Mouse NRH:quinone oxidoreductase (NQO2): cloning of cDNA and gene- and tissue-specific expression

Genetic and metabolic insights into sexual dimorphism in the flexor carpi radialis of Asiatic toads (Bufo gargarizans) associated with amplexus behavior

BACKGROUND

Sexual dimorphism, a widespread phenomenon across the animal kingdom, encompasses differences between sexes in size, morphology, and physiological traits. In this study, we investigated sexual dimorphism in the flexor carpi radialis (FCR) muscle, which is critical for amplexus in Asiatic toads (Bufo gargarizans), using integrated transcriptomic and metabolomic approaches.

RESULTS

Male toads exhibited significantly larger FCR muscles, reflecting enhanced muscle function required for sustained amplexus. Transcriptomic analysis identified 818 differentially expressed genes (DEGs) between sexes, with 389 upregulated and 429 downregulated in males, predominantly associated with muscle contraction, sarcomere organization, and energy metabolism. Metabolomic profiling revealed 69 differentially expressed metabolites (DEMs), with male-biased enrichment in pathways involved in protein synthesis and degradation, energy metabolism, and material transport. Integrated analysis pinpointed key metabolic pathways-such as glycine, serine, and threonine metabolism; alanine, aspartate, and glutamate metabolism; fatty acid degradation; and the tricarboxylic acid (TCA) cycle-as central to the observed sexual dimorphism. Among these, the genes AGXT, ACADL, ACAT1, MDH2, and SUCLG2 emerged as pivotal regulators.

CONCLUSIONS

Collectively, these findings provide novel insights into the genetic and metabolic basis of sexual dimorphism in B. gargarizans, offering a deeper understanding of the evolutionary mechanisms driving sex-specific traits in vertebrates.

Molecular Pharmacology of NRH:Quinone Oxidoreductase 2: A Detoxifying Enzyme Acting as an Undercover Toxifying Enzyme

N-ribosyldihydronicotinamide:quinone oxidoreductase 2 (NQO2/QR2, Enzyme Commission number 1.10.99.2) is a cytosolic enzyme, abundant in the liver and variably expressed in mammalian tissues. Cloned 30 years ago, it was characterized as a flavoenzyme catalyzing the reduction of quinones and pseudoquinones. To do so, it uses exclusively N-alkyl nicotinamide derivatives, without being able to recognize NADH, the reference hydrure donor compound, in contrast to its next of a kind, NAD(P)H:quinone oxidoreductase 1 (NQO1). For a long time both enzymes have been considered as key detoxifying enzymes in quinone metabolism, but more recent findings point to a more toxifying function of NQO2, particularly with respect to ortho-quinones. In fact, during the reduction of substrates, NQO2 generates fairly unstable intermediates that reoxidize immediately back to the original quinone, creating a futile cycle, the byproducts of which are deleterious reactive oxygen species. Beside this peculiarity, it is a target for numerous drugs and natural compounds such as melatonin, chloroquine, imiquimod, resveratrol, piceatannol, quercetin, and other flavonoids. Most of these enzyme-ligand interactions have been documented by numerous crystallographic studies, and now NQO2 is one of the best represented proteins in the structural biology database. Despite evidence for a causative role in several important diseases, the functional role of NQO2 remains poorly explored. In the present review, we aimed at detailing the main characteristics of NQO2 from a molecular pharmacology perspective. By drawing a clear border between facts and speculations, we hope to stimulate the future research toward a better understanding of this intriguing drug target. SIGNIFICANCE STATEMENT: Evidence is reviewed on the prevalent toxifying function of N-ribosyldihydronicotinamide:quinone oxidoreductase 2 while catalyzing the reduction of ortho-quinones such as dopamine quinone. The product of this reaction is unstable and generates a futile but harmful cycle (substrate/product/substrate) associated with reactive oxygen species generation.

In cellulo monitoring of quinone reductase activity and reactive oxygen species production during the redox cycling of 1,2 and 1,4 quinones

Quinones are highly reactive molecules that readily undergo either one- or two-electron reduction. One-electron reduction of quinones or their derivatives by enzymes such as cytochrome P450 reductase or other flavoproteins generates unstable semiquinones, which undergo redox cycling in the presence of molecular oxygen leading to the formation of highly reactive oxygen species. Quinone reductases 1 and 2 (QR1 and QR2) catalyze the two-electron reduction of quinones to form hydroquinones, which can be removed from the cell by conjugation of the hydroxyl with glucuronide or sulfate thus avoiding its autoxidation and the formation of free radicals and highly reactive oxygen species. This characteristic confers a detoxifying enzyme role to QR1 and QR2, even if this character is strongly linked to the excretion capacity of the cell. Using EPR spectroscopy and confocal microscopy we demonstrated that the amount of reactive oxygen species (ROS) produced by Chinese hamster ovary (CHO) cells overexpressing QR1 or QR2 compared to naive CHO cells was determined by the quinone structural type. Indeed, whereas the amount of ROS produced in the cell was strongly decreased with para-quinones such as menadione in the presence of quinone reductase 1 or 2, a strong increase in ROS was recorded with ortho-quinones such as adrenochrome, aminochrome, dopachrome, or 3,5-di-tert-butyl-o-benzoquinone in cells overexpressing QR, especially QR2. These differences could originate from the excretion process, which is different for para- and ortho-quinones. These results are of particular interest in the case of dopamine considering the association of QR2 with various neurological disorders such as Parkinson disease.

RNA-sequencing quantification of hepatic ontogeny of phase-I enzymes in mice

Phase-I drug metabolizing enzymes catalyze reactions of hydrolysis, reduction, and oxidation of drugs and play a critical role in drug metabolism. However, the functions of most phase-I enzymes are not mature at birth, which markedly affects drug metabolism in newborns. Therefore, characterization of the expression profiles of phase-I enzymes and the underlying regulatory mechanisms during liver maturation is needed for better estimation of using drugs in pediatric patients. The mouse is an animal model widely used for studying the mechanisms in the regulation of developmental expression of phase-I genes. Therefore, we applied RNA sequencing to provide a "true quantification" of the mRNA expression of phase-I genes in the mouse liver during development. Liver samples of male C57BL/6 mice at 12 different ages from prenatal to adulthood were used for defining the ontogenic mRNA profiles of phase-I families, including hydrolysis: carboxylesterase (Ces), paraoxonase (Pon), and epoxide hydrolase (Ephx); reduction: aldo-keto reductase (Akr), quinone oxidoreductase (Nqo), and dihydropyrimidine dehydrogenase (Dpyd); and oxidation: alcohol dehydrogenase (Adh), aldehyde dehydrogenase (Aldh), flavin monooxygenases (Fmo), molybdenum hydroxylase (Aox and Xdh), cytochrome P450 (P450), and cytochrome P450 oxidoreductase (Por). Two rapidly increasing stages of total phase-I gene expression after birth reflect functional transition of the liver during development. Diverse expression patterns were identified, and some large gene families contained the mRNA of genes that are enriched at different stages of development. Our study reveals the mRNA abundance of phase-I genes in the mouse liver during development and provides a valuable foundation for mechanistic studies in the future.

Oral toxicity of okadaic acid in mice: study of lethality, organ damage, distribution and effects on detoxifying gene expression

In vivo, after administration by gavage to mice and rats, okadaic acid has been reported to produce lesions in liver, small intestine and forestomach. Because several reports differ in the damage detected in different organs, and on okadaic acid distribution after consumption, we determined the toxicity of this compound after oral administration to mice. After 24 hours, histopathological examination showed necrotic foci and lipid vacuoles in the livers of intoxicated animals. By immunohistochemical analysis, we detected this toxin in the liver and kidneys of intoxicated animals. Okadaic acid induces oxidative stress and can be activated in vitro into reactive compounds by the post-mitochondrial S9 fraction, so we studied the okadaic effect on the gene expression of antioxidant and phase II detoxifying enzymes in liver. We observed a downregulation in the expression of these enzymes and a reduction of protein expression of catalase and superoxide dismutase 1 in intoxicated animals.

NAD(P)H: quinone oxidoreductase 1 and NRH:quinone oxidoreductase 2 polymorphisms in papillary thyroid microcarcinoma: correlation with phenotype

PURPOSE

NAD(P)H:Quinone Oxidoreductase 1 (NQO1) C609T missense variant (NQO1*2) and 29 basepair (bp)-insertion/deletion (I29/D) polymorphism of the NRH:Quinone Oxidoreductase 2 (NQO2) gene promoter have been proposed as predictive and prognostic factors for cancer development and progression. The purpose of this study is to investigate the relationship between NQO1/NQO2 genotype and clinico-pathological features of papillary thyroid microcarcinoma (PTMC).

MATERIALS AND METHODS

Genomic DNA was isolated from 243 patients; and clinical data were retrospectively analyzed. NQO1*2 and tri-allelic polymorphism of NQO2 were investigated by polymerase chain reaction (PCR) and restriction fragment length polymorphism (RFLP) analysis.

RESULTS

PTMC with NQO1*2 frequently exhibited extra-thyroidal extension as compared to PTMC with wild-type NQO1 (p=0.039). There was a significant relationship between I29/I29 homozygosity of NQO2 and lymph node metastasis (p=0.042). Multivariate analysis showed that the I29/I29 genotype was associated with an increased risk of lymph node metastasis (OR, 2.24; 95% CI, 1.10-4.56; p=0.026).

CONCLUSION

NQO1*2 and I29 allele of the NQO2 are associated with aggressive clinical phenotypes of PTMC, and the I29 allele represents a putative prognostic marker for PTMC.

In silico identification and biochemical evaluation of novel inhibitors of NRH:quinone oxidoreductase 2 (NQO2)

The NCI chemical database has been screened using in silico docking to identify novel inhibitors of NRH:quinone oxidoreductase 2 (NQO2). Compounds identified from the screen exhibit a diverse range of scaffolds and inhibitory potencies are generally in the micromolar range. Some of the compounds also have the ability to inhibit NQO1. The modes of binding of the different compounds to the two enzymes are illustrated and discussed.

Deficiency of NRH:quinone oxidoreductase 2 differentially regulates TNF signaling in keratinocytes: up-regulation of apoptosis correlates with down-regulation of cell survival kinases

NRH:quinone oxidoreductase 2 (NQO2) is a cytosolic flavoprotein that catalyzes the two-electron reduction of quinones and quinoid compounds to hydroquinones. Although the role of a homologue, NAD(P)H:quinone oxidoreductase 1 (NQO1), is well defined in oxidative stress, neoplasia, and carcinogenesis, little is known about the mechanism of actions of NQO2 in these cellular responses. Whether NQO2 has any role in tumor necrosis factor (TNF) signaling was investigated using keratinocytes derived from wild-type and NQO2 knockout (NQO2-/-) mice. Although exposure of wild-type cells to TNF led to activation of nuclear factor-kappaB (NF-kappaB) and IkappaBalpha kinase, IkappaBalpha degradation, p65 phosphorylation, and p65 nuclear translocation, this cytokine had no effect on NQO2-/- cells. Deletion of NQO2 also abolished TNF-induced c-Jun NH2-terminal kinase, Akt, p38, and p44/p42 mitogen-activated protein kinase activation. The induction of various antiapoptotic gene products (MMP-9, cyclin D1, COX-2, IAP1, IAP2, Bcl-2, cFLIP, and XIAP) by TNF was also abolished in NQO2-/- cells. This correlated with potentiation of TNF-induced apoptosis as indicated by cell viability, Annexin V staining, and caspase activation. In agreement with this, we also found that TNF activated NQO2, and NQO2-specific small interfering RNA abrogated the TNF-induced NQO2 activity and NF-kappaB activation. Overall, our results indicate that deletion of NQO2 plays a differential role in TNF signaling pathway: by suppressing cell survival signals and potentiating TNF-induced apoptosis.

The role of aryl hydrocarbon receptor in the pathogenesis of cardiovascular diseases

Numerous experimental and epidemiological studies have demonstrated that polycyclic aromatic hydrocarbons (PAHs), which are major constituents of cigarette tobacco tar, are strongly involved in the pathogenesis of the cardiovascular diseases (CVDs). Knowing that PAH-induced toxicities are mediated by the activation of a cytosolic receptor, aryl hydrocarbon receptor (AhR), which regulates the expression of a group of xenobiotic metabolizing enzymes (XMEs) such as CYP1A1, CYP1A2, CYP1B1, NQO1, and GSTA1, suggests a direct link between AhR-regulated XMEs and CVDs. Therefore, identifying the localization and expression of the AhR and its regulated XMEs in the cardiovascular system (CVS) is of major importance in understanding their physiological and pathological roles. Generally, it was believed that the levels of AhR-regulated XMEs are lower in the CVS than in the liver; however, it has been shown that similar or even higher levels of expression are demonstrated in the CVS in a tissue- and species-specific manner. Moreover, most, if not all, AhR-regulated XMEs are differentially expressed in most of the CVS, particularly in the endothelium cells, aorta, coronary arteries, and ventricles. Although the exact mechanisms of PAH-mediated cardiotoxicity are not fully understood, several mechanisms are proposed. Generally, induction of CYP1A1, CYP1A2, and CYP1B1 is considered cardiotoxic through generating reactive oxygen species (ROS), DNA adducts, and endogenous arachidonic acid metabolites. However the cardioprotective properties of NQO1 and GSTA1 are mainly attributed to the antioxidant effect by decreasing ROS and increasing the levels of endogenous antioxidants. This review provides a clear understanding of the role of AhR and its regulated XMEs in the pathogenesis of CVDs, in which imbalance in the expression of cardioprotective and cardiotoxic XMEs is the main determinant of PAH-mediated cardiotoxicity.

NRH:quinone oxidoreductase 2 (NQO2) catalyzes metabolic activation of quinones and anti-tumor drugs

NRH:quinone oxidoreductase 2 (NQO2) is a cytosolic flavoprotein that utilizes NRH as electron donor. The present studies investigate the role of NQO2 in metabolic detoxification/activation of quinones and quinone based anti-tumor drugs. Chinese hamster ovary (CHO) cells stably overexpressing cDNA derived mouse NQO2 and mouse keratinocytes from DMBA-induced skin tumors in wild-type and NQO2-null mice were generated. The CHO cells overexpressing NQO2 and mouse keratinocytes expressing or deficient in NQO2 were treated with varying concentrations of mitomycin C (MMC), CB1954, MMC analog BMY25067, EO9, menadione and BP-3,6-quinone, in the absence and presence of NRH. The cytotoxicity of the drugs was evaluated by colony formation. The CHO cells overexpressing higher levels of mouse NQO2 showed significantly increased cytotoxicity to menadione, BP-3,6-quinone and to the anti-tumor drugs MMC and CB1954 when compared to CHO cells expressing endogenous NQO2. The cytotoxicity increased in presence of NRH. Similar results were also observed with BMY25067 and EO9 treatments, but to a lesser extent. The results on keratinocytes deficient in NQO2 supported the data from CHO cells. The inclusion of NRH had no effect on cytotoxicity of quinones and drugs in keratinocytes deficient in NQO2. Mouse NQO2 protein was expressed in bacteria, purified and used to study the role of NQO2 in MMC-induced DNA cross-linking. Bacterially expressed and purified NQO2 efficiently catalyzed MMC activation that led to DNA cross-linking. These results concluded that NQO2 plays a significant role in the metabolic activation of both quinones and anti-tumor drugs leading to cytotoxicity and cell death.

Nuclear factor Nrf2 and antioxidant response element regulate NRH:quinone oxidoreductase 2 (NQO2) gene expression and antioxidant induction

Human NRH:quinone oxidoreductase 2 (NQO2) is a cytosolic protein that catalyzes the metabolic reduction of quinones and provides protection against myelogenous hyperplasia and chemical carcinogenesis. NQO2 gene expression is induced in response to antioxidant tert-butylhydroquinone (tBHQ). Sequence analysis revealed six putative antioxidant response elements (ARE1 through 6) in the human NQO2 gene promoter. Deletion mutagenesis and transfection studies suggested that the ARE region between nucleotides -1433 and -1424 is essential for basal expression and antioxidant induction of NQO2 gene expression. Mutation of this ARE from 3.8 kb NQO2 gene promoter significantly repressed expression and abrogated the induction in response to antioxidant in transfected cells. Band shift, supershift, and chromatin immunoprecipitation (ChIP) assays demonstrated binding of nuclear factors Nrf2 and JunD with human NQO2 gene ARE. Coimmunoprecipitation experiments revealed an association between Nrf2 and JunD. Overexpression of Nrf2 upregulated and overexpression of Nrf2 dominant-negative mutant downregulated ARE-mediated NQO2 gene expression. The treatment of Hep-G2 cells with Nrf2-specific RNAi significantly reduced Nrf2 and NQO2 gene expression and tBHQ induction. The results combined demonstrated that Nrf2 associates with JunD, binds to ARE at nucleotide -1433, and regulates human NQO2 gene expression and induction in response to antioxidants.

Characterization of the melatoninergic MT3 binding site on the NRH:quinone oxidoreductase 2 enzyme

Melatonin acts through a series of molecular targets: the G-protein coupled receptors, MT1 and MT2, and a third binding site, MT3, recently identified as the enzyme NRH:quinone oxydoreductase 2 (QR2). The relationship between the multiple physiological functions of melatonin and this enzyme remains unclear. Because of the relationship of QR2 with the redox status of cells, these studies could bring the first tools for a molecular rationale of the antioxidant effects of melatonin. In the present paper, we used a QR2-stably expressing cell line and hamster kidneys to compare the 2-[125I]-iodomelatonin and 2-[125I]-iodo-5-methoxycarbonylamino-N-acetyltryptamine binding data, and to characterize the MT3 binding site. We designed and tested compounds from two distinct chemicals series in a displacement assay of the two MT3 ligands, 2-[125I]-iodomelatonin and 2-[125I]-iodo-5-methoxycarbonylamino-N-acetyltryptamine from their cloned target. We also tested their ability to inhibit QR2 catalytic activity. These compounds were separated into two classes: those that bind within the catalytic site (and being inhibitors) and those that bind outside it (and therefore not being inhibitors). Compounds range from potent ligands (K(i) = 1 nM) to potent inhibitors (14 nM), and include one compound [NMDPEF: N-[2-(2-methoxy-6H-dipyrido[2,3-a:3,2-e]pyrrolizin-11-yl)ethyl]-2-furamide] active on both parameters in the low nanomolar range. To dissect the physio-pathological pathways in which QR2, MT3 and melatonin meet, one needs more compounds binding to MT3 and/or inhibitors of QR2 enzymatic activity. The compounds described in the present paper are new tools for such a task.

NRH:quinone reductase 2: an enzyme of surprises and mysteries

Quinone reductase 2 has been discovered in 1961 and rediscovered in 1997. Because of its sequence homology with quinone reductase 1, it has been suspected to detoxify quinones. Ten years later, evidences begin to point to a versatile role of this enzyme. Indeed, QR2 is strongly suspected to be the molecular target of anti-malarian drugs such as chloroquin or paraquine, and of red wine-derived resveratrol that might be responsible for the so-called French paradox. It also is identical to the melatonin binding site MT3, and might therefore be a rationale explanation for the antioxidant role of melatonin. Finally QR2 might be implicated in the toxicity, in vivo, of quinones such as menadione. The present commentary attempts to summarize this information and discusses a series of hypotheses.

Quinone reductase 2 substrate specificity and inhibition pharmacology

Quinone reductase 2 is a mammalian cytosolic FAD-dependent enzyme, the activity of which is not supported by conventional nicotinamide nucleotides. An endobiotic substrate has never been reported for this enzyme nor a set of molecular tools, such as inhibitors. In the present work, we used the recombinant human enzyme, expressed in CHO cells for the systematic screening of both co-substrates and substrates. The co-substrates survey showed that the natural occurring compound, N-ribosylnicotinamide, was a poor co-substrate. The synthetic N-benzylnicotinamide is a better one compared to any other compounds tested. We found that tetrahydrofolic acid acted as a co-substrate for the reduction of menadione catalysed by quinone reductase 2, although with poor potency (Km approximately 2 mM). Among a series of commercially available quinones, a single one was found to be substrate of quinone reductase 2, in the presence of N-benzyldihydronicotinamide: coenzyme Q0. Finally, we tested a series of 197 flavonoids as potential inhibitors. We found apigenin, genistein or kaempferol as good inhibitor of quinone reductase 2 activity with IC50 in the 100 nM range. These compounds, co-substrate, substrate and inhibitors will permit to better know this enzyme, the role of which is still poorly understood.

Circadian rhythms, oxidative stress, and antioxidative defense mechanisms

Endogenous circadian and exogenously driven daily rhythms of antioxidative enzyme activities and of low molecular weight antioxidants (LMWAs) are described in various phylogenetically distant organisms. Substantial amplitudes are detected in several cases, suggesting the significance of rhythmicity in avoiding excessive oxidative stress. Mammalian and/or avian glutathione peroxidase and, as a consequence, glutathione reductase activities follow the rhythm of melatonin. Another hint for an involvement of melatonin in the control of redox processes is seen in its high-affinity binding to cytosolic quinone reductase 2, previously believed to be a melatonin receptor. Although antioxidative protection by pharmacological doses of melatonin is repeatedly reported, explanations of these findings are still insufficient and their physiological and chronobiological relevance is not yet settled. Recent data indicate a role of melatonin in the avoidance of mitochondrial radical formation, a function which may prevail over direct scavenging. Rhythmic changes in oxidative damage of protein and lipid molecules are also reported. Enhanced oxidative protein modification accompanied by a marked increase in the circadian amplitude of this parameter is detected in the Drosophila mutant rosy, which is deficient in the LMWA urate. Preliminary evidence for the significance of circadian rhythmicity in diminishing oxidative stress comes from clock mutants. In Drosophila, moderately enhanced protein damage is described for the arrhythmic and melatonin null mutant per0, but even more elevated, periodic damage is found in the short-period mutant per(s), synchronized to LD 12:12. Remarkably large increases in oxidative protein damage, along with impairment of tissue integrity and--obviously insufficient--compensatory elevations in protective enzymes are observed in a particularly vulnerable organ, the Harderian gland, of another short-period mutant tau, in the Syrian hamster. Mice deficient in the per2 gene homolog are reported to be cancer-prone, a finding which might also relate to oxidative stress. In the dinoflagellate Lingulodinium polyedrum [Gonyaulax polyedra], various treatments that cause oxidative stress result in strong suppressions of melatonin and its metabolite 5-methoxytryptamine (5-MT) and to secondary effects on overt rhythmicity. The glow maximum, depending on the presence of elevated 5-MT at the end of subjective night, decreases in a dose-dependent manner already under moderate, non-lethal oxidative stress, but is restored by replenishing melatonin. Therefore, a general effect of oxidative stress may consist in declines of easily oxidizable signaling molecules such as melatonin, and this can have consequences on the circadian intraorganismal organization and expression of overt rhythms. Recent findings on a redox-sensitive input into the core oscillator via modulation of NPAS2/BMAL1 or CLK/BMAL1 heterodimer binding to DNA indicate a direct influence of cellular redox balance, including oxidative stress, on the circadian clock.

Disruption of dihydronicotinamide riboside:quinone oxidoreductase 2 (NQO2) leads to myeloid hyperplasia of bone marrow and decreased sensitivity to menadione toxicity

Dihydronicotinamide riboside (NRH):quinone oxidoreductase 2 (NQO2) is a flavoenzyme that catalyzes the reductive metabolism of quinones. To examine the in vivo role of NQO2, NQO2-null (NQO2-/-) mice were generated using targeted gene disruption. Mice lacking NQO2 gene expression showed no detectable developmental abnormalities and were indistinguishable from wild-type (NQO2+/+) mice. However, NQO2-null mice exhibited myeloid hyperplasia of the bone marrow and increased neutrophils, basophils, eosinophils, and platelets in the peripheral blood. Decreased apoptosis of bone marrow cells and circulating granulocytes contributed to myeloid hyperplasia and hyperactivity of bone marrow in NQO2-null mice. The hematological changes in NQO2-/- mice were specifically associated with loss of the NQO2 gene because histological analysis of various tissues including spleen, thymus, blood cultures, and urine analysis demonstrated no sign of infection. NQO2-null mice also demonstrated decreased toxicity when exposed to menadione or menadione with NRH. These results establish a role for NQO2 in protection against myelogenous hyperplasia and in metabolic activation of menadione, leading to hepatic toxicity. The NQO2-null mice are a model for NQO2 deficiency in humans and can be used to determine the role of this enzyme in sensitivities to toxicity and carcinogenesis.

CNS neurons express two distinct plasma membrane electron transport systems implicated in neuronal viability

Trans-plasma membrane electron transport is critical for maintaining cellular redox balance and viability, yet few, if any, investigations have studied it in intact primary neurons. In this investigation, extracellular reduction of 2,6-dichloroindophenol (DCIP) and ferricyanide (FeCN) were measured as indicators of trans-plasma membrane electron transport by chick forebrain neurons. Neurons readily reduced DCIP, but not FeCN unless CoQ(1), an exogenous ubiquinone analog, was added to the assays. CoQ(1) stimulated FeCN reduction in a dose-dependent manner but had no effect on DCIP reduction. Reduction of both substrates was totally inhibited by epsilon-maleimidocaproic acid (MCA), a membrane-impermeant thiol reagent, and slightly inhibited by superoxide dismutase. Diphenylene iodonium, a flavoenzyme inhibitor, completely inhibited FeCN reduction but had no affect on DCIP reduction, suggesting that these substrates are reduced by distinct redox pathways. The relationship between plasma membrane electron transport and neuronal viability was tested using the inhibitors MCA and capsaicin. MCA caused a dose-dependent decline in neuronal viability that closely paralleled its inhibition of both reductase activities. Similarly capsaicin, a NADH oxidase inhibitor, induced a rapid decline in neuronal viability. These results suggest that trans-plasma membrane electron transport helps maintain a stable redox environment required for neuronal viability.

Correlation of simultaneous differential gene expression in the blood and heart with known mechanisms of adriamycin-induced cardiomyopathy in the rat

As the genomes of mammalian species become sequenced and gene functions are ascribed, the use of differential gene expression (DGE) to evaluate organ function will become common in the experimental evaluation of new drug therapies. The ability to translate this technology into useful information for human exposures depends on tissue sampling that is impractical or generally not possible in man. The possibility that the DGE of nucleated cells, reticulocytes, or platelets in blood may present the necessary link with target organ toxicity provides an opportunity to correlate preclinical with clinical outcomes. Adriamycin is highly effective alone and more frequently in combination with other chemotherapeutic agents in the treatment of a variety of susceptible malignancies. Adriamycin-induced cardiomyopathy was examined as an endpoint to measure the utility of DOE on whole blood as a predictor of cardiac toxicity. Statistically significant gene changes were observed between relevant blood and cardiac gene profiles that corroborated the accepted mechanisms of toxicity (oxidative stress, effects on carnitine transport, DNA intercalation). There were, however, clear indications that other target organs (bone marrow and intestinal tract) were affected. The divergent expression of some genes between the blood and the heart on day 7 may also indicate the timing and mechanism of development of the cardiomyopathy and confirm current therapeutic approaches for its prevention. The data demonstrate that whole blood gene expression particularly in relation to oxidative stress, in conjunction with standard hematology and clinical chemistry, may be useful in monitoring and predicting cardiac damage secondary to adriamycin administration. Appendices A & B, referenced in this paper, are not printed in this issue of Toxicologic Pathology. They are available as downloadable text files at http://taylorandfrancis.metapress.com/openurl.asp?genre=journal&issn=0192-6233. To access them, click on the issue link for 30(4), then select this article. A download option appears at the bottom of this abstract. In order to access the full article online, you must either have an individual subscription or a member subscription accessed through www.toxpath.org.

Comparative pharmacological studies of melatonin receptors: MT1, MT2 and MT3/QR2. Tissue distribution of MT3/QR2

The neurohormone melatonin is the central switch of the circadian rhythm and presumably exerts its activities through a series of receptors among which MT1 and MT2 have been widely studied. The third binding site of melatonin, MT3, has been recently characterized as a melatonin-sensitive form of the quinone reductase 2 (QR2, EC 1.6.99.2). In the present work, we showed that the binding of melatonin at MT3/QR2 was better described with 2-[125I]-iodomethoxy-carbonylamino-N-acetyltryptamine (2-[125I]-I-MCA-NAT) and, most importantly, that it was measurable at 20 degrees while it has been initially described and thoroughly studied using 2-[125I]-iodomelatonin at 4 degrees. Under these novel conditions, binding to MT3 could be traced without cross-reactivity with MT1 and MT2 receptors and, moreover, under conditions similar to those used to measure MT3/QR2 catalytic activity. The pharmacology established here on hamster kidney samples using the reference compounds remained essentially as already described using other experimental conditions. A new series of compounds with nanomolar affinity for the MT3 binding site and a high MT3 selectivity versus MT1 and MT2 is reported. In addition, we further document the MT3/QR2 binding site by demonstrating that it was widely distributed among mammals, although inter-species and inter-tissues differences exist. The present report details new experimental conditions for the pharmacological study of melatonin-sensitive QR2 isoforms, and suggests that, in addition to an already demonstrated inter-species difference, inter-tissues differences in QR2 sensitivity to melatonin may exist in primates and, therefore, represent an original and interesting route of investigation on the effect of melatonin on MT3/QR2.

NRH:quinone oxidoreductase2 (NQO2)

The quinone oxidoreductases [NAD(P)H:quinone oxidoreductase1 (NQO1) and NRH:quinone oxidoreductase2 (NQO2)] are flavoproteins. NQO1 is known to catalyse metabolic detoxification of quinones and protect cells from redox cycling, oxidative stress and neoplasia. NQO2 is a 231 amino acid protein (25956 mw) that is 43 amino acids shorter than NQO1 at its carboxy-terminus. The human NQO2 cDNA and protein are 54 and 49% similar to the human liver cytosolic NQO1 cDNA and protein. Recent studies have revealed that NQO2 differs from NQO1 in its cofactor requirement. NQO2 uses dihydronicotinamide riboside (NRH) rather than NAD(P)H as an electron donor. Another difference between NQO1 and NQO2 is that NQO2 is resistant to typical inhibitors of NQO1, such as dicoumarol, Cibacron blue and phenindone. Flavones, including quercetin and benzo(a)pyrene, are known inhibitors of NQO2. Even though overlapping substrate specificities have been observed for NQO1 and NQO2, significant differences exist in relative affinities for the various substrates. Analysis of the crystal structure of NQO2 revealed that NQO2 contains a specific metal binding site, which is not present in NQO1. The human NQO2 gene has been precisely localized to chromosome 6p25. The human NQO2 gene locus is highly polymorphic. The NQO2 gene is ubiquitously expressed and induced in response to TCDD. Nucleotide sequence analysis of the NQO2 gene promoter revealed the presence of several cis-elements, including SP1 binding sites, CCAAT box, xenobiotic response element (XRE) and an antioxidant response element (ARE). The complement of these elements regulates tissue specific expression and induction of the NQO2 gene in response to xenobiotics and antioxidants. The in vivo role of NQO2 and its role in quinone detoxification remains unknown.