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.     

Structure basis of bigelovin as a selective RXR agonist with a distinct binding mode

The nuclear receptor retinoid X receptor (RXR) functions potently in the regulation of homeostasis and cell development, while rexinoids as RXR agonists have proved their therapeutic potential in the treatment of metabolic diseases and cancer. Here, the natural product bigelovin was identified as a selective RXRα agonist. Interestingly, this compound could not transactivate RXRα:RXRα homodimer but could enhance the transactivation of RXRα:peroxisome proliferator-activated receptor γ heterodimer and repress that of RXRα:liver X receptor (LXR) α heterodimer, while it had no effects on RXRα:farnesoid X receptor heterodimer. Considering that the effective role of LXR response element involved transactivation of sterol regulatory element-binding protein-1c mediated by RXRα:LXRα in triglyceride elevation, such LXR response element repressing by bigelovin has obviously addressed its potency for further research. Moreover, our determined crystal structure of the bigelovin-activated RXRα ligand-binding domain with the coactivator human steroid receptor coactivator-1 peptide revealed that bigelovin adopted a distinct binding mode. Compared with the known RXR ligands, bigelovin lacks the acidic moiety in structure, which indicated that the acidic moiety rendered little effects on RXR activation. Our results have thereby provided new insights into the structure-based selective rexinoids design with bigelovin as a potential lead compound.     

Retinoid X receptor alpha transactivates the hepatitis B virus enhancer 1 element by forming a heterodimeric complex with the peroxisome proliferator-activated receptor.

The hepatitis B virus enhancer 1 contains a retinoic acid responsive element (RARE). We have previously demonstrated that retinoid X receptor alpha (RXR alpha) transactivates enhancer 1 by binding to the RARE. The present study has revealed that a heterodimeric complex composed of RXR alpha and peroxisome proliferator-activated receptor (PPAR) interacts with the hepatitis B virus RARE. Transient transfection studies, in conjunction with in vitro DNA binding data, support the hypothesis that the RXR alpha-PPAR heterodimer transactivates enhancer 1.     

Isolation and characterization of unsaturated fatty acids as natural ligands for the retinoid-X receptor

The retinoid-X receptor (RXR) is a ligand activated nuclear receptor that is the heterodimer partner for many class II nuclear receptors. Previously identified natural ligands for this receptor include 9-cis retinoic acid (9cRA), docosahexaenoic acid, and phytanic acid. Our studies were performed to determine if there are any unidentified, physiologically important RXR ligands. Agonists for RXR were purified from rat heart and testes lipid extracts with the use of a cell-based reporter assay to monitor RXR activation. Purified active fractions contained a variety of unsaturated fatty acids and components were quantified by gas-liquid chromatography of derivatized samples. The corresponding fatty acid standards elicited a similar response in the reporter cell assay. Competition binding analysis revealed that the active fatty acids compete with [3H]9cRA for binding to RXR. Non-esterified fatty acids were analyzed from lipid extracts of isolated heart and testes nuclei and endogenous concentrations were found to be within the range of their determined binding affinities. Our studies reveal tissue dependent profiles of RXR agonists and support the idea of unsaturated fatty acids as physiological ligands of RXR.     

Autonomous rexinoid death signaling is suppressed by converging signaling pathways in immature leukemia cells.

On their own, retinoid X receptor (RXR)-selective ligands (rexinoids) are silent in retinoic acid receptor (RAR)-RXR heterodimers, and no selective rexinoid program has been described as yet in cellular systems. We report here on the rexinoid signaling capacity that triggers apoptosis of immature promyelocytic NB4 cells as a default pathway in the absence of survival factors. Rexinoid-induced apoptosis displays all features of bona fide programmed cell death and is inhibited by RXR, but not RAR antagonists. Several types of survival signals block rexinoid-induced apoptosis. RARalpha agonists switch the cellular response toward differentiation and induce the expression of antiapoptosis factors. Activation of the protein kinase A pathway in the presence of rexinoid agonists induces maturation and blocks immature cell apoptosis. Addition of nonretinoid serum factors also blocks cell death but does not induce cell differentiation. Rexinoid-induced apoptosis is linked to neither the presence nor stability of the promyelocytic leukemia-RARalpha fusion protein and operates also in non-acute promyelocytic leukemia cells. Together our results support a model according to which rexinoids activate in certain leukemia cells a default death pathway onto which several other signaling paradigms converge. This pathway is entirely distinct from that triggered by RAR agonists, which control cell maturation and postmaturation apoptosis.     

Retinoid X receptor agonists impair arterial mononuclear cell recruitment through peroxisome proliferator-activated receptor-γ activation

Mononuclear cell migration into the vascular subendothelium constitutes an early event of the atherogenic process. Because the effect of retinoid X receptor (RXR)α on arterial mononuclear leukocyte recruitment is poorly understood, this study investigated whether RXR agonists can affect this response and the underlying mechanisms involved. Decreased RXRα expression was detected after 4 h stimulation of human umbilical arterial endothelial cells with TNF-α. Interestingly, under physiological flow conditions, TNF-α-induced endothelial adhesion of human mononuclear cells was concentration-dependently inhibited by preincubation of the human umbilical arterial endothelial cells with RXR agonists such as bexarotene or 9-cis-retinoid acid. RXR agonists also prevented TNF-α-induced VCAM-1 and ICAM-1 expression, as well as endothelial growth-related oncogene-α and MCP-1 release. Suppression of RXRα expression with a small interfering RNA abrogated these responses. Furthermore, inhibition of MAPKs and NF-κB pathways were involved in these events. RXR agonist-induced antileukocyte adhesive effects seemed to be mediated via RXRα/peroxisome proliferator-activated receptor (PPAR)γ interaction, since endothelial PPARγ silencing abolished their inhibitory responses. Furthermore, RXR agonists increased RXR/PPARγ interaction, and combinations of suboptimal concentrations of both nuclear receptor ligands inhibited TNF-α-induced mononuclear leukocyte arrest by 60-65%. In vivo, bexarotene dose-dependently inhibited TNF-α-induced leukocyte adhesion to the murine cremasteric arterioles and decreased VCAM-1 and ICAM-1 expression. Therefore, these results reveal that RXR agonists can inhibit the initial inflammatory response that precedes the atherogenic process by targeting different steps of the mononuclear recruitment cascade. Thus, RXR agonists may constitute a new therapeutic tool in the control of the inflammatory process associated with cardiovascular disease.     

Metalloproteinase expression in PMA-stimulated THP-1 cells. Effects of peroxisome proliferator-activated receptor-gamma (PPAR gamma) agonists and 9-cis-retinoic acid

The PPAR gamma agonists, thiazolidinediones (TZDs), have anti-inflammatory properties as well as increasing insulin sensitivity. This has widened their therapeutic scope to treat inflammatory diseases such as atherosclerosis in addition to Type 2 Diabetes. TZDs are known to reduce monocyte/macrophage expression of Matrix metalloproteinase (MMP)-9, which is implicated in atherosclerotic plaque destabilization. This study aims to identify other metalloproteinase genes of the ADAM (A Disintegin And Metalloproteinase) and ADAMTS families that are regulated by PPAR gamma or RXR agonists, which are potentially important in type 2 diabetes and/or related atherosclerosis. The synthetic PPAR gamma agonist, GW7845, and the natural agonist 15d-PGJ2, suppressed PMA stimulated MMP-9 in human monocyte-like cells (THP-1) only in the presence of 9-cis-retinoic acid. Quantitative Real-Time PCR showed that this reduction was regulated at the mRNA level. Expression of ADAMs 8, 9, and 17 were increased, and ADAM15 was decreased by stimulation of THP-1 with PMA, although these ADAMs were not regulated by PPAR gamma or RXR agonists. PMA-induced ADAM28 expression was further enhanced by the addition of 9-cis-retinoic acid. ADAMTS4, implicated in rheumatoid arthritis, was expressed in THP-1 cells, and significantly increased after 24 h of PMA stimulation. ADAMTS4 expression was suppressed by both PPAR gamma and RXR agonists and was undetectable when the agonists were combined. Pretreatment of THP-1 cells with the PPAR gamma antagonist, GW9662, suggests that PPAR gamma plays subtly different roles in the regulation of MMP-9, ADAMTS4 and ADAM28 gene expression. These results indicate that PPAR gamma and RXR agonists have complex effects on monocyte metalloproteinase expression, which may have implications for therapeutic strategies.     

The atypical interaction of peroxisome proliferator-activated receptor α with liver X receptor α antagonizes the stimulatory effect of their respective ligands on the murine cholesterol 7α-hydroxylase gene promoter

Cholesterol 7α-hydroxylase (cyp7a) mediates cholesterol elimination in the liver by catalyzing the first and rate-limiting step in the conversion of cholesterol into bile acids. Peroxisome proliferator-activated receptor α (PPARα; NR1C1) and liver X receptor α (LXRα; NR1H3) are two nuclear receptors that stimulate the murine Cyp7a1 gene. Here we report that co-expression of PPARα and LXRα in hepatoma cells abolishes the stimulation of Cyp7a1 gene promoter in response to their respective agonists. PPARα and LXRα form an atypical heterodimer that binds to two directly adjacent hexameric sequences localized within overlapping PPARα and LXRα response elements (termed Site I), antagonizing the interaction of PPARα:retinoid X receptor α (RXRα) or RXRα:LXRα with the Cyp7a1 gene promoter. Mutations within either hexameric sequences that specifically abolished LXRα:PPARα heterodimer binding to the murine Cyp7a1 Site I also relieved promoter inhibition. The LXRα:PPARα heterodimer may be important in coordinating the expression of genes that encode proteins involved in metabolism of fats and cholesterol.