Environmentally relevant concentrations of arsenite and monomethylarsonous acid inhibit IL-7/STAT5 cytokine signaling pathways in mouse CD3+CD4-CD8- double negative thymus cells

Environmental arsenic exposure is a public health issue. Immunotoxicity induced by arsenic has been reported in humans and animal models. The purpose of this study was to evaluate mechanisms of As(+3) and MMA(+3) toxicity in mouse thymus cells. Because we know that MMA(+3) inhibits IL-7 signaling in mouse bone marrow pre-B cells, we studied the influence of As(+3) and MMA(+3) on T cell development in the thymus at the earliest stage of T cell development (CD4-CD8-, double negative, DN) which requires IL-7 dependent signaling. We found in a DN thymus cell line (D1) that a low concentration of MMA(+3) (50 nM) suppressed IL-7 dependent JAK1, 3 and STAT5 signaling. As(+3) suppressed STAT5 and JAK3 at higher concentrations (500 nM). Cell surface expression of the IL-7 receptor (CD127) was also suppressed by 50 nM MMA(+)3, but was increased by 500 NM As(+3), indicating possible differences in the mechanisms of action of these agents. A decrease in cyclin D1 protein expression was observed in D1 cells exposed to As(+3) at 500 nM and MMA(+3) starting at 50 nM, suggesting that arsenic at these environmentally-relevant doses suppresses early T cell development through the inhibition of IL-7 signaling pathway.

Evaluation of Toxicity in Mouse Bone Marrow Progenitor Cells

Development of blood cells through hematopoiesis occurs in the bone marrow (BM), and can be adversely impacted by various substances and/or conditions ranging from known therapeutic, intentionally administered xenobiotics to unintentional food additives and exposure to environmental chemicals. The principles underlying the techniques for evaluating toxicity to BM progenitors (erythroid, myeloid, and lymphoid) exploit changes in the normal hematopoietic process, biochemical cell surface and intracellular markers, as well as components of the BM microenvironment. Toxicological investigations following in vivo exposures of mice or in vitro exposures of mouse primary BM cell cultures allow the assessment of the developmental and functional integrity of BM cells, cell population shifts, and adverse biochemical effects due to toxicity. Colony forming unit (CFU) assays and flow cytometry are indispensable techniques in these toxicity studies.

Monomethylarsonous acid (MMA+3) Inhibits IL-7 Signaling in Mouse Pre-B Cells

Our previously published data show that As(+3) in vivo and in vitro, at very low concentrations, inhibits lymphoid, but not myeloid stem cell development in mouse bone marrow. We also showed that the As(+3) metabolite, monomethylarsonous acid (MMA(+3)), was responsible for the observed pre-B cell toxicity caused by As(+3). Interleukin-7 (IL-7) is the primary growth factor responsible for pre-lymphoid development in mouse and human bone marrow, and Signal Transducer and Activator of Transcription 5 (STAT5) is a transcriptional factor in the IL-7 signaling pathway. We found that MMA(+3) inhibited STAT5 phosphorylation at a concentration as low as 50 nM in mouse bone marrow pre-B cells. Inhibition of STAT5 phosphorylation by As(+3) occurred only at a concentration of 500 nM. In the IL-7 dependent mouse pre-B 2E8 cell line, we also found selective inhibition of STAT5 phosphorylation by MMA(+3), and this inhibition was dependent on effects on JAK3 phosphorylation. IL-7 receptor expression on 2E8 cell surface was also suppressed by 50 nM MMA(+3) at 18 h. As further evidence for the inhibition of STAT5, we found that the induction of several genes required in B cell development, cyclin D1, E2A, EBF1, and PAX5, were selectively inhibited by MMA(+3). Since 2E8 cells lack the enzymes responsible for the conversion of As(+3) to MMA(+3) in vitro, the results of these studies suggest that As(+3) induced inhibition of pre-B cell formation in vivo is likely dependent on the formation of MMA(+3) which in turn inhibits IL-7 signaling at several steps in mouse pre-B cells.

Environmentally Relevant Concentrations of Arsenite Induce Dose-Dependent Differential Genotoxicity Through Poly(ADP-Ribose) Polymerase Inhibition and Oxidative Stress in Mouse Thymus Cells

Inhibition of DNA repair and oxidative stress are 2 common mechanisms associated with arsenic-induced genotoxicity. The purpose of this study was to examine mechanisms of genotoxicity induced by environmentally relevant doses of arsenite (As(+3)) in mouse thymus cells. An increase in DNA damage and a decrease in poly(ADP-ribose) polymerase (PARP) activity were seen in vitro following exposure to 50 nM As(+3) in primary mouse thymus cells and a murine thymus pre-T cell line, D1. 3,4-Dihydro-5[4-(1-piperindinyl) butoxyl]-1(2H)-isoquinoline, a well-characterized PARP inhibitor, also produced DNA damage in D1 cells, confirming the correlation between PARP inhibition and DNA damage increase. As(+3) at 500 nM induced double strand breaks (DSBs) in DNA and oxidative stress at 4 h in D1 cells, which was reversed at 18 h. No apoptosis or decrease of viability was observed in these exposures. 4-Hydroxy-2,2,6,6-tetramethylpiperidin-1-oxyl, a widely-used antioxidant, was utilized to confirm that oxidative stress is partially responsible for the increase of strand breaks in 500 nM As(+3) exposure at 4 h. Expression of As(+3) exporters, Mdr1 and Mrp1, were found to be induced by 500 nM As(+3) in D1 cells, suggesting a possible mechanism for reversal of oxidative stress and DSBs at the 18-h timepoint. Finally, we showed that DNA damage and PARP inhibition by As(+3) were reversed by zinc (Zn(+2)) at approximate equimolar doses. Collectively, these results demonstrate that As(+3) at doses within the nanomolar range induce genotoxicity by inhibiting PARP, and produces oxidative stress at higher concentrations, which can be reversed by a Zn(+2) treatment.

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.

Conference Summary and Recent Advances: the 8th Conference on Metal Toxicity and Carcinogenesis

Diseases caused by occupational and environmental exposure to metals are a public health concern. The underlying molecular mechanisms of metal toxicity and carcinogenicity remain largely unknown. Over 130 scientists attended the 8th Conference on Metal Toxicity and Carcinogenesis, presenting their various research concerns and recent findings to stimulate interactions and collaborations among scientists in the field. Several major areas were emphasized, including human & population studies, molecular & cellular mechanisms, biological targets, epigenetic effects, metabolism, and metal mixtures. Here we summarize presentations at the conference sessions and highlight the attendees’ latest work published in this special issue of Biological Trace Element Research .

Analysis of dibenzo[def,p]chrysene-deoxyadenosine adducts in wild-type and cytochrome P450 1b1 knockout mice using stable-isotope dilution UHPLC-MS/MS

The polycyclic aromatic hydrocarbon (PAH), dibenzo[def,p]chrysene (DBC; also known as dibenzo[a,l]pyrene), is a potent carcinogen in animal models and a class 2A human carcinogen. Recent investigations into DBC-mediated toxicity identified DBC as a potent immunosuppressive agent similar to the well-studied immunotoxicant 7,12-dimethylbenz[a]anthracene (DMBA). DBC, like DMBA, is bioactivated by cytochrome P450 (CYP) 1B1 and forms the reactive metabolite DBC-11,12-diol-13,14-epoxide (DBCDE). DBCDE is largely responsible for the genotoxicity associated with DBC exposure. The immunosuppressive properties of several PAHs are also linked to genotoxic mechanisms. Therefore, this study was designed to identify DBCDE-DNA adduct formation in the spleen and thymus of wild-type and cytochrome P450 1b1 (Cyp1b1) knockout (KO) mice using a highly sensitive stable-isotope dilution UHPLC-MS/MS method. Stable-isotope dilution UHPLC-MS/MS identified the major DBC adducts (±)-anti-cis-DBCDE-dA and (±)-anti-trans-DBCDE-dA in the lung, liver, and spleen of both WT and Cyp1b1 KO mice. However, adduct formation in the thymus was below the level of quantitation for our method. Additionally, adduct formation in Cyp1b1 KO mice was significantly reduced compared to wild-type (WT) mice receiving DBC via oral gavage. In conclusion, the current study identifies for the first time DBCDE-dA adducts in the spleen of mice supporting the link between genotoxicity and immunosuppression, in addition to supporting previous studies identifying Cyp1b1 as the primary CYP involved in DBC bioactivation to DBCDE. The high levels of DBC-DNA adducts identified in the spleen, along with the known high levels of Cyp1b1 expression in this organ, supports further investigation into DBC-mediated immunotoxicity.

Differential susceptibility of human peripheral blood T cells to suppression by environmental levels of sodium arsenite and monomethylarsonous acid

Human exposure to arsenic in drinking water is known to contribute to many different health outcomes such as cancer, diabetes, and cardiopulmonary disease. Several epidemiological studies suggest that T cell function is also altered by drinking water arsenic exposure. However, it is unclear how individual responses differ to various levels of exposure to arsenic. Our laboratory has recently identified differential responses of human peripheral blood mononuclear cell (HPMBC) T cells as measured by polyclonal T cell activation by mitogens during sodium arsenite exposure. T cells from certain healthy individuals exposed to various concentrations (1-100 nM) of arsenite in vitro showed a dose-dependent suppression at these extremely low concentrations (∼ 0.1-10 ppb) of arsenite, whereas other individuals were not suppressed at low concentrations. In a series of more than 30 normal donors, two individuals were found to be sensitive to low concentration (10 nM equivalent ∼ 1 ppb drinking water exposure) to sodium arsenite-induced inhibition of T cell proliferation produced by phytohemagglutinin (PHA) and anti-CD3/anti-CD28. In an arsenite-susceptible individual, arsenite suppressed the activation of Th1 (Tbet) cells, and decreased the percentage of cells in the double positive Th17 (RORγt) and Treg (FoxP3) population. While the majority of normal blood donors tested were not susceptible to inhibition of proliferation at the 1-100 nM concentrations of As(+3), it was found that all donors were sensitive to suppression by 100 nM monomethylarsonous acid (MMA+3), a key metabolite of arsenite. Thus, our studies demonstrate for the first time that low ppb-equivalent concentrations of As(+3) are immunosuppressive to HPBMC T cells in some individuals, but that most donor HPBMC are sensitive to suppression by MMA(+3) at environmentally relevant exposure levels.

Arsenite selectively inhibits mouse bone marrow lymphoid progenitor cell development in vivo and in vitro and suppresses humoral immunity in vivo

It is known that exposure to As⁺³ via drinking water causes a disruption of the immune system and significantly compromises the immune response to infection. The purpose of these studies was to assess the effects of As⁺³ on bone marrow progenitor cell colony formation and the humoral immune response to a T-dependent antigen response (TDAR) in vivo. In a 30 day drinking water study, mice were exposed to 19, 75, or 300 ppb As⁺³. There was a decrease in bone marrow cell recovery, but not spleen cell recovery at 300 ppb As⁺³. In the bone marrow, As⁺³ altered neither the expression of CD34+ and CD38+ cells, markers of early hematopoietic stem cells, nor CD45−/CD105+, markers of mesenchymal stem cells. Spleen cell surface marker CD45 expression on B cells (CD19+), T cells (CD3+), T helper cells (CD4+) and cytotoxic T cells (CD8+), natural killer (NK+), and macrophages (Mac 1+) were not altered by the 30 day in vivo As⁺³ exposure. Functional assays of CFU-B colony formation showed significant selective suppression (p<0.05) by 300 ppb As⁺³ exposure, whereas CFU-GM formation was not altered. The TDAR of the spleen cells was significantly suppressed at 75 and 300 ppb As⁺³. In vitro studies of the bone marrow revealed a selective suppression of CFU-B by 50 nM As⁺³ in the absence of apparent cytotoxicity. Monomethylarsonous acid (MMA⁺³) demonstrated a dose-dependent and selective suppression of CFU-B beginning at 5 nM (p<0.05). MMA⁺³ suppressed CFU-GM formation at 500 nM, a concentration that proved to be nonspecifically cytotoxic. As⁺⁵ did not suppress CFU-B and/or CFU-GM in vitro at concentrations up to 500 nM. Collectively, these results demonstrate that As⁺³ and likely its metabolite (MMA⁺³) target lymphoid progenitor cells in mouse bone marrow and mature B and T cell activity in the spleen.

Differential binding of monomethylarsonous acid compared to arsenite and arsenic trioxide with zinc finger peptides and proteins

Arsenic is an environmental toxin that enhances the carcinogenic effect of DNA-damaging agents, such as ultraviolet radiation and benzo[a]pyrene. Interaction with zinc finger proteins has been shown to be an important molecular mechanism for arsenic toxicity and cocarcinogenesis. Arsenicals such as arsenite, arsenic trioxide (ATO), and monomethylarsonous acid (MMA(III)) have been reported to interact with cysteine residues of zinc finger domains, but little is known about potential differences in their selectivity of interaction. Herein we analyzed the interaction of arsenite, MMA(III), and ATO with C2H2, C3H1, and C4 configurations of zinc fingers using UV-vis, cobalt, fluorescence, and mass spectrometry. We observed that arsenite and ATO both selectively bound to C3H1 and C4 zinc fingers, while MMA(III) interacted with all three configurations of zinc finger peptides. Structurally and functionally, arsenite and ATO caused conformational changes and zinc loss on C3H1 and C4 zinc finger peptide and protein, respectively, whereas MMA(III) changed conformation and displaced zinc on all three types of zinc fingers. The differential selectivity was also demonstrated in zinc finger proteins isolated from cells treated with these arsenicals. Our results show that trivalent inorganic arsenic compounds, arsenite and ATO, have the same selectivity and behavior when interacting with zinc finger proteins, while methylation removes the selectivity. These findings provide insights on the molecular mechanisms underlying the differential effects of inorganic versus methylated arsenicals, as well as the role of in vivo arsenic methylation in arsenic toxicity and carcinogenesis.

Dibenzo[def,p]chrysene (DBC) suppresses antibody formation in spleen cells following oral exposures of mice

Dibenzo[def,p]chrysene (DBC) is a potent environmental carcinogen in rodents, fish, and human cells examined in culture. There are numerous similarities between the patterns of cytochrome P-450 (P450) activation of DBC and its covalent binding to DNA and proteins with another polycyclic aromatic hydrocarbon (PAH), 7,12-dimethylbenz[a]anthracene (DMBA). Our lab has previously shown that DMBA produces immunosuppression in rodents and human cell systems. Therefore, the purpose of these studies was to examine the immunotoxicity of DBC in a rodent model that was found to be sensitive to the immunosuppressive effects of DMBA. Data showed that DBC had similar potency to DMBA in producing suppression of a T-dependent antibody response (TDAR) and altered spleen cell subsets in a similar manner as DMBA when DMBA was given by gavage for 5 d in corn oil to mice at doses of 1-100 mg/kg total cumulative doses. T-cell-independent antigen (TNP-Ficoll) responses were quantitatively less sensitive to DBC suppression. It was also found that as with DMBA, DBC produced a persistent immunosuppression, which lasted for at least 4 wk following dosing with a novel pill method for self-administration of DBC. In conclusion, DBC appears to possess many of the same characteristics of DMBA in terms of its immunotoxicity.

Potential risks resulting from fruit/vegetable-drug interactions: effects on drug-metabolizing enzymes and drug transporters

It has been well established that complex mixtures of phytochemicals in fruits and vegetables can be beneficial for human health. Moreover, it is becoming increasingly apparent that phytochemicals can influence the pharmacological activity of drugs by modifying their absorption characteristics through interactions with drug transporters as well as drug-metabolizing enzyme systems. Such effects are more likely to occur in the intestine and liver, where high concentrations of phytochemicals may occur. Alterations in cytochrome P450 and other enzyme activities may influence the fate of drugs subject to extensive first-pass metabolism. Although numerous studies of nutrient-drug interactions have been published and systematic reviews and meta-analyses of these studies are available, no generalizations on the effect of nutrient-drug interactions on drug bioavailability are currently available. Several publications have highlighted the unintended consequences of the combined use of nutrients and drugs. Many phytochemicals have been shown to have pharmacokinetic interactions with drugs. The present review is limited to commonly consumed fruits and vegetables with significant beneficial effects as nutrients and components in folk medicine. Here, we discuss the phytochemistry and pharmacokinetic interactions of the following fruit and vegetables: grapefruit, orange, tangerine, grapes, cranberry, pomegranate, mango, guava, black raspberry, black mulberry, apple, broccoli, cauliflower, watercress, spinach, tomato, carrot, and avocado. We conclude that our knowledge of the potential risk of nutrient-drug interactions is still limited. Therefore, efforts to elucidate potential risks resulting from food-drug interactions should be intensified in order to prevent undesired and harmful clinical consequences.

Principal component analysis optimization of a PM2.5 land use regression model with small monitoring network

The use of land-use regression (LUR) techniques for modeling small-scale variations of intraurban air pollution has been increasing in the last decade. The most appealing feature of LUR techniques is the economical monitoring requirements. In this study, principal component analysis (PCA) was employed to optimize an LUR model for PM2.5. The PM2.5 monitoring network consisted of 13 sites, which constrained the regression model to a maximum of one independent variable. An optimized surrogate of vehicle emissions was produced by PCA and employed as the predictor variable in the model. The vehicle emissions surrogate consisted of a linear combination of several traffic variables (e.g., vehicle miles traveled, speed, traffic demand, road length, and time) obtained from a road network used for traffic modeling. The vehicle-emissions surrogate produced by the PCA had a predictive capacity greater (R2=.458) than the traffic variable, Traffic Demand summarized for a 1 km buffer, with best predictive capacity (R2=.341). The PCA-based method employed in this study was effective at increasing the fit of an ordinary LUR model by optimizing the utilization of a PM2.5 dataset from small-n monitoring network. In general, the method used can contribute to LUR techniques in two major ways: 1) by improving the predictive power of the input variable, by substituting a principal component for a single variable and 2) by creating an orthogonal set of predictor variables, and thus fulfilling the no colinearity assumption of the linear regression methods. The proposed PCA method, should be universally applicable to LUR methods and will expand their economical attractiveness.

Ethanol-induced oxidative stress is associated with EGF receptor phosphorylation in MCF-10A cells overexpressing CYP2E1

Breast cancer is the most common cancer and the second leading cause of cancer-related mortality worldwide. The etiology of breast cancer is very diverse and ethanol (EtOH) consumption is a well-established risk factor for breast cancer in women. However, the mechanism by which EtOH exerts its carcinogenic activity in breast tissue remains unknown. CYP2E1 is known to metabolize ethanol and produce reactive oxygen species (ROS), including superoxide in epithelial cells. Therefore, in the present studies, we investigated whether there is an increase in ROS following overexpression of CYP2E1 in MCF-10A cells. We found that 30 and 100 mM EtOH increased ROS levels after 2 h treatment in CYP2E1 overexpressing cells. Based on these results and our previous studies with ROS-producing chemicals, we also examined epidermal growth factor receptor (EGFR) activation following exposure to ethanol. We found that there was an increase in phosphorylation of pY1086 EGFR after 18 h EtOH treatment in CYP2E1 overexpressing cells. These studies support a hypothesis that EtOH might increase human mammary cell activation, via an EGFR-dependent signaling mechanism associated with oxidative stress.

Immunotoxicity and biodistribution analysis of arsenic trioxide in C57Bl/6 mice following a 2-week inhalation exposure

In these studies the immunotoxicity of arsenic trioxide (ATO, As(2)O(3)) was evaluated in mice following 14 days of inhalation exposures (nose only, 3 h per day) at concentrations of 50 microg/m(3) and 1 mg/m(3). A biodistribution analysis performed immediately after inhalation exposures revealed highest levels of arsenic in the kidneys, bladder, liver, and lung. Spleen cell levels were comparable to those found in the blood, with the highest concentration of arsenic detected in the spleen being 150 microg/g tissue following the 1 mg/m(3) exposures. No spleen cell cytotoxicity was observed at either of the two exposure levels. There were no changes in spleen cell surface marker expression for B cells, T cells, macrophages, and natural killer (NK) cells. There were also no changes detected in the B cell (LPS-stimulated) and T cell (Con A-stimulated) proliferative responses of spleen cells, and no changes were found in the NK-mediated lysis of Yac-1 target cells. The primary T-dependent antibody response was, however, found to be highly susceptible to ATO suppression. Both the 50 microg/m(3) and 1 mg/m(3) exposures produced greater than 70% suppression of the humoral immune response to sheep red blood cells. Thus, the primary finding of this study is that the T-dependent humoral immune response is extremely sensitive to suppression by ATO and assessment of humoral immune responses should be considered in evaluating the health effects of arsenic containing agents.

Temporal-spatial analysis of U.S.-Mexico border environmental fine and coarse PM air sample extract activity in human bronchial epithelial cells

Particulate matter less than 10 microm (PM10) has been shown to be associated with aggravation of asthma and respiratory and cardiopulmonary morbidity. There is also great interest in the potential health effects of PM2.5. Particulate matter (PM) varies in composition both spatially and temporally depending on the source, location and seasonal condition. El Paso County which lies in the Paso del Norte airshed is a unique location to study ambient air pollution due to three major points: the geological land formation, the relatively large population and the various sources of PM. In this study, dichotomous filters were collected from various sites in El Paso County every 7 days for a period of 1 year. The sampling sites were both distant and near border crossings, which are near heavily populated areas with high traffic volume. Fine (PM2.5) and Coarse (PM10-2.5) PM filter samples were extracted using dichloromethane and were assessed for biologic activity and polycyclic aromatic (PAH) content. Three sets of marker genes human BEAS2B bronchial epithelial cells were utilized to assess the effects of airborne PAHs on biologic activities associated with specific biological pathways associated with airway diseases. These pathways included in inflammatory cytokine production (IL-6, IL-8), oxidative stress (HMOX-1, NQO-1, ALDH3A1, AKR1C1), and aryl hydrocarbon receptor (AhR)-dependent signaling (CYP1A1). Results demonstrated interesting temporal and spatial patterns of gene induction for all pathways, particularly those associated with oxidative stress, and significant differences in the PAHs detected in the PM10-2.5 and PM2.5 fractions. Temporally, the greatest effects on gene induction were observed in winter months, which appeared to correlate with inversions that are common in the air basin. Spatially, the greatest gene expression increases were seen in extracts collected from the central most areas of El Paso which are also closest to highways and border crossings.

EGF-receptor phosphorylation and downstream signaling are activated by benzo[a]pyrene 3,6-quinone and benzo[a]pyrene 1,6-quinone in human mammary epithelial cells

Benzo[a]pyrene (BaP) is activated by xenobiotic-metabolizing enzymes to highly mutagenic and carcinogenic metabolites. Previous studies in this laboratory have shown that benzo[a]pyrene quinones (BPQs), 1,6-BPQ and 3,6-BPQ, are able to induce epidermal growth factor receptor (EGFR) cell signaling through the production of reactive oxygen species. Recently, we have reported that BPQs have the potential to induce the expression of genes involved in numerous pathways associated with cell proliferation and survival in human mammary epithelial cells. In the present study we demonstrated that BPQs not only induced EGFR tyrosine autophosphorylation, but also induced EGFR-dependent tyrosine phosphorylation of phospholipase C-gamma1 and several signal transducers and activators of transcription (STATs). The effects of BPQs were evaluated in a model of EGF withdrawal in MCF10-A cells. We found that BPQs (1 muM), induced EGFR tyrosine phosphorylation at positions Y845, Y992, Y1068, and Y1086. PLC-gamma1 phosphorylation correlated with the phosphorylation of tyrosine-Y992, a proposed docking site for PLC-gamma1 on the EGFR. Additionally, we found that BPQs induced the activation of STAT-1, STAT-3, STAT-5a and STAT-5b. STAT5 was shown to translocate to the nucleus following 3,6-BPQ and 1,6-BPQ exposures. Although the patterns of phosphorylation at EGFR, PLC-gamma1 and STATs were quite similar to those induced by EGF, an important difference between BPQ-mediated signaling of the EGFR was observed. Signaling produced by EGF ligand produced a rapid disappearance of EGFR from the cell surface, whereas BPQ signaling maintained EGFR receptors on the cell membrane. Thus, the results of these studies show that 1,6-BPQ and 3,6-BPQ can produce early events as evidenced by EGFR expression, and a prolonged transactivation of EGFR leading to downstream cell signaling pathways.

A multifunctional androgen receptor screening assay using the high-throughput Hypercyt flow cytometry system

The androgen receptor (AR) is a steroid hormone receptor which regulates transcription of androgen-sensitive genes and is responsible for the development and maintenance of male secondary sexual characteristics. Chemicals that interfere with AR activity may lead to pathological conditions in androgen-sensitive tissues. A variety of reporter systems have been developed, driven by androgen-sensitive promoters, which screen for chemicals that modulate androgenic activity. We have developed a flexible, high-throughput AR transcriptional activation assay, designated the Multifunctional Androgen Receptor Screening (MARS) assay, to facilitate the identification of novel modulators of AR transcriptional activity using flow cytometry. Androgen-independent human prostate cancer-derived PC3 cells were transiently cotransfected with an expression vector for the wild-type human AR and an androgen-sensitive promoter regulating the expression of destabilized enhanced GFP (dsEGFP). The transfected cells were stimulated with established androgenic and antiandrogenic compounds and assessed for increased or decreased dsEGFP expression. To screen for antagonists of AR transcription, the AR agonist R1881 was coadministered at submaximal concentrations with potential AR antagonists. The assay was formatted for high-throughput screening using the HyperCyt flow cytometry system. Agents with established androgenic and antiandrogenic activity were used for validation of the MARS assay. AR agonists were found to potently induce dsEGFP. Furthermore, AR agonists induced dsEGFP expression in a dose-dependent manner. Alternatively, AR antagonists blocked dsEGFP expression when coadministered with low-dose R1881, which also occurred in a dose-dependent manner. Modulators of AR transcriptional activity can be successfully identified by the MARS assay, utilizing a rapid, flexible, sensitive, and high-throughput format. Dose-response curves can be successfully generated for these compounds, allowing for an assessment of potency. Because of its simplicity and high-throughput compatibility, the MARS assay and HyperCyt system combined with flow cytometric analysis represents a valuable and novel addition to the current repertoire of AR transcriptional activation screening assays.

p53 and ATM/ATR regulate 7,12-dimethylbenz[a]anthracene-induced immunosuppression

The tumor suppressor protein p53 is a transcription factor that regulates apoptotic responses produced by genotoxic agents. Previous studies have reported that 7,12-dimethylbenz[a]anthracene (DMBA)-induced bone marrow toxicity is p53-dependent in vivo. Our laboratory has shown that DMBA-induced splenic immunosuppression is CYP1B1- and microsomal epoxide hydrolase (mEH)-dependent, demonstrating that the DMBA-3,4-dihydrodiol-1,2-epoxide metabolite (DMBA-DE) is probably responsible for DMBA-induced immunosuppression. DMBA-DE is known to bind to DNA leading to strand breaks. Therefore, we postulated that a p53 pathway is required for DBMA-induced immunosuppression. In the present studies, our data show that activated p53 accumulated in the nuclei of spleen cells in WT and AhR-null mice after DMBA treatment, but not in CYP1B1-null or mEH-null mice. These results suggest that DMBA activates p53 in a CYP1B1- and mEH-dependent manner in vivo but is not AhR-dependent. Ataxia telangiectasia mutated (ATM) and ATM and Rad3-related protein (ATR) are sensors for DNA damage that signal p53 activation. Increased ATM, phospho-ATM (Ser(1987)), and ATR levels were observed after DMBA treatment in WT, p53-null, and AhR-null mice but not in CYP1B1-null or mEH-null mice. Therefore, ATM and ATR seem to act upstream of p53 as sensors of DNA damage. Ex vivo immune function studies demonstrated that DMBA-induced splenic immunosuppression is p53-dependent at doses of DMBA that produce immunosuppression in the absence of cytotoxicity. High-dose DMBA cytotoxicity may be associated with p53-independent pathways. This study provides new insights into the requirement of genotoxicity for DMBA-induced immunosuppression in vivo and highlights the roles of ATM/ATR in signaling p53.

Pulmonary and Systemic Immune Response to Inhaled Multiwalled Carbon Nanotubes

Inhalation of multiwalled carbon nanotubes (MWCNTs) at particle concentrations ranging from 0.3 to 5 mg/m3 did not result in significant lung inflammation or tissue damage, but caused systemic immune function alterations. C57BL/6 adult (10- to 12-week) male mice were exposed by whole-body inhalation to control air or 0.3, 1, or 5 mg/m3 respirable aggregates of MWCNTs for 7 or 14 days (6 h/day). Histopathology of lungs from exposed animals showed alveolar macrophages containing black particles; however, there was no inflammation or tissue damage observed. Bronchial alveolar lavage fluid also demonstrated particle-laden macrophages; however, white blood cell counts were not increased compared to controls. MWCNT exposures to 0.3 mg/m3 and higher particle concentrations caused nonmonotonic systemic immunosuppression after 14 days but not after 7 days. Immunosuppression was characterized by reduced T-cell-dependent antibody response to sheep erythrocytes as well as T-cell proliferative ability in presence of mitogen, Concanavalin A. Assessment of nonspecific natural killer (NK) cell activity showed that animals exposed to 1 mg/m(3) had decreased NK cell function. Gene expression analysis of selected cytokines and an indicator of oxidative stress were assessed in lung tissue and spleen. No changes in gene expression were observed in lung; however, interleukin-10 (IL-10) and NAD(P)H oxidoreductase 1 mRNA levels were increased in spleen.

Microsomal Expoxide Hydrolase Is Required for 7,12-Dimethylbenz[a]anthracene (DMBA)-Induced Immunotoxicity in Mice

Microsomal epoxide hydrolase (mEH, EPHX1) is involved in the metabolism of chemicals to generate dihydrodiol intermediates in the presence of the cytochrome P450. We have previously shown that 7,12-dimethylbenz[a]anthracene (DMBA) can suppress both cell-mediated and humoral immune responses in wild-type (WT) C57BL/6N mice but not in CYP1B1 null mice. In the present studies, we hypothesized the critical metabolite responsible for DMBA-induced immunotoxicity is likely to be the 3,4-dihydrodiol-1,2-epoxide metabolite of DMBA, which requires mEH for formation. Mice were gavaged orally with DMBA (0, 17, 50, and 150 mg/kg) once a day for 5 days. Immune function and other assays were performed on day 7. Our data showed that unlike WT mice, DMBA treatment of mEH null mice produced no alterations in the body weight, spleen weight, or spleen cellularity. Similarly, DMBA treatments did not affect the PFC response in mEH null mice. Natural killer activity was not altered by DMBA treatment in mEH null mice. T-cell mitogenesis was partially suppressed by 50 and 150 mg/kg DMBA treatments of mEH null mice, but B-cell mitogenesis was not affected. Finally, we assessed the biodistribution of DMBA in both C57BL/6N WT and mEH null mice in spleen, thymus, and liver after 24 h and 7 days oral gavage. The concentrations of DMBA in each organ were not significantly different in WT and in mEH null mice. Collectively, these results demonstrate that mEH (EPHX1 gene) is a crucial enzyme for metabolic activation of DMBA in vivo leading to immunosuppression of spleen cells.

Activation of dioxin response element (DRE)-associated genes by benzo(a)pyrene 3,6-quinone and benzo(a)pyrene 1,6-quinone in MCF-10A human mammary epithelial cells

Benzo(a)pyrene (BaP) is a known human carcinogen and a suspected breast cancer complete carcinogen. BaP is metabolized by several metabolic pathways, some having bioactivation and others detoxification properties. BaP-quinones (BPQs) are formed via cytochrome P450 and peroxidase dependent pathways. Previous studies by our laboratory have shown that BPQs have significant growth promoting and anti-apoptotic activities in human MCF-10A mammary epithelial cells examined in vitro. Previous results suggest that BPQs act via redox-cycling and oxidative stress. However, because two specific BPQs (1,6-BPQ and 3,6-BPQ) differed in their ability to produce reactive oxygen species (ROS) and yet both had strong proliferative and EGF receptor activating activity, we utilized mRNA expression arrays and qRT-PCR to determine potential pathways and mechanisms of gene activation. The results of the present studies demonstrated that 1,6-BPQ and 3,6-BPQ activate dioxin response elements (DRE, also known as xenobiotic response elements, XRE) and anti-oxidant response elements (ARE, also known as electrophile response elements, EpRE). 3,6-BPQ had greater DRE activity than 1,6-BPQ, whereas the opposite was true for the activation of ARE. Both 3,6-BPQ and 1,6-BPQ induced oxidative stress-associated genes (HMOX1, GCLC, GCLM, and SLC7A11), phase 2 enzyme genes (NQO1, NQO2, ALDH3A1), PAH metabolizing genes (CYP1B1, EPHX1, AKR1C1), and certain EGF receptor-associated genes (EGFR, IER3, ING1, SQSTM1 and TRIM16). The results of these studies demonstrate that BPQs activate numerous pathways in human mammary epithelial cells associated with increased cell growth and survival that may play important roles in tumor promotion.

PYK2 mediates anti-apoptotic AKT signaling in response to benzo[a]pyrene diol epoxide in mammary epithelial cells

Polycyclic aromatic hydrocarbons, such as benzo[a]pyrene (BaP), are known mammary carcinogens in rodents and may be involved in human breast cancer. The carcinogenicity of BaP has been partially attributed to the formation of the BaP diol epoxide (BPDE), which has been shown to stably bind DNA and act as an initiator. BaP is a complete carcinogen, but the mechanisms for tumor promotion are less well characterized. Previous studies have demonstrated that BPDE enhanced anti-apoptotic signaling through Akt; however, mechanisms for Akt activation by BPDE are not well defined. In the current studies, we found that BPDE increased intracellular Ca2+ concentration in the human mammary epithelial cell line MCF-10A. A peak in Ca2+ concentration at 20 min was followed by increased phosphorylation of Pyk2 at Tyr881 and increased total tyrosine phosphorylation of the epidermal growth factor receptor (EGFR). Consistent with activation of the EGFR, Akt and ERK1/2 phosphorylation was detected in MCF-10A cells treated with BPDE. Pharmacological methods to prevent Ca2+ elevation and EGFR activity, and small-interfering RNA against Pyk2, prevented Akt phosphorylation by BPDE, which suggested that Ca2+, Pyk2 and EGFR activation lay upstream of Akt. In addition, we found that BPDE increased p53 activity and apoptosis in MCF-10A; however, transient transfection of constitutively active Akt attenuated both BPDE-dependent apoptosis and p53 activity. In contrast, apoptosis was enhanced by inhibitors of phosphatidyl inositol 3-kinase (PI3-K). This work demonstrates a novel mechanism for Akt activation by BPDE that occurs through increased Ca2+ concentration, and implicates Ca2+, Pyk2, EGFR and Akt as a potential pathway by which BPDE can inhibit apoptosis and act as a promoter of carcinogenesis.

Ryanodine Receptor-Mediated Rapid Increase in Intracellular Calcium Induced by 7,8-Benzo(a)Pyrene Quinone in Human and Murine Leukocytes

Benzo(a)pyrene (BaP) is an environmentally prevalent polycyclic aromatic hydrocarbon (PAH) known to produce immunotoxicity in murine and human lymphocytes. Previous studies by our lab have shown that certain BaP metabolites increase intracellular Ca(2+) in human and murine lymphocytes. The mechanism by which these BaP metabolites increase Ca(2+) may involve src kinase activation and mitochondrial oxidative stress. We have implicated a new pathway of Ca(2+) elevation in lymphocytes produced by a novel BaP metabolite, BaP-7,8-dione (7,8-BPQ). This ortho quinone is produced from BaP-7,8-dihydrodiol by aldoketoreductase 1C1 (AKR1C) isoforms in human cells. We have previously shown that 7,8-BPQ increases Ca(2+) levels in an in vitro rabbit skeletal muscle sarcoplasmic reticulum (SR) vesicle model via interaction with ryanodine receptors (RyR). In the present study, we found that 7,8-BPQ produced a RyR-dependent rapid increase in intracellular Ca(2+) in the Daudi human B cell line. However, other BP-diones including 1,6-, 3,6-, and 6,12-BPQs failed to produce a rapid increase in Ca(2+). Instead they produced a late increase in intracellular Ca(2+), presumably via a redox-cycling-dependent loss of Ca(2+) buffering capacity by mitochondria. Functional RyR were detected in Daudi using a (3)H-ryanodine binding assay. The studies were extended to normal human peripheral blood and murine spleen cells, where it was found that 7,8-BPQ rapidly elevated intracellular Ca(2+) in B cells and T cells in both species. The Ca(2+)-elevating effect of 7,8-BPQ was prevented by pretreatment with a high concentration of ryanodine (500 muM). Collectively, these results demonstrate a novel mechanism of Ca(2+) elevation by an environmentally relevant metabolite of BaP in murine and human lymphocytes.