Expression of Ah receptor (TCDD receptor) during human monocytic differentiation

Leukocyte activation induces aryl hydrocarbon receptor up-regulation, DNA binding, and increased Cyp1a1 expression in the absence of exogenous ligand

The aryl hydrocarbon receptor (AhR) functions as a transcription factor after ligand binding by halogenated aromatic hydrocarbons. 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), the most toxic halogenated aromatic hydrocarbon, is dependent on binding to the AhR to mediate a broad range of toxic effects. Immune suppression is one of the most sensitive sequela associated with TCDD exposure, yet, paradoxically, resting leukocytes express a relatively low amount of AhR. Here we report that activation of leukocytes produced a 6-fold increase in AhR steady state mRNA levels and a concordant increase in AhR protein expression. Furthermore, leukocyte activation induced AhR translocation, DNA binding to a dioxin response element, and CYP1A1 transcription in the absence of TCDD. Activated leukocytes exhibited an even greater enhancement of dioxin response element binding by the AhR in the presence of TCDD than in the absence of TCDD. These studies suggest that the mechanism responsible for the sensitivity of immunocompetent cells to TCDD may be directly associated with a marked increase in AhR expression, which accompanies leukocyte activation.

Characterization of the aromatic hydrocarbon receptor gene and its expression in Atlantic tomcod

Cytochrome P4501A1 (CYP1A1) mRNA is not inducible in Atlantic tomcod from the Hudson River that are treated with halogenated aromatic hydrocarbons (HAHs). In contrast, CYP1A1 mRNA is inducible in Hudson River tomcod that are treated with polycyclic aromatic hydrocarbons (PAHs) and in tomcod that are collected from cleaner rivers and treated with HAHs or PAHs. We hypothesize that CYP1A1 transcription is inhibited in Hudson River tomcod because of down-regulation of the aromatic hydrocarbon receptor (AhR) pathway and that separate molecular pathways modulate CYP1A1 transcription in fish treated with HAHs and PAHs. We initially evaluated levels of hepatic nuclear protein binding at enhancer elements (DREs) in the regulatory region of tomcod CYP1A1. No difference in levels of protein binding was observed between tomcod from the Hudson and Miramichi (cleaner) rivers that were untreated or were treated with benzo[a]pyrene. In contrast, levels of protein binding were lower in tomcod from the Hudson River that were treated with TCB than in similarly treated fish from the Miramichi River, suggesting differences between the populations in the structure or expression of AhR pathway molecules. To address this possibility, AhR DNA sequences were characterized from tomcod cDNA and genomic DNA libraries. In tomcod and mammals, AhR is represented by 11 exons, overall peptide sizes are similar, and amino acid sequences at basic, helix-loop-helix, PAAS A, and PAAS B domains are highly conserved. In contrast, little similarity was observed between tomcod and mammals in the sizes or sequences of AhR exons 10 and 11, including the absence in tomcod of glutamine-rich domains. No differences in levels of hepatic AhR mRNA were observed between the two populations or treatment groups when tomcod were untreated or were treated with aromatic hydrocarbons. In contrast, variation in levels of AhR mRNA expression was observed among tomcod tissues; however, no relationship was observed between levels of AhR mRNAs and CYP1A1 mRNAs in tissues from chemically or vehicle control-treated fish. RFLP analysis revealed extensive variation in exons 10 and 11 of AhR cDNA among tomcod from different rivers. Our results suggest that variation between tomcod populations in CYP1A1 mRNA inducibility is reflected by differences in levels of inducible hepatic protein binding to DREs. However, levels of hepatic AhR mRNA are not down-regulated in the Hudson River population, are not affected by AH treatments, and levels of AhR mRNA expression are not responsible for the differential inducibility of CYP1A1 transcription.

Environmental toxins and endometriosis

The effect of natural, environmental, or manufactured substances on endocrine organs is a current topic of discussion. Recently, some toxicants have been suggested to promote endometriosis; therefore, attention has turned to the role of environmental factors as a stimulant for endometriosis growth and maintenance. This article reviews the influence of various toxicants on endometriosis.