Syndecan-4 signals cooperatively with integrins in a Rho-dependent manner in the assembly of focal adhesions and actin stress fibers

The assembly of focal adhesions and actin stress fibers by cells plated on fibronectin depends on adhesion-mediated signals involving both integrins a nd cell-surface heparan sulfate proteoglycans. These two cell-surface recep tors interact with different domains of fibronectin, To attempt to identify the heparan sulfate proteoglycans involved, we used fibronectin-null (FN-/ -) mouse fibroblasts to eliminate the contribution of endogenous fibronecti n during the analysis. FN-/- fibroblasts plated on the cell-binding domain of fibronectin or on antibodies directed against mouse beta 1 integrin chai ns attach but fail to spread and do not form focal adhesions or actin stres s fibers. When such cells are treated with antibodies directed against the ectodomain of mouse syndecan-4, they spread fully and assemble focal adhesi ons and actin stress fibers indistinguishable from those seen in cells plat ed on intact fibronectin, These results identify syndecan-4 as a heparan su lfate proteoglycan involved in the assembly process. The antibody-stimulate d assembly of focal adhesions and actin stress fibers in cells plated on th e cell-binding domain of fibronectin can be blocked with C3 exotransferase, an inhibitor of the small GTP-binding protein Rho, Treatment of cells with lysophosphatidic acid, which activates Rho, results in full spreading and assembly of focal adhesions and actin stress fibers in fibroblasts plated o n the cell-binding domain of fibronectin. We conclude that syndecan-4 and i ntegrins can act cooperatively in generating signals for cell spreading and for the assembly of focal adhesions and actin stress fibers. We conclude f urther that these joint signals are regulated in a Rho-dependent manner.