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

Blockade of rapid versus prolonged extracellularly regulated kinase 1/2 activation has differential effects on insulin-induced gene expression

In the present work, insulin's regulation of expression of activating transcription factor 3 (ATF-3), the putative transcription factor proline-rich induced protein (Pip)92, and insulin-inducible gene-1 (Insig-1) (an ER resident protein involved in regulation of sterol-responsive element-binding protein 1 activation) have been examined in a liver-derived cell line (rat H4IIE hepatoma cells). We report that: 1) insulin-induced transcription of ATF-3, Pip92, and Insig-1 required MEK-ERK activation; 2) insulin-induced transcription of ATF-3 and Pip92 reached maximum levels within 15 min and was blocked by wortmannin but not LY294002; 3) in contrast, the maximum level of insulin-induced transcription of Insig-1 was delayed and was not blocked by either wortmannin or LY294002; 4) insulin activated ERK1/2 in two distinct phases, a rapid peak and a later plateau; 5) the delayed plateau phase of insulin-induced ERK1/2 activation was partially phosphatidylinositol 3-OH-kinase dependent; and 6) however, the rapid, insulin-induced peak of ERK1/2 activation was blocked by wortmannin but not LY294002.

A Ras activation pathway dependent on Syk phosphorylation of protein kinase C

Protein kinase C (PKC) and Syk protein tyrosine kinase play critical roles in immune cell activation including that through the high-affinity IgE receptor, FcepsilonRI. Mechanisms by which PKC activation leads to the activation of Ras, a family of GTPases essential for immune cell activation, have been elusive. We present evidence that Tyr-662 and Tyr-658 of PKCbetaI and PKCalpha, respectively, are phosphorylated by Syk in the membrane compartment of FcepsilonRI-stimulated mast cells. These phosphorylations require prior PKC autophosphorylation of the adjacent serine residues (Ser-661 and Ser-657, respectively) and generate a binding site for the SH2 domain of the adaptor protein Grb-2. By recruiting the Grb-2/Sos complex to the plasma membrane, these conventional PKC isoforms contribute to the full activation of the Ras/extracellular signal-regulated kinase signaling pathway in FcepsilonRI-stimulated mast cells.

Calmodulin prevents activation of Ras by PKC in 3T3 fibroblasts

We have shown previously (Villalonga, P., López- Alcalá, C., Bosch, M., Chiloeches, A., Rocamora, N., Gil, J., Marais, R., Marshall, C. J., Bachs, O., and Agell, N. (2001) Mol. Cell. Biol. 21, 7345-7354) that calmodulin negatively regulates Ras activation in fibroblasts. Hence, anti-calmodulin drugs (such as W13, trifluoroperazine, or W7) are able to induce Ras/ERK pathway activation under low levels of growth factors. We show here that cell treatment with protein kinase C (PKC) inhibitors abolishes W13-induced activation of Ras, Raf-1, and ERK. Consequently, PKC activity is essential for achieving the synergism between calmodulin inhibition and growth factors to activate Ras. Furthermore, whereas the activation of PKC by 12-O-tetradecanoylphorbol-13-acetate (TPA) does not induce Ras activation in 3T3 cells, activation is observed if calmodulin is simultaneously inhibited. This indicates that calmodulin is preventing Ras activation by PKC. Treatment of cells with epidermal growth factor receptor or platelet-derived growth factor receptor tyrosine kinase inhibitors does not abrogate the activation of Ras by calmodulin inhibition. This implies that epidermal growth factor receptor and platelet-derived growth factor receptor tyrosine kinase activities are dispensable for the activation of Ras by TPA plus W13, and, therefore, Ras activation is not a consequence of the transactivation of those receptors by the combination of the anti-calmodulin drug plus TPA. Furthermore, K-Ras, the isoform previously shown to bind to calmodulin, is the only one activated by TPA when calmodulin is inhibited. These data suggest that direct interaction between K-Ras and calmodulin may account for the inability of PKC to activate Ras in 3T3 fibroblasts. In vitro experiments showed that the phosphorylation of K-Ras by PKC was inhibited by calmodulin, suggesting that calmodulin-dependent modulation of K-Ras phosphorylation by PKC could be the mechanism underlying K-Ras activation in fibroblasts treated with TPA plus W13.

Insulin-induced expression of the activity-regulated cytoskeleton-associated gene (ARC) in human neuroblastoma cells requires p21(ras), mitogen-activated protein kinase/extracellular regulated kinase and src tyrosine kinases but is protein kinase C-independent

We examined the effect of insulin on the expression of the activity-regulated cytoskeleton-associated gene (ARC), an effector immediate early gene with a proposed role in memory formation. In human SH-SY5Y neuroblastoma cells, application of insulin leads to dramatic increase in ARC mRNA and protein levels. Inhibition experiments reveal, that p21(ras), mitogen-activated protein kinase/extracellular regulated kinase and tyrosine kinase (src) activity are required for the insulin-induced ARC expression in SH-SY5Y cells, whereas protein kinase C is not involved in the signal transduction pathway. Our data indicated for the first time a correlation of the insulin-controlled signal cascade and the induction of synaptic plasticity-associated immediate early genes.

Insulin, insulin-like growth factor-I, and platelet-derived growth factor activate extracellular signal-regulated kinase by distinct pathways in muscle cells

We have investigated the signaling pathways initiated by insulin, insulin-like growth factor-1 (IGF-I), and platelet-derived growth factor (PDGF) leading to activation of the extracellular signal-regulated kinase (ERK) in L6 myotubes. Insulin but not IGF-I or PDGF-induced ERK activation was abrogated by Ras inhibition, either by treatment with the farnesyl transferase inhibitor FTP III, or by actin disassembly by cytochalasin D, previously shown to inhibit Ras activation. The protein kinase C (PKC) inhibitor bisindolylmaleimide abolished PDGF but not IGF-I or insulin-induced ERK activation. ERK activation by insulin, IGF-I, or PDGF was unaffected by the phosphatidylinositol 3-kinase inhibitor wortmannin but was abolished by the MEK inhibitor PD98059. In contrast, activation of the pathway involving phosphatidylinositol 3-kinase (PI3k), protein kinase B, and glycogen synthase kinase 3 (GSK3) was mediated similarly by all three receptors, through a PI 3-kinase-dependent but Ras- and actin-independent pathway. We conclude that ERK activation is mediated by distinct pathways including: (i) a cytoskeleton- and Ras-dependent, PKC-independent, pathway utilized by insulin, (ii) a PKC-dependent, cytoskeleton- and Ras-independent pathway used by PDGF, and (iii) a cytoskeleton-, Ras-, and PKC-independent pathway utilized by IGF-I.

Insulin-activated protein kinase Cbeta bypasses Ras and stimulates mitogen-activated protein kinase activity and cell proliferation in muscle cells

In L6 muscle cells expressing wild-type human insulin receptors (L6hIR), insulin induced protein kinase Calpha (PKCalpha) and beta activities. The expression of kinase-deficient IR mutants abolished insulin stimulation of these PKC isoforms, indicating that receptor kinase is necessary for PKC activation by insulin. In L6hIR cells, inhibition of insulin receptor substrate 1 (IRS-1) expression caused a 90% decrease in insulin-induced PKCalpha and -beta activation and blocked insulin stimulation of mitogen-activated protein kinase (MAPK) and DNA synthesis. Blocking PKCbeta with either antisense oligonucleotide or the specific inhibitor LY379196 decreased the effects of insulin on MAPK activity and DNA synthesis by >80% but did not affect epidermal growth factor (EGF)- and serum-stimulated mitogenesis. In contrast, blocking c-Ras with lovastatin or the use of the L61,S186 dominant negative Ras mutant inhibited insulin-stimulated MAPK activity and DNA synthesis by only about 30% but completely blocked the effect of EGF. PKCbeta block did not affect Ras activity but almost completely inhibited insulin-induced Raf kinase activation and coprecipitation with PKCbeta. Finally, blocking PKCalpha expression by antisense oligonucleotide constitutively increased MAPK activity and DNA synthesis, with little effect on their insulin sensitivity. We make the following conclusions. (i) The tyrosine kinase activity of the IR is necessary for insulin activation of PKCalpha and -beta. (ii) IRS-1 phosphorylation is necessary for insulin activation of these PKCs in the L6 cells. (iii) In these cells, PKCbeta plays a unique Ras-independent role in mediating insulin but not EGF or other growth factor mitogenic signals.

A G1 cell cycle arrest induced by ligands of the reovirus type 3 receptor is secondary to inactivation of p21ras and mitogen-activated protein kinase

The reovirus type 3 S1 gene product (type 3 hemagglutinin; HA3) is the viral protein responsible for binding to a mammalian cell-surface receptor. It has been shown that HA3 binding to its receptor inhibits cell growth, even in the continuous presence of serum mitogens. Here, receptor-mediated signal transduction leading to growth arrest was studied after binding with synthetic or recombinant ligands in the absence of viral infection. Receptor ligation caused rapid inactivation of p21(ras), a decrease in Raf phosphorylation and in mitogen-activated protein kinase (MAPK) enzymatic activity, and G1 cell cycle arrest. Transfection and expression of constitutively active v-Has-ras prevented the G1 arrest, indicating that inactivation of p21(ras) is causative. Interestingly, v-Has-ras expression also decreased the efficiency of reoviridae replication, suggesting that inactivation of p21(ras) signals is required at some step of the viral cycle. This study may define new mechanisms regulating cell growth and support the approach of using viral proteins to identify and study cellular receptors. Synthetic receptor ligands with antiproliferative properties may be useful in drug development with the aim of blocking mitosis.

Regulation of Ras.GTP loading and Ras-Raf association in neonatal rat ventricular myocytes by G protein-coupled receptor agonists and phorbol ester. Activation of the extracellular signal-regulated kinase cascade by phorbol ester is mediated by Ras

The small G protein Ras has been implicated in hypertrophy of cardiac myocytes. We therefore examined the activation (GTP loading) of Ras by the following hypertrophic agonists: phorbol 12-myristate 13-acetate (PMA), endothelin-1 (ET-1), and phenylephrine (PE). All three increased Ras.GTP loading by 10-15-fold (maximal in 1-2 min), as did bradykinin. Other G protein-coupled receptor agonists (e.g. angiotensin II, carbachol, isoproterenol) were less effective. Activation of Ras by PMA, ET-1, or PE was reduced by inhibition of protein kinase C (PKC), and that induced by ET-1 or PE was partly sensitive to pertussis toxin. 8-(4-Chlorophenylthio)-cAMP (CPT-cAMP) did not inhibit Ras.GTP loading by PMA, ET-1, or PE. The association of Ras with c-Raf protein was increased by PMA, ET-1, or PE, and this was inhibited by CPT-cAMP. However, only PMA and ET-1 increased Ras-associated mitogen-activated protein kinase kinase 1-activating activity, and this was decreased by PKC inhibition, pertussis toxin, and CPT-cAMP. PMA caused the rapid appearance of phosphorylated (activated) extracellular signal-regulated kinase in the nucleus, which was inhibited by a microinjected neutralizing anti-Ras antibody. We conclude that PKC- and Gi-dependent mechanisms mediate the activation of Ras in myocytes and that Ras activation is required for stimulation of extracellular signal-regulated kinase by PMA.

Protein kinase C regulates integrin-induced activation of the extracellular regulated kinase pathway upstream of Shc

Adhesion of fibroblasts to extracellular matrices via integrin receptors is accompanied by extensive cytoskeletal rearrangements and intracellular signaling events. The protein kinase C (PKC) family of serine/threonine kinases has been implicated in several integrin-mediated events including focal adhesion formation, cell spreading, cell migration, and cytoskeletal rearrangements. However, the mechanism by which PKC regulates integrin function is not known. To characterize the role of PKC family kinases in mediating integrin-induced signaling, we monitored the effects of PKC inhibition on fibronectin-induced signaling events in Cos7 cells using pharmacological and genetic approaches. We found that inhibition of classical and novel isoforms of PKC by down-regulation with 12-0-tetradeconoyl-phorbol-13-acetate or overexpression of dominant-negative mutants of PKC significantly reduced extracellular regulated kinase 2 (Erk2) activation by fibronectin receptors in Cos7 cells. Furthermore, overexpression of constitutively active PKCalpha, PKCdelta, or PKCepsilon was sufficient to rescue 12-0-tetradeconoyl-phorbol-13-acetate-mediated down-regulation of Erk2 activation, and all three of these PKC isoforms were activated following adhesion. PKC was required for maximal activation of mitogen-activated kinase kinase 1, Raf-1, and Ras, tyrosine phosphorylation of Shc, and Shc association with Grb2. PKC inhibition does not appear to have a generalized effect on integrin signaling, because it does not block integrin-induced focal adhesion kinase or paxillin tyrosine phosphorylation. These results indicate that PKC activity enhances Erk2 activation in response to fibronectin by stimulating the Erk/mitogen-activated protein kinase pathway at an early step upstream of Shc.

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.

From preadipocyte to adipocyte: differentiation-directed signals of insulin from the cell surface to the nucleus

An alarming rise in obesity, and the accompanying threat of type 2 diabetes mellitus and cardiovascular disease, have attracted worldwide attention. The pathogenic mechanism(s) underlying obesity remains obscure. However, new cellular and molecular insights about the development of adipose tissue, with respect to adipocyte number (hyperplasia) and size (hypertrophy), are occurring at a rapid pace. Specialized fibroblasts (preadipocytes) committed to the adipocyte lineage are present throughout life. Primary cell culture systems and immortalized cell line models of preadipocytes have advanced the study of adipocyte differentiation (adipogenesis). Differentiation-inducing cues are able to trigger a complex network of intracellular signaling pathways in the preadipocyte, allowing signals from cell-surface receptors to reach nuclear transcription factors that regulate the genetic program of adipocyte differentiation. The extracellular matrix environment of the preadipocyte, known to modulate adipogenesis, may act by altering some of these signaling events.

Comparison of the signal transduction pathways activated by gastrin in enterochromaffin-like and parietal cells

BACKGROUND & AIMS

Gastrin stimulates acid secretion from parietal cells and histamine release from enterochromaffin-like (ECL) cells through identical gastrin receptors. However, gastrin has been shown to have a trophic effect only on ECL cells. The aim of this study was to compare gastrin-induced signal transduction pathways in the ECL and parietal cells of Mastomys natalensis, an African rodent.

METHODS

Both ECL and parietal cells were isolated from the gastric mucosa of M. natalensis, and intracellular signal transduction events in response to gastrin were investigated.

RESULTS

Gastrin elicited histamine release from ECL cells and acid secretion from parietal cells in association with enhanced inositol phospholipid turnover. Although gastrin increased [3H]thymidine incorporation into ECL cells, it had no effect on parietal cells. Moreover, tyrosine phosphorylation and activation of mitogen-activated protein (MAP) kinase as well as c-fos and c-jun gene expression were augmented only in ECL cells. In addition, gastrin increased the formation of guanosine triphosphate-Ras with a simultaneous decrease in guanosine diphosphate-Ras levels in ECL but not in parietal cells.

CONCLUSIONS

Although gastrin receptors are present in both ECL and parietal cells, they activate the Ras-MAP kinase pathway only in ECL cells.

Insulin induction of protein kinase C alpha expression is independent of insulin receptor Tyr1162/1163 residues and involves mitogen-activated protein kinase kinase 1 and sustained activation of nuclear p44MAPK

We examined the effect of insulin on protein kinase C alpha (PKCalpha) expression and the implication of the mitogen-activated protein kinase kinase 1 mitogen-activated protein kinase (MAPK) pathway in this effect. PKCalpha expression was measured by quantitative RT-PCR and Western blotting using Chinese hamster ovary (CHO) cells overexpressing human insulin receptors of the wild type (CHO-R) or insulin receptors mutated at Tyr1162/1163 autophosphorylation sites (CHO-Y2). In CHO-R cells, insulin caused a time- and concentration-dependent increase in PKCalpha messenger RNA, with a maximum at 6 h and 10-(8)M insulin. This increase involved a transcriptional mechanism, as it was not due to stabilization of PKCalpha messenger RNA and was associated with a similar increase in the immunoreactive PKCalpha level. Insulin induction of PKCalpha expression involved the MEK1MAPK pathway, as it was 1) almost completely suppressed by the potent MEK1 inhibitor PD98059, 2) mimicked by the dominant-active MEK1 (S218D/S222D) mutant, and 3) associated with sustained MAPK activation. In CHO-Y2 cells in which the early phase of MAPK activation by insulin was lost and only the late and sustained phase of activation was observed, insulin signaling of PKCalpha expression was preserved and again involved the MEK1-MAPK pathway. Moreover, we show that in both CHO-R and CHO-Y2 cells, insulin stimulation of PKCalpha gene expression was associated with prolonged activation of nuclear p44MAPK. These results indicate that induction of PKCalpha gene expression by insulin is independent of Tyr1162/1163 autophosphorylation sites and correlates with sustained activation of p44MAPK at the nuclear level.

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.

Modulation of insulin-like growth factor I mitogenic signaling in 3T3-L1 preadipocyte differentiation

Insulin-like growth factor I (IGF-I) stimulates mitogenesis in proliferating 3T3-L1 preadipocytes. However, IGF-I functions to stimulate differentiation once growth arrest occurs at confluence. Epidermal growth factor (EGF) is also a potent mitogen in these cells, but inhibits differentiation of preadipocytes. We compared mitogenic signaling via the mitogen-activated protein kinase (MAPK) pathway in response to IGF-I or EGF in proliferating, growth-arrested, and differentiating 3T3-L1 cells. IGF-I stimulation of MAPK was rapid and maximal in proliferating 3T3-L1 preadipocytes, but decreased greatly in differentiating cells. EGF was more potent than IGF-I in stimulating MAPK activity in 3T3-L1 cells, and activation of MAPK was maintained in differentiating cells. These results suggest an uncoupling of MAPK activation specific to IGF-I-mediated 3T3-L1 preadipocyte differentiation. Studies of proximal signaling revealed Shc phosphorylation and Shc/Grb2 complex formation in IGF-I-treated proliferating, but not differentiating, cells. Insulin receptor substrate-1 phosphorylation after IGF-I treatment was present in proliferating, quiescent, and differentiating preadipocytes. Shc phosphorylation and Grb2 association after EGF treatment were present throughout all phases of growth. The change in IGF-I signaling via Shc phosphorylation and MAPK activity during 3T3-L1 differentiation indicates that loss of IGF-I mitogenic signaling via the MAPK pathway is part of the differentiation process.

The effect of glucose concentration on insulin-induced 3T3-L1 adipose cell differentiation

We examined the effect of glucose concentration on insulin-induced 3T3-L1 adipose cell differentiation. Oil Red O staining of neutral lipid, cellular triglyceride mass, and glycerol phosphate dehydrogenase (GPDH) activity, were greater in 3T3-L1 cells cultured at 5 mM vs. 25 mM glucose. GPDH activity was 2- to 4-fold higher at 5 mM vs. 25 mM glucose over a range of insulin concentrations (0.1 to 100 nM). Insulin-stimulated tyrosine phosphorylation of insulin receptor substrate-1 (IRS-1) was 1.7-fold greater, and insulin-stimulated phosphoinositide 3-kinase association with IRS-1 was 2.3-fold higher, at 5 mM vs. 25 mM glucose. These effects of glucose were not caused by alterations in IRS-1 mass or cell-surface insulin binding. In preadipose cells at 5 mM glucose, expression of the leukocyte antigen-related (LAR) protein tyrosine phosphatase (negative regulator of insulin signaling) was 63% of the level at 25 mM glucose. Our data demonstrate that glucose concentration affects insulin-induced 3T3-L1 adipose cell differentiation as well as differentiation-directed insulin signaling pathways. Alterations in LAR expression potentially may be involved in modulating these responses.

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.

Chemotactic peptide-induced activation of Ras in human neutrophils is associated with inhibition of p120-GAP activity

The monomeric G-protein Ras is now considered to function as an initial regulator of multiple signaling pathways in both normal and transformed cell types. Adhesion and chemoattractant receptors are known to trigger activation of Ras in human neutrophils, but the signaling mechanism that activates Ras has only been partially elucidated. The present results show that in neutrophils, a time- and dose-dependent f-Met-Leu-Phe (FMLP)-induced activation of Ras is mediated by Gi2-proteins, because such activation is inhibited by pertussis toxin and because direct stimulation of heterotrimeric G-proteins with AlF4- is sufficient to activate Ras. Pretreatment of neutrophils with tyrosine kinase inhibitors, i.e. genistein or erbstatin that completely block FMLP-stimulated protein tyrosine phosphorylations, did not affect the FMLP-induced activation of Ras. Moreover, FMLP did not induce any detectable translocation of Grb2 and Sos to the plasma membrane of neutrophils. Other signaling molecules, such as protein kinase C, phosphatidylinositol 3-kinase and Ca2+, do not appear to be involved in the FMLP-induced Ras activation. Instead, stimulation of neutrophils with FMLP or C5a, the latter of which also activates Gi2-proteins, resulted in transient inhibition of the activity of Ras GTPase-activating proteins (GAP) with kinetics that correlated well with the kinetics of Ras activation. Moreover, decreased Ras-GAP activity was found in p120-GAP but not in neurofibromin immunoprecipitates of FMLP-stimulated cells. These results suggest that tyrosine kinase-dependent Ras exchange factors do not contribute to the FMLP-induced activation of Ras but that such activation is mediated via inhibition of p120-GAP in neutrophils.

Requirement of protein kinase C zeta for stimulation of protein synthesis by insulin

The ability of insulin to stimulate protein synthesis and cellular growth is mediated through the insulin receptor (IR), which phosphorylates Tyr residues in the insulin receptor substrate-signaling proteins (IRS-1 and IRS-2), Gab-1, and Shc. These phosphorylated substrates directly bind and activate enzymes such as phosphatidylinositol 3'-kinase (PI3K) and the guanine nucleotide exchange factor for p21Ras (GRB-2/SOS), which are in turn required for insulin-stimulated protein synthesis, cell cycle progression, and prevention of apoptosis. We have now shown that one or more members of the atypical protein kinase C group, as exemplified by the zeta isoform (PKC zeta), are downstream of IRS-1 and P13K and mediate the effect of insulin on general protein synthesis. Ectopic expression of constitutively activated PKC zeta eliminates the requirement of IRS-1 for general protein synthesis but not for insulin-stimulated activation of 70-kDa S6 kinase (p70S6K), synthesis of growth-regulated proteins (e.g., c-Myc), or mitogenesis. The fact that PKC zeta stimulates general protein synthesis but not activation of p70S6K indicates that PKC zeta activation does not involve the proto-oncogene Akt, which is also activated by PI3K. Yet insulin is still required for the stimulation of general protein synthesis in the presence of constitutively active PKC zeta and in the absence of IRS-1, suggesting a requirement for the convergence of the IRS-1/PI3K/PKC zeta pathway with one or more additional pathways emanating from the IR, e.g., Shc/SOS/p21Ras/mitogen-activated protein kinase. Thus, PI3K appears to represent a bifurcation in the insulin signaling pathway, one branch leading through PKC zeta to general protein synthesis and one, through Akt and the target of rapamycin (mTOR), to growth-regulated protein synthesis and cell cycle progression.

Insulin stimulates mitogen-activated protein kinase by a Ras-independent pathway in 3T3-L1 adipocytes

To characterize tissue-specific differences in insulin signaling, we compared the mechanisms of mitogen-activated protein (MAP) kinase activation by insulin in the mitogenically active 3T3-L1 fibroblasts with the metabolically active 3T3-L1 adipocytes. In both cell lines, insulin significantly increased p21(ras).GTP loading (1.5-2-fold) and MAP kinase activity (5-8-fold). Inhibition of Ras farnesylation with lovastatin blocked activation of p21(ras) and Raf-1 kinase in both 3T3-L1 fibroblasts and 3T3-L1 adipocytes. In 3T3-L1 fibroblasts, this was accompanied by an inhibition of the stimulatory effect of insulin on MAP kinase. In contrast, in 3T3-L1 adipocytes, despite an inhibition of activation of p21(ras) and Raf-1 by lovastatin, insulin continued to stimulate MAP kinase activity. Fractionation of the cell lysates on the FPLC Mono-Q column revealed that lovastatin inhibited insulin stimulation of ERK2 (and, to a lesser extent, ERK1) in 3T3-L1 fibroblasts and had no effect on the insulin-stimulated ERK2 in 3T3-L1 adipocytes. These results demonstrate an important distinction between the mechanism of insulin signaling in the metabolically and mitogenically active cells. Insulin activates MAP kinase by the Ras-dependent pathway in the 3T3-L1 fibroblasts and by the Ras-independent pathway in the 3T3-L1 adipocytes.