Mechanical injury potentiates proteoglycan catabolism induced by interleukin-6 with soluble interleukin-6 receptor and tumor necrosis factor alpha in immature bovine and adult human articular cartilage.

OBJECTIVE: Traumatic joint injury can damage cartilage and release inflammatory cytokines from adjacent joint tissue. The present study was undertaken to study the combined effects of compression injury, tumor necrosis factor alpha (TNFalpha), and interleukin-6 (IL-6) and its soluble receptor (sIL-6R) on immature bovine and adult human knee and ankle cartilage, using an in vitro model, and to test the hypothesis that endogenous IL-6 plays a role in proteoglycan loss caused by a combination of injury and TNFalpha. METHODS: Injured or uninjured cartilage disks were incubated with or without TNFalpha and/or IL-6/sIL-6R. Additional samples were preincubated with an IL-6-blocking antibody Fab fragment and subjected to injury and TNFalpha treatment. Treatment effects were assessed by histologic analysis, measurement of glycosaminoglycan (GAG) loss, Western blot to determine proteoglycan degradation, zymography, radiolabeling to determine chondrocyte biosynthesis, and Western blot and enzyme-linked immunosorbent assay to determine chondrocyte production of IL-6. RESULTS: In bovine cartilage samples, injury combined with TNFalpha and IL-6/sIL-6R exposure caused the most severe GAG loss. Findings in human knee and ankle cartilage were strikingly similar to those in bovine samples, although in human ankle tissue, the GAG loss was less severe than that observed in human knee tissue. Without exogenous IL-6/sIL-6R, injury plus TNFalpha exposure up-regulated chondrocyte production of IL-6, but incubation with the IL-6-blocking Fab significantly reduced proteoglycan degradation. CONCLUSION: Our findings indicate that mechanical injury potentiates the catabolic effects of TNFalpha and IL-6/sIL-6R in causing proteoglycan degradation in human and bovine cartilage. The temporal and spatial evolution of degradation suggests the importance of transport of biomolecules, which may be altered by overload injury. The catabolic effects of injury plus TNFalpha appeared partly due to endogenous IL-6, since GAG loss was partially abrogated by an IL-6-blocking Fab.

What next for rheumatoid arthritis therapy?

Introduction of biological agents for the treatment of the chronic inflammatory joint disease rheumatoid arthritis has reinvigorated research into this debilitating disease. These agents have been shown to both act on the signs and symptoms of disease, as well as retard the progression of joint destruction. However, these agents are not efficacious in all cases and their expense and route of administration can severely limit their use. Therefore the search continues not only for additional targets to help those individuals refractive to current therapy but also for more affordable orally active small molecule alternatives to biological agents.

Proteoglycan degradation after injurious compression of bovine and human articular cartilage in vitro: interaction with exogenous cytokines.

OBJECTIVE: Traumatic joint injury leads to an increased risk of osteoarthritis (OA), but the progression to OA is not well understood. We undertook this study to measure aspects of proteoglycan (PG) degradation after in vitro injurious mechanical compression, including up-regulation of enzymatic degradative expression and cytokine-stimulated degradation. METHODS: Articular cartilage tissue explants were obtained from newborn bovine femoropatellar groove and from adult normal human donor knee and ankle tissue. Following injurious compression of the cartilage, matrix metalloproteinase 3 (MMP-3) and MMP-13 messenger RNA (mRNA) expression levels were measured by Northern analysis, and PG loss to the medium after cartilage injury was measured in the presence and absence of added exogenous cytokine (interleukin-1alpha [IL-1alpha] or tumor necrosis factor alpha [TNFalpha]). RESULTS: During the first 24 hours after injury in bovine cartilage, MMP-3 mRNA levels increased 10-fold over the levels in control cartilage (n = 3 experiments), whereas MMP-13 mRNA levels were unchanged. PG loss was significantly increased after injury, but only by 2% of the total PG content and only for the first 3 days following injury. However, compared with injury alone or cytokine treatment alone, treatment of injured tissue with either 1 ng/ml IL-1alpha or 100 ng/ml TNFalpha caused marked increases in PG loss (35% and 54%, respectively, of the total cartilage PG content). These interactions between cytokine treatment and injury were statistically significant. In human knee cartilage, the interaction was also significant for both IL-1alpha and TNFalpha, although the magnitude of increase in PG loss was lower than that in bovine cartilage. In contrast, in human ankle cartilage, there was no significant interaction between injury and IL-1alpha. CONCLUSION: The cytokines IL-1alpha and TNFalpha can cause a synergistic loss of PG from mechanically injured bovine and human cartilage. By attempting to incorporate interactions with other joint tissues that may be sources of cytokines, in vitro models of mechanical cartilage injury may explain aspects of the interactions between mechanical forces and degradative pathways which lead to OA progression.

Small-molecule inhibitors of NF-kappaB for the treatment of inflammatory joint disease.

Recent advances in our understanding of the role of cytokine networks in inflammatory processes have led to the development of novel biological agents for the treatment of chronic inflammatory diseases. At the present time, significant efforts are focused on characterizing the complex signal transduction cascades that are activated by these cytokines and, in turn, regulate their expression. The transcription factor NF-kappaB is a pivotal regulator of the inducible expression of key proinflammatory mediators, and activated NF-kappaB has been observed in several debilitating inflammatory disorders, including rheumatoid arthritis and osteoarthritis. In light of its central role in inflammation, the identification of inhibitors of NF-kappaB should provide novel therapeutics for the treatment of chronic joint disease.