Glycosphingolipid-Glycosphingolipid Interaction: A Model for a New Type of Cell Recognition System

Abstract

Endogenous lectins (including selectins) are expected to define carbohydrate(CHO)-dependent adhesion between a cell showing surface expression of lectin and a different cell expressing the target CHO epitope. This mechanism, however, is apparently not universal because surface expression of lectins is seen only in certain cell types, and the structures defined by known lectins and selectins are highly restricted. In contrast, surface expression of a great number of CHOs [including glycosphingolipids(GSLs)] is known to change dramatically during ontogenesis and oncogenesis, and cell adhesion occurring at defined stages of ontogenesis can be inhibited by specific CHOs or GSLs. Our early studies focused on the mechanism of tight cell adhesion(compaction) of morula-stage embryo cells, as mediated by Le^x, and led to the discovery of Le^x-Le^x interaction in the presence of bivalent cation. Later, more systematic studies on adhesion of various GSLs (incorporated in [¹⁴C]cholesterol-liposomes) to GSLs coated on plastic plates showed strong H-H, H-Le^y, GM₃-Gg₃Cer, GM₃-LacCer, and sulfatide-GalCer interaction. We also demonstrated (i) specific interaction between GM₃-expressing B16 mouse melanoma cells and Gg₃Cer-expressing L5178 mouse leukemia cells based on GM₃-Gg₃Cer interaction, and (ii) adhesion of B16 variants to endothelial cells(ECs) based on GM₃-LacCer interaction. Relative degree of GM₃ expression in four B16 variants, and degree of adhesion to ECs or LacCer-coated plates, was found to be in the same order as degree of metastatic potential of the B16 variants. Initial adhesion of B16 cells to non-activated ECs is mediated by GM₃-LacCer interaction. Experimental data from a dynamic flow adhesion system supported this hypothesis. The GSL-GSL interactions demonstrated in these experimental systems may reflect a generalized phenomenon, i.e., involvement of specific CHO-CHO interaction in the earliest stages of cell-cell recognition and adhesion, which triggers subsequent (and stronger) adhesion events mediated by integrin and immunoglobulin family receptors.