Elucidating the Role of Organic Solute Transporter (OST) Alpha/Beta in Bile Acid Transport and Drug Interactions

项目总结/文摘

Project Summary/Abstract Organic solute transporter (OST) α/β, a heteromeric protein localized on the basolateral membrane of hepatic, intestinal and kidney epithelial cells, is an important transporter involved in the homeostasis of bile acids (BAs) and other steroid hormones. OSTα/β-deficient mice exhibit features of BA malabsorption. Hepatic expression of OSTα/β is significantly increased in obstructive cholestasis and primary biliary cholangitis, suggesting that this transporter provides an excretion pathway for BAs in cholestatic liver disease. OSTα/β is overexpressed in patients with nonalcoholic steatohepatitis, and a link has been found between genetic defects in this transporter and congenital diarrhea and cholestasis, implying a key role for OSTα/β in health and disease. Dysregulation of BA disposition, particularly by drug-mediated inhibition of hepatic efflux transporters, can lead to hepatocellular accumulation of lipophilic BAs resulting in hepatotoxicity. This is an important mechanism of drug-induced liver injury (DILI), a major safety issue in drug development. However, very little is known about the role of OSTα/β in BA-mediated DILI. A few BAs and drugs are known OSTα/β substrates and/or inhibitors, but a systematic study of OSTα/β-mediated BA transport is lacking. Therefore, identifying which BAs are OSTα/β substrates, identifying drugs that interact with OSTα/β-mediated BA transport, and elucidating OSTα/β residues that are crucial for BA transport is vital to understanding the role of this transporter in human health, disease and DILI. Specific aims will test the following hypotheses: 1) BA affinity for OSTα/β is a function of BA lipophilicity/hepatotoxicity; 2) drug- induced alterations in OSTα/β-mediated BA transport are dependent on the BA species; and 3) evolutionarily conserved, charged and hydrophilic/polar residues in the transmembrane domains of OSTα/β are crucial for OSTα/β-mediated BA transport. Using a stable, OSTα/β-overexpressing human cell line and the clinically relevant sandwich-cultured human hepatocyte model in the proposed project, the most prevalent hydrophilic and lipophilic/hepatotoxic BAs will be analyzed to identify OSTα/β substrates and elucidate the interaction of drugs with OSTα/β-mediated transport of these BA substrates. In Aim 1 of this project, OSTα/β-mediated transport kinetics of specific BAs will be studied by overexpression or induction of OSTα/β. In Aim 2, drug-induced alterations in OSTα/β-mediated BA disposition will be elucidated. To further reveal OSTα/β-mediated BA transport mechanisms, critical amino acid residues in OSTα/β’s transmembrane domains will be investigated (Aim 3). The results of the proposed studies will provide fundamental, and mechanistic information about the role of OSTα/β in human health and disease, and the effect of DILI-associated drugs on OSTα/β-mediated BA transport. These data will improve predictions of BA-mediated DILI using computational modeling. The results are anticipated to provide novel insights regarding biomarker candidates of OSTα/β dysfunction, and novel therapeutic targets for BA-related disorders (e.g., nonalcoholic fatty liver disease, obesity, diabetes).