Connective tissue remodeling is, currently, a poorly understood process that occurs during growth and development as well as in the chronic inflammatory diseases (the arthritides and periodontitis) and tumor metastasis. In the past decade a family of vertebrate enzymes called matrix metalloproteinases (MMPs) were found to be associated with these remodeling processes. The MMPs are a family of nine or more zinc-metalloproteinases secreted as zymogens and when activated can, collectively, degrade most or all the known extracellular matrix (ECM) proteins. Periodontitis is a unique model for ECM remodeling because degradation of the hard and soft tissues supporting the teeth results from a chronic bacterial infection. During the disease process, tooth-supporting collagen fibers close to the bacterial infection are degraded while the epithelium surrounding the tooth migrates into the space left by the dissolved collagen, ultimately resulting in loss of the tooth. The source of the enzymes responsible for this ECM degradation is not known. We have previously shown that live epithelial cells (keratinocytes) growing on type I collagen fibers can be stimulated to degrade the collagen substrate by the addition of bacterial culture fluid from Porphyromonas gingivalis, a microorganism highly associated with the incidence and severity of periodontitis in adults. The factor within the P. gingivalis culture medium responsible for this stimulating effect was identified as a thiol-proteinase and characterized as an aggregate of peptides all derived from internal hydrolysis of a 140 kDa precursor proteinase. The P. gingivalis proteinase demonstrated the ability to activate MMPs, including collagenase, and when applied to live keratinocytes in culture stimulated the induction of MMP mRNA levels resulting in highly localized collagen--substrate degradation by the keratinocytes. These results are some of the first to define the potential relationship between proteinase activity released by P. gingivalis and epithelial cells with regard to matrix remodeling. Given the close proximity that the bacteria, the epithelium, and the collagenous attachment apparatus share in periodontitis, these data suggest that activation and induction of MMPs in the epithelial cells by pathogenic bacteria such as P. gingivalis may promote the accelerated tissue destruction in periodontitis.
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