A novel Arnt-interacting protein Ainp2 enhances the aryl hydrocarbon receptor signalling

Selective suppression of the human aryl hydrocarbon receptor function can be mediated through binding interference at the C-terminal half of the receptor

The human aryl hydrocarbon receptor is a cytosolic signaling molecule which affects immune response and aberrant cell growth. Canonical signaling of the receptor requires the recruitment of coactivators to the promoter region to remodel local chromatin structure. We predicted that interference of this recruitment would block the aryl hydrocarbon receptor function. To prove that, we employed phage display to identify nine peptides of twelve-amino-acid in length which target the C-terminal half of the human aryl hydrocarbon receptor, including the region where coactivators bind. Eight 12mer peptides, in the form of GFP fusion, suppressed the ligand-dependent transcription of six AHR target genes (cyp1a1, cyp1a2, cyp1b1, ugt1a1, nqo1, and ahrr) in different patterns in Hep3B cells, whereas the AHR antagonist CH-223191 suppressed all these target genes similarly. Three of the 12mer peptides (namely 11-3, 1-7, and 7-3) suppressed the 3MC-induced, CYP1A1-dependent EROD activity and the ROS production caused by benzo[a]pyrene. These 12mer peptides suppressed the AHR function synergistically with CH-223191. In conclusion, we provide evidence that targeting the C-terminal half of the human aryl hydrocarbon receptor is a viable, new approach to selectively block the receptor function.

Differential suppression of the aryl hydrocarbon receptor nuclear translocator-dependent function by an aryl hydrocarbon receptor PAS-A-derived inhibitory molecule

The aryl hydrocarbon receptor (AhR) heterodimerizes with the aryl hydrocarbon receptor nuclear translocator (Arnt) for transcriptional regulation. We generated three N-terminal deletion constructs of the human AhR of 12-24 kDa in size--namely D1, D2, and D3--to suppress the Arnt function. We observed that all three deletions interact with the human Arnt with similar affinities. D2, which contains part of the AhR PAS-A domain and interacts with the PAS-A domain of Arnt, inhibits the formation of the AhR gel shift complex. D2 suppresses the 3-methylcholanthrene-induced, dioxin response element (DRE)-driven luciferase activity in Hep3B cells and exogenous Arnt reverses this D2 suppression. D2 suppresses the induction of CYP1A1 at both the message and protein levels in Hep3B cells; however, the CYP1B1 induction is not affected. D2 suppresses the recruitment of Arnt to the cyp1a1 promoter but not to the cyp1b1 promoter, partly because the AhR/Arnt heterodimer binds better to the cyp1b1 DRE than to the cyp1a1 DRE. Interestingly, D2 has no effect on the cobalt chloride-induced, hypoxia inducible factor-1 (HIF-1)-dependent expression of vegf, aldolase c, and ldh-a messages. Our data reveal that the flanking sequences of the DRE contribute to the binding affinity of the AhR/Arnt heterodimer to its endogenous enhancers and the function of AhR and HIF-1 can be differentially suppressed by the D2 inhibitory molecule.

A cell-penetrating peptide suppresses the hypoxia inducible factor-1 function by binding to the helix-loop-helix domain of the aryl hydrocarbon receptor nuclear translocator

The heterodimeric hypoxia inducible factor-1 (HIF-1) complex is composed of the hypoxia inducible factor-1 alpha (HIF-1α) and the aryl hydrocarbon receptor nuclear translocator (ARNT). Activation of the HIF-1 function is essential for tumor growth and metastasis. We previously showed that transfection of a plasmid containing an ARNT-interacting peptide (Ainp1) cDNA suppresses the HIF-1 signaling in Hep3B cells. Here we generated TAT fusion of the Ainp1 peptide (6His-TAT-Ainp1) to determine whether and how the Ainp1 peptide suppresses the HIF-1 function. The bacterially expressed 6His-TAT-Ainp1 was purified under denatured condition and then refolded by limited dialysis. The refolded 6His-TAT-Ainp1 interacts with the helix-loop-helix (HLH) domain of ARNT in a similar fashion as the native 6His-Ainp1. 6His-TAT-Ainp1 colocalizes with ARNT in the nucleus of HeLa and Hep3B cells after protein transduction. The transduced protein reaches the maximum intracellular levels within 2 h while remains detectable up to 96 h in HeLa cells. At 2 μM concentration, 6His-TAT-Ainp1 is not cytotoxic in HeLa cells but suppresses the cobalt chloride-activated, hypoxia responsive enhancer-driven luciferase expression in a dose-dependent manner. In addition, it decreases the cobalt chloride-dependent induction of the HIF-1 target genes at both the message (vascular endothelial growth factor and aldolase C) and protein (carbonic anhydrase IX and glucose transporter 1) levels. The protein levels of HIF-1α and ARNT are not altered in the presence of 6His-TAT-Ainp1. In summary, we provided evidence to support that the Ainp1 peptide directly suppresses the HIF-1 function by interacting with the ARNT HLH domain, and in turn interfering with the heterodimerization of HIF-1α and ARNT.

Suppression of the hypoxia inducible factor-1 function by redistributing the aryl hydrocarbon receptor nuclear translocator from nucleus to cytoplasm

The aryl hydrocarbon receptor nuclear translocator (ARNT) heterodimerizes with hypoxia inducible factor-1α (HIF-1α), followed by upregulation of genes that are essential for carcinogenesis. We utilized a novel peptide (Ainp1) to address whether the HIF-1α signaling could be suppressed by an ARNT-mediated mechanism. Ainp1 suppresses the HIF-1α-dependent luciferase expression in Hep3B cells and this suppression can be reversed by ARNT. Ainp1 reduces the interaction between ARNT and HIF-1α, suppresses the formation of the HIF-1 gel shift complex, and suppresses the ARNT recruitment to the vegf promoter. These effects are partly mediated by redistribution of the nuclear ARNT contents to the cytoplasm.

The aryl hydrocarbon receptor nuclear translocator-interacting protein 2 suppresses the estrogen receptor signaling via an Arnt-dependent mechanism

We explored whether modulation of the estrogen receptor (ER) signaling is possible through an aryl hydrocarbon receptor nuclear translocator (Arnt)-dependent mechanism. We utilized the Arnt-interacting protein 2 (Ainp2) to examine whether the presence of Ainp2 in MCF-7 cells would interfere with the Arnt-mediated ER signaling. We found that Arnt increased the 17 beta-estradiol (E2)-dependent luciferase activity and Ainp2 significantly suppressed this Arnt-mediated luciferase activity. Ainp2 significantly suppressed 25% of the E2- and Arnt-dependent up-regulation of the GREB1 message. No suppression of the ER target gene expression by Ainp2 was detected in Arnt-knockdown MCF-7 cells and in Arnt-independent ER signaling. Although Ainp2 did not interact with ER alpha and ER beta, it suppressed the ER alpha::Arnt interaction and reduced the E2-driven recruitment of Arnt to the GREB1 promoter. We concluded that Ainp2 suppresses the ER signaling by not allowing Arnt to participate in the ER-dependent, Arnt-mediated activation of gene transcription.