Visit our Expo - Redox and Inflammation signaling 2012

Session XIV : Transcriptional and translational control Poster XIV, 27
Arylhydrocarbon Receptor Repressor (AhRR) controls AhR regulated genes: clues for
recruitment of HDACs
Thomas Haarmann-Stemmann, Hanno Bothe, Ellen Fritsche, Josef Abel
Institut für umweltmedizinische Forschung, Auf’m Hennekamp 50, 40225 Düsseldorf,
Germany; E-mail: haarmann@uni-duesseldorf.de
The AhRR is a recently discovered new member of the bHLH-PAS superfamily of
transcription factors, closely related to the arylhydrocarbon receptor (AhR) and ARNT. These
proteins are key regulators of drug metabolizing enzymes in response to xenobiotics like
benzo(a)pyrene (B(a)P) or 3-methylcholanthrene (3-MC). Here we report about the regulation
and function of the AhRR in different human cell lines, varying in expression and inducibility
of the AhRR: the responsive HepG2 cells, the low-responsive A549 cells, and the nonresponsive
human primary fibroblast and HeLa cells. Expression analyses revealed a high
expression of the AhRR in fibroblast and HeLa cells and low expression in HepG2 and A549
cells. Short term treatment with 10µM B(a)P enhanced the AhRR mRNA only in HepG2
cells. No significant increases were found in A549, fibroblast and HeLa cells. In order to test
whether methylation or acetylation status of the AhRR promoter account for the cell-specific
response of the AhRR, non-responsive cell lines were treated with either Azacytidine (Aza) or
Na-butyrate with or without B(a)P. Aza-treatment did not change the constitutive expression
of the AhRR, whereas co-treatment with Aza and B(a)P enhanced the AhRR mRNA only in
A549 cells. Bisulfit sequencing revealed no remarkable differences in methylation status of
AhRR promoter between responsive and non-responsive cell lines. Studies with the HDACinhibitor
indicate the importance of acetylation status in the regulation of AhRR and CYP1A1
expression. Chromatin immunoprecipitation (ChIP) revealed a strong XRE-binding of AhRR
in the non-responsive cell lines which disappears after Na-butyrate treatment. Coimmunoprecpitation
analyses imply an AhRR-HDAC interaction. These findings indicate that
the AhRR recruits HDACs and thereby controls expression of AhR-regulated genes. This
model is confirmed by siRNA analyses showing that with decreasing amount of AhRR the
expression of CYP1A1 increased. In summary, our data provide evidence that the AhRR is an
important regulator of transcription of AhR regulated genes via its ability to recruit HDACs.
From our study we suggest that the AhRR exerts a nuclear co-repressor function, but details
of this mechanism remain to be elucidated.
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