The role of the Keap1/Nrf2 signaling pathway in the pathogenesis of non-small cell lung cancer and induction of cancer treatment resistance

Allen Jacob

 Introduction. Lung Cancer has been identified by the National Cancer Institute of the NIH as the leading cause of mortality among all cancers in the United States, with an estimated 155,870 deaths in 2017 alone. Non-small cell lung cancer (NSCLC) encompasses 85% of lung cancer diagnoses, and a normal pathway that is often mutated in most NSCLC cases is the Keap1/Nrf2 signaling pathway. Nrf2 is a transcription factor located within the cell cytoplasm that maintains reduction-oxidation homeostasis by regulating many protective enzymes, such as antioxidants, phase I and phase II detoxification enzymes, and drug efflux membrane transporters. Under homeostatic conditions, most Nrf2 is bound by Keap1, which is a negative regulator of Nrf2 activity. In tissue stains of tumors in patients with NSCLC, a common observation in the overexpression of Nrf2 and the under expression of Keap1. Further studies in Nrf2 knockout mice have shown that while Nrf2 functions to prevent the initiation of lung carcinogenesis, its overexpression paradoxically leads to the progression of lung cancer. Methods. A high-throughput screen was first used to screen out useful probe molecules out of 400,000 total molecules. The molecule identified that can bind to Nrf2 and inhibit it was ML385. Then clonogenic assays, treated with either doxorubicin or taxol, were created using ML385 and platinum-based drugs. Preclinical models of NSCLC cells with gain of Nrf2 function were used as targets of ML385 in combination with carboplatin. Statistical analyses were then conducted using a one-way ANOVA and post hoc test. Results. ML385 inhibits Nrf2 signaling in lung cancer cells and was shown to reduce Nrf2-mediated transcription and even reduce Nrf2 itself when administered against A549 cells. Interestingly, ML385 was also shown to be selectively toxic to the H460 NSCLC cell line with a point mutation in Keap1 but exert no effect on the growth of H460-Keap1 Knock-in H460 cells expressing wild-type Keap1. ML385 potentiated the toxicity of the chemotherapeutic drugs paclitaxel, doxorubicin, or carboplatin on H460 and A549 NSCLC cells with Keap1 mutations, leading to a reduction in tumor volume and weight. Conclusions. Targeting mutations in the Keap1/Nrf2 pathway is a necessary direction for the future of NSCLC treatments due to the variability and high mortality of this form of cancer. ML385 is a promising small molecule inhibitor of Nrf2 that should be used in combination with chemotherapeutic drugs to elicit the greatest reduction in NSCLC tumors.

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