Synthesis of cytochrome C oxidase 2: a p53-dependent metabolic regulator that promotes respiratory function and protects glioma and colon cancer cells from hypoxia-induced cell death

P53 has an important role in the processing of starvation signals. P53-dependent molecular mediators of the Warburg effect reduce glucose consumption and promote mitochondrial function. We therefore hypothesized that the retention of wild-type p53 characteristic of primary glioblastomas limits metabolic demands induced by deregulated signal transduction in the presence of hypoxia and nutrient depletion. Here we report that short hairpin RNA-mediated gene suppression of wild-type p53 or ectopic expression of mutant temperature-sensitive dominant-negative p53(V135A) increased glucose consumption and lactate production, decreased oxygen consumption and enhanced hypoxia-induced cell death in p53 wild-type human glioblastoma cells. Similarly, genetic knockout of p53 in HCT116 colon carcinoma cells resulted in reduced respiration and hypersensitivity towards hypoxia-induced cell death. Further, wild-type p53 gene silencing reduced the expression of synthesis of cytochrome c oxidase 2 (SCO2), an effector necessary for respiratory chain function. An SCO2 transgene reverted the metabolic phenotype and restored resistance towards hypoxia in p53-depleted and p53 mutant glioma cells in a rotenone-sensitive manner, demonstrating that this effect was dependent on intact oxidative phosphorylation. Supplementation with methyl-pyruvate, a mitochondrial substrate, rescued p53 wild-type but not p53 mutant cells from hypoxic cell death, demonstrating a p53-mediated selective aptitude to metabolize mitochondrial substrates. Further, SCO2 gene silencing in p53 wild-type glioma cells sensitized these cells towards hypoxia. Finally, lentiviral gene suppression of SCO2 significantly enhanced tumor necrosis in a subcutaneous HCT116 xenograft tumor model, compatible with impaired energy metabolism in these cells. These findings demonstrate that glioma and colon cancer cells with p53 wild-type status can skew the Warburg effect and thereby reduce their vulnerability towards tumor hypoxia in an SCO2-dependent manner. Targeting SCO2 may therefore represent a valuable strategy to enhance sensitivity towards hypoxia and may complement strategies targeting glucose metabolism.     

GRP78 enhances the glutamine metabolism to support cell survival from glucose deficiency by modulating the β-catenin signaling

To support the high rates of proliferation, cancer cells undergo the metabolic reprogramming: aerobic glycolysis and glutamine addiction. Though glucose regulated protein 78 (GRP78) is a glucose-sensing protein and frequently highly expressed in tumor cells, its roles in glucose and glutamine metabolic regulation remain poorly unknown. We report here that glucose deficiency-induced GRP78 enhances β-catenin signaling and consequently promotes its downstream c-Myc-mediated glutamine metabolism in colorectal cancer cells. Mechanistically, GRP78 elevates intracellular free β-catenin level via disruption of adenomatous polyposis coli (APC)-β-catenin and E-cadherin-β-catenin protein complexes. Notably, overexpression of GRP78 causes APC protein downregulation in proteasome- and lysosome-independent manners. Further mechanistic studies reveal that GRP78 facilitates the extracellular release of APC, thereby rendering the liberation of β-catenin from APC. Furthermore, GRP78 acts through both hindering E-cadherin expression and impairing the interaction of E-cadherin with β-catenin to indirectly and directly influence E-cadherin-β-catenin complex stability. Our study reveals that GRP78 is a novel molecular link between metabolic alterations and signal transduction during tumor progression.     

Transglutaminase 2 reprogramming of glucose metabolism in mammary epithelial cells via activation of inflammatory signaling pathways

Aberrant glucose metabolism characterized by high levels of glycolysis, even in the presence of oxygen, is an important hallmark of cancer. This metabolic reprogramming referred to as the Warburg effect is essential to the survival of tumor cells and provides them with substrates required for biomass generation. Molecular mechanisms responsible for this shift in glucose metabolism remain elusive. As described herein, we found that aberrant expression of the proinflammatory protein transglutaminase 2 (TG2) is an important regulator of the Warburg effect in mammary epithelial cells. Mechanistically, TG2 regulated metabolic reprogramming by constitutively activating nuclear factor (NF)‐κB, which binds to the hypoxia‐inducible factor (HIF)?1α promoter and induces its expression even under normoxic conditions. TG2/NF‐κB‐induced increase in HIF‐1α expression was associated with increased glucose uptake, increased lactate production and decreased oxygen consumption by mitochondria. Experimental suppression of TG2 attenuated HIF‐1α expression and reversed downstream events in mammary epithelial cells. Moreover, downregulation of p65/RelA or HIF‐1α expression in these cells restored normal glucose uptake, lactate production, mitochondrial respiration and glycolytic protein expression. Our results suggest that aberrant expression of TG2 is a master regulator of metabolic reprogramming and facilitates metabolic alterations in epithelial cells even under normoxic conditions. A TG2‐induced shift in glucose metabolism helps breast cancer cells to survive under stressful conditions and promotes their metastatic competence.     

Tumor cell metabolism: an integral view

Cancer is a genetic disease that is caused by mutations in oncogenes, tumor suppressor genes and stability genes. The fact that the metabolism of tumor cells is altered has been known for many years. However, the mechanisms and consequences of metabolic reprogramming have just begun to be understood. In this review, an integral view of tumor cell metabolism is presented, showing how metabolic pathways are reprogrammed to satisfy tumor cell proliferation and survival requirements. In tumor cells, glycolysis is strongly enhanced to fulfill the high ATP demands of these cells; glucose carbons are the main building blocks in fatty acid and nucleotide biosynthesis. Glutaminolysis is also increased to satisfy NADPH regeneration, whereas glutamine carbons replenish the Krebs cycle, which produces metabolites that are constantly used for macromolecular biosynthesis. A characteristic feature of the tumor microenvironment is acidosis, which results from the local increase in lactic acid production by tumor cells. This phenomenon is attributed to the carbons from glutamine and glucose, which are also used for lactic acid production. Lactic acidosis also directs the metabolic reprogramming of tumor cells and serves as an additional selective pressure. Finally, we also discuss the role of mitochondria in supporting tumor cell metabolism.     

胶质瘤代谢异常调控细胞自噬的研究进展

细胞代谢异常是肿瘤的基本特征之一。随着研究的不断深入,人们逐渐认识到肿瘤代谢重编程在肿瘤发生和发展中扮演着极其重要的角色,自胶质瘤中IDH1突变被发现以来,代谢异常在胶质瘤中也越来越引起科学家的兴趣和重视。研究表明胶质瘤细胞代谢重编程可导致肿瘤细胞产生大量异常的代谢产物,这些代谢产物不但对胶质瘤细胞的增殖等自身生物学行为发挥重要作用,而且可重塑其肿瘤微环境。自噬是肿瘤细胞有效利用自身代谢产物和适应代谢应激微环境的重要途径,因此针对肿瘤细胞代谢和自噬之间的研究可能为胶质瘤的治疗提供新的策略。本综述根据最新的研究证据和进展,对胶质瘤代谢重编程和自噬之间的关系进行总结。     

Myc and Ras oncogenes engage different energy metabolism programs and evoke distinct patterns of oxidative and DNA replication stress

Both Myc and Ras oncogenes impact cellular metabolism, deregulate redox homeostasis and trigger DNA replication stress (RS) that compromises genomic integrity. However, how are such oncogene‐induced effects evoked and temporally related, to what extent are these kinetic parameters shared by Myc and Ras, and how are these cellular changes linked with oncogene‐induced cellular senescence in different cell context(s) remain poorly understood. Here, we addressed the above‐mentioned open questions by multifaceted comparative analyses of human cellular models with inducible expression of c‐Myc and H‐RasV12 (Ras), two commonly deregulated oncoproteins operating in a functionally connected signaling network. Our study of DNA replication parameters using the DNA fiber approach and time‐course assessment of perturbations in glycolytic flux, oxygen consumption and production of reactive oxygen species (ROS) revealed the following results. First, overabundance of nuclear Myc triggered RS promptly, already after one day of Myc induction, causing slow replication fork progression and fork asymmetry, even before any metabolic changes occurred. In contrast, Ras overexpression initially induced a burst of cell proliferation and increased the speed of replication fork progression. However, after several days of induction Ras caused bioenergetic metabolic changes that correlated with slower DNA replication fork progression and the ensuing cell cycle arrest, gradually leading to senescence. Second, the observed oncogene‐induced RS and metabolic alterations were cell‐type/context dependent, as shown by comparative analyses of normal human BJ fibroblasts versus U2‐OS sarcoma cells. Third, the energy metabolic reprogramming triggered by Ras was more robust compared to impact of Myc. Fourth, the detected oncogene‐induced oxidative stress was due to ROS (superoxide) of non‐mitochondrial origin and mitochondrial OXPHOS was reduced (Crabtree effect). Overall, our study provides novel insights into oncogene‐evoked metabolic reprogramming, replication and oxidative stress, with implications for mechanisms of tumorigenesis and potential targeting of oncogene addiction.     

Glutamine at focus: versatile roles in cancer

During the past decade, a heightened understanding of metabolic pathways in cancer has significantly increased. It is recognized that many tumor cells are genetically programmed and have involved an abnormal metabolic state. Interestingly, this increased metabolic autonomy generates dependence on various nutrients such as glucose and glutamine. Both of these components participate in various facets of metabolic activity that allow for energy production, synthesis of biomass, antioxidant defense, and the regulation of cell signaling. Here, we outline the emerging data on glutamine metabolism and address the molecular mechanisms underlying glutamine-induced cell survival. We also discuss novel therapeutic strategies to exploit glutamine addiction of certain cancer cell lines.     

Metabolic rewiring of pancreatic ductal adenocarcinoma: New routes to follow within the maze

Pancreatic ductal adenocarcinoma (PDAC) is a debilitating and almost universally fatal malignancy. Despite advances in understanding of the oncogenetics of the disease, very few clinical benefits have been shown. One of the main characteristics of PDAC is the tumor architecture where tumor cells are surrounded by a firm desmoplasia. By reducing vascularization, thus both oxygen and nutrients delivery to the tumor, this stroma causes the appearance of hypoxic zones driving metabolic adaptation in surviving tumor cells in order to cope with challenging conditions. This metabolic reprogramming promoted by environmental constraints enhances PDAC aggressiveness. In this review, we provide a brief overview of previous works regarding the importance of glucose and glutamine addiction of PDAC cells. In particular we aim to highlight the need for exploring the impact of metabolites other than glucose and glutamine, such as non‐essential amino acids and oncometabolites, to find new treatments. We also discuss the need for progress in methodology for metabolites detection. The overall purpose of our review is to emphasize the need to look beyond what is currently known, with a focus on amino acid availability, in order to improve our understanding of PDAC biology.     

Altered glycolysis results in drug-resistant in clinical tumor therapy

Cancer cells undergo metabolic reprogramming, including increased glucose metabolism, fatty acid synthesis and glutamine metabolic rates. These enhancements to three major metabolic pathways are closely associated with glycolysis, which is considered the central component of cancer cell metabolism. Increasing evidence suggests that dysfunctional glycolysis is commonly associated with drug resistance in cancer treatment, and aberrant glycolysis plays a significant role in drug-resistant cancer cells. Studies on the development of drugs targeting these abnormalities have led to improvements in the efficacy of tumor treatment. The present review discusses the changes in glycolysis targets that cause drug resistance in cancer cells, including hexokinase, pyruvate kinase, pyruvate dehydrogenase complex, glucose transporters, and lactate, as well the underlying molecular mechanisms and corresponding novel therapeutic strategies. In addition, the association between increased oxidative phosphorylation and drug resistance is introduced, which is caused by metabolic plasticity. Given that aberrant glycolysis has been identified as a common metabolic feature of drug-resistant tumor cells, targeting glycolysis may be a novel strategy to develop new drugs to benefit patients with drug-resistance.     

Glutamine fuels a vicious cycle of autophagy in the tumor stroma and oxidative mitochondrial metabolism in epithelial cancer cells: Implications for preventing chemotherapy resistance

Glutamine metabolism is crucial for cancer cell growth via the generation of intermediate molecules in the tricarboxylic acid (TCA) cycle, antioxidants and ammonia. The goal of the current study was to evaluate the effects of glutamine on metabolism in the breast cancer tumor microenvironment, with a focus on autophagy and cell death in both epithelial and stromal compartments. For this purpose, MCF7 breast cancer cells were cultured alone or co-cultured with nontransformed fibroblasts in media containing high glutamine and low glucose (glutamine +) or under control conditions, with no glutamine and high glucose (glutamine −). Here, we show that MCF7 cells maintained in co-culture with glutamine display increased mitochondrial mass, as compared with control conditions. Importantly, treatment with the autophagy inhibitor chloroquine abolishes the glutamine-induced augmentation of mitochondrial mass. It is known that loss of caveolin-1 (Cav-1) expression in fibroblasts is associated with increased autophagy and an aggressive tumor microenvironment. Here, we show that Cav-1 downregulation which occurs in fibroblasts maintained in co-culture specifically requires glutamine. Interestingly, glutamine increases the expression of autophagy markers in fibroblasts, but decreases expression of autophagy markers in MCF7 cells, indicating that glutamine regulates the autophagy program in a compartment-specific manner. Functionally, glutamine protects MCF7 cells against apoptosis, via the upregulation of the anti-apoptotic and anti-autophagic protein TIGAR. Also, we show that glutamine cooperates with stromal fibroblasts to confer tamoxifen-resistance in MCF7 cancer cells. Finally, we provide evidence that co-culture with fibroblasts (1) promotes glutamine catabolism, and (2) decreases glutamine synthesis in MCF7 cancer cells. Taken together, our findings suggest that autophagic fibroblasts may serve as a key source of energy-rich glutamine to fuel cancer cell mitochondrial activity, driving a vicious cycle of catabolism in the tumor stroma and anabolic tumor cell expansion.     

Metabolic reprogramming in glioblastoma: the influence of cancer metabolism on epigenetics and unanswered questions

A defining hallmark of glioblastoma is altered tumor metabolism. The metabolic shift towards aerobic glycolysis with reprogramming of mitochondrial oxidative phosphorylation, regardless of oxygen availability, is a phenomenon known as the Warburg effect. In addition to the Warburg effect, glioblastoma tumor cells also utilize the tricarboxylic acid cycle/oxidative phosphorylation in a different capacity than normal tissue. Altered metabolic enzymes and their metabolites are oncogenic and not simply a product of tumor proliferation. Here we highlight the advantages of why tumor cells, including glioblastoma cells, require metabolic reprogramming and how tumor metabolism can converge on tumor epigenetics and unanswered questions in the field.     

CUL7 is a novel antiapoptotic oncogene

Using an expression cloning approach, we identify CUL7, a member of the cullin family, as a functional inhibitor of Myc-induced apoptosis. Deregulated expression of the Myc oncogene drives cellular proliferation yet also sensitizes cells to undergo p53-dependent and p53-independent apoptosis. Here, we report that CUL7 exerts its antiapoptotic function through p53. CUL7 binds directly to p53, and small interfering RNA-mediated knockdown of CUL7 results in the elevation of p53 protein levels. This antiapoptotic role of CUL7 enables this novel oncogene to cooperate with Myc to drive transformation. Deregulated ectopic expression of c-Myc and CUL7 promotes Rat1a cell growth in soft agar, and knockdown of CUL7 significantly blocks human neuroblastoma SHEP cell growth in an anchorage-independent manner. Furthermore, using public microarray data sets, we show that CUL7 mRNA is significantly overexpressed in non-small cell lung carcinoma and is associated with poor patient prognosis. We provide experimental evidence to show CUL7 is a new oncogene that cooperates with Myc in transformation by blocking Myc-induced apoptosis in a p53-dependent manner.     

Bcl-xL gain of function and p19 ARF loss of function cooperate oncogenically with Myc in vivo by distinct mechanisms

Overexpression of Bcl-xL, loss of p19 ARF, and loss of p53 all accelerate Myc oncogenesis. All three lesions are implicated in suppressing Myc-induced apoptosis, suggesting that this is a common mechanism by which they synergize with Myc. However, using an acutely switchable model of Myc-induced tumorigenesis, we demonstrate that each lesion cooperates with Myc in vivo by a distinct mechanism. While Bcl-xL blocks Myc-induced apoptosis, inactivation of p19 ARF enhances it. However, this increase in apoptosis is matched by increased Myc-induced proliferation. p53 inactivation shares features of both lesions, partially suppressing apoptosis while augmenting proliferation. Bcl-xL and p19 ARF loss together synergize to further accelerate Myc oncogenesis. Thus, differing lesions cooperate oncogenically with Myc by discrete mechanisms that can themselves synergize with each other.     

Targeting cancer cell metabolism in pancreatic adenocarcinoma

Pancreatic ductal adenocarcinoma (PDAC) is expected to become the second leading cause of cancer death by 2030. Current therapeutic options are limited, warranting an urgent need to explore innovative treatment strategies. Due to specific microenvironment constraints including an extensive desmoplastic stroma reaction, PDAC faces major metabolic challenges, principally hypoxia and nutrient deprivation. Their connection with oncogenic alterations such as KRAS mutations has brought metabolic reprogramming to the forefront of PDAC therapeutic research. The Warburg effect, glutamine addiction, and autophagy stand as the most important adaptive metabolic mechanisms of cancer cells themselves, however metabolic reprogramming is also an important feature of the tumor microenvironment, having a major impact on epigenetic reprogramming and tumor cell interactions with its complex stroma. We present a comprehensive overview of the main metabolic adaptations contributing to PDAC development and progression. A review of current and future therapies targeting this range of metabolic pathways is provided.     

Generation of combined hepatocellular-cholangiocarcinoma through transdifferentiation and dedifferentiation in p53-knockout mice

The two principal histological types of primary liver cancers, hepatocellular carcinoma (HCC) and cholangiocarcinoma, can coexist within a tumor, comprising combined hepatocellular-cholangiocarcinoma (cHCC-CCA). Although the possible involvement of liver stem/progenitor cells has been proposed for the pathogenesis of cHCC-CCA, the cells might originate from transformed hepatocytes that undergo ductular transdifferentiation or dedifferentiation. We previously demonstrated that concomitant introduction of mutant HRAS^V12 (HRAS) and Myc into mouse hepatocytes induced dedifferentiated tumors that expressed fetal/neonatal liver genes and proteins. Here, we examine whether the phenotype of HRAS- or HRAS/Myc-induced tumors might be affected by the disruption of the Trp53 gene, which has been shown to induce biliary differentiation in mouse liver tumors. Hepatocyte-derived liver tumors were induced in heterozygous and homozygous p53-knockout (KO) mice by hydrodynamic tail vein injection of HRAS- or Myc-containing transposon cassette plasmids, which were modified by deleting loxP sites, with a transposase-expressing plasmid. The HRAS-induced and HRAS/Myc-induced tumors in the wild-type mice demonstrated histological features of HCC, whereas the phenotype of the tumors generated in the p53-KO mice was consistent with cHCC-CCA. The expression of fetal/neonatal liver proteins, including delta-like 1, was detected in the HRAS/Myc-induced but not in the HRAS-induced cHCC-CCA tissues. The dedifferentiation in the HRAS/Myc-induced tumors was more marked in the homozygous p53-KO mice than in the heterozygous p53-KO mice and was associated with activation of Myc and YAP and suppression of ERK phosphorylation. Our results suggest that the loss of p53 promotes ductular differentiation of hepatocyte-derived tumor cells through either transdifferentiation or Myc-mediated dedifferentiation.     

Understanding the Warburg effect and the prognostic value of stromal caveolin-1 as a marker of a lethal tumor microenvironment

Cancer cells show a broad spectrum of bioenergetic states, with some cells using aerobic glycolysis while others rely on oxidative phosphorylation as their main source of energy. In addition, there is mounting evidence that metabolic coupling occurs in aggressive tumors, between epithelial cancer cells and the stromal compartment, and between well-oxygenated and hypoxic compartments. We recently showed that oxidative stress in the tumor stroma, due to aerobic glycolysis and mitochondrial dysfunction, is important for cancer cell mutagenesis and tumor progression. More specifically , increased autophagy/mitophagy in the tumor stroma drives a form of parasitic epithelial-stromal metabolic coupling. These findings explain why it is effective to treat tumors with either inducers or inhibitors of autophagy, as both would disrupt this energetic coupling. We also discuss evidence that glutamine addiction in cancer cells produces ammonia via oxidative mitochondrial metabolism. Ammonia production in cancer cells, in turn, could then help maintain autophagy in the tumor stromal compartment. In this vicious cycle, the initial glutamine provided to cancer cells would be produced by autophagy in the tumor stroma. Thus, we believe that parasitic epithelial-stromal metabolic coupling has important implications for cancer diagnosis and therapy, for example, in designing novel metabolic imaging techniques and establishing new targeted therapies. In direct support of this notion, we identified a loss of stromal caveolin-1 as a marker of oxidative stress, hypoxia, and autophagy in the tumor microenvironment, explaining its powerful predictive value. Loss of stromal caveolin-1 in breast cancers is associated with early tumor recurrence, metastasis, and drug resistance, leading to poor clinical outcome.     

Cibler le métabolisme tumoral : efficacité et perspectives

Metabolic reprogramming is a hallmark of cancer which will provide tumor cells with all metabolites needed for growth and proliferation. Numerous oncogenes are responsible for a cellular metabolic reprogramming as part of their transforming role which illustrates the potential therapeutic advantage of targeting cancer metabolism. Anti-metabolites drugs, such as methotrexate or L-asparignase, have proven to be effective in many cancers and brought the proof of concept of cancer metabolism targeting. This justifies the development of new agents in this domain. When developing such molecules, many challenges are faced: to avoid toxicity linked to metabolism inhibition of normal cells, to anticipate redundancy of metabolic pathways which could abrogate efficacy, and to identify new targets and biomarkers. Therefore, cancer metabolism targeting is part of oncologic personalized medicine. Clinical and mostly pre-clinical researches are ongoing for glucose, glutamine, fatty acids, amino acids metabolism, and TCA cycle targeting. Genome scale Metabolic Modeling (GSMM) has been proven to be effective in identifying new metabolic targets. Cancer metabolism targeting is a promising way to treat cancer which has already proven efficacy with anti-metabolite drugs. At the moment, most of the data are preclinical but clinical trial of such agents will undoubtedly be set in the future.