AHR in the intestinal microenvironment: safeguarding barrier function

Abstract

Mammalian aryl hydrocarbon receptor (AHR) is a ligand-dependent transcription factor that belongs to the basic helix–loop–helix (bHLH)–PAS family of transcription factors, which are evolutionarily conserved environmental sensors. In the absence of ligands, AHR resides in the cytoplasm in a complex with molecular chaperones such as HSP90, XAP2 and p23. Upon ligand binding, AHR translocates into the nuclear compartment, where it dimerizes with its partner protein, AHR nuclear translocator (ARNT), an obligatory partner for the DNA-binding and functional activity. Historically, AHR had mostly been considered as a key intermediary for the detrimental effects of environmental pollutants on the body. However, following the discovery of AHR-mediated functions in various immune cells, as well as the emergence of non-toxic ‘natural’ AHR ligands, this view slowly began to change, and the study of AHR-deficient mice revealed a plethora of important beneficial functions linked to AHR activation. This Review focuses on regulation of the AHR pathway and the barrier-protective roles AHR has in haematopoietic, as well as non-haematopoietic, cells within the intestinal microenvironment. It covers the nature of AHR ligands and feedback regulation of the AHR pathway, outlining the currently known physiological functions in immune, epithelial, endothelial and neuronal cells of the intestine. Aryl hydrocarbon receptor (AHR) signalling has important roles in the intestine. In this Review, Stockinger, Shah and Wincent discuss AHR regulation, its role in various intestinal cell types and in intestinal inflammation and tumorigenesis. Aryl hydrocarbon receptor (AHR) is widely expressed in the intestinal microenvironment, and its activation by natural ligands results in barrier-protective effects. In addition to well-documented functions in intestinal immune cells, AHR has important and less well-studied functions in epithelial and endothelial cells as well as in neurons of the enteric nervous system. AHR involvement can appear subtle under homeostatic conditions but becomes prominent in regeneration following tissue injury. AHR acts as a tumour suppressor for development of colonic malignancy. New models and tools enabling the dissection of cell type-specific and tissue type-specific AHR functions in vivo are needed to understand the effect of environmental signals on inflammation and tissue repair in the gastrointestinal barrier. Aryl hydrocarbon receptor (AHR) is widely expressed in the intestinal microenvironment, and its activation by natural ligands results in barrier-protective effects. In addition to well-documented functions in intestinal immune cells, AHR has important and less well-studied functions in epithelial and endothelial cells as well as in neurons of the enteric nervous system. AHR involvement can appear subtle under homeostatic conditions but becomes prominent in regeneration following tissue injury. AHR acts as a tumour suppressor for development of colonic malignancy. New models and tools enabling the dissection of cell type-specific and tissue type-specific AHR functions in vivo are needed to understand the effect of environmental signals on inflammation and tissue repair in the gastrointestinal barrier.