Glycobiology of Sialyl 6-Sulfo Lewis x, a New Carbohydrate Ligand for Selectins

Selectin is a family of cell adhesion molecules expressed on endothelial cells, leukocytes and platelets. It is implicated in extravasation of leukocytes, homing of lymphocytes, and infiltration or metastasis of malignant cells including leukemia cells and cancer cells. Sialyl Lewis x, sialyl Lewis a and several other carbohydrate determinants have been known to serve as carbohydrate ligands for selectins. The most-recently described selectin ligand is sialyl 6-sulfo Lewis x. This determinant was first described as a major lymphocyte homing receptor for L-selectin expressed on high endothelial venules of human lymph nodes, and later found to be expressed on certain subsets of lymphocytes and on some cells of epithelial origin, calso serving as a ligand for E- and/or P-selectin. Expression of sialyl 6-sulfo Lewis x is regulated differently from that of conventional sialyl Lewis x in that it is metabolized through a distinct metabolic pathway involving the cyclization of sialic acid moiety. In this article we will review the distribution, csynthesis and metabolic fate of this new selectin ligand, and discuss its physiological significance.