ABSTRACT (Project 3) Role of Proline Metabolism in Regulation of Mammalian Cardiomyocyte Proliferation Heart failure progression is a complex biological process that is precipitated by the maladaptive myocardial response to injury, compounded by failure of the adult heart to replace lost or damaged cardiomyocytes. Our lab has previously outlined the regenerative capacity of the newborn mammalian heart and outlined several mechanisms that regulate this process. Specifically, we demonstrate that the endogenous regenerative capacity of the newborn heart is mediated by proliferation of preexisting cardiomyocytes and is lost when cardiomyocytes exit cell cycle within a few days after birth. We described several fundamental mechanisms that regulate cell cycle exit of cardiomyocytes, including spontaneous DNA damage that occurs as a result of increased mitochondrial oxidative phosphorylation. Subsequently, we demonstrated that gradual severe systemic hypoxia can induce cardiomyocyte proliferation in the adult mouse heart and is associated with decreased DNA damage. These finding suggest that oxygen metabolism is an upstream signal that mediates postnatal cardiomyocyte cell cycle in the postnatal heart. Intriguingly, we found that proline metabolism was markedly upregulated in regenerative cardiomyocytes under hypoxic conditions. From a mechanistic standpoint, we want to better understand the factors that regulate cardiomyocyte proliferation under hypoxia. Therefore, this proposal will examine the role of proline metabolism in regulation of cardiomyocyte adaptation and proliferation under hypoxia in both mice and pigs. In addition, we will examine the role of Hypoxia inducible factors (Hifs) in regulation of proline metabolism and cardiomyocyte proliferation in the neonatal heart and under hypoxic conditions.