Post-Transcriptional Regulation of Nrf2: Novel Mechanisms beyond Keap1

Abstract

Nrf2 (NF-E2-related factor 2) is a transcription factor that regulates a battery of downstream genes that contain the antioxidant response element (ARE) in their promoter regions, including intracellular redox-balancing proteins, phase II detoxifying enzymes, and transporters. These Nrf2-dependent proteins work in collaboration to protect against many diseases where oxidative stress plays an essential role in disease onset and progression. Consequently, it is imperative to understand the basic molecular mechanisms of how Nrf2 is regulated so that this pathway can be targeted for disease prevention and treatment.Nrf2 is mainly regulated at the protein level by the ubiquitin proteasome system. Under basal conditions Nrf2 is constantly ubiquitinated by the Keap1-Cul3-E3 ubiquitin ligase complex and subsequently degraded by the 26S proteasome. Currently, regulation of the Nrf2-Keap1 pathway by ubiquitination is largely understood. However, other mechanism responsible for modulating Nrf2-ARE signal remains to be explored. This dissertation identifies three molecular mechanisms that are important in understanding how the Nrf2-Keap1 pathway is regulated: (i) In Chapter 2, KPNA6 was identified and characterized as a negative regulatory mechanism of the Nrf2 pathway, which mediates Keap1 nuclear import and represses the Nrf2-dependent antioxidant response at post-induction phase. (ii) In Chapter 3, I identified PARP-1 as a new transcription co-activator of Nrf2, which augments ARE-specific DNA binding of Nrf2 and enhances the transcription of Nrf2 target genes. This indicates a novel function of PARP-1 and reveals another layer of regulation of Nrf2. (iii) In Chapter 4, I demonstrated that XBP1 and SYVN1 are involved in regulating the Nrf2 pathway in a Keap1-independent mechanism. During ER stress, XBP1s upregulates transcription of SYVN1, which is an ubiquitin E3 ligase. SYVN1 accelerates the clearance of Nrf2 protein through promoting ubiquitination of Nrf2, and subsequent proteasomal degradation. Moreover, we observed an inverse correlation between XBP1s/SYVN1 and Nrf2 expression in the end stage alcoholic cirrhosis liver samples, implying a pathological role of ER stress-oxidative stress crosstalk. Taken together, these findings further our understanding of how the Nrf2-Keap1 pathway is regulated, providing novel targets of chemoprevention or chemotherapy.