Objective: To determine whether the focal susceptibility to cartilage degeneration in joints is related to a differential response to cytokine stimulation.
Methods: Compare aggrecan and collagen catabolism in in-vitro models of cartilage degradation induced by retinoic acid (RA), interleukin-1 (IL-1), tumor necrosis factor alpha (TNF) and IL-1 plus oncostatin M (OSM). Glycosaminoglycan (GAG) and hydroxyproline (HyPro) quantification and Western immunoblot analyses of aggrecan and collagen degradation products were undertaken in explant cultures of normal cartilage from regions of equine joints with a known high and low susceptibility to degeneration in disease. RNA isolation and semi quantitative reverse transcription-polymerase chain reaction (RT-PCR) analysis were performed to determine the expression of aggrecanases, matrix metalloproteinases (MMPs) and their inhibitors.
Results: Although the rate of basal cartilage aggrecan turnover was dependent on joint region there was no difference in the response of different cartilages to cytokines. Individual animals did show a significant difference in the response of certain cartilages to cytokines, with both decreased and increased aggrecan loss in cartilage with a low susceptibility to degeneration. Aggrecan release in both short- and long-term cultures from all cartilages was associated with increased cleavage by aggrecanases rather than MMPs. There was a poor correlation between expression of aggrecanases, MMPs or their inhibitors and cytokine induced aggrecan catabolism. IL-1 alone was able to stimulate collagen breakdown in equine articular cartilage and surprisingly, significantly more collagen loss was induced in cartilage from regions less susceptible to degeneration.
Conclusions: Collectively, these studies suggest that a regional difference in response to catabolic cytokines is unlikely to be a factor in the initiation of focal cartilage degeneration in osteoarthritis (OA).