Role of mitochondria and caspases in vitamin D-mediated apoptosis of MCF-7breast cancer cells

Vitamin D-3 compounds are currently in clinical trials for human breast can cer and offer an alternative approach to anti-hormonal therapies for this d isease. 1 alpha ,25-Dihydroxyvitamin D-3 (1 alpha ,25(OH)(2)D-3), the activ e form of vitamin D-3, 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 i n 1 alpha ,25(OH)(2)D-3-mediated apoptosis in breast cancer cells (MCF-7) i n vitro. The effect of 1 alpha ,25(OH)(2)D-3 on MCF-7 cells was compared wi th that of tumor necrosis factor alpha, which induces apoptosis via a caspa se-dependent pathway. Our major findings are that 1 alpha ,25(OH)(2)D-3 ind uces apoptosis in MCF-7 cells by disruption of mitochondrial function, whic h is associated with Bax translocation to mitochondria, cytochrome c releas e, and production of reactive oxygen species. Moreover, we show that Bax tr anslocation and mitochondrial disruption do not occur after 1 alpha ,25(OH) (2)D-3 treatment of a MCF-7 cell clone selected for resistance to 1 alpha , 25(OH)(2)D-3-mediated apoptosis. These mitochondrial effects of 1 alpha ,25 (OH)(2)D-3 do not require caspase activation, since they are not blocked by the cell-permeable caspase inhibitor z-Val-Ala-Asp-fluoromethylketone. Alt hough caspase inhibition blocks 1 alpha ,25(OH)(2)D-3-mediated events downs tream of mitochondria such as poly(ADP-ribose) polymerase cleavage, externa l display of phosphatidylserine, and DNA fragmentation, MCF-7 cells still e xecute apoptosis in the presence of z-Val-Ala-Asp-fluoromethylketone, indic ating that the commitment to 1 alpha ,25(OH)(2)D-3-mediated cell death is c aspase-independent.