Enhancement of apoptotic damage of squamous cell carcinoma cells by inhibition of the mitochondrial DNA repairing system

Mitochondrial DNA (mtDNA) repair systems are thought to be associated with the susceptibility of cancer cells to anticancer agents. The present study investigated the relationship between the susceptibility to γ‐rays and the mtDNA repair ability of oral squamous cell carcinoma (OSC) cell lines. The levels of 8‐hydroxy‐2′‐deoxyguanosine (8‐OHdG) and mtDNA common deletion in both nuclear and mitochondrial DNA of OSC‐2, OSC‐3 and OSC‐6 cells (radio‐sensitive cell lines) after γ‐ray‐irradiation were higher than those of OSC‐1, OSC‐4 and OSC‐5 cells (radio‐resistant cell lines). Compared with OSC‐2, OSC‐3 and OSC‐6 cells, OSC‐1, OSC‐4 and OSC‐5 cells had higher levels of activity of phosphoinositide‐3 kinase (PI‐3K)/Akt and more strongly expressed 8‐hydroxyguanine DNA glycosylase (OGG1), DNA polymerase γ (POLG) and mitochondrial transcription factor A (Tfam). Down‐regulation of these mtDNA‐repair‐associated molecules by the RNA interference technique enhanced the susceptibility of OSC‐2 and OSC‐5 cells to γ‐rays, and the expression of Tfam and POLG was down‐regulated by inhibitors of PI‐3K/Akt signaling. These results indicate that the inhibition of mtDNA repair capacity by PI‐3K/Akt signal inhibitors and OGG1 down‐regulator in cancer cells may be a useful strategy for cancer treatment when combined with ionizing irradiation and chemotherapeutic drugs. (Cancer Sci 2008; 99: 2230–2237)