Proteoglycan aggregate synthesis in normal and chronically hydrocortisone-suppressed rabbit articular cartilage

Mature rabbits were given eight weekly injections of hydrocortisone into both knee joints. The articular cartilage was then removed and its biochemistry compared with that of normal controls. Chronically suppressed tissue had significantly lower water and proteoglycan content while hydroxyproline and DNA values were only slighly depressed. Samples were pulsed in vitro with [³⁵S]sulfate and [³H]serine, the proteoglycans extracted with 4 m guanidine hydrochloride in the presence of protease inhibitors, purified under associative conditions in a cesium chloride density gradient, and the fractions further characterized. The rate of sulfate incorporation was 8% of the control values. For both normal and suppressed, the proteolgycan monomer had a K_av on Sepharose CL-2B of 0.31 and was smaller than typical hyaline cartilage proteoglycans. Amino acid composition of the monomer was similar to that found in other hyaline cartilages with a high content of serine, glycine, proline, and glutamic acid. The glucosamine/galactosamine ratio was 0.37. For both the cold and newly synthesized proteoglycans, the chondroitin chain length was M_r = 8 × 10³ with a ratio of the 4-sulfated to 6-sulfated isomers of 0.4. The values were similar in the hydrocortisone-treated animals. In the suppressed tissue, both the newly synthesized and cold unlabeled proteoglycans were less extractable and less aggregated than in the control tissue. Aggregate fractions isolated from both the control and hydrocortisone-suppressed cartilages were hyaluronic acid deficient; exogenous added hyaluronic acid increased the percentage aggregate three- to fivefold. In both control and suppressed cartilages, newly synthesized material was less aggregated than the unlabeled proteoglycans.