Kinetics of proteoglycan turnover in bovine articular cartilage explants

The turnover of proteoglycans was studied in explant cultures of mature bovine articular cartilage. The aim of the study was to compare the in vitro turnover rates of newly synthesized proteoglycans and endogenous proteoglycans. Cartilage was maintained in the presence of various serum concentrations in order to determine the conditions of steady-state proteoglycan metabolism. The steady state was achieved in medium containing 20% fetal calf serum. The proteoglycan synthesis rate and the half-life of labeled proteoglycans in steady-state cultures were used to calculate the size of the metabolic pool of newly synthesized proteoglycans in steady state. This metabolic pool was shown to be equal to the total amount of proteoglycans in the matrix. It is concluded that all of the proteoglycans in the matrix have the same half-life in vitro. Taking another approach, aggrecan was isolated from the cartilage and the medium of steady-state cultures prelabeled with [35S]sulfate. The specific activity of the glycosaminoglycans from cartilage aggrecan were compared with that of glycosaminoglycans from medium aggrecan. These proved to be the same throughout the culture period. This shows that newly synthesized aggrecan and endogenous aggrecan have the same turnover rate in vitro. The significance of explant culture systems for the study of proteoglycan turnover is discussed.