Protein kinase C alpha enhances sodium-calcium exchange during store-operated calcium entry in mouse platelets

School of Physiology and Pharmacology, Medical Sciences Building, University of Bristol, Bristol BS8 1TD, United Kingdom.
Cell calcium (Impact Factor: 3.51). 12/2010; 48(6):333-40. DOI: 10.1016/j.ceca.2010.10.006
Source: PubMed


A rise in intracellular calcium concentration ([Ca(2+)](i)) is necessary for platelet activation. A major component of the [Ca(2+)](i) elevation occurs through store-operated Ca(2+) entry (SOCE). The aim of this study was to understand the contribution of the classical PKC isoform, PKCα to platelet SOCE, using platelets from PKCα-deficient mice. SOCE was reduced by approximately 50% in PKCα(-/-) platelets, or following treatment with bisindolylmaleimide I, a PKC inhibitor. However, TG-induced Mn(2+) entry was unaffected, which suggests that divalent cation entry through store-operated channels is not directly regulated. Blocking the autocrine action of secreted ADP or 5-HT on its receptors did not reproduce the effect of PKCα deficiency. In contrast, SN-6, a Na(+)/Ca(2+) exchanger inhibitor, did reduce SOCE to the same extent as loss of PKCα, as did replacing extracellular Na(+) with NMDG(+). These treatments had no further effect in PKCα(-/-) platelets. These data suggest that PKCα enhances the extent of SOCE in mouse platelets by regulating Ca(2+) entry through the Na(+)/Ca(2+) exchanger.

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    • "The role of individual isoforms in platelet activation has been investigated using isoform-specific inhibitors and mice deficient in single isoforms [4–7,9–19]. This has led to the conclusion that the classical isoforms play positive roles in platelet activation, with PKCα playing the predominant role, supported by PKCβ, while the novel isoforms play minor or inhibitory roles [10]. "
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    ABSTRACT: 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. To measure the levels of PKCθ and PKCε and to investigate the phenotype of mice deficient in both novel PKC isoforms. Tail bleeding and platelet activation assays were monitored in mice and platelets from mice deficient in both PKCθ and PKCε. 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. 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.
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    ABSTRACT: Cytosolic calcium concentration is a critical regulator of platelet activation, and so platelet Ca(2+) signaling must be tightly controlled. Thrombin-induced Ca(2+) signaling is enhanced by inhibitors of protein kinase C (PKC), suggesting that PKC negatively regulates the Ca(2+) signal, although the mechanisms by which this occurs and its physiological relevance are still unclear. To investigate the mechanisms by which PKC inhibitors enhance thrombin-induced Ca(2+) signaling, and to determine the importance of this pathway in platelet activation. Cytosolic Ca(2+) signaling was monitored in fura-2-loaded human platelets. Phosphatidylserine (PS) exposure, a marker of platelet procoagulant activity, was measured by annexin V binding and flow cytometry. PKC inhibition by bisindolylmaleimide-I (BIM-I) enhanced α-thrombin-induced Ca(2+) signaling in a concentration-dependent manner. PAR1 signaling, activated by SFLLRN, was enhanced much more strongly than PAR4, activated by AYPGKF or γ-thrombin, which is a potent PAR4 agonist but a poor activator of PAR1. BIM-I had little effect on α-thrombin-induced signaling following treatment with the PAR1 antagonist, SCH-79797. BIM-I enhanced Ca(2+) release from intracellular stores and Ca(2+) entry, as assessed by Mn(2+) quench. However, the plasma membrane Ca(2+) ATPase inhibitor, 5(6)-carboxyeosin, did not prevent the effect of BIM-I. PKC inhibition strongly enhanced α-thrombin-induced PS exposure, which was reversed by blockade of PAR1. Together, these data show that when PAR1 is stimulated, PKC negatively regulates Ca(2+) release and Ca(2+) entry, which leads to reduced platelet PS exposure.
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