Phorbol ester stimulates phosphorylation on serine 1327 of the human insulin receptor

Molecular mechanism of insulin resistance in obesity and type 2 diabetes

Insulin resistance is a major risk factor for developing type 2 diabetes caused by the inability of insulin-target tissues to respond properly to insulin, and contributes to the morbidity of obesity. Insulin action involves a series of signaling cascades initiated by insulin binding to its receptor, eliciting receptor autophosphorylation and activation of the receptor tyrosine kinase, resulting in tyrosine phosphorylation of insulin receptor substrates (IRSs). Phosphorylation of IRSs leads to activation of phosphatidylinositol 3-kinase (PI3K) and, subsequently, to activation of Akt and its downstream mediator AS160, all of which are important steps for stimulating glucose transport induced by insulin. Although the mechanisms underlying insulin resistance are not completely understood in skeletal muscle, it is thought to result, at least in part, from impaired insulin-dependent PI3K activation and downstream signaling. This review focuses on the molecular basis of skeletal muscle insulin resistance in obesity and type 2 diabetes. In addition, the effects of insulin-sensitizing agent treatment and lifestyle intervention of human insulin-resistant subjects on insulin signaling cascade are discussed. Furthermore, the role of Rho-kinase, a newly identified regulator of insulin action in insulin control of metabolism, is addressed.

Regulation of the insulin receptor by protein kinase C isoenzymes: preferential interaction with beta isoenzymes and interaction with the catalytic domain of betaII

We analysed the effects of high glucose in rat1 cells overexpressing insulin receptor. High (25 mM) glucose inhibited insulin-stimulated tyrosine kinase activity completely at insulin concentrations of 1 and 5 ng/ml. Decapeptides modelled on insulin receptor sequences surrounding serines 1035 and 1270 were found to inhibit protein kinase C activity in vitro and after microinjection into cells blocked the inhibition of mitogenesis induced by glucose. Purification of receptor from 3T3L1 adipocytes revealed that only the isoenzymes beta1, betaII and delta were detected. The site of the interaction was mapped to the catalytic domain of betaII. These results demonstrate that the inhibition of insulin receptor tyrosine kinase activity can be ameliorated using insulin receptor peptide sequences and there is constitutive and differential interaction of individual PKC isoenzymes with the insulin receptor, and in the case of betaII, this interaction maps to the catalytic domain rather than the regulatory domain.

Morphine induces desensitization of insulin receptor signaling

Morphine analgesia is mediated principally by the micro -opioid receptor (MOR). Since morphine and other opiates have been shown to influence glucose homeostasis, we investigated the hypothesis of direct cross talk between the MOR and the insulin receptor (IR) signaling cascades. We show that prolonged morphine exposure of cell lines expressing endogenous or transfected MOR, IR, and the insulin substrate 1 (IRS-1) protein specifically desensitizes IR signaling to Akt and ERK cascades. Morphine caused serine phosphorylation of the IR and impaired the formation of the signaling complex among the IR, Shc, and Grb2. Morphine also resulted in IRS-1 phosphorylation at serine 612 and reduced tyrosine phosphorylation at the YMXM p85-binding motifs, weakening the association of the IRS-1/p85 phosphatidylinositol 3-kinase complex. However, the IRS-1/Grb2 complex was unaffected by chronic morphine treatment. These results suggest that morphine attenuates IR signaling to Akt by disrupting the IRS-1-p85 interaction but inhibits signaling to ERK by disruption of the complex among the IR, Shc, and Grb2. Finally, we show that systemic morphine induced IRS-1 phosphorylation at Ser612 in the hypothalamus and hippocampus of wild type, but not MOR knockout, mice. Our results demonstrate that opiates can inhibit insulin signaling through direct cross talk between the downstream signaling pathways of the MOR and the IR.

Troglitazone but not metformin restores insulin-stimulated phosphoinositide 3-kinase activity and increases p110beta protein levels in skeletal muscle of type 2 diabetic subjects

Insulin stimulation of phosphatidylinositol (PI) 3-kinase activity is defective in skeletal muscle of type 2 diabetic individuals. We studied the impact of antidiabetic therapy on this defect in type 2 diabetic subjects who failed glyburide treatment by the addition of troglitazone (600 mg/day) or metformin (2,550 mg/day) therapy for 3-4 months. Improvement in glycemic control was similar for the two groups, as indicated by changes in fasting glucose and HbA(1c) levels. Insulin action on whole-body glucose disposal rate (GDR) was determined before and after treatment using the hyperinsulinemic (300 mU x m(-2) x min(-1)) euglycemic (5.0-5.5 mmol/l) clamp technique. Needle biopsies of vastus lateralis muscle were obtained before and after each 3-h insulin infusion. Troglitazone treatment resulted in a 35 +/- 9% improvement in GDR (P < 0.01), which was greater than (P < 0.05) the 22 +/- 13% increase (P < 0.05) after metformin treatment. Neither treatment had any effect on basal insulin receptor substrate-1 (IRS-1)-associated PI 3-kinase activity in muscle. However, insulin stimulation of PI 3-kinase activity was augmented nearly threefold after troglitazone treatment (from 67 +/- 22% stimulation over basal pre-treatment to 211 +/- 62% post-treatment, P < 0.05), whereas metformin had no effect. The troglitazone effect on PI 3-kinase activity was associated with a 46 +/- 22% increase (P < 0.05) in the amount of the p110beta catalytic subunit of PI 3-kinase. Insulin-stimulated Akt activity also increased after troglitazone treatment (from 32 +/- 8 to 107 +/- 32% stimulation, P < 0.05) but was unchanged after metformin therapy. Protein expression of other key insulin signaling molecules (IRS-1, the p85 subunit of PI 3-kinase, and Akt) was unaltered after either treatment. We conclude that the mechanism for the insulin-sensitizing effect of troglitazone, but not metformin, involves enhanced PI 3-kinase pathway activation in skeletal muscle of obese type 2 diabetic subjects.

Identification of Ser(1275) and Ser(1309) as autophosphorylation sites of the human insulin receptor in intact cells

In a previous report we described Ser(1275) and Ser(1309) as autophosphorylation sites of the human insulin receptor (IR) tyrosine kinase (TK) in vitro. The question remained whether the observed phosphorylation was exclusive for the in vitro activated receptor or a more general, mechanism of the activated receptor in situ. In this study, we determined the intrinsic activity of the IR to phosphorylate both serine residues in intact cells. For this purpose CHO-09 and NIH-3T3 derived cell-lines expressing the human IR were metabolically labelled with [(32)P]orthophosphate, followed by hormone stimulation of the receptor. The IR was isolated by immunoprecipitation and SDS-PAGE and subsequently analysed for serine phosphorylation by phosphopeptide mapping of HPLC-purified tryptic phosphopeptides. Activation of the IR in the intact cell appeared to result in phosphate incorporation into Ser(1275) and Ser(1309), providing strong evidence that both serine residues are phosphorylation sites of the activated receptor in intact cells.

A single substitution of the insulin receptor kinase inhibits serine autophosphorylation in vitro: evidence for an interaction between the C-terminus and the activation loop

We examined the effects of mutations of tyrosine and serine autophosphorylation sites on the dual specificity of the insulin receptor kinase (IRKD) in vitro using autophosphorylation and substrate phosphorylation and phosphopeptide mapping. For comparable studies, the recombinant kinases were overexpressed in the baculovirus system, purified, and analyzed. The phosphate incorporation into the enzymes was in the range of 3-4.5 mol/mol, and initial velocities of autophosphorylation were reduced up to 2-fold. However, the mutation Y1151F in the activation loop inhibited phosphate incorporation in the C-terminal serine residues 1275 and 1309, due to a 10-fold decrease of the initial velocity of serine autophosphorylation. Although the K(M) and V(MAX) values of this mutant were only slightly altered in substrate phosphorylation reactions using a recombinant C-terminal insulin receptor peptide (K(M): Y1151F, 9.9 +/- 0.4 microM; IRKD, 6.1 +/- 0.2 microM; V(MAX): Y1151F, 72 +/- 4 nmol min(-)(1) mg(-)(1); IRKD, 117 +/- 6 nmol min(-)(1) mg(-)(1)), diminished phosphate incorporation into serine residues of the peptide was observed. In contrast, the phosphorylation of a recombinant IRS-1 fragment, which was shown to be phosphorylated markedly on serine residues by IRKD, was not affected by any kinase mutation. These results underline that IRKD is a kinase with dual specificity. The substrate specificity toward C-terminal serine phosphorylation sites can be modified by a single amino acid substitution in the activation loop, whereas the specificity toward IRS-1 is not affected, suggesting that the C-terminus and the activation loop interact.

Protein kinase C modulates insulin action in human skeletal muscle

There is good evidence from cell lines and rodents that elevated protein kinase C (PKC) overexpression/activity causes insulin resistance. Therefore, the present study determined the effects of PKC activation/inhibition on insulin-mediated glucose transport in incubated human skeletal muscle and primary adipocytes to discern a potential role for PKC in insulin action. Rectus abdominus muscle strips or adipocytes from obese, insulin-resistant, and insulin-sensitive patients were incubated in vitro under basal and insulin (100 nM)-stimulated conditions in the presence of GF 109203X (GF), a PKC inhibitor, or 12-deoxyphorbol 13-phenylacetate 20-acetate (dPPA), a PKC activator. PKC inhibition had no effect on basal glucose transport. GF increased (P < 0.05) insulin-stimulated 2-deoxyglucose (2-DOG) transport approximately twofold above basal. GF plus insulin also increased (P < 0.05) insulin receptor tyrosine phosphorylation 48% and phosphatidylinositol 3-kinase (PI 3-kinase) activity approximately 50% (P < 0.05) vs. insulin treatment alone. Similar results for GF on glucose uptake were observed in human primary adipocytes. Further support for the hypothesis that elevated PKC activity is related to insulin resistance comes from the finding that PKC activation by dPPA was associated with a 40% decrease (P < 0.05) in insulin-stimulated 2-DOG transport. Incubation of insulin-sensitive muscles with GF also resulted in enhanced insulin action ( approximately 3-fold above basal). These data demonstrate that certain PKC inhibitors augment insulin-mediated glucose uptake and suggest that PKC may modulate insulin action in human skeletal muscle.

Serine phosphorylation of the ligand-activated beta-platelet-derived growth factor receptor by casein kinase I-gamma2 inhibits the receptor's autophosphorylating activity

Platelet-derived growth factor (PDGF) receptors (PDGFRs) are membrane protein-tyrosine kinases that, upon activation, become tyrosine-phosphorylated and associate with numerous SH2 domain-containing molecules involved in mediating signal transduction. In Rat-2 fibroblasts, we have characterized the phosphorylation of the beta-PDGFR following its activation by PDGF. In contrast to tyrosine phosphorylation, which was transient and returned to near basal levels by 30 min, PDGF-stimulated Ser/Thr phosphorylation of the beta-PDGFR was increased by 5 min and remained elevated after 30 min. In vivo, after 5 min of PDGF stimulation, serine phosphorylation of the beta-PDGFR was greatly reduced by CKI-7, a specific inhibitor of casein kinase I (CKI). In vitro, recombinant CKI-gamma2 phosphorylated the ligand-activated beta-PDGFR on serine residues in a CKI-7-sensitive manner and resulted in a marked inhibition of the receptor's autophosphorylating activity. Furthermore, in Rat-2 fibroblasts, expression of hemagglutinin epitope-tagged active CKI-gamma2 resulted in a dramatic decrease in the tyrosine phosphorylation state of the beta-PDGFR in response to PDGF, consistent with receptor inactivation. Our data suggest that upon PDGF stimulation, CKI-gamma2 is activated and/or translocated in proximity to the beta-PDGFR, whereby it phosphorylates the beta-PDGFR on serine residues and negatively regulates its tyrosine kinase activity, leading to receptor inactivation.

Analysis of potential phosphorylation sites in human T cell leukemia virus type 1 Tax

The human T cell leukemia virus type 1 (HTLV-1) Tax is a phosphoprotein, however, the contribution of phosphorylation to Tax activity is unknown. Previous studies have shown that phosphorylation of Tax occurs on serine residue(s), within one tryptic fragment, in response to 4beta-phorbol-12beta-myristate-13alpha-acetate, in both mouse and human cells. Studies were conducted in multiple cell lines to identify the specific phosphorylated serines as a prelude to functional analysis. The phosphorylation pattern of Tax was found to be different in 293T and COS-7 cells in comparison with MT-4 and Px-1 cells. However, one tryptic fragment remained consistent in comigration analyses among all cell lines. Using selected Tax serine mutants a tryptic fragment containing a serine at residue 113 believed to be the site of phosphorylation of Tax did not comigrate with the common phosphorylated tryptic fragment. Analysis of selected Tax mutants for ability to trans-activate the cytomegalovirus promoter demonstrated mutation of serine 77 to alanine reduced trans-activation by 90% compared to wild-type Tax. However, examination of the phosphorylation pattern of the serine 77 mutant demonstrated that it is not the site of phosphorylation. These studies demonstrate the importance of using relevant cell lines to characterize the role of phosphorylation in protein function.

Role of protein kinase C in the translational regulation of lipoprotein lipase in adipocytes

The hypertriglyceridemia of diabetes is accompanied by decreased lipoprotein lipase (LPL) activity in adipocytes. Although the mechanism for decreased LPL is not known, elevated glucose is known to increase diacylglycerol, which activates protein kinase C (PKC). To determine whether PKC is involved in the regulation of LPL, we studied the effect of 12-O-tetradecanoyl phorbol 13-acetate (TPA) on adipocytes. LPL activity was inhibited when TPA was added to cultures of 3T3-F442A and rat primary adipocytes. The inhibitory effect of TPA on LPL activity was observed after 6 h of treatment, and was observed at a concentration of 6 nM. 100 nM TPA yielded maximal (80%) inhibition of LPL. No stimulation of LPL occurred after short term addition of TPA to cultures. To determine whether TPA treatment of adipocytes decreased LPL synthesis, cells were labeled with [35S]methionine and LPL protein was immunoprecipitated. LPL synthetic rate decreased after 6 h of TPA treatment. Western blot analysis of cell lysates indicated a decrease in LPL mass after TPA treatment. Despite this decrease in LPL synthesis, there was no change in LPL mRNA in the TPA-treated cells. Long term treatment of cells with TPA is known to down-regulate PKC. To assess the involvement of the different PKC isoforms, Western blotting was performed. TPA treatment of 3T3-F442A adipocytes decreased PKC alpha, beta, delta, and epsilon isoforms, whereas PKC lambda, theta, zeta, micro, iota, and gamma remained unchanged or decreased minimally. To directly assess the effect of PKC inhibition, PKC inhibitors (calphostin C and staurosporine) were added to cultures. The PKC inhibitors inhibited LPL activity rapidly (within 60 min). Thus, activation of PKC did not increase LPL, but inhibition of PKC resulted in decreased LPL synthesis by inhibition of translation, indicating a constitutive role of PKC in LPL gene expression.

ATP-dependent desensitization of insulin binding and tyrosine kinase activity of the insulin receptor kinase. The role of endosomal acidification

Incubating endosomes with ATP decreased binding of 125I-insulin but not 125I-labeled human growth hormone. Increasing ATP concentrations from 0.1 to 1 mM increased beta-subunit tyrosine phosphorylation and insulin receptor kinase (IRK) activity assayed after partial purification. At higher (5 mM) ATP concentrations beta-subunit tyrosine phosphorylation and IRK activity were markedly decreased. This was not observed with nonhydrolyzable analogs of ATP, nor with plasma membrane IRK, nor with endosomal epidermal growth factor receptor kinase autophosphorylation. The inhibition of endosomal IRK tyrosine phosphorylation and activity was completely reversed by bafilomycin A1, indicating a role for endosomal proton pump(s). The inhibition of IRK was not due to serine/threonine phosphorylation nor was it influenced by the inhibition of phosphotyrosyl phosphatase using bisperoxo(1,10-phenanthroline)oxovanadate anion. Prior phosphorylation of the beta-subunit with 1 mM ATP did not prevent the inhibition of IRK activity on incubating with 5 mM ATP. To evaluate conformational change we incubated endosomes with dithiothreitol (DTT) followed by SDS-polyacrylamide gel electrophoresis under nonreducing conditions. Without DTT the predominant species of IRK observed was alpha2 beta2. With DTT the alpha beta dimer predominated but on co-incubation with 5 mM ATP the alpha2 beta2 form predominated. Thus, ATP-dependent endosomal acidification contributes to the termination of transmembrane signaling by, among other processes, effecting a deactivating conformational change of the IRK.

In NIH-3T3 fibroblasts, insulin receptor interaction with specific protein kinase C isoforms controls receptor intracellular routing

Insulin increased protein kinase C (PKC) activity by 2-fold in both membrane preparations and insulin receptor (IR) antibody precipitates from NIH-3T3 cells expressing human IRs (3T3hIR). PKC-alpha, -delta, and -zeta were barely detectable in IR antibody precipitates of unstimulated cells, while increasing by 7-, 3.5-, and 3-fold, respectively, after insulin addition. Preexposure of 3T3hIR cells to staurosporine reduced insulin-induced receptor coprecipitation with PKC-alpha, -delta, and -zeta by 3-, 4-, and 10-fold, respectively, accompanied by a 1.5-fold decrease in insulin degradation and a similar increase in insulin retroendocytosis. Selective depletion of cellular PKC-alpha and -delta, by 24 h of 12-O-tetradecanoylphorbol-13-acetate (TPA) exposure, reduced insulin degradation by 3-fold and similarly increased insulin retroendocytosis, with no change in PKC-zeta. In lysates of NIH-3T3 cells expressing the R1152Q/K1153A IRs (3T3Mut), insulin-induced coprecipitation of PKC-alpha, -delta, and -zeta with the IR was reduced by 10-, 7-, and 3-fold, respectively. Similar to the 3T3hIR cells chronically exposed to TPA, untreated 3T3Mut featured a 3-fold decrease in insulin degradation, with a 3-fold increase in intact insulin retroendocytosis. Thus, in NIH-3T3 cells, insulin elicits receptor interaction with multiple PKC isoforms. Interaction of PKC-alpha and/or -delta with the IR appears to control its intracellular routing.

Potentiation of insulin-induced phosphatidylinositol-3 kinase activity by phorbol ester is mediated by protein kinase C epsilon

Our previous results have demonstrated that phorbol 12-myristate 13-acetate (TPA) and insulin synergistically stimulate the activity of phosphatidylinositol-3 kinase (PI-3 kinase) and PI-3 kinase plays an important role in both of TPA-induced AP-1 activation and cell transformation in tumour promotion sensitive (P+) JB6 cells. In the present study, we investigated the role of PKC and its isozymes in the synergistic induction of PI-3 kinase by TPA and insulin. Bisindolylmaleimide inhibits TPA- and TPA+ insulin-induced PI-3 kinase activity. Pretreatment of cells for 24 h with TPA has significant inhibitory effects on TPA-induced PI-3 kinase activity and abolishes the synergistic effect of TPA and insulin-stimulated PI-3 kinase activity. Furthermore, overexpression of a dominant negative PKC epsilon, but not dominant negative PKC alpha, blocks the synergistic effect of TPA and insulin-induced PI-3 kinase activity. These results indicate that the potentiation effect of TPA on insulin-induced PI-3 kinase activity is specific through PKC epsilon in JB6 cells.

Protein kinase C modulation of insulin receptor substrate-1 tyrosine phosphorylation requires serine 612

Activation of the endogenous protein kinase Cs in human kidney fibroblast (293) cells was found in the present study to inhibit the subsequent ability of insulin to stimulate the tyrosine phosphorylation of an expressed insulin receptor substrate-1. This inhibition was also observed in an in vitro phosphorylation reaction if the insulin receptor and its substrate were both isolated from cells in which the protein kinase C had been activated. To test whether serine phosphorylation of the insulin receptor substrate-1 was contributing to this process, serine 612 of this molecule was changed to an alanine. The insulin-stimulated tyrosine phosphorylation and the associated phosphatidylinositol 3-kinase activity of the expressed mutant were found to be comparable to those of the expressed wild-type substrate. However, unlike the wild-type protein, activation of protein kinase C did not inhibit the insulin-stimulated tyrosine phosphorylation of the S612A mutant nor its subsequent association with phosphatidylinositol 3-kinase. Tryptic peptide mapping of in vivo labeled IRS-1 and the S612A mutant revealed that PMA stimulates the phosphorylation of a peptide from wild-type IRS-1 that is absent from the tryptic peptide maps of the S612A mutant. Moreover, a synthetic peptide containing this phosphoserine and its nearby tyrosine was found to be phosphorylated by the insulin receptor to a much lower extent than the same peptide without the phosphoserine. Activation of protein kinase C was found to stimulate by 10-fold the ability of a cytosolic kinase to phosphorylate this synthetic peptide as well as the intact insulin receptor substrate-1. Finally, cytosolic extracts from the livers of ob/ob mice showed an 8-fold increase in a kinase activity capable of phosphorylating this synthetic peptide, compared to extracts of livers from lean litter mates. These results indicate that activation of protein kinase C stimulates a kinase which can phosphorylate insulin receptor substrate-1 at serine 612, resulting in an inhibition of insulin signaling in the cell, posing a potential mechanism for insulin resistance in some models of obesity.

Brain-derived neurotrophic factor binding to the p75 neurotrophin receptor reduces TrkA signaling while increasing serine phosphorylation in the TrkA intracellular domain

We have examined whether the low affinity neurotrophin receptor p75NTR modulates TrkA function by intracellular signaling. Using ligands that selectively bind p75NTR or TrkA, we found that a p75NTR-derived signal reduces TrkA activation. Short term treatment of PC12 cells with ceramide analogues also resulted in reduced NGF-stimulated TrkA activation, suggesting that p75-mediated increases in sphingomyelinase activity may contribute to this modulatory effect. Phosphoamino acid analysis was performed to determine if brain-derived neurotrophic factor- or ceramide-mediated phosphorylation of the TrkA intracellular domain correlated with a reduction in its ligand-induced activation. A specific increase in TrkA phosphoserine content was observed in response to both C2-ceramide and brain-derived neurotrophic factor. These results suggest that ligand binding of p75NTR can activate a signaling cascade that results in reduced TrkA activity through phosphorylation of its intracellular domain.

CSF-1 receptor/insulin receptor chimera permits CSF-1-dependent differentiation of 3T3-L1 preadipocytes

A chimeric growth factor receptor (CSF1R/IR) was constructed by splicing cDNA sequences encoding the extracellular ligand binding domain of the human colony stimulating factor-1 (CSF-1) receptor to sequences encoding the transmembrane and cytoplasmic domains of the human insulin receptor. The addition of CSF-1 to cells transfected with the CSF1R/IR chimera cDNA stimulated the tyrosine phosphorylation of a protein that was immunoprecipitated by an antibody directed against the carboxyl terminus of the insulin receptor. Phosphopeptide maps of the 32P-labeled CSF1R/IR protein revealed the same pattern of phosphorylation observed in 32P-labeled insulin receptor beta subunits. CSF-1 stimulated the tyrosine phosphorylation of insulin receptor substrate-1 (IRS-1) and Shc in cells expressing the CSF1R/IR chimera. Lipid accumulation and the expression of a differentiation-specific marker demonstrated that 3T3-L1 preadipocytes undergo CSF-1-dependent differentiation when transfected with the CSF1R/IR chimera cDNA but not when transfected with the expression vector alone. A 12-amino acid deletion within the juxtamembrane region of the CSF1R/IR (CSF1R/IRDelta960) blocked CSF-1-stimulated phosphorylation of IRS-1 and Shc but did not inhibit CSF-1-mediated differentiation of 3T3-L1 preadipocytes. These observations indicate that adipocyte differentiation can be initiated by intracellular pathways that do not require tyrosine phosphorylation of IRS-1 or Shc.

A synthetic peptide derived from a COOH-terminal domain of the insulin receptor specifically enhances insulin receptor signaling

The role of the insulin receptor COOH-terminal domain in the regulation of insulin signal transduction was explored with a variety of synthetic peptides. One of the peptides, termed peptide HC, whose structure corresponds to residues 1293-1307 of the insulin proreceptor sequence, enhanced insulin-stimulated autophosphorylation of the insulin receptor in cell-free systems and in semipermeabilized Chinese hamster ovary (CHO) cells that had been transfected with an expression plasmid encoding the human insulin receptor (CHO/HIRc) at concentrations where there was no detectable effect on basal autophosphorylation levels or on receptor dephosphorylation. A lipophilic analogue of peptide HC, stearyl peptide HC, added to intact CHO/HIRc cells enhanced significantly insulin-stimulated insulin receptor autophosphorylation while having no effect on ligand-stimulated receptor phosphorylation in CHO cells overexpressing either the IGF-1 receptor or epidermal growth factor receptor. Addition of stearyl peptide HC to CHO/HIRc cells resulted in a 2.4 +/- 0.3-fold increase in the amount of insulin-stimulated phosphatidylinositol 3-kinase detected in anti-IRS-1 immunoprecipitates and a 2.1 +/- 0.6-fold increase in the levels of tyrosine phosphorylation of mitogen-activated protein kinase in response to insulin. Finally, a derivative of peptide HC coupled to a biotin moiety was prepared and showed to bind with the beta-subunit of the wild-type insulin receptor and a truncated receptor that lacks 43 amino acids from its carboxyl terminus. However, there was little binding, if any, of the peptide with the IGF-1 receptors or the epidermal growth factor receptors. Taken together, our data demonstrate that a pentadecapeptide related to the carboxyl terminus of the insulin receptor binds to the insulin receptor beta-subunit and that this interaction may contribute to the increased receptor's intrinsic activity and signal transduction.

Interactions of protein kinase C with insulin signaling. Influence on GAP and Sos activities

In this study, we investigated the influence of the protein kinase C (PKC)-dependent system upon the ability of insulin to stimulate p21(ras).GTP loading in 3T3-L1 adipocytes. Activation of PKC by 12-0-tetradecanoylphorbol-13-acetate (TPA) did not affect the basal amount of p21(ras).GTP but significantly reduced insulin-induced increases in p21(ras).GTP. This reduction was due to inhibition of the insulin's ability to stimulate guanine nucleotide exchange activity of Sos in cells incubated with 100 nM TPA for either 30 min or 3 h. TPA had no effect on basal activity of Sos. Depletion of PKC by an 18-h incubation with TPA or inhibition by bisindolylmaleimide resulted in profound inhibition of the insulin-induced p21(ras).GTP loading. In contrast to PKC activation, removal of PKC did not influence Sos activity but resulted in a 2-fold stimulation of GTPase activating protein (GAP). This effect of PKC depletion is unique to 3T3-L1 adipocytes and was not observed in either 3T3-L1 fibroblasts or Rat-1 fibroblasts. Thus, it appears that in 3T3-L1 adipocytes, PKC has a constitutive inhibitory effect on GAP that permits insulin to activate Sos and p21(ras). Removal of this inhibitory influence activates GAP and reduces insulin-stimulated p21(ras).GTP loading.

Synergistic effects of insulin and phorbol ester on mitogen-activated protein kinase in Rat-1 HIR cells

Regulation of the activity of the extracellular signal regulated kinase (ERK) mitogen-activated protein kinases was examined in Rat-1 HIR, a fibroblast cell line overexpressing the human insulin receptor. Insulin or phorbol ester induced partial activations of ERKs, while a combination of insulin and phorbol ester resulted in a synergistic activation. Preincubation with phorbol ester increased the subsequent response to insulin. Phorbol ester did not enhance tyrosine phosphorylation of the insulin receptor. Insulin did not enhance activation of phospholipase D in response to phorbol ester. Lysophosphatidic acid also acted synergistically with insulin to induce ERK activation. Lysophosphatidic acid alone had little effect on ERK, and did not activate phospholipase D. The combination of phorbol ester and insulin maintained tyrosine phosphorylation of focal adhesion kinase, while insulin alone decreased its tyrosine phosphorylation. Phorbol ester induced phosphorylation of She on serine/threonine, while insulin induced tyrosine phosphorylation of She and She-Grb2 binding. These results suggest that full activation of ERKs in fibroblasts can require the cooperation of at least two signaling pathways, one of which may result from a protein kinase C-dependent phosphorylation of effectors regulating ERK activation. In this manner, phorbol esters may enhance mitogenic signals initiated by growth factor receptors.

The synthesis and compositional analysis of phosphopeptides

Protein phosphorylation plays a critical role in the regulation of cell growth and differentiation. There is considerable interest, therefore, in the facile synthesis of peptides that possess selectively phosphorylated residues for use as molecular probes in mechanistic studies of the biological consequences of phosphorylation. This work will review the various synthetic protocols used in the generation of phosphopeptides and will discuss their characterization by amino acid compositional analysis.

Activation of protein kinase C alpha inhibits signaling by members of the insulin receptor family

Stimulation of the activity of protein kinase C by pretreatment of cells with phorbol esters was tested for its ability to inhibit signaling by four members of the insulin receptor family, including the human insulin and insulin-like growth factor-I receptors, the human insulin receptor-related receptor, and the Drosophila insulin receptor. Activation of overexpressed protein kinase C alpha resulted in a subsequent inhibition of the ligand-stimulated increase in antiphosphotyrosine-precipitable phosphatidylinositol 3-kinase mediated by the kinase domains of all four receptors. This inhibition varied from 97% for the insulin receptor-related receptor to 65% for the Drosophila insulin receptor. In addition, the activation of protein kinase C alpha inhibited the in situ ligand-stimulated increase in tyrosine phosphorylation of the GTPase-activating protein-associated p60 protein as well as Shc mediated by these receptors. The mechanism for this inhibition was further studied in the case of the insulin-like growth factor-I receptor. Although the in situ phosphorylation of insulin-receptor substrate-1 and p60 by this receptor was inhibited by prior stimulation of protein kinase C alpha, the in vitro tyrosine phosphorylation of these two substrates by this receptor was not decreased by prior stimulation of the protein kinase C alpha in the cells that served as a source of the substrates. Finally, the insulin-like growth factor-I-stimulated increase in cell proliferation was found to be inhibited by prior activation of protein kinase C alpha.(ABSTRACT TRUNCATED AT 250 WORDS)

Insulin stimulates the tyrosine dephosphorylation of pp125 focal adhesion kinase

The phosphorylation state of pp125 focal adhesion kinase in response to insulin was examined in parental and transfected Rat-1 fibroblasts expressing both wild-type (HIRc cells) and mutant human insulin receptor cDNAs lacking the C-terminal twin tyrosine phosphorylation sites (YF2 cells) or a deletion mutant lacking the distal 43 amino acids of the beta-subunit (delta CT cells). In HIRc cells insulin stimulated the tyrosine dephosphorylation of pp125fak, whereas IGF-I did not. In contrast, the tyrosine phosphorylation state of pp125fak was unchanged in the parental Rat-1 fibroblasts and the YF2 or delta CT mutant cell lines in response to insulin. Analysis of the supernatants revealed that pp125fak was only one component of the major M(r), 120-130-kDa phosphotyrosine band seen in HIRc cells. We conclude that: 1) in contrast to other growth factors, insulin stimulates the dephosphorylation of pp125fak; 2) the presence of the insulin receptor C-terminal tyrosines 1328 and 1334 is required for the insulin-stimulated tyrosine dephosphorylation of pp125fak, suggesting a possible SH2 domain-dependent interaction; 3) insulin may modulate integrin-mediated signaling through pp125fak by altering the phosphorylation state of pp125fak.