The relation of protein synthesis to chondroitin sulphate biosynthesis in cultured bovine cartilage

Abstract

The effect of cycloheximide on chondroitin sulphate biosynthesis was studied in bovine articular cartilage maintained in culture. Addition of 0.4 mM-cycloheximide to the culture medium was followed, over the next 4h, by a first-order decrease in the rate of incorporation of [35S]sulphate into glycosaminoglycan (half-life, t 1/2 = 32 min), which is consistent with the depletion of a pool of proteoglycan core protein. Addition of 1.0 mM-benzyl beta-D-xyloside increased the rate of incorporation of [35S]sulphate and [3H]acetate into glycosaminoglycan, but this elevated rate was also diminished by cycloheximide. It was concluded that cycloheximide exerted two effects on the tissue; not only did it inhibit the synthesis of the core protein, but it also lowered the tissue's capacity for chondroitin sulphate chain synthesis. Similar results were obtained with chick chondrocytes grown in high-density cultures. Although the exact mechanism of this secondary effect of cycloheximide is not known, it was shown that there was no detectable change in cellular ATP concentration or in the amount of three glycosyltransferases (galactosyltransferase-I, N-acetylgalactosaminyltransferase and glucuronosyltransferase-II) involved in chondroitin sulphate chain synthesis. The sizes of the glycosaminoglycan chains formed in the presence of cycloheximide were larger than those formed in control cultures, whereas those synthesized in the presence of benzyl beta-D-xyloside were consistently smaller, irrespective of the presence of cycloheximide. These results suggest that beta-D-xylosides must be used with caution to study chondroitin sulphate biosynthesis as an event entirely independent of proteoglycan core-protein synthesis, and they also indicate a possible involvement of the core protein in the activation of the enzymes of chondroitin sulphate synthesis.