NO Better Way to Protect the Heart during Ischemia-Reperfusion: To be in the Right Place at the Right Time.

Abstract

Acute myocardial infarction (MI) is one of the leading causes of mortality worldwide. MI is the heart muscle irreversible death secondary to prolonged ischemia. Over the last few decades, medical progress in how and when to restore blood flow to the ischemic area have markedly improved patient survival. Although early heart reperfusion is acknowledged to be the most effective way to limit infarct size, post-ischemic reperfusion is associated with detrimental effects, such as myocardial stunning, ventricular arrhythmias, microvascular dysfunction, and cell death. The molecular mechanisms of these reperfusion injuries remain to be elucidated and their management is very challenging. Among the various therapeutic molecular approaches proposed by experimental studies, nitric oxide (NO) role in protecting heart against MI and reperfusion injuries has been widely assessed and discussed (1–4). NO is a gasotransmitter that is abundantly produced in the cardiovascular system mainly by the NO synthase (NOS) enzymes system. Two isoforms, endothelial NOS (eNOS) and neuronal NOS (nNOS), are constitutively expressed in both myocardium and vessels, whereas inducible NOS (iNOS) is detected only in pathological conditions, such as inflammatory and/or oxidative stress. Both eNOS and nNOS are low-NO output Ca2+-dependent enzymes, while iNOS is a high-NO output Ca2+-independent enzyme. In physiological conditions, NOS form homodimers (“coupled” NOS) that catalyze NO production from l-arginine and O2 through electron transfer from NADPH on the reductase domain of one monomer to the oxidase domain of the second monomer. In pathological conditions, such as in the absence of the essential cofactor tetrahydrobiopterin (BH4), eNOS can be “uncoupled” to produce O2− instead of NO. In stress conditions, NO protects tissues through two distinct pathways. In the first one, NO activates the soluble guanylate cyclase (sGC) that initiates cyclic guanosine monophosphate (cGMP) production, leading to the activation of protein kinase G (PKG). As sGC is the major cell receptor for NO and the NO/sGC/cGMP/PKG pathway plays a critical role in both myocardium excitation–contraction coupling and cardiovascular function regulation (5–8), NO cardioprotective role was first attributed to PKG activation (9–11). However, a second pathway in which proteins are directly modified by NO addition to sulfhydryl residues, a process known as S-nitrosylation (SNO), has recently emerged in the scientific literature. Although PKG activation pathway has been largely involved in NO-mediated cardioprotection (11–13), SNO is now taking the front stage and is considered to be a key player in cardioprotection through (i) the transient modification of protein activity and/or (ii) their protection from irreversible oxidation (14–17). Indeed, Sun et al. (18) showed that reduced heart vulnerability to ischemia–reperfusion (IR) following acute ischemic preconditioning is mainly related to SNO signaling and not to PKG activation through the NO–SGC–cGMP pathway. Accordingly, we found that in exercise training-induced cardioprotection against IR injuries, protein SNO level, but not cGMP level, increased during early reperfusion (19). The same year, Methner et al. (20), using a Cre/loxP approach to selectively ablate type I PKG in cardiomyocytes, demonstrated that ischemic post-conditioning reduced infarct size in these mice like in wild type controls. Moreover, they showed that the cardioprotective effect against IR injury of mitochondria-targeted S-nitrosothiol (MitoSNO), which allows NO and S-nitrosothiol accumulation in mitochondria, was comparable in mice that specifically lack PKG in cardiomyocytes and in controls. This indicates that MitoSNO cardioprotective effect is independent of PKG. The current literature strongly supports NO implication in cardioprotection. However, the mechanism is still debated and whether increased NO availability during IR is cytoprotective remains to be demonstrated. Here, we discuss how NO might contribute to protect heart and particularly the importance of NO (i) localization, (ii) concentration, and (iii) time of availability during IR.