ABCB4

ABCB4 Full Name

ATP-binding cassette, sub-family B (MDR/TAP), member 4

ABCB4 Introduction

ATP Binding Cassette Subfamily B Member 4 (ABCB4), also known as Multidrug Resistance Protein 3 (MDR3), is an integral membrane protein belonging to the ATP-binding cassette (ABC) transporter superfamily. Primarily expressed in the canalicular membrane of hepatocytes, ABCB4 functions as a phospholipid flippase that mediates the translocation of phosphatidylcholine (PC) from the inner to the outer leaflet of the hepatocyte canalicular membrane. This process is essential for the formation and secretion of bile, as phosphatidylcholine acts as a detergent to solubilize bile salts and prevent their cytotoxic effects on hepatocytes. ABCB4 contains two nucleotide-binding domains (NBDs) that hydrolyze ATP to provide energy for phospholipid transport, and two transmembrane domains (TMDs) that form the phospholipid translocation channel. Today, ABCB4 is recognized not only as a key regulator of bile formation but also as a critical gene associated with hepatic diseases, with profound implications for hepatology, pharmacology, and clinical medicine.

Figure 1.The structure of ABCB4.

Molecular Structure and Transport Mechanism
ABCB4 shares the canonical ABC transporter architecture: four core domains comprising two transmembrane domains (TMD1 and TMD2, each with six membrane-spanning helices) and two nucleotide-binding domains (NBD1 and NBD2). The TMDs form the substrate translocation pathway, while the NBDs bind and hydrolyze ATP to drive conformational changes. Unlike the closely related ABCB1 (which transports a broad range of hydrophobic xenobiotics), ABCB4 has high substrate specificity for phosphatidylcholine, particularly species with unsaturated fatty acyl chains (e.g., PC 18:1/18:1 and PC 16:0/18:1). ABCB4 likely functions as a homodimer or higher-order oligomer, as observed for other ABC exporters. The transporter is regulated by post-translational modifications including phosphorylation (by PKC and PKA), glycosylation at Asn residues in the extracellular loops, and interactions with PDZ-domain proteins that anchor it to the canalicular cytoskeleton.

Clinical Significance of ABCB4 Dysregulation
Dysregulation of ABCB4 expression or function is closely associated with a spectrum of hepatic diseases, primarily cholestatic disorders. Genetic mutations in ABCB4 are the primary cause of progressive familial intrahepatic cholestasis type 3 (PFIC3), an autosomal recessive disorder characterized by impaired bile formation, cholestasis, pruritus, and progressive liver fibrosis, which often leads to end-stage liver disease in childhood or adolescence. Heterozygous ABCB4 mutations are also linked to other cholestatic conditions, including intrahepatic cholestasis of pregnancy (ICP), primary biliary cholangitis (PBC), and primary sclerosing cholangitis (PSC). Reduced ABCB4 expression is observed in chronic cholestatic diseases and hepatocellular carcinoma (HCC), where it contributes to bile duct damage and disease progression. The estimated incidence of PFIC3 is approximately 1 in 100,000–200,000 individuals worldwide, while heterozygous ABCB4 mutations are more common, affecting up to 1% of the population in some regions. ABCB4 has emerged as a key diagnostic biomarker for cholestatic diseases, and its targeting holds promise for the development of novel therapies for these conditions.

Alternate Names for ABCB4

ABCB4; ATP-binding cassette, sub-family B (MDR/TAP), member 4; MDR3, PGY3; multidrug resistance protein 3; GBD1; MDR2; PFIC 3; P-glycoprotein 3; multiple drug resistance 3; ATP-binding cassette sub-family B member 4

• Immunohistochemistry Antibodies for Pathology