The role of PKC in X-ray-induced megakaryocyte apoptosis and thrombocytopenia.

Thrombocytopenia is a critical complication after radiation therapy and exposure. Dysfunction of megakaryocyte development and platelet production are key pathophysiological stages in ionizing radiation (IR)-induced thrombocytopenia. Protein kinase C (PKC) plays an important role in regulating megakaryocyte development and platelet production. However, it remains unclear how PKC regulates IR-induced megakaryocyte apoptosis. In this study, we found that pretreatment of PKC pan-inhibitor Go6983 delayed IR-induced megakaryocyte apoptosis, and inhibited IR-induced mitochondrial membrane potential and ROS production in CMK cells. Moreover, suppressing PKC activation inhibited cleaved caspase3 expression and reduced p38 phosphorylation levels, and IR-induced PKC activation might be regulated by p53. In vivo experiments confirmed that Go6983 promoted platelet count recovery after 21 days of 3 Gy total body irradiation. Furthermore, Go6983 reduced megakaryocyte apoptosis, increased the number of megakaryocyte and polyploid formation in bone marrow, and improved the survival rate of 6 Gy total body irradiation. In conclusion, our results provided a potential therapeutic target for IR-induced thrombocytopenia.

Essential role of glycoprotein Ibα in platelet activation.

ABSTRACT: Glycoprotein Ibα (GPIbα), the ligand-binding subunit of platelet GPIb-IX complex, interacts with von Willebrand factor (VWF) exposed at the injured vessel wall, initiating platelet adhesion, activation, hemostasis, and thrombus formation. The cytoplasmic tail of GPIbα interacts with 14-3-3ζ, regulating the VWF-GPIbα-elicited signal transduction and VWF binding function of GPIbα. However, we unexpectedly found that the GPIbα-14-3-3ζ association, beyond VWF-dependent function, is essential for general platelet activation. We found that the myristoylated peptide of GPIbα C-terminus MPαC, a potential GPIbα inhibitor, by itself induced platelet aggregation, integrin αIIbß3 activation, granule secretion, and phosphatidylserine (PS) exposure. Conversely, the deletion of the cytoplasmic tail of GPIbα in mouse platelets (10aa-/-) decreased platelet aggregation, integrin αIIbß3 activation, granule secretion, and PS exposure induced by various physiological agonists. Phosphoproteome-based kinase activity profiling revealed significantly upregulated protein kinase C (PKC) activity in MPαC-treated platelets. MPαC-induced platelet activation was abolished by the pan-PKC inhibitor and PKCα deletion. Decreased PKC activity was observed in both resting and agonist-stimulated 10aa-/- platelets. GPIbα regulates PKCα activity by sequestering 14-3-3ζ from PKCα. In vivo, the deletion of the GPIbα cytoplasmic tail impaired mouse hemostasis and thrombus formation and protected against platelet-dependent pulmonary thromboembolism. Therefore, our findings demonstrate an essential role for the GPIbα cytoplasmic tail in regulating platelet general activation and thrombus formation beyond the VWF-GPIbα axis.