Crosstalk between the urokinase-type plasminogen activator receptor and EGF receptor variant III supports survival and growth of glioblastoma cells

摘要

A truncated and constitutively active form of the EGF receptor, variant III (EGFRvlll), is a major determinant of tumor growth and progression in glioblastoma multiforme (GBM). Extensive bidirectional crosstalk occurs in the cell-signaling pathways downstream of the EGFR and the urokinase-type plasminogen activator receptor (uPAR); however, crosstalk between EGFRvlll and uPAR has not been examined. Here, we show that uPAR does not regulate ERK activation in EGFRvlll-expressing GBM cells; however, in GBM cells isolated from four separate xenografts in which EGFRvlll expression was down-regulated in vivo, uPAR assumed a major role in sustaining ERK activation. Phosphorylation of Tyr-845 in the EGFR, which is mediated by Src family kinases, depended on uPAR in EGFRvlll-expressing GBM cells. Activation of the mitogenic and prosurvival transcription factor, STAT5b, downstream of EGFRvlll, also required uPAR. The EGFR-selective tyrosine kinase inhibitors, erlotinib and gefitinib, blocked not only EGFRvlll signaling to ERK but also uPAR-dependent STAT5b activation. uPAR gene silencing in EGFRvlll-expressing GBM cells and in cells from tumors that escaped dependency on EGFRvlll decreased cell survival and proliferation. Xenografts of EGFRvlll-expressing cancer cell lines and a human GBM, which was propagated as a xenograft, were robustly immunopositive for uPAR and phospho-Tyr-845 by immunohisto-chemistry. A human GBM in which the EGFR gene was amplified without truncation was immunonegative for both uPAR and phospho-Tyr-845. These studies identify distinct cell-signaling activities for uPAR in GBM cells that express EGFRvlll and in cells released from dormancy when EGFRvlll is neutralized. uPAR and its crosstalk pathways with EGFRvlll emerge as logical targets for therapeutics development in GBM.