Differential effects of 1,25-dihydroxyvitamin D-3 and tetradecanoylphorbol acetate on cell cycle and apoptosis of MCF-7 cells and a vitamin D-3-resistant variant

1,25-Dihydroxyvitamin D-3 (1,25-(OH)(2)D-3), the active form of vitamin D-3, and tetradecanoylphorbol acetate (TPA) are potent negative growth regulators of breast cancer cells. In this study, we compared the mechanism of action of these two compounds in MCF-7 cells and a Vitamin D-3-resistant variant (MCF-7(DSRes)). In parental MCF-7 cells, 1,25-(OH)(2)D-3 induced morphological and biochemical markers of apoptosis (chromatin and nuclear matrix condensation and DNA fragmentation), whereas TPA induced growth arrest without apoptosis. Both 1,25-(OH)(2)D-3 and TPA independently up-regulated the vitamin D receptor, p21, and the hypophosphorylated form of retinoblastoma (Rb) protein. The growth regulatory effects of 1,25-(OH)(2)D-3 and TPA did not correlate with induction of p53 protein expression. When both compounds were added simultaneously, synergistic effects on MCF-7 cell number were observed, and cell cycle regulatory proteins were down-regulated. The MCF-7(D3Res) cells, which are not sensitive to 1,25-(OH)(2)D-3, were growth inhibited by TPA, and TPA partially sensitized MCF-7(D3Res) cells to the growth inhibitory effects of 1,25(OH)(2)D-3. In MCF-7(D3Res) cells, 1,25-(OH)(2)D-3 treatment had minimal effects on p21 or Rb protein expression, whereas TPA downregulated Rb protein and transiently up-regulated p21. These studies indicate dissociation between the pathways triggered by 1,25-(OH)(2)D-3 and TPA, which mediate growth regulation in MCF-7 cells. Because both compounds induce growth arrest, but only 1,25(OH)(2)D-3 mediates apoptosis, we conclude that cell cycle arrest is not sufficient to trigger cell death of MCF-7 cells, and that 1,25(OH)(2)D-3 generates distinct signals which lead to induction of apoptosis in breast cancer cells.