An overview on the biological activity and anti-cancer mechanism of lovastatin

Lovastatin, a secondary metabolite isolated from fungi, is often used as a representative drug to reduce blood lipid concentration and treat hypercholesterolemia. Its structure is similar to that of HMG-CoA. Lovastatin inhibits the binding of the substrate to HMG-CoA reductase, and strongly competes with HMG-CoA reductase (HMGR), thereby exerting a hypolipidemic effect. Further, its safety has been confirmed in vivo and in vitro. Lovastatin also has anti-inflammatory, anti-cancer, and neuroprotective effects. Therefore, the biological activity of lovastatin, especially its anti-cancer effect, has garnered research attention. Several in vitro studies have confirmed that lovastatin has a significant inhibitory effect on cancer cell viability in a variety of cancers (such as breast, liver, cervical, lung, and colon cancer). At the same time, lovastatin can also increase the sensitivity of some types of cancer cells to chemotherapeutic drugs and strengthen their therapeutic effect. Lovastatin inhibits cell proliferation and regulates cancer cell signaling pathways, thereby inducing apoptosis and cell cycle arrest. This article reviews the structure, biosynthetic pathways, and applications of lovastatin, focusing on the anti-cancer effects and mechanisms of action.     

Juvenile hormone acid synthesis and HMG-CoA reductase activity in corpora allata of Manduca sexta prepupae

Corpora allata (CA) of last instar larvae of Manduca sexta switch from juvenile hormone (JH) to JH acid secretion just before the onset of wandering behavior. JH acid secretion peaked during the prepupal period and ceased prior to pupal ecdysis. HMG-CoA reductase activity also peaked during the prepupal period and then declined. However, substantial enzyme activity was present in pupal and pharate adult glands. Removal of the brain at the wandering stage caused a reduction in JH acid secretion by prepupal CA. The profile of HMG-CoA activity in CA of debrained larvae resembled that of sham-operated larvae except that the prepupal peak was smaller than in control larvae. Addition of brain extracts to CA maintained in vitro neither stimulated not inhibited JH acid secretion and HMG-CoA reductase activity. It is suggested that the brain regulates CA activity in post-wandering stages via intact nerves.     

Studies on plant growth substances,IAA metabolism and nitrogenase activity in root nodules of phaseolus aureus Roxb. var.mungo

The mature nodules ofPhaseolus aureus Roxb. var.mungo possessed, in comparison with young and old nodules, higher activities of nitrogenase (N_2ase), and indol-3-ylacetic acid (IAA) metabolic enzymes like IAA oxidase, methylene oxindole reductase and peroxidase; higher levels of IAA-like, gibberellic acid-like (GA), and cytokinin-like (CK) substances and tryptophan, and lower level of phenol. The abscisic acid-like (ABA) substance level was higher in the old nodules. The N_2ase activity in the mature nodules changed parallelly with IAA and OK. butoppositely with GA and ABA. The changes in tryptophan level, IAA oxidizing enzymes, and phenol metabolism controlled the IAA level in the nodules. The nodules developed similarly throughout the year, but they had variable hormone levels in different seasons. This indicated that the formation and growth of the nodules was controlled not only by the nodular hormones.     

Hydroxymethylglutaryl coenzyme A reductase activity of adult Hymenolepis diminuta

The occurrence of hydroxymethylglutaryl coenzyme A (HMG-CoA) reductase in adult Hymenolepis diminuta was demonstrated. This activity was negligible in the cestode's cytosolic fraction but was noted when the mitochondrial or microsomal fraction served as the enzyme source. The predominant localization of HMG-CoA reductase activity was with the microsomal fraction. This fraction did not contain appreciable mitochondrial contamination based on the distribution of marker enzymes. The enzymatic nature of HMG-CoA conversion to mevalonic acid by either fraction was apparent because the reaction was heat labile and responded linearly to time of assay and protein content. The enzymatic reduction of HMG-CoA absolutely required NADPH when either fraction was assayed. The lesser activity of the mitochondrial fraction was membrane-associated. The predominant localization of HMG-CoA reductase activity with microsomal membranes and its separation with the membranous component of the mitochondrial fraction suggest that mitochondrial activity reflects the presence of microsomal membranes. In its predominant localization and pyridine nucleotide requirement, the cestode's HMG-CoA reductase activity resembles that of mammalian systems. The finding of HMG-CoA reductase provides an enzymatic mechanism for the intermediate conversion of HMG-CoA to mevalonic acid that would be needed for acetate-dependent isoprenoid lipid synthesis by adult H. diminuta.     

Inhibition of human leukocyte 3-hydroxy-3-methylglutaryl coenzyme A reductase activity by ascorbic acid. An effect mediated by the free radical monodehydroascorbate

3-Hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase activity in microsomes isolated from cultured lymphoid (IM-9) cells or freshly isolated human leukocytes was markedly decreased by either ascorbic acid or its oxidized derivative, dehydroascorbate. Inhibition of IM-9 leukocyte HMG-CoA reductase activity was log linear between 0.01 and 10 mM ascorbic acid (25 and 81% inhibition, respectively) and 0.1 and 10 mM dehydroascorbate (5 and 75% inhibition, respectively). Inhibition was noncompetitive with respect to HMG-CoA (Km = 10.2 microM (RS); ascorbic acid, Ki = 6.4 mM; dehydroascorbate, Ki = 15 mM) and competitive with respect to NADPH (Km = 16.3 microM; acetic acid, Ki = 6.3 mM; dehydroascorbate, Ki = 3.1 mM). Ascorbic acid and dehydroascorbate are interconverted through the free radical intermediate monodehydroascorbate. Reducing agents are required to convert dehydroascorbate to monodehydroascorbate, but prevent formation of the free radical from ascorbate. In microsomes from IM-9 cells, the reducing agent, dithiothreitol, abolished HMG-CoA reductase inhibition by ascorbate but enhanced inhibition by dehydroascorbate. In addition, the concentration of monodehydroascorbate present in ascorbate solutions was directly proportional to the degree of HMG-CoA reductase inhibition by 1.0 mM ascorbate. Fifty per cent inhibition of enzyme activity occurred at a monodehydroascorbate concentration of 14 microM. These data indicate that monodehydroascorbate mediates inhibition of HMG-CoA reductase by both ascorbate and dehydroascorbate. This effect does not appear to be due to free radical-induced membrane lipid modification, however, since both ascorbate and dehydroascorbate inhibited the protease-solubilized, partially purified human liver enzyme. Since inhibition of HMG-CoA reductase occurs at physiological concentrations of ascorbic acid in the human leukocyte (0.2-1.72 mM), this vitamin may be important in the regulation of endogenous cholesterol synthesis in man.     

The activity of 3-hydroxy-3-methylglutaryl-CoA reductase and acyl-CoA: cholesterol acyltransferase in hepatic microsomes from male, female and pregnant rats. The effect of cholestyramine treatment and the relationship of enzyme activity to microsomal lipid composition

The relationship of microsomal cholesterol and phospholipid fatty acid composition to the activities of 3-hydroxy-3-methylglutaryl-coenzyme A (HMG-CoA) reductase and acyl-CoA: cholesterol acyltransferase was investigated in male, female virgin and pregnant rats when hepatic cholesterogenesis was stimulated by cholestyramine. Cholestyramine increased HMG-CoA reductase activity in both sexes but had no effect on microsomal free cholesterol level or acyl-CoA: cholesterol acyltransferase activity. The data suggest that during cholestyramine treatment high rates of bile acid synthesis are supported by preferential channelling of cholesterol into this pathway, whilst the substrate pool and activity of acyl-CoA:cholesterol acyltransferase are maintained unaltered. The lack of a consistent relationship among enzyme activities and microsomal lipid composition infers that HMG-CoA reductase and acyl-CoA:cholesterol acyltransferase are regulated in vivo by independent mechanisms which are unlikely to involve modulation by the physical properties of the microsomal lipid.     

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.     

Regulation of 3-hydroxy-3-methylglutaryl coenzyme A reductase activity in human fibroblasts by reversible phosphorylation: modulation of enzymatic activity by low density lipoprotein, sterols, and mevalonolactone

3-Hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase exists in interconvertible active and inactive forms in cultured fibroblasts from normal and familial hypercholesterolemic subjects. The inactive form can be activated by endogenous or added phosphoprotein phosphatase. Active or partially active HMG-CoA reductase in cell extracts was inactivated by a ATP-Mg-dependent reductase kinase. Incubation of phosphorylated (inactive) HMG-CoA reductase with purified phosphoprotein phosphatase was associated with dephosphorylation (reactivation) and complete restoration of HMG-CoA reductase activity. Low density lipoprotein, 25-hydroxycholesterol, 7-ketocholesterol, and mevalonolactone suppressed HMG-CoA reductase activity by a short-term mechanism involving reversible phosphorylation. 25-Hydroxycholesterol, which enters cells without the requirement of low density lipoprotein-receptor binding, inhibited the HMG-CoA reductase activity in familial hypercholesterolemic cells by reversible phosphorylation. Measurement of the short-term effects of inhibitors on the rate of cholesterol synthesis from radiolabeled acetate revealed that HMG-CoA reductase phosphorylation was responsible for rapid suppression of sterol synthesis. Reductase kinase activity of cultured fibroblasts was also affected by reversible phosphorylation. The active (phosphorylated) reductase kinase can be inactivated by dephosphorylation with phosphatase. Inactive reductase kinase can be reactivated by phosphorylation with ATP-Mg and a second protein kinase from rat liver, designated reductase kinase kinase. Reductase kinase kinase activity has been shown to be present in the extracts of cultured fibroblasts. The combined results represent the initial demonstration of a short-term regulation of HMG-CoA reductase activity and cholesterol synthesis in normal and receptor-negative cultured fibroblasts involving reversible phosphorylation of both HMG-CoA reductase and reductase kinase.     

Lack of correlation between 3-hydroxy-3-methylglutaryl coenzyme A reductase activity and lovastatin resistance in nerve growth factor treated PC-12 cells

Summary 1. The relationships among the mevalonic acid (MVA) forming enzyme, 3-hydroxy-3-methylglutaryl coenzyme A (CoA) reductase, cell growth and differentiation, and the cytotoxic effects of the reductase inhibitor lovastatin were studied in PC-12 cells, exposed to growth factors.2. When added individually, nerve growth factor (NGF), basic fibroblast growth factor, and epidermal growth factor induce an increase in HMG-CoA reductase activity in cells grown in serum-containing medium. In the presence of serum, the effect of NGF on HMG-CoA reductase is persistent.3. Short-term serum starvation and long-term NGF treatment, in combination, have an additive effect, resulting in a high reductase activity.4. Unlike serum and MVA, which downregulate levels of HMG-CoA reductase by accelerating its degradation, NGF upregulates reductase by slowing the rate of its degradation. This mechanism, however, appears to operate only in the presence of serum, as after prolonged growth with NGF in serum-free medium, cells have a low reductase activity.5. PC-12 cells grown in the absence of NGF are highly sensitive to lovastatin (25 µM) and more than 70% of the cells die after 48 hr. NGF confers lovastatin resistance on cells grown in the presence or in the absence of serum (only 30–40% cell death after 48 hr with lovastatin).6. NGF-induced resistance on lovastatin develops with time and is apparent only in the well-differentiated PC-12 cells whether or not the cells express a high reductase activity.7. Thus, levels of HMG-CoA reductase activity and lovastatin resistance in PC-12 cells are not directly correlated, though clearly inversed lovastatin cytotoxicity and elevated reductase activities are expressed during the period of cell proliferation.8. These data suggest that fully differentiated neuronal cells may not be affected by prolonged high doses of lovastatin.     

Effect of docosahexaenoic acid on brain 3-hydroxy-3-methylglutaryl-coenzyme A reductase activity in male ICR mice

We investigated the influence of docosahexaenoic acid ethyl ester (DHA-EE) on 3-hydroxy-3-methylglutaryl-coenzyme A (HMG-CoA) reductase activity in the brains of adult and aged mice. Male mice (Crlj:CD-1) were fed diets containing 3% lard plus 2% linoleic acid ethyl ester (LA-EE), or 2% DHA-EE, for 3 months. The brain HMG-CoA reductase activity of 8-month-old (adult) mice was not significantly influenced by dietary intake of DHA-EE. However, in 18-month-old (aged) mice, its activity was enhanced with dietary intake of DHA-EE. Brain HMG-CoA reductase activity and brain cholesterol content significantly increased with age. Hepatic HMG-CoA reductase activity and the cholesterol content of both adult and aged mice were reduced in DHA-EE diet groups, compared with LA-EE diet groups. The DHA percentages of brain and liver microsomal fractions increased with the intake of DHA-EE in adult and aged mice. These results suggest that DHA may enhance brain HMG-CoA reductase activity in aged mice.     

Studies on the relationships between 7α-hydroxylation and the ability of 25- and 27-hydroxycholesterol to suppress the activity of HMG-CoA reductase

The metabolism of 25-hydroxycholesterol in different cell types was studied and the role of 7α-hydroxylation for the effect of 25-hydroxycholesterol on the activity of HMG-CoA reductase was determined. Human diploid fibroblasts (HDF) and the human melanoma cell line SK-MEL-2 converted 25-hydroxycholesterol into 7α,25-dihydroxycholesterol and 7α,25-dihydroxy-4-cholesten-3-one while the virus-transformed fibroblast line 90VA-VI, the colon carcinoma cell line WiDr and the breast cancer cell line MDA-231 did not express 7α-hydroxylase activity. The 7α-hydroxylation of 25-hydroxycholesterol in HDF could be stimulated by dexamethasone and cortisol and inhibited by metyrapone. An unidentified, possibly 4-hydroxylated, metabolite was formed by 90VA-VI cells and a polar, probably conjugated, metabolite was formed by WiDr cells. The 7α-hydroxylated metabolites of 25-hydroxycholesterol suppressed the activity of HMG-CoA reductase to a similar extent as 25-hydroxycholesterol in HDF but not in 90VA-VI cells, while the 7α-hydroxylated metabolites of 27-hydroxycholesterol suppressed the activity of HMG-CoA reductase also in 90VA-VI cells. The suppression of HMG-CoA reductase activity by 25- and 27-hydroxycholesterol was decreased or abolished by dehydroepiandrosterone or pregnenolone which have little or no effect on the 7α-hydroxylation. The results indicate that 7α-hydroxylation is not directly involved, positively or negatively, in the action of 25- or 27-hydroxycholesterol as suppressors of HMG-CoA reductase activity.     

Mode of interaction of beta-hydroxy-beta-methylglutaryl coenzyme A reductase with strong binding inhibitors: compactin and related compounds

The sodium salts of compactin (1) and trans-6-[2-(2,4- dichloro-6-hydroxyphenyl)ethyl]-3,4,5,6-tetrahydro-4-hydroxy-2H-pyran- 2-one (3) are inhibitors of yeast beta-hydroxy-beta-methylglutaryl coenzyme A (HMG-CoA) reductase. The dissociation constants are 0.24 X 10(-9) and 0.28 X 10(-9) M, respectively. Similar values have been reported for HMG-CoA reductase from mammalian sources [Endo, A., Kuroda, M., & Tanzawa, K. (1976) FEBS Lett. 72, 323; Alberts, A. W., et al. (1980) Proc. Natl. Acad. Sci. U.S.A. 77, 3957]. The structures of these compounds marginally resemble that of any substrates of HMG-CoA reductase. We, therefore, investigated the basis for the strong interaction between HMG-CoA reductase and these inhibitors. HMG-CoA and coenzyme A (CoASH), but not reduced nicotinamide adenine dinucleotide phosphate (NADPH), prevent binding of compactin to the enzyme. HMG-CoA, but not CoASH or NADPH, prevents binding of 3 to the enzyme. We also investigated the inhibitory activity of molecules that resemble structural components of compactin. Compactin consists of a moiety resembling 3,5-dihydroxyvaleric acid that is attached to a decalin structure. The sodium salt of DL-3,5-dihydroxyvaleric acid inhibits HMG-CoA reductase competitively with respect to HMG-CoA and noncompetitively with respect to NADPH. The dissociation constant for DL-3,5-dihydroxyvaleric acid, derived from protection against inactivation of enzyme by iodoacetic acid, is (2.1 +/- 0.9) X 10(-2) M. Two decalin derivatives (structurally identical with or closely related to the decalin moiety of compactin) showed no detectable inhibition. If the lack of inhibition is due to their limited solubility, the dissociation constant of these decalin derivatives may be conservatively estimated to be greater than or equal to 0.5 mM. Simultaneous addition of decalin derivatives and DL-3,5-dihydroxyvaleric acid does not lead to enhanced inhibition. The sodium salt of (E)-6-[2-(2-methoxy-1-naphthalenyl)ethenyl]-3,4,5,6- tetrahydro-4-hydroxy-2H-pyran-2-one (6) inhibits HMG-CoA reductase competitively with respect to HMG-CoA and noncompetitively with respect to NADPH. The inhibition constant (vs. HMG-CoA) is 0.8 microM. CoASH does not prevent binding of 6 to enzyme. Compound 6, therefore, behaves analogously to compound 3. We propose that these inhibitors occupy two sites on the enzyme: one site is the hydroxymethylglutaryl binding domain of the enzyme active site and the other site is a hydrophobic pocket located adjacent to the active site.(ABSTRACT TRUNCATED AT 400 WORDS)     

Regulation of 3-hydroxyl-3-methylglutaryl-coenzyme A reductase activity in rat uterine tissues

Initiation of uterine DNA synthesis and mitosis in response to estrogen appears to depend upon the stimulation of protein synthesis. 3-Hydroxy-3-methylglutaryl-coenzyme A (HMG-CoA) reductase could have a key function in controlling uterine mitosis through its control of mevalonic acid and cholesterol synthesis as the rate-limiting enzyme in their synthetic pathways. These studies were initiated to examine the kinetics of the uterine increases in HMG-CoA reductase activity in response to estradiol. In the uterus of the ovariectomized mature rat, estradiol increased levels of enzyme activity in both the luminal epithelium and stroma-myometrium up to 12 h after estradiol treatment. Levels of HMG-CoA reductase activity decreased after 12 h in the luminal epithelium and further increased in the stroma-myometrium. Previous studies have shown that estradiol does not increase DNA synthesis and mitosis in the stroma-myometrium of the uterus of the ovariectomized mature rat. Since estradiol increased HMG-CoA reductase activity in both the luminal epithelium and stroma-myometrium, we conclude that even though increased HMG-CoA reductase activity may be a prerequisite for increased DNA synthesis, increases in uterine HMG-CoA reductase activity are not necessarily followed by increased DNA synthesis.     

GA,ABA, phenol interaction in the control of growth: Phenolic compounds as effective modulators of GA-ABA interaction in radish seedlings

Abscisic acid, a potent growth inhibitor inhibits hypocotyl growth ofRaphanus sativus seedlings. Phenolic compounds,viz., trans-cinnamic acid, chlorogenic acid, ferulic acid, salicylic acid, tannic acid and quercetin when applied with ABA, antagonize ABA action and restore normal seedling growth. Gibberellic acid promotes hypocotyl growth and on combined application with ABA, the ratio of their concentrations determines the course of the resultant growth. This interaction can be modulated by phenolic compounds. Phenolic compounds in low concentrations when present together with GA and ABA, favour GA-induced growth by antagonizing the inhibitory influence of ABA. The inhibitory action of abscisic acid on a wide range of growth processes is so far known to be reversed only by growth promoting hormones,viz., IAA, GA and cytokinins. Antagonistic action of phenolic compounds towards ABA, and increasing the action of GA when present together with GA and ABA, establishes a dual role to this class of compounds; balancing the effect of both growth promoting and growth inhibiting hormones. Part I.     

Regulation of 3-hydroxy-3-methylglutaryl coenzyme A reductase activity in avian myeloblasts. Mode of action of 25-hydroxycholesterol

25-Hydroxycholesterol inhibits cholesterol biosynthesis by inhibiting the activity of 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase. Addition of 25-hydroxycholesterol to chicken myeloblasts caused a rapid inhibition of HMG-CoA reductase activity, producing approximately an 80% decrease in enzyme activity after 60 min. The mode of action of 25-hydroxycholesterol was determined by immunoprecipitating radiolabeled enzyme from 25-hydroxycholesterol-treated myeloblasts. The decline in enzyme activity due to addition of 25-hydroxycholesterol was not associated with increased levels of [32P]PO4 incorporation into the immunoprecipitated reductase polypeptide (Mr = 94,000). Hence, 25-hydroxycholesterol did not appear to regulate reductase activity by enzyme phosphorylation, as observed for other modulators of HMG-CoA reductase. However, 25-hydroxycholesterol was shown to inhibit reductase activity by causing a 350% increase in the relative rate of reductase degradation and a 72% decrease in the relative rate of reductase synthesis. These alterations in the rates of degradation and synthesis occurred rapidly (within 10-30 min after addition of 25-hydroxycholesterol) and can account completely for the 25-hydroxycholesterol-induced inhibition of enzyme activity. The rapid decline in the rate of synthesis of HMG-CoA reductase in 25-hydroxycholesterol-treated cells was not associated with concomitant changes in the levels of reductase mRNA; therefore, suggesting that 25-hydroxycholesterol must inhibit the rate of reductase synthesis by translational regulation. We also present evidence that mRNA purified from chicken myeloblasts codes for two reductase polypeptides of Mr = 94,000 and 102,000.     

Involvement of 3-hydroxy-3-methylglutaryl coenzyme a reductase in the regulation of sesquiterpenoid phytoalexin synthesis in potato

The importance of 3-hydroxy-3-methylglutaryl coenzyme A reductase (HMG-CoA reductase) in the regulation of sesquiterpenoid phytoalexin accumulation in potato (Solanum tuberosum L. cv Kennebec) was examined. Wounding of potato tubers produced a large temporary increase in HMG-CoA reductase activity of the microsomal and organelle fractions. Treatment of wounded tuber tissue with the sesquiterpenoid phytoalexin elicitor arachidonic acid further increased and prolonged the HMG-CoA reductase activity in the microsomal but not the organelle fraction. Incubation of elicitor-treated tuber tissue in white light reduced organelle and microsomal HMG-CoA reductase activity to 50% and 10%, respectively, of the activity of tissues held in darkness. Constant light also reduced overall phytoalexin accumulation 58% by greatly reducing levels of lubimin. Rishitin accumulation was not significantly altered by light. Application of nanomolar amounts of mevinolin, a highly specific inhibitor of HMG-CoA reductase, to elicitor-treated tuber tissue produced a large decline in lubimin accumulation and did not markedly alter rishitin accumulation. These results indicate that HMG-CoA reductase has a role in the complex regulation of sesquiterpenoid phytoalexin accumulation in potato.     

Effects of dietary alpha- or gamma-linolenic acid on levels and fatty acid compositions of serum and hepatic lipids, and activity and mRNA abundance of 3-hydroxy-3-methylglutaryl CoA reductase in rats.

The effects of diets containing equal amounts of alpha (alpha)- or gamma (gamma)-linolenic acid on lipid metabolism were compared in rats. Four groups of male Wistar rats were given the diets containing 20% perilla/corn mixed oil or borage oil in the absence (PO- and BO-diets, respectively) or presence (CPO- and CBO-diets) of cholesterol for 20 days. The PO-diet yielded lower serum cholesterol than the BO-diet, although the difference was not observed between the CPO and CBO groups. The PO and CPO groups showed lower high-density lipoprotein cholesterol than the BO and CBO groups, respectively. A similar tendency was observed in serum phospholipids. The CPO-diet gave markedly lower hepatic triglycerides than the CBO-diet. The activity of hepatic 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase was much lower on the PO-diet than on the BO-diet. mRNA abundance of HMG-CoA reductase was lower in rats on the PO-diet than on the BO-diet, though there was no significant difference between the CPO and CBO groups. The present results indicate that alpha-linolenic acid exhibits a larger hypocholesterolemic effect than gamma-linolenic acid, and it may be displayed mainly through the repression of the activity and mRNA expression of HMG-CoA reductase.     

3-Hydroxy-3-Methylglutaryl Coenzyme A Reductase Activity in Ochromonas malhamensis: A System to Study the Relationship between Enzyme Activity and Rate of Steroid Biosynthesis

3-Hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase, the key regulatory enzyme of the isoprenoid pathway, was found to be predominantly microsomal in Ochromonas malhamensis, a chrysophytic alga. Detection of HMG-CoA reductase requires the presence of 1% bovine serum albumin during cell homogenization, and the activity is stimulated by the presence of Triton X-100. The enzyme has a pH optimum of 8.0 and an absolute requirement for NADPH. When grown in 10 micromolar mevinolin, a competitive inhibitor of HMG-CoA reductase, O. malhamensis shows a 10- to 15-fold increase in HMG-CoA reductase activity (after washing) with little or no effect on cell growth rate. Cultures can be maintained in 10 micromolar mevinolin for months. O. malhamensis produces a large amount (1% dry weight) of poriferasterol, a product of the isoprenoid pathway. The addition of 10 micromolar mevinolin initially blocked poriferasterol biosynthesis by >90%; within 2 days the rate of synthesis returned to normal levels. Immediately after mevinolin was washed from the 2-day culture, there was a transient 2.5-fold increase in the rate of poriferasterol biosynthesis. The rate of poriferasterol biosynthesis and the level of HMG-CoA reductase activity both fell to control levels within hours.     

The regulation of activity of main mevalonic acid pathway enzymes: farnesyl diphosphate synthase, 3-hydroxy-3-methylglutaryl-CoA reductase, and squalene synthase in yeast Saccharomyces cerevisiae

The co-regulation of the main mevalonic acid pathway enzymes was investigated in the yeast Saccharomyces cerevisiae. It was found that a 6-fold increase in FPPS activity compared with that of the wild-type strain FL100 did not cause significant changes in HMG-CoA reductase activity, while the amounts of synthesized dolichols and ergosterol increased by 80 and 32%, respectively. The disruption of the SQS gene in the strain grown in the presence of ergosterol repressed the activities of both FPP synthase and HMG-CoA reductase to a comparable degree, whereas in the same strain starved for ergosterol the activity of FPPS was 10-fold higher and HMG-CoA reductase activity was practically unchanged. We show that FPPS is the enzyme that regulates the flow rate of synthesized mevalonic acid pathway products independent of HMG-CoA reductase and SQS.