Synergism and antagonism of two distinct, but confused, Nrf1 factors in integral regulation of the nuclear-to mitochondrial respiratory and antioxidant transcription networks

Abstract

There is hitherto no literature available for explaining two distinct, but confused Nrf1 transcription factors, because they shared the same abbreviations from nuclear factor erythroid 2-related factor 1 (also called Nfe2l1) and nuclear respiratory factor (originally designated -Pal). Thus, we have here identified that Nfe2l1Nrf1 and -PalNRF1 exert synergistic and antagonistic roles in integrative regulation of the nuclear-to-mitochondrial respiratory and antioxidant transcription profiles. In mouse embryonic fibroblasts (MEFs), knockout of Nfe2l1-/- leads to substantial decreases in expression levels of -PalNRF1 and Nfe2l2, together with TFAM (mitochondrial transcription factor A) and other target genes. Similar inhibitory results were determined in Nfe2l2-/- MEFs, with an exception that GSTa1 and Aldh1a1 were distinguishably up-regulated in Nfe2l1-/- MEFs. Such synergistic contributions of Nfe2l1 and Nfe2l2 to the positive regulation of -PalNRF1 and TFAM were validated in Keap1-/- MEFs. However, human -PalNRF1 expression was unaltered by hNfe2l1-/-, hNfe2l2-/-{Delta}TA or even hNfe2l1-/-+siNrf2, albeit TFAM was activated by Nfe2l1 but inhibited by Nfe2l2; such an antagonism occured in HepG2 cells. Conversely, almost all of mouse Nfe2l1, Nfe2l2 and co-target genes were down-expressed in -PalNRF1+/- MEFs. On the contrary, up-regulation of human Nfe2l1, Nfe2l2 and relevant reporter genes took place after silencing of -PalNRF1, but their down-regulation occurred upon ectopic expression of -PalNRF1. Furtherly, Pitx2 (pituitary homeobox 2) was also identified as a direct upstream regulator of Nfe2l1 and TFAM, besides -PalNRF1. Overall, these across-talks amongst Nfe2l1, Nfe2l2 and -PalNRF1, along with Pitx2, are integrated from the endoplasmic reticulum to the nuclear-to-mitochondrial communication for targeting TFAM, in order to finely tune the cellular respiratory and antioxidant gene transcription networks, albeit they differ between the mouse and the human.