Exposure to inorganic mercury in vivo attenuates extrinsic apoptotic signaling in Staphylococcal aureus enterotoxin B stimulated T-cells

Mercury intoxication disrupts tonic signaling in B cells, and may promote autoimmunity due to abnormal phosphorylation of STIM-1 and other autoimmunity risk associated phosphoproteins involved in BCR signaling

Epidemiological studies link exposure to mercury with autoimmune disease. Unfortunately, in spite of considerable effort, no generally accepted mechanistic understanding of how mercury functions with respect to the etiology of autoimmune disease is currently available. Nevertheless, autoimmune disease often arises because of defective B cell signaling. Because B cell signaling is dependent on phosphorylation cascades, in this report, we have focused on how mercury intoxication alters phosphorylation of B cell proteins in antigen-non stimulated (tonic) mouse (BALB/c) splenic B cells. Specifically, we utilized mass spectrometric techniques to conduct a comprehensive unbiased global analysis of the effect of inorganic mercury (Hg2+) on the entire B cell phosphoproteome. We found that the effects were pleotropic in the sense that large numbers of pathways were impacted. However, confirming our earlier work, we found that the B cell signaling pathway stood out from the rest, in that phosphoproteins which had sites which were affected by Hg2+, exhibited a much higher degree of connectivity, than components of other pathways. Further analysis showed that many of these BCR pathway proteins had been previously linked to autoimmune disease. Finally, dose response analysis of these BCR pathway proteins showed STIM1_S575, and NFAT2_S259 are the two most Hg2+ sensitive of these sites. Because STIM1_S575 controls the ability of STIM1 to regulate internal Ca2+, we speculate that STIM1 may be the initial point of disruption, where Hg2+ interferes with B cell signaling leading to systemic autoimmunity, with the molecular effects pleiotropically propagated throughout the cell by virtue of Ca2+ dysregulation.

At low levels, inorganic mercury interference with antigen signaling is associated with modifications to a panel of novel phosphoserine sites in B cell receptor pathway proteins

Epidemiological studies indicate that human and animal exposure to environmental mercury (Hg) disrupts normal immune system function, but the molecular mechanism responsible for this is still unresolved. We have previously utilized phospho-proteomic mass spectrometry to demonstrate that in the absence of B Cell Receptor (BCR) stimulation, exposure of B cells to Hg induces significant changes to a great many elements of the BCR signaling pathway in a concentration dependent manner. In this report, we have extended those initial findings by utilizing mass spectrometry to evaluate in detail the effect of low-level Hg exposure on BCR induced phospho-proteomic changes. Specifically, murine WEHI-231 B lymphoma cells were exposed to environmentally relevant levels of Hg with or without concomitant BCR stimulation. The cellular phospho-proteomes were then profiled by LC-MS/MS. We found that for low-level exposures, Hg interference with signal transduction across the BCR pathway was predominantly associated with modification of phosphorylation of 12 phosphosites located on seven different proteins. Nine sites were serine, two sites tyrosine and one site threonine. Most of these sites are novel, in the sense that only the two tyrosine and one of the serine sites have previously been reported to be associated with BCR signaling.

Exposure of pregnant rats to staphylococcal enterotoxin B attenuates the response of increased Tregs to re-exposure to SEB in the thymus of adult offspring

Regulatory T cells (Tregs) play an essential role during homeostasis and tolerance of the immune system. Based on our previous study that exposure of pregnant rats to staphylococcal enterotoxin B (SEB) can alter the percentage of CD4/CD8 subsets in the thymus of the offspring, in this study, we focus on the influence of exposure of pregnant rats to SEB on number, function and response of Tregs in the thymus of the offspring. Pregnant rats at gestational day of 16 were intravenously injected with 15 μg SEB and the thymuses of the neonatal and adult offspring were harvested for this study. We found that exposure of pregnant rats to SEB could significantly increase the absolute number of Tregs and the FoxP3 expression level in the thymus of not only neonatal but also adult offspring. Re-exposure of adult offspring to SEB remarkably reduced the suppressive capacity of Tregs to CD4⁺ T cells and the expression levels of TGF-β and IL-10 in the thymus, but had no effect on production of IL-4 and IFN-γ. Furthermore, it also notedly decreased the absolute number of Tregs and the FoxP3 expression level. These data suggest that prenatal exposure of pregnant rats to SEB attenuates the response of increased Tregs to re-exposure to SEB in the thymus of adult offspring.

Prenatal exposure of staphylococcal enterotoxin B attenuates the development and function of blood regulatory T cells to repeated staphylococcal enterotoxin B exposure in adult offspring rats

Introduction. Staphylococcal enterotoxin B (SEB) is an extensively studied super-antigen. A previous study by us suggested that SEB exposure during pregnancy could alter the percentage of CD4+ and CD8+ T cells in the peripheral blood of neonatal offspring rats.Aim. It is unknown whether SEB exposure during pregnancy can influence the development of regulatory T cells (Tregs) in the peripheral blood of neonatal offspring rats.Methodology. Pregnant rats at gestational day 16 were intravenously injected with 15 µg SEB. Peripheral blood was acquired from neonatal offspring rats on days 1, 3 and 5 after delivery and from adult offspring rats for determination of Treg number by cytometry, cytokines by ELISA, and FoxP3 expression by real-time PCR and western blot.Results. SEB given to pregnant rats significantly increased the absolute number of Tregs and the expression levels of FoxP3, IL-10 and TGF-β (P<0.05, P<0.01) in the peripheral blood of not only neonatal but also adult offspring rats. Furthermore, repeated SEB exposure in adult offspring rats significantly decreased the absolute number of Tregs (P<0.01), and the expression levels of FoxP3, IL-10 and TGF-β (P<0.05, P<0.01) in their peripheral blood.Conclusion. Prenatal SEB exposure attenuates the development and function of Tregs to repeated SEB exposure in the peripheral blood of adult offspring rats.

Low level Hg2+ exposure modulates the B-cell cytoskeletal phosphoproteome

Exposure of Wehi-231 B-cells to Hg²⁺ for 5min resulted in concentration dependent changes in protein phosphorylations. Phosphorylation was quantified using mass spectrometry to analyze TiO₂ and anti-pTyr antibody selected phosphopeptides from Wehi-231 digests. The most frequent and largest amplitude responses to Hg²⁺ exposure were increased phosphorylation although a decrease was observed for 1% of phosphoproteins detected in the untreated cells. A subset of proteins responded with an increase in phosphorylation to Hg²⁺ exposure at low micromolar concentrations. The majority of proteins required Hg²⁺ over 20μM in order to increase phosphorylation. Ser/Thr phosphorylations are prominent in the cytoskeletal organization and the GTPase signaling systems and these systems are notable as the primary ones responding to the lowest concentrations of Hg²⁺. Systems that required higher concentrations of Hg²⁺ to increase phosphorylation included immune receptor signaling. The proteins for which an increase in phosphorylation occurred at Hg²⁺ above 20μM have a higher proportion of pTyr sites. Anti Ig stimulation of Wehi-231 cells confirmed that cytoskeletal protein phosphorylation and GTPase signaling are modulated in physiologically relevant B-cell receptor activation. Candidate kinases that respond to Hg²⁺ exposure at the low μM concentrations include MAP Kinase 1, CaM Kinase II delta and PAK2.

SIGNIFICANCE

Mercury (Hg) is a wide spread environmental toxicant. Epidemiological and laboratory studies suggest that exposure to environmental Hg at current levels, which have been perceived to be non-toxic, may contribute to immune system dysfunction and autoimmune disease in humans and animals respectively. While we have previously shown that exposure of B lymphocytes to low levels of mercury interferes with B-cell receptor signaling mediated by post transcriptional phosphorylation events, overall the mechanism that is responsible for increased autoimmunity in mercury exposed human or animal populations is not well understood. The current study evaluated the dose dependent actions of mercury to change phosphorylation in the Wehi-231 cell line, an immature B-cell model in which actions of mercury on development of cell function can be evaluated. The study identified the cytoskeletal proteins as the most sensitive to modulation by mercury with changes in Ser/Thr phosphorylation being observed at the lowest concentrations of mercury. These findings indicate that the actions of mercury on B-cell immune function and development are at least in part likely mediated through changes in cytoskeletal protein phosphorylation.

Lead induces apoptosis in mouse TM3 Leydig cells through the Fas/FasL death receptor pathway

The study was aimed to investigate the effect of Pb toxicity on mouse Leydig cells and its molecular mechanism. The TM3 cells were cultured in vitro and exposed to Pb at different concentrations for 24h. The effects of Pb on cell proliferation and apoptosis were analyzed with MTT and Annexin V-FITC/PI via flow cytometry, respectively. Expression levels of Fas, Fas-L and caspase-8 in TM3 cells were determined by western blot. As well as the inhibitory effect of the caspase-8 inhibitor Z-IETD-FMK on cell apoptosis. We found that Pb treatment significantly decreased the cellar viability (P<0.05), increased the apoptosis (P<0.01) and the Fas, FasL, and caspase-8 expression levels in Pb-treated cells as compared to the control cells (P<0.05 or P<0.01). Furthermore, the caspase-8 inhibitor effectively block the Pb-induced cell apoptosis. Taken together, our data suggest that Pb-induced TM3 cell toxic effect may involve in the Fas/FasL death receptor signaling pathway.

Low level exposure to inorganic mercury interferes with B cell receptor signaling in transitional type 1 B cells

Mercury (Hg) has been implicated as a factor contributing to autoimmune disease in animal models and humans. However the mechanism by which this occurs has remained elusive. Since the discovery of B cells it has been appreciated by immunologists that during the normal course of B cell development, some immature B cells must be generated that produce immunoglobulin reactive to self-antigens (auto-antibodies). However in the course of normal development, the vast majority of immature auto-reactive B cells are prevented from maturing by processes collectively known as tolerance. Autoimmune disease arises when these mechanisms of tolerance are disrupted. In the B cell compartment, it is firmly established that tolerance depends in part upon negative selection of self-reactive immature (transitional type 1) B cells. In these cells negative selection depends upon signals generated by the B Cell Receptor (BCR), in the sense that those T1 B cells who's BCRs most strongly bind to, and so generate the strongest signals to self-antigens are neutralized. In this report we have utilized multicolor phosphoflow cytometry to show that in immature T1 B cells Hg attenuates signal generation by the BCR through mechanisms that may involve Lyn, a key tyrosine kinase in the BCR signal transduction pathway. We suggest that exposure to low, environmentally relevant levels of Hg, disrupts tolerance by interfering with BCR signaling in immature B cells, potentially leading to the appearance of mature auto-reactive B cells which have the ability to contribute to auto-immune disease.

Dietary n-3 PUFAs augment caspase 8 activation in Staphylococcal aureus enterotoxin B stimulated T-cells

Epidemiological studies have linked consumption of n-3 PUFAs with a variety of beneficial health benefits, particularly with respect to putative anti-inflammatory effects. Unfortunately, many of these results remain somewhat controversial because in most instances there has not been a linkage to specific molecular mechanisms. For instance, dietary exposure to low levels of mercury has been shown to be damaging to neural development, but concomitant ingestion of n-3 PUFAs as occurs during consumption of fish, has been shown to counteract the detrimental effects. As the mechanisms mediating the neurotoxicity of environmental mercury are not fully delineated, it is difficult to conceptualize a testable molecular mechanism explaining how n-3 PUFAs negate its neurotoxic effects. However, environmental exposure to mercury also has been linked to increased autoimmunity. By way of a molecular understanding of this immuno-toxic association, disruption of CD95 signaling is well established as a triggering factor for autoimmunity, and we have previously shown that environmentally relevant in vitro and dietary exposures to mercury interfere with CD95 signaling. In particular we have shown that activation of caspase 8, as well as downstream activation of caspase 3, in response to CD95 agonist stimulation is depressed by mercury. More recently we have shown in vitro that the n-3 PUFA docosahexaenoic acid counteracts the negative effect of mercury on CD95 signaling by restoring caspase activity. We hypothesized that concomitant ingestion of n-3 PUFAs with mercury might be protective from the immuno-toxic effects of mercury, as it is with mercury's neuro-toxic effects, and in the case of immuno-toxicity this would be related to restoration of CD95 signal strength. We now show that dietary ingestion of n-3 PUFAs generally promotes CD95 signaling by upregulating caspase 8 activation. Apart from accounting for the ability of n-3 PUFAs to specifically counteract autoimmune sequelae of mercury exposure, this novel finding for the first time suggests a testable molecular mechanism explaining the overall anti-inflammatory properties of n-3 PUFAs.

Developmental exposure to 2,3,7,8 tetrachlorodibenzo-p-dioxin attenuates later-life Notch1-mediated T cell development and leukemogenesis

Over half of T cell acute lymphoblastic leukemia (T-ALL) patients have activating mutations in the Notch gene. Moreover, the contaminant 2,3,7,8 tetrachlorodibenzo-p-dioxin (TCDD) is a known carcinogen that mediates its toxicity through the aryl hydrocarbon receptor (AHR), and crosstalk between activated AHR and Notch signaling pathways has previously been observed. Given the importance of Notch signaling in thymocyte development and T-ALL disease progression, we hypothesized that the activated AHR potentiates disease initiation and progression in an in vivo model of Notch1-induced thymoma. This hypothesis was tested utilizing adult and developmental exposure paradigms to TCDD in mice expressing a constitutively active Notch1 transgene (Notch(ICN-TG)). Following exposure of adult Notch(ICN-TG) mice to a single high dose of TCDD, we observed a significant increase in the efficiency of CD8 thymocyte generation. We next exposed pregnant mice to 3μg/kg of TCDD throughout gestation and lactation to elucidate effects of developmental AHR activation on later-life T cell development and T-ALL-like thymoma susceptibility induced by Notch1. We found that the vehicle-exposed Notch(ICN-TG) offspring have a peripheral T cell pool heavily biased toward the CD4 lineage, while TCDD-exposed Notch(ICN-TG) offspring were biased toward the CD8 lineage. Furthermore, while the vehicle-exposed NotchICN-TG mice showed increased splenomegaly and B to T cell ratios indicative of disease, mice developmentally exposed to TCDD were largely protected from disease. These studies support a model where developmental AHR activation attenuates later-life Notch1-dependent impacts on thymocyte development and disease progression.

Docosahexaenoic acid counteracts attenuation of CD95-induced cell death by inorganic mercury

In the United States the principal environmental exposure to mercury is through dietary consumption of sea food. Although the mechanism by which low levels of mercury affect the nervous system is not well established, epidemiological studies suggest that low level exposure of pregnant women to dietary mercury can adversely impact cognitive development in their children, but that Docosahexaenoic acid (DHA), the most prominent n-polyunsaturated fatty acid (n-PUFA) present in fish may counteract negative effects of mercury on the nervous system. Aside from effects on the nervous system, epidemiological and animal studies have also suggested that low level mercury exposure may be a risk factor for autoimmune disease. However unlike the nervous system where a mechanism linking mercury to impaired cognitive development remains elusive, we have previously suggested a potential mechanism linking low level mercury exposures to immune system dysfunction and autoimmunity. In the immune system it is well established that disruption of CD95 mediated apoptosis leads to autoimmune disease. We have previously shown in vitro as well as in vivo that in lymphocytes burdened with low levels of mercury, CD95 mediated cell death is impaired. In this report we now show that DHA counteracts the negative effect of mercury on CD95 signaling in T lymphocytes. T cells which have been pre-exposed to DHA are able to cleave pro-caspase 3 and efficiently signal programmed cell death through the CD95 signaling pathway, whether or not they are burdened with low levels of mercury. Thus DHA may lower the risk of autoimmune disease after low level mercury exposures.

Elements of the B cell signalosome are differentially affected by mercury intoxication

It has been suggested that environmental exposures to mercury contribute to autoimmune disease. Disruption of BCR signaling is associated with failure of central tolerance and autoimmunity, and we have previously shown that low levels of Hg²⁺ interfere with BCR signaling. In this report we have employed multiparametric phosphoflow cytometry, as well as a novel generalization of the Overton algorithm from one- to two-dimensional unimodal distributions to simultaneously monitor the effect of low level Hg²⁺ intoxication on activation of ERK and several upstream elements of the BCR signaling pathway in WEHI-231 B cells. We have found that, after exposure to low levels of Hg²⁺, only about a third of the cells are sensitive to the metal. For those cells which are sensitive, we confirm our earlier work that activation of ERK is attenuated but now report that Hg²⁺ has little upstream effect on the Btk tyrosine kinase. On the other hand, we find that signaling upstream through the Syk tyrosine kinase is actually augmented, as is upstream activation of the B cell signalosome scaffolding protein BLNK.

Coexposure to mercury increases immunotoxicity of trichloroethylene

We have shown previously that chronic (32 weeks) exposure to occupationally relevant concentrations of the environmental pollutant trichloroethylene (TCE) induced autoimmune hepatitis (AIH) in autoimmune-prone MRL+/+ mice. In real-life, individuals are never exposed to only one chemical such as TCE. However, very little is known about the effects of chemical mixtures on the immune system. The current study examined whether coexposure to another known immunotoxicant, mercuric chloride (HgCl(2)), altered TCE-induced AIH. Female MRL+/+ mice were treated for only 8 weeks with TCE (9.9 or 186.9 mg/kg/day in drinking water) and/or HgCl(2) (260 μg/kg/day, sc). Unlike mice exposed to either TCE or HgCl(2) alone, mice exposed to both toxicants for 8 weeks developed significant liver pathology commensurate with early stages of AIH. Disease development in the coexposed mice was accompanied by a unique pattern of anti-liver and anti-brain antibodies that recognized, among others, a protein of approximately 90 kDa. Subsequent immunoblotting showed that sera from the coexposed mice contained antibodies specific for heat shock proteins, a chaperone protein targeted by antibodies in patients with AIH. Thus, although TCE can promote autoimmune disease following chronic exposure, a shorter exposure to a binary mixture of TCE and HgCl(2) accelerated disease development. Coexposure to TCE and HgCl(2) also generated a unique liver-specific antibody response not found in mice exposed to a single toxicant. This finding stresses the importance of including mixtures in assessments of chemical immunotoxicity.

Correlations between gene expression and mercury levels in blood of boys with and without autism

Gene expression in blood was correlated with mercury levels in blood of 2- to 5-year-old boys with autism (AU) compared to age-matched typically developing (TD) control boys. This was done to address the possibility that the two groups might metabolize toxicants, such as mercury, differently. RNA was isolated from blood and gene expression assessed on whole genome Affymetrix Human U133 expression microarrays. Mercury levels were measured using an inductively coupled plasma mass spectrometer. Analysis of covariance (ANCOVA) was performed and partial correlations between gene expression and mercury levels were calculated, after correcting for age and batch effects. To reduce false positives, only genes shared by the ANCOVA models were analyzed. Of the 26 genes that correlated with mercury levels in both AU and TD boys, 11 were significantly different between the groups (P(Diagnosis*Mercury) ≤ 0.05). The expression of a large number of genes (n = 316) correlated with mercury levels in TD but not in AU boys (P ≤ 0.05), the most represented biological functions being cell death and cell morphology. Expression of 189 genes correlated with mercury levels in AU but not in TD boys (P ≤ 0.05), the most represented biological functions being cell morphology, amino acid metabolism, and antigen presentation. These data and those in our companion study on correlation of gene expression and lead levels show that AU and TD children display different correlations between transcript levels and low levels of mercury and lead. These findings might suggest different genetic transcriptional programs associated with mercury in AU compared to TD children.