No abstract

The biological effects of dioxins are mediated by the aryl hydrocarbon receptor (AhR) and its dimerization partner, the AhR nuclear translocator (ARNT), and include interference with hormonal signaling pathways like the response to estrogens. The effects of estrogens are mediated by two estrogen receptor (ER) isoforms, ERalpha and ERbeta, which belong to the family of nuclear receptors. We have previously shown that ARNT can act as coactivator of the ERs. In this study, we show that recruitment of ARNT to AhR or hypoxia-inducible factor-1alpha signaling pathways as well as small interfering RNA-mediated down-regulation of ARNT levels lead to a reduction in ER transcriptional activity. Using chromatin immunoprecipitation assays, we demonstrate that this decrease coincides with reduced recruitment of ARNT to estradiol-regulated promoters. We show further that coactivation by ARNT as well as inhibition by dioxin acts stronger on ERbeta than on ERalpha activity. Additionally, we demonstrate that the effects of ARNT are dependent on the A/B domain of the ERs with the A/B domain of ERbeta being considerably stronger in mediating the coactivating effects of ARNT. Taken together, our studies show that recruitment of ARNT to the AhR after dioxin treatment can account for the antiestrogenic effect of dioxins. Moreover, we show for the first time that the inhibitory effects of dioxin are more pronounced on ERbeta than on ERalpha.

Retinoic acid receptor (RAR) and retinoid X receptor are ligand-induced transcription factors that belong to the nuclear receptor family. The receptors are activated by small hydrophobic compounds, such as all-trans-retinoic acid and 9-cis-retinoic acid, respectively. Interestingly, these receptors are also targets for a number of exogenous compounds. In this study, we characterized the biological activity of the 9-cissubstituted retinoic acid metabolite, S-4-oxo-9-cis-13,14-dihydro-retinoic acid (S-4o9cDH-RA). The endogenous levels of this metabolite in wild-type mice and rats were found to be higher than those of alltrans-retinoic acid, especially in the liver. Using cell-based luciferase reporter systems, we showed that S-4o9cDH-RA activates the transcription of retinoic acid response element-containing genes in several cell types, both from a simple 2xDR5 element and from the promoter of the natural retinoid target gene RARb2. In addition, quantitative RT-PCR analysis demonstrated that S-4o9cDH-RA treatment significantly increases the endogenous mRNA levels of the RAR target gene RARb2. Utilizing a limited proteolytic digestion assay, we showed that S-4o9cDH-RA induces conformational changes to both RARa and RARb in the same manner as does all-trans-retinoic acid, suggesting that S-4o9cDH-RA is indeed an endogenous ligand for these receptors. These in vitro results were corroborated in an in vivo system, where S-4o9cDH-RA induced morphological changes similar to those of all-trans-retinoic acid in the developing chicken wing bud. When locally applied to the wing bud, S-4o9cDH-RA induced digit pattern duplications in a dose-dependent fashion. The results illustrate that S-4o9cDH-RA closely mimics alltrans-retinoic acid with regard to pattern respecification. Finally, using quantitative RT-PCR analysis, we showed that S-4o9cDH-RA induces the transcription of several retinoic acid-regulated genes in chick wing buds, including Hoxb8, RARb2, shh, Cyp26 and bmp2. Although