Hydrogen peroxide - production, fate and role in redox signaling of tumor cells

Abstract

Hydrogen peroxide (H2O2) is involved in various signal transduction pathways and cell fate decisions. The mechanism of the so called "redox signaling" includes the H2O2-mediated reversible oxidation of redox sensitive cysteine residues in enzymes and transcription factors thereby altering their activities. Depending on its intracellular concentration and localization, H2O2 exhibits either pro- or anti-apoptotic activities. In comparison to normal cells, cancer cells are characterized by an increased H2O2 production rate and an impaired redox balance thereby affecting the microenvironment as well as the anti-tumoral immune response. This article reviews the current knowledge about the intracellular production of H2O2 along with redox signaling pathways mediating either the growth or apoptosis of tumor cells. In addition it will be discussed how the targeting of H2O2-linked sources and/or signaling components involved in tumor progression and survival might lead to novel therapeutic targets.

Fig. 1

Interplay between physiological/pathophysiological H₂O₂ generation and the anti-oxidative response mechanism. a H₂O₂ is produced, e.g. in response to growth factors by the NOX/SOD system and enters cells through simple diffusion and facilitated diffusion through AQPs, respectively, leading to increased intracellular H₂O₂ levels. b Peroxiredoxins (Prx) act as highly active redox sensors and are part of one of the main H₂O₂ detoxifying systems. Hyperoxidation inactivates Prxs allowing c the oxidation of sensitive cysteine residues in cellular proteins including transcription factors. d The Nrf2 system is activated in response to increased H₂O₂ levels leading to the anti-oxidative response. AQP, aquaporin; GF, growth factor; GFR, growth factor receptor.

Fig. 2

Redox modifications of reactive cysteine residues by H₂O₂. Redox-sensitive proteins contain cysteine residues, which are partially ionized under physiological pH. Oxidation of this thiolate anion (1) results in a sulfenic acid or rather its salt (2), which is relatively reactive and forms intra-/intermolecular disulfide bonds in the presence of thiolate. This sulfenylation can be intramolecular or intermolecular (3), the latter predominantly with GSH to form glutathionylated intermediates (5), or sulfenylamides with oxidizable amines (4) and glutathionylated intermediates (5), respectively. These redox modifications result in altered functions of their target proteins and can be reversed by the Trx- or GSH-based anti-oxidative systems. Under excessive H₂O₂ concentration the sulfonate or sulfonamide intermediates can be further irreversibly oxidized to sulfinic (6) and sulfonic acids (7) forming the respective anions under physiological pH thus also shifting the isoelectric points of affected proteins.

Fig. 3

The Nrf2/Keap1 signaling pathway. Under basal conditions Nrf2 is bound by two molecules of Keap1, poly-ubiquitinylated by the Cul3 system and thereby marked for proteasomal degradation. Only a small portion of Nrf2 escapes from this degradation process and translocates to the nucleus to maintain the basal expression of anti-oxidant response genes. Under stress conditions like elevated levels of H₂O₂ Keap1 is modified at redox sensitive cysteine residues leading to an impaired conformation and inactivation of Keap1. Newly translated Nrf2 escapes ubiquitinylation, translocates to the nucleus and induces the anti-oxidative stress response. Mechanisms for the continuously accumulation of Nrf2 in the nucleus of several cancer cells can be triggered by (i) mutations of Keap1 associated with its inactivation, (ii) epigenetic silencing of Keap1 and (iii) mutations of oncogenes such as K-ras, B-raf and c-myc leading to the transcriptional induction of Nrf2.

Fig. 4

Maintenance of redox homeostasis by Nrf2. Nrf2 induces the expression of genes coding for enzymes involved in (a) hydrogen peroxide detoxification and (b) redox signaling. (c) High levels of H₂O₂ activate Nrf2 resulting in the induction of the anti-oxidative stress response. The red boxes symbolize Nrf2 inducible enzymes

Fig. 5

Trx-based upregulation of anti-oxidative systems by Nrf2. Oxidized Trxs are reduced by TrxRs and maintained in their active form. Reduced Trxs can reduce oxidized Prxs, which under physiological conditions detoxify H₂O₂. Reduced Trxs can interact with redox-sensitive proteins, such as ASK1, PTEN, AP-1 and p53 suggesting that different cellular processes such as proliferation, the cellular metabolism and apoptosis and might be regulated by Trxs.

Fig. 6

Components of anti-oxidative systems involved in tumor development. Cancer cells are characterized by high levels of ROS (H₂O₂). To prevent cell damage and cell death cancer cells induce the expression of anti-oxidative enzymes via the activation of the transcription factor Nrf2. Despite high H₂O₂ levels cancer cells maintain the capacity to promote cell survival, differentiation and proliferation by undergoing metabolic adaption processes thereby relying on the redox regulation of cancer-related redox sensors.

Fig. 7

Redox control of the cellular energy metabolism. In cancer cells the shift of the metabolism into anaerobic glycolysis is mainly mediated by the PI3K/AKT pathway. AKT activates mTOR, which subsequently activates HIF1α resulting in an induction of GLUT1, enzymes of the glycolysis and the mitochondrial PDK, which inhibits the pyruvate flux into the TCA. The AMPK is able to block this mechanism by inhibition of mTOR to conserve energy. Cancer cells exhibit high ROS (H₂O₂) levels leading to an inhibition of the AMPK and of PTPs, which can inactivate AKT. Even through high H₂O₂ levels DSBs could occur leading to the activation of ATM accompanied with cell cycle arrest. The interaction of ATM and AMPK might enhance the DNA damage response. In addition H₂O₂ might inactivate the PKM2 leading to an altered flux of glucose in the pentose phosphate pathway for the generation of reductions equivalents to detoxify ROS. PDK, pyruvate dehydrogenase kinase; PKM2, pyruvate kinase M2; TCA, tricarboxylic acid.