Differential distribution of sialic acid in alpha2,3 and alpha2,6 linkages in the apical membrane of cultured epithelial cells and tissues.

We used lectin cytochemistry and confocal microscopy to examine the distribution of sialic acid in epithelial cells. Maackia amurensis lectin and Sambuccus nigra agglutinin were used to detect alpha2,3 and alpha2,6 sialic acid, respectively. In Caco-2, HT-29 5M12, and MCF-7 cells, which express sialic acid mainly in one type of linkage, the majority of the signal was observed in the apical membrane. In cells that bound both lectins, alpha2,3 sialic acid was distributed apically, whereas alpha2,6 sialic acid showed a broader distribution. In IMIM-PC-1 cultures, alpha2,3 sialic acid was detected mainly in the apical membrane, whereas alpha2,6 sialic acid was more abundant in the basolateral domain of polarized cells. In these cells, treatment with GalNAc-O-benzyl led to reduced alpha2,3 levels and to an increase and redistribution of alpha2,6 to the apical domain. Similarly, sialic acid was predominantly expressed apically in all epithelial tissues examined. In conclusion, (a) sialic acid is mainly distributed to the apical membrane of epithelial cells, (b) there is a hierarchy in the distribution of sialic acids in polarized epithelial cells, i.e., alpha2,3 is preferred to alpha2,6 in the apical membrane, and (c) IMIM-PC-1 cells are a good model in which to study the regulation of the levels and distribution of sialic acids.