Regulatory crosstalk and interference between the xenobiotic and hypoxia sensing pathways at the AhR-ARNT-HIF1α signaling node

Abstract

The aryl hydrocarbon receptor (AhR) is a ligand-activated transcription factor that mediates many of the responses to toxic environmental chemicals such as TCDD or dioxin-like PCBs. To regulate gene expression, the AhR requires its binding partner, the aryl hydrocarbon receptor nuclear translocator (ARNT). ARNT is also required by the hypoxia-inducible factor-1α (HIF-1α), a crucial regulator of responses to conditions of reduced oxygen. The important role of ARNT in both the AhR and HIF-1α signaling pathways establishes a meaningful foundation for a possible crosstalk between these two vitally important signaling pathways. This crosstalk might lead to interference between the two signaling pathways and thus might play a role in the variety of cellular responses after exposure to AhR ligands and reduced oxygen availability. This review focuses on studies that have analyzed the effect of low oxygen environments and hypoxia-mimetic agents on AhR signaling and conversely, the effect of AhR ligands, with a special emphasis on PCBs, on HIF-1α signaling. We highlight studies that assess the role of ARNT, elucidate the mechanism of the crosstalk, and discuss the physiological implications for exposure to AhR-inducing compounds in the context of hypoxia.

Keywords: ARNT; Crosstalk; Environmental toxicants; Metabolism; Oxygen sensing.

Figure 1. Domain structures of various basic helix-loop-helix/Per-Arnt-Sim (bHLH/PAS) proteins

The AhR, HIF-1α (class I), and ARNT (class II) proteins each have three distinct domains that are crucial for their function. The bHLH domain plays a role in dimerization and DNA binding. The PAS A and PAS B domains function as secondary dimerization domains. The transactivation region plays a role in interactions with other transcription coactivators. aa = amino acids.

Figure 2. The aryl hydrocarbon receptor (AhR) signaling pathway

In its quiescent state, the AhR is bound to various repressive factors in the cytoplasm. Ligand (L) binding to the AhR activates the AhR and induces a conformational change which exposes a nuclear localization signal. After nuclear translocation, the AhR dimerizes with ARNT. The AhR: ARNT transcription factor complex binds to XRE sequences in target gene promoters. Interaction with multiple transcriptional coactivators is necessary to induce transcription. GTF: general transcription factors; hsp90: heat shock protein 90; PolII: RNA polymerase II; p23: prostaglandin E synthase 3/ hsp90 co-chaperone; TBP: TATA-box-binding protein; XAP2: AH receptor-interacting protein/ hepatitis B virus X-associated protein 2; XRE: xenobiotic response element.

Figure 3. The hypoxia-inducible HIF-1α signaling pathway

Under normal oxygen conditions (21% oxygen), HIF-1α proteins are proline hydroxylated by PHDs, ubiquitinated by VHL proteins and ultimately degraded via the proteaseome. Hypoxic conditions (< 6% oxygen) diminish PHD function and thus lead to HIF-1α stabilization. Non-hydroxylated HIF-1α translocates to the nucleus and dimerizes with ARNT. The HIF-1α:ARNT transcription factor complex binds to HRE sequences in target gene promoters. HRE: hypoxia response element; PHD: prolyl hydroxylase domain-containing enzymes; p300/CBP: histone actelytransferase p300/CREB-binding protein; VHL: von Hippel-Lindau tumor suppressor.

Figure 4. Crosstalk at the AhR-ARNT-HIF1α signaling node

Top of figure: the AhR is quiescent in the cytoplasm. Ligand (L) binding activates the AhR and initiates translocation to the nucleus. In the nucleus the AhR binds to ARNT to induce transcription of XRE-containing genes. Bottom of figure: normoxic conditions lead to HIF-1α hydroxylation and degradation. In contrast, hypoxic environments cause HIF-1α stabilization and translocation to the nucleus. In the nucleus HIF-1α binds to ARNT to induce transcription of HRE-containing genes.