Purine metabolism and the microaerophily of Helicobacter pylori

The requirements for purine nucleotide synthesis, the effects of purine analogues, and the metabolism of adenine in the bacterium Helicobacter pylori were investigated employing cell culture techniques and one-dimensional NMR spectroscopy. Bacterial cells grew and proliferated in media lacking preformed purines, indicating that H. pylori can synthesize purine nucleotides de novo to meet its requirements. Blocking of this pathway in the absence of sufficient preformed purines for salvage nucleotide synthesis led to cell death. Analogues of purine nucleobases and nucleosides taken up by the cells were cytotoxic, suggesting that salvage routes could be exploited for therapy. Adenine or hypoxanthine were able to substitute for catalase in supporting cell growth and proliferation, suggesting a role for these bases in maintaining the microaerophilic conditions essentially required by the bacterium. Received: 23 May 1997 / Accepted: 17 July 1997     

Effect of simvastatin on the uptake and metabolic conversion of palmitic, dihomo-gamma-linoleic and alpha-linolenic acids in A549 cells

It is well known that simvastatin affects cholesterol synthesis. Furthermore it inhibits growth and proliferation and perturbs fatty acid metabolism in some cell lines. We have studied the effects of simvastatin on the uptake and metabolism of exogenous fatty acid in the human lung adenocarcinoma A549 cells. Simvastatin inhibited the proliferation of A549, and caused an increment in phospholipid/cholesterol ratio due to an increment in phospholipid content without affecting cholesterol content. All the fatty acids were uptaken and metabolized in both control and treated cells. The conversion of palmitic, linoleic and dihomo-gamma-linoleic acids to their metabolites and products/precursor ratios for the desaturation and elongation reactions showed that simvastatin enhanced the Delta5 desaturation step and altered some elongating steps. The machinery for unsaturated fatty acid synthesis in A549 is quite sensitive to simvastatin and its effects could have important implication taking into account that highly unsaturated fatty acids are involved in the regulation of diverse cellular functions by themselves or through their metabolites.     

PHGDH amplification and altered glucose metabolism in human melanoma

The metabolic requirements of cancer cells differ from that of their normal counterparts. To support their proliferation, cancer cells switch to a fermentative metabolism that is thought to support biomass production. Instances where metabolic enzymes promote tumorigenesis remain rare. However, an enzyme involved in the de novo synthesis of serine, 3-phosphoglycerate dehydrogenase (PHGDH), was recently identified as a putative oncogene. The potential mechanisms by which PHGDH promotes cancer are discussed.     

Inhibition of 3-phosphoglycerate dehydrogenase (PHGDH) by indole amides abrogates de novo serine synthesis in cancer cells

Cancer cells reprogram their metabolism to support growth and to mitigate cellular stressors. The serine synthesis pathway has been identified as a metabolic pathway frequently altered in cancers and there has been considerable interest in developing pharmacological agents to target this pathway. Here, we report a series of indole amides that inhibit human 3-phosphoglycerate dehydrogenase (PHGDH), the enzyme that catalyzes the first committed step of the serine synthesis pathway. Using X-ray crystallography, we show that the indole amides bind the NAD⁺ pocket of PHGDH. Through structure-based optimization we were able to develop compounds with low nanomolar affinities for PHGDH in an enzymatic IC₅₀ assay. In cellular assays, the most potent compounds inhibited de novo serine synthesis with low micromolar to sub-micromolar activities and these compounds successfully abrogated the proliferation of cancer cells in serine free media. The indole amide series reported here represent an important improvement over previously published PHGDH inhibitors as they are markedly more potent and their mechanism of action is better defined.     

SLC1A3 promotes gastric cancer progression via the PI3K/AKT signalling pathway

Gastric cancer is a major cause of mortality worldwide. The glutamate/aspartate transporter SLC1A3 has been implicated in tumour metabolism and progression, but the roles of SLC1A3 in gastric cancer remain unclear. We used bioinformatics approaches to analyse the expression of SLC1A3 and its role in gastric cancer. The expression levels of SLC1A3 were examined using RT‐qPCR and Western bolting. SLC1A3 overexpressing and knock‐down cell lines were constructed, and the cell viability was evaluated. Glucose consumption, lactate excretion and ATP levels were determined. The roles of SLC1A3 in tumour growth were evaluated using a xenograft tumour growth model. SLC1A3 was found to be overexpressed in gastric cancer, and this overexpression was associated with poor prognosis. In vitro and in vivo assays showed that SLC1A3 affected glucose metabolism and promoted gastric cancer growth. GSEA analysis suggested that SLC1A3 was positively associated with the up‐regulation of the PI3K/AKT pathway. SLC1A3 overexpression activated the PI3K/AKT pathway and up‐regulated GLUT1, HK II and LDHA expression. The PI3K/AKT inhibitor LY294002 prevented SLC1A3‐induced glucose metabolism and cell proliferation. Our findings indicate that SLC1A3 promotes gastric cancer progression via the PI3K/AKT signalling pathway. SLC1A3 is therefore a potential therapeutic target in gastric cancer.     

Metabolic rewiring in cancer cells overexpressing the glucocorticoid-induced leucine zipper protein (GILZ): Activation of mitochondrial oxidative phosphorylation and sensitization to oxidative cell death induced by mitochondrial targeted drugs

Cancer cell metabolism is largely controlled by oncogenic signals and nutrient availability. Here, we highlighted that the glucocorticoid-induced leucine zipper (GILZ), an intracellular protein influencing many signaling pathways, reprograms cancer cell metabolism to promote proliferation. We provided evidence that GILZ overexpression induced a significant increase of mitochondrial oxidative phosphorylation as evidenced by the augmentation in basal respiration, ATP-linked respiration as well as respiratory capacity. Pharmacological inhibition of glucose, glutamine and fatty acid oxidation reduced the activation of GILZ-induced mitochondrial oxidative phosphorylation. At glycolysis level, GILZ-overexpressing cells enhanced the expression of glucose transporters in their plasmatic membrane and showed higher glycolytic reserve. ¹H NMR metabolites quantification showed an up-regulation of amino acid biosynthesis. The GILZ-induced metabolic reprograming is present in various cancer cell lines regardless of their driver mutations status and is associated with higher proliferation rates persisting under metabolic stress conditions. Interestingly, high levels of OXPHOS made GILZ-overexpressing cells vulnerable to cell death induced by mitochondrial pro-oxidants. Altogether, these data indicate that GILZ reprograms cancer metabolism towards mitochondrial OXPHOS and sensitizes cancer cells to mitochondria-targeted drugs with pro-oxidant activities.     

Regulation of the pentose phosphate pathway in cancer

Energy metabolism is significantly reprogrammed in many human cancers, and these alterations confer many advantages to cancer cells, including the promotion of biosynthesis, ATP generation, detoxification and support of rapid proliferation. The pentose phosphate pathway (PPP) is a major pathway for glucose catabolism. The PPP directs glucose flux to its oxidative branch and produces a reduced form of nicotinamide adenine dinucleotide phosphate (NADPH), an essential reductant in anabolic processes. It has become clear that the PPP plays a critical role in regulating cancer cell growth by supplying cells with not only ribose-5-phosphate but also NADPH for detoxification of intracellular reactive oxygen species, reductive biosynthesis and ribose biogenesis. Thus, alteration of the PPP contributes directly to cell proliferation, survival and senescence. Furthermore, recent studies have shown that the PPP is regulated oncogenically and/or metabolically by numerous factors, including tumor suppressors, oncoproteins and intracellular metabolites. Dysregulation of PPP flux dramatically impacts cancer growth and survival. Therefore, a better understanding of how the PPP is reprogrammed and the mechanism underlying the balance between glycolysis and PPP flux in cancer will be valuable in developing therapeutic strategies targeting this pathway.     

Translation initiation factor eIF3a regulates glucose metabolism and cell proliferation via promoting small GTPase Rheb synthesis and AMPK activation

Eukaryotic translation initiation factor 3 subunit A (eIF3a), the largest subunit of the eIF3 complex, has been shown to be overexpressed in malignant cancer cells, potentially making it a proto-oncogene. eIF3a overexpression can drive cancer cell proliferation but contributes to better prognosis. While its contribution to prognosis was previously shown to be due to its function in suppressing synthesis of DNA damage repair proteins, it remains unclear how eIF3a regulates cancer cell proliferation. In this study, we show using genetic approaches that eIF3a controls cell proliferation by regulating glucose metabolism via the phosphorylation and activation of AMP-activated protein kinase alpha (AMPKα) at Thr¹⁷² in its kinase activation loop. We demonstrate that eIF3a regulates AMPK activation mainly by controlling synthesis of the small GTPase Rheb, largely independent of the well-known AMPK upstream liver kinase B1 and Ca²⁺/calmodulin-dependent protein kinase kinase 2, and also independent of mammalian target of rapamycin signaling and glucose levels. Our findings suggest that glucose metabolism in and proliferation of cancer cells may be translationally regulated via a novel eIF3a–Rheb–AMPK signaling axis.     

Antitumor effects of curcumin: A lipid perspective

Lipid metabolism plays an important role in cancer development due to the necessities of rapidly dividing cells to increase structural, energetic, and biosynthetic demands for cell proliferation. Basically, obesity, type 2 diabetes, and other related diseases, and cancer are associated with a common hyperactivated “lipogenic state.” Recent evidence suggests that metabolic reprogramming and overproduction of enzymes involved in the synthesis of fatty acids are the new hallmarks of cancer, which occur in an early phase of tumorigenesis. As the first evidence to confirm dysregulated lipid metabolism in cancer cells, the overexpression of fatty acid synthase (FAS) was observed in breast cancer patients and demonstrated the role of FAS in cancer. Other enzymes of fatty acid synthesis have recently been found to be dysregulated in cancer, including ATP-dependent citrate lyase and acetyl-CoA carboxylase, which further underscores the connection of these metabolic pathways with cancer cell survival and proliferation. The degree of overexpression of lipogenic enzymes and elevated lipid utilization in tumors is closely associated with cancer progression. The question that arises is whether the progression of cancer can be suppressed, or at least decelerated, by modulating gene expression related to fatty acid metabolism. Curcumin, due to its effects on the regulation of lipogenic enzymes, might be able to suppress, or even cause regression of tumor growth. This review discusses recent evidence concerning the important role of lipogenic enzymes in the metabolism of cancer cells and whether the inhibitory effects of curcumin on lipogenic enzymes is therapeutically efficacious.     

Origin of fatty acid synthesis: Thermodynamics and kinetics of reaction pathways

Summary The primitiveness of contemporary fatty acid biosynthesis was evaluated by using the thermodynamics and kinetics of its component reactions to estimate the extent of its dependence on powerful and selective catalysis by enzymes. Since this analysis indicated that the modern pathway is not primitive because it requires sophisticated enzymatic catalysis, we here propose an alternative pathway of primitive fatty acid synthesis that uses glycolaldehyde as a substrate. In contrast to the modern pathway, this primitive pathway is not dependent on an exogenous source of phosphoanhydride energy (ATP). Furthermore, the chemical spontaneity of its reactions suggests that it could have been readily catalyzed by the rudimentary biocatalysts available at an early stage in the origin of life.     

SCD1 inhibition causes cancer cell death by depleting mono-unsaturated fatty acids

Increased metabolism is a requirement for tumor cell proliferation. To understand the dependence of tumor cells on fatty acid metabolism, we evaluated various nodes of the fatty acid synthesis pathway. Using RNAi we have demonstrated that depletion of fatty-acid synthesis pathway enzymes SCD1, FASN, or ACC1 in HCT116 colon cancer cells results in cytotoxicity that is reversible by addition of exogenous fatty acids. This conditional phenotype is most pronounced when SCD1 is depleted. We used this fatty-acid rescue strategy to characterize several small-molecule inhibitors of fatty acid synthesis, including identification of TOFA as a potent SCD1 inhibitor, representing a previously undescribed activity for this compound. Reference FASN and ACC inhibitors show cytotoxicity that is less pronounced than that of TOFA, and fatty-acid rescue profiles consistent with their proposed enzyme targets. Two reference SCD1 inhibitors show low-nanomolar cytotoxicity that is offset by at least two orders of magnitude by exogenous oleate. One of these inhibitors slows growth of HCT116 xenograft tumors. Our data outline an effective strategy for interrogation of on-mechanism potency and pathway-node-specificity of fatty acid synthesis inhibitors, establish an unambiguous link between fatty acid synthesis and cancer cell survival, and point toward SCD1 as a key target in this pathway.     

High Metastaticgastric and Breast Cancer Cells Consume Oleic Acid in an AMPK Dependent Manner

Gastric cancer and breast cancer have a clear tendency toward metastasis and invasion to the microenvironment predominantly composed of adipocytes. Oleic acid is an abundant monounsaturated fatty acid that releases from adipocytes and impinges on different energy metabolism responses. The effect and underlying mechanisms of oleic acid on highly metastatic cancer cells are not completely understood. We reported that AMP-activated protein kinase (AMPK) was obviously activated in highly aggressive carcinoma cell lines treated by oleic acid, including gastric carcinoma HGC-27 and breast carcinoma MDA-MB-231 cell lines. AMPK enhanced the rates of fatty acid oxidation and ATP production and thus significantly promoted cancer growth and migration under serum deprivation. Inactivation of AMPK attenuated these activities of oleic acid. Oleic acid inhibited cancer cell growth and survival in low metastatic carcinoma cells, such as gastric carcinoma SGC7901 and breast carcinoma MCF-7 cell lines. Pharmacological activation of AMPK rescued the cell viability by maintained ATP levels by increasing fatty acid β-oxidation. These results indicate that highly metastatic carcinoma cells could consume oleic acid to maintain malignancy in an AMPK-dependent manner. Our findings demonstrate the important contribution of fatty acid oxidation to cancer cell function.     

PKM2, cancer metabolism,and the road ahead

A major metabolic aberration associated with cancer is a change in glucose metabolism. Isoform selection of the glycolytic enzyme pyruvate kinase has been implicated in the metabolic phenotype of cancer cells, and specific pyruvate kinase isoforms have been suggested to support divergent energetic and biosynthetic requirements of cells in tumors and normal tissues. PKM2 isoform expression has been closely linked to embryogenesis, tissue repair, and cancer. In contrast, forced expression of the PKM1 isoform has been associated with reduced tumor cell proliferation. Here, we discuss the role that PKM2 plays in cells and provide a historical perspective for how the study of PKM2 has contributed to understanding cancer metabolism. We also review recent studies that raise important questions with regard to the role of PKM2 in both normal and cancer cell metabolism.     

Development of protein kinase activators: AMPK as a target in metabolic disorders and cancer

AMP-activated protein kinase (AMPK) is a cellular energy sensor activated by metabolic stresses that either inhibit ATP synthesis or accelerate ATP consumption. Activation of AMPK in response to an increase in the cellular AMP:ATP ratio results in inhibition of ATP-consuming processes such as gluconeogenesis and fatty acid synthesis, while stimulating ATP-generating processes, including fatty acid oxidation. These alterations in lipid and glucose metabolism would be expected to ameliorate the pathogenesis of obesity, type 2 diabetes and other metabolic disorders. Recently, AMPK has also been identified as a potential target for cancer prevention and/or treatment. Cell growth and proliferation are energetically demanding, and AMPK may act as an “energy checkpoint” that permits growth and proliferation only when energy reserves are sufficient. Thus, activators of AMPK could have potential as novel therapeutics both for metabolic disorders and for cancer, which together constitute two of the most prevalent groups of diseases worldwide.     

Effect of Fatty Acids on Human Bone Marrow Mesenchymal Stem Cell Energy Metabolism and Survival

Successful stem cell therapy requires the optimal proliferation, engraftment, and differentiation of stem cells into the desired cell lineage of tissues. However, stem cell therapy clinical trials to date have had limited success, suggesting that a better understanding of stem cell biology is needed. This includes a better understanding of stem cell energy metabolism because of the importance of energy metabolism in stem cell proliferation and differentiation. We report here the first direct evidence that human bone marrow mesenchymal stem cell (BMMSC) energy metabolism is highly glycolytic with low rates of mitochondrial oxidative metabolism. The contribution of glycolysis to ATP production is greater than 97% in undifferentiated BMMSCs, while glucose and fatty acid oxidation combined only contribute 3% of ATP production. We also assessed the effect of physiological levels of fatty acids on human BMMSC survival and energy metabolism. We found that the saturated fatty acid palmitate induces BMMSC apoptosis and decreases proliferation, an effect prevented by the unsaturated fatty acid oleate. Interestingly, chronic exposure of human BMMSCs to physiological levels of palmitate (for 24 hr) reduces palmitate oxidation rates. This decrease in palmitate oxidation is prevented by chronic exposure of the BMMSCs to oleate. These results suggest that reducing saturated fatty acid oxidation can decrease human BMMSC proliferation and cause cell death. These results also suggest that saturated fatty acids may be involved in the long-term impairment of BMMSC survival in vivo.     

‘Why do tumour cells glycolyse?’: From glycolysis through citrate to lipogenesis

The re-emergence of interest in intermediary metabolism and the development of metabolomics in relation to cancer and other diseases provide a timely reason to revisit issues of tumour cell metabolism. In this review, we address the issue of the role of high aerobic glycolysis, which is commonly associated with the metabolism of many tumour cells. The concept presented emphasises the importance of the glycolysis-citrate-lipogenesis pathway in providing the synthetic and bioenergetic requirements that are essential for the growth and proliferation of tumour cells. We hope that our discussion will be informative and instructive, and will stimulate interest and research regarding the intermediary metabolism and its regulation in tumour cells. We express our appreciation to the many pioneering and contemporary researchers whose studies provide much of the basis for this presentation. (Mol Cell Biochem xxx: 1–8, 2005)     

Yeast and cancer cells – common principles in lipid metabolism

One of the paradigms in cancer pathogenesis is the requirement of a cell to undergo transformation from respiration to aerobic glycolysis – the Warburg effect – to become malignant. The demands of a rapidly proliferating cell for carbon metabolites for the synthesis of biomass, energy and redox equivalents, are fundamentally different from the requirements of a differentiated, quiescent cell, but it remains open whether this metabolic switch is a cause or a consequence of malignant transformation. One of the major requirements is the synthesis of lipids for membrane formation to allow for cell proliferation, cell cycle progression and cytokinesis. Enzymes involved in lipid metabolism were indeed found to play a major role in cancer cell proliferation, and most of these enzymes are conserved in the yeast, Saccharomyces cerevisiae. Most notably, cancer cell physiology and metabolic fluxes are very similar to those in the fermenting and rapidly proliferating yeast. Both types of cells display highly active pathways for the synthesis of fatty acids and their incorporation into complex lipids, and imbalances in synthesis or turnover of lipids affect growth and viability of both yeast and cancer cells. Thus, understanding lipid metabolism in S. cerevisiae during cell cycle progression and cell proliferation may complement recent efforts to understand the importance and fundamental regulatory mechanisms of these pathways in cancer.