Urokinase and Inhibitor in Human Monocytes

Piasminogen activators (PA) l are thought to play a role in the proteolytic events associated with cell migration and tissue destruction and remodeling (1), in particular in the context of inflammation (2). Indeed, monocytes and macrophages from both human and murine origin produce PA (3, 4), and the secretion of these enzymes has been linked with the participation of mononu¢lear phagocytes in inflammatory reactions: peritoneal macrophages collected from inflammatory exudates synthesize and secrete such enzymes (reference 3, D. Belin et al., manuscript in preparation); furthermore, PA production is enhanced by inflammatory agents (5, 6) and suppressed by antiinflammatory glucocorticoids (2). However, little is known at the molecular level about the mechanisms by which PA activity is regulated. Recently progress has been made in the characterization of PA. Two different enzymes, urokinase (Uk) and tissue activator (TA), have been identified. Their primary structures have been elucidated at the protein or mRNA level (7-9), and it is now clear that they are the products of different genes. Aspects of their biosynthesis have been investigated. Uk has been shown to be secreted by certain human and murine cells as a one-chain inactive zymogen, with a Mr of 55,000 for the human protein (10-12). Active Mr 55,000 human Uk is generated through a single intramolecular proteolytic cleavage, and consists of two disulfidelinked polypeptide chains. Mr 33,000 human Uk is a proteolytic product of the Mr 55,000 enzyme with similar specific catalytic activity. The cellular production of PA inhibitors has also been described: in particular the secretion of proteasenexin (13) by human fibroblasts has been suggested to play a role in the regulation of extracellular PA activity (14); mouse peritoneal macrophages have been reported to be a source of such inhibitors (15, 16). In view of these developments, it was of interest to determine the type of PA produced in cultures of human monocytes-macrophages, as well as to investigate