Jun 22, 2007 - decreased overall patient survival.13 Because hypoxic tumor cells are ... embryonic cell line (HEK293) for cell death response in hypoxia.
[Autophagy 4:2, 195-204; 16 February 2008]; ©2008 Landes Bioscience
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Hypoxia induces autophagic cell death in apoptosis-competent cells through a mechanism involving BNIP3
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Research Paper
Meghan B. Azad,1-3 Yongqiang Chen,1,2 Elizabeth S. Henson,1-3 Jeannick Cizeau,1 Eileen McMillan-Ward,1,2 Sara J. Israels1,2 and Spencer B. Gibson1-3 Institute of Cell Biology; 2CancerCare Manitoba; 3Department of Biochemistry and Medical Genetics; University of Manitoba; Winnipeg, Manitoba, Canada
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1Manitoba
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Abbreviations: BNIP3, Bcl-2/adenovirus E1B 19kDa-interacting protein 3; ATG, autophagy-related gene; 3-MA, 3-methyladenine; PCD, programmed cell death; LC3, microtubule-associated light chain 3 (mammalian ATG8); AVO, acidic vacuole; TM, transmembrane; siRNA, small interfering RNA
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Introduction
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Autophagy is a regulated lysosomal pathway used in the degradation and recycling of long‑lived proteins and organelles. During autophagy, cytoplasmic constituents are sequestered into double‑membraned autophagosomes which are delivered to lyso‑ somes, where degradation occurs.1 Genetic screening in yeast has identified more than 30 ATG (autophagy‑related) genes, many of which have mammalian homologs.2 Conserved from yeast to humans, autophagy functions in basic cellular homeostasis and is known to be essential for survival during starvation.3 More recently it has been shown that under certain conditions, autophagy can also promote cell death. For example, chemical agents such as arsenic trioxide4 and over‑expression of tumor suppressor proteins such as p19ARF5 have been shown to induce cell death through an autoph‑ agic mechanism independent of apoptosis. In other cases, autophagic cell death has been achieved in systems where apoptosis is blocked, either though the use of caspase inhibitors6,7 or by elimination of the pro‑apoptotic Bcl‑2 family members Bax and Bak.8 Hypoxia is a physiological stress encountered during various pathologies including cancer, myocardial infarction, and stroke.9 Cells deprived of oxygen will initially employ adaptive and survival strategies, but if hypoxia is sustained, cell death will eventually occur. The precise mechanisms of hypoxia‑induced cell death remain unclear as apoptosis, necrosis and autophagy have all been reported in response to hypoxic stress.10‑12 Chronic hypoxia is typical of tumor development as rapid proliferation causes the tumor to outgrow its available oxygen supply. The extent of tumor hypoxia correlates with neoplastic aggression, resistance to therapy‑induced apoptosis and decreased overall patient survival.13 Because hypoxic tumor cells are difficult to target effectively, it is important to understand the cell death mechanisms involved (and evaded) during sustained oxygen deprivation in order to develop better strategies to treat cancers. Bcl‑2/E1B 19kDa interacting protein (BNIP3) is a pro‑cell death Bcl‑2 family member that is upregulated under hypoxic conditions.14 In transformed and cancer cells, forced over‑expression of BNIP3 induces ‘non-apoptotic’ cell death that is characterized by localiza‑ tion to the mitochondria, opening of the permeability transition
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Hypoxia (lack of oxygen) is a physiological stress often associated with solid tumors. Hypoxia correlates with poor prognosis since hypoxic regions within tumors are considered apoptosisresistant. Autophagy (cellular “self digestion”) has been associated with hypoxia during cardiac ischemia and metabolic stress as a survival mechanism. However, although autophagy is best characterized as a survival response, it can also function as a mechanism of programmed cell death. Our results show that autophagic cell death is induced by hypoxia in cancer cells with intact apoptotic machinery. We have analyzed two glioma cell lines (U87, U373), two breast cancer cell lines (MDA-MB-231, ZR75) and one embryonic cell line (HEK293) for cell death response in hypoxia (
