High levels of cytosolic free calcium inhibit dephosphorylation of insulin receptor and glycogen synthase

Dysregulation of glycogen synthase COOH- and NH2-terminal phosphorylation by insulin in obesity and type 2 diabetes mellitus

CONTEXT

Insulin-stimulated glucose disposal is impaired in obesity and type 2 diabetes mellitus (T2DM) and is tightly linked to impaired skeletal muscle glucose uptake and storage. Impaired activation of glycogen synthase (GS) by insulin is a well-established defect in both obesity and T2DM, but the underlying mechanisms remain unclear.

DESIGN AND PARTICIPANTS

Insulin action was investigated in a matched cohort of lean healthy, obese nondiabetic, and obese type 2 diabetic subjects by the euglycemic-hyperinsulinemic clamp technique combined with muscle biopsies. Activity, site-specific phosphorylation, and upstream signaling of GS were evaluated in skeletal muscle.

RESULTS

GS activity correlated inversely with phosphorylation of GS site 2+2a and 3a. Insulin significantly decreased 2+2a phosphorylation in lean subjects only and induced a larger dephosphorylation at site 3 in lean compared with obese subjects. The exaggerated insulin resistance in T2DM compared with obese subjects was not reflected by differences in site 3 phosphorylation but was accompanied by a significantly higher site 1b phosphorylation during insulin stimulation. Hyperphosphorylation of another Ca(2+)/calmodulin-dependent kinase-II target, phospholamban-Thr17, was also evident in T2DM. Dephosphorylation of GS by phosphatase treatment fully restored GS activity in all groups.

CONCLUSIONS

Dysregulation of GS phosphorylation plays a major role in impaired insulin regulation of GS in obesity and T2DM. In obesity, independent of T2DM, this is associated with impaired regulation of site 2+2a and likely site 3, whereas the exaggerated insulin resistance to activate GS in T2DM is linked to hyperphosphorylation of at least site 1b. Thus, T2DM per se seems unrelated to defects in the glycogen synthase kinase-3 regulation of GS.

Abnormal cell calcium homeostasis in type 2 diabetes mellitus: a new look on old disease

Cumulative evidence reveals that diabetes is a condition in which cell Ca2+ homeostasis is impaired. Defects in cell Ca2+ regulation were found in erythrocytes, cardiac muscle, platelets, skeletal muscle, kidney, aorta, adipocytes, liver, osteoblasts, arteries, lens, peripheral nerves, brain synaptosomes, retinal tissue, and pancreatic beta cells, confirming that this defect in cell Ca2+ metabolism is a basic pathology associated with the diabetic state. Though different defects in a variety of functions that regulate cell Ca2+ homeostasis were described in diabetes, the most common finding is an increase in [Ca2+]i levels. However, it is not clear whether the defect in cell Ca2+ metabolism in diabetes precedes or succeeds the overt diabetic condition. It is also not clear which of the multiple functions involved in cell Ca2+ regulation has the primary defect. Defects in cell Ca2+ metabolism may be significant for the observed pathologies in insulin secretion and insulin action in diabetes. They may also play an important role in the vascular complications seen in this condition, such as hypertension, atherosclerosis, and microangiopathy. Therefore, better understanding of the impairment in cell Ca2+ metabolism in diabetes may markedly enhance our understanding of this condition.

Fuel oxidation in skeletal muscle is increased by nitric oxide/cGMP--evidence for involvement of cGMP-dependent protein kinase

The cyclic guanosine-3',5'-monophosphate (cGMP) analogue, 8-bromo-cGMP (1 mM), increased glucose oxidation in isolated soleus muscle. The nitric oxide (NO) donor, sodium nitroprusside (SNP) (15 mM), increased glucose, pyruvate, palmitate and leucine oxidation. Removal of extracellular Ca2+ did not affect SNP-stimulated glucose oxidation (or other glucose utilization parameters), thus eliminating the influx of Ca2+ as a mechanism for the increases. The guanylate cyclase inhibitor, LY-83583 (10 microM), inhibited SNP-stimulated palmitate oxidation and activation of cGMP-dependent protein kinase (PKG). Activation of PKG might supersede any inhibitory effects of NO on respiration to stimulate metabolic fuel oxidation in skeletal muscle.

Activation of protein kinase C by intracellular free calcium in the motoneuron cell line NSC-19

The relationship between intracellular free calcium ([Ca2+]i) and the activation of protein kinase C (PKC) and Ca2+/calmodulin-dependent protein kinase II (CaMKII) was investigated in the NSC-19 motoneuron cell line. Increased extracellular calcium ([Ca2+]o) up to 10 mM resulted in sustained elevations of [Ca2+]i. Control cell cultures (1.3 mM [Ca2+]o, [Ca2+]i = 83 +/- 17 nM) contained Ca2+- and PS/DO lipid-dependent PKC activity predominantly in the cytosol. However, elevation of [Ca2+]o up to 5 mM ([Ca2+]i = 232 +/- 24 nM) resulted in almost complete loss of cytosolic PKC activity. Cells incubated in 10 mM [Ca2+]o ([Ca2+]i = 365 +/- 13 nM) showed increased levels of both cytosolic and membrane PKC activity compared to control. These alterations in PKC activity appeared to be translocation-independent, since PKC protein levels were unchanged as demonstrated by Western blotting analysis. When cells were exposed to 25 or 50 mM [Ca2+]o, [Ca2+]i rose transiently to over 600 and 900 nM, respectively, and then returned to near basal values. Under these conditions, total PKC activity decreased, and increased amounts of the catalytic fragment of PKC, protein kinase M, were generated. Extracts from cells exposed to [Ca2+]o between 1.3 and 25 mM did not differ significantly in the levels of measurable CaMKII activity 10 min following the change in [Ca2+]o.

Agouti regulation of intracellular calcium: role of melanocortin receptors

Several dominant mutations at the murine agouti locus cause a syndrome of marked obesity and insulin resistance. We have recently reported that intracellular free Ca2+ concentration ([Ca2+]i) is elevated in viable yellow mice. Because [Ca2+]i has a key role in the pathogenesis of insulin resistance, obesity, and hypertension, the role of the purified agouti gene product in regulating [Ca2+]i was evaluated in a number of cell types. Purified murine agouti induced slow, sustained increases in [Ca2+]i in A7r5 vascular smooth muscle cells and 3T3-L1 adipocytes in a dose-dependent fashion. In L6 skeletal myocytes, agouti stimulated an increase in [Ca2+]i with an apparent concentration eliciting 50% of the maximal response (EC50) of 62 nM. This response was substantially inhibited by Ca2+ entry blockade with nitrendipine. To determine whether melanocortin receptors play a role in agouti regulation of [Ca2+]i, we examined the effect of melanocortin peptides and agouti in cells stably transfected with human melanocortin receptors. Human embryonic kidney cells (HEK-293 cells) transfected with either the human melanocortin 1 receptor (MC1R) or melanocortin 3 receptor responded to human agouti with slow, sustained increases in [Ca2+]i, whereas nontransfected HEK-293 cells with no melanocortin receptors did not respond to agouti. Dose-response curves in the MC1R line showed that agouti had an EC50 of 18 nM, which is comparable to that for agouti antagonism of (125)I-Nle,D-Phe-alpha-melanocyte-stimulating hormone binding in the same cell line. This direct effect of agouti on stimulating increases in [Ca2+]i suggests a potential mechanism for agouti-induced insulin resistance.

Agouti regulation of intracellular calcium: role in the insulin resistance of viable yellow mice

Several dominant mutations at the agouti locus in the mouse cause a syndrome of marked obesity, hyperinsulinemia, and insulin resistance. Although it is known that the agouti gene is expressed in an ectopic manner in these mutants, the precise mechanism by which the agouti gene product mediates these effects is unclear. Since intracellular Ca2+ is believed to play a role in mediating insulin action and dysregulation of Ca2+ flux is observed in diabetic animals and humans, we examined the status of intracellular Ca2+ in mice carrying the dominant agouti allele, viable yellow (Avy). We show here that in mice carrying this mutation, the intracellular free calcium concentration ([Ca2+]i) is elevated in skeletal muscle, and the degree of elevation is closely correlated with the degree to which the mutant traits are expressed in individual animals. Moreover, we demonstrate that the agouti gene product is capable of inducing increased [Ca2+]i in cultured and freshly isolated skeletal muscle myocytes from wild-type mice. Based on these findings, we present a model in which we propose that the agouti polypeptide promotes insulin resistance in mutant animals through its ability to increase [Ca2+]i.

Phenylarsine oxide inhibits insulin-stimulated protein phosphatase 1 activity and GLUT-4 translocation

Phenylarsine oxide (PAO) has previously been shown to inhibit insulin-stimulated glucose transport without affecting insulin binding and tyrosine kinase activity of insulin receptor (S. C. Frost and M. D. Lane. J. Biol. Chem. 260: 2646-2652, 1985). This study examines the effect of PAO on insulin's ability to activate adipocyte protein phosphatase 1 (PP-1) and dephosphorylate GLUT-4, the insulin-sensitive glucose transporter. In particulate fractions, insulin stimulated PP-1 activity (40% increase over basal with phosphorylase a) in a time- and dose-dependent manner (half-maximal effect of 0.89 nM in 1 min). Insulin did not alter cytosolic PP-1 activity. With GLUT-4 as a substrate, insulin caused more than twofold stimulation of particulate PP-1 activity. Addition of PAO (5 microM) before or after insulin treatment abolished insulin's effect on PP-1 activation. The presence of 2,3-dimercaptopropanol (200 microM) prevented the effect of PAO on PP-1 activation and glucose uptake. In addition, PAO significantly increased GLUT-4 phosphorylation, blocked insulin-stimulated dephosphorylation, and partially diminished insulin-stimulated translocation of GLUT-4. We conclude that PAO may interfere with the components of insulin signal transduction pathways that lead to the activation of PP-1 and this may be responsible for the observed inhibition in insulin action.

Diabetes mellitus: a disease of abnormal cellular calcium metabolism?

Although the pathogenesis of the diabetes mellitus syndrome remains poorly understood, both insulin-dependent diabetes mellitus and non-insulin-dependent diabetes mellitus predispose the individual to a similar spectrum of complications, including hypertension, macrovascular and microvascular disease, cataracts cardiomyopathy, neuropathy, and premature aging, suggesting that these complications develop along a pathway common to both diabetic conditions. Yet not all diabetic persons are affected by all of these complications or to the same degree. What causes this marked variability in the clinical manifestations of the diabetes syndrome remains an enigma. Accumulating data from animal models of diabetes and from studying patients with diabetes reveal that intracellular calcium levels are increased in most tissues. The activities of the membrane, adenosine triphosphatase (ATPase) associated cation pumps, which determine intracellular calcium level (i.e., calcium-ATPase and [sodium + potassium]-ATPase), are also altered. The nature of the alteration is often tissue specific and may depend on the level of blood glucose or insulin, or both. In this review we discuss the potential contribution of these changes in intracellular calcium regulation, whether acquired or genetically determined, to the pathogenesis of the diabetes syndrome, to the abnormalities in insulin secretion and action (mainly in non-insulin-dependent diabetes), and to the complications of both diabetes syndromes. Altered intracellular calcium metabolism may represent a common, underlying abnormality linking the metabolic, cardiovascular, ocular, and neural manifestations of the diabetic disease process.

Inhibition of protein tyrosine phosphatase activity in HER14 cells by melittin and Ca2+ ionophore A23187

We investigated the effect of melittin and Ca2+ ionophore A23187 on protein tyrosine phosphatase activity in HER14 cells (NIH-3T3 cells transfected with human epidermal growth factor 'EGF' receptor). Cell fractions were used to measure protein tyrosine phosphatase activity in vitro using 32P-labeled poly(Glu/Tyr) (4:1) peptide as a substrate. Treatment of HER14 cells with melittin or with A23187, inhibited protein tyrosine phosphatase activity in the cell sonicate and homogenate, as well as in cytosolic and particulate fractions of these cells. The inhibitory effect of both drugs was prevented by preincubating cells with EGTA (ethyleneglycolbis(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid). The cyclooxygenase inhibitor indomethacin enhanced the inhibitory effect of A23187, but not that of melittin. Addition of arachidonic acid to the cells partially prevented the inhibition of protein tyrosine phosphatase activity by melittin or A23187. Preexposure of cells to EGF enhanced the inhibitory effect of melittin--but not that of A23187. Addition of CaCl2, or MgCl2 to the cell homogenate inhibited protein tyrosine phosphatase activity. These results show that protein tyrosine phosphatase activity in HER14 cells is inhibited by melittin and Ca2+ ionophore A23187 through a Ca(2+)-dependent mechanism, and is regulated by arachidonic acid metabolism and EGF receptor activation.

Cytosolic calcium and insulin resistance

The presence of insulin resistance in many patients with hypertension has become a well-recognized phenomenon. However, the mechanism of this association remains enigmatic. We have hypothesized that abnormal cellular calcium handling, particularly elevations in cytosolic free calcium concentrations, may represent a common intracellular abnormality (a missing link) that is responsible for the frequent co-existence of insulin resistance and hypertension. We have shown recently that sustained elevations of cytosolic free calcium in insulin target cells, such as are observed in patients with obesity and non-insulin-dependent diabetes mellitus and in some patients with hypertension, may lead to the development of insulin resistance. Although the mechanisms that lead to such increases are not yet well understood, they appear to include an enhanced influx of calcium via calcium channels. We found that the presence of the calcium antagonist nitrendipine in the incubation medium prevented increases in cytosolic free calcium concentration and ameliorated the insulin resistance induced by various mechanisms. To further evaluate the existence of an association between elevated levels of cytosolic calcium and diminished cellular sensitivity to insulin in patients with essential hypertension, we studied insulin sensitivity in vivo and in vitro in isolated adipocytes from older hypertensive, nondiabetic subjects. Obese hypertensive individuals demonstrated marked hyperinsulinemia and significantly reduced submaximally stimulated adipocyte 2-deoxyglucose uptake. One month of therapy with nitrendipine (10 mg twice daily) reduced blood pressure in hypertensive subjects, reduced plasma insulin to control values in obese hypertensive individuals, and restored adipocyte 2-deoxyglucose uptake at at submaximally effective insulin concentrations to control values in both obese hypertensive subjects and those of normal weight.(ABSTRACT TRUNCATED AT 250 WORDS)

Insulin receptor beta-subunit serine phosphorylation in permeabilized cultured fetal rat hepatocytes

Regulation of cellular protein phosphorylation by insulin was investigated after short exposure at 37 degrees C prior to applying the permeabilization/phosphorylation step in the presence of digitonin and [gamma-32P]ATP for 30 min at 4 degrees C. The results revealed major 32P incorporation into a limited number of membrane polypeptides exhibiting a molecular mass of 95, 58 and 51 kDa. Phosphorylation of 95 kDa protein was selectively inhibited with Ca(2+)-free EGTA-containing permeabilization/phosphorylation buffer and became predominant in the presence of Ca2+. Considering in particular its immunoprecipitation by a monoclonal antibody directed against insulin receptor, the 32P-labeled 95 kDa protein represented the beta-subunit of the insulin receptor. Its phosphorylation was transiently stimulated after exposure to insulin (35% after 2 min), and concerned mostly serine residues under both basal and stimulated conditions. Vanadate had a similar effect and both agents favored glycogenesis, whereas heparin which inhibited 95 kDa protein phosphoseryl phosphorylation had an opposite effect on glycogenesis. These results suggest a biological role for the membrane-associated phosphoseryl-protein kinase(s) and phosphatase(s) acting on the insulin receptor beta-subunit in cultured fetal hepatocytes.

Effect of streptozotocin-induced diabetes on GLUT-4 phosphorylation in rat adipocytes

We have examined the regulation of GLUT-4 phosphorylation in adipocytes isolated from diabetic rats. Despite progressive (40-70%) reductions in GLUT-4 protein contents on the 2nd, 7th, and 14th day of diabetes, the phosphorylation of GLUT-4 was increased two- to fourfold. These alterations were accompanied by concomitant reductions (40-66%) in the insulin-stimulated 2-deoxyglucose transport. Insulin treatment of diabetic animals for 5 d restored glucose transport activity, GLUT-4 protein, and GLUT-4 phosphorylation to control levels whereas vanadate and phlorizin were ineffective. In control adipocytes, insulin promoted GLUT-4 translocation from the low density microsomal (LDM) pool to the plasma membranes (PM) and decreased the state of GLUT-4 phosphorylation. In adipocytes isolated from the diabetic rats, insulin failed to stimulate GLUT-4 translocation and to decrease GLUT-4 phosphorylation. To explore the mechanism of the diabetes-induced increases in the GLUT-4 phosphorylation, we investigated phosphoserine phosphatase (PSPase) activities using 32P-labeled GLUT-4 and phosphorylase "a" as substrates. Diabetes resulted in 50-60% increase in the particulate PSPase activity and concomitant reductions in cytosolic PSPase activities. Although reduced cytosolic PSPase activity correlated with an inadequate dephosphorylation of LDM GLUT-4, the existence of highly phosphorylated PM GLUT-4 in the presence of increased particulate PSPase activity required additional explanation. To address this problem, we used PM GLUT-4 from diabetic rats as a substrate of particulate PSPase. Highly active diabetic particulate PSPase, which dephosphorylated control GLUT-4 and phosphorylase a, failed to dephosphorylate PM GLUT-4 from diabetic rats. These data suggest that PM GLUT-4 from diabetic rats is unable to interact with PSPase or that its phosphorylation sites are not accessible to PSPase action. In summary, an induction of diabetes with streptozotocin resulted in significant increases in GLUT-4 phosphorylation. In contrast to normal cells, insulin failed to promote GLUT-4 recruitment to the plasma membranes and its dephosphorylation in diabetic adipocytes. At the same time, diabetes appears to induce redistribution of PSPases, resulting in lower cytosolic activity and higher particulate activity. It also appears that the existence of highly phosphorylated GLUT-4 in the plasma membranes of diabetic adipocytes resulted from its inability to interact with particulate PSPases.