Integrated stress response and lipid remodeling are adaptive mechanisms triggered by Complex I deficiency in cancer cells

Respiratory Complex I (CI) is a pivotal enzyme for cellular bioenergetics and its functional alterations trigger metabolic and molecular adaptive responses that help cancer cells survive despite the energetic deficit. Since such mechanisms are far from being completely understood, we exploited multiple cancer cell lines of different tissue origins lacking CI, in which we knocked out the core subunit NDUFS3 (NDUFS3-/-), to elucidate their response to energetic stress. During glucose restriction, NDUFS3-/- cells showed significant mitochondrial membrane depolarization, that in turn triggered the activation of the mitochondrial protease OMA1 leading to the processing of its targets, including those responsible for network fragmentation. These mitochondrial alterations were accompanied by a consistent activation of Integrated Stress Response (ISR) as proved by the phosphorylation of eIF2α, the block of protein synthesis, the nuclear re-localization of ATF4 and the increased expression of CHOP. These results were also observed in vivo and upon pharmacological CI inhibition by using EVP 4593. This molecular mechanism is responsible for the survival of CI-defective cells, and it is directed by OMA1 and suppressed when the protease expression is prevented. We also found that glucose deprivation triggered the accumulation of lipid droplets in close proximity to deranged mitochondria in CI-defective cells both in vitro and in vivo, accompanied by a profound alteration of lipid content, in particular we found an increase in triacylglycerols and in cholesteryl esters amount. In this context lipid droplets could sequester misfolded proteins, excess lipids and prevent free calcium overload in the cytosol in order to alleviate endoplasmic reticulum stress. In conclusion, loss of CI and glucose restriction induce a striking mitochondrial distress with depolarization and network fragmentation and trigger OMA1-mediated ISR to overcome the stress status and survive.