Abstract
Akt1 is known to promote non-homologous end-joining (NHEJ)-mediated DNA double-strand break (DSB) repair by stimulation of DNA-PKcs. In the present study, we investigated the effect of Akt1 on homologous recombination (HR)-dependent repair of radiation-induced DSBs in non-small cell lung cancer (NSCLC) cells A549 and H460. Akt1-knockdown (Akt1-KD) significantly reduced Rad51 protein level, Rad51 foci formation and its colocalization with γH2AX foci after irradiation. Moreover, Akt1-KD decreased clonogenicity after treatment with Mitomycin C and HR repair, as tested by an HR-reporter assay. Double knockdown of Akt1 and Rad51 did not lead to a further decrease in HR compared to the single knockdown of Rad51. Consequently, Akt1-KD significantly increased the number of residual DSBs after irradiation partially independent of the kinase activity of DNA-PKcs. Likewise, the number of residual BRCA1 foci, indicating unsuccessful HR events, also significantly increased in the irradiated cells after Akt1-KD. Together, the results of the study indicate that Akt1 seems to be a regulatory component in the HR repair of DSBs in a Rad51-dependent manner. Thus, based on this novel role of Akt1 in HR and the previously described role of Akt1 in NHEJ, we propose that targeting Akt1 could be an effective approach to selectively improve the killing of tumor cells by DSB-inducing cytotoxic agents, such as ionizing radiation.
Highlights
Non-small cell lung cancer (NSCLC) accounts for the largest subgroup of lung cancers, resulting in the highest portion of cancer-related mortality worldwide [1,2,3,4]
The results of Rad51 protein expression and foci formation together with the data from the homologous recombination (HR)-reporter assays strongly suggest that Akt1 stimulates HR-dependent double-strand break (DSB) repair
In contrast to A549 cells, the H460 cell line shows an activating mutation of PIK3CA coding for PI3K, which acts upstream of Akt1 [55,56]
Summary
Non-small cell lung cancer (NSCLC) accounts for the largest subgroup of lung cancers, resulting in the highest portion of cancer-related mortality worldwide [1,2,3,4]. Treatment resistance can occur, resulting in 3-year survival rates of 15–20% [5,6]. A high level of activated Akt in tumor specimens is a prognostic factor for poor outcomes in NSCLC [12]. In addition to stimulating tumor cell proliferation, growth and survival [7], Akt, especially Akt, promotes DNA double-strand break (DSB) repair and clonogenic survival after irradiation [13,14,15,16]. Various Akt inhibitors are currently being tested in clinical studies, demonstrating overall tolerable toxicities and promising anti-tumor activities [17,18,19]
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