Rat liver NAD(P)H:quinone oxidoreductase: cDNA expression and site-directed mutagenesis

Attenuation of catalase activity in the malignant phenotype plays a functional role in an in vitro model for tumor progression

We have developed an in vitro model to study the molecular mechanisms of tumor progression. Using repeated treatments with ionizing radiation or N-methyl-N'-nitro-N-nitrosoguanidine (MNNG), we caused malignant progression of a papilloma producing mouse keratinocyte cell line, 308 cells. In a previous study we have shown that the malignant variants of 308 cells have elevated reactive oxygen species (ROS) levels, and have established a functional role for the pro-oxidant state in the progressed phenotype (Carcinogenesis 20 (1999) 2063). In this study, we have evaluated the status of intracellular defense mechanisms for ROS scavenging in the progressed phenotype to identify sources that contribute to their pro-oxidant state. Our results demonstrate that a reduction in several anti-oxidant defense mechanisms, including catalase and glutathione S-transferase mu, correlates with the emergence of the malignant phenotype. We provide evidence that attenuation of catalase activity may play a functional role in the malignant progression of mouse keratinocytes.

Structure-function studies of DT-diaphorase (NQO1) and NRH: quinone oxidoreductase (NQO2)

DT-diaphorase, also referred to as NQO1 or NAD(P)H: quinone acceptor oxidoreductase, is a flavoprotein that catalyzes the two-electron reduction of quinones and quinonoid compounds to hydroquinones, using either NADH or NADPH as the electron donor. NRH (dihydronicotinamide riboside): quinone oxidoreductase, also referred to as NQO2, has a high nucleotide sequence identity to DT-diaphorase and is considered to be an isozyme of DT-diaphorase. These enzymes transfer two electrons to a quinone, resulting in the formation of a hydroquinone product without the accumulation of a dissociated semiquinone. Steady and rapid-reaction kinetic experiments have been performed to determine the reaction mechanism of DT-diaphorase. Furthermore, chimeric and site-directed mutagenesis experiments have been performed to determine the molecular basis of the catalytic differences between the two isozymes and to identify the critical amino acid residues that interact with various inhibitors of the enzymes. In addition, functional studies of a natural occurring mutant Pro-187 to Ser (P187S) have been carried out. Results obtained from these investigations are summarized and discussed.

The importance of DT-diaphorase in mitomycin C resistance in human colon cancer cell lines

BACKGROUND

Prior studies have suggested the multifactorial nature of mitomycin C (MMC) resistance. However, the relative importance of the different resistance mechanisms is unknown.

MATERIALS AND METHODS

A panel of colon cancer cell lines with levels of MMC resistance from 2- to 15-fold compared to the parent line HT-29 was produced by repeated MMC exposure. Cell survival was measured using clonogenic assay. Glutathione and related enzymes and DT-diaphorase were measured using biochemical assays. P-glycoprotein expression was measured using flow cytometry. Topoisomerase II activity was measured using the pBR322 DNA relaxation assay.

RESULTS

Multiple drug resistance mechanisms were altered in the resistant cell lines (glutathione reductase, glutathione peroxidase, topoisomerase II). However, the level of DT-diaphorase correlated best with the degree of MMC resistance. The importance of DT-diaphorase was confirmed by using BMY 25282, an MMC analogue which is less dependent on DT-diaphorase for activation. Resistance in the HT-29R54 cell line was 15-fold with MMC compared to 5-fold with BMY 25282. P-glycoprotein-mediated resistance does not appear important in this model.

CONCLUSIONS

Although MMC resistance appears to be multifactorial, the results of this study strongly suggest that DT-diaphorase is the major contributor to MMC resistance under aerobic conditions. Strategies to enhance drug activation may therefore be useful for reversing MMC resistance.

Exploring the mechanistic aspects of mitomycin antibiotic bioactivation in Chinese hamster ovary cells overexpressing NADPH:cytochrome C (P-450) reductase and DT-diaphorase

We have directly demonstrated the involvement of human NADPH: cytochrome c (P-450) reductase in the aerobic/hypoxic differential toxicity of mitomycin C and porfiromycin in living cells by varying only this enzyme in a transfected cell line. In the same manner, we have implicated rat DT-diaphorase in the aerobic and hypoxic activation of mitomycin C, but found only a minor role for this enzyme in the aerobic activation of porfiromycin. DT-Diaphorase does not cause the production of an aerobic/hypoxic differential toxicity by mitomycin C, but rather activates this agent through an oxygen insensitive pathway. The evidence suggests that DT-diaphorase activates mitomycin C more effectively than porfiromycin, with porfiromycin being preferentially activated through a one-electron reductive pathway. The therapeutic potential of mitomycin antibiotics in the treatment of cancer can be envisioned to be enhanced for those tumors containing elevated levels of the bioreductive enzymes. However, cytogenetic heterogeneity within the tumor cell population and the various environmental factors which impact on bioreductive enzyme function, including pH and oxygen tension, may subvert this approach. Moreover, if high tumor levels of a drug activating enzyme reflect high levels in the normal tissues of the patient, normal tissue damage may also be enhanced with possibly no improvement in the therapeutic ratio. Approaches utilizing gene therapy, whereby a specific bioreductive catalyst is introduced into the tumor cell population via a targeting vehicle to activate a particular prodrug, may be more effective in that not only will the prodrug of choice be specifically activated in the tumor, but the source of the catalyst, be it bacterial, rodent, or human, will not be important. In fact, in the case of DT-diaphorase and mitomycin C, the rat form of the enzyme could be advantageous because it is more effective in activating mitomycin C than is the human form of this enzyme. Assuming targeted gene delivery to malignant cells, a non-host enzyme which is more effective at activating mitomycin C than the analogous host enzyme might also result in less drug activation in normal tissue and, hence, less normal tissue toxicity.

Bioactivation of mitomycin antibiotics by aerobic and hypoxic Chinese hamster ovary cells overexpressing DT-diaphorase

DT-Diaphorase catalyzes a two-electron reduction of mitomycin C (MC) and porfiromycin (POR) to reactive species. Many cell lines that overexpress DT-diaphorase and are sensitive to the mitomycins are protected from the aerobic cytotoxicity of these drugs by the DT-diaphorase inhibitor dicumarol. The cytoprotective properties of this relatively non-specific inhibitor, however, vanish under hypoxic conditions. To ascertain the role of DT-diaphorase in mitomycin bioactivation and cytotoxicity in living cells, a rat liver DT-diaphorase cDNA was transfected into Chinese hamster ovary cells. MC was equitoxic to the parental cells under oxygenated and hypoxic conditions. In contrast, POR was less toxic than MC to these cells under aerobic conditions, but significantly more toxic than MC under hypoxia. Two DT-diaphorase-transfected clones displayed increases in DT-diaphorase activity of 126- and 133-fold over parental cells. The activities of other oxidoreductases implicated in mitomycin bioreduction were unchanged. MC was more toxic to both DT-diaphorase-transfected lines than to parental cells; the toxicity of MC to the transfected lines was similar in air and hypoxia. POR was also more toxic to the DT-diaphorase-elevated clones than to parental cells under oxygenated conditions. Under hypoxia, however, the toxicity of POR to the transfected clones was unchanged from that of parental cells. The findings implicate DT-diaphorase in mitomycin bioactivation in living cells, but suggest that this enzyme does not contribute to the differential toxicity of MC or POR in air and hypoxia.

Active site studies of DT-diaphorase employing artificial flavins

NAD(P)H:quinone oxidoreductase (EC 1.6.99.2) (DT-diaphorase) is an FAD-containing enzyme that catalyzes the 2-electron reduction of quinones to hydroquinones using either NADH or NADPH as the electron donor. In this study, FAD was removed by dialyzing the holoprotein against 2 M KBr, and synthetic analogs of FAD were substituted in the flavin binding site as structural probes. Spectral analysis indicates that the benzoquinoid forms of 8-mercapto-FAD and 6-mercapto-FAD are stabilized on binding to the enzyme. This is consistent with the fact that the native flavoprotein forms the anion flavin radical upon photoreduction and suggests the presence of a positive charge near the N(1)C(2)O position of the isoalloxazine ring. Reactivity studies using 8-chloro- and 8-mercapto-flavins suggest that the 8 position of the FAD is accessible to the solvent. However, the rates of the reactions were dramatically decreased in the presence of the competitive inhibitor, dicumarol. 6-Mercapto-, 6-thiocyanato-, 6-azido-, and 6-amino-flavins were also used as structural probes. The results indicate that the 6 position is accessible to solvent. Dicumarol binding increases the pK alpha of the enzyme-bound 6-mercapto-flavin from below pH 5.0 to higher than pH 9.0. The results suggest that DT-diaphorase shows the same properties as the C-C transhydrogenases, and the binding of dicumarol elicits a conformational change or an adjustment in the polarity of the FAD pocket. The enzyme reconstituted with oxidized 5-deaza-FAD has significant catalytic activity, confirming that DT-diaphorase is an obligatory 2-electron transfer enzyme and plays a role in the detoxification of quinones and quinoid compounds by reducing them to the relatively stable hydroquinones.

Mouse liver NAD(P)H:quinone acceptor oxidoreductase: protein sequence analysis by tandem mass spectrometry, cDNA cloning, expression in Escherichia coli, and enzyme activity analysis

The amino acid sequence of mouse liver NAD(P)H:quinone acceptor oxidoreductase (EC 1.6.99.2) has been determined by tandem mass spectrometry and deduced from the nucleotide sequence of the cDNA encoding for the enzyme. The electrospray mass spectral analyses revealed, as previously reported (Prochaska HJ, Talalay P, 1986, J Biol Chem 261:1372-1378), that the 2 forms--the hydrophilic and hydrophobic forms--of the mouse liver quinone reductase have the same molecular weight. No amino acid sequence differences were found by tandem mass spectral analyses of tryptic peptides of the 2 forms. Moreover, the amino-termini of the mouse enzymes are acetylated as determined by tandem mass spectrometry. Further, only 1 cDNA species encoding for the quinone reductase was found. These results suggest that the 2 forms of the mouse quinone reductase have the same primary sequences, and that any difference between the 2 forms may be attributed to a labile posttranslational modification. Analysis of the mouse quinone reductase cDNA revealed that the enzyme is 273 amino acids long and has a sequence homologous to those of rat and human quinone reductases. In this study, the mouse quinone reductase cDNA was also ligated into a prokaryotic expression plasmid pKK233.2, and the constructed plasmid was used to transform Escherichia coli strain JM109. The E. coli-expressed mouse quinone reductase was purified and characterized. Although mouse quinone reductase has an amino acid sequence similar to those of the rat and human enzymes, the mouse enzyme has a higher NAD(P)H-menadione reductase activity and is less sensitive to flavones and dicoumarol, 2 known inhibitors of the enzyme.(ABSTRACT TRUNCATED AT 250 WORDS)

A two-domain structure for the two subunits of NAD(P)H:quinone acceptor oxidoreductase

NAD(P)H:quinone acceptor oxidoreductase (EC 1.6.99.2) (DT-diaphorase) is a FAD-containing reductase that catalyzes a unique 2-electron reduction of quinones. It consists of 2 identical subunits. In this study, it was found that the carboxyl-terminal portion of the 2 subunits can be cleaved by various proteases, whereas the amino-terminal portion cannot. It was also found that proteolytic digestion of the enzyme can be blocked by the prosthetic group FAD, substrates NAD(P)H and menadione, and inhibitors dicoumarol and phenindione. Interestingly, chrysin and Cibacron blue, 2 additional inhibitors, cannot protect the enzyme from proteolytic digestion. The results obtained from this study indicate that the subunit of the quinone reductase has a 2-domain structure, i.e., an amino-terminal compact domain and a carboxyl-terminal flexible domain. A structural model of the quinone reductase is generated based on results obtained from amino-terminal and carboxyl-terminal protein sequence analyses and electrospray mass spectral analyses of hydrolytic products of the enzyme generated by trypsin, chymotrypsin, and Staphylococcus aureus protease. Furthermore, based on the data, it is suggested that the binding of substrates involves an interaction between 2 structural domains.

Expression of rat liver NAD(P)H:quinone-acceptor oxidoreductase in Escherichia coli and mutagenesis in vitro at Arg-177

A prokaryotic expression plasmid, pKK-DT2, containing the cDNA of rat liver NAD(P)H:quinone-acceptor oxidoreductase (EC 1.6.99.2; DT-diaphorase) was constructed and used to transform Escherichia coli strain JM109. The rat liver quinone reductase was expressed in strain in JM109 and was inducible with isopropyl beta-D-thiogalactopyranoside (IPTG). The expressed rat protein was purified by affinity chromatography and had kinetic and physical properties identical with the protein purified from rat liver in that it could utilize either NADH or NADPH as the electron donor and its activity was inhibited by dicoumarol. In addition, we have generated four mutants, Arg-177----His (R177H), Arg-177----Ala (R177A), Arg-177----Cys (R177C) and Arg-177----Leu (R177L), using this expression system. Several of the mutants behaved anomalously on SDS/PAGE, but all of the mutant proteins had the expected M(r) as determined by electrospray m.s. These results and those obtained from enzyme kinetic analysis, u.v./visible absorption spectral analysis, and flavin and tryptophan fluorescence analysis of the wild-type enzyme and four mutants indicated that mutations at Arg-177 changed the conformation of the enzyme, resulting in a decrease in enzyme activity. Replacing Arg-177 with leucine altered the protein conformation and decreased FAD incorporation.

The role of NAD(P)H:quinone oxidoreductase in mitomycin C- and porfiromycin-resistant HCT 116 human colon-cancer cells

A mitomycin C (MMC)- and porfiromycin (PFM)-resistant subline of the HCT 116 human colon-cancer cell line was isolated after repeated exposure of HCT 116 cells to increasing concentrations of MMC under aerobic conditions. The MMC-resistant subline (designated HCT 116-R30A) was 5 times more resistant than the parent cells to MMC and PFM under aerobic conditions. Both the MMC-resistant cells and the parent HCT 116 cells accumulated similar amounts of PFM by passive diffusion, but levels of macromolecule-bound PFM were about 50% lower in the resistant cell line, implying a decrease in PFM reductive activation in the resistant cells. The finding that microsomes from either sensitive or resistant cells showed an equal ability to reduce MMC and PFM indicated that the activity of NADPH cytochrome P-450 reductase (EC 1.6.2.4) was not changed in the resistant subline. Soluble extracts of HCT 116 cells reduced MMC and PFM more effectively at pH 6.1, and NADH and NADPH were utilized equally well as electron donors under both aerobic and anaerobic conditions. These data suggest that quinone reductase (EC 1.6.99.2; DT-diaphorase) in soluble extracts is responsible for the reduction of MMC. Quinone reductase activities in soluble extracts of HCT 116-R30A cells for the reduction of dichlorophenol indophenol (DCPIP) and menadione-cytochrome c at optimal pHs were decreased by 95% as compared with those obtained in parent cells. However, the MMC-reducing activity of HCT 116-R30A soluble extracts was only 50% lower than that of the parent cell extracts. The kinetic constants (Km, Vmax) found for quinone reductase in the two cell lines with respect to the substrates DCPIP and menadione differed. Two species of mRNA for quinone reductase (2.7 and 1.2 kb) were detected in both cell lines, and there was no detectable difference between parent and resistant cells in the steady-state level of either of these mRNA species. Furthermore, incubation with the quinone reductase inhibitor dicoumarol rendered HCT 116 cells more resistant to MMC. Alteration of the quinone reductase activity in HCT 116-R30A cells appears to be the mechanism responsible for their resistance to MMC and PFM.

NAD(P)H:quinone oxidoreductase (DT-diaphorase) activity and mRNA content in normal and neoplastic mouse lung epithelia

DT-diaphorase (DTD) is a flavoprotein which catalyzes obligate two-electron reduction of a diverse group of substrates. We have reported previously that non-tumorigenic mouse lung alveolar type-II pneumocytes have high DTD activity, while cell lines derived from lung tumors do not. In contrast, other investigators, using human lung tissue, reported increased DTD activity in tumors compared with normal tissue. We therefore investigated DTD associated with mouse lung neoplasia in vivo as well as in vitro. Pulmonary tumors had far less DTD activity compared with normal mouse lung. Correspondingly, a tumorigenic mouse lung cell line which arose as a spontaneous transformant of a normal cell line had very low DTD activity compared with non-tumorigenic lung cells. DTD-specific mRNA levels were also much higher in normal cell lines than in neoplastic ones. DTD was localized histochemically in type-II pneumocytes in situ, but was not observed by this technique in normal bronchiolar epithelia or in tumor cells. These data show that, unlike what has been observed in human lung cancer, a marked decrease in DTD content and activity accompanied mouse lung tumorigenesis in vivo and neoplastic transformation in vitro.