Abstract 501: Nuclear Factor-κB Is Involved in Platelet CD40 Signaling and Activation

Introduction and hypothesis: CD40 ligand (CD40L) is a thrombo-inflammatory molecule that predicts cardiovascular events. Platelets constitute the major source of soluble CD40L (sCD40L), which has been shown to influence platelet activation. We have previously shown that upon ligation, CD40 potentiates platelet activation and aggregation via p38 mitogen activated protein kinase (MAPK) and Rac1 signaling. In B lymphocytes, CD40 induces activation and nuclear translocation of nuclear factor kappa B (NF-κB), which is dependent on the phosphorylation and dissociation of the inhibitor of kappa B α (IκBα). Given that platelets contain NF-κB, we hypothesized that it may be involved in platelet CD40 signaling. Methods and results: In human platelets, sCD40L induced association of tumor necrosis factor receptor associated factor 2 to CD40, and a time-dependant phosphorylation of IκBα, which is indicative of NF-κB activation. Activation of NF-κB in platelets treated with sCD40L was abolished by CD40L blockade. Pretreatment of platelets with the IκBα inhibitor, BAY 11-7082, reversed IκBα phosphorylation induced by sCD40L, without affecting p38 MAPK activation. On the other hand, pretreatment of platelets with the p38 MAPK phosphorylation inhibitor, SB203580, had no effect on IκBα phosphorylation, indicating a divergence in the signaling pathway originating from CD40 upon its ligation. Finally, inhibition of IκBα phosphorylation by either BAY 11-7082 or the IκB kinase inhibitor VII reversed sCD40L induced platelet activation, as measured by P-selectin expression, and the potentiation of platelet aggregation induced by a priming dose of collagen. Conclusion: This study demonstrates the implication of NF-κB in platelet signaling downstream of CD40, where it plays a role in platelet activation and aggregation upon sCD40L stimulation.