Degradation of estrogen receptor α induced by ionizing radiation contributes to senescence of ER⁺ breast cancer cells.

e12544 Background: According to the clinical and pathological characteristics, breast cancer is usually classified into estrogen receptor positive (ER+), human epidermal growth factor receptor 2 positive and triple negative. Among them, ER+ breast cancer accounts for more than two-thirds of total breast cancer. Nowadays, both ionizing radiation and endocrine therapy are used for the treatment of ER+ breast cancer. In our previous studies, it was found that significant cellular senescence and reduction of endogenous estradiol (E2) were induced by carbon ion or X-ray exposure in ER+ breast cancer cells. However, the underlying mechanism remains unclear. Methods: Two human ER+ breast cancer cell lines, MCF7 and T47D, were chosen to investigate the senescence-associated indications after 1-10 Gy of X-ray exposure. Since the elevated estrogen receptor α (ERα) is the most significant characteristic of ER+ breast cancer, the levels of ERα were detected by qRT-PCR and West Blotting analysis. In addition, MCF7 tumors were established in BALB/C nude mice and discerned using immunohistochemistry after local exposure of tumors to 2 Gy of X-rays. Results: It was confirmed that the cell survival decreased in a dose-dependent manner after 1-10 Gy of X-ray exposure in both cell lines. Increased p21 protein expression which was considered as a senescence biomarker and enhanced senescence-associated β-galactosidase activity were also observed after X-ray exposure. Meanwhile, distinct cell cycle arrest was induced by 2 Gy of X-rays in T47D cells or by 8 Gy of X-rays in MCF7 cells, implying that T47D cells were prone to enter senescence. Furthermore, both the mRNA levels and protein levels of ERα were significantly decreased at 24 hours in the two cell lines when the exposure dose was up to 4 Gy of X-rays. Reduction of glucocorticoid-regulated kinase 3 (SGK3), a transcriptional target for ERα, was also observed in both cell lines, in according with the reported SGK3-dependent survival mediated by ERα/E2 dimer in ER+ breast cancer cells. In vitro experiments, the results verified the ERα degradation and the proliferating inhibition indicated by suppression of Ki67 protein in MCF7 tumor after 2 Gy exposure. Conclusions: These findings reveal that the degradation of estrogen receptor α induced by ionizing radiation contributes to the senescence of ER+ breast cancer cells and thereby inhibits the tumor growth. The results provide a potential therapeutic strategy to enhance the radiosensitivity of ER+ breast cancer cells through regulations of ERα or ERα-regulated kinase SGK3 in radiotherapy.