Regulation of 3-hydroxy-3-methylglutaryl coenzyme A reductase by reversible phosphorylation-dephosphorylation

Preventive or Curative Administration of Taurine Regulates Lipid Metabolism in the Liver of Rats with Alcoholic Liver Disease

Excessive consumption causes alcoholic liver disease (ALD), which injures hepatocytes and induces imbalance of lipid metabolism. Taurine is known to protect the liver from various liver injuries, and relieve lipid profile. Our previous studies also found that taurine can prevent or cure ALD, reduce fat deposition, but the mechanism remains unclear. In the present study, ALD rat model was established by administration of alcohol, pyrazole and high fat diet. Two percent taurine was administered at the same time or after ALD model establishment. Serum activities of alanine aminotransferase (ALT) and aspartate aminotransferase (AST), serum and hepatic TC, TG, HDL-C and LDL-C were analyzed. Real-Time RT-PCR was conducted to detect the mRNA expressions of fatty acid synthetase (FAS), acetyl-CoA catboxylase (ACC), carnitine palmitoyl transferase 1 (CPT-1), 3-Hydroxy-3-methyl glutaric acid acyl Coenzyme A reductase (HMGCR), peroxisome proliferators activated receptor α (PPARα) and sterol regulatory element-binding protein 1c (SREBP-1c). The results showed that serum ALT, AST, serum and hepatic TC, TG and LDL-C were higher, while HDL-C in ALD model rats was lower than normal rats, the changes of which can be significantly relieved by taurine administration. mRNA expressions of ACC, FAS, CPT-1, HMGCR, PPARα and SREBP-1c which were significantly changed by ethanol can also be regulated by taurine. The results indicated that taurine can prevent and repair hepatic injury of ALD rats and balance lipid metabolism indexes in the liver, the mechanisms may involves in the regulation of related enzymes and transcriptional regulators participated in lipid metabolism.

Amoxillin- and pefloxacin-induced cholesterogenesis and phospholipidosis in rat tissues

BACKGROUND

To investigate whether amoxillin and pefloxacin perturb lipid metabolism.

METHODS

Rats were treated with therapeutic doses of each antibiotic for 5 and 10 days respectively. Twenty four hours after the last antibiotic treatment and 5 days after antibiotic withdrawal, blood and other tissues (liver, kidney, brain, heart and spleen) were removed from the animals after an overnight fast and analysed for their lipid contents.

RESULTS

Both antibiotics produced various degrees of compartment-specific dyslipidemia in the animals. While plasma and erythrocyte dyslipidemia was characterised by up-regulation of the concentrations of the major lipids (cholesterol, triglycerides, phospholipids and free fatty acids), hepatic and renal dyslipidemia was characterised by cholesterogenesis and phospholipidosis. Splenic dyslipidemia was characterised by cholesterogenesis and decreased phospholipid levels. Cardiac and brain cholesterol contents were not affected by the antibiotics. A transient phospholipidosis was observed in the brain whereas cardiac phospholipids decreased significantly. Lipoprotein abnormalities were reflected as down-regulation of HDL cholesterol. Furthermore, the two antibiotics increased the activity of hepatic HMG-CoA reductase. Although erythrocyte phospholipidosis was resolved 5 days after withdrawing the antibiotics, dyslipidemia observed in other compartments was still not reversible.

CONCLUSION

Our findings suggest that induction of cholesterogenesis and phospholipidosis might represent additional adverse effects of amoxillin and pefloxacin.

Alteration of enzyme expressions in mevalonate pathway: possible role for cardiovascular remodeling in spontaneously hypertensive rats

BACKGROUND

The mevalonate pathway is an important metabolic pathway that plays a key role in multiple cellular processes. The aim of this study was to define whether the enzyme expression in mevalonate pathway changes during cardiovascular remodelling in spontaneously hypertensive rats (SHR).

METHODS AND RESULTS

Hearts and thoracic aortas were removed for the study of cardiovascular remodeling in SHR and Wistar-Kyoto rats (WKY). The protein expression of the enzymes in hearts, aortas and livers was analyzed by western blot. The histological measurements showed that the mass and the size of cardiomyocytes, the media thickness and the media cross-sectional area (MCSA) of the thoracic aorta were all increased in SHR since 3 weeks of age. In the heart, there was overexpression of some enzymes, including 3-hydroxy-3-methylglutaryl-coenzyme A reductase (HMGR), farnesyl diphosphate synthase (FDPS), and geranylgeranyltransferase type I (GGTase-I), and downregulation of squalene synthetase (SQS) in SHR since 3 weeks of age. In the aorta, besides similar expressions of HMGR, SQS, FDPS and GGTase-I as in the heart, there was upregulation of farnesyltransferase α at 16 and 25 weeks of age and of farnesyltransferase β in 25-weeks-old SHR. Western blot demonstrated overexpression of HMGR and downregulation of SQS in SHR livers at all ages tested.

CONCLUSIONS

The cardiovascular remodeling of SHR preceded the development of hypertension, and altered expression of several key enzymes in the mevalonate pathway may play a potential pathophysiological role in cardiovascular remodeling.

Tissue-specific effects of atorvastatin on 3-hydroxy-3-methylglutarylcoenzyme A reductase expression and activity in spontaneously hypertensive rats

AIM

Cardiovascular remodeling is closely associated with cholesterol and is attenuated by statins. The spontaneously hypertensive rat (SHR) has a low serum cholesterol level and evident cardiovascular remodeling. The aims of the present study were to characterize the effects of atorvastatin on tissue cholesterol content and 3-hydroxy-3-methylglutaryl-coenzyme A (HMG-CoA) reductase expression and activity in four tissues from SHR: liver, heart, aorta and kidney.

METHODS

SHR and normotensive Wistar-Kyoto rats (WKY) were treated daily with atorvastatin (50 mg/kg) for 8 weeks. Cholesterol levels of serum and tissues (liver, heart, aorta and kidney) were determined by commercial enzymatic methods. Western blot analysis and high performance liquid chromatogram (HPLC) were used to assay the expression and activity of enzyme respectively.

RESULTS

Treatment with atorvastatin decreased cholesterol content and HMGCoA reductase expression and activity in all four tissues of SHR. However, in WKY, atorvastatin only altered HMG-CoA reductase in liver, where the protein expression was upregulated but the enzyme activity was decreased.

CONCLUSION

The present study demonstrates that the effects of atorvastatin on tissue cholesterol content and HMG-CoA reductase are strain- and tissue-specific.

Adult sterol metabolism is not affected by a positive sterol balance in the neonatal Golden Syrian hamster

Dietary components impact metabolism early in life. Some of the diet-induced effects are long lasting and can lead to various adult-based diseases. In the current studies, we examined the short-term effects of dietary cholesterol on neonatal hepatic sterol metabolism and the long-term effects that those early-life diets had on sterol metabolism in adulthood. Neonatal hamsters began consuming solid food as a supplement to milk by 5 days of age; diets contained 0 or 2% added cholesterol (wt/wt). By 10 days of age, plasma and liver cholesterol concentrations were 3.2- and 2.5-fold greater, respectively, in the neonates fed cholesterol. Hepatic sterol synthesis rates were suppressed 65% in cholesterol-fed neonates compared with control neonates. By 20 days of age, plasma and liver cholesterol concentrations were still greater and sterol synthesis rates were now suppressed maximally in neonates fed cholesterol compared with control neonates. The expression level of an apolipoprotein B-containing lipoprotein receptor (low-density lipoprotein receptor-related protein) was greater and the mature form of the sterol regulatory element-binding protein-2 was similar in livers of 20-day-old control neonates compared with control neonates at 10 days of age. To test whether the change in sterol balance in the neonatal period had a lasting effect on hepatic sterol metabolism, all animals were weaned on a low-cholesterol diet. At 70 days of age, hepatic sterol synthesis rates, plasma lipoprotein and liver cholesterol concentrations, and bile acid pool sizes and compositions were measured. Sterol balance in the adults was similar between animals fed either diet early in life, as demonstrated by a lack of difference in any parameter measured. Thus, even though dietary cholesterol suppressed hepatic sterol synthesis rates dramatically in the neonatal hamster, the change has little impact on sterol balance later in life.

Probucol Decreases Mevalonate Pyrophosphate Decarboxylase in the Rat Liver

It is known that cholesterol biosynthesis in the liver is inhibited by probucol. This inhibition by probucol is caused at least in part by a decrease in 3-hydroxy-3-methylglutaryl (HMG)-CoA reductase activity. In this study, we examined serum cholesterol and the change in the activity or protein level of mevalonate pyrophosphate decarboxylase (MPD), which is involved in cholesterol biosynthesis, in the livers of rats fed probucol. The results indicated that serum cholesterol, MPD activity and MPD protein were decreased by 70, 50 and 60% by probucol, respectively, as compared with those in rats fed normal chow. These data show for the first time that probucol decreases the level of an enzyme involved in cholesterol biosynthesis other than HMG-CoA reductase.

Role of the cyclic AMP-dependent pathway in free radical-induced cholesterol accumulation in vascular smooth muscle cells

We have previously reported that free radical-treated vascular smooth muscle cells (SMC) lead to cholesterol accumulation in vitro. In the current study, we investigated the effects of oxidative stress on cyclic AMP concentration and cAMP-dependent enzymes involved in cholesterol homeostasis in A7r5 cells. Under our conditions of a mild oxidative stress, namely with no change in cell viability, we found that free radicals, initiated using azobis-amidinopropane dihydrochloride (AAPH), resulted in a dose-dependent decrease in cellular cAMP which was opposed by vitamin E preincubation. Although the addition of adenylate cyclase activators (carbacyclin and forskolin) increased cAMP levels it did not succeed in restoring the AAPH-induced decrease. The oxidative stress-induced increase in activities of 3-hydroxy-3-methylglutaryl coenzyme A reductase and of acyl coenzyme A: cholesterol acyltransferase and the decrease in neutral cholesteryl ester hydrolase activity were suppressed by addition of dibutyryl cAMP. Taken together, these results strongly suggest that free radicals reduce cAMP concentrations by altering cell membrane adenylate cyclase activity. The changes of cAMP-dependent enzymes induced by oxidative stress resulting in cholesterol accumulation might be one of the processes leading to SMC-derived foam cells depicted in atheroma plaque. Moreover, if extrapolated to in vivo, these data may explain in part the beneficial effects of antioxidants in the reduction of cardiovascular diseases.

SR-12813 lowers plasma cholesterol in beagle dogs by decreasing cholesterol biosynthesis

SR-12813 inhibits cholesterol biosynthesis in Hep G2 cells via an enhanced degradation of 3-hydroxy-3-methylglutaryl-coenzyme A (HMG-CoA) reductase. Here we also show that SR-12813 inhibits cholesterol biosynthesis in vivo. A sterol balance study was performed in normolipemic beagle dogs. The dogs were given SR-12813 orally at dosages of 10 and 25 mg/kg/day for a period of 9 days. After 7 days plasma cholesterol was decreased by 15% in the 10 mg/kg/day group and by 19% in the 25 mg/kg/day group. Using a dual isotope technique no effects on intestinal cholesterol absorption were observed. The sterol balance indicated that endogenous synthesis of cholesterol was reduced by 23% in the 10 mg/kg/day group and by 37% in the 25 mg/kg/day group. Plasma lathosterol-cholesterol levels in dogs treated with 25 mg/kg/day SR-12813 were reduced by 56%, confirming a reduction of the cholesterol biosynthesis. Treatment with SR-12813 or the HMG-CoA reductase inhibitor lovastatin resulted in a large decrease in low density lipoprotein (LDL) cholesterol. It is concluded that SR-12813 reduces cholesterol biosynthesis in the dog model which results in a decrease of bile acid excretion, cholesterol excretion and plasma cholesterol level. The in vivo profile of SR-12813 is very similar to that of direct HMG-CoA reductase inhibitors, although the mode of action of the compound is unique.

The AMP-activated protein kinase--fuel gauge of the mammalian cell?

A single entity, the AMP-activated protein kinase (AMPK), phosphorylates and regulates in vivo hydroxymethylglutaryl-CoA reductase and acetyl-CoA carboxylase (key regulatory enzymes of sterol synthesis and fatty acid synthesis, respectively), and probably many additional targets. The kinase is activated by high AMP and low ATP via a complex mechanism, which involves allosteric regulation, promotion of phosphorylation by an upstream protein kinase (AMPK kinase), and inhibition of dephosphorylation. This protein-kinase cascade represents a sensitive system, which is activated by cellular stresses that deplete ATP, and thus acts like a cellular fuel gauge. Our central hypothesis is that, when it detects a 'low-fuel' situation, it protects the cell by switching off ATP-consuming pathways (e.g. fatty acid synthesis and sterol synthesis) and switching on alternative pathways for ATP generation (e.g. fatty acid oxidation). Native AMP-activated protein kinase is a heterotrimer consisting of a catalytic alpha subunit, and beta and gamma subunits, which are also essential for activity. All three subunits have homologues in budding yeast, which are components of the SNF1 protein-kinase complex. SNF1 is activated by glucose starvation (which in yeast leads to ATP depletion) and genetic studies have shown that it is involved in derepression of glucose-repressed genes. This raises the intriguing possibility that AMPK may regulate gene expression in mammals. AMPK/SNF1 homologues are found in higher plants, and this protein-kinase cascade appears to be an ancient system which evolved to protect cells against the effects of nutritional or environmental stress.

Protein engineering of the HMG-CoA reductase of Pseudomonas mevalonii. Construction of mutant enzymes whose activity is regulated by phosphorylation and dephosphorylation

The activity of Pseudomonas mevalonii HMG-CoA reductase (EC 1.1.1.88) is not regulated by phosphorylation, presumably due to the absence of a suitable target serine and protein kinase recognition motif. We have engineered P. mevalonii HMG-CoA reductase to a form whose activity, like that of mammalian HMG-CoA reductases, is regulated by phosphorylation/dephosphorylation. We substituted serine for arginine 387, the residue that corresponds to the regulatory serine of the HMG-CoA reductases of higher eukaryotes. A recognition motif for cAMP-dependent protein kinase was added by replacing leucine 384 by histidine (enzyme L384H/R387S) and also valine 391 by leucine (enzyme L384H/R387S/V391L). The activity of P. mevalonii HMG-CoA reductase mutant enzymes L384H/R387S and L384H/R387S/V391L was attenuated by phosphorylation. Restoration of activity accompanied subsequent dephosphorylation catalyzed by lambda protein phosphatase. Incorporation and subsequent release of phosphate paralleled the attenuation and restoration of catalytic activity. Incorporation of 0.5 mol of phosphate per subunit was accompanied by an approximately 50% decrease in initial activity. As in the analogous Syrian hamster mutant enzyme S871D, P. mevalonii mutant enzyme R387D exhibited 10% wild-type activity, suggesting that the attenuation of activity that accompanies phosphorylation results at least in part from the introduction of negative charge. Engineering of P. mevalonii HMG-CoA reductase to forms whose activity is reversibly regulated by phosphorylation/dephosphorylation provides an attractive model for future structure-based mechanistic studies. Solution of the X-ray structure of phosphorylated and dephosphorylated forms of engineered P. mevalonii HMG-CoA reductase should then reveal interactions of the active site phosphoseryl residue that result in attenuation of catalytic activity.

Identification of elements critical for phosphorylation of 3-hydroxy-3-methylglutaryl coenzyme A reductase by adenosine monophosphate-activated protein kinase: protein engineering of the naturally nonphosphorylatable 3-hydroxy-3-methylglutaryl coenzyme A reductase from Pseudomonas mevalonii

The initially nonphosphorylatable 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase of Pseudomonas mevalonii (E.C. 1.1.1.88) was engineered to phosphorylatable forms in order to identify elements critical for phosphorylation of HMG-CoA reductase by AMP-activated protein kinase. P. mevalonii, mutant enzymes phosphorylatable by AMP-activated protein kinase were engineered by substituting cognate residues from the kinase recognition sequence of Syrian hamster HMG-CoA reductase (E.C. 1.1.1.34). Various combinations of residues 381-391, which correspond to the kinase recognition sequence of the hamster enzyme, were mutated. P. mevalonii mutant enzyme R387S, in which a serine had been inserted at position P, which corresponds to that of the regulatory serine of the hamster enzyme, was only weakly phosphorylated. Genes that encoded thirty-six additional mutant enzymes containing various portions of the hamster kinase recognition sequence were constructed. Following expression, purified mutant enzymes were assayed as substrates for AMP-activated protein kinase. Identified as critical for phosphorylation was the simultaneous presence of aspartate or asparagine at position P+3 and of leucine at position P+4, three and four residues on the C-terminal side of the phosphorylatable serine, respectively. Two basic residues at positions P-1, P-2, or P-3 also appeared to be critical for phosphorylation when present in combination with aspartate or asparagine at P+3 and leucine at P+4.

3-Hydroxy-3-methylglutaryl-coenzyme A reductase of Haloferax volcanii: role of histidine 398 and attenuation of activity by introduction of negative charge at position 404

Mutant 3-hydroxy-3-methylglutaryl-coenzyme A (HMG-CoA) reductases of the halophilic archaeon Haloferax volcanii were constructed to test the proposed mechanism that phosphorylation downregulates the activity of higher eukarya HMG-CoA reductases via charge-charge interaction with the active site histidine. To first verify the sequence-based inference that His 398 is the catalytic histidine of the H. volcanii enzyme, enzyme H398Q was constructed, purified, and assayed for catalysis of three reactions: [1] reductive deacylation of HMG-CoA, [2] reduction of mevaldehyde, and [3] oxidative acylation of mevaldehyde. Enzyme H398Q had low activity for catalysis of reaction [1] or [3], but readily catalyzed mevaldehyde reduction. By analogy to hamster HMG-CoA reductase, we conclude that His 398 is the active site histidine. Mutant forms of the 403-residue H. volcanii enzyme were constructed to model phosphorylation and infer whether attenuated activity involved interaction with His 398. Chimeric H. volcanii-hamster enzymes constructed in an effort to create an active, phosphorylatable chimeric enzyme were inactive or not phosphorylated. We therefore added Asp at position 404 to mimic the introduction of negative charge that would accompany phosphorylation. Enzyme 404D/H398Q was inactive for reaction [1] or [3], but catalyzed reaction [2] at 35% the wild-type rate. These observations are consistent with the model that attenuation of catalytic activity results from an ionic interaction between the imidazolium cation of His 398 and the carboxylate anion of Asp 404.

The novel cholesterol-lowering drug SR-12813 inhibits cholesterol synthesis via an increased degradation of 3-hydroxy-3-methylglutaryl-coenzyme A reductase

SR-12813 (tetra-ethyl 2-(3,5-di-tert-butyl-4-hydroxyphenyl)ethenyl-1, 1-bisphosphonate) lowers plasma cholesterol in five species. In this paper we investigate the underlying mechanism using Hep G2 cells. SR-12813 inhibited incorporation of tritiated water into cholesterol with an IC50 of 1.2 microM but had no effect on fatty acid synthesis. Furthermore, SR-12813 reduced cellular 3-hydroxy-3-methylglutaryl-coenzyme A (HMG-CoA) reductase activity with an IC50 of 0.85 microM. The inhibition of HMG-CoA reductase activity was rapid with a T1/2 of 10 min. After a 16-h incubation with SR-12813, mRNA levels of HMG-CoA reductase and low density lipoprotein (LDL) receptor were increased. The increased expression of LDL receptor translated into a higher LDL uptake, which can explain the primary hypocholesterolemic effect of SR-12813 in vivo. Western blot analysis indicated that the amount of HMG-CoA reductase protein rapidly decreased in the presence of SR-12813. Pulse-chase experiments with [35S]methionine showed that the T1/2 of HMG-CoA reductase degradation decreased in the presence of SR-12813 from 90 to 20 min. Pre-incubation with 50 microM of lovastatin did not prevent the effects of SR-12813 on HMG-CoA reductase degradation, indicating that the compound does not need mevalonate-derived regulators for its action. It is concluded that SR-12813 inhibits cholesterol synthesis mainly by an enhanced degradation of HMG-CoA reductase.

Comparison of the effects of cyclic AMP analogues on cholesterol metabolism in cultured rat and hamster hepatocytes

The effects of two cell-permeable cyclic AMP analogues, 8-chloro cyclic AMP (8-Cl cAMP) and 8-(4-chlorophenylthio) cyclic AMP (8-CPT cAMP), on cholesterol esterification, cholesteryl ester hydrolysis and bile acid synthesis were compared in cultured rat and hamster hepatocytes. Cholesterol esterification, as measured by the incorporation of [3H]oleate into cholesteryl ester, was increased by 58-88% by the analogues in rat hepatocytes and by 33-43% in hamster cells. The response in rat hepatocytes, however, was observed after a relatively short incubation time (28% increase after 1 hr), whereas that in hamster cells required a longer period (36% after 12 hr) to become apparent. The activity of the cytosolic neutral cholesteryl ester hydrolase in rat hepatocytes was also stimulated by both cyclic AMP analogues (31-37%, but the microsomal activity was unaffected. In hamster hepatocytes, however, microsomal cholesteryl ester hydrolase activity was increased (47-80%) in the presence of 8-Cl cAMP or 8-CPT cAMP. Bile acid synthesis was increased by 8-CPT cyclic AMP in rat cells (approximately 25%) but was unchanged by both analogues in hamster hepatocytes. These results indicate significant differences in the way in which cholesterol metabolism responds to cyclic AMP in cultured rat and hamster hepatocytes.

Effects of lovastatin on hepatic fatty acid metabolism

The in vitro and in vivo effects of lovastatin on fatty acid metabolism were studied in isolated rat hepatocytes. When added in vitro to cell incubations, lovastatin stimulated de novo fatty acid synthesis and acetyl-CoA carboxylase activity, whereas fatty acid synthase activity was unaffected. Lovastatin depressed palmitate, but not octanoate, oxidation. This may be attributed to the lovastatin-induced increase in intracellular malonyl-CoA levels, as no concomitant change of carnitine palmitoyltransferase I (CPT-I) specific activity was detected. Lovastatin had no effect on the synthesis and secretion of triacylglycerols and phospholipids in the form of very low density lipoproteins (VLDL). When rats were fed a diet supplemented with 0.1% (w/w) lovastatin for one week, both acetyl-CoA carboxylase activity and de novo fatty acid synthesis were reduced compared to pair-fed controls, whereas fatty acid synthase activity was unaffected. Palmitate oxidation was enhanced in the lovastatin-fed group. There was an increase in CPT-I activity but no change in intracellular concentration of malonyl-CoA. Lovastatin feeding had no significant effect either on the esterification of exogenous palmitic acid into both cellular and VLDL triacylglycerols and phospholipids or on hepatic lipid accumulation. The in vitro and in vivo effects of lovastatin were not significantly different between periportal and perivenous hepatocytes.(ABSTRACT TRUNCATED AT 250 WORDS)

Lipoprotein-X fails to inhibit hydroxymethylglutaryl coenzyme A reductase in HepG2 cells

Abnormalities of lipid composition and metabolism are frequently observed in patients with cholestatic liver disease. Both elevated low-density lipoprotein (LDL) levels and the appearance of lipoprotein-X (LP-X) in plasma underlie the high incidence of hypercholesterolemia in this population. We tested the hypothesis that the hypercholesterolemia of cholestasis may reflect a failure of normal feedback regulation of hepatic cholesterogenesis by determining the influence of LP-X on the rate-limiting enzyme of cholesterol synthesis, hydroxymethylglutaryl coenzyme A (HMG CoA) reductase. Cultured human hepatoma (HepG2) cells were incubated in purified lipoprotein for 24 hours, harvested, and then assayed for HMG CoA reductase activity and mass. LDL isolated from either normal controls or patients with cholestasis decreased reductase activity in a dose-dependent fashion (2 to 30 micrograms cholesterol/mL media) to a level approximately 50% of that measured in cells incubated in lipid-deficient serum. LP-X failed to downregulate enzyme activity compared with LDL, with little change in reductase activity at cholesterol concentrations (30 micrograms/mL media) that produced maximal reductase inhibition by LDL. Three distinct LP-X subspecies were purified from the plasma of a patient with primary biliary cirrhosis (PBC) and tested in an analogous manner. All LP-X subspecies were similar in their inability to decrease reductase activity as compared with LDL. HMG CoA reductase mass was increased approximately twofold in cells incubated with LP-X, as estimated by Western blot analysis. These results suggest that LP-X may contribute to hypercholesterolemia in the cholestatic patient by not effectively downregulating hepatic cholesterol synthesis.

Barrier function regulates epidermal lipid and DNA synthesis

The stratum corneum, the permeability barrier between the internal milieu and the environment, is composed of fibrous protein-enriched corneocytes and a lipid-enriched intercellular matrix. The lipids are a mixture of sphingolipids, cholesterol and free fatty acids, which form intercellular membrane bilayers. Lipid synthesis occurs in the keratinocytes in all nucleated layers of the epidermis, and the newly synthesized lipids are delivered by lamellar bodies to the interstices of the stratum corneum during epidermal differentiation. Disruption of barrier function by topical acetone treatment results in an increase in the synthesis of free fatty acids, sphingolipids and cholesterol in the living layers of the epidermis, leading to barrier repair. Cholesterol and sphingolipid synthesis are regulated by the rate-limiting enzymes HMG CoA reductase and serine palmitoyl transferase (SPT), respectively. Acute barrier disruption leads to an increase in both enzymes, but with a different time curve: increase in HMG CoA reductase activity begins at 1.5 h, whereas the increase in SPT activity occurs 6 h after barrier impairment. Topical application of HMG CoA reductase or SPT inhibitors after acetone treatment delays barrier repair, providing further evidence for a role of cholesterol and sphingolipids in epidermal barrier function. Repeated application of lovastatin to untreated skin results in disturbed barrier function accompanied by increased DNA synthesis and epidermal hyperplasia. Therefore, we have examined the specific relationship between barrier function and epidermal DNA synthesis. After acute and chronic disturbances not only lipid, but also DNA synthesis, is stimulated. Thus, stimulation of DNA synthesis leading to epidermal hyperplasia may be a second mechanism by which the epidermis repairs defects in barrier function. The link between barrier function and both lipid and DNA synthesis is supported further by occlusion studies. Artificial barrier repair by latex occlusion prevents an increase in both lipid and DNA synthesis. In addition, increased epidermal lipid and DNA synthesis in essential fatty-acid deficiency can be reversed by topical applications of the n-6 unsaturated fatty acids, linoleic or columbinic acid. These studies may be of relevance in understanding the pathogenesis of hyperproliferative skin diseases, such as ichthyosis, psoriasis, atopic dermatitis, and irritant contact dermatitis.

Isopentenoid synthesis in isolated embryonic Drosophila cells: absolute, basal mevalonate synthesis rate determination

Embryonic Drosophila cells (Kc cells) and [5-3H]mevalonate (less than or equal to 10 microM) were used to determine the absolute basal in vivo rate of total mevalonic acid synthesis/utilization. An absolute in vivo mevalonic acid synthesis rate of 0.69 nmol/h/mg total cell protein was measured. Absolute mevalonate utilization was obtained by correcting for the extent of endogenous dilution of exogenous [3H]mevalonate at isotopic equilibrium. Cellular [3H]farnesol specific radioactivity was used as representative of a rapidly turning over isopentenoid pool. Although our previous Kc cell study (Havel, C. M., Rector, E. R. II, Watson, J. A., 1986, J. Biol. Chem. 261, 10,150-10,156) demonstrated that greater than or equal to 40% of the metabolized [3H]mevalonate appeared as 3H-labeled media water, this report established that t,t-3,7,11-[3H]trimethyl-2,6,10-dodecatriene-1,12 dioic acid was also secreted. Media accumulation of the C15-alpha,omega-prenyl dioic acid and 3H2O was related directly to [3H]mevalonic acid availability. This is the first mevalonate carbon balance study reported for a eukaryotic organism. It was concluded that (i) Kc cells synthesized more mevalonate than needed for normal growth and essential isopentenoids and (ii) excess mevalonate carbon accumulated intra- and extracellularly as isopentenoid compounds distal to C5 products. Finally, this study emphasized the need to measure total mevalonate utilization and not mevalonate conversion to a single isopentenoid end product in carbon balance investigations.

Preparation of a soluble 58 kDa-3-hydroxy-3-methylglutaryl CoA reductase from liver microsomes and its inhibition by ethoxysilatrane, a hypocholesterolemic compound

On repeated thawing at room temperature of frozen preparations of heavy microsomes from rat livers, HMGCoA reductase activity was solubilized due to limited proteolysis. This soluble enzyme was partially purified by fractionation with ammonium sulfate and filtration on Sephacryl S-200 column. The active enzyme was coeluted with a major 92 kDa-protein and was identified as a 58 kDa-protein after separation by SDS-PAGE and immunoblotting. Ethoxysilatrane, a hypocholesterolemic compound, which decreased the liver-microsomal activity of HMGCoA reductase on intra-peritonial treatment of animals, showed little effect on the enzyme activity with isolated microsomes or the 50 kDa-soluble enzyme when added in the assay. But it was able to inhibit the activity of the soluble 58 kDa-enzyme in a concentration-dependent, reversible manner. Cholesterol and an oxycholesterol were without effect whereas chlorophenoxyisobutyrate and ubiquinone showed small inhibition under these conditions. The extra region that links the active site domain (50 kDa protein) to the membrane, present in the 58 kDa-protein appears to be involved in mediating the inhibition by silatrane.

Lysosome inhibitors enhance the ability of cyclic AMP-elevating agents to induce the LDL receptor in human vascular smooth muscle cells

In human vascular smooth muscle cells cyclic AMP elevation by forskolin increases synthesis of the LDL receptor by a mechanism which appears independent of sterol control. This increased receptor synthesis is further enhanced by chloroquine. Both forskolin and prostaglandin E1 increase the number of cell surface LDL receptors indicating that prostaglandins could exert physiological control over LDL metabolism. This effect is enhanced synergistically by chloroquine. The stimulation by forskolin of LDL receptor synthesis and expression leads to increased metabolism of apo-B and increased hydrolysis of LDL-borne cholesteryl ester. These effects of cyclic AMP on the activity of the LDL pathway are enhanced more than additively by preincubation with the reversible lysosomal inhibitor NH4Cl. Thus cyclic AMP causes up-regulation of the LDL receptor pathway resulting in increased rates of LDL metabolism but this effect can be damped or masked in cell culture by a cyclic AMP-sensitive lysosomal event, probably the acute stimulation of lysosomal cholesterol ester hydrolase.

Localization and regulation of epidermal 3-hydroxy-3-methylglutaryl-coenzyme A reductase activity by barrier requirements

Recent studies have shown that epidermal cholesterol synthesis is regulated by HMG CoA reductase activity and that this activity is modulated by changes in the cutaneous permeability barrier. Here, we quantitated HMG CoA reductase activity after acute and chronic barrier disruption in the upper and lower layers of murine epidermis. In unperturbed epidermis, 13 and 87% of enzyme activity localized to the upper and lower epidermis, respectively, with the majority of activity in the stratum basale. Acute barrier disruption with either acetone or sodium dodecylsulfate provoked an increase in HMG CoA reductase activity (54% and 30%) in the lower layers, but only a small change in the upper layers. However, the activation state of the enzyme was increased 50% in the upper epidermis. Correction of barrier function by occlusion with an impermeable Latex wrap prevented the increase both in enzyme activity and activation state. After chronic barrier disruption; i.e., essential fatty acid deficient (EFAD) diet, HMG CoA reductase activity was increased in the upper epidermis (161%); a change prevented by occlusion. These results show: (1) that HMG CoA reductase activity is present in both the upper and lower cell layers; (2) that acute insults to barrier integrity stimulate enzyme activity in both the upper and lower epidermis; and (3) that chronic insults provoke an increase in enzyme activity in the upper layers. These studies provide further insights into the linkage of the permeability barrier with epidermal cholesterol metabolism.

The specific protein phosphatase inhibitor okadaic acid differentially modulates insulin action

The pleiotropic nature of insulin action suggests diverse mechanisms of signal transduction for the hormone. The specific protein phosphatase inhibitor, okadaic acid, is utilized to differentiate metabolic pathways that may be regulated by phosphorylation or dephosphorylation of key enzymes. In H-35 hepatoma cells, okadaic acid inhibits insulin-stimulated glycogen synthesis with an IC50 of 400 nM. In contrast, activation of lipogenesis by insulin is inhibited with an IC50 of 50 nM okadaic acid. The toxin also inhibits stimulation of lipogenesis in these cells by the insulin-sensitive inositol glycan enzyme modulator. In isolated rat adipocytes, insulin-stimulated lipogenesis is also inhibited by okadaic acid with an IC50 of approximately 1,700 nM. The antilipolytic effect of insulin in these cells is more sensitive to okadaic acid, exhibiting an IC50 of 150 nM. Maximal activation of lipogenesis by insulin is dramatically reduced by okadaic acid with no effect on the concentration required for half-maximal activation, whereas the sensitivity of insulin-induced antilipolysis is attenuated by okadaic acid, with no apparent reduction in the maximal effect of the hormone. Taken together, these data suggest that specific phosphatases may be differentially involved in some of the metabolic pathways regulated by insulin.

3-Hydroxy-3-methylglutaryldithio-coenzyme A: a potent inhibitor of Pseudomonas mevalonii HMG-CoA reductase

3-Hydroxy-3-methyl-1-thionoglutaryl-coenzyme A, a dithioester analog of 3-hydroxy-3-methylglutaryl-CoA, has been enzymatically synthesized using the HMG-CoA synthase catalyzed condensation of acetyl-CoA with 3-oxo-1-thionobutyryl-CoA. HMGdithio-CoA is a potent inhibitor of Pseudomonas mevalonii HMG-CoA reductase. Inhibition was mainly competitive with respect to HMG-CoA with a Kis of 0.086 +/- .01 microM and noncompetitive with respect to NADH with a Kis of 3.7 +/- 1.5 microM and a Kii of 0.65 +/- .05 microM in the presence of 110 microM (R.S)-HMG-CoA.

Effect of ketanserin tartrate on HMG CoA reductase and LDL receptor activity in cultured human skin fibroblasts

In man ketanserin tartrate reduces plasma LDL cholesterol. To clarify the mechanism of this effect the effect of ketanserin on 3-hydroxy-3-methylglutaryl (HMG) CoA reductase and LDL receptor activity in cultured human skin fibroblasts has been examined. After incubation with ketanserin for 14 h HMG CoA reductase activity was decreased in a dose-dependent manner up to 300 ng/ml (550 nM) without changing the free cholesterol content in the cells. Ketanserin increased specific binding and specific internalization of 125I-LDL dose-dependently. There was a significant inverse relationship between the percentage changes in HMG CoA reductase and LDL receptor activity. It appears that ketanserin induces up-regulation of LDL receptor activity by direct suppression of HMG CoA reductase, and this may be one mechanism by which plasma LDL-cholesterol is reduced by ketanserin.

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)

(S)-3-hydroxy-3-methylglutaryl coenzyme A reductase, a product of the mva operon of Pseudomonas mevalonii, is regulated at the transcriptional level

We have cloned and sequenced a 505-base-pair (bp) segment of DNA situated upstream of mvaA, the structural gene for (S)-3-hydroxy-3-methylglutaryl coenzyme A reductase (EC 1.1.1.88) of Pseudomonas mevalonii. The DNA segment that we characterized includes the promoter region for the mva operon. Nuclease S1 mapping and primer extension analysis showed that mvaA is the promoter-proximal gene of the mva operon. Transcription initiates at -56 bp relative to the first A (+1) of the translation start site. Transcription in vivo was induced by mevalonate. Structural features of the mva promoter region include an 80-bp A + T-rich region, and -12, -24 consensus sequences that resemble sequences of sigma 54 promoters in enteric organisms. The relative amplitudes of catalytic activity, enzyme protein, and mvaA mRNA are consistent with a model of regulation of this operon at the transcriptional level.

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.

Modulation of HMG-CoA reductase activity by pantetheine/pantethine

The ability of pantetheine/pantethine to modulate the activity of HMG-CoA reductase (EC 1.1.1.34) was determined in vitro with rat liver microsomes. The decay of the activity was obtained with pantethine in the 10(-5)-10(-4) M range, whereas stimulation by pantetheine occurred at 10(-3)-10(-2) M, as previously reported for GSSG and GSH, respectively. Inhibition of HMG-CoA by pantethine in isolated liver cells was also investigated by measuring the enzyme activity in microsomes isolated from hepatocytes incubated without or with 1 mM pantethine under conditions previously shown by us to induce inhibition of cholesterol synthesis from acetate. The enzyme amount was not modified by pantethine, but in cells treated with the disulphide, the relative amounts of the thiolic active forms of the enzyme, both phosphorylated and dephosphorylated, were decreased to about half compared to controls.

The function of glycosyl phosphoinositides in hormone action

The molecular events involved in the cellular actions of insulin remain unexplained. Some of the acute actions of the hormone may be due to the intracellular generation of a chemical substance which modulates certain enzyme activities. Such an enzyme-modulating substance has been identified as an inositol phosphate-glycan, produced by the insulin-sensitive hydrolysis of a glycosyl-phosphatidylinositol (glycosyl-PtdIns) precursor. This precursor glycolipid is structurally similar to the glycosyl-phosphoinositide membrane protein anchor. The exposure of fat, liver or muscle cells to insulin results in the hydrolysis of glycosyl-PtdIns, giving rise to the inositol phosphate glycan and diacylglycerol. This hydrolysis reaction is catalysed by a glycosyl-PtdIns-specific phospholipase C. This enzyme has been characterized and purified from a plasma membrane fraction of liver. This reaction also results in the acute release of certain glycosyl-PtdIns-anchored proteins from the cell surface. Elucidation of the functional role of glycosyl-phosphoinositides in the generation of second messengers or the release of proteins may provide further insights into the pleiotropic nature of insulin action.

Regulation of squalene epoxidase activity by membrane fatty acid composition in yeast

Depriving Saccharomyces cerevisiae strain GL7 of exogenous unsaturated fatty acid supplements causes this sterol biosynthetic mutant to accumulate squalene at the expense of squalene epoxide and squalene diepoxide. To further characterize the apparent relationship between squalene epoxidase activity and membrane fatty acid composition, a variety of unsaturated fatty acids differing in their chain lengths and in the positions and orientation (cis or trans) of their double bonds were tested for their ability to promote turnover of endogenous squalene in cells previously deprived of olefinic supplements. All of the unsaturated fatty acids tested were found to restore squalene epoxidase activity but there were marked differences in their efficacies that best were correlated with the extent to which they reduced the medium chain (C-10 + C-12) saturated fatty acid content of cellular phospholipids. Additional studies demonstrated that de novo protein synthesis was required for the restoration of squalene epoxidase activity in unsaturated fatty acid-deprived cells.

Reaction of 5'-p-fluorosulfonylbenzoyladenosine with the catalytic and AMP allosteric sites of microsomal HMG-CoA reductase kinase

The nucleotide analogue 5-p-fluorosulfonylbenzoyladenosine reacts with rat liver microsomal 3-hydroxy-3-methylglutaryl-CoA reductase kinase, causing a rapid loss of the AMP activation capacity and a slower inactivation of the catalytic activity. The rate constant for loss of AMP activation is eleven times higher (K1 = 0.107 min-1) than the rate constant for inactivation (K2 = 0.0094 min-1). Mg-ATP protects preferentially against inactivation, while Mg-AMP at a low concentration (7.5/0.05 mM) protects preferentially against loss of the AMP activation capacity. Oppositely, Mg-ADP at a low concentration (7.5/0.05 mM) hardly protects against loss of AMP activation capacity. We conclude that microsomal reductase kinase has distinct sites for activation and catalysis.

Characterization and regulation of HMG-CoA reductase during a cycle of juvenile hormone synthesis

The enzyme 3-hydroxy-3-methylglutaryl-coenzyme A (HMG-CoA) reductase was characterized in cockroach corpora allata which produce insect juvenile hormone III (methyl-(10R)10,11-epoxy-3,7,11-tri-methyl-2E,6E-dodecadienoate ). HMG-CoA reductase is a microsomal enzyme dependent on NADPH and dithiothreitol (or glutathione) for activity. The enzyme selectively reduced (3S)-HMG-CoA to (3R)-mevalonate with an apparent KM of 7.6 microM. Mevinolin was a competitive inhibitor of HMG-CoA reductase with a KI of 2.4 nM. No evidence for a modulation of enzyme activity by phosphorylation was obtained. Levels of HMG-CoA reductase were not altered after incubation of the corpora allata with either mevinolin (to decrease isoprenoid flux) or with mevalonate or farnesol (to increase isoprenoid flux). Split pairs of corpora allata were used to compare JH III synthetic activity with HMG-CoA reductase activity during the cycle of JH III synthesis that controls vitellogenesis and oocyte growth in adult females. Both activities changed over 10-fold and peaked on day 5 after emergence/mating, but JH III synthesis did not parallel HMG-CoA reductase activity precisely thereafter. The half-life of HMG-CoA reductase measured in the presence of cycloheximide was significantly different between low and high activity glands and was not related to the half-life of JH III synthesis. The results suggest that HMG-CoA reductase should not be considered 'the rate-limiting enzyme' in juvenile hormone synthesis by Diploptera punctata corpora allata.

Modulation of the enzymic activity of 3-hydroxy-3-methylglutaryl coenzyme A reductase by multiple kinase systems involving reversible phosphorylation: a review

This report summarizes the current concepts regarding the in vitro and in vivo modulation of the enzymic activity of HMG-CoA reductase and mevalonate formation in rat and human liver, as well as in cultured fibroblasts from normal and familial hypercholesterolemic subjects. Three separate mechanisms for the short-term modulation of hepatic HMG-CoA reductase activity by covalent phosphorylation have been described. These mechanisms involved three separate specific kinase systems including reductase kinase, protein kinase C, and a Ca+2, calmodulin-dependent kinase. The conceptual schemes presented in this report will provide a basis for future research as well as an overview for improved understanding of the complex and multifaceted short-term regulation of this key enzyme in the biosynthetic pathways of mevalonate, ubiquinones, dolichols, isopentenyl-tRNAs, and cholesterol.

HMG CoA reductase activity of lens epithelial cells: compared with true rates of sterol synthesis

The regulation of sterol synthesis in the lens was addressed in the present study by comparing changes in the activity of HMG CoA reductase to changes in true rates of sterol synthesis for bovine lens epithelial cells in culture. The lens cells possessed very high levels of reductase activity (165 to 241 units/10(6) cells) which doubled when the cells were grown in media depleted of lipoproteins. True rates of sterol synthesis were simultaneously measured from incorporation of tritiated water into digitonin-precipitable sterols. Rates of sterol synthesis increased an average 37% more than the increase in reductase activity when the cells were deprived of exogenous cholesterol. Although not perfect, the results indicate a close correlation between HMG CoA reductase activity and rates of sterol synthesis in lens epithelial cells. We conclude that the activity of HMG CoA reductase is a major determinant of the rate of cholesterol synthesis in the lens.

Hydroxymethylglutaryl-coenzyme A reductase exhibits graded distribution in normal and mevinolin-treated ileum

Because the small bowel is a site of significant cholesterol synthesis, we determined the ileal distribution of 3-hydroxy-3-methylglutaryl-coenzyme A reductase (HMG-CoA reductase), the rate-limiting enzyme of the cholesterol biosynthetic pathway. Immunofluorescence microscopy on unfixed snap-frozen sections of ileum and jejunum from untreated rats or dogs showed HMG-CoA reductase in the absorptive villus epithelial cells and this appeared to be strikingly localized in their apical cytoplasm. This pattern of HMG-CoA reductase staining approximated a gradient along the villus-crypt axis with the distal villi labeling most intensely. Treatment of rats with mevinolin and/or cholestyramine for 12 days induced a 5- to 11-fold increase in ileal HMG-CoA reductase activity, and yielded a higher intensity of immunostaining without altering the pattern of enzyme distribution observed in control intestines. Also, rats with maximal induction of ileal HMG-CoA reductase exhibited a twofold increase in the number of epithelial villus cells containing prominent stacks of smooth-surfaced membranes in their apical cytoplasm as seen with electron microscopy. These observations suggest that the distal villus absorptive epithelial cells of the ileum contain high concentrations of HMG-CoA reductase, and therefore might be capable of contributing significant quantities of cholesterol to the circulation.