Canine ABCB4: Tissue expression and cDNA structure

The ABCB gene subfamily of ABC (ATP-binding cassette) transporters is responsible for transporting a wide spectrum of molecules including peptides, iron, bile salts, drugs, and phospholipids. In humans, ABCB4 appears to be exclusively expressed on the apical membrane of hepatocytes where it translocates phosphatidylcholine from the inner to the outer leaflet of the canalicular membrane. Functional alterations in the ABCB4 transporter are associated with a number of cholestatic syndromes in humans. Because of its role in biliary lipid homeostasis in humans, investigation of the ABCB4 gene in dogs is warranted. Thus, the full cDNA sequence of canine ABCB4 was elucidated and its mRNA and protein expression levels in tissues were determined. Canine ABCB4 consists of 3804 nucleotides spanning 26 exons and is 89% identical to human ABCB4. Expression of ABCB4 in canine liver supports a potential role for the protein in normal biliary function similar to that in humans. The function of ABCB4 expressed in brain tissue has yet to be determined.