Metabolism of the matrix by chondrocytes is sensitive to alterations in cell volume that occur, for example, during static loading and osteoarthritis. The ability of chondrocytes to respond to changes in volume could be important, and this study was aimed at testing the hypothesis that chondrocytes can regulate their volume following cell shrinking by regulatory volume increase (RVI). We used single cell fluorescence imaging of in situ bovine articular chondrocytes, cells freshly isolated into 280 or 380 mOsm, or 2-D cultured chondrocytes loaded with calcein or fura-2, to investigate RVI and changes to [Ca2+]i during shrinkage. Following a 42% hyperosmotic challenge, chondrocytes rapidly shrunk, however, only approximately 6% of the in situ or freshly isolated chondrocytes demonstrated RVI. This contrasted with 2D-cultured chondrocytes where approximately 54% of the cells exhibited RVI. The rate of RVI was the same for all preparations. During the 'post-RVD/RVI protocol', approximately 60% of the in situ and freshly isolated chondrocytes demonstrated RVD, but only approximately 5% showed RVI. There was no relationship between [Ca2+]i and RVI either during hyperosmotic challenge, or during RVD suggesting that changes to [Ca2+]i were not required for RVI. Depolymerisation of the actin cytoskeleton by latrunculin, increased RVI by freshly isolated chondrocytes, in a bumetanide-sensitive manner. The results showed that in situ and freshly isolated articular chondrocytes have only limited RVI capacity. However, RVI was stimulated by treating freshly isolated chondrocytes with latrunculin B and following 2D culture of chondrocytes, suggesting that cytoskeletal integrity plays a role in regulating RVI activity which appears to be mediated principally by the Na+ - K+ -2Cl- cotransporter.

BACKGROUND AND PURPOSE Melanocortin MC1 and MC3 receptors, mediate the anti‐inflammatory effects of melanocortin peptides. Targeting these receptors could therefore lead to development of novel anti‐inflammatory therapeutic agents. We investigated the expression of MC1 and MC3 receptors on chondrocytes and the role of α‐melanocyte‐stimulating hormone (α‐MSH) and the selective MC3 receptor agonist, [DTRP8]‐γ‐MSH, in modulating production of inflammatory cytokines, tissue‐destructive proteins and induction of apoptotic pathway(s) in the human chondrocytic C‐20/A4 cells. EXPERIMENTAL APPROACH Effects of α‐MSH, [DTRP8]‐γ‐MSH alone or in the presence of the MC3/4 receptor antagonist, SHU9119, on TNF‐α induced release of pro‐inflammatory cytokines, MMPs, apoptotic pathway(s) and cell death in C‐20/A4 chondrocytes were investigated, along with their effect on the release of the anti‐inflammatory cytokine IL‐10. KEY RESULTS C‐20/A4 chondrocytes expressed functionally active MC1,3 receptors. α‐MSH and [DTRP8]‐γ‐MSH treatment, for 30 min before TNF‐α stimulation, provided a time‐and‐bell‐shaped concentration‐dependent decrease in pro‐inflammatory cytokines (IL‐1β, IL‐6 and IL‐8) release and increased release of the chondroprotective and anti‐inflammatory cytokine, IL‐10, whilst decreasing expression of MMP1, MMP3, MMP13 genes.α‐MSH and [DTRP8]‐γ‐MSH treatment also inhibited TNF‐α‐induced caspase‐3/7 activation and chondrocyte death. The effects of [DTRP8]‐γ‐MSH, but not α‐MSH, were abolished by the MC3/4 receptor antagonist, SHU9119. CONCLUSION AND IMPLICATIONS Activation of MC1/MC3 receptors in C‐20/A4 chondrocytes down‐regulated production of pro‐inflammatory cytokines and cartilage‐destroying proteinases, inhibited initiation of apoptotic pathways and promoted release of chondroprotective and anti‐inflammatory cytokines. Developing small molecule agonists to MC1/MC3 receptors could be a viable approach for developing chondroprotective and anti‐inflammatory therapies in rheumatoid and osteoarthritis.

The data suggested that in freshly isolated and 2D cultured chondrocytes, the rise in [Ca2+]i occurring during hypotonic challenge could be related to RVD, but only in some cells. However, with 2D culture, the Ca2+ response switched to being Gd3+-sensitive, suggesting that as a result of changes to chondrocyte shape, stretch-activated cation channels although present, do not appear to play a role in volume regulation.

Arthritic pathologies are a major cause of morbidity within the western world, with rheumatoid arthritis affecting approximately 1% of adults. This review highlights the therapeutic potential of naturally occurring hormones and their peptides, in both arthritic models of disease and patients. The arthritides represent a group of closely related pathologies in which cytokines, joint destruction, and leukocytes play a causal role. Here we discuss the role of naturally occurring pro-opiomelanocortin (POMC)-derived melanocortin peptides (e.g., alpha melanocyte stimulating hormone [a-MSH]) and synthetic derivatives in these diseases. Melanocortins exhibit their biological efficacy by modulating proinflammatory cytokines and subsequent leukocyte extravasation. Their biological effects are mediated via seven transmembrane G-protein-coupled receptors, of which five have been cloned, identified, and termed MC1 to MC5. Adrenocorticotrophic hormone represents the parent molecule of the melanocortins; the first 13 amino acids of which (termed a-MSH) have been shown to be the most pharmacologically active region of the parent hormone. The melanocortin peptides have been shown to display potent anti-inflammatory effects in both animal models of disease and patients. The potential anti-inflammatory role for endogenous peptides in arthritic pathologies is in its infancy. The ability to inhibit leukocyte migration, release of cytokines, and induction of anti-inflammatory proteins appears to play an important role in affording protection in arthritic injury, and thus may lead to potential therapeutic targets.

Conclusion:Melanocortin peptide pre-treatment prevented chondrocyte death following mechanical impact to cartilage and led to a marked reduction of pro-inflammatory cytokines, whilst prompting the production of anti-inflammatory/pro-resolving cytokine IL-10.Development of small molecule agonists towards melanocortin receptors could thus be a viable approach for preventing chondrocyte inflammation and death within cartilage and represent an alternative approach for the treatment of osteoarthritis.