Insulin-induced glycerolipid mediators and the stimulation of glucose transport in BC3H-1 myocytes

Insulin and the stimulation of glycogen synthesis. The road from glycogen structure to glycogen synthase to cyclic AMP-dependent protein kinase to insulin mediators

The enhanced phosphorylations via cAMP, Ca2+ mobilization, and diacyl glycerol formation via the activation of the respective kinases is now classical. The decreased phosphorylation via inhibition of adenylate cyclase via the alpha adrenergic receptor is also becoming understood. What the insulin studies on the control of glycogen synthesis have taught us is that the rate limiting enzyme glycogen synthase is regulated by multiple covalent phosphorylation in an elegant but complex manner. The overall pattern of dephosphorylation is influenced by effecting both phosphatase and kinase activities in a set of interrelated mechanisms. In the presence of glucose, in muscle, fat, and liver under physiological conditions G-6-P acts as a signal to stimulate the phosphatase. An additional stimulation could occur via a novel insulin phosphatase stimulatory mediator. The phosphatase is also stimulated by at least three covalent mechanisms involving altered phosphorylation state. In one there is a decreased phosphorylation of the phosphatase inhibitor 1 potentially related to decreased cAMP-dependent protein kinase activity. In the second, there is decreased phosphorylation of the deinhibitor also potentially related to decreased cAMP-dependent protein kinase phosphorylation. In the third, an increased activity of casein kinase 2 could activate the ATP-Mg dependent phosphatase by an increased phosphorylation of phosphatase inhibitor 2 (modulatory subunit). In the liver, allosteric control of the phosphatase by G-6-P and nucleotides is of great importance. Insulin also stimulates the phosphatase in long-term experiments via increased protein synthesis. It is clear that future work will be required to determine which species of the various classes of phosphatases are regulated in short-term and long-term regulation by insulin. In terms of kinases, the effects of insulin to inactivate and desensitize the cAMP-dependent protein kinase are established. The molecular mechanisms of this effect remain to be worked out. The enhanced activity of MAP and S-6 kinase would appear to be part of a cascade of reactions perhaps originating in the autophosphorylation and activation of the insulin receptor tyrosine kinase. The mechanism of the short-term activation of casein kinase 2 remains to be elucidated. A cAMP-dependent protein kinase inhibitory mediator, which also inhibits adenylate cyclase is an important element in the regulation of kinase and adenylate cyclase activity by insulin. Its physiological significance must be established in the future, in terms of its control of glycogen synthase activation by insulin. Clearly this kinase inhibitor as well as the phosphatase stimulator are potential regulators of glycogen synthase activity by insulin.

Phorbol esters affect skeletal muscle glucose transport in a fiber type-specific manner

Recent evidence has shown that activation of lipid-sensitive protein kinase C (PKC) isoforms leads to skeletal muscle insulin resistance. However, earlier studies demonstrated that phorbol esters increase glucose transport in skeletal muscle. The purpose of the present study was to try to resolve this discrepancy. Treatment with the phorbol ester 12-deoxyphorbol-13-phenylacetate 20-acetate (dPPA) led to an approximately 3.5-fold increase in glucose transport in isolated fast-twitch epitrochlearis and flexor digitorum brevis muscles. Phorbol ester treatment was additive to a maximally effective concentration of insulin in fast-twitch skeletal muscles. Treatment with dPPA did not affect insulin signaling in the epitrochlearis. In contrast, phorbol esters had no effect on basal glucose transport and inhibited maximally insulin-stimulated glucose transport approximately 50% in isolated slow-twitch soleus muscle. Furthermore, dPPA treatment inhibited the insulin-stimulated tyrosine phosphorylation of insulin receptor substrate (IRS)-1 and the threonine and serine phosphorylation of PKB by approximately 50% in the soleus. dPPA treatment also caused serine phosphorylation of IRS-1 in the slow-twitch soleus muscle. In conclusion, our results show that phorbol esters stimulate glucose transport in fast-twitch skeletal muscles and inhibit insulin signaling in slow-twitch soleus muscle of rats. These findings suggest that mechanisms other than PKC activation mediate lipotoxicity-induced whole body insulin resistance.

Knockout of PKC alpha enhances insulin signaling through PI3K

Insulin stimulates glucose transport and certain other metabolic processes by activating atypical PKC isoforms (lambda, zeta, iota) and protein kinase B (PKB) through increases in D3-polyphosphoinositides derived from the action of PI3K. The role of diacylglycerol-sensitive PKC isoforms is less clear as they have been suggested to be both activated by insulin and yet inhibit insulin signaling to PI3K. Presently, we found that insulin signaling to insulin receptor substrate 1-dependent PI3K, PKB, and PKC lambda, and downstream processes, glucose transport and activation of ERK, were enhanced in skeletal muscles and adipocytes of mice in which the ubiquitous conventional diacylglycerol-sensitive PKC isoform, PKC alpha, was knocked out by homologous recombination. On the other hand, insulin provoked wortmannin-insensitive increases in immunoprecipitable PKC alpha activity in adipocytes and skeletal muscles of wild-type mice and rats. We conclude that 1) PKC alpha is not required for insulin-stimulated glucose transport, and 2) PKC alpha is activated by insulin at least partly independently of PI3K, and largely serves as a physiological feedback inhibitor of insulin signaling to the insulin receptor substrate 1/PI3K/PKB/PKC lambda/zeta/iota complex and dependent metabolic processes.

Insulin-sensitive phospholipid signaling systems and glucose transport. Update II

Insulin provokes rapid changes in phospholipid metabolism and thereby generates biologically active lipids that serve as intracellular signaling factors that regulate glucose transport and glycogen synthesis. These changes include: (i) activation of phosphatidylinositol 3-kinase (PI3K) and production of PIP3; (ii) PIP3-dependent activation of atypical protein kinase Cs (PKCs); (iii) PIP3-dependent activation of PKB; (iv) PI3K-dependent activation of phospholipase D and hydrolysis of phosphatidylcholine with subsequent increases in phosphatidic acid (PA) and diacylglycerol (DAG); (v) PI3K-independent activation of glycerol-3-phosphate acylytansferase and increases in de novo synthesis of PA and DAG; and (vi) activation of DAG-sensitive PKCs. Recent findings suggest that atypical PKCs and PKB serve as important positive regulators of insulin-stimulated glucose metabolism, whereas mechanisms that result in the activation of DAG-sensitive PKCs serve mainly as negative regulators of insulin signaling through PI3K. Atypical PKCs and PKB are rapidly activated by insulin in adipocytes, liver, skeletal muscles, and other cell types by a mechanism requiring PI3K and its downstream effector, 3-phosphoinositide-dependent protein kinase-1 (PDK-1), which, in conjunction with PIP3, phosphorylates critical threonine residues in the activation loops of atypical PKCs and PKB. PIP3 also promotes increases in autophosphorylation and allosteric activation of atypical PKCs. Atypical PKCs and perhaps PKB appear to be required for insulin-induced translocation of the GLUT 4 glucose transporter to the plasma membrane and subsequent glucose transport. PKB also appears to be the major regulator of glycogen synthase. Together, atypical PKCs and PKB serve as a potent, integrated PI3K/PDK-1-directed signaling system that is used by insulin to regulate glucose metabolism.

Ectopic expression of protein kinase CbetaII, -delta, and -epsilon, but not -betaI or -zeta, provide for insulin stimulation of glucose uptake in NIH-3T3 cells

Insulin regulates a diverse array of signaling pathways involved in the control of growth, differentiation, proliferation, and metabolism. Insulin increases in glucose uptake via a protein kinase C-dependent pathway in target tissues such as fat and muscle are well documented. Insulin-regulated events, however, occur in all cells. The utilization of glucose as a preferred energy source is a ubiquitous event in eukaryotic cells. In NIH-3T3 fibroblasts, insulin treatment increased levels of the cPKC and nPKC activator, diacylglycerol. Insulin-responsive 2-[(3)H]deoxyglucose uptake was stimulated in a dose-dependent manner. The overexpression of protein kinase C (PKC)betaI, -betaII, -delta, -epsilon, and -zeta was used to investigate the specificity of PKC isozymes for insulin-sensitive glucose uptake. The stable overexpression of PKCbetaII, -delta, and -epsilon resulted in increases in insulin-stimulated 2-[(3)H]deoxyglucose uptake compared to vector control cells, while basal 2-deoxyglucose uptake levels were not elevated. Overexpression of PKCbetaI and PKCzeta isozymes had no further effect on basal or insulin-stimulated 2-deoxyglucose uptake. The PKC-specific inhibitor, CGP41251, blocked insulin effects on 2-deoxyglucose uptake but not its effects on tyrosine phosphorylation of cellular substrates. Insulin-stimulated 3-O-methylglucose uptake was also greater in cells overexpressing PKCbetaII, -delta, and -epsilon, compared to control cells. The increased responsiveness was not accompanied by conversion of 3T3 cells to the adipocyte phenotype or the increased expression of insulin receptors or glucose transporters (GLUT1-type). Insulin-stimulated recruitment of GLUT1 to plasma membranes of cells overexpressing PKCbetaII, -delta, and -epsilon, was greater than that in control cells. The data suggest that more than one PKC isozyme is involved in insulin signaling pathways in fibroblasts, resulting in increased GLUT1 transporter recruitment to cell membranes.

Characterization of glucose transport activity reconstituted from heart and other tissues

We examined several aspects of glucose transport reconstituted in liposomes, with emphasis on transporters of rat heart (mostly GLUT4) compared to those of human erythrocytes (GLUT1), and on effects of agents that modulate transport in intact cells. Several types of samples gave higher reconstituted activity using liposomes of egg lipids rather than soybean lipids. Diacylglycerol, proposed to activate transporters directly as part of the mechanism of insulin action, increased the intrinsic activity of heart transporters by only 25%, but increased the size of the reconstituted liposomes by 90%. The dipeptide Cbz-Gly-Phe-NH2 inhibited GLUT4 with a Ki of 0.2 mM, compared to 2.5 mM for GLUT1, which explains its preferential inhibition of insulin-stimulated glucose transport in adipocytes. Verapamil, which inhibits insulin- and hypoxia-stimulated glucose transport in muscle, had no effect on reconstituted transporters. Heart transporters had a higher Km for glucose uptake (13.4) than did GLUT1 (1.6 mM), in agreement with a recent study of GLUT1 and GLUT4 expressed in yeast and reconstituted in liposomes. Transporters reconstituted from heart and adipocytes were 40-70% inactivated by external trypsin, suggesting the presence of trypsin-sensitive sites on the cytoplasmic domain of GLUT4. NaCl and KCl both reduced reconstituted transport activity, but KCl had a much smaller effect on the size of the liposomes.

Evidence for involvement of protein kinase C (PKC)-zeta and noninvolvement of diacylglycerol-sensitive PKCs in insulin-stimulated glucose transport in L6 myotubes

We examined the question of whether insulin activates protein kinase C (PKC)-zeta in L6 myotubes, and the dependence of this activation on phosphatidylinositol (PI) 3-kinase. We also evaluated a number of issues that are relevant to the question of whether diacylglycerol (DAG)-dependent PKCs or DAG-insensitive PKCs, such as PKC-zeta, are more likely to play a role in insulin-stimulated glucose transport in L6 myotubes and other insulin-sensitive cell types. We found that insulin increased the enzyme activity of immunoprecipitable PKC-zeta in L6 myotubes, and this effect was blocked by PI 3-kinase inhibitors, wortmannin and LY294002; this suggested that PKC-zeta operates downstream of PI 3-kinase during insulin action. We also found that treatment of L6 myotubes with 5 microM tetradecanoyl phorbol-13-acetate (TPA) for 24 h led to 80-100% losses of all DAG-dependent PKCs (alpha, beta1, beta2, delta, epsilon) and TPA-stimulated glucose transport (2-deoxyglucose uptake); in contrast, there was full retention of PKC-zeta, as well as insulin-stimulated glucose transport and translocation of GLUT4 and GLUT1 to the plasma membrane. Unlike what has been reported in BC3H-1 myocytes, TPA treatment did not elicit increases in PKCbeta2 messenger RNA or protein in L6 myotubes, and selective retention of this PKC isoform could not explain the retention of insulin effects on glucose transport after prolonged TPA treatment. Of further interest, TPA acutely activated membrane-associated PI 3-kinase in L6 myotubes, and acute effects of TPA on glucose transport were inhibited, not only by the PKC inhibitor, LY379196, but also by both wortmannin and LY294002; this suggested that DAG-sensitive PKCs activate glucose transport through cross-talk with phosphatidylinositol (PI) 3-kinase, rather than directly through PKC. Also, the cell-permeable, myristoylated PKC-zeta pseudosubstrate inhibited insulin-stimulated glucose transport both in non-down-regulated and PKC-depleted (TPA-treated) L6 myotubes; thus, the PKC-zeta pseudosubstrate appeared to inhibit a protein kinase that is required for insulin-stimulated glucose transport but is distinct from DAG-sensitive PKCs. In keeping with the latter dissociation of DAG-sensitive PKCs and insulin-stimulated glucose transport, LY379196, which inhibits PKC-beta (preferentially) and other DAG-sensitive PKCs at relatively low concentrations, inhibited insulin-stimulated glucose transport only at much higher concentrations, not only in L6 myotubes, but also in rat adipocytes, BC3H-1 myocytes, 3T3/L1 adipocytes and rat soleus muscles. Finally, stable and transient expression of a kinase-inactive PKC-zeta inhibited basal and insulin-stimulated glucose transport in L6 myotubes. Collectively, our findings suggest that, whereas PKC-zeta is a reasonable candidate to participate in insulin stimulation of glucose transport, DAG-sensitive PKCs are unlikely participants.

Phorbol esters stimulate muscle glucose transport by a mechanism distinct from the insulin and hypoxia pathways

Glucose transport in skeletal muscle can be stimulated by insulin and also by contractions and hypoxia. Activation of protein kinase C (PKC) stimulates glucose transport in muscle and other insulin-responsive cells. This study was performed to determine if the diacylglycerol (DAG)/phorbol ester-sensitive PKC isoforms participate in insulin and/or hypoxia-stimulated glucose transport in skeletal muscle. The phorbol ester 12-deoxyphorbol 13-phenylacetate 20-acetate (dPPA) induced a three- to fourfold increase in glucose transport in rat epitrochlearis muscle. The effects of dPPA on glucose transport and on cell surface GLUT-4 were completely additive to the maximal effects of insulin or hypoxia. Phorbol ester treatment induced 5- to 10-fold increases in phosphorylation of the myristoylated alanine-rich C kinase substrate protein in muscle, whereas insulin and hypoxia had negligible effects. Calphostin C, an inhibitor of DAG-sensitive PKC isoforms, decreased glucose transport stimulation by dPPA but not by insulin or hypoxia. These results provide evidence that activation of DAG/phorbol ester-sensitive PKCs is not involved in the pathways by which either insulin or hypoxia stimulates muscle glucose transport. They also show that activation of this group of PKCs increases glucose transport by a mechanism that is independent of and additive to the effects of insulin or hypoxia.

Insulin second messengers

The molecular pathways for insulin's signal transduction from its cell surface receptor to the cell's interior metabolic machinery remain in many ways uncharted. Lately two molecules have been proposed as second messengers transducing the insulin signal into the target cell. One is a phospho-oligosaccharide/inositolphosphoglycan and the other is diacylglycerol, both deriving from the same plasma membrane glycolipid, which is hydrolysed in response to insulin treatment. The phospho-oligosaccharide appears to mediate many metabolic effects of insulin through control of the phosphorylation state of key regulatory metabolic enzymes. Diacylglycerol may mediate insulin's stimulation of glucose transport over the plasma membrane. The glycolipid precursor of these putative second messengers, as well as the receptor for insulin, appear to be localized in caveolae microdomains of the plasma membrane, and glucose transporters accumulate in caveolae in response to insulin treatment, suggesting a focal role for caveolae in insulin signalling.

Insulin translocates PKC-epsilon and phorbol esters induce and persistently translocate PKC-beta 2 in BC3H-1 myocytes

Initial studies suggested that insulin increases diacylglycerol and activates protein kinase C (PKC) in BC3H-1 myocytes. In these earlier studies, insulin was found to translocate PKC-beta, but the presence of PKC-epsilon was not appreciated. More recently, the presence of PKC-epsilon was documented, but PKC-beta was not detected, and it was questioned whether insulin activates PKC in BC3H-1 myocytes [Stumpo, D.J., Haupt, D.M. and Blackshear, P.J. (1994) J. Biol. Chem. 269:21184-21190]. We questioned whether insulin translocates PKC-epsilon in BC3H-1 myocytes, and re-evaluated the question of whether myocytes truly contain a PKC-beta isoform whose existence can be verified by its response to phorbol ester treatment. We found that PKC-epsilon was acutely translocated by insulin and phorbol esters from the cytosol to the membrane fraction in BC3H-1 myocytes; in addition, PKC-epsilon, like PKC-alpha, was depleted by chronic phorbol ester treatment. We also found that BC3H-1 myocytes containing a 76,000 Mr PKC-beta isoform that is acutely translocated and subsequently depleted by phorbol esters. Moreover, chronic phorbol ester treatment induced an 84,000 Mr PKC-beta 2 isoform that appeared to be persistently translocated and activated, as suggested by studies of myristoylated arginic-rich C kinase substrate (MARCKS) phosphorylation. We conclude that: (1) insulin acutely translocates PKC-epsilon, as well as PKC-beta, in BC3H-1 myocytes; and (2) PKC-beta is not truly downregulated by phorbol esters in BC3H-1 myocytes.

Regulation of alternative splicing of protein kinase C beta by insulin

Insulin regulates a diverse array of cellular signaling processes involved in the control of growth, differentiation, and cellular metabolism. Insulin increases glucose transport via a protein kinase C (PKC)-dependent pathway in BC3H-1 myocytes, but the function of specific PKC isozymes in insulin action has not been elucidated. Two isoforms of PKC beta result via alternative splicing of precursor mRNA. As now shown, both isoforms are present in BC3H-1 myocytes, and insulin induces alternative splicing of the PKC beta mRNA thereby switching expression from PKC beta I to PKC beta II mRNA. This effect occurs rapidly (15 min after insulin treatment) and is dose-dependent. The switch in mRNA is reflected by increases in the protein levels of PKC beta II. High levels of 12-0-tetradecanoylphorbol-13-acetate, which are commonly used to deplete or down-regulate PKC in cells, also induce the switch to PKC beta II mRNA following overnight treatment, and protein levels of PKC beta II reflected mRNA increases. To investigate the functional importance of the shift in PKC beta isoform expression, stable transfectants of NIH-3T3 fibroblasts overexpressing PKC beta I and PKC beta II were established. The overexpression of PKC beta II but not PKC beta I in NIH-3T3 cells significantly enhanced insulin effects on glucose transport. This suggests that PKC beta II may be more selective than PKC beta I for enhancing the glucose transport effects of insulin in at least certain cells and, furthermore, that insulin can regulate the expression of PKC beta II by alternative mRNA splicing.

Insulin action on cardiac glucose transport: studies on the role of protein kinase C

Isolated ventricular cardiomyocytes from adult rat have been used to elucidate a possible relationship between protein kinase C (PKC) and the stimulatory action of insulin on cardiac glucose transport. Cells were incubated in the presence of either insulin or phospholipase C from Clostridium perfringens (PLC-Cp) and intracellular sn-1,2-diacylglycerol (DAG) levels and initial rates of 3-O-methylglucose transport were determined. Insulin had no effect on the DAG mass level, whereas it was elevated by PLC-Cp to 200% of control. Under these conditions the hormone produced a 2.7-fold stimulation of glucose transport with no significant effect of PLC-Cp. Insulin was unable to produce a redistribution of PKC, whereas phorbol 12-myristate 13-acetate (PMA) increased membrane associated PKC twofold. The PKC inhibitors tamoxifen and staurosporine did not interfere with glucose transport stimulation by insulin. Furthermore, cells treated with PMA exhibited unaltered basal and maximally insulin stimulated rates of glucose transport. In contrast, at physiological concentrations of insulin the stimulatory action of the hormone was significantly reduced. We conclude from our data that PKC is not involved in insulin action on cardiac glucose transport. However, activation of this enzyme may lead to a modified insulin sensitivity of the cardiac cell.

Insulin stimulates turnover of phosphatidylcholine in rat adipocytes

The present study investigated the effect of insulin on phosphatidylcholine turnover in rat adipocytes labelled to equilibrium with [14C]-choline. Insulin induced a rapid turnover of this major phospholipid that was maximal by 1 min and transient in nature. Following a 1 min stimulation of the cells with insulin at a maximally effective concentration (7 nM), a 4-6% decrease in the percentage of total cellular choline associated with this phospholipid was observed. This reflected a significant transient increase in the percentage of total cellular choline associated with phosphorylcholine, which together with diacylglycerol are the phospholipase C cleavage products of phosphatidylcholine. These effects were observed over a physiological range of insulin concentrations. No effect of insulin on any other choline phospholipid or metabolite (sphingomyelin, lysophophatidylcholine, glycerophosphocholine or choline) was seen. These results suggest that insulin stimulates a phospholipase C-mediated turnover of phosphatidylcholine in rat adipocytes. The rapid nature of this turnover suggests a potential role in signal transduction.

Growth factors, mitogens, oncogenes and the regulation of glucose transport

The erythrocyte (or HepG2/brain) type glucose transporter (GLUT 1) was the first of the family of facilitative glucose transporter proteins to be cloned [M. Mueckler et al., Science 229, 941-945, 1985]. GLUT 1 is expressed in most tissue types, all cell lines, transformed cells and tumour cells. It is thought to be responsible for "housekeeping" levels of glucose transport, i.e. the uptake of glucose required for oxidative phosphorylation. The rate of glucose transport via GLUT 1 can be regulated under conditions in which the metabolic rate must be adjusted such as cell division (mitosis and meiosis), differentiation, transformation and nutrient starvation. Here we review the recent literature on the control of glucose transport of mitogens, growth factors and oncogenes, and discuss some of the implications for the integration of cellular signalling pathways and cell growth.

Insulin and platelet-derived growth factor acutely stimulate glucose transport in 3T3-L1 fibroblasts independently of protein kinase C

Insulin and platelet-derived growth factor (PDGF) are mitogenic for murine 3T3-L1 fibroblasts. Both these mitogens acutely stimulate glucose transport by 2-4-fold in these cells, evident within minutes of agonist exposure. The tumour promoter and protein kinase C activator, phorbol 12-myristate 13-acetate (PMA) also stimulates glucose transport by 2-3-fold over a similar time frame, suggesting that protein kinase C may be involved in the mitogenic action of insulin and PDGF in this cell line. In an attempt to address this, we have measured intracellular sn-1,2-diacylglycerol (DAG) levels in response to insulin, PDGF and PMA. We show that PDGF and PMA induce a rapid elevation in intracellular diacylglycerol levels, but insulin was without effect. In addition, we have shown that PMA and PDGF, but not insulin, stimulate protein kinase C activity. However, depletion of protein kinase C by overnight exposure to PMA, abolished PMA-stimulated glucose transport but had no effect on insulin- and PDGF-stimulated glucose transport, suggesting that the stimulation of glucose transport by these mitogens does not involve protein kinase C. The use of the selective protein kinase C inhibitor, Roche 31-8220, which inhibited PMA-stimulated glucose transport, but was without effect on insulin- and PDGF-stimulated glucose transport further supports this conclusion. Taken together, these data argue against a role for protein kinase C in the stimulation of glucose transport in 3T3-L1 fibroblasts caused by acute exposure to insulin or PDGF.

Direct evidence for protein kinase C involvement in insulin-stimulated hexose uptake

Insulin has been reported to translocate protein kinase C (PKC) in rat adipocytes, and activation of PKC by phorbol esters is known to increase hexose uptake in these cells (1.2). To test the hypothesis that PKC may participate in insulin-stimulated hexose uptake, adipocytes were partially depleted of protein kinase C by overnight phorbol ester treatment, thereby impairing insulin effects on hexose uptake. Purified PKC was then introduced into these PKC-depleted adipocytes by electropermeabilization, and this fully restored insulin-stimulated hexose uptake. These findings provide direct evidence that PKC is required for insulin-stimulated hexose uptake.

Correlation of insulin receptor level with both insulin action and breakdown of a potential insulin mediator precursor; studies in CHO cell-lines transfected with insulin receptor cDNA

A phosphatidylinositol-glycan (PI-glycan) has been described previously that may serve as the precursor for a mediator of some of insulins actions. The present study further addresses the potential relevance of this compound by correlating its breakdown with other insulin actions in Chinese hamster ovary (CHO) cells which express different levels of insulin receptor. Comparisons were drawn between parent CHO cells expressing 3 x 10(3) receptors/cells and two cell-lines transfected with human insulin receptor cDNA, that expressed 600 (CHO.TH) and 300 (CHO.T) times the parent receptor level. A PI-glycan was isolated from all cells that incorporated [3H]glucosamine, [3H]galactose and [3H]inositol and was rapidly turned over upon insulin stimulation. Maximal turnover by insulin of approx. 20% was achieved in each cell line consistent with the fibroblastic nature of these cells. The effect of increased insulin receptor expression was to increase the sensitivity of the PI-glycan response to insulin. Increasing receptor number from 3 x 10(3) to 0.88 x 10(6) also increased the sensitivity of response of other insulin actions measured in this study, namely activation of pyruvate dehydrogenase (PDH), glucose utilization and transport. Thus turnover of the PI-glycan is linked closely to both metabolic actions of insulin and to cell surface insulin receptor expression further supporting its potential role in insulin action.

The insulin receptor: signalling mechanism and contribution to the pathogenesis of insulin resistance

The insulin receptor is a heterotetrameric structure consisting of two alpha-subunits of Mr 135 kilodalton on the outside of the plasma membrane connected by disulphide bonds to beta-subunits of Mr 95 kilodalton which are transmembrane proteins. Insulin binding to the alpha-subunit induces conformational changes which are transduced to the beta-subunit. This leads to the activation of a tyrosine kinase activity which is intrinsic to the cytoplasmatic domains of the beta-subunit. Activation of the tyrosine kinase activity of the insulin receptor represents an essential step in the transduction of an insulin signal across the plasma membrane of target cells. Signal transduction on the post-kinase level is not yet understood in detail, possible mechanisms involve phosphorylation of substrate proteins at tyrosine residues, activation of serine kinases, the interaction with G-proteins, phospholipases and phosphatidylinositol kinases. Studies in multiple insulin-resistant cell models have demonstrated that an impaired response of the tyrosine kinase to insulin stimulation is one potential mechanism causing insulin resistance. An impairment of the insulin effect on tyrosine kinase activation in all major target tissues of insulin, in particular the skeletal muscle was demonstrated in Type 2 (non-insulin-dependent) diabetic patients. There is no evidence that the impaired tyrosine kinase response in the skeletal muscle is a primary defect, however, it is likely that this abnormality of insulin signal transduction contributes significantly to the pathogenesis of the insulin-resistant state in Type 2 diabetes.

Effects of insulin and phospholipase C in control and denervated rat skeletal muscle

Phospholipase C (PLC), an enzyme that increases endogenous 1,2-diacylglycerol (DAG), caused dose-dependent stimulation of 2-deoxy-D-glucose (2-DG) uptake in rat soleus muscles; the maximal effect was less than that of insulin. In denervated muscles the effect of insulin on 2-DG uptake was markedly reduced, whereas the response to PLC was identical to that of control muscles. Both PLC and insulin stimulated glucose incorporation into glycogen in control but not in denervated solei. Amino acid transport was unaffected by PLC; however, the enzyme completely inhibited the stimulation of amino acid transport by insulin. PLC did not activate the insulin receptor tyrosine kinase but decreased activation of the receptor by insulin in vivo. Basal muscle DAG content increased after denervation. Incubation with PLC markedly increased DAG in control and in denervated muscle. Insulin increased total DAG mass less than PLC in control muscles and did not affect DAG in denervated muscles. In media without added Ca2+, PLC stimulation of DAG production was impaired, and 2-DG uptake was unresponsive to PLC. The data are consistent with, but do not prove, that a subpopulation of DAGs may participate in insulin-mediated stimulation of glucose transport. They also suggest that the denervation-induced insulin resistance of glucose transport may reflect impaired generation of certain DAGs involved in the signaling cascade.

Protein kinase C inhibitors block insulin and PMA-stimulated hexose transport in isolated rat adipocytes and BC3H-1 myocytes

Effects of protein kinase C (PKC) inhibitors and "down-regulation" on insulin and PMA-stimulated 2-deoxyglucose transport were determined in isolated rat adipocytes or BC3H-1 myocytes. In both model systems, H-7, sangivamycin, and staurosporine, inhibitors of the catalytic domain of PKC, each effectively blocked insulin and PMA-stimulated hexose uptake at similar concentrations. In the myocytes, staurosporine completely blocked the insulin effect retained post-chronic phorbol myristate acetate (PMA)-induced "down-regulation." These findings indicate (1) that chronic pretreatment with PMA may not lead to a complete loss of PKC activity in the myocyte, and (2) that PKC is involved in insulin-stimulated hexose transport in both isolated rat adipocytes and BC3H-1 myocytes.

Insulin activates glycerol-3-phosphate acyltransferase (de novo phosphatidic acid synthesis) through a phospholipid-derived mediator. Apparent involvement of Gi alpha and activation of a phospholipase C

We studied the mechanism whereby insulin activates de novo phosphatidic acid synthesis in BC3H-1 myocytes. Insulin rapidly activated glycerol-3-phosphate acyltransferase (G3PAT) in intact and cell-free preparations of myocytes in a dose-related manner. The apparent Km of the enzyme was decreased by treatment with insulin, whereas the Vmax was unaffected. No activation was found by ACTH, insulin-like growth factor-I, angiotensin II, or phenylephrine, but epidermal growth factor, which, like insulin, is known to activate de novo phosphatidic acid synthesis in intact myocytes, also stimulated G3PAT activity. In homogenates or membrane fractions, the effect of insulin on G3PAT was fully mimicked by nonspecific or phosphatidylinositol (PI)-specific phospholipase C (PLC). An antiserum raised against PI-glycan-PLC completely blocked the effect of insulin on G3PAT. Although the above findings suggested involvement of a PLC in insulin-induced activation of G3PAT, neither diacylglycerol nor protein kinase C activation appeared to be involved. On the other hand, insulin stimulated the release of a cytosolic factor, which activated membrane-associated G3PAT. This cytosolic factor had a molecular weight of less than 5K as determined by Sephadex G-25 chromatography. NaF, a phosphatase inhibitor, blocked the activation of G3PAT by insulin, suggesting involvement of a phosphatase. Insulin-induced activation of G3PAT was also blocked by pretreatment of intact myocytes with pertussis toxin and by prior addition, to homogenates, of an antiserum that recognizes the C-terminal decapeptide of Gi alpha.(ABSTRACT TRUNCATED AT 250 WORDS)

Phosphorylation control by insulin in adipocytes is interfered with at a post-receptor step by phosphoinositol and glucosamine

Inositol-phosphates, glucosamine and glucose-6-phosphate blocked the effects of insulin on target protein phosphorylation in adipocytes, but the unsubstituted or sulphated derivatives of inositol or of glucose, or N-acetyl-glucosamine were without effect. The insulin stimulated tyrosine phosphorylation of the insulin receptor was not affected. The sugar-phosphates inositol-phosphate and glucose-6-phosphate did not enter into the cells. They also blocked the insulin-like effects of a potential second messenger of insulin, a phospho-oligosaccharide (POS), which has previously been shown to mimic the effects of insulin on protein phosphorylation in intact cells.

Diacylglycerols modulate phosphorylation of the insulin receptor from human mononuclear cells

It has been found that 1,2- but not 1,3-diacylglycerols stimulated phosphorylation of the insulin receptor of cultured human monocyte-like (U-937) and lymphoblastoid (IM-9) cells both in the intact- and broken-cell systems. The stimulation of the receptor's beta-subunit phosphorylation was dose-dependent, with optimal effect at 100 micrograms/ml of diacylglycerol. The effects of insulin and 1,2-diacylglycerols on the phosphorylation of partially purified insulin receptors were additive. Phosphoamino acid analysis showed a major effect of diacylglycerols on phosphorylation of tyrosine residues. The diacylglycerols also stimulated tyrosine kinase activity of the partially purified U-937 and IM-9 insulin receptors 2.5-3.5-fold when measured by phosphorylation of an exogenous substrate, poly(Glu80Tyr20) in the absence of any added insulin, calcium or phospholipid. Since this diacylglycerol effect could not be reproduced under conditions optimal for protein kinase C activation and the purified protein kinase C did not stimulate phosphorylation of the beta-subunit of the insulin receptor in this system, it is unlikely that the diacylglycerol effect was mediated by protein kinase C. Since these exogenous 1,2-diacylglycerols at the same high concentration also inhibited 125I-insulin binding to the insulin receptor of the intact U-937 and IM-9 cells, diacylglycerols could modulate the function of the insulin receptor and insulin action in human mononuclear cells.

Inositol uptake in rat aorta

The purpose of this study was to investigate the mechanism of inositol uptake into rat thoracic aorta. 3H-inositol uptake into deendothelialized aorta was linear for at least 2 h and was composed of both a saturable, Na(+)-dependent, and a nonsaturable, Na(+)-independent component. The Na(+)-dependent component of inositol uptake had a Km of 50 microM and a Vmax of 289 pmol/mg prot/h. Exposure to LiCl, ouabain, or Ca2(+)-free Krebs-Ringer bicarbonate solution inhibited uptake. Metabolic poisoning with dinitrophenol, as well as incubation with phloretin, an inhibitor of carrier-mediated hexose transport, also inhibited uptake. Exposure to norepinephrine decreased inositol uptake, while phorbol myristate acetate was without effect. Isobutylmethylxanthine significantly increased inositol uptake, while the increased uptake due to dibutyryl cyclic AMP and forskolin were not statistically significant. Sodium nitroprusside, an activator of guanylate cyclase, and 8-bromo cyclic GMP, were without effect on uptake, as was methylene blue, an inhibitor of guanylate cyclase. Inositol uptake into the aorta was increased when the endothelium was allowed to remain intact, although this effect was likely due to uptake into both the endothelial and smooth muscle cells. These results suggest that the uptake of inositol into vascular smooth muscle is: (1) dependent upon an inward Na(+)-gradient; (2) carrier mediated, and (3) inhibited by alpha 1 adrenoceptor agonists.

A further comparison of insulin- and phorbol ester-stimulated glucose transport in adipocytes

Insulin and 4 beta-phorbol 12 beta-myristate 13 alpha-acetate (PMA) stimulatory effects on adipocyte glucose transport were compared for their sensitivity to: (1) sphingosine and staurosporine, two potent protein kinase C (PKC) inhibitors; and (2) phenylarsine oxide (PhAsO), a dithiol reagent blocking insulin-stimulated glucose transport. None affected basal 2-deoxyglucose transport, cell viability, cellular ATP content, or insulin binding. Insulin- and PMA-stimulated 2-deoxyglucose transport were both markedly inhibited by sphingosine (5-50 microM) and staurosporine (0.1-2 microM), although with differences in the extents of maximal inhibitions (65 and 48% vs. 88 and 98%) and the concentrations of the drugs causing the half-maximal inhibitions observed in the experiments (2- to 3-fold higher for insulin). Insulin and PMA both altered PKC along with glucose transport, either by increasing its activity in the cytosol or by promoting its translocation to membrane. Insulin- and PMA-stimulated 2-deoxyglucose transport were both inhibited selectively by PhAsO (0.1-1 microM), at almost identical maximal inhibitions (84 and 90%) and IC50 values (0.18 and 0.16 microM). Furthermore, insulin- and PMA-induced increases in transport Vmax (6.5- and 3.4-fold) were both reduced by 89% by PhAsO, which, however, failed to affect the decrease in transport Km (1.7-fold) exclusively induced by insulin. Likewise, PhAsO did not affect insulin or PMA activation of PKC. The results suggest that insulin activates adipocyte glucose transport through: (1) a PKC-dependent mechanism requiring cellular dithiols, responsible for a part of the hormone-induced increase in transport Vmax; and (2) a PKC-independent mechanism responsible for both a further increase in transport Vmax and a decrease in transport Km.

Retention of specific protein kinase C isozymes following chronic phorbol ester treatment in BC3H-1 myocytes

Since insulin effects on glucose transport persist in phorbol ester "desensitized" or "down-regulated" BC3H-1 myocytes, we reexamined the evidence for protein kinase C (PKC) depletion. After 24 hrs of 5 microM 12-0-tetradecanoyl phorbol-13-acetate (TPA) treatment, PKC-directed histone phosphorylation and acute TPA effects on glucose transport were lost, but PKC-dependent vinculin phosphorylation was still evident. Hydroxylapatite (HAP) chromatography revealed loss of a type III, but not a type II, PKC-dependent vinculin phosphorylation. Immunoblots of cytosolic preparations of PKC-"depleted" myocytes confirmed the retention of PKC. Our findings indicate that TPA "down-regulated" BC3H-1 myocytes contain immunoreactive and functionally active PKC. The latter may explain the continued effectiveness of both insulin and diacylglycerol (DiC8) for stimulating glucose transport in "down-regulated" cells.

Regulation of glucose transport by insulin, bombesin, and bradykinin in Swiss 3T3 fibroblasts: involvement of protein kinase C-dependent and -independent mechanisms

Glucose transport stimulation by insulin, bombesin, and bradykinin in Swiss 3T3 fibroblasts was compared with the phosphoinositide hydrolysis effects of the same stimulants in a variety of experimental paradigms known to affect generation and/or functioning of intracellular second messengers: short- and long-term treatments with phorbol dibutyrate, that cause activation and down-regulation of protein kinase C, respectively; cell loading with high [quin2], that causes clamping of [Ca2+]i near the resting level; poisoning with pertussis toxin, that affects the GTP binding proteins of the Go/Gi class; treatment with Ca2+ ionophores. Glucose transport stimulation by maximal [insulin] was affected by neither pertussis toxin nor protein kinase C down-regulation. The latter, however, partially blocked the action of suboptimal [insulin]; moreover, acute phorbol dibutyrate treatment caused responses more than additive at all [insulin]. Thus, the insulin action on glucose transport in 3T3 cells appears to be synergistically potentiated by a protein kinase C-dependent mechanism, and not directly mediated by the enzyme. This result correlates with the lack of effect of insulin on phosphoinositide hydrolysis. In contrast, part of the glucose transport responses induced by bombesin and bradykinin appeared to be mediated by protein kinase C in proportion with the stimulation induced by these peptides on the phosphoinositide hydrolysis. The protein kinase C-independent portion of the response to bradykinin was found to be inhibitable by pertussis toxin. This latter result might suggest an interaction between the bradykinin receptor and a glucose transporter, mediated by a protein of the Go/Gi class.

Phospholipid signaling systems in insulin action

Insulin is known to control a number of anabolic metabolic processes in a variety of target tissues through activation of cell surface receptors. It is clear that insulin receptor activation provokes increases in tyrosine kinase activity and autophosphorylation of the insulin receptor, but subsequent events have not been elucidated. Recently, it has become clear that insulin provokes the following rapid changes in phospholipid metabolism, which result in the generation of several intercellular signaling substances (or mediators): (1) hydrolysis of a phosphatidylinositol-glycan; (2) stimulation of de novo synthesis of phosphatidic acid; and (3) hydrolysis of phosphatidylcholine by a phospholipase C and/or D. Hydrolysis of the phosphatidylinositol-glycan leads to the release of polar headgroups, which serve as mediators to activate phosphatases, and may thereby account for a number of insulin effects on carbohydrate metabolism, lipid metabolism, and regulation of cyclic nucleotide metabolism. All three phospholipid effects of insulin also generate diacylglycerol, which activates protein kinase C, and this may contribute to insulin effects on glucose transport, ion and amino acid transport, protein synthesis, and gene expression (messenger RNA synthesis). Combined, the headgroup mediators and diacylglycerol-protein kinase C signaling systems may account for many, or perhaps most, of insulin's actions. Moreover, the three phospholipid effects of insulin appear to be coordinated, and may function as an integrated cycle to ensure the continued synthesis of lipids, which are the sources of the signaling substances during insulin action.