A potent antidiabetic thiazolidinedione with unique peroxisome proliferator-activated receptor gamma-activating properties

Bone is a target for the antidiabetic compound rosiglitazone

Rosiglitazone is an FDA-approved oral antidiabetic agent for the treatment of type 2 diabetes. This compound improves insulin sensitivity through the activation of the nuclear receptor, peroxisome proliferator-activated receptor-gamma (PPAR-gamma). In addition to sensitizing cells to insulin, the PPAR-gamma2 isoform appears to be critical for the regulation of osteoblast and adipocyte differentiation from common mesenchymal bone marrow progenitors. We have demonstrated previously that PPAR-gamma2 activated with rosiglitazone acts as a dominant inhibitor of osteoblastogenesis in murine bone marrow in vitro. Here, we show that in vivo, rosiglitazone administration results in significant bone loss. When rosiglitazone (20 microg/g body weight/d) was given to 6-month-old, nondiabetic C57BL/6 mice for 7 wk, a significant decrease in total body bone mineral density was observed. Analysis of bone microarchitecture, using micro-computed tomography, demonstrated a decrease in bone volume, trabecular width, and trabecular number and an increase in trabecular spacing. Histomorphometric analysis showed a decrease in bone formation rate, with a simultaneous increase in fat content in the bone marrow. Changes in bone morphology and structure were accompanied by changes in the expression of osteoblast- and adipocyte-specific marker genes; the expression of the osteoblast-specific genes Runx2/Cbfa1, Dlx5, and alpha1(I)collagen were decreased, whereas the expression of the adipocyte-specific fatty acid binding protein aP2, was increased. These in vivo data suggest that rosiglitazone therapy may pose a significant risk of adverse skeletal effects in humans.

Progress in Methodology Improved Reporter Gene Assays Used to Identify Ligands Acting on Orphan Seven-Transmembrane Receptors

Seven-transmembrane G-protein-coupled receptors play a central role in physiology by facilitating cell communication through recognition of a wide range of ligands. Even more important, they represent important drug targets. Unfortunately, for many of these receptors the endogenous ligands, and hence their functions, remain to be identified. These receptors are referred to as "orphan" receptors. A pre-requisite for the identification of ligands activating orphan receptors is powerful assay systems. Until now, reporter gene assays have not been in common use in this process. Here, we summarize our development of improved reporter gene assays. We optimized reporter gene assays in respect of (i) the promoter region of the construct, (ii) the reporter enzyme used, (iii) and the assay procedure. Furthermore, an unique fluorescence-based clone selection step was introduced, allowing rapid selection of the most sensitive reporter cell clones when establishing stable reporter cell lines. Mathematical formulae are provided to enable a simple and reliable comparison between different cell lines, when tested with a compound of interest. The resulting reporter cell lines responded in a very sensitive way to the stimulation of various test receptors. The reporter system was termed HighTRACE (high-throughput reporter assay with clone election). Its high assay quality makes it suitable as a primary screening tool. Ligands for two recently unknown 7TM receptors were identified using the HighTRACE system i.e., two cell surface free fatty acid receptors, GPR40 (FFA1R) and GPR43 (FFA2R). The identification was accomplished using a reverse pharmacology approach.

Novel peroxisome proliferator-activated receptor ligands for Type 2 diabetes and the metabolic syndrome

As the incidence of Type 2 diabetes has reached near epidemic proportions, the quest for novel therapies to combat this disorder has intensified dramatically. In recent years, the peroxisome proliferator-activated receptor (PPAR) family has received tremendous attention as perhaps an ideal target class to address the multiple metabolic anomalies associated with the diabetic state. This review focuses on a variety of novel PPAR approaches currently being investigated for Type 2 diabetes or the metabolic syndrome, including the highly potent selective PPAR agonists, PPAR combination agonists and alternative PPAR ligands.

A novel peroxisome proliferator-activated gamma (PPAR gamma) agonist, CLX-0921, has potent antihyperglycemic activity with low adipogenic potential

Agonists of the nuclear receptor peroxisome proliferator-activated receptor gamma (PPAR gamma) are pharmacologically active antihyperglycemic agents that act by increasing peripheral tissue sensitivity to insulin. Many of these agonists have antihyperglycemic activity that is directly proportional to their ability to bind and activate PPAR gamma; however, recent data bring this relationship into question. In this report we describe a new PPAR gamma agonist, CLX-0921, that is derived from a natural product. This thiazolidinedione (TZD) has a spectrum of activity that differs from commercially available TZDs. It is a weak activator of PPAR gamma (EC(50) of 0.284 micromol/L) compared to rosiglitazone (EC(50) 0.009 micromol/L). Despite this difference, the drug maintains potent glucose uptake activity in vitro and glucose-lowering activity in vivo that is equipotent to that of rosiglitazone. Moreover, CLX-0921 showed a 10-fold reduction in in vitro adipogenic potential compared to rosiglitazone. CLX-0921 also increases glycogen synthesis, an activity not typically associated with rosiglitazone or pioglitazone. Thus CLX-0921 appears to have a distinct spectrum of activity relative to other TZDs.

PAT5A: a partial agonist of peroxisome proliferator-activated receptor gamma is a potent antidiabetic thiazolidinedione yet weakly adipogenic

PAT5A [5-[4-[N-(2-pyridyl)-(2S)-pyrrolidine-2-methoxyl]phenylmethylene[thiazolidine-2,4-dione, malic acid salt]], a chemically distinct unsaturated thiazolidinedione, activates peroxisome proliferator-activated receptor gamma (PPARgamma) submaximally in vitro with the binding affinity approximately 10 times less than that of rosiglitazone, a highly potent thiazolidinedione. PAT5A reduces plasma glucose level and improves insulin sensitivity in insulin resistant db/db mice, similar to that of rosiglitazone, while exerting a relatively weak adipogenic effect. In contrast to rosiglitazone, PAT5A inhibits cholesterol and fatty acid biosynthesis suggesting that PAT5A possesses a unique receptor-independent non-PPAR related property. PAT5A induces qualitatively similar but quantitatively different protease digestion patterns and interacts with PPARgamma differently than rosiglitazone. PAT5A shows differential cofactor recruitment and gene activation than that of rosiglitazone. Thus, the partial agonism of PAT5A to PPARgamma together with its receptor independent effects may contribute to its antidiabetic potency similar to rosiglitazone in vivo despite reduced affinity for PPARgamma. These biological effects suggest that PAT5A is a PPARgamma modulator that activates some (insulin sensitization), but not all (adipogenesis), PPARgamma-signaling pathways.

Thiazolidinediones -- some recent developments

The role of thiazolidinediones (currently rosiglitazone and pioglitazone) in the treatment of Type 2 diabetes is firmly established. The mechanism of action involves binding to the peroxisome proliferator-activated receptor-gamma, a transcription factor that regulates the expression of specific genes especially in fat cells but also other cell types such as endothelial cells, macrophages and monocytes, vascular smooth muscle cells and colonic epithelium. Thiazolidinediones have been shown to interfere with expression and release of mediators of insulin resistance originating in adipose tissue (e.g., increased free fatty acids, decreased adiponectin) in a way that results in net improvement of insulin sensitivity (i.e., in muscle and liver). A direct or indirect effect on AMP-dependent protein kinase may also be involved. Prevention of lipid accumulation in tissues critical to glycaemia such as visceral adipocytes, liver, muscle and beta-cells at the expense of lipids accumulating at the less harmful subcutaneous site may be central to their net metabolic effect. The sustained beneficial effect of troglitazone on beta-cell function in women with previous gestational diabetes in addition to the insulin-sensitising properties point to an important role of this class of drugs in the prevention of Type 2 diabetes. Original safety concerns based on animal and in vitro studies (e.g., fatty bone marrow transformation, colonic cancer, adipogenic transdifferentiation of blood cells) remain theoretical issues but become less pressing practically with prolonged uneventful clinical use. Hepatotoxicity for troglitazone and fluid retention, which can aggravate pre-existing heart failure, are the most important side effects. In summary, with the thiazolidinediones, a novel concept for the treatment of insulin resistance and possibly preservation of beta-cell function is available that could become effective in the prevention of Type 2 diabetes. Moreover, their anti-inflammatory properties also make them interesting in the prevention and treatment of atherosclerosis and possibly other inflammatory conditions (e.g., inflammatory bowel disease). Long-term data will be necessary for a final risk-benefit assessment of these substances.

Nuclear receptors and the control of metabolism

The metabolic nuclear receptors act as metabolic and toxicological sensors, enabling the organism to quickly adapt to environmental changes by inducing the appropriate metabolic genes and pathways. Ligands for these metabolic receptors are compounds from dietary origin, intermediates in metabolic pathways, drugs, or other environmental factors that, unlike classical nuclear receptor ligands, are present in high concentrations. Metabolic receptors are master regulators integrating the homeostatic control of (a) energy and glucose metabolism through peroxisome proliferator-activated receptor gamma (PPARgamma); (b) fatty acid, triglyceride, and lipoprotein metabolism via PPARalpha, beta/delta, and gamma; (c) reverse cholesterol transport and cholesterol absorption through the liver X receptors (LXRs) and liver receptor homolog-1 (LRH-1); (d) bile acid metabolism through the farnesol X receptor (FXR), LXRs, LRH-1; and (e) the defense against xeno- and endobiotics by the pregnane X receptor/steroid and xenobiotic receptor (PXR/SXR). The transcriptional control of these metabolic circuits requires coordination between these metabolic receptors and other transcription factors and coregulators. Altered signaling by this subset of receptors, either through chronic ligand excess or genetic factors, may cause an imbalance in these homeostatic circuits and contribute to the pathogenesis of common metabolic diseases such as obesity, insulin resistance and type 2 diabetes, hyperlipidemia and atherosclerosis, and gallbladder disease. Further studies should exploit the fact that many of these nuclear receptors are designed to respond to small molecules and turn them into therapeutic targets for the treatment of these disorders.

Differential effects of YM440 a hypoglycemic agent on binding to a peroxisome proliferator-activated receptor gamma and its transactivation

Peroxisome proliferator-activated receptor (PPAR) gamma is a ligand-inducible transcription factor mediating glucose and lipid metabolism. Prior studies showed that YM440 ameliorated hyperglycemia in diabetic mice without affecting body fat weight or PPARgamma transactivation. In this study we have examined further the effects of YM440 on PPARgamma binding, transactivation and conformational change. YM440, pioglitazone and rosiglitazone displaced [3H]rosiglitazone from PPARgamma with K(i) values of 4.0, 3.1, and 0.20 microM, indicating that YM440 was comparable to pioglitazone and 20-fold less potent than rosiglitazone. Although pioglitazone and rosiglitazone increased both PPARgamma transactivation in cells expressing human full-length PPARgamma2 or GAL4-PPARgamma and mRNA expression of PPARgamma responsive genes in 3T3-L1 cells, YM440 had weak effects on PPARgamma transactivation and mRNA expression being 550- to 790-fold and 36- to 110-fold less active than rosiglitazone, respectively. YM440 and rosiglitazone induced interaction between PPARgamma and the transcriptional cofactor, p300 or SRC-1, but YM440 was 151- and 1091-fold less potent than rosiglitazone, respectively. The weak transcriptional activity of YM440 was not due to poor cell permeability. Limited trypsin digestion of the full-length human PPARgamma2 with YM440 or rosiglitazone showed distinct patterns of digestion, suggesting a difference in the conformational change of PPARgamma. When db/db mice were treated with YM440 (100mg/kg) for 28 days, YM440 increased hepatic glucokinase expression but not adipose tissue FABP and UCP1 expression, indicating a tissue selective expression of PPARgamma-related genes. Unique properties regarding the binding-transactivation of PPARgamma by YM440 may lead to the hypoglycemic activity without affecting body fat weight in diabetic mice.

A human cell surface receptor activated by free fatty acids and thiazolidinedione drugs

Fatty acids, which are essential nutritional components, are also involved in cardiovascular and metabolic diseases. Here we report a human cell surface receptor that we name free fatty acid receptor (FFAR), because it is specifically activated by medium to long-chain free fatty acids. The receptor belongs to the class of seven-transmembrane, G-protein coupled receptors (GPCRs) and also mediates responses to antidiabetic drugs of the thiazolidinedione type. It is expressed in skeletal muscle, heart, liver, and pancreatic beta-cells. Stimulation of FFAR increases the intracellular calcium concentration in cells expressing the receptor in a native (pancreatic beta-cell line) or in a recombinant form. In view of the nature of the activating substances, their physiological role in the body, and the tissue distribution of FFAR we suggest the term "nutrient sensing receptor" for receptors acting at the interface between dietary components and signalling molecules.

Studies on non-thiazolidinedione antidiabetic agents. 2. Novel oxyiminoalkanoic acid derivatives as potent glucose and lipid lowering agents

We previously reported that (Z)-2-(4-[(5-methyl-2-phenyl-1, 3-oxazol-4-yl)methoxy]benzyloxyimino)-2-(4-phenoxyphenyl)acetic acid (3) showed potent glucose and lipid lowering effects in genetically obese and diabetic mice, KKA(y). This compound also showed transcriptional activity for peroxisome proliferator-activated receptor (PPAR)-gamma. We expanded on the structure-activity relationships of oxyiminoalkanoic acid derivatives based on this transcriptional activity (in vitro). Insertion of a carbon chain between the imino carbon and the carboxyl moiety of (Z)-2-(4-[(5-methyl-2-phenyl-1, 3-oxazol-4-yl)methoxy]benzyloxyimino)-2-phenylacetic acid (2) resulted in a marked increase in transcriptional activity at PPARgamma. In vivo potencies of synthesized compounds, which showed strong functional activity at PPARgamma, were tested using KKA(y) mice. Among these compounds, (E)-4-(4-[(5-methyl-2-phenyl-1, 3-oxazol-4-yl)methoxy]benzyloxyimino)-4-phenylbutyric acid (27) exhibited marked glucose and lipid lowering activity while showing no significant body weight gain. Compound (27) (TAK-559) showed favorable pharmacokinetic properties with good absorption and duration, and was considered as an attractive candidate for further evaluation.

Stimulation of proximal tubular cell apoptosis by albumin-bound fatty acids mediated by peroxisome proliferator activated receptor-gamma

In nephrotic syndrome, large quantities of albumin enter the kidney tubule. This albumin carries with it a heavy load of fatty acids to which the proximal tubule cells are exposed at high concentration. It is postulated that exposure to fatty acids in this way is injurious to proximal tubule cells. This study has examined the ability of fatty acids to interact with peroxisome proliferator-activated receptors (PPAR) in primary cultures of human proximal tubule cells. Luciferase reporter assays in transiently transfected human proximal tubule cells were used to show that albumin bound fatty acids and other agonists activate PPARgamma in a dose-dependent manner. One of the consequences of this activation is apoptosis of the cells as determined by changes in cell morphology, evidence of PARP cleavage, and appearance of DNA laddering. Overexpression of PPARgamma in these cells also results in enhanced apoptosis. Both fatty acid-induced PPAR activation and apoptosis in these cells can be blocked by PPAR response element decoy oligonucleotides. Activation of PPARgamma by the specific agonist PGJ(2) is associated with inhibition of cell proliferation, whereas activation by albumin bound fatty acids is accompanied by increased proliferation. However, the net balance of apoptosis/proliferation favors deletion of cells. These results implicate albumin-bound fatty acids as important mediators of tubular injury in nephrosis and provide fresh impetus for pursuit of lipid-lowering strategies in proteinuric renal disease.

SRC-1 and TIF2 control energy balance between white and brown adipose tissues

We have explored the effects of two members of the p160 coregulator family on energy homeostasis. TIF2-/- mice are protected against obesity and display enhanced adaptive thermogenesis, whereas SRC-1-/- mice are prone to obesity due to reduced energy expenditure. In white adipose tissue, lack of TIF2 decreases PPARgamma activity and reduces fat accumulation, whereas in brown adipose tissue it facilitates the interaction between SRC-1 and PGC-1alpha, which induces PGC-1alpha's thermogenic activity. Interestingly, a high-fat diet increases the TIF2/SRC-1 expression ratio, which may contribute to weight gain. These results reveal that the relative level of TIF2/SRC-1 can modulate energy metabolism.

Studies on non-thiazolidinedione antidiabetic agents. 1. Discovery of novel oxyiminoacetic acid derivatives

A novel series of oxyiminoacetic acid derivatives were synthesized in an effort to develop a potent antidiabetic agent, which does not contain the 2,4-thiazolidinedione moiety. These compounds were evaluated for glucose and lipid lowering effects in genetically obese and diabetic KKA(y) mice. Several of the compounds showed strong antidiabetic activity, including functional potency at peroxisome proliferator-activated receptor (PPAR)-gamma. (Z)-2-[4-[(5-Methyl-2-phenyl-1,3-oxazol-4-yl)methoxy]benzyloxyimino]-2-(4-phenoxyphenyl)acetic acid (25) significantly reduced plasma glucose (33%, p<0.01) and plasma triglycelide levels (43%, p<0.01) even at a dosage of 0.001% in diet. Pharmacokinetic analyses of 25 are also reported.

PMT13, a pyrimidone analogue of thiazolidinedione improves insulin resistance-associated disorders in animal models of type 2 diabetes

AIM

To evaluate the antidiabetic and hypolipidaemic potential of a novel thiazolidinedione, PMT13, in different animal models of insulin resistance.

METHODS

PPAR transactivation study was performed in HEK293T cells using ligand binding domains of PPARalpha, gamma and delta. Insulin-resistant db/db and ob/ob mice were treated orally with different doses of PMT13 at 0.3-10 mg/kg/day for 15 and 14 days respectively. Zucker fa/fa rats were treated with 3 mg/kg (p.o.) dose of the compound. Plasma glucose, triglyceride, free fatty acid and insulin levels were measured. Liver glucose 6-phosphatase (G6-Ptase) and adipose lipoprotein lipase activity was measured in treated mice. Isolated rat aortic preparations preconstricted with phenylephrine were used to study the vascular relaxation potential of PMT13 in presence of insulin. A 28-day oral toxicity study was performed in Wistar rats.

RESULTS

PMT13 showed similar PPARgamma activation as rosiglitazone, but failed to show any activity against PPARalpha or PPARdelta. In obese and diabetic db/db and ob/ob mice, PMT13 showed better reduction in plasma glucose, triglyceride and insulin levels than rosiglitazone and an improvement in glucose tolerance. In insulin-resistant Zucker fa/fa rat model, PMT13 treatment showed better reduction in plasma triglyceride, free fatty acid and insulin levels than that of rosiglitazone. Treated mice showed decreased G6-Ptase activity in liver. The LPL activity was increased in post-heparin plasma and epididymal fat of treated db/db mice. In an isolated, precontracted rat aortic preparation, PMT13 treatment significantly increased insulin-induced relaxation. A 28-day oral toxicity study in rats showed no treatment-related adverse effects.

CONCLUSION

Our studies indicate that PMT13 is a potent activator of PPARgamma with antidiabetic, hypolipidaemic and insulin-sensitizing properties. Additionally, PMT13 inhibited liver G6-Ptase activity and increased lipoprotein lipase activity. It showed improvement in insulin-induced vasorelaxation. The compound also showed a good safety margin. Therefore, PMT13 can be a potential drug candidate for future development.

The peroxisome proliferator-activated receptor-gamma2 Pro12Ala polymorphism

Peroxisome proliferator-activated receptor (PPAR)-gamma is a transcription factor with a key role in adipocyte differentiation. The Ala allele of the common Pro12Ala polymorphism in the isoform PPAR-gamma2 is associated with reduced risk for type 2 diabetes. The effect on the individual is weak, but because of a prevalence of >75% of the high-risk Pro allele, the population-attributable risk is enormous. The in vivo effects of the polymorphism are secondary to alterations in adipose tissue, where PPAR-gamma2 is predominantly expressed. Moderate reduction in transcriptional activity of PPAR-gamma as a result of the polymorphism modulates production and release of adipose-derived factors. Both decreased release of insulin-desensitizing free fatty acids, tumor necrosis factor-alpha, and resistin and increased release of the insulin-sensitizing hormone adiponectin result in secondary improvement of insulin sensitivity of glucose uptake and suppression of glucose production. The population effect of this polymorphism may be modulated by environmental or genetic factors such as obesity, ethnicity, ratio of unsaturated to saturated fatty acids, and genetic background. Once diabetes has developed, the protective effect of the Ala allele may be lost, since increased vascular complications and more pronounced beta-cell dysfunction have been reported. These observations, however, are currently unexplained. In conclusion, the Pro12Ala polymorphism in PPAR-gamma2 represents the first genetic variant with a broad impact on the risk of common type 2 diabetes. The precise understanding of its mechanism may lead to novel diagnostic, preventive, and therapeutic approaches for improving the management of type 2 diabetes.

Biology and toxicology of PPARgamma ligands

The peroxisome proliferator activated receptor-gamma (PPARgamma) is an attractive target for therapeutic intervention, as modulation of PPARgamma-regulated pathways is potentially beneficial in a number of disease areas. This review provides an overview of what is known about the biology of PPARgamma, and an indication of what progress has been made towards drug development in several therapy areas. As well as efficacy, the safety of drugs is of course an important issue, and a substantial volume of preclinical and clinical information has already accumulated for PPARgamma agonists. Here we discuss some of the major toxicology issues with PPARgamma agonists, and give a perspective on likely issues concerning the development of PPARgamma modulators in the future.

Overexpression and ribozyme-mediated targeting of transcriptional coactivators CREB-binding protein and p300 revealed their indispensable roles in adipocyte differentiation through the regulation of peroxisome proliferator-activated receptor gamma

The cAMP-response element-binding protein-binding protein (CBP) and p300 are common coactivators for several transcriptional factors. It has been reported that both CBP and p300 are significant for the activation of peroxisome proliferator-activated receptor gamma (PPARgamma), which is a crucial nuclear receptor in adipogenesis. However, it remains unclear whether CBP and/or p300 is physiologically essential to the activation of PPARgamma in adipocytes and adipocyte differentiation. In this study, we investigated the physiological significance of CBP/p300 in NIH3T3 cells transiently expressing PPARgamma and CBP and in 3T3-L1 preadipocytes stably expressing CBP- or p300-specific ribozymes. In PPARgamma-transfected NIH3T3 cells, induction of expression of PPARgamma target genes such as adipocyte fatty acid-binding protein (aP2) and lipoprotein lipase (LPL) by adding thiazolidinedione was enhanced, depending on the amount of a CBP expression plasmid transfected. Expression of aP2 and LPL genes, as well as glycerol-3-phosphate dehydrogenase activity and triacylglyceride accumulation after adipogenic induction, was largely suppressed in 3T3-L1 adipocytes expressing either the CBP- or p300-specific active ribozyme, but not in inactive ribozyme-expressing cells. These data suggest that both CBP and p300 are indispensable for the full activation of PPARgamma and adipocyte differentiation and that CBP and p300 do not mutually complement in the process.

Differential gene regulation by PPARgamma agonist and constitutively active PPARgamma2

The PPARgamma is a key adipogenic determination factor. Ligands for PPARgamma such as antidiabetic thiazolidinedione (TZD) compounds are adipogenic, and many adipocyte genes that are activated by TZDs contain binding sites for PPARgamma. Like ligands for other nuclear receptors, TZDs can regulate genes positively or negatively. Here, we sought to understand the importance of positive regulation of gene expression by PPARgamma in adipogenesis. Fusion of the potent viral transcriptional activator VP16 to PPARgamma2 (VP16-PPARgamma) created a transcription factor that constitutively and dramatically activated transcription of PPARgamma-responsive genes in the absence of ligand. Forced expression of VP16-PPARgamma in 3T3-L1 preadipocytes using retroviral vectors led to adipogenesis in the absence of standard differentiating medium or any exogenous PPARgamma ligand. Gene microarray analysis revealed that VP16-PPARgamma induced many of the genes associated with adipogenesis and adipocyte function. Thus, direct up-regulation of gene expression by PPARgamma is sufficient for adipogenesis. TZD-induced adipogenesis up-regulated many of the same genes, although some were divergently regulated, including resistin, whose gene expression was reduced inVP16-PPARgamma adipocytes treated with TZDs. These results show that, although activation of PPARgamma by a heterologous activation domain is sufficient for adipogenesis, it is not equivalent to TZD treatment. This conclusion has important implications for understanding biological effects of the TZDs on adipogenesis and insulin sensitization.

Nuclear receptor corepressor-dependent repression of peroxisome-proliferator-activated receptor delta-mediated transactivation

The nuclear receptor corepressor (NCoR) was isolated as a peroxisome-proliferator-activated receptor (PPAR) delta interacting protein using the yeast two-hybrid system. NCoR interacted strongly with the ligand-binding domain of PPAR delta, whereas interactions with the ligand-binding domains of PPAR gamma and PPAR alpha were significantly weaker. PPAR-NCoR interactions were antagonized by ligands in the two-hybrid system, but were ligand-insensitive in in vitro pull-down assays. Interaction between PPAR delta and NCoR was unaffected by coexpression of retinoid X receptor (RXR) alpha. The PPAR delta-RXR alpha heterodimer bound to an acyl-CoA oxidase (ACO)-type peroxisome-proliferator response element recruited a glutathione S-transferase-NCoR fusion protein in a ligand-independent manner. Contrasting with most other nuclear receptors, PPAR delta was found to interact equally well with interaction domains I and II of NCoR. In transient transfection experiments, NCoR and the related silencing mediator for retinoid and thyroid hormone receptor (SMRT) were shown to exert a marked dose-dependent repression of ligand-induced PPAR delta-mediated transactivation; in addition, transactivation induced by the cAMP-elevating agent forskolin was efficiently reduced to basal levels by NCoR as well as SMRT coexpression. Our results suggest that the transactivation potential of liganded PPAR delta can be fine-tuned by interaction with NCoR and SMRT in a manner determined by the expression levels of corepressors and coactivators.

Peroxisome proliferator-activated receptor gamma and metabolic disease

The nuclear peroxisome proliferator-activated receptor gamma (PPAR gamma) is a transcription factor that is activated by polyunsaturated fatty acids and their metabolites and is essential for fat cell formation. Although obesity is a strong risk factor for type 2 diabetes mellitus and other metabolic diseases, potent PPAR gamma activators such as the glitazone drugs lower glucose and lipid levels in patients with type 2 diabetes and also have antiatherosclerotic and antihypertensive effects. We review recent studies providing insight into the paradoxical relationship between PPAR gamma and metabolic disease. We also review recent advances in understanding the structural basis for PPAR gamma activation by ligands. The unusual ligand-binding properties of PPAR gamma suggest that it will be possible to discover new chemical classes of receptor "modulators" with distinct pharmacological activities for the treatment of type 2 diabetes and other metabolic diseases.

The dawn of the SPPARMs?

Thiazolidinediones (TZDs) are used as antidiabetic agents in the treatment of type II diabetes. These compounds are ligands for the nuclear hormone receptor PPARgamma, which is highly expressed in adipose tissue. PPARgamma acts as a molecular switch in the process of fat cell development. The quest for the ideal antidiabetic agent is challenged by the need to develop PPARgamma ligands that improve insulin sensitivity, but do not promote fat cell formation. A newly described PPARgamma ligand may represent an initial step in this direction and could lead to improved agents for treating insulin resistance in type II diabetes.

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.

A novel potent antagonist of peroxisome proliferator-activated receptor gamma blocks adipocyte differentiation but does not revert the phenotype of terminally differentiated adipocytes

The antidiabetic thiazolidinediones, which include troglitazone and rosiglitazone, are ligands for the nuclear receptor peroxisome proliferator-activated receptor gamma (PPARgamma). Their antihyperglycemic effects seem to be linked to the regulation of PPARgamma-responsive genes. Here, we report the characterization of a specific PPARgamma antagonist that blocks several of the biological activities of the PPARgamma agonist rosiglitazone. PD068235 inhibited rosiglitazone-dependent PPARgamma transcriptional activity with an IC(50) of 0.8 microM and rosiglitazone-stimulated in vitro coactivator association. The role of PPARgamma in the initiation of differentiation is well documented. In this study, we used PD068235 as a tool to evaluate the functional role of PPARgamma in the maintenance of the terminally differentiated state. Treatment of confluent, growth-arrested 3T3-L1 preadipocytes with PD068235 blocked adipocyte differentiation induced by the standard adipogenic hormonal mixture (insulin/dexamethasone/isobutylmethylxanthin) and fully antagonized rosiglitazone-induced adipogenesis. In contrast, long-term treatment of terminally differentiated 3T3-L1 adipocytes with PD068235 did not induce any obvious morphological changes and had no effect on basal lipolysis rates. In addition, in fully differentiated adipocytes PD068235 did not alter the basal expression of PPARgamma target genes aP2 and CAP, but it effectively blocked rosiglitazone-induced expression of both genes. These results suggest that in terminally differentiated adipocytes, the PPARgamma activity is minimal and may not be required for the maintenance of PPARgamma target gene expression.

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.

Adipose tissue resistin expression is severely suppressed in obesity and stimulated by peroxisome proliferator-activated receptor gamma agonists

Elevated levels of the hormone resistin, which is secreted by fat cells, are proposed to cause insulin resistance and to serve as a link between obesity and type 2 diabetes. In this report we show that resistin expression is significantly decreased in the white adipose tissue of several different models of obesity including the ob/ob, db/db, tub/tub, and KKA(y) mice compared with their lean counterparts. Furthermore, in response to several different classes of antidiabetic peroxisome proliferator-activated receptor gamma agonists, adipose tissue resistin expression is increased in both ob/ob mice and Zucker diabetic fatty rats. These data demonstrate that experimental obesity in rodents is associated with severely defective resistin expression, and decreases in resistin expression are not required for the antidiabetic actions of peroxisome proliferator-activated receptor gamma agonists.

Oxidized alkyl phospholipids are specific, high affinity peroxisome proliferator-activated receptor gamma ligands and agonists

Synthetic high affinity peroxisome proliferator-activated receptor (PPAR) agonists are known, but biologic ligands are of low affinity. Oxidized low density lipoprotein (oxLDL) is inflammatory and signals through PPARs. We showed, by phospholipase A(1) digestion, that PPARgamma agonists in oxLDL arise from the small pool of alkyl phosphatidylcholines in LDL. We identified an abundant oxidatively fragmented alkyl phospholipid in oxLDL, hexadecyl azelaoyl phosphatidylcholine (azPC), as a high affinity ligand and agonist for PPARgamma. [(3)H]azPC bound recombinant PPARgamma with an affinity (K(d)((app)) approximately 40 nm) that was equivalent to rosiglitazone (BRL49653), and competition with rosiglitazone showed that binding occurred in the ligand-binding pocket. azPC induced PPRE reporter gene expression, as did rosiglitazone, with a half-maximal effect at 100 nm. Overexpression of PPARalpha or PPARgamma revealed that azPC was a specific PPARgamma agonist. The scavenger receptor CD36 is encoded by a PPRE-responsive gene, and azPC enhanced expression of CD36 in primary human monocytes. We found that anti-CD36 inhibited azPC uptake, and it inhibited PPRE reporter induction. Results with a small molecule phospholipid flippase mimetic suggest azPC acts intracellularly and that cellular azPC accumulation was efficient. Thus, certain alkyl phospholipid oxidation products in oxLDL are specific, high affinity extracellular ligands and agonists for PPARgamma that induce PPAR-responsive genes.

Troglitazone induces GLUT4 translocation in L6 myotubes

A number of studies have demonstrated that insulin resistance in the skeletal muscle plays a pivotal role in the insulin resistance associated with obesity and type 2 diabetes. A decrease in GLUT4 translocation from the intracellular pool to the plasma membranes in skeletal muscles has been implicated as a possible cause of insulin resistance. Herein, we examined the effects of an insulin-sensitizing drug, troglitazone (TGZ), on glucose uptake and the translocation of GLUT4 in L6 myotubes. The prolonged exposure (24 h) of L6 myotubes to TGZ (10(-5) mol/l) caused a substantial increase in the 2-deoxy-[3H]D-glucose (2-DG) uptake without changing the total amount of the glucose transporters GLUT4, GLUT1, and GLUT3. The TGZ-induced 2-DG uptake was completely abolished by cytochalasin-B (10 micromol/l). The ability of TGZ to translocate GLUT4 from light microsomes to the crude plasma membranes was greater than that of insulin. Both cycloheximide treatment (3.5 x 10(-6) mol/l) and the removal of TGZ by washing reversed the 2-DG uptake to the basal level. Moreover, insulin did not enhance the TGZ-induced 2-DG uptake additively. The TGZ-induced 2-DG uptake was only partially reversed by wortmannin to 80%, and TGZ did not change the expression and the phosphorylation of protein kinase B; the expression of protein kinase C (PKC)-lambda, PKC-beta2, and PKC-zeta; or 5'AMP-activated protein kinase activity. a-Tocopherol, which has a molecular structure similar to that of TGZ, did not increase 2-DG uptake. We conclude that the glucose transport in L6 myotubes exposed to TGZ for 24 h is the result of an increased translocation of GLUT4. The present results imply that the effects of troglitazone on GLUT4 translocation may include a new mechanism for improving glucose transport in skeletal muscle.

Peroxisome proliferator-activated receptor gamma (PPARgamma) activation and its consequences in humans

Peroxisome proliferator-activated receptor gamma (PPARgamma) is a member of the superfamily of nuclear receptors. It binds and is activated by natural polyunsaturated fatty acids, eicosanoids, synthetic thiazolidinediones and related analogues. Biological effects exerted by PPARgamma ligands are mostly concerned with differentiation processes, sensitization to insulin and atherogenesis, and are paradigmatically ascribed to PPARgamma transactivation of PPARgamma-responsive genes. The PPARgamma paradigm and its consequences in humans are analyzed here in terms of the tissue specificity of PPARgamma, loss and gain of function mutants of PPARgamma, PPARgamma-responsive genes and clinical effects of PPARgamma ligands. Differentiation, as well as some of the atherogenic effects induced by PPARgamma ligands, does conform to the PPARgamma paradigm. However, sensitization to insulin as well as some of the antiatherogenic effects of PPARgamma ligands is not accounted for by PPARgamma activation, thus calling for an alternative target for insulin sensitizers.

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.

Solution- and solid-phase synthesis of peptide-substituted thiazolidinediones as potential PPAR ligands

Solution- and solid-phase methods for the preparation of peptide-substituted thiazolidinediones have been developed as an approach towards the preparation of a library of these compounds as potential ligands for the peroxisome proliferator-activated receptors (PPARs).

Bisphenol A diglycidyl ether (BADGE) is a PPARgamma agonist in an ECV304 cell line

Peroxisome proliferator activated receptors (PPAR)s are nuclear transcription factors of the steroid receptor super-family. One member, PPARgamma, a critical transcription factor in adipogenesis, is expressed in ECV304 cells, and when activated participates in the induction of cell death by apoptosis. Here we describe a clone of ECV304 cells, ECV-ACO.Luc, which stably expresses a reporter gene for PPAR activation. ECV-ACO.Luc respond to the PPARgamma agonists, 15-deoxy-Delta(12,14) PGJ(2), and ciglitizone, by inducing luciferase expression. Furthermore, using ECV-ACO.Luc, we demonstrate that a newly described PPARgamma antagonist, bisphenol A diglycidyl ether (BADGE) has agonist activities. Similar to 15-deoxy-Delta(12,14) PGJ(2), BADGE induces PPARgamma activation, nuclear localization of the receptor, and induces cell death.

New approaches in the treatment of type 2 diabetes

Type 2 diabetes is a chronic metabolic derangement that results from defects in both insulin action and secretion. New thiazolidinedione insulin sensitizers have been recently launched. New approaches with mechanisms different from current therapies are being explored, including novel ligands of peroxisome proliferator-activated receptor, glucagon receptor antagonists, dipeptidyl peptidase IV inhibitors, and insulin receptor activators.

PPARgamma: observations in the hematopoietic system

Human Peroxisome Proliferator-Activated Receptor gamma (PPARgamma) was originally cloned from a human bone marrow library. What role does this ligand activated transcription factor play in hematopoiesis and the immune system? We note that: a) PPARgamma has potential to interact/interfere or synergize with retinoid biology, b) fatty acids and a prostaglandin have been identified as ligands, and c) lymphocytes, monocytes and neutrophils use fatty acids as a major source of energy production, d) PPARgamma has been shown to oppose TNFalpha and down regulate cytokine production in monocytes. Therefore, we undertook a review of the literature and an expression survey of PPARgamma in a number of major organs and cells involved in the hematopoietic system, for the purpose of building a database towards understanding the role and function of PPARgamma gene regulation in the developing blood and immune systems. PPARgamma is expressed before mesodermal induction in tissue in and around Speymann's organizer in the xenopus blastocyst, in erythroid precursors of blood islands and in the circulation of the day 10.0 murine embryo, in human 19 week fetal liver, in some but not all murine and human bone marrow erythroid, myeloid, and monocytoid progenitors, bone marrow stromal cells and adipocytes, osteoblasts, endothelial cells, some T, and B lymphocytes, monocytes, macrophages, and other monocytic derivatives. It can be found in the cells of Peyer's patches, lymphoid follicles, spleen, and thymus. It is not clear if it is ever or transiently expressed in megakaryocytes, mast cells, or neutrophils. Based on the above data and a review of the literature, PPARgamma seems to play a role during the elicitation of immune responses. We propose PPARgamma may be involved in changes in energy states required during activation and development of many cell types involved, and has additional immunologically relevant effects in erythroid, myeloid, monocytic, T and B lymphocytic, stromal, and endothelial cell function.

Peroxisome proliferator-activated receptors in the cardiovascular system

Peroxisome proliferator-activated receptor (PPAR)s are a family of three nuclear hormone receptors, PPARalpha, -delta, and -gamma, which are members of the steriod receptor superfamily. The first member of the family (PPARalpha) was originally discovered as the mediator by which a number of xenobiotic drugs cause peroxisome proliferation in the liver. Defined functions for all these receptors, until recently, mainly concerned their ability to regulate energy balance, with PPARalpha being involved in beta-oxidation pathways, and PPARgamma in the differentiation of adipocytes. Little is known about the functions of PPARdelta, though it is the most ubiquitously expressed. Since their discovery, PPARs have been shown to be expressed in monocytes/macrophages, the heart, vascular smooth muscle cells, endothelial cells, and in atherosclerotic lesions. Furthermore, PPARs can be activated by a vast number of compounds including synthetic drugs, of the clofibrate, and anti-diabetic thiazoldinedione classes, polyunsaturated fatty acids, and a number of eicosanoids, including prostaglandins, lipoxygenase products, and oxidized low density lipoprotein. This review will aim to introduce the field of PPAR nuclear hormone receptors, and discuss the discovery and actions of PPARs in the cardiovascular system, as well as the source of potential ligands.

The novel hypoglycemic agent YM440 normalizes hyperglycemia without changing body fat weight in diabetic db/db mice

To determine the relationship between hypoglycemic activity and body weight gain induced by insulin sensitizers, we compared the effects of thiazolidinedione analogs (troglitazone and pioglitazone) and the oxadiazolidinedione analog (Z)-1,4-bis4[(3,5-dioxo-1,2,4-oxadiazolidin-2-yl)methyl]phen oxy¿but-2-ene (YM440) in diabetic db/db mice. Oral treatment with YM440(100 mg/kg) for 28 days decreased the blood glucose concentration (control v YM440, 418 +/- 12 v243 +/- 44 mg/dL). The hypoglycemic activity of this agent was comparable to that of troglitazone (300 mg/kg) and pioglitazone (100 mg/kg). There were no changes in food intake among the groups. Troglitazone and pioglitazone, but not YM440, significantly increased body weight gain during treatment (control, 7.2 +/- 0.5 g; YM440, 7.5 +/- 0.8 g; troglitazone, 10.9 +/- 0.8 g; and pioglitazone, 14.5 +/- 1.1 g). To further assess whether the increase in body weight by troglitazone or pioglitazone was due to adipogenesis, the weight of intraabdominal fat tissue (epididymal, retroperitoneal, and perirenal) was determined. There were no differences in the total weight of visceral fat between the control and YM440 treatment (3.53 +/- 0.23 and 3.60 +/- 0.16 g). In contrast, troglitazone and pioglitazone significantly increased the fat weight (4.31 +/- 0.13 and 4.66 +/- 0.19 g). Thiazolidinediones are known as ligands for peroxisome proliferator-activated receptor gamma (PPARgamma), a nuclear receptor responsible for adipogenesis. Troglitazone and pioglitazone activated PPARgamma and increased triglyceride accumulation and mRNA expression of fatty acid-binding protein (FABP) in 3T3-L1 cells. However, YM440 had no effect on these indices for adipocyte differentiation. These results suggest that the mechanism is different for the hypoglycemic action of YM440 versus the thiazolidinediones. YM440 ameliorates hyperglycemia without changing PPARgamma activity, adipocyte differentiation, or fat weight. Thus, YM440 could be a useful hypoglycemic agent for the treatment of non-insulin-dependent diabetes mellitus (NIDDM) without affecting body weight.

Improved insulin-sensitivity in mice heterozygous for PPAR-gamma deficiency

The thiazolidinedione class of insulin-sensitizing, antidiabetic drugs interacts with peroxisome proliferator-activated receptor gamma (PPAR-gamma). To gain insight into the role of this nuclear receptor in insulin resistance and diabetes, we conducted metabolic studies in the PPAR-gamma gene knockout mouse model. Because homozygous PPAR-gamma-null mice die in development, we studied glucose metabolism in mice heterozygous for the mutation (PPAR-gamma(+/-) mice). We identified no statistically significant differences in body weight, basal glucose, insulin, or FFA levels between the wild-type (WT) and PPAR-gamma(+/-) groups. Nor was there a difference in glucose excursion between the groups of mice during oral glucose tolerance test, but insulin concentrations of the WT group were greater than those of the PPAR-gamma(+/-) group, and insulin-induced increase in glucose disposal rate was significantly increased in PPAR-gamma(+/-) mice. Likewise, the insulin-induced suppression of hepatic glucose production was significantly greater in the PPAR-gamma(+/-) mice than in the WT mice. Taken together, these results indicate that - counterintuitively - although pharmacological activation of PPAR-gamma improves insulin sensitivity, a similar effect is obtained by genetically reducing the expression levels of the receptor.

The nuclear receptor PPAR gamma and immunoregulation: PPAR gamma mediates inhibition of helper T cell responses

The peroxisome proliferator-activated receptors (PPARs) are a family of transcription factors belonging to the nuclear receptor superfamily. Until recently, the genes regulated by PPARs were those believed to be predominantly associated with lipid metabolism. Recently, an immunomodulatory role for PPAR gamma has been described in cells critical to the innate immune system, the monocyte/macrophage. In addition, evidence for an antiinflammatory role of the PPAR gamma ligand, 15-deoxy-Delta 12,14-PGJ2 (15d-PGJ2) has been found. In the present studies, we demonstrate, for the first time, that murine helper T cell clones and freshly isolated splenocytes express PPAR gamma 1. The PPAR gamma expressed is of functional significance in that two ligands for PPAR gamma, 15d-PGJ2 and a thiazolidinedione, ciglitazone, mediate significant inhibition of proliferative responses of both the T cell clones and the freshly isolated splenocytes. This inhibition is mediated directly at the level of the T cell and not at the level of the macrophage/APC. Finally, we demonstrate that the two ligands for PPAR gamma mediate inhibition of IL-2 secretion by the T cell clones while not inhibiting IL-2-induced proliferation of such clones. The demonstration of the expression and function of PPAR gamma in T cells reveals a new level of immunoregulatory control for PPARs and significantly increases the role and importance of PPAR gamma in immunoregulation.

A novel role for helix 12 of retinoid X receptor in regulating repression

Nutrients, drugs, and hormones influence transcription during differentiation and metabolism by binding to high-affinity nuclear receptors. In the absence of ligand, some but not all nuclear receptors repress transcription as a heterodimer with retinoid X receptor (RXR). Here we define a novel role for helix 12 (H12) in sterically masking the corepressor (CoR) binding site in apo-RXR. Removing H12 converts RXR to a potent transcriptional repressor. The length but not the specific sequence of H12 is critical for masking RXR's intrinsic repression function. This contrasts with the amphipathic character required for mediating ligand-dependent activation and coactivator recruitment. Physiologically, we show that heterodimerization of RXR with apo-thyroid hormone receptor (TR) unmasks the CoR binding site in RXR and allows the TR-RXR heterodimer to repress. A molecular mechanism that involves sequence-specific interaction between RXR H12 and the coactivator-binding surface of the nuclear receptor is proposed for this heterodimerization-mediated unmasking. Peroxisome proliferator-activated receptor gamma does not interact as well with RXR H12, thus explaining its inability to repress transcription as an RXR heterodimer. The requirement to unmask RXR's latent repression function explains why only certain RXR partners repress transcription.