Activation of the Nrf2-Keap1-ARE pathway is a master defense mechanism protecting against oxidative and electrophilic stress. Nrf2 was traditionally considered cancer-preventing, as Nrf2 deficiency enhances susceptibility to carcinogens in a variety of mouse models. However, increasing evidence suggests a tumor-promoting role for Nrf2 in established tumors: gain of function mutations in NFE2L2, which activates the Nrf2 pathway, or loss of function mutations in its negative regulator, Keap1, have been found most frequently in lung cancer. Constitutive activation of the Nrf2 pathway is associated with chemoresistance and poor survival. Nrf2 inhibitors, therefore, become potential novel drugs for cancer treatment and needed tool compounds to study the Nrf2 pathway in cancer. To identify Nrf2 inhibitors, we conducted a high throughput screen with a diverse set of small molecules (~6500 compounds). 890 compounds were identified as hits, and a funnel strategy was applied to filter through the hits and select lead compounds. Although other known Nrf2 inhibitors were identified, a potentially novel compound, MSU225, was chosen as a lead for further evaluation. MSU225 downregulated tBHQ (tert-butyl hydroquinone)-induced Nrf2 transcriptional activity in a luciferase assay without inducing cell death. When A549 human lung cancer cells, in which Keap1 is mutated inducing constitutive activation of Nrf2, were treated with 5 µM MSU225, the mRNA expression of HMOX1, NQO1, GST1A1, GCLC, GCLM, and UGT1A6, all downstream targets of Nrf2, were decreased. Moreover, MSU225 significantly (p<0.05) decreased the protein level of Nrf2 in A549 cells. The effect of MSU225 on Nrf2 protein levels was blocked by the proteasome inhibitor MG132, suggesting MSU225 enhances Nrf2 degradation through the proteasome system. In addition, MSU225 inhibited the growth of human lung cancer cells (A549, H460, A427) in soft agar. Cells addicted to Nrf2 were more susceptible to MSU225 for suppression of cell proliferation than cells not addicted to Nrf2. MSU225 also sensitized A549 cells to carboplatin treatment. Our results suggest that MSU225 is a novel Nrf2 inhibitor, and additional in vivo studies will be done to determine if it can be used to treat experimental lung cancer.

Citation Format: Di Zhang, Corbin Livingston, Thomas Dexheimer, Edmund Ellsworth, Aaron Odom, Karen Liby. A small molecule inhibits Nrf2 expression and activity in cancer cells [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 1.