A role of chondroitin sulfate glycosaminoglycan binding site in α₄β₁ integrin-mediated melanoma cell adhesion

We have previously reported that α₄β₁ (but not α₅β₁) integrin- mediated melanoma cell adhesion is inhibited by removal of cell surface chondroitin sulfate glycosaminoglycan (CSGAG), suggesting that melanoma chondroitin sulfate proteoglycan plays a role in modulating the adhesive function of α₄β₁ integrin. In the current study, we demonstrated that α₄β₁ integrin binds to CSGAG. We have identified a peptide from within α₄ integrin termed SG1 (KKEKDIMKKTI) that binds to cell surface melanoma chondroitin sulfate proteoglycan, indicating that SG1 represents a CSGAG binding site within the α₄ integrin subunit. Soluble SG1 inhibits α₄β₁ integrin-mediated human melanoma cell adhesion to CS1. Polyclonal antibody generated against the peptide inhibits melanoma cell adhesion to CS1, and the inhibition is reversed by Mn²⁺ and an activating monoclonal antibody anti- β₁ (8A2). Additionally, pretreatment of cells with anti-SG1 IgG inhibits the expression of the monoclonal antibody 15/7 epitope in the presence of soluble CS1 peptide, suggesting that anti-SG1 IgG prevents ligand binding by α₄β₁ integrin. These results demonstrate that α₄β₁ integrin interacts directly with CSGAG through SG1 site, and that this site can affect the ligand binding properties of the integrin.