Progressive familial intrahepatic cholestasis type 1 (PFIC1, Byler disease, OMIM 211600) is a severe inherited liver disease caused by mutations in ATP8B1. ATP8B1 is a member of the type 4 subfamily of P-type ATPases, which are phospholipid flippases. PFIC1 patients generally develop end-stage liver disease before the second decade of life. The disease is characterized by impaired biliary bile salt excretion, but the mechanism whereby impaired ATP8B1 function results in cholestasis is unclear. In a mouse model for PFIC1, we observed decreased resistance of the hepatocanalicular membrane to hydrophobic bile salts as evidenced by enhanced biliary recovery of phosphatidylserine, cholesterol, and ectoenzymes. In liver specimens from PFIC1 patients, but not in those from control subjects, ectoenzyme expression at the canalicular membrane was markedly deficient. In isolated mouse livers Atp8b1 deficiency impaired the transport of hydrophobic bile salts into bile. In conclusion, our study shows that Atp8b1 deficiency causes loss of canalicular phospholipid membrane asymmetry that in turn renders the canalicular membrane less resistant toward hydrophobic bile salts. The loss of phospholipid asymmetry may subsequently impair bile salt transport and cause cholestasis. Supplementary material for this article can be found on the HEPATOLOGY website (http://interscience.wiley.com/jpages/0270-9139/suppmat/ index.html). (HEPATOLOGY 2006;44:195-204.)

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MRP3 is an ABC transporter localized in the basolateral membrane of epithelial cells such as hepatocytes and enterocytes. In this study, the role of Mrp3 in drug disposition was investigated. Because Mrp3 preferentially transports glucuronide conjugates, we investigated the in vivo disposition of acetaminophen (APAP) and its metabolites. Mrp3 ؉/؉ and Mrp3 ؊/؊ knockout mice received APAP (150 mg/kg), and bile was collected. Basolateral and canalicular excretion of APAP was also assessed in the isolated perfused liver. In separate studies, mice received 400 mg APAP/kg for assessment of hepatotoxicity. No differences were found in the biliary excretion of APAP, APAP-sulfate, and APAPglutathione between Mrp3 ؉/؉ and Mrp3 ؊/؊ mice. However, 20-fold higher accumulation of APAPglucuronide (APAP-GLUC) was found in the liver of Mrp3 ؊/؊ mice. Concomitantly, plasma APAP-GLUC content in Mrp3 ؊/؊ mice was less than 10% of that in Mrp3 ؉/؉ mice. In addition, APAP-GLUC excretion in bile of Mrp3 ؊/؊ mice was tenfold higher than in Mrp3 ؉/؉ mice. In the isolated perfused liver, we also found a strong decrease of APAP-GLUC secretion into the perfusate of Mrp3 ؊/؊ livers. Plasma alanine aminotransferase (ALT), aspartate aminotransferase (AST), and histopathology showed that Mrp3 ؊/؊ mice are more resistant to APAP hepatotoxicity than Mrp3 ؉/؉ mice, which is most likely a result of the faster repletion of hepatic GSH. Substrates exported from hepatocytes by MRP1-3 include conjugates of glutathione, glucuronide, and sulfate as well as organic amphiphilic anions. MRP3, which is expressed in liver and gastrointestinal tract, 2,3 prefers glucuronide over glutathione conjugates. 4,5 Mice lacking Mrp3 were recently developed; these mice are viable, fertile, and have no apparent phenotype. 2 Because of the potential role of Mrp3 in basolateral secretion of glucuronidated drugs, the disposition of acetaminophen (APAP) in Mrp3 Ϫ/Ϫ mice was investigated.Acetaminophen is a popular analgesic and antipyretic that can produce acute liver failure with excessive dosing. 6,7 This drug is metabolized in the liver by conjugation with glucuronic acid and sulfate, while a reactive metabolite generated by CYP450 is detoxified by conjugation Abbreviations: MRP, multidrug resistance protein; APAP, acetaminophen; GSH,