ABC-transporters are localized in caveolin-1-positive and reggie-1-negative and reggie-2-negative microdomains of the canalicular membrane in rat hepatocytes

Abstract

The canalicular plasma membrane is constantly exposed to bile acids acting as detergents. Bile acids are essential to mediate release of biliary lipids from the canalicular membrane. Membrane microdomains (previously called lipid rafts) are biochemically defined by their resistance to detergent solubilization at cold temperature. We aimed to investigate the canalicular plasma membrane for the presence of microdomains, which could protect this membrane against the detergent action of bile acids. Highly purified rat liver canalicular plasma membrane vesicles were extracted with 1% Triton X-100 or 1% Lubrol WX at 4°C and subjected to flotation through sucrose step gradients. Both detergents yielded detergent-resistant membranes containing the microdomain markers alkaline phosphatase and sphingomyelin. However, cholesterol was resistant to Lubrol WX solubilization, whereas it was only marginally resistant to solubilization by Triton X-100. The microdomain marker caveolin-1 was localized to the canalicular plasma membrane domain and was resistant to Lubrol WX, but to a large extent solubilized by Triton X-100. The two additional microdomain markers, reggie-1 and reggie-2, were localized to the basolateral and canalicular plasma membrane and were partially resistant to Lubrol WX but resistant to Triton X-100. The canalicular transporters bile salt export pump, multidrug resistance protein 2, multidrug resistance-associated protein 2, and Abcg5 were largely resistant to Lubrol WX but were solubilized by Triton X-100. Conclusion: These results indicate the presence of two different types of microdomains in the canalicular plasma membrane: "Lubrol-microdomains" and "Triton-microdomains". "Lubrol-microdomains" contain the machinery for canalicular bile formation and may be the starting place for canalicular lipid secretion. (HEPATOLOGY 2009;49:1673-1682

The three-dimensional microanatomy of the pancreatic duct system in the Japanese monkey, Macaca fuscata, with special reference to fine proximal passages of a high species-specificity

“…Our observations confirmed the ample expansion of the contact surface of the duct epithelium with canaliculi closely resemble the liver bile canaliculi with regard to their constitution in the narrow grooves on lateral cell surfaces, dense coverage with microvilli, and frequent continuation into intracellular canaliculi (5,24). Recent membrane fraction analyses have shown the bile canaliculi to comprise lipid-enriched microdomains that accumulate channels and transporters essential for bile secretion (11,26). Similar membrane microdomains may operate stress (18).…”

The Role of the Sodium-Taurocholate Cotransporting Polypeptide (NTCP) and of the Bile Salt Export Pump (BSEP) in Physiology and Pathophysiology of Bile Formation

“…Hence, Bsep could partition into subdomains of the canalicular plasma membrane. This assumption is supported by the observation that Bsep is concentrated in canalicular microdomains, which are resistant to extraction by Lubrol WX [141]. Bsep is also expressed in subapical vesicles in close proximity to the canalicular plasma membrane [62,94].…”

Lipid Regulation of the ABCB1 and ABCG2 Multidrug Transporters

“…Both ABCB1 and ABCG2 have been reported to reside in detergentinsoluble rafts (Ismair et al, 2009;Orlowski, Martin, & Escargueil, 2006;Radeva, Perabo, & Sharom, 2005;Storch, Ehehalt, Haefeli, & Weiss, 2007) and in close interaction with cholesterol and caveolin. In the canalicular membranes of hepatocytes, several ABC transporters, including ABCB4 (MDR3), ABCB11 (BSEP, S-P-gp), ABCC2 (MRP2), and ABCG5/G8, have also been shown to reside in raft domains (Ismair et al, 2009).…”

“…In the canalicular membranes of hepatocytes, several ABC transporters, including ABCB4 (MDR3), ABCB11 (BSEP, S-P-gp), ABCC2 (MRP2), and ABCG5/G8, have also been shown to reside in raft domains (Ismair et al, 2009). The properties of transmembrane regions of ABC transporters, such as the length of transmembrane helices and/or the interaction of transmembrane helices with cholesterol, may actually favor the localization of these proteins in the raft domains.…”

“…ABCB1 has been reported to reside in Triton X-100-resistant rafts (Ismair et al, 2009;Orlowski et al, 2006) or in Brij 96-resistant rafts (Radeva et al, 2005), although ABCB1 was found to be an active drug transporter both in raft and in nonraft regions (Bucher, Besse, Kamau, Wunderli-Allenspach, & Kramer, 2005). Still, recent studies, examining the ratio of active state conformations of ABCB1 by the conformation sensitive UIC2 antibody, indicate that this protein may be more active in cholesterol-rich, caveolin-positive regions and thus may be regulated by factors altering raft assembly (Bacso et al, 2004).…”