Metastasis of
circulating tumor cells requires a multistep cascade of events initiated by adhesion of
tumor cells to the vascular endothelium of involved tissues. This process occurs under the forces of blood flow and is promoted by adhesion molecules specialized to interact under shear conditions. The endothelial molecule
E-selectin is a major mediator of these adhesive events, and there is strong evidence that
E-selectin receptor-
ligand interactions contribute to the formation of
metastasis. However, little is known about the identity of
E-selectin ligand(s) expressed on
cancer cells. To address this issue, we did SDS-PAGE analysis of
membrane proteins, metabolic inhibition studies, and blot rolling assays of LS174T, a colon
carcinoma cell line known to interact with
E-selectin under physiologic flow conditions. Our studies show that LS174T cells express the hematopoietic cell E/
L-selectin (HCELL) glycoform of CD44, which functions as a high-affinity
E-selectin glycoprotein ligand on these cells. However, in contrast to the HCELL glycoform on human hematopoietic progenitor cells, which expresses
carbohydrate-binding determinant(s) for
E-selectin primarily on N-
glycans of standard CD44, the relevant determinant(s) on LS174T cells is expressed on O-
glycans and is predominantly found on variant
isoforms of CD44 (CD44v). Our finding that
tumor-associated CD44 splice variant(s) express
E-selectin ligand activity provides novel perspectives on the biology of CD44 in
cancer metastasis.