1596 Review Diet, Autophagy, and Cancer: A Review Keith Singletary1 and John Milner2 1Department of Food Science and Human Nutrition, University of Illinois, Urbana, Illinois and 2Nutritional Science Research Group, Division of Cancer Prevention, National Cancer Institute, Bethesda, Maryland Abstract A host of dietary factors can influence various cellular standing of the interactions among bioactive food processes and thereby potentially influence overall constituents, autophagy, and cancer. Whereas a variety cancer risk and tumor behavior. In many cases, these of food components including vitamin D, selenium, factors suppress cancer by stimulating programmed curcumin, resveratrol, and genistein have been shown to cell death. However, death not only can follow the stimulate autophagy vacuolization, it is often difficult to well-characterized type I apoptotic pathway but also can determine if this is a protumorigenic or antitumorigenic proceed by nonapoptotic modes such as type II (macro- response. Additional studies are needed to examine autophagy-related) and type III (necrosis) or combina- dose and duration of exposures and tissue specificity tions thereof. In contrast to apoptosis, the induction of in response to bioactive food components in transgenic macroautophagy may contribute to either the survival or and knockout models to resolve the physiologic impli- death of cells in response to a stressor. This review cations of early changes in the autophagy process. highlights current knowledge and gaps in our under- (Cancer Epidemiol Biomarkers Prev 2008;17(7):1596–610) Introduction A wealth of evidence links diet habits and the accompa- degradation. Paradoxically, depending on the circum- nying nutritional status with cancer risk and tumor stances, this process of ‘‘self-consumption’’ may be behavior (1-3). Characterizing how essential and nones- involved in both health and disease progression (9-13). sential dietary constituents can modulate single or This review will highlight the current knowledge and multiple hallmark processes of carcinogenesis (4) is gaps in our understanding about the ingestion of fundamental to identifying more effective mechanistic- bioactive food constituents, autophagy, and cancer and based strategies for cancer prevention and therapy. To will suggest issues and opportunities for future study. date, numerous structurally diverse biologically active food components have been reported to interfere with intercellular and intracellular signaling pathways that Functions and Detection of Autophagy may activate critical cellular proto-oncogenes or inacti- vate tumor suppressor genes (3, 4). Consequently, these In eukaryotic cells, two major strategies for protein altered pathways can modify growth autonomy, prolif- degradation exist: the ubiquitin-proteasome system for eration, angiogenesis, and metastasis and dysregulate disposal of short-lived proteins and the lysosomal system cellular processes. Furthermore, a host of dietary factors for degradation of intracellular and extracellular pro- can stimulate programmed cell death (5). In this regard, teins. As a part of the lysosomal system, autophagy is an it is now apparent that the demise of cancer cells not evolutionarily conserved cellular strategy to engulf and only can follow the well-characterized type I apoptotic degrade long-lived cytosolic proteins and organelles to pathway but also can proceed by stimulation of non- provide substrates for energy metabolism and to recycle apoptotic modes such as type II (macroautophagy- amino acids, fatty acids, and nucleotides for the related) and type III (necrosis) or combinations thereof biosynthetic needs of the cell. Typically, these metabolic (6-8). However, in contrast to apoptosis, the induction of changes are constitutively active at low levels in cells and macroautophagy (hereafter referred to as autophagy) also may occur in the face of nutrient deprivation, may contribute to either cell survival or death in growth factor withdrawal, or other stressors (14, 15). response to a stressor. As described in more detail The initiation stage of autophagy involves formation below, autophagy is a degradative process by which of a double-membrane vacuole, the autophagosome, cytoplasmic constituents of cells are engulfed within a which nonselectively sequesters as cargo cytoplasmic cytoplasmic vacuole and delivered to the lysosome for proteins, mitochondria, endoplasmic reticulum, and ribosomes(Fig.1).Thesecomponents,aswellas endocytosed materials, are transported during the mat- Received 12/19/07; revised 3/28/08; accepted 4/18/08. uration stage of autophagy for eventual degradation by Requests for reprints: Keith Singletary, Department of Food science and Human lysosomal hydrolases following fusion with the lysosome. Nutrition, University of Illinois, 905 South Goodwin Avenue, Urbana, IL 61801. Phone: 217-244-4497; Fax: 217-265-0925. E-mail: [email protected] The resulting degraded cargo components can then be Copyright D 2008 American Association for Cancer Research. returned to the cytoplasm by permeases in the lysosomal doi:10.1158/1055-9965.EPI-07-2917 membrane for reuse. A similar process of ‘‘self eating,’’ Cancer Epidemiol Biomarkers Prev 2008;17(7). July 2008 Downloaded from cebp.aacrjournals.org on October 2, 2021. © 2008 American Association for Cancer Research. Cancer Epidemiology,Biomarkers & Prevention 1597 Figure 1. Steps in the for- mation of autolysosome dur- ing autophagy. (Modified from ref. 214). referred to as chaperone-mediated autophagy, involves Autophagic cell death is identified by extensive inclu- selectively delivering soluble cytosolic proteins that sion of cytoplasm and organelles within autophago- contain a chaperone-mediated autophagy targeting motif somes and localization of microtubule-associated light for incorporation into lysosomes (16). In this case, a chain 3 (LC3) protein into autophagosomal membranes. chaperone-cochaperone complex transports the specific There is little, if any, caspase activation and DNA substrate to the lysosomal membrane for translocation fragmentation, and the cytoskeleton initially remains into the lysosome following binding to lysosomal- intact. This is in contrast to apoptosis during which associated membrane protein 2A. A third process of there is chromosome condensation, DNA fragmentation, self-cannibalization is microautophagy, whereby cyto- usually caspase activation, and membrane rearrange- plasmic constituents are engulfed by invaginations of the ment and blebbing. In apoptosis and autophagy, the lysosomal membrane and subsequently degraded (17). remains of cells are disposed of by phagocytosis The engulfment of peroxisomes and mitochondria by without an inflammatory response, in contrast to the autophagosomes for transport to lysosomal degradation massive cellular disintegration and subsequent inflam- is referred to as pexophagy and mitophagy, respectively. mation accompanying necrotic programmed cell death Autophagy can serve a variety of functions. It is a means (8, 12, 25). Morphologic features of more than one type by which cells can respond to metabolic stress or adapt to of cell death program can be observed in the same cell changing environment. Thus, it is a basal housekeeping (25-28). process present in most cells, sometimes referred to as a The occurrence of autophagy can be confirmed by garbage disposal mechanism (18). Its purpose is to dispose several methodologies (17). Foremost is the appearance of of defective organelles and macromolecular structures, as autophagic vacuoles. These vacuoles may be identified by well as cytosolic components such as damaged and transmission electron microscopy, which is considered aggregate-prone proteins. It has cell-specific developmen- one of the gold standards used to determine double- tal functions such as elimination of the ribosomes and membrane vacuole structures that engulf cytoplasmic mitochondria from erythrocytes following removal of the components (such as endoplasmic reticulum and ribo- nucleus. Autophagy also functions during initial postpar- somes) and organelles (such as mitochondria). Additional tum survival of the neonate, normal tissue remodeling, confirmation of autophagy can include immunohisto- embryogenesis, and counteracting age-related physiolog- chemical evidence (29) that there is punctuate localization ic changes such as those due to oxidative stress. It can of the LC3 protein within the membranes of the putative contribute during infections to presentation of foreign autophagic vacuoles. LC3 protein is normally dispersed antigens for recognition by the immune system and to diffusely throughout the cytoplasm in healthy cells and killing of bacteria ingested by cells. Furthermore, autoph- accumulates within the autophagosomal membrane in a agy has been associated with development of pathologies form covalently linked to phosphatidylethanolamine such as neurodegenerative disease and carcinogenesis during stimulation of autophagic vacuolization (30). The (10, 11, 19-24). degradation rate of long-lived proteins in response to a As a type of programmed cell death, autophagy is treatment may also be used as an indicator that morphologically distinct from apoptosis and necrosis. autophagy is triggered (17, 31). Cancer Epidemiol Biomarkers Prev 2008;17(7). July 2008 Downloaded from cebp.aacrjournals.org on October 2, 2021. © 2008 American Association for Cancer Research. 1598 Diet,Autophagy,and Cancer Induction and Regulation of Autophagy mTOR activity. Alternatively,