Vitamin D3 compounds offer an alternative approach to anti-hormonal therapies for human breast cancer. l,25-Dihydroxyvitrnain D3 1 ,25-D3 acts through the nuclear vitamin D receptor VDR, a phosphoprotein, which can be phosphorylated by protein kinase C PKC and casein kinase II to modulate gene expression. Our lab has shown that 1 ,2S-D3 induces apoptosis in MCF-7 cells by disruption of initochondrial function which is associated with Bax translocation to mitochondria, cytocinome C release, and production of reactive oxygen species ROS. Fluorescence microscopy showed direct evidence of Bax translocation to the initochondria after treatment with 1 ,25-D3 in MCF-7 cells. TPA a PKC activator did not induce cytocinome C release or Bax translocation, thus demonstrating that it has no effect on mitochondria. However, when the MCF-7D3Res cells a vitamin D3-resistant variant were treated with 1 ,25-D3 in the presence of TPA, the cells displayed apoptotic morphology and redistribution of both cytochrome C and Bax. The observation that the phorbol ester TPA can sensitize the vitamin D3-resistant variant to the effects of 1 ,25-D3 suggests an important role for phosphorylation in dictating sensitivity to vitamin D3 mediated apoptosis. This study indicates that the effects of 1 ,25-D3 on mitochondrial disruption might be sensitized through activators of PKC.