Turnover of proteoglycans in articular-cartilage cultures. Characterization of proteoglycans released into the medium

By using an e.l.i.s.a. method it was demonstrated that the majority of proteoglycans released into the medium of both control and retinoic acid-treated explant cultures of bovine articular cartilage did not contain a hyaluronate-binding region. This supports our previous findings [Campbell & Handley (1987) Arch. Biochem. Biophys. 258, 143-155] that proteoglycans released into the medium of both cultures were of smaller hydrodynamic size, more polydisperse and unable to form aggregates with hyaluronate. Analysis of 35S-labelled core proteins associated with proteoglycans released into the medium of both cultures by using SDS/polyacrylamide-gel electrophoresis and fluorography indicated the presence of a series of core-protein bands (Mr approx. 300,000, 230,000, 215,000, 200,000, 180,000, 140,000, 135,000, 105,000, 85,000 and 60,000) compared with three core proteins derived from the proteoglycans remaining in the matrix (Mr 300,000, 230,000 and 215,000). Further analysis of the core proteins released into the medium indicated that the larger core proteins associated with medium proteoglycans contain both chondroitin sulphate and keratan sulphate glycosaminoglycans whereas the smaller core proteins contain only chondroitin sulphate chains. These experiments provide definitive evidence that the loss of proteoglycans from the matrix involves proteolytic cleavage at various sites along the proteoglycan core protein.