Emerging role of nuclear factor erythroid 2-related factor 2 in the mechanism of action and resistance to anticancer therapies

Abstract

Nuclear factor E2-related factor 2 (NRF2), a transcription factor, is a master regulator of an array of genes related to oxidative and electrophilic stress that promote and maintain redox homeostasis. NRF2 function is well studied in in vitro, animal and general physiology models. However, emerging data has uncovered novel functionality of this transcription factor in human diseases such as cancer, autism, anxiety disorders and diabetes. A key finding in these emerging roles has been its constitutive upregulation in multiple cancers promoting pro-survival phenotypes. The survivability pathways in these studies were mostly explained by classical NRF2 activation involving KEAP-1 relief and transcriptional induction of reactive oxygen species (ROS) neutralizing and cytoprotective drug-metabolizing enzymes (phase I, II, III and 0). Further, NRF2 status and activation is associated with lowered cancer therapeutic efficacy and the eventual emergence of therapeutic resistance. Interestingly, we and others have provided further evidence of direct NRF2 regulation of anticancer drug targets like receptor tyrosine kinases and DNA damage and repair proteins and kinases with implications for therapy outcome. This novel finding demonstrates a renewed role of NRF2 as a key modulatory factor informing anticancer therapeutic outcomes, which extends beyond its described classical role as a ROS regulator. This review will provide a knowledge base for these emerging roles of NRF2 in anticancer therapies involving feedback and feed forward models and will consolidate and present such findings in a systematic manner. This places NRF2 as a key determinant of action, effectiveness and resistance to anticancer therapy.

Keywords: Cancer; DNA damage and repair response; anticancer therapeutics; drug resistance; nuclear factor E2-related factor 2; receptor tyrosine kinase inhibitor; targeted therapy.

Figure 1

Plausible mechanisms of cancer drug resistance. The mechanisms can generally be classified as pharmacological and cellular physiological mechanisms, which either feed into or feed out of the well-known and characterised hallmarks of cancer^[12]

Figure 2

Structural and functional domains of NRF2. A Schematic representation of the human/mammalian NRF2 structure and function domains. There are 7 highly conserved regions in NRF2 that are referred to as NEH domains. From the N-terminal to the C-terminal of NRF2, the NEH2 domain contains the DLG/ETGE motifs that facilitate NRF2 interaction with KEAP1 and for KEAP1-dependent NRF2 proteasomal degradation. The NEH2 domain also contain a lysine residues rich site that is directly ubiquitylated by the Cul3/Rbx1/E3 cullin-based E3 ubiquitin ligase substrate adaptor complex, as well as a first NLS sequence between the amino acids 42 and 53. The NEH4-5 domains facilitate the interaction of NRF2 with Hdr1 and other proteins like p300 and CBP to activate NRF2-dependent transcription; also, a NES is located between amino acids 191-202 in the NEH5 region. The NEH7 domain contains sites for interaction with the RARs (RXR-α and RAR-α) that facilitates NRF2 transcriptional repression. The NEH6 domain contains two specific sites of interaction with the β-transducing repeat-containing protein (βTrCP ubiquitin ligase; the binding by βTrCP to the DSGIS motif requires the prior phosphorylation of NRF2 in Ser344 and Ser347 by Gsk-3β, but the interaction of βTrCP with the DSPAGS motif of NRF2 is direct. The association of NRF2 with βTrCP leads to Cul1-mediated ubiquitination, followed by NRF2 proteasome degradation. The NEH1 domain contains the CNC bZIP region, which is required for DNA binding and dimerization with small Maf proteins and other TFs; also, there is a second NES sequence between amino acids 553 and 562. Finally, the NEH3 region is another transactivation domain that contains a second NLS sequence between amino acids 595 and 601. NRF2: nuclear factor E2-related factor 2; NLS: nuclear localisation signal; NES: nuclear export signal; CNC bZIP: Cap'n'Collar basic leucine zipper; TFs: transcription factors; Maf: musculoaponeurotic fibrosarcoma

Figure 3

Redox regulation of NRF2-KEAP1 signalling. A: Under normal physiological (homeostatic) conditions, KEAP1 interacts with NRF2 in the cytosol, promoting its polyubiquitylation and subsequent proteasomal degradation by the substrate adaptor for cullin-based E3 ubiquitin ligase complex, resulting in little NRF2 that is sufficient for the maintenance of cellular homeostasis or absence of NRF2 transactivation; B: In contrast, under oxidative stress conditions, the binding of KEAP1 to NRF2 is greatly impaired to compromise the likelihood of NRF2 ubiquitylation. Consequentl, a greater fraction of NRF2 molecules in the cytosolic pool can translocate into the nucleus, wherein NRF2 interacts with sMAF proteins, then binds to DNA and other transcription partners to form a heterodimeric nuclear complex, which induces the transcription of several antioxidant and cytoprotective genes. NRF: nuclear factor E2-related factor 2; sMAF: small musculoaponeurotic fibrosarcoma

Figure 4

Cooperativity of NRF2 and HER family receptors in cellular proliferation and cancer. The NRF2-ARE pathway cross-talks with the HER family receptor and other signalling pathways. This provides rationale and justification for the use and design of anticancer drugs and/or molecules that target HER family receptors and/or target NRF2 to enhance the action and effectiveness of HER targeting anticancer therapies and delay or overcome resistance. NRF2: nuclear factor E2-related factor 2

Figure 5

NRF2 modulators with anticancer activity. Small molecule compound or drugs that directly or indirectly modulate NRF2 activity by either activating or inhibiting NRF2 activity and functions. The benefits and risks of modulating NRF2 pathway activity in patients are not fully captured and understood. However, the development of novel NRF2 inhibitors used in combination with existing anticancer drugs could be rational strategy to arrest and mitigate the emergence of chemoresistance to anticancer agents. NRF2: nuclear factor E2-related factor 2

Figure 6

Positive and negative feedback loops in NRF2 signalling. A: The positive feedback regulatory loop of NRF2 and other proteins and signalling pathways that upregulate NRF2 and functions. Showing, in part, the mutual regulatory loop between NRF2 and HER family receptors; B: The negative feedback regulatory loop of NRF2 and other proteins and signalling pathways that repress NRF2 and functions. NRF2: nuclear factor E2-related factor 2