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Identification and characterization of an alternative canalicular bile acid transporter Lam, Ping

Abstract

In vertebrates, bile flow is essential for movement of water and solutes across the liver canalicular membrane. The formation of bile is contributed by the active transport of bile acids and other organic solutes (Coleman 1987; Nathanson and Boyer 1991; Erlinger 1996). In recent years, the molecular motor of canalicular bile acid secretion has been identified to be a member of the ATP Binding Cassette transporter superfamily, known as Sister of p-glycoprotein (Spgp) or Bile Salt Efflux Pump (Bsep) (Childs et al. 1995; Gerloff et al. 1998). In humans, mutations in spgp cause a severe liver disease known as type 2 intrahepatic cholestasis (PFIC2), which is characterized by low bile acid output (Strautnieks et al. 1998; Jansen et al. 1999). Spgp[sup -/-] mice produce a cholestatic phenotype that is less severe than human PFIC2 (Wang et al. 2001). The work in this thesis investigated the molecular mechanism(s) of bile acid secretion in spgp[sup -/-] mice. Hepatic responses of bile flow and bile acid secretion were significantly stimulated by cholic acid (CA) feeding in mutant mice. In addition, the capacity of total bile acid secretion was comparable to wildtype mice, compatible with the hypothesis that additional molecular mechanisms exist for bile acid secretion. To investigate whether another ATP-dependent bile acid transport may account for the bile acid clearance in spgp[sup -/-] mice, mRNA and protein expression data were studied. The expression of Mdr1a associated with resistance to multiple anticancer drugs, was dramatically increased in spgp[sup -/-] mice. Kinetic studies showed ATP-dependent bile acid transport in canalicular membrane vesicles isolated from control mice but not in those from spgp[sup -/-] mice. To reconcile this difference between in vivo data and in vitro canalicular membrane transport measurement, membrane vesicles containing highly overexpressed hamster Pgp (a homolog of mouse mdr1a) was used to demonstrate ATP-dependent bile acid transport. The kinetic parameters of Pgp-mediated ATP-dependent taurocholate transport were compatible with a low-affinity transporter. Taken together, these studies have provided evidence that Mdrla is capable of transporting bile acids, which has not been recognized previously.

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