Studies on the feed-back regulation of cholesterol synthesis

Suppression in mevalonate synthesis mediates antitumor effects of combined statin and gamma-tocotrienol treatment

Statins directly inhibit 3-hydroxy-3-methylglutaryl-coenzyme A reductase (HMGR) activity, while gamma-tocotrienol, an isoform of vitamin E, enhances the degradation and reduces cellular levels of HMGR in various tumor cell lines. Since treatment with statins or gamma-tocotrienol alone induced a dose-responsive inhibition, whereas combined treatment with subeffective doses of these agents resulted in a synergistic inhibition in +SA mammary tumor cell growth, studies were conducted to investigate the role of the HMGR pathway in mediating the antiproliferative effects of combined low dose statin and gamma-tocotrienol. Treatment with 8 microM simvastatin inhibited cell growth and isoprenylation of Rap1A and Rab6, and supplementation with 2 microM mevalonate reversed these effects. However, the growth inhibitory effects of 4 microM gamma-tocotrienol were not dependent upon suppression in mevalonate synthesis. Treatment with subeffective doses of simvastatin (0.25 microM), lovastatin (0.25 microM), mevastatin (0.25 microM), pravastatin (10 microM), or gamma-tocotrienol (2 muM) alone had no effect on protein prenylation or mitogenic signaling, whereas combined treatment with these agents resulted in a significant inhibition in +SA cell growth, and a corresponding decrease in total HMGR, Rap1A and Rab6 prenylation, and MAPK signaling, and mevalonate supplementation reversed these effects. These findings demonstrate that the synergistic antiproliferative effects of combined low dose statin and gamma-tocotrienol treatment are directly related to an inhibition in HMGR activity and subsequent suppression in mevalonate synthesis.

Plant-derived monoterpenes suppress hamster kidney cell 3-hydroxy-3-methylglutaryl coenzyme a reductase synthesis at the post-transcriptional level

The rate-limiting enzyme for mevalonate and cholesterol synthesis in mammalian cells is 3-hydroxy-3-methylglutaryl-CoA (HMG-CoA) reductase. Control occurs through both transcriptional and post-transcriptional actions signaled by the end product, cholesterol, and by isoprenoid intermediates. End products of plant mevalonate metabolism, i.e., plant-derived isoprenoids, also suppress mammalian HMG-CoA reductase. Previous studies reported that isoprenoids suppress reductase synthesis at a post-transcriptional level. We tested the hypothesis that plant-derived isoprenoids also regulate mammalian HMG-CoA reductase synthesis at a post-transcriptional level by incubating lovastatin-treated C100 cells with mevalonate or a plant-derived isoprenoid (the monoterpenes, limonene, perillyl alcohol or geraniol) either alone or combined with the oxysterol, 25-hydroxycholesterol (25-OH C). Mevalonate decreased HMG-CoA reductase synthesis and mRNA levels by 65 and 66%, respectively (P < 0.05). The cyclic monoterpenes, limonene and perillyl alcohol, lowered HMG-CoA reductase synthesis by 70 and 89%, respectively (P < 0.05); although neither reduced HMG-CoA reductase mRNA levels (P = 0.88). Geraniol, an acyclic monoterpene, suppressed HMG-CoA reductase synthesis by 98% and lowered mRNA levels by 66% (P < 0.05). A combination of 25-OH C and either mevalonate or any three monoterpenes reduced HMG-CoA reductase mRNA levels (P < 0.05) compared with lovastatin-only treated cells. However, the dual combination of 25-OH C and either mevalonate or a monoterpene resulted in a greater decrease in HMG-CoA reductase synthesis than in mRNA levels. The difference between changes in HMG-CoA reductase synthesis and mRNA levels reflects a specific effect of isoprenoids on HMG-CoA reductase synthesis at the translational level. Mevalonate enhanced HMG-CoA reductase degradation, but no such effect was observed for the monoterpenes. These results indicate that the three plant-derived isoprenoids primarily suppress HMG-CoA reductase synthesis at a post-transcriptional level by attenuating HMG-CoA reductase mRNA translational efficiency.

Stimulation of hepatic cholesterol biosynthesis by fatty acids. Effects of oleate on cytoplasmic acetoacetyl-CoA thiolase, acetoacetyl-CoA synthetase and hydroxymethylglutaryl-CoA synthase

The effects of oleic acid on the activities of cytosolic HMG-CoA (3-hydroxy-3-methylglutaryl-CoA) synthase, AcAc-CoA (acetoacetyl-CoA) thiolase and AcAc-CoA synthetase, as well as microsomal HMG-CoA reductase, all enzymes in the pathway of cholesterol biosynthesis, were studied in the isolated perfused rat liver. Oleic acid bound to bovine serum albumin, or albumin alone, was infused for 4 h at a rate sufficient to sustain an average concentration of 0.61 +/- 0.05 mM fatty acid during the perfusion. Hepatic cytosol and microsomal fractions were isolated at the termination of the perfusion. Oleic acid simultaneously increased the activities of the cytosolic cholesterol-biosynthetic enzymes 1.4-2.7-fold in livers from normal fed rats and from animals fasted for 24 h. These effects were accompanied by increased net secretion by the liver of cholesterol and triacylglycerol in the very-low-density lipoprotein (VLDL). We confirmed the observations reported previously from this laboratory of the stimulation by oleic acid of microsomal HMG-CoA reductase. In cytosols from perfused livers, the increase in AcAc-CoA thiolase activity was characterized by an increase in Vmax. without any change in the apparent Km of the enzyme for AcAc-CoA. In contrast, oleic acid decreased the Km of HMG-CoA synthase for Ac-CoA, without alteration of the Vmax. of the enzyme. The Vmax. of AcAc-CoA synthetase was increased by oleic acid, and there was a trend towards a small increase in the Km of the enzyme for acetoacetate. These data allow us to conclude that the enzymes that supply the HMG-CoA required for hepatic cholesterogenesis are stimulated, as is HMG-CoA reductase, by a physiological substrate, fatty acid, that increases rates of hepatic cholesterol synthesis and cholesterol secretion. Furthermore, we suggest that these effects of fatty acid on hepatic cholesterol metabolism result from stimulation of secretion of triacylglycerol in the VLDL by fatty acids, and the absolute requirement of cholesterol as an important structural surface component of the VLDL necessary for transport of triacylglycerol from the liver.

Cholesterol and cancer: comparative biochemistry and selective toxicity

There is abundant evidence in the literature for the existence of a difference between normal and neoplastic cells with regard to the regulation of cholesterol biosynthesis. More specifically, the loss of negative feedback control of this pathway seems to be a property of transformed cells. Herein we describe the relationship between cholesterol and cancer, and we describe a model which attempts to employ Adrien Albert's second principle of selectivity--that of comparative biochemistry--by exploiting the above mentioned difference in regulation.

Regulation of hepatic cholesterol biosynthesis by fatty acids: effect of feeding olive oil on cytoplasmic acetoacetyl-coenzyme A thiolase, beta-hydroxy-beta-methylglutaryl-CoA synthase, and acetoacetyl-coenzyme A ligase

We reported previously that, in the perfused rat liver, oleic acid increased the specific activity of cytosolic enzymes of cholesterol biosynthesis. In this study, we examined the effects of oral administration of olive oil on the activities of HMG-CoA synthase, AcAc-CoA thiolase, AcAc-CoA ligase and HMG-CoA reductase. Olive oil feeding increased the specific activity of hepatic HMG-CoA synthase by 50%, AcAc-CoA thiolase by 2-fold, and AcAc-CoA ligase by 3-fold. Olive oil had no effect on HMG-CoA reductase activity. These data suggest that the enzymes that supply the HMG-CoA required for hepatic cholesterogenesis are regulated in parallel by a physiological substrate, fatty acid, independent of HMG-CoA reductase under these conditions.

Loss of the cholesterol feedback system in the intact hepatoma-bearing rat

By means of the desmosterol suppression technique described in the previous paper, the influence of hepatomas on sterol metabolism has been studied in the intact rat. The major finding of this study is that all hepatoma-bearing rats demonstrate a consistent in vivo loss of the cholesterol feedback system that is characteristic of normal liver. The results also demonstrate that such tumors retain only minor amounts of the sterol they synthesize, releasing over 90% of such endogenous sterol into the circulation. Finally, the in vivo loss of cholesterol feedback control was found to occur in at least two minimal deviation hepatomas and in one highly malignant adenocarcinoma of hepatic origin. These findings indicate that even tumors that are capable of only very limited cholesterol synthesis in vitro, can contribute significant quantities of sterol to the bloodstream. It is concluded that as a result of their lack of normal cholesterol feedback control, hepatomas may represent a physiologically important source of sterols in the tumor-bearing animal, and that the absence of feedback control of sterol synthesis may provide a means of detecting the presence of such tumors in the intact animal.

Loss of feedback control of hydroxymethylglutaryl coenzyme A reductase in hepatomas

THE FEEDBACK RESPONSE OF MEVALONATE: NADP oxidoreductase (acylating CoA; EC 1.1.1.34) in two varieties of hepatoma has been examined. In marked contrast to the feedback inhibition of this enzyme that is regularly observed in normal liver, the feedback control is completely lost in minimal-deviation hepatomas 9121 and 3924A. This finding provides the first specific biochemical localization of the defect in cholesterol feedback control, which has been shown previously to be a characteristic of all hepatomas.

Th biosynthesis of rubber from beta-hydroxy-beta-methylgluarylcoenzyme A in Hevea brasiliensis latex

1. beta-Hydroxy-beta-methyl[3-(14)C]glutaryl-CoA is efficiently incorporated into rubber on incubation with Hevea brasiliensis latex, and the incorporation is diminished in the presence of unlabelled mevalonate. beta-Hydroxy-beta-methylglutaric acid is not utilized for rubber synthesis, but inhibits the formation of rubber from beta-hydroxy-beta-methylglutaryl-CoA. 2. The incorporation of beta-hydroxy-beta-methylglutaryl-CoA into rubber is stimulated equally by NADP(+) and NADPH and less so by NAD(+) and NADH. ATP is slightly stimulatory and CoA is inhibitory. 3. beta-Hydroxy-beta-methylglutaryl-CoA reductase is concentrated in the sediment (bottom fraction) formed by centrifuging latex at low speed and the enzyme is unstable in the absence of cysteine or GSH. The formation of NADPH takes place in the latex serum. 4. There is a marked seasonal variation in the extent of beta-hydroxy-beta-methylglutaryl-CoA incorporation into rubber in latex, but mevalonate incorporation is relatively constant. This observation, together with the finding that beta-hydroxy-beta-methylglutaryl-CoA reduction is the rate-limiting step in the formation of rubber from beta-hydroxy-beta-methylglutaryl-CoA, suggests that the conversion of beta-hydroxy-beta-methylglutaryl-CoA into mevalonate is of importance in the regulation of rubber synthesis. 5. Evidence suggesting that beta-hydroxy-beta-methylglutaryl-CoA lyase is present in H. brasiliensis latex has been obtained.

Stimulation of liver cholesterol synthesis by actinomycin D

1. An eightfold increase in the incorporation of [2-(14)C]acetate into liver cholesterol in vivo was observed 24hr. after starved rats had been given actinomycin D (0.5mg./kg. of body wt.). Liver cholesterol radioactivity declined faster in the treated animals, suggesting a greater rate of cholesterol turnover. 2. Liver slices from treated animals showed a tenfold increase in the incorporation of [2-(14)C]acetate into cholesterol; conversion into CO(2) and into fatty acids was less markedly increased, and conversion into ketone bodies was not significantly affected. 3. The patterns of conversion into liver cholesterol in vivo of the lactone and the sodium salt of mevalonic acid differed markedly. The former was converted at a faster rate and to a greater extent than the latter. Treatment with actinomycin D increased the conversion of both forms of mevalonic acid into liver cholesterol, but only to a small extent. 4. Stimulation of the incorporation of acetate into cholesterol occurred at 4hr. after the administration of actinomycin D but not at 2hr. The response was abolished by the simultaneous administration of dl-ethionine or puromycin. 5. Pre-feeding with a cholesterol-rich diet greatly diminished the stimulation of conversion of acetate into cholesterol caused by actinomycin D, though it did not completely suppress it. Adrenalectomized animals responded to the drug, but much less markedly. 6. It is concluded that actinomycin D stimulates the synthesis of cholesterol in the liver at a stage in the pathway before mevalonic acid, by a mechanism that probably requires protein synthesis. A likely site would be the beta-hydroxy-beta-methylglutaryl-CoA reductase, the rate-limiting enzyme in cholesterol biosynthesis. Some possible mechanisms by which the drug may lead to increased activity of this enzyme are considered.

Mechanism of suckling-rat hypercholesteremia. II. Cholesterol biosynthesis and cholic acid turnover studies

Cholesterol biosynthesis from acetate-2-(14)C by the livers of suckling rats, which is known to be relatively slow, was increased 2-3-fold within 24 hours after severing the bile duct. Cholesterol synthesis by sham-operated litter mates showed no change under similar treatment. Mevalonate biosynthesis from acetate-2-(14)C in vitro by recombined liver microsomal supernatant (105,000xg) fractions from suckling rats also was only 10% of that of comparable recombined fractions from normal controls (young adult rats which were consuming colony diet). Activity was not improved by combining either the microsomal or supernatant fraction from suckling rat livers with the complementary fraction from normal adult livers. On the other hand, activity was restored to 100% when microsomes from livers of duct-served suckling rats were combined with the supernatant fraction from normal controls. Likewise, recombined liver fractions prepared from adult rats fed synthetic diets exhibited low activity for mevalonate biosynthesis. Activity was restored by bile duct cannulation, but inhibited when cholic acid was infused into the cannulated animal. Therefore, surgical procedures which interrupt the enterohepatic recirculation of bile components lead to a restoration of cholesterol biosynthesis and, at least in the adult animal where cannulation studies are practicable, this effect can be reversed readily by bile acid infusion.A slow rate of fecal excretion of(14)C-cholic acid was observed in suckling rats and rats fed synthetic diets, apparently reflecting an efficient enterohepatic recirculation of bile salts. The data suggest that under these dietary conditions bile salt retention either directly or indirectly influences hepatic synthesis of cholesterol.

Activities of some enzymes concerned with citrate and glucose metabolism in transplanted rat hepatomas

1. Certain enzymes concerned with citrate and glucose metabolism have been measured in two transplanted rat hepatomas, one induced with ethionine (minimal deviation type) and one induced with dimethylaminoazobenzene. In these hepatomas both citrate-cleavage enzyme and NADP-linked isocitrate dehydrogenase in the soluble fraction of the cell were approximately one-third of the values for normal rat liver. These changes have been discussed in relation to the increased citric acid content of tumours and depressed rate of fatty acid synthesis. 2. The glucose-ATP-phosphotransferase activity was below normal liver values in the ethionine-induced tumour but greater than normal in the dimethylaminoazobenzene-induced hepatoma. The apparent K(m) values for the glucose-ATP phosphotransferases of these hepatomas were approx. 8x10(-5)m; no evidence was found for an enzyme with a high K(m) for glucose equivalent to liver glucokinase. 3. Of the enzymes of the pentose phosphate pathway, glucose 6-phosphate-dehydrogenase activity was three to five times as great whereas 6-phosphogluconate-dehydrogenase activity was the same or lower than normal liver in the ethionine-and dimethylaminoazobenzene-induced tumours respectively.

agr-D-Glucose: Precise Determination of Crystal and Molecular Structure by Neutron-Diffraction Analysis

This analysis provides the first precise crystal and molecular structural parameters for alpha-D-glucose. The C-C, C-H, and O-H bond lengths deviate only slightly from their means of 1.523 A, 1.098 A, and 0.968 A. The C(1)-O(1) bond length, 1.389 A, is significantly shorter than the mean value, 1.420 A, of the other C-O bonds. The valence angle of the ring oxygen is 113.8 degrees.