The emerging role of nuclear receptor RORalpha and its crosstalk with LXR in xeno- and endobiotic gene regulation.

Retinoid-related orphan receptors (RORs), including the alpha, beta and gamma isoforms (NR1F1-3), are orphan nuclear receptors that have been implicated in tissue development, immune responses, and circadian rhythm. Although RORalpha and RORgamma have been shown to be expressed in the liver, the hepatic function of these two RORs remains unknown. We have recently shown that loss of RORalpha and/or RORgamma can positively or negatively influence the expression of multiple Phase I and Phase II drug metabolizing enzymes and transporters in the liver. Among ROR responsive genes, we identified oxysterol 7alpha-hydroxylase (Cyp7b1), which plays a critical role in the homeostasis of cholesterol, as a RORalpha target gene. We showed that RORalpha is both necessary and sufficient for Cyp7b1 activation. Studies of mice deficient of RORalpha or liver X receptors (LXRs) revealed an interesting and potentially important functional crosstalk between RORalpha and LXR. The respective activation of LXR target genes and ROR target genes in RORalpha null mice and LXR null mice led to our hypothesis that these two receptors are mutually suppressive in vivo. LXRs have been shown to regulate a battery of metabolic genes. We conclude that RORs participate in the xeno- and endobiotic regulatory network by regulating gene expression directly or through crosstalk with LXR, which may have broad implications in metabolic homeostasis.