MMP9 and chronic obstructive pulmonary disease: In development of COPD, MMP-9 was shown to be activated by various stimuli, such as CS, oxidative stress, and other airway proteases in the airway, and was involved in the destruction of normal alveolar structure, production of proinflammatory mediators, and inflammatory cell migration (Cane et al., 2016; Grzela et al., 2016).