EFFECTS OF CELL-SURFACE GANGLIOSIDE SIALIDASE INHIBITION ON GROWTH-CONTROL AND DIFFERENTIATION OF HUMAN NEUROBLASTOMA-CELLS

Gangliosides on the external side of the plasma membrane are important modulators of cellular functions, In previous work we had found that in cultured human SK-N-MC neuroblastoma cells a cell surface sialidase activity specifically cleaved terminal sialic acids from gangliosides , leading to a shift from higher sialylated species to GM1 and a decre ase of GM3, To further elucidate the function of the enzyme, we have n ow examined the consequences of ganglioside sialidase inhibition, When present in the culture medium, the ganglioside sialidase inhibitors 2 -deoxy-2,3-dehydro-N-acetylneuraminic acid (NeuAc2en), heparin, and he paran sulfate caused dramatic changes in cell behavior, Thus, the inhi bitors uniformly led to a complete release from contact inhibition of growth, and to the loss of the differentiation markers neuron-specific enolase and neurofilaments, and a decrease of cyclic AMP, In presence of NeuAc2en, cells that normally were spread out evenly and were firm ly attached, appeared smaller, rounded, and only loosely adherent to t he culture vessel, Exogenous addition of vibrio cholerae sialidase mim icked the action of the plasma membrane ganglioside sialidase by retar ding cell proliferation and increasing intracellular acetylcholinester ase. That the ganglioside sialidase inhibitors in the culture medium i ndeed affected solely the cell surface enzyme and not also a lysosomal sialidase, was demonstrated in an experiment where the desialylation of exogenously added radioactive gangliosides was determined in absenc e and presence of NeuAc2en and NH4Cl, an inhibitor of lysosomal functi on, Taken together, our results suggest that the ganglioside sialidase on the surface of SK-N-MC cells is responsible for growth control and differentiation in this neuronal cell line.