Expression of functional aromatic hydrocarbon receptor and aromatic hydrocarbon nuclear translocator proteins in murine bone marrow stromal cells

Expression of aryl hydrocarbon receptor in rat brain lesions following traumatic brain injury

Background

Aryl Hydrocarbon Receptor (AhR) is a ligand-activated transcription factor with multiple functions operating in a variety of organs, including the brain. Recent studies have revealed that AhR played a functional role in traumatic injuries. This paper aims to study the expression of AhR during the early phase following a traumatic brain injury (TBI) in rat brains by immunohistochemistry.

Methods

Weight-drop induced TBI was performed in rats. The expression of AhR in brain of TBI rats were examined by immunohistochemistry.

Results

Neuron expression of AhR in the rat brains of experiment group had been upregulated since day 3 in lesional hemisphere compared to that of the control group and mainly located in the cytoplasm, indicating an inactivated state. Interestingly, the accumulation of AhR⁺ non-neuron cells became significant as early as 18 h after injury, which had kept increasing until 24 h post injury and then decreased slowly. For AhR⁺ non-neuron cells, the AhR mainly located in cell nucleus, indicating a reactive status. Furthermore, double staining showed that most AhR⁺ non-neuron cells co-localized with W3/13, a marker for T lymphocytes, but not with ED-1 (for activated microglia/macrophages) or GFAP (for activated astrocytes), suggesting that most AhR⁺ non-neuron cells were T lymphocytes.

Conclusion

This is the first study concerning AhR expression in brains following TBI, and our data demonstrated that AhR was upregulated and activated in T lymphocytes following TBI. More research is needed to make a more conclusive conclusion.

The long winding road toward understanding the molecular mechanisms for B-cell suppression by 2,3,7,8-tetrachlorodibenzo-p-dioxin

Suppression of humoral immune responses by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) was first reported in the mid-1970s. Since this initial observation, much effort has been devoted by many laboratories toward elucidation of the cellular and molecular mechanisms responsible for the profound impairment of humoral immune responses by TCDD, which is characterized by decreased B cell to plasma cell differentiation and suppression of immunoglobulin production. These efforts have led to a significant body of research demonstrating a direct effect of TCDD on B-cell maturation and function as well as a requisite but as yet undefined role of the aryl hydrocarbon receptor (AhR) in these effects. Likewise, a number of molecular targets putatively involved in mediating B-cell dysfunction by TCDD, and other AhR ligands, have been identified. However, our current understanding has primarily relied on findings from mouse models, and the translation of this knowledge to effects on human B cells and humoral immunity in humans is less clear. Therefore, a current challenge is to determine how TCDD and the AhR affect human B cells. Efforts have been made in this direction but continued progress in developing adequate human models is needed. An in-depth discussion of these advances and limitations in elucidating the cellular and molecular mechanisms putatively involved in the suppression of B-cell function by TCDD as well as the implications on human diseases associated in epidemiological studies with exposure to TCDD and dioxin-like compounds is the primary focus of this review.

The aryl hydrocarbon receptor: regulation of hematopoiesis and involvement in the progression of blood diseases

The aryl hydrocarbon receptor (AhR) is a basic helix-loop-helix protein that belongs to the superfamily of environment-sensing PAS (Per-ARNT-Sim) proteins. A large number of ligands have been described to bind AhR and promote its nuclear translocation. In the nucleus, the AhR and its dimerization partner the AhR nuclear translocator (ARNT) form a DNA-binding complex that acts as a transcriptional regulator. Animal and human data suggest that, beyond its mediating responses to xenobiotic and/or unknown endogenous ligands, the AhR has a role, although as yet undefined, in the regulation of cell cycle and inflammation. The AhR also appears to regulate the hematopoietic and immune systems during development and adult life in a cell-specific manner. While accidental exposure to xenobiotic AhR ligands has been associated with leukemia in humans, the specific mechanisms of AhR involvement are still not completely understood. However, recent data are consistent with a functional role of the AhR in the maintenance of hematopoietic stem and/or progenitor cells (HSCs/HPCs). Studies highlighting AhR regulation of HSCs/HPCs provide a rational framework to understand their biology, a role of the AhR in hematopoietic diseases, and a means to develop interventions for these diseases.

Treatment of mice with the Ah receptor agonist and human carcinogen dioxin results in altered numbers and function of hematopoietic stem cells

The aryl hydrocarbon receptor (AhR) mediates the carcinogenicity of a family of environmental contaminants, the most potent being 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). Increased incidence of lymphoma and leukemia in humans is associated with TCDD exposure. Although AhR activation by TCDD has profound effects on the immune system, precise cellular and molecular mechanisms have yet to be determined. These studies tested the hypothesis that alteration of marrow populations following treatment of mice with TCDD is due to an effect on hematopoietic stem cells (HSCs). Treatment with TCDD resulted in an increased number and proliferation of bone marrow (BM) populations enriched for HSCs. There was a time-dependent decrease in B-lineage cells with a concomitant increase in myeloid populations. The decrease in the B-cell lineage colony-forming unit-preB progenitors along with a transient increase in myeloid progenitors were consistent with a skewing of lineage development from lymphoid to myeloid populations. However, HSCs from TCDD-treated mice exhibited diminished capacity to reconstitute and home to marrow of irradiated recipients. AhR messenger RNA was expressed in progenitor subsets but is downregulated during HSC proliferation. This result was consistent with the lack of response following the exposure of 5-fluorouracil-treated mice to TCDD. The direct exposure of cultured BM cells to TCDD inhibited the growth of immature hematopoietic progenitor cells, but not more mature lineage-restricted progenitors. Overall, these data are consistent with the hypothesis that TCDD, through AhR activation, alters the ability of HSCs to respond appropriately to signals within the marrow microenvironment.

Differentiating human multipotent mesenchymal stromal cells regulate microRNAs: prediction of microRNA regulation by PDGF during osteogenesis

OBJECTIVE

Human multipotent mesenchymal stromal cells (MSC) have the potential to differentiate into multiple cell types, although little is known about factors that control their fate. Differentiation-specific microRNAs may play a key role in stem cell self-renewal and differentiation. We propose that specific intracellular signaling pathways modulate gene expression during differentiation by regulating microRNA expression.

MATERIALS AND METHODS

Illumina mRNA and NCode microRNA expression analyses were performed on MSC and their differentiated progeny. A combination of bioinformatic prediction and pathway inhibition was used to identify microRNAs associated with platelet-derived growth factor (PDGF) signaling.

RESULTS

The pattern of microRNA expression in MSC is distinct from that in pluripotent stem cells, such as human embryonic stem cells. Specific populations of microRNAs are regulated in MSC during differentiation targeted toward specific cell types. Complementary mRNA expression analysis increases the pool of markers characteristic of MSC or differentiated progeny. To identify microRNA expression patterns affected by signaling pathways, we examined the PDGF pathway found to be regulated during osteogenesis by microarray studies. A set of microRNAs bioinformatically predicted to respond to PDGF signaling was experimentally confirmed by direct PDGF inhibition.

CONCLUSION

Our results demonstrate that a subset of microRNAs regulated during osteogenic differentiation of MSCs is responsive to perturbation of the PDGF pathway. This approach not only identifies characteristic classes of differentiation-specific mRNAs and microRNAs, but begins to link regulated molecules with specific cellular pathways.

Aryl Hydrocarbon Receptor Activation Impairs the Priming but Not the Recall of Influenza Virus-Specific CD8+T Cells in the Lung

The response of CD8+ T cells to influenza virus is very sensitive to modulation by aryl hydrocarbon receptor (AhR) agonists; however, the mechanism underlying AhR-mediated alterations in CD8+ T cell function remains unclear. Moreover, very little is known regarding how AhR activation affects anamnestic CD8+ T cell responses. In this study, we analyzed how AhR activation by the pollutant 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) alters the in vivo distribution and frequency of CD8+ T cells specific for three different influenza A virus epitopes during and after the resolution of a primary infection. We then determined the effects of TCDD on the expansion of virus-specific memory CD8+ T cells during recall challenge. Adoptive transfer of AhR-null CD8+ T cells into congenic AhR(+/+) recipients, and the generation of CD45.2AhR(-/-)-->CD45.1AhR(+/+) chimeric mice demonstrate that AhR-regulated events within hemopoietic cells, but not directly within CD8+ T cells, underlie suppressed expansion of virus-specific CD8+ T cells during primary infection. Using a dual-adoptive transfer approach, we directly compared the responsiveness of virus-specific memory CD8+ T cells created in the presence or absence of TCDD, which revealed that despite profound suppression of the primary response to influenza virus, the recall response of virus-specific CD8+ T cells that form in the presence of TCDD is only mildly impaired. Thus, the delayed kinetics of the recall response in TCDD-treated mice reflects the fact that there are fewer memory cells at the time of reinfection rather than an inherent defect in the responsive capacity of virus-specific memory CD8+ cells.

Benzo(a)pyrene and 7,12-dimethylbenz(a)anthrecene differentially affect bone marrow cells of the lymphoid and myeloid lineages

Polycyclic aromatic hydrocarbons (PAHs) are common environmental contaminants that are carcinogenic and immunosuppressive. Benzo(a)pyrene (BP) and 7,12-dimethylbenz(a)anthracene (DMBA) are two prototypic PAHs known to impair the cell-mediated and humoral immune responses. We have previously shown that, in C57BL/6J mice, total bone marrow (BM) cellularity decreased two-fold following intraperitoneal DMBA treatment but not BP treatment. Here, we have used flow cytometry to demonstrate that BP and DMBA differentially alter the lymphoid and myeloid lineages. Following DMBA treatment, the pro/pre B-lymphocytes (B220(lo)/IgM(-)) and the immature B-lymphocytes (B220(lo)/IgM(+)) significantly decreased, while the mature B-lymphocytes (B220(hi)/IgM(+)) remained unaffected. In contrast, BP treatment decreased the pro/pre B-lymphocytes, and did not affect the immature B-lymphocytes or mature B-lymphocytes. The Gr-1(+) cells of the myeloid lineage were depleted 50% following DMBA treatment and only minimally depleted following BP treatment. Interestingly, the monocytes (7/4(+)1A8(lo)) and neutrophils (7/4(+)1A8(hi)) within this Gr-1(+) population were differentially affected by these PAHs. Monocytes and neutrophils were depleted following DMBA treatment whereas neutrophils decreased and monocytes increased following BP treatment. Although TNFalpha and CYP1B1 are implicated as essential mediators of hypocellularity, the similar induction of TNFalpha mRNA and CYP1B1 mRNA in the BM by BP and DMBA suggests that they are not limiting factors in mediating the different effects of these PAHs. Given that similar amounts of BP and DMBA reach the BM when administered intraperitoneally, their differential effects on the lymphoid and myeloid lineages probably stem from differences in reactive metabolites such as PAH quinones and PAH-dihydrodiol-epoxides.

The HIF family member EPAS1/HIF-2alpha is required for normal hematopoiesis in mice

Hypoxic stress plays a role in pathophysiologic states such as myocardial infarction and cerebral vascular events as well as in normal physiologic conditions including development and hematopoiesis. Members of the hypoxia inducible factor (HIF) family function as transcriptional regulators of genes involved in the hypoxic response. After generating adult mice that globally lack endothelial PAS domain protein 1 (EPAS1, also known as HIF-2alpha/HRF/HLF/MOP3), the second member of the HIF family, characterization of the hematopoietic cell population indicated that the loss of EPAS1/HIF-2alpha resulted in pancytopenia. Using bone marrow reconstitution experiments of lethally irradiated hosts, we have defined the extent and site of hematopoietic impairment in the EPAS1/HIF-2alpha null mice. These data suggest a critical role for EPAS1/HIF-2alpha in maintaining a functional microenvironment in the bone marrow for effective hematopoiesis.

A constitutively active aryl hydrocarbon receptor causes loss of peritoneal B1 cells

The dioxin/aryl hydrocarbon (Ah) receptor functions as a ligand-activated transcription factor that mediates toxicity of dioxins and related environmental pollutants. We have developed a transgenic mouse model that expresses a constitutively active Ah receptor. The immune system is one of the most sensitive target organs for dioxin toxicity and we have therefore investigated alterations of different lymphocyte populations in these mice. The population of mature bone-marrow derived B cells was enlarged, consistent with previous findings in dioxin exposed mice. In contrast, the peritoneal population of CD5-expressing B cells (B1 cells) was significantly diminished. This is the first study that demonstrates the effect of an activated Ah receptor on B1 cells. Since these cells are important mediators of innate immunity against pathogens such as Influenza virus, these results may explain the decreased resistance against infections that has been documented after dioxin exposure.

Estrogenic and antiproliferative properties of soy sapogenols in human breast cancer cells in vitro

Two soy sapogenols, soyasapogenol A (SA) and soyasapogenol B (SB) were tested for their estrogenic activities in estrogen responsive MCF-7 or estrogen-insensitive MDA-MB-231 (MDA) human breast cancer cells. SB and SA had differential actives on cell proliferation with 10 microM SB being growth inhibitory to MDA cells with no significant effect at any concentration on MCF-7 cells. SA also inhibited MDA cell proliferation at 10 micro, but at this same dose stimulated a 2.5-fold increase in MCF-7 proliferation. SA (0.1-10 microM) induced pS2 mRNA levels and the induction was blocked by co-treatment of cells with the anti-estrogen ICI 182,780. SA also induced the formation of an ER-ERE DNA complex measured by electrophoretic mobility shift assay. In summary, these results show that soyasapogenol A is estrogenic, whereas soyasapogenol B is growth inhibitory.

2,3,7,8-tetrachlorodibenzo-p-dioxin does not directly alter the phenotype of maturing B cells in a murine coculture system

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD), acting through the aromatic hydrocarbon receptor (AhR), elicits numerous toxicological effects, including immunosuppression. Previous work from our laboratory has suggested that TCDD exposure in mice is associated with altered lymphopoietic development, in particular altered B-cell phenotype in the bone marrow. It remains to be determined which specific hematopoietic populations or subpopulations within the marrow cavity are directly targeted by TCDD. To examine the effects of TCDD on developing B cells in vitro, a stromal coculture model was used. Primary bone marrow cells from male, 6- to 7-week-old C57Bl/6 mice were cocultured separately on two AhR-containing stromal cell lines (M2-10B4 and S17) that support B-cell development in the presence of IL-7. The cocultures were treated with 0 to 10 nM TCDD. Shifts in phenotype were quantified by cell surface marker staining and flow cytometry. Four populations in the maturing B cell (very early pre-pro-B, pre-pro-B, pro-B, and pre-B) were defined for quantification. The results show that the only statistically significant effect of TCDD was within the pre-pro-B population in cultures with the S17 stromal cell line. The increase in number of cells with this phenotype was seen in cultures with both wild-type and AhR-/- primary bone marrow cells. These results suggest that the maturing B220+ B cell is not the direct target for TCDD-induced bone marrow B-cell alterations.

The PAS superfamily: sensors of environmental and developmental signals

Over the past decade, PAS domains have been identified in dozens of signal transduction molecules and various forms have been found in animals, plants, and prokaryotes. In this review, we summarize this rapidly expanding research area by providing a detailed description of three signal transduction pathways that utilize PAS protein heterodimers to drive their transcriptional output. It is hoped that these model pathways can provide a framework for use in understanding the biology of the less well-understood members of this emerging superfamily, as well as of those to be characterized in the days to come. We use this review to develop the idea that most eukaryotic PAS proteins can be classified by functional similarities, as well as by predicted phylogenetic relationships. We focus on the alpha-class proteins, which often act as sensors of environmental signals, and the beta-class proteins, which typically act as broad-spectrum partners that target these heterodimers to their genomic targets.

The aryl hydrocarbon receptor has a role in the in vivo maturation of murine bone marrow B lymphocytes and their response to 2,3,7,8-tetrachlorodibenzo-p-dioxin

The ligand-activated aryl hydrocarbon receptor (AHR) is a cytosolic DNA binding protein. Although no biologic role for AHR has been elucidated, it mediates the immunotoxicity of xenobiotics such as 2, 3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), and its targeted inactivation produces abnormal immune system development. While investigators have demonstrated AHR's involvement in TCDD-induced B lymphocyte functional alterations, little is known about the receptor's possible role in early B cell maturation and whether exogenous ligands change this process. The purpose of this study was to determine, (1) whether bone marrow B lymphocyte maturation is affected by AHR presence, (2) if so, its relative importance in hematopoietic and/or nonhematopoietic elements and, (3) whether TCDD alters this process. Radiation chimeras were produced that were AHR positive (Ahr+/+) or negative (Ahr-/-) in either their nonhematopoietic or hematopoietic elements, or both. Marrow cells were analyzed for alterations in B lymphocyte maturation stage cell numbers in both vehicle- and TCDD-treated animals. Our results showed that (1) Ahr-/- animals had significantly higher numbers of pro/pre-B cells than Ahr+/+ animals, (2) TCDD treatment of Ahr+/+ animals produced a decrease in pro/pre-B cell numbers, whereas no effect was observed on Ahr-/- animals, and (3) AHR is required in both hematopoietic and stromal elements for maintenance of B cell subset maturation profiles.

Estrogen receptor reduces CYP1A1 induction in cultured human endometrial cells

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) exerts its toxic action via the aryl hydrocarbon (Ah) receptor, which induces a battery of xenobiotic-metabolizing enzymes, including the cytochrome P450 isozyme, CYP1A1. TCDD-induced 7-ethoxycoumarin-O-deethylase activity was reduced 75% in cultured human endometrial ECC-1 cells exposed to various concentrations of 17beta-estradiol for up to 72 h, with a half-maximal effective concentration (EC50) of 0.9 nM. Reduced enzyme activity was correlated with decreased CYP1A1 mRNA levels, and transcription. Exposure to TCDD plus 17beta-estradiol also reduced CYP1A1 activity in MCF-7 breast cancer cells but not in Hep-3B human liver cells or HuE primary human keratinocytes, suggesting that the effect was specific to estrogen-regulated cells. Estrogen receptor antagonists 4-hydroxytamoxifen and 7alpha-[9-(4,4, 5,5,5-pentafluoro-pentylsulfinyl)nonyl]estra-1,3,5(10)-tr iene3, 17beta-diol restored TCDD-induced CYP1A1 transcription, steady-state mRNA levels, and enzymatic activity in ECC-1 cells. Gel mobility shift assay showed that 17beta-estradiol had little effect on Ah receptor binding to its DNA-responsive element. 17beta-Estradiol did not alter the induction of another Ah receptor-regulated gene, CYP1B1, suggesting that altered Ah receptor binding to DNA does not mediate reduced CYP1A1 transcription. Transfecting ECC-1 cells with a general transcription factor involved in CYP1A1 induction, nuclear factor-1, reversed 17beta-estradiol antagonism of dioxin induced-CYP1A1. The data suggest that 17beta-estradiol reduced CYP1A1 expression at the transcriptional level by squelching available nuclear factor-1, a transcription factor that interacts with both Ah and estrogen receptors.

Bone marrow stromal cells constitutively express high levels of cytochrome P4501B1 that metabolize 7,12-dimethylbenz[a]anthracene

The polycyclic aromatic hydrocarbon 7,12-dimethylbenz[a]anthracene (DMBA) is a potent carcinogen that produces immunotoxic effects in bone marrow. Here, we show that bone marrow stromal cells metabolize DMBA to such products as 3,4-dihydrodiol, the precursor to the most mutagenic DMBA metabolite. The BMS2 bone marrow stromal cell line constitutively expressed higher levels of CYP1B1 protein and mRNA than C3H10T1/2 mouse embryo fibroblasts. BMS2 cells also produced a DMBA metabolite profile that was consistent with CYP1B1 activity. Treatment with the potent aryl hydrocarbon receptor (AhR) ligand 2,3, 7,8-tetrachlorodibenzo-p-dioxin (TCDD) induced a approximately 2-fold increase in CYP1B1 mRNA, protein, and activity in BMS2 cells. Two forms of the AhR (97 and 104 kDa) and the AhR nuclear translocator were detected in BMS2 cells. The AhR translocated to the nucleus after treatment with TCDD or DMBA but was approximately 5 times slower with DMBA. Primary bone marrow stromal (BMS) cell cultures established from AhR-/- mice showed similar basal CYP1B1 expression and activity as cell cultures established from heterozygous littermates or C57BL/6 mice. However, primary BMS cells from AhR-/- mice did not exhibit increased CYP1B1 protein expression after incubation with TCDD. BMS cells therefore constitutively express functional CYP1B1 that is not dependent on the AhR. This contrasts with embryo fibroblasts from the same mouse strain, in which basal CYP1B1 expression is AhR dependent. We therefore conclude that bone marrow toxicity may be mediated by CYP1B1-dependent DMBA metabolism, which is regulated by factors other than the AhR.