Mechanism by Which U937 Promonocytic Cells Inactivate Human Interferon-γ

Abstract

Mononuclear phagocytes produce proteinases that are thought to play a role in regulating the activity of cytokines. Activated macrophages secrete urokinase-type plasminogen activator (uPA), which mediates the formation of the serine proteinase plasmin from the ubiquitous zymogen plasminogen. We previously observed a correlation between in vitro plasminogen activation by the promonocytic cell line U937 and the apparent ability of these cells to inactivate recombinant interferon-gamma (rIFN-gamma) by proteolysis. The present study was designed to test the hypothesis that plasmin, generated in U937 cell cultures, is both necessary and sufficient to inactivate rIFN-gamma by limited proteolysis. The following observations are consistent with this hypothesis: (1) inactivation of rIFN-gamma was prevented by inhibitors of serine proteinases or an antibody that specifically immunodepleted plasmin activity; (2) purified plasmin inactivated rIFN-gamma as efficiently as U937 culture supernatants and was similarly sensitive to serine proteinase inhibitors; and (3) plasmin removed an 11 amino acid carboxyl-terminal peptide from rIFN-gamma, which included a region known to be required for bioactivity. Overall, these data indicate that plasminogen activation by U937 promonocytic cells leads to the proteolytic inactivation of IFN-gamma by a process that only requires the production of plasmin.

Monocyte-expressed urokinase inhibits vascular smooth muscle cell growth by activating Stat1

“…The short period of cell stimulation with exogenous IFN-␥ used in our experiments excludes uPA proteolysis reported in other systems. 37 Although more work is needed to explore the possibility in greater detail, these data suggest that IFN-␥ might use the Stat1-independent pathway in the physiologic situation in which Stat1 activation by uPA is predominant. Recent evidence suggests that non-Stat pathways are important in the generation of signals for interferons and may be primary mediators of their biologic consequences.…”

Conversion of Glu-Plasminogen to Lys-Plasminogen Is Necessary for Optimal Stimulation of Plasminogen Activation on the Endothelial Cell Surface

“…This result allowed us to distinguish between two potential mechanisms for stimulation of Pm formation when Glu-Pg is bound to the cell surface (1)(2)(3)(4)(5)(6)(7)(8)(9)(10). In the first mechanism, the direct activation of Glu-Pg is promoted on the cell surface.…”

“…1 the native circulating form of the zymogen, is bound to cell surfaces, plasmin (Pm) generation by plasminogen (Pg) activators is markedly stimulated compared with the reaction in solution (1)(2)(3)(4)(5)(6)(7)(8)(9)(10). This results in arming cell surfaces with the proteolytic activity of Pm.…”

Crystallization of soluble urokinase receptor (suPAR) in complex with urokinase amino-terminal fragment (1–143)

“…uPAR is anchored to the plasma membrane of cells by a glycosylphosphatidylinositol (GPI) moiety that is added concurrently with the post-translational removal of the COOH-terminal signal sequence at Gly283 (Ploug et al, 1991;Moller et al, 1992). uPAR localizes and promotes the proteolytic activities of uPA on the surface of cells (Ellis et al, 1989;Stephens et al, 1989;Duval-Jobe & Parmely, 1994). In addition to converting plasminogen to plasmin, uPA is capable of cleaving fibronectin, hepatocyte growth factor and other substrates that may be implicated in cell adhesion and migration.…”