Pathologies of the cardiac and pulmonary systems in their broad spectrum of etiology, including among others heart failure, cardiomyopathies, lung cancer, and acute lung injury, rank among the leading causes of mortality and have a significant impact on health and healthcare of patients. Tremendous efforts and advancements in understanding critical cellular and molecular processes during the development and pathogenesis of these conditions have established an increased contribution of oxidative stress. Over the past few years, numerous studies have reported concomitant expression signature of reactive oxygen species (ROS) and small noncoding microRNAs (miRNAs), which are key players in many pathophysiological processes [1
]. This recent focus set the premise to study the crosstalk between redox signaling and disease-specific miRNAs in the cardiac and pulmonary systems.
ROS are among the primary oxidants endogenously produced by eukaryotic cells, mostly as by-products in processes that include aerobic metabolism and cellular respiration, and host response to infection and injury; they also seem to be pivotal in both intra- and intercellular redox signaling pathways. The major source of ROS originates from mitochondrial production [3
], mainly due to complexes I and III function in the respiratory chain [4
]. However, other non-mitochondrial enzymes and protein complexes also produce ROS: among these are nicotinamide adenine dinucleotide phosphate oxidase (NOX) and nitric oxide synthases (NOSs) [7
], xanthine oxidase [8
], the α-ketoglutarate dehydrogenase complex [10
], including dihydrolipoamide dehydrogenase [12
], and d-amino acid oxidases [16
]. Nitric oxide (NO), for instance, produced from sources such as endothelial NOS, contributes to vascular homeostasis, whereas under conditions of oxidative stress such as inflammation, NO interaction with other ROS potentiates cellular damage. Furthermore, these highly reactive particles can be exogenous as well, as they can be freely found in the gaseous environment. Stable ROS, such as H2
, due to their non-radicality and the capacity to migrate across membranes, are cardinal to redox biology by activating signaling pathways to initiate biological processes. For instance, essential transcription factors such as activator protein-1 (AP-1), nuclear factor kappa B (NF-κB), and/or nuclear factor erythroid 2 like 2 (Nrf2) have been shown to be affected by the redox state with subsequent nuclear translocation and binding to DNA [19
]. During homeostasis, ROS are kept in check by antioxidant molecules, which can either neutralize their oxidation potential or inhibit the steps that generate them.
MiRNAs are 20–22-nucleotide long noncoding RNA molecules that act as post-transcriptional regulators of gene expression. Since their initial discovery in Caenorhabditis elegans
], thousands of miRNAs have been discovered [28
] and their function investigated. However, the ‘miRBase’ database represents just the tip of the iceberg and most of the miRNA repertoire remains unexplored. MiRNAs are transcribed by RNA polymerase II into primary miRNAs (pri-miRNAs), then cleaved within the nucleus to hairpin-structured precursor miRNAs (pre-miRNAs) by a protein complex including Drosha and DGCR8, and finally moved to the cytoplasm by Exportin 5 (a Ran-GTP dependent transporter) where they are further cleaved by Dicer to unstable double-stranded miRNA duplexes. The duplex is further processed into a single-stranded guide RNA and becomes part of the RNA-induced silencing complex (RISC) together with Argonaute proteins. Indeed, two mature miRNAs can be generated from the same pre-miRNA precursor, referred as -5p and -3p, respectively originated from the 5′ or 3′ arm of the precursor. The RISC complex recognizes and silences target genes based on the coupling of the miRNA with the 3′ untranslated region (UTR) of the gene mRNA. The partial sequence complementarity between miRNA and target is sufficient to achieve gene silencing, with most of the ‘seed’ sequence of the miRNA (from nucleotide 2 to nucleotides 7 or 8) pairing to the 3′ UTR of the mRNA. Post-transcriptional gene silencing is achieved through either the inhibition of translation, or the degradation of the target mRNA, resulting in both cases in protein downregulation.
Due to broad target recognition, miRNAs are involved in all cellular processes including redox balance and ROS production. MiRNAs that are regulated by oxidative stress originated from several sources, such as UV, hydrogen peroxide, ionizing radiation, and antitumoral agents [29
], have been termed ROSmirs and regulate the expression of their targets in response to ROS [30
]. The activation of inflammatory cells under pathological conditions and release of cytokines induce ROS production, which can ultimately cause DNA damage and lead to cell death. Cellular events resulting in oxidative stress, including the downregulation of antioxidants, have been associated directly or indirectly with miRNA expression changes in diseases of both heart and lung compartments.
With this review, we aim to highlight the most recent and relevant literature on the crosstalk between miRNAs and ROS in several cardiac and pulmonary pathologies. We will discuss studies describing either mechanistic correlation between miRNAs and oxidative stress-related genes or modulation of miRNA expression by ROS. Thus, studies in which ROS production have only been used as a read-out of oxidative stress following miRNA expression changes will not be included. In Figure 1
, we show selected key players of oxidative stress that interact with miRNAs in cardiac and pulmonary diseases, providing a summary of the molecules mentioned in the present review.
4. MiRNAs and Oxidative Stress in Both Cardiac and Pulmonary Diseases
Several miRNAs have been reported to crosstalk with oxidative stress in both cardiac and pulmonary systems. In Figure 2
, we summarize the findings for the ten miRNAs shared by cardiac and pulmonary settings. Three of them, miR-155, miR-21, and miR1/206, have been extensively studied in cardiac and pulmonary diseases, thus we will highlight them in the following sub-sections.
In the lung, miR-144 and miR-16 have been reported to work synergistically with miR-34a under oxidative stress conditions. As reported above, miR-144 and miR-16 cooperated with miR-34a to downregulate the antioxidant transcription factor Nrf2 [139
] and the anti-apoptotic mitochondrial protein Bcl2, respectively, contributing to oxidative stress [172
]. Their functions were very similar in the context of cardiac disease. In a cell line of cardiomyocytes, as well as in vivo, in the myocardium of rats subjected to I/R, miR-144 has been found to target another antioxidant transcription factor, i.e., FOXO1, preventing the myocardial I/R injury to some extent [176
]. In AMI rat models and neonatal rat ventricular cardiomyocytes, miR-16 has been shown to be increased and to target beta2-adrenergic receptor (β2-AR), which is known to protect injured cardiomyocytes from hypoxia- and oxidative stress-induced apoptosis [177
]. Indeed, high miR-16 levels caused decreased β2-AR expression and, consequently, induction of hypoxia and oxidative stress. Thus, both miR-144 and miR-16 were found to promote oxidative stress in cardiac and pulmonary diseases; however, there is no evidence of a potential cooperation of these two miRNAs with miR-34a in the heart. Of note, as described above, miR-34a was also reported to target Nrf2 in synergy with miR-27a and 28a in CHF [51
Furthermore, in the lung, miR-34a has been correlated to the antioxidant function of FS in radiotherapy, and, more in general, the miR-34 family members have been suggested to be a promising therapeutic target for the treatment of lung cancer, as well as to be involved in COPD [142
]. In the heart, an increase of miR-34a has been observed in pre-diabetic and diabetic patients [178
], where HG levels are known to induce an accumulation of ROS. MiR-34a was found upregulated in diabetic mouse hearts and to regulate redox signaling pathways [179
]. Moreover, it was reported that miR-34a upregulation in diabetic mice led to dysregulation of endothelial cells by targeting Sirt1 [180
]. Interestingly, in an independent study, in vitro experiments using cardiomyocytes in HG conditions confirmed the induction of high levels of miR-34a and targeting of Sirt1 [181
], ultimately leading to oxidative stress. Thus, while induction of miR-34a can exert an anti-tumoral therapeutic effect, its downregulation could be of potential benefit for the treatment of DCM.
A contrasting function between cardiac and pulmonary systems has been reported for three miRNAs, i.e., miR-421, miR-129 and miR-181c.
MiR-421 was shown to promote oxidative stress in the heart, by targeting Sirt3, which plays an antioxidative role in cardiomyocytes, and conversely to induce the gene expression of antioxidants by targeting KEAP1, the Nrf2 binding partner [182
], in the lungs. Inhibition of Sirt3 expression or suppression of its activity was associated with an increase of ROS and activation of Akt signaling [71
]. In the cardiac I/R settings, Liu et al. demonstrated that silencing of miR-421 in vitro decreased apoptosis, and both MDA and LDH levels, and increased SOD levels, ultimately reverting I/R effects. Sirt3, the miR-421 target, was found downregulated after I/R and its expression inversely correlated to increased expression of miR-421 [62
]. In lung cancer, the regulation of ROS by miR-421 has been associated with resistance to the chemotherapy drug paclitaxel [183
]. MiR-421 was reported upregulated and associated with poor prognosis in NSCLC patients [184
] which was further correlated with the targeting of KEAP1. Under oxidative stress, Nrf2 is generally released from the KEAP1-Cul3 E3 ligase complex [185
] and can translocate into the nucleus and activate antioxidant response elements. Knock-down of miR-421 caused levels of KEAP to rise, inhibiting Nrf2-dependent antioxidant expression, and ultimately increasing intracellular levels of ROS [183
]. This study, therefore, suggests that the inhibition of miR-421, whose expression was found to be induced by beta-catenin, could increase the sensitivity of cancer cells to paclitaxel by inducing ROS-dependent apoptosis.
MiR-129-5p has been found downregulated in cardiac diseases, allowing the expression of its target HMGB1, and it was suggested that its restoration could be protective from oxidative stress. HMGB1 regulates cellular responses to stress, inflammation and tissue damage, and oxidative stress induces translocation, release, and activity of HMGB1 during inflammation and cell death [186
]. HMGB1 regulation of downstream apoptosis or survival requires TNFα for its secretion and HMGB1 accumulation at sites of oxidative DNA damage can induce repair of the DNA [55
]. In the serum of patients with CHF, lower levels of miR-129-5p were observed to be inversely correlated with HMGB1 levels [187
]. Accordingly, when miR-129-5p was overexpressed in vivo, levels of HMBG1 were lower in the myocardial tissue. MiR-129-5p was shown to directly target HMBG1, reducing inflammatory response (TNFα, Interleukin 6) and MDA levels and increasing SOD levels in both serum of rats with CHF and cardiac cells [187
]. Thus, restoration of miR-129-5p levels could play a protective role in CHF, decreasing oxidative stress, and inflammation in cardiac cells [187
]. In contrast, miR-129-5p has been reported to have a pro-oxidative stress role in the lung. HDAC inhibitors, such as suberoylanilide hydroxamic acid (SAHA), have been shown to induce apoptosis in various cancer cells by increasing the production of ROS [188
]. You et al. [190
] found that miR-129-5p was upregulated by SAHA-treatment in cancer cells and, at least in part, mediated ROS increased production by targeting the antioxidant protein thioredoxin1 (Trx1). More recently and in line with this, miR-129-5p has been shown to function as a tumor suppressor when overexpressed in lung cancer cells [191
]. Furthermore, chidamide, an oral benzamide-type selective HDAC inhibitor, has been shown to induce cancer cell cycle arrest by inducing upregulation of miR-129-3p together with ROS production to inhibit telomerase activity [192
MiR-181c has been found to be deleterious in the cardiac setting, but protective in the pulmonary system. Das et al. delivered miR-181c into rats by using nanoparticles and found that it targeted the mitochondrial cytochrome c oxidase subunit 1 (mt-COX1). COX is the last enzyme of the mitochondrial respiratory chain and the major oxygen consumer enzyme in the cells [193
]. Indeed, by delivering miR-181c, the authors observed a significantly aberrant consumption of oxygen, ROS production, and mitochondrial membrane potential in cardiac mitochondria isolated from miR-181c-nanoparticle-treated animals, suggesting that miR-181c targets mitochondrial genes, therefore causing cardiac dysfunction [195
]. In the lung, miR-181c expression levels were found to be low in the tissue of COPD patients and overexpression of this miRNA was shown to inhibit cigarette smoke-induced COPD in mice. MiR-181c was found to target CNN1 (Cysr61) and its overexpression to decrease the inflammatory response, neutrophil infiltration, and inflammatory cytokines induced by cigarette smoking, as well as the reactive oxygen species (ROS) generation [196
]. However, the exact mechanism by which miR-181c regulates ROS in COPD has not yet been elucidated. Of note, another member of the miR-181 family was also found to cause a reduction of the levels of ROS in the pulmonary system. Jiang et al. discovered that the expression of miR-181a was downregulated in lungs of LPS-challenged mice and that the Toll-Like Receptor 4 (TLR4) was a target of miR-181a. When miR-181a was overexpressed through a mimic transfection, the LPS-induced inflammatory response was alleviated. The authors found that overexpression of miR-181a reduced the LPS-induced intracellular ROS accumulation, similarly to what happened by siTLR4 transfection. Finally, this study suggested that miR-181a could reduce LPS-induced inflammation by targeting TLR4 and subsequently reduce ROS accumulation [197
Finally, while the expression of miR-31-5p in the heart has been found to be induced through hypoxia and oxidative stress, a protective function for miR-31-3p was reported in the lung. In the heart, miR-31-5p has been shown to target cardiac troponin-T (Tnnt2), mineralcorticoid receptor (Nr3c2), E2F transcription factor 6 (E2f6), and metalloproteinase inhibitor 4 (Timp4). The silencing of miR-31 after myocardial infarction has been found to be beneficial and to ameliorate the left ventricle dysfunction [198
]. In contrast, the silencing of miR-31-3p was reported to induce ROS production in lung cells treated with LPS. Guo et al. treated MRC-5 lung fibroblasts with LPS showing that LPS can induce cell death, cytokines expression, and ROS production in lung fibroblasts. Interestingly, they found that this phenotype was due to the overexpression of a circular RNA, circANKRD36, that acted as a sponge for miR-31-3p. CircRNAs are noncoding RNAs that bind to miRNAs to inhibit their binding to the targets. MiR-31-3p was found to target MyD88, therefore downregulating the NF-κB pathway. Thus, apoptosis and ROS production observed in LPS-treated MRC5 were caused, at least in part, by the circANKRD36 binding with miR-31-3p that impeded the miRNA downregulation of the NF-κB pathway [199
4.1. MiR-155 and Oxidative Stress in Cardiac and Pulmonary Diseases
The extensive role of miR-155 in cancer (e.g., lung cancer), infections, nervous system disorders, immune system-associated diseases, and cardiovascular conditions has been recently reviewed by Gulei et al. [200
]. MiR-155 was initially identified as a B-cell integration cluster (bic) capable of inducing leukosis in chickens under viral infection [201
], and was subsequently found expressed in humans and mice. In the immune system, miR-155 has been shown to travel between immune cells via exosomes contributing to the regulation of inflammation [202
], while in cancer it has been identified as a biomarker and associated with drug resistance [203
]. Hence, numerous in vitro and in vivo studies focused on the modulation of miR-155 expression and preclinical results indicate that inhibition of miR-155, either in combination with other therapeutic approaches or by itself, might indeed be beneficial in several pathological conditions [200
As previously mentioned, transcription factors of the FoxO family are involved in the regulation of cellular stress responses and promotion of the antioxidant defense. Herein, we underline the negative regulation of FoxO transcription factors by miR-155 in both lung and cardiac pathologies. In cardiomyocytes, miR-155 was found to directly target FoxO3a [204
], ultimately leading to cardiac hypertrophy. Resveratrol (Rsv), a natural phenol, could counteract this process by directly acting on cardiomyocytes or indirectly by reducing pulmonary hypertension [205
]. In line with this, Fan et al. demonstrated that miR-155 was decreased in cardiomyocytes treated with Rsv, suggesting that the beneficial anti-hypertrophic effect of Rsv is mediated by the miR-155 downregulation together with the enhanced expression of BRCA1, a key regulator of cardiac function [206
], also known to be involved in lung cancer [207
]. Similarly, miR-155 overexpression has been shown to inhibit FoxO1 in NSCLC [208
]. Likun et al. demonstrated that FoxO1 is a direct target of miR-155, and that a miR-155/FoxO1/ROS axis exists and that it promotes NSCLC growth. In fact, antioxidant NAC treatment induced miR-155 expression, targeting FoxO1 and significantly reducing cell proliferation, therefore suggesting that the miR-155/FoxO1/ROS axis might be a novel therapeutic target for the inhibition of NSCLC growth.
Besides silencing genes from the FoxO family, miR-155 is a major player in inflammatory events, as introduced above. In an inflammatory-related context during I/R injury, increased levels of miR-155 were found to lead to the downregulation of its direct target, the suppressor of cytokines signaling 1 (SOCS-1), whose aberrant regulation contributed to the progression from hypertrophy to heart failure [209
]. Using a mouse model, in vivo depletion of miR-155 caused a reduced necrosis after induction of MI and a decreased inflammatory response to injury. Interestingly, a reduced generation of ROS was measured in immune cells in this mouse model as well. The authors concluded that miR-155 could increase the inflammatory response during tissue damage by targeting SOCS-1 and, finally, by regulating the generation of ROS [210
]. In line with its role in cardiac inflammation, miR-155 could play a prominent role in asthma. Indeed, elevated levels of miR-155 have also been reported in the serum of asthmatic patients [211
], and it has been suggested that miR-155 modulates cockroach allergen and oxidative stress-induced COX-2 in asthma. However, although the authors showed that cockroach extract (CRE) could induce ROS production through overexpression of COX-2 and that increased ROS levels were significantly reduced in miR-155 knock out (KO) mice, it remains unclear how miR-155 regulates COX-2. In fact, they also proved direct targeting of COX-2 by miR-155 using luciferase assay and miRNA-mRNA pull-down assays and observed a lower recruitment of inflammatory cells in the lungs of CRE-treated miR-155 KO mice. They concluded that miR-155 may act to exacerbate Th2-associated lung inflammation by inducing the upregulation of its target COX-2. However, while in some cases mRNA overexpression of a miRNA target can happen to compensate for the protein downregulation, to the best of our knowledge there is no evidence for protein overexpression as a result of miRNA binding to the target 3’UTR. A better explanation for the protein overexpression of a miRNA target is that in some specific contexts the miRNA does not target the gene. In fact, each miRNA is capable of recognizing target sequences on different genes and based on cellular and molecular conditions it may or may not bind to the target. Although it is controversial whether COX-2 direct targeting underlies the correlation between miR-155 and oxidative stress during CRE exposure, overexpression of miR-155 is associated with oxidative stress in allergen-induced lung inflammation, making miR-155 inhibition a therapeutic target for the treatment of asthma.
4.2. MiR-21 and Oxidative Stress in Cardiac and Pulmonary Diseases
MiR-21 is a highly expressed miRNA in mammalian cells, being associated with different types of cancer, and several studies have reported a major contribution of miR-21 to apoptosis in both heart and lung tissues in oxidative stress.
MiR-21 has been shown to play a role in oxidative stress associated with DCM. Gao et al. showed that the expression of LAZ3 (also known as BCL6), an oncogene capable of inhibiting ROS production and apoptosis during chemotherapy treatments [212
], decreased in in vitro and in vivo models of DCM, i.e., cardiomyocytes stimulated with HG and the heart of a DCM mouse model [213
], respectively. Furthermore, the authors proved that silencing of LAZ3 in vitro led to decreased cell viability due to increased apoptosis. Specifically, the lack of LAZ3 increased oxidative stress by upregulation of NOX expression, by MDA production and by decreased activity of SOD. In line with this, Gao et al. also showed that overexpression of LAZ3 in cells or in the heart of diabetic mice attenuated oxidative stress and cell death. The mechanism leading to the LAZ3-dependent oxidative stress involved miR-21, which was shown to be decreased by LAZ3, resulting in PPARα activation, which in turn increased PGC-1α and subsequent Nrf2 expression and nuclear translocation [213
]. Altogether, the authors elegantly demonstrated that miR-21 acts upstream of the PPARα/PGC-1α/Nrf2 to prevent the activation of the Nrf2 antioxidant program, leading to a deteriorating phenotype. LAZ3 inhibitory effect on miR-21 counteracts DCM progression, suggesting a potential therapeutic role of the oncogene in DCM treatment.
In contrast with a pro-oxidative role for miR-21, another study reported its protective function in the heart. Gelsolin, an actin-binding protein, inhibits the enzymatic activity of Cu/Zn SOD resulting in the accumulation of oxygen free radicals [214
], and its deficiency can improve cardiac systolic function [215
]. By directly targeting Gelsolin, miR-21 was able to reduce ROS production and upregulate the levels of NO available. Moreover, miR-21 alleviated palmitate-induced injury, protecting against cardiac hypertrophy in diabetic mice exhibiting diastolic dysfunction [216
In line with this, in pulmonary vascular smooth muscle cells (VSMC) undergoing oxidative stress, miR-21 has been reported to target PDCD4, exerting a protective role as it does in cardiac myocytes [217
] and human aortic endothelial cells (HAEC) [218
]. In the lung, chronic hypoxia causes a massive ROS production leading to pulmonary oxidative stress, which results in pulmonary vascular remodeling [219
]. Sarkar et al. found that hypoxia could induce the proliferation of pulmonary arterial smooth muscle cells (PASMC) through the upregulation of miR-21 [221
]. Therefore, miR-21 was upregulated and actively participated in ROS response during pulmonary remodeling [220
As reported above, it is well established that cancer cells evade cell death by taking advantage of ROS-mediated signaling pathways that allow for cancer progression by promoting proliferation [222
]. In contrast with its protective role in VSMCs, miR-21 was also found to play a pro-oxidative function in cancer. Three independent studies on lung cancer have demonstrated that ROS-dependent upregulation of miR-21 is associated with poor prognosis [223
] and that it occurs through the AKT [226
], ERK/NF-kB [227
] and MAPK [228
] pathways. Zhang et al. [223
] showed that a TLR4/ROS/miR-21 pathway promoted tumor progression in human lung cancer cells treated with LPS. Pulmonary infections of Gram-negative bacteria are common in lung cancer patients and elevated ROS production is critical for LPS to be able to induce miR-21 expression, promoting primary lung cancer outgrowth through miR-21 targeting of PTEN and PDCD4 [223
]. Poyil et al. [224
] reported that lung cells exposed to hexavalent chromium compounds Cr(VI), widely used in industry (e.g., plating, paint, steel, tanning and chrome ore processing) and classified as human carcinogens, exhibited increased levels of miR-21 expression with a consequent inhibition of PDCD4 and malignant cell transformation. Interestingly, antioxidants such as CAT were able to inhibit chronic Cr(VI)-induced miR-21 elevation and PDCD4 suppression, demonstrating that ROS have a crucial role in the regulation of the miR-21/PDCD4 signaling in lung cancer, which the authors suggested to happen through IL-6/STAT3 [224
]. In a follow-up study, Poyil et al. [225
] further demonstrated that quercetin, the most abundant plant food flavonoid, inhibited the Cr(VI)-induced miR-21/PDCD4 regulation by inhibiting the Cr(VI)-induced ROS generation as well [225
Another aspect of lung cancer that has been associated with the ROS/miR-21 regulation is relative to the radiotherapy for cancer treatment. The radiation-induced bystander effects (RIBEs) is a phenomenon that may interest normal and/or cancer cells. Understanding RIBEs regulation may have important clinical implications in the context of radiotherapy. TGF-β1 cytokine has been identified as a soluble signaling molecule that mediates the RIBEs between the signaling and bystander cells [229
] leading to increased ROS levels in the bystander cell [233
]. Jiang et al. found that mir-21 mediated this signaling by being expressed in the bystander cells in response to the TGF-β1 pathways activated in both the signaling and the bystander cells, leading in turn to increased ROS levels and DNA damage. Interestingly, the authors found that the time at which the radiation conditioned media (RCM) from the signaling cells was harvested post-irradiation determined the effect on the expression of miR-21 in the bystander cells. Short-term RCM induced elevated miR-21, and ROS levels and elevated DNA damage in the bystander cells, while miR-21 levels were reduced and proliferation inhibited in bystander cells following exposure to the long-term RCM [233
]. Thus, timing and regulation of miR-21 expression, from which subsequently the oxidative stress in bystander cells depends, might be crucial in radiotherapy. Finally, miR-21 has also been reported to target SOD3 and TNF-α [234
] to modulate the levels of ROS, suggesting that a complex crosstalk between miR-21 and ROS is likely to exist.
4.3. MiR-1/206 and Oxidative Stress in the Cardiac and Pulmonary Diseases
MiR-1 and miR-206 are evolutionarily conserved miRNAs exhibiting a high sequence and expression similarity in the muscle from C. elegans
to human, even if they are not located on the same genomic region. Although these two miRNAs originate from different pre-miRNA sequences, their mature sequences vary in only four nucleotides, exhibiting identical seed sequence and thus sharing many target genes. However, their role is controversial since they can act either separately, being implicated in multiple regulatory pathways, or together to regulate the same genes [235
Both miR-1 and miR-206 have a prominent role in the regulation of the antioxidant system. Wang et al. [236
] were interested in studying whether miR-1 levels could affect proteins related to oxidative stress and directly influence heart dysfunction. In miR1-overexpressing mice, ROS levels were elevated and the activity of enzymes such as LDH and CK in plasma were found to be increased. Accordingly, the authors also found that inhibiting miR-1 levels in rat cardiomyocytes could revert these results. On the contrary, overexpression of miR-1 on such cells led to increased ROS levels and apoptosis, following treatment with H2
. The authors also found that the redox-related proteins, SOD1, glutamate-cysteine ligase catalytic subunit (Gclc), and glucose-6-phosphate dehydrogenase (G6PD) were targets of miR-1. In the lung, SOD1 is also targeted by miR-206 that shares the same locus with miR-1. Specifically, Wanga et al. demonstrated that miR-206 expression levels are increased upon ROS production induced by PM2.5 in an asthmatic mouse model. They identified SOD1 as the genuine target of mir-206, suggesting that the PM2.5/miR-206/SOD1 regulation axis causes ROS accumulation and disease progression in asthmatic mice [237
Metabolism homeostasis is critical in cardiac and lung tissue. For instance, cardiac metabolism includes various biochemical reactions necessary to sustain cardiomyocyte growth or contraction, while the lung, besides its function of gas exchange machinery, can metabolize vasoactive molecules (i.e., angiotensin). Therefore, a fine-tuning of metabolic signaling pathways is paramount for maintaining a proper cardiac and pulmonary function. Here, we summarize some elegant studies that shed light on the critical impact of miR-1 and miR-206 on cardiac or pulmonary metabolism. High-fat diet or Liver X Receptors (LXRs) are key regulators of lipid and glucose metabolism and also have an anti-inflammatory role. Activation of LXRs in the heart attenuates cardiovascular diseases, such as DCM in diabetic mice [238
]. A synthetic agonist of LXRα has been shown to attenuate oxidative stress, mitochondrial damage, and apoptosis in cardiomyocytes in HG conditions. What adds to the interest is that in glucose-induced cells, miR-1 has been found to be overexpressed and to target LXRα, thus enhancing apoptosis and ROS production; it increased mitochondrial membrane potential and aggravated the cleavage of PARP, caspase-3, and caspase-9 [97
]. All such effects could be prevented by IGF-1, which has been shown to be itself a direct target of miR-1 [239
]. Within the pulmonary system, gain of function of Nrf2 has been shown to promote tumorigenesis. Singh et al. showed that, in response to metabolic stress, lung cancer cells activated Nrf2 signaling through an autoregulatory feedback loop involving miRNA-dependent regulation of genes involved in glucose metabolism pathways, i.e., the pentose phosphate pathway (PPP), the tricarboxylic acid (TCA) cycle, and fatty acid synthesis. Prolonged Nrf2 activation reduced downstream miR-1 and miR-206, that in turn enhanced the expression of metabolic genes such as G6PD, transketolase (TKT), phosphogluconate dehydrogenase (PGD), and glycerol-3-phosphate dehydrogenase (GPD2) in both malignant and nonmalignant cells [240
Finally, the amount of ROS is a determining factor to modulate expression of miRNAs and their target genes that are important effectors during pathogenesis. During cardiac hypertrophy or I/R, Lee et al. [241
] showed that varying the intensity and the frequency of ROS dosages resulted in differential expressions of miR-1 and in decreasing levels of its target, myocardin. In vivo, miR-1 overexpression attenuated TAC-induced cardiac hypertrophy, while its inhibition attenuated I/R-induced cardiac apoptosis.
The present review extensively summarizes the importance of the crosstalk between miRNAs and ROS in cardiac and pulmonary diseases. Interestingly, in both cardiac and pulmonary settings, miRNAs have been shown to interfere with the generation of ROS and oxidative stress using a dual mechanism of action: (1) directly, by targeting oxidase and antioxidant enzymes (e.g., NOX, SOD, CAT), antioxidant genes (e.g., Sirt, Trx1) and their transcription factors (e.g., Nrf2, FOXOs), as well as mitochondrial genes (e.g., COX, Bnip3), and (2) indirectly, mainly by targeting genes involved in apoptosis (e.g., Bcl2, NFκB), tumor–suppressor genes (e.g., p53), and interfering with pro-survival signaling pathways (e.g., Akt, IGF-1). Furthermore, miRNA expression itself can be either induced directly by ROS or modulated in response to inflammatory pathways and contribute ultimately to oxidative stress. It is also worth considering that, among the miRNAs linked to oxidative stress in the pulmonary system, there are some known to play a role in cardiac diseases; however, not yet correlated with oxidative stress. The same is true for some miRNAs involved in pulmonary diseases that were found to be correlated to oxidative stress in the cardiac settings. Finally, among the miRNAs playing a role in oxidative stress in both systems, miR-421, miR-129, and miR-181c were found to have an opposite role in cardiac vs. pulmonary diseases, supporting the existence of a complex crosstalk between miRNAs and ROS that may depend on tissue specificity.