NRH:quinone oxidoreductase 2 (NQO2) and glutaminase (GLS) both play a role in large extracellular vesicles (LEV) formation in preclinical LNCaP-C4-2B prostate cancer model of progressive metastasis

In the course of studies aimed at the role of oxidative stress in the development of metastatic potential in the LNCaP-C4-2B prostate cancer progression model system, we found a relative decrease in the level of expression of the cytoplasmic nicotinamide riboside: quinone oxidoreductase (NQO2) and an increase in the oxidative stress in C4-2B cells compared to that in LNCaP or its derivatives C4 and C4-2. It was also found that C4-2B cells specifically shed large extracellular vesicles (LEVs) suggesting that these LEVs and their cargo could participate in the establishment of the osseous metastases. The level of expression of caveolin-1 increased as the system progresses from LNCaP to C4-2B. Since NQO2 RNA levels were not changed in LNCaP, C4, C4-2, and C4-2B, we tested an altered cellular distribution hypothesis of NQO2 being compartmentalized in the membrane fractions of C4-2B cells which are rich in lipid rafts and caveolae. This was confirmed when the detergent resistant membrane fractions were probed on immunoblots. Moreover, when the LEVs were analyzed for membrane associated caveolin-1 as possible cargo, we noticed that the enzyme NQO2 was also a component of the cargo along with caveolin-1 as seen in double immunofluorescence studies. Molecular modeling studies showed that a caveolin-1 accessible site is present in NQO2. Specific interaction between NQO2 and caveolin-1 was confirmed using deletion constructs of caveolin-1 fused with glutathione S-transferase (GST). Interestingly, whole cell lysate and mitochondrial preparations of LNCaP, C4, C4-2, and C4-2B showed an increasing expression of glutaminase (GLS, kidney type). The extrusion of LEVs appears to be a specific property of the bone metastatic C4-2B cells and this process could be inhibited by a GLS specific inhibitor BPTES, suggesting the critical role of a functioning glutamine metabolism. Our results indicate that a high level of expression of caveolin-1 in C4-2B cells contributes to an interaction between caveolin-1 and NQO2 and to their packaging as cargo in the shed LEVs. These results suggest an important role of membrane associated oxidoreductases in the establishment of osseous metastases in prostate cancer.

The vertebrate homolog of sulfide-quinone reductase is expressed in mitochondria of neuronal tissues

Hydrogen sulfide (H₂S) can be consumed by both invertebrates and vertebrates as an inorganic substrate. The pathway metabolizing H₂S probably involves three mitochondrial enzymes, one of which is sulfide-quinone oxidoreductase (SQR), known as sulfide-quinone reductase-like protein (SQRDL) in vertebrates. Evidence from fission yeast suggests that SQR might have a role in regulating sulfide levels in the cell. Regulation might be essential for H₂S to act as a gaseous transmitter (gasotransmitter). The brain is an organ with high activity of gasotransmitters, like nitric oxide (NO) and H₂S, which are known to affect synaptic transmission. In this study, we provide evidence that SQRDL is expressed in the mammalian brain. Real-time polymerase chain reaction (PCR) showed an increase in the number of Sqrdl transcripts in the brain with increasing age. Cellular fractionation and subsequent analysis by Western blotting indicated that the protein is located in mitochondria, which is the site of sulfide consumption in the cell. With an immunohistochemical approach, we demonstrated that the SQRDL protein is expressed in neurons, oligodendrocytes, and endothelial cells. Taken together, our data suggest that brain tissue harbors the machinery required for local regulation of sulfide levels.

gamma-Tocotrienol controls proliferation, modulates expression of cell cycle regulatory proteins and up-regulates quinone reductase NQO2 in MCF-7 breast cancer cells

BACKGROUND

Tocotrienols, a subgroup of the vitamin E family, have demonstrated antioxidant and anticancer properties. Differential growth responses among different types of tocotrienols have been observed in breast cancer cells; however, specific bioactivity of each individual tocotrienol remains to be elucidated.

MATERIALS AND METHODS

In this study, the effects of gamma-tocotrienol were examined with regard to its ability to suppress cell proliferation via modulation of cell cycle regulatory protein expression, and also from the perspective of control of cellular oxidoreductive status through regulation of detoxification enzymes, e.g., quinone reductase NQO2, using estrogen receptor-positive MCF-7 human breast cancer cells.

RESULTS

It was shown that treatment by gamma-tocotrienol suppressed MCF-7 cell proliferation in a dose- and time-dependent manner. Growth suppression by gamma-tocotrienol was accompanied by changes in the levels of cell cycle regulatory proteins, notably, Rb/E2F complex, cyclin D1/cdk4 and cyclin B1/cdk1, as exemplified by loss of cyclin D1, inhibition of specific Rb phosphorylation (pRb-p at Thr821), and by the time- and dose-dependent increase in the expression of NQO2.

CONCLUSION

By exerting control on expression of specific cell cycle regulatory proteins in concomitance with suppression of cell proliferation, as well as the induction of NQO2, gamma-tocotrienol offers promise as an added chemopreventive and/or chemotherapeutic agent against breast cancer carcinogenesis.

[Induction of phase II enzymes as a strategy in the chemoprevention of cancer and other degenerative diseases]

The long interval which occurs between the initiation and the development of clinically defined cancer is a characteristic feature of carcinogenesis and other degenerative diseases in experimental models and in humans. This makes early intervention possible. In the last half century, the role of xenobiotic metabolism in the induction carcinogenesis has been thoroughly described. Recent research using new tools provide data helpful in understanding the mechanism of induction and convincing proof that selective induction of phase II enzymes is an effective way of protecting cells against reactive carcinogenic metabolites and reactive oxygen species. These results create a foundation for applying this approach as a chemopreventive and chemoprotective strategy. S-transferase glutathione and reductase NAD(P)H-quinone are enzymes of concern. In this paper the characteristics and function of these two enzymatic systems are presented as well as the mechanism of induction of compounds, which induce phase II enzymes and act as potential chemopreventive agents.

NRH:quinone oxidoreductase 2 and NAD(P)H:quinone oxidoreductase 1 protect tumor suppressor p53 against 20s proteasomal degradation leading to stabilization and activation of p53

Tumor suppressor p53 is either lost or mutated in several types of cancer. MDM2 interaction with p53 results in ubiquitination and 26S proteasomal degradation of p53. Chronic DNA damage leads to inactivation of MDM2, stabilization of p53, and apoptotic cell death. Here, we present a novel MDM2/ubiquitination-independent mechanism of stabilization and transient activation of p53. The present studies show that 20S proteasomes degrade p53. The 20S degradation of p53 was observed in ubiquitin-efficient and -deficient cells, indicating that this pathway of degradation did not require ubiquitination of p53. The cytosolic quinone oxidoreductases [NRH:quinone oxidoreductase 2 (NQO2) and NAD(P)H:quinone oxidoreductase 1 (NQO1)] interacted with p53 and protected p53 against 20S proteasomal degradation. Further studies revealed that acute exposure to radiation or chemical leads to induction of NQO1 and NQO2 that stabilizes and transiently activates p53 and downstream genes. These results suggest that stress-induced NQO1 and NQO2 transiently stabilize p53, which leads to protection against adverse effects of stressors.

Regulation of expression of Na+ -translocating NADH:quinone oxidoreductase genes in Vibrio harveyi and Klebsiella pneumoniae

The expression of genes encoding sodium-translocating NADH:quinone oxidoreductase (Na(+)-NQR) was studied in the marine bacterium Vibrio harveyi and in the enterobacterium Klebsiella pneumoniae. It has been shown that such parameters as NaCl concentration, pH value, and presence of an uncoupler in the growth media do not influence significantly the level of nqr expression. However, nqr expression depends on the growth substrates used by these bacteria. Na(+)-NQR is highly repressed in V. harveyi during anaerobic growth, and nqr expression is modulated by electron acceptors and values of their redox potentials. The latter effect was shown to be independent of the ArcAB regulatory system.

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.

Exfoliated buccal and microdissected lung cell expression of antioxidant enzymes

INTRODUCTION

An exfoliated buccal cell biomarker assay for antioxidant gene transcript levels was used to measure inter-tissue concordance with lung, and inter-subject variability in a lung cancer case-control study.

METHODS

First, qualitative RNA-specific RT-PCR was used to compare expression in exfoliated buccal cells with that in laser microdissected lung tissue remote from the tumor from 14 individuals providing both specimens.

RESULTS

There was complete [100% for quinone oxidoreductase 1 (NQO1), glutathione peroxidase (GPX), and superoxide dismutase 1 (SOD1)], or predominant [85.7% for catalase (CAT)] inter-tissue concordance for qualitative expression. Second, quantitative real-time RT-PCR for antioxidant enzyme transcript levels was performed in exfoliated buccal samples from these same 14 individuals, as well as 28 additional individuals providing buccal cells only, for a total of 42 buccal specimens (19 current smokers and 23 ex- or never-smokers), of whom 26 (61.39%) had a new diagnosis of lung cancer.

DISCUSSION

Wide inter-individual expression differences for each gene transcript (>10(1)-10(4)-fold) were observed in the exfoliated buccal cells, unrelated to smoking and case-control status. In multivariate analyses, family history of tobacco-related malignancy correlated inversely with buccal NQO1 and CAT mRNA levels (p=0.003, p<0.001, respectively). This antioxidant expression trait may relate to family risk of cancer, but is notably unrelated to oxidant challenges inherent in cigarette smoke.

Induction of NAD(P)H Quinone Oxidoreductase by Vegetables Widely Consumed in Catalonia, Spain

Monofunctional inducers (MIs) enhance phase 2 enzymes such as nicotinamide-adenine-dinucleotide-phosphate [NAD(P)H] quinone oxidoreductase (NQO1) without modifying oxidation enzymes. The induction of these protective enzymes appears to be mediated by genetic regulatory elements in their promoter regions known as the antioxidant response element (ARE). The aim of this study was to identify, through an in vitro study, which of the 30 fruits and vegetables commonly consumed in Catalonia, Spain, contain MIs of NQO1. We assayed the capacity of extracts of these fruits and vegetables to induce NQO1 [by more than 1.5-fold: ratio of induction (cells treated/control) >1.5, 8-mg/ml dose] in two murine hepatoma cell lines: Hepa 1c1c7 and BPrC1, a modified cell line that possesses a nonfunctional aryl hydrocarbon receptor nuclear translocator system and is thus nonresponsive to bifunctional inducers. We also used a third cell line, papiloma (PE) murine keratinocytes, a stably transfected cell line with an ARE-luc+ plasmid (AREPE cell line) for verifying induction through the ARE with a simple luminescence screening assay. Broccoli (Hepa 1c1c7, ratio=5.5; BPrC1, ratio=2.3), calcot (Allium cepa L.) (Hepa 1c1c7, ratio=4.7; BPrC1, ratio=.5), green onion (Hepa 1c1c7, ratio=4.6; BPrC1, ratio=2), green cabbage (Hepa 1c1c7, ratio=3.6; BPrC1, ratio=2.7), purple cabbage (Hepa 1c1c7, ratio=3.4; BPrC1, ratio=2), and black cabbage (Hepa 1c1c7, ratio=3; BPrC1, ratio=3) were active NQO1 inducers in both murine hepatoma cell lines. Extracts from broccoli (ratio=3.5), calcot (ratio=4.8), cauliflower (ratio=4.2), cabbage (ratio=2.2), green onion (ratio=3.2), green cabbage (ratio=3.6), black cabbage (ratio=4.5), and purple cabbage (ratio=3.7) were confirmed to contain MIs in the AREPE cell line. These results are very similar to those described for vegetables consumed in the United States, with the exception of calcot, which is common in Catalonia but is not grown or consumed widely in the United States.

Evidence for the existence of PAH-quinone reductase and catechol-O-methyltransferase in Mycobacterium vanbaalenii PYR-1

Polycyclic aromatic hydrocarbon (PAH) quinone reductase (PQR) and catechol-O-methyltransferase (COMT), from the PAH-degrading Mycobacterium vanbaalenii PYR-1, were demonstrated to be constitutive enzymes located in the soluble fraction of cell extracts. PQR activities for the reduction of 9,10-phenanthrenequinone and 4,5-pyrene- quinone were 1.40+/-0.13 and 0.12+/-0.01 micromol min(-1) mg-protein(-1), respectively. The exogenous catechols alizarin, anthrarobin, 2,3-dihydroxynaphthalene and esculetin inhibited PQR activity. Anthrarobin (100 microM) and esculetin (100 microM) inhibited 4,5-pyrenequinone reduction by 64-92%. COMT was involved in the O-methylation of 1,2-dihydroxyphenanthrene to form 1-methoxy-2-hydroxyphenanthrene and 1,2-dimethoxyphenanthrene. Both pyrene and 1-hydroxypyrene were metabolized by M. vanbaalenii PYR-1 to form 1-methoxypyrene, 1-methoxy-2-hydroxypyrene, 1-hydroxy-2-methoxypyrene and 1,2-dimethoxypyrene. Among the catechols tested, anthrarobin showed the highest COMT activity (1.06+/-0.04 nmol/30 min(-1) mg-protein(-1)). These results suggest that the PQR and COMT activities of M. vanbaalenii PYR-1 may play an important role in the detoxification of PAH catechols.

Crystal structure of quinone reductase 2 in complex with resveratrol

Resveratrol has been shown to have chemopreventive, cardioprotective, and antiaging properties. Here, we report that resveratrol is a potent inhibitor of quinone reductase 2 (QR2) activity in vitro with a dissociation constant of 35 nM and show that it specifically binds to the deep active-site cleft of QR2 using high-resolution structural analysis. All three resveratrol hydroxyl groups form hydrogen bonds with amino acids from QR2, anchoring a flat resveratrol molecule in parallel with the isoalloxazine ring of FAD. The unique active-site pocket in QR2 could potentially bind other natural polyphenols such as flavonoids, as proven by the high affinity exhibited by quercetin toward QR2. K562 cells with QR2 expression suppressed by RNAi showed similar properties as resveratrol-treated cells in their resistance to quinone toxicity. Furthermore, the QR2 knockdown K562 cells exhibit increased antioxidant and detoxification enzyme expression and reduced proliferation rates. These observations could imply that the chemopreventive and cardioprotective properties of resveratrol are possibly the results of QR2 activity inhibition, which in turn, up-regulates the expression of cellular antioxidant enzymes and cellular resistance to oxidative stress.

Induction of NAD(P)H quinone oxidoreductase and glutathione S-transferase activities in livers of female August-Copenhagen Irish rats treated chronically with estradiol: comparison with the Sprague-Dawley rat

Estradiol (E2) has been linked to both, protection against damage associated with chronic diseases or exposure to chemicals, and to the incidence of cancer. In its protective role, E2 appears to attenuate oxidative stress while as a carcinogen, E2 damages macromolecules via formation of reactive catechol metabolites. Alterations in the expression of antioxidant and xenobiotic metabolizing enzymes upon administration of pharmacological doses of E2 have been previously identified, but the effect of chronic exposure to low concentrations of E2 on activities of those enzymes in liver is unclear. The August-Copenhagen Irish (ACI) rat is more sensitive to estrogen-induced carcinogenesis than the Sprague-Dawley rat. Accordingly, the effect of treatment of female ACI and Sprague-Dawley rats for 6 weeks with E2 on activities of NAD(P)H quinone oxidoreductase 1 (NQO1), glutathione peroxidase, glutathione S-transferase (GST), phenol sulfotransferase (SULT1A1), cytochrome P450 (CYP450) and UDP-glucuronosyltransferase (UGT) was studied. Basal expression of these enzymes was similar in livers from both strains prior to exposure to E2. However, only NQO1 and GST activity was increased (3- and 2.5-fold, respectively) in liver cytosol of ACI rats treated with E2. In contrast, only NQO1 activity was increased modestly in livers of Sprague-Dawley rats. Other enzymes were not significantly affected in the livers of ACI or Sprague-Dawley rats following chronic treatment with E2. The selective induction of NQO1 and GST activity suggests that under physiological conditions, E2 may protect against oxidative stress via elevation of these antioxidant enzymes. The marked induction of NQO1 and GST in the ACI rat indicates a potential for this strain to be used as a model to study the E2-mediated modulation of these enzymes in tissues that are either sensitive to E2 carcinogenesis or to its protective effects.

Two polycyclic aromatic hydrocarbon o-quinone reductases from a pyrene-degrading Mycobacterium

Polycyclic aromatic hydrocarbon (PAH) o-quinone reductase (PQR) plays a crucial role in the detoxification of PAH o-quinones by reducing them to catechols. Two constitutive PQRs were found in cell extracts of a pyrene-degrading Mycobacterium sp. strain PYR100. The enzymes had an activity towards 9,10-phenanthrenequinone (PQ) and/or 4,5-pyrenequinone (PyQ), and the relative amounts varied with the pH of the culture media. PQR1, containing an FAD cofactor, was a monomer (20.1 kDa), and PQR2, with no flavin cofactor, was a homodimer (26.5 kDa subunits). There was no homology between the N-terminal sequences of PQR1 and PQR2. Dicumarol and quercetin inhibited PQR2 more strongly than PQR1. PQR1 had much lower specificity constants (k(cat)/K(m), 10(5)M(-1)s(-1)) for menadione (0.80) and PQ (5.19) than PQR2 (13.9 for menadione and 176 for PQ). Additionally, PQR2 exhibited a broad substrate specificity with high specificity constants for 1,4-naphthalenequinone, 1,2-naphthalenequinone, and PyQ.

Identification of potential prostate cancer preventive agents through induction of quinone reductase in vitro

Human prostate cancer is characterized by an early and near-universal loss of expression of the phase 2 enzyme glutathione S-transferase-pi (GSTP1). We hypothesize that a mechanism-based prostate cancer preventive strategy could involve induction of phase 2 enzymes within the prostate to compensate for the loss of GSTP1 expression. NAD[P]H:(quinone-acceptor) oxidoreductase (quinone reductase or QR) enzymatic activity, a surrogate of phase 2 enzyme response, was measured after treating the human prostate cancer cell line LNCaP with known phase 2 enzyme-inducing agents from 10 distinct chemical classes. QR enzymatic activity was assayed in microtiter plates using the menadione-coupled reduction of tetrazolium dye. Degree of induction was expressed as fold-increase over control and corrected for toxicity. Compounds were also tested in LNCaP-5-aza-C, an LNCaP subline selected in 5-aza-cytidine that expresses GSTP1, and in the human liver cell line HepG2. LNCaP showed robust induction of QR enzymatic activity after treatment with a subset of the phase 2 enzyme-inducing agents. All Michael acceptors were effective at inducing QR activity in LNCaP. Some phenolic antioxidants, heavy metal salts, and quinones also significantly increased QR activity, although inducer potency varied widely within these classes of compounds. Some of the isothiocyanates, mercaptans, bifunctional inducers, and trivalent arsenicals also produced modest QR induction, but peroxides and dithiolethiones were inactive. LNCaP-5-aza-C and LNCaP responded similarly to all compounds, but the pattern of response for HepG2 differed significantly. The differences in QR responsiveness between the prostate cell lines and HepG2 suggest that prostate tissues may have a unique pattern of response to phase 2-inducing agents distinct from other tissue types. Our data suggest that measurement of QR induction in prostate cancer cell lines may help identify potential cancer chemopreventive agents effective in the prostate.

Induction of quinone reductase by withanolides

Thirty-seven naturally occurring withanolides (1-37), previously isolated in our laboratories, were evaluated for their potential to induce quinone reductase with cultured murine hepatoma cells (Hepa 1c1c7). Spiranoid (29, 32) and 18-functionalized withanolides (2-5, 7-9, 24) were found to be potent inducers of the enzyme, while 5alpha-substituted derivatives exhibited weak activity. Preliminary studies were performed with compound 29 to evaluate enzyme-inducing capacity in multiple organ sites of BALB/c mice. Significant induction was observed in liver and colon, but not in lung, stomach, or mammary gland.

Characterization of the membrane domain Nqo11 subunit of the proton-translocating NADH-quinone oxidoreductase of Paracoccus denitrificans

The proton-translocating NADH-quinone oxidoreductase (NDH-1) of Paracoccus denitrificans consists of at least 14 unlike subunits (designated Nqo1-14). The NDH-1 is composed of two segments (the peripheral and membrane segments). The membrane domain segment appears to be made up of seven subunits (Nqo7, -8, -10-14). In this report, the characterization of the Paracoccus Nqo11 subunit has been investigated. An antibody against the C-terminal 12 amino acid residues of the Paracoccus Nqo11 subunit (Nqo11c) has been raised. The Nqo11c antibody reacted with a single band (11 kDa) of the Paracoccus membranes and cross-reacted with Rhodobactor capsulatus membranes. The Nqo11 subunit was not able to be extracted from the Paracoccus membranes by NaI or alkaline treatment, unlike the peripheral subunits (Nqo1 and Nqo6). The C-terminal region of the Paracoccus Nqo11 is exposed to the cytoplasmic phase. For further characterization of the Paracoccus Nqo11 subunit, the subunit was overexpressed in Escherichia coli by using the maltose-binding protein (MBP) fusion system. The MBP-fused Nqo11 subunit was expressed in the E. coli membranes (but not in soluble phase) and was extracted by Triton X-100. The isolated MBP-fused Nqo11 subunit interacted with the phospholipid vesicles and suppressed their membrane fluidity. Topological studies of the Nqo11 subunit expressed in E. coli membranes have been performed by using cysteine mapping and immunochemical analyses. The data suggest that the Nqo11 subunit has three transmembrane segments and its C-terminus protrudes into the cytoplasmic phase.

Differential gene expression of NAD(P)H:quinone oxidoreductase and NRH:quinone oxidoreductase in human hepatocellular and biliary tissue

NAD(P)H:quinone oxidoreductase (NQO1) and dihydronicotinamide riboside:quinone oxidoreductases (NQO2) are cytosolic flavoproteins that catalyze the two-electron reduction of quinones and quinoid compounds to hydroquinones, thereby promoting detoxification and preventing the formation of highly reactive oxygen species, which lead to DNA and cell damage. Two NQO isoforms, designated NQO1 and NQO2, have been cloned and sequenced. To elucidate their role in carcinogenesis, the gene expression of human NQO1 and NQO2 in paired normal and tumor tissue samples was examined. Quantitative triplex reverse transcriptase polymerase chain reaction was employed to analyze NQO1 and NQO2 mRNA expression in normal hepatic and biliary tissue as well as in cholangiocellular carcinomas (CCC), hepatocellular carcinomas (HCC), and focal nodular hyperplasias (FNH). Coexpression of beta-actin RNA was used as an internal reference standard and linear ranges of transcript amplification were established for each sample. In normal hepatocellular tissue, the two NQO isoforms were differentially regulated, with a higher expression of NQO2 than NQO1. Malignant hepatocellular tissue (HCC), however, displayed up-regulation of NQO1 and down-regulation of NQO2. Regulation of either transcript was not seen in benign hepatocellular tumor tissue (FNH), which indicates a reciprocal control of NQO genes in hepatocarcinogenesis. Normal biliary tissue expressed a significantly higher level of NQO1 transcripts compared with normal liver, whereas biliary NQO2 levels were significantly lower than in hepatocellular tissue. Comparing the levels of expression in normal and malignant biliary tissue (CCC), no significant differences were noted between the expression levels of either transcript. Thus, this study provides evidence for differential hepatic and biliary regulation of both NQO1 and NQO2.

Role of phase 2 enzyme induction in chemoprotection by dithiolethiones

One of the major mechanisms of protection against carcinogenesis, mutagenesis, and other forms of toxicity mediated by carcinogens is the induction of enzymes involved in their metabolism, particularly phase 2 enzymes such as glutathione S-transferases (GSTs), UDP-glucuronosyl transferases, and quinone reductases. Animal studies indicate that induction of phase 2 enzymes is a sufficient condition for obtaining chemoprevention and can be achieved by administering any of a diverse array of naturally-occurring and synthetic chemopreventive agents. Indeed, monitoring of enzyme induction has led to the recognition or isolation of novel, potent chemopreventive agents such as 1,2-dithiole-3-thiones, terpenoids and the isothiocyanate sulforaphane. For example, oltipraz, a substituted 1,2-dithiole-3-thione originally developed as an antischistosomal agent, possesses chemopreventive activity against different classes of carcinogens targeting multiple organs. Mechanistic studies in rodent models for chemoprevention of aflatoxin B(1) (AFB(1))-induced hepatocarcinogenesis by oltipraz indicates that increased expression of phase 2 genes is of central importance, although inhibition of phase 1 activation of AFB(1) can also contribute to protection. Exposure of rodents to 1,2-dithiole-3-thiones triggers nuclear accumulation of the transcription factor Nrf2 and its enhanced binding to the "antioxidant response element" (ARE), leading to transcriptional activation of a score of genes involved in carcinogen detoxication and attenuation of oxidative stress. Nrf2-deficient mice fail to induce many of these genes in response to dithiolethiones; moreover, basal expression of these genes is typically repressed. To test the hypothesis that enzyme induction is a useful strategy for chemoprevention in humans, three key elements are necessary: a candidate agent, an at-risk population and modulatable intermediate endpoints. Towards this end, a placebo-controlled, double blind clinical trial of oltipraz was conducted in residents of Qidong, PR China who are exposed to dietary aflatoxins and who are at high risk for the development of liver cancer. Oltipraz significantly enhanced excretion of a phase 2 product, aflatoxin-mercapturic acid, a derivative of the aflatoxin-glutathione conjugate, in the urine of study participants administered 125 mg oltipraz by mouth daily. Administration of 500 mg oltipraz once a week led to a significant reduction in the excretion of the primary oxidative metabolite of AFB(1), AFM(1), when measured shortly after drug administration. While this study highlighted the general feasibility of inducing phase 2 enzymes in humans, a longer term intervention is addressing whether protective alterations in aflatoxin metabolism can be sustained for extended periods of time in this high-risk population.

Sulfide oxidation in gram-negative bacteria by expression of the sulfide-quinone reductase gene of Rhodobacter capsulatus and by electron transport to ubiquinone

The oxidation of sulfide was studied in recombinant bacteria expressing the sulfide-quinone reductase gene (sqr) from Rhodobacter capsulatus. Sulfide was oxidized by the Escherichia coli strain W3110 harboring the sqr construct (pKKSQ) under anaerobic conditions and nitrate was utilized as a terminal electron acceptor. Following the oxidation, elemental sulfur and nitrite were produced as the final reaction products. This activity was retained in the membrane preparation and was sensitive towards antimycin A, stigmatellin, and azide. As a consequence of the ubiquinone deficiency, this activity was markedly decreased. In additon, by recovery of ubiquinone, the oxidation was also restored to rates similar to those of the wild-type strain. These results indicate that sulfide oxidation in this strain occurs via the quinone pool in vivo, and that this sulfide-quinone reductase (SQR) in particular utilizes ubiquinone as a more appropriate electron acceptor than menaquinone or demetylmenaquinone. To our knowledge, this is the first study to show a direct interaction between SQR and ubiquinone in cells. When expressed in Pseudomonas putida and Rhizobium meliloti, the SQR conferred on these organisms the ability to oxidize sulfide as well as E. coli in vivo.

The RegB/RegA two-component regulatory system controls synthesis of photosynthesis and respiratory electron transfer components in Rhodobacter capsulatus

Recently, we demonstrated that the RegB/RegA two-component regulatory system from Rhodobacter capsulatus functions as a global regulator of metabolic processes that either generate or consume reducing equivalents. For example, the RegB/RegA system controls expression of such energy generating processes as photosynthesis and hydrogen utilization. In addition, RegB/RegA also control nitrogen and carbon fixation pathways that utilize reducing equivalents. Here, we use a combination of DNase I protection and plasmid-based reporter expression studies to demonstrate that RegA directly controls synthesis of cytochrome cbb3 and ubiquinol oxidases that function as terminal electron acceptors in a branched respiratory chain. We also demonstrate that RegA controls expression of cytochromes c2, c(y) and the cytochrome bc1 complex that are involved in both photosynthetic and respiratory electron transfer events. These data provide evidence that the RegB/RegA two-component system has a major role in controlling the synthesis of numerous processes that affect reducing equivalents in Rhodobacter capsulatus.

Expression and mutagenesis of the NqrC subunit of the NQR respiratory Na(+) pump from Vibrio cholerae with covalently attached FMN

The Na(+)-translocating NADH:quinone oxidoreductase (Na(+)-NQR) is present in the membranes of a number of marine bacteria and pathogenic bacteria. Two of the six subunits of the Na(+)-NQR, NqrB and NqrC, have been previously shown to contain covalently bound flavin adenine mononucleotide (FMN). In the current work, the cloning of nqrC from Vibrio cholerae is reported. The gene has been expressed in V. cholerae and shown to contain one equivalent of covalently bound FMN. In contrast, no covalent flavin was detected when threonine-225 was replaced by leucine. The data show that the FMN attachment does not require assembly of the enzyme and are consistent with the unusual threonine attachment site.

Tyrphostin [correction of Tryphostin] AG879, a tyrosine kinase inhibitor: prevention of transcriptional activation of the electrophile and the aromatic hydrocarbon response elements

To investigate a possible role of phosphorylation in the signal transduction pathways responsible for transcriptional regulation of drug-metabolizing enzymes, we tested seven specific tyrosine kinase inhibitors (tyrphostins) for their effects on NAD(P)H:quinone oxidoreductase-1 (NQO1) mRNA levels in mouse hepatoma Hepa-1c1c7 (Hepa-1) cells and chose to study AG879 further. The potent electrophile tert-butylhydroquinone (tBHQ) is known to activate NQO1 gene transcription via the electrophile response element (EPRE). Among the tyrphostins tested, tyrphostin AG879 was unique in preventing the accumulation of tBHQ-induced NQO1 mRNA; this effect was dependent on the AG879 dose and was also sensitive to the time when AG879 was added relative to the beginning of tBHQ treatment. 2,3,7,8-Tetrachlorodibenzo-p-dioxin (dioxin; TCDD) is known to activate Cyp1a1 gene transcription by way of aromatic hydrocarbon response elements (AHREs). We found that AG879 also prevents, to a lesser extent, the AHRE-mediated induction of CYP1A1 and NQO1 mRNA by dioxin. Zinc or cadmium is known to activate metallothionein (Mt1) gene transcription via the metal response element (MRE). AG879 induced MT1 mRNA, and AG879 did not block zinc- or cadmium-induced MT1 mRNA, indicating that the effects of AG879 on NQO1 or CYP1A1 mRNA levels cannot be generalized to all transcripts. Using transient transfection of EPRE-, AHRE-, or MRE-driven luciferase reporter gene constructs in Hepa-1 cells, we showed that the inhibitory effects of AG879 occurred at the level of EPRE- and AHRE-mediated transcription, but that AG879 did not affect the MRE-driven transcriptional response. These data suggest that AG879 might inhibit an unknown tyrosine kinase(s) whose activity is essential for EPRE- and AHRE-mediated trans-activation of certain mammalian genes. These results also indicate that some sharing of common signal transduction pathways might exist in the regulation of genes involved in drug metabolism that also respond to oxidative stress.

Transcriptional regulation of nicotinamide adenine dinucleotide phosphate: quinone oxidoreductase in murine hepatoma cells by 6-(methylsufinyl)hexyl isothiocyanate, an active principle of wasabi (Eutrema wasabi Maxim)

Wasabi is a very popular pungent spice in Japan. This study examined the ability of 6-(methylsufinyl)hexyl isothiocyanate (6-MITC), an active principle of wasabi, to induce the cellular expression of nicotinamide adenine dinucleotide phosphate: quinone oxidoreductase (QR) in Hepa 1c1c7 cells. The cells were treated with various concentrations of 6-MITC, and were then assessed for cell growth, QR activity and QR mRNA expression. The induction of QR activity and QR mRNA expression was time- and dose-responsive over a narrow range of 0.1-5 microM, with declining induction at higher concentrations due to cell toxicity. Furthermore, transfection studies demonstrated that the induction of transcription of the QR gene by 6-MITC involved an antioxidant/electrophile-responsive element (ARE/EpRE) activation. Our results suggest a novel mechanism by which dietary wasabi 6-MITC may be implicated in cancer chemoprevention.

Quinone reductase (NQO1), a sensitive redox indicator, is increased in Alzheimer's disease

NAD(P)H:quinone oxidoreductase 1 (NQO1), a redox-regulated flavoenzyme, plays a central role in monitoring cellular redox state. NQO1 acts to protect against oxidative stress induced by a variety of metabolic situations, including metabolism of quinones and other xenobiotics, by: (i) functioning as a two electron donor to provide a shunt that competes with the formation of reactive oxygen species; (ii) maintaining reduced coenzyme Q; and (iii) regulating the stress activated kinase pathway. In Alzheimer's disease, while there is abundant evidence for the involvement of oxidative stress, the cause or the consequences are largely unresolved. We suspected that increased NQO1 could signal a major shift in redox balance in Alzheimer's disease and, in this study, found that NQO1 is localized not only to neurofibrillary tangles but also the cytoplasm of hippocampal neurons. By marked contrast, there is very little NQO1 in the same neuronal populations in young and age-matched controls. This novel association of NQO1 further buttresses the nexus of oxidative stress, via free radicals, with selective neuronal vulnerability and also supports a fundamental abnormality in redox balance in Alzheimer's disease.

Chemoprevention of aflatoxin B1 hepatocarcinogenesis by coumarin, a natural benzopyrone that is a potent inducer of aflatoxin B1-aldehyde reductase, the glutathione S-transferase A5 and P1 subunits, and NAD(P)H:quinone oxidoreductase in rat liver

Structurally diverse compounds can confer resistance to aflatoxin B1 (AFB1) hepatocarcinogenesis in the rat. Treatment with either phytochemicals [benzyl isothiocyanate, coumarin (CMRN), or indole-3-carbinol] or synthetic antioxidants and other drugs (butylated hydroxyanisole, diethyl maleate, ethoxyquin, beta-naphthoflavone, oltipraz, phenobarbital, or trans-stilbene oxide) has been found to increase hepatic aldo-keto reductase activity toward AFB1-dialdehyde and glutathione S-transferase (GST) activity toward AFB1-8,9-epoxide in both male and female rats. Under the conditions used, the natural benzopyrone CMRN was a major inducer of the AFB1 aldehyde reductase (AFAR) and the aflatoxin-conjugating class-alpha GST A5 subunit in rat liver, causing elevations of between 25- and 35-fold in hepatic levels of these proteins. Induction was not limited to AFAR and GSTA5: treatment with CMRN caused similar increases in the amount of the class-pi GST P1 subunit and NAD(P)H: quinone oxidoreductase in rat liver. Immunohistochemistry demonstrated that the overexpression of AFAR, GSTA5, GSTP1, and NAD(P)H:quinone oxidoreductase affected by CMRN is restricted to the centrilobular (periacinar) zone of the lobule, sometimes extending almost as far as the portal tract. This pattern of induction was also observed with ethoxyquin, oltipraz, and trans-stilbene oxide. By contrast, induction of these proteins by beta-naphthoflavone and diethyl maleate was predominantly periportal. Northern blotting showed that induction of these phase II drug-metabolizing enzymes by CMRN was accompanied by similar increases in the levels of their mRNAs. To assess the biological significance of enzyme induction by dietary CMRN, two intervention studies were performed in which the ability of the benzopyrone to inhibit either AFB1-initiated preneoplastic nodules (at 13 weeks) or AFB1-initiated liver tumors (at 50 weeks) was investigated. Animals pretreated with CMRN for 2 weeks prior to administration of AFB1, and with continued treatment during exposure to the carcinogen for a further 11 weeks, were protected completely from development of hepatic preneoplastic lesions by 13 weeks. In the longer-term dietary intervention, treatment with CMRN before and during exposure to AFB1 for a total of 24 weeks was found to significantly inhibit the number and size of tumors that subsequently developed by 50 weeks. These data suggest that consumption of a CMRN-containing diet provides substantial protection against the initiation of AFB1 hepatocarcinogenesis in the rat.

Modulation of oxidative phosphorylation of human kidney 293 cells by transfection with the internal rotenone-insensitive NADH-quinone oxidoreductase (NDI1) gene of Saccharomyces cerevisiae

In contrast to the mitochondrial proton-translocating NADH-quinone oxidoreductase (complex I), which consists of at least 43 different subunits, the internal rotenone-insensitive NADH-quinone oxidoreductase (Ndi1) of Saccharomyces cerevisiae is a single polypeptide enzyme. The NDI1 gene was stably transfected into the human embryonal kidney 293 (HEK 293) cells. The transfected NDI1 gene was then transcribed and translated in the HEK 293 cells to produce the functional enzyme. The immunochemical and immunofluorescence analyses indicated that the expressed Ndi1 polypeptide was located to the inner mitochondrial membranes. The expression of Ndi1 did not alter the content of existing complex I in the HEK 293 mitochondria, suggesting that the expressed Ndi1 enzyme does not displace the endogenous complex I. The NADH oxidase activity of the NDI1-transfected HEK 293 cells was not affected by rotenone but was inhibited by flavone. The ADP/O ratios coupled to NADH oxidation were lowered from 2.4 to 1.8 by NDI1-transfection while the ADP/O ratios coupled to succinate oxidation (1.6) were not changed. The NDI1-transfected HEK 293 cells were able to grow in media containing a complex I inhibitor such as rotenone and 1-methyl-4-phenylpyridinium ion. The potential usefulness of incorporating the Ndi1 protein into mitochondria of human cells is discussed.

Impact of dioxin-type induction of drug-metabolizing enzymes on the metabolism of endo- and xenobiotics

The induction of a number of drug-metabolizing enzymes is among the best-understood biochemical effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and related agonists of the aryl hydrocarbon receptor (AhR). Among the cytochrome P450s (CYPs), the genes encoding CYP1A1, 1A2, and 1B1 are responsive to AhR agonists, i.e. their expression is inducible in various mammalian tissues and organs as well as in many types of cell lines and primary cells in culture. In addition, an aldehyde dehydrogenase, an NADPH-quinone-oxidoreductase, and the phase II conjugating enzymes glutathione-S-transferase (GST) Ya and UDP-glucuronosyltransferase 1A1 have been identified as responsive to AhR agonists. Induced expression of these members of the AhR gene battery is thought to be aimed at an improved elimination of the inducing agent and its metabolites. However, the identity of the physiological ligand(s) of the AhR is still obscure. The consequences of induced expression of AhR-regulated genes encoding drug-metabolizing enzymes have been investigated in human populations, e.g. in smokers, and in various experimental models. A prominent example of increased adverse effects due to the induction of CYP1A isozymes is the metabolic activation of carcinogenic aromatic amines and polycyclic aromatic hydrocarbons. An increasing amount of data is also available on the impact of dioxin-type induction on the metabolism of drugs, food constituents, and endogenous substrates. For example, the hepatic clearance of the drug theophylline, which is widely used in asthma therapy, is enhanced significantly in smokers. Increased glucuronidation of thyroxine in rats treated with TCDD or other potent AhR agonists is thought to result in hypothyroxinemia and its biological consequences, such as sustained hyperplasia of the thyroid, bearing a higher risk of thyroid cancer. The relevance of these observations for humans exposed to dioxin-type inducers is discussed.

Rotenone-insensitive internal NADH-quinone oxidoreductase of Saccharomyces cerevisiae mitochondria: the enzyme expressed in Escherichia coli acts as a member of the respiratory chain in the host cells

The NDI1 gene encodes the internal rotenone-insensitive NADH-quinone oxidoreductase localized in the inner mitochondrial membranes of Saccharomyces cerevisiae. The T7 tag-fused mature NDI1 was overexpressed in Escherichia coli. The overexpressed NDI1 was exclusively found in the membrane fraction. The NDI1-overexpressed membranes showed significantly increased activities of NADH oxidase and NADH-ubiquinone-1 (UQ1) reductase when compared with the control membranes. Flavone, which is a specific inhibitor of the S. cerevisiae NDI1, inhibited almost completely NADH oxidase and NADH-UQ1 reductase activities of NDI1-overexpressed membranes but scarcely inhibited these activities of the control membranes. In addition, the NADH oxidase activity of the NDI1-overexpressed membranes was also inhibited by KCN as well as the control membranes. These results indicate that the overexpressed NDI1 worked as a member of the respiratory chain in the host cells, even though E. coli membranes are different from S. cerevisiae inner mitochondrial membranes in terms of quinones and lipid composition.

Expression and characterization of the flavoprotein subcomplex composed of 50-kDa (NQO1) and 25-kDa (NQO2) subunits of the proton-translocating NADH-quinone oxidoreductase of Paracoccus denitrificans

This study reports the expression of the flavoprotein (FP) subcomplex of the proton-translocating NADH-quinone oxidoreductase (NDH-1) from Paracoccus denitrificans, which is composed of the NQO1 (50 kDa) and the NQO2 (25 kDa) subunits. The two subunits are co-expressed in Escherichia coli using a double expression plasmid system. The expressed subunits form a water-soluble heterodimer complex with 1:1 stoichiometry. The expressed complex contained one [2Fe 2S] cluster but almost no FMN or [4Fe 4S] cluster. The two latter prosthetic groups could be partially reconstituted with FMN, Na2S, and (NH4)2Fe(SO4)2 in vitro under anaerobic conditions. The reconstituted FP subcomplex showed EPR signals from two distinct species of iron-sulfur cluster. One resonance transition originates from a [2Fe-2S] cluster with g values of gx,y,z = 1.92, 1.95, and 2.00 and slow spin relaxation, which was tentatively assigned to the cluster N1a. These EPR properties are very similar to those reported for the NQO2 subunit expressed alone (Yano, T., Sled', V. D., Ohnishi, T., and Yagi, T. (1994) Biochemistry 33, 494-499). The other originates from a [4Fe 4S] cluster with g values of gx,y, z = 1.87, 1.94, and 2.04 and fast relaxing behavior, which are reminiscent of the cluster N3 in the membrane bound enzyme complex. After reconstitution with FMN, the FP subcomplex catalyzed electron transfer from NADH and from deamino-NADH to a variety of electron acceptors. The enzymatic properties of the FP subcomplex, reconstituted with FMN and iron-sulfur, correspond to those of the isolated P. denitrificans NADH-dehydrogenase complex.

Expression and characterization of the 66-kilodalton (NQO3) iron-sulfur subunit of the proton-translocating NADH-quinone oxidoreductase of Paracoccus denitrificans

The proton-translocating NADH-quinone oxidoreductase (NDH-1) of Paracoccus denitrificans is composed of at least 14 dissimilar subunits which are designated NQO1-14 and contains one noncovalently bound FMN and at least five EPR-visible iron-sulfur clusters (N1a, N1b, N2, N3, and N4) as prosthetic groups. Comparison of the deduced primary structures of the subunits with consensus sequences for the cofactor binding sites has predicted that NQO1, NQO2, NQO3, NQO9, and probably NQO6 subunits are cofactor binding subunits. Previously, we have reported that the NQO2 (25 kDa) subunit was overexpressed as a water-soluble protein in Escherichia coli and was found to ligate a single [2Fe-2S] cluster with rhombic symmetry (gx,y,z = 1.92, 1.95, and 2.00) (Yano, T., Sled', V.D., Ohnishi, T., and Yagi, T. (1994) Biochemistry 33, 494-499). In the present study, the NQO3 (66 kDa) subunit, which is equivalent to the 75-kDa subunit of bovine heart Complex I, was overexpressed in E. coli. The expressed NQO3 subunit was found predominantly in the cytoplasmic phase and was purified by ammonium sulfate fractionation and anion-exchange chromatography. The chemical analyses and UV-visible and EPR spectroscopic studies showed that the expressed NQO3 subunit contains at least two distinct iron-sulfur clusters: a [2Fe-2S] cluster with axial EPR signals (g perpendicular, parallel = 1.934 and 2.026, and L perpendicular parallel = 1.8 and 3.0 millitesla) and a [4Fe-4S] cluster with rhombic symmetry (gx,y,z = 1.892, 1.928, and 2.063, and Lx,y,z = 2.40, 1.55, and 1.75 millitesla). The midpoint redox potentials of [2Fe-2S] and [4Fe-4S] clusters at pH 8.6 are -472 and -391 mV, respectively. The tetranuclear cluster in the isolated NQO3 subunit is sensitive toward oxidants and converts into [3Fe-4S] form. The assignment of these iron-sulfur clusters to those identified in the P. denitrificans NDH-1 enzyme complex and the possible functional role of the NQO3 subunit is discussed.

Cloning of the Na(+)-translocating NADH-quinone reductase gene from the marine bacterium Vibrio alginolyticus and the expression of the beta-subunit in Escherichia coli

The Na(+)-translocating NADH-quinone reductase purified from the marine bacterium Vibrio alginolyticus is composed of three subunits, alpha, beta and gamma. From the N-terminal amino acid sequences of each subunit and its polypeptide fragment obtained by partial digestion with V8 protease, oligonucleotides corresponding to forward and reverse primers for each gene (NQR A, B and C) encoding the alpha, beta and gamma subunit, respectively, were synthesized. Using these primers, a part of each gene was amplified from the chromosomal DNA of V. alginolyticus by a PCR method, and the PCR products were used for the cloning of the NQR gene in lambda phage. Among the subclones selected by probe C, the expression of the beta-subunit as a gene product was detected in Escherichia coli membranes by activity staining and Western blotting.

Cloning and sequencing of four structural genes for the Na(+)-translocating NADH-ubiquinone oxidoreductase of Vibrio alginolyticus

Oligonucleotide probes based on the N-terminal amino acid sequences of the NqrA and NqrC subunits were used to clone genes for the Na(+)-dependent NADH-ubiquinone oxidoreductase complex from Vibrio alginolyticus. Four consecutive ORFs were identified encoding subunit proteins of 48.6, 46.8, 27.7 and 22.6 kDa, respectively (NqrA-D). A further ORF, showing 71% homology to the BolA protein of Escherichia coli, was located upstream. From sequence comparisons, we conclude that the Na(+)-dependent NADH-ubiquinone oxidoreductase complex of V. alginolyticus is clearly distinct from the corresponding H(+)-dependent enzymes of both prokaryotes and eukaryotes.

Increased activity and expression of NAD(P)H:quinone acceptor oxidoreductase in confluent cell cultures and within multicellular spheroids

NAD(P)H:quinone acceptor oxidoreductase (NQO1, EC 1.6.99.2) is an enzyme that is believed to play a central role in the bioreductive activation of several compounds, particularly quinones. The results of this study demonstrate that the activity of NQO1 is significantly elevated (2.5-fold) in HT-29 human colon cells that are in the plateau phase of the growth curve as opposed to cells in the exponential phase. Analysis of gene expression using semiquantitative reverse transcription-polymerase chain reaction and Northern blot analysis demonstrates that the increased enzyme activity is associated with increased NQO1 mRNA levels. Sequential trypsinization of layers of cells from HT-29 multicellular spheroids and analysis of gene expression by reverse transcription-polymerase chain reaction demonstrate that NQO1 expression is elevated in cells close to the necrotic center. Maximum expression occurs at a depth of 90-110 microns, with reduced expression as the distance toward both the surface and the necrotic center decreases. HT-29 spheroids were significantly more responsive than monolayers (concentration producing 50% inhibition, 124.6 and 364 nM, respectively) to the experimental drug, 2,5-dimethyl-3,6 diaziridinyl-1,4 benzoquinone. While the environmental stimulus responsible for causing elevated NQO1 expression has not been identified, the fact that NQO1 expression is influenced by microenvironmental conditions will have important implications for those drugs that are activated by NQO1.

Purification and characterization of sulfide-quinone reductase, a novel enzyme driving anoxygenic photosynthesis in Oscillatoria limnetica

An enzyme catalyzing sulfide quinone oxido-reduction (E.C.1.8.5.'.; SQR) has been purified in an active form, from thylakoids of the cyanobacterium Oscillatoria limnetica. It is composed of a single polypeptide of about 57 kDa. The catalytic activity of the purified enzyme is similar to the membrane-bound form in its kinetic parameters: apparent Km for sulfide equals 8 microM; Vmax of 100-150 mumol of plastoquinone-1 reduced/mg protein/h; quinone-substrate specificity; differential sensitivity to quinone analog inhibitors, the most potent of which being aurachin C (I50 = 7 nM), and specific inducibility by sulfide. Taken together, they suggest that the purified SQR is the enzyme catalyzing anoxygenic photosynthesis in cyanobacteria. The UV and visible absorption and fluorescence spectra of the purified SQR are typical of a flavoprotein. Both the absorption and fluorescence intensities are reduced by sulfide. The SQR activity is inhibited by KCN, a flavoprotein inhibitor. We have sequenced so far 29 amino acid residues of the SQR NH2 terminus and found that from the second residue, this sequence contains the highly conserved fingerprint of the NAD/FAD-binding domain of many NAD/FAD-binding enzymes (Wierenga, R. K., Terpstra, P., and Hol, W. G. S. (1986) J. Mol. Biol. 187, 101-107). This suggests that the SQR enzyme is a flavoprotein which contains binding sites for sulfide and quinone and that the electron transfer between the two is mediated by FAD.

Design and synthesis of bifunctional isothiocyanate analogs of sulforaphane: correlation between structure and potency as inducers of anticarcinogenic detoxication enzymes

Thirty-five bifunctional isothiocyanates were synthesized as structural analogs of sulforaphane [(-)-1-isothiocyanato-4(R)-(methylsulfinyl)butane] that was recently isolated from broccoli as the principal and very potent inducer of detoxication (phase 2) enzymes in mouse tissues and murine hepatoma cells (Hepa 1c1c7) in culture (Zhang, Y.; Talalay, P.; Cho, C.-G.; Posner, G.H. Proc. Natl. Acad. Sci. U.S.A. 1992, 89, 2399-2403). Determination of the potency of each analog in inducing NAD(P)H:quinone reductase, a phase 2 detoxication enzyme, has allowed generalizations concerning the relation of structure and activity. The most potent analogs were bifunctional derivatives in which the isothiocyanate group was separated from a methylsulfonyl or an acetyl group by three or four carbon atoms, and in some of which these groups were conformationally restricted. Among these analogs, the bicyclic ketoisothiocyanate (+/-)-exo-2-acetyl-6-isothiocyanatonorbornane (30) was a very potent inducer (comparable to sulforaphane) of quinone reductase in hepatoma cells, and it also induced both quinone reductase and glutathione transferases in several mouse organs in vivo. This and related bicyclic ketoisothiocyanates represent potent phase 2 enzyme inducers that are relatively easily synthesized and that may be more stable metabolically than the natural sulfoxide sulforaphane.

Effects of arsenite treatment on NAD(P)H:quinone acceptor oxidoreductase activity in liver, lung, kidney, and heart of the rat. Comparison to induction by the polyaromatic hydrocarbon, beta-naphthoflavone

Arsenite is a potent toxin, a carcinogen, and an inducer of heat shock proteins. In this study we found that arsenite is also a novel inducer of NAD(P)H:quinone acceptor oxidoreductase (QOR) [EC 1.6.99.2] in both liver and kidney. The increases in activity were unlinked to those caused by prior treatment with the polyaromatic hydrocarbon inducer, beta-naphthoflavone suggesting different mechanisms of induction. A single dose of sodium arsenite (75 mumol/kg sc) caused a 4-fold and 2-fold increase in activity in kidney and liver, respectively, whereas beta-naphthoflavone (60 mg/kg ip once daily for 4 days) caused a 10-fold and 4.7-fold increase in kidney and liver, respectively. This is the first study of a metalloid inducing QOR activity. Arsenite is chemically unlike any other inducer described for QOR, which include phenolic antioxidants and Michael acceptors, polyaromatic hydrocarbons, and hydrogen peroxide. Arsenite also increased glutathione S-transferase [EC 2.5.1.18] activity in rat kidney. Arsenite could be inducing QOR in liver and kidney and the glutathione S-transferase activity in kidney by an oxidant stress mechanism.

Induction of quinone reductase and glutathione in bone marrow cells by 1,2-dithiole-3-thione: effect on hydroquinone-induced cytotoxicity

Stromal cells from bone marrow are susceptible to toxicity induced by several redox-active metabolites of benzene, including hydroquinone (HQ). We have previously shown that tert-butyl-hydroquinone (tBHQ) can induce quinone reductase (QR) in bone marrow stroma as well as protect stromal cells against HQ-induced toxicity. Current studies investigate the underlining mechanisms of chemoprotection against HQ in DBA/2- and C57Bl/6-derived bone marrow stromal cells. The chemoprotector 1,2-dithiole-3-thione (DTT) has been used in these studies due to tBHQ toxicity to stromal cells at higher concentrations. Pretreatment of cells with DTT prior to HQ administration protected cells against HQ-induced toxicity. DTT induced QR activity in a dose-dependent manner in stromal cells from both strains of mice. However, there were no corresponding changes in glutathione transferase activity. DTT also increased cytosolic glutathione (GSH) concentrations by approximately 85% in both strains. Since bone marrow stroma consists primarily of fibroblasts and macrophages, we also evaluated QR activity in the separate cell types from the two strains of mice. There were differences in basal and DTT-induced QR activity between fibroblasts and macrophage cells derived from the same strain of mice, as well as the expected differences between strains. Additionally, dicoumarol, an inhibitor of QR activity, potentiated HQ-induced toxicity in both strains of bone marrow stromal cells. Thus, cellular glutathione, QR activity, and their inducibility by chemoprotective agents such as DTT may prove to be important factors in chemically induced bone marrow toxicity and carcinogenicity.

Dinitropyrene nitroreductase activity of purified NAD(P)H-quinone oxidoreductase: role in rat liver cytosol and induction by Aroclor-1254 pretreatment

Dinitropyrenes (DNP) are potent bacterial mutagens in the Ames test and genotoxins in cultured mammalian cells. Rat liver cytosol contains nitroreductases that are critical in the activation of DNP to the ultimate DNA-binding species. In order to study the nature and inducibility of liver cytosolic enzymes involved in the activation of DNP, cytosolic nitroreductase activities towards three DNP isomers (1,3-, 1.6- and 1,8-DNP) were determined in Aroclor-pretreated and untreated rats. Aroclor-1254 pretreatment resulted in up to 5-fold induction of cytosolic DNP nitroreductase. This induction was reflected in at least a 15-fold increase in cytosolic NAD(P)H-quinone oxidoreductase (NQOR) (E.C. 1.6.99.2) activity. The rates of nitroreduction for the three DNP isomers followed the order 1,6- greater than 1,8- greater than 1,3-DNP in all cases studied. 1,6-DNP nitroreductase coeluted with NQOR activity upon affinity purification. Highly purified NQOR catalyzed the NADH- and NADPH-dependent reduction of each of the three DNP isomers and displayed the same stereospecificity as the cytosolic activity. These results provide evidence that NQOR participates in the cytosolic nitroreduction of DNP and constitutes a major part of the total DNP nitroreductase activity upon induction of NQOR by Aroclor-1254 pretreatment. Thus, the role of NQOR in the metabolism of these mutagens depends significantly upon the degree to which this enzyme is induced.

Induction of cytochrome P450 and other drug metabolizing enzymes in rat liver following dietary exposure to Aroclor 1254

Selected drug metabolizing activities were measured in female F344/NCr rats exposed to graded dietary concentrations of Aroclor 1254 (1 to 1000 ppm) for 7 days or to lower concentrations of Aroclor (1 to 10 ppm) for up to 28 days. Following the 7-day exposure, the hepatic O-dealkylation of ethoxyresorufin (ETR), mediated primarily by cytochrome P450IA, was increased 60-, 10-, and 4-fold by 33, 10, and 3 ppm Aroclor, respectively. In rats exposed to 10 and 3 ppm Aroclor for 28 days, this activity was increased approximately 30- and 10-fold, respectively. Hepatic ETR O-dealkylase activities correlated with Aroclor concentrations in the livers of exposed rats (r = 0.99, p less than 0.01). Although the O-dealkylation of benzyloxyresorufin was highly increased by 7-days dietary exposure to 1000 ppm Aroclor, the levels of Aroclor necessary for detection of induction were substantially higher than those required for detection of ETR O-dealkylase induction. Examination of the non-P450-mediated drug metabolizing activities, epoxide hydrolase and DT-diaphorase, similarly showed limited (approximately 10-fold) increases. In contrast, aldehyde dehydrogenase (benzaldehyde, NADP+) activity was highly increased (greater than 40-fold) at 1000 ppm, however this activity was increased to only a limited extent at lower Aroclor concentrations (e.g. approximately 3-fold at 33 ppm). These results support the potential use of cytochrome P450 activities as potential biomarkers for environmental exposure to PCBs and related compounds.

The potency of inducers of NAD(P)H:(quinone-acceptor) oxidoreductase parallels their efficiency as substrates for glutathione transferases. Structural and electronic correlations

Induction of glutathione transferases (EC. 2.5.1.18), NAD(P)H:(quinone-acceptor) oxidoreductase (EC 1.6.99.2; quinone reductase) and other detoxification enzymes is a major mechanism for protecting cells against the toxicities of electrophiles, including many carcinogens. Although inducers of these two enzymes belong to many different chemical classes, they nevertheless contain (or acquire by metabolism) electrophilic centres that appear to be essential for inclusive activity, and many inducers are Michael reaction acceptors [Talalay, De Long & Prochaska (1988) Proc. Natl. Acad. Sci. U.S.A., 85, 8261-8265]. The inducers therefore share structural and electronic features with glutathione transferase substrates. To define these features more precisely, we examined the inductive potencies (by measuring quinone reductase in murine hepatoma cells) of two types of glutathione transferase substrates: a series of 1-chloro-2-nitrobenzenes bearing para-oriented electron-donating or -withdrawing substituents and a wide variety of other commonly used and structurally unrelated glutathione transferase substrates. We conclude that virtually all glutathione transferase substrates are inducers, and their potencies in the nitrobenzene series correlate linearly with the Hammett sigma or sigma- values of the aromatic substituents, precisely as previously reported for their efficiencies as glutathione transferase substrates. More detailed information on the electronic requirements for inductive activity was obtained with a series of methyl trans-cinnamates bearing electron-withdrawing or -donating substituents on the aromatic ring, and in which the electronic densities at the olefinic and adjacent carbon atoms were measured by 13C n.m.r. Electron-withdrawing meta-substituents markedly enhance inductive potency in parallel with their increased non-enzymic reactivity with GSH. Thus, methyl 3-bromo-, 3-nitro- and 3-chloro-cinnamates are 21, 14 and 8 times more potent inducers than the parent methyl cinnamate. This finding permits the design of more potent inducers, which are important for elucidation of the molecular mechanisms of induction.

Cutaneous NAD(P)H: quinone reductase: a xenobiotica-metabolizing enzyme with potential cancer and oxidation stress-protecting properties

The toxicity of quinones--including certain chemotherapeutic agents such as doxorubicin--have been related to the enzymatic or nonenzymatic formation of the corresponding semiquinones and their subsequent reaction with molecular oxygen yielding superoxide anion radicals by spontaneous regenerating of the quinones. This semiquinone redox cycling is prevented by the NAD(P)H:quinone reductase (NQR; EC 1.6.99.2) because it mediates a 2-electron reduction which results in the formation of hydroquinones instead of semiquinones. Interestingly, inducers of this enzyme such as butylated hydroxytoluene protect against the severe ulceration of accidental infiltration of doxorubicin into the area around the intravenous infusion. Recently, it has been shown that this highly protective enzyme has a very high basal activity in the epidermis which is in the same range as in the liver. The human gene of the NQR is localized on chromosome 16 and has been cloned recently as well as the gene of the murine liver NQR. We determined NQR in the cytoplasma of murine skin, liver, and human keratinocytes using 2,6-dichlorophenol-indophenol as substrate. In order to characterize this enzyme, induction by polycyclic hydrocarbones and inhibition with several known inhibitors of dihydrodiol dehydrogenase, aldo-keto and carbonyl reductase activities were determined. There was a similar pattern of inhibition of the basal and induced activity in all tissues so far investigated. Pyrazole, progesterone and phenobarbital did not inhibit; however, rutin and indomethacin inhibited dose-dependently. The most potent inhibitor was dicoumarol. These findings suggest that the same enzymatic form is present in liver and skin, and in murine skin and human keratinocytes.

Induction of glutathione transferases and NAD(P)H:quinone reductase by fumaric acid derivatives in rodent cells and tissues

Dimethyl fumarate and dimethyl maleate are potent inducers of cytosolic NAD(P)H:(quinone acceptor) oxidoreductase (here designated quinone reductase) activity in Hepa 1c1c7 murine hepatoma cells in culture, whereas fumaric and maleic acids are much less potent, in agreement with the much greater reactivity of the esters as Michael reaction acceptors (P. Talalay, M. J. De Long, and H. J. Prochaska, Proc. Natl. Acad. Sci. USA, 85:8261-8265, 1988). Dimethyl fumarate also induced quinone reductase in mutants of the Hepa 1c1c7 cell line that were either defective in the Ah receptor or in cytochrome P1-450 activity, thereby establishing that this compound is a monofunctional inducer (H. J. Prochaska and P. Talalay, Cancer Res., 48: 4776-4782, 1988). Addition of dimethyl fumarate to the diet of female CD-1 mice and female Sprague-Dawley rats at 0.2-0.5% concentrations elevated cytosolic glutathione transferases and quinone reductase activities in a variety of organs, whereas much higher concentrations of fumaric acid were only marginally active. The widespread induction of such detoxication enzymes by dimethyl fumarate suggests the potential value of this compound as a protective agent against chemical carcinogenesis and other forms of electrophile toxicity. This proposal is supported by the finding that the concentrations of dimethyl fumarate required to obtain substantial enzyme inductions were well tolerated by rodents. Furthermore, the parent fumaric acid has low chronic toxicity and is a naturally occurring metabolic intermediate that is already in the food chain as an additive, and fumarate salts and esters are used for therapeutic purposes in man.

Characterization of FMN-dependent NADH-quinone reductase induced by menadione in Escherichia coli

It was found that when Escherichia coli is grown in the presence of 0.2-0.3 mM menadione (2-methyl-1,4-naphthoquinone), an FMN-dependent NADH-quinone reductase increases more than 20-fold in the cytoplasmic fraction. The menadione-induced quinone reductase was isolated from the cytoplasmic fraction of induced cells. The purified enzyme had an Mr of 24 kDa on SDS-polyacrylamide gel electrophoresis. The enzyme required flavin as a cofactor and a half-maximum activity was obtained with 0.54 microM FMN or 16.5 microM FAD. The enzyme had a broad pH optimum at pH 7.0-8.0 and reacted with NADH, but not with NADPH. The reaction followed a ping-pong mechanism and the intrinsic Km values for NADH and menadione were estimated to be 132 microM and 2.0 microM, respectively. Dicoumarol was a simple competitive inhibitor with respect to NADH with a Ki value of 0.22 microM. The electron acceptor specificity of this enzyme was very similar to that of NAD(P)H: (quinone acceptor) oxidoreductase (EC 1.6.99.2, DT-diaphorase) from rat liver. Since menadione is reduced by the two-electron reduction pathway to menadiol, the induction of this enzyme is likely to be an adaptive response of E. coli to partially alleviate the toxicity of menadione.

Glucocorticoid regulation of polycyclic aromatic hydrocarbon induction of cytochrome P450IA1, glutathione S-transferases, and NAD(P)H:quinone oxidoreductase in cultured fetal rat hepatocytes

The regulation of polycyclic aromatic hydrocarbon-inducible enzymes, cytochrome P450IA1, NAD(P)H:quinone oxidoreductase, and glutathione S-transferases, by glucocorticoids was investigated using primary fetal rat hepatocyte culture. Treatment of cells in culture with 1,2-benzanthracene (100 microM, 72 hr) resulted in 60-, 2-, and 6-fold increases in cytochrome P450IA1, glutathione S-transferase, and NAD(P)H:quinone reductase activities, respectively. The inductive effect of 1,2-benzanthracene on cytochrome P450IA1 and glutathione S-transferase (1-chloro-2,4-dinitrobenzene conjugation) activities was potentiated approximately 3- and 2- to 3-fold, respectively, when dexamethasone (0.01-1 microM) was included in the culture medium. In contrast, 1 microM dexamethasone was found not to potentiate the induction of NAD(P)H:quinone oxidoreductase activity by 1,2-benzanthracene. Treatment of cultured hepatocytes with dexamethasone alone, at concentrations of up to 100 microM, resulted in a 2- to 4-fold increase in glutathione S-transferase and NAD(P)H:quinone oxidoreductase activity. Both the induction of glutathione S-transferase activity by high concentrations of dexamethasone alone and the potentiation of 1,2-benzanthracene induction by lower concentrations of dexamethasone were observed for other steroids of the glucocorticoid class in conjunction with a variety of polycyclic aromatic hydrocarbons. Western immunoblot analyses indicated that low concentrations of dexamethasone (0.1-1 microM) potentiated 1,2-benzanthracene-dependent induction of cytochrome P450IA1, glutathione S-transferase Ya/Yc subunit and NAD(P)H:quinone oxidoreductase content. Additionally, increased glutathione S-transferase activity in response to concentrations of dexamethasone exceeding 1 microM was associated with concomitant increases in Ya/Yc and Yb subunit content. Potentiation of polycyclic aromatic hydrocarbon induction of cytochrome P450IA1, glutathione S-transferase, and NAD(P)H:quinone oxidoreductase protein content by low concentrations of glucocorticoids and induction of glutathione S-transferase and NAD(P)H:quinone oxidoreductase by high concentrations of glucocorticoids alone indicates the importance of these endogenous compounds in the regulation of some hepatic enzymes involved in xenobiotic metabolism.

Assembly kinetics and identification of precursor proteins of complex I from Neurospora crassa

Complex I from Neurospora crassa was fractionated using chaotropic agents and various chromatographic techniques. Several subunits were isolated. Polyclonal antibodies directed against the holocomplex or individual subunits were raised in rabbits, and employed to analyse the composition and assembly of this respiratory chain enzyme in vivo. N. crassa cells were pulse-labelled with radioactive amino acids. The time course of incorporation of radioactivity into complex-I polypeptides was studied by immunoprecipitation. The labelling kinetics of whole complex I was found to be similar to that of cytochrome oxidase, displaying a half-maximal labelling time of 10 min. Newly synthesized individual polypeptide subunits (about 23 species) assembled into the holoenzyme at markedly different rates. Two mitochondrially synthesized proteins, a 29-kDa polypeptide (the ND-1 gene product) and a 12-kDa polypeptide were the fastest components to appear in the enzyme. We estimate that the precursor pool sizes of all components range between 1-25% of the amounts present in the final complex. Precursors of polypeptides of complex I were synthesized in an heterologous cell-free system and immunoprecipitated with subunit specific antibodies. Six isolated precursors were compared with the corresponding mature proteins. It appears that four subunits (apparent molecular masses of 22, 25, 31 and 33 kDa) are initially synthesized as larger-molecular-mass precursors. Two subunits (apparent molecular masses of 12.5 and 14 kDa) are made with the same size as their mature forms.

A small isoform of NADH:ubiquinone oxidoreductase (complex I) without mitochondrially encoded subunits is made in chloramphenicol-treated Neurospora crassa

In mitochondria of Neurospora crassa grown in the presence of chloramphenicol a small form of NADH:ubiquinone reductase is made in place of the normal electron-transfer-complex I. This smaller enzyme has a molecular mass of approximately 350 kDa and consists of (at least) 13 different subunits which are all synthesized in the cytoplasm. The complex I which is normally found in Neurospora has a molecular mass of approximately 700 kDa and consists of around 30 different subunits, of which at least six are made in the mitochondria. Immunoblotting and peptide mapping suggest that the subunits of the small enzyme are homologous to subunits of the large enzyme, one subunit might even be identical. The small and the large NADH:ubiquinone reductases have the same high-affinity binding site for NADH but the two enzymes differ in the affinity and inhibitor sensitivity of the ubiquinone-binding site. The possibility is discussed that the small NADH:ubiquinone reductase is primitive isoform of complex I.

Mouse strain variations in the magnitude of induction of liver DT-diaphorase and hereditary transmission of the trait

1. Strain variations among mice in terms of cytosolic DT-diaphorase activity were studied in liver, kidney, stomach and heart tissues with or without the administration of 3-tert-butyl-4-hydroxyanisole (BHA). 2. BHA induced DT-diaphorase activity in all strains examined, and the magnitude of induction varied depending on the strain and tissue. Among the 10 inbred strains tested, BALB/c and C57BL mice showed relatively large magnitudes of induction for liver DT-diaphorase, whereas C3H and CBA mice showed relatively small magnitudes. 3. Results of examinations of BALB/c-C3H-F1, -F2 and C57BL-CBA-F1 mice revealed that smaller magnitudes of induction of liver DT-diaphorase were inherited essentially as a dominant trait. The hereditary trait could be adequately explained by postulating two gene loci that regulate the magnitude of induction. 4. The possible significance of DT-diaphorase activity in chemical carcinogenesis was discussed.