Retinoic acid receptors (RAR), thyroid hormone receptors (TR), peroxisome proliferator activated receptors (PPARs) and the orphan receptor, LXR, bind preferentially to DNA as heterodimers with a common partner, retinoid X receptor (RXR), to regulate transcription. We investigated whether RXR-selective agonists replicate the activity of ligands for several of these receptors? We demonstrate here that RXR-selective ligands (referred to as rexinoids) function as RXR heterodimer-selective agonists, activating RXR: PPARgamma and RXR:LXR dimers but not RXR:RAR or RXR:TR heterodimers. Because PPARgamma is a target for antidiabetic agents, we investigated whether RXR ligands could alter insulin and glucose signalling. In mouse models of noninsulin-dependent diabetes mellitus (NIDDM) and obesity, RXR agonists function as insulin sensitizers and can decrease hyperglycaemia, hypertriglyceridaemia and hyperinsulinaemia. This antidiabetic activity can be further enhanced by combination treatment with PPARgamma agonists, such as thiazolidinediones. These data suggest that the RXR:PPARgamma heterodimer is a single-function complex serving as a molecular target for treatment of insulin resistance. Activation of the RXR:PPARgamma dimer with rexinoids may provide a new and effective treatment for NIDDM.

We describe the cloning, characterization, and tissue distribution of the two human peroxisome proliferator activated receptor isoforms hPPAR␥2 and hPPAR␥1. In cotransfection assays the two isoforms were activated to approximately the same extent by known PPAR␥ activators. Human PPAR␥ binds to DNA as a heterodimer with the retinoid X receptor (RXR). This heterodimer was activated by both RXR agonists and antagonists and the addition of PPAR␥ ligands with retinoids resulted in greater than additive activation. Such heterodimer-selective modulators may have a role in the treatment of PPAR␥/RXR-modulated diseases like diabetes. Northern blot analysis indicated the presence of PPAR␥ in skeletal muscle, and a sensitive RNase protection assay confirmed the presence of only PPAR␥1 in muscle that was not solely due to fat contamination. However, both PPAR␥1 and PPAR␥2 RNA were detected in fat, and the ratio of PPAR␥1 to PPAR␥2 RNA varied in different individuals. The presence of tissue-specific distribution of isoforms and the variable ratio of PPAR␥1 to PPAR␥2 raised the possibility that isoform expression may be modulated in disease states like non-insulin-dependent diabetes mellitus. Interestingly, a third protected band was detected with fat RNA indicating the possible existence of a third human PPAR␥ isoform.

A common feature of many metabolic pathways is their control by retinoid X receptor (RXR) heterodimers. Dysregulation of such metabolic pathways can lead to the development of atherosclerosis, a disease influenced by both systemic and local factors. Here we analyzed the effects of activation of RXR and some of its heterodimers in apolipoprotein E ؊͞؊ mice, a well established animal model of atherosclerosis. An RXR agonist drastically reduced the development of atherosclerosis. In addition, a ligand for the peroxisome proliferator-activated receptor (PPAR)␥ and a dual agonist of both PPAR␣ and PPAR␥ had moderate inhibitory effects. Both RXR and liver X receptor (LXR) agonists induced ATP-binding cassette protein 1 (ABC-1) expression and stimulated ABC-1-mediated cholesterol efflux from macrophages from wild-type, but not from LXR␣ and ␤ double ؊͞؊, mice. Hence, activation of ABC-1-mediated cholesterol efflux by the RXR͞LXR heterodimer might contribute to the beneficial effects of rexinoids on atherosclerosis and warrant further evaluation of RXR͞LXR agonists in prevention and treatment of atherosclerosis. R etinoid X receptors (RXRs) are ubiquitously expressed nuclear receptors that heterodimerize with a number of other receptors (e.g., thyroid hormone receptor, vitamin D receptor, retinoic acid receptors, etc.; refs. 1 and 2). The elucidation of RXRs biological activity has advanced significantly through the characterization of rexinoids, high-affinity selective synthetic ligands for RXRs (1, 2). The use of rexinoids led to the demonstration that RXRs are active and permissive signaling molecules in heterodimers with the farnesol X receptor (FXR) [or bile acid receptor (BAR)], the liver X receptors (LXRs), and the peroxisome proliferatoractivated receptors (PPARs). The simultaneous activation of several permissive heterodimers underlies in part RXRs pleiotropic functions, affecting numerous receptor signaling pathways, and ranges from the control of cell proliferation, differentiation, and apoptosis (3) to the regulation of glucose and lipid metabolism (4). This pivotal role of the various permissive RXR heterodimers is in fact an emerging theme in multiple metabolic pathways. Heterodimers between RXR and PPAR␥, PPAR␣, LXR␣, and FXR͞BAR respectively inf luence glucose, triglyceride, cholesterol, and bile acid homeostasis. Dysregulation of these homeostatic control pathways can result in common metabolic disorders such as obesity, type 2 diabetes, and hyperlipidemia, which are often complicated by the development of atherosclerosis.In view of the potential implication of RXRs in various metabolic pathways implicated in atherosclerosis, we studied whether modulating the activity of these receptors or of some of their heterodimers affects the development of atherosclerosis in apolipoprotein (apo)E-deficient mice. We demonstrate here that the administration of rexinoids significantly attenuates atherosclerosis development, and we show that this effect might be linked to the activation of reverse cholesterol transpor...