Regulation of Sterol Regulatory Element-binding Proteins in Hamster Intestine by Changes in Cholesterol Flux

Abstract

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 cholesterol-depleted 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.