Cholesterol and its oxysterol congeners are important constituents of cell membranes and function as intermediates in several crucial biosynthetic pathways. These compounds autoregulate their metabolic fate by end-product repression and activation of downstream catabolism. Although end-product repression by oxysterols is relatively well understood, the mechanism by which these compounds act as positive transcription signalling molecules is unknown. Here we identify a specific group of endogenous oxysterols that activate transcription through the nuclear receptor LXR alpha. Transactivation of LXR alpha by oxysterols occurs at concentrations at which these compounds exist in vivo. The most potent activators also serve as intermediary substrates in the rate-limiting steps of three important metabolic pathways: steroid hormone biosynthesis, bile acid synthesis, and conversion of lanosterol to cholesterol. Our results demonstrate the existence of a nuclear receptor signalling pathway for oxysterols and suggest that LXR alpha may be important as a sensor of cholesterol metabolites.

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.

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.