Inhibition of aryl hydrocarbon receptor transactivation and DNA adduct formation by CYP1 isoform-selective metabolic deactivation of benzo[a]pyrene

3D-QSAR assisted identification of selective CYP1B1 inhibitors: an effective bioisosteric replacement/molecular docking/electrostatic complementarity analysis

Cytochrome P450-1B1 is a majorly overexpressed drug-metabolizing enzyme in tumors and is responsible for inactivation and subsequent resistance to a variety of anti-cancer drugs, i.e., docetaxel, tamoxifen, and cisplatin. In the present study, a 3D quantitative structure-activity relationship (3D-QSAR) model has been constructed for the identification, design, and optimization of novel CYP1B1 inhibitors. The model has been built using a set of 148 selective CYP1B1 inhibitors. The developed model was evaluated based on certain statistical parameters including q2 and r2 which showed the acceptable predictive and descriptive capability of the generated model. The developed 3D-QSAR model assisted in understanding the key molecular fields which were firmly related to the selective CYP1B1 inhibition. A theoretic approach for the generation of new lead compounds with optimized CYP1B1 receptor affinity has been performed utilizing bioisosteric replacement analysis. These generated molecules were subjected to a developed 3D-QSAR model to predict the inhibitory activity potentials. Furthermore, these compounds were scrutinized through the activity atlas model, molecular docking, electrostatic complementarity, molecular dynamics, and waterswap analysis. The final hits might act as selective CYP1B1 inhibitors which could address the issue of resistance. This 3D-QSAR includes several chemically diverse selective CYP1B1 receptor ligands and well accounts for the individual ligand's inhibition affinities. These features of the developed 3D-QSAR model will ensure future prospective applications of the model to speed up the identification of new potent and selective CYP1B1 receptor ligands.

BPDE-DNA adduct formation and alterations of mRNA, protein, and DNA methylation of CYP1A1, GSTP1, and GSTM1 induced by benzo[a]pyrene and the intervention of aspirin in mice

Benzo[a]pyrene (B[a]P), one typical environmental pollutant, the toxicity mechanisms, and potential prevention remain perplexing. Available evidence suggests cytochrome P450 1A1 (CYP1A1) and glutathione S-transferases (GSTs) metabolize B[a]P, resulting in metabolic activation and detoxification of B[a]P. This study aimed to reveal the impact of B[a]P exposure on trans-7,8-diol-anti-9,10-epoxide DNA (BPDE-DNA) adduct formation, level of CYP1A1, glutathione S-transferase pi (GSTP1) and glutathione S-transferase mu1 (GSTM1) mRNA, protein and DNA methylation in mice, and the potential prevention of aspirin (ASP). This study firstly determined the BPDE-DNA adduct formation in an acute toxicity test of a large dose in mice induced by B[a]P, which subsequently detected CYP1A1, GSTP1, and GSTM1 at levels of mRNA, protein, and DNA methylation in the organs of mice in a subacute toxicity test at appropriate doses and the potential prevention of ASP, using the methods of real-time quantitative PCR (QPCR), western blotting, and real-time methylation-specific PCR (MSP), respectively. The results verified that B[a]P induced the formation of BPDE-DNA adduct in all the organs of mice in an acute toxicity test, and the order of concentration of which was lung > kidney > liver > brain. In a subacute toxicity test, following B[a]P treatment, mice showed a dose-dependent slowdown in body weight gain and abnormalities in behavioral and cognitive function and which were alleviated by ASP co-treatment. Compared to the controls, following B[a]P treatment, CYP1A1 was significantly induced in all organs in mice at mRNA level (P < 0.05), was suppressed in the lung and cerebrum of mice at protein level, and inhibited at DNA methylation level in the liver, lung, and cerebrum, whereas GSTP1 and GSTM1 at mRNA, protein, and DNA methylation levels showed organ-specific changes in mice following B[a]P treatment, which was generally alleviated by ASP intervention. In conclusion, B[a]P induced BPDE-DNA adduct formation in all organs in mice and altered the mRNA, protein, and DNA methylation levels in CYP1A1, GSTP1, and GSTM1 in an organ-dependent pattern, which could be related to the organ toxicity and mechanism of B[a]P. ASP intervention may be an effective measure to prevent B[a]P toxicity. The findings provide scientific evidence for further study on the organ toxicity and mechanisms of B[a]P.

Butyrate Enhances γ-H2AX Induced by Benzo[a]pyrene

Benzo[a]pyrene (BaP) is known to form DNA adduct following metabolic activation, which causes phosphorylation of histone H2AX (γ-H2AX). Recent studies have shown that histone deacetylase (HDAC) inhibitors enhanced BaP-induced CYP1A1 gene expression. In this study, we examined the relationship between the HDAC inhibitor-augmented metabolic activation and BaP-induced γ-H2AX. Sodium butyrate (SB), a typical HDAC inhibitor, enhanced BaP-induced γ-H2AX. The enhanced DNA damage was further confirmed by biased sinusoidal field gel electrophoresis, which detects DNA double-strand breaks. SB remarkably augmented BaP-induced CYP1A1 gene expression, and CYP1A1-overexpressing cells showed elevated generation of γ-H2AX. Furthermore, SB enhanced intracellular oxidation after treatment with BaP. These results suggested that SB-induced CYP1A1 upregulation facilitated BaP metabolism, which might result in excess DNA adducts or oxidative DNA damages, leading to augmentation of γ-H2AX.

Machine Learning Enabled Structure-Based Drug Repurposing Approach to Identify Potential CYP1B1 Inhibitors

Drug-metabolizing enzyme (DME)-mediated pharmacokinetic resistance of some clinically approved anticancer agents is one of the main reasons for cancer treatment failure. In particular, some commonly used anticancer medicines, including docetaxel, tamoxifen, imatinib, cisplatin, and paclitaxel, are inactivated by CYP1B1. Currently, no approved drugs are available to treat this CYP1B1-mediated inactivation, making the pharmaceutical industries strive to discover new anticancer agents. Because of the extreme complexity and high risk in drug discovery and development, it is worthwhile to come up with a drug repurposing strategy that may solve the resistance problem of existing chemotherapeutics. Therefore, in the current study, a drug repurposing strategy was implemented to find the possible CYP1B1 inhibitors using machine learning (ML) and structure-based virtual screening (SB-VS) approaches. Initially, three different ML models were developed such as support vector machines (SVMs), random forest (RF), and artificial neural network (ANN); subsequently, the best-selected ML model was employed for virtual screening of the selleckchem database to identify potential CYP1B1 inhibitors. The inhibition potency of the obtained hits was judged by analyzing the crucial active site amino acid interactions against CYP1B1. After a thorough assessment of docking scores, binding affinities, as well as binding modes, four compounds were selected and further subjected to in vitro analysis. From the in vitro analysis, it was observed that chlorprothixene, nadifloxacin, and ticagrelor showed promising inhibitory activity toward CYP1B1 in the IC₅₀ range of 0.07-3.00 μM. These new chemical scaffolds can be explored as adjuvant therapies to address CYP1B1-mediated drug-resistance problems.

Alterations of DNA methylation and mRNA levels of CYP1A1, GSTP1, and GSTM1 in human bronchial epithelial cells induced by benzo[a]pyrene

Benzo[a]pyrene (B[a]P) is a known human carcinogen and plays a major function in the initiation of lung cancer at its first proximity. However, the underlying molecular mechanisms are less well understood. In this study, we investigated the impact of B[a]P treatment on the DNA methylation and mRNA levels of CYP1A1, GSTP1, and GSTM1 in human bronchial epithelial cells (16HBEs), and provide scientific evidence for the mechanism study on the carcinogenesis of B[a]P. We treated 16HBEs with DMSO or concentrations of B[a]P at 1, 2, and 5 mmol/L for 24 h, observed the morphological changes, determined the cell viability, DNA methylation, and mRNA levels of CYP1A1, GSTP1, and GSTM1. Compared to the DMSO controls, B[a]P treatment had significantly increased the neoplastic cell number and cell viability in 16HBEs at all three doses (1, 2, and 5 mmol/L), and had significantly reduced the CYP1A1 and GSTP1 DNA promoter methylation levels. Following B[a]P treatment, the GSTM1 promoter methylation level in 16HBEs was profoundly reduced at low dose group compared to the DMSO controls, yet it was significantly increased at both middle and high dose groups. The mRNA levels of CYP1A1, GSTP1, and GSTM1 were significantly decreased in 16HBEs following B[a]P treatment at all three doses. The findings demonstrate that B[a]P promoted cell proliferation in 16HBEs, which was possibly related to the altered DNA methylations and the inhibited mRNA levels in CYP1A1, GSTP1, and GSTM1.

Phenotypic and transcriptomic changes in the corneal epithelium following exposure to cigarette smoke

Cigarette smoke extract (CSE), a complex mixture of compounds, contributes to a range of eye diseases; however, the underlying pathophysiological responses to tobacco smoke remain ambiguous. The purpose of the present study was to evaluate the cigarette smoke-induced phenotypic and transcriptomic changes in the corneal epithelium with a view to elucidating the likely underlying mechanism. Accordingly, for the first time, we characterized the genome-wide effects of CSE on the corneal epithelium. The ocular surface of the mice in the experimental groups was exposed to CSE for 1 h per day for a period of one week, while mice in the control group were exposed to preservative-free artificial tears. Corneal fluorescein staining, in vivo confocal microscopy and scanning electron microscopy were performed to examine the corneal ultrastructure. Transcriptome sequencing and bioinformatics analysis were performed followed by RT-qPCR to validate gene expression changes. The results indicate that CSE exposure disrupted the structural integrity of the superficial epithelium, decreased the density of microvilli, and compromised the corneal epithelial barrier intactness. RNA-seq revealed 667 differentially expressed genes, and functional analysis highlighted the enhancement of several biological processes such as antioxidant activity and the response to oxidative stress. Moreover, the Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway enrichment analysis showed that glutathione metabolism and drug metabolism cytochrome P450 were the most relevant pathways contributing to the effects of CSE on the corneal epithelium. Protein-protein interaction (PPI) network analysis illustrated that GCLC, NQO1, and HMOX1 were the most relevant nodes. In conclusion, the present study indicates that CSE exposure induces changes in the phenotype and genotype of the corneal epithelium. The antioxidant response element is essential for counteracting the effects of cigarette smoke on this tissue layer. These results shed novel insights into how cigarette smoke damages this ocular surface.

Benzo[a]pyrene-Induced Genotoxicity in Rats Is Affected by Co-Exposure to Sudan I by Altering the Expression of Biotransformation Enzymes

The environmental pollutant benzo[a]pyrene (BaP) is a human carcinogen that reacts with DNA after metabolic activation catalysed by cytochromes P450 (CYP) 1A1 and 1B1 together with microsomal epoxide hydrolase. The azo dye Sudan I is a potent inducer of CYP1A1/2. Here, Wistar rats were either treated with single doses of BaP (150 mg/kg bw) or Sudan I (50 mg/kg bw) alone or with both compounds in combination to explore BaP-derived DNA adduct formation in vivo. Using 32P-postlabelling, DNA adducts generated by BaP-7,8-dihydrodiol-9,10-epoxide were found in livers of rats treated with BaP alone or co-exposed to Sudan I. During co-exposure to Sudan I prior to BaP treatment, BaP-DNA adduct levels increased 2.1-fold in comparison to BaP treatment alone. Similarly, hepatic microsomes isolated from rats exposed to Sudan I prior to BaP treatment were also the most effective in generating DNA adducts in vitro with the activated metabolites BaP-7,8-dihydrodiol or BaP-9-ol as intermediates. DNA adduct formation correlated with changes in the expression and/or enzyme activities of CYP1A1, 1A2 and 1B1 in hepatic microsomes. Thus, BaP genotoxicity in rats in vivo appears to be related to the enhanced expression and/or activity of hepatic CYP1A1/2 and 1B1 caused by exposure of rats to the studied compounds. Our results indicate that the industrially employed azo dye Sudan I potentiates the genotoxicity of the human carcinogen BaP, and exposure to both substances at the same time seems to be hazardous to humans.

Discovery of heterocycle-containing α-naphthoflavone derivatives as water-soluble, highly potent and selective CYP1B1 inhibitors

Cytochrome P450 1B1 (CYP1B1) has been well validated as an attractive target for cancer prevention and drug resistance reversal. In continuation of our interest in this area, herein, a set of forty-six 6,7,10-trimethoxy-α-naphthoflavone derivatives varying in B ring was synthesized and screened against CYP1 enzymes, leading to the identification of fluorine-containing compound 15i as the most potent and selective CYP1B1 inhibitor (IC₅₀ value of 0.07 nM), being 84-fold more potent than that of the template molecule ANF. Alternatively, the amino-substituted derivative 13h not only possessed a potent inhibitory effect on CYP1B1 (IC₅₀ value of 0.98 nM), but also had a substantially increased water solubility as compared with the lead ANF (311 μg/mL for 13h and <5 μg/mL for ANF). The current study expanded the structural diversity of CYP1B1 inhibitors, and compound 13h could be considered as a promising starting point with great potential for further studies.

Potential of Inactivated Bifidobacterium Strain in Attenuating Benzo(A)Pyrene Exposure-Induced Damage in Colon Epithelial Cells In Vitro

Long-term exposure to benzo(a)pyrene (BaP) poses a serious genotoxic threat to human beings. This in vitro study investigated the potential of inactivated Bifidobacterium animalis subsp. lactis BI-04 in alleviating the damage caused by BaP in colon epithelial cells. A concentration of BaP higher than 50 μM strongly inhibited the growth of colon epithelial cells. The colon epithelial cells were treated with 50 μM BaP in the presence or absence of inactivated strain BI-04 (~5 × 10⁸ CFU/mL). The BaP-induced apoptosis of the colon epithelial cells was retarded in the presence of B. lactis BI-04 through activation of the PI3K/ AKT signaling pathway, and p53 gene expression was decreased. The presence of the BI-04 strain reduced the intracellular oxidative stress and DNA damage incurred in the colon epithelial cells by BaP treatment due to the enhanced expression of antioxidant enzymes and metabolism-related enzymes (CYP1A1). The data from comet assay, qRT-PCR, and western blot analysis showed that the cytotoxic effects of BaP on colon epithelial cells were largely alleviated because the bifidobacterial strain could bind to this carcinogenic compound. The in vitro study highlights that the consumption of commercial probiotic strain BI-04 might be a promising strategy to mitigate BaP cytotoxicity.

Khellinoflavanone, a Semisynthetic Derivative of Khellin, Overcomes Benzo[a]pyrene Toxicity in Human Normal and Cancer Cells That Express CYP1A1

Cytochrome P450 family 1 (CYP1) enzymes catalyze the metabolic activation of environmental procarcinogens such as benzo[a]pyrene, B[a]P, into carcinogens, which initiates the process of carcinogenesis. Thus, stopping the metabolic activation of procarcinogens can possibly prevent the onset of cancer. Several natural products have been reported to show unique ability in inhibiting CYP1 enzymes. We found that khellin, a naturally occurring furanochromone from Ammi visnaga, inhibits CYP1A1 enzyme with an IC50 value of 4.02 μM in CYP1A1-overexpressing human HEK293 suspension cells. To further explore this natural product for discovery of more potent and selective CYP1A1 inhibitors, two sets of semisynthetic derivatives were prepared. Treatment of khellin with alkali results in opening of a pyrone ring, yielding khellinone (2). Claisen-Schmidt condensation of khellinone (2) with various aldehydes in presence of potassium hydroxide, at room temperature, provides a series of furanochalcones 3a-v (khellinochalcones). Treatment of khellinone (2) with aryl aldehydes in the presence of piperidine, under reflux, affords the flavanone series of compounds 4a-p (khellinoflavanones). The khellinoflavanone 4l potently inhibited CYP1A1 with an IC50 value of 140 nM in live cells, with 170-fold selectivity over CYP1B1 (IC50 for CYP1B1 = 23.8 μM). Compound 4l at 3× IC50 concentration for inhibition of CYP1A1 completely protected HEK293 cells from CYP1A1-mediated B[a]P toxicity. Lung cancer cells, A549 (p53+) and Calu-1 (p53-null), blocked in growth at the S-phase by B[a]P were restored into the cell cycle by compound 4l. The results presented herein strongly indicate the potential of these khellin derivatives for further development as cancer chemopreventive agents.

Cytochrome P450 1A1 (CYP1A1) protects against nonalcoholic fatty liver disease caused by Western diet containing benzo[a]pyrene in mice

The Western diet contributes to nonalcoholic fatty liver disease (NAFLD) pathogenesis. Benzo[a]pyrene (BaP), a prototypical environmental pollutant produced by combustion processes, is present in charcoal-grilled meat. Cytochrome P450 1A1 (CYP1A1) metabolizes BaP, resulting in either detoxication or metabolic activation in a context-dependent manner. To elucidate a role of CYP1A1-BaP in NAFLD pathogenesis, we compared the effects of a Western diet, with or without oral BaP treatment, on the development of NAFLD in Cyp1a1(-/-) mice versus wild-type mice. A Western diet plus BaP induced lipid-droplet accumulation in liver of Cyp1a1(-/-) mice, but not wild-type mice. The hepatic steatosis observed in Cyp1a1(-/-) mice was associated with increased cholesterol, triglyceride and bile acid levels. Cyp1a1(-/-) mice fed Western diet plus BaP had changes in expression of genes involved in bile acid and lipid metabolism, and showed no increase in Cyp1a2 expression but did exhibit enhanced Cyp1b1 mRNA expression, as well as hepatic inflammation. Enhanced BaP metabolic activation, oxidative stress and inflammation may exacerbate metabolic dysfunction in liver of Cyp1a1(-/-) mice. Thus, Western diet plus BaP induces NAFLD and hepatic inflammation in Cyp1a1(-/-) mice in comparison to wild-type mice, indicating a protective role of CYP1A1 against NAFLD pathogenesis.

Overexpression of Catalase Enhances Benzo(a)pyrene Detoxification in Endothelial Microsomes

We previously reported that overexpression of catalase upregulated xenobiotic- metabolizing enzyme (XME) expression and diminished benzo(a)pyrene (BaP) intermediate accumulation in mouse aortic endothelial cells (MAECs). Endoplasmic reticulum (ER) is the most active organelle involved in BaP metabolism. To examine the involvement of ER in catalase-induced BaP detoxification, we compared the level and distribution of XMEs, and the profile of BaP intermediates in the microsomes of wild-type and catalase transgenic endothelial cells. Our data showed that endothelial microsomes were enriched in cytochrome P450 (CYP) 1A1, CYP1B1 and epoxide hydrolase 1 (EH1), and contained considerable levels of NAD(P)H: quinone oxidoreductase-1 (NQO1) and glutathione S-transferase-pi (GSTP). Treatment of wild-type MAECs with 1μM BaP for 2 h increased the expression of microsomal CYP1A1, 1B1 and NQO1 by ~300, 64 and 116%, respectively. However, the same treatment did not significantly alter the expression of EH1 and GSTP. Overexpression of catalase did not significantly increase EH1, but upregulated BaP-induced expression of microsomal CYP1A1, 1B1, NQO1 and GSTP in the following order: 1A1>NQO1>GSTP>1B1. Overexpression of catalase did not alter the distribution of each of these enzymes in the microsomes. In contrast to our previous report showing lower level of BaP phenols versus BaP diols/diones in the whole-cell, this report demonstrated that the sum of microsomal BaP phenolic metabolites were ~60% greater than that of the BaP diols/diones after exposure of microsomes to BaP. Overexpression of catalase reduced the concentrations of microsomal BaP phenols and diols/diones by ~45 and 95%, respectively. This process enhanced the ratio of BaP phenol versus diol/dione metabolites in a potent manner. Taken together, upregulation of phase II XMEs and CYP1 proteins, but not EH1 in the ER might be the mechanism by which overexpression of catalase reduces the levels of all the BaP metabolites, and enhances the ratio of BaP phenolic metabolites versus diol/diones in endothelial microsomes.

Inflammation-associated extracellular β-glucuronidase alters cellular responses to the chemical carcinogen benzo[a]pyrene

Neutrophils infiltrate tissues during inflammation, and when activated, they release β-glucuronidase. Since inflammation is associated with carcinogenesis, we investigated how extracellular β-glucuronidase changed the in vitro cellular response to the chemical carcinogen benzo(a)pyrene (B[a]P). For this we exposed human liver (HepG2) and lung (A549) cells to B[a]P in the presence or absence of β-glucuronidase. β-Glucuronidase reduced B[a]P-induced expression of CYP1A1 and CYP1B1 at 6 h after exposure, which did not depend on β-glucuronidase activity, because the inhibitor D-saccharic acid 1,4-lactone monohydrate did not antagonize the effect of β-glucuronidase. On the other hand, the inhibitory effect of β-glucuronidase on CYP expression was dependent on signalling via the insulin-like growth factor receptor (IGF2R, a known receptor for β-glucuronidase), because co-incubation with the IGF2R inhibitor mannose-6-phosphate completely abolished the effect of β-glucuronidase. Extracellular β-glucuronidase also reduced the formation of several B[a]P metabolites and B[a]P-DNA adducts. Interestingly, at 24 h of exposure, β-glucuronidase significantly enhanced CYP expression, probably because β-glucuronidase de-glucuronidated B[a]P metabolites, which continued to trigger the aryl hydrocarbon receptor (Ah receptor) and induced expression of CYP1A1 (in both cell lines) and CYP1B1 (in A549 only). Consequently, significantly higher concentrations of B[a]P metabolites and DNA adducts were found in β-glucuronidase-treated cells at 24 h. DNA adduct levels peaked at 48 h in cells that were exposed to B[a]P and treated with β-glucuronidase. Overall, these data show that β-glucuronidase alters the cellular response to B[a]P and ultimately enhances B[a]P-induced DNA adduct levels.

The impact of individual cytochrome P450 enzymes on oxidative metabolism of benzo[a]pyrene in human livers

Benzo[a]pyrene (BaP) is a human carcinogen that covalently binds to DNA after metabolic activation by cytochrome P450 (CYP) enzymes. In this study human recombinant CYPs (CYP1A1, 1A2, 1B1, 2A6, 2B6, 2C8, 2C9, 2C19, 2E1, 3A4, and 3A5) were expressed in Supersomes™ together with their reductases, NADPH:CYP oxidoreductase, epoxide hydrolase and cytochrome b5 , to investigate BaP metabolism. Human CYPs produced up to eight BaP metabolites. Among these, BaP-7,8-dihydrodiol and BaP-9-ol, which are intermediates in BaP-derived DNA adduct formation, were mainly formed by CYP1A1 and 1B1, and to a lesser extent by CYP2C19 and 3A4. BaP-3-ol, a metabolite that is a 'detoxified' product of BaP, was formed by most human CYPs tested, although CYP1A1 and 1B1 produced it the most efficiently. Based on the amounts of the individual BaP metabolites formed by these CYPs and their expression levels in human liver, we determined their contributions to BaP metabolite formation in this organ. Our results indicate that hepatic CYP1A1 and CYP2C19 are most important in the activation of BaP to BaP-7,8-dihydrodiol, whereas CYP2C19, 3A4, and 1A1 are the major enzymes contributing to the formation of BaP-9-ol. BaP-3-ol is predominantly formed by hepatic CYP3A4, while CYP1A1 and 2C19 are less active.

Induced expression of microsomal cytochrome b5 determined at mRNA and protein levels in rats exposed to ellipticine, benzo[a]pyrene, and 1-phenylazo-2-naphthol (Sudan I)

ABSTRACT

The microsomal protein cytochrome b5 , which is located in the membrane of the endoplasmic reticulum, has been shown to modulate many reactions catalyzed by cytochrome P450 (CYP) enzymes. We investigated the influence of exposure to the anticancer drug ellipticine and to two environmental carcinogens, benzo[a]pyrene (BaP) and 1-phenylazo-2-naphthol (Sudan I), on the expression of cytochrome b5 in livers of rats, both at the mRNA and protein levels. We also studied the effects of these compounds on their own metabolism and the formation of DNA adducts generated by their activation metabolite(s) in vitro. The relative amounts of cytochrome b5 mRNA, measured by real-time polymerase chain reaction analysis, were induced by the test compounds up to 11.7-fold in rat livers. Western blotting using antibodies raised against cytochrome b5 showed that protein expression was induced by up to sevenfold in livers of treated rats. Microsomes isolated from livers of exposed rats catalyzed the oxidation of ellipticine, BaP, and Sudan I and the formation of DNA adducts generated by their reactive metabolite(s) more effectively than hepatic microsomes isolated from control rats. All test compounds are known to induce CYP1A1. This induction is one of the reasons responsible for increased oxidation of these xenobiotics by microsomes. However, induction of cytochrome b5 can also contribute to their enhanced metabolism.

GRAPHICAL ABSTRACT

Dopamine D2-Receptor Antagonists Down-Regulate CYP1A1/2 and CYP1B1 in the Rat Liver

Dopaminergic systems regulate the release of several hormones including growth hormone (GH), thyroid hormones, insulin, glucocorticoids and prolactin (PRL) that play significant roles in the regulation of various Cytochrome P450 (CYP) enzymes. The present study investigated the role of dopamine D₂-receptor-linked pathways in the regulation of CYP1A1, CYP1A2 and CYP1B1 that belong to a battery of genes controlled by the Aryl Hydrocarbon Receptor (AhR) and play a crucial role in the metabolism and toxicity of numerous environmental toxicants. Inhibition of dopamine D₂-receptors with sulpiride (SULP) significantly repressed the constitutive and benzo[a]pyrene (B[a]P)-induced CYP1A1, CYP1A2 and CYP1B expression in the rat liver. The expression of AhR, heat shock protein 90 (HSP90) and AhR nuclear translocator (ARNT) was suppressed by SULP in B[a]P-treated livers, whereas the AhRR expression was increased by the drug suggesting that the SULP-mediated repression of the CYP1 inducibility is due to inactivation of the AhR regulatory system. At signal transduction level, the D₂-mediated down-regulation of constitutive CYP1A1/2 and CYP1B1 expression appears to be mediated by activation of the insulin/PI3K/AKT pathway. PRL-linked pathways exerting a negative control on various CYPs, and inactivation of the glucocorticoid-linked pathways that positively control the AhR-regulated CYP1 genes, may also participate in the SULP-mediated repression of both, the constitutive and induced CYP1 expression. The present findings indicate that drugs acting as D₂-dopamine receptor antagonists can modify several hormone systems that regulate the expression of CYP1A1, CYP1A2 and CYP1B1, and may affect the toxicity and carcinogenicity outcome of numerous toxicants and pre-carcinogenic substances. Therefore, these drugs could be considered as a part of the strategy to reduce the risk of exposure to environmental pollutants and pre-carcinogens.

Protective role of cytochrome P450 1A1 (CYP1A1) against benzo[a]pyrene-induced toxicity in mouse aorta

Benzo[a]pyrene (BaP) is an environmental pollutant produced by combustive processes, such as cigarette smoke and coke ovens, and is implicated in the pathogenesis of atherosclerosis. Cytochrome P450 1A1 (CYP1A1) plays a role in both metabolic activation and detoxication of BaP in a context-dependent manner. The role of CYP1A1 in BaP-induced toxicity in aorta remains unknown. First, we fed Apoe⁻/⁻ mice an atherogenic diet plus BaP and found that oral BaP-enhanced atherosclerosis is associated with increased reactive oxygen species (ROS) and inflammatory markers, such as plasma tumor necrosis factor levels and aortic mRNA expression of vascular endothelial growth factor A (Vegfa). We next examined the effect of an atherogenic diet plus BaP on ROS and inflammatory markers in Cyp1a1⁻/⁻ mice. Although this treatment was not sufficient to induce atherosclerotic lesions in Cyp1a1⁻/⁻ mice, plasma antioxidant levels were decreased in Cyp1a1⁻/⁻ mice even in the absence of BaP treatment. The atherogenic diet plus BaP effectively elevated plasma ROS levels and expression of atherosclerosis-related genes, specifically Vegfa, in Cyp1a1⁻/⁻ mice compared with wild-type mice. BaP treatment increased Vegfa mRNA levels in mouse embryonic fibroblasts from Cyp1a1⁻/⁻ mice but not from wild-type mice. BaP-induced DNA adduct formation was increased in the aorta of Cyp1a1⁻/⁻ mice, but not wild-type or Apoe⁻/⁻ mice, and the atherogenic diet decreased BaP-induced DNA adducts in Cyp1a1⁻/⁻ mice compared with mice on a control diet. These data suggest that ROS production contributes to BaP-exacerbated atherosclerosis and that CYP1A1 plays a protective role against oral BaP toxicity in aorta.

Polycyclic aromatic hydrocarbons stimulate human CYP3A4 promoter activity via PXR

Metabolic activation of polycyclic aromatic hydrocarbons (PAH) is mediated mainly by cytochrome P₄₅₀ monooxygenases (CYP) CYP1A1, 1A2 and 1B1. Several PAH are known to induce these CYP via aryl hydrocarbon receptor (AhR) signaling. Recently, it was shown that the PAH benzo[a]pyrene (BaP) can induce CYP3A4 as well. The induction was suggested to be mediated by the pregnane X receptor (PXR) rather than AhR. Metabolism by CYP3A4 is only known for dihydrodiol metabolites of PAH but not for their parent compounds. In the present study, a CYP3A4 reporter gene assay, requiring the overexpression of PXR, was used to investigate whether the PAH parent compounds BaP, benzo[c]phenanthrene (BcP) and dibenzo[a,l]pyrene (DBalP) as well as their corresponding phase I metabolites, the respective dihydrodiols and diol epoxides, can induce CYP3A4 promoter activity. BaP, BcP and their dihydrodiols were found to significantly activate the CYP3A4 promoter. Moreover, activation of PXR by all four compounds was detected by using a PXR transactivation assay, supporting that PXR mediates CYP3A4 induction by PAH. Taken together, these results show that both PAH parent compounds as well as their phase I metabolites induce CYP3A4 promoter via the transcription factor PXR.

Silymarin prevents benzo(a)pyrene-induced toxicity in Wistar rats by modulating xenobiotic-metabolizing enzymes

Benzo(a)pyrene (B(a)P), which is commonly used as an indicator species for polycyclic aromatic hydrocarbon (PAH) contamination, has a large number of hazardous consequences on human health. In the presence of the enzyme cytochrome-P-450 1A1 (CYP1A1), it undergoes metabolic activation to form reactive intermediates that are capable of inducing mutagenic, cytotoxic, teratogenic and carcinogenic effects in various species and tissues. Research within the last few years has shown that flavonoids exhibit chemopreventive effect against these toxins. In the present study, the protective effect of silymarin (a flavonoid) against B(a)P-induced toxicity was monitored in Wistar rats by evaluating the levels of hepatic phase I (CYP1A1), phase II enzymes (glutathione-S-transferase, epoxide hydroxylases, uridinediphosphate glucuronosyltransferases, NAD(P)H: quinone oxidoreductase 1, sulfotransferases), cellular antioxidant enzyme heme oxygenase and total glutathione. The results reveal that silymarin possesses substantial protective effect against B(a)P-induced damages by inhibiting phase I detoxification enzyme CYP1A1 and modulating phase II conjugating enzymes, which were confirmed by histopathological analysis. Overall, the inhibition of CYP1A1 and the modulation of phase II enzymes may provide, in part, the molecular basis for the effect of silymarin against B(a)P.

Dioxin suppresses benzo[a]pyrene-induced mutations and DNA adduct formation through cytochrome P450 1A1 induction and (±)-anti-benzo[a]pyrene-7,8-diol-9,10-epoxide inactivation in human hepatoma cells

Benzo[a]pyrene (BaP) is metabolically activated by cytochrome P450 enzymes, and forms DNA adduct leading to mutations. Cytochrome P450 1A1 plays a central role in this activation step, and this enzyme is strongly induced by chemical agents that bind to the aryl hydrocarbon receptor (AhR), which is also known as a dioxin receptor. 2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD), a potent AhR ligand has not been shown to form any DNA adduct, but has a possibility to aggravate the toxicity of precarcinogenic polycyclic hydrocarbons through the induction of metabolic enzymes. We treated human hepatoma cells (HepG2) with TCDD, and subsequently exposed them to BaP to elucidate the synergistic effects on mutations. Surprisingly, mutant frequency induced by BaP at the hypoxanthine-guanine phosphribosyltransferase (HPRT) locus was decreased by pretreatment with TCDD. In correlation with decrease in the mutant frequencies, BaP-DNA adduct formation was also decreased by TCDD pretreatment. This suppressive effect of TCDD was more potent when the cells were exposed to (±)-anti-benzo[a]pyrene-7,8-diol-9,10-epoxide (BPDE), a reactive metabolic intermediate of BaP. Among the enzymes catalyzing BaP oxidation and conjugation, cytochrome P450 1A1, 1A2, 3A4 and UDP-glucuronosyltransferase 1A1 mRNAs were induced by the exposure to TCDD. In cytochrome P450 1A1-deficient murine cells and cytochrome P450 1A1-uninducible human cells, TCDD could not suppress BPDE-DNA adduct formation. Further experiments using "Tet-On" cytochrome P450 1A1-overexpressing cells and a recombinant cytochrome P450 1A1 enzyme demonstrated that this is the key enzyme involved in the biotransformation of BaP, that is, both production and inactivation of BPDE. We conclude that TCDD-induced cytochrome P450 catalyzes the metabolism of BPDE to as yet-unidentified products that are not apparently DNA-reactive, thereby reducing mutations in hepatoma cells.

Vitamin D receptor activation enhances benzo[a]pyrene metabolism via CYP1A1 expression in macrophages

Benzo[a]pyrene (BaP) activates the aryl hydrocarbon (AHR) and induces the expression of genes involved in xenobiotic metabolism, including CYP1A1. CYP1A1 is involved not only in BaP detoxification but also in metabolic activation, which results in DNA adduct formation. Vitamin D receptor (VDR) belongs to the NR1I subfamily of the nuclear receptor superfamily, which also regulates expression of xenobiotic metabolism genes. We investigated the cross-talk between AHR and VDR signaling pathways and found that 1α,25-dihydroxyvitamin D(3) [1,25(OH)(2)D(3)], a potent physiological VDR agonist, enhanced BaP-induced transcription of CYP1A1 in human monocytic U937 cells and THP-1 cells, breast cancer cells, and kidney epithelium-derived cells. 1,25(OH)(2)D(3) alone did not induce CYP1A1, and 1,25(OH)(2)D(3) plus BaP did not increase CYP1A2 or CYP1B1 mRNA expression in U937 cells. The combination of 1,25(OH)(2)D(3) and BaP increased CYP1A1 protein levels, BaP hydroxylation activity, and BaP-DNA adduct formation in U937 cells and THP-1 cells more effectively than BaP alone. The combined effect of 1,25(OH)(2)D(3) and BaP on CYP1A1 mRNA expression in U937 cells and/or THP-1 cells was inhibited by VDR knockdown, VDR antagonists, and α-naphthoflavone, an AHR antagonist. Electrophoretic mobility shift assays and chromatin immunoprecipitation assays showed that VDR directly bound to an everted repeat (ER) 8 motif in the human CYP1A1 promoter. Thus, CYP1A1 is a novel VDR target gene involved in xenobiotic metabolism. Induction of CYP1A1 by the activation of VDR and AHR may contribute to BaP-mediated toxicity and the physiological function of this enzyme.

Biotransformation of benzo[a]pyrene in Ahr knockout mice is dependent on time and route of exposure

Benzo[a]pyrene (BP) is an ubiquitous environmental pollutant with potent mutagenic and carcinogenic properties. The Ah receptor (Ahr) is important in the metabolic activation of BP and is therefore central to BP-induced carcinogenesis. Although Ahr(-/-) mice are refractory to BP-induced carcinogenesis, higher levels of BP-DNA and -protein adducts were formed in them than in wild-type mice. These results indicated the presence of an Ahr-independent and/or a slower biotransformation of BP in Ahr knockout mice. To address this issue further, we have now performed a time-course experiment, with mice receiving a single oral dose of BP (100 mg/kg). Wild-type mice have an effective clearance of BP metabolites, mainly through 3-hydroxybenzo[a]pyrene and 9-hydroxybenzo[a]pyrene in the feces with reduced levels of DNA and protein adducts in the examined tissues. On the other hand, the Ahr(-/-) mice appear to have a lower metabolic clearance of BP resulting in increased levels of DNA and protein adducts and of unmetabolized BP. In addition, we have performed an administration route experiment and found that skin-exposed Ahr(-/-) mice showed lower levels of protein adducts along with markedly reduced P450 1B1 expression, but only in the exposed area, as compared with the wild-type mice. In addition, the systemic uptake of BP is increased in the Ahr(-/-) mice as compared with the wild-type mice. Hence, the lack of a functional Ah receptor results in an Ahr-independent biotransformation of BP with a slower clearance of BP and higher levels of DNA and protein adducts, but the distribution and levels of BP and BP-protein adducts are clearly dependent on the route of exposure.

Aryl hydrocarbon receptor ligand 2,3,7,8-tetrachlorodibenzo-p-dioxin enhances liver damage in bile duct-ligated mice

The environmental pollutant 2,3,7,8-tetracholorodibenzo-p-dioxin (TCDD) is known to cause a wide variety of toxic effects, including hepatotoxicity, by way of the aryl hydrocarbon receptor (AHR). Although inducible expression of cytochrome P450 (CYP) 1A1 and CYP1A2 is associated with liver injury caused by high-dose TCDD, the specific role of the AHR-CYP1 cascade in hepatotoxicity remains unclear. We investigated the effects of AHR activation under conditions of cholestasis. We administered oral TCDD to mice at a dose that can effectively induce Cyp1 gene expression without overt liver toxicity and then ligated their bile ducts. TCDD pretreatment enhanced bile duct ligation (BDL)-induced increases in liver and plasma bile acids, bilirubin, and aminotransferases. Histology of TCDD-pretreated BDL mice revealed massive hepatic necrosis without any increase in number of apoptotic cells. Whereas induction of AHR-target genes by TCDD was observed similarly in sham-operated as well as in BDL mice, TCDD pretreatment of BDL mice altered the expression of hepatic genes involved in bile acid synthesis and transport. Increased plasma proinflammatory cytokines, tumor necrosis factor and interleukin-1β, in BDL mice were further elevated by TCDD pretreatment. Liver injury by TCDD plus BDL, such as increased plasma bile acids, bilirubin and aminotransferases, liver necrosis, and increased tumor necrosis factor production, was exaggerated in Cyp1a1/1a2(-/-) double knockout mice. These findings indicate that TCDD aggravates cholestatic liver damage and that the presence of CYP1A1 and CYP1A2 plays a protective role in liver damage caused by TCDD and BDL.

Nonviral gene transfer as a tool for studying transcription regulation of xenobiotic metabolizing enzymes

Numerous xenobiotic metabolizing enzymes are regulated by nuclear receptors at transcriptional level. The challenge we currently face is to understand how a given nuclear receptor interacts with its xenobiotics, migrates into nucleus, binds to the xenobiotic response element of a target gene, and regulates transcription. Toward this end, new methods have been developed to introduce the nuclear receptor gene into appropriate cells and study its activity in activating reporter gene expression under the control of a promoter containing xenobiotic response elements. The goal of this review is to critically examine the gene transfer methods currently available. We concentrate on the gene transfer mechanism, advantages and limitations of each method when employed for nuclear receptor-mediated gene regulation studies. It is our hope that the information provided highlights the importance of gene transfer in studying the mechanisms by which our body eliminates the potentially harmful substances and maintains the homeostasis.

Malondialdehyde-modified low density lipoprotein (MDA-LDL)-induced cell growth was suppressed by polycyclic aromatic hydrocarbons (PAHs)

Malondialdehyde-modified low-density lipoprotein (MDA-LDL) and oxidized LDL (Ox-LDL), which accelerate the pathogenesis of arteriosclerosis, are thought to be involved in parthenogenesis caused by smooth muscle cell proliferation. In this study, we investigated the suppression mechanism of polycyclic aromatic hydrocarbons (PAHs) on the growth of an MDA-LDL-induced human acute monocyte leukemia suspension cell line (THP-1 cells). We found that PAHs suppressed MDA-LDL-induced THP-1 cell growth. Cotreatment with benzo[a]pyrene (BaP) or 3-methylchoranthrene (3-MC) decreased MDA-LDL-induced THP-1 cell growth, whereas treatment with benzo[e]pyrene (BeP) or pyrene, which is not a ligand for the arylhydrocarbon receptor (AhR), did not decrease THP-1 cell growth. Our findings clearly demonstrated that THP-1 cell growth, which was suppressed by PAHs, was restored by the addition of alpha-naphtoflavone, which is a partial antagonist to AhR. Moreover, it was shown that cotreatment with MDA-LDL and BaP markedly induced the expression of human cytochrome P4501A1 (hCYP1A1) messenger ribonucleic acid (mRNA) and significantly induced the expressions of p53 and p21 mRNAs. In support of these findings, AhR small interfering RNA suppressed the induced level of p21 mRNA and by BaP and the overexpression of hCYP1A1 significantly induced levels of p21 mRNA. On the other hand, the uptake rate of [(14)C]BaP into cells was increased more significantly by cotreatment with MDA-LDL than by treatment with [(14)C]BaP alone. These results strongly suggest that the suppression of MDA-LDL-induced THP-1 cell growth is caused by the increased uptake of PAHs, which strongly activate the AhR signal pathway accompanying DNA damage.

Overexpression of Cu/Zn-superoxide dismutase and/or catalase accelerates benzo(a)pyrene detoxification by upregulation of the aryl hydrocarbon receptor in mouse endothelial cells

A reduction in endogenously generated reactive oxygen species in vivo delays benzo(a)pyrene (BaP)-accelerated atherosclerosis, as revealed in hypercholesterolemic mice overexpressing Cu/Zn-superoxide dismutase (SOD) and/or catalase. To understand the molecular events involved in this protective action, we studied the effects of Cu/Zn-SOD and/or catalase overexpression on BaP detoxification and on aryl hydrocarbon receptor (AhR) expression and its target gene expression in mouse aortic endothelial cells (MAECs). Our data demonstrate that overexpression of Cu/Zn-SOD and/or catalase leads to an 18- to 20-fold increase in the expression of AhR protein in MAECs. After BaP exposure, the amount of AhR binding to the cytochrome P450 (CYP) 1A1 promoter was significantly greater, and the concentrations of BaP reactive intermediates were significantly less in MAECs overexpressing Cu/Zn-SOD and/or catalase than in wild-type cells. In addition, the BaP-induced CYP1A1 and 1B1 protein levels and BaP-elevated glutathione S-transferase (GST) activity were significantly higher in these transgenic cells, in parallel with elevated GSTp1, CYP1A1, and CYP1B1 mRNA levels, compared to wild-type MAECs. Moreover, knockdown of AhR with RNA interference diminished the Cu/Zn-SOD and catalase enhancement of CYP1A1 expression, GST activity, and BaP detoxification. These data demonstrate that overexpression of Cu/Zn-SOD and/or catalase is associated with upregulation of AhR and its target genes, such as xenobiotic-metabolizing enzymes.

The aryl hydrocarbon receptor activator benzo[a]pyrene enhances vitamin D3 catabolism in macrophages

Benzo[a]pyrene (BaP), a polycyclic aromatic hydrocarbon produced by cigarette combustion, is implicated as a causative agent in smoking-related cancer and atherosclerosis. 1,25-Dihydroxyvitamin D3 [1,25(OH)2D3], a potent ligand for the nuclear receptor vitamin D receptor (VDR), has been shown to decrease the risk of osteoporosis, some types of cancer and cardiovascular disease, suggesting an opposing effect of vitamin D3 to cigarette smoking. In this study, we investigated the effects of BaP on the vitamin D3 signaling pathway. BaP effectively enhanced the 1,25(OH)2D3-dependent induction of cytochrome P450 24A1 (CYP24A1) in human monocyte/macrophage-derived THP-1 cells and breast cancer MCF-7 cells. BaP combination was less or not effective on mRNA expression of CD14, arachidonate 5-lipoxygenase, and cathelicidin antimicrobial peptide in THP-1 cells. BaP also increased the expression of CYP24A1 induced by a non-vitamin D VDR ligand, lithocholic acid acetate. Another aryl hydrocarbon receptor (AhR) ligand, 2,3,7,8-tetrachlorodibenzo-p-dioxin, enhanced CYP24A1 expression by 1,25(OH)2D3 in THP-1 cells. Treatment of cells with an AhR antagonist and a protein synthesis inhibitor inhibited the enhancing effect of BaP on CYP24A1 induction, indicating that the effects of BaP are mediated by AhR activation and de novo protein synthesis. BaP pretreatment increased 1,25(OH)2D3-dependent recruitment of VDR and retinoid X receptor to the CYP24A1 promoter. Analysis of 1,25(OH)2D3 metabolism showed that BaP enhanced the hydroxylation of 1,25(OH)2D3 by CYP24A1 in THP-1 cells. Thus, AhR activation by BaP stimulates vitamin D3 catabolism. Modulation of vitamin D signaling by AhR may represent a mechanism underlying cigarette smoking-related diseases.