PROJECT SUMMARY Humans and rodents have constitutive brown adipose tissue (BAT), a tissue that responds to fatty acid overload by dissipating heat, an adaptive mechanism called diet-induced thermogenesis (DIT). DIT contributes to energy homeostasis and restricts obesity development. Activation of DIT by fat overload depends on thyroid hormones and is induced by glutathione (GSH) depletion via increases in reactive oxygen species (ROS). Two selenoproteins, iodothyronine deiodinase 2 (Dio2) and GSH peroxidase 1 (Gpx1), are involved in the regulation of adaptive thermogenesis. Selenoproteins have in their primary structure the micronutrient selenium (Se) incorporated as the amino acid selenocysteine (Sec). Sec is decomposed by the enzyme Sec lyase (Scly) into selenide, used for de novo synthesis of Sec, triggering a Se recycling process required to maintain optimal levels of selenoproteins, particularly when Se is limiting. Se recycling is key for energy balance, as disruption of the Scly gene in mice (Scly KO) leads to weight gain worsened by Se deficiency. It is unknown if Se recycling modulates BAT Se levels, impacting Dio2 and Gpx1 activities, ultimately contributing to DIT activation. While most investigations into the role that selenoproteins play in obesity and metabolism have focused on dietary Se intake, this project will employ innovative models that couple the understudied mechanisms of Se recycling along with Se intake. Our long-term research goal is to determine how molecular mechanisms regulated by bioavailable Se contribute to obesity development, enabling tailored nutritional strategies for improving human health status via modulation of micronutrient intake. The overall objective of this proposal is to determine how Se intake and Se recycling modulate brown adipocyte physiology and activation of DIT. In Aim 1, we will determine the role of Se recycling in BAT activation in obesity. In Aim 2, we will determine if Se supplementation rescues brown adipocyte DIT response after the loss of Scly. In Aim 3 we will determine how Scly controls selenoprotein degradation in palmitate-treated brown adipocytes. These results will clarify the modulatory effect of Se on brown adipocyte physiology. By including both Se intake and recycling, this innovative project will provide significant new insights into the role that Se plays in brown adipocyte physiology in the pathological context of obesity.