Bile acids are secreted by the liver, stored in the gallbladder, and efficiently reabsorbed from the intestine. This enterohepatic cycling is crucial for maintaining the pool of bile acids and for restricting these potentially cytotoxic detergents to specific compartments such as the liver and Gl tract. Many of the transporters that function to maintain the enterohepatic circulation of bile acids have been identified over the past fifteen years. Notably absent from that list is the identity of the ileal basolateral bile acid transporter. The goal of the studies in this application is to determine the relative roles of the recently identified Organic solute transporter alpha-beta, Osta-Ostp, and the multidrug resistance protein-3, Mrp3, in the basolateral transport of bile acids in the small intestine and colon. The studies in Specific Aim 1 will directly test the hypothesis that ileal basolateral bile acid transport is mediated primarily by Osta-Ostp, with Mrp3 serving only a secondary back-up role. These experiments will use recently developed Osta KO, Mrp3 KO and Mrp3/Osta double KO mouse models. In addition to analyzing the gastrointestinal phenotype of these models, the adaptive changes in hepatic and intestinal expression of genes important for bile acid biosynthesis, transport, and regulation will be determined. The direct effects of these specific gene deletions on intestinal bile acid transport will be determined in vivo by measuring changes in bile acid fecal excretion and pool size, and in vitro using ileal everted gut sacs. The studies in Specific Aim 2 will directly test the hypothesis that Mrp3 and Osta-Ostp play important roles in protecting the colonic epithelium from the cytotoxic actions of bile acids. These experiments will use KO mice fed deoxycholic acid rich diets. The endpoints to be measured include colonic macroscopic and microcopic tissue damage, and cytokine levels as markers for the extent of injury. Relevance: A single layer of epithelial cells separates the gut lumen from the underlying immune cells of the gastrointestinal tract. Transporters such as Osta-Ostp and Mrp3 may play important roles in protecting that critical barrier by preventing cellular accumulation of bile acids and toxins. Since diseases such as colon cancer, inflammatory bowel disease, and colitis have been linked to defects in intestinal defense, it will be important to determine the specific functions of transporters such as Osta-Ostp and Mrp3 and if loss of these transporters contributes to the development or progression of gastrointestinal diseases.
