Radiation-induced molecular changes in rat mammary tissue: possible implications for radiation-induced carcinogenesis.
OBJECTIVE
Ionizing radiation is a potent mammary gland carcinogen, yet the exact molecular etiology of radiation-induced breast cancer remains unknown.


METHODS
Our study utilized a rat model of breast carcinogenesis to analyse the molecular and epigenetic changes induced in mammary gland tissue upon exposure to ionizing radiation (IR). Using a methylation-sensitive cytosine extension assay we studied the IR-induced changes in DNA methylation. In parallel, we analysed the expression of proteins involved in DNA methylation, DNA repair and cell proliferation control. Molecular changes were related to cellular proliferation and apoptosis.


RESULTS
We found that IR led to a loss of genomic cytosine methylation in the exposed mammary tissue. Global DNA hypomethylation was paralleled by reduction in the levels of maintenance (DNMT1) and de novo (DNMT3a and 3b) DNA methyltransferases and methyl-binding protein MeCP2. The observed DNA hypomethylation was linked, at least in part, to activation of DNA repair processes. Concurrently, we observed increased levels of phosphorylated extracellular signal-regulated kinase (p-ERK1/2), phosphorylated AKT kinase (p-AKT), cyclin D1 and proliferating cells nuclear antigen (PCNA) proteins, suggesting IR alters intra-cellular signaling and cell cycle control mechanisms in mammary tissue. We also noted a significant induction of apoptosis in the exposed tissue 6 hours after irradiation. The observed apoptosis levels were paralleled by the slight elevation of cellular proliferation.


CONCLUSIONS
We have demonstrated that a single exposure to 5 Gy of X rays leads to noticeable epigenetic changes in the rat mammary gland that occurred in the context of activation of DNA damage repair and alterations in the pro-survival growth-stimulatory cellular signaling pathways. The possible cellular repercussions of the observed changes in relationship to breast carcinogenesis are discussed.
Loree
Jonathan
J
Department of Biological Sciences, University of Lethbridge, Lethbridge, Alberta, Canada.
Koturbash
Igor
I
Kutanzi
Kristy
K
Baker
Mike
M
Pogribny
Igor
I
Kovalchuk
Olga
O
D000818:Animals; D017209:Apoptosis / Q000528:radiation effects; D049109:Cell Proliferation / Q000528:radiation effects; D000076123:DNA (Cytosine-5-)-Methyltransferase 1; D004248:DNA (Cytosine-5-)-Methyltransferases / Q000032:analysis; D019175:DNA Methylation; D004260:DNA Repair; D005260:Female; D008321:Mammary Glands, Animal / Q000378:metabolism / Q000528:radiation effects*; D008325:Mammary Neoplasms, Experimental / Q000209:etiology*; D009381:Neoplasms, Radiation-Induced / Q000209:etiology*; D051135:Rad51 Recombinase / Q000096:biosynthesis; D051381:Rats; D020318:Rats, Long-Evans; D000097584:DNA Methyltransferase 3B
D000076123:DNA (Cytosine-5-)-Methyltransferase 1; D004248:DNA (Cytosine-5-)-Methyltransferases; D051135:Rad51 Recombinase; D000097584:DNA Methyltransferase 3B; C000619382:Dnmt1 protein, rat; C495155:Rad51 protein, rat