The Role of the Aryl Hydrocarbon Receptor in Tumor Growth and Chemoresistance

Abstract

The aryl hydrocarbon receptor (AHR) has been shown to play a role in cancer initiation and progression in oral squamous cell carcinomas (OSCC), and other cancers. The AHR is activated by environmental toxins, including polycyclic aromatic hydrocarbons, which are commonly found in cigarette smoke. It is hypothesized that activation of the AHR by these environmental toxins can contribute to the growth and chemoresistance of OSCCs. Nude mice tongues were injected with a human OSCCs cell line, SCC2s, and treated with an AHR antagonist at 25mg/kg daily via oral gavage. Primary tumor growth was measured via calipers and IVIS imaging. RT-qPCR analysis of the harvested tongue tumors and livers was used to examine the activity of the AHR by quantifying the expression levels of Cyp1b1 and Cyp1a1. Based on the results of the in vivo experiments, continued testing was conducted to examine the role of AHR inhibition in chemoresistance. Using MTT cell viability assays coupled with dosing of commonly used chemotherapeutics, the effects of the AHR on the chemo-resistance of SCC2s was tested. Three commonly used chemotherapeutics were tested at various dose ranges: Cisplatin (0-10uM), doxorubicin (0-1uM), and 5-Fluorouracil (0-10uM). In addition, cells were co-treated with an AHR antagonist (5uM CH223191) and the chemotherapeutic to determine if decreasing AHR activity increased chemotherapeutic efficiency. ANOVAs were used to evaluate the significance of AHR activity on the effectiveness of the chemotherapeutics. It was determined that AHR antagonism with CB7993113 significantly affected OSCC primary tumor growth in vivo. Additionally, it was found that both Cyp1a1 and Cyp1b1 expression decreased after treatment with CB7993113 when compared to vehicle alone in the tongue. In the liver, it was found that both Cyp1a1 and Cyp1b1 expression also decreased after treatment with CB7993113 when compared to vehicle alone. Interestingly, we also found that decreasing AHR activity with an AHR antagonist CH223191 in addition to treatment with a chemotherapeutic lead to a significant increase in cell death when compared to treatment with the chemotherapeutic alone. This phenomenon was observed in three different frontline OSCC therapeutics. These novel findings implicate the AHR in OSCC initiation and growth, also supporting the development of AHR modulators as potential chemotherapeutics. Overall, these findings support the hypothesis that the activation of the AHR is linked to tumor growth of oral squamous cell carcinomas as well as contributing to the potential chemoresistance of these cells.