PKCtheta in platelet activation

A novel di-peptide Met-Glu from collagen hydrolysates inhibits platelet aggregation and thrombus formation via regulation of Gq-mediated signaling

Increasing evidence has shown that collagen peptides had various biological activities. In this study, a novel antiplatelet peptide Met-Glu (ME) was separated and identified from silver carp skin by YMC ODS-A C18 separation and ESI-MS/MS analysis. Peptide ME inhibited platelet aggregation and secretion of platelet granules induced by ADP, thrombin and collagen, and significantly attenuated ferric chloride-induced thrombus formation in rats. It did not prolong the bleeding time in mice even at the dose of 300 μmol/kg body weight that showed potent anti-thrombosis effects. Additionally, peptide ME targeted at Gq-protein to downregulate the phosphorylation of PLCβ, an important upstream effector of PI3K/Akt and Erk/MAPK signaling to inhibit intracellular calcium ion mobilization. These results suggest that peptide ME inhibited thrombosis in vivo and inhibited Gq-mediated signaling in platelets, indicating the possibility that ME could potentially be developed as a novel therapeutic agent in the prevention and treatment of thrombotic diseases. PRACTICAL APPLICATIONS: Cardiovascular diseases (CVDs) are the leading cause of mortality and morbidity worldwide. The proximal cause of CVDs is intravascular thrombosis formation, which mostly results from platelet activation, aggregation, and granules secretion. Traditional drugs in the prevention of thrombotic disease, such as aspirin and clopidogrel, are still limited for their side effects, especially bleeding complications. Collagen is a natural source for bioactive peptides and our previous study has shown that collagen peptides could inhibit platelet aggregation in vitro. Understanding the mechanism of collagen peptides on regulation of platelet activation and their in vivo anti-thrombosis activities were important for the development of novel-specific medical food in the prevention of thrombotic diseases.

PKCs in thrombus formation

The protein kinase C (PKC) family has been implicated in several physiological processes regulating platelet activation. Each isoform of PKC expressed on platelets, may have a positive and/or negative role depending on the nature and concentration of the agonist. Mice lacking PKCα show much reduced thrombus formation in vivo, while PKCθ(-/-) showed inhibition of aggregation in response to PAR4. On the other hand, PKCδ by associating with Fyn, inhibits platelet aggregation. In addition, PKCβ by interacting with its receptor RACK1 has been implicated in the primary phases of signaling via the αIIbβ3 and finally PKCɛ appears to be involved in platelet function downstream GPVI. The present review discusses the latest observations relevant to the role of individual PKC isoforms in platelet activation and thrombus formation.

Secrets of platelet exocytosis - what do we really know about platelet secretion mechanisms?

Upon activation by extracellular matrix components or soluble agonists, platelets release in excess of 300 active molecules from intracellular granules. Those factors can both activate further platelets and mediate a range of responses in other cells. The complex microenvironment of a growing thrombus, as well as platelets' roles in both physiological and pathological processes, require platelet secretion to be highly spatially and temporally regulated to ensure appropriate responses to a range of stimuli. However, how this regulation is achieved remains incompletely understood. In this review we outline the importance of regulated secretion in thrombosis as well as in 'novel' scenarios beyond haemostasis and give a detailed summary of what is known about the molecular mechanisms of platelet exocytosis. We also discuss a number of theories of how different cargoes could be released in a tightly orchestrated manner, allowing complex interactions between platelets and their environment.

Tyrosine phosphorylation on spleen tyrosine kinase (Syk) is differentially regulated in human and murine platelets by protein kinase C isoforms

Protein kinase C (PKC) isoforms differentially regulate platelet functional responses downstream of glycoprotein VI (GPVI) signaling, but the role of PKCs regulating upstream effectors such as Syk is not known. We investigated the role of PKC on Syk tyrosine phosphorylation using the pan-PKC inhibitor GF109203X (GFX). GPVI-mediated phosphorylation on Syk Tyr-323, Tyr-352, and Tyr-525/526 was rapidly dephosphorylated, but GFX treatment inhibited this dephosphorylation on Tyr-525/526 in human platelets but not in wild type murine platelets. GFX treatment did not affect tyrosine phosphorylation on FcRγ chain or Src family kinases. Phosphorylation of Lat Tyr-191 and PLCγ2 Tyr-759 was also increased upon treatment with GFX. We evaluated whether secreted ADP is required for such dephosphorylation. Exogenous addition of ADP to GFX-treated platelets did not affect tyrosine phosphorylation on Syk. FcγRIIA- or CLEC-2-mediated Syk tyrosine phosphorylation was also potentiated with GFX in human platelets. Because potentiation of Syk phosphorylation is not observed in murine platelets, PKC-deficient mice cannot be used to identify the PKC isoform regulating Syk phosphorylation. We therefore used selective inhibitors of PKC isoforms. Only PKCβ inhibition resulted in Syk hyperphosphorylation similar to that in platelets treated with GFX. This result indicates that PKCβ is the isoform responsible for Syk negative regulation in human platelets. In conclusion, we have elucidated a novel pathway of Syk regulation by PKCβ in human platelets.

Protein kinase Cε and protein kinase Cθ double-deficient mice have a bleeding diathesis

BACKGROUND

In comparison to the classical isoforms of protein kinase C (PKC), the novel isoforms are thought to play minor or inhibitory roles in the regulation of platelet activation and thrombosis.

OBJECTIVES

To measure the levels of PKCθ and PKCε and to investigate the phenotype of mice deficient in both novel PKC isoforms.

METHODS

Tail bleeding and platelet activation assays were monitored in mice and platelets from mice deficient in both PKCθ and PKCε.

RESULTS

PKCε plays a minor role in supporting aggregation and secretion following stimulation of the collagen receptor GPVI in mouse platelets but has no apparent role in spreading on fibrinogen. PKCθ, in contrast, plays a minor role in supporting adhesion and filopodial generation on fibrinogen but has no apparent role in aggregation and secretion induced by GPVI despite being expressed at over 10 times the level of PKCε. Platelets deficient in both novel isoforms have a similar pattern of aggregation downstream of GPVI and spreading on fibrinogen as the single null mutants. Strikingly, a marked reduction in aggregation on collagen under arteriolar shear conditions is observed in blood from the double but not single-deficient mice along with a significant increase in tail bleeding.

CONCLUSIONS

These results reveal a greater than additive role for PKCθ and PKCε in supporting platelet activation under shear conditions and demonstrate that, in combination, the two novel PKCs support platelet activation.

Platelet signaling-a primer

OBJECTIVE

To review the receptors and signal transduction pathways involved in platelet plug formation and to highlight links between platelets, leukocytes, endothelium, and the coagulation system.

DATA SOURCES

Original studies, review articles, and book chapters in the human and veterinary medical fields.

DATA SYNTHESIS

Platelets express numerous surface receptors. Critical among these are glycoprotein VI, the glycoprotein Ib-IX-V complex, integrin α(IIb) β(3) , and the G-protein-coupled receptors for thrombin, ADP, and thromboxane. Activation of these receptors leads to various important functional events, in particular activation of the principal adhesion receptor α(IIb) β(3) . Integrin activation allows binding of ligands such as fibrinogen, mediating platelet-platelet interaction in the process of aggregation. Signals activated by these receptors also couple to 3 other important functional events, secretion of granule contents, change in cell shape through cytoskeletal rearrangement, and procoagulant membrane expression. These processes generate a stable thrombus to limit blood loss and promote restoration of endothelial integrity.

CONCLUSIONS

Improvements in our understanding of how platelets operate through their signaling networks are critical for diagnosis of unusual primary hemostatic disorders and for rational antithrombotic drug design.

Submaximal inhibition of protein kinase C restores ADP-induced dense granule secretion in platelets in the presence of Ca2+

Protein kinase C (PKC) is a family of serine/threonine kinases that play isoform-specific inhibitory and stimulatory roles in platelet activation. We show here that the pan-PKC inhibitor Ro31-8220 can be used to dissect these events following platelet activation by ADP. Submaximal concentrations of Ro31-8220 potentiated aggregation and dense granule secretion to ADP in plasma anticoagulated with citrate, in d-Phe-Pro-Arg-chloromethyl ketone-anticoagulated plasma, which has physiological levels of Ca²⁺, and in washed platelets. Potentiation was retained on inhibition of cyclooxygenase and was associated with an increase in intracellular Ca²⁺. Potentiation of aggregation and secretion was abolished by a maximally effective concentration of Ro31-8220, consistent with a critical role of PKC in secretion. ADP-induced secretion was potentiated in the presence of an inhibitor of PKCβ but not in the presence of available inhibitors of other PKC isoforms in human and mouse platelets. ADP-induced secretion was also potentiated in mouse platelets deficient in PKCϵ but not PKCθ. These results demonstrate that partial blockade of PKC potentiates aggregation and dense granule secretion by ADP in association with increased Ca²⁺. This provides a molecular explanation for the inability of ADP to induce secretion in plasma in the presence of physiological Ca²⁺ concentrations, and it reveals a novel role for PKC in inhibiting platelet activation by ADP in vivo. These results also demonstrate isoform-specific inhibitory effects of PKC in platelets.

Functional divergence of platelet protein kinase C (PKC) isoforms in thrombus formation on collagen

Arterial thrombosis, a major cause of myocardial infarction and stroke, is initiated by activation of blood platelets by subendothelial collagen. The protein kinase C (PKC) family centrally regulates platelet activation, and it is becoming clear that the individual PKC isoforms play distinct roles, some of which oppose each other. Here, for the first time, we address all four of the major platelet-expressed PKC isoforms, determining their comparative roles in regulating platelet adhesion to collagen and their subsequent activation under physiological flow conditions. Using mouse gene knock-out and pharmacological approaches in human platelets, we show that collagen-dependent α-granule secretion and thrombus formation are mediated by the conventional PKC isoforms, PKCα and PKCβ, whereas the novel isoform, PKCθ, negatively regulates these events. PKCδ also negatively regulates thrombus formation but not α-granule secretion. In addition, we demonstrate for the first time that individual PKC isoforms differentially regulate platelet calcium signaling and exposure of phosphatidylserine under flow. Although platelet deficient in PKCα or PKCβ showed reduced calcium signaling and phosphatidylserine exposure, these responses were enhanced in the absence of PKCθ. In summary therefore, this direct comparison between individual subtypes of PKC, by standardized methodology under flow conditions, reveals that the four major PKCs expressed in platelets play distinct non-redundant roles, where conventional PKCs promote and novel PKCs inhibit thrombus formation on collagen.

Protein kinase Ctheta negatively regulates store-independent Ca2+ entry and phosphatidylserine exposure downstream of glycoprotein VI in platelets

Platelet activation must be tightly controlled to provide an effective, but not excessive, response to vascular injury. Cytosolic calcium is a critical regulator of platelet function, including granule secretion, integrin activation, and phosphatidylserine (PS) exposure. Here we report that the novel protein kinase C isoform, PKCtheta, plays an important role in negatively regulating Ca(2+) signaling downstream of the major collagen receptor, glycoprotein VI (GPVI). This limits PS exposure and so may prevent excessive platelet procoagulant activity. Stimulation of GPVI resulted in significantly higher and more sustained Ca(2+) signals in PKCtheta(-/-) platelets. PKCtheta acts at multiple distinct sites. PKCtheta limits secretion, reducing autocrine ADP signaling that enhances Ca(2+) release from intracellular Ca(2+) stores. PKCtheta thereby indirectly regulates activation of store-operated Ca(2+) entry. However, PKCtheta also directly and negatively regulates store-independent Ca(2+) entry. This pathway, activated by the diacylglycerol analogue, 1-oleoyl-2-acetyl-sn-glycerol, was enhanced in PKCtheta(-/-) platelets, independently of ADP secretion. Moreover, LOE-908, which blocks 1-oleoyl-2-acetyl-sn-glycerol-induced Ca(2+) entry but not store-operated Ca(2+) entry, blocked the enhanced GPVI-dependent Ca(2+) signaling and PS exposure seen in PKCtheta(-/-) platelets. We propose that PKCtheta normally acts to restrict store-independent Ca(2+) entry during GPVI signaling, which results in reduced PS exposure, limiting platelet procoagulant activity during thrombus formation.

Diverse functions of protein kinase C isoforms in platelet activation and thrombus formation

Platelet activation is a complex balance of positive and negative signaling pathways. The protein kinase C (PKC) family is a major regulator of platelet granule secretion, integrin activation, aggregation, spreading and procoagulant activity. As broad-spectrum PKC inhibitors reduce secretion and aggregation, the PKC family is generally considered to be a positive regulator of platelet activation. However, the individual members of the PKC family that are expressed in platelets are regulated in different ways, and an increasing body of evidence indicates that they have distinct, and often opposing, roles. Many of the recent advances in understanding the contributions of individual PKC isoforms have come from mouse gene knockout studies. PKCalpha, a classic isoform, is an essential positive regulator of granule secretion and thrombus formation, both in vitro and in vivo. Mice lacking PKCalpha show much reduced thrombus formation in vivo but do not have a bleeding defect, suggesting that PKCalpha could be an attractive antithrombotic target. Important, apparently non-redundant, roles, both positive and negative, for the novel PKC isoforms delta, theta and epsilon in granule secretion have also been proposed, indicating highly complex regulation of this essential process. Similarly, PKCbeta, PKCdelta and PKCtheta have non-redundant roles in platelet spreading, as absence of either PKCbeta or PKCtheta reduces spreading, whereas PKCdelta negatively regulates filopodial formation. This negative signaling by PKCdelta may reduce platelet aggregation and so restrict thrombus formation. In this review, we discuss the current understanding of the regulation and functions of individual PKC isoforms in platelet activation and thrombus formation.