The Molecular Evolution of the Aryl Hydrocarbon Receptor and the Effects of IFNγ on Tryptophan Metabolism in Oral Squamous Carcinoma Cells
Open Access
- Author:
- Yap, Melanie Jane
- Graduate Program:
- Biochemistry, Microbiology, and Molecular Biology
- Degree:
- Master of Science
- Document Type:
- Master Thesis
- Date of Defense:
- May 30, 2018
- Committee Members:
- Gary H Perdew, Thesis Advisor/Co-Advisor
Andrew David Patterson, Committee Member
Kumble Sandeep Prabhu, Committee Member - Keywords:
- Aryl hydrocarbon receptor
AHR
Oral Squamous Carcinoma
OSC19
Head and Neck Squamous Cell Carcinoma
HNSCC
Tryptophan
Kynurenine
Tryptophan Metabolism
IDO1 - Abstract:
- The aryl hydrocarbon receptor (AHR) is the only ligand-activated transcription factor in the basic helix-loop-helix Per-Arnt-Sim family. It was first discovered as the mediator for 2,3,7,8-tetrachlorodibenzo-p¬¬-dioxin (TCDD)-induced xenobiotic metabolism and acute toxicity. The AHR has been implicated in cell cycle progression, organogenesis, carcinogenesis, gastrointestinal barrier function and immune regulation. AHR can be activated by a wide array of exogenous and endogenous ligands. Evolutionary studies have revealed a difference in AHR ligand selectivity and sensitivity between species. In this thesis, we examined guinea pig AHR as a potential evolutionary link between rodent AHR and Neanderthal AHR. Our findings suggest that guinea pig AHR does not exhibit higher endogenous ligand binding sensitivity that is observed in hominids. Thus, guinea pig AHR appears to more closely resemble the mouse AHR. The AHR has been of interest in cancer development and progression especially in terms of its role in tryptophan catabolism, through the induction of indolamine-2,3-dioxygenase (IDO1) expression and the production of the metabolite kynurenine, an AHR ligand. IFNγ-induced overexpression of IDO1 in human oral squamous carcinoma cells (OSC19) resulted in an increase in the production of kynurenine; while there was no congruent increase in AHR activation in OSC19 cells. However, the application of conditioned media taken from IFNγ-treated OSC19 cells induced AHR activation in reporter cells, suggesting rapid efflux of tryptophan metabolites. Kynurenine and other L-kynurenine metabolites in OSC19 media were identified using ELISA and LC-MS techniques. Furthermore, OSC19 cells incubated in 10% amino acid levels resulted in elevated secretion of AHR ligands into the media and reporter activity relative to incubation with 100% amino acid media. This result would argue that ligand production is not dependent on tryptophan levels in the media but is dependent on the effects of limiting the amino acid pool size. Moreover, these observations challenge the recent theory implicating kynurenine as a driver of AHR activity in tumor cells, instead our results would suggest that other cell types in the tumor microenvironment may exhibit AHR activation dependent on kynurenine import/export homeostasis. This would argue against the concept that the AHR ligand in the media is kynurenine. Future studies will be needed to identify the AHR ligand produced.