The mean time to reperfusion of acute myocardial infarction patients in the USA is 3-4h. The metabolic activity within the ischemic tissue over this period is a major determinant of outcome and is critically influenced by the glycemic and insulin responsive status of the subject. Inadequate glucose-stimulated insulin production and/or insulin resistance of muscle and liver cause hyperglycemia. We hypothesize that the exacerbated injury that accompanies myocardial ischemia/reperfusion in hyperglycemic subjects is at least in part the consequence of an early switch in the function of c-Jun-N-terminal kinase (JNK) from pro-survival to pro-death. We previously described an energy-dependent switch in the function of JNK in cultured cardiac myocytes. The switch occurred when cardiac myocyte ATP level fell by below 50%, a condition that sensitized myocytes to oxidative stress, and coincided with increased phosphorylation of the insulin receptor substrate-1 (IRS-1) on the inhibitory serine-307. We have now confirmed that the switch also operates in vivo when the myocardium is subjected to ischemia/reperfusion and encounters similar metabolic stress. The in vivo effect is dramatic, JNK inhibition during prolonged ischemia reduced both apoptosis and infarction by ~50%, but the same inhibition imposed on a short ischemic episode increased apoptosis >3-fold and infarct >2-fold. Because >50% of injury caused by severe ischemia/reperfusion is regulated by JNK, the switch is potentially a major contributor to the outcome of AMI. In this revised application we will determine the degree to which the etiology of hyperglycemia affects JNK metabolic switching, characterize the metabolic signals that mediate switching and determine the death/survival pathways that are affected.
In Aim 1 we will determine infarction and remodeling over time by high-definition echocardiography and micro-MRI. This will be implemented in 3 models of hyperglycemia: (i) db/db mice with monogenetic susceptibility to obesity and insulin resistance. (ii) NONcNZO10 mice with polygenic susceptibility to obesity and insulin resistance. (iii) Wild type mice infused with lipid to induce insulin resistance and hyperglycemia.
In Aim 2 we will use advanced imaging techniques including micro-NMR/MRI to determine the precise metabolic parameters that regulate JNK switching in normal and hyperglycemic hearts subjected to ischemia/reperfusion.
In Aim 3 we will define the death pathways that are affected by the JNK switch and the long-term effects on remodeling and gene expression in the affected region of the myocardium.
Fasting hyperglycemia is associated with increased myocardial infarction in both animal models and patients. Fasting hyperglycemia is also associated with insulin resistance. The mechanism for the poor outcome of hyperglycemic patients to acute myocardial infarction is not known. Here we present a new hypothesis where stress kinases are activated specifically in hyperglycemic patients undergoing AMI and cause poor outcome. Experiments are proposed to test this hypothesis in mouse models. If our hypothesis is correct we will described the molecular basis for this an begin to develop new approaches for treatment.
