AN EXAMINATION OF THE ARYL HYDROCARBON RECEPTOR IN LIVER METABOLISM
Open Access
- Author:
- Girer, Nathaniel Gabriel
- Graduate Program:
- Biochemistry, Microbiology, and Molecular Biology
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- November 10, 2016
- Committee Members:
- Gary Perdew, Dissertation Advisor/Co-Advisor
Gary Perdew, Committee Chair/Co-Chair
Andrew Patterson, Committee Member
Jeffrey Peters, Committee Member
Connie Rogers, Outside Member
Ross Hardison, Committee Member
Ross Hardison, Committee Member - Keywords:
- Aryl Hydrocarbon Receptor
Fibroblast growth factor 21
Liver Metabolism - Abstract:
- The aryl hydrocarbon receptor (AHR) is a ligand-activated transcription factor from the basic helix-loop-helix PER/ARNT/SIM family of proteins that is evolutionarily conserved in both vertebrates and invertebrates. Known as a "promiscuous" receptor, AHR can bind to several different classes of chemical compounds such as polycyclic aromatic hydrocarbons (PAH) and flavonoids. When not bound to ligand, AHR resides in a cytosolic complex containing two molecules of heat shock protein 90, one molecule of X-associated protein 2, and a molecule of p23. Upon ligand binding, this complex is transported to the nucleus via nuclear importins and AHR dissociates to form a heterodimer with aryl hydrocarbon nuclear translocator (ARNT). The AHR/ARNT heterodimer then binds to specific DNA sequences known as dioxin response elements (DRE) within the promoter region of target genes (e.g. cytochrome P450 enzyme 1A1, CYP1A1) to activate their transcription. Previous studies have primarily examined AHR within the context of ligand-mediated transcriptional activation of its prototypical target gene, Cyp1a1. However, the AHR can also influence gene transcription in the absence of exogenous ligand and/or Cyp1a1 expression. Using a conditional AHR knockout mouse model that lacks hepatocyte-specific AHR expression (Ahrfx/fxAlbCre), this dissertation examines how basal AHR activity in the absence of Cyp1a1 transcription can influence metabolic homeostasis. In particular, the data reveal that the loss of hepatocyte-specific AHR expression correlates with reduced body and liver mass relative to congenic AHR-expressing mice (Ahrfx/fx). Additionally, Ahrfx/fxAlbCre mice maintained on purified AIN-93M diet display impaired glucose tolerance without any perturbations of insulin sensitivity. Conversely, Ahrfx/fxAlbCre mice challenged with a high-sucrose dietary modification of AIN-93M exhibit reduced insulin sensitivity without any difference in glucose tolerance. Most notably, Ahrfx/fxAlbCre mice challenged with a high-fat/high-sucrose (HF/HS) diet exhibit significantly decreased gene/protein expression of key enzymes involved in de novo fatty acid synthesis and fatty acid import, as well as significantly increased expression of key fatty acid export genes. Furthermore, inflammatory gene expression is also significantly reduced in HF/HS-fed Ahrfx/fxAlbCre mice relative to Ahrfx/fx. Together, the data suggest that basal hepatocyte-specific AHR signaling may promote diet-induced steatohepatitis in Ahrfx/fx mice. Utilizing the Ahrfx/fxAlbCre mouse model, this dissertation also explores the role of AHR in regulating hepatic fibroblast growth factor 21 (FGF21) production. FGF21 is an important metabolic hormone and regulator of the fasting response. Notably, FGF21 can attenuate obesity-associated morbidities when administered to various genetic and diet-induced mouse models of the disease. In the absence of exogenous AHR ligand, non-fasted Ahrfx/fxAlbCre mice exhibit 4-fold greater hepatic Fgf21 expression relative to Ahrfx/fx, along with elevated expression of the FGF21-target gene Igfbp1. Furthermore, in vivo agonist activation of AHR reduces hepatic Fgf21 expression during a fast. The Fgf21 promoter contains several putative dioxin response elements (DREs) and utilizing electromobility shift assays, we demonstrate that the AHR/ARNT heterodimer binds to a specific DRE which overlaps binding sequences for peroxisome proliferator-activated receptor α (PPARα), carbohydrate response element-binding protein (ChREBP), and cAMP response element-binding protein hepatocyte specific (CREBH). In addition, agonist-activated AHR impairs PPARα-, ChREBP-, and CREBH-mediated Fgf21 promoter activity in Hepa-1 cells. Similarly, treatment of Hepa-1 cells with AHR agonist ablates potent ER stress-driven Fgf21 expression, while pre-treatment with AHR antagonist blocks this effect. Finally, the data demonstrate that pre-treatment of primary human hepatocytes with AHR agonist attenuates PPARα-, glucose-, and ER stress-driven induction of FGF21 expression, indicating this phenomenon is not mouse-specific. Overall, the data show that AHR contributes to hepatic energy homeostasis partly through the constitutive repression of FGF21 expression and signaling. Therefore, future studies should examine the potential use of AHR antagonists, such as the flavonoid compounds readily found within edible plants, to increase FGF21 expression and subsequently produce therapeutic outcomes. In summary, the data from this dissertation indicate that basal hepatocyte-specific AHR activity plays a critical role in the regulation of liver metabolism and that AHR agonists are not only capable of activating DRE-dependent transcription, but can also suppress or interfere with certain metabolic gene pathways. Nevertheless, the exact mechanisms through which AHR exerts these effects remain unclear. Ultimately, this dissertation presents contrasting roles for AHR when activated with ligands and when exerting its regulation in the absence of exogenous ligand. Therefore, future research should aim to define the role of AHR within these separate contexts to gain a better understanding of the complex mechanisms through which AHR regulates transcription.