uPAR regulation of fibronectin matrix formation in MG-63 cells. A – MG-63 cells were incubated with 125I-fibronectin for 6 hours in the presence of the indicated concentrations of P25 in DMEM containing 0.02% BSA. Cell layers were extracted with 1% DOC, and 125I-fibronectin that was incorporated into the detergent-insoluble matrix fibronectin was recovered by centrifugation and measured by gamma scintillation. Control wells contained no added peptide. B – MG-63 cells were preincubated for 2 h with 20 μg/ml of normal mouse IgG or clone 6S6 antibody, a function-blocking monoclonal antibody against the β1 integrin, and then treated with 50 μM P25 or S25 in the presence of 125I-fibronectin (FN) for 6 hours. Cell layers were extracted in 1% DOC, and 125I-Fn that was incorporated into the detergent-insoluble matrix was recovered by centrifugation and measured by gamma scintillation.