Effect of Genetic Variation on Human Cytochrome P450 Reductase-Mediated Paraquat Cytotoxicity

Multifaceted interplays between the essential players and lipid peroxidation in ferroptosis

Ferroptosis, a type of programmed cell death, represents a distinct paradigm in cell biology. It is characterized by the iron-dependent accumulation of reactive oxygen species, which induce lipid peroxidation (LPO), and is orchestrated by the interplay between iron, lipid peroxides, and glutathione. In this review, we emphasize the frequently overlooked role of iron in LPO beyond the classical iron-driven Fenton reaction in several crucial processes that regulate cellular iron homeostasis, including iron intake and export as well as ferritinophagy, and the emerging roles of endoplasmic reticulum-resident flavoprotein oxidoreductases, especially P450 oxidoreductases, in modulating LPO. We summarize how various types of fatty acids (FAs), including saturated, monounsaturated, and polyunsaturated FAs, differentially influence ferroptosis when incorporated into phospholipids. Furthermore, we highlight the therapeutic potential of targeting LPO to mitigate ferroptosis and discuss the regulatory mechanisms of endogenous lipophilic radical-trapping antioxidants that confer resistance to ferroptosis, shedding light on therapeutic avenues for ferroptosis-associated diseases.

The POR rs10954732 polymorphism decreases susceptibility to hepatocellular carcinoma and hepsin as a prognostic biomarker correlated with immune infiltration based on proteomics

The effect of the cytochrome P450 oxidoreductase (POR) rs10954732 (G > A) polymorphism on hepatocellular carcinoma (HCC) susceptibility is unknown. Here we found that A allele carriers showed a 69% decrease in susceptibility to HCC with overall survival (OS) prolonged to 199%, accompanied by lower activity for cytochrome P450 2E1. A total of 222 differentially expressed proteins were mainly enriched in neutrophil and T cell activation and involved in the immune and inflammatory responses, constituting the altered immune tumor microenvironment related with A allele by proteomics analysis. Hepsin (HPN) showed significant down-regulation in HCC and up-regulation in A allele carriers. A lower HPN level was associated with increased susceptibility to HCC and a worse prognosis. Moreover, HPN is a potential independent prognostic biomarker for HCC and is strongly associated with clinicopathological features, tumor-infiltrating status of immune cells both in our discovery cohort and database surveys. Our findings provide a new potential mechanism by which HPN may play an important role in the susceptibility of rs10954732 A allele carriers to HCC and their prognosis through tumor immune infiltration, thus offering potential insights for future studies on tumor immunotherapy.

ADME/T-based strategies for paraquat detoxification: Transporters and enzymes

Paraquat (PQ) is a toxic, organic herbicide for which there is no specific antidote. Although banned in some countries, it is still used as an irreplaceable weed killer in others. The lack of understanding of the precise mechanism of its toxicity has hindered the development of treatments for PQ exposure. While toxicity is thought to be related to PQ-induced oxidative stress, antioxidants are limited in their ability to ameliorate the untoward biological responses to this agent. Summarized in this review are data on the absorption, distribution, metabolism, excretion, and toxicity (ADME/T) of PQ, focusing on the essential roles of individual transporters and enzymes in these processes. Based on these findings, strategies are proposed to design and test specific and effective antidotes for the clinical management of PQ poisoning.

CD34+ derived macrophage and dendritic cells display differential responses to paraquat

Paraquat (PQ) is a redox cycling herbicide known for its acute toxicity in humans. Airway parenchymal cells have been identified as primary sites for PQ accumulation, tissue inflammation and cellular injury. However, the role of immune cells in PQ induced tissue injury is largely unknown. To explore this further, primary cultures of human CD34+ stem cell derived macrophages (MC^cd34) and dendritic cells (DC^cd34) were established and characterised using RNA-Seq profiling. The impact of PQ on DC^cd34 and MC^cd34 cytotoxicity revealed increased effect within DC^cd34 cultures. PQ toxicity mechanisms were examined using sub-cytotoxic concentrations and TempO-seq transcriptomic assays. Comparable increases for several stress response pathway (NFE2L2, NF-kB and HSF) dependent genes were observed across both cell types. Interestingly, PQ induced unfolded protein response (UPR), p53, Irf and DC maturation genes in DC^cd34 but not in MC^cd34. Further exploration of the immune modifying potential of PQ was performed using the common allergen house dust mite (HD). Co-treatment of PQ and HD resulted in enhanced inflammatory responses within MC^cd34 but not DC^cd34. These results demonstrate immune cell type differential responses to PQ, that may underlie aspects of acute toxicity and susceptibility to inflammatory disease.

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Paraquat induces inflammatory and oxidative events in immune cells.

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Paraquat prompts selective induction of several pathways in dendritic cells.

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Paraquat and dust mite co-exposure enhances inflammatory response in macrophages.

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These results provide insight into paraquat mechanisms of toxicity.

Gallic acid exerts anti-inflammatory, anti-oxidative stress, and nephroprotective effects against paraquat-induced renal injury in male rats

Paraquat (PRQ) is a toxic chemical compound that is very noxious to animals and humans. Gallic acid is a phenolic compound that has antioxidant properties. In this study, we evaluated the ameliorative effect of gallic acid against PRQ-induced renal injury and oxidative stress. In this research, the rats were segregated into six groups. Group 1 is the control group; group 2 received paraquat only; group 3 received gallic acid only; and groups 4, 5, and 6 received paraquat plus gallic acid at doses of 25, 50, and 100 mg/kg bw respectively. Findings of this work displayed that the renal contents of the vitamin C, superoxide dismutase (SOD), and catalase (CAT) significantly reduced and the levels of the serum protein carbonyl, creatinine, serum glutamate pyruvate transaminase (sGPT), urea, serum glutamate oxaloacetate transaminase (sGOT), uric acid, MDA, serum IL-1β, and the kidney IL-1β gene expression were remarkably increased in the group receiving PRQ only compared with that in the control group. On the other hand, treatment with gallic acid after exposure to PRQ led to a significant elevation in renal vitamin C, SOD, and CAT levels plus a remarkable decrease in the serum protein carbonyl, creatinine, sGPT, urea, sGOT, uric acid, MDA, IL-1β, and renal gene expression of IL-1β in comparison with the PRQ-only-treated rats. Histological changes were also ameliorated by gallic acid administration. The data approve that gallic acid diminished the deleterious effects of PRQ exposure. In this regard, our results indicated that the administration of gallic acid could alleviate the noxious effects of PRQ on the antioxidant defense system and renal tissue.

Paraquat exposure delays late-stage Leydig cell differentiation in rats during puberty

Paraquat is a fast and non-selective herbicide that is widely used in crop cultivation and conservation tillage systems. Animal experiments have shown that paraquat decreases sperm quality and testicular organ coefficient, but its effects on the development of Leydig cells remain unclear. The objective of the current study was to investigate the effects of paraquat exposure on the Leydig cell development in rats during puberty. Twenty-eight male 35-day-old Sprague-Dawley rats were divided into 4 groups: 0, 0.5, 2.0, and 8 mg kg^-1 d^-1 paraquat. Paraquat was gavaged for 10 d. Adult Leydig cells were isolated and treated with paraquat for 24 h. Paraquat in vivo significantly decreased body and testis weights at 8 mg kg^-1 and lowered serum testosterone levels at 2 and 8 mg kg^-1 without affecting the levels of serum luteinizing hormone and follicle-stimulating hormone. Paraquat did not alter Leydig cell number and PCNA labeling index. Real-time PCR showed that paraquat down-regulated the expression of Lhcgr, Scarb1, Cyp11a1, Cyp17a1, and Hsd17b3 genes and their proteins at 2 or 8 mg kg^-1, while it up-regulated the expression of Srd5a1 at 8 mg kg^-1. Paraquat increased ROS and decreased testosterone production by Leydig cells at 1 and 10 μM after in vitro 24-h exposure. Vitamin E (40 μg/ml) reversed paraquat-induced ROS and suppression of testosterone synthesis in vitro. In conclusion, paraquat directly delays Leydig cell differentiation to block testosterone synthesis via down-regulating the expression of critical testosterone synthesis-related genes and up-regulating the expression of testosterone metabolic enzyme (Srd5a1) gene and possibly via increasing ROS production.

Paraquat exposure delays stem/progenitor Leydig cell regeneration in the adult rat testis

Paraquat, a widely used nonselective herbicide, is a serious hazard to human health. However, the effects of paraquat on the male reproductive system remain unclear. In this study, adult male Sprague Dawley rats were intraperitoneally injected ethane dimethane sulfonate (EDS, 75 mg/kg) to initiate a regeneration of Leydig cells. EDS-treated rats were orally exposed to paraquat (0.5, 2, 8 mg/kg/day) from post-EDS day 17 to day 28 and effects of paraquat on Leydig and Sertoli cell functions on post-EDS day 35 and day 56 were investigated. Paraquat significantly decreased serum testosterone levels at 2 and 8 mg/kg. Paraquat lowered Leydig cell Hsd17b3, Srd5a1, and Hsd11b1 mRNA levels but increased Hsd3b1 on post-EDS day 35. Paraquat lowered Cyp11a1, Cyp17a1, and Hsd11b1 but increased Srd5a1 on post-EDS day 56. However, paraquat did not alter Leydig cell number and PCNA labeling index. Epididymal staining showed that few sperms were observed in paraquat-treated rats. Primary culture of adult Leydig cells showed that paraquat diminished testosterone output and induced reactive oxygen species generation at 1 and 10 μM and apoptosis rate at 10 μM. In conclusion, a short-term exposure to paraquat delays Leydig cell regeneration from stem/progenitor Leydig cells, causing low production of testosterone and an arrest of spermatogenesis.

A CRISPR screen identifies a pathway required for paraquat-induced cell death

Paraquat, a herbicide linked to Parkinson's disease, generates reactive oxygen species (ROS), which causes cell death. Because the source of paraquat-induced ROS production remains unknown, we conducted a CRISPR-based positive-selection screen to identify metabolic genes essential for paraquat-induced cell death. Our screen uncovered three genes, POR (cytochrome P450 oxidoreductase), ATP7A (copper transporter), and SLC45A4 (sucrose transporter), required for paraquat-induced cell death. Furthermore, our results revealed POR as the source of paraquat-induced ROS production. Thus, our study highlights the use of functional genomic screens for uncovering redox biology.

Functional POR A503V is associated with the risk of bladder cancer in a Chinese population

Human cytochrome P450 oxidoreductase (POR) plays important roles in the metabolism of exogenous carcinogens and endogenous sterol hormones. However, few studies have explored the association between POR variants and the risk of bladder cancer. In this study, we first sequenced all 16 POR exons among 50 randomly selected controls, and found three variants, rs1135612, rs1057868 (A503V) and rs2228104, which were then assessed the relation to risk of bladder cancer in a case-control study of 1,050 bladder cancer cases and 1,404 cancer-free controls in a Chinese population. People with A503V TT genotype have a decreased risk of bladder cancer in a recessive model (TT vs. CC/CT, OR = 0.73, 95% CI = 0.57–0.93), which was more pronounced among elderly male, non-smoking, subjects. Especially, A503V TT genotype showed a protective effect in the invasive tumor stage. Functional analysis revealed that A503V activity decreased in cytochrome c reduction (50.5 units/mg vs. 135.4 units/mg), mitomycin C clearance (38.3% vs. 96.8%), and mitomycin C-induced colony formation (78.0 vs 34.3 colonies per dish). The results suggested that POR A503V might decrease the risk of bladder cancer by reducing its metabolic activity, and should be a potential biomarker for predicting the susceptibility to human bladder cancer.

The mechanism of rapamycin in the intervention of paraquat-induced acute lung injury in rats

1. Paraquat (PQ) is an organic nitrogen heterocyclic herbicide that is widely used in agriculture throughout the world. Numerous studies have reported PQ intoxication on humans. 2. In this study, we established a rat lung injury model induced by PQ and evaluated the intervention effect of rapamycin on the model, exploring the pathogenesis of PQ on lung injury as well as therapeutic effects of rapamycin on PQ-induced lung injury. 3. A rat lung injury model was established by gavage of PQ, and rapamycin was used to treat the model animals with PQ-induced lung injury. Different physiological indices were measured through Western blot and real-time polymerase chain reaction to evaluate the effect of rapamycin on the PQ-induced lung injury. 4. The analyses showed that application of rapamycin could significantly reduce the lung injury damage caused by PQ, with lung tissue wet-dry weight ratio, pathological features, compositions in serum, protein in bronchoalveolar lavage fluid and other indices being significantly improved after the injection of rapamycin. 5. It was inferred that the use of rapamycin could improve the PQ-induced lung injury through inhibiting the activity of mTOR. And we expected the use of rapamycin to be a potential treatment method for the PQ intoxication in future.

Role of protein-protein interactions in cytochrome P450-mediated drug metabolism and toxicity

Through their unique oxidative chemistry, cytochrome P450 monooxygenases (CYPs) catalyze the elimination of most drugs and toxins from the human body. Protein–protein interactions play a critical role in this process. Historically, the study of CYP–protein interactions has focused on their electron transfer partners and allosteric mediators, cytochrome P450 reductase and cytochrome b5. However, CYPs can bind other proteins that also affect CYP function. Some examples include the progesterone receptor membrane component 1, damage resistance protein 1, human and bovine serum albumin, and intestinal fatty acid binding protein, in addition to other CYP isoforms. Furthermore, disruption of these interactions can lead to altered paths of metabolism and the production of toxic metabolites. In this review, we summarize the available evidence for CYP protein–protein interactions from the literature and offer a discussion of the potential impact of future studies aimed at characterizing noncanonical protein–protein interactions with CYP enzymes.

Protective effects of exogenous β-hydroxybutyrate on paraquat toxicity in rat kidney

In this study, we demonstrated the protective effects of β-hydroxybutyrate (β-HB) against paraquat (PQ)-induced kidney injury and elucidated the underlying molecular mechanisms. By histological examination and renal dysfunction specific markers (serum BUN and creatinine) assay, β-HB could protect the PQ-induced kidney injury in rat. PQ-induced kidney injury is associated with oxidative stress, which was measured by increased lipid peroxidation (MDA) and decreased intracellular anti-oxidative abilities (SOD, CAT and GSH). β-HB pretreatment significantly attenuated that. Caspase-mediated apoptosis pathway contributed importantly to PQ toxicity, as revealed by the activation of caspase-9/-3, cleavage of PARP, and regulation of Bcl-2 and Bax, which were also effectively blocked by β-HB. Moreover, treatment of PQ strongly decreased the nuclear Nrf2 levels. However, pre-treatment with β-HB effectively suppressed this action of PQ. This may imply the important role of β-HB on Nrf2 pathway. Taken together, this study provides a novel finding that β-HB has a renoprotective ability against paraquat-induced kidney injury.

Influence of various polymorphic variants of cytochrome P450 oxidoreductase (POR) on drug metabolic activity of CYP3A4 and CYP2B6

Cytochrome P450 oxidoreductase (POR) is known as the sole electron donor in the metabolism of drugs by cytochrome P450 (CYP) enzymes in human. However, little is known about the effect of polymorphic variants of POR on drug metabolic activities of CYP3A4 and CYP2B6. In order to better understand the mechanism of the activity of CYPs affected by polymorphic variants of POR, six full-length mutants of POR (e.g., Y181D, A287P, K49N, A115V, S244C and G413S) were designed and then co-expressed with CYP3A4 and CYP2B6 in the baculovirus-Sf9 insect cells to determine their kinetic parameters. Surprisingly, both mutants, Y181D and A287P in POR completely inhibited the CYP3A4 activity with testosterone, while the catalytic activity of CYP2B6 with bupropion was reduced to approximately ∼70% of wild-type activity by Y181D and A287P mutations. In addition, the mutant K49N of POR increased the CLint (Vmax/Km) of CYP3A4 up to more than 31% of wild-type, while it reduced the catalytic efficiency of CYP2B6 to 74% of wild-type. Moreover, CLint values of CYP3A4-POR (A115V, G413S) were increased up to 36% and 65% of wild-type respectively. However, there were no appreciable effects observed by the remaining two mutants of POR (i.e., A115V and G413S) on activities of CYP2B6. In conclusion, the extent to which the catalytic activities of CYP were altered did not only depend on the specific POR mutations but also on the isoforms of different CYP redox partners. Thereby, we proposed that the POR-mutant patients should be carefully monitored for the activity of CYP3A4 and CYP2B6 on the prescribed medication.

Redox cycling and increased oxygen utilization contribute to diquat-induced oxidative stress and cytotoxicity in Chinese hamster ovary cells overexpressing NADPH-cytochrome P450 reductase

Diquat and paraquat are nonspecific defoliants that induce toxicity in many organs including the lung, liver, kidney, and brain. This toxicity is thought to be due to the generation of reactive oxygen species (ROS). An important pathway leading to ROS production by these compounds is redox cycling. In this study, diquat and paraquat redox cycling was characterized using human recombinant NADPH-cytochrome P450 reductase, rat liver microsomes, and Chinese hamster ovary (CHO) cells constructed to overexpress cytochrome P450 reductase (CHO-OR) and wild-type control cells (CHO-WT). In redox cycling assays with recombinant cytochrome P450 reductase and microsomes, diquat was 10-40 times more effective at generating ROS compared to paraquat (K(M)=1.0 and 44.2μM, respectively, for H(2)O(2) generation by diquat and paraquat using recombinant enzyme, and 15.1 and 178.5μM, respectively for microsomes). In contrast, at saturating concentrations, these compounds showed similar redox cycling activity (V(max)≈6.0nmol H(2)O(2)/min/mg protein) for recombinant enzyme and microsomes. Diquat and paraquat also redox cycle in CHO cells. Significantly more activity was evident in CHO-OR cells than in CHO-WT cells. Diquat redox cycling in CHO cells was associated with marked increases in protein carbonyl formation, a marker of protein oxidation, as well as cellular oxygen consumption, measured using oxygen microsensors; greater activity was detected in CHO-OR cells than in CHO-WT cells. These data demonstrate that ROS formation during diquat redox cycling can generate oxidative stress. Enhanced oxygen utilization during redox cycling may reduce intracellular oxygen available for metabolic reactions and contribute to toxicity.

Distinct roles of cytochrome P450 reductase in mitomycin C redox cycling and cytotoxicity

Mitomycin c (MMC), a quinone-containing anticancer drug, is known to redox cycle and generate reactive oxygen species. A key enzyme mediating MMC redox cycling is cytochrome P450 reductase, a microsomal NADPH-dependent flavoenzyme. In the present studies, Chinese hamster ovary (CHO) cells overexpressing this enzyme (CHO-OR cells) and corresponding control cells (CHO-WT cells) were used to investigate the role of cytochrome P450 reductase in the actions of MMC. In lysates from both cell types, MMC was found to redox cycle and generate H(2)O(2); this activity was greater in CHO-OR cells (V(max) = 1.2 +/- 0.1 nmol H(2)O(2)/min/mg protein in CHO-WT cells versus 32.4 +/- 3.9 nmol H(2)O(2)/min/mg protein in CHO-OR cells). MMC was also more effective in generating superoxide anion and hydroxyl radicals in CHO-OR cells, relative to CHO-WT cells. Despite these differences in MMC redox cycling, MMC-induced cytotoxicity, as measured by growth inhibition, was similar in the two cell types (IC(50) = 72 +/- 20 nmol/L for CHO-WT and 75 +/- 23 nmol/L for CHO-OR cells), as was its ability to induce G(2)-M and S phase arrest. Additionally, in nine different tumor cell lines, although a strong correlation was observed between MMC-induced H(2)O(2) generation and cytochrome P450 reductase activity, there was no relationship between redox cycling and cytotoxicity. Hypoxia, which stabilizes MMC radicals generated by redox cycling, also had no effect on the sensitivity of tumor cells to MMC-induced cytotoxicity. These data indicate that NADPH cytochrome P450 reductase-mediated MMC redox cycling is not involved in the cytotoxicity of this chemotherapeutic agent.

Delivery of NADPH-cytochrome P450 reductase antisense oligos using avidin-biotin approach

Although avidin-mediated intracellular delivery of oligonucleotides or proteins has been shown before, the efficacy studies are lacking. Here, we tested the effectiveness of avidin for delivery of a cytochrome P450 reductase (CPR) antisense oligo in rat liver epithelial cells. A phosphorodiamidate morpholino oligo (PMO) against CPR was biotinylated using four reagents with short, cleavable, or long linkers, followed by conjugation with avidin. The dose-inhibitory response of the unmodified PMO in the presence of a transfection reagent (Endoporter, EP) and the effectiveness of the EP-assisted and avidin-assisted delivery of biotinylated PMOs were tested by Western blot analysis. Additionally, in a preliminary study, the avidin-biotin PMO with a long linker was also tested in vivo in rats. The biotinylated oligos were at least as effective as the unmodified oligo. Whereas the avidin conjugate of biotinylated PMO with the short linker was ineffective, those with the long linkers showed significant reductions in CPR protein expression. Finally, the in vivo study showed modest, but significant, reductions in CPR activity. In conclusion, these studies show for the first time that avidin-mediated intracellular delivery of biotinylated oligos can effectively knock down target genes in vitro, depending on the length of the linker. Additionally, the avidin-biotin approach may be of potential value for in vivo gene knockdown.

Age-dependent endothelial nitric oxide synthase uncoupling in pulmonary arteries of endoglin heterozygous mice

Endoglin is a TGF-β superfamily receptor critical for endothelial cell function. Mutations in this gene are associated with hereditary hemorrhagic telangiectasia type I (HHT1), and clinical signs of disease are generally more evident later in life. We previously showed that systemic vessels of adult Eng heterozygous ( Eng+/−) mice exhibit increased vasorelaxation due to uncoupling of endothelial nitric oxide synthase (eNOS). We postulated that these changes may develop with age and evaluated pulmonary arteries from newborn and adult Eng+/− mice for eNOS-dependent, acetylcholine (ACh-induced) vasorelaxation, compared with that of age-matched littermate controls. While ACh-induced vasorelaxation was similar in all newborn mice, it was significantly increased in the adult Eng+/− vs. control vessels. The vasodilatory responses were inhibited by l-NAME suggesting eNOS dependence. eNOS uncoupling was observed in lung tissues of adult, but not newborn, heterozygous mice and was associated with increased production of reactive O2 species (ROS) in adult Eng +/− vs. control lungs. Interestingly, ROS generation was higher in adult than newborn mice and so were the levels of NADPH oxidase 4 and SOD 1, 2, 3 isoforms. However, enzyme protein levels and NADPH activity were normal in adult Eng+/− lungs indicating that the developmental maturation of ROS generation and scavenging cannot account for the increased vasodilatation observed in adult Eng+/− mice. Our data suggest that eNOS-dependent H2O2 generation in Eng+/− lungs accounts for the heightened pulmonary vasorelaxation. To the extent that these mice mimic human HHT1, age-associated pulmonary vascular eNOS uncoupling may explain the late childhood and adult onset of clinical lung manifestations.

Preparation of methacrylic acid-modified rice husk improved by an experimental design and application for paraquat adsorption

Methacrylic acid (MAA) grafted rice husk was synthesized using graft copolymerization with Fenton's reagent as the redox initiator and applied to the adsorption of paraquat. The highest grafting percentage of 44.3% was obtained using the traditional kinetic method. However, a maximum grafting percentage of 65.3% was calculated using the central composite design. Experimental results based on the recipes predicted from the statistical analysis are consistent with theoretical calculations. A representative polymethacrylic acid-g-rice husk (PMAA-g-rice husk) copolymer was hydrolyzed to a salt type and applied to the adsorption of paraquat. The adsorption equilibrium data correlate more closely with the Langmuir isotherm than with the Freundlich equation. The maximum adsorption capacity of modified rice husk is 292.5mg/g-adsorbent. This value exceeds those for Fuller's earth and activated carbon, which are the most common binding agents used for paraquat. The samples at various stages were characterized by solid-state (13)C NMR spectroscopy.

Roles of DNA repair and reductase activity in the cytotoxicity of the hypoxia-activated dinitrobenzamide mustard PR-104A

PR-104 is a dinitrobenzamide mustard currently in clinical trial as a hypoxia-activated prodrug. Its major metabolite, PR-104A, is metabolized to the corresponding hydroxylamine (PR-104H) and amine (PR-104M), resulting in activation of the nitrogen mustard moiety. We characterize DNA damage responsible for cytotoxicity of PR-104A by comparing sensitivity of repair-defective hamster Chinese hamster ovary cell lines with their repair-competent counterparts. PR-104H showed a repair profile similar to the reference DNA cross-linking agents chlorambucil and mitomycin C, with marked hypersensitivity of XPF(-/-), ERCC1(-/-), and Rad51D(-/-) cells but not of XPD(-/-) or DNA-PK(CS)(-/-) cells. This pattern confirmed the expected dependence on the ERCC1-XPF endonuclease, implicated in unhooking DNA interstrand cross-links at blocked replication forks, and homologous recombination repair (HRR) in restarting collapsed forks. However, even under anoxia, the hypersensitivity of XPF(-/-), ERCC1(-/-), and Rad51D(-/-) cells to PR-104A itself was lower than for chlorambucil. To test whether this reflects inefficient PR-104A reduction, a soluble form of human NADPH:cytochrome P450 oxidoreductase was stably expressed in Rad51D(-/-) cells and their HRR-restored counterpart. This expression increased hypoxic metabolism of PR-104A to PR-104H and PR-104M as well as hypoxia-selective cytotoxicity of PR-104A and its dependence on HRR. We conclude that PR-104A cytotoxicity is primarily due to DNA interstrand cross-linking by its reduced metabolites, although under conditions of inefficient PR-104A reduction (low reductase expression or aerobic cells), a second mechanism contributes to cell killing. This study shows that hypoxia, reductase activity, and DNA interstrand cross-link repair proficiency are key variables that interact to determine PR-104A sensitivity.

P450 oxidoreductase: genetic polymorphisms and implications for drug metabolism and toxicity

BACKGROUND

Cytochrome P450 oxidoreductase (POR) is the only electron donor for all microsomal cytochrome P450 monooxygenases (CYP), some of which are phase I drug-metabolizing enzymes, responsible for oxidation of more than 80% of drugs.

OBJECTIVES

To provide a more thorough understanding of the genetic factors influencing drug metabolism, we address the role of genetic polymorphisms in the POR gene, and their implications for drug metabolism and cytotoxicity.

METHODS

The scope of this review is intended to cover polymorphisms currently identified in the POR gene, assess their functional significance on POR activity, and address their impact on CYP-mediated drug metabolism. POR is also responsible for directly metabolizing several anticancer prodrugs via a 1-electron reduction reaction, so the effect of POR polymorphisms on the direct bioactivation of drugs is also considered.

RESULTS/CONCLUSION

POR is a polymorphic enzyme that can affect CYP-mediated drug metabolism as well as direct bioactivation of prodrugs. Genetic polymorphisms in the POR gene may help to explain altered drug-metabolizing phenotypes.

Impairment of human CYP1A2-mediated xenobiotic metabolism by Antley-Bixler syndrome variants of cytochrome P450 oxidoreductase

Y459H and V492E mutations of cytochrome P450 reductase (CYPOR) cause Antley-Bixler syndrome due to diminished binding of the FAD cofactor. To address whether these mutations impaired the interaction with drug-metabolizing CYPs, a bacterial model of human liver expression of CYP1A2 and CYPOR was implemented. Four models were generated: POR(null), POR(wt), POR(YH), and POR(VE), for which equivalent CYP1A2 and CYPOR levels were confirmed, except for POR(null), not containing any CYPOR. The mutant CYPORs were unable to catalyze cytochrome c and MTT reduction, and were unable to support EROD and MROD activities. Activity was restored by the addition of FAD, with V492E having a higher apparent FAD affinity than Y459H. The CYP1A2-activated procarcinogens, 2-aminoanthracene, 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone, and 2-amino-3-methylimidazo(4,5-f)quinoline, were significantly less mutagenic in POR(YH) and POR(VE) models than in POR(wt), indicating that CYP1A2, and likely other drug-metabolizing CYPs, are impaired by ABS-related POR mutations as observed in the steroidogenic CYPs.

Metabolism of 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP) by human CYP1B1 genetic variants

Human cytochrome P450 1B1 (CYP1B1) plays a critical role in the metabolic activation of a variety of procarcinogens, including 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP). The existence of human CYP1B1 missense genetic variants has been demonstrated, but their activities in metabolizing PhIP are unknown. In this study, we expressed 15 naturally occurring CYP1B1 variants (with either single or multiple amino acid substitutions) and determined their activity changes in metabolizing PhIP to its two major metabolites, 2-hydroxyamino-PhIP and 4'-hydroxy-PhIP. Although the PhIP-metabolizing activities of four variants (Ala(119)Ser, Pro(379)Leu, Ala(443)Gly, Arg(48)Gly/Leu(432)Val) were comparable with that of the expressed wild-type CYP1B1, five variants (Trp(57)Cys, Gly(61)Glu, Arg(48)Gly/Ala(119)Ser, Arg(48)Gly/Ala(119)Ser/Leu(432)Val, Arg(48)Gly/Ala(119)Ser/Leu(432)Val/Ala(443)Gly) exhibited more than 2-fold decrease in activity and a reduction in the catalytic efficiency (V(max)/K(m)) for both N- and 4-hydroxylation of PhIP. Six variants (Gly(365)Trp, Glu(387)Lys, Arg(390)His, Pro(437)Leu, Asn(453)Ser, Arg(469)Trp) showed little activity in PhIP metabolism, but the molecular mechanisms involved are apparently different. The microsomal CYP1B1 protein level was significantly decreased for the Trp(365), Lys(387), and His(390) variants and was not detectable for the Ser(453) variant. In contrast, there was no difference between the Trp(469) variant and the wild-type in the microsomal CYP1B1 protein level and P450 content but the Trp(469) variant totally lost its metabolic activity toward PhIP. The Leu(437) variant also had a substantial amount of CYP1B1 protein in the microsomes, but there was a lack of detectable P450 peak and activity. Our results should be useful in selecting appropriate CYP1B1 variants as cancer susceptibility biomarkers for human population studies related to PhIP exposure.

Role of cytochrome P450 reductase in nitrofurantoin-induced redox cycling and cytotoxicity

The one-electron reduction of redox-active chemotherapeutic agents generates highly toxic radical anions and reactive oxygen intermediates (ROI). A major enzyme catalyzing this process is cytochrome P450 reductase. Because many tumor cells highly express this enzyme, redox cycling of chemotherapeutic agents in these cells may confer selective antitumor activity. Nitrofurantoin is a commonly used redox-active antibiotic that possesses antitumor activity. In the present studies we determined whether nitrofurantoin redox cycling is correlated with cytochrome P450 reductase activity and cytotoxicity in a variety of cell lines. Recombinant cytochrome P450 reductase was found to support redox cycling of nitrofurantoin and to generate superoxide anion, hydrogen peroxide, and, in the presence of redox-active iron, hydroxyl radicals. This activity was NADPH dependent and inhibitable by diphenyleneiodonium, indicating a requirement for the flavin cofactors in the reductase. Nitrofurantoin-induced redox cycling was next analyzed in different cell lines varying in cytochrome P450 reductase activity including Chinese hamster ovary cells (CHO-OR) constructed to overexpress the enzyme. Nitrofurantoin-induced hydrogen peroxide production was 16-fold greater in lysates from CHO-OR cells than from control CHO cells. A strong correlation between cytochrome P450 reductase activity and nitrofurantoin-induced redox cycling among the cell lines was found. Unexpectedly, no correlation between nitrofurantoin-induced ROI production and cytotoxicity was observed. These data indicate that nitrofurantoin-induced redox cycling and subsequent generation of ROI are not sufficient to mediate cytotoxicity and that cytochrome P450 reductase is not a determinant of sensitivity to redox-active chemotherapeutic agents.

Oxygen, the lead actor in the pathophysiologic drama: enactment of the trinity of normoxia, hypoxia, and hyperoxia in disease and therapy

Aerobic life has evolved a dependence on molecular oxygen for its mere survival. Mitochondrial oxidative phosphorylation absolutely requires oxygen to generate the currency of energy in aerobes. The physiologic homeostasis of these organisms is strictly maintained by optimal cellular and tissue-oxygenation status through complex oxygen-sensing mechanisms, signaling cascades, and transport processes. In the event of fluctuating oxygen levels leading to either an increase (hyperoxia) or decrease (hypoxia) in cellular oxygen, the organism faces a crisis involving depletion of energy reserves, altered cell-signaling cascades, oxidative reactions/events, and cell death or tissue damage. Molecular oxygen is activated by both nonenzymatic and enzymatic mechanisms into highly reactive oxygen species (ROS). Aerobes have evolved effective antioxidant defenses to counteract the reactivity of ROS. Although the ROS are also required for many normal physiologic functions of the aerobes, overwhelming production of ROS coupled with their insufficient scavenging by endogenous antioxidants will lead to detrimental oxidative stress. Needless to say, molecular oxygen is at the center of oxygenation, oxidative phosphorylation, and oxidative stress. This review focuses on the biology and pathophysiology of oxygen, with an emphasis on transport, sensing, and activation of oxygen, oxidative phosphorylation, oxygenation, oxidative stress, and oxygen therapy.

An RNAi-based genetic screen for oxidative stress resistance reveals retinol saturase as a mediator of stress resistance

Oxidative stress has been implicated in the pathogenesis of numerous late-onset diseases as well as organismal longevity. Nevertheless, the genetic components that affect cellular sensitivity to oxidative stress have not been explored extensively at the genome-wide level in mammals. Here we report an RNA interference (RNAi) screen for genes that increase resistance to an organic oxidant, tert-butylhydroperoxide (tert-BHP), in cultured fibroblasts. The loss-of-function screen allowed us to identify several short hairpin RNAs (shRNAs) that elevated the cellular resistance to tert-BHP. One of these shRNAs strongly protected cells from tert-BHP and H(2)O(2) by specifically reducing the expression of retinol saturase, an enzyme that converts all-trans-retinol (vitamin A) to all-trans-13,14-dihydroretinol. The protective effect was well correlated with the reduction in mRNA level and was observed in both primary fibroblasts and NIH3T3 cells. The results suggest a novel role for retinol saturase in regulating sensitivity to oxidative stress and demonstrate the usefulness of large-scale RNAi screening for elucidating new molecular pathways involved in stress resistance.

Paraquat Increases Cyanide-insensitive Respiration in Murine Lung Epithelial Cells by Activating an NAD(P)H:Paraquat Oxidoreductase

Pulmonary fibrosis is one of the most severe consequences of exposure to paraquat, an herbicide that causes rapid alveolar inflammation and epithelial cell damage. Paraquat is known to induce toxicity in cells by stimulating oxygen utilization via redox cycling and the generation of reactive oxygen intermediates. However, the enzymatic activity mediating this reaction in lung cells is not completely understood. Using self-referencing microsensors, we measured the effects of paraquat on oxygen flux into murine lung epithelial cells. Paraquat (10-100 microm) was found to cause a 2-4-fold increase in cellular oxygen flux. The mitochondrial poisons cyanide, rotenone, and antimycin A prevented mitochondrial- but not paraquat-mediated oxygen flux into cells. In contrast, diphenyleneiodonium (10 microm), an NADPH oxidase inhibitor, blocked the effects of paraquat without altering mitochondrial respiration. NADPH oxidases, enzymes that are highly expressed in lung epithelial cells, utilize molecular oxygen to generate superoxide anion. We discovered that lung epithelial cells possess a distinct cytoplasmic diphenyleneiodonium-sensitive NAD(P)H:paraquat oxidoreductase. This enzyme utilizes oxygen, requires NADH or NADPH, and readily generates the reduced paraquat radical. Purification and sequence analysis identified this enzyme activity as thioredoxin reductase. Purified paraquat reductase from the cells contained thioredoxin reductase activity, and purified rat liver thioredoxin reductase or recombinant enzyme possessed paraquat reductase activity. Reactive oxygen intermediates and subsequent oxidative stress generated from this enzyme are likely to contribute to paraquat-induced lung toxicity.

Genetic variation of human cytochrome p450 reductase as a potential biomarker for mitomycin C-induced cytotoxicity

The importance of genetic variation in clinical response to various drugs is now well recognized. Identification of genetic biomarkers that can predict efficacy and toxicity of chemotherapeutic drugs in cancer patients holds great promise in treatment improvement and cost reduction. Mitomycin C (MMC) is a common anticancer drug used for the treatment of numerous types of tumors. Metabolism-mediated activation, by either one-electron or two-electron reduction, plays a critical role in the chemotherapeutic action of MMC. NADPH-cytochrome P450 (oxido)reductase (POR) is a major enzyme responsible for MMC activation through the one-electron reductive pathway, which leads to the production of semiquinone anion radicals and subsequent DNA damage in the cells. Recently, a total of six naturally occurring human POR variants with single amino acid changes (Y181D, A287P, R457H, V492E, C569Y, and V608F) have been identified. Although the catalytic efficiency of these variants in reduction of cytochrome c was reported to be altered, their capability in activating MMC, a direct substrate of POR, has not been examined. In the present study, we demonstrated that except for the C569Y variant, MMC-induced toxicity assayed as cell viability and proliferative capability was significantly decreased in the Flp-In Chinese hamster ovary cells stably expressing all the other POR variants in comparison with the cells expressing wild-type human POR. Cells expressing the V608F and Y181D variants had a complete loss of the capability to activate MMC. Our finding suggests that these functional POR genetic variations may serve as a potential biomarker to predict the chemotherapeutic response to MMC.