WY14,643, a peroxisome proliferator-activated receptor alpha (PPARalpha ) agonist, improves hepatic and muscle steatosis and reverses insulin resistance in lipoatrophic A-ZIP/F-1 mice

A growing burden: the pathogenesis, investigation and management of non-alcoholic fatty liver disease

Non-alcoholic fatty liver disease (NAFLD) is the most common hepatic disorder in western countries, and its incidence is increasing. This review outlines the significant health burden posed by NAFLD and discusses what is presently known about its pathogenesis, including the roles of the metabolic syndrome, obesity, insulin resistance, hepatic steatosis, reactive oxygen species, inflammatory cytokines and adipocytokines. The way in which NAFLD is clinically diagnosed is described, and areas of uncertainty surrounding its investigation are identified, before discussing the relative merits of the limited treatment options available and looking ahead to potential therapeutic strategies for the future.

PPAR-alpha agonism improves whole body and muscle mitochondrial fat oxidation, but does not alter intracellular fat concentrations in burn trauma children in a randomized controlled trial

Background

Insulin resistance is often associated with increased levels of intracellular triglycerides, diacylglycerol and decreased fat β-oxidation. It was unknown if this relationship was present in patients with acute insulin resistance induced by trauma.

Methods

A double blind placebo controlled trial was conducted in 18 children with severe burn injury. Metabolic studies to assess whole body palmitate oxidation and insulin sensitivity, muscle biopsies for mitochondrial palmitate oxidation, diacylglycerol, fatty acyl Co-A and fatty acyl carnitine concentrations, and magnetic resonance spectroscopy for muscle and liver triglycerides were compared before and after two weeks of placebo or PPAR-α agonist treatment.

Results

Insulin sensitivity and basal whole body palmitate oxidation as measured with an isotope tracer increased significantly (P = 0.003 and P = 0.004, respectively) after PPAR-α agonist treatment compared to placebo. Mitochondrial palmitate oxidation rates in muscle samples increased significantly after PPAR-α treatment (P = 0.002). However, the concentrations of muscle triglyceride, diacylglycerol, fatty acyl CoA, fatty acyl carnitine, and liver triglycerides did not change with either treatment. PKC-θ activation during hyper-insulinemia decreased significantly following PPAR-α treatment.

Conclusion

PPAR-α agonist treatment increases palmitate oxidation and decreases PKC activity along with reduced insulin sensitivity in acute trauma, However, a direct link between these responses cannot be attributed to alterations in intracellular lipid concentrations.

Activation of PPARα lowers synthesis and concentration of cholesterol by reduction of nuclear SREBP-2

To elucidate the mechanisms underlying the cholesterol lowering effects of PPARalpha agonists we investigated key regulators of cholesterol synthesis and uptake in rats and in the rat hepatoma cell line Fao after treatment with the PPARalpha agonists clofibrate and WY 14,643, respectively. In rat liver as well as in Fao cells, PPARalpha activation led to a decrease of transcriptionally active nuclear SREBP-2. mRNA concentrations of the key regulators of SREBP processing, Insig-1 in rat liver and Insig-1 and Insig-2a in Fao cells, were increased upon PPARalpha activation. Thus we suggest, that the observed reduction of the amount of nuclear SREBP-2 was due to an inhibition of the processing of the precursor protein. Both, in rat liver and in Fao cells, mRNA concentrations of the SREBP-2 target genes HMG-CoA reductase (EC1.1.1.34) and LDL receptor were reduced after treatment with the PPARalpha agonists. Furthermore, treatment of Fao cells with WY 14,643 reduced cholesterol synthesis. As a result, the amount of total cholesterol in liver, plasma and lipoproteins of clofibrate treated rats and in WY 14,643 treated Fao cells was decreased compared to control animals and cells, respectively. In conclusion, we could show a novel link between PPARalpha and cholesterol metabolism by demonstrating that PPARalpha activation lowers cholesterol concentration by reducing the abundance of nuclear SREBP-2.

International Union of Pharmacology. LXI. Peroxisome proliferator-activated receptors

The three peroxisome proliferator-activated receptors (PPARs) are ligand-activated transcription factors of the nuclear hormone receptor superfamily. They share a high degree of structural homology with all members of the superfamily, particularly in the DNA-binding domain and ligand- and cofactor-binding domain. Many cellular and systemic roles have been attributed to these receptors, reaching far beyond the stimulation of peroxisome proliferation in rodents after which they were initially named. PPARs exhibit broad, isotype-specific tissue expression patterns. PPARalpha is expressed at high levels in organs with significant catabolism of fatty acids. PPARbeta/delta has the broadest expression pattern, and the levels of expression in certain tissues depend on the extent of cell proliferation and differentiation. PPARgamma is expressed as two isoforms, of which PPARgamma2 is found at high levels in the adipose tissues, whereas PPARgamma1 has a broader expression pattern. Transcriptional regulation by PPARs requires heterodimerization with the retinoid X receptor (RXR). When activated by a ligand, the dimer modulates transcription via binding to a specific DNA sequence element called a peroxisome proliferator response element (PPRE) in the promoter region of target genes. A wide variety of natural or synthetic compounds was identified as PPAR ligands. Among the synthetic ligands, the lipid-lowering drugs, fibrates, and the insulin sensitizers, thiazolidinediones, are PPARalpha and PPARgamma agonists, respectively, which underscores the important role of PPARs as therapeutic targets. Transcriptional control by PPAR/RXR heterodimers also requires interaction with coregulator complexes. Thus, selective action of PPARs in vivo results from the interplay at a given time point between expression levels of each of the three PPAR and RXR isotypes, affinity for a specific promoter PPRE, and ligand and cofactor availabilities.

PPARalpha activation elevates blood pressure and does not correct glucocorticoid-induced insulin resistance in humans

Fibrates, activators of the nuclear receptor PPARalpha, improve dyslipidemia, but their effects on insulin resistance and vascular disease are unresolved. To test the hypothesis that PPARalpha activation improves insulin resistance and vascular function, we determined the effects of fenofibrate in healthy adults with insulin resistance induced by short-term glucocorticoid administration. Eighteen normal-weight subjects were studied in four stages: at baseline, after 21 days of fenofibrate (160 mg/day) alone, after 3 days of dexamethasone (8 mg/day) added to fenofibrate, and after 3 days of dexamethasone added to placebo (dexamethasone alone). Dexamethasone alone caused hyperinsulinemia, increased glucose, decreased glucose disposal, and reduced insulin-induced suppression of hepatic glucose production as determined by hyperinsulinemic euglycemic clamp and increased systolic blood pressure as determined by ambulatory monitoring, features associated with an insulin-resistant state. Fenofibrate improved fasting LDL and total cholesterol in the setting of dexamethasone treatment but had no significant effect on levels of insulin or glucose, insulin-stimulated glucose disposal, or insulin suppression of glucose production during clamps, or ambulatory monitored blood pressure. In the absence of dexamethasone, fenofibrate lowered fasting triglycerides and cholesterol but unexpectedly increased systolic blood pressure by ambulatory monitoring. These data suggest that PPARalpha activation in humans does not correct insulin resistance induced by glucocorticoids and may adversely affect blood pressure.

Improvement of insulin sensitivity after peroxisome proliferator-activated receptor-alpha agonist treatment is accompanied by paradoxical increase of circulating resistin levels

We studied the effect of peroxisome proliferator-activated receptor-alpha (PPAR-alpha) activation on serum concentrations and tissue expression of resistin, adiponectin, and adiponectin receptor-1 and -2 (AdipoR1 and AdipoR2) mRNA in normal mice and mice with insulin resistance induced by lipogenic, simple-carbohydrate diet (LD). Sixteen weeks of LD feeding induced obesity with liver steatosis and increased insulin levels but did not significantly affect circulating adiponectin or resistin. Treatment with PPAR-alpha agonist fenofibrate decreased body weight and fat pad weight and ameliorated liver steatosis in LD-fed mice with concomitant reduction in blood glucose, free fatty acid, triglyceride, serum insulin levels, and homeostasis model assessment index values. Euglycemic-hyperinsulinemic clamp demonstrated the development of whole-body and liver insulin resistance in LD-fed mice, which were both normalized by fenofibrate. Fenofibrate treatment markedly increased circulating resistin levels on both diets and adiponectin levels in chow-fed mice only. Fat adiponectin mRNA expression was not affected by fenofibrate treatment. Resistin mRNA expression increased in subcutaneous but not gonadal fat after fenofibrate treatment. In addition to fat, a significant amount of adiponectin mRNA was also expressed in the muscle. This expression markedly increased after fenofibrate treatment in chow- but not in LD-fed mice. Adipose tissue expression of AdipoR1 mRNA was significantly reduced in LD-fed mice and increased after fenofibrate treatment. In conclusion, PPAR-alpha activation ameliorated the development of insulin resistance in LD-fed mice despite a major increase in serum resistin levels. This effect could be partially explained by increased AdipoR1 expression in adipose tissue after fenofibrate treatment.

Peroxisome proliferator-activated receptor-alpha-null mice have increased white adipose tissue glucose utilization, GLUT4, and fat mass: Role in liver and brain

Activation of the peroxisome proliferator-activated receptor (PPAR)-alpha increases lipid catabolism and lowers the concentration of circulating lipid, but its role in the control of glucose metabolism is not as clearly established. Here we compared PPARalpha knockout mice with wild type and confirmed that the former developed hypoglycemia during fasting. This was associated with only a slight increase in insulin sensitivity but a dramatic increase in whole-body and adipose tissue glucose use rates in the fasting state. The white sc and visceral fat depots were larger due to an increase in the size and number of adipocytes, and their level of GLUT4 expression was higher and no longer regulated by the fed-to-fast transition. To evaluate whether these adipocyte deregulations were secondary to the absence of PPARalpha from liver, we reexpresssed this transcription factor in the liver of knockout mice using recombinant adenoviruses. Whereas more than 90% of the hepatocytes were infected and PPARalpha expression was restored to normal levels, the whole-body glucose use rate remained elevated. Next, to evaluate whether brain PPARalpha could affect glucose homeostasis, we activated brain PPARalpha in wild-type mice by infusing WY14643 into the lateral ventricle and showed that whole-body glucose use was reduced. Hence, our data show that PPARalpha is involved in the regulation of glucose homeostasis, insulin sensitivity, fat accumulation, and adipose tissue glucose use by a mechanism that does not require PPARalpha expression in the liver. By contrast, activation of PPARalpha in the brain stimulates peripheral glucose use. This suggests that the alteration in adipocyte glucose metabolism in the knockout mice may result from the absence of PPARalpha in the brain.

Investigational PPAR-gamma agonists for the treatment of Type 2 diabetes

The tremendous increase in the global prevalence of Type 2 diabetes (T2D) and its conglomeration of metabolic disorders has dramatically intensified the search for innovative therapies to fight this emerging epidemic. Over the last decade, the family of nuclear receptors, especially the peroxisome proliferator-activated receptors (PPARs), has emerged as one of the most important drug targets aimed at combating the metabolic syndrome. Consequently, compounds that activate the PPARs have served as potential therapeutics for the treatment of T2D and the metabolic anomalies associated with this disorder. This review focuses on the currently marketed compounds and also describes the discovery and development of the next generation of PPAR ligands that are under investigation for the potential treatment of T2D and the metabolic syndrome.

Targeting foxo1 in mice using antisense oligonucleotide improves hepatic and peripheral insulin action

Fasting hyperglycemia, a prominent finding in diabetes, is primarily due to increased gluconeogenesis. The transcription factor Foxo1 links insulin signaling to decreased transcription of PEPCK and glucose-6-phosphatase (G6Pase) and provides a possible therapeutic target in insulin-resistant states. Synthetic, optimized antisense oligonucleotides (ASOs) specifically inhibit Foxo1 expression. Here we show the effect of such therapy on insulin resistance in mice with diet-induced obesity (DIO). Reducing Foxo1 mRNA expression with ASO therapy in mouse hepatocytes decreased levels of Foxo1 protein and mRNA expression of PEPCK by 48 +/- 4% and G6Pase by 64 +/- 3%. In mice with DIO and insulin resistance, Foxo1 ASO therapy lowered plasma glucose concentration and the rate of basal endogenous glucose production. In addition, Foxo1 ASO therapy lowered both hepatic triglyceride and diacylglycerol content and improved hepatic insulin sensitivity. Foxo1 ASO also improved adipocyte insulin action. At a tissue-specific level, this manifested as improved insulin-mediated 2-deoxyglucose uptake and suppression of lipolysis. On a whole-body level, the result was improved glucose tolerance after an intraperitoneal glucose load and increased insulin-stimulated whole-body glucose disposal during a hyperinsulinemic-euglycemic clamp. In conclusion, Foxo1 ASO therapy improved both hepatic insulin and peripheral insulin action. Foxo1 is a potential therapeutic target for improving insulin resistance.

Insulin resistance and atherosclerosis

Considerable evidence supports the association between insulin resistance and vascular disease, and this has led to wide acceptance of the clustering of hyperlipidemia, glucose intolerance, hypertension, and obesity as a clinical entity, the metabolic syndrome. While insulin resistance, by promoting dyslipidemia and other metabolic abnormalities, is part of the proatherogenic milieu, it is possible that insulin resistance itself in the vascular wall does not promote atherosclerosis. Recent findings suggest that insulin resistance and atherosclerosis could represent independent and ultimately maladaptive responses to the disruption of cellular homeostasis caused by the excess delivery of fuel.

The PPARalpha/gamma dual agonist chiglitazar improves insulin resistance and dyslipidemia in MSG obese rats

1. The aim of this study was to investigate the capacity of chiglitazar to improve insulin resistance and dyslipidemia in monosodium L-glutamate (MSG) obese rats and to determine whether its lipid-lowering effect is mediated through its activation of PPARalpha. 2. Chiglitazar is a PPARalpha/gamma dual agonist. 3. The compound improved impaired insulin and glucose tolerance; decreased plasma insulin level and increased the insulin sensitivity index and decreased HOMA index. Euglycemic hyperinsulinemic clamp studies showed chiglitazar increased the glucose infusion rate in MSG obese rats. 4. Chiglitazar inhibited alanine gluconeogenesis, lowered the hepatic glycogen level in MSG obese rats. Like rosiglitazone, chiglitazar promoted the differentiation of adipocytes and decreased the maximal diameter of adipocytes. In addition, chiglitazar decreased the fibrosis and lipid accumulation in the islets and increased the size of islets. 5. Chiglitazar reduced plasma triglyceride, total cholesterol (TCHO), nonesterified fatty acids (NEFA) and low density lipoprotein-cholesterol levels; lowered hepatic triglyceride and TCHO contents; decreased muscular NEFA level. Unlike rosiglitazone, chiglitazar showed significant increase of mRNA expression of PPARalpha, CPT1, BIFEZ, ACO and CYP4A10 in the liver of MSG obese rats. 6. These data suggest that PPARalpha/gamma coagonist, such as chiglitazar, affect lipid homeostasis with different mechanisms from rosiglitazone, chiglitazar may have better effects on lipid homeostasis in diabetic patients than selective PPARgamma agonists.

Peroxisome proliferator-activated receptor alpha improves pancreatic adaptation to insulin resistance in obese mice and reduces lipotoxicity in human islets

Peroxisome proliferator-activated receptor (PPAR) alpha is a transcription factor controlling lipid and glucose homeostasis. PPARalpha-deficient (-/-) mice are protected from high-fat diet-induced insulin resistance. However, the impact of PPARalpha in the pathophysiological setting of obesity-related insulin resistance is unknown. Therefore, PPARalpha(-/-) mice in an obese (ob/ob) background were generated. PPARalpha deficiency did not influence the growth curves of the obese mice but surprisingly resulted in a severe, age-dependent hyperglycemia. PPARalpha deficiency did not aggravate peripheral insulin resistance. By contrast, PPARalpha(-/-) ob/ob mice developed pancreatic beta-cell dysfunction characterized by reduced mean islet area and decreased insulin secretion in response to glucose in vitro and in vivo. In primary human pancreatic islets, PPARalpha agonist treatment prevented fatty acid-induced impairment of glucose-stimulated insulin secretion, apoptosis, and triglyceride accumulation. These results indicate that PPARalpha improves the adaptative response of the pancreatic beta-cell to pathological conditions. PPARalpha could thus represent a promising target in the prevention of type 2 diabetes.

Fenofibrate, a peroxisome proliferator-activated receptor alpha agonist, reduces hepatic steatosis and lipid peroxidation in fatty liver Shionogi mice with hereditary fatty liver

BACKGROUND AND AIMS

The fatty liver Shionogi (FLS) mouse, a unique model for nonalcoholic fatty liver disease (NAFLD), is an inbred strain that develops spontaneous hepatic steatosis without obesity or diabetes mellitus. Peroxisome proliferator-activated receptor (PPAR) alpha controls fatty acid metabolism. In the present study, we investigated the effect of fenofibrate, a PPARalpha agonist, on hepatic steatosis in FLS mice.

METHODS

Thirteen-week-old FLS mice were fed a diet with 0.1% fenofibrate (w/w) for 12 days. The degree of hepatic steatosis was estimated by histological examination and hepatic triglyceride levels. Expression levels of genes involved in fatty acid turnover, including Acox1, Cpt1a, Fabp1, Acadl, and Acadm, were determined by Northern blot analyses. We measured levels of lipid peroxidation, glutathione, and anti-oxidative enzymes, such as superoxide dismutase, catalase, and glutathione peroxidase, in the liver.

RESULT

Treatment of FLS mice with fenofibrate improved hepatic steatosis by activating expression of genes involved in fatty acid turnover and decreased hepatic lipid peroxidation. Fenofibrate increased the activity of catalase by upregulating its mRNA levels.

CONCLUSION

Fenofibrate, which is currently used in therapy of hyperlipidemia, might also be useful for treating patients with NAFLD even in cases where NAFLD is not associated with obesity or diabetes mellitus.

Sorting out the roles of PPAR alpha in energy metabolism and vascular homeostasis

PPARalpha is a nuclear receptor that regulates liver and skeletal muscle lipid metabolism as well as glucose homeostasis. Acting as a molecular sensor of endogenous fatty acids (FAs) and their derivatives, this ligand-activated transcription factor regulates the expression of genes encoding enzymes and transport proteins controlling lipid homeostasis, thereby stimulating FA oxidation and improving lipoprotein metabolism. PPARalpha also exerts pleiotropic antiinflammatory and antiproliferative effects and prevents the proatherogenic effects of cholesterol accumulation in macrophages by stimulating cholesterol efflux. Cellular and animal models of PPARalpha help explain the clinical actions of fibrates, synthetic PPARalpha agonists used to treat dyslipidemia and reduce cardiovascular disease and its complications in patients with the metabolic syndrome. Although these preclinical studies cannot predict all of the effects of PPARalpha in humans, recent findings have revealed potential adverse effects of PPARalpha action, underlining the need for further study. This Review will focus on the mechanisms of action of PPARalpha in metabolic diseases and their associated vascular pathologies.

Hepatitis C infection and diabetes

Since the discovery of the hepatitis C virus (HCV) in 1989, attention has been paid to the association of chronic HCV infection and the development of diabetes. The risk factors for diabetes include older age, HCV genotype 3, severe liver fibrosis, family history of diabetes, and liver/kidney transplantation. Emerging evidence in animals and humans has shown that HCV infection induces hepatic steatosis and increases tumor necrosis factor-alpha level, both resulting in the development of insulin resistance and subsequent type 2 diabetes. It is suggested that the presence of diabetes and hepatic steatosis may enhance fibrosis progression, hepatocellular carcinoma, and atherosclerosis. Interferon is reportedly associated with improved glucose tolerance. However, interferon might enhance underlying autoimmunity against beta cells, leading to overt type 1 diabetes that is genetically predisposed or give rise to hyperglycemia, resulting in the development of type 2 diabetes. In light of the national epidemic of type 2 diabetes, the link between HCV and diabetes would be a major public health problem. Further clinical researches are awaited in order to effectively detect, prevent, and treat HCV-associated type 2 diabetes, which would also slow the progression of hepatitis C itself.

PEROXISOME PROLIFERATOR-ACTIVATED RECEPTORS: How Their Effects on Macrophages Can Lead to the Development of a New Drug Therapy Against Atherosclerosis

Peroxisome proliferator-activated receptors (PPARs) alpha (alpha), beta/delta (beta/delta), and gamma (gamma) are members of the nuclear receptor superfamily, which also includes the estrogen, androgen, and glucocorticoid receptors. Recent evidence suggests that PPARs regulate genes involved in lipid metabolism, glucose homeostasis, and inflammation in various tissues; however, the mechanisms involved are not completely understood. Anti-diabetic drugs, called glitazones, can selectively activate PPARgamma, and hypolipidemic drugs, called fibrates, can weakly activate PPARalpha. Both classes of drugs can decrease insulin resistance and dyslipidemias, which also makes them attractive for treating the metabolic syndrome. The metabolic syndrome exhibits a constellation of risk factors for atherosclerosis that include obesity, insulin resistance, dyslipidemias, and hypertension. Interestingly, all three PPARs are present in macrophages and can therefore have a profound effect on several disease processes, including atherosclerosis. Macrophages are key players in atherosclerotic lesion development. Currently, the first line of defense in reducing the risk of atherosclerosis is aimed at lowering low-density lipoproteins (LDL) and raising high-density lipoproteins (HDL), but a large percentage of patients on statins still succumb to coronary artery disease. However, with the development of drugs selectively activating PPARs, a new arsenal of drugs specifically targeting to the macrophage/foam cell may potentially have a profound impact on how we treat cardiovascular disease.

Mitochondrial dysfunction in NASH: causes, consequences and possible means to prevent it

Calorie-enriched diet and lack of exercise are causing a worldwide surge of obesity, insulin resistance and lipid accretion in liver (i.e. hepatic steatosis), which can lead to steatohepatitis. Steatosis and nonalcoholic steatohepatitis (NASH) can also be induced by drugs such as amiodarone, tamoxifen and some antiretroviral drugs, including stavudine and zidovudine. There is accumulating evidence that mitochondrial dysfunction (more particularly respiratory chain deficiency) plays a key role in the physiopathology of NASH whatever its initial cause. In contrast, the mitochondrial beta-oxidation of fatty acids can be either increased (as in insulin resistance-associated NASH) or decreased (as in drug-induced NASH). However, in both circumstances, generation of reactive oxygen species (ROS) by the damaged respiratory chain can be augmented. ROS generation in an environment enriched in lipids in turn induces lipid peroxidation which releases highly reactive aldehydic derivatives (e.g. malondialdehyde) that have diverse detrimental effects on hepatocytes and other hepatic cells. In hepatocytes, ROS, reactive nitrogen species and lipid peroxidation products further impair the respiratory chain, either directly or indirectly through oxidative damage to the mitochondrial genome. This consequently leads to the generation of more ROS and a vicious cycle occurs. Mitochondrial dysfunction can also lead to apoptosis or necrosis depending on the energy status of the cell. ROS and lipid peroxidation products also increase the generation of several cytokines (TNF-alpha, TGF-beta, Fas ligand) playing a key role in cell death, inflammation and fibrosis. Recent investigations have shown that some genetic polymorphisms can significantly increase the risk of steatohepatitis and that several drugs can prevent or even reverse NASH. Interestingly, most of these drugs could exert their beneficial effects by improving directly or indirectly mitochondrial function in liver. Finding a drug, which could fully prevent oxidative stress and mitochondrial dysfunction in NASH is a major challenge for the next decade.

Peroxisome proliferator-activated receptor (PPAR)alpha activation increases adiponectin receptors and reduces obesity-related inflammation in adipose tissue: comparison of activation of PPARalpha, PPARgamma, and their combination

We examined the effects of activation of peroxisome proliferator-activated receptor (PPAR)alpha, PPARgamma, and both of them in combination in obese diabetic KKAy mice and investigated the mechanisms by which they improve insulin sensitivity. PPARalpha activation by its agonist, Wy-14,643, as well as PPARgamma activation by its agonist, rosiglitazone, markedly improved insulin sensitivity. Interestingly, dual activation of PPARalpha and -gamma by a combination of Wy-14,643 and rosiglitazone showed increased efficacy. Adipocyte size in Wy-14,643-treated KKAy mice was much smaller than that of vehicle- or rosiglitazone-treated mice, suggesting that activation of PPARalpha prevents adipocyte hypertrophy. Moreover, Wy-14,643 treatment reduced inflammation and the expression of macrophage-specific genes in white adipose tissue (WAT). Importantly, Wy-14,643 treatment upregulated expression of the adiponectin receptor (AdipoR)-1 and AdipoR2 in WAT, which was decreased in WAT of KKAy mice compared with that in nondiabetic control mice. Furthermore, Wy-14,643 directly increased expression of AdipoRs and decreased monocyte chemoattractant protein-1 expression in adipocytes and macrophages. Rosiglitazone increased serum adiponectin concentrations and the ratio of high molecular weight multimers of adiponectin to total adiponectin. A combination of rosiglitazone and Wy-14,643 increased both serum adiponectin concentrations and AdipoR expression in WAT. These data suggest that PPARalpha activation prevents inflammation in WAT and that dual activation of PPARalpha and -gamma enhances the action of adiponectin by increasing both adiponectin and AdipoRs, which can result in the amelioration of obesity-induced insulin resistance.

PPARalpha activation increases triglyceride mass and adipose differentiation-related protein in hepatocytes

Adipose differentiation-related protein (ADRP) is a lipid droplet-associated protein that is expressed in various tissues. In mice treated with the peroxisome proliferator-activated receptor alpha (PPARalpha) agonist Wy14,643 (Wy), hepatic mRNA and protein levels of ADRP as well as hepatic triglyceride content increased. Also in primary mouse hepatocytes, Wy increased ADRP expression and intracellular triglyceride mass. The triglyceride mass increased in spite of unchanged triglyceride biosynthesis and increased palmitic acid oxidation. However, Wy incubation decreased the secretion of newly synthesized triglycerides, whereas apolipoprotein B secretion increased. Thus, decreased availability of triglycerides for VLDL assembly could help to explain the cellular accumulation of triglycerides after Wy treatment. We hypothesized that this effect could be mediated by increased ADRP expression. Similar to PPARalpha activation, adenovirus-mediated ADRP overexpression in mouse hepatocytes enhanced cellular triglyceride mass and decreased the secretion of newly synthesized triglycerides. In ADRP-overexpressing cells, Wy incubation resulted in a further decrease in triglyceride secretion. This effect of Wy was not attributable to decreased cellular triglycerides after increased fatty acid oxidation because the triglyceride mass in Wy-treated ADRP-overexpressing cells was unchanged. In summary, PPARalpha activation prevents the availability of triglycerides for VLDL assembly and increases hepatic triglyceride content in part by increasing the expression of ADRP.

Transgenic expression of mutant peroxisome proliferator-activated receptor gamma in liver precipitates fasting-induced steatosis but protects against high-fat diet-induced steatosis in mice

Steatosis is one of the most common liver diseases and is associated with the metabolic syndrome. A line of evidence suggests that peroxisome proliferator-activated receptor (PPAR) alpha and PPARgamma are involved in its pathogenesis. Hepatic overexpression of PPARgamma1 in mice provokes steatosis, whereas liver-specific PPARgamma disruption ameliorates steatosis in ob/ob mice, suggesting that hepatic PPARgamma functions as an aggravator of steatosis. In contrast, PPARalpha-null mice are susceptible to steatosis because of reduced hepatic fatty acid oxidation. PPARgamma with mutations in its C-terminal ligand-binding domain (L468A/E471A mutant PPARgamma1) have been reported as a constitutive repressor of both PPARalpha and PPARgamma activities in vitro. To elucidate the effect of co-suppression of PPARalpha and PPARgamma on steatosis, we generated mutant PPARgamma transgenic mice (Liver mt PPARgamma Tg) under the control of liver-specific human serum amyloid P component promoter. In the liver of transgenic mice, PPARalpha and PPARgamma agonist-induced augmentation of the expression of downstream target genes of PPARalpha and PPARgamma, respectively, was significantly attenuated, suggesting PPARalpha and PPARgamma co-suppression in vivo. Suppression of PPARalpha and PPARgamma target genes was also observed in the fasted and high-fat-fed conditions. Liver mt PPARgamma Tg were susceptible to fasting-induced steatosis while being protected against high-fat diet-induced steatosis. The opposite hepatic outcomes in Liver mt PPARgamma Tg as a result of fasting and high-fat feeding may indicate distinct roles of PPARalpha and PPARgamma in 2 different types of nutritionally provoked steatosis.

Obesity, peroxisome proliferator-activated receptor, and atherosclerosis in type 2 diabetes

Peroxisome proliferator-activated receptors (PPARs) are ligand-activated transcription factors belonging to the nuclear hormone receptor superfamily. The 3 PPAR isotypes, PPAR-alpha, PPAR-gamma, and PPAR-delta, play a key role in the regulation of lipid and glucose metabolism. Obesity and the interrelated disorders of the metabolic syndrome have become a major worldwide health problem. In this review, we summarize the critical role of PPARs in regulating inflammation, lipoprotein metabolism, and glucose homeostasis and their potential implications for the treatment of obesity, diabetes, and atherosclerosis.

Prevention of hepatic steatosis and hepatic insulin resistance in mitochondrial acyl-CoA:glycerol-sn-3-phosphate acyltransferase 1 knockout mice

In order to investigate the role of mitochondrial acyl-CoA:glycerol-sn-3-phosphate acyltransferase 1 (mtGPAT1) in the pathogenesis of hepatic steatosis and hepatic insulin resistance, we examined whole-body insulin action in awake mtGPAT1 knockout (mtGPAT1(-/-)) and wild-type (wt) mice after regular control diet or three weeks of high-fat feeding. In contrast to high-fat-fed wt mice, mtGPAT1(-/-) mice displayed markedly lower hepatic triacylglycerol and diacylglycerol concentrations and were protected from hepatic insulin resistance possibly due to a lower diacylglycerol-mediated PKC activation. Hepatic acyl-CoA has previously been implicated in the pathogenesis of insulin resistance. Surprisingly, compared to wt mice, mtGPAT1(-/-) mice exhibited increased hepatic insulin sensitivity despite an almost 2-fold elevation in hepatic acyl-CoA content. These data suggest that mtGPAT1 might serve as a novel target for treatment of hepatic steatosis and hepatic insulin resistance and that long chain acyl-CoA's do not mediate fat-induced hepatic insulin resistance in this model.

Potential regulatory role of the farnesoid X receptor in the metabolic syndrome

Dyslipidemia and gallbladder diseases are two current anomalies observed in patients suffering from the metabolic syndrome and type 2 diabetes. The bile acid-activated nuclear receptor farnesoid X receptor (FXR) controls bile acid as well as lipid metabolism. Recent observations indicate a role for FXR also in carbohydrate metabolism. Hepatic FXR expression is altered in diabetic animal models in vivo and regulated by hormones and nutrients in vitro. At the molecular level, FXR activation modifies the transcriptional activity of different transcription factors controlling gluconeogenesis and lipogenesis, thus affecting in concert bile acid, lipid and carbohydrate metabolism. The present review focuses on recent advances in our understanding of the modulation of carbohydrate metabolism by FXR. These observations raise the intriguing possibility for a modulatory role of this receptor also in the metabolic syndrome.

Effects of dietary mulberry, Korean red ginseng, and banaba on glucose homeostasis in relation to PPAR-alpha, PPAR-gamma, and LPL mRNA expressions

Despite lack of scientific evidences to support its therapeutic efficacy, the use of herbal supplements has significantly increased. The purpose of this study was to evaluate the effects of traditional anti-diabetic herbs on the progress of diabetes in db/db mice, a typical non-insulin-dependent model. Five different experimental diets were as follows: control diet, 0.5% mulberry leaf water extract diet, 0.5% Korean red ginseng diet, 0.5% banaba leaf water extract diet, and 0.5% combination diet (mulberry leaf water extract/Korean red ginseng/banaba leaf water extract, 1:1:1). Blood levels of glucose, insulin, HbA1c, and triglyceride were measured every 2 weeks. At 12 weeks of age, animals were sacrificed, and tissue mRNA levels of PPAR-alpha, PPAR-gamma, and LPL were determined. Results indicated that mulberry leaf water extract, Korean red ginseng, banaba leaf water extract, and the combination of above herbs effectively reduced blood glucose, insulin, TG, and percent HbA1c in study animals (p<0.05). We also observed that the increased expressions of liver PPAR-alpha mRNA and adipose tissue PPAR-gamma mRNA in animals fed diets supplemented with test herbs. The expression of liver LPL mRNA was also increased with experimental diets containing herbs. The efficacy was highest in animals fed the combination diet for all of the markers used. These results suggest that mulberry leaf water extract, Korean red ginseng, banaba leaf water extract, and the combination of these herbs fed at the level of 0.5% of the diet significantly increase insulin sensitivity, and improve hyperglycemia possibly through regulating PPAR-mediated lipid metabolism.

Induction of overlapping genes by fasting and a peroxisome proliferator in pigs: evidence of functional PPARα in nonproliferating species

Peroxisome proliferator-activated receptor α (PPARα), a key regulator of fatty acid oxidation, is essential for adaptation to fasting in rats and mice. However, physiological functions of PPARα in other species, including humans, are controversial. A group of PPARα ligands called peroxisome proliferators (PPs) causes peroxisome proliferation and hepatocarcinogenesis only in rats and mice. To elucidate the role of PPARα in adaptation to fasting in nonproliferating species, we compared gene expressions in pig liver from fasted and clofibric acid (a PP)-fed groups against a control diet-fed group. As in rats and mice, fasting induced genes involved with mitochondrial fatty acid oxidation and ketogenesis in pigs. Those genes were also induced by clofibric acid feeding, indicating that PPARα mediates the induction of these genes. In contrast to rats and mice, little or no induction of genes for peroxisomal or microsomal fatty acid oxidation was observed in clofibric acid-fed pigs. Histology showed no significant hyperplasia or hepatomegaly in the clofibric acid-fed pigs, whereas it showed a reduction of glycogen by clofibric acid, an effect of PPs also observed in rats. Copy number of PPARα mRNA was higher in pigs than in mice and rats, suggesting that peroxisomal proliferation and hyperresponse of several genes to PPs seen only in rats and mice are unrelated to the abundance of PPARα. In conclusion, PPARα is likely to play a central role in adaptation to fasting in pig liver as in rats and mice.

Ertiprotafib improves glycemic control and lowers lipids via multiple mechanisms

Ertiprotafib belongs to a novel class of insulin sensitizers developed for treatment of type 2 diabetes. In insulin-resistant rodent models, ertiprotafib and a close analog lowered both fasting blood glucose and insulin levels and improved glycemic excursion during an oral glucose tolerance test. In addition, treatment of rodents improved lipid profiles, with significantly lowered triglyceride and free fatty acid levels. These results suggested that this therapeutic activity might involve mechanisms in addition to PTP1b inhibition. In this study, we demonstrate that ertiprotafib activates peroxisome proliferator-activated receptor (PPAR)alpha and PPARgamma at concentrations comparable with those of known agonists of these regulators. Furthermore, it is able to drive adipocyte differentiation of C3H10T(1/2) cells, a hallmark of PPARgamma activation. Livers from ertiprotafib-treated animals showed significant induction of acyl-CoA oxidase activity, probably caused by PPARalpha engagement in these animals. We also show that ertiprotafib inhibits PTP1b in vitro with nonclassic kinetics at concentrations above its EC(50) for PPAR agonism. Thus, the complete mechanism of action for ertiprotafib and related compounds in vivo may involve multiple independent mechanisms, including (but not necessarily limited to) PTP1b inhibition and dual PPARalpha/PPARgamma agonism. Ertiprotafib pharmacology and interpretation of clinical results must be seen in light of this complexity.

Stress- (and diet-) related regulation of hepatic nuclear receptors and its relevance for ABC-transporter functions

Nuclear receptors (NRs) play an important role in maintaining cellular homeostasis. With clearly established roles in fatty acid metabolism and inflammation, peroxisome proliferator activated receptors (PPARs) and other nuclear receptors are essential in liver functioning. However, much less is known about the regulation of NRs themselves during inflammatory processes in the liver. Interestingly PPARs and other NRs are negative acute phase proteins because they become rapidly downregulated during the acute phase response. However, PPARs have important roles in modulating inflammatory responses. One of the mechanisms by which dietary or inflammatory stress is relieved involves the hepatic adenosine triphosphate-binding cassette (ABC) transporter proteins, which import and export a wide variety of substrates. These ABC transporters are under close control of several NRs. Because NRs play important roles in fatty acid metabolism and inflammation as well as in the regulation of bile production, they are reviewed here with respect to their role in dietary and stress-related responses of the liver and their impact on the regulation and function of hepatic ABC transporters.

PPAR- and LXR-dependent pathways controlling lipid metabolism and the development of atherosclerosis

The nuclear receptor superfamily is composed of transcription factors that positively and negatively regulate gene expression in response to the binding of a diverse array of lipid-derived hormones and metabolites. Intense efforts are currently being directed at defining the biological roles and mechanisms of action of liver X receptors (LXRs) and peroxisome proliferator-activated receptors (PPARs). LXRs have been found to play essential roles in the regulation of whole body cholesterol absorption and excretion, in the efflux of cholesterol from peripheral cells, and in the biosynthesis and metabolism of very low density lipoproteins. PPARs have been found to regulate diverse aspects of lipid metabolism, including fatty acid oxidation, fat cell development, lipoprotein metabolism, and glucose homeostasis. Intervention studies indicate that activation of PPARalpha, PPARgamma, and LXRs by specific synthetic ligands can inhibit the development of atherosclerosis in animal models. Here, we review recent studies that provide new insights into the mechanisms by which these subclasses of nuclear receptors act to systemically influence lipid and glucose metabolism and regulate gene expression within the artery wall.

PPARalpha governs glycerol metabolism

Glycerol, a product of adipose tissue lipolysis, is an important substrate for hepatic glucose synthesis. However, little is known about the regulation of hepatic glycerol metabolism. Here we show that several genes involved in the hepatic metabolism of glycerol, i.e., cytosolic and mitochondrial glycerol 3-phosphate dehydrogenase (GPDH), glycerol kinase, and glycerol transporters aquaporin 3 and 9, are upregulated by fasting in wild-type mice but not in mice lacking PPARalpha. Furthermore, expression of these genes was induced by the PPARalpha agonist Wy14643 in wild-type but not PPARalpha-null mice. In adipocytes, which express high levels of PPARgamma, expression of cytosolic GPDH was enhanced by PPARgamma and beta/delta agonists, while expression was decreased in PPARgamma(+/-) and PPARbeta/delta(-/-) mice. Transactivation, gel shift, and chromatin immunoprecipitation experiments demonstrated that cytosolic GPDH is a direct PPAR target gene. In line with a stimulating role of PPARalpha in hepatic glycerol utilization, administration of synthetic PPARalpha agonists in mice and humans decreased plasma glycerol. Finally, hepatic glucose production was decreased in PPARalpha-null mice simultaneously fasted and exposed to Wy14643, suggesting that the stimulatory effect of PPARalpha on gluconeogenic gene expression was translated at the functional level. Overall, these data indicate that PPARalpha directly governs glycerol metabolism in liver, whereas PPARgamma regulates glycerol metabolism in adipose tissue.

Biochemical and morphological effects of K-111, a peroxisome proliferator-activated receptor (PPAR)alpha activator, in non-human primates

K-111 has been characterized as a potent peroxisome proliferator-activated receptor (PPAR)alpha activator. Antidiabetic potency and amelioration of disturbed lipid metabolism were demonstrated in rodents, which were accompanied by elevations of peroxisomal enzymes and liver weight. To examine the possible therapeutic application of K-111 we have now assessed its efficacy in non-human primates with high transferability to humans. For this purpose obese, hypertriglyceridaemic, hyperinsulinaemic prediabetic rhesus monkeys were dosed sequentially with 0, 1, 3 and 10mg/kg per day orally over a period of 4 weeks each. In addition, the effect of K-111 on the peroxisome compartment was analyzed in cynomolgus monkeys using liver samples obtained following a 13-week oral toxicity study. In prediabetic monkeys, the reduction of hyperinsulinaemia and improvement of insulin-stimulated glucose uptake rate indicated amelioration of insulin resistance. These effects were nearly maximal at a dose of 3mg/kg per day, while triglycerides and body weight were lowered significantly in a dose-dependent manner. This reduction of body weight contrasts sharply with the adipogenic response observed with thiazolidinediones, another family of insulin-sensitizing agents. In young cynomolgus monkeys at a dosage of 5mg/kg per day and more, K-111 induced an up to three-fold increase in lipid beta-oxidation enzymes with an 1.5- to 2-fold increase in peroxisome volume density. This moderate increase in peroxisomal activity by K-111 in monkeys is consistent with its role as an PPARalpha activator and corresponds to the observations with fibrates in other low responder mammalian species. The increase in beta-oxidation may explain, at least in part, the lipid modulating effect as well as the antidiabetic potency of K-111. This pharmacological profile makes K-111 a highly promising drug candidate for clinical applications in the treatment of type 2 diabetes, dyslipidaemia, obesity and the metabolic syndrome.

Metabolic Adaptations in the Absence of Perilipin

Targeted disruption of the lipid droplet protein, perilipin, in mice leads to constitutional lipolysis associated with marked reduction in white adipose tissue as a result of unbridled lipolysis. To investigate the metabolic adaptations in response to the constitutive lipolysis, we studied perilipin-null (plin(-/-)) mice in terms of their fatty acid oxidation and glycerol and glucose metabolism homeostasis by using dynamic biochemical testing and clamp and tracer infusion methods. plin(-/-) mice showed increased beta-oxidation in muscle, liver, and adipose tissue resulting from a coordinated regulation of the enzymes and proteins involved in beta-oxidation. The increased beta-oxidation helped remove the extra free fatty acids created by the constitutive lipolysis. An increase in the expression of the transcripts for uncoupling proteins-2 and -3 also accompanied this increase in fatty acid oxidation. Adult plin(-/-) mice had normal plasma glucose but a reduced basal hepatic glucose production (46% that of plin(+/+)). Insulin infusion during low dose hyperinsulinemic-euglycemic clamp further lowered the glucose production in plin(-/-) mice, but plin(-/-) mice also showed a 36% decrease (p < 0.007) in glucose disposal rate during the low dose insulin clamp, indicating peripheral insulin resistance. However, compared with plin(+/+) mice, 14-week-old plin(-/-) mice showed no significant difference in glucose disposal rate during the high dose hyperinsulinemic clamp, whereas 42-week-old plin(-/-) mice displayed significant insulin resistance on high dose hyperinsulinemic clamp. Despite increasing insulin resistance with age, plin(-/-) mice at different ages maintained a normal glucose response during an intraperitoneal glucose tolerance curve, being compensated by the increased beta-oxidation and reduced hepatic glucose production. These experiments uncover the metabolic adaptations associated with the constitutional lipolysis in plin(-/-) mice that allowed the mice to continue to exhibit normal glucose tolerance in the presence of peripheral insulin resistance.

PPARs and the complex journey to obesity

Obesity and the related disorders of dyslipidemia and diabetes (components of syndrome X) have become global health epidemics. Over the past decade, the elucidation of key regulators of energy balance and insulin signaling have revolutionized our understanding of fat and sugar metabolism and their intimate link. The three 'lipid-sensing' peroxisome proliferator-activated receptors (PPAR-alpha, PPAR-gamma and PPAR-delta) exemplify this connection, regulating diverse aspects of lipid and glucose homeostasis, and serving as bona fide therapeutic targets. With molecular underpinnings now in place, new pharmacologic approaches to metabolic disease and new questions are emerging.

Administration of the potent PPARalpha agonist, Wy-14,643, reverses nutritional fibrosis and steatohepatitis in mice

Administration of a methionine and choline deficient (MCD) diet to rodents causes progressive fibrosing steatohepatitis pathologically similar to human metabolic steatohepatitis. We have previously shown that the peroxisome proliferator-activated receptor-alpha (PPARalpha) agonist, Wy-14,643, prevented the development of MCD diet-induced steatohepatitis. We have now tested whether Wy-14,643 ameliorates established steatohepatitis and fibrosis. Male C57BL6 mice were fed the MCD diet for 51 days to induce severe steatohepatitis. They were then treated with Wy-14,643 together with the MCD diet for 5 or 12 days; positive controls continued on the MCD diet for 5 or 12 days. After 5 days of Wy-14,643 treatment, alanine aminotransferase (ALT) levels were significantly decreased, steatohepatitis less severe, and hepatic lipoperoxides significantly reduced. After 12 days, hepatic triglycerides were normalized and there was near resolution of histological changes. MCD dietary feeding was associated with increased expression of vascular cell adhesion molecule (VCAM)-1, and increased numbers of activated macrophages in the liver. Treatment with Wy-14,643 reduced VCAM-1 expression and macrophage numbers. MCD diet-fed mice developed hepatic fibrosis with increased hepatic collagen alpha1(I), tissue inhibitor of metalloproteinases (TIMP)-1, TIMP-2, and matrix metalloproteinase (MMP)-13 mRNA levels. After treatment with Wy-14,643, expression of these genes was reduced in a manner that paralleled the reduction in activated hepatic stellate cells and near resolution of liver fibrosis. In conclusion, the present study shows that MCD diet-induced fibrosing steatohepatitis can be reversed by treatment with Wy-14,643. It is likely that activation of PPARalpha reverses fibrosis indirectly by reducing stimuli, such as lipid peroxides, and activation of cells responsible for promoting hepatic fibrosis.

No effect of rosiglitazone for treatment of HIV-1 lipoatrophy: randomised, double-blind, placebo-controlled trial

BACKGROUND

Lipodystrophy commonly complicates antiretroviral therapy of HIV-1 infection. Thiazolidinediones such as rosiglitazone promote subcutaneous fat growth in type 2 diabetics and adults with congenital lipodystrophy, and can prevent HIV-1 protease inhibitor toxicity to adipocytes in vitro. We postulated that rosiglitazone would improve HIV lipoatrophy.

METHODS

108 HIV-1-infected lipoatrophic adults on antiretroviral therapy were randomised to rosiglitazone 4 mg twice daily (n=53) or matching placebo (n=55) for 48 weeks. The study had 80% power to detect a 0.5 kg difference in changes in limb fat (using dual-energy X-ray absorptiometry) between groups at week 48 by intention-to-treat analysis, and a 0.7 kg difference within each protease inhibitor stratum.

FINDINGS

Limb fat increased by 0.14 kg in the rosiglitazone group and 0.18 kg in the placebo group (mean difference -0.04 kg [95%CI -0.29 to 0.21]; p=0.74 by t test), with three participants (one on rosiglitazone and two controls), lost to follow-up. Rosiglitazone had no significant benefit on any other measure of lipodystrophy, despite large relative increases in plasma adiponectin (4.2 mmol/L [102%]; p<0.0001) and in three markers of insulin sensitivity (p=0.01 to 0.02). Six participants ceased study drug in each group, four participants (three on rosiglitazone and one control) for related adverse events. The main adverse effects, which seem to be almost unique to this population, were asymptomatic hypertriglyceridaemia (mean relative increase 0.9 mmol/L at week 48; p=0.04) and hypercholesterolaemia (1.5 mmol/L; p=0.001).

INTERPRETATION

Rosiglitazone for 48 weeks did not improve lipoatrophy in HIV-1-infected adults receiving antiretroviral therapy. Use of less toxic antiretroviral treatment is necessary to prevent lipoatrophy.

The biology of peroxisome proliferator-activated receptors: relationship with lipid metabolism and insulin sensitivity

Peroxisome proliferator-activated receptors (PPARs) are transcription factors belonging to the superfamily of nuclear receptors. Three isoforms (alpha, delta, and gamma) have been described. They act on DNA response elements as heterodimers with the nuclear retinoic acid receptor. Their natural activating ligands are fatty acids and lipid-derived substrates. PPAR-alpha is present in liver, heart, and, to a lesser extent, skeletal muscle. When activated, it promotes fatty acid oxidation, ketone body synthesis, and glucose sparing. Fibrates, which are used as hypolipidemic drugs, are ligands of PPAR-alpha. PPAR-delta is ubiquitous and could also favor fatty acid oxidation in tissues in which PPAR-alpha is absent or less expressed. PPAR-gamma is expressed in adipose tissue, lower intestine, and cells involved in immunity. Activation of PPAR-gamma induces the differentiation of preadipocytes into adipocytes and stimulates triglyceride storage. Thiazolidinediones are compounds used as hypoglycemic, muscle insulin-sensitizing agents in type 2 diabetes. Unexpectedly, they are activators of PPAR-gamma. Their action on muscle insulin sensitivity may be secondary to the lowering of circulating lipids on PPAR-gamma activation and to the secretion by adipocytes of insulin-sensitizing hormones such as adiponectin, all promoting glucose utilization. The PPARs are thus major regulators of lipid and glucose metabolism, allowing adaptation to the prevailing nutritional environment.

Liver peroxisome proliferator-activated receptor gamma contributes to hepatic steatosis, triglyceride clearance, and regulation of body fat mass

Peroxisome proliferator-activated receptor gamma (PPAR gamma) is a nuclear receptor that mediates the antidiabetic effects of thiazolidinediones. PPAR gamma is present in adipose tissue and becomes elevated in fatty livers, but the roles of specific tissues in thiazolidinedione actions are unclear. We studied the function of liver PPAR gamma in both lipoatrophic A-ZIP/F-1 (AZIP) and wild type mice. In AZIP mice, ablation of liver PPAR gamma reduced the hepatic steatosis but worsened the hyperlipidemia, triglyceride clearance, and muscle insulin resistance. Inactivation of AZIP liver PPAR gamma also abolished the hypoglycemic and hypolipidemic effects of rosiglitazone, demonstrating that, in the absence of adipose tissue, the liver is a primary and major site of thiazolidinedione action. In contrast, rosiglitazone remained effective in non-lipoatrophic mice lacking liver PPAR gamma, suggesting that adipose tissue is the major site of thiazolidinedione action in typical mice with adipose tissue. Interestingly, mice without liver PPAR gamma, but with adipose tissue, developed relative fat intolerance, increased adiposity, hyperlipidemia, and insulin resistance. Thus, liver PPAR gamma regulates triglyceride homeostasis, contributing to hepatic steatosis, but protecting other tissues from triglyceride accumulation and insulin resistance.

Central role of PPARalpha-dependent hepatic lipid turnover in dietary steatohepatitis in mice

We have proposed that steatohepatitis results from reactive oxygen species (ROS) acting on accumulated fatty acids to form proinflammatory lipoperoxides. Cytochrome P450 4a (Cyp4a) and Cyp2e1 are potential hepatic sources of ROS. We tested the hypothesis that increasing Cyp4a through activation of peroxisome proliferator-activated receptor alpha (PPARalpha) should aggravate steatohepatitis produced by feeding a methionine and choline deficient (MCD) diet. Conversely, we assessed dietary steatohepatitis in PPARalpha(-/-) mice that cannot up-regulate Cyp4a. Male wild type (wt) or PPARalpha(-/-) mice (C57BL6 background) were fed the MCD diet with or without Wy-14,643 (0.1% wt/wt), a potent PPARalpha agonist. Controls were fed the same diet supplemented with methionine and choline. After 5 weeks, wt mice fed the MCD diet developed moderate steatohepatitis and alanine aminotransferase (ALT) levels were increased. Wy-14,643 prevented rather than increased liver injury; ALT levels were only mildly elevated whereas steatohepatitis was absent. Wy-14,643 up-regulated mRNA for liver fatty acid binding protein and peroxisomal beta-oxidation enzymes (acyl-CoA oxidase, bifunctional enzyme, and ketothiolase), thereby reducing hepatic triglycerides and preventing steatosis. In wt mice, dietary feeding up-regulated Cyp4a14 mRNA 2.7-fold and increased hepatic lipoperoxides compared with controls. Wy-14,643 prevented hepatic lipoperoxides from accumulating despite an 18-fold increase in both Cyp4a10 and Cyp4a14 mRNA. PPARalpha(-/-) mice fed the MCD diet developed more severe steatohepatitis than wt mice, and were unaffected by Wy-14,643. In conclusion, PPARalpha activation both increases Cyp4a expression and enhances hepatic lipid turnover; the latter effect removes fatty acids as substrate for lipid peroxidation and is sufficiently powerful to prevent the development of dietary steatohepatitis.

Peroxisome proliferator-activated receptor-alpha agonist treatment in a transgenic model of type 2 diabetes reverses the lipotoxic state and improves glucose homeostasis

Abnormalities in insulin action are the characteristics of type 2 diabetes. Dominant-negative muscle-specific IGF-I receptor (MKR) mice exhibit elevated lipid levels at an early age and eventually develop type 2 diabetes. To evaluate the role of elevated lipids in the progression of the diabetic state, MKR mice were treated with WY14,643, a peroxisome proliferator-activated receptor (PPAR)-alpha agonist. WY14,643 treatment markedly reduced serum fatty acid and triglyceride levels within a few days, as well as muscle triglyceride levels, and subsequently normalized glucose and insulin levels in MKR mice. Hyperinsulinemic-euglycemic clamp analysis showed that WY14,643 treatment enhanced muscle and adipose tissue glucose uptake by improving whole-body insulin sensitivity. Insulin suppression of endogenous glucose production by the liver of MKR mice was also improved. The expression of genes involved in fatty acid oxidation was increased in liver and skeletal muscle, whereas gene expression levels of hepatic gluconeogenic enzymes were decreased in WY14,643-treated MKR mice. WY14,643 treatment also improved the pattern of glucose-stimulated insulin secretion from the perfused pancreata of MKR mice and reduced the beta-cell mass. Taken together, these findings suggest that the reduction in circulating or intracellular lipids by activation of PPAR-alpha improved insulin sensitivity and the diabetic condition of MKR mice.

Lipotoxicity: when tissues overeat

PURPOSE OF REVIEW

This review will provide the reader with an update on our understanding of the adverse effects of fatty acid accumulation in non-adipose tissues, a phenomenon known as lipotoxicity. Recent studies will be reviewed. Cellular mechanisms involved in the lipotoxic response will be discussed. Physiologic responses to lipid overload and therapeutic approaches to decreasing lipid accumulation will be discussed, as they add to our understanding of important pathophysiologic mechanisms.

RECENT FINDINGS

Excess lipid accumulation in non-adipose tissues may arise in the setting of high plasma free fatty acids or triglycerides. Alternatively, lipid overload results from mismatch between free fatty acid import and utilization. Evidence from human studies and animal models suggests that lipid accumulation in the heart, skeletal muscle, pancreas, liver, and kidney play an important role in the pathogenesis of heart failure, obesity and diabetes. Excess free fatty acids may impair normal cell signaling, causing cellular dysfunction. In some circumstances, excess free fatty acids induce apoptotic cell death.

SUMMARY

Recent studies provide clues regarding the cellular mechanisms that determine whether excess lipid accumulation is well tolerated or cytotoxic. Critical in this process are physiologic mechanisms for directing excess free fatty acids to specific tissues as well as cellular mechanisms for channeling excess fatty acid to particular metabolic fates. Insight into these mechanisms may contribute to the development of more effective therapies for common human disorders in which lipotoxicity contributes to pathogenesis.

Transplantation of adipose tissue lacking leptin is unable to reverse the metabolic abnormalities associated with lipoatrophy

Severe adipose tissue deficiency (lipoatrophy) causes insulin-resistant diabetes, elevated serum triglyceride and fatty acid levels, and massive triglyceride deposition in the liver. In lipoatrophic A-ZIP/F-1 mice, transplantation of normal adipose tissue greatly improved these parameters, whereas 1 week of leptin infusion had more modest effects. In contrast, leptin infusion was strikingly more effective in the aP2-n sterol response element binding protein 1 lipoatrophic mouse. Here we show that a longer duration of leptin infusion further improves the metabolic status of the A-ZIP/F-1 mice and that genetic background does not make a major contribution to the effect of leptin on glucose and insulin levels. Adipose transplantation using leptin-deficient ob/ob fat had no effect on the phenotype of the A-ZIP/F-1 mice. Moreover, the presence of ob/ob adipose tissue did not enhance the effects of leptin infusion. Serum adiponectin levels were 2% of control levels in the A-ZIP/F-1 mouse and increased only twofold with adipose transplantation and not at all after leptin infusion, suggesting that adiponectin deficiency is not a major contributor to the diabetic phenotype. Taken together, these results suggest that sequestration of triglycerides into fat may not be enough to restore a nondiabetic phenotype and that leptin deficiency plays a major role in causing the metabolic complications of lipoatrophy.