Lau A,* Wang H, Zhang DD. Arsenic induces chronic Nrf2 activation. Abstract for poster presentation, American College of Toxicology, 32nd Annual Meeting of the American College of Toxicology, Phoenix, AZ, November 2011.
The Nrf2-Keap1 signaling pathway is an endogenous protective mechanism promoting detoxification and cell survival against cellular stress and environmental insults. Previous research has demonstrated that the Nrf2-dependent defense response is imperative in protecting cells against arsenic toxicity. However, chronic activation of Nrf2 has been shown to be present in many types of cancers, contributing to chemoresistance. Here, we demonstrate that arsenic-mediated Nrf2 activation is dependent on p62, a specific substrate of autophagy, and can be attenuated when expression of p62 is silenced by siRNA. This non-canonical Nrf2 pathway results in a chronic, sustained increase of Nrf2 activation. Moreover, we demonstrate by live cell imaging that arsenic causes deregulation of autophagy, specifically resulting in accumulation of autophagosomes and leading to colocalization of Keap1, p62, and LC3 (a marker of autophagosomes). Utilizing a tandem RFP-GFP-LC3 construct, arsenic was shown to inhibit the later stages of autophagy, thereby affecting autophagic flux in in vitro models. Interestingly, the aforementioned arsenic-induced effects on autophagy were reversible by sulforaphane, a well-characterized chemopreventive compound that specifically activates Nrf2 through the Keap1-mediated pathway. Collectively these findings provide experimental evidence that arsenic causes prolonged activation of Nrf2 through autophagy dysfunction, which may lead to a novel mechanism for arsenic carcinogenicity in humans.