ATR inhibition potentiates ionizing radiation‐induced interferon response via cytosolic nucleic acid‐sensing pathways

Mechanistic understanding of how ionizing radiation induces type I interferon signaling and how to amplify this signaling module should help to maximize the efficacy of radiotherapy. In the current study, we report that inhibitors of the DNA damage response kinase ATR can significantly potentiate ionizing radiation‐induced innate immune responses. Using a series of mammalian knockout cell lines, we demonstrate that, surprisingly, both the cGAS/STING‐dependent DNA‐sensing pathway and the MAVS‐dependent RNA‐sensing pathway are responsible for type I interferon signaling induced by ionizing radiation in the presence or absence of ATR inhibitors. The relative contributions of these two pathways in type I interferon signaling depend on cell type and/or genetic background. We propose that DNA damage‐elicited double‐strand DNA breaks releases DNA fragments, which may either activate the cGAS/STING‐dependent pathway or—especially in the case of AT‐rich DNA sequences—be transcribed and initiate MAVS‐dependent RNA sensing and signaling. Together, our results suggest the involvement of two distinct pathways in type I interferon signaling upon DNA damage. Moreover, radiation plus ATR inhibition may be a promising new combination therapy against cancer. Synopsis Type I interferon signaling plays key roles in cancer radiotherapy with ionizing radiation (IR). Here, exploration of combinatory effects of IR and DNA damage kinase inhibitors reveals surprising involvement of distinct cytosolic nucleic acid‐sensing pathways in interferon response induction. Inhibition of DNA damage response kinase ATR (ATRi) significantly potentiates IR‐induced type I interferon response in multiple human and murine cancer cells. MAVS‐dependent RNA sensing pathway is indispensable for interferon signaling induced by combined IR+ATRi in some human cells. Both cGAS/STING‐dependent cytosolic DNA‐ and MAVS‐dependent cytosolic RNA‐sensing pathways contribute to IR+ATRi‐induced interferon signaling to varying extent in different cell lines. DNA damage‐elicited AT‐rich DNA mediates type I interferon signaling in a subset of human cells. Graphical Abstract Surprisingly, both cGAS/STING‐dependent DNA‐ and MAVS‐dependent RNA‐sensing pathways contribute to the effects of combined radiotherapy and blocked DNA damage signaling, depending on cellular context.