Gene/Protein Disease Symptom Drug Enzyme Compound
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Query: UNIPROT:P01275 (glucagon)
 26,492 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.
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PMID:Phosphorylation and degradation of HMG CoA reductase. 262 76

The fraction of 3-hydroxy-3-methylglutaryl-CoA (HMG-CoA) reductase in the dephosphorylated (active) form in rat liver in vivo was measured after various experimental treatments of animals. Intraperitoneal injection of glucose (to raise serum insulin concentrations) into rats 4 h into the light phase (L-4) resulted in a transient (30 min) increase in the expressed (E)/total (T) activity ratio of HMG-CoA reductase without any change in total activity (obtained after complete dephosphorylation of the enzyme). Conversely, intravenous injection of guinea-pig anti-insulin serum into rats 4 h into the dark phase (D-4) significantly depressed the E/T ratio within 20 min. Intravenous injection of glucagon into normal rats at this time point did not affect the degree of phosphorylation of the enzyme, in spite of a 10-fold increase in hepatic cyclic AMP concentration induced by the hormone treatment. A 3-fold increase in the concentration of the cyclic nucleotide induced by adrenaline infusion was similarly ineffective in inducing any change in expressed or total activities of hepatic HMG-CoA reductase. However, when insulin secretion was inhibited, either by the induction of streptozotocin-diabetes or by simultaneous infusion of somatostatin, glucagon treatment was able to depress the expressed activity of HMG-CoA reductase (i.e. it increased the phosphorylation of the enzyme). Therefore insulin appears to have a dominant role in the regulation of the phosphorylation state of hepatic HMG-CoA reductase. In apparent corroboration of this suggestion, short-term 4 h food deprivation of animals before D-4 resulted in a marked decrease in the E/T activity ratio of reductase, which was not affected further by an additional 8 h starvation. By contrast, the total activity of the enzyme was not significantly affected by 4 h starvation, but was markedly diminished after 12 or 24 h starvation. Longer-term starvation also produced a chronic increase in the degree of phosphorylation of the enzyme. These results are discussed in relation to the role of reversible phosphorylation in the control of hepatic HMG-CoA reductase activity in vivo.
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PMID:Acute effects of starvation and treatment of rats with anti-insulin serum, glucagon and catecholamines on the state of phosphorylation of hepatic 3-hydroxy-3-methylglutaryl-CoA reductase in vivo. 288 48

Rat hepatocytes were used to demonstrate rapid, transient effects on the modulation state (defined as the fraction of the enzyme present in the catalytically active form) of 3-hydroxy-3-methylglutaryl-CoA reductase (HMG-CoA reductase, E.C. 1.1.1.34). Insulin elevated, while glucagon, cAMP or cGMP lowered HMG-CoA reductase modulation state within 10 to 15 min. These changes were accompanied by a parallel change in sterol synthesis. Total HMG-CoA reductase activity was not altered. Rapid modulation of HMG-CoA reductase activity therefore constitutes a viable in vivo control mechanism. By contrast to the hormones and second messengers, mevalonolactone lowered both HMG-CoA reductase modulation state and total reductase quantity.
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PMID:Rapid modulation of rat hepatocyte HMG-CoA reductase activity by cyclic AMP or cyclic GMP. 299 25

Vasoactive intestinal peptide (VIP) has been identified in ovarian nerves and stimulates steroid secretion from immature ovaries. To gain insight into its mechanism of action, the effect of VIP on the synthesis of the cholesterol side-chain cleavage enzyme complex was studied in ovarian granulosa cells from immature estrogen-primed rats. The cells were cultured for 48 hr in serum-free medium; the proteins were labeled with [35S]methionine; and the synthesis of cytochrome P-450, iron-sulfur protein, and NADPH:iron-sulfur protein reductase was evaluated by electrophoretic analysis after immunoisolation with polyclonal antibodies directed against the bovine adrenal enzymes. VIP at concentrations ranging from 0.001 to 1 microM stimulated 3- to 5-fold the synthesis of cytochrome P-450 and iron-sulfur protein. Peptide NH2-terminal histidine, COOH-terminal isoleucine, which has greater than 50% sequence homology of VIP, stimulated the synthesis of both proteins at approximately 50% of VIP effectiveness. Secretin, another member of the glucagon-secretin family of peptides, which has only 30% sequence homology to VIP, was without effect. Similar results were observed with the NADPH:iron-sulfur protein reductase. VIP-induced synthesis of the cholesterol side-chain cleavage enzyme complex was accompanied by a dose-related increase in cAMP accumulation and progestin formation. It is concluded that VIP regulates the synthesis of the ovarian cholesterol side-chain cleavage enzyme complex, which catalyzes the rate-limiting reaction in progesterone biosynthesis, and that the VIP effect is at least partially mediated through cAMP. It is suggested that a stimulatory action of VIP on the synthesis of ovarian progesterone may contribute to regulating the functional development of the ovary.
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PMID:Vasoactive intestinal peptide induces the synthesis of the cholesterol side-chain cleavage enzyme complex in cultured rat ovarian granulosa cells. 302 May 46

The expressed catalytic activity of liver microsomal HMG CoA reductase, the limiting enzyme in cholesterol synthesis, is reversibly diminished by phosphorylation in vitro. In intact hepatocytes the expressed activity of HMG CoA reductase is enhanced by incubation of cells with insulin, and diminished by treatment with glucagon or with mevalonate. In the latter situations the level of total reductase activity falls following initial inactivation (phosphorylation) of the enzyme. This observation suggested that the phosphorylated form of HMG CoA reductase is more sensitive to proteolysis. HMG CoA reductase is a 97,000 dalton (97 K) integral protein of the endoplasmic reticulum with a cytosolic domain that includes the catalytic site and serine residues that may be reversibly phosphorylated. In vitro the Ca2+-activated proteolytic enzyme, calpain, generates two catalytically-active fragments: a membrane bound 62 K and a soluble 53 K form of the enzyme which are quantified by specific immunoblot procedures. Cleavage of the native 97 K HMG CoA reductase is enhanced by pretreatment (inactivation) of microsomes with ATP (Mg2+) and liver reductase kinase compared to microsomes pretreated with protein phosphatase. This is reflected in a loss of 97 K reductase and an increase in the soluble 53 K form of the enzyme. Degradation of HMG CoA reductase in hepatocytes is partially blocked by lysosomotropic agents and insulin. A steady state model for the turnover of proteins subject to reversible phosphorylation has been developed which recognizes fractional degradative rate constants for the phosphorylated and dephosphorylated species.
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PMID:Phosphorylation state of HMG CoA reductase affects its catalytic activity and degradation. 302 50

Incubation of rat hepatocytes with glucagon results in a time- and dose-dependent decrease in the activity of 3-hydroxy-3-methylglutaryl coenzyme A reductase. We demonstrate, using immunoprecipitation of radiolabeled enzyme, that 10 nM glucagon inhibits the synthesis of the enzyme by approximately 50%, but that the apparent rate of degradation of the enzyme is not affected by the hormone. We also demonstrate that the intact reductase polypeptide contained phosphoserine. We conclude that glucagon inhibits the activity of the reductase by inhibition of enzyme synthesis.
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PMID:The effect of glucagon on the synthesis and degradation of 3-hydroxy-3-methylglutaryl coenzyme A reductase. 372 13

The effect of glucagon and insulin on the incorporation of 1-14C-acetate into cholesterol and fatty acids and on the enzymes involved in the first steps of cholesterol synthesis (3-hydroxy-3-methyl-glutaryl-coenzyme A reductase, 3-hydroxy-3-methyl-glutaryl-coenzyme A synthase, and acetoacetyl-coenzyme A thiolase) was investigated. Isolated rat hepatocytes at different stages of fetal and postnatal development were employed. Data obtained show the appearance of hormonal control on the 18th day of fetal life, indicating the same pattern, as regards acetate incorporation and HMGCoA reductase prepared and assayed in the presence of NaF. On the contrary, HMGCoA reductase, prepared without NaF, HMGCoA synthase, and acetoacetyl CoA thiolase, does not appear to respond to hormonal stimulation. In the perinatal period, the hormonal effect is no longer detectable, probably because of a hormone resistance of this metabolic pathway.
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PMID:Hormonal control of cholesterogenesis and related enzymes in isolated rat hepatocytes during pre- and postnatal development. 390 31

We have previously reported that the enzymic activity of rat liver-3-hydroxy-3-methyl-glutaryl-CoA reductase (NADPH) (HMG-CoA reductase) is modulated in vitro by a phosphorylation-dephosphorylation reaction sequence. The in vitro phosphorylation of HMG-CoA reductase was further studied by utilizing purified HMG-CoA reductase and reductase kinase. Analysis of 32P-labeled HMG-CoA reductase revealed 1 mol of phosphate per subunit. Purified [32P]HMG-CoA reductase could be dephosphorylated with phosphoprotein phosphatase. To demonstrate the in vivo phosphorylation, rats were injected with 32P and hepatic HMG-CoA reductase was isolated by immunoprecipitation and also by purification of the enzyme to homogeneity. Analysis of [32P]HMG-CoA reductase by sodium dodecyl sulfate gel electrophoresis revealed a single peak of radioactivity comigrating with HMG-CoA reductase. Administration of glucagon enhances the in vivo phosphorylation of both HMG-CoA reductase and reductase kinase. In response to glucagon, HMG-CoA reductase activity is decreased whereas reductase kinase activity is increased. These results support our concept that the enzymic activity of HMG-CoA reductase is modulated by a bicyclic cascade system involving phosphorylation-dephosphorylation. The enzymic activity of HMG-CoA reductase has also been shown to be modulated by cholesterol and mevalonolactone by both short-term and long-term mechanisms. The effects of cholesterol and mevalonolactone are twofold. Rapid inhibition of HMG-CoA reductase activity is due to increased phosphorylation of the enzyme; the long-term effect of HMG-CoA reductase is achieved by reduction in enzyme concentration by modulation of enzyme synthesis and/or degradation. Regulation of HMG-CoA reductase by mevalonolactone is of major importance in cellular metabolism because mevalonate serves as precursor for four separate metabolic pathways, including the formation of cholesterol, ubiquinone, dolichols, and isopentenyl tRNA.
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PMID:Modulation of rat liver 3-hydroxy-3-methylglutaryl-CoA reductase activity by reversible phosphorylation. 628 63

Within the last few years considerable evidence has accumulated which indicates that changes in HMG-CoA reductase are due primarily, if not solely, to changes in enzyme quantity. These include the changes caused by insulin, glucagon, cholesterol, mevalonolactone, cholestyramine, compactin, cyclic monoterpenes and the diurnal rhythm. Investigations of whether these changes in enzyme quantity primarily represent changes in rates of enzyme synthesis or degradation are just beginning. Regulation of reductase activity by reversible phosphorylation seems unlikely when all the available data are critically examined.
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PMID:Regulation of 3-hydroxy-3-methylglutaryl coenzyme A reductase. 631 84

The effects of insulin, glucagon, pyruvate, and lactate on the rate of sterol synthesis and 3-hydroxy-3-methylglutaryl (HMG)-CoA reductase activity were determined in hepatocytes obtained at different times of the day from rats maintained on a controlled lighting and feeding schedule. In hepatocytes from animals killed immediately before the start of the feeding period (D0 hepatocytes), the initially low activity of HMG-CoA reductase increased during incubation while that in hepatocytes prepared 6 h later (D6 hepatocytes) remained constantly high. The rates of sterol synthesis followed similar patterns of change. In both D0 and D6 cells, insulin stimulated HMG-CoA reductase but had little or no effect on the rates of sterol synthesis. In both types of cell preparation glucagon maximally suppressed HMG-CoA reductase activity at a concentration of 10(-7) M, but there was relatively little change in the rates of sterol synthesis. Both pyruvate and lactate mitigated the glucagon-mediated inhibition of HMG-CoA reductase. Each of these lipogenic precursors alone suppressed the rate of sterol synthesis in a dose-dependent manner. These changes were more apparent in the simultaneous presence of insulin and were greater in the D0 compared to the D6 hepatocytes. In the presence of lactate or pyruvate, the activity of HMG-CoA reductase was elevated, and the increase was greater when insulin was simultaneously present. In general, changes in the rate of fatty acid synthesis were positively correlated with changes in the activity of HMG-CoA reductase. These observations suggest that the latter changes are required to compensate for variations in the availability of simple precursors for sterol synthesis.
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PMID:Evidence that changes in hepatic 3-hydroxy-3-methylglutaryl coenzyme A reductase activity are required partly to maintain a constant rate of sterol synthesis. 638 48


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