Expression of cathepsin K in lung epithelial cells

Alveolar and bronchial epithelial cells have been shown to have regulatory functions in the maintenance of lung structure and function. Recent evidenc e supports the premise that these cells can synthesize a variety of extrace llular matrix components in vitro, suggesting an active participation in co nnective tissue remodeling. Their possible role in extracellular matrix deg radation, however, is less clear. This study addresses the question of whet her alveolar and bronchial epithelial cells express the highly collagenolyt ic and elastinolytic cysteine proteinase cathepsin K, which has recently be en newly described. We provide evidence that the epithelial cell lines A549 and BEAS-2B are capable of expressing cathepsin K messenger RNA. Furthermo re, we show that cathepsin K is expressed in normal bronchial epithelial ce lls. Western blot analyses of human lung-tissue lysates revealed specific i mmunoreactivity at molecular weights of 46 and 27 kD, corresponding to the procathepsin and the mature cathepsin K. Immunohistochemical analyses showe d a pronounced staining of bronchial epithelial cells and in single alveola r epithelial cells. Using a specific fluorogenic cytochemical assay, the in tracellular activity of the enzyme was localized. These findings demonstrat e that bronchial and alveolar epithelial cells are capable of expressing ca thepsin K, which could be of considerable importance for remodeling process es of the extracellular matrix in the lung.