Gene/Protein Disease Symptom Drug Enzyme Compound
Pivot Concepts:   Target Concepts:
Query: EC:2.7.11.1 (protein kinase)
 81,284 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

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.
 ...
PMID:Phosphorylation and degradation of HMG CoA reductase. 262 76

We have demonstrated previously that cultured rat ovarian granulosa cells synthesize and secrete apoE, and this production of apoE is increased by agents that stimulate protein kinase A (cyclic AMP-dependent enzyme) (for example, cholera toxin) and protein kinase C (Ca2+/phospholipid-dependent enzyme) (for example, 12-O-tetradecanoylphorbol-13-acetate, a phorbol ester). In the studies presented in this report, we have examined the effect of changes in cell cholesterol synthesis on the production of apoE by rat ovarian granulosa cells. Mevinolin, an inhibitor of hydroxymethylglutaryl (HMG)-CoA reductase (the rate-limiting enzyme in cholesterol synthesis), and 4,4,10 beta-trimethyl-trans-decal-3 beta-ol, an inhibitor of squalene cyclization, both attenuate the cholera toxin or 12-O-tetradecanoylphorbol-13-acetate stimulation of granulosa cell apoE secretion and apoE mRNA content in a dose-responsive manner. The inhibitory effect of mevinolin is reversed by the concomitant administration of mevalolactone, which provides the cells with the product of the reaction catalyzed by HMG-CoA reductase. Steroidogenesis per se has no effect on apoE production. Aminoglutethimide, which blocks the rate-limiting step in steroidogenesis, has no effect on apoE or apoE mRNA. The data indicate that products of HMG-CoA reductase (isoprenes, cholesterol and/or cholesterol metabolites) are required along with stimulators of protein kinases A and C, to regulate ovarian granulosa cell apoE production.
 ...
PMID:Rat granulosa cell apolipoprotein E secretion. Regulation by cell cholesterol. 277 96

The type-1 protein phosphatase associated with hepatic microsomes has been distinguished from the glycogen-bound enzyme in five ways. (1) The phosphorylase phosphatase/synthase phosphatase activity ratio of the microsomal enzyme (measured using muscle phosphorylase a and glycogen synthase (labelled in sites-3) as substrates) was 50-fold higher than that of the glycogen-bound enzyme. (2) The microsomal enzyme had a greater sensitivity to inhibitors-1 and 2. (3) Release of the catalytic subunit from the microsomal type-1 phosphatase by tryptic digestion was accompanied by a 2-fold increase in synthase phosphatase activity, whereas release of the catalytic subunit from the glycogen-bound enzyme decreased synthase phosphatase activity by 60%. (4) 95% of the synthase phosphatase activity was released from the microsomes with 0.3 M NaCl, whereas little activity could be released from the glycogen fraction with salt. (5) The type-1 phosphatase separated from glycogen by anion-exchange chromatography could be rebound to glycogen, whereas the microsomal enzyme (separated from the microsomes by the same procedure, or by extraction with NaCl) could not. These findings indicate that the synthase phosphatase activity of the microsomal enzyme is not explained by contamination with glycogen-bound enzyme. The microsomal and glycogen-associated enzymes may contain a common catalytic subunit complexed to microsomal and glycogen-binding subunits, respectively. Thiophosphorylase a was a potent inhibitor of the dephosphorylation of ribosomal protein S6, HMG-CoA reductase and glycogen synthase, by the glycogen-associated type-1 protein phosphatase. By contrast, thiophosphorylase a did not inhibit the dephosphorylation of S6 or HMG-CoA reductase by the microsomal enzyme, although the dephosphorylation of glycogen synthase was inhibited. The I50 for inhibition of synthase phosphatase activity by thiophosphorylase a catalysed by either the glycogen-associated or microsomal type-1 phosphatases, or for inhibition of S6 phosphatase activity catalysed by the glycogen-associated enzyme, was decreased 20-fold to 5-10 nM in the presence of glycogen. The results suggest that the physiologically relevant inhibitor of the glycogen-associated type-1 phosphatase is the phosphorylase a-glycogen complex, and that inhibition of the microsomal type-1 phosphatase by phosphorylase a is unlikely to play a role in the hormonal control of cholesterol or protein synthesis. Protein phosphatase-1 appears to be the principal S6 phosphatase in mammalian liver acting on the serine residues phosphorylated by cyclic AMP-dependent protein kinase.
 ...
PMID:Distinct type-1 protein phosphatases are associated with hepatic glycogen and microsomes. 284 6

A highly purified rat liver protein kinase phosphorylates and inactivates acetyl-CoA carboxylase, and causes rapid inactivation of microsomal HMG-CoA reductase in the presence of MgATP. Both effects are stimulated in an identical manner by AMP, and are greatly reduced by prior treatment of the kinase with purified protein phosphatase. The dephosphorylated kinase can be reactivated in the presence of MgATP, apparently due to a distinct kinase kinase, and this reactivation is stimulated by nanomolar concentrations of palmitoyl-CoA. These results show that a common, bicyclic protein kinase cascade can potently inactivate the regulatory enzymes of both fatty acid and cholesterol biosynthesis.
 ...
PMID:A common bicyclic protein kinase cascade inactivates the regulatory enzymes of fatty acid and cholesterol biosynthesis. 2462 16

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.
 ...
PMID: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. 300 40

A calcium-activated and phospholipid-dependent protein kinase (protein kinase C) catalyzes the phosphorylation of both insoluble microsomal (Mr approximately 100,000) and purified soluble (Mr = 53,000) 3-hydroxy-3-methylglutaryl coenzyme A reductase. The phosphorylation and concomitant inactivation of enzymic activity of HMG-CoA reductase was absolutely dependent on Ca2+, phosphatidylserine, and diolein. Dephosphorylation of phosphorylated HMG-CoA reductase was associated with the loss of protein bound radioactivity and reactivation of enzymic activity. Maximal phosphorylation of purified HMG-CoA reductase was associated with the incorporation of 1.05 +/- 0.016 mol of phosphate/mol of native form of HMG-CoA reductase (Mr approximately 100,000). The apparent Km for purified HMG-CoA reductase and histone H1 was 0.08 mg/ml, and 0.12 mg/ml, respectively. The tumor-promoting phorbol ester, phorbol 12-myristate 13-acetate stimulated the protein kinase C-catalyzed phosphorylation of HMG-CoA reductase. Increased phosphorylation of HMG-CoA reductase by phorbol 12-myristate 13-acetate suggests a possible in vivo protein kinase C-mediated mechanism for the short-term regulation of HMG-CoA reductase activity. The identification of the protein kinase C system in addition to the reductase kinase-reductase kinase kinase bicyclic cascade systems for the modulation of the enzymic activity of HMG-CoA reductase may provide new insights into the molecular mechanisms involved in the regulation of cholesterol biosynthesis.
 ...
PMID:Phosphorylation of hepatic 3-hydroxy-3-methylglutaryl coenzyme A reductase and modulation of its enzymic activity by calcium-activated and phospholipid-dependent protein kinase. 315 37

A protein kinase which phosphorylates and inactivates acetyl-CoA carboxylase has been purified to apparent homogeneity from rat liver. The kinase was found to exist in two forms: bound to carboxylase in a complex or in a free form that is in different stages of aggregation over a wide range of molecular weights. The purification of the kinase involved first partial purification of acetyl-CoA carboxylase through polyethylene glycol precipitation and DEAE-cellulose chromatography. The kinase was then separated from acetyl-CoA carboxylase by Sepharose 2B chromatography. The molecular weight of the kinase subunit was 170,000 as determined by sodium dodecyl sulfate-gel electrophoresis. The incorporation of 1 mol of phosphate/mole of carboxylase subunit caused complete inactivation of the carboxylase. Acetyl-CoA carboxylase, inactivated by the kinase, can be dephosphorylated and reactivated when incubated with phosphorylase phosphatase. The Km values of the kinase for acetyl-CoA carboxylase and ATP are 90 nM and 20 microM, respectively. The kinase was found to be cyclic AMP-independent, but activated by CoA. The protein kinase can phosphorylate acetyl-CoA carboxylase, protamine, and histones, but could not act on hydroxymethylglutaryl-CoA reductase or phosphorylase b.
 ...
PMID:Purification and properties of a kinase which phosphorylates and inactivates acetyl-CoA carboxylase. 612 Jan 70

A model proposed for regulation of steroidogenesis, lipoprotein utilization and cholesterol metabolism in HFA tissue is presented in Fig 17. We envision that the role of ACTH and cAMP in steroidogenesis and cholesterol metabolism is as follows. ACTH binds to specific receptors on the surface of the cells of the HFA gland and as a consequence, adenylate cyclase is activated, leading to increased formation of cAMP. cAMP causes activation of protein kinase that leads, presumably, to phosphorylation of specific proteins. This leads to the initiation of reactions that give rise to increased activity of key enzymes and levels of proteins involved in adrenal cholesterol metabolism. Presumably, the action of ACTH causes an increase in the activity of cholesterol side chain cleavage, the rate-limiting step in the conversion of cholesterol to steroid hormones. We suggest that once the mitochondrial cholesterol side-chain cleavage system is fully activated by ACTH, the supply of cholesterol to the mitochondria becomes rate-limiting for steroidogenesis. To meet this demand for cholesterol, a further action of ACTH results in an increase in the number of LDL receptors. LDL binds to specific receptors on the cell surface that are localized in coated pits. LDL is internalized by a process of adsorptive endocytosis and the internalized vesicles fuse with lysosomes and the protein component of LDL is hydrolyzed by lysosomal proteolytic enzymes to amino acids. The cholesteryl esters of LDL also are hydrolyzed to give rise to fatty acids and cholesterol. The liberated cholesterol is available for utilization in the biosynthesis of steroid hormones and other cellular processes. In addition, ACTH stimulates the activity of HMG CoA reductase and, thus, the rate of de novo cholesterol biosynthesis. In this way sufficient cholesterol is obtained to provide for precursor cholesterol to maintain the high rate of steroid synthesis by the HFA. HDL is not utilized as a source of cholesterol by the HFA. Because of the rapid rate of utilization of LDL by the HFA, fetal plasma levels of LDL are low and the activity of the HFA is a primary determinant of these levels. Thus, in the case of anencephaly, in which the activity of the adrenal is very low, plasma levels of LDL are 2--3 times higher than in normal fetuses, whereas plasma HDL levels are similar. In addition, in the normal neonate plasma LDL levels rise rapidly after birth, and this event is coincident with the involution of the fetal zone of the adrenal. The fetal liver is likely to be the major source ultimately of the LDL-cholesterol utilized by the HFA. Consequently, factors that regulate cholesterol and lipoprotein synthesis in the fetal liver may, in turn, affect the steroidogenic activity of the HFA through regulation of the supply of cholesterol precursor. Thus, if trophic factors for the HFA other than ACTH exist, an important site of their action might be the fetal liver, rather than a direct action to influence the rate of synthesis of steroids by the fetal adrenal.
 ...
PMID:Lipoprotein utilization and cholesterol synthesis by the human fetal adrenal gland. 626 97

Acyl-coenzyme A:cholesterol O-acyltransferase (ACATase; EC 2.3.1.26) is a membrane-bound microsomal enzyme that catalyzes the formation of long-chain fatty-acyl cholesterol esters in rat liver and other tissues. This enzyme is important in regulating the concentration of unesterified cholesterol in the cell. Having recently demonstrated that rat liver 3-hydroxy-3-methylglutaryl-coenzyme A reductase (HMG-CoA reductase; EC 1.1.1.34), the major regulatory enzyme in cholesterol biosynthesis, undergoes in vivo phosphorylation and inactivation after a single cholesterol meal, we decided to test the hypothesis that the enzyme ACATase, important in cholesterol utilization and storage, is also subject to regulation by phosphorylation/dephosphorylation. The results show that rat liver ACATase can be reversibly inactivated/activated, in vitro, by incubation conditions that favor dephosphorylation/phosphorylation. Activation was also achieved by using a partially purified protein kinase extracted from microsomes. It is significant that HMG-CoA reductase is inactivated by phosphorylation whereas ACATase is activated by phosphorylation. ACATase is, therefore, regulated by phosphorylation in a manner exactly opposite to that of HMG-CoA reductase. We propose that the coordinate regulation of ACATase and HMG-CoA reductase by phosphorylation/dephosphorylation provides a mechanism for short-term intracellular cholesterol homeostasis.
 ...
PMID:Evidence for phosphorylation/dephosphorylation of rat liver acyl-CoA:cholesterol acyltransferase. 630 Aug 97

The control of androgen production by the Leydig cell is dependent upon the episodic secretion of hormone (LH), which is released from the anterior pituitary gland in pulses of high biological activity. This mode of episodic LH secretion supports steroidogenic enzyme activity in the testis through interaction with LH receptors and stimulation of the adenylate cyclase/protein kinase sequence, leading to phosphorylation of key intermediates in the steroid biosynthetic pathway. The plasma membrane events that are rapidly activated by the specific interaction of LH or hCG with Leydig cell receptors include increased binding of guanyl nucleotide, and stimulation of cAMP-independent, Ca2+-dependent phosphorylation of a 44,500 Mr protein, with the characteristics of the adenylate cyclase nucleotide regulatory unit. Hormonal activation of adenylate cyclase is affected by Ca2+ with the same concentration-dependence, suggesting that nucleotide-induced phosphorylation is related to activation of the catalytic cyclase unit. In addition to the characteristic increases in pregnenolone synthesis and androgen production, gonadotropin-stimulated Leydig cells show prominent changes in LH receptor content and steroidogenic activity that modify their subsequent responses to hormonal signals. Thus, after exposure to increased LH and hCG levels in vivo and in vitro, LH receptors show an initial transient increase (up-regulation) followed by a marked decrease (down-regulation) and a prolonged depletion of LH receptor sites. Large doses of hCG cause "early" (prior to pregnenolone) and "late" steroidogenic lesions (17 alpha-hydroxylase, 17-20 desmolase) that are independent of receptor loss. The early lesion is partly due to reduced activity of HMG CoA reductase, and is mainly attributable to the increased activity of an inhibitory protein factor that modulates the activity of cholesterol side chain cleavage enzyme in Leydig cell mitochondria. In contrast, the late steroidogenic lesion is related to the nuclear actions of E2 produced during hormonal action. After hCG stimulation, an increase in nuclear E2 binding was accompanied by an early rise of RNA polymerase activities within 45 min coincident with the maximal increases in circulating testosterone and estradiol levels. These events were followed by the emergence of an E2-induced protein of Mr 27,000 at 3-6 h, and by reduction in the activity of 17 alpha-hydroxylase/17-20 desmolase, and a decrease in microsomal cytochrome P-450.(ABSTRACT TRUNCATED AT 400 WORDS)
 ...
PMID:Hormonal regulation of androgen production by the Leydig cell. 632 62


<< Previous 1 2 3 4 5 6 7 8 Next >>