We have previously shown that the genotoxin-induced apoptosis in mouse embryo fibroblasts was enhanced by the extracellular matrix protein fibronectin (FN). In the present study, we tested the hypothesis that FN regulates the DNA damage response (DDR) signaling pathways in HCT116 (p53-wt) and HT29 (p53-mut) human colon cancer cells and tumor-derived myofibroblasts. DNA damage recognition mechanisms were analyzed by immunofluorescence staining, cell cycle analysis and immunoblotting addressed at specific molecular sensors and executors involved in the DDR pathways. The results show that FN, but not collagen type IV or Matrigel, initiates and potentiates the DDR to the anticancer drug cisplatin in an α5 integrin and cell cycle-dependent manner (S and G2/M phases) in human colon cancer cells. Accordingly, we demonstrate that adhesion of HCT116 cells to FN upregulated the expression of α5 integrin FN receptors at the cell surface. These FN-induced DDR pathways include the concerted phosphorylation of histone H2AX on Ser139 detected as nuclear foci (γ-H2AX, 15 and 25 kDa forms), of ataxia telangiectasia mutated (ATM-Ser1981), checkpoint kinase 2 (CHK2-Thr68, 62 and 67 kDa) and p53-Ser15. These FN-induced γ-H2AX signals were interrupted or attenuated by selective inhibitors acting on the DDR pathway kinases, including wortmannin (targeting the phosphatidylinositol-3-kinase-related protein kinases, PIKK), KU55933 (ATM), NU7026 (DNA-dependent protein kinase catalytic subunit, DNA-PK-cs) and SP600125 (JNK2, stress activated protein kinase/c-Jun N-terminal kinase-2). Adhesion to FN also potentiated the γ-H2AX signals and the cytotoxic effects of cisplatin in human colon tumor-derived myofibroblasts. These data provide evidence that FN promotes DNA damage recognition and chemosensitization to cisplatin via the potentiation of the DNA damage signaling responses in human colon cancer cells and tumor-derived myofibroblasts.

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