Paradoxical roles of dual oxidases in cancer biology

Abstract

Dysregulated oxidative metabolism is a well-recognized aspect of cancer biology, and many therapeutic strategies are based on targeting cancers by altering cellular redox pathways. The NADPH oxidases (NOXes) present an important enzymatic source of biological oxidants, and the expression and activation of several NOX isoforms are frequently dysregulated in many cancers. Cell-based studies have demonstrated a role for several NOX isozymes in controlling cell proliferation and/or cell migration, further supporting a potential contributing role for NOX in promoting cancer. While various NOX isoforms are often upregulated in cancers, paradoxical recent findings indicate that dual oxidases (DUOXes), normally prominently expressed in epithelial lineages, are frequently suppressed in epithelial-derived cancers by epigenetic mechanisms, although the functional relevance of such DUOX silencing has remained unclear. This review will briefly summarize our current understanding regarding the importance of reactive oxygen species (ROS) and NOXes in cancer biology, and focus on recent observations indicating the unique and seemingly opposing roles of DUOX enzymes in cancer biology. We will discuss current knowledge regarding the functional properties of DUOX, and recent studies highlighting mechanistic consequences of DUOX1 loss in lung cancer, and its consequences for tumor invasiveness and current anticancer therapy. Finally, we will also discuss potentially unique roles for the DUOX maturation factors. Overall, a better understanding of mechanisms that regulate DUOX and the functional consequences of DUOX silencing in cancer may offer valuable new diagnostic insights and novel therapeutic opportunities.

Keywords: DUOX; EGFR; Epigenetics; Epithelial; NADPH oxidase; NOX; Reactive oxygen species.

Figure 1. Importance of redox homeostasis for cellular function

Cellular production of reactive oxygen species (ROS) from various sources is controlled by various antioxidant networks. Dysregulated ROS production or antioxidant systems can disturb cellular redox homeostasis, and contribute to pathology.

Figure 2. Biochemical reactions associated with oxidation-dependent contributions to cancer biology

Enhanced levels of reactive oxygen species (e.g. O₂, H₂O₂) promotes oxidation events such as direct DNA oxidation (e.g. 8-oxo-guanine) (A), or lipid peroxidation (B), which can both lead to genomic instability and drive tumor initiation and/or progression. ROS imbalance can also perturb redox dependent cellular processes through oxidation of sensitive amino acid residues such methionine or cysteine (C), leading to potential alternative protein function and/or dysregulated cell signaling.

Figure 3. DUOX1 and DUOX2 display differential mRNA expression in various epithelial derived cancers

Dot blots represent mRNA expression of DUOX1 (A) or DUOX2 (B) extracted from either normal (N) healthy tissue (blue symbols), or tumor (T) tissue (red symbols). Data extracted from Tissue Cancer Genome Atlas (TCGA); results shown as AVG ± SEM. Statistical significance determined by one-way ANOVA, n.s. = not significant; * p-value<0.05; ***p-value<0.001. RPKM = Reads Per Kilobase of transcript per Million mapped reads.

Figure 4. Comparison of DUOX1 mRNA expression lung squamous cell carcinomas (SCC) and lung adenocarcinomas (AC) compared to corresponding matched controls

RNASeq data for SSC (n=50; A) and AC (n=58; B) and matched controls were downloaded from the Genomic Data Commons Data Portal (https://portal.gdc.cancer.gov/) and converted to z-scores as described: Cheadle C, Vawter MP, Freed WJ, Becker KG. Analysis of Microarray Data Using Z Score Transformation. The Journal of molecular diagnostics: JMD. 2003;5(2):73–81. *P<0.0001 based on Wilcoxon matched-pairs signed rank test (Prism v7.01).

Figure 5. Topology of DUOX1-DUOXA1 proteins

DUOX1 utilizes reducing equivalents from NADPH to transfer electrons through the two heme molecules, reducing molecular O₂ to H₂O₂. Regions highlighted in RED and YELLOW indicate regions with relatively contain low sequence homology between DUOX1 and DUOX2 and between DUOXA1 and DUOXA2.

Figure 6. Loss of DUOX1 and local redox signaling has potential implications for EGFR regulation and signaling

(A) Schematic diagram of DUOX1-dependent redox signaling in response to epithelial stimuli. DUOX1-derived H₂O₂ oxidizes target protein cysteines (e.g. Src/EGFR) in a localized fashion near or at the plasma membrane, and promotes features consistent with normal epithelial redox signaling. (B) Loss of DUOX1 in cancer removes a local oxidant source, and is associated with oncogenic EGFR signaling, potentially due to perturbed redox signaling mechanisms e.g. contributions from other NOXes or mitochondria. (C) DUOX1 suppression promotes EMT, which is accompanied with altered EGFR spatial distribution, although how altered redox regulation contributes to these consequences are unknown. (D) Associations between altered EGFR subcellular localization and oncogenic outcomes.

Figure 7. DUOXA1 and DUOXA2 mRNA expression in various epithelial derived cancers

Dot blots represent mRNA expression of DUOXA1 (A) or DUOXA2 (B) extracted from either normal (N) healthy tissue (blue symbols), or tumor (T) tissue (red symbols). Data extracted from Tissue Cancer Genome Atlas (TCGA); results shown as AVG ± SEM. Statistical significance determined by one-way ANOVA, n.s. = not significant; * p-value<0.05; ***p-value<0.001. RPKM = Reads Per Kilobase of transcript per Million mapped reads.