Effect of synthetic sialyl 2 -> 1 sphingosine and other glycosylsphingosines on the structure and function of the "glycosphingolipid signaling domain(GSD)" in mouse melanoma B16 cells

Mouse melanoma B16 cells are characterized by a high concentration of GM3 g anglioside, which has been identified as a melanoma-associated antigen and is present as a clustered microdomain organized with major signal transduce rs, c-Src, small G-protein (Rho A), and focal adhesion kinase (FAK), to for m a "glycosphingolipid signaling domain" or "glycosignaling domain" (GSD) s eparable from cholesterol- and caveolin-enriched microdomain, "caveolae." C holesterol-binding reagents, filipin and nystatin, disrupt the structure an d function of caveolae, but have no effect on GSD function [Iwabuchi, K., e t al. (1998) J. Biol. Chem. 273, 33766-33773]. In this study, we searched f or compounds which disrupt the structure and function of GSD in B16 cells. Such compounds should have structural features analogous to those of GM3, d estroy or reduce clustering of GM3 in GSD, and inhibit GM3-dependent adhesi on and signaling. The simplest compound so far found with these properties is sialyl alpha 2-->1 sphingosine (Sph). We describe the synthesis of this compound and its analogues, and their effects on GM3 expression pattern and GSD function, in comparison with effects of lyso-GM3 and other lyse compou nds, in B16 cells. Incubation of B16 cells with 0.5-10 mu M sialyl alpha 2- ->1 Sph or 1-5 mu M lyso-GM3 reduced GM3 clustering and GM3-dependent adhes ion, and inhibited adhesion-dependent cellular FAK activity. The c-Src acti vation response of GSD isolated from B16 cells was inhibited strongly by si alyl alpha 2-->1 Sph. Substitution of the Sph amino group with a chloroacet yl or N,N-dimethyl group strongly reduced the inhibitory effect of sialyl a lpha 2-->1 Sph on GM3-dependent adhesion, FAK, and c-Src response. Other ly se compounds such as lyso-phosphatidylcholine, galactosyl-Sph (psychosine), and lactosyl-Sph at 0.5-10 mu M did not show the same effect as sialyl alp ha 2-->1 Sph. Thus, adhesion coupled with signal transduction, initiated by clusters of GM3 in GSD, is blocked by sialyl alpha 2-->1 Sph or lyso-GM3. Analogues with N-substitution of Sph in sialyl alpha 2-->1 Sph, other lyso- phospholipids, and galactosyl- or lactosyl-Sph did not block such adhesion, coupled with activation of c-Src and FAK.