REPRESSION OF THE LEWIS FUCOSYL-TRANSFERASE BY RETINOIC ACID INCREASES APICAL SIALOSYL LEWIS(A) SECRETION IN COLORECTAL-CARCINOMA CULTURES

The rate of polarised secretion of sialosyl Lewis(a)(19-9) molecular s pecies (SiaLeams) by SW1116 colorectal carcinoma cells is stimulated a t least ninefold by the presence of 3 mu M retinoic acid (RA). In orde r to investigate the intracellular origins of this augmentation, carci noma cell membranes, membrane subfractions, and media were studied to determine alterations in sialosyl Lewis(a) levels, oligosaccharide com position, and core structures accompanying the capacity to increase ex port of this epitope. We observed a nine- to twentyfold increase in si alosyl Lewis(a) epitope levels in a light membrane subfraction from RA -treated cells. Antigenic molecules of <200,000 Mr on acrylamide gradi ent gels were concentrated in two doublets in the apparent Mr range 10 6,000-152,000 on Western blots. Carbohydrate analyses of oligosacchari des from SiaLeams of membrane subfractions and apical media indicated much higher fucose/mannose, fucose/sialic, fucose/sialosyl Lewis(a), f ucose/total CHO, and (H-3) fucose incorporation in control samples tha n RA samples. Western blots of samples from membrane subfractions and media indicated that, in contrast to the effect of RA on the sialosyl Lewis(a) epitope, RA treatment did not augment cysteine-rich, PDTRP, b lood group H-2, blood group A, and ECF receptor-like region epitopes i n the media. In addition, Northern blots using the Lewis fucosyl trans ferase (FTIII) cDNA showed a dramatic diminution of mRNA encoding FTII I but apparently unaltered levels of sialyl transferase (ST4) mRNA. Si nce subterminal fucosylation of lactosyl termini blocks terminal sialy lation, we conclude that one mechanism of sialosyl Lewis(a) induction in this culture system is the lower expression of the Lewis fucosyl tr ansferase mRNA. Therefore less subterminal fucosylation of GlcNAc perm its the prior sialylation of terminal Gal beta 1-3 moieties at oligosa ccharide termini destined for export from the Golgi. (C) 1995 Wiley-Li ss, Inc.