ERK phosphorylation mediates sildenafil-induced myocardial protection against ischemia-reperfusion injury in mice

Mitochondrial kinase signalling pathways in myocardial protection from ischaemia/reperfusion-induced necrosis

Multiple cardioprotective signal pathways that are activated by ischaemic preconditioning (IPC) and those by IPC mimetics converge on mitochondria. Recent studies have shown that pools of Akt, protein kinase C-ε, extracellular-regulated kinases, glycogen synthase kinase-3beta (GSK-3beta), and hexokinases (HK) I and II, are localized in mitochondria in addition to their pools in the cytosol. Accumulating evidence indicates that such 'mitochondrial protein kinases' receive signals from cytosolic molecules and enhance tolerance of myocytes to injury. Proteomic analyses suggest that these kinases form complexes with each other and with subunit proteins of the mitochondrial permeability transition pore (mPTP). Functional relationships between the protein kinases in mitochondria have not been fully clarified, but GSK-3beta and HKs appear to be at the end of the signal pathways and directly responsible for inhibition of opening of the mPTP and, thus, for myocyte protection from necrosis. In this review, recent findings supporting roles of mitochondrial protein kinases in protection from myocardial necrosis after ischaemia/reperfusion are summarized and discussed.

Single administration of the CXC chemokine-binding protein Evasin-3 during ischemia prevents myocardial reperfusion injury in mice

OBJECTIVE

Evasins (chemokine-binding proteins) have been shown to selectively neutralize chemokine bioactivity. We investigated the potential benefits of Evasin-3 on mouse myocardial ischemia/reperfusion injury.

METHODS AND RESULTS

In vivo and ex vivo (Langendorff model) left coronary artery ligature was performed in C57Bl/6 mice. Coronary occlusion was maintained for 30 minutes, followed by different times (up to 24 hours) of reperfusion. Five minutes after coronary occlusion, mice received 1 intraperitoneal injection of Evasin-3 or vehicle. Infarct size was assessed histologically and by serum cardiac troponin I ELISA. In vitro neutrophil chemotaxis, immunohistology, oxidative stress quantification, real-time RT-PCR analysis of leukocyte chemoattractants, and Western blots for cardioprotective intracellular pathway activation were performed. Evasin-3 reduced infarct size and cardiac troponin I levels compared with vehicle. This effect was associated with the reduction of neutrophil infiltration and reactive oxygen species production within the infarcted myocardium. Evasin-3 did not reduce infarct size in the absence of circulating neutrophils (Langendorff model). Evasin-3 did not influence the activation of intracellular cardioprotective pathways or the expression of leukocyte chemoattractants during early phases of reperfusion.

CONCLUSIONS

Single administration of Evasin-3 during myocardial ischemia significantly reduced infarct size by preventing CXC chemokine-induced neutrophil recruitment and reactive oxygen species production in myocardial ischemia/reperfusion.

Periodic acceleration (pGz) prior to whole body ischemia reperfusion injury provides early cardioprotective preconditioning

AIMS

Periodic acceleration (pGz) is a method that applies repetitive sinusoidal head-to-foot motion to the horizontally positioned body. pGz adds pulses to the circulation as a function of frequency, thereby increasing shear stress to the endothelium. Pulsatile shear stress increases release of cardioprotective endothelial-derived nitric oxide prostaglandin E-2 and prostacyclin into the circulation. We investigated whether pGz may be effective as an early preconditioning strategy when applied one hour prior to whole body ischemia reperfusion injury induced by ventricular fibrillation (VF).

MAIN METHODS

Twenty anesthetized and paralyzed male swine were randomized to one hour of pGz and conventional mechanical ventilation [PC] or solely conventional mechanical ventilation [Control] prior to VF and resuscitation. After eight minutes of unsupported VF, cardiopulmonary resuscitation was carried out followed by defibrillation. Hemodynamics, electrocardiogram, echocardiogram, regional blood flows, and markers of global myocardial injury were measured. Protein expression of endothelial-derived nitric oxide synthase (eNOS), phosphorylated eNOS (p-eNOS), serine/threonine kinase Akt total (t-Akt), and phosphorylated (p-Akt) were determined by immunoblotting.

KEY FINDINGS

All animals had spontaneous return of circulation after cardiopulmonary resuscitation (CPR) and defibrillation. Preconditioned animals had less hemodynamically significant arrhythmias, less myocardial stunning, and greater regional blood flows to the brain, heart, kidneys, and ileum than Controls. Troponin I and creatine phosphokinase values in PC were 65% of the values present in Controls. In addition, preconditioned animals had higher protein expression of cardiac eNOS, p-eNOS, t-Akt, and p-Akt than Controls.

SIGNIFICANCE

pGz preconditioning confers early cardioprotection in a model of whole body ischemia reperfusion injury.

Transient hypoxia induces ERK-dependent anti-apoptotic cell survival in endothelial cells

Ischemia-induced apoptosis of endothelial cells may contribute to tissue injury, organ failure, and transplantation rejection. However, little is known about survival mechanisms capable to counteract endothelial apoptosis. This study investigated the potential role of an endogenous anti-apoptotic response elicited by transient hypoxia, capable to avert ongoing apoptosis in endothelial cells. Experiments were carried out in three different types of cultured endothelial cells (human umbilical vein, pig aorta, and from rat coronary microvasculature). As a pro-apoptotic challenge endothelial cells were cultured in serum-free medium and subjected to hypoxia for 2 h. We found that transient hypoxia reduced caspase 3 activation within 1 h of hypoxia. Accordingly, the number of apoptotic cells was reduced after 24 h of reoxygenation. This was true for all three cell types analyzed. Analysis of Akt and mitogen-activated protein kinase kinase (MEK)/extracellular signal-regulated kinase (ERK) pathways revealed that hypoxia induced a transient activation of ERK 2 but not of Akt. ERK 2 phosphorylation preceded the phosphorylation of pro-apoptotic molecule Bad at Ser112, an inhibitory phosphorylation site specific for ERK. The protective effects of hypoxia regarding Bad phosphorylation, caspase 3 activation, and apoptosis were abolished by MEK 1/2 inhibitors, PD98059 or UO126, as well as by antisense oligonucleotides directed against ERK 1/2. Furthermore, inhibition of this pathway inhibited hypoxia-induced increase in mitochondrial membrane potential. The present study demonstrates that transient hypoxia induces a novel survival mechanism that protects endothelial cells against apoptosis. This endogenous process involves MEK/ERK-mediated inhibition of the pro-apoptotic molecule Bad and caspase 3.

Netrin-1 prevents ischemia/reperfusion-induced myocardial infarction via a DCC/ERK1/2/eNOS s1177/NO/DCC feed-forward mechanism

We have recently shown that a novel endothelial mitogen netrin-1 potently stimulates nitric oxide (NO()) production via a DCC-ERK1/2 dependent mechanism. In view of the well-established cardioprotective role of NO(), the present study investigated whether netrin-1 is cardioprotective via NO(*) signaling in the heart. Netrin-1 receptor DCC was abundantly expressed in the C57BL/6J mouse hearts. Perfusion of heart with netrin-1 (100 ng/mL) using a Langendorff system significantly increased NO(*) production. Under ischemia/reperfusion (I/R), netrin-1 induced a substantial reduction in infarct size (21.8+/-4.9% from 42.5+/-3.6% in the controls), which was accompanied by an augmented production of NO(*). Pre-perfusion with DCC-antibody, U0126 (MEK1/2 inhibitor), L-NAME or PTIO (NO(*) scavenger) attenuated protective effects of netrin-1 on infarct size and NO(*) production, indicating upstream roles of DCC and ERK1/2 in NO(*) production, as well as an essential role of NO(*) in cardioprotection. Netrin-1 induced reduction in infarct size was significantly attenuated in DCC+/- mice, confirming an intermediate role of DCC. In additional experiments we found netrin-1 increased ERK1/2 and eNOS(s1177) phosphorylation, and DCC protein expression, which was diminished by I/R. Furthermore, netrin-1-induced DCC upregulation was NO(*) and ERK1/2-dependent, implicating a feed-forward mechanism. DAF-AM staining revealed enhanced NO(*) production in both cardiac endothelial cells (ECs) and myocytes. In primarily isolated cardiomyocytes, netrin-1 also increased NO(*) production, DCC abundance and ERK1/2 phosphorylation. Of note, cardiac apoptosis was significantly attenuated by netrin-1, which was reversed by DCC-antibody, U0126, L-NAME or PTIO. In summary, our data clearly demonstrate that netrin-1 potently protects the heart from I/R injury by stimulating NO(*) production from cardiac ECs and myocytes. This potent effect is mediated by a DCC/ERK1/2/eNOS(s1177)/NO(*)/DCC feed-forward mechanism in both cell types.

Alterations in mitochondrial function as a harbinger of cardiomyopathy: lessons from the dystrophic heart

While compelling evidence supports the central role of mitochondrial dysfunction in the pathogenesis of heart failure, there is comparatively less information available on mitochondrial alterations that occur prior to failure. Building on our recent work with the dystrophin-deficient mdx mouse heart, this review focuses on how early changes in mitochondrial functional phenotype occur prior to overt cardiomyopathy and may be a determinant for the development of adverse cardiac remodelling leading to failure. These include alterations in energy substrate utilization and signalling of cell death through increased permeability of mitochondrial membranes, which may result from abnormal calcium handling, and production of reactive oxygen species. Furthermore, we will discuss evidence supporting the notion that these alterations in the dystrophin-deficient heart may represent an early "subclinical" signature of a defective nitric oxide/cGMP signalling pathway, as well as the potential benefit of mitochondria-targeted therapies. While the mdx mouse is an animal model of Duchenne muscular dystrophy (DMD), changes in the structural integrity of dystrophin, the mutated cytoskeletal protein responsible for DMD, have also recently been implicated as a common mechanism for contractile dysfunction in heart failure. In fact, altogether our findings support a critical role for dystrophin in maintaining optimal coupling between metabolism and contraction in the heart.

Pretreatment of sildenafil attenuates ischemia-reperfusion renal injury in rats

Sildenafil was the first selective inhibitor of phosphodiesterase-5 (PDE5) to be widely used for treating erectile dysfunction. Many recent studies have investigated the cardioprotective role of sildenafil in animal models. We evaluated the protective effects of sildenafil in experimental renal ischemia-reperfusion (IR) injury in two studies. In study 1, male Sprague-Dawley rats were divided into four groups: sham, sildenafil-treated sham, vehicle-treated IR, and sildenafil-treated IR groups. In study 2, we divided the rats into two groups: sildenafil-treated IR rats and PD98059 (ERK inhibitor)+sildenafil-treated IR rats. Functional parameters of the kidney were evaluated at the molecular and structural levels. Blood urea nitrogen (BUN) and serum creatinine levels were lower in sildenafil-treated IR rats than in vehicle-treated IR rats. The expression of inducible (iNOS) and endothelial nitric oxide synthase (eNOS) proteins in sildenafil-treated IR rats was significantly higher than in vehicle-treated IR rats. Pretreatment with sildenafil in IR rats increased ERK phosphorylation and reduced the renal Bax/Bcl-2 ratio, renal caspase-3 activity, and terminal dUTP nick end-labeling-positive apoptotic cells. In contrast, PD98059 treatment increased BUN and serum creatinine levels and attenuated the sildenafil-induced expression of pERK, iNOS, eNOS, and Bcl-2. PD98059 also increased caspase-3 activity but did not decrease the sildenafil-induced accumulation of cGMP. In conclusion, this study suggests that sildenafil has antiapoptotic effects in experimental IR renal injury via ERK phosphorylation, induction of iNOS and eNOS production, and a decrease in the Bax/Bcl-2 ratio.

Phosphodiesterase 5 inhibitors: are they cardioprotective?

A growing body of animal studies provides evidence for potential cardioprotective effects of inhibitors of the enzyme phosphodiesterase isoform 5. Infarct size reduction by administration of phosphodiesterase 5 inhibitors was described in various experimental models of ischaemia and reperfusion. Furthermore, potential beneficial effects were demonstrated in experimental models of congestive heart failure and left ventricular hypertrophy. Some of the observed effects resemble the basic mechanisms of ischaemic pre-conditioning, mimicking both acute and delayed effects. Other effects may be due to action on systemic and cardiac haemodynamics. Mechanisms and signalling pathways, characterized in some of the experimental models, appear to be complex: for instance, the rate of cyclic guanosine monophosphate (cGMP) synthesis and the functional compartmentalization of intracellular cGMP metabolism as well as interaction with ss-adrenergic and nitric oxide signalling may influence effects in different experimental settings. In this review, we discuss mechanisms, signalling pathways, and experimental limitations and touch on considerations for translation into potentially useful applications in the clinical arena.