Decrease In HMG-CoA Reductase Activity Research Articles

Inhibitory effects on the HMG-CoA Reductase in the chemical constituents of the Cassia mimosoides Linn

Abstract Cassia mimosoides Linn has been used from ancient times and used for treating hepatitis for its supposedly medically beneficial properties. In this study, different constituents of the Cassia mimosoides Linn (β-Sitosterol, Oleanolic Acid, Emodin, Carotene, Resorcinol, Luteolin, and α-L-Rhamnose) were evaluated for potential anti-HMG-CoA reductase effect. The inhibitory effects of HMG-CoA reductase of Cassia mimosoides Linn extracts and Pravastatin inhibitor at different concentrations (at doses of 1, 5, 25 or 125 μg/mL, respectively) in reaction system (70 mmol/L phosphate buffer, 200mmol/L NADPH, 5 μg HMG-CoA reductase, 2 mmol/L EDTA, 2 mmol/L cysteamine, 0.06% BSA) into 37°C preheat HMG-CoA for initiating this reaction, and then determined the change of HMG-CoA reductase activity (ΔAΔt) at 340 nm, the inhibition ratio of HMG-CoA reductase activity and its dynamic change of inhibitory effect within 15 min and the descent rate of NADPH. Emodin, Luteolin, β-Sitosterol, Oleanolic Acid, α-L-Rhamnose and Carotene showed good inhibition of HMG-CoA reductase activity. Among them, only the Emodin (1 and 5 μg/mL) groups showed a significant decrease of HMG-CoA reductase activity compared to the Pravastatin (1 and 5 μg/mL) groups respectively. In addition, the HMG-CoA reductase activity in the Emodin and Luteolin (25 and 125 μg/mL) groups was clearly lower than the Pravastatin (25 and 125 μg/mL) groups respectively. And the Emodin and Luteolin (1, 5, 25 or 125 μg/mL) groups exhibited a stable effect on inhibiting the HMG-CoA reductase within 15 min. These findings further support the exploration of Cassia mimosoides Linn as a potential agent for the treatment of hepatitis in future studies.

Geranyl Favonoid from Breadfruit Regulate Dyslipidemia in Hypercholesterolemic Rat

Hpercholesterolemia is closely related to oxidative damages from high fat and cholesterol intakes. The geranyl flavonoid (GF) from Artocarpus altilis (Breadfruit) has potential antioxidant, anti-hyperlipidemia and anti-atherogenic properties. This work was to investigate the influence of GF on synthesis of cholesterol and lipogenesis in hypercholesterolemic rat fed with high-cholesterol diet. Treatment with GF significantly increased SOD, CAT, GPx and GR activities in organ tissues, as well as decreased TBARS activity compared to the untreated group. Total glutathione, NO and α-tocopherol contents in liver, kidney and adipose tissues in GF treated group were also higher than that in untreated group. In addition, the GF treatment increased ACAT and fatty acid synthase activities, and decreased HMG-CoA reductase activity. The results suggest that treatment of GF could inhibit the synthesis of cholesterol and the lipogenesis by regulating ACAT, HMG-CoA reductase and fatty acid synthase activities and improving the antioxidant defense status in hypercholesterolemic rat.

Antihyperlipidemic Activity of Aloe succotrina in Rats: Possibly Mediated by Inhibition of HMG-CoA Reductase.

The present study was designed to investigate antihyperlipidemic activity of dried pulp of Aloe succotrina leaves in Wistar albino rats. Hyperlipidemia was induced in rats by feeding them high fat diet (HFD) or D-fructose (25% w/v) for 4 successive weeks. From 15th to 28th day, dried pulp (100 and 200 mg/kg, p.o) and atorvastatin (10 mg/kg, p.o.) per se were administered 2 h prior to feeding rats with HFD or fructose. Aloe succotrina did not significantly decrease the body weight of rats. The dried pulp and atorvastatin per se significantly decreased relative liver weight but did not significantly affect relative heart weight. HFD or fructose significantly increased serum total cholesterol, triglycerides, LDL-c, and VLDL, and decreased HDL-c; significantly increased liver MDA and decreased GSH levels. The dried pulp (200 mg/kg p.o.) significantly reversed high fat diet-induced and fructose-induced hyperlipidemia and atherogenic index. Aloe succotrina significantly decreased HMG Co-A reductase activity. Antihyperlipidemic effect of the dried pulp was comparable to atorvastatin. Thus, Aloe succotrina produced significant antihyperlipidemic activity in both HFD and fructose-induced hyperlipidemic rats, possibly through normalization of serum lipid profile, HMG-CoA reductase inhibitory activity, and amelioration of oxidative stress in liver.

Acrolein‐induced dyslipidemia and acute‐phase response are independent of HMG‐CoA reductase

Aldehydes are ubiquitous natural constituents of foods, water and beverages. Dietary intake represents the greatest source of exposure to acrolein and related aldehydes. Oral acrolein induces dyslipidemia acutely and chronically increases atherosclerosis in mice, yet the mechanisms are unknown. Because lipid synthesis and trafficking are largely under hepatic control, we examined hepatic genes in murine models of acute and chronic oral acrolein exposure. Changes in hepatic gene expression were examined using a Steroltalk microarray. Acute acrolein feeding modified plasma and hepatic proteins and increased plasma triglycerides within 15 min. By 6 h, acrolein altered hepatic gene expression including Insig1, Insig2 and Hmgcr genes and stimulated an acute-phase response (APR) with up-regulation of serum amyloid A genes (Saa) and systemic hypoalbuminemia. To test if decreased HMG-CoA reductase activity could modify acrolein-induced dyslipidemia or the APR, mice were pretreated with simvastatin. Statin treatment, however, did not alter acrolein-induced dyslipidemia or hypoalbuminemia associated with an APR. Few hepatic genes were dysregulated by chronic acrolein feeding in apoE-null mice. These studies confirmed that acute acrolein exposure altered expression of hepatic genes involved with lipid synthesis and trafficking and APR, and thus, indicated a hepatic locus of acrolein-induced dyslipidemia and APR that was independent of HMG CoA-reductase. Dietary intake of acrolein could contribute to cardiovascular disease risk by disturbing hepatic function.

Antihyperlipidemic activity of Clitoria ternatea and Vigna mungo in rats

Context: Hyperlipidemia is one of the major risk factors for atherosclerosis and cardiovascular diseases. Some plants are effective in controlling hyperlipidemia.Objective: To investigate the anti-hyperlipidemic effect of Clitoria ternatea L. and Vigna mungo L. (Fabaceae) on experimentally induced hyperlipidemia in rats.Materials and methods: The poloxamer 407-induced acute hyperlipidemia and diet-induced hyperlipidemia models were used for this investigation.Results: Oral administration of the hydroalcoholic extract of the roots and seeds of C. ternatea and the hydroalcoholic extract of the seeds of V. mungo resulted in a significant (p < 0.05) reduction of serum total cholesterol, triglycerides, very low-density lipoprotein cholesterol, and low-density lipoprotein cholesterol levels. The atherogenic index and the HDL/LDL ratio were also normalized after treatment in diet-induced hyperlipidemic rats. The effects were compared with atorvastatin (50 mg/kg, p.o.) and gemfibrozil (50 mg/kg, p.o.), reference standards.Discussion: The cholesterol-lowering effect of C. ternatea might be attributed to increased biliary excretion and decreased absorption of dietary cholesterol. The cholesterol-lowering effects of V. mungo seeds might be because of decreased HMG-CoA reductase activity, increased biliary excretion, and decreased absorption of dietary cholesterol. Additionally, they improved natural antioxidant defense mechanisms.Conclusion: The findings of the investigation suggest that C. ternatea and V. mungo have significant antihyperlipidemic action against experimentally-induced hyperlipidemia.

Inhibition of 3-hydroxy-3-methylglutaryl-coenzyme A reductase activity and of Ras farnesylation mediate antitumor effects of anandamide in human breast cancer cells

The endocannabinoid system regulates cell proliferation in human breast cancer cells. Recently, we described that a metabolically stable anandamide analog, 2-methyl-2'-F-anandamide, by activation of CB1 receptors significantly inhibited cell proliferation of human breast cancer cell lines. In this study, we observed that the activation of the CB1 receptor, in two human mammary carcinoma cell lines, MDA-MB-231 and MCF7, caused the inhibition of 3-hydroxy-3-methylglutaryl-coenzyme A (HMG-CoA) reductase activity due to a reduction of HMG-CoA reductase transcript levels. The decrease of HMG-CoA reductase activity induced the inhibition of the prenylation of proteins, in particular of the farnesylation of Ras oncogenic protein involved in cell proliferation of these cell lines. We suggest that the inhibitory effect of anandamide analog on tumor cell proliferation could be related to the inhibition of Ras farnesylation.

Endoplasmic reticulum stress mediates γ-tocotrienol-induced apoptosis in mammary tumor cells

gamma-Tocotrienol, a member of the vitamin E family of compounds, induces apoptosis in a variety of cancer cell types. However, previous studies have clearly demonstrated that gamma-tocotrienol-induced apoptosis in neoplastic mouse +SA mammary epithelial cells is not mediated through mitochondrial stress or death receptor apoptotic signaling. Therefore, studies were conducted to determine the role of endoplasmic reticulum (ER) stress in mediating gamma-tocotrienol-induced apoptosis in +SA mammary tumor cells. Treatment with 15-40 microM gamma-tocotrienol induced +SA cell death in a dose-responsive manner, and these effects were associated with a corresponding increase in poly (ADP-ribose) polymerase (PARP)-cleavage and activation of protein kinase-like endoplasmic reticulum kinase/eukaryotic translational initiation factor/activating transcription factor 4 (PERK/eIF2alpha/ATF-4) pathway, a marker of ER stress response. These treatments also caused a large increase in C/EBP homologous protein (CHOP) levels, a key component of ER stress mediated apoptosis that increases expression of tribbles 3 (TRB3). Knockdown of CHOP by specific siRNAs attenuated gamma-tocotrienol-induced PARP-cleavage, CHOP and TRB3 expression. gamma-Tocotrienol treatment also reduced full-length caspase-12 levels, an indication of caspase-12 cleavage and activation. Intracellular levels of 3-hydroxy-3-methylglutaryl-coenzyme A (HMGCoA) reductase, an ER-transmembrane enzyme catalyzing the synthesis of mevalonate, decreased following gamma-tocotrienol treatment, but combined treatment with mevalonate did not reverse gamma-tocotrienol-induced apoptosis, suggesting that a decrease in HMGCoA reductase activity is not required for gamma-tocotrienol induced apoptosis. These results demonstrate that ER stress apoptotic signaling is associated with gamma-tocotrienol-induced apoptosis in +SA mammary tumor cells.

Evaluation of Ayurvedic plants for anti-atherosclerotic and anti-hyperlipidemic effects

AIM: To study effects of Ayurvedic plants, Plumbago zeylanicus (Pz), Phyllanthus emblica (Pe), Cyperus rotundus (Cr) and Embelia ribes (Er) on prevention of atherosclerosis and hyperlipidemia. Methodology: The study was carried out in two parts after Animal Ethics Committee approval. In both the parts, animals were assigned to groups viz. Normal control, Disease control, Atorvastatin (AT), Pe, Pz, Cr and Er. Standardized aqueous extracts of test drugs in doses equivalent to human therapeutic dose were administered orally. In Part 1, male adult New-Zealand strain rabbits were administered high cholesterol diet pellets along with test drugs/vehicle for 45 days. On 46th day, lipid profile and histopathology of aorta were studied. In Part 2, male wistar rats (150–200 gm) received test drug/vehicle with cholesterol supplementation (CS: 400mg/kg with choline in coconut oil p.o.) for 35 days. Lipid profile and liver HMG-CoA reductase activity and aortic MDA were estimated on 36th day. Data was analysed using ANOVA followed by post hoc tests. Results: In part 1, only AT and Pe prevented hyperlipidemia induced by HCD. However, all the test drugs exhibited anti-atherosclerotic potential as seen from significant reduction in mean plaque area and percent luminal encroachment by the plaques. The best results were observed with AT and Pz, followed by Cr. In Part 2, AT prevented hyperlipidemia and increased HDL-C (p<0.01). Of the test drugs, Pz reduced triglycerides and VLDL-C (p<0.05) and increased HDL-C (p<0.05). Pe reduced LDL-C (p<0.01) and showed trend to reduce total cholesterol. AT, Pz and Pe also decreased HMG-CoA reductase activity and prevented rise in vascular MDA in response to CS. Conclusion:Pz showed the best anti-atherosclerotic effect comparable to AT. Anti-hyperlipidemic effects of Pz and Pe appear to be responsible for the anti-atherosclerotic activity. The mechanisms responsible for anti-atherosclerotic potential of Cr and Er need to be explored. Acknowledgement: 1. Dept. of AYUSH Govt. of India 2. Zandu Pharmaceuticals Works Ltd.

Effect of ethanol on cell growth and cholesterol metabolism in cultured Hep G2 cells.

The Hep G2 human hepatoma cell line has been recognized as an excellent in vitro human model system. For this reason, this line was used to study the effect of ethanol on HMG-CoA reductase activity concerning cell growth and cholesterol metabolism. Cells were incubated in ethanol-containing medium (0-400 mmol/L) for up to 102 h. Ethanol caused an inhibition in the growth rate and in HMG-CoA reductase activity that could be reverted by the removal of ethanol from the culture medium, indicating no cellular damage. These changes cannot be ascribed to the regulatory effect of cholesterol levels, since its content was not modified either in the cells or in the medium. The addition of mevalonate to the culture medium could not revert the growth rate inhibition evoked by ethanol. Moreover, ethanol produced an increment in the cholesterol efflux in [3H]cholesterol-prelabeled cells. We conclude that the decrease in HMG-CoA reductase activity evoked by ethanol treatment on Hep G2 cells would not be the cause but the consequence of the impairment in cellular growth, since this impairment could not be reverted by the addition of mevalonate to the culture medium.

THE CONCEPT OF CEREBRAL CHEMICAL DOMINANCE

The study assessed the biochemical differences between right hemispheric dominant and left hemispheric dominant individuals detected by handedness and the dichotic listening test. The isoprenoid metabolites--digoxin, dolichol, and ubiquinone, glycoconjugate metabolism, free radical metabolism, and the RBC membrane composition were studied in individuals with differing hemispheric dominance. The results showed that all right hemispheric dominant individuals and 50% of left hemispheric dominant individuals had increased HMG CoA reductase activity, elevated digoxin and dolichol levels. The serum magnesium, RBC membrane Na+-K+ ATPase activity and serum ubiquinone levels were reduced in all right hemispheric dominant individuals and 50% of left hemispheric dominant individuals. The tryptophan-derived neurotrans­mittors--serotonin, quinolinic acid, strychnine, and nicotine--were increased while the tyrosine derived neurotransmittors--dopamine, noradrena1ine, and morphine--were reduced in all right hemispheric dominant individuals­ and 50% of left hemispheric dominant individuals. The other 50% of left hemispheric dominant individuals had decreased HMG CoA reductase activity, reduced digoxin, and dolichol levels. The serum magnesium, RBC membrane Na+-K+ ATPase activity, and serum ubiquinone levels were increased in this group. The tryptophan derived neurotrans­mittors--serotonin, quinolinic acid, strychnine, and nicotine were reduced,­ while the tyrosine-derived neurotransmittors--dopamine, noradrenaline, and morphine--­were increased in the rest (50% of left hemispheric dominant individuals). Hemispheric dominance detected by the dichotic listening test and handedness has no correlation with cerebral chemical dominance.­

A Review : Dietary Fiber Biochemical Inductions and Its Hypocholesterolemic Effects

Most people already know that dietary_ fiber can have detrimental or beneficial effects on health, primarily with regard to nutritional status. In this writing one of the beneficial effects of the consumption ofdietary fiber especially its hypocholesterolemic effect will be discussed. Many research papers have already showed regarding the connection between the biochemical induction of soluble fiber and the hypocholesterolemic effect of soluble fiber. In these investigations treatments were given to various kinds of experimental animals and to humans as experimental subjects. Kinds of foods were used as sources of fiber. The researchers were able to discover a variety of biochemical inductions that led to a hypocholesterolomic effect and were able to lower the cholesterol concentration in various kinds of lipoprotein fractions. Biochemical inductions discovered were, among others: fiber making food difficult to be digested, thereby inhibiting lipid absorption, decreasing cholesterol synthesis, increase of cholesterol excretion through the bile, decreasing of Apo B synthesis in the liver, decrease of the mRNA Apo B concentration of the liver, decrease in mRNA Apo A-I and mRNA Apo A-IV concentration in intestinal cells, decrease in HMG-CoA reductase activity and decrease in insulin secretion. Every soluble .fiber from specific sources of dietary fiber produced a certain biochemical induction and also hypocholesterolemic effect to a certain lipoprotein fraction. The concentration of total cholesterol and cholesterol in lipoproteins containing Apo B, and the LDL-C: HDL-C ratio are all lowered. Thus the patient is able to choose as a source offiber, the food of his or her preference appropriate to his or her kind of hypercholesterolemic disorder, with the goal of reduction of cholesterol concentration.

Scavenger receptor class B type I affects cholesterol homeostasis by magnifying cholesterol flux between cells and HDL

Results from several laboratories clearly indicate that expression of scavenger receptor class B type I (SR-BI) enhances the bidirectional flux of cholesterol between cells and lipoproteins. Because the activity of HMG-CoA reductase, the key enzyme in cholesterol biosynthesis, is regulated by cell cholesterol content, we designed experiments to investigate the effect of SR-BI expression on the activity of this enzyme and on net cellular cholesterol mass. In addition, we compared the function of SR-BI with its human homolog, CD36 and LIMPII analogous 1. Our experiments demonstrate that both receptors enhance the flux of unesterified or free cholesterol bidirectionally, down a concentration gradient. Receptor-mediated cholesterol flux can effectively modulate multiple aspects of cellular cholesterol metabolism, including the pool that regulates the activity of HMG-CoA reductase. We also found that constitutive expression of SR-BI alters the steady state level of cellular cholesterol and phospholipid when SR-BI-expressing cells are maintained in medium containing serum lipoproteins. All of these effects are proportional to the level of receptor on the cell surface. These data indicate that the level of SR-BI expression determines both the rate of free cholesterol flux and the steady state level of cellular cholesterol.

Effects of CYP7A1 overexpression on cholesterol and bile acid homeostasis.

The initial and rate-limiting step in the classic pathway of bile acid biosynthesis is 7alpha-hydroxylation of cholesterol, a reaction catalyzed by cholesterol 7alpha-hydroxylase (CYP7A1). The effect of CYP7A1 overexpression on cholesterol homeostasis in human liver cells has not been examined. The specific aim of this study was to determine the effects of overexpression of CYP7A1 on key regulatory steps involved in hepatocellular cholesterol homeostasis, using primary human hepatocytes (PHH) and HepG2 cells. Overexpression of CYP7A1 in HepG2 cells and PHH was accomplished by using a recombinant adenovirus encoding a CYP7A1 cDNA (AdCMV-CYP7A1). CYP7A1 overexpression resulted in a marked activation of the classic pathway of bile acid biosynthesis in both PHH and HepG2 cells. In response, there was decreased HMG-CoA-reductase (HMGR) activity, decreased acyl CoA:cholesterol acyltransferase (ACAT) activity, increased cholesteryl ester hydrolase (CEH) activity, and increased low-density lipoprotein receptor (LDLR) mRNA expression. Changes observed in HMGR, ACAT, and CEH mRNA levels paralleled changes in enzyme specific activities. More specifically, LDLR expression, ACAT activity, and CEH activity appeared responsive to an increase in cholesterol degradation after increased CYP7A1 expression. Conversely, accumulation of the oxysterol 7alpha-hydroxycholesterol in the microsomes after CYP7A1 overexpression was correlated with a decrease in HMGR activity.

Epidermal Growth Factor Stimulates 3-Hydroxy-3-Methylglutaryl-coenzyme A Reductase Expression via the ErbB-2 Pathway in Human Breast Adenocarcinoma Cells

HMG-CoA reductase is the key enzyme for the biosynthesis of isoprenoid compounds essential for cell growth and differentiation. Its tyrosine kinase-dependent modulation has recently been suggested and described in the ErbB-2 overexpressing cell line SKBR-3 [Asslan et al. (1998) Biochem. J. 330, 241–246]. Epidermal growth factor (EGF) increased the HMG-CoA reductase activity, protein, and mRNA levels only in ErbB-2-expressing cells (SKBR-3 and MCF-7) but not in MDA-MB-468 cells that do not express ErbB-2 even though their EGF receptor was efficiently phosphorylated. Tyrphostin AG 879, a specific inhibitor of ErbB-2 tyrosine kinase activity, decreased HMG-CoA reductase activity only in cells that expressed ErbB-2. A functional EGF receptor appeared to be necessary since its inhibition by the specific tyrphostin AG 1478 abolished the EGF effects. Phosphatidylinositol 3-kinase (PI 3-kinase) might be a crucial enzyme in the signaling pathway since the specific inhibitor, LY 294002, was shown to inhibit HMG-CoA reductase activity and to completely abolish the stimulation by EGF in SKBR-3 cells.

Dexamethasone-induced decrease in HMG-CoA reductase and protein-farnesyl transferase activities does not impair ras processing in AR 4-2J cells.

Rat pancreatic acinar cells AR 4-2J respond to dexamethasone by differentiation and a decreased proliferation rate. Protein labelling by [3H]-mevalonolactone, used as a precursor of farnesyl and geranylgeranyl isoprenoid groups, was increased in the presence of dexamethasone. In these same conditions, dexamethasone decreased HMG-CoA reductase activity, leading to a diminished isotopic dilution of the mevalonate precursor. As ras proteins, known to be involved in the regulation of proliferation and differentiation, need to be farnesylated for full biological function, we also measured the level of farnesyl transferase activity and found a dose-dependent decrease in dexamethasone treated cells. Despite these negative effects of dexamethasone on mevalonate pathway, there was no appearance of non-isoprenylated forms of ras, indicating that the level of isoprenoid precursors and farnesyl transferase activity were not limiting in this model.

Effects of beta-blockers on HMG CoA reductase and LDL receptor activity in cultured human skin fibroblasts

Previous reports, based on clinical trials and animal experiments, suggest that beta-blockers may be useful in the prevention of atherosclerosis. Betaxolol, a new beta1-selective blocker, was shown to decrease plasma total and LDL cholesterol levels or to have no adverse effect on those [1-4]. While many reports deal with metabolism of triglyceride and high density lipoprotein, fewer publications about cholesterol metabolism are currently available. To clarify the mechanism by which beta-blockers affect lipid metabolism, we examined the effects of beta-blockers on HMG CoA reductase and LDL receptor activity in cultured human skin fibroblasts. L-propranolol, a nonselective beta-blocker, increased HMG CoA reductase activity and decreased LDL receptor activity. However, d-propranolol had no major effects on HMG CoA reductase activity. These results suggest that beta-blockers act on HMG CoA reductase through the beta receptors. Beta1-blocking action should decrease HMG CoA reductase activity and increase LDL receptor activity. In fact, betaxolol, a beta1-selective blocker, decreased HMG CoA reductase activity and increased LDL receptor activity, but metoprolol had no major effect. We speculate that the discrepancy between betaxolol and metoprolol in the effect on HMG CoA reductase and the LDL receptor might be due to the difference of the extent of beta1-selectivity. We conclude that beta1-selective blockers are antihypertensive agents potentially valuable in the prevention of atherosclerosis.

Regulation of low density lipoprotein receptor and 3-hydroxy-3-methyl-glutaryl-CoA reductase activities are differentially affected in Niemann-Pick type C and type D fibroblasts.

Defective regulation of intracellular cholesterol metabolism has been investigated in cultured fibroblasts from two subtypes of Niemann-Pick type II disease: the panethnic Niemann-Pick type C (NPC) and the Nova Scotia type D (NPD). Cell extracts from NPC and NPD fibroblasts cultured in lipoprotein-deficient medium exhibited activities of 3-hydroxy-3-methylglutaryl-CoA (HMG-CoA) reductase that were two-fold greater than in normal cells. Addition of serum resulted in only a 15% decrease in HMG-CoA reductase activity within 6 h in these cells, compared with a decrease of 80% in normal fibroblasts. The initial rate of return to maximal values for the first 6 h after removal of serum was similar in all three cell types; thereafter, the rate was faster in the mutant fibroblasts. Binding and internalization of 125I-labeled low density lipoprotein (LDL) was not decreased within 12 h of incubation of NPC fibroblasts with serum, while a decrease of 50% was observed for both NPD and normal fibroblasts over this time period. Northern blot analysis also indicated a slower decrease in steady-state LDL receptor mRNA in NPC relative to normal and NPD cells. In all three cell types, inhibition of HMG-CoA reductase with mevinolin had no effect on serum-stimulated cholesterol esterification, while inhibition of acyl-CoA:cholesterol acyltransferase with Sandoz 58-035 did not influence HMG-CoA reductase activity, indicating that defects in these regulatory mechanisms are independent.(ABSTRACT TRUNCATED AT 250 WORDS)

Homeostatic restoration of microsomal lipids and enzyme changes in HMG-CoA reductase and acyl-CoA: cholesterol acyltransferase in chick liver.

We have studied the correlation between changes in the lipid composition in chick liver microsomes and the activities of 3-hydroxy-3-methylglutaryl-CoA reductase (HMG-CoA reductase) and acyl-CoA: cholesterol acyltransferase (ACAT) by in vivo and in vitro experiments with 21-day-old chicks. A 5% cholesterol diet for 3 hr produced an increase in the microsomal and plasmatic cholesterol content, a decrease in HMG-CoA reductase activity and a concomitant increase in ACAT activity. The effect produced by the short-term treatment virtually disappeared 27 hr after ending the cholesterol diet. In vitro experiments were carried out by using vesicles constituted by phosphatidylcholine/cholesterol and phosphatidylcholine.

Regulation of gene expression and synthesis and degradation of 3-hydroxy-3-methylglutaryl coenzyme A reductase by micellar cholesterolin CaCo-2 cells.

To investigate whether, and by what mechanisms, luminal (dietary) cholesterol regulates cholesterol synthesis in human intestinal cells, HMG-CoA reductase activity, gene expression, synthesis, and degradation were investigated in CaCo-2 cells exposed to taurocholate micelles containing cholesterol. In cells incubated with cholesterol solubilized in 5 mM taurocholate and 30 microM monoolein, HMG-CoA reductase activity was decreased. 25-Hydroxycholesterol, delivered to the cells in the same manner as native cholesterol, was significantly more potent in inhibiting reductase activity and was used, therefore, to investigate mechanisms for sterol regulation. Cells incubated with taurocholate micelles without cholesterol lost cellular cholesterol into the medium causing an increase in HMG-CoA reductase activity and enzyme mass. Although steady-state levels of HMG-CoA reductase mRNA were increased under conditions of cholesterol efflux, synthesis rates of reductase protein were not increased. An increase in activity and enzyme mass in cells incubated with micelles alone, however, was accompanied by a significant decrease in the rate of degradation of reductase protein. In contrast, sterol influx from taurocholate micelles was associated with a marked decrease in HMG-CoA reductase activity and mass without altering mRNA levels except at high concentrations of the polar sterol which did decrease reductase mRNA levels by 50%. The absorption of apical sterol resulted in a significant decrease in the translational efficiency of reductase mRNA and a modest increase in the rate of degradation of the enzyme. Thus, although the primary function of the enterocyte is to transport luminal (dietary) cholesterol to other tissues of the body, apically derived cholesterol enters metabolic pools within the cell which regulates its own cholesterol synthesis. Dietary cholesterol, therefore, will regulate the contribution to the total body cholesterol pool of endogenously derived cholesterol from the intestine. The mechanism for this regulation of intestinal HMG-CoA reductase by luminal cholesterol occurs primarily at the post-transcriptional level.

Bezafibrate fails to directly modulate HMG-CoA reductase or LDL catabolism in human mononuclear cells

The effect of bezafibrate on HMG-CoA reductase, the key enzyme of cholesterol synthesis, and LDL metabolism was studied in human mononuclear cells. Bezafibrate at concentrations achieved during administration in patients did not suppress preformed reductase in mononuclear cells. Similarly, the drug was ineffective in regulating reductase when added to the medium of cultured cells. Also, the fibrate did not modulate the enzyme suppression mediated by LDL. At very high concentrations bezafibrate enhanced LDL binding, but both total cell association and degradation were unchanged. Thus, the previously observed decrease of HMG-CoA reductase activity in mononuclear cells of patients treated with fibrates is likely to be indirect and probably due to changes in LDL structure.