Differential yet integral contributions of Nrf1 and Nrf2 in the human antioxidant cytoprotective response against tert-butylhydroquinone as a pro-oxidative stressor

Abstract

In the past 25 years, Nrf2 had been preferentially parsed as a master hub of regulating antioxidant, detoxification and cytoprotective genes, albeit as a matter of fact that Nrf1, rather than Nrf2, is indispensable for cell homeostasis and organ integrity during normal growth and development. Here, distinct genotypic cell lines (Nrf1α−/−, Nrf2−/−ΔTA and caNrf2ΔN) are employed to determine differential yet integral roles of Nrf1 and Nrf2 in mediating antioxidant responsive genes to tBHQ as a pro-oxidative stressor. In Nrf1α−/− cells, Nrf2 was highly accumulated but also cannot fully compensate specific loss of Nrf1α's function in its basal cytoprotective response against endogenous oxidative stress, though it exerted partially inducible antioxidant response, as the hormetic effect of tBHQ, against apoptotic damages. By contrast, Nrf2−/−ΔTA cells gave rise to a substantial reduction of Nrf1 in both basal and tBHQ-stimulated expression and hence resulted in obvious oxidative stress, but can still be allowed to mediate a potent antioxidant response, as accompanied by a significantly decreased ratio of GSSG to GSH. Conversely, a remarkable increase of Nrf1 expression was resulted from the constitutive active caNrf2ΔN cells, which were not manifested with oxidative stress, no matter if it was intervened with tBHQ. Such inter-regulatory effects of Nrf1 and Nrf2 on antioxidant and detoxification genes (encoding HO-1, NQO1, GCLC, GCLM, GSR, GPX1, TALDO, MT1E and MT2), as well on the ROS-scavenging activities of SOD and CAT, were further investigated. The collective results unraveled that Nrf1 and Nrf2 make distinctive yet cooperative contributions to finely tuning basal constitutive and/or tBHQ-inducible expression levels of antioxidant cytoprotective genes in the inter-regulatory networks. Overall, Nrf1 acts as a brake control for Nrf2's functionality to be confined within a certain extent, whilst its transcription is regulated by Nrf2. Keywords: Nrf1; Nrf2; antioxidant; oxidative stress; reactive oxygen species; tert-butylhydroquinone (tBHQ).