Partial or complete occlusion of a coronary artery creates ischemia, thereby exposing the myocardium to either an insufficient supply of oxygen (hypoxia) or a complete lack of oxygen (anoxia). Consequently, myocardial injury and necrosis occur that are dependent on multiple variables such as the extent and duration of oxygen deprivation and the adverse effects produced in cardiomyocytes during reoxygenation of the myocardium (ischemia/reperfusion). The overall energetic capacity of cardiomyocytes under normal physiological conditions is coupled to fatty acid oxidation for metabolic fuel and to mitochondrial oxidative phosphorylation for ATP generation. To minimize the adverse effects of insufficient oxygen on cardiac structure and function, the myocardium adapts by activating glycolytic pathways to fuel energy metabolism in the cardiomyocyte. This switch to glucose metabolism is a common feature of pathological remodeling that develops during ischemic heart disease and other causes of heart failure. This gene program for energy metabolism is regulated by Estrogen-Related Receptors (ERR), a member of the nuclear receptor superfamily that activates transcription of a wide array of target genes required for mitochondrial biogenesis, fatty acid metabolism and oxidative phosphorylation. Specifically, these studies will define the causes that result in down-regulation of ERR target genes by examining molecular mechanisms that regulate translational efficiency and/or stability of ERR mRNA isoforms in response to hypoxia and reoxygenation of adult cardiocytes. ERR mRNA has both structural features and sequence elements in the 54-UTR and 34-UTR that are characteristic of translationally regulated mRNAs. These include a 54-UTR that is G-C rich (82%) with extensive secondary structure in the form of a large, stable hairpin loop near the 54-cap, and a relatively long 34-UTR that has possible AU-rich elements involved in translation-linked mRNA stability. The preliminary data have revealed that ERR1 expression is regulated in adult cardiomyocytes during hypoxia and reoxygenation at the level of translation. These studies will examine the effects of hypoxia and reoxygenation in regulating translational efficiency and stability of ERR mRNA isoforms in adult feline cardiomyocytes that are electrically stimulated to contract continuously at defined physiological parameters. Companion studies will be done using a murine model of myocardial ischemia and ischemia/reperfusion injury establish the physiological significance of the adult cardiomyocyte model to oxidative stress in vivo. The hypothesis is that expression of ERR1 is controlled during hypoxia and reoxygenation in the adult cardiomyocyte by mechanisms that modify translation and/or stability of ERR1 mRNA.
The specific aims are: 1) to determine the mechanism(s) that regulate expression of ERRs during hypoxia and hypoxia/reoxygenation of adult cardiomyocytes in vitro and during myocardial remodeling produced in response to ischemia and ischemia/reperfusion injury in a murine model in vivo;2) to examine the role of specific elements in the 54-UTR and/or 34-UTR in regulating translation and stability of ERR1 mRNA in adult cardiomyocytes;3) to examine how modifying activity of initiation factors can alter expression of ERR1 and its activity as determined by downstream effects on target genes. The long term goal of these studies to gain a better understanding of the mechanisms that contribute to adaptive changes in myocardial gene expression during adverse remodeling, and ultimately devise therapeutic strategies that can reverse the transition to heart failure and improve outcomes for Veterans and other patients in the U.S. population.
Coronary heart disease is directly responsible for about 1 of every 5 deaths in the United States, and is a major cause of morbidity and mortality in VA patients. Blockage of coronary arteries leads to myocardial infarction (heart attack) that injures the heart muscle because of an insufficient supply of oxygen and the subsequent generation of harmful metabolites. Consequently, damage to the heart muscle causes adverse adaptive changes in structure and function that ultimately causes heart failure. A key signature of heart failure is an inability to sustain its energy requirements by normal metabolic pathways. The molecular mechanisms underlying defects in energy metabolism of the failing heart are not fully understood. The goal of these studies to gain a better understanding of how these adaptive changes occur, and ultimately devise therapeutic strategies that can reverse the transition to heart failure and improve outcomes for these patients.