The peroxisome proliferator activated receptors (PPARS) and their effects on lipid metabolism and adipocyte differentiation

Differentiation of human marrow stromal precursor cells: bone morphogenetic protein-2 increases OSF2/CBFA1, enhances osteoblast commitment, and inhibits late adipocyte maturation

Because regulation of the differentiation to osteoblasts and adipocytes from a common progenitor in bone marrow stroma is poorly understood, we assessed effects of bone morphogenetic protein-2 (BMP-2) on a conditionally immortalized human marrow stromal cell line, hMS(2-6), which is capable of differentiation to either lineage. BMP-2 did not affect hMS(2-6) cell proliferation but enhanced osteoblast differentiation as assessed by a 1.8-fold increase in expression of OSF2/CBFA1 (a gene involved in commitment to the osteoblast pathway), by increased mRNA expression and protein secretion for alkaline phosphatase (ALP), type I procollagen and osteocalcin (OC) (except for OC protein), and by increased mineralized nodule formation. Transient transfection with Osf2/Cbfa1 antisense oligonucleotide substantially reduced BMP-2-stimulated expression of ALP mRNA and protein. The effects of BMP-2 on adipocyte differentiation varied: expression of peroxisome proliferator-activated receptor gamma2 (a gene involved in commitment to the adipocyte pathway) was unchanged, mRNA expression of the early differentiation marker, lipoprotein lipase, was increased, and mRNA and protein levels of the late differentiation marker, leptin, and the formation of cytoplasmic lipid droplets were decreased. Thus, by enhancing osteoblast commitment and by inhibiting late adipocyte maturation, BMP-2 acts to shunt uncommitted marrow stromal precursor cells from the adipocyte to the osteoblast differentiation pathway.

Opposite regulation of PPAR-alpha and -gamma gene expression by both their ligands and retinoic acid in brown adipocytes

The peroxisome proliferator-activated receptors (PPARs) are lipid-activated transcription factors involved in the regulation of lipid metabolism and adipocyte differentiation. Little is known, however, about the control of the expression of the genes encoding each of all three receptor subtypes: alpha, delta, and gamma. We have addressed this question in the brown adipocyte, the only cell type that co-expresses high levels of the three PPAR subtypes. Differentiation of brown adipocytes is associated with enhanced expression of PPAR genes. However, whereas PPARgamma and PPARdelta genes are already expressed in preadipocytes, the mRNA for PPARalpha appears suddenly in association with the acquisition of the terminally differentiated phenotype. Both retinoic acid isomers and PPAR agonists, specific for either PPARalpha or PPARgamma, regulate expression of each PPAR subtype gene in the opposite way: they up-regulate PPARalpha and down-regulate PPARgamma. The effects on PPARalpha mRNA are independent of protein synthesis, whereas inhibition of PPARgamma mRNA expression depends on protein synthesis, except when its specific ligand prostaglandin J2 is used. Our results indicate a strictly opposite autoregulation of PPAR subtypes, which supports specific physiological roles for them in controlling brown fat differentiation and thermogenic activity.

Chemical probes that differentially modulate peroxisome proliferator-activated receptor alpha and BLTR, nuclear and cell surface receptors for leukotriene B(4)

Peroxisome proliferator-activated receptor alpha (PPARalpha)is a nuclear receptor for various fatty acids, eicosanoids, and hypolipidemic drugs. In the presence of ligand, this transcription factor increases expression of target genes that are primarily associated with lipid homeostasis. We have previously reported PPARalpha as a nuclear receptor of the inflammatory mediator leukotriene B(4) (LTB(4)) and demonstrated an anti-inflammatory function for PPARalpha in vivo (Devchand, P. R., Keller, H., Peters, J. M., Vazquez, M., Gonzalez, F. J., and Wahli, W. (1996) Nature 384, 39-43). LTB(4) also has a cell surface receptor (BLTR) that mediates proinflammatory events, such as chemotaxis and chemokinesis (Yokomizo, T., Izumi, T., Chang, K., Takuwa, Y., and Shimizu, T. (1997) Nature 387, 620-624). In this study, we report on chemical probes that differentially modulate activity of these two LTB(4) receptors. The compounds selected were originally characterized as synthetic BLTR effectors, both agonists and antagonists. Here, we evaluate the compounds as effectors of the three PPAR isotypes (alpha, beta, and gamma) by transient transfection assays and also determine whether the compounds are ligands for these nuclear receptors by coactivator-dependent receptor ligand interaction assay, a semifunctional in vitro assay. Because the compounds are PPARalpha selective, we further analyze their potency in a biological assay for the PPARalpha-mediated activity of lipid accumulation. These chemical probes will prove invaluable in dissecting processes that involve nuclear and cell surface LTB(4) receptors and also aid in drug discovery programs.

Cloning and gene structure of rat phosphatidylcholine transfer protein, Pctp

Phosphatidylcholine transfer protein (PC-TP) is a cytosolic lipid transfer protein that promotes intermembrane transfer of phosphatidylcholines but no other phospholipids. Although its physiological function remains unknown, phosphatidylcholine transfer protein is enriched in liver and evidence from model systems suggests a role in hepatocellular selection and transport of biliary phospholipids. To facilitate in vivo studies, a cDNA encoding rat PC-TP was cloned by library screening and 5'-rapid amplification of cDNA ends. Genomic cloning demonstrated the rat Pctp gene spans 10. 8kb and is comprised of six exons. The putative transcription initiation site was identified 50bp upstream of the translation initiation site. Nucleotide sequence analysis of the 5'-flanking region revealed a CAAT- but no TATA-box. Transient transfection of a series of 5'-deleted Pctp-promoter-firefly luciferase constructs into Reuber H35 rat hepatoma cells, which express Pctp mRNA, and Gunn rat fibroblasts, which do not, suggest that cis-acting elements in a 637bp promoter region contribute to enhanced expression of PC-TP in liver.

Effect of peroxisome proliferator-activated receptor alpha activators on tumor necrosis factor expression in mice during endotoxemia

Inflammatory mediators orchestrate the host immune and metabolic response to acute bacterial infections and mediate the events leading to septic shock. Tumor necrosis factor (TNF) has long been identified as one of the proximal mediators of endotoxin action. Recent studies have implicated peroxisome proliferator-activated receptor alpha (PPARalpha) as a potential target to modulate regulation of the immune response. Since PPARalpha activators, which are hypolipidemic drugs, are being prescribed for a significant population of older patients, it is important to determine the impact of these drugs on the host response to acute inflammation. Therefore, we examined the role of PPARalpha activators on the regulation of TNF expression in a mouse model of endotoxemia. CD-1 mice treated with dietary fenofibrate or Wy-14,643 had fivefold-higher lipopolysaccharide (LPS)-induced TNF plasma levels than LPS-treated control-fed animals. Higher LPS-induced TNF levels in drug-fed animals were reflected physiologically in significantly lower glucose levels in plasma and a significantly lower 50% lethal dose than those in LPS-treated control-fed animals. Utilizing PPARalpha wild-type (WT) and knockout (KO) mice, we showed that the effect of fenofibrate on LPS-induced TNF expression was indeed mediated by PPARalpha. PPARalpha WT mice fed fenofibrate also had a fivefold increase in LPS-induced TNF levels in plasma compared to control-fed animals. However, LPS-induced TNF levels were significantly decreased and glucose levels in plasma were significantly increased in PPARalpha KO mice fed fenofibrate compared to those in control-fed animals. Data from peritoneal macrophage studies indicate that Wy-14,643 modestly decreased TNF expression in vitro. Similarly, overexpression of PPARalpha in 293T cells decreased activity of a human TNF promoter-luciferase construct. The results from these studies suggest that any anti-inflammatory activity of PPARalpha in vivo can be masked by other systemic effects of PPARalpha activators.

Peroxisome proliferator-activated receptors: mediators of a fast food impact on gene regulation

Peroxisome proliferator-activated receptors are nuclear receptors with pleiotropic effects on intra- and extracellular lipid metabolism, glucose homeostasis, inflammation control, and cell proliferation. This review addresses the respective roles of the different peroxisome proliferator-activated receptor isoforms in these different processes.

Hemostatic factors in hypertriglyceridemic men: effects of a fatty meal before and after triglyceride-lowering treatment with etofibrate

The aims of this double-blind study were to examine whether in hypertriglyceridemic men the ingestion of a standardized fatty meal alters hemostasis negatively and whether triglyceride-lowering treatment with etofibrate for 6 weeks alters fasting and postprandial hemostasis positively, thus reversing the potential negative effects of a fatty meal on postprandial hemostasis. To answer these questions, we measured markers of hemostasis immediately before a standardized fatty meal, and 4, 6, 8, and 10 hours after the meal in 21 hypertriglyceridemic men both before and after treatment with etofibrate. We found that the concentration of plasmin alpha2antiplasmin complex markedly increased for at least 10 hours after the fatty meal, but that the activation of factor XII and the concentration of prothrombin activation fragment1+2 decreased after the fatty meal. These results on factor XII contradict reported in vitro data. Triglyceride-lowering treatment with etofibrate in 10 of these men for 6 weeks increased fasting and postprandial protein C and plasminogen and also slightly decreased the activation of fXII; however, it did not reverse the postprandial increase of PAP or change the decrease of prothrombin activation fragment1+2. Our findings indicate that postprandial lipoproteins alter markers of hemostasis positively in an antithrombotic and profibrinolytic direction. In addition, triglyceride-lowering treatment with etofibrate only slightly improves markers of fasting and postprandial hemostasis in an antithrombotic and profibrinolytic direction.

Cyclo-oxygenase-2-derived prostacyclin mediates embryo implantation in the mouse via PPARdelta

We have demonstrated previously that cyclo-oxygenase-2 (COX2), the rate-limiting enzyme in the biosynthesis of prostaglandins (PGs), is essential for blastocyst implantation and decidualization. However, the candidate PG(s) that participates in these processes and the mechanism of its action remain undefined. Using COX2-deficient mice and multiple approaches, we demonstrate herein that COX2-derived prostacyclin (PGI2) is the primary PG that is essential for implantation and decidualization. Several lines of evidence suggest that the effects of PGI2 are mediated by its activation of the nuclear hormone receptor PPARdelta, demonstrating the first reported biologic function of this receptor signaling pathway.

Peroxisome proliferator-activated receptor-alpha activators regulate genes governing lipoprotein metabolism, vascular inflammation and atherosclerosis

The peroxisome proliferator-activated receptors (PPARs) [alpha, delta (beta) and gamma] form a subfamily of the nuclear receptor gene family. All PPARs are, albeit to different extents, activated by fatty acids and derivatives; PPAR-alpha binds the hypolipidemic fibrates whereas antidiabetic glitazones are ligands for PPAR-gamma. PPAR-alpha activation mediates pleiotropic effects such as stimulation of lipid oxidation, alteration in lipoprotein metabolism and inhibition of vascular inflammation. PPAR-alpha activators increase hepatic uptake and the esterification of free fatty acids by stimulating the fatty acid transport protein and acyl-CoA synthetase expression. In skeletal muscle and heart, PPAR-alpha increases mitochondrial free fatty acid uptake and the resulting free fatty acid oxidation through stimulating the muscle-type carnitine palmitoyltransferase-I. The effect of fibrates on the metabolism of triglyceride-rich lipoproteins is due to a PPAR-alpha dependent stimulation of lipoprotein lipase and an inhibition of apolipoprotein C-III expressions, whereas the increase in plasma HDL cholesterol depends on an overexpression of apolipoprotein A-I and apolipoprotein A-II. PPARs are also expressed in atherosclerotic lesions. PPAR-alpha is present in endothelial and smooth muscle cells, monocytes and monocyte-derived macrophages. It inhibits inducible nitric oxide synthase in macrophages and prevents the IL-1-induced expression of IL-6 and cyclooxygenase-2, as well as thrombin-induced endothelin-1 expression, as a result of a negative transcriptional regulation of the nuclear factor-kappa B and activator protein-1 signalling pathways. PPAR activation also induces apoptosis in human monocyte-derived macrophages most likely through inhibition of nuclear factor-kappa B activity. Therefore, the pleiotropic effects of PPAR-alpha activators on the plasma lipid profile and vascular wall inflammation certainly participate in the inhibition of atherosclerosis development observed in angiographically documented intervention trials with fibrates.

Identification of nuclear receptor corepressor as a peroxisome proliferator-activated receptor alpha interacting protein

Nuclear receptor corepressor (NCoR) was demonstrated to interact strongly with peroxisome proliferator-activated receptor alpha (PPARalpha), and PPARalpha ligands suppressed this interaction. In contrast to the interaction of PPARalpha with the coactivator protein, p300, association of the receptor with NCoR did not require any part of the PPARalpha ligand binding domain. NCoR was found to suppress PPARalpha-dependent transcriptional activation in the context of a PPARalpha.retinoid X receptor alpha (RXRalpha) heterodimeric complex bound to a peroxisome proliferator-responsive element in human embryonic kidney 293 cells. This repression was reversed agonists of either receptor demonstrating a functional interaction between NCoR and PPARalpha.RXRalpha heterodimeric complexes in mammalian cells. NCoR appears to influence PPARalpha signaling pathways and, therefore, may modulate tissue responsiveness to peroxisome proliferators.

Regulation of alpha-foetoprotein gene expression by fatty acids and fibrates

Alpha-foetoprotein (AFP), the major plasma protein in the foetus, is mainly synthesized by yolk sac and foetal liver. It binds polyunsaturated fatty acids and probably controls their metabolism and action. We investigated the effects of fatty acids and fibrates on expression of the AFP gene using two complementary approaches. Treatment with 5-8-11-14 eicosatetraynoic acid (ETYA), an analogue of arachidonic acid, specifically led to lower AFP mRNA levels in cultured rat yolk sac explants whereas treatment with palmitic or oleic acid did not. Clofibric acid and fenofibrate also gave lower AFP mRNA levels. Transient transfection experiments with HepG2 hepatoma cells showed that ETYA and clofibric acid decreased the transcriptional activity of the 7 kb regulatory region of the rat AFP gene. The 330 bp AFP promoter was identified as a target for these down regulating effects.

Up-regulation of liver uncoupling protein-2 mRNA by either fish oil feeding or fibrate administration in mice

Fish oil feeding showed less obesity in rodents, relative to other dietary oils. N-3 fatty acids rich in fish oil and fibrate compounds are peroxisome proliferator-activated receptor alpha (PPARalpha) ligands that stimulate beta-oxidation of fatty acids in liver and are used for treatment of hypertriglycemic patients. Since UCP-2, a member of an uncoupling protein family, has been shown to express in hepatocytes, the effects of these agents on the expression of UCP2 mRNA were investigated. C57BL/6J mice were divided into three groups; the first group was given a high-carbohydrate diet, and the other two groups were given a high-fat diet (60% of total energy) as safflower oil or fish oil for 5 months. Safflower oil diet fed mice developed obesity, but those fed fish oil diet did not. Therefore, the effects of fish oil feeding on the expression of UCP1, UCP2 and UCP3 in liver, skeletal muscle (gastrocnemius), white adipose tissue (WAT) and brown adipose tissue (BAT) were assessed by Northern blotting. Compared with safflower oil feeding, fish oil feeding up-regulated liver UCP2, BAT UCP2 and skeletal muscle UCP3 mRNA, while down-regulated WAT UCP2 and BAT UCP3 mRNA. Among these alterations, 5-fold up-regulation of liver UCP2 mRNA, relative to carbohydrate feeding, was noteworthy. Fenofibrate administration (about 500 mg/kg BW/d) for 2 wks also induced liver UCP2 expression by 9-fold. These data indicated that fish oil feeding and fibrate administration each up-regulated UCP2 mRNA expression in liver possibly via PPARalpha and hence each has the potential of increasing energy expenditure for prevention of obesity.

Tissue-specific effect of clofibrate on rat lipogenic enzyme gene expression

Fibrate derivatives are commonly used to treat hyperlipidaemia; however, the mechanism of the antilipidaemic action of these drugs is still unknown. The effect of clofibrate (fibrate derivative) administration for 14 days on lipogenesis and on malic enzyme (EC 1.1.1.40) and fatty acid synthase (EC 2.3.1.85) gene expression in brown and white adipose tissues and in the liver was examined in rats. The rate of brown adipose tissue lipogenesis in the clofibrate-treated animals was significantly lower than that of the control rats. The rate of liver and white adipose tissue lipogenesis was not affected significantly by clofibrate. In brown adipose tissue, the drug treatment resulted in a depression of fatty acid synthase and malic enzyme mRNA levels. The fatty acid synthase mRNA level did not change significantly in the liver, whereas the malic enzyme mRNA level increased approximately 6-fold in this organ after clofibrate treatment. The malic enzyme mRNA level in white adipose tissue increased about 2-fold, while the fatty acid synthase mRNA level was unchanged after clofibrate feeding. The results presented in this paper provide further evidence that the hypolipidaemia caused by treatment of rats with clofibrate cannot be related to the inhibition of fatty acid synthesis in the liver and white adipose tissue. These data also indicate that clofibrate exhibits tissue specificity.

Increased peroxisomal fatty acid beta-oxidation and enhanced expression of peroxisome proliferator-activated receptor-alpha in diabetic rat liver

To determine whether the increased fatty acid beta-oxidation in the peroxisomes of diabetic rat liver is mediated by a common peroxisome proliferation mechanism, we measured the activation of long-chain (LC) and very long chain (VLC) fatty acids catalyzed by palmitoyl CoA ligase (PAL) and lignoceryl CoA ligase and oxidation of LC (palmitic acid) and VLC (lignoceric acid) fatty acids by isotopic methods. Immunoblot analysis of acyl-CoA oxidase (ACO), and Northern blot analysis of peroxisome proliferator-activated receptor (PPAR-alpha), ACO, and PAL were also performed. The PAL activity increased in peroxisomes and mitochondria from the liver of diabetic rats by 2.6-fold and 2.1 -fold, respectively. The lignoceroyl-CoA ligase activity increased by 2.6-fold in diabetic peroxisomes. Palmitic acid oxidation increased in the diabetic peroxisomes and mitochondria by 2.5-fold and 2.7-fold, respectively, while lignoceric acid oxidation increased by 2.0-fold in the peroxisomes. Immunoreactive ACO protein increased by 2-fold in the diabetic group. The mRNA levels for PPAR-alpha, ACO and PAL increased 2.9-, 2.8- and 1.6-fold, respectively, in the diabetic group. These results suggest that the increased supply of fatty acids to liver in diabetic state stimulates the expression of PPAR-alpha and its target genes responsible for the metabolism of fatty acids.

Peroxisome proliferator-activated receptor alpha in metabolic disease, inflammation, atherosclerosis and aging

Peroxisome proliferator-activated receptors (PPARs) are nuclear receptors which are activated by fatty acids and derivatives. The PPAR alpha form has been shown to mediate the action of the hypolipidemic drugs of the fibrate class on lipid and lipoprotein metabolism. PPAR alpha activators furthermore improve glucose homeostasis and influence body weight and energy homeostasis. It is likely that these actions of PPAR alpha activators on lipid, glucose and energy metabolism are, at least in part, due to the increase of hepatic fatty acid beta-oxidation resulting in an enhanced fatty acid flux and degradation in the liver. Moreover, PPARs are expressed in different immunological and vascular wall cell types where they exert anti-inflammatory and proapoptotic activities. The observation that these receptors are also expressed in atherosclerotic lesions suggests a role in atherogenesis. Finally, PPAR alpha activators correct age-related dysregulations in redox balance. Taken together, these data indicate a modulatory role for PPAR alpha in the pathogenesis of age-related disorders, such as dyslipidemia, insulin resistance and chronic inflammation, predisposing to atherosclerosis.

Perfluorooctanoic acid, a peroxisome-proliferating hypolipidemic agent, dissociates apolipoprotein B48 from lipoprotein particles and decreases secretion of very low density lipoproteins by cultured rat hepatocytes

The hypolipidemic effect is evoked by various peroxisome proliferators. Modulation of gene transcription via peroxisome proliferator-activated receptor (PPAR) is generally responsible for this effect. In addition, we have found a PPAR-independent mechanism in which fibrates, known peroxisome proliferators, decrease hepatic secretion of very low density lipoproteins (VLDL) through inhibition of phosphatidylcholine synthesis via methylation of phosphatidylethanolamine (PE) (T. Nishimaki-Mogami et al., Biochim. Biophys. Acta 1304 (1996) 21-31). In the present study, we show a novel mechanism by which perfluorooctanoic acid (PFOA), a potent peroxisome proliferator and inhibitor of PE methylation, exerts its hypolipidemic effect. PFOA (100 microM) added to the medium rapidly decreased the secretion of triglyceride by cultured rat hepatocytes, which was independent of the activity of cellular PE methylation. Analysis of the density of apoB secreted into the medium showed that PFOA decreased apoB48 in VLDL, but increased apoB48 in the bottom d>1.21 fraction. This lipid-poor apoB48 was also generated by incubating medium that had been harvested from control cells with PFOA, indicating that PFOA has the ability to dissociate apoB48 from lipoprotein particles. Exposure of cells to PFOA for 2 h prior to the experiment was sufficient to generate lipid-poor apoB48, indicating that PFOA exerted its effect intracellularly. Taken together, the data suggest that a strong interaction of PFOA with apoB48 disturbs the association of apoB48 with lipids in the process of intracellular VLDL assembly, thereby inhibiting VLDL secretion. This study shows that the mechanisms of hypolipidemic effect caused by various classes of peroxisome proliferators are diverse.

Reconstitution of insulin-sensitive glucose transport in fibroblasts requires expression of both PPARgamma and C/EBPalpha

Adipocyte differentiation is regulated by at least two major transcription factors, CCAAT/enhancer-binding protein alpha (C/EBPalpha) and peroxisome proliferator-activated receptor gamma (PPARgamma). Expression of PPARgamma in fibroblasts converts them to fat-laden cells with an adipocyte-like morphology. Here, we investigate the ability of PPARgamma to confer insulin-sensitive glucose transport to a variety of murine fibroblast cell lines. When cultured in the presence of a PPARgamma ligand, Swiss-3T3 and BALB/c-3T3 cells ectopically expressing PPARgamma accumulate lipid droplets, express C/EBPalpha, aP2, insulin-responsive aminopeptidase, and glucose transporter isoform 4 (GLUT4), and exhibit highly insulin-responsive 2-deoxyglucose uptake. In contrast, PPARgamma-expressing NIH-3T3 cells, despite similar lipid accumulation, adipocyte morphology, and aP2 expression, do not express C/EBPalpha or GLUT4 and fail to acquire insulin sensitivity. In cells ectopically expressing PPARgamma, the development of insulin-responsive glucose uptake correlates with C/EBPalpha expression. Furthermore, ectopic expression of C/EBPalpha in NIH-3T3 cells converts them to the adipocyte phenotype and restores insulin-sensitive glucose uptake. We propose that the pathway(s) leading to fat accumulation and morphological changes are distinct from that leading to insulin-dependent glucose transport. Our results suggest that although PPARgamma is sufficient to trigger the adipogenic program, C/EBPalpha is required for establishment of insulin-sensitive glucose transport.

Novel peroxisome proliferator-activated receptor (PPAR) gamma and PPARdelta ligands produce distinct biological effects

The peroxisome proliferator-activated receptors (PPARs) include three receptor subtypes encoded by separate genes: PPARalpha, PPARdelta, and PPARgamma. PPARgamma has been implicated as a mediator of adipocyte differentiation and the mechanism by which thiazolidinedione drugs exert in vivo insulin sensitization. Here we characterized novel, non-thiazolidinedione agonists for PPARgamma and PPARdelta that were identified by radioligand binding assays. In transient transactivation assays these ligands were agonists of the receptors to which they bind. Protease protection studies showed that ligand binding produced specific alterations in receptor conformation. Both PPARgamma and PPARdelta directly interacted with a nuclear receptor co-activator (CREB-binding protein) in an agonist-dependent manner. Only the PPARgamma agonists were able to promote differentiation of 3T3-L1 preadipocytes. In diabetic db/db mice all PPARgamma agonists were orally active insulin-sensitizing agents producing reductions of elevated plasma glucose and triglyceride concentrations. In contrast, selective in vivo activation of PPARdelta did not significantly affect these parameters. In vivo PPARalpha activation with WY-14653 resulted in reductions in elevated triglyceride levels with minimal effect on hyperglycemia. We conclude that: 1) synthetic non-thiazolidinediones can serve as ligands of PPARgamma and PPARdelta; 2) ligand-dependent activation of PPARdelta involves an apparent conformational change and association of the receptor ligand binding domain with CREB-binding protein; 3) PPARgamma activation (but not PPARdelta or PPARalpha activation) is sufficient to potentiate preadipocyte differentiation; 4) non-thiazolidinedione PPARgamma agonists improve hyperglycemia and hypertriglyceridemia in vivo; 5) although PPARalpha activation is sufficient to affect triglyceride metabolism, PPARdelta activation does not appear to modulate glucose or triglyceride levels.

Reversing adipocyte differentiation: implications for treatment of obesity

Conventional treatment of obesity reduces fat in mature adipocytes but leaves them with lipogenic enzymes capable of rapid resynthesis of fat, a likely factor in treatment failure. Adenovirus-induced hyperleptinemia in normal rats results in rapid nonketotic fat loss that persists after hyperleptinemia disappears, whereas pair-fed controls regain their weight in 2 weeks. We report here that the hyperleptinemia depletes adipocyte fat while profoundly down-regulating lipogenic enzymes and their transcription factor, peroxisome proliferator-activated receptor (PPAR)gamma in epididymal fat; enzymes of fatty acid oxidation and their transcription factor, PPARalpha, normally low in adipocytes, are up-regulated, as are uncoupling proteins 1 and 2. This transformation of adipocytes from cells that store triglycerides to fatty acid-oxidizing cells is accompanied by loss of the adipocyte markers, adipocyte fatty acid-binding protein 2, tumor necrosis factor alpha, and leptin, and by the appearance of the preadipocyte marker Pref-1. These findings suggest a strategy for the treatment of obesity by alteration of the adipocyte phenotype.

Fatty acids: links between genes involved in fatty acid and cholesterol metabolism

Fatty acids are a major constituent of dietary fats and form an integral part of the cellular membrane and lipoproteins. The gene regulatory potential of fatty acids has long been recognized, but the precise regulatory mechanisms are unknown. The regulatory ability of fatty acids on the expression of a number of genes together with potential mechanisms and pathways of regulation are reviewed. In this review, we emphasize a key aspect of regulation mediated by the sterol regulatory element binding-protein, and its effects on sterol regulatory elements.

Peroxisome proliferator activated receptor-gamma, leptin and tumor necrosis factor-alpha mRNA expression during very low calorie diet in subcutaneous adipose tissue in obese women

BACKGROUND

PPAR gamma, leptin and TNF alpha are three major factors that play a key role in influencing adipocyte differentiation and both adipose tissue function and metabolism. However, the regulation of these three genes during a dynamic period of weight loss is unknown. We therefore investigated the concomitant regulation of the mRNA expression of PPAR gamma, leptin and TNF alpha in adipose tissue during a 21-day very low calorie diet (VLCD) in 12 non-diabetic obese women.

METHODS

The mRNA levels of PPAR gamma, leptin and TNF alpha were quantified by quantitative RT-competitive PCR in abdominal subcutaneous adipose tissue before and during VLCD (940 kcal/day).

RESULTS

VLCD induced weight loss (approximately 6 kg) and improved insulin sensitivity. Simultaneously, VLCD induced the reduction in the adipose tissue mRNA abundances of PPAR gamma (-13%, p < 0.05) and of leptin (-58%, p < 0.005), whereas TNF alpha mRNA levels increased (+78%, p < 0.005). PPAR gamma and leptin mRNA levels were correlated before (r = 0.778, p < 0.01) and after VLCD (r = 0.797, p < 0.01). Serum HDL-cholesterol concentrations were positively associated with PPAR gamma (r = 0.696, p < 0.03) and leptin (r = 0.806, p < 0.01) mRNA levels.

CONCLUSIONS

The increase in TNF alpha mRNA levels suggested that a local increased expression of this cytokine in adipose tissue might play a role in the control of the fat mass during weight loss. PPAR gamma and leptin mRNA levels were positively associated both before and after VLCD, suggesting that common regulatory mechanism(s) might control their expression. More strikingly, we found strong positive correlations between circulating HDL-cholesterol and both PPAR gamma and leptin mRNA levels, suggesting the existence of physiological links between circulating lipoprotein metabolism and adipose tissue function.

Plasma homocysteine concentration related to diet, endothelial function and mononuclear cell gene expression among male hyperlipidaemic smokers

BACKGROUND

Elevated plasma concentration of homocysteine is an independent risk factor for development of cardiovascular diseases.

MATERIALS AND METHODS

We evaluated potential links between homocysteine and atherothrombogenesis by relating the plasma concentration of homocysteine to (i) dietary antioxidants and omega-3 fatty acids (and determined influence of intervention with antioxidants or omega-3 fatty acids); (ii) markers of endothelial cell function; and (iii) peripheral blood mononuclear cell mRNA levels.

RESULTS

We observed an inverse relationship between the plasma homocysteine concentration and dietary intake of vegetables, vitamin C and beta-carotene and between homocysteine and the serum concentration of folate, vitamin B12 and omega-3 fatty acids. Intervention with antioxidants or omega-3 fatty acids did not affect plasma homocysteine concentration. The plasma levels of cysteinylglycine and vitamin B12 correlated positively with circulating E-selectin and VCAM-1, respectively, whereas folate in serum and blood correlated negatively with P-selectin. A negative correlation was found between the concentrations of homocysteine and von Willebrand factor. Negative and positive correlations were found between plasma homocysteine and the mononuclear cell mRNA levels of peroxisome proliferator activated receptor delta (PPAR delta) and c-myc respectively. A negative correlation was also found between plasma homocysteine and mononuclear cell mRNA levels of the proteoglycan serglycin. Homocysteine was not correlated with serum activity of glutathione peroxidase or with the mRNA level of glutathione peroxidase in mononuclear cells.

CONCLUSION

The plasma homocysteine level was negatively correlated with dietary intake of vegetables, including vitamins C and E, and serum omega-3 fatty acids, whereas supplementation with antioxidants or omega-3 fatty acids did not affect plasma homocysteine concentration. Homocysteine was not associated with circulating adhesion molecules or increased procoagulant activity, but homocysteine may alter mononuclear cell gene expression. Cysteine showed no significant correlation with these parameters.

Possible mechanisms underlying the cholesterol-raising effect of the coffee diterpene cafestol

Cafestol, a coffee diterpene present in unfiltered coffee brews, potently raises serum lipids in humans. The mechanism through which this dietary compound influences liporotein metabolism is largely unknown. Unravelling the mechanism of action might lead to new insights into the regulation of serum cholesterol levels in humans. This review summaries ways in which cafestol may act on serum lipids.

Rat preputial sebocyte differentiation involves peroxisome proliferator-activated receptors

The hallmark of sebaceous epithelial cell (sebocyte) differentiation is the accumulation of fused neutral fat droplets. Very little sebocyte differentiation occurs, however, in primary or organ culture, even upon incubating with androgens, which are required for maturation in vivo. We hypothesized that sebocyte cell culture systems lack activators of the peroxisome proliferator-activated receptors that are involved in adipocyte differentiation. We here report that activation of peroxisome proliferator-activated receptor gamma and alpha by their respective specific ligands, a thiazolidinedione and a fibrate, induced lipid droplet formation in sebocytes but not epidermal cells. Linoleic acid and carbaprostacyclin, both peroxisome proliferator-activated receptor delta and alpha ligand-activators, were more effective but less specific, stimulating lipid formation in both types of cells. Either was more effective than the combination of peroxisome proliferator-activated receptor gamma and alpha activation, suggesting that peroxisome proliferator-activated receptor delta is involved in this lipid formation. Linoleic acid 0.1 mM stimulated significantly more advanced sebocyte maturation than any other treatment, including carbaprostacyclin, which suggests a distinct role of long chain fatty acids in sebocyte differentiation. Peroxisome proliferator-activated receptor gammal mRNA was demonstrated in sebocytes, but not in epidermal cells; it was more strongly expressed in freshly dispersed than in cultured sebocytes. In contrast, peroxisome proliferator-activated receptor delta mRNA was expressed to a similarly high extent before and after culture in both sebocytes and epidermal cells. These findings are compatible with the concepts that peroxisome proliferator-activated receptor gamma1 gene expression plays a unique role in the differentiation of sebocytes, while peroxisome proliferator-activated receptor delta activation and long chain fatty acids finalize sebocyte maturation and are capable of stimulating epidermal lipid formation. These findings have implications for the development of new modalities of treatment for acne vulgaris.

Cholesteryl ester transfer protein gene expression during differentiation of human preadipocytes to adipocytes in primary culture

The expression pattern of the CETP gene in relationship to that of LPL, adipsin, PPARgamma, C/EBPalpha, ADD1/SREBPI and actin was examined by RT-PCR during differentiation of human fibroblastic preadipocytes to adipocytes in primary culture. Preadipocytes were isolated from subcutaneous fat obtained from healthy female subjects undergoing mammary reduction procedures, and induced to differentiate in culture. Morphologically, adipogenesis was confirmed by the accumulation of lipid droplets in cells. We show that the gene encoding CETP is expressed in preadipocytes and is present throughout differentiation as compared to LPL and adipsin which were detected in the majority of samples by day 2 or 3 of adipogenesis. The transcription factors, PPARgamma, ADD1/SREBP1 and C/EBPalpha were expressed by day 2, concomitant with the appearance of LPL and adipsin but subsequent to the appearance of CETP. CETP mRNA was not detectable in human skin fibroblasts. These studies demonstrate that CETP. expression is induced at an early stage of commitment to the adipocyte lineage and may be activated by transcription factor(s), which are not members of the PPAR, ADD1/SREBP1 or C/EBP families.

Phytanic acid activates the peroxisome proliferator-activated receptor alpha (PPARalpha) in sterol carrier protein 2-/ sterol carrier protein x-deficient mice

We showed recently that a targeted null mutation in the murine sterol carrier protein 2-/sterol carrier protein x-gene (Scp2) leads to defective peroxisomal catabolism of 3,7,11, 15-tetramethylhexadecanoic acid (phytanic acid), peroxisome proliferation, hypolipidemia, and enhanced hepatic expression of several genes that have been demonstrated to be transcriptionally regulated by the peroxisome proliferator-activated receptor alpha (PPARalpha). As a broad range of fatty acids activates PPARalpha in vitro, we examined whether the latter effects could be because of phytanic acid-induced activation of this transcription factor. Dietary phytol supplementation was used to modulate the concentration of phytanic acid in C57Bl/6 and Scp2 (-/-) mice. We found that the serum concentrations of phytanic acid correlated well with the expression of genes encoding peroxisomal beta-oxidation enzymes and liver fatty acid-binding protein, which have all been demonstrated to contain functionally active peroxisome proliferator response elements in their promoter regions. In accordance with these findings, a stimulating effect on acyl-CoA oxidase gene expression was also observed after incubation of the rat hepatoma cell line MH1C1 with phytanic acid. Moreover, reporter gene studies revealed that phytanic acid induces the expression of a peroxisome proliferator response element-driven chloramphenicol transferase reporter gene comparable with strong peroxisome proliferators. In addition, the ability of phytanic acid to act as an inductor of PPARalpha-dependent gene expression corresponded with high affinity binding of this dietary branched chain fatty acid to recombinant PPARalpha. We conclude that phytanic acid can be considered as a bona fide physiological ligand of murine PPARalpha.

Pro12Ala missense mutation of the peroxisome proliferator activated receptor gamma and diabetes mellitus

Peroxisome proliferator activated receptor-gamma (PPARgamma) is a nuclear receptor that regulates adipocyte differentiation and possibly lipid metabolism and insulin sensitivity. Therefore, PPARgamma is a promising candidate gene for several disorders including diabetes, obesity, and dyslipoproteinemia. Screening for mutations in the entire coding region of the PPARgamma gene yielded a missense C --> G mutation at codon 12, resulting in the substitution of proline with alanine (Pro12Ala). The objective of our study was to examine the relationship between this genetic variant and diabetes and associated diseases in a large group of patients with type 1 (n = 522) and type 2 (n = 503) diabetes. Allelic frequencies of the PPARgamma2 12Ala allele were similar between patients with either type of diabetes and comparable to that in healthy controls (n = 310). There was also no significant relationship between dyslipoproteinemia or obesity and the PPARgamma2 Pro12Ala genotype. Thus, our data, in this large and ethnically homogenous group of patients, do not support the hypothesis that this genetic variant is strongly associated with diabetes, obesity, or dyslipidemia in patients with type 1 or type 2 diabetes mellitus. This genetic marker is therefore unlikely to serve as a clinically useful predictor of these disorders in Caucasian patients with diabetes mellitus.

Expression of peroxisome proliferator activated receptor mRNA in normal and tumorigenic rodent mammary glands

The peroxisome proliferator activated receptors (PPARs) alpha, beta/delta, and gamma are novel nuclear hormone receptors activated by long chain fatty acids and synthetic ligands and which regulate lipid metabolism. Recent studies have detected PPARgamma mRNA in human mammary tumor cell lines. The current study examined the expression profile of PPAR mRNAs in normal and malignant rodent mammary tissues. Virgin murine mammary glands contained PPAR alpha, beta/delta, and gamma mRNAs based on northern blot analysis. The PPARgamma isoform was predominantly gamma2 based on quantitative PCR analysis. During pregnancy and lactation, the PPARalpha and gamma mRNAs decreased while the PPAR beta/delta mRNA remained relatively unchanged. NMuMG cells, an epithelial line derived from normal murine mammary gland, expressed PPAR alpha, beta/delta, and gamma mRNAs, independent of the presence or absence of compounds modifying PPAR activity. In rats, the physiologic expression pattern of PPARgamma mRNA paralleled the murine model; levels were detected in virgin but not lactating mammary glands. In addition, the PPARgamma mRNA was not detected in several histologically distinct 7,12-dimethylbenz(a)anthracene induced mammary tumors. These findings suggest that PPARs may regulate mammary epithelial and stromal cell function in response to physiologic or pathologic stimuli that profoundly alter lipid metabolism.

Expression and cDNA cloning of porcine peroxisome proliferator-activated receptor gamma (PPARgamma)

Peroxisome proliferator-activated receptor gamma (PPARgamma) is a member of the PPAR subfamily of nuclear hormone receptors. In rodents and humans, expression of PPARgamma is predominantly found in adipose tissue where it regulates adipocyte differentiation and the expression of multiple adipocyte genes. The primary aim of this work was to clone the porcine PPARgamma cDNA and examine the regulation of gene expression in porcine subcutaneous adipose tissue. The porcine PPARgamma gene encodes a 1.8-kb mRNA transcript and shares 99, 96 and 97% amino acid sequence identity to the human, mouse and cow PPARgamma molecules, respectively. Both isoforms of PPARgamma (gamma1 and gamma2) are highly expressed in porcine adipose tissue. The gamma2 isoform is expressed in low abundance in porcine spleen, whereas the gamma1 isoform is highly expressed in spleen and lung and at a low abundance in several other tissues. Western blot analysis confirmed a high level of PPARgamma protein expression in porcine adipose tissue compared to other tissues. Both caloric restriction and fasting significantly reduced PPARgamma2 but not gamma1 mRNA and PPARgamma protein abundance in subcutaneous adipose tissue compared to ad-libitum fed controls. We provide the first evidence that PPARgamma is abundantly expressed in porcine subcutaneous adipose tissue, and that expression is regulated by caloric intake. Thus, PPARgamma may play an important role in adipogenesis and hormone action in porcine adipocytes.

Early modulation of genes encoding peroxisomal and mitochondrial beta-oxidation enzymes by 3-thia fatty acids

The aim of the present study was to elucidate the effects of a single dose of 3-thia fatty acids (tetradecylthioacetic acid and 3-thiadicarboxylic acid) over a 24-hr study period on the expression of genes related to peroxisomal and mitochondrial beta-oxidation in liver of rats. The plasma triglyceride level decreased at 2-4 hr, 4-8 hr, and 8-24 hr, respectively, after a single dose of 150, 300, or 500 mg of 3-thia fatty acids/kg body weight. Four to eight hours after administration of 3-thia fatty acids, a several-fold-induced gene expression of peroxisomal multifunctional protein, fatty acyl-CoA oxidase (EC 1.3.3.6), fatty acid binding protein, and 2,4-dienoyl-CoA reductase (EC 1.3.1.43) resulted, concomitant with increased activity of 2,4-dienoyl-CoA reductase and fatty acyl-CoA oxidase. The expression of carnitine palmitoyltransferase-I and carnitine palmitoyltransferase-II increased at 2 and 4 hr, respectively, although at a smaller scale. In cultured hepatocytes, 3-thia fatty acids stimulated fatty acid oxidation after 4 hr, and this was both L-carnitine- and L-aminocarnitine-sensitive. The hepatic content of eicosapentaenoic acid and docosahexaenoic acid decreased throughout the study period. In contrast, the hepatic content of oleic acid tended to increase after 24 hr and was significantly increased after repeated administration of 3-thia fatty acids. Similarly, the expression of delta9-desaturase was unchanged during the 24-hr study, but increased after feeding for 5 days. To conclude, carnitine palmitoyltransferase-I expression seemed to be induced earlier than 2,4-dienoyl-CoA reductase and fatty acid binding protein, and not later than the peroxisomal fatty acyl-CoA oxidase. The expression of delta9-desaturase showed a more delayed response.

A mitochondrial ketogenic enzyme regulates its gene expression by association with the nuclear hormone receptor PPARalpha

Mitochondrial 3-hydroxy-3-methylglutaryl-CoA synthase (mHMG-CoAS) is a key enzyme in ketogenesis, catalyzing the condensation of acetyl-CoA and acetoacetyl-CoA to generate HMG-CoA, which is eventually converted to ketone bodies. Transcription of the nuclear-encoded gene for mHMG-CoAS is stimulated by peroxisome proliferator-activated receptor (PPAR) alpha, a fatty acid-activated nuclear hormone receptor. Here we show that the mHMG-CoAS protein physically interacts with PPARalpha in vitro, and potentiates PPARalpha-dependent transcriptional activation via the cognate PPAR response element of the mHMG-CoAS gene in vivo. Immunofluorescence of transiently transfected cells demonstrated that in the presence of PPARalpha, mHMG-CoAS is translocated into the nucleus. Binding to PPARalpha, stimulation of PPARalpha activity and nuclear penetration require the integrity of the sequence LXXLL in mHMG-CoAS, a motif known to mediate the interaction between nuclear hormone receptors and coactivators. These findings reveal a novel mechanism of gene regulation whereby the product of a PPARalpha-responsive gene, normally resident in the mitochondria, directly interacts with this nuclear hormone receptor to autoregulate its own nuclear transcription.

Receptor-interacting protein 140 interacts with and inhibits transactivation by, peroxisome proliferator-activated receptor alpha and liver-X-receptor alpha

Receptor interacting protein 140 (RIP140), a previously identified putative ligand-dependent coactivator of nuclear hormone receptors, was isolated by yeast two-hybrid cloning as a factor that interacts with peroxisome proliferator-activated receptor alpha (PPARalpha). This interaction in yeast required the integrity of the carboxyl-terminal, ligand-dependent activation domain of PPARalpha. However, protein binding studies carried out in vitro showed that full-length RIP140 bound efficiently to PPARalpha in the absence of exogenously added ligand. RIP140 also bound strongly to the liver-X-receptor (LXRalpha) in the absence of an activator for this receptor. In contrast, a strong interaction of RIP140 with the PPARalpha and LXRalpha heterodimerization partner retinoid-X-receptor alpha (RXRalpha) required the presence of its cognate ligand, 9-cis retinoic acid. Transfection analysis in mammalian cells demonstrated that RIP140 antagonized PPARalpha/RXRalpha- and LXRalpha/RXRalpha-mediated signaling. Our findings identify RIP140 as a novel modulator of transcriptional activation mediated by PPARalpha and LXRalpha and indicate that RIP140 can also bind to nuclear hormone receptors in a ligand-independent manner and repress their activity.

Mechanism of action of fibrates on lipid and lipoprotein metabolism

Treatment with fibrates, a widely used class of lipid-modifying agents, results in a substantial decrease in plasma triglycerides and is usually associated with a moderate decrease in LDL cholesterol and an increase in HDL cholesterol concentrations. Recent investigations indicate that the effects of fibrates are mediated, at least in part, through alterations in transcription of genes encoding for proteins that control lipoprotein metabolism. Fibrates activate specific transcription factors belonging to the nuclear hormone receptor superfamily, termed peroxisome proliferator-activated receptors (PPARs). The PPAR-alpha form mediates fibrate action on HDL cholesterol levels via transcriptional induction of synthesis of the major HDL apolipoproteins, apoA-I and apoA-II. Fibrates lower hepatic apoC-III production and increase lipoprotein lipase--mediated lipolysis via PPAR. Fibrates stimulate cellular fatty acid uptake, conversion to acyl-CoA derivatives, and catabolism by the beta-oxidation pathways, which, combined with a reduction in fatty acid and triglyceride synthesis, results in a decrease in VLDL production. In summary, both enhanced catabolism of triglyceride-rich particles and reduced secretion of VLDL underlie the hypotriglyceridemic effect of fibrates, whereas their effect on HDL metabolism is associated with changes in HDL apolipoprotein expression.

Enhanced differentiation of HL-60 leukemia cells to macrophages induced by ciprofibrate

Ciprofibrate, an hypolipidaemic peroxisome proliferator, induced differentiation of HL-60 cells. The effect was greatly potentiated by phorbol 12-myristate 13-acetate at a concentration where neither phorbol ester nor ciprofibrate alone had any effect on these cells. As occurs for HL-60 cell differentiation induced by high phorbol ester concentration, the ciprofibrate-induced phorbol ester-dependent differentiation of HL-60 cells proceeded through the monocytic/macrophage pathway and induced the phosphorylation of proteins with similar molecular weights suggesting that increased protein kinase C activity may be involved in the effect. The peroxisome proliferator-activated receptor (PPARalpha) transcription factor is expressed in HL-60 cells, but no changes were observed in its expression upon HL-60 cell differentiation.

Global regulatory functions of Oaf1p and Pip2p (Oaf2p), transcription factors that regulate genes encoding peroxisomal proteins in Saccharomyces cerevisiae

Two transcription factors, Oaf1p and Pip2p (Oaf2p), are key components in the pathway by which several Saccharomyces cerevisiae genes encoding peroxisomal proteins are activated in the presence of a fatty acid such as oleate. By searching the S. cerevisiae genomic database for the consensus sequence that acts as a target for these transcription factors, we identified 40 genes that contain a putative Oaf1p-Pip2p binding site in their promoter region. Quantitative Northern analysis confirmed that the expression of 22 of the genes identified is induced by oleate and that either one or both of these transcription factors are required for the activation. In addition to known peroxisomal proteins, the regulated genes encode novel peroxisomal proteins, a mitochondrial protein, and proteins of unknown location and function. We demonstrate that Oaf1p regulates certain genes in the absence of Pip2p and that both of these transcription factors play a role in maintaining the glucose-repressed state of one gene. Furthermore, we provide evidence that the defined consensus binding site is not required for the regulation of certain oleate-responsive genes.

Maturation of peroxisomes in differentiating human hepatoblastoma cells (HepG2): possible involvement of the peroxisome proliferator-activated receptor alpha (PPAR alpha)

We have studied the alterations of peroxisomes in the human hepatoblastoma cell line HepG2, induced to differentiate by long-term cultivation (20 days without passaging) using morphological and biochemical techniques as well as mRNA analysis. Ultrastructural studies revealed alterations in shape and size of peroxisomes, with significant increases in mean diameter and formation of small clusters exhibiting heterogeneous staining for catalase after 20 days in culture. These alterations of peroxisomes correspond to the changes described during the maturation process from prenatal to adult human hepatocytes. As revealed by Northern and Western blotting there was marked elevation of the mRNA (190%) and protein (180%) of the peroxisomal branched-chain acyl-CoA oxidase. This protein is the key regulatory enzyme for the side chain oxidation of cholesterol for bile acid synthesis, a pathway associated with mature hepatocytes. Concomitantly a marked increase of bile canaliculi was noted by light and electron microscopy. This differentiation process was confirmed also by the increase of albumin synthesis (mRNA: 160%; protein: 190%) which is generally used as a differentiation marker of hepatocytes in culture. Interestingly, the mRNA for peroxisome proliferator-activated receptor alpha (PPAR alpha) increased drastically by almost 390% and its corresponding protein by 150%, suggesting its involvement in maturation of the peroxisomal compartment in differentiating HepG2 cells. In contrast to the wellknown increases during the drug-induced peroxisome proliferation of cytochrome P450 4A, multifunctional enzyme 1, palmitoyl-CoA oxidase and the 70-kDa peroxisomal membrane protein, those proteins were either not altered or only slightly elevated during the differentiation process, suggesting that peroxisome proliferation and maturation are two distinct and differentially regulated processes.

Leptin and its receptors: regulators of whole-body energy homeostasis

Leptin is the adipocyte-specific product of the ob gene. Expression of leptin in fully fed animals reflects adipocyte size and body-fat mass. Leptin signals the status of body energy stores to the brain, where signals emanate to regulate food intake and whole-body energy expenditure. The leptin gene was identified in the leptin-deficient, obese ob/ob mouse by positional cloning techniques. Recently, leptin has been cloned in domestic species including pigs, cattle, and chickens. The leptin receptor has at least five splice variants; the long form of the receptor is primarily expressed in the hypothalamus and is thought to be the predominant signaling isoform. Leptin receptors are members of the cytokine family of receptors and signal via janus-activated kinases (JAK)/signal transducers and activators of transcription (STAT) and mitogen-activated protein kinase (MAPK) pathways. Mutations in the leptin or leptin receptor genes results in morbid obesity, infertility, and insulin resistance in rodents and humans. Leptin regulates food intake and energy expenditure via central and peripheral mechanisms. Leptin receptors are expressed in most tissues, and in vitro evidence suggests that leptin may have direct effects on some tissues such as adipose tissue, the adrenal cortex, and the pancreatic beta-cell. Leptin is thought to influence whole-body glucose homeostasis and insulin action. Studies are underway to determine the role that leptin plays in the biology of domestic animals.

The nuclear receptors peroxisome proliferator-activated receptor alpha and Rev-erbalpha mediate the species-specific regulation of apolipoprotein A-I expression by fibrates

Fibrates are widely used hypolipidemic drugs which activate the nuclear peroxisome proliferator-activated receptor (PPAR) alpha and thereby alter the transcription of genes controlling lipoprotein metabolism. Fibrates influence plasma high density lipoprotein and its major protein, apolipoprotein (apo) A-I, in an opposite manner in man (increase) versus rodents (decrease). In the present study we studied the molecular mechanisms of this species-specific regulation of apoA-I expression by fibrates. In primary rat and human hepatocytes fenofibric acid, respectively, decreased and increased apoA-I mRNA levels. The absence of induction of rat apoA-I gene expression by fibrates is due to 3 nucleotide differences between the rat and the human apoA-I promoter A site, rendering a positive PPAR-response element in the human apoA-I promoter nonfunctional in rats. In contrast, rat, but not human, apoA-I transcription is repressed by the nuclear receptor Rev-erbalpha, which binds to a negative response element adjacent to the TATA box of the rat apoA-I promoter. In rats fibrates increase liver Rev-erbalpha mRNA levels >10-fold. In conclusion, the opposite regulation of rat and human apoA-I gene expression by fibrates is linked to differences in cis-elements in their respective promoters leading to repression by Rev-erbalpha of rat apoA-I and activation by PPARalpha of human apoA-I. Finally, Rev-erbalpha is identified as a novel fibrate target gene, suggesting a role for this nuclear receptor in lipid and lipoprotein metabolism.

Identification of a novel peroxisome proliferator responsive cDNA isolated from rat hepatocytes as the zinc-finger protein ZFP-37

The implementation of a rat hepatocyte model system and differential display-polymerase chain reaction resulted in the isolation of ZFP-37 as a peroxisome proliferator-responsive gene. In addition to being responsive to peroxisome proliferators, rat ZFP-37 (rZFP-37) mRNA accumulates rapidly after treating cells with several other hepatic tumor promoters, serum, and cycloheximide, indicating that this gene belongs to the immediate-early growth responsive gene family. Although rZFP-37 and mouse ZFP-37 (mZFP-37) are both members of the Krüppel-associated box and C2H2 zinc finger superfamily of proteins, there are several features that distinguish the two proteins. The primary protein sequences of rat and mouse ZFP-37 are highly conserved, especially within the region encoding the 12 C2H2 zinc finger motifs; however, a region believed to be involved in DNA binding in mZFP-37 is divergent in rZFP-37. Mouse ZFP-37 mRNA is expressed almost exclusively in testes and brain, whereas rZFP-37 mRNA is expressed in testes, brain, kidney, spleen, thymus, lung, and at low levels in liver. A major difference between regulation of ZFP-37 in the two species exists as rZFP-37 is induced, while mZFP-37 is repressed, in liver by the administration of the potent peroxisome proliferator Wy 14,643. Despite the fact that mZFP-37 is believed to be important in cell growth and differentiation in testes and brain, the pronounced differences in regulation of this gene in two closely related species preclude an extrapolation to rZFP-37's biological role. Nonetheless, the effects of tumor promoters and mitogens on its expression and the inclusion of rZFP-37 into the immediate-early growth gene families raise the possibility that this gene plays a role in hepatocyte proliferation and/or differentiation.

Characterisation of porcine peroxisome proliferator-activated receptors gamma 1 and gamma 2: detection of breed and age differences in gene expression

Two isoforms of peroxisome proliferator-activated receptor gamma (PPAR gamma) cDNAs, gamma 1 and gamma 2, have been isolated and characterised in swine. The relative expression of the two transcripts was studied by northern blot analysis using total RNA isolated from several porcine tissues taken at three different ages (day 1, after 5 weeks and at 100 kg weight). Hybridisation were carried out with two different probes, one binding to both PPAR gamma transcripts and the other being PPAR gamma 2 specific. Strongest hybridisation signals with the PPAR gamma probe binding both variants were detected in adipose tissues and spleen at all three ages, whereas only faint or no signals were detected in other tissues. The tissue distribution pattern of PPAR gamma 1 and gamma 2 suggests a modulation of tissue distribution for the two transcripts and obvious age and breed differences in gene expression in swine.

Troglitazone effects on gene expression in human skeletal muscle of type II diabetes involve up-regulation of peroxisome proliferator-activated receptor-gamma

Troglitazone, besides improving insulin action in insulin-resistant subjects, is also a specific ligand for the nuclear receptor peroxisome proliferator-activated receptor-gamma (PPARgamma). To determine whether troglitazone might enhance insulin action by stimulation of PPARgamma gene expression in muscle, total PPARgamma messenger RNA (mRNA), and protein were determined in skeletal muscle cultures from nondiabetic control and type II diabetic subjects before and after treatment of cultures with troglitazone (4 days +/- troglitazone, 11.5 microM). Troglitazone treatment increased PPARgamma mRNA levels up to 3-fold in muscle cultures from type II diabetics (277 +/- 63 to 630 +/- 100 x 10(3) copies/microg total RNA, P = 0.003) and in nondiabetic control subjects (200 +/- 42 to 490 +/- 81, P = 0.003). PPARgamma protein levels in both diabetic (4.7 +/- 1.6 to 13.6 +/- 3.0 AU/10 microg protein, P < 0.02) and nondiabetic cells (7.4 +/- 1.0 to 12.7 +/- 1.8, P < 0.05) were also upregulated by troglitazone treatment. Increased PPARgamma was associated with stimulation of human adipocyte lipid binding protein (ALBP) and muscle fatty acid binding protein (mFABP) mRNA, without change in the mRNA for glycerol-3-phosphate dehydrogenase, PPARdelta, myogenin, uncoupling protein-2, or sarcomeric alpha-actin protein. In summary, we showed that troglitazone markedly induces PPARgamma, ALBP, and mFABP mRNA abundance in muscle cultures from both nondiabetic and type II diabetic subjects. Increased expression of PPARgamma protein and other genes involved in glucose and lipid metabolism in skeletal muscle may account, in part, for the insulin sensitizing effects of troglitazone in type II diabetes.

Characterization of the inhibitory effect of growth hormone on primary preadipocyte differentiation

GH exerts adipogenic activity in several preadipocyte cell lines, whereas in primary rat preadipocytes, GH has an antiadipogenic activity. To better understand the molecular mechanism involved in adipocyte differentiation, the expression of adipocyte-specific genes was analyzed in differentiating preadipocytes in response to GH. We found that the expression of both adipocyte determination and differentiation factor 1 (ADD1) and peroxisome proliferator activated receptor gamma(PPARgamma) was induced in preadipocytes during differentiation. In the presence of GH, which markedly inhibited triglyceride accumulation, no reduction in the expression level of ADD1 was observed in response to GH, whereas there was a 50% reduction in the expression of PPARgamma. The DNA binding activity of the PPARgamma/retinoid X receptor-alpha(RXRalpha) to the ARE7 element from the aP2 gene was also reduced by approximately 50% in response to GH. GH inhibited the expression of late markers of adipocyte differentiation, fatty acid synthase, aP2, and hormone-sensitive lipase by 70-80%. The antiadipogenic effect of GH was not affected by the mitogen-activated protein (MAP) kinase/ extracellular-regulated protein (ERK) kinase inhibitor PD 98059, indicating that the mitogen-activated protein kinase pathway was not involved in GH inhibition of preadipocyte differentiation. The expression of preadipocyte factor-1/fetal antigen 1 was decreased during differentiation, and GH treatment prevented this down-regulation of Pref1/FA1. A possible role for Pref-1/FA1 in mediating the antiadipogenic effect of GH was indicated by the observation that FA1 inhibited differentiation as effectively as GH. These data suggest that GH exerts its inhibitory activity in adipocyte differentiation at a step after the induction of ADD1 but before the induction of genes required for terminal differentiation.

cis-parinaric acid is a ligand for the human peroxisome proliferator activated receptor gamma: development of a novel spectrophotometric assay for the discovery of PPARgamma ligands

Peroxisome proliferator activated receptor gamma (PPARgamma) is the subject of intense investigation as a target for drugs against diabetes, atherosclerosis and cancer. For this reason there is considerable interest in the spectrum of compounds that bind this receptor. In this paper we have identified cis-parinaric acid (CPA) as a novel hPPARgamma ligand. The binding of this fatty acid to the receptor increases its fluorescence and causes a shift in the UV spectrum. This spectral shift is reversible by competition with other known ligands for PPARgamma. This report represents the first direct demonstration of a fatty acid binding to PPARgamma.

Inhibition of Inducible Nitric Oxide Synthase by Peroxisome Proliferator-Activated Receptor Agonists: Correlation with Induction of Heme Oxygenase 1

Genetic knock-out in mice of peroxisome proliferator-activated receptor-α (PPARα) can prolong inflammation in response to leukotriene B4. Although cyclooxygenase 2 has been shown to be induced by PPAR activation, the effect of PPAR agonists on the key inflammatory enzyme systems of nitric oxide synthase (NOS) and stress proteins has not been investigated. The effect on these of naturally occurring eicosanoid PPAR agonists (leukotriene B4 and 8(S)-hydroxyeicosatetraenoic acid, which are PPARα selective; PGA2, PGD2, PGJ2, and Δ12PGJ2, which are PPARγ selective) and the synthetic PPARα agonist Wy14,643 was examined in activated RAW264.7 murine macrophages. Leukotriene B4 and 8(S)-hydroxyeicosatetraenoic acid stimulated nitrite accumulation, indicative of enhanced NOS activity. PGA2, PGD2, PGJ2, Δ12PGJ2, and Wy14,643 reduced nitrite accumulation, with Δ12PGJ2 being the most effective. The mechanism behind this reduction was examined using Western blotting. Inhibition of nitrite accumulation was associated with a fall in inducible NOS protein and an induction of heme oxygenase 1, correlating both dose dependently and temporally. Other proteins examined (cyclooxygenase 2, heme oxygenase 2, heat shock protein 70, and glucose-regulated protein 78) were unaffected. The data suggest that naturally occurring PPAR agonists can inhibit the inducible NOS enzyme pathway. This inhibition may be mediated by modulation of the stress protein, heme oxygenase 1. Thus, the generation of eicosanoid breakdown products during inflammation may contribute to its eventual resolution by activation of the PPAR system. This system may thus represent a novel target for therapeutic intervention in inflammatory disease.

Understanding adipocyte differentiation

The adipocyte plays a critical role in energy balance. Adipose tissue growth involves an increase in adipocyte size and the formation of new adipocytes from precursor cells. For the last 20 years, the cellular and molecular mechanisms of adipocyte differentiation have been extensively studied using preadipocyte culture systems. Committed preadipocytes undergo growth arrest and subsequent terminal differentiation into adipocytes. This is accompanied by a dramatic increase in expression of adipocyte genes including adipocyte fatty acid binding protein and lipid-metabolizing enzymes. Characterization of regulatory regions of adipose-specific genes has led to the identification of the transcription factors peroxisome proliferator-activated receptor-gamma (PPAR-gamma) and CCAAT/enhancer binding protein (C/EBP), which play a key role in the complex transcriptional cascade during adipocyte differentiation. Growth and differentiation of preadipocytes is controlled by communication between individual cells or between cells and the extracellular environment. Various hormones and growth factors that affect adipocyte differentiation in a positive or negative manner have been identified. In addition, components involved in cell-cell or cell-matrix interactions such as preadipocyte factor-1 and extracellular matrix proteins are also pivotal in regulating the differentiation process. Identification of these molecules has yielded clues to the biochemical pathways that ultimately result in transcriptional activation via PPAR-gamma and C/EBP. Studies on the regulation of the these transcription factors and the mode of action of various agents that influence adipocyte differentiation will reveal the physiological and pathophysiological mechanisms underlying adipose tissue development.

Subtype- and response element-dependent differences in transactivation by peroxisome proliferator-activated receptors alpha and gamma

Peroxisome proliferator-activated receptors (PPAR) modulate transcription by binding to specific peroxisome proliferator-response elements (PPRE) through heterodimerization with the 9-cis retinoic acid receptor (RXR). To investigate potential subtype- and response element-dependent differences in transcriptional activation by PPARs, we expressed PPARalpha or PPARgamma2, along with RXRalpha, in the yeast Saccharoromyces cerevisiae and compared their ability to activate transcription of reporter genes containing a PPRE from either the rat acyl-CoA oxidase (AOx) or hydratase-dehydrogenase (HD) gene. PPARgamma2 and RXRalpha, when coexpressed from low copy vectors, potently and synergistically activated transcription of the AOx-PPRE reporter gene, but only weakly stimulated transcription of the HD-PPRE reporter gene. This response element preference, which was also observed in mammalian cells, could not be attributed to differences in binding affinity of PPARgamma2/RXRalpha heterodimers to these elements in vitro. Interestingly, PPARgamma2 expressed from a high copy vector was able to strongly activate transcription of the HD-PPRE reporter gene, even in the absence of coexpressed RXRalpha. In comparison to the findings with PPARgamma2, the HD-PPRE served as a significantly more robust response element for PPARalpha as compared to the AOx-PPRE. PPRE-dependent transcriptional activation by PPARalpha correlated with binding efficiencies of PPARalpha/RXRalpha to the response element. Our findings demonstrate that the transactivation potential of PPAR subtypes can be differentially modulated by distinct PPREs.