Stimulatory effect of pseudomonal elastase on collagen degradation by cultured keratocytes

PURPOSE. The pathobiology of corneal ulceration induced by Pseudomonas aeru ginosa was investigated by characterization of the pseudomonal pathogenic f actors responsible for degradation of the collagen matrix. METHODS. Three-dimensional gels of type I collagen containing (Or not) rabb it keratocytes were incubated in the presence of either culture supernatant of P. aeruginosa strain PAO1 or pseudomonal pathogenic factors (elastase, lipopolysaccharide, or exotoxin A), and the extent of collagen degradation was assessed after 24 hours by measurement of released hydroxyproline. Acti vation of matrix metalloproteinases (MMPs) produced by keratocytes was also examined by gelatin zymography and immunoblot analysis. RESULTS. In the absence of keratocytes, the PAO1-conditioned medium increas ed the extent of collagen degradation. The conditioned medium also promoted keratocyte-mediated collagen degradation. Of the pseudomonal pathogenic fa ctors examined, only elastase degraded collagen directly as well as stimula ted keratocyte-mediated collagen degradation. Culture supernatant of elasta se-deficient P. aeruginosa (lasR or lasB) mutants had no effect on collagen degradation in the absence or presence of keratocytes. Elastase also induc ed the conversion of the inactive precursors of MMP-1, -2, -3, and -9 produ ced by keratocytes to the active forms of the enzymes. CONCLUSIONS. These results suggest that pseudomonal elastase both degrades type I collagen directly and promotes collagen degradation mediated by kera tocytes, the latter effect being likely attributable, at least in part, to the activation of proMMPs.