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 alphainfinity 5beta infinity 1wedge alpha infinity: IIb& beta infinity 3, and that for CS1 has been defined as alpha infinity 4& beta infinity 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 infinity 3& beta 1 infinity, alpha4 & beta infinity 1, and alpha infinity 5 beta 1, and not expression of alpha infinity 2 beta infinity 1. Adhesion of U937 cells to fibronectin-coated wells is specific and is inhibited by anti-alpha infinity 4 beta infinity 1 and anti- alpha infinity 5 beta infinity 1 monoclonal antibodies. The IC-50 for anti-alpha infinity 5 beta infinity 1 antibody was almost a log lower than the value for anti-alpha infinity 4 beta infinity 1 antibody. These results demonstrated that interactions of RGDS and CS1 sequence of fibronectin with alpha infinity 5 beta infinity 1 and alpha infinity 4 beta infinity 1& on U937 cells were 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.