Endogenously produced urokinase-type plasminogen activator is a major determinant of the basal level of activated ERK/MAP kinase and prevents apoptosis in MDA-MB-231 breast cancer cells

Urokinase-type plasminogen activator (uPA) binds to the uPA receptor (uPAR) and activates the Ras-extracellular signal-regulated kinase (ERK) signalin g pathway in many different cell types. In this study, we demonstrated that endogenously produced uPA functions as a major determinant of the basal le vel of activated ERK in MDA-MB-231 breast cancer cells. When these cells we re cultured in the presence of antibodies that block the binding of uPA to uPAR, the level of phosphorylated ERK decreased substantially. Furthermore, conditioned medium from MDA-MB-231 cells activated ERK in MCF-7 cells and this response was blocked by uPA-specific antibody. The mitogen-activated p rotein kinase kinase inhibitor, PD098059, decreased expression of uPA and u PAR in MDA-MB-231 cells. Thus, uPA and the uPAR-ERK signaling pathway form a positive feedback loop in these cells. When this feedback loop was disrup ted with uPA- or uPAR-specific antibody, uPA mRNA-specific antisense oligod eoxynucleotides or PD098059, cell growth was inhibited and apoptosis was pr omoted, as determined by the increase in cytoplasmic nucleosomes and caspas e-3 activity. Treating the cells simultaneously with PD098059 and uPA- or u PAR-specific antibody did not further promote apoptosis, compared with eith er reagent added separately, supporting the hypothesis that uPAR and ERK ar e components of the same cell growth/survival-regulatory pathway. The abili ty of uPA to signal through uPAR, maintain an elevated basal level of activ ated ERK and inhibit apoptosis represents a novel mechanism whereby the uPA -uPAR system may affect breast cancer progression in vivo.