Aryl hydrocarbon receptor signaling in Airway Inflammation

Abstract Asthma is influenced by multiple factors including genetics, infections, diet, and environmental exposures, with the immune response and subsequent regulatory mechanisms influencing pathophysiology, disease progression and severity. The aryl hydrocarbon receptor (AhR) is an intracellular sensor for organic compounds and toxins that regulates cellular homeostasis and physiological responses. Recent studies suggest an important role for AhR in regulating airway epithelial and immune cell responses to lung inflammation. While interest in AhR signaling has centered around immune cells and epithelium, how AhR activation influences airway smooth muscle (ASM) per se, particularly in inflammation/asthma, are not known. In this regard, some unresolved questions are A) Is ASM AhR signaling important, and if so, what is its expression and function during inflammation and asthma? B) What roles do ASM AhR play in regulating mechanism that promotes airway hyperresponsiveness and remodeling? Our novel preliminary studies show: 1) Human ASM expresses AhR, which is enhanced during asthma; 2) AhR activation represses gene expression, particularly those involved in proliferation and remodeling pathways; 3) AhR activation inhibits mitogen and/or diesel exhaust particle (DEP)- induced ASM cell proliferation and migration; 4) AhR activation also reduces TGFβ-mediated ECM production and deposition; 5) In a mixed allergen (MA)-induced asthma mouse model, smooth muscle specific AhR knockout mice (AhRfl/fl/smMHCCre/0) show exacerbated hyperresponsiveness and worsening of lung function compared to WT mice; 6) TCDD (AhR agonist) inhibits airway hyperresponsiveness and remodeling in MA- challenged mice; 7) ASM AhR activation in vivo confirmed by increased ASM CYP1B1 expression. Thus, our overall hypothesis is that activation of ASM AhR signaling inhibits inflammation-induced hyperresponsiveness and remodeling in asthma. This will be tested via the following Specific Aims. 1) In human ASM, determine the mechanisms of AhR expression and regulation in inflammation and asthma. 2) In human ASM, determine the role of AhR signaling in proliferation and remodeling in the context of inflammation and asthma. 3) To determine the in vivo importance of ASM-specific AhR signaling in airway remodeling in a chronic allergen mouse model of asthma. Aim 1 explores AhR and ARNT (AhR nuclear translocator) expression and their underlying mechanisms in the presence/absence of pro-inflammatory cytokines relevant to airway inflammation/asthma, MA, and DEP. Aim 2 explores the AhR effects on ASM cell proliferation, migration, ECM, and their underlying mechanisms relevant to airway remodeling. Aim 3 will integrate in vitro results in a clinically-relevant MA adult mouse model of allergic asthma with/without chronic inhaled AhR agonist. Using AhRfl/fl/smMHCCre/0 mice, studies will assess airway function, structure (remodeling), and changes in ASM proteins for mechanisms explored in Aims 1 and 2 (laser capture microdissection, RNA-seq; immunofluorescence). Clinical significance lies in establishing a protective role of AhR in ASM towards novel therapeutic approaches to alleviate remodeling in asthma.