Isoform-selective upregulation of mast cell chymase in the development of skin fibrosis in scleroderma model mice

The involvement of connective-tissue-type mast cells and chymase, a proteas e unique to their secretory granules, has been implicated in fibrotic disea ses. To elucidate the role of chymase in fibroproliferative inflammation, i n this study we examined the enzymatic activity and mRNA expression of chym ase in the sclerotic skin of tight-skin mice; syngeneic Pallid mice served as the control. Dorsal skin specimens from mice aged 5, 10, and 20 wk were evaluated by morphometric and biochemical analyses. At ages 10 and 20 wk, t he hydroxyproline concentration in tight-skin dermis was higher than that i n Pallid, At any age, the subcutaneous fibrous layer was thicker in tight-s kin than in Pallid. In accordance with these fibrous changes, both connecti ve-tissue-type mast cell counts and chymase activity were higher in tight-s kin skin than in Pallid skin up to 20 wk of age. Age-matched (10-wk-old) ti ght-skin and Pallid were quantified for their mRNA of connective-tissue-typ e mast-cell-specific chymase, mouse mast cell protease-4, by the competitiv e reverse transcriptase polymerase chain reaction technique, which revealed its higher level in tight-skin than Pallid, In contrast, the mRNA level of mouse mast cell protease-5, the chymase isoform of undifferentiated mast c ells, in tight-skin skin was only a tenth that of mouse mast cell protease- 4 and no different from the mouse mast cell protease-5 mRNA level of Pallid mice. An in situ hybridization study confirmed the higher expression of mo use mast cell protease-4 by connective-tissue-type mast cells in tight-skin skin than Pallid skin. These results strongly support the contention that the connective-tissue-type mast cell chymase plays a crucial role in fibrop roliferative remodeling of the skin.