TISSUE-PLASMINOGEN ACTIVATOR INCREASES CANINE ENDOTHELIAL-CELL PROLIFERATION RATE THROUGH A PLASMIN-INDEPENDENT, RECEPTOR-MEDIATED MECHANISM

Background: Tissue plasminogen activator (tPA) is elevated in cancer p atients and is thought to promote tumor angiogenesis by facilitating e ndothelial cell. migration through plasmin-mediated degradation of ext racellular matrix. Due to the presence of an epidermal growth factor ( EGF)-finger domain in the tPA A-chain and the existence of an endothel ial cell (EC) receptor that binds this domain, it was hypothesized tha t tPA has a direct receptor-mediated effect on EC proliferation, indep endent of plasmin. Methods and Results: Using cultured canine ECs, tPA (7.25 mu g/ml, similar to 107 nM) increased proliferation as much as 50 and 170% in the absence and presence of growth factors, respectivel y. tPA-induced increases in EC proliferation occurred independent of p lasmin generation, as the plasmin inhibitor, aprotinin (10 mu g/ml) di d not inhibit tPA-induced proliferation. However, tPA-induced prolifer ation was inhibited dose-dependently to a maximum of 78% using a monoc lonal antibody against the tPA EGF-finger domain. This antibody, known to inhibit tPA binding to its receptor, did not inhibit tPA-induced p lasmin generation. To investigate the role of potential signal transdu ction pathways, ECs were exposed to lavendustin A, a tyrosine kinase i nhibitor, at 33.5 mu M (IC50 for basic fibroblast growth factor). Lave ndustin A did not inhibit tPA-induced EC proliferation. However, Rp-cA MP, an inhibitor of cAMP-dependent kinases, specifically inhibited tPA -induced EC proliferation in a dose-dependent manner (IC50 = 50.5 mu m ). Pertussis toxin at maximal concentrations for this system (0.5 ng/m l) did not inhibit tPA-induced EC proliferation. Conclusion: These res ults lend support to the hypothesis that tPA may have a direct recepto r-mediated effect on EC proliferation and that this effect occurs inde pendent of plasmin and may be dependent upon protein kinase A activity . (C) 1996 Academic Press, Inc.