Nitrogen anabolism underlies the importance of glutaminolysis in proliferating cells

Glutaminolysis and the Warburg effect are the two most noticeable metabolic features of tumor cells, whereas their biological significance in cell proliferation remains elusive. A widely accepted current hypothesis is that tumor cells use glutamine as a preferred carbon source for energy and reducing power, which has been used to explain both glutaminolysis and the Warburg effect. Here we provide evidence to show that supplying nitrogen, not the carbon skeleton, underlies the major biological importance of glutaminolysis for proliferating cells. We show that alternative nitrogen supplying mechanisms rescue cell proliferation in glutamine-free media. Particularly, we show that ammonia is sufficient to maintain a long-term survival and proliferation of Hep3B in glutamine-free media. We also observed that nitrogen source restriction repressed carbon metabolic pathways, including glucose utilization. Based on these new observations and metabolic pathways well-established in published literature, we propose an alternative model that cellular demand for glutamate is a key molecule in nitrogen anabolism, which is the driving force of glutaminolysis in proliferating cells. Our model suggests that the Warburg effect may be a metabolic consequence secondary to the nitrogen anabolism.Key words: glutaminolysis, cancer, Warburg effect, transamination, glycolysis, hypoxia     

Nitrogen anabolism underlies the importance of glutaminolysis in proliferating cells

Warburg effect and glutaminolysis are the two most noticeable metabolic features of tumor cells. A recent hypothesis is that tumor cells use glutamine as a preferred carbon source for energy and reducing power, which has been used to explain both Warburg effect and glutaminolysis. Here we provide evidence to demonstrate that supplying nitrogen, not the carbon skeleton, underlies the biological importance of glutaminolysis for proliferating cells. We show alternative nitrogen supplying mechanisms rescue Hep3B and HeLa cell proliferation in glutamine-free media. We conclude that cellular demand for nitrogen is a driving force of glutaminolysis for proliferating cells, and propose that glutamine addiction and Warburg effect are metabolic consequences of coordinated utilization of nitrogen and carbon sources for biosynthesis.     

Targeting respiratory complex I to prevent the Warburg effect

In the last 10 years, studies of energetic metabolism in different tumors clearly indicate that the definition of Warburg effect, i.e. the glycolytic shift cells undergo upon transformation, ought to be revisited considering the metabolic plasticity of cancer cells. In fact, recent findings show that the shift from glycolysis to re-established oxidative metabolism is required for certain steps of tumor progression, suggesting that mitochondrial function and, in particular, respiratory complex I are crucial for metabolic and hypoxic adaptation. Based on these evidences, complex I can be considered a lethality target for potential anticancer strategies. In conclusion, in this mini review we summarize and discuss why it is not paradoxical to develop pharmacological and genome editing approaches to target complex I as novel adjuvant therapies for cancer treatment.This article is part of a Directed Issue entitled: Energy Metabolism Disorders and Therapies.     

Curbing cancer's sweet tooth: Is there a role for MnSOD in regulation of the Warburg effect?

Reactive oxygen species (ROS), while vital for normal cellular function, can have harmful effects on cells, leading to the development of diseases such as cancer. The Warburg effect, the shift from oxidative phosphorylation to glycolysis, even in the presence of adequate oxygen, is an important metabolic change that confers many growth and survival advantages to cancer cells. Reactive oxygen species are important regulators of the Warburg effect. The mitochondria-localized antioxidant enzyme manganese superoxide dismutase (MnSOD) is vital to survival in our oxygen-rich atmosphere because it scavenges mitochondrial ROS. MnSOD is important in cancer development and progression. However, the significance of MnSOD in the regulation of the Warburg effect is just now being revealed, and it may significantly impact the treatment of cancer in the future.     

Downregulation of the Werner syndrome protein induces a metabolic shift that compromises redox homeostasis and limits proliferation of cancer cells

The Werner syndrome protein (WRN) is a nuclear protein required for cell growth and proliferation. Loss‐of‐function mutations in the Werner syndrome gene are associated with the premature onset of age‐related diseases. How loss of WRN limits cell proliferation and induces replicative senescence is poorly understood. Here, we show that WRN depletion leads to a striking metabolic shift that coordinately weakens the pathways that generate reducing equivalents for detoxification of reactive oxygen species and increases mitochondrial respiration. In cancer cells, this metabolic shift counteracts the Warburg effect, a defining characteristic of many malignant cells, resulting in altered redox balance and accumulation of oxidative DNA damage that inhibits cell proliferation and induces a senescence‐like phenotype. Consistent with these findings, supplementation with antioxidant rescues at least in part cell proliferation and decreases senescence in WRN‐knockdown cancer cells. These results demonstrate that WRN plays a critical role in cancer cell proliferation by contributing to the Warburg effect and preventing metabolic stress.     

Analysis of CHO cells metabolic redistribution in a glutamate-based defined medium in continuous culture.

The effect of glutamine replacement by glutamate and the balance between glutamate and glucose metabolism on the redistribution of t-PA-producing recombinant CHO cells metabolism is studied in a series of glucose shift down and shift up experiments in continuous culture. These experiments reveal the existence of multiple steady states, and experimental data are used to perform metabolic flux analysis to gain a better insight into cellular metabolism and its redistribution. Regulation of glucose feed rate promotes a higher efficiency of glucose and nitrogen source utilization, with lower production of metabolic byproducts, but this reduces t-PA specific production rate. This reduction under glucose limitation can be attributed to the fact that the cells are forced to efficiently utilize the carbon and energy source for growth, impairing the production of dispensable metabolites. It is, therefore, the combination of growth rate and carbon and energy source availability that determines the level of t-PA production in continuous culture.     

New aspects of the Warburg effect in cancer cell biology

Altered cellular metabolism is a defining feature of cancer [1]. The best studied metabolic phenotype of cancer is aerobic glycolysis--also known as the Warburg effect--characterized by increased metabolism of glucose to lactate in the presence of sufficient oxygen. Interest in the Warburg effect has escalated in recent years due to the proven utility of FDG-PET for imaging tumors in cancer patients and growing evidence that mutations in oncogenes and tumor suppressor genes directly impact metabolism. The goals of this review are to provide an organized snapshot of the current understanding of regulatory mechanisms important for Warburg effect and its role in tumor biology. Since several reviews have covered aspects of this topic in recent years, we focus on newest contributions to the field and reference other reviews where appropriate.     

The Warburg effect and its cancer therapeutic implications

Increased aerobic glycolysis in cancer, a phenomenon known as the Warburg effect, has been observed in various tumor cells and represents a major biochemical alteration associated with malignant transformation. Although the exact molecular mechanisms underlying this metabolic change remain to be elucidated, the profound biochemical alteration in cancer cell energy metabolism provides exciting opportunities for the development of therapeutic strategies to preferentially kill cancer cells by targeting the glycolytic pathway. Several small molecules capable of inhibiting glycolysis in experimental systems have been shown to have promising anticancer activity in vitro and in vivo. This review article provides a brief summary of our current understanding of the Warburg effect, the underlying mechanisms, and its influence on the development of therapeutic strategies for cancer treatment.     

Cellular energetics as a target for tumor cell elimination

Investigation of cancer cell metabolism has revealed variability of the metabolic profiles among different types of tumors. According to the most classical model of cancer bioenergetics, malignant cells primarily use glycolysis as the major metabolic pathway and produce large quantities of lactate with suppressed oxidative phosphorylation even in the presence of ample oxygen. This is referred to as aerobic glycolysis, or the Warburg effect. However, a growing number of recent studies provide evidence that not all cancer cells depend on glycolysis, and, moreover, oxidative phosphorylation is essential for tumorigenesis. Thus, it is necessary to consider distinctive patterns of cancer metabolism in each specific case. Chemoresistance of cancer cells is associated with decreased sensitivity to different types of antitumor agents. Stimulation of apoptosis is a major strategy for elimination of cancer cells, and therefore activation of mitochondrial functions with direct impact on mitochondria to destabilize them appears to be an important approach to the induction of cell death. Consequently, the design of combination therapies using acclaimed cytotoxic agents directed to induction of apoptosis and metabolic agents affecting cancer cell bioenergetics are prospective strategies for antineoplastic therapy.     

乳酸代谢——肿瘤治疗新靶点

恶性肿瘤严重危害人类健康,其治疗目前主要有手术、放疗和化疗三种方式,但疗效尚无法达到令人满意的程度,因此寻找肿瘤治疗新靶点、实现肿瘤的靶向治疗非常迫切.Warburg效应普遍存在于多种肿瘤中,其重要特征是在氧气充足的条件下,癌细胞的能量代谢仍以糖酵解为主.Warburg效应是糖酵解的典型过程,葡萄糖被大量吸收并通过糖酵解转化为乳酸.糖酵解产物乳酸可以激活癌细胞中许多重要的信号通路,促进癌细胞的存活、侵袭、免疫逃逸、转移和血管生成.因此,靶向乳酸代谢过程及其关键酶可能为肿瘤治疗提供新的靶点.本文对肿瘤细胞代谢方式的改变,乳酸对肿瘤细胞免疫逃逸、肿瘤转移、肿瘤血管生成的影响,以及以乳酸为靶点的肿瘤治疗等方面进行综述.     

Oncogenic viral infection and amino acid metabolism in cancer progression: Molecular insights and clinical implications

Viruses lack essential living system, so they must hijack host cell metabolism for its survival and reproduction. Interestingly, the metabolic reprogramming induced by oncovirus is critical for the malignant transformation. Amino acid can supply the source of nitrogen and carbon for biosynthesis or fulfill the energy requirement for the rapid growth of tumor cells. Amino acid metabolism caused by oncogenic viral infection often mirrors metabolic changes observed in cancer cells, such as glutamine addiction, asparagine dependence, arginine auxotrophy and active serine/ proline metabolism. In this review, we describe amino acid metabolism reprogramming in tumors. We also discuss how oncogenic viruses hijack amino acid metabolism in the stress status. Further research on the metabolic profile of virus-related cancers will not only provide new targets for tumor prevention and treatment, but novel diagnostic and therapeutic strategies as well.     

An update on therapeutic opportunities offered by cancer glycolytic metabolism

Almost all invasive cancers, regardless of tissue origin, are characterized by specific modifications of their cellular energy metabolism. In fact, a strong predominance of aerobic glycolysis over oxidative phosphorylation (Warburg effect) is usually associated with aggressive tumour phenotypes. This metabolic shift offers a survival advantage to cancer cells, since they may continue to produce energy and anabolites even when they are exposed to either transient or permanent hypoxic conditions. Moreover, it ensures a high production rate of glycolysis intermediates, useful as building blocks for fast cell proliferation of cancer cells. This peculiar metabolic profile may constitute an ideal target for therapeutic interventions that selectively hit cancer cells with minimal residual systemic toxicity. In this review we provide an update about some of the most recent advances in the discovery of new bioactive molecules that are able to interfere with cancer glycolysis.     

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.     

Cancer cell metabolism: Warburg and beyond

Described decades ago, the Warburg effect of aerobic glycolysis is a key metabolic hallmark of cancer, yet its significance remains unclear. In this Essay, we re-examine the Warburg effect and establish a framework for understanding its contribution to the altered metabolism of cancer cells.     

Metabolic regulation of suppressive myeloid cells in cancer

Cancer cells rewire their metabolism to promote growth, survival, proliferation and long-term maintenance. The common feature of this altered metabolism is the increased glucose uptake and fermentation of glucose to lactate, which is observed even in the presence of completely functioning mitochondria. This effect is known as the ‘Warburg Effect’ and its intensive investigation in the last decade has partially established either its causes or its functions. It is now emerging that a major side effect of the Warburg Effect is immunosuppression, which limits the immunogenicity of cancer cells and therefore restricts the therapeutic efficacy of anticancer immunotherapy. Here we discuss how the metabolic communication between cancer and infiltrating myeloid cells contributes to cancer immune evasion and how the understanding of these mechanisms may improve current immunotherapies.     

Combination of Sulindac and Dichloroacetate Kills Cancer Cells via Oxidative Damage

Sulindac is an FDA-approved non-steroidal anti-inflammatory drug with documented anticancer activities. Our recent studies showed that sulindac selectively enhanced the killing of cancer cells exposed to oxidizing agents via production of reactive oxygen species (ROS) resulting in mitochondrial dysfunction. This effect of sulindac and oxidative stress on cancer cells could be related to the defect in respiration in cancer cells, first described by Warburg 50 years ago, known as the Warburg effect. We postulated that sulindac might enhance the selective killing of cancer cells when combined with any compound that alters mitochondrial respiration. To test this hypothesis we have used dichloroacetate (DCA), which is known to shift pyruvate metabolism away from lactic acid formation to respiration. One might expect that DCA, since it stimulates aerobic metabolism, could stress mitochondrial respiration in cancer cells, which would result in enhanced killing in the presence of sulindac. In this study, we have shown that the combination of sulindac and DCA enhances the selective killing of A549 and SCC25 cancer cells under the conditions used. As predicted, the mechanism of killing involves ROS production, mitochondrial dysfunction, JNK signaling and death by apoptosis. Our results suggest that the sulindac-DCA drug combination may provide an effective cancer therapy.