A novel mechanism of plasmin‐induced mitogenesis in fibroblasts

Summary.  The plasminogen activator/plasmin system is believed to play an important role in diverse pathophysiological processes, including wound healing, vascular remodeling and pulmonary fibrosis. Our recent studies show that plasmin upregulates the expression of Cyr61, a growth factor‐like gene that has been implicated in cell proliferation and migration. In the present study, we investigated whether plasmin promotes fibroblast proliferation and, if so, determine the role of Cyr61 in the plasmin‐induced response. Human lung fibroblasts were exposed to varying concentrations of plasmin and DNA synthesis was monitored by measuring the incorporation of 3 H‐thymidine into DNA. Plasmin increased DNA synthesis of fibroblasts in a dose‐dependent manner. Protease‐activated receptor‐1 (PAR‐1)‐specific antibodies, but not PAR‐2‐specific antibodies, reduced the plasmin‐induced DNA synthesis. Consistent with this, plasmin had no substantial effect on the DNA synthesis in PAR‐1‐deficient mouse fibroblasts. Plasmin activated both p38 and p44/42 MAPKs and specific inhibitors of these pathways inhibited the plasmin‐induced DNA synthesis. Plasmin‐induced increase in the DNA synthesis was completely abrogated by anti‐Cyr61 antibodies. Interestingly, thrombin, which is a potent inducer of Cyr61, had only a minimal effect on fibroblast proliferation. Additional experiments suggested that plasmin cleaved cell/extracellular matrix‐associated Cyr61 and the conditioned media from plasmin‐treated cells could support the cell proliferation. Overall, these data suggest that plasmin promotes fibroblast proliferation by a novel pathway, involving two independent steps. In the first step, plasmin induces Cyr61 expression via activation of PAR‐1, and in the second step, plasmin releases Cyr61 deposited in the extracellular matrix, thus making it accessible to act on cells.