Oxidative stress is a condition characterized by elevated levels of intracellular reactive oxygen species (ROS). ROS either are, or break down to form, free radicals. ROS include superoxide anion (O2-
), hydrogen peroxide (H2
), and hydroxyl radicals (OH-
) that are capable of reacting with, and damaging DNA, proteins, and lipids. Under normal conditions, ROS are cleared from the cell by reducing agents or by enzymatic reactions that remove reactive species. Low levels of intracellular ROS have been identified as second messengers in signaling pathways and implicated in transcriptional regulation to promote cell proliferation, but high concentrations of ROS result in cell cycle arrest and cell death. Whether the result is cell survival or apoptosis, oxidative stress activates numerous intracellular signaling pathways such as Akt, NF-kappa B, p53, Jak/Stat, and MAPK to affect changes in gene expression.