Context: Our previous data showed that reactive oxygen species generation might be ascribed to the cytotoxic response of thyroid cancer cells to proteasome inhibition and the ability of cancer cells to induce catalytic subunit for glutamate cysteine ligase (GCLC) and subsequent production of glutathione, thereby scavenging reactive oxygen species was partly ascribed to the cytotoxic responses of thyroid cancer cells to proteasome inhibition. Nuclear factor erythroid 2-related factor 2 (Nrf2) is a transcription factor responsible for transcriptional activation of various cytoprotective genes including GCLC.
Objective: The purpose of this study was to determine the involvement of Nrf2 in GCLC induction and cytotoxicity of thyroid cancer cells mediated by proteasome inhibition.
Design: The effects of proteasome inhibition on the expression and distribution of Nrf2 were analyzed using immunocytochemistry and Western blot. To ascertain the effect of Nrf2 and p38 MAPK, cells were transfected with Nrf2 plasmid or small interfering RNA against Nrf2 or p38 MAPK. Apoptotic cells, production of glutathione, and induction of GCLC mediated by proteasome inhibition were investigated using flow cytometry, spectrophotometry, and real-time RT-PCR, respectively.
Results: Proteasome inhibition caused accumulation and nuclear translocation of Nrf2, which compromised the cytotoxic effects of proteasome inhibition, at least in part, via induction of GCLC. In addition, nuclear translocation of Nrf2 was p38 MAPK dependent, and p38 MAPK inhibition augmented the cytotoxic effects of proteasome, at least partly, via suppression of transactivation of Nrf2.
Conclusions: These studies support the hypothesis that proteasome inhibitors activate an antiapoptotic survival program through p38 MAPK that involves transcriptional activity of Nrf2.