Exposure to estrogen and ionizing radiation causes epigenetic dysregulation, activation of mitogen-activated protein kinase pathways, and genome instability in the mammary gland of ACI rats

Abstract

The impact of environmental mutagens and carcinogens on the mammary gland has recently received a lot of attention. Among the most generally accepted carcinogenic agents identified as factors that may increase breast cancer incidence are ionizing radiation and elevated estrogen levels. However, the molecular mechanisms of mammary gland aberrations associated with radiation and estrogen exposure still need to be further elucidated, especially the interplay between elevated hormone levels and radiation. Therefore, in the present study, we investigated molecular changes induced in rat mammary gland tissue by estrogen, ionizing radiation, and the combined action of these two carcinogens using a well-established ACI rat model. We found that continuous exposure of intact female ACI rats to elevated levels of estrogen or to both estrogen and radiation resulted in significant hyperproliferative changes in rat mammary glands. In contrast, radiation exposure alone did not induce hyperplasia. Interestingly, despite the obvious disparity in mammary gland morphology, we did not detect significant differences in the levels of genomic methylation among animals exposed to estrogen, radiation, or both agents together. Specifically, we observed a significant global genomic hypomethylation at 6 weeks of exposure. However, by 12 and 18 weeks, the levels of global DNA methylation returned to those of age-matched controls. We also found that combined exposure to radiation and estrogen significantly altered the levels of histone H3 and H4 methylation and acetylation. Most importantly, we for the first time demonstrated that estrogen and radiation exposure caused a significant induction of p42/44 MAPK and p38 pathways that was paralleled by elevated levels of H3S10 phosphorylation, a well-established biomarker of genome and chromosome instability. The precise role of MAPK pathways and their inter-relationship with H3S10 phosphorylation and genome instability in mammary gland tissues needs to be explored further.

Keywords: :

• epigenetics
• estrogen
• ionizing radiation
• DNA methylation
• histones
• breast cancer
• mitogen-activated protein kinase pathways

Disclosure of Potential Conflicts of Interest

No potential conflict of interest was disclosed.

Acknowledgments

Work was supported by the Alberta Cancer Research Institute and Canadian Breast Cancer Foundation Operating grants to OK. KK was a recipient of the Vanier Canada Graduate Scholarship, National Science and Engineering Research Council Graduate Scholarship, Queen Elizabeth II Graduate Scholarship, and the Alberta Heritage for Medical Research Scholarship. We are thankful to Rocio Rodriguez-Juarez and Igor Koturbash for technical assistance, to Dr Roderick T Bronson and Dr Langxing Pan for their help and advice with histopathology, to Dr Igor Pogribny for useful discussions, and to Dr Valentina Titova for careful proofreading of this manuscript.