Rapid stimulation of tyrosine phosphorylation signals downstream of G-protein-coupled receptors for thromboxane A 2} in human platelets
Résumé
Signals ensuing from trimeric G protein-coupled receptors synergize to induce platelet activation. At low doses, the thromboxane A2 analogue U46619 does not activate the {alpha} IIb}{beta} 3} integrin nor trigger platelet aggregation, but it induces shape changes. We addressed whether low U46619 doses trigger tyrosine phosphorylation signals independently of {alpha} IIb}{beta} 3} integrin activation and ADP secretion, and synergize with epinephrine to induce aggregation in aspirin-treated platelets. Low U46619 doses triggered tyrosine phosphorylation of different proteins including the focal adhesion kinase, Src and Syk independently of signals ensuing from the {alpha} IIb}{beta} 3} integrin or ADP receptors engaged by secreted ADP. The G12/13-mediated Rho/Rho kinase pathway was also increased by low U46619 doses. However, this pathway was not upstream of tyrosine phosphorylation because this occurred in the presence of the Rho kinase inhibitor Y-27632. Whereas low U46619 doses or epinephrine alone were unable to trigger platelet aggregation and {alpha} IIb}{beta} 3} activation, the combination of the two stimuli effectively induced these responses. PP2, a tyrosine kinase inhibitor, and Y-27632 inhibited platelet activation induced by low U46619 doses plus epinephrine and when used in combination totally suppressed this platelet response. However, the two inhibitors selectively blocked the tyrosine kinase or the Rho/Rho kinase pathway, respectively. These findings suggest that both tyrosine phosphorylation signals and the Rho/Rho kinase pathway are required to activate platelet aggregation via G 12/13} plus G z} signaling.
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