Metabolic syndrome is characterized by a prothrombotic state and platelets from patients with type I or type II diabetes are hyper-reactive suggesting a tight link between insulin resistance and platelet activation. The AMP-activated protein kinase (AMPK) a2 catalytic subunit has been shown to determine whole-body insulin sensitivity. However, the role of AMPK in platelet activation has not yet been investigated. Human and murine platelets express both the AMPKa1 and a2 isoforms and the phosphorylation of the AMPK on Thr¹⁷² was time- and concentration-dependently stimulated by thrombin (0,01 to 0,3 U/ml thrombin). Both the tumor promoter LKB1 and the Ca²⁺/calmodulin kinase kinase (CaMKK) are reported to phosphorylate the AMPK in other cells. We found that thrombin markedly increased the phosphorylation of LKB1 in platelets. Moreover, the thrombin-induced phosphorylation of the AMPK in platelets was not affected by the CAMKK inhibitor KN93 indicating the involvement of LKB1 but not of CAMKK. In washed human platelets the AMPK inhibitors iodotubericidin and compound C significantly inhibited thrombin-induced aggregation without affecting the concomitant increase in [Ca²⁺]_i. Moreover, in human samples clot retraction was significantly inhibited by the AMPK inhibitors, and AMPKa2^-/- mice demonstrated an impaired clot retraction when compared with their wild-type littermates. Furthermore, thrombus stability was also decreased in an in vivo model of FeCl₃-induced injury in the AMPKa2^-/- mice even though bleeding time was not significantly different between the AMPKa2^-/- and wild-type animals. Mechanistically, AMPKa2 phosphorylated in vitro the Src-family kinase, Fyn on Thr12. Moreover, stimulation of washed human or murine platelets with thrombin led to increased threonine phosphorylation of Fyn as well as the subsequent phosphorylation of its substrate b3 integrin on Tyr747, which was inhibited by AMPK inhibitors or by deletion of the AMPKa2 isoform in mice. These data indicate that AMPKa2, by affecting Fyn phosphorylation and activity, plays a key role in platelet aIIbb3 integrin signaling, leading to clot retraction and thrombus stability.

Figure 1