Background

Several studies have indicated the potential value of targeting PI3K/Akt signaling to enhance the anti-tumor activity of ionizing radiation due to apoptosis as well as impaired repair of DNA-double strand breaks (DNA-DSB). Here, we investigated a mechanism by which targeting Akt affects DNA-double strand break repair and induces radiosensitization. Likewise, function of apoptosis in radiosensitization following Akt targeting was investigated.

Method

Function of Akt was investigated in accumulation of DNA-dependent protein kinase catalytic subunit (DNA-PKcs) to DNA damage site, DNA-PKcs activity/phosphorylation, Ku/DNA-PKcs complex formation, DNA-DSB repair and post-irradiation survival.

Results

In colon and lung carcinoma cell lines it was shown that ionizing radiation (IR)-induced DNA-PKcs activity is markedly Akt dependent. This could be substantiated by experiments which proved that the Akt inhibitor API- as well as AKT1-siRNA significantly inhibited micro-beam laser-induced accumulation of GFP-tagged DNA-PKcs to double strand break sites. Akt targeting by siRNA or API impaired repair of IR-induced DNA-DSB in a DNA-PKcs dependent manner. API did not affect DNA-PKcs kinase activity and DNA binding of Ku70/80 directly as tested by electrophoretic mobility shift assay and in vitro kinase assay. Combination of API with single dose irradiation (1 to 5 Gy) enhanced cellular radiation sensitivity in a DNA-PK dependent manner. In contrast to the described role of  apoptosis on cell death following Akt targeting in the presence or absence of irradiation our data indicates that the differential ability of cells from solid tumors i.e. NSCLC to undergo apoptosis does not play an important role in determining clonogenic cell survival .

Conclusion

This study provides novel evidence that Akt is directly involved in DNA-DSBs repair primarily through the activation and accumulation of DNA-PKcs at sites of DNA damage and, thus, offers new options for anticancer strategies.