Title: Oxidative damage to the endoplasmic reticulum by hypericin-mediated photodynamic therapy as inducer of apoptosis and autophagy pathways
Authors: Dewaele, Michael
Verfaillie, Tom
Buytaert, Esther
Martinet, Wim
Agostinis, Patrizia #
Issue Date: 2008
Conference: European Calcium Society on Calcium-binding Proteins in Normal and Transformed Cells edition:10 date:17-20 September
Abstract: Oxidative damage to the endoplasmic reticulum by hypericin-mediated photodynamic therapy as inducer of apoptosis and autophagy pathways

Michael Dewaele1, Tom Verfaillie1, Esther Buytaert1, Wim Martinet2, and Patrizia Agostinis1

1Dept. Molecular Cell Biology, Faculty of Medicine, Catholic University of Leuven, Belgium 2Division of Pharmacology, University of Antwerp, Belgium.

Photodynamic therapy (PDT) is an anticancer therapy utilizing cytotoxic reactive oxygen species (ROS) produced after irradiation of a photosensitizer with visible light, to kill the cancer cells1. Light activation of the ER-associated photosensitizer hypericin causes immediate photooxidative damage to the SERCA pump, due to the local generation of 1O2., This event results in the rapid loss of ER Ca2+ homeostasis which leads to ER-stress and a subsequent stress-response, triggering both apoptotic cell death and autophagy. While a previous study indicated an involvement of autophagy in the execution of apoptosis-incompetent Bax/Bak double knock-out MEFs2, the exact role of PDT-induced autophagy in apoptosis-competent cells remains largely unclear. In this study we show that in apoptosis-competent cells, the induction of apoptotic cell death occurs concomitantly with the classical characteristics of autophagy, suggesting that both pathways are simultaneously propagated in response to PDT-mediated ER stress. To study the role of autophagy in apoptosis-competent cells we used MEFs conditionally expressing the essential atg-gene Atg5. Interestingly, a knock-out of Atg5, does not only inhibit autophagy but also hampers the release of cytochrome c and caspase-activation in response to PDT. Intriguingly, Atg5 deficient cells exhibit an increased capacity to remove oxidative damaged proteins as compared to their wild type counterparts. On the other hand, pharmacological inhibition of autophagy with 3-methyladenine, also suppresses autophagy but increases the apoptotic fraction due to a hampered removal of oxidized proteins.
These data point at a complex level of cross-communication between apoptosis and autophagy in PDT treated cells and suggest that either key autophagy genes have a potential role in propagating apoptosis or that other compensatory and protective mechanisms exist in cells in which autophagy has been permanently knocked down. We are currently investigating the involvement of the ubiquitin-proteasome system (UPS) and chaperone-mediated autophagy (CMA) as compensatory mechanisms, potentially involved in the enhanced clearance of photomodified proteins of macroautophagy deficient MEFs.

1Dolmans et al, (2003) Nat Rev Cancer 3:380-7
2Buytaert et al, (2006) FASEB J. 20:756-8
Publication status: accepted
KU Leuven publication type: IMa
Appears in Collections:Laboratory of Cell Death Research & Therapy
Laboratory for Molecular Cancer Biology
# (joint) last author

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