David D. Moore

Androstane metabolites bind to and deactivate the nuclear receptor CAR-β

The orphan receptor CAR-β (ref. 1) binds DNA as a heterodimer with the retinoid-X receptor and activates gene transcription in a constitutive manner. Here we show that, in contrast to the classical nuclear receptors, the constitutive activity of CAR-β results from a ligand-independent recruitment of transcriptional co-activators. While searching for potential ligands of CAR-β, we found that the steroids androstanol and androstenol inhibit the constitutive activity of CAR-β. This effect is stereospecific: only 3α-hydroxy, 5α-reduced androstanes are active. These androstanes do not interfere with heterodimerization or DNA binding of CAR-β; instead, they promote co-activator release from the ligand-binding domain. These androstane ligands are examples of naturally occurring inverse agonists, that reverse transcriptional activation by nuclear receptors. CAR-β (constitutive androstane receptor-β), therefore, defines an unanticipated steroidal signalling pathway that functions in a manner opposite to that of the conventional nuclear receptor pathways.

Interactions between Hepatic Mrp4 and Sult2a as Revealed by the Constitutive Androstane Receptor and Mrp4 Knockout Mice

The ABC transporter, Mrp4, transports the sulfated steroid DHEA-s, and sulfated bile acids interact with Mrp4 with high affinity. Hepatic Mrp4 levels are low, but increase under cholestatic conditions. We therefore inferred that up-regulation of Mrp4 during cholestasis is a compensatory mechanism to protect the liver from accumulation of hydrophobic bile acids. We determined that the nuclear receptor CAR is required to coordinately up-regulate hepatic expression of Mrp4 and an enzyme known to sulfate hydroxy-bile acids and steroids, Sult2a1. CAR activators increased Mrp4 and Sult2a1 expression in primary human hepatocytes and HepG2, a human liver cell line. Sult2a1 was down-regulated in Mrp4-null mice, further indicating an inter-relation between Mrp4 and Sult2a1 gene expression. Based on the hydrophilic nature of sulfated bile acids and the Mrp4 capability to transport sulfated steroids, our findings suggest that Mrp4 and Sult2a1 participate in an integrated pathway mediating elimination of sulfated steroid and bile acid metabolites from the liver.