Dual Regulation of the Transcriptional Activity of Nrf1 by β-TrCP- and Hrd1-Dependent Degradation Mechanisms

Abstract

A growing body of evidence suggests that Nrf1 is an inducible transcription factor that maintains cellular homeostasis. Under physiological conditions, Nrf1 is targeted to the endoplasmic reticulum (ER), implying that it translocates into the nucleus in response to an activating signal. However, the molecular mechanisms by which the function of Nrf1 is modulated remain poorly understood. Here, we report that two distinct degradation mechanisms regulate Nrf1 activity and the expression of its target genes. In the nucleus, ␤-TrCP, an adaptor for the SCF (Skp1-Cul1-F-box protein) ubiquitin ligase, promotes the degradation of Nrf1 by catalyzing its polyubiquitination. This activity requires a DSGLS motif on Nrf1, which is similar to the canonical ␤-TrCP recognition motif. The short interfering RNA (siRNA)-mediated silencing of ␤-TrCP markedly augments the expression of Nrf1 target genes, such as the proteasome subunit PSMC4, indicating that ␤-TrCP represses Nrf1 activation. Meanwhile, in the cytoplasm, Nrf1 is degraded and suppressed by the ER-associated degradation (ERAD) ubiquitin ligase Hrd1 and valosin-containing protein (VCP) under normal conditions. We identified a cytoplasmic degradation motif on Nrf1 between the NHB1 and NHB2 domains that exhibited species conservation. Thus, these results clearly suggest that both ␤-TrCP-and Hrd1-dependent degradation mechanisms regulate the transcriptional activity of Nrf1 to maintain cellular homeostasis.

The Casein Kinase 2-Nrf1 Axis Controls the Clearance of Ubiquitinated Proteins by Regulating Proteasome Gene Expression

“…1A). MG132 stabilized the Nrf1 protein as previously reported (17)(18)(19), and the siRNA-mediated knockdown of Nrf1 efficiently suppressed the accumulation of Nrf1 protein (Fig. 1B).…”

“…Plasmids and recombinant proteins. The 3ϫFlag mouse Nrf1 expression plasmid was described previously (19). Human CK2␣ (hCK2␣) and hCK2␤ cDNAs were subcloned into pcDNA3 (HA).…”

Hrd1 suppresses Nrf2-mediated cellular protection during liver cirrhosis

“…Until now, Keap1, b-TrCP, and Hrd1 are three E3 ubiquitin ligases of Nrf2 that have been discovered (Supplemental Table 1). It is also worth mentioning that only b-TrCP and Hrd1, but not Keap1, are the E3 ubiquitin ligases for Nrf1, another member of the CNC family, even though Nrf1 also contains Keap1-binding DLG and ETGE motifs (Supplemental Table 1; Zhang et al 2006;Steffen et al 2010;Tsuchiya et al 2011). Understanding how Nrf2 is controlled in distinct pathophysiological conditions and developing Nrf2 modulators to inactivate the appropriate E3 ligases are crucial for targeted disease prevention and intervention.…”

“…It also plays a crucial role in the pathogenesis of arthropathy and liver cirrhosis/fibrosis (Amano et al 2003;Hasegawa et al 2010). Although Hrd1 was initially characterized as an E3 ubiquitin ligase controlling ERassociated degradation (ERAD), recent reports demonstrate that Hrd1 can also control the turnover of non-ERAD substrates such as p53, IRE1, and Nrf1, a member of the cap'n'collar (CNC) transcription factor family (Yamasaki et al 2007;Gao et al 2008;Steffen et al 2010;Tsuchiya et al 2011). In addition to the UPR, reactive oxygen species (ROS) have been shown to play a major role in the pathogenesis of liver cirrhosis (Lotersztajn et al 2005;Wynn and Ramalingam 2012).…”

The Proteasome: From Basic Mechanisms to Emerging Roles

“…Intriguingly, β-TrCP, an adaptor for the Skp1-Cul1-F-box protein (SCF) ubiquitin ligase, promotes Nrf1 degradation by catalyzing its polyubiquitylation in the nucleus. 44 On the other hand, it has also been reported that Nrf1 is a shortlived protein that is targeted by the F-box protein Fbw7, which is another substrate-specifying component of SCFtype ubiquitin ligase that mediates degradation via the ubiquitin−proteasome pathway. 45 Together, these findings indicate that Nrf1 plays a central role in upregulating the proteasome system through an ERAD-dependent feedback loop.…”