Abstract
MicroRNAs (miRNAs) are molecules increasingly investigated for both diagnostic and therapeutic strategies. Whereas information about their role in the left ventricle has been studied for many years, there is scarce information about the right ventricle. We thus here review known details about the expression, regulation, and function of miRNAs in right heart diseases. Current identified therapeutic strategies using miRNA modulators to treat pulmonary hypertension and thus also having beneficial effects on the right ventricle are also discussed. Finally, the current knowledge about the diagnostic and predictive use of circulating miRNAs in patients with pulmonary hypertension and right ventricular failure is presented. There is strong hope that the increasing knowledge about miRNAs in the right heart will finally help to improve the treatment of patients with pulmonary and right ventricular heart diseases.
MicroRNAs (miRNA/miR) are short, endogenous, regulatory RNA molecules that orchestrate a large fraction of the genome, thus having fundamental importance in the regulation of individual cellular function in health and disease. During diseases many miRNAs are deregulated, with dramatic consequences for cell type–specific gene expression and its regulation. It is thus not surprising that many miRNA-related therapeutic strategies have arisen with the idea of modulating a single miRNA for the purpose of normalizing derailed gene expression in diseased tissues. Many vascular and cardiac approaches have been developed in mouse and some large-animal models.1–3 Surprisingly, approaches to target the right heart and associated diseases remain scarce, and it is likely that different approaches, specific to the right ventricle, should be applied. This may be of clinical importance, because many clinical trials suggest that medications for left ventricular heart failure have no clear benefit when used for patients with right heart failure.4,5 Thus, we here summarize the current knowledge about the role of miRNAs, with special emphasis on the right heart and its associated diseases, such as pulmonary hypertension (Fig. 1).
miRNA EXPRESSION IN THE RIGHT HEART AND RESPONSES TO STRESS
It has been known for quite some time that there are differences in gene expression between the left and right ventricles. 6 Likewise, a lot of expression and sequencing data exist about expression of miRNAs in both healthy and diseased left ventricles.7,8 In contrast, data involving miRNA expression in the right ventricle are less frequent. The expression and distribution of miRNAs in various cardiac regions, including the right ventricle and the right atrium, have recently been reported in various animals. 9 Hierarchical Euclidean linkage clustering of cardiac miRNA expression profiles revealed different clusters when right ventricles and right atria were compared. For instance, miR-208b levels were high in ventricles but low in atria of the three different species investigated (rat, dog, and monkey). Surprisingly, this report did not find many differences in miRNA expression when right and left ventricles or left and right atria were compared. However, it could be likely that miRNA expression responses to various stress and disease conditions could be different in the right versus the left heart. Recently, the dynamics of miRNA expression during the transition from right ventricular hypertrophy to failure has been investigated (Table 1). 10 Right ventricular hypertrophy and subsequent failure are often the consequences of pulmonary hypertension in patients with systemic diseases of the right ventricle and those after repair of right-sided obstructive congenital heart diseases such as tetralogy of Fallot (TOF) or pulmonary atresia. In a murine model of pulmonary artery constriction, the expression patterns of miRNAs were investigated over time (0–10 days). Right ventricular mass increased after 2 days of banding, with subsequent further increase at 4 and 10 days after banding. Whereas many changes in right ventricular miRNA expression were similar to those in the left ventricle after transaortic constriction, there also were important differences, which may indeed be of importance. Four miRNAs were found to be selectively increased in right ventricular hypertrophy, which was defined by a band gradient of >35 mmHg. These miRNAs were downregulated or unaltered in left ventricular hypertrophy after increase of afterload. The miRNAs selectively increased after pulmonary banding were miR-34a, miR-28, miR-148a, and miR-93. Especially the increase in miR-34a expression is of interest, as this miRNA has recently been described as playing a key role in cardiac apoptosis and aging. 11 Other miRNAs that typically increased during cardiac fibrosis of the left ventricle, such as miR-21, 7 were also increased in right heart hypertrophy–associated cardiac fibrosis. 10
Changed microRNA (miR) profiles of the right ventricle during transition from hypertrophy to failure
Note: PA: pulmonary artery; VEGFR: vascular endothelial growth factor.
MicroRNA (miR) signaling in pulmonary arterial hypertension
Note: TGFβ: transforming growth factor β; BMPR2: bone morphogenic protein receptor, type 2; KLF: Krüppel-like factor; BMP4: bone morphogenic protein 4; SHP2: STAT3: signal transducer and activator of transcription 3; NFAT: nuclear factor of activated T-cells; IL-6: interleukin 6; CDK: cyclin-dependent kinase; IGF-1: insulin-like growth factor; FGF2: basic fibroblast growth factor; FGFR1: fibroblast growth factor receptor 1.
Another study used two different mouse models of compensated right ventricular hypertrophy (pulmonary artery banding and chronic hypoxia) and right ventricular failure. 12 This group also compared potential differences in miRNAs between left and right ventricles in a rat model by microarray analysis and sequencing approaches. Although the differences were small, a unique set of miRNAs differed between the ventricles (miR27b and miR-34c*). In addition, there were differences in expression profile between right ventricular hypertrophy and right ventricular failure (Table 1). Chronic hypoxia–induced hypertrophy decreased the right ventricular expression of miR-21 and miR-34c*, whereas in right ventricular failure, expression of miR-133a, miR-139-3p, miR-21, and miR-34c* was decreased. When comparing the available miRNA expression data in the right ventricle,10,12 there are few overlaps between the various studies. Reasons are likely the different approaches used to induce right ventricular hypertrophy and failure (e.g., banding vs. chronic hypoxia), different surgical operations leading to different banding gradients, and use of different species (mouse, rat, dog, monkey). Thus, future investigations are needed to gain a broader insight into disease-specific miRNA changes in the right ventricle. Beyond preclinical animal models, there are few clinical data available. Changes in miRNA expression in human right ventricular hypertrophy due to the TOF have been described. 13 The authors of these studies investigated miRNA expression in right ventricular tissue from 16 infants with nonsyndromic TOF and compared it to that in tissue from fetal myocardium and tissue from normal developing infants. In total, 61 miRNAs were identified as significantly altered in TOF samples versus healthy controls. Interestingly, the patterns of miRNA in diseased right ventricular tissue resembled to a remarkable extent that in fetal heart tissue, suggesting reactivation of a fetal miRNA program in right ventricular disease. A similar reactivation of a fetal miRNA program is also evident in left ventricular heart failure. 14
miRNA IN PULMONARY HYPERTENSION AND CONSEQUENCES FOR THE RIGHT HEART
The reasons for the development of pulmonary hypertension in patients are diverse, and the pathomechanism is not completely understood. Pulmonary hypertension leads to pulmonary arterial remodeling and increased right ventricular afterload, subsequent hypertrophy, and failure, which may finally lead to the death of the patient. The roles of miRNAs in the process of pulmonary arterial remodeling have been reviewed recently (Table 2). 15 This review mainly focused on the role of miRNAs in regulating the biology of endothelial cells and vascular smooth muscle cells as the most important players in this disease process. Thus, it is not surprising that in a rodent model of pulmonary hypertension in lung tissue, several miRNAs also important in the biology of endothelial cells and smooth muscle cells were deregulated. 16 A typical characteristic of severe pulmonary hypertension in humans is the presence of plexiform and concentric obliterative lesions. Using laser-assisted microdissection with miRNA expression analysis, Bockmeyer and colleagues 21 found that in patients with pulmonary hypertension, miR-143/145 levels were significantly higher in concentric lesions, whereas miR-126 and miR-21 were upregulated and miR-204 downregulated in plexiform-type vascular lesions. In patients with pulmonary hypertension in isolated pulmonary arteries, significant changes in miRNA expression were also found. 12 Thus, a better understanding of the spatiotemporal miRNA regulation in both hypertensive lung disease and the stressed right ventricle is important for the development of future miRNA-based diagnostic and therapeutic strategies, some of which are discussed in the next section.
THERAPEUTIC APPROACHES TO PULMONARY HYPERTENSION AND THE STRESSED RIGHT VENTRICLE
MicroRNAs can now be targeted with several chemistries in vivo. 1 While strategies about the use of miRNA modulators to affect directly the right ventricle are currently lacking, there are several reports about the successful use of miRNA modulators as a therapy for pulmonary hypertension that is indirectly beneficial for the right heart. For instance, the miR-145/143 cluster was described as playing a significant role in the biology of smooth muscle cells,28,29 which in the lung play a dominant role in the development of pulmonary resistance and hypertension. Mice with genetic or pharmacologic ablation of miR-145 were significantly protected from pulmonary artery hypertension when exposed to hypoxia. 16 Indeed, right ventricular pressure and right ventricular hypertrophy were reduced by miR-145 reduction, in comparison to wild-type mice or mice treated with control oligonucleotides, demonstrating significant indirect therapeutic effects in the right ventricle. As these authors also found miR-145 to be highly increased in plexiform and concentric lesions of patients with pulmonary hypertension, the approach of silencing miR-145 in lung tissue of patients may be of great future therapeutic relevance. 16 Another study found expression of miR-204 to be lower in lung tissue of patients, mice, and rats with pulmonary hypertension. 19 Importantly, delivery of synthetic miR-204 to lungs of animals with pulmonary hypertension (monocrotaline model) significantly reduced disease severity, including reduction of right ventricular hypertrophy. In mice with chronic hypoxia, treatment with inhibitors of miR-17 also reduced right ventricular pressure and right ventricular remodeling. 30 In a recent, very elegant study, Lee et al. 23 demonstrated the paracrine role of miRNAs in mesenchymal stromal cell (MSC)–induced protection in hypoxic pulmonary hypertension. In the study, MSC-derived exosomes inhibited pulmonary vascular remodeling and hypoxic pulmonary hypertension by suppressing the hypoxic activation of signal transducer and activator of transcription 3 (STAT3) and upregulating the miR-17 superfamily cluster and miR-204, which is typically decreased in human pulmonary hypertension. 23 It is thus now evident that several miRNAs, most importantly miR-145, miR-204, and miR-17, modulate pathologic right ventricular processes that must be explored in more detail in the future to develop potential clinical uses in patients with pulmonary hypertension.
CIRCULATING MiRNAs AS POTENTIAL BIOMARKERS FOR RIGHT VENTRICULAR FUNCTION AND PULMONARY HYPERTENSION
Information about circulating miRNAs as a marker for right ventricular function and associated diseases of right ventricular dysfunction are so far also scarce. Whereas miR-423-5p has been suggested to be a potential diagnostic biomarker of left ventricular function, 31 this seems not to be the case for the right heart. Patients with transposition of the great arteries, where the right ventricle becomes the systemic ventricle after surgical correction and then develops failure do not show an increase of circulating miR-423-5p. 32 These results suggest that distinct miRNA regulatory pathways or secretion mechanisms may exist for the right ventricle and that other circulating miRNAs should also be explored that may be useful in determining right ventricular function and/or predicting outcomes. In patients with pulmonary hypertension, circulating levels of miR-1, miR26a, miR-29c, miR-34b, miR-451, and miR-1246 were lower and those of miR-21, miR-130a, miR-133b, miR-191, miR-204, and miR-208b were higher than those in normal control subjects. 33 The increased levels of miR-21 and lower levels of miR-29c could indicate higher level of fibrosis in subjects with pulmonary hypertension.7,8 Increased levels of the cardiomyocyte-enriched miRNAs miR-133b and miR-208b may reflect increased right ventricular stress and/or damage. Interestingly, there was a correlation with the degree of pulmonary hypertension, as determined by mean pulmonary arterial pressure and levels of those miRNAs. 33 Another study found circulating miR-150 levels to be strongly reduced in patients with pulmonary arterial hypertension and that this was associated with poor survival, showing potential prognostic power for this particular circulating miRNA. 34
CONCLUSION AND OUTLOOK
Information and interest about the distinct expression, regulation, and function of miRNAs in physiology and pathophysiology of the right heart and the pulmonary system are growing. Interesting first therapeutic approaches that demonstrated the efficacy of miRNA-modulating strategies in several animal models of pulmonary hypertension are also having beneficial indirect effects on the right ventricle. Also, targeted effects of miRNA therapeutics to prevent cardiac remodeling in the right heart should be explored. The diagnostic and prognostic value of circulating miRNAs and miRNA patterns should be further explored and validated in large patient cohorts. With this in mind, there is strong and solid hope that miRNA tools will, in the future, help our patients with pulmonary hypertension and related heart diseases.