Cooperation between uPAR signaling with β₁ integrin to ERK-MAPK activation and β₃ integrin–dependent Rac activation. (A) Cell surface expression of α_vβ₃ in uPAR- or vector-transfected HEK 293T cells. Top three left panels, siRNA-transfected cells; top right panel, 2 μM PD184352 or vehicle (DMSO) treatment. Red, IgG control; blue, α_vβ₃ vector transfected; green, uPAR–α_vβ₃ transfected. (bottom) β₃ integrin immunoblot of siRNA-transfected or PD184352-treated HEK 293T cells. (B) HEK 293T cells were transfected as in A and ERK1/2 activation was measured (mean + SEM; n = 6). *, P < 0.05; unpaired Student's t test. (C) Rac activation in HEK 293T cells transfected with uPAR or vector and treated with MEK inhibitors or vehicle (DMSO) for 24 h (mean + SEM; n = 6). *, P < 0.05; unpaired Student's t test. (D) Surface expression of α_vβ₃ on BE cells. Left and middle, siRNA transfections; right, MEK inhibition. Data are representative of three independent experiments. (E) ERK1/2 activation in cells transfected with siRNAs. Left, representative immunoblot (BE), total ERK1/2 (red), phospho-ERK1/2 (green); right, quantitation (mean + SEM; n = 4). Closed bars, BE cells; open bars, MDA-MB-231; shaded bars, SNB19.