Diabetes Mellitus is a growing epidemic, affecting 8% percent of population in the US. Type 2 diabetes mellitus, accounting for 90-95% of diabetes cases, is a well characterized risk factor for cardiomyopathy and heart failure. Diabetic Cardiomyopathy (DCM) is characterized by both systolic and diastolic dysfunction, resulting in decreased cardiac function, in the absence of coronary artery disease (CAD) and hypertension. Interestingly, DCM is associated with hyperinsulinemia and depressed adrenergic signaling leading to decreased contractility despite elevated catecholamine levels. The lack of CAD and hypertension in DCM, even in the presence of elevated catecholamines, suggests that hyperinsulinemia elicits direct cardiomyocyte insult, possibly by modulating adrenergic function. The consequence of concomitant insulin receptor (IR) and adrenergic receptor (AR) stimulation within the heart, however, remains unknown. Here, our major hypothesis is that insulin signaling attenuates the 2AR-induced elevation in cardiac performance in response to elevated catecholamines. Our preliminary data show that chronic insulin treatment impairs 2AR stimulation of cAMP/protein kinase A (PKA) signaling, PKA phosphorylation of phospholamban (PLN), and myocyte contraction rate response in cardiac myocytes. This inhibitory effect is mediated by both enhanced 2AR/Gi coupling and an increased expression of phosphodiesterase 4 (PDE4);together these contribute to a down-regulation of 2AR-induced cAMP signaling. The signaling mechanism underlying the cross-talk between IRs and 2ARs, as well as the physiological consequence, in the myocardium remains unknown. We hypothesize that decreased cardiac performance observed in DCM results, in part, from chronic insulin stimulation which impairs 2AR signaling via increased 2AR/Gi coupling and increased PDE4 expression. By combining our well-established model system based on myocytes isolated from 2AR gene deficient mice with our newly-developed FRET-based cell imaging techniques to analyze the dynamics of cAMP/PKA activities, we will determine the mechanism and consequence of 2AR/IR cross-talk in cardiomyocytes. The combination of functional myocyte contraction measurements and real-time measurement of 2AR-induced cAMP/PKA activities with recombinant adenoviral-mediated delivery of mutant signaling components allows for a unique yet powerful approach to uncover the mechanism underlying the functional integration of the insulin and 2AR signaling network. By elucidating the mechanism by which insulin attenuates adrenergic-mediated cardiac contractility both in vitro and in vivo, new therapeutic strategies may be developed to treat diabetic patients with heart failure.
This proposal seeks to characterize the molecular mechanisms underlying 2 adrenergic- and insulin receptor cross-talk under pathophysiological conditions (e.g. hyperinsulinemia in type II diabetes) and to understand how this signaling integration modulates cardiac contractility in the disease state. Our preliminary studies suggest that insulin receptor activation attenuates 2 adrenergic-mediated increases in cardiac performance. By elucidating the mechanism by which insulin inhibits adrenergic-mediated cardiac contractility both in vitro and in vivo, new therapeutic strategies may be developed to treat diabetic patients with heart failure.
