Adhesion of U937 cells on vitronectin in response to divalent cations and uPA. (A) Differentiated U937 cells were allowed to adhere on VN in response to Zn²⁺ (•), Mn²⁺ (▪), Co²⁺ (□), Ni²⁺(○), or Cu²⁺ (▴). The data are expressed as percent of control as represented by adhesion of U937 cells in the absence of any stimulus (mean ± SEM, n = 3). Similar results were obtained in three separate experiments. (B) The adhesion of U937 on VN was performed without stimulus (−) or in the presence of 50 μmol/L Zn²⁺ or 50 nmol/L uPA respectively. In parallel wells, no other additives (▩) or 20 μg/mL of MoAb R3 against uPAR (▨), 10 μg/mL of MoAb 13H1 against VN (▥), or 100 nmol/L PAI-1 () were present. A control antibody (20 μg/mL) did not affect adhesion (not shown). Data are the mean ± SEM (n = 3) of a typical experiment. Similar results were obtained in three independent experiments.