The Critical Roles of SIN1 in Platelet Activation and Myocardial Infarction

Mammalian stress-activated protein kinase interacting protein 1 (SIN1) is an essential subunit of the mTORC2 complex, which regulates Akt activation by phosphorylation of Akt at Ser473 residue. Despite the function of Akt in platelet activation and thrombosis was well studied, the role of SIN1 in platelet activation and thrombosis remains unknown. In this study, we observed that megakaryocyte/platelet specific SIN1 deficiency caused 30% reduction of platelet counts in peripheral blood probably by blockage of megakaryocyte differentiation and enhancing platelet apoptosis, suggesting that SIN1 had an important role in thrombopoiesis. More importantly, SIN1 deficiency caused a defect in platelet aggregation in response to low level of thrombin, U46619, ADP and collagen. SIN1 deficiency also exhibited diminished ability of platelet to spread on immobilized fibrinogen and the decreased rate of clot retraction in platelet-rich plasma containing SIN1 deficient platelets. mTORC2 complex analysis revealed that the expression levels of Rictor, another mTORC2 component, were significantly diminished in SIN1 deficient platelet. And SIN1 deficiency attenuated agonist-induced phosphorylation of Akt at Ser473, Thr308 and Thr450, and Gsk3β at Ser9 in platelet. SIN1 could be phosphorylated at Thr86, which correlated with the phosphorylation of Akt at Ser473 in activated platelets. Further study demonstrated that the phosphorylation levels of SIN1 at Thr86 and Akt at Ser473 and Thr450, but not at Thr308 were enhanced in the platelets collected from ST-segment elevation myocardial infarction (STEMI) patients, indicating that SIN1 activation correlated with myocardial infarction process. A mouse model of chronic myocardial infarction (MI) was performed and the results demonstrated that platelet-specific SIN1 deficient mice had less platelet activation, reduced MI size, and improved post-MI heart function. In conclusion, SIN1 plays critical roles in platelet activation, MI and post-MI heart failure, therefore serves as a target for therapeutic intervention in the thrombosis and myocardial infarction.