MECHANISMS OF ARSENITE-MEDIATED DECREASES IN BENZO[K]FLUORANTHENE-INDUCED HUMAN CYTOCHROME P4501A1 LEVELS IN HEPG2 CELLS

Arsenic: Various species with different effects on cytochrome P450 regulation in humans

Arsenic is well-recognized as one of the most hazardous elements which is characterized by its omnipresence throughout the environment in various chemical forms. From the simple inorganic arsenite (iAsIII) and arsenate (iAsV) molecules, a multitude of more complex organic species are biologically produced through a process of metabolic transformation with biomethylation being the core of this process. Because of their differential toxicity, speciation of arsenic-based compounds is necessary for assessing health risks posed by exposure to individual species or co-exposure to several species. In this regard, exposure assessment is another pivotal factor that includes identification of the potential sources as well as routes of exposure. Identification of arsenic impact on different physiological organ systems, through understanding its behavior in the human body that leads to homeostatic derangements, is the key for developing strategies to mitigate its toxicity. Metabolic machinery is one of the sophisticated body systems targeted by arsenic. The prominent role of cytochrome P450 enzymes (CYPs) in the metabolism of both endobiotics and xenobiotics necessitates paying a great deal of attention to the possible effects of arsenic compounds on this superfamily of enzymes. Here we highlight the toxicologically relevant arsenic species with a detailed description of the different environmental sources as well as the possible routes of human exposure to these species. We also summarize the reported findings of experimental investigations evaluating the influence of various arsenicals on different members of CYP superfamily using human-based models.

Artificial turf infill associated with systematic toxicity in an amniote vertebrate

Artificial athletic turf containing crumb rubber (CR) from shredded tires is a growing environmental and public health concern. However, the associated health risk is unknown due to the lack of toxicity data for higher vertebrates. We evaluated the toxic effects of CR in a developing amniote vertebrate embryo. CR water leachate was administered to fertilized chicken eggs via different exposure routes, i.e., coating by dropping CR leachate on the eggshell; dipping the eggs into CR leachate; microinjecting CR leachate into the air cell or yolk. After 3 or 7 d of incubation, embryonic morphology, organ development, physiology, and molecular pathways were measured. The results showed that CR leachate injected into the yolk caused mild to severe developmental malformations, reduced growth, and specifically impaired the development of the brain and cardiovascular system, which were associated with gene dysregulation in aryl hydrocarbon receptor, stress-response, and thyroid hormone pathways. The observed systematic effects were probably due to a complex mixture of toxic chemicals leaching from CR, such as metals (e.g., Zn, Cr, Pb) and amines (e.g., benzothiazole). This study points to a need to closely examine the potential regulation of the use of CR on playgrounds and artificial fields.

An Overview of Human Immunodeficiency Virus Type 1-Associated Common Neurological Complications: Does Aging Pose a Challenge?

With increasing survival of patients infected with human immunodeficiency virus type 1 (HIV-1), the manifestation of heterogeneous neurological complications is also increasing alarmingly in these patients. Currently, more than 30% of about 40 million HIV-1 infected people worldwide develop central nervous system (CNS)-associated dysfunction, including dementia, sensory, and motor neuropathy. Furthermore, the highly effective antiretroviral therapy has been shown to increase the prevalence of mild cognitive functions while reducing other HIV-1-associated neurological complications. On the contrary, the presence of neurological disorder frequently affects the outcome of conventional HIV-1 therapy. Although, both the children and adults suffer from the post-HIV treatment-associated cognitive impairment, adults, especially depending on the age of disease onset, are more prone to CNS dysfunction. Thus, addressing neurological complications in an HIV-1-infected patient is a delicate balance of several factors and requires characterization of the molecular signature of associated CNS disorders involving intricate cross-talk with HIV-1-derived neurotoxins and other cellular factors. In this review, we summarize some of the current data supporting both the direct and indirect mechanisms, including neuro-inflammation and genome instability in association with aging, leading to CNS dysfunction after HIV-1 infection, and discuss the potential strategies addressing the treatment or prevention of HIV-1-mediated neurotoxicity.

Arsenite Interacts with Dibenzo[def,p]chrysene (DBC) at Low Levels to Suppress Bone Marrow Lymphoid Progenitors in Mice

Arsenite (As(+3)) and dibenzo[def,p]chrysene (DBC), a polycyclic aromatic hyrdrocarbon (PAH), are found in nature as environmental contaminants. Both are known to individually suppress the immune system of humans and mice. In order to determine their potential interactive and combined immunosuppressive effects, we examined murine bone marrow (BM) immune progenitor cells' responses following combined oral exposures at very low levels of exposure to As(+3) and DBC. Oral 5-day exposure to DBC at 1 mg/kg (cumulative dose) was found to suppress mouse BM lymphoid progenitor cells, but not the myeloid progenitors. Previously established no-effect doses of As(+3) in drinking water (19 and 75 ppb for 30 days) produced more lymphoid suppression in the bone marrow when mice were concomitantly fed a low dose of DBC during the last 5 days. The lower dose (19 ppb) As(+3) had a stronger suppressive effect with DBC than the higher dose (75 ppb). Thus, the interactive toxicity of As(+3) and DBC in vivo could be As(+3) dose dependent. In vitro, the suppressive interaction of As(+3) and DBC was also evident at low concentrations (0.5 nM), but not at higher concentrations (5 nM) of As(+3). These studies show potentially important interactions between As(+3) and DBC on mouse BM at extremely low levels of exposure in vivo and in vitro.

Evaluation of the interactive effects of lead, zinc and benzo(k)fluoranthene on crucian carp, Carassius carassius, using a multiple biomarker approach

The aim of this study was to evaluate the interactive effects of the metals lead (Pb) and zinc (Zn) and the polycyclic aromatic hydrocarbon benzo(k)fluoranthene (BkF) on crucian carp (Carassius carassius). To this end, various biomarkers in the fish [7-ethoxyresorufin-O-deethylase (EROD); superoxide dismutase (SOD); malondialdehyde (MDA) and metallothioneins (MT)] were measured after 96 h of exposure. When compared to either treatment with Pb and Zn mixtures or BkF-only treatment, the treatment with BkF combined with Pb and Zn resulted in lower responses of EROD and MT synthesis activity, and higher SOD and MDA increases. These results indicated that the co-exposure of metals and polycyclic aromatic hydrocarbons may cause more severe oxidative stress on fish. However, the effect of these interactions on EROD and MT may lead to an underestimation of the actual exposure in the field and thereby should be carefully considered.

Arsenic trioxide co-exposure potentiates benzo(a)pyrene genotoxicity by enhancing the oxidative stress in human lung adenocarcinoma cell

Although both arsenic trioxide (As2O3) and benzo(a)pyrene (BaP) are well-established human carcinogens, the interaction between As2O3 and BaP is synergistic or antagonistic remains controversial in terms of the existing studies. In addition, the mechanisms responsible for the combined effects are still unclear. In this study, we examined the potential interactive effects between As2O3 (1, 5, and 10 μM) and BaP (5, 10, and 20 μM) in cultured A549 cells by treating with BaP and As2O3 alone or in combination at various concentrations for 24 h. The single and combined effects of As2O3 and BaP on the cytotoxicity, DNA/chromosomal damage, and oxidative stress were examined by using tetrazolium (3-(4,5-dimethyithiazol-2-yl)-2,5-diphenyl-tetrazolium bromide) dye colorimetric assay, colony formation assay, fluorescence probe, chemical colorimetry, comet assay as well as micronucleus test. Our results showed that As2O3 synergistically enhanced the cytotoxicity, genotoxicity, and level of oxidative stress induced by BaP at various tested concentrations. Also, our experimental results showed that intracellular glutathione (GSH) contents were increased by various doses of BaP, but single or cotreatment with As2O3 significantly decreased the GSH level in the cells at all tested concentrations. Taken together, our results suggest that As2O3 may exert its synergistic cyto- and genotoxic effects with BaP mainly via elevated intracellular reactive oxygen species and reduced GSH contents and superoxide dismutase activities, thus promoting high level of oxidative stress, which may be a pivotal mechanism underlying As2O3 cocarcinogenic action.

Associations between CYP1A1 and CYP2E1 polymorphisms and susceptibility to esophageal cancer in Chaoshan and Taihang areas of China

PURPOSE

To study the causes of esophageal cancer in Chaoshan and Taihang areas.

METHODS

By using gel-based DNA microarray genotyping method, four cancer-related polymorphisms including CYP1A1 m2, CYP1A1 m4, CYP2E1 Pst I and CYP2E1 Rsa I were studied with 565 (CYP1A1) or 482 (CYP2E1) cases and 468 (CYP1A1) or 466 (CYP2E1) controls.

RESULTS

For CYP1A1 m2, the mutant allele frequencies were 21.3% (Chaoshan) and 19.6% (Taihang), and OR for AG versus AA genotype (Chaoshan) was 1.855 (95% CI [1.227-2.805]). For CYP1A1 m4, no mutant allele was detectable. For CYP2E1 Pst I, the mutant allele frequencies were 27.3% (Chaoshan) and 29.4% (Taihang), and OR for GG versus CC genotype (Taihang) was 3.263 (95% CI [1.059-10.052]). For CYP2E1 Rsa I, the mutant allele frequencies were 27.3% (Chaoshan) and 29.6% (Taihang), and OR for CC versus TT genotype (Taihang) was 3.167 (95% CI [1.026-9.776]).

CONCLUSION

The results suggest that AG genotype of CYP1A1 in Chaoshan area and GG (CC) genotype of CYP2E1 in Taihang area are significantly associated with esophageal cancer susceptibility.

Activity of Phase I and Phase II enzymes of the benzo[a]pyrene transformation pathway in zebrafish (Danio rerio) following waterborne exposure to arsenite

The environmental pollutants inorganic arsenic (iAs) and benzo[a]pyrene (B[a]P) are carcinogens often found together in groundwater. The hepatic metabolism of B[a]P is a multi-step process requiring several Phase I and Phase II enzymes, notably cytochrome p450 1A (CYP1A), epoxide hydrolase (EH), and glutathione S-transferase (GST). The purpose of this study was to examine the effect of arsenite (As(III)) on the activity of these enzymes in vivo utilizing adult zebrafish (Danio rerio). Zebrafish were exposed to either 0.4 microM B[a]P, 0.4 microM B[a]P+0.4 microM As(III), 0.4 microM B[a]P+8 microM As(III), 0.4 microM As(III), or 8 microM As(III) for 7 days. Co-exposures to As(III) and B[a]P led to significant decreases in CYP1A enzyme activity (approximately 3-fold) when compared to exposure to B[a]P alone. No similar effects occurred with EH or GST, although B[a]P exposure did significantly increase EH activity. Furthermore As(III) and B[a]P co-exposures significantly decreased CYP1A transcript levels (up to 35-fold) when compared to B[a]P. However, B[a]P-induced CYP1A protein levels remained elevated following co-exposures to As(III). This evidence suggests that As(III) has the potential to modify components of the B[a]P biotransformation pathway in vivo via a disruption of CYP1A activity by way of both pre- and post-translational mechanisms.

Long-term estrogen exposure promotes carcinogen bioactivation, induces persistent changes in gene expression, and enhances the tumorigenicity of MCF-7 human breast cancer cells

The cumulative exposure to estrogens is an important determinant in the risk of breast cancer, yet the full range of mechanisms involving estrogens in the genesis and progression of breast cancer remains a subject of debate. Interactions of estrogens and environmental toxicants have received attention as putative factors contributing to carcinogenesis. Mechanistic studies have demonstrated interactions between estrogen receptor alpha (ERalpha) and the aryl hydrocarbon receptor (AhR), with consequences on the genes that they regulate. Many studies of ERalpha and AhR-mediated effects and crosstalk between them have focused on the initial molecular events. In this study, we investigated ERalpha- and AhR-mediated effects in long-term estrogen exposed (LTEE) MCF-7 human breast cancer cells, which were obtained by continuous culturing for at least 12 weeks in medium supplemented with 1 nM of 17beta-estradiol (E(2)). With these LTEE cells and with parallel control cells cultured without E(2) supplementation, we performed an extensive study of cytochrome P450 (CYP) induction, carcinogen bioactivation, global gene expression, and tumorigenicity in immunocompromised mice. We found that LTEE cells, in comparison with control cells, had higher levels of AhR mRNA and protein, greater responsiveness for AhR-regulated CYP1A1 and CYP1B1 induction, a 6-fold higher initial level of benzo(a)pyrene-DNA adducts as determined by liquid chromatography tandem mass spectrometry, marked differences in the expression of numerous genes, and a higher rate of E(2)-dependent tumor growth as xenografts. These studies indicate that LTEE causes adaptive responses in MCF-7 cells, which may reflect processes that contribute to the overall carcinogenic effect of E(2).

The Role of Trace Metals in Cytochrome P4501 Regulation

Trace metals and polycyclic aromatic hydrocarbons (PAHs) are ubiquitous environmental co-contaminants and the trace metals could influence the carcinogenicity of the PAHs by altering their extent of induction of cytochromes P4501A1, 1A2, and 1B1 (CYP). Studies in cell lines from humans, rodents, chickens, and fish, and in cell culture generally indicate that trace metals diminish the inductive potency of PAHs for these CYPs. The extent of the effect is species-, metal-, PAH-, and metal dose-dependent. Both transcriptional and post-translational mechanisms are involved in the trace metal-mediated down regulation of the CYP1 forms. The latter mechanism incorporates induction of heme oxygenase-1 by the metals, with resultant heme catabolism. Thus, trace metals could diminish the carcinogenicity of PAHs.

Down-regulation of the carcinogen-metabolizing enzyme cytochrome P450 1a1 by vanadium

Vanadium (V(5+)), a heavy metal contaminant with important toxicological consequences, has received considerable attention as an anticancer agent, although the mechanisms remain unknown. As a first step to investigate these mechanisms, we examined the effect of V(5+) (as ammonium metavanadate, NH(4)VO(3)) on the expression of the aryl hydrocarbon receptor (AhR)-regulated gene: cytochrome P450 1a1 (Cyp1a1) at each step of the AhR signal transduction pathway, using Hepa 1c1c7 cells. Our results showed a significant reduction in 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD)-mediated induction of Cyp1a1 mRNA, protein and activity levels after V(5+) treatments in a dose-dependent manner. Investigation of the effect of coexposure to V(5+) and TCDD at transcriptional levels revealed that V(5+) significantly inhibited TCDD-mediated induction of AhR-dependent luciferase reporter gene expression. Furthermore, despite not affecting the direct activation of the cytosolic AhR by TCDD and subsequently transforming it to a DNA-binding form, V(5+) inhibited the nuclear accumulation of liganded AhR and subsequent formation of the AhR/aryl hydrocarbon nuclear translocator (Arnt)/xenobiotic responsive element (XRE) complex. Importantly, the V(5+)-mediated inhibition of AhR/Arnt/XRE complex formation coincided with a significant decrease in ecto-ATPase activity. Looking at the post-transcriptional and post-translational effects of V(5+) on existing Cyp1a1 mRNA and protein levels, we showed that V(5+) did not affect Cyp1a1 mRNA or protein stability, thus eliminating possible role of V(5+) in modifying Cyp1a1 gene expression through these mechanisms. This study provides the first evidence that V(5+) down-regulates the expression of Cyp1a1 at the transcriptional level through an ATP-dependent mechanism.

Induction of CYP1A1 and CYP1B1 by benzo(k)fluoranthene and benzo(a)pyrene in T-47D human breast cancer cells: roles of PAH interactions and PAH metabolites

The interactions of polycyclic aromatic hydrocarbons (PAH) and cytochromes P450 (CYP) are complex; PAHs are enzyme inducers, substrates, and inhibitors. In T-47D breast cancer cells, exposure to 0.1 to 1 microM benzo(k)fluoranthene (BKF) induced CYP1A1/1B1-catalyzed 17beta-estradiol (E(2)) metabolism, whereas BKF levels greater than 1 muM inhibited E(2) metabolism. Time course studies showed that induction of CYP1-catalyzed E(2) metabolism persisted after the disappearance of BKF or co-exposed benzo(a)pyrene, suggesting that BKF metabolites retaining Ah receptor agonist activity were responsible for prolonged CYP1 induction. BKF metabolites were shown, through the use of ethoxyresorufin O-deethylase and CYP1A1-promoter-luciferase reporter assays to induce CYP1A1/1B1 in T-47D cells. Metabolites formed by oxidation at the C-2/C-3 region of BKF had potencies for CYP1 induction exceeding those of BKF, whereas C-8/C-9 oxidative metabolites were somewhat less potent than BKF. The activities of expressed human CYP1A1 and 1B1 with BKF as substrate were investigated by use of HPLC with fluorescence detection, and by GC/MS. The results showed that both enzymes efficiently catalyzed the formation of 3-, 8-, and 9-OHBKF from BKF. These studies indicate that the inductive effects of PAH metabolites as potent CYP1 inducers are likely to be additional important factors in PAH-CYP interactions that affect metabolism and bioactivation of other PAHs, ultimately modulating PAH toxicity and carcinogenicity.

A Common Regulatory Region Functions Bidirectionally in Transcriptional Activation of the HumanCYP1A1andCYP1A2Genes

The human CYP1A1 and CYP1A2 genes on chromosome 15 are orientated head-to-head and are separated by a 23-kilobase (kb) intergenic spacer region. Thus, the possibility exists for sharing common regulatory elements contained in the spacer region responsible for transcriptional activation and regulation of the CYP1A1 and CYP1A2 genes. In the present study, a reporter gene construct containing -22.4 kb of the 5'-flanking region of the CYP1A2 gene was found to support beta-naphthoflavone (BNF) and 3-methylchoranthrene (3-MC)-mediated transcriptional activation. The responsive region was also functional in directing activation of the CYP1A1 promoter, indicating that the region works bidirectionally to govern transcriptional activation of both CYP1A1 and CYP1A2. To simultaneously evaluate transcriptional activation of both genes, a dual reporter vector was developed in which the spacer region was inserted between two different reporter genes, firefly luciferase and secreted alkaline phosphatase. Transient transfection of the dual reporter vector in HepG2 cells revealed increases in both reporter activities after exposure of the cells to BNF and 3-MC. Deletion studies of the spacer region indicated that a region from -464 to -1829 of the CYP1A1 gene works bidirectionally to enhance the transcriptional activation of not only CYP1A1 but also CYP1A2. In addition, a negative bidirectional regulatory region was found to exist from -18,989 to -21,992 of the CYP1A1 gene. These data established that induction of human CYP1A1 and CYP1A2 is simultaneously controlled through bidirectional and common regulatory elements.

Regulatory mechanisms modulating the expression of cytochrome P450 1A1 gene by heavy metals

We recently demonstrated that heavy metals, Hg2+, Pb2+, and Cu2+ induced Cyp1a1 gene expression, yet the mechanisms involved remain unknown. To explore the molecular mechanisms involved in the modulation of Cyp1a1 by heavy metals, Hepa 1c1c7 cells were treated with the metals in the presence and absence of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), a potent Cyp1a1 inducer. Time-dependent effect study showed that all metals significantly induced the basal Cyp1a1 mRNA. This was apparent 3 h after treatment, and levels remained elevated for at least 24 h. At the inducible level, Hg2+ and Pb2+ further increased, while Cu2+ decreased, the TCDD-mediated induction of Cyp1a1 mRNA. The RNA synthesis inhibitor, actinomycin D, completely blocked the Cyp1a1 induction by heavy metals. The protein synthesis inhibitor, cycloheximide, and 26S proteasome inhibitor, carbobenzoxy-L-leucyl-L-leucyl-leucinal (MG-132), super-induced the metal-mediated induction of Cyp1a1 mRNA. In addition, all three metals induced aryl hydrocarbon receptor/xenobiotic-responsive element (AhR/XRE) binding, suggesting an AhR-dependent mechanism. Cyp1a1 mRNA and protein decay experiments showed that the three metals did not significantly affect the half-life of mRNA; however, they significantly decreased the degradation rate of its protein, implying a posttranslational regulation of the Cyp1a1 by the heavy metals. A significant decrease in TCDD-mediated induction of Cyp1a1 activity associated with an increase in HO-1 mRNA and a decrease in cellular heme content was observed after all metals treatment. This suggests that heme degradation plays a role in reducing Cyp1a1 activity. This is the first demonstration that heavy metals can directly induce Cyp1a1 gene expression in an AhR-dependent manner through transcriptional and posttranslational mechanisms.

Arsenite-Induced Aryl Hydrocarbon Receptor Nuclear Translocation Results in Additive Induction of Phase I Genes and Synergistic Induction of Phase II Genes

Complex mixtures of carcinogenic metalloids, such as arsenic, and polycyclic aromatic hydrocarbons or halogenated aromatic hydrocarbons are common environmental contaminants. The biological consequences of exposure to these mixtures are unpredictable and, although the health effects of individual chemicals may be known, the toxicity of environmental mixtures is largely unexplored. Arsenic, not a potent mutagen by itself, is co-mutagenic with many DNA-damaging agents. Mixtures of arsenite plus benzo[a]pyrene (B[a]P) augment B[a]P mutagenicity, suggesting that arsenite might uncouple expression of phase I and II genes responsible for detoxification. We have studied the effects of arsenite exposure on the activation of the aryl hydrocarbon receptor (AHR) and its subsequent role in gene transactivation. Treatment of mouse Hepa-1 cells with arsenite induces AHR nuclear translocation and binding to the Cyp1a1 gene promoter with the same efficiency as tetrachlorodibenzo-p-dioxin (TCDD), the most potent ligand of the AHR; however, TCDD and B[a]P are an order of magnitude more potent than arsenite in up-regulating Cyp1a1 transcription. Global profiling analyses of cells treated with arsenite plus B[a]P indicate that several phase I and II detoxification genes are in some cases additively and in others synergistically deregulated by the mixtures. Real-time reverse transcription-polymerase chain reaction analyses of mouse embryonic fibroblasts showed that the mixtures had an additive effect on the mRNA levels of Cyp1b1, a prototypical phase I detoxification gene, and an AHR-dependent synergistic effect on the corresponding levels of Nqo1, a prototypical phase II gene. We conclude that exposure to arsenite/B[a]P mixtures causes regulatory changes in the expression of detoxification genes that ultimately affect the metabolic activation and disposition of toxicants.

Arsenite decreases CYP3A4 and RXRalpha in primary human hepatocytes

Arsenic is a naturally occurring, worldwide contaminant implicated in numerous pathological conditions in humans, including cancer and several forms of liver disease. One of the contributing factors to these disorders may be the alteration of cytochrome P450 (P450) levels by arsenic. P450s are involved in the oxidative metabolism and elimination of numerous toxic chemicals. CYP3A4, a major P450 in humans, is involved in the metabolism of half of all currently used drugs. Acute exposure to arsenite decreases the induction of CYP1A1/2 proteins and activities in cultured human hepatocytes, as well as CYP3A23 in cultured rat hepatocytes. Here, in primary cultures of human hepatocytes, we assessed the effects of acute arsenite exposure on CYP3A4 and several transcription factors involved in CYP3A4 expression. The concentrations of arsenite used in these studies were nontoxic to the hepatocytes and failed to elicit an oxidative response. Treatment with arsenite in the presence of CYP3A4 inducers, rifampicin (Rif) or phenobarbital, caused major decreases in CYP3A4 mRNA, protein, and activity. In addition, the levels of CYP3A4 in untreated cells were decreased following arsenite treatment. Transcription of the CYP3A4 gene is primarily regulated by heterodimers of the retinoid X receptor alpha (RXRalpha) and the pregnane X receptor (PXR). We found that arsenite failed to affect expression of PXR or the transcription factor Sp1, yet caused a significant decrease in PXR responsiveness to Rif. Arsenite caused a large decrease in nuclear RXRalpha protein and, to a lesser extent, RXRalpha mRNA. These results suggest that arsenite inhibits both untreated and induced CYP3A4 transcription in primary human hepatocytes by decreasing the activity of PXR, as well as expression of the nuclear receptor RXRalpha.