Under the basal resting conditions, cytosolic Nuclear factor erythroid 2-related NFE2L2 (NRF2) is maintained at low basal levels by constitutive proteasomal degradation. Kelch-like ECH associated protein 1 (KEAP1), which is a substrate adaptor protein for the Cullin 3 (CUL3)-dependent E3 ubiquitin ligase complex binds with and represses NFE2L2 by promoting its ubiquitination and subsequent proteasomal degradation (Itoh et al. 1999, Cullinan et al. 2004, Kobayashi et al. 2004, Zhang et al. 2004, Furukawa & Xiong 2005). Therefore, the KEAP1–CUL3–E3 ubiquitin ligase complex tightly regulates NFE2L2 protein to maintain it at a low level. NFE2L2 contains seven functional domains, known as Neh1-Neh7. Neh2 domain contains two motifs termed ETGE and DLG that are involved in interacting with KEAP1. The ETGE and the DLG motifs have overlapping binding sites on KEAP1, with the ETGE motif mediating a higher affinity interaction with KEAP1 than the DLG motif. One molecule of NFE2L2 interacts simultaneously with two KEAP1 molecules, with the DLG motif and the ETGE motif on NFE2L2 contacting similar sites on each member of the KEAP1 dimer (Tong et al, 2006; McMahon et al, 2006; Baird et al, 2013; Fukutomi et al, 2014). This complex assembly positions NFE2L2 appropriately to be ubiquitinated by the CUL3/RBX1 ubiquitin ligase, targeting it for degradation. In the presence of electrophiles or other NFE2L2 inducers, conformational changes within KEAP1 occur as inducers interact with KEAP1 'sensor cysteines'. These conformational changes disrupt the KEAP1-DLG motif interaction, repositioning NFE2L2 within the KEAP1 complex in such a way as to prevent its ubiquitination. In this 'hinge and latch model', saturation of the KEAP1:CUL3:RBX1 complex with mal-positioned and thus not degradable NFE2L2 allows newly translated NFE2L2 to accumulate and translocate into the nucleus to stimulate transcription (Tong et al, 2006; Tong et al, 2007; reviewed in Baird and Yamamoto, 2020). This NFE2L2-KEAP1 interaction is also known to be regulated negatively by multiple proteins like PALB2, DPP3 and AMER(WTX1) which competitively bind with NFE2L2/KEAP1 and allow NFE2L2 nuclear translocation. They play an important role in NFE2L2 regulation to other pathways (Mizukami et al, 2012; Lu et al, 2017; Camp et al, 2012). In addition to that, this NFE2L2-KEAP1 binding is also regulated by succinylation of KEAP1 as cysteine residue and inhibits KEAP1 binding with NRF2 in FH mutant (LOF) condition (Ooi et al, 2011).
KEAP1 and NFE2L2 mutations occur in several tumor types and KEAP1 and NFE2L2 mutations occur at a frequency of around 25% in lung cancer. The NFE2L2 pathway has multiple pro-tumorigenic functions, and NFE2L2 levels are increased in head and neck squamous cell carcinoma (HNSCC). KEAP1 somatic mutant C23Y is observed in tumors from approximately 15% of patients with lung cancer (Hayes & McMahon 2009).
