ROS in Cancer: The Burning Question

Abstract

An unanswered question in human health is whether antioxidation prevents or promotes cancer. Antioxidation has historically been viewed as chemopreventive, but emerging evidence suggests that antioxidants may be supportive of neoplasia. We posit this contention to be rooted in the fact that ROS do not operate as one single biochemical entity, but as diverse secondary messengers in cancer cells. This cautions against therapeutic strategies to increase ROS at a global level. To leverage redox alterations towards the development of effective therapies necessitates the application of biophysical and biochemical approaches to define redox dynamics and to functionally elucidate specific oxidative modifications in cancer versus normal cells. An improved understanding of the sophisticated workings of redox biology is imperative to defeating cancer.

Figure 1. Sources of Cellular Reactive Oxygen Species and Antioxidant Defense Mechanisms

The major source of cellular ROS is the mitochondria. Leakage through the electron transport chain releases mitochondrial superoxides (O₂⁻), which can be converted by mitochondrial superoxide dismutase (SOD2) into hydrogen peroxide (H₂O₂). Likewise, activity of oxidases such as NADPH oxidases (NOXs) at membranes, cytochrome p450 at the endoplasmic reticulum, lipoxygenases, cyclooxygenase and xanthine oxidase, can all lead to the production of O₂, which can be converted by cytoplasmic SOD1 into H₂O₂. SOD3 mediates a similar reaction in the extracellular space. H₂O₂ can further undergo a Fenton chemical reaction to yield hydroxyl radicals (OH•). Peroxiredoxin (PRX), glutathione peroxidase (GPX) and catalase (CAT) enzymatically mediate reduction of H₂O₂ into water (H₂O). A number of metabolic enzymes generate reducing equivalents of nicotinamide adenine dinucleotide phosphate (NADPH) as a byproduct, thus contributing to the maintenance of redox homeostasis. Examples of these include Glucose-6-phosphate Dehydrogenase (G6PD) and 6-Phosphogluconate Dehydrogenase (PGD) of the Pentose phosphate pathway, Methylenetetrahydrofolate Dehydrogenase (MTHFD1), Isocitrate Dehydrogenase (cytoplasmic IDH1 and mitochondrial IDH2) and Malic Enzyme (cytoplasmic ME1 and mitochondrial ME2/3). Biosynthesis of reduced glutathione (GSH) is mediated through Glutamate-cysteine Ligase (GCL) while Glutathione Reductase (GR) catalyzes the reduction of glutathione disulfide (GSSG) back to the sulfhydryl form GSH. GSSG can be exported out of the cell through multidrug resistant proteins to maintain intracellular redox homeostasis. The oxidized dimer of cysteine, cystine, is a substrate for the cystine-glutamate antiporter (system xc^–). This transport system increases the concentration of cystine inside the cell, which is quickly reduced to cysteine. Extracellular GSH can be hydrolyzed by Gamma-glutamyl Transferase (GGT) to yield products that are imported into cells as individual amino acids or as dipeptides. This salvage pathway represents a means by which GSH can be produced independently of GCL over the short term. Dotted lines denote H₂O₂ diffusion.

Figure 2. Oxidative Modifications of the Amino Acid Cysteine

Top: Reactive cysteine thiols (R-SH) can be oxidized by hydrogen peroxide (H₂O₂), organic hydroperoxides, hypohalous acids or peroxynitrite to form sulfenic acids, which can go on to form reversible disulfides (R-SS-R') or irreversible oxidation products such as sulfinic acid or sulfonic acids. Sulfinic acid forms of some peroxiredoxins can be enzymatically recovered through the action of sulfinic acid reductases (sulfiredoxins).: Bottom: Disulfide reduction by the dithiol reductant thioredoxin (TRX) or glutaredoxin (GRX). Two electrons are required in a two-step reaction to reduce the two proximal cysteines from the disulfide to the dithiol form. Reduction of a regulatory disulfide by TRX or GRX creates an unstable intermediate mixed disulfide form. The close proximity of a second cysteine in these dithiol reductases drives the forward reaction to reduce the disulfide into the dithiol form with high efficiency. (For a detailed discussion of cysteine-based modifications please refer to (Klomsiri et al., 2011)).

Key Figure, Figure 3. Redox and Cancer: Comprehensive Outlook

In this review, we attempted to provide some insight into the diversity of redox signaling events in cancer. As cysteinyl thiol-based oxidative modifications do not exclusively mediate macromolecular damage, but also mediate intracellular signaling events, the field must advance towards investigating individual redox couples with spatial, temporal and chemical specificity. The advent of novel biochemical and biophysical tools will facilitate the elucidation of each oxidative modification in growth, viability and migration. This will move the field towards the goal of identifying cancer-specific redox vulnerabilities that can be targeted therapeutically.