Distinct roles of Nrf1 and Nrf2 in monitoring the reductive stress response to dithiothreitol (DTT)

Transcription factor Nrf2 (nuclear factor, erythroid 2-like 2, encoded by Nfe2l2) has been accepted as a key player in redox regulatory responses to oxidative or reductive stresses. However, it is less or not known about the potential role for Nrf1 (nuclear factor, erythroid 2-like 1, encoded by Nfe2l1) in the redox responses, particularly to reductive stress, albeit this fossil-like factor is indispensable for cell homeostasis and organ integrity during life process. Here, we examine distinct roles of Nrf1 and Nrf2 in monitoring the defense response to 1,4-dithiothreitol (DTT, serving as a reductive stressor), concomitantly with unfolded protein response being induced by this chemical (also as an endoplasmic reticulum stressor). The results revealed that intracellular reactive oxygen species (ROS) were modestly increased in DTT-treated wild-type (WT) and Nrf1-/- cell lines, but almost unaltered in Nrf2-/-{Delta}TA or caNrf2{Delta}N cell lines (with a genetic loss of its transactivation or N-terminal Keap1-binding domains, respectively). This chemical treatment also enabled the rate of oxidized to reduced glutathione (i.e., GSSG to GSH) to be amplified in WT and Nrf2-/-{Delta}TA cells, but diminished in Nrf1-/- cells, along with no changes in caNrf2{Delta}N cells. Consequently, Nrf1-/-, but not Nrf2-/-{Delta}TA or caNrf2{Delta}N, cell viability was reinforced by DTT against its cytotoxicity, as accompanied by decreased apoptosis. Further experiments unraveled that Nrf1 and Nrf2 differentially, and also synergistically, regulated DTT-inducible expression of critical genes for defending redox stress and endoplasmic reticulum stress. In addition, we have also identified that Cys342 and Cys640 of Nrf1 (as redox-sensing sites within its N-glycodomain and DNA-binding domain, respectively) are required for its protein stability and transcription activity.