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Biomedical subjects

J Avruch

Publications and source records attributed to J Avruch.

At least 109 records · Page 6Linked to original sources
PubMed1983

Insulin and growth factors stimulate the phosphorylation of a Mr-22000 protein in 3T3-L1 adipocytes.

Insulin, epidermal growth factor (EGF), platelet-derived growth factor, multiplication-stimulating activity and 10% foetal-calf serum each stimulated the phosphorylation of a cytosolic Mr-22000 acidic heat-stable protein in Swiss mouse 3T3-L1 adipocytes. Phosphorylation of this protein was not stimulated by isoprenaline or dibutyryl cyclic AMP. The effect of insulin was maximal (3-fold increase) by 10 min; half-maximal stimulation was observed at 70 pM-insulin. Both [32P]phosphoserine and [32P]phosphothreonine residues were present in the Mr-22000 protein after insulin- and growth-factor-stimulated phosphorylation, but no [32P]phosphotyrosine. The major site of insulin- and EGF-stimulated phosphorylation appeared to be a threonine residue, in contrast with previously studied insulin-stimulated phosphorylation of serine residues. Insulin treatment appeared to result in a shift of the protein toward the anode on isoelectric focusing. Insulin and EGF present simultaneously did not lead to phosphorylation beyond that seen with each hormone singly. We surmise that insulin, EGF and perhaps other growth factors may activate a common protein kinase or inhibit a common protein phosphatase in 3T3-L1 adipocytes which acts on the Mr-22000 protein.

Adipose Tissue↗
PubMed1982

Insulin-stimulated tyrosine phosphorylation of the insulin receptor in detergent extracts of human placental membranes. Comparison to epidermal growth factor-stimulated phosphorylation.

Addition of insulin to Triton-solubilized extracts of human placental membranes selectively stimulates the incorporation of 32P from [gamma-32P]ATP into an endogenous 95,000-dalton protein, which is identified as a component of the insulin receptor by immunoprecipitation. The insulin-stimulated increment in 32P is recovered largely in [32P]tyrosine after acid hydrolysis. E Epidermal growth factor (EGF) stimulates the phosphorylation of a 150,000-dalton protein in these detergent extracts. This reaction differs in several respects from the insulin-stimulated phosphorylation of the 95,000-dalton protein. Insulin-stimulated phosphorylation exhibits an absolute requirement for Mn2+ as the sole divalent cation, whereas EGF-stimulated phosphorylation is supported by Mg2+ and Co2+ as well as Mn2+. In the presence of Mn2+, insulin-stimulated phosphorylation is not detected at less than 50 microM ATP, whereas EGF-stimulated phosphorylation is well expressed at 5 microM ATP. Thus, in detergent-solubilized membrane extracts, insulin stimulates the phosphorylation of its own receptor on tyrosine residues. This reaction has enzymatic properties distinct from those of the EGF-stimulated phosphorylation in these same extracts. The role of this insulin-stimulated phosphorylation reaction in the initiation of insulin's many biologic actions merits further study.

Adenosine Triphosphate↗
PubMed1982

The insulin-directed phosphorylation site on ATP-citrate lyase is identical with the site phosphorylated by the cAMP-dependent protein kinase in vitro.

32P-labeled ATP-citrate lyase isolated from 32P-labeled hepatocytes treated with insulin contained 1.6-1.8-fold greater 32P-radioactivity per mg protein than control enzyme. Both enzyme preparations were digested in parallel with trypsin until 94% of all 32P-radioactivity was rendered acid soluble. Quantitative high performance liquid chromatographic peptide mapping of the tryptic digests revealed a principal 32P-peptide which accounted for at least 80% of the insulin induced increment in 32P-radioactivity of native lyase. This peptide was purified, sequenced, and the site of 32P-phosphorylation assigned by two methods: electrophoresis (pH 6.5) of residual peptide after each step of Edman degradation and solid phase sequencing. The site of insulin-directed phosphorylation of ATP-citrate lyase (Thr-Ala-Ser(32P)-Phe-Ser-Glu-Ser-Arg) is the same as that directed by glucagon, and, in turn, identical with that phosphorylated by the cAMP-dependent protein kinase in vitro.

ATP Citrate (pro-S)-Lyase↗
PubMed1982

Preliminary characterization of a heat-stable protein from rat adipose tissue whose phosphorylation is stimulated by insulin.

Exposure of 32P-labelled isolated rat adipocytes or epididymal fat-pads to insulin resulted in an increase in the phosphorylation of a heat-stable acid-soluble protein of Mr 22 000. The phosphorylation of this protein was unaffected by isoprenaline (isoproterenol) in intact cells, nor was its phosphorylation catalysed by exposure in vitro to the cyclic AMP-dependent protein kinase or smooth-muscle myosin light-chain kinase. The properties of the Mr-22 000 protein include: heat-stability; solubility in 1% trichloroacetic acid; pI 4.9; elution at apparent Mr 37 500 on gel filtration; and it contains both phosphoserine and phosphothreonine. It can be distinguished from the heat-stable phosphatase inhibitor 1 of adipose tissue (inhibitor 1A) and the phosphorylated form of adipose-tissue myosin light chain by several criteria. Its identity, and the possible functional significance of the insulin-stimulated phosphorylation, remain problems for future study.

Adipose Tissue↗
PubMed1982

Insulin-stimulated phosphorylation of ATP-citrate lyase in isolated hepatocytes. Stoichiometry and relation to the phosphoenzyme intermediate.

We have estimated the insulin-stimulated phosphorylation of ATP-citrate lyase by two methods. Isolated hepatocytes incorporate extracellular 32P into [gamma-35P] ATP and immunoprecipitated ATP-citrate lyase to steady state levels by 1 h. The content of acid-stable 32P in hepatocyte ATP-citrate lyase at steady state is 0.33 +/- 0.038 mol of P/mol (tetrameric) holoenzyme. Insulin (1 milliunit/ml) increases the 32P content of immunoprecipitated lyase 2- to 3-fold in 10 min. Over 90% of acid-stable 32P on lyase is 32P-serine in enzyme isolated from both control and insulin-treated cells. ATP-citrate lyase isolated from hepatocytes contains 0.95 +/- 0.1 mol of alkali-labile phosphate/mol of holoenzyme. Insulin treatment of hepatocytes (1 milliunit/ml for 10 min) increases the alkali-labile P content by 45%. Evidence is presented which indicates that the insulin-stimulated phosphorylation does not arise by intramolecular migration from the catalytic phosphoenzyme intermediate. These observations support the conclusion that insulin-stimulated phosphorylation of ATP-citrate lyase is mediated either by an insulin-induced increase in the activity of lyase kinase and/or decrease in a lyase phosphatase. The functional role of the substoichiometric phosphorylation of ATP-citrate lyase remains unknown.

ATP Citrate (pro-S)-Lyase↗
PubMed1982

Intensive conventional and insulin pump therapies in adult type I diabetes. A crossover study.

We examined the relative efficacy of two modes of insulin therapy in five patients with insulin-dependent diabetes. Using a crossover protocol, therapy with multiple insulin injections was compared with the use of continuous subcutaneous insulin infusion (pump therapy); both therapies were regulated by home blood glucose monitoring. Although both therapies were equally effective during 2 to 4 days of inpatient management, pump therapy was clearly superior in the outpatient setting. Mean plasma glucose and hemoglobin A1c concentrations and the frequency of hypoglycemic reactions were significantly less with pump therapy despite apparently equal compliance with both therapies. We conclude that in compliant adult patients with type I diabetes continuous insulin infusion is more effective in achieving normalization of plasma glucose concentration than intensified conventional therapy in the outpatient setting. Although these patients found pump therapy preferable to intensive conventional therapy, the superiority of pump therapy is not explained by this bias. Whether these results are applicable in a more standard treatment setting remains to be established.

Adult↗
PubMed1982

Role of insulin-stimulated protein phosphorylation in insulin action.

Insulin promotes both the phosphorylation and dephosphorylation of proteins in its target cells. Insulin-induced dephosphorylation has long been thought to serve an important regulatory function; the role of insulin-stimulation phosphorylation is less certain. The proteins known to be substrates for this reaction are ATP citrate (pro-3S)-lyase, acetyl-CoA carboxylase, and the ribosomal subunit S6. The evidence as to the physiological role and mechanism underlying the insulin-stimulated phosphorylation of these proteins is summarized. Present information suggests that insulin-stimulated phosphorylation may serve an important regulatory role in certain actions of insulin.

ATP Citrate (pro-S)-Lyase↗
PubMed1981

ATP-citrate lyase. Structure of a tryptic peptide containing the phosphorylation site directed by glucagon and the cAMP-dependent protein kinase.

[32P]ATP-citrate lyase phosphorylated by the cAMP-dependent protein kinase was partially digested by trypsin. Two tryptic 32P-labeled phosphopeptides containing more than 90% of the 32P radioactivity present on the phosphorylated enzyme were purified and found to have overlapping amino acid sequences around the same phosphorylated site (Thr-Ala-Ser(32P)-Phe-Ser-Glu-Ser-Arg). Tryptic digestion of 32P-labeled ATP-citrate lyase purified from 32P-labeled hepatocytes exposed to glucagon yielded a major 32P-labeled peptide of identical amino acid composition with that indicated above. Thus, the site on ATP-citrate lyase phosphorylated by the cAMP-dependent protein kinase in vitro resides on the same octapeptide as the site of glucagon-stimulated phosphorylation in intact hepatocytes.

ATP Citrate (pro-S)-Lyase↗
PubMed1981

The role of the cyclic AMP-dependent protein kinase in the glucagon-stimulated phosphorylation of ATP-citrate lyase.

We have examined the mechanism whereby glucagon stimulates the phosphorylation of ATP-citrate lyase in intact rat hepatocytes. Purified ATP-citrate lyase is phosphorylated in vitro by the catalytic subunit of the cyclic AMP-dependent protein kinase, in a reaction wherein 2-3 mol phosphate/mol lyase are incorporated, at an initial rate that approaches that observed for mixed histone. This reaction is completely abolished by the protein kinase inhibitor protein. Limited tryptic digestion of ATP-citrate lyase phosphorylated in vitro by the cyclic AMP-dependent protein kinase yields a pattern of 32P-labeled peptides, indistinguishable from those observed in parallel digests of lyase isolated from 32P-labeled, glucagon-stimulated hepatocytes. Phosphorylase b kinase catalyzes the incorporation of 1 mol phosphate/mol lyase, albeit at less than 1/160 the rate observed for phosphorylase b. The phosphorylation of purified ATP-citrate lyase is also catalyzed by homogenates of hepatocytes. This reaction is stimulated by cyclic AMP. At 30 degrees C, in the presence of maximally stimulating concentrations of cyclic AMP, the addition of excess protein kinase inhibitor protein inhibits the phosphorylation of ATP-citrate lyase by 67%. Thus, hepatocytes contain both cyclic AMP-dependent and cyclic AMP-independent ATP-citrate lyase kinase activities. Pretreatment of hepatocytes with glucagon (10(-8) M for 2 min) prior to homogenization results in activation of an endogenous hepatocyte ATP-citrate lyase kinase, as well as histone kinase and phosphorylase b kinase; the glucagon-stimulated increment in lyase kinase (and histone kinase) is observed only when homogenates are assayed in the absence of added cyclic AMP, and is completely abolished by an excess of the protein kinase inhibitor protein. We conclude that the glucagon-stimulated phosphorylation of ATP-citrate lyase in intact hepatocytes is catalyzed directly by the cyclic AMP-dependent protein kinase.

ATP Citrate (pro-S)-Lyase↗
PubMed1979

Glucagon regulation of protein phosphorylation. Identification of acetyl coenzyme A carboxylase as a substrate.

A hormonally induced change in the covalent phosphorylation state of several enzymes is generally regarded as an important mechanism for hormonal modulation of enzyme activity. We have previously demonstrated that epinephrine stimulates the phosphorylation of a peptide of Mr = 220,000 in adipocytes. Incubation of 32P-labeled cytosolic proteins from adipocytes and hepatocytes with antisera raised against homogeneous chicken and rat liver acetyl coenzyme A carboxylase results in the specific and complete precipitation of the same phosphopeptide. No other major phosphopeptide is specifically precipitated. In hepatocytes, glucagon stimulates the incorporation of 32P into this peptide associated with an inhibition of enzyme activity. These data, coupled with previous studies in adipocytes, suggest that cyclic AMP-dependent protein phosphorylation plays a major role in the regulation of acetyl-CoA carboxylase activity and of fatty acid biosynthesis in adipose tissue and liver.

Acetyl-CoA Carboxylase↗
PubMed1979

Insulin and the phosphorylation of intracellular proteins.

Protein phosphorylation is a ubiquitous form of posttranslational protein modification in mammalian cells which often serves to regulate protein function. Insulin alters the activity of a number of enzymes known to be regulated via phosphorylation. With the premise that altered protein phosphorylation might be an obligatory intermediate step in insulin action, we have examined the effects of insulin on the phosphorylation of the major phosphopeptides in adipocytes and hepatocytes. Insulin affects overall protein phosphorylation in two ways: 1) Insulin selectively stimulates the phosphorylation of a major peptide in adipose tissue (MW 123,000) and liver (MW 46,000) through a mechanism independent of cAMP and the cAMP-dependent protein kinase. Net dephosphorylation is not observed with insulin as the sole hormone. 2) Insulin antagonizes cAMP-directed protein phosphorylation. The mechanism of insulin-stimulated phosphorylation and the possible role of this phenomenon in overall insulin action is discussed.

Adipose Tissue↗
PubMed1978

Insulin regulation of hepatic glycogen synthase and phosphorylase.

The relative roles of insulin and glucose in the regulation of hepatic glycogen synthase and phosphorylase were studied in hepatocytes from fed rats. Elevation of extra-cellular glucose led to a rapid decrease in phosphorylase a activity followed by a slower increase in glycogen synthase I activity. A reciprocal and coordinate relationship between phosphorylase inactivation and synthase activation in response to glucose was observed; following initial glucose-induced inactivation of phosphorylase, there was a highly significant linear inverse relationship between residual phosphorylase activity and glycogen synthase activation. Insulin led to a further decrease in phosphorylase activity and a 30-50% additional increase in glycogen synthase activity over that caused by glucose. The effects of insulin required the presence of glucose and served to augment acute glucose stimulation of glycogen synthase and inhibition of phosphorylase. Insulin did not perturb the reciprocal and coordinate relationship between phosphorylase inactivation and synthase activation in response to glucose. The results suggest that the ability of insulin to activate hepatic glycogen synthase can be entirely accounted for by its ability to inactivate phosphorylase.

Animals↗
PubMed1978

Phosphorylation and dephosphorylation of spectrin.

The phosphorylation of spectrin polypeptide 2 is thought to be involved in the metabolically dependent regulation of red cell shape and deformability. Spectrin phosphorylation is not affected by cAMP. The reaction in isolated membranes resembles the cAMP-independent, salt-stimulated phosphorylation of an exogenous substrate, casein, by enzyme(s) present both in isolated membranes and cytoplasmic extracts. Spectrin kinase is selectively eluted from membranes by 0.5 M NaCl and co-fractionates with eluted casein kinase. Phosphorylation of band 3 in the membrane is inhibited by salt, but the band 3 kinase is otherwise indistinguishable operationally from spectrin kinase. The membrane-bound casein (spectrin) kinase is not eluted efficiently with spectrin at low ionic strength; about 80% of the activity is apparently bound at sites (perhaps on or near band 3) other than spectrin. Partitioning of casein kinase between cytoplasm and membrane is metabolically dependent; the proportion of casein kinase on the membrane can range from 25% to 75%, but for fresh cells is normally about 40%. Dephosphorylation of phosphorylated spectrin has not been studied intensively. Slow release of 32Pi from [32P] spectrin on the membrane can be demonstrated, but phosphatase activity measured against solubilized [32P] spectrin is concentrated in the cytoplasm. The crude cytoplasmic phosphospectrin phosphatase is inhibited by various anions--notably, ATP and 2,3-DPG at physiological concentrations. Regulation of spectrin phosphorylation in intact cells has not been studied. We speculate that spectrin phosphorylation state may be regulated 1) by metabolic intermediates and other internal chemical signals that modulate kinase and phosphatase activities per se or determine their intracellular localization and 2) by membrane deformation that alters enzyme-spectrin interaction locally. Progress in the isolation and characterization of spectrin kinase and phosphospectrin phosphatase should lead to the resolution of major questions raised by previous work: the relationships between membrane-bound and cytoplasmic forms of the enzymes, the nature of their physical interactions with the membrane, and the regulation of their activities in defined cell-free systems.

Adenosine Triphosphate↗