SirT3 Regulates the Mitochondrial Unfolded Protein Response

The mitochondria of cancer cells are characterized by elevated oxidative stress caused by reactive oxygen species (ROS). Such an elevation in ROS levels contributes to mitochondrial reprogramming and malignant transformation. However, high levels of ROS can cause irreversible damage to proteins, leading to their misfolding, mitochondrial stress, and ultimately cell death. Therefore, mechanisms to overcome mitochondrial stress are needed. The unfolded protein response (UPR) triggered by accumulation of misfolded proteins in the mitochondria (UPRmt) has been reported recently. So far, the UPRmt has been reported to involve the activation of CHOP and estrogen receptor alpha (ER alpha). The current study describes a novel role of the mitochondrial deacetylase SirT3 in the UPRmt. Our data reveal that SirT3 acts to orchestrate two pathways, the antioxidant machinery and mitophagy. Inhibition of SirT3 in cells undergoing proteotoxic stress severely impairs the mitochondrial network and results in cellular death. These observations suggest that SirT3 acts to sort moderately stressed from irreversibly damaged organelles. Since SirT3 is reported to act as a tumor suppressor during transformation, our findings reveal a dual role of SirT3. This novel role of SirT3 in established tumors represents an essential mechanism of adaptation of cancer cells to proteotoxic and mitochondrial stress.