Metabolic processing of newly synthesized link protein in bovine articular cartilage explant cultures

In explant cultures of articular cartilage from cattle of different ages radiolabeled leucine was shown to be incorporated into link proteins 1, 2 and 3. The newly synthesized link proteins were incorporated into and lost from the cartilage extracellular matrix with time. The levels of radiolabeled link proteins 1 and 2 remaining in the matrix declined over the culture period, but there was an initial increase in the amount of radiolabeled link protein 3, before its level declined. The turnover time of the radiolabeled link proteins 1 and 2 were similar, indicating that neither link protein was preferentially processed to generate link protein 3, nor lost from the extracellular matrix. The majority of the radiolabeled link protein lost from the cartilage matrix could not be recovered from the culture medium, suggesting that turnover of the radiolabeled aggrecan complexes involves the newly synthesized link protein being internalized by the chondrocytes. Inclusion of cytotoxic proteinase inhibitors to the culture medium resulted in a marked decrease in the rate of loss of link protein from the cartilage, suggesting that the catabolism of link protein is cell-mediated and dependent on metabolically active cells.