Role Of Carbohydrate Modification in the Production and Secretion of Human Granulocyte Macrophage Colony-Stimulating Factor in Genetically Engineered and Normal Mesenchymal Cells

Colony-stimulating factors (CSFs) are a group of acidic glycoproteins which stimulate the proliferation and differentiation of hematopoietic progenitor cells in vitro and stimulate hemopoiesis in vivo. Human GM-CSF contains two N-linked carbohydrate side chains of the complex acidic type and several sites of O-linked carbohydrate clustered on serine and threonine residues near the N-terminus of the molecule. Previous studies have failed to detect a significant functional role for the carbohydrate modification characteristic of human GM-CSF. Using permanent cell lines and transient expression systems which produce moderate to high levels of native or carbohydrate-deficient forms of the growth factor, the role of carbohydrate modification in the biosynthesis and secretion of GM-CSF was studied. Unlike a number of other secreted glycoproteins, the transit time and secretory efficiency of several carbohydrate-deficient mutants of GM-CSF are indistinguishable from those of the native growth factor in BHK, 293, COS, and ldlD cells. Furthermore, normal human endothelial cells and fibroblasts, which normally produce the growth factor, can synthesize and secrete GM-CSF that lacks all forms of carbohydrate modification. These studies help to point out the range of roles played by carbohydrate modification in the biosynthesis, assembly, and secretion of glycoprotein hormones.