**Abstract**

Aryl hydrocarbon receptor (AhR) is a biological sensor that integrates environmental, metabolic, and endogenous signals to control complex cellular responses in physiological and pathophysiological functions. The full-length AhR encompasses various domains, including a bHLH, a PAS A, a PAS B, and transactivation domains. With the exception of the PAS B and transactivation domains, the available 3D structures of AhR revealed structural details of its subdomains interactions as well as its interaction with other protein partners. Towards screening for novel AhR modulators homology modeling was employed to develop AhR-PAS B domain models. These models were validated using molecular dynamics simulations and binding site identification methods. Furthermore, docking of well-known AhR ligands assisted in confirming these binding pockets and discovering critical residues to host these ligands. In this context, virtual screening utilizing both ligand-based and structure-based methods screened large databases of small molecules to identify novel AhR agonists or antagonists and suggest hits from these screens for validation in an experimental biological test. Recently, machine-learning algorithms are being explored as a tool to enhance the screening process of AhR modulators and to minimize the errors associated with structure-based methods. This chapter reviews all *in silico* screening that were focused on identifying AhR modulators and discusses future perspectives towards this goal.

**Keywords:** human AhR, in silico, in vitro, AhR modulator, crystal structure, AhR modeling

## **1. Introduction**

Six decades ago, researchers made extensive studies to answer a puzzling question. That was how administrating exogenous substances such as polycyclic aromatic hydrocarbons (PAHs) had a potent induction on xenobiotic-metabolizing enzymes in rats' livers [1, 2]. It was finally Alan Poland and his colleagues who finally answered this question in the early 1970s. Poland discovered a novel hepatic protein in complex with the polycyclic aromatic hydrocarbons compound, 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) [2]. The new protein was bound to TCDD in a potent affinity and was isolated from hepatic cytosolic fractions of mice C57BL/6, a mice model strain for studying aromatic hydrocarbon responsiveness.

This protein was later termed as the aryl hydrocarbon receptor (AhR) [2] and was identified as a ligand-activated transcription factor.

Later studies showed that AhR is expressed in several tissues including but not limited to; liver, lung, placenta, and heart and different cell types throughout the developmental periods of organ growth [3]. Further knockout studies in mice revealed essential functions for AhR in multiple physiological and pathophysiological pathways [4–6]. This accumulated knowledge over the last decades defined AhR as an environmental sensor for air pollutants and as a ligand-activated transcriptional factor, which regulates the expression of various genes, including enzymes responsible for xenobiotic metabolism [7].

AhR-mediates the toxicity of uncountable xenobiotics, and their triggered toxicity is accompanied by an overexpression and overactivation of AhR in cells. Thus, it increases the pathophysiological functions of AhR and could develop cancer in different organs such as the breast and liver. In addition, it can also lead to cardiovascular diseases, among other diseases [8, 9]. Thus, targeting AhR with a small molecule agonist/antagonist could efficiently inhibit several of the important hallmarks of various cancers [10].

Computational modeling and computer simulations continue to be an important tool for studying various biological mechanisms and for analyzing the interactions between biomolecular entities (*e.g.* proteins, DNA, and drugs). These methods use complex physiochemical and mathematical algorithms to predict the interactions between ligands and proteins at the atomistic level [11]. For instance, in the case of AhR, several computational studies have been reported to screen the chemical space for potential AhR modulators and to investigate how these ligands interact with the individual residues within AhR [12, 13]. This chapter will focus on summarizing the findings and computational approaches described in these studies. The chapter first discusses current knowledge related to AhR functions and describes all reported AhR crystal structures. It will then provide an overview on the different computational studies reported in the literature, which focused on investigating AhR dynamics and on identifying novel AhR modulators. We hope the information provided in this chapter can advance the development of AhR ligands and contribute to progress in this important research area.

#### **2. AhR structure and functions**

#### **2.1 AhR domain structure**

AhR is a member of the basic helix–loop–helix (bHLH)-PER- ARNT-SIM (PAS) family of transcription factors. The "PAS" term is an abbreviation for three proteins, namely, the Drosophila circadian rhythm protein period (Per), the mammalian AhR nuclear translocator (ARNT), and Drosophila neurogenic protein single-minded (Sim) [7, 14, 15]. Human AhR is a 848 amino acid with a molecular weight of ~96 kDa [16]. It includes two PAS domains, namely PAS A and PAS B, and interacts with the Aryl hydrocarbon nuclear tranlocator (ARNT) protein. Moreover, the PAS B domain involves two interactions sites: a ligand-binding site in which a bound ligand can modulate the AhR activity; and a direct binding interface for the HSP 90-chaperone protein. Additionally, AhR includes a basic helix loop helix motif located near its N-terminal domain, which is responsible for DNA binding as well as contributing to other protein–protein interactions. Finally, the transactivation (*i.e.,* glutamine-rich region) domain is located close to the C-terminal of AhR and binds to a co-activator [7] as shown in **Figure 1**.

*Targeting the Aryl Hydrocarbon Receptor (AhR): A Review of the In-Silico Screening… DOI: http://dx.doi.org/10.5772/intechopen.99228*

**Figure 1.**

*AhR domain structure and sub-domains' functions.*

## **2.2 The AhR ligands and their modes of action**

The AhR PAS B domain can interact with both exogenous and endogenous chemicals from various origins. These interactions can induce different effects on AhR activity, leading to a wide range of physiological and toxicological downstream consequences. For example, several studies showed that environmental pollutants have been associated with developing cardiovascular diseases, cancer, and other diseases through AhR modulation [7, 17, 18]. Exogenous AhR ligands include various aromatic hydrocarbon molecules such as dioxins. One can be exposed to such ligands through contaminated food or environmental pollutants. Following exposure, their interaction with AhR can lead to several toxic effects, including organ dysfunctions, immunotoxicity, and carcinogenicity. On the other hand, endogenous AhR ligands are usually metabolic derivatives derived from cellular processes such as 6-Formylindolo (3,2- b) carbazole (FICZ). The interaction of these ligands with AhR is part of a normal functional response through AhR modulation [7, 19, 20].

### **2.3 AhR physiological and pathophysiological roles**

AhR is an essential protein that contributes to countless biological pathways to establish its physiological role in developing the immune system and regulating xenobiotic enzymes [7, 15, 21]. AhR knockout mice models showed abnormal female reproductive functions and impairment in managing blood pressure [7]. The overactivation and constitutive activation of AhR have been associated with the initiation, promotion, progression, and invasion of cancer cells. For example, activating AhR by exogenous AhR ligands can have several effects, which includes inducing cell proliferation in the G1-S phase, silencing tumor suppressor genes, and activating proto-oncogenes in cancer cell lines.

Earlier findings showed that the exogenous AhR ligand, 2,3,7,8-tetrachlorodibenzo-p-dioxin TCDD promoted the degradation of cell–cell adhesion and expansion of cancer cells' motility by separating the Src kinase from the AhR protein complex. Furthermore, the activation of AhR via environmental pollutants can lead to a significant induction of xenobiotic-metabolizing enzymes, including CYP1A, which produces reactive intermediate metabolites and reactive oxygen species to promote tumor growth [14, 22]. In a nutshell, AhR resembles a machinery of genes, which controls xenobiotic-metabolizing enzymes in phases I and II, as shown in **Table 1**. Also, known AhR agonists such as TCDD and β -naphthoflavone have been shown to induce cellular hypertrophic actions on H9c2 cardiomyoblast cells. This was correlated with an increase in the levels of numerous cytochrome P450 genes, which could overcome by using an AhR antagonist [31].

On the positive side, experiments on a mouse model of induced colitis showed that the endogenous AhR agonist (FICZ), which has a strong binding affinity towards AhR, could block IL-6 and claudin-2 expression, and prevent any induced disorders in the intestinal barrier function through AhR activation [32].


**Table 1.**

*Xenobiotic metabolizing enzymes genes regulate via AhR pathway.*

Further protein knockout studies showed that AhR ligands play a fundamental role in autoimmune diseases through regulating Tregs and TH17 cell differentiation in the immune system. For example, FICZ inhibited Treg and TH17 cell development, accelerating experimental autoimmune encephalomyelitis in mice models [21, 33].
