Novel NRF2-activated cancer treatments utilizing synthetic lethality

Abstract

The KEAP1-NRF2 pathway regulates the main inducible cellular response to oxidative and electrophilic stresses. Activating mutations in the KEAP1-NRF2 pathway occur commonly in human cancer, where they contribute to the formation of aggressive tumours that are associated with a poor prognosis for patients. An important clinical feature of these tumours is their defiance to all current anti-cancer treatment regimens, highlighting the need for the development of new therapeutic strategies to target NRF2-activated cancers. In this review, we discuss the mechanisms through which acquired NRF2 hyperactivation can result in resistance of tumours to immune checkpoint inhibitor therapies in addition to classical chemotherapeutics, and propose with examples that using a synthetic lethal strategy mediated by NRF2-target gene-dependent bioactivation of prodrugs represents a promising strategy to specifically enhance toxicity to heretofore untreatable NRF2-hyperactivated human tumours.

Keywords: KEAP1; NFE2L2; NRF2; bioactivation; cancer; mitomycin C; oxidative stress; prodrug; stress response; synthetic lethal.

FIGURE 1

An overview of the KEAP1‐NRF2 oxidative stress response pathway

FIGURE 2

A summary of the mechanisms through which NRF2 activation can protect cells from the cytotoxic effects of anti‐cancer drugs

FIGURE 3

Examples of the mechanisms through which anti‐cancer prodrugs can be bioactivated within patients

FIGURE 4

An overview of the synthetic lethal strategy used to target NRF2 activation in human cancer. The synthetic lethal “compound X" is only toxic to cells in combination with a pre‐existing mutation in KEAP1 leading to enhanced NRF2 signalling and an amplified NRF2 transcriptome

FIGURE 5

An overview of the isogenic cell screening assay used to identify compounds that are synthetic lethal with NRF2. WT cells are labelled with GFP, while KEAP1 KO cells are labelled with mCherry. The ratio between green and red fluorescence can be used to assay the toxicity of a compound based on the levels of Nrf2 activation

FIGURE 6

A scheme showing the intracellular bioactivation of mitomycin C (MMC), from the inert prodrug form to the final DNA damaging agent. Note that the NRF2 target genes NQO1, CYPOR and the components of the pentose phosphate pathway (PPP), which generate NADPH, all play a critical role in this bioactivation process

FIGURE 7

High levels of NRF2 activity make lung cancer cells extremely sensitive to the cytotoxic effects of mitomycin C (MMC). MMC sensitivity data were obtained from the Genomics of Drug Sensitivity in Cancer (GDSC) database. Cells were characterized based on their NRF2 activity levels as defined by Saigusa et al. The mechanisms through which NRF2 is activated are described in the following sources: * = COSMIC database, ** = Klijn et al., *** = Goldstein et al.

FIGURE 8

Across the entire Genomics of Drug Sensitivity in Cancer (GDSC) database, activation of NRF2 is a critical feature of mitomycin C (MMC) toxicity. The top 1% of cell lines from the GDSC database that are the most sensitive to MMC were selected and analyzed for their NRF2 activation status based on the expression of six well‐characterized NRF2 target genes, based on gene expression data from the Cell Model Passport. This analysis revealed that seven of the nine cell lines that are most sensitive to MMC showed high expression of at least three of these NRF2 target genes, which suggests that across all cancer cell types, level of NRF2 activation is a major determinant of MMC sensitivity

FIGURE 9

(a) The chemical structures of the four geldanamycin‐derived HSP90 inhibitors that are discussed in this review. (b) The mechanism through which the NRF2 target enzyme NQO1 catalyzes the bioactivation on 17‐AAG into the more potent compound 17‐AAGH₂

FIGURE 10

NRF2‐activated lung cancer cell lines that are sensitive to mitomycin C (MMC) are also sensitive to 17‐AAG treatment. (a) Data derived from the Genomics of Drug Sensitivity in Cancer (GDSC) database reveal that lung cancer cells with high levels of NRF2 activity that are sensitive to MMC treatment, also display enhanced sensitivity to 17‐AAG (Tanespimycin), when compared to lung cancer cell lines with low levels of NRF2 activity. This suggests that the co‐treatment of MMC with 17‐AAG could produce enhanced toxicity in cells with NRF2 activation. (b) The combination of synthetic lethal targeting of NRF2 activity through the parallel bioactivation of MMC and 17‐AAG by NRF2 target genes is named “concurrent synthetic lethality.” MMC is bioactivated by the NRF2 target genes NQO1 and Cytochrome P450 reductase (CYPOR), while 17‐AAG is bioactivated by NQO1. In both cases, the electrons required for the reductive bioactivation are donated from NADPH, which is generated through the pentose phosphate pathway (PPP). NRF2 directly regulates the gene expression of the enzymes which function in the PPP