The obesity pandemic is accelerating the risk of diabetes, non-alcoholic fatty liver disease (NAFLD), cardio- vascular diseases and other ailments. PPARa agonist drugs are prescribed to promote fat loss and amelio- rate hepatic steatosis and insulin resistance by activating gene programs driving mitochondrial b-oxidation. Effects of PPARa agonists in the liver are enhanced by remodeling the action of MAMs, cytoplasmic struc- tures that foster exchange of Ca2+ and phospholipids between the mitochondrion and endoplasmic reticu- lum. The master mTORC2/Akt signaling node localizes to the MAM where it coordinates PPARa and MAM actions through an unknown mechanism. Therefore, understanding how mTORC2/Akt regulates PPARa and MAMs in the liver is crucial to developing new approaches to combat the obesity epidemic. Our long- term goal is to understand how nuclear gene expression and cytoplasmic ER/mitochondrial activity are coor- dinated to control energy homeostasis in humans. The objective of this particular application is to determine how mTORC2/Akt-mediated phosphorylation of the multi-functional protein PACS-2 Ser437 combines with PACS-2 nuclear trafficking signals to coordinate PPARa transcriptional activity with MAM-dependent calci- um exchange in response to fasting or a high fat diet. Our central hypothesis is that in response to overeat- ing, mTORC2/Akt-phosphorylated PACS-2 sequesters PPARa in the cytoplasm and increases MAMs, there- by repressing genes controlling fatty acid oxidation while inducing calcium overload in mitochondria. These combined effects cause steatosis and insulin resistance. By contrast, fasting silences mTORC2/Akt signal- ing, triggering PACS-2 dephosphorylation. Consequently, PPARa is liberated and MAMs are remodeled, which combine to increase fatty acid oxidation and support fasting-induced autophagy. Guided by strong preliminary data, we will test our hypothesis by pursuing three specific aims: 1) Determine how liver PACS- 2 coordinates PPARa-dependent gene expression with MAM remodeling to regulate fatty acid oxidation; 2) Determine how mTOR/Akt controls PACS-2 regulation of PGC-1a/PPARa activity; and 3) Determine how PACS-2 regulates access of PPARa to the nucleus. The approach is innovative because it will combine in vivo models of overnutrition and fasting together with studies on gene expression, ER-mitochondria commu- nication, mitochondrial oxygen consumption and live-cell imaging to describe a novel and previously unrec- ognized pathway controlling the response to fasting or a high fat diet. This research is significant because it will advance our understanding of how mTORC2/Akt controls PACS-2 Ser437 to act as a molecular switch to coordinate vital, homeostatic transcriptional and mitochondrial responses to nutrient stresses.
The proposed research is relevant to public health because understanding the molecular pathways controlling diet-induced obesity and its comorbidity may lead to novel therapeutic interventions that can be used to treat NAFLD, dyslipidemia and diabetes. Thus, the proposed research is relevant to the part of NIH's mission supporting fundamental research that will ultimately cure disease.