Summary
Little is known about the signaling cascades that eventually regulate the activity
of the endothelial nitric oxide synthase (eNOS) in platelets. Here, we investigated
the effects of insulin on the phosphorylation and activation of eNOS in washed human
platelets and in endothelial cells.
Insulin activated the protein kinase Akt in cultured endothelial cells and increased
the phosphorylation of eNOS on Ser1177but failed to increase endothelial cyclic GMP levels or to elicit the relaxation of
endothelium-intact porcine coronary arteries. In platelets, insulin also elicited
the activation of Akt as well as the phosphorylation of eNOS and initiated NO production
which was associated with increased cyclic GMP levels and the inhibition of thrombin-induced
aggregation. The insulin-induced inhibition of aggregation was accompanied by a decreased
Ca2+response to thrombin and was also prevented by Nωnitro-L- arginine. In platelets, but not in endothelial cells, insulin induced the
activation of the AMP-activated protein kinase (AMPK), a metabolic stress-sensing
kinase which was sensitive to the phosphatidylinositol 3-kinase (PI3-K) inhibitor
wortmannin and the AMPK inhibitor iodotubercidin. Moreover, the insulin-mediated inhibition
of thrombin-induced aggregation was prevented by iodotubercidin. Insulin-independent
activation of the AMPK using 5-aminoimidazole-4-carboxamide ribonucleoside, increased
platelet eNOS phosphorylation, increased cyclic GMP levels and attenuated platelet
aggregation.
These results highlight the differences in the signal transduction cascade activated
by insulin in endothelial cells and platelets, and demonstrate that insulin stimulates
the formation of NO in human platelets, in the absence of an increase in Ca2+, by activating PI3-K and AMPK which phosphorylates eNOS on Ser1177.
Keywords
Aggregation - 5-aminoimidazole-4-carboxamide ribonucleoside - iodotubercidin - endothelium
- calcium