Nuclear receptors regulate gene expression by direct activation of target genes and inhibition of AP-1. Here we report that, unexpectedly, activation by nuclear receptors requires the actions of CREB-binding protein (CBP) and that inhibition of AP-1 activity is the apparent result of competition for limiting amounts of CBP/p300 in cells. Utilizing distinct domains, CBP directly interacts with the ligand-binding domain of multiple nuclear receptors and with the p160 nuclear receptor coactivators, which upon cloning have proven to be variants of the SRC-1 protein. Because CBP represents a common factor, required in addition to distinct coactivators for function of nuclear receptors, CREB, and AP-1, we suggest that CBP/p300 serves as an integrator of multiple signal transduction pathways within the nucleus.

Peroxisomes are cytoplasmic organelles which are important in mammals in modulation of lipid homeostasis, including the metabolism of long-chain fatty acids and conversion of cholesterol to bile salts (reviewed in refs 1 and 2). Amphipathic carboxylates such as clofibric acid have been used in man as hypolipidaemic agents and in rodents they stimulate the proliferation of peroxisomes. These agents, termed peroxisome proliferators, and all-trans retinoic acid activate genes involved in peroxisomal-mediated beta-oxidation of fatty acids. Here we show that the receptor activated by peroxisome proliferators and the retinoid X receptor-alpha (ref. 6) form a heterodimer that activates acyl-CoA oxidase gene expression in response to either clofibric acid or the retinoid X receptor-alpha ligand, 9-cis retinoic acid, an all-trans retinoic acid metabolite; simultaneous exposure to both activators results in a synergistic induction of gene expression. These data demonstrate the coupling of the peroxisome proliferator and retinoid signalling pathways and provide evidence for a physiological role for 9-cis retinoic acid in modulating lipid metabolism.

Several nuclear hormone receptors involved in lipid metabolism form obligate heterodimers with retinoid X receptors (RXRs) and are activated by RXR agonists such as rexinoids. Animals treated with rexinoids exhibited marked changes in cholesterol balance, including inhibition of cholesterol absorption and repressed bile acid synthesis. Studies with receptor-selective agonists revealed that oxysterol receptors (LXRs) and the bile acid receptor (FXR) are the RXR heterodimeric partners that mediate these effects by regulating expression of the reverse cholesterol transporter, ABC1, and the rate-limiting enzyme of bile acid synthesis, CYP7A1, respectively. Thus, these RXR heterodimers serve as key regulators of cholesterol homeostasis by governing reverse cholesterol transport from peripheral tissues, bile acid synthesis in liver, and cholesterol absorption in intestine.

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We have identified a new retinoid response pathway through which 9-cis retinoic acid (9cRA) activates transcription in the presence of LXRot, a member of the nuclear receptor superfamily. LXRe~ shows a specific pattern of expression in visceral organs, thereby restricting the response to certain tissues. Retinoid trans-activation occurs selectively on a distinct response element termed an LXRE. Significantly, neither RXR homodimers nor RXR/RAR heterodimers are able to substitute for LXR~ in mediating this retinoid response. We provide evidence that the retinoid response on the LXRE is the result of a unique interaction between LXRot and endogenous RXR, which, unlike in the RXR/RAR heterodimer, makes RXR competent to respond to retinoids. Thus, the interaction with LXRot shifts RXR from its role described previously as a silent, DNA-binding partner to an active ligand-binding subunit in mediating retinoid responses through target genes defined by LXREs.

We explored the effects of bile acids on triglyceride (TG) homeostasis using a combination of molecular, cellular, and animal models. Cholic acid (CA) prevents hepatic TG accumulation, VLDL secretion, and elevated serum TG in mouse models of hypertriglyceridemia. At the molecular level, CA decreases hepatic expression of SREBP-1c and its lipogenic target genes. Through the use of mouse mutants for the short heterodimer partner (SHP) and liver X receptor (LXR) α and β, we demonstrate the critical dependence of the reduction of SREBP-1c expression by either natural or synthetic farnesoid X receptor (FXR) agonists on both SHP and LXRα and LXRβ. These results suggest that strategies aimed at increasing FXR activity and the repressive effects of SHP should be explored to correct hypertriglyceridemia. 1408The Nonstandard abbreviations used: acetyl-CoA carboxylase (ACC); acetyl-CoA synthetase (AceCS); angiopoietin-like protein 3 (ANGPTL3); carnitine palmitoyltransferase I (CPT-I); chenodeoxycholic acid (CDCA); cholesterol 7α-hydroxylase (CYP7A1); cholic acid (CA); farnesoid X receptor (FXR); fatty acid synthase (FAS); LDL receptor (LDL-R); liver receptor homolog-1 (LRH-1); liver receptor homolog-1 response element (LRH-1RE); liver X receptor (LXR); liver X receptor response element (LXRRE); long-chain acyl-CoA dehydrogenase (LCAD); malic enzyme (ME); medium-chain acyl-CoA dehydrogenase (MCAD); retinoid X receptor (RXR); short heterodimer partner (SHP); stearoyl-CoA desaturase-1 (SCD-1); triglyceride (TG). Conflict of interest:The authors have declared that no conflict of interest exists. Plasmids. pCMX-SHP was obtained by insertion of a PCR product corresponding to the mouse SHP cDNA into the pCMX vector. pCMX-liver receptor homolog-1 (pCMX-LRH-1) was produced by insertion of a PCR product corresponding to the mouse LRH-1 cDNA into pCMX. The pCMX-LXRα expression vector was as described (10); the pSG5-retinoid X receptor α (pSG5-RXRα) expression vector was a gift of P. Chambon (Institut Clinique de la Souris, Illkirch, France). The SREBP-1c promoter luciferase reporter plasmids were generated by PCR amplification of promoter fragments corresponding to sequences located between -1070 to -51 of the mouse SREBP-1c gene. The PCR product was ligated into the pGL3 basic vector (Promega, Madison, Wisconsin, USA). The reverse primer used for each promoter construct is 5′-CTTCCGCGCCGATTTCACCTG-3′. The different forward primers used for each construct are as follows: pSREBP-1c1070-Luc (5′-ACCCCTCAGACTGTGTGAGT-3′), pSREBP-1c571-Luc (5′-CTA GCTAGATGACCCTGCACCACCAA-3′), pSREBP-1c327-Luc (5′-TTGCCTGTGCGGCAGGGGTTGGGACGA-3′), pSREBP1c276-Luc (5′-CGCGCTGGCGCAGACGCGGTTAAA-3′), and pSREBP-1c151-Luc (5′-CTGCTGATTGGCCATGTGCGCTCA-3′). The primers were tailed with either a KpnI site (forward) or a BglII site (reverse). The LXR response elements in pSREBP-1c324-Luc were mutated for promoter analysis using the Quick Change SiteDirected Mutagenesis Kit (Stratagene, La Jolla, California, USA). All constructs were verified by sequence analysis.Ani...

Recent studies have shown that genes involved in oxidative phosphorylation (OXPHOS) exhibit reduced expression in skeletal muscle of diabetic and prediabetic humans. Moreover, these changes may be mediated by the transcriptional coactivator peroxisome proliferator-activated receptor γ coactivator-1α (PGC-1α). By combining PGC-1α-induced genome-wide transcriptional profiles with a computational strategy to detect cis-regulatory motifs, we identified estrogen-related receptor α (Errα) and GA repeat-binding protein α as key transcription factors regulating the OXPHOS pathway. Interestingly, the genes encoding these two transcription factors are themselves PGC-1α-inducible and contain variants of both motifs near their promoters. Cellular assays confirmed that Errα and GA-binding protein a partner with PGC-1α in muscle to form a double-positive-feedback loop that drives the expression of many OXPHOS genes. By using a synthetic inhibitor of Errα, we demonstrated its key role in PGC-1α-mediated effects on gene regulation and cellular respiration. These results illustrate the dissection of gene regulatory networks in a complex mammalian system, elucidate the mechanism of PGC-1α action in the OXPHOS pathway, and suggest that Errα agonists may ameliorate insulin-resistance in individuals with type 2 diabetes mellitus.

Continued reliance on the androgen receptor (AR) is now understood as a core mechanism in castration-resistant prostate cancer (CRPC), the most advanced form of this disease. While established and novel AR-pathway targeting agents display clinical efficacy in metastatic CRPC, dose-limiting side effects remain problematic for all current agents. In this study, we report the discovery and development of ARN-509, a competitive AR inhibitor this is fully antagonistic to AR overexpression, a common and important feature of CRPC. ARN-509 was optimized for inhibition of AR transcriptional activity and prostate cancer cell proliferation, pharmacokinetics and in vivo efficacy. In contrast to bicalutamide, ARN-509 lacked significant agonist activity in preclinical models of CRPC. Moreover, ARN-509 lacked inducing activity for AR nuclear localization or DNA binding. In a clinically valid murine xenograft model of human CRPC, ARN-509 showed greater efficacy than MDV3100. Maximal therapeutic response in this model was achieved at 30 mg/kg/day of ARN-509, whereas the same response required 100 mg/kg/day of MDV3100 and higher steady-state plasma concentrations. Thus, ARN-509 exhibits characteristics predicting a higher therapeutic index with a greater potential to reach maximally efficacious doses in man than current AR antagonists. Our findings offer preclinical proof of principle for ARN-509 as a promising therapeutic in both castration-sensitive and castration-resistant forms of prostate cancer.