Articular cartilage chondrocytes are reported to synthesize many MMPs, namely, MMPs 1, 2, 3, 7, 8, 13, and 14 [44
], as well as a variety of other serine and cysteine proteinases [47
]. Most of these enzyme activities are increased in OA, whether by the mechanism of increased synthesis, increased activation of proenzymes by other MMPs or plasmin, or decreased inhibitor activity. In nearly all OA cells, MMP-3 (stromelysin), MMP-8 (collagenase-2), and MMP-13 (collagenase-3) were elevated. Many of these MMPs are stimulated by exposure of the cells to inflammatory cytokines [48
]. To agonize the effects of MMPs, expression levels of inhibitors such as tissue inhibitor of metalloproteinases (TIMP)-1 are reduced in OA and rheumatoid arthritis [49
], although the ratio of total MMPs to total inhibitors is not really known. In 92% of OA cases in one study [51
], MMP-7 (matrilysin), an enzyme with a wide range of susceptible proteins, was localized in chondrocytes, mainly those in the superficial and transitional zones. Approximately 30% of the total chondrocytes were immunostained in the positive OA cartilage samples. The results of mRNA analysis were consistent with the localization of protein. The noncollagenase enzymes could act to disrupt the matrix, rendering it weaker and more susceptible to hydration.
The degradation of type II collagen has been studied extensively by the team of Dr Robin Poole, who have shown that MMP-13 is the enzyme responsible for most of the collagen degradation [52
]. In addition, MMP-3 can cleave in the nonhelical telopeptide of type II and type IX collagens [53
], leading to the disruption of a collagen crosslink. This cleavage could result in a disrupted fibril structure and, consequently, disrupted fibril function. Indeed, Bonassar and associates have shown that treatment of cartilage plugs in vitro
with stromelysin causes marked swelling of the tissue, whereas treatment with trypsin does not [54
]. We have recently shown that the type II collagen telopeptide can also be cleaved by MMPs 7, 9, 13, and 14; this finding indicates the presence in OA of a host of enzyme candidates capable of disrupting the collagen network [55
]. Disruption of this network will eventually lead to destabilization of the joint. Evidence for disrupted collagen structure in the pathophysiology of OA also comes from genetic studies showing that mutations in type II collagen lead to an unstable collagen network and eventually to premature OA [56
Two new families of degradative enzymes have been detected in articular cartilage. Protein and mRNA for ADAM-10 (A Disintegrin-like And Metalloproteinase-like domain) was found in the most fibrillated areas of OA cartilage, especially in the cell clusters. Probably more importantly, two new enzymes, called aggrecanase 1 and 2, have been isolated that are ADAMs enzymes with an additional thrombospondin domain (ADAM-TS) capable of binding to chondroitin sulfate. The MMPs and aggrecanases cleave aggrecan at distinct sites in the core protein [58
Cysteine peptidases, primarily cathepsins, have recently been found in OA cartilage and subchondral bone. Cathepsins L and K were localized subchondrally in association with cathepsin B, in osteophytes, in zones undergoing bone remodeling and at sites of inflammation, whereas cathepsin B was present and active in cartilage, particularly at sites where matrix neosynthesis takes place [59
]. Inhibition of these cysteine enzymes had an effect on cartilage breakdown, indicating that they may play a role in the cascade of events leading to matrix degradation.