Reactive Oxygen Species As Mediators-Signal Transduction

Heart, Lung, And Blood Institute

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Abstract

For many years it was generally believed that the production of reactive oxygen species (ROS) was an unwanted byproduct of aerobic metabolism and other cellular enzymatic processes and that ROS were uniformly deleterious in nature. We have spent the majority of our energies pursuing an alternative hypothesis; that production of ROS are tightly regulated, the targets of ROS are specific, and that oxidants contribute to disease progression, at least in part, through the redox-regulation of specific pathways. Nearly ten years ago we observed that certain cells produce high levels of ROS when stimulated by peptide growth factors. The production of ROS was transient, peaking in the first few minutes following ligand stimulation and returning to baseline within 30 minutes after stimulation. Our initial observation was in vascular smooth muscle cells stimulated with the growth factor PDGF, but subsequently it has become clear that similar events transpire in a wide variety of different cell types stimulated by a host of different ligands. Interestingly we found that inhibiting this rise in ROS blocked the initial signaling events demonstrating an essential role in ROS generation for normal physiological signal transduction. We have also previously demonstrated that the source of the ligand stimulated ROS-generator in non-phagocytic cells shared certain molecular and biochemical similarities with the phagocytic NADPH oxidase. In particular, we were able to demonstrate a role for the small GTPase Rac1 in the regulation of the intracellular redox state. We have also shown with the help of our collaborators that the related GTPase Ras also plays an important role in redox regulation within cells. Interestingly, the ability of Ras proteins to induce transformation in the context of immortalized cell, or to induce senescence in the context of primary cells, appears dependent in some fashion on the ability of Ras proteins to induce a change in the level of ROS. These observations have been extended recently in our lab in an attempt to understand the molecular regulators of mitochondrial ROS production as well as mitochondrial oxygen consumption. We have recently been able to demonstrate a connection between three pathways that regulate lifespan and the production of mitochondrial oxidants. These pathways include the NAD-dependent deacetylase SIRT1, the adapter molecule p66shc and the the mTOR pathway. Together these observations therefore raise fundamental question as to how oxidants participate in disease processes. In particular, do oxidants contribute to aging or to diseases such as atherosclerosis, neurodegeneration or cancer through random non-specific damage or instead do they produce disease phenotypes through the activation of specific redox-sensitive processes. We are actively pusuing the answers to these questions.