Human tryptase as a potent, cell-specific mitogen: role of signaling pathways in synergistic responses

Mast cells are hypothesized to participate in processes leading to tissue fibrosis in human lung and skin. To explore the possible involvement of mast cell mediators in fibrogenesis, the mitogenic activity of mast cell tryptase from human lung was examined in vitro. The results indicate that human tryptase is a potent inducer of DNA synthesis in fibroblasts from multiple sources, including human lung. As demonstrated by mitogenic responses in fibroblasts, but not in vascular smooth muscle cells, tryptase is a mitogen with target cell specificity. Additionally, specificity is demonstrated by the differences in mitogenic activity of tryptase in comparison with thrombin, a structurally related mitogenic proteinase. Examination of the mitogenic effects of tryptase in the presence of other mitogens reveals synergy with mitogens that act through receptors coupled to intrinsic tyrosine kinases (insulin, epidermal growth factor, and basic fibroblast growth factor) or to G proteins (thrombin and serotonin). In the latter case, studies in Chinese hamster lung fibroblasts using specific receptor agonists and antagonists or receptor-transfected cell lines reveal a requirement for the activation of a G protein (Gi) negatively coupled to adenylate cyclase to act synergistically with tryptase. These data establish that human tryptase is a potent and specific mitogen in vitro and suggest that mitogenic signals generated by tryptase can interact synergistically with signals generated by both tyrosine kinase-coupled and G protein-coupled growth factor receptors.