RANKL–RANK signaling promotes cigarette smoke–induced emphysema with MMP-9 upregulation in alveolar macrophages

BackgroundChronic obstructive pulmonary disease (COPD) is a leading global health problem, with pulmonary emphysema as one of its hallmark pathological features. Matrix metalloproteinase-9 (MMP-9), predominantly derived from alveolar macrophages, has been implicated in extracellular matrix degradation. However, the upstream regulatory signals responsible for MMP-9 induction in cigarette smoke (CS)–related COPD remain incompletely understood. We investigated whether receptor activator of nuclear