Urokinase‐type plasminogen activator and its inhibitor secreted by cultured human monocyte‐macrophages

Abstract

Human blood monocytes in culture differentiate to macrophagelike cells within 1 week. Coinciding with this morphological transition the cells started releasing increasing amounts of the serine proteinase plasminogen activator (PA; Mr 56,000) of the urokinase (u‐PA) type and the proteinase inhibitor alpha‐2‐macroglobulin (α₂M). Unlike the cell‐associated PA activity, which was also readily detected in fresh monocytes, the activity secreted into the serum‐free culture medium could be measured only after treatment of the samples with sodium dodecyl sulphate. Heat or acid treatment of the medium was not sufficient to reveal the PA activity, suggesting that, apart from α₂M, another PA‐inhibiting substance was present in the culture medium. The inhibitor (Mr 65,000) was found to be synthesized by macrophages and specifically inhibited u‐PA activity but not tissue‐type PA (t‐PA) or plasmin activity. Dexamethasone decreased the secretion of PA by differentiated macrophages without affecting the production of α₂M or the PA inhibitor. Dexamethasone also inhibited the morphological differentiation of the cells when added to the monocyte‐phase cells.