A Non-Canonical IRAK Signaling Pathway Triggered by DNA Damage.

Interleukin-1 receptor (IL-1R)-associated kinases (IRAKs) are core effectors of Toll-like receptor (TLR) and IL-1R signaling, with no reported roles outside of innate immunity. We find that vertebrate cells exposed to ionizing radiation (IR) sequentially activate IRAK4 and IRAK1 through a phosphorylation cascade mirroring that induced by TLR/IL-1R, resulting in a potent anti-apoptotic response. However, IR-induced IRAK1 activation does not require the receptors or the IRAK4/1 adaptor protein MyD88, and instead of remaining in the cytoplasm, the activated kinase is immediately transported to the nucleus via a conserved nuclear localization signal. We identify: double-strand DNA breaks (DSBs) as the biologic trigger for this pathway; the E3 ubiquitin ligase Pellino1 as the scaffold enabling IRAK4/1 activation in place of TLR/IL-1R-MyD88; and the pro-apoptotic PIDDosome (PIDD1-RAIDD-caspase-2) as a critical downstream target in the nucleus. The data delineate a non-canonical IRAK signaling pathway derived from, or ancestral to, TLR signaling. This DSB detection pathway, which is also activated by genotoxic chemotherapies, provides multiple actionable targets for overcoming tumor resistance to mainstay cancer treatments.

A noncanonical IRAK4-IRAK1 pathway counters DNA damage-induced apoptosis independently of TLR/IL-1R signaling.

Interleukin-1 receptor (IL-1R)-associated kinases (IRAKs) are core effectors of Toll-like receptors (TLRs) and IL-1R in innate immunity. Here, we found that IRAK4 and IRAK1 together inhibited DNA damage-induced cell death independently of TLR or IL-1R signaling. In human cancer cells, IRAK4 was activated downstream of ATR kinase in response to double-strand breaks (DSBs) induced by ionizing radiation (IR). Activated IRAK4 then formed a complex with and activated IRAK1. The formation of this complex required the E3 ubiquitin ligase Pellino1, acting structurally but not catalytically, and the activation of IRAK1 occurred independently of extracellular signaling, intracellular TLRs, and the TLR/IL-1R signaling adaptor MyD88. Activated IRAK1 translocated to the nucleus in a Pellino2-dependent manner. In the nucleus, IRAK1 bound to the PIDD1 subunit of the proapoptotic PIDDosome and interfered with platform assembly, thus supporting cell survival. This noncanonical IRAK signaling pathway was also activated in response to other DSB-inducing agents. The loss of IRAK4, of IRAK4 kinase activity, of either Pellino protein, or of the nuclear localization sequence in IRAK1 sensitized p53-mutant zebrafish to radiation. Thus, the findings may lead to strategies for overcoming tumor resistance to conventional cancer treatments.

FANCI functions as a repair/apoptosis switch in response to DNA crosslinks.

Cells counter DNA damage through repair or apoptosis, yet a direct mechanism for this choice has remained elusive. When facing interstrand crosslinks (ICLs), the ICL-repair protein FANCI heterodimerizes with FANCD2 to initiate ICL excision. We found that FANCI alternatively interacts with a pro-apoptotic factor, PIDD1, to enable PIDDosome (PIDD1-RAIDD-caspase-2) formation and apoptotic death. FANCI switches from FANCD2/repair to PIDD1/apoptosis signaling in the event of ICL-repair failure. Specifically, removing key endonucleases downstream of FANCI/FANCD2, increasing ICL levels, or allowing damaged cells into mitosis (when repair is suppressed) all suffice for switching. Reciprocally, apoptosis-committed FANCI reverts from PIDD1 to FANCD2 after a failed attempt to assemble the PIDDosome. Monoubiquitination and deubiquitination at FANCI K523 impact interactor selection. These data unveil a repair-or-apoptosis switch in eukaryotes. Beyond ensuring the removal of unrepaired genomes, the switch's bidirectionality reveals that damaged cells can offset apoptotic defects via de novo attempts at lesion repair.