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      Autophagy and cancer

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          Abstract

          Basal autophagy plays a critical role in maintaining cellular homeostasis and genomic integrity by degrading aged or malfunctioning organelles and damaged or misfolded proteins. However, autophagy also plays a complicated role in tumorigenesis and treatment responsiveness. It can be tumor-suppressing during the early stages of tumorigenesis (i.e., it is an anti-tumor mechanism), as reduced autophagy is found in tumor cells and may be associated with malignant transformation. In this case, induction of autophagy would seem to be beneficial for cancer prevention. In established tumors, however, autophagy can be tumor-promoting (i.e., it is a pro-tumor mechanism), and cancer cells can use enhanced autophagy to survive under metabolic and therapeutic stress. The pharmacological and/or genetic inhibition of autophagy was recently shown to sensitize cancer cells to the lethal effects of various cancer therapies, including chemotherapy, radiotherapy and targeted therapies, suggesting that suppression of the autophagic pathway may represent a valuable sensitizing strategy for cancer treatments. In contrast, excessive stimulation of autophagy may also provide a therapeutic strategy for treating resistant cancer cells having high apoptotic thresholds. In order for us to develop successful autophagy-modulating strategies against cancer, we need to better understand how the roles of autophagy differ depending on the tumor stage, cell type and/or genetic factors, and we need to determine how specific pathways of autophagy are activated or inhibited by the various anti-cancer therapies.

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          Most cited references 87

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          Beclin 1, an autophagy gene essential for early embryonic development, is a haploinsufficient tumor suppressor.

          The biochemical properties of beclin 1 suggest a role in two fundamentally important cell biological pathways: autophagy and apoptosis. We show here that beclin 1-/- mutant mice die early in embryogenesis and beclin 1+/- mutant mice suffer from a high incidence of spontaneous tumors. These tumors continue to express wild-type beclin 1 mRNA and protein, establishing that beclin 1 is a haploinsufficient tumor suppressor gene. Beclin 1-/- embryonic stem cells have a severely altered autophagic response, whereas their apoptotic response to serum withdrawal or UV light is normal. These results demonstrate that beclin 1 is a critical component of mammalian autophagy and establish a role for autophagy in tumor suppression. They both provide a biological explanation for recent evidence implicating beclin 1 in human cancer and suggest that mutations in other genes operating in this pathway may contribute to tumor formation through deregulation of autophagy.
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            How to interpret LC3 immunoblotting.

            Microtubule-associated protein light chain 3 (LC3) is now widely used to monitor autophagy. One approach is to detect LC3 conversion (LC3-I to LC3-II) by immunoblot analysis because the amount of LC3-II is clearly correlated with the number of autophagosomes. However, LC3-II itself is degraded by autophagy, making interpretation of the results of LC3 immunoblotting problematic. Furthermore, the amount of LC3 at a certain time point does not indicate autophagic flux, and therefore, it is important to measure the amount of LC3-II delivered to lysosomes by comparing LC3-II levels in the presence and absence of lysosomal protease inhibitors. Another problem with this method is that LC3-II tends to be much more sensitive to be detected by immunoblotting than LC3-I. Accordingly, simple comparison of LC3-I and LC3-II, or summation of LC3-I and LC3-II for ratio determinations, may not be appropriate, and rather, the amount of LC3-II can be compared between samples.
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              Life and death partners: apoptosis, autophagy and the cross-talk between them.

              It is not surprising that the demise of a cell is a complex well-controlled process. Apoptosis, the first genetically programmed death process identified, has been extensively studied and its contribution to the pathogenesis of disease well documented. Yet, apoptosis does not function alone to determine a cell's fate. More recently, autophagy, a process in which de novo-formed membrane-enclosed vesicles engulf and consume cellular components, has been shown to engage in a complex interplay with apoptosis. In some cellular settings, it can serve as a cell survival pathway, suppressing apoptosis, and in others, it can lead to death itself, either in collaboration with apoptosis or as a back-up mechanism when the former is defective. The molecular regulators of both pathways are inter-connected; numerous death stimuli are capable of activating either pathway, and both pathways share several genes that are critical for their respective execution. The cross-talk between apoptosis and autophagy is therefore quite complex, and sometimes contradictory, but surely critical to the overall fate of the cell. Furthermore, the cross-talk is a key factor in the outcome of death-related pathologies such as cancer, its development and treatment.
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                Author and article information

                Journal
                Exp Mol Med
                EMM
                Experimental & Molecular Medicine
                Korean Society for Biochemistry and Molecular Biology
                1226-3613
                2092-6413
                29 February 2012
                19 January 2012
                : 44
                : 2
                : 109-120
                Affiliations
                Department of Molecular Science and Technology, Institute for Medical Sciences, Ajou University School of Medicine, Suwon 443-721, Korea.
                Author notes
                Corresponding author: Tel, 82-31-219-4552; Fax, 82-31-219-4530; kschoi@ 123456ajou.ac.kr
                Article
                10.3858/emm.2012.44.2.033
                3296807
                22257886
                Copyright © 2012 by The Korean Society for Biochemistry and Molecular Biology

                This is an open-access article distributed under the terms of the Creative Commons Attribution Non-Commercial License ( http://creativecommons.org/licenses/by-nc/3.0) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.

                Categories
                Review
                Featured Review: Autophagy and Human Diseases

                Molecular medicine

                autophagy, cell transformation, neoplastic, therapeutics, neoplasms, cell death

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