The farnesoid X receptor (FXR) is a bile acid/alcohol-activated nuclear receptor that regulates lipid homeostasis. Unlike other steroid receptors, FXR binds bile acids in an orientation that allows the steroid nucleus A ring to face helix 12 in the receptor, a Crucial domain for coactivator-recruitment. Because Most naturally occurring bile acids and alcohols contain a cis-oriented A ring, which is distinct from that of other steroids and cholesterol metabolites, we investigated the role of this 5 beta-configuration in FXR activation. The results showed that the 5 beta-(A/B cis) bile alcohols 5 beta-cyprinol and bufol are potent FXR agonists, whereas their 5 alpha-(A/B trans) counterparts antagonize FXR transactivation and target gene expression. Both isomers bound to FXR, but their ability to induce coactivator-recruitment and thereby induce transactivation differed. These findings suggest a critical role for the A-ring orientation of bile salts in agonist/antagonist function. (c) 2005 Elsevier Inc. All rights reserved.