Bile salt–dependent efflux of cellular phospholipids mediated by ATP binding cassette protein B4
✍ Scribed by Shin-ya Morita; Aya Kobayashi; Yasukazu Takanezawa; Noriyuki Kioka; Tetsurou Handa; Hiroyuki Arai; Michinori Matsuo; Kazumitsu Ueda
- Publisher
- John Wiley and Sons
- Year
- 2007
- Tongue
- English
- Weight
- 450 KB
- Volume
- 46
- Category
- Article
- ISSN
- 0270-9139
No coin nor oath required. For personal study only.
✦ Synopsis
Human ABCB4 (multidrug resistance [MDR]3 P-glycoprotein) is expressed in the canalicular membrane of the hepatocyte. ABCB4 has been shown to be required for phosphatidylcholine (PC) secretion into the bile and to translocate PC across the plasma membrane. To further investigate the function of ABCB4, we established a cell line stably expressing ABCB4 (human embryonic kidney [HEK]/ABCB4). The efflux of phospholipids from HEK/ ABCB4 cells was remarkably increased by the addition of taurocholate. In addition, the cholesterol efflux from HEK/ABCB4 cells was also enhanced in the presence of taurocholate. Light scattering measurements suggested that the taurocholate monomer plays an important role in ABCB4-mediated lipid secretion. On the other hand, the efflux of phospholipids and cholesterol was not mediated by ABCB1 (MDR1) even in the presence of taurocholate. Taurocholate promoted the efflux of phospholipids and cholesterol from HEK/ABCB4 cells more efficiently than glycocholate and cholate. ABCB4-K435M and ABCB4-K1075M, Walker A lysine mutants, did not mediate the phospholipid and cholesterol efflux in the presence of taurocholate, suggesting that ATP hydrolysis is essential for the efflux. Verapamil completely inhibited the taurocholate-dependent efflux of phospholipids and cholesterol from HEK/ABCB4 cells. Mass spectrometry revealed that, in the presence of taurocholate, HEK/ABCB4 cells preferentially secreted PC compared to sphingomyelin. PC vesicles induced cholesterol diffusion from cell membrane, but did not accept cholesterol from ABCB4. Conclusion: ABCB4 mediates the efflux of phospholipids into the canalicular lumen in the presence of bile salts, and plays a crucial role in bile formation and lipid homeostasis. (HEPATOLOGY 2007;46:188-199.) B ile formation is important for cholesterol excretion from body as well as cholesterol absorption from the intestine, and thus it is critical for overall cholesterol homeostasis. 1 Several ABC proteins, expressed on the apical membranes of hepatocytes, are involved in the secretion of bile salts, phospholipids and cholesterol, and thus in canalicular bile formation. ABCB4, also called multidrug resistance 3 (MDR3) in humans or mdr2 in mice, is essential for the secretion of phosphatidylcholine (PC) into bile. 2 ABCB11, also called bile salt export pump or sister of P-glycoprotein, secretes bile salts into bile. ABCG5 and ABCG8 are considered to be the main transporters for secretion of biliary cholesterol. The function of ABCB4 is required for proper bile formation. Mice with homozygous disruption of the Abcb4 gene show almost complete absence of PC from their bile, 2,8-10 which causes segmental biliary strictures due to periductal fibrosis, fibro-obliteration of bile ducts, and spontaneous gallstone formation. Human ABCB4 mutations result in a wide spectrum of phenotypes, ranging from progressive familial intrahepatic cholestasis type 3 to adult cholestatic liver disorders