Role of Mitochondria and Caspases in Vitamin D-mediated Apoptosis of MCF-7 Breast Cancer Cells

Abstract

Vitamin D3 compounds are currently in clinical trials for human breast cancer and offer an alternative approach to anti-hormonal therapies for this disease. 1α,25-Dihydroxyvitamin D3 (1α,25(OH) 2D3), the active form of vitamin D3, induces apoptosis in breast cancer cells and tumors, but the underlying mechanisms are poorly characterized. In these studies, we focused on the role of caspase activation and mitochondrial disruption in 1α,25(OH)2D 3-mediated apoptosis in breast cancer cells (MCF-7) in vitro. The effect of 1α,25(OH)2D3 on MCF-7 cells was compared with that of tumor necrosis factor α, which induces apoptosis via a caspase-dependent pathway. Our major findings are that 1α,25(OH) 2D3 induces apoptosis in MCF-7 cells by disruption of mitochondrial function, which is associated with Bax translocation to mitochondria, cytochrome c release, and production of reactive oxygen species. Moreover, we show that Bax translocation and mitochondrial disruption do not occur after 1α,25(OH)2D3 treatment of a MCF-7 cell clone selected for resistance to 1α,25(OH)2D 3-mediated apoptosis. These mitochondrial effects of 1α,25(OH)2D3 do not require caspase activation, since they are not blocked by the cell-permeable caspase inhibitor z-ValAla-Asp-fluoromethylketone. Although caspase inhibition blocks 1α,25(OH)2D3-mediated events downstream of mitochondria such as poly(ADP-ribose) polymerase cleavage, external display of phosphatidylserine, and DNA fragmentation, MCF-7 cells still execute apoptosis in the presence of z-Val-Ala-Asp-fluoromethylketone, indicating that the commitment to 1α,25(OH)2D3-mediated cell death is caspase-independent.