| Estrogen has long been implicated in the formation and progression of cancer of the breast. Estrogens promote tumourigenesis by stimulating cell growth and proliferation in addition to increasing the frequency of mutations and chromosomal rearrangements, although the mechanism behind this is not fully understood. Mutations in several proteins involved in the response to and repair of DNA double strand breaks (DSBs) correlate with an increased risk of developing breast cancer. The most notable of these genes, BRCA1 and BRCA2, are involved in the homologous recombination repair (HRR) pathway and signalling in response to DSB formation. Here, I show treatment of estrogen responsive breast cancer cells with 17beta-estradiol (E2) results in the formation of DNA DSBs as indicated by the formation of punctate foci consisting of the DSB marker H2AX phosphorylated on serine 139 (gammaH2AX), and other proteins involved in the DNA damage response, including 53BP1, MDC1, ATM and ubiquitin. I investigated the mechanism of E2-induced foci formation and show this occurred by a process dependent on estrogen receptor alpha-mediated transcription and the decatenating enzyme, topoisomerase IIbeta. Moreover, gammaH2AX was enriched at the promoter of the E2-inducible trefoil factor 1 gene, suggesting DSBs occur at sites of E2-mediated transcription. In addition, E2-induced gammaH2AX foci formation occurred in a cell cycle-dependent manner and foci were repaired by the HRR pathway. A deficiency of the HRR pathway resulted in an accumulation of breaks, G2 arrest and spontaneous cell death. Exposure of hormone-responsive MCF-7 cells to E2 prevented ionizing radiation-induced cell cycle arrest and promoted radioresistance, suggesting E2 not only induces the formation of DNA damage, but E2 signalling also counteracts the growth inhibitory and cell death response normally induced by DSBs. I propose the presence of persistent DSBs in response to E2 is a source of genome instability whilst the pro-survival and growth signal mediated by E2 reduces the cell cycle arrest and death response normally associated with DSB formation. These observations shed light on the mechanism of E2-mediated genotoxicity and carcinogenesis. |
