Dissociation of vitamin D₃ and anti-estrogen mediated growth regulation in MCF-7 breast cancer cells

Our studies have identified 1,25(OH)₂D₃ as a coordinate regulator of proliferation and apoptosis in breast cancer cells. In MCF-7 cells, 1,25(OH)₂D₃ down regulates the estrogen receptor (ER), suggesting that the effects of 1,25(OH)₂D₃ may be linked to disruption of estrogen regulated survival signals. Although studies have demonstrated that 1,25(OH)₂D₃ inhibits growth of ER negative breast cancer cells, previous data were generated by comparison of cell lines derived from heterogeneous human tumors and harboring diverse genetic alterations. To provide more conclusive evidence for independent growth regulatory pathways mediated by antiestrogens and 1,25(OH)₂D₃, we examined vitamin D₃ sensitivity in MCF-7 cells selected for resistance to ICI 182,780 (Zeneca, Macclesfield, UK). The clones we selected for resistance to ICI 182,780 retain functional VDR and undergo 1,25(OH)₂D₃ mediated growth arrest and apoptosis, in vitro and in vivo, despite loss of estrogen dependence. Cell cycle data indicate that treatment of parental or anti-estrogen resistant MCF-7 clones with 1,25(OH)₂D₃, in the presence or absence of ICI 182,780, increases the percentage of cells in G₀/G₁ while reducing the number of cells in S phase. In addition, 1,25(OH)₂D₃ induces characteristic features of apoptosis, including DNA fragmentation, in both parental and anti-estrogen resistant MCF-7 cells. Furthermore, we report that cells selected for vitamin D₃ resistance retain sensitivity to ICI 182,780 mediated growth arrest and apoptosis. This work emphasizes that vitamin D₃ compounds and anti-estrogens trigger growth arrest and apoptosis in breast cancer cells by distinct mechanisms, and that breast cancer cell sensitivity to 1,25(OH)₂D₃ is not diminished during the progression to estrogen independence.