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[摘要]：Integrin alpha(IIb)beta(3) signaling mediated by kinases and phosphatases participate in hemostasis and thrombosis, in part, by supporting stable platelet adhesion. Our previous studies indicate that the genetic manipulation of PP2Ac alpha (alpha isoform of the catalytic subunit of protein phosphatase 2A) negatively regulate the adhesion of human embryonal kidney 293 cells expressing alpha(IIb)beta(3) to fibrinogen. Here, we demonstrated that small interference RNA (siRNA) mediated knockdown of PP2Ac alpha in 293 alpha(IIb)beta(3) cells led to the dephosphorylation of Src Tyr-529, phosphorylation of Src Tyr-418 and an increased Src kinase activity. Conversely, overexpression of PP2Ac alpha decreased the basal Src activity. Pharmacological inhibition of PP2Ac in human platelets or PP2Ac alpha knockdown in primary murine megakaryocytes resulted in Src activation. PP2Ac alpha-depleted 293 alpha(IIb)beta(3) cells did not alter the serine (Ser) phosphorylation of Src but enhanced the Ser-50 phosphorylation of protein tyrosine phosphatase 1B (PTP-1B) with a concomitant increase in the PTP-1B activity. Src activation in the PP2Ac alpha-depleted 293 alpha(IIb)beta(3) cells was abolished by siRNA mediated knockdown of PTP-1B. Pharmacological inhibition of Src or knockdown of Src, PTP-1B blocked the enhanced activation of extracellular signal-regulated kinase (ERK1/2) and the increased adhesiveness of PP2Ac alpha-depleted 293 alpha(IIb)beta(3) cells to fibrinogen, respectively. Thus, inactivation of PP2Ac alpha promotes hyperphosphorylation of PTP-1B Ser-50, elevates PTP-1B activity, which dephosphorylates Src Tyr-529 to activate Src and its downstream ERK1/2 signaling pathways that regulate alpha(IIb)beta(3) adhesion. Moreover, these studies extend the notion that a cross-talk between Ser/Thr and Tyr phosphatases can fine-tune alpha(IIb)beta(3) outside-in signaling. |
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