correct substrates for obese subjects with a predominance of visceral fat, increased manufacture of TAGs by the liver [21]. Release increased lipolytic activity triggers the release of NEFAs, of NEFAs by adipocytes is greater in central obesity prompting the subsequent hormonal and metabolic than in lower-body obesity, with no concomitant increase changes observed in obesity, such as hyperinsulinaemia, in oxidation by peripheral tissues [22,23]. The ‘portal hypoadiponectinaemia and hypertriglyceridaemia [6]. theory’ posits one of the major mechanisms behind the However, as stated by Smith et al. [7], sustained changes dyslipidaemia, which is the increased flux of NEFAs in lipolysis could never occur dissociated from sustained from adipose tissue to the liver via the portal vein when changes in synthesis. Otherwise, the mass of TAGs within visceral TAG stores are increased, which is related to the adipocyte would soon be depleted. IR and the lack of inhibition of HSL [24]. Individuals Obesity is associated with the so-called MS (metabolic afflicted by the MS are often viscerally obese and insulin- syndrome) [6] characterized by hyperinsulinaemia and resistant. Under these circumstances, the failure of insulin peripheral resistance to the action of insulin, glucose to suppress HSL stimulates the release of NEFAs from intolerance or Type 2 diabetes, hypertriglyceridaemia, lipolytically active visceral fat. This increased flux of decreased HDL-C [HDL (high-density lipoprotein) NEFAs, channelled to the liver via the portal circulation, cholesterol] and other changes associated with risks stimulates hepatic TAG synthesis, apoB100 formation of cardiovascular disease, such as arterial hypertension and, ultimately, the assembly and secretion of VLDL [25]. [8–10]. IR (insulin resistance) would appear to be NEFAs promote increased TAG synthesis in the liver, the major common finding in subjects with obesity, which can lead to the secretion of VLDL (Figure 1). glucose intolerance or Type 2 diabetes, hypertension or In peripheral tissues, VLDL particles are exposed to dyslipidaemia, and it has been claimed to be the initial LPL, which hydrolyses the TAG of VLDL particles, factor triggering a metabolic cascade which is also in- generating NEFAs. Under normal conditions, these fluenced by genetic and environmental factors [8,11–13]. NEFAs are taken up by muscle and adipose tissue for The combination of IR and hyperinsulinaemia greatly energy use or storage. The resulting remnant particles are increases the likelihood of developing a cluster of closely hydrolysed further by HL (hepatic lipase) to form LDL. related abnormalities, and the resulting clinical diagnoses In contrast, in individuals afflicted by the MS, the failure can be considered to make up the IR syndrome [8]. of insulin to activate LPL favours the accumulation of Findings common to central obesity and the MS TAG-rich lipoproteins in the circulation [23,26], which include, in addition to increased VLDL and indeed TAG, in turn enhances the exchange of TAG from TAG- the presence of small-dense LDL particles, increased rich lipoproteins to LDL and HDL, with a reciprocal apoB (apolipoprotein B), decreased apoA-I (apolipopro- transfer of cholesteryl esters to TAG-rich lipoproteins. tein A-I) and impaired haemostasis due to increased TAG-enriched cholesteryl ester-depleted LDL are better fibrinogen and PAI-1 (plasminogen-activator inhibitor) substrates for HL-mediated TAG lipolysis, which in [14-17]. Likewise, the cholesterol content of HDL turn leads to the formation of small-dense LDL declines, whereas that of VLDL and LDL increases [18]. (Figure 2). Mechanistically, small-dense LDL particles The present review focuses on some of the major enter the arterial wall more easily, bind to arterial wall alterations in plasma lipids in obesity and their involve- proteoglycans more avidly and are highly susceptible to ment in the MS, and also addresses the main cellular oxidative modification, leading to macrophage uptake, all lipid alterations, especially FA oxidation, occurring in of which may contribute to increased atherogenesis [27].

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Figure 1 Increased synthesis of VLDL associated with IR The ﬂux of NEFAs (free FAs) from adipose tissue to the liver via the portal vein is increased when visceral TAG stores are increased, which is related to IR and lack of inhibition of HSL. NEFAs promote increased TAG (TG) synthesis in the liver. In addition, an increased rate of hepatic NEFA uptake stimulates the secretion of apoB-100, leading to increased numbers of apoB-containing particles and, indeed, the secretion of VLDL is enhanced.

Figure 2 Mechanisms of NEFAs involved in dyslipidaemia associated with the MS Increased plasma VLDL and TAG is usually associated with decreased HDL levels. CETP mediates the transfer of cholesterol from HDL to the apoB-containing lipoproteins, and HL and endothelial lipase are up-regulated in the MS, thus promoting hypercatabolism of HDL and leading to the generation of small-dense LDL (Sd-LDL) particles and a decrease in HDL2-C. FFA, NEFA; LDL-R, LDL receptor; TG, TAG.

Elevated LC-CoA (long-chain acyl-CoA) tissue levels are A low HDL-C level is even more common in patients involved in the increase in plasma TAG concentrations with IR than hypertriglyceridaemia. Two mechanisms associated with IR. In the liver, LC-CoA stimulates explain why increased plasma TAG is almost always the synthesis of TAG-rich lipoproteins, whereas in associated with reduced HDL levels: (i) CETP [CE peripheral tissue they reduce plasma lipoprotein clearance (cholesterol ester) transfer protein] mediates the transfer via inhibition of LPL [28]. Elevated plasma apoC-III of cholesterol from HDL to the apoB-containing concentration is also a feature of dyslipidaemia in the MS lipoproteins; and (ii) enzymes, such as HL and endothelial that contributes to the kinetic defects in apoB metabolism lipase, are up-regulated in the MS and, therefore, promote [29]. ApoC-III is a protein that inhibits LPL, favouring hypercatabolism of HDL, which leads to the generation the accumulation of TAG-rich lipoproteins. (Figure 2). A of small-dense LDL particles and a decrease in HDL2-C recent study has shown that some polymorphisms in the [28] (Figure 2). APOC3 and LPL genes might have a small effect on apoB The ‘portal theory’ proposes a link between visceral levels in the Central European Caucasian population with adipose tissue to IR and the MS and is based on the direct dyslipidaemia associated with the MS [30]. effects of NEFAs on the liver. Intra-abdominal tissue

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Figure 3 Inﬂuence of NEFAs on the development of some typical features of the MS In the liver, NEFAs (FFA) provide enough energy for glucose production. In addition, high levels of NEFAs are toxic to pancreatic β-cells and promote insulin resistance in muscle.

adipocytes are much more insulin-resistant than their that these compounds are the equivalent of NEFAs at the subcutaneous counterparts, suggesting, as we commented intracellular level [36,37]. Accumulation of free radicals before, that NEFA delivery to the liver via the portal derived from mitochondrial oxidation of LC-CoA gives vein is increased when visceral TAG stores are increased rise to endothelial dysfunction and the gradual decline in [24]. Elevated NEFA levels increase hepatic glucon- insulin production by β-cells [38,39]. Functional defects eogenesis and lower peripheral tissue glucose uptake, in pancreatic β-cells have been identiﬁed even prior to prompting a further increase in the hyperinsulinaemia diabetes diagnosis, especially in individuals with central typically found in the MS [31,32] (Figure 3). Studies obesity and the MS. β-Cell regulation is governed by involving healthy adult volunteers undergoing short- central processes and by signals such as NEFAs. Elevated term starvation have shown that NEFAs inhibit the de- NEFA concentrations are toxic to pancreatic β-cells, gradation, but not the synthesis, of hepatic glycogen and inducing their apoptosis, accelerating pancreas failure stimulate gluconeogenesis [31]. Moreover, it was found and favouring progression to diabetes [24,40]. In some in the early 1960s that NEFAs restrain glucose use in cases, increased insulin secretion aimed at maintaining muscle, as increased production of acetyl-CoA in muscle blood sugar levels within a normal range may prompt tissue mitochondria inhibits pyruvate dehydrogenase, a a functional decrease in β-cells, leading eventually to glucose-oxidation-limiting enzyme [13]. Although cer- irreversible failure [41] (Figure 3). tain NEFA levels are believed to stimulate insulin secretion, elevated levels are reported to have few effects on in vivo insulin secretion [33]. It has been reported CHANGES IN PLASMA AND TISSUE FA previously that NEFAs interfere with insulin signalling, COMPOSITION AS RELATED TO THE MS thereby stimulating various PKC (protein kinase C) isoforms, which block cellular insulin signalling mechan- The serum FA composition has been shown to predict the isms and inhibit glucose transport [34]. Moreover, an risk of diabetes and cardiovascular disease, and has been inverse correlation between PI3K (phosphoinositide 3- closely related to components of the MS. Prospective kinase) activity, a key enzyme in the insulin signalling studies are, however, needed to investigate the role of FA cascade, and plasma NEFA concentrations has been composition in the development of the MS. A number reported [35]. Also, it has been shown that elevated of studies in both animal obesity models and adult obese intracellular LC-CoA levels are associated with IR, and humans report changes in FA composition in plasma,

C The Authors Journal compilation C 2008 Biochemical Society Alterations in plasma and tissue lipids associated with obesity and metabolic syndrome 187 as well as in liver, muscle and adipose tissue lipids. (stearoyl-CoA desaturase 1) and D6D, and low activity of Higher levels of SFAs (saturated FAs) and lower levels of D5D (5 desaturase), have been associated with the MS n − 3 PUFAs (polyunsaturated FAs) are associated with [51]. Thus, although the quantity and quality of dietary obesity in the MS [42–47]. fat clearly contribute to changes in FA composition, other Rossner et al. [42] reported that, in obese patients, hormonal or genetic factors may also be involved. the most marked differences were decreased relative Given the fact that individuals with MS features might contents of C18:2n−6 (γ -linoleic acid) in serum TAGs, PLs beneﬁt more from a diet that is low in carbohydrate,

(phospholipids) and CEs, whereas C18:3n−3 (linolenic acid) but with a greater proportion of fat, the qualitative was decreased in TAGs and CEs, with reciprocal increases composition of dietary fat may be of importance for in C16:0 (palmitic acid) and C16:1n−7 (palmitoleic acid) in individuals with the MS. Diets rich in FAs, mainly SFAs TAGs and CEs. Similarly, an increased proportion of and trans-FAs, as well as carbohydrate-rich diets, favour

C16:0 and a low proportion of C18:2n−6 have been reported an acute increase in IR, independent of adiposity [52]. in obese subjects with the MS [46]. Interestingly, the In addition, C18:2n−6 and C20:4n−6, which are consumed in proportion of C16:0 in the TAG fraction has been posit- relatively higher amounts in Western diets, may ively associated with plasma fasting insulin and DBP (dia- contribute to IR in adulthood [53]. Moreover it has been stolic blood pressure) and SBP (systolic blood pressure). suggested that dietary n − 3 PUFAs, although they do not

On the other hand, the proportion of C18:3n−3 has appear to influence IR, due to the anti-inflammatory and been associated negatively with apoB concentrations and anti-atherogenic properties, may benefit individuals with positively with LDL diameter, whereas the proportion the MS [54]. of C18:3n−6 was associated negatively with plasma TAG, DBP, SBP and plasma fasting insulin and positively with HDL-C and LDL diameter [47]. A study by Klein- Platat et al. [48] in overweight adolescents found similar CELLULAR FA OXIDATION AND THE MS correlations between FA composition and some MS- related parameters. In that study [48], normal-weight Role of adipokines adolescents had lower SFA levels in PLs and CEs, Ravussin and Smith [55] have put forward an alternative and higher C22:6n−3 (docosahexaenoic acid) levels than to the classical paradigm or ‘portal hypothesis’ to explain obese adolescents, who also had an increase in C16:1n−7 the features of the MS. The ‘endocrine’ paradigm, attributed to de novo lipogenesis. In obese subjects, developed in parallel with the ectopic fat storage the PUFA/SFA ratio and C18:2n−6 levels were associated syndrome hypothesis, posits that adipose tissue secretes a positively with HDL-C, whereas the PUFA/SFA ratio wide variety of endocrine hormones and adipocytokines in PLs and CEs was associated inversely with IL-6 that regulate energy metabolism and, especially, lipid

(interleukin-6). C18:2n−6,C20:5n−3 (eicosapentaenoic acid) metabolism. From this viewpoint, adipose tissue plays a and CEs were also inversely related to CRP (C-reactive critical role as an endocrine gland, affecting the functions protein). This would suggest that a high intake of n − 3 of distant organs including the CNS (central nervous PUFAs may protect obese subjects against the MS and system), skeletal muscle and liver. Hormone changes may inflammation associated with obesity [49]. Moreover, thus precede any change in metabolites such as NEFAs lower proportions of the PL C20:4n−6 (arachidonic acid) or plasma glucose. have been reported in serum from obese humans and in Leptin is currently considered the major liporegulat- liver from obese Zucker rats, implying impaired D6D (6 ory hormone [56], maintaining normal intracellular lipid desaturase) activity or, alternatively, an abnormality in homoeostasis in the same way that insulin is required for the catabolism or tissue distribution of this FA in obesity glucose homoeostasis. Almost 10 years ago, leptin was [43,44]. identified as the circulating hormone that informs the Changes in tissue FA composition largely reflect brain about the abundance of body fat, thus enabling food changes in plasma PL composition. However, there may intake, metabolism and endocrine physiology to match be site-specific differences in FA composition of adipose the body’s nutritional status [57,58]. It has been suggested tissue. A study in obese adults found that SFAs were that leptin promotes weight loss by suppressing appetite higher and MUFAs (monounsaturated FAs) perivisceral and stimulating metabolic activity. In theory, however, than in subcutaneous fat. Moreover, central obesity was there appear to be sound reasons for doubting that its positively associated with n − 6 PUFAs and inversely primary function is to prevent obesity. In the first place, associated with n − 3 PUFAs, which in turn had a negative there is little evidence to show that the pressures to which correlation with apoB and TAG. Thus adipose tissue humans have been subjected in the course of evolution composition may govern alterations in certain obesity were aimed at preventing an accumulation of body fat. risk biomarkers [50]. Certain changes in the composition In fact, evidence appears to suggest the reverse, that and endogenous synthesis of tissue FAs may predict increased body fat might be seen as a survival mechanism the development of the MS. High activity of SCD-1 or a defence against periods of scarcity or famine [59].

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It appears that the primary function of leptin, the antisteatotic effect of leptin. Inhibition of malonyl- protein product of the ob gene, is not to prevent obesity CoA formation, in turn, activates expression of CPT- by acting on the hypothalamus, but to avoid metabolic 1, thus prompting adequate mitochondrial FA oxidation damage to non-adipose tissues by allowing body fat [21,68] (Figure 4). Leptin also enhances the intracellular to accumulate in fat cells during excess caloric intake expression of PGC-1α (PPAR-γ co-activator 1α), through a direct effect on leptin receptors, as adipocytes thus increasing mitochondrial enzyme activity for FA are the only cells adapted to this purpose. This points oxidation and mitochondrial biogenesis. In cases of to the critical role of leptin as an antisteatotic hormone leptin resistance, as it occurs in obesity and the MS, [60,61]. AMPK fails to inhibit ACC, leading to overexpression If it were not for this normal and physiological of malonyl-CoA and increased synthesis of TAGs and antisteatotic activity of leptin, surplus FAs during excess FAs, and simultaneous blocking of FA oxidation through calorie intake would flood non-adipose tissues, mainly inhibition of CPT-1 [62,68]. pancreatic islet β-cells and heart/muscle cells, causing Studies involving incubation of muscle cells with organ dysfunction, lipotoxicity and lipoapoptosis. Just leptin have shown that leptin stimulates FA oxidation as insulin regulates tolerance of a carboydrate-rich diet in skeletal muscle by activating AMPK, which leads to by directing glucose to the relevant target cells, leptin the activation of CPT-1, thus enhancing the access of increases tolerance of a fat-rich diet, protecting key FAs to mitochondria [68]. Early activation of AMPK non-adipose tissues against potential lipotoxicity by occurs by leptin acting directly on muscle, whereas increasing FA oxidation [62]. Research suggests that, later activation depends on leptin functioning through at the molecular level, the end product of the HPA the hypothalamic–nervous system axis [21,69,70], as (hypothalamic–pituitary–adrenal) axis (cortisol) is the discussed below. major local factor involved in inducing the expression of Adiponectin is another peptide hormone secreted in the so-called leptin-resistance factors, which inhibit leptin large amounts by adipocytes, which also displays clear signalling in non-adipose tissues (muscle, liver, β-cells, antisteatotic activity in non-adipose tissues, together heart and kidney) [63]. Indeed, tissue steatosis appears to with major insulin-sensitizing, anti-atherogenic and anti- be reverted or prevented by appropriate leptin signalling inflammatory properties [71–73]. Plasma concentrations [64,65]. are inversely correlated with the amount of body Subcutaneous adipose tissue synthesizes more leptin fat in obesity, Type 2 diabetes and, in general, in and displays a greater affinity for insulin, exerting all states characterized by IR, including coronary greater antilipolytic effects; visceral adipose tissue is heart disease [74]. Plasma adiponectin levels correlate more metabolically active, has a higher rate of TAG negatively with BMI (body mass index), insulin and turnover, releases more NEFAs, is more insulin-resistant TAG levels, and positively with HDL-C, in obese adults and has more adrenergic, androgenic and glucocorticoid [75]. Adiponectin also enhances whole-body insulin receptors. Thus impaired lipid metabolism may be an sensitivity by increasing FA oxidation, prompting a early manifestation of IR and the MS, subsequently giving decline in circulating FA levels as well as in muscle and rise to changes in blood sugar levels [66,67]. liver TAGs [76] (Figure 5). Leptin, by binding to its cell-membrane receptor OB- Adiponectin stimulates insulin receptor tyrosine R, induces the phosphorylation of a protein known as kinase activity by activating oxidative phosphorylation STAT-3 (signal transducer and activator of transcription- mediated by UCPs (uncoupling proteins). Adiponectin- 3) which, when activated, penetrates the nucleus and knockout mice fed on a carbohydrate-rich diet develop regulates the transcriptional activity of leptin-controlled IR and display impaired PI3K activity [77]. Like genes. Leptin therefore down-regulates the activity leptin, adiponectin also activates AMPK, stimulating of lipogenic transcription factors, mainly PPAR-γ 2 glucose utilization and FA oxidation ([21,78–80], but (peroxisome-proliferator-activated receptor-γ 2) and, in see [79a]) (Figure 4). Administration of full-length or liver cells, the sterol-regulatory-element-binding protein globular adiponectin in mice increases AMPK-dependent SREBP-1c [4]. It thus induces decreased expression of phosphorylation in skeletal muscle; in the liver, this the lipogenic enzymes ACC (acetyl-CoA carboxylase) activation is achieved only with the full-length form and FAS (FA synthase), and increased expression of [81]. As indicated above, AMPK activation prompts key enzymes involved in FA oxidation, such as ACO an increase in acyl-CoA oxidation. The adiponectin (acyl-CoA oxidase) and CPT-1 (carnitine-palmitoyl receptors adipoR1 and adipoR2 have been identified and transferase-1), particularly in adipose tissue. At the same cloned in muscle and liver cells respectively; expression of time, leptin activates AMPK (AMP-activated protein adipoR1 is associated with increased phosphorylation of kinase) which, in turn, inactivates ACC, an enzyme AMPK, ACC and p38 MAPK (mitogen-activated protein involved in the initial phase of TAG and FA synthesis, kinase), whereas, in liver cells, there is an increase in by phosphorylation and blocks its expression, lowering phosphorylation of AMPK and ACC. Adiponectin also the formation of malonyl-CoA. This is the key to the increases the activity of PPAR-α, a transcription factor

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Figure 4 Regulation of leptin- and adiponectin-mediated cellular FA oxidation Leptin and adiponectin activate AMPK, which, in turn, inactivates ACC by phosphorylation and blocks its expression, lowering the formation of malonyl-CoA. Inhibition of malonyl-CoA formation, in turn, activates CPT-1, thus prompting adequate mitochondrial FA oxidation. P, phosphorylation; TG, TAG. expressed in the liver, which plays an essential role in in intracellular LCFA-CoAs in the hypothalamus, and regulating FA oxidation ([21,79], but see [79a]). the alteration of this homoeostatic mechanism may be related to central obesity and the MS. LCFA-CoAs Role of LCFAs in the hypothalamus appear to initiate a hypothalamic satiety signal by LCFAs in certain areas of the hypothalamus may act activating neuronal pathways to reduce food intake and as a sensor for nutrient availability, integrating multiple glucose production. In a study carried out in baboons, hormonal and nutritional signals. LCFAs are transported intravenous injection of a lipid emulsion was sufﬁcient through the bloodstream bound to albumin, or packaged to reduce food intake; circulating lipids (TAGs, glycerol into chylomicrons or other lipoproteins. LCFA can and LCFAs) prompted a satiety signal, regardless of cross the blood–brain barrier by simple diffusion changes in plasma insulin levels or intestinal absorption of and, once inside the cell, are transformed into CoA nutrients [82]. Intracerebroventricular administration esters (LCFA-CoA) by ACS (acyl-CoA synthetase). of C18:1n−9 (oleic acid) markedly inhibits food intake Intracellular LCFA-CoA content depends on the amount and liver glucose production. Administration of this formed and its use, either for lipid biosynthesis or for LCFA also inhibits the hypothalamic expression of mitochondrial β-oxidation. This latter process requires orexigenic peptides, such as NPY (neuropeptide Y) CPT-1 to transport LCFA-CoA into the mitochondria. and AgRP (agouti protein), and the expression of β-Oxidation is regulated by the availability of malonyl- glucose-6-phosphatase in the liver [83,84]. Interestingly,

CoA, a potent CPT-1 inhibitor. Malonyl-CoA derives administration of the short-chain FA C8:0 (octanoic acid), largely from acetyl-CoA, which is, in turn, the end- which does not require CPT-1 for mitochondrial access product of glycolysis [37,81], and, by this means, cell lipid and subsequent oxidation, fails to reproduce the potent and carbohydrate availability are controlled. As indicated effect of intracerebroventricular C18:1n−9 [85]. above, malonyl-CoA formation from acetyl-CoA is Elevation of circulating LCFAs may duplicate the catalysed by ACC, which is allosterically inhibited via hypothalamic LCFA-CoA pool, thus creating a satiety phosphorylation of AMPK. signal. This hypothesis is borne out by the ﬁnding Circulating lipids, particularly LCFAs, may regulate that the increase in LCFA-CoA can be inhibited appetite and glucose production by prompting an increase by intrahypothalamic infusion of the pharmacological

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Figure 5 Inﬂuence of obesity on adiponectin secretion, decreased clearance of NEFAs and IR Low plasma levels of adiponectin decreases the activities of FATP-1 (FA transporter protein-1) and AMPK, leading to lower FA oxidation. Likewise, insulin signalling mediated though IRS-1 (insulin receptor substrate-1) is altered and glucose uptake is impaired. In addition, low levels of adiponectin activate phosphoenolpyruvate carboxykinase, a key enzyme of glyconeogenesis, resulting in hyperglycaemia. FFA, NEFA.

ACS inhibitor Tri-C (triacsin C); moreover, central food intake, it would appear that decreased AMPK administration of Tri-C prevents circulating LCFAs from activity gives rise to an increase in hypothalamic malonyl- restricting liver glucose production [86]. It may be CoA levels through activation of ACC. Increased concluded that hypothalamic accumulation of LCFA- malonyl-CoA levels might inhibit CPT-1 activity, and CoA, rather than delivery of FAs to mitochondria for thus inhibit food intake due to the accumulation β-oxidation, constitutes the first hypothalamic signal of LCFA-CoA [83,85] (Figure 6). AMPK may also leading to the inhibition of food intake and glucose modulate hypothalamic transcription of neuropeptides, production (Figure 6). Therefore the availability of regardless of its effect on ACC or malonyl-CoA [70]. cellular LCFA-CoA in the hypothalamus may modulate Leptin resistance, as occurring in obesity and the MS, these hypothalamic signals, so that either genetic or bio- would contribute to a lack of inhibition of food intake chemical inhibition of CPT-1 will increase hypothalamic through AMPK. LCFA-CoA levels while decreasing the expression FAS is an enzymatic complex required for FA synthesis of peptides NPY and AgRP, leading to the eventual that utilizes malonyl-CoA. Double fluorescence in situ inhibition of food intake and diminished liver glucose has shown that FAS co-localizes with NPY in neurons in production [87]. the arcuate nucleus, thus demonstrating its presence AMPK plays a major role in hypothalamic mechanisms in the hypothalamus and its role in modulating food for controlling energy homoeostasis. A number of studies intake at neuronal level [88]. Two FAS inhibitors, have shown that hypothalamic AMPK may regulate food cerulenin and C-75, have been shown to reduce food intake. Manipulation of hypothalamic nucleus activity intake and body weight in rodents [89,90]. Moreover, in rats has proven sufficient to alter feeding behaviour central administration of C-75 in fasted mice increases [69]. Leptin, like other anorexigenic compounds such hypothalamic levels of malonyl-CoA and blocks the as insulin and C-75, blocks AMPK activity in the expression of fasting-induced NPY and AgrP [91]. The hypothalamus, whereas the orexigenic hormone ghrelin increase in malonyl-CoA levels prompted by C-75 increases AMPK expression [70]. Although little is appears to take place through two mechanisms: known about the mechanisms by which hypothalamic (i) accumulation of substrate due to FAS or (ii) inhibition inhibition of AMPK prompts decreased expression of of AMPK activity, giving rise to increased conversion of orexigenic peptides (NPY and AgRP) leading to reduced acetyl-CoA into malonyl-CoA by ACC [91].

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Figure 6 LCFA-mediated hypothalamic mechanisms for controlling energy homoeostasis Hypothalamic accumulation of LCFA-CoA constitutes the ﬁrst hypothalamic signal leading to the inhibition of food intake and glucose production. Cellular LCFA-CoA in the hypothalamus inhibit CPT-1 whilst decreasing the expression of peptides NPY and AgRP, leading to the eventual inhibition of food intake and diminished liver glucose production.

CONCLUSIONS AND FUTURE DIRECTIONS ACKNOWLEDGMENTS

Central obesity is the main cause of the MS, which, in This study was financed by the Spanish Ministry of turn, is associated with numerous alterations in plasma Health and Consumer Affairs, the Spanish National lipids and tissue lipid metabolism. In the liver, increased Programme for Scientific Research, Development and synthesis of VLDL and thus of TAG is accompanied Technological Innovation (I + D + I), the Instituto de by decreased HDL synthesis. This, in conjunction with Salud Carlos III, and the Spanish Health Research Fund decreased TAG clearance by peripheral tissues, leads to (Project No. PI 051968). M.G.-C. is a research scientist increased plasma TAG levels. There are also changes in appointed on a training contract financed by the Spanish plasma FA profiles, with a decrease in n − 6andn − 3 Health Research Institute Carlos III.

PUFAs relative to SFAs and an increase in C16:1n−7 due to endogenous synthesis. At the same time, obesity and MS- related hormonal changes, including increased insulin REFERENCES synthesis and peripheral IR, increased leptin synthesis and decreased adiponectin synthesis by adipose tissue, 1 Gil-Hernandez,´ A. (2002) Obesidad y genes. Vox. Paediatrica 10, 40–45 lead to diminished FA oxidation. AMPK is a key enzyme 2 Astrup, A., Buemann, B., Christensen, N. J. and Toubro, in regulating FA metabolism not only in adipose and S. (1994) Failure to increase lipid oxidation in response to muscle tissue, but also in the hypothalamus, where increasing dietary fat content in formerly obese women. Am. J. Physiol. 266, E592–E599 it appears to play a critical role in regulating energy 3 Filozof, C. M., Murua, C., Sanchez,´ M. P. et al. (2000) Low homoeostasis. Further research is needed to establish the plasma leptin concentration and low rates of fat oxidation in weight-stable post-obese subjects. Obes. Res. 8, relationship between hormones, adipokines and lipids 205–210 in the development of obesity and the MS. Findings 4 Sampath, H. and Ntambi, J. M. (2006) Stearoyl-coenzyme emerging from other studies about the behaviour of lipid A desaturase 1, sterol regulatory element binding protein- 1c and peroxisome proliferator-activated receptor-α: metabolism and adipose tissue could modify the future independent and interactive roles in the regulation of lipid evolution and treatment in obesity and the MS. metabolism. Curr. Opin. Clin. Nutr. Metab. Care 9, 84–88

C The Authors Journal compilation C 2008 Biochemical Society 192 C. M. Aguilera and others

5 Rankinen, T., Zuberi, A., Chagnon, Y. C. et al. (2006) The 29 Chan, D. C., Watts, G. F., Redgrave, T. G., Mori, T. A. human obesity gene map: the 2005 update. Obes. Res. 14, and Barrett, P. H. (2002) Apolipoprotein B-100 kinetics in 529–644 visceral obesity: associations with plasma apolipoprotein 6 Reaven, G. M. (1995) Pathophysiology of insulin C-III concentration. Metab. Clin. Exp. 51, resistance in human disease. Physiol. Rev. 75, 473–486 1041–1046 7 Smith, J., Al-Amri, M., Dorairaj, P. and Sniderman, A. 30 Stancakova, A., Baldaufova, L., Javorsky, M., Kozarova, (2006) The adipocyte life cycle hypothesis. Clin. Sci. 110, M., Salagovic, J. and Tkac, I. (2006) Effect of gene 1–9 polymorphisms on lipoprotein levels in patients with 8 Reaven, G. M. (2005) The insulin resistance syndrome: dyslipidemia of metabolic syndrome. Physiol. Res. 55, definition and dietary approaches to treatment. 483–490 Annu. Rev. Nutr. 25, 391–406 31 Wajchenberg, B. L. (2000) Subcutaneous and visceral 9 Alberti, K., Zimmet, P. and Consultation, W. (1998) adipose tissue: their relation to the metabolic syndrome. Definition diagnosis and classification of diabetes mellitus Endocr. Rev. 21, 697–738 and its complications. Part 1: diagnosis and classification 32 Bergman, R. N., Kim, S. P., Hsu, I. R. et al. (2007) of diabetes mellitus, provisional report of a WHO Abdominal obesity: role in the pathophysiology of consultation. Diabetic Med. 15, 539–553 metabolic disease and cardiovascular risk. Am. J. Med. 10 Weiss, R., Dufour, S., Taksali, S. et al. (2003) Prediabetes 120, 3–8 in obese youth: a syndrome of impaired glucose tolerance, 33 Grill, V. and Qvigstad, E. (2000) Fatty acids and insulin severe insulin resistance, and altered myocellular and secretion. Br. J. Nutr. 83, 79–84 abdominal fat partitioning. Lancet 362, 951–957 34 Kohen-Avramoglu, R., Theriault, A. and Adeli, K. (2003) 11 Haslam, D. W. and James, W. P. T. (2005) Obesity. Lancet Emergence of the metabolic syndrome in childhood: an 366, 1197–1209 epidemiological overview and mechanistic link to 12 Reinehr, T., Holl, R. W., Roth, C. L. et al. (2005) Insulin dyslipidemia. Clin. Biochem. 36, 413–420 resistance in children and adolescents with type 1 diabetes 35 Belfort, R., Mandarino, L., Kashyap, S. et al. (2005) mellitus: relation to obesity. Pediatr. Diabetes 6, 3–4 Dose-response effect of elevated plasma free fatty acid on 13 Goldstein, B. J. (2002) Insulin resistance as the core defect insulin signaling. Diabetes 54, 1640–1648 in type 2 diabetes mellitus. Am. J. Cardiol. 90, (Suppl.), 36 Ruderman, N. B., Saha, A. K. and Kraegen, E. W. (2003) 3G–10G Minireview: malonyl CoA, AMP-activated protein kinase, 14 Kopelman, P. G. (2000) Obesity as a medical problem. and adiposity. Endocrinology 144, 5166–5171 Nature 404, 635–643 37 Ruderman, N. B., Saha, A. K., Vavvas, D., Heydrick, S. J. 15 Valle, M., Gascon,´ F., Martos, R., Bermudo, F., and Kurowski, T. G. (1997) Lipid abnormalities in muscle De Torres, G. and Canete,˜ R. (1999) Apolipoproteins of insulin-resistant rodents. The malonyl CoA in the obese child: Correlation with antropometric hypothesis. Ann. N.Y. Acad. Sci. 827, 221–230 measures and insulin levels. Clin. Chem. Lab. Med. 37, 38 Nolan, C. J., Madiraju, M. S., Delghingaro-Augusto, V., 276 Peyot, M. L. and Prentki, M. (2006) Fatty acid signaling in 16 Valle, M., Gascon,´ F., Martos, R. et al. (2000) Infantile the β-cell and insulin secretion. Diabetes 55, 16–23 obesity: A situation of atherothrombotic risk? 39 Hosokawa, H., Corkey, B. E. and Leahy, J. L. (1997) Metab. Clin. Exp. 49, 672–675 β-Cell hypersensitivity to glucose following 24-h 17 Valle, M., Gascon,´ F., Martos, R., Ruiz, F., Rıos,´ R. and exposure of rat islets to fatty acids. Diabetologia 40, Canete,˜ R. (2001) Apo A-I decrease and tryglicerides 392–397 increase serum levels in hiperinsulinemic children. 40 Shimabukuro, M., Zhou, Y. T., Levi, M. and Unger, R. H. Clin. Chem. Lab. Med. 39, 247 (1998) Fatty acid-induced β cell apoptosis: a link between 18 Brunzell, J. D. and Hokanson, J. E. (1999) Low-density obesity and diabetes. Proc. Natl. Acad. Sci. U.S.A. 95, and high-density lipoprotein subspecies and risk for 2498–2502 premature coronary artery disease. Am. J. Med. 107, 41 Kahn, S., Prigeon, R., Schwartz, R. et al. (2001) Obesity, 16S–18S body fat distribution, insulin sensitivity and islet β-cell 19 Belfort, R., Mandarino, L., Kashyap, S. et al. (2005) function as explanations for metabolic diversity. J. Nutr. Dose-response effect of elevated plasma free fatty acid on 131, 354–360 insulin signaling. Diabetes 54, 1640–1648 42 Rossner, S., Walldius, G. and Bjorvell, H. (1989) Fatty 20 Zammit, V. A., Waterman, I. J., Topping, D. and McKay, acid composition in serum lipids and adipose tissue in G. (2001) Insulin stimulation of hepatic triacylglycerol severe obesity before and after six weeks of weight loss. secretion and the etiology of insulin resistance. J. Nutr. Int. J. Obes. 13, 603–612 131, 2074–2077 43 Phinney, S. D., Tang, A. B., Thurmond, D. C., Nakamura, 21 Gil-Campos, M., Canete,˜ R. and Gil, A. (2004) Hormones M. T. and Stern, J. S. (1993) Abnormal polyunsaturated regulating lipid metabolism and plasma lipids in childhood lipid metabolism in the obese Zucker rat, with partial obesity. Int. J. Obes. Relat. Metab. Disord. 3, S75–S80 metabolic correction by gamma-linolenic acid 22 Jensen, M. D. (1997) Health consequences of fat administration. Metab. Clin. Exp. 42, 1127–1140 distribution. Horm. Res. 48, 88–92 44 Phinney, S. D., Fisler, J. S., Tang, A. B. and Warden, C. 23 Sprangers, F., Romijn, J. A., Endert, E., Ackermans, M. T. (1994) Liver fatty acid composition correlates with body and Sauerwein, H. P. (2001) The role of free fatty acids fat and gender in a multigenic mouse model of obesity. (FFA) in the regulation of intrahepatic fluxes of glucose Am.J.Clin.Nutr.60, 61–67 and glycogen metabolism during short-term starvation in 45 Kunesova, M., Hainer, V., Tvrzicka, E. et al. (2002) Serum healthy volunteers. Clin. Nutr. 20, 177–179 and adipose tissue fatty acid composition in female obese 24 Bergman, R. N. and Ader, M. (2000) Free fatty acids and identical twins. Lipids 37, 27–32 pathogenesis of type 2 diabetes mellitus. Trends 46 Vessby, B. (2003) Dietary fat, fatty acid composition in Endocrinol. Metab. 11, 351–356 plasma and the metabolic syndrome. Curr. Opin. Lipidol. 25 Marsh, J. B. (2003) Lipoprotein metabolism in obesity and 14, 15–19 diabetes: insights from stable isotope kinetic studies in 47 Tremblay, A. J., Despres, J. P., Piche, M. E. et al. (2004) humans. Nutr. Rev. 61, 363–375 Associations between the fatty acid content of 26 Chen, X., Iqbal, N. and Boden, G. (1999) The effects of triglyceride, visceral adipose tissue accumulation, and free fatty acids on gluconeogenesis and glycogenolysis in components of the insulin resistance syndrome. Metab. normal subjects. J. Clin. Invest. 103, 365–372 Clin. Exp. 53, 310–317 27 Grundy, S. M., Cleeman, J. I., Daniels, S. R. et al. (2005) 48 Klein-Platat, K., Drai, J., Oujaa, M., Schlienger, J. L. and Diagnosis and management of the metabolic syndrome: Simon, C (2005) Plasma fatty acid composition is an American Heart Association/National Heart, Lung, associated with the metabolic syndrome and low-grade and Blood Institute Scientific Statement. Circulation 112, inflammation in overweight adolescents. Am. J. Clin. 2735–2752 Nutr. 82, 1178–1184 28 Syvanne, M. and Taskinen, M. R. (1997) Lipids and 49 Phinney, S. D. (2005) Fatty acids, inflammation and lipoproteins as coronary risk factors in non-insulin- metabolic syndrome. Am. J. Clin. Nutr. 82, dependent diabetes mellitus. Lancet 350, SI20–SI23 1151–1152

C The Authors Journal compilation C 2008 Biochemical Society Alterations in plasma and tissue lipids associated with obesity and metabolic syndrome 193

50 Garaulet, M., Perez-Llamas,´ F. and Perez-Ayala,´ M. 72 Beltowski, J. (2003) Adiponectin and resistin: new (2001) Site-specific differences in the fatty acid hormones of white adipose tissue. Med. Sci. Monit. 9, composition of abdominal adipose tissue in an obese RA55–RA61 population from a Mediterranean area: relation with 73 Stefan, N. and Stumvoll, M. (2002) Adiponectin: its role in dietary fatty acids, plasma lipid profile, serum insulin, and metabolism and beyond. Horm. Metab. Res. 34, 469–474 central obesity. Am. J. Clin. Nutr. 74, 585–591 74 Arita, Y., Kihara, S., Ouchi, N. et al. (1999) Paradoxical 51 Warensjo, E., Riserus, U. and Vessby, B. (2005) Fatty acid decrease of an adipose-specific protein, adiponectin, in composition of serum lipids predicts the development of obesity. Biochem. Biophys. Res. Commun. 257, 79–83 the metabolic syndrome in men. Diabetologia 48, 75 Diez, J. J. and Iglesias, P. (2003) The role of the novel 1999–2005 adipocyte-derived hormone adiponectin in human 52 Canete,˜ R., Gil-Campos, M., Aguilera, C. M. and Gil, A. disease. Eur. J. Endocrinol. 148, 293–300 (2007) Development of insulin resistance and its relation 76 Stefan, N., Vozarova, B., Funahashi, T. et al. (2002) Plasma to diet in the obese child. Eur. J. Nutr. 46, 181–187 adiponectin concentration is associated with skeletal 53 Simopoulus, A. P. (1994) Is insulin resistance influenced muscle insulin receptor tyrosine phosphorylation, and by dietary linoleic acid and trans fatty acids? Free Radical low plasma concentration precedes a decrease in Biol. Med. 17, 367–372 whole-body insulin sensitivity in humans. Diabetes 51, 1884–1888 54 McAuley, K. A. and Mann, J. L. (2006) Nutritional 77 Yamauchi, T., Kamon, J., Minokoshi, Y. et al. (2002) determinants of insulin resistance. J. Lipid Res. 47, Adiponectin stimulates glucose utilization and fatty-acid 1668–1676 oxidation by activating AMP-activated protein kinase. 55 Ravussin, E. and Smith, S. R. (2002) Increased fat intake, Nat. Med. 8, 1288–1295 impaired fat oxidation, and failure of fat cell proliferation 78 Maeda, N., Shimomura, I., Kishida, K. et al. (2002) result in ectopic fat storage, insulin resistance, and type 2 Diet-induced insulin resistance in mice lacking diabetes mellitus. Ann. N.Y. Acad. Sci. 967, 363–378 adiponectin/ACRP30. Nat. Med. 8, 731–737 56 Unger, R. H. and Orci, L. (2001) Diseases of 79 Yamauchi, T., Kamon, J., Ito, Y. et al. (2003) Cloning of liporegulation: new perspective on obesity and related adiponectin receptors that mediate antidiabetic metabolic disorders. FASEB J. 15, 312–321 effects. Nature 423, 762–769 57 Baile, C. A., Della-Fera, M. A. and Martin, R. J. (2000) 79a Erratum (2004) Nature 431, 1123 Regulation of metabolism and body fat mass by leptin. 80 Fruebis, J., Tsao, T. S., Javorschi, S. et al. (2001) Annu. Rev. Nutr. 20, 105–127 Proteolytic cleavage product of 30-kDa adipocyte 58 Friedman, J. M. (1998) Leptin, leptin receptors, and the complement-related protein increases fatty acid oxidation control of body weight. Nutr. Rev. 56, 38–46 in muscle and causes weight loss in mice. Proc. Natl. 59 Neel, J. V., Weder, A. B. and Julius, S. (1998) Type II Acad. Sci. U.S.A. 98, 2005–2010 diabetes, essential hypertension, and obesity as 81 Lam, T. K., Schwartz, G. J. and Rossetti, L. (2005) ‘syndromes of impaired genetic homeostasis’: the ‘thrifty Hypothalamic sensing of fatty acids. Nat. Neurosci. 8, genotype’ hypothesis enters the 21st century. Perspect. 579–584 Biol. Med. 42, 44–74 82 Woods, S. C., Stein, L. J., McKay, L. D. and Porte, Jr, D. 60 Lee, Y., Wang, M. Y. and Kakuma, T. (2001) (1984) Suppression of food intake by intravenous Liporegulation in diet-induced obesity. The antisteatotic nutrients and insulin in the baboon. Am. J. Physiol. 247, role of hyperleptinemia. J. Biol. Chem. 276, 5629–5635 R393–R401 61 Steinberg, G. R., Parolin, M. L., Heigenhauser, G. J. and 83 Obici, S., Feng, Z., Karkanias, G., Baskin, D. G. and Dyck, D. J. (2002) Leptin increases FA oxidation in lean Rossetti, L. (2002) Decreasing hypothalamic insulin but not obese human skeletal muscle: evidence of receptors causes hyperphagia and insulin resistance in peripheral leptin resistance. Am. J. Physiol. rats. Nat. Neurosci. 5, 566–572 Endocrinol. Metab. 283, E187–E192 84 Morgan, K., Obici, S. and Rossetti, L. (2004) 62 Unger, R. H. (2003) The physiology of cellular Hypothalamic responses to long-chain fatty acids are liporegulation. Annu. Rev. Physiol. 65, 333–347 nutritionally regulated. J. Biol. Chem. 279, 3139–3148 63 Unger, R. H. (2005) Hyperleptinemia: protecting the 85 Obici, S., Feng, Z., Morgan, K., Stein, D., Karkanias, G. heart from lipid overload. Hypertension 45, 1031–1034 and Rossetti, L. (2002) Central administration of oleic 64 Oral, E. A., Ruiz, E. and Andewelt, A. (2002) Effect of acid inhibits glucose production and food intake. Diabetes 51, 271–275 leptin replacement on pituitary hormone regulation in 86 Lam, T. K., Pocai, A. and Gutierrez-Juarez,´ R. (2005) patients with severe lipodystrophy. J. Clin. Endocrinol. Hypothalamic sensing of circulating fatty acids is required Metab. 87, 3110–3117 for glucose homeostasis. Nat. Med. 11, 320–327 65 Javor, E. D., Cochran, E. K., Musso, C., Young, J. R., 87 Obici, S., Feng, Z., Arduini, A., Conti, R. and Rossetti, L. Depaoli, A. M. and Gorden, P. (2005) Long-term efficacy (2003) Inhibition of hypothalamic carnitine of leptin replacement in patients with generalized palmitoyltransferase-1 decreases food intake and glucose lipodystrophy. Diabetes 54, 1994–2002 production. Nat. Med. 9, 756–761 66 Canete,˜ R., Gil, M. and Poyato, J. L. (2003) Obesidad en 88 Kim, E. K., Miller, I., Landree, L. E. et al. (2002) el nino:˜ nuevos conceptos de etiopatogenia y tratamiento. Expression of FAS within hypothalamic neurons: a model Pediatr. Integral. 7, 480–490 for decreased food intake after C75 treatment. Am. J. 67 Groop, L. (2000) Pathogenesis of type 2 diabetes: the Physiol. Endocrinol. Metab. 283, E867–E879 relative contribution of insulin resistance and impaired 89 Clegg, D. J., Wortman, M. D., Benoit, S. C., McOsker, insulin secretion. Int. J. Clin. Pract. 113, 3–13 C. C. and Seeley, R. J. (2002) Comparison of central and 68 Minokoshi, Y. and Kahn, B. B. (2003) Role of peripheral administration of C75 on food intake, body AMP-activated protein kinase in leptin-induced fatty acid weight, and conditioned taste aversion. Diabetes 51, oxidation in muscle. Biochem. Soc. Trans. 31, 196–201 3196–3201 69 Andersson, U., Filipsson, K., Abbott, C. R. et al. (2004) 90 Makimura, H., Mizuno, T. M., Yang, X. J., Silverstein, J., AMP-activated protein kinase plays a role in the control Beasley, J. and Mobbs, C. V. (2001) Cerulenin mimics of food intake. J. Biol. Chem. 279, 1205–1208 effects of leptin on metabolic rate, food intake, and body 70 Minokoshi, Y., Alquier, T., Furukawa, N. et al. (2004) weight independent of the melanocortin system, but AMP-kinase regulates food intake by responding to unlike leptin, cerulenin fails to block neuroendocrine hormonal and nutrient signals in the hypothalamus. effects of fasting. Diabetes 50, 733–739 Nature 428, 569–574 91 Loftus, T. M., Jaworsky, D. E. and Frehywot, G. L. (2000) 71 Gil-Campos, M., Canete,˜ R. and Gil, A. (2004) Reduced food intake and body weight in mice treated Adiponectin, the missing link in insulin resistance and with fatty acid synthase inhibitors. Science 288, obesity. Clin. Nutr. 23, 963–974 2379–2381

Received 2 April 2007/20 June 2007; accepted 20 July 2007 Published on the Internet 8 January 2008, doi:10.1042/CS20070115

C The Authors Journal compilation C 2008 Biochemical Society