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^ω‐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.