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

Thyroid hormone receptors regulate adipogenesis and carcinogenesis via crosstalk signaling with peroxisome proliferator-activated receptors

Peroxisome proliferator-activated receptors (PPARs) and thyroid hormone receptors (TRs) are members of the nuclear receptor superfamily. They are ligand-dependent transcription factors that interact with their cognate hormone response elements in the promoters to regulate respective target gene expression to modulate cellular functions. While the transcription activity of each is regulated by their respective ligands, recent studies indicate that via multiple mechanisms PPARs and TRs crosstalk to affect diverse biological functions. Here, we review recent advances in the understanding of the molecular mechanisms and biological impact of crosstalk between these two important nuclear receptors, focusing on their roles in adipogenesis and carcinogenesis.

Therapeutic Implications of PPARgamma in Human Osteosarcoma

Osteosarcoma (OS) is the most common nonhematologic malignancy of bone in children and adults. Although dysregulation of tumor suppressor genes and oncogenes, such as Rb, p53, and the genes critical to cell cycle control, genetic stability, and apoptosis have been identified in OS, consensus genetic changes that lead to OS development are poorly understood. Disruption of the osteogenic differentiation pathway may be at least in part responsible for OS tumorigenesis. Current OS management involves chemotherapy and surgery. Peroxisome proliferator-activated receptor (PPAR) agonists and/or retinoids can inhibit OS proliferation and induce apoptosis and may inhibit OS growth by promoting osteoblastic terminal differentiation. Thus, safe and effective PPAR agonists and/or retinoid derivatives can be then used as adjuvant therapeutic drugs for OS therapy. Furthermore, these agents have the potential to be used as chemopreventive agents for the OS patients who undergo the resection of the primary bone tumors in order to prevent local recurrence and/or distal pulmonary metastasis.

Novel bisaryl substituted thiazoles and oxazoles as highly potent and selective peroxisome proliferator-activated receptor delta agonists

The discovery, synthesis, and optimization of compound 1 from a high-throughput screening hit to highly potent and selective peroxisome proliferator-activated receptor delta (PPARdelta) agonists are reported. The synthesis and structure-activity relationship in this series are described in detail. On the basis of a general schematic PPAR pharmacophore model, scaffold 1 was divided into headgroup, linker, and tailgroup and successively optimized for PPAR activation using in vitro PPAR transactivation assays. A (2-methylphenoxy)acetic acid headgroup, a flexible linker, and a five-membered heteroaromatic center ring with two hydrophobic aryl substituents were required for efficient and selective PPARdelta activation. The fine-tuning of these aryl substituents led to an array of highly potent and selective compounds such as compound 38c, displaying an excellent pharmacokinetic profile in mouse. In an in vivo acute dosing model, selected members of this array were shown to induce the expression of pyruvate dehydrogenase kinase-4 (PDK4) and uncoupling protein-3 (UCP3), genes that are known to be involved in energy homeostasis and regulated by PPARdelta in skeletal muscle.

Endothelium-dependent vasodilator effects of peroxisome proliferator-activated receptor beta agonists via the phosphatidyl-inositol-3 kinase-Akt pathway

Peroxisome proliferator-activated receptor beta/delta (PPAR-beta) is a ligand-activated transcription factor belonging to the nuclear hormone receptor superfamily that regulates the transcription of many target genes. More recently, acute, nongenomic effects of PPAR-beta agonists have also been described. In the present study, we hypothesized that PPAR-beta agonists might exert acute nongenomic effects on vascular tone. Here, we report that the structurally unrelated PPAR-beta ligands [4-[3-(4-acetyl-3-hydroxy-2-propylphenoxy)propoxy]phenoxy]acetic acid (L-165041) and 4-[[[2-[3-fluoro-4-(trifluoromethyl)phenyl]-4-methyl-5-thiazolyl] methyl]thio]-2-methylphenoxy]acetic acid (GW0742) induced vascular relaxation in phenylephrine-precontracted endothelium-intact rat aortic rings, which was significantly inhibited by endothelial denudation or nitric-oxide synthase (NOS) inhibition with N(G)-nitro-l-arginine methylester. These relaxant effects reached steady state within 15 min. The relaxation induced by L-165041 and GW0742 in aortic rings precontracted with the thromboxane A(2) analog 9,11-dideoxy-11alpha,9alpha-epoxymethanoprostaglandin F2alpha (U-46619) was unaffected either by removal of extracellular calcium or by incubation with calcium-free solution containing the intracellular calcium chelator 1,2-bis-(o-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid tetra(acetoxymethyl) ester. However, the phosphatidylinositol 3-kinase (PI3K) inhibitor 2-(4-morpholinyl)-8-phenyl-1(4H)-benzopyran-4-one hydrochloride (LY-294002) inhibited the endothelium-dependent relaxant responses induced by both PPAR-beta agonists. Blockade of PPAR-beta with 3-[[[2-methoxy-4-(phenylamino)phenyl]amino]sulfonyl]-2-thiophenecarboxylic acid methyl ester (GSK0660) also partially inhibited these relaxant responses, although PPAR-gamma blockade with 2-chloro-5-nitro-N-phenylbenzamide (GW9662) had no effect. In human umbilical vein endothelial cells, L-165041 and GW0742 increased nitric oxide (NO) production and Akt and endothelial NOS (eNOS) phosphorylation, which were sensitive to PI3K inhibition and PPAR-beta blockade. In conclusion, the PPAR-beta agonists acutely caused vasodilatation, which was partially dependent on endothelial-derived NO. The eNOS activation is calcium-independent and seems to be related to activation of the PI3K-Akt-eNOS pathway.

Synthesis and biological activities of novel indole derivatives as potent and selective PPARgamma modulators

Starting from the structure of Telmisartan, a new series of potent and selective PPARgamma modulators was identified. The synthesis, in vitro and in vivo evaluation of the most potent compounds are reported and the X-ray structure of compound 7b bound to the PPARgamma ligand binding domain is described.

Peroxisome proliferator-activated receptor beta/delta (PPARbeta/delta) acts as regulator of metabolism linked to multiple cellular functions

Peroxisome proliferator-activated receptors (PPARs) are nuclear receptors. They function as ligand activated transcription factors. They exist in three isoforms, PPARalpha, PPARbeta (formerly PPARdelta), and PPARgamma. For all PPARs lipids are endogenous ligands, linking them directly to metabolism. PPARs form heterodimers with retinoic X receptors, and, upon ligand binding, modulate gene expression of downstream target genes dependent on the presence of co-repressors or co-activators. This results in cell-type specific complex regulations of proliferation, differentiation and cell survival. Specific synthetic agonists for all PPARs are available. PPARalpha and PPARgamma agonists are already in clinical use for the treatment of hyperlipidemia and type 2 diabetes, respectively. More recently, PPARbeta activation came into focus as an interesting novel approach for the treatment of metabolic syndrome and associated cardiovascular diseases. Although the initial notion was that PPARbeta is expressed ubiquitously, more recently extensive investigations have been performed demonstrating high PPARbeta expression in a variety of tissues, e.g. skin, skeletal muscle, adipose tissue, inflammatory cells, heart, and various types of cancer. In addition, in vitro and in vivo studies using specific PPARbeta agonists, tissue-specific over-expression or knockout mouse models have demonstrated a variety of functions of PPARbeta in adipose tissue, muscle, skin, inflammation, and cancer. We will focus here on functions of PPARbeta in adipose tissue, skeletal muscle, heart, angiogenesis and cancer related to modifications in metabolism and the identified underlying molecular mechanisms.

Cross-Talk between PPARs and the Partners of RXR: A Molecular Perspective

The PPARs are integral parts of the RXR-dependent signaling networks. Many other nuclear receptor subfamily 1 members also require RXR as their obligatory heterodimerization partner and they are often co-expressed in any given tissue. Therefore, the PPARs often complete with other RXR-dependent nuclear receptors and this competition has important biological implications. Thorough understanding of this cross-talk at the molecular level is crucial to determine the detailed functional roles of the PPARs. At the level of DNA binding, most RXR heterodimers bind selectively to the well-known “DR1 to 5” DNA response elements. As a result, many heterodimers share the same DR element and must complete with each other for DNA binding. At the level of heterodimerization, the partners of RXR share the same RXR dimerization interface. As a result, individual nuclear receptors must complete with each other for RXR to form functional heterodimers. Cross-talk through DNA binding and RXR heterodimerization present challenges to the study of these nuclear receptors that cannot be adequately addressed by current experimental approaches. Novel tools, such as engineered nuclear receptors with altered dimerization properties, are currently being developed. These tools will enable future studies to dissect specific RXR heterodimers and their signaling pathways.

Role of protease inhibitors and acylation stimulating protein in the adipogenesis in 3T3-L1 cells

Treatment of AIDS (HIV) and hepatitis C virus needs protease inhibitors (PI) to prevent viral replication. Uses of PI in therapy are usually associated with a decrease in body weight and dyslipidemia. Acylation stimulating protein (ASP) is a protein synthesized in adipocytes to increase triglycerides biosynthesis, for that the relation of PI and ASP to adipogenesis is tested in this work. ASP expression was increased during 3T3-L1 differentiation and reached a peak at day 8 with cell maturation. Addition of PI during adipocytes differentiation dose dependently and significantly (p < 0.5) inhibited the degree of triglycerides (TG) accumulation. Moreover, presence of ASP (450 ng/mL) in media significantly (p < 0.5) stimulated the degree of TG accumulation and there was additive stimulation for ASP when added with insulin (10 µg/mL). Finally, when ASP in different doses (Low, 16.7; Medium, 45 and High, 450 ng/mL) incubated with a dose of ×150 PI, ASP partially inhibited the PI-inhibited adipogenesis and TG accumulation. The results in this study show that PI inhibit lipids accumulation and confirm role of ASP in TG biosynthesis and adipogenesis.

Sorting out the functional role(s) of peroxisome proliferator-activated receptor-beta/delta (PPARbeta/delta) in cell proliferation and cancer

Peroxisome proliferator-activated receptor-beta/delta (PPARbeta/delta) has many beneficial physiological functions ranging from enhancing fatty acid catabolism, improving insulin sensitivity, inhibiting inflammation and increasing oxidative myofibers allowing for improved athletic performance. Thus, given the potential for targeting PPARbeta/delta for the prevention and/or treatment of diseases including diabetes, dyslipidemias, metabolic syndrome and cancer, it is critical to clarify the functional role of PPARbeta/delta in cell proliferation and associated disorders such as cancer. However, there is considerable controversy whether PPARbeta/delta stimulates or inhibits cell proliferation. This review summarizes the literature describing the influence of PPARbeta/delta on cell proliferation, with an emphasis toward dissecting the data that give rise to opposing hypotheses. Suggestions are offered to standardize measurements associated with these studies so that interlaboratory comparisons can be accurately assessed.

Emodin suppresses cell proliferation and fibronectin expression via p38MAPK pathway in rat mesangial cells cultured under high glucose

Our previous findings demonstrated that emodin could improve the renal function in rats with diabetic nephropathy, but little is known about its molecular mechanisms. In this study, we investigated the effects of emodin on high glucose (HG)-induced cell proliferation and fibronectin (FN) protein expression in rat mesangial cells, and explored the possible mechanism. Cell proliferation and cell cycle were determined by 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) and flow cytometry assay, respectively. The protein levels of FN, p-p38MAPK, t-p38MAPK, p-CREB, PPARgamma, and CTGF in rat mesangial cells were detected by Western blot. Our results demonstrated that emodin significantly suppressed HG-induced cell proliferation and arrested cell cycle progress. Protein expression of FN, phospho-p38MAPK, phospho-CREB and CTGF was markedly reduced, and PPARgamma protein level was significantly increased after emodin treatment. In conclusion, emodin suppressed HG-induced cell proliferation and FN expression in rat mesangial cells through inhibiting the p38MAPK pathway involved CREB, PPAPgamma and CTGF, suggesting a potential role of emodin in the treatment of diabetic nephropathy.

Emerging roles of peroxisome proliferator-activated receptor-beta/delta in inflammation

Peroxisome proliferator-activated receptor (PPAR)-beta/delta is a member of the PPAR nuclear hormone receptor family. The PPARs are a family of 3 ligand-activated transcription factors: PPARalpha (NR1C1), PPARbeta/delta (NR1C2), and PPARgamma (NR1C3). All the PPARs play important roles in the regulation of metabolic pathways, including those of lipid of biosynthesis and glucose metabolism, as well as in a variety of cell differentiation, proliferation, and apoptosis pathways. Recently, there has been a great deal of interest in the involvement of PPARs in the inflammatory processes. In particular, PPARalpha and PPARgamma inhibit the activation of inflammatory gene expression and can negatively interfere with pro-inflammatory transcription factor signalling pathways in vascular and inflammatory cells. In contrast, the roles of PPARbeta/delta regulating inflammation and immunity are only just emerging. This review will focus on these emerging roles of PPARbeta/delta in regulating inflammatory processes.

Activation of PPARbeta/delta inhibits leukocyte recruitment, cell adhesion molecule expression, and chemokine release

The infiltration of PMNs into tissues is a prominent feature in inflammation. The mechanism underlying PMN recruitment depends on the release of chemotactic mediators and CAM expression on endothelial cells. The nuclear receptor PPARbeta/delta is widely expressed in many tissues, including the vascular endothelium; however, its role in acute inflammation remains unclear. Using intravital microscopy in the mouse cremasteric microcirculation, we have shown that activation of PPARbeta/delta by its selective ligand GW501516 inhibits TNF-alpha-induced leukocyte rolling flux, adhesion, and emigration in a dose-dependant manner. Moreover, GW501516 reduced the expression of adhesion molecules such as ICAM-1, VCAM-1, and E-selectin in the cremasteric postcapillary venules. Similarly, rolling and adhesion of hPMNs under physiological flow on TNF-alpha-activated HUVECs were also inhibited markedly by GW501516. These inhibitory responses of GW501516 on activated endothelium were accompanied by a reduction in TNF-alpha-induced endothelial GRO-alpha release and VCAM-1, E-selectin, and ICAM-1 mRNA expression. Taken together, our results show that PPARbeta/delta modulates acute inflammation in vivo and in vitro under flow by targeting the neutrophil-endothelial cell interaction.

PPARs and the cardiovascular system

Peroxisome proliferator-activated receptors (PPARs) belong to the nuclear hormone-receptor superfamily. Originally cloned in 1990, PPARs were found to be mediators of pharmacologic agents that induce hepatocyte peroxisome proliferation. PPARs also are expressed in cells of the cardiovascular system. PPAR gamma appears to be highly expressed during atherosclerotic lesion formation, suggesting that increased PPAR gamma expression may be a vascular compensatory response. Also, ligand-activated PPAR gamma decreases the inflammatory response in cardiovascular cells, particularly in endothelial cells. PPAR alpha, similar to PPAR gamma, also has pleiotropic effects in the cardiovascular system, including antiinflammatory and antiatherosclerotic properties. PPAR alpha activation inhibits vascular smooth muscle proinflammatory responses, attenuating the development of atherosclerosis. However, PPAR delta overexpression may lead to elevated macrophage inflammation and atherosclerosis. Conversely, PPAR delta ligands are shown to attenuate the pathogenesis of atherosclerosis by improving endothelial cell proliferation and survival while decreasing endothelial cell inflammation and vascular smooth muscle cell proliferation. Furthermore, the administration of PPAR ligands in the form of TZDs and fibrates has been disappointing in terms of markedly reducing cardiovascular events in the clinical setting. Therefore, a better understanding of PPAR-dependent and -independent signaling will provide the foundation for future research on the role of PPARs in human cardiovascular biology.

Significance of peroxisome proliferator-activated receptors in the cardiovascular system in health and disease

Peroxisome proliferator-activated receptors (PPARs) are ligand-activated nuclear transcription factors that belong to the nuclear receptor superfamily. Three isoforms of PPAR have been identified, alpha, delta and gamma, which play distinct roles in the regulation of key metabolic processes, such as glucose and lipid redistribution. PPARalpha is expressed predominantly in the liver, kidney and heart, and is primarily involved in fatty acid oxidation. PPARgamma is mainly associated with adipose tissue, where it controls adipocyte differentiation and insulin sensitivity. PPARdelta is abundantly and ubiquitously expressed, but as yet its function has not been clearly defined. Activators of PPARalpha (fibrates) and gamma (thiazolidinediones) have been used clinically for a number of years in the treatment of hyperlipidaemia and to improve insulin sensitivity in diabetes. More recently, PPAR activation has been found to confer additional benefits on endothelial function, inflammation and thrombosis, suggesting that PPAR agonists may be good candidates for the treatment of cardiovascular disease. In this regard, it has been demonstrated that PPAR activators are capable of reducing blood pressure and attenuating the development of atherosclerosis and cardiac hypertrophy. This review will provide a detailed discussion of the current understanding of basic PPAR physiology, with particular reference to the cardiovascular system. It will also examine the evidence supporting the involvement of the different PPAR isoforms in cardiovascular disease and discuss the current and potential future clinical applications of PPAR activators.

PPARdelta agonist attenuates alcohol-induced hepatic insulin resistance and improves liver injury and repair

BACKGROUND/AIMS

Chronic ethanol exposure impairs liver regeneration due to inhibition of insulin signaling and oxidative injury. PPAR agonists function as insulin sensitizers and anti-inflammatory agents. We investigated whether treatment with a PPARdelta agonist could restore hepatic insulin sensitivity, survival signaling, and regenerative responses vis-a-vis chronic ethanol feeding.

METHODS

Adult rats were fed isocaloric liquid diets containing 0% or 37% ethanol, and administered a PPARdelta agonist by i.p. injection. We used liver tissue to examine histopathology, gene expression, oxidative stress, insulin signaling, and regenerative responses to 2/3 hepatectomy.

RESULTS

Chronic ethanol feeding caused insulin resistance, increased oxidative stress, lipid peroxidation, DNA damage, and hepatocellular injury in liver. These effects were associated with reduced insulin receptor binding and affinity, impaired survival signaling through PI3K/Akt/GSK3beta, and reduced expression of insulin responsive genes mediating energy metabolism and tissue remodeling. PPARdelta agonist treatment reduced ethanol-mediated hepatic injury, oxidative stress, lipid peroxidation, and insulin resistance, increased signaling through PI3K/Akt/GSK3beta, and enhanced the regenerative response to partial hepatectomy.

CONCLUSIONS

PPARdelta agonist administration may attenuate the severity of chronic ethanol-induced liver injury and ethanol's adverse effects on the hepatic repair by restoring insulin responsiveness, even in the context of continued high-level ethanol consumption.

Dominant-negative loss of PPARgamma function enhances smooth muscle cell proliferation, migration, and vascular remodeling

OBJECTIVE

The peroxisome proliferator activated receptor-gamma (PPARgamma) protein is a nuclear transcriptional activator with importance in diabetes management as the molecular target for the thiazolidinedione (TZD) family of drugs. Substantial evidence indicates that the TZD family of PPARgamma agonists may retard the development of atherosclerosis. However, recent clinical data have suggested that at least one TZD may increase the risk of myocardial infarction and death from cardiovascular disease. In this study, we used a genetic approach to disrupt PPARgamma signaling to probe the protein's role in smooth muscle cell (SMC) responses that are important for atherosclerosis.

METHODS AND RESULTS

SMC isolated from transgenic mice harboring the dominate-negative P465L mutation in PPARgamma (PPARgamma(L/+)) exhibited greater proliferation and migration then did wild-type cells. Upregulation of ETS-1, but not ERK activation, correlated with enhanced proliferative and migratory responses PPARgamma(L/+) SMCs. After arterial injury, PPARgamma(L/+) mice had a approximately 4.3-fold increase in the development of intimal hyperplasia.

CONCLUSIONS

These findings are consistent with a normal role for PPARgamma in inhibiting SMC migration and proliferation in the context of restenosis or atherosclerosis.

PPARδ activity in cardiovascular diseases: A potential pharmacological target

Activation of peroxisome proliferator-activated receptors (PPARs), and particularly of PPARα and PPARγ, using selective agonists, is currently used in the treatment of metabolic diseases such as hypertriglyceridemia and type 2 diabetes mellitus. PPARα and PPARγ anti-inflammatory, antiproliferative and antiangiogenic properties in cardiovascular cells were extensively clarified in a variety of in vitro and in vivo models. In contrast, the role of PPARδ in cardiovascular system is poorly understood. Prostacyclin, the predominant prostanoid released by vascular cells, is a putative endogenous agonist for PPARδ, but only recently PPARδ selective synthetic agonists were found, improving studies about the physiological and pathophysiological roles of PPARδ activation. Recent reports suggest that the PPARδ activation may play a pivotal role to regulate inflammation, apoptosis, and cell proliferation, suggesting that this transcriptional factor could become an interesting pharmacological target to regulate cardiovascular cell apoptosis, proliferation, inflammation, and metabolism.

Enhancement of PPAR-beta activity by repetitive low-grade H(2)O(2) stress protects human umbilical vein endothelial cells from subsequent oxidative stress-induced apoptosis

Repetitive stress has been shown to up-regulate antioxidant defense and increase survival after subsequent oxidative injury. The up-regulation of antioxidant defense has been identified as an underlying cause of the apoptosis-inhibitory effects exerted by repetitive stress. However, it remains unclear what the important signaling mechanisms are by which cells preexposed to low-grade stress deal with apoptosis-inducing stress. In this study, we repetitively stressed human umbilical vein endothelial cells (HUVECs) through multiple exposures to a low dose (30 microM) of H(2)O(2) in culture for 4 weeks. We then examined the effects of repetitive stress on PPAR-beta expression and activity as well as the role of PPAR-beta in the protective potency of repetitive stress. Our results show that repetitive stress enhances PPAR-beta expression and activity, thereby inhibiting oxidative stress-induced apoptosis. Further, PPAR-beta-directed antisense oligonucleotides reduced the PPAR-beta protein content, enhanced the H(2)O(2)-mediated apoptosis, and ablated the protective effect of repetitive low-grade H(2)O(2) stress. The specific PPAR-beta agonist L-165041 significantly potentiated the apoptosis induced by H(2)O(2) (p<0.05) and increased the protective effect of repetitive stress. These findings indicate that repetitive low-grade H(2)O(2) stress protects HUVECs from subsequent oxidative stress-induced apoptosis by enhancing PPAR-beta expression and activity.

Obesity and diabetes: lipids, 'nowhere to run to'

Although specific pathogenic entities contributing to diabetic risk, such as central adiposity, ectopic fat accumulation, hyperlipidaemia and inflammation, are well-characterized, the response of cellular systems to such insults are less well understood. This short review highlights the effect of increasing fat mass on ectopic fat accumulation, the role of triacylglycerols (triglycerides) in Type 2 diabetes mellitus and cardiovascular disease pathogenesis, and selected current therapeutic strategies used to ameliorate these risk factors.

PPAR alpha and PPAR delta transactivity and p300 binding activity induced by arachidonic acid in colorectal cancer cell line Caco-2

It is reported that arachidonic acid strongly induces the conformational change in vitro and transactivity of PPAR alpha in colorectal cancer cell line Caco-2. In this study, we demonstrated that the induction of conformational change and transactivity of PPAR delta by arachidonic acid, as well as other polyunsaturated fatty acids, was considerably lower than that of PPAR alpha. Mammalian two-hybrid assay showed that arachidonic acid enhanced binding of one of the coactivators, p300, to PPAR alpha but not to PPAR delta. Additionally, arachidonic acid induced in vitro binding of both PPAR alpha-RXR alpha and PPAR delta-RXR alpha heterodimers to several PPREs on CRBPII, L-FABP and ACO genes. Our results suggest that the lower transactivity of PPAR delta for arachidonic acid in Caco-2 cells, compared with PPAR alpha, is associated with the binding activity of p300 to the receptor.

Ligand activation of peroxisome proliferator-activated receptor-beta/delta inhibits cell proliferation in human HaCaT keratinocytes

Although there is strong evidence that ligand activation of peroxisome proliferator-activated receptor (PPAR)-beta/delta induces terminal differentiation and attenuates cell growth, some studies suggest that PPARbeta/delta actually enhances cell proliferation. For example, it was suggested recently that retinoic acid (RA) is a ligand for PPARbeta/delta and potentiates cell proliferation by activating PPARbeta/delta. The present study examined the effect of ligand activation of PPARbeta/delta on cell proliferation, cell cycle kinetics, and target gene expression in human HaCaT keratinocytes using two highly specific PPARbeta/delta ligands [4-[[[2-[3-fluoro-4-(trifluoromethyl)phenyl]-4-methyl-5-thiazolyl]methyl]thio]-2-methylphenoxy acetic acid (GW0742) and 2-methyl-4-((4-methyl-2-(4-trifluoromethylphenyl)-1,3-thiazol-5-yl)-methylsulfanyl)phenoxy-acetic acid (GW501516)] and RA. Both PPARbeta/delta ligands and RA inhibited cell proliferation of HaCaT keratinocytes. GW0742 and GW501516 increased expression of known PPARbeta/delta target genes, whereas RA did not; RA increased the expression of known retinoic acid receptor/retinoid X receptor target genes, whereas GW0742 did not affect these genes. GW0742, GW501516, and RA did not modulate the expression of 3-phosphoinositide-dependent protein kinase or alter protein kinase B phosphorylation. GW0742 and RA increased annexin V staining as quantitatively determined by flow cytometry. The effects of GW0742 and RA were also examined in wild-type and PPARbeta/delta-null primary mouse keratinocytes to determine the specific role of PPARbeta/delta in modulating cell growth. Although inhibition of keratinocyte proliferation by GW0742 was PPARbeta/delta-dependent, inhibition of cell proliferation by RA occurred in both genotypes. Results from these studies demonstrate that ligand activation of PPARbeta/delta inhibits keratinocyte proliferation through PPARbeta/delta-dependent mechanisms. In contrast, the observed inhibition of cell proliferation in mouse and human keratinocytes by RA is mediated by PPARbeta/delta-independent mechanisms and is inconsistent with the notion that RA potentiates cell proliferation by activating PPARbeta/delta.

PPARdelta Agonism for the Treatment of Obesity and Associated Disorders: Challenges and Opportunities

The prevalence of obesity in the USA and worldwide has reached epidemic proportions during the last two decades. Drugs currently available for the treatment of obesity provide no more than 5% placebo-adjusted weight loss and are associated with undesirable side effects. Peroxisome proliferator-activated receptor (PPAR) modulators offer potential benefits for the treatment of obesity and its associated complications but their development has been complicated by biological, technical, and regulatory challenges. Despite significant challenges, PPAR modulators are attractive targets for the treatment of obesity and could offer a viable alternative to the millions of patients who fail to lose weight following rigorous dieting and exercise protocols. In addition, PPAR modulators have the potential-added benefit of ameliorating the associated comorbidities.

Role of Oxidative Stress in Peroxisome Proliferator-Mediated Carcinogenesis

In this review, the evidence about the role of oxidative stress in the induction of hepatocellular carcinomas by peroxisome proliferators is examined. The activation of PPAR-alpha by peroxisome proliferators in rats and mice may produce oxidative stress, due to the induction of enzymes like fatty acyl coenzyme A (CoA) oxidase (AOX) and cytochrome P-450 4A1. The effect of peroxisome proliferators on the antioxidant defense system is reviewed, as is the effect on endpoints resulting from oxidative stress that may be important in carcinogenesis, such as lipid peroxidation, oxidative DNA damage, and transcription factor activation. Peroxisome proliferators clearly inhibit several enzymes in the antioxidant defense system, but studies examining effects on lipid peroxidation and oxidative DNA damage are conflicting. There is a profound species difference in the induction of hepatocellular carcinomas by peroxisome proliferators, with rats and mice being sensitive, whereas species such as nonhuman primates and guinea pigs are not susceptible to the effects of peroxisome proliferators. The possible role of oxidative stress in these species differences is also reviewed. Overall, peroxisome proliferators produce changes in oxidative stress, but whether these changes are important in the carcinogenic process is not clear at this time.

Ligand activation of peroxisome proliferator-activated receptor beta/delta (PPAR beta/delta) inhibits chemically induced skin tumorigenesis

Peroxisome proliferator-activated receptor (PPAR)beta/delta-null mice exhibit enhanced tumorigenesis in a two-stage chemical carcinogenesis model as compared with wild-type mice. Previous work showed that ligand activation of PPARbeta/delta induces terminal differentiation and inhibits proliferation of primary keratinocytes, and this effect does not occur in the absence of PPARbeta/delta expression. In the present studies, the effect of ligand activation of PPARbeta/delta on skin tumorigenesis was examined using both in vivo and ex vivo skin carcinogenesis models. Inhibition of chemically induced skin tumorigenesis was observed in wild-type mice administered GW0742, and this effect was likely the result of ligand-induced terminal differentiation and inhibition of replicative DNA synthesis. These effects were not found in similarly treated PPARbeta/delta-null mice. Ligand activation of PPARbeta/delta also inhibited cell proliferation and induced terminal differentiation in initiated/neoplastic keratinocyte cell lines representing different stages of skin carcinogenesis. These studies suggest that topical administration of PPARbeta/delta ligands may be useful as both a chemopreventive and/or a chemotherapeutic approach to inhibit skin cancer.

Role of peroxisome-proliferator-activated receptor beta/delta (PPARbeta/delta) in gastrointestinal tract function and disease

PPARbeta/delta (peroxisome-proliferator-activated receptor beta/delta) is one of three PPARs in the nuclear hormone receptor superfamily that are collectively involved in the control of lipid homoeostasis among other functions. PPARbeta/delta not only acts as a ligand-activated transcription factor, but also affects signal transduction by interacting with other transcription factors such as NF-kappaB (nuclear factor kappaB). Constitutive expression of PPARbeta/delta in the gastrointestinal tract is very high compared with other tissues and its potential physiological roles in this tissue include homoeostatic regulation of intestinal cell proliferation/differentiation and modulation of inflammation associated with inflammatory bowel disease and colon cancer. Analysis of mouse epithelial cells in the intestine and colon has clearly demonstrated that ligand activation of PPARbeta/delta induces terminal differentiation. The PPARbeta/delta target genes mediating this effect are currently unknown. Emerging evidence suggests that PPARbeta/delta can suppress inflammatory bowel disease through PPARbeta/delta-dependent and ligand-independent down-regulation of inflammatory signalling. However, the role of PPARbeta/delta in colon carcinogenesis remains controversial, as conflicting evidence suggests that ligand activation of PPARbeta/delta can either potentiate or attenuate this disease. In the present review, we summarize the role of PPARbeta/delta in gastrointestinal physiology and disease with an emphasis on findings in experimental models using both high-affinity ligands and null-mouse models.

The influence of thiazolidinediones on adipogenesis in vitro and in vivo: Potential modifiers of intramuscular adipose tissue deposition in meat animals1,2

Thiazolidinediones (TZD) are insulin sensitizing agents currently used for the treatment of type 2 diabetes and are widely used as adipogenic agents because they are ligands of peroxisome proliferator-activated receptor gamma (PPARgamma), a key adipogenic transcription factor. In vivo and in vitro studies of TZD as potential modifiers of intramuscular or marbling adipogenesis are reviewed. Thiazolidinedione-induced adipogenesis has been reported in numerous cell culture systems, including rodent, human, bovine, and porcine adipose tissue stromal-vascular (S-V) cell cultures. Studies of porcine S-V cell cultures derived from semitendinosus muscle show that TZD can potentially modify intramuscular or marbling adipogenesis. Preadipocyte recruitment was TZD-dependent in muscle S-V cultures but TZD-independent in adipose S-V cultures. There appear to be differences between adipocytes in muscle and subcutaneous adipose tissue, reminiscent of differences observed in adipocytes from different adipose tissue depots. Troglitazone, a TZD, induces marbling adipogenesis without inhibiting myogenesis when cells are grown on laminin precoated culture dishes. Additionally, troglitazone treatment does not increase lipid content in porcine adipose tissue or muscle S-V cell cultures. Thiazolidinedione treatment increases lipid content of muscle in rodents and humans; however, rosiglitazone treatment for 49 d in pigs did not influence muscle lipid content and meat quality, but several significant changes in muscle fatty acid composition were observed. Although timing of treatment with TZD needs to be optimized, evidence suggests these compounds may enhance marbling deposition in swine.

Peroxisome proliferator-activated receptors (PPARs) and the human skin: importance of PPARs in skin physiology and dermatologic diseases

Peroxisome proliferator-activated receptors (PPARs) are members of the nuclear receptor superfamily that regulate lipid, glucose, and amino acid metabolism. More recently, PPARs and corresponding ligands have been shown in skin and other organs to regulate important cellular functions, including cell proliferation and differentiation, as well as inflammatory responses. These new functions identify PPARs and corresponding ligands as potential targets for the treatment of various skin diseases and other disorders. It has been shown that in inflammatory skin disorders, including hyperproliferative psoriatic epidermis and the skin of patients with atopic dermatitis, the expression of both PPARalpha and PPARgamma is decreased. This observation suggests the possibility that PPARalpha and PPARgamma activators, or compounds that positively regulate PPAR gene expression, may represent novel NSAIDs for the topical or systemic treatment of common inflammatory skin diseases such as atopic dermatitis, psoriasis, and allergic contact dermatitis. Moreover, recent findings indicate that PPAR-signaling pathways may act as a promising therapeutic target for the treatment of hyperproliferative skin diseases including skin malignancies. Studies in non-diabetic patients suggest that oral thiazolidinediones, which are synthetic ligands of PPARgamma, not only exert an antidiabetic effect but also may be beneficial for moderate chronic plaque psoriasis by suppressing proliferation and inducing differentiation of keratinocytes; furthermore, they may even induce cell growth arrest, apoptosis, and terminal differentiation in various human malignant tumors. It has been reported that PPARalpha immunoreactivity is reduced in human keratinocytes of squamous cell carcinoma (SCC) and actinic keratosis (AK), while PPARdelta appears to be upregulated. Additionally, the microvessel density is significantly higher in AK and SCC that express high levels of PPARdelta. PPARdelta has been demonstrated to have an anti-apoptotic role and to maintain survival and differentiation of epithelial cells, whereas PPARalpha and PPARgamma activators induce differentiation and inhibit proliferation and regulate apoptosis. In melanoma, the growth inhibitory effect of PPARgamma activation is independent of apoptosis and seems to occur primarily through induction of cell cycle arrest in the G1 phase of the cell cycle or induction of re-differentiation. PPARalpha activation causes inhibition of migration of melanoma cells and anchorage-independent growth, whereas primary tumor growth remains unaltered. In clinical trials of gemfibrozil, a PPARalpha ligand, significantly fewer patients treated with this lipid-lowering drug were diagnosed with melanoma as compared to those in the control group. In conclusion, an increasing body of evidence indicates that PPAR signaling pathways may represent interesting therapeutic targets for a broad variety of skin disorders, including inflammatory skin diseases such as psoriasis and atopic dermatitis, and skin malignancies.

Cilostazol increases 3T3-L1 preadipocyte differentiation with improved glucose uptake associated with activation of peroxisome proliferator-activated receptor-gamma transcription

In the present study, we assessed that cilostazol stimulates differentiation of 3T3-L1 fibroblasts into adipocytes, and to improve insulin sensitivity in conjunction with PPARgamma transcriptional activity. Upon treatment of COS-7 cells and human umbilical vein endothelial cells (HUVECs) with cilostazol (10 and 30 microM), endogenous PPARgamma transcriptional activity was significantly elevated in both cells as did rosiglitazone (10 microM), and these effects were suppressed by 5 microM GW9662, an antagonist of PPARgamma activity. Cilostazol-induced 3T3-L1 fibroblast differentiation into adipocytes in concert with increases in expression of PPARgamma responsive genes such as CCAAT enhancer binding protein alpha (C-EBPalpha), aP2, which were accompanied by increased adiponectin and decreased resistin expressions as did rosiglitazone. These variables were strongly suppressed by GW9662, indicative of a PPARgamma-mediated signaling. GLUT4 protein expression and glucose uptake were significantly elevated by cilostazol as was by rosiglitazone, which were also attenuated by GW9662, indicative of improvement of insulin sensitivity. Signaling pathways involved in the cilostazol-stimulated PPARgamma transcription activity in HUVECs included phosphatidylinositol 3-kinase (PI3-kinase)/AKT. Taken together, it is suggested that cilostazol increases differentiation of 3T3-L1 fibroblasts into adipocytes, and improves insulin sensitivity by stimulating PPARgamma transcription.

Peroxisome proliferator-activated receptor-beta/delta (PPARbeta/delta) ligands inhibit growth of UACC903 and MCF7 human cancer cell lines

The development of peroxisome proliferator-activated receptor-beta/delta (PPARbeta/delta) ligands for the treatment of diseases including metabolic syndrome, diabetes and obesity has been hampered due to contradictory findings on their potential safety. For example, while some reports show that ligand activation of PPARbeta/delta promotes the induction of terminal differentiation and inhibition of cell growth, other reports suggest that PPARbeta/delta ligands potentiate tumorigenesis by increasing cell proliferation. Some of the contradictory findings could be due in part to differences in the ligand examined, the presence or absence of serum in cell cultures, differences in cell lines or differences in the method used to quantify cell growth. For these reasons, this study examined the effect of ligand activation of PPARbeta/delta on cell growth of two human cancer cell lines, MCF7 (breast cancer) and UACC903 (melanoma) in the presence or absence of serum using two highly specific PPARbeta/delta ligands, GW0742 or GW501516. Culturing cells in the presence of either GW0742 or GW501516 caused upregulation of the known PPARbeta/delta target gene angiopoietin-like protein 4 (ANGPTL4). Inhibition of cell growth was observed in both cell lines cultured in the presence of either GW0742 or GW501516, and the presence or absence of serum had little influence on this inhibition. Results from the present studies demonstrate that ligand activation of PPARbeta/delta inhibits the growth of both MCF7 and UACC903 cell lines and provide further evidence that PPARbeta/delta ligands are not mitogenic in human cancer cell lines.

Ectopic expression of porcine peroxisome proliferator-activated receptor δ regulates adipogenesis in mouse myoblasts1

Peroxisome proliferator-activated receptor gamma (PPARgamma) plays a critical role in regulating adipogenesis. The expression of peroxisome proliferator-activated receptor delta (PPARdelta) precedes that of PPARgamma during adipocyte differentiation in rodents. The current experiment was designed to study the function of porcine PPARdelta and the interaction of PPARdelta and PPARgamma in adipocyte differentiation. Inhibition of myogenesis was observed in mouse myoblasts expressing porcine PPARdelta, similar to myoblasts expressing PPARgamma. Treatment of myoblasts expressing PPARdelta with ligands for both PPARdelta and PPARgamma enhanced lipogenesis and adipogenesis to a greater extent than treatment with a PPARgamma ligand alone, suggesting that both genes were involved in regulating lipogenesis and adipogenesis. The ability to transdifferentiate myoblasts into adipocytes was decreased in myoblasts coexpressing PPARdelta with either wild type or mutated PPARgamma (Ser 112 was mutated to Ala; the mutated PPARgamma is more active than the wild type) compared with myoblasts expressing PPARgamma alone. Adipocyte differentiation in myoblasts coexpressing PPARdelta and mutated PPARgamma was greater than in myoblasts coexpressing PPARdelta and wild type PPARgamma, confirming that Ser 112 is important for the function of PPARgamma. Taken together, our results demonstrate that overexpression of PPARdelta inhibits myotube formation and also enhances adipocyte differentiation. However, the complexity and interaction of PPARdelta and PPARgamma in adipogenesis are not clearly understood.

The differential interactions of peroxisome proliferator-activated receptor gamma ligands with Tyr473 is a physical basis for their unique biological activities

Despite their proven antidiabetic efficacy, widespread use of peroxisome proliferator-activated receptor (PPAR)gamma agonists has been limited by adverse cardiovascular effects. To overcome this shortcoming, selective PPARgamma modulators (SPPARgammaMs) have been identified that have antidiabetic efficacy comparable with full agonists with improved tolerability in preclinical species. The results of structural studies support the proposition that SPPARgammaMs interact with PPARgamma differently from full agonists, thereby providing a physical basis for their novel activities. Herein, we describe a novel PPARgamma ligand, SPPARgammaM2. This compound was a partial agonist in a cell-based transcriptional activity assay, with diminished adipogenic activity and an attenuated gene signature in cultured human adipocytes. X-ray cocrystallography studies demonstrated that, unlike rosiglitazone, SPPARgammaM2 did not interact with the Tyr473 residue located within helix 12 of the ligand binding domain (LBD). Instead, SPPARgammaM2 was found to bind to and activate human PPARgamma in which the Tyr473 residue had been mutated to alanine (hPPARgammaY473A), with potencies similar to those observed with the wild-type receptor (hPPARgammaWT). In additional studies, we found that the intrinsic binding and functional potencies of structurally distinct SPPARgammaMs were not diminished by the Y473A mutation, whereas those of various thiazolidinedione (TZD) and non-TZD PPARgamma full agonists were reduced in a correlative manner. These results directly demonstrate the important role of Tyr473 in mediating the interaction of full agonists but not SPPARgammaMs with the PPARgamma LBD, thereby providing a precise molecular determinant for their differing pharmacologies.

Importance of peroxisome proliferator-activated receptor-gamma in hepatic ischemia/reperfusion injury in mice

BACKGROUND/AIMS

Peroxisome proliferator-activated receptor-gamma (PPARgamma) is a transcriptional factor belonging to the nuclear receptor superfamily. Recent studies have suggested that PPARgamma regulates inflammatory responses and PPARgamma specific agonists have beneficial effects on several disease conditions in the various organs. However, the precise role of PPARgamma in acute liver injury remains unknown.

METHODS

We investigated the pathophysiological role of PPARgamma and the effect of the selective PPARgamma agonist, pioglitazone, on the hepatic ischemia/reperfusion (I/R) injury.

RESULTS

PPARgamma expression in the liver was upregulated after reperfusion following ischemia. Pioglitazone treatment significantly inhibited hepatic I/R injury as determined by serological and histological analyses. The protective effect was associated with downregulation of the local expression of several potent proinflammatory cytokines, chemokines and adhesion molecules after reperfusion. The neutrophil accumulation was also inhibited by the treatment. Furthermore, the treatment inhibited the induction of apoptosis on hepatocytes. Finally, pioglitazone significantly improved the mouse survival in a lethal model of hepatic I/R injury.

CONCLUSIONS

PPARgamma plays an inhibitory role in hepatic I/R injury and the stimulation by selective agonist has a significant beneficial effect. Thus, PPARgamma may be a new therapeutic target for the protection of the liver against acute injury.

Human immunodeficiency virus (HIV)-1 viral protein R suppresses transcriptional activity of peroxisome proliferator-activated receptor {gamma} and inhibits adipocyte differentiation: implications for HIV-associated lipodystrophy

HIV-1-infected patients may develop lipodystrophy and insulin resistance. We investigated the effect of the HIV-1 accessory protein viral protein R (Vpr) on the activity of the peroxisome proliferator-activating receptor-gamma (PPARgamma), a key regulator of adipocyte differentiation and tissue insulin sensitivity. We studied expression of PPARgamma-responsive reporter genes in 3T3-L1 mouse adipocytes. We investigated Vpr interaction with the PPAR/retinoid X receptor (RXR)-binding site of the c-Cbl-associating protein (CAP) gene using the chromatin immunoprecipitation assay as well as the interaction of Vpr and PPARgamma using coimmunoprecipitation. Finally, we studied the ability of exogenous Vpr protein to enter cultured adipocytes and retard differentiation. We found that Vpr suppressed PPARgamma-induced transactivation in both undifferentiated and differentiated 3T3-L1 cells. Transcriptional suppression by Vpr required an intact LXXLL coactivator motif. Vpr suppressed mRNA expression of PPARgamma-responsive genes in undifferentiated 3T3-L1 cells and associated with the PPAR/RXR-binding site located in the promoter region of the CAP gene. Vpr interacted with the ligand-binding domain of PPARgamma in an agonist-dependent fashion in vitro. Vpr delivered either by an expression plasmid or as protein added to media suppressed PPARgamma agonist-induced adipocyte differentiation, assessed as lipid accumulation and mRNA expression of the adipocyte differentiation marker adipocyte P2 in 3T3-L1 cells. In conclusion, circulating Vpr or, alternatively, Vpr produced as a consequence of direct infection of adipocytes could suppress in vivo differentiation of preadipocytes by acting as a corepressor of PPARgamma-mediated gene transcription. Vpr may alter sensitivity to insulin and thereby contribute to the development of lipodystrophy and insulin resistance observed in HIV-1-infected patients.

Peroxisome proliferator-activated receptor-beta/delta (PPARbeta/delta) ligands do not potentiate growth of human cancer cell lines

Ligands for peroxisome proliferator-activated receptor-beta/delta (PPARbeta/delta) increase skeletal muscle fatty acid catabolism, improve insulin sensitivity, increase serum high-density lipoprotein cholesterol, elicit anti-inflammatory activity and induce terminal differentiation. Contradictory findings are also reported suggesting that PPARbeta/delta ligands potentiate tumorigenesis by increasing cell proliferation, by inhibiting apoptosis through phosphorylation of Akt and by increasing cyclooxygenase-2 (COX2) and vascular endothelial growth factor (VEGF) expression. The contradictory findings could be due to differences in the model system (cancer cell line versus in vivo), differences in cell culture conditions (with and without serum) or differences in ligands. The present study examined the effect of two different PPARbeta/delta ligands (GW0742 and GW501516) in human cancer cell lines (HT29, HCT116, LS-174T, HepG2 and HuH7) cultured in the presence or absence of serum and compared in vitro analysis with in vivo analysis. Neither PPARbeta/delta ligand increased cell growth or phosphorylation of Akt and no increase in the expression of VEGF or COX2 were detected in any cancer cell line in the presence or absence of serum. Similarly, liver, colon and colon polyps from mice administered these PPARbeta/delta ligands in vivo did not exhibit changes in these markers. Results from these studies demonstrate that serum withdrawal and/or differences in ligands do not underlie the disparity in responses reported in the literature. The quantitative nature of the present findings are inconsistent with the hypothesis that cancer cell lines respond differentially as compared with normal cells, and provide further evidence that PPARbeta/delta ligands do not potentiate tumorigenesis.

Peroxisome proliferator-activated receptor-alpha is required for the neurotrophic effect of oleic acid in neurons

Oleic acid synthesized by astrocytes behaves as a neurotrophic factor for neurons, up-regulating the molecular markers of axonal and dendritic outgrowth, growth-associated protein 43 and microtubule-associated protein 2. In this work, the nature of the receptor involved in this neurotrophic effect was investigated. As oleic acid has been reported to be a ligand and activator of the peroxisome proliferator-activated receptor (PPAR), we focus on this family of receptors. Our results show that PPARalpha, beta/delta, and gamma are expressed in neurons in culture. However, only the agonists of PPARalpha, Wy14643, GW7647 and oleoylethanolamide, promoted neuronal differentiation, while PPAR beta/delta and gamma agonists did not modify neuronal differentiation. Consequently, we investigated the involvement of PPARalpha (Nr1c1) in oleic acid-induced neuronal differentiation. Our results indicate that oleic acid activates PPARalpha in neurons. In addition, the effect of oleic acid on neuronal morphology, growth-associated protein 43 and microtubule-associated protein 2 expression decreases in neurons after PPARalpha has been silenced by small interfering RNA. Taken together, our results suggest that PPARalpha could be the receptor for oleic acid in neurons, further broadening the range of functions attributed to this family of transcription factors. Although several works have reported that PPARalpha could be involved in neuroprotection, the present work provides the first evidence suggesting a role of PPARalpha in neuronal differentiation.

Endocannabinoid system and pathophysiology of adipogenesis: current management of obesity

The endocannabinoids are now known as novel and important regulators of energy metabolism and homeostasis. The endocrine functions of white adipose are chiefly involved in the control of whole-body metabolism, insulin sensitivity and food intake. Adipocytes produce hormones, such as leptin and adiponectin, that can improve insulin resistance or peptides, such as TNF-α, that elicit insulin resistance. Adipocytes express specific receptors, such as peroxisome proliferator-activated receptor (PPAR)-γ, which serve as adipocyte targets for insulin sensitizers such as thiazolidinediones. Recently, endocannabinoids and related compounds were identified in human fat cells. The endocannabinoid system consists primarily of two receptors, cannabinoid (CB)1 and CB2, their endogenous ligands termed endocannabinoids and the enzymes responsible for ligand biosynthesis and degradation. The endocannabinoids 2-arachidonylglycerol and anandamide or N-arachidonoylethanolamine increase food intake and promote weight gain in animals. Rimonabant, a selective CB1 blocker, reduces food intake and body weight in animals and humans.

Present concepts and future outlook: function of peroxisome proliferator-activated receptors (PPARs) for pathogenesis, progression, and therapy of cancer

Peroxisome proliferator-activated receptors (PPARs) are members of the nuclear receptor superfamily of transcriptional regulators that regulate lipid, glucose, and amino acid metabolism. In recent studies it also has been shown that these receptors are implicated in tumor progression, cellular differentiation, and apoptosis and modulation of their function is therefore considered as a potential target for cancer prevention and treatment. PPAR ligands and other agents influencing PPAR signalling pathways have been shown to reveal chemopreventive potential by mediating tumor suppressive activities in a variety of human cancers and could represent a potential novel strategy to inhibit tumor carcinogenesis and progression. This review summarizes the currently available data on the roles of PPARs in relation to the processes of cell differentiation and carcinogenesis as well as their role as promising future therapeutic targets.

Peroxisome proliferator-activated receptor-delta upregulates 14-3-3 epsilon in human endothelial cells via CCAAT/enhancer binding protein-beta

Peroxisome proliferator-activated receptor delta (PPARdelta) agonists are promising new agents for treatment of the metabolic syndrome. Although they possess antiatherosclerotic properties in vivo and promote endothelial cell survival, their mechanism of action is incompletely understood. 14-3-3epsilon is a critical component of the endothelial cell antiapoptotic machinery, which is essential to maintain homeostasis of the vascular wall. To test the hypothesis that PPARdelta targets 14-3-3epsilon in endothelial cells, we studied the response of the gene that encodes 14-3-3epsilon in humans, YWHAE, to PPARdelta ligands (L-165,041 and GW501516). We found that PPARdelta activates YWHAE promoter in a concentration and time-dependent manner. Consistent with these findings, L-165,041 increased 14-3-3epsilon mRNA and protein level, whereas PPARdelta small interfering RNA suppressed both basal and L-165,041-dependent YWHAE transcription and 14-3-3epsilon protein expression. Surprisingly, PPAR response elements in YWHAE promoter were not required for upregulation by PPARdelta, whereas a CCAAT/enhancer binding protein (C/EBP) site located at -160/-151 bp regulated both basal and PPARdelta-dependent promoter activity. Intriguingly, activation or knock down of endogenous PPARdelta regulated C/EBPbeta protein expression. Chromatin immunoprecipitation assays demonstrated that L-165,041 determines the localization of C/EBPbeta to the region spanning this C/EBP response element, whereas sequential chromatin immunoprecipitation analysis revealed that C/EBPbeta and PPARdelta form a transcriptional activating complex on this C/EBP site. Our work uncovers a novel role for C/EBPbeta as a mediator of PPARdelta-dependent 14-3-3epsilon gene regulation in human endothelial cells and provides insight into the mechanism by which PPARdelta agonists may be beneficial in atherosclerosis.

Regulation of inflammation by PPARs: a future approach to treat lung inflammatory diseases?

Lung inflammatory diseases, such as acute lung injury (ALI), asthma, chronic obstructive pulmonary disease (COPD) and lung fibrosis, represent a major health problem worldwide. Although glucocorticoids are the most potent anti-inflammatory drug in asthma, they exhibit major side effects and have poor activity in lung inflammatory disorders such as ALI or COPD. Therefore, there is growing need for the development of alternative or new therapies to treat inflammation in the lung. Peroxisome proliferator-activated receptors (PPARs), including the three isotypes PPARalpha, PPARbeta (or PPARdelta) and PPARgamma, are transcription factors belonging to the nuclear hormone receptor superfamily. PPARs, and in particular PPARalpha and PPARgamma, are well known for their critical role in the regulation of energy homeostasis by controlling expression of a variety of genes involved in lipid and carbohydrate metabolism. Synthetic ligands of the two receptor isotypes, the fibrates and the thiazolidinediones, are clinically used to treat dyslipidaemia and type 2 diabetes, respectively. Recently however, PPARalpha and PPARgamma have been shown to exert a potent anti-inflammatory activity, mainly through their ability to downregulate pro-inflammatory gene expression and inflammatory cell functions. The present article reviews the current knowledge of the role of PPARalpha and PPARgamma in controlling inflammation, and presents different findings suggesting that PPARalpha and PPARgamma activators may be helpful in the treatment of lung inflammatory diseases.

Design and synthesis of oxime ethers of α-acyl-β-phenylpropanoic acids as PPAR dual agonists

Oxime ethers of alpha-acyl-beta-phenylpropanoic acids were prepared to apply as PPARalpha and gamma dual agonists. Among them, compound 11l proved to exhibit potent in vitro activities with EC(50) of 19 and 13nM in PPARalpha and gamma, respectively. It showed better glucose lowering effects than rosiglitazone 1 and ameliorated the lipid profile like plasma triglyceride in db/db mice model.