Structural Adaptive Response of the Aryl hydrocarbon Receptor (AhR) to Xenobiotics: Implication in predicting the toxicity of pollutants in both humans and ecosystems – PlasticAhR

Contamination of ecosystems and organisms by environmental pollutants are among the major health concerns of the twenty first century. Over the last decades, toxicities of some xenobiotics have been characterized. However, despite intensive research, our ability to predict the toxicity of persistent organic pollutants (POP) or of other toxic compounds (coming from food derivatives, other combustion products, etc…) is still limited.
Cellular receptors for xenobiotics are critical components of the cellular adaptative pathway as well as being intimately related to the toxicity of these compounds. The AhR (Aryl hydrocarbon Receptor) which is the focus of this application is one of the three major xenobiotic receptors. It is unique in that it is a member of the bHLH-PAS (basic Helix Loop Helix – Per Arnt Sim) protein family. Following ligand binding, the cytoplasmic Ah receptor translocates into the nucleus and associates with the ARNT protein (AhR Nuclear Translocator), another bHLH-PAS family member. The N-terminal halves of the AhR/ARNT complex are essential and sufficient for xenobiotic and DNA binding. They contain a PAS-A domain sandwiched by a DNA binding domain (DBD, bHLH) and a ligand binding domain (LBD, PAS-B). The PAS-B domain of the AhR can bind a wide spectrum of xenobiotics including a variety of pollutants (like dioxins, cigarette smoke compounds, etc…) as well as food components such as polyphenols. Remarkably, depending upon the type of ligand, AhR activation triggers adaptive, detrimental or protective transcription responses (cytotoxicity, apoptosis or cardio protecting effects).
We hypothesize that different ligands elicit different structural conformations of the receptor thus leading to selective binding to one of the three xenobiotic responsive elements families, called XRE (initially identified in the CYP1A1, PON1 and BAX promoters). However, despite years of study, the molecular mechanisms leading to the triggering of a particular transcriptional pathway by a ligand remain elusive. To date, there is no structural information available on the AhR and many issues remain to be addressed: for example, what are the xenobiotic chemical determinants that lead to a specific AhR-bound conformation? What are the amino acids responsible for species-dependent AhR responses? How is the signal transmitted towards the bHLH domain upon ligand binding to the AhR PAS-B domain? How can AhR and cytochrome P450s share the same wide ligand binding spectrum without any sequence similarities? Can we anticipate the effects of xenobiotics on a whole organism from the unique knowledge of their chemical structures?
We present here an ambitious but realistic project in which we will address all of these questions in three specific tasks. Several ligand or DNA-bound structures will be determined by X-ray crystallography, namely cytochrome P450s and LBD, DBD, LBD-DBD domains from the Ah Receptor and ARNT. We will attempt to solve those complex structures using proteins from different organisms. We will then use molecular modeling and a cellular assay to identify differences between species. By combining these data, we will define precise species-dependent AhR xenobiotic binding determinants in order to predict in silico, which chemicals will bind to the AhR and activate one or several characteristic AhR XRE. This information will be essential for industrial and/or governmental organizations to estimate the toxicity of new developing or existing compounds (REACH regulation). Patenting this in silico model will be the end product of this project.