ESTRO 2021 Abstract Book

S340

ESTRO 2021

Conclusion Together, these results demonstrate how the cGAS-STING pathway, most frequently associated with abscopal immune responses, also plays a key role in mediating in vitro RIBEs. Additionally, cytosolic dsDNA, as the primary activator of this pathway, appears to be an important factor in the transmission of RIBEs between cells. In vitro RIBEs may be the manifestation of inflammation-driven genotoxic stress caused by downstream products of the cGAS-STING pathway. PH-0441 DNA- and mitochondrial damage may be involved in tamoxifen-induced radioresistance F. Naumann 1 , G. Adema 1 , F. Sweep 2 , J. Bussink 1 , P. Span 1 1 Radboud university medical center, Department of Radiation Oncology, Nijmegen, The Netherlands; 2 Radboud university medical center, Department of Laboratory medicine, Nijmegen, The Netherlands Purpose or Objective For estrogen receptor (ER) positive breast cancer patients, adjuvant systemic endocrine treatment using the ER antagonist tamoxifen has been an important part of treatment for decades. Besides, local treatment using external beam irradiation is applied in a majority of patients. Recently we described that prolonged tamoxifen treatment in vitro led to cross-resistance to radiotherapy, but not chemotherapy (Post et al. 2018). Tamoxifen induced an interferon (IFN) response that was also found to be associated with both tamoxifen- and radioresistance in retrospective cohorts of breast cancer patients. IFN responses related to cGAS-STING signaling have also been described to be involved in resistance to radiotherapy (Deng et al. 2014, Khodarev et al. 2004). Here, we assessed the effects of high concentrations of tamoxifen that may explain subsequent radioresistance. We hypothesized that tamoxifen-induced DNA damage may induce an IFN response independent of the ER through effects on mitochondrial reactive oxygen species (ROS) production, and subsequent activation of DNA sensors such as cGAS and STING. Materials and Methods MCF7, T47D and MDA-MB-231 breast cancer cells were treated with 5uM of 4-hydroxy-tamoxifen (4OHT). ROS production was measured by live cell imaging with the ROS-tracker CellROX. After 24h of treatment with 4OHT, cells were immunofluorescently stained for 53BP1, PINK1 and cGAS. RNA was extracted with and without prior treatment of the STING inhibitor CCCP for qPCR to determine gene expression of IFN-pathway and mitochondrial genes. Pathway analysis was performed with gProfiler on gene expression data of patient material treated neo-adjuvantly with tamoxifen acquired by Kastrati et al. (2020).