Tangeretin regulates platelet function through inhibition of phosphoinositide 3-kinase and cyclic nucleotide signaling.

OBJECTIVE: Dietary flavonoids have long been appreciated in reducing cardiovascular disease risk factors, but their mechanisms of action are complex in nature. In this study, the effects of tangeretin, a dietary flavonoid, were explored on platelet function, signaling, and hemostasis. APPROACH AND RESULTS: Tangeretin inhibited agonist-induced human platelet activation in a concentration-dependent manner. It inhibited agonist-induced integrin αIIbß3 inside-out and outside-in signaling, intracellular calcium mobilization, and granule secretion. Tangeretin also inhibited human platelet adhesion and subsequent thrombus formation on collagen-coated surfaces under arterial flow conditions in vitro and reduced hemostasis in mice. Further characterization to explore the mechanism by which tangeretin inhibits platelet function revealed distinctive effects of platelet signaling. Tangeretin was found to inhibit phosphoinositide 3-kinase-mediated signaling and increase cGMP levels in platelets, although phosphodiesterase activity was unaffected. Consistent with increased cGMP levels, tangeretin increased the phosphorylation of vasodilator-stimulated phosphoprotein at S239. CONCLUSIONS: This study provides support for the ability and mechanisms of action of dietary flavonoids to modulate platelet signaling and function, which may affect the risk of thrombotic disease.

Connexin40 regulates platelet function.

The presence of multiple connexins was recently demonstrated in platelets, with notable expression of Cx37. Studies with Cx37-deficient mice and connexin inhibitors established roles for hemichannels and gap junctions in platelet function. It was uncertain, however, whether Cx37 functions alone or in collaboration with other family members through heteromeric interactions in regulation of platelet function. Here we report the presence and functions of an additional platelet connexin, Cx40. Inhibition of Cx40 in human platelets or its deletion in mice reduces platelet aggregation, fibrinogen binding, granule secretion and clot retraction. The effects of the Cx37 inhibitor (37,43)Gap27 on Cx40(-/-) mouse platelets and of the Cx40 inhibitor (40)Gap27 on Cx37(-/-) mouse platelets revealed that each connexin is able to function independently. Inhibition or deletion of Cx40 reduces haemostatic responses in mice, indicating the physiological importance of this protein in platelets. We conclude that multiple connexins are involved in regulating platelet function, thereby contributing to haemostasis and thrombosis.