Role of Adenosine Receptors in Cardiac Failure and Protection

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Heart failure due to systolic dysfunction is a disease of epidemic proportions affecting over 5 million patients in the US. Although present treatments improve survival and decrease hospitalizations, the disease continues to be characterized by a progressive decrease in cardiac contractility due at least in part to cellular hypertrophy, apoptosis and extracellular matrix remodeling. Activation of G protein-coupled receptors (GPCRs) - in particular the (3-adrenergic receptors -plays a significant role in the heart's initial response to damage as well as providing important signals for activation of the cascade of proteins that mediate maladaptive remodeling. Over the past two decades, our laboratory has focused on the role of G protein signaling and downstream signaling through tumor necrosis factor-a (TNF) on maladaptive remodeling in the heart. By contrast with (3-adrenergic signaling, it has been proposed that the ligand adenosine and its cognate GPCRs, protect the heart against injury during cardiac stress. Four known adenosine receptor subtypes (A^, A2A-, A2B-, and A3-R's) have been identified and are expressed in a tissue specific fashion. Indeed, activation of these receptors inhibits TNF expression and limits adrenergic signaling. However, specific adenosine receptor subtypes activate pathways that have diametrically opposite effects: The A-,-, and A3-Rs inhibit adenylyl cyclase through activation of Gj, whereas the A2A-Rs activate adenylyl cyclase through activation of Gs. Early studies assessing the role of these selective adenosine receptor subtypes in cardiac physiology and pathophysiology were limited by the absence of truly "selective" sub-type specific agonists or antagonists. However, it is well described that adenosine levels increase in the ischemic heart and studies using transgenic mouse models in which the receptors are constitutively expressed or ablated demonstrate that the Ar and A3- Rs are key mediators of cardioprotection during ischemia/reperfusion. Importantly, recent studies from our laboratory using transgenic mouse models in which transgene expression can be "controlled" have resulted in a reassessment of the current dogma regarding the role of selective adenosine receptors in the heart and in particular their role during cardiac injury and repair. These studies have demonstrated that: 1) by contrast with adenosine levels in ischemic myocardium, adenosine levels decrease substantially in the failing murine heart; 2) both constitutive and controlled overexpression of the ArR results in the development of heart failure; 3) constitutive and controlled overexpression of the A^-R enhances cardiac contractility without the development of cellular hypertropohy; and 4) overexpression of the A2A-R prevents the heart failure phenotype in mice overexpressing the ArR. Our preliminary data also suggests that the marked differences in the effects of A^-R signaling and (3-adrenergic signaling in the heart might be due to receptor sub-type specific effects on downstream signaling through Akt (protein kinase B), GRK5 and G|. Furthermore, the disparate effects of Ar and A^-R signaling in the heart appear to be due to disparate effects on calcium (Ca2+) handling by the sarcoplasmic reticulum. Taken together, these results have led us to hypothesize that the individual adenosine receptor subtypes play unique roles in cardiac signaling and function during normal cardiac physiology and in the physiologic response to stressors that cause cardiac injury and progress to heart failure. If true, this hypothesis has important safety implications for ongoing clinical studies assessing the efficacy in humans of a variety of adenosine receptor sub-type specific agonists and antagonists. To test this hypothesis we will pursue three Specific Aims that will test whether: 1) A^-R-mediated signaling has unique effects on myocardial physiology and affords both cardiac protection and inotropic support through distinct signaling pathways; 2) changes in intracellular Ca2+ handling modifies the cardiac phenotype after overexpression of adenosine receptors; and 3) downstream signaling through G, GRK5 and/or Akt modulates the adaptive effects of AaA-R signaling in the heart. These studies will be facilitated by the unique models developed in our own laboratory, gene transfer technology, surgical expertise and sophisticated imaging available through the Core facilities, and the expertise in Ca2+ homeostasis and GPCR signaling that is present within our PPG group.