Signaling Mechanisms of Vitamin D3 Mediated Apoptosis in MCF-7 Breast Cancer Cells

The active metabolite of vitamin D3, 1,25D, induces growth arrest and apoptosis in MCF-7 breast cancer cells through a mechanism involving mitochondrial disruption, reactive oxygen species (ROS) generation and autophagy. The purpose of these studies was to further investigate the signaling pathways impacted by 1,25D in MCF-7 cells and a 1,25D resistant variant, MCF-7DR. Characterization of MCF-7DR cells revealed lower expression and activity of VDR which probably underlies resistance of these cells to 1,25D. Disruption of redox balance and ROS generation is characteristic of 1,25D induced apoptosis in MCF-7 cells. VDUP1 is a 1,25D regulated protein that binds to the small redox protein thioredoxin, which is responsible for neutralizing ROS. In MCF-7 cells 1,25D altered thioredoxin localization and redox status. To further elucidate the pathways affected by 1,25D a large screen for apoptosis related proteins was carried out. This screen identified Grim19 as a 1,25D regulated protein in MCF-7 cells. In follow-up analysis 1,25D significantly reduced Grim19 protein in MCF-7 cells and induced its nuclear localization. Grim19 binds to and negatively regulates Stat3, a member of the signal transducer and activator of transcription family of proteins. 1,25D treatment reduced Stat3 phosphorylation at tyrosine 705, which is necessary for Stat3 activity. Through these studies a newly characterized protein, Grim19, was identified as a 1,25D target and the Stat3 pathway was implicated in 1,25D mediated apoptosis and signaling.