In vivo HIF-mediated reductive carboxylation is regulated by citrate levels and sensitizes VHL-deficient cells to glutamine deprivation

Regulation of Stem Cell Fate by ROS-mediated Alteration of Metabolism

Stem cells have attracted much attention due to their distinct features that support infinite self-renewal and differentiation into the cellular derivatives of three lineages. Recent studies have suggested that many stem cells both embryonic and adult stem cells reside in a specialized niche defined by hypoxic condition. In this respect, distinguishing functional differences arising from the oxygen concentration is important in understanding the nature of stem cells and in controlling stem cell fate for therapeutic purposes. ROS act as cellular signaling molecules involved in the propagation of signaling and the translation of environmental cues into cellular responses to maintain cellular homeostasis, which is mediated by the coordination of various cellular processes, and to adapt cellular activity to available bioenergetic sources. Thus, in this review, we describe the physiological role of ROS in stem cell fate and its effect on the metabolic regulation of stem cells.

The metabolic alterations of cancer cells

Cancer cells exhibit profound metabolic alterations, allowing them to fulfill the metabolic needs that come with increased proliferation and additional facets of malignancy. Such a metabolic transformation is orchestrated by the genetic changes that drive tumorigenesis, that is, the activation of oncogenes and/or the loss of oncosuppressor genes, and further shaped by environmental cues, such as oxygen concentration and nutrient availability. Understanding this metabolic rewiring is essential to elucidate the fundamental mechanisms of tumorigenesis as well as to find novel, therapeutically exploitable liabilities of malignant cells. Here, we describe key features of the metabolic transformation of cancer cells, which frequently include the switch to aerobic glycolysis, a profound mitochondrial reprogramming, and the deregulation of lipid metabolism, highlighting the notion that these pathways are not independent but rather cooperate to sustain proliferation. Finally, we hypothesize that only those genetic defects that effectively support anabolism are selected in the course of tumor progression, implying that cancer-associated mutations may undergo a metabolically convergent evolution.

Grade-Dependent Metabolic Reprogramming in Kidney Cancer Revealed by Combined Proteomics and Metabolomics Analysis

Kidney cancer [or renal cell carcinoma (RCC)] is known as "the internist's tumor" because it has protean systemic manifestations, suggesting that it utilizes complex, nonphysiologic metabolic pathways. Given the increasing incidence of this cancer and its lack of effective therapeutic targets, we undertook an extensive analysis of human RCC tissue employing combined grade-dependent proteomics and metabolomics analysis to determine how metabolic reprogramming occurring in this disease allows it to escape available therapeutic approaches. After validation experiments in RCC cell lines that were wild-type or mutant for the Von Hippel-Lindau tumor suppressor, in characterizing higher-grade tumors, we found that the Warburg effect is relatively more prominent at the expense of the tricarboxylic acid cycle and oxidative metabolism in general. Further, we found that the glutamine metabolism pathway acts to inhibit reactive oxygen species, as evidenced by an upregulated glutathione pathway, whereas the β-oxidation pathway is inhibited, leading to increased fatty acylcarnitines. In support of findings from previous urine metabolomics analyses, we also documented tryptophan catabolism associated with immune suppression, which was highly represented in RCC compared with other metabolic pathways. Together, our results offer a rationale to evaluate novel antimetabolic treatment strategies being developed in other disease settings as therapeutic strategies in RCC.

Metabolic consequences of oncogenic IDH mutations

Specific point mutations in isocitrate dehydrogenase 1 and 2 (IDH1 and IDH2) occur in a variety of cancers, including acute myeloid leukemia (AML), low-grade gliomas, and chondrosarcomas. These mutations inactivate wild-type enzymatic activity and convey neomorphic function to produce d-2-hydroxyglutarate (d-2HG), which accumulates at millimolar levels within tumors. d-2HG can impact α-ketoglutarate-dependent dioxygenase activity and subsequently affect various cellular functions in these cancers. Inhibitors of the neomorphic activity of mutant IDH1 and IDH2 are currently in Phase I/II clinical trials for both solid and blood tumors. As IDH1 and IDH2 represent key enzymes within the tricarboxylic acid (TCA) cycle, mutations have significant impact on intermediary metabolism. The loss of some wild-type metabolic activity is an important, potentially deleterious and therapeutically exploitable consequence of oncogenic IDH mutations and requires continued investigation in the future. Here we review how IDH1 and IDH2 mutations influence cellular metabolism, epigenetics, and other biochemical functions, discussing these changes in the context of current efforts to therapeutically target cancers bearing these mutations.

In vitro expansion of U87-MG human glioblastoma cells under hypoxic conditions affects glucose metabolism and subsequent in vivo growth

Hypoxia is a characteristic feature of solid tumors leading to the over expression of hypoxia-inducible factor (HIF)-1α protein and therefore to a specific cellular behavior. However, even though the oxygen tension in tumors is low (<5 %), most of the cell lines used in cancer studies are grown under 21 % oxygen tension. This work focuses on the impact of oxygen conditions during in vitro cell culture on glucose metabolism using 1-(13)C-glucose. Growing U87-MG glioma cells under hypoxic conditions leads to a two- to threefold reduction of labeled glutamine and an accumulation of fructose. However, under both hypoxic and normoxic conditions, glucose is used for de novo synthesis of pyrimidine since the (13)C label is found both in the uracil and ribose moieties. Labeling of the ribose ring demonstrates that U87-MG glioma cells use the reversible branch of the non-oxidative pentose phosphate pathway. Interestingly, stereotactic implantation of U87-MG cells grown under normoxia or mild hypoxia within the striatum of nude mice led to differential growth; the cells grown under hypoxia retaining an imprint of the oxygen adaptation as their development is then slowed down.

Pharmacological HIF2α inhibition improves VHL disease-associated phenotypes in zebrafish model

Patients with a germline mutation in von Hippel-Lindau (VHL) develop renal cell cancers and hypervascular tumors of the brain, adrenal glands, and pancreas as well as erythrocytosis. These phenotypes are driven by aberrant expression of HIF2α, which induces expression of genes involved in cell proliferation, angiogenesis, and red blood cell production. Currently, there are no effective treatments available for VHL disease. Here, using an animal model of VHL, we report a marked improvement of VHL-associated phenotypes following treatment with HIF2α inhibitors. Inactivation of vhl in zebrafish led to constitutive activation of HIF2α orthologs and modeled several aspects of the human disease, including erythrocytosis, pathologic angiogenesis in the brain and retina, and aberrant kidney and liver proliferation. Treatment of vhl(-/-) mutant embryos with HIF2α-specific inhibitors downregulated Hif target gene expression in a dose-dependent manner, improved abnormal hematopoiesis, and substantially suppressed erythrocytosis and angiogenic sprouting. Moreover, pharmacologic inhibition of HIF2α reversed the compromised cardiac contractility of vhl(-/-) embryos and partially rescued early lethality. This study demonstrates that small-molecule targeting of HIF2α improves VHL-related phenotypes in a vertebrate animal model and supports further exploration of this strategy for treating VHL disease.

Chicken or the egg: Warburg effect and mitochondrial dysfunction

Compared with normal cells, cancer cells show alterations in many cellular processes, including energy metabolism. Studies on cancer metabolism started with Otto Warburg's observation at the beginning of the last century. According to Warburg, cancer cells rely on glycolysis more than mitochondrial respiration for energy production. Considering that glycolysis yields much less energy compared with mitochondrial respiration, Warburg hypothesized that mitochondria must be dysfunctional and this is the initiating factor for cancer formation. However, this hypothesis did not convince every scientist in the field. Some believed the opposite: the reduction in mitochondrial activity is a result of increased glycolysis. This discrepancy of opinions is ongoing. In this review, we will discuss the alterations in glycolysis, pyruvate metabolism, and the Krebs cycle in cancer cells and focus on cause and consequence.

Analyses of the transcriptome and metabolome demonstrate that HIF1α mediates altered tumor metabolism in clear cell renal cell carcinoma

Hypoxia inducible factor 1 alpha (HIF1α) is a transcription factor that is frequently stabilized and active in human clear cell renal cell carcinoma (ccRCC). We have found that constitutively active HIF1α is sufficient to cause neoplastic transformation in a murine model of ccRCC termed the TRACK model. RNA sequencing (RNAseq) and untargeted metabolomics analyses of samples from TRACK kidneys demonstrate that HIF1α activates the transcription of genes that cause increased glucose uptake, glycolysis, and lactate production, as well as a decrease in the flux of pyruvate entering the tricarboxylic acid (TCA) cycle and a decrease in oxidative phosphorylation; these changes are identical to those observed in human ccRCC samples. These studies show that a constitutively active HIF1α promotes tumorigenesis in TRACK mice by mediating a metabolic switch to aerobic glycolysis, i.e., the Warburg effect, and suggest that TRACK mice are a valid model to test novel therapies targeting metabolic changes to inhibit human ccRCC.

Genomics of chromophobe renal cell carcinoma: implications from a rare tumor for pan-cancer studies

Chromophobe Renal Cell Carcinoma (ChRCC) is a rare subtype of the renal cell carcinomas, a heterogenous group of cancers arising from the nephron. Recently, The Cancer Genome Atlas (TCGA) profiled this understudied disease using multiple data platforms, including whole exome sequencing, whole genome sequencing (WGS), and mitochondrial DNA (mtDNA) sequencing. The insights gained from this study would have implications for other types of kidney cancer as well as for cancer biology in general. Global molecular patterns in ChRCC provided clues as to this cancer's cell of origin, which is distinct from that of the other renal cell carcinomas, illustrating an approach that might be applied towards elucidating the cell of origin of other cancer types. MtDNA sequencing revealed loss-of-function mutations in NADH dehydrogenase subunits, highlighting the role of deregulated metabolism in this and other cancers. Analysis of WGS data led to the discovery of recurrent genomic rearrangements involving TERT promoter region, which were associated with very high expression levels of TERT, pointing to a potential mechanism for TERT deregulation that might be found in other cancers. WGS data, generated by large scale efforts such as TCGA and the International Cancer Genomics Consortium (ICGC), could be more extensively mined across various cancer types, to uncover structural variants, mtDNA mutations, themes of tumor metabolic properties, as well as noncoding point mutations. TCGA's data on ChRCC should continue to serve as a resource for future pan-cancer as well as kidney cancer studies, and highlight the value of investigations into rare tumor types to globally inform principals of cancer biology.

Analysis and interpretation of transcriptomic data obtained from extended Warburg effect genes in patients with clear cell renal cell carcinoma

Background

Many cancers adopt a metabolism that is characterized by the well-known Warburg effect (aerobic glycolysis). Recently, numerous attempts have been made to treat cancer by targeting one or more gene products involved in this pathway without notable success. This work outlines a transcriptomic approach to identify genes that are highly perturbed in clear cell renal cell carcinoma (CCRCC).

Methods

We developed a model of the extended Warburg effect and outlined the model using Cytoscape. Following this, gene expression fold changes (FCs) for tumor and adjacent normal tissue from patients with CCRCC (GSE6344) were mapped on to the network. Gene expression values with FCs of greater than two were considered as potential targets for treatment of CCRCC.

Results

The Cytoscape network includes glycolysis, gluconeogenesis, the pentose phosphate pathway (PPP), the TCA cycle, the serine/glycine pathway, and partial glutaminolysis and fatty acid synthesis pathways. Gene expression FCs for nine of the 10 CCRCC patients in the GSE6344 data set were consistent with a shift to aerobic glycolysis. Genes involved in glycolysis and the synthesis and transport of lactate were over-expressed, as was the gene that codes for the kinase that inhibits the conversion of pyruvate to acetyl-CoA. Interestingly, genes that code for unique proteins involved in gluconeogenesis were strongly under-expressed as was also the case for the serine/glycine pathway. These latter two results suggest that the role attributed to the M2 isoform of pyruvate kinase (PKM2), frequently the principal isoform of PK present in cancer: i.e. causing a buildup of glucose metabolites that are shunted into branch pathways for synthesis of key biomolecules, may not be operative in CCRCC. The fact that there was no increase in the expression FC of any gene in the PPP is consistent with this hypothesis. Literature protein data generally support the transcriptomic findings.

Conclusions

A number of key genes have been identified that could serve as valid targets for anti-cancer pharmaceutical agents. Genes that are highly over-expressed include ENO2, HK2, PFKP, SLC2A3, PDK1, and SLC16A1. Genes that are highly under-expressed include ALDOB, PKLR, PFKFB2, G6PC, PCK1, FBP1, PC, and SUCLG1.

Coactivator SRC-2-dependent metabolic reprogramming mediates prostate cancer survival and metastasis

Metabolic pathway reprogramming is a hallmark of cancer cell growth and survival and supports the anabolic and energetic demands of these rapidly dividing cells. The underlying regulators of the tumor metabolic program are not completely understood; however, these factors have potential as cancer therapy targets. Here, we determined that upregulation of the oncogenic transcriptional coregulator steroid receptor coactivator 2 (SRC-2), also known as NCOA2, drives glutamine-dependent de novo lipogenesis, which supports tumor cell survival and eventual metastasis. SRC-2 was highly elevated in a variety of tumors, especially in prostate cancer, in which SRC-2 was amplified and overexpressed in 37% of the metastatic tumors evaluated. In prostate cancer cells, SRC-2 stimulated reductive carboxylation of α-ketoglutarate to generate citrate via retrograde TCA cycling, promoting lipogenesis and reprogramming of glutamine metabolism. Glutamine-mediated nutrient signaling activated SRC-2 via mTORC1-dependent phosphorylation, which then triggered downstream transcriptional responses by coactivating SREBP-1, which subsequently enhanced lipogenic enzyme expression. Metabolic profiling of human prostate tumors identified a massive increase in the SRC-2-driven metabolic signature in metastatic tumors compared with that seen in localized tumors, further implicating SRC-2 as a prominent metabolic coordinator of cancer metastasis. Moreover, SRC-2 inhibition in murine models severely attenuated the survival, growth, and metastasis of prostate cancer. Together, these results suggest that the SRC-2 pathway has potential as a therapeutic target for prostate cancer.

Mitochondria as biosynthetic factories for cancer proliferation

Unchecked growth and proliferation is a hallmark of cancer, and numerous oncogenic mutations reprogram cellular metabolism to fuel these processes. As a central metabolic organelle, mitochondria execute critical biochemical functions for the synthesis of fundamental cellular components, including fatty acids, amino acids, and nucleotides. Despite the extensive interest in the glycolytic phenotype of many cancer cells, tumors contain fully functional mitochondria that support proliferation and survival. Furthermore, tumor cells commonly increase flux through one or more mitochondrial pathways, and pharmacological inhibition of mitochondrial metabolism is emerging as a potential therapeutic strategy in some cancers. Here, we review the biosynthetic roles of mitochondrial metabolism in tumors and highlight specific cancers where these processes are activated.

Metabolic Signatures of Human Breast Cancer

Metabolomics has emerged as a new discovery tool with the promise of identifying therapeutic targets in cancer. Recent discoveries have described essential metabolomic pathways in breast cancer and characterized oncometabolites that drive tumor growth and progression. Oncogenes like MYC and tumor suppressor genes like TP53 prominently affect breast cancer biology through regulation of cell metabolism and mitochondrial biogenesis. These findings indicate that tumors with dominant mutations could be susceptible to inhibitors of disease metabolism. Moreover, various preclinical and clinical studies have linked tumor metabolism to therapeutic response and patient survival. Thus, recent advances suggest that metabolic profiling provides new opportunities to improve outcomes in breast cancer. In this review we summarize some of the identified roles of oncometabolites in breast cancer biology and highlight their clinical utility.

Metabolic evaluations of cancer metabolism by NMR-based stable isotope tracer methodologies

BACKGROUND

Cancer cells are widely recognized for being able to adapt their metabolism towards converting available nutrients into biomass to increase proliferation rates.

MATERIALS AND METHODS

We will review a series of nuclear magnetic resonance (NMR)-based stable isotope tracer methodologies for probing cancer metabolism.

RESULTS

The monitoring of such adaptations is of the utmost importance to unravel cancer metabolism and tumour growth. Several major metabolic targets have been recognized as promising foci and have been addressed by multiple studies in recent years. In this work are presented strategies to quantify glycolysis, pentose phosphate pathway, Krebs cycle turnover and de novo lipogenesis by NMR isotopomer analysis.

CONCLUSIONS

Being able to adequately define the interplay between metabolic pathways allows the monitoring of their prevalence in tissues and such information is critical for an accurate knowledge of the metabolic distinctive nature of tumours towards devising more efficient antitumorigenic strategies. Discussed methodologies are currently available in the literature, but to date, no single review has compiled all their possible uses, particularly in an interdependent perspective.

Mesenchymal phenotype predisposes lung cancer cells to impaired proliferation and redox stress in response to glutaminase inhibition

Recent work has highlighted glutaminase (GLS) as a key player in cancer cell metabolism, providing glutamine-derived carbon and nitrogen to pathways that support proliferation. There is significant interest in targeting GLS for cancer therapy, although the gene is not known to be mutated or amplified in tumors. As a result, identification of tractable markers that predict GLS dependence is needed for translation of GLS inhibitors to the clinic. Herein we validate a small molecule inhibitor of GLS and show that non-small cell lung cancer cells marked by low E-cadherin and high vimentin expression, hallmarks of a mesenchymal phenotype, are particularly sensitive to inhibition of the enzyme. Furthermore, lung cancer cells induced to undergo epithelial to mesenchymal transition (EMT) acquire sensitivity to the GLS inhibitor. Metabolic studies suggest that the mesenchymal cells have a reduced capacity for oxidative phosphorylation and increased susceptibility to oxidative stress, rendering them unable to cope with the perturbations induced by GLS inhibition. These findings elucidate selective metabolic dependencies of mesenchymal lung cancer cells and suggest novel pathways as potential targets in this aggressive cancer type.

MicroRNA-126 suppresses mesothelioma malignancy by targeting IRS1 and interfering with the mitochondrial function

AIMS

MiR126 was found to be frequently lost in many types of cancer, including malignant mesothelioma (MM), which represents one of the most challenging neoplastic diseases. In this study, we investigated the potential tumor suppressor function of MiR126 in MM cells. The effect of MiR126 was examined in response to oxidative stress, aberrant mitochondrial function induced by inhibition of complex I, mitochondrial DNA (mtDNA) depletion, and hypoxia.

RESULTS

MiR126 was up-regulated by oxidative stress in nonmalignant mesothelial (Met5A) and MM (H28) cell lines. In Met5A cells, rotenone inhibited MiR126 expression, but mtDNA depletion and hypoxia up-regulated MiR126. However, these various stimuli suppressed the levels of MiR126 in H28 cells. MiR126 affected mitochondrial energy metabolism, reduced mitochondrial respiration, and promoted glycolysis in H28 cells. This metabolic shift, associated with insulin receptor substrate-1 (IRS1)-modulated ATP-citrate lyase deregulation, resulted in higher ATP and citrate production. These changes were linked to the down-regulation of IRS1 by ectopic MiR126, reducing Akt signaling and inhibiting cytosolic sequestration of Forkhead box O1 (FoxO1), which promoted the expression of genes involved in gluconeogenesis and oxidative stress defense. These metabolic changes induced hypoxia-inducible factor-1α (HIF1α) stabilization. Consequently, MiR126 suppressed the malignancy of MM cells in vitro, a notion corroborated by the failure of H28(MiR126) cells to form tumors in nude mice.

INNOVATION AND CONCLUSION

MiR126 affects mitochondrial energy metabolism, resulting in MM tumor suppression. Since MM is a fatal neoplastic disease with a few therapeutic options, this finding is of potential translational importance.

Glutamine oxidation maintains the TCA cycle and cell survival during impaired mitochondrial pyruvate transport

Alternative modes of metabolism enable cells to resist metabolic stress. Inhibiting these compensatory pathways may produce synthetic lethality. We previously demonstrated that glucose deprivation stimulated a pathway in which acetyl-CoA was formed from glutamine downstream of glutamate dehydrogenase (GDH). Here we show that import of pyruvate into the mitochondria suppresses GDH and glutamine-dependent acetyl-CoA formation. Inhibiting the mitochondrial pyruvate carrier (MPC) activates GDH and reroutes glutamine metabolism to generate both oxaloacetate and acetyl-CoA, enabling persistent tricarboxylic acid (TCA) cycle function. Pharmacological blockade of GDH elicited largely cytostatic effects in culture, but these effects became cytotoxic when combined with MPC inhibition. Concomitant administration of MPC and GDH inhibitors significantly impaired tumor growth compared to either inhibitor used as a single agent. Together, the data define a mechanism to induce glutaminolysis and uncover a survival pathway engaged during compromised supply of pyruvate to the mitochondria.

Targeting metabolic changes in cancer: novel therapeutic approaches

Therapeutic strategies designed to target cancer metabolism are an area of intense research. Antimetabolites, first used to treat patients in the early twentieth century, served as an early proof of concept for such therapies. We highlight strategies that attempt to improve on the anti-metabolite approach as well as new metabolic drug targets. Some of these targets have the advantage of a strong genetic anchor to drive patient selection (isocitrate dehydrogenase 1/2, Enolase 2). Additional approaches described here derive from hypothesis-driven and systems biology efforts designed to exploit tumor cell metabolic dependencies (fatty acid oxidation, nicotinamide adenine dinucleotide synthesis, glutamine biology).

L-2-Hydroxyglutarate: an epigenetic modifier and putative oncometabolite in renal cancer

Through unbiased metabolomics, we identified elevations of the metabolite 2-hydroxyglutarate (2HG) in renal cell carcinoma (RCC). 2HG can inhibit 2-oxoglutaratre (2-OG)-dependent dioxygenases that mediate epigenetic events, including DNA and histone demethylation. 2HG accumulation, specifically the d enantiomer, can result from gain-of-function mutations of isocitrate dehydrogenase (IDH1, IDH2) found in several different tumors. In contrast, kidney tumors demonstrate elevations of the l enantiomer of 2HG (l-2HG). High-2HG tumors demonstrate reduced DNA levels of 5-hydroxymethylcytosine (5hmC), consistent with 2HG-mediated inhibition of ten-eleven translocation (TET) enzymes, which convert 5-methylcytosine (5mC) to 5hmC. l-2HG elevation is mediated in part by reduced expression of l-2HG dehydrogenase (L2HGDH). L2HGDH reconstitution in RCC cells lowers l-2HG and promotes 5hmC accumulation. In addition, L2HGDH expression in RCC cells reduces histone methylation and suppresses in vitro tumor phenotypes. Our report identifies l-2HG as an epigenetic modifier and putative oncometabolite in kidney cancer.

SIGNIFICANCE

Here, we report elevations of the putative oncometabolite l-2HG in the most common subtype of kidney cancer and describe a novel mechanism for the regulation of DNA 5hmC levels. Our findings provide new insight into the metabolic basis for the epigenetic landscape of renal cancer.

Prolyl hydroxylase domain enzymes: important regulators of cancer metabolism

The hypoxia-inducible factor (HIF) prolyl hydroxylase domain enzymes (PHDs) regulate the stability of HIF protein by post-translational hydroxylation of two conserved prolyl residues in its α subunit in an oxygen-dependent manner. Trans-4-prolyl hydroxylation of HIFα under normal oxygen (O₂) availability enables its association with the von Hippel-Lindau (VHL) tumor suppressor pVHL E3 ligase complex, leading to the degradation of HIFα via the ubiquitin-proteasome pathway. Due to the obligatory requirement of molecular O₂ as a co-substrate, the activity of PHDs is inhibited under hypoxic conditions, resulting in stabilized HIFα, which dimerizes with HIFβ and, together with transcriptional co-activators CBP/p300, activates the transcription of its target genes. As a key molecular regulator of adaptive response to hypoxia, HIF plays important roles in multiple cellular processes and its overexpression has been detected in various cancers. The HIF1α isoform in particular has a strong impact on cellular metabolism, most notably by promoting anaerobic, whilst inhibiting O₂-dependent, metabolism of glucose. The PHD enzymes also seem to have HIF-independent functions and are subject to regulation by factors other than O₂, such as by metabolic status, oxidative stress, and abnormal levels of endogenous metabolites (oncometabolites) that have been observed in some types of cancers. In this review, we aim to summarize current understandings of the function and regulation of PHDs in cancer with an emphasis on their roles in metabolism.

Akt-dependent metabolic reprogramming regulates tumor cell histone acetylation

Histone acetylation plays important roles in gene regulation, DNA replication, and the response to DNA damage, and it is frequently deregulated in tumors. We postulated that tumor cell histone acetylation levels are determined in part by changes in acetyl coenzyme A (acetyl-CoA) availability mediated by oncogenic metabolic reprogramming. Here, we demonstrate that acetyl-CoA is dynamically regulated by glucose availability in cancer cells and that the ratio of acetyl-CoA:coenzyme A within the nucleus modulates global histone acetylation levels. In vivo, expression of oncogenic Kras or Akt stimulates histone acetylation changes that precede tumor development. Furthermore, we show that Akt's effects on histone acetylation are mediated through the metabolic enzyme ATP-citrate lyase and that pAkt(Ser473) levels correlate significantly with histone acetylation marks in human gliomas and prostate tumors. The data implicate acetyl-CoA metabolism as a key determinant of histone acetylation levels in cancer cells.

A low carbohydrate, high protein diet combined with celecoxib markedly reduces metastasis

We recently demonstrated that both murine and human carcinomas grow significantly slower in mice on low carbohydrate (CHO), high protein diets than on isocaloric Western diets and that a further reduction in tumor growth rates occur when the low CHO diets are combined with the cyclooxygenase-2 inhibitor, celecoxib. Following upon these studies, we asked herein what effect low CHO, high protein diets, with or without celecoxib, might have on tumor metastasis. In the highly metastatic 4T1 mouse mammary tumor model, a 15% CHO, high protein diet supplemented with celecoxib (1 g/kg chow) markedly reduced lung metastases. Moreover, in longer-term studies using male Transgenic Adenocarcinoma of the Mouse Prostate mice, which are predisposed to metastatic prostate cancer, the 15% CHO diet, with and without celecoxib (0.3 g/kg chow), gave the lowest incidence of metastases, but a more moderate 25% CHO diet containing celecoxib led to the best survival. Metabolic studies with 4T1 tumors suggested that the low CHO, high protein diets may be forcing tumors to become dependent on amino acid catabolism for survival/growth. Taken together, our results suggest that a combination of a low CHO, high protein diet with celecoxib substantially reduces metastasis.

IDH1 mutations alter citric acid cycle metabolism and increase dependence on oxidative mitochondrial metabolism

Oncogenic mutations in isocitrate dehydrogenase 1 and 2 (IDH1/2) occur in several types of cancer, but the metabolic consequences of these genetic changes are not fully understood. In this study, we performed (13)C metabolic flux analysis on a panel of isogenic cell lines containing heterozygous IDH1/2 mutations. We observed that under hypoxic conditions, IDH1-mutant cells exhibited increased oxidative tricarboxylic acid metabolism along with decreased reductive glutamine metabolism, but not IDH2-mutant cells. However, selective inhibition of mutant IDH1 enzyme function could not reverse the defect in reductive carboxylation activity. Furthermore, this metabolic reprogramming increased the sensitivity of IDH1-mutant cells to hypoxia or electron transport chain inhibition in vitro. Lastly, IDH1-mutant cells also grew poorly as subcutaneous xenografts within a hypoxic in vivo microenvironment. Together, our results suggest therapeutic opportunities to exploit the metabolic vulnerabilities specific to IDH1 mutation.

HIF1α and HIF2α exert distinct nutrient preferences in renal cells

BACKGROUND

Hypoxia Inducible Factors (HIF1α and HIF2α) are commonly stabilized and play key roles related to cell growth and metabolic programming in clear cell renal cell carcinoma. The relationship of these factors to discretely alter cell metabolic activities has largely been described in cancer cells, or in hypoxic conditions, where other confounding factors undoubtedly compete. These transcription factors and their specific roles in promoting cancer metabolic phenotypes from the earliest stages are poorly understood in pre-malignant cells.

METHODS

We undertook an analysis of SV40-transformed primary kidney epithelial cells derived from newborn mice genetically engineered to express a stabilized HIF1α or HIF2α transgene. We examined the metabolic profile in relation to each gene.

RESULTS

Although the cells proliferated similarly, the metabolic profile of each genotype of cell was markedly different and correlated with altered gene expression of factors influencing components of metabolic signaling. HIF1α promoted high levels of glycolysis as well as increased oxidative phosphorylation in complete media, but oxidative phosphorylation was suppressed when supplied with single carbon source media. HIF2α, in contrast, supported oxidative phosphorylation in complete media or single glucose carbon source, but these cells were not responsive to glutamine nutrient sources. This finding correlates to HIF2α-specific induction of Glul, effectively reducing glutamine utilization by limiting the glutamate pool, and knockdown of Glul allows these cells to perform oxidative phosphorylation in glutamine media.

CONCLUSION

HIF1α and HIF2α support highly divergent patterns of kidney epithelial cell metabolic phenotype. Expression of these factors ultimately alters the nutrient resource utilization and energy generation strategy in the setting of complete or limiting nutrients.

Oxidation of alpha-ketoglutarate is required for reductive carboxylation in cancer cells with mitochondrial defects

Mammalian cells generate citrate by decarboxylating pyruvate in the mitochondria to supply the tricarboxylic acid (TCA) cycle. In contrast, hypoxia and other impairments of mitochondrial function induce an alternative pathway that produces citrate by reductively carboxylating α-ketoglutarate (AKG) via NADPH-dependent isocitrate dehydrogenase (IDH). It is unknown how cells generate reducing equivalents necessary to supply reductive carboxylation in the setting of mitochondrial impairment. Here, we identified shared metabolic features in cells using reductive carboxylation. Paradoxically, reductive carboxylation was accompanied by concomitant AKG oxidation in the TCA cycle. Inhibiting AKG oxidation decreased reducing equivalent availability and suppressed reductive carboxylation. Interrupting transfer of reducing equivalents from NADH to NADPH by nicotinamide nucleotide transhydrogenase increased NADH abundance and decreased NADPH abundance while suppressing reductive carboxylation. The data demonstrate that reductive carboxylation requires bidirectional AKG metabolism along oxidative and reductive pathways, with the oxidative pathway producing reducing equivalents used to operate IDH in reverse.

The tumor suppressor folliculin regulates AMPK-dependent metabolic transformation

The Warburg effect is a tumorigenic metabolic adaptation process characterized by augmented aerobic glycolysis, which enhances cellular bioenergetics. In normal cells, energy homeostasis is controlled by AMPK; however, its role in cancer is not understood, as both AMPK-dependent tumor-promoting and -inhibiting functions were reported. Upon stress, energy levels are maintained by increased mitochondrial biogenesis and glycolysis, controlled by transcriptional coactivator PGC-1α and HIF, respectively. In normoxia, AMPK induces PGC-1α, but how HIF is activated is unclear. Germline mutations in the gene encoding the tumor suppressor folliculin (FLCN) lead to Birt-Hogg-Dubé (BHD) syndrome, which is associated with an increased cancer risk. FLCN was identified as an AMPK binding partner, and we evaluated its role with respect to AMPK-dependent energy functions. We revealed that loss of FLCN constitutively activates AMPK, resulting in PGC-1α-mediated mitochondrial biogenesis and increased ROS production. ROS induced HIF transcriptional activity and drove Warburg metabolic reprogramming, coupling AMPK-dependent mitochondrial biogenesis to HIF-dependent metabolic changes. This reprogramming stimulated cellular bioenergetics and conferred a HIF-dependent tumorigenic advantage in FLCN-negative cancer cells. Moreover, this pathway is conserved in a BHD-derived tumor. These results indicate that FLCN inhibits tumorigenesis by preventing AMPK-dependent HIF activation and the subsequent Warburg metabolic transformation.

Therapeutic strategies impacting cancer cell glutamine metabolism

The metabolic adaptations that support oncogenic growth can also render cancer cells dependent on certain nutrients. Along with the Warburg effect, increased utilization of glutamine is one of the metabolic hallmarks of the transformed state. Glutamine catabolism is positively regulated by multiple oncogenic signals, including those transmitted by the Rho family of GTPases and by c-Myc. The recent identification of mechanistically distinct inhibitors of glutaminase, which can selectively block cellular transformation, has revived interest in the possibility of targeting glutamine metabolism in cancer therapy. Here, we outline the regulation and roles of glutamine metabolism within cancer cells and discuss possible strategies for, and the consequences of, impacting these processes therapeutically.

Hypoxia and cancer cell metabolism

The past decade has witnessed a rapid accumulation of evidence showing that hypoxic microenvironment, which is typical during cancer development, plays key roles in regulating cancer cell metabolism. In this review, we will focus on the role of hypoxic response, particularly, its master regulator hypoxia-inducible factor-1, in regulating glucose, lipid, as well as amino acid metabolism in cancer cells. We will also discuss the therapeutic opportunities by targeting specific pathways that facilitate metabolic reprogramming in cancer cells.

Vascular endothelial growth factor signaling in hypoxia and inflammation

Infection, cancer and cardiovascular diseases are the major causes for morbidity and mortality in the United States according to the Center for Disease Control. The underlying etiology that contributes to the severity of these diseases is either hypoxia induced inflammation or inflammation resulting in hypoxia. Therefore, molecular mechanisms that regulate hypoxia-induced adaptive responses in cells are important areas of investigation. Oxygen availability is sensed by molecular switches which regulate synthesis and secretion of growth factors and inflammatory mediators. As a consequence, tissue microenvironment is altered by re-programming metabolic pathways, angiogenesis, vascular permeability, pH homeostasis to facilitate tissue remodeling. Hypoxia inducible factor (HIF) is the central mediator of hypoxic response. HIF regulates several hundred genes and vascular endothelial growth factor (VEGF) is one of the primary target genes. Understanding the regulation of HIF and its influence on inflammatory response offers unique opportunities for drug development to modulate inflammation and ischemia in pathological conditions.

Hypoxic regulation of glutamine metabolism through HIF1 and SIAH2 supports lipid synthesis that is necessary for tumor growth

Recent reports have identified a phenomenon by which hypoxia shifts glutamine metabolism from oxidation to reductive carboxylation. We now identify the mechanism by which HIF-1 activation results in a dramatic reduction in the activity of the key mitochondrial enzyme complex α ketoglutarate dehydrogenase (αKGDH). HIF-1 activation promotes SIAH2 targeted ubiquitination and proteolysis of the 48 kDa splice variant of the E1 subunit of the αKGDH complex (OGDH2). Knockdown of SIAH2 or mutation of the ubiquitinated lysine residue on OGDH2 (336KA) reverses the hypoxic drop in αKGDH activity, stimulates glutamine oxidation, and reduces glutamine-dependent lipid synthesis. 336KA OGDH2-expressing cells require exogenous lipids or citrate for growth in hypoxia in vitro and fail to grow as model tumors in immunodeficient mice. Reversal of hypoxic mitochondrial function may provide a target for the development of next-generation anticancer agents targeting tumor metabolism.

Hypoxia signaling pathways in cancer metabolism: the importance of co-selecting interconnected physiological pathways

Both tumor hypoxia and dysregulated metabolism are classical features of cancer. Recent analyses have revealed complex interconnections between oncogenic activation, hypoxia signaling systems and metabolic pathways that are dysregulated in cancer. These studies have demonstrated that rather than responding simply to error signals arising from energy depletion or tumor hypoxia, metabolic and hypoxia signaling pathways are also directly connected to oncogenic signaling mechanisms at many points. This review will summarize current understanding of the role of hypoxia inducible factor (HIF) in these networks. It will also discuss the role of these interconnected pathways in generating the cancer phenotype; in particular, the implications of switching massive pathways that are physiologically 'hard-wired’ to oncogenic mechanisms driving cancer.

13C isotope-assisted methods for quantifying glutamine metabolism in cancer cells

Glutamine has recently emerged as a key substrate to support cancer cell proliferation, and the quantification of its metabolic flux is essential to understand the mechanisms by which this amino acid participates in the metabolic rewiring that sustains the survival and growth of neoplastic cells. Glutamine metabolism involves two major routes, glutaminolysis and reductive carboxylation, both of which begin with the deamination of glutamine to glutamate and the conversion of glutamate into α-ketoglutarate. In glutaminolysis, α-ketoglutarate is oxidized via the tricarboxylic acid cycle and decarboxylated to pyruvate. In reductive carboxylation, α-ketoglutarate is reductively converted into isocitrate, which is isomerized to citrate to supply acetyl-CoA for de novo lipogenesis. Here, we describe methods to quantify the metabolic flux of glutamine through these two routes, as well as the contribution of glutamine to lipid synthesis. Examples of how these methods can be applied to study metabolic pathways of oncological relevance are provided.

From molecular understanding to clinical advances

Big data and computational biology brought to the forefront a number of potential actionable mutations and drug targets in clear cell renal cell carcinoma in 2013. As we continue to unravel the molecular underpinnings of tumorigenesis and progression, the clinical benefits will eventually be reaped.

Fatty acid biosynthesis from glutamate and glutamine is specifically induced in neuronal cells under hypoxia

Hypoxia is involved in many neuronal and non-neuronal diseases, and defining the mechanisms for tissue adaptation to hypoxia is critical for the understanding and treatment of these diseases. One mechanism for tissue adaptation to hypoxia is increased glutamine and/or glutamate (Gln/Glu) utilization. To address this mechanism, we determined incorporation of Gln/Glu and other lipogenic substrates into lipids and fatty acids in both primary neurons and a neuronal cell line under normoxic and hypoxic conditions and compared this to non-neuronal primary cells and non-neuronal cell lines. Incorporation of Gln/Glu into total lipids was dramatically and specifically increased under hypoxia in neuronal cells including both primary (2.0- and 3.0-fold for Gln and Glu, respectively) and immortalized cultures (3.5- and 8.0-fold for Gln and Glu, respectively), and 90% to 97% of this increase was accounted for by incorporation into fatty acids (FA) depending upon substrate and cell type. All other non-neuronal cells tested demonstrated decreased or unchanged FA synthesis from Gln/Glu under hypoxia. Consistent with these data, total FA mass was also increased in neuronal cells under hypoxia that was mainly accounted for by the increase in saturated and monounsaturated FA with carbon length from 14 to 24. Incorporation of FA synthesized from Gln/Glu was increased in all major lipid classes including cholesteryl esters, triacylglycerols, diacylglycerols, free FA, and phospholipids, with the highest rate of incorporation into triacylglycerols. These results indicate that increased FA biosynthesis from Gln/Glu followed by esterification may be a neuronal specific pathway for adaptation to hypoxia. We identified a novel neuronal specific pathway for adaptation to hypoxia through increased fatty acid biosynthesis from glutamine and glutamate (Gln/Glu) followed by esterification into lipids. All other non-neuronal cells tested demonstrated decreased or unchanged lipid synthesis from Gln/Glu under hypoxia. Incorporation of other lipogenic substrates into lipids was decreased under hypoxia in neuronal cells. We believe that this finding will provide a novel strategy for treatment of oxygen and energy deficient conditions in the neuronal system.

Metabolic targets for cancer therapy

Malignant cells exhibit metabolic changes, when compared to their normal counterparts, owing to both genetic and epigenetic alterations. Although such a metabolic rewiring has recently been indicated as yet another general hallmark of cancer, accumulating evidence suggests that the metabolic alterations of each neoplasm represent a molecular signature that intimately accompanies and allows for different facets of malignant transformation. During the past decade, targeting cancer metabolism has emerged as a promising strategy for the development of selective antineoplastic agents. Here, we discuss the intimate relationship between metabolism and malignancy, focusing on strategies through which this central aspect of tumour biology might be turned into cancer's Achilles heel.

MicroRNAs as regulators of mitochondrial function: role in cancer suppression

BACKGROUND

Mitochondria, essential to the cell homeostasis maintenance, are central to the intrinsic apoptotic pathway and their dysfunction is associated with multiple diseases. Recent research documents that microRNAs (miRNAs) regulate important signalling pathways in mitochondria, and many of these miRNAs are deregulated in various diseases including cancers.

SCOPE OF REVIEW

In this review, we summarise the role of miRNAs in the regulation of the mitochondrial bioenergetics/function, and discuss the role of miRNAs modulating the various metabolic pathways resulting in tumour suppression and their possible therapeutic applications.

MAJOR CONCLUSIONS

MiRNAs have recently emerged as key regulators of metabolism and can affect mitochondria by modulating mitochondrial proteins coded by nuclear genes. They were also found in mitochondria. Reprogramming of the energy metabolism has been postulated as a major feature of cancer. Modulation of miRNAs levels may provide a new therapeutic approach for the treatment of mitochondria-related pathologies, including neoplastic diseases.

GENERAL SIGNIFICANCE

The elucidation of the role of miRNAs in the regulation of mitochondrial activity/bioenergetics will deepen our understanding of the molecular aspects of various aspects of cell biology associated with the genesis and progression of neoplastic diseases. Eventually, this knowledge may promote the development of innovative pharmacological interventions. This article is part of a Special Issue entitled Frontiers of Mitochondrial Research.

Cofactor balance by nicotinamide nucleotide transhydrogenase (NNT) coordinates reductive carboxylation and glucose catabolism in the tricarboxylic acid (TCA) cycle

Cancer and proliferating cells exhibit an increased demand for glutamine-derived carbons to support anabolic processes. In addition, reductive carboxylation of α-ketoglutarate by isocitrate dehydrogenase 1 (IDH1) and 2 (IDH2) was recently shown to be a major source of citrate synthesis from glutamine. The role of NAD(P)H/NAD(P)(+) cofactors in coordinating glucose and glutamine utilization in the tricarboxylic acid (TCA) cycle is not well understood, with the source(s) of NADPH for the reductive carboxylation reaction remaining unexplored. Nicotinamide nucleotide transhydrogenase (NNT) is a mitochondrial enzyme that transfers reducing equivalents from NADH to NADPH. Here, we show that knockdown of NNT inhibits the contribution of glutamine to the TCA cycle and activates glucose catabolism in SkMel5 melanoma cells. The increase in glucose oxidation partially occurred through pyruvate carboxylase and rendered NNT knockdown cells more sensitive to glucose deprivation. Importantly, knocking down NNT inhibits reductive carboxylation in SkMel5 and 786-O renal carcinoma cells. Overexpression of NNT is sufficient to stimulate glutamine oxidation and reductive carboxylation, whereas it inhibits glucose catabolism in the TCA cycle. These observations are supported by an impairment of the NAD(P)H/NAD(P)(+) ratios. Our findings underscore the role of NNT in regulating central carbon metabolism via redox balance, calling for other mechanisms that coordinate substrate preference to maintain a functional TCA cycle.

Long-lasting supersensitivity of the rat vas deferens to norepinephrine after chronic guanethidine administration

Metabolic alterations, driven by genetic and epigenetic factors, have long been known to be associated with the etiology of cancer. Furthermore, accumulating evidence suggest that cancer metabolism is intimately linked to drug resistance, which is currently one of the most important challenges in cancer treatment. Altered metabolic pathways help cancer cells to proliferate at a rate higher than normal, adapt to nutrient limited conditions, and develop drug resistance phenotypes. Application of systems biology, boosted by recent advancement of novel high-throughput technologies to obtain cancer-associated, transcriptomic, proteomic and metabolomic data, is expected to make a significant contribution to our understanding of metabolic properties related to malignancy. Indeed, despite being at a very early stage, quantitative data obtained from the omics platforms and through applications of ¹³C metabolic flux analysis (MFA) in in vitro studies, researchers have already began to gain insight into the complex metabolic mechanisms of cancer, paving the way for selection of molecular targets for therapeutic interventions. In this review, we discuss some of the major findings associated with the metabolic pathways in cancer cells and also discuss new evidences and achievements on specific metabolic enzyme targets and target-directed small molecules that can potentially be used as anti-cancer drugs.