Purinergic Signaling
Purinergic system has been recognized as a main pathway to regulate immune response and it is critical to prevent the collateral damage produced by the exacerbation of the immune response in several diseases including cardiac pathologies. In normal conditions, ATP is localized intracellularly where it is the main source of energy for cell functions and it accomplishes indispensable functions in cellular metabolism such as proliferation, migration, motility, biosynthesis and contraction functions within the cardiomyocytes. In addition to its metabolic function, ATP also acts as an important signaling molecule when it is released to the extracellular compartment. After it is released, the extracellular ATP hydrolysis is governed by the activity of two membrane ectoenzymes, the CD39 that catalyzes the phosphohydrolysis of ATP to ADO monophosphate (AMP) and CD73 that hydrolyzes AMP into ADO and inorganic phosphate (45). CD39 and CD73 expression on immune and vascular cells and their enzymatic activities play decisive role in the regulation of the magnitude and duration of purinergic signals that modulate cellular function in an autocrine or paracrine manner (160). These effects are mediated by two types of nucleotide/nucleoside receptors: P1 receptors activated by extracellular ADO and P2 receptors activated by ATP and other nucleotides such as ADP and AMP. The first group includes 4 ADO receptors (ADORA 1, ADORA 2A, ADORA 2B, and ADORA 3), and the second group includes several P2X and P2Y subtypes of receptors (161).
Purinergic signaling is highly described in tumoral context (162) and in autoimmune diseases (163) but poorly studied in the context of cardiac pathologies. Given the high production rate of ATP and the turnover required to maintain its continuous mechanical work, disruption of heart tissue generates the release of high amounts of ATP to the extracellular space. Bonner et al. have reported that cardiomyocytes and erythrocytes do not detectably express the ATP catabolic machinery, but coronary endothelial cells were highly positive for CD39 under basal conditions and a low proportion of these cells express CD73. Furthermore, they stated that resident cardiac APCs and monocytes were the responsible of the first step of ATP degradation in the heart after ischemic injury because they were highly positive for the CD39 while CD73 was absent. Likewise, the dephosphorylating step of AMP to immunosuppressive ADO seems to take place on T lymphocytes and granulocytes because CD73 was mainly expressed in those populations (164).
In contrast to the pro-inflammatory action of ATP, ADO exhibits diverse immunoregulatory roles. Bonner and coworkers found that macrophages stimulated through ADORA 2A and ADORA 2B receptors with ADO undergo M2 phenotype in infarcted mice. In addition, in CD73KO mice there was augmented expression of microbicidal M1 genes, while the expression of M2 genes like arginase-1, IL-10, and TGF-β decreased. This monocyte imbalance toward inflammatory Ly6Chigh-expressing monocytes in mice lacking CD73 had been related to adverse myocardial healing and ventricular dilatation after MI (165). Furthermore, Haskó et al. reported that ADO diminish the oxidative burst by downregulating NO production and inhibit TNF release from monocytes and macrophages (166). A parallel mechanism has also been reported by Cain group in murine isquemia/reperfusion model where ADO pretreatment decreased myocardial TNF production (167). There is strong evidence that ADO can modulate leukocyte adhesion to vascular endothelium in vivo. In this sense, Koszalka et al. demonstrated that ADO is an important endogenous pathway to modulate the inflammatory vascular response. After a period of ischemia-reperfusion, they observed significant increase of leukocyte adherence to the vascular endothelium only in the CD73 mutant (168). It has also been suggested that purinergic signals play a role in cellular migration. Analysis of atherosclerotic lesions of P2Y6 KO mice revealed fewer macrophages, diminished RNA expression of IL-6 and VCAM-1, suggesting that deficiency in this ATP receptor limits atherosclerosis and plaque inflammation (169). Recently, it was reported that lack of P2X7 receptor resolute plaque destabilization by inhibiting inflammasome activation and consequently ameliorates experimental atherosclerosis. ATP binding to P2 receptors is crucial to NLRP3 assembly and the subsequent IL-1β release. P2X7 null mice had decreased amount of macrophages in the wound and, in consequence, smaller and less inflamed atherosclerotic lesions than respective control animals (170). Altogether, the data indicate that purinergic signaling can modulate macrophage polarization and infiltration; and that ADO is a key metabolite in suppressing inflammation following injury or infection.
In addition, ADO induces the production of vascular endothelial growth factor (VEGF) in human macrophages, associated with increased hypoxia inducible factor-1α (HIF-1α) expression, the main transcriptional inducer of VEGF in hypoxic milieu. VEGF promotes blood vessel formation, induces cell proliferation and initiates immune cell migration to stimulate vasculogenesis and angiogenesis (171). On the other hand, extracellular ATP contributes to tissue repair by the stimulation of P2X7 receptor and release of the pro-angiogenic factor VEGF (172). Summing up, activation of purinergic receptors in macrophages can promote wound healing and may be targeted to improve cardiac repair mechanisms.