Targeting the aryl hydrocarbon receptor in stem cells to improve the use of food as medicine

Abstract

Purpose of review:

Intestinal stem cells, the most rapidly proliferating adult stem cells, are exquisitely sensitive to extrinsic dietary factors. Uncontrolled regulation of intestinal stem cells is closely linked to colon tumorigenesis. This review focuses on how dietary and microbial derived cues regulate intestinal stem cell functionality and colon tumorigenesis in mouse models by targeting the aryl hydrocarbon receptor (AhR).

Recent findings:

AhR, a ligand activated transcription factor, can integrate environmental, dietary and microbial cues to modulate intestinal stem cell proliferation, differentiation and their microenvironment, affecting colon cancer risk. Modulation of AhR activity is associated with many chronic diseases, including inflammatory bowel diseases where AhR expression is protective.

Summary:

AhR signaling controls the maintenance and differentiation of intestinal stem cells, influences local niche factors, and plays a protective role in colon tumorigenesis. Mounting evidence suggests that extrinsic nutritional/dietary cues which modulate AhR signaling may be a promising approach to colon cancer chemoprevention.

Introduction

Over the past decade, exciting advances have been made in the identification of stem cells, which replenish the intestinal epithelium every 3–5 days [1]. Lgr5 (leucine-rich-repeat-containing G-protein-coupled receptor 5, also known as Gpr49), a receptor for R-spondins [2], marks a long-lived pool of rapidly cycling stem cells in the small intestine and colon (approximately 6 per crypt) [3]. The distribution of Lgr5⁺ cells within stem cell-derived adenomas indicates that a stem cell/progenitor cell hierarchy is maintained in early neoplastic lesions [4]. By crossing stem-cell-specific Lgr5-EGFP-IRES-creER^T2 knockin mice to Apc^flox/flox mice, Barker et al unequivocally demonstrated that crypt stem cells are the cells-of-origin of intestinal cancer [4]. Indeed, with respect to irrefutable stem cell characterization, a single Lgr5⁺ intestinal stem cell can generate a continuously expanding, self-organizing epithelial structure reminiscent of normal gut [5]. In addition, it has been demonstrated that intestinal crypt homeostasis results from neutral competition between symmetrically dividing Lgr5⁺ stem cells [6]. Consistent with findings that tissue stem cells act as tumor-initiating “cancer stem cells”, cancer therapy has been used to promote the elimination of Lgr5⁺ stem cells inappropriately activated by oncogenic events [7]. Collectively, these data provide evidence indicating that Lgr5⁺ is a marker of adult stem cells, and that perturbations in their behavior drives cancer initiation and/or progression by regulating tumor growth and metastasis [8]. Interestingly, some of these properties are shared by other slow cycling crypt cells expressing Musashi-1, DCAMKL1, BMI1 and Lrig1[9].

Over the last several years, emerging data have shown that signals from both the local stem cell niche/microenvironment as well as circulating, systemic factors contribute to the regulation of stem cells [10, 11]. In this context, intestinal stem cell/ cancer stem cell fate, i.e., the balance between self-renewal and differentiation, is influenced by both the intrinsic metabolic state and extrinsic nutritional/dietary cues. In depth reviews on the metabolic requirements of stem cells and cancer stem cells have previously been reported [12–15]. With respect to nutritional and metabolic control of stem cell fate, recent studies have examined the effects of calorie restriction/fasting/ketogenic diet, obesigenic high fat diet, vitamin D and Ca²⁺; vitamin C, dietary fiber/butyrate and curcumin/omega-3 fatty acids [16–20]. One of the novel findings of these studies is the fact that rapidly cycling Lgr5⁺ stem cells are exquisitely sensitive to extrinsic dietary factors that modulate colon cancer risk. For example, dietary curcumin combined with omega-3 fatty acids synergistically reduced carcinogen-induced nuclear β-catenin levels in aberrant crypt foci, in part, by promoting p53-dependent signaling and targeted apoptosis in damaged colonic Lgr5⁺ stem cells at the cancer initiation stage [19]. In addition, mice with DNA-damaged Lgr5⁺ stem cells were highly responsive to this combination diet compared with DNA-damaged differentiated cells [19].

We recently demonstrated that some of the effects of diet on stem cells in the gut, e.g., modulation of Lgr5⁺ energy metabolism and cell number, are mediated by ligand induced activation of the aryl hydrocarbon receptor (AhR) [21, 22]. Thus, this review will focus on recent studies describing the effects of AhR signaling and precision nutrition on colonic stem cells and colon tumorigenesis.

AhR signaling pathway: direct vs indirect effects of diet

AhR is a ligand activated basic helix-loop-helix transcription factor that senses environmental xenobiotics and dietary- or microbiota-derived small molecules. Generally, AhR ligands are generated by combustion and chemical synthesis and are produced by plants and microorganisms. The synthetic AhR ligands, including halogenated aromatic hydrocarbons, such as 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), and polycyclic aromatic hydrocarbons, such as 3-methylcholanthrene (3MC), are generated from incomplete combustion or are formed as industrial by-products, and TCDD binds with high affinity to the AhR [23]. Naturally occurring ligands include phytochemicals such as polyphenolics, tryptophan metabolites, indole-3 carbinol related compounds, gut microbial metabolites, and photoproducts derived from tryptophan. Naturally occurring AhR ligands typically exhibit modest AhR binding affinities (K_D ~ 10⁻⁶ M) with variable metabolic half-lives; however, high physiological concentrations of some ligands, such as indole, indole-3-aldehyde, and kynurenine, can compensate for the low binding affinities resulting in AhR activation [24–26].

In the absence of ligands, the AhR is bound to several chaperone proteins in the cytoplasm including heat shock proteins 90 (hsp90), p23 and immunophilin related protein XAP2. Upon ligand binding, XAP2 dissociates from the cytosolic AhR complex, and the AhR-ligand complex is then translocated into the nucleus. Once in the nucleus, hsp90 and p23 are displaced by the AhR nuclear translocator (ARNT) [27, 28] to form a heterodimeric AhR-ARNT complex, which then interacts with cis-acting dioxin response elements (DREs) with a core sequence of 5’-TNGCGTG-3’ or 5’-CACGCNA-3’ on promoters of AhR-responsive genes, including several forms of cytochrome P450 (CYP1A1, CYP1A2, CYP1B1), AhR repressor (AHRR), glucuronosyl transferases (UGT1A1) and other phase II drug metabolizing enzymes [29–31]. CYP1A1, CYP1A2, and CYP1B1 limit the availability of AhR ligands by catalyzing their metabolism. AHRR can competitively bind to ARNT, preventing AhR-ARNT formation. Following transcription, AhR is then exported from the nucleus and degraded by the cytoplasmic proteasome [32]. The AhR can also regulate expression of genes that lack canonical DREs in their promoter regions, such as plasminogen activator inhibitor 1 (PAI-1) [33], independent of ARNT. For example, the recognition of non-canonical DREs in the PAI-1 promoter requires the interaction between AhR and the Kruppel-like factor (KLF) family member KLF6 [34]. In addition, AhR may act as an E3 ligase mediating protein degradation, of which, β-catenin is one of the most characterized targets, although this remains controversial [35–37]. Since β-catenin plays a pivotal role in Wnt signaling and upregulated Wnt signaling is commonly observed in colon cancer, ongoing studies are probing the relationship between AhR and β-catenin.

AhR mediated regulation of colonic stem cells

Trillions of microorganisms (microbiota), including bacteria, archaea and fungus, inhabit the large intestine [38], and produce many bioactive metabolites, including short chain fatty acids and tryptophan metabolites, which serve as endogenous AhR ligands [39]. Reduced levels of AhR ligands, such as indole-3-acetic acid and indole-3-sulfate, are observed in the feces and blood of germ free or antibiotic treated mice compared to control mice [40, 41]. Due to close proximity, gut microbiota-derived tryptophan metabolites are readily absorbed by colonic stem cells, thereby regulating their self-renewal, differentiation and functionality (Figure 1A). Importantly, recent findings suggest that intestinal epithelial cells serve as a gatekeeper, regulating the availability of AhR ligands to the host [42]. However, at present it is not known whether differentiated epithelial cells further metabolize gut microbiota derived AhR ligands to create a physiological concentration gradient along the crypt axis, including the colonic stem cell zone.

Figure 1.

AhR mediated regulation of intestinal stem cells. (A) Direct regulation of intestinal stem cells by AhR signaling. AhR activation by dietary or gut microbiota derived metabolites/ligands controls intestinal stem cell (ISC) proliferation and self-renewal and promotes their differentiation into enterocytes or goblet cells. (B) AhR mediated indirect regulation of intestinal stem cells by controlling the production of their niche factors. For example, AhR activation promotes the production of IL10, IL22 and PGE₂, and inhibits the secretion of IL13. These cytokines and PGE₂ serve as growth mediators, capable of modulating the maintenance of intestinal stem cells.

Colonic stem cells, marked by Lgr5, reside at the bottom region of crypts, are intermingled with Reg4⁺ deep crypt secretory cells [43]. Lgr5-EGFP-IRES-Cre^ERT2 knockin mice are a commonly used mouse model to study colonic stem cells, where green fluorescent protein (GFP) is a proxy for colonic stem cells. However, Lgr5-GFP reporter mice are mosaic and not all colonic stem cells are marked with GFP, sometimes causing inaccurate or even confusing assessment of treatment-related effects on colonic stem cells. To reduce the confounding effect of mosacism, our lab introduced a temporal controlled Rosa26^LSL-Tdtomato reporter into these mice. Cell lineage tracing revealed that AhR activation by TCDD (a high affinity AhR ligand) reduced the percentage of colonic stem cells, while intestinal stem cell specific AhR deletion promoted the expansion of colonic stem cells, which was consistent with a previous study [44]. Moreover, similar conclusions were drawn using an organoid culture model, which is deprived of submucosal stromal and immune cells, demonstrating that colonic stem cells are intrinsically regulated by AhR signaling.

In complementary experiments, AhR signaling modulated the functionality of colonic stem cells. For example, AhR activation decreased the clonogenic capacity and organoid growth of colonic stem or progenitor cells, while AhR KO promoted their stemness and inhibited differentiation of colonic stem cells toward goblet cells and enterocytes (Figure 1A) [44, 22]. Further analysis revealed that intestinal specific AhR KO promoted colonic stem cell proliferation and cell cycle progression [22]. From a mechanistic perspective, AhR acted as a transcriptional suppressor of FoxM1, a master regulator of cell proliferation and cell cycle progression. Inhibition of FoxM1 phenocopied the effects of AhR activation and attenuated AhR KO mediated phenotypes. It is noteworthy that 3,3’-diindolylmethane (DIM), an AhR agonist [45], effectively downregulates FoxM1 in various breast cancer cell lines and inhibits breast cancer cell growth [46]. In terms of the effects of AhR on stem cell maintenance, Metidji et al found that AhR KO potentiated Wnt signaling by downregulating the expression of Znrf3 and Rnf43 [44], which act as E3 ubiquitin ligases to target Wnt receptors for degradation [47]. Furthermore, AhR may serve as an E3 ligase for β-catenin degradation, suppressing Wnt signaling [36]. These observations are noteworthy, because Wnt signaling is required for maintanance of colonic stem cell renewal [48]. Interestingly, we did not detect altered Wnt signaling and β-catenin levels in colonic Lgr5⁺ stem cells following AhR activation or AhR KO, which is consistent with other groups [49, 37]. The discrepancies observed with respect to AhR and Wnt signaling pathways may be explained by differences in cell context, e.g., normal vs malignant transformed state. Future work is needed to determine whether AhR signaling directly regulates Wnt signaling and/or how AhR signaling affects Wnt signaling in the context of normal regenerative homeostasis vs tumorigenesis.

Even though AhR is a highly conserved transcription factor between mouse and human (82% similarity in sequence), some species related differences have been observed. For example, human AhR contains an alanine to valine substitution at codon 375 in the ligand binding domain, compared with murine AhR^b1 isoform, and this substitution results in a lower binding affinity to some ligands, including TCDD [50]. Importantly, it is noted that human AhR regulates different gene expression profiles [51]. Therefore, caution should be taken when translating mouse related data to humans, particularly with respect to AhR downstream targets. For example, inherent physiological differences between human and mouse immune cells can influence how endogenous ligands to AhR can modulate lymphocyte responses and anti-inflammatory sequelae in the intestine [52]. Consistent with mouse based studies, AhR can directly regulate the expression of FoxM1, and AhR activation reduces the percentage of human colonic stem cells and decreases the clonogenic capacity and organoid growth of human colonocytes, which are independent of β-catenin and pERK1/2 levels [22]. This is noteworthy, because lower levels of AhR ligands are observed in individuals with inflammatory bowel diseases, metabolic syndrome and obesity [53, 41]. It will be interesting to determine whether reduced AhR activity can affect the expansion of colonic stem cells and barrier permeability due to compromized cell differentiation and whether impaired AhR signaling accelerates premature colonic stem cell exhaustion in the aged. In addition, emerging studies have reported that AhR signaling plays an important role in modulating the dynamics and functionality of other stem cells, including hematopoietic stem cells (HSCs), neural stem cells and hepatic stem cells [54–56], implying that AhR broadly functions to regulate multiple stem cell populations in the body.

The regulation of colonic stem cell niche factors

Colonic stem cells are in close proximity to gut immune cells and mucosal stroma cells. This is noteworthy, because neighboring cells in the stem cell niche play an important role in shaping colonic AhR signaling. In particular, how AhR signaling regulates soluble mediators, such as IL22, IL10, and PGE₂, has attracted significant attention (Figure 1B). For example, AhR acts as a key transcription factor in controlling the differentiation of Th17, Treg cells, group 2 innate lymphoid cells (ILC2s) and group 3 innate lymphoid cells (ILC3s), in which Treg and ILC3 are two major IL22-producing immune cells in the gut, and Foxp3+ Treg cells are the major source of IL10 secretion [57]. Specifically, AhR activation by TCDD favorably induces Treg cell differentiation by directly promoting Foxp3 expression [58], a key lineage-specific transcription factor of Treg cells. Similarly, AhR signaling is required for ILC3 generation, even though the molecular mechanism remains unknown [59, 60]. Recently, it has been demonstrated that AhR activation intrinsically inhibits ILC2 cell differentiation and function [61]. AhR regulates chromatin accessibility at select gene loci, such as AhR and Interleukin 1 Receptor Like 1 (Il1rl1), which encodes IL33 receptor ST2. Moreover, AhR can bind to Il1rl1 promoter and directly suppress its expression, as evidenced by upregulated Il1rl1 mRNA in AhR^−/− ILC2s. The production of type 2 cytokines, such as IL5 and IL13, is partially dependent on the IL33-ST2 pathways [61]. Importantly, ILC2 derived IL13 has been shown to promote the differentiation of intestinal stem cells towards tuft and goblet cells [62].

AhR can directly control the production of IL10 and IL22. Upon binding to DREs in the IL22 promoter, AhR cooperatively interacts with RORγt and STAT3 to promote IL22 expression [63, 64] and the AhR synergistically interacts with c-Maf to directly control expression of IL10[65, 66]. Consequently, IL10 and IL22 derived from gut immune cells can modulate intestinal stem cell renewal and differentiation. For example, IL10 treatment or coculture of Treg cells with intestinal stem cells (ISCs) promotes the expansion of ISCs within organoids [57]. The regulation of ISC by IL22 remains controversial, where some studies indicate that IL22 treatment promotes ISC expansion and organoid growth [67, 57], while other groups report IL22 actually inhibits ISC expansion [68, 69]. Interestingly, in addition to IL10 and IL22, other cytokines have been shown to affect ISCs. For example, proinflammatory cytokines, such as IL17A, IFNγ and IL13, promote ISC differentiation [57]. However, little is known regarding how crypt neighboring immune cells or their cytokines, such as IL10 and IL22, influence other cell types, such as progenitor cells, and how they contribute to epithelial plasticity and crypt regeneration in the context of inflammation.

Several studies suggest that prostaglandin E₂ (PGE₂) plays an important role in tumorigenesis, crypt regeneration and inflammation [70–72]. AhR activation increases the production of PGE₂ in the colon, and inhibition of PGE₂ production abrogates the beneficial effects of TCDD on DSS-induced colitis [73]. The production of PGE₂ is primarily synthesized by pericryptal fibroblasts and/or colonic mesenchymal stem cells (cMSCs) [71, 70]. It will be interesting to explore whether and how AhR signaling regulates fibroblasts or cMSCs to enhance PGE₂ synthesis in the colon. Interestingly, AhR activation promotes the expression of prostaglandin-endoperoxide synthase 2 (COX-2) and microsomal prostaglandin E synthase-1 (mPGES) to enhance PGE₂ production in human lung fibroblasts [74]. However, little in known regarding whether PGE₂ signaling affects intestinal stem cells. Evidence from recent studies show that PGE₂ drives the expansion of Sca-1⁺ reserve-like stem cells by promoting yes-associated protein 1 (YAP) signaling, which promotes colon cancer stem cell expansion by activating NF-κB [71, 75]. In contrast, PGE₂-receptor 4 (EP₄) signaling reduces Lgr5⁺ intestinal stem cell pools both in human and mouse organoids [76, 77] and modulates intestinal stem cells, progenitor cells, and immature enterocytes to form wound associated epithelial cells [77]. Similarly, our lab demonstrated that PGE₂ signaling had no effect on the clonogenic capacity and organoid growth from sorted colonic stem cells [22]. Thus, PGE₂ exerts diverse effects on different) intestinal reserve noncycling vs rapidly cycling and normal vs cancer) stem cells.

Colon cancer

Since AhR signaling closely regulates colonic stem cells and their niches, and dysregulated intestinal stem cells are the cells-of-origin of intestinal cancer [4], it is not surprising that AhR signaling modulates colon tumorigenesis. Currently, three types of colon tumor models are utilized to assess the role of AhR signaling in regulating colon tumorigenesis (Table 1).

Table 1.

Summary of effects of AhR signaling on intestinal tumorigenesis in mouse models

Colon cancer models	AhR status	Effects	References
	Global KO	Spontaneously develop cecum tumors	[36]
ApcMin/+	Global KO or heterozygotes	Accelerates cecum tumorigenesis and promotes intestinal tumor multiplicity	[36]
	Activation by I3C and DIM	Indole-3-carbinol (I3C) and 3,3’-Diindoylmethane (DIM) delay cecum tumorigenesis and inhibits intestinal tumor multiplicity	[36]
AOM	Intestinal specific KO	Increases aberrant crypt foci or colon tumor multiplicity	[44, 85]
AOM/DSS	Global KO	Azoxymethane (AOM) and dextran sodium sulphate (DSS) combination promotes colon tumor multiplicity	[49]
	Activation by I3C	I3C reduces colon tumor incidence Inhibits colon tumor multiplicity	[49]
	Intestinal specific KO	Promotes colon tumor incidence, multiplicity, and size	[22, 44]

i. Genetically induced gastrointestinal tumor models.

Several colon genetic tumor models are utilized to recapitulate human hereditary and/or sporadic colorectal tumorigenesis [78, 79]. Typically, the adenomatous polyposis coli (Apc) gene is targeted since ~80% of sporadic colorectal tumors contain Apc mutations [80]. Apc^min/+ mice encode a nonsense mutation at codon 850, and are predisposed to developing intestinal adenomas [79]. Interestingly, Kawajiri et al reported that spontaneous cecal tumors were observed in AhR null mice and haploinsufficiency of AhR accelerated tumorigenesis in cecum and small intestinal in AhR^+/−; Apc^min/+ mice. The increased tumorigenesis in AhR^+/−; Apc^min/+ was associated with increased stability of β-catenin [36]. Interestingly, AhR activation by feeding dietary AhR ligands, e.g., indole-3-carbinol (I3C) and 3,3’-diindoylmethane (DIM), robustly decreased β-catenin level and significantly reduced cecal tumorigenesis and tumor burdens in Apc^min/+ mice [36].

Contradictory claims have surfaced regarding the functions of the AhR as an E3 ligase to degrade β-catenin [36, 37]. For example, AhR does not interact with β-catenin even in the presence of AhR interacting proteins, e.g. Arnt, CUL4B and DDB1, and AhR activation has no effect on TCF/β-catenin dependent transcription in human colon cancer cell lines [37]. Additional findings indicate that AhR null mice do not spontaneously develop tumors in the cecum or colon, and β-catenin and its target c-myc expression are not altered [49]. Contributing factors for these distinct outcomes include model related differences in AhR exon deletion (exon 1 vs exon 2), and the potential contribution of distinct communities of gut microbiota, since gut microbiota plays an important role in colon tumorigenesis [81, 82]. Moreover, germ free AhR null mice or mice lacking both the AhR and apoptosis-associated speck-like protein containing a CARD (ASC), exhibit a reduction in cecal tumorigenesis compared with conventional AhR KO mice [83], implying that inflammation plays a role in cecal and colon tumorigenesis in AhR null mice. In the future, it will be interesting to determine the epithelial role of AhR in different genetic colon tumor models, and whether AhR ligand supplementation suppresses colon tumorigenesis.

ii. Carcinogen induced colon tumorigenesis.

Azoxymethane (AOM) is one of the most commonly used inducers of colorectal cancer. AOM induced colorectal tumors recapitulate the multistage progression and histopathological characteristics of human colorectal cancer [84]. Intestinal specific AhR KO promotes stem cell proliferation and AOM induced aberrant crypt foci, premalignant colon tumor lesions, and colon tumors [85, 44]. Further analysis revealed that accumulated nuclear β-catenin is observed in AhR KO tumor masses. However, it is unclear if this accumulated nuclear β-catenin is the cause or result of more tumors in AhR KO mice. Metidji et al reported that intestinal specific KO or depletion of AhR ligands by overexpressing Cyp1a1 also increased the expression of IL-6, implying that impaired ligand-dependent AhR signaling in epithelial cells enhances gut inflammation [44].

iii. Colitis associated colon tumorigenesis.

Compared with carcinogen induced colon tumorigenesis, the preclinical colitis associated tumor model (AOM with DSS) accelerates the progression and multiplicity of colon tumors. In this context, AhR signaling also plays a protective role. For example, global AhR knockout promotes colitis-associated colon tumorigenesis, and AhR activation by feeding I3C decreases tumor incidence [49]. From a mechanistic perspective, the protective role of AhR in this colitis associated tumor model is not associated with altered Wnt signaling, but with decreased DNA damage and DSS induced inflammation [49]. Interestingly, global AhR KO mice are deficient with respect to IL22 production [42, 64], and IL22 signaling is required to initiate efficient DNA damage response after exposure to AOM [86]. In addition, studies using intestinal specific AhR KO mice unequivocally show that the loss of AhR in the gut epithelium increases colon tumor incidence and multiplicity [22, 44]. Metidji et al found that AhR KO potentiated Wnt signaling, as evidenced by increased β-catenin levels and Wnt target genes. In addition, dietary I3C supplementation decreased tumor multiplicity in R26^LSL-Cyp1a1; Villin-Cre mice [44], in which overexpression of Cyp1a1 in intestinal epithelial cells potentially depleted AhR ligands, effectively suppressing AhR activation [42]. Interestingly, I3C has no effect on colon tumorigenesis in intestinal specific AhR KO mice [44], in which I3C can potentially activate AhR in gut immune cells. In complementary studies, it has been demonstrated that AhR activation suppresses DSS induced inflammation and increases the production of IL22 and IL10, which can act on intestinal epithelial cells to suppress colon tumorigenesis [87–89, 86]. Hence, it is important to determine whether epithelial AhR KO indirectly affects AhR activation in non-epithelial immune cells.

Precision nutrition

AhR ligands, such as dietary- and intestinal microbiota-derived compounds may uniquely modulate gastrointestinal stem cells and the immune system. Importantly, the chronic reduction in cellular AhR activation has been linked to the suppression in AhR ligand production in patients with numerous chronic diseases, including inflammatory bowel disease, obesity, Type 2 diabetes and high blood pressure [87, 90, 53]. The defect in AhR agonist production appears to be in part, the result of an impaired capacity of gut microbiota to metabolize tryptophan into AhR agonists in mice and humans [53, 90]. These findings suggest that the substantial inter-individual variability in AhR-mediated response to dietary exposures is likely the result of microbiome influences. Moreover, polymorphisms of human AhR may also contribute to individual sensitivity to AhR ligand exposure [91–93], and several AhR single nucleotide polymorphisms (SNPs) have been significantly associated with many diseases [94–96]. Since “Precision Nutrition” aspires to offer individual tools to personalize dietary and lifestyle practices for optimal health [97], future research should explore relationships between host AhR biology and microbiome changes in response to specific probiotics and dietary prebiotic intervention.

Conclusion

Intestinal epithelial cells co-exist with gut microbiota and are frequently exposed to external environmental stimuli, where bioactive components from diet or gut microbiota derived metabolites can uniquely modulate crypt stem cells. AhR acts as an environmental sensor that can integrate environmental, dietary and gut microbial cues to modulate intestinal stem cells and their microenvironment, affecting colon cancer risk. AhR signaling regulates intestinal stem cells both intrinsically and extrinsically and constitutes an important axis to mediate interaction between gut microbiota, intestinal stem cells and immune cells in the stem cell niche. However, precisely how AhR signaling regulates this multi-cellular tripartite interaction in the context of homeostasis and inflammatory pathogenesis is still under investigation. In summary, current studies provide rationale for AhR as a potential therapeutic target to optimize and reduce the burden of chronic disease, and this is particularly true for the colon.