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      Dual role of autophagy in hallmarks of cancer

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          Abstract

          Evolutionarily conserved across eukaryotic cells, macroautophagy (herein autophagy) is an intracellular catabolic degradative process targeting damaged and superfluous cellular proteins, organelles, and other cytoplasmic components. Mechanistically, it involves formation of double-membrane vesicles called autophagosomes that capture cytosolic cargo and deliver it to lysosomes, wherein the breakdown products are eventually recycled back to the cytoplasm. Dysregulation of autophagy often results in various disease manifestations, including neurodegeneration, microbial infections, and cancer. In the case of cancer, extensive attention has been devoted to understanding the paradoxical roles of autophagy in tumor suppression and tumor promotion. In this review, while we summarize how this self-eating process is implicated at various stages of tumorigenesis, most importantly, we address the link between autophagy and hallmarks of cancer. This would eventually provide a better understanding of tumor dependence on autophagy. We also discuss how therapeutics targeting autophagy can counter various transformations involved in tumorigenesis. Finally, this review will provide a novel insight into the mutational landscapes of autophagy-related genes in several human cancers, using genetic information collected from an array of cancers.

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          Most cited references66

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          Cancer. p53, guardian of the genome.

          D P Lane (1992)
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            Pancreatic cancers require autophagy for tumor growth.

            Macroautophagy (autophagy) is a regulated catabolic pathway to degrade cellular organelles and macromolecules. The role of autophagy in cancer is complex and may differ depending on tumor type or context. Here we show that pancreatic cancers have a distinct dependence on autophagy. Pancreatic cancer primary tumors and cell lines show elevated autophagy under basal conditions. Genetic or pharmacologic inhibition of autophagy leads to increased reactive oxygen species, elevated DNA damage, and a metabolic defect leading to decreased mitochondrial oxidative phosphorylation. Together, these ultimately result in significant growth suppression of pancreatic cancer cells in vitro. Most importantly, inhibition of autophagy by genetic means or chloroquine treatment leads to robust tumor regression and prolonged survival in pancreatic cancer xenografts and genetic mouse models. These results suggest that, unlike in other cancers where autophagy inhibition may synergize with chemotherapy or targeted agents by preventing the up-regulation of autophagy as a reactive survival mechanism, autophagy is actually required for tumorigenic growth of pancreatic cancers de novo, and drugs that inactivate this process may have a unique clinical utility in treating pancreatic cancers and other malignancies with a similar dependence on autophagy. As chloroquine and its derivatives are potent inhibitors of autophagy and have been used safely in human patients for decades for a variety of purposes, these results are immediately translatable to the treatment of pancreatic cancer patients, and provide a much needed, novel vantage point of attack.
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              LC3 conjugation system in mammalian autophagy

              Autophagy is the bulk degradation of proteins and organelles, a process essential for cellular maintenance, cell viability, differentiation and development in mammals. Autophagy has significant associations with neurodegenerative diseases, cardiomyopathies, cancer, programmed cell death, and bacterial and viral infections. During autophagy, a cup-shaped structure, the preautophagosome, engulfs cytosolic components, including organelles, and closes, forming an autophagosome, which subsequently fuses with a lysosome, leading to the proteolytic degradation of internal components of the autophagosome by lysosomal lytic enzymes. During the formation of mammalian autophagosomes, two ubiquitylation-like modifications are required, Atg12-conjugation and LC3-modification. LC3 is an autophagosomal ortholog of yeast Atg8. A lipidated form of LC3, LC3-II, has been shown to be an autophagosomal marker in mammals, and has been used to study autophagy in neurodegenerative and neuromuscular diseases, tumorigenesis, and bacterial and viral infections. The other Atg8 homologues, GABARAP and GATE-16, are also modified by the same mechanism. In non-starved rats, the tissue distribution of LC3-II differs from those of the lipidated forms of GABARAP and GATE-16, GABARAP-II and GATE-16-II, suggesting that there is a functional divergence among these three modified proteins. Delipidation of LC3-II and GABARAP-II is mediated by hAtg4B. We review the molecular mechanism of LC3-modification, the crosstalk between LC3-modification and mammalian Atg12-conjugation, and the cycle of LC3-lipidation and delipidation mediated by hAtg4B, as well as recent findings concerning the other two Atg8 homologues, GABARAP and GATE-16. We also highlight recent findings regarding the pathobiology of LC3-modification, including its role in microbial infection, cancer and neuromuscular diseases.
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                Author and article information

                Journal
                Oncogene
                Oncogene
                Springer Nature
                0950-9232
                1476-5594
                December 2017
                :
                :
                Article
                10.1038/s41388-017-0046-6
                29255248
                ddd03adb-5b37-45e9-8a53-3ee2fc266fa9
                History

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