Urokinase receptor primes cells to proliferate in response to epidermal growth factor

Epidermal growth factor (EGF) express es mitogenic activity by a mechanism that requires the EGF receptor (EGFR). We report that murine embryonic. broblasts ( MEFs) prolifer ate in response to EGF only when these cells express the urokinase receptor (uPAR). EGFR expression was equivalent in uPAR-/- and uPAR+/+ MEFs. In response to EGF, these cells demonstrated equivalent overall EGFR tyrosine phosphorylation and ERK/MAP kinase activation; however, phosphorylation of Tyr-845 in the EGFR, which has been implicated in cell growth, was substantially decreased in uPAR-/-MEFs. STAT5b activation also was decreased. As Tyr- 845 is a c- Src target, we overexpressed c- Src in uPAR-/- MEFs and rescued EGF mitogenic activity. Rescue also was achieved by expressing murine but not human uPAR, suggesting a role for autocrine uPAR cell- signaling. In MDA-MB 231 breast cancer cells, EGF mitogenic activity was blocked by uPAR gene silencing, with antibodies that block uPA-binding to uPAR, and with a synthetic peptide that disrupts uPAR-dependent cell signaling. Again, c-Src overexpression rescued the mitogenic activity of EGF. We conclude that uPAR-dependent cell-signaling may prime cells to proliferate in response to EGF by promoting Tyr-845 phosphorylation and STAT5b activation. The importance of this pathway depends on the c-Src level in the cell.