Authors: Noemi Rubio, Isabelle Coupienne, Emmanuel Di Valentin, Ingeborg Heirman, Johan Grooten, Jacques Piette and Patrizia Agostinis

Noemi Rubio

Virology and Immunology Unit; GIGA-R, GIGA B34; University of Liège; Liège, Belgium; Cell Death Research & Therapy Laboratory; Cellular and Molecular Medicine Department; KU Leuven; Leuven, Belgium

Isabelle Coupienne

Virology and Immunology Unit; GIGA-R, GIGA B34; University of Liège; Liège, Belgium

Emmanuel Di Valentin

Virology and Immunology Unit; GIGA-R, GIGA B34; University of Liège; Liège, Belgium

Ingeborg Heirman

Molecular Immunology Laboratory; Ghent University; Ghent, Belgium

Johan Grooten

Molecular Immunology Laboratory; Ghent University; Ghent, Belgium

Jacques Piette

Corresponding author: jpiette@ulg.ac.be
Virology and Immunology Unit; GIGA-R, GIGA B34; University of Liège; Liège, Belgium

Patrizia Agostinis

Corresponding author: Patricia.Agostinis@med.kuleuven.be
Cell Death Research & Therapy Laboratory; Cellular and Molecular Medicine Department; KU Leuven; Leuven, Belgium

Abstract:
Although reactive oxygen species (ROS) have been reported to evoke different autophagic pathways, how ROS or their secondary products modulate the selective clearance of oxidatively damaged organelles is less explored. To investigate the signaling role of ROS and the impact of their compartmentalization in autophagy pathways, we used murine fibrosarcoma L929 cells overexpressing different antioxidant enzymes targeted to the cytosol or mitochondria and subjected them to photodynamic (PD) stress with the endoplasmic reticulum (ER)-associated photosensitizer hypericin. We show that following apical ROS-mediated damage to the ER, predominantly cells overexpressing mitochondria-associated glutathione peroxidase 4 (GPX4) and manganese superoxide dismutase (SOD2) displayed attenuated kinetics of autophagosome formation and overall cell death, as detected by computerized time-lapse microscopy. Consistent with a primary ER photodamage, kinetics and colocalization studies revealed that photogenerated ROS induced an initial reticulophagy, followed by morphological changes in the mitochondrial network that preceded clearance of mitochondria by mitophagy. Overexpression of cytosolic and mitochondria-associated GPX4 retained the tubular mitochondrial network in response to PD stress and concomitantly blocked the progression toward mitophagy. Preventing the formation of phospholipid hydroperoxides and H₂O₂ in the cytosol as well as in the mitochondria significantly reduced cardiolipin peroxidation and apoptosis. All together, these results show that in response to apical ER photodamage ROS propagate to mitochondria, which in turn amplify ROS production, thereby contributing to two antagonizing processes, mitophagy and apoptosis.

Received: February 8, 2012; Accepted: May 15, 2012; Published Online: August 14, 2012

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