Role of reactive oxygen species in the regulation of physiological functions.

Bio-organisms possess numerous systems that produce reactive oxygen species (ROS). Severe oxidative stress induces cellular damage that can lead to apoptosis or necrosis, but moderate ROS levels constitute and modulate normal and critical physiological pathways in the regulation of cellular functions, including signaling cascades and transcriptional/post-transcriptional control of gene expression. ROS are also found to mimic some of the physiological stimuli by direct modification of factors or indirect mechanisms via change in the oxidative and reductive status inside/outside cells. This review will describe the biological relevance and essential roles of ROS in animal cells, focusing on signal transduction, gene expression, apoptosis and aging.