Deletion of Nrf1α promotes glutamine addiction of HepG2 cells and thus enhances its apoptosis caused by glutamine deprivation

Abstract

Glutamine metabolism plays a central role in regulating uncontrolled growth of tumor (exhibiting glutamine addiction) by modulating bioenergetics and redox homeostasis, as well as serving as a precursor for biomass synthesis. Thus, limiting its availability is becoming a potential therapeutic strategy. Since Nrf1 is an indispensable determinant of mitochondrial homeostasis through the integration of a multilevel regulatory network for redox balance, no reports on the regulatory role of Nrf1 in glutamine addiction emerged hitherto. In this study, we found that glutamine deprivation leads to mitochondrial morphological and functional damage, reduced GSH levels, increased ROS, and rapid death of Nrf1-knockout HepG2 cells. A series of further experiments revealed that such rapid death of Nrf1-deficient cells from glutamine deprivation is caused by different ways. Loss of Nrf1 results in an obvious enhancement of glycolysis and fatty acid synthesis, leading to extensive catabolism of glutamine into -KG within mitochondria; the -KG reverses TCA cycles to generate citrate, which exits to the cytosol for fatty acid synthesis. Simultaneously, increased expression of xCT together with decreased expression of GLUL leads to elevated extracellular efflux of glutamate and reduced capacity for glutamine synthesis. These factors intensify glutamine addiction of Nrf1-null cells, with reducing GSH synthesis and increasing ROS levels due to dysfunctional mitochondria, ultimately leading to enhanced cell death upon glutamine deprivation. Overall, such insights into Nrf1s role in cancer glutamine metabolism are conducive to developing much specific preventive and therapeutic strategies by precision Nrf1-targeting anti-tumorigenesis.