Defining the Role of Nuclear Hormone Receptors in Response to Radiation Therapy: Targeted Therapies for Radiosensitization in Androgen Receptor-Positive and Estrogen Receptor-Positive Breast Cancers

Abstract

Breast cancer is the most common form of invasive cancer diagnosed in women, and despite advances in therapeutic strategies, approximately 10% of women with breast tumors will experience locoregional recurrence. Therefore, we have identified the presence of nuclear hormone receptors, including the androgen and estrogen receptors, as potential therapeutic targets that may be aiding in the response to ionizing radiation. While the androgen receptor (AR) and the estrogen receptor (ER) are expressed both alone and together in breast cancer, the potential roles of AR or ER alone or in tandem have not been investigated. Previous work demonstrated that AR may be a mediator of radioresistance in AR-positive, ER-negative models of breast cancer, but little is known about the underlying mechanism of radiosensitization. First, we demonstrate that in AR+ triple negative breast cancer (TNBC) models, AR inhibition with the novel AR inhibitor and CYP17 lyase inhibitor, seviteronel, or AR knockdown results in radiosensitization through a delay in dsDNA break repair following treatment with radiation therapy (RT). While seviteronel is sufficient to radiosensitize AR+ TNBC cells, seviteronel appears to have a different mechanism of radiosensitization in comparison to the second-generation anti-androgen, enzalutamide. To further investigate the mechanism of radiosensitization with AR inhibition, we transcriptomic and proteomic data to nominate the MAPK/ERK signaling pathway as a mediator of radioresistance in AR+ TNBC models. Our data demonstrates an increase in p-ERK1/2 signaling in response to AR activation using synthetic androgens. This work suggests that AR may be activating the MAPK/ERK signaling cascade to promote DNA repair following RT, and inhibition of AR may be sufficient to block this phenotype. ER-positive (ER+) breast cancers account for 67-80% of all breast cancer diagnoses, and women with breast cancer receive multimodal therapies including surgery, radiation therapy, and endocrine therapies targeting estrogens and ER signaling. We have shown that combination treatment of endocrine therapies with RT results in an increase in radiosensitivity in AR-negative (AR-)/ER+ breast cancer models. This radiosensitization is due to a decrease in non-homologous end joining efficiency resulting in a delay in dsDNA break repair with endocrine therapy treatment. Co-expression of ER and AR has been observed in 70-90% of all ER+ breast cancers, and we sought to understand whether AR and ER had independent roles when co-expressed together in AR+/ER+ breast cancers. Abrogation of AR alone in AR+/ER+ breast cancer models, using pharmacologic or genetic approaches, had no effect on radiosensitivity, while targeting the ER had a limited effect on radiosensitization in some models. To understand whether there may be a compensation mechanism between AR and ER signaling, we assessed radiosensitization in AR+/ER+ models, seeing no added benefit from combined abrogation of AR and ER compared to monotherapy alone, suggesting that the role of AR in response to RT may be dependent on co-expression of other proteins, most notably, the estrogen receptor. Collectively, our data begin to uncover the nuances of nuclear hormone receptor signaling in response to ionizing radiation in breast cancer models, suggesting that AR and ER may play independent roles when expressed alone or in tandem. Together these findings will inform clinical trial design for patients with breast cancer, increasing the translational relevance of our work.