Regulation of protein kinase C zeta by PI 3-kinase and PDK-1

Modulation of DNA synthesis by muscarinic cholinergic receptors

Acetylcholine muscarinic receptors are a family of five G-protein-coupled receptors widely distributed in the central nervous system and in peripheral organs. Activation of certain subtypes of muscarinic receptors (M1, M3, M5) has been found to modulate DNA synthesis in a number of cell types. In several cell types acetylcholine, by activating endogenous or transfected muscarinic receptors, can indeed elicit cell proliferation. In other cell types, however, or under different experimental conditions, activation of muscarinic receptors has no effect, or inhibits DNA synthesis. A large number of intracellular pathways are being investigated to define the mechanisms involved in these effects of muscarinic receptors; these include among others, phospholipase D, protein kinases C and mitogen-activated-protein kinases. The ability of acetylcholine to modulate DNA synthesis through muscarinic receptors may be relevant in the context of brain development and neoplastic growth.

Activated R-ras, Rac1, PI 3-kinase and PKCepsilon can each restore cell spreading inhibited by isolated integrin beta1 cytoplasmic domains

Attachment of many cell types to extracellular matrix proteins triggers cell spreading, a process that strengthens cell adhesion and is a prerequisite for many adhesion-dependent processes including cell migration, survival, and proliferation. Cell spreading requires integrins with intact beta cytoplasmic domains, presumably to connect integrins with the actin cytoskeleton and to activate signaling pathways that promote cell spreading. Several signaling proteins are known to regulate cell spreading, including R-Ras, PI 3-kinase, PKCepsilon and Rac1; however, it is not known whether they do so through a mechanism involving integrin beta cytoplasmic domains. To study the mechanisms whereby cell spreading is regulated by integrin beta cytoplasmic domains, we inhibited cell spreading on collagen I or fibrinogen by expressing tac-beta1, a dominant-negative inhibitor of integrin function, and examined whether cell spreading could be restored by the coexpression of either V38R-Ras, p110alpha-CAAX, myr-PKCepsilon, or L61Rac1. Each of these activated signaling proteins was able to restore cell spreading as assayed by an increase in the area of cells expressing tac-beta1. R-Ras and Rac1 rescued cell spreading in a GTP-dependent manner, whereas PKCstraightepsilon required an intact kinase domain. Importantly, each of these signaling proteins required intact beta cytoplasmic domains on the integrins mediating adhesion in order to restore cell spreading. In addition, the rescue of cell spreading by V38R-Ras was inhibited by LY294002, suggesting that PI 3-kinase activity is required for V38R-Ras to restore cell spreading. In contrast, L61Rac1 and myr-PKCstraightepsilon each increased cell spreading independent of PI 3-kinase activity. Additionally, the dominant-negative mutant of Rac1, N17Rac1, abrogated cell spreading and inhibited the ability of p110alpha-CAAX and myr-PKCstraightepsilon to increase cell spreading. These studies suggest that R-Ras, PI 3-kinase, Rac1 and PKCepsilon require the function of integrin beta cytoplasmic domains to regulate cell spreading and that Rac1 is downstream of PI 3-kinase and PKCepsilon in a pathway involving integrin beta cytoplasmic domain function in cell spreading.

Activation of atypical protein kinase C zeta by caspase processing and degradation by the ubiquitin-proteasome system

Atypical protein kinase C zeta (PKCzeta) is known to transduce signals that influence cell proliferation and survival. Here we show that recombinant human caspases can process PKCzeta at three sites in the hinge region between the regulatory and catalytic domains. Caspase-3, -6, -7, and -8 chiefly cleaved human PKCzeta at EETD downward arrowG, and caspase-3 and -7 also cleaved PKCzeta at DGMD downward arrowG and DSED downward arrowL, respectively. Processing of PKCzeta expressed in transfected cells occurred chiefly at EETD downward arrowG and DGMD downward arrowG and produced carboxyl-terminal polypeptides that contained the catalytic domain. Epitope-tagged PKCzeta that lacked the regulatory domain was catalytically active following expression in HeLa cells. Induction of apoptosis in HeLa cells by tumor necrosis factor alpha plus cycloheximide evoked the conversion of full-length epitope-tagged PKCzeta to two catalytic domain polypeptides and increased PKCzeta activity. A caspase inhibitor, zVAD-fmk, prevented epitope-tagged PKCzeta processing and activation following the induction of apoptosis. Induction of apoptosis in rat parotid C5 cells produced catalytic domain polypeptides of endogenous PKCzeta and increased PKCzeta activity. Caspase inhibitors prevented the increase in PKCzeta activity and production of the catalytic domain polypeptides. Treatment with lactacystin, a selective inhibitor of the proteasome, caused polyubiquitin-PKCzeta conjugates to accumulate in cells transfected with the catalytic domain or full-length PKCzeta, or with a PKCzeta mutant that was resistant to caspase processing. We conclude that caspases process PKCzeta to carboxyl-terminal fragments that are catalytically active and that are degraded by the ubiquitin-proteasome pathway.

Coordinate control of muscle cell survival by distinct insulin-like growth factor activated signaling pathways

Peptide growth factors control diverse cellular functions by regulating distinct signal transduction pathways. In cultured myoblasts, insulin-like growth factors (IGFs) stimulate differentiation and promote hypertrophy. IGFs also maintain muscle cell viability. We previously described C2 skeletal muscle lines lacking expression of IGF-II. These cells did not differentiate, but underwent progressive apoptotic death when incubated in differentiation medium. Viability could be sustained and differentiation enabled by IGF analogues that activated the IGF-I receptor; survival was dependent on stimulation of phosphatidylinositol 3-kinase (PI3-kinase). We now find that IGF action promotes myoblast survival through two distinguishable PI3-kinase-regulated pathways that culminate in expression of the cyclin-dependent kinase inhibitor, p21. Incubation with IGF-I or transfection with active PI3-kinase led to rapid induction of MyoD and p21, and forced expression of either protein maintained viability in the absence of growth factors. Ectopic expression of MyoD induced p21, and inhibition of p21 blocked MyoD-mediated survival, thus defining one PI3-kinase-dependent pathway as leading first to MyoD, and then to p21 and survival. Unexpectedly, loss of MyoD expression did not impede IGF-mediated survival, revealing a second pathway involving activation by PI3-kinase of Akt, and subsequent induction of p21. Since inhibition of p21 caused death even in the presence of IGF-I, these results establish a central role for p21 as a survival factor for muscle cells. Our observations also define a MyoD-independent pathway for regulating p21 in muscle, and demonstrate that distinct mechanisms help ensure appropriate expression of this key protein during differentiation.

Pathophysiology and pharmacological treatment of insulin resistance

Diabetes mellitus type 2 is a world-wide growing health problem affecting more than 150 million people at the beginning of the new millennium. It is believed that this number will double in the next 25 yr. The pathophysiological hallmarks of type 2 diabetes mellitus consist of insulin resistance, pancreatic beta-cell dysfunction, and increased endogenous glucose production. To reduce the marked increase of cardiovascular mortality of type 2 diabetic subjects, optimal treatment aims at normalization of body weight, glycemia, blood pressure, and lipidemia. This review focuses on the pathophysiology and molecular pathogenesis of insulin resistance and on the capability of antihyperglycemic pharmacological agents to treat insulin resistance, i.e., a-glucosidase inhibitors, biguanides, thiazolidinediones, sulfonylureas, and insulin. Finally, a rational treatment approach is proposed based on the dynamic pathophysiological abnormalities of this highly heterogeneous and progressive disease.

Possible role of protein kinase C zeta in muscarinic receptor-induced proliferation of astrocytoma cells

Recent studies have shown that protein kinase C zeta (PKC zeta) is part of a pathway that plays a key role in a wide range of physiological processes including mitogenesis, cell survival, and transcriptional regulation. Most studies on PKC zeta have been done by stimulating cells with tyrosine kinase receptor agonists, or by transfecting the cells with either constitutively active PKC zeta or negative mutants of PKC zeta. Less is known about the ability of endogenous G-protein-coupled receptors to generate a mitogenic signal through activation of endogenous PKC zeta. In the present paper, we showed that in 123-1N1 human astrocytoma cells, which express the G-protein-coupled M2, M3, and M5 muscarinic receptors, PKC zeta is activated by carbachol in a concentration-dependent manner, resulting in the translocation of PKC zeta from the cytoplasm to granules in the perinuclear region. The effect of carbachol was long-lasting (up to 24 hr) and appeared to be mediated by activation of M3 muscarinic receptors. A selective PKC zeta inhibitor peptide (peptide Z) inhibited PKC zeta translocation as well as carbachol-induced DNA synthesis. Inhibition of both phosphatidylinositol 3-kinase and phospholipase D decreased carbachol-induced [(3)H]thymidine incorporation and blocked carbachol-induced PKC zeta translocation, suggesting an involvement of both pathways in these effects.

Further evidence that 3-phosphoinositide-dependent protein kinase-1 (PDK1) is required for the stability and phosphorylation of protein kinase C (PKC) isoforms

The multi-site phosphorylation of the protein kinase C (PKC) superfamily plays an important role in the regulation of these enzymes. One of the key phosphorylation sites required for the activation of all PKC isoforms lies in the T-loop of the kinase domain. Recent in vitro and transfection experiments indicate that phosphorylation of this residue can be mediated by the 3-phosphoinositide-dependent protein kinase-1 (PDK1). In this study, we demonstrate that in embryonic stem (ES) cells lacking PDK1 (PDK1-/- cells), the intracellular levels of endogenously expressed PKCalpha, PKCbetaI, PKCgamma, PKCdelta, PKCepsilon, and PKC-related kinase-1 (PRK1) are vastly reduced compared to control ES cells (PDK1+/+ cells). The levels of PKCzeta and PRK2 protein are only moderately reduced in the PDK1-/- ES cells. We demonstrate that in contrast to PKCzeta expressed PDK1+/+ ES cells, PKCzeta in ES cells lacking PDK1 is not phosphorylated at its T-loop residue. This provides the first genetic evidence that PKCzeta is a physiological substrate for PDK1. In contrast, PRK2 is still partially phosphorylated at its T-loop in PDK1-/- cells, indicating the existence of a PDK1-independent mechanism for the phosphorylation of PRK2 at this residue.

Association of immature hypophosphorylated protein kinase cepsilon with an anchoring protein CG-NAP

Protein kinase C (PKC) family requires phosphorylation of itself to become competent for responding to second messengers. Much attention has been focused on elucidating the role of phosphorylation in PKC activity; however, it remains unknown where this modification takes place in the cells. This study examines whether anchoring protein is involved in the regulation of PKC phosphorylation. A certain population of PKC epsilon in rat brain extracts as well as that expressed in COS7 cells was associated with an endogenous anchoring protein CG-NAP (centrosome and Golgi localized PKN- associated protein). Pulse chase experiments revealed that the associated PKC epsilon was an immature species at the hypophosphorylated state. In vitro binding studies confirmed that non- or hypophosphorylated PKC epsilon directly bound to CG-NAP via its catalytic domain, whereas sufficiently phosphorylated PKC epsilon did not. PKC epsilon mutant at a potential phosphorylation site of Thr-566 or Ser-729 to Ala, possessing almost no catalytic activity, was associated and co-localized with CG-NAP at Golgi/centrosome area. On the other hand, wild type and a phosphorylation-mimicking mutant at Thr-566 were mainly distributed in cytosol and represented second messenger-dependent catalytic activation. These results suggest that CG-NAP anchors hypophosphorylated PKCepsilon at the Golgi/centrosome area during maturation and serves as a scaffold for the phosphorylation reaction.

Synthesis of the translational apparatus is regulated at the translational level

The synthesis of many mammalian proteins associated with the translational apparatus is selectively regulated by mitogenic and nutritional stimuli, at the translational level. The apparent advantages of the regulation of gene expression at the translational level are the speed and the readily reversible nature of the response to altering physiological conditions. These two features enable cells to rapidly repress the biosynthesis of the translational machinery upon shortage of amino acids or growth arrest, thus rapidly blocking unnecessary energy wastage. Likewise, when amino acids are replenished or mitogenic stimulation is applied, then cells can rapidly respond in resuming the costly biosynthesis of the translational apparatus. A structural hallmark, common to mRNAs encoding many components of the translational machinery, is the presence of a 5' terminal oligopyrimidine tract (5'TOP), referred to as TOP mRNAs. This structural motif comprises the core of the translational cis-regulatory element of these mRNAs. The present review focuses on the mechanism underlying the translational control of TOP mRNAs upon growth and nutritional stimuli. A special emphasis is put on the pivotal role played by ribosomal protein S6 kinase (S6K) in this mode of regulation, and the upstream regulatory pathways, which might be engaged in transducing external signals into activation of S6K. Finally, the possible involvement of pyrimidine-binding proteins in the translational control of TOP mRNAs is discussed.

Unique structural and functional properties of the ATP-binding domain of atypical protein kinase C-iota

Atypical protein kinase C-iota (aPKCiota) plays an important role in mitogenic signaling, actin cytoskeleton organization, and cell survival. Apart from the differences in the regulatory domain, the catalytic domain of aPKCiota differs considerably from other known kinases, because it contains a modification within the glycine-rich loop motif (GXGXXG) that is found in the nucleotide-binding fold of virtually all nucleotide-binding proteins including PKCs, Ras, adenylate kinase, and the mitochondrial F1-ATPase. We have used site-directed mutagenesis and kinetic analysis to investigate whether these sequence differences affect the nucleotide binding properties and catalytic activity of aPKCiota. When lysine 274, a residue essential for ATP binding and activity conserved in most protein kinases, was replaced by arginine (K274R mutant), aPKCiota retained its normal kinase activity. This is in sharp contrast to results published for any other PKC or even distantly related kinases like phosphoinositide 3-kinase gamma, where the same mutation completely abrogated the kinase activity. Furthermore, the sensitivity of aPKCiota for inhibition by GF109203X, a substance acting on the ATP-binding site, was not altered in the K274R mutant. In contrast, replacement of Lys-274 by tryptophan (K274W) completely abolished the kinase activity of PKCiota. In accordance with results obtained with other kinase-defective PKC mutants, in cultured cells aPKCiota-K274W acted in a dominant negative fashion on signal transduction pathways involving endogenous aPKCiota, whereas the effect of the catalytically active K274R mutant was identical to the wild type enzyme. In summary, aPKCiota differs from classical and novel PKCs also in the catalytic domain. This information could be of significant value for the development of specific inhibitors of aPKCiota as a key factor in central signaling pathways.

Lipopolysaccharide induces Jun N-terminal kinase activation in macrophages by a novel Cdc42/Rac-independent pathway involving sequential activation of protein kinase C ζ and phosphatidylcholine-dependent phospholipase C

The activation of kinases of the mitogen-activated protein kinase superfamily initiated by lipopolysaccharide (LPS) plays an important role in transducing inflammatory signals. The pathway leading to the induction of stress-activated protein kinases in macrophages stimulated with LPS was investigated. The activation of Jun N-terminal kinases (JNK) by LPS is herbimycin sensitive. Using specific inhibitors, it was shown that the pathway involves the activation of phosphoinositide 3-kinase (PI 3-K). However, in contrast to previous reports, the small GTPases Cdc42 and Rac are not required downstream of PI 3-K for JNK activation. Instead, the phosphoinositides produced by PI 3-K stimulate protein kinase C (PKC) ζ activation through PDK1. In turn, activation of this atypical PKC leads to the stimulation of phosphatidylcholine phospholipase C (PC-PLC) and acidic sphingomyelinase (ASMase). It is therefore proposed that PKCζ regulates the PC-PLC/ASMase pathway, and it is hypothesized that the resultant ceramide accumulation mediates the activation of the SEK/JNK module by LPS.

Lipopolysaccharide induces Jun N-terminal kinase activation in macrophages by a novel Cdc42/Rac-independent pathway involving sequential activation of protein kinase C ζ and phosphatidylcholine-dependent phospholipase C

The activation of kinases of the mitogen-activated protein kinase superfamily initiated by lipopolysaccharide (LPS) plays an important role in transducing inflammatory signals. The pathway leading to the induction of stress-activated protein kinases in macrophages stimulated with LPS was investigated. The activation of Jun N-terminal kinases (JNK) by LPS is herbimycin sensitive. Using specific inhibitors, it was shown that the pathway involves the activation of phosphoinositide 3-kinase (PI 3-K). However, in contrast to previous reports, the small GTPases Cdc42 and Rac are not required downstream of PI 3-K for JNK activation. Instead, the phosphoinositides produced by PI 3-K stimulate protein kinase C (PKC) ζ activation through PDK1. In turn, activation of this atypical PKC leads to the stimulation of phosphatidylcholine phospholipase C (PC-PLC) and acidic sphingomyelinase (ASMase). It is therefore proposed that PKCζ regulates the PC-PLC/ASMase pathway, and it is hypothesized that the resultant ceramide accumulation mediates the activation of the SEK/JNK module by LPS.

TNF-alpha stimulation of MCP-1 expression is mediated by the Akt/PKB signal transduction pathway in vascular endothelial cells

MCP-1 is expressed in a variety of cell types including vascular endothelial cells following induction by different stimuli such as tumor necrosis factor (TNF)-alpha. Although TNF-alpha stimulates MCP-1 expression and secretion, the mechanism by which TNF-alpha stimulates expression of the MCP-1 gene is not known. In this study, we examine the involvement of the phosphatidylinositol-3-OH kinase (PI3-kinase)-Akt/PKB pathway. Exposure of human umbilical vein endothelial cells (HUVECs) to TNF-alpha elicited the rapid phosphorylation of Akt/PKB. In HUVECs, wortmannin, a PI3-kinase inhibitor, inhibits TNF-alpha-mediated MCP-1 secretion at a dose-dependent manner. Constitutively active form of Akt/PKB induces transcription of the MCP-1 gene, and cotransfection of dominant negative Akt/PKB suppressed the activation of the MCP-1 promoter induced by TNF-alpha. These findings show that Akt/PKB participates in the TNF-alpha induction of MCP-1 gene transcription in endothelial cells.

Functional dichotomy of protein kinase C (PKC) in tumor necrosis factor-alpha (TNF-alpha ) signal transduction in L929 cells. Translocation and inactivation of PKC by TNF-alpha

Tumor necrosis factor-alpha (TNF-alpha) is capable of inducing a variety of biologic responses through multiple signaling pathways. Because of the potential role of protein kinase C (PKC) in apoptosis, we examined the effects and mechanisms of TNF-alpha on PKC regulation, specifically on PKC alpha. In L929 murine fibroblasts, TNF-alpha (0.5- 5 nm) caused potent inhibition of PKC alpha activity and induced translocation of PKC alpha from the cytosol to the membrane. Treatment of cells with TNF-alpha also induced dephosphorylation of PKC alpha as detected by a mobility shift on SDS-polyacrylamide gel and inhibition of PKC phosphorylation as probed by anti-phospho-PKC antibodies. Since PKC is activated directly by diacylglycerol and inactivated indirectly by ceramide, we next examined the roles of these lipid mediators in the regulation of PKC alpha. Addition of TNF-alpha led to accumulation of both ceramide and diacylglycerol. Fumonisin B(1), an inhibitor of ceramide synthase, and glutathione, an inhibitor of neutral sphingomyelinase, both reversed the effect of TNF-alpha on PKC alpha activity, suggesting that ceramide production is necessary for the action of TNF-alpha. The diacylglycerol mimic phorbol 12-myristate 13-acetate was sufficient to cause translocation of PKC alpha, but not the mobility shift. Okadaic acid at 2 nm, a potent protein phosphatase inhibitor, blocked the effects of TNF-alpha on PKC alpha activity, but not on PKC alpha translocation, thus demonstrating that dephosphorylation and translocation are independent processes. These results demonstrate that PKC alpha acts as a downstream target for TNF-alpha and that different lipid-mediated pathways in TNF-alpha signaling lead to opposing signals in the regulation of PKC alpha activity.

Cross-talk between receptors with intrinsic tyrosine kinase activity and alpha1b-adrenoceptors

The effect of epidermal growth factor (EGF) and platelet-derived growth factor (PDGF) on the phosphorylation and function of alpha(1b)-adrenoceptors transfected into Rat-1 fibroblasts was studied. EGF and PDGF increased the phosphorylation of these adrenoceptors. The effect of EGF was blocked by tyrphostin AG1478 and that of PDGF was blocked by tyrphostin AG1296, inhibitors of the intrinsic tyrosine kinase activities of the receptors for these growth factors. Wortmannin, an inhibitor of phosphoinositide 3-kinase, blocked the alpha(1b)-adrenoceptor phosphorylation induced by EGF but not that induced by PDGF. Inhibition of protein kinase C blocked the adrenoceptor phosphorylation induced by EGF and PDGF. The ability of noradrenaline to increase [(35)S]guanosine 5'-[gamma-thio]triphosphate ([(35)S]GTP[S]) binding in membrane preparations was used as an index of the functional coupling of the alpha(1b)-adrenoceptors and G-proteins. Noradrenaline-stimulated [(35)S]GTP[S] binding was markedly decreased in membranes from cells pretreated with EGF or PDGF. Our data indicate that: (i) activation of EGF and PDGF receptors induces phosphorylation of alpha(1b)-adrenoceptors, (ii) phosphatidylinositol 3-kinase is involved in the EGF response, but does not seem to play a major role in the action of PDGF, (iii) protein kinase C mediates this action of both growth factors and (iv) the phosphorylation of alpha(1b)-adrenoceptors induced by EGF and PDGF is associated with adrenoceptor desensitization.

Deficiency of PTEN in Jurkat T cells causes constitutive localization of Itk to the plasma membrane and hyperresponsiveness to CD3 stimulation

Pleckstrin homology (PH) domain binding to D3-phosphorylated phosphatidylinositides (PI) provides a reversible means of recruiting proteins to the plasma membrane, with the resultant change in subcellular localization playing a key role in the activation of multiple intracellular signaling pathways. Previously we found that the T-cell-specific PH domain-containing kinase Itk is constitutively membrane associated in Jurkat T cells. This distribution was unexpected given that the closely related B-cell kinase, Btk, is almost exclusively cytosolic. In addition to constitutive membrane association of Itk, unstimulated JTAg T cells also exhibited constitutive phosphorylation of Akt on Ser-473, an indication of elevated basal levels of the phosphatidylinositol 3-kinase (PI3K) products PI-3,4-P(2) and PI-3,4,5-P(3) in the plasma membrane. Here we describe a defect in expression of the D3 phosphoinositide phosphatase, PTEN, in Jurkat and JTAg T cells that leads to unregulated PH domain interactions with the plasma membrane. Inhibition of D3 phosphorylation by PI3K inhibitors, or by expression of PTEN, blocked constitutive phosphorylation of Akt on Ser-473 and caused Itk to redistribute to the cytosol. The PTEN-deficient cells were also hyperresponsive to T-cell receptor (TCR) stimulation, as measured by Itk kinase activity, tyrosine phosphorylation of phospholipase C-gamma1, and activation of Erk compared to those in PTEN-replete cells. These data support the idea that PH domain-mediated association with the plasma membrane is required for Itk activation, provide evidence for a negative regulatory role of PTEN in TCR stimulation, and suggest that signaling models based on results from Jurkat T-cell lines may underestimate the role of PI3K in TCR signaling.

Protein kinase C isozymes and the regulation of diverse cell responses

Individual protein kinase C (PKC) isozymes have been implicated in many cellular responses important in lung health and disease, including permeability, contraction, migration, hypertrophy, proliferation, apoptosis, and secretion. New ideas on mechanisms that regulate PKC activity, including the identification of a novel PKC kinase, 3-phosphoinositide-dependent kinase-1 (PDK-1), that regulates phosphorylation of PKC, have been advanced. The importance of targeted translocation of PKC and isozyme-specific binding proteins (like receptors for activated C-kinase and caveolins) is well established. Phosphorylation state and localization are now thought to be key determinants of isozyme activity and specificity. New concepts on the role of individual PKC isozymes in proliferation and apoptosis are emerging. Opposing roles for selected isozymes in the same cell system have been defined. Coupling to the Wnt signaling pathway has been described. Phenotypes for PKC knockout mice have recently been reported. More specific approaches for studying PKC isozymes and their role in cell responses have been developed. Strengths and weaknesses of different experimental strategies are reviewed. Future directions for investigation are identified.

Ischemic preconditioning activates phosphatidylinositol-3-kinase upstream of protein kinase C

The present study is designed to test whether phosphatidylinositol 3-kinase (PI3-kinase) has a role in the signaling pathway in ischemic preconditioning (PC) and whether it is proximal or distal to protein kinase C (PKC). Before 20 minutes of global ischemia, Langendorff-perfused rat hearts were perfused for 20 minutes (control); preconditioned with 4 cycles of 5-minute ischemia and 5-minute reflow (PC); treated with either wortmannin (WM) or LY 294002 (LY), each of which is a PI3-kinase inhibitor, for 5 minutes before and throughout PC; treated with 1,2-dioctanoyl-sn-glycerol (DOG), an activator of PKC for 10 minutes (DOG); treated identically to the DOG group except with WM added 10 minutes before and during perfusion with DOG; or treated with either WM or LY for 25 minutes. Recovery of left ventricular developed pressure (LVDP; percentage of initial preischemic LVDP), measured after 30 minutes of reflow, was improved by PC (72+/-2% versus 36+/-4% in control; P<0.001), and this was blocked by WM and LY (41+/-4% and 43+/-5%, respectively; P<0.05 compared with PC). DOG addition improved postischemic LVDP (67+/-6%; P<0.001 compared with control), but in contrast to its effect on PC, WM did not completely eliminate the protective effect of DOG (52+/-4%; P>0.05 compared with DOG; P<0.05 compared with control). PC induced phosphorylation of protein kinase B and translocation of PKC epsilon, and it increased NO production, and these effects were blocked by WM, which suggests a role for PI3-kinase in PC upstream of PKC and NO.

Different protein kinase C isoforms determine growth factor specificity in neuronal cells

Although mitogenic and differentiating factors often activate a number of common signaling pathways, the mechanisms leading to their distinct cellular outcomes have not been elucidated. In a previous report, we demonstrated that mitogen-activated protein (MAP) kinase (ERK) activation by the neurogenic agents fibroblast growth factor (FGF) and nerve growth factor is dependent on protein kinase Cdelta (PKCdelta), whereas MAP kinase activation in response to the mitogen epidermal growth factor (EGF) is independent of PKCdelta in rat hippocampal (H19-7) and pheochromocytoma (PC12) cells. We now show that EGF activates MAP kinase through a PKCzeta-dependent pathway involving phosphatidylinositol 3-kinase and PDK1 in H19-7 cells. PKCzeta, like PKCdelta, acts upstream of MEK, and PKCzeta can potentiate Raf-1 activation by EGF. Inhibition of PKCzeta also blocks EGF-induced DNA synthesis as monitored by bromodeoxyuridine incorporation in H19-7 cells. Finally, in embryonic rat brain hippocampal cell cultures, inhibitors of PKCzeta or PKCdelta suppress MAP kinase activation by EGF or FGF, respectively, indicating that these factors activate distinct signaling pathways in primary as well as immortalized neural cells. Taken together, these results implicate different PKC isoforms as determinants of growth factor signaling specificity within the same cell. Furthermore, these data provide a mechanism whereby different growth factors can differentially activate a common signaling intermediate and thereby generate biological diversity.

Constitutive PI3-K activity is essential for proliferation, but not survival, of Theileria parva-transformed B cells

Theileria is an intracellular parasite that causes lymphoproliferative disorders in cattle, and infection of leucocytes induces a transformed phenotype similar to tumour cells, but the mechanisms by which the parasite induces this phenotype are not understood. Here, we show that infected B lymphocytes display constitutive phosphoinositide 3-kinase (PI3-K) activity, which appears to be necessary for proliferation, but not survival. Importantly, we demonstrate that one mechanism by which PI3-K mediates the proliferation of infected B lymphocytes is through the induction of a granulocyte-monocyte colony-stimulating factor (GM-CSF)-dependent autocrine loop. PI3-K induction of GM-CSF appears to be at the transcriptional level and, consistently, we demonstrate that PI3-K is also involved in the constitutive induction of AP-1 and NF-kappaB, which characterizes Theileria-infected leucocytes. Taken together, our results highlight a novel strategy exploited by the intracellular parasite Theileria to induce continued proliferation of its host leucocyte.

AKT/PKB and other D3 phosphoinositide-regulated kinases: kinase activation by phosphoinositide-dependent phosphorylation

The protein kinase Akt/PKB is activated via a multistep process by a variety of signals. In the early steps of this process, PI-3 kinase-generated D3-phosphorylated phosphoinositides bind the Akt PH domain and induce the translocation of the kinase to the plasma membrane where it co-localizes with phosphoinositide-dependent kinase-1. By binding to the PH domains of both Akt and phosphoinositide-dependent kinase-1, D3-phosphorylated phosphoinositides appear to also induce conformational changes that permit phosphoinositide-dependent kinase-1 to phosphorylate the activation loop of Akt. The paradigm of Akt activation via phosphoinositide-dependent phosphorylation provided a framework for research into the mechanism of activation of other members of the AGC kinase group (p70S6K, PKC, and PKA) and members of the Tec tyrosine kinase family (TecI, TecII, Btk/Atk, Itk/Tsk/Emt, Txk/Rlk, and Bm/Etk). The result was the discovery that these kinases and Akt are activated by overlapping pathways. In this review, we present our current understanding of the regulation and function of the Akt kinase and we discuss the common and unique features of the activation processes of Akt and the AGC and Tec kinase families. In addition, we present an overview of the biosynthesis of phosphoinositides that contribute to the regulation of these kinases.

A 3-phosphoinositide-dependent protein kinase-1 (PDK1) docking site is required for the phosphorylation of protein kinase Czeta (PKCzeta ) and PKC-related kinase 2 by PDK1

Members of the AGC subfamily of protein kinases including protein kinase B, p70 S6 kinase, and protein kinase C (PKC) isoforms are activated and/or stabilized by phosphorylation of two residues, one that resides in the T-loop of the kinase domain and the other that is located C-terminal to the kinase domain in a region known as the hydrophobic motif. Atypical PKC isoforms, such as PKCzeta, and the PKC-related kinases, like PRK2, are also activated by phosphorylation of their T-loop site but, instead of possessing a phosphorylatable Ser/Thr in their hydrophobic motif, contain an acidic residue. The 3-phosphoinositide-dependent protein kinase (PDK1) activates many members of the AGC subfamily of kinases in vitro, including PKCzeta and PRK2 by phosphorylating the T-loop residue. In the present study we demonstrate that the hydrophobic motifs of PKCzeta and PKCiota, as well as PRK1 and PRK2, interact with the kinase domain of PDK1. Mutation of the conserved residues of the hydrophobic motif of full-length PKCzeta, full-length PRK2, or PRK2 lacking its N-terminal regulatory domain abolishes or significantly reduces the ability of these kinases to interact with PDK1 and to become phosphorylated at their T-loop sites in vivo. Furthermore, overexpression of the hydrophobic motif of PRK2 in cells prevents the T-loop phosphorylation and thus inhibits the activation of PRK2 and PKCzeta. These findings indicate that the hydrophobic motif of PRK2 and PKCzeta acts as a "docking site" enabling the recruitment of PDK1 to these substrates. This is essential for their phosphorylation by PDK1 in cells.

Thapsigargin-induced degranulation of mast cells is dependent on transient activation of phosphatidylinositol-3 kinase

Thapsigargin, which elevates cytosolic calcium levels by inhibiting the sarcoplasmic/endoplasmic reticulum calcium-dependent ATPase, was tested for its ability to degranulate bone marrow-derived mast cells (BMMCs) from src homology 2-containing inositol phosphatase +/+ (SHIP+/+) and SHIP-/- mice. As was found previously with steel factor, thapsigargin stimulated far more degranulation in SHIP-/- than in SHIP+/+ BMMCs, and this was blocked with the phosphatidylinositol-3 (PI-3) kinase inhibitors, LY294002 and wortmannin. In contrast to steel factor, however, this heightened degranulation of SHIP-/- BMMCs was not due to a greater calcium influx into these cells, nor was the thapsigargin-induced calcium influx inhibited by LY294002, suggesting that the heightened thapsigargin-induced degranulation of SHIP-/- BMMCs was due to a PI-3 kinase-regulated step distinct from that regulating calcium entry. An investigation of thapsigargin-stimulated pathways in both cell types revealed that MAPK was heavily but equally phosphorylated. Interestingly, the protein kinase C inhibitor, bisindolylmaleimide (compound 3), totally blocked thapsigargin-induced degranulation in both SHIP+/+ and SHIP-/- BMMCs. As well, thapsigargin stimulated a PI-3 kinase-dependent, transient activation of protein kinase B, and this activation was far greater in SHIP-/- than in SHIP+/+ BMMCs. Consistent with this, thapsigargin was found to be a potent survival factor, following cytokine withdrawal, for both cell types and was more potent with SHIP-/- cells. These studies have both identified an additional PI-3 kinase-dependent step within the mast cell degranulation process, possibly involving 3-phosphoinositide-dependent protein kinase-1 and a diacylglycerol-independent protein kinase C isoform, and shown that the tumor-promoting activity of thapsigargin may be due to its activation of protein kinase B and subsequent promotion of cell survival.

Increased nuclear translocation of catalytically active PKC-zeta during mouse colonocyte hyperproliferation

Protein kinase (PK) C-zeta is implicated in the control of colonic epithelial cell proliferation in vitro. However, less is known about its physiological role in vivo. Using the transmissible murine colonic hyperplasia (TMCH) model, we determined its expression, subcellular localization, and kinase activity during native crypt hyperproliferation. Enhanced mitosis was associated with increased cellular 72-kDa holoenzyme (PKC-zeta, 3.2-fold), 48-kDa catalytic subunit (PKM-zeta, 3- to 9-fold), and 24-kDa membrane-bound fragment (M(f)-zeta, >10-fold) expression. Both PKC-zeta and PKM-zeta exhibited intrinsic kinase activity, and substrate phosphorylation increased 4.5-fold. No change in cellular PKC-iota/PKM-iota expression occurred. The subcellular distribution of immunoreactive PKC-zeta changed significantly: neck cells lost their basal subcellular pole filamentous staining, whereas proliferating cell nuclear antigen-positive cells exhibited elevated cytoplasmic, lateral membrane, and nuclear staining. Subcellular fractionation revealed increased PKC-zeta and PKM-zeta expression and activity within nuclei, which preferentially accumulated PKM-zeta. These results suggest separate cellular and nuclear roles, respectively, for PKC-zeta in quiescent and mitotically active colonocytes. PKM-zeta may specifically act as a modulator of proliferation during TMCH.

Nuclear mitogen-activated protein kinase activation by protein kinase czeta during reoxygenation after ischemic hypoxia

We examined the upstream kinases for mitogen-activated protein kinase (MAPK) activation during ischemic hypoxia and reoxygenation using H9c2 cells derived from rat cardiomyocytes. Protein kinase C (PKC)zeta, an atypical PKC isoform mainly expressed in rat heart, has been shown to act as an upstream kinase of MAPK during ischemic hypoxia and reoxygenation by analyses with PKC inhibitors, antisense DNA, a dominant negative kinase defective mutant, and constitutively active mutants of PKCzeta. Immunocytochemical observations show PKCzeta staining in the nucleus during ischemic hypoxia and reoxygenation when phosphorylated MAPK is also detected in the nucleus. This nuclear localization of PKCzeta is inhibited by treatment with wortmannin, a phosphoinositide 3-kinase inhibitor that also inhibits MAPK activation in a dose-dependent manner. This is supported by the inhibition of MAPK phosphorylation by another blocker of phosphoinositide 3-kinase, LY294002. An upstream kinase of MAPK, MEK1/2, is significantly phosphorylated 15 min after reoxygenation and observed mainly in the nucleus, whereas it is present in the cytoplasm in serum stimulation. The phosphorylation of MEK is blocked by PKC inhibitors and phosphoinositide 3-kinase inhibitors, as observed in the case of MAPK phosphorylation. These observations indicate that PKCzeta, which is activated by phosphoinositide 3-kinase, induces MAPK activation through MEK in the nucleus during reoxygenation after ischemic hypoxia.

Axonal regulation of Schwann cell proliferation and survival and the initial events of myelination requires PI 3-kinase activity

In this report, we have investigated the signaling pathways that are activated by, and mediate the effects of, the neuregulins and axonal contact in Schwann cells. Phosphatidylinositol 3-kinase (PI 3-kinase) and mitogen-activated protein kinase kinase (MAPK kinase) are strongly activated in Schwann cells by glial growth factor (GGF), a soluble neuregulin, and by contact with neurite membranes; both kinase activities are also detected in Schwann cell-DRG neuron cocultures. Inhibition of the PI 3-kinase, but not the MAP kinase, pathway reversibly inhibited Schwann cell proliferation induced by GGF and neurites. Cultured Schwann cells undergo apoptosis after serum deprivation and can be rescued by GGF or contact with neurites; these survival effects were also blocked by inhibition of PI 3-kinase. Finally, we have examined the role of these signaling pathways in Schwann cell differentiation in cocultures. At early stages of coculture, inhibition of PI 3-kinase, but not MAPK kinase, blocked Schwann cell elongation and subsequent myelination but did not affect laminin deposition. Later, after Schwann cells established a one-to-one relationship with axons, inhibition of PI 3-kinase did not block myelin formation, but the myelin sheaths that formed were shorter, and the rate of myelin protein accumulation was markedly decreased. PI 3-kinase inhibition had no observable effect on the maintenance of myelin sheaths in mature myelinated cocultures. These results indicate that activation of PI 3-kinase by axonal factors, including the neuregulins, promotes Schwann cell proliferation and survival and implicate PI 3-kinase in the early events of myelination.

A human homolog of the C. elegans polarity determinant Par-6 links Rac and Cdc42 to PKCzeta signaling and cell transformation

BACKGROUND

Rac and Cdc42 are members of the Rho family of small GTPases. They modulate cell growth and polarity, and contribute to oncogenic transformation by Ras. The molecular mechanisms underlying these functions remain elusive, however.

RESULTS

We have identified a novel effector of Rac and Cdc42, hPar-6, which is the human homolog of a cell-polarity determinant in Caenorhabditis elegans. hPar-6 contains a PDZ domain and a Cdc42/Rac interactive binding (CRIB) motif, and interacts with Rac1 and Cdc42 in a GTP-dependent manner. hPar-6 also binds directly to an atypical protein kinase C isoform, PKCzeta, and forms a stable ternary complex with Rac1 or Cdc42 and PKCzeta. This association results in stimulation of PKCzeta kinase activity. Moreover, hPar-6 potentiates cell transformation by Rac1/Cdc42 and its interaction with Rac1/Cdc42 is essential for this effect. Cell transformation by hPar-6 involves a PKCzeta-dependent pathway distinct from the pathway mediated by Raf.

CONCLUSIONS

These findings indicate that Rac/Cdc42 can regulate cell growth through Par-6 and PKCzeta, and suggest that deregulation of cell-polarity signaling can lead to cell transformation.

Thiazolidinedione treatment enhances insulin effects on protein kinase C-zeta /lambda activation and glucose transport in adipocytes of nondiabetic and Goto-Kakizaki type II diabetic rats

We evaluated effects of the thiazolidinedione, rosiglitazone, on insulin-induced activation of protein kinase C (PKC)-zeta/lambda and glucose transport in adipocytes of Goto-Kakizaki (GK)-diabetic and nondiabetic rats. Insulin effects on PKC-zeta/lambda and 2-deoxyglucose uptake were diminished by approximately 50% in GK adipocytes, as compared with control adipocytes. This defect in insulin-induced PKC-zeta/lambda activation was associated with diminished activation of IRS-1-dependent phosphatidylinositol (PI) 3-kinase, and was accompanied by diminished phosphorylation of threonine 410 in the activation loop of PKC-zeta; in contrast, protein kinase B (PKB) activation and phosphorylation were not significantly altered. Rosiglitazone completely reversed defects in insulin-stimulated 2-deoxyglucose uptake, PKCzeta/lambda enzyme activity and PKC-zeta threonine 410 phosphorylation, but had no effect on PI 3-kinase activation or PKB activation/phosphorylation in GK adipocytes. Similarly, in adipocytes of nondiabetic rats, rosiglitazone provoked increases in insulin-stimulated 2-deoxyglucose uptake, PKC-zeta/lambda enzyme activity and phosphorylation of both threonine 410 activation loop and threonine 560 autophosphorylation sites in PKC-zeta, but had no effect on PI 3-kinase activation or PKB activation/phosphorylation. Our findings suggest that (a) decreased effects of insulin on glucose transport in adipocytes of GK-diabetic rats are due at least in part to diminished phosphorylation/activation of PKC-zeta/lambda, and (b) thiazolidinediones enhance glucose transport responses to insulin in adipocytes of both diabetic and nondiabetic rats through increases in phosphorylation/activation of PKC-zeta/lambda.

Phosphorylation of protein kinase N by phosphoinositide-dependent protein kinase-1 mediates insulin signals to the actin cytoskeleton

Growth factors such as insulin regulate phosphatidylinositol 3-kinase-dependent actin cytoskeleton rearrangement in many types of cells. However, the mechanism by which the insulin signal is transmitted to the actin cytoskeleton remains largely unknown. Yeast two-hybrid screening revealed that the phosphatidylinositol 3-kinase downstream effector phosphoinositide-dependent protein kinase-1 (PDK1) interacted with protein kinase N (PKN), a Rho-binding Ser/Thr protein kinase potentially implicated in a variety of cellular events, including phosphorylation of cytoskeletal components. PDK1 and PKN interacted in vitro and in intact cells, and this interaction was mediated by the kinase domain of PDK1 and the carboxyl terminus of PKN. In addition to a direct interaction, PDK1 also phosphorylated Thr(774) in the activation loop and activated PKN. Insulin treatment or ectopic expression of the wild-type PDK1 or PKN, but not protein kinase Czeta, induced actin cytoskeleton reorganization and membrane ruffling in 3T3-L1 fibroblasts and Rat1 cells that stably express the insulin receptor (Rat1-IR). However, the insulin-stimulated actin cytoskeleton reorganization in Rat1-IR cells was prevented by expression of kinase-defective PDK1 or PDK1-phosphorylation site-mutated PKN. Thus, phosphorylation by PDK1 appears to be necessary for PKN to transduce signals from the insulin receptor to the actin cytoskeleton.

The PI3K-PDK1 connection: more than just a road to PKB

Phosphoinositide 3-kinases (PI3Ks) generate specific inositol lipids that have been implicated in the regulation of cell growth, proliferation, survival, differentiation and cytoskeletal changes. One of the best characterized targets of PI3K lipid products is the protein kinase Akt or protein kinase B (PKB). In quiescent cells, PKB resides in the cytosol in a low-activity conformation. Upon cellular stimulation, PKB is activated through recruitment to cellular membranes by PI3K lipid products and phosphorylation by 3'-phosphoinositide-dependent kinase-1 (PDK1). Here we review the mechanism by which PKB is activated and the downstream actions of this multifunctional kinase. We also discuss the evidence that PDK1 may be involved in the activation of protein kinases other than PKB, the mechanisms by which this activity of PDK1 could be regulated and the possibility that some of the currently postulated PKB substrates targets might in fact be phosphorylated by PDK1-regulated kinases other than PKB.

The role of 3-phosphoinositide-dependent protein kinase 1 in activating AGC kinases defined in embryonic stem cells

BACKGROUND

Protein kinase B (PKB), and the p70 and p90 ribosomal S6 kinases (p70 S6 kinase and p90 Rsk, respectively), are activated by phosphorylation of two residues, one in the 'T-loop' of the kinase domain and, the other, in the hydrophobic motif carboxy terminal to the kinase domain. The 3-phosphoinositide-dependent protein kinase 1 (PDK1) activates many AGC kinases in vitro by phosphorylating the T-loop residue, but whether PDK1 also phosphorylates the hydrophobic motif and whether all other AGC kinases are substrates for PDK1 is unknown.

RESULTS

Mouse embryonic stem (ES) cells in which both copies of the PDK1 gene were disrupted were viable. In PDK1(-/-) ES cells, PKB, p70 S6 kinase and p90 Rsk were not activated by stimuli that induced strong activation in PDK1(+/+) cells. Other AGC kinases - namely, protein kinase A (PKA), the mitogen- and stress-activated protein kinase 1 (MSK1) and the AMP-activated protein kinase (AMPK) - had normal activity or were activated normally in PDK1(-/-) cells. The insulin-like growth factor 1 (IGF1) induced PKB phosphorylation at its hydrophobic motif, but not at its T-loop residue, in PDK1(-/-) cells. IGF1 did not induce phosphorylation of p70 S6 kinase at its hydrophobic motif in PDK1(-/-) cells.

CONCLUSIONS

PDK1 mediates activation of PKB, p70 S6 kinase and p90 Rsk in vivo, but is not rate-limiting for activation of PKA, MSK1 and AMPK. Another kinase phosphorylates PKB at its hydrophobic motif in PDK1(-/-) cells. PDK1 phosphorylates the hydrophobic motif of p70 S6 kinase either directly or by activation of another kinase.

Rho GTPase control of protein kinase C-related protein kinase activation by 3-phosphoinositide-dependent protein kinase

The protein kinase C-related protein kinases (PRKs) have been shown to be under the control of the Rho GTPases and influenced by autophosphorylation. In analyzing the relationship between these inputs, it is shown that activation in vitro and in vivo involves the activation loop phosphorylation of PRK1/2 by 3-phosphoinositide-dependent protein kinase-1 (PDK1). Rho overexpression in cultured cells is shown to increase the activation loop phosphorylation of endogenous PRKs and is demonstrated to influence this process by controlling the ability of PRKs to bind to PDK1. The interaction of PRK1/2 with PDK1 is shown to be dependent upon Rho. Direct demonstration of ternary (Rho.PRK.PDK1) complex formation in situ is provided by the observation that PDK1 is recruited to RhoB-containing endosomes only if PRK is coexpressed. Furthermore, this in vivo complex is maintained after phosphoinositide 3-kinase inhibition. The control of PRKs by PDK1 thus evidences a novel strategy of substrate-directed control involving GTPases.

Dual role of pseudosubstrate in the coordinated regulation of protein kinase C by phosphorylation and diacylglycerol

The activity of protein kinase C is reversibly regulated by an autoinhibitory pseudosubstrate, which blocks the active site of the enzyme in the absence of activators. However, before it can be allosterically regulated, protein kinase C must first be processed by three ordered phosphorylations, the first of which is modification of the activation loop catalyzed by the phosphoinositide-dependent kinase-1 (PDK-1). Here we use limited proteolysis to show that 1) newly synthesized protein kinase C adopts a conformation in which its pseudosubstrate sequence is removed from the active site, and 2) this exposure is essential to allow PDK-1 to phosphorylate the enzyme. Precursor (unphosphorylated) protein kinase C betaII obtained by 1) in vitro transcription and translation, 2) expression of a phosphorylation-deficient mutant (T500V), or 3) in vivo labeling with a pulse of [(35)S]cysteine/methionine is cleaved at the amino-terminal pseudosubstrate by the endoproteinase Arg-C. In marked contrast to mature (phosphorylated) enzyme, proteolysis occurs in the absence of lipid activators, revealing that precursor protein kinase C has its pseudosubstrate sequence removed constitutively. Additionally, we show that PDK-1 is unable to phosphorylate protein kinase C when the active site is sterically blocked by a peptide substrate. Neither can mature enzyme be dephosphorylated when the active site is blocked by binding either the pseudosubstrate sequence or a heterologous substrate. Thus, the accessibility of the activation loop to both phosphorylation and dephosphorylation requires an exposed pseudosubstrate. In summary, newly synthesized protein kinase C adopts a conformation in which its pseudosubstrate sequence is removed from the active site, rendering the activation loop accessible to phosphorylation by PDK-1. Phosphorylation serves as a conformational switch to position the pseudosubstrate so that it blocks the active site, a conformation that is maintained until stimulus-dependent membrane binding releases it, thus activating the enzyme.

Effect of pertussis toxin on insulin-induced signal transduction in rat adipocytes and soleus muscles

It has been reported that pertussis toxin (PTX) suppresses the function of trimeric guanine nucleotide binding protein (G-protein). We examined the effect of PTX on insulin-induced glucose uptake, diacylglycerol (DG)-protein kinase C (PKC) signalling, phosphatidylinositol (PI) 3-kinase and PKC zeta activation and insulin-induced tyrosine phosphorylation of Gialpha to clarify the role of G-protein for insulin-mediated signal transduction mechanism in rat adipocytes and soleus muscles. Isolated adipocytes and soleus muscles were preincubated with 0.01 approximately 1 ng/ml PTX for 2 hours, followed by stimulation with 10-100 nM insulin or 1 microM tetradecanoyl phorbol-13-acetate (TPA). Pretreatment with PTX resulted in dose-responsive decreases in insulin-stimulated [3H]2-deoxyglucose (DOG) uptake, and unchanged TPA-stimulated [3H]2-DOG uptake, without affecting basal [3H]2-DOG uptake. In adipocytes, insulin-induced DG-PKC signalling, PI 3-kinase activation and PKC zeta translocation from cytosol to the membrane were suppressed when treated with PTX, despite no changes in [125I]insulin-specific binding and insulin receptor tyrosine kinase activity. Moreover, to elucidate insulin-stimulated tyrosine phosphorylation of 40 kDa alpha-subunit of G-protein (Gialpha-2), adipocytes were stimulated with 10 nM insulin for 10 minutes, homogenized, immunoprecipitated with anti-phosphotyrosine antibody, and immunoblotted with anti-Gialpha-2 antibody. Insulin-induced tyrosine phosphorylation of Gialpha-2 was found by immunoblot analysis with anti-Gialpha-2 antibody. These results suggest that G-protein regulates DG-PKC signalling by binding of Gialpha-2 with GTP and PI 3-kinase-PKC zeta signalling by releasing of Gbetagamma via dissociation of trimeric G-protein after insulin receptor tyrosine phosphorylation in insulin-sensitive tissues.

Akt/protein kinase B is regulated by autophosphorylation at the hypothetical PDK-2 site

The function of Akt (protein kinase B) is regulated by phosphorylation on two sites conserved within the AGC kinase family: the activation loop (Thr-308) in the kinase core and a hydrophobic phosphorylation site on the carboxyl terminus (Ser-473). Thr-308 is phosphorylated by the phosphoinositide-dependent kinase-1, (PDK-1), whereas the mechanism of phosphorylation of the hydrophobic site, tentatively referred to as the PDK-2 site, is unknown. Here we report that phosphorylation of the hydrophobic motif requires catalytically competent Akt. First we show that a kinase-inactive construct of Akt fails to incorporate phosphate at Ser-473 following IGF-1 stimulation in vivo but does incorporate phosphate at Thr-308 and a second carboxyl-terminal site, Thr-450; this ligand triggers the phosphorylation of both sites in wild-type enzyme. Neither does a catalytically inactive construct in which phosphorylation at the activation loop is blocked, T308A, become phosphorylated on the hydrophobic site in response to stimulation. Second, we show that Akt autophosphorylates on the hydrophobic site in vitro: phosphorylation of the activation loop by PDK-1 triggers the phosphorylation of the hydrophobic site in kinase-active, but not thermally inactivated, Akt alpha. Thus, Akt is regulated by autophosphorylation at the Ser-473 hydrophobic site.

A critical role for PI 3-kinase in cytokine-induced Fcα-receptor activation

Fc-receptors, such as FcR and FcγRII, play an important role in leukocyte activation, and rapid modulation of ligand binding (“activation”) is critical for receptor regulation. We have previously demonstrated that ligand binding to Fc-receptors on human eosinophils is dependent on cytokine stimulation. Utilization of pharmacological inhibitors provided evidence that the phenomenon of interleukin (IL)-5 induced immunoglobulin A (IgA) binding to human eosinophils requires activation of phosphatidylinositol 3-kinase (PI3K). However, eosinophils are refractory to manipulation by molecular techniques such as DNA transfection or viral infection. Here we utilize an IL-3 dependent pre-B cell line to investigate the molecular mechanism of cytokine-mediated ligand binding to FcR. In this system, IgA binding is dependent on IL-3, similarly to the requirement for IL-5 of eosinophils. We show that IL-3-mediated activation of FcR (CD89) requires the activation of PI3K, independent of p21ras activation. Co-expression of dominant negative (▵p85) and active (p110_K227E) forms of PI3K demonstrate that the affinity switch regulating FcR activation requires PI3K. Moreover, overexpression of PI3K is both necessary and sufficient for activation of FcR. Furthermore, we show that IL-3/IL-5/GM-CSF induced inside-out signaling pathways activating FcR require the involvement of protein kinase C downstream of PI3K. Finally, we show that these inside-out signaling pathways responsible for Fc-receptor modulation require CD89, independent of its association with the FcRγ chain.

Identification of a pocket in the PDK1 kinase domain that interacts with PIF and the C-terminal residues of PKA

The 3-phosphoinositide-dependent protein kinase-1 (PDK1) phosphorylates and activates a number of protein kinases of the AGC subfamily. The kinase domain of PDK1 interacts with a region of protein kinase C-related kinase-2 (PRK2), termed the PDK1-interacting fragment (PIF), through a hydrophobic motif. Here we identify a hydrophobic pocket in the small lobe of the PDK1 kinase domain, separate from the ATP- and substrate-binding sites, that interacts with PIF. Mutation of residues predicted to form part of this hydrophobic pocket either abolished or significantly diminished the affinity of PDK1 for PIF. PIF increased the rate at which PDK1 phosphorylated a synthetic dodecapeptide (T308tide), corresponding to the sequences surrounding the PDK1 phosphorylation site of PKB. This peptide is a poor substrate for PDK1, but a peptide comprising T308tide fused to the PDK1-binding motif of PIF was a vastly superior substrate for PDK1. Our results suggest that the PIF-binding pocket on the kinase domain of PDK1 acts as a 'docking site', enabling it to interact with and enhance the phosphorylation of its substrates.

Rap1 is a potent activation signal for leukocyte function-associated antigen 1 distinct from protein kinase C and phosphatidylinositol-3-OH kinase

To identify the intracellular signals which increase the adhesiveness of leukocyte function-associated antigen 1 (LFA-1), we established an assay system for activation-dependent adhesion through LFA-1/intercellular adhesion molecule 1 ICAM-1 using mouse lymphoid cells reconstituted with human LFA-1 and then introduced constitutively active forms of signaling molecules. We found that the phorbol myristate acetate (PMA)-responsive protein kinase C (PKC) isotypes (alpha, betaI, betaII, and delta) or phosphatidylinositol-3-OH kinase (PI 3-kinase) itself activated LFA-1 to bind ICAM-1. H-Ras and Rac activated LFA-1 in a PI 3-kinase-dependent manner, whereas Rho and R-Ras had little effect. Unexpectedly, Rap1 was demonstrated to function as the most potent activator of LFA-1. Distinct from H-Ras and Rac, Rap1 increased the adhesiveness independently of PI 3-kinase, indicating that Rap1 is a novel activation signal for the integrins. Rap1 induced changes in the conformation and affinity of LFA-1 and, interestingly, caused marked LFA-1/ICAM-1-mediated cell aggregation. Furthermore, a dominant negative form of Rap1 (Rap1N17) inhibited T-cell receptor-mediated LFA-1 activation in Jurkat T cells and LFA-1/ICAM-1-dependent cell aggregation upon differentiation of HL-60 cells into macrophages, suggesting that Rap1 is critically involved in physiological processes. These unique functions of Rap1 in controlling cellular adhesion through LFA-1 suggest a pivotal role as an immunological regulator.

Ca(2+)-evoked serotonin secretion by parafollicular cells: roles in signal transduction of phosphatidylinositol 3'-kinase, and the gamma and zeta isoforms of protein kinase C

Parafollicular (PF) cells secrete 5-HT in response to stimulation of a G-protein-coupled Ca(2+) receptor (CaR) by increased extracellular Ca(2+) (upward arrow[Ca(2+)](e)). We tested the hypothesis that protein kinase C (PKC) participates in stimulus-secretion coupling. Immunoblots from membrane and cytosolic fractions of isolated PF cells revealed conventional (alpha, betaI, and gamma), novel (delta and epsilon), and atypical (iota/lambda and zeta) PKCs. Only PKCgamma was found to have been translocated to the membrane fraction when secretion of 5-HT was evoked by upward arrow[Ca(2+)](e) or phorbol esters. Although phorbol downregulation caused PKCgamma to disappear, secretion was only partially inhibited. A similar reduction of upward arrow[Ca(2+)](e)-evoked secretion was produced by inhibitors of conventional and/or novel PKCs (Gö 6976, calphostin C, and pseudoA), and these compounds did not inhibit secretion at all when applied to phorbol-downregulated cells. In contrast, the phorbol downregulation-resistant component of secretion was abolished by pseudoZ, which inhibits the atypical PKCzeta. Stimulation of PF cells with upward arrow[Ca(2+)](e) increased the activity of immunoprecipitated PKCzeta (but not PKCiota/lambda), and the activity of this PKCzeta was inhibited by pseudoZ. PF cells were found to express regulatory (p85) and catalytic (p110alpha and p110beta) subunits of phosphatidylinositol 3'-kinase (PI3'-kinase). upward arrow[Ca(2+)](e) increased the activity of immunoprecipitated PI3'-kinase; moreover, PI3'-kinase inhibitors (wortmannin and LY294002) antagonized secretion. We suggest that PKC isoforms mediate secretion of 5-HT by PF cells in response to stimulation of the CaR. PKC involvement can be accounted for by PKCgamma and an isoform sensitive to inhibition by pseudoZ, probably PKCzeta, which is activated via PI3'-kinase.

Evidence that 3-phosphoinositide-dependent protein kinase-1 mediates phosphorylation of p70 S6 kinase in vivo at Thr-412 as well as Thr-252

Protein kinase B and p70 S6 kinase are members of the cyclic AMP-dependent/cyclic GMP-dependent/protein kinase C subfamily of protein kinases and are activated by a phosphatidylinositol 3-kinase-dependent pathway when cells are stimulated with insulin or growth factors. Both of these kinases are activated in cells by phosphorylation of a conserved residue in the kinase domain (Thr-308 of protein kinase B (PKB) and Thr-252 of p70 S6 kinase) and another conserved residue located C-terminal to the kinase domain (Ser-473 of PKB and Thr-412 of p70 S6 kinase). Thr-308 of PKBalpha and Thr-252 of p70 S6 kinase are phosphorylated by 3-phosphoinositide-dependent protein kinase-1 (PDK1) in vitro. Recent work has shown that PDK1 interacts with a region of protein kinase C-related kinase-2, termed the PDK1 interacting fragment (PIF). Interaction with PIF converts PDK1 from a form that phosphorylates PKB at Thr-308 alone to a species capable of phosphorylating Ser-473 as well as Thr-308. This suggests that PDK1 may be the enzyme that phosphorylates both residues in vivo. Here we demonstrate that PDK1 is capable of phosphorylating p70 S6 kinase at Thr-412 in vitro. We study the effect of PIF on the ability of PDK1 to phosphorylate p70 S6 kinase. Surprisingly, we find that PDK1 bound to PIF is no longer able to interact with or phosphorylate p70 S6 kinase in vitro at either Thr-252 or Thr-412. The expression of PIF in cells prevents insulin-like growth factor 1 from inducing the activation of the p70 S6 kinase and its phosphorylation at Thr-412. Overexpression of PDK1 in cells induces the phosphorylation of p70 S6 kinase at Thr-412 in unstimulated cells, and a catalytically inactive mutant of PDK1 prevents the phosphorylation of p70 S6K at Thr-412 in insulin-like growth factor 1-stimulated cells. These observations indicate that PDK1 regulates the activation of p70 S6 kinase and provides evidence that PDK1 mediates the phosphorylation of p70 S6 kinase at Thr-412.

Mammalian TOR controls one of two kinase pathways acting upon nPKCdelta and nPKCepsilon

There are three conserved phosphorylation sites in protein kinase C (PKC) isotypes that have been termed priming sites and play an important role in PKC function. The requirements and pathways involved in novel (nPKC) phosphorylation have been investigated here. The evidence presented for nPKCdelta shows that there are two independent kinase pathways that act upon the activation loop (Thr-505) and a C-terminal hydrophobic site (Ser-662) and that the phosphorylation of the Ser-662 site is protected from dephosphorylation by the Thr-505 phosphorylation. Both phosphorylations require C1 domain-dependent allosteric activation of PKC. The third site (Ser-643) appears to be an autophosphorylation site. The serum-dependent phosphorylation of the Thr-505 and Ser-662 sites increases nPKCdelta activity up to 80-fold. Phosphorylation at the Ser-662 site is independently controlled by a pathway involving mammalian TOR (mTOR) because the rapamycin-induced block of its phosphorylation is overcome by co-expression of a rapamycin-resistant mutant of mTOR. Consistent with this role of mTOR, amino acid deprivation selectively inhibits the serum-induced phosphorylation of the Ser-662 site in nPKCdelta. It is established that nPKCepsilon behaves in a manner similar to nPKCdelta with respect to phosphorylation at its C-terminal hydrophobic site, Ser-729. The results define the regulatory inputs to nPKCdelta and nPKCepsilon and establish these PKC isotypes downstream of mTOR and on an amino acid sensing pathway. The multiple signals integrated in PKC are discussed.

Regulation of integrin function by T cell activation: points of convergence and divergence

Lymphocyte adhesiveness is dynamically regulated in response to conditions in the extracellular environment. One mechanism of regulation of integrin adhesion receptors involves a rapid, but transient, increase in integrin function upon T lymphocyte activation. These integrin activating signals can be initiated either via ligation of Ig superfamily members that are coupled to tyrosine kinase cascades, such as the CD3/T cell receptor, CD2, and CD28, or by G protein-coupled receptors for chemokines. Analysis of integrin activation induced by CD3/TCR, CD2 and CD28 suggests a critical role for phosphoinositide 3-OH kinase (PI 3-K). This review summarizes recent insights into PI 3-K-dependent regulation of integrin function in leukocytes, including the mechanisms by which these receptors are coupled to PI 3-K, and potential downstream effectors of PI 3-K that regulate integrin-mediated adhesion in leukocytes.

Ribosomal S6 kinase signaling and the control of translation

The highly homologous 40S ribosomal protein S6 kinases (S6K1 and S6K2) play a key role in the regulation of cell growth by controlling the biosynthesis of translational components which make up the protein synthetic apparatus, most notably ribosomal proteins. In the case of S6K1, at least eight phosphorylation sites are believed to mediate kinase activation in a hierarchical fashion. Activation is initiated by phosphatidylinositide-3OH kinase (PI3K)-mediated phosphorylation of key residues in the carboxy-terminus of the kinase, allowing phosphorylation of a critical residue residing in the activation loop of the catalytic domain by phosphoinositide-dependent kinase 1 (PDK1). The kinases responsible for phosphorylating the carboxy-terminal sites have yet to be identified. Additionally, S6 kinases are under the control of the PI3K relative, mammalian Target Of Rapamycin (mTOR), which may serve an additional function as a checkpoint for amino acid availability. In this review we set out to discuss the present state of knowledge regarding upstream signaling components which have been implicated in the control of S6K1 activation and the role of the kinase in controlling cell growth through regulating ribosome biogenesis at the translational level.

Signaling by distinct classes of phosphoinositide 3-kinases

Many signaling pathways converge on and regulate phosphoinositide 3-kinase (PI3K) enzymes whose inositol lipid products are key mediators of intracellular signaling. Different PI3K isoforms generate specific lipids that bind to FYVE and pleckstrin homology (PH) domains in a variety of proteins, affecting their localization, conformation, and activities. Here we review the activation mechanisms of the different types of PI3Ks and their downstream actions, with focus on the PI3Ks that are acutely triggered by extracellular stimulation.

Potential role of 3-phosphoinositide-dependent protein kinase 1 (PDK1) in insulin-stimulated glucose transporter 4 translocation in adipocytes

Insulin stimulation of Glut 4 translocation requires the activation of phosphatidylinositol 3-kinase (PI 3-kinase) but the downstream pathway remains ill-defined. We demonstrated that the overexpression of PDK1 (3-phosphoinositide-dependent protein kinase 1), a downstream effector of PI 3-kinase, stimulated Glut 4 translocation in adipocytes. This effect does not require the PH domain of PDK1, but expression of the pleckstrin homology domain-deleted PDK1 inhibits the effect of insulin, but not okadaic acid, on Glut 4 translocation. These results support a role of the PDK1 pathway in the transmission of insulin signal to Glut translocation.

Requirement for Ras and phosphatidylinositol 3-kinase signaling uncouples the glucocorticoid-induced junctional organization and transepithelial electrical resistance in mammary tumor cells

In Con8 rat mammary epithelial tumor cells, the synthetic glucocorticoid dexamethasone stimulates the remodeling of the apical junction (tight and adherens junctions) and the transepithelial electrical resistance (TER), which reflects tight junction sealing. Indirect immunofluorescence revealed that dexamethasone induced the recruitment of endogenous Ras and the p85 regulatory subunit of phosphatidylinositol (PI) 3-kinase to regions of cell-cell contact, concurrently with the stimulation of TER. Expression of dominant-negative RasN17 abolished the dexamethasone stimulation in TER, whereas, dexamethasone induced the reorganization of tight junction and adherens junction proteins, ZO-1 and beta-catenin, as well as F-actin, to precise regions of cell-cell contact in a Ras-independent manner. Confocal microscopy revealed that RasN17 and the p85 regulatory subunit of PI 3-kinase co-localized with ZO-1 and F-actin at the tight junction and adherens junction, respectively. Treatment with either of the PI 3-kinase inhibitors, wortmannin or LY294002, or the MEK inhibitor PD 098059, which prevents MAPK signaling, attenuated the dexamethasone stimulation of TER without affecting apical junction remodeling. Similar to dominant-negative RasN17, disruption of both Ras effector pathways using a combination of inhibitors abolished the glucocorticoid stimulation of TER. Thus, the glucocorticoiddependent remodeling of the apical junction and tight junction sealing can be uncoupled by their dependence on Ras and/or PI 3-kinase-dependent pathways, implicating a new role for Ras and PI 3-kinase cell signaling events in the steroid control of cell-cell interactions.

Distinct Mechanisms Target Stress and Extracellular Signal-Activated Kinase 1 and Jun N-Terminal Kinase During Infection of Macrophages with Salmonella

The interaction between bacteria and macrophages is central to the outcome of Salmonella infections. Salmonella can escape killing by these phagocytes and survive and multiply within them, giving rise to chronic infections. Cytokines produced by infected macrophages are involved in the early gastrointestinal pathology of the infection as well as in the induction and maintenance of the immune response against the invaders. Jun N-terminal kinases (JNK) are activated by inflammatory stimuli and play a role in cytokine production. We have investigated the signaling routes leading to JNK activation in Salmonella-infected macrophages and have discovered that they differ radically from the mechanisms operating in epithelial cells. In particular, activation of the JNK kinase stress and extracellular-activated kinase 1 (SEK1) and of JNK in macrophages occurs independently of actin rearrangements and of the GTPases Cdc42 and Rac, essential mediators in other cells. Activation of JNK is effected by a novel pathway comprising tyrosine kinase(s), phosphoinositide 3-kinase and, likely, atypical protein kinase C ζ. SEK1 is stimulated by a distinct mechanism involving phosphatidylcholine-phospholipase C and acidic sphingomyelinase. Dominant-negative SEK1 can block JNK activation by LPS, but not by Salmonella. These data demonstrate that SEK1 and JNK are activated independently in Salmonella-infected macrophages and offer experimental support for the concept that incoming signals can direct the selective coupling of downstream pathways to elicit highly specific responses. Inhibitors of stress kinase pathways are receiving increasing attention as potential anti-inflammatory drugs. The precise reconstruction of stimulus-specific pathways will be instrumental in predicting/evaluating the effects of the inhibitors on a given pathological condition.