The aryl hydrocarbon receptor (AhR) in the regulation of cell-cell contact and tumor growth

Abstract

The aryl hydrocarbon receptor (AhR) is a ligand-dependent transcription factor, which is activated by a large group of environmental pollutants including polycyclic aromatic hydrocarbons, dioxins and planar polychlorinated biphenyls. Ligand binding leads to dimerization of the AhR with aryl hydrocarbon receptor nuclear translocator and transcriptional activation of several xenobiotic phase I and phase II metabolizing enzymes, such as cytochrome P4501A1 and glutathione-S-transferase, respectively. Since phase I enzymes convert inert carcinogens to active genotoxins, the AhR plays a key role in tumor initiation. Besides this classical route, the AhR mediates tumor promotion and recent evidence suggests that the AhR also plays a role in tumor progression. To date, no mechanistic link could be established between the canonical pathway involving xenobiotic metabolism and AhR-dependent tumor promotion and progression. A hallmark of tumor promotion is unbalanced proliferation, whereas tumor progression is characterized by dedifferentiation, increased motility and metastasis of tumor cells. Tumor progression and presumably also tumor promotion are triggered by loss of cell-cell contact. Cell-cell contact is known to be a critical regulator of proliferation, differentiation and cell motility in vitro and in vivo. Increasing evidence suggests that activation of the AhR may lead to deregulation of cell-cell contact, thereby inducing unbalanced proliferation, dedifferentiation and enhanced motility. In line with this is the finding of increased AhR expression and malignancy in some animal and human cancers. Here, we summarize our current knowledge on non-canonical AhR-driven pathways being involved in deregulation of cell-cell contact and discuss the data with respect to tumor initiation, promotion and progression.

Fig. 1.

Canonical and non-canonical signaling pathways of the AhR. ( A ) Schematic representation of canonical AhR signaling pathway. The cytosolic AhR is complexed by two molecules of Hsp90, XAP2 and the co-chaperone p23. Binding of a ligand, e.g. TCDD, leads to a conformational change, thereby allowing nuclear translocation of the AhR complex. In the nucleus, the AhR dissociates from the complex and dimerizes with ARNT. The AhR–ARNT heterodimer then binds to xenobiotic-responsive elements (XREs) in the promoters of genes encoding for several phase I and phase II metabolizing enzymes but also several other genes, e.g. CYP2S1, COX2 or Slug ( 139 ). Recruitment of additional co-factors and factors of the basal transcription machinery (not shown) finally allows transcription of these genes. GSTM, glutathione- S -transferase M; NQO1, NAD(P)H:quinone oxidoreductase 1; UGT1A, uridine 5′-diphosphate-glucuronosyltransferase 1A; ALDH, aldehyde dehydrogenase ( B – D ) Schematic representation of examples of non-canonical AhR signaling. (B) In pRB-proficient cell lines, activation of the AhR by exogenous ligands may lead to direct interaction with pRB and via several mechanisms to inhibition of the transcription factor E2F ( 99 ). As a consequence, progression from G ₁ - to S-phase is blocked. Cell cycle arrest may also be induced by additional mechanisms, such as induction of p27 ( 36 , 140 ). (C) Reciprocal inhibitory effects between the nuclear factor kappa B (NF-κB) and the AhR pathway have been described. One consequence of inhibition of AhR signaling by NF-κB is attenuation of ligand-induced CYP1A1 expression. However, also cooperative effects of the AhR and NF-κB pathways are known ( 37 ). (D) The ligand-activated AhR directly associates with estrogen or androgen receptors (ERα or AR) and modulates their function both positively and negatively. Recently, it was shown that the AhR promotes the proteolysis of ERα/AR through assembling a ubiquitin ligase complex, CUL4B(AhR) ( 38 ).

Fig. 2.

Signaling cascade of contact inhibition in fibroblasts. Cell–cell contacts lead to a rapid activation of protein kinase Cδ (PKCδ) and a persistent activation of p38α MAPK resulting in accumulation of the KIP inhibitor p27, hence inhibiting cyclin E/Cdk2 activity. Additionally, the INK inhibitor p16 is up-regulated, thereby blocking activity of Cdk4. As a result, pRB remains in its hypophosphorylated state and does not allow transcription of S-phase-specific genes, such as cyclin A. If PKCδ plays a role in activation of p38α MAPK or p38α MAPK is involved in p16 up-regulation remains to be elucidated.

Fig. 3.

Schematic illustration of EMT. In epithelial cells, cell–cell adhesion is mediated by homophilic interactions of E-cadherin. One consequence is sequestration of β-catenin, thereby preventing its nuclear translocation. Over-expression of master regulators of EMT, such as Snail and Slug (and others) lead to down-regulation of E-cadherin, hence allowing nuclear translocation of β-catenin. In association with transcription factors (TF) of the TCF/LEF family, β-catenin induces transcription of proliferative and mesenchymal genes (see text for detail). This process is referred to as EMT.

Fig. 4.

Proposed novel non-canonical pathway of the AhR. According to the proposed novel non-canonical pathway, non-genotoxic ligands, such as TCDD, lead to activation of the AhR, and very likely independent from ARNT, to induction of JunD, which after heterodimerization with ATF2 results in transcriptional activation of cyclin A finally leading to a release from contact inhibition. If genotoxic ligands may also induce this novel pathway is currently not known.

Fig. 5.

Potential role of the AhR in EMT and tumor progression. Tumor progression is generally characterized by dedifferentiation of a cell from an epithelial to a mesenchymal phenotype and increased motility. A central process is down-regulation of E-cadherin. Transcriptional inhibition of E-cadherin may either be mediated by AhR-dependent activation of the transcriptional repressor Slug and/or activation of JNK. Breakdown of E-cadherin may also be achieved through activation of the NFAT/autotaxin/LPA pathway (see text for detail).

Fig. 6.

Hypothetical model of the role of AhR in tumor initiation, promotion and progression. Procarcinogens such as PAHs are known to activate the canonical xenobiotic-responsive element-dependent pathway, thereby leading to their conversion to genotoxic metabolites forming DNA adducts. Mutations are fixed by clonal expansion of initiated cells. Non-genotoxic AhR agonists, such as TCDD, are known to increase cell number, either by inhibition of apoptosis (not included in the figure) ( 24 ) or possibly by enhanced proliferation due to loss of contact inhibition providing a mechanistic basis for their tumor-promoting effects. AhR ligands may further lead to breakdown of E-cadherin function by regulating several key players of EMT, thereby driving the process of tumor progression.