Role of MMP-2 in alveolar epithelial cell repair after bleomycin administration in rabbits

Matrix metalloproteinases (MMPs) have been implicated in the pathological p rocesses of interstitial lung diseases. However, underlying mechanisms, par ticularly for activity levels and distribution of activated MMP-2 in the di sease process, are yet to be elucidated. The present study investigated the immunolocalization of MMP-2, membrane type 1-matrix metalloproteinase (MT1 -MMP), tissue inhibitor of metalloproteinase (TIMP)-2, p53, and Ki-67 in a rabbit model of bleomycin-induced pulmonary fibrosis. Gelatin zymography an d in situ zymography were used to examine the activity and the localization of MMP-2. Furthermore, we performed Western blot and in situ hybridization for MT1-MMP, an activator for MMP-2. The total MMP-2 level estimated by ge latin zymography increased significantly at 3, 7, and 14 days after bleomyc in administration, compared with controls. In the immunohistochemical study , immunoreaction for MMP-2 was strongest in alveolar epithelial cells among the cell populations. Swollen and/or elongated type II alveolar epithelia[ cells showed strong immunoreactions for MMP-2, MT1-MMP, and TIMP-2. After bleomycin administration, immunoreaction for p53 was observed in bronchiola r and alveolar epithelia[ cells. The proportion of p53-positive cells was h igh in epithelial cells from 1 to 14 days as MMP-2 levels were increased, s uggesting that p53 may be responsible, at least in part, for the increase o f MMP-2. The ratio of activated MMP-2 to total MMP-2 estimated by gelatin z ymography increased significantly at 3, 7, 14, and 28 days after bleomycin treatment. In situ zymography revealed that type II alveolar epithelial cel ls degraded gelatin. An increased expression of MT1-MMP protein was observe d by Western blot following administration of bleomycin. In situ hybridizat ion demonstrated that type II alveolar epithelial cells gave intense signal for MT1-MMP mRNA. These results suggest that type II alveolar epithelia[ c ells express MT1-MMP and activate MMP-2 on their cell surfaces, which may l ead to the elongation and migration of alveolar epithelial cells in the rep air process of bleomycin-induced pulmonary fibrosis.