A unity of opposites in between Nrf1- and Nrf2-mediated responses to the endoplasmic reticulum stressor tunicamycin

Abstract

The water-soluble Nrf2 is accepted as a master regulator of antioxidant responses to cellular stress, it was also identified as a direct target of the endoplasmic reticulum (ER)-anchored PERK. However, the membrane-bound Nrf1 response to ER stress remains elusive. Herein, we report a unity of opposites in both Nrf1- and Nrf2-coordinated responses to the ER stressor tunicamycin (TU). The TU-inducible transcription of Nrf1 and Nrf2, as well as GCLM and HO-1, was accompanied by activation of ER stress signaling networks. The unfolded protein response (UPR) mediated by ATF6, IRE1 and PERK was significantly suppressed by Nrf1-specific knockout, but hyper-expression of Nrf2, GCLM and HO-1 was retained in Nrf1-/- cells. By contrast, Nrf2-/-{Delta}TA cells with a genomic deletion of its transactivation domain resulted in significant decreases of GCLM, HO-1 and Nrf1; this was accompanied by partial decreases of IRE1 and ATF6, but not PERK, along with an obvious increase of ATF4. Notably, Nrf1 glycosylation and its trans-activity to mediate transcriptional expression of 26S proteasomal subunits were repressed by TU. This inhibitory effect was enhanced by Nrf1-/- and Nrf2-/-{Delta}TA, but not by a constitutive activator caNrf2{Delta}N (that increased abundances of non-glycosylated and processed Nrf1). Furthermore, caNrf2{Delta}N also enhanced induction of PERK and IRE1 by TU, but reduced expression of ATF4 and HO-1. Such distinct roles of Nrf1 and Nrf2 are unified to maintain cell homeostasis by a series of coordinated ER-to-nuclear signaling responses to TU. Overall, Nrf1 acts in a cell-autonomous manner to determine transcription of most of UPR-target genes, albeit Nrf2 is also partially involved in this process.