Properties of purified rat hepatic 3-hydroxy-3-methylglutaryl coenzyme A reductase and regulation of enzyme activity

Cosmc regulates O-glycan extension in murine hepatocytes

Hepatocytes synthesize a vast number of glycoproteins found in their membranes and secretions, many of which contain O-glycans linked to Ser/Thr residues. As the functions and distribution of O-glycans on hepatocyte-derived membrane glycoproteins and blood glycoproteins are not well understood, we generated mice with a targeted deletion of Cosmc (C1Galt1c1) in hepatocytes. Liver glycoproteins in WT mice express typical sialylated core 1 O-glycans (T antigen/CD176) (Galβ1-3GalNAcα1-O-Ser/Thr), whereas the Cosmc knockout hepatocytes (HEP-Cosmc-KO) lack extended O-glycans and express the Tn antigen (CD175) (GalNAcα1-O-Ser/Thr). Tn-containing glycoproteins occur in the sera of HEP-Cosmc-KO mice but not in WT mice. The LDL-receptor (LDLR), a well-studied O-glycosylated glycoprotein in hepatocytes, behaves as a ∼145kD glycoprotein in WT liver lysates, whereas it is reduced to ∼120 kDa in lysates from HEP-Cosmc-KO mice. Interestingly, the expression of the LDLR, as well as HMG-CoA reductase, which is typically altered in response to dysregulated cholesterol metabolism, are similar between WT and HEP-Cosmc-KO mice, indicating no significant effect by Cosmc deletion on either LDLR stability or cholesterol metabolism. Consistent with this, we observed no detectable phenotype in the HEP-Cosmc-KO mice regarding development, appearance or aging compared to WT. These results provide surprising, novel information about the pathway of O-glycosylation in the liver.

Eupatilin Alleviates Hyperlipidemia in Mice by Inhibiting HMG-CoA Reductase

Artemisia princeps (family Asteraceae) is a natural product broadly used as an antioxidative, hepatoprotective, antibacterial, and anti-inflammatory agent in East Asia. In the present study, eupatilin, the main constituent of Artemisia princeps, was investigated as an antihyperlipidemic agent. Eupatilin inhibited 3-hydroxy-3-methylglutaryl (HMG)-CoA reductase (HCR), an enzyme that is a therapeutic target for hyperlipidemia, in an ex vivo assay using rat liver. In addition, oral administration of eupatilin significantly lowered the serum levels of total cholesterol (TC) and triglycerides (TG) in corn oil-induced and Triton WR-1339-induced hyperlipidemic mice. These results suggest that eupatilin can alleviate hyperlipidemia by inhibiting HCR.

Effects and Mechanisms of Chitosan and ChitosanOligosaccharide on Hepatic Lipogenesis and Lipid Peroxidation, Adipose Lipolysis, and Intestinal Lipid Absorption in Rats with High-Fat Diet-Induced Obesity

Chitosan and its derivative, chitosan oligosaccharide (CO), possess hypolipidemic and anti-obesity effects. However, it is still unclear if the mechanisms are different or similar between chitosan and CO. This study was designed to investigate and compare the effects of CO and high-molecular-weight chitosan (HC) on liver lipogenesis and lipid peroxidation, adipose lipolysis, and intestinal lipid absorption in high-fat (HF) diet-fed rats for 12 weeks. Rats were divided into four groups: normal control diet (NC), HF diet, HF diet+5% HC, and HF diet+5% CO. Both HC and CO supplementation could reduce liver lipid biosynthesis, but HC had a better effect than CO on improving liver lipid accumulation in HF diet-fed rats. The increased levels of triglyceride decreased lipolysis rate, and increased lipoprotein lipase activity in the perirenal adipose tissue of HF diet-fed rats could be significantly reversed by both HC and CO supplementation. HC, but not CO, supplementation promoted liver antioxidant enzymes glutathione peroxidase and superoxide dismutase activities and reduced liver lipid peroxidation. In the intestines, CO, but not HC, supplementation reduced lipid absorption by reducing the expression of fabp2 and fatp4 mRNA. These results suggest that HC and CO have different mechanisms for improving lipid metabolism in HF diet-fed rats.

The regulatory effects of fish oil and chitosan on hepatic lipogenic signals in high-fat diet-induced obese rats

The present study investigated the regulatory effects of fish oil and chitosan on the signals of hepatic lipid metabolism and the postulated mechanism in high-fat diet-induced obese rats. Diet supplementation of chitosan and fish oil efficiently suppressed the increased weights in body and livers of high-fat diet-fed rats. Supplementation of chitosan and fish oil significantly decreased the activities of hepatic lipid biosynthesis-related enzymes and efficiently regulated plasma lipoprotein homeostasis. Both chitosan and fish oil significantly ameliorated the alterations in the protein expressions of hepatic lipogenic transcription factors (LXRα and PPARα), and could also significantly regulate the downstream hepatic lipogenic genes (FAS, HMGCR, CYP7A1, FATP, FABP, AOX, and ABCA) expressions in high-fat diet-fed rats. These results suggest that both fish oil and chitosan exerts downregulative effects on hepatic lipid metabolism in high-fat diet-induced obese rats via the LXRα inhibition and PPARα activation, which further affect the expressions of hepatic lipogenesis-associated genes.

Supplementation of chitosan alleviates high-fat diet-enhanced lipogenesis in rats via adenosine monophosphate (AMP)-activated protein kinase activation and inhibition of lipogenesis-associated genes

This study investigated the role of chitosan in lipogenesis in high-fat diet-induced obese rats. The lipogenesis-associated genes and their upstream regulatory proteins were explored. Diet supplementation of chitosan efficiently decreased the increased weights in body, livers, and adipose tissues in high-fat diet-fed rats. Chitosan supplementation significantly raised the lipolysis rate; attenuated the adipocyte hypertrophy, triglyceride accumulation, and lipoprotein lipase activity in epididymal adipose tissues; and decreased hepatic enzyme activities of lipid biosynthesis. Chitosan supplementation significantly activated adenosine monophosphate (AMP)-activated protein kinase (AMPK) phosphorylation and attenuated high-fat diet-induced protein expressions of lipogenic transcription factors (PPAR-γ and SREBP1c) in livers and adipose tissues. Moreover, chitosan supplementation significantly inhibited the expressions of downstream lipogenic genes (FAS, HMGCR, FATP1, and FABP4) in livers and adipose tissues of high-fat diet-fed rats. These results demonstrate for the first time that chitosan supplementation alleviates high-fat diet-enhanced lipogenesis in rats via AMPK activation and lipogenesis-associated gene inhibition.

HMG-CoA Reductase Inhibitors fromMonascus-Fermented Rice

Seven compounds were isolated fromMonascus-fermented rice by column chromatography with silica gel and semiprep HPLC. Their structures were elucidated by extensive spectroscopic methods. All compounds displayed HMG-CoA reductase inhibitory potential, among them compound7exhibited strong inhibition with IC50value comparable with lovastatin. In this study, two compounds (1and2) were obtained from natural source for the first time.

Epigallocatechin-3-gallate potently inhibits the in vitro activity of hydroxy-3-methyl-glutaryl-CoA reductase

Hydroxy-3-methyl-glutaryl-CoA reductase (HMGR) is the rate-controlling enzyme of cholesterol synthesis, and owing to its biological and pharmacological relevance, researchers have investigated several compounds capable of modulating its activity with the hope of developing new hypocholesterolemic drugs. In particular, polyphenol-rich extracts were extensively tested for their cholesterol-lowering effect as alternatives, or adjuvants, to the conventional statin therapies, but a full understanding of the mechanism of their action has yet to be reached. Our work reports on a detailed kinetic and equilibrium study on the modulation of HMGR by the most-abundant catechin in green tea, epigallocatechin-3-gallate (EGCG). Using a concerted approach involving spectrophotometric, optical biosensor, and chromatographic analyses, molecular docking, and site-directed mutagenesis on the cofactor site of HMGR, we have demonstrated that EGCG potently inhibits the in vitro activity of HMGR (K(i) in the nanomolar range) by competitively binding to the cofactor site of the reductase. Finally, we evaluated the effect of combined EGCG-statin administration.

Regulation of isoprenoid/cholesterol biosynthesis in cells from mevalonate kinase-deficient patients

Mevalonic aciduria (MA) and hyper-IgD and periodic fever syndrome (HIDS) are two inherited disorders both caused by depressed mevalonate kinase (MK) activity. MK is the first enzyme to follow the highly regulated 3-hydroxy-3-methylglutaryl (HMG)-CoA reductase (HMGR), which catalyzes the rate-limiting step in the isoprenoid/cholesterol biosynthesis pathway. In fibroblasts of MA patients, but not of HIDS patients, HMGR activity is elevated under normal growth conditions. This activity is down-regulated when cells are supplemented with the isoprenoid precursors geraniol, farnesol, and geranylgeraniol, and a mixture of 25-hydroxycholesterol and cholesterol. This indicates that the regulation of the pathway in these cells is not disturbed. The elevated HMGR activity is probably due to a shortage of non-sterol isoprenoid end products, as indicated by normal HMGR mRNA levels in MA fibroblasts. Furthermore, the HMGR activity in MA cells was more sensitive to geranylgeraniol suppression and less sensitive to sterol suppression than the HMGR activity in low density lipoprotein receptor-deficient cells. HMGR activity in MA cells was down-regulated also by addition of its product mevalonate to the culture medium. Thus, it appears that the elevation of mevalonate levels, which are high in MA patients and moderate in HIDS patients, allows the cells to compensate for the depressed MK activity. Indeed, the isoprenylation of Ras and RhoA protein appeared normal in HIDS and MA fibroblasts under normal conditions but showed increased sensitivity toward inhibition of HMGR by simvastatin. Our results indicate that MK-deficient cells maintain the flux through the isoprenoid/cholesterol biosynthesis pathway by elevating intracellular mevalonate levels.

Daio-Orengedokuto inhibits HMG-CoA reductase and pancreatic lipase

To evaluate the antihyperlipidemic activities of Orengedokuto (OT) and Daio-Orengedokuto (DOT), the inhibitory effects of these polyprescriptions on HMG-CoA reductase and pancreatic lipase and on the rat hyperlipidemic model induced by Triton WR-1339 were measured. OT potently inhibited HMG-CoA reductase but did not inhibit lipase. Among their ingredients, Coptidis Rhizoma was the most potent inhibitor, followed by Rhei Rhizoma. The HMG-CoA reductase-inhibitory activity of 80% EtOH extract was superior to that of water extract. However, DOT potently inhibited HMG CoA-reductase as well as pancreatic lipase. In the rat hyperlipidemic model induced by Triton WR-1339, OT and DOT decreased serum total cholesterol and low-density lipoprotein cholesterol levels. DOT also decreased serum triglyceride levels, but OT did not decrease it. These results suggest that the antihyperlipidemic activity of DOT may originate from the inhibition of pancreatic lipase as well as HMG-CoA reductase.

Ovarian 3-hydroxy-3-methylglutaryl-CoA reductase in Blattella germanica (L.): pattern of expression and critical role in embryogenesis

In the ovary of adult Blattella germanica, the enzyme 3-hydroxy-3-methylglutaryl-CoA reductase (HMG-CoA reductase) is highly expressed in mid-late vitellogenesis, suggesting a functional link of the mevalonate pathway with choriogenesis. The inhibitor of HMG-CoA reductase, fluvastatin, applied in females in late vitellogenesis, inhibits the activity of the enzyme in the ovary and in the developing embryos within the ootheca. This does not affect choriogenesis or ootheca formation but reduces the number of larvae per ootheca. Our results suggest that fluvastatin is incorporated into the oocytes and has delayed inhibitory effects on the oviposited eggs. HMG-CoA reductase is essential for embryogenesis, but not for chorion formation.

Coordinated expression and activity of 3-hydroxy-3-methylglutaryl coenzyme A synthase and reductase in the fat body of Blattella germanica (L.) during vitellogenesis

Levels of mRNA for the two 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) synthases, (HMG-S1 and HMG-S2), and for HMG-CoA reductase (HMG-R) of Blattella germanica were analyzed in the fat body during the first gonadotrophic cycle. HMG-S2 and HMG-R showed the highest mRNA levels on day 0 and decreased thereafter, whereas HMG-S1, showed faint expression. Western blot using specific antibodies for HMG-S1 and HMG-S2 showed no detectable levels for HMG-S1 but a clear pattern for HMG-S2. Both results point to a very limited role for HMG-CoA synthase-1 in B. germanica fat body that the functional enzyme in this organ is HMG-CoA synthase-2. HMG-CoA reductase and synthase proteins shared a cyclic pattern (maximum levels at day 4 and minimum levels on days 0 and 8), which was coincident with the pattern of activity. The delay between gene transcription and protein synthesis suggests a finely regulated translation mechanism. Moreover, the pattern of mevalonate synthesis parallels that of vitellogenin production, suggesting a coordinate mechanism between the mevalonate pathway and the production of vitellogenin.

Characterization of the response of growth and differentiation to lipoproteins and agents affecting cholesterol metabolism in murine neuroblastoma cells

Treatment with mevinolin, a competitive inhibitor of HMGCoAR, the key enzyme of isoprenoid metabolism, causes the arrest of proliferation and the differentiation of a neuroblastoma cell line (N18TG2). Mevalonate and high density lipoproteins partially restore growth. Cholesterol synthesis in the presence of mevinolin remains active, because in these cells the key enzyme HMG-CoA reductase is not completely inhibited by this drug. The fact that cell growth is reduced, while cholesterogenesis remains active, suggests that mevinolin acts by interfering with the synthesis of some unknown compound, other than cholesterol, which is necessary for proliferation.

Unstable amplification of the chromosomal gene for 3-hydroxy-3-methylglutaryl coenzyme A reductase in compactin-resistant CR200 cells

CR200 cells, a compactin-resistant clone of mouse FM3A cells, overaccumulate 3-hydroxy-3-methylglutaryl coenzyme A reductase. The elevated reductase activity is not regulated normally by low-density lipoprotein, mevalonate and 25-hydroxycholesterol. The amounts of reductase protein and mRNA were elevated in CR200 cells by 40- to 60-fold, as compared to those in parental FM3A cells and the rate of reductase transcription and the number of copies of reductase gene were increased in CR200 cells by 20- to 50-fold. In the parental cells, mevalonate and 25-hydroxycholesterol suppressed reductase transcription by greater than 90%, while that in the mutant cells was suppressed by only 20-50%, suggesting a regulatory alteration in the gene transcription in CR200 cells. When CR200 cells were grown for 10-20 weeks in the absence of compactin, levels of the gene amplification were reduced from approx. 50-fold to approx. 2-fold, along with a marked decrease in reductase activity and compactin-resistance of the cells. While the gene amplification was unstable, minute chromosomes were not seen in the cells and centrifugal fractionation and in situ analysis demonstrated that the amplified reductase gene was present on 2-3 chromosomes in the pseudotetraploidal CR200 cells having approx. 78 chromosomes. From these results it was concluded that the amplified reductase gene, which is responsible for overaccumulation of reductase, is located on chromosomes but is unstable in CR200 cells.

Drug design based on biosynthetic studies: synthesis, biological activity, and kinetics of new inhibitors of 2,3-oxidosqualene cyclase and squalene epoxidase

Various classes of inhibitor of 2,3-oxido squalene cyclase have been synthesized and tested on rat liver and Saccharomyces cerevisiae microsomes, 3T3 fibroblast cultures, and various bacteria, fungi, and yeasts. The compounds include azasqualenes, azasqualanes, bis-azasqualenes, bis-azasqualanes, and N-oxide and ammonium derivatives of squalene. In order to better mimic the transition state involved in the SN2-like opening of 2,3-oxidosqualene, we synthesized squalene N-methyloxaziridine. Other derivatives tested were N-methylimine, aminalic hydroperoxide, and N-methylamide. We also attempted to produce new "suicide" inhibitors of SO cyclase, such as a squalenoid epoxide vinyl ether. Many of the products described inhibited the various cyclases, the best having an IC50 of 0.3 microM on plants and 1.5 microM on rat liver microsomes, and good antibacterial and antifungal activity. In a search for inhibitors of squalene epoxidase, a series of mono- and bifunctional squalenoid acetylenes and allenes were synthesized. Some of them proved to be inhibitors of squalene epoxidase.

Biochemical aspect of HMG CoA reductase inhibitors

Subsequent to the discovery of compactin (ML-236B) as a specific inhibitor of HMG CoA reductase, a series of compactin analogs have been either isolated or synthesized. Several of these compounds, which include compactin, mevinolin (monacolin K) and CS-514, have been extensively studied. The inhibition of HMG CoA reductase by these compounds is reversible and competitive (Ki = approximately 1 nM). The 3', 5'-dihydroxypentanoic acid portion of the acid form of compactin analogs, which resembles the HMG portion of HMG CoA, plays a crucial role in inhibitory activity. These inhibitors block sterol synthesis both in cultured mammalian cells and in animals. Strong inhibition of sterol synthesis results in a marked increase in HMG CoA reductase activity both in vitro and in vivo. These compounds strongly lower plasma LDL-cholesterol levels in animals and humans. The lowering of LDL-cholesterol levels occurs by an inhibition of LDL synthesis and/or by an elevation of the receptor-mediated LDL catabolism in the liver.

Phosphorylation and degradation of HMG CoA reductase

3-Hydroxy-3-methylglutaryl coenzyme A (HMG CoA) reductase is the limiting enzyme step in cholesterol formation in mammalian liver and other tissues. It is a glycoprotein of 97,000 daltons embedded in the endoplasmic reticulum with a long cytoplasmic extension that is the site of catalytic conversion of HMG CoA to mevalonate. The enzyme is subject to both long-term (induction/repression; degradation) and short-term control (reversible phosphorylation) mediated by endocrine signaling (insulin, glucagon) and through negative feedback by metabolic products of mevalonate (e.g., cholesterol). The catalytic capacity of microsomal reductase falls rapidly in the presence of several protein kinases (reductase kinase, protein kinase-C, calmodulin-dependent protein kinase). Activity is restored with various protein phosphatases. Increased phosphorylation of reductase in intact cells after addition of glucagon or mevalonate is followed by enhanced degradation of the enzyme. In an in vitro model system, phosphorylated, native microsomal reductase is more rapidly cleaved by the calcium-dependent, neutral protease calpain than the dephosphorylated from of reductase. Our present research which centers on the mechanism of the in vitro model system is reviewed. Calpain in the presence of Ca2+ cleaves the cytosolic domain of phosphorylated 97 kDa reductase at two points giving rise to two fragments of nearly the same size that appear as a 52-56,000 dalton doublet by electrophoresis and immunoblotting. In the same system native reductase labeled with [gamma-32P]ATP generates a doublet with 32P solely in the upper (heavier) band. This indicates that serine phosphorylation sites lie between the two calpain cleavage loci. These are positioned in the "linker" region of the long carboxy-terminal cytosolic domain near the membrane. This segment possesses five invariant serine residues and two PEST sequences (constellations of proline, glutamate, serine and threonine) that are characteristic of proteins with short half-lives. If phosphorylation of HMG CoA reductase is confined to the linker region, we must look to this domain in order to interpret the resulting conformational changes that markedly influence reductase catalytic activity and prepare the enzyme for degradation.

Effect of ACTH on cholesterol and steroid synthesis in adrenocortical tissues

Previous studies have established that under normal conditions, adrenal HMG-CoA reductase activity is higher in hamsters than in rats and humans. The hamster reductase activity follows a diurnal rhythm corresponding to that of plasma ACTH and glucocorticoids [Endocrinology 107 (1980) 215] but not to that of aldosterone. ACTH treatments to hamsters increased reductase activity after a latency of 60 min; this enhancement was prevented by cycloheximide [J. steroid Biochem. 24 (1986) 325]. Immunotitration and immunoblotting studies confirmed that ACTH caused an increase in reductase protein synthesis. In rats, long-term (1-9 days) and short-term (3 h) treatments with ACTH also induced increase in adrenal HMG-CoA reductase activity and reductase protein. In the presence of iodoacetamide and inhibitors of proteolytic enzyme, a main specific band of enzyme was evinced in the area of 102 +/- 6 kDaMr, by Western blotting, for both hamster homogenate and microsomal preparations (Endocrinology, 120 (1987]. Similarly Mr values were found with rat adrenal preparations. The concentration of mRNA, analyzed using the c-DNA pRed-10 coding for the Chinese hamster ovary reductase, was increased in adrenals of hamsters treated with ACTH. The reductase mRNA levels also fluctuated during the day in parallel with those of reductase activity and reductase protein. In conclusion, these results indicate that ACTH and other conditions inducing a change in hamster adrenal HMG-CoA reductase activity provoke parallel changes in reductase mRNA and reductase protein content. ACTH acts on the adrenal reductase of species synthesizing large as well as small quantities of cholesterol, thus indicating the general importance of this hormonal control.

The squalene-2,3-epoxide cyclase as a model for the development of new drugs

The 2,3-oxido squalene (SO) cyclases represent a group of enzymes which convert SO into polycyclic triterpenoids such as lanosterol, cycloartenol, cucurbitadienol and beta-amyrin. Taking into account the postulated model of the enzymatic cyclization of SO, we have investigated the possibility of designing compounds that would be selective and potent inhibitors of SO cyclases. Due to the fundamental role of sterols in animal, higher plant and fungal tissues, these inhibitors might behave as very selective (ipocholesterolemic, antifungal or phytotoxic) drugs. Our first approach was the synthesis and biological evaluation of 2-aza-2,3-dihydrosqualene and its derivatives which, being protonated at physiological pH, would present some similarities to the C-2 carbon ion generated by the opening of the oxirane ring of SO. Microsomes from different sources (germinated pea cotyledons, maize seedlings, rat liver and yeasts) were utilized to determine the inhibition values (I50: concentration of inhibitor producing 50% inhibition at a given substrate concentration). From the results obtained so far we conclude that 2-aza-2-dihydrosqualene and its derivatives strongly inhibited the cyclases, the site of the enzyme responsible for binding to the inhibitor is quite sensitive to the steric hindrance, and the degree of the inhibitory activity is greater in higher plants than in rat liver or fungi.

Phosphorylation state of HMG CoA reductase affects its catalytic activity and degradation

The expressed catalytic activity of liver microsomal HMG CoA reductase, the limiting enzyme in cholesterol synthesis, is reversibly diminished by phosphorylation in vitro. In intact hepatocytes the expressed activity of HMG CoA reductase is enhanced by incubation of cells with insulin, and diminished by treatment with glucagon or with mevalonate. In the latter situations the level of total reductase activity falls following initial inactivation (phosphorylation) of the enzyme. This observation suggested that the phosphorylated form of HMG CoA reductase is more sensitive to proteolysis. HMG CoA reductase is a 97,000 dalton (97 K) integral protein of the endoplasmic reticulum with a cytosolic domain that includes the catalytic site and serine residues that may be reversibly phosphorylated. In vitro the Ca2+-activated proteolytic enzyme, calpain, generates two catalytically-active fragments: a membrane bound 62 K and a soluble 53 K form of the enzyme which are quantified by specific immunoblot procedures. Cleavage of the native 97 K HMG CoA reductase is enhanced by pretreatment (inactivation) of microsomes with ATP (Mg2+) and liver reductase kinase compared to microsomes pretreated with protein phosphatase. This is reflected in a loss of 97 K reductase and an increase in the soluble 53 K form of the enzyme. Degradation of HMG CoA reductase in hepatocytes is partially blocked by lysosomotropic agents and insulin. A steady state model for the turnover of proteins subject to reversible phosphorylation has been developed which recognizes fractional degradative rate constants for the phosphorylated and dephosphorylated species.

Differentiation of neuroblastoma cells induced by an inhibitor of mevalonate synthesis: relation of neurite outgrowth and acetylcholinesterase activity to changes in cell proliferation and blocked isoprenoid synthesis

Mevinolin, a competitive inhibitor of 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase, stimulates neurite outgrowth and acetylcholinesterase (ACE) activity in C1300 (Neuro-2A) murine neuroblastoma cells. Sprouting of neurites began within 4-8 h, before changes in cell proliferation could be detected by [3H]thymidine incorporation or flow cytometry. In contrast, the increase in ACE activity was temporally correlated with suppression of DNA synthesis, which occurred after 8 h. The activity of the membrane marker enzyme phosphodiesterase I was not stimulated by mevinolin. Suppression of protein synthesis with cycloheximide blocked the induction of ACE activity but only partially inhibited neurite outgrowth in the mevinolin-treated cultures. When mevinolin was removed from the culture medium, most of the cells retracted their neurites within 2 h, but ACE activity did not decline until DNA synthesis began to return to control levels after 10 h. Similarly, retraction of neurites in differentiated cells exposed to colchicine was not accompanied by a decrease in ACE activity. DNA histograms suggested that mevinolin arrests neuroblastoma cells in both the G1 and G2/M compartments of the cell cycle. Other cytostatic drugs that arrest cells at different stages of the cell cycle did not cause Neuro-2A cells to form neurites such as those seen in the mevinolin-treated cultures. When incorporation of [3H]acetate into isoprenoid compounds was studied in cultures containing mevinolin in concentrations ranging from 0.25 microM to 25 microM, the labeling of cholesterol, dolichol, and ubiquinone was suppressed by 90% or more at all concentrations. However, significant growth arrest and cell differentiation were observed only at the highest concentrations of mevinolin. Supplementing the medium with 100 microM mevalonate prevented the cellular response to mevinolin, but additions of cholesterol, dolichol, ubiquinone, or isopentenyl adenine were generally ineffective. The cholesterol content of neuroblastoma cells incubated with 25 microM mevinolin for 24 h was not diminished, and protein glycosylation, measured by [3H]mannose incorporation, was decreased only after 24 h at high mevinolin concentration. These studies suggest that the stimulation of neurite outgrowth and the increase in ACE activity induced by mevinolin are independent phenomena. Whereas neurite outgrowth is not related directly to the effects of mevinolin on cell cycling, the induction of ACE is correlated with the inhibition of cell proliferation.(ABSTRACT TRUNCATED AT 400 WORDS)

The nucleotide sequence of Syrian hamster HMG-CoA reductase cDNA

We have determined the nucleotide sequence of Syrian hamster 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase cDNA. Comparison of the deduced amino acid sequence with the homologous sequence from Chinese hamster reveals highly conserved domains which appear to have functional significance. The amino-terminal membrane domain of HMG-CoA reductase exhibits 100% homology. This region may span the endoplasmic reticulum seven times and is thought to be involved in the sterol-regulated degradation of HMG-CoA reductase (Gil et al., 1985; Liscum et al., 1985). The carboxyl terminus contains the active site of the enzyme and exhibits greater than 99% homology. A central region linking these two conserved domains exhibits greater divergence. In this region there is only 85% homology between the two hamster lines, suggesting that this linkage domain has a less stringent structural requirement.

Isoprenoid biosynthesis in multiple sclerosis

Recently discovered metabolites in urine have suggested a defect of isoprenoid metabolism in multiple sclerosis. Lymphocyte HMG-CoA reductase was found unaffected however, and so was lymphocyte biosynthesis of geraniol, farnesol and squalene from mevalonolactone. The level of dolichol in white matter of an MS brain was similar to that of a control sample. Serum ubiquinone, on the other hand, was decreased in multiple sclerosis. Ubiquinone in serum was both age-dependent and related to serum cholesterol. Active as well as stable MS displayed a decreased level of serum ubiquinone, and a reduced ubiquinone-cholesterol ratio. These results are compatible with a deficient ubiquinone biosynthesis in multiple sclerosis.

Ascorbic acid regulation of 3-hydroxy-3-methylglutaryl coenzyme A reductase activity and cholesterol synthesis in guinea pig liver

Guinea pigs fed a normal diet show the expected diurnal variation in 3-hydroxy-3-methylglutaryl coenzyme A reductase activity. Vitamin C deficiency, however, suppresses the diurnal peak activity of reductase, due to a decrease in active (unphosphorylated) enzyme. Inhibition of reductase is paralleled by both a fall in hepatic cholesterol synthesis and a rise in serum cholesterol. Incubation of normal guinea pig hepatic microsomes with physiologic and supraphysiologic concentrations of sodium ascorbate also leads to a concentration-dependent inhibition of reductase activity. Thus, dietary extremes of vitamin C may exert similar effects on reductase activity and cholesterolgenesis. Moreover, the changes in enzyme activity induced by ascorbic acid appear to be due in part to a direct effect of the vitamin on the microsomally bound enzyme.

Phosphorylation of microsomal HMG CoA reductase increases susceptibility to proteolytic degradation in vitro

Conversion of native, 97-100 kDa rat liver microsomal HMG CoA reductase to membrane-bound 62 kDa and soluble 52-56 kDa catalytically active forms was catalyzed in vitro by the calcium-dependent, leupeptin- and calpastatin-sensitive protease calpain-II purified from rat liver cytosol. Cleavage of the native 97-100 kDa reductase was enhanced by pretreatment (inactivation) of microsomes with ATP(Mg2+) and liver reductase kinase (compared to protein phosphatase-pretreated controls). This was reflected in a loss of the 97-100 kDa species and an increase in the soluble 52-56 kDa species (total enzyme activity and specific immunoblot recovery).

Antibody preference for the catalytically active form of beta-hydroxy-beta-methylglutaryl coenzyme A reductase

The catalytically inactivating subset within rabbit serum polyclonal antibody to the solubilized, purified 55,000 to 60,000 dalton active fragment of rat liver microsomal beta-hydroxy-beta-methylglutaryl coenzyme A reductase immunoinactivates this enzyme with little or no diminution of effect by enzyme catalytically inactivated by incubation of microsomes with ATP,Mg++. Reactivation of inactive enzyme with ethanol-treated rat liver phosphatase restores antibody affinity showing that the catalytically inactivating subset of antibody exhibits marked or complete affinity for the active enzyme over the ATP,Mg++- inactivated form. This means that immunoinactivation using this antibody is not a valid way of measuring changes in the specific activity of the enzyme via phosphorylation-dephosphorylation. Preference for the active enzyme has not been obvious because when different amounts of enzyme activity are used in immunotitrations of samples of low activity, apparent differences in specific activity are observed when none actually exist. If precautions are not taken, results are obtained supporting phosphorylation by using an antibody that is not capable of distinguishing it.

Effects of compactin on the levels of 3-hydroxy-3-methylglutaryl coenzyme A reductase in compactin-resistant C100 and wild-type cells

A cell line, C100, resistant to 225 microM compactin, has been isolated which overproduces 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase approximately 100-fold compared to the parental cell line [E. Hardeman, H. Jenke and R. Simoni (1983) Proc. Natl. Acad. Sci. U.S.A. 80, 1516-1520]. It is demonstrated that the overproduction of HMG-CoA reductase in these cells is the result of increased enzyme synthesis due to elevated levels of translatable mRNA. Furthermore, the apparent molecular weight of the in vitro translation product is 94,000, which agrees with the molecular weight of the in vivo synthesized HMG-CoA reductase protomer in C100 cells. However, a comparison of the Staphylococcus aureus V8 proteolysis patterns between the in vitro and in vivo translation products reveals structural differences which suggests in vivo post-translation modification(s). It is also demonstrated unequivocally, by comparing proteolytic cleavage patterns and pulse-chase experiments, that the previously reported 63,000-, 52,000-, and 38,000-Da polypeptides recognized by HMG-CoA reductase antiserum derive from the 94,000-Da protomer as a result of nonphysiological proteolysis. Finally, the types of regulatory mechanisms involved in both the induction and repression of the enzyme in the presence or absence of compactin were determined. Four biochemical parameters of HMG-CoA reductase were examined in variant and parental cells grown in the presence and absence of compactin: enzymatic activity, degradation rate, synthesis rate, and concentration of translatable mRNA. These studies revealed that changes in cellular HMG-CoA reductase content are a function of concurrent changes in the rates of enzyme degradation and synthesis. Changes in enzyme synthesis are due to alterations in the level of translatable mRNA.