Objective. Increasing evidence implicates serine proteinases in pathologic tissue turnover. The aim of this study was to assess the role of the transmembrane serine proteinase matriptase in cartilage destruction in osteoarthritis (OA).Methods. Serine proteinase gene expression in femoral head cartilage obtained from either patients with hip OA or patients with fracture to the neck of the femur (NOF) was assessed using a low-density array. The effect of matriptase on collagen breakdown was determined in cartilage degradation models, while the effect on matrix metalloproteinase (MMP) expression was analyzed by real-time polymerase chain reaction. ProMMP processing was determined using sodium dodecyl sulfate-polyacrylamide gel electrophoresis/Nterminal sequencing, while its ability to activate proteinase-activated receptor 2 (PAR-2) was determined using a synovial perfusion assay in mice.Results. Matriptase gene expression was significantly elevated in OA cartilage compared with NOF cartilage, and matriptase was immunolocalized to OA chondrocytes. We showed that matriptase activated proMMP-1 and processed proMMP-3 to its fully active form. Exogenous matriptase significantly enhanced cytokine-stimulated cartilage collagenolysis, while matriptase alone caused significant collagenolysis from OA cartilage, which was metalloproteinase-dependent. Matriptase also induced MMP-1, MMP-3, and MMP-13 gene expression. Synovial perfusion data confirmed that matriptase activates PAR-2, and we demonstrated that matriptase-dependent enhancement of collagenolysis from OA cartilage is blocked by PAR-2 inhibition.Conclusion. Elevated matriptase expression in OA and the ability of matriptase to activate selective proMMPs as well as induce collagenase expression make this serine proteinase a key initiator and inducer of cartilage destruction in OA. We propose that the indirect effects of matriptase are mediated by PAR-2, and a more detailed understanding of these mechanisms may highlight important new therapeutic targets for OA treatment.Metalloproteinases, especially matrix metalloproteinases (MMPs), are considered to be the most important class of proteinase in terms of cartilage degradation, because collectively they can degrade all components of this complex extracellular matrix (ECM) (1). Indeed, type II collagen is a major structural component of this ECM, and collagenolysis is an essentially irreversible step (2), making such proteolysis a major therapeutic target (3). Collagens are remarkably resistant to prote-

Conclusion. We confirm that both synovial fibroblasts and articular chondrocytes express MMP-10 following treatment with procatabolic stimuli. Furthermore, the detectable levels of synovial fluid MMP-10 and the histologic detection of this proteinase in diseased joint tissues strongly implicate MMP-10 in the cartilage degradome during arthritis. The ability of MMP-10 to superactivate procollagenases that are relevant to cartilage degradation suggests that this activation represents an important mechanism by which this MMP contributes to tissue destruction in arthritis.

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Objective Bovine and human cartilages in explant culture respond to proinflammatory cytokines with the up‐regulation of procollagenases. In stimulated bovine nasal cartilage (BNC), >90% of collagen is released by day 14 of culture, but collagen release is rarely seen before day 7. The aim of this study was to investigate if activation of procollagenases is a rate‐limiting step in cartilage collagen breakdown. Methods BNC and human articular cartilage explants were cultured with interleukin‐1α (IL‐1α) and/or oncostatin M (OSM) with or without test reagents. Collagen levels were determined by assay of hydroxyproline. Collagenase activity was measured using the diffuse fibril assay. Results The addition of procollagenase activators, matrix metalloproteinase 3 (MMP‐3), and APMA to IL‐1α/OSM–stimulated BNC resulted in early release of collagen. The release with APMA was completely blocked by the addition of tissue inhibitor of metalloproteinases 1. This shows that procollagenases are present early in the culture period, but cartilage collagen breakdown does not happen until activation occurs. The addition of plasminogen to IL‐1α/OSM–stimulated cartilage produced early collagen release in bovine and a significant increase in human cartilage. Thus, plasminogen activators (PAs) are present and convert plasminogen to plasmin, a known activator of several MMPs, including collagenases. Addition of α1‐proteinase inhibitor or a urokinase‐type PA inhibitor, 7‐amino‐4‐chloro‐3‐(3‐isothiureidopropoxy) isocoumarin, partially blocked the breakdown of collagen from IL‐1α/OSM–treated bovine cartilage. This suggests that serine proteinases are involved in the activation cascades of procollagenases that result in cartilage collagen breakdown. Conclusion The activation of procollagenases is a key control point in cartilage collagen breakdown, and serine proteinase pathways activate MMPs.

The matrix metalloproteinases (MMPs) comprise a family of enzymes that collectively can degrade all components of the extracellular matrix (ECM). MMPs play an important role in many physiological processes such as embryonic development and growth, tissue remodelling and repair. Overexpression and activation of MMPs contributes to many pathologies, including arthritis, cardiovascular disease, tumour progression and lung disease. Targeted mutagenesis has allowed investigators to examine the contribution of MMPs to these physiological and pathologic processes. In this manuscript, we will present an up-to date review of these studies. Rheumatoid arthritis (RA) and osteoarthritis (OA) are chronic diseases that result in cartilage degradation and loss of joint function. MMPs have been implicated in the collagen breakdown that contributes to joint destruction. Current available drugs to treat arthritis are predominantly directed towards the control of pain and/or the inflammation associated with joint synovitis but they do little to reduce joint destruction. Synthetic MMP inhibitors have been developed and in animal models of OA and/or RA, these agents have shown chondroprotective effects. However, results from clinical trials in RA have been equivocal, with some studies being terminated because of lack of efficacy or safety concerns. Increased understanding of the structure, regulation and function of individual MMPs may lead to more effective strategies. Approaches aimed at multiple steps of the pathogenesis of arthritis may be needed to break the chronic cycle of joint destruction. In the future, it will be important to have drugs that prevent the structural damage caused by bone and cartilage breakdown.

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Dysregulated proteolysis of the extracellular matrix of articular cartilage represents a unifying hallmark of the arthritides, and has been a target for therapeutic intervention for some time, although clinical efficacy has been elusive. Members of the 'A disintegrin and metalloprotease with thrombospondin motifs' and matrix metalloprotease families are considered to be collectively responsible for cartilage catabolism, such that inhibition of these activities is theoretically a highly attractive strategy for preventing further proteolytic damage. This review outlines the biology of these metalloproteases and what we have learnt from inhibition studies and transgenics, and highlights the important questions that this information raises for the future development of therapeutics directed towards metalloproteases for arthritis treatment.

Increasing evidence implicates serine proteinases in the proteolytic cascades leading to the pathological destruction of extracellular matrices such as cartilage in osteoarthritis (OA). We have previously demonstrated that the type II transmembrane serine proteinase (TTSP) matriptase acts as a novel initiator of cartilage destruction via the induction and activation of matrix metalloproteinases (MMPs). Hepsin is another TTSP expressed in OA cartilage such that we hypothesized this proteinase may also contribute to matrix turnover. Herein, we demonstrate that addition of hepsin to OA cartilage in explant culture induced significant collagen and aggrecan release and activated proMMP-1 and proMMP-3. Furthermore, hepsin directly cleaved the aggrecan core protein at a novel cleavage site within the interglobular domain. Hepsin expression correlated with synovitis as well as tumour necrosis factor α expression, and was induced in cartilage by a pro-inflammatory stimulus. However, a major difference compared to matriptase was that hepsin demonstrated markedly reduced capacity to activate proteinase-activated receptor-2. Overall, our data suggest that hepsin, like matriptase, induces potent destruction of the extracellular matrix whilst displaying distinct efficiencies for the cleavage of specific substrates.