Radiation-sensitization in the absence of Bax/Bak

4953

Abstract Bax and Bak act as a mitochondrial gateway for various apoptotic signals. However, cells lacking both Bax and Bak are resistant to apoptosis induced by various apoptotic stimuli. Instead, they undergo autophagic cell death. Resistance to apoptosis has been associated with radiation resistance, although little is known regarding the relationship between Bax/Bak -/- and irradiation-induced autophagy. In the present study, we investigated radiation-induced autophagy in the presence or absence of Bax and Bak. We found that mouse embryonic fibroblasts (MEFs) lacking both Bax and Bak are much more sensitive to radiation than the wild-type (WT) MEFs as demonstrated by clonogenic assay. Following radiation, increased levels of cleaved caspase 3 and higher apoptotic index were detected in the WT cells, but not in the Bax/Bak -/- double knockout (DKO) cells. Punctate fluorescence of GFP-LC3, characteristic of autophagy, was significantly increased in the irradiated DKO cells. Electron microscopy revealed that these cells had numerous autophagosomes and lysosomes. Inhibition of autophagy by 3-methyl adenine (3MA) increased the radiation resistance of Bax/Bak-/- MEF cells. In addition, expression of the pro-autophagic proteins APG5 and Beclin 1 was induced by radiation in the DKO cells. These results indicate that deficiency of Bax and Bak sensitized MEF cells to radiation and upregulated non-apoptotic cell death in response to radiation.