Plasminogen inhibits TNFα-induced apoptosis in monocytes

Monocytes are major mediators of inflammation, and apoptosis provides a mechanism for regulating the inflammatory response by eliminating activated macrophages. Furthermore, as a consequence of apoptosis, plasminogen binding is markedly increased on monocytoid cells. Therefore, we investigated the ability of plasminogen to modulate monocyte apoptosis. Apoptosis of monocytoid cells (human monocytes and U937 cells) was induced with either TNF alpha or cycloheximide. When apoptosis was induced in the presence of increasing concentrations of plasminogen, apoptosis was inhibited in a dose-dependent manner with full inhibition achieved at 2 mu M plasminogen. Plasminogen treatment also markedly reduced internucleosomal DNA fragmentation and reduced levels of active caspase 3, caspase 8, and caspase 9 induced by TNF alpha or by cycloheximide. We examined the requirement for plasmin proteolytic activity in the cytoprotective function of plasminogen. A plasminogen active site mutant, [D(646)E]Plg, failed to recapitulate the cytoprotective effect of wild-type plasminogen. Further-more, antibodies against PAR1 blocked the antiapoptotic effect of plasminogen. Our results suggest that plasminogen inhibits monocyte apoptosis. The cytoprotective effect of plasminogen requires plasmin proteolytic activity and requires PAR1. Because apoptosis of monocytes plays a key role in inflammation and atherosclerosis, these results provide insight into a novel role of plasminogen in these processes.