REGULATION OF SIALIC-ACID 9-O-ACETYLATION DURING THE GROWTH AND DIFFERENTIATION OF MURINE ERYTHROLEUKEMIA-CELLS

Sialic acids are typically found at the terminal position on vertebrat e oligosaccharides. They are sometimes modified by an O-acetyl ester a t the 9-position, potentially altering recognition of sialic acid by a ntibodies, lectins, and viruses, 9-O-Acetylation is known to be select ively expressed on gangliosides in melanoma cells and on N-linked chai ns in hepatocytes, Using a recently developed probe, we show here that in murine erythroleukemia cells, this modification is selectively exp ressed on another class of oligosaccharides, O-linked chains carried o n cell surface sialomucins, These cells also express 9-O-acetylation o n the ganglioside G(Da), but this modification appears to be undetecta ble on the cell surface, Increasing cell density in culture is associa ted with a decrease in cell surface 9-O-acetylation of sialomucins. Th is change correlates with the spontaneous differentiation toward a mat ure erythroid phenotype, This down-regulation upon differentiation and entry into the G(0)/G(1) stage of the cell cycle is confirmed by diff erentiation-inducing agents, In contrast, cells arrested in G(2)/M by the microtubule depolymerizing agent nocodazole show increased express ion of cell surface 9-O-acetylated sialomucins (but not the 9-O-acetyl ated ganglioside), However, the microtubule stabilizer taxol does not induce this increase, showing that the nocodazole effect is independen t of cell cycle stage, Indeed, direct analysis showed no correlation o f 9-O-acetylation with cell cycle stage in rapidly growing cells, and shorter treatments with nocodazole also increased expression, Western blots of cell extracts confirmed that changes caused by differentiatio n and nocodazole are not due to redistribution of molecules from the c ell surface, Indeed, follow ing selective removal of 9-O-acetyl groups from the cell surface by a specific esterase, the recovery of express ion is mediated by new synthesis rather than by redistribution from an internal pool, Thus, 9-O-acetylation on these sialomucins appears to be primarily regulated by the rate of synthesis, and the increase with nocodazole treatment is likely due to the inhibition of turnover of c ell surface molecules, These data show that 9-O-acetylation of sialic acids in murine erythroleukemia cells is a highly regulated modificati on, being selectively expressed in a cell type-specific manner on cert ain classes of oligosaccharides and differentially regulated with rega rd to subcellular localization and to the state of cellular differenti ation.