Abstract
The main player in biliary cholesterol secretion is the heterodimeric transporter complex, ABCG5/ABCG8, the function of which is necessary for the majority of sterols secreted into bile. It is not clear whether the primary step in this process is flopping of cholesterol from the inner to the outer leaflet of the canalicular membrane, with desorption by mixed micelles, or decreasing of the activation energy required for cholesterol desorption from the outer membrane leaflet. In this study, we investigated these mechanisms by infusing Abcg8(+/+), Abcg8(+/-), and Abcg8(-/-) mice with hydrophilic and hydrophobic bile salts. In Abcg8(-/-) mice, this failed to substantially stimulate biliary cholesterol secretion. Infusion of the hydrophobic bile salt taurodeoxycholate also resulted in cholestasis, which was induced in Abcg8(-/-) mice at a much lower infusion rate compared with Abc8(-/-) and Abcg8(+/-) mice, suggesting a reduced cholesterol content in the outer leaflet of the canalicular membrane. Indeed, isolation of canalicular membranes revealed a reduction of 45% in cholesterol content under these conditions in Abcg8(-/-) mice. Our data support the model that ABCG5/ABCG8 primarily play a role in flopping cholesterol (and sterols) from the inner leaflet to the outer leaflet of the canalicular membrane.
Highlights
The main player in biliary cholesterol secretion is the heterodimeric transporter complex, ABCG5/ABCG8, the function of which is necessary for the majority of sterols secreted into bile
Wittenburg and Carey [15] suggested that the ABCG5/ABCG8 heterodimer acts as a floppase, defined as the translocation of cholesterol from the inner to the outer leaflet of the canalicular membrane, in a manner similar to the floppase mechanism shown for the translocation and secretion of phosphatidylcholine (PC) into bile mediated by ABCB4 [16,17,18]
To investigate whether biliary cholesterol secretion could be induced by hydrophobic bile salts in the absence of Abcg8, experiments were performed in which Abcg81/1, Abcg81/2, and Abcg82/2 mice were infused with increasing concentrations of the hydrophobic bile salt taurocholic acid (TC) or taurodeoxycholic acid (TDC)
Summary
The main player in biliary cholesterol secretion is the heterodimeric transporter complex, ABCG5/ABCG8, the function of which is necessary for the majority of sterols secreted into bile. Times ranging from 50 min [19] to ,1 s [20], depending on the method used to quantify the flip-flop rates This spontaneous translocation activity of cholesterol led Small [21] to propose another role for ABCG5/ABCG8 in cholesterol secretion. He suggested that the heterodimer is involved in decreasing the energy required to release cholesterol from the outer leaflet of the canalicular membrane [21]. On the other hand, when the heterodimer is a floppase, the outer leaflet of the canalicular membrane should be low in cholesterol in the absence of Abcg5/Abcg and sensitive to the detergent action of hydrophobic bile salts
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