1,25-dihydroxyvitamin D-3 enhances the susceptibility of breast cancer cells to doxorubicin-induced oxidative damage

1,25-Dihydroxyvitamin D-3 (1,25(OH)(2)D-3), the hormonal form of vitamin D, has anticancer activity in vivo and in vitro. Doxorubicin exerts its cytot oxic effect on tumor cells mainly by two mechanisms: (a) generation of reac tive oxygen species (ROS); and (b) inhibition of topoisomerase II. We studi ed the combined cytotoxic action of 1,25(OH)(2)D-3 and doxorubicin on MCF-7 breast cancer cells. Pretreatement with 1,25(OH)(2)D-3 resulted in enhance d cytotoxicity of doxorubicin, The average enhancing effect after a 72-h pr etreatment with 1,25(OH)(2)D-3 (10 nM) followed by a 24-h treatment with 1 mu g/ml doxorubicin was 74 +/- 9% (mean +/- SE). Under these experimental c onditions, 1,25(OH)(2)D-3 on its own did not affect cell number or viabilit y. 1,25(OH)(2)D-3 also enhanced the cytotoxic activity of another ROS gener ating quinone, menadione, but did not affect cytotoxicity induced by the to poisomerase inhibitor etoposide, The antioxidant N-acetylcysteine slightly reduced the cytotoxic activity of doxorubicin but had a marked protective e ffect against the combined action of 1,25(OH)(2)D-3 and doxorubicin. These results indicate that ROS are involved in the interaction between 1,25(OH)( 2)D-3 and doxorubicin. 1,25(OH)(2)D-3 also increased doxorubicin cytotoxici ty in primary cultures of rat cardiomyocytes. Treatment of MCF-7 cells with 1,25(OH)(2)D-3 alone markedly reduced the activity, protein, and mRNA leve ls of the cytoplasmic antioxidant enzyme Cu/Zn superoxide dismutase, which indicated that the hormone inhibits its biosynthesis. This reduction in the antioxidant capacity of the cells could account for the synergistic intera ction between 1,25(OH)(2)D-3 and doxorubicin and may also suggest increased efficacy of 1,25(OH)(2)D-3 or its analogues in combination with other ROS- generating anticancer therapeutic modalities.