Project Summary/Abstract Organic solute transporter (OST) ?/?, 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. OST?/?-deficient mice exhibit features of BA malabsorption. Hepatic expression of OST?/? is significantly increased in obstructive cholestasis and primary biliary cholangitis, suggesting that this transporter provides an excretion pathway for BAs in cholestatic liver disease. OST?/? 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 OST?/? 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 OST?/? in BA-mediated DILI. A few BAs and drugs are known OST?/? substrates and/or inhibitors, but a systematic study of OST?/?-mediated BA transport is lacking. Therefore, identifying which BAs are OST?/? substrates, identifying drugs that interact with OST?/?-mediated BA transport, and elucidating OST?/? 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 OST?/? is a function of BA lipophilicity/hepatotoxicity; 2) drug- induced alterations in OST?/?-mediated BA transport are dependent on the BA species; and 3) evolutionarily conserved, charged and hydrophilic/polar residues in the transmembrane domains of OST?/? are crucial for OST?/?-mediated BA transport. Using a stable, OST?/?-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 OST?/? substrates and elucidate the interaction of drugs with OST?/?-mediated transport of these BA substrates. In Aim 1 of this project, OST?/?-mediated transport kinetics of specific BAs will be studied by overexpression or induction of OST?/?. In Aim 2, drug-induced alterations in OST?/?-mediated BA disposition will be elucidated. To further reveal OST?/?-mediated BA transport mechanisms, critical amino acid residues in OST?/??s transmembrane domains will be investigated (Aim 3). The results of the proposed studies will provide fundamental, and mechanistic information about the role of OST?/? in human health and disease, and the effect of DILI-associated drugs on OST?/?-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 OST?/? dysfunction, and novel therapeutic targets for BA-related disorders (e.g., nonalcoholic fatty liver disease, obesity, diabetes).