Metabolism regulated by NRF2
As a key regulator of redox metabolism, NRF2 directly regulates many enzymes and antioxidant proteins involved in the redox regulation. Enzymes and transporters supporting glutathione synthesis and utilization are widely regulated by NRF2, which includes catalytic and regulatory subunits of gamma-glutamylcysteine ligase (GCLC and GCLM), glutathione reductase (GSR), glutathione peroxidases (e.g. , GPX2), glutathione-S-transferase (e.g. , GSTM1 and GSTP1), and a cystine transporter (xCT) (Figure 4) (Malhotra et al., 2010; Chorley et al., 2012). Thioredoxin system is also under the regulation of NRF2. In NRF2-activated cancer cells possessing hyperactivation of NRF2 and consequently exhibiting NRF2 addiction, which is often caused by somatic mutations of KEAP1 or NFE2L2 gene, glutathione synthesis is greatly enhanced and thereby, cysteine, glutamate and glycine are highly consumed and required for glutathione. In the NRF2-activated cancer cells, the demand for cysteine is fulfilled by increased expression of xCT (Sasaki et al., 2002), and the requirement of glycine is covered by increased de novo synthesis from serine (DeNicola et al., 2015) and increased dependency on the uptake of extracellular serine and glycine (LeBoeuf et al., 2020). In contrast, glutamate is decreased and short due to glutamate export by xCT and glutamate consumption for the glutathione synthesis, which results in the metabolic vulnerability of NRF2-activated cancer cells (Romero et al., 2017; Sayin et al., 2017).
Another metabolic activity regulated by NRF2 is NADPH synthesis (Figure 4). Pentose phosphate pathway contains two enzymes for the NADPH synthesis, glucose-6-phosphate dehydrogenase (G6PD) and phosphogluconate dehydrogenase (PGD), both of which are target genes of NRF2 (Mutsuishi et al., 2012; Ding et al., 2021). Other NADPH synthesis steps are regulated by isocitrate dehydrogenase 1 (IDH1) and malic enzyme 1 (ME1), which are also regulated by NRF2 (Mutsuishi et al., 2012). Folate metabolism-coupled NADPH production mediated by methylenetetrahydrofolate dehydrogenase, cyclohydrolase and formyltetrahydrofolate synthetase 1 (MTHFD1) and MTHFD2 appears to be partly and indirectly regulated by NRF2, especially in NRF2-activated cancer cells where NRF2 cooperates with ATF4 (Mutsuishi et al., 2012; Fan et al., 2014).
As mentioned above, NRF2 enhances NADPH production by re-wiring metabolism pathway, and offer strong reducing condition. The NRF2-mediated reducing condition is beneficial for maintaining the high efficiency of translation because many ribosomal subunits are rather susceptible to oxidation of cysteine residues and subsequent decline of their functionality (Chio et al., 2016). On the other hand, continuous stabilization of NRF2 is considered to cause excessive cellular reducing force, that is, reductive stress. Inheritable missense mutations in small molecular weight heat-shock proteins promotes hypertrophic cardiomyopathy by forming protein aggregate containing KEAP1, which causes persistent activation of NRF2 (Rajasekaran et al., 2007; Rajasekaran et al., 2011). Under this condition, NRF2-induced reductive stress is regarded to further promotes protein aggregation, which exacerbates cardiomyopathy, as NRF2 suppression mitigates protein aggregation and improves the cardiac function (Kannan et al., 2013). Comprehensive analysis of various lung cancer cell lines also showed that NRF2 activation increases NADH vs. NAD+, leading to reductive stress (Weiss-Sadan et al., 2023). Consistent with these studies, NRF2-activated cancers exhibit dependency on SLC33A1, which is related to unfolded protein response and autophagy, possibly to avoid protein aggregation under reductive cellular environment caused by NRF2 (Romero et al., 2020).