Gene expression of sterol regulatory element-binding proteins in hamster small intestine

Gene expression of sterol regulatory elementbinding proteins 1a, 1c, and 2 (SREBP-1a, -1c, and -2) and of 3-hydroxy-3-methylglutaryl-coenzyme A (HMG-CoA) synthase, HMG-CoA reductase, and the low density lipoprotein (LDL) receptor was examined in hamster small intestine. SREBP-1c transcript predominated over SREBP-1a. mRNA levels for SREBP-1a, -1c, and -2, LDL receptor, and HMGCoA synthase were highest in jejunum and ileum. Expression of SREBP-1a and SREBP-1c was highest in cells of the upper villus and decreased in cells of the lower villus. Gene expression of SREBP-2 was lowest in cells of the upper villus and increased in cells of the lower villus. Ileal SREBP-2 gene expression was highest in cells of the midvillus. mRNA levels for HMG-CoA synthase and the LDL receptor followed a pattern similar to that of SREBP-2. A positive correlation existed between SREBP-2 gene expression and rates of cholesterol synthesis. Fatty acid synthesis was highest in jejunum and ileum, correlating positively with the expression of SREBP-1c. Sterol influx into intestinal cells decreased mRNA levels of SREBP-2, HMG-CoA reductase, HMG-CoA synthase, and LDL receptor. In ileum, sterol influx decreased gene expression of SREBP-1a and increased expression of SREBP-1c. The results suggest that SREBP-2 regulates cholesterol synthesis in the small intestine. SREBP-1a is a minor transcript and its expression does not correlate with cholesterol-synthesizing activity. SREBP-1c is a major transcript in small intestine and its expression along the length of the gut correlates with fatty acid synthesis. Sterols regulate gene expression of sterol-responsive genes, including SREBP-2, in small intestine. — Field, F. J., E. Born, S. Murthy, and S. N. Mathur. Gene expression of sterol regulatory element-binding proteins in hamster small intestine. J. Lipid Res. 2001. 42: 1–8. Supplementary key words cholesterol synthesis • fatty acid synthesis • HMG-CoA reductase • HMG-CoA synthase • LDL receptor The small intestine is important in maintaining total body cholesterol homeostasis. It is second only to the liver in the rate of de novo cholesterol synthesis, and in some animal models the intestine actually contributes more cholesterol to total body stores than does liver (1, 2). Newly synthesized cholesterol from the small intestine contributes to the plasma pool of cholesterol, suggesting that changes in intestinal cholesterol synthesis will directly alter plasma cholesterol levels (3). Moreover, by absorbing cholesterol and secreting triacylglycerol-rich lipoproteins, the small intestine is responsible for regulating the amount of dietary and biliary cholesterol entering the body. In addition to cholesterol synthesis, cholesterol absorption, and lipoprotein synthesis, the small intestine also takes up and degrades circulating low density lipoprotein (LDL) particles (4). Because of the complexity of the organ, understanding the regulation of cholesterol metabolism in the small intestine is not a simple matter. For example, cholesterol and fat-soluble vitamins are absorbed in the upper part of the small intestine; whereas structurally related molecules, such as bile acids, are absorbed in the distal ileum (5, 6). Fatty acid influx and lipoprotein assembly and secretion, which likely affect cholesterol metabolism, predominantly occur in the midgut. Functional differences also exist along the villus axis. Most nutrients, including cholesterol, are absorbed in the midto upper villus. In the lower regions of the villus and within the crypts, cells are rapidly dividing to replenish cells being lost in the upper regions of the villus. Cells of the crypts are not exposed to lumenal contents and thus, do not actively absorb cholesterol. These cells, however, do require sufficient cholesterol to meet their increased demands for new membrane synthesis. Thus, when investigating the regulation of cholesterol metabolism in the small intestine, the regional and cellular differences that exist within this organ must be considered. Our understanding of how a cell senses and regulates the amount of cholesterol has been advanced by the description of two transcription factors designated sterol regulatory element-binding proteins 1 and 2 (SREBP-1 and -2) (7). These proteins, derived from separate genes, regulate the transcription of genes that encode enzymes in the biosynthetic pathways of cholesterol and fatty acids (8–12). There are two isoforms of SREBP-1, SREBP-1a and SREBPAbbreviations: EDTA, ethylenediaminetetraacetic acid; HEPES, N -2-hydroxyethylpiperazineN 9 -2-ethanesulfonic acid; HMG-CoA, 3-hydroxy-3-methylglutaryl-coenzyme A; LDL, low density lipoproteins; SREBP, sterol regulatory element-binding proteins. 1 To whom correspondence should be addressed. at P E N N S T A T E U N IV E R S IT Y , on F ebuary 3, 2013 w w w . j.org D ow nladed fom