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, β-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 β-TrCP recognition motif. The short interfering RNA (siRNA)-mediated silencing of β-TrCP markedly augments the expression of Nrf1 target genes, such as the proteasome subunit PSMC4, indicating that β-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 β-TrCP- and Hrd1-dependent degradation mechanisms regulate the transcriptional activity of Nrf1 to maintain cellular homeostasis.

ACKNOWLEDGMENTS

We are grateful to Keiko Nakayama, Raymond J. Deshaies, and Dirk Bohmann for the β-TrCP2 plasmids, 3×PSMA4-ARE-Luc, and HA ubiquitin, respectively. We also thank Tomoki Chiba, Takako Tsukide, Akiko Matsumoto, and Jun Takai for research support.

This work was supported in part by grants-in-aid from the Ministry of Education, Sports, Science and Technology (A.K., Y.T., and M.Y.), the Mochida Memorial Foundation (A.K.), the Naito Foundation (A.K.), the Suzuken Memorial Foundation (A.K.), the Takeda Science Foundation (A.K.), the Inamori Foundation (Y.T.), and the Uehara Memorial Foundation (Y.T.).