Reversible modulation of rat liver 3-hydroxy 3-methyl glutaryl coenzyme A reductase. Evidence for an enzyme-catalyzed phosphorylation-dephosphorylation system

Stimulation of microsomal cholesterol ester hydrolase by glucagon, cyclic AMP analogues, and vasopressin in isolated rat hepatocytes

Short-term activation of microsomal cholesterol ester hydrolase by glucagon, cAMP analogues, and vasopressin in isolated rat hepatocytes is described. Glucagon led to a dose- and time-dependent activation of cholesteryl oleate hydrolysis, but values returned to basal levels within 120 min. Exposure of isolated hepatocytes to 0.5 mM concentrations of dibutyryl-cAMP or 8-[4-chlorophenylthio]-cAMP, or 25 microM forskolin caused persistent activation of cholesterol ester hydrolase activity after a lag period of 30 min. The three agents resulted in early marked intracellular accumulation of cAMP that declined progressively, and moderate and sustained reductions in the diacylglycerol content. The actions of glucagon on hepatocytes were inhibited by pretreatment of cells with 10 nM [8-arginine] vasopressin. Vasopressin elicited a consistent and sustained increase in cholesterol ester hydrolase activity and diacylglycerol without affecting cAMP while reducing the effect of glucagon on cAMP. Furthermore, the effects of glucagon and vasopressin on the activation of cholesterol ester hydrolase were not additive despite the similarity of their stimulation of diacylglycerol formation. Blockade of vasopressin-mediated activation of cholesterol ester hydrolase and diacylglycerol content were induced by excess prazosin. These data suggest that stimulation of microsomal cholesterol ester hydrolase in isolated liver cells may involve at least two signal transduction systems.

Regulation of rat liver microsomal cholesterol ester hydrolase by reversible phosphorylation

The regulation of neutral cholesterol ester hydrolase activity by changes in its phosphorylation state was studied in rat liver microsomes. Treatment with cAMP-dependent protein kinase resulted in increased enzyme activity, which was further enhanced by the addition of cAMP and MgATP. Consistent activations were also achieved with MgCl2 and MgATP, the magnesium effect being abolished by ethylenediaminetetraacetic acid and adenosine triphosphate. Cholesterol ester hydrolase was activated twofold by free calcium and Ca2+/calmodulin; this latter effect was blocked by the chelator ethylene-glycol-bis(beta-aminoethyl ether)N,N,N',N'-tetraacetic acid and the calmodulin antagonist trifluoperazine. The phosphatase inhibitors pyrophosphate and glycerophosphate led to marked and dose-dependent increases in esterase activity, whereas okadaic acid elicited no effect. Furthermore, pyrophosphate and okadaic acid did not change the increases in enzyme activity promoted by Ca2+, Ca2+/calmodulin, Mg2+ and MgATP. Cholesterol ester hydrolase was inactivated in a concentration-dependent manner by nonspecific alkaline phosphatases. In cAMP-dependent protein kinase/cAMP- or Ca2+/calmodulin-activated microsomes, a time-dependent loss of activation in cholesteryl oleate hydrolysis was caused by alkaline phosphatase. These findings suggest that microsomal cholesterol ester hydrolase is activated through cAMP and Ca2+/calmodulin phosphorylation, whereas enzyme deactivation is dependent on phosphatase action.

Diurnal rhythm of rat liver cytosolic 3-hydroxy-3-methylglutaryl-CoA synthase

Rat liver cytosolic 3-hydroxy-3-methylglutaryl-CoA (HMG-CoA) synthase exhibits a diurnal rhythm of enzyme activity which coincides with the diurnal rhythm of HMG-CoA synthase protein. The peaks of activity and protein (determined by SDS/PAGE and immunoblotting) both occur at D10 (the tenth hour of the daily 12 h dark cycle). The peak of mRNA levels (measured by slot-blot hybridization of liver RNA) is slightly advanced with respect to that of protein, by about 4 h, and shows a maximum at D6. Cytosolic HMG-CoA synthase activity and protein in rats fed on a normal diet were approx. 2-fold higher during the peak at D10 than in the nadir at D2. HMG-CoA synthase mRNA levels were approx. 4-fold higher during the peak at D6 than in the nadir at D2. These results point to a transcriptional and translational regulation of the cytosolic HMG-CoA synthase.

Phosphorylation of HMG-CoA reductase induced by mevalonate accelerates its rate of degradation in isolated rat hepatocytes

Incubation of rat hepatocytes with 10 mM mevalonate produces a decrease in HMG-CoA reductase activity and in the rate of synthesis of both monomeric and dimeric HMG-CoA reductase, and an increase in the rate of degradation of the monomeric form without significant change in that of the dimeric form. Since mevalonate promotes a short-term phosphorylation of the monomeric form without affecting the dimeric form, it is suggested that the mechanism of degradation of reductase is controlled by its phosphorylation state.

Activation of rat liver cytosolic 3-hydroxy-3-methylglutaryl coenzyme A reductase kinase by adenosine 5'-monophosphate

Inactivation of 3-hydroxy-3-methylglutaryl Coenzyme A reductase by reductase kinase and ATP-Mg needs either ADP or 5'-AMP as cofactors. 5'-AMP is a more potent activator of cytosolic reductase kinase than ADP. This capacity is expressed by increasing not only the rate of reductase inactivation, but also the rate of reductase phosphorylation from [gamma-32P]ATP. Activation constants, Ka, for 5'-AMP and ADP are 20 microM and 420 microM respectively. Neither 3'-AMP nor 2'-AMP activate reductase kinase. Other nucleoside monophosphates like UMP, CMP and GMP cannot replace 5'-AMP as activators of reductase kinase.

Modulation of rat liver hydroxymethylglutaryl-CoA reductase by protein phosphatases: purification of nonspecific hydroxymethylglutaryl-CoA reductase phosphatases

Four phosphoprotein phosphatases, with the ability to act upon hydroxymethylglutaryl (HMG)-CoA reductase, phosphorylase, and glycogen synthase have been purified from rat liver cytosol through a process that involves DEAE-cellulose, aminohexyl-Sepharose-4B, and Bio-Gel A 1.5 m chromatographies. Protein phosphatase II (Mr 180,000) was the major enzyme (68%) with a very broad substrate specificity, showing similar activity toward the three substrates. Phosphatases I1 (Mr 180,000) and I3 (Mr 250,000) accounted for only 12 and 15% of the total activity, respectively, and they were also able to dephosphorylate the three substrates. In contrast, phosphatase I2 (Mr 200,000) showed only phosphorylase phosphatase activity with insignificant dephosphorylating capacity toward HMG-CoA reductase and glycogen synthase. Upon ethanol treatment at room temperature, the Mr of all phosphatases changed; protein phosphatases I2, I3, and II were brought to an Mr of 35,000, while phosphatase I1 was reduced to an Mr of 69,000. Glycogen synthase phosphatase activity was decreased in all four phosphatases. There was also a decrease in phosphatase I1 activity toward HMG-CoA reductase and phosphorylase as substrates. The HMG-CoA reductase phosphatase and phosphorylase phosphatase activities of phosphatases I2, I3, and II were increased after ethanol treatment. Each protein phosphatase showed a different optimum pH, which changed depending on the substrate. The four phosphatases increased their activity in the presence of Mn2+ and Mg2+. In general, Mn2+ was a better activator than Mg2+, and phosphatase I1 showed a stronger dependency on these cations than any other phosphatase. Phosphorylase was a competitive substrate in the HMG-CoA reductase phosphatase and glycogen synthase phosphatase reactions of protein phosphatases I1, I3, and II. HMG-CoA reductase was also able to compete with phosphorylase and glycogen synthase for phosphatase activity. Glycogen synthase phosphatase activity presented less inhibition in the low-Mr forms. A comparison has been made with other protein phosphatases previously reported in the literature.

Phosphorylation of 3-hydroxy-3-methylglutaryl coenzyme A reductase by microsomal 3-hydroxy-3-methylglutaryl coenzyme A reductase kinase

Microsomal 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase kinase has been purified to apparent homogeneity by a process involving the following steps: solubilization from microsomes and chromatography on Affi-Gel Blue, phosphocellulose, Bio-Gel A 1.5m, and agarose-hexane-ATP. The apparent Mr of the purified enzyme as judged by gel-filtration chromatography is 205,000 and by sodium dodecyl sulfate-gel electrophoresis is 105,000. Immunoprecipitation of homogeneous reductase phosphorylated by reductase kinase and [gamma-32P]ATP produces a unique band containing 32P bound to protein which migrates at the same Rf as the reductase subunit. Incubation of 32P-labeled HMG-CoA reductase with reductase phosphatase results in a time-dependent loss of protein-bound 32P radioactivity, as well as an increase in enzymic activity. Reductase kinase, when incubated with ATP, undergoes autophosphorylation, and a simultaneous increase in its enzymatic activity is observed. Tryptic treatment of immunoprecipitated, 32P-labeled HMG-CoA reductase phosphorylated with reductase kinase produces only one 32P-labeled phosphopeptide with the same Rf as one of the two tryptic phosphopeptides that have been reported in a previous paper. The possible existence of a second microsomal reductase kinase is discussed.

Regulation of three key enzymes in cholesterol metabolism by phosphorylation/dephosphorylation

Our laboratories have investigated the role of phosphorylation/dephosphorylation in the regulation of three key enzymes in cholesterol metabolism. 3-Hydroxy-3-methylglutarylcoenzyme A (HMG-CoA) reductase (EC 1.1.1.34), the major regulatory enzyme in cholesterol biosynthesis, is inhibited by phosphorylation. Acyl-CoA:cholesterol O-acyltransferase (ACATase; EC 2.3.1.26) and cholesterol 7 alpha-hydroxylase (EC 1.14.13.7), key regulatory enzymes in the utilization of cholesterol, are activated by phosphorylation. In view of these results, we propose that short-term regulation of the concentration of intracellular unesterified cholesterol is achieved by a coordinate phosphorylation/dephosphorylation of these three enzymes. For example, if cholesterol enters the liver cell, HMG-CoA reductase would be inhibited by phosphorylation and biosynthesis of cholesterol would be reduced; however, reactions utilizing cholesterol would be activated, due to the phosphorylation of ACATase and cholesterol 7 alpha-hydroxylase. Thus, the phosphorylation/dephosphorylation of these three enzymes provides an elegant short-term mechanism for the homeostasis of intracellular unesterified cholesterol.

Regulation of short-term changes in hepatic beta-hydroxy-beta-methylglutaryl-CoA reductase activity

Immunotitrations of rat liver hydroxymethylglutaryl-CoA (HOMeGlt-CoA) reductase activity were performed before and after short-term changes in the nutritional or hormonal state of the animals. Changes in enzyme activity (increase or decrease) within 1 h following cholesterol feeding or glucagon or mevalonolactone administration to normal rats, or insulin administration to diabetic rats were accompanied by no change in the specific activity of the enzyme, as determined from the quantity of enzyme activity inactivated by a fixed quantity of antibody. These results support the conclusion that the loss in enzyme activity was due to conversion of the enzyme to immuno-unreactive products. In agreement with this conclusion the enzyme activity lost after these short-term physiological changes was not restorable by phosphoprotein phosphatase action. On the other hand, incubation of rat liver microsomes with ATP and Mg2+ decreased the specific activity of HOMeGlt-CoA reductase about tenfold, as determined by immunotitration. The low specific activity produced under these conditions was increased by phosphatase action to nearly the original level. The above evidence suggests that the changes in HOMeGlt-CoA reductase activity that resulted from short-term physiological changes in hormonal or nutritional states of an animal were brought about by a change in the quantity of enzyme, and not by reversible phosphorylation of pre-existing enzyme.

Purification and properties of rat liver hydroxymethylglutaryl coenzyme A reductase phosphatases

A procedure for the isolation and purification of two rat liver hydroxymethylglutaryl coenzyme A reductase phosphatases is described for the first time. Each of the preparations was obtained in two molecular forms of different molecular weights. The molecular weights of the holoenzymes were 480,000 and 310,000, respectively, while the molecular forms obtained after an ethanol treatment were in both cases 35,000. Several kinetic measurements were made which showed that the protein of Mr 35,000 was identical in both cases, irrespective of the holoenzymatic starting preparation used. The optimum pH of the three phosphatases ranged between 6.0 and 6.5. The Km of the phosphatases ranged between 6.5 and 19.5 nM when hydroxymethylglutaryl coenzyme A (HMG-CoA) reductase was the substrate. The three HMG-CoA reductase phosphatases, upon incubation, released 32P from 32P-labelled HMG-CoA reductase. This dephosphorylation also produces an activation of the HMG-CoA reductase activity.

Effect of unsaturated fatty acids on sterol biosynthesis in yeast

Lipid-depleted yeast, grown anaerobically, contains only very low amounts of sterols. The hydroxymethylglutaryl-CoA reductase activity, the regulatory enzyme of sterol synthesis in yeast, is also low. Aeration of such cells in a buffer containing a carbon source induces hydroxymethylglutaryl-CoA reductase activity and increases sterol synthesis. The velocity of the increase depends on the carbon source present during the aeration period. Glucose and sugars that are easily converted to glucose were found to be most effective. A supplement of unsaturated fatty acids during anaerobic growth causes a several-fold greater velocity of the enzyme induction and of sterol biosynthesis. Linolenic acid (30 microM) accelerated sterol biosynthesis about 7-fold. Activities of galactokinase and galactose-1-phosphate uridyltransferase, which are involved in the conversion of galactose to glucose, increased several-fold in the supplemented cells within 6 h of aeration, concomitantly with stimulation of sterol synthesis from galactose. It is suggested that the stimulation of enzyme induction and sterol biosynthesis in fatty acid supplemented cells is due to a completion of the protein-synthesizing apparatus during cell growth. A markedly enhanced capacity of these cells to incorporate leucine into acid-precipitable protein supports this assumption.

In vivo regulation of rat liver 3-hydroxy-3-methylglutaryl-coenzyme A reductase: enzyme phosphorylation as an early regulatory response after intragastric administration of mevalonolactone

Although substantial evidence supports the conclusion that 3-hydroxy-3-methylglutaryl-coenzyme A (HMG-CoA) reductase [mevalonate:NADP+ oxidoreductase (CoA-acylating), EC 1.1.1.34] is the major regulatory enzyme in cholesterol biosynthesis, the molecular events involved in the in vivo regulation of this enzyme have remained obscure. To study this problem, rats were given a single 100-mg dose of mevalonolactone by intragastric tube. The rats were sacrificed 20 or 60 min later, and liver microsomes were prepared by ultracentrifugation. Two phases of inhibition of microsomal HMG-CoA reductase were observed. The first phase of inhibition, observed 20 min after mevalonolactone administration, was completely reversed by preincubation of the microsomes with purified phosphoprotein phosphatase. The second phase of inhibition, observed 60 min after mevalonolactone administration, was not reversed by phosphoprotein phosphatase. The reactivation of liver microsomal HMG-CoA reductase by phosphoprotein phosphatase was blocked by potassium fluoride or by phosphoprotein phosphatase inhibitor. Results obtained by immunotitration also showed that microsomal HMG-CoA reductase obtained from animals killed 20 min after mevalonolactone administration was significantly activated by phosphoprotein phosphatase treatment of the microsomes. These findings demonstrate that phosphorylation of rat liver HMG-CoA reductase is an early in vivo regulatory response after intragastric administration of mevalonolactone.

Studies on the mechanisms of the rapid modulation of 3-hydroxy-3-methylglutaryl coenzyme A reductase in intact liver by mevalonolactone and 25-hydroxycholesterol

A rapid, biphasic inhibition of rat hepatic 3-hydroxy-3-methylglutaryl coenzyme A reductase (mevalonate:NADP+ oxidoreductase (CoA-acylating), EC 1.1.1.34) was induced by intragastric administration of R,S-mevalonolactone. The initial phase had a t1/2 of 5.3 min. 30 min after drug administration the inhibition could be reversed in vitro by cytosol or a partially purified cytosolic activator. The reactivation was prevented by 50mM NaF. Thus the initial inhibition appeared to be the result of reversible inactivation possibly by phosphorylation of the enzyme. Consistent with this was the finding that the net reductase activator (phosphatase) activity present in cytosol was decreased 64% in these animals. The rapid reversible inhibition could not be reproduced in vitro by incubating microsomes or postmitochondrial supernatants with mevalonate suggesting the intact cell was necessary for expression of the effect. The second phase of inhibition due to mevalonate administration had a t1/2 of 1.3 h and was not reversible. It was attributed to inhibition of synthesis of reductase probably as the result of sterol accumulation in the cell. Perfusion of 25-hydroxycholesterol through livers isolated from animals at the circadian peak of cholesterol biosynthesis resulted in a rapid, 75-80% inhibition of 3-hydroxy-3-methylglutaryl coenzyme A reductase. This inhibition was not reversed by incubation with cytosol or partially purified activator. Further, there was no apparent change in net activator levels in cytosol from the livers perfused with 25-hydroxycholesterol. This suggests the effect of this sterol on reductase does not involve reversible phosphorylation-dephosphorylation. On the basis of this study it is postulated that there are at least two mechanisms by which 3-hydroxy-3-methylglutaryl coenzyme A reductase activity can be rapidly suppressed in the intact liver. One is reversible and appears to be the result of alteration in the reductase kinase-phosphatase system. The second is irreversible and may be due to acceleration of the normal degradation system.

Inactive 3-hydroxy-3-methylglutaryl-coenzyme A reductase in broken cell preparations of various mammalian tissues and cell cultures

Preincubation of broken cell preparations from a variety of tissues and cell cultures resulted in an apparent increase in the level of 3-hydroxy-3-methylglutaryl-CoA reductase activity. However, apparent activation of the reductase in mouse liver, hepatomas and primary liver cell cultures was attributed largely to the loss, during the preincubation period, of an interfering enzyme, 3-hydroxy-3-methylglutaryl-CoA lyase. Among non hepatic cells and tissues (which did not contain appreciable lyase activity) the proportion of latent reductase was high in sonicates of fetal brain and in L cells and was independent of the level of total enzyme activity present. Activation of the reductase was blocked by hydroxymethylglutaryl-CoA and NADPH as well as by KF so that activation did not occur under the conditions of the enzyme assay. The enzyme was activated slowly at 4 degrees C, so that partial activation of the latent form occurred during isolation of the microsomal fraction by differential centrifugation. The reductase present in sonicates of cells with either a high or low proportion of the latent enzyme was inactivated by incubation with ATP and Mg2+. Suppression of reductase activity in L cell cultures by treatment with 25-hydroxycholesterol and an age-related decline in brain enzyme activity did not involve reversible conversion of the reductase to an inactive form.