The adenosine A2A receptor is a G-protein-coupled receptor (GPCR) that has been extensively studied during the past few decades because it offers numerous possibilities for therapeutic applications. Herein we describe adenosine A2A receptor distribution, signaling pathways, pharmacology, and molecular structure, followed by a summary and SAR discussion of the most relevant series of adenosine A2A agonists and antagonists. This review also provides an update of the A2A ligands that are undergoing or have undergone clinical studies, including the two currently marketed agonists adenosine and regadenoson.

Background The volume overload of isolated mitral regurgitation (MR) in the dog results in left ventricular (LV) dilatation and interstitial collagen loss. To better understand the mechanism of collagen loss we performed a gene array and overlaid regulated genes into Ingenuity Pathway Analysis (IPA). Methods and Results Gene arrays from LV tissue were compared in 4 dogs prior to and 4 months after MR. Cine-magnetic resonance-derived LV end-diastolic volume increased 2-fold (p=0.005) and LV ejection fraction increased from 41 to 53% (p < 0.001). LV interstitial collagen decreased 40% (p<0.05) compared to controls and replacement collagen was in short strands and in disarray. IPA identified Marfan’s syndrome, aneurysm formation, LV dilatation, and myocardial infarction, all of which have extracellular matrix (ECM) protein defects and/or degradation. MMP-1 and -9 mRNA increased 5- (p=0.01) and 10-fold (0.003), while collagen I did not change and collagen III mRNA increased 1.5-fold (p=0.02). However, noncollagen genes important in ECM structure were significantly downregulated, including decorin, fibulin 1, and fibrillin 1. Decorin mRNA downregulation correlated with LV dilatation (r= 0.83 p<0.05). In addition, connective tissue growth factor and plasminogen activator inhibitor were downregulated, along with multiple genes in TGF-β signaling pathway, resulting decreased LV TGF-β1 activity (p=0.03). Conclusions LV collagen loss in isolated, compensated MR is chiefly due to post-translational processing and degradation. The downregulation of multiple noncollagen genes important in global ECM structure, coupled with decreased expression of multiple profibrotic factors, explain the failure to replace interstitial collagen in the MR heart.

Left ventricular (LV) volume overload (VO) causes eccentric remodeling with inflammatory cell infiltration and extracellular matrix (ECM) degradation, for which there is currently no proven therapy. To uncover new pathways that connect inflammation and ECM homeostasis with cellular dysfunction, we determined the cardiac transciptome in subacute, compensated, and decompensated stages based on in vivo hemodynamics and echocardiography in the rat with aortocaval fistula (ACF). LV dilatation at 5 wk was associated with a normal LV end-diastolic dimension-to-posterior wall thickness ratio (LVEDD/PWT; compensated), whereas the early 2-wk (subacute) and late 15-wk (decompensated) ACF groups had significant increases in LVEDD/PWT. Subacute and decompensated stages had a significant upregulation of genes related to inflammation, the ECM, the cell cycle, and apoptosis. These changes were accompanied by neutrophil and macrophage infiltration, nonmyocyte apoptosis, and interstitial collagen loss. At 15 wk, there was a 40-fold increase in the matricellular protein periostin, which inhibits connections between collagen and cells, thereby potentially mediating a side-to-side slippage of cardiomyocytes and LV dilatation. The majority of downregulated genes was composed of mitochondrial enzymes whose suppression progressed from 5 to 15 wk concomitant with LV dilatation and systolic heart failure. The profound decrease in gene expression related to fatty acid, amino acid, and glucose metabolism was associated with the downregulation of peroxisome proliferator associated receptor (PPAR)-α-related and bioenergetic-related genes at 15 wk. In VO, an early phase of inflammation subsides at 5 wk but reappears at 15 wk with marked periostin production along with the suppression of genes related to PPAR-α and energy metabolism.

BackgroundThe clinical problem of a “pure volume overload” as in isolated mitral or aortic regurgitation currently has no documented medical therapy that attenuates collagen loss and the resultant left ventricular (LV) dilatation and failure. Here, we identify a potential mechanism related to upregulation of the kallikrein-kinin system in the volume overload of aortocaval fistula (ACF) in the rat.Methodology/Principal FindingsLV interstitial fluid (ISF) collection, hemodynamics, and echocardiography were performed in age-matched shams and 4 and 15 wk ACF rats. ACF rats had LV dilatation and a 2-fold increase in LV end-diastolic pressure, along with increases in LV ISF bradykinin, myocardial kallikrein and bradykinin type-2 receptor (BK2R) mRNA expression. Mast cell numbers were increased and interstitial collagen was decreased at 4 and 15 wk ACF, despite increases in LV ACE and chymase activities. Treatment with the kallikrein inhibitor aprotinin preserved interstitial collagen, prevented the increase in mast cells, and improved LV systolic function at 4 wk ACF. To establish a cause and effect between ISF bradykinin and mast cell-mediated collagen loss, direct LV interstitial bradykinin infusion in vivo for 24 hrs produced a 2-fold increase in mast cell numbers and a 30% decrease in interstitial collagen, which were prevented by BK2R antagonist. To further connect myocardial stretch with cellular kallikrein-kinin system upregulation, 24 hrs cyclic stretch of adult cardiomyocytes and fibroblasts produced increased kallikrein, BK2R mRNA expressions, bradykinin protein and gelatinase activity, which were all decreased by the kallikrein inhibitor-aprotinin.Conclusions/SignificanceA pure volume overload is associated with upregulation of the kallikrein-kinin system and ISF bradykinin, which mediates mast cell infiltration, extracellular matrix loss, and LV dysfunction–all of which are improved by kallikrein inhibition. The current investigation provides important new insights into future potential medical therapies for the volume overload of aortic and mitral regurgitation.

Xanthine oxidoreductase (XOR) is increased in the left ventricle (LV) of humans with volume overload (VO) and mitochondrial inhibition of the respiratory chain occurs in animal models of VO. Since mitochondria are both a source and target of reactive oxygen and nitrogen species, we hypothesized that activation of XOR and mitochondrial dysfunction are interdependent. To test this we used the aortocaval fistula (ACF) rat model of VO and a simulation of the stretch response in isolated adult cardiomyocytes with and without the inhibitor of XOR, allopurinol, or the mitochondrially targeted antioxidant MitoQ. XO activity was increased in cardiomyocytes from ACF vs. sham rats (24h) without an increase in XO protein. A two-fold increase in LV end-diastolic pressure/wall stress and a decrease in LV systolic elastance with ACF were improved with allopurinol (100 mg/kg) started at ACF induction. Subsarcolemmal state 3 mitochondrial respiration was significantly decreased in ACF and normalized by allopurinol. Cardiomyocytes subjected to 3 hour cyclical stretch resulted in an increase in XO activity and mitochondrial swelling, which was prevented by allopurinol or MitoQ pretreatment. These studies establish an early interplay between cardiomyocyte XO activation and bioenergetic dysfunction that may provide a new target that prevents progression to heart failure in VO.

Volume overload (VO) caused by aortocaval fistula (ACF) is associated with oxidative/inflammatory stress. The resulting inflammation, matrix metalloproteinase (MMP) activation, and collagen degradation is thought to play a pivotal role in left ventricular (LV) dilatation and failure. Since mitochondria are also targets for inflammation and oxidative stress, we hypothesized that there would be bioenergetic dysfunction with acute VO. In Sprague-Dawley rats subjected to 24 hrs of ACF, there was a two-fold increase in LV pressure-volume area in vivo, consistent with increased LV myocardial oxygen usage and increased bioenergetic demand in cardiomyocytes. Isolated cardiomyocytes from ACF LVs demonstrated increased hydrogen peroxide and superoxide formation and increased MMP activity. Subsarcolemmal mitochondria (SSM) showed a 40% decrease in state 3 respiration and proteomic analysis of SSM demonstrated decreased levels of complexes I-V in ACF. Immunohistochemical analysis revealed disruption of the subsarcolemmal location of the SSM network in ACF. To test for a potential link between SSM dysfunction and loss of interstitial collagen, rats were treated with the MMP-inhibitor PD166793 prior to ACF. MMP-inhibitor preserved interstitial collagen, integrin-α5 and the SSM structural arrangement. In addition, the decrease in state 3 mitochondrial respiration with ACF was prevented by PD166793. These studies established an important interaction between degradation of interstitial collagen in acute VO and the disruption of SSM structure and function which could contribute to progression to heart failure.

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Acute stretch caused by volume overload (VO) of aorto-caval fistula (ACF) induces a variety of myocardial responses including mast cell accumulation, matrix metalloproteinase (MMP) activation and collagen degradation, all of which are critical in dictating long term left ventricle (LV) outcome to VO. Meanwhile, these responses can be part of myocardial inflammation dictated by tumor necrosis factor-α (TNF-α) which is elevated after acute ACF. However, it is unknown whether TNF-α mediates a major myocardial inflammatory response to stretch in early VO. In 24 hour ACF and sham rats, microarray gene expression profiling and subsequent Ingenuity Pathway Analysis identified a predominant inflammatory response and a gene network of biologically interactive genes strongly linked to TNF-α. Western blot demonstrated increased local production of TNF-α in the LV (1.71- and 1.66-fold in pro- and active-TNF-α over control, respectively, P < 0.05) and cardiomyocytes (2- and 4-fold in pro- and active-TNF-α over control, respectively, P < 0.05). TNF-α neutralization with infliximab (5.5 mg/kg) attenuated the myocardial inflammatory response to acute VO, as indicated by inhibition of inflammatory gene upregulation, myocardial infiltration (total CD45+ cells, mast cells and neutrophils), MMP-2 activation, collagen degradation and cardiac cell apoptosis, without improving LV remodeling and function. These results indicate that TNF-α produced by cardiomyocytes mediates a predominant inflammatory response to stretch in the early VO in the ACF rat, suggesting an important role of TNF-α in initiating pathophysiological response of myocardium to VO.