Lxr Signaling Regulates Macrophage Survival and Inflammation in Response to Ionizing Radiation

Abstract Purpose To evaluate the role of Liver X Receptors (LXR) nuclear receptors on irradiation-induced cell death and polarization of macrophages, and their potential implications in the context of radiotherapy treatment of cancer. Methods and Materials Primary and immortalized murine bone marrow macrophages (BMDMs) from wild type or LXR-double knock-out mice were exposed to gamma irradiation. Subsequently, analysis of LXR signaling on cell proliferation and cytotoxicity induced by ionizing radiation was determined by time-lapse photomicroscopy. Genotoxic cell damage was evaluated by western blot of γ-H2AX and p53. Pyroptosis was analyzed through cell viability assay, lactate dehydrogenase (LDH) release assay and western-blot of caspase-1 active protein. Expression of inflammatory markers was measured by real-time quantitative polymerase chain reaction. Results Genetic and pharmacological inactivation of LXR induced radiosensitivity of macrophages. LXR deficiency decreased cell proliferation and enhanced cytotoxicity induced by ionizing radiation, in both immortalized and primary BMDMs. Protein levels of γ-H2AX and p53, both involved in response to cell damage, were exacerbated in LXR-deficient macrophages exposed to irradiation. Cell membrane damage augmented and cell viability decreased in LXR-deficient macrophages compared to LXR-WT macrophages in response to irradiation. In addition, LXR deficiency enhanced both caspase-1 activation and LDH release in BMDM exposed inflammasome activators. LXR inactivation or deficiency markedly increased the expression of pro-inflammatory markers IL-1β, IL-6 and iNOS in irradiated macrophages. Conclusions The present work identifies LXR transcription factors as potential therapeutic targets to enhance the suppressive effects of radiotherapy on tumor growth, through induction of macrophage cell death and activation of the inflammatory cascade.