Abstract

Tryptase, the major secretory product of human mast cells, is emerging as a new target for therapeutic intervention in allergic airways disease. We have investigated the ability of tryptase and inhibitors of tryptase to modulate histamine release from human lung mast cells and have examined the potential contribution of proteinase-activated receptor 2 (PAR2). The tryptase inhibitor APC366 [N-(1-hydroxy-2-naphthoyl)-l-arginyl-l-prolinamide hydrochloride] was highly effective at inhibiting histamine release stimulated by anti-IgE antibody or calcium ionophore from enzymatically dispersed human lung cells. A concentration of APC366 as low as 10 μM was able to inhibit anti-IgE-dependent histamine release by some 50%. Addition of leupeptin or the tryptic substrate N-benzoyl-d,l-arginine-p-nitroanilide also inhibited IgE-dependent histamine release. Purified tryptase in the presence of heparin stimulated a small but significant release of histamine from lung cells, suggesting that tryptase may provide an amplification signal from activated cells that may be susceptible to proteinase inhibitors. Trypsin was also able to induce histamine release apparently by a catalytic mechanism. Moreover, pretreatment of cells with metabolic inhibitors or with pertussis toxin reduced responses, indicating a noncytoxic pertussis toxin-sensitive G proteinmediated signaling process. Addition to cells of the PAR2 agonists SLIGKV-NH₂ or tc-LIGRLO-NH₂ or appropriate control peptides were without effect on histamine release, and PAR2 was not detected by immunohistochemistry in tissue mast cells. The potent actions of tryptase inhibitors as mast cell-stabilizing agents could be of value in the treatment of allergic inflammation of the respiratory tract, possibly by targeting the non-PAR2-mediated actions of tryptase.