Roles of Mast Cell Proteases in Airways

8 Nov

Roles of Mast Cell Proteases in AirwaysMast cells have been implicated in asthma and in other inflammatory diseases. Multiple physiologic and pathologic effects have been ascribed to mediators that have been shown to be released from these cells, including histamine, prostaglandin D2, leukotrienes, and platelet-activating factor. During degranulation, cells also release tryptic and chymo-tryptic proteases (“tryptases” and “chymases”) extracellularly. These proteases are very abundant (they constitute the major protein in mast cell granules and more than 20 percent of all protein in the cell), they are active outside of the cell, and they are distinct from other members of their class, the serine estero-proteases. Evidence has been presented suggesting that they could play a pathologic role in various inflammatory diseases, but no unique functions had been ascribed to these enzymes.

A major reason for the lack of complete structural information and the existence of unresolved controversies regarding the function of these enzymes is the difficulty in purifying significant amounts of the enzymes. Recently, Caughey and his associates have been able to isolate and purify to homogeneity both tryptase and chymase and to begin to examine their functions. Our hypothesis is that the mast cell proteases play a pathologic role in immunologic and inflammatory airway diseases. Our studies were facilitated by the fact that our colleagues, Dr. Lazarus et al, have succeeded in propagating dog mastocytoma cells in nude mice, so large numbers of pure, morphologically and functionally identical cells are available for study. These cells resemble normal mast cells of dogs and humans, and they release chymase and tryptase during degranulation. We disaggregated the cells and activated them with calcium ionophore A23187 using a two-stage reaction, so that ionophore would not be present in the supernatant. This causes the release of tryptase and chymase together with histamine. Below we will describe some of our studies demonstrating interactions of potential biologic importance with airway smooth muscle, neuropeptides, submucosal gland secretion, and epithelial cell glycocalyx. In these studies, we have examined the effects of supernatants of mastocytoma cells, as described above. Caughey and associates have been able to purify both tryptase and chymase, and this has enabled us to study the effects of the purified enzymes in various biologic systems.