Novel peptide ligands for integrin α₄β₁ overexpressed in cancer cells

Using the "one-bead one-peptide" combinatorial technology, a library of random cyclic octapeptides and nonapeptides, consisting of natural and unnatural amino acids, was synthesized on polystyrene beads. This library was used to screen for peptides that promoted attachment and proliferation of bronchioloalveolar carcinoma cells (H1650), employing a "cell growth on bead" assay. Consensus peptide sequences of cNIeDXXXXc and cXNIeDXXXXc (where NIe is norleucine) were identified. With alanine scanning and site-directed deletion, a typical ligand consisted of a motif of -NIeDI/V/NIe- with two flanking cysteines. These peptide ligands were specific for promoting cell attachment of the H1650 cells and the cells of lymphoid cancers (Jurkat and Raji) but not other selected human cell lines of lung cancer and fibroblast. In an antibody blocking assay, integrin α₄β₁ which was overexpressed in H1650, Jurkat, and Raji, was identified as a putative receptor for these peptide ligands. Using Chinese hamster ovary cells transfected with either wild-type or mutant integrin ≤₄, a critical binding site for these peptides was localized to the glycine residue at position 190 of integrin α₄.