Autophagy and Tumor Metabolism.pdf

Autophagy and Tumor MetabolismAlec C.Kimmelman1 and Eileen White2,31Perlmutter Cancer Center,Department of Radiation Oncology,NYU Medical School,New York,NY 100162Rutgers Cancer Institute of New Jersey,New Brunswick,NJ 089033Department of Molecular Biology and Biochemistry,Rutgers University,Piscataway,NJ,08854AbstractAutophagy is a critical cellular process that generally protects cells and organisms from stressors such as nutrient deprivation.In addition to its role in normal physiology,autophagy plays a role in pathological processes such as cancer.Indeed,there has been substantial work exploring the complex and context-dependent role of autophagy in cancer.One of the emerging themes is that in certain cancer types,autophagy is important to support tumor growth and therefore inhibiting autophagy as a therapeutic approach is actively being tested in clinical trials.A key mechanism of how autophagy promotes the growth and survival of various cancers is its ability to support cellular metabolism.The diverse metabolic fuel sources that can be produced by autophagy provides tumors with metabolic plasticity and can allow them to thrive in what can be an austere microenvironment.Therefore,understanding how autophagy can fuel cellular metabolism will enable more effective combinatorial therapeutic strategies.IntroductionMacroautophagy(hereafter referred to as autophagy)is a catabolic process whereby intracellular components(cargo)are enveloped in double-membraned vesicles,known as an autophagososomes,which ultimately fuse with lysosomes where the contents are degraded and recycled into the cytosol(Levine and Klionsky,2004).In addition to macroautophagy,there are two additional types of autophagy,microautophagy and chaperone-mediated autophagy,which differ in how the particular cargo is delivered to lysosomes and are the subject of several excellent reviews(Bejarano and Cuervo,2010;Kon and Cuervo,2010;Mijaljica et al.,2011).Corresponding Authors:Dr.Alec Kimmelman,Perlmutter Cancer Center,Department of Radiation Oncology,NYU Medical School,New York,NY 10016.V:646.501.8940.Alec.Kimmelmannyumc.org,(lead contact);Dr.Eileen White,Rutgers Cancer Institute of New Jersey,195 Little Albany Street,New Brunswick,NJ 08903-2681,V:732-235-5329,epwhitecinj.rutgers.edu.Disclosure of Potential Conflicts of Interest:ACK and EW have financial interests in Vescor Therapeutics,LLC.A.C.K.is an inventor on patents pertaining to Kras regulated metabolic pathways,redox control pathways in pancreatic cancer,targeting GOT1 as a therapeutic approach,and the autophagic control of iron metabolism.A.C.K is on the SAB of Cornerstone Pharmaceuticals.EW is on the SAB of Forma TherapeuticsPublishers Disclaimer:This is a PDF file of an unedited manuscript that has been accepted for publication.As a service to our customers we are providing this early version of the manuscript.The manuscript will undergo copyediting,typesetting,and review of the resulting proof before it is published in its final citable form.Please note that during the production process errors may be discovered which could affect the content,and all legal disclaimers that apply to the journal pertain.HHS Public AccessAuthor manuscriptCell Metab.Author manuscript;available in PMC 2018 May 02.Published in final edited form as:Cell Metab.2017 May 02;25(5):10371043.doi:10.1016/j.cmet.2017.04.004.Author ManuscriptAuthor ManuscriptAuthor ManuscriptAuthor ManuscriptWhile initially thought to be only a mechanism of cell death,it is now recognized through multiple in vivo experiments that autophagy is a major reactive survival mechanism,although there are situations where it can contribute to cell death(Debnath et al.,2005;Levy and Thorburn,2011;White,2016).Autophagy promotes cellular and mammalian survival during periods of stress,particularly metabolic stress induced by nutrient deprivation(Figure 1).While most tissues have low levels of basal autophagy,it is significantly stimulated by the stressed state the most well studied is starvation and this is tightly regulated by the mTOR and AMP kinase pathways(Kim and Guan,2015;Mihaylova and Shaw,2011;Neufeld,2010).Through the seminal work of multiple groups,we have a significant understanding of the autophagic machinery and how it functions in the formation of autophagosomes through the fusion to lysosomes and culminating in the degradation of the autophagosome cargo and recycling of the breakdown products.Indeed,this has been the subject of multiple excellent reviews(Feng et al.,2014;Klionsky and Codogno,2013;Ktistakis and Tooze,2016).The process is directed and executed by a series of proteins encoded by autophagy-related genes(ATG)and there are currently more than 30 ATG genes that are involved in all aspects of the process(Klionsky et al.,2011;Mizushima,2007).The process begins with the formation of the autophgosome from various sources of cellular membranes.During this process,cytosol,organelles and proteins become trapped in the forming autophagosome.Initially,autophagy was thought to be a non-selective degradative process whereby bulk cytoplasm and the proteins and organelles contained within it were sequestered by proximity into forming autophagosomes.More recently,it has been shown that there is significant selectivity for cargo and there are multiple selective autophagy pathways that are named for the particular cargo that is degraded(e.g.mitochondria mitophagy;ferritin ferritinophagy;ER reticulophagy;bacteria xenophagy)(Khaminets et al.,2016;Mancias and Kimmelman,2016).Cargo is recognized by specific receptor proteins that bind to cargo and then often interact with ATG8 proteins that are inserted into the autophagsomal membrane.Once formed,autophagosomes fuse with lysosomes where the cargo is degraded via lysosomal enzymes and the resultant degraded products are recycled into the cytosol for use as substrates in metabolic and biosynthetic pathways.Autophagy and CancerGiven its important role in tissue homeostasis,it is not surprising that the dysregulation of autophagy has been linked to multiple disease states such as cancer and neurodegenerative disease.The role of autophagy in cancer has been of particular interest and the work in this area has greatly expanded over the past several years(reviewed in(Amaravadi et al.,2016;Galluzzi et al.,2015;Guo and White,2017;Liu and Debnath,2016;White,2015).Autophagy has a complex role in cancer and its function can be dependent on biological factors such as the tumor type,driving oncogene and tumor suppressor gene constellation of a tumor,as well as technical aspects such as the model system used to investigate its function(Amaravadi et al.,2016;Nyfeler and Eng,2016).Initially,autophagy was thought to have a tumor suppressive role.This was based on two major lines of evidence.First,many of the activating mutations in oncogenes(e.g.PIK3CA)or inactivation of tumor suppressor genes(e.g.PTEN)would be predicted to inhibit autophagy.Second,deletion of autophagy Kimmelman and WhitePage 2Cell Metab.Author manuscript;available in PMC 2018 May 02.Author ManuscriptAuthor ManuscriptAuthor ManuscriptAuthor Manuscriptgenes in the setting of certain mouse models can result in the initiation of neoplasia.The initial identification of this phenotype was in Beclin1(ATG6 ortholog)heterozygous mice,whereby these mice developed various neoplasms(Qu et al.,2003;Yue et al.,2003).An important aspect to note is that in this case autophagy was only partly attenuated as a functional copy of Beclin1 was intact in the mice.In contrast,when ATG5 was completely deleted in a mosaic fashion in the whole mouse,thereby completely inhibiting autophagy in those cells with the deletion,the results were different(Takamura et al.,2011).Interestingly,the only tissue that developed any neoplastic change was the liver,indicating that there are significant susceptibilities based on tissue type.Additionally,the lesions that developed were benign liver tumors,which indicates that autophagy is required for progression to malignancy and explains why Beclin1 heterozygous mice with diminished but intact autophagy can develop malignant tumors.Similar results were obtained when Atg7 deletion was targeted to the liver(Inami et al.,2011).The need for intact autophagy to progress to the malignant state may explain the apparent lack of mutations in canonical autophagy genes in human cancers.Beclin1 was initially identified as a haploinsufficient tumor suppressor gene given that multiple breast and ovarian tumors demonstrated loss of one allele(Liang et al.,1999),although indication that these are passenger deletions given its proximity to the BRCA1 tumor suppressor has recently been suggested(Laddha et al.,2014).In contrast to its role in constraining tumor initiation,autophagy has been shown to have a critical pro-tumorigenic role in multiple cancer types.Initial studies demonstrated that autophagy was elevated in hypoxic regions of tumors and that the process could promote tumor cell survival upon a variety of stressors such as nutrient and oxygen deprivation(Degenhardt et al.,2006).These findings have been extended to show that,in many cases,autophagy can promote survival during the stress of therapies(chemotherapy,radiotherapy,and targeted agents)and thus promotes therapeutic resistance(Amaravadi et al.,2011;Rebecca and Amaravadi,2016;Thorburn et al.,2014).In addition to therapeutic resistance,autophagy has been shown to play a critical role in tumor maintenance.Many tumor types demonstrate elevated basal autophagy and this can be seen in a cell autonomous manner,even under nutrient replete conditions(Kimmelman,2011;White,2015).Indeed,inhibition of autophagy genetically or pharmacologically can slow tumor growth in various model systems.Genetically engineered mouse models(GEMMs)of cancers have been particularly informative and helped define the critical role of autophagy in multiple cancers,including pancreatic cancer(Rosenfeldt et al.,2013;Yang and Kimmelman,2014),lung cancer(Guo et al.,2013;Karsli-Uzunbas et al.,2014;Rao et al.,2014;Strohecker et al.,2013;Strohecker and White,2014),prostate cancer(Santanam et al.,2016),melanoma(Xie et al.,2015),glioblastoma(Gammoh et al.,2016),and breast cancer(Huo et al.,2013;Wei et al.,2011).Additionally,the contribution of non-cell autonomous autophagy in tumor growth was also demonstrated in model organisms such as Drosophila(Katheder et al.,2017).This body of work has defined important cell autonomous and non-autonomous roles of autophagy in tumor maintenance.While the role of autophagy in promoting tumor growth is likely multifaceted,one of its key functions,and the subject of this review,is its contribution to the metabolism of tumors.Kimmelman and WhitePage 3Cell Metab.Author manuscript;available in PMC 2018 May 02.Author ManuscriptAuthor ManuscriptAuthor ManuscriptAuthor ManuscriptMetabolism and CancerIt has long been known that cancers have altered metabolism to help meet the needs of cells that have the potential for unconstrained proliferation(Pavlova and Thompson,2016;Vander Heiden and DeBerardinis,2017).This includes a significant shift towards anabolic metabolism to biosynthesize the building blocks needed to support proliferation.Tumor cells have been shown to use a variety of fuel sources to accomplish these goals.Indeed,work has shown that some tumors have increased aerobic glycolysis(Warburg Effect)(Vander Heiden et al.,2009).Additionally,mitochondrial metabolism has been demonstrated to be critical for ATP production,redox balance,as well as the biosynthesis of other key metabolites in various tumor types(Weinberg and Chandel,2015).Because of these changes in metabolism,there is potential for therapeutic targeting and preclinical studies have supported this approach with a variety of targets.Some of these efforts are making their way into early-phase clinical trials in human patients(Kishton and Rathmell,2015;Ross and Critchlow,2014;Vander Heiden,2011).Two major concerns with this approach are the metabolic plasticity of cancer cells,allowing rapid metabolic rewiring as a compensatory response,as well as potential toxicity of targeting fundamental metabolic pathways in rapidly proliferating“normal”cells which also may rely on them for growth and survival.However,there is a long track record of targeting metabolic pathways in cancer therapy with anti-metabolites continuing to be a major component of the successful treatment of multiple cancer types.Autophagy and cancer metabolismGiven the diverse substrates that can be degraded via autophagy,it is not surprising that autophagy has the potential to fuel nearly all aspects of central carbon metabolism(Guo and White,2017;Rabinowitz and White,2010)(Figure 1).For example,degradation of carbohydrates into sugars can fuel glycolysis;proteins into amino acids can fuel the TCA cycle;DNA into nucleosides can fuel glycolysis;and lipids into fatty acids can fuel the TCA cycle.Given the multitude of metabolic pathways that autophagy can feed into,it can provide normal and tumor cells with tremendous metabolic plasticity.This is particularly relevant to the multiple metabolic stresses a growing tumor faces ranging from hypoxia,to nutrient limitation,and even that from the therapies themselves.Early studies that examined the importance of autophagy in cancer implicated its potential role in supporting tumor metabolism.Debnath and colleagues demonstrated that autophagy is important for Ras transformation and this is in part,due to its maintenance of glycolytic capacity(Lock et al.,2011).A similar role for autophagy in glycolysis was shown in polyoma middle T antigen(PMyT)driven breast cancer(Wei et al.,2011).The importance of autophagy in sustaining glycolysis was also demonstrated in hematological malignancies including chronic myeloid leukemia(Karvela et al.,2016).In a related fashion,our labs demonstrated that autophagy was important in growth of Ras-driven tumors(Guo et al.,2011;Yang et al.,2011).However,in the systems we used,maintenance of mitochondrial metabolism was significantly impaired in the setting of autophagy loss.Indeed,in both pancreatic cancer cells where autophagy was inhibited either pharmacologically with the lysosomal inhibitor chloroquine(CQ)or genetically using RNAi to essential autophagy Kimmelman and WhitePage 4Cell Metab.Author manuscript;available in PMC 2018 May 02.Author ManuscriptAuthor ManuscriptAuthor ManuscriptAuthor Manuscriptgenes,oxidative phosphorylation was significantly decreased.Consistent with this,growth of these cells could be partially rescued by adding mitochondrial fuel sources,such as pyruvate.In a similar fashion,oxidative phosphorylation and a variety of TCA cycle metabolites were decreased in autophagy incompetent,Hras transformed immortalized mouse kidney cells.Here,autophagy was also critical for maintaining mitochondrial quality control through mitophagy and the autophagy deficient transformed cells accu