The importance of this pathway resides in its participation in the correct development of most organs [48,49,50]. It has been described that Wnt-mediated signaling needs to be in balance in order to obtain a proper development program, as both downregulation and over-signaling of Wnt can lead to several defects [51,52,53], including lung, breast and skin cancer [54].

Despite AHR and Wnt signaling have been long studied independently, numerous evidence of crosstalk between the two pathways have emerged in recent years [55]. Interestingly, it has been described that AHR and Wnt/β-catenin cooperate in the induction of AHR transcriptional targets, such as Cyp1a1 and Cyp1b1, with the persistent AHR activation triggering reduced levels of active β-catenin, affecting the phenotype of hepatic progenitor cells and leading them towards other more differentiated types [56]. Furthermore, AHR agonists such as indole-3-carbinol (I3C), 3,3′-diindolylmethane (DIM) and indirubin-3′-monoxime were able to downregulate CTNNB1(β-catenin) expression [57,58,59].

Subsequently, AHR was identified as an inhibitor of the canonical Wnt signaling pathway in mouse intestine, being able to suppress intestinal carcinogenesis by degradation of β-catenin [60], with activated AHR expression also downregulating CTNNB1 expression in human colon cancer cell lines [55]. In the human breast cancer cell line, constitutive expression of AHR by introduction of a mutation was able to negatively regulate CTNNB1 [55]. Furthermore, in mouse liver, elevated levels of β-catenin were detected in AHR -/- preweaning mice [61]. This alteration could be possible due to AHR being part of the repressive complex that binds to CTNNB1, promoting its ubiquitination and subsequent degradation [55,60], since both proteins co-immunoprecipitated under basal conditions in adult liver [61]. AHR ligands also decrease ABC levels and generate further alterations in the Wnt pathway, such as Dvl dephosphorylation [56]. On the other hand, TCDD-induced reduction of the canonical Wnt pathway has been associated with a decrease in R-Spondin2 (Rspo2) and R-Spondin3 (Rspo3) activators [62]. However, in zebrafish, AHR activation by TCDD was shown to upregulate canonical Wnt signaling by overexpression of R-Spondin1 (Rspo1) [63]. Such Rspo1 signaling is mediated by the co-receptor LRP6 (low-density lipoprotein receptor-related protein 6), which is required for TCDD to upregulate the Wnt pathway and inhibit regeneration in zebrafish. This is consistent with data indicating that the role of AHR is cell type, tissue, organ and animal model dependent [64,65,66].

The phosphatidylinositol 3-kinase (PI3K)/protein kinase B (AKT) signaling pathway is one of the key regulators of cell proliferation, cell cycle and apoptosis [67]. Moreover, its over activation and de-regulation is a common feature in human malignancies, making it an ideal candidate for drug-based cancer therapies [68].

The importance of AHR in such processes is notorious, presenting a relevant series of interactions with the pathway. Hence, AHR has been found to reduce apoptosis in a mouse hepatoma cell line due to increased AKT activation [69]. However, a recent in vivo study evidenced that there was more AKT activity in AHR -/- mouse liver, thus p-AKT target GSK3β is more efficiently phosphorylated in the liver of such knockout animals [61]. In fact, GSK3β constitutes a link between the Wnt-β-Catenin and PI3K/AKT pathways, being able to inhibit itself in the absence of AHR and ensuring the maintenance of cellular homeostasis. Furthermore, PI3K also was found to have a connection with the Ras pathway via regulation of the activation of mitogen-activated kinases (MAPKs), presenting a sustained PI3K-dependent ERK1/2 activation in AHR -/- mice liver [61,70].

AHR has also been reported to control AKT phosphorylation under basal conditions without the necessity to respond to growth factors, cigarette smoke extract (CSE) or AHR ligands. Due to the finding that in the absence of AHR there is differential phosphorylation of several proteins such as fibrillin and fibronectin, it is speculated that the increased phosphorylation of AKT in AHR-/-MLF (mouse lung fibroblast) could be associated with extracellular matrix (ECM) deregulation [71]. This apparent discordance between the results of different studies suggests that AHR regulation of basal AKT activity may be cell-type specific and/or reflect differences between primary and cancer cells, a well-known feature of AHR [17].

Transforming growth factors β (TGF-β) are cytokines that have an important role in proliferation, development, homeostasis and tumorigenesis [72,73]. It is known that TGF-β can be activated by mechanisms involving proteolytic cleavage of latent TGF-β binding protein (LTBP-1) and the release of thrombospondin-1 (TSP-1) [72,74]. Once activated, it binds to its membrane receptors, initiating the signaling pathway [75]. Due to the processes that present TGF-β participation, its crosstalk with AHR has been widely studied. Regarding this, in a mouse fibrosis liver model, AHR-/- mice showed higher levels of LTBP-1 protein, co-localizing with TGF-β1 and collagen accumulation. This LTBP-1 could be responsible for TGF-β activation through alterations of the proteases PA/plasmin, elastase and TSP-1 (Thrombospondin 1), all of them affected by the presence or absence of AHR [75]. In fact, in mouse liver and mouse embryonic fibroblasts (MEFs) from AHR-/- mice, a higher level of total and active TGF-β versus the AHR+/+ ones was reported, leading to a reduced proliferation rate and increased apoptosis [14,75,76]. Furthermore, the AHR knockout mice also presented a better skin wound healing process than the wild type ones, which could be a consequence of the higher cell migration pattern triggered by the over-activation of TGF-β pathway [77,78].

Interestingly, the interaction between AHR and TGF-β seems to be tissue-dependent. In fact, while AHR is able to repress TGF-β signaling in brain tumors [79], TGF-β is also needed for the maintenance of proper AHR expression levels in lymphocytes [80]. Moreover, the tandem TGF-β/Smad can dissociate the AHR/ARNT complex through the inhibition of CYP1A1-mediated metabolic activation of polycyclic aromatic hydrocarbons (PAHs), resulting in cell protection against carcinogenesis [81].

The modulation exerted by AHR in the inflammatory response via the nuclear factor kappa enhancer of activated B-cell light chains (NF-κβ) has been widely studied. The acute inflammatory response in macrophages is mediated by the recognition of microbial products by toll-like receptors (TLRs), and its activity is controlled by NF-κβ and RelA/p65 [82]. It has been shown that the RelB subunit of NF-κβ is physically associated with AHR in U937 macrophage cell line, whereas in TCDD-treated Hepa1c1c7 cells it was seen that the physical association involved AHR and RelA/p65 [83,84]. In fact, it has been revealed the ability of AHR to suppress NF-κβ/p65 signaling pathway in intestinal epithelium [85]. Interestingly, a similar pattern was found in a model of inflammatory response in lung, where AHR presented a negative regulatory capacity of such response through direct modulation of NF-κβ signaling [86]. Additionally, this association of AHR with RelA has been found to activate NF-κβ activity in order to upregulate interleukin-6 (IL-6) expression [87]. Consequently, there is thus a crosstalk between the NF-κβ and AHR pathways, in such a way AHR activation favors RelA/p65 protein degradation by ubiquitin–proteasome system (UPS) and lysosomes, resulting in decreased levels of proinflammatory cytokines in mouse macrophages [88]. In fact, such crosstalk between AHR and NF-κβ pathways could contribute to a variety of AHR responses during the different types and stages of chronic kidney disease (CKD) [89].

Therefore, the interaction between AHR, RelA, RelB and NF-κβ signaling pathways continues to be an interesting field of study since it was only initially unveiled [90].

Additionally, there are multiple proteins which has been described to affect AHR activity and vice versa. Their main interactions are highlighted in the Table 1.