Mechanism of Nutrient-Regulated mTOR Signaling

Abstract Our long-term goal is to increase knowledge on mTOR signaling mechanisms pertaining to metabolic diseases such as diabetes, obesity, and insulin resistance. Despite the important link of mTOR to metabolic diseases, the mechanism of mTOR regulation in adipose metabolism has barely been explored. This proposal is intended to define crucial functions of newly-identified components of the mTOR pathway and their post- translational modifications in adipose metabolism and insulin resistance. mTOR complex 1 (mTORC1), mTOR complex 2 (mTORC2) and Akt are the crucial regulators of insulin signaling that need to be balanced in their activities. When the tight regulation of mTOR is lost or mTOR is hyper-active, it can cause metabolic problems such as the development of obesity and diabetes due to insensitivity of cells to insulin. The key components, mTORC1, mTORC2, and Akt, in insulin signaling are not in a linear relation but interact through complicated networks of regulatory interactions. An underlying concept in this applied research is that mTORC1 and mTORC2 intimately crosstalk upon nutritional and insulin stimulation in order to adjust the relative activities of mTORC1, mTORC2, and Akt. The central hypothesis for the proposed research is that the dynamic regulation of the stability and abundance of mTORC1 and mTORC2 is an important mechanism through which adipose cells respond to insulin and nutrients for appropriate metabolic regulation and homeostasis. The preliminary study supports the crosstalk involved in the regulation of mTORC1 and mTORC2 activities. However, how this regulation is involved in insulin sensitivity determination in adipose cells has not been clarified. Aim #1 will address this question by defining the nutrient regulation of mTORC1 and mTORC2 abundance by testing nutrient or disease conditions that can cause insulin resistance. Given the cellular conditions defined from aim #1, the studies in Aim #2 will be oriented to understand the mechanism underlying the regulation. This aim will be focused on raptor phoshorylation and ubiquitination that regulate raptor stability and how these modifications are involved in adipose metabolism. Aim #3 will be focused to understand the roles of PRAS40, the newest member of mTORC1, and its phosphorylations in the regulation of mTORC1 activity, raptor stability, and adipose metabolism. Aim #4 will be focused to understand the roles of PRR5, the newest member of mTORC2, and its phosphorylation in the regulation of the mTORC1-mTORC2 crosstalk through which PRR5 regulates adipose insulin signaling and metabolism. This proposed research addresses the importance of balanced regulation of mTORC1 and mTORC2 activities in response to nutrients and insulin in adipose metabolism and will provide new insight into the development of insulin resistance.