1,25-dihydroxyvitamin D-3 and all-trans-retinoic acid sensitize breast cancer cells to chemotherapy-induced cell death

We investigated the capacity of 1,25-dihydroxyvitamin D-3 [1,25(OH)(2)D-3] and all-trans-retinoic acid (ATRA) to sensitize three breast cancer cell li nes to the cell killing effects of paclitaxel (Taxol) and Adriamycin, two c hemotherapeutic agents commonly used in the treatment of breast cancer. In tissue culture colony assays 1,25(OH)(2)D-3 and ATRA were synergistic in in hibiting the clonogenicity of MCF-7 and T-47D cells that expressed estrogen receptor; vitamin D receptor; retinoic acid receptors (RARs) alpha, beta, and gamma; and retinoid X receptors alpha, beta, and gamma but were not add itive in MDA-MB-231 cells that lacked expression of estrogen receptor, RAR alpha, and RAR beta. The hormones used individually or in combination induc ed up to 40-50% cell death by a trypan blue exclusion assay in a dose-depen dent manner up to concentrations of 10(-7) M in MCF-7 and T-47D cells, more modestly in MDA-MB-231 cells, and not at all in MCF-IO and MCF-12 nontrans formed mammary epithelial cells. Pretreating the cancer cell lines with 1,2 5(OH)(2)D-3 and ATRA individually or in combination for 3 days prior to a 1 -h incubation with paclitaxel or Adriamycin decreased the ED,, for inhibiti on of colony formation or for cell death by trypan blue by up to 2 logs for paclitaxel and up to 1 log fur Adriamycin in all three cell lines but had no effect on chemotherapy-induced MCF-12 cell death. The effects of the hor mones were synergistic with those of the chemotherapy agents in all of the breast cancer cell lines, generally at the higher concentrations. Cell deat h took place by apoptosis, To determine one potential reason for the greate r potentiation of the effects of paclitaxel than those of Adriamycin, we de termined the effects of preincubation of MCF-7 cells on paclitaxel-induced phosphorylation of Bcl-2, Pretreatment of MCF-7 cells with either 1,25(OH)( 2)D-3 or ATRA increased the phosphorylation of Eel-2 by variable concentrat ions of paclitaxel, These data suggest that pretreatment of breast cancer w ith 1,25(OH)(2)D-3 or ATRA lowers the threshold For cell killing by chemoth erapy agents and may provide a novel treatment option for this disease.