The Environmental Sensor AHR Protects from Inflammatory Damage by Maintaining Intestinal Stem Cell Homeostasis and Barrier Integrity

Abstract

The epithelium and immune compartment in the intestine are constantly exposed to a fluctuating external environment. Defective communication between these compartments at this barrier surface underlies susceptibility to infections and chronic inflammation. Environmental factors play a significant, but mechanistically poorly understood, role in intestinal homeostasis. We found that regeneration of intestinal epithelial cells (IECs) upon injury through infection or chemical insults was profoundly influenced by the environmental sensor aryl hydrocarbon receptor (AHR). IEC-specific deletion of Ahr resulted in failure to control C. rodentium infection due to unrestricted intestinal stem cell (ISC) proliferation and impaired differentiation, culminating in malignant transformation. AHR activation by dietary ligands restored barrier homeostasis, protected the stem cell niche, and prevented tumorigenesis via transcriptional regulation of of Rnf43 and Znrf3, E3 ubiquitin ligases that inhibit Wnt-β-catenin signaling and restrict ISC proliferation. Thus, activation of the AHR pathway in IECs guards the stem cell niche to maintain intestinal barrier integrity.

Keywords: AHR; IBD; Wnt-β-catenin; colon cancer; crypt stem cell; diet; goblet cells; gut barrier; indole-3-carbinol; inflammation; intestinal epithelial cell.

Graphical abstract

Figure 1

Ahr Deficiency in IEC Impairs Resistance to Citrobacter rodentium (A) Survival plot of bone marrow chimeras infected with C. rodentium (WT→ WT; Ahr^−/− → WT; Ahr^−/− → Ahr^−/−; WT → Ahr^−/−); n = 5 per group. (B) Absolute numbers of colonic RORγt⁺ ILC3 and IL-17A-producing TCRβ⁺CD4⁺ T cells (WT, n = 7; Villin^CreAhr^fl/fl, n = 7) at day 7. (C) IL-22 protein content in colon explant cultures (WT, n = 12; Villin^CreAhr^fl/fl, n = 12). Data represent pooled results of at least two independent experiments. (D) qPCR analysis of antimicrobial (IL-22, Reg3g, S100a9) in colon from WT and Villin^CreAhr^fl/fl mice at day 7. (E) C. rodentium burdens in colon, liver, and spleen. Bars show the median and each symbol represents an individual mouse. (F) Colon sections stained for E-cadherin (green), C. rodentium (red), and DAPI (blue). Scale bars, 50 μm. (G) Survival plot of mice infected with C. rodentium (WT, n = 7; Villin^CreAhr^fl/fl, n = 7). (H) qPCR analysis of the goblet cell marker Muc2 and enterocyte marker Car4 (WT, n = 7; Villin^CreAhr^fl/fl, n = 7). (I) Representative image of AB-PAS staining in WT and Villin^CreAhr^fl/fl. Scale bars, 50 μm. (J) Quantification of the number of AB-PAS-positive cells per 20 crypts in WT and Villin^CreAhr^fl/fl. Error bars, mean + SEM. ^∗p < 0.05, ^∗∗p < 0.01, ^∗∗∗p < 0.001, ^∗∗∗∗p < 0.0001 as calculated by one-way ANOVA with Tukey post-test.

Figure 2

AHR Pathway Is Required for Epithelial Cell Differentiation from Crypt Stem Cells (A) Images of stem cell (SC) organoids from WT, Ahr^−/−, and R26^Cyp1a1 mice. Scale bars, 50 μm. (B) Percentage of EdU⁺ cells in SC organoid cultures. (C) Percentage of EdU⁺ cells in SC organoid cultures treated with 5 nM FICZ or 1 μM ICZ. (D and E) qPCR analysis of the goblet cell marker Muc2 and enterocyte marker Car4 in SC or differentiated (diff) organoids treated or not with 5 nM FICZ for 4 days. Data represent pooled results of at least two independent experiments (n = 6). Error bars, mean + SEM. ^∗p < 0.05, ^∗∗p < 0.01, ^∗∗∗p < 0.001, ^∗∗∗∗p < 0.0001 as calculated by one-way and two-way ANOVA with Tukey post-test.

Figure 3

Dysregulated Ahr in IECs Leads to Enhanced Stem Cell Proliferation and Increased Inflammation (A) Flow cytometry analysis of Lgr5 and Ki-67 expression in EpCam⁺CD45⁻ cells and absolute number of Lgr5⁺Ki-67⁺ cells at steady state from 5- to 8-week-old mice. (B and C) qPCR analysis of the goblet cell marker Muc2 and enterocyte marker Car4 from sorted EpCam⁺ cells (WT, n = 6; Villin^CreAhr^fl/fl, n = 6; Villin^CreR26^LSL-Cyp1a1, n = 6) in young (5 to 9 weeks) and old (14 to 16 weeks) mice. (D and E) IL-6 protein content in colon explant cultures at steady state (WT, n = 6; Villin^CreAhr^fl/fl, n = 6; Villin^CreR26^LSL-Cyp1a1, n = 6) from young and old mice. (F) Number of colon tumors in WT (n = 8), Villin^CreAhr^fl/fl (n = 8), and Villin^CreR26^LSL-Cyp1a1 (n = 8) mice injected with 10 mg/kg of azoxymethane once a week for 6 weeks. Representative image of colon in mice 22 weeks after the first azoxymethane injection. Error bars, mean + SEM., ^∗∗p < 0.01, ^∗∗∗p < 0.001, ^∗∗∗∗p < 0.0001 as calculated by one-way ANOVA with Tukey post-test. See also Figures S1A and S1B.

Figure 4

Inflammation-Induced Tumorigenesis in Mice with Ahr-Deficient Epithelium (A) Number of colon tumors in WT (n = 10), Villin^CreAhr^fl/fl (n = 10), and Villin^CreR26^LSL-Cyp1a1 (n = 9) mice. (B) Representative image of colon after AOM/DSS treatment. (C) Size of colon tumors in WT (n = 10), Villin^CreAhr^fl/fl (n = 10), and Villin^CreR26^LSL-Cyp1a1 (n = 9) mice. (D) Scoring of colon tumors Villin^CreAhr^fl/fl (n = 10) and Villin^CreR26^LSL-Cyp1a1 (n = 9) mice. (E) Representative images of hematoxylin and eosin (H&E) (top) and β-catenin staining of whole intestine (middle) and focus (bottom) of the indicated mice 60 days after injection of azoxymethane. Scale bars, 50 μm. (F) qPCR analysis of Wnt-negative regulators (Znrf3 and Rnf43) and WNT target genes (Axin2, cMyc, and Ephb2) in the colon (WT, n = 5; Villin^CreAhr^fl/fl, n = 5; Villin^CreR26^LSL-Cyp1a1, n = 5) after AOM/DSS treatment. (G) Chromatin immunoprecipitation (ChIP) analysis of AHR interaction with Rnf43 promoter from lanima propria of WT, WT treated with FICZ, Ahr^−/−, and Ahr^−/− treated with FICZ. Error bars, mean + SEM. ^∗p < 0.05, ^∗∗p < 0.01, ^∗∗∗p < 0.001, ^∗∗∗∗p < 0.0001 as calculated by one-way and two-way ANOVA with Tukey post-test. See also Figures S2A–S2D.

Figure 5

Dietary AHR Ligands Prevent Tumorigenesis and Restore Regulation of the WNT Pathway (A) qPCR analysis of WNT negative regulators (Znrf3 and Rnf43) and WNT target genes (Axin2, cMyc, and Ephb2) in the colon of Villin^CreR26^LSL-Cyp1a1 mice fed purified or I3C diet (n = 5). (B) Number of colon tumors in WT (n = 10), Villin^CreAhr^fl/fl, and Villin^CreR26^LSL-Cyp1a1 (n = 10), fed purified or I3C diet after AOM/DSS treatment. (C) Representative images of hematoxylin and eosin (H&E) and β-catenin of colon tumors in Villin^CreR26^LSL-Cyp1a1 fed purified or I3C diet. Scale bars, 50 μm. (D) qPCR analysis of Muc2 and Car4 in colon from WT (n = 5) and Villin^CreR26^LSL-Cyp1a1 (n = 5), fed purified or I3C diet. (E) Representative images of Ki-67 of colon tumors in Villin^CreR26^LSL-Cyp1a1 mice fed purified or I3C diet. Scale bars, 50 μm. (F) Quantification of the number of Ki-67⁺ cells in Villin^CreR26^LSL-Cyp1a1 mice fed purified or I3C diet. Error bars, mean + SEM. ^∗p < 0.05, ^∗∗p < 0.01, ^∗∗∗∗p < 0.0001, as calculated by unpaired t test and two-way ANOVA with Tukey post-test. See also Figures S3A and S3B.

Figure 6

Dietary AHR Ligands Can Halt Progression of Tumorigenesis (A) Villin^CreR26^LSL-Cyp1a1 were injected with 10 mg/kg of AOM followed with one cycle of 1% DSS on standard chow diet. For the second cycle of DSS, mice were put either on a purified diet or I3C diet until the end of the experiment. (B) Number of colon tumors in WT (n = 10), Villin^CreAhr^fl/fl, and Villin^CreR26^LSL-Cyp1a1 (n = 10) mice, fed purified or I3C diet after AOM/DSS treatment. (C) Representative image of colon after AOM/DSS treatment. (D and E) Size (D) and score (E) of tumors in Villin^CreR26^LSL-Cyp1a1 mice fed purified or I3C diet. (F) Representative images of hematoxylin and eosin (H&E) colon tumors in Villin^CreR26^LSL-Cyp1a1 mice fed purified or I3C diet. Scale bars, 50 μm. Error bars, mean + SEM. ^∗∗∗p < 0.001, as calculated by paired t test.