NAD(P)H:quinone oxidoreductase1 (DT-diaphorase) expression in normal and tumor tissues

Modulating endogenous NQO1 levels identifies key regulatory mechanisms of action of β-lapachone for pancreatic cancer therapy

PURPOSE

Pancreatic cancer is the fourth leading cause of cancer-related deaths, in which the 5-year survival rate is less than 5%. Current standard of care therapies offer little selectivity and high toxicity. Novel, tumor-selective approaches are desperately needed. Although prior work suggested that β-lapachone (β-lap) could be used for the treatment of pancreatic cancers, the lack of knowledge of the compound's mechanism of action prevented optimal use of this agent.

EXPERIMENTAL DESIGN

We examined the role of NAD(P)H:quinone oxidoreductase-1 (NQO1) in β-lap-mediated antitumor activity, using a series of MIA PaCa-2 pancreatic cancer clones varying in NQO1 levels by stable shRNA knockdown. The antitumor efficacy of β-lap was determined using an optimal hydroxypropyl-β-cyclodextran (HPβ-CD) vehicle formulation in metastatic pancreatic cancer models.

RESULTS

β-Lap-mediated cell death required ∼90 enzymatic units of NQO1. Essential downstream mediators of lethality were as follows: (i) reactive oxygen species (ROS); (ii) single-strand DNA breaks induced by ROS; (iii) poly(ADP-ribose)polymerase-1 (PARP1) hyperactivation; (iv) dramatic NAD(+)/ATP depletion; and (v) programmed necrosis. We showed that 1 regimen of β-lap therapy (5 treatments every other day) efficaciously regressed and reduced human pancreatic tumor burden and dramatically extended the survival of athymic mice, using metastatic pancreatic cancer models.

CONCLUSIONS

Because NQO1 enzyme activities are easily measured and commonly overexpressed (i.e., >70%) in pancreatic cancers 5- to 10-fold above normal tissue, strategies using β-lap to efficaciously treat pancreatic cancers are indicated. On the basis of optimal drug formulation and efficacious antitumor efficacy, such a therapy should be extremely safe and not accompanied with normal tissue toxicity or hemolytic anemia.

Hsp90 inhibition: elimination of shock and stress

The 90 kDa heat shock proteins (Hsp90) represent a class of molecular chaperones responsible for the maturation and stabilization of many oncogenic proteins. Disrupting the ability of ATP to bind and facilitate the operation of Hsp90 has emerged as a promising approach toward cancer chemotherapeutic development. While numerous Hsp90 inhibitory scaffolds have been identified, progress through the clinic has revealed many obstacles that should be addressed in future analogue development. Recent reports of the complications, pitfalls, and downstream effects associated with Hsp90 inhibition are discussed herein, in hopes of providing a reference that can be used to guide the future design of Hsp90 inhibitory scaffolds.

Beta-lapachone micellar nanotherapeutics for non-small cell lung cancer therapy

Lung cancer is the leading cause of cancer-related deaths with current chemotherapies lacking adequate specificity and efficacy. Beta-lapachone (beta-lap) is a novel anticancer drug that is bioactivated by NAD(P)H:quinone oxidoreductase 1, an enzyme found specifically overexpressed in non-small cell lung cancer (NSCLC). Herein, we report a nanotherapeutic strategy that targets NSCLC tumors in two ways: (a) pharmacodynamically through the use of a bioactivatable agent, beta-lap, and (b) pharmacokinetically by using a biocompatible nanocarrier, polymeric micelles, to achieve drug stability, bioavailability, and targeted delivery. Beta-lap micelles produced by a film sonication technique were small ( approximately 30 nm), displayed core-shell architecture, and possessed favorable release kinetics. Pharmacokinetic analyses in mice bearing subcutaneous A549 lung tumors showed prolonged blood circulation (t(1/2), approximately 28 h) and increased accumulation in tumors. Antitumor efficacy analyses in mice bearing subcutaneous A549 lung tumors and orthotopic Lewis lung carcinoma models showed significant tumor growth delay and increased survival. In summary, we have established a clinically viable beta-lap nanomedicine platform with enhanced safety, pharmacokinetics, and antitumor efficacy for the specific treatment of NSCLC tumors.

Prevalence of GSTT1, GSTM1 and NQO1 (609C>T) in Filipino children with ALL (acute lymphoblastic leukaemia)

In the present paper, we examined the incidence of polymorphic genes involved with the detoxification of exogenous chemicals, including carcinogens, namely GSTT1 (glutathione transferase theta1), GSTM1 (glutathione transferase micro1) and NQO1 (NAD(P)H:quinone oxidoreductase 1) in 60 Filipino paediatric patients with ALL (acute lymphoblastic leukaemia). We found a significantly high incidence of the GSTM1 null genotype in ALL children (71.7%) compared with 51.7% in the control group of children (P<0.05). The GSTT1 null genotype was observed in 35.0% and 33.3% of the ALL cases and the control subjects respectively, with no significant difference. Screening for NQO1 (609C>T) mutant alleles showed a high incidence of the NQO1 C/C genotype (NQO1 homozygous wild-type allele genotype) in 60.0% of ALL cases and was significantly higher than in the control group (23.3%) (P<0.01). These GSTM1 null and NQO1 wild-type genotypes are independently associated with the risk of ALL in Filipino patients. When these two genotypes, GSTM1 null and NQO1 C/C, were combined, the hazard rate for childhood leukaemia was significantly increased (P<0.001). We also noticed that the incidences of GSTM1 null mutations and the NQO1 C/C genotype were significantly higher among Filipinos. These findings suggest a possible role of the GSTM1 null and NQO1 C/C genotypes in the susceptibility of paediatric ALL cases in the Philippines.

A Model for NAD(P)H:Quinoneoxidoreductase 1 (NQO1) Targeted Individualized Cancer Chemotherapy

NQO1 (NAD(P)H:quinoneoxidoreductase 1) is a reductive enzyme that is an important activator of bioreductive antitumor agents. NQO1 activity varies in individual tumors but is generally higher in tumor cells than in normal cells. NQO1 has been used as a target for tumor specific drug development. We investigated a series of bioreductive benzoquinone mustard analogs as a model for NQO1 targeted individualized cancer chemotherapy. We compared the tumor cell growth inhibitory activity of benzoquinone mustard analogs with sterically bulky groups of different size and placed at different positions on the benzoquinone ring, using tumor cell lines with different levels of NQO1. We demonstrated that functional groups of different steric size could be used to produce a series of bioreductive antitumor agents that were activated by different levels of NQO1 in tumor cells. This series of drugs could then be used to target cells with specific levels of NQO1 for growth inhibition and to avoid damage to normal cells, like bone marrow cells, that have low levels of NQO1. This approach could be used to develop new bioreductive antitumor agents for NQO1 targeted individualized cancer chemotherapy.

Role of Oxidative Stress in Peroxisome Proliferator-Mediated Carcinogenesis

In this review, the evidence about the role of oxidative stress in the induction of hepatocellular carcinomas by peroxisome proliferators is examined. The activation of PPAR-alpha by peroxisome proliferators in rats and mice may produce oxidative stress, due to the induction of enzymes like fatty acyl coenzyme A (CoA) oxidase (AOX) and cytochrome P-450 4A1. The effect of peroxisome proliferators on the antioxidant defense system is reviewed, as is the effect on endpoints resulting from oxidative stress that may be important in carcinogenesis, such as lipid peroxidation, oxidative DNA damage, and transcription factor activation. Peroxisome proliferators clearly inhibit several enzymes in the antioxidant defense system, but studies examining effects on lipid peroxidation and oxidative DNA damage are conflicting. There is a profound species difference in the induction of hepatocellular carcinomas by peroxisome proliferators, with rats and mice being sensitive, whereas species such as nonhuman primates and guinea pigs are not susceptible to the effects of peroxisome proliferators. The possible role of oxidative stress in these species differences is also reviewed. Overall, peroxisome proliferators produce changes in oxidative stress, but whether these changes are important in the carcinogenic process is not clear at this time.

Mediator-assisted simultaneous probing of cytosolic and mitochondrial redox activity in living cells

This work describes an electron transfer mediator-assisted amperometric flow injection method for assessing redox enzyme activity in different subcellular compartments of the phosphoglucose isomerase deletion mutant strain of Saccharomyces cerevisiae, EBY44. The method is demonstrated using the ferricyanide-menadione double mediator system to study the effect of dicoumarol, an inhibitor of cytosolic and mitochondrial oxidoreductases and an uncoupler of the electron transport chain. Evaluation of the role of NAD(P)H-producing pathways in mediating biological effects is facilitated by introducing either fructose or glucose as the carbon source, yielding either NADH or NADPH through the glycolytic or pentose phosphate pathway, respectively. Respiratory noncompetent cells show greater inhibition of cytosolic menadione-reducing enzymes when NADH rather than NADPH is produced. Spectrophotometric in vitro assays show no difference between the cofactors. Respiratory competent cells show cytosolic inhibition only when NADPH is produced, whereas production of NADH reveals uncoupling at low dicoumarol concentrations and inhibition of complexes III and IV at higher concentrations. Spectrophotometric assays only indicate the presence of cytosolic inhibition regardless of the reduced cofactor used. This article shows the applicability of the amperometric method and emphasizes the significance of determining biological effects of chemicals in living cells.

Cigarette smoke induces endoplasmic reticulum stress and the unfolded protein response in normal and malignant human lung cells

Background

Although lung cancer is among the few malignancies for which we know the primary etiological agent (i.e., cigarette smoke), a precise understanding of the temporal sequence of events that drive tumor progression remains elusive. In addition to finding that cigarette smoke (CS) impacts the functioning of key pathways with significant roles in redox homeostasis, xenobiotic detoxification, cell cycle control, and endoplasmic reticulum (ER) functioning, our data highlighted a defensive role for the unfolded protein response (UPR) program. The UPR promotes cell survival by reducing the accumulation of aberrantly folded proteins through translation arrest, production of chaperone proteins, and increased degradation. Importance of the UPR in maintaining tissue health is evidenced by the fact that a chronic increase in defective protein structures plays a pathogenic role in diabetes, cardiovascular disease, Alzheimer's and Parkinson's syndromes, and cancer.

Methods

Gene and protein expression changes in CS exposed human cell cultures were monitored by high-density microarrays and Western blot analysis. Tissue arrays containing samples from 110 lung cancers were probed with antibodies to proteins of interest using immunohistochemistry.

Results

We show that: 1) CS induces ER stress and activates components of the UPR; 2) reactive species in CS that promote oxidative stress are primarily responsible for UPR activation; 3) CS exposure results in increased expression of several genes with significant roles in attenuating oxidative stress; and 4) several major UPR regulators are increased either in expression (i.e., BiP and eIF2α) or phosphorylation (i.e., phospho-eIF2α) in a majority of human lung cancers.

Conclusion

These data indicate that chronic ER stress and recruitment of one or more UPR effector arms upon exposure to CS may play a pivotal role in the etiology or progression of lung cancers, and that phospho-eIF2α and BiP may have diagnostic and/or therapeutic potential. Furthermore, we speculate that upregulation of UPR regulators (in particular BiP) may provide a pro-survival advantage by increasing resistance to cytotoxic stresses such as hypoxia and chemotherapeutic drugs, and that UPR induction is a potential mechanism that could be attenuated or reversed resulting in a more efficacious treatment strategy for lung cancer.

Investigation of an NQO1 polymorphism as a possible risk and prognostic factor for chronic lymphocytic leukemia

NAD(P)H:quinoneoxidoreductase 1 (NQO1) inhibits some cancers and increases p53 and apoptosis in cells. Due to an inactivating polymorphism, 10% of humans have no NQO1 activity. A case:control study suggested that chronic lymphocytic leukemia (CLL) patients may have an increased incidence of the NQO1 null genotype compared with controls. NQO1 genotype did not correlate with various CLL prognostic factors, but we observed a trend toward lower drug response in patients with the NQO1 null genotype. Inhibiting NQO1 activity decreased p53 levels and drug induced apoptosis in CLL cells. These results raise the possibility that the NQO1 polymorphism may be a risk factor for CLL and a predictor of response to chemotherapy.

NQO1 expression in pancreatic cancer and its potential use as a biomarker

Pancreatic ductal adenocarcinoma (PDA) is rarely curable due to regional/metastatic spread at diagnosis. Identification of molecular markers may enhance diagnosis and early detection of PDA. The 2-electron reductase, NAD(P)H:quinone oxidoreductase (NQO1) has been found to be overexpressed in many solid tumors including PDA, and may be a useful clinically relevant diagnostic marker of malignancy. For this study, we used 37 surgical resection cases: 24 PDAs and 13 benign pancreatic tissue specimens. An additional 16 specimens from pancreatic endoscopic ultrasound-guided fine needle aspiration (EUS-FNA) were included as a pilot series. NQO1 was detected by avidin-biotin based immunohistochemical and immunocytochemical methods. Both staining intensity and proportion of NQO1 positive tumor cells were scored. Moderate to strong (2 to 3+) staining for NQO1 was detected in 22/24 (92%) surgically resected PDAs, 9/9 (100%) EUS-FNAs with malignant diagnoses, one cytologically atypical but not diagnostic for malignancy EUS-FNA, and 1/6 (17%) EUS-FNAs initially diagnosed as negative for malignancy. Subsequent histologic assessment confirmed malignancy in all 9 cytologically positive EUS-FNAs and in the atypical case. The NQO1 positive case initially diagnosed as negative for malignancy showed no evidence of carcinoma on subsequent tissue biopsy. NQO1 staining was also observed in some benign ducts/cells; however, correlation of NQO1 expression with cellular morphology assessment minimizes the risk of false positive diagnosis. NQO1 is consistently overexpressed in PDA. Although NQO1 is observed in some benign tissue components, this marker may be a clinically useful diagnostic adjunct for detection of PDA, independent of tumor grade/stage.

Proteomic profiling of endothelial cells in human lung cancer

Genomic and proteomic analysis of normal and diseased tissues have yielded an abundance of molecular information for diagnostic and potential therapeutic targets. Changing the target of analysis from poorly accessible cells within tissues to easily accessible vascular endothelium has theoretical advantages in tissue-specific targeting. In this study, we sought to map a large-scale proteome of microvascular endothelium in human non-small cell lung cancer (NSCLC) and normal lung tissues, and identify lung cancer-related endothelial cell (EC)-selective proteins. Endothelial cells were isolated within NSCLC tissues and adjacent-normal lung tissue of lung cancer patients by using CD31-immunomagnetic beads. The complex proteins from the ECs were separated by one-dimensional gel electrophoresis, and the proteins in each gel band were digested by trypsin. Peptides were separated by online reverse-phase liquid-chromatography and analyzed by electrospray ionization (ESI) ion trap tandem mass spectrometry. Approximately 600-1000 proteins were identified in each individual sample. Five patient cases of paired individual data, extracted from the protein identification data sets of both normal- and cancer-derived ECs, were analyzed by subtractive proteomics. An average of 300 proteins was specifically identified from each lung cancer-derived EC isolate, compared to normal lung-derived ECs. With the use of several comparative analyses, we identified among those 300 proteins, 16 common candidate proteins that were detected in at least 3 of 5 cases specific to lung cancer-derived ECs. Proteins selectively identified in cancer-derived ECs, including coatomer protein complex, subunit gamma (COPG), and peroxiredoxin 4 (PRDX4), were validated by Western blot analysis. In an additional experiment in which 16 cancer samples were analyzed by immunohistochemistry, PRDX4, thymopoietin (TMPO), and COPG were confirmed to be abundantly expressed in lung cancer-derived ECs and in cancerous lung cells. Further ongoing analysis of these 16 candidate proteins will determine their potential applicability to NSCLC-specific diagnosis and therapeutics.

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

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

Heat shock protein 90: the cancer chaperone

Heat shock protein 90 (Hsp90) is a molecular chaperone required for the stability and function of a number of conditionally activated and/or expressed signalling proteins, as well as multiple mutated, chimeric, and/or over-expressed signalling proteins, that promote cancer cell growth and/or survival. Hsp90 inhibitors are unique in that, although they are directed towards a specific molecular target, they simultaneously inhibit multiple cellular signalling pathways. By inhibiting nodal points in multiple overlapping survival pathways utilized by cancer cells, combination of an Hsp90 inhibitor with standard chemotherapeutic agents may dramatically increase the in vivo efficacy of the standard agent. Hsp90 inhibitors may circumvent the characteristic genetic plasticity that has allowed cancer cells to eventually evade the toxic effects of most molecularly targeted agents. The mechanism-based use of Hsp90 inhibitors, both alone and in combination with other drugs, should be effective toward multiple forms of cancer. Further, because Hsp90 inhibitors also induce Hsf-1-dependent expression of Hsp70, and because certain mutated Hsp90 client proteins are neurotoxic, these drugs display ameliorative properties in several neurodegenerative disease models, suggesting a novel role for Hsp90 inhibitors in treating multiple pathologies involving neurodegeneration.

In vivo role of NAD(P)H:quinone oxidoreductase 1 in metabolic activation of mitomycin C and bone marrow cytotoxicity

NAD(P)H:quinone oxidoreductase 1(-/-) (NQO1(-/-)), NQO1(+/-) along with NRH:quinone oxidoreductase 2(-/-) (NQO2(-/-)), and wild-type (WT) mice were exposed to five once weekly doses of mitomycin C. The mice were euthanized 15 weeks after the first dose. Blood cell counts and histologic analyses were done. WT and NQO2(-/-) mice showed hypocellularity and a significant increase in adipocytes in bone marrow. They also showed anemia because of the loss of RBC and hemoglobin. The neutrophils and platelets were reduced, whereas other blood cell types and tissues were normal. Interestingly, NQO1(-/-) mice showed a complete resistance to mitomycin C-induced bone marrow cytotoxicity and reduction in RBC, hemoglobin, and neutrophils. NQO1(+/-) mice also showed limited resistance to mitomycin C-induced bone marrow cytotoxicity. These data show a major in vivo role of NQO1 in metabolic activation of mitomycin C with implications in mitomycin C chemotherapy.

Upregulation of NAD(P)H:quinone oxidoreductase by radiation potentiates the effect of bioreductive beta-lapachone on cancer cells

We found that beta-lapachone (beta-lap), a novel bioreductive drug, caused rapid apoptosis and clonogenic cell death in A549 human lung epithelial cancer cells in vitro in a dose-dependent manner. The clonogenic cell death caused by beta-lap could be significantly inhibited by dicoumarol, an inhibitor of NAD(P)H:quinone oxido-reductase (NQO1), and also by siRNA for NQO1, demonstrating that NQO1-induced bioreduction of beta-lap is an essential step in beta-lap-induced cell death. Irradiation of A549 cells with 4 Gy caused a long-lasting upregulation of NQO1, thereby increasing NQO1-mediated beta-lap-induced cell deaths. Although the direct cause of beta-lap-induced apoptosis is not yet clear, beta-lap treatment reduced the expression of p53 and NF-kappaB, whereas it increased cytochrome C release, caspase-3 activity, and gammaH2AX foci formation. Importantly, beta-lap treatment immediately after irradiation enhanced radiation-induced cell death, indicating that beta-lap sensitizes cancer cells to radiation, in addition to directly killing some of the cells. The growth of A549 tumors induced in immunocompromised mice could be markedly suppressed by local radiation therapy when followed by beta-lap treatment. This is the first study to demonstrate that combined radiotherapy and beta-lap treatment can have a significant effect on human tumor xenografts.

Epidemiology of carcinogen metabolism genes and risk of squamous cell carcinoma of the head and neck

The risk association between tobacco and alcohol use with squamous cell carcinoma of the head and neck (SCCHN) is well recognized. However, clearly not all individuals who smoke or drink develop SCCHN. Individual genetic susceptibility differences in carcinogen-metabolizing enzyme function, mutagen sensitivity, apoptosis, and chromosomal aberrations either alone or in combination have been theorized to modify the risk of SCCHN. Nearly all carcinogens and procarcinogens require activation by metabolizing enzymes. Similarly, detoxifying enzymes exist and deactivate carcinogens as well as their intermediate by-products. Together these enzymes are termed xenobiotic-metabolizing enzymes; genetic polymorphisms of these enzymes can modify an individual's response to carcinogens and hence the carcinogenic potential of such exposures. In this review, we explore the available evidence in recent literature regarding the risk association between SCCHN and various xenobiotic-metabolizing enzymes, including cytochrome P450s, glutathione S-transferases, N-acetyltransferases, NAD(P)H:quinone oxidoreductase 1, alcohol dehydrogenase, and aldehyde dehydrogenase.

Beta-lapachone-containing PEG-PLA polymer micelles as novel nanotherapeutics against NQO1-overexpressing tumor cells

Beta-lapachone (beta-lap) is a novel anticancer agent that is bioactivated by NADP(H): quinone oxidoreductase 1 (NQO1), an enzyme overexpressed in a variety of tumors. Despite its therapeutic promise, the poor aqueous solubility of beta-lap hinders its preclinical evaluation and clinical translation. Our objective was to develop beta-lap-containing poly(ethylene glycol)-block-poly(D,L-lactide) (PEG-PLA) polymer micelles for the treatment of NQO1-overexpressing tumors. Several micelle fabrication strategies were examined to maximize drug loading. A film sonication method yielded beta-lap micelles with relatively high loading density (4.7+/-1.0% to 6.5+/-1.0%) and optimal size (29.6+/-1.5 nm). Release studies in phosphate-buffered saline (pH 7.4) showed the time (t(1/2)) for 50% of drug release at 18 h. In vitro cytotoxicity assays were performed in NQO1-overexpressing (NQO1+) and NQO1-null (NQO1-) H596 lung, DU-145 prostate, and MDA-MB-231 breast cancer cells. Cytotoxicity data showed that after a 2 h incubation with beta-lap micelles, a marked increase in toxicity was shown in NQO1+ cells over NQO1- cells, resembling free drug both in efficacy and mechanism of cell death. In summary, these data demonstrate the potential of beta-lap micelles as an effective therapeutic strategy against NQO1-overexpressing tumor cells.

A NAD(P)H:quinone oxidoreductase 1 polymorphism is a risk factor for human colon cancer

Colon cancer is one of the most common cancers in North America and generally develops from colonic epithelial cells following initiation by carcinogens. We have shown that the phase II detoxifying enzyme, NAD(P)H:quinone oxidoreductase 1 (NQO1) contributes to the inhibition of carcinogen-induced colon cancer in rats at both the initiation and postinitiation stages. An inactivating polymorphism at base 609 of the NQO1 gene, (609)C (NQO1 *1) --> (609)T (NQO1 *2), occurs at high frequency in the human population. Thus, we carried out a case-control study to determine if this polymorphism is associated with an increased risk of developing colon cancer. A total of 298 patients with colon cancer and 349 healthy controls matched for age, gender, and ethnic origin were enrolled in the study. There was an increased incidence of the NQO1 *2/*2 genotype in patients with colon cancer, with a gender and age-adjusted odds ratio of 2.68 (95% confidence intervals, 1.14-6.28). However, the incidence of the NQO1 *1/*2 genotype was not increased in patients with colon cancer compared with controls. When the patient and control groups were stratified by tobacco and alcohol use, the incidences of the NQO1 *2/*2 genotype were increased in patients with colon cancer for tobacco and alcohol users and nonusers, suggesting that there is no interaction between the NQO1 base 609 polymorphism and tobacco or alcohol use. These results strongly suggest that NQO1 plays a significant role in preventing the development of colon cancer, and individuals with an NQO1 *2/*2 genotype are at an increased risk of developing this disease.

Development of 17-allylamino-17-demethoxygeldanamycin hydroquinone hydrochloride (IPI-504), an anti-cancer agent directed against Hsp90

Heat shock protein 90 (Hsp90) is an emerging therapeutic target of interest for the treatment of cancer. Its role in protein homeostasis and the selective chaperoning of key signaling proteins in cancer survival and proliferation pathways has made it an attractive target of small molecule therapeutic intervention. 17-Allylamino-17-demethoxygeldanamycin (17-AAG), the most studied agent directed against Hsp90, suffers from poor physical-chemical properties that limit its clinical potential. Therefore, there exists a need for novel, patient-friendly Hsp90-directed agents for clinical investigation. IPI-504, the highly soluble hydroquinone hydrochloride derivative of 17-AAG, was synthesized as an Hsp90 inhibitor with favorable pharmaceutical properties. Its biochemical and biological activity was profiled in an Hsp90-binding assay, as well as in cancer-cell assays. Furthermore, the metabolic profile of IPI-504 was compared with that of 17-AAG, a geldanamycin analog currently in clinical trials. The anti-tumor activity of IPI-504 was tested as both a single agent as well as in combination with bortezomib in myeloma cell lines and in vivo xenograft models, and the retention of IPI-504 in tumor tissue was determined. In conclusion, IPI-504, a potent inhibitor of Hsp90, is efficacious in cellular and animal models of myeloma. It is synergistically efficacious with the proteasome inhibitor bortezomib and is preferentially retained in tumor tissues relative to plasma. Importantly, it was observed that IPI-504 interconverts with the known agent 17-AAG in vitro and in vivo via an oxidation-reduction equilibrium, and we demonstrate that IPI-504 is the slightly more potent inhibitor of Hsp90.

Xenobiotic-metabolizing enzymes in canine mammary tumours

Mammary tumours are the most common neoplasms in female dogs. The present study was designed to evaluate the relationship between different clinical stages with activities of phase I and phase II carcinogen-metabolizing enzymes in canine mammary tumours. The levels of cytochrome P450 and cytochrome b5 and the activities of glutathione S-transferase (GST), gamma-glutamyl transpeptidase (GGT), DT-diaphorase (DTD) and NADPH diaphorase in tumour tissues of 25 bitches was estimated. Enhanced levels of cytochrome P450 and b5 and phase II enzyme activities were observed in tumour tissues compared to the corresponding uninvolved adjacent tissues. The magnitude of the changes in phase I and phase II enzyme status was, however, more pronounced in stages I and II compared to stages III and IV. The results suggest that the balance between phase I carcinogen activation and phase II detoxification systems may play an important role in canine mammary tumour development.

New tricks for old drugs: the anticarcinogenic potential of DNA repair inhibitors

Defective or abortive repair of DNA lesions has been associated with carcinogenesis. Therefore it is imperative for a cell to accurately repair its DNA after damage if it is to return to a normal cellular phenotype. In certain circumstances, if DNA damage cannot be repaired completely and with high fidelity, it is more advantageous for an organism to have some of its more severely damaged cells die rather than survive as neoplastic transformants. A number of DNA repair inhibitors have the potential to act as anticarcinogenic compounds. These drugs are capable of modulating DNA repair, thus promoting cell death rather than repair of potentially carcinogenic DNA damage mediated by error-prone DNA repair processes. In theory, exposure to a DNA repair inhibitor during, or immediately after, carcinogenic exposure should decrease or prevent tumorigenesis. However, the ability of DNA repair inhibitors to prevent cancer development is difficult to interpret depending upon the system used and the type of genotoxic stress. Inhibitors may act on multiple aspects of DNA repair as well as the cellular signaling pathways activated in response to the initial damage. In this review, we summarize basic DNA repair mechanisms and explore the effects of a number of DNA repair inhibitors that not only potentiate DNA-damaging agents but also decrease carcinogenicity. In particular, we focus on a novel anti-tumor agent, beta-lapachone, and its potential to block transformation by modulating poly(ADP-ribose) polymerase-1.

Synergistic Effects of Radiation and β-Lapachone in DU-145 Human Prostate Cancer CellsIn Vitro

It has been reported that beta-lapachone (beta-lap), a bioreductive anti-cancer drug, synergistically interacts with ionizing radiation and that the sensitivity of cells to beta-lap is closely related to the activity of NAD(P)H:quinone oxidoreductase 1 (NQO1). Here we report the results of our studies of mechanisms underlying the synergistic interaction of beta-lap and radiation in killing cancer cells using the DU-145 human prostate cancer cell line. The clonogenic cell death caused by the combination of radiation and beta-lap was synergistic when beta-lap was administered 0-10 h after irradiation but not when it was given before irradiation. The expression and activity of NQO1 increased significantly and remained elevated for longer than 12 h after 4 Gy irradiation, suggesting that the long-lasting elevation of NQO1 sensitized the cells to beta-lap. Studies with split-dose irradiation demonstrated that beta-lap given immediately after irradiation effectively inhibited sublethal radiation damage (SLD) repair. Taken together, these results lead us to conclude that the synergistic interaction between beta-lap and radiation in killing cells is the result of two distinct mechanisms: First, radiation sensitizes cells to beta-lap by up-regulating NQO1, and second, beta-lap sensitizes cells to radiation by inhibiting SLD repair. The combination of beta-lap and radiotherapy is potentially promising modality for the treatment of cancer in humans.

The plasma membrane redox system in aging

Oxidative stress over time leads to the accumulation of damaged macromolecules and to profound physiological changes that are associated with several age-related diseases. The plasma membrane redox system (PMRS) appears to attenuate oxidative stress acting as a compensatory mechanism during the aging process. The PMRS appears to play a protective role during mitochondrial dysfunction to provide cells with a survival mechanism by lowering oxidative stress. The PMRS accomplishes this by producing more NAD(+) for glycolytic ATP production via transfer of electrons from intracellular reducing equivalents to extracelluar acceptors. Ubiquinone and alpha-tocopherol are key antioxidant molecules in the plasma membrane that are affected by aging and can be up-regulated by dietary interventions such as calorie restriction (CR). Up-regulation of PMRS activity leads to cell survival and membrane homeostasis under stress conditions and during calorie restriction. Further studies of the PMRS may provide not only additional information on the mechanisms involved in aging and CR, but may provide therapeutic targets for the prevention and treatment of age-related diseases.

Role of NQO1 polymorphisms as risk factors for squamous cell carcinoma of the head and neck

A case: control study was carried out to determine if inactivating polymorphisms of the NQO1 gene at bases 609 and 465 are associated with altered risk of developing squamous cell carcinoma of the head and neck (SCCHN). Genotyping was carried out by PCR RFLP analysis on whole blood samples. The frequency of the inactive 609T and active 609C forms, and the inactive 465T and active 465C forms, of NQO1 were compared in patient and control groups by a logistic regression analysis and odds ratios (ORs) were calculated. Participants were stratified by tobacco and alcohol use, and genotype distributions in these sub-groups were compared. There were no significant differences in genotype distribution between SCCHN patients and the control population for the base 609 or 465 polymorphisms. There were also no significant differences in genotype distributions between patient and control groups for tobacco and/or alcohol users and non-users. Genotype distributions were similar for SCCHN patients at all disease sites with the exception of the nasopharynx where there was a higher incidence of the 609C:609T and 609T:609T genotypes. These results suggest that individuals having either 609T or 465T alleles generally do not have an altered risk of developing SCCHN.

Xenobiotic-metabolising enzymes in patients with adenocarcinoma of the breast: Correlation with clinical stage and menopausal status

Adenocarcinoma of the breast is the most common cancer in women worldwide and its incidence is increasing in most countries. The present study was designed to evaluate the relationship between different clinical stages and menopausal status using the activity of phase I and II carcinogen-metabolising enzymes in breast cancer patients. Fifty breast cancer patients clinically categorized as stage I, II and III, and as pre- and postmenopausal were chosen for the study. The levels of cytochrome P450 and b(5) and the activity of glutathione S-transferase (GST), gamma-glutamyl transpeptidase (GGT), DT-diaphorase (DTD) and NADPH diaphorase in tumour tissues and adjacent normal tissues were estimated. Enhanced levels of cytochrome P450 and b(5) and phase II enzyme activity were observed in breast tumour tissues compared with the corresponding uninvolved adjacent tissues irrespective of clinical stage and menopausal status of the patients. The magnitude of the changes in phase I and II enzyme status was, however, more pronounced in stage I and in premenopausal patients than in stage II and III and postmenopausal patients respectively. Our results suggest that the balance between phase I carcinogen activation and phase II detoxification systems may play an important role in the development of breast tumours.

Heat-induced up-regulation of NAD(P)H:quinone oxidoreductase potentiates anticancer effects of beta-lapachone

PURPOSE

The purpose of the present study was to evaluate the efficacy of mild hyperthermia to potentiate the anticancer effects of beta-lapachone (3,4-dihydro-2,2-dimethyl-2H-naphthol[1,2-b]pyran-5,6-dione) by up-regulating NAD(P)H:quinone oxidoreductase (NQO1) in cancer cells.

EXPERIMENTAL DESIGN

Effects of beta-lapachone alone or in combination with mild heating on the clonogenic survival of FSaII fibrosarcoma cells of C3H mice and A549 human lung tumor cells in vitro was determined. Effects of heating on the NQO1 level in the cancer cells in vitro were assessed using Western blot analysis for NQO1 expression, biochemical determination of NQO1 activity, and immunofluorescence microscopy for NQO1 expression. Growth of FSaII tumors in the hind legs of C3H mice was determined after treating the host mice with i.p. injection of 45 mg/kg beta-lapachone followed by heating the tumors at 42 degrees C for 1 hour every other day for four times.

RESULTS

Incubation of FSaII tumor cells and A549 tumor cells with beta-lapachone at 37 degrees C reduced clonogenic survival of the cells in dose-dependent and incubation time-dependent manner. NQO1 level in the cancer cells in vitro increased within 1 hour after heating at 42 degrees C for 1 hour and remained elevated for >72 hours. The clonogenic cell death caused by beta-lapachone increased in parallel with the increase in NQO1 levels in heated cells. Heating FSaII tumors in the legs of C3H mice enhanced the effect of i.p.-injected beta-lapachone in suppressing tumor growth.

CONCLUSION

We observed for the first time that mild heat shock up-regulates NQO1 in tumor cells. The heat-induced up-regulation of NQO1 enhanced the anticancer effects of beta-lapachone in vitro and in vivo.

Identification of NADH dehydrogenase 1 α subcomplex 5 capable to transform murine fibroblasts and overexpressed in human cervical carcinoma cell lines

The human papillomavirus (HPV) E6 and E7 proteins play essential roles in HPV-associated cervical carcinogenesis. However, cells transformed by E6 or E7 rarely grow into tumors in nude mice, indicating that the carcinogenesis involves additional molecular events. The highly efficient retroviral cDNA expression system derived from HeLa cells identified two cDNA species coding NADH dehydrogenase 1 alpha subcomplex 5 (NDUFA5) and zinc finger protein 9 (ZNF9), exhibiting the potential to transform murine fibroblast cell line, NIH3T3. The real-time RT-PCR analysis revealed that the expressions of the NDUFA5 mRNA, but not the ZNF9 mRNA level, were significantly up-regulated in all the tested cell lines derived from HPV-positive cervical cancer, HeLa, SW576, and CaSKi. The NDUFA5 expression may contribute to the multi-step carcinogenesis in human cervical cancer.

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

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

Targeting NAD(P)H:quinone oxidoreductase (NQO1) in pancreatic cancer

NAD(P)H:quinone oxidoreductase (NQO1) functions as an important part of cellular antioxidant defense by detoxifying quinones, thus preventing the formation of reactive oxygen species (ROS). The aim of our study was to determine if NQO1 is elevated in pancreatic cancer specimens and pancreatic cancer cell lines and if so, would compounds previously demonstrated to redox cycle with NQO1 be effective in killing pancreatic cancer cells. Immunohistochemistry of resected pancreatic specimens demonstrated an increased immunoreactivity for NQO1 in pancreatic cancer and pancreatic intraepithelial neoplasia (PanIN) specimens versus normal human pancreas. Immunocytochemistry and Western immunoblots demonstrated increased immunoreactivity in pancreatic cancer cells when compared to a near normal immortalized human pancreatic ductal epithelial cell line and a colonic epithelial cell line. Streptonigrin, a compound known to cause redox cycling in the presence of NQO1, decreased clonogenic survival and decreased anchorage-independent growth in soft agar. Streptonigrin had little effect on cell lines with absent or reduced levels of NQO1. The effects of streptonigrin were reversed in pancreatic cancer cells pretreated with dicumarol, a known inhibitor of NQO1. NQO1 may be a therapeutic target in pancreatic cancer where survival is measured in months.

Development of beta-lapachone prodrugs for therapy against human cancer cells with elevated NAD(P)H:quinone oxidoreductase 1 levels

beta-Lapachone, an o-naphthoquinone, induces a novel caspase- and p53-independent apoptotic pathway dependent on NAD(P)H:quinone oxidoreductase 1 (NQO1). NQO1 reduces beta-lapachone to an unstable hydroquinone that rapidly undergoes a two-step oxidation back to the parent compound, perpetuating a futile redox cycle. A deficiency or inhibition of NQO1 rendered cells resistant to beta-lapachone. Thus, beta-lapachone has great potential for the treatment of specific cancers with elevated NQO1 levels (e.g., breast, non-small cell lung, pancreatic, colon, and prostate cancers). We report the development of mono(arylimino) derivatives of beta-lapachone as potential prodrugs. These derivatives are relatively nontoxic and not substrates for NQO1 when initially diluted in water. In solution, however, they undergo hydrolytic conversion to beta-lapachone at rates dependent on the electron-withdrawing strength of their substituent groups and pH of the diluent. NQO1 enzyme assays, UV-visible spectrophotometry, high-performance liquid chromatography-electrospray ionization-mass spectrometry, and nuclear magnetic resonance analyses confirmed and monitored conversion of each derivative to beta-lapachone. Once converted, beta-lapachone derivatives caused NQO1-dependent, mu-calpain-mediated cell death in human cancer cells identical to that caused by beta-lapachone. Interestingly, coadministration of N-acetyl-l-cysteine, prevented derivative-induced cytotoxicity but did not affect beta-lapachone lethality. Nuclear magnetic resonance analyses indicated that prevention of beta-lapachone derivative cytotoxicity was the result of direct modification of these derivatives by N-acetyl-l-cysteine, preventing their conversion to beta-lapachone. The use of beta-lapachone mono(arylimino) prodrug derivatives, or more specifically a derivative converted in a tumor-specific manner (i.e., in the acidic local environment of the tumor tissue), should reduce normal tissue toxicity while eliciting tumor-selective cell killing by NQO1 bioactivation.

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.

Effect of NQO1 induction on the antitumor activity of RH1 in human tumors in vitro and in vivo

NQO1 is a reductive enzyme that is important for the activation of many bioreductive agents and is a target for an enzyme-directed approach to cancer therapy. It can be selectively induced in many tumor types by a number of compounds including dimethyl fumarate and sulforaphane. Mitomycin C is a bioreductive agent that is used clinically for treatment of solid tumors. RH1 (2,5-diaziridinyl-3-(hydroxymethyl)- 6-methyl-1,4-benzoquinone) is a new bioreductive agent currently in clinical trials. We have shown previously that induction of NQO1 can enhance the antitumor activity of mitomycin C in tumor cells in vitro and in vivo. As RH1 is activated selectively by NQO1 while mitomycin C is activated by many reductive enzymes, we investigated whether induction of NQO1 would produce a greater enhancement of the antitumor activity of RH1 compared with mitomycin C. HCT116 human colon cancer cells and T47D human breast cancer cells were incubated with or without dimethyl fumarate or sulforaphane followed by mitomycin C or RH1 treatment, and cytotoxic activity was measured by a clonogenic (HCT116) or MTT assay (T47D). Dimethyl fumarate and sulforaphane treatment increased NQO1 activity by 1.4- to 2.8-fold and resulted in a significant enhancement of the antitumor activity of mitomycin C, but not of RH1. This appeared to be due to the presence of a sufficient constitutive level of NQO1 activity in the tumor cells to fully activate the RH1. Mice were implanted with HL60 human promyelocytic leukemia cells, which have low levels of NQO1 activity. The mice were fed control or dimethyl fumarate-containing diet and were treated with RH1. NQO1 activity in the tumors increased but RH1 produced no antitumor activity in mice fed control or dimethyl fumarate diet. This is consistent with a narrow window of NQO1 activity between no RH1 activation and maximum RH1 activation. This study suggests that selective induction of NQO1 in tumor cells is not likely to be an effective strategy for enhancing the antitumor activity of RH1. In addition, we found that RH1 treatment produced significant leukopenia in mice that may be of concern in the clinic. These results suggest that the ease of reduction of RH1 by NQO1 makes it a poor candidate for an enzyme-directed approach to cancer therapy.

Dietary induction of NQO1 increases the antitumour activity of mitomycin C in human colon tumours in vivo

The bioreductive antitumour agent, mitomycin C (MMC), requires activation by reductive enzymes like NAD(P)H:quinone oxidoreductase 1 (NQO1). We used a novel approach to increase MMC efficacy by selectively inducing NQO1 in tumour cells in vivo. CD-1 nude mice were implanted with HCT116 cells, and fed control diet or diet containing 0.3% of the NQO1 inducer, dimethyl fumarate (DMF). The mice were then treated with saline, 2.0, 3.5 or 2.0 mg kg⁻¹ MMC and dicoumarol, an NQO1 inhibitor. The DMF diet increased NQO1 activity by 2.5-fold in the tumours, but had no effect in marrow cells. Mice given control diet/2.0 mg kg⁻¹ MMC had tumours with the same volume as control mice; however, mice given DMF diet/2.0 mg kg⁻¹ MMC had significantly smaller tumours. Tumour volumes in mice given DMF/2.0 mg kg⁻¹ MMC were similar to those in mice given control diet/3.5 mg kg⁻¹ MMC. Tumour inhibition was partially reversed in mice given DMF/2.0 mg kg⁻¹ MMC and dicoumarol. DMF diet/2.0 mg kg⁻¹ MMC treatment did not increase myelosuppression and did not produce any organ toxicity. These results provide strong evidence that dietary inducers of NQO1 can increase the antitumour activity of bioreductive agents like MMC without increasing toxicity.

Enhancement of solubility and bioavailability of beta-lapachone using cyclodextrin inclusion complexes

PURPOSE

To explore the use of cyclodextrins (CD) to form inclusion complexes with beta-lapachone (beta-lap) to overcome solubility and bioavailability problems previously noted with this drug.

METHODS

Inclusion complexes between beta-lap and four cyclodextrins (alpha-, beta-, gamma-, and HPbeta-CD) in aqueous solution were investigated by phase solubility studies, fluorescence, and 1H-NMR spectroscopy. Biologic activity and bioavailability of beta-lap inclusion complexes were investigated by in vitro cytotoxicity studies with MCF-7 cells and by in vivo lethality studies with C57Blk/6 mice (18-20 g).

RESULTS

Phase solubility studies showed that beta-lap solubility increased in a linear fashion as a function of alpha-, beta-, or HPbeta-CD concentrations but not gamma-CD. Maximum solubility of beta-lap was achieved at 16.0 mg/ml or 66.0 mM with HPbeta-CD. Fluorescence and 1H-NMR spectroscopy proved the formation of 1:1 inclusion complexes between beta-CD and HPbeta-CD with beta-lap. Cytotoxicity assays with MCF-7 cells showed similar biologic activities of beta-lap in beta-CD or HPbeta-CD inclusion complexes (TD50 = 2.1 microM). Animal studies in mice showed that the LD50 value of beta-lap in an HPbeta-CD inclusion complex is between 50 and 60 mg/kg.

CONCLUSIONS

Complexation of beta-lap with HPbeta-CD offers a major improvement in drug solubility and bioavailability.

Induction of NAD(P)H:quinone reductase by probucol: a possible mechanism for protection against chemical carcinogenesis and toxicity

Dietary antioxidants protect laboratory animals against induction of tumours by a variety of chemical carcinogens. Among possible mechanism, protection against chemical carcinogenesis could be mediated via antioxidant-dependent induction of detoxifying enzymes, including quinone reductase and glutathione S-transferase (GSH transferase). Probucol is used cholesterol-lowering drug used in the clinic, with pronounced antioxidant effect that protect against chemical carcinogenesis and toxicity. In the present study we therefore examined the ability of probucol to induce activities of quinone reductase in the cytosolic fractions of various tissues of mice. Quinone reductase activity was increased significantly in 6 of 8 tissues examined from probucol-fed mice. The greatest proportionate increase, to 1.8 times control levels, was observed in liver. Probucol also increased quinone reductase activities of forestomach, heart, kidney, lungs and spleen. Quinone reductase is a major enzyme of xenobiotic metabolism that carries out obligatory two-electron reductions and thereby protects cells against toxicity of quinones. It is induced in many tissues coordinately with other enzymes that protect against electrophilic toxicity. The protective effects of probucol appear to be due, at least in part, to the ability of this antioxidant to increase the activities in rodent tissues of several enzymes involved in the non-oxidative metabolism of a wide variety of xenobiotics. The induction of such enzyme, quinone reductase by probucol suggests the potential value of this compound as a protective agent against chemical carcinogenesis and other forms of electrophilic toxicity. The significance of these results can be implicated in relation to cancer chemopreventive effects of probucol in various target organs.

P53 hot-spot mutants are resistant to ubiquitin-independent degradation by increased binding to NAD(P)H:quinone oxidoreductase 1

Proteasomal degradation of p53 is mediated by two alternative pathways that are either dependent or independent of both Mdm2 and ubiquitin. The ubiquitin-independent pathway is regulated by NAD(P)H: quinone oxidoreductase 1 (NQO1) that stabilizes p53. The NQO1 inhibitor dicoumarol induces ubiquitin-independent p53 degradation. We now show that, like dicoumarol, several other coumarin and flavone inhibitors of NQO1 activity, which compete with NAD(P)H for binding to NQO1, induced ubiquitin-independent p53 degradation and inhibited wild-type p53-mediated apoptosis. Although wild-type p53 and several p53 mutants were sensitive to dicoumarol-induced degradation, the most frequent "hot-spot" p53 mutants in human cancer, R175H, R248H, and R273H, were resistant to dicoumarol-induced degradation, but remained sensitive to Mdm2-ubiquitin-mediated degradation. The two alternative pathways for p53 degradation thus have different p53 structural requirements. Further mutational analysis showed that arginines at positions 175 and 248 were essential for dicoumarol-induced p53 degradation. NQO1 bound to wild-type p53 and dicoumarol, which induced a conformational change in NQO1, inhibited this binding. Compared with wild-type p53, the hot-spot p53 mutants showed increased binding to NQO1, which can explain their resistance to dicoumarol-induced degradation. NQO1 thus has an important role in stabilizing hot-spot p53 mutant proteins in human cancer.

Human aldehyde dehydrogenases: potential pathological, pharmacological, and toxicological impact

Aldehyde dehydrogenases catalyze the pyridine nucleotide-dependent oxidation of aldehydes to acids. Seventeen enzymes are currently viewed as belonging to the human aldehyde dehydrogenase superfamily. Summarized herein, insofar as the information is available, are the structural composition, physical properties, tissue distribution, subcellular location, substrate specificity, and cofactor preference of each member of this superfamily. Also summarized are the chromosomal locations and organization of the genes that encode these enzymes and the biological consequences when enzyme activity is lost or substantially diminished. Broadly, aldehyde dehydrogenases can be categorized as critical for normal development and/or physiological homeostasis (1). even when the organism is in a friendly environment or (2). only when the organism finds itself in a hostile environment. The primary, if not sole, evolved raison d'être of first category aldehyde dehydrogenases appears to be to catalyze the biotransformation of a single endobiotic for which they are relatively specific and of which the resultant metabolite is essential to the organism. Most of the human aldehyde dehydrogenases for which the relevant information is available fall into this category. Second category aldehyde dehydrogenases are relatively substrate nonspecific and their evolved raison d'être seems to be to protect the organism from potentially harmful xenobiotics, specifically aldehydes or xenobiotics that give rise to aldehydes, by catalyzing their detoxification. Thus, the lack of a fully functional first category aldehyde dehydrogenase results in a gross pathological phenotype in the absence of any insult, whereas the lack of a functional second category aldehyde dehydrogenase is ordinarily of no consequence with respect to gross phenotype, but is of consequence in that regard when the organism is subjected to a relevant insult.

Transplasma membrane electron transport: enzymes involved and biological function

The notion of transmembrane electron transport is usually associated with mitochondria and chloroplasts. However, since the early 1970s, it has been known that this phenomenon also occurs at the level of the plasma membrane. Ever since, evidence has accumulated for the existence of a plethora of transplasma membrane electron transport enzymes. In this review, we discuss the various enzymes known, their molecular characteristics and their biological functions.

Cytotoxicity of RH1 and related aziridinylbenzoquinones: involvement of activation by NAD(P)H:quinone oxidoreductase (NQO1) and oxidative stress

It is supposed that the main cytotoxicity mechanism of antitumour aziridinyl-substituted benzoquinones is their two-electron reduction to alkylating products by NAD(P)H:quinone oxidoreductase (NQO1, DT-diaphorase, EC 1.6.99.2). However, other possible cytotoxicity mechanisms, e.g., oxidative stress, are studied insufficiently. In the single-electron reduction of quinones including a novel compound RH1 (2,5-diaziridinyl- 3-(hydroxymethyl)-6-methyl-1,4-benzoquinone), by NADPH:cytochrome P-450 reductase (EC 1.6.2.4, P-450R), their reactivity increased with an increase in the redox potential of quinone/semiquinone couple (E(1)7), reaching a limiting value at E(1)7> or =-0.1V. The reactivity of quinones towards NQO1 did not depend on their E(1)7. The cytotoxicity of aziridinyl-unsubstituted quinones in bovine leukemia virus-transformed lamb kidney fibroblasts (line FLK) mimics their reactivity in P-450R-catalyzed reactions, exhibiting a parabolic dependence on their E(1)7. The toxicity of aziridinyl-benzoquinones, although being higher, also followed this trend and did not depend on their reactivity towards NQO1. The action of aziridinylbenzoquinones in FLK cells was accompanied by an increase in lipid peroxidation, their toxicity decreased by desferrioxamine and the antioxidant N,N'-diphenyl-p-phenylene diamine, and potentiated by 1,3-bis-(2-chloroethyl)-1-nitrosourea. The inhibitor of NQO1, dicumarol, protected against the toxicity of aziridinyl-benzoquinones except of 2,5-bis-(2'-hydroxyethylamino)-3,6-diaziridinyl-1,4-benzoquinone (BZQ), which was almost inactive as NQO1 substrate. The same events except the absence of pronounced effect of dicumarol were characteristic in the cytotoxicity of aziridinyl-unsubstituted quinones. These findings indicate that in addition to the activation by NQO1, the oxidative stress presumably initiated by single-electron transferring enzymes may be an important factor in the cytotoxicity of aziridinylbenzoquinones. The information obtained may contribute to the understanding of the molecular mechanisms of aziridinylquinone cytotoxicity and may be useful in the design of future bioreductive drugs.