The rexinoid LG100754 is a novel RXR:PPARgamma agonist and decreases glucose levels in vivo.

The RXR serves as a heterodimer partner for the PPARgamma and the dimer is a molecular target for insulin sensitizers such as the thiazolidinediones. Ligands for either receptor can activate PPAR-dependent pathways via PPAR response elements. Unlike PPARgamma agonists, however, RXR agonists like LG100268 are promiscuous and activate multiple RXR heterodimers. Here, we demonstrate that LG100754, a RXR:RXR antagonist and RXR:PPARalpha agonist, also functions as a RXR:PPARgamma agonist. It does not activate other LG100268 responsive heterodimers like RXR:liver X receptoralpha, RXR:liver X receptorbeta, RXR:bile acid receptor/farnesoid X receptor and RXR:nerve growth factor induced gene B. This unique RXR ligand triggers cellular RXR:PPARgamma-dependent pathways including adipocyte differentiation and inhibition of TNFalpha-mediated hypophosphorylation of the insulin receptor, but does not activate key farnesoid X receptor and liver X receptor target genes. Also, LG100754 treatment of db/db animals leads to an improvement in insulin resistance in vivo. Interestingly, activation of RXR:PPARgamma by LG100268 and LG100754 occurs through different mechanisms. Therefore, LG100754 represents a novel class of insulin sensitizers that functions through RXR but exhibits greater heterodimer selectivity compared with LG100268. These results establish an approach to the design of novel RXR-based insulin sensitizers with greater specificity.     

Effects of rexinoids on glucose transport and insulin-mediated signaling in skeletal muscles of diabetic (db/db) mice

Rexinoids and thiazolidinediones (TZDs) are two classes of nuclear receptor ligands that induce insulin sensitization in diabetic rodents. TZDs are peroxisome proliferator-activated receptor gamma (PPARgamma) activators, whereas rexinoids are selective ligands for the retinoid X receptors (RXRs). Activation of both the insulin receptor substrates (IRSs)/Akt and the c-Cbl-associated protein (CAP)/c-Cbl pathways are important in regulating insulin-stimulated glucose transport. We have compared the effects of a rexinoid (LG268) and a TZD (rosiglitazone) on these two signal pathways in skeletal muscle of diabetic (db/db) mice. The results we have obtained show that treatment of db/db mice with either LG268 or rosiglitazone for 2 weeks results in a significant increase in insulin-stimulated glucose transport activity in skeletal muscle. Treatment with LG268 increases insulin-stimulated IRS-1 tyrosine phosphorylation and Akt phosphorylation in skeletal muscle without affecting the activity of the CAP/c-Cbl pathway. In contrast, rosiglitazone increases the levels of CAP expression and insulin-stimulated c-Cbl phosphorylation without affecting the IRS-1/Akt pathway. The effects of LG268 on the IRS-1/Akt pathway were associated with a decrease in the level of IRS-1 Ser(307) phosphorylation. Taken together, these data suggest that rexinoids improve insulin sensitivity via changes in skeletal muscle metabolism that are distinct from those induced by TZDs. Rexinoids represent a novel class of insulin sensitizers with potential applications in the treatment of insulin resistance.     

A selective peroxisome proliferator-activated receptor-gamma (PPARgamma) modulator blocks adipocyte differentiation but stimulates glucose uptake in 3T3-L1 adipocytes

Peroxisome proliferator-activated receptor-gamma (PPARgamma) agonists such as the thiazolidinediones are insulin sensitizers used in the treatment of type 2 diabetes. These compounds induce adipogenesis in cell culture models and increase weight gain in rodents and humans. We have identified a novel PPARgamma ligand, LG100641, that does not activate PPARgamma but selectively and competitively blocks thiazolidinedione-induced PPARgamma activation and adipocyte conversion. It also antagonizes target gene activation as well as repression in agonist-treated 3T3-L1 adipocytes. This novel PPARgamma antagonist does not block adipocyte differentiation induced by a ligand for the retinoid X receptor (RXR), the heterodimeric partner for PPARgamma, or by a differentiation cocktail containing insulin, dexamethasone, and 1-methyl-3-isobutylxanthine. Surprisingly, LG100641, like the PPARgamma agonist rosiglitazone, increases glucose uptake in 3T3-L1 adipocytes. Such selective PPARgamma antagonists may help determine whether insulin sensitization by thiazolidinediones is mediated solely through PPARgamma activation, and whether there are PPARgamma-ligand-independent pathways for adipocyte differentiation. If selective PPARgamma modulators block adipogenesis in vivo, they may prevent obesity, lower insulin resistance, and delay the onset of type 2 diabetes.     

PPARadigms and PPARadoxes: expanding roles for PPARgamma in the control of lipid metabolism

The nuclear receptor PPARgamma is a central regulator of adipose tissue development and an important modulator of gene expression in a number of specialized cell types including adipocytes, epithelial cells, and macrophages. PPARgamma signaling pathways impact both cellular and systemic lipid metabolism and have links to obesity, diabetes, and cardiovascular disease. The ability to activate this receptor with small molecule ligands has made PPARgamma an attractive target for intervention in human metabolic disease. As our understanding of PPARgamma biology has expanded, so has the therapeutic potential of PPARgamma ligands. Recent studies have provided insight into the paradoxical relationship between PPARgamma and metabolic disease and established new paradigms for the control of lipid metabolism. This review focuses on recent advances in PPARgamma biology in the areas of adipocyte differentiation, insulin resistance, and atherosclerosis.     

Retinoid signalling and gene expression in neuroblastoma cells: RXR agonist and antagonist effects on CRABP-II and RARbeta expression

9-cis Retinoic acid (RA) induces gene expression in neuroblastoma cells more effectively and with different kinetics than other RA isomers, and could be acting in part through Retinoid X Receptors (RXRs). The aim of this study was to characterise the effects of an RXR agonist and RXR homodimer antagonist on the induction of cellular RA binding protein II (CRABP-II) and RA receptor-beta (RARbeta) in neuroblastoma cells in response to different retinoids. The RXR agonist, LDG1069, was as effective as all-trans RA in inducing gene expression, but less effective than 9-cis RA. The RXR-homodimer antagonist, LG100754, inhibited the induction of CRABP-II mRNA in SH-SY5Y neuroblastoma cells by 9-cis RA or the RXR-specific agonist LGD1069, but had no effect when used with all-trans RA. Conversely, LG100754 did not inhibit induction of RARbeta mRNA by 9-cis or all-trans RA, or by LGD1069. RAR- and RXR-specific ligands used together induced CRABP-II and RARbeta as effectively as 9-cis RA. These results demonstrate the value of combining RXR- and RAR-specific ligands to regulate RA-inducible gene expression. The possibility that RXR-homodimers mediate, in part, the induction of CRABP-II by 9-cis RA and RXR-specific ligands is discussed.     

The small molecule harmine is an antidiabetic cell-type-specific regulator of PPARgamma expression

PPARgamma is the master regulator of adipogenesis and the molecular target of the thiazolidinedione antidiabetic drugs. By screening for compounds that promote adipogenesis, we identified a small molecule that targets the PPARgamma pathway by a distinct mechanism. This molecule, harmine, is not a ligand for the receptor; rather, it acts as a cell-type-specific regulator of PPARgamma expression. Administration of harmine to diabetic mice mimics the effects of PPARgamma ligands on adipocyte gene expression and insulin sensitivity. Unlike thiazolidinediones, however, harmine does not cause significant weight gain or hepatic lipid accumulation. Molecular studies indicate that harmine controls PPARgamma expression through inhibition of the Wnt signaling pathway. This work validates phenotypic screening of adipocytes as a promising strategy for the identification of bioactive small molecules and suggests that regulators of PPARgamma expression may represent a complementary approach to PPARgamma ligands in the treatment of insulin resistance.     

Genetic modulation of PPARgamma phosphorylation regulates insulin sensitivity

Obesity-associated diabetes is epidemic in industrialized societies. The nuclear receptor peroxisome proliferator-activated receptor gamma (PPARgamma) is highly expressed in adipose tissue and the presumed molecular target for antidiabetic thiazolidinedione drugs that reverse insulin resistance but also promote weight gain. Phosphorylation reduces the activity of PPARgamma in vitro, but physiological relevance has not been demonstrated. We have studied mice homozygous for a mutation (S112A) that prevents PPARgamma phosphorylation. Surprisingly, the weights and adipose mass of PPARgamma-S112A mice are not greater than wild-type. Remarkably, however, genetic prevention of PPARgamma phosphorylation preserves insulin sensitivity in the setting of diet-induced obesity. Underlying this protection are smaller fat cells, elevated serum adiponectin, and reduced free fatty acid levels. Thus, the phosphorylation state of PPARgamma modulates insulin sensitivity. Compounds that prevent PPARgamma phosphorylation or ligands that induce the conformation of nonphosphorylated PPARgamma may selectively enhance insulin sensitivity without increasing body weight.     

Role of peroxisome proliferator-activated receptor gamma in glucose-induced insulin secretion

Peroxisome proliferator-activated receptor (PPAR) isoforms (α and γ) are known to beexpressed in pancreatic islets as well as in insulin-producing cell lines.Ligands of PPAR have been shoWn toenhance glucose-induced insulin secretion in rat pancreatic islets.However,their effect on insulin secretionis still unclear.To understand the molecular mechanism by which PPAR7 exerts its effect on glucose-induced insulin secretion,we examined the endogenous activity of PPAR isoforms,and studied the PPARyfunction and its target gene expression in INS-1 cells.We found that:(1)endogenous PPARγ was activatedin a ligand-dependent manner in INS-1 cells;(2)overexpression of PPARy in the absence of PPARγ ligandsenhanced glucose-induced insulin secretion,which indicates that the increased glucose-induced insulin secretionis a PPARγ-mediated event;(3)the addition of both PPARγ and retinoid X receptor (RXR) ligands showed asynergistic effect on the augmentation of reporter activity,suggesting that the hetero-dimerization of PPAR7and RXR is required for the regulation of the target genes;(4)PPARs upregulated both the glucose transporter2 (GLUT2) and Cbl-associated protein (CAP) genes in INS-1 cells.Our findings suggest an importantmechanistic pathway in which PPARγ enhances glucose-induced insulin secretion by activating the expressionof GLUT2 and CAP genes in a ligand-dependent manner.     

A role of PPARgamma in reproduction?

Synthetic molecules of the glitazone family are currently used in the treatment of type II diabetes. Glitazones also improve secondary pathologies that are frequently associated with insulin resistance such as the polycystic ovary syndrome (PCOS). Glitazones bind to the peroxysome proliferator-activated receptor gamma (PPARgamma), a nuclear receptor which is highly expressed in adipose tissue. PPARgamma also binds natural ligands such as long-chain fatty acids. Recently, several groups have shown that PPARgamma is also highly expressed in ovarian granulosa cells, and that glitazones are able to modulate in vitro granulosa cell proliferation and steroidogenesis in several species. These recent data raise new questions concerning the underlying mechanism of the effect of glitazones on PCOS. One might hypothesize, as for other < glucophage > molecules such as metformin, that it is the general improvement of glucose metabolism and insulin sensitivity by glitazones which indirectly, and via an unknown mechanism, ameliorates ovarian functionality. The data discussed here suggest now an alternative possibility, that glitazones act directly at the ovarian level. Moreover, PPARgamma also seems to play a key role in the maturation of the placenta. In particular, inactivation of PPARgamma in mice is lethal, since the foetus is unable to develop because of alterations of placental maturation. In women, the activation of PPARgamma in placenta leads to an increase of placental hormone secretion. Overall, these results raise some questions about the role of natural ligands of PPARgamma such as long chain fatty acids on female fertility and the interactions between energy metabolism and reproduction in general.     

Effect of heterozygous PPARgamma deficiency and TZD treatment on insulin resistance associated with age and high-fat feeding

Peroxisome proliferator-activated receptor-gamma (PPARgamma) is the target receptor for thiazolidinedione (TZD) compounds, which are a class of insulin-sensitizing drugs used in the treatment of type 2 diabetes. Paradoxically, however, mice deficient in PPARgamma (PPARgamma(+/-)) are more insulin sensitive than their wild-type (WT) littermates, not less, as would be predicted. To determine whether PPARgamma deficiency could prevent the development of the insulin resistance associated with increasing age or high-fat (HF) feeding, insulin sensitivity was assessed in PPARgamma(+/-) and WT mice at 2, 4, and 8 mo of age and in animals fed an HF diet. Because TZDs elicit their effect through PPARgamma receptor, we also examined the effect of troglitazone (a TZD) in these mice. Glucose metabolism was assessed by hyperinsulinemic euglycemic clamp and oral glucose tolerance test. Insulin sensitivity declined with age for both groups. However, the decline in the PPARgamma(+/-) animals was substantially less than that of the WT animals, such that, by 8 mo of age, the PPARgamma(+/-) mice were markedly more insulin sensitive than the WT mice. This greater sensitivity in PPARgamma(+/-) mice was lost with TZD treatment. HF feeding led to marked adipocyte hypertrophy and peripheral tissue and hepatic insulin resistance in WT mice but also in PPARgamma(+/-) mice. Treatment of these mice with troglitazone completely prevented the adipocyte hypertrophy and normalized insulin action. In conclusion, PPARgamma deficiency partially protects against age-related insulin resistance but does not protect against HF diet-induced insulin resistance.     

A unique PPARgamma ligand with potent insulin-sensitizing yet weak adipogenic activity.

FMOC-L-Leucine (F-L-Leu) is a chemically distinct PPARgamma ligand. Two molecules of F-L-Leu bind to the ligand binding domain of a single PPARgamma molecule, making its mode of receptor interaction distinct from that of other nuclear receptor ligands. F-L-Leu induces a particular allosteric configuration of PPARgamma, resulting in differential cofactor recruitment and translating in distinct pharmacological properties. F-L-Leu activates PPARgamma with a lower potency, but a similar maximal efficacy, than rosiglitazone. The particular PPARgamma configuration induced by F-L-Leu leads to a modified pattern of target gene activation. F-L-Leu improves insulin sensitivity in normal, diet-induced glucose-intolerant, and in diabetic db/db mice, yet it has a lower adipogenic activity. These biological effects suggest that F-L-Leu is a selective PPARgamma modulator that activates some (insulin sensitization), but not all (adipogenesis), PPARgamma-signaling pathways.     

Peroxisome proliferator-activated receptor gamma (PPARgamma) and immunoregulation: enhancement of regulatory T cells through PPARgamma-dependent and -independent mechanisms

Peroxisome proliferator-activated receptor (PPAR)gamma is a nuclear hormone receptor primarily characterized for its effect on insulin metabolism. PPARgamma ligands, used to treat human type 2 diabetes, also down-regulate most immune system cells including APCs and pathogenic T cells. These effects putatively underlie the efficacy of PPARgamma ligands in treating animal models of autoimmunity, leading to projections of therapeutic potential in human autoimmunity. However, the relationship between PPARgamma ligands and CD4+CD25+ regulatory T cells (Tregs) has not been examined. Specifically, no studies have examined the role of Tregs in mediating the in vivo immunoregulatory effects of PPARgamma ligands, and there have been no investigations of the use of PPARgamma ligands to treat autoimmunity in the absence of Tregs. We now characterize the novel relationship between ciglitazone, a thiazolidinedione class of PPARgamma ligand, and both murine natural Tregs (nTregs) and inducible Tregs (iTregs). In vitro, ciglitazone significantly enhances generation of iTregs in a PPARgamma-independent manner. Surprisingly, and contrary to the current paradigm, we find that, in a model of graft-vs-host disease, the immunotherapeutic effect of ciglitazone requires the presence of nTregs that express PPARgamma. Overall, our results indicate that, unlike its down-regulatory effect on other cells of the immune system, ciglitazone has an enhancing effect on both iTregs and nTregs, and this finding may have important implications for using PPARgamma ligands in treating human autoimmune disease.