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1.  Nrf2 protects human bladder urothelial cells from arsenite and monomethylarsonous acid toxicity 
Toxicology and applied pharmacology  2007;225(2):206-213.
Arsenic is widely spread in our living environment and imposes a big challenge on human health worldwide. Arsenic damages biological systems through multiple mechanisms including the generation of reactive oxygen species. The transcription factor Nrf2 regulates the cellular antioxidant response that protects cells from various insults. In this study, the protective role of Nrf2 in arsenic toxicity was investigated in a human bladder urothelial cell line, UROtsa. Using an UROtsa cell line stably infected with Nrf2-siRNA, we clearly demonstrate that compromised Nrf2 expression sensitized the cells to As(III)- and MMA(III)-induced toxicity. On the other hand, the activation of the Nrf2 pathway by tert-butylhydroquinone (tBHQ) and sulforaphane (SF), the known Nrf2-inducers, rendered UROtsa cells more resistant to As(III)- and MMA(III). Furthermore, the wild type mouse embryo fibroblast (WT-MEF) cells were protected from As(III)- and MMA(III)-induced toxicity following Nrf2 activation by tBHQ or SF whereas neither tBHQ nor SF conferred protection in the Nrf2−/−-MEF cells, demonstrating that tBHQ- or SF-mediated protection against As(III)- and MMA(III)-induced toxicity depends on Nrf2 activation. These results, obtained by both loss of function and gain of function analyses, clearly demonstrate the protective role of Nrf2 in arsenic-induced toxicity. The current work lays the groundwork for using Nrf2 activators for therapeutic and dietary interventions against adverse effects of arsenic.
doi:10.1016/j.taap.2007.07.016
PMCID: PMC2610476  PMID: 17765279
Nrf2; Keap1; arsenic; arsenite; MMA(III); UROtsa
2.  Keap1 Controls Postinduction Repression of the Nrf2-Mediated Antioxidant Response by Escorting Nuclear Export of Nrf2▿  
Molecular and Cellular Biology  2007;27(18):6334-6349.
The transcription factor Nrf2 regulates cellular redox homeostasis. Under basal conditions, Keap1 recruits Nrf2 into the Cul3-containing E3 ubiquitin ligase complex for ubiquitin conjugation and subsequent proteasomal degradation. Oxidative stress triggers activation of Nrf2 through inhibition of E3 ubiquitin ligase activity, resulting in increased levels of Nrf2 and transcriptional activation of Nrf2-dependent genes. In this study, we identify Keap1 as a key postinduction repressor of Nrf2 and demonstrate that a nuclear export sequence (NES) in Keap1 is required for termination of Nrf2-antioxidant response element (ARE) signaling by escorting nuclear export of Nrf2. We provide evidence that ubiquitination of Nrf2 is carried out in the cytosol. Furthermore, we show that Keap1 nuclear translocation is independent of Nrf2 and the Nrf2-Keap1 complex does not bind the ARE. Collectively, our results suggest the following mechanism of postinduction repression: upon recovery of cellular redox homeostasis, Keap1 translocates into the nucleus to dissociate Nrf2 from the ARE. The Nrf2-Keap1 complex is then transported out of the nucleus by the NES in Keap1. Once in the cytoplasm, the Keap1-Nrf2 complex associates with the E3 ubiquitin ligase, resulting in degradation of Nrf2 and termination of the Nrf2 signaling pathway. Hence, postinduction repression of the Nrf2-mediated antioxidant response is controlled by the nuclear export function of Keap1 in alliance with the cytoplasmic ubiquitination and degradation machinery.
doi:10.1128/MCB.00630-07
PMCID: PMC2099624  PMID: 17636022

Results 1-2 (2)