The arthritides are debilitating diseases in which degradation of articular cartilage is a major feature. The cartilage extracellular matrix (ECM) is composed primarily of type II collagen and proteoglycan (aggrecan), structural components that provide tensile strength and resistance to compressive forces. The matrix metalloproteinases (MMPs) are a key family of potent enzymes in tissue degradomics (1), including the cartilage degradome, since collectively they can degrade all the ECM components and have been strongly implicated in arthritic disease (2, 3). Aggrecanolysis is thought to be predominantly mediated by ADAMTS proteinases (4), although this ECM component can be replaced relatively rapidly once the stimulus, such as interleukin-1 (IL-1), has been removed (5). In contrast, collagenolysis is much less readily achieved, but when degradation does occur, the structural integrity of the tissue is irreversibly lost (6).
The collagenolytic MMPs (MMPs 1, 8, and 13) have all been implicated in pathologic collagenolysis. These proteinases require activation of their latent pro forms via the proteolytic removal of a propeptide, which can be mediated by serine proteinases (7–9) or by MMPs. For example, MMP-3 (also called stromelysin 1) activates the pro forms of MMPs 1, 7, 8, and 13 (10–14), and such activation represents a key step in cartilage collagenolysis (8, 9). MMP-3 expression is a marker of disease activity and joint damage progression in early rheumatoid arthritis (RA) (15) and may play a role in other degenerative arthritides, including osteoarthritis (OA) (16) and juvenile idiopathic arthritis (JIA) (17).
MMP-10 (stromelysin 2) has a structure and substrate specificity similar to that of MMP-3, also activates MMPs 1, 7, 8, and 9 pro forms (18), and its expression correlates with the invasive properties of RA fibroblast-like synoviocytes (19). However, there are relatively few data concerning the potential role of MMP-10 in the context of the arthritides. In the present study, we demonstrate that MMP-10 expression in human articular chondrocytes is induced by proinflammatory stimuli known to promote cartilage catabolism (20–24), that MMP-10 is detectable in synovial fluids and joint tissues of arthritis patients, and that MMP-10 potentiates cartilage collagenolysis via the activation of procollagenase(s).
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- MATERIALS AND METHODS
During previous gene array analyses of IL-1 plus OSM–stimulated chondrocytes, we observed a synergistic induction of MMP-10 (25). In the present study, we confirmed MMP-10 induction in primary human articular chondrocytes as well as synovial fibroblasts following stimuli known to promote cartilage collagenolysis (20, 22) and arthritis in vivo (21, 23). Relatively few data exist on MMP-10 expression in arthritis, and it was reported by Hembry and coworkers (32) to be absent in RA and OA synovial tissues, although in vitro, synovial fibroblasts do express MMP-10, which correlates with an invasive phenotype (19). Herein we confirm synovial MMP-10 expression in both an animal model of inflammatory arthritis and in synovial fluids and tissues from RA and OA patients. Like many MMPs, MMP-10 is induced by proinflammatory cytokines such as IL-1 and TNFα, and the increasing data that support a role for such mediators of inflammation in OA as well as RA (41, 42) explain the presence of MMP-10 in OA synovial tissues with evidence of synovitis. It is likely, therefore, that MMP-10 will be less abundant in noninflamed OA tissues (32).
Data on MMP-10 expression in cartilage are limited, although both normal and OA cartilage have low levels of mRNA expression (43). The present findings clearly demonstrate the ability of chondrocytes to also produce this proteinase following a proinflammatory stimulus. Thus, the synovial fluid levels found in 3 different arthropathies examined in the present study are probably a combined result of both synovium- and cartilage-derived production, although it is probable that synovial production will predominate during the early stages in any inflamed joint, since this is the major site of proinflammatory cytokine production. There is clear evidence in both RA and OA that chondrocytes contribute significantly to cartilage degradation (25, 39, 44), which may be partly due to the release of factors that are stimulatory for synoviocytes (44). The present study confirms that both RA and OA tissues express MMP-10, such that procollagenase activation is possible, and hence collagenolysis is likely to occur, both within the cartilage and at pannus–cartilage interfaces.
MMP-10 expression has also been reported in osteoblasts and osteophytes and in growth plate chondrocytes during development (45, 46); these observations provide further evidence of a role of MMP-10 in ECM remodeling events. Of significance is the induction of MMP-10 by 2 potent cytokine combinations already known to induce a highly degradative arthritis with both aggrecanolysis and collagenolysis (20–23). Moreover, inclusion of MMP-10 in an in vitro model of cartilage degradation significantly (up to 50%) exacerbated the extent of collagen release at a time point when collagenolysis is rarely observed (8, 20). This extent of collagen release is very similar to the findings of our previous study using equivalent amounts of MMP-3 (8), which indicated that these stromelysins have comparable potencies in terms of procollagenase activation. Indeed, MMP-10 has previously been shown to “superactivate” MMP-1 as does MMP-3 (38).
This “superactivation” results in a collagenase form that has ≥10-fold higher specific activity than that of procollagenases activated by APMA, trypsin, or plasmin (10, 38, 47), and we observed this phenomenon with MMP-10 for proMMP-1, proMMP-8, and proMMP-13. We therefore conclude that once active, MMP-10 could help to superactivate the procollagenase reservoir known to be present by day 7 of culture in this model (8, 20). Such superactivation may well have significant implications for tissue integrity once the endogenous pool of tissue inhibitors of metalloproteinases has been exceeded, resulting in rapid collagenolysis (8, 9, 20–22).
Furthermore, the present study is the first to confirm that MMP-10 activates proMMP-13, which is highly significant, since we have previously shown that this collagenase is a major collagenolytic MMP in this degradation assay (9), as well as MMP-1 (20, 21). While stimulation with IL-1 plus OSM only weakly induces MMP-8 expression in bovine chondrocytes (21), the ability of MMP-10 to activate this collagenase may still be pathophysiologically relevant, especially in reactive arthritis (48). Indeed, we have proposed that procollagenase activation is a key and rate-limiting step in cartilage collagenolysis (8, 9), and the present study clearly implicates MMP-10 as such an activator. Thus, the activation cascades for the procollagenases are numerous and complex and include the closely related MMP-3 (stromelysin 1) as well as MMP-14 (2) and possibly 2 different serine proteinase–dependent mechanisms (8, 9).
For either MMP-10 or MMP-3 to be a pathophysiologic activator of procollagenases, the pro forms of these proteinases would also require proteolytic activation. The serine proteinase plasmin is known to activate proMMP-3, and it may be that the proposed serine proteinase activation cascades for the procollagenases (8, 9) involve intermediates such as MMP-3 and/or MMP-10. However, the identity of specific serine proteinases involved in the proteolytic events that lead to cartilage collagenolysis will need to be elucidated. These observations could imply a level of redundancy in terms of procollagenase activation, and could help to explain in part the lack of efficacy of MMP inhibitors that have been used therapeutically to date (49).
It is becoming increasingly apparent that the degradome that drives cartilage destruction in arthritis is complex and involves multiple proteinases. For many of these proteinases, we are only just beginning to understand their substrate specificities and preferences in relation to the cartilage ECM. Thus, our data clearly demonstrate the presence of MMP-10 in arthritic tissues and its ability to activate procollagenases. This suggests that it will be prudent to include MMP-10 in the ever more diverse list of proteinases that are likely to be active players in the proteolytic disassembly of the cartilage ECM during degenerative joint diseases.