Phosphatidylinositol (3,4,5)-trisphosphate stimulates phosphorylation of pleckstrin in human platelets

Protein kinase C and P2Y12 take center stage in thrombin-mediated activation of mammalian target of rapamycin complex 1 in human platelets

BACKGROUND

Rapamycin, an inhibitor of mammalian target of rapamycin complex-1 (mTORC1), reduces platelet spreading, thrombus stability, and clot retraction. Despite an important role of mTORC1 in platelet function, little is known about how it is regulated. The objective of this study was to determine the signaling pathways that regulate mTORC1 in human platelets.

METHODS

Mammalian target of rapamycin complex-1 activation was assessed by measuring the phosphorylation of its downstream substrate ribosomal S6 kinase 1 (p70S6K).

RESULTS

Thrombin or the protein kinase C (PKC) activator phorbal 12-myristate 13-acetate stimulated activation of mTORC1 in a PKC-dependent, Akt-independent manner that correlated with phosphorylation of tuberin/tuberous sclerosis 2 (TSC2) (Ser939 and Thr1462). In contrast, insulin-like growth factor 1 (IGF-1)-stimulated TSC2 phosphorylation was completely dependent on phosphoinositide 3 kinase (PI3 kinase)/Akt but did not result in any detectable mTORC1 activation. Early (Ser939 and Thr1462) and late (Thr1462) TSC2 phosphorylation in response to thrombin were directly PKC dependent, whereas later TSC2 (Ser939) and p70S6K phosphorylation were largely dependent on paracrine signaling through P2Y(12). PKC-mediated adenosine diphosphate (ADP) secretion was essential for thrombin-stimulated mTORC1 activation, as (i) ADP rescued p70S6K phosphorylation in the presence of a PKC inhibitor and (ii) P2Y(12) antagonism prevented thrombin-mediated mTORC1 activation. Rescue of mTORC1 activation with exogenous ADP was completely dependent on the Src family kinases but independent of PI3 kinase/Akt. Interestingly, although inhibition of Src blocked the ADP rescue, it had little effect on thrombin-stimulated p70S6K phosphorylation under conditions where PKC was not inhibited.

CONCLUSION

These results demonstrate that thrombin activates the mTORC1 pathway in human platelets through PKC-mediated ADP secretion and subsequent activation of P2Y(12), in a manner largely independent of the canonical PI3 kinase/Akt pathway.

Dysfunction of the PI3 kinase/Rap1/integrin α(IIb)β(3) pathway underlies ex vivo platelet hypoactivity in essential thrombocythemia

Patients with myeloproliferative disorders (MPDs), such as essential thrombocythemia (ET) have increased risk of thrombosis and bleeding, which are major sources of morbidity and mortality. Most MPD patients have a gain of function mutation in Janus kinase 2 (JAK2V617F), but little is known how JAK2V617F affects platelet function. Here, we demonstrate that platelets from ET patients have impaired SFLLRN-mediated fibrinogen binding and have lost the potentiating effect of thrombopoietin (which couples to JAK2) on this pathway. In contrast, SFLLRN-mediated P-selectin expression, ATP secretion, phosphorylation of the PKC substrate pleckstrin, and Ca(2+) mobilization were unaffected in JAK2V617F positive platelets. In addition, thrombopoietin-mediated JAK2 phosphorylation was unchanged, suggesting that signaling pathways activated downstream of JAK2 are impaired. Indeed, we found that platelets from JAK2V617F positive ET patients have significantly reduced phosphorylation of the PI3 kinase substrate Akt, and have reduced activation of Rap1 in response to thrombopoietin, IGF-1,ADP, SFLLRN, and thrombin. This effect was independent of Giα P2Y12 purinergic receptor function as ADP-mediated inhibition of VASP phosphorylation was unchanged. These results demonstrate that the PI3 kinase/Rap1 pathway is intrinsically impaired in platelets from JAK2V617F-positive ET patients, resulting in diminished thrombin and thrombopoietin-mediated integrin α(IIb)β(3) activation.

Antioxidative properties of hydrogen sulfide may involve in its antiadhesive action on blood platelets

BACKGROUND

Hydrogen sulfide (H(2)S) is a signaling molecule in different systems, including the cardiovascular system. However, mechanisms involved in the relationship between the action of H(2)S and hemostasis process are still unclear.

OBJECTIVE AND METHODS

The present work was designed to study the effects of hydrogen sulfide on adhesion of blood platelets in vitro. Platelet suspensions were preincubated (5-30 min) with NaHS as a hydrogen sulfide donor at the final concentrations of 0.00001-10 mM. Then, for platelet activation thrombin (0.1 U/mL) or TRAP, peptide with the sequence Ser-Phe-Leu-Leu-Arg-Asn (SFLLRN; 20 μM) was used. We also measured the effects of H(2)S on superoxide anion radicals (O(2)(-•)) production in blood platelets.

RESULTS

We observed that adhesion to collagen and to fibrinogen of resting platelets preincubated with NaHS was changed, and this process was statistically significant (for 0.00001-5mM NaHS, p<0.05; 10 mM, p<0.01). The inhibitory effect of NaHS on adhesion of thrombin - or TRAP - stimulated platelets to collagen was found (for 0.00001 and 0.0001 mM NaHS, p<0.05; 0.001-1 mM NaHS, p<0.01; 5 and 10 mM NaHS, p<0.001). Hydrogen sulfide reduced also the thrombin- or TRAP-induced platelet adhesion to fibrinogen (for 0.00001 and 0.0001 mM NaHS, p<0.05; 0.001-1 mM NaHS, p<0.01; 5 and 10 mM NaHS, p<0.001). Moreover, H(2)S caused a dose-dependent reduction of O(2)(-•) produced in platelets (p<0.05).

CONCLUSION

The results obtained that the antioxidative activity of H(2)S may involve in its antiadhesive properties on blood platelets.

The roles and mechanisms of PAR4 and P2Y12/phosphatidylinositol 3-kinase pathway in maintaining thrombin-induced platelet aggregation

BACKGROUND AND PURPOSE

Activation of human platelets by thrombin is mediated predominately through two proteinase-activated receptors (PARs), PAR1 and PAR4. Phosphatidylinositol 3-kinase (PI3K) inhibition leads to reversible PAR1-mediated platelet aggregation, but has no effect on the stability of platelet aggregation induced by thrombin. In the present study, the molecular mechanisms underlying this difference were investigated.

EXPERIMENTAL APPROACH

The functions of PI3K and PAR4 were assessed using specific inhibitors and aggregometry. The duration of platelet glycoprotein (GP) IIb/IIIa exposure was determined by flow cytometry with the antibody PAC-1. Western blotting and fluo-3 was used to evaluate the activation of Akt and protein kinase C (PKC) and intracellular Ca(2+) mobilization respectively.

KEY RESULTS

When PAR4 function was inhibited either by the PAR4 antagonist YD-3 [1-benzyl-3-(ethoxycarbonylphenyl)-indazole] or by receptor desensitization, the PI3K inhibitor wortmannin turned thrombin-elicited platelet aggregation from an irreversible event to a reversible event. Moreover, wortmannin plus YD-3 markedly accelerated the inactivation of GPIIb/IIIa in thrombin-stimulated platelets. The aggregation-reversing activity mainly resulted from inhibition of both PI3K-dependent PKC activation and PAR4-mediated sustained intracellular Ca(2+) rises. Blockade of ADP P2Y(12) receptor with 2-methylthioadenosine 5'-monophosphate triethylammonium salt mimicked the inhibitory effect of wortmannin on PI3K-dependent PKC activation and its ability to reverse PAR1-activating peptide-induced platelet aggregation. Co-administration of 2-methylthioadenosine 5'-monophosphate triethylammonium salt with YD-3 also decreased the stability of thrombin-induced platelet aggregation.

CONCLUSIONS AND IMPLICATIONS

These results suggest that PAR4 acts in parallel with the P2Y(12)/PI3K pathway to stabilize platelet aggregates, and provide new insights into the mechanisms of thrombus stabilization and potential applications for antithrombotic therapy.

Structure and dynamics of the human pleckstrin DEP domain: distinct molecular features of a novel DEP domain subfamily

Pleckstrin1 is a major substrate for protein kinase C in platelets and leukocytes, and comprises a central DEP (disheveled, Egl-10, pleckstrin) domain, which is flanked by two PH (pleckstrin homology) domains. DEP domains display a unique alpha/beta fold and have been implicated in membrane binding utilizing different mechanisms. Using multiple sequence alignments and phylogenetic tree reconstructions, we find that 6 subfamilies of the DEP domain exist, of which pleckstrin represents a novel and distinct subfamily. To clarify structural determinants of the DEP fold and to gain further insight into the role of the DEP domain, we determined the three-dimensional structure of the pleckstrin DEP domain using heteronuclear NMR spectroscopy. Pleckstrin DEP shares main structural features with the DEP domains of disheveled and Epac, which belong to different DEP subfamilies. However, the pleckstrin DEP fold is distinct from these structures and contains an additional, short helix alpha4 inserted in the beta4-beta5 loop that exhibits increased backbone mobility as judged by NMR relaxation measurements. Based on sequence conservation, the helix alpha4 may also be present in the DEP domains of regulator of G-protein signaling (RGS) proteins, which are members of the same DEP subfamily. In pleckstrin, the DEP domain is surrounded by two PH domains. Structural analysis and charge complementarity suggest that the DEP domain may interact with the N-terminal PH domain in pleckstrin. Phosphorylation of the PH-DEP linker, which is required for pleckstrin function, could regulate such an intramolecular interaction. This suggests a role of the pleckstrin DEP domain in intramolecular domain interactions, which is distinct from the functions of other DEP domain subfamilies found so far.

Pharmacological characterization of protease activated receptor-1 by a serum responsive element-dependent reporter gene assay: Major role of calmodulin

We studied the protease activated receptor-1 coupling to a serum response element (SRE)-dependent luciferase activity readout in transfected COS-7 cells. Thrombin, with a pEC50 of 10.5, was 3000-fold more potent than the peptide agonists SFLLR and its derived compound C721-40 in stimulating luciferase activity, although the three agonists exhibited similar efficacy at the maximal concentration tested. Interestingly, SFLLR- and C721-40-induced luciferase activity was biphasic, suggesting that at least two populations of G proteins couple to the receptor. Further pharmacological characterization of this system was performed using selective protease activated receptor-1 antagonists. SCH203099 and ER-112787 blocked SFLLR-induced luciferase activity with similar potencies (pK(B) of 7), slightly higher than that exhibited by an arylisoxazole derivative compound from Merck (pK(B) of 6.1). These values correlated with their affinities established by competition binding experiments using [3H]-C721-40 as radioligand for protease activated receptor-1. Transduction mechanisms of protease activated receptor-1 coupling to SRE-dependent luciferase activity were examined using specific inhibitors. The Ca2+ chelator BAPTA-AM, as well as the calmodulin inhibitors W-7 and ophiobolin A, robustly inhibited SFLLR-induced SRE activation. Overexpression of RGS2 and a dominant negative rhoA protein abolished the SFLLR signal in an additive manner, suggesting a major role of Gq and G12/13 proteins. Furthermore, inhibition of phospholipase C, MAP-kinases, phosphatidyl inositol-3 kinase, rho-kinase and Ca2+/calmodulin-dependent protein kinases, all downstream effectors of Gq and G12/13, partially blocked the SFLLR-induced luciferase signal. Taken together, this SRE-luciferase assay reveals a complex network of transduction pathways of protease activated receptor-1 in accordance with the pleiotrophic action of thrombin.

Intraplatelet signaling mechanisms of the priming effect of matrix metalloproteinase-2 on platelet aggregation

OBJECTIVE

Platelets contain and release some matrix metalloproteinases (MMPs), enzymes involved in the degradation of extracellular matrix, and one of these (MMP-2) exerts a proaggregatory effect. We explored the signal transduction mechanisms activated by MMP-2 in human blood platelets.

METHODS AND RESULTS

Recombinant, human MMP-2, added before stimulation with subthreshold doses of different agonists, potentiated platelet activation, calcium influx, IP3 formation, and pleckstrin phosphorylation. Wortmannin and LY29400, two PI3-K inhibitors, suppressed the potentiating effects of MMP-2 and preincubation with MMP-2 enhanced the thrombin-induced association of the p85alpha PI3-K subunit with the cytoskeleton and increased the phosphorylation of PKB. Protein tyrosine kinase inhibitors, MAP kinase inhibitors, PLA2 inhibitors, cyclooxygenase inhibitors and antagonists of the P2Y1 and P2Y12 receptors did not affect the potentiating activity of MMP-2 on platelets.

CONCLUSION

Our data show that MMP-2, at a concentration released by activated platelets, facilitates platelet activation acting at the level of a second messenger system common to different agonists and related to the activation of PI3-K. Platelet-released MMP-2 may contribute to platelet activation in vivo.

Inhibition of agonist-induced p42 and p38 mitogen-activated protein kinase phosphorylation and CD40 ligand/P-selectin expression by cyclic nucleotide-regulated pathways in human platelets

Platelet activation and adhesion to endothelial cells and extracellular matrix proteins are crucial events in the development of arterial cardiovascular diseases. Platelet activation is initiated by stimulation of intracellular signaling cascades, including the p42 mitogen-activated protein kinase (MAPK) and p38 MAPK pathways, followed by major changes in the platelet cytoskeleton and expression and activation of platelet surface receptors, such as P-selectin (CD62P) and CD40 ligand (CD40L). Activated platelets directly bind to vascular endothelial cells via CD40L/CD40 interactions and induce inflammatory reactions that initiate or aggravate atherosclerotic lesions. The aim of this study was to investigate effects of two known platelet inhibitors-the cAMP-elevating prostaglandin E(1) (PG-E(1)) and the cGMP-elevating sodium nitroprusside (SNP)-on platelet p42 MAPK and p38 MAPK activation as well as on surface expression of CD62P and CD40L. MAPK activation was analyzed by Western blot experiments using phosphorylation-specific antibodies, and surface CD40L and CD62P expression was determined by flow cytometry analysis. PG-E(1) and SNP strongly inhibited p42 and p38 MAPK phosphorylation as well as CD40L and CD62P expression in response to thrombin, a thromboxane A(2) analog, and ADP. These data indicate that adenosine and guanosine 3',5'-cyclic monophosphate-dependent protein kinases not only inhibit platelet pathways leading to activation and aggregation, but also those resulting in enhanced surface expression of protein ligands involved in inflammation. Expression of CD40L and CD62P was found to be independent of MAPK activation, since it was not inhibited by specific MAPK inhibitors. Inhibition of platelet-induced inflammatory responses including CD62P- and CD40L-mediated interaction of platelets with leukocytes and endothelial cells, respectively, is suggested to be an important component of the long-term vasoprotective effects of cyclic nucleotide-elevating prostaglandins and NO donors.

Human platelet thrombin receptors. Roles in platelet activation

Platelets are essential participants in hemostasis and thrombosis. Platelets normally circulate in blood as discoid resting cells that become critical constituents of hemostatic plugs or arterial thrombi only after specific receptors on platelet membranes interact with their ligands (agonists) to initiate the reactions that lead to platelet activation. The well-characterized events associated with platelet activation include activation of membrane receptors, shape change, granular secretion, cytoskeletal reassembly, platelet cohesion, and aggregation. The plasma protease alpha-thrombin is the most potent physiologic platelet agonist; this enzyme has other key roles in hemostasis, in the genesis of arterial thrombi, and in embryonic development, inflammation, wound healing, and cell proliferation.

Activation of protein kinase C is required for the stable attachment of adherent platelets to collagen but is not needed for the initial rapid adhesion under flow conditions

We have investigated the role of protein kinase C (PKC) in the initial events of alpha(2)beta(1)-integrin-mediated platelet adhesion to collagen under flow conditions. Although adhesion caused activation of PKC, as evidenced by pleckstrin phosphorylation, the PKC inhibitors GF 109203X and Gö 6976 had no effect on adhesion, even though they prevented pleckstrin phosphorylation. The initial kinetics and extent of platelet adhesion to collagen (<5 seconds) and tyrosine phosphorylation of p125(FAK) and p72(syk) were not influenced by the PKC inhibitors, whereas adhesion to polylysine was prevented. These results indicate that adhesion to collagen and polylysine involve different mechanisms and requirements for PKC activation. Pretreatment with GF 109203X destabilized collagen-adherent platelets, accelerating their detachment, which was associated with tyrosine dephosphorylation of p125(FAK). Thus, although PKC activation was not required for rapid platelet adhesion to collagen, it appears to play an important role in stabilizing the attachment of adherent platelets to collagen. We also examined the effect of PKC activation by the phorbol ester phorbol 12-myristate 13-acetate (PMA) on platelet adhesion to collagen. PMA at 100 nmol/L strongly potentiated adhesion and tyrosine phosphorylation of p125(FAK) and p72(syk) and activated beta(1)-integrins, as determined by increased exposure of the 15/7 epitope. The PMA-stimulated adhesion was partially blocked by an anti-alpha(2)beta(1) antibody, was completely inhibited by GF 109203X, and was not correlated with the extent of pleckstrin phosphorylation. Therefore, strong PKC activation may lead to inside-out signaling, enhancing the role of beta(1)-integrins in adhesion. Pleckstrin phosphorylation does not appear to be involved in the initial phase of basic or PMA-stimulated adhesion but may help stabilize the adherent platelets.

A role for phosphatidylinositol 3-kinase in platelet aggregation in response to low, but not high, concentrations of PAF or thrombin

In this study we show that platelet activating factor (PAF) activates PI 3-kinase over a rapid time course that correlates closely with the aggregation response. Tyrosine kinases are involved in this response, since there is increased PI 3-kinase activity associated with tyrosine-phosphorylated proteins. PI 3-kinase inhibitors were used to probe the dependence of PAF-induced aggregation on PI 3-kinase. Both wortmannin and LY-294002 inhibited PAF-induced aggregation that correlated with PI 3-kinase inhibition only when using lower concentrations of PAF giving reversible aggregation (primary phase). Similar results were obtained with human platelets using thrombin or thrombin receptor activating peptide. The same pattern of response was observed when activation of GPIIbIIIa was assessed by flow cytometry, i.e., PI 3-kinase inhibitors blocked integrin activation only when lower concentrations of agonist were used. We suggest that PI 3-kinase is important for reversible (primary) aggregation of platelets in response to PAF or thrombin, perhaps by contributing to the 'inside-out' activation of the platelet integrin GPIIbIIIa, only when submaximal concentrations of agonists are used. The lack of effect of PI 3-kinase inhibitors, when high concentrations of agonist are used, suggests that PI 3-kinase-independent pathways contribute to aggregation under these conditions.

Expression of the Protein Kinase C Substrate Pleckstrin in Macrophages: Association with Phagosomal Membranes

Despite evidence suggesting that protein kinase C (PKC) isoforms are important in phagocytosis by Fcγ receptors, the mechanisms by which the substrates of these kinases act are largely unknown. We have investigated the role of one PKC substrate, pleckstrin, in cells of the monocyte/macrophage lineage. Pleckstrin expression in mouse macrophages was induced severalfold in response to bacterial LPS and IFN-γ. In unstimulated cells, the protein was largely confined to the cytosol. Upon ingestion of IgG-opsonized zymosan particles (OPZ), however, pleckstrin accumulated on the phagosomal membrane. This association was transient, being maximal after 15 min and declining thereafter. Similar kinetics of association was also seen for both filamentous actin and the δ isoform of PKC. Ingestion of OPZ was found to induce phosphorylation of pleckstrin. To examine whether phosphorylation was required for phagosomal association, pleckstrin was expressed in CHO-IIA cells that stably express the FcγRIIA receptor and are competent for phagocytosis of OPZ. In these cells, both wild-type pleckstrin and mutants in which the phosphoacceptor sites had been mutated to either alanine (nonphosphorylatable) or glutamine (pseudophosphorylated) were found to accumulate on OPZ phagosomes. Thus, association of pleckstrin with phagosomes is independent of its phosphorylation. Our findings suggest that pleckstrin may serve as an intracellular adaptor/targeting protein in response to particulate stimuli. By targeting interacting ligands to the phagosomal compartment, pleckstrin may serve to regulate phagocytosis and/or early steps during maturation of the phagosome.

Role of PI3-kinase in the development of interstitial cells and pacemaking in murine gastrointestinal smooth muscle

1. Development of the pacemaker system in the small intestine depends upon signalling via tyrosine kinase (Kit) receptors. The downstream pathways initiated by Kit in interstitial cells of Cajal (ICC) have not been investigated. Wortmannin and 2-(4-morpholinyl)-8-phenyl-4H-1-benzopyran-4-one (LY 294002), inhibitors of phosphatidylinositol 3'-kinase (PI3-kinase), were used to test the involvement of this pathway in the development and maintenance of ICC and electrical rhythmicity in the murine small intestine. 2. ICC and electrical slow waves were present in the murine jejunum at birth. ICC and electrical rhythmicity continued to develop in neonates such that adult activity was recorded after 1 week. Development of ICC and rhythmicity were maintained in organ culture. 3. Wortmannin or LY 294002 inhibited the development of slow waves and blocked rhythmicity within 2-4 days. Loss of slow waves was preceded by disappearance of Kit-positive cells from the myenteric (IC-MY) and deep muscular plexus (IC-DMP) regions. Wortmannin or LY 294002 had no acute effect on slow waves. 4. Muscles from older animals (day 10-day 30) developed resistance to wortmannin treatment, but when the exposure to wortmannin was increased to 35 days, damage to ICC networks and electrical dysrhythmias were observed. 5. PI3-kinase appears to be a critical downstream signalling element linking Kit receptors to ICC development and maintenance of phenotype. ICC are more sensitive to Kit or PI3-kinase blockade at birth, but the importance of the PI3-kinase signalling in the maintenance of ICC persists into adulthood. Interference with PI3-kinase signalling in immature or adult animals could result in disruption of ICC and gastrointestinal dysrhythmias.

Abnormalities of protein kinases in neurodegenerative diseases

In neurodegenerative diseases such as ALS and AD there is evidence for abnormal regulation of protein kinases. In these diseases, altered activities and protein levels of several specific kinases suggest that abnormal phosphorylation is present and this aberrant phosphorylation may be involved in the pathogenesis of these diseases. The observation that regulation of the NMDA receptor ion channel is altered in tissue from ALS patients may arise from the abnormal phosphorylation state of the protein kinase regulating NMDA receptor function. Whether the abnormalities of these protein kinases is a primary event leading to altered receptor regulation or vice versa is still poorly understood. The seemingly multiple pathogenic mechanisms of ALS and AD create complexity in assessing a primary cause that may lead to cell death. The mechanisms causing cell death (apoptosis or necrosis) may be overlapping with integrated events among the components interacting and contributing to a final pathway for neuron death. Thus, evidence of impairment in protein kinase signalling in these diseases may be a primary cause, a secondary event, or a compensatory mechanism. To further study this issue, different model systems could be beneficial to obtain a better understanding of these diseases.

Biphasic activation of PKBalpha/Akt in platelets. Evidence for stimulation both by phosphatidylinositol 3,4-bisphosphate, produced via a novel pathway, and by phosphatidylinositol 3,4,5-trisphosphate

Stimulation of platelet thrombin receptors or protein kinase C causes fibrinogen-dependent aggregation that is a function of integrin alphaIIb beta3 activation. Such platelets rapidly and transiently form phosphatidylinositol 3,4,5-trisphosphate (PtdIns(3,4,5)P3) and a small amount of phosphatidylinositol 3,4-bisphosphate (PtdIns(3,4)P2). After aggregation, a larger amount of PtdIns(3,4)P2 is generated. We report that this latter PtdIns(3,4)P2 arises largely through wortmannin-inhibitable generation of PtdIns3P and then phosphorylation by PtdIns3P 4-kinase (PtdIns3P 4-K), a novel pathway apparently contingent upon the activation of the Ca2+-dependent protease calpain. Elevation of cytosolic Ca2+ by ionophore, without integrin/ligand binding, is insufficient to activate the pathway. PtdIns3P 4-K is not the recently described "PIP5KIIalpha." Cytoskeletal activities of phosphatidylinositol 3-kinase and PtdIns3P 4-K increase after aggregation. Prior to aggregation, PtdIns3P 4-K can be regulated negatively by the beta gamma subunit of heterotrimeric GTP-binding protein. After aggregation, PtdIns3P 4-K calpain-dependently loses its susceptibility to Gbeta gamma and is, in addition, activated. Both PtdIns(3,4,5)P3 and PtdIns(3,4)P2 have been shown to stimulate PKBalpha/Akt phosphorylation and activation by phosphoinositide-dependent kinase 1. We find that activation of PKBalpha/Akt in platelets is phosphorylation-dependent and biphasic; the initial phase is PtdIns(3,4,5)P3-dependent and more efficient, whereas the second phase depends upon PtdIns(3,4)P2 generated after aggregation. There is thus potential for both pre- and post-aggregation-dependent signaling by PKBalpha/Akt.

Analysis of Signal Transduction Pathways in Human Eosinophils Activated by Chemoattractants and the T-Helper 2–Derived Cytokines Interleukin-4 and Interleukin-5

Activation and recruitment of eosinophils in allergic inflammation is in part mediated by chemoattractants and T-helper 2 (Th2)-derived cytokines. However, little is known concerning the signal transduction mechanisms by which this activation occurs. We have investigated tyrosine kinase-mediated activation of phosphatidylinositol 3-kinase (PI3K) and compared this with the activation of the p21ras-ERK signaling pathway in human eosinophils. The related cytokines interleukin-3 (IL-3), IL-5, and granulocyte-macrophage colony-stimulating factor (GM-CSF), all induced PI3K activity detected in antiphosphotyrosine immunoprecipitates. Furthermore, the chemoattractants platelet-activating factor (PAF), RANTES, and C5a were also able to induce phosphotyrosine-associated PI3K activity. Protein kinase B (PKB) is a downstream target of PI3K activation by growth factors. Induction of PKB phosphorylation in human eosinophils was transiently induced on activation with the cytokines IL-4 and IL-5, as well as the chemoattractants PAF, C5a, and RANTES showing a broad activation profile. Surprisingly, analysis of the activation of the mitogen-activated protein (MAP) kinases p44ERK1 and p42ERK2, showed that ERK2, but not ERK1, was transiently activated in human eosinophils after stimulation with IL-5 or PAF. Activation kinetics correlated with activation of p21ras by both cytokines and chemoattractants as measured by a novel assay for guanosine triphosphate (GTP)-loading. Finally, using specific inhibitors of both the p21ras-ERK and PI3K signaling pathways, a role was demonstrated for PI3K, but not p21ras-ERK, in activation of the serum-treated zymosan (STZ)-mediated respiratory burst in IL-5 and PAF-primed eosinophils. In summary, these data show that in human eosinophils, Th2-derived cytokines differentially activate both PI3K and MAP kinase signal transduction pathways with distinct functional consequences showing complex regulation of eosinophil effector functions.

Downstream signalling events regulated by phosphatidylinositol 3-kinase activity

The phosphatidylinositol (PI) 3-kinase family of enzymes is now known to be regulated by several different upstream pathways in response to virtually all growth factors and cytokines. In the past few years, the phosphoinositides phosphorylated at the 3-OH position of the inositol ring have been shown to be lipid second messengers that may directly or indirectly regulate the activity of several different serine/threonine kinases. Consistent with the many different cellular events in which PI 3-kinase plays an important role, a diverse group of serine/threonine kinases are regulated downstream of PI 3-kinases, including protein kinase C (PKC) isoforms, p70 S6 kinase, and PKB/Akt. This review summarises studies done primarily in the past few years that have begun to unravel these targets of PI 3-kinase activity.

Analysis of Signal Transduction Pathways in Human Eosinophils Activated by Chemoattractants and the T-Helper 2–Derived Cytokines Interleukin-4 and Interleukin-5

Activation and recruitment of eosinophils in allergic inflammation is in part mediated by chemoattractants and T-helper 2 (Th2)-derived cytokines. However, little is known concerning the signal transduction mechanisms by which this activation occurs. We have investigated tyrosine kinase-mediated activation of phosphatidylinositol 3-kinase (PI3K) and compared this with the activation of the p21ras-ERK signaling pathway in human eosinophils. The related cytokines interleukin-3 (IL-3), IL-5, and granulocyte-macrophage colony-stimulating factor (GM-CSF), all induced PI3K activity detected in antiphosphotyrosine immunoprecipitates. Furthermore, the chemoattractants platelet-activating factor (PAF), RANTES, and C5a were also able to induce phosphotyrosine-associated PI3K activity. Protein kinase B (PKB) is a downstream target of PI3K activation by growth factors. Induction of PKB phosphorylation in human eosinophils was transiently induced on activation with the cytokines IL-4 and IL-5, as well as the chemoattractants PAF, C5a, and RANTES showing a broad activation profile. Surprisingly, analysis of the activation of the mitogen-activated protein (MAP) kinases p44ERK1 and p42ERK2, showed that ERK2, but not ERK1, was transiently activated in human eosinophils after stimulation with IL-5 or PAF. Activation kinetics correlated with activation of p21ras by both cytokines and chemoattractants as measured by a novel assay for guanosine triphosphate (GTP)-loading. Finally, using specific inhibitors of both the p21ras-ERK and PI3K signaling pathways, a role was demonstrated for PI3K, but not p21ras-ERK, in activation of the serum-treated zymosan (STZ)-mediated respiratory burst in IL-5 and PAF-primed eosinophils. In summary, these data show that in human eosinophils, Th2-derived cytokines differentially activate both PI3K and MAP kinase signal transduction pathways with distinct functional consequences showing complex regulation of eosinophil effector functions.

Conditional inhibition of the mitogen-activated protein kinase cascade by wortmannin. Dependence on signal strength

Phosphoinositide (PI) 3-kinase and the mitogen-activated protein (MAP) kinase cascades are activated by many of the same ligands. Several groups have reported involvement of PI 3-kinase in the activation of Erk1 and Erk2, whereas many other groups have shown that activation of Erk1 and Erk2 is not sensitive to inhibitors of PI 3-kinase such as wortmannin. Here we show that wortmannin inhibition of the MAP kinase pathway is cell type- and ligand-specific. Wortmannin blocks platelet-derived growth factor (PDGF)-dependent activation of Raf-1 and the MAP kinase cascade in Chinese hamster ovary cells, which have few PDGF receptors, but has no significant effect on Erk activation in Swiss 3T3 cells, which have high levels of PDGF receptors. However, wortmannin blocks activation of Erk proteins if Swiss 3T3 cells are stimulated with lower, physiological levels of PDGF. These results suggest that PI 3-kinase is in an efficient pathway for activation of MAP kinase, but that MAP kinase can be stimulated by a redundant pathway when a large number of receptors are activated. We present evidence that a protein kinase C family member downstream of phospholipase Cgamma is involved in the redundant pathway.

Tyrosine phosphorylation and relocation of SHIP are integrin-mediated in thrombin-stimulated human blood platelets

The SH2 domain-containing inositol 5-phosphatase, SHIP, known to dephosphorylate inositol 1,3,4,5-tetrakisphosphate and phosphatidylinositol 3,4,5-trisphosphate has recently been shown to be expressed in a variety of hemopoietic cells. This 145-kDa protein is induced to associate with Shc by multiple cytokines and may play an important role in the negative regulation of immunocompetent cells mediated by FcgammaRIIB receptor. We report here that SHIP is present in human blood platelets and may be involved in platelet activation evoked by thrombin. Platelet SHIP was identified by Western blotting as a single 145-kDa protein. Both phosphatidylinositol 3,4,5-trisphosphate and inositol 1,3,4, 5-tetrakisphosphate 5-phosphatase activities could be demonstrated in anti-SHIP immunoprecipitates of platelet lysate. Thrombin stimulation induced a tyrosine phosphorylation of SHIP, this effect being prevented if platelets were not shaken or if RGD-containing peptides were present, indicating an aggregation-dependent, integrin-mediated event. Moreover, although the intrinsic phosphatase activity of SHIP did not appear to be significantly increased, tyrosine-phosphorylated SHIP was relocated to the actin cytoskeleton upon activation in an aggregation- and integrin engagement-dependent manner. Finally, the striking correlation observed between phosphatidylinositol 3,4-bisphosphate production and the tyrosine phosphorylation of SHIP, as well as its relocation to the cytoskeleton upon thrombin stimulation, suggest a role for SHIP in the aggregation-dependent and GpIIb-IIIa-mediated accumulation of this important phosphoinositide.

SIP/SHIP inhibits Xenopus oocyte maturation induced by insulin and phosphatidylinositol 3-kinase

SIP (signaling inositol phosphatase) or SHIP (SH2-containing inositol phosphatase) is a recently identified SH2 domain-containing protein which has been implicated as an important signaling molecule. SIP/SHIP becomes tyrosine phosphorylated and binds the phosphotyrosine-binding domain of SHC in response to activation of hematopoietic cells. The signaling pathways and biological responses that may be regulated by SIP have not been demonstrated. SIP is a phosphatidylinositol- and inositol-polyphosphate 5-phosphatase with specificity in vitro for substrates phosphorylated at the 3' position. Phosphatidylinositol 3'-kinase (PI 3-kinase) is an enzyme which is involved in mitogenic signaling and whose phosphorylated lipid products are predicted to be substrates for SIP. We tested the hypothesis that SIP can modulate signaling by PI 3-kinase in vivo by injecting SIP cRNAs into Xenopus oocytes. SIP inhibited germinal vesicle breakdown (GVBD) induced by expression of a constitutively activated form of PI 3-kinase (p110*) and blocked GVBD induced by insulin. SIP had no effect on progesterone-induced GVBD. Catalytically inactive SIP had little effect on insulin- or PI 3-kinase-induced GVBD. Expression of SIP, but not catalytically inactive SIP, also blocked insulin-induced mitogen-activated protein kinase phosphorylation in oocytes. SIP specifically and markedly reduced the level of phosphatidylinositol (3,4,5) triphosphate [PtdIns(3,4,5)P3] generated in oocytes in response to insulin. These results demonstrate that a member of the phosphatidylinositol polyphosphate 5-phosphatase family can inhibit signaling in vivo. Further, our data suggest that the generation of PtdIns(3,4,5)P3 by PI 3-kinase is necessary for insulin-induced GVBD in Xenopus oocytes.

Inositol polyphosphate 4-phosphatase is inactivated by calpain-mediated proteolysis in stimulated human platelets

Inositol polyphosphate 4-phosphatase (4-phosphatase), an enzyme that catalyzes the hydrolysis of the 4-position phosphate of phosphatidylinositol 3,4-bisphosphate, was shown to be a substrate for the calcium-dependent protease calpain in vitro and in stimulated human platelets. Stimulation of platelets with the calcium ionophore, A23187, resulted in complete proteolysis of 4-phosphatase and a 75% reduction in enzyme activity. Thrombin stimulation of platelets resulted in partial proteolysis of 4-phosphatase and a 41% reduction in enzyme activity (n = 8, range of 36-51%). In addition, preincubation with the calpain inhibitor, calpeptin, suppressed the accumulation of phosphatidylinositol 3, 4-bisphosphate in thrombin-stimulated platelets by 36% (n = 2, range = 35-37%). These data suggest that the calpain-mediated inhibition of 4-phosphatase is involved in the phosphatidylinositol 3, 4-bisphosphate accumulation in thrombin-stimulated platelets.

Inhibition of phosphatidylinositol 3-kinase activity blocks depolarization- and insulin-like growth factor I-mediated survival of cerebellar granule cells

Depolarizing concentrations of potassium promote the survival of many neuronal cell types including cerebellar granule cells. To begin to understand the intracellular mediators of neuronal survival, we have tested whether the survival-promoting effect of potassium depolarization on cerebellar granule cells is dependent on either mitogen-activated protein (MAP) kinase or phosphatidylinositol 3-kinase (PI-3-K) activity. In 7-day cerebellar granule cell cultures, potassium depolarization activated both MAP kinase and PI-3-K. Preventing the activation of MAP kinase with the MEK1 inhibitor PD98059 did not affect potassium saving. In contrast, the survival-promoting effect of 25 mM potassium was negated by the addition of 30 microM LY 294002 or 1 microM wortmannin, two distinct inhibitors of PI-3-K. The cell death induced by PI-3-K inhibition was indistinguishable from the cell death caused by potassium deprivation; LY 294002-induced death included nuclear condensation, was blocked by cycloheximide, and had the same time course as potassium deprivation-induced cell death. Cerebellar granule cells can also be maintained in serum-free medium containing either 100 ng/ml insulin-like growth factor I (IGF-I) or 800 microM cAMP. PI-3-K inhibition completely blocked the survival-promoting activity of IGF-I, but had no effect on cAMP-mediated survival. These data indicate that the survival-promoting effects of depolarization and IGF-I, but not cAMP, require PI-3-K activity.

Regulation of AP-3 function by inositides. Identification of phosphatidylinositol 3,4,5-trisphosphate as a potent ligand

As part of the growing effort to understand the role inositol phosphates and inositol lipids play in the regulation of vesicle traffic within nerve terminals, we determined whether or not the synapse-specific clathrin assembly protein AP-3 can interact with inositol lipids. We found that soluble dioctanoyl-phosphatidylinositol 3,4,5-trisphosphate (DiC8PtdIns(3,4, 5)P3) was only 7.5-fold weaker a ligand than D-myo-inositol hexakisphosphate in assays that measured the displacement of D-myo-[3H]inositol hexakisphosphate. In functional assays we found that both of these ligands inhibited clathrin assembly, but DiC8-PtdIns(3,4,5)P3 was more potent and exhibited a larger maximal effect. We also examined the structural features of DiC8-PtdIns(3,4, 5)P3 that establish specificity. Dioctanoyl-phosphatidylinositol 3, 4-bisphosphate, which does not have a 5-phosphate, and 4, 5-O-bisphosphoryl-D-myo-inosityl 1-O-(1, 2-O-diundecyl)-sn-3-glycerylphosphate, which does not have a 3-phosphate, were, respectively, 2-fold and 4-fold less potent than DiC8-PtdIns(3,4,5)P3 as inhibitors of clathrin assembly. Deacylation of DiC8-PtdIns(3,4,5)P3 reduced its affinity for AP-3 almost 20-fold, and also dramatically lowered its ability to inhibit clathrin assembly. The deacylated products of the soluble derivatives of phosphatidylinositol 3,4-bisphosphate and phosphatidylinositol 4, 5-bisphosphate were both not significant inhibitors of clathrin assembly. It therefore appears that the interactions of inositides with AP-3 should not be considered simply in terms of electrostatic effects of the highly charged phosphate groups. Ligand specificity appears also to be mediated by hydrophobic interactions with the fatty-acyl chains of the inositol lipids.

Growth factor-dependent phosphoinositide signalling

A wide variety of messages, in the form of diffusible growth factors, hormones and cytokines, are carried throughout multicellular organisms to coordinate important physiological properties of target cells, such as proliferation, differentiation, migration, apoptosis and metabolism. Most messengers bind to cognate receptors on target cells, which initiate a characteristic cascade of reactions within the cell, ultimately leading to the desired response. The cellular response is defined by the combination of signalling components whose individual activity depends upon the number and type of surface receptors. Consequently the responses of different cell types to one or more stimuli can be quite disparate. A molecular understanding of the signalling pathways employed by each type of receptor therefore underlies the ability to rationalize many cellular functions and to correct disfunctions. As a well studied example of the primary signalling events that take place on the cytoplasmic leaflet of the plasma membrane following receptor activation, we will discuss how the widely expressed receptor for epidermal growth factor (EGF) causes the phosphorylation and hydrolysis of a signalling precursor, the membrane lipid phosphatidylinositol. This paradigm will be used to illustrate certain general principles of signalling, including formation of multienzyme complexes, compartmentation of second messengers and intermediates, and cross-talk between different signalling pathways.

Signaling inositol polyphosphate-5-phosphatase. Characterization of activity and effect of GRB2 association

An inositol polyphosphate-5-phosphatase (SIP-110) that binds the SH3 domains of the adaptor protein GRB2 was produced in Sf9 cells and characterized. SIP-110 binds to GRB2 in vitro with a stoichiometry of 1 mol of GRB2/0.7 mol of SIP-110. GRB2 binding does not affect enzyme activity implying that GRB2 serves mainly to localize SIP-110 within cells. SIP-110 hydrolyses inositol (Ins)(1,3,4,5)P4 to Ins(1, 3,4)P3. The enzyme does not hydrolyze Ins(1,4,5)P3 that is a substrate for previously described 5-phosphatases nor does it hydrolyze phosphatidylinositol (PtdIns)(4,5)P2. SIP-110 also hydrolyzed PtdIns(3,4,5)P3 to PtdIns(3,4)P2 as did recombinant forms of two other 5-phosphatases designated as inositol polyphosphate-5- phosphatase II, and OCRL (the protein that is mutated in oculocerebrorenal syndrome). The inositol polyphosphate-5-phosphatase enzyme family now is represented by at least 9 distinct genes and includes enzymes that fall into 4 subfamilies based on their activities toward various 5-phosphatase substrates.

A specific product of phosphatidylinositol 3-kinase directly activates the protein kinase Akt through its pleckstrin homology domain

Phosphatidylinositol (PI) 3-kinase is a cytoplasmic signaling molecule that is recruited to activated growth factor receptors after growth factor stimulation of cells. Activation of PI 3-kinase results in increased intracellular levels of 3' phosphorylated inositol phospholipids and the induction of signaling responses, including the activation of the protein kinase Akt, which is also known as RAC-PK or PKB. We tested the possibility that the phospholipid products of PI 3-kinase directly mediate the activation of Akt. We have previously described a constitutively active PI 3-kinase, p110, which can stimulate Akt activity. We used purified p110 protein to generate a series of 3' phosphorylated inositol phospholipids and tested whether any of these lipids could activate Akt in vitro. Phospholipid vesicles containing PI3,4 bisphosphate (P2) specifically activated Akt in vitro. By contrast, the presence of phospholipid vesicles containing PI3P or PI3,4,5P3 failed to increase the kinase activity of Akt. Akt could also be activated by synthetic dipalmitoylated PI3,4P2 or after enzymatic conversion of PI3,4,5P3 into PI3,4P2 with the signaling inositol polyphosphate 5' phosphatase SIP. We show that PI3,4P2-mediated activation is dependent on a functional pleckstrin homology domain in Akt, since a point mutation in the pleckstrin homology domain abrogated the response to PI3,4P2. Our findings show that a phospholipid product of PI 3-kinase can directly stimulate an enzyme known to be an important mediator of PI 3-kinase signaling.

Specific activation of p85-p110 phosphatidylinositol 3'-kinase stimulates DNA synthesis by ras- and p70 S6 kinase-dependent pathways

We have developed a polyclonal antibody that activates the heterodimeric p85-p110 phosphatidylinositol (PI) 3'-kinase in vitro and in microinjected cells. Affinity purification revealed that the activating antibody recognized the N-terminal SH2 (NSH2) domain of p85, and the antibody increased the catalytic activity of recombinant p85-p110 dimers threefold in vitro. To study the role of endogenous PI 3'-kinase in intact cells, the activating anti-NSH2 antibody was microinjected into GRC + LR73 cells, a CHO cell derivative selected for tight quiescence during serum withdrawal. Microinjection of anti-NSH2 antibodies increased bromodeoxyuridine (BrdU) incorporation fivefold in quiescent cells and enhanced the response to serum. These data reflect a specific activation of PI 3'-kinase, as the effect was blocked by coinjection of the appropriate antigen (glutathione S-transferase-NSH2 domains from p85 alpha), coinjection of inhibitory anti-p110 antibodies, or treatment of cells with wortmannin. We used the activating antibodies to study signals downstream from PI 3'-kinase. Although treatment of cells with 50 nM rapamycin only partially decreased anti-NSH2-stimulated BrdU incorporation, coinjection with an anti-p70 S6 kinase antibody effectively blocked anti-NSH2-stimulated DNA synthesis. We also found that coinjection of inhibitory anti-ras antibodies blocked both serum- and anti-NSH2-stimulated BrdU incorporation by approximately 60%, and treatment of cells with a specific inhibitor of MEK abolished antibody-stimulated BrdU incorporation. We conclude that selective activation of physiological levels of PI 3'-kinase is sufficient to stimulate DNA synthesis in quiescent cells. PI 3'-kinase-mediated DNA synthesis requires both p70 S6 kinase and the P21ras/MEK pathway.

D3 phosphoinositides and outside-in integrin signaling by glycoprotein IIb-IIIa mediate platelet actin assembly and filopodial extension induced by phorbol 12-myristate 13-acetate

Phorbol 12-myristate 13-acetate (PMA) uncaps a small number of the fast-growing (barbed) ends of actin filaments, thereby eliciting slow actin assembly and extension of filopodia in human blood platelets. These reactions, which also occur in response to immunologic perturbation of the integrin glycoprotein (GP) IIb-IIIa, are sensitive to the phosphoinositide 3-kinase inhibitor wortmannin. Platelets deficient in GPIIb-IIIa integrins or with GPIIb-IIIa function inhibited by calcium chelation or the peptide RGDS have diminished PMA responsiveness. The effects of PMA contrast with thrombin receptor stimulation by >/=5 microM thrombin receptor-activating peptide (TRAP), which causes rapid and massive wortmannin-insensitive actin assembly and lamellar and filopodial extension. However, we show here that wortmannin can inhibit filopod formation if the thrombin receptor is ligated using suboptimal doses (<1 microM) of TRAP. Phosphatidylinositol 3,4-bisphosphate inhibits actin filament severing and capping by human gelsolin in vitro. The findings implicate D3 polyphosphoinositides and integrin signaling in PMA-mediated platelet stimulation and implicate D3 containing phosphoinositides generated in response to protein kinase C activation and GPIIb-IIIa signaling as late-acting intermediates leading to filopodial actin assembly.

Phosphopleckstrin inhibits gbetagamma-activable platelet phosphatidylinositol-4,5-bisphosphate 3-kinase

Pleckstrin, the prototypic protein containing two copies of the pleckstrin homology domain, is a prominent substrate of protein kinase C in platelets and neutrophils. Both cell types have p85 subunit-containing phosphoinositide 3-kinase (p85/PI3K) and non-p85-containing PI3K (PI3Kgamma) that is activated by betagamma subunits of heterotrimeric GTP-binding proteins. We have shown that a PI3K product, phosphatidylinositol (PI) 3,4,5-trisphosphate, promotes pleckstrin phosphorylation in platelets. Since pleckstrin homology domains are thought to interact with Gbetagamma heterodimers and/or PI(4,5)P2, we have examined the effects of recombinant pleckstrins on platelet PI3Kgamma and p85/PI3K activities. Depending upon its phosphorylation/charged state, pleckstrin inhibits PI3Kgamma, but not p85/PI3K. Pleckstrin-mediated inhibition of PI3Kgamma is overcome by excess Gbetagamma and is restricted to PI(4,5)P2 as substrate, i.e. pleckstrin does not inhibit phosphorylation of PI()P or PI. Consistent with this, activation of protein kinase C by exposure of platelets to beta-phorbol diester (to increase endogenous pleckstrin phosphorylation) prior to platelet lysis causes inhibition of Gbetagamma-stimulatable PI3K activity only with respect to PI(4,5)P2 substrate. This phosphopleckstrin-mediated inhibition is overcome by increasing concentrations of Gbetagamma. We propose that phosphorylation of pleckstrin may constitute an important inhibitory mechanism for PI3Kgamma-mediated cell signaling.

Identification and cloning of centaurin-alpha. A novel phosphatidylinositol 3,4,5-trisphosphate-binding protein from rat brain

Using an affinity resin and photoaffinity label based on phospholipid analogs of inositol 1,3,4,5-tetrakisphosphate (InsP4), we have isolated, characterized, and cloned a 46-kDa protein from rat brain, which we have named centaurin-alpha. Binding specificity was determined using displacement of 1-O-[3H](3-[4-benzoyldihydrocinnamidyl]propyl)-InsP4 photoaffinity labeling. Centaurin-alpha displayed highest affinity for phosphatidylinositol 3,4,5-trisphosphate (PtdInsP3) (IC50 = 120 nM), whereas InsP4, PtdInsP2, and InsP3 bound with 5-, 12-, and >50-fold lower affinity, respectively. Screening a rat brain cDNA library with a polymerase chain reaction product, generated using partial amino acid sequence from tryptic peptides, yielded a full-length clone. The 2,450-base pair cDNA contained an open reading frame (ORF) encoding a novel protein of 419 amino acids. Northern analysis revealed a 2.5-kilobase transcript that is highly expressed in brain. The deduced sequence contains a novel putative zinc finger motif, 10 ankyrin-like repeats, and shows homology to recently identified yeast and mammalian Arf GTPase-activating proteins. Given the specificity of binding and enrichment in brain, centaurin-alpha is a candidate PtdInsP3 receptor that may link the activation of phosphoinositide 3-kinase to downstream responses in the brain.

alphaIIb beta 3-integrin mediated adhesion of human platelets to a fibrinogen matrix triggers phospholipase C activation and phosphatidylinositol 3',4'-biphosphate accumulation

This study focused on the variations in phosphoinositide metabolism depending upon alphaIIbbeta3-integrin/fibrinogen interaction without previous activation of platelet agonist receptors. We found that adhesion of resting human platelets to immobilized fibrinogen stimulates phosphatidic acid production and a concomitant decrease in phosphatidylinositol 4',5'-bisphosphate. These results, and the absence of a transphosphatidylation reaction, argue in favor of the activation of a phospholipase C. Moreover, we observed the accumulation of phosphatidylinositol 3',4'-bisphosphate in adherent platelets as a consequence of the activation of a phosphatidylinositol 3-kinase. This effect was inhibited by ADP scavengers. Our results demonstrate that in adherent platelets, whereas phosphatidylinositol 3-kinase activation is controlled by both alphaIIbbeta-integrin engagement and released ADP, phospholipase C stimulation is triggered only by alphaIIbbeta-integrin/fibrinogen interaction.

Phosphoinositide 3-kinase and p72syk noncovalently associate with the low affinity Fc gamma receptor on human platelets through an immunoreceptor tyrosine-based activation motif. Reconstitution with synthetic phosphopeptides

Previously, we have demonstrated that the cytoplasmic tyrosine kinase p72syk is coupled to the platelet Fc receptor for IgG (Fc gamma RIIA) (Chacko, G. W., Duchemin, A. M., Coggeshall, K. M., Osborne, J. M., Brandt, J. T., and Anderson, C. L. (1994) J. Biol. Chem. 269, 32435-32440). Further analysis of the platelet activation by Fc gamma RIIA demonstrated that Fc gamma RIIA is also inducibly coupled to the serine/threonine and lipid kinase, phosphoinositide 3-kinase (PI 3-K). activation of platelets with anti-Fc gamma RIIA antibodies resulted in the noncovalent association of PI 3-K with Fc gamma RIIA as well as an increase in Fc gamma RIIA-associated PI 3-K activity. Binding of both p72syk and PI 3-K to Fc gamma RIIA was reconstituted with synthetic phosphopeptides corresponding to the sequence of the atypical immunoreceptor tyrosine-based activation motif (ITAM) in the cytoplasmic domain of Fc gamma RIIA. Our findings demonstrate that coupling of both p72syk and PI. 3-K activities to Fc gamma RIIA is regulated by tyrosine phosphorylation of the ITAM, and we speculate that p72syk might act as an adapter to recruit PI 3-K to activated Fc gamma RIIA.

Phosphoinositide kinases

Recently, a number of cDNA clones with homology to the catalytic subunit of phosphoinositide 3-kinase have been identified, and the sequence of the first cDNA clone encoding a phosphatidylinositol 4-phosphate 5-kinase has been published. Use of both dominant-negative mutants of phosphoinositide 3-kinase and the inhibitors wortmannin and LY294002 has identified a number of processes in which phosphoinositide 3-kinase participates, including cell motility, the Ras pathway, vesicle trafficking and secretion, and apoptosis. Several possible biochemical targets of phosphoinositides have been found.

Thrombin stimulates wortmannin-inhibitable phosphoinositide 3-kinase and membrane blebbing in CHRF-288 cells

We have investigated thrombin-stimulated morphological changes and the activation of phosphoinositide 3-kinase (PI 3-K), as manifested by the accumulation of PtdIns(3,4)P2 and PtdIns(3,4,5)P3 (labelled with 32P or myo-[3H]inositol), in CHRF-288 cells, a leukaemic cell line derived from a platelet progenitor cell. We report that these cells, when exposed to thrombin or SFLLRN (the peptide Ser-Phe-Leu-Leu-Arg-Asn, a thrombin-receptor ligand) rapidly change shape, forming membrane 'blebs', detectable by differential interference contrast or confocal microscopy, as well as labelled 3-phosphorylated phosphoinositides. The 'blebs' are distinguishable from 'ruffles' or lamellae, since they do not contain phalloidin-detectable actin. Studies with permeabilized cells indicate that PI 3-K is activated synergistically by thrombin+guanosine 5'[gamma-thio]triphosphate. Two forms of PI 3-K, i.e. PI 3-K(gamma) and p85/PI 3-K, regulated by G beta gamma subunits of heterotrimeric G-protein and the small G-protein Rho, respectively, are present in these cells, as is true for platelets. Wortmannin, a known potent and specific inhibitor of PI 3-K activities, inhibits thrombin-stiumlated accumulation of 3-phosphorylated phosphoinositides in a dose-dependent manner (IC50 approximately 10nM), without affecting phospholipase C activation. Pretreatment of CHRF-288 cells with either wortmannin (100 nM) or an unrelated synthetic PI 3-K inhibitor, LY294002 (50 microM), abolishes thrombin-receptor-stimulated blebbing. These results suggest that thrombin-stimulated accumulation of 3-phosphorylated phosphoinositide(s) is required for the shape-change response in CHRF-288 cells.

Phosphoinositide 3-kinase gamma and p85/phosphoinositide 3-kinase in platelets. Relative activation by thrombin receptor or beta-phorbol myristate acetate and roles in promoting the ligand-binding function of alphaIIbbeta3 integrin

Platelets exposed to thrombin or thrombin receptor agonist peptide (SFLLRN) activate phospholipase C and protein kinase C (PKC), and accumulate 3-phosphorylated phosphoinositides (3-PPI) as a function of the activation and relocalization of two cytoskeletally-associated phosphoinositide 3-kinases (PI 3-K): p85/PI 3-K and PI 3-Kgamma. We now report that exposure of platelets to PKC-activating beta-phorbol myristate acetate (betaPMA) does not stimulate PI 3-Kgamma, but rather stimulates p85/PI 3-K, which associates with the cytoskeleton. Wortmannin is an inhibitor of both PI 3-Ks, known to act with more potency on p85/PI 3-K. betaPMA-stimulated 3-PPI accumulation is more sensitive to wortmannin (IC50 = 1.3 nM) than is SFLLRN- or thrombin-stimulated 3-PPI accumulation (IC50 = 10 nM). The activity of p85/PI 3-K in immunoprecipitates or in cytoskeletal fractions is inhibited more potently by exposure of platelets to wortmannin than is the activity of PI 3-Kgamma. betaPMA or SFLLRN promotes the conversion of platelet integrin alphaIIb/beta3 into a fibrinogen-binding form required for platelet aggregation. Activation of alphaIIb/beta3 in response to betaPMA or SFLLRN is inhibited by wortmannin with an IC50 of 1 nM in each case. Wortmannin inhibits neither activation of alphaIIb/beta3 by ligand-induced binding site antibody (anti-LIBS6 Fab) nor anti-LIBS6 Fab-induced platelet aggregation in the presence of fibrinogen, indicating that this type of "outside-in" signaling by alphaIIb/beta3 is largely PI 3-K-independent. We conclude that p85/PI 3-K, in preference to PI 3-Kgamma, contributes to activation of alphaIIb/beta3 when the thrombin receptor or PKC is stimulated.