Macrophage Metalloelastase Mediates Acute Cigarette Smoke–induced Inflammation via Tumor Necrosis Factor-α Release

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Abstract
The cells and proteases that mediate cigarette smoke–induced emphysema are controversial, with evidence favoring either neutrophils and neutrophil-derived serine proteases or macrophages and macrophage-derived metalloproteases as the important effectors. We recently reported that both macrophage metalloelastase (MMP-12) and neutrophils are required for acute cigarette smoke-induced connective tissue breakdown, the precursor of emphysema. Here we show how these disparate observations can be linked. Both wild-type (MMP-12 +/+) mice and mice lacking MMP-12 (MMP-12 −/−) demonstrated rapid increases in whole-lung nuclear factor-κB activation and gene expression of proinflammatory cytokines after cigarette smoke exposure, indicating that a lack of MMP-12 does not produce a global failure to upregulate inflammatory mediators. However, only MMP-12 +/+ mice demonstrated increased whole-lung tumor necrosis factor-α (TNF-α) protein or release of TNF-α from cultured alveolar macrophages exposed to smoke in vitro. Levels of whole-lung E-selectin, an endothelial activation marker, were increased in only MMP-12 +/+ mice. These findings suggest that, acutely, MMP-12 mediates smoke-induced inflammation by releasing TNF-α from macrophages, with subsequent endothelial activation, neutrophil influx, and proteolytic matrix breakdown caused by neutrophil-derived proteases. TNF-α release may be a general mechanism whereby metalloproteases drive cigarette smoke–induced inflammation.