Activation of myelin basic protein and S6 peptide kinases in phorbol ester- and PAF-treated sheep platelets

Protein kinase C regulation of 12-lipoxygenase-mediated human platelet activation

Platelet activation is important in the regulation of hemostasis and thrombosis. Uncontrolled activation of platelets may lead to arterial thrombosis, which is a major cause of myocardial infarction and stroke. After activation, metabolism of arachidonic acid (AA) by 12-lipoxygenase (12-LOX) may play a significant role in regulating the degree and stability of platelet activation because inhibition of 12-LOX significantly attenuates platelet aggregation in response to various agonists. Protein kinase C (PKC) activation is also known to be an important regulator of platelet activity. Using a newly developed selective inhibitor for 12-LOX and a pan-PKC inhibitor, we investigated the role of PKC in 12-LOX-mediated regulation of agonist signaling in the platelet. To determine the role of PKC within the 12-LOX pathway, a number of biochemical endpoints were measured, including platelet aggregation, calcium mobilization, and integrin activation. Inhibition of 12-LOX or PKC resulted in inhibition of dense granule secretion and attenuation of both aggregation and αIIbβ(3) activation. However, activation of PKC downstream of 12-LOX inhibition rescued agonist-induced aggregation and integrin activation. Furthermore, inhibition of 12-LOX had no effect on PKC-mediated aggregation, indicating that 12-LOX is upstream of PKC. These studies support an essential role for PKC downstream of 12-LOX activation in human platelets and suggest 12-LOX as a possible target for antiplatelet therapy.

Glycoprotein IIb/IIIa inhibition attenuates platelet-activating factor-induced platelet activation by reducing protein kinase C activity

Glycoprotein (GP)IIb/IIIa inhibition may abolish activated leukocyte-induced platelet activation, in which leukocyte-released platelet-activating factor (PAF) is a major mediator. The present study thus investigated if and how GPIIb/IIIa inhibitors interfere with PAF-induced platelet activation. Platelet and leukocyte activation were monitored by flow cytometry and immunoblotting. GPIIb/IIIa inhibitors (c7E3, non-peptide SR121566, and MAb RFGP56) attenuated PAF-induced, but not adenosine diphosphate (ADP)- or thrombin receptor activating peptide (TRAP)-induced platelet P-selectin expression in whole blood. GPIIb/IIIa blockade enhanced ADP- or TRAP-induced leukocyte CD11b expression, but not the response to PAF. GPIIb/IIIa blockade attenuated PAF-induced, but enhanced ADP- or TRAP-induced platelet-leukocyte aggregation. Under the present experimental conditions, thromboxane A2 receptor antagonism did not significantly influence PAF-induced platelet activation, and GPIIb/IIIa inhibition did not interfere with calcium mobilization/influx in platelets. Protein kinase C (PKC) blockade inhibited PAF-induced platelet P-selectin expression, and PAF-induced PKC activity was reduced by GPIIb/IIIa inhibition. PAF (=1 micro m) did not induce MEK 1/2 or ERK 1/2 phosphorylation, whilst thrombin induced marked responses, which were enhanced by GPIIb/IIIa blockade. Thus, GPIIb/IIIa inhibition attenuates PAF-induced platelet activation via inhibiting PKC activity. GPIIb/IIIa blockade enhances thrombin-induced platelet MEK 1/2 and ERK 1/2 activation, and augments ADP- and TRAP-induced leukocyte activation by enhancing platelet-leukocyte aggregation.

Platelet ERK2 activation by thrombin is dependent on calcium and conventional protein kinases C but not Raf-1 or B-Raf

Extracellular signal-regulated kinase (ERK) activation pathways have been well characterized in a number of cell types but very few data are available for platelets. The thrombin-induced signaling pathway leading to ERK2 activation in platelets is largely uncharacterized. In this study, we investigated the kinases involved in thrombin-induced ERK2 activation in conditions of maximal ERK2 activation. We found that thrombin-induced mitogen-activated protein kinase/ERK kinase (MEK)1/2 activation was necessary for ERK2 phosphorylation. We obtained strong evidence that conventional protein kinase Cs (PKCs) and calcium are involved in thrombin-induced ERK2 activation. First, ERK2 and MEK1/2 phosphorylation was totally inhibited by low concentrations (1 microM) of RO318425, a specific inhibitor of conventional PKCs. Second, Ca(2+), from either intracellular pools or the extracellular medium, was necessary for ERK2 activation and conventional PKC activation, excluding the involvement of a new class of calcium-insensitive PKCs. Third, LY294002 and wortmannin had no significant effect on ERK2 activation, even at concentrations that inhibit phosphatidylinositol (PI)3-kinase (5 microM to 25 microM and 50 nM, respectively). This suggests that PI3-kinase was not necessary for ERK2 activation and therefore, that PI3-kinase-dependent atypical PKCs were not involved. Surprisingly, in contrast to proliferative cells, we found that the serine/threonine kinases Raf-1 and B-Raf were not an intermediate kinase between conventional PKCs and MEK1/2. After immunoprecipitation of Raf-1 and B-Raf, the basal glutathione S-transferase-MEK1 phosphorylation observed in resting platelets was not upregulated by thrombin and was still observed in the absence of anti-Raf-1 or anti-B-Raf antibodies. In these conditions, the in vitro cascade kinase assay did not detect any MEK activity. Thus in platelets, thrombin-induced ERK2 activation is activated by conventional PKCs independently of Raf-1 and B-Raf activation.

Phosphorylation of STAT-3 in response to basic fibroblast growth factor occurs through a mechanism involving platelet-activating factor, JAK-2, and Src in human umbilical vein endothelial cells. Evidence for a dual kinase mechanism

Platelet-activating factor (PAF) is a potent proinflammatory phospholipid with multiple pathological and physiological effects. We have shown that basic fibroblast growth factor (bFGF) supplementation induces rapid proliferation of human umbilical vein endothelial cells (HUVEC), which is reduced upon removal of bFGF or by bFGF immunoneutralization. The PAF receptor antagonist LAU-8080 inhibited bFGF-stimulated HUVEC proliferation, indicating the involvement of PAF in the bFGF-mediated signaling of HUVEC. Although FGF receptor phosphorylation was not affected by LAU-8080, the bFGF-mediated prolonged phosphorylation, and activation of Erk-1 and -2 were attenuated. Phosphorylation of STAT-3 was observed in the presence of PAF or bFGF, which was attenuated by PAFR antagonists. PAF-induced STAT-3 phosphorylation observed in HUVEC pretreated with either Src inhibitor PP1 or JAK-2 inhibitor AG-490 indicated (i) immediate (1 min) phosphorylation of STAT-3 is dependent on Src, (ii) JAK-2-dependent STAT-3 phosphorylation occurs after the delayed (30 min) PAF exposure, and (iii) prolonged (60 min) STAT-3 phosphorylation may be either through Src and/or JAK-2. Attenuation of the STAT-3 phosphorylation by the PAFR antagonists indicated signaling through the PAF receptor. Taken together, these findings suggest the production of PAF is important for bFGF-mediated signaling and that a dual kinase mechanism is involved in the PAF-mediated signal transduction cascade.

Reverse relationship between mitogen activated protein kinase and human platelet aggregation

The role of MAPK in platelets was investigated. In human platelets maintained at 4 degrees C for 2 hr, the MAPK activity increased (approximately 2 fold) when compared to those maintained at 37 degrees C. When aggregation was monitored under these conditions, the platelets maintained at 4 degrees C or 15 degrees C showed an 85% and 71% decrease respectively to thrombin (0.5 U/ml for 1 min) induced aggregation. When the platelet cytosol was maintained at 4 degrees C and assayed for MAPK activity, the MAPK activity decreased significantly, indicating that the observed effects are seen in intact platelets only, and are not due to temperature effects on the assay. When platelets maintained at 4 degrees C or 15 degrees C (for 2 hrs) were transferred to 37 degrees C, the MAPK activity decreased to levels observed in platelets maintained throughout at 37 degrees C and was thus reversible. Therefore, it is concluded that a possible reverse relationship between MAPK and platelet aggregation plays a role in platelet responses.

Involvement of different protein kinases and phospholipases A2 in phorbol ester (TPA)-induced arachidonic acid liberation in bovine platelets

The effect of various phospholipase A2 and protein kinase inhibitors on the arachidonic acid liberation in bovine platelets induced by the protein kinase activator 12-O-tetradecanoylphorbol-13-acetate (TPA) was studied. TPA stimulates arachidonic acid release mainly by activating group IV cytosolic PLA2 (cPLA2), since inhibitors of this enzyme markedly inhibited arachidonic acid formation. However, group VI Ca2+-independent PLA2 (iPLA2) seems to contribute to the arachidonic acid liberation too, since the relatively specific iPLA2 inhibitor bromoenol lactone (BEL) decreased arachidonic acid generation in part. The pronounced inhibition of the TPA-induced arachidonic acid release by the protein kinase C (PKC) inhibitors GF 109203X and Ro 31-82220, respectively, and by the p38 MAP kinase inhibitor SB 202190 suggests that the activation of the PLA2s by TPA is mediated via PKC and p38 MAP kinase.

Negative regulation of mitogen-activated protein kinase activation by integrin alphaIIbbeta3 in platelets

Activation of the mitogen-activated protein (MAP) kinase pathway in nucleated cells is dependent on both growth factor receptors and integrins engaged in cell adhesion. Human platelets are an interesting model for studying cell adhesion and the involvement of integrin engagement on extracellular signal-regulated kinase (ERK) activation, independently from the nuclear-DNA signal pathway. Maximal phosphorylation and activity of ERK2 occurred late during thrombin-induced platelet aggregation (90 s and later), an alphaIIbbeta3 integrin-dependent event. Surprisingly, alphaIIbbeta3 inhibition by the RGDS ligand peptide, or (Fab')2 fragments of the AP-2 monoclonal antibody, resulted in a 2-fold enhancement in ERK2 phosphorylation and activity. A similar 2-fold enhancement of ERK2 activation was observed in thrombasthenic platelets which are defective in alphaIIbbeta3 and do not aggregate. This suggests that ERK2 activation in thrombin-induced platelet aggregation is dependent on thrombin rather than on alphaIIbbeta3 and is down-regulated by alphaIIbbeta3 engaged in ligand (fibrinogen) binding and/or aggregation. Finally, in the absence of stirring which allows fibrinogen binding to alphaIIbbeta3 but prevents aggregation, ERK2 was again overactivated. This overactivation appears to be consecutive to inhibition of aggregation itself and to alphaIIbbeta3 ligand binding. We conclude that in platelets, alphaIIbbeta3 engaged in aggregation down-regulates thrombin-induced ERK2 activation. To our knowledge, this is the first report of a down-regulation of the MAP kinase pathway by integrin engagement.

Ras activation in platelets after stimulation of the thrombin receptor, thromboxane A2 receptor or protein kinase C

Several reports have indicated that the small G-protein Ras is not present immunologically in platelets. However, here we report the identification of Ras in platelets by immunoprecipitation with the Ras-specific monoclonal antibodies Y13-259 or Y13-238, followed by Western blotting. The presence of Ras was not due to contamination of samples with erythrocytes or leucocytes. Immunofluorescence studies indicated that Ras was present in a peripheral rim pattern in fixed, permeabilized platelets, suggesting an intracellular, plasma membrane location. Activation of platelets with the thrombin receptor peptide42-50, the prostaglandin H2/thromboxane A2 mimetic U46619 or phorbol 12-myristate 13-acetate induced a rapid increase in GTP-bound, activated Ras. In each case, this increase was inhibited by the protein kinase C (PKC) inhibitor bisindolylmaleimide GF 109203X, suggesting that Ras is activated downstream of PKC in platelets. Thus the activation of Ras in platelets by agonists will now allow consideration of multiple potential Ras-dependent signal transduction pathways in platelet activation processes.

Thromboxane A2 stimulates mitogen-activated protein kinase and arachidonic acid liberation in rabbit platelets

U46619, a thromboxane A2 mimetic, caused tyrosine phosphorylation of several proteins in rabbit platelets. Among them, 42 kDa protein was identified as a mitogen-activated protein kinase (MAPK). U46619 activated MAPK in a concentration-dependent manner, measured by incorporation of 32P to a specific substrate for MAPK. U46619 also liberated [3H] arachidonic acid in a concentration-dependent manner. The U46619-induced MAPK activation and [3H]arachidonic acid liberation were inhibited by SQ29548 and by the removal of external Ca2+ ions. This is a first demonstration that TXA2 activates MAPK accompanied with arachidonic acid liberation in rabbit platelets.

Stimulation of mitogen-activated protein kinase and Na+/H+ exchanger in human platelets. Differential effect of phorbol ester and vasopressin

Treatment of human platelets with phorbol 12-myristate 13-acetate (PMA) and arginine vasopressin (AVP) increase the phosphorylation and activation of mitogen-activated protein kinase (MAPK). Electrophoretic retardation of MAPK mobility on SDS-polyacrylamide gels was used for determination of MAPK phosphorylation. The activity of MAPK was tested in myelin basic protein (MBP)-containing polyacrylamide gels. In this study we compared the PMA and AVP signal transduction pathways leading to the activation of MAPKs and Na+/H+ exchanger (NHE). Both agonists stimulate MAPK and NHE activities in a similar time frame and concentration dependence. The MAPK and NHE activities induced by PMA were inhibited by staurosporine, a potent inhibitor for protein kinase C (PKC), and by MAPK kinase (MEK) inhibitor, PD98059, but were not affected by the tyrosine kinase inhibitor genistein. In contrast, both AVP-induced MAPK and NHE activities were inhibited by genistein and MEK inhibitor but were not affected by staurosporine. Immunoprecipitation studies demonstrate that PMA, but not AVP, enhances the basal phosphorylation of the NHE-1. In this study, MAPKs are suggested to be a part of converging signaling leading to NHE activation by PKC-dependent and AVP-tyrosine kinase-dependent pathways. We propose that the MAPK activation of the NHE-1 does not involve phosphorylation of this exchanger protein. On the other hand, PKC can lead to phosphorylation and to additional activation of the NHE-1 through a MAPK-independent pathway.

Wortmannin inhibits mitogen-activated protein kinase activation by platelet-activating factor through a mechanism independent of p85/p110-type phosphatidylinositol 3-kinase

We have shown previously that wortmannin partially inhibits mitogen-activated protein kinase (MAPK) activated by platelet-activating factor (PAF) in guinea pig neutrophils (Ferby, M. I., Waga, I., Sakanaka, C., Kume, K., and Shimizu, T. (1994) J. Biol. Chem. 269, 30485-30488). To identify whether p85-dependent phosphatidylinositol 3-kinase is a target molecule of wortmannin in this inhibitory process, we established a murine macrophage cell line (P388D1), inducibly expressing a dominant-negative p85, delta p85. Upon induction of delta p85 by isopropyl-beta-D-thiogalactopyranoside, PAF still induced unaltered activation of MAPK, which was inhibited completely by wortmannin and 1,2-bis-(O-aminophenoxy)-ethane-N,N,N',N'-tetraacetic acid acetoxymethyl ester in an additive manner. Thus, PAF activates MAPK in P388D1 cells via two distinct pathways, one calcium-dependent and another calcium-independent, but wortmannin-sensitive. The inhibition of calcium-independent activation of MAPK by wortmannin does not involve p85-dependent phosphatidylinositol 3-kinase.

Cytosolic phospholipase A2 is phosphorylated in collagen- and thrombin-stimulated human platelets independent of protein kinase C and mitogen-activated protein kinase

Human platelets pretreated with indomethacin release arachidonic acid predominantly through the activity of cytosolic phospholipase A2 (cPLA2), an 85-kDa protein. This enzyme is regulated by an increase in intracellular Ca2+, a necessary condition of for arachidonic acid liberation, and by phosphorylation. Phosphorylation of cPLA2 enhanced the Ca(2+)-induced arachidonic acid release in platelets stimulated by the ionophore A23187 and phorbol ester (phorbol 12,13-dibutyrate (PDBu)). In thrombin-stimulated platelets, however, phosphorylation appeared not to be necessary for arachidonic acid release since the latter response was not impaired in the presence of staurosporine, which inhibited phosphorylation. Collagen, thrombin, and PDBu induced phosphorylation of platelet cPLA2 as well as activation of mitogen-activated protein kinase (MAPK; p42mapk and p44mapk). cPLA2 activation was not dependent on protein kinase C (PKC) in thrombin- and collagen-stimulated platelets, as preincubation with the PKC inhibitor Ro 31-8220 neither interfered with cPLA2 phosphorylation nor reduced arachidonic acid release. However, collagen- and thrombin-induced activation of MAPK was inhibited by Ro 31-8220, indicating that PKC is necessary for MAPK stimulation in platelets. Although MAPK may underlie phosphorylation of cPLA2 in PDBu-activated human platelets, our results provide evidence for PKC- and MAPK-independent phosphorylation of cPLA2 in platelets stimulated by the physiological activators collagen and thrombin.

Activation of mitogen-activated protein kinase by arachidonic acid in rat liver epithelial WB cells by a protein kinase C-dependent mechanism

Arachidonic acid (20:4(n-6)), which is released by cells responding to a wide range of stimuli, may play an important role in intracellular signaling. We now report that incubation of WB cells with 20:4(n-6) resulted in the appearance of several tyrosine-phosphorylated cytosolic proteins. Two of the phosphotyrosine-containing proteins, migrating in SDS-polyacrylamide gels of approximately 43 and 45 kDa, corresponded in mobility to phosphorylated species of the 42- and 44-kDa mitogen-activated protein kinase (MAPK) isoforms. Immunoblots of soluble fractions from unstimulated WB cells with anti-MAPK antibodies revealed the presence of the 42- and 44-kDa isoforms of MAPK. Upon incubation with 20:4(n-6), the mobility of both isoforms was retarded, consistent with their activation by phosphorylation. Chromatography of soluble fractions from these cells on Mono Q columns revealed early and late eluting peaks of myelin basic protein kinase activity, which contained the 42- and 44-kDa MAPK isoforms, respectively. Activation of MAPK was transient, peaking at 5 min, and was detectable at 5 microM 20:4(n-6). Further studies into the mechanisms by which MAPK was activated by 20:4(n-6) strongly suggested the involvement of protein kinase C (PKC). Not only did incubation of WB cells with 20:4(n-6) result in the translocation of PKC alpha, delta, and epsilon to a particulate fraction, it was found that the fatty acid failed to activate MAPK in cells pretreated for 26 h with phorbol 12-myristate 13-acetate, which depleted WB cells of PKC alpha, delta and epsilon. In addition, fatty acids of the n-3 series were effective activators of MAPK. The present study, to our knowledge, is the first to report that polyunsaturated fatty acids can cause the activation of MAPK.

Activation of mitogen-activated protein kinase and arachidonate release via two G protein-coupled receptors expressed in the rat hippocampus

Platelet-activating factor and somatostatin receptors, two G protein-coupled receptors expressed in the rat hippocampus, were analyzed for the downstream signaling pathways in Chinese hamster ovary cells stably expressing each receptor. Ligand stimulation to each CHO cell line induced (1) inhibition of forskolin-induced accumulation of cAMP, (2) arachidonate release, and (3) activation of mitogen-activated protein kinase and MAP kinase kinase. In contrast, inositol phosphate breakdown was seen only in the PAF-stimulated CHO cells. The induction of these signals accompanied no detectable Ras activation. Suppression of the signals by pertussis toxin was almost complete for the somatostatin receptor but partial for the PAF receptor, suggesting that the somatostatin receptor couples only with PTX-sensitive G protein, while the PAF receptor couples with both PTX-sensitive and -insensitive G proteins. A model of G protein-mediated signaling pathways was proposed in which the signals from Gi and those from Gq converge at MAP kinase kinase and lead to arachidonate release. The present system using CHO cells is useful for analyzing signaling pathways from G proteins to MAP kinase kinase and will thereby provide clues for understanding the mechanisms underlying the physiological and pathological events mediated by PAF, somatostatin, and other G protein-coupled receptors in the central nervous system and other tissues.

Lysophosphatidic acid inhibits gap-junctional communication and stimulates phosphorylation of connexin-43 in WB cells: possible involvement of the mitogen-activated protein kinase cascade

Lysophosphatidic acid (LPA) was shown to be a powerful inhibitor of gap-junctional communication between cultured rat liver WB cells, as determined by the transfer of Lucifer Yellow, with 50% inhibition obtained at about 0.3 microM LPA. Inhibition of communication was rapid (5 min) and was maintained for at least 80 min. After incubation for 3 h with LPA, communication competence was partially restored and dye transfer was refractory to further addition of LPA. Communication in LPA-refractory cells retained sensitivity to inhibition by phorbol ester and by epidermal growth factor (EGF). LPA-induced inhibition was associated with phosphorylation of connexin-43 protein, as detected by slower migration of the protein detected on Western blots, which could be eliminated by incubation of samples with alkaline phosphatase. A close correspondence was observed between the time- and dose-dependency of LPA effects on communication and the induction of mitogen-activated protein kinase (MAP kinase). Activation of both the 42 kDa and 44 kDa subspecies were confirmed by mobility shifts on Western blots using an anti-(MAP kinase R1) (erk 1-III) antibody and by fractionation on Mono Q columns. Cells pretreated with phorbol ester for 24 h were insensitive to phorbol ester inhibition of communication or activation of MAP kinase, but retained their sensitivity to LPA. The results indicate that LPA initiates the activation of protein kinase cascades in WB cells that are probably independent of protein kinase C and identifies connexin-43 as one substrate for the activated kinases.

p42 mitogen-activated protein kinase and p90 ribosomal S6 kinase are selectively phosphorylated and activated during thrombin-induced platelet activation and aggregation

Human platelets provide an excellent model system for the study of phosphorylation events during signal transduction and cell adhesion. Platelets are terminally differentiated cells that exhibit rapid phosphorylation of many proteins upon agonist-induced activation and aggregation. We have sought to identify the kinases as well as the phosphorylated substrates that participate in thrombin-induced signal transduction and platelet aggregation. In this study, we have identified two forms of mitogen-activated protein kinase (MAPK), p42mapk and p44mapk, in platelets. The data demonstrate that p42mapk but not p44mapk becomes phosphorylated on serine, threonine, and tyrosine during platelet activation. Immune complex kinase assays, gel renaturation assays, and a direct assay for MAPK activity in platelet extracts all support the conclusion that p42mapk but not p44mapk shows increased kinase activity during platelet activation. The activation of p42mapk, independently of p44mapk, in platelets is unique since in other systems, both kinases are coactivated by a variety of stimuli. We also show that platelets express p90rsk, a ribosomal S6 kinase that has previously been characterized as a substrate for MAPK. p90rsk is phosphorylated on serine in resting platelets, and this phosphorylation is enhanced upon thrombin-induced platelet activation. Immune complex kinase assays demonstrate that the activity of p90rsk is markedly increased during platelet activation. Another ribosomal S6 protein kinase, p70S6K, is expressed by platelets but shows no change in kinase activity upon platelet activation with thrombin. Finally, we show that the increased phosphorylation and activity of both p42mapk and p90rsk does not require integrin-mediated platelet aggregation. Since platelets are nonproliferative cells, the signal transduction pathways that include p42mapk and p90rsk cannot lead to a mitogenic signal and instead may regulate cytoskeletal or secretory changes during platelet activation.

p42 mitogen-activated protein kinase and p90 ribosomal S6 kinase are selectively phosphorylated and activated during thrombin-induced platelet activation and aggregation

Human platelets provide an excellent model system for the study of phosphorylation events during signal transduction and cell adhesion. Platelets are terminally differentiated cells that exhibit rapid phosphorylation of many proteins upon agonist-induced activation and aggregation. We have sought to identify the kinases as well as the phosphorylated substrates that participate in thrombin-induced signal transduction and platelet aggregation. In this study, we have identified two forms of mitogen-activated protein kinase (MAPK), p42mapk and p44mapk, in platelets. The data demonstrate that p42mapk but not p44mapk becomes phosphorylated on serine, threonine, and tyrosine during platelet activation. Immune complex kinase assays, gel renaturation assays, and a direct assay for MAPK activity in platelet extracts all support the conclusion that p42mapk but not p44mapk shows increased kinase activity during platelet activation. The activation of p42mapk, independently of p44mapk, in platelets is unique since in other systems, both kinases are coactivated by a variety of stimuli. We also show that platelets express p90rsk, a ribosomal S6 kinase that has previously been characterized as a substrate for MAPK. p90rsk is phosphorylated on serine in resting platelets, and this phosphorylation is enhanced upon thrombin-induced platelet activation. Immune complex kinase assays demonstrate that the activity of p90rsk is markedly increased during platelet activation. Another ribosomal S6 protein kinase, p70S6K, is expressed by platelets but shows no change in kinase activity upon platelet activation with thrombin. Finally, we show that the increased phosphorylation and activity of both p42mapk and p90rsk does not require integrin-mediated platelet aggregation. Since platelets are nonproliferative cells, the signal transduction pathways that include p42mapk and p90rsk cannot lead to a mitogenic signal and instead may regulate cytoskeletal or secretory changes during platelet activation.