Abstract Body: Energetic homeostasis in the heart is indispensable, as its perturbation results in an energy starved myocardium and drives dysfunction. For example, pathologies such as Type II Diabetes and ischemic injury are characterized by altered fatty acid and glucose metabolism. Regulated in development and DNA damage (REDD)1 is a negative regulator of mammalian target of rapamycin complex (mTORC)1, and its global deletion leads to systemic glucose intolerance in mice. Additionally, REDD1 has been shown to be induced in models of diabetes and ischemia, suggesting a role for REDD1 in these disease states. Thus, we hypothesized that REDD1 is a critical regulator of glucose oxidation in cardiomyocytes. We observed an increase in the expression of pyruvate dehydrogenase kinase 4 (PDK4) (9.28-fold±1.15 n=3), as well as an increase in its glucose oxidation-inactivating phosphorylation at S300 (2.01-fold±0.13, n=9) in REDD1-null cardiomyocytes as compared to control cardiomyocytes. In addition, we observed reductions in PDH activity (0.83-fold±0.03, n=10) and maximal respiration (0.75-fold±0.05, n=57), as well as an increase in extracellular acidification rate (1.19-fold±0.05, n=54), each indicating reduced glucose oxidation. Increased Pdk4 expression was also observed in the hearts of mice with global or cardiomyocyte-specific deletion of REDD1 (1.35-fold±0.14, n=12; 3.11-fold±0.46, n=4). Utilizing mTORC1 inhibitor Everolimus, we found no rescue of PDK4 levels, pPDH (S300), or PDH activity in REDD1-null cardiomyocytes, indicating an mTORC1-independent mechanism. We then assessed the activity of peroxisome proliferator activated receptor alpha (PPARα), a known transcriptional regulator of PDK4 and fatty acid metabolic genes. We found its activity significantly to be significantly upregulated, both in vitro and in vivo (1.35-fold±0.06, n=11; 1.25-fold±0.08, n=14). Bulk RNA-Seq data also demonstrated significant upregulation of PPARα-regulated fatty acid catabolic genes in the hearts of mice with cardiomyocyte-specific deletion of REDD1. Notably, elevated PDK4 was rescued both in vitro and in vivo with PPAR antagonist GW6471 (0.62-fold±0.04, n=6; 0.63-fold±0.11, n=11). Importantly, the increased activity of PPARα was not rescued by Everolimus. Together, these data demonstrate that cardiac REDD1 is critical for enhancing glucose oxidation and suppressing fatty acid catabolism via an mTORC1-independent and PPARα-dependent mechanism.