Disruption of Abcg5 and Abcg8 in mice reveals their crucial role in biliary cholesterol secretion

Abstract

Cholesterol and other sterols exit the body primarily by secretion into bile. In patients with sitosterolemia, mutations in either of two ATP-binding cassette (ABC) half-transporters, ABCG5 or ABCG8, lead to reduced secretion of sterols into bile, implicating these transporters in this process. To elucidate the roles of ABCG5 and ABCG8 in the trafficking of sterols, we disrupted Abcg5 and Abcg8 in mice (G5G8 ؊/؊ ). The G5G8 ؊/؊ mice had a 2-to 3-fold increase in the fractional absorption of dietary plant sterols, which was associated with an Ϸ30-fold increase in plasma sitosterol. Biliary cholesterol concentrations were extremely low in the G5G8 ؊/؊ mice when compared with wild-type animals (mean ‫؍‬ 0.4 vs. 5.5 mol͞ml) and increased only modestly with cholesterol feeding. Plasma and liver cholesterol levels were reduced by 50% in the chow-fed G5G8 ؊/؊ mice and increased 2.4-and 18-fold, respectively, after cholesterol feeding. These data indicate that ABCG5 and ABCG8 are required for efficient secretion of cholesterol into bile and that disruption of these genes increases dramatically the responsiveness of plasma and hepatic cholesterol levels to changes in dietary cholesterol content.ATP-binding cassette transporters ͉ sitosterolemia ͉ bile ͉ knockout mice S itosterolemia is a rare autosomal recessive disorder characterized by the accumulation of plant and animal sterols in blood and tissues (1, 2). Affected subjects with this disorder develop large deposits of cholesterol in their skin, tendons, and coronary arteries. The accumulation of plant and animal sterols in the blood is caused by an increase in the fractional absorption of sterols from the diet and a decrease in the secretion of sterols into the bile, which is the major route of exit of sterols from the body (3, 4).A striking feature of sitosterolemia is the precipitous fall in plasma cholesterol that follows reductions in dietary cholesterol intake, especially in young patients (5, 6). When normal individuals are switched from a high cholesterol, high fat diet to a low cholesterol, low fat diet, plasma levels of cholesterol fall Ϸ10-20%; in patients with sitosterolemia, plasma cholesterol can fall by Ͼ45% (5, 6). The hypercholesterolemia of sitosterolemia is also sensitive to treatment with bile-acid resins, which stimulate the conversion of cholesterol to bile acids, another pathway for removal of cholesterol from the body.The pathognomonic feature of sitosterolemia is the elevation in plasma sitosterol, the most abundant plant sterol (1). Sitosterolemic patients also accumulate other sterols in plasma including a variety of plant sterols (campesterol, stigmasterol, and avenasterol) and shellfish sterols (brassicasterol, 24-methylene cholesterol, and 22-dehydrocholesterol) (1, 7). In normal individuals these sterols are poorly absorbed and preferentially secreted into the bile (8-10). These sterols comprise only Ϸ1% of plasma and tissue sterols in normal individuals but Ϸ15% of circulating and tissue sterols in sitosterolemia (11).Studies of sitoste...

Down-regulation of hepatic and intestinal Abcg5 and Abcg8 expression associated with altered sterol fluxes in rats with streptozotocin-induced diabetes

“…Reduced hepatic expression of these "sitosterolaemia genes" coincided with a reduction of hepatobiliary cholesterol secretion, whereas their reduced intestinal expression was associated with an increased absorption of cholesterol as deduced from an indirect measure of the process (plasma plant sterol concentrations) and from calculation of the apparent absorption efficiency. Similar changes in cholesterol transport have been reported in sitosterolaemia patients [24] and in Abcg5/Abcg8-deficient mice [8]. Over-expression of the human genes in transgenic mice [7] and pharmacological induction of expression of the endogenous genes in wild-type mice [13] have been reported to have opposite effects, i.e., to stimulate biliary cholesterol excretion and to reduce intestinal cholesterol absorption.…”

“…Membranes were washed thrice in TTBS and incubated with horseradish peroxidase-conjugated V. W. Bloks et al: Down-regulation of hepatic and intestinal Abcg5 and Abcg8 expression associated 105 secretion of cholesterol into bile is coupled to that of phospholipids in a process that is, in part, controlled by bile salt secretion [6]. Recent studies, however, indicate that specific ABC transporters, i.e., Abcg5 and Abcg8, are involved in biliary cholesterol secretion [7,8]. The genes encoding these transporters are highly expressed in the liver [9].…”

Scavenger receptor class B type I mediates biliary cholesterol secretion independent of ATP-binding cassette transporter g5/g8 in mice

“…Thus far, the Abcg5/Abcg8 obligate heterodimer is considered the rate-controlling transporter system mediating biliary cholesterol secretion. 18,19 To investigate whether SR-BI would work in concert with or independent from Abcg5/Abcg8, Abcg5 knockout mice on a mixed genetic background were injected either with the SR-BI-expressing adenovirus or the control adenovirus AdNull. SR-BI overexpression resulted in decreased plasma cholesterol levels (84 Ϯ 2 mg/dL versus 10 Ϯ 8 mg/dL, P Ͻ 0.001; Fig.…”

“…It is generally accepted that phospholipid secretion mediated by ATP-binding cassette transporter b4 (Abcb4) is required for cholesterol to be secreted into bile as evidenced by the phenotype of the Abcb4 knockout mouse model, in which hepatic cholesterol secretion is virtually absent. 17 Regarding direct cholesterol secretion under steady-state conditions, the quantitatively highest contribution comes from the heterodimer Abcg5/Abcg8: Abcg5/Abcg8 knockout mice have a significant reduction in biliary cholesterol secretion, 18 whereas Abcg5/Abcg8 overexpression results in elevated biliary cholesterol levels. 19 However, in the absence of Abcb4, even high-level overexpression of Abcg5/Abcg8 does not increase biliary cholesterol secretion, 20 delineating the crucial role for biliary phospholipids in mixed micelles with bile salts as acceptors for cholesterol solubilization.…”

The sterol transporting heterodimer ABCG5/ABCG8 requires bile salts to mediate cholesterol efflux

“…The discovery that defects in the ABC transporters Abcg5 and Abcg8 were responsible for the disease sitosterolemia has provided considerable new insight [1,2]. Abrogating the activity of both or just one of the half transporters not only influenced plant sterol transport but also strongly decreased secretion of cholesterol into the bile [3][4][5]. The transporters require co-expression and probably accessory proteins to invoke trafficking from their site of synthesis in the ER to the apical site of hepatocytes and only the heterodimer shows catalytic activity [6,7].…”