Aryl hydrocarbon receptor ligands in cancer: friend and foe

Abstract

The aryl hydrocarbon receptor (AHR) is a ligand-activated transcription factor that is best known for mediating the toxicity and tumour-promoting properties of the carcinogen 2,3,7,8-tetrachlorodibenzo-p-dioxin, commonly referred to as ‘dioxin’. AHR influences the major stages of tumorigenesis — initiation, promotion, progression and metastasis — and physiologically relevant AHR ligands are often formed during disease states or during heightened innate and adaptive immune responses. Interestingly, ligand specificity and affinity vary between rodents and humans. Studies of aggressive tumours and tumour cell lines show increased levels of AHR and constitutive localization of this receptor in the nucleus. This suggests that the AHR is chronically activated in tumours, thus facilitating tumour progression. This Review discusses the role of AHR in tumorigenesis and the potential for therapeutic modulation of its activity in tumours.

Figure 1. Agonist-mediated activation of the AHR

The unliganded AHR resides in the cytoplasm of a cell, complexed with a dimer of HSP90 and the co-chaperone protein X-associated protein 2. The AHR contains both a nuclear localization and a nuclear export signal sequence and undergoes nucleocytoplasmic shuttling. Upon binding an agonist, the AHR complex translocates into the nucleus and ARNT mediates HSP90 displacement, leading to AHR/ARNT heterodimer formation. This heterodimer is capable of binding to a dioxin responsive element (DRE) with the sequence 5′-T/G/TCGTGA/CG/TA/T-3′. Both the AHR and ARNT can recruit coactivators, leading to transcription of a wide variety of genes. The AHR target gene CYP1A1 is almost totally dependent on AHR activity for expression and is highly induced by AHR activation through multiple DREs. CYP1A1 metabolizes a number of pro-carcinogens, such as benzo(a)pyrene, to intermediates that can react with DNA to form adducts, resulting in subsequent mutagenesis.

Figure 2. AHR activity within the tumor micro-environment

Tumor-associated AHR activity is elevated when compared to surrounding tissue, suggesting that AHR may influence tumor development. The mechanisms that govern enhanced tumor-associated AHR expression are unclear; notwithstanding, increased AHR expression is likely to elevate basal AHR activity within tumors, especially given the systemic omnipresence of AHR agonist ligands derived from xenobiotic, dietary and microbial sources. Indeed, malignant tissue exhibits enhanced nuclear localization of AHR together with increased expression of the prototypical AHR target gene CYP1A1, indicative of higher AHR transcriptional activity. The presence of immune cells (e.g. antigen-presenting cells, APC) within the tumor microenvironment, in conjunction with malignant cells, often exhibit enhanced expression of IDO and TDO ^, . These enzymes generate agonistic AHR ligands from tryptophan within the tumor micro-environment, thus adding to the AHR activation potential within tumors. The consequences of enhanced AHR expression and activities arising from systemic and tumor-derived AHR agonists have not been thoroughly investigated; however, such activation is likely to promote tumor growth. AHR activation and inflammatory cytokine signaling, a common feature of tumors, results in synergistic induction of pro-inflammatory factors, including IL6, exacerbating inflammation while simultaneously promoting diffentiation of immune-suppressive Treg cells through increased IL10, TGF, and VEGF expression. Systemic and tumor-localized generation of AHR ligands, heightened AHR expression/activity may establish a pro-inflammatory yet immune-suppressive tumor micro-environment, favoring tumor survival and escape from immune surveillance, which results in tumor progression.

Figure 3. Proposed mechanisms of cell cycle modulation by the AHR

Multiple mechanisms are proposed to account for the pro-/anti-proliferative action of AHR agonists observed with tumor cells in vitro. 1) Binding to AHR response elements in promoters of their respective genes, AHR acts as a direct transcriptional activator stimulating expression of growth factors epiregulin and amphiregulin. As mitogens, these contribute to the proliferation of tumor cells and may account for AHR agonist-mediated tumor cell expansion. 2) Agonist-activated AHR binds to the promoter of p27 (CDKN1B) enhancing its expression ^, . As an inhibitor of cyclin-dependent kinase activity, increased p27 limits phosphorylation of retinoblastoma protein (Rb) thus restricting E2F-dependent gene expression and progression through the cell cycle. 3) Association of AHR with Rb in an agonist-dependent manner attenuates both phosphorylation of Rb and liberation of E2F, resulting in cell cycle arrest and inhibition of proliferation . 4) Agonist activation of AHR promotes association with β-catenin and stimulation of a previously unrecognized ubiquitin ligase function of AHR ^, . Ubiquitination of β-catenin in an AHR-dependent fashion promotes proteosomal degradation, restricting cell cycle-dependent gene expression and proliferation. 5) In the absence of ligand, AHR forms a complex with cyclin D and the cyclin-dependent kinases CDK4/6, suppressing phosphorylation of Rb and subsequent E2F-mediated gene expression to promote cell cycle arrest . Exposure to AHR agonist favors the dissociation of the AHR/cyclinD/CDK complex to permit cell cycle progression and tumor cell proliferation. The contradictory nature of these mechanisms may reflect cell-type/culture-dependent differences and emphasize the need to investigate the effect of AHR ligands in the context of in vivo proliferation models.

Figure 4. Proposed role of the AHR in tumor metastasis

The dual effect of elevated AHR expression and localized AHR agonist generation by tumor and tumor-associated immune cells increases AHR activity, leading to initiation of epithelial-mesenchymal transition (EMT) and facilitating tumor cell migration, invasiveness, and metastasis. Heightened AHR activity promotes the expression of SLUG, which then inhibits E-cadherin expression thus decreasing cell adhesion. AHR-dependent expression of matrix metalloproteases (MMPs) and intra-cellular signaling factors, which promote cytoskeletal rearrangement (e.g. VAV3) render tumor cells increasingly motile . The presence of tumor-associated inflammatory cytokine signaling results in a self-sustaining synergistic loop in combination with AHR activation, which enhances cell motility . Elevated growth and pro-angiogenic factor gene expression elicited in an AHR-dependent manner provides an escape and proliferative route for motile tumor cells. The generation of AHR agonists by tumor-associated immune cells facilitates the differentiation of immune-suppressive Treg cells, which dampens the immune response to isolated motile tumor cells, thus allowing metastasis and the establishment of distant secondary tumors.