Tryptanthrins and other tryptophan-derived agonists of the dioxin receptor

Aryl hydrocarbon receptor: current perspectives on key signaling partners and immunoregulatory role in inflammatory diseases

The aryl hydrocarbon receptor (AhR) is a versatile environmental sensor and transcription factor found throughout the body, responding to a wide range of small molecules originating from the environment, our diets, host microbiomes, and internal metabolic processes. Increasing evidence highlights AhR's role as a critical regulator of numerous biological functions, such as cellular differentiation, immune response, metabolism, and even tumor formation. Typically located in the cytoplasm, AhR moves to the nucleus upon activation by an agonist where it partners with either the aryl hydrocarbon receptor nuclear translocator (ARNT) or hypoxia-inducible factor 1β (HIF-1β). This complex then interacts with xenobiotic response elements (XREs) to control the expression of key genes. AhR is notably present in various crucial immune cells, and recent research underscores its significant impact on both innate and adaptive immunity. This review delves into the latest insights on AhR's structure, activating ligands, and its multifaceted roles. We explore the sophisticated molecular pathways through which AhR influences immune and lymphoid cells, emphasizing its emerging importance in managing inflammatory diseases. Furthermore, we discuss the exciting potential of developing targeted therapies that modulate AhR activity, opening new avenues for medical intervention in immune-related conditions.

Ligand activation of the Ah receptor contributes to gastrointestinal homeostasis

The Ah receptor (AHR) is capable of binding a structurally diverse group of compounds that can be found in the diet, produced by bacteria in the gut and through endogenous metabolism. The gastrointestinal tract is a rich source of AHR ligands, which have been shown to protect the gut upon challenge with either pathogenic bacteria or toxic chemicals. The human AHR can be activated by a broader range of ligands compared to the mouse AHR, suggesting that studies in mice may underestimate the impact of AHR ligands in the human gut. The protective effect of AHR activation appears to be due to modulating the immune system within the gut. While several mechanisms have been established, due to the increasingly pleotropic nature of the AHR, other mechanisms of action likely exist that remain to be identified. The major contributors to AHR function in the gut and the most appropriate level of receptor activation that maintains intestinal homeostasis warrants further investigation.

Indole and Tryptophan Metabolism: Endogenous and Dietary Routes to Ah Receptor Activation

The aryl hydrocarbon receptor (AHR) is a ligand-activated transcription factor recognized for its role in xenobiotic metabolism. The physiologic function of AHR has expanded to include roles in immune regulation, organogenesis, mucosal barrier function, and the cell cycle. These functions are likely dependent upon ligand-mediated activation of the receptor. High-affinity ligands of AHR have been classically defined as xenobiotics, such as polychlorinated biphenyls and dioxins. Identification of endogenous AHR ligands is key to understanding the physiologic functions of this enigmatic receptor. Metabolic pathways targeting the amino acid tryptophan and indole can lead to a myriad of metabolites, some of which are AHR ligands. Many of these ligands exhibit species selective preferential binding to AHR. The discovery of specific tryptophan metabolites as AHR ligands may provide insight concerning where AHR is activated in an organism, such as at the site of inflammation and within the intestinal tract.

Effects of tryptophan photoproducts in the circadian timing system: searching for a physiological role for aryl hydrocarbon receptor

The aryl hydrocarbon receptor (AhR) mediates adverse effects of dioxins, but its physiological role remains ambiguous. The similarity between AhR and canonical circadian clock genes suggests potential involvement of AhR in regulation of circadian timing. Photoproducts of tryptophan (TRP), including 6-formylindolo[3,2-b]carbazole (FICZ), have high affinity for AhR and are postulated as endogenous ligands. Although TRP photoproducts activate AhR signaling in vitro, their effects in vivo have not been investigated in mammals. Because TRP photoproducts may act as transducers of light, we examined their effects on the circadian clock. Intraperitoneal injection of TRP photoproducts or FICZ to C57BL/6J mice dose dependently induced AhR downstream targets, cytochrome P4501A1 (CYP1A1) and cytochrome P4501B1 mRNA expression, in liver. c-fos mRNA, a commonly used marker for light responses, was also induced with FICZ, and all responses were AhR dependent. A rat-immortalized suprachiasmatic nucleus (SCN) cell line, SCN 2.2, was used to examine the direct effect of TRP photoproducts on the molecular clock. Both TRP photoproducts and FICZ-increased CYP1A1 expression and prolonged FICZ incubation altered the circadian expression of clock genes (Per1, Cry1, and Cry2) in SCN 2.2 cells. Furthermore, FICZ inhibited glutamate-induced phase shifting of the mouse SCN electrical activity rhythm. Circadian light entrainment is critical for adjustment of the endogenous rhythm to environmental light cycle. Our results reveal a potential for TRP photoproducts to modulate light-dependent regulation of circadian rhythm through triggering of AhR signaling. This may lead to further understanding of toxicity of dioxins and the role of AhR in circadian rhythmicity.

Inhibition of Hepatocyte Growth Factor Induction in Human Dermal Fibroblasts by Tryptanthrin

In addition to regulation of normal cell functions, hepatocyte growth factor (HGF) has also been shown to be involved in malignant cell transformation and in growth, invasion and metastasis in cancer cells. Inhibitors of HGF production have a potential for interfering with malignant cell transformation and progression of tumors. We found that tryptanthrin, one of the major compounds extracted from the medicinal plant Polygonum tinctorium, which is known for its antitumor activity, strongly inhibited HGF production stimulated by various HGF inducers in human dermal fibroblasts. HGF production induced by phorbol 12-myristate 13-acetate (PMA) was potently inhibited by tryptanthrin without any appreciable cytotoxic effect. Tryptanthrin also inhibited HGF production induced by epidermal growth factor (EGF) and platelet-derived growth factor. Moreover, proliferation of the fibroblasts induced by the two growth factors was potently suppressed by tryptanthrin to the level of proliferation of unstimulated fibroblasts. However, tryptanthrin did not inhibit HGF production induced by the protein kinase A-activating agents cholera toxin and 8-bromo-cAMP. These effects of tryptanthrin were different from the effects of transforming growth factor beta1 and dexamethasone, both of which inhibit HGF production induced by all the above inducers. Upregulations of HGF gene expression by PMA and EGF were also inhibited by tryptanthrin. Activation of the mitogen-activated protein kinase (MAPK) signaling pathway is crucial for PMA-induced HGF production, but tryptanthrin did not attenuate phosphorylation of MAPK induced by PMA. These results indicate that tryptanthrin potently inhibited induction of HGF production probably through events downstream of MAPK activation.

Fate of Ah-receptor agonists in organic household waste during anaerobic degradation--estimation of levels using EROD induction in organ cultures of chick embryo livers

The fate of 7-ethoxyresorufin O-deethylase (EROD)-inducing compounds in source-separated organic household waste subject to anaerobic degradation (i.e. mesophilic/thermophilic anaerobic treatment) was investigated using organ cultures of embryonic chicken livers from fertilised hen eggs. This bioassay reflects the combined effect of all EROD-inducing, possibly dioxin-like compounds in a sample, including chemicals that are seldom or never analysed. All samples tested induced EROD in the bioassay, indicating the presence of dioxin-like compounds. In the anaerobic processes, the amounts of acid-resistant EROD-inducing compounds coming out of the reactors were considerably higher than the incoming amounts, especially for the low-temperature (mesophilic) process. This apparent production of EROD-inducing compounds may be due to de novo synthesis or to an increase in the EROD-inducing potency of the compounds in the material.