The long term objective of this project is to enhance our understanding of the physiological regulation of cholesterol biosynthesis and degradation. Regulation of these processes play major roles in achieving cholesterol homeostasis. Elevated serum cholesterol is a major risk factor in the development of atherosclerotic vascular diseases. The regulation of hepatic HMG-CoA reductase and cholesterol 7alpha hydroxylase gene expression by hormones and dietary factors will be investigated using various rat models. These two enzymes catalyze the rate-limiting reactions of cholesterol and bile acid synthesis. The basis for the rapid increase in cholesterol 7alpha hydroxylase mRNA levels in response to thyroid hormone will be investigated. Rates of transcription will be determined. The promoter region of the cholesterol 7alpha hydroxylase gene will be analyzed to determine whether putative thyroid responsive elements actually function as such. The effects of other hormones such as insulin, glucagon and glucocorticoids on cholesterol 7alpha hydroxylase mRNA levels will be examined. Enzyme activity and protein mass will be measured to determine whether changes in mRNA levels fully account for changes in cholesterol 7alpha hydroxylase activity. The effects of cholesterol and bile acid feeding on rates of transcription will be determined. In terms of HMG-CoA reductase, the possible effects of insulin and glucagon on transcription will be determined. The mechanism(s) by which thyroid hormone and glucocorticoids affect HMG-CoA reductase mRNA stability will be studied. The basis for the marked decrease in HMG-CoA reductase activity without a significant change in mRNA levels that occurs in response to cholesterol feeding will be examined. Possible effects on reductase protein levels and activation/inactivation of the enzyme by phosphorylation or thiol modification will be investigated. Certain other enzymes in the cholesterol biosynthetic pathway such as HMG-CoA synthetase, farnesyl pyrophosphate synthetase and mevalonate kinase which are felt to be coordinately regulated with HMG-CoA reductase will be investigated. The degree of similarity of mechanisms of hormonal and dietary regulation of these enzymes will be determined.

National Institute of Health (NIH)
National Heart, Lung, and Blood Institute (NHLBI)
Research Project (R01)
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Metabolism Study Section (MET)
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University of South Florida
Schools of Medicine
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Li, Hai; Chen, Frank; Shang, Quan et al. (2005) FXR-activating ligands inhibit rabbit ASBT expression via FXR-SHP-FTF cascade. Am J Physiol Gastrointest Liver Physiol 288:G60-6
Xu, Guorong; Li, Hai; Pan, Lu-Xing et al. (2003) FXR-mediated down-regulation of CYP7A1 dominates LXRalpha in long-term cholesterol-fed NZW rabbits. J Lipid Res 44:1956-62
Xu, Guorong; Pan, Lu-xing; Erickson, Sandra K et al. (2002) Removal of the bile acid pool upregulates cholesterol 7alpha-hydroxylase by deactivating FXR in rabbits. J Lipid Res 43:45-50
Xu, Guorong; Pan, Lu-Xing; Li, Hai et al. (2002) Regulation of the farnesoid X receptor (FXR) by bile acid flux in rabbits. J Biol Chem 277:50491-6
Xu, G; Salen, G; Shneider, B L et al. (2001) Cholecystectomy prevents expansion of the bile acid pool and inhibition of cholesterol 7alpha-hydroxylase in rabbits fed cholesterol. J Lipid Res 42:1438-43
Xu, G; Salen, G; Shefer, S et al. (2001) Plant stanol fatty acid esters inhibit cholesterol absorption and hepatic hydroxymethyl glutaryl coenzyme A reductase activity to reduce plasma levels in rabbits. Metabolism 50:1106-12
Xu, G; Shneider, B L; Shefer, S et al. (2000) Ileal bile acid transport regulates bile acid pool, synthesis, and plasma cholesterol levels differently in cholesterol-fed rats and rabbits. J Lipid Res 41:298-304
Xu, G; Salen, G; Shefer, S et al. (1999) Increasing dietary cholesterol induces different regulation of classic and alternative bile acid synthesis. J Clin Invest 103:89-95
Xu, G; Salen, G; Shefer, S et al. (1998) Regulation of classic and alternative bile acid synthesis in hypercholesterolemic rabbits: effects of cholesterol feeding and bile acid depletion. J Lipid Res 39:1608-15
Xu, G; Servatius, R J; Shefer, S et al. (1998) Relationship between abnormal cholesterol synthesis and retarded learning in rats. Metabolism 47:878-82

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