Abstract
Protein tyrosine phosphorylation is a fundamental mechanism for diverse physiological processes, which is regulated by protein tyrosine kinases and protein tyrosine phosphatases (PTPs). In this study, we searched for protein substrates of PTP-MEG2 (also called PTPN9), a nonreceptor PTP, and investigated its function in endothelial cells (ECs). By using a PTP-MEG2 substrate-trapping DA mutant, we found that a couple of tyrosine-phosphorylated proteins were associated with the DA mutant but not wild-type PTP-MEG2 and that the association was enhanced by vascular endothelial growth factor (VEGF) in ECs. We further found that VEGF receptor 2 (VEGFR2) was coimmunopricipitated with the DA mutant but not wild-type PTP-MEG2. The VEGF-induced phosphorylation of VEGFR2 on Tyr1175, a critical autophosphorylation site for VEGFR2 signaling, was inhibited 70% by overexpression of wild-type PTP-MEG2 but was enhanced (2.2-fold) by the DA mutant of PTP-MEG2. We also found that PTP-MEG2 DA mutant preferentially associated with Janus kinase 1 (JAK1) but not with other JAK kinases (Tyk2 and JAK2) present in ECs and regulated JAK1 tyrosine phosphorylation. Lastly, the VEGF-induced signal transduction and the production of interleukin (IL)-6 were significantly enhanced by PTP-MEG2 knockdown in ECs, whereas the VEGF-induced IL-6 production was inhibited 50% by PTP-MEG2 overexpression. Thus we have indentified VEGFR2 as a PTP-MEG2 substrate, and our findings indicate that PTP-MEG2 is a negative regulator of VEGFR2 signaling and function in ECs.