Overexpression of ABCG5 and ABCG8 promotes biliary cholesterol secretion and reduces fractional absorption of dietary cholesterol

Abstract

Introduction Multiple physiological mechanisms limit the entry of dietary sterols into the bloodstream. Excess cholesterol is removed from the body either by direct secretion into the bile or after conversion to bile acids. Although the mechanisms by which dietary sterols are absorbed by the intestine and secreted into the bile have been well characterized physiologically, the machinery responsible for these two processes has not been molecularly defined. The most abundant sterols in the human diet are cholesterol, the principal animal-derived sterol, and sitosterol, the major plant sterol. These two sterols, although structurally very similar, are handled quite differently by the intestine and liver of normal mammals. The absorption of sitosterol is significantly more limited than that of cholesterol. Humans absorb less than 5% of dietary sitosterol (1), and the small amount of sitosterol that reaches the liver is preferentially secreted into the bile (2). Consequently, plasma levels of sitosterol are very low (<1 mg/dl) in normal individuals. A much higher proportion (45-55%) of dietary cholesterol is absorbed by the proximal small intestine in humans, and the fractional excretion of sterols into the bile is lower for cholesterol than for sitosterol (3). Recently, a critical component of the transport machinery for dietary sterols was revealed by the finding that mutations in the genes encoding the ATP-binding cassette (ABC) half-transporters ABCG5 and ABCG8 cause sitosterolemia, a rare autosomal reces-sive disorder of sterol metabolism (4, 5). Patients with sitosterolemia have increased fractional absorption and decreased biliary secretion of all dietary neutral sterols (3, 6-8), which invariably leads to dramatically elevated plasma levels of sitosterol and other plant sterols. Most sitosterolemic individuals are also hyper-cholesterolemic (9). The disease phenotypically resembles homozygous familial hypercholesterolemia in that both diseases are characterized by the development of xanthomas in childhood accompanied by premature coronary atherosclerosis (6, 9, 10). ABCG5 and ABCG8 are expressed predominantly in hepatocytes and enterocytes in the proximal small intestine of Two ATP-binding cassette (ABC) transporters, ABCG5 and ABCG8, have been proposed to limit sterol absorption and to promote biliary sterol excretion in humans. To test this hypothesis, a P1 clone containing the human ABCG5 and ABCG8 genes was used to generate transgenic mice. The transgenes were expressed primarily in the liver and small intestine, mirroring the expression pattern of the endogenous genes. Transgene expression only modestly affected plasma and liver cholesterol levels but profoundly altered cholesterol transport. The fractional absorption of dietary cholesterol was reduced by about 50%, and biliary cholesterol levels were increased more than fivefold. Fecal neutral sterol excretion was increased three-to sixfold and hepatic cholesterol synthesis increased two-to fourfold in the transgenic mice. No significant changes in the pool size, composition, and fecal excretion of bile acids were observed in the transgenic mice. Transgene expression attenuated the increase in hepatic cholesterol content induced by consumption of a high cholesterol diet. These results demonstrate that increased expression of ABCG5 and ABCG8 selectively drives biliary neutral sterol secretion and reduces intestinal cholesterol absorption, leading to a selective increase in neutral sterol excretion and a compensatory increase in cholesterol synthesis.