Swainsonine induced modification of N-linked oligosaccharides on human phagocytic cell formyl peptide chemotactic receptors.

Swainsonine, a Golgi mannosidase II inhibitor, causes a 15,000 dalton decrease in the apparent molecular weight of formyl peptide chemotactic receptor (FPCR) synthesized by three types of human phagocytic cells (differentiated HL-60 cells, differentiated U-937 cells, and cultured human macrophages derived from peripheral blood monocytes). This indicates that some of the asparagine-linked (N-linked) oligosaccharide of FPCR is of the complex type. Studies with endoglycosidase F and endoglycosidase H show that unaltered FPCR contains both complex and high mannose content N-linked oligosaccharide chains, and that FPCR synthesized in the presence of swainsonine contains only high mannose N-linked oligosaccharide chains. Cycloheximide prevents the swainsonine effect on FPCR molecular weight. This shows that swainsonine only affects the carbohydrate structure of FPCR which is newly synthesized in the presence of swainsonine. Since swainsonine decreases the molecular weight of FPCR from human peripheral blood monocytes in culture, these cells must be synthesizing new FPCR. The abnormally low molecular weight FPCR synthesized in the presence of swainsonine is converted to the normal, higher molecular weight form of FPCR when swainsonine is removed from the culture. This occurs even when new protein synthesis is inhibited by cycloheximide. This requires that previously synthesized, swainsonine-affected FPCR at the cell surface must come in contact with Golgi enzymes which complete the processing of the abnormal, high mannose oligosaccharide. This implies a potential Golgi-related repair mechanism for altered or "damaged" cell surface receptors.