Plasmin induces smooth muscle cell proliferation

Background. Plasminogen activators are routinely used for thrombolysis. They lead to the generation of the protease, plasmin, which can induce smooth muscle cell proliferation and may thus promote further intimal hyperplasia in the thrombolysed vessel. The signaling pathways used by plasmin are not understood. Methods. Murine aortic smooth muscle cells were cultured in vitro. Assays of DNA synthesis, cell proliferation, MAPKK and MAPK activation were examined in response to plasmin alone and in the presence of plasmin inhibitors (ε-aminocaproic acid and aprotinin), pertussis toxin (Gαi inhibitor, PTx), GP-2A (Gαq inhibitor), wortmannin (PI3-K inhibitor, Wn), LY294002, (PI3-K inhibitor, LY), PD98059 (MEK inhibitor, PD), and SB203580 (p38 ^MAPK inhibitor, SB). Results. Plasmin produced concentration dependent smooth muscle cells DNA synthesis and proliferation and induced ERK1/2 and p38^MAPK phosphorylation. Inhibition of the proteolytic activity of plasmin prevented these responses. The ERK1/2 inhibitor, PD, but not the p38^MAPK inhibitors, SB, blocked cell proliferation. The activation of the MEK1/2 and ERK1/2 pathway was both Gαi dependent (PTx-sensitive) and Gαq dependent (GP-2A-sensitive). It was blocked by the PI3-K inhibitors, Wn and LY. PI3-K activation as measured by akt phosphorylation was dependent on Gαi, but was independent of Gαq. Conclusion. Plasmin induces smooth muscle cell proliferation. Plasmin induced ERK1/2 phosphorylation occurs through two pathways: one which is Gαi mediated/PI3-K dependent and a second which is Gαq mediated/PI3K independent. p38^MAPK appears not to be involved in plasmin-mediated cell proliferation. This pattern of activation is distinct from that seen with urokinase plasminogen activator.