SDF-1α/CXCR4 Signaling in Lipid Rafts Induces Platelet Aggregation via PI3 Kinase-Dependent Akt Phosphorylation

Stromal cell-derived factor-1 alpha (SDF-1 alpha)-induced platelet aggregation is mediated through its G protein-coupled receptor CXCR4 and phosphatidylinositol 3 kinase (P13K). Here, we demonstrate that SDF-1 alpha induces phosphorylation of Akt at Thr308 and Ser473 in human platelets. SDF-1 alpha-induced platelet aggregation and Akt phosphorylation are inhibited by pretreatment with the CXCR4 antagonist AMD3100 or the P13K inhibitor LY294002. SDF-1 alpha also induces the phosphorylation of PDK1 at Ser241 (an upstream activator of Akt), GSK3 beta at Ser9 (a downstream substrate of Akt), and myosin light chain at Ser19 (a downstream element of the Akt signaling pathway). SDF-1 alpha-induced platelet aggregation is inhibited by pretreatment with the Akt inhibitor MK-2206 in a dose-dependent manner. Furthermore, SDF-1 alpha-induced platelet aggregation and Akt phosphorylation are inhibited by pretreatment with the raft-disrupting agent methyl-beta-cyclodextrin. Sucrose density gradient analysis shows that 35% of CXCR4, 93% of the heterotrimeric G proteins G alpha i-1, 91% of G alpha i-2, 50% of G beta and 4.0% of PI3K beta, and 4.5% of Akt2 are localized in the detergent-resistant membrane raft fraction. These findings suggest that SDF-1 alpha/CXCR4 signaling in lipid rafts induces platelet aggregation via P13K-dependent Akt phosphorylation.