AKT reduces NHEJ efficiency by phosphorylating XLF, which dissociates XLF from LIG4 and XRCC4.
Major finding: AKT reduces NHEJ efficiency by phosphorylating XLF, which dissociates XLF from LIG4 and XRCC4.
Mechanism: Phosphorylation of XLF promotes its binding to 14-3-3β, which sequesters XLF in the cytoplasm.
Impact: Oncogenic AKT activation may promote genomic instability in part through impairment of NHEJ.
Aberrant accumulation of DNA double-strand breaks (DSB) caused by impairment of homologous recombination (HR)– or nonhomologous end-joining (NHEJ)–mediated repair can lead to genomic instability and ultimately promote tumorigenesis. Liu, Gan, and colleagues found that hyperactive AKT signaling increased DSB accumulation and markedly reduced NHEJ-mediated DSB repair efficiency. AKT activation specifically led to phosphorylation of X-ray repair cross-complementing protein 4 (XRCC4)–like factor (XLF), which forms a complex with XRCC4 and DNA ligase IV (LIG4) that mediates the ligation of damaged DNA ends. AKT1, but not AKT2 or other related kinases, directly phosphorylated XLF on threonine 181 (T181), which subsequently led to dissociation of XLF from XRCC4 and LIG4 and retention of XLF in the cytoplasm by inducing binding to 14-3-3β and reducing binding to importin complexes. In the cytoplasm, casein kinases further phosphorylated XLF to facilitate its association with an E3 ubiquitin protein ligase complex containing β-transducin repeat containing protein 1 (β-TRCP1), which led to ubiquitination of XLF and its subsequent proteasomal degradation. NHEJ efficiency and cell survival after irradiation were significantly reduced in XLF-deficient cells expressing a phosphomimetic T181E mutant, providing further evidence that AKT1-mediated phosphorylation of XLF negatively regulates the NHEJ repair pathway. Of note, an XLFR178Q mutation within the AKT phosphorylation motif identified in a patient with colorectal cancer blocked T181 phosphorylation, which enhanced NHEJ efficiency and conferred resistance to DSB-inducing chemotherapy, suggesting that evasion of AKT-dependent NHEJ inhibition could possibly represent a chemoresistance mechanism. Although further characterization of the dynamics of XLF regulation by AKT1 in both normal and tumor cells is needed, these findings highlight the connection between AKT signaling and DNA repair, which may have implications for tumorigenesis and response to cancer therapies.
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