The aryl hydrocarbon receptor has a normal function in the regulation of hematopoietic and other stem/progenitor cell populations

Aryl hydrocarbon receptor activation alters immune cell populations in the lung and bone marrow during coronavirus infection

Respiratory viral infections are one of the major causes of illness and death worldwide. Symptoms associated with respiratory infections can range from mild to severe, and there is limited understanding of why there is large variation in severity. Environmental exposures are a potential causative factor. The aryl hydrocarbon receptor (AHR) is an environment-sensing molecule expressed in all immune cells. Although there is considerable evidence that AHR signaling influences immune responses to other immune challenges, including respiratory pathogens, less is known about the impact of AHR signaling on immune responses during coronavirus (CoV) infection. In this study, we report that AHR activation significantly altered immune cells in the lungs and bone marrow of mice infected with a mouse CoV. AHR activation transiently reduced the frequency of multiple cells in the mononuclear phagocyte system, including monocytes, interstitial macrophages, and dendritic cells in the lung. In the bone marrow, AHR activation altered myelopoiesis, as evidenced by a reduction in granulocyte-monocyte progenitor cells and an increased frequency of myeloid-biased progenitor cells. Moreover, AHR activation significantly affected multiple stages of the megakaryocyte lineage. Overall, these findings indicate that AHR activation modulates multiple aspects of the immune response to a CoV infection. Given the significant burden of respiratory viruses on human health, understanding how environmental exposures shape immune responses to infection advances our knowledge of factors that contribute to variability in disease severity and provides insight into novel approaches to prevent or treat disease.NEW & NOTEWORTHY Our study reveals a multifaceted role for aryl hydrocarbon receptor (AHR) signaling in the immune response to coronavirus (CoV) infection. Sustained AHR activation during in vivo mouse CoV infection altered the frequency of mature immune cells in the lung and modulated emergency hematopoiesis, specifically myelopoiesis and megakaryopoiesis, in bone marrow. This provides new insight into immunoregulation by the AHR and extends our understanding of how environmental exposures can impact host responses to respiratory viral infections.

Indole-3-carboxaldehyde ameliorates ionizing radiation-induced hematopoietic injury by enhancing hematopoietic stem and progenitor cell quiescence

Indole-3-carboxaldehyde (I3A), one of tryptophan metabolites derived from gut microbiota, extends the lifespan of mice after high-dose ionizing radiation exposure. Persistent myelosuppression is the most common and fatal complication for victims of nuclear accidents and patients undergoing radiotherapy, with few therapeutic options available. However, whether and how I3A protects ionizing radiation-induced hematopoietic toxicity remain unknown. In this study, we demonstrated that I3A treatment effectively ameliorated radiation-induced hematopoietic injury through accelerating peripheral blood cells recovery, promoting bone marrow cellularity restoration and enhancing functional HSPC regeneration. Additionally, I3A also suppressed intracellular reactive oxygen species production and inhibited apoptosis in irradiated HSPCs. Mechanistically, I3A treatment significantly increased HSPC quiescence, thus conferring HSPCs with resistance against radiation injury. Finally, I3A treatment could improve survival of lethally irradiated mice. Taken together, our data suggest that I3A acts as a gut microbiota-derived paracrine factor that regulates HSPC regeneration and may serve as a promising therapeutic agent for ionizing radiation-induced myelosuppression.

Influential factors for optimizing and strengthening mesenchymal stem cells and hematopoietic stem cells co-culture

Mesenchymal stem cells (MSCs) and Hematopoietic stem cells (HSCs) are two types of bone marrow stem cells that can proliferate and differentiate into different cell lineages. HSCs interact with MSCs under protective conditions, called niche. Numerous studies have indicated supportive effects of MSCs on HSCs proliferation and differentiation. Furthermore, HSCs have many clinical applications and could treat different hematologic and non-hematologic diseases. For this purpose, there is a need to perform in vitro studies to optimize their expansion. Therefore, various methods including co-culture with MSCs are used to address the limitations of HSCs culture. Some parameters that might be effective for improving the MSC/ HSC co-culture systems. Manipulating culture condition to enhance MSC paracrine activity, scaffolds, hypoxia, culture medium additives, and the use of various MSC sources, have been examined in different studies. In this article, we investigated the potential factors for optimizing HSCs/ MSCs co-culture. It might be helpful to apply a suitable approach for providing high-quality HSCs and improving their therapeutic applications.

The Aryl Hydrocarbon Receptor: Impact on the Tumor Immune Microenvironment and Modulation as a Potential Therapy

The aryl hydrocarbon receptor (AhR) is a ubiquitous nuclear receptor with a broad range of functions, both in tumor cells and immune cells within the tumor microenvironment (TME). Activation of AhR has been shown to have a carcinogenic effect in a variety of organs, through induction of cellular proliferation and migration, promotion of epithelial-to-mesenchymal transition, and inhibition of apoptosis, among other functions. However, the impact on immune cell function is more complicated, with both pro- and anti-tumorigenic roles identified. Although targeting AhR in cancer has shown significant promise in pre-clinical studies, there has been limited efficacy in phase III clinical trials to date. With the contrasting roles of AhR activation on immune cell polarization, understanding the impact of AhR activation on the tumor immune microenvironment is necessary to guide therapies targeting the AhR. This review article summarizes the state of knowledge of AhR activation on the TME, limitations of current findings, and the potential for modulation of the AhR as a cancer therapy.

The Aryl Hydrocarbon Receptor, Epigenetics and the Aging Process

The aryl hydrocarbon receptor (AhR) is a ligand-dependent transcription factor, classically associated with the regulation of xenobiotic metabolism in response to environmental toxins. In recent years, transgenic rodent models have implicated AhR in aging and longevity. Moreover, several AhR ligands, such as resveratrol and quercetin, are compounds proven to extend the lifespan of model organisms. In this paper, we first review AhR biology with a focus on aging and highlight several AhR ligands with potential anti-aging properties. We outline how AhR-driven expression of xenobiotic metabolism genes into old age may be a key mechanism through which moderate induction of AhR elicits positive benefits on longevity and healthspan. Furthermore, via integration of publicly available datasets, we show that liver-specific AhR target genes are enriched among genes subject to epigenetic aging. Changes to epigenetic states can profoundly affect transcription factor binding and are a hallmark of the aging process. We suggest that the interplay between AhR and epigenetic aging should be the subject of future research and outline several key gaps in the current literature. Finally, we recommend that a broad range of non-toxic AhR ligands should be investigated for their potential to promote healthspan and longevity.

Efficient expansion of rare human circulating hematopoietic stem/progenitor cells in steady-state blood using a polypeptide-forming 3D culture

Although widely applied in treating hematopoietic malignancies, transplantation of hematopoietic stem/progenitor cells (HSPCs) is impeded by HSPC shortage. Whether circulating HSPCs (cHSPCs) in steady-state blood could be used as an alternative source remains largely elusive. Here we develop a three-dimensional culture system (3DCS) including arginine, glycine, aspartate, and a series of factors. Fourteen-day culture of peripheral blood mononuclear cells (PBMNCs) in 3DCS led to 125- and 70-fold increase of the frequency and number of CD34⁺ cells. Further, 3DCS-expanded cHSPCs exhibited the similar reconstitution rate compared to CD34⁺ HSPCs in bone marrow. Mechanistically, 3DCS fabricated an immunomodulatory niche, secreting cytokines as TNF to support cHSPC survival and proliferation. Finally, 3DCS could also promote the expansion of cHSPCs in patients who failed in HSPC mobilization. Our 3DCS successfully expands rare cHSPCs, providing an alternative source for the HSPC therapy, particularly for the patients/donors who have failed in HSPC mobilization.

Isolation, Maintenance and Expansion of Adult Hematopoietic Stem/Progenitor Cells and Leukemic Stem Cells

Simple Summary

Transplantation of adult hematopoietic stem cells is an important therapeutic tool to help patients suffering from diverse hematological disorders. All types of blood cells can develop from a single hematopoietic stem cell underlining their enormous potential. Intense efforts are ongoing to generate “engraftable” human hematopoietic stem cells to treat hematopoietic diseases and to understand the molecular machinery driving them. Leukemic stem cells represent a low frequency subpopulation of leukemia cells that possess stem cell properties. They can instigate, maintain, and serially propagate leukemia in vivo, while they retain the capacity to differentiate into committed progenitors. Leukemic stem cells are unaffected by many therapeutic strategies and represent the major cause of relapse. We here describe all methods to maintain and expand murine and human hematopoietic cells in culture and describe their specific advantages. These methods are also employed to understand the biology of leukemic stem cells and to identify novel therapeutic strategies.

Abstract

Hematopoietic stem cells (HSCs) are rare, self-renewing cells that perch on top of the hematopoietic tree. The HSCs ensure the constant supply of mature blood cells in a tightly regulated process producing peripheral blood cells. Intense efforts are ongoing to optimize HSC engraftment as therapeutic strategy to treat patients suffering from hematopoietic diseases. Preclinical research paves the way by developing methods to maintain, manipulate and expand HSCs ex vivo to understand their regulation and molecular make-up. The generation of a sufficient number of transplantable HSCs is the Holy Grail for clinical therapy. Leukemia stem cells (LSCs) are characterized by their acquired stem cell characteristics and are responsible for disease initiation, progression, and relapse. We summarize efforts, that have been undertaken to increase the number of long-term (LT)-HSCs and to prevent differentiation towards committed progenitors in ex vivo culture. We provide an overview and compare methods currently available to isolate, maintain and enrich HSC subsets, progenitors and LSCs and discuss their individual advantages and drawbacks.

OUP accepted manuscript

Stem cell therapies, including stem cell transplantation and rejuvenation of stem cells in situ, are promising avenues for tackling a broad range of diseases. Stem cells can both self-renew and differentiate into other cell types, and play a significant role in the regulation of tissue homeostasis and regeneration after cell degeneration or injury. However, stem cell exhaustion or dysfunction increases with age and impedes the normal function of multiple tissues and systems. Thus, stem cell therapies could provide a solution to aging and age-associated diseases. Here, we discuss recent advances in understanding the mechanisms that regulate stem cell regeneration. We also summarize potential strategies for rejuvenating stem cells that leverage intrinsic and extrinsic factors. These approaches may pave the way toward therapeutic interventions aiming at extending both health and life span.

The Aryl Hydrocarbon Receptor Modulates Murine Hematopoietic Stem Cell Homeostasis and Influences Lineage-Biased Stem and Progenitor Cells

The core function of hematopoietic stem and progenitor cells (HSPCs) is to provide lifelong production of all lineages of the blood and immune cells. The mechanisms that modulate HSPC homeostasis and lineage biasing are not fully understood. Growing evidence implicates the aryl hydrocarbon receptor (AHR), an environment-sensing transcription factor, as a regulator of hematopoiesis. AHR ligands modulate the frequency of mature hematopoietic cells in the bone marrow and periphery, while HSPCs from mice lacking AHR (AHR KO) have increased proliferation. Yet, whether AHR modulates HSPC lineage potential and directs differentiation toward specific lineage-biased progenitors is not well understood. This study revealed that AHR KO mice have an increased proportion of myeloid-biased HSCs and myeloid-biased multipotent progenitor (MPP3) cells. Utilizing inducible AHR knockout mice (iAHR KO), it was discovered that acute deletion of AHR doubled the number of MPP3 cells and altered the composition of downstream lineage-committed progenitors, such as increased frequency of pregranulocyte/premonocyte committed progenitors. Furthermore, in vivo antagonism of the AHR led to a 2.5-fold increase in the number of MPP3 cells and promoted myeloid-biased differentiation. Using hematopoietic-specific conditional AHR knockout mice (AHRVav1) revealed that increased frequency of myeloid-biased HSCs and myeloid-biased progenitors is driven by AHR signaling that is intrinsic to the hematopoietic compartment. These findings demonstrate that the AHR plays a pivotal role in regulating steady-state hematopoiesis, influencing HSPC homeostasis and lineage potential. In addition, the data presented provide potential insight into how deliberate modulation of AHR signaling could help with the treatment of a broad range of diseases that require the hematopoietic compartment.

Immunotoxicity of Xenobiotics in Fish: A Role for the Aryl Hydrocarbon Receptor (AhR)?

The impact of anthropogenic contaminants on the immune system of fishes is an issue of growing concern. An important xenobiotic receptor that mediates effects of chemicals, such as halogenated aromatic hydrocarbons (HAHs) and polyaromatic hydrocarbons (PAHs), is the aryl hydrocarbon receptor (AhR). Fish toxicological research has focused on the role of this receptor in xenobiotic biotransformation as well as in causing developmental, cardiac, and reproductive toxicity. However, biomedical research has unraveled an important physiological role of the AhR in the immune system, what suggests that this receptor could be involved in immunotoxic effects of environmental contaminants. The aims of the present review are to critically discuss the available knowledge on (i) the expression and possible function of the AhR in the immune systems of teleost fishes; and (ii) the impact of AhR-activating xenobiotics on the immune systems of fish at the levels of immune gene expression, immune cell proliferation and immune cell function, immune pathology, and resistance to infectious disease. The existing information indicates that the AhR is expressed in the fish immune system, but currently, we have little understanding of its physiological role. Exposure to AhR-activating contaminants results in the modulation of numerous immune structural and functional parameters of fish. Despite the diversity of fish species studied and the experimental conditions investigated, the published findings rather uniformly point to immunosuppressive actions of xenobiotic AhR ligands in fish. These effects are often associated with increased disease susceptibility. The fact that fish populations from HAH- and PAH-contaminated environments suffer immune disturbances and elevated disease susceptibility highlights that the immunotoxic effects of AhR-activating xenobiotics bear environmental relevance.

How the AHR Became Important in Cancer: The Role of Chronically Active AHR in Cancer Aggression

For decades, the aryl hydrocarbon receptor (AHR) was studied for its role in environmental chemical toxicity i.e., as a quirk of nature and a mediator of unintended consequences of human pollution. During that period, it was not certain that the AHR had a “normal” physiological function. However, the ongoing accumulation of data from an ever-expanding variety of studies on cancer, cancer immunity, autoimmunity, organ development, and other areas bears witness to a staggering array of AHR-controlled normal and pathological activities. The objective of this review is to discuss how the AHR has gone from a likely contributor to genotoxic environmental carcinogen-induced cancer to a master regulator of malignant cell progression and cancer aggression. Particular focus is placed on the association between AHR activity and poor cancer outcomes, feedback loops that control chronic AHR activity in cancer, and the role of chronically active AHR in driving cancer cell invasion, migration, cancer stem cell characteristics, and survival.

Anti-Aging Physiological Roles of Aryl Hydrocarbon Receptor and Its Dietary Regulators

The vast majority of the literature on the aryl hydrocarbon receptor is concerned with its functions in xenobiotic detoxification. However, in the course of evolution, this receptor had to have physiological (rather than toxicological) functions. Our aim was to review the aryl hydrocarbon receptor’s role in the physiological functions involved in aging. This study was performed by searching the MEDLINE and Google Academic databases. A total of 34 articles were selected that focused specifically on the aryl hydrocarbon receptor and aging, the aryl hydrocarbon receptor and physiological functions, and the combination of both. This receptor’s main physiological functions (mediated by the modulation of gene expression) were cell regeneration, the immune reaction, intestinal homeostasis, and cell proliferation. Furthermore, it was shown that the loss of this receptor led to premature aging. This process may be caused by the dysregulation of hematopoietic stem cells, loss of glucose and lipid homeostasis, increase in inflammation, and deterioration of the brain. We conclude that the aryl hydrocarbon receptor, apart from its well-established role in xenobiotic detoxication, plays an important role in physiological functions and in the aging process. Modulation of the signaling pathway of this receptor could be a therapeutic target of interest in aging.

Cellular and Molecular Mechanisms of Environmental Pollutants on Hematopoiesis

Hematopoiesis is a complex and intricate process that aims to replenish blood components in a constant fashion. It is orchestrated mostly by hematopoietic progenitor cells (hematopoietic stem cells (HSCs)) that are capable of self-renewal and differentiation. These cells can originate other cell subtypes that are responsible for maintaining vital functions, mediate innate and adaptive immune responses, provide tissues with oxygen, and control coagulation. Hematopoiesis in adults takes place in the bone marrow, which is endowed with an extensive vasculature conferring an intense flow of cells. A myriad of cell subtypes can be found in the bone marrow at different levels of activation, being also under constant action of an extensive amount of diverse chemical mediators and enzymatic systems. Bone marrow platelets, mature erythrocytes and leukocytes are delivered into the bloodstream readily available to meet body demands. Leukocytes circulate and reach different tissues, returning or not returning to the bloodstream. Senescent leukocytes, specially granulocytes, return to the bone marrow to be phagocytized by macrophages, restarting granulopoiesis. The constant high production and delivery of cells into the bloodstream, alongside the fact that blood cells can also circulate between tissues, makes the hematopoietic system a prime target for toxic agents to act upon, making the understanding of the bone marrow microenvironment vital for both toxicological sciences and risk assessment. Environmental and occupational pollutants, therapeutic molecules, drugs of abuse, and even nutritional status can directly affect progenitor cells at their differentiation and maturation stages, altering behavior and function of blood compounds and resulting in impaired immune responses, anemias, leukemias, and blood coagulation disturbances. This review aims to describe the most recently investigated molecular and cellular toxicity mechanisms of current major environmental pollutants on hematopoiesis in the bone marrow.

Strategies for elevating hematopoietic stem cells expansion and engraftment capacity

Hematopoietic stem cells (HSCs) are a rare cell population in adult bone marrow, mobilized peripheral blood, and umbilical cord blood possessing self-renewal and differentiation capability into a full spectrum of blood cells. Bone marrow HSC transplantation has been considered as an ideal option for certain disorders treatment including hematologic diseases, leukemia, immunodeficiency, bone marrow failure syndrome, genetic defects such as thalassemia, sickle cell anemia, autoimmune disease, and certain solid cancers. Ex vivo proliferation of these cells prior to transplantation has been proposed as a potential solution against limited number of stem cells. In such culture process, MSCs have also been shown to exhibit high capacity for secretion of soluble mediators contributing to the principle biological and therapeutic activities of HSCs. In addition, endothelial cells have been introduced to bridge the blood and sub tissues in the bone marrow, as well as, HSCs regeneration induction and survival. Cell culture in the laboratory environment requires cell growth strict control to protect against contamination, symmetrical cell division and optimal conditions for maximum yield. In this regard, microfluidic systems provide culture and analysis capabilities in micro volume scales. Moreover, two-dimensional cultures cannot fully demonstrate extracellular matrix found in different tissues and organs as an abstract representation of three dimensional cell structure. Microfluidic systems can also strongly describe the effects of physical factors such as temperature and pressure on cell behavior.

Transcriptional Regulation of gga-miR-451 by AhR:Arnt in Mycoplasma gallisepticum (HS Strain) Infection

MicroRNAs (miRNAs) have been determined to be important regulators for pathogenic microorganism infection. However, it is largely unclear how miRNAs are triggered during pathogen infection. We previously reported that the up-regulation of gga-miR-451 negatively regulates the Mycoplasma gallisepticum (MG)-induced production of inflammatory cytokines via targeting tyrosine3-monooxygenase/tryptophan5-monooxygenase activation protein zeta (YWHAZ). The aim of this study was to investigate the mechanism regulating gga-miR-451 in MG infection in chickens. Analysis of gga-miR-451 precursor, pri-miR-451, and pre-miR-451 indicated that the regulation occurred transcriptionally. We also identified the transcriptional regulatory region of gga-miR-451 that contained consensus-binding motif for aryl hydrocarbon receptor (AhR) and aryl hydrocarbon receptor nuclear translocator (Arnt) complex, which is known as the transcription factor that regulates gene expression. Luciferase reporter assays combined with chromatin immunoprecipitation (ChIP) demonstrated that AhR:Arnt bound directly to the promoter elements of gga-miR-451, which were responsible for gga-miR-451 transcription in the context of MG infection. Furthermore, upregulation of AhR:Arnt significantly induced gga-miR-451 and inhibited YWHAZ expression, suggesting that AhR:Arnt may play an anti-inflammatory role in MG infection. This discovery suggests that induced gga-miR-451 expression is modulated by AhR:Arnt in response to MG infection.

PAX3 Confers Functional Heterogeneity in Skeletal Muscle Stem Cell Responses to Environmental Stress

Muscle satellite cells (MuSCs) are the quiescent muscle stem cells required for adult skeletal muscle repair. The impact of environmental stress such as pollution on MuSC behavior remains unexplored. We evaluated the impact of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) exposure, a ubiquitous and highly toxic pollutant, on MuSCs by combining in vivo mouse molecular genetic models with ex vivo studies. While all MuSCs express the transcription factor PAX7, we show that a subset also express PAX3 and exhibit resistance to environmental stress. Upon systemic TCDD treatment, PAX3-negative MuSCs display impaired survival, atypical activation, and sporadic differentiation through xenobiotic aryl hydrocarbon receptor signaling. We further show that PAX3-positive MuSCs become sensitized to environmental stress when PAX3 function is impaired and that PAX3-mediated induction of mTORC1 is required for protection. Our study, therefore, identifies a functional heterogeneity of MuSCs in response to environmental stress controlled by PAX3.

The tryptophan derivative 6-formylindolo[3,2-b]carbazole, FICZ, a dynamic mediator of endogenous aryl hydrocarbon receptor signaling, balances cell growth and differentiation

The aryl hydrocarbon receptor (AHR) is not essential to survival, but does act as a key regulator of many normal physiological events. The role of this receptor in toxicological processes has been studied extensively, primarily employing the high-affinity ligand 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). However, regulation of physiological responses by endogenous AHR ligands remains to be elucidated. Here, we review developments in this field, with a focus on 6-formylindolo[3,2-b]carbazole (FICZ), the endogenous ligand with the highest affinity to the receptor reported to date. The binding of FICZ to different isoforms of the AHR seems to be evolutionarily well conserved and there is a feedback loop that controls AHR activity through metabolic degradation of FICZ via the highly inducible cytochrome P450 1A1. Several investigations provide strong evidence that FICZ plays a critical role in normal physiological processes and can ameliorate immune diseases with remarkable efficiency. Low levels of FICZ are pro-inflammatory, providing resistance to pathogenic bacteria, stimulating the anti-tumor functions, and promoting the differentiation of cancer cells by repressing genes in cancer stem cells. In contrast, at high concentrations FICZ behaves in a manner similar to TCDD, exhibiting toxicity toward fish and bird embryos, immune suppression, and activation of cancer progression. The findings are indicative of a dual role for endogenously activated AHR in barrier tissues, aiding clearance of infections and suppressing immunity to terminate a vicious cycle that might otherwise lead to disease. There is not much support for the AHR ligand-specific immune responses proposed, the differences between FICZ and TCDD in this context appear to be explained by the rapid metabolism of FICZ.

Chemistry and Properties of Indolocarbazoles

The indolocarbazoles are an important class of nitrogen heterocycles which has evolved significantly in recent years, with numerous studies focusing on their diverse biological effects, or targeting new materials with potential applications in organic electronics. This review aims at providing a broad survey of the chemistry and properties of indolocarbazoles from an interdisciplinary point of view, with particular emphasis on practical synthetic aspects, as well as certain topics which have not been previously accounted for in detail, such as the occurrence, formation, biological activities, and metabolism of indolo[3,2- b]carbazoles. The literature of the past decade forms the basis of the text, which is further supplemented with older key references.

Aryl hydrocarbon receptor (AHR) is a novel druggable pathway controlling malignant progenitor proliferation in chronic myeloid leukemia (CML)

Aryl Hydrocarbon Receptor (AHR) is an ubiquitous basic helix-loop-helix transcription factor, which is ligand-activated and involved in numerous biological processes including cell division, cell quiescence and inflammation. It has been shown that AHR is involved in normal hematopoietic progenitor proliferation in human cells. In addition, loss of AHR in knockout mice is accompanied by a myeloproliferative syndrome-like disease, suggesting a role of AHR in hematopoietic stem cell (HSC) maintenance. To study the potential role of AHR pathway in CML progenitors and stem cells, we have first evaluated the expression of AHR in UT-7 cell line expressing BCR-ABL. AHR expression was highly reduced in UT-7 cell expressing BCR-ABL as compared to controls. AHR transcript levels, quantified in primary peripheral blood CML cells at diagnosis (n = 31 patients) were found to be significantly reduced compared to healthy controls (n = 15). The use of StemRegenin (SR1), an AHR antagonist, induced a marked expansion of total leukemic cells and leukemic CD34+ cells by about 4- and 10-fold respectively. SR1-treated CML CD34+ cells generated more colony-forming cells and long-term culture initiating cell (LTC-IC)-derived progenitors as compared to non-SR1-treated counterparts. Conversely, treatment of CML CD34+ cells with FICZ, a natural agonist of AHR, induced a 3-fold decrease in the number of CD34+ cells in culture after 7 days. Moreover, a 4-day FICZ treatment was sufficient to significantly reduce the clonogenic potential of CML CD34+ cells and this effect was synergized by Imatinib and Dasatinib treatments. Similarly, a 3-day FICZ treatment contributed to hinder significantly the number of LTC-IC-derived progenitors without synergistic effect with Imatinib. The analysis of molecular circuitry of AHR signaling in CML showed a transcriptional signature in CML derived CD34+ CD38- primitive cells with either low or high levels of AHR, with an upregulation of myeloid genes involved in differentiation in the "AHR low" fraction and an upregulation of genes involved in stem cell maintenance in the "AHR high" fraction. In conclusion, these findings demonstrate for the first time that down-regulation of AHR expression, a major cell cycle regulator, is involved in the myeloproliferative phenotype associated with CML. AHR agonists inhibit clonogenic and LTC-IC-derived progenitor growth and they could be used in leukemic stem cell targeting in CML.

Serotonin is an endogenous regulator of intestinal CYP1A1 via AhR

Aryl hydrocarbon receptor (AhR) is a nuclear receptor that controls xenobiotic detoxification via induction of cytochrome P450 1A1 (CYP1A1) and regulates immune responses in the intestine. Metabolites of L-tryptophan activate AhR, which confers protection against intestinal inflammation. We tested the hypothesis that serotonin (5-HT) is an endogenous activator of AhR in intestinal epithelial cells. Treatment of Caco-2 monolayers with 5-HT induced CYP1A1 mRNA in a time- and concentration-dependent manner and also stimulated CYP1A1 activity. CYP1A1 induction by 5-HT was dependent upon uptake via serotonin transporter (SERT). Antagonism of AhR and knockdown of AhR and its binding partner aryl hydrocarbon receptor nuclear translocator (ARNT) attenuated CYP1A1 induction by 5-HT. Activation of AhR was evident by its nuclear translocation after 5-HT treatment and by induction of an AhR-responsive luciferase reporter. In vivo studies showed a dramatic decrease in CYP1A1 expression and other AhR target genes in SERT KO ileal mucosa by microarray analysis. These results suggest that intracellular accumulation of 5-HT via SERT induces CYP1A1 expression via AhR in intestinal epithelial cells, and SERT deficiency in vivo impairs activation of AhR. Our studies provide a novel link between the serotonergic and AhR pathways which has implications in xenobiotic metabolism and intestinal inflammation.

Notch and Aryl Hydrocarbon Receptor Signaling Impact Definitive Hematopoiesis from Human Pluripotent Stem Cells

Induced pluripotent stem cells (iPSCs) stand to revolutionize the way we study human development, model disease, and eventually, treat patients. However, these cell sources produce progeny that retain embryonic and/or fetal characteristics. The failure to mature to definitive, adult‐type cells is a major barrier for iPSC‐based disease modeling and drug discovery. To directly address these concerns, we have developed a chemically defined, serum and feeder‐free–directed differentiation platform to generate hematopoietic stem‐progenitor cells (HSPCs) and resultant adult‐type progeny from iPSCs. This system allows for strict control of signaling pathways over time through growth factor and/or small molecule modulation. Through direct comparison with our previously described protocol for the production of primitive wave hematopoietic cells, we demonstrate that induced HSPCs are enhanced for erythroid and myeloid colony forming potential, and strikingly, resultant erythroid‐lineage cells display enhanced expression of adult β globin indicating definitive pathway patterning. Using this system, we demonstrate the stage‐specific roles of two key signaling pathways, Notch and the aryl hydrocarbon receptor (AHR), in the derivation of definitive hematopoietic cells. We illustrate the stage‐specific necessity of Notch signaling in the emergence of hematopoietic progenitors and downstream definitive, adult‐type erythroblasts. We also show that genetic or small molecule inhibition of the AHR results in the increased production of CD34⁺CD45⁺ HSPCs while conversely, activation of the same receptor results in a block of hematopoietic cell emergence. Results presented here should have broad implications for hematopoietic stem cell transplantation and future clinical translation of iPSC‐derived blood cells. Stem Cells2018;36:1004–1019

Early Life Exposure to Low Levels of AHR Agonist PCB126 (3,3',4,4',5-Pentachlorobiphenyl) Reprograms Gene Expression in Adult Brain

Early life exposure to environmental chemicals can have long-term consequences that are not always apparent until later in life. We recently demonstrated that developmental exposure of zebrafish to low, nonembryotoxic levels of 3,3',4,4',5-pentachlorobiphenyl (PCB126) did not affect larval behavior, but caused changes in adult behavior. The objective of this study was to investigate the underlying molecular basis for adult behavioral phenotypes resulting from early life exposure to PCB126. We exposed zebrafish embryos to PCB126 during early development and measured transcriptional profiles in whole embryos, larvae and adult male brains using RNA-sequencing. Early life exposure to 0.3 nM PCB126 induced cyp1a transcript levels in 2-dpf embryos, but not in 5-dpf larvae, suggesting transient activation of aryl hydrocarbon receptor with this treatment. No significant induction of cyp1a was observed in the brains of adults exposed as embryos to PCB126. However, a total of 2209 and 1628 genes were differentially expressed in 0.3 and 1.2 nM PCB126-exposed groups, respectively. KEGG pathway analyses of upregulated genes in the brain suggest enrichment of calcium signaling, MAPK and notch signaling, and lysine degradation pathways. Calcium is an important signaling molecule in the brain and altered calcium homeostasis could affect neurobehavior. The downregulated genes in the brain were enriched with oxidative phosphorylation and various metabolic pathways, suggesting that the metabolic capacity of the brain is impaired. Overall, our results suggest that PCB exposure during sensitive periods of early development alters normal development of the brain by reprogramming gene expression patterns, which may result in alterations in adult behavior.

Aryl hydrocarbon receptor (AHR) is a potential tumour suppressor in pituitary adenomas

Pituitary adenomas (PA) represent the largest group of intracranial neoplasms and yet the molecular mechanisms driving this disease remain largely unknown. The aim of this study was to use a high-throughput screening method to identify molecular pathways that may be playing a significant and consistent role in PA. RNA profiling using microarrays on eight local PAs identified the aryl hydrocarbon receptor (AHR) signalling pathway as a key canonical pathway downregulated in all PA types. This was confirmed by real-time PCR in 31 tumours. The AHR has been shown to regulate cell cycle progression in various cell types; however, its role in pituitary tissue has never been investigated. In order to validate the role of AHR in PA behaviour, further functional studies were undertaken. Over-expression of AHR in GH3 cells revealed a tumour suppressor potential independent of exogenous ligand activation by benzo α-pyrene (BαP). Cell cycle analysis and quantitative PCR of cell cycle regulator genes revealed that both unstimulated and BαP-stimulated AHR reduced E2F-driven transcription and altered expression of cell cycle regulator genes, thus increasing the percentage of cells in G₀/G₁ phase and slowing the proliferation rate of GH3 cells. Co-immunoprecipitation confirmed the interaction between AHR and retinoblastoma (Rb1) protein supporting this as a functional mechanism for the observed reduction. Endogenous Ahr reduction using silencing RNA confirmed the tumour suppressive function of the Ahr. These data support a mechanistic pathway for the putative tumour suppressive role of AHR specifically in PA, possibly through its role as a cell cycle co-regulator, even in the absence of exogenous ligands.

A small-molecule/cytokine combination enhances hematopoietic stem cell proliferation via inhibition of cell differentiation

Background

Accumulated evidence supports the potent stimulating effects of multiple small molecules on the expansion of hematopoietic stem cells (HSCs) which are important for the therapy of various hematological disorders. Here, we report a novel, optimized formula, named the SC cocktail, which contains a combination of three such small molecules and four cytokines.

Methods

Small-molecule candidates were individually screened and then combined at their optimal concentration with the presence of cytokines to achieve maximum capacity for stimulating the human CD34⁺ cell expansion ex vivo. The extent of cell expansion and the immunophenotype of expanded cells were assessed through flow cytometry. The functional preservation of HSC stemness was confirmed by additional cell and molecular assays in vitro. Subsequently, the expanded cells were transplanted into sublethally irradiated NOD/SCID mice for the assessment of human cell viability and engraftment potential in vivo. Furthermore, the expression of several genes in the cell proliferation and differentiation pathways was analyzed through quantitative polymerase chain reaction (qPCR) during the process of CD34⁺ cell expansion.

Results

The SC cocktail supported the retention of the immunophenotype of hematopoietic stem/progenitor cells remarkably well, by yielding purities of 86.6 ± 11.2% for CD34⁺ cells and 76.2 ± 10.5% for CD34⁺CD38^– cells, respectively, for a 7-day culture. On day 7, the enhancement of expansion of CD34⁺ cells and CD34⁺CD38^– cells reached a maxima of 28.0 ± 5.5-fold and 27.9 ± 4.3-fold, respectively. The SC cocktail-expanded CD34⁺ cells preserved the characteristics of HSCs by effectively inhibiting their differentiation in vitro and retained the multilineage differentiation potential in primary and secondary in vivo murine xenotransplantation trials. Further gene expression analysis suggested that the small-molecule combination strengthened the ability of the cytokines to enhance the Notch pathway for the preservation of HSC stemness, and inhibited the ability of the cytokines to activate the Wnt pathway for HSC differentiation.

Conclusions

We developed an optimal small-molecule/cytokine combination for the enhancement of HSC expansion via inhibition of differentiation. This approach indicates promising application for preparation of both the HSCs and the mature, functional hematopoietic cells for clinical transplantation.

Electronic supplementary material

The online version of this article (doi:10.1186/s13287-017-0625-z) contains supplementary material, which is available to authorized users.

The Complex Biology of the Aryl Hydrocarbon Receptor and Its Role in the Pituitary Gland

The aryl hydrocarbon receptor (AHR) is a ligand-activated transcription factor best known for its ability to mediate the effects of environmental toxins such as 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD or dioxin), polycyclic aromatic hydrocarbons (PAHs), benzene, and polychlorinated biphenyls (PCBs) through the initiation of transcription of a number of metabolically active enzymes. Therefore, the AHR has been studied mostly in the context of xenobiotic signaling. However, several studies have shown that the AHR is constitutively active and plays an important role in general cell physiology, independently of its activity as a xenobiotic receptor and in the absence of exogenous ligands. Within the pituitary, activation of the AHR by environmental toxins has been implicated in disruption of gonadal development and fertility. Studies carried out predominantly in mouse models have revealed the detrimental influence of several environmental toxins on specific cell lineages of the pituitary tissue mediated by activation of AHR and its downstream effectors. Activation of AHR during fetal development adversely affected pituitary development while adult models exposed to AHR ligands demonstrated varying degrees of pituitary dysfunction. Such dysfunction may arise as a result of direct effects on pituitary cells or indirect effects on the hypothalamic-pituitary-gonadal axis. This review offers in-depth analysis of all aspects of AHR biology, with a particular focus on its role and activity within the adenohypophysis and specifically in pituitary tumorigenesis. A novel mechanism by which the AHR may play a direct role in pituitary cell proliferation and tumor formation is postulated. This review therefore attempts to cover all aspects of the AHR's role in the pituitary tissue, from fetal development to adult physiology and the pathophysiology underlying endocrine disruption and pituitary tumorigenesis.

Human and rodent aryl hydrocarbon receptor (AHR): from mediator of dioxin toxicity to physiologic AHR functions and therapeutic options

Metabolism of aryl hydrocarbons and toxicity of dioxins led to the discovery of the aryl hydrocarbon receptor (AHR). Tremendous advances have been made on multiplicity of AHR signaling and identification of endogenous ligands including the tryptophan metabolites FICZ and kynurenine. However, human AHR functions are still poorly understood due to marked species differences as well as cell-type- and cell context-dependent AHR functions. Observations in dioxin-poisoned individuals may provide hints to physiologic AHR functions in humans. Based on these observations three human AHR functions are discussed: (1) Chemical defence and homeostasis of endobiotics. The AHR variant Val381 in modern humans leads to reduced AHR affinity to aryl hydrocarbons in comparison with Neanderthals and primates expressing the Ala381 variant while affinity to indoles remains unimpaired. (2) Homeostasis of stem/progenitor cells. Dioxins dysregulate homeostasis in sebocyte stem cells. (3) Modulation of immunity. In addition to microbial defence, AHR may be involved in a ‘disease tolerance defence pathway’. Further characterization of physiologic AHR functions may lead to therapeutic options.

The aryl hydrocarbon receptor: a multifunctional chemical sensor for host defense and homeostatic maintenance

The aryl hydrocarbon receptor (AHR) is a pivotal chemical sensor that transduces extrinsic and intrinsic signals into cellular responses. AHR was originally thought to be involved in not only drug metabolism but also carcinogenic and toxicological responses against environmental contaminants, such as 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and polycyclic aromatic hydrocarbons. However, recent studies demonstrate that the AHR plays multiple intrinsic roles in host defense and homeostasis as well, including immunity, stem cell maintenance, and cell differentiation, upon binding with an increasing number of newly defined dietary, cellular, and microbe-derived ligands. In addition, AHR is a convergence point for several signaling cascades, which may be involved in the diverse diseases caused by binding of the persistent ligand TCDD with extremely high affinity to AHR. A comprehensive understanding of physiological and pathological processes initiated by endogenous AHR agonists and antagonists may allow for the therapeutic regulation of AHR activity. Thus, the AHR can be a valuable diagnostic marker and therapeutic target for human diseases.

The Aryl Hydrocarbon Receptor and Its Ligands Inhibit Myofibroblast Formation and Activation: Implications for Thyroid Eye Disease

Thyroid eye disease (TED) is a degenerative disease that manifests with detrimental tissue remodeling, myofibroblast accumulation, and scarring in the orbit of affected individuals. Currently, there are no effective therapies for TED that target or prevent the excessive tissue remodeling caused by myofibroblast formation and activation. The canonical cytokine that induces myofibroblast formation is transforming growth factor (TGF)-β. The TGF-β signaling pathway is influenced by aryl hydrocarbon receptor (AHR) signaling pathways. We hypothesized that AHR agonists can prevent myofibroblast formation in fibroblasts from patients with TED, and thus AHR ligands are potential therapeutics for the disease. Orbital fibroblasts explanted from patients with TED were treated with TGF-β to induce myofibroblast formation, contraction, and proliferation. We found that AHR ligands prevent TGF-β-dependent myofibroblast formation, and this ability is dependent on AHR expression. The AHR and AHR ligands block profibrotic Wnt signaling by inhibiting the phosphorylation of GSK3β to prevent myofibroblast formation. These results provide new insight into the molecular pathways underlying orbital scarring in TED. These novel studies highlight the potential of the AHR and AHR ligands as future therapeutic options for eye diseases and possibly also for other scarring conditions.

The Aryl Hydrocarbon Receptor in Immunity: Tools and Potential

The signaling pathway of the evolutionary old transcription factor AhR is inducible by a number of small molecular weight chemicals, including toxicants such as polycyclic aromatic hydrocarbons, bacterial toxic pigments, and physiological compounds such as tryptophan derivatives or dietary indoles. AhR activation is of immunological importance, but at the same time mediates toxicity of environmental pollutants, such as immunosuppression by dioxins. Measuring AhR activity and identification of ligands is thus of great interest for a variety of research fields. In this chapter, I briefly introduce the AhR signaling pathway, its role in immunology, and the tools and assays needed to analyze AhR signaling. Both are also needed when therapeutic applications are envisioned.

Environmental Immunology: Lessons Learned from Exposure to a Select Panel of Immunotoxicants

Exposure to environmental contaminants can produce profound effects on the immune system. Many classes of xenobiotics can significantly suppress or enhance immune responsiveness depending on the levels (i.e., dose) and context (i.e., timing, route) of exposure. Although defining the effects that toxicants can have on the immune system is a valuable component to improving public health, environmental immunology has greatly enhanced our understanding of how the immune system functions and has provided innovative avenues to explore new immunotherapies. This Brief Review focuses on three examples of how immunotoxicology has benefitted the field of immunology, presenting information on the aryl hydrocarbon receptor signaling pathway, the immunomodulatory effects of nanomaterials, and the impact of xenobiotic exposure on the developing immune system. Collectively, contributions from immunotoxicology have significantly enhanced public health and spurred seminal advances in both basic and applied immunology.

Effects of Developmental Activation of the Aryl Hydrocarbon Receptor by 2,3,7,8-Tetrachlorodibenzo-p-dioxin on Long-term Self-renewal of Murine Hematopoietic Stem Cells

BACKGROUND

Human epidemiological and animal studies suggest that developmental exposure to contaminants that activate the aryl hydrocarbon receptor (AHR) lead to suppression of immune system function throughout life. The persistence of immune deficiency throughout life suggests that the cellular target of AHR activation is a fetal hematopoietic progenitor or stem cell.

OBJECTIVES

The aim of this study was to identify the effects of transplacental exposure to an AHR agonist on long-term self-renewal of fetal hematopoietic stem cells.

METHODS

Pregnant C57BL/6 or AHR+/- mice were exposed to the AHR agonist, 2,3,7,8-tetra-​chlorodibenzo-p-dioxin (TCDD). On day 14 of gestation, hematopoietic progenitors from wild-type or AHR-deficient fetuses were placed into in vitro T-lymphocyte differentiation cultures to identify the effects of transplacental TCDD on AHR activation in the fetus. We next analyzed the fetal hematopoietic progenitor cells for changes in reactive oxygen species (ROS). Finally, hematopoietic progenitors from fetuses exposed transplacentally to TCDD were mixed 1:1 with cells from congenic controls and used to reconstitute lethally irradiated recipients for analysis of long-term self-renewal potential.

RESULTS

Our findings suggested that the effects of TCDD on the developing hematopoietic system were mediated by direct AHR activation in the fetus. Furthermore, developmental AHR activation by TCDD increased ROS in the fetal hematopoietic stem cells, and the elevated ROS was associated with a reduced capacity of the TCDD-exposed fetal cells to compete with control cells in a mixed competitive irradiation/reconstitution assay.

CONCLUSIONS

Our findings indicate that AHR activation by TCDD in the fetus during pregnancy leads to impairment of long-term self-renewal of hematopoietic stem cells.

CITATION

Laiosa MD, Tate ER, Ahrenhoerster LS, Chen Y, Wang D. 2016. Effects of developmental activation of the aryl hydrocarbon receptor by 2,3,7,8-tetrachlorodibenzo-p-dioxin on long-term self-renewal of murine hematopoietic stem cells. Environ Health Perspect 124:957-965; http://dx.doi.org/10.1289/ehp.1509820.

β-naphthoflavone interferes with cyp1c1, cox2 and IL-8 gene transcription and leukotriene B4 secretion in Atlantic cod (Gadus morhua) head kidney cells during inflammation

The objective of this study was to evaluate how β-naphthoflavone interacts with lipopolysaccharide (LPS) and polyinosinic acid: polycytidylic acid (poly I: C) induced innate immune parameters as well as phase I and phase II detoxification enzymes in head kidney cells isolated from Atlantic cod. β-naphthoflavone is a pure agonist of aryl hydrocarbon receptor (AhR) while LPS and poly I: C are not. β-naphthoflavone was added to head kidney leukocytes alone or together with LPS or poly I: C and the responses were evaluated in terms of protein and gene expression. The results showed that β-naphthoflavone (25 nM), with and without LPS, significantly induced cytochrome P450 (cyp1c) transcription in cod head kidney cells. β-naphthoflavone (100 nM) in the presence of the virus mimic, poly I: C, also increased cyp1c1transcription. LPS induced cyp1c1, cyclooxygenase 2 (cox2), interleukin 1β (IL-1β), interleukin 6 (IL-6) and interleukin 8 (IL-8) transcription, genes that were not affected by the tested β-naphthoflavone concentrations alone. However, β-naphthoflavone (25 and 50 nM) strengthened LPS induced cox2 and IL-8 transcription. Cod head kidney cells exposed to β-naphthoflavone concentrations ranging from 25 to 100 nM, with and without LPS or poly I: C, expressed AhR protein. LPS or β-naphthoflavone (5-50 nM) significantly induced leukotriene B4 (LTB4) secretion compared to control. In conclusion, this study suggests that β-naphthoflavone could interfere with LPS induced immune cell signaling in cod head kidney cells.

Improvement of Chicken Primordial Germ Cell Maintenance In Vitro by Blockade of the Aryl Hydrocarbon Receptor Endogenous Activity

Primordial germ cells (PGCs) are the undifferentiated progenitors of gametes. Germline competent PGCs can be developed as a cell-based system for genetic modification in chickens, which provides a valuable tool for transgenic technology with both research and industrial applications. This implies manipulation of PGCs, which, in recent years, encouraged a lot of research focused on the study of PGCs and the way of improving their culture. The aryl hydrocarbon receptor (AHR) is a ligand-activated transcription factor that besides mediating toxic responses to environmental contaminants plays pivotal physiological roles in various biological processes. Since a novel compound that acts as an antagonist of this receptor has been reported to promote expansion of hematopoietic stem cells, we conducted the present study with the aim of determining whether addition of an established AHR antagonist to the standard culture medium used nowadays for in vitro chicken PGCs culture improves ex vivo expansion. We have found that addition of α-naphthoflavone in culture medium promotes the amplification of undifferentiated cells and that this effect is exerted by the blockade of AHR action. Our results constitute the first report of the successful use of a readily available AHR antagonist to improve avian PGCs expansion, and they further extend the knowledge of the effects of AHR modulation in undifferentiated cells.

Toward elucidation of dioxin-mediated chloracne and Ah receptor functions

Target cells and molecular targets responsible for dioxin-mediated chloracne, the hallmark of dioxin toxicity, are reviewed. The dioxin TCDD accumulates in sebum, and thereby persistently activates the Ah receptor (AhR), expressed in bipotential stem/progenitor cells of the sebaceous gland. AhR operates in cooperation with other transcription factors including c-Myc, Blimp1 and ß-Catenin/TCF: c-Myc stimulates exit of stem cells from quiescence to proliferating sebocyte progenitors; Blimp1 is a major c-Myc repressor, and ß-Catenin/TCF represses sebaceous gland differentiation and stimulates differentiation to interfollicular epidermis. TCDD has been demonstrated to induce Blimp1 expression in the sebocyte stem/progenitor cell line SZ95, leading to sebocyte apoptosis and proliferation of interfollicular epidermis cells. These findings explain observations in TCDD-poisoned individuals, and identify target cells and molecular targets of dioxin-mediated chloracne. They clearly demonstrate that the AhR operates in a cell context-dependent manner, and provide hints to homeostatic functions of AhR in stem/progenitor cells.

The aryl hydrocarbon receptor in barrier organ physiology, immunology, and toxicology

The aryl hydrocarbon receptor (AhR) is an evolutionarily old transcription factor belonging to the Per-ARNT-Sim-basic helix-loop-helix protein family. AhR translocates into the nucleus upon binding of various small molecules into the pocket of its single-ligand binding domain. AhR binding to both xenobiotic and endogenous ligands results in highly cell-specific transcriptome changes and in changes in cellular functions. We discuss here the role of AhR for immune cells of the barrier organs: skin, gut, and lung. Both adaptive and innate immune cells require AhR signaling at critical checkpoints. We also discuss the current two prevailing views-namely, 1) AhR as a promiscuous sensor for small chemicals and 2) a role for AhR as a balancing factor for cell differentiation and function, which is controlled by levels of endogenous high-affinity ligands. AhR signaling is considered a promising drug and preventive target, particularly for cancer, inflammatory, and autoimmune diseases. Therefore, understanding its biology is of great importance.

Fetal Hematopoietic Stem Cells Are the Canaries in the Coal Mine That Portend Later Life Immune Deficiency

Disorders of the blood system are a significant and growing global health concern and include a spectrum of diseases ranging from aplastic anemia and leukemias to immune suppression. This array of hematological disorders is attributed to the fact that the blood system undergoes a perpetual cycle of turn over with aged and exhausted red and white blood cells undergoing daily replacement. The foundational cells of this replenishment process are comprised of rare hematopoietic stem cells (HSCs) located in the bone marrow that possess the dual function of long-term self-renewal and multilineage differentiation. This constant turnover makes the hematopoietic system uniquely vulnerable to changes in the environment that impact multilineage differentiation, self-renewal, or both. Notably, environmental endocrine-disrupting exposures occurring during development, when HSCs are first emerging, can lead to alterations in HSC programming that impacts the blood and immune systems throughout life. In this review, we describe the process of fetal hematopoiesis and provide an overview of the intrauterine environmental and endocrine-disrupting compounds that disrupt this process. Finally, we describe research opportunities for fetal HSCs as potential sentinels of later-life blood and immune system disorders.

Comparison of Different Cytokine Conditions Reveals Resveratrol as a New Molecule for Ex Vivo Cultivation of Cord Blood-Derived Hematopoietic Stem Cells

Human cord blood (CB)-derived hematopoietic stem cells (HSCs) are an interesting source for HSC transplantation. However, the number of collected CB-HSCs is often too low for one transplantation; therefore, ex vivo expansion of CB-HSCs is desirable. Current expansion protocols are based on the use of cytokine combinations, including insulin-like growth factor-binding protein 2 (IGFBP2) and angiopoietin-like proteins, or combinations with "small molecules" such as stemregenin-1. The aim of our project was to compare the potential of different CB-HSC expansion strategies side-by-side by phenotypical analysis in vitro and serial engraftment properties in NOD/SCID/IL2rg-/- (NSG) immunodeficient mice. We further identified resveratrol, a naturally occurring polyphenol, as a new, alternative small molecule combined with cytokines to facilitate serum-free ex vivo expansion of human CB-HSCs. The cultivation in resveratrol preserved the CB-HSC phenotype in vitro most efficiently and was ∼2 times more potent than commonly used cytokine conditions (including stem cell factor, thrombopoietin, Fms-related tyrosine kinase 3 ligand, interleukin-6) and the recently established serum-free culture, including IGFBP2 and angiopoietin-like 5. Serial transplantation studies further confirmed resveratrol to support robust multilineage engraftment in primary and secondary NSG recipients. Therefore, our work proposes resveratrol as a new small molecule for improved ex vivo culture and modification of human HSCs based on an efficient ex vivo propagation of the HSC fate.

SIGNIFICANCE

Human cord blood (CB)-derived hematopoietic stem cells (HSCs) are an important source for HSC transplantations but restricted in their usage because of their low numbers. In gene therapy, modifications of HSCs relies on their ex vivo modification without losing their stemness properties. Therefore, ex vivo cultivation and expansion of CB-HSCs is important for their effective application in HSC transplantation and gene therapy. Several promising protocols for serum-free cultivation of HSCs using different combinations of cytokines or so-called small molecules are described. A direct comparison was performed of three described serum-free cytokine conditions, demonstrating that the natural occurring polyphenol resveratrol is able to support ex vivo cultivation of CB-HSCs. The results show that resveratrol is an additional candidate for improving ex vivo cultures of HSCs for transplantation and gene therapeutic applications in the future.

Engraftment and lineage potential of adult hematopoietic stem and progenitor cells is compromised following short-term culture in the presence of an aryl hydrocarbon receptor antagonist

Hematopoietic stem cell gene therapy for HIV/AIDS is a promising alternative to lifelong antiretroviral therapy. One of the limitations of this approach is the number and quality of stem cells available for transplant following in vitro manipulations associated with stem cell isolation and genetic modification. The development of methods to increase the number of autologous, gene-modified stem cells available for transplantation would overcome this barrier. Hematopoietic stem and progenitor cells (HSPC) from adult growth factor-mobilized peripheral blood were cultured in the presence of an aryl hydrocarbon receptor antagonist (AhRA) previously shown to expand HSPC from umbilical cord blood. Qualitative and quantitative assessment of the hematopoietic potential of minimally cultured (MC-HSPC) or expanded HSPC (Exp-HSPC) was performed using an immunodeficient mouse model of transplantation. Our results demonstrate robust, multilineage engraftment of both MC-HSPC and Exp-HSPC although estimates of expansion based on stem cell phenotype were not supported by a corresponding increase in in vivo engrafting units. Bone marrow of animals transplanted with either MC-HSPC or Exp-HSPC contained secondary engrafting cells verifying the presence of primitive stem cells in both populations. However, the frequency of in vivo engrafting units among the more primitive CD34+/CD90+ HSPC population was significantly lower in Exp-HSPC compared with MC-HSPC. Exp-HSPC also produced fewer lymphoid progeny and more myeloid progeny than MC-HSPC. These results reveal that in vitro culture of adult HSPC in AhRA maintains but does not increase the number of in vivo engrafting cells and that HSPC expanded in vitro contain defects in lymphopoiesis as assessed in this model system. Further investigation is required before implementation of this approach in the clinical setting.

Aryl hydrocarbon receptor protein and Cyp1A1 gene induction by LPS and phenanthrene in Atlantic cod (Gadus morhua) head kidney cells

The objective of this study was to evaluate interactions between environmental toxicants and cod immune cells during inflammation. Phenanthrene is abundant in plant oils (rapeseed, palm, and soya oil) as compared to fish oils, and consequently constitute an undesirable element in plant replacement diets in aquaculture. Phenanthrene was added to head kidney cell cultures, alone or together with LPS (lipopolysaccharide) or poly I: C (polyinosinic acid: polycytidylic acid), and the responses were evaluated in terms of protein and gene expression. The results showed that LPS, poly I: C or phenanthrene, added to the cultures separately, induced aryl hydrocarbon receptor (AhR) protein expression. Phenanthrene treatment in combination with LPS induced AhR protein expression and Cyp1A1 gene transcription, which not was observed combining poly I: C and phenanthrene. Phenanthrene exposure up regulated the transcription of common stress and detoxification enzymes like catalase, caspase 3 and glutathione S-transferase alfa 3 subunit B (GSTAB3), while LPS exposure alone or combined with phenanthrene down regulated GSTAB3 and catalase in cod leukocytes. It seems clear that immune regulation and phenanthrene induced signaling pathways interact; transcriptional down regulation of detoxification and antioxidant enzymes by LPS could indicate that combating bacterial infections is the number one priority in these cells, and that AhR and Cyp1A1 is somehow involved in this signaling cascade. LPS seems to affect the mitogen activated protein kinases (MAPKs) pathways (P-p38 and ERK1/2) thus modulating the AhR protein and Cyp1A1 gene transcription, while phenanthrene possibly activates AhR by ligand binding.

Aryl hydrocarbon receptor (AhR) attenuation of subchronic cigarette smoke-induced pulmonary neutrophilia is associated with retention of nuclear RelB and suppression of intercellular adhesion molecule-1 (ICAM-1)

Cigarette smoke is associated with chronic and enhanced pulmonary inflammation characterized by increased cytokine production and leukocyte recruitment to the lung. Although the aryl hydrocarbon receptor (AhR) is well-known to mediate toxic effects of manmade environmental contaminants, the AhR has emerged as a suppressor of acute cigarette smoke-induced neutrophilia by a mechanism involving the NF-κB protein RelB. Yet individuals who smoke often smoke for many years and vary in their cigarette consumption. As there is currently no information on the AhR prevention of lung inflammation, including neutrophilia, due to varied and prolonged exposure regimes, we exposed control and AhR(-/-) mice to cigarette smoke for 2 weeks (subchronic exposure) utilizing low and high exposure protocols and evaluated pulmonary inflammation. Subchronic cigarette smoke exposure significantly increased pulmonary neutrophilia dose-dependently in AhR(-/-) mice. Surprisingly, there was no difference between smoke-exposed AhR(+/-) and AhR(-/-) mice in the expression of cytokines associated with neutrophil recruitment. Expression of pulmonary intercellular adhesion molecule-1 (ICAM-1), an adhesion molecule involved in neutrophil migration and retention, was higher in pulmonary endothelial cells from AhR(-/-) mice. Although total lung RelB expression was increased by cigarette smoke, nuclear RelB was significantly lower in subchronically exposed AhR(-/-) mice. Inhibition of AhR activity by CH-223191 in endothelial cells potentiated ICAM-1 expression and prevented RelB nuclear translocation but had no effect on neutrophil adhesion. These data support that genetic absence of the AhR contributes to heightened pulmonary neutrophilia in response to ongoing cigarette smoke exposure. Interindividual variations in AhR expression may enhance the susceptibility to cigarette smoke-induced diseases.

New insights into the role of the aryl hydrocarbon receptor in the function of CD11c⁺ cells during respiratory viral infection

The aryl hydrocarbon receptor (AHR) has garnered considerable attention as a modulator of CD4(+) cell lineage development and function. It also regulates antiviral CD8(+) T-cell responses, but via indirect mechanisms that have yet to be determined. Here, we show that during acute influenza virus infection, AHR activation skews dendritic-cell (DC) subsets in the lung-draining lymph nodes, such that there are fewer conventional CD103(+) DCs and CD11b(+) DCs. Sorting DC subsets reveals AHR activation reduces immunostimulatory function of CD103(+) DCs in the mediastinal lymph nodes, and decreases their frequency in the lung. DNA-binding domain Ahr mutants demonstrate that alterations in DC subsets require the ligand-activated AHR to contain its inherent DNA-binding domain. To evaluate the intrinsic role of AHR in DCs, conditional knockouts were created using Cre-LoxP technology, which revealed that AHR in CD11c(+) cells plays a key role in controlling the acquisition of effector CD8(+) T cells in the infected lung. However, AHR within other leukocyte lineages contributes to diminished naïve CD8(+) T-cell activation in the draining lymphoid nodes. These findings indicate DCs are among the direct targets of AHR ligands in vivo, and AHR signaling modifies host responses to a common respiratory pathogen by affecting the complex interplay of multiple cell types.