2B), which appeared to be mainly due to an increase in lymphocytes (Fig. cells showed increased expression ofCebpeand decreased expression of several hematopoiesis-associated genes. These data indicate that AhR has a physiological and functional role in hematopoiesis. The AhR appears to play a role in maintaining the normal quiescence of HSCs. == Introduction == The aryl hydrocarbon receptor(AhR) belongs to the Per-Arnt-Sim superfamily of DNA-binding proteins that play roles in regulating responses to signals in the tissue environment, such as hypoxia, oxidationreduction status, and circadian rhythms [1]. Upon ligand binding, the AhR translocates to the nucleus where it heterodimerizes with the AhR nuclear translocation protein (Arnt, also known as HIF1). This interaction increases the AhRArnt complex’s ability to bind to specific enhancer sequences, Ah-responsive elements, near target promoters, leading to the recruitment of coactivators and basal transcription factors to alter the expression of target genes [2]. The AhR may also alter signaling pathways by nongenomic means through its direct interaction with proteins such as Rb and Rel-B [3,4]. Many of the molecular events leading to modulation of gene transcription following AhR activation by the potent xenobiotic ligand 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) have been characterized. For example, induction ofCYP1A1, a member of the cytochrome P450 drug-metabolizing enzyme family, and other genes includingHES-1has been ascribed to the recognition by the Amisulpride hydrochloride AhRArnt complex of Ah-responsive elements located in the upstream regions of responsive genes [5,6]. However, direct relationships between the modulation of known responsive genes and specific functional outcomes Amisulpride hydrochloride in cells/tissues have to be yet determined. Likewise, the normal function of the AhR is not known and true endogenous ligands have not been clearly identified, although recent data suggest likely candidates [79]. Most of our understanding of AhR biology has come from studies using TCDD. This chemical causes, through its high affinity binding to and prolonged activation of the AhR, a wide range of biochemical and pathological changes in mammalian and nonmammalian species. TCDD has been shown to be a potent immunotoxicant, eliciting immune suppression in all animal species tested [10]. In humans, TCDD exposure has been associated with an increased incidence of lymphomas and leukemias [11,12]. Recently, AhR activity has been found to have a role in Th-17 cell development and autoimmune responses [13,14]. Given the profound effects of persistent AhR activation on the immune system, we have been investigating the cellular and molecular events underlying induced alterations in bone marrow (BM). The number of immature and mature B cells in mouse BM decreases following exposure to a single dose of TCDD [15]. Pre- or postnatal TCDD exposure results in a significant reduction in lymphoid-specific TdT and RAG-1 mRNA in BM, and thymic seeding by BM cells from TCDD-treated animals is substantially decreased [16]. However, TCDD treatment produces a significant increase in the number of HDACA hematopoietic stem cell (HSC)-enriched LinSca-1+c-Kit+(LSK) cells [17,18]. The use of radiation chimeras demonstrated that the presence of the AhR in hematopoietic cells, but not supporting stroma, was responsible for these effects [19]. It was Amisulpride hydrochloride also shown that marrow cells from TCDD-treated animals had lost the ability to fully reconstitute the immune Amisulpride hydrochloride system of irradiated mice Amisulpride hydrochloride [18,20]. Overall, these data indicate that both the numbers and functionality of HSCs and/or hematopoietic progenitor cells (HPCs) are directly modulated by AhR activation. To better understand the physiological role of the AhR,Ahr-null allele (AhR/or.
