Activation of the membrane-bound Nrf1 transcription factor by USP19, a ubiquitinspecific protease C-terminally tail-anchored in the endoplasmic reticulum

Abstract

The membrane-bound transcription factor Nrf1 (i.e., encoded by Nfe2l1) is activated by sensing glucose deprivation, cholesterol excess, proteasomal inhibition and oxidative stress, and then mediates distinct signaling responses in order to maintain cellular homeostasis. Herein, we found that Nrf1 stability and transactivity are both enhanced by USP19, a ubiquitin-specific protease tail-anchored in the endoplasmic reticulum (ER) through its C-terminal transmembrane domain. Further experiments revealed that USP19 directly interacts with Nrf1 in proximity to the ER and topologically acts as a deubiquitinating enzyme to remove ubiquitin moieties from this protein, and hence allows it to circumvent the potential proteasomal degradation. Such USP19-mediated effect takes place only after Nrf1 is retrotranslocated by p97 out of ER membranes to dislocate the cytoplasmic side. Conversely, knockout of USP19 causes significant decreases in the Nrf1 abundance and its active isoform entering the nucleus, resulting in down-regulation of its target proteasomal subunits. This led to a modest reduction of USP19-/--derived tumor growth in xenograft mice, when compared with wild-type controls. Altogether, these demonstrate that USP19 serves as a novel mechanistic modulator of Nrf1, but not Nrf2, enabling Nrf1 to be rescued from putative ubiquitin-directed ER-associated degradation pathway. In turn, our additional experimental evidence has unraveled that transcriptional expression of endogenous USP19 and its promoter-driven reporter genes is differentially regulated by Nrf2, as well by Nrf1, at distinct layers within a complex hierarchical regulatory network.