Abstract

A senescence-like growth arrest succeeded by rapid recovery of proliferative capacity is observed in MCF-7 breast tumor cells exposed to fractionated radiation (5 x 2Gy) alone. Exposure to the vitamin D3 analog EB 1089 (100nM) prior to irradiation converts the initial growth arrest response to cell death in part through the inhibition of radiation-induced senescence and promotion of both apoptotic and autophagic cell death. More importantly, EB 1089 was shown to profoundly reduce the rate of recovery following fractionated irradiation. The effect of EB 1089 on the temporal response to radiation is also observed in MCF-7 cells expressing caspase 3, but not in cells where p53 function is abrogated. EB 1089 does not increase radiation-induced DNA damage or inhibit DNA repair, as measured by both the alkaline unwinding assay and 53BP1 foci formation. EB 1089 inhibits radiation-induced down-regulation of myc; however, doxycyclin-induction of myc does not mimic the radiosensitizing effects of EB 1089. EB 1089 increases radiation-induced reactive oxygen species (ROS) generation; however, both free radical scavengers, N-acetyl cysteine (NAC) and reduced glutathione (GSH), fail to attenuate the radio-sensitizing effects of EB 1089. These studies rule out increased radiation-induced DNA damage, inhibition of DNA repair, inhibition of myc suppression, or increased ROS generation as the mechanism(s) responsible for the radiosensitizing effects of EB 1089. However, EB 1089 does cause an increase in radiation-induced ceramide generation in the cell, while the ceramide synthase inhibitor, fumonisin B1, inhibits apoptosis and increases cell viability following treatment with EB 1089 plus radiation. Signaling pathways that promote ceramide generation and autophagic cell death may provide insights as to the mechanisms underlying the interaction(s) between EB 1089 or vitamin D3 and radiation in breast tumor cells.