Oxidative Stress and Cancer Heterogeneity Orchestrate NRF2 Roles Relevant for Therapy Response

Abstract

Oxidative stress and its end-products, such as 4-hydroxynonenal (HNE), initiate activation of the Nuclear Factor Erythroid 2-Related Factor 2 (NRF2)/Kelch Like ECH Associated Protein 1 (KEAP1) signaling pathway that plays a crucial role in the maintenance of cellular redox homeostasis. However, an involvement of 4-HNE and NRF2 in processes associated with the initiation of cancer, its progression, and response to therapy includes numerous, highly complex events. They occur through interactions between cancer and stromal cells. These events are dependent on many cell-type specific features. They start with the extent of NRF2 binding to its cytoplasmic repressor, KEAP1, and extend to the permissiveness of chromatin for transcription of Antioxidant Response Element (ARE)-containing genes that are NRF2 targets. This review will explore epigenetic molecular mechanisms of NRF2 transcription through the specific molecular anatomy of its promoter. It will explain the role of NRF2 in cancer stem cells, with respect to cancer therapy resistance. Additionally, it also discusses NRF2 involvement at the cross-roads of communication between tumor associated inflammatory and stromal cells, which is also an important factor involved in the response to therapy.

Keywords: 4-hydroxynonenal; KEAP-1; NFE2L2 promoter; cancer stem cells; micro RNA; polarization; therapy resistance; tumor associated macrophages (TAMs); tumor associated neutrophils (TANs).

Figure 1

Interactions of HNE with cellular proteins, lipids and DNA. HNE is an end-product of n6-polyunsaturated fatty acids (n6-PUFA) peroxidation which acts in a concentration dependent manner. Low concentrations: interaction with DNA (black arrow) results in forming exocyclic guanine adducts [24]. High concentrations: occurrence of sister chromatid exchange, DNA fragmentation [19], and inhibition of nucleotide excision repair [25]. HNE also binds to membrane lipids (purple arrow) [20]. Interactions with proteins are much more complex, as HNE directly or indirectly causes an increase (green arrows) or decrease (red arrow) in the activity or expression of Nuclear Factor Kappa-Light-Chain-Enhancer of Activated B Cells (NF-κB), Cyclooxygenase 2 (COX2), Hypoxia-Inducible Factor (HIF1) Vascular Endothelial Growth Factor (VEGF), TP53 and Epidermal Growth Factor Receptor (EGFR) [17,26]. Interactions with NRF2 will be reviewed separately.

Figure 2

NRF2 promoter, presented from −1342. The most current TSS is presented in red highlight (NM_006164.5), and the previous version is presented in turquois (NM_006164.4). The TSS described by Yamamoto is shown in green highlight [89]. Differentially methylated CG spots are presented in pink, including the one in the ARE element. Transcription factor binding sites for AHR/ARNT (Aryl Hydrocarbon Receptor/Aryl Hydrocarbon Receptor Nuclear Translocator), NF-κB, and MYC are underlined. ATG: first coding triplet. Polymorphism are presented in a blue highlight.

Figure 3

The involvement of cytokines, ROS, HNE, and NRF2 in TAM polarization. In response to TNF-α, LPS or IFN-γ, TAMs are differentiated into M1 phenotype. M1 TAMs express high levels of iNOS, ROS, and TNF-α promoting HNE, immunostimulation, inflammation, and inhibiting tumor growth. In contrast, in response to IL-4, IL-10, IL-13 or TGF-β, TAMs are polarized to M2 phenotype. Although M2 TAMs have lower ROS compared to M1, ROS are essential for NRF2 activation and M2 polarization. M2 TAMs produce TGF-β, MMP, IL-10, and VEGF promoting matrix remodeling, EMT, tumor growth, and metastasis.