Functional glycoconjugates involved in cellular interaction

Abstract

We first examined the involvement of the complex sphingolipids in cell-substratum adhesion using GM-95, a mutant cell line deficient in glycosphingolipids (GSLs) due to the lack of ceramide glucosyltransferase activity. We determined the adhesion of the mutant cells and stable transfectants expressing GSLs, which were established by transfection of GlcT-1 cDNA into GM-95 cells under neutral sphingomyelinase (sm) treatment. We confirmed that complex sphingolipids play critical roles in cell-substratum adhesion, and the presence of either GSLs or SM is sufficient for the adhesion. We also investigated intracellular signaling (glycosignaling) mediated by endogenous GM1a involved in the neuronal differentiation of PC12 cells using the cholera toxin B subunit (CTB) that specifically binds to ganglioside GM1a. Treatment with CTB induced neuron-like differentiation of PC12 cells. Biochemical analyses demonstrated that the tyrosine phosphorylation induced by CTB was responsible for neuron-like differentiation of PC12 cells and that the MEK-ERK cascade is a part of the biological signals mediated by endogenous ganglioside GM1a on PC12 cells. We further demonstrated that glycosignaling is mediated through a high-affinity ligand, PSGL-1, for P-selection on neutrophils. In this case, engagement of PSGL-1 on the cell surface strongly induced tyrosine phosphorylation of several cellular proteins including ERKs and activated a canonical MAP kinase pathway. Tyrosine phosphorylation induced by engagement of PSGL-1 is responsible for the secretion of interleukin-8 from neutrophils, suggesting that PSGL-1-mediated glycosignals are involved in the progression of the inflammatory response. In this review, we mainly discuss the biological and pathological significance of glycoconjugates in relation to the above issues.