Doxycycline inhibits type X collagen synthesis in avian hypertrophic chondrocyte cultures

Doxycycline, a member of the tetracycline family, has been shown to reduce a type X collagen epitope as detected by immunohistochemistry with a monoclonal antibody in an avian explant culture system (1). It was also shown to decrease collagenase and gelatinase activities and thus matrix degradation, This study investigates the effect of doxycycline on type X collagen synthesis in monolayer cultures of hypertrophic chondrocytes. Protein synthesis was evaluated by radioisotopic labeling during doxycycline, tetracycline, or minocycline treatment, Radiolabeled proteins were analyzed by gel electrophoresis, and total collagen was quantitated by hydroxyproline analysis, Additionally, the synthesis of type X collagen was measured by immunoprecipitation, Doxycycline was found to inhibit type X production more effectively than either of the other tetracyclines at comparable dose levels, Furthermore, type X collagen was inhibited more than other collagens, non-collagenous proteins and proteoglycans, with maximal inhibition at 80 mu g/ml and an IC50 of 7 mu g/ml. This inhibition by doxycycline was specific for type X collagen at 10 mu g/ml, and the pattern was distinct from cycloheximide, a recognized inhibitor of protein translation, This suppression of type X collagen could not be overcome by excess extracellular calcium, conditions that have been demonstrated to induce synthesis of this protein (2).