Feedback regulation of hepatic 3-hydroxy-3-methylglutaryl-CoA reductase activity by dietary chlolesterol is not due to altered mRNA levels (original) (raw)
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Journal of Biological Chemistry
Feeding rats diets containing 2% cholesterol markedly reduced hepatic 3-hydroxy-3-methylglutaryl (HMG)-CoA reductase activity but had little effect on mRNA levels. Addition of mevalonolactone to the diet further decreased reductase activity independent of a change in mRNA levels. In contrast, farnesyl pyrophosphate synthetase mRNA levels and enzyme activity were decreased to similar degrees in response to dietary cholesterol. Addition of mevalonolactone to the diet did not further decrease farnesyl pyrophosphate synthetase activity. Dietary cholesterol and mevalonolactone had no effect on mRNA levels for "cellular nucleic acid-binding protein" which has been demonstrated to bind the sterol regulatory elements in the HMG-CoA reductase and farnesyl pyrophosphate synthetase promoters. Dietary cholesterol increased cholesterol 7 alpha-hydroxylase mRNA levels as expected. These results suggest that cholesterol-mediated feed-back regulation of hepatic HMG-CoA reductase gene expr...
Archives of Biochemistry and Biophysics, 1998
The level of gene expression at which dietary cholesterol exerts feedback regulation on hepatic 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase was investigated using young male Sprague-Dawley rats. Previous studies suggested that this regulation might be exerted posttranscriptionally. Thus, possible regulation at the levels of catalytic efficiency, protein turnover, and translation was investigated. To examine possible regulation at the level of catalytic efficiency, rats were placed on chow diets supplemented with 2% cholesterol and the rates of decline in hepatic HMG-CoA reductase activity and immunoreactive protein levels were determined. Both decreased slowly over a 72-h period. The catalytic efficiency did not change. These observations are inconsistent with phosphorylation-dephosphorylation or thiol-disulfide interchange as possible mechanisms. The possibility that dietary cholesterol might act by increasing the rate of turnover of HMG-CoA reductase protein was examined by determining the half-life of the enzyme in livers from rats consuming chow or chow supplemented with 2% cholesterol. The half-life of HMG-CoA reductase protein was not decreased in the animals receiving cholesterol, thus ruling out this possibility. Regulation at the level of translation was investigated by measuring the rate of HMG-CoA reductase protein synthesis in liver slices using [ 35 S]methionine and [ 35 S]cysteine. It was found that the rate of synthesis was reduced by over 80% in liver slices from rats fed a diet supplemented with 2% cholesterol. Similar results were obtained with liver slices from rats given mevalonolactone, which supplies both ste-rol and nonsterol endproducts. These results indicate that cholesterol regulates hepatic HMG-CoA reductase gene expression in rats primarily at the level of translation. . 2 Abbreviations used: HMG-CoA, 3-hydroxy-3-methylglutaryl coenzyme A; HMGR, HMG-CoA reductase; TCA, trichloroacetic acid; SRE, sterol response element; SREBP, sterol response element binding protein; PVDF, polyvinylidene fluoride; PMSF, phenylmethylsulfonyl fluoride; BSA, bovine serum albumin; CHO, Chinese hamster ovary.
Translational Regulation of Hepatic HMG-CoA Reductase by Dietary Cholesterol
Biochemical and Biophysical Research Communications, 1997
would be expected in animals fed cholesterol. Northern The question of whether dietary cholesterol exerts blotting analysis revealed that feeding 2% cholesterol feedback regulation on hepatic HMG-CoA reductase at to rats had little affect on hepatic HMG-CoA reductase the level of translation was examined by performing mRNA levels despite a reduction in enzyme activity polysome profile analysis. Liver polysomes from rats to three percent of control (9). This observation was fed 2% cholesterol in their diets for 3 days were comconfirmed by others . This implies that dietary chopared with those isolated from rats fed a normal chow lesterol exerts feedback regulation on hepatic HMGdiet. Northern blotting analysis of the individual frac-CoA reductase at a post-transcriptional level. If regula-
Experimental biology and medicine (Maywood, N.J.), 2006
The effect of the potent inhibitor of cholesterol absorption, ezetimibe, on serum cholesterol levels was tested in diabetic and thyroidectomized male Sprague-Dawley rats. Feeding diets supplemented with 1% cholesterol to the diabetic rats raised serum cholesterol levels from 132 to 514 mg/dl while decreasing hepatic 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase protein and mRNA levels. Addition of 10 mg/kg/day of ezetimibe to the diets of these animals lowered the serum cholesterol level to 90 mg/dl and produced a large compensatory increase in hepatic HMG-CoA reductase protein without significantly increasing mRNA levels, indicating a post-transcriptional effect. Hepatic LDL receptor protein levels in these diabetic rats were unaffected by ezetimibe treatment. In contrast, ezetimibe treatment of these young normal Sprague-Dawley rats, known to express high levels of hepatic HMG-CoA reductase, did not lower serum cholesterol levels. In thyroidectomized rats, dietary chol...
Hepatic HMG-CoA reductase expression and resistance to dietary cholesterol
Experimental biology and medicine (Maywood, N.J.), 2004
The premise that the intrinsic level of expression of hepatic 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase determines the relative sensitivity to the serum cholesterol raising action of dietary cholesterol was examined in 9 strains of rat. For further comparison purposes, hamsters were also examined. The basal expression of hepatic HMG-CoA reductase, extent of feedback regulation by cholesterol, and changes in serum cholesterol levels and the hepatic low-density lipoprotein (LDL) receptor in response to cholesterol challenge were determined in these animals. The Sprague-Dawley, Wistar-Furth, Spontaneously Hypertensive, Lewis, and Wistar-Kyoto rats were all very resistant to dietary cholesterol and exhibited hepatic HMG-CoA reductase activities above 150 pmol / min(-1) / mg(-1). The Buffalo, Brown Norway, and Copenhagen 2331 rats had hepatic HMG-CoA reductase activities below 90 pmol / min(-1) / mg(-1) and had increases in serum cholesterol levels ranging from 12 to 33 m...
We investigated the effects of cholesterol, cholestyramine, and taurocholate feeding on steady state specific activities and mRNA levels of hepatic 3-hydroxy-3-methylglutaryl (HMG)-CoA reductase and cholesterol 7a-hydroxylase in the rat. Interruption of the enterohepatic circulation of bile acids (cholestyramine feeding) increased total HMG-CoA reductase activity 5-fold. Cholesterol and taurocholate administration suppressed total microsomal HMG-CoA reductase activities 87 % and 65%, respectively. HMG-CoA reductase mRNA levels increased 3-fold with cholestyramine, did not decrease significantly with cholesterol feeding, but were markedly decreased after taurocholate treatment. Cholesterol 7a-hydroxylase activity increased 4-fold with cholestyramine and 29% during cholesterol feeding, but decreased 64% with taurocholate. Cholesterol 7ahydroxylase mRNA levels rose 150% and 50% with cholestyramine and cholesterol feeding, respectively, but decreased 73% with taurocholate. The administration of cholesterol together with taurocholate prevented the decline in cholesterol 7ahydroxylase mRNA levels, but inhibition of enzyme activity persisted ( -76%). Hepatic microsomal cholesterol concentrations increased 2-fold with cholesterol feeding but did not change with taurocholate or cholestyramine treatment.
Coordinate Regulation of Cholesterol 7α-Hydroxylase and HMG-CoA Reductase in the Liver
Subcellular Biochemistry, 1997
Cholesterol homeostasis in the liver is dependent upon the activity of a number of key structures: 3-hydroxy-3-methylglutaryl-CoA, HMG-CoA reductase (HMGR), acy 1 cholesterol acyltransferase, cholesterol 7 a-hydroxylase, and the LDL receptor (for a review, see Goldstein and Brown, 1990). The regulation of all these proteins is steroid dependent. This steroid-dependent regulation has been characterized in some detail at the gene level for HMG CoA synthase and the LDL receptor, and involves sophisticated coordinate transcriptional control, with interaction of transacting factors with sterol regulatory elements (SRE-l) sequences present in the promoter regions of these genes. In addition to this transcriptional control, there may also be regulatory control at different posttranscriptionallevels. The detailed mechanism behind the sterol-dependent regulation of the other key enzymes is not as well characterized at present.
2000
The possibility that the serum concentrations of various cholesterol precursors may reflect the activity of the hepatic HMG-CoA reductase was investigated in humans under different conditions. The serum levels of squalene, free and es- terified lanosterol, (4a, 48, 14a-trimethyl-5a-cholest-8, 24-dien- 38-01), two dimethylsterols (4a, 4&dimethyl-5/3-cholest-8-en- 38-01 and 4a, 48-dimethyl-5a-cholest-8, 24-dien-3#?-01), two methostenols (4a-methyl-5a-cholest-7-en-3/3-ol and 4a-methyl- 5a-cholest-8-en-3/3-01), two lathosterols (5a-cholest-7-en-38-01 and