Topological probes of monoamine oxidases A and B in rat liver mitochondria: inhibition by TEMPO-substituted pargyline analogues and inactivation by proteolysis

Novel treatments against airway inflammation in COPD based on drug repurposing

Chronic obstructive pulmonary disease (COPD) is a major cause of death and reduces quality of life that contributes to a health problem worldwide. Chronic airway inflammation is a hallmark of COPD, which occurs in response to exposure of inhaled irritants like cigarette smoke. Despite accessible to the most up-to-date medications, none of the treatments is currently available to decrease the disease progression. Therefore, it is believed that drugs which can reduce airway inflammation will provide effective disease modifying therapy for COPD. There are many broad-range anti-inflammatory drugs including those that inhibit cell signaling pathways like inhibitors of p38 mitogen-activated protein kinase (MAPK), nuclear factor-κB (NF-κB), and phosphoinositide-3-kinase (PI3K), are now in phase III development for COPD. In this chapter, we review recent basic research data in the laboratory that may indicate novel therapeutic pathways arisen from currently used drugs such as selective monoamine oxidase (MAO)-B inhibitors and drugs targeting peripheral benzodiazepine receptors [also known as translocator protein (TSPO)] to reduce airway inflammation. Considering the impact of chronic airway inflammation on the lives of COPD patients, the potential pharmacological candidates for new anti-inflammatory targets should be further investigated. In addition, it is crucial to consider the phenotypes/molecular endotypes of COPD patients together with specific outcome measures to target novel therapies. This review will enhance our knowledge on how cigarette smoke affects MAO-B activity and TSPO activation/inactivation with specific ligands through regulation of mitochondrial function, and will help to identify new potential treatment for COPD in future.

Questions in the Chemical Enzymology of MAO

We have structure, a wealth of kinetic data, thousands of chemical ligands and clinical information for the effects of a range of drugs on monoamine oxidase activity in vivo. We have comparative information from various species and mutations on kinetics and effects of inhibition. Nevertheless, there are what seem like simple questions still to be answered. This article presents a brief summary of existing experimental evidence the background and poses questions that remain intriguing for chemists and biochemists researching the chemical enzymology of and drug design for monoamine oxidases (FAD-containing EC 4.1.3.4).

Monoamine Oxidase Inhibitors: A Review of Their Anti-Inflammatory Therapeutic Potential and Mechanisms of Action

Chronic inflammatory diseases are debilitating, affect patients' quality of life, and are a significant financial burden on health care. Inflammation is regulated by pro-inflammatory cytokines and chemokines that are expressed by immune and non-immune cells, and their expression is highly controlled, both spatially and temporally. Their dysregulation is a hallmark of chronic inflammatory and autoimmune diseases. Significant evidence supports that monoamine oxidase (MAO) inhibitor drugs have anti-inflammatory effects. MAO inhibitors are principally prescribed for the management of a variety of central nervous system (CNS)-associated diseases such as depression, Alzheimer's, and Parkinson's; however, they also have anti-inflammatory effects in the CNS and a variety of non-CNS tissues. To bolster support for their development as anti-inflammatories, it is critical to elucidate their mechanism(s) of action. MAO inhibitors decrease the generation of end products such as hydrogen peroxide, aldehyde, and ammonium. They also inhibit biogenic amine degradation, and this increases cellular and pericellular catecholamines in a variety of immune and some non-immune cells. This decrease in end product metabolites and increase in catecholamines can play a significant role in the anti-inflammatory effects of MAO inhibitors. This review examines MAO inhibitor effects on inflammation in a variety of in vitro and in vivo CNS and non-CNS disease models, as well as their anti-inflammatory mechanism(s) of action.

A cytochrome c is the natural electron acceptor for nicotine oxidoreductase

Nicotine oxidoreductase (NicA2), a member of the flavin-containing amine oxidase family, is of medical relevance as it shows potential as a therapeutic to aid cessation of smoking due to its ability to oxidize nicotine into a non-psychoactive metabolite. However, the use of NicA2 in this capacity is stymied by its dismal O₂-dependent activity. Unlike other enzymes in the amine oxidase family, NicA2 reacts very slowly with O₂, severely limiting its nicotine-degrading activity. Instead of using O₂ as an oxidant, we discovered that NicA2 donates electrons to a cytochrome c, which means that NicA2 is actually a dehydrogenase. This is surprising, as enzymes of the flavin-containing amine oxidase family were invariably thought to use O₂ as an electron acceptor. Our findings establish new perspectives for engineering this potentially useful therapeutic and prompt a reconsideration of the term 'oxidase' in referring to members of the flavin-containing amine 'oxidase' family.

Vasoconstrictor Mechanisms in Chronic Hypoxia-Induced Pulmonary Hypertension: Role of Oxidant Signaling

Elevated resistance of pulmonary circulation after chronic hypoxia exposure leads to pulmonary hypertension. Contributing to this pathological process is enhanced pulmonary vasoconstriction through both calcium-dependent and calcium sensitization mechanisms. Reactive oxygen species (ROS), as a result of increased enzymatic production and/or decreased scavenging, participate in augmentation of pulmonary arterial constriction by potentiating calcium influx as well as activation of myofilament sensitization, therefore mediating the development of pulmonary hypertension. Here, we review the effects of chronic hypoxia on sources of ROS within the pulmonary vasculature including NADPH oxidases, mitochondria, uncoupled endothelial nitric oxide synthase, xanthine oxidase, monoamine oxidases and dysfunctional superoxide dismutases. We also summarize the ROS-induced functional alterations of various Ca2+ and K+ channels involved in regulating Ca2+ influx, and of Rho kinase that is responsible for myofilament Ca2+ sensitivity. A variety of antioxidants have been shown to have beneficial therapeutic effects in animal models of pulmonary hypertension, supporting the role of ROS in the development of pulmonary hypertension. A better understanding of the mechanisms by which ROS enhance vasoconstriction will be useful in evaluating the efficacy of antioxidants for the treatment of pulmonary hypertension.

Cysteine residues in mitochondrial intermembrane space proteins: more than just import

The intermembrane space (IMS) is a very small mitochondrial sub-compartment with critical relevance for many cellular processes. IMS proteins fulfil important functions in transport of proteins, lipids, metabolites and metal ions, in signalling, in metabolism and in defining the mitochondrial ultrastructure. Our understanding of the IMS proteome has become increasingly refined although we still lack information on the identity and function of many of its proteins. One characteristic of many IMS proteins are conserved cysteines. Different post-translational modifications of these cysteine residues can have critical roles in protein function, localization and/or stability. The close localization to different ROS-producing enzyme systems, a dedicated machinery for oxidative protein folding, and a unique equipment with antioxidative systems, render the careful balancing of the redox and modification states of the cysteine residues, a major challenge in the IMS. In this review, we discuss different functions of human IMS proteins, the involvement of cysteine residues in these functions, the consequences of cysteine modifications and the consequences of cysteine mutations or defects in the machinery for disulfide bond formation in terms of human health. LINKED ARTICLES: This article is part of a themed section on Chemical Biology of Reactive Sulfur Species. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v176.4/issuetoc.

Monoamine Oxidases

Monoamine oxidases A and B (MAO A and B) are mammalian flavoenzymes bound to the outer mitochondrial membrane. They were discovered almost a century ago and they have been the subject of many biochemical, structural and pharmacological investigations due to their central role in neurotransmitter metabolism. Currently, the treatment of Parkinson's disease involves the use of selective MAO B inhibitors such as rasagiline and safinamide. MAO inhibition was shown to exert a general neuroprotective effect as a result of the reduction of oxidative stress produced by these enzymes, which seems to be relevant also in non-neuronal contexts. MAOs were successfully expressed as recombinant proteins in Pichia pastoris, which allowed a thorough biochemical and structural characterization. These enzymes are characterized by a globular water-soluble main body that is anchored to the mitochondrial membrane through a C-terminal α-helix, similar to other bitopic membrane proteins. In both MAO A and MAO B the enzyme active site consists of a hydrophobic cavity lined by residues that are conserved in the two isozymes, except for few details that determine substrate and inhibitor specificity. In particular, human MAO B features a dual-cavity active site whose conformation depends on the size of the bound ligand. This article provides a comprehensive and historical review of MAOs and the state-of-the-art of these enzymes as membrane drug targets.

Comparative In Vitro Study of 11C-Methionine and 11C-Deuterodeprenyl Uptake in Three Human Glioma Cell Lines

AIM

To compare the uptake of ¹¹C-deuterodeprenyl (¹¹C-DED) and ¹¹C-methionine (¹¹C-MET) in three human glioma cell lines and study the relationship with glial fibrillary acid protein (GFAP) and monoamine oxidase B (MAO B) expression. ¹¹C-DED is used in positron emission tomography imaging as a marker of astrocytosis in various central nervous system pathologies. It binds irreversibly to MAO B, a glial dimeric enzyme with increased activity in some neurological pathologies.

MATERIALS AND METHODS

Binding and internalization studies of ¹¹C-MET and ¹¹C-DED were performed in astrocytoma grade III, glioblastoma grade IV, and radio-resistant glioblastoma grade IV cells. Immunofluorescence was used.

RESULTS

¹¹C-MET specific activity bound to membrane was 9.0%-11.1% and that internalized was 88.9%-91.0%. ¹¹C-DED specific activity bound to membrane was 34.8%-58.0% and that internalized was 38.7%-65.2%. Immunocytochemistry revealed GFAP and MAO B expression.

CONCLUSIONS

The expression of MAO B measured by ¹¹C-DED uptake or immunocytochemistry was not significantly different in grade III or IV cells. The GFAP signal was higher for grade IV compared to grade III. ¹¹C-MET uptake was high in all the tumor cells. ¹¹C-DED is a dopamine analogue and the transport across cell membranes is expected to be mediated by DAT receptors present in astrocytes. Reactive astrocytes surround tumor lesions; so the authors suggest that the ¹¹C-DED uptake might be caused by the reactive astrocytosis and not by MAO B expression in tumor cells.

Fluorescent Mechanism-Based Probe for Aerobic Flavin-Dependent Enzyme Activity

Diversity in non-ribosomal peptide and polyketide secondary metabolism is facilitated by interactions between biosynthetic domains with discrete monomer loading and their cognate tailoring enzymes, such as oxidation or halogenation enzymes. The cooperation between peptidyl carrier proteins and flavin-dependent enzymes offers a specialized strategy for monomer selectivity for oxidization of small molecules from within a complex cellular milieu. In an effort to study this process, we have developed fluorescent probes to selectively label aerobic flavin-dependent enzymes. Here we report the preparation and implementation of these tools to label oxidase, monooxygenase, and halogenase flavin-dependent enzymes.

Successful Treatment of Intracranial Glioblastoma Xenografts With a Monoamine Oxidase B-Activated Pro-Drug

The last major advance in the treatment of glioblastoma multiforme (GBM) was the introduction of temozolomide in 1999. Treatment with temozolomide following surgical debulking extends survival rate compared to radiotherapy and debulking alone. However, virtually all glioblastoma patients experience disease progression within 7 to 10 months. Although many salvage treatments, including bevacizumab, rechallenge with temozolomide, and other alkylating agents, have been evaluated, none of these clearly improves survival. Monoamine oxidase B (MAOB) is highly expressed in glioblastoma cell mitochondria, and mitochondrial function is intimately tied to treatment-resistant glioblastoma progression. These glioblastoma properties provide a strong rationale for pursuing a MAOB-selective pro-drug treatment approach that, upon drug activation, targets glioblastoma mitochondria, especially mitochondrial DNA. MP-MUS is the lead compound in a family of pro-drugs designed to treat GBM that is converted into the mature, mitochondria-targeting drug, P⁺-MUS, by MAOB. We show that MP-MUS can successfully kill primary gliomas in vitro and in vivo mouse xenograft models.

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Monoamine oxidase B (MAOB) is upregulated in glioblastoma.

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MAOB is used to convert a nitrogen mustard pro-drug into a mtDNA targeting drug.

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In vitro data show MAOB dependent, mitochondrial-based, toxicity.

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In vivo primary glioblastoma mouse models show efficacy.

Glioblastoma multiforme (GBM) is an inevitably fatal disease with inadequate treatment. MP-MUS, the lead compound in a family of pro-drugs specifically designed to treat GBM, is converted to the mature drug P⁺-MUS by the enzyme MAOB. This lipophilic cation accumulates in mitochondria, attacking mitochondrial DNA and promoting oxidative stress. GBM has markedly increased levels of MAOB compared to that in other cells; thus, mitochondrial and cell toxicity is restricted to GBM cells. MP-MUS significantly reduced tumor size in flank and intracranial xenograft models of GBM and induced permanent changes in the phenotype of surviving cells without causing systemic or organ-specific toxicities.

Monoamine oxidase A and B substrates: probing the pathway for drug development

Drug-discovery and -development efforts focused on the MAOs have increased at an accelerated rate over the past decade. Since the first crystal structure of human MAO-B was solved in 2002, over 40 additional structures have been reported and have helped define new, or confirm speculative, binding modes of inhibitors. The detailed mechanism of the MAO-catalyzed oxidation of amine substrates has not been fully elucidated, but its significance is central in the development of new mechanism-based inactivators. Novel fungal MAO-N variants derived from directed evolution strategies are enabling the production of new chiral amine products. Robust assays have been established for measuring MAO status in tissue and cells, while improved MAO radioligands are being deployed for PET imaging studies. This review will attempt to highlight the more recent and salient aspects of MAO research in drug discovery and development, with emphasis on substrates 'probing the pathway'.

Revelation in the neuroprotective functions of rasagiline and selegiline: the induction of distinct genes by different mechanisms

In Parkinson's disease, cell death of dopamine neurons in the substantia nigra progresses and neuroprotective therapy is required to halt neuronal loss. In cellular and animal models, selegiline [(-)deprenyl] and rasagiline, inhibitors of type B monoamine oxidase (MAO)-B, protect neuronal cells from programmed cell death. In this paper, the authors review their recent results on the molecular mechanisms by which MAO inhibitors prevent the cell death through the induction of antiapoptotic, prosurvival genes. MAO-A mediates the induction of antiapoptotic bcl-2 and mao-a itself by rasagiline, whereas a different mechanism is associated with selegiline. Rasagiline and selegiline preferentially increase GDNF and BDNF in nonhuman primates and Parkinsonian patients, respectively. Enhanced neurotrophic factors might be applicable to monitor the neurorescuing activity of neuroprotection.

Monoamine oxidases as sources of oxidants in the heart

Oxidative stress can be generated at several sites within the mitochondria. Among these, monoamine oxidase (MAO) has been described as a prominent source. MAOs are mitochondrial flavoenzymes responsible for the oxidative deamination of catecholamines, serotonin and biogenic amines, and during this process they generate H2O2 and aldehyde intermediates. The role of MAO in cardiovascular pathophysiology has only recently gathered some attention since it has been demonstrated that both H2O2 and aldehydes may target mitochondrial function and consequently affect function and viability of the myocardium. In the present review, we will discuss the role of MAO in catecholamine and serotonin clearance and cycling in relation to cardiac structure and function. The relevant contribution of each MAO isoform (MAO-A or -B) will be discussed in relation to mitochondrial dysfunction and myocardial injury. Finally, we will examine both beneficial effects of their pharmacological or genetic inhibition along with potential adverse effects observed at baseline in MAO knockout mice, as well as the deleterious effects following their over-expression specifically at cardiomyocyte level. This article is part of a Special Issue entitled "Redox Signalling in the Cardiovascular System".

Monoamine oxidase-A knockdown in human neuroblastoma cells reveals protection against mitochondrial toxins

The study examined how the mitochondrial enzyme monoamine oxidase-A (MAO-A), which produces hydrogen peroxide as a catalytic by-product, influences death and survival mechanisms. Targeted microRNA (miRNA) was used to stably knock down MAO-A mRNA, protein, and catalytic activity by 60-70% in SH-SY5Y human neuroblastoma cells. The effects of MAO-A knockdown (KD) on ATP, oxidative stress, electron transport chain, and survival following exposure to mitochondrial toxins were assessed. In control cells, complex I inhibition resulted in caspase-mediated cell death linked with ROS production and reduced ATP, followed by up-regulation of MAO-A mRNA, protein, and enzyme activity levels. Inhibition of complex III and IV resulted in a similar increase in MAO-A expression, while up-regulation of MAO-A was lower following complex II inhibition. MAO-A KD decreased basal reactive oxygen species levels by 50% and increased levels of ATP and reduced glutathione and Bcl-2. MAO-A KD specifically increased the activity of complex I but had no effect on complex II-IV activities. Furthermore, MAO-A KD protected against inhibitors of complex I, III, and IV. In summary, endogenous MAO-A levels influence mitochondrial function, notably complex I activity, and MAO-A may be a target for protection against neurodegenerative conditions that involve oxidative stress and mitochondrial dysfunction as underlying pathogenic factors.

Rasagiline and selegiline, inhibitors of type B monoamine oxidase, induce type A monoamine oxidase in human SH-SY5Y cells

Type B monoamine oxidase (MAO-B) is proposed to be involved in the pathogenesis of neurodegenerative disorders, such as Parkinson's disease, through oxidative stress and synthesis of neurotoxins. MAO-B inhibitors, rasagiline and selegiline [(-)deprenyl], protect neuronal cells by direct intervention in mitochondrial death signaling and induction of pro-survival Bcl-2 and neurotrophic factors. Recently, type A MAO (MAO-A) was found to mediate the induction of anti-apoptotic Bcl-2 by rasagiline, whereas MAO-A increases in neuronal death and also serves as a target of neurotoxins. These controversial results suggest that MAO-A may play a decisive role in neuronal survival and death. This paper reports that rasagiline and selegiline increased the mRNA, protein and catalytic activity of MAO-A in SH-SY5Y cells. Silencing MAO-A expression with small interfering (si)RNA suppressed rasagiline-dependent MAO-A expression, but MAO-B overexpression in SH-SY5Y cells did not affect, suggesting that MAO-A, not MAO-B, might be associated with MAO-A upregulation. Rasagiline reduced R1, a MAO-A specific repressor, but selegiline did not. Mithramycin-A, an inhibitor of Sp1 binding, and actinomycin-D, a transcriptional inhibitor, reduced the rasagiline-dependent upregulation of MAO-A mRNA, indicating that rasagiline induced MAO-A transcriptionally through R1-Sp1 pathway, whereas selegiline by another non-defined pathway. These results are discussed in relation to the role of MAO-A and these MAO-B inhibitors in neuronal death and neuroprotection.

Activity-based probes for studying the activity of flavin-dependent oxidases and for the protein target profiling of monoamine oxidase inhibitors

High profile: new activity-based protein profiling (ABPP) probes have been designed that target exclusively monoamine oxidases A and B within living cells (see picture; FAD=flavin adenine dinucleotide, FMN=flavin monodinucleotide). With these probes it could be shown that the MAO inhibitor deprenyl, which is in clinical use against Parkinson's disease, shows unique protein specificity despite its covalent mechanism of action.

Geranylgeraniol oxidase activity involved in oxidative formation of geranylgeranoic acid in human hepatoma cells

Geranylgeranoic acid (GGA), a 20-carbon acyclic polyprenoic acid (all-trans 3,7,11,15-tetramethyl- 2,4,6,10,14-hexadecatetraenoic acid) and its derivatives were developed as synthetic "acyclic retinoids" for cancer chemoprevention. Previously, we have shown the natural occurrence of GGA in various medicinal herbs and reported enzymatic formation of GGA from geranylgeraniol (GGOH) through geranylgeranial (GGal) by rat liver homogenates. Here, we present several lines of evidence that a putative GGOH oxidase is involved in GGA synthesis by human hepatoma cell lysates. First, conversion of GGOH to GGal did not require exogenous NAD(+), whereas the conversion from GGal to GGA absolutely required additional NAD(+). Second, GGal synthesis from GGOH was coupled with consumption of oxygen from the reaction mixture. Third, GGOH-dependent GGal synthesis activity was proteinase K-resistant and even enhanced by proteinase K treatment; GGOH oxidase activity was enriched in the mitochondrial fraction. Finally, recombinant human monoamine oxidase (MAO)-B, but not MAO-A catalyzed oxidation of GGOH to GGal. These data suggest that a putative mitochondrial GGOH oxidase is involved in the initial step of GGA synthesis from GGOH.