Four classes of cell-associated proteoglycans in suspension cultures of articular-cartilage chondrocytes

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Abstract
The characteristics of cell-associated proteoglycans were studied and compared with those from the medium in suspension cultures of calf articular-cartilage chondrocytes. By including hyaluronic acid or proteoglycan in the medium during [35S]sulphate labelling the proportion of cell-surface-associated proteoglycans could be decreased from 34% to about 15% of all incorporated label. A pulse-chase experiment indicated that this decrease was probably due to blocking of the reassociation with the cells of proteoglycans exported to the medium. Three peaks of [35S]sulphate-labelled proteoglycans from cell extracts and two from the medium were isolated by gel chromatography on Sephacryl S-500. These were characterized by agarose/polyacrylamide-gel electrophoresis, by SDS/polyacrylamide-gel electrophoresis of core proteins, by glycosaminoglycan composition and chain size as well as by distribution of glycosaminoglycans in proteolytic fragments. The results showed that associated with the cells were (a) large proteoglycans typical for cartilage, apparently bound to hyaluronic acid at the cell surface, (b) an intermediate-size proteoglycan with chondroitin sulphate side chains (this proteoglycan, which had a large core protein, was only found associated with the cells and is apparently not related to the large proteoglycans), (c) a small proteoglycan with dermatan sulphate side chains with a low degree of epimerization, and (d) a somewhat smaller proteoglycan containing heparan sulphate side chains. The medium contained a large aggregating proteoglycan of similar nature to the large cell-associated proteoglycan and small proteoglycans with dermatan sulphate side chains with a higher degree of epimerization than those of the cells, i.e. containing some 20% iduronic acid.