The mechanism of aggrecan release from cartilage differs with tissue origin and the agent used to stimulate catabolism.

The mechanisms of aggrecan degradation in adult human articular, adult bovine nasal and fetal bovine epiphyseal cartilage in response to either interleukin-1beta (IL-1beta) or retinoic acid were compared using an explant culture system. Bovine nasal cartilage cultured with either IL-1beta or retinoic acid exhibited significant release of glycosaminoglycan (GAG). For both factors, aggrecan proteolysis occurred predominantly at the 'aggrecanase' site, with no evidence for the action of matrix metalloproteinases, and resulted in the appearance of the corresponding G1 fragment in tissue extracts and in culture media. In human cartilage, little effect of IL-1beta was seen, but abundant release of GAG occurred in the presence of retinoic acid, with evidence of aggrecanase action. Treatment of fetal epiphyseal cartilage with retinoic acid resulted in significant GAG release, whereas treatment with IL-1beta did not. In the retinoic acid-treated tissue, however, no evidence for the cleavage of aggrecan in the interglobular region was apparent. Thus, in the fetal system, agents in addition to aggrecanase and matrix metalloproteinases appear to be active. Taken together, these data demonstrate that the pathways utilized for aggrecan catabolism may vary between different cartilages for a given stimulatory agent, and that, for a given tissue, different factors may elicit aggrecan release via different pathways.