GALNAC-ALPHA-O-BENZYL INHIBITS NEUAC-ALPHA-2-3 GLYCOSYLATION AND BLOCKS THE INTRACELLULAR-TRANSPORT OF APICAL GLYCOPROTEINS AND MUCUS IN DIFFERENTIATED HT-29 CELLS

Exposure for 24 h of mucus-secreting HT-29 cells to the sugar analogue GalNAc-alpha-O-benzyl results in inhibition of Gal beta 1-3GalNAc:alp ha 2,3-sialyltransferase, reduced mucin sialylation, and inhibition of their secretion (Huet, G., I. Kim, C. de Bolos, J.M. Loguidice, O. Mo reau, B. Hemon, C. Richet, P. Delannoy, F.X. Real., and P. Degand. 199 5. J. Cell Sci. 108..275-1285). To determine the effects of prolonged inhibition of sialylation, differentiated HT-29 populations were grown under permanent exposure to GalNAc-alpha-O-benzyl. This results in no t only inhibition of mucus secretion, but also in a dramatic swelling of the cells and the accumulation in intracytoplasmic vesicles of brus h border-associated glycoproteins like dipeptidylpeptidase-IV, the muc in-like glycoprotein MUCl and carcinoembryonic antigen which are no lo nger expressed at the apical membrane. The block occurs beyond the cis -Golgi as substantiated by endoglycosidase treatment and biosynthesis analysis. In contrast, the polarized expression of the basolateral gly coprotein GP 120 is not modified. Underlying these effects we found th at (a) like in mucins, NeuAc alpha 2-3Gal-R is expressed in the termin al position of the oligosaccharide species associated with the apical. but not the basolateral glycoproteins of the cells, and (b) treatment with GalNAc-alpha-O-benzyl results in an impairment of their sialylat ion. These effects are reversible upon removal of the drug. It is sugg ested that alpha 2-3 sialylation is involved in apical targeting of br ush border membrane glycoproteins and mucus secretion in HT-29 cells.