Breast cancer cells with decreased fibronectin-integrin adhesion strength attenuate the malignancy of its microenvironment.

Fibronectin (FN), a key component of many solid tumors microenvironment, increases their mechanical stiffening and promotes cancer progression and metastasis. We studied Fn1syn/syn;PyMT mice carrying the breast cancer-inducing PyMT oncogene and a Fn1 gene with a nonfunctional synergy site (FNsyn), which impairs the reinforcement of the α5β1 integrin adhesion to the FN-RGD motif. Here, we show that Fn1syn/syn;PyMT mice exhibit delayed tumor onset in the mammary glands and reduced lung metastasis associated with decreased tumor vascularization, reduced cancer cell proliferation and lower abundance of ECM fibrillar proteins within the tumor stroma. Isolated cancer-associated fibroblasts with high αSMA expression levels (H-αSMA CAFs) and tumor cells reveal that FNsyn reduces their response when the substrate stiffness rises. Consequently, Fn1syn/syn CAFs express low αSMA and reduced Lysyl-2 and -3 oxidases leading to the assembly of loose fibrotic ECMs. Moreover, luminal breast cancer cells grown as organoids showed reduced incidence of invasive traits on stiff FNsyn substrates. Importantly, these key alterations were strongly correlated with the phenotypic changes observed in the stroma of Fn1syn/syn;PyMT tumors, contributing to the establishment of a microenvironment characterized by a reduced abundance of immunosuppressive cytokines and an increased infiltration of immune cells. Therapeutic targeting the FN synergy site is sufficient to reprogram the tumor microenvironment toward a less fibrotic and immunoactive state.