Meta-iodobenzylguanidine inhibits complex I and III of the respiratory chain in the human cell line Molt-4

Research Progress on Improving the Efficiency of CDT by Exacerbating Tumor Acidification

In recent years, chemodynamic therapy (CDT) has received extensive attention as a novel means of cancer treatment. The CDT agents can exert Fenton and Fenton-like reactions in the acidic tumor microenvironment (TME), converting hydrogen peroxide (H₂O₂) into highly toxic hydroxyl radicals (·OH). However, the pH of TME, as an essential factor in the Fenton reaction, does not catalyze the reaction effectively, hindering its efficiency, which poses a significant challenge for the future clinical application of CDT. Therefore, this paper reviews various strategies to enhance the antitumor properties of nanomaterials by modulating tumor acidity. Ultimately, the performance of CDT can be further improved by inducing strong oxidative stress to produce sufficient ·OH. In this paper, the various acidification pathways and proton pumps with potential acidification functions are mainly discussed, such as catalytic enzymes, exogenous acids, CAIX, MCT, NHE, NBCn1, etc. The problems, opportunities, and challenges of CDT in the cancer field are also discussed, thereby providing new insights for the design of nanomaterials and laying the foundation for their future clinical applications.

The nature compound dehydrocrenatidine exerts potent antihepatocellular carcinoma by destroying mitochondrial complexes in vitro and in vivo

Cumulative evidence indicates that mitochondria dysfunction plays an important role in tumour treatment. Given the limited efficacy and toxicity of current mitochondria-targeted drugs, research into effective mitochondria-targeted anticancer agents remains an irresistible general trend. In this study, it was found that dehydrocrenatidine (DEC), a β-carbolin alkaloid isolated from Picrasma quassiodes, displays a promising growth inhibitory effect in vitro and in vivo by inducing apoptosis of hepatocellular carcinoma (HCC) cells. Mechanistically, we provided that the possible target of DEC against HCC cells was determined by isobaric labels for relative and absolute quantification assay and validated them using further experiments. The results suggested that DEC can target and regulate the function of mitochondrial complexes I, III and IV, affecting oxidative phosphorylation and ultimately leading to mitochondrial dysfunction to exert its anti-HCC effects. In addition, the combination of DEC and sorafenib showed a synergistic effect and was also associated with mitochondrial dysfunction. Importantly, DEC did not show significant toxicity in mice. This study provided a new insight into underlying mechanisms in DEC-treated HCC cells, suggesting that DEC might be a mitochondrial targeting lead compound.

Mitochondria: The metabolic switch of cellular oncogenic transformation

Mitochondria, well recognized as the "powerhouse" of cells, are maternally inherited organelles with bacterial ancestry that play essential roles in a myriad of cellular functions. It has become profoundly evident that mitochondria regulate a wide array of cellular and metabolic functions, including biosynthetic metabolism, cell signaling, redox homeostasis, and cell survival. Correspondingly, defects in normal mitochondrial functioning have been implicated in various human malignancies. Cancer development involves the activation of oncogenes, inactivation of tumor suppressor genes, and impairment of apoptotic programs in cells. Mitochondria have been recognized as the site of key metabolic switches for normal cells to acquire a malignant phenotype. This review outlines the role of mitochondria in human malignancies and highlights potential aspects of mitochondrial metabolism that could be targeted for therapeutic development.

Oxidative Phosphorylation as an Emerging Target in Cancer Therapy

Cancer cells have upregulated glycolysis compared with normal cells, which has led many to the assumption that oxidative phosphorylation (OXPHOS) is downregulated in all cancers. However, recent studies have shown that OXPHOS can be also upregulated in certain cancers, including leukemias, lymphomas, pancreatic ductal adenocarcinoma, high OXPHOS subtype melanoma, and endometrial carcinoma, and that this can occur even in the face of active glycolysis. OXPHOS inhibitors could therefore be used to target cancer subtypes in which OXPHOS is upregulated and to alleviate therapeutically adverse tumor hypoxia. Several drugs including metformin, atovaquone, and arsenic trioxide are used clinically for non-oncologic indications, but emerging data demonstrate their potential use as OXPHOS inhibitors. We highlight novel applications of OXPHOS inhibitors with a suitable therapeutic index to target cancer cell metabolism. Clin Cancer Res; 24(11); 2482-90. ©2018 AACR.

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.

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.

Generation of reactive oxygen species is an early event in dolichyl phosphate-induced apoptosis

The mechanism of induction of apoptosis by dolichyl phosphate (Dol-P) was investigated in U937 cells. Studies using isolated mitochondria revealed that the respiratory complex II activity was almost completely inhibited by 20 microg/ml of Dol-P but not by the same concentration of dolichol. Activities of complex I and III were also inhibited by Dol-P, but nearly 50% of activity still remained at 20 microg/ml. Dol-P induced release of cytochrome-c from the isolated mitochondria. Fluorometric microtiter plate assay revealed that generation of reactive oxygen species (ROS) increased in a time-dependent manner. Flow cytometric analysis also indicated that Dol-P caused loss of mitochondrial membrane potential (Deltapsi(m)) and increased ROS generation. The addition of the antioxidant pyrrolidine dithiocarbamate (PDTC) significantly inhibited Dol-P-induced ROS generation and activation of caspase-3. A specific inhibitor of respiratory complex II, thenoyltrifluoroacetone (TTFA), increased ROS generation, potentially mimicking the consequence of inhibition of electron flow at complex II by Dol-P in U937 cells. Electron microscopy revealed that mitochondria became swollen and spherical in shape by the treatment with Dol-P. Neither the tyrosine kinase inhibitor k252a nor mitogen activated protein kinase/extracellular signal-regulated kinase kinase (MEK) inhibitors PD98059 and U0126 inhibited the Dol-P-induced apoptosis. Together, these results suggest that the direct disruption of mitochondrial respiratory complexes and the consequent ROS generation play a critical role in the initiation of Dol-P-induced apoptosis.

T-2 toxin initially activates caspase-2 and induces apoptosis in U937 cells

T-2 toxin, which belongs to a group of mycotoxins synthesized by Fusarium fungi that are widely encountered as natural contaminants, induced apoptosis with distinct morphological and biological features in U937 cells. The concentration of more than 10nM T-2 toxin affected cell viability, induced nuclear and DNA fragmentation and caspase-3 activation. Caspase-2, -3, -8, and -9 were activated during T-2 toxin-induced apoptosis. T-2 toxin neither inhibited mitochondrial respiratory chain complexes I-IV in isolated mitochondria nor decreased ATP levels in U937 cells. Both enzyme activity assay and Western blot analysis revealed that T-2 toxin activated caspase-2 earlier than caspase-3, -8, and -9. Caspase-2 inhibitor (VDVAD-CHO/fmk) and caspase-8 inhibitor (IETD-CHO/fmk) completely blocked the T-2 toxin-induced process of procaspase-3, while caspase-9 inhibitor (LEHD-CHO/fmk) did so less effectively. Caspase-2 inhibitor entirely blocked T-2 toxin-induced caspase-8, and -9 activation. These results clearly indicate that activation of caspase-2 is essential to T-2 toxin-induced apoptosis and that apoptotic signals are mainly transmitted via caspase-8 and caspase-3 rather than mitochondrial pathway.

Potential Antitumor Agents. 37. Synthesis and Antitumor Activity of Guanylhydrazones from Imidazo[2,1-b]thiazoles and from the New Heterocyclic System Thiazolo[2‘,3‘:2,3]imidazo[4,5-c]quinoline

This paper reports synthesis and antitumor activity of new guanylhydrazones from imidazo[2,1-b]thiazoles and from the new heterocyclic system thiazolo[2',3':2,3]imidazo[4,5-c]quinoline. The compounds were tested as potential antitumor agents at the National Cancer Institute. The effect of the guanylhydrazone of 2-chloro-6-(2,5-dimethoxy-4-nitrophenyl)imidazo[2,1-b]thiazole-5-carbaldehyde (41) was investigated, and it was found to be an inhibitor of Complex III of the mitochondrial respiratory chain and is able to induce apoptosis in the cell lines HT29 and HL60.

Molecular Mechanism of Cell Death Induced by the Antioxidant tert-Butylhydroxyanisole in Human Monocytic Leukemia U937 Cells

A phenolic antioxidant 3-tert-butyl-4-hydroxyanisole (BHA) is a widely used food additive. BHA had cytotoxicity in human monocytic leukemia U937 cells. BHA at 0.75 mM caused nuclear condensation and fragmentation, structural damage in mitochondria, decrease in mitochondrial transmembrane potential, and internucleosomal DNA cleavage. It induced the activities of caspase-3 and/or -7, -6, -8 and -9, especially high when DEVD-MCA was the substrate (caspase-3 and/or -7). DEVDase activity increased in time- and dose-dependent manner and high activity was observed in lysates of cells treated for 3 h at 0.75 mM. Addition of GSH (reduced glutathione) during the treatment of cells with BHA inhibited the induction of DEVDase activity, and the intracellular GSH level decreased as the concentration of BHA was raised. Intracellular ATP levels decreased in time- and dose-dependent manner when the cells were treated with BHA in the presence or absence of glucose. Enzyme activities involved in the respiratory chain were assayed with the mitochondrial fraction prepared from U937 cells. BHA distinctly inhibited NADH-ubiquinone oxidoreductase (complex I) and cytochrome c oxidase (complex IV) at low concentrations. Succinate-ubiquinone oxidoreductase (complex II) was also inhibited, but to somewhat less extent. Without mitochondrial enzymes, BHA stimulated the ubiquinol-dependent reduction of cytochrome c (complex III), but it might have some detrimental effects on the mitochondrial enzyme reaction of complex III. The inhibition of mitochondrial oxidative phosphorylation might corroborate the mechanistic evidence for apoptosis of leukemia cells by BHA. Cell death induced by BHA is primarily ascribable to apoptosis.

Meta-iodobenzylguanidine induces growth inhibition and apoptosis of neuroendocrine gastrointestinal tumor cells

Neuroendocrine gastrointestinal tumors take up, decarboxylate and store large amounts of monoamines. Radioactive-labeled monoamines like the norepinephrine analogue meta-iodobenzylguanidine (MIBG) have been used for the imaging of neuroendocrine tumors for many years. MIBG is selectively taken up via norepinephrine transporters (NETs) localized in the plasma membrane of neuroendocrine gastrointestinal tumor cells and thereby offers the possibility for specific and innovative therapeutic approaches. We investigated the antiproliferative, cytotoxic, cell cycle-arresting and apoptosis-inducing effects of MIBG in the neuroendocrine gastrointestinal tumor cell line STC-1 and for control in the nonneuroendocrine colorectal cancer cell line HT-29. RT-PCR revealed the expression of NET in STC-1 but not in HT-29 cells. MIBG dose-dependently induced cytotoxicity and growth inhibition of STC-1 cells. It potently induced apoptosis in STC-1 cells as assessed by changes in the mitochondrial membrane potential, activation of caspase-3 and DNA fragmentation. Moreover, MIBG altered the expression of several genes involved in proliferation, apoptosis and stress responses as shown by cDNA arrays. In contrast, neither cytotoxicity, nor growth inhibition nor induction of apoptosis were detected in response to MIBG in the NET-deficient colorectal cancer cell line HT-29. Our data show that MIBG induces growth inhibition and apoptosis in neuroendocrine gastrointestinal tumor cells. MIBG did not arrest the cell cycle in either cell line. Thus, monoamine transporters in the plasma membrane of neuroendocrine gastrointestinal tumor cells are promising targets for innovative and specific treatment strategies of these tumors.

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.

MIBG inhibits respiration: potential for radio- and hyperthermic sensitization

INTRODUCTION

Meta-iodobenzylguanidine (MIBG) in its 131I-labeled form is clinically used as a tumor-targeted radiopharmaceutical in the diagnosis and treatment of adrenergic tumors. This well established drug may have additional clinical applications as a radiosensitizer or hyperthermic agent, ie., MIBG reportedly inhibits mitochondrial respiration in vitro. The mechanism for MIBG inhibition of cellular oxygen consumption is uncertain. Moreover, MIBG reportedly stimulates glycolysis both in vitro and in vivo. Our studies show the effect of MIBG on 9L glioma oxygen consumption and redox status with tumors cells in vitro and in vivo.

MATERIALS AND METHODS

The effects on electron transfer were determined by following oxygen consumption with a Clark oxygen electrode. Fluorescence measurements were used to determine effects of MIBG on intracellular electron acceptors, NADPH and flavoproteins, in vitro and in vivo. 31P-NMR was used to determine alterations in tumor cell pH in vivo.

RESULTS

Our results show the inhibition of oxygen utilization with MIBG for cell suspensions in vitro. The same results were demonstrated for tumor cell suspensions rapidly isolated from tumors grown in rats. Moreover, NAD(P)H and flavoprotein (Fp) fluorescence changes were observed to rapidly occur following MIBG addition in vitro. Changes in intracellular pH measured with 31P-NMR, in vivo, precede the changes in fluorescence of NAD(P)H and Fp obtained with frozen sections of tumor.

CONCLUSIONS

We conclude that 31P-NMR measurements and fluorescence changes, following MIBG injection, can be used as criterion for selecting the proper time to treat tumors with ionizing radiation or hyperthermia.

The role of phosphometabolites in cell proliferation, energy metabolism, and tumor therapy

A common characteristic of tumor cells is the constant overexpression of glycolytic and glutaminolytic enzymes. In tumor cells the hyperactive hexokinase and the partly inactive pyruvate kinase lead to an expansion of all phosphometabolites from glucose 6-phosphate to phosphoenolpyruvate. In addition to the glycolytic phosphometabolites, synthesis of their metabolic derivatives such as P-ribose-PP, NADH, NADPH, UTP, CTP, and UDP-N-acetyl glucosamine is also enhanced during cell proliferation. Another phosphometabolite derived from P-ribose-PP, AMP, inhibits cell proliferation. The accumulation of AMP inhibits both P-ribose-PP-synthetase and the increase in concentration of phosphometabolites derived from P-ribose-PP. In cells with low glycerol 3-phosphate and malate-aspartate shuttle capacities the inhibition of the lactate dehydrogenase by low NADH levels leads to an inhibition of glycolytic ATP production. Several tumor-therapeutic drugs reduce NAD and NADH levels, thereby inhibiting glycolytic energy production. The role of AMP, NADH, and NADPH levels in the success of chemotherapeutic treatment is discussed.

The role of phosphometabolites in cell proliferation, energy metabolism, and tumor therapy

A common characteristic of tumor cells is the constant overexpression of glycolytic and glutaminolytic enzymes. In tumor cells the hyperactive hexokinase and the partly inactive pyruvate kinase lead to an expansion of all phosphometabolites from glucose 6-phosphate to phosphoenolpyruvate. In addition to the glycolytic phosphometabolites, synthesis of their metabolic derivatives such as P-ribose-PP, NADH, NADPH, UTP, CTP, and UDP-N-acetyl glucosamine is also enhanced during cell proliferation. Another phosphometabolite derived from P-ribose-PP, AMP, inhibits cell proliferation. The accumulation of AMP inhibits both P-ribose-PP-synthetase and the increase in concentration of phosphometabolites derived from P-ribose-PP. In cells with low glycerol 3-phosphate and malate-aspartate shuttle capacities the inhibition of the lactate dehydrogenase by low NADH levels leads to an inhibition of glycolytic ATP production. Several tumor-therapeutic drugs reduce NAD and NADH levels, thereby inhibiting glycolytic energy production. The role of AMP, NADH, and NADPH levels in the success of chemotherapeutic treatment is discussed.

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.

Characterization of the respiratory chain from cultured Crithidia fasciculata

Mitochondrial mRNAs encoding subunits of respiratory-chain complexes in kinetoplastids are post-transcriptionally edited by uridine insertion and deletion. In order to identify the proteins encoded by these mRNAs, we have analyzed respiratory-chain complexes from cultured cells of Crithidia fasciculata with the aid of 2D polyacrylamide gel electrophoresis (PAGE). The subunit composition of F0F1-ATPase (complex V), identified on the basis of its activity as an oligomycin-sensitive ATPase, is similar to that of bovine mitochondrial F0F1-ATPase. Amino acid sequence analysis, combined with binding studies using dicyclohexyldiimide and azido ATP allowed the identification of two F0 subunits (b and c) and all of the F1 subunits. The F0 b subunit has a low degree of similarity to subunit b from other organisms. The F1 alpha subunit is extremely small making the beta subunit the largest F1 subunit. Other respiratory-chain complexes were also analyzed. Interestingly, an NADH: ubiquinone oxidoreductase (complex I) appeared to be absent, as judged by electron paramagnetic resonance (EPR), enzyme activity and 2D PAGE analysis. Cytochrome c oxidase (complex IV) displayed a subunit pattern identical to that reported for the purified enzyme, whereas cytochrome c reductase (complex III) appeared to contain two extra subunits. A putative complex II was also identified. The amino acid sequences of the subunits of these complexes also show a very low degree of similarity (if any) to the corresponding sequences in other organisms. Remarkably, peptide sequences derived from mitochondrially encoded subunits were not found in spite of the fact that sequences were obtained of virtually all subunits of complex III, IV and V.

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.

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

In this paper, we report on our studies of the effects of MIBG, a structural analogue of norepinephrine, on SK-N-BE(2c) cells. In micromolar concentrations, MIBG caused almost complete inhibition of the proliferation of SK-N-BE(2c) cells. In intact SK-N-BE(2c) cells, addition of MIBG led to a decrease of the ATP to ADP ratio. A progressive increase of the lactate to pyruvate ratio (due to increased lactate production) was observed after incubation of the cells with glucose and increasing concentrations of MIBG. In cells treated with digitonin, MIBG inhibited malate driven ATP synthesis. Comparable inhibition of ATP synthesis with succinate as a substrate required higher concentrations of MIBG. These results indicate that, apart from inhibition of complex I, MIBG was capable of inhibiting at least one other complex of the respiratory chain. Although maximal inhibition of ATP synthesis was observed at a concentration of 10 microM, optimal inhibition of cell proliferation occurred at a MIBG concentration > 25 microM. This suggests that MIBG also influences other cellular processes apart from mitochondrial ATP synthesis, resulting in additional inhibition of cell proliferation.