Mineko Terao

Mammalian molybdo-flavoenzymes, an expanding family of proteins: structure, genetics, regulation, function and pathophysiology

The molybdo-flavoenzymes are structurally related proteins that require a molybdopterin cofactor and FAD for their catalytic activity. In mammals, four enzymes are known: xanthine oxidoreductase, aldehyde oxidase and two recently described mouse proteins known as aldehyde oxidase homologue 1 and aldehyde oxidase homologue 2. The present review article summarizes current knowledge on the structure, enzymology, genetics, regulation and pathophysiology of mammalian molybdo-flavoenzymes. Molybdo-flavoenzymes are structurally complex oxidoreductases with an equally complex mechanism of catalysis. Our knowledge has greatly increased due to the recent crystallization of two xanthine oxidoreductases and the determination of the amino acid sequences of many members of the family. The evolution of molybdo-flavoenzymes can now be traced, given the availability of the structures of the corresponding genes in many organisms. The genes coding for molybdo-flavoenzymes are expressed in a cell-specific fashion and are controlled by endogenous and exogenous stimuli. The recent cloning of the genes involved in the biosynthesis of the molybdenum cofactor has increased our knowledge on the assembly of the apo-forms of molybdo-flavoproteins into the corresponding holo-forms. Xanthine oxidoreductase is the key enzyme in the catabolism of purines, although recent data suggest that the physiological function of this enzyme is more complex than previously assumed. The enzyme has been implicated in such diverse pathological situations as organ ischaemia, inflammation and infection. At present, very little is known about the pathophysiological relevance of aldehyde oxidase, aldehyde oxidase homologue 1 and aldehyde oxidase homologue 2, which do not as yet have an accepted endogenous substrate.

Mammalian aldehyde oxidases : genetics, evolution and biochemistry

Abstract.Mammalian aldehyde oxidases are a small group of proteins belonging to the larger family of molybdo-flavoenzymes along with xanthine oxidoreductase and other bacterial enzymes. The two general types of reactions catalyzed by aldehyde oxidases are the hydroxylation of heterocycles and the oxidation of aldehydes into the corresponding carboxylic acids. Different animal species are characterized by a different complement of aldehyde oxidase genes. Humans contain a single active gene, while marsupials and rodents are characterized by four such genes clustering at a short distance on the same chromosome. At present, little is known about the physiological relevance of aldehyde oxidases in humans and other mammals, although these enzymes are known to play a role in the metabolism of drugs and compounds of toxicological importance in the liver. The present article provides an overview of the current knowledge of genetics, evolution, structure, enzymology, tissue distribution and regulation of mammalian aldehyde oxidases.

The role of aldehyde oxidase in drug metabolism

Introduction: Aldehyde oxidases (AOXs) are molybdo-flavoenzymes with complex evolutionary profiles, as the number and types of active AOX genes vary according to the animal species considered. Humans and higher primates have a single functional AOX1 gene, while rodents are endowed with four AOXs. Along with the endoplasmic cytochrome P450 system (CYP450), cytoplasmic AOX1 is the major enzyme involved in the hepatic phase I metabolism of numerous xenobiotics. Areas covered: The authors review literature to highlight the fact that aldehydes are not the only AOX substrates, as aza- and oxo-heterocycles, that represent the scaffold of many drugs, are also oxidized efficiently by these enzymes. Additionally, the ndefine the different complements of AOX isoenzymes expressed in humans and animal models used in drug metabolism studies and discuss the implications. Furthermore, the authors report on human AOX1 allelic variants that alter the activity of this enzyme. Finally, they discuss the factors of potential importance in controlling the functional activity of AOX1. Expert opinion: There is evidence for an increasing relevance of AOX1 in the metabolism and clearance of new drugs, as measures aiming at controlling CYP450-dependent metabolism of prospective therapeutic agents are becoming routine. This calls for investigations into the biology, catalytic properties and substrate specificity of human AOX1.

Increasing recognition of the importance of aldehyde oxidase in drug development and discovery

Aldehyde oxidases are molybdoflavoenzymes with broad substrate specificity, oxidizing different types of aldehydes, and heterocyclic rings. The physiological function of aldehyde oxidases is largely unknown, although the enzymes play an important role in the metabolism of numerous compounds of medicinal and toxicological interest, as they oxidize a wide range of aldehydes and heterocyclic compounds. In this article, we review the significance of aldehyde oxidases for the design and development of new drugs and discuss associated problems. These include species-specific differences in the complement of isoenzymes synthesized and general difficulties in studying the enzymatic characteristics of purified or recombinant aldehyde oxidases. We highlight the potential offered by human aldehyde oxidase targeting for the development of new pharmacological agents, limiting our attention to the realms of antiobesity and anti-cancer drugs. The last point is discussed in the context of the design of novel anticancer drugs, selectively activated or inactivated by this enzyme, with the final aim of achieving organ and/or tumor selectivity.

Induction of miR-21 by Retinoic Acid in Estrogen Receptor-positive Breast Carcinoma Cells BIOLOGICAL CORRELATES AND MOLECULAR TARGETS

Retinoids are promising agents for the treatment/prevention of breast carcinoma. We examined the role of microRNAs in mediating the effects of all-trans-retinoic acid (ATRA), which suppresses the proliferation of estrogen receptor-positive (ERα+) breast carcinoma cells, such as MCF-7, but not estrogen receptor-negative cells, such as MDA-MB-231. We found that pro-oncogenic miR-21 is selectively induced by ATRA in ERα+ cells. Induction of miR-21 counteracts the anti-proliferative action of ATRA but has the potentially beneficial effect of reducing cell motility. In ERα+ cells, retinoid-dependent induction of miR-21 is due to increased transcription of the MIR21 gene via ligand-dependent activation of the nuclear retinoid receptor, RARα. RARα is part of the transcription complex present in the 5′-flanking region of the MIR21 gene. The receptor binds to two functional retinoic acid-responsive elements mapping upstream of the transcription initiation site. Silencing of miR-21 enhances ATRA-dependent growth inhibition and senescence while reverting suppression of cell motility afforded by the retinoid. Up-regulation of miR-21 results in retinoid-dependent inhibition of the established target, maspin. Knockdown and overexpression of maspin in MCF-7 cells indicates that the protein is involved in ATRA-induced growth inhibition and contributes to the ATRA-dependent anti-motility responses. Integration between whole genome analysis of genes differentially regulated by ATRA in MCF-7 and MDA-MB-231 cells, prediction of miR-21 regulated genes, and functional studies led to the identification of three novel direct miR-21 targets: the pro-inflammatory cytokine IL1B, the adhesion molecule ICAM-1 and PLAT, the tissue-type plasminogen activator. Evidence for ICAM-1 involvement in retinoid-dependent inhibition of MCF-7 cell motility is provided.

Axonal-SMN (a-SMN), a protein isoform of the survival motor neuron gene, is specifically involved in axonogenesis

Spinal muscular atrophy (SMA) is an autosomal recessive disease of childhood due to loss of the telomeric survival motor neuron gene, SMN1. The general functions of the main SMN1 protein product, full-length SMN (FL-SMN), do not explain the selective motoneuronal loss of SMA. We identified axonal-SMN (a-SMN), an alternatively spliced SMN form, preferentially encoded by the SMN1 gene in humans. The a-SMN transcript and protein are down-regulated during early development in different tissues. In the spinal cord, the a-SMN protein is selectively expressed in motor neurons and mainly localized in axons. Forced expression of a-SMN stimulates motor neuron axonogenesis in a time-dependent fashion and induces axonal-like growth in non-neuronal cells. Exons 2b and 3 are essential for the axonogenic effects. This discovery indicates an unexpected complexity of the SMN gene system and may help in understanding the pathogenesis of SMA.

Retinoids as differentiating agents in oncology: a network of interactions with intracellular pathways as the basis for rational therapeutic combinations.

Retinoic acid and natural as well as synthetic derivatives (retinoids) are promising anti-neoplastic agents endowed with both therapeutic and chemopreventive potential. Although the treatment of acute promyelocic leukemia with all-trans retinoic acid is an outstanding example, the full potential of retinoids in oncology has not yet been exploited and a more generalized use of these compounds is not yet a reality. This may be the result of issues such as natural and induced resistance as well as local and systemic toxicity. One way to enhance the therapeutic and chemopreventive activity of retinoic acid and derivatives is to identify rational combinations between these compounds and other pharmacological agents. This is now possible given the wealth of information available on the biochemical and molecular mechanisms underlying the biological activity of retinoids. At the cellular level, the anti-leukemia and anti-cancer activity of retinoids is the result of three main actions, cell-differentiation, growth inhibition and apoptosis. At the molecular level, retinoids act through the activation of nuclear-retinoic-acid-receptor-dependent and-independent pathways. The cellular pathways and molecular networks relevant for retinoid activity are modulated by a panoply of other intra-cellular and extra-cellular pathways that may be targeted by known drugs and other experimental therapeutics. The review article aims to summarize and critically discuss the available knowledge in the field and provide a rational framework that may be useful for the design of effective drug combinations with the potential to enhance the therapeutic index of retinoids.

The First Mammalian Aldehyde Oxidase Crystal Structure: Insights Into Substrate specificity

Background: Aldehyde oxidases have pharmacological relevance, and AOX3 is the major drug-metabolizing enzyme in rodents. Results: The crystal structure of mouse AOX3 with kinetics and molecular docking studies provides insights into its enzymatic characteristics. Conclusion: Differences in substrate and inhibitor specificities can be rationalized by comparing the AOX3 and xanthine oxidase structures. Significance: The first aldehyde oxidase structure represents a major advance for drug design and mechanistic studies. Aldehyde oxidases (AOXs) are homodimeric proteins belonging to the xanthine oxidase family of molybdenum-containing enzymes. Each 150-kDa monomer contains a FAD redox cofactor, two spectroscopically distinct [2Fe-2S] clusters, and a molybdenum cofactor located within the protein active site. AOXs are characterized by broad range substrate specificity, oxidizing different aldehydes and aromatic N-heterocycles. Despite increasing recognition of its role in the metabolism of drugs and xenobiotics, the physiological function of the protein is still largely unknown. We have crystallized and solved the crystal structure of mouse liver aldehyde oxidase 3 to 2.9 Å. This is the first mammalian AOX whose structure has been solved. The structure provides important insights into the protein active center and further evidence on the catalytic differences characterizing AOX and xanthine oxidoreductase. The mouse liver aldehyde oxidase 3 three-dimensional structure combined with kinetic, mutagenesis data, molecular docking, and molecular dynamics studies make a decisive contribution to understand the molecular basis of its rather broad substrate specificity.

The Impact of Single Nucleotide Polymorphisms on Human Aldehyde Oxidase

Aldehyde oxidase (AO) is a complex molybdo-flavoprotein that belongs to the xanthine oxidase family. AO is active as a homodimer, and each 150-kDa monomer binds two distinct [2Fe2S] clusters, FAD, and the molybdenum cofactor. AO has an important role in the metabolism of drugs based on its broad substrate specificity oxidizing aromatic aza-heterocycles, for example, N1-methylnicotinamide and N-methylphthalazinium, or aldehydes, such as benzaldehyde, retinal, and vanillin. Sequencing the 35 coding exons of the human AOX1 gene in a sample of 180 Italian individuals led to the identification of relatively frequent, synonymous, missense and nonsense single-nucleotide polymorphisms (SNPs). Human aldehyde oxidase (hAOX1) was purified after heterologous expression in Escherichia coli. The recombinant protein was obtained with a purity of 95% and a yield of 50 μg/l E. coli culture. Site-directed mutagenesis of the hAOX1 cDNA allowed the purification of protein variants bearing the amino acid changes R802C, R921H, N1135S, and H1297R, which correspond to some of the identified SNPs. The hAOX1 variants were purified and compared with the wild-type protein relative to activity, oligomerization state, and metal content. Our data show that the mutation of each amino acid residue has a variable impact on the ability of hAOX1 to metabolize selected substrates. Thus, the human population is characterized by the presence of functionally inactive hAOX1 allelic variants as well as variants encoding enzymes with different catalytic activities. Our results indicate that the presence of these allelic variants should be considered for the design of future drugs.

Cloning of the cDNAs coding for two novel molybdo-flavoproteins showing high similarity with aldehyde oxidase and xanthine oxidoreductase.

The cDNAs coding for two novel mouse molybdo-flavoproteins, AOH1 and AOH2 (aldehydeoxidase homolog 1 and 2), were isolated. The AOH1 and AOH2 cDNAs code for polypeptides of 1336 amino acids. The two proteins have similar primary structure and show striking amino acid identity with aldehyde oxidase and xanthine oxidoreductase, two other molybdo-flavoenzymes. AOH1 and AOH2 contain consensus sequences for a molybdopterin-binding site and two distinct 2Fe-2S redox centers. In its native conformation, AOH1 has a molecular weight consistent with a homotetrameric structure. Transfection of the AOH1 and AOH2 cDNAs results in the production of proteins with phenanthridine but not hypoxanthine oxidizing activity. Furthermore, the AOH1 protein has benzaldehyde oxidizing activity with electrophoretic characteristics identical to those of a previously identified aldehyde oxidase isoenzyme (Holmes, R. S. (1979) Biochem. Genet. 17, 517–528). The AOH1 transcript is expressed in the hepatocytes of the adult and fetal liver and in spermatogonia. In liver, the AOH1 protein is synthesized in a gender-specific fashion. The expression of AOH2 is limited to keratinized epithelia and the basal layer of the epidermis and hair folliculi. The selective cell and tissue distribution of AOH1 and AOH2 mRNAs is consistent with the localization of the respective protein products.