1,25-Dihydroxyvitamin D₃ enhances the susceptibility of breast cancer cells to doxorubicin-induced oxidative damage

1,25-Dihydroxyvitamin D₃ (1,25(OH)₂D₃), the hormonal form of vitamin D, has anticancer activity in vivo and in vitro. Doxorubicin exerts its cytotoxic effect on tumor cells mainly by two mechanisms: (a) generation of reactive oxygen species (ROS); and (b) inhibition of topoisomerase II. We studied the combined cytotoxic action of 1,25(OH)₂D₃ and doxorubicin on MCF-7 breast cancer cells. Pretreatment with 1,25(OH)₂D₃ resulted in enhanced cytotoxicity of doxorubicin. The average enhancing effect after a 72-h pretreatment with 1,25(OH)₂D₃ (10 nM) followed by a 24-h treatment with 1 μg/ml doxorubicin was 74 ± 9% (mean ± SE). Under these experimental conditions, 1,25(OH)₂D₃ on its own did not affect cell number or viability. 1,25(OH)₂D₃ also enhanced the cytotoxic activity of another ROS generating quinone, menadione, but did not affect cytotoxicity induced by the topoisomerase inhibitor etoposide. The antioxidant N-acetylcysteine slightly reduced the cytotoxic activity of doxorubicin but had a marked protective effect against the combined action of 1,25(OH)₂D₃ and doxorubicin. These results indicate that ROS are involved in the interaction between 1,25(OH)₂D₃ and doxorubicin. 1,25(OH)₂D₃ also increased doxorubicin cytotoxicity in primary cultures of rat cardiomyocytes. Treatment of MCF-7 cells with 1,25(OH)₂D₃ alone markedly reduced the activity, protein, and mRNA levels 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 interaction between 1,25(OH)₂D₃ and doxorubicin and may also suggest increased efficacy of 1,25(OH)₂D₃ or its analogues in combination with other ROS-generating anticancer therapeutic modalities.