REGULATION OF GENE TRANSCRIPTION BY THE ARYL HYDROCARBON RECEPTOR –NEW TARGETS AND MECHANISMS OF REGULATION.
Open Access
- Author:
- Patel, Rushang Dilipkumar
- Graduate Program:
- Integrative Biosciences
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- April 29, 2008
- Committee Members:
- Gary H Perdew, Committee Chair/Co-Chair
Curtis John Omiecinski, Committee Member
Jeffrey Maurice Peters, Committee Member
Robert Bruce Mitchell, Committee Member
Naomi S Altman, Committee Member
Peter John Hudson, Committee Member - Keywords:
- Aryl hydrocarbon receptor
Dioxin
Inflammation
Toxicology
Nuclear receptor
Gene regulation - Abstract:
- Adaptation in response to changes in internal as well as external environment is imperative to sustenance of life. Modulation of gene expression is a critical component of this adaptive response and is mediated by activation of various transcription factors. Individual signaling pathways have been well characterized for many transcription factor systems. Aryl hydrocarbon receptor (AHR) is a transcription factor that is activated by a variety of structurally diverse ligands, including the environmental contaminant dioxin, the cigarette smoke constituent benzo[a]pyrene and the therapeutically prescribed drug omeprazole. Prior to activation, AHR is primarily located in a cytoplasmic complex with chaperone and co-chaperone proteins. Ligand-binding is believed to initiate a conformational change that leads to nuclear translocation, dissociation from the chaperones and heterodimerization with AHR-nuclear translocator (ARNT). AHR-ARNT heterodimer recognizes and binds to a consensus DNA sequence (TNGCGTG), commonly referred to as a dioxin response element (DRE), to drive transcription of target genes. Phase I and II xenobiotic metabolism enzymes have been the well-characterized targets of AHR-mediated transactivation. This sequence of coordinate events has been described as the classical pathway of AHR activity. Different lines of evidence suggest that AHR serves physiologically relevant functions, though the details have not been elucidated. The goal of this research project was to identify previously uncharacterized targets of AHR-mediated gene regulation and to investigate the hypothesis that AHR functions through mechanisms that are independent of DNA-binding. The advances in performing genome-wide transcriptional profiling at the time of commencement of this project, encouraged the use of DNA-microarray technology for identifying new target genes. Epiregulin, a potent mitogen belonging to the epidermal growth factor family, was discovered to be regulated by AHR in immortalized murine hepatocytes. The fact that a number of AHR ligands have been associated with carcinogenesis signifies that the induction of growth factors like epiregulin might be a potential mechanism for AHR-mediated tumor enhancement. The next phase of this project led to the identification of the constitutive androstane receptor (CAR), another receptor involved in drug metabolism, as an in vivo target of AHR activation. This association between AHR-CAR highlights the possibility of crosstalk between AHR and other pathways. Exposure to divergent stimuli leads to simultaneous activation of multiple signaling pathways. This suggests that it is essential to study the networking of various pathways to be able to predict the biological outcomes. The third phase of this project focuses on the ability of AHR to modulate the inflammatory pathway and on the involved mechanism. AHR activation can repress the acute-phase response (APR) gene expression, implicated in disorders like septic shock and Alzheimer’s, partly by antagonizing NF-êB mediated gene regulation through a non-classical mechanism not involving DRE. Serum amyloid family members, C-reactive protein and haptoglobin were found to be repressed by AHR, signifying that AHR regulates multiple members of the APR. Thus, this research has led to the identification of multiple AHR-regulated genes. It also presents a model to study AHR-mediated gene repression, an aspect that has therapeutic potential.