Influence of monensin on biosynthesis, processing and secretion of proteodermatan sulfate by skin fibroblasts

The influence of monensin on biosynthesis, processing and secretion of proteodermatan sulfate from human skin fibroblasts was studied with the aid of a specific immunological procedure. Double-labeling experiments with [3H]leucine and [35S]sulfate indicated that monensin caused a dose-dependent parallel decrease of sulfate incorporation into total and of secretion of 3H-labeled proteodermatan sulfate. Compared with the untreated control, a greater proportion of incorporated [35S]sulfate than of incorporated [3H]leucine became secreted. Other monensin effects were a moderate intracellular accumulation of glycosaminoglycan-free core protein, a reduced chain length and a greatly reduced epimerization of D-glucuronic to L-iduronic acid residues. In contrast to the formation of N-acetylgalactosamine 4-sulfate residues 6-sulfation was not affected. Conversion of high-mannose-type oligosaccharides to complex-type N-glycans which normally occurred concomitantly with glycosaminoglycan biosynthesis was inhibited. Withdrawal of monensin made possible an additional sulfation of intracellularly accumulated proteodermatan sulfate. The newly formed sulfate esters did not cluster at the non-reducing ends of the glycosaminoglycan chains. Cells preexposed to monensin and labeled with [3H]glucosamine either in the absence or continuous presence of the drug incorporated similar amounts of 3H radioactivity into proteodermatan sulfate. The results suggest that epimerization of D-glucuronic acid residues and 4-sulfation occur predominantly in the trans cisternae of the Golgi apparatus whereas chain polymerisation and 6-sulfation take place predominantly in the cis Golgi complex.