Markus Frericks

Promoter analysis of TCDD-inducible genes in a thymic epithelial cell line indicates the potential for cell-specific transcription factor crosstalk in the AhR response

Activation of the aryl hydrocarbon receptor (AhR(1)) by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) elicits severe immunosuppression accompanied by thymic atrophy. Previous evidence suggests that TCDD targets both thymocytes and thymic epithelial cells. The AhR induces cell-specific changes in gene transcription via binding to the dioxin response element DRE; however, the underlying specificity-mechanisms, in particular with regard to the role of promoter element context, and possible transcription factor crosstalk remain poorly understood. Global gene expression in the cortical thymic epithelial cell line ET at 2, 4, and 6 h following 5 nM TCDD exposure resulted in differential regulation of 201 genes. JASPAR and TRANSFAC mapped the statistically over-represented promoter elements in the regulated genes to specific transcription factor binding sites, suggesting a regulatory role in AhR signaling. Over-represented elements included the xenobiotic response element XRE, NF kappaB-Rel, HRE, PPAR gamma, GR, PAX-4 and estrogen receptor binding sites. Co-treatment experiments with TCDD and CoCl(2), to induce hypoxia, or TCDD and 17-beta-estradiol (E2) indicated crosstalk between AhR and Hif or ER, in agreement with other experimental models. The computational identification of TFBS and the demonstration of interaction confirm their interactions with AhR signaling and suggest that the other over-represented elements may also be important in the immunosuppressive effects elicited by TCDD. In conclusion, we demonstrated the importance of promoter element cooperation in the shaping of a cell-specific AhR response. Our findings regarding the transcriptional changes in cortical epithelial cells are congruent with the well-known thymotoxic TCDD-phenotype, and useful in new hypothesis generation of the role of cortical TECs in TCDD toxicity.

Role of the aryl hydrocarbon receptor in thymocyte emigration in vivo

The aryl hydrocarbon receptor (AHR) is a ligand‐dependent member of the PAS‐bHLH‐family of nuclear receptors. Anthropogenic ligands include environmental contaminants such as 2,3,7,8‐tetrachlorodibenzo‐p‐dioxin (TCDD). Over‐activation of the AHR causes thymus atrophy and immunosuppression. Signaling via the AHR changes the thymocyte differentiation program at several checkpoints, in particular within the CD4–CD8– double‐negative (DN) thymocyte subset. Here, we show that AHR over‐activation led to the preferential emigration of DN thymocytes to the periphery and accumulation in the spleen. Some of these recent thymic emigrants (RTE) had a novel “activated immature” phenotype (CD3–TCRβ–CD25+/intCD44–CD45RB+/intCD62L+CD69– cells). Gene expression profiling of DN RTE revealed 15 genes that were up‐regulated more than threefold by TCDD, including the S100A9 gene. Exposure of S100A9 null mice to TCDD showed a role for this protein in AHR‐mediated thymic egress.

Transcriptional signatures of immune cells in aryl hydrocarbon receptor (AHR)-proficient and AHR-deficient mice

Abstract The ligand-activated aryl hydrocarbon receptor (AHR) is known to modulate many genes in a highly cell-specific manner, either directly or indirectly via secondary effects. In contrast, little is known about the effects of AHR deficiency on gene expression balance. We compared the transcriptome of CD4 T cells from AHR-/- mice and wild-type mice; 390 genes, many of them immunotypic, were deregulated in AHR-deficient CD4 cells. TCDD-induced transcriptome changes correlated with the AHR expression level in immune cells. However, there was little overlap in AHR-dependent transcripts found in T lineage cells or dendritic cells. Our results demonstrate flexible gene accessibility for the AHR in immune cells. The idea of a universal battery of AHR-responsive genes is not tenable.

Uterine adenocarcinoma in the rat induced by afidopyropen. An analysis of the lesion's induction, progression and its relevance to humans

ABSTRACT Afidopyropen is a novel insecticide that acts as a TRPV channel modulator in chordotonal organs of target insects. In two carcinogenicity studies with Fischer rats, an increased incidence of uterine adenocarcinomas was observed at 1000 and 3000 ppm. This finding prompted an investigation of the mechanism of the tumor formation as well as the relevance of this mechanism to humans. The mechanistic work took parallel paths: one path investigated the pharmacokinetic properties of the test substance at the doses where the tumors were found; while the second path examined the key mechanistic events that culminated in uterine adenocarcinomas. The results of the investigation indicated that the tumors only occurred at doses where excretion of test substance was saturated – indicating that homeostatic biological and/or physiological processes were overwhelmed. At the doses where these processes were overwhelmed, the test substance acted through a mechanism of dopamine agonism, triggering a cascade key events that resulted in uterine adenocarcinomas. An analysis of these mechanisms observed in rat showed that they are both quantitatively (pharmacokinetic mechanism) and qualitatively (dopamine agonism mechanism) not relevant to humans. Therefore the uterine adenocarcinomas observed in the rat associated with high doses of Afidopyropen are not expected to pose a carcinogenic risk to humans. HighlightsAfidopyropen induces rat uterine adenocarcinomas at high doses.Excretion of Afidopyropen was saturated at high doses.Dopamine agonism identified as MoA for induction of rat uterine adenocarcinomas.