Stimulation of protein phosphatase-1 activity by phorbol esters. Evaluation of the regulatory role of protein kinase C in insulin action

Cofilin activation in pancreatic acinar cells plays a pivotal convergent role for mediating CCK-stimulated enzyme secretion and growth

Introduction: The actin regulatory protein, cofilin plays a key signaling role in many cells for numerous cellular responses including in proliferation, development, motility, migration, secretion and growth. In the pancreas it is important in islet insulin secretion, growth of pancreatic cancer cells and in pancreatitis. However, there are no studies on its role or activation in pancreatic acinar cells. Methods: To address this question, we studied the ability of CCK to activate cofilin in pancreatic acinar cells, AR42J cells and CCK1-R transfected Panc-1 cells, the signaling cascades involved and its effect on enzyme secretion and MAPK activation, a key mediator of pancreatic growth. Results: CCK (0.3 and 100 nM), TPA, carbachol, Bombesin, secretin and VIP decreased phospho-cofilin (i.e., activate cofilin) and both phospho-kinetic and inhibitor studies of cofilin, LIM kinase (LIMK) and Slingshot Protein Phosphatase (SSH1) demonstrated these conventional activators of cofilin were not involved. Serine phosphatases inhibitors (calyculin A and okadaic acid), however inhibited CCK/TPA-cofilin activation. Studies of various CCK-activated signaling cascades showed activation of PKC/PKD, Src, PAK4, JNK, ROCK mediated cofilin activation, but not PI3K, p38, or MEK. Furthermore, using both siRNA and cofilin inhibitors, cofilin activation was shown to be essential for CCK-mediated enzyme secretion and MAPK activation. Conclusion: These results support the conclusion that cofilin activation plays a pivotal convergent role for various cell signaling cascades in CCK mediated growth/enzyme secretion in pancreatic acini.

Insulin treatment promotes tyrosine phosphorylation of PKR and inhibits polyIC induced PKR threonine phosphorylation

Tyrosine phosphorylation of insulin receptor beta (IRβ) in insulin treated HepG2 cells is inversely correlated to ser(51) phosphorylation in the alpha-subunit of eukaryotic initiation factor 2 (eIF2α) that regulates protein synthesis. Insulin stimulates interaction between IRβ and PKR, double stranded RNA-dependent protein kinase, also known as EIF2AK2, and phosphorylation of tyrosine residues in PKR, as analyzed by immunoprecipitation and pull down assays using anti-IRβ and anti-phosphotyrosine antibodies, recombinant IRβ and immunopurified PKR. Further polyIC or synthetic double stranded RNA-induced threonine phosphorylation or activation of immunopurified and cellular PKR is suppressed in the presence of insulin treated purified IRβ and cell extracts. Acute, but not chronic, insulin treatment enhances tyrosine phosphorylation of IRβ, its interaction with PKR and tyrosine phosphorylation of PKR. In contrast, lipopolysaccharide that stimulates threonine phosphorylation of PKR and eIF2α phosphorylation and AG 1024, an inhibitor of the tyrosine kinase activity of IRβ, reduces PKR association with the receptor, IRβ in HepG2 cells. These findings therefore may suggest that tyrosine phosphorylated PKR plays a role in the regulation of insulin induced protein synthesis and in maintaining insulin sensitivity, whereas, suppression of polyIC-mediated threonine phosphorylation of PKR by insulin compromises its ability to fight against virus infection in host cells.

PKA and CaMKII mediate PI3K activation in bovine sperm by inhibition of the PKC/PP1 cascade

To enable fertilization, spermatozoa must undergo several biochemical processes in the female reproductive tract, collectively called capacitation. These processes involve protein kinase A (PKA)-dependent protein tyrosine phosphorylation including phosphatidylinositol-3-kinase (PI3K). It is not known how PKA, a serine/threonine (S/T) kinase, mediates tyrosine phosphorylation of proteins. We recently showed that inhibition of S/T phosphatase 1 (PP1) causes a significant increase in phospho-PI3K. In this study, we propose a mechanism by which PKA and PP1 mediate an increase in PI3K tyrosine phosphorylation and implicate calmodulin-dependent kinase II (CaMKII) in this process. Inhibition of sperm PP1 or PKC, stimulated CaMKII phosphorylation/activation, and inhibition of PKC enhanced PP1 phosphorylation/inactivation. Inhibition of CaMKII, using KN-93, caused significant reduction in phospho-PP1, indicating its activation. Moreover, KN-93 prevented the dephosphorylation/inactivation of PKC. We therefore suggest that CaMKII inhibits PKC, leading to PP1 inhibition and the reciprocal auto-activation of CaMKII. Thus, CaMKII can regulate its own activation by inhibiting the PKC/PP1 cascade. Inhibition of Src family kinases (SFK) caused significant inhibition of CaMKII and PP1 phosphorylation, suggesting that SFK activity results in PP1 inhibition and CaMKII activation. Activation of sperm PKA by 8Br-cAMP revealed an increase in phospho-CaMKII, which was inhibited by PKA inhibitor. Tyrosine phosphorylation of PI3K was stimulated by 8Br-cAMP and by PKC or PP1 inhibition and was abrogated by CaMKII inhibition. Furthermore, phosphorylation/activation of the tyrosine kinase Pyk2 was enhanced by PP1 inhibition, and this activation is blocked by CaMKII inhibition. Thus, PKA activates Src, which inhibits PP1, leading to CaMKII and Pyk2 activation, resulting in PI3K tyrosine phosphorylation/activation.

Protein kinase A and protein kinase C(alpha)/PPP1CC2 play opposing roles in the regulation of phosphatidylinositol 3-kinase activation in bovine sperm

In order to acquire fertilization competence, spermatozoa have to undergo biochemical changes in the female reproductive tract, known as capacitation. Signaling pathways that take place during the capacitation process are much investigated issue. However, the role and regulation of phosphatidylinositol 3-kinase (PI3K) in this process are still not clear. Previously, we reported that short-time activation of protein kinase A (PRKA, PKA) leads to PI3K activation and protein kinase C(alpha)(PRKCA, PKC(alpha)) inhibition. In the present study, we found that during the capacitation PI3K phosphorylation/activation increases. PI3K activation was PRKA dependent, and down-regulated by PRKCA. PRKCA is found to be highly active at the beginning of the capacitation, conditions in which PI3K is not active. Moreover, inhibition of PRKCA causes significant activation of PI3K. Similar activation of PI3K is seen when the phosphatase PPP1 is blocked suggesting that PPP1 regulates PI3K activity. We found that during the capacitation PRKCA and PPP1CC2 (PP1gamma2) form a complex, and the two enzymes were degraded during the capacitation, suggesting that this degradation enables the activation of PI3K. This degradation is mediated by PRKA, indicating that in addition to the direct activation of PI3K by PRKA, this kinase can enhance PI3K phosphorylation indirectly by enhancing the degradation and inactivation of PRKCA and PPP1CC2.

K-Cl cotransport function and its potential contribution to cardiovascular disease

K-Cl cotransport is the coupled electroneutral movement of K and Cl ions carried out by at least four protein isoforms, KCC1-4. These transporters belong to the SLC12A family of coupled cotransporters and, due to their multiple functions, play an important role in the maintenance of cellular homeostasis. Significant information exists on the overall function of these transporters, but less is known about the role of the specific isoforms. Most functional studies were done on K-Cl cotransport fluxes without knowing the molecular details, and only recently attention has been paid to the isoforms and their individual contribution to the fluxes. This review summarizes briefly and updates the information on the overall functions of this transporter, and offers some ideas on its potential contribution to the pathophysiological basis of cardiovascular disease. By virtue of its properties and the cellular ionic distribution, K-Cl cotransport participates in volume regulation of the nucleated and some enucleated cells studied thus far. One of the hallmarks in cardiovascular disease is the inability of the organism to maintain water and electrolyte balance in effectors and/or target tissues. Oxidative stress is another compounding factor in cardiovascular disease and of great significance in our modern life styles. Several functions of the transporter are modulated by oxidative stress, which in turn may cause the transporter to operate in either "overdrive" with the purpose to counteract homeostatic changes, or not to respond at all, again setting the stage for pathological changes leading to cardiovascular disease. Intracellular Mg, a second messenger, acts as an inhibitor of K-Cl cotransport and plays a crucial role in regulating the activity of protein kinases and phosphatases, which, in turn, regulate a myriad of cellular functions. Although the role of Mg in cardiovascular disease has been dealt with for several decades, this chapter is evolving nowadays at a faster pace and the relationships between Mg, K-Cl cotransport, and cardiovascular disease is an area that awaits further experimentation. We envision that further studies on the role of K-Cl cotransport, and ideally on its specific isoforms, in mammalian cells will add missing links and help to understand the cellular mechanisms involved in the pathophysiology of cardiovascular disease.

Protein Kinase C-induced Activation of a Ceramide/Protein Phosphatase 1 Pathway Leading to Dephosphorylation of p38 MAPK

Recently we showed that, in human breast cancer cells, activation of protein kinase C by 4beta-phorbol 12-myristate 13-acetate (PMA) produced ceramide formed from the salvage pathway (Becker, K. P., Kitatani, K., Idkowiak-Baldys, J., Bielawski, J., and Hannun, Y. A. (2005) J. Biol. Chem. 280, 2606-2612). In this study, we investigated intracellular signaling events mediated by this novel activated pathway of ceramide generation. PMA treatment resulted in transient activation of mitogen-activated protein kinases (ERK1/2, JNK1/2, and p38) followed by dephosphorylation/inactivation. Interestingly, fumonisin B1 (FB1), an inhibitor of the salvage pathway, attenuated loss of phosphorylation of p38, suggesting a role for ceramide in p38 dephosphorylation. This was confirmed by knock-down of longevity-assurance homologue 5, which partially suppressed the formation of C(16)-ceramide induced by PMA and increased the phosphorylation of p38. These results demonstrate a role for the salvage pathway in feedback inhibition of p38. To determine which protein phosphatases act in this pathway, specific knock-down of serine/threonine protein phosphatases was performed, and it was observed that knock-down of protein phosphatase 1 (PP1) catalytic subunits significantly increased p38 phosphorylation, suggesting activation of PP1 results in an inhibitory effect on p38. Moreover, PMA recruited PP1 catalytic subunits to mitochondria, and this was significantly suppressed by FB1. In addition, phospho-p38 resided in PMA-stimulated mitochondria. Upon PMA treatment, a mitochondria-enriched/purified fraction exhibited significant increases in C(16)-ceramide, a major ceramide specie, which was suppressed by FB1. Taken together, these data suggest that accumulation of C(16)-ceramide in mitochondria formed from the protein kinase C-dependent salvage pathway results at least in part from the action of longevity-assurance homologue 5, and the generated ceramide modulates the p38 cascade via PP1.

Protein kinase Cdelta participates in insulin-induced activation of PKB via PDK1

PKCdelta has been shown to be activated by insulin and to interact with insulin receptor and IRS. PKB(Akt) plays an important role in glucose transport and glycogen synthesis. In this study, we investigated the possibility that PKCdelta may be involved in insulin-induced activation of PKB. Studies were conducted on primary cultures of rat skeletal muscle. PKB was activated by insulin stimulation within 5min and reached a peak by 15-30min. Insulin also increased the physical association between PKCdelta with PKB and with PDK1. The insulin-induced PKCdelta-PKB association was PI3K dependent. PKB-PKCdelta association was accounted for by the involvement of PDK1. Overexpression of dominant negative PKCdelta abrogated insulin-induced association of PKCdelta with both PKB and PDK1. Blockade of PKCdelta also decreased insulin-induced Thr308 PKB phosphorylation and PKB translocation. Moreover, PKCdelta inhibition reduced insulin-induced GSK3 phosphorylation. The results indicate that insulin-activated PKCdelta interacts with PDK1 to regulate PKB.

Protein kinase Calpha regulates insulin receptor signaling in skeletal muscle

Certain PKC isoforms are stimulated by insulin and interact with IR as well as with IRS, but it is still not clear if specific PKC isoforms regulate IR signaling directly or through IRS-1. PKCalpha may regulate IRS activity in response to insulin. We investigated the possibility that PKCalpha may be important in insulin signaling. Studies were conducted on skeletal muscle in adult mice and on L6 skeletal cells. PKCalpha is constitutively associated with IRS-1, and insulin stimulation of PKCalpha causes disassociation of the two proteins within 5 min. Blockade of PKCalpha inhibited insulin-induced disassociation of PKCalpha from IRS1. Selective inhibition of PKCalpha increased the ability of insulin to reduce blood glucose levels. Insulin stimulation activates PKB and increases the association of PKCalpha with PKB. Blockade of PKCalpha increased threonine phosphorylation of PKB. We suggest that PKCalpha regulates insulin signaling in skeletal muscle through its disassociation from IRS-1 and association with PKB.

Signal transduction mechanisms of K+-Cl- cotransport regulation and relationship to disease

The K+-Cl- cotransport (COT) regulatory pathways recently uncovered in our laboratory and their implication in disease state are reviewed. Three mechanisms of K+-Cl- COT regulation can be identified in vascular cells: (1) the Li+-sensitive pathway, (2) the platelet-derived growth factor (PDGF)-sensitive pathway and (3) the nitric oxide (NO)-dependent pathway. Ion fluxes, Western blotting, semi-quantitative RT-PCR, immunofluorescence and confocal microscopy were used. Li+, used in the treatment of manic depression, stimulates volume-sensitive K+-Cl- COT of low K+ sheep red blood cells at cellular concentrations <1 mM and inhibits at >3 mM, causes cell swelling, and appears to regulate K+-Cl- COT through a protein kinase C-dependent pathway. PDGF, a potent serum mitogen for vascular smooth muscle cells (VSMCs), regulates membrane transport and is involved in atherosclerosis. PDGF stimulates VSM K+-Cl- COT in a time- and concentration-dependent manner, both acutely and chronically, through the PDGF receptor. The acute effect occurs at the post-translational level whereas the chronic effect may involve regulation through gene expression. Regulation by PDGF involves the signalling molecules phosphoinositides 3-kinase and protein phosphatase-1. Finally, the NO/cGMP/protein kinase G pathway, involved in vasodilation and hence cardiovascular disease, regulates K+-Cl- COT in VSMCs at the mRNA expression and transport levels. A complex and diverse array of mechanisms and effectors regulate K+-Cl- COT and thus cell volume homeostasis, setting the stage for abnormalities at the genetic and/or regulatory level thus effecting or being affected by various pathological conditions.

Insulin rapidly upregulates protein kinase Cdelta gene expression in skeletal muscle

Recent studies in our laboratories have shown that Protein Kinase C delta (PKCdelta) is essential for insulin-induced glucose transport in skeletal muscle, and that insulin rapidly stimulates PKCdelta activity skeletal muscle. The purpose of this study was to examine mechanisms of regulation of PKCdelta protein availability. Studies were done on several models of mammalian skeletal muscle and utilized whole cell lysates of differentiated myotubes. PKCdelta protein levels were determined by Western blotting techniques, and PKCdelta RNA levels were determined by Northern blotting, RT-PCR and Real-Time RT-PCR. Insulin stimulation increased PKCdelta protein levels in whole cell lysates. This effect was not due to an inhibition by insulin of the rate of PKCdelta protein degradation. Insulin also increased 35S-methionine incorporation into PKCdelta within 5-15 min. Pretreatment of cells with transcription or translation inhibitors abrogated the insulin-induced increase in PKCdelta protein levels. We also found that insulin rapidly increased the level of PKCdelta RNA, an effect abolished by inhibitors of transcription. The insulin-induced increase in PKCdelta expression was not reduced by inhibition of either PI3 Kinase or MAP kinase, indicating that these signaling mechanisms are not involved, consistent with insulin activation of PKCdelta. Studies on cells transfected with the PKCdelta promoter demonstrate that insulin activated the promoter within 5 min. This study indicates that the expression of PKCdelta may be regulated in a rapid manner during the course of insulin action in skeletal muscle and raise the possibility that PKCdelta may be an immediate early response gene activated by insulin.

Interleukin-6 increases rat metalloproteinase-13 gene expression through Janus kinase-2-mediated inhibition of serine/threonine phosphatase-2A

Interleukin-6 (IL-6) increases metalloproteinase-13 (MMP-13) gene expression by increasing phosphorylated c-Jun and by inhibiting serine/threonine phosphatase-2A (PP2A) activity. We investigated the mechanisms by which IL-6 induces c-Jun phosphorylation and PP2A inactivation in Rat-1 fibroblasts. We show that IL-6 increased MMP-13 mRNA, phosphorylated c-Jun, and activator protein 1 (AP1) binding activity without increasing c-Jun-N-terminal kinase (JNK) activity. These effects did not seem to be mediated by ERK, p38 MAP kinase, phosphatidylinositol-3-kinase, calmoduline-dependent protein kinase, protein kinase C (PKC) or protein kinase A since inhibition with specific inhibitors did not abrogate these effects. IL-6 increases PP2A catalytic subunit tyrosine phosphorylation. Inhibition of the tyrosine kinase Jak2, with the specific inhibitor AG490, abrogated this effect. Likewise, this Jak2 inhibitor blocked the effects of IL-6 on c-Jun phosphorylation, AP1 binding activity and metalloproteinase-13 gene expression. We conclude that IL-6 increases MMP-13 gene expression by activation of Jak2, resulting in tyrosine phosphorylation of the catalytic subunit of PP2A, which in turn decreases PP2A activity and prolongs c-Jun phosphorylation.

Phorbol esters increase MLC phosphorylation and actin remodeling in bovine lung endothelium without increased contraction

Direct protein kinase C (PKC) activation with phorbol myristate acetate (PMA) results in the loss of endothelial monolayer integrity in bovine lung endothelial cells (EC) but produces barrier enhancement in human lung endothelium. To extend these findings, we studied EC contractile events and observed a 40% increase in myosin light chain (MLC) phosphorylation in bovine endothelium following PMA challenge. The increase in PMA-mediated MLC phosphorylation occurred at sites distinct from Ser19/Thr18, sites catalyzed by MLC kinase (MLCK), and immunoblotting with antibodies specific to phosphorylated Ser19/Thr18 demonstrated profound time-dependent Ser19/Thr18 dephosphorylation. These events occurred in conjunction with rearrangement of stress fibers into a grid-like network, but without an increase in cellular contraction as measured by silicone membrane wrinkling assay. The PMA-induced MLC dephosphorylation was not due to kinase inhibition but, rather, correlated with rapid increases in myosin-associated phosphatase 1 (PPase 1) activity. These data suggest that PMA-mediated EC barrier regulation may involve dual mechanisms that alter MLC phosphorylation. The increase in bovine MLC phosphorylation likely occurs via direct PKC-dependent MLC phosphorylation in conjunction with decreases in Ser19/Thr18 phosphorylation catalyzed by MLCK due to PMA-induced increases in PPase 1 activity. Together, these events result in stress fiber destabilization and profound actin rearrangement in bovine endothelium, which may result in the physiological alterations observed in these models.

Initiation factor 2B activity is regulated by protein phosphatase 1, which is activated by the mitogen-activated protein kinase-dependent pathway in insulin-like growth factor 1-stimulated neuronal cells

We have previously demonstrated that insulin-like growth factor 1 (IGF1) induces eukaryotic initiation factor 2B (eIF2B) activation in neuronal cells through the phosphatidylinositol 3 kinase/glycogen synthase kinase 3 pathway as well as by activation of the mitogen-activated protein kinase (MAPK)-activating kinase (MEK)/MAPK signaling pathway (Quevedo, C., Alcázar, A., and Salinas, M. (2000) J. Biol. Chem. 275, 19192-19197). This paper addresses the mechanism involved in IGF1-induced eIF2B activation via the MEK/MAPK cascade in cultured neurons treated with IGF1 and demonstrates that extracellular signal-regulated MAP kinase 1 and 2 (ERK1 and -2) immunoprecipitates of IGF1-treated neuronal cells promote this activation. This effect did not directly result from eIF2B phosphorylation by ERK immunoprecipitates. In addition, recombinant ERK1 and -2 neither activate eIF2B nor phosphorylate it. Endogenous protein phosphatase 1 and 2A catalytic subunits (PP1C and PP2AC, respectively) were co-immunoprecipitated with ERK1 and -2, and the association of ERK with PP1C was stimulated by IGF1 treatment, resulting in increased PP1 activity. ERK immunoprecipitates incubated with PP1 inhibitors did not activate eIF2B, indicating that PP1C activates eIF2B. In vitro experiments with phosphorylated eIF2B showed that recombinant PP1C (alpha isoform) dephosphorylates and activates eIF2B. Paralleling eIF2B activation, IGF1 treatment induced PP1 activation in a MEK/MAPK-dependent fashion. Moreover, the treatment of neurons with the PP1 inhibitor tautomycin inhibited PP1 activation and prevented IGF1-induced eIF2B activation. These findings strongly suggest that IGF1-induced eIF2B activation in neurons is effected by PP1, the activation of which is mediated by the MEK/MAPK signaling pathway.

Differential effects of tumor necrosis factor-alpha on protein kinase C isoforms alpha and delta mediate inhibition of insulin receptor signaling

Tumor necrosis factor-alpha (TNF-alpha) is a multifunctional cytokine that interferes with insulin signaling, but the molecular mechanisms of this effect are unclear. Because certain protein kinase C (PKC) isoforms are activated by insulin, we examined the role of PKC in TNF-alpha inhibition of insulin signaling in primary cultures of mouse skeletal muscle. TNF-alpha, given 5 min before insulin, inhibited insulin-induced tyrosine phosphorylation of insulin receptor (IR), IR substrate (IRS)-1, insulin-induced association of IRS-1 with the p85 subunit of phosphatidylinositol 3-kinase (PI3-K), and insulin-induced glucose uptake. Insulin and TNF-alpha each caused tyrosine phosphorylation and activation of PKCs delta and alpha, but when TNF-alpha preceded insulin, the effects were less than that produced by each substance alone. Insulin induced PKCdelta specifically to coprecipitate with IR, an effect blocked by TNF-alpha. Both PKCalpha and -delta are constitutively associated with IRS-1. Whereas insulin decreased coprecipitation of IRS-1 with PKCalpha, it increased coprecipitation of IRS-1 with PKCdelta. TNF-alpha blocked the effects of insulin on association of both PKCs with IRS-1. To further investigate the involvement of PKCs in inhibitory actions of TNF-alpha on insulin signaling, we overexpressed specific PKC isoforms in mature myotubes. PKCalpha overexpression inhibited basal and insulin-induced IR autophosphorylation, whereas PKCdelta overexpression increased IR autophosphorylation and abrogated the inhibitory effect of TNF-alpha on IR autophosphorylation and signaling to PI3-K. Blockade of PKCalpha antagonized the inhibitory effects of TNF-alpha on both insulin-induced IR tyrosine phosphorylation and IR signaling to PI3-K. We suggest that the effects of TNF-alpha on IR tyrosine phosphorylation are mediated via alteration of insulin-induced activation and association of PKCdelta and -alpha with upstream signaling molecules.

Protein kinase C (PKC)-alpha activation inhibits PKC-zeta and mediates the action of PED/PEA-15 on glucose transport in the L6 skeletal muscle cells

Overexpression of the PED/PEA-15 protein in muscle and adipose cells increases glucose transport and impairs further insulin induction. Like glucose transport, protein kinase C (PKC)-alpha and -beta are also constitutively activated and are not further stimulatable by insulin in L6 skeletal muscle cells overexpressing PED (L6(PED)). PKC-zeta features no basal change but completely loses insulin sensitivity in L6(PED). In these cells, blockage of PKC-alpha and -beta additively returns 2-deoxy-D-glucose (2-DG) uptake to the levels of cells expressing only endogenous PED (L6(WT)). Blockage of PKC-alpha and -beta also restores insulin activation of PKC-zeta in L6(PED) cells, with that of PKC-alpha sixfold more effective than PKC-beta. Similar effects on 2-DG uptake and PKC-zeta were also achieved by 50-fold overexpression of PKC-zeta in L6(PED). In L6(WT), fivefold overexpression of PKC-alpha or -beta increases basal 2-DG uptake and impairs further insulin induction with no effect on insulin receptor or insulin receptor substrate phosphorylation. In these cells, overexpression of PKC-alpha blocks insulin induction of PKC-zeta activity. PKC-beta is 10-fold less effective than PKC-alpha in inhibiting PKC-zeta stimulation. Expression of the dominant-negative K(281)-->W PKC-zeta mutant simultaneously inhibits insulin activation of PKC-zeta and 2-DG uptake in the L6(WT) cells. We conclude that activation of classic PKCs, mainly PKC-alpha, inhibits PKC-zeta and may mediate the action of PED on glucose uptake in L6 skeletal muscle cells.

A dual role of protein kinase C in insulin signal transduction via adenylyl cyclase signaling system in muscle tissues of vertebrates and invertebrates

Further decoding of a novel adenylyl cyclase signaling mechanism (ACSM) of the action of insulin and related peptides detected earlier (Pertseva et al. Comp Biochem Physiol B Biochem Mol Biol 1995;112:689-95 and Pertseva et al. Biochem Pharmacol 1996;52:1867-74) was carried out with special attention given to the role of protein kinase C (PKC) in the ACSM. It was shown for the first time that transduction of the insulin signal via the ACSM followed by adenylyl cyclase (AC, EC 4.6.1.1) activation was blocked in the muscle tissues of rat and mollusc Anodonta cygnea in the presence of pertussis toxin, inducing the impairment of G(i)-protein function, wortmannin, an inhibitor of phosphatidylinositol 3-kinase (PI3-K), and calphostin C, a blocker of PKC. The cholera toxin treatment of muscle membranes led to an increase in basal AC activity and a decrease in enzyme insulin reactivity. Phorbol ester and diacylglycerol activation of PKC (acute treatment) induced the inhibition of the insulin AC activating effect. This negative influence was also observed in the case of the AC system activated by biogenic amines. It was first concluded that the ACSM of insulin action involves the following signaling chain: receptor tyrosine kinase => G(i) (betagamma) => PI3-K => PKCzeta (?) => G(s) => AC => adenosine 3',5'-cyclic monophosphate. It was also concluded that the PKC system has a dual role in the ACSM: (1) a regulatory role (PKC sensitive to phorbol esters) that is manifested as a negative feedback modulation of insulin signal transduction via the ACSM; (2) a transductory role, which consists in direct participation of atypical PKC (PKCzeta) in the process of insulin signal transduction via the ACSM.

Down-regulation of Na-K-Cl cotransport by protein kinase C is mediated by protein phosphatase 1 in pigmented ciliary epithelial cells

The role of protein phosphatases in the regulation of Na-K-Cl cotransport was examined in human pigmented ciliary epithelial (PE) cells. Both a 37 kDa form and a 72 kDa form of protein phosphatase 1 (PP1) could be immunologically detected. The protein phosphatase inhibitor calyculin A stimulated Na-K-Cl cotransport by 89 +/- 12% at 10 n M, whereas okadaic acid had no effect at concentrations less than 100 n M. Calyculin A had no significant effect on either Na-K ATPase or ouabain-insensitive, bumetanide-insensitive 86Rb+uptake. These data suggest that PP1 plays a role in the inhibition of Na-K-Cl cotransport in PE cells. Treatment of cells with phorbol 12-myristate, 13-acetate (PMA), a protein kinase C (PKC) activator caused an 82% inhibition of Na-K-Cl cotransport. When cells were first treated for 5 min with PMA, 10 n M calyculin A stimulated Na-K-Cl cotransport by 53% compared to 101% by calyculin A alone. Treatment of cells with PMA after stimulation of Na-K-Cl cotransport by calyculin A resulted in a prompt 56% drop in cotransport activity. These data suggest that maximal inhibition of Na-K-Cl cotransport by PKC requires PP1 activity, but that a part of PKCs inhibitory effect is independent of PP1. The effect of PKC activation on PP1 was further examined by determining PP1 activity in cells pretreated with PMA. PP1 activity increased 38+/-8% in cells exposed to 1 microM PMA for 5 min. This stimulation was blocked by 100 n M staurosporine or 1 microM bisindolylmaleimide, two PKC inhibitors. An isomer which does not activate PKC (4 alpha phorbol didecanoate), did not stimulate PP1 activity. Thus PKC activation leads to an increase in PP1 activity in PE cells. Pretreatment of cells with the protein kinase A (PKA) inhibitor PHI 14-22 resulted in a partial reduction in calyculin A stimulation of cotransport, suggesting that PP1 and PKA function in a kinase-phosphatase regulatory loop. To determine whether other protein kinases might also be involved, several protein kinase inhibitors were tested, including KT5823 (protein kinase G, type II-specific), KN62 (calmodulin activated kinase-specific) and ML7 (myosin light chain kinase-specific). None prevented activation of Na-K-Cl cotransport by calyculin A, suggesting that these kinases are not involved in the activation of Na-K-Cl cotransport.

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.

The transcription factor nuclear factor I mediates repression of the GLUT4 promoter by insulin

Insulin represses GLUT4 expression in 3T3-L1 adipocytes through an insulin response element located at bases -706 to -676 in the 5'-flanking sequence. Nuclear proteins related to the nuclear factor I (NF1) family of transcription factors bind to this insulin response element. Mutations that disrupt binding of NF1 proteins to the insulin response element impair the insulin response in reporter gene assays. Insulin treatment of 3T3-L1 adipocytes induces a rapid change in the level of phosphorylation of NF1 proteins, providing a potential mechanism for insulin's ability to regulate gene expression through NF1. Another as yet unidentified protein, not related to NF1, also binds to the GLUT4 insulin response element and is able to mediate partial repression of the GLUT4 promoter in reporter gene assays.

Activation of a metabotropic glutamate receptor and protein kinase C reduce the extent of inactivation of the K+ channel Kv1.1/Kvbeta1.1 via dephosphorylation of Kv1.1

Various brain K+ channels, which may normally exist as complexes of alpha (pore-forming) and beta (auxiliary) subunits, were subjected to regulation by metabotropic glutamate receptors. Kv1.1/Kvbeta1.1 is a voltage-dependent K+ channel composed of alpha and beta proteins that are widely expressed in the brain. Expression of this channel in Xenopus oocytes resulted in a current that had fast inactivating and noninactivating components. Previously we showed that basal and protein kinase A-induced phosphorylation of the alpha subunit at Ser-446 decreases the fraction of the noninactivating component. In this study we investigated the effect of protein kinase C (PKC) on the channel. We showed that a PKC-activating phorbol ester (phorbol 12-myristate 13-acetate (PMA)) increased the noninactivating fraction via activation of a PKC subtype that was inhibited by staurosporine and bisindolylmaleimide but not by calphostin C. However, it was not a PKC-induced phosphorylation but rather a dephosphorylation that mediated the effect. PMA reduced the basal phosphorylation of Ser-446 significantly in plasma membrane channels and failed to affect the inactivation of channels having an alpha subunit that was mutated at Ser-446. Also, the activation of coexpressed mGluR1a known to activate phospholipase C mimicked the effect of PMA on the inactivation via induction of dephosphorylation at Ser-446. Thus, this study identified a potential neuronal pathway initiated by activation of metabotropic glutamate receptor 1a coupled to a signaling cascade that possibly utilized PKC to induce dephosphorylation and thereby to decrease the extent of inactivation of a K+ channel.

Regulation of prostaglandin H2 synthase-2 expression in primary human amnion cells by tyrosine kinase dependent mechanisms

Prostaglandin H2 synthase (PGHS)-1 and PGHS-2 expression was examined in primary cultures of human amnion cells, an in vitro model of amnion tissue. Epidermal growth factor (EGF), the protein kinase C (PKC) activating phorbol ester TPA, and the protein phosphatase inhibitor, okadaic acid (OA), stimulated PGHS activity and the level of PGHS-2 mRNA, but did not affect the level of PGHS-1 mRNA. In situ hybridization suggested that the same population of cells responded to EGF, TPA and OA. Okadaic acid promoted PGHS activity independently of PKC. EGF stimulated the activity of extracellular signal-regulated protein kinase (Erk) and N-terminal c-Jun kinase (Jnk). OA increased Jnk activity but had no effect on Erk activity, while TPA had no influence on either Erk or Jnk activity. PD098059, a selective inhibitor of the Erk-activating kinase MEK, blocked the stimulation of PGHS expression by EGF, but did not decrease stimulation in response to OA. Herbimycin A, a tyrosine kinase inhibitor, suppressed the stimulation of PGHS activity and PGHS-2 mRNA abundance by all three stimulants, and blocked signalling via the Erk and Jnk mitogen-activated protein kinase pathways. Thus, growth factor stimulation, PKC activation and protein phosphatase inhibition induced the expression of PGHS-2 in primary amnion cells by distinct regulatory mechanisms involving tyrosine kinase(s). Tyrosine kinase inhibitors may constitute a new category of PGHS-2 inhibitors that act by blocking the expression of the enzyme.

Insulin regulates protein kinase CbetaII expression through enhanced exon inclusion in L6 skeletal muscle cells. A novel mechanism of insulin- and insulin-like growth factor-i-induced 5' splice site selection

The protein kinase Cbeta (PKCbeta) gene encodes two isoforms, PKCbetaI and PKCbetaII, as a result of alternative splicing. The unique mechanism that underlies insulin-induced alternative splicing of PKCbeta pre-mRNA was examined in L6 myotubes. Mature PKCbetaII mRNA and protein rapidly increased >3-fold following acute insulin treatment, while PKCbetaI mRNA and protein levels remained unchanged. Mature PKCbetaII mRNA resulted from inclusion of the PKCbetaII-specific exon rather than from selection of an alternative polyadenylation site. Increased PKCbetaII expression was also not likely accounted for by transcriptional activation of the gene or increased stabilization of the PKCbetaII mRNA, and suggest that PKCbetaII expression is regulated primarily at the level of alternative splicing. Insulin effects on exon inclusion were observed as early as 15 min after insulin treatment; by 20 min, a new 5'-splice site variant of PKCbetaII was also observed. After 30 min, the longer 5'-splice site variant became the predominate species through activation of a downstream 5' splice site. Similar results were obtained using IGF-I. Although the role of this new PKCbetaII mRNA species is presently unknown, inclusion of either PKCbetaII-specific exon results in the same PKCbetaII protein.

Altered regulation of insulin signaling components in adipocytes of insulin-resistant type II diabetic Goto-Kakizaki rats

We investigated the cellular mechanism(s) of insulin resistance associated with non-insulin-dependent diabetes mellitus (NIDDM) using adipocytes isolated from non-obese, insulin-resistant type II diabetic Goto-Kakizaki (GK) rats, a well-known genetic rat model for type II diabetic humans. In adipocytes isolated from control rats, insulin (5 nmol/L) stimulated particulate serine/threonine protein phosphatase-1 (PP-1) activity (56% increase over the basal value after 5 minutes). In contrast, adipocytes from diabetic GK rats exhibited a 32% decrease in basal (P < .05) and a 65% decrease in insulin-stimulated PP-1 activity compared with values in control Wistar rats. Conversely, cytosolic PP-2A activity was elevated in diabetic GK rats in the basal state (twofold increase v controls, P < .05). Insulin treatment resulted in a 50% to 60% inhibition in PP-2A activity in control rats, but failed to inhibit PP-2A activity in diabetic GK rat adipocytes. The defects in PP-1/PP-2A activation/inactivation were accompanied by inhibition of insulin's effect on mitogen-activated protein kinase (MAPK) activation. In addition, insulin-stimulated tyrosine phosphorylation of insulin receptor (IR) substrate-1 (IRS-1) was decreased more than 90% compared with control values, while a twofold increase in basal IRS-1 phosphorylation status was observed in diabetic GK rats. The abnormalities in IRS-1 phosphorylation were accompanied by a severe impairment of insulin-mediated targeting of the Grb2/Sos complex to the plasma membrane. We conclude that (1) a rapid activation of PP-1 along with concomitant inhibition of cytosolic PP-2A may be important in the mechanism of insulin action in a normal cell, and (2) the resistance to insulin in terms of glucose uptake and glycogen synthesis observed in diabetic GK rats is partly due to defective regulation of PP-1, PP-2A, and MAPK caused by multiple defects in the upstream insulin signaling components (IRS-1/phosphatidylinositol-3-kinase [PI3-kinase] and Grb2/Sos) that participate in insulin-mediated activation of PP-1 and inactivation of PP-2A.

The possible role of protein phosphatase 2A in the sodium sensitivity of the receptor binding of opiate antagonists naloxone and naltrindole

In striatal membrane preparation used for receptor binding experiments high levels of protein phosphatase 1 and 2A activities were detected using [32P]phosphorylase a as substrate. Sodium chloride decreased the activity of protein phosphatase 2A and increased the activity of protein phosphatase 1 in a concentration-dependent manner. Sodium chloride facilitated the saturation binding of naloxone and naltrindole in rat striatal membrane preparation preincubated with ATP (50 microM) and MgCl2 (5 mM). Preincubation with calyculin A (1 nM) further increased the binding of naloxone. Addition of okadaic acid in a concentration of 2 nM, which is specific for the inhibition of protein phosphatase 2A, augmented the number of binding sites of naloxone or naltrindole. The results suggest a protein phosphatase-dependent regulation of the binding of opiate ligands in the striatum.

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.

Role of serine/threonine protein phosphatases in insulin regulation of Na+/K+-ATPase activity in cultured rat skeletal muscle cells

In this study, we examined the potential role of serine/threonine protein phosphatase-1 (PP-1) and PP-2A in the mechanism of Na+/K+-ATPase activation by insulin in the rat skeletal muscle cell line L6. Incubation of L6 cells with insulin caused a time- and dose-dependent stimulation of ouabain-sensitive plasma membrane Na+/K+-ATPase activity. Pretreatment with okadaic acid (OA; 0.1-1 microM) or calyculin A (1 microM) blocked insulin's effect on Na+/K+-ATPase activation. Low concentrations of OA that specifically inhibit PP-2A were ineffective. Immunoprecipitation of the enzyme from 32P-labeled cells with an antibody directed against the alpha-1 subunit of the enzyme revealed a 60% decrease in 110-kDa protein phosphorylation in insulin-treated cells. The presence of calyculin A blocked insulin-mediated dephosphorylation of Na+/K+-ATPase, whereas low concentrations of OA were ineffective. To further confirm the role of PP-1, we used L6 cell lines that overexpress the glycogen/SR-associated regulatory subunit of PP-1, PP-1G. Overexpression of PP-1G resulted in a 3-fold increase in insulin-stimulated PP-1 catalytic activity. This was accompanied by a 30% increase in basal Na+/K+-ATPase activity and a >2-fold increase in insulin's effect on pump activity. Inhibition of phosphatidylinositol-3 kinase with wortmannin blocked insulin-stimulated PP-1 activation as well as the dephosphorylation and activation of Na+/K+-ATPase. We conclude that insulin regulates the activity of Na+/K+-ATPase by promoting dephosphorylation of the alpha subunit via an insulin-stimulated PP-1 and that phosphatidylinositol-3 kinase-generated signals may mediate insulin activation of PP-1 and Na+/K+-ATPase.

Tumor necrosis factor-alpha and interleukin-1 induce activation of MAP kinase and SAP kinase in human neuroma fibroblasts

Two cytokines, tumor necrosis factor-alpha (TNF-alpha) and interleukin-1 (IL-1), which are released by macrophages during the early inflammatory phase of nerve injury, are known to induce activation of mitogen-activated protein kinase (MAPK) and stress-activated protein kinase (SAPK), which locate at different signal transduction pathways and are involved in cell cycle G0/G1 transition and cellular proliferation in human fibroblasts. Activation of these two protein kinases by the cytokines may stimulate fibroblast proliferation in damaged nerves and thereby play a role in the formation of a neuroma, a disorganized mass of tissue that interferes with neural regeneration and repair. To investigate the possibility that this mechanism is operative in neuroma formation, we used cultured, serum-starved fibroblasts from surgically removed human neuromas stimulated with TNF-alpha and/or IL-1 alpha and IL-1 beta, and measured the activation of MAPK and SAPK using myelin basic protein (MBP) and human c-Jun (1-169) glutathione S-agarose transferase (GST) fusion protein as substrates. For comparison, neuroma fibroblast cultures were also stimulated with phorbol 12-myristate 13-acetate (PMA) and platelet-derived growth factor-AB (PDGF-AB), a potent activator for MAPK. TNF-alpha and both forms of IL-1 produced a rapid activation of MAPK, with a peak at 15 min for TNF-alpha stimulation, and a peak at 30 min for IL-1 stimulation. TNF-alpha combined with either IL-1 alpha or IL-1 beta produced a synergistic effect on the activation of MAPK. The increases in MAPK induced by TNF-alpha and IL-1 were similar to the increases induced by PMA and PDGF-AB. To confirm the presence of MAPK, immunoprecipitation and immunoblotting were carried out on experimental and control lysates. TNF-alpha and IL-1 also increased activation of SAPK, but to a lesser extent than MAPK. PMA and PDGF-AB were also much less effective in stimulating activation of SAPK. Our findings indicate that TNF-alpha and IL-1 activate parallel signal transduction pathways in human neuroma fibroblasts, and that they are relatively stronger activators of MAPK than of SAPK. Previous studies have convincingly demonstrated that MAPK and SAPK are involved in human fibroblast proliferation. The results of our study suggest that TNF-alpha and IL-1 may play a role in frustrating functional nerve regeneration after injury by stimulating these two kinases, which, in turn, leads to fibroblast proliferation and formation of neuromas.

Contribution of protein kinase C to the phosphorylation of rhodopsin in intact retinas

Similar to other G protein-coupled receptors, the visual receptor, rhodopsin, is phosphorylated by both a substrate-regulated kinase, rhodopsin kinase, and a second messenger-regulated kinase, protein kinase C. In the present study, the extent of involvement of protein kinase C in the light-dependent phosphorylation of rhodopsin in intact retinas was assessed using a specific activator (phorbol ester) and specific inhibitor (calphostin C) of protein kinase C. Kinetic analysis of rhodopsin phosphorylation following different illumination conditions revealed that hyperactivation of protein kinase C with phorbol ester resulted in a relative increase in rhodopsin phosphorylation that peaked 10-15 min after the onset of illumination. Following this period, the rate of rhodopsin dephosphorylation was increased in the phorbol ester-treated retinas, so that by about 30 min the amount of phosphorylation was similar to that in control retinas. Treatment of retinas with calphostin C, a potent regulatory domain-directed inhibitor of protein kinase C, resulted in an approximately 50% reduction in the light-dependent phosphorylation of rhodopsin. This inhibitor had no effect on the activity of rhodopsin kinase in vitro. Last, we show that frog rhodopsin is phosphorylated in vitro by protein kinase C from frog rod outer segments, indicating that this kinase could directly modulate rhodopsin in vivo. In conclusion, the present results reveal that the kinetics of rhodopsin phosphorylation/dephosphorylation differ markedly, depending on whether protein kinase C or rhodopsin kinase activity dominates, and that, under the conditions studied, protein kinase C contributes to approximately half of the phosphorylation of rhodopsin in intact frog retinas.

The role of protein kinase C in cellular tolerance to ethanol

We have shown that ethanol inhibits uptake of adenosine by a specific nucleoside transporter in NG108-15 neuroblastoma x glioma cells and that cAMP-dependent protein kinase (PKA) activity is required for this inhibition. After chronic exposure to ethanol, adenosine uptake is no longer inhibited on rechallenge with ethanol, i.e. transport has become tolerant to ethanol. Here we show that protein kinase C (PKC) contributes to ethanol-induced tolerance of adenosine transport. Activation of PKC by phorbol esters in control cells results in an ethanol-tolerant phenotype, similar to that produced by chronic ethanol exposure. In addition, chronic exposure to ethanol increases the amounts of alpha, delta, and epsilon PKC. However, reducing PKC activity by inhibition with chelerythrine during chronic exposure to ethanol or down-regulation by phorbol esters prevents the development of ethanol-induced tolerance of adenosine transport. By contrast, the inhibition of PKA activity produces tolerance to ethanol inhibition of adenosine uptake. When protein phosphatase inhibitors are present, inhibiting PKA activity has no effect on ethanol sensitivity of adenosine uptake, suggesting a role for protein phosphatases in the regulation of ethanol sensitivity of uptake. Taken together, our results suggest that PKA and PKC have opposing effects on the ethanol sensitivity of adenosine transport; PKA activity is required for ethanol sensitivity, and PKC activation produces tolerance. Based on these data, we propose that chronic ethanol exposure increases PKC activity, leading to the activation of a protein phosphatase (1 or 2A). This phosphatase then dephosphorylates a PKA-phosphorylated site, which is required for ethanol to inhibit adenosine uptake. Therefore, the sensitivity of adenosine transport to ethanol appears to be maintained by a balance of PKA and protein phosphatase activities, and PKC may regulate phosphatase activity.

Phosphorylation/dephosphorylation steps are key events in the phytochrome-mediated enhancement of nitrate reductase mRNA levels and enzyme activity in maize

We provide evidence to show that the increase in nitrate reductase (NR) transcript level stimulated by red light is mediated via a phosphorylation-dependent step. The light-stimulated enhancement of NR transcript level was significantly inhibited by H-7, a protein kinase inhibitor, whereas okadaic acid (OKA), a phosphatase inhibitor, had no effect. Phorbol myristate acetate (PMA), an activator of protein kinase C (PKC) enhanced the NR transcript level in dark-grown leaves. No correlation between changes in NR transcript level and NR activity (NRA) was observed. Inhibition of NRA by OKA and stimulation by H-7 indicated that NRA is increased by dephosphorylating the enzyme. We have identified a protein kinase (C type) that can phosphorylate the purified NR in vitro without the involvement of other accessory proteins. By in vivo labelling with 32P and immunoprecipitation of NR with NR antibodies it was found that in the presence of OKA most NR protein (NRP) was present in phosphorylated state, while with H-7 the reverse was seen. The red (R) and far-red (FR) light reversible experiments suggested that phytochrome (Pfr, an active form) stimulation of NRA is mediated by dephosphorylation of the enzyme, suggesting that Pfr regulates both NR transcription and NRA via phosphorylation/dephosphorylation steps controlled by separate signal transduction pathways.

Effect of tumor necrosis factor-alpha on insulin-stimulated mitogen-activated protein kinase cascade in cultured rat skeletal muscle cells

Tumor necrosis factor-alpha (TNF-alpha) is a proposed mediator of insulin resistance in obese/diabetic animals through its effects on tyrosine phosphorylation of the insulin receptor and its substrate, insulin receptor substrate-1. In this study, the acute effects of TNF-alpha on the mitogen-activated protein kinase (MAPK) signalling cascade were examined in cultured rat skeletal muscle cell line, L6. Insulin treatment of L6 cells resulted in a rapid increase in MAPK activity (> twofold in 5 min with 10 nM insulin). Prior treatment with TNF-alpha for 60 min blocked subsequent insulin-induced activation of MAPK in a dose- and time-dependent manner. Metabolic labelling studies with inorganic [32P]phosphate followed by immuno-precipitation of MAPK and its upstream activator, mitogen-activated protein kinase kinase, indicated decreased phosphorylation of MAPK and its kinase in response to insulin in cells exposed to TNF-alpha. This effect of TNF-alpha was not due to inhibition of insulin-stimulated p21ras-GTP loading or Raf-1 phosphorylation. Low concentrations (2 nM) of okadaic acid, a serine/threonine phosphatase inhibitor, prevented TNF-alpha-induced inhibition of MAPK and restored insulin's effect on MAPK activity, while orthovanadate (a tyrosine phosphatase inhibitor), inhibitor 2 (phosphatase-1 inhibitor) and FK506 (phosphatase-2B inhibitor) were ineffective. These results suggested an involvement of an okadaic-acid-sensitive serine/threonine phosphatase in TNF-alpha-induced blockade of insulin's effect on MAPK and/or its kinase. Therefore, we examined the effect of TNF-alpha on protein phosphatase-1 (PP-1) and protein phosphatase-2A (PP-2A) activities. As reported by us earlier, insulin rapidly stimulated PP-1 and concomitantly inhibited PP-2A activities in control cells. TNF-alpha treatment blocked insulin-induced activation of PP-1. In contrast to PP-1, TNF-alpha caused a 60% increase in PP-2A activity and insulin failed to prevent this TNF-alpha effect. The time course of PP-2A activation by TNF-alpha preceded the kinetics of inhibition of MAPK. Cell-permeable ceramide analogs mimicked the TNF-alpha effect on MAPK inhibition and PP-2A activation. We conclude that TNF-alpha abrogates the insulin effect on MAPK activation by increasing dephosphorylation of MAPK kinase via an activated phosphatase.

Phospho-serine/threonine phosphatases in rat islets of Langerhans: identification and effect on insulin secretion

Stimulation of insulin secretion is accompanied by changes in the phosphorylation state of several islet polypeptides. Protein (de)phosphorylation is mediated by the action of protein kinases and phosphoprotein phosphatases. In this study we have investigated expression of phospho-serine/threonine phosphatases (PPs) in rat islets of Langerhans and studied the role of these enzymes in the regulation of insulin secretion. PP1, PP2A and PP2B were identified in rat islets and high levels of PP1/2A activities were detected. Inhibition of PP1/2A markedly inhibited glucose-stimulated insulin secretion, whilst glucose increased islet PP1/2A activities in situ. Insulin secretion at basal glucose was unaffected by inhibitors of PP1/2A. Inhibition of PP2B had no effect on either basal or glucose stimulated insulin secretion. These results suggest that PP1/2A are stimulated by glucose in rat islets and the presence of active PP1/2A is required for stimulation of insulin secretion by glucose.

Differential activation of mitogen-activated protein kinase and S6 kinase signaling pathways by 12-O-tetradecanoylphorbol-13-acetate (TPA) and insulin. Evidence for involvement of a TPA-stimulated protein-tyrosine kinase

AG-18, an inhibitor of protein-tyrosine kinases, was employed to study the role of tyrosine-phosphorylated proteins in insulin- and phorbol ester-induced signaling cascades. When incubated with Chinese hamster ovary cells overexpressing the insulin receptor, AG-18 reversibly inhibited insulin-induced tyrosine phosphorylation of insulin receptor substate-1, with minimal effects either on receptor autophosphorylation or on phosphorylation of Shc64. Under these conditions, AG-18 inhibited insulin-stimulated phosphorylation of the ribosomal protein S6, while no inhibition of insulin-induced activation of mitogen-activated protein kinase (MAPK) kinase or MAPK was detected. In contrast, 12-O-tetradecanoylphorbol-13-acetate (TPA)-induced activation of MAPK kinase and MAPK and phosphorylation of S6 were inhibited by AG-18. This correlated with inhibition of TPA-stimulated tyrosine phosphorylation of several proteins, the most prominent ones being pp114 and pp120. We conclude that Tyr-phosphorylated insulin receptor substrate-1 is the main upstream regulator of insulin-induced S6 phosphorylation by p70s6k, whereas MAPK signaling seems to be activated in these cells primarily through the adaptor molecule Shc. In contrast, TPA-induced S6 phosphorylation is mediated by the MAPK/p90rsk cascade. A key element of this TPA-stimulated signaling pathway is an AG-18-sensitive protein-tyrosine kinase.

Insulin signaling in chick embryos exposed to alcohol

Although insulin is known to be an important generator of regulatory signals during fetal growth and development, neither the immediate nor long-term effects of alcohol (ethanol) on insulin action are well understood. In the rat, fetal exposure to alcohol has been shown to be correlated with a subsequent abnormal response to a glucose load in the neonate and adult. Further, fetal hypoplasia secondary to maternal alcohol consumption is correlated with decreased placental glucose transport and with a lowering of the glucose levels in fetal tissues. However, the fetal effects of alcohol cannot be completely overcome by glucose/caloric supplementation, suggesting that factors other than glucose transport are involved. Using an embryonic chick model that negates the factors of maternal/placental metabolism and transport, the current study found that fetal alcohol exposure markedly increased insulin binding in developing tissue, but had little effect on the binding of the insulin-like growth factors. Competitive binding experiments revealed a marked increase in insulin receptor numbers, but no change in binding affinity as a result of the alcohol exposure. Basal uptake of 2-deoxyglucose by fetal tissue was lowered by alcohol exposure, but incubation with exogenous porcine insulin (1 x 10(-7) M) resulted in a significant increase in glucose uptake by the alcohol-exposed embryos. The increases in insulin binding and in insulin-dependent glucose uptake notwithstanding, exogenous insulin could not induce normal levels of ornithine decarboxylase activity in embryonic cells previously exposed to alcohol.(ABSTRACT TRUNCATED AT 250 WORDS)

Long term phorbol ester treatment down-regulates the beta 3-adrenergic receptor in 3T3-F442A adipocytes

The role of protein kinase C (PKC) in the regulation of the beta 3-adrenergic receptor (beta 3-AR) gene was examined in murine 3T3-F442A adipocytes, which express this receptor subtype at a high level. We also investigated the involvement of this kinase in the modulation of beta 3-AR gene expression by insulin. Long term exposure of 3T3-F442A adipocytes to phorbol 12-myristate 13-acetate (PMA) decreased beta 3-AR mRNA content in a time- and concentration-dependent manner, with maximal changes observed at 6 h (6.5-fold decrease) and at 100 nM PMA. This inhibition was selective for beta 3-AR transcripts, since beta 1- and beta 2-AR mRNA content remained unchanged. Also, (-)-[125I]cyanopindolol saturation and competition binding experiments on adipocyte membranes indicated that PMA induced an approximately 2-fold decrease in beta 3-AR expression, while that of the two other subtypes was not affected. This correlated with a lower efficacy of beta 3-AR agonists to stimulate adenylyl cyclase. Conversely, long term exposure to PMA did not alter adenylyl cyclase activity in response to guanosine 5'-O-(3-thiotriphosphate) or forskolin. The inactive phorbol ester 4 alpha-phorbol 12,13-didecanoate did not repress beta 3-AR mRNA levels. Inhibition of beta 3-AR mRNA by PMA was suppressed by the PKC-selective inhibitor bisindolylmaleimide, and was not observed in PKC-depleted cells, indicating that PKC was involved in this response. mRNA turnover experiments showed that the half-life of beta 3-AR transcripts was not affected by long term PMA exposure. When 3T3-F442A adipocytes were pretreated with PMA for 24 h to down-regulate PKC, or with bisindolylmaleimide, the insulin-induced inhibition of beta 3-AR mRNA levels was reduced by 44-67%. These findings demonstrate that sustained PKC activation exerts a specific control of beta 3-AR gene expression and is involved, at least in part, in the modulation by insulin of this adrenergic receptor subtype.

Insulin regulates transcription of the CCAAT/enhancer binding protein (C/EBP) alpha, beta, and delta genes in fully-differentiated 3T3-L1 adipocytes

The effect of insulin on expression of CCAAT/enhancer binding protein (C/EBP) alpha, beta, and delta was investigated in fully-differentiated 3T3-L1 adipocytes. Treatment of adipocytes with insulin stimulated rapid dephosphorylation of C/EBP alpha, and repressed the expression of C/EBP alpha within 2-4 h, with > 90% suppression occurring at 24 h. While insulin induced expression of C/EBP beta and C/EBP delta within 1 h and caused a > 20-fold increase by 4 h, expression returned to nearly pretreatment levels by 24 h. The insulin concentration dependence of these effects was consistent with involvement of the insulin receptor. Gel shift analysis revealed that 6 h of insulin treatment decreased the binding of nuclear C/EBP alpha while increasing binding of nuclear C/EBP beta and C/EBP delta. The reciprocal effects of insulin on the steady-state levels of C/EBP transcription factors can be accounted for kinetically and quantitatively by changes in their mRNA levels, which can be accounted for by effects on gene transcription. The effects of insulin on adipocyte gene transcription (e.g. GLUT4) may be mediated, at least in part, by down-regulation of C/EBP alpha and/or its dephosphorylation.

Stimulation of protein phosphatase-1 activity by insulin in rat adipocytes. Evaluation of the role of mitogen-activated protein kinase pathway

In this study, we examined the distribution of protein serine/threonine phosphatase-1 (PP-1) and analyzed the effect of insulin on PP-1 and its mechanism of activation in freshly isolated rat adipocytes. The adipocyte particulate fraction (PF) constituted approximately 80% of cellular PP-1 activity, while PP-2A was entirely cytosolic. Insulin rapidly stimulated PF PP-1 in a time- and dose-dependent manner (maximum stimulation at 5 min with 4 nM insulin). Immunoprecipitation of PF with an antibody against the site-1 sequence of rabbit skeletal muscle glycogen-associated PP-1 (PP-1G) subunit indicated that approximately 40% of adipocyte PP-1 activity was due to PP-1G form of the enzyme. Insulin stimulated PP-1G (120% over basal levels) without affecting the other forms of PP-1 in the PF. Insulin activation of PP-1 was accompanied by > 2-fold increase in the phosphorylation state of the 160-kDa regulatory subunit of PP-1. Stimulation of p21Ras/mitogen-activated protein kinase pathway (MAP) with GTP analogues also resulted in stimulation of PP-1 similar to insulin. The insulin effect on MAP kinase and PP-1 activation was blocked by a GTP antagonist, guanyl-5'-yl thiophosphate. The inhibitors of MAP kinase activation (viz. cAMP agonists, SpcAMP and ML-9) also blocked PP-1 stimulation by insulin. The time course of MAP kinase activation preceded the phosphorylation of PP-1 regulatory subunit and PP-1 activation. We conclude that insulin rapidly activates a membrane-associated PP-1 in adipocytes, which may be similar to rabbit skeletal muscle PP-1G, and the activation is mediated by p21Ras/MAP kinase pathway.