Oxidative stress contributes substantially to numerous human diseases and aging. Dietary restriction (DR), a dietary regimen that ameliorates various diseases and extends lifespan, reduces the steady state levels of oxidative stress and damage. DR has been postulated to reduce oxidative stress by slowing metabolism, and thereby reducing mitochondrial generation of reactive oxygen species (ROS). Recent findings, however, indicate that mitochondrial activity increases during DR. Thus, the molecular mechanism underlying DR- induced reduction of oxidative stress remains elusive. The central hypothesis of this proposal is that, instead of passively slowing metabolism, DR triggers an active defense program involving a cascade of molecular regulators to reduce oxidative stress. Our recent finding that DR activates a nutrient sensor to reduce oxidative stress supports this hypothesis. We plan to greatly expand this initial finding to delineate the cascade that senses nutrient input to regulate the cellular oxidative stress status. We have established a DR mouse model and a cell culture model to study nutrient sensing and the oxidative stress response. Using the established system, we will elucidate the molecular events that result in either reduced production or improved removal of cellular ROS during DR. Using a gain-of-function approach, we will test the feasibility of activating these molecules to mimic the DR response and reduce oxidative stress. Collectively, these studies challenge the paradigm that DR reduces oxidative stress by passively slowing metabolism and highlight an active defense program provoked by a low calorie diet to reduce oxidative stress. The factors identified in our study are likely to play paramount roles in maintaining the cellular oxidative stress status and suggest new approaches to treat oxidative stress-related diseases and slow aging. The proposed studies will build a solid foundation for a dynamic research program that has a great potential for development: nutrient sensing and the oxidative stress response.