Coordination of gut-liver bile acid signaling by FXR

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DESCRIPTION (provided by applicant): The overall goal of this project is to determine whether the nuclear bile acid receptor, Farnesoid X Receptor (FXR), functions as a master metabolic regulator by coordinating the responses of the liver and intestine to bile acid signaling in vivo. Bile acids are important integrative signaling molecules that control metabolism and energy balance and FXR, as the primary biosensor for bile acids, plays an important role in controlling bile acid, cholesterol, and glucose levels through regulation of gene expression. After a meal, bile acid-activated FXR induces the synthesis and secretion of a recently identified intestinal hormone, fibroblast growth factor 19 (FGF19), which acts at the liver to control fed-state metabolism. Beta-Klotho (bKL) is the membrane coreceptor for FGF19 and, thus, is crucial for the FGF19 signaling in the liver. Our recent genome-wide hepatic FXR binding and microRNA (miR) studies have suggested that FXR may regulate hepatic expression of FGF19 signaling pathway genes, particularly bKL, both by induction of gene expression and post-transcriptionally through the inhibition of miR-34a. In FXR null mice, the mRNA and protein levels of bKL were substantially decreased, whereas miR-34a levels were increased. Moreover, miR-34a likely targets bKL by directly binding to the 3'UTR of the bKL mRNA. Remarkably, hepatic bKL levels were substantially reduced, miR-34a levels were elevated, and FGF19-activated ERK signaling was defective in obese mice. Therefore, we hypothesize that FXR primes the liver for signaling by intestinal FGF19 both by transcriptionally regulating expression of key components of the FGF19 pathway, particularly bKL, and by post-transcriptionally up-regulating bKL indirectly through miR-34a inhibition. Further, an aberrant FXR/miR-34a/bKL regulatory axis in obesity contributes to defective FGF19 signaling and abnormal metabolic responses. To test these hypotheses, we will employ multiple approaches from in vitro and cell culture, including primary hepatocytes, studies to in vivo studies using normal, obese, and FXR-null mice. We will first ask whether FXR transcriptionally regulates hepatic expression of key components of FGF19 signaling, particularly bKL, and therefore, primes the liver to respond to FGF19 signaling. Second we will ask whether miR-34a, known to be negatively regulated by FXR, directly targets and regulates the levels of bKL. We will also examine whether this FXR/miR-34a/bKL regulatory pathway is abnormal in obese mice, and, if so, whether targeting components of this pathway, particularly the aberrantly elevated miR-34, might be useful in the treatment of metabolic disease. Our proposed studies should provide new insights into the mechanisms by which FXR coordinates gut-liver bile acid signaling and may reveal novel therapeutic targets to treat metabolic disorders.