Immunohistochemical analysis of NAD(P)H:quinone oxidoreductase and NADPH cytochrome P450 reductase in human superficial bladder tumours: Relationship between tumour enzymology and clinical outcome following intravesical mitomycin C therapy

Inactivation of apaziquone by haematuria: implications for the design of phase III clinical trials against non-muscle invasive bladder cancer

Purpose

Despite positive responses in phase II clinical trials, the bioreductive prodrug apaziquone failed to achieve statistically significant activity in non-muscle invasive bladder cancer in phase III trials. Apaziquone was administered shortly after transurethral resection and here we test the hypothesis that haematuria inactivates apaziquone.

Methods

HPLC analysis was used to determine the ability of human whole blood to metabolise apaziquone ex vivo. An in vitro model of haematuria was developed and the response of RT112 and EJ138 cells following a 1-h exposure to apaziquone was determined in the presence of urine plus or minus whole blood or lysed whole blood.

Results

HPLC analysis demonstrated that apaziquone is metabolised by human whole blood with a half-life of 78.6 ± 23.0 min. As a model for haematuria, incubation of cells in media containing up to 75% buffered (pH 7.4) urine and 25% whole blood was not toxic to cells for a 1-h exposure period. Whole blood (5% v/v) significantly (p < 0.01) reduced the potency of apaziquone in this experimental model. Lysed whole blood also significantly (p < 0.05) reduced cell growth, although higher concentrations were required to achieve an effect (15% v/v).

Conclusions

The results of this study demonstrate that haematuria can reduce the potency of apaziquone in this experimental model. These findings impact upon the design of further phase III clinical trials and strongly suggest that apaziquone should not be administered immediately after transurethral resection of non-muscle invasive bladder cancer when haematuria is common.

Synthesis, characterization and antineoplastic activity of bis-aziridinyl dimeric naphthoquinone - A novel class of compounds with potent activity against acute myeloid leukemia cells

The synthesis, characterization and antileukemic activity of rationally designed amino dimeric naphthoquinone (BiQ) possessing aziridine as alkylating moiety is described. Bis-aziridinyl BiQ decreased proliferation of acute myeloid leukemia (AML) cell lines and primary cells from patients, and exhibited potent (nanomolar) inhibition of colony formation and overall cell survival in AML cells. Effective production of reactive oxygen species (ROS) and double stranded DNA breaks (DSB) induced by bis-aziridinyl BiQ is reported. Bis-dimethylamine BiQ, as the isostere of bis-aziridinyl BiQ but without the alkylating moiety did not show as potent anti-AML activity. Systemic administration of bis-aziridinyl BiQ was well tolerated in NSG mice.

EO9 (Apaziquone): from the clinic to the laboratory and back again

EO9 (Apaziquone) is a bioreductive drug that has a chequered history. It underwent clinical trial but failed to show activity in phase II clinical trials when administered i.v. Poor drug delivery to tumours caused by a combination of rapid pharmacokinetic elimination and poor penetration through avascular tissue were the major factors responsible for EO9's poor efficacy. Based upon an understanding of why EO9 failed, a further clinical trial against patients with superficial transitional cell carcinoma of the bladder was conducted. The rationale for this was that intravesical administration directly into the bladder would circumvent the drug delivery problem, and any drug reaching the blood supply would be rapidly cleared thereby reducing the risk of systemic exposure. EO9 was well tolerated, and clinical activity against marker lesions was recorded in both phase I and II clinical trials. This article charts the pharmacological history of EO9 and discusses the potential implications that 'the EO9 story' has for the development of other loco-regional therapies.

NAD(P)H:quinone oxidoreductase 1 (NQO1) in the sensitivity and resistance to antitumor quinones

Quinones represent a large and diverse class of antitumor drugs and many quinones are approved for clinical use or are currently undergoing evaluation in clinical trials. For many quinones reduction to the hydroquinone has been shown to play a key role in their antitumor activity. The two-electron reduction of quinones by NQO1 has been shown to be an efficient pathway to hydroquinone formation. NQO1 is expressed at high levels in many human solid tumors making this enzyme ideally suited for intracellular drug activation. Cellular levels of NQO1 are influenced by the NQO1*2 polymorphism. Individuals homozygous for the NQO1*2 allele are NQO1 null and homozygous NQO1*2*2 cell lines have been shown to be more resistant to antitumor quinones when compared to isogenic cell lines overexpressing NQO1. In this review we will discuss the role of NQO1 in the sensitivity and resistance of human cancers to the quinone antitumor drugs mitomycin C, β-lapachone and the benzoquinone ansamycin class of Hsp90 inhibitors including 17-AAG. The role of NQO1 in the bioreductive activation of mitomycin C remains controversial but pre-clinical data strongly suggests a role for NQO1 in the activation of β-lapachone and the benzoquinone ansamycin class of Hsp90 inhibitors. Despite a large volume of preclinical data demonstrating that NQO1 is an important determinant of sensitivity to these antitumor quinones there is little information on whether the clinical response to these agents is influenced by the NQO1*2 polymorphism. The availability of simple assays for the determination of the NQO1*2 polymorphism should facilitate clinical testing of this hypothesis.

Synthesis and antitumor evaluation of novel bis-triaziquone derivatives

Aziridine-containing compounds have been of interest as anticancer agents since late 1970s. The design, synthesis and study of triaziquone (TZQ) analogues with the aim of obtaining compounds with enhanced efficacy and reduced toxicity are an ongoing research effort in our group. A series of bis-type TZQ derivatives has been prepared and their cytotoxic activities were investigated. The cytotoxicity of these bis-type TZQ derivatives were tested on three cancer lines, including breast cancer (BC-M1), oral cancer (OEC-M1), larynx epidermal cancer (Hep2) and one normal skin fibroblast (SF). Most of these synthetic derivatives displayed significant cytotoxic activities against human carcinoma cell lines, but weak activities against SF. Among tested analogues the bis-type TZQ derivative 1a showed lethal effects on larynx epidermal carcinoma cells (Hep2), with an LC₅₀ value of 2.02 mM, and also weak cytotoxic activity against SF cells with an LC₅₀ value over 10 mM for 24 hr treatment. Comparing the viability of normal fibroblast cells treated with compound 1a and TZQ, the LC₅₀ value of the latter was 2.52 mM, indicating more toxicity than compound 1a. This significantly decreased cytotoxicity of compound 1a towards normal SF cells, while still maintaining the anticancer activity towards Hep2 cells is an interesting feature. Among the seven compounds synthesized, compound 1c has similar toxicity effects on the three cancer cell lines and SF normal cells as the TZQ monomer.

Genetic epidemiology of bladder cancer: scaling up in the identification of low-penetrance genetic markers of bladder cancer risk and progression

Bladder cancer is an increasingly important international public health problem. As a multifactorial disease, both environmental and genetic factors are involved in its development and progression. This neoplasm is a paradigm for the participation of low-penetrance genetic variants (GSTM1-null and NAT2-slow) and provides the best established gene-environment interaction in cancer (NAT2-slow * tobacco). Genetic variants in nucleotide excision and double strand break DNA repair pathways have provided promising results, ERCC2-XPD rs238406 being the most consistent variant associated with an increased of bladder cancer risk, by itself and by interacting with tobacco. Variants in other pathways such as cell-cycle control, 1-C metabolism and inflammation have been studied, although the results are inconsistent. Three very large whole genome association studies are being undertaken using the same genotyping platform. Their results will be available soon. Genetic variants have seldom been considered as markers of prognosis or response to therapy in this tumour. The results of these studies are inconclusive. Other issues that need to be addressed are the role of genetic variants in different population subgroups--defined by ethnicity, gender and age, among others--and the association with bladder cancer subphenotypes according to clinical, pathological and molecular characteristics of the tumour. This endeavour can only be achieved by integrating multidisciplinary tools and information. Can this information be applied better to identify high-risk populations? Can the information be used to better assess prognosis or predict response to therapy? These questions require large, well-designed, multicentre studies to be conducted. Funding agencies should be aware of these needs.

Dual roles of Nrf2 in cancer

In response to oxidative stress, the transcription factor NF-E2-related factor 2 (Nrf2) controls the fate of cells through transcriptional upregulation of antioxidant response element (ARE)-bearing genes, including those encoding endogenous antioxidants, phase II detoxifying enzymes, and transporters. Expression of the Nrf2-dependent proteins is critical for ameliorating or eliminating toxicants/carcinogens to maintain cellular redox homeostasis. As a result, activation of the Nrf2 pathway, by naturally-occurring compounds or synthetic chemicals at sub-toxic doses, confers protection against subsequent toxic/carcinogenic exposure. Thus, the use of dietary compounds or synthetic chemicals to boost the Nrf2-dependent adaptive response to counteract environmental insults has emerged to be a promising strategy for cancer prevention. Interestingly, recent emerging data has revealed the "dark" side of Nrf2. Nrf2 and its downstream genes are overexpressed in many cancer cell lines and human cancer tissues, giving cancer cells an advantage for survival and growth. Furthermore, Nrf2 is upregulated in resistant cancer cells and is thought to be responsible for acquired chemoresistance. Therefore, it may be necessary to inhibit the Nrf2 pathway during chemotherapy. This review is primarily focused on the role of Nrf2 in cancer, with emphasis on the recent findings indicating the cancer promoting function of Nrf2 and its role in acquired chemoresistance.

Identification of human reductases that activate the dinitrobenzamide mustard prodrug PR-104A: a role for NADPH:cytochrome P450 oxidoreductase under hypoxia

Hypoxia is a common trait found in many solid tumours and thus represents a therapeutic target with considerable potential. PR-104, a hypoxia-activated prodrug currently in clinical trial, is a water-soluble phosphate ester which is converted in vivo to the corresponding alcohol, PR-104A. This 3,5-dinitrobenzamide-2-nitrogen mustard is activated by reduction to the corresponding 5-hydroxylamine (PR-104H) and 5-amine (PR-104M) in hypoxic cells. The clinical effectiveness of PR-104 will depend in part on the expression of reductases within tumours that can effect this reduction. Here, we evaluate the roles of NADPH:cytochrome P450 oxidoreductase (CYPOR; E.C.1.6.2.4) and NAD(P)H:quinone oxidoreductase (NQO1; E.C.1.6.99.2) as candidate PR-104A reductases. A weak correlation was observed between NQO1 activity and aerobic cytotoxicity in a panel of eight tumour cell lines. However, overexpression of human NQO1 did not increase cytotoxicity of PR-104A or the formation of PR-104H/M, showing that PR-104A is not a substrate for NQO1. Overexpression of human CYPOR did, however, increase the hypoxic cytotoxicity of PR-104A, and its metabolism to PR-104H and PR-104M, demonstrating it to be a PR-104A reductase. To assess the contribution of CYPOR to overall activation of PR-104A in hypoxic SiHa cells, a combination of siRNA transfection and antisense expression were used to suppress CYPOR protein by 91% (+/-3%), a phenotype which conferred 45% (+/-7%) decrease in cytotoxic potency of PR-104A. Regression analysis of all CYPOR depletion data was found to correlate with cytoprotection and metabolism (p<0.001). Residual PR-104A reductase activity could be inhibited by the flavoprotein inhibitor diphenyliodonium. We conclude that CYPOR is an important PR-104A reductase, but that other flavoenzymes also contribute to its activation in hypoxic SiHa cells.

Epidemiology of urinary bladder cancer: from tumor development to patient’s death

Urinary bladder cancer (UBC) ranks ninth in worldwide cancer incidence. It is more frequent in men than in women. We review the main established/proposed factors, both environmental and genetic, associated with bladder cancer etiology and prognosis. Data were extracted from previous reviews and original articles identified from PubMed searches, reference lists, and book chapters dealing with the reviewed topics. Evaluation and consensus of both the contribution of each factor in bladder cancer burden and the appropriateness of the available evidences was done during an ad hoc meeting held during the 18th Congress of the European Society for Urological Research. Cigarette smoking and specific occupational exposures are the main known causes of UBC. Phenacetin, chlornaphazine and cyclophosphamide also increase the risk of bladder cancer. Chronic infection by Schistosoma haematobium is a cause of squamous cell carcinoma of the bladder. NAT2 slow acetylator and GSTM1 null genotypes are associated with an increased risk of this cancer. Vegetables and fresh fruits protect against this tumor. Regarding prognosis, there is little knowledge on the predictive role of environmental exposures and genetic polymorphisms on tumor recurrence and progression and patient's death. Although active tobacco smoking is the most commonly studied factor, no definitive conclusion can be drawn from the literature. More research is needed regarding the effect of complex etiological factors in bladder carcinogenesis. Subgroup analysis according to stage, grade, and molecular features may help in identifying specific etiological and prognostic factors involved in different bladder cancer progression pathways.

NAD(P)H:Quinone Oxidoreductase 1 and NRH:Quinone Oxidoreductase 2 Activity and Expression in Bladder and Ovarian Cancer and Lower NRH:Quinone Oxidoreductase 2 Activity Associated with an NQO2 Exon 3 Single-Nucleotide Polymorphism

PURPOSE

NRH:quinone oxidoreductase 2 (NQO2) is a homologue of NAD(P)H:quinone oxidoreductase 1 (NQO1). Despite 54% homology with human NQO1, NQO2 has little endogenous enzymatic activity. However, NQO2 has potential as a therapeutic target because the addition of the nonbiogenic electron donor dihydronicotinamide riboside (NRH) selectively potentiates the bioactivation of the alkylating agent tretazicar (CB 1954). The NQO activity of ovarian and bladder tumors was determined and the effect of NQO polymorphisms on NQO activity was investigated.

EXPERIMENTAL DESIGN

Intraperitoneal ovarian metastases and bladder tumor clinical samples were analyzed for NQO1 and NQO2 activity, mRNA expression by semiquantitative reverse transcription-PCR, and genotype by RFLP analysis.

RESULTS

NQO1 activity was higher in the bladder cohort than in the ovarian cohort (0-283 and 0-30 nmol/min/mg, respectively; P < 0.0001). In contrast, NQO2 activity was higher in the ovarian tissue than in the bladder samples (0.15-2.27 and 0-1.14 nmol/min/mg, respectively; P = 0.0004). In both cohorts, the NQO1 C609T single-nucleotide polymorphism (SNP) was associated with approximately 7-fold lower NQO1 activity. The NQO2 exon 3 T14055C SNP was associated with lower NQO2 activity relative to wild-type [median values of 0.18 and 0.37 nmol/min/mg in the bladder samples (P = 0.007) and 0.82 and 1.16 nmol/min/mg in the ovarian cohort (P = 0.034)].

CONCLUSION

This is the first observation reporting an apparent association between an NQO2 exon 3 SNP and lower enzymatic activity. The high NQO2 activity of intraperitoneal ovarian metastases relative to other tissues indicates a potential for tretazicar therapy in the treatment of this disease. In contrast, the low level of NQO1 activity and expression relative to other tissues suggests that NQO1-directed therapies would not be appropriate.

Phase I/II Pilot Study of Intravesical Apaziquone (EO9) for Superficial Bladder Cancer

PURPOSE

The quinone based bioreductive drug apaziquone (EO9) failed to demonstrate efficacy in previous phase II studies following intravenous administration. We determined the dose of apaziquone that can be safely administered intravesically and explored its activity for superficial bladder transitional cell carcinoma.

MATERIALS AND METHODS

Six patients with multifocal, Ta/T1 and G1/G2 transitional cell carcinoma of the bladder received escalating doses of apaziquone formulated as EOquintrade mark (0.5 mg/40 ml up to 16 mg/40 ml) weekly for 6 weeks. A further 6 patients received weekly apaziquone at the highest nontoxic dose established. Pharmacokinetic parameters were determined in urine and blood, and the pharmacodynamic markers NQO1 (reduced nicotinamide adenine dinucleotide phosphate:quinone oxidoreductase-1) and glucose transporter 1 were also characterized. Efficacy was determined against a marker lesion.

RESULTS

Local toxicity (grades 2 and 3 dysuria, and hematuria) was observed at doses of 8 mg/40 ml and above but 4 mg/40 ml was well tolerated with no systemic or local side effects. Apaziquone in urine increased linearly with the dose but no apaziquone was detected in plasma. In 8 of 12 patients complete macroscopic and histological disappearance of the marker lesion occurred. A correlation between response and NQO1 and/or glucose transporter 1 expression could not be established.

CONCLUSIONS

Intravesical administration of 4 mg/40 ml apaziquone was well tolerated and had ablative activity against superficial bladder cancer marker lesions.

The lethal effect of bis-type azridinylnaphthoquinone derivative on oral cancer cells (OEC-M1) associated with anti-apoptotic protein bcl-2

Several drugs of aziridinylbenzoquinone analogs have undergone clinical trials as potential antitumor drugs. These bioreductive compounds are designed to kill tumor cells preferentially within the hypoxic microenvironment. From our previous reported data, it was found that the synthesized 2-aziridin-1-yl-3-[(2-[2-[(3-aziridin-1-yl-1,4-dioxo-1,4-dihydronaphthalen-2-yl)thio]ethoxy]ethyl)thio]naphthoquinone (AZ-1) is a bioreductive compound with potent lethal effect on oral cancer cell, OEC-M1. It was found in this study that the lethal effect of the oral cancer cell lines OEC-M1 induced by AZ-1 was mediated through the cell cycle arrest and apoptosis pathway. The LC50 values of OEC-M1 and KB cells induced by AZ-1 compound were 0.72 and 1.02 microM, respectively, which were much lower than that of normal fibroblast cells (SF with LC50 = 5.6 microM) with more than 90% of normal fibroblasts surviving as compared to control at a concentration of AZ-1 as high as 2 microM. It was interesting to note that the LC50 of monotype diaziridinylbenzoquinone compound, diaziquone (AZQ), was 50 microM on OEC-M1 cells. Comparing the cytotoxicity of AZ-1 and AZQ on OEC-M1 cells, AZ-1 is approximately 70 times more potent than AZQ. By using Western blot, both G2/M phase cell cycle arresting protein, cyclin B, and anti-apoptotic protein, bcl-2, were expressed in OEC-M1 cell when the concentrations of AZ-1 were increased from 0.125 to 0.5 microM and then decreased from 1 to 2 microM of AZ-1 treatment as compared with control for 24 h. Both proteins were expressed most abundantly at 0.5 microM AZ-1. However, the expression of bcl-2 protein in OEC-M1 was significantly decreasing in a dose-dependent manner and was only about 50% protein level at 2 microM AZ-1 for 48h as compared with control. The cell survival check protein p53 increased from 1.72- to 2.8-fold and 1.36- to 2.16-fold at concentrations of AZ-1 from 0.125 to 2.0 microM in a dose-dependently increasing manner on OEC-M1 as compared with control for 24 and48 h treatments, respectively. The apoptotic-related phenomena were observed, which included apoptotic body formation and the enzyme activity change of caspase-3. The apoptotic bodies and caspase-3 activity of OEC-M1 were induced only at 2 microM AZ-1 for a 24h treatment, yet apoptotic body formation was observed at as low as 0.5 microM AZ-1 and in a dose-dependently increasing manner for a 48 h treatment. The caspase-3 activity was increased 20.6%, 26.8%, and 84.2%, respectively, at 0.5, 1, and 2muM concentrations of AZ-1 for a 48 h treatment as compared with control. These results indicate that AZ-1 induced the cell death of OEC-M1 through the G2/M phase arrest of cell cycle and anti-apoptosis first and then apoptosis following a 48 h treatment. All of the pathway might be associated with bcl-2 and p53 protein expression. We propose that the AZ-1 could be used as anti-oral cancer drug for future studies with animal models.

Preclinical evaluation of the pharmacodynamic properties of 2,5-diaziridinyl-3-hydroxymethyl-6-methyl-1,4-benzoquinone

PURPOSE

The purpose of our study was to investigate the cellular accumulation, DNA cross-linking ability, and cellular toxicity of RH1 (2,5-diaziridinyl-3-[hydroxymethyl[-6-methyl-1,4-benzoquinone), a novel DNA alkylating agent currently in clinical trials. In addition, the in vivo efficacy of RH1 formulated in different vehicles was also compared.

EXPERIMENTAL DESIGN

RH1 is activated by the two-electron reducing enzyme NQO1 [NADPH:quinone oxidoreductase] forming a potent cytotoxic agent that cross-links DNA. We have used whole blood, cell lines, and primary explanted tumor cultures to measure both the cellular accumulation, DNA cross-linking, and cytotoxicity of RH1. Furthermore, the pharmacokinetic and pharmacodynamic characteristics of RH1 formulated in different vehicles were measured in vivo using the validated comet-X assay in mice bearing human tumor xenografts.

RESULTS

Accumulation of RH1 was shown to be both time and concentration dependent, reaching a maximum after 2 hours and correlated well with DNA cross-linking measurements. DNA cross-linking in vitro could be detected at low (1-10 nmol/L) concentrations after as little as 2 hours exposure. In primary tumor cultures, RH1 induces much higher levels of DNA cross-links at lower doses than either mitomycin C or cisplatin. In vivo efficacy testing using polyvinyl pyrrolidone, saline, or cyclodextrin as vehicles showed DNA cross-links readily detectable in all tissues examined and was enhanced when given in cyclodextrin compared with polyvinyl pyrrolidone or saline.

CONCLUSIONS

RH1 represents a potent bioreductive anticancer drug, which may prove effective in the treatment of cancers, particularly those that overexpress NQO1. DNA cross-linking can be reliably measured in tissue using the validated comet-X assay.

Formation of DNA interstrand cross-links as a marker of Mitomycin C bioreductive activation and chemosensitivity

Tumour response to Mitomycin C (MMC) is heterogenous and past attempts to predict clinical response based on enzyme activities have proven unsatisfactory. Using in vitro techniques, the aim of this study was to determine if the induction of DNA interstrand cross-links correlated with cellular response and to assess if DNA repair and induction of apoptosis influenced MMC chemosensitivity. Poor correlations were found between sensitivity and both DNA repair and induction of apoptosis suggesting that these processes do not play a major role in determining cellular response to MMC. In contrast, there was good correlation between the induction of DNA interstrand cross-links as determined by the alkaline comet assay and cellular response, suggesting that the biochemical events leading to DNA damage are the key factors that determine cellular response in vitro. Further studies are required to assess whether this approach as a mean of prediction has practical applications in vivo.

Tumoral drug metabolism: overview and its implications for cancer therapy

Drug-metabolizing enzymes (DME) in tumors are capable of biotransforming a variety of xenobiotics, including antineoplastics, resulting in either their activation or detoxification. Many studies have reported the presence of DME in tumors; however, heterogeneous detection methodology and patient cohorts have not generated consistent, firm data. Nevertheless, various gene therapy approaches and oral prodrugs have been devised, taking advantage of tumoral DME. With the need to target and individualize anticancer therapies, tumoral processes such as drug metabolism must be considered as both a potential mechanism of resistance to therapy and a potential means of achieving optimal therapy. This review discusses cytotoxic drug metabolism by tumors, through addressing the classes of the individual DME, their relevant substrates, and their distribution in specific malignancies. The limitations of preclinical models relative to the clinical setting and lack of data on the changes of DME with disease progression and host response will be discussed. The therapeutic implications of tumoral drug metabolism will be addressed-in particular, the role of DME in predicting therapeutic response, the activation of prodrugs, and the potential for modulation of their activity for gain are considered, with relevant clinical examples. The contribution of tumoral drug metabolism to cancer therapy can only be truly ascertained through large-scale prospective studies and supported by new technologies for tumor sampling and genetic analysis such as microarrays. Only then can efforts be concentrated in the design of better prodrugs or combination therapy to improve drug efficacy and individualize therapy.

Contribution of NAD(P)H:quinone oxidoreductase 1 to protection against carcinogenesis, and regulation of its gene by the Nrf2 basic-region leucine zipper and the arylhydrocarbon receptor basic helix-loop-helix transcription factors

NAD(P)H:quinone oxidoreductase 1 (NQO1) is a key enzyme involved in defence against reactive forms of oxygen and inhibition of neoplasia. Under conditions of oxidative stress, expression of NQO1 is induced, and the resulting increase in oxidoreductase protein provides the cell with multiple layers of protection against environmental insults. Firstly, the catalytic activity of NQO1 is directed towards the complete reduction and detoxication of highly reactive quinones. Secondly, the oxidoreductase maintains the endogenous lipid-soluble antioxidants, alpha-tocopherol-hydroquinone and ubiquinol in their reduced and active forms. Thirdly, NQO1 is required for the stabilisation of p53 protein in response to DNA-damaging stimuli, and it thereby influences cell fate decisions. In view of the anticarcinogenic actions of NQO1, an understanding of the mechanisms that govern its expression is desirable. The redox sensitivity of NQO1 transcription occurs through a cis-acting antioxidant response element (ARE) located within the regulatory region of the mouse, rat and human genes. This element recruits the positively acting basic leucine zipper (bZip) transcription factor NF-E2 p45-related factor 2 (Nrf2). Under normal constitutive conditions, Nrf2 associates with the cytoskeletal-binding protein Keap1, which regulates the subcellular distribution of the bZip factor and also targets it for proteasome-dependent degradation. Oxidative stress inhibits the Nrf2-Keap1 interaction, thus promoting nuclear accumulation of the transcription factor and transactivation of NQO1 and other ARE-driven genes. Mouse, rat and human NQO1 can also be induced by planar aromatic hydrocarbons through a cis-acting xenobiotic response element (XRE) located in their gene promoters. The XRE recruits the arylhydrocarbon receptor (AhR) and AhR nuclear translocator. Cross-talk may occur between Nrf2 and AhR, but the details of this process remain to be elucidated.