A Molecular Mechanism To Switch the Aryl Hydrocarbon Receptor from a Transcription Factor to an E3 Ubiquitin Ligase

Abstract

The aryl hydrocarbon receptor (AhR) is a ligand-activated transcription factor that is known as a mediator of toxic responses. Recently, it was shown that the AhR has dual functions. Besides being a transcription factor, it also possesses an intrinsic E3 ubiquitin ligase function that targets, e.g., the steroid receptors for proteasomal degradation. The aim of this study was to identify the molecular switch that determines whether the AhR acts as a transcription factor or an E3 ubiquitin ligase. To do this, we used the breast cancer cell line MCF7, which expresses a functional estrogen receptor alpha (ER␣) signaling pathway. Our data suggest that aryl hydrocarbon receptor nuclear translocator (ARNT) plays an important role in the modulation of the dual functions of the AhR. ARNT knockdown dramatically impaired the transcriptional activation properties of the ligand-activated AhR but did not affect its E3 ubiquitin ligase function. The availability of ARNT itself is modulated by another basic helix-loop-helix (bHLH)-Per-ARNT-SIM (PAS) protein, the repressor of AhR function (AhRR). MCF7 cells overexpressing the AhRR showed lower ER␣ protein levels, reduced responsiveness to estradiol, and reduced growth rates. Importantly, when these cells were used to produce estrogendependent xenograft tumors in SCID mice, we also observed lower ER␣ protein levels and a reduced tumor mass, implying a tumor-suppressive-like function of the AhR in MCF7 xenograft tumors.KEYWORDS aryl hydrocarbon receptor, molecular switch, E3 ubiquitin ligase, transcription factor, aryl hydrocarbon receptor nuclear translocator, aryl hydrocarbon receptor repressor B asic helix-loop-helix (bHLH)-Per-aryl hydrocarbon receptor (AhR) nuclear translocator (ARNT)-SIM (PAS) proteins function as biological sensors that respond to physiological stimuli and environmental signals to mediate adaptive cellular responses. In general, these proteins form heterodimers that consist of a signal-induced subunit and an unregulated, ubiquitously expressed subunit (1). A member of the family of bHLH-PAS proteins is the AhR. Initially identified by Poland and Knutson (2) as a binding site for planar, nonhalogenated, and halogenated xenobiotics, in particular the highly toxic 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), the AhR was viewed for a long time as a mediator of xenobiotic-induced toxicity and carcinogenesis. However, mouse knockdown studies have revealed a role for the AhR beyond xenobiotic-induced toxicity and carcinogenesis. For example, the AhR has been shown to be involved in fetal liver development (3), immune cell modulation and differentiation (4-6), and gastrointestinal homeostasis (7, 8). The developmental defects observed in AhR Ϫ/Ϫ mice and the strong conservation of the AhR throughout evolution (for a review, see reference 9)

The Environmental Sensor AHR Protects from Inflammatory Damage by Maintaining Intestinal Stem Cell Homeostasis and Barrier Integrity

“…The progression from inflammation to tumorigenesis is well established ( Schäfer and Werner, 2008 ) and our mice readily developed mixed colon adenomas/adenocarcinomas in the standard AOM/DSS model in line with previous data in global Ahr −/− mice ( Díaz-Díaz et al., 2016 ) or even with the mutagen AOM alone. In vitro studies on cell lines proposed that AHR functions as part of an E3 ubiquitin ligase complex that serves to ubiquinate β-catenin and target it for degradation ( Luecke-Johansson et al., 2017 , Ohtake et al., 2007 ). Our results provide evidence that Wnt-β-catenin pathway dysregulation in Ahr -deficient IECs occurred upstream of β-catenin degradation, affecting the response to WNT signals themselves through the expression of the two related E3 ubiquitin ligases (ZNRF3 and RNF43) that degrade the Wnt frizzled receptors ( Koo et al., 2012 ) in LGR5 + stem cells.…”

AhR controls redox homeostasis and shapes the tumor microenvironment in BRCA1-associated breast cancer

“…1C). Compared with IgG antibody control, AhR recruitment peaked at 1 h posttreatment, suggesting that BSO can trigger AhR transcriptional activity as rapidly as the well-characterized AhR ligand, 2,3,7,8-tetrachlorodibenzo-p-dioxin (18,19). Immunocytochemistry assay showed a high frequency of cells positive for nuclear AhR after 2 h of exposure to BSO compared with control conditions ( Fig.…”

Insight into the relationship between aryl-hydrocarbon receptor and β-catenin in human colon cancer cells

“…A recent detailed study on the interaction between AhR and CUL4B using human cell lines presented conflicting data compared to work by Ohtake and colleagues [15]. Luecke-Johansson and colleagues could not identify Arnt as a part of the CUL4B/AhR complex when performing co-immunoprecipitations with a CUL4B antibody, even after treatment with TCDD, although Arnt had been shown to be essential for the binding between AhR and CUL4B in the previous report [35]. Moreover, a recent study showed that over-expression of CUL4B increased β-catenin accumulation.…”