Cartilage is a highly organised tissue where the arrangement of collagen and proteoglycan and minor components is carefully controlled. The chondrocytes maintain a precise balance between anabolic and catabolic processes that maintains this extracellular matrix. Proteoglycan removal is rapid and reversible while collagen removal is slower but, once removed, is difficult to replace. Our main research focus is to discover the mechanism of collagen turnover and we study the members of the matrix metalloproteinase (MMP) family that specifically cleave type II collagen to give characteristic one-quarter and three-quarter fragments; MMP-1, MMP-8, MMP-13 and also MMP-2 and MMP-14 cleave in this way.
Oncostatin M (OSM) is a member of the IL-6 family that synergises with IL-1 to induce cartilage proteoglycan and collagen degradation in a cartilage explant culture system . A significant finding is the synergistic induction of the collagenase, MMP-1, which occurs via interplay between the JAK/STAT, AP-1 and MAPK pathways. Other collagenases such as MMP-8 and MMP-13 are also upregulated along with MMP-14 and MMP-3. This latter enzyme can activate the collagenases, and an important feature of OSM may be its ability to promote the activation of enzymes that initiate activation cascades that lead to the production of active collagenases. OSM can also exacerbate the effects of other important proinflammatory mediators such as tumour necrosis factor alpha (TNF-α) and IL-17.
Molecular and cellular studies aim to discover the mechanism of action that leads to synergy. Affymetrix microarrays show that a specific cohort of genes are upregulated by these cytokine mixtures including MMPs, ADAMs (A disintegrin and metalloproteinases), activators, cell surface proteins and cytokines. Two-dimensional gel electrophoresis and proteomic analysis confirm that many of the corresponding proteins are made by chondrocytes after stimulation. Purification of specific proteins from conditioned culture medium has been undertaken to try and determine the enzymes responsible for collagen turnover.
In order to assess the effects of these cytokine combinations in vivo, we have injected OSM in combination with either IL-1 or TNF-α intra-articularly into murine knee joints using recombinant adenovirus. Engineered adenoviruses were administered for only 7 days, after which time joints were fixed, decalcified and sectioned. Histological analyses indicated marked synovial hyperplasia and inflammatory cell infiltration for IL-1, TNF-α and OSM treated joints but not in controls (joints treated with an 'empty' adenovirus). The inflammation was more pronounced for both OSM + IL-1 and OSM + TNF-α combinations with evidence of extensive cartilage and bone destruction. Significant loss of both proteoglycan and collagen was also seen for these combinations, and an increased expression of MMPs with decreased tissue inhibitors of metalloproteinases was found in both articular cartilage and synovium. The effects of these combinations were significantly greater than those seen with any of the cytokines alone. Cytokine combinations also upregulated RANK/RANKL, and increased the number of TRAP-positive cells showing an increase in osteoclast formation and bone damage. Taken together, these data confirm that, in vivo, OSM can significantly exacerbate the effects of both IL-1 and TNF-α resulting in inflammation and tissue destruction characteristic of that seen in rheumatoid arthritis.
These studies provide further evidence to implicate the upregulation of collagenases as a key factor in the destruction of collagen that occurs in joint pathology, and suggests that OSM is a potent mediator when found in the joint with other proinflammatory cytokines.