Protein kinase C-delta activation and tyrosine phosphorylation in platelets

Role and Mechanism of PKC-δ for Cardiovascular Disease: Current Status and Perspective

Protein kinase C (PKC) is a protein kinase with important cellular functions. PKC-δ, a member of the novel PKC subfamily, has been well-documented over the years. Activation of PKC-δ plays an important regulatory role in myocardial ischemia/reperfusion (IRI) injury and myocardial fibrosis, and its activity and expression levels can regulate pathological cardiovascular diseases such as atherosclerosis, hypertension, cardiac hypertrophy, and heart failure. This article aims to review the structure and function of PKC-δ, summarize the current research regarding its activation mechanism and its role in cardiovascular disease, and provide novel insight into further research on the role of PKC-δ in cardiovascular diseases.

Fyn Kinase-Mediated PKCδ Y311 Phosphorylation Induces Dopaminergic Degeneration in Cell Culture and Animal Models: Implications for the Identification of a New Pharmacological Target for Parkinson's Disease

Oxidative stress, neuroinflammation and apoptosis are some of the key etiological factors responsible for dopamin(DA)ergic degeneration during Parkinson's disease (PD), yet the downstream molecular mechanisms underlying neurodegeneration are largely unknown. Recently, a genome-wide association study revealed the FYN gene to be associated with PD, suggesting that Fyn kinase could be a pharmacological target for PD. In this study, we report that Fyn-mediated PKCδ tyrosine (Y311) phosphorylation is a key event preceding its proteolytic activation in a 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) model of Parkinsonism. MPP+/MPTP induced Fyn kinase activation in N27 DAergic neuronal cells and the mouse substantia nigra. PKCδ-Y311 phosphorylation by activated Fyn initiates the apoptotic caspase-signaling cascade during DAergic degeneration. Pharmacological attenuation of Fyn activity protected DAergic neurons from MPP+-induced degeneration in primary mesencephalic neuronal cultures. We further employed Fyn wild-type and Fyn knockout (KO) mice to confirm whether Fyn is a valid pharmacological target of DAergic neurodegeneration. Primary mesencephalic neurons from Fyn KO mice were greatly protected from MPP+-induced DAergic cell death, neurite loss and DA reuptake loss. Furthermore, Fyn KO mice were significantly protected from MPTP-induced PKCδ-Y311 phosphorylation, behavioral deficits and nigral DAergic degeneration. This study thus unveils a mechanism by which Fyn regulates PKCδ's pro-apoptotic function and DAergic degeneration. Pharmacological inhibitors directed at Fyn activation could prove to be a novel therapeutic target in the delay or halting of selective DAergic degeneration during PD.

Characterization of Integrin αIIbβ3-Mediated Outside-in Signaling by Protein Kinase Cδ in Platelets

Engagement of integrin αIIbβ3 promotes platelet-platelet interaction and stimulates outside-in signaling that amplifies activation. Protein kinase Cδ (PKCδ) is known to play an important role in platelet activation, but its role in outside-in signaling has not been established. In the present study, we determined the role of PKCδ and its signaling pathways in integrin αIIbβ3-mediated outside-in signaling in platelets using PKCδ-deficient platelets. Platelet spreading to immobilized fibrinogen resulted in PKCδ phosphorylation, suggesting that αIIbβ3 activation caused PKCδ activation. αIIbβ3-mediated phosphorylation of Akt was significantly inhibited in PKCδ -/- platelets, indicating a role of PKCδ in outside-in signaling. αIIbβ3-mediated PKCδ phosphorylation was inhibited by proline-rich tyrosine kinase 2 (Pyk2) selective inhibitor, suggesting that Pyk2 contributes to the regulation of PKCδ phosphorylation in outside-in signaling. Additionally, Src-family kinase inhibitor PP2 inhibited integrin-mediated Pyk2 and PKCδ phosphorylation. Lastly, platelet spreading was inhibited in PKCδ -/- platelets compared to the wild-type (WT) platelets, and clot retraction from PKCδ -/- platelets was markedly delayed, indicating that PKCδ is involved in the regulation of αIIbβ3-dependent interactivities with cytoskeleton elements. Together, these results provide evidence that PKCδ plays an important role in outside-in signaling, which is regulated by Pyk2 in platelets.

Distinctive roles of PKC delta isozyme in platelet function

Platelet activation is a complex balance of positive and negative signaling pathways. Several protein kinase C (PKC) isoforms are expressed in human platelets. They are a major regulator of platelet granule secretion, activation and aggregation activity. One of those isoforms is the PKCδ isozyme, it has a central yet complex role in platelets such as opposite signaling functions depending on the nature of the agonist, it concentration and pathway. In fact, it has been shown that PKCδ has an overall negative influence on platelet function in response to collagen, while, following PAR stimulation, PKCδ has a positive effect on platelet function. Understanding the crucial role of PKCδ in platelet functions is recently emerging in the literature, therefore, further investigations should shed light into its specific role in hemostasis. In this review, we focus on the different roles of PKCδ in platelet activation, aggregation and thrombus formation.

Cross talk between serine/threonine and tyrosine kinases regulates ADP-induced thromboxane generation in platelets

ADP-induced thromboxane generation depends on Src family kinases (SFKs) and is enhanced with pan-protein kinase C (PKC) inhibitors, but it is not clear how these two events are linked. The aim of the current study is to investigate the role of Y311 phosphorylated PKCδ in regulating ADP-induced platelet activation. In the current study, we employed various inhibitors and murine platelets from mice deficient in specific molecules to evaluate the role of PKCδ in ADP-induced platelet responses. We show that, upon stimulation of platelets with 2MeSADP, Y311 on PKCδ is phosphorylated in a P2Y1/Gq and Lyn-dependent manner. By using PKCδ and Lyn knockout murine platelets, we also show that tyrosine phosphorylated PKCδ plays a functional role in mediating 2MeSADP-induced thromboxane generation. 2MeSADP-induced PKCδ Y311 phosphorylation and thromboxane generation were potentiated in human platelets pre-treated with either a pan-PKC inhibitor, GF109203X or a PKC α/β inhibitor and in PKC α or β knockout murine platelets compared to controls. Furthermore, we show that PKC α/β inhibition potentiates the activity of SFK, which further hyper-phosphorylates PKCδ and potentiates thromboxane generation. These results show for the first time that tyrosine phosphorylated PKCδ regulates ADP-induced thromboxane generation independent of its catalytic activity and that classical PKC isoforms α/β regulate the tyrosine phosphorylation on PKCδ and subsequent thromboxane generation through tyrosine kinase, Lyn, in platelets.

Protein kinase C ε expression in platelets from patients with acute myocardial infarction

Objective

Platelets play crucial roles in the pathophysiology of thrombosis and myocardial infarction. Protein kinase C ε (PKCε) is virtually absent in human platelets and its expression is precisely regulated during human megakaryocytic differentiation. On the basis of what is known on the role of platelet PKCε in other species, we hypothesized that platelets from myocardial infarction patients might ectopically express PKCε with a pathophysiological role in the disease.

Methods and Results

We therefore studied platelet PKCε expression from 24 patients with myocardial infarction, 24 patients with stable coronary artery disease and 24 healthy subjects. Indeed, platelets from myocardial infarction patients expressed PKCε with a significant frequency as compared to both stable coronary artery disease and healthy subjects. PKCε returned negative during patient follow-up. The forced expression of PKCε in normal donor platelets significantly increased their response to adenosine diphosphate-induced activation and adhesion to subendothelial collagen.

Conclusions

Our data suggest that platelet generations produced before the acute event retain PKCε-mRNA that is not down-regulated during terminal megakaryocyte differentiation. Results are discussed in the perspective of peri-infarctual megakaryocytopoiesis as a critical component of myocardial infarction pathophysiology.

The role of C2 domains in PKC signaling

More than two decades ago, the discovery of the first C2 domain in conventional Protein Kinase Cs (cPKCs) and of its role as a calcium-binding motif began to shed light on the activation mechanism of this family of Serine/Threonine kinases which are involved in several critical signal transduction pathways. In this chapter, we review the current knowledge of the structure and the function of the different C2 domains in PKCs. The C2 domain of cPKCs is a calcium sensor and its calcium-dependent binding to phospholipids is crucial for kinase activation. While the functional role of the cPKC C2 domain is better understood, phylogenetic analysis revealed that the novel C2 domain is more ancient and related to the C2 domain in the fungal PKC family, while the cPKC C2 domain is first associated with PKC in metazoans. The C2 domain of novel PKCs (nPKCs) does not contain a calcium-binding motif but still plays a critical role in nPKCs activation by regulating C1-C2 domain interactions and consequently C2 domain-mediated inhibition in both the nPKCs of the epsilon family and the nPKCs of the delta family. Moreover, the C2 domain of the nPKCs of the delta family was shown to recognize phosphotyrosines in a novel mode different from the ones observed for the Src Homology 2 (SH2) and the phosphotyrosine binding domains (PTB). By binding to phosphotyrosines, the C2 domain regulates the activation of this subclass of PKCs. The C2 domain was also shown to be involved in protein-protein interactions and binding to the receptor for activated C-kinase (RACKs) thus contributing to the subcellular localization of PKCs. In summary, the C2 domain is a critical player that can sense the activated signaling pathway in response to external stimuli to specifically regulate the different conventional and novel PKC isoforms.

Possible Involvement of Ca Activated K Channels, SK Channel, in the Quercetin-Induced Vasodilatation

Effects of quercetin, a kind of flavonoids, on the vasodilating actions were investigated. Among the mechanisms for quercetin-induced vasodilatation in rat aorta, the involvement with the Ca(2+) activated K(+) (K(Ca)) channel was examined. Pretreatment with NE (5 microM) or KCl (60 mM) was carried out and then, the modulation by quercetin of the constriction was examined using rat aorta ring strips (3 mm) at 36.5. Quercetin (0.1 to 100 microM) relaxed the NE-induced vasoconstrictions in a concentration-dependent manner. NO synthesis (NOS) inhibitor, NG-monomethyl-L-arginine acetate (L-NMMA), at 100 microM reduced the quercetin (100 microM)-induced vasodilatation from 97.8+/-3.7% (n=10) to 78.0+/-11.6% (n=5, p<0.05). Another NOS inhibitor, L-NG-nitro arginine methyl ester (L-NAME), at 100 microM also had the similar effect. In the presence of both 100 microM L-NMMA and 10 microM indomethacin, the quercetin-induced vasodilatation was further attenuated by 100 microM tetraethylammonium (TEA, a K(Ca) channel inhibitor). Also TEA decreased the quercetin-induced vasodilatation in endothelium-denuded rat aorta. Used other K(Ca) channel inhibitors, the quercetin-induced vasodilatation was attenuated by 0.3 microM apamin (a SK channel inhibitor), but not by 30 nM charybdotoxin (a BK and IK channel inhibitor). Quercetin caused a concentration-dependent vasodilatation, due to the endothelium-dependent and -independent actions. Also quercetin contributes to the vasodilatation selectively with SK channel on smooth muscle.

Differential regulation of threonine and tyrosine phosphorylations on protein kinase Cdelta by G-protein-mediated pathways in platelets

Phosphorylation of activation loop threonine (Thr(505)) and regulatory domain tyrosine (Tyr(311)) residues are key regulators of PKC (protein kinase C) delta function in platelets. In the present study, we show that G(q) and G(12/13) pathways regulate the Thr(505) and Tyr(311) phosphorylation on PKCdelta in an interdependent manner. DiC8 (1,2-dioctanoylglycerol), a synthetic analogue of DAG (diacylglycerol), caused Thr(505), but not Tyr(311), phosphorylation on PKCdelta, whereas selective activation of G(12/13) pathways by the YFLLRNP peptide failed to cause phosphorylation of either residue. However, simultaneous activation by DiC8 and YFLLRNP resulted in Thr(505) and Tyr(311) phosphorylation on PKCdelta. In addition, we found that the activation of SFKs (Src family tyrosine kinases) is essential for G(12/13)-mediated Tyr(311) phosphorylation of PKCdelta. These results were confirmed using G(q)-deficient mouse platelets. Finally, we investigated whether Thr(505) phosphorylation is required for Tyr(311) phosphorylation. A T505A PKCdelta mutant failed to be phosphorylated at Tyr(311), even upon stimulation of both G(q) and G(12/13) pathways. We conclude that (i) PKCdelta binding to DAG, downstream of G(q) pathways, and its translocation results in Thr(505) phosphorylation, (ii) G(12/13) pathways activate SFKs required for the phosphorylation of Tyr(311) on Thr(505)-phosphorylated PKCdelta, and (iii) Thr(505) phosphorylation is a prerequisite for Tyr(311) phosphorylation on PKCdelta.

Differential roles of the PKC novel isoforms, PKCdelta and PKCepsilon, in mouse and human platelets

BACKGROUND

Increasing evidence suggests that individual isoforms of protein kinase C (PKC) play distinct roles in regulating platelet activation.

METHODOLOGY/PRINCIPAL FINDINGS

In this study, we focus on the role of two novel PKC isoforms, PKCdelta and PKCepsilon, in both mouse and human platelets. PKCdelta is robustly expressed in human platelets and undergoes transient tyrosine phosphorylation upon stimulation by thrombin or the collagen receptor, GPVI, which becomes sustained in the presence of the pan-PKC inhibitor, Ro 31-8220. In mouse platelets, however, PKCdelta undergoes sustained tyrosine phosphorylation upon activation. In contrast the related isoform, PKCepsilon, is expressed at high levels in mouse but not human platelets. There is a marked inhibition in aggregation and dense granule secretion to low concentrations of GPVI agonists in mouse platelets lacking PKCepsilon in contrast to a minor inhibition in response to G protein-coupled receptor agonists. This reduction is mediated by inhibition of tyrosine phosphorylation of the FcRgamma-chain and downstream proteins, an effect also observed in wild-type mouse platelets in the presence of a PKC inhibitor.

CONCLUSIONS

These results demonstrate a reciprocal relationship in levels of the novel PKC isoforms delta and epsilon in human and mouse platelets and a selective role for PKCepsilon in signalling through GPVI.

PKCdelta-dependent functional switch of rpS3 between translation and DNA repair

Ribosomal protein S3 (rpS3) is critically involved in translation as a component of the 40S ribosomal subunit and participates in the processing of DNA damage, functioning as a damage DNA endonuclease. However, it is not yet known how the function of rpS3 switches between translation and DNA repair. Here we show that PKCdelta phosphorylates rpS3 resulting in its mobilization in the nucleus to repair damaged DNA. Phosphorylated rpS3 was only detected in non-ribosomal rpS3 and the repair endonuclease activity of rpS3 was increased by its phosphorylation. In addition, rpS3 knock-down cells showed more sensitivity to genotoxic stress than control cells, and this sensitivity was corrected by overexpressed wild-type rpS3 but not by phosphorylation defective rpS3. In conclusion, we propose that the destiny of rpS3 molecules between translation and DNA repair is regulated by PKCdelta-dependent phosphorylation.

Coincident regulation of PKCdelta in human platelets by phosphorylation of Tyr311 and Tyr565 and phospholipase C signalling

PKC (protein kinase C)d plays a complex role in platelets, having effects on both positive and negative signalling functions. It is phosphorylated on tyrosine residues in response to thrombin and collagen, and it has recently been shown that Tyr311 is phosphorylated in response to PAR (protease-activated receptor) 1 and PAR4 receptor activation. In the present study, we show that Tyr311 and Tyr565 are phosphorylated in response to thrombin, and have examined the interplay between phosphorylation and the classical lipid-mediated activation of PKCd. Phosphorylation of both Tyr311 and Tyr565 is dependent on Src kinase and PLC (phospholipase C) activity in response to thrombin. Importantly, direct allosteric activation of PKCd with PMA also induced phosphorylation of Tyr311 and Tyr565, and this was dependent on the activity of Src kinases, but not PLC. Membrane recruitment of PKCd is essential for phosphorylation of this tyrosine residue, but tyrosine phosphorylation is not required for membrane recruitment of PKCd. Both thrombin and PMA induce recruitment of PKCd to the membrane, and for thrombin, this recruitment is a PLC-dependent process. In order to address the functional role of tyrosine residue phosphorylation of PKCd, we demonstrate that phosphorylation can potentiate the activity of the kinase, although phosphorylation does not play a role in membrane recruitment of the kinase. PKCd is therefore regulated in a coincident fashion, PLC-dependent signals recruiting it to the plasma membrane and by phosphorylation on tyrosine residues, potentiating its activity.

PKCdelta signaling: mechanisms of DNA damage response and apoptosis

The cellular response to genotoxic stress that damages DNA includes cell cycle arrest, activation of DNA repair, and in the event of irreparable damage, induction of apoptosis. However, the signals that determine cell fate, that is, survival or apoptosis, are largely unknown. The delta isoform of protein kinase C (PKCdelta) has been implicated in many important cellular processes, including regulation of apoptotic cell death. The available information supports a model in which certain sensors of DNA lesions activate PKCdelta. This activation is triggered in part by tyrosine phosphorylation of PKCdelta by c-Abl tyrosine kinase. PKCdelta is further proteolytically activated by caspase-3. The cleaved catalytic fragment of PKCdelta translocates to the nucleus and induces apoptosis. Importantly, accumulating data have revealed the nuclear targets for PKCdelta in the induction of apoptosis. A pro-apoptotic function of activated PKCdelta is mediated by at least several downstream effectors known to be associated with the elicitation of apoptosis. Recent findings also demonstrated that PKCdelta is involved in cell cycle-specific activation and induction of apoptotic cell death. Moreover, previous studies have shown that PKCdelta regulates transcription by phosphorylating various transcription factors, including the p53 tumor suppressor that is critical for cell cycle arrest and apoptosis in response to DNA damage. These findings collectively support a pivotal role for PKCdelta in the induction of apoptosis with significant impact. This review is focused on the current views regarding the regulation of cell fate by PKCdelta signaling in response to DNA damage.

Essential Role of Protein Kinase Cδ in Platelet Signaling, αIIbβ3 Activation, and Thromboxane A2 Release

The protein kinase C (PKC) family is an essential signaling mediator in platelet activation and aggregation. However, the relative importance of the major platelet PKC isoforms and their downstream effectors in platelet signaling and function remain unclear. Using isolated human platelets, we report that PKCdelta, but not PKCalpha or PKCbeta, is required for collagen-induced phospholipase C-dependent signaling, activation of alpha(IIb)beta(3), and platelet aggregation. Analysis of PKCdelta phosphorylation and translocation to the membrane following activation by both collagen and thrombin indicates that it is positively regulated by alpha(IIb)beta(3) outside-in signaling. Moreover, PKCdelta triggers activation of the mitogen-activated protein kinase-kinase (MEK)/extracellular-signal regulated kinase (ERK) and the p38 MAPK signaling. This leads to the subsequent release of thromboxane A(2), which is essential for collagen-induced but not thrombin-induced platelet activation and aggregation. This study adds new insight to the role of PKCs in platelet function, where PKCdelta signaling, via the MEK/ERK and p38 MAPK pathways, is required for the secretion of thromboxane A(2).

Molecular mechanism and functional implications of thrombin-mediated tyrosine phosphorylation of PKCdelta in platelets

Thrombin has been known to cause tyrosine phosphorylation of protein kinase C delta (PKCdelta) in platelets, but the molecular mechanisms and function of this tyrosine phosphorylation is not known. In this study, we investigated the signaling pathways used by protease-activated receptors (PARs) to cause tyrosine phosphorylation of PKCdelta and the role of this event in platelet function. PKCdelta was tyrosine phosphorylated by either PAR1 or PAR4 in a concentration- and time-dependent manner in human platelets. In particular, the tyrosine 311 residue was phosphorylated downstream of PAR receptors. Also the tyrosine phosphorylation of PKCdelta did not occur in Galpha(q)-deficient mouse platelets and was inhibited in the presence of a phospholipase C (PLC) inhibitor U73122 and calcium chelator BAPTA (5,5'-dimethyl-bis(o-aminophenoxy)ethane-N, N, N ', N '-tetraacetic acid), suggesting a role for Galpha(q) pathways and calcium in this event. Both PAR1 and PAR4 caused a time-dependent activation of Src (pp60c-src) tyrosine kinase and Src tyrosine kinase inhibitors completely blocked the tyrosine phosphorylation of PKCdelta. Inhibition of tyrosine phosphorylation or the kinase activity of PKCdelta dramatically blocked PAR-mediated thromboxane A2 generation. We conclude that thrombin causes tyrosine phosphorylation of PKCdelta in a calcium- and Src-family kinase-dependent manner in platelets, with functional implications in thromboxane A2 generation.

Activation of integrin alpha IIb beta 3 in the glycoprotein Ib-high population of a megakaryocytic cell line, CMK, by inside-out signaling

Affinity/avidity state of integrin alpha IIb beta 3 is regulated by intracellular inside-out signaling. Although several megakaryocytic cell lines have been established, soluble ligand binding to alpha IIb beta 3 expressed in these cells by cellular agonists has not been demonstrated. We have re-examined agonist-induced alpha IIb beta 3 activation on megakaryocytic cell lines with a marker of the late stage of megakaryocytic differentiation, glycoprotein Ib (GPIb). Activation of alpha IIb beta 3 was assessed by PAC1 and soluble fibrinogen binding to the cells. We found that alpha IIb beta 3 expressed in CMK cells with high GPIb expression was activated by a phorbor ester, phorbol myristate acetate (PMA). Although the population of the GPIbhigh cells was <0.5% of the total cells, incubation with a nucleoside analog, ribavirin, efficiently increased the PMA-reactive GPIbhigh cells. Not only PMA but also a calcium ionophore, A23187, induced alpha IIb beta 3 activation, and PMA and A23187 had an additive effect on alpha IIb beta 3 activation. Ligand binding to the activated alpha IIb beta 3 in the GPIbhigh CMK cells is totally abolished by an alpha IIb beta 3-specific antagonist, and inhibited by wortmannin, cytochalasin-D and prostaglandin E1, and the effects of these inhibitors on alpha IIb beta 3 activation in the GPIbhigh CMK cells were compatible with those in platelets. We have also demonstrated that the ribavirin-treated CMK cells express PKC-alpha, -beta, -delta and -theta, and suggested that PKC-alpha and/or -beta appear to be responsible for PMA-induced activation of alpha IIb beta 3 in CMK cells.

Cholecystokinin-stimulated protein kinase C-delta kinase activation, tyrosine phosphorylation, and translocation are mediated by Src tyrosine kinases in pancreatic acinar cells

Protein kinase C-delta (PKC-delta) is involved in growth, differentiation, tumor suppression, and regulation of other cellular processes. PKC-delta activation causes translocation, tyrosine phosphorylation, and serine-threonine kinase activity. However, little is known about the ability of G protein-coupled receptors to activate these processes or the mediators involved. In the present study, we explored the ability of the neurotransmitter/hormone, CCK, to stimulate these changes in PKC-delta and explored the mechanisms. In rat pancreatic acini under basal conditions, PKC-delta is almost exclusively located in cytosol. CCK and TPA stimulated a rapid PKC-delta translocation to membrane and nuclear fractions, which was transient with CCK. CCK stimulated rapid tyrosine phosphorylation of PKC-delta and increased kinase activity. Using tyrosine kinase (B44) and a tyrosine phosphatase inhibitor (orthovanadate), changes in both CCK- and TPA-stimulated PKC-delta tyrosine phosphorylation were shown to correlate with changes in its kinase activity but not translocation. Both PKC-delta tyrosine phosphorylation and activation occur exclusively in particulate fractions. The Src kinase inhibitors, SU6656 and PP2, but not the inactive related compound, PP3, inhibited CCK- and TPA-stimulated PKC-delta tyrosine phosphorylation and activation. In contrast, PP2 also had a lesser effect on CCK- but not TPA-stimulated PKC-delta translocation. CCK stimulated the association of Src kinases with PKC-delta, demonstrated by co-immunoprecipitation. These results demonstrate that CCKA receptor activation results in rapid translocation, tyrosine phosphorylation, and activation of PKC-delta. Stimulation of PKC-delta translocation precedes tyrosine phosphorylation, which is essential for activation to occur. Activation of Src kinases is essential for the tyrosine phosphorylation and kinase activation to occur and plays a partial role in translocation.

Physical and functional interaction between protein kinase C delta and Fyn tyrosine kinase in human platelets

An increasing number of tyrosine kinases have been shown to associate with isoforms of the protein kinase C (PKC) family. Here, we show evidence for physical and functional interaction between PKCdelta and the Src family kinase Fyn in human platelets activated by alboaggregin-A, a snake venom capable of activating both GPIb-V-IX and GPVI adhesion receptors. This interaction involves phosphorylation of PKCdelta on tyrosine and is specific in that other isoforms of PKC, PKCepsilon and lambda, which also become tyrosine-phosphorylated, do not interact with Fyn. In addition, PKCdelta does not interact with other platelet-expressed tyrosine kinases Syk, Src, or Btk. Stimulation also leads to activation of both Fyn and PKCdelta and to serine phosphorylation of Fyn within a PKC consensus sequence. Alboaggregin-A-dependent activation of Fyn is blocked by bisindolylmaleimide I, suggesting a role for PKC isoforms in regulating Fyn activity. Platelet activation with alboaggregin-A induces translocation of the two kinases from cytoplasm to the plasma membrane of platelets, as observed by confocal immunofluorescence microscopy. Translocation of Fyn and PKCdelta are blocked by PP1 and bisindolylmaleimide I, showing a dependence upon Src and PKC kinase activities. Although PKC activity is required for translocation, it is not required for association between the two kinases, because this was not blocked by bisindolylmaleimide I. Rottlerin, which inhibited PKCdelta activity, did not block translocation of either PKCdelta or Fyn but potentiated platelet aggregation, 5-hydroxytryptamine secretion, and the calcium response induced by alboaggregin-A, indicating that this kinase plays a negative role in the control of these processes.

Thrombin rapidly induces protein kinase D phosphorylation, and protein kinase C delta mediates the activation

Thrombin plays a critical role in hemostasis, thrombosis, and inflammation. However, the responsible intracellular signaling pathways triggered by thrombin are still not well defined. We report here that thrombin rapidly and transiently induces activation of protein kinase D (PKD) in aortic smooth muscle cells. Our data demonstrate that protein kinase C (PKC) inhibitors completely block thrombin-induced PKD activation, suggesting that thrombin induces PKD activation via a PKC-dependent pathway. Furthermore, our results show that thrombin rapidly induces PKC delta phosphorylation and that the PKC delta-specific inhibitor rottlerin blocks thrombin-induced PKD activation, suggesting that PKC delta mediates the thrombin-induced PKD activation. Using dominant negative approaches, we demonstrated that expression of a dominant negative PKC delta inhibits the phosphorylation and activation of PKD induced by thrombin, whereas neither PKC epsilon nor PKC zeta affects thrombin-induced PKD activation. In addition, our results of co-immunoprecipitation assays showed that PKD forms a complex with PKC delta in smooth muscle cells. Taken together, the findings of the present study demonstrate that thrombin induces activation of PKD and reveal a novel role of PKC delta in mediating thrombin-induced PKD activation in vascular smooth muscle cells.

Cholecystokinin-stimulated tyrosine phosphorylation of PKC-delta in pancreatic acinar cells is regulated bidirectionally by PKC activation

PKC-delta is important in cell growth, apoptosis, and secretion. Recent studies show its stability is regulated by tyrosine phosphorylation (TYR-P), which can be stimulated by a number of agents. Many of these stimuli also activate phospholipase C (PLC) cascades and little is known about the relationship between these cascades and PKC-delta TYR-P. Cholecystokinin (CCK) stimulates PKCs but it is unknown if it causes PKC-delta TYR-P and if so, the relationship between these cascades is unknown. In rat pancreatic acini, CCK-8 stimulated rapid PKC-delta TYR-P by activation of the low affinity CCK(A) receptor state. TPA had a similar effect. BAPTA did not decrease CCK-stimulated PKC-delta TYR-P but instead, increased it. A23187 did not stimulate PKC-delta TYR-P. Wortmannin and LY 294002 did not alter CCK-stimulated PKC-delta TYR-P. GF 109203X, at low concentrations, increased PKC-delta TYR-P stimulated by CCK or TPA and at higher concentrations, inhibited it. The cPKC inhibitors, Gö 6976 and safingol, caused a similar increase in TPA- and CCK-stimulated PKC-delta TYR-P. These results demonstrate that CCK(A) receptor activation causes PKC-delta TYR-P through activation of only one of its two receptor affinity states. This PKC-delta TYR-P is not directly influenced by changes in [Ca(2+)](i); however, the resultant activation of PKC-alpha has an inhibitory effect. Therefore, CCK activates both stimulatory and inhibitory PKC cascades regulating PKC-delta TYR-P and, hence, likely plays an important role in regulating PKC-delta degradation and cellular abundance.

Carbachol stimulates TYR phosphorylation and association of PKCdelta and PYK2 in pancreas

Carbachol treatment resulted in increased phosphorylation on tyrosine of PKCdelta immunoprecipitated from rat pancreatic acinar cells. The Ca2+-dependent tyrosine kinase PYK2 coimmunoprecipitated with PKCdelta from carbachol-exposed cells and also exhibited increased tyrosine phosphorylation. Tyrosine phosphorylation of both PKCdelta and PYK2 was concentration-dependent with respect to carbachol, and rapid, reaching maximal levels by 5 min of treatment. Exposure of acinar cells to phorbol myristate acetate (PMA), a phorbol ester activator of PKCdelta, also resulted in increased phosphorylation of PKCdelta and PYK2 isolated using anti-PKCdelta immunoprecipitation. These results are suggestive of a physical and functional interaction between PKCdelta and PYK2 following muscarinic stimulation in the pancreatic acinar cell.

Junctional adhesion molecule (JAM) is phosphorylated by protein kinase C upon platelet activation

Junctional adhesion molecule (JAM) is a member of the immunoglobulin superfamily (IgSF) expressed in tight junctions of epithelial cells and endothelial cells, and implicated in transendothelial migration of leukocytes. Recently, JAM is reported to be constitutively expressed on circulating monocytes, neutrophils, lymphocytes subsets, and platelets. However, the role of JAM is not known. Here, we examined how phosphorylaton of JAM is regulated upon platelet activation. Phosphorylation of JAM was induced by thrombin, collagen, but not by ADP. The phosphorylated amino acids were shown to be serine residues by phosphoamino acid analysis. Inhibition of JAM's phosphorylation by PKC inhibitors and Ca(++) chelator suggests the involvement of conventional types of PKCs. By in vitro kinase assays, we demonstrated that JAM could be directly phosphorylated by cPKCs. We also demonstrated phosphorylation of Ser 284, a putative PKC phosphorylation site, by immunoblotting with anti-phosphoserine-JAM antibody in thrombin-stimulated platelets. In addition to the phosphorylation, JAM seemed to form clusters at several sites of cell-cell contact in aggregated platelets by immunoelectron microscopic study. We speculate that JAM may be directly phosphorylated by cPKC(s)upon platelet activation and that the phosphorylationmight be involved in platelet activation.

Regulation of protein kinase Ctheta function during T cell activation by Lck-mediated tyrosine phosphorylation

Protein kinase C theta (PKCtheta) is a novel Ca(2+)-independent PKC isoform, which is selectively expressed in skeletal muscle and hematopoietic cells, especially T cells. In T cells, it colocalizes with the T cell antigen receptor (TCR).CD3 complex in antigen-stimulated T cells and is involved in the transcriptional activation of the interleukin-2 gene. In the present study, we report that PKCtheta is tyrosine phosphorylated in Jurkat T cells upon TCR.CD3 activation. The Src family protein-tyrosine kinase, Lck, was critical in TCR-induced tyrosine phosphorylation of PKCtheta. Lck phosphorylated and was associated with the regulatory domain of PKCtheta both in vitro and in intact cells. This association was constitutive, but it was enhanced by T cell activation, with both Src-homology 2 and Src-homology 3 domains of Lck contributing to it. Tyrosine 90 (Tyr-90) in the regulatory domain of PKCtheta was identified as the major phosphorylation site by Lck. A constitutively active mutant of PKCtheta (A148E) could enhance proliferation of Jurkat T cells and synergized with ionomycin to induce nuclear factor of T cells activity. However, mutation of Tyr-90 into phenylalanine markedly reduced (or abolished) these activities. These results suggest that Lck plays an important role in tyrosine phosphorylation of PKCtheta, which may in turn modulate the physiological functions of PKCtheta during TCR-induced T cell activation.

Thrombopoietin acts synergistically on Ca(2+) mobilization in platelets caused by ADP or thrombin receptor agonist peptide

Thrombopoietin (TPO) is the main regulator of megakaryopoiesis and influences also the function of mature platelets. TPO has been shown to synergize in multiple platelet activation processes induced by various agonists. Our aim was to elucidate whether TPO affects calcium signaling during platelet activation processes. TPO demonstrated a synergistic effect on the exocytosis induced by suboptimal doses of adenosine diphosphate (ADP) and the thrombin receptor agonist peptide (TRAP). We detected synergistic effects of TPO on the ADP or TRAP induced Ca(2+) mobilization in a small range of very low agonist concentrations. The TPO synergism on Ca(2+) mobilization and CD62P expression was measurable in different, nonoverlapping ranges of ADP or TRAP concentrations. Sustaining the agonist-induced calcium signal with thapsigargin led to a detectable TPO synergism in CD62P expression even in agonist concentrations in which the synergism only occurs in Ca(2+) signaling without thapsigargin.

Integrin alpha IIb beta 3-mediated pp125FAK phosphorylation and platelet spreading on fibrinogen are regulated by PI 3-kinase

Activation of the focal adhesion kinase pp125FAK correlates with its phosphorylation on tyrosine residues and is mediated by multiple receptor-ligand pairs. In platelets, pp125FAK phosphorylation is triggered by alpha IIb beta 3 integrin or Fc gamma RII receptor interaction with immobilized fibrinogen and IgG, respectively. In this study we used platelets as a model system to explore the role of PI 3-kinase relative to pp125FAK phosphorylation. Treatment of the platelets with two PI 3-kinase inhibitors, wortmannin and LY294002, inhibited in a dose-dependent manner alpha IIb beta 3-mediated platelet spreading on fibrinogen having no effect on platelet spreading on IgG. Both inhibitors also completely abolished alpha IIb beta 3-mediated pp125FAK phosphorylation but not pp72syk phosphorylation. Furthermore, Fc gamma RII- and thrombin-induced pp125FAK phosphorylation were not affected by wortmannin and LY294002. Finally, the PI 3-kinase inhibitors' effect on alpha IIb beta 3-mediated spreading and pp125FAK phosphorylation was reversed by phorbol ester treatment. These results establish that the role of PI 3-kinase relative to pp125FAK phosphorylation in platelets is receptor type-specific yet essential for alpha IIb beta 3-mediated cell spreading and pp125FAK phosphorylation.