FIC1, the protein affected in two forms of hereditary cholestasis, is localized in the cholangiocyte and the canalicular membrane of the hepatocyte

Background/Aims : FIC1 (familial intrahepatic cholestasis 1) is affected in two clinically distinct forms of hereditary cholestasis, namely progressive familial intrahepatic cholestasis type 1 (PFIC1) and benign recurrent intrahepatic cholestasis. Here we examined the subcellular localization of this protein within the liver. Methods : Antibodies raised against different epitopes of human FIC1 were used for immunoblot analysis and immunohistochemical detection of FIC1. Results : Immunoblot analysis of intestine and liver tissue extracts from human, rat and mouse origin indicated that the antibodies raised against FIC1 specifically detected FIC1 as a 140-kDa protein. In the liver homogenate of a PFIC1 patient, FIC1 could not be detected. Analysis of isolated rat liver membrane vesicles indicated that this protein is predominantly present in the canalicular membrane fraction. Immunohistochemical detection of the protein in liver sections confirmed that FIC1 was present in the canalicular membrane, whereas no staining was observed in the PFIC1 patients liver. Double label immunofluorescence of murine liver revealed that FIC1 colocalized with cytokeratin 7 in cholangiocytes. Conclusions : The localization of FIC1 in the canalicular membrane and cholangiocytes suggests that it may directly or indirectly play a role in bile formation since mutations in FIC1 are associated with severe symptoms of cholestasis.