Fasting has long been a part of many religions and cultures and is now being used in the clinic to promote weight loss and potentially reduce the risk of cardiovascular disease. The heart is a highly metabolic tissue and only stores enough energy for five heartbeats. It is unclear how the myocardium choses a metabolic source when nutrient supply is limited during fasting or what effects this has on the heart long term. Bile acids increase in the serum after a meal and may serve as a systemic fed-state marker. Bile acids are a ligand for the nuclear receptor farnesoid X receptor (FXR). FXR is a transcriptional regulator of several genes involved in metabolism and indirectly regulates autophagic gene transcription. Autophagy is the recycling of damaged or excess cellular proteins and supplies a cell with macromolecules and nutrients. Activation of FXR decreases autophagy in the liver. Further, antagonism of FXR in the myocardium is protective in ischemia reperfusion (IR). The preliminary data in this proposal show that FXR is present in cardiomyocytes, its activation reduces autophagy and suggests a switch to glucose oxidation, regulating metabolic substrate choice. This represent a potential new mechanism for the myocardium to sense and respond to systemic nutrient availability and is in line with literature from the liver and intestine. Further, we demonstrate that induction of autophagy during IR is protective. In preliminary studies, autophagy was induced by a peptide and we will expand upon these studies to evaluate the role of FXR-induced autophagy in protection from IR injury. The proposed work aims to evaluate FXR as a fed-state regulator of autophagy and metabolism through complementary in vivo experiments with FXR knockout mice and in vitro experiments in adult, neonatal and embryonic stem cell-derived cardiomyocytes. We will use FXR knockout mice to assess the role of FXR and autophagy in protection from IR injury. We will use both natural and synthetic agonists of FXR, as well as siRNA knockdown in vitro to confirm the specificity of the effects. Collectively, these studies will provide insight into the potential coordination of autophagy and metabolism by FXR in the nutrient deprived myocardium, the effects of fasting on the myocardium and the protective role of autophagy in IR. The insights will offer new avenues for IR intervention.
Proposal Narrative The farnesoid x receptor regulates autophagy and metabolic substrate choice in the gastrointestinal tract, but has not been evaluated for these functions in the myocardium. The objective of this proposal is to test if the farnesoid x receptor serves as a fed-state regulator of autophagy and metabolism in the myocardium. The proposed studies will improve our understanding of the effects of fasting on the myocardium and elucidate potential therapeutic targets for ischemia reperfusion injury.