Repression of the Lewis fucosyl transferase by retinoic acid increases apical sialosyl Lewisa secretion in colorectal carcinoma cultures

The rate of polarised secretion of sialosyl Lewisa(19‐9) molecular species (SiaLeams) by SW1116 colorectal carcinoma cells is stimulated at least ninefold by the presence of 3 μM retinoic acid (RA). In order to investigate the intracellular origins of this augmentation, carcinoma cell membranes, membrane subfractions, and media were studied to determine alterations in sialosyl Lewisa levels, oligosaccharide composition, and core structures accompanying the capacity to increase export of this epitope. We observed a nine‐ to twentyfold increase in sialosyl Lewisa epitope levels in a light membrane subfraction from RA‐treated cells. Antigenic molecules of < 200,000 Mr on acrylamide gradient gels were concentrated in two doublets in the apparent Mr range 106,000–152,000 on Western blots. Carbohydrates analyses of oligosaccharides from SiaLeams of membrane subfraction and apical media indicated much higher fucose/mannose, fucose/sialic, fucose/sialosyl Lewisa, fucose/total CHO, and (3H) fucose incorporation in control samples than RA samples. Western blots of samples from membranes subfractions and media indicated that, in contrast to the effect of RA on the sialosyl Lewisa epitope, RA treatment did not augment cysteine‐rich, PDTRP, blood group H‐2, blood group A, and EGF receptor‐like region epitopes in the media. In addition, Northern blots using the Lewis fucosyl transferase (FTIII) cDNA showed a dramatic diminution of mRNA encoding FTIII but apparently unaltered levels of sialyl transferase (ST4) mRNA. Since subterminal fucosylation of lactosyl termini blocks terminal sialylation, we conclude that one mechanism of sialosyl Lewisa induction in this culture system is the lower expression of the Lewis fucosyl transferase mRNA. Therefore less subterminal fucosylation of GlcNAc permits the prior sialylation of terminal Galβ1‐3 moieties at oligosaccharide termini destined for export from the Golgi.