Interactions at a dioxin responsive element (DRE) and an overlapping kappaB site within the hs4 domain of the 3'alpha immunoglobulin heavy chain enhancer

The aryl hydrocarbon receptor differentially modulates the expression profile of antibody isotypes in a human B-cell line

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) is a persistent environmental contaminant and high affinity ligand for the aryl hydrocarbon receptor (AhR). In animal models, AhR activation by TCDD generally inhibits antibody secretion. However, it is less clear if this translates to human antibody production. Using a human Burkitt lymphoma B-cell line (CL-01) that can be stimulated to secrete Ig and undergo class switch recombination to other Ig isotypes, the current study evaluated the effects of AhR activation or antagonism on the human Ig isotypic expression profile with CD40L+IL-4 stimulation. Our results suggest that AhR agonists (TCDD and indirubin) have little to no effect on IgM or IgA secretion, which were also not induced with stimulation. However, AhR activation significantly inhibited stimulation-induced IgG secretion, an effect reversed by the AhR antagonist CH223191. Evaluation of Ig heavy chain (IgH) constant region gene expression (ie Cμ, Cγ1-4, Cα1-2, and Cε that encode for IgM, IgG1-4, IgA1-2, and IgE, respectively) demonstrated differential effects. While Cμ and Cα2 transcripts were unaffected by stimulation or AhR agonists, AhR activation significantly inhibited stimulation-induced Cγ2-4 and Cε mRNA transcripts, which was reversed by AhR antagonism. Notably, AhR antagonism in the absence of exogenous AhR ligands significantly increased IgG and IgA secretion as well as the expression of Cγ2-4 and Cε. These results suggest that modulation of AhR activity differentially alters the IgH isotypic expression profile and antibody secretion that may be partly dependent on cellular stimulation. Since a variety of chemicals from anthropogenic, industrial, pharmaceutical, dietary, and bacterial sources bind the AhR, the ability of environmental exposures to alter AhR activity (i.e. activate or inhibit) may have a direct influence on immune function and antibody-relevant disease conditions.

Functions of the aryl hydrocarbon receptor (AHR) beyond the canonical AHR/ARNT signaling pathway

The aryl hydrocarbon receptor (AHR) is a ligand-dependent transcription factor regulating adaptive and maladaptive responses toward exogenous and endogenous signals. Research from various biomedical disciplines has provided compelling evidence that the AHR is critically involved in the pathogenesis of a variety of diseases and disorders, including autoimmunity, inflammatory diseases, endocrine disruption, premature aging and cancer. Accordingly, AHR is considered an attractive target for the development of novel preventive and therapeutic measures. However, the ligand-based targeting of AHR is considerably complicated by the fact that the receptor does not always follow the beaten track, i.e. the canonical AHR/ARNT signaling pathway. Instead, AHR might team up with other transcription factors and signaling molecules to shape gene expression patterns and associated physiological or pathophysiological functions in a ligand-, cell- and micromilieu-dependent manner. Herein, we provide an overview about some of the most important non-canonical functions of AHR, including crosstalk with major signaling pathways involved in controlling cell fate and function, immune responses, adaptation to low oxygen levels and oxidative stress, ubiquitination and proteasomal degradation. Further research on these diverse and exciting yet often ambivalent facets of AHR biology is urgently needed in order to exploit the full potential of AHR modulation for disease prevention and treatment.

Aryl Hydrocarbon Receptor Signaling Synergizes with TLR/NF-κB-Signaling for Induction of IL-22 Through Canonical and Non-Canonical AhR Pathways

Interleukin 22 (IL-22) is critically involved in gut immunity and host defense and primarily produced by activated T cells. In different circumstances IL-22 may contribute to pathological conditions or act as a cancer promoting cytokine secreted by infiltrating immune cells. Here we show that bone marrow-derived macrophages (BMM) express and produce IL-22 after activation of the aryl hydrocarbon receptor (AhR) when cells are activated through the Toll-like receptor (TLR) family. The additional activation of AhR triggered a significant induction of IL-22 in TLR-activated BMM. Deletion and mutation constructs of the IL-22 promoter revealed that a consensus DRE and RelBAhRE binding element are necessary to mediate the synergistic effects of AhR and TLR ligands. Inhibitor studies and analysis of BMM derived from knockout mice confirmed that the synergistic induction of IL-22 by AhR and TLR ligands depend on the expression of AhR and Nuclear Factor-kappa B (NF-κB) member RelB. The exposure to particulate matter (PM) collected from traffic related air pollution (TRAP) and wildfires activated AhR as well as NF-κB signaling and significantly induced the expression of IL-22. In summary this study shows that simultaneous activation of the AhR and NF-κB signaling pathways leads to synergistic and prolonged induction of IL-22 by integrating signals of the canonical and non-canonical AhR pathway.

Aryl Hydrocarbon Receptor Connects Inflammation to Breast Cancer

Aryl hydrocarbon receptor (AhR), an evolutionary conserved transcription factor, is a pleiotropic signal transductor. Thanks to its promiscuous ligand binding domain, during the evolution of eukaryotic cells its developmental functions were integrated with biosensor functions. Its activation by a multitude of endogenous and exogenous molecules stimulates its participation in several pathways, some of which are linked to inflammation and breast cancer (BC). Over time, the study of this malignancy has led to the identification of several therapeutic targets in cancer cells. An intense area of study is dedicated to BC phenotypes lacking adequate targets. In this context, due to its high constitutive activation in BC, AhR is currently gaining more and more attention. In this review, I have considered its interactions with: 1. the immune system, whose dysregulation is a renowned cancer hallmark; 2. interleukin 6 (IL6) which is a pivotal inflammatory marker and is closely correlated to breast cancer risk; 3. NF-kB, another evolutionary conserved transcription factor, which plays a key role in immunoregulatory functions, inflammatory response and breast carcinogenesis; 4. kynurenine, a tryptophan-derived ligand that activates and bridges AhR to chronic inflammation and breast carcinogenesis. Overall, the data here presented form an interesting framework where AhR is an interesting connector between inflammation and BC.

Aryl hydrocarbon receptor-induced activation of the human IGH hs1.2 enhancer: Mutational analysis of putative regulatory binding motifs

The human hs1.2 enhancer within the Ig heavy chain gene (IGH) is polymorphic and associated with a number of autoimmune diseases. The polymorphic region is characterized by tandem repeats of an ∼53-bp invariant sequence containing possible binding sites for several transcription factors. Our previous studies suggest the human hs1.2 enhancer is sensitive to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), an environmental toxicant and high affinity ligand of the aryl hydrocarbon receptor (AhR). TCDD induced hs1.2 enhancer activity in an AhR-dependent manner and the number of invariant sequences influenced the magnitude of activity. To better understand the regulation of human hs1.2 enhancer activity, the objective of the current study was to utilize mutational analysis and luciferase reporter constructs to evaluate the contribution of putative transcription factor binding sites to overall hs1.2 enhancer activity and modulation by TCDD. Basal and LPS-induced activity of the hs1.2 enhancer appeared to be most affected by mutation of sites outside of the invariant sequence or deletion of the entire invariant sequence; whereas sites influencing the effect of TCDD were dependent on the cellular activation state (i.e. unstimulated vs. LPS stimulation) and relatively independent of the putative AhR binding site within the invariant sequence. These results suggest that AhR activation affects human hs1.2 activity through an as yet undetermined non-canonical pathway. A better understanding regarding the role of the hs1.2 enhancer in human Ig expression and how AhR ligands modulate its activity may lead to insights into overall Ig regulation and mechanisms of dysfunction.

The AhR and NF-κB/Rel Proteins Mediate the Inhibitory Effect of 2,3,7,8-Tetrachlorodibenzo-p-Dioxin on the 3' Immunoglobulin Heavy Chain Regulatory Region

Transcriptional regulation of the murine immunoglobulin (Ig) heavy chain gene (Igh) involves several regulatory elements including the 3'Igh regulatory region (3'IghRR), which is composed of at least 4 enhancers (hs3A, hs1.2, hs3B, and hs4). The hs1.2 and hs4 enhancers exhibit the greatest transcriptional activity and contain binding sites for several transcription factors including nuclear factor kappaB/Rel (NF-κB/Rel) proteins and the aryl hydrocarbon receptor (AhR). Interestingly, the environmental immunosuppressant 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), which potently inhibits antibody secretion, also profoundly inhibits 3'IghRR and hs1.2 enhancer activation induced by the B-lymphocyte activator lipopolysaccharide (LPS), but enhances LPS-induced activation of the hs4 enhancer. Within the hs1.2 and hs4 enhancers, the AhR binding site is in close proximity or overlaps an NF-κB/Rel binding site suggesting a potential reciprocal modulation of the 3'IghRR by AhR and NF-κB/Rel. The objective of the current study was to evaluate the role of NF-κB/Rel and the AhR on the 3'IghRR and its enhancers using the AhR ligand TCDD, the AhR antagonist CH223191, and toll-like receptor agonists LPS, Resiquimod (R848), or cytosine-phosphate-guanine-oligodeoxynucleotides (CpG). Utilizing the CH12.LX B-lymphocyte cell line and variants expressing either a 3'IghRR-regulated transgene reporter or an inducible IκBα (inhibitor kappa B-alpha protein) superrepressor (IκBαAA), we demonstrate an AhR- and NF-κB/Rel-dependent modulation of 3'IghRR and hs4 activity. Additionally, in mouse splenocytes or CH12.LX cells, binding within the hs1.2 and hs4 enhancer of the AhR and the NF-κB/Rel proteins RelA and RelB was differentially altered by the cotreatment of LPS and TCDD. These results suggest that the AhR and NF-κB/Rel protein binding profile within the 3'IghRR mediates the inhibitory effects of TCDD on Ig expression and therefore antibody levels.

The aryl hydrocarbon receptor regulates an essential transcriptional element in the immunoglobulin heavy chain gene

Ig heavy chain (Igh) transcription involves several regulatory elements including the 3'Igh regulatory region (3'IghRR). 3'IghRR activity is modulated by several transcription factors, including NF-κB and AP-1 and potentially the aryl hydrocarbon receptor (AhR). The prototypical AhR ligand 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) inhibits antibody secretion and 3'IghRR activity. However, the exact mechanism is unknown and TCDD can modulate NF-κB and AP-1 in an AhR-independent manner. To determine if the AhR is a significant regulator of the 3'IghRR, we utilized a mouse B-cell line that stably expresses a 3'IghRR-regulated transgene and either an AhR antagonist or shRNA targeting the AhR. Disruption of the AhR pathway reversed TCDD-induced suppression of the 3'IghRR-regulated transgene and of endogenous Ig demonstrating a biologically significant effect of the AhR on 3'IghRR activation. Altered human 3'IGHRR activity by AhR ligands, which include dietary, environmental, and pharmaceutical chemicals, may have significant implications to human diseases previously associated with the 3'IGHRR.

Aryl hydrocarbon receptor signaling regulates NF-κB RelB activation during dendritic-cell differentiation

How the aryl hydrocarbon receptor (AhR) regulates dendritic-cell (DC) differentiation is unknown. We show that activation of AhR by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) caused enhanced differentiation from immature DCs (IDCs) to mature DCs (MDCs) in the bone-marrow-derived DCs (BMDC) from B6 wild-type mice but not in the BMDCs from AhR-null mice as indicated by the expression of CD11c and class II major histocompatibility complex (MHC). Enhanced maturation of BMDCs was associated with elevated levels of CD86 and an increased AhR-dependent nuclear accumulation of nuclear factor-kappa-light-chain enhancer of activated B cell (NF-κB) member RelB in BMDCs. The expression of interleukin (IL) 10 and chemokine DC-CK1 was suppressed, whereas that of CXCL2, CXCL3 and IL-22 was significantly increased in AhR-activated BMDCs. Furthermore, TCDD induced expression of the regulatory enzymes indoleamine 2,3-dioxygenase (IDO1) and indoleamine 2,3-dioxygenase-like 1 (IDO2). Increased expression of IDO2 was associated with coexpression of the cell-surface marker CCR6. Interestingly, mRNA expression of the chemokine receptor CCR6 was drastically decreased in AhR-null IDCs and MDCs. Overall, these data demonstrate that AhR modifies the maturation of BMDCs associated with the induction of the regulatory enzyme IDO and altered expression of cytokine, chemokines and DC-specific surface markers and receptors.

The role of the aryl hydrocarbon receptor in normal and malignant B cell development

The aryl hydrocarbon receptor (AhR) is a ligand-activated transcription factor historically studied for its role in environmental chemical-mediated toxicity and carcinogenicity. In the last 5 years, however, it has become clear that the AhR, presumably activated by endogenous ligand(s), plays an important role in immune system development and function. Other articles in this edition summarize AhR function during T cell and antigen-presenting cell development and function, including the effects of AhR activation on dendritic cell function, T cell skewing, inflammation, and autoimmune disease. Here, we focus on AhR expression and function during B cell differentiation. Studies exploiting immunosuppressive environmental chemicals to probe the role of the AhR in humoral immunity are also reviewed to illustrate the multiple levels at which a “nominally activated” AhR could control B cell differentiation from the hematopoietic stem cell through the pro-B cell, mature B cell, and antibody-secreting plasma cell stages. Finally, a putative role for the AhR in the basic biology of B cell malignancies, many of which have been associated with exposure to environmental AhR ligands, is discussed.

The AhR twist: ligand-dependent AhR signaling and pharmaco-toxicological implications

The aryl hydrocarbon receptor (AhR) is a transcription factor which is activated by diverse compounds and regulates the expression of xenobiotic metabolism genes. Recent studies have unraveled unsuspected physiological roles and novel alternative ligand-specific pathways for this receptor. In this review, we discuss these novel aspects and focus on the different responses elicited by the diverse endogenous and/or exogenous AhR ligands. In addition to challenging the relevance of the 'agonist/antagonist' classification of ligands, we introduce the concept of AhR plasticity as a primordial factor in the generation of these pathways. Finally, we suggest several promising perspectives for the pharmacological modulation of these responses.

A dioxin response element in the multiple cloning site of the pGL3 luciferase reporter influences transcriptional activity

Luciferase reporter plasmids (pGL3 backbone, Promega) have been utilized to characterize the transcriptional effects of the environmental contaminant 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and other aryl hydrocarbon receptor (AhR) ligands. Following ligand activation, the AhR and its dimerization partner AhR nuclear translocator (ARNT) regulate transcription by binding dioxin response elements (DREs) in regulatory regions of dioxin-sensitive genes. Upon sequencing of our luciferase reporters, we unexpectedly identified a DRE core motif within the multiple cloning site (mcsDRE) of the pGL3 luciferase plasmid backbone in a subset of our reporters. Therefore, the objective of this study was to determine if the mcsDRE inadvertently influences reporter activity. Utilizing deletional analysis we determined that the mcsDRE did significantly alter the transcriptional effect induced by TCDD. Since many chemicals have been shown to interact with the AhR and influence transcription through the DRE, the presence of the mcsDRE in the pGL3 luciferase plasmid may inappropriately influence promoter and enhancer analysis. As such, insertion of regulatory elements into pGL3 reporters should be designed to avoid retaining the mcsDRE core motif (GCGTG) and currently utilized pGL3 reporters should be evaluated for the presence of the mcsDRE.

2,3,7,8-tetrachlorodibenzo-p-dioxin induces transcriptional activity of the human polymorphic hs1,2 enhancer of the 3'Igh regulatory region

2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) is an environmental toxicant known to inhibit Ab secretion and Ig expression. Inhibition of Ig expression may be partially mediated through repression of the 3'Igh regulatory region (3'IghRR). TCDD inhibits mouse 3'IghRR activation and induces aryl hydrocarbon receptor binding to dioxin response elements within the 3'IghRR enhancers hs1,2 and hs4. The human hs1,2 enhancer (hu-hs1,2) is polymorphic as the result of the presence of one to four invariant sequences (ISs), which have been correlated with several autoimmune diseases. The IS also contains a dioxin response element core motif. Therefore, the objective was to determine whether hu-hs1,2 activity is sensitive to TCDD. Using a mouse B cell line (CH12.LX), we compared the effects of TCDD on mouse hs1,2 versus hu-hs1,2 activity. TCDD inhibited mouse hs1,2 similarly to the mouse 3'IghRR. In contrast, hu-hs1,2 was activated by TCDD, and antagonist studies supported an aryl hydrocarbon receptor-dependent activation, which was replicated in a human B cell line (IM-9). Absence of Pax5 binding sites is a major difference between the human and mouse hs1,2 sequence. Insertion of the high-affinity Pax5 site in hu-hs1,2 markedly blunted reporter activity but did not alter TCDD's effect (i.e., no shift from activation to inhibition). Additionally, deletional analysis demonstrated a significant IS contribution to hu-hs1,2 basal activity, but TCDD-induced activity was not strictly IS number dependent. Taken together, our results suggest that hu-hs1,2 is a significant target of TCDD and support species differences in hs1,2 regulation. Therefore, sensitivity of hu-hs1,2 to chemical-induced modulation may influence the occurrence and/or severity of human diseases associated with hu-hs1,2.

Regulation of Bach2 by the aryl hydrocarbon receptor as a mechanism for suppression of B-cell differentiation by 2,3,7,8-tetrachlorodibenzo-p-dioxin

Exposure to the aryl hydrocarbon receptor (AHR) agonist, 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) alters B-cell differentiation and suppresses antibody production. Previous genomic studies in mouse B cells identified Bach2 as a direct target of the AHR. Bach2 is known to repress expression of Prdm1, a key transcription factor involved in B-cell differentiation, by binding to Maf elements (MAREs) in the regulatory regions of the gene. Chromatin immunoprecipitation followed by quantitative PCR in TCDD-treated lipopolysaccharide (LPS)-activated B cells showed increased binding of the AHR within the first intron in the Bach2 gene. The binding was further confirmed by electrophoretic mobility shift assay (EMSA). TCDD also induced expression of Bach2 in activated as well as resting B cells from 2 to 24h post-treatment in a time- and concentration-dependent manner. Expression of Prdm1 was decreased by TCDD at 24h and was consistent with repression by Bach2. Increased DNA binding activity to the intron 5 MARE with increasing TCDD concentrations was observed by EMSA. Supershifts identified the presence of Bach2 in the DNA binding complex associated with the intron 5 MARE of Prdm1. Functional validation of the role of Bach2 in the suppression of B-cell differentiation by TCDD was performed using RNA interference (RNAi). Knockdown of Bach2 showed approximately 40% reversal in the TCDD-induced suppression of IgM secretion when compared to controls. The results suggest that the transcriptional regulation of Bach2 by the AHR is one of the mechanisms involved in the suppression of B-cell differentiation by TCDD.

The IgH locus 3' regulatory region: pulling the strings from behind

Antigen receptor gene loci are among the most complex in mammals. The IgH locus, encoding the immunoglobulin heavy chain (IgH) in B-lineage cells, undergoes major transcription-dependent DNA remodeling events, namely V(D)J recombination, Ig class-switch recombination (CSR), and somatic hypermutation (SHM). Various cis-regulatory elements (encompassing promoters, enhancers, and chromatin insulators) recruit multiple nuclear factors in order to ensure IgH locus regulation by tightly orchestrated physical and/or functional interactions. Among major IgH cis-acting regions, the large 3' regulatory region (3'RR) located at the 3' boundary of the locus includes several enhancers and harbors an intriguing quasi-palindromic structure. In this review, we report progress insights made over the past decade in order to describe in more details the structure and functions of IgH 3'RRs in mouse and human. Generation of multiple cellular, transgenic and knock-out models helped out to decipher the function of the IgH 3' regulatory elements in the context of normal and pathologic B cells. Beside its interest in physiology, the challenge of elucidating the locus-wide cross talk between distant cis-regulatory elements might provide useful insights into the mechanisms that mediate oncogene deregulation after chromosomal translocations onto the IgH locus.

Hypoxia activates constitutive luciferase reporter constructs

Hypoxia has been identified as a contributing factor in the pathophysiology of several diseases and oxygen regulation is important during stem cell development, particularly in early embryogenesis. One aspect that has emerged is the role of hypoxia-inducible factors, or HIFs in regulating the effect of hypoxia. Studies in our laboratory sought to examine the hypoxic regulation of HIF activity in placental trophoblast cells, through the use of dual-reporter luciferase assays. Our study demonstrates that hypoxic conditions cause a significant increase in the level of constitutive luciferase reporter activity. We also show that this induction is not a cell type or species-specific phenomenon and provides an alternative method for normalizing transfection efficiency in luciferase assays under hypoxic conditions. Our results suggest that in studies dealing with hypoxic conditions, caution should be used when interpreting measurements of transcriptional activity by traditional dual-reporter assays.

An integrated genomic analysis of aryl hydrocarbon receptor-mediated inhibition of B-cell differentiation

The aryl hydrocarbon receptor (AHR) agonist 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) alters differentiation of B cells and suppresses antibody production. A combination of whole-genome, microarray-based chromatin immunoprecipitation (ChIP-on-chip), and time course gene expression microarray analysis was performed on the mouse B-cell line CH12.LX following exposure to lipopolysaccharide (LPS) or LPS and TCDD to identify the primary and downstream transcriptional elements of B-cell differentiation that are altered by the AHR. ChIP-on-chip analysis identified 1893 regions with a significant increase in AHR binding with TCDD treatment. Transcription factor binding site analysis on the ChIP-on-chip data showed enrichment in AHR response elements. Other transcription factors showed significant coenrichment with AHR response elements. When ChIP-on-chip regions were compared with gene expression changes at the early time points, 78 genes were identified as potential direct targets of the AHR. AHR binding and expression changes were confirmed for a subset of genes in primary mouse B cells. Network analysis examining connections between the 78 potential AHR target genes and three transcription factors known to regulate B-cell differentiation indicated multiple paths for potential regulation by the AHR. Enrichment analysis on the differentially expressed genes at each time point evaluated the downstream impact of AHR-regulated gene expression changes on B-cell-related processes. AHR-mediated impairment of B-cell differentiation occurred at multiple nodes of the B-cell differentiation network and potentially through multiple mechanisms including direct cis-acting effects on key regulators of B-cell differentiation, indirect regulation of B-cell differentiation-related pathways, and transcriptional coregulation of target genes by AHR and other transcription factors.

2,3,7,8-tetrachlorodibenzo-p-dioxin-mediated suppression of toll-like receptor stimulated B-lymphocyte activation and initiation of plasmacytic differentiation

2,3,7,8-Tetrachlordibenzo-p-dioxin (TCDD) is a potent suppressor of humoral immunity, disrupting antibody production in response to both T cell-dependent and T cell-independent antigens. Among the cell types required for humoral responses, the B cell is highly, and directly, sensitive to TCDD. B cells become antibody-secreting cells via plasmacytic differentiation, a process regulated by several transcription factors, including activator protein-1, B-cell CLL/lymphoma 6 (BCL-6), and B lymphocyte-induced maturation protein 1 (Blimp-1). The overarching conceptual framework guiding experimentation is that TCDD disrupts plasmacytic differentiation by altering the expression or activity for upstream regulators of Blimp-1. Multiparametric flow cytometry was used to investigate TCDD-induced alterations in both activation marker and transcription factor expression following lipopolysaccharide (LPS) activation of purified B cells. TCDD significantly impaired LPS-activated expression of major histocompatibility complex class II, cluster of differentiation (CD)69, CD80, and CD86. Immunosuppressive concentrations of TCDD also suppressed LPS-activated Blimp-1 and phosphorylated c-Jun expression, whereas elevating BCL-6 expression. Because BCL-6 and c-Jun are directly and indirectly regulated by the kinases AKT, extracellular signal-regulated kinase (ERK), and Jun N-terminal kinase (JNK), it was hypothesized that TCDD alters toll-like receptor-activated kinase phosphorylation. TCDD at 0.03 and 0.3 nM significantly impaired phosphorylation of AKT, ERK, and JNK in CH12.LX B cells activated with LPS, CpG oligonucleotides, or resiquimod (R848). In primary B cells, R848-activated phosphorylation of AKT, ERK, and JNK was also impaired by TCDD at 30 nM. These results suggest that impairment of plasmacytic differentiation by TCDD involves altered transcription factor expression, in part, by suppressed kinase phosphorylation.

Simultaneous in vivo time course and dose response evaluation for TCDD-induced impairment of the LPS-stimulated primary IgM response

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) is a potent suppressor of humoral immunity but the specific molecular mechanisms responsible for immunosuppression by TCDD are poorly understood. In vivo and in vitro studies of the primary humoral IgM response demonstrated that the B cell is a sensitive cell type to modulation by TCDD. We hypothesized that in vivo administration of TCDD disrupts expression of transcription factors controlling B cell to plasma cell differentiation. Female C57BL6 mice were treated with a single dose of TCDD (3, 10, or 30 microg/kg) and/or vehicle (sesame oil). On day 4 post-TCDD administration mice were sensitized with 25 microg lipopolysacchride (LPS) by intraperitioneal injection to stimulate an immune response. Splenocytes were isolated on subsequent days following LPS, up to 3 days post-LPS, and the expression of IgM, XBP-1, PAX5, BCL-6, and Blimp-1 was assessed. TCDD treatment dose-dependently suppressed LPS-induced IgM antibody-forming cell number, which was correlated with decreased frequency of CD19+ CD138+ cells. Gene expression analysis revealed that TCDD caused a dose-dependent suppression of Igmicro chain, Igkappa chain, IgJ chain, XBP-1, and Blimp-1. TCDD also dose-dependently suppressed LPS-stimulated increases in Blimp-1 protein expression in CD19+ B cells. The deregulation of Blimp-1 expression by TCDD provides a partial explanation for the concomitant suppression of the IgM response and confirms previous observations established in vitro.

Involvement of Blimp-1 and AP-1 dysregulation in the 2,3,7,8-Tetrachlorodibenzo-p-dioxin-mediated suppression of the IgM response by B cells

B cell differentiation and humoral immune responses are markedly suppressed by the persistent environmental contaminant, 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). The suppression of humoral immune responses by TCDD occurs by direct actions on the B cell and involves activation of the aryl hydrocarbon receptor. Transcriptional regulation of paired box gene 5 (Pax5), an important regulator of B cell differentiation, is altered by TCDD in concordance with the suppression of B cell differentiation and humoral immunoglobulin M response. We hypothesized that TCDD treatment leads to dysregulation of Pax5 transcription by interfering with the basic B cell differentiation mechanisms and aimed to determine the effects of TCDD on upstream regulators of Pax5. A critical regulator of B cell differentiation, B lymphocyte-induced maturation protein-1 (Blimp-1) acts as a transcriptional repressor of Pax5. In lipopolysaccharide (LPS)-activated murine B cell lymphoma, CH12.LX, Blimp-1 messenger RNA, and DNA-binding activity within the Pax5 promoter were suppressed by TCDD. Furthermore, LPS activation of CH12.LX cells upregulated DNA-binding activity of activator protein 1 (AP-1) at three responsive element-like motifs within the Blimp-1 promoter. TCDD treatment of LPS-activated CH12.LX cells suppressed AP-1 binding to these motifs between 24 and 72 h, in concordance with the suppression of Blimp-1 by TCDD. A more comprehensive analysis at 72 h demonstrated that the suppression of AP-1 binding within the Blimp-1 promoter by TCDD was concentration dependent. In summary, our findings link the TCDD-mediated suppression of Blimp-1 through AP-1 to the dysregulation of Pax5, which ultimately leads to the suppression of B cell differentiation and humoral immune responses.

Diverse chemicals including aryl hydrocarbon receptor ligands modulate transcriptional activity of the 3'immunoglobulin heavy chain regulatory region

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD), a known disruptor of B-cell differentiation and a ligand for the aryl hydrocarbon receptor (AhR), induces binding of the AhR to dioxin responsive elements (DRE) in sensitive genes. The Ig heavy chain (IgH) gene is a sensitive target of TCDD and may be transcriptionally inhibited by TCDD through inhibition of the 3'IgH transcriptional regulatory region (3'IgHRR). While the 3'IgHRR contains binding sites for several transcription factors, two DRE motifs were also identified which may be responsible for TCDD-induced inhibition of 3'IgHRR activation and may implicate the AhR as an important regulator of IgH expression. The objectives of the present study were to determine if 3'IgHRR modulation is limited to TCDD or if structurally diverse chemicals (AhR ligands and non-AhR ligands) from environmental, industrial, dietary or pharmaceutical origin are also capable of modulating the 3'IgHRR and to verify a correlation between effects on a stable 3'IgHRR reporter and the endogenous IgH protein. Utilizing a CH12.LX mouse B-cell line that stably expresses a 3'IgHRR-regulated transgene, we identified an inhibition of both 3'IgHRR activation and IgH protein expression by the non-dioxin AhR activators indolo(3,2-b)carbazole, primaquine, carbaryl, and omeprazole which followed a rank order potency for AhR activation supporting a role of the AhR in the transcriptional regulation of the 3'IgHRR and IgH expression. However, modulation of the 3'IgHRR and IgH expression was not limited to AhR activators or to suppressive effects. Hydrogen peroxide and terbutaline had an activating effect and benzyl isothiocyanate was inhibitory. These chemicals are not known to influence the AhR signaling pathway but have been previously shown to modulate humoral immunity and/or transcription factors that regulate the 3'IgHRR. Taken together these results implicate the 3'IgHRR as a sensitive immunological target and are the first to identify altered 3'IgHRR activation by a diverse range of chemicals.

Activation of Multifarious Transcription of an Adhesion Protein ap65-1 Gene by a Novel Myb2 Protein in the Protozoan Parasite Trichomonas vaginalis

Multifarious transcription of the adhesion protein ap65-1 gene in the human pathogen, Trichomonas vaginalis, is critically regulated by the coordination of two similar but opposite oriented DNA regulatory regions, MRE-1/MRE-2r and MRE-2f, both of which are binding sites for multiple Myb-like proteins. In the present study, MRE-1/MRE-2r was demonstrated to be composed of multiple overlapping promoter elements, among which the entire region is required for growth-related ap65-1 transcription, and the 5'-MRE-1 antagonizes the suppressive activity of the 3'-MRE-2r in iron-inducible transcription. The recombinant Myb2 protein derived from a previously identified myb2 gene was demonstrated to recognize distinct sequence contexts in MRE-2r and MRE-2f, whereas Myb2 in the nuclear lysate preferentially binds to MRE-2f to MRE-2r. Iron repletion resulted in persistent repression of the myb2 gene, and temporal activation/deactivation of Myb2 promoter entry, which was also activated by prolonged iron depletion. The hemagglutinintagged Myb2 when overexpressed during iron-depleted conditions facilitated basal and growth-related ap65-1 transcription to a level that was achieved in iron-replete cells, whereas ironinducible ap65-1 transcription was abolished with knockdown of Myb2. These findings demonstrated that Myb2 is involved in activation of growth-related and iron-inducible transcription of the ap65-1 gene, possibly through differential promoter selection in competition with other Myb proteins.

Role of dioxin response element and nuclear factor-kappaB motifs in 2,3,7,8-tetrachlorodibenzo-p-dioxin-mediated regulation of Fas and Fas ligand expression

We have demonstrated previously that 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) up-regulates Fas and FasL in immune cells, although the molecular mechanisms remain unknown. We investigated the regulation of Fas or FasL promoter by TCDD in EL4 T cells using luciferase reporter constructs. We observed 20 +/- 5- and 14 +/- 4-fold induction of promoter activity for Fas and FasL, respectively, after TCDD exposure. The induction of luciferase was significantly reduced (2 +/- 1-fold) in the presence of alpha-naphthoflavone, an aryl hydrocarbon receptor (AhR) antagonist. We noted the presence of a dioxin response element (DRE) and five nuclear factor-kappaB (NF-kappaB) motifs on Fas promoter, and no DRE but two NF-kappaB motifs on FasL promoter. When we investigated the role of DRE and NF-kappaB, we observed varying levels of luciferase induction (9 +/- 2-fold for DRE and 8 +/- 2-fold for NF-kappaBs of Fas promoter and 6 +/- 3-fold for NF-kappaBs of FasL promoter). Mutations in DRE of Fas promoter or NF-kappaBs of FasL promoter led to decreased luciferase induction, further supporting our results. Probes for DRE or NF-kappaB motifs of Fas and/or FasL promoters demonstrated mobility shift in the presence of nuclear extract from TCDD-treated EL4 cells. Furthermore, we observed supershift in mobility when DRE and NF-kappaB probes were incubated in the presence of anti-mouse AhR, and anti-NF-kappaB (RelA/p65 and p50) antibodies, respectively. Administration of TCDD into mice caused significant increase in Fas and FasL transcripts in thymus and liver. These data demonstrate that TCDD regulates Fas and FasL promoters through DRE and/or NF-kappaB motifs via AhR.

The p53 inhibitor pifithrin-alpha is a potent agonist of the aryl hydrocarbon receptor

The tumor suppressor protein p53 is currently a target of emerging drug therapies directed toward neurodegenerative diseases, such as Alzheimer's and Parkinson's, and side effects associated with cancer treatments. Of this group of drugs, the best characterized is pifithrin-alpha, a small molecule that inhibits p53-dependent apoptosis through an undetermined mechanism. In this study, we have used a number of molecular approaches to test the hypothesis that pifithrin-alpha acts as an aryl hydrocarbon receptor (AhR) agonist and, in this manner, inhibits the actions of p53. Toward this end, we have found that pifithrin-alpha is a potent AhR agonist as determined by its ability to bind the AhR, induce formation of its DNA binding complex, activate reporter activity, and up-regulate the classic AhR target gene CYP1A1. However, examination of its ability to inhibit p53-mediated gene activation and apoptosis revealed that these actions occurred via an AhR-independent manner. The significance of this study is based on the fact that activation of the AhR is typically associated with an increase in phase I and phase II metabolizing enzymes and adverse biological events such as tumor promotion that may contribute to untoward effects of pifithrin-alpha. Hence, this work will aid in the future design of more specific members of this important class of p53 inhibitors for use in a clinical setting.