PPARγ agonists negatively regulate αIIbβ3 integrin outside-in signaling and platelet function through up-regulation of protein kinase A activity.

Background: Agonists for the peroxisome proliferator-activated receptor (PPARc) have been shown to have inhibitory effects on platelet activity following stimulation by GPVI and GPCR agonists. Objectives: Profound effects on thrombus formation led us to suspect a role for PPARc agonists in the regulation of integrin alpha IIb beta 3 mediated signaling. Both GPVI and GPCR signaling pathways lead to alpha IIb beta 3 activation, and signaling through alpha IIb beta 3 plays a critical role in platelet function and normal hemostasis. Methods: The effects of PPARc agonists on the regulation of alpha IIb beta 3 outside-in signaling was determined by monitoring the ability of platelets to adhere and spread on fibrinogen and undergo clot retraction. Effects on signaling components downstream of alpha IIb beta 3 activation were also determined following adhesion to fibrinogen by Western blotting. Results: Treatment of platelets with PPARc agonists inhibited platelet adhesion and spreading on fibrinogen and diminished clot retraction. A reduction in phosphorylation of several components of alpha IIb beta 3 signaling, including the integrin beta 3 subunit, Syk, PLC gamma 2, focal adhesion kinase (FAK) and Akt, was also observed as a result of reduced interaction of the integrin b3 subunit with G alpha 13. Studies of VASP phosphorylation revealed that this was because of an increase in PKA activity following treatment with PPARc receptor agonists. Conclusions: This study provides further evidence for antiplatelet actions of PPARc agonists, identifies a negative regulatory role for PPARc agonists in the control of integrin alpha IIb beta 3 outside-in signaling, and provides a molecular basis by which the PPARc agonists negatively regulate platelet activation and thrombus formation.