Regulation of gene expression of ATP-binding cassette transporter (ABC)A1 and ABCG1 by liver X receptor/retinoid X receptor (LXR/RXR) ligands was investigated in the human intestinal cell line CaCo-2. Neither the RXR ligand, 9-cis retinoic acid, nor the natural LXR ligand 22-hydroxycholesterol alone altered ABCA1 mRNA levels. When added together, ABCA1 and ABCG1 mRNA levels were increased 3-and 7-fold, respectively. T0901317, a synthetic non-sterol LXR agonist, increased ABCA1 and ABCG1 gene expression 11-and 6-fold, respectively. ABCA1 mass was increased by LXR/RXR activation. T0901317 or 9-cis retinoic acid and 22-hydroxycholesterol increased cholesterol efflux from basolateral but not apical membranes.

A control chow diet or diets containing 1% cholesterol (cholesterol-enriched) or 4% cholestyramine and 0.15% lovastatin (cholesterol-depletion) were fed to hamsters for 2 weeks. Sterol regulatory element-binding protein (SREBP)-1a, SREBP-1c, SREBP-2, 3-hydroxy-3-methylglutaryl-coenzyme A reductase, 3-hydroxy-3-methylglutaryl-coenzyme A synthase, and LDL receptor mRNA levels and SREBP-1 and -2 protein expression were estimated in villus cell populations from duodenum, jejunum, and ileum. SREBP-1a was a minor transcript in hamster intestine, and its gene expression was not altered by changes in dietary cholesterol flux. In contrast, SREBP-1c gene expression was increased by dietary cholesterol and decreased by cholesterol depletion. mRNA levels for SREBP-2 and the other sterol-responsive genes were increased in intestines of animals on the cholesterol depletion diet but minimally suppressed if at all, by the diet enriched in cholesterol. In general, the amount of the precursor form of SREBP-1 was higher in cells of the upper villus and lower in cells of the lower villus. SREBP-2 precursor was higher in cells of the lower villus and lower in cells of the upper villus. Protein expression of precursor correlated with the location of gene expression for SREBPs. The amount of precursor mass of SREBP-2 was not altered by cholesterol feeding but was increased by cholesterol depletion. The mature form of SREBP-2 was in very low abundance and difficult to detect in intestines of animals fed control chow or cholesterol. It was readily detectable and increased in intestines of animals on the cholesterol-depletion diet. The diets did not significantly alter the amount of precursor or mature forms of SREBP-1. Cholesterol feeding had no effect on cholesterol or fatty acid synthesis, whereas synthesis of these lipids was increased in intestines of hamsters on the cholesteroldepleted diet. These results suggest that SREBP-1a has little or no role in regulating intestinal cholesterol synthesis. It is postulated that under basal conditions, SREBP-1c regulates intestinal fatty acid synthesis and SREBP-2 regulates cholesterol synthesis. Following marked changes in cholesterol flux across the intestine, SREBP-2 assumes the role of SREBP-1 and regulates both cholesterol and fatty acid synthesis in intestine.The cholesterol content of all cells is tightly controlled. When cholesterol is in excess, cells decrease both lipoprotein cholesterol uptake and cholesterol synthesis to prevent accumulation of this potentially toxic sterol. In contrast, when cells have insufficient cholesterol to meet their sterol requirements, lipoprotein cholesterol uptake is increased as are rates of cholesterol biosynthesis. This end product feedback mechanism of regulating the amount of cellular cholesterol has been extensively studied and characterized (for a review, see Ref. 1). Understanding of how a cell senses and regulates the amount of cholesterol has been advanced by the characterization of two transcription factors called sterol regulatory element-bi...

Possible mechanisms for the cholesterol-lowering effects of plant stanol esters were addressed by feeding hamsters diets containing stanol esters, cholesterol, or cholestyramine/lovastatin. ABCA1, ATP binding cassette G1 (ABCG1), ABCG5, ABCG8, and Niemann-Pick C1-like 1 (NPC1L1) mRNA levels were then estimated in duodenum, jejunum, and ileum. Plasma cholesterol was decreased by 36% and 94% in animals fed stanol esters and cholestyramine/lovastatin, respectively. Cholesterol feeding increased plasma cholesterol by 2.5-fold. Plasma plant sterols were unchanged by stanol ester feeding but became undetectable by feeding cholestyramine/lovastatin. Cholesterol and stanols accumulated in enterocytes of animals fed cholesterol and stanol esters, respectively. ABCG5 and ABCG8 mRNA levels were decreased by stanol esters and cholestyramine/lovastatin. Cholesterol feeding markedly increased ABCA1 and ABCG1 expression and modestly increased ABCG5/ABCG8. NPC1L1 mRNA was not significantly altered by any of the diets. Most evidence, however, points to an effect at the level of the intestine leading to a decrease in cholesterol absorption (4-7). In the lumen of the intestine, plant sterols compete with cholesterol for space within bile salt micelles. Without micellar solubilization, cholesterol is poorly absorbed, if at all. By displacing cholesterol from the micelle, plant sterols impede the absorption of cholesterol (8-10). Because dietary cholesterol contributes to LDL cholesterol and decreasing LDL cholesterol is proven to be "cardioprotective," the food industry is taking advantage of this property by packaging plant sterols into a variety of different foodstuffs.Recommending the ingestion of plant sterols would not be a sensible way to decrease plasma cholesterol and thus, the risk of atherosclerosis, if plant sterols themselves were readily absorbed. Fortunately, this is not the case. Despite structurally differing from cholesterol by only a methyl (campesterol) or an ethyl ( ␤ -sitosterol) group attached to the C-24 position of the side chain, these major plant sterols are absorbed to a much lesser extent than cholesterol (for reviews, see Refs. 11,12). The reasons for this are not clear. Recently, mutations in the tandem genes ATP binding cassette G5 (Abcg5) and Abcg8 were shown to cause the rare autosomal recessive disorder ␤ -sitosterolemia (13,14). Individuals with this disorder have impaired biliary secretion of sterols, "hyperabsorb" sterols, have increased plasma and tissue levels of plant sterols, and have an increased risk of atherosclerosis. It is speculated that in the intestine, the proteins encoded by these genes function as sterol transporters, facilitating the efflux of absorbed plant and animal sterols into the lumen of the gut (13). Although an attractive hypothesis, this has not been demonstrated at the cellular level.

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Regulation of sterol regulatory element-binding proteins (SREBPs) by fatty acid flux was investigated in CaCo-2 cells. Cells were incubated with 1 mM taurocholate with or without 250 microM 18:0, 18:1, 18:2, 20:4, 20:5 or 22:6 fatty acids. Fatty acid synthase (FAS) and acetyl-CoA carboxylase mRNA levels and gene and protein expression of SREBPs were estimated. 18:2, 20:4, 20:5 and 22:6 fatty acids decreased the amount of mature SREBP-1 and mRNA levels of SREBP-1c, SREBP-1a, FAS and acetyl-CoA carboxylase. SREBP-2 gene or mature protein expression was not altered. Liver X receptor (LXR) activation by T0901317 increased gene expression of SREBP-1c, SREBP-1a, FAS and acetyl-CoA carboxylase without altering SREBP-2. 20:5, but not 18:1, prevented the full expression of SREBP-1c mRNA by T0901317. T0901317 increased SREBP-1 mass without altering the mass of mature SREBP-2. Although only 18:2, 20:4, 20:5 and 22:6 suppressed SREBP-1, acetyl-CoA carboxylase and FAS expression, all fatty acids decreased the rate of fatty acid synthesis. T0901317 increased endogenous fatty acid synthesis yet did not increase secretion of triacylglycerol-rich lipoproteins. In CaCo-2 cells, polyunsaturated fatty acids decrease gene and protein expression of SREBP-1 and FAS mRNA, probably through interference with LXR activity. Since all fatty acids decreased fatty acid synthesis, mechanisms other than changes in SREBP-1c expression must be entertained. Increased endogenous fatty acid synthesis does not promote triacylglycerol-rich lipoprotein secretion.