Acquisition of stable inducible up-regulation of nuclear factor-κB by tumor necrosis factor exposure confers increased radiation resistance without increased transformation in breast cancer cells

High-grade breast cancers are better adapted to hypoxia and more resistant to chemotherapy and radiotherapy. Constitutive activation of the transcription factor nuclear factor-kappa B (NF-kappa B) increases in breast tumors and in breast cancer cell lines, where it promotes chemoradiation resistance, in part by activation of antiapoptotic genes. The role for up-regulation of NF-kappa B in breast cancer progression is less clear. Here, we first show that whereas the constitutive activity of NF-kappa B is incrementally elevated from immortalized breast epithelial to frank transformed invasive ductal breast cancer cell lines (similar to 3-fold, +/- 0.1-fold, P < 0.05), inflammatory cytokine-inducible activity is further increased (up to 9-fold, +/- 0.9-fold, P < 0.05). We then show that inhibition of NF-kappa B activity selectively sensitizes transformed but not immortalized cells to killing by ionizing radiation or low levels of tumor necrosis factor (TNF) by up to 10-fold (+/- 1-fold, P < 0.05) but has little effect on hypoxia-mediated cell death. Prolonged cultivation of immortalized and partially transformed cells in TNF selected for cells displaying stable constitutive and strongly inducible overexpression of NF-kappa B even in the absence of TNF. Stable acquisition of increased NF-kappa B activity conferred resistance to ionizing radiation or inflammatory cytokines, which was dependent on elevated NF-kappa B activity, but had no effect on transformation potential measured by in vitro and in vivo parameters. Thus, TNF and possibly other inflammatory cytokines in the tumor-stroma matrix likely select for breast cancer cells that stably overexpress NF-kappa B, leading to greater cancer cell survival. Greater cell survival despite increased genomic injury may permit increased acquisition of malignant genetic alterations as well as resistance to chemoradiation therapy.