[Development of new therapeutic strategy for transporter-related diseases]

Significant technological advances in gene sequence analysis and construction of genetically-modified animals during the last two decades made it possible to reveal that many transporters are associated with diseases. The bile salt export pump (BSEP/ABCB11), a member of the family of ATP-binding cassette transporters, is localized on the canalicular membrane of hepatocytes and predominantly mediates the biliary excretion of bile salts. A hereditary defect of BSEP results in severe cholestasis called progressive familial intrahepatic cholestasis type 2 (PFIC2). Without liver transplantation, this disease progresses to liver failure and death before adulthood; therefore the development of new, less invasive medical therapy for PFIC2 is of the highest priority. We have previously shown that in many cases of PFIC2 patients, the dysfunction of BSEP is attributable to decreased BSEP expression on the hepatocanalicular membrane and that 4-phenylbutyrate (4PB), an approved drug for urea cycle disorder, may be a compound with potential to restore BSEP expression. This drug inhibits ubiquitination of cell surface-resident BSEP and thereby its clathrin-mediated endocytosis through the AP2 adaptor complex, leading to increase in BSEP expression on the canalicular membrane. Clinical studies to investigate the efficacy of 4PB in the treatment of PFIC2 revealed that 4PB therapy biochemically and histologically improved liver function without any side effect. Therefore, 4PB therapy may become the preferred choice, instead of liver transplantation, for PFIC2 patients. The strategy employed and findings in this study would be valuable for the drug development of transporter-related disorders.