Reactive oxygen species produced by altered tumor metabolism impacts cancer stem cell maintenance

Abstract

Controlling reactive oxygen species (ROS) at sustainable levels can drive multiple facets of tumor biology, including within the cancer stem cell (CSC) population. Tight regulation of ROS is one key component in CSCs that drives disease recurrence, cell signaling, and therapeutic resistance. While ROS are well-appreciated to need oxygen and are a product of oxidative phosphorylation, there are also important roles for ROS under hypoxia. As hypoxia promotes and sustains major stemness pathways, further consideration of ROS impacts on CSCs in the tumor microenvironment is important. Furthermore, glycolytic shifts that occur in cancer and may be promoted by hypoxia are associated with multiple mechanisms to mitigate oxidative stress. This altered metabolism provides survival advantages that sustain malignant features, such as proliferation and self-renewal, while producing the necessary antioxidants that reduce damage from oxidative stress. Finally, disease recurrence is believed to be attributed to therapy resistant CSCs which can be quiescent and have changes in redox status. Effective DNA damage response pathways and/or a slow-cycling state can protect CSCs from the genomic catastrophe induced by irradiation and genotoxic agents. This review will explore the delicate, yet complex, relationship between ROS and its pleiotropic role in modulating the CSC.

Keywords: Cancer stem cell; Metabolism; Reactive oxygen species; Tumor initiating cell.

Fig. 1

The malignant role of reactive oxygen species (ROS). ROS such as hydrogen peroxide (H₂O₂) and superoxide (O₂^.-) are produced from various cellular processes, including NADPH oxidase (NOX) and oxidative phosphorylation in the mitochondria. Superoxide dismutase (SOD) converts O₂^.- into H₂O₂, which can then be used in protein functional group modification (not shown) or converted into H₂O by peroxiredoxin (Prdx) or glutathione (GPx). H₂O₂ also reversibly inactivates tyrosine phosphatases, which catalyze phosphate removal from cell membrane receptors, including epidermal growth factor receptor (EGFR). Growth-related genes and pathways are thus elevated, contributing to the uncontrolled proliferation that is characteristic of malignancy. Increased H₂O₂ leads to dissociation of nuclear factor erythroid 2-related factor 2 (NRF2) from Kelch-like ECH associated protein 1 (KEAP1), stabilizing NRF2 and allowing it to increase transcription of antioxidants. NRF2 and/or NRF2 target genes have been shown to be elevated in CSCs. Hypoxia inducible factors (HIF1α and HIF2α) can be stabilized by hypoxic conditions or ROS and translocate into the nucleus to activate HIF target genes. Hypoxia and HIF are important regulators of cancer cell growth, angiogenesis, metastasis and pluripotency.

Fig. 2

ROS induced stabilization of HIF signaling. Induction of hypoxia/HIF signaling is known to promote and maintain CSC phenotypes. Under normoxic conditions, HIF-1α is hydroxylated by prolyl hydroxylase domain proteins (PHD) which targets HIF-1α for ubiquitination by von-Hippel Lindau (VHL) for proteasomal degradation. During malignancies, mitochondrial ROS production can be elevated. Hydrogen peroxide (H₂O₂) can stabilize HIF-1α, in the absence of hypoxia, to facilitate its translocation into the nucleus to dimerized, with HIF-1β. Once dimerized, HIF-1α and HIF-1β can bind to the hypoxia response element (HRE) to induce gene expression.

Fig. 3

Enhanced glycolysis in CSCs and control of oxidative stress. The transcription factor MYC is an important regulator of CSC phenotypes and promotes the production of key glycolytic enzymes. MYC can also lead to alternative splicing of pyruvate kinase into its M2 isoform (PKM2) to further regulate metabolism. Glucose-6-phosphate (G6P) can shuttle into the Pentose Phosphate Pathway (PPP) for NADPH production to fuel antioxidant pathways, thus promoting a reductive environment for CSCs. Cluster of Differentiation 44 (CD44) is a common CSC marker with a role in enhancing glycolysis and interacts with PKM2. In addition, CD44 variant can regulate the cystine/glutamate antiporter system x_c via stabilization of the xCT subunit. Overall, glucose can be shuttled through glycolysis with reduced oxidative phosphorylation in the mitochondria and multiple mechanisms utilized to reduce overall ROS.

Fig. 4

Quiescence and CSC modes for therapeutic resistance. Heterogenous cell cycling in CSCs may confer different characteristics for therapeutic resistance. Proliferative CSCs may upregulate antioxidants, thus preventing cell death induced by high oxidative stress during cancer treatment. Increased DNA repair activity resolves single-strand breaks, double-strand breaks, and replicative stress caused by irradiation, chemotherapy, or rapid proliferation. During quiescence, CSCs have low intercellular ROS. By not entering the cell cycle, G₀ CSCs could avoid cancer treatment induced death at cell cycle checkpoints. Quiescence is a transient state and CSCs may re-enter the cell cycle for cancer homeostasis.