Synergistic anticancer activity of 1,25-dihydroxyvitamin D3 and immune cytokines:: the involvement of reactive oxygen species

It was previously shown that 1,25-dihydroxyvitamin D-3 (1,25(OH)(2)D-3) enhances the cytotoxic activity of tumor necrosis factor alpha (TNF alpha), doxorubicin and menadione. A feature shared by these anticancer agents is the involvement of reactive oxygen species (ROS) in their action. In this work we found that 1,25(OH)(2)D-3 acted synergistically with interleukin 1 beta (IL-1 beta) or interleukin 6 (IL-6) to inhibit the proliferation of MCF-7 breast cancer cells. The extent of the synergism was maximal at 1 nM, a concentration at which 1,25(OH)(2)D-3, acting singly, only marginally reduced the cell number. The thiol antioxidant, N-acetylcysteine (NAC) abolished the synergism between IL-1 beta or IL-6 and 1,25(OH)(2)D-3, but had only a small protective effect when the cytokines acted alone. NAC and reduced glutathione (GSH) protected MCF-7 cells from cytotoxicity induced both by TNF alpha alone and by TNF alpha and 1,25(OH)(2)D-3. A two-day exposure to TNF alpha caused a 27.7 +/- 3.1% (mean +/- SEM) reduction in GSH content. This effect increased to 46.4 +/- 5.5% by co-treatment with 1,25(OH)(2)D-3 which did not affect GSH levels on it own. We conclude that 1,25(OH)(2)D-3 can act synergistically with anticancer cytokines present in the tumor milieu and that ROS plays a mediatory role in this interaction. (C) 2000 Elsevier Science Ltd. All rights reserved.