Estrogen and aryl hydrocarbon receptor signaling

Abstract

The biological effects of estrogens are mediated by the two estrogen receptor (ER) isoforms, ERa and ERb. The ERs are ligand-inducible transcription factors that belong to the nuclear receptor (NR) superfamily. Ligand-activated ERs bind to specific DNA elements in promoters of target genes, thereby inducing transcription. Due to the promiscuous ligand-binding cavity, ERs are prone to interference by environmental pollutants. Chemicals that disturb hormonal systems are known as endocrine disruptive chemicals (EDCs). Many EDCs are ligands for the aryl hydrocarbon receptor (AhR) that regulates the cellular response to xenobiotics. 2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD; dioxin) is one example of an extremely toxic contaminant with endocrine disruptive properties that binds with high affinity to AhR. AhR and its heterodimerization partner AhR Nuclear Translocator (ARNT) belong to the bHLH-PAS superfamily, another class of transcription factors. Members of this family serve as sensors of various environmental stimuli, such as low oxygen tension and xenobiotic insult. Crosstalk between the ER and AhR systems leads to inhibition of estrogenic signaling both in vitro and in experimental animals. In addition, reports of human exposure to dioxin describe adverse endocrine effects, such as alterations in sex ratio in children of exposed parents.

In this thesis, we have studied molecular mechanisms of crosstalk between ER and AhR signaling pathways. We have identified ARNT, the binding partner of AhR, as a coactivator of the ERs. ARNT coactivation function relies on the N-terminal region of ARNT, and the mechanism differs from that described for the classical p160 coactivators. In addition, ARNT shows a preference for ERb. We could also show that the antiestrogenic effect of TCDD is partly due to competition between ER and AhR for its common cofactor ARNT and that ERbeta activity is more susceptible to TCDD.

Furthermore, we have compared the biological effects of two prototypical AhR ligands, 3-methylcholanthrene (3-MC) and TCDD, on ER transcriptional activation. We show that these compounds exert distinct effects on ER signaling. TCDD and 3-MC differ in structure and stability: while TCDD is refractory to biotransformation, 3-MC is metabolized by enzymes in the exposed cells. We describe that 3-MC is converted into metabolites with estrogenic properties depending on cell type. The metabolites are active via the ligand-binding pocket of ERalpha.

The differences between 3-MC and TCDD have also been characterized on a genome-wide level in HepG2 cells. The results show that 3-MC, in contrast to TCDD, induces several known estrogen target genes. Gene induction is correlated with recruitment of ERalpha to the respective promoters. Furthermore, the estrogenic activities exerted by these compounds depend on ERalpha and do not function through ERbeta, suggesting an ER isoform-specific mechanism of action.

In summary, we have identified a novel point of convergence between ER and AhR signaling. Our data also suggest that the antiestrogenic effects of TCDD exposure might be influenced by ER isoform expression. The studies of 3-MC highlight the importance of taking metabolism into account when evaluating the endocrine disruptive potency of chemicals.