Meta-iodobenzylguanidine (MIBG) inhibits malate and succinate driven mitochondrial ATP synthesis in the human neuroblastoma cell line SK-N-BE(2c)

The Plasma Membrane Monoamine Transporter is Highly Expressed in Neuroblastoma and Functions as an mIBG Transporter

Neuroblastoma (NB) is a pediatric cancer with low survival rates in high-risk patients. 131I-mIBG has emerged as a promising therapy for high-risk NB and kills tumor cells by radiation. Consequently, 131I-mIBG tumor uptake and retention are major determinants for its therapeutic efficacy. mIBG enters NB cells through the norepinephrine transporter (NET), and accumulates in mitochondria through unknown mechanisms. Here we evaluated the expression of monoamine and organic cation transporters in high-risk NB tumors and explored their relationship with MYCN amplification and patient survival. We found that NB mainly expresses NET, the plasma membrane monoamine transporter (PMAT), and the vesicular membrane monoamine transporter 1/2 (VMAT1/2), and that the expression of these transporters is significantly reduced in MYCN-amplified tumor samples. PMAT expression is the highest and correlates with overall survival in high-risk NB patients without MYCN amplification. Immunostaining showed that PMAT resides intracellularly in NB cells and co-localizes with mitochondria. Using cells expressing PMAT, mIBG was identified as a PMAT substrate. In mitochondria isolated from NB cell lines, mIBG uptake was reduced by ∼50% by a PMAT inhibitor. Together, our data suggest that PMAT is a previously unrecognized transporter highly expressed in NB and could impact intracellular transport and therapeutic response to 131I-mIBG. SIGNIFICANCE STATEMENT: This study identified that plasma membrane monoamine transporter (PMAT) is a novel transporter highly expressed in neuroblastoma and its expression level is associated with overall survival rate in high-risk patients without MYCN amplification. PMAT is expressed intracellularly in neuroblastoma cells, transports meta-iodobenzylguanidine (mIBG) and thus could impact tumor retention and response to 131I-mIBG therapy. These findings have important clinical implications as PMAT could represent a novel molecular marker to help inform disease prognosis and predict response to 131I-mIBG therapy.

Selective elimination of neuroblastoma cells by synergistic effect of Akt kinase inhibitor and tetrathiomolybdate

Neuroblastoma is the most common extracranial solid tumour of infancy. Pathological activation of glucose consumption, glycolysis and glycolysis‐activating Akt kinase occur frequently in neuroblastoma cells, and these changes correlate with poor prognosis of patients. Therefore, several inhibitors of glucose utilization and the Akt kinase activity are in preclinical trials as potential anti‐cancer drugs. However, metabolic plasticity of cancer cells might undermine efficacy of this approach. In this work, we identified oxidative phosphorylation as compensatory mechanism preserving viability of neuroblastoma cells with inhibited glucose uptake/Akt kinase. It was oxidative phosphorylation that maintained intracellular level of ATP and proliferative capacity of these cells. The oxidative phosphorylation inhibitors (rotenone, tetrathiomolybdate) synergized with inhibitor of the Akt kinase/glucose uptake in down‐regulation of both viability of neuroblastoma cells and clonogenic potential of cells forming neuroblastoma spheroids. Interestingly, tetrathiomolybdate acted as highly specific inhibitor of oxygen consumption and activator of lactate production in neuroblastoma cells, but not in normal fibroblasts and neuronal cells. Moreover, the reducing effect of tetrathiomolybdate on cell viability and the level of ATP in the cells with inhibited Akt kinase/glucose uptake was also selective for neuroblastoma cells. Therefore, efficient elimination of neuroblastoma cells requires inhibition of both glucose uptake/Akt kinase and oxidative phosphorylation activities. The use of tetrathiomolybdate as a mitochondrial inhibitor contributes to selectivity of this combined treatment, preferentially targeting neuroblastoma cells.

Nifurtimox reduces N-Myc expression and aerobic glycolysis in neuroblastoma

Neuroblastoma is one of the most common solid tumors in childhood and usually accompanied with poor prognosis and rapid tumor progression when diagnosed with amplification of the proto-oncogene N-Myc. The amplification of N-Myc has major influence on the maintenance of aerobic glycolysis, also known as the Warburg effect. This specific switch in the conversion of pyruvate to lactate instead of the conversion of pyruvate to acetyl-coenzyme A even in the presence of oxygen has important benefits for the tumor, e.g. increased production of enzymes and enzyme substrates that are involved in tumor progression, angiogenesis and inhibition of apoptosis. The antiprotozoal drug nifurtimox, which is generally used for the treatment of infections with the parasitic protozoan Trypanosoma cruzi, has been reported to have cytotoxic properties in the therapy of neuroblastoma. However, its action of mechanism has not been described in detail yet. The presented in vitro study on the neuroblastoma cell lines LA-N-1, IMR-32, LS and SK-N-SH shows an increased production of oxidative stress, a reduced lactate dehydrogenase enzyme activity and reduced lactate production after nifurtimox treatment. Furthermore, nifurtimox leads to reduced mRNA and protein levels of the proto-oncogene protein N-Myc. Thus, the current work gives new insights into the effect of nifurtimox on tumor metabolism revealing a shifted glucose metabolism from production of lactate to oxidative phosphorylation and a reduced expression of the major molecular prognostic factor in neuroblastoma N-Myc, presenting nifurtimox as a possible adjuvant therapeutic agent against (high risk) neuroblastoma.

Energy metabolism in neuroblastoma and Wilms tumor

To support high proliferation, the majority of cancer cells undergo fundamental metabolic changes such as increasing their glucose uptake and shifting to glycolysis for ATP production at the expense of far more efficient mitochondrial energy production by oxidative phosphorylation (OXPHOS), which at first glance is a paradox. This phenomenon is known as the Warburg effect. However, enhanced glycolysis is necessary to provide building blocks for anabolic growth. Apart from the generation of ATP, intermediates of glycolysis serve as precursors for a variety of biosynthetic pathways essential for cell proliferation. In the last 10-15 years the field of tumor metabolism has experienced an enormous boom in interest. It is now well established that tumor suppressor genes and oncogenes often play a central role in the regulation of cellular metabolism. Therefore, they significantly contribute to the manifestation of the Warburg effect. While much attention has focused on adult solid tumors, so far there has been comparatively little effort directed at elucidation of the mechanism responsible for the Warburg effect in childhood cancers. In this review we focus on metabolic pathways in neuroblastoma (NB) and Wilms tumor (WT), the two most frequent solid tumors in children. Both tumor types show alterations of the OXPHOS system and glycolytic features. Chromosomal alterations and activation of oncogenes like MYC or inactivation of tumor suppressor genes like TP53 can in part explain the changes of energy metabolism in these cancers. The strict dependence of cancer cells on glucose metabolism is a fairly common feature among otherwise biologically diverse types of cancer. Therefore, inhibition of glycolysis or starvation of cancer cells through glucose deprivation via a high-fat low-carbohydrate diet may be a promising avenue for future adjuvant therapeutic strategies.

Nothing but NET: a review of norepinephrine transporter expression and efficacy of 131I-mIBG therapy

Neuroblastoma is unique amongst common pediatric cancers for its expression of the norepinephrine transporter (NET), enabling tumor-selective imaging and therapy with radioactive analogues of norepinephrine. The majority of neuroblastoma tumors are avid for (123)I-metaiodobenzaguanidine (mIBG) on imaging, yet the therapeutic response to (131) I-mIBG is only 30% in clinical trials, and off-target effects cause short- and long-term morbidity. We review the contemporary understanding of the tumor-selective uptake, retention, and efflux of meta-iodobenzylguanidine (mIBG) and strategies currently in development for improving its efficacy. Combination treatment strategies aimed at enhancing NET are likely necessary to reach the full potential of (131)I-mIBG therapy.

Synthesis and biological effects of new hybrid compounds composed of benzylguanidines and the alkylating group of busulfan on neuroblastoma cells

(131)Iodine-labelled (meta-iodobenzyl)guanidine ([(131)I]-mIBG) and busulfan [butane-1,4-diylbis(methanesulfonate)] are well-established pharmaceuticals in neuroblastoma therapy. We report the design, synthesis, and testing of hybrid molecules-mBBG and pBBG-which combine key structural features of (meta-iodobenzyl)guanidine and busulfan: they contain a benzylguanidine moiety for accumulating in neuroblastoma cells via the noradrenaline transporter and, in the meta- or para-position, respectively, one of the two identical alkylating motives of busulfan for killing cells. Uptake and toxicity of hybrids mBBG and pBBG in human neuroblastoma cells compared favorably to their ancestors [(131)I]-mIBG and busulfan.

Relative contributions of mitochondria and NADPH oxidase to deoxycorticosterone acetate-salt hypertension in mice

We assessed the relative contribution of the mitochondrial respiratory chain and NADPH (nicotinamide adenine dinucleotide phosphate) oxidase to deoxycorticosterone acetate (DOCA)-salt hypertension in mice. The daily mean arterial pressure was monitored by radiotelemetry in DOCA-salt-treated mice given vehicle or the mitochondrial respiratory chain complex I inhibitor rotenone. This treatment produced remarkable attenuation of DOCA-salt hypertension. Similar results were obtained with other inhibitors of mitochondrial function, including 5-hydroxydecanoate (specific for mitochondrial potassium-ATP channels), benzylguanidine (complexes I and III), and the cell-permeable manganese tetrakis (4-benzoic acid) porphyrin (a mimic of mitochondrial superoxide dismutase). In parallel with the blood pressure-lowering effect of rotenone, the DOCA-salt-induced increases in urinary 8-isoprostane excretion and in reactive oxygen species production of isolated kidney mitochondria were both significantly attenuated. Conversely, the DOCA-salt-induced reduction of urinary nitrate/nitrite excretion was significantly elevated. Following DOCA-salt treatment, mice deficient in NADPH oxidase subunits gp91(phox) or p47(phox) exhibited a partial attenuation of the hypertensive response at early but not later time points. Thus, the mitochondrial respiratory chain is a major source of oxidative stress in DOCA-salt hypertension, whereas NADPH oxidase may have a relatively minor role during the early stage of hypertension.

Radiolabeled metaiodobenzylguanidine for the treatment of neuroblastoma

INTRODUCTION

Neuroblastoma is the most common pediatric extracranial solid cancer. This tumor is characterized by metaiodobenzylguanidine (MIBG) avidity in 90% of cases, prompting the use of radiolabeled MIBG for targeted radiotherapy in these tumors.

METHODS

The available English language literature was reviewed for original research investigating in vitro, in vivo and clinical applications of radiolabeled MIBG for neuroblastoma.

RESULTS

MIBG is actively transported into neuroblastoma cells by the norepinephrine transporter. Preclinical studies demonstrate substantial activity of radiolabeled MIBG in neuroblastoma models, with (131)I-MIBG showing enhanced activity in larger tumors compared to (125)I-MIBG. Clinical studies of (131)I-MIBG in patients with relapsed or refractory neuroblastoma have identified myelosuppression as the main dose-limiting toxicity, necessitating stem cell reinfusion at higher doses. Most studies report a response rate of 30-40% with (131)I-MIBG in this population. More recent studies have focused on the use of (131)I-MIBG in combination with chemotherapy or myeloablative regimens.

CONCLUSIONS

(131)I-MIBG is an active agent for the treatment of patients with neuroblastoma. Future studies will need to define the optimal role of this targeted radiopharmaceutical in the therapy of this disease.

Potential mechanisms for the inhibition of tumor cell growth by manganese superoxide dismutase

Studies from many laboratories have shown that overexpression of manganese superoxide dismutase (MnSOD) inhibits the growth of numerous tumor cell types. The inhibition of tumor cell growth can be attributed to the increase in the steady-state levels of H2O2 as a result of the increased dismuting activity of MnSOD. Here we demonstrate that overexpression of MnSOD enhances the activity of the superoxide (O2*-)-sensitive enzyme aconitase, decreases the intracellular GSH/GSSG ratio, and dose-dependently inhibits pyruvate carboxylase activity. Thus, alterations in the steady-state concentrations of mitochondrial O2*- and H2O2 as a result of MnSOD overexpression can alter the metabolic capacity of the cell leading to inhibition of cell growth. Furthermore, we propose that MnSOD overexpression can modulate the activity of nitric oxide (*NO) by preventing its reaction with O2*-. This hypothesis suggests that the redox environment of the mitochondria can be altered to favor the activity of *NO rather than peroxynitrite (ONOO-) and may explain the enhanced toxicity of *NO-generating compounds toward MnSOD-overexpressing cell lines. These findings indicate that therapeutic strategies targeted at overexpressing MnSOD in tumor tissue may be more effective when used in combination with agents that deplete the oxidant-buffering and enhance the *NO-generating capacity of the tumor and host, respectively.

Guanidino-containing drugs in cancer chemotherapy: biochemical and clinical pharmacology

The pharmacology and clinical application of three guanidino-containing compounds are reviewed in this commentary with special focus on a new member of this group of drugs, CHS 828 [N-(6-(4-chlorophenoxy)hexyl)-N'-cyano-N"-4-pyridylguanidine]. m-Iodobenzylguanidine (MIBG) and methylglyoxal bis(guanylhydrazone) (MGBG) have been extensively studied, preclinically as well as clinically, and have established use as anticancer agents. MIBG has structural similarities to the neurotransmitter, norepinephrine, and MGBG is a structural analog of the natural polyamine spermidine. CHS 828 is a pyridyl cyanoguanidine newly recognized as having cytotoxic effects when screening antihypertensive compounds. Apart from having the guanidino groups in common, there are many differences between these drugs in both structure and their mechanisms of action. However, they all inhibit mitochondrial function, a seemingly unique feature among chemotherapeutic drugs. In vitro in various cell lines and primary cultures of patient tumor cells and in vivo in various tumor models, CHS 828 has cytotoxic properties unlike any of the standard cytotoxic drugs with which it has been compared. Among these are non-cross-resistance to standard drugs and pronounced activity in tumor models acknowledged to be highly drug-resistant. Similar to MIBG, CHS 828 induces an early increase in extracellular acidification, due to stimulation of the glycolytic flux. Furthermore, ATP levels decrease, and the syntheses of DNA and protein are shut off after approximately 30 hr of exposure, indicating active cell death. CHS 828 is now in early clinical trials, the results of which are eagerly awaited.

Cellular pharmacodynamics of the cytotoxic guanidino-containing drug CHS 828. Comparison with methylglyoxal-bis(guanylhydrazone)

N-(6-(4-chlorophenoxy)hexyl)-N'-cyano-N"-4-pyridylguanidine (CHS 828) is a new guanidino-containing compound with antitumoral activity both in vitro and in vivo. Its activity profile differs from those of standard cytotoxic drugs but the mechanism of action is not yet fully understood. CHS 828 is presently in early phase I and II clinical trials. In the present study, the pharmacodynamic effects at the cellular level of CHS 828 was compared to another compound containing two guanidino groups, methylglyoxal-bis(guanylhydrazone) (MGBG). MGBG is known to inhibit the synthesis of polyamines, which are important in, e.g., proliferation and macromolecular synthesis. The concentration-response relationship of CHS 828 closely resembled that of MGBG and the drugs were similar with respect to inhibition of DNA and protein synthesis. On the other hand, CHS 828 induced a significant increase in cellular metabolism while MGBG did not. The cytotoxic effect of MGBG was reversed by the addition of exogenous polyamines, while that of CHS 828 was unaffected. Unlike MGBG, there was also no effect of CHS 828 on the levels of decarboxylating enzymes in the polyamine biosynthesis. In conclusion, CHS 828 does not appear to share any major mechanisms of action with the polyamine synthesis inhibitor MGBG. Further studies will be required to define the exact mechanism of action of CHS 828.

Mitochondrial or cytosolic catalase reverses the MnSOD-dependent inhibition of proliferation by enhancing respiratory chain activity, net ATP production, and decreasing the steady state levels of H(2)O(2)

Manganese superoxide dismutase (MnSOD) overexpression has been shown to reverse the malignant phenotype in a variety of tumor cell lines. The inhibition of proliferation and reversal of the malignant phenotype has been attributed to an increase in H(2)O(2) production as a result of the dismutation reaction. However, direct evidence in support of this hypothesis has not been forthcoming. To evaluate the contribution of H(2)O(2) in the regulation of cell growth in response to MnSOD overexpression, control and MnSOD-overexpressing HT-1080 fibrosarcoma cells were transfected with constructs that direct catalase to either the mitochondrial or cytosolic compartments. Overexpression of catalase in either compartment reversed the proliferative and clonogenic inhibition associated with MnSOD overexpression, blocked the increase in the steady state levels of H(2)O(2) as measured by flow cytometric analysis of 2', 7'-dichlorofluorescein diacetate, and increased protection from the cytotoxicity of H(2)O(2). In addition, mitochondrial or cytosolic catalase enhances respiration through complex I and II in both control and MnSOD overexpressing cell lines and reverses a MnSOD-dependent decrease in net ATP production. Thus, catalase reverses the proliferative inhibition associated with MnSOD overexpression and may also play an important role in metabolic regulation.

Early stimulation of acidification rate by novel cytotoxic pyridyl cyanoguanidines in human tumor cells: comparison with m-iodobenzylguanidine

CHS 828, a newly recognized pyridyl cyanoguanidine, has shown promising antitumor activity both in vitro and in vivo and is presently in early phase I clinical trial in collaboration with EORTC. In this study, the effects of CHS 828 and a series of analogues on extracellular acidification and cytotoxicity were compared with those of m-iodobenzylguanidine (MIBG) in human tumor cells. The extracellular acidification rate was measured using the Cytosensor microphysiometer, and determination of cytotoxicity and proliferation was [(14)C] performed by the fluorometric microculture cytotoxicity assay (FMCA) and measurement of [(14)C]thymidine and leucine uptake. CHS 828 significantly increased the acidification rate during the first 15-24 hr in a concentration-dependent manner. This effect was abolished by removal of glucose from the medium, substituted with 10 mM of pyruvate, indicating stimulated glycolysis as the source of the increased acidification rate. However, CHS 828 induced cytotoxicity at concentrations well below those that affected the rate of acidification; when a series of closely related pyridylguanidine analogues were tested and compared, no apparent relationship between cytotoxicity and acidification could be discerned. Furthermore, comparable increases in the acidification rate were evident in one subline with high-grade resistance to the cytotoxic actions of CHS 828. The results indicate that CHS 828 may share the inhibitory actions of MIBG on mitochondrial respiration with a subsequent increase in glycolysis and acidification rate. However, this mechanism of action appears neither necessary nor sufficient to fully explain the cytotoxic actions of CHS 828 in human tumor cells, actions which remain to be mechanistically clarified.

MIBG causes oxidative stress and up-regulation of anti-oxidant enzymes in the human neuroblastoma cell line SK-N-BE(2c)

We report the effects of meta-iodobenzylguanidine (MIBG), a neuroblastoma-seeking agent, on cell proliferation and several oxidative stress-related parameters in the human neuroblastoma cell line SK-N-BE(2c). MIBG inhibited the proliferation of this cell line in micromolar concentrations. Measurements of the malondialdehyde (MDA) concentrations (a measure of the extent of lipid peroxidation) of cells treated with MIBG showed that increasing concentrations of MIBG led to an increase in MDA levels of the cells. This effect was most pronounced after one day of cellular exposure to MIBG and disappeared after 3 days. Disappearance of the elevated MDA levels caused by MIBG is probably the result of increased activity of the H2O2 detoxifying enzymes, catalase and glutathion peroxidase (GPx). The catalase- and GPx-enzyme activity of cells exposed to MIBG steadily increased with time, reaching a maximum after 4 days. Oxidative stress caused by MIBG thus at first leads to cellular damage (lipid peroxidation) but over a longer period does not lead to decreased proliferation rate of the cells, most likely because of cellular adaptation to increased oxidative stress by up-regulation of the H2O2 detoxifying enzymes catalase and GPx.

The effect of the neuroblastoma-seeking agent meta-iodobenzylguanidine (MIBG) on NADH-driven superoxide formation and NADH-driven lipid peroxidation in beef heart submitochondrial particles

In this paper we report the effects of the neuroblastoma-seeking agent meta-iodobenzylguanidine (MIBG) on NADH-driven superoxide formation and NADH-driven lipid peroxidation in beef heart submitochondrial particles. MIBG is a structural analogue of noradrenaline and is capable of inhibiting complex I and complex III of the respiratory chain. The results of our studies show that MIBG enhanced both NADH-driven superoxide formation and NADH-driven lipid peroxidation at concentrations that are likely to exist inside mitochondria of the target cells of neuroblastoma patients treated with [131I]MIBG. The effect of MIBG is comparable to that of rotenone (an inhibitor of complex I) rather than that of antimycin (an inhibitor of complex III). These results suggest that the formation of superoxide and lipid peroxidation contributes to the cytotoxicity of [131I]MIBG.