Dual functions of autophagy in the response of breast tumor cells to radiation

Abstract

In MCF-7 breast tumor cells, ionizing radiation promoted autophagy that was cytoprotective; pharmacological or genetic interference with autophagy induced by radiation resulted in growth suppression and/or cell killing (primarily by apoptosis). The hormonally active form of vitamin D, 1,25D₃, also promoted autophagy in irradiated MCF-7 cells, sensitized the cells to radiation and suppressed the proliferative recovery that occurs after radiation alone. 1,25D₃ enhanced radiosensitivity and promoted autophagy in MCF-7 cells that overexpress Her-2/neu as well as in p53 mutant Hs578t breast tumor cells. In contrast, 1,25D₃ failed to alter radiosensitivity or promote autophagy in the BT474 breast tumor cell line with low-level expression of the vitamin D receptor. Enhancement of MCF-7 cell sensitivity to radiation by 1,25D₃ was not attenuated by a genetic block to autophagy due largely to the promotion of apoptosis via the collateral suppression of protective autophagy. However, MCF-7 cells were protected from the combination of 1,25D₃ with radiation using a concentration of chloroquine that produced minimal sensitization to radiation alone. The current studies are consistent with the premise that while autophagy mediates a cytoprotective function in irradiated breast tumor cells, promotion of autophagy can also confer radiosensitivity by vitamin D (1,25D₃). As both cytoprotective and cytotoxic autophagy can apparently be expressed in the same experimental system in response to radiation, this type of model could be utilized to distinguish biochemical, molecular and/or functional differences in these dual functions of autophagy.

Keywords: :

• apoptosis
• autophagy
• breast cancer
• ionizing radiation
• vitamin D₃

Disclosure of Potential Conflicts of Interest

No potential conflicts of interest were disclosed.

Acknowledgments

This work was supported, in part, by American Institute for Cancer Research Grant #06A058-REV. Molly Bristol was supported, in part, by a predoctoral training grant (W81XWH-09-1-0020) from the Department of Defense. Electron microscopy was performed at the VCU—Department of Neurobiology and Anatomy Microscopy Facility, supported, in part, with funding from NIH-NINDS Center core grant P30NS047463. The RFP-LC3 vector was generously provided by Dr. Keith Miskimins at the University of South Dakota and was originally developed by the laboratory of Dr. A.M. Tolkovsky. The MCF7/ATG7^−/− cells were a generous gift from the lab of Dr. Ameeta Kelekar at University of Minnesota, Minneapolis, MN.

Notes

* Although this representative experiment appears to indicate differences in the extent of p62 degradation between IR alone and 1,25D₃ + IR, densitometry analysis and pooling of the data from multiple western blots has determined that the level of p62 degradation is not significantly different between these treatments.