Arsenic is classified as a human carcinogen that has been associated with increased risk of developing cancers of the lung, skin, and bladder. However, the mechanism of arsenic tumorgenicity remains incompletely described. Arsenic has been shown to upregulate NRF2, the master regulator of the cellular stress response in vitro and in vivo. Under non-stress conditions, the basal level of NRF2 is kept low by KEAP1-CUL3-RBX1- mediated ubiquitylation and subsequent 26S proteasomal degradation. During canonical NRF2 activation KEAP1-Cys151 is modified, preventing NRF2 ubiquitylation and increasing the level of NRF2 and its downstream genes to protect cells from environmental insults. Once cellular homeostasis is restored, NRF2 returns to low basal level, ensuring transient NRF2 upregulation. However, we discovered arsenic activates NRF2 by a non- canonical mechanism. Environmentally relevant doses of arsenic block autophagosome maturation, which leads to p62-mediated sequestration of KEAP1 into autophagosomes, blocking NRF2 ubiquitylation and degradation. This leads to sustained hyperexpression of NRF2 and its antioxidant response element (ARE) containing target genes, conferring a cellular survival advantage. Interestingly, our novel unpublished results demonstrate that the ATPase associated with various cellular activities (AAA+) chaperone, p97, contains a functional ARE in its promoter. Functionally, p97 uses the binding and hydrolysis of ATP to generate a force to segregate ubiquitylated proteins from other biomolecules, often for subsequent degradation by the proteasome. We have demonstrated that p97 negatively regulates NRF2 by extracting ubiquitinated NRF2 from the KEAP1-CUL3-RBX1 E3 ubiquitin ligase complex that facilitates NRF2 degradation by proteasome. This segregase activity of p97 plays a prominent role in protein homeostasis ? proteostasis. Our finding of p97 as a novel NRF2 target gene indicates that NRF2 controls not only oxidative stress but also proteotoxic stress. Like NRF2, hyperexpression of p97 in human tumors has been reported and p97 is considered a target for treating cancer. We also observed that the protein levels of both NRF2 and p97 were increased in arsenic transformed cell lines and in human lung tumor tissues (preliminary data). Based on these findings, we propose the following model: Under basal conditions, low level expression of NRF2 and p97 is maintained through the NRF2-p97-NRF2 negative feedback loop. However, during chronic arsenic exposure, this NRF2-p97-NRF2 negative feed bask loop is disabled since NRF2 is no longer ubiquitylated, resulting in sustained upregulation of both NRF2 and p97. Thus, arsenic increases both the oxidative stress response and the proteotoxic stress response. We will test our hypothesis that sustained NRF2 and NRF2-mediated p97 upregulation in response to environmental arsenic exposure provides a survival advantage that enables arsenic-exposed cells to survive both oxidative and proteotoxic stress and to accumulate sufficient molecular events that drive malignant transformation.
Arsenic is an environmental carcinogen the effects 200 million people worldwide. Yet, the underlying cause of arsenic mediated carcinogenesis remains without a molecular explanation. These studies will explore the relationship between redox and protein homeostasis and how this can lead to arsenic-derived tumors.
