APICAL RELEASE OF BASE-LABILE FATTY ACYL-GROUPS COMMENSURATE WITH STIMULATION OF GLYCOPROTEIN SIALOSYL LEWIS(A) SECRETION IN COLORECTAL-CARCINOMA CELLS

The rate of polarized secretion of a putative adhesion ligand, sialosy l Lewis(a)(19-9), by SWI116 colorectal carcinoma cells is stimulated a t least 20-fold after pre-incubation with, and the incorporation of, r etinoic acid (RA). In order to investigate the possible involvement of fatty acylation in the export of the epitope, purified ligands from c arcinoma-cell membranes, membrane subfractions and media were analyzed during RA-induced secretion. Incorporation of radioactivity from (H-3 )palmitate into membrane subfractions and purified sialosyl Lewis(a) a ntigenic molecular species of Mr > 150,000 (SiaLeams) was stimulated b y RA treatment. Most of the intracellular lipid radioactivity which bo und to solid-phase 19-9 antibody behaved chromato-graphically, either like ganglioside or like NH2 OH-labile acyl groups, but most of the (H -3) bound to SiaLeams of post-incubation media behaved like base-labil e fatty acyl groups, or free fatty acid. Release of base-labile lipid radioactivity after 3 hr (associated with antigen) was almost exclusiv ely into the apical media of membrane inserts. Gas-liquid chromatograp hy/mass spec. analyses of purified Sialeams revealed the presence of p almitate (16:0), as well as stearate (18:0) and oleate (18:1) fatty ac yl groups. Our results suggest that fatty acylation of SiaLeams may be co-ordinated with alterations in glycosylation and participate in dir ecting these molecules to the apical surface. Lipid analyses were cons istent with ganglioside chaperonage of SiaLeams to the apical surface, where N-fatty-acytated gangliosides remain for the most part integrat ed into the bilayer, but some oxyester or thioester bonds may be cleav ed to permit release of SiaLeams to the apical medium. (C) 1995 Wiley- Liss, Inc.