Glycosaminoglycan Addition to Proteoglycans by Articular Chondrocytes - Evidence for Core Protein-Specific Pathways

Abstract

The intracellular compartmentalization of enzyme activities involved in the elongation and sulfation of glycosaminoglycans on aggrecan, decorin, and fibromodulin was investigated using brefeldin A, a compound with known inhibitory action on normal vesicular transport and secretion of macromolecules. Treatment of bovine chondrocyte cultures with the compound resulted in greater than 98% inhibition of Na35SO4 incorporation into macromolecules, whereas [3H]leucine or [3H]glucosamine continued at 60-70% of the levels measured in control cultures. The release of newly synthesized products into the medium was also decreased markedly by brefeldin A to 7 and 2% of control levels for [3H]leucine- and [3H]glucosamine-labeled macromolecules, respectively. Analysis of [3H]glucosamine-labeled products in these cultures showed that synthesis of sulfated glycosaminoglycans (chondroitin/dermatan sulfate and keratan sulfate) was inhibited in response to brefeldin A, whereas hyaluronan synthesis was essentially unaffected. Significant amounts of elongated chondroitin continued to be synthesized in the presence of brefeldin A. Immunoprecipitation of [3H]leucine-labeled decorin, aggrecan, and fibromodulin from cells showed that aggrecan and fibromodulin were not substituted with glycosaminoglycans, whereas all decorin molecules synthesized under these conditions were substituted with chondroitin. The results suggest that in articular chondrocytes, elongation of the glycosaminoglycan chains on decorin, but not their sulfation, occurs in a Golgi compartment unaffected by disruption of vesicular core protein transport. This is in contrast to glycosaminoglycan elongation and sulfation on aggrecan and fibromodulin, where both processes apparently occur in the trans-Golgi network, which becomes inaccessible to these core proteins in the presence of brefeldin A. The results further suggest that in brefeldin A-treated cells decorin is contained in a discrete ER-Golgi compartment separated from aggrecan; this compartment is accessible to p-nitrophenyl-β-D-xylosides, since β-xylosides become elongated with chondrotin even in the presence of brefeldin A.