Proteolytic degradation of articular cartilage is a hallmark of osteoarthritic (OA) joint destruction. Articular cartilage contains chondrocytes embedded in an avascular matrix composed primarily of type II collagen (CII) fibrils and the proteoglycan, aggrecan. Collagen fibrils provide tensile strength and serve as a lattice to anchor water-laden proteoglycans, which deform while absorbing impact loads (1). OA chondrocytes and the adjacent synovial tissue secrete the inflammatory cytokine interleukin-1␤ (IL-1␤), which stimulates chondrocytes to secrete neutral proteases that degrade both collagen and proteoglycans (2,3). The newly cloned aggrecanase (4) and multiple matrix metalloproteinases (MMP) are secreted (5) and cleave aggrecan, the predominant proteoglycan, while other MMP known as the collagenases are the only enzymes able to degrade CII. MMPs are a family of zinc-containing, calciumdependent neutral proteases that share a common domain structure. Collectively these enzymes can degrade the components of the extracellular matrix (6,7).

AP-1 = activating protein-1; bp = base pairs; CDDO = 2-cyano-3,12-dioxoolean-1,9,dien-28-oic acid; ERK = extracellular signal-regulated kinase; Ets = erythroblastosis twenty-six; GR = glucocorticoid receptor; IκB = inhibitor of κB; IKK = inhibitor of κB kinase; IL = interleukin; JNK = c-Jun Nterminal kinase; MAPK = mitogen-activated protein kinase; MAPKK = MAPK kinase; MAPKKK = MAPKK kinase; MMP = matrix metalloproteinase; NF-κB = nuclear factor κB; NIK = NF-κB-inducing kinase; OA = osteoarthritis; PPAR-γ = peroxisome proliferator-activated receptor-γ; RA = rheumatoid arthritis; Runx-2 = Runt domain factor-2; TNF-α = tumor necrosis factor-α.Available online http://arthritis-research.com/content/4/3/157 IntroductionThe matrix metalloproteinase (MMP) family members are the major enzymes that degrade the components of the extracellular matrix [1,2]. At the time of writing this article, 20 members of this family have been identified [3]. All are active at neutral pH, require Ca 2+ for activity and contain a central zinc atom as part of their structure. Most MMPs are secreted into the extracellular space in a latent proform, and require proteolytic cleavage for enzymatic activity. A few MMPs, however, are activated intracellularly by a furin-like mechanism and therefore, these enzymes are fully active when they reach the extracellular space [2].Most cells in the body express MMPs, even though some enzymes are often associated with a particular cell type. For example, the principle substrate of MMP-2 (also known as gelatinase A) and MMP-9 (also known as gelatinase B) is the type IV collagen in basement membrane and thus, these enzymes are usually expressed by endothelial cells, although other cells (e.g. stromal fibroblasts, macrophages, tumor cells) also express them [1,4]. AbstractMatrix metalloproteinase (MMP)-1, MMP-8 and MMP-13 are interstitial collagenases that degrade type II collagen in cartilage; this is a committed step in the progression of rheumatoid arthritis and osteoarthritis. Of these enzymes, the expression of MMP-1 and MMP-13 is substantially increased in response to IL-1 and tumor necrosis factor-α, and elevated levels of these collagenases are observed in arthritic tissues. Therefore, cytokine-mediated MMP-1 and MMP-13 gene regulation is an important issue in arthritis research. In this review, we discuss current models of MMP-1 and MMP-13 transcriptional regulation, with a focus on signaling intermediates and transcription factors that may be future targets for the development of new arthritis drugs.

Matrix metalloproteinase-1 (MMP-1) is one of three collagenases that can degrade the interstitial collagens, types I, II, and III at neutral pH. As these collagens are the most abundant proteins in the body, collagenase plays a critical role in modeling and remodeling the extracellular matrix. Therefore, it is not surprising that MMP-1 gene expression can be regulated at multiple points. Procollagenase can be activated by mechanisms that generate an active enzyme with differing specific activities, and the active enzyme can be inhibited by complexing with either the tissue inhibitor of metalloproteinases (TIMPs) or alpha 2 macroglobulin. The activator protein-1 (AP-1) site in the collagenase promoter plays a prominent role in the transcriptional control of the collagenase gene. It is essential for basal transcription, and contributes to induction by phorbol esters, although other sites in the proximal promoter are essential. In contrast, transactivation by cytokines such as Interleukin-1 depends on sequences in more distal regions of the promoter. Posttranscriptional mechanisms also regulate gene expression, and several cytokines and growth factors increase the stability of the collagenase transcript. Finally, glucocorticoid hormones repress transcription of the collagenase gene by the interaction of glucocorticoid receptors with the AP-1 proteins, Fos and Jun. Retinoids also suppress transcription by mechanisms that involve down-regulation of fos and jun mRNA, sequestration of Fos and Jun proteins, and the formation of complexes of retinoic acid receptors (RAR/RXR heterodimers) and AP-1 proteins on the DNA. These multiple points of regulation assure precise control of collagenolytic activity in a variety of physiologic and pathologic conditions.

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Matrix metalloproteinases (MMPs) degrade collagen and mediate tissue remodeling. The novel cytokine IL-17 is expressed during various inflammatory conditions and modulates MMP expression. We investigated the effect of IL-17 on MMP-1 expression in primary human cardiac fibroblasts (HCF) and delineated the signaling pathways involved. HCF were treated with recombinant human IL-17. MMP-1 expression was analyzed by Northern blotting, RTquantitative PCR, Western blotting, and ELISA; transcriptional induction and transcription factor binding by EMSA, ELISA, and reporter assay; and p38 MAPK and ERK1/2 activation by protein kinase assays and Western blotting. Signal transduction pathways were investigated using pharmacological inhibitors, small interfering RNA (siRNA), and adenoviral dominant-negative expression vectors. IL-17 stimulated MMP-1 gene transcription, net mRNA levels, protein, and promoter-reporter activity in HCF. This response was blocked by IL-17 receptor-Fc chimera and IL-17 receptor antibodies, but not by IL-6, TNF-␣, or IL-1␤ antibodies. IL-17-stimulated type I collagenase activity was inhibited by the MMP inhibitor GM-6001 and by siRNA-mediated MMP-1 knockdown. IL-17 stimulated activator protein-1 [AP-1 (c-Fos, c-Jun, and Fra-1)], NF-B (p50 and p65), and CCAAT enhancer-binding protein (C/EBP)-␤ DNA binding and reporter gene activities, effects attenuated by antisense oligonucleotides, siRNA-mediated knockdown, or expression of dominantnegative signaling proteins. Inhibition of AP-1, NF-B, or C/EBP activation attenuated IL-17-stimulated MMP-1 expression. IL-17 induced p38 MAPK and ERK1/2 activation, and inhibition by SB-203580 and PD-98059 blunted IL-17-mediated transcription factor activation and MMP-1 expression. Our data indicate that IL-17 induces MMP-1 in human cardiac fibroblasts directly via p38 MAPKand ERK-dependent AP-1, NF-B, and C/EBP-␤ activation and suggest that IL-17 may play a critical role in myocardial remodeling. cytokines; interleukins; matrix metalloproteinases; fibrosis EXTRACELLULAR MATRIX (ECM) turnover in the normal heart is a tightly regulated process. The alteration in the delicate balance between matrix metalloproteinases (MMPs) and their tissue inhibitors (TIMPs) during myocardial injury and inflammation results in enhanced ECM degradation and remodeling (29,30).