Autophagy and cancer: Dynamism of the ... - ResearchGate

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ARTICLE IN PRESS2 K. Tsuchihara et al. / Cancer Letters xxx (2008) xxx–xxxFinally, maturated autophagosomes fuse with the lysosome.The engulfed components, as well as the inner membraneof the autophagosome, are degraded by lysosomalhydrolases such as cathepsins. Autophagic processes havebeen well characterized in yeast, and more than 30autophagy-related genes (ATG) that encode the proteinsexecuting autophagy have been identified in the field ofyeast genetics [6]. Similar autophagic machineries havebeen observed in mammalian cells, and mammalian orthologsfor ATGs and other autophagy regulating moleculeshave also been identified [7]. The detailed molecularevents involved in these processes are described in anothercomprehensive review [8].Compared with the ubiquitin–proteasome system,which recognizes specific targets for degradation, autophagyhas been thought to engulf cytoplasmic constituentsnon-selectively. Recently, selective sequestration targetingspecific organelles or invasive microbes has also attractedattention [9].A very concise description of the function of autophagyis recycling; reducing waste and yielding resources. Damagedorganelles and misfolded proteins accumulate insenescent cells or cells under various stresses, such as oxidativestress and infection. The autophagic machinery degradesand reduces this cellular ‘‘garbage”. This functionis particularly effective in postmitotic cells, like neuronsand cardiac myocytes, since these cells cannot dilute suchsuperfluous components by undergoing cell division. Basalautophagy is constitutively observed in these postmitoticcells and may have an important function for the qualitycontrol of cellular components [10]. On the other hand,self-digested components provide nutrients and materialsfor newly constructed cellular structures. The amino acidsand fatty acids generated by autophagic degradation areused by the tricarboxylic acid (TCA) cycle to produce ATP,a main energy source for various cellular events. Methylpyruvate,a membrane-permeable derivative of pyruvatethat serves as a substrate for the TCA cycle, restored ATPproduction in autophagy-deficient cells. Furthermore, supplementationwith methylpyruvate rescued these cellsfrom metabolic stress-induced cell death [11].Observations of clinical samples and animal and cellularmodels have suggested a variety of physiological and pathologicalroles of autophagy, such as development, aging,host defense system, neurodegenerative diseases, muscleand cardiac diseases and cancer [12,13]. However, the relevanceof autophagy in tumor formation and progression isstill controversial. Although previous findings stronglysuggest that autophagy contributes to sustainable cell survival,anti-tumorigenic roles of autophagy have been alsomentioned. Here, we will summarize and discuss recentstudies of autophagy in cancer biology with the goal ofclarifying this issue.2. Autophagy protects cancer cells from starvationCancer cells in solid tumors obtain their necessarynutrients from blood flow. Aberrantly proliferating cancercells may have high bioenergetic demands and requiremore nutrients than non-cancerous cells. Tumor angiogenesisis a reasonable way to increase blood flow. During theinitial phase of tumorigenesis, tumor vessels have not yetbeen induced and the nutritional demands of tumor cellsare likely to surpass the supply from normal vasculature.Moreover, even after tumor vessels have been established,the oxygen tension and glucose concentration in locallyadvanced tumors remain at a low level. This suggests thatthe tumor microvasculature is functionally and structurallyimmature to support sufficient blood flow [14–16].Or, even once functionally and structurally adequate tumorvessels were established, soon or later, the balance of supplyand demand would be ruined by disorganized tumorcell proliferation resulting in the disastrous dysfunctionof the tumor vessels. The reduction of functional bloodflow is significant in clinically hypovascular tumors suchas pancreatic cancers [17]. Under these conditions, cancercells are likely to encounter chronic ischemia leading to ashortage of nutrients. Cancer cells might adapt themselvesto such a harsh microenvironment. In experimental cellculture systems, several cancer cells were resistant tonutrient-deprived conditions. For example, several pancreaticcancer- and colorectal cancer-derived cell linesshowed a survival rate of more than 50% after 48 h of culturein a medium completely lacking carbon and nitrogensources. Contrastingly, non-transformed fibroblasts werecompletely abolished within a day under the same conditions[18]. These findings suggest that cancer cells usealternative metabolic processes for their survival understarved conditions.Metabolic stresses reportedly induce apoptosis [19]. Inmany tumor cells, apoptosis is suppressed by the overexpressionof anti-apoptotic molecules or by the lack ofpro-apoptotic molecules. Less apoptosis may explain thelimited death of cancer cells under metabolic stresses,but the manner in which cancer cells obtain their necessarynutrients remains a mystery. One possible solutionis that the cells digest their own components and obtainamino acids as an alternative energy source. Autophagyseems ideal for cancer cells to maintain energy homeostasisin such an autonomous fashion. Autophagy is known tobe induced by different forms of metabolic stress, includingnutrient deprivation, growth factor deprivation, andhypoxia [13,20]. Since these conditions are often observedin physiological tumor microenvironments, autophagy islikely activated in cancer cells.We should also remind the apparent discrepancy betweenthe tumor ‘‘cell” doubling time and the tumor ‘‘volume”doubling time. The potential tumor cell doublingtime estimated by in vivo measurement of S-phase durationwas remarkably rapid in many solid tumors, a medianvalue of the order of 5 days. However, tumor volume doublingtime determined by radiological measurement of thesize of these tumors was much slower, months or years[21,22]. It suggests that about half of the proliferating cancercells are subsequently lost under tumor microenvironmentthough many of them have already acquired antiapoptoticfunction. Is there any physiological relevance ofsuch dynamism of cell kinetics? A potential scenario is thatthe vanishing cells serve nutritional sources for surroundingsurviving cells. In this sense, inducing cell death andsupporting cell survival under metabolic stresses are notPlease cite this article in press as: K. Tsuchihara et al., Autophagy and cancer: Dynamism of the metabolism of tumor cells..., Cancer Lett. (2008), doi:10.1016/j.canlet.2008.09.040
ARTICLE IN PRESSK. Tsuchihara et al. / Cancer Letters xxx (2008) xxx–xxx 3contradicting. Autophagy is also known to induce celldeath and possibly sacrifice some part of individual tumorcell for supporting a prolonged survival and continuousgrowth of other cells in the tumor mass.Several experimental methods have been used to identifythe activation of autophagy (Table 1) [23]. Measuringthe turnover of long-lived proteins provides a biochemicalclue to autophagy. Transmission electron microscopy revealsthe ultrastructure characteristics of the autophagosome.Autophagosome formation is also convenientlymonitored by following a phosphatidylethanolamine (PE)-conjugated form of yeast Atg8 or mammalian LC3, one ofthe mammalian orthologs of Atg8. During autophagy, LC3shifts from a soluble form to a membrane-bound form(LC3-II) and is incorporated into the autophagosomal innermembrane. The existence of autophagosome-incorporatedLC3-II can be detected biochemically (immunoblottingagainst LC3) or microscopically (immunocytochemistryagainst LC3 or exogenously expressed, fluorescently taggedLC3).The ultrastructure of autophagosomes in tumor cellshas been observed in several experimental systems,including a rat pancreatic cancer model [24]. A high potentialfor autophagic protein degradation was observed in anundifferentiated colon cancer cell line, HT29, and othertransformed cells [25]. Nutrient deprivation-induced LC3-II turnover was observed in colorectal cancer-derived celllines that showed resistance to starvation [26]. In additionto these findings in cultured cancer cells, the increasedexpression of autophagy-related proteins, including BNIP3and LC3, was observed specifically in colorectal and gastriccancer epithelia in surgically-resected specimens [26,27].Increasing evidence implies that autophagy has a cytoprotectiverole in cancer cells under metabolic stress.Transplanted epithelial tumors in which the Beclin 1 orAtg5 alleles were deleted showed a reduction in autophagyand an increase in cell death in regions exposed to metabolicstress [28–30]. Similarly, a cytoprotective functionof autophagy was observed in cultured cancer cells. The geneticinactivation of autophagy by the suppression of ATGexpression using RNA interference or the constitutive activationof PI3K induced cell death in response to metabolicstresses [28–30]. RNA interference of ATG5, Beclin 1 andATG7 enhanced tamoxifen-induced apoptosis in tamoxifen-resistantbreast cancer cell lines [31]. ATG5 knockdownalso enhanced the effect of alkylating drug-inducedcell death [32]. The pharmacological inhibition of autophagyalso induced nutrient deprivation-induced cell death[26]. Chloroquine, which interferes with lysosomal function,inhibited autophagy and suppressed Myc-inducedlymphomagenesis in a transgenic mouse model [32].Though the above findings suggest that autophagy maycontribute to tumor cell survival and tumor formation, themolecular mechanisms underlying the acquisition of vigorousautophagic activity in cancer cells have remained unclear.Recently, Kroemer and his colleagues reported apotential anti-autophagic function of cytoplasm-localizingp53 [33]. They observed the degradation of p53 under metabolicstresses followed by the induction of autophagy.Contrastingly, the loss of p53 resulted in the consistentactivation of autophagy in a series of cell lines. While overexpressionof wild-type p53 reduced the aberrant autophagosomeformation, a mutant p53 protein whichharbors a codon 175 mutation (R175H), which is frequentlyidentified in clinical human cancers, did not affectthe higher basal autophagic activity of p53-null cells.Though most of their observation was limited in the culturecell system, further studies which clarify its relevanceto tumor formation and progression are awaited.3. Autophagy and tumor suppressionAn opposite perspective was presented by a studyexamining a Bcl-2-binding protein, Beclin 1. Levine andTable 1Recommanded methods for monitoring autophagy in higher eukaryotes [23].CriteriaMethodsMonitoring phagophore and autophagosome formation by steady state methods1. Electron microscopy (increase in autophagosome quantity) Quantitative electron microscopy, immunoelectron microscopy2. Atg8/LC3 Western blotting and ubiquitin-like protein conjugation Western blotsystems(increase in the amount of LC3-II, and Atg12–Atg5 conjugation)3. Fluorescence microscopy (increase in punctate LC3 (or Atg18)) Fluorescence, immunofluorescence and immunoelectron microscopy4. TOR and Atg1 kinase activity Western blot, immunoprecipitation or kinase assays5. Transcriptional regulation Northern blot or qRT-PCRMonitoring autophagy by flux measurements1. Autophagic protein degradation Turnover of long-lived proteins2. Turnover of LC3-II Western blot +/ lysosomal fusion or degradation inhibitors3. GFP-Atg8/LC3 lysosomal delivery, and proteolysisFluorescence microscopy, FACS Western blot +/ lysosomal fusion or degradation(to generate free GFP)inhibitors4. p62 Western blot Western blot with qRT-PCR or Northern blot to assess transcription5. Autophagic sequestration assays Lysosomal accumulation by biochemical or multilabel fluorescence techniques6. Turnover of autophagic compartments Electron microscopy morphometry/stereology7. Autophagosome-lysosome colocalization and dequenching assay Fluorescence microscopy8. Sequestration and processing assays in plants Chimeric RFP fluorescence and processing, light and electron microscopy9. Tandem mRFP-GFP fluorescence microscopy Fluorescence microscopy of tandem mRFP-GFP-LC310. Tissue fractionation Centrifugation, Western blot and electron microscopy11. Analyses in vivo Fluorescence microscopy and immunohistochemistryPlease cite this article in press as: K. Tsuchihara et al., Autophagy and cancer: Dynamism of the metabolism of tumor cells..., Cancer Lett. (2008), doi:10.1016/j.canlet.2008.09.040
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