Aryl hydrocarbon receptor knockout accelerates PanIN formation and fibro-inflammation in a mutant Kras-driven pancreatic cancer model.

OBJECTIVES: The pathogenesis of pancreas cancer (PDAC) remains poorly understood, hindering efforts to develop a more effective therapy for PDAC. Recent discoveries show the aryl hydrocarbon receptor (AHR) plays a crucial role in the development of several cancers, and can be targeted for therapeutic effect. However, its involvement in the pathogenesis of PDAC remains unclear. To address this gap, we evaluated the role of AHR in the development of PDAC pre-cancerous lesions in vivo. METHODS: We created a global AHR-null, mutant Kras-driven PDAC mouse model (A-/-KC) and evaluated the changes in PDAC precursor lesion formation (Pan-IN 1, 2, and 3) and associated fibro-inflammation between KC and A-/-KC at 5 months of age. We then examined the changes in the immune microenvironment followed by single-cell RNA-sequencing analysis to evaluate concomitant transcriptomic changes. RESULTS: We identified a significant increase in PanIN-1 lesion formation and PanIN-1 associated fibro-inflammatory infiltrate in A-/-KC vs KC mice. This was associated with significant changes in the adaptive immune system, particularly a decrease in the CD4+/CD8+ T-cell ratio, as well as a decrease in the T-regulatory/Th17 T-cell ratio suggesting unregulated inflammation. CONCLUSION: These findings show the loss of AHR results in heightened Kras-induced PanIN formation, through modulation of immune cells within the pancreatic tumor microenvironment.

DNAJB1-PRKACA fusion protein-regulated LINC00473 promotes tumor growth and alters mitochondrial fitness in fibrolamellar carcinoma.

Fibrolamellar carcinoma (FLC) is a rare liver cancer that disproportionately affects adolescents and young adults. Currently, no standard of care is available and there remains a dire need for new therapeutics. Most patients harbor the fusion oncogene DNAJB1-PRKACA (DP fusion), but clinical inhibitors are not yet developed and it is critical to identify downstream mediators of FLC pathogenesis. Here, we identify long noncoding RNA LINC00473 among the most highly upregulated genes in FLC tumors and determine that it is strongly suppressed by RNAi-mediated inhibition of the DP fusion in FLC tumor epithelial cells. We show by loss- and gain-of-function studies that LINC00473 suppresses apoptosis, increases the expression of FLC marker genes, and promotes FLC growth in cell-based and in vivo disease models. Mechanistically, LINC00473 plays an important role in promoting glycolysis and altering mitochondrial activity. Specifically, LINC00473 knockdown leads to increased spare respiratory capacity, which indicates mitochondrial fitness. Overall, we propose that LINC00473 could be a viable target for this devastating disease.

The circadian clock is disrupted in pancreatic cancer.

Disruption of the circadian clock is linked to cancer development and progression. Establishing this connection has proven beneficial for understanding cancer pathogenesis, determining prognosis, and uncovering novel therapeutic targets. However, barriers to characterizing the circadian clock in human pancreas and human pancreatic cancer-one of the deadliest malignancies-have hindered an appreciation of its role in this cancer. Here, we employed normalized coefficient of variation (nCV) and clock correlation analysis in human population-level data to determine the functioning of the circadian clock in pancreas cancer and adjacent normal tissue. We found a substantially attenuated clock in the pancreatic cancer tissue. Then we exploited our existing mouse pancreatic transcriptome data to perform an analysis of the human normal and pancreas cancer samples using a machine learning method, cyclic ordering by periodic structure (CYCLOPS). Through CYCLOPS ordering, we confirmed the nCV and clock correlation findings of an intact circadian clock in normal pancreas with robust cycling of several core clock genes. However, in pancreas cancer, there was a loss of rhythmicity of many core clock genes with an inability to effectively order the cancer samples, providing substantive evidence of a dysregulated clock. The implications of clock disruption were further assessed with a Bmal1 knockout pancreas cancer model, which revealed that an arrhythmic clock caused accelerated cancer growth and worse survival, accompanied by chemoresistance and enrichment of key cancer-related pathways. These findings provide strong evidence for clock disruption in human pancreas cancer and demonstrate a link between circadian disruption and pancreas cancer progression.

Pdx1 expression in hematopoietic cells activates Kras-mutation to drive leukemia in KC (Pdx1-Cre; LSL-KrasG12D/+) mice.

The highly utilized KC model has a reported lethality rate of about 30%, which has been attributed to pancreas cancer. However, a competing cause of lethality in KC mice is due to the activation of mutant-Kras gene (KrasG12D/+) in the multipotent progenitor cells (MPP), and subsequent development of Kras-mutant T-cell acute lymphoblastic leukemia (T-ALL). Overall, 20% (5/25) of KC mice developed T-ALL by 9 months of age. Transplantation of pooled bone marrow from KC mice into CD45 congenic mice caused T-ALL in 100% of recipient mice, confirming that mutant-Kras expression in the hematologic compartment is driving the development of T-ALL in the KC mouse model. These results are an essential consideration for investigators using this model. Further, the lower penetrance of T-ALL in KC mice (versus existing leukemia models) suggests this model could be considered as an alternative research model to evaluate onset and factors that exacerbate the development of T-ALL.

Chronic jetlag accelerates pancreatic neoplasia in conditional Kras-mutant mice.

Misalignment of the circadian clock compared to environmental cues causes circadian desynchrony, which is pervasive in humans. Clock misalignment can lead to various pathologies including obesity and diabetes, both of which are associated with pancreatic ductal adenocarcinoma - a devastating cancer with an 80% five-year mortality rate. Although circadian desynchrony is associated with an increased risk of several solid-organ cancers, the correlation between clock misalignment and pancreas cancer is unclear. Using a chronic jetlag model, we investigated the impact of clock misalignment on pancreas cancer initiation in mice harboring a pancreas-specific activated Kras mutation. We found that chronic jetlag accelerated the development of pancreatic cancer precursor lesions, with a concomitant increase in precursor lesion grade. Cell-autonomous knock-out of the clock in pancreatic epithelial cells of Kras-mutant mice demonstrated no acceleration of precursor lesion formation, indicating non-cell-autonomous clock dysfunction was responsible for the expedited tumor development. Therefore, we applied single-cell RNA sequencing over time and identified fibroblasts as the cell population manifesting the greatest clock-dependent changes, with enrichment of specific cancer-associated fibroblast pathways due to circadian misalignment.

Aryl hydrocarbon receptor knockout accelerates PanIN formation and fibro-inflammation in a mutant Kras-driven pancreatic cancer model.

Objectives: The pathogenesis of pancreas cancer (PDAC) remains poorly understood, hindering efforts to develop a more effective therapy for PDAC. Recent discoveries show the aryl hydrocarbon receptor (AHR) plays a crucial role in the pathogenesis of several cancers, and can be targeted for therapeutic effect. However, its involvement in PDAC remains unclear. Therefore, we evaluated the role of AHR in the development of PDAC in vivo. Methods: We created a global AHR-null, mutant Kras-driven PDAC mouse model (A-/-KC) and evaluated the changes in PDAC precursor lesion formation (Pan-IN 1, 2, and 3) and associated fibro-inflammation between KC and A-/-KC at 5 months of age. We then examined the changes in the immune microenvironment followed by single-cell RNA-sequencing analysis to evaluate concomitant transcriptomic changes. Results: We found a significant increase in PanIN-1 lesion formation and PanIN-1 associated fibro-inflammatory infiltrate in A-/-KC vs KC mice. This was associated with significant changes in the adaptive immune system, particularly a decrease in the CD4+/CD8+ T-cell ratio, as well as a decrease in the T-regulatory/Th17 T-cell ratio suggesting unregulated inflammation. Conclusion: These findings show the loss of AHR results in heightened Kras-induced PanIN formation, through modulation of immune cells within the pancreatic tumor microenvironment.

Sex-dependent development of Kras-induced anal squamous cell carcinoma in mice.

Anal squamous cell carcinoma (SCC) will be diagnosed in an estimated 9,080 adults in the United States this year, and rates have been rising over the last several decades. Most people that develop anal SCC have associated human papillomavirus (HPV) infection (~85-95%), with approximately 5-15% of anal SCC cases occurring in HPV-negative patients from unknown etiology. This study identified and characterized the Kras-driven, female sex hormone-dependent development of anal squamous cell carcinoma (SCC) in the LSL-KrasG12D; Pdx1-Cre (KC) mouse model that is not dependent on papillomavirus infection. One hundred percent of female KC mice develop anal SCC, while no male KC mice develop tumors. Both male and female KC anal tissue express Pdx1 and Cre-recombinase mRNA, and the activated mutant KrasG12D gene. Although the driver gene mutation KrasG12D is present in anus of both sexes, only female KC mice develop Kras-mutant induced anal SCC. To understand the sex-dependent differences, KC male mice were castrated and KC female mice were ovariectomized. Castrated KC males displayed an unchanged phenotype with no anal tumor formation. In contrast, ovariectomized KC females demonstrated a marked reduction in anal SCC development, with only 15% developing anal SCC. Finally, exogenous administration of estrogen rescued the tumor development in ovariectomized KC female mice and induced tumor development in castrated KC males. These results confirm that the anal SCC is estrogen mediated. The delineation of the role of female sex hormones in mediating mutant Kras to drive anal SCC pathogenesis highlights a subtype of anal SCC that is independent of papillomavirus infection. These findings may have clinical applicability for the papillomavirus-negative subset of anal SCC patients that typically respond poorly to standard of care chemoradiation.

Chronic jetlag-induced alterations in pancreatic diurnal gene expression.

Cell-autonomous circadian clocks exist in nearly every organ and function to maintain homeostasis through a complex series of transcriptional-translational feedback loops. The response of these peripheral clocks to external perturbations, such as chronic jetlag and shift work, has been extensively investigated. However, an evaluation of the effects of chronic jetlag on the mouse pancreatic transcriptome is still lacking. Herein, we report an evaluation of the diurnal variations encountered in the pancreatic transcriptome following exposure to an established chronic jetlag protocol. We found approximately 5.4% of the pancreatic transcriptome was rhythmic. Following chronic jetlag, we found the number of rhythmic transcripts decreased to approximately 3.6% of the transcriptome. Analysis of the core clock genes, which orchestrate circadian physiology, revealed that nearly all exhibited a shift in the timing of peak gene expression-known as a phase shift. Similarly, over 95% of the rhythmically expressed genes in the pancreatic transcriptome exhibited a phase shift, many of which were found to be important for metabolism. Evaluation of the genes involved in pancreatic exocrine secretion and insulin signaling revealed many pancreas-specific genes were also rhythmically expressed and several displayed a concomitant phase shift with chronic jetlag. Phase differences were found 9 days after normalization, indicating a persistent failure to reentrain to the new light-dark cycle. This study is the first to evaluate the endogenous pancreatic clock and rhythmic gene expression in whole pancreas over 48 h, and how the external perturbation of chronic jetlag affects the rhythmic expression of genes in the pancreatic transcriptome.

Generation of an Allelic Series at the Ahr Locus Using an Edited Recombinant Approach.

The aryl hydrocarbon receptor (AHR) is a ligand-activated transcription factor and a member of the PER-ARNT-SIM (PAS) superfamily of environmental sensors. The AHR is involved in a series of biological processes including adaptive metabolism of xenobiotics, toxicity of certain environmental pollutants, vascular development, fertility, and immune function. Mouse models, including the Ahr null and Ahr conditional null (Ahrfx) mice, are widely used for the study of AHR-mediated biology and toxicity. The Ahr conditional null mouse harbors the low-affinity Ahrd allele that exhibits approximately a 10-fold lower binding affinity for certain xenobiotic AHR ligands than the widely used C57BL/6 mouse that harbors the higher affinity Ahrb1 allele. Here, we report a novel mouse model that introduces a V375A polymorphism that converts the low-affinity allele into a high-affinity allele, offering a more sensitive conditional model. In the generation of this novel conditional allele, two additional mutants arose, including a 3-bp deletion in the PAS-B domain (AhrNG367R) and an early termination codon in the PAS-B domain (AhrTer383). The AhrNG367R allele presents as a phenocopy of the null and the AhrTer383 allele presents as an antimorph when assessing for the ductus venosus and liver lobe weight endpoints. These new models represent a series of tools that will be useful in further characterizing AHR biology.

Curcumin Conjugates of Non-steroidal Anti-Inflammatory Drugs: Synthesis, Structures, Anti-proliferative Assays, Computational Docking, and Inflammatory Response.

In an effort to combine the anti-proliferative effect of CUR-BF2 and CUR compounds with anti-inflammatory benefits of non-steroidal anti-inflammatory drugs (NSAIDs), a library of the bis- and mono-NSAID/CUR-BF2 and NSAID/CUR conjugates were synthesized by coupling flufenamic acid, flurbiprofen, naproxen, indomethacin, and ibuprofen to diversely substituted hydroxy-benzaldehydes via an ester linkage, and by subsequent reaction with acetylacetone-BF2 to form the bis- and the mono-NSAID/CUR-BF2 adducts. Since conversion to NSAID/CUR by the previously developed decomplexation protocol showed limited success, a set of NSAID/CUR conjugates were independently prepared by directly coupling the NSAIDs with parent curcumin. The bis-NSAID/CUR-BF2 and bis-NSAID-CUR hybrids exhibited low cytotoxicity in NCI-60 assay, and in independent cell viability assay on colorectal cancer (CRC) cells (HCT116, HT29, DLD-1, RKO, SW837, CaCo2) and in normal CR cells (CCD841CoN). By contrast, the mono-naproxin and mono-flurbiprofen CUR-BF2 adducts exhibited remarkable anti-proliferative and apoptopic activity in NCI-60 assay most notably against HCT-116 (colon), OVCAR-3 (ovarian), and ACHN (renal) cells. Computational molecular docking calculations showed favorable binding energies to HER2, VEGFR2, BRAF, and Bcl-2 as well as to COX-1 and COX-2, which in several cases exceeded known inhibitors. The main interactions between the ligands and the proteins were hydrophobic, although several hydrogen bonds were also observed. A sub-set of six compounds that had exhibited little or no cytotoxicity were tested for their anti-inflammatory response with THP-1 human macrophages in comparison to parent NSAIDs or parent curcumin.

Deuterated Curcuminoids: Synthesis, Structures, Computational/Docking and Comparative Cell Viability Assays against Colorectal Cancer.

A series of deuterated curcuminoids (CUR) were synthesized, bearing two to six OCD3 groups, in some cases in combination with methoxy groups, and in others together with fluorine or chlorine atoms. A model ring-deuterated hexamethoxy-CUR-BF2 and its corresponding CUR compound were also synthesized from a 2,4,6-trimethoxybenzaldehyde-3,5-d2 precursor. As with their protio analogues, the deuterated compounds were found to remain exclusively in the enolic form. The antiproliferative activities of these compounds were studied by in vitro bioassays against a panel of 60 cancer cell lines, and more specifically in human colorectal cancer (CRC) cells (HCT116, HT29, DLD-1, RKO, SW837, and Caco2) and in normal colon cells (CCD841CoN). The deuterated CUR-BF2 adducts exhibited better overall growth inhibition by NCI-60 assay, while for other CUR-BF2 adducts the non-deuterated analogues were more cytotoxic. Results of the more focused comparative cell viability assays followed the same trend, but with some variation depending on cell lines. The CUR-BF2 adducts exhibited significantly higher cytotoxicity than CURs. Structural studies (X-ray and DFT) and computational molecular docking calculations comparing their inhibitory efficacy with those of known anticancer agents used in chemotherapy are also reported.

Synthesis, Computational Docking Study, and Biological Evaluation of a Library of Heterocyclic Curcuminoids with Remarkable Antitumor Activity.

In a continuing search for curcuminoid (CUR) compounds with antitumor activity, a novel series of heterocyclic CUR-BF2 adducts and CUR compounds based on indole, benzothiophene, and benzofuran along with their aryl pyrazoles were synthesized. Computational docking studies were performed to compare binding efficiency to target proteins involved in specific cancers, namely HER2, proteasome, VEGFR, BRAF, and Bcl-2, versus known inhibitor drugs. The majority presented very good binding affinities, similar to, and even more favorable than those of known inhibitors. The indole-based CUR-BF2 and CUR compounds and their bis-thiocyanato derivatives exhibited high anti-proliferative and apoptotic activity by in vitro bioassays against a panel of 60 cancer cell lines, more specifically against multiple myeloma (MM) cell lines (KMS11, MM1.S, and RPMI-8226) with significantly lower IC50 values versus healthy PBMC cells; they also exhibited higher anti-proliferative activity in human colorectal cancer cells (HCT116, HT29, DLD-1, RKO, SW837, and Caco2) than the parent curcumin, while showing notably lower cytotoxicity in normal colon cells (CCD112CoN and CCD841CoN).

The aryl hydrocarbon receptor as an antitumor target of synthetic curcuminoids in colorectal cancer.

BACKGROUND: Curcumin has proven to be a potent antitumor agent in both preclinical and clinical models of colorectal cancer (CRC). It has also been identified as a ligand of the transcription factor known as the aryl hydrocarbon receptor (AHR). Our laboratory has identified the AHR as a mechanism which contributes to both tumorigenesis in a mouse model of inflammatory CRC as well an apoptotic target in vitro. Curcumin's role as an AHR ligand may modulate its effects to induce colon cancer cell death, and this role may be enhanced via structural modification of the curcumin backbone. We sought to determine if the two piperidone analogs of curcumin, RL66 and RL118, exhibit more robust antitumor actions than their parent compound in the context of colorectal cancer in vitro. Moreover, to ascertain the ability of curcumin, RL66 and RL118 to activate the AHR and evaluate if this activation has any effect on CRC cell death. MATERIALS AND METHODS: DLD1, HCT116, LS513, and RKO colon cell lines were propagated in vitro. Natural curcumin was obtained commercially, whereas RL66 and RL118 were synthesized and characterized de novo. Multiwell fluorescent/luminescent signal detection was used to simultaneously ascertain cell viability, cell cytonecrosis, and relative amounts of apoptotic activity. AHR activity was measured with a dual luciferase reporter gene system. Stable expression of small interfering RNA interference was established in the HCT116 cell lines to create AHR "knock down" cell lines. RESULTS: Both RL66 and RL118 proved to be more potent antitumor agents than their parent compound curcumin in all cell lines tested. The majority of this cell death was due to induction of apoptosis, which occurred earlier and to a greater degree following RL66 and RL118 treatment as opposed to curcumin. Also, RL66 and RL118 were found to be activators of AHR, and a portion of their ability to cause cell death was dependent on this induction. Curcumin was found unable to activate the AHR, and levels of AHR messenger RNA did not change their effects on cell death. CONCLUSIONS: Piperidone analogs of curcumin exhibited enhanced antitumor effects in vitro as opposed to their parent compound. Even more, this enhanced cell death profile may be partially attributed to the ability of these compounds to activate the AHR. Further study of synthetic curcumin analogs as chemopreventives and chemoadjuncts in CRC is warranted. Also, more generally, the AHR may represent a potential putative target for novel anticancer agents for CRC.

Indole-3-carbinol induces tumor cell death: function follows form.

BACKGROUND: Even with colonoscopy screening and preventive measures becoming more commonplace, colorectal cancer (CRC) remains the third leading cause of oncologic death in the United States as of 2014. Many chemotherapeutics exist for the treatment of colorectal cancer, though they often come with significant side effect profiles or narrow efficacy ranges in terms of patient profile. Dietary phytochemicals such as glucobrassicin and its metabolite indole-3-carbinol (I3C) have been implicated in tumor prevention in many preclinical models across a variety of gastrointestinal tumors and represent an intriguing new class of natural chemotherapeutics for CRC. I3C has been identified as a ligand of the aryl hydrocarbon receptor (AHR), and we aimed to characterize this AHR activation in relation to its cytotoxic properties. METHODS: Human colorectal cancer cell lines DLD1, HCT116, HT-29, LS513, and RKO were treated with indole-3-carbinol or vehicle. Cell viability was assessed via a fluorescent product assay, and apoptotic activity was assessed via a luminescent signal tied to a ratio of caspase-3 and caspase-7 activity. Gene expression of AHR and CYP1A1 messenger ribonucleic acid (mRNA) was measured using quantitative real-time polymerase chain reaction. Small interfering RNA stable expression lines were established on a HCT116 background using a laboratory-developed transfection protocol as published elsewhere. RESULTS: Multiple colorectal cancer cell types express increased CYP1A1 mRNA levels (a specific marker of AHR-driven activity) after treatment with I3C, characterizing I3C treatment as agonistic of this pathway. Also, I3C induced a dose-dependent decrease in cell viability as well as inducing apoptosis. Furthermore, using small interfering RNA interference to knockdown AHR responsiveness generated a significant resistance to the chemotherapeutic actions of indole-3-carbinol regarding both cell viability and apoptotic activity. CONCLUSIONS: Some degree of the cytotoxic and proapoptotic effects of indole-3-carbinol on colon cancer cells is dependent on activation of the aryl hydrocarbon receptor. This represents a novel mechanism for the molecular action of indole-3-carbinol and enhances our understanding of its effects in the context of colorectal cancer. Continued preclinical study of both indole-3-carbinol and the aryl hydrocarbon receptor pathway is warranted, which may one day lead to novel diet-derived colon cancer treatments that enlist the AHR.

The Aryl Hydrocarbon Receptor is a Repressor of Inflammation-associated Colorectal Tumorigenesis in Mouse.

OBJECTIVE: To determine the role of the aryl hydrocarbon receptor (AHR) in colitis-associated colorectal tumorigenesis. BACKGROUND: Colorectal cancer (CRC) is the third most commonly diagnosed cancer in United States. Chronic intestinal inflammation increases the risk for the development of CRC. We investigated the involvement of AHR, a ligand-activated transcriptional regulator, in colitis-associated colorectal tumorigenesis. METHODS: We used a mouse model of colitis-associated colorectal tumorigenesis that employs treatment with azoxymethane and dextran sodium sulfate. We examined the role of AHR using both an Ahr-deletion mouse model (Ahr) and treatment with the AHR pro-agonist indole-3-carbinol (I3C). Incidence, multiplicity, and location of tumors were visually counted. Tumors were defined as neoplasms. Intestinal inflammation was assessed by quantitative PCR for proinflammatory markers and colon length. Data were evaluated and compared using GraphPad Prism software (version 6, La Jolla, CA). RESULTS: Tumor incidence was increased 32% in Ahr null mice and tumor multiplicity was approximately increased 3-fold compared with wild-type mice (2.4 vs 7; P < 0.05). Furthermore, tumor multiplicity was reduced 92% by treatment of I3C in wild-type mice, whereas the suppressor effect of I3C was not observed in Ahr null mice (P < 0.05). CONCLUSIONS: We found that AHR plays a protective role in colitis-associated colorectal tumorigenesis. This conclusion is based on the observations that Ahr null mice showed increased number of colorectal tumors, and mice treated with I3C exhibited fewer tumors. This study supports the use of AHR agonists such as I3C as a chemopreventive therapy for IBD-associated CRC in human patients.

Aryl hydrocarbon receptor-dependent apoptotic cell death induced by the flavonoid chrysin in human colorectal cancer cells.

The polyphenolic flavone chrysin has been evaluated as a natural chemopreventive agent due to its anti-cancer effects in a variety of cancer cell lines. However, the mechanism of the chemopreventive effect has been not well established, especially in human colorectal cancer cells. We evaluated the chemopreventive effect of chrysin in three different human colorectal cancer cell lines. We found that chrysin treatment consequently reduced cell viability via induction of apoptosis. We identified that the involvement of up-regulation of pro-apoptotic cytokines tumor necrosis factor (Tnf) α and ß genes and consequent activation of the TNF-mediated transcriptional pathway in chrysin-induced apoptosis. Using our generated AHR siRNA expressing colorectal cancer cells, we demonstrated that the chrysin-induced up-regulation of Tnfα and ß gene expression was dependent on the aryl hydrocarbon receptor (AHR), which is a ligand-receptor for chrysin. Subsequently, we found that the AHR siRNA expressing colorectal cancer cells were resistant to chrysin-induced apoptosis. Therefore, we concluded that AHR is required for the chrysin-induced apoptosis and the up-regulation of Tnfα and ß gene expression in human colorectal cancer cells.

Liver tumor promotion by 2,3,7,8-tetrachlorodibenzo-p-dioxin is dependent on the aryl hydrocarbon receptor and TNF/IL-1 receptors.

We set out to better understand the signal transduction pathways that mediate liver tumor promotion by 2,3,7,8-tetrachlorodibenzo-p-dioxn ("dioxin"). To this end, we first employed congenic mice homozygous for either the Ahr(b1) or Ahr(d) alleles (encoding an aryl hydrocarbon receptor (AHR) with high or low binding affinity for dioxin, respectively) and demonstrated that hepatocellular tumor promotion in response to dioxin segregated with the Ahr locus. Once we had genetic evidence for the importance of AHR signaling, we then asked if tumor promotion by dioxin was influenced by "interleukin-1 (IL-1)-like" inflammatory cytokines. The importance of this question arose from our earlier observation that aspects of the acute hepatocellular toxicity of dioxin are dependent upon IL1-like cytokine signaling. To address this issue, we employed a triple knock-out (TKO) mouse model with null alleles at the loci encoding the three relevant receptors for tumor necrosis factors α and ß and IL-1α and IL-1ß (i.e., null alleles at the Tnfrsf1a, Tnfrsf1b, and Il-1r1 loci). The observation that TKO mice were resistant to the tumor promoting effects of dioxin in liver suggests that inflammatory cytokines play an important step in dioxin mediated liver tumor promotion in the mouse. Collectively, these data support the idea that the mechanism of dioxin acute hepatotoxicity and its activity as a promoter in a mouse two stage liver cancer model may be similar, i.e., tumor promotion by dioxin, like acute hepatotoxicity, are mediated by the linked action of two receptor systems, the AHR and the receptors for the "IL-1-like" cytokines.

SU5416, a VEGF receptor inhibitor and ligand of the AHR, represents a new alternative for immunomodulation.

The experimental compound SU5416 went as far as Phase III clinical trials as an anticancer agent, putatively because of its activity as a VEGFR-2 inhibitor, but showed poor results. Here, we show that SU5416 is also an aryl hydrocarbon receptor (AHR) agonist with unique properties. Like TCDD, SU5416 favors induction of indoleamine 2,3 dioxygenase (IDO) in immunologically relevant populations such as dendritic cells in an AHR-dependent manner, leading to generation of regulatory T-cells in vitro. These characteristics lead us to suggest that SU5416 may be an ideal clinical agent for treatment of autoimmune diseases and prevention of transplant rejection, two areas where regulatory ligands of the AHR have shown promise. At the same time, AHR agonism might represent a poor characteristic for an anticancer drug, as regulatory T-cells can inhibit clearance of cancer cells, and activation of the AHR can lead to upregulation of xenobiotic metabolizing enzymes that might influence the half-lives of co-administered chemotherapeutic agents. Not only does SU5416 activate the human AHR with a potency approaching 2,3,7,8-tetrachlorodibenzo-p-dioxin, but it also activates polymorphic murine receptor isoforms (encoded by the Ahr(d) and Ahr(b1) alleles) with similar potency, a finding that has rarely been described and may have implications in identifying true endogenous ligands of this receptor.

Identification of the Ah-receptor structural determinants for ligand preferences.

The aryl hydrocarbon receptor (AHR) is a transcription factor that responds to diverse ligands and plays a critical role in toxicology, immune function, and cardiovascular physiology. The structural basis of the AHR for ligand promiscuity and preferences is critical for understanding AHR function. Based on the structure of a closely related protein HIF2α, we modeled the AHR ligand binding domain (LBD) bound to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and benzo(a)pyrene (BaP) and identified residues that control ligand preferences by shape and H-bond potential. Mutations to these residues, particularly Q377 and G298, resulted in robust and opposite changes in the potency of TCDD and BaP and up to a 20-fold change in the ratio of TCDD/BaP efficacy. The model also revealed a flexible "belt" structure; molecular dynamic (MD) simulation suggested that the "belt" and several other structural elements in the AHR-LBD are more flexible than HIF2α and likely contribute to ligand promiscuity. Molecular docking of TCDD congeners to a model of human AHR-LBD ranks their binding affinity similar to experimental ranking of their toxicity. Our study reveals key structural basis for prediction of toxicity and understanding the AHR signaling through diverse ligands.

AHR drives the development of gut ILC22 cells and postnatal lymphoid tissues via pathways dependent on and independent of Notch.

Innate lymphoid cells (ILCs) of the ILC22 type protect the intestinal mucosa from infection by secreting interleukin 22 (IL-22). ILC22 cells include NKp46(+) and lymphoid tissue-inducer (LTi)-like subsets that express the aryl hydrocarbon receptor (AHR). Here we found that Ahr(-/-) mice had a considerable deficit in ILC22 cells that resulted in less secretion of IL-22 and inadequate protection against intestinal bacterial infection. Ahr(-/-) mice also lacked postnatally 'imprinted' cryptopatches and isolated lymphoid follicles (ILFs), but not embryonically 'imprinted' Peyer's patches. AHR induced the transcription factor Notch, which was required for NKp46(+) ILCs, whereas LTi-like ILCs, cryptopatches and ILFs were partially dependent on Notch signaling. Thus, AHR was essential for ILC22 cells and postnatal intestinal lymphoid tissues. Moreover, ILC22 subsets were heterogeneous in their requirement for Notch and their effect on the generation of intestinal lymphoid tissues.