Starvation-induced autophagy is regulated by mitochondrial reactive oxygen species leading to AMPK activation

Starvation is the most extensively studied condition that induces autophagy. Previous studies have demonstrated that starvation-induced autophagy is regulated by reactive oxygen species (ROS) such as superoxide (O-2(center dot-)) but the source for ROS under starvation conditions and the downstream signaling pathways regulating autophagy are unclear. In this study, a cervical cancer HeLa cell line was generated that was deficient in mitochondrial electron transport chain (mETC) (Hela rho degrees cells). This resulted in endogenous levels of O-2(center dot-) being significantly reduced and failed to be induced under starvation of glucose, L-glutamine, pyruvate, and serum (GP) or of amino acids and serum (AA) compared to wild type (wt) Hela cells. In contrast, H2O2 production failed to increase under GP starvation in both wild type and rho degrees cells whereas it increased in wt cells but not in rho degrees cells under AA starvation. GP or AA starvation induced autophagy was blocked in rho degrees cells as determined by the amount of autophagosomes and autolysosomes. Autophagy is regulated by 5' adenosine monophosphate-activated protein kinase (AMPK) activation and AMPK is activated under starvation conditions. We demonstrate that rho degrees cells and HeLa cells over expressing manganese-superoxide dismutase 2 (SOD2) cells fail to activate AMPK activation following starvation. This indicates that mitochondrial ROS might regulate AMPK activation. In addition, inhibiting AMPK activation either by siRNA or compound C resulted in reduced autophagy during starvation. Using a ROS scavenger NAC, AMPK activation is reduced under starvation condition and mTOR signaling is increased. Taken together, mitochondria-generated ROS induces autophagy mediated by the AMPK pathway under starvation conditions. (C) 2012 Elsevier Inc. All rights reserved.