Novel regulation of lipid homeostasis by the Ah receptor

Open Access
Author:
Tanos , Rachel
Graduate Program:
Biochemistry and Molecular Biology
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
May 30, 2012
Committee Members:
  • Gary H Perdew, Dissertation Advisor/Co-Advisor
  • Jeffrey Maurice Peters, Committee Member
  • Richard John Frisque, Committee Member
  • Ross Cameron Hardison, Committee Member
  • Joshua D Lambert, Committee Member
Keywords:
  • Ah receptor
  • AHR
  • cholesterol
  • Fatty acid
  • lipid
  • SREBP
Abstract:
The aryl hydrocarbon receptor (AHR) is a ligand activated transcription factor. Activation of AHR has been associated with toxicity through its binding to dioxin response element (DRE) sequences in its target genes (e.g. CYP1A1). Evolutionary conservation of the receptor and the phenotype of mice lacking the AHR suggest an endogenous role for the receptor beyond responding to xenobiotic exposure. Based on previous work in our lab showing the ability of the receptor to alter gene expression independent of DRE sequences, we set out to explore a possible involvement of the receptor in key cellular pathways. We performed a microarray analysis on liver isolated from ligand-treated transgenic mice expressing a wild-type Ahr or a DRE-binding mutant Ahr (A78D). The results revealed that AHR ligand treatment suppressed cholesterol synthesis but did not require DRE binding. In order to confirm this finding in humans, primary human hepatocytes were administered an AHR ligand and subsequent analysis of mRNA levels revealed a significant trend of repression in fatty acid and cholesterol synthesis genes. Our lab has also established the ability of the ligand SGA360 to activate AHR without inducing DRE-mediated activity. Since toxicity associated with hyperactivation of the AHR is a DRE-mediated event and our microarray data revealed that DRE-binding is not essential for the regulation of our genes of interest, we tested the effect of SGA360 in primary human hepatocytes. Selective activation of the receptor showed a significant attenuation in the expression of our target genes and the ability of the receptor to inhibit the compensatory mechanism of statins on cholesterol synthesis gene expression. In addition, SGA360 exhibited a higher degree of repression of fatty acid and cholesterol biosynthesis gene expression than the AHR agonist BNF. Mirroring our gene expression results, a significant repression of fatty acid and cholesterol secretion was observed in human cells. In an effort to elucidate the mechanism of this regulation by AHR, we investigated the activity of AHR on the sterol element binding proteins (SREBPs), the key regulators of both pathways. Our results indicated the targeted proteosomal degradation of the cleaved active form of those transcription factors when AHR is activated causing the attenuation of their transcriptional signaling. The discovery of AHR as a regulator of fatty acid and cholesterol biosynthesis pathways independently of its DRE-binding and the ability to selectively activate the receptor for this purpose suggests that AHR may be a previously unrecognized therapeutic target.