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

Inhibition of cytokine production by the herbicide atrazine. Search for nuclear receptor targets

The hematological toxicity of the commonly used triazine herbicides is a cause for concern. In a search for molecular targets of these compounds, as their effects paralleled those seen with dexamethasone (DEX), we first looked for interaction with the glucocorticoid receptor. In contrast to the effects on proliferation and cytokine production of DEX, those induced by atrazine were not prevented by the glucocorticoid antagonist RU486. Also, whereas DEX was able to inhibit the promoter activity of genes regulated by NF-kappaB, atrazine failed to do so. We next looked for interaction with members of the peroxisome proliferator-activated receptor (PPAR) family. No peroxisome proliferation was observed in the liver or kidneys of mice treated with atrazine. Moreover, no PPAR-mediated induction of promoter activity was seen on targets of PPARalpha, PPARgamma, or PPARdelta. Similarly, neither atrazine nor simazine were able to stimulate RORalpha-mediated promoter activity. Finally, no binding of atrazine to the AR was observed. In conclusion, the effects of atrazine-type herbicides most probably do not result from interaction with the above-mentioned nuclear receptors.

Peroxisome proliferator-activated receptor-beta signaling contributes to enhanced proliferation of hepatic stellate cells

BACKGROUND & AIMS

The peroxisome proliferator-activated nuclear receptors (PPAR-alpha, PPAR-beta, and PPAR-gamma), which modulate the expression of genes involved in energy homeostasis, cell cycle, and immune function, may play a role in hepatic stellate cell activation. Previous studies focused on the decreased expression of PPAR-gamma in hepatic stellate cell activation but did not investigate the expression and role of the PPAR-alpha and -beta isotypes. The aim of this study was to evaluate the expression of the different PPARs during hepatic stellate cell activation in vitro and in situ and to analyze possible factors that might contribute to their expression. In a second part of the study, the effect of a PPAR-beta agonist on acute liver injury was evaluated.

METHODS

The effects of PPAR isotype-specific ligands on hepatic stellate cell transition were evaluated by bromodeoxyuridine incorporation, gel shifts, immunoprecipitation, and use of antisense PPAR-beta RNA-expressing adenoviruses. Tumor necrosis factor alpha-induced PPAR-beta phosphorylation and expression was evaluated by metabolic labeling and by using specific P38 inhibitors.

RESULTS

Hepatic stellate cells constitutively express high levels of PPAR-beta, which become further induced during culture activation and in vivo fibrogenesis. No significant expression of PPAR-alpha or -gamma was found. Stimulation of the P38 mitogen-activated protein kinase pathway modulated the expression of PPAR-beta. Transcriptional activation of PPAR-beta by L165041 enhanced hepatic stellate cell proliferation. Treatment of rats with a single bolus of CCl(4) in combination with L165041 further enhanced the expression of fibrotic markers.

CONCLUSIONS

PPAR-beta is an important signal-transducing factor contributing to hepatic stellate cell proliferation during acute and chronic liver inflammation.

New insights in the transcriptional activity and coregulator molecules in the arterial wall

A number of vascular diseases are associated with abnormal expression of genes that contribute to their pathophysiological and clinical manifestations, but at the same time offer potential therapeutic targets. One of the promising therapeutic approaches targets the pathophysiological pathways leading to aberrant gene activation, namely transcriptional activity and its molecular modulators (agonists, antagonists, coregulators, and nuclear receptors). The transcription factors can be divided into four classes (I-IV) classified by structural elements, like basic leucine zipper (bZIP) or basic helix-loop-helix (bHLH), which mediate their DNA binding activity but also determine the classes of drugs that can affect their activity. For example, statins modulate activation of the class-I transcription factor sterol responsive element-binding protein (SREBP), whose target genes including hydroxyl-methyl-glutaryl acetyl Coenzyme-A (HMG-CoA) reductase, HMG-CoA synthase, and the low-density lipoprotein receptor, all of which are involved in cholesterol and fatty acid metabolism. Similarly, insulin-like drugs target the nuclear receptor peroxisome-proliferator-activator-receptor (PPAR)-gamma (class-II), several anti-inflammatory drugs inhibit activation of nuclear factor kappa B (NFkappaB) (class-IV), while others (e.g. flavopiridol, rapamycin, and paclitaxel) target regulation of cell-cycle proteins. Increased understanding of the genetic and molecular basis of disease (e.g. transcriptional activity and its coregulation) will potentially enhance future diagnosis, treatment, and prevention of vascular diseases.

PPAR(gamma) and glucose homeostasis

Peroxisome proliferator-activated receptor gamma (PPARgamma) is a nuclear receptor involved in the control of metabolism. Research on PPARgamma is oriented towards understanding its role in insulin sensitization, which was inspired by the discovery that antidiabetic agents, the thiazolidinediones, were agonists for PPARgamma. PPARgamma stimulation improves glucose tolerance and insulin sensitivity in type 2 diabetic patients and in animal models of insulin resistance through mechanisms that are incompletely understood. Upon activation, PPARgamma heterodimerizes with retinoid X receptor, recruits specific cofactors, and binds to responsive DNA elements, thereby stimulating the transcription of target genes. Because PPARgamma is highly enriched in adipose tissue and because of its major role in adipocyte differentiation, it is thought that the effects of PPARgamma in adipose tissue are crucial to explain its role in insulin sensitization, but recent studies have highlighted the contribution of other tissues as well. Although relatively potent for their insulin-sensitizing action, currently marketed PPARgamma activators have some important undesirable side effects. These concerns led to the discovery of new ligands with potent antidiabetic properties but devoid of certain of these side effects. Data from human genetic studies and from PPARgamma heterozygous knockout mice indicate that a reduction in PPARgamma activity could paradoxically improve insulin sensitivity. These findings suggest that modulation of PPARgamma activity by partial agonists or compounds that affect cofactor recruitment might hold promise for the treatment of insulin resistance.

Insulin resistance, diabetes, and atherosclerosis: thiazolidinediones as therapeutic interventions

The insulin resistance syndrome, a cluster of metabolic abnormalities involving dyslipidemia, hypertension, diabetes, impaired glucose tolerance, and hypercoagulability, carries an increased risk of atherosclerosis. Although interventions targeting elements of this syndrome have dramatically reduced cardiovascular risk, the impact of glucose-lowering has been more disappointing. Thiazolidinediones (TZDs) are a new class of insulin-sensitizing agents that activate the nuclear receptor peroxisome proliferator-activated receptor-g. TZDs may improve not only glucose levels but also other metabolic parameters associated with insulin resistance. The TZD data are reviewed, with a focus on their potential cardiovascular effects.

Cardiac function and metabolism in Type 2 diabetic mice after treatment with BM 17.0744, a novel PPAR-alpha activator

Hearts from diabetic db/db mice, a model of Type 2 diabetes, exhibit left ventricular failure and altered metabolism of exogenous substrates. Peroxisome proliferator-activated receptor-alpha (PPAR-alpha) ligands reduce plasma lipid and glucose concentrations and improve insulin sensitivity in db/db mice. Consequently, the effect of 4- to 5-wk treatment of db/db mice with a novel PPAR-alpha ligand (BM 17.0744; 25-38 mg x kg(-1) x day(-1)), commencing at 8 wk of age, on ex vivo cardiac function and metabolism was determined. Elevated plasma concentrations of glucose, fatty acids, and triacylglycerol (34.0 +/- 3.6, 2.0 +/- 0.4, and 0.9 +/- 0.1 mM, respectively) were reduced to normal after treatment with BM 17.0744 (10.8 +/- 0.6, 1.1 +/- 0.1, and 0.6 +/- 0.1 mM). Plasma insulin was also reduced significantly in treated compared with untreated db/db mice. Chronic treatment of db/db mice with the PPAR-alpha agonist resulted in a 50% reduction in rates of fatty acid oxidation, with a concomitant increase in glycolysis (1.7-fold) and glucose oxidation (2.3- fold). Correction of the diabetes-induced abnormalities in systemic and cardiac metabolism after BM 17.0744 treatment did not, however, improve left ventricular contractile function.

Expression of PPAR and RXR isoforms in the developing rat and human term placentas

Placental fatty acid transfer is critical to meet the foetal requirements necessary for the biosynthesis of biological membranes, myelin, and various signaling molecules. The primary objective of this research was to elucidate the placental expression patterns of genes that may potentially regulate placental fatty acid transfer and homeostasis. In this study, we have elucidated the temporal and spatial patterns of expression of peroxisome proliferator-activated receptor (PPAR) and 9-cis retinoic acid receptor (RXR) isoforms in the junctional and labyrinth zones of the developing rat chorioallantoic placenta and in human term placenta. PPAR (alpha, beta, and gamma) and RXR (alpha, beta, and gamma) isoforms are nuclear hormone receptors that are known to regulate gene transcription and protein expression levels of fatty acid transport and metabolism mediating proteins through the formation of a DNA binding heterodimer complex. In the present study, the expression patterns of PPAR and RXR isoforms were determined in developing rat placenta and human term placenta using RT-PCR and immunohistochemical analyses. PPARalpha, beta, gamma, RXRalpha, beta and gamma were expressed in both junctional (invasive/endocrine function) and labyrinth (transport barrier) zones of the rat placenta, from day 13 to day 21 of gestation. In the human term placenta, PPARalpha, beta, gamma, RXRalpha and gamma were observed, while RXRbeta was not detected. Immunocytochemistry staining results determined the presence of PPARalpha, beta, gamma, RXRalpha and gamma to be specific to the syncytial trophoblast layer of the human chorionic villi. The presence of PPAR and RXR isoforms in both the rat and human placentas suggest that PPAR and RXR isoforms are potential regulators of placental lipid transfer and homeostasis. Our work provides a framework for the further investigation of PPAR and RXR isoform specific regulation of placental fatty acid uptake, transport and metabolism.

Potentiation of insulin signaling in tissues of Zucker obese rats after acute and long-term treatment with PPARgamma agonists

Thiazolidinediones (TZDs), agonists of peroxisome proliferator-activated receptor-gamma (PPARgamma), improve insulin sensitivity in vivo, and the mechanism remains largely unknown. In this study, we showed that, in Zucker obese (fa/fa) rats, acute (1-day) treatment with both rosiglitazone (a TZD) and a non-TZD PPARgamma agonist (nTZD) reduced plasma free fatty acid and insulin levels and, concomitantly, potentiated insulin-stimulated Akt phosphorylation at threonine 308 (Akt-pT308) in adipose and muscle tissues. A similar effect on Akt was observed in liver after a 7-day treatment. The increase in Akt-pT308 was correlated with an increase in Akt phosphorylation at serine 473 (Akt-pS473), tyrosine phosphorylation of insulin receptor beta subunit and insulin receptor substrate-1, and serine phosphorylation of glycogen synthase kinase-3alpha/beta. The agonists appeared to potentiate Akt1 phosphorylation in muscle and liver and both Akt1 and Akt2 in adipose. Finally, potentiation of insulin signaling was also observed in isolated adipose tissue ex vivo and differentiated 3T3 L1 adipocytes in vitro, but not in rat primary hepatocytes in vitro. These results suggest that 1) PPARgamma agonists acutely potentiate insulin signaling in adipose and muscle tissues and such regulation may be physiologically relevant to insulin sensitization in vivo; 2) the agonists directly target adipose tissues; and 3) the metabolic and signaling effects of the agonists are mediated by structurally distinct PPARgamma agonists.

Ligand and coactivator recruitment preferences of peroxisome proliferator activated receptor alpha

The mechanism by which ligands of nuclear receptors show differential effects on gene transcription is not fully understood, but is believed to result in part from the preferential recruitment and/or displacement of coactivators and corepressors. We have explored the interaction of several known ligands and the nuclear receptor (peroxisome proliferator activated receptor alpha, PPARalpha) using scintillation proximity assay (SPA) and the interaction of LXXLL containing peptides derived from three coactivators (SRC-1, CBP and PGC-1) with PPARalpha in the presence of PPARalpha agonist ligands using fluorescence resonance energy transfer (FRET). The EC(50)s of the individual ligands for recruitment showed the same rank order regardless of the coactivator peptide used, with GW2331<WY14643=ciprofibrate<L165041<gemfibrozil. Similarly, for all ligands tested, the rank order of EC(50) for peptide recruitment was CBP<PGC-1<SRC-1. These data suggest that for these LXXLL coactivator peptides, the ligands do not substantially differ in their preferences. Partial agonism was observed with ciprofibrate and PGC-1 and gemfibrozil and CBP giving a lower FRET at saturation than with the other ligands. This suggests that ciprofibrate and gemfibrozil induce a different conformation to the receptor-PGC-1 and receptor-CBP complex, respectively. In cotransfection assays, unexpected differences in potencies and efficacies were observed and the rank order of EC(50)s for activation differed from that predicted by FRET assays. In most cases, the presence of a coactivator peptide led to decrease in the EC(50)s seen in FRET assays compared to the K(i)s observed in binding to receptor only, consistent with the lower EC(50)s obtained in the transfection assays. Our data demonstrate that ligand induced coactivator preferences of PPARalpha contribute to transcription potency and efficacy.

A novel liver X receptor agonist establishes species differences in the regulation of cholesterol 7alpha-hydroxylase (CYP7a)

The liver X receptors, LXRalpha and LXRbeta, are members of the nuclear receptor superfamily. Originally identified as orphans, both receptor subtypes have since been shown to be activated by naturally occurring oxysterols. LXRalpha knockout mice fail to regulate cyp7a mRNA levels upon cholesterol feeding, implicating the role of this receptor in cholesterol homeostasis. LXR activation also induces the expression of the lipid pump involved in cholesterol efflux, the gene encoding ATP binding cassette protein A1 (ABCA1). Therefore, LXR is believed to be a sensor of cholesterol levels and a potential therapeutic target for atherosclerosis. Here we describe a synthetic molecule named F(3)MethylAA [3-chloro-4-(3-(7-propyl-3-trifluoromethyl-6-(4,5)-isoxazolyl)propylthio)-phenyl acetic acid] that is more potent than 22(R)-hydroxycholesterol in LXR in vitro assays. F(3)MethylAA is capable not only of inducing ABCA1 mRNA levels, but also increasing cholesterol efflux from THP-1 macrophages. In rat hepatocytes, F(3)MethylAA induced cyp7a mRNA, confirming conclusions from the knockout mouse studies. Furthermore, in rat in vivo studies, F(3)MethylAA induced liver cyp7a mRNA and enzyme activity. A critical species difference is also reported in that neither F(3)MethylAA nor 22(R)-hydroxycholesterol induced cyp7a in human primary hepatocytes. However, other LXR target genes, ABCA1, ABCG1, and SREBP1, were regulated.

Differential regulation of vascular endothelial growth factor expression by peroxisome proliferator-activated receptors in bladder cancer cells

The growth of any solid tumor depends on angiogenesis. Vascular endothelial growth factor (VEGF) plays a prominent role in vesical tumor angiogenesis regulation. Previous studies have shown that the peroxisome proliferator-activated receptor gamma (PPARgamma) was involved in the angiogenesis process. Here, we report for the first time that in two different human bladder cancer cell lines, RT4 (derived from grade I tumor) and T24 (derived from grade III tumor), VEGF (mRNA and protein) is differentially up-regulated by the three PPAR isotypes. Its expression is increased by PPARalpha, beta, and gamma in RT4 cells and only by PPARbeta in T24 cells via a transcriptional activation of the VEGF promoter through an indirect mechanism. This effect is potentiated by an RXR (retinoid-X-receptor), selective retinoid LG10068 providing support for a PPAR agonist-specific action on VEGF expression. While investigating the downstream signaling pathways involved in PPAR agonist-mediated up-regulation of VEGF, we found that only the MEK inhibitor PD98059 reduced PPAR ligand-induced expression of VEGF. These data contribute to a better understanding of the mechanisms by which PPARs regulate VEGF expression. They may lead to a new therapeutic approach to human bladder cancer in which excessive angiogenesis is a negative prognostic factor.

Opposing effects of fatty acids and acyl-CoA esters on conformation and cofactor recruitment of peroxisome proliferator-activated receptors

The peroxisome proliferator-activated receptors (PPARs) bind and are activated by a variety of fatty acids and derivatives thereof. Agonist binding enhances PPAR-mediated transactivation via release of corepressors and recruitment of coactivator complexes. Recently, we and others have reported that acyl-CoA esters act as PPAR antagonists in vitro. Here, we show that the binding of the nonhydrolyzable acyl-CoA analogue, S-hexadecyl-CoA, differentially affected conformation and coactivator recruitment of the individual PPAR subtypes. In protease protection assays, S-hexadecyl CoA increased the sensitivity of PPARalpha and PPARdelta towards chymotrypsin, whereas the action of chymotrypsin on PPARgamma was only marginally affected, suggesting distinct subtype-dependent differences in the effects of S-hexadecyl-CoA on conformation of the PPARs. In keeping with these findings, S-hexadecyl-CoA abrogated ligand-induced recruitment of coactivators to PPARalpha and PPARdelta, whereas coactivator recruitment to PPARgamma was unaffected by S-hexadecyl-CoA.

Gene expression profile of adipocyte differentiation and its regulation by peroxisome proliferator-activated receptor-gamma agonists

PPAR gamma is an adipocyte-specific nuclear hormone receptor. Agonists of PPAR gamma, such as thiazolidinediones (TZDs), promote adipocyte differentiation and have insulin-sensitizing effects in animals and diabetic patients. Affymetrix oligonucleotide arrays representing 6347 genes were employed to profile the gene expression responses of mature 3T3-L1 adipocytes and differentiating preadipocytes to a TZD PPAR gamma agonist in vitro. The expression of 579 genes was significantly up- or down-regulated by more than 1.5-fold during differentiation and/or by treatment with TZD, and these genes were organized into 32 clusters that demonstrated concerted changes in expression of genes controlling cell growth or lipid metabolism. Quantitative PCR was employed to further characterize gene expression and led to the identification of beta-catenin as a new PPAR gamma target gene. Both mRNA and protein levels for beta-catenin were down-regulated in 3T3-L1 adipocytes compared with fibroblasts and were further decreased by treatment of adipocytes with PPAR gamma agonists. Treatment of db/db mice with a PPAR gamma agonist also resulted in reduction of beta-catenin mRNA levels in adipose tissue. These results suggest that beta-catenin plays an important role in the regulation of adipogenesis. Thus, the transcriptional patterns revealed in this study further the understanding of adipogenesis process and the function of PPAR gamma activation.

PPARgamma, an X-ceptor for Xs

Evidence from both human genetic studies and characterization of peroxisome proliferator-activated receptor gamma (PPARgamma) knockout mice suggested that the prime function of PPARgamma is fat formation and that its role in insulin sensitization might be secondary to this function. The thrifty function of PPARgamma was most likely evolutionary beneficial, but might in "times of plenty" contribute to the pathogenesis of disorders, such as obesity, insulin resistance, type 2 diabetes, and hyperlipidemia, often commonly referred to as "syndrome X". This role of PPARgamma in these diseases also questions the eventual therapeutic benefits of pure PPARgamma activation, which is associated with an increase in adipose tissue mass. We characterized a new chemical class of PPARgamma agonists, that is, FMOC-l-leucine (FLL). FLL induces a different conformation of PPARgamma relative to classical PPARgamma ligands. Mass spectrometry indicates that two molecules of FLL bind to a single PPARgamma molecule, making its mode of receptor interaction distinctive. FLL recruits a different set of coactivators and activates PPARgamma with a lower potency, but a similar maximal efficacy, relative to known PPARgamma ligands. In contrast, FLL is a more effective insulin sensitizer than current PPARgamma agonists, an effect potentially linked to its weak adipogenic activity. These data make a strong point for potential therapeutic benefits of PPARgamma modulation rather than activation.

Peroxisome proliferator-activated receptor gamma agonists inhibit HIV-1 replication in macrophages by transcriptional and post-transcriptional effects

Previous studies have demonstrated that cyclopentenone prostaglandins (cyPG) inhibit human immunodeficiency virus type 1 (HIV-1) replication in various cell types. We investigated the role of PG in the replication of HIV-1 in primary macrophages. The cyPG, PGA(1) and PGA(2), inhibited HIV-1 replication in acutely infected human monocyte-derived macrophages (MDM). Because PGA(1) and PGA(2) have previously been shown to be peroxisome proliferator-activated receptor gamma (PPARgamma) agonists, we examined the effect of synthetic PPARgamma agonists on HIV replication. The PPARgamma agonist ciglitazone inhibited HIV-1 replication in a dose-dependent manner in acutely infected human MDM. In addition, cyPG and ciglitazone reduced HIV replication in latently infected and viral entry-independent U1 cells, suggesting an effect at the level of HIV gene expression. Ciglitazone also suppressed HIV-1 mRNA levels as measured by reverse transcriptase PCR, in parallel with the decrease in reverse transcriptase activity. Co-transfection of PPARgamma wild type vectors and treatment with PPARgamma agonists inhibited HIV-1 promoter activity in U937 cells. Activation of PPARgamma also decreased HIV-1 mRNA stability following actinomycin D treatment. In summary, our experimental findings implicate PPARgamma as an important factor in the suppression of HIV-1 gene expression in MDM by cyPG. Thus natural and synthetic PPARgamma agonists may play a role in controlling HIV-1 infection in macrophages.

The mechanisms of action of PPARs

The peroxisome proliferator-activated receptors (PPARs) are a group of three nuclear receptor isoforms, PPAR gamma, PPAR alpha, and PPAR delta, encoded by different genes. PPARs are ligand-regulated transcription factors that control gene expression by binding to specific response elements (PPREs) within promoters. PPARs bind as heterodimers with a retinoid X receptor and, upon binding agonist, interact with cofactors such that the rate of transcription initiation is increased. The PPARs play a critical physiological role as lipid sensors and regulators of lipid metabolism. Fatty acids and eicosanoids have been identified as natural ligands for the PPARs. More potent synthetic PPAR ligands, including the fibrates and thiazolidinediones, have proven effective in the treatment of dyslipidemia and diabetes. Use of such ligands has allowed researchers to unveil many potential roles for the PPARs in pathological states including atherosclerosis, inflammation, cancer, infertility, and demyelination. Here, we present the current state of knowledge regarding the molecular mechanisms of PPAR action and the involvement of the PPARs in the etiology and treatment of several chronic diseases.

Octanoate attenuates adipogenesis in 3T3-L1 preadipocytes

Preadipocytes exposed to octanoate accumulate less lipid than cells exposed to long-chain fatty acids. This effect of octanoate involves significant attenuation of expression of key adipogenic transcription factors, including peroxisome proliferator-activated receptor (PPAR)gamma, steroid regulatory binding element protein (SREBP)-1c and CCAAT element binding protein (C/EBPalpha) at both the mRNA and protein levels. Expression of differentiation markers, including adipocyte fatty acid binding protein (ALBP), glycerol-3-phosphate dehydrogenase (GPDH) and leptin, was also significantly diminished by octanoate. However, octanoate did not prevent the decrease in preadipocyte factor-1 (Pref-1) expression that occurs during adipogenesis, nor did it inhibit the early induction of C/EBPbeta,delta. Treatment with synthetic PPARgamma ligands partially offset the inhibitory effect of octanoate on differentiation. Ectopic expression of PPARgamma2 in 3T3-L1 cells partially restored lipid accretion and GPDH activity in octanoate-treated cells. Adding octanoate together with troglitazone attenuated the effects of troglitazone on adipocyte differentiation in both normal 3T3-L1 cells and engineered 3T3-L1 cells that expressed ectopic PPARgamma2, implying that octanoate might compete against troglitazone for its binding to PPARgamma. These results suggest that octanoate may block adipogenesis at least in part by its influence on the expression/activation of PPARgamma.

Development of a homogeneous, fluorescence resonance energy transfer-based in vitro recruitment assay for peroxisome proliferator-activated receptor delta via selection of active LXXLL coactivator peptides

The peroxisome proliferator-activated receptors (PPARs) are nuclear receptors activated by fatty acids and their metabolites. The PPARdelta subtype is believed to be involved in lipoprotein regulation and may have a role in reverse cholesterol transport. While the range of biological roles of PPARdelta still remains unclear, it is of therapeutic interest in cardiovascular diseases. Here we report a homogeneous in vitro assay for studying ligand activation of PPARdelta. We surveyed a panel of peptides containing the LXXLL motifs derived from coactivator protein sequences. Peptides with the best response were used to develop a sensitive and homogeneous recruitment assay for PPARdelta. The optimized assay has a signal-to-background ratio of about 8:1 and an assay quality parameter Z'-factor value of 0.8. The assay signal generated is stable for hours to even overnight. This simple recruitment assay can provide agonist and/or antagonist information that cannot be assessed by receptor-binding assay, and can be used for characterization and screening of ligands that modulate the activation of PPARdelta.

Induction of adipocyte complement-related protein of 30 kilodaltons by PPARgamma agonists: a potential mechanism of insulin sensitization

Adipocyte complement-related protein of 30 kDa (Acrp30, adiponectin, or AdipoQ) is a fat-derived secreted protein that circulates in plasma. Adipose tissue expression of Acrp30 is lower in insulin-resistant states and it is implicated in the regulation of in vivo insulin sensitivity. Here we have characterized the ability of PPARgamma agonists to modulate Acrp30 expression. After chronic treatment of obese-diabetic (db/db) mice with PPARgamma agonists (11 d), mean plasma Acrp30 protein levels increased (>3x). Similar effects were noted in a nongenetic type 2 diabetes model (fat-fed and low-dose streptozotocin-treated mice). In contrast, treatment of mice (db/db or fat-fed) with metformin or a PPARalpha agonist did not affect plasma Acrp30 protein levels. In a cohort of normal human subjects, 14-d treatment with rosiglitazone also produced a 130% increase in circulating Acrp30 levels vs. placebo. In addition, circulating Acrp30 levels were suppressed 5-fold in patients with severe insulin resistance in association with dominant-negative PPARgamma mutations. Thus, induction of adipose tissue Acrp30 expression and consequent increases in circulating Acrp30 levels represents a novel potential mechanism for PPARgamma-mediated enhancement of whole-body insulin sensitivity. Furthermore, Acrp30 is likely to be a biomarker of in vivo PPARgamma activation.

Characterization of the mouse peroxisome proliferator-activated receptor delta gene

Peroxisome proliferator-activated receptors (alpha, delta and gamma) are ligand-activated transcription factors that are involved in multiple cellular responses. The PPARdelta subtype is the least understood of all PPAR subtypes. PPARdelta is activated by unsaturated fatty acids, PGI2, and by synthetic ligands. PPARdelta-regulated genes have not been identified and the factors that control PPARdelta expression are not known. The gene that encodes the mouse PPARdelta gene is contained in >30 kb DNA sequence and organized in eight exons, six of which encode the PPARdelta receptor. A PPARdelta-luciferase reporter containing 694 bp 5' upstream regulatory and 127 bp untranslated was introduced to primary brain cultures to begin a characterization of the DNA sequences that mediate transcriptional regulation of PPARdelta. PPARdelta-luciferase expression was 10 times higher in oligodendrocyte-containing mature cultures than in immature cultures, indicating that PPARdelta may play a role during oligodendrocyte migration, proliferation, and/or maturation.

Peroxisome proliferator-activated receptors

An exciting and rapidly evolving area in vascular biology and atherosclerosis research over the past 3 years has been the establishment of peroxisome proliferator-activated receptor (PPAR) expression in the vascular and inflammatory cells, and the emerging picture of the roles these ligand-activated nuclear receptor/transcription factors might play in vascular biology and atherosclerosis. Such work is all the more compelling given the ongoing clinical use of PPAR activators in patients. Thiazolidinediones (PPAR-g agonists) are used as insulin sensitizers in diabetic patients known to be at extraordinarily high risk for cardiovascular disease, whereas fibrates (PPAR-a agonists) are used to treat dyslipidemia, particularly in the case of high triglycerides and low high-density lipoprotein cholesterol.

Physiological and therapeutic roles of peroxisome proliferator-activated receptors

The peroxisome proliferator-activated receptors (PPARs) are a group of nuclear receptor isoforms, including PPARgamma, PPARalpha, and PPARdelta, encoded by different genes. PPARs are ligand-regulated transcription factors that control gene expression by binding to specific response elements (PPREs) within promoters. PPARs bind as heterodimers with a retinoid X receptor and, upon binding agonist, interact with cofactors increasing the rate of transcription initiation. The PPARs play a critical physiological role as lipid sensors and regulators of lipid metabolism. Natural ligands for the PPARs include fatty acids and eicosanoids. More potent synthetic PPAR ligands, including the fibrates and thiazolidinediones, are effective in the treatment of dyslipidemia and diabetes. Use of selective ligands led to the discovery of additional potential roles for the PPARs in pathological states, including atherosclerosis, inflammation, and hypertension. This review provides an overview of the molecular mechanisms of PPAR action and the involvement of the PPARs in the etiology and treatment of several chronic diseases.

Sebaceous epithelial cell differentiation requires cyclic adenosine monophosphate generation

The cyclic adenosine monophosphate (cAMP) generator choleratoxin is known to promote the growth of sebaceous epithelial cells (sebocytes) in monolayer culture in classical serum-containing media. Now that sebocytes can be grown in serum-free medium, we have examined whether choleratoxin or other cAMP generators are required for differentiation of rat preputial sebocytes in response to specific ligand activators of peroxisome proliferator-activated receptors (PPARs). Unexpectedly, choleratoxin reduced sebocyte proliferation. However, sebocyte differentiation in response to specific PPARalpha and PPARgamma agonists required a cAMP generator such as choleratoxin, and this response was suppressed by a protein kinase A inhibitor. In contrast, the stable prostacyclin analog, carbaprostacyclin (cPGI2), a PPARalpha,delta agonist that also generates cAMP, stimulated differentiation independently of choleratoxin. Furthermore, unlike the selective PPARalpha and PPARgamma agonists, cPGI2 stimulated both sebocyte DNA synthesis and proliferation. These data are compatible with the evidence that prostacyclin has the additional effect of generating cAMP. In addition, we addressed the possibility that choleratoxin may act as a surrogate for beta-adrenergic catecholamines in generating cAMP. In contrast with choleratoxin, both alpha- and beta-adrenergic catecholamines stimulated sebocyte growth and interfered with the choleratoxin effect on differentiation. These data suggest ligand-dependent, complex interactions between cAMP and the other signal transduction pathways involved in sebocyte growth and development.

Novel insulin sensitizers: pharmacogenomic aspects

Thiazolidinediones (TZD, glitazones) are a new class of oral antidiabetic drugs which exert their insulin sensitizing action by stimulation of the nuclear transcription factor peroxisome proliferator-activated receptor gamma (PPAR-gamma). At present pioglitazone and rosiglitazone are available for clinical use. Different activation levels of PPAR-gamma and of co-factors determine the binding of PPAR-gamma to distinct target genes, which in turn regulates their transcriptional activity. TZD lower blood glucose levels, partly by influencing glucose transporters and the insulin-signaling pathway. In this review the molecular and cellular mechanisms as well as the metabolic effects of PPAR activation by TZD are discussed. Knowledge regarding the influence of genetic variations of PPAR-gamma on the effects of TZD is so far limited to in vitro studies. The results of these studies are reviewed.

Adipocyte differentiation: from fibroblast to endocrine cell

Recent advances regarding the biology of adipose tissue have demonstrated that white adipose tissue (WAT) plays a central role in the regulation of energy balance and acts as a secretory/endocrine organ that mediates numerous physiological and pathological processes. Dysregulation of WAT mass causes obesity or lipoatrophy, two disorders associated with life-threatening pathologies, including cardiovascular diseases and diabetes. Alterations in WAT mass result from changes in adipocyte size and/or number. Change in adipocyte number is achieved through a complex interplay between proliferation and differentiation of preadipocytes. Adipocyte differentiation or adipogenesis is a highly controlled process that has been extensively studied for the last 25 years. In vitro preadipocyte culture systems that recapitulate most of the critical aspects of fat cell formation in vivo have allowed a meticulous dissection of the cellular and molecular events involved in the adipogenesis process. The adipogenic transcription factors peroxisome proliferator-activated receptor-gamma and CCAAT/enhancer binding protein-alpha play a key role in the complex transcriptional cascade that occurs during adipogenesis. Hormonal and nutritional signaling affects adipocyte differentiation in a positive or negative manner, and components involved in cell-cell or cell-matrix interactions are also pivotal in regulating the differentiation process. This knowledge provides a basis for understanding the physiological and pathophysiological mechanisms that underlie adipose tissue formation and for the development of novel and sound therapeutic approaches to treat obesity and its related diseases.

The peroxisome proliferator-activated receptor delta promotes lipid accumulation in human macrophages

The peroxisome proliferator-activated receptors (PPARs) are a family of fatty acid-activated transcription factors which control lipid homeostasis and cellular differentiation. PPARalpha (NR1C1) controls lipid oxidation and clearance in hepatocytes and PPARgamma (NR1C3) promotes preadipocyte differentiation and lipogenesis. Drugs that activate PPARalpha are effective in lowering plasma levels of lipids and have been used in the management of hyperlipidemia. PPARgamma agonists increase insulin sensitivity and are used in the management of type 2 diabetes. In contrast, there are no marketed drugs that selectively target PPARdelta (NR1C2) and the physiological roles of PPARdelta are unclear. In this report we demonstrate that the expression of PPARdelta is increased during the differentiation of human macrophages in vitro. In addition, a highly selective agonist of PPARdelta (compound F) promotes lipid accumulation in primary human macrophages and in macrophages derived from the human monocytic cell line, THP-1. Compound F increases the expression of genes involved in lipid uptake and storage such as the class A and B scavenger receptors (SRA, CD36) and adipophilin. PPARdelta activation also represses key genes involved in lipid metabolism and efflux, i.e. cholesterol 27-hydroxylase and apolipoprotein E. We have generated THP-1 sublines that overexpress PPARdelta and have confirmed that PPARdelta is a powerful promoter of macrophage lipid accumulation. These data suggest that PPARdelta may play a role in the pathology of diseases associated with lipid-filled macrophages, such as atherosclerosis, arthritis, and neurodegeneration.

Tetradecylthioacetic acid inhibits growth of rat glioma cells ex vivo and in vivo via PPAR-dependent and PPAR-independent pathways

The peroxisome proliferator-activated receptors (PPARs) are transcription factors involved in fatty acid metabolism and energy homeostasis. The PPARs also play crucial roles in the control of cellular growth and differentiation. Especially, the recently emerged concept of ligand-dependent PPARgamma-mediated inhibition of cancer cell proliferation through induction of G(1)-phase arrest and differentiation is of clinical interest to cancer therapy. Tetradecylthioacetic acid (TTA) is a sulphur-substituted saturated fatty acid analog with unique biochemical properties. In this study, we investigated the effects of TTA-administration on cell proliferation in glioma cancer models. The rat glioma cell line BT4Cn, whether grown in culture or implanted in rats, expressed significant levels of PPARgamma and PPARdelta, with PPARgamma being the predominant PPAR subtype. In BT4Cn cells, TTA activated all PPAR subtypes in a dose-dependent manner. In cell culture experiments, the PPARgamma-selective ligand BRL49653 moderately inhibited growth of BT4Cn cells, whereas administration of TTA resulted in a marked growth inhibition. Administration of the PPARgamma-selective antagonist GW9662 abolished BRL49653-induced growth inhibition, but only marginally reduced the effect of TTA. TTA reduced tumor growth and increased the survival time of rats with implanted BT4Cn tumor. TTA-induced apoptosis in BT4Cn cells, and the administration of TTA led to cytochrome c release from mitochondria and increased the glutathione content in glioma cells. In conclusion, our results indicate that TTA inhibits proliferation of glioma cancer cells through both PPARgamma-dependent and PPARgamma-independent pathways, of which the latter appears to predominate.

Peroxisome proliferator-activated receptor (PPAR) modulators: diabetes and beyond

Peroxisome proliferator-activated receptors (PPARs) are a class of ligand modulated transcription factors with a prominent role in the regulation of metabolic processes. This report is intended to provide a limited introduction to the PPAR field, sketched with reference to one early series of PPAR ligands.

Peroxisome proliferator-activated receptor gamma ligands differentially modulate muscle cell differentiation and MyoD gene expression via peroxisome proliferator-activated receptor gamma -dependent and -independent pathways

The effects of distinct classes of peroxisome proliferator-activated receptor gamma (PPARgamma) ligands on myogenesis and MyoD gene expression were examined in mouse skeletal muscle C2C12 myoblasts. Treatment of C2C12 cells with the PPARgamma ligand, 15-deoxy-Delta12,14-prostaglandin J2 (15d-PGJ2), repressed morphologically defined myogenesis and reduced endogenous mRNA levels of the myogenic differentiation markers MyoD, myogenin, and alpha-actin. In contrast, two synthetic PPARgamma ligands, L-805645 and ciglitazone, exhibited no effects. In transient transfection assays, 15d-PGJ2 specifically inhibited the expression of a MyoD promoter-luciferase reporter gene (MyoDLuc) in a cell type- and promoter-specific manner, indicating that 15d-PGJ2 functions in part by repressing MyoD gene transcription. The inhibition of MyoD gene expression by 15d-PGJ2 is mediated by the distal region of the MyoD gene promoter. PPARgamma on its own also inhibited MyoDLuc expression and further augmented the 15d-PGJ2 response. In contrast, L-805645 and ciglitazone did not inhibit MyoDLuc expression on their own but did so in the presence of ectopically expressed PPARgamma. Interestingly, a transdominant inhibitor of PPARgamma (hPPARgamma2Delta500) had no effect on the 15d-PGJ2-dependent repression of MyoDLuc expression but overcame L-805645/PPARgamma-dependent repression. Finally, saturating concentrations of L-805645, which did not affect myogenesis, failed to ablate 15d-PGJ2-mediated repression of the myogenic program. Thus, distinct PPARgamma ligands may repress MyoD gene expression through PPARgamma-dependent and -independent pathways, and 15d-PGJ2 can inhibit the myogenic program independent of its cognate receptor, PPARgamma.

Direct thiazolidinedione action on isolated rat skeletal muscle fuel handling is independent of peroxisome proliferator-activated receptor-gamma-mediated changes in gene expression

Thiazolidinediones (TZDs) are believed to induce insulin sensitization by modulating gene expression via agonistic stimulation of the nuclear peroxisome proliferator-activated receptor-gamma (PPAR-gamma). We have shown earlier that the TZD troglitazone inhibits mitochondrial fuel oxidation in isolated rat skeletal muscle. In the present study, rat soleus muscle strips were exposed to TZDs to examine whether the inhibition of fuel oxidation is mediated by PPAR-gamma activation. Our findings consistently indicated direct, acute, and PPAR-gamma-independent TZD action on skeletal muscle fuel metabolism. Rapid stimulation of lactate release by 20 micromol/l troglitazone within 30 min suggested that direct TZD action on skeletal muscle in vitro does not rely on changes in gene expression rates (12.6 +/- 0.6 [control] vs. 16.0 +/- 0.8 micromol. g(-1). h(-1) [troglitazone]; P < 0.01). This conclusion was supported by the failure of actinomycin D and cycloheximide to block the effects of troglitazone. Mitochondrial fuel oxidation was consistently inhibited by six different TZDs (percent inhibition of CO(2) production from palmitate after 25 h: troglitazone, -61 +/- 2%; pioglitazone, -43 +/- 7%; rosiglitazone, -22 +/- 6%; BM13.1258, -47 +/- 9%; BM15.2054, -51 +/- 4%; and T-174, -59 +/- 4% [P < 0.005 each]), but not by PPAR-gamma agonistic compounds not belonging to the TZD class (JTT-501, -5 +/- 7% [NS]; prostaglandin J(2), 17 +/- 7% [P < 0.05]), which further argues against dependence on PPAR-gamma activation. In summary, our findings provided good evidence that direct inhibition of mitochondrial fuel oxidation in isolated skeletal muscle is a group-specific effect of TZDs and is independent of PPAR-gamma-mediated gene expression.

Up-regulation of uncoupling protein 3 gene expression by fatty acids and agonists for PPARs in L6 myotubes

Uncoupling protein 3 (UCP3), which uncouples electron transport from ATP synthesis, is expressed at high levels in the skeletal muscle, an important organ in glucose and lipid metabolism. Because several reports proposed that fatty acids induced UCP3 gene expression in skeletal muscle in vivo, in the present study we examined the regulation of UCP3 gene expression by various fatty acids using L6 myotubes. UCP3 gene expression was increased in L6 myotubes by various fatty acids or by alpha-bromopalmitate, a nonmetabolized derivative of palmitic acid. Because fatty acids are also known as agonists for PPARs, we examined the involvement of PPARs in the regulation of the UCP3 gene expression. L-165041, a PPAR delta agonist, increased UCP3 gene expression in L6 myotubes, whereas neither Wy 14,643, a PPAR alpha agonist, nor Pioglitazone, a PPAR gamma agonist, increased it. Therefore, we conclude that UCP3 gene expression is increased by the activation of PPAR delta in L6 myotubes and postulate that PPAR delta mediates at least some part of the increased UCP3 gene expression by fatty acids in skeletal muscle in vivo.

Peroxisome proliferator-activated receptor-gamma-independent inhibition of macrophage activation by the non-thiazolidinedione agonist L-796,449. Comparison with the effects of 15-deoxy-delta(12,14)-prostaglandin J(2)

The effects of L-796,449 (3-chloro-4-(3-(3-phenyl-7-propylbenzofuran-6-yloxy)propylthio)phenylacetic acid; referred to henceforth as compound G), a thiazolidinedione-unrelated peroxisome proliferator activated-receptor-gamma (PPAR-gamma) agonist, on early signaling in lipopolysaccharide-activated RAW 264.7 macrophages were analyzed and compared with those elicited by 15-deoxy-Delta(12,14)-prostaglandin J(2) and the thiazolidinedione rosiglitazone. Compound G inhibited the activation of nuclear factor kappa B through the impairment of the targeting and degradation of I kappa B proteins and promoted a redistribution of I kappa B alpha and I kappa B beta in the nucleus of activated cells. Compound G inhibited I kappa B kinase (IKK) activity both in vivo and in vitro, suggesting a direct mechanism of interaction between this molecule and the IKK complex. The effect of compound G on IKK activity was independent of PPAR-gamma engagement because RAW 264.7 cells expressed negligible levels of this nuclear receptor, and rosiglitazone failed to mimic these actions. Moreover, treatment of activated macrophages with compound G enhanced the synthesis of superoxide anion, which, in combination with the NO produced under activation conditions, triggered apoptosis through the intracellular synthesis of peroxynitrite. These results suggest that compound G might contribute to the resolution of inflammation by favoring the induction of apoptosis through mechanisms independent of PPAR-gamma engagement.

Differential gene regulation in human versus rodent hepatocytes by peroxisome proliferator-activated receptor (PPAR) alpha. PPAR alpha fails to induce peroxisome proliferation-associated genes in human cells independently of the level of receptor expresson

We compared the ability of rat and human hepatocytes to respond to fenofibric acid and a novel potent phenylacetic acid peroxisome proliferator-activated receptor (PPAR) alpha agonist (compound 1). Fatty acyl-CoA oxidase (FACO) activity and mRNA were increased after treatment with either fenofibric acid or compound 1 in rat hepatocytes. In addition, apolipoprotein CIII mRNA was decreased by both fenofibric acid and compound 1 in rat hepatocytes. Both agonists decreased apolipoprotein CIII mRNA in human hepatocytes; however, very little change in FACO activity or mRNA was observed. Furthermore, other peroxisome proliferation (PP)-associated genes including peroxisomal 3-oxoacyl-CoA thiolase (THIO), peroxisomal enoyl-CoA hydratase/3-hydroxyacyl-CoA dehydrogenase (HD), peroxisomal membrane protein-70 (PMP-70) were not regulated by PPAR alpha agonists in human hepatocytes. Moreover, other genes that are regulated by PPAR alpha ligands in human hepatocytes such as mitochondrial HMG-CoA synthase and carnitine palmitoyl transferase-1 (CPT-1) were also regulated in HepG2 cells by PPAR alpha agonists. Several stably transfected HepG2 cell lines were established that overexpressed human PPAR alpha to levels between 6- and 26-fold over normal human hepatocytes. These PPAR alpha-overexpressing cells had higher basal mRNA levels of mitochondrial HMG-CoA synthase and CPT-1; however, basal FACO mRNA levels and other PP-associated genes including THIO, HD, or PMP-70 mRNA were not substantially affected. In addition, FACO, THIO, HD, and PMP-70 mRNA levels did not increase in response to PPAR alpha agonist treatment in the PPAR alpha-overexpressing cells, although mitochondrial HMG-CoA synthase and CPT-1 mRNAs were both induced. These results suggest that other factors besides PPAR alpha levels determine the species-specific response of human and rat hepatocytes to the induction of PP.

Modulation of keratinocyte gene expression and differentiation by PPAR-selective ligands and tetradecylthioacetic acid

Peroxisome proliferator-activated receptors (PPARs) are pleiotropic regulators of growth and differentiation of many cell types. We have performed a comprehensive analysis of the expression of PPARs, transcriptional cofactors, and marker genes during differentiation of normal human keratinocytes using a combination of reverse transcriptase polymerase chain reaction, Northern and Western blotting, and immunohistochemistry. PPARdelta was the predominant PPAR subtype in human keratinocytes and highly expressed in basal cells and suprabasal cells. Induction of PPARalpha and PPARgamma expression was linked to differentiation, and accordingly, expression of PPARalpha and PPARgamma was in essence confined to suprabasal cells. Differentiation was not accompanied by significant changes in the expression of the coactivators CREB-binding protein, p300, steroid receptor coactivator 1, or the corepressors nuclear receptor corepressor and silence mediator for retinoid and thyroid hormone receptors. We critically evaluated the effects of selective PPAR ligands and a synthetic fatty acid analog, tetradecylthioacetic acid. Tetradecylthioacetic acid activated all human PPAR subtypes in the ranking order PPARdelta >> PPARalpha > PPARgamma. All selective PPAR ligands marginally induced transglutaminase-1 expression with the PPARdelta-selective ligand L165041 being the most potent. The PPARalpha- and PPARgamma-selective ligands Wy14643 and BRL49653 had negligible effect on involucrin expression, whereas a dose-dependent induction was observed with L165041. Simultaneous addition of L165041 and BRL49653 synergistically induced strong involucrin expression. Additionally, L165041 potently induced CD36 mRNA expression. Administration of tetradecylthioacetic acid resulted in a dramatic decrease in proliferation and a robust upregulation of the expression of involucrin and transglutaminase. Our results indicate that tetradecylthioacetic acid may affect keratinocyte gene expression and differentiation via PPAR-dependent and PPAR-independent pathways, and that the latter play an important role.

Peroxisome proliferator-activated receptors (PPARs): novel therapeutic targets in renal disease

Peroxisome proliferator-activated receptors (PPARs): Novel therapeutic targets in renal disease. Peroxisome proliferator-activated receptors (PPARs) are members of the nuclear hormone receptor superfamily of ligand-dependent transcription factors. PPARs play an important role in the general transcriptional control of numerous cellular processes, including lipid metabolism, glucose homeostasis, cell cycle progression, cell differentiation, inflammation and extracellular matrix remodeling. Three PPAR isoforms, designated PPARalpha, PPARbeta and PPARgamma, have been cloned and are differentially expressed in several tissues including the kidney. PPARalpha primary regulates lipid metabolism and modulates inflammation. PPARalpha is the molecular target of the hypolipidemic fibrates including bezafibrate and clofibrate. PPARbeta participates in embryonic development, implantation and bone formation. PPARgamma is a key factor in adipogenesis and also plays an important role in insulin sensitivity, cell cycle regulation and cell differentiation. Antidiabetic thiazolidinediones (TZDs) such as troglitazone and rosiglitazone are specific ligands of PPARgamma, and this interaction is responsible for the insulin-sensitizing and hypoglycemic effect of these drugs. The kidney has been shown to differentially express all PPAR isoforms. PPARalpha is predominantly expressed in proximal tubules and medullary thick ascending limbs, while PPARgamma is expressed in medullary collecting ducts, pelvic urothelium and glomerular mesangial cells. PPARbeta is ubiquitously expressed at low levels in all segments of nephron. Accumulating data has begun to emerge suggesting physiological and pathophysiological roles of PPARs in several tissues including the kidney. The availability of PPAR-selective agonists and antagonists may provide a new approach to modulate the renal response to diseases including glomerulonephritis, glomerulosclerosis and diabetic nephropathy.

Peroxisome proliferator-activated receptors: from transcriptional control to clinical practice

Peroxisome proliferator-activated receptors (PPARs) are lipid-activated transcription factors that control energy homeostasis through genomic actions. Over the past few years significant advances have been made in unravelling the pathways that are modulated by PPARs. Gene targeting experiments in mice and genetic studies in humans have demonstrated a physiological role for these receptors in adipocyte function, glucose homeostasis, and lipid and lipoprotein metabolism. Recent data indicate that PPARs enhance the reverse cholesterol transport pathway by regulating genes that control macrophage cholesterol efflux, cholesterol transport in plasma and bile acid synthesis. Clinical and experimental evidence suggest that PPAR activation decreases the incidence of cardiovascular disease not only by correcting metabolic disorders, but also through direct actions at the level of the vascular wall. Thus, dysregulation of PPAR activity modulates the onset and evolution of metabolic disorders such as dyslipidaemia, obesity and insulin resistance, predisposing to atherosclerosis.

Therapeutic modulation of transcription factor activity

Aberrant gene expression is a fundamental cause of many disease-associated pathophysiologies. The pharmacological modulation of transcription factor activity therefore represents an attractive therapeutic approach to such disorders. With the exception of nuclear receptors, which are the direct targets of pharmaceuticals, other known classes of transcription factors are largely regulated indirectly by drugs that impact upon those signal transduction cascades that alter transcription factor phosphorylation and dephosphorylation and/or nuclear import. However, recent advances in drug discovery technologies now enable high-throughput screens that can identify molecules that act directly at the level of transcription factor complexes.

A selective peroxisome proliferator-activated receptor delta agonist promotes reverse cholesterol transport

The peroxisome proliferator-activated receptors (PPARs) are dietary lipid sensors that regulate fatty acid and carbohydrate metabolism. The hypolipidemic effects of the fibrate drugs and the antidiabetic effects of the glitazone drugs in humans are due to activation of the alpha (NR1C1) and gamma (NR1C3) subtypes, respectively. By contrast, the therapeutic potential of the delta (NR1C2) subtype is unknown, due in part to the lack of selective ligands. We have used combinatorial chemistry and structure-based drug design to develop a potent and subtype-selective PPARdelta agonist, GW501516. In macrophages, fibroblasts, and intestinal cells, GW501516 increases expression of the reverse cholesterol transporter ATP-binding cassette A1 and induces apolipoprotein A1-specific cholesterol efflux. When dosed to insulin-resistant middle-aged obese rhesus monkeys, GW501516 causes a dramatic dose-dependent rise in serum high density lipoprotein cholesterol while lowering the levels of small-dense low density lipoprotein, fasting triglycerides, and fasting insulin. Our results suggest that PPARdelta agonists may be effective drugs to increase reverse cholesterol transport and decrease cardiovascular disease associated with the metabolic syndrome X.

Peroxisome proliferator-activated receptor-gamma ligands inhibit adipocyte 11beta -hydroxysteroid dehydrogenase type 1 expression and activity

Peroxisome proliferator-activated receptor-gamma (PPARgamma) has been shown to play an important role in the regulation of expression of a subclass of adipocyte genes and to serve as the molecular target of the thiazolidinedione (TZD) and certain non-TZD antidiabetic agents. Hypercorticosteroidism leads to insulin resistance, a variety of metabolic dysfunctions typically seen in diabetes, and hypertrophy of visceral adipose tissue. In adipocytes, the enzyme 11beta-hydroxysteroid dehydrogenase type 1 (11beta-HSD-1) converts inactive cortisone into the active glucocorticoid cortisol and thereby plays an important role in regulating the actions of corticosteroids in adipose tissue. Here, we show that both TZD and non-TZD PPARgamma agonists markedly reduced 11beta-HSD-1 gene expression in 3T3-L1 adipocytes. This diminution correlated with a significant decrease in the ability of the adipocytes to convert cortisone to cortisol. The half-maximal inhibition of 11beta-HSD-1 mRNA expression by the TZD, rosiglitazone, occurred at a concentration that was similar to its K(d) for binding PPARgamma and EC(50) for inducing adipocyte differentiation thereby indicating that this action was PPARgamma-dependent. The time required for the inhibitory action of the TZD was markedly greater for 11beta-HSD-1 gene expression than for leptin, suggesting that these genes may be down-regulated by different molecular mechanisms. Furthermore, whereas regulation of PPARgamma-inducible genes such as phosphoenolpyruvate carboxykinase was maintained when cellular protein synthesis was abrogated, PPARgamma agonist inhibition of 11beta-HSD-1 and leptin gene expression was ablated, thereby supporting the conclusion that PPARgamma affects the down-regulation of 11beta-HSD-1 indirectly. Finally, treatment of diabetic db/db mice with rosiglitazone inhibited expression of 11beta-HSD-1 in adipose tissue. This decrease in enzyme expression correlated with a significant decline in plasma corticosterone levels. In sum, these data indicate that some of the beneficial effects of PPARgamma antidiabetic agents may result, at least in part, from the down-regulation of 11beta-HSD-1 expression in adipose tissue.

Differential involvement of peroxisome-proliferator-activated receptors alpha and delta in fibrate and fatty-acid-mediated inductions of the gene encoding liver fatty-acid-binding protein in the liver and the small intestine

Liver fatty-acid-binding protein (L-FABP) is a cytoplasmic polypeptide that binds with strong affinity especially to long-chain fatty acids (LCFAs). It is highly expressed in both the liver and small intestine, where it is thought to have an essential role in the control of the cellular fatty acid (FA) flux. Because expression of the gene encoding L-FABP is increased by both fibrate hypolipidaemic drugs and LCFAs, it seems to be under the control of transcription factors, termed peroxisome-proliferator-activated receptors (PPARs), activated by fibrate or FAs. However, the precise molecular mechanism by which these regulations take place remain to be fully substantiated. Using transfection assays, we found that the different PPAR subtypes (alpha, gamma and delta) are able to mediate the up-regulation by FAs of the gene encoding L-FABP in vitro. Through analysis of LCFA- and fibrate-mediated effects on L-FABP mRNA levels in wild-type and PPARalpha-null mice, we have found that PPARalpha in the intestine does not constitute a dominant regulator of L-FABP gene expression, in contrast with what is known in the liver. Only the PPARdelta/alpha agonist GW2433 is able to up-regulate the gene encoding L-FABP in the intestine of PPARalpha-null mice. These findings demonstrate that PPARdelta can act as a fibrate/FA-activated receptor in tissues in which it is highly expressed and that L-FABP is a PPARdelta target gene in the small intestine. We propose that PPARdelta contributes to metabolic adaptation of the small intestine to changes in the lipid content of the diet.

The regulation of uncoupling protein-2 gene expression by omega-6 polyunsaturated fatty acids in human skeletal muscle cells involves multiple pathways, including the nuclear receptor peroxisome proliferator-activated receptor beta

Fatty acids have been postulated to regulate uncoupling protein (UCP) gene expression in skeletal muscle in vivo. We have identified, at least in part, the mechanism by which polyunsaturated fatty acids increase UCP-2 expression in primary culture of human muscle cells. omega-6 fatty acids and arachidonic acid induced a 3-fold rise in UCP-2 mRNA levels possibly through transcriptional activation. This effect was prevented by indomethacin and mimicked by prostaglandin (PG) E(2) and carbaprostacyclin PGI(2), consistent with a cyclooxygenase-mediated process. Incubation of myotubes for 6 h with 100 micrometer arachidonic acid resulted in a 150-fold increase in PGE(2) and a 15-fold increase in PGI(2) in the culture medium. Consistent with a role of cAMP and protein kinase A, both prostaglandins induced a marked accumulation of cAMP in human myotubes, and forskolin reproduced the effect of arachidonic acid on UCP-2 mRNA expression. Inhibition of protein kinase A with H-89 suppressed the effect of PGE(2), whereas cPGI(2) and arachidonic acid were still able to increase ucp-2 gene expression, suggesting additional mechanisms. We found, however, that the MAP kinase pathway was not involved. Prostaglandins, particularly PGI(2), are potent activators of the peroxisome proliferator-activated receptors. A specific agonist of peroxisome proliferator-activated receptor (PPAR) beta (L165041) increased UCP-2 mRNA levels in myotubes, whereas activation of PPARalpha or PPARgamma was ineffective. These results suggest thus that ucp-2 gene expression is regulated by omega-6 fatty acids in human muscle cells through mechanisms involving at least protein kinase A and the nuclear receptor PPARbeta.

Peroxisome proliferators and fatty acids negatively regulate liver X receptor-mediated activity and sterol biosynthesis

Peroxisome proliferators (PPs) are potent tumor promoters in rodents. The mechanism of hepatocarcinogenesis requires the nuclear receptor peroxisome proliferator activated receptor-alpha (PPARalpha), but might also involve the PPARalpha independent alteration of signaling pathways that regulate cell growth. Here, we studied the effects of PPs on the mevalonate pathway, a critical pathway that controls cell proliferation. Liver X receptors (LXRs) are nuclear receptors that act as sterol sensors in the mevalonate pathway. In gene reporter assays in COS-7 cells, the basal activity of the LXR responsive reporter gene (LXRE-luc) was suppressed by 10 microM lovastatin and zaragozic acid A, suggesting that this activity was attributed to the activation of native LXRs, by endogenously produced mevalonate products. The potent PP and rodent tumor promoter, pirinixic acid (WY-14643) also inhibited LXR-mediated transcription in a dose related manner (approximate IC(50) of 100 microM). As did several other PPs including ciprofibric acid and mono-ethylhexylphthalate. Polyunsaturated and medium to long chain fatty acids at 100 microM were also potent inhibitors; the arachidonic acid analogue eicosatetraynoic acid being the most active (approximate IC(50) of 10 microM). Of the PPs and fatty acids tested, there was a strong correlation between the ability of these agents to suppress de novo sterol synthesis in a rat hepatoma cell line, H4IIEC3, and inhibit LXR-mediated transcription in COS-7 cells, but a discordance between these endpoints and PPARalpha activation and fatty acid acyl-CoA oxidase induction. Taken together, these results suggest that PPs and fatty acids negatively regulate the mevalonate pathway through a mechanism that is not entirely dependent on PPARalpha activation. Because of the importance of the mevalonate pathway in regulating cell proliferation, the modulation of this pathway by PPs and fatty acids might contribute to their actions on cell growth/differentiation.

Molecular regulation of adipogenesis

Adipogenesis, or the development of fat cells from preadipocytes, has been one of the most intensely studied models of cellular differentiation. In part this has been because of the availability of in vitro models that faithfully recapitulate most of the critical aspects of fat cell formation in vivo. More recently, studies of adipogenesis have proceeded with the hope that manipulation of this process in humans might one day lead to a reduction in the burden of obesity and diabetes. This review explores some of the highlights of a large and burgeoning literature devoted to understanding adipogenesis at the molecular level. The hormonal and transcriptional control of adipogenesis is reviewed, as well as studies on a less well known type of fat cell, the brown adipocyte. Emphasis is placed, where possible, on in vivo studies with the hope that the results discussed may one day shed light on basic questions of cellular growth and differentiation in addition to possible benefits in human health.

Transcriptional control of adipogenesis

The major transcriptional factors involved in the adipogenic process include proteins belonging to the CCAAT/enhancer binding protein family, peroxisome proliferator-activated receptor gamma, and adipocyte determination and differentiation dependent factor 1, also known as sterol regulatory element-binding protein 1. This process has been characterized with the aid of cell lines that represent various stages in the path of adipocyte commitment, ranging from pluripotent mesodermal fibroblasts to preadipocytes. Molecular analyses have led to a cascade model for adipogenesis based on timed expression of CCAAT/enhancer-binding proteins and peroxisome proliferator-activated receptor gamma. Gene targeting and transgenic-mouse technologies, which allow the manipulation of endogenous genes for these transcription factors, have also contributed to the understanding of adipogenesis. This review aims to integrate this information to gain an understanding of the transcriptional regulation of fat cell formation.

Peroxisome proliferator activated receptor agonists

The peroxisome proliferator activated receptors (PPARs) represent a group of ligand-activated transcription factors that mediate the biological effects of various drugs, such as fibrates and thiazolidinediones. Three PPAR subtypes can be distinguished, alpha, beta and gamma, each of which has an unique pattern of expression among vertebrate tissues. The PPAR alpha receptor is activated by hypolipidemic drugs of the fibrate class, and regulates the expression of numerous genes involved in fatty acid catabolism. The PPAR gamma receptor is activated by hypoglycaemic drugs of the thiazolidinedione class, and is an important determinant of adipocyte differentiation. Little is currently known about PPAR beta. A heavy research effort is currently directed towards the identification of novel high-affinity, high-specificity agonists and antagonists that may be used in the treatment of hyperglycaemia, hyperlipidemia, and other diseases of metabolic origin.

Peroxisome proliferator-activated receptor delta (PPARdelta )-mediated regulation of preadipocyte proliferation and gene expression is dependent on cAMP signaling

The peroxisome proliferator-activated receptor gamma (PPARgamma) is a key regulator of terminal adipocyte differentiation. PPARdelta is expressed in preadipocytes, but the importance of this PPAR subtype in adipogenesis has been a matter of debate. Here we present a critical evaluation of the role of PPARdelta in adipocyte differentiation. We demonstrate that treatment of NIH-3T3 fibroblasts overexpressing PPARdelta with standard adipogenic inducers led to induction of PPARgamma2 expression and terminal adipocyte differentiation in a manner that was strictly dependent on simultaneous administration of a PPARdelta ligand and methylisobutylxanthine (MIX) or other cAMP elevating agents. We further show that ligands and MIX synergistically stimulated PPARdelta-mediated transactivation. In 3T3-L1 preadipocytes, simultaneous administration of a PPARdelta-selective ligand and MIX significantly enhanced the early expression of PPARgamma and ALBP/aP2, but only modestly promoted terminal differentiation as determined by lipid accumulation. Finally, we provide evidence that synergistic activation of PPARdelta promotes mitotic clonal expansion in 3T3-L1 cells with or without forced expression of PPARdelta. In conclusion, our results suggest that PPARdelta may play a role in the proliferation of adipocyte precursor cells, whereas activation of endogenous PPARdelta in 3T3-L1 cells appears to have only minor impact on the processes leading to terminal adipocyte differentiation.

Peroxisome proliferator-activated receptor (PPAR)-alpha activation lowers muscle lipids and improves insulin sensitivity in high fat-fed rats: comparison with PPAR-gamma activation

Peroxisome proliferator-activated receptor (PPAR)-alpha agonists lower circulating lipids, but the consequences for muscle lipid metabolism and insulin sensitivity are not clear. We investigated whether PPAR-alpha activation improves insulin sensitivity in insulin-resistant rats and compared the effects with PPAR-gamma activation. Three-week high fat-fed male Wistar rats were untreated or treated with the specific PPAR-alpha agonist WY14643 or the PPAR-gamma agonist pioglitazone (both 3 mg x kg(-1) x day(-1)) for the last 2 weeks of high-fat feeding. Like pioglitazone, WY14643 lowered basal plasma levels of glucose, triglycerides (-16% vs. untreated), and leptin (-52%), and also muscle triglyceride (-34%) and total long-chain acyl-CoAs (LCACoAs) (-41%) (P < 0.05). In contrast to pioglitazone, WY14643 substantially reduced visceral fat weight and total liver triglyceride content (P < 0.01) without increasing body weight gain. WY14643 and pioglitazone similarly enhanced whole-body insulin sensitivity (clamp glucose infusion rate increased 35 and 37% and glucose disposal 22 and 15%, respectively, vs. untreated). Both agents enhanced insulin-mediated muscle glucose metabolic index (Rg') and reduced muscle triglyceride and LCACoA accumulation (P < 0.05). Although pioglitazone had more potent effects than WY14643 on muscle insulin sensitization, this was associated with its greater effect to reduce muscle LCACoA accumulation. Overall insulin-mediated muscle Rg' was inversely correlated with the content of LCACoAs (r = -0.74, P = 0.001) and with plasma triglyceride levels (r = -0.77, P < 0.001). We conclude that even though WY14643 and pioglitazone, representing PPAR-alpha and PPAR-gamma activation, respectively, may alter muscle lipid supply by different mechanisms, both significantly improve muscle insulin action in the high fat-fed rat model of insulin resistance, and this effect is proportional to the degree to which they reduce muscle lipid accumulation.