Phenotypic changes induced in human breast cancer cells by overexpression of manganese-containing superoxide dismutase

Human manganese containing superoxide dismutase (MnSOD) cDNA was transfected into a human breast cancer cell line (MCF-7) in order to examine the effect of increased functional MnSOD on the cellular phenotype. A MnSOD-overexpressing clone was compared to control vector-transfected cells and to wild type MCF-7 cells. Southern blotting indicated incorporation of MnSOD cDNA into genomic DNA in the MnSOD overexpressing cell line. The MnSOD overexpressing cell line showed a 5.7-fold increase in MnSOD activity compared to wild type MCF-7 cells. Similar increases in MnSOD immunoreactive protein and mRNA levels were observed by Western and Northern blotting as well as using RT-PCR. The plating efficiency of cells grown in different concentrations of serum (1 to 20%) was decreased in the MnSOD overexpressing cell line. The clonogenic fraction in soft agar culture was also decreased after MnSOD cDNA transfection. When inoculated in nude mice, tumor growth was markedly inhibited in MnSOD overexpressing cells compared to wild type MCF-7 cells or plasmid control cells. These results support the hypothesis that increased MnSOD expression suppresses the malignant phenotype of human breast cancer cells and suggests that the MnSOD gene is a tumor suppressor gene in human breast cancer.