Activation of messenger-independent protein kinases in wild-type and phorbol ester-resistant EL4 thymoma cells

Time-resolved Phosphoproteome and Proteome Analysis Reveals Kinase Signalling on Master Transcription Factors During Myogenesis

Myogenesis is governed by signaling networks that are tightly regulated in a time-dependent manner. Although different protein kinases have been identified, knowledge of the global signaling networks and their downstream substrates during myogenesis remains incomplete. Here, we map the myogenic differentiation of C2C12 cells using phosphoproteomics and proteomics. From these data, we infer global kinase activity and predict the substrates that are involved in myogenesis. We found that multiple mitogen-activated protein kinases (MAPKs) mark the initial wave of signaling cascades. Further phosphoproteomic and proteomic profiling with MAPK1/3 and MAPK8/9 specific inhibitions unveil their shared and distinctive roles in myogenesis. Lastly, we identified and validated the transcription factor nuclear factor 1 X-type (NFIX) as a novel MAPK1/3 substrate and demonstrated the functional impact of NFIX phosphorylation on myogenesis. Altogether, these data characterize the dynamics, interactions, and downstream control of kinase signaling networks during myogenesis on a global scale.

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Phosphoproteomic and proteomic maps of myogenic differentiation of C2C12 cells

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Myogenic kinome activity and kinase-substrates prediction using machine learning

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MAPK1/3 and MAPK8/9 inhibition unveil shared and distinctive effects on myogenesis

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Validation of NFIX phosphorylation by MAPK1/3 and its impact on myogenesis

Differential expression of FAK and Pyk2 in metastatic and non-metastatic EL4 lymphoma cell lines

The murine EL4 lymphoma cell line exists in variants that are either sensitive or resistant to phorbol 12-myristate 13-acetate (PMA). In sensitive cells, PMA causes Erk MAPK activation and Erk-mediated growth arrest. In resistant cells, PMA induces a low level of Erk activation, without growth arrest. A relatively unexplored aspect of the phenotypes is that resistant cells are more adherent to culture substrate than are sensitive cells. In this study, the roles of the protein tyrosine kinases FAK and Pyk2 in EL4 phenotype were examined, with a particular emphasis on the role of these proteins in metastasis. FAK is expressed only in PMA-resistant (or intermediate phenotype) EL4 cells, correlating with enhanced cell-substrate adherence, while Pyk2 is more highly expressed in non-adherent PMA-sensitive cells. PMA treatment causes modulation of mRNA for FAK (up-regulation) and Pyk2 (down-regulation) in PMA-sensitive but not PMA-resistant EL4 cells. The increase in Pyk2 mRNA is correlated with an increase in Pyk2 protein expression. The roles of FAK in cell phenotype were further explored using transfection and knockdown experiments. The results showed that FAK does not play a major role in modulating PMA-induced Erk activation in EL4 cells. However, the knockdown studies demonstrated that FAK expression is required for proliferation and migration of PMA-resistant cells. In an experimental metastasis model using syngeneic mice, only FAK-expressing (PMA-resistant) EL4 cells form liver tumors. Taken together, these studies suggest that FAK expression promotes metastasis of EL4 lymphoma cells.

Phosphorylation of paxillin via the ERK mitogen-activated protein kinase cascade in EL4 thymoma cells

Intracellular signals can regulate cell adhesion via several mechanisms in a process referred to as "inside-out" signaling. In phorbol ester-sensitive EL4 thymoma cells, phorbol-12-myristate 13-acetate (PMA) induces activation of extracellular signal-regulated kinase (ERK) mitogen-activated protein kinases and promotes cell adhesion. In this study, clonal EL4 cell lines with varying abilities to activate ERKs in response to PMA were used to examine signaling events occurring downstream of ERK activation. Paxillin, a multifunctional docking protein involved in cell adhesion, was phosphorylated on serine/threonine residues in response to PMA treatment. This response was correlated with the extent and time course of ERK activation. PMA-induced phosphorylation of paxillin was inhibited by compounds that block the ERK activation pathway in EL4 cells, primary murine thymocytes, and primary murine splenocytes. Paxillin was phosphorylated in vitro by purified active ERK2. Two-dimensional electrophoresis revealed that PMA treatment generated a complex pattern of phosphorylated paxillin species in intact cells, some of which were generated by ERK-mediated phosphorylation in vitro. An ERK pathway inhibitor interfered with PMA-induced adhesion of sensitive EL4 cells to substrate. These findings describe a novel inside-out signaling pathway by which the ERK cascade may regulate events involved in adhesion.

Mechanisms of MAPK activation by bradykinin in vascular smooth muscle cells

Vascular smooth muscle cell (VSMC) proliferation is a prominent feature of the atherosclerotic process occurring after endothelial injury. A vascular wall kallikrein-kinin system has been described. The contribution of this system to vascular disease is undefined. In the present study we characterized the signal transduction pathway leading to mitogen-activated protein kinase (MAPK) activation in response to bradykinin (BK) in VSMC. Addition of 10(-10)-10(-7) M BK to VSMC resulted in a rapid and concentration-dependent increase in tyrosine phosphorylation of several 144- to 40-kDa proteins. This effect of BK was abolished by the B(2)-kinin receptor antagonist HOE-140, but not by the B(1)-kinin receptor antagonist des-Arg(9)-Leu(8)-BK. Immunoprecipitation with anti-phosphotyrosine antibodies followed by immunoblot revealed that 10(-9) M BK induced tyrosine phosphorylation of focal adhesion kinase (p125(FAK)). BK (10(-8) M) promoted the association of p60(src) with the adapter protein growth factor receptor binding protein-2 and also induced a significant increase in MAPK activity. Pertussis and cholera toxins did not inhibit BK-induced MAPK tyrosine phosphorylation. Protein kinase C downregulation by phorbol 12-myristate 13-acetate and/or inhibitors to protein kinase C, p60(src) kinase, and MAPK kinase inhibited BK-induced MAPK tyrosine phosphorylation. These findings provide evidence that activation of the B(2)-kinin receptor in VSMC leads to generation of multiple second messengers that converge to activate MAPK. The activation of this crucial kinase by BK provides a strong rationale to investigate the mitogenic actions of BK on VSMC proliferation in disease states of vascular injury.

Different sensitivities of p42 mitogen-activated protein kinase to phorbol ester and okadaic acid tumor promoters among cell types

The operational equivalence of different types of tumor promoters was studied by comparing immediate, early, and late effects of okadaic acid (OA) and 12-O-tetradecanoylphorbol-13-acetate (TPA) on the phosphorylation state of p42 mitogen-activated protein kinase isoform (ERK2) in eight different cell lines. In normal human and mouse fibroblasts, both agents stimulated immediate/early (15-60 min) phosphorylation of ERK2. In mouse 3T3 cells, enhanced phosphorylation of ERK2 was detected only within the first hour of treatment with TPA but not with OA. The early response to both TPA and OA, in turn, was lost in another established cell line, the PNT2 prostate epithelial cells, where we could detect increased levels of phosphorylated ERK2 only after a 24-hr treatment with OA. When the effect of OA was evaluated in different PNT cell strains, we observed that their capacity to respond to this agent, by stabilizing phosphorylated forms of ERK2, was lost in less differentiated strains. In HeLa S3 and HTC tumor cells, however, neither TPA nor OA treatment led to any detectable increase in ERK2 phosphorylation at any time point analyzed. We conclude that the effects of OA and TPA on the phosphorylation states of ERK2 could be related to the cell type, and that the operational equivalence between these two different tumor promoters is maximal in normal cells.

Follicle-stimulating hormone promotes histone H3 phosphorylation on serine-10

FSH promoted the rapid phosphorylation of the nuclear protein histone H3 in immature rat ovarian granulosa cells under experimental conditions that lead to cellular differentiation and not proliferation. FSH-stimulated histone H3 phosphorylation correlated with cAMP-dependent protein kinase A (PKA) activation and translocation of the PKA catalytic subunit to a nuclear-enriched fraction and was inhibited by the PKA inhibitor H89, and histone H3 phosphorylation was stimulated in cells treated with agents that raise intracellular cAMP levels such as forskolin and 8-bromo-cAMP. FSH-stimulated histone H3 phosphorylation in granulosa cells mapped to ser-10, a site previously identified as the PKA phosphorylation site in various mitotically active cells as the mitosis-specific phosphorylation site. Injection of the FSH analog PMSG to immature rats, which is known to stimulate granulosa cell proliferation as well as differentiation, also promoted histone H3 phosphorylation on ser-10 in granulosa cells. These results establish that FSH-stimulated histone H3 phosphorylation in granulosa cells is linked not only to granulosa cell mitosis but also to granulosa cell differentiation and that FSH-stimulated histone H3 phosphorylation on ser-10 in isolated granulosa cells is mediated by PKA. These results also identify the PKA-dependent histone H3 phosphorylation as an early nuclear protein marker for FSH-stimulated differentiation of granulosa cells. Based on the recently described function of histone H3 as a coactivator of transcription, these results are consistent with the hypothesis that phosphorylated histone H3 may facilitate PKA-dependent gene transcription in granulosa cells leading to the preovulatory phenotype.

H-7-induced apoptosis in the cells of a Drosophila neuronal cell line through affecting unidentified H-7-sensitive substance(s)

The present study was undertaken to reveal underlying mechanisms of apoptosis in neurons using clonal neuronal cells, ML-DmBG2-c2, derived from Drosophila larval central nervous system 1-(5-Isoquinolinesulfonyl)-2-methylpiperazine (H-7), a protein kinase inhibitor, induced cell death with typical features of apoptosis such as internucleosomal DNA fragmentation, nuclear condensation and apoptotic bodies in the cells. Though H-7 is known to inhibit cAMP-dependent protein kinase (PKA), protein kinase C (PKC), cGMP-dependent protein kinase (PKG), myosin light chain kinase (MLCK), and casein kinase I (CKI), specific inhibitors for these kinases such as H-89, calphostin C, ML-9, or CKI-7 did not induce apoptosis in the cells. Other kinases such as tyrosine kinase. PI3-kinase and Ca2+/CaM kinase II so far examined in the present study were interpreted not to be involved in the apoptotic cascade. Therefore, it is concluded that an H-7-sensitive substance(s) other than these kinases is responsible for the apoptosis in the neuronal cells. Caspase inhibitors prevented apoptosis in the cells treated with H-7. These results suggest that caspase(s) is involved downstream of the H-7-sensitive point in the cascade of the apoptosis.

MAP kinase-dependent pathways in cell cycle control

Mitogen-activated protein kinases such as Erk1 and Erk2 serve as a paradigm for a growing family of proline-directed protein kinases that mediate entry, progression and exit from the cell cycle in diverse eukaryotic cells. These enzymes function within highly conserved modules of sequentially activating protein kinases that transduce signals from diverse extracellular stimuli. In vertebrates, at least three distinct kinases modules have been characterized. Mitogens induce the sequential activation of the kinases Raf1-->Mek1-->Erk2-->Rsk via the G-protein Ras. Stress factors stimulate c-Jun activation through a related kinase pathway involving Mekk-->Sek-->SAPK c-Jun, and hsp27 phosphorylation via the MKK3-->Hog-->MAPKAPK-2 hsp27 route. Genetic and biochemical studies, for example from budding yeast, imply the existence of several related protein kinase modules that can operate in parallel or within integrated systems.

Redox regulation of signal transduction in smooth muscle cells: distinct effects of PKC-down regulation and PKC inhibitors on oxidant induced MAP kinase

Reactive oxygen species function as signaling molecules, and are known to be generated under both normal and pathological conditions. Using vascular smooth muscle cells, we have demonstrated an increase in mitogen activated protein kinase activity in response to oxidants. Mitogen activated protein kinase activity increased linearly with time in cells treated with pervanadate. Hydrogen peroxide also caused rapid induction of mitogen activated protein kinase. Protein kinase C down regulation partially decreased induction of mitogen activated protein kinase activity by oxidants, and the Ca2+ ionophore, ionomycin. Protein kinase C inhibitors, compound-3 and bisindolylmaleimide did not inhibit oxidant induced mitogen activated protein kinase activity, where as calphostin C activated it. The tyrosine kinase inhibitors genistein, herbimycin A and tyrphostin caused 50% inhibition of oxidant induced mitogen activated protein kinase activation. These results suggest that oxidant-induced mitogen activated protein kinase is protein kinase C independent.

The protein kinase C pseudosubstrate peptide (PKC19-36) inhibits insulin-stimulated protein kinase activity and insulin-mediated translocation of the glucose transporter glut 4 in streptolysin-O permeabilized adipocytes

The effect of insulin on protein kinase activity and plasma membrane translocation of the glucose transporter GLUT 4 has been studied in adipocytes permeabilized by Streptolysin-O. Insulin increased protein kinase activity, and this was completely inhibited by the PKC pseudosubstrate inhibitor peptide (PKC19-36). Insulin-mediated translocation of GLUT 4 was also inhibited by the PKC inhibitor peptide. Both these insulin effects were blocked by a PKCbeta neutralizing antibody. Our results are consistent with the hypothesis that insulin activates PKCbeta activity in adipocytes in situ, and that this PKC activation is a component of the system whereby insulin regulates translocation of GLUT 4 to the plasma membrane.

Orthovanadate stimulates cyclic guanosine monophosphate-inhibited cyclic adenosine monophosphate phosphodiesterase activity in isolated rat fat pads through activation of particulate myelin basic protein kinase by protein tyrosine kinase

Involvement of protein kinases in the stimulation of cGMP-inhibited cAMP phosphodiesterase (PDE) activity by orthovanadate (vanadate) was studied. When the fat pads were incubated with 2 mM vanadate or 10 nM insulin, the stimulation of myelin basic protein kinase (MBPK) activity in the particulate by vanadate reached a maximum at 60 min. In contrast, insulin showed a transient increase at 20 min. A 60-min incubation of the fat pads with vanadate stimulated all activities of protein tyrosine kinase (PTK), MBPK, and PDE in the particulate, in a similar dose-dependent manner. Amiloride, a PTK inhibitor, inhibited the stimulations of three enzymes by vanadate in a similar concentration range. Enzyme fractions, which were separated from the solubilized particulate, were subjected to the immunoblot analysis. A fraction of MBPK was identified to contain a major protein of mol wt (44K) and a minor one (42K), both of which are immunoreactive with a mitogen-activated protein kinase (MAPK) antibody. The partially purified PDE activity was stimulated by the addition of the partially purified MBPK. The further stimulation was observed with the PTK-activated MBPK. These results suggest that vanadate stimulates in part the PDE activity through the activation of the particulate MBPK, probably MAPKs, by PTK sensitive to vanadate.

Factors affecting the cytokine production of human T cells stimulated by different modes of activation

According to the widely accepted classification, human TH cell clones can be divided into two mutually exclusive subsets, TH1 and TH2, based on their profile of cytokine production. The intracellular difference between these clones is not clear. To characterize the biochemical nature of T-cell receptor (TCR)/CD3 complex-mediated signal transduction pathways, we introduced several human TH cell clones of THO- or TH1-like phenotype and analyzed the effects of various drugs and antibodies on cytokine production or proliferation of these clones. The tyrosine kinase inhibitor herbimycin inhibited the production of interferon-gamma (IFN-gamma) by THO-like clone, after stimulation with anti-CD3 monoclonal antibody alpha CD3-mAb) or with phorbol 12-myristate 13-acetate (PMA) and the calcium ionophore A23187. However, whereas herbimycin strongly inhibited the production of IL-4 and IL-5 by alpha CD3 mAb stimulated T cells, it did not affect the production of these cytokines after PMA/A23187 stimulation. Cyclosporin A inhibited the proliferation as well as the production of the cytokines, including that of IL-2, IL-4, IL-5, and IFN-gamma, irrespective of the mode of stimulation. A23187, which synergizes with PMA in the induction of IL-4 and IFN-gamma, inhibited PMA-induced IL-10 production in a dose-dependent manner. Transforming growth factor-beta and anti-IL-2 receptor monoclonal antibody partially inhibited alpha CD3 mAb-mediated T-cell proliferation, but had no effect on the proliferation induced by PMA and A23187. Cyclic adenosine monophosphate (cAMP)-elevating drugs, like prostaglandin E2 and dibutyryl cAMP, inhibited the TCR-mediated cytokine production but shifted the cytokine production profile from a TH0 to a TH2 type after stimulation with PMA and A23187. Finally, we analyzed the induction of activity of two transcription factors, nuclear factor-kappa B (NF-kappa B) and nuclear factor of activated T cells, involved in the regulation of cytokine gene expression, after a different mode of activation. The induction of NF-kappa B (p50/p65 heterodimer) by using alpha CD3-mAb stimulation but not by using PMA/A23187 stimulation was found to be inhibited by using cAMP-elevating drugs.

Effects of phorbol ester on phospholipase D and mitogen-activated protein kinase activities in T-lymphocyte cell lines

The effects of phorbol 12-myristate 13-acetate (PMA) on the activities of phospholipase D (PLD3), mitogen-activated protein kinase (ERK), and c-Jun N-terminal kinase (JNK) were studied in Jurkat, a human T cell line, and EL4, a murine T-cell line. PMA treatment rapidly activated PLD in Jurkat, as detected either in intact or broken cells. In contrast, PMA did not stimulate PLD activity in EL4 cells. PLD activity was not detected in membranes prepared from EL4 cells. Jurkat, but not EL4, expresses a 120-kDa protein recognized by an anti-PLD antibody. In both Jurkat and EL4 cells, PMA caused activation of ERKs. Incubation of EL4 cells with bacterial PLD increased phosphatidic acid levels, but did not activate ERK. In both EL4 and Jurkat cells, co-stimulation with PMA and ionomycin stimulated JNK activity. These results show that activation of PLD is not required for activation of ERKs or JNKs by PMA in T-cell lines. Thus, while PLD activity is expressed in some T-cell lines, the role of this enzyme and its products in T-cell activation remain to be elucidated.

Effects of endothelin on mitogen-activated protein kinase activity and protein synthesis in isolated adult feline cardiac myocytes

The growth-promoting effects of endothelin-1 (ET-1) were examined in adult heart cells. The activity of mitogen-activated protein kinases (MAPKs) was measured in cytosolic extracts of isolated adult feline cardiac myocytes incubated with and without ET-1. Kinase activity was assessed by phosphorylation of the exogenous substrate, myelin basic protein. ET-1 stimulated the activity of MAPK up to 4-fold, with peak activation occurring between five and ten minutes after addition of ET-1. Polyclonal antisera raised against a 14-amino acid sequence of the erk-2 gene product, a MAPK isoform, identified two major bands in cytosolic extracts of the cardiac myocytes. Partial purification of kinase activities using Mono Q anion-exchange chromatography demonstrated two major peaks of myelin basic protein kinase activity. Subsequent immunoblots of the eluted fractions demonstrated that the immunoreactive bands observed in the cytosolic extracts eluted in those fractions possessing kinase activity. Overnight pretreatment of the cardiac myocytes with 100 ng/ml pertussis toxin inhibited the ET-1 stimulated increase in MAPK activity by 50 - 70%, but did not alter stimulation by 100 nM phorbol 12-myristate 13-acetate (PMA). These data suggest that stimulation of MAPK by ET-1 may be mediated by more than one pathway. MAPK has been shown to be activated in the intracellular transmission of growth factor signals. Indicative of a growth effect in this adult heart cell model, myocytes exposed to increasing concentrations of ET-1 demonstrated a dose dependent increase in [3H]-phenylalanine incorporation into cellular protein. This response was blocked by staurosporine and partially inhibited by pretreatment with pertussis toxin, again suggesting the possible involvement of multiple early signals. These data from isolated adult cardiac myocytes further support the hypothesis that ET-1 has a role in the regulation of cardiac growth.

Sequential activation of MAP kinase cascade by angiotensin II in opossum kidney cells

Angiotensin II (Ang II) is a potent regulator of proximal tubule functions, including transport, metabolism, and cell proliferation. The opossum kidney (OK) cell line is a useful model of renal proximal tubule. Mitogen-activated protein (MAP) kinases are rapidly phosphorylated and activated in response to various agonists. We investigated Ang II effects on serine/threonine kinase cascades in OK cells. The major findings of the present study are that Ang II stimulated MAP kinase kinase (MAPKK), MAP kinase (MAPK), and S6 kinase activities, and that it increased phosphorylation of Raf-1 kinase and p42 MAP kinase in OK cells. These stimulations of kinases were dose-dependent (from 10(-6) to 10(-11) M). The time course of activation was sequential; the peak stimulation was reached at 5 to 10 minutes for Raf-1 kinase, MAPKK and MAPK, and at 20 minutes for S6 kinase. The activation of MAPK was inhibited by approximately 70% with prolonged 24-hour PMA pretreatment or in the presence of calphostin C or H-7. Tyrosine kinase inhibitors (genistein and herbimycin) did not inhibit AngII-induced MAPK activity. This activation of MAPK was also inhibited via AT1 receptor antagonist, Dup753 and pertussis toxin. This evidence suggests that the activation of serine/threonine cascades by Ang II is largely dependent on PMA-sensitive PKC, and is not dependent on tyrosine kinase and pertussis toxin.

c-Jun N-terminal kinase but not mitogen-activated protein kinase is sensitive to cAMP inhibition in T lymphocytes

The molecular mechanism underlying the cAMP inhibition of nuclear activation events in T lymphocytes is unknown. Recently, the activation of fibroblasts and muscle cells are shown to be antagonized by cAMP through the inhibition of mitogen-activated protein (MAP) kinases signaling pathway. Whether a similar antagonism may account for the late inhibitory effect of cAMP in T cell was examined. Surprisingly, extracellular signal regulated kinase 2 (ERK1, ERK2, and ERK3) of MAP kinase were poorly inhibited by cAMP. High concentration of cAMP also only weakly antagonized Raf-1 in T cells. The resistance of ERK and Raf-1 to cAMP clearly distinguishes T cells from fibroblasts. In contrast, another MAP kinase homologue c-Jun N-terminal kinase (JNK) was inhibited by cAMP in good correlation with that of IL-2 suppression. Moreover, JNK was antagonized by a delayed kinetics which is characteristic of cAMP inhibition. Despite that both ERK and JNK are essential for T cell activation, selective inhibition by cAMP further supports the specific role of JNK in T cell activation.

Iloprost-induced inhibition of proliferation of coronary artery smooth muscle cells is abolished by homologous desensitization

In addition to inhibition of platelet function, prostacyclin and its stable analogues are reported to attenuate vascular smooth muscle cell proliferation. However, desensitization of prostacyclin responsiveness is a known phenomenon in platelets. In this study we investigated the time-dependent effects of the prostacyclin-mimetic iloprost and of PGE1, respectively, on PDGF-induced proliferation of cultured coronary artery smooth muscle cells. Proliferation, assessed by [3H]thymidine incorporation was markedly inhibited by coincubation with iloprost (100 nM) and PGE1 (100 nM) for 4 h. In contrast, addition of iloprost (100 nM) for 24 h did not decrease smooth muscle cell proliferation, whereas inhibition by PGE1 or by forskolin was not diminished. These results suggest a homologous desensitization of anti-mitogenic effects of iloprost in coronary artery smooth muscle cells, probably at receptor-level.

Involvement of protein kianse C and Ca2+ in angiotensin II-induced mitogenesis of cardiac fibroblasts

Angiotensin (ANG) II has been previously shown to stimulate proliferation of neonatal rat cardiac fibroblasts via AT1 receptors. Here we conducted studies to assess involvement in this process of two second messengers linked to AT1 receptors, protein kinase C (PKC) and Ca2+. Several findings argue against a dominant role for PKC in ANG II-induced mitogenesis: 1) [Sar1]ANG II, which produced a modest, transient increase in PKC activity, was equally effective in inducing thymidine incorporation into DNA in PKC-depleted cells, whereas the effect of platelet-derived growth factor (PDGF)-BB on thymidine incorporation was reduced to the level observed with [Sar1]ANG II; 2) phorbol 12-myristate 13-acetate (PMA), a potent PKC stimulator, was ineffective in stimulating thymidine incorporation; and 3) PKC downregulation or the highly specific PKC inhibitor, compound 3, eliminated PMA-induced mitogen-activated protein (MAP) kinase activity but did not affect comparable increases induced by [Sar1]ANG II or PDGF-BB. Increased intracellular Ca2+ may be sufficient to account for [Sar1]ANG II-induced MAP kinase activity because ionomycin also increased MAP kinase activity and chelation of intracellular Ca2+ eliminated [Sar1]ANG II-induced activity in PKC-depleted fibroblasts. However, Ca2+ chelation did not prevent [Sar1]ANG II-induced MAP kinase activity in non-PKC-depleted fibroblasts. Thus ANG II can activate MAP kinase in cardiac fibroblasts by either Ca(2+)- or PKC-dependent pathways, and whereas the full effect of PDGF-BB on thymidine incorporation and cell proliferation requires a phorbol ester-sensitive PKC, the hyperplastic growth effect of ANG II does not.

Lipopolysaccharide-induced cytokine production in human monocytes: role of tyrosine phosphorylation in transmembrane signal transduction

The signal transduction events that follow the binding of lipopolysaccharide (LPS) to the macrophage cell surface are not well defined. In the current studies LPS was found to induce alterations in phosphorylation of monocyte proteins on tyrosine. Herbimycin A and genistein, inhibitors of tyrosine kinases, markedly attenuated LPS-induced tumor necrosis factor-alpha (TNF-alpha) and interleukin-6 (IL-6) protein and mRNA production. Reciprocally, the tyrosine phosphatase inhibitor sodium orthovanadate enhanced LPS-induced production of TNF-alpha. LPS induced a concentration-dependent increase in tyrosine phosphorylation of several proteins, which paralleled and preceded the onset of LPS-induced TNF-alpha production. LPS stimulation had different but reproducible effects on three members of the src family of tyrosine kinases. Both Hck and Lyn kinase activity increased before the onset of TNF-alpha production, consistent with their participation in the observed LPS-induced tyrosine phosphoprotein accumulation. In contrast, Yes kinase activity was not affected. These observations were made at concentrations of LPS that required serum rich in LPS-binding protein and the monocyte surface antigen CD14 for TNF-alpha production. These data indicate that tyrosine kinases and phosphatases are involved in the signal transduction cascade by which LPS induces production of TNF-alpha and IL-6 by human monocytes, and suggest that Lyn and Hck are candidate participants in this process.

Signalling pathways as targets for anticancer drug development

Intracellular signalling pathways mediating the effects of oncogenes on cell growth and transformation offer novel targets for the development of anticancer drugs. With this approach, it may be sufficient to target a component of the signalling pathway activated by the oncogene rather than the oncogene product itself. In this review, the abilities of some antiproliferative drugs to inhibit signalling targets are considered. There are some anticancer drugs already in clinical trial that may act by inhibiting signalling targets, as well as drugs in preclinical development. Some problems that may be encountered in developing this new class of anticancer drugs are discussed.

Activation of peripheral large granular lymphocytes with the serine/threonine phosphatase inhibitor, okadaic acid

The murine polyether fatty acid, okadaic acid, is a potent inhibitor of serine/threonine phosphatases in eukaryotic cells. This compound inhibits both protein phosphatase 1 (PP1) and phosphatase 2A (PP2A). Here we have examined the potential of okadaic acid as an activator of fresh peripheral CD3- large granular lymphocytes (LGL). We demonstrate that overnight exposure of LGL to as little as 1 nM okadaic acid induced an increase in natural killing against the K562 cell line, but does not induce LAK activity. Optimal cytotoxic activation (2-fold) occurred at 0.01-1.0 nM okadaic acid, with a return to baseline levels at 10-20 nM, and inhibition, likely due to toxicity, at 40 nM. In addition, okadaic acid at doses > or = 20 nM induced LGL but not T cells to produce interferon-gamma. Similar to phorbol esters, overnight incubation with okadaic acid causes a dose-dependent reduction in expression of the low-affinity receptor for the Fc portion of IgG (CD16). However, unlike phorbol ester, short-term (5 min) okadaic acid treatment did not block CD16-mediated Ca2+ mobilization in LGL. To address the underlying biochemical mechanisms of okadaic acid activities, the levels of several as-yet-unidentified serine/threonine kinases were assayed after renaturation. Under these conditions, okadaic acid induced similar increases in kinase levels in both T cells and LGL. Taken together, these data suggest an important role for PP1 and PP2A in LGL physiology, and define okadaic acid as a potentially important biological response modifier for the study of LGL and T cell biochemistry, signal transduction, and transcriptional regulation.

Effect of UCN-01, a selective inhibitor of protein kinase C, on the cell-cycle distribution of human epidermoid carcinoma, A431 cells

UCN-01 (7-hydroxy-staurosporine), a selective inhibitor of protein kinase C (PKC), was shown to exhibit antitumor activity in murine and human tumor cell lines in vitro and in vivo. On the other hand, staurosporine, a non-selective protein kinase inhibitor, was not shown to exert antitumor activity in vivo despite its potent antiproliferative activity in vitro. To compare the modes of action of UCN-01 and staurosporine in vitro, the effects of both drugs on the cell cycle progression of human epidermoid carcinoma A431 cells were examined by flow cytometry using propidium iodide (PI) staining. At 50% growth inhibitory concentrations, both UCN-01 and staurosporine induced G1 phase accumulation in the cell cycle. At 80% growth inhibitory concentrations, UCN-01 also induced preferential G1 phase accumulation, but staurosporine mostly induced G2M phase accumulation. Staurosporine also induced higher DNA ploidy when the cells were exposed to the drug for more than one generation time of A431 cells. An analysis of cell kinetics by 5-bromo-2-deoxyuridine incorporation versus DNA content confirmed that the G1 phase block by UCN-01 and the G1 and G2M phase block by staurosporine at the respective doses, as was the case for PI staining. Additionally, DNA synthesis of the cells, which was determined by the uptake of 3H-TdR, was not suppressed at least 8 h after the treatment with UCN-01. These results suggested that UCN-01 could affect the G1 phase of cell cycle in A431 cells in quite different manners from staurosporine. The G1 phase block induced by UCN-01 might be important for the growth inhibitory activity of UCN-01 against A431 cells in vitro and in vivo.

Networking with mitogen-activated protein kinases

Mitogen activated protein (MAP) kinases and their target ribosomal protein S6 (RSK) kinases have been recognized as shared components in the intracellular signaling pathways of many diverse cytokines. Recent studies have extended this protein kinase cascade by identifying the major activator of vertebrate MAP kinases as a serine/threonine/tyrosine-protein kinase called MEK, which is related to yeast mating factor-regulated protein kinases encoded by the STE7 and byr1 genes. MEK, in turn, may be activated following its phosphorylation on serine by either of the kinases encoded by proto-oncogenes raf1 or mos, as well as by p78mekk, which is related to the yeast STE11 and byr2 gene products. Isoforms of all of these protein kinases may specifically combine to assemble distinct modules for intracellular signal transmission. However, the fundamental architecture of these protein kinase cascades has been highly conserved during eukaryotic evolution.

Ras is essential for nerve growth factor- and phorbol ester-induced tyrosine phosphorylation of MAP kinases

Treatment of PC12 cells with nerve growth factor (NGF) induces a rapid increase in tyrosine phosphorylation of multiple cellular proteins. Expression of a dominant inhibitory Ras mutant specifically blocked NGF- and TPA-induced tyrosine phosphorylation of two proteins of approximately 42 and 44 kd. Conversely, expression of an oncogenic variant of Ras induced tyrosine phosphorylation of the same 42 and 44 kd proteins. The 44 kd protein was immunoprecipitated with an antibody directed against extracellular signal-regulated kinase 1/mitogen-activated protein kinase (MAPK) and the 42 kd protein comigrated with a 42 kd MAPK, indicating that at least one and probably both Ras-regulated phosphoproteins are MAPKs. In addition, MAPK activation, as measured by in vitro phosphorylation of myelin basic protein, was also regulated by Ras. Ras was not required for NGF-induced activation of Trk or tyrosine phosphorylation of PLC-gamma 1. Thus, NGF-induced tyrosine phosphorylation occurs both prior to and following Ras action, and Ras plays a critical role in the NGF- and TPA-induced tyrosine phosphorylation of MAPKs.

Recent progress in characterization of protein kinase cascades for phosphorylation of ribosomal protein S6

Ribosomal protein S6 is phosphorylated in response to mitogens by activation of one or more protein kinase cascades. Phosphorylation of S6 in vivo is catalyzed by (at least) two distinct mitogen-activated S6 kinase families distinguishable by size, the 70 kDa and 90 kDa S6 kinases. Both S6 kinases are activated by serine/threonine phosphorylation. Members of each family have been cloned. The 90 kDa S6 kinases are activated more rapidly than the 70 kDa S6 kinase, and may have other intracellular targets. The 70 kDa S6 kinase is relatively specific for 40 S ribosomal subunits. No kinase capable of activating the 70 kDa S6 kinase has been identified. Members of the 90 kDa S6 kinases are activated in vitro by 42 kDa and 44 kDa MAP kinases, which are in turn activated by mitogen-dependent activators. The pathways for mitogen-stimulated S6 phosphorylation are discussed.

Protein kinase C(19-31) pseudosubstrate inhibition of insulin action in rat adipocytes

Treatment of intact adipocytes with the autoregulatory PKC pseudosubstrate PKC(19-31) inhibited insulin-stimulated hexose uptake and lipogenesis, with no effect on basal values. The effect was dose-dependent with respective IC50 values of 30 microM and 600 microM for insulin-stimulated hexose uptake in electroporated and intact adipocytes. These studies indicate that PKC may play a role in the mediation of insulin action in adipocytes.