Disruption of mitochondrial dynamics alters DNA repair capacity in colon cancer

Mitochondrial dynamics refers to a collection of mitochondrial movements, including fission, fusion, and transport. Cancer cells are known to adjust mitochondrial dynamics to provide the metabolic plasticity needed for cell growth, proliferation and migration. We have shown previously that Drp1, a key regulator of mitochondrial fission, plays an important role in mediating fatty acid-induced activation of Wnt/beta-catenin signaling in colon cancer. In this study, we determined the functional interaction between mitochondrial dynamics and DNA damage response in colon cancer cells. To disrupt mitochondrial fission, inducible Drp1 knockdown colon cancer cell lines were generated using lentivirus-mediated RNAi. Control and Drp1 knockdown cells were treated with radiation or chemotherapy drug irinotecan to induce DNA damage response. The extent of DNA damage as well as the activation of DNA repair signaling were analyzed by western blot and RT-qPCR. We found that Drp1 was activated upon irinotecan or radiation treatment as shown by increased phosphorylation at S616 site. Interestingly, the expression of gammaH2AX, a DNA damage marker, induced by irinotecan or radiation treatment was increased in Drp1 knockdown cells, whereas the expression and activation of DNA damage sensing proteins (e.g., ATM and ATR) remained unchanged. To determine if increased fatty acid uptake alters DNA damage, we showed that the presence of exogenous fatty acids attenuated gammaH2AX expression induced by radiation in control cells whereas knockdown of Drp1 blunted this response. Functionally, the addition of fatty acids increased, while silencing Drp1 decreased, cell survival post radiation as measured by colony formation assays. However, the pro-survival effect of fatty acids was largely abolished in Drp1 knockdown cells. Taken together, our results suggest that fatty acid uptake may enhance DNA repair via a mitochondrial fission-dependent mechanism. This study establishes a functional link between fatty acid metabolism, mitochondrial dynamics, and DNA damage response in colon cancer cells.