Overexpression of nitric oxide synthase by the endothelium attenuates bleomycin-induced lung fibrosis and impairs MMP-9/TIMP-1 balance

Background: Nitric oxide (NO) produced by endothelial NO synthase (eNOS) is thought to effect an anti-inflammatory response, but its mechanism is still unknown. Methods: eNOS transgenic (eNOS-TG) mice and their littermate controls (C57/BL6) were used to clarify the role of NO derived from eNOS. Bleomycin hydrochloride (1 U/body/day) or PBS was injected intraperitoneally. Results: Subpleural fibrotic changes and hydroxyproline content in the eNOS-TG mice were significantly reduced compared with those of the wild-type (WT) mice by day 56. Administration of N-omega-nitro-L-arginine methyl ester, a potent inhibitor of NO synthase, worsened the fibrotic response in bleomycin-treated eNOS-TG mice. Gelatinolytic activity in lung homogenates, corresponding to metalloproteinase-9 (MMP-9), was significantly increased in bleomycin-injured WT mice on day 14. In contrast, the level of tissue inhibitor of metalloproteinases-1 (TIMP-1), an endogenous MMP-9 inhibitor, was increased in the bleomycin-treated eNOS-TG mice compared with WT. Immunohistochemical analysis demonstrated that MMP-9 and TIMP-1 were strongly expressed in inflammatory cells, including subpleural fibrotic lesions. Conclusion: These data suggested that eNOS overexpression attenuates bleomycin-induced lung injury by ameliorating the MMP-9/TIMP-1 balance.