Inhibition of AP-1 and NF-κB by manganese-containing superoxide dismutase in human breast cancer cells

One of the primary antioxidant enzymes, manganese-containing superoxide dismutase (MnSOD), has shown the ability to reverse malignant phenotypes in a variety of human tumor cells that are low or absent in MnSOD expression. We have observed that overexpression of human MnSOD in human breast cancer MCF-7 cells inhibits tumor growth both in vitro and in vivo. The signaling pathway underlying the MnSOD induced tumor suppression is unknown. We demonstrate here that transcriptional and DNA binding ability of AP-1 and NF-kappa B, but not SP-1, were inhibited (by 50%) in the MCF-7 cell line overexpressing MnSOD, When transiently expressing, MnSOD inhibited AP-I but increased NF-kappa B transactivation, which can be abolished by sodium pyruvate, a hydrogen peroxide scavenger. To analyze the target genes responsible for MnSOD-induced tumor suppression, genes related to tumor growth and responsive to AP-1 or NF-kappa B were analyzed. AP-1 responsive collagenase I, stromelysin I, and NF-kappa B responsive IL-1 and IL-6 were down-regulated in the MnSOD stable transfectants compared to the control cell lines. Since TPA induces differentiation in human breast cancer cells and up-regulates MnSOD gene in HeLa cells, MnSOD expression and AP-1 and NF-kappa B activity were measured under TPA treatment. The results showed that TPA induced endogenous MnSOD expression and inhibited both AP-1 and NF-kappa B, Together, these results suggest that tumor suppression by overexpressing MnSOD is related to a modulation of AP-1 and NF-kappa B, which causes a downregulation of genes responsible for tumor malignant phenotype.