Advertisement

NRF2 Controls the Competitive Fitness of Squamous Epithelial Cells in the Mouse Esophagus

  • Zhaohui Xiong
    Affiliations
    Cancer Research Program, Julius L. Chambers Biomedical Biotechnology Research Institute, North Carolina Central University, Durham, North Carolina
    Search for articles by this author
  • Xiaoxin Chen
    Correspondence
    Correspondence Address correspondence to: Xiaoxin Chen, MD, PhD, Cancer Research Program, Julius L. Chambers Biomedical Biotechnology Research Institute, North Carolina Central University, 700 George Street, Durham, North Carolina 27707.
    Affiliations
    Cancer Research Program, Julius L. Chambers Biomedical Biotechnology Research Institute, North Carolina Central University, Durham, North Carolina
    Center for Gastrointestinal Biology and Disease, Division of Gastroenterology and Hepatology, Department of Medicine, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina
    Search for articles by this author
Open AccessPublished:November 06, 2022DOI:https://doi.org/10.1016/j.jcmgh.2022.10.010
      In the human esophagus, somatic cancer-associated mutations can occur within distinct clonal populations, some of which are selected over time.
      • Martincorena I.
      • Fowler J.C.
      • Wabik A.
      • et al.
      Somatic mutant clones colonize the human esophagus with age.
      In mice in which Cre-mediated deletion of the transcription factor NRF2 (nuclear factor erythroid 2-related factor) was designed to yield approximately equal levels of esophageal epithelial cells with Nrf2 deletion and those with Nrf2 intact, the Nrf2-deleted cells were eliminated rapidly on exposure to a chemical carcinogen,
      • Horiuchi M.
      • Taguchi K.
      • Hirose W.
      • et al.
      Cellular Nrf2 levels determine cell fate during chemical carcinogenesis in esophageal epithelium.
      suggesting a similar selection process. Functionally, NRF2 regulates stress responses and is itself regulated at the protein level by KEAP1 (Kelch-like ECH-associated protein 1), which promotes NRF2 degradation. Global Keap1 deletion leads to hyperproliferation and hyperkeratosis in the esophageal squamous epithelium, whereas deletion of Nrf2 along with Keap1 completely rescues the phenotype.
      In this issue of Cellular and Molecular Gastroenterology and Hepatology, Hirose et al
      • Hirose W.
      • Horiuchi M.
      • Li D.
      • et al.
      Selective elimination of NRF2-activated cells by competition with neighboring cells in the esophageal epithelium.
      addressed the selective extinction of Nrf2-deleted cells in the esophageal epithelium by investigating whether cells with higher NRF2 levels had a competitive advantage over cells with normal NRF2 levels. To test this, the authors activated NRF2 in a subgroup of esophageal squamous epithelial cells by tamoxifen-induced recombination using K5CreERT2;Keap1floxB/floxB mice. Expression of NRF2 in esophageal epithelial cells following conditional Keap1 deletion (Keap1-cKO mice) was assessed by staining for the NRF2 transcriptional target NQO1. Mild hyperkeratosis was observed in the esophagus of these Keap1-cKO mice, in contrast to severe hyperkeratosis seen in global Keap1 deletion. Interestingly, KEAP1-deleted cells (ie, NQO1 positive/NRF2-high) and KEAP1-normal cells (NRF2-normal) were committed to different fates, with the former being hyperproliferative and detached from the basement membrane for terminal differentiation and the latter undergoing compensatory hyperproliferation. As a result, KEAP1-deleted cells were outcompeted by KEAP1-normal cells and mostly eliminated from the esophageal epithelium.
      Cell competition was further tested in K5CreERT2;Keap1floxB/floxB;Nrf2SA/SA (Keap1 cKO::Nrf2SAmice), in which a knock-in mutation prevents degradation of NRF2 by KEAP1 and another protein, β-transducin repeat containing 21 protein (βTRCP). Because the Nrf2SA mutation was global, Keap1-cKO::Nrf2SA mice had higher levels of NQO1 (suggesting stronger NRF2 activation) than Keap1-cKO mice, and consistent with the prior results, the disappearance of NQO1-positive cells was even more rapid in Keap1-cKO::Nrf2SA mice than in Keap1-cKO mice. Although replicative stress and DNA damage took place in proliferating KEAP1-normal epithelial cells, inflammatory cells also migrated to the submucosal and intraepithelial layers, particularly in the regions surrounding the KEAP1-normal epithelial cells. Furthermore, Keap1-cKO mice developed many more chemically induced tumors than wild-type mice, and 26/27 of these tumors (96%) were KEAP1 positive. These data suggest that even though KEAP1-deleted cells lost the competition with KEAP1-normal cells, KEAP1-deleted cells still contributed to tumor formation of KEAP1-normal cells; potential mechanisms include residual inflammation and compensatory growth pressure resulting from the KEAP1-deleted cells that promoted proliferation and carcinogenesis in the KEAP1-normal cells or the generation of toxic metabolites, paracrine signals, or mechanical stress by KEAP1-deletion.
      • Hirose W.
      • Horiuchi M.
      • Li D.
      • et al.
      Selective elimination of NRF2-activated cells by competition with neighboring cells in the esophageal epithelium.
      Notably, NRF2 activation caused inflammation by compromising the epithelial barrier in the mouse skin, so this may also play a role in the esophageal epithelium.
      • Schafer M.
      • Willrodt A.H.
      • Kurinna S.
      • et al.
      Activation of Nrf2 in keratinocytes causes chloracne (MADISH)-like skin disease in mice.
      Manipulation of the competitive fitness of various cell populations may have important implications for esophageal cancer prevention and therapy. For example, a recent study using a chemical carcinogenesis model demonstrated that Notch1-inactivating mutations conferred a competitive advantage in the histologically normal esophageal epithelium and resulted in the elimination of microtumors.
      • Colom B.
      • Herms A.
      • Hall M.W.J.
      • et al.
      Mutant clones in normal epithelium outcompete and eliminate emerging tumours.
      The study by Hirose et al
      • Hirose W.
      • Horiuchi M.
      • Li D.
      • et al.
      Selective elimination of NRF2-activated cells by competition with neighboring cells in the esophageal epithelium.
      implicates NRF2 signaling as another potential target regulating clonal selection in esophageal squamous epithelial cells. In human esophageal squamous cell carcinoma, NRF2 may be mutated, overexpressed, and hyperactive, whereas KEAP1 mutations are much less frequent. In those cancers with hyperactivating NRF2 mutations, NRF2-normal cells might be induced to outcompete neighboring NRF2-high cells before treatment with chemotherapy and radiation. Moderately high NRF2, which can be achieved through treatment with chemical NRF2 activators, but not high NRF2, would seem most desirable for esophageal cancer prevention.
      Nevertheless, this study raises new questions about how NRF2 regulates homeostasis and carcinogenesis in the esophageal epithelium. For example, what molecules are essential to enhance the competitive fitness of KEAP1-normal/NRF2-normal cells? Do cells with activating mutations in NRF2 have different fitness than wild-type NRF2-high cells when competing against KEAP1-normal cells/NRF2-normal cells? Is there a way to optimize NRF2 activity in the esophageal epithelium to a favorable level? Because the normal human esophagus is not keratinized like the mouse esophagus and hyperkeratosis is rarely seen in the human esophagus, are the findings in this study directly applicable to human esophagus? Further studies along these directions will be of great interest to those studying esophageal biology and cancer.

      References

        • Martincorena I.
        • Fowler J.C.
        • Wabik A.
        • et al.
        Somatic mutant clones colonize the human esophagus with age.
        Science. 2018; 362: 911-917
        • Horiuchi M.
        • Taguchi K.
        • Hirose W.
        • et al.
        Cellular Nrf2 levels determine cell fate during chemical carcinogenesis in esophageal epithelium.
        Mol Cell Biol. 2021; 41 (e00536-20)
        • Hirose W.
        • Horiuchi M.
        • Li D.
        • et al.
        Selective elimination of NRF2-activated cells by competition with neighboring cells in the esophageal epithelium.
        Cell Mol Gastroenterol Hepatol. 2022; XX (XXX–XXX)
        • Schafer M.
        • Willrodt A.H.
        • Kurinna S.
        • et al.
        Activation of Nrf2 in keratinocytes causes chloracne (MADISH)-like skin disease in mice.
        EMBO Mol Med. 2014; 6: 442-457
        • Colom B.
        • Herms A.
        • Hall M.W.J.
        • et al.
        Mutant clones in normal epithelium outcompete and eliminate emerging tumours.
        Nature. 2021; 598: 510-514

      Linked Article

      • Selective Elimination of NRF2-Activated Cells by Competition With Neighboring Cells in the Esophageal Epithelium
        Cellular and Molecular Gastroenterology and HepatologyVol. 15Issue 1
        • Preview
          NF-E2-related factor 2 (NRF2) is a transcription factor that regulates cytoprotective gene expression in response to oxidative and electrophilic stresses. NRF2 activity is mainly controlled by Kelch-like ECH-associated protein 1 (KEAP1). Constitutive NRF2 activation by NRF2 mutations or KEAP1 dysfunction results in a poor prognosis for esophageal squamous cell carcinoma (ESCC) through the activation of cytoprotective functions. However, the detailed contributions of NRF2 to ESCC initiation or promotion have not been clarified.
        • Full-Text
        • PDF
        Open Access