1. Oxidative Stress
2. Redox regulation
3. ROS in cancer
Enhanced oxidative stress is now well documented to occur in a number of degenerative diseases including including Parkinson's disease, Alzheimer's and diabetes.
Mitochondria are a primary site for the production of reaction oxygen species (ROS). More than 98% of the molecular oxygen taken up by cells is utilized by cytochrome c oxidase (cyt C) to form water, however, cyt C can also release partly reduced oxygen species. Other enzymes of the respiratory chain, and in particular complexes I and III, also produce partly reduced oxygen species including superoxide. ROS can react with nitric oxide to produce reactive nitrogen species including peroxynitrite. While a significant proportion of the reactive oxygen and nitrogen species diffuse into the cytosol, a major portion remains in the mitochondria.
Reactive oxygen and nitrogen species produced within the mitochondrion is found to directly correlate with dysfunction of the oxidative phosphorylation (OXPHOS) chain. Reduced electron transfer within OXPHOS chain can result in the production of ROS, which leads to the damage of cytosolic proteins and DNA. Furthermore, oxidative and nitrative damage of mitochondrial proteins adds to OXPHOS dysfunction, further exacerbating ROS production.
Nicotinamide nucleotide transhydrogenase (NNT) uses the energy of the mitochondrial membrane potential to transfer reducing equivalents from NADP+ to NADPH.
- The increased reducing potential of NADPH is then used to reduce oxidized glutathione for the reduction of H2O2 to H2O, change the thiol-disulfide balance of amino acids in proteins to regulate transcription factors and enzymes and to drive macromolecular synthesis.
- Cytosolic NADPH can be generated by the pentose phosphate shunt, by the conversion of malate to pyruvate and by the action of IDH1.
- Alteration of these pathways can increase ROS levels and alter cellular metabolism and growth.
ROS in cancer
Evidence suggests that increased ROS levels can play a pivotal role in the generation of the hallmarks of cancer, such as immortilization, cell proliferation, mitogenic signalling, cell survival and the disruption of cell death pathways, angiogenesis and drug resistance. Increased generation of reactive oxygen species and an altered redox status are observed in cancer cells both in vitro and in vivo, with increased ROS stress in cancer cells correlating with increased aggressiveness of tumours and poor prognosis.
Moderate increases in ROS can promote cell proliferation and differentiation whereas excessive amounts of ROS can result in damage to DNA, lipids and proteins, thus resulting in the formation of apurinic/apyrymidinic DNA bases and protein aggregation.
Cells control ROS levels by balancing ROS generation with their elimination by ROS-scavenging systems such as catalase, thioredoxin, glutathione peroxidase, glutaredoxin, superoxide dismutases (SOD1-3)and peroxiredoxin.
Increased expression of oncogenes such as Bcr-Abl, c-Myc, and Ras are found to result in ROS production. For instance in H-Ras transformed NIH3T3 fibroblasts are found to produce superoxide through the activation of the ROS-producing enzyme NADPH oxidase (NOX).
In advanced diseased stages cancer cells exhibit genetic instability and show a significant increase in ROS generation. P53 plays a crucial role in sensing and removing oxidative damage to DNA, thus preventing oxidative DNA damage thus preventing genetic instability. Furthermore, p53 regulates the transcription of many pro- and anti-oxidant genes.
Adaptation of cancer cells to increased ROS stress has been shown to occur. Mechanism of adaptation of high ROS levels involve multiple transcription factors such as NF-kB, Nrf2, c-Jun, and HIF1 which lead to increased expression of antioxidant molecules such as GSH antioxidant system, catalase, thioredoxin and super oxide dismutases. Redox sensitive transcription factors are also found to promote expression of cell-survival molecules such as Bcl-2 family proteins and the Akt survival pathway, thus conferring drug-resistance capacity and cell survival. Identification of key molecules which regulate ROS production and scavenging may result in reduced levels in genetic instability and protein aggregation leading to curative therapeutics for cancers and neurodegenerative diseases.