The Unfolded Protein Response and Bile Acid Metabolism

PROJECT SUMMARY/ABSTRACT Cholestatic liver disorders affect over 200,000 Americans and there are few effective and no curative therapies to prevent disease progression. The accumulation of intracellular misfolded and unfolded proteins causes a form of cellular stress termed endoplasmic reticulum (ER) stress. The X-box binding protein 1 (XBP1) pathway is a highly conserved unfolded protein response signaling pathway that acts to reduces endoplasmic reticulum (ER) stress and return cells to normal homeostasis. During cholestasis, elevated intrahepatic bile acid concentrations can cause protein misfolding and ER stress in the liver, and we have previously shown in murine models and human liver specimens from patients with cholestatic liver disease that the inability to properly activate the hepatic XBP1s pathway is associated with enhanced cholestatic liver injury. Hepatic XBP1s reduces intracellular misfolded protein and regulates lipid and glucose metabolism, and we have demonstrated using murine models that hepatic XBP1s also regulates hepatic bile acid metabolism, although the mechanisms are essentially unknown. The central hypothesis of this grant proposal is that during cholestasis, hepatic XBP1s reduces bile acid synthesis and regulates bile acid metabolism by transcriptionally regulating Cyp7a1, SHP and other bile acid metabolic genes. We present preliminary data demonstrating that acute XBP1s activation transcriptionally regulates hepatic Cyp7a1 to reduce its gene expression, protein expression and function. Therefore, we will determine the mechanisms by which acute liver XBP1s activation transcriptionally regulates Cyp7a1 and the resultant effects on bile acid metabolism (Aim 1). We also provide data demonstrating that XBP1s and FXR both regulate hepatic SHP and will investigate the transcriptional regulation of bile acid metabolism by XBP1s and FXR/SHP and determine the functional consequences (Aim 2). Finally, the bile acid pool and hepatic gene expression differ between mice and humans so we will determine the effects of hepatic XBP1s on human bile acid metabolism using mice with humanized bile acid pools, human hepatocytes and human liver samples (Aim 3). This proposal uses a team of investigators with extensive expertise in transcriptional gene regulation, bile acid metabolism, bioinformatics and medicinal chemistry, combined with state-of-the-art technologies. By determining the hepatic XBP1s regulation of bile acid metabolism, we can identify new molecular targets to develop therapies for cholestatic liver diseases.