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Experiments using purified recombinant human NAD(P)H:quinone oxidoreductase 1 (NQO1) revealed that the auto-oxidation of fully reduced protein resulted in a 1:1 stoichiometry of oxygen consumption to NADH oxidation with the production of hydrogen peroxide. The rate of auto-oxidation of fully reduced NQO1 was markedly accelerated in the presence of superoxide (O 2 . ), whereas the addition of superoxide dismutase greatly inhibited the rate of auto-oxidation.

The quinone pharmacophore is present in many drug classes but is particularly common among antitumor drugs. Many quinones serve essentially as pro-drugs and exert their activities after reduction. Reduction of quinones may generate semiquinones or hydroquinones with subsequent generation of reactive oxygen radicals and oxidative stress, quinones can be designed so they lose a leaving group when reduced to the hydroquinone generating a reactive electrophile or the hydroquinone form of the molecule may have greater pharmacological activity than the parent quinone against a particular target. Enzyme systems that reduce quinones therefore become critically important in the pharmacological activity of this class of drugs. There are a number of enzyme systems that can catalyze reduction of quinones including cytochrome P450 reductase, cytochrome b5 reductase, NAD(P)H:quinone oxidoreductase 1 (NQO1), NAD(P)H:quinone oxidoreductase 2 (NQO2), carbonyl reductases, and thioredoxin reductase. In this context, one of the most extensively studied reductases has been NAD(P)H:quinone oxidoreductase 1 (NQO1). In this review we will focus on the role of NQO1 in the bioactivation of clinically important quinones mitomycin C, β-lapachone and 17AAG as well as the influence of the NQO1*2 polymorphism on the sensitivity and resistance to these agents.

We have examined the role of NAD(P)H:quinone oxidoreductase 1 (NQO1) in the bioreductive metabolism of 17-allylamino-demethoxygeldanamycin (17-AAG). High-performance liquid chromatography (HPLC) analysis of the metabolism of 17-AAG by recombinant human NQO1 revealed the formation of a more polar metabolite 17-AAGH 2 . The formation of 17-AAGH 2 was NQO1 dependent, and its formation could be inhibited by the addition of 5-methoxy-1,2-dimethyl-3-[(4-nitrophenoxy)methyl]indole-4,7-dione (ES936), a mechanismbased (suicide) inhibitor of NQO1. The reduction of 17-AAG to the corresponding hydroquinone 17-AAGH 2 was confirmed by tandem liquid chromatography-mass spectrometry. 17-AAGH 2 was relatively stable and only slowly underwent autooxidation back to 17-AAG over a period of hours. To examine the role of NQO1 in 17-AAG metabolism in cells, we used an isogenic pair of human breast cancer cell lines differing only in NQO1 levels. MDA468 cells lack NQO1 due to a genetic polymorphism, and MDA468/NQ16 cells are a stably transfected clone that express high levels of NQO1 protein. HPLC analysis of 17-AAG metabolism using cell sonicates and intact cells showed that 17-AAGH 2 was formed by MDA468/NQ16 cells, and formation of 17-AAGH 2 could be inhibited by ES936. No 17-AAGH 2 was detected in sonicates or intact MDA468 cells. Following a 4-hour treatment with 17-AAG, the MDA468/NQ16 cells were 12-fold more sensitive to growth inhibition compared with MDA468 cells. More importantly, the increased sensitivity of MDA468/NQ16 cells to 17-AAG could be abolished if the cells were pretreated with ES936. Cellular markers of heat shock protein (Hsp) 90 inhibition, Hsp70 induction, and Raf-1 degradation were measured by immunoblot analysis. Marked Hsp70 induction and Raf-1 degradation was observed in MDA468/NQ16 cells but not in MDA468 cells. Similarly, downstream Raf-1 signaling molecules mitogen-activated protein kinase/extracellular signal-regulated kinase (ERK) kinase and ERK also showed decreased levels of phosphorylation in MDA468/NQ16 cells but not in MDA468 cells. The ability of 17-AAG and 17-AAGH 2 to inhibit purified yeast and human Hsp90 ATPase activity was examined. Maximal 17-AAG-induced ATPase inhibition was observed in the presence of NQO1 and could be abrogated by ES936, showing that 17-AAGH 2 was a more potent Hsp90 inhibitor compared with 17-AAG. Molecular modeling studies also showed that due to increased hydrogen bonding between the hydroquinone and the Hsp90 protein, 17-AAGH 2 was bound more tightly to the ATP-binding site in both yeast and human Hsp90 models. In conclusion, these studies have shown that reduction of 17-AAG by NQO1 generates 17-AAGH 2 , a relatively stable hydroquinone that exhibits superior Hsp90 inhibition. (Cancer Res 2005; 65(21): 10006-15)

The NAD(P)H:quinone oxidoreductase 1 (NQO1)*2 polymorphism is characterized by a single proline-to-serine amino acid substitution. Cell lines and tissues from organisms genotyped as homozygous for the NQO1*2 polymorphism are deficient in NQO1 activity. In studies with cells homozygous for the wild-type allele and cells homozygous for the mutant NQO1*2 allele, no difference in the half-life of NQO1 mRNA transcripts was observed. Similarly, in vitro transcription/translation studies showed that both wild-type and mutant NQO1 coding regions were transcribed and translated into full-length protein with equal efficiency. Protein turnover studies in NQO1 wild-type and mutant cell lines demonstrated that the half-life of wild-type NQO1 was greater than 18 h, whereas the half-life of mutant NQO1 was 1.2 h. Incubation of NQO1 mutant cell lines with proteasome inhibitors increased the amount of immunoreactive NQO1 protein, suggesting that mutant protein may be degraded via the proteasome pathway. Additional studies were performed using purified recombinant NQO1 wild-type and mutant proteins incubated in a rabbit reticulocyte lysate system. In these studies, no degradation of wild-type NQO1 protein was observed; however, mutant NQO1 protein was completely degraded in 2 h. Degradation of mutant NQO1 was inhibited by proteasome inhibitors and was ATP-dependent. Mutant NQO1 incubated in rabbit reticulocyte lysate with MG132 resulted in the accumulation of proteins with increased molecular masses that were immunoreactive for both NQO1 and ubiquitin. These data suggest that wild-type NQO1 persists in cells whereas mutant NQO1 is rapidly degraded via ubiquitination and proteasome degradation.

Summary NAD(P)H:quinone oxidoreductase (NQO1, EC 1.6.99.2) is an obligate two-electron reductase that can either bioactivate or detoxify quinones and has been proposed to play an important role in chemoprevention. We have previously characterized a homozygous point mutation in the BE human colon carcinoma cell line that leads to a loss of NOO1 activity. Sequence analysis showed that this mutation was at position 609 of the NQO1 cDNA, conferring a proline to serine substitution at position 187 of the NQO1 enzyme. Using polymerase chain reaction (PCR) analysis, we have found that the H596 human non-small-cell lung cancer (NSCLC) cell line has elevated NQO1 mRNA, but no detectable enzyme activity. Sequencing of the coding region of NQO1 from the H596 cells showed the presence of the identical homozygous point mutation present in the BE cell line. Expression and purification of recombinant wild-type and mutant protein from E. coli showed that mutant protein could be detected using immunoblot analysis and had 2% of the enzymatic activity of the wild-type protein. PCR and Northern blot analysis showed moderate to low levels of expression of the correctly sized transcript in the mutant cells. Immunoblot analysis also revealed that recombinant mutant protein was immunoreactive; however, the mutant protein was not detected in the cytosol of either BE or H596 cells, suggesting that the mutant proteins were either not translated or were rapidly degraded. The absence of any detectable, active protein, therefore, appears to be responsible for the lack of NQO1 activity in cells homozygous for the mutation. A polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP) analysis for the mutation at position 609 conducted on 90 human lung tissue samples (45 matched sets of tumour and uninvolved tissue) revealed a 7% incidence of individuals homozygous for the mutation, and 42% heterozygous for the mutation. These data suggest that the mutation at position 609 represents a polymorphism in an important xenobiotic metabolizing enzyme, which has implications for cancer therapy, chemoprevention and chemoprotection.

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