The Role of the Nrf2-Keap1 Pathway in Autophagy and How it Contributes to Arsenic Carcinogenicity

Abstract

NF-E2-related factor 2 (Nrf2) is a transcription factor that is responsible for maintaining cellular homeostasis by controlling the fate of cells through transcriptional upregulation of antioxidant response element-bearing genes critical for eliminating toxicants and carcinogens. Under quiescent conditions, basal levels of Nrf2 are relatively low due to tight regulation by Keap1, a substrate adaptor protein for a Cullin 3 (Cul3)-E3 ubiquitin ligase complex that facilitates the ubiquitination and degradation of Nrf2. It is thought that when cells are exposed to oxidative stress, naturally-occurring compounds, or synthetic chemicals, cysteine residues in Keap1, particularly cysteine 151 (C151), are modified causing a conformational change that compromises the ability of the Keap1-Cul3-E3 ubiquitin ligase complex to properly ubiquitinate Nrf2. It is then stabilized and allowed to translocate into the nucleus to transcriptionally activate downstream genes. Interestingly, recent emerging data has revealed the "dark side" of Nrf2. Epigentic alterations and somatic mutations in either Nrf2 or Keap1 disrupting the Nrf2-Keap1 axis and causing constitutive activation of Nrf2 have been found in many human cancer cell lines and tumors. Thus, Nrf2 provides mutated cells a protective advantage against cytotoxic chemotherapeutics, allowing for further cell survival and growth. It is well known that arsenic is a human carcinogen and can activate the Nrf2 pathway through a Keap1-C151 independent mechanism. It has also been shown that arsenic can activate autophagy, a bulk-lysosomal degradation pathway. In this dissertation, we establish the cross-talk between the Nrf2-Keap1 pathway and autophagy by elucidating a novel non-canonical mechanism of Nrf2 activation. We found that deregulation of autophagy causes accumulation of p62, a substrate adaptor protein, which sequesters Keap1 into autophagosomes and activates the Nrf2 pathway. Moreover, we also demonstrate how arsenic blocks autophagic flux and prolongs Nrf2 activation through this novel mechanism. Additionally, activation of the Nrf2 pathway has been shown to confer protection against arsenic-induced toxicity and carcinogenicity. We demonstrate that co-treatment with sulforaphane alleviates arsenic-mediated autophagy. These studies suggest that the Keap1-C151 dependent mechanism triggers the chemopreventive role of Nrf2 while activation through p62 elicits the dark side. Therefore, the use of Keap1-C151-dependent compounds to counteract environmental insults continuous to be a promising strategy for cancer prevention.