PROJECT SUMMARY Succinate, a mitochondrial metabolite, has recently emerged as a major regulator of metabolic physiology and inflammation. Much of this succinate-dependent signaling occurs through its release from cells and subsequent activation of its cognate GPCR, SUCNR1. We discovered that diminished brown adipocyte activity, and the obesogenic Western Diet, lowers succinate uptake capacity by brown fat and leads to chronic accumulation of extracellular succinate in the liver, leading to SUCNR1-mediated tissue fibrosis and inflammation. We also defined a succinate transport mechanism in muscle and adipose cells via plasma membrane monocarboxylate transporter 1 (MCT1). The major discovery of our previous grant period is that secretion of succinate from metabolic tissues can play both protective and pathogenic roles in metabolic health, both via SUCNR1. What is unclear is which characteristics determine whether succinate will act in pro- or anti-pathogenic manner? Therefore, in the next stage of our project, we ask what mechanisms determine protective versus pathogenic regulation by extracellular succinate? We hypothesize that succinate activates divergent molecular mechanisms within specific cell types depending on length of exposure (acute vs. chronic), leading to distinct physiological consequences. To test this hypothesis and answer the proposed question, we aim to (1) Determine the cell- type specific consequences of acute versus chronic succinate-mediated SUCNR1 activation in the liver. (2) Determine the intracellular, downstream mechanism of SUCNR1 activation; and (3) Investigate the MCT1- mediated mechanism of succinate transport in the liver. Proposed aims will address the critical gap in our understanding of the differential mechanisms that are activated by chronic and acute exposure to succinate, a major metabolic regulator of metabolic health and disease. Building on the critical discoveries in our initial grant period, the current proposal will define the consequences of succinate exposure in the liver, but it is likely that similar pathways will be activated in other tissues of interest in metabolic disorders. Therefore, our work will address how obesity and associated pathologies develop and may lead to identification of candidates for further pharmacological development.