Proteolysis in the lung.

Abstract

Pulmonary emphysema is a major component of the morbidity and mortality of chronic obstructive pulmonary disease (COPD), a condition that has become the fourth leading cause of death in the USA, and is becoming epidemic worldwide. Emphysema is defined as enlargement of peripheral airspaces of the lung including respiratory bronchioles, alveolar ducts and alveoli, accompanied by destruction of the walls of these structures. The pathogenesis of emphysema can be dissected into three interrelated events: 1) chronic exposure to cigarette smoke may lead to inflammatory cell recruitment within the terminal airspaces of the lung; 2) these inflammatory cells release elastolytic proteinases that damage the extracellular matrix of the lung; and 3) ineffective repair of elastin and perhaps other extracellular matrix components result in pulmonary emphysema. Inherited deficiency of α1‐antitrypsin (α1‐AT), the primary inhibitor of neutrophil elastase (NE), predisposes individuals to early onset emphysema, and intrapulmonary instillation of elastolytic enzymes in experimental animals causes emphysema. Together, these findings led to the elastase/antielastase hypothesis for the pathogenesis of emphysema, which was proposed ∼40 yrs ago and remains the prevailing concept today. While the capacity of NE to initiate emphysema in patients deficient in α1‐AT is clear, the proteinases involved in the pathogenesis of the common form of emphysema associated with cigarette smoking is more complicated. In addition to NE, neutrophil primary granules contain other elastolytic serine proteinases including, cathepsin G and proteinase 3, and secondary granules possess matrix metalloproteinases; MMP‐8, a collagenase and MMP‐9, a 92 kDa gelatinase. While, macrophages are prominent inflammatory cells in smokers' lungs, the capacity of macrophages to degrade elastin was controversial until cysteine proteinases including cathepsins S and L were identified (K the most potent elastase has not been identified in lung macrophages). The author's group identified several MMPs produced by alveolar and interstitial …