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...

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.

IntroductionMultiple 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 ATPbinding cassette (ABC) half-transporters ABCG5 and ABCG8 cause sitosterolemia, a rare autosomal recessive 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 hypercholesterolemic (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 twoto fourfold in the transgenic mice. No significant changes in the pool s...

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Mice without oxysterol 7␣-hydroxylase, an enzyme of the alternate bile acid synthesis pathway with a sexually dimorphic expression pattern, were constructed by the introduction of a null mutation at the Cyp7b1 locus. Animals heterozygous (Cyp7b1 ؉/؊ ) and homozygous (Cyp7b1 ؊/؊ ) for this mutation were grossly indistinguishable from wild-type mice. Plasma and tissue levels of 25-and 27-hydroxycholesterol, two oxysterol substrates of this enzyme with potent regulatory actions in cultured cells, were markedly elevated in Cyp7b1 ؊/؊ knockout animals. Parameters of bile acid metabolism as well as plasma cholesterol and triglyceride levels in male and female Cyp7b1 ؊/؊ mice were normal. The cholesterol contents of major tissues were not altered. In vivo sterol biosynthetic rates were unaffected in multiple tissues with the exception of the male kidney, which showed a ϳ40% decrease in de novo synthesis versus controls. We conclude that the major physiological role of the CYP7B1 oxysterol 7␣-hydroxylase is to metabolize 25-and 27-hydroxycholesterol and that loss of this enzyme in the liver is compensated for by increases in the synthesis of bile acids by other pathways. A failure to catabolize oxysterols in the male kidney may lead to a decrease in de novo sterol synthesis.

The synthesis of 7␣-hydroxylated bile acids from oxysterols requires an oxysterol 7␣-hydroxylase encoded by the Cyp7b1 locus. As expected, mice deficient in this enzyme have elevated plasma and tissue levels of 25-and 27-hydroxycholesterol; however, levels of another major oxysterol, 24-hydroxycholesterol, are not increased in these mice, suggesting the presence of another oxysterol 7␣-hydroxylase. Here, we describe the cloning and characterization of murine and human cDNAs and genes that encode a second oxysterol 7␣-hydroxylase. The genes contain 12 exons and are located on chromosome 6 in the human (CYP39A1 locus) and in a syntenic position on chromosome 17 in the mouse (Cyp39a1 locus). CYP39A1 is a microsomal cytochrome P450 enzyme that has preference for 24-hydroxycholesterol and is expressed in the liver. The levels of hepatic CYP39A1 mRNA do not change in response to dietary cholesterol, bile acids, or a bile acid-binding resin, unlike those encoding other sterol 7␣-hydroxylases. Hepatic CYP39A1 expression is sexually dimorphic (female > male), which is opposite that of CYP7B1 (male > female). We conclude that oxysterol 7␣-hydroxylases with different substrate specificities exist in mice and humans and that sexually dimorphic expression patterns of these enzymes in the mouse may underlie differences in bile acid metabolism between the sexes.Two metabolic pathways that differ in their initial steps produce 7␣-hydroxylated bile acids (1). In one pathway, cholesterol (5-cholesten-3␤-ol) is first converted into 7␣-hydroxycholesterol (cholest-5-ene-3␤,7␣-diol) by the enzyme cholesterol 7␣-hydroxylase, which is encoded by the Cyp7a1 gene in mice. In the other pathway, cholesterol is first converted into one of several oxysterols prior to being 7␣-hydroxylated by oxysterol 7␣-hydroxylase, which is encoded by the Cyp7b1 gene. The 7␣-hydroxylated intermediates produced by these different initiating steps are subsequently converted into primary bile acids by a series of shared enzymes in the liver (2).Cholesterol 7␣-hydroxylase shows a marked preference for cholesterol as a substrate and is only weakly active against other sterols (3), whereas oxysterol 7␣-hydroxylase prefers 25-hydroxycholesterol (cholest-5-ene-3␤,25-diol) and 27-hydroxycholesterol (cholest-5-ene-3␤,27-diol) (4, 5). In agreement with the latter preference, Cyp7b1 knockout mice accumulate these two oxysterols in their plasma and tissues (6). The levels of the other major oxysterol, 24-hydroxycholesterol (cholest-5-ene-3␤,24-diol), are near normal in these animals (6). In contrast, a human with a complete absence of oxysterol 7␣-hydroxylase activity accumulated 24-hydroxycholesterol as well as 25-and 27-hydroxycholesterol in his plasma (7). These observations suggest that the human enzyme, unlike the mouse enzyme, may act on all three oxysterol substrates.The observed differences in the oxysterol accumulation patterns in mice and humans that express no oxysterol 7␣-hydroxylase are not due to differences in the biosynthesis of oxysterols, as cholester...

Cholesterol is converted into dozens of primary and secondary bile acids through pathways subject to negative feedback regulation mediated by the nuclear receptor farnesoid X receptor (FXR) and other effectors. Disruption of the sterol 12α-hydroxylase gene (Cyp8b1) in mice prevents the synthesis of cholate, a primary bile acid, and its metabolites. Feedback regulation of the rate-limiting biosynthetic enzyme cholesterol 7α-hydroxylase (CYP7A1) is lost in Cyp8b1–/– mice, causing expansion of the bile acid pool and alterations in cholesterol metabolism. Expression of other FXR target genes is unaltered in these mice. Cholate restores CYP7A1 regulation in vivo and in vitro. The results implicate cholate as an important negative regulator of bile acid synthesis and provide preliminary evidence for ligand-specific gene activation by a nuclear receptor

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