Nrf1 is not a direct target gene of SREBP1, albeit both are integrated into the rapamycin-responsive regulatory network in human hepatoma cells

Abstract

It is questionable why no further experimental evidence confirming those findings, since being reported by Mannings group in 2014s Nature (doi: 10.1038/nature13492), has been provided in the hitherto known literature. They found that Growth factors stimulate an increase in Nrf1 through mTORC1, which induces NRF1 transcription in an SREBP1-dependent manner and asserted that Nrf1 is directly regulated by SREBP1. Thereby, a key issue arising from their work is of particular concern about whether the mTORC1 signaling to upregulation of Nrf1-targeted proteasomal expression profiles occurs directly by SREBP1. In this study, our experiment evidence revealed that Nrf1 is not a direct target of SREBP1, although both are involved in the rapamycin-responsive regulatory networks. Closely scrutinizing two distinct transcriptomic datasets unraveled no significant changes in transcriptional expression of Nrf1 and almost all proteasomal subunits in either siSREBP2-silencing cells or SREBP1-/- MEFs, when compared to equivalent controls. However, distinct upstream signaling to Nrf1 dislocation by p97 and its processing by DDI1/2, along with downstream proteasomal expression, may be monitored by mTOR signaling, to various certain extents, depending on distinct experimental settings in different types of cells. Our further evidence has been obtained from DDI1-/-(DDI2insC) cells, demonstrating that putative effects of mTOR on the rapamycin-responsive signaling to Nrf1 and proteasomes may also be executed partially through a DDI1/2-independent mechanism, albeit the detailed regulatory events remain to be determined.