Abstract B16: Activation of NRF2 and adaptive resistance to chemotherapy

Nuclear factor-erythroid-2-related factor 2 (NRF2), a member of the cap ‘n’ collar family of bZIP transcription factors, confers protection against oxidative and electrophilic stress. NRF2 is of great interest in cancer research, due to its role in response to chemotherapy, including the class of drugs targeting thymidylate synthase (TYMS). It has long been known that inhibition of TYMS leads to depletion of thymidine levels and the onset of programmed cell death, deriving from the enzyme's function as the sole de novo source of thymidine for DNA replication and repair. Exposing cells to TYMS inhibitors such as fluoropyrimidine antimetabolites (5-fluorouracil, or FUra; 5'-fluoro-2'-deoxyuridine, or FdUrd), as well as anti-folate analogs (raltitrexed, or RTX), induce intracellular concentrations of reactive oxygen species, which are a primary cause of drug-mediated toxicity. This prompted our focus on assessing the impact of NRF2 on cellular response to TYMS inhibitors. Using human colon tumor-derived cell line HCT116, we have shown by gene expression profiling that drug exposure induces expression of a number of genes that are modulated by NRF2. Quantitative PCR assays of several colon tumor cell lines verified that FUra, FdUrd, and RTX induce transcription of several genes known to be NRF2-targets, including AKR1B10, ALDH3A1, HSPB8, HMOX1, and SERPINE1, among others. Such induction mirrors that in response to the known NRF2 activator tert-butylhydroquinone (tBHQ). NRF2 protein concentration and nuclear localization are increased by FdUrd treatment, though not to the same extent as with tBHQ. Reporter gene constructs were used to show that both FdUrd and tBHQ induce transcription mediated by the NRF2-binding antioxidant response element (ARE). Furthermore, chromatin-immunoprecipitation experiments revealed that TYMS inhibitors promote occupancy of the ARE regions of several genes by NRF2; again, tBHQ had a much greater effect. Further analysis showed an association between gene induction by TYMS inhibitors and patterns of histone acetylation within the NRF2-binding ARE regions. Finally, we observed that increases in the apoptotic index following exposure to TYMS inhibitors were greater in cells in which the transcription factor was subjected to siRNA-mediated “knockdown” or CRISPR/Cas9-mediated “knockout”, indicating that reduced NRF2 expression sensitizes cells to TYMS inhibitors. Overall, we conclude that TYMS inhibitors activate NRF2 and its downstream