Cardioprotection: Where to from here?

A Review of the Evidence for Tryptophan and the Kynurenine Pathway as a Regulator of Stem Cell Niches in Health and Disease

Stem cells are ubiquitously found in various tissues and organs in the body, and underpin the body's ability to repair itself following injury or disease initiation, though repair can sometimes be compromised. Understanding how stem cells are produced, and functional signaling systems between different niches is critical to understanding the potential use of stem cells in regenerative medicine. In this context, this review considers kynurenine pathway (KP) metabolism in multipotent adult progenitor cells, embryonic, haematopoietic, neural, cancer, cardiac and induced pluripotent stem cells, endothelial progenitor cells, and mesenchymal stromal cells. The KP is the major enzymatic pathway for sequentially catabolising the essential amino acid tryptophan (TRP), resulting in key metabolites including kynurenine, kynurenic acid, and quinolinic acid (QUIN). QUIN metabolism transitions into the adjoining de novo pathway for nicotinamide adenine dinucleotide (NAD) production, a critical cofactor in many fundamental cellular biochemical pathways. How stem cells uptake and utilise TRP varies between different species and stem cell types, because of their expression of transporters and responses to inflammatory cytokines. Several KP metabolites are physiologically active, with either beneficial or detrimental outcomes, and evidence of this is presented relating to several stem cell types, which is important as they may exert a significant impact on surrounding differentiated cells, particularly if they metabolise or secrete metabolites differently. Interferon-gamma (IFN-γ) in mesenchymal stromal cells, for instance, highly upregulates rate-limiting enzyme indoleamine-2,3-dioxygenase (IDO-1), initiating TRP depletion and production of metabolites including kynurenine/kynurenic acid, known agonists of the Aryl hydrocarbon receptor (AhR) transcription factor. AhR transcriptionally regulates an immunosuppressive phenotype, making them attractive for regenerative therapy. We also draw attention to important gaps in knowledge for future studies, which will underpin future application for stem cell-based cellular therapies or optimising drugs which can modulate the KP in innate stem cell populations, for disease treatment.

Novel therapeutic strategies to reduce reperfusion injury after acute myocardial infarction

For almost 30 years, urgent revascularization termed primary percutaneous coronary intervention (pPCI) has been a cornerstone of modern care for acute myocardial infarction (AMI). It lowers mortality and improved cardiovascular outcome compared to conservative therapy including thrombolysis. Reperfusion injury, which occurs after successful re-opening of the formerly occluded coronary artery, had been exploited as a potential therapeutic target. When revascularisation became faster and pPCI was successfully performed within 60-90 minutes of symptom onset, the interest in a potential additive effect of targeting reperfusion injury vanished. More recently, several meta-analyses indicated that limiting reperfusion injury prevents microvascular obstruction and reduces final infarct size, thereby lowering the probability of heart failure events and improving quality of life in AMI survivors. Here, we describe the current strategies to limit reperfusion injury and to improve post-AMI outcomes such as systemic or intracoronary hypothermia, left-ventricular unloading, intracoronary infusion of super-saturated oxygen, intermittent coronary sinus occlusion, and C-reactive protein apharesis.

Intracoronary Application of Super-Saturated Oxygen to Reduce Infarct Size Following Myocardial Infarction

Optimal medical therapy for secondary prevention following acute myocardial infarction reduces non-fatal ischaemic events. Intensive antithrombotic or lipid-lowering approaches have failed to significantly lower mortality. In the past, reduction of infarct size in patients undergoing primary percutaneous revascularisation for acute myocardial infarction had been considered as a surrogate outcome marker. However, infarct size measured by magnetic resonance imaging or SPECT is strongly associated with all-cause mortality and hospitalization for heart failure within the first year after an acute myocardial infarction. Intracoronary administration of super-saturated oxygen (SSO₂) immediately after revascularisation is an approach that can be used to reduce infarct size and, therefore, improve cardiovascular outcome in patients with acute myocardial infarction. In this article, we describe the modulation of pathophysiology by SSO₂, review the existing trial data and present our first impressions with the technique in real clinical practice.

Update on Cardioprotective Strategies for STEMI: Focus on Supersaturated Oxygen Delivery

Despite the fact that door-to-balloon times have been greatly reduced, the rates of death and the incidence of heart failure in patients with ST-segment elevation myocardial infarction (MI) have plateaued. There is still an unmet need to further reduce MI size in the reperfusion era. Most adjunctive therapies to enhance myocardial salvage have failed, but some have shown promise. Currently, the only adjunctive therapy in a pivotal trial that has demonstrated reductions in infarct size is localized delivery of supersaturated oxygen (SSO₂) therapy. This review provides background on prior infarct size reduction efforts. The authors describe the preclinical data that shows the effectiveness of SSO₂ in reducing MI size, improving regional myocardial blood flow and cardiac function, and reducing adverse left ventricular remodeling-presumably by reducing patchy areas of residual ischemia within the reperfused risk zone. Potential mechanisms by which SSO₂ is beneficial are described, including the delivery of high levels of dissolved oxygen through plasma to ischemic, but viable, vascular and myocardial cells, thus allowing their survival and function. The authors then describe the SSO₂ clinical trials, demonstrating that in patients with anterior ST-segment elevation MI, SSO₂ therapy safely and effectively reduces infarct size, improves cardiac function, and reduces adverse left ventricular remodeling.

Preclinical models of myocardial infarction: from mechanism to translation

Approximately 7 million people are affected by acute myocardial infarction (MI) each year, and despite significant therapeutic and diagnostic advancements, MI remains a leading cause of mortality worldwide. Preclinical animal models have significantly advanced our understanding of MI and have enabled the development of therapeutic strategies to combat this debilitating disease. Notably, some drugs currently used to treat MI and heart failure (HF) in patients had initially been studied in preclinical animal models. Despite this, preclinical models are limited in their ability to fully reproduce the complexity of MI in humans. The preclinical model must be carefully selected to maximise the translational potential of experimental findings. This review describes current experimental models of MI and considers how they have been used to understand drug mechanisms of action and support translational medicine development.

DHA Supplementation Attenuates MI-Induced LV Matrix Remodeling and Dysfunction in Mice

Objective

Myocardial ischemia and reperfusion (I/R) injury is associated with oxidative stress and inflammation, leading to scar development and malfunction. The marine omega-3 fatty acids (ω-3 FA), eicosapentaenoic acid (EPA), and docosahexaenoic acid (DHA) are mediating cardioprotection and improving clinical outcomes in patients with heart disease. Therefore, we tested the hypothesis that docosahexaenoic acid (DHA) supplementation prior to LAD occlusion-induced myocardial injury (MI) confers cardioprotection in mice.

Methods

C57BL/6N mice were placed on DHA or control diets (CD) beginning 7 d prior to 60 min LAD occlusion-induced MI or sham surgery. The expression of inflammatory mediators was measured via RT-qPCR. Besides FACS analysis for macrophage quantification and subtype evaluation, macrophage accumulation as well as collagen deposition was quantified in histological sections. Cardiac function was assessed using a pressure-volume catheter for up to 14 d.

Results

DHA supplementation significantly attenuated the induction of peroxisome proliferator-activated receptor-α (PPAR-α) (2.3 ± 0.4 CD vs. 1.4 ± 0.3 DHA) after LAD occlusion. Furthermore, TNF-α (4.0 ± 0.6 CD vs. 1.5 ± 0.2 DHA), IL-1β (60.7 ± 7.0 CD vs. 11.6 ± 1.9 DHA), and IL-10 (223.8 ± 62.1 CD vs. 135.5 ± 38.5 DHA) mRNA expression increase was diminished in DHA-supplemented mice after 72 h reperfusion. These changes were accompanied by a less prominent switch in α/β myosin heavy chain isoforms. Chemokine mRNA expression was stronger initiated (CCL2 6 h: 32.8 ± 11.5 CD vs. 78.8 ± 13.6 DHA) but terminated earlier (CCL2 72 h: 39.5 ± 7.8 CD vs. 8.2 ± 1.9 DHA; CCL3 72 h: 794.3 ± 270.9 CD vs. 258.2 ± 57.8 DHA) in DHA supplementation compared to CD mice after LAD occlusion. Correspondingly, DHA supplementation was associated with a stronger increase of predominantly alternatively activated Ly6C-positive macrophage phenotype, being associated with less collagen deposition and better LV function (EF 14 d: 17.6 ± 2.6 CD vs. 31.4 ± 1.5 DHA).

Conclusion

Our data indicate that DHA supplementation mediates cardioprotection from MI via modulation of the inflammatory response with timely and attenuated remodeling. DHA seems to attenuate MI-induced cardiomyocyte injury partly by transient PPAR-α downregulation, diminishing the need for antioxidant mechanisms including mitochondrial function, or α- to β-MHC isoform switch.

Efficacy of a Novel Mitochondrial-Derived Peptide in a Porcine Model of Myocardial Ischemia/Reperfusion Injury

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A mitochondrial-derived peptide therapy, HNG, was safe and was delivered as adjunctive therapy with standard-of-care reperfusion in a translational large animal model of myocardial ischemia/reperfusion injury.

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HNG reduced infarct size per area-at-risk by 41% with an ischemic time of 60 min followed by 48 h of reperfusion.

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The infarct-sparing effects of HNG were abolished when the ischemic time was increased to 75 min followed by 48 h of reperfusion.

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The use of rigorous translational large animal models that account for clinically relevant variables is a prerequisite to better predict the clinical efficacy and outcomes of novel therapeutic strategies.

With the complexities that surround myocardial ischemia/reperfusion (MI/R) injury, therapies adjunctive to reperfusion that elicit beneficial pleiotropic effects and do not overlap with standard of care are necessary. This study found that the mitochondrial-derived peptide S14G-humanin (HNG) (2 mg/kg), an analogue of humanin, reduced infarct size in a large animal model of MI/R. However, when ischemic time was increased, the infarct-sparing effects were abolished with the same dose of HNG. Thus, although the 60-min MI/R study showed that HNG cardioprotection translates beyond small animal models, further studies are needed to optimize HNG therapy for longer, more patient-relevant periods of cardiac ischemia.

Minimal Invasive Pericardial Perfusion Model in Swine: A Translational Model for Cardiac Remodeling After Ischemia/Reperfusion Injury

Rationale

Adverse remodeling leads to heart failure after myocardial infarction (MI), with important impact on morbidity and mortality. New therapeutic approaches are needed to further improve and broaden heart failure therapy. We established a minimally invasive, reproducible pericardial irrigation model in swine, as a translational model to study the impact of temperature on adverse cardiac remodeling and its molecular mechanisms after MI.

Objective

Chronic heart failure remains a leading cause of death in western industrialized countries, with a tremendous economic impact on the health care system. Previously, many studies have investigated mechanisms to reduce infarct size after ischemia/reperfusion injury, including therapeutic hypothermia. Nonetheless, the molecular mechanisms of adverse remodeling after MI remain poorly understood. By deciphering the latter, new therapeutic strategies can be developed to not only reduce rehospitalization of heart failure patients but also reduce or prevent adverse remodeling in the first place.

Methods and Results

After 90 min of MI, a 12Fr dual lumen dialysis catheter was place into the pericardium via minimal invasive, sub-xiphoidal percutaneous puncture. We performed pericardial irrigation with cold or warm saline for 60 min in 25 female farm pigs after ischemia and reperfusion. After one week of survival the heart was harvested for further studies. After cold pericardial irrigation we observed a significant decrease of systemic body temperature measured with a rectal probe in the cold group, reflecting that the heart was chilled throughout its entire thickness. The temperature remained stable in the control group during the procedure. We did not see any difference in arrhythmia or hemodynamic stability between both groups.

Conclusion

We established a minimally invasive, reproducible and translational model of pericardial irrigation in swine. This method enables the investigation of mechanisms involved in myocardial adverse remodeling after ischemia/reperfusion injury in the future.

Iodide Improves Outcome After Acute Myocardial Infarction in Rats and Pigs

OBJECTIVES

In this study, we tested whether iodide would reduce heart damage in rat and pig models of acute myocardial infarction as a risk analysis for a human trial.

DESIGN

Prospective blinded and randomized laboratory animal investigation.

SETTING

Animal research laboratories.

SUBJECTS

Sexually mature rats and pigs.

INTERVENTIONS

Acute myocardial infarction was induced by temporary ligation of the coronary artery followed by reperfusion. Iodide was administered orally in rats or IV in rats and pigs just prior to reperfusion.

MEASUREMENTS AND MAIN RESULTS

Damage was assessed by blood cardiac troponin and infarct size; heart function was determined by echocardiography. Blood peroxide scavenging activity was measured enzymatically, and blood thyroid hormone was determined using radioimmune assay. Iodide administration preserved heart function and reduced blood cardiac troponin and infarct size by approximately 45% in pigs and approximately 60% in rats. Iodide administration also increased blood peroxide scavenging activity and maintained thyroid hormone levels.

CONCLUSIONS

Iodide administration improved the structure and function of the heart after acute myocardial infarction in rats and pigs.

Left Ventricular Unloading Before Reperfusion Promotes Functional Recovery After Acute Myocardial Infarction

BACKGROUND

Heart failure after an acute myocardial infarction (AMI) is a major cause of morbidity and mortality worldwide. We recently reported that activation of a transvalvular axial-flow pump in the left ventricle and delaying myocardial reperfusion, known as primary unloading, limits infarct size after AMI. The mechanisms underlying the cardioprotective benefit of primary unloading and whether the acute decrease in infarct size results in a durable reduction in LV scar and improves cardiac function remain unknown.

OBJECTIVES

This study tested the importance of LV unloading before reperfusion, explored cardioprotective mechanisms, and determined the late-term impact of primary unloading on myocardial function.

METHODS

Adult male swine were subjected to primary reperfusion or primary unloading after 90 min of percutaneous left anterior descending artery occlusion.

RESULTS

Compared with primary reperfusion, 30 min of LV unloading was necessary and sufficient before reperfusion to limit infarct size 28 days after AMI. Compared with primary reperfusion, primary unloading increased expression of genes associated with cellular respiration and mitochondrial integrity within the infarct zone. Primary unloading for 30 min further reduced activity levels of proteases known to degrade the cardioprotective cytokine, stromal-derived factor (SDF)-1α, thereby increasing SDF-1α signaling via reperfusion injury salvage kinases, which limits apoptosis within the infarct zone. Inhibiting SDF-1α activity attenuated the cardioprotective effect of primary unloading. Twenty-eight days after AMI, primary unloading reduced LV scar size, improved cardiac function, and limited expression of biomarkers associated with heart failure and maladaptive remodeling.

CONCLUSIONS

The authors report for the first time that first mechanically reducing LV work before coronary reperfusion with a transvalvular pump is necessary and sufficient to reduce infarct size and to activate a cardioprotective program that includes enhanced SDF-1α activity. Primary unloading further improved LV scar size and cardiac function 28 days after AMI.

Two pharmacological epoxyeicosatrienoic acid-enhancing therapies are effectively antihypertensive and reduce the severity of ischemic arrhythmias in rats with angiotensin II-dependent hypertension

OBJECTIVE

We examined the effects of treatment with soluble epoxide hydrolase inhibitor (sEHi) and epoxyeicosatrienoic acids (EETs) analogue (EET-A), given alone or combined, on blood pressure (BP) and ischemia/reperfusion myocardial injury in rats with angiotensin II (ANG II)-dependent hypertension.

METHODS

Ren-2 transgenic rats (TGR) were used as a model of ANG II-dependent hypertension and Hannover Sprague-Dawley rats served as controls. Rats were treated for 14 days with sEHi or EET-A and BP was measured by radiotelemetry. Albuminuria, cardiac hypertrophy and concentrations of ANG II and EETs were determined. Separate groups were subjected to acute myocardial ischemia/reperfusion injury and the infarct size and ventricular arrhythmias were determined.

RESULTS

Treatment of TGR with sEHi and EET-A, given alone or combined, decreased BP to a similar degree, reduced albuminuria and cardiac hypertrophy to similar extent; only treatment regimens including sEHi increased myocardial and renal tissue concentrations of EETs. sEHi and EET-A, given alone or combined, suppressed kidney ANG II levels in TGR. Remarkably, infarct size did not significantly differ between TGR and Hannover Sprague-Dawley rats, but the incidence of ischemia-induced ventricular fibrillations was higher in TGR. Application of sEHi and EET-A given alone and combined sEHi and EET-A treatment were all equally effective in reducing life-threatening ventricular fibrillation in TGR.

CONCLUSION

The findings indicate that chronic treatment with either sEHi or EET-A exerts distinct antihypertensive and antiarrhythmic actions in our ANG II-dependent model of hypertension whereas combined administration of sEHi and EET-A does not provide additive antihypertensive or cardioprotective effects.

Beyond Reperfusion: Acute Ventricular Unloading and Cardioprotection During Myocardial Infarction

Heart failure is a major cause of morbidity and mortality around the world, and myocardial infarction is its leading cause. Myocardial infarction destroys viable myocardium, and this dead tissue is replaced by a non-contractile scar that results in impaired cardiac function and a significantly increased likelihood of the patient developing heart failure. Limiting infarct scar size has been the target of pre-clinical and clinical investigations for decades. However, beyond reperfusion, few therapies have translated into the clinic that limit its formation. New approaches are needed. This review will focus on new clinical and pre-clinical data demonstrating that acute ventricular unloading prior to reperfusion by means of percutaneous left ventricular support devices reduces ischemia-reperfusion injury and limits infarct scar size. Emphasis will be given to summarizing our current mechanistic understanding of this new therapeutic approach to treating myocardial infarction.

Reduction of myocardial ischaemia-reperfusion injury by inactivating oxidized phospholipids

Aims

Myocardial ischaemia followed by reperfusion (IR) causes an oxidative burst resulting in cellular dysfunction. Little is known about the impact of oxidative stress on cardiomyocyte lipids and their role in cardiac cell death. Our goal was to identify oxidized phosphatidylcholine-containing phospholipids (OxPL) generated during IR, and to determine their impact on cell viability and myocardial infarct size.

Methods and results

OxPL were quantitated in isolated rat cardiomyocytes using mass spectrophotometry following 24 h of IR. Cardiomyocyte cell death was quantitated following exogenously added OxPL and in the absence or presence of E06, a 'natural' murine monoclonal antibody that binds to the PC headgroup of OxPL. The impact of OxPL on mitochondria in cardiomyocytes was also determined using cell fractionation and Bnip expression. Transgenic Ldlr-/- mice, overexpressing a single-chain variable fragment of E06 (Ldlr-/--E06-scFv-Tg) were used to assess the effect of inactivating endogenously generated OxPL in vivo on myocardial infarct size. Following IR in vitro, isolated rat cardiomyocytes showed a significant increase in the specific OxPLs PONPC, POVPC, PAzPC, and PGPC (P < 0.05 to P < 0.001 for all). Exogenously added OxPLs resulted in significant death of rat cardiomyocytes, an effect inhibited by E06 (percent cell death with added POVPC was 22.6 ± 4.14% and with PONPC was 25.3 ± 3.4% compared to 8.0 ± 1.6% and 6.4 ± 1.0%, respectively, with the addition of E06, P < 0.05 for both). IR increased mitochondrial content of OxPL in rat cardiomyocytes and also increased expression of Bcl-2 death protein 3 (Bnip3), which was inhibited in presence of E06. Notably cardiomyocytes with Bnip3 knock-down were protected against cytotoxic effects of OxPL. In mice exposed to myocardial IR in vivo, compared to Ldlr-/- mice, Ldlr-/--E06-scFv-Tg mice had significantly smaller myocardial infarct size normalized to area at risk (72.4 ± 21.9% vs. 47.7 ± 17.6%, P = 0.023).

Conclusions

OxPL are generated within cardiomyocytes during IR and have detrimental effects on cardiomyocyte viability. Inactivation of OxPL in vivo results in a reduction of infarct size.

Exosomes and cardioprotection - A critical analysis

Exosomes are nano-sized vesicles released by numerous cell types that appear to have diverse beneficial effects on the injured heart. Studies using exosomes from stem cells or from the blood have indicated that they are able to protect the heart both in models of acute ischaemia and reperfusion, and during chronic ischaemia. In addition to decreasing initial infarct size, they are able to stimulate angiogenesis, reduce fibrosis and remodelling, alter immune cell function and improve long-term cardiac contractile function. However, since the technology and techniques used for the study of exosomes is relatively immature and continually evolving, there remain many important caveats to the interpretation of studies. This review presents a critical analysis of the field of exosomes and cardioprotection. We analyse the effects of exosomes from all types of stem cells investigated to date, summarize the major effects observed and their potential mechanism, and offer our perspective on the major outstanding issues.

The Oxygen Paradox, the French Paradox, and age-related diseases

A paradox is a seemingly absurd or impossible concept, proposition, or theory that is often difficult to understand or explain, sometimes apparently self-contradictory, and yet ultimately correct or true. How is it possible, for example, that oxygen "a toxic environmental poison" could be also indispensable for life (Beckman and Ames Physiol Rev 78(2):547-81, 1998; Stadtman and Berlett Chem Res Toxicol 10(5):485-94, 1997)?: the so-called Oxygen Paradox (Davies and Ursini 1995; Davies Biochem Soc Symp 61:1-31, 1995). How can French people apparently disregard the rule that high dietary intakes of cholesterol and saturated fats (e.g., cheese and paté) will result in an early death from cardiovascular diseases (Renaud and de Lorgeril Lancet 339(8808):1523-6, 1992; Catalgol et al. Front Pharmacol 3:141, 2012; Eisenberg et al. Nat Med 22(12):1428-1438, 2016)?: the so-called, French Paradox. Doubtless, the truth is not a duality and epistemological bias probably generates apparently self-contradictory conclusions. Perhaps nowhere in biology are there so many apparently contradictory views, and even experimental results, affecting human physiology and pathology as in the fields of free radicals and oxidative stress, antioxidants, foods and drinks, and dietary recommendations; this is particularly true when issues such as disease-susceptibility or avoidance, "healthspan," "lifespan," and ageing are involved. Consider, for example, the apparently paradoxical observation that treatment with low doses of a substance that is toxic at high concentrations may actually induce transient adaptations that protect against a subsequent exposure to the same (or similar) toxin. This particular paradox is now mechanistically explained as "Adaptive Homeostasis" (Davies Mol Asp Med 49:1-7, 2016; Pomatto et al. 2017a; Lomeli et al. Clin Sci (Lond) 131(21):2573-2599, 2017; Pomatto and Davies 2017); the non-damaging process by which an apparent toxicant can activate biological signal transduction pathways to increase expression of protective genes, by mechanisms that are completely different from those by which the same agent induces toxicity at high concentrations. In this review, we explore the influences and effects of paradoxes such as the Oxygen Paradox and the French Paradox on the etiology, progression, and outcomes of many of the major human age-related diseases, as well as the basic biological phenomenon of ageing itself.