Human plasma fibronectin mediates adhesion of U937 cells by RGD and CS1

Fibronectin specifically binds to U937 cells (monocytic cell line) in a dose-dependent manner. The specific receptors for the RGD sequence have been identified as alpha(5)beta(1) and alpha(IIb)beta(2), and that for CS1 has been defined as alpha(4)beta(1). RGDS, CS1 peptide, and two peptides together showed similar inhibitory activities on this adhesion, while the 29-kD dispase-digested fragment of the C-terminal heparin-binding domain did not. Thus, the adhesion of fibronectin to U937 cells is mainly mediated by RGDS in the cell-binding domain and CS1 in the alternatively spliced region. Flow cytometry using monoclonal antibodies revealed expressions of alpha(3)beta(1), alpha(4)beta(1), and alpha(5)beta(1), and not expression of alpha(2)beta(1). Adhesion of U937 cells to fibronectin-coated wells is specific and is inhibited by anti-alpha(4)beta(1), and anti-alpha(5)beta(1) monoclonal antibodies. The IC50 for anti-alpha(5)beta(1) antibody was almost a log lower than the value for anti-alpha(4)beta(1) antibody. These results demonstrated that interactions of RGDS and CS1 sequence of fibronectin with alpha(5)beta(1) and alpha(4)beta(1) on U937 cells mere required for the adhesion of U937 cells to fibronectin. These results may provide further information to understand the mechanism(s) of tumor cell adhesion and atherogenesis.