Mechanism of platelet inhibition by nitric oxide: In vivo phosphorylation of thromboxane receptor by cyclic GMP-dependent protein kinase

Nitric oxide (NO) is a potent vasodilator and inhibitor of platelet activation. NO stimulates production of cGMP and activates cGMP-dependent protein kinase (G kinase), which by an unknown mechanism leads to inhibition of Gα(q)-phospholipase C-inositol 1,4,5-triphosphate signaling and intracellular calcium mobilization for several important agonists, including thromboxane A(2) (TXA(2)). To explore the mechanism of platelet inhibition by NO, activation of platelet TXA(2) receptors in the presence of cGMP was studied. The nonhydrolyzable analog 8-bromo-cyclic GMP (8-Br-cGMP) potently inhibited activation of the TXA(2)-specific GTPase in platelet membranes in a concentration-dependent fashion, suggesting that G kinase catalyzes the phosphorylation of some proximal component of the receptor–G protein signaling pathway. Nanomolar concentrations of G kinase were found to catalyze the phosphorylation of platelet TXA(2) receptors in vitro, but not Gα(q) copurifying with the TXA(2) receptors in these experiments. Using immunoaffinity methods, in vivo phosphorylation of TXA(2) receptors by cyclic GMP was demonstrated from (32)P-labeled cells treated with 8-Br-cGMP. Peptide mapping studies of in vivo phosphorylated TXA(2) receptors demonstrated cGMP mediates phosphorylation of the carboxyl terminus of the TXA(2) receptor. G kinase also catalyzed the phosphorylation of peptides corresponding to the cytoplasmic tails of both α and β forms of the receptor but not control peptide or a peptide corresponding to the third intracytoplasmic loop of the TXA(2) receptor. These data identify TXA(2) receptors as cGMP-dependent protein kinase substrates and support a novel mechanism for the inhibition of cell function by NO in which activation of G kinase inhibits signaling by G protein-coupled receptors by catalyzing their phosphorylation.