Suppression of oxidative phosphorylation in mouse embryonic fibroblast cells deficient in apurinic/apyrimidinic endonuclease

• Isolation of mouse embryonic fibroblast (MEF) with extremely low APE1 activity.
• The APE1 deficient MEF grew normally and showed little sign of DNA damage stress.
• Yet the base excision repair activity of AP sites was almost undetectable in the APE1 deficient MEF, indicating surprisingly low demand for repair of endogenous DNA damage.
• Low APE1 was associated with low oxidative phosphorylation and high glycolysis activities.

The mammalian apurinic/apyrimidinic (AP) endonuclease 1 (APE1) is an essential DNA repair/gene regulatory protein. Decrease of APE1 in cells by inducible shRNA knockdown or by conditional gene knockout caused apoptosis. Here we succeeded in establishing a unique mouse embryonic fibroblast (MEF) line expressing APE1 at a level far lower than those achieved with shRNA knockdown. The cells, named MEFla (MEFlowAPE1), were hypersensitive to methyl methanesulfonate (MMS), and showed little activity for repairing AP-sites and MMS induced DNA damage. While these results were consistent with the essential role of APE1 in repair of AP sites, the MEFla cells grew normally and the basal activation of poly(ADP-ribose) polymerases in MEFla was lower than that in the wild-type MEF (MEFwt), indicating the low DNA damage stress in MEFla under the normal growth condition. Oxidative phosphorylation activity in MEFla was lower than in MEFwt, while the glycolysis rates in MEFla were higher than in MEFwt. In addition, we observed decreased intracellular oxidative stress in MEFla. These results suggest that cells with low APE1 reversibly suppress mitochondrial respiration and thereby reduce DNA damage stress and increases the cell viability.