Characterization of proteolytic agents involved in the degradation of human articular cartilage proteoglycan during aging and arthritis

Some of the age-related changes in the structure of human proteoglycan aggregate, and the depletion of proteoglycan in arthritic cartilage have been attributed to the action of proteolytic agents. However, the identity of these agents is still uncertain. In this study, the cartilage matrix protein, link protein, has been used as an in situ probe of endogenous proteolysis, due to its relative resistance to proteolysis and the accumulation of its proteolytically-modified form with age in the matrix. Stromelysin, a metalloproteinase secreted in a latent form by human articular cartilage in organ culture, was shown to play a major role in the matrix degradation of neonatal cartilage. In adult cartilage, additional proteolytic agents, besides stromelysin, are involved. The data are compatible with the action of cathepsins B and G, and probably cathepsin L and hydroxyl radicals. The same multiple proteolytic agents appear to have acted on the cartilage remaining in both osteoarthritic and rheumatoid arthritic joints. To further investigate the role of the stromelysin in the degradation of arthritic cartilage, relative steady-state mRNA levels of prostromelysin in normal and arthritic cartilage were also examined. Northern blots indicated that abundant amounts of prostromelysin mRNA were present in normal adult cartilage, whereas relatively lower levels were found in arthritic and neonatal cartilage. Relative to prostromelysin mRNA, little procollagenase and tissue inhibitor of metalloproteinases (TIMP) mRNA were expressed in vivo by chondrocytes in human articular cartilage. In situ hybridization revealed that the metalloproteinase message was being expressed predominantly in the superficial chondrocytes of normal adult and osteoarthritic cartilage. If the observed low levels of latent metalloproteinase precursor are characteristic of all phases of the disease process, then it would appear that the level of activation rather than the amount of the metalloproteinase pr