The Nrf1 CNC-bZIP Protein Is Regulated by the Proteasome and Activated by Hypoxia

Department of Biology, Carleton University, Ottawa, Ontario, Canada.
PLoS ONE (Impact Factor: 3.23). 12/2011; 6(12):e29167. DOI: 10.1371/journal.pone.0029167
Source: PubMed


Nrf1 (nuclear factor-erythroid 2 p45 subunit-related factor 1) is a transcription factor mediating cellular responses to xenobiotic and pro-oxidant stress. Nrf1 regulates the transcription of many stress-related genes through the electrophile response elements (EpREs) located in their promoter regions. Despite its potential importance in human health, the mechanisms controlling Nrf1 have not been addressed fully.
We found that proteasomal inhibitors MG-132 and clasto-lactacystin-β-lactone stabilized the protein expression of full-length Nrf1 in both COS7 and WFF2002 cells. Concomitantly, proteasomal inhibition decreased the expression of a smaller, N-terminal Nrf1 fragment, with an approximate molecular weight of 23 kDa. The EpRE-luciferase reporter assays revealed that proteasomal inhibition markedly inhibited the Nrf1 transactivational activity. These results support earlier hypotheses that the 26 S proteasome processes Nrf1 into its active form by removing its inhibitory N-terminal domain anchoring Nrf1 to the endoplasmic reticulum. Immunoprecipitation demonstrated that Nrf1 is ubiquitinated and that proteasomal inhibition increased the degree of Nrf1 ubiquitination. Furthermore, Nrf1 protein had a half-life of approximately 5 hours in COS7 cells. In contrast, hypoxia (1% O(2)) significantly increased the luciferase reporter activity of exogenous Nrf1 protein, while decreasing the protein expression of p65, a shorter form of Nrf1, known to act as a repressor of EpRE-controlled gene expression. Finally, the protein phosphatase inhibitor okadaic acid activated Nrf1 reporter activity, while the latter was repressed by the PKC inhibitor staurosporine.
Collectively, our data suggests that Nrf1 is controlled by several post-translational mechanisms, including ubiquitination, proteolytic processing and proteasomal-mediated degradation as well as by its phosphorylation status.

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    • "Accumulating evidence reveals that over eleven Nrf1 isoforms [46] are produced from the single nfe2l1 gene, though they are differentially expressed in different mammalian species [1], [57]–[60]. Upon translation, the mouse Nrf1 is biosynthesized as an unglycosylated 95-kDa polypeptide. During the membrane-topogenic vectorial process, the nascent 95-kDa polypeptide is co-translationally integrated within the ER, where it is glycosylated to become a 120-kDa glycoprotein [45], [46]. "
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    ABSTRACT: e C-terminal domain (CTD, aa 686–741) of nuclear factor-erythroid 2 p45-related factor 1 (Nrf1) shares 53% amino acid sequence identity with the equivalent Neh3 domain of Nrf2, a homologous transcription factor. The Neh3 positively regulates Nrf2, but whether the Neh3-like (Neh3L) CTD of Nrf1 has a similar role in regulating Nrf1-target gene expression is unknown. Herein, we report that CTD negatively regulates the full-length Nrf1 (i.e. 120-kDa glycoprotein and 95-kDa deglycoprotein) and its shorter isoform LCR-F1/Nrf1β (55-kDa). Attachment of its CTD-adjoining 112-aa to the C-terminus of Nrf2 yields the chimaeric Nrf2-C112Nrf1 factor with a markedly decreased activity. Live-cell imaging of GFP-CTD reveals that the extra-nuclear portion of the fusion protein is allowed to associate with the endoplasmic reticulum (ER) membrane through the amphipathic Neh3L region of Nrf1 and its basic c-tail. Thus removal of either the entire CTD or the essential Neh3L portion within CTD from Nrf1, LCR-F1/Nrf1β and Nrf2-C112Nrf1, results in an increase in their transcriptional ability to regulate antioxidant response element (ARE)-driven reporter genes. Further examinations unravel that two smaller isoforms, 36-kDa Nrf1γ and 25-kDa Nrf1δ, act as dominant-negative inhibitors to compete against Nrf1, LCR-F1/Nrf1β and Nrf2. Relative to Nrf1, LCR-F1/Nrf1β is a weak activator, that is positively regulated by its Asn/Ser/Thr-rich (NST) domain and acidic domain 2 (AD2). Like AD1 of Nrf1, both AD2 and NST domain of LCR-F1/Nrf1β fused within two different chimaeric contexts to yield Gal4D:Nrf1β607 and Nrf1β:C270Nrf2, positively regulate their transactivation activity of cognate Gal4- and Nrf2-target reporter genes. More importantly, differential expression of endogenous ARE-battery genes is attributable to up-regulation by Nrf1 and LCR-F1/Nrf1β and down-regulation by Nrf1γ and Nrf1δ.
    Full-text · Article · Oct 2014 · PLoS ONE
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    • "Once the 120-kDa Nrf1 glycoprotein enters the cytoplasm and/or nucleoplasm, it is subject to N-linked deglycosylation by PNGase [29], before it is modified by calpain- and/or proteasome-mediated proteolysis [30], [63], [64] to yield distinct fragments of between 25-kDa and 95-kDa, each with a distinct function. It should be noted that we have yet to demonstrate that Nrf1 is enzymatically deglycosylated following its retrotranslocation from the ER lumen, but this seems to be probable. "
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    ABSTRACT: The membrane-bound Nrf1 transcription factor regulates critical homeostatic and developmental genes. The conserved N-terminal homology box 1 (NHB1) sequence in Nrf1 targets the cap'n'collar (CNC) basic basic-region leucine zipper (bZIP) factor to the endoplasmic reticulum (ER), but it is unknown how its activity is controlled topologically within membranes. Herein, we report a hitherto unknown mechanism by which the transactivation activity of Nrf1 is controlled through its membrane-topology. Thus after Nrf1 is anchored within ER membranes, its acidic transactivation domains (TADs), including the Asn/Ser/Thr-rich (NST) glycodomain situated between acidic domain 1 (AD1) and AD2, are transiently translocated into the lumen of the ER, where NST is glycosylated in the presence of glucose to yield an inactive 120-kDa Nrf1 glycoprotein. Subsequently, portions of the TADs partially repartition across membranes into the cyto/nucleoplasmic compartments, whereupon an active 95-kDa form of Nrf1 accumulates, a process that is more obvious in glucose-deprived cells and may involve deglycosylation. The repartitioning of Nrf1 out of membranes is monitored within this protein by its acidic-hydrophobic amphipathic glucose-responsive domains, particularly the Neh5L subdomain within AD1. Therefore, the membrane-topological organization of Nrf1 dictates its post-translational modifications (i.e. glycosylation, the putative deglycosylation and selective proteolysis), which together control its ability to transactivate target genes.
    Full-text · Article · Apr 2014 · PLoS ONE
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    • "Onetherapeuticstrategycouldbetotargettheproteosomal pathwaythatmayregulatep65-Nrf1processinganditssubcellular localization.Chepelevetal.(2011),recentlydescribedseveral mechanismsbywhichthefulllengthNrf1(p120-Nrf1)maybe processedintoitssmallerisoformsviathe26Sproteasome pathway[35].Theproteasomalinhibitor,MG-132canstabilize theexpressionoffull-lengthNrf1.Interestingly,theproteasome pathwayisknowntoregulateARactivityinPCacells[43]and Celastrol,apotentproteasomeinhibitor,cansuppressgrowthof PCaxenograftsinnudemice[44].SincetheNTDofp120-Nrf1is knowntofacilitateitsintegrationintotheERandnuclear membranes[27] [38].Strategiestosuppressthisnuclearexport signalintheNTDofp120-Nrf1couldincreaseitsnuclear localizationandreduceARactivityinCRPCcells[45]. "
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    Full-text · Article · Jan 2014 · PLoS ONE
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