Dissecting the role of Aryl Hydrocarbon Receptor in adipose and vascular homeostasis

Abstract

Aryl Hydrocarbon Receptor (AHR) is a ligand activated transcription factor, which was initially characterised as an environmental sensor, that mediates toxic effects in response to harmful chemicals. Early research therefore focussed on exogenous ligands such as 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), which elicit a sustained activation of the AHR pathway, rather than a transient one associated with natural ligands. More recently, AHR has been implicated in obesity. The incidence of obesity has increased dramatically over recent decades, and carries a high burden to health care systems, along with increased prevalence of cardiovascular disease and cancer. Continued work surrounding obesity has shown that thermogenesis contributes to energy balance, and is now an attractive therapeutic target to attempt to reduce obesity rates. Findings show that AHR is not required for activation, nor regulation of thermogenesis through the characterisation of the global AHR knock out (KO) and adipocyte-specific AHR KO mouse models when exposed to cold. Both AHR KO and adipocyte-specific KO animals are able to maintain body temperature when exposed to acute cold challenge. No differences were observed in oxygen consumption nor energy expenditure. These mice were also able to maintain body temperature upon fasting prior to cold exposure. Endothelial cells (ECs) were also selected as an important cell-type to investigate AHR function due to the severe vascular defects of global AHR KO animals. Vascular disease is again associated with a high burden on healthcare, especially in the ageing population and in individuals who are obese. A human model system of human umbilical vein endothelial cells (HUVECs) was used to investigate novel functions of AHR in ECs in vitro. Bulk RNA sequencing revealed that AHR activation caused a downregulation of many cell cycle-related pathways. Functional data using EdU staining also verified that activation of AHR with various concentrations of FICZ for 6Hrs caused a transient accumulation of cells in G0/1, whilst S and G2/M remained unchanged when compared to DMSO control. Further protein expression analysis at a single cell resolution using Operetta indicated that these effects were mediated through E2F. In addition, AHR was knocked down in HUVECs, which saw a decrease in cells at G1/0 compared to control siKD cells, and an increase in S phase when stained with EdU. Cells reached confluency more rapidly in siAHR group, compared to siKD control group using Incucyte. Increases in E2F were also observed using the Operetta system.