Characteristics of rat liver microsomal 3-hydroxy-3-methylglutaryl-coenzyme A reductase

Hepatic HMG-CoA Reductase Expression and Resistance to Dietary Cholesterol

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 mg/dl when given a 4-day, 1% cholesterol challenge. The extent of feedback regulation was reduced to only 3-fold in the Fisher 344 and Brown Norway rats that exhibited significant increases in serum cholesterol levels when given a cholesterol challenge. The Golden Syrian hamsters exhibited the largest increase (197 mg/dl) in serum cholesterol levels in response to dietary cholesterol and the lowest basal expression of hepatic HMG-CoA reductase (3.3 pmol / min-1 / mg-1). Hepatic LDL receptor levels were not significantly decreased by dietary cholesterol in any of the animals. The data from these inbred rats and the hamsters strongly support the conclusion that the animals expressing the highest levels of hepatic HMG-CoA reductase are the most resistant to the serum cholesterol raising action of dietary cholesterol.

Increased Sensitivity to Dietary Cholesterol in Diabetic and Hypothyroid Rats Associated with Low Levels of Hepatic HMG-CoA Reductase Expression

We recently postulated that hepatic 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase functions as a cholesterol buffer to protect against the serum and tissue cholesterol raising action of dietary cholesterol. This postulate predicts that diminished basal expression of hepatic HMG-CoA reductase results in increased sensitivity to dietary cholesterol. Because diabetic and hypothyroid animals are known to have markedly reduced hepatic HMG-CoA reductase, these animals were selected as models to test our postulate. When rats were rendered diabetic with streptozotocin, their hepatic HMG-CoA reductase activity decreased from 314 to 22 pmol • min-1 • mg-1, and their serum cholesterol levels increased slightly. When the diabetic animals were challenged with a diet containing 1% cholesterol, their serum cholesterol levels doubled, and their hepatic reductase activity decreased further to 0.9 pmol • min-1 • mg-1. Hepatic low-density lipoprotein (LDL) receptor immunoreactive protein levels were unaffected in the diabetic rats whether fed cholesterol-supplemented diets or not. In rats rendered hypothyroid by thyroparathyroidectomy, serum cholesterol levels rose from 100 to 386 mg/dl in response to the 1% cholesterol challenge, whereas HMG-CoA reductase activity dropped from 33.8 to 3.4 pmol • min-1 • mg-1. Hepatic LDL receptor immunoreactive protein levels decreased only slightly in the hypothyroid rats fed cholesterol-supplemented diets. Taken together, these results show that rats deficient in either insulin or thyroid hormone are extremely sensitive to dietary cholesterol largely due to low basal expression of hepatic HMG-CoA reductase.

Identification of Novel Tau Interactions with Endoplasmic Reticulum Proteins in Alzheimer's Disease Brain

Alzheimer's disease (AD) is a progressive neurodegenerative disorder that is pathologically characterized by the formation of extracellular amyloid plaques and intraneuronal tau tangles. We recently identified that tau associates with proteins known to participate in endoplasmic reticulum (ER)-associated degradation (ERAD); consequently, ERAD becomes dysfunctional and causes neurotoxicity. We hypothesized that tau associates with other ER proteins, and that this association could also lead to cellular dysfunction in AD. Portions of human AD and non-demented age matched control brains were fractionated to obtain microsomes, from which tau was co-immunoprecipitated. Samples from both conditions containing tau and its associated proteins were analyzed by mass spectrometry. In total, we identified 91 ER proteins that co-immunoprecipitated with tau; 15.4% were common between AD and control brains, and 42.9% only in the AD samples. The remainder, 41.8% of the proteins, was only seen in the control brain samples. We identified a variety of previously unreported interactions between tau and ER proteins. These proteins participate in over sixteen functional categories, the most abundant being involved in RNA translation. We then determined that association of tau with these ER proteins was different between the AD and control samples. We found that tau associated equally with the ribosomal protein L28 but more robustly with the ribosomal protein P0. These data suggest that the differential association between tau and ER proteins in disease could reveal the pathogenic processes by which tau induces cellular dysfunction.

A critical role of mevalonate for peptidoglycan synthesis in Staphylococcus aureus

3-hydroxy-3-methyl-glutaryl-CoA (HMG-CoA) reductase, a mevalonate synthetase, is required for the growth of Staphylococcus aureus. However, the essential role of the enzyme in cell growth has remained unclear. Here we show that three mutants possessed single-base substitutions in the mvaA gene, which encodes HMG-CoA reductase, show a temperature-sensitive phenotype. The phenotype was suppressed by the addition of mevalonate or farnesyl diphosphate, which is a product synthesized from mevalonate. Farnesyl diphosphate is a precursor of undecaprenyl phosphate that is required for peptidoglycan synthesis. The rate of peptidoglycan synthesis was decreased in the mvaA mutants under the non-permissive conditions and the phenotype was suppressed by the addition of mevalonate. HMG-CoA reductase activities of mutant MvaA proteins in the temperature sensitive mutants were lower than that of wild-type MvaA protein. Our findings from genetic and biochemical analyses suggest that mevalonate produced by HMG-CoA reductase is required for peptidoglycan synthesis for S. aureus cell growth.

Deciphering molecular mechanism underlying hypolipidemic activity of echinocystic Acid

Our previous study showed that a triterpene mixture, consisting of echinocystic acid (EA) and oleanolic acid (OA) at a ratio of 4 : 1, dose-dependently ameliorated the hyperlipidemia and atherosclerosis in rabbits fed with high fat/high cholesterol diets. This study was aimed at exploring the mechanisms underlying antihyperlipidemic effect of EA. Molecular docking simulation of EA was performed using Molegro Virtual Docker (version: 4.3.0) to investigate the potential targets related to lipid metabolism. Based on the molecular docking information, isotope labeling method or spectrophotometry was applied to examine the effect of EA on the activity of 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase, acyl-CoA:cholesterol acyltransferase (ACAT), and diacylglycerol acyltransferase (DGAT) in rat liver microsomes. Our results revealed a strong affinity of EA towards ACAT and DGAT in molecular docking analysis, while low binding affinity existed between EA and HMG-CoA reductase as well as between EA and cholesteryl ester transfer protein. Consistent with the results of molecular docking, in vitro enzyme activity assays showed that EA inhibited ACAT and DGAT, with IC₅₀ values of 103 and 139 μM, respectively, and exhibited no significant effect on HMG-CoA reductase activity. The present findings suggest that EA may exert hypolipidemic effect by inhibiting the activity of ACAT and DGAT.

Mechanism of resistance to dietary cholesterol

Background. Alterations in expression of hepatic genes that could contribute to resistance to dietary cholesterol were investigated in Sprague-Dawley rats, which are known to be resistant to the serum cholesterol raising action of dietary cholesterol. Methods. Microarray analysis was used to provide a comprehensive analysis of changes in hepatic gene expression in rats in response to dietary cholesterol. Changes were confirmed by RT-PCR analysis. Western blotting was employed to measure changes in hepatic cholesterol 7α hydroxylase protein. Results. Of the 28,000 genes examined using the Affymetrix rat microarray, relatively few were significantly altered. As expected, decreases were observed for several genes that encode enzymes of the cholesterol biosynthetic pathway. The largest decreases were seen for squalene epoxidase and lanosterol 14α demethylase (CYP 51A1). These changes were confirmed by quantitative RT-PCR. LDL receptor expression was not altered by dietary cholesterol. Critically, the expression of cholesterol 7α hydroxylase, which catalyzes the rate-limiting step in bile acid synthesis, was increased over 4-fold in livers of rats fed diets containing 1% cholesterol. In contrast, mice, which are not resistant to dietary cholesterol, exhibited lower hepatic cholesterol 7α hydroxylase (CYP7A1) protein levels, which were not increased in response to diets containing 2% cholesterol.

LDLR expression and localization are altered in mouse and human cell culture models of Alzheimer's disease

Background

Alzheimer's disease (AD) is a chronic neurodegenerative disorder and the most common form of dementia. The major molecular risk factor for late-onset AD is expression of the ε-4 allele of apolipoprotein E (apoE), the major cholesterol transporter in the brain. The low-density lipoprotein receptor (LDLR) has the highest affinity for apoE and plays an important role in brain cholesterol metabolism.

Methodology/Principal Findings

Using RT-PCR and western blotting techniques we found that over-expression of APP caused increases in both LDLR mRNA and protein levels in APP transfected H4 neuroglioma cells compared to H4 controls. Furthermore, immunohistochemical experiments showed aberrant localization of LDLR in H4-APP neuroglioma cells, Aβ-treated primary neurons, and in the PSAPP transgenic mouse model of AD. Finally, immunofluorescent staining of LDLR and of γ- and α-tubulin showed a change in LDLR localization preferentially away from the plasma membrane that was paralleled by and likely the result of a disruption of the microtubule-organizing center and associated microtubule network.

Conclusions/Significance

These data suggest that increased APP expression and Aβ exposure alters microtubule function, leading to reduced transport of LDLR to the plasma membrane. Consequent deleterious effects on apoE uptake and function will have implications for AD pathogenesis and/or progression.

Thioesterase superfamily member 2 (Them2)/acyl-CoA thioesterase 13 (Acot13): a homotetrameric hotdog fold thioesterase with selectivity for long-chain fatty acyl-CoAs

Them2 (thioesterase superfamily member 2) is a 140-amino-acid protein of unknown biological function that comprises a single hotdog fold thioesterase domain. On the basis of its putative association with mitochondria, accentuated expression in oxidative tissues and interaction with StarD2 (also known as phosphatidylcholine-transfer protein, PC-TP), a regulator of fatty acid metabolism, we explored whether Them2 functions as a physiologically relevant fatty acyl-CoA thioesterase. In solution, Them2 formed a stable homotetramer, which denatured in a single transition at 59.3 degrees C. Them2 exhibited thioesterase activity for medium- and long-chain acyl-CoAs, with Km values that decreased exponentially as a function of increasing acyl chain length. Steady-state kinetic parameters for Them2 were characteristic of long-chain mammalian acyl-CoA thioesterases, with minimal values of Km and maximal values of kcat/Km observed for myristoyl-CoA and palmitoyl-CoA. For these acyl-CoAs, substrate inhibition was observed when concentrations approached their critical micellar concentrations. The acyl-CoA thioesterase activity of Them2 was optimized at physiological temperature, ionic strength and pH. For both myristoyl-CoA and palmitoyl-CoA, the addition of StarD2 increased the kcat of Them2. Enzymatic activity was decreased by the addition of phosphatidic acid/phosphatidylcholine small unilamellar vesicles. Them2 expression, which was most pronounced in mouse heart, was associated with mitochondria and was induced by activation of PPARalpha (peroxisome-proliferator-activated receptor alpha). We conclude that, under biological conditions, Them2 probably functions as a homotetrameric long-chain acyl-CoA thioesterase. Accordingly, Them2 has been designated as the 13th member of the mammalian acyl-CoA thioesterase family, Acot13.

Target size analysis by radiation inactivation: the use of free radical scavengers

Several model systems were employed to assess indirect effects that occur in the process of using radiation inactivation analysis to determine protein target sizes. In the absence of free radical scavengers, such as mannitol and benzoic acid, protein functional unit sizes can be drastically overestimated. In the case of glutamate dehydrogenase, inclusion of free radical scavengers reduced the apparent target size from that of a hexamer to that of a trimer based on enzyme activity determinations. For glucose-6-phosphate dehydrogenase, the apparent target size was reduced from a dimer to a monomer. The target sizes for both glutamate dehydrogenase and glucose-6-phosphate dehydrogenase in the presence of free radical scavengers corresponded to subunit sizes when determinations of protein by sodium dodecyl sulfate-polyacrylamide gel electrophoresis or immunoblotting were done rather than enzyme activity. The free radical scavengers appear to compete with proteins for damage by secondary radiation products, since irradiation of these compounds can result in production of inhibitory species. Addition of benzoic acid/mannitol to samples undergoing irradiation was more effective in eliminating secondary damage than were 11 other potential free radical scavenging systems. Addition of a free radical scavenging system enables more accurate functional unit size determinations to be made using radiation inactivation analysis.

Degradation of HMG-CoA reductase in rat liver is cholesterol and ubiquitin independent

In contrast with the accelerated degradation observed in tumor cells in response to sterols, hepatic 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase turnover in whole animals was not increased by dietary cholesterol. Furthermore, treating rats with lovastatin to lower hepatic cholesterol levels did not decrease the rate of degradation. The half-life remained in the 6 h range. Co-immunoprecipitation studies revealed that the amount of ubiquitin associated with the reductase was entirely dependent upon the amount of microsomal protein subjected to immunoprecipitation. The results indicate that in liver, neither the rate of reductase protein degradation nor the ubiquitin-proteasome system appear to play roles in mediating changes in HMG-CoA reductase protein levels in response to dietary cholesterol.

S-alk(en)yl cysteines of garlic inhibit cholesterol synthesis by deactivating HMG-CoA reductase in cultured rat hepatocytes

The effects of water-soluble organosulfur compounds of garlic on hepatic cholesterol biosynthesis in cultured rat hepatocytes were studied. S-Alk(en)yl cysteines, i.e., S-allyl cysteine (SAC), S-ethyl cysteine (SEC) and S-propyl cysteine (SPC) inhibited cholesterol synthesis from [(14)C]acetate but not from [(14)C]mevalonate. The activity of 3-hydroxy-3-methylglutaryl CoA (HMG-CoA) reductase in the cells treated with SAC, SEC and SPC was 30-40% lower than that of the untreated cells. S-Alk(en)yl cysteines did not alter abundance of mRNA coded for HMG-CoA reductase or protein concentration of the enzyme. The ratio of expressed to total activity (E/T) of HMG-CoA reductase was then determined as an index of phosphorylation status of the enzyme. The E/T ratio was reduced 18-29% by SAC, SEC and SPC, resulting primarily from decreased expressed activity. The results suggest that S-alk(en)yl cysteines inhibit cholesterol synthesis by deactivating HMG-CoA reductase via enhanced phosphorylation, but not changing levels of mRNA or the amount of the enzyme. Additionally, of the three S-alk(en)yl cysteines tested, only SAC appears to further decrease the activity of HMG-CoA reductase by increasing sulfhydryl oxidation of the enzyme.

Regulation of hepatic lanosterol 14 alpha-demethylase gene expression by dietary cholesterol and cholesterol-lowering agents

Binding of sterol response element binding protein 1a to sterol response element-1 (SRE-1) in the promoter region of lanosterol 14 alpha-demethylase (14DM) has been demonstrated previously. Decreased 14DM activity has been shown to result in accumulation of the intermediate, 3 beta-hydroxy-lanost-8-en-32-al, a known translational downregulator of 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase. Since it has also been demonstrated that feedback regulation of hepatic HMG-CoA reductase occurs primarily at the level of translation, the effects of dietary cholesterol and cholesterol lowering agents on levels of hepatic 14DM mRNA and immunoreactive protein were investigated. Addition of 1% cholesterol to a chow diet markedly decreased hepatic 14DM mRNA and protein levels in Sprague-Dawley rats. The extent and time course of this decrease in 14DM immunoreactive protein closely paralleled that of HMG-CoA reductase. Supplementation of the diet with the HMG-CoA reductase inhibitor, Lovastatin, to a level of 0.02%, raised 14DM mRNA and protein levels 2- to 3-fold. Addition of 2% Colestipol, a bile acid binding resin, to the chow diet caused smaller increases. The highest level of 14DM protein expression was observed in liver, the major site of feedback regulation of HMG-CoA reductase by cholesterol. Taken together, these observations suggest a critical role for 14DM in the feedback regulation of hepatic HMG-CoA reductase.

7-Dehydrocholesterol-dependent proteolysis of HMG-CoA reductase suppresses sterol biosynthesis in a mouse model of Smith-Lemli-Opitz/RSH syndrome

Smith-Lemli-Opitz/RSH syndrome (SLOS), a relatively common birth-defect mental-retardation syndrome, is caused by mutations in DHCR7, whose product catalyzes an obligate step in cholesterol biosynthesis, the conversion of 7-dehydrocholesterol to cholesterol. A null mutation in the murine Dhcr7 causes an identical biochemical defect to that seen in SLOS, including markedly reduced tissue cholesterol and total sterol levels, and 30- to 40-fold elevated concentrations of 7-dehydrocholesterol. Prenatal lethality was not noted, but newborn homozygotes breathed with difficulty, did not suckle, and died soon after birth with immature lungs, enlarged bladders, and, frequently, cleft palates. Despite reduced sterol concentrations in Dhcr7(-/-) mice, mRNA levels for 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase, the rate-controlling enzyme for sterol biosynthesis, the LDL receptor, and SREBP-2 appeared neither elevated nor repressed. In contrast to mRNA, protein levels and activities of HMG-CoA reductase were markedly reduced. Consistent with this finding, 7-dehydrocholesterol accelerates proteolysis of HMG-CoA reductase while sparing other key proteins. These results demonstrate that in mice without Dhcr7 activity, accumulated 7-dehydrocholesterol suppresses sterol biosynthesis posttranslationally. This effect might exacerbate abnormal development in SLOS by increasing the fetal cholesterol deficiency.

Mechanism of aminobisphosphonate action: characterization of alendronate inhibition of the isoprenoid pathway

Alendronate (ALN), an aminobisphosphonate compound used for the treatment of osteoporosis and other disorders of bone resorption, has been suggested to act by inhibition of the formation of GGPP. In the present study we used an S(10) homogenate fraction of rat liver to show that ALN causes a dose-dependent inhibition of [(3)H]MVA incorporation into sterols and a concomitant increase in incorporation of radiolabel into IPP and DMAPP. We further show that ALN is a potent inhibitor of cytosolic trans-prenyltransferase (FPP synthase). The inhibition is competitive with respect to allylic pyrophosphate substrates, but not IPP, suggesting that ALN acts as an allylic pyrophosphate analog and binds to the free enzyme. The K(i) is in the 0.5 microM range.

Regulation of pathways determining cholesterol availability in the baboon placenta with advancing gestation

Low density lipoprotein (LDL) is accepted as the primary source of cholesterol for progesterone biosynthesis in the primate placental syncytiotrophoblast. We hypothesized that the syncytiotrophoblast may, however, derive significant amounts of cholesterol from sources in addition to the LDL pathway, especially during early pregnancy or when faced with a paucity of lipoprotein-cholesterol. To test this, alternate cholesterol-providing pathways were assessed in placentae at early (Days 60-61), mid (Days 98-102), and late (Days 160-167) gestation in the baboon (Papio sp., term approximately 184 days). Expression of LDL receptor mRNA transcripts in an enriched fraction of syncytiotrophoblast cells was approximately 13-fold greater (P < 0.05) in mid and late gestation than in early pregnancy, although no differences were observed in whole villous tissue. The abundance of transcripts for 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase, the enzyme responsible for de novo cholesterol synthesis, remained unchanged in syncytiotrophoblast cells; however, HMG-CoA reductase activity declined approximately 2-fold from early to late pregnancy (P < 0.01), with a commensurate decline in immunoreactive HMG-CoA reductase protein. Activities for acyl-coenzyme A:cholesterol acyl transferase (ACAT), a rate-limiting enzyme for cholesterol esterification, were greater (P < 0.05) at early and mid pregnancy in placental homogenates than in those from late pregnancy, while ACAT-1 mRNA concentrations and cholesterol ester hydrolase activity remained unchanged. These results, taken together, suggest that although de novo synthesis has the potential to provide a measure of the cholesterol used for placental progesterone production during early baboon pregnancy, its contribution declines with advancing gestational age as LDL receptor-derived cholesterol becomes the major source of substrate. Changes in LDL receptor mRNA abundance suggest differences in mechanisms regulating cholesterol homeostasis in steroidogenically active syncytiotrophoblasts vs. proliferative nonendocrine cell types in the placenta.

Enhancement of sterol synthesis by the monoterpene perillyl alcohol is unaffected by competitive 3-hydroxy-3-methylglutaryl-CoA reductase inhibition

Monoterpenes such as limonene and perillyl alcohol (PA) are currently under investigation for their chemotherapeutic properties which have been tied to their ability to affect protein isoprenylation. Because PA affects the synthesis of isoprenoids, such as ubiquinone, and cholesterol is the end product of the synthetic pathway from which this isoprenoid pathway branches, we investigated the effects of this compound upon cholesterol metabolism in the colonic adenocarcinoma cell line SW480. PA (1 mM) inhibited incorporation of 14C-mevalonate into 21-26 kDa proteins by 25% in SW480 cells. Cholesterol (CH) biosynthesis was assessed by measuring the incorporation of 14C-acetate and 14C-mevalonate into 27-carbon-sterols. Cells treated with PA (1 mM) exhibited a fourfold increase in the incorporation of 14C-acetate but not 14C-mevalonate into cholesterol. Mevinolin (lovastatin), an inhibitor of 3-hydroxy-3-methylglutaryl-CoA(HMG-CoA) reductase, at 2 microM concentration, inhibited CH synthesis from 14C-acetate by 80%. Surprisingly, concurrent addition of mevinolin and PA did not significantly alter the stimulatory effects of PA. As observed differences in 14C-acetate and 14C-mevalonate precursor labeling could indicate PA affects early pathway events, the effects of this monoterpene on HMG-CoA reductase activity were evaluated. Unexpectedly, 1 mM PA did not stimulate activity of this enzyme. Consistent with its action as a reversibly bound inhibitor, in washed microsomes, 2 microM mevinolin pretreatment increased reductase protein expression causing a 12.7 (+/- 2.4)-fold compensatory HMG-CoA reductase activity increase; concurrent treatment with 1 mM PA attenuated this to a 5.3 (+/- 0.03)-fold increase. Gas chromatographic analysis confirmed CH was the major lipid present in the measured thin-layer chromatography spot. Since 14C-acetate incorporation into free fatty acid and phospholipid pools was not significantly affected by PA treatment, nonspecific changes in whole acetate pool sizes were not indicated. Because increases in endogenous CH synthesis should result in compensatory changes in exogenous sterol utilization, the effects of PA upon low density lipoprotein (LDL) receptor activity were evaluated. Consistent with the observed increases in CH synthesis, 1 mM PA decreased 125I-LDL internalization to 50% of the fetal bovine serum control; concurrent addition of 2 microM mevinolin attenuated this effect to a reduction of 80% of the control value. Data suggest that in certain colonic tumor cells PA strongly affects cholesterol metabolism via a mechanism of action that is insensitive to the HMG-CoA reductase inhibitor mevinolin.

Effects of 15-oxa-32-vinyl-lanost-8-ene-3 beta,32 diol on the expression of 3-hydroxy-3-methylglutaryl coenzyme A reductase and low density lipoprotein receptor in rat liver

The mechanisms by which oxylanosterols regulate expression of hepatic 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase and lower serum cholesterol levels were examined by using a novel nonmetabolizable oxylanosterol mimic, 15-oxa-32-vinyl-lanost-8-ene-3 beta, 32 diol (DMP 565). This compound, unlike other nonmetabolizable oxylanosterols, is not a substrate for lanosterol 14 alpha-methyl demethylase. Feeding rats a diet supplemented with 0.02% DMP 565 markedly decreased HMG-CoA reductase immunoreactive protein and enzyme activity levels without affecting mRNA levels. The rate of reductase protein degradation was unaffected. However, the rate of translation was reduced to less than 20% of control. Thus, DMP 565 appears to regulate hepatic HMG-CoA reductase gene expression primarily at the level of translation. The pronounced inhibition of HMG-CoA reductase by DMP 565 resulted in a compensatory increase in the functioning of the hepatic low density lipoprotein (LDL) receptor, possibly by increased cycling, as evidenced by a marked increase in the rate of degradation of the LDL receptor. The half-life of the receptor was decreased from over 7 h to only 1 h in animals receiving DMP 565. This increase in the rate of degradation occurred without a change in the steady state level of the receptor. Addition of dietary cholesterol attenuated the increased turnover of the LDL receptor. These effects on the hepatic LDL receptor have also been observed with HMG-CoA reductase inhibitors (G. C. Ness et al., 1996, Arch. Biochem, Biophys. 325, 242-248). However, the effect of DMP 565 on the rate of degradation of the hepatic LDL receptor was of a greater magnitude when equal doses of the drugs were used. These regulatory actions of DMP 565 provide, in part, an explanation for the observed hypocholesterolemic action of this compound.

The activity of HMG-CoA reductase and acetyl-CoA carboxylase in human apocrine sweat glands, sebaceous glands, and hair follicles is regulated by phosphorylation and by exogenous cholesterol

Human apocrine and sebaceous glands function to secrete lipids, predominantly triglycerides, fatty acids, cholesterol and its esters, and, in the sebaceous gland, squalene. The enzymes that catalyze the important regulatory steps in cholesterol and fatty acid biosyntheses, 3-hydroxy-3-methylglutaryl-coenzyme A (HMG-CoA) reductase and acetyl-CoA carboxylase, respectively, were therefore studied in isolated human skin appendages, and their relevant kinetic parameters determined. The enzyme activities that were observed can account for previously described rates of incorporation of radiolabeled substrates into the appropriate lipids by glands in vitro. Reduced enzyme activities following homogenization in the presence of fluoride indicated that both of these enzymes in skin appendages are inactivated by phosphorylation. The activity of the enzyme known to catalyze this phosphorylation, the AMP-activated protein kinase, was also measured. Compactin was shown to inhibit HMG-CoA reductase in homogenates of these appendages. Conversely, incubation of whole sebaceous glands with compactin resulted in the stimulation of enzyme activity, which suggests that these appendages can respond to diminishing cholesterol levels. The effect of exogenous low density lipoprotein and 25-hydroxycholesterol on HMG-CoA reductase activity from skin appendages was investigated. HMG-CoA reductase activity in both apocrine and sebaceous glands was reduced following incubation with either low density lipoprotein or 25-hydroxycholesterol. Low density lipoprotein receptor and lipoprotein lipase mRNA expression was also detected in skin appendages. These results indicate that apocrine and sebaceous glands have the capacity to sequester dietary cholesterol and fatty acids that may have important implications for the understanding of both acne and axillary odor.

Effects of L-triiodothyronine and the thyromimetic L-94901 on serum lipoprotein levels and hepatic low-density lipoprotein receptor, 3-hydroxy-3-methylglutaryl coenzyme A reductase, and apo A-I gene expression

The mechanisms by which thyroid hormone (triiodothyronine (T3)) and a thyromimetic, 2-amino-3-(3,5-dibromo-4-[4-hydroxy-3-(6-oxo-1,6-dihydro-pyridazin -3-ylmethyl)-phenoxyl]-phenyl)propionic acid (L-94901), lower plasma low density lipoprotein (LDL) cholesterol and raise plasma high density lipoprotein (HDL) cholesterol levels was investigated in thyroidectomized and sham-operated rats. Thyroidectomy resulted in a 77% increase in plasma LDL cholesterol, a 60% decrease in plasma triglycerides, and a modest reduction in HDL cholesterol. Daily oral dosing with T3 (10-170 nmol/kg) or L94901 (100-1000 nmol/kg) for 7 days decreased plasma LDL cholesterol in thyroidectomized rats by 60-80%, respectively. This reduction in LDL cholesterol was accompanied by a dose-dependent increase in HDL cholesterol levels of up to 60%. Thus, the ratio of LDL to HDL was decreased from 1.01 to 0.12 after treatment with L-94901 and to 0.25 after dosing with T3. In sham-operated animals, T3 and L-94901 lowered LDL cholesterol by 61 and 46%, respectively, and increased HDL cholesterol by 25 and 53%, respectively. Immunoblotting analysis of liver membranes prepared from thyroidectomized or sham-operated rats demonstrated that LDL receptor protein levels were increased by up to eight-fold. Northern blotting analysis revealed similar large increases in hepatic LDL receptor mRNA levels that accounted for the increases in LDL receptor protein levels. Hepatic 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase mRNA, protein, and activity were increased 2- to 3-fold. The T3- and L-94901-mediated increases in serum HDL levels were associated with 2- to 3-fold increases in apo A-I mRNA levels. In contrast with most other hypocholesterolemic agents, T3 and L-94901 significantly increase HDL cholesterol levels in addition to decreasing LDL cholesterol levels due to induction of hepatic apo A-I and LDL receptor gene expression.

Dietary cholesterol regulates hepatic 3-hydroxy-3-methylglutaryl coenzyme A reductase gene expression in rats primarily at the level of translation

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 [35S]methionine and [35S]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 sterol and nonsterol endproducts. These results indicate that cholesterol regulates hepatic HMG-CoA reductase gene expression in rats primarily at the level of translation.

Compensatory responses to inhibition of hepatic squalene synthase

The mechanism by which depletion of hepatic cholesterol levels, achieved by inhibition of squalene synthase, alters hepatic LDL receptor, HMG-CoA reductase, and cholesterol 7alpha-hydroxylase gene expression was investigated by measuring transcription rates, mRNA stability, rates of translation, translational efficiency, and levels of sterol response element binding proteins. It was found that the transcription of both hepatic LDL receptor and HMG-CoA reductase were increased about twofold. The increase in LDL receptor transcription occurred within 2 h after giving 2 mg/kg zaragozic acid A, a potent inhibitor of squalene synthase. This preceded the increase in transcription of HMG-CoA reductase that occurred at 4 h. Increases in the stability of both of these mRNAs were also observed. These changes account for the increases in LDL receptor and HMG-CoA reductase mRNA levels previously observed. The rate of transcription of hepatic cholesterol 7alpha-hydroxylase was decreased to about 25% of control within 3 h after administration of zaragozic acid A, which correlates with the decrease in this mRNA. The rates of translation, as determined by pulse labeling, of both hepatic HMG-CoA reductase and LDL receptor were increased two- to threefold. The translational efficiency of these two mRNAs was also increased as judged by polysome profile analysis. There was an increase in mRNA associated with the heaviest polysome fraction and a decrease in that associated with monosomes. No significant change was observed in the levels of sterol response element binding protein 2, the form that mediates induced transcription, in response to zaragozic acid A treatment, indicating that this protein might not be involved in mediating the observed transcriptional changes. An increase in sterol response element binding protein -1 was observed 30 min after giving zaragozic acid A. The results suggest that compensatory responses to depletion of squalene-derived products involve alterations in the rates of transcription, mRNA stability, and translational of key proteins involved in cholesterol homeostasis.

Squalene synthase inhibition alters metabolism of nonsterols in rat liver

We have used the potent squalene synthase inhibitor squalestatin I to investigate the regulation of isoprenoid metabolism in rat liver Fresh-frozen liver pieces from normal rats and rats infused with squalestatin I at 16 micrograms h-1 for 16 h were assayed for farnesyl pyrophosphate (FPP) and geranylgeranyl pyrophosphate (GGPP) by HPLC after dephosphorylation. Levels of FPP and GGPP were 5.4 +/- 1.6 nmol g-1 and 1.6 +/- 0.7 nmol g-1 (n = 13) wet wt., respectively, in control livers and 110 + 41 nmol g-1 and 3.0 +/- 2.2 nmol g-1 (n = 13) in livers from squalestatin I infused rats. In order to determine the relative level of isopentenyl pyrophosphate, liver slices from normal and squalestatin I infused rats were labeled to steady-state with [3H]acetate. Analysis of isoprenoid pyrophosphate intermediates by radio-HPLC after dephosphorylation indicated that squalestatin I brought about a 20-fold increase in the relative level of FPP (confirming direct analysis) and a 5-fold increase in the relative level of IPP. No change in either of these compounds was observed in livers from cholesterol-fed rats. To determine if squalestatin I altered the synthesis of nonsterol products, rats were subjected to long term subcutaneous infusion. After 14 days of infusion of 15 micrograms h-1, the median chain length of hepatic dolichol and dolichyl phosphate increased from C95 to C115 and the levels of these lipids increased approximately 3-fold. In addition, dolichyl phosphate mannose synthase activity in microsomes from squalestatin I treated rats was increased relative to controls when assayed in the absence of dolichyl phosphate. Squalestatin I affected ubiquinone metabolism to a lesser extent: chain lengths shifted from a Q10/Q9 ratio of 0.118 +/- 0.021 in the normal rat to 0.185 +/- 0.016 in the squalestatin I treated animals, and levels rose by approximately 90%. These results suggest that the isoprenoid pyrophosphate intermediates are shared by the cholesterol, dolichol and ubiquinone pathways and further show that the dolichol and ubiquinone pathways are not saturated. Apparently, under normal conditions, the levels of these intermediates are maintained relatively constant by coordinate enzyme regulation, thereby ensuring a constant rate of synthesis of nonsterols.

Gene expression of hepatic cholesterol 7 alpha-hydroxylase in the course of puromycin-induced nephrosis

Cholesterol conversion to and biliary excretion of bile acids represents the principal pathway of cholesterol catabolism in mammals. Cholesterol 7 alpha-hydroxylase (Ch-7 alpha-H) is the first and the rate limiting step in bile acid production. Recently, Ch-7 alpha-H enzymatic activity has been shown to be normal in rats with established puromycin aminonucleoside-induced nephrosis (NS). To our knowledge, the gene expression of Ch-7 alpha-H in NS has not been investigated. We measured hepatic Ch-7 alpha-H mRNA and protein (by Northern and Western blot analyses) in rats at baseline and longitudinally during the course of induction and chronic phase of puromycin (PAN) induced NS. Groups of placebo-treated (controls) and diet-induced hypercholesterolemic (DHC) rats were included for comparison. The NS and DHC animals exhibited severe hypercholesterolemia of similar magnitude. Hepatic Ch-7 alpha-H transcript and protein remained virtually unchanged throughout the study period in the NS group. In contrast, Ch-7 alpha-H gene expression was markedly up-regulated in the DHC group. These observations suggest that hepatic Ch-7 alpha-H gene expression may be inappropriately low for the degree of the associated hypercholesterolemia in the NS group. It should be noted, however, that hepatic tissue cholesterol concentration was normal in the NS group and greatly increased in the DHC group. This can account for the disparity in Ch-7 alpha-H mRNA levels between the two groups since intracellular rather than extracellular cholesterol modulates Ch-7 alpha-H gene expression. In conclusion, the present study revealed that hepatic Ch-7 alpha-H gene expression remains unchanged during the course of PAN-induced NS in rats. It thus appears that generation and maintenance of hypercholesterolemia in this model of NS does not involve significant alteration of Ch-7 alpha-H gene expression.

Barrier disruption stimulates interleukin-1 alpha expression and release from a pre-formed pool in murine epidermis

Previous studies have shown that barrier disruption increases epidermal mRNA levels of interleukin-1 alpha (IL-1 alpha). We used immunohistochemistry to examine IL-1 alpha expression in hairless mouse skin under basal conditions and following barrier abrogation. In untreated mice, IL-1 alpha was present in the dermis and nucleated epidermal layers in a diffuse, generalized pattern. In essential fatty acid deficient mice IL-1 alpha was present in all epidermal layers and the dermis, with prominent staining in the stratum corneum. After acute barrier disruption with tape-stripping, IL-1 alpha increased in the epidermis and dermis within 10 min, remained elevated at 2 and 4 h, and decreased to near basal levels by 24 h. Moreover, intense, perinuclear, basal cell staining appeared at 10 min, persisting until 4 h after barrier disruption. Since the increase in IL-1 alpha immunostaining after acute barrier abrogation precedes the increase in mRNA, we hypothesized that the IL-1 alpha might derive from a pre-formed pool. Prolonged occlusion of normal skin, a treatment that specifically reduces epidermal mRNA levels of IL-1 alpha, decreased basal immunostaining for IL-1 alpha and blunted the increase in IL-1 alpha usually seen following barrier disruption. Moreover, tape-stripping of skin, maintained ex vivo at 4 degrees C, resulted in increased IL-1 alpha immunostaining within the upper nucleated epidermal layers, as well as release of mature IL-1 alpha into the medium, as measured by Western blotting and enzyme-linked immunosorbent assay. In addition, the stratum corneum attached to the tape contained IL-1 alpha. These studies show that acute barrier disruption induces both the immediate release and dispersion of IL-1 alpha from a pre-formed, epidermal pool, as well as increased IL-1 alpha synthesis; both mechanisms are consistent with a role for IL-1 alpha in the regulation of proinflammatory and homeostatic processes in the skin.

Effects of monoterpenes and mevinolin on murine colon tumor CT-26 in vitro and its hepatic "metastases" in vivo

Tumors derived from the colonic epithelium exhibit cholesterol metabolism which is clearly different from that in fibroblasts, hepatocytes, adrenals, and ovaries. In hepatocytes and fibroblasts MEV inhibition of the rate limiting step in cholesterol synthesis HMG Co A reductase can be overcome by the uptake of LDL. Colon cancer cells however do not overcome MEV inhibition by LDL uptake but rather exhibit further growth suppression Mevinolin (Mevacor), a drug used to lower serum cholesterol levels has the advantage of accumulating in the liver to approximately 95% with the first pass. A small but variable percentage of non-sterol precursors may escape inhibition and be utilized for other pathways in the isoprenylation of certain proteins, among them members of the ras family. Mutated ras, an oncogene, is found in 40-50% of colon tumors and the expression of a functional gene product is dependent on isoprenylation for anchorage to the tumor cell membrane. d-Limonene, a relatively non-toxic monoterpene found in orange skin oil, selectively inhibits isoprenylation and also accumulates to some extent in the liver. It was hypothesized that the differences in mevalonate metabolism between hepatocytes and colon tumor cells could provide a chemotherapeutic advantage in which MEV and/or d-limonene could effectively inhibit cholesterol synthesis and post-translational modification of proteins with non-sterol cholesterol precursors in colon tumor derived hepatic metastases and thus inhibit their growth. Since each drug affects aspects of mevalonate synthesis at different points, the effects of the combination of their agents on inhibiting tumor metastases was investigated to ascertain if these could be additive. In tissue culture, MEV and d-limonene significantly inhibited the growth of CT-26, a murine transplantable colon tumor. Cholesterol synthesis assessed in these cells indicated that in lipid deficient media the following additions-25-hydroxycholesterol, and LDL significantly reduced cholesterol synthesis. Conversely, perillyl alcohol increased cholesterol synthesis 2.5 fold. In cells cultured in FBS based medium, which have an FBS control, MEV treatment reduced cholesterol synthesis to 65% of control. Perillyl alcohol increased synthesis 1.4 fold and when given in conjunction with MEV, it abolished the effects of this inhibitor. In isoprenylation studies of 14C-mevalonate incorporation into proteins, MEV impaired isoprenylation by restricting synthesis of mevalonate derived intermediates. Results of CT-26 treatment with perillyl alcohol are inconsistent with its putative role as a protein isoprenylation inhibitor. The combination of these agents indicates an additive action which requires additional investigation to elucidate their mechanism(s). Dietary MEV and d-limonene were evaluated alone and in combination for their chemotherapeutic potential in a hepatic "metastasis" model. Using splenic colonization in which CT-26 was implanted into the spleen and ultimately seeded the liver, each of these compounds were found to inhibit the growth of resultant tumors both alone and in combination by approximately 80% versus controls at 35 days post-implantation. Assessment of HMGCoA reductase in liver and tumor indicated that these agents were effective in reaching these target sites. The findings to date indicate that while d-limonene and MEV may differentially affect the same pathway, and their individual actions may appear antagonistic in vitro, their overall action individually or together, appears promising as a chemotherapeutic modality for the possible management of hepatic metastases from colon cancer.

Hepatic HMG-CoA reductase gene expression during the course of puromycin-induced nephrosis

Increased production and depressed catabolism of lipoproteins play major roles in the pathogenesis of hypercholesterolemia of nephrotic syndrome (NS). However, the effect, if any, of NS on cholesterol biosynthetic capacity is uncertain. We examined the gene expression of hepatic 3-hydroxy-3-methylglutaryl-CoA reductase (HMG-CoAR, the rate limiting step in cholesterol biosynthesis) during the induction and chronic phase of puromycin (PAN)-induced NS in rats. The rats were randomized to NS (given i.p. puromycin aminonucleoside 130 mg/kg on day 1 and 60 mg/kg on day 14) and placebo-treated control groups. Subgroups of animals were sacrificed at days 5, 10, 20 and 30. The liver was harvested between 7 and 9 p.m. for measurements of HMG-CoAR and actin mRNAs, HMG-CoAR enzymatic activity and microsomal cholesterol concentration. In separate experiments, subgroups of animals with chronic NS (day 30) were studied in fed and 20-hour fasting states. A marked but transient rise in hepatic HMG-CoAR mRNA and HMG-CoAR enzymatic activity was observed following the onset and exacerbation of proteinuria within a few days after each puromycin injection. On each occasion, HMG-CoAR fell to the baseline level despite persistent severe hypercholesterolemia. In an attempt to examine the possible acute effect of PAN per se, experiments were repeated before and at short intervals (8 and 24 hr) after puromycin injection when proteinuria was absent and the drug exposure prominent. The HMG-CoAR mRNA and activity were virtually unchanged during this period, suggesting the lack of an acute effect of puromycin. Twenty-hour fasting led to a marked rise in HMG-CoAR mRNA and activity in animals with chronic NS but not in the controls. Microsomal cholesterol remained unchanged and comparable in the two groups at all points. Thus, the marked but transient rise in hepatic HMG-CoAR gene expression observed during the induction phase and with fasting during the chronic phase of PAN-induced NS may contribute to the generation and maintenance of hypercholesterolemia in this animal model.

A fraction derived from brewer's yeast inhibits cholesterol synthesis by rat liver preparations in vitro

Brewer's yeast was grown on a defined medium containing tracer 51Cr with or without added chromium. The two batches of yeast contained 10 microgram/g (high-Cr) or 80 ng/g (low-Cr). Extracts were prepared and fractionated. A third batch of yeast (third batch) was grown with added Cr, and fractionated. Rats were reared on either rat cubes (normal diet) or on a low-Cr diet (low-Cr), or on rat cubes with added cholestyramine (cholestyramine diet). Preparations of rat liver, both cell-free and intact hepatocytes, incorporated acetate-carbon into fatty acids and cholesterol. These processes were inhibited by a yeast fraction containing small, neutral, water-soluble compounds. The degree of inhibition was the same whether the liver came from normal rats or rats fed on the low-Cr diet. Similarly the inhibitory effect was found with identical amounts of extracts from low- or high-Cr yeasts. Therefore, Cr compounds do not appear to account for the inhibitory effects of brewer's yeast. Use of other substrates indicated that the site of inhibition of sterol synthesis was apparently between acetyl-CoA and mevalonate. One inhibitory substance was isolated from yeast and was found to be nicotinamide riboside. This may have been produced from NAD(P) during the preparation of yeast extracts, and it may be produced from dietary yeast supplements during digestion in vivo. Nicotinamide riboside may be partly responsible for the reported effects of yeast supplements on plasma lipids in humans.

Decreased cholesterol biosynthesis in sitosterolemia with xanthomatosis: diminished mononuclear leukocyte 3-hydroxy-3-methylglutaryl coenzyme A reductase activity and enzyme protein associated with increased low-density lipoprotein receptor function

We investigated the mechanism for reduced cholesterol biosynthesis in sitosterolemia with xanthomatosis. The conversion of acetate to cholesterol and total and active hydroxymethylglutaryl (HMG) coenzyme A (CoA) reductase activities, enzyme protein mass, and catalytic efficiency were related to low-density lipoprotein (LDL) receptor function in freshly isolated mononuclear leukocytes collected at 9 AM after a 12-hour fast from two affected sisters and 12 control subjects. Active HMG-CoA reductase activity was determined in mononuclear leukocyte microsomes prepared and assayed in the presence of sodium fluoride, while total HMG-CoA reductase activity was determined in the absence of the phosphatase inhibitor. Enzyme protein was assayed using rabbit polyclonal anti-rat liver microsomal HMG-CoA reductase serum. The rates at which [14C]acetate was transformed to cholesterol by sitosterolemic mononuclear leukocytes were decreased 29% and 41%, respectively, compared with the mean value for mononuclear leukocytes from 12 control subjects. Similarly, total HMG-CoA reductase activities were 71% and 68% lower in sitosterolemic mononuclear leukocyte microsomes and were associated with 62% and 65% less enzyme protein than the mean for the control microsomal preparations. This marked decrease in HMG-CoA reductase protein mass in sitosterolemic microsomes was partially compensated for by an increase in the proportion of active enzyme. Sitosterolemic plasma and mononuclear leukocyte cholesterol concentrations were not significantly different from control values, although total sterol levels were increased about 20% because of abundant plant sterols. In contrast, receptor-mediated LDL degradation by sitosterolemic mononuclear leukocytes was increased 50% over control.(ABSTRACT TRUNCATED AT 250 WORDS)

Identification of a cholesterol-regulated 180-kDa microsomal protein in rat hepatocytes

The immunoprecipitation by antibodies to 3-hydroxy-3-methylglutaryl-coenzyme A (HMG-CoA) reductase of extracts of [35S]methionine-pulse-labelled isolated hepatocytes, followed by electrophoresis and fluorography, showed the presence not only of 97-kDa HMG-CoA reductase, but also of another protein of 180 kDa. Boiling the immunoprecipitates both in the presence and in the absence of 2-mercaptoethanol, followed by SDS/polyacrylamide gel electrophoresis both in the presence and in the absence of 8 M urea, was not found to change the ratio of 180-kDa/97-kDa proteins. These facts suggest that the 180-kDa protein is not an aggregated form of HMG-CoA reductase. A different batch of antibodies obtained from a newly purified HMG-CoA reductase fully titrated the reductase activity, but did not immunoprecipitate the 180-kDa protein, showing that there is no cross-reactivity between these proteins. The 180-kDa polypeptide is a glycoprotein of N-linked high-mannose oligosaccharide chains, which is not processed on the Golgi system. The apparent molecular mass of the carbohydrate is 16 kDa. The incubation of rat hepatocytes with sterols produces, on the one hand, a decrease in the rate of synthesis, and on the other hand, an acceleration in the turnover rate of the 180-kDa protein. In addition, mevalonate is known to decrease its rate of synthesis. The carbohydrate-free 164-kDa protein was found to degrade only a tenth as fast as the glycoprotein and, furthermore, the degradation was no longer accelerated by sterols. These results support the notion that the 180-kDa protein is not a modified form of 97-kDa reductase, but probably a different protein related to cholesterol metabolism, and also that the N-linked, high-mannose chains, which are bound to the glycoprotein, are required for rapid and controlled degradation of the protein.

Purification and characterization of the AMP-activated protein kinase. Copurification of acetyl-CoA carboxylase kinase and 3-hydroxy-3-methylglutaryl-CoA reductase kinase activities

1. We have purified the AMP-activated protein kinase 4800-fold from rat liver. The acetyl-CoA carboxylase kinase and 3-hydroxy-3-methylglutaryl-CoA(HMG-CoA) reductase kinase activities copurify through all six purification steps and are inactivated with similar kinetics by treatment with the reactive ATP analogue fluorosulphonylbenzoyladenosine. 2. The final preparation contains several polypeptides detectable by SDS/polyacrylamide gel electrophoresis, but only one of these, with an apparent molecular mass of 63 kDa, is labelled using [14C]fluorosulphonylbenzoyladenosine. This is also the only polypeptide in the preparation that becomes significantly labelled during incubation with [gamma 32P]ATP. This autophosphorylation reaction did not affect the AMP-stimulated kinase activity. 3. In the absence of AMP the purified kinase has apparent Km values for ATP and acetyl-CoA carboxylase of 86 microM and 1.9 microM respectively. AMP increases the Vmax 3-5-fold without a significant change in the Km for either protein or ATP substrates. 4. The response to AMP depends on the ATP concentration in the assay, but at a near-physiological ATP concentration the half-maximal effect of AMP occurs at 14 microM. Studies with a range of nucleoside monophosphates and diphosphates, and AMP analogues showed that the allosteric activation by AMP was very specific. ADP gave a small stimulation at low concentrations but was inhibitory at high concentrations. 5. These results show that the AMP-activated protein kinase is the major HMG-CoA reductase kinase detectable in rat liver under our assay conditions and that it is therefore likely to be identical to previously described HMG-CoA reductase kinase(s) which are activated by adenine nucleotides and phosphorylation. The AMP-binding and catalytic domains of the kinase are located on a 63-kDa polypeptide which is subject to autophosphorylation.

A discoordinate increase in the cellular amount of 3-hydroxy-3-methylglutaryl-CoA reductase results in the loss of rate-limiting control over cholesterogenesis in a tumour cell-free system

Cholesterol biosynthesis was characterized in cell-free post-mitochondrial supernatant systems prepared from both normal rat liver and Morris hepatoma 3924A. The rate of cholesterol synthesis per cell was 9-fold greater in the tumour system than in that from normal liver, and the tumour systems showed the loss of rate-limiting control at the hydroxymethylglutaryl-CoA reductase (HMGR)-catalysed step. The apparent absence of rate-limiting control over cell-free tumour cholesterogenesis was traced primarily to a discoordinate and dramatic increase in the amount of HMGR in the tumour relative to the liver system. Preliminary evidence for an altered control of the post-lanosterol portion of the pathway was also obtained with the tumour system.

Distinct type-1 protein phosphatases are associated with hepatic glycogen and microsomes

The type-1 protein phosphatase associated with hepatic microsomes has been distinguished from the glycogen-bound enzyme in five ways. (1) The phosphorylase phosphatase/synthase phosphatase activity ratio of the microsomal enzyme (measured using muscle phosphorylase a and glycogen synthase (labelled in sites-3) as substrates) was 50-fold higher than that of the glycogen-bound enzyme. (2) The microsomal enzyme had a greater sensitivity to inhibitors-1 and 2. (3) Release of the catalytic subunit from the microsomal type-1 phosphatase by tryptic digestion was accompanied by a 2-fold increase in synthase phosphatase activity, whereas release of the catalytic subunit from the glycogen-bound enzyme decreased synthase phosphatase activity by 60%. (4) 95% of the synthase phosphatase activity was released from the microsomes with 0.3 M NaCl, whereas little activity could be released from the glycogen fraction with salt. (5) The type-1 phosphatase separated from glycogen by anion-exchange chromatography could be rebound to glycogen, whereas the microsomal enzyme (separated from the microsomes by the same procedure, or by extraction with NaCl) could not. These findings indicate that the synthase phosphatase activity of the microsomal enzyme is not explained by contamination with glycogen-bound enzyme. The microsomal and glycogen-associated enzymes may contain a common catalytic subunit complexed to microsomal and glycogen-binding subunits, respectively. Thiophosphorylase a was a potent inhibitor of the dephosphorylation of ribosomal protein S6, HMG-CoA reductase and glycogen synthase, by the glycogen-associated type-1 protein phosphatase. By contrast, thiophosphorylase a did not inhibit the dephosphorylation of S6 or HMG-CoA reductase by the microsomal enzyme, although the dephosphorylation of glycogen synthase was inhibited. The I50 for inhibition of synthase phosphatase activity by thiophosphorylase a catalysed by either the glycogen-associated or microsomal type-1 phosphatases, or for inhibition of S6 phosphatase activity catalysed by the glycogen-associated enzyme, was decreased 20-fold to 5-10 nM in the presence of glycogen. The results suggest that the physiologically relevant inhibitor of the glycogen-associated type-1 phosphatase is the phosphorylase a-glycogen complex, and that inhibition of the microsomal type-1 phosphatase by phosphorylase a is unlikely to play a role in the hormonal control of cholesterol or protein synthesis. Protein phosphatase-1 appears to be the principal S6 phosphatase in mammalian liver acting on the serine residues phosphorylated by cyclic AMP-dependent protein kinase.

In situ determination of the functional size of hepatic 3-hydroxy-3-methylglutaryl-CoA reductase by radiation inactivation analysis

Radiation inactivation analysis of liver pieces yielded a target size of 210 kDa for hepatic 3-hydroxy-3-methylglutaryl CoA (HMG-CoA) reductase [S)-mevalonate:NADP+ oxidoreductase (CoA-acylating), EC 1.1.1.34) from rats fed a normal diet. Feeding a diet containing mevinolin and colestipol, which causes a marked increase in enzyme activity, resulted in a reduction of the target size to 120 kDa. These results are consistent with those obtained by radiation inactivation and immunoblotting analysis of isolated microsomes and suggest that the increase in HMG-CoA reductase activity caused by these dietary agents is accompanied by a change from a dimer to a monomer form of the enzyme.

In vivo regulation of human mononuclear leukocyte 3-hydroxy-3-methylglutaryl coenzyme A reductase. Decreased enzyme catalytic efficiency in familial hypercholesterolemia

3-Hydroxy-3-methylglutaryl coenzyme A reductase (HMG CoA reductase) controls the rate of cholesterol biosynthesis and is itself modulated through feedback suppression by internalized low density lipoprotein (LDL) cholesterol. We measured HMG CoA reductase protein concentration and microsomal enzyme activity in freshly isolated mononuclear leukocytes from normal individuals and patients with heterozygous or homozygous familial hypercholesterolemia (FH). Reductase protein concentration was similar in normal and heterozygous subjects, but was over twofold elevated in patients with homozygous FH. Reductase protein concentration was inversely related to LDL receptor status. Total activity and catalytic efficiency of reductase, however, were decreased in heterozygous and homozygous FH patients. The decrease in catalytic efficiency was not due to enzyme phosphorylation or thiol-disulfide formation. Reduction of plasma cholesterol concentration over 2 h by plasmapheresis increased reductase activity, the degree of which was directly proportional to the LDL-receptor status of the subjects. Decreased HMG CoA reductase activity and catalytic efficiency in mononuclear leukocytes and perhaps other cells in FH may represent a fundamental abnormality in the regulation of this enzyme independent of that induced by the LDL-receptor defect and may provide new insight into the control of cholesterol metabolism in FH.

Turnover rates of HMG-CoA reductase mRNA: role of pituitary and thyroid hormones

When normal or hypophysectomized rats maintained on a diet containing mevinolin and colestipol were switched to a normal chow diet, HMG-CoA reductase mRNA fell rapidly with a half-life of 3 hrs. After a 90 min lag period, reductase activity and immunoreactive protein fell in parallel with the mRNA in normal rats. In hypophysectomized rats, reductase activity and protein required 10 and 18 hrs, respectively to fall to 50% of their original levels. Administration of thyroid hormones to hypophysectomized rats resulted in a 3 to 4 fold stabilization of reductase mRNA suggesting that the increased mRNA levels are due in part to a posttranscriptional regulatory effect of thyroid hormones.

Loss of NADPH during assays of HMG-CoA reductase: implications and approaches to minimize errors

In assays of 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase activity, preincubation of isolated washed microsomes with NADPH led to a time- and protein concentration-dependent loss of enzyme activity. This occurred despite the presence of an NADPH regenerating system. Addition of fresh NADP, glucose 6-phosphate and glucose 6-phosphate dehydrogenase restored activity. Of the individual components, only NADP was effective. Errors due to loss of NADPH are most pronounced in assays using high microsomal protein, low NADPH levels and preincubation with NADPH and when glutathione rather than dithiothreitol is present. To minimize the effects of NADPH depletion, it is recommended that (i) NADP and NADPH not be present during the preincubation period; (ii) incubation periods be relatively short; (iii) microsomal protein concentrations be less than 1 mg; and (iv) NADPH concentrations be 1 to 2 mM.