Canonical and new generation anticancer drugs also target energy metabolism

Energy Metabolism Behavior and Response to Microenvironmental Factors of the Experimental Cancer Cell Models Differ from that of Actual Human Tumors

Analysis of the biochemical differences in the energy metabolism among bi-dimensional (2D) and tri-dimensional (3D) cultured cancer cell models and actual human tumors was undertaken. In 2D cancer cells, the oxidative phosphorylation (OxPhos) fluxes range is 2.5-19 nmol O2/min/mg cellular protein. Hypoxia drastically decreased OxPhos flux by 2-3 times in 2D models, similar to what occurs in mature multicellular tumor spheroids (MCTS), a representative 3D cancer cell model. However, mitochondrial protein contents and enzyme activities were significantly different between both models. Moreover, glycolytic fluxes were also significantly different between 2D and MCTS. The glycolytic flux range in 2D models is 1-34 nmol lactate/min/mg cellular protein, whereas in MCTS the range of glycolysis fluxes is 60-80 nmol lactate/min/mg cellular. In addition, sensitivity to anticancer canonical and metabolic drugs was greater in MCTS than in 2D. Actual solid human tumor samples show lower (1.6-4.5 times) OxPhos fluxes compared to normoxic 2D cancer cell cultures. These observations indicate that tridimensional organization provides a unique microenvironment affecting tumor physiology, which has not been so far faithfully reproduced by the 2D environment. Thus, the analysis of the resemblances and differences among cancer cell models undertaken in the present study raises caution on the interpretation of results derived from 2D cultured cancer cells when they are extended to clinical settings. It also raises awareness about detecting which biological and environmental factors are missing in 2D and 3D cancer cell models to be able to reproduce the actual human tumor behavior.

A new strategy for the rapid identification and validation of direct toxicity targets of psoralen-induced hepatotoxicity

The interaction between small-molecule compounds of traditional Chinese medicine and their direct targets is the molecular initiation event, which is the key factor for toxicity efficacy. Psoralen, an active component of Fructus Psoraleae, is toxic to the liver and has various pharmacological properties. Although the mechanism of psoralen-induced hepatotoxicity has been studied, the direct target of psoralen remains unclear. Thus, the aim of this study was to discover direct targets of psoralen. To this end, we initially used proteomics based on drug affinity responsive target stability (DARTS) technology to identify the direct targets of psoralen. Next, we used surface plasmon resonance (SPR) analysis and verified the affinity effect of the 'component-target protein'. This method combines molecular docking technology to explore binding sites between small molecules and proteins. SPR and molecular docking confirmed that psoralen and tyrosine-protein kinase ABL1 could be stably combined. Based on the above experimental results, ABL1 is a potential direct target of psoralen-induced hepatotoxicity. Finally, the targets Nrf2 and mTOR, which are closely related to the hepatotoxicity caused by psoralen, were predicted by integrating proteomics and network pharmacology. The direct target ABL1 is located upstream of Nrf2 and mTOR, Nrf2 can influence the expression of mTOR by affecting the level of reactive oxygen species. Immunofluorescence experiments and western blot results showed that psoralen could affect ROS levels and downstream Nrf2 and mTOR protein changes, whereas the ABL1 inhibitor imatinib and ABL1 agonist DPH could enhance or inhibit this effect. In summary, we speculated that when psoralen causes hepatotoxicity, it acts on the direct target ABL1, resulting in a decrease in Nrf2 expression, an increase in ROS levels and a reduction in mTOR expression, which may cause cell death. We developed a new strategy for predicting and validating the direct targets of psoralen. This strategy identified the toxic target, ABL1, and the potential toxic mechanism of psoralen.

Interplay Among Metabolism, Epigenetic Modifications, and Gene Expression in Cancer

Epigenetic modifications and metabolism are two fundamental biological processes. During tumorigenesis and cancer development both epigenetic and metabolic alterations occur and are often intertwined together. Epigenetic modifications contribute to metabolic reprogramming by modifying the transcriptional regulation of metabolic enzymes, which is crucial for glucose metabolism, lipid metabolism, and amino acid metabolism. Metabolites provide substrates for epigenetic modifications, including histone modification (methylation, acetylation, and phosphorylation), DNA and RNA methylation and non-coding RNAs. Simultaneously, some metabolites can also serve as substrates for nonhistone post-translational modifications that have an impact on the development of tumors. And metabolic enzymes also regulate epigenetic modifications independent of their metabolites. In addition, metabolites produced by gut microbiota influence host metabolism. Understanding the crosstalk among metabolism, epigenetic modifications, and gene expression in cancer may help researchers explore the mechanisms of carcinogenesis and progression to metastasis, thereby provide strategies for the prevention and therapy of cancer. In this review, we summarize the progress in the understanding of the interactions between cancer metabolism and epigenetics.

Target identification of a novel unsymmetrical 1,3,4-oxadiazole derivative with antiproliferative properties

1,3,4-Oxadiazole derivatives are widely used in research on antineoplastic drugs. Recently, we discovered a novel unsymmetrical 1,3,4-oxadiazole compound with antiproliferative properties called 2j. To further investigate its possible targets and molecular mechanisms, RNA-seq was performed and the differentially expressed genes (DEGs) were obtained after treatment. Data were analyzed using functional (Gene Ontology term) and pathway (Kyoto Encyclopedia of Genes and Genomes) enrichment of the DEGs. The hub genes were determined by the analysis of protein-protein interaction networks. The connectivity map (CMap) information provided insight into the model action of antitumor small molecule drugs. Hub genes have been identified through function gene networks using STRING analysis. The small molecular targets obtained by CMap comparison showed that 2j is a tubulin inhibitor and it acts mainly affecting tumor cells through the cell cycle, FoxO signaling pathway, apoptotic, and p53 signaling pathways. The possible targets of 2j could be TUBA1A and TUBA4A. Molecular docking results indicated that 2j interacts at the colchicine-binding site on tubulin.

Quantitative analysis of amino acid metabolism in liver cancer links glutamate excretion to nucleotide synthesis

We used a combination of experimental measurements and computer simulations to understand how liver cancer cells rewire their metabolism to grow faster. We observed that glutamate is excreted by the cells, and our simulations suggest that this occurs because glutamate is formed in excess in the cytoplasm, when cells rapidly synthesize nucleotides, which are required for growth. Meanwhile, glutamate that is formed in the mitochondria is, on the other hand, not excreted. Treating glutamate as two distinct pools, a cytosolic and a mitochondrial, is useful to better understand why many cancer cells rapidly consume glutamine, the precursor of glutamate. The results point toward potential drug targets that could be used to reduce growth of liver cancer cells.

Many cancer cells consume glutamine at high rates; counterintuitively, they simultaneously excrete glutamate, the first intermediate in glutamine metabolism. Glutamine consumption has been linked to replenishment of tricarboxylic acid cycle (TCA) intermediates and synthesis of adenosine triphosphate (ATP), but the reason for glutamate excretion is unclear. Here, we dynamically profile the uptake and excretion fluxes of a liver cancer cell line (HepG2) and use genome-scale metabolic modeling for in-depth analysis. We find that up to 30% of the glutamine is metabolized in the cytosol, primarily for nucleotide synthesis, producing cytosolic glutamate. We hypothesize that excreting glutamate helps the cell to increase the nucleotide synthesis rate to sustain growth. Indeed, we show experimentally that partial inhibition of glutamate excretion reduces cell growth. Our integrative approach thus links glutamine addiction to glutamate excretion in cancer and points toward potential drug targets.

A Therapeutic Role for the F1FO-ATP Synthase

Recently, the F1FO-ATP synthase, due to its dual role of life enzyme as main adenosine triphosphate (ATP) maker and of death enzyme, as ATP dissipator and putative structural component of the mitochondrial permeability transition pore (mPTP), which triggers cell death, has been increasingly considered as a drug target. Accordingly, the enzyme offers new strategies to counteract the increased antibiotic resistance. The challenge is to find or synthesize compounds able to discriminate between prokaryotic and mitochondrial F1FO-ATP synthase, exploiting subtle structural differences to kill pathogens without affecting the host. From this perspective, the eukaryotic enzyme could also be made refractory to macrolide antibiotics by chemically produced posttranslational modifications. Moreover, because the mitochondrial F1FO-ATPase activity stimulated by Ca2+ instead of by the natural modulator Mg2+ is most likely involved in mPTP formation, effectors preferentially targeting the Ca2+-activated enzyme may modulate the mPTP. If the enzyme involvement in the mPTP is confirmed, Ca2+-ATPase inhibitors may counteract conditions featured by an increased mPTP activity, such as neurodegenerative and cardiovascular diseases and physiological aging. Conversely, mPTP opening could be pharmacologically stimulated to selectively kill unwanted cells. On the basis of recent literature and promising lab findings, the action mechanism of F1 and FO inhibitors is considered. These molecules may act as enzyme modifiers and constitute new drugs to kill pathogens, improve compromised enzyme functions, and limit the deathly enzyme role in pathologies. The enzyme offers a wide spectrum of therapeutic strategies to fight at the molecular level diseases whose treatment is still insufficient or merely symptomatic.

α-Ketoglutarate-Activated NF-κB Signaling Promotes Compensatory Glucose Uptake and Brain Tumor Development

The rapid proliferation of cancer cells and dysregulated vasculature within the tumor leads to limited nutrient accessibility. Cancer cells often rewire their metabolic pathways for adaption to nutrient stress, and the underlying mechanism remains largely unknown. Glutamate dehydrogenase 1 (GDH1) is a key enzyme in glutaminolysis that converts glutamate to α-ketoglutarate (α-KG). Here, we show that, under low glucose, GDH1 is phosphorylated at serine (S) 384 and interacts with RelA and IKKβ. GDH1-produced α-KG directly binds to and activates IKKβ and nuclear factor κB (NF-κB) signaling, which promotes glucose uptake and tumor cell survival by upregulating GLUT1, thereby accelerating gliomagenesis. In addition, GDH1 S384 phosphorylation correlates with the malignancy and prognosis of human glioblastoma. Our finding reveals a unique role of α-KG to directly regulate signal pathway, uncovers a distinct mechanism of metabolite-mediated NF-κB activation, and also establishes the critical role of α-KG-activated NF-κB in brain tumor development.

Connections between metabolism and epigenetics in cancers

In the past half century, our version on cancer, from tumor initiation, growth, to metastasis, is dominated by genetic mutation. The importance of metabolism and epigenetics was not recognized until most recently. Extensive cell proliferation is one of the hallmarks of cancers. To support the energetic and anabolic demands of enhanced proliferation, tumors reprogram the pathways of nutrient procurement and metabolism. In this context, a new link between metabolic alterations and cancer progression has been unraveled over the last decade by the studies conducted in the area of cancer cell metabolism. Cancer cells are known to alter their metabolic profile during the course of tumorigenesis and metastasis thereby exhibiting a tightly regulated program of metabolic plasticity. Noteworthy, certain metabolic alteration are known to occur at the epigenetic level, thus making epigenetics and metabolism highly interwoven in a reciprocal manner. Metabolites that are generated during metabolic pathways, such as in glycolytic cycle and oxidative phosphorylation, serve as cofactors or substrates for the enzymatic reactions that catalyze the epigenetic modifications and transcriptional regulation. Several studies also indicate that the epigenome is sensitive to cellular metabolism. Since many of the metabolic alterations and consequently aberrated epigenetic regulation are common to a wide range of cancer types, they serve as promising targets for anti-cancer therapies. Here we discuss the latest findings in cancer cell metabolism, elucidating the major anabolic, catabolic and energetic demands required for sustaining cancer growth, and the influence of altered metabolism on epigenetics and vice versa. A comprehensive research pertaining to metabolomic profiling and epigenome interactors/mediators in malignant neoplasias is imperative in deciphering the potential targets that can be exploited for the development of robust anti-cancer therapies.

3-Bromopyruvate induces expression of antioxidant genes

An alkylating compound, 3-bromopyruvic acid (3-3-bromopyruvic acid (BP)) is a promising anti-cancer agent, potentially able to act on multidrug-resistant cells. Its action has been attributed mainly to inhibition of glycolysis. This compound induces also oxidative stress at a cellular level. The effects of 3-BP on gene expression have not been studied although they may determine the survival of cells exposed to 3-BP. The aim of this paper was to examine the effect 3-BP on gene expression pattern in breast MCF-7 cancer cells. Detection of the differences in gene expression was performed using microarrays and dysregulated genes were validated by reverse transcription-quantitative polymerase chain reaction (RT-qPCR). Exposure of cells to 100 µM 3-BP for 6, 12 and 24 increased expression and diminished expression of 39 and 6 genes, respectively. Among the induced genes, 22 belong to general cellular stress response genes, maintenance genes involved in redox homeostasis, responding to oxidative stress (among them metallothioneins, low-molecular-weight thiol homeostasis enzymes and genes coding for NAD(P)H-dependent oxidoreductases operating on complex organic substrates, including aldo-keto reductases). These results demonstrate that transient oxidative stress in cells exposed to 3-BP is followed by antioxidant response.

Potent antiproliferative effect of fatty-acid derivative AIC-47 on leukemic mice harboring BCR-ABL mutation

Therapy based on targeted inhibition of BCR‐ABL tyrosine kinase has greatly improved the prognosis for patients with Philadelphia chromosome (Ph)‐positive leukemia and tyrosine kinase inhibitors (TKI) have become standard therapy. However, some patients acquire resistance to TKI that is frequently associated with point mutations in BCR‐ABL. We previously reported that a medium‐chain fatty‐acid derivative AIC‐47 induced transcriptional suppression of BCR‐ABL and perturbation of the Warburg effect, leading to growth inhibition in Ph‐positive leukemia cells. Herein, we showed that AIC‐47 had anti‐leukemic effects in either wild type (WT)‐ or mutated‐BCR‐ABL‐harboring cells. AIC‐47 suppressed transcription of BCR‐ABL gene regardless of the mutation through downregulation of transcriptional activator, c‐Myc. Reprogramming of the metabolic pathway has been reported to be associated with resistance to anti‐cancer drugs; however, we found that a point mutation of BCR‐ABL was independent of the profile of pyruvate kinase muscle (PKM) isoform expression. Even in T315I‐mutated cells, AIC‐47 induced switching of the expression profile of PKM isoforms from PKM2 to PKM1, suggesting that AIC‐47 disrupted the Warburg effect. In a leukemic mouse model, AIC‐47 greatly suppressed the increase in BCR‐ABLmRNA level and improved hepatosplenomegaly regardless of the BCR‐ABL mutation. Notably, the improvement of splenomegaly by AIC‐47 was remarkable and might be equal to or greater than that of TKI. These findings suggest that AIC‐47 might be a promising agent for overcoming the resistance of Ph‐positive leukemia to therapy.

Targeting glycometabolic reprogramming to restore the sensitivity of leukemia drug-resistant K562/ADM cells to adriamycin

AIMS

Mounting studies have confirmed that cancer cells reprogram their metabolism during early carcinogenesis to develop many other hallmarks, and demonstrated a relationship between aerobic glycolysis and the occurrence of drug resistance. However, the molecular mechanisms and role in tumor drug resistance of aerobic glycolysis remain unclear.

MAIN METHODS

We analyzed differentially expressed genes (DEGs) at the RNA level between the multi-drug resistance (MDR) leukemia cell line K562/adriamycin (ADM) and its parental, drug-sensitive K562 cell line. Clustering and enrichment analysis of DEGs was performed. Oxamate, a lactic dehydrogenase inhibitor were used to assess the effect of glycolysis inhibition on ADM susceptibility and the expression of the enriched DEGs in K562/ADM cells.

KEY FINDINGS

A total of 1742 DEGs were detected between the K562/ADM and K562 cell lines. The differential expression of unigenes encoding enzymes involved in glycometabolism signifies that there was a greater aerobic glycolysis flux in K562/ADM cells. The PI3K-AKT signaling pathway, which is related to glucose metabolism, showed representative differential enrichment and up-regulation in K562/ADM cells. Oxamate improved and re-sensitized the therapeutic effect of ADM in ADM-resistant cells by inhibiting aerobic glycolysis either directly or indirectly by down-regulation of the AKT-mTOR pathway.

SIGNIFICANCE

Our findings suggest that ADM resistance mediated by the increase of aerobic glycolysis, which related to the over-activation of the AKT-mTOR-c-Myc pathway in MDR leukemia cells. Inhibition of aerobic glycolysis and down-regulation of signaling pathways involved in aerobic glycolysis represent a potential chemotherapeutic strategy for sensitizing leukemic cells and thereby overcoming MDR.

The phosphoglycerate kinase 1 variants found in carcinoma cells display different catalytic activity and conformational stability compared to the native enzyme

Cancer cells are able to survive in difficult conditions, reprogramming their metabolism according to their requirements. Under hypoxic conditions they shift from oxidative phosphorylation to aerobic glycolysis, a behavior known as Warburg effect. In the last years, glycolytic enzymes have been identified as potential targets for alternative anticancer therapies. Recently, phosphoglycerate kinase 1 (PGK1), an ubiquitous enzyme expressed in all somatic cells that catalyzes the seventh step of glycolysis which consists of the reversible phosphotransfer reaction from 1,3-bisphosphoglycerate to ADP, has been discovered to be overexpressed in many cancer types. Moreover, several somatic variants of PGK1 have been identified in tumors. In this study we analyzed the effect of the single nucleotide variants found in cancer tissues on the PGK1 structure and function. Our results clearly show that the variants display a decreased catalytic efficiency and/or thermodynamic stability and an altered local tertiary structure, as shown by the solved X-ray structures. The changes in the catalytic properties and in the stability of the PGK1 variants, mainly due to the local changes evidenced by the X-ray structures, suggest also changes in the functional role of PGK to support the biosynthetic need of the growing and proliferating tumour cells.

The role of the ATPase inhibitor factor 1 (IF1) in cancer cells adaptation to hypoxia and anoxia

The physiological role of the mitochondrial ATP synthase complex is to generate ATP through oxidative phosphorylation. Indeed, the enzyme can reverse its activity and hydrolyze ATP under ischemic conditions, as shown in isolated mitochondria and in mammalian heart and liver. However, what occurs when cancer cells experience hypoxia or anoxia has not been well explored. In the present study, we investigated the bioenergetics of cancer cells under hypoxic/anoxic conditions with particular emphasis on ATP synthase, and the conditions driving it to work in reverse. In this context, we further examined the role exerted by its endogenous inhibitor factor, IF1, that it is overexpressed in cancer cells. Metabolic and bioenergetic analysis of cancer cells exposed to severe hypoxia (down to 0.1% O2) unexpectedly showed that Δψm is preserved independently of the presence of IF1 and that ATP synthase still phosphorylates ADP though at a much lower rate than in normoxia. However, when we induced an anoxia-mimicking condition by collapsing ΔμΗ+ with the FCCP uncoupler, the IF1-silenced clones only reversed the ATP synthase activity hydrolyzing ATP in order to reconstitute the electrochemical proton gradient. Notably, in cancer cells IF1 overexpression fully prevents ATP synthase hydrolytic activity activation under uncoupling conditions. Therefore, our results suggest that IF1 overexpression promotes cancer cells survival under temporary anoxic conditions by preserving cellular ATP despite mitochondria dysfunction.

Teaching the basics of cancer metabolism: Developing antitumor strategies by exploiting the differences between normal and cancer cell metabolism

This review of the basics of cancer metabolism focuses on exploiting the metabolic differences between normal and cancer cells. The first part of the review covers the different metabolic pathways utilized in normal cells to generate cellular energy, or ATP, and the glycolytic intermediates required to build the cellular machinery. The second part of the review discusses aerobic glycolysis, or the Warburg effect, and the metabolic reprogramming involving glycolysis, tricarboxylic acid cycle, and glutaminolysis in the context of developing targeted inhibitors in cancer cells. Finally, the selective targeting of cancer mitochondrial metabolism using positively charged lipophilic compounds as potential therapeutics and their ability to mitigate the toxic side effects of conventional chemotherapeutics in normal cells are discussed. I hope this graphical review will be useful in helping undergraduate, graduate, and medical students understand how investigating the basics of cancer cell metabolism could provide new insight in developing potentially new anticancer treatment strategies.

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Exploiting biochemical and metabolic differences between normal and cancer cells.

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Mitigating reverse Warburg effect in the tumor stroma or microenvironment to hinder tumor growth.

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Dual targeting of glycolysis and mitochondrial metabolism to inhibit tumor cell proliferation.

Hypoglycemia Enhances Epithelial-Mesenchymal Transition and Invasiveness, and Restrains the Warburg Phenotype, in Hypoxic HeLa Cell Cultures and Microspheroids

The accelerated growth of solid tumors leads to episodes of both hypoxia and hypoglycemia (HH) affecting their intermediary metabolism, signal transduction, and transcriptional activity. A previous study showed that normoxia (20% O₂ ) plus 24 h hypoglycemia (2.5 mM glucose) increased glycolytic flux whereas oxidative phosphorylation (OxPhos) was unchanged versus normoglycemia in HeLa cells. However, the simultaneous effect of HH on energy metabolism has not been yet examined. Therefore, the effect of hypoxia (0.1-1% O₂ ) plus hypoglycemia on the energy metabolism of HeLa cells was analyzed by evaluating protein content and activity, along with fluxes of both glycolysis and OxPhos. Under hypoxia, in which cell growth ceased and OxPhos enzyme activities, ΔΨm and flux were depressed, hypoglycemia did not stimulate glycolytic flux despite increasing H-RAS, p-AMPK, GLUT1, GLUT3, and HKI levels, and further decreasing mitochondrial enzyme content. The impaired mitochondrial function in HH cells correlated with mitophagy activation. The depressed OxPhos and unchanged glycolysis pattern was also observed in quiescent cells from mature multicellular tumor spheroids, suggesting that these inner cell layers are similarly subjected to HH. The principal ATP supplier was glycolysis for HH 2D monolayer and 3D quiescent spheroid cells. Accordingly, the glycolytic inhibitors iodoacetate and gossypol were more effective than mitochondrial inhibitors in decreasing HH-cancer cell viability. Under HH, stem cell-, angiogenic-, and EMT-biomarkers, as well as glycoprotein-P content and invasiveness, were also enhanced. These observations indicate that HH cancer cells develop an attenuated Warburg and pronounced EMT- and invasive-phenotype. J. Cell. Physiol. 232: 1346-1359, 2017. © 2016 Wiley Periodicals, Inc.

Assessment of the low inhibitory specificity of oxamate, aminooxyacetate and dichloroacetate on cancer energy metabolism

BACKGROUND

Exceedingly high therapeutic/experimental doses of metabolic drugs such as oxamate, aminooxyacetate (AOA) and dichloroacetate (DCA) are required to diminish growth, glycolysis and oxidative phosphorylation (OxPhos) of different cancer cells. To identify the mechanisms of action of these drugs on cancer energy metabolism, a systematic analysis of their specificities was undertaken.

METHODS

Hepatocarcinoma AS-30D cells were treated with the inhibitors and glycolysis and OxPhos enzyme activities, metabolites and fluxes were analyzed. Kinetic modeling of glycolysis was used to identify the regulatory mechanisms.

RESULTS

Oxamate (i) not only inhibited LDH, but also PYK and ENO activities inducing an increase in the cytosolic NAD(P)H, Fru1,6BP and DHAP levels in AS-30D cells; (ii) it slightly inhibited HPI, ALD and Glc6PDH; and (iii) it inhibited pyruvate-driven OxPhos in isolated heart mitochondria. AOA (i) strongly inhibited both AAT and AlaT, and 2-OGDH and glutamate-driven OxPhos; and (ii) moderately affected GAPDH and TPI. DCA slightly affected pyruvate-driven OxPhos and Glc6PDH. Kinetic modeling of cancer glycolysis revealed that oxamate inhibition of LDH, PYK and ENO was insufficient to achieve glycolysis flux inhibition. To do so, HK, HPI, TPI and GAPDH have to be also inhibited by the accumulated Fru1,6BP and DHAP induced by oxamate.

CONCLUSION

Oxamate, AOA, and DCA are not specific drugs since they inhibit several enzymes/transporters of the glycolytic and OxPhos pathways through direct interaction or indirect mechanisms.

GENERAL SIGNIFICANCE

These data explain why oxamate or AOA, through their multisite inhibitory actions on glycolysis or OxPhos, may be able to decrease the proliferation of cancer cells.

Early effects of the antineoplastic agent salinomycin on mitochondrial function

Salinomycin, isolated from Streptomyces albus, displays antimicrobial activity. Recently, a large-scale screening approach identified salinomycin and nigericin as selective apoptosis inducers of cancer stem cells. Growing evidence suggests that salinomycin is able to kill different types of non-stem tumor cells that usually display resistance to common therapeutic approaches, but the mechanism of action of this molecule is still poorly understood. Since salinomycin has been suggested to act as a K(+) ionophore, we explored its impact on mitochondrial bioenergetic performance at an early time point following drug application. In contrast to the K(+) ionophore valinomycin, salinomycin induced a rapid hyperpolarization. In addition, mitochondrial matrix acidification and a significant decrease of respiration were observed in intact mouse embryonic fibroblasts (MEFs) and in cancer stem cell-like HMLE cells within tens of minutes, while increased production of reactive oxygen species was not detected. By comparing the chemical structures and cellular effects of this drug with those of valinomycin (K(+) ionophore) and nigericin (K(+)/H(+) exchanger), we conclude that salinomycin mediates K(+)/H(+) exchange across the inner mitochondrial membrane. Compatible with its direct modulation of mitochondrial function, salinomycin was able to induce cell death also in Bax/Bak-less double-knockout MEF cells. Since at the concentration range used in most studies (around 10 μM) salinomycin exerts its effect at the level of mitochondria and alters bioenergetic performance, the specificity of its action on pathologic B cells isolated from patients with chronic lymphocytic leukemia (CLL) versus B cells from healthy subjects was investigated. Mesenchymal stromal cells (MSCs), proposed to mimic the tumor environment, attenuated the apoptotic effect of salinomycin on B-CLL cells. Apoptosis occurred to a significant extent in healthy B cells as well as in MSCs and human primary fibroblasts. The results indicate that salinomycin, when used above μM concentrations, exerts direct, mitochondrial effects, thus compromising cell survival.

Antiproliferative effects of mitochondria-targeted cationic antioxidants and analogs: Role of mitochondrial bioenergetics and energy-sensing mechanism

One of the proposed mechanisms for tumor proliferation involves redox signaling mediated by reactive oxygen species such as superoxide and hydrogen peroxide generated at moderate levels. Thus, the antiproliferative and anti-tumor effects of certain antioxidants were attributed to their ability to mitigate intracellular reactive oxygen species (ROS). Recent reports support a role for mitochondrial ROS in stimulating tumor cell proliferation. In this study, we compared the antiproliferative effects and the effects on mitochondrial bioenergetic functions of a mitochondria-targeted cationic carboxyproxyl nitroxide (Mito-CP), exhibiting superoxide dismutase (SOD)-like activity and a synthetic cationic acetamide analog (Mito-CP-Ac) lacking the nitroxide moiety responsible for the SOD activity. Results indicate that both Mito-CP and Mito-CP-Ac potently inhibited tumor cell proliferation. Both compounds altered mitochondrial and glycolytic functions, and intracellular citrate levels. Both Mito-CP and Mito-CP-Ac synergized with 2-deoxy-glucose (2-DG) to deplete intracellular ATP, inhibit cell proliferation and induce apoptosis in pancreatic cancer cells. We conclude that mitochondria-targeted cationic agents inhibit tumor proliferation via modification of mitochondrial bioenergetics pathways rather than by dismutating and detoxifying mitochondrial superoxide.