A peptide mimic of E-selectin ligand inhibits sialyl Lewis X-dependent lung colonization of tumor cells

Selectins bind to carbohydrate ligands in a calcium-dependent manner and pl ay critical roles in host defense and possibly in tumor metastasis, To isol ate peptides that mimic E-selectin ligands, we screened a phage peptide lib rary using E-selectin as a target molecule. This attempt unexpectedly faile d, probably because the binding affinity of E-selectin to its ligand is low . We then took an approach that is analogous to the isolation of anti-idiot ype antibodies and were able to isolate peptides that bound to anticarbohyd rate antibodies recognizing E-selectin ligands. These peptides, enriched fo r their binding to anti-Lewis A antibody, were found to bind to E-, P- and L-selectins in a calcium-dependent manner. Phage harboring the identified p eptide IELLQAR and synthetic peptides having the same sequence inhibited th e binding of sialyl Lewis X or sialyl Lewis A oligosaccharides to E-selecti n. The adhesion of HL-60 and B16 melanoma cells expressing sialyl Lewis X t o E-selectin was also inhibited by the phage-displaying IELLQAR peptide. Mo reover, i.v. injected IELLQAR peptide inhibited the lung colonization of mo use B16 melanoma and human lung tumor cells expressing sialyl Lewis X. Thes e results demonstrate that it is possible to isolate peptides mimicking car bohydrate ligands by screening the peptides for binding to anticarbohydrate antibodies and then using them to inhibit carbohydrate-dependent experimen tal tumor metastasis.