Cardiac fibroblasts protect cardiomyocytes against lethal ischemia-reperfusion injury

Contribution of apoptosis in myocardial reperfusion injury and loss of cardioprotection in diabetes mellitus

Ischemic heart disease is one of the major causes of death worldwide. Ischemia is a condition in which blood flow of the myocardium declines, leading to cardiomyocyte death. However, reperfusion of ischemic regions decreases the rate of mortality, but it can also cause later complications. In a clinical setting, ischemic heart disease is always coincident with other co-morbidities such as diabetes. The risk of heart disease increases 2-3 times in diabetic patients. Apoptosis is considered to be one of the main pathophysiological mechanisms of myocardial ischemia-reperfusion injury. Diabetes can disrupt the anti-apoptotic intracellular signaling cascades involved in myocardial protection. Therefore, targeting these changes may be an effective cardioprotective approach in the diabetic myocardium against ischemia-reperfusion injury. In this article, we review the interaction of diabetes with the pathophysiology of myocardial ischemia-reperfusion injury, focusing on the contribution of apoptosis in this context, and then discuss the alterations of pro-apoptotic or anti-apoptotic pathways probably responsible for the loss of cardioprotection in diabetes.

Ischemia and reperfusion related myocardial inflammation: A network of cells and mediators targeting the cardiomyocyte

Occlusion of a coronary artery if maintained for longer period of time results in damage of the cardiac tissue. However, restoration of blood flow to previously ischemic tissue can itself induce further cardiac damage, a phenomenon known as myocardial reperfusion injury. Cardiac homoeostasis is supported by a network of direct and indirect interactions between cardiomyocytes and resident cell types such as fibroblasts, adipocytes, and endothelial cells or invading blood cells. This review will discuss the role of the cellular interplay in ischemia-reperfusion injury from a cardiomyocyte-centered view, although we are aware that other cellular interactions are equally important. We will try to work out currently unresolved questions and potential future directions in the field.

Vitamin D receptor activation protects against myocardial reperfusion injury through inhibition of apoptosis and modulation of autophagy

AIMS

To determine the roles of vitamin D receptor (VDR) in ischemia/reperfusion-induced myocardial injury and to investigate the underlying mechanisms involved.

RESULTS

The endogenous VDR expression was detected in the mouse heart, and myocardial ischemia/reperfusion (MI/R) upregulated VDR expression. Activation of VDR by natural and synthetic agonists reduced myocardial infarct size and improved cardiac function. Mechanistically, VDR activation inhibited endoplasmic reticulum (ER) stress (determined by the reduction of CCAAT/enhancer-binding protein homologous protein expression and caspase-12 activation), attenuated mitochondrial impairment (determined by the decrease of mitochondrial cytochrome c release and caspase-9 activation), and reduced cardiomyocyte apoptosis. Furthermore, VDR activation significantly inhibited MI/R-induced autophagy dysfunction (determined by the inhibition of Beclin 1 over-activation, the reduction of autophagosomes, the LC3-II/LC3-I ratio, p62 protein abundance, and the restoration of autophagy flux). Moreover, VDR activation inhibited MI/R-induced oxidative stress through a metallothionein-dependent mechanism. The cardioprotective effects of VDR agonists mentioned earlier were impaired in the setting of cardiac-specific VDR silencing. In contrast, adenovirus-mediated cardiac VDR overexpression decreased myocardial infarct size and improved cardiac function through attenuating oxidative stress, and inhibiting apoptosis and autophagy dysfunction.

INNOVATION AND CONCLUSION

Our data demonstrate that VDR is a novel endogenous self-defensive and cardioprotective receptor against MI/R injury, via mechanisms (at least in part) reducing oxidative stress, and inhibiting apoptosis and autophagy dysfunction-mediated cell death.

Combined hypoxia and sodium nitrite pretreatment for cardiomyocyte protection in vitro

Methods that increase cardiomyocyte survival upon exposure to ischemia, hypoxia and reoxygenation injuries are required to improve the efficacy of cardiac cell therapy and enhance the viability and function of engineered tissues. We investigated the effect of combined hypoxia/NaNO2 pretreatment on rat neonatal cardiomyocyte (CM), cardiac fibroblast, and human embryonic stem cell-derived CM (hESC-CM) survival upon exposure to hypoxia/reoxygenation (H/R) injury in vitro. Cells were pretreated with and without hypoxia and/or various concentrations of NaNO2 for 20 min, then incubated for 2 h under hypoxic conditions, followed by 2 h in normoxia. The control cells were maintained under normoxia for 4 h. Pretreatment with either hypoxia or NaNO2 significantly increased CM viability but had no effect on cardiac fibroblast viability. Combined hypoxia/NaNO2 pretreatment significantly increased CM viability but significantly decreased cardiac fibroblast viability. In rat neonatal CMs, cell death, as determined by lactate dehydrogenase (LDH) activity, was significantly reduced with hypoxia/NaNO2 pretreatment; and in hESC-CMs, hypoxia/NaNO2 pretreatment increased the BCL-2/BAX gene expression ratio, suggesting that hypoxia/NaNO2 pretreatment promotes cell viability by downregulating apoptosis. Additionally, we found a correlation between the prosurvival effect of hypoxia/NaNO2 pretreatment and the myoglobin content of the cells by comparing neonatal rat ventricular and atrial CMs, which express high and low myoglobin respectively. Functionally, hypoxia/NaNO2 pretreatment significantly improved the excitation threshold upon H/R injury to the level observed for uninjured cells, whereas pretreatment did not affect the maximum capture rate. Hence, hypoxia/NaNO2 pretreatment may serve as a strategy to increase CM survival in cardiac regenerative therapy applications and tissue engineering.

Matrix metalloproteinase 9 secreted by hypoxia cardiac fibroblasts triggers cardiac stem cell migration in vitro

Cessation of blood supply due to myocardial infarction (MI) leads to complicated pathological alteration in the affected regions. Cardiac stem cells (CSCs) migration plays a major role in promoting recovery of cardiac function and protecting cardiomyocytes in post-MI remodeling. Despite being the most abundant cell type in the mammalian heart, cardiac fibroblasts (CFs) were underestimated in the mechanism of CSCs migration. Our objective in this study is therefore to investigate the migration related factors secreted by hypoxia CFs in vitro and the degree that they contribute to CSCs migration. We found that supernatant from hypoxia induced CFs could accelerate CSCs migration. Four migration-related cytokines were reported upregulated both in mRNA and protein levels. Upon adding antagonists of these cytokines, the number of migration cells significantly declined. When the cocktail antagonists of all above four cytokines were added, the migration cells number reduced to the minimum level. Besides, MMP-9 had an important effect on triggering CSCs migration. As shown in our results, MMP-9 induced CSCs migration and the underlying mechanism might involve TNF-α signaling which induced VEGF and MMP-9 expression.

Atrial remodeling, fibrosis, and atrial fibrillation

The fundamental mechanisms governing the perpetuation of atrial fibrillation (AF), the most common arrhythmia seen in clinical practice, are poorly understood, which explains in part why AF prevention and treatment remain suboptimal. Although some clinical parameters have been identified as predicting a transition from paroxysmal to persistent AF in some patients, the molecular, electrophysiological, and inflammation changes leading to such a progression have not been described in detail. Oxidative stress, atrial dilatation, calcium overload, inflammation, microRNAs, and myofibroblast activation are all thought to be involved in AF-induced atrial remodeling. However, it is unknown to what extent and at which time points such alterations influence the remodeling process that perpetuates AF. Here we postulate a working model that might open new pathways for future investigation into mechanisms of AF perpetuation. We start from the premise that the progression to AF perpetuation is the result of interplay among manifold signaling pathways with differing kinetics. Some such pathways have relatively fast kinetics (e.g., oxidative stress-mediated shortening of refractory period); others likely depend on molecular processes with slower kinetics (e.g., transcriptional changes in myocyte ion channel protein expression mediated through inflammation and fibroblast activation). We stress the need to fully understand the relationships among such pathways should one hope to identify novel, truly effective targets for AF therapy and prevention.

Molecular candidates for cardiac stretch-activated ion channels

The heart is a mechanically-active organ that dynamically senses its own mechanical environment. This environment is constantly changing, on a beat-by-beat basis, with additional modulation by respiratory activity and changes in posture or physical activity, and further overlaid with more slowly occurring physiological (e.g. pregnancy, endurance training) or pathological challenges (e.g. pressure or volume overload). Far from being a simple pump, the heart detects changes in mechanical demand and adjusts its performance accordingly, both via heart rate and stroke volume alteration. Many of the underlying regulatory processes are encoded intracardially, and are thus maintained even in heart transplant recipients. Over the last three decades, molecular substrates of cardiac mechanosensitivity have gained increasing recognition in the scientific and clinical communities. Nonetheless, the processes underlying this phenomenon are still poorly understood. Stretch-activated ion channels (SAC) have been identified as one contributor to mechanosensitive autoregulation of the heartbeat. They also appear to play important roles in the development of cardiac pathologies – most notably stretch-induced arrhythmias. As recently discovered, some established cardiac drugs act, in part at least, via mechanotransduction pathways suggesting SAC as potential therapeutic targets. Clearly, identification of the molecular substrate of cardiac SAC is of clinical importance and a number of candidate proteins have been identified. At the same time, experimental studies have revealed variable–and at times contrasting–results regarding their function. Further complication arises from the fact that many ion channels that are not classically defined as SAC, including voltage and ligand-gated ion channels, can respond to mechanical stimulation. Here, we summarise what is known about the molecular substrate of the main candidates for cardiac SAC, before identifying potential further developments in this area of translational research.