Project Summary The overall goal of this application is to determine the interplay between the liver and intestine in the regulation of fed-state metabolism with emphasis on bile acid (BA) metabolism. BAs function as physiological detergents to aid digestion of lipid nutrients but also function as signaling molecules that profoundly impact metabolism by activating nuclear and membrane BA receptors and also by mediating induction of intestinal FGF15/19 (human FGF19; mouse FGF15). BAs in excess are toxic so that their levels must be tightly controlled and the BA nuclear receptor FXR plays a key role in this regulation. Despite recent advances, the mechanisms underlying the interplay between the liver and intestine mediated by BAs and FGF15 to regulate BA levels, in particular hepatic expression of Cyp7a1, the rate-limiting BA synthetic enzyme, are not well understood. We have preliminary evidence that FGF15-mediated phosphorylation by Src of FXR plays an important role in this interplay. Based on our preliminary data, we hypothesize that Src phosphorylation of FXR at Tyr-67 is mediated by FGF15 signaling under physiological conditions and that Y67FXR phosphorylation is important for gene-selective transcriptional regulation to reduce liver BA levels and terminate FGF15 signaling. To test this hypothesis, we propose the following Specific Aims: 1) Examine the in vivo role of Src phosphorylation of FXR in regulating BA levels, protecting against hepatobiliary defects in normal mice and in mice challenged with biliary insults, and human relevance by examining FXR and Src and their phosphorylated forms in liver samples from primary cholestasis patients. 2) Define the function and mechanisms of FXR phosphorylation by FGF15- and BA-activated Src in the regulation of BA metabolism and 3) determine the role of FGF15-mediated phosphorylation of FXR, in coordination with PPAR?, in terminating FGF15 signaling. Multiple in vitro and in vivo approaches will be used, including genetic and virally generated mouse models and ChIP-seq genomic analysis to identify FXR binding sites globally in mice treated with FGF19. Our expertise on post-translational modifications of nuclear receptors and transcriptional regulation of BA metabolism uniquely qualifies us to address this important research question of the role of posttranslational phosphorylation of FXR in regulation of BA metabolism. Our proposal will determine the novel function and mechanisms of FGF15- induced phosphorylation of FXR, and provide new potential therapeutics and diagnostic markers for BA- and FGF15-related diseases, including cholestatic liver diseases.