The farnesoid X receptor regulates adipocyte differentiation and function by promoting peroxisome proliferator-activated receptor-gamma and interfering with the Wnt/beta-catenin pathways.

The bile acid receptor farnesoid X receptor (FXR) is expressed in adipose tissue, but its function remains poorly defined. Peroxisome proliferator-activated receptor-γ (PPARγ) is a master regulator of adipocyte differentiation and function. The aim of this study was to analyze the role of FXR in adipocyte function and to assess whether it modulates PPARγ action. Therefore, we tested the responsiveness of FXR-deficient mice (FXR(-/-)) and cells to the PPARγ activator rosiglitazone. Our results show that genetically obese FXR(-/-)/ob/ob mice displayed a resistance to rosiglitazone treatment. In vitro, rosiglitazone treatment did not induce normal adipocyte differentiation and lipid droplet formation in FXR(-/-) mouse embryonic fibroblasts (MEFs) and preadipocytes. Moreover, FXR(-/-) MEFs displayed both an increased lipolysis and a decreased de novo lipogenesis, resulting in reduced intracellular triglyceride content, even upon PPARγ activation. Retroviral-mediated FXR re-expression in FXR(-/-) MEFs restored the induction of adipogenic marker genes during rosiglitazone-forced adipocyte differentiation. The expression of Wnt/ß-catenin pathway and target genes was increased in FXR(-/-) adipose tissue and MEFs. Moreover, the expression of several endogenous inhibitors of this pathway was decreased early during the adipocyte differentiation of FXR(-/-) MEFs. These findings demonstrate that FXR regulates adipocyte differentiation and function by regulating two counteracting pathways of adipocyte differentiation, the PPARγ and Wnt/ß-catenin pathways.

Obesity resistant mechanisms in the Lean polygenic mouse model as indicated by liver transcriptome and expression of selected genes in skeletal muscle.

BACKGROUND: Divergently selected Lean and Fat mouse lines represent unique models for a polygenic form of resistance and susceptibility to obesity development. Previous research on these lines focused mainly on obesity-susceptible factors in the Fat line. This study aimed to examine the molecular basis of obesity-resistant mechanisms in the Lean line by analyzing various fat depots and organs, the liver transcriptome of selected metabolic pathways, plasma and lipid homeostasis and expression of selected skeletal muscle genes. RESULTS: Expression profiling using our custom Steroltalk v2 microarray demonstrated that Lean mice exhibit a higher hepatic expression of cholesterol biosynthesis genes compared to the Fat line, although this was not reflected in elevation of total plasma or liver cholesterol. However, FPLC analysis showed that protective HDL cholesterol was elevated in Lean mice. A significant difference between the strains was also found in bile acid metabolism. Lean mice had a higher expression of Cyp8b1, a regulatory enzyme of bile acid synthesis, and the Abcb11 bile acid transporter gene responsible for export of acids to the bile. Additionally, a higher content of blood circulating bile acids was observed in Lean mice. Elevated HDL and upregulation of some bile acids synthesis and transport genes suggests enhanced reverse cholesterol transport in the Lean line--the flux of cholesterol out of the body is higher which is compensated by upregulation of endogenous cholesterol biosynthesis. Increased skeletal muscle Il6 and Dio2 mRNA levels as well as increased activity of muscle succinic acid dehydrogenase (SDH) in the Lean mice demonstrates for the first time that changes in muscle energy metabolism play important role in the Lean line phenotype determination and corroborate our previous findings of increased physical activity and thermogenesis in this line. Finally, differential expression of Abcb11 and Dio2 identifies novel strong positional candidate genes as they map within the quantitative trait loci (QTL) regions detected previously in crosses between the Lean and Fat mice. CONCLUSION: We identified novel candidate molecular targets and metabolic changes which can at least in part explain resistance to obesity development in the Lean line. The major difference between the Lean and Fat mice was in increased liver cholesterol biosynthesis gene mRNA expression, bile acid metabolism and changes in selected muscle genes' expression in the Lean line. The liver Abcb11 and muscle Dio2 were identified as novel positional candidate genes to explain part of the phenotypic difference between the Lean and Fat lines.

Human apolipoprotein A-II determines plasma triglycerides by regulating lipoprotein lipase activity and high-density lipoprotein proteome.

INTRODUCTION: Apolipoprotein (apo) A-II is the second most abundant high-density lipoprotein (HDL) apolipoprotein. We assessed the mechanism involved in the altered postprandial triglyceride-rich lipoprotein metabolism of female human apoA-II-transgenic mice (hapoA-II-Tg mice), which results in up to an 11-fold increase in plasma triglyceride concentration. The relationships between apoA-II, HDL composition, and lipoprotein lipase (LPL) activity were also analyzed in a group of normolipidemic women. METHODS AND RESULTS: Triglyceride-rich lipoprotein catabolism was decreased in hapoA-II-Tg mice compared to control mice. This suggests that hapoA-II, which was mainly associated with HDL during fasting and postprandially, impairs triglyceride-rich lipoprotein lipolysis. HDL isolated from hapoA-II-Tg mice impaired bovine LPL activity. Two-dimensional gel electrophoresis, mass spectrometry, and immunonephelometry identified a marked deficiency in the HDL content of apoA-I, apoC-III, and apoE in these mice. In normolipidemic women, apoA-II concentration was directly correlated with plasma triglyceride and inversely correlated with the HDL-apoC-II+apoE/apoC-III ratio [corrected]. HDL-mediated induction of LPL activity was inversely correlated with apoA-II and directly correlated with the HDL-apoC-II+apoE/apoC-III ratio [corrected]. Purified hapoA-II displaced apoC-II, apoC-III, and apoE from human HDL2. Human HDL3 was, compared to HDL2, enriched in apoA-II but poorer in apoC-II, apoC-III, and apoE. CONCLUSIONS: ApoA-II plays a crucial role in triglyceride catabolism by regulating LPL activity, at least in part, through HDL proteome modulation.

Atheroprotective effect of human apolipoprotein A5 in a mouse model of mixed dyslipidemia.

Hypertriglyceridemia is an independent risk factor for coronary artery disease. Because apolipoprotein (Apo)A5 regulates plasma triglyceride levels, we investigated the impact of human (h)ApoA5 on atherogenesis. The influence of hApoA5 transgenic expression was studied in the ApoE2 knock-in mouse model of mixed dyslipidemia. Our results demonstrate that hApoA5 lowers plasma triglyceride levels in Western diet-fed ApoE2 knock-in mice. Moreover, atherosclerotic lesion development was significantly decreased in the hApoA5 transgenic mice. Finally, pharmacologic activation of hApoA5 expression by the peroxisome proliferator-activated receptor-alpha agonist fenofibrate resulted in an enhanced atheroprotection. These results identify an atheroprotective role of hApoA5 in a mouse model of mixed dyslipidemia.

Tissue-specific roles of ABCA1 influence susceptibility to atherosclerosis.

OBJECTIVE: The ATP-binding cassette transporter, subfamily A, member 1 (ABCA1) plays a key role in HDL cholesterol metabolism. However, the role of ABCA1 in modulating susceptibility to atherosclerosis is controversial. METHODS AND RESULTS: We investigated the role of ABCA1 in atherosclerosis using a combination of overexpression and selective deletion models. First, we examined the effect of transgenic overexpression of a full-length human ABCA1-containing bacterial artificial chromosome (BAC) in the presence or absence of the endogenous mouse Abca1 gene. ABCA1 overexpression in the atherosclerosis-susceptible Ldlr(-/-) background significantly reduced the development of atherosclerosis in both the presence and absence of mouse Abca1. Next, we used mice with tissue-specific inactivation of Abca1 to dissect the discrete roles of Abca1 in different tissues on susceptibility to atherosclerosis. On the Apoe(-/-) background, mice lacking hepatic Abca1 had significantly reduced HDL cholesterol and accelerated atherosclerosis, indicating that the liver is an important site at which Abca1 plays an antiatherogenic role. In contrast, mice with macrophage-specific inactivation of Abca1 on the Ldlr(-/-) background displayed no change in atherosclerotic lesion area. CONCLUSIONS: These data indicate that physiological expression of Abca1 modulates the susceptibility to atherosclerosis and establish hepatic Abca1 expression as an important site of atheroprotection. In contrast, we show that selective deletion of macrophage Abca1 does not significantly modulate atherogenesis.

Rexinoid bexarotene modulates triglyceride but not cholesterol metabolism via gene-specific permissivity of the RXR/LXR heterodimer in the liver.

OBJECTIVE: Bexarotene (Targretin) is a clinically used antitumoral agent which exerts its action through binding to and activation of the retinoid-X-receptor (RXR). The most frequent side-effect of bexarotene administration is an increase in plasma triglycerides, an independent risk factor of cardiovascular disease. The molecular mechanism behind this hypertriglyceridemia remains poorly understood. METHODS AND RESULTS: Using wild-type and LXR alpha/beta-deficient mice, we show here that bexarotene induces hypertriglyceridemia and activates hepatic LXR-target genes of lipogenesis in an LXR-dependent manner, hence exerting a permissive effect on RXR/LXR heterodimers. Interestingly, RNA analysis and Chromatin Immunoprecipitation assays performed in the liver reveal that the in vivo permissive effect of bexarotene on the RXR/LXR heterodimer is restricted to lipogenic genes without modulation of genes controlling cholesterol homeostasis. CONCLUSIONS: These findings demonstrate that the hypertriglyceridemic action of bexarotene occurs via the RXR/LXR heterodimer and show that RXR heterodimers can act with a selective permissivity on target genes of specific metabolic pathways in the liver.

Despite antiatherogenic metabolic characteristics, SCD1-deficient mice have increased inflammation and atherosclerosis.

OBJECTIVE: Absence of stearoyl-CoA desaturase-1 (SCD1) in mice reduces plasma triglycerides and provides protection from obesity and insulin resistance, which would be predicted to be associated with reduced susceptibility to atherosclerosis. The aim of this study was to determine the effect of SCD1 deficiency on atherosclerosis. METHODS AND RESULTS: Despite an antiatherogenic metabolic profile, SCD1 deficiency increases atherosclerosis in hyperlipidemic low-density lipoprotein receptor (LDLR)-deficient mice challenged with a Western diet. Lesion area at the aortic root is significantly increased in males and females in two models of SCD1 deficiency. Inflammatory changes are evident in the skin of these mice, including increased intercellular adhesion molecule (ICAM)-1 and ulcerative dermatitis. Increases in ICAM-1 and interleukin-6 are also evident in plasma of SCD1-deficient mice. HDL particles demonstrate changes associated with inflammation, including decreased plasma apoA-II and apoA-I and paraoxonase-1 and increased plasma serum amyloid A. Lipopolysaccharide-induced inflammatory response and cholesterol efflux are not altered in SCD1-deficient macrophages. In addition, when SCD1 deficiency is limited to bone marrow-derived cells, lesion size is not altered in LDLR-deficient mice. CONCLUSIONS: These studies reinforce the crucial role of chronic inflammation in promoting atherosclerosis, even in the presence of antiatherogenic biochemical and metabolic characteristics.

Combination therapy of statins and fibrates in the management of cardiovascular risk.

PURPOSE OF REVIEW: Despite the fact that statin treatment substantially reduces cardiovascular morbidity and mortality, many treated patients still experience a high residual risk. Statins lower LDL cholesterol (LDL-C), with limited effects on other lipid parameters. Fibrates improve atherogenic dyslipidemia characterized by high triglyceride and/or low HDL cholesterol levels and elevated concentrations of small dense LDL particles, with or without high LDL-C levels. Fibrates decrease cardiovascular morbidity, especially in patients with the metabolic syndrome. The purpose of this review is to provide a rationale for the combined use of statins and fibrates in the management of patients with high residual cardiovascular risk related to atherogenic dyslipidemia and persisting after single therapy. RECENT FINDINGS: A meta-analysis from 14 randomized trials conducted in high-risk patients reported that statin therapy is effective in reducing the proportional risk for major vascular events by 21% for each mmol/l lowering of LDL-C. However, on an average, 14% of patients still experienced an event despite being allocated to statin. Beyond LDL-C, other factors, including triglycerides, non-HDL cholesterol, HDL cholesterol, and apolipoprotein B, have been identified as factors determining residual risk, and normalization of these parameters may further decrease cardiovascular disease in patients treated with statins. Data from fibrate trials indicate that these drugs are particularly effective in reducing cardiovascular morbidity in patients with atherogenic dyslipidemia. SUMMARY: Reducing the residual cardiovascular risk in patients treated with statins requires addressing multiple lipid goals. In this context, future therapeutic interventions based on combination therapy, such as statins and fibrates, appear particularly promising.

Intestinal ABCA1 directly contributes to HDL biogenesis in vivo.

Plasma HDL cholesterol levels are inversely related to risk for atherosclerosis. The ATP-binding cassette, subfamily A, member 1 (ABCA1) mediates the rate-controlling step in HDL particle formation, the assembly of free cholesterol and phospholipids with apoA-I. ABCA1 is expressed in many tissues; however, the physiological functions of ABCA1 in specific tissues and organs are still elusive. The liver is known to be the major source of plasma HDL, but it is likely that there are other important sites of HDL biogenesis. To assess the contribution of intestinal ABCA1 to plasma HDL levels in vivo, we generated mice that specifically lack ABCA1 in the intestine. Our results indicate that approximately 30% of the steady-state plasma HDL pool is contributed by intestinal ABCA1 in mice. In addition, our data suggest that HDL derived from intestinal ABCA1 is secreted directly into the circulation and that HDL in lymph is predominantly derived from the plasma compartment. These data establish a critical role for intestinal ABCA1 in plasma HDL biogenesis in vivo.

Efficacy of peroxisome proliferator-activated receptor agonists in diabetes and coronary artery disease.

The greatest clinical challenge in type 2 diabetes mellitus is the prevention of its long-term complications, many of which are of a cardiovascular nature. Despite the progress in cardiovascular risk management of diabetes patients using lipid-lowering and antihypertensive drugs, a substantial residual risk persists. Indeed, treated diabetes patients have a similar risk as untreated nondiabetic individuals. Although glycemic control through the use of antihyperglycemic agents improves microvascular complications, macrovascular disease risk is not reduced. These observations point to the need for additional therapeutic approaches in order to better control global cardiovascular risk. The peroxisome proliferator-activated receptor (PPAR) family members play major roles in the regulation of lipid and glucose metabolism and immune-inflammatory processes, making these transcription factors ideal targets for such therapeutic strategies. This review discusses our current knowledge of the effectiveness of PPAR-based therapeutics, focusing exclusively on cardiovascular disease in type 2 diabetes mellitus and the future prospects for novel generation of PPAR agonists.

Reduced plasma high-density lipoprotein cholesterol in hyperthyroid mice coincides with decreased hepatic adenosine 5'-triphosphate-binding cassette transporter 1 expression.

The aim of the study was to investigate the influence of severe hyperthyroidism on plasma high-density lipoprotein cholesterol (HDL-C). Recently, it was shown in mice that increasing doses of T(3) up-regulate hepatic expression of scavenger receptor class B, type I, resulting in increased clearance of plasma HDL-C. Here, we show that severe hyperthyroidism in mice did not affect hepatic expression of scavenger receptor class B, type I, but reduced hepatic expression of ATP-binding cassette transporter 1, accompanied by a 40% reduction of HDL-C. The sterol content of bile, liver, and feces was markedly increased, accompanied by up-regulation of hepatic cholesterol 7alpha-hydroxylase, and ATP-binding cassette transporter 5, which is known to promote biliary sterol secretion upon dimerization with ATP-binding cassette transporter 8. Both control and hyperthyroid mice exerted identical plasma clearance of iv injected [(3)H]HDL-C, supporting the view that severe hyperthyroidism does not affect HDL-C clearance but, rather, its formation via hepatic ATP-binding cassette transporter 1.

Both hepatic and extrahepatic ABCA1 have discrete and essential functions in the maintenance of plasma high-density lipoprotein cholesterol levels in vivo.

BACKGROUND: Extrahepatic tissues have long been considered critical contributors of cholesterol to nascent HDL particles in the reverse cholesterol transport pathway, in which ABCA1 plays the crucial role. Recent studies, however, including both overexpression and deletion of ABCA1 selectively in the liver, have highlighted the primary role of the liver in the maintenance of HDL levels in vivo. METHODS AND RESULTS: The availability of mice with complete deletion of ABCA1 (total knockout [TKO]) and with liver-specific deletion of ABCA1 (LSKO) has enabled us to dissect the discrete roles of hepatic relative to extrahepatic ABCA1 in HDL biogenesis. Delivery of adenoviral ABCA1 resulted in selective expression of physiological levels of ABCA1 in the livers of both LSKO and TKO mice, resulting in increased HDL cholesterol (HDL-C). Expression of ABCA1 in the liver of LSKO mice resulted in plasma HDL-C levels that were similar to those in wild-type mice and significantly above those seen in similarly treated TKO mice. HDL particles from ABCA1-expressing LSKO mice were larger and contained significantly increased cholesterol compared with TKO mice. Infusion of human apolipoprotein A-I/phospholipid reconstituted HDL particles normalized plasma HDL-C levels in LSKO mice but had no effect on HDL-C levels in TKO mice. CONCLUSIONS: Although hepatic ABCA1 appears crucial for phospholipid transport, extrahepatic tissues play an important role in cholesterol transfer to nascent HDL particles. These data highlight the discrete and specific roles of both liver and extrahepatic ABCA1 in HDL biogenesis in vivo and indicate that ABCA1 shows lipid cargo selectivity depending on its site of expression.

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.

Increased ABCA1 activity protects against atherosclerosis.

The ABC transporter ABCA1 plays a key role in the first steps of the reverse cholesterol transport pathway by mediating lipid efflux from macrophages. Previously, it was demonstrated that human ABCA1 overexpression in vivo in transgenic mice results in a mild elevation of plasma HDL levels and increased efflux of cholesterol from macrophages. In this study, we determined the effect of overexpression of ABCA1 on atherosclerosis development. Human ABCA1 transgenic mice (BAC(+)) were crossed with ApoE(-/-) mice, a strain that spontaneously develop atherosclerotic lesions. BAC(+)ApoE(-/-) mice developed dramatically smaller, less-complex lesions as compared with their ApoE(-/-) counterparts. In addition, there was increased efflux of cholesterol from macrophages isolated from the BAC(+)ApoE(-/-) mice. Although the increase in plasma HDL cholesterol levels was small, HDL particles from BAC(+)ApoE(-/-) mice were significantly better acceptors of cholesterol. Lipid analysis of HDL particles from BAC(+)ApoE(-/-) mice revealed an increase in phospholipid levels, which was correlated significantly with their ability to enhance cholesterol efflux.

Genetically-engineered animals as research models for atherosclerosis: their use for the characterization of PPAR agonists in the treatment of cardiometabolic disorders.

Experimental approaches to understand the pathogenesis and to develop treatments of atherosclerosis involve studies in animal and cellular models. However, relevant animal models are rare since atherosclerosis is a disease that naturally affects only humans and one or two other species (pigs and certain primates). For a long time, atherosclerosis studies were carried out using diet-induced atherosclerosis models, even though the diets were unphysiological and the arterial lesions that developed were often limited in size, composition and location. During the last decade, with the advent of molecular genetics and genetic manipulation techniques, the development of genetically-engineered animals, mainly mice, allowed an explosion in the number of models resulting in a tremendous progress in atherosclerosis research and enhancement of our understanding of the disease. Atherosclerosis is a multifactorial disease which normally develops very slowly and asymptomatically during several decades, leading to atheromatous plaque formation. Once the plaque is weakened, its rupture or erosion induces severe clinical complications, such as myocardial infarction or cerebrovascular accidents. Several risk factors predispose to atherosclerosis including hypertension and abnormalities in lipoprotein metabolism and glucose homeostasis. The formation of the atherosclerotic lesion is a complex process, characterized by the presence of lipid-laden monocyte-derived macrophages (called foam cells), establishing therefore a status of chronic inflammation. The dysregulated expression of genes encoding proteins involved in the control of metabolic pathways contributes to vascular inflammation and the development of atherosclerosis. The expression of these genes is controlled by different transcription factors amongst which are the Peroxisome Proliferator-Activated Receptor (PPAR) family of nuclear receptors. This review focuses on the use of genetically-engineered animals as models for experimental atherosclerosis research, pointing out their contribution to investigate the implication of PPARs and their ligands in regulating metabolic and inflammatory abnormalities predisposing to atherosclerosis development.

The RXR agonist bexarotene improves cholesterol homeostasis and inhibits atherosclerosis progression in a mouse model of mixed dyslipidemia.

OBJECTIVE: The activity of the antitumoral agent bexarotene (Targretin, Bexarotene) depends on its binding to the nuclear retinoid-X receptor (RXR) and subsequent transcriptional regulation of target genes. Through RXR activation, bexarotene may modulate numerous metabolic pathways involved in atherosclerosis. Here, we investigated the effect of bexarotene on atherosclerosis progression in a dyslipidemic murine model, the human apolipoprotein E2 knockin mouse, that develops essentially macrophage-laden lesions. METHODS AND RESULTS: Atherosclerotic lesions together with different metabolic pathways involved in atherosclerosis were investigated in mice treated or not with bexarotene. Bexarotene protects from atherosclerosis development in mice, at least in part by improving the circulating cholesterol distribution profile likely via a marked decrease of dietary cholesterol absorption caused by modulation of intestinal expression of genes recently identified as major players in this process, Niemann-Pick-C1-Like1 (NPC1L1) and CD13. This atheroprotection appears despite a strong hypertriglyceridemia. Moreover, bexarotene treatment only modestly modulates inflammatory gene expression in the vascular wall, but markedly enhanced the capacity of macrophages to efflux cellular lipids. CONCLUSIONS: These data provide evidence of a favorable pharmacological effect of bexarotene on atherosclerosis despite the induction of hypertriglyceridemia, likely via a beneficial action on intestinal absorption and macrophage efflux.

Cholesterol 7alpha-hydroxylase deficiency in mice on an APOE*3-Leiden background increases hepatic ABCA1 mRNA expression and HDL-cholesterol.

OBJECTIVE: High-density lipoprotein (HDL) plays a key role in protection against development of atherosclerosis by reducing inflammation, protecting against LDL oxidation, and promoting reverse cholesterol transport from peripheral tissues to the liver for secretion into bile. Cholesterol 7alpha-hydroxylase (Cyp7a1) catalyzes the rate-limiting step in the intrahepatic conversion of cholesterol to bile acids that may have a role in HDL metabolism. We investigated the effect of Cyp7a1 deficiency on HDL metabolism in APOE*3-Leiden transgenic mice. METHODS AND RESULTS: Reduced bile acid biosynthesis in Cyp7a1-/-.APOE*3-Leiden mice versus APOE*3-Leiden mice did not affect total plasma cholesterol levels, but the distribution of cholesterol over various lipoproteins was different. Cholesterol was decreased in apoB-containing lipoproteins (ie, VLDL and IDL/LDL), whereas cholesterol was increased in HDL. The activity of PLTP and LCAT, which play a role in HDL catabolism, were not changed, and neither was HDL clearance. However, the hepatic cholesterol content was 2-fold increased, which was accompanied by a 2-fold elevated expression of hepatic ABCA1 and increased rate of cholesterol efflux from the liver to HDL. CONCLUSIONS: Strongly reduced bile acid synthesis in Cyp7a1-/-.APOE*3-Leiden mice leads to increased plasma HDL-cholesterol levels, as related to an increased hepatic expression of ABCA1.

PPARalpha and PPARgamma dual agonists for the treatment of type 2 diabetes and the metabolic syndrome.

The discovery of the crucial role of peroxisome proliferator-activated receptors (PPARs) as regulators of lipid and glucose metabolism has raised interest in the development of synthetic ligands as potential tools for therapeutic intervention in type 2 diabetes and the metabolic syndrome. PPARalpha activators primarily improve dyslipidemia, whereas thiazolidinediones are potent PPARgamma activators that improve insulin resistance. Important research programs to develop agonists that combine the therapeutic effects of both PPARalpha- and PPARgamma-selective agonists, creating the expectation of greater efficacy and other advantages in the treatment of type 2 diabetes and the metabolic syndrome, have therefore been undertaken. Among these dual PPARalpha/gamma agonists, compounds that belong to the glitazar class are in the most advanced stage of development. However, although they demonstrated beneficial impact over selective PPAR agonists by improving both lipid and glucose homeostasis, safety has been a critical issue and has led to the discontinuation of their development because of adverse toxicity profiles. However, the target-related mechanism responsible for the identified safety issues and the relevance of rodent toxicities to the human situation are unclear. Therefore, future development of dual PPARalpha/gamma agonists with selective PPAR modulator activity appears appropriate and should be feasible.

Paradoxical exacerbation of combined hyperlipidemia in human apolipoprotein A-II transgenic mice treated with fenofibrate.

Apolipoprotein (apo) A-II has been biochemically and genetically linked to familial combined hyperlipidemia. Human ApoA-II transgenic mice and peroxisome proliferator-activated receptor alpha (PPARalpha)-deficient mice share some similar phenotypic characteristics. The aim of this study was to determine whether a fibrate-induced PPARalpha activation corrects the combined hyperlipidemia present in human apoA-II transgenic mice. ApoA-II transgenic mice were treated with fenofibrate (250 mg/kg) for 13 days. After this period, they presented a remarkable 8-fold increase in plasma triglycerides. This was concomitant with a 4-fold increase in non-high-density lipoprotein (non-HDL) cholesterol, a quantitatively similar decrease in HDL cholesterol and a severe reduction in mouse plasma apoA-I and apoA-II. Fenofibrate stimulated liver fatty acid beta-oxidation, increased the transcriptional expression of carnitine palmitoyltransferase 1 and phospholipid transfer protein, and decreased expression of apoA-I and apoC-III. However, very-low-density lipoprotein (VLDL)-triglyceride production and lipoprotein lipase (LPL) activities and the expression of other PPARalpha target genes were similar in mice treated with vehicle and fenofibrate. Further, fenofibrate-treated mice presented decreased in vivo [3H]VLDL catabolism and decreased VLDL-triglyceride hydrolysis by exogenous LPL. Therefore, the paradoxical enhancement of hyperlipidemia in fenofibrate-treated apoA-II transgenic mice is mainly due to decreased VLDL catabolism and, also, to a partial impairment in PPARalpha-signaling.

PPARalpha, but not PPARgamma, activators decrease macrophage-laden atherosclerotic lesions in a nondiabetic mouse model of mixed dyslipidemia.

OBJECTIVE: Peroxisome proliferator-activated receptor (PPAR) alpha and gamma are nuclear receptors that may modulate atherogenesis, not only by correcting metabolic disorders predisposing to atherosclerosis but also by directly acting at the level of the vascular wall. The accumulation of lipid-laden macrophages in the arterial wall is an early pivotal event participating in the initiation and promotion of atherosclerotic lesion formation. Because PPARalpha and gamma modulate macrophage gene expression and cellular function, it has been suggested that their ligands may modulate atherosclerosis development via direct effects on macrophages. In this report, we investigated the effect of a PPARalpha ligand (fenofibrate) and 2 PPARgamma ligands (rosiglitazone and pioglitazone) on atherogenesis in a dyslipidemic nondiabetic murine model that develops essentially macrophage-laden lesions. METHODS AND RESULTS: Mice were fed a Western diet supplemented or not with fenofibrate (100 mpk), rosiglitazone (10 mpk), or pioglitazone (40 mpk) for 10 weeks. Atherosclerotic lesions together with metabolic parameters were measured after treatment. Fenofibrate treatment significantly improved lipoprotein metabolism toward a less atherogenic phenotype but did not affect insulin sensitivity. Contrarily, rosiglitazone and pioglitazone improved glucose homeostasis, whereas they did not improve lipoprotein metabolism. Fenofibrate treatment significantly decreased the accumulation of lipids and macrophages in the aortic sinus. However, surprisingly, neither rosiglitazone nor pioglitazone had an effect on lesion lipid accumulation or macrophage content. CONCLUSIONS: These results indicate that in a dyslipidemic nondiabetic murine model, PPARalpha, but not PPARgamma, activators protect against macrophage foam cell formation.