Fibronectin polymerization stimulates cell growth by RGD-dependent and -independent mechanisms

Many aspects of cell behavior are regulated by cell-extracellular matrix in teractions, including cell migration and cell growth, We previously showed that the addition of soluble fibronectin to collagen-adherent fibronectin-n ull cells enhances cell growth, This growth-promoting effect of fibronectin depended upon the deposition of fibronectin into the extracellular matrix; occupancy and clustering of fibronectin-binding integrins was not sufficie nt to trigger enhanced cell growth, To determine whether the binding of int egrins to fibronectin's RGD site is required for fibronectin-enhanced cell growth, the ability of fibronectin lacking the integrin-binding RGD site (F N Delta RGD) to promote cell growth was tested, FN Delta RGD promoted cell growth when used as an adhesive substrate or when added in solution to coll agen-adherent fibronectin-null cells. Addition of FN Delta RGD to collagen- adherent fibronectin-null cells resulted in a 1.6-1.8x increase in cell gro wth in comparison with cells grown in the absence of fibronectin, The growt h-promoting effects of FN Delta RGD and wild-type hbronectin were blocked b y inhibitors of fibronectin polymerization, including the anti-fibronectin antibody, L8 In addition, FN Delta RGD-induced cell growth was completely i nhibited by the addition of heparin, and was partially blocked by either he paritinase-treatment or by addition of recombinant fibronectin heparin-bind ing domain, Heparin and heparitinase-treatment also partially blocked the g rowth-promoting effects of wild-type fibronectin, as well as the deposition of wild-type fibronectin into the extracellular matrix, These data suggest that ceh surface heparan-sulfate proteoglycans contribute to the growth-pr omoting effects of FN Delta RGD and wild-type fibronectin, Addition of hepa rin, treatment with heparitinase, or incubation with monoclonal antibody L8 all inhibited the formation of short linear FN Delta RGD fibrils on the ce ll surface, Inhibitory pi integrin antibodies had no effect on FN Delta RGD fibril formation, FN Delta RGD-induced cell growth, or cell adhesion on FN Delta RGD-coated substrates. These data suggest that fibronectin fibril fo rmation can promote cell growth by a novel mechanism that is independent of RGD-integrin binding, and that involves cell surface proteoglycans.