Role of Nrf2 Signaling in Therapeutic Resistance and Cancer Metastasis

Abstract

The stress response transcription factor Nrf2 is a master regulator of cytoprotective genes. In non-stress situation, INrf2 (Keap1), constantly sequesters Nrf2 and facilitates its ubiquitination-mediated proteasomal degradation. Upon exposure to stress, Nrf2 stabilizes, translocates to nucleus and binds to antioxidant response elements (ARE) in the promoter of its target genes and activates transcription of many cellular defense genes, resulting in cell survival and cytoprotection. Consequently, upregulation of Nrf2 results in development of therapeutic resistance. Aromatase inhibitors (AI) have been used for treatment of ERα positive breast cancer in post-menopausal women. However, persistent treatment with AI can lead to resistance. Little is known about the underlying mechanisms for the development of AI-resistance. In this study, we examined the involvement of Nrf2 signaling in AI-resistant cells and found downregulation of INrf2 and upregulation of Nrf2 and its target genes such as anti-apoptotic proteins, antioxidants and drug efflux transporters. Importantly, Nrf2- knock down AI-resistant cells showed fewer tumor-initiating cells (TIC), formed fewer mammospheres and become more sensitive to doxorubicin and etoposide. A stable cell line containing a luciferase reporter under the control of an ARE-promoter was generated to identify Nrf2 inhibitors by high-throughput screening to develop an adjuvant therapy that contains an Nrf2 inhibitor to enhance the therapeutic efficacy of aromatase inhibitors. We also investigated the role of Nrf2 in prostate cancer metastasis by generating stable cell lines expressing different levels of Nrf2. We investigated the role of Nrf2 signaling in colony formation, cell proliferation, migration and invasion. Our findings suggested that loss of Nrf2 leads to increased anchorage independent cell growth, proliferation, migration and invasion. In conclusion, persistent AI-treatment downregulated INrf2 leading to higher expression of Nrf2 and cytoprotective proteins that may, in part, result in increased AI-resistance. As knocking down Nrf2 decreased TIC, the inhibition of Nrf2 signaling appears to be beneficial to reduce AI-resistance. However, lowering the levels of Nrf2 might program cancer cells towards invasion and migration. Thus, understanding the levels of Nrf2 in tumors would be important to determine whether inhibition or activation of Nrf2 is beneficial to a cancer therapy.