Phosphoinositide 3-Kinases p110α and p110β Have Differential Roles in Insulin-Like Growth Factor-1–Mediated Akt Phosphorylation and Platelet Priming

Abstract

Objective— Platelet hyperactivity is a contributing factor in the pathogenesis of cardiovascular disease and can be induced by elevated levels of circulating growth factors, such as insulin-like growth factor-1 (IGF-1). IGF-1 is a primer that cannot stimulate platelet activation by itself, but in combination with physiological stimuli can potentiate platelet functional responses via a phosphoinositide 3-kinase–dependent mechanism. In this study, we explored the role of the phosphoinositide 3-kinase p110α isoform in IGF-1–mediated enhancement of platelet function. Approach and Results— Using a platelet-specific p110α knockout murine model, we demonstrate that genetic deletion, similar to pharmacological inactivation of p110α, did not affect proteinase-activated receptor 4 signaling to Akt/protein kinase B but significantly reduced IGF-1–mediated Akt phosphorylation. The p110β inhibitor TGX-221 abolished IGF-1–induced Akt phosphorylation in p110α-deficient platelets, demonstrating that both p110α and p110β contribute to IGF-1–mediated Akt phosphorylation. Genetic deletion of p110α had no effect on IGF-1–mediated increases in thrombus formation on collagen and enhancement of proteinase-activated receptor 4–mediated integrin activation and α-granule secretion. In contrast, pharmacological inhibition of p110α blocked IGF-1–mediated potentiation of integrin activation and α-granule secretion. Functional enhancement by IGF-1 in p110α knockout samples was lost after TGX-221 treatment, suggesting that p110β drives priming in the absence of the p110α isoform. Conclusions— Together, these results demonstrate that both p110α and p110β are involved in Akt signaling by IGF-1, but that it is the p110α isoform that is responsible for IGF-1–mediated potentiation of platelet function.