Introduction

MicroRNAs (miRNAs) are a class of small non-coding RNAs that mediate posttranscriptional gene silencing. Recently, the work on miRNAs renovates our understanding about the gene regulation in cardiac physiology and pathology. The cardiac-specific knockout of Dicer, an RNAse III endonuclease critical for processing of pre-miRNAs into mature miRNAs, leads to rapidly progressive dilated cardiomyopathy, heart failure, and postnatal lethality [1, 2]. Knockout of an individual miRNA may result in pathological disorders. Targeted deletion of the muscle-specific miRNA, miR-1-2, reveals numerous functions in the heart including regulation of cardiac morphogenesis, electrical conduction, and cell-cycle control [3]. These results suggest that miRNAs play critical roles in maintaining normal cardiac structure and function. Cardiomyocyte apoptosis is related to cardiac disorders such as myocardial infarction, cardiomyopathy, cardiac hypertrophy, and anthracycline-induced cardiotoxicity [4–10]. The growing evidence demonstrates that miRNAs can regulate apoptosis [11–14]. This article reviews the role of miRNAs in regulating cardiac apoptosis. Specifically, it discusses the potential therapeutic approaches for apoptosis-related cardiac diseases by modulating miRNAs.

How do miRNAs Regulate Apoptosis?

Apoptosis can be initiated by a variety of miRNAs. miR-200a is able to target the E-cadherin and Wnt/β-catenin signaling pathways [15]. E2F1 is negatively regulated by miR-330, the latter induces apoptosis through E2F1-mediated suppression of Akt phosphorylation [16]. miR-122 modulates cyclin G1 silencing and increases sensitivity to doxorubicin challenge [17]. Upregulation of miR-23a-27a-24-2 cluster induces caspase-dependent and caspase-independent apoptosis [18]. Histone deacetylases are related to cell survival, and their expression can be repressed by miR-449a [19]. miR-101 and miR-29b exert their proapoptotic function via targeting Mcl-1 [20, 21]; the latter is a member of Bcl-2 family and can inhibit apoptosis [22, 23]. miR-34a inhibits silent information regulator 1 (SIRT1) expression leading to an increase in acetylated p53 and expression of p21 and p53 upregulated modulator of apoptosis [24]. These results suggest that miRNAs can provoke apoptosis.

Many miRNAs have been found to be able to inhibit apoptosis. miR-206 prevents apoptosis by targeting notch3 [25]. miR-17-92 cluster is a novel target for p53-mediated transcriptional repression under hypoxia, and overexpression of miR-17-92 cluster markedly inhibits hypoxia-induced apoptosis [26]. In addition, p53 is also regulated by miR-125b [27]. A summary of miRNAs functions in apoptosis is shown in Table 1.

Table 1 The miRNAs functions in apoptosis
Full size table

Although the functions of the above miRNAs in apoptosis have been tested in other cell types, it can be speculated that they may have an impact in cardiomyocyte apoptosis. This is because their targets such as E2F1 and p53 have been proven to participate in the apoptotic program of cardiomyocytes [34–36].

miRNAs in Apoptosis-Related Cardiac Pathology

Cardiac hypertrophy is an early milestone of many heart diseases [37, 38] and associated with changes in gene expression [39]. It is related to apoptosis that plays a driving role in the transition from compensated hypertrophy to failure in the work-overloaded myocardium [7, 8, 40–42]. Recently, miRNAs are shown to regulate cardiac hypertrophy [43, 44]. Functional studies reveal that different miRNAs have distinct effects on cardiac hypertrophy. For example, inhibition of miR-133 causes significant cardiac hypertrophy [45]. In contrast, miR-208 is required for cardiomyocyte hypertrophy in response to stress and hypothyroidism [46, 47]. Cardiac-specific overexpression of miR-195 in mice results in severe cardiac hypertrophy [48]. Overexpression of miR-214 in the cardiomyocytes causes significant hypertrophy [48]. miR-1 negatively regulates the expression of hypertrophy-associated calmodulin and Mef2a genes [49]. Isoproterenol and aldosterone stimulate miR-23a expression. Knockdown of miR-23a could attenuate hypertrophy, suggesting that miR-23a is able to convey the hypertrophic signal. Nuclear factor of activated T cells can directly activate miR-23a expression through the transcriptional machinery. The muscle-specific ring finger protein 1, an anti-hypertrophic protein [50], is identified to be a target of miR-23a, and its translation could be suppressed by miR-23a [51]. Thus, it appears that miRNAs play multiple and essential roles in the regulation of cardiac hypertrophy. Future studies are required to elucidate whether miRNAs play a role for the transition from hypertrophy to apoptosis.

Heart failure is characterized by the occurrence of apoptosis [6, 52]. Dgcr8 controls microRNA biogenesis, and cardiomyocyte-specific deletion of dgcr8 leads to a fully penetrant phenotype that begins with left ventricular malfunction progressing to a dilated cardiomyopathy and premature lethality [53]. Intriguingly, miRNA expression patterns are distinct in two types of heart failure: idiopathic dilated cardiomyopathy and ischemic cardiomyopathy [54].

The excessive apoptosis has been identified in myocardial infarction [55, 56]. The growing evidence shows that miRNAs are related to acute myocardial infarction. In analyzing the expression patterns of miRNAs in noninfarcted and infarcted areas, 38 miRNAs are differentially expressed in infarcted areas, and 33 miRNAs are aberrantly expressed in the border areas [57]. It is of note that miR-21 expression is decreased in infarcted areas but is elevated in border areas. Enforced expression of miR-21 leads to a reduction in myocardial infarct size [57]. In response to ischemia/reperfusion, miR-320 expression is decreased in the hearts. Transgenic mice with cardiac-specific overexpression of miR-320 demonstrate an increased extent of apoptosis and infarction size in the hearts upon ischemia/reperfusion [33]. Acute myocardial infarction is related to the reduction of miR-29 [58]. A recent report shows that miR-1, miR-133a, miR-133b, and miR-208 are dysregulated in human myocardial infarction [59]. The broad involvement of miRNAs in apoptosis-related cardiac pathology necessitates the elucidation of the molecular mechanism by which they control apoptosis and the identification of their direct and indirect targets.

miRNAs Control Apoptotic Program of the Heart

miRNAs have been characterized to regulate cardiac apoptotic program (Fig. 1). In response to ischemia/reperfusion, miR-320 expression is decreased in the heart. The functional study reveals that overexpression of miR-320 enhances apoptosis in cardiomyocytes, whereas knockdown of miR-320 can attenuate cell death upon ischemia/reperfusion. In searching for the targets of miR-320, heat-shock protein 20 (Hsp20) known as a cardioprotective protein, is proved to be a target for miR-320 in regulating apoptosis [33].

Fig. 1
figure 1

Summary of the apoptotic targets of microRNAs in the heart. See text for detailed explanations. Plus sign indicates that the final effect is the activation of apoptosis. Minus sign indicates that the final effect is the inhibition of apoptosis

Full size image

miR-21 can regulate the ERK-MAP kinase signaling pathway in cardiac fibroblasts, which has impacts on global cardiac structure and function [60]. Programmed cell death 4 also is a direct target of miR-21 [29]. miR-199a inhibits hypoxia-inducible factor (Hif)-1α and its stabilization of p53 thereby reducing apoptosis. Knockdown of miR-199a during normoxia results in the upregulation of Hif-1α and SIRT1 and reproduces hypoxia preconditioning [32]. miR-1 participates in the activation of apoptosis by reducing the expression levels of heat shock protein-60 and heat shock protein-70, whereas miR-133 antagonizes apoptosis by repressing caspase-9 expression [28].

miRNAs inhibit apoptosis through multiple manners including the mitochondrial apoptotic pathway. Mitochondrial morphology is an important determinant of mitochondrial function [61]. Mitochondria constantly undergo fusion and fission that are necessary for the maintenance of organelle fidelity [62]. However, abnormal mitochondrial fusion and fission participate in the regulation of apoptosis. Mitochondrial fusion is able to inhibit apoptosis, while mitochondrial fission is involved in the initiation of apoptosis [63]. Mitochondrial fission is initiated by dynamin-related protein-1 (Drp1) [62, 64, 65]. miR-30 family members are able to inhibit mitochondrial fission and apoptosis by suppressing the expression of p53 that can upregulate Drp1 [30]. Taken together, this body of evidence demonstrates that miRNAs are able to regulate cardiac apoptotic machinery. It is expected that more miRNAs and their targets in the apoptotic program of cardiomyocytes will be delineated in the coming years.

miRNAs as the Therapeutic Targets for Apoptosis-Related Cardiac Diseases

Given the importance of miRNAs in cardiac physiology and pathology, can miRNAs be employed for the therapy of apoptosis-related heart diseases? Stem cells have been demonstrated to be a promising therapeutic approach for myocardial ischemic injury. Recently, there is a report showing that microRNAs can regulate the fate of stem cells. Ischemic preconditioning of bone marrow-derived mesenchymal stem cells with ischemia/re-oxygenation supports their survival under subsequent longer exposure to anoxia, and this effect is mediated by miR-210 that can target caspase-8-associated protein-2 [66].

miR-92a controls angiogenesis [67]. Enforced overexpression of miR-92a in endothelial cells blocks angiogenesis in vitro and in vivo. In mouse models of limb ischemia and myocardial infarction, systemic administration of a miR-92a antagomir leads to enhanced blood vessel growth and functional recovery of the damaged tissue. Thus, miR-92a may serve as a valuable therapeutic target in the setting of ischemic diseases [67].

To employ miRNAs as therapeutic targets, it is necessary to modulate the levels of endogenous miRNAs. To this end, the engineered oligonucleotides termed as “antagomirs” have been widely used as silencers of endogenous miRNAs. These chemically modified or cholesterol-conjugated antagomirs are quite stable and, therefore, can be efficiently delivered to the heart [45, 51] and liver [31]. Nevertheless, the effective delivery into target tissues remains a major hurdle for the applications of antagomirs as well as the synthetic miRNA duplexes [68]. For example, the systematic injection of antagomirs is incapable of silencing miRNAs in the central nervous system, and a local injection into the mouse cortex is required for efficiently targeting miRNAs [69]. It remains to be solved as to how the antagomirs can be maximally delivered to the heart. Finally, the in vivo safety should be well-defined in preclinical animal models.

Future Prospects

It has become quite clear that miRNAs regulate a diverse set of cardiovascular events including cardiac development, hypertrophy, and angiogenesis [70–74]. Especially in the last few years, the emerging evidence has shown that miRNAs play an important role in regulating apoptosis in the heart [75, 76]. Around 800 human miRNAs have been so far identified [12], but few of them have been studied in apoptosis in the heart. It is apparent that future studies are required to elucidate the role of each miRNA in cardiac apoptosis. A characteristic of miRNAs is that they may have multiple targets. It is thus necessary to explore the molecular targets of miRNAs in the cardiac apoptotic machinery. The crucial goals in coming years would be finding out the therapeutic approaches for apoptosis-related cardiac diseases by modulating miRNAs.