Pharmacological inhibition of S6K1 facilitates platelet activation by enhancing Akt phosphorylation

Platelet activation and thrombus formation is a delicate process involving a series of crosstalk between different pathways. P70 ribosomal S6 kinase1 (S6K1) is a member of serine/threonine kinases and can be phosphorylated by 3-phosphoinositide-dependent protein kinase 1 (PDK1). S6K1 is widely reported to play important roles in cancers and metabolic diseases, but the role of S6K1 and the importance of phosphorylation on Thr229 in platelet activation have not been defined. PF-4708671 is a recently synthesized highly specific inhibitor of S6K1. In this study, we tested PF-4708671 to assess the role of S6K1 in platelet. PF-4708671 facilitated mouse and human platelet aggregation and ATP secretion induced by collagen, thrombin, and adenosine diphosphate through enhanced Akt and Gsk3β phosphorylation. PF-4708671 also accelerated integrin αIIbβ3-mediated clot retraction and spreading. Intravenously given PF-4708671 shortened the occlusion time in arterial thrombosis model. Further results demonstrated that S6K1 was phosphorylated by PDK1 on Thr229 in the resting platelets and dephosphorylated in response to agonist stimulation. PDK1-deficient mice showed higher aggregation when PI3K-Akt-Gsk3β signaling was blocked by the Gsk3β-inhibitor SB216763. Thus, S6K1 Thr229 phosphorylation might function as a regulator that prevents platelets from activation. S6K1 inhibition may yield potential pro-thrombotic effects and should be used cautiously when considered as a therapy.