Effects of AntagomiRs on Different Lung Diseases in Human, Cellular, and Animal Models

Introduction: MiRNAs have been shown to play a crucial role among lung cancer, pulmonary fibrosis, tuberculosis (TBC) infection, and bronchial hypersensitivity, thus including chronic obstructive pulmonary disease (COPD) and asthma. The oncogenic effect of several miRNAs has been recently ruled out. In order to act on miRNAs turnover, antagomiRs have been developed. Materials and methods: The systematic review was conducted under the PRISMA guidelines (registration number is: CRD42019134173). The PubMed database was searched between 1 January 2000 and 30 April 2019 under the following search strategy: (((antagomiR) OR (mirna antagonists) OR (mirna antagonist)) AND ((lung[MeSH Terms]) OR (“lung diseases”[MeSH Terms]))). We included original articles, published in English, whereas exclusion criteria included reviews, meta-analyses, single case reports, and studies published in a language other than English. Results and Conclusions: A total of 68 articles matching the inclusion criteria were retrieved. Overall, the use of antagomiR was seen to be efficient in downregulating the specific miRNA they are conceived for. The usefulness of antagomiRs was demonstrated in humans, animal models, and cell lines. To our best knowledge, this is the first article to encompass evidence regarding miRNAs and their respective antagomiRs in the lung, in order to provide readers a comprehensive review upon major lung disorders.


Introduction
MicroRNAs (miRNAs) are small molecules made of 21 nucleotides, which modulate several biological processes through post-transcriptional gene expression regulation [1]. In addition, many miRNA knockout strains have differential responses to models of several disorders such as neuronal, cardiac, pulmonary, vascular, renal, immunological, while some of them have altered susceptibility to fungal or bacterial infections, or altered propensity to develop tumors in cancer models [2]. In fact, miRNAs may control tumor development, both acting as tumor-promoting miRNAs (oncomiRNAs and metastamiRNAs) or as tumor suppressor miRNAs [3,4]. Furthermore, it was demonstrated that several human miRNAs are located in specific genomic sites which are involved in cancer [5]. During the last decade, researchers investigated miRNAs functioning and regulation. More recently, they focused on miRNA stability and turnover. Indeed, it was established that miRNAs are very dynamic molecules, presenting with a rapid turnover, which depends on their activation. On these bases, researchers investigated miRNA-induced silencing complex (miRISC) and Argonaute (AGO) proteins, which directly interact with miRNAs and are key factors in the assembly and function of Int. J. Mol. Sci. 2019, 20, 3938 2 of 32 miRISCs [1]. In order to act on miRNAs turnover, antagomiRs have been developed. They are a novel class of chemically engineered oligonucleotides, which are specific silencers of endogenous miRNAs. Specifically, two major molecular changings have been developed in order to increase chemical stability. Thus, including switching of the phosphodiester support with a phosphorothioate linkage between nucleotides or including a 2 O-methyl group. Additionally, antagomiRs with a cholesterol moiety are thought to promote cellular uptake [6]. Considering the complex role of miRNAs, these new molecules can be powerful tools to silence specific miRNAs in vivo and may represent a therapeutic strategy for silencing miRNAs in disease. Indeed, Krützfeldt et al. [7] conducted a study on mice in order to study the biological significance of miR-122, which is abundant in the liver. Analysis of gene expression of messenger RNA from antagomiR-treated animals disclosed that the 3 untranslated areas of upregulated genes were extremely enriched with miR-122 recognition motives, while down-regulated genes were poor of these motives. Moreover, researchers found that cholesterol biosynthesis genes would have been modulated by miR-122; in fact, plasma cholesterol levels were reduced in antagomiR-122-treated mice. So far, evidence on antagomiR function has been collected on cellular, animal, and human models. Moreover, as described below, data in these three groups are often coherent. This in support of the hypothesis that miRNAs are molecular factors capable of influencing the expression of several disorders. To date, new studies regarding the use of miRNAs as therapeutic targets are ongoing, especially in the treatment of HCV infection, atherosclerosis, and oncologic diseases. As Gambari et al. [3] highlighted in their review, until now, several antagomiRs have been studied among oncologic diseases as therapeutic agents, both alone or in combination with standard drugs. The promising results explain the reason why these agents will improve the therapy of several tumors, such as gastric cancer, gliomas, and breast cancer [8][9][10]. Indeed, the role of miRNAs upon lung disorders has been extensively studied. MiRNAs have proven to play a crucial role among lung cancer, pulmonary fibrosis, tuberculosis (TBC) infection, and bronchial hypersensitivity, thus including chronic obstructive pulmonary disease (COPD) and asthma. The oncogenic effect of several miRNAs has been recently ruled out. Herein, a study of our group recently identified for the first time new mechanisms, supporting the crucial role of cigarette smoke-induced miR-21 expression in the amplification of inflammatory responses and in tumorigenesis processes within the airways [11]. COPD is a complex disease with a high rate of morbidity and mortality, especially in Western countries. Disease exacerbations and the associated hospitalizations often represent a considerable expense at a socio-economic level. Reduced lung function predicts mortality and is key to the diagnosis of COPD. Shrine et al. [12] conducted a genome-wide associated study in order to highlight new genetic mechanisms in order to improve future preventive and therapeutic strategies for COPD. More recently, many authors investigated miRNAs expression in COPD, noticing that wide networks composed of miRNA and messenger RNA (mRNAs) cooperate in COPD pathogenesis [13]. Moreover, Faiz et al. [14] investigated whether miRNA expression was modulated by inhaled corticosteroid (ICS) treatment and identified miR-320d as a novel mediator of ICS, regulating the pro-inflammatory response of the airway epithelium. Nowadays, among chronic inflammatory lung diseases, asthma is one of the most prevalent. Pathological mechanisms rely on activation of mast cells and eosinophils and dysregulation of Th 2 response. Asthmatic patients often have a strong genetic background, however recent studies demonstrated the role of epigenetic factors such as miRNAs [15]. Gold standard therapies for asthma control include inhaled β-agonists, both short and long acting, and steroids. On this background, Yu at al. [16] recently demonstrated that a specific miRNA, miR-16, may be used as a predictive biomarker of therapeutic response in asthma, thus, suggesting the role of miRNAs not only upon disease physiopathology but also upon drug response. MiRNAs have shown to have an effect also upon pulmonary arterial hypertension (PAH). PAH is a complex disease with different clinical manifestations and high genetic variability. ESC/ERS Guidelines outlined some of the most frequently involved genetic factors such as nuclear factor of activated T cells (NFAT), hypoxia-inducible factor 1α (HIF-1α), and signal transducer and activator of transcription 3 (STAT3) [17]. New research showed that miRNAs may play a crucial role in vascular remodeling, thus inhibiting or promoting pulmonary As shown in Table 1-Table 5, the vast majority of them included research carried out on cell lines and on animals, whereas only a few (20, corresponding to 29.4%) investigated the selected topic on humans. As expected, the condition most widely investigated was lung cancer (21/68), followed by pulmonary hypertension (7/68). All of the selected papers took into account the efficacy of the antagomiR treatment on the various diseases, whereas safety was rarely investigated (only in two cases out of 68). Overall, the use of antagomiR was seen to be efficient in downregulating the specific miRNA they are conceived for, with quite similar results either in vivo and in vitro and independently from the disease and the cell line they are used for. Overall, the usefulness of antagomiRs was demonstrated in humans, as well as in animals and cell lines without particular differences between those three aggregated groups. According to our results, the miRNA most frequently retrieved in lung conditions, therefore considered as "organ-specific," although not "disease-specific," appears to be miR-21, which has been found to be somewhat involved in lung conditions according to six of the selected papers [8,[24][25][26][27][28]. However, miR-155 was also largely investigated and found to be involved in five works retrieved [29][30][31][32]72]. Further miRNAs frequently involved in lung conditions include miR-7 and miR-34, included in four articles, and miR-92 and miR-374, with three hits each. As evidenced, despite a handful miRNAs with multiple hits across the works included in our research, the only critical issue eventually identified for our approach concerns the right choice for the miRNA to be downregulated to achieve the expected result of the whole As shown in Tables 1-5, the vast majority of them included research carried out on cell lines and on animals, whereas only a few (20, corresponding to 29.4%) investigated the selected topic on humans. As expected, the condition most widely investigated was lung cancer (21/68), followed by pulmonary hypertension (7/68). All of the selected papers took into account the efficacy of the antagomiR treatment on the various diseases, whereas safety was rarely investigated (only in two cases out of 68). Overall, the use of antagomiR was seen to be efficient in downregulating the specific miRNA they are conceived for, with quite similar results either in vivo and in vitro and independently from the disease and the cell line they are used for. Overall, the usefulness of antagomiRs was demonstrated in humans, as well as in animals and cell lines without particular differences between those three aggregated groups. According to our results, the miRNA most frequently retrieved in lung conditions, therefore considered as "organ-specific," although not "disease-specific," appears to be miR-21, which has been found to be somewhat involved in lung conditions according to six of the selected papers [8,[24][25][26][27][28]. However, miR-155 was also largely investigated and found to be involved in five works retrieved [29][30][31][32]72]. Further miRNAs frequently involved in lung conditions include miR-7 and miR-34, included in four articles, and miR-92 and miR-374, with three hits each. As evidenced, despite a handful miRNAs with multiple hits across the works included in our research, the only critical issue eventually identified for our approach concerns the right choice for the miRNA to be downregulated to achieve the expected result of the whole process. Indeed, even taking into account a single disease (e.g., lung cancer) on a similar population (e.g., on humans), a plethora of miRNAs with different functions can be identified as associated with the pathological process, therefore the selection of the right process to be blocked or, conversely, enhanced, is critical for good outcome of the process. Taking into account some specific disease categories, the first one to be analyzed, due to its numerical prevalence, was lung cancer. Inhibition of elevated miR-570-3p in COPD small airway epithelial cells, using an antagomir, restores sirtuin-1, and suppresses markers of cellular senescence, restoring cellular growth (p < 0.05) Inhibition of elevated miR-570-3p in COPD small airway epithelial cells, using an antagomir, restores sirtuin-1 and suppresses markers of cellular senescence, restoring cellular growth (p < 0.05)    Antagomirs for miR-323a-3p augment murine lung fibrosis after bleomycin injury (p < 0.05)

Lung Cancer
All the works retrieved showed an excellent efficacy of antagomiRs in blocking the action of the respective miRNAs both in vivo (on both humans and animals) and in vitro. The studies, taken singularly, have shown a wide heterogeneity of the mechanisms investigated and challenged with a respective wide difference in terms of miRNAs studied and antagomiRs employed. This applies both analyzing separately single sub-groups of samples and investigating the whole amount of studies together, making it difficult to identify a "principal" microRNA to be challenged in this specific domain.

Humans
Eleven studies have specifically assessed the role of antagomiRs in human subjects with lung cancer, all of which demonstrating the usefulness of such compounds in blocking cancer-related cellular mechanisms, and even restraining the growth of tumors even in vivo (see, for example, He et al., 2019 [33] or Liu et al., 2011 [34]). The extreme heterogeneity of the miRNAs (and, consequently, antagomiRs) taken into account reveals the complexity of biological and cellular patterns of the lung cancer in vivo, suggesting the specific role of each compound for a given mechanism to be studied and tailored.

Animals
Ten works investigating lung cancer in animals were retrieved, all of which used mice models for this assessment. AntagomiRs were demonstrated to be extremely efficient in modulating cancer growth, preventing it in most cases (see, for example, anti-miR135b, in Lin et al., 2013 [73]; anti-miR19a, in Liu et al., 2011 [34]; anti-miR494, in Mao et al., 2015 [74]), or even promoting tumor growth when experimentally needed (see anti-miR30c, in McCann et al., 2019 [75]).

Cell lines
As expected, studies investigating lung cancer in cell lines are the vast majority with respect to the other sub-groups (20 studies were retrieved). Such studies confirmed in vitro the evidence arising in vivo, with antagomiRs able to modulate cancer growth depending on the outcome desired. Even on cell lines, the heterogeneity in terms of miRNAs investigated and antagomiRs employed was present, as already seen in humans and animals.

Bronchial Hypersensitivity
Under this category, studies dealing with asthma, COPD, allergic airway disease, airway hyper-responsiveness were included. Overall, eight studies were found, the majority of which investigating COPD (four studies) or asthma (three articles). Despite the heterogeneity in terms of miRNAs investigated, in this case promoted by both the relatively low number of studies retrieved and by the slightly different kind of diseases included in this category, miR-21 was included in several studies and its activity successfully challenged by its corresponding antagomiR. In summary, despite the differences in terms of category of samples studied and antagomiRs used, these compounds have demonstrated their usefulness also in this class of clinical conditions.

Humans
Only four works investigated such diseases in humans, all of which dealing with COPD. Overall, the studies highlighted a positive contribution of antagomiRs in decreasing the clinical symptoms of the disease. In some cases, these molecules have even shown to improve clinical outcomes with the support of other compounds, like protein kinase R (see Hsu et al. 2016 [76]).

Animals
Six studies were published on animal models, of which three investigated asthma, and one each COPD, airway disease, and airway hyper-responsiveness. All those studies took into account mice models, with miR-21 that was investigated in three out of the six works. Overall, the results were encouraging, with minor differences. Indeed, as observed by Collison et al. [24], just the inhibition of miR-145, but not miR-21 or let-7b, inhibited eosinophilic inflammation, mucus hypersecretion, TH2 cytokine production, and airway hyper-responsiveness, and also Plank et al. [30] on asthma observed that the use of the respective antagomiR (anti-miR-155-5p) reduced miR-155-5p expression, but failed to alter the disease phenotype.

Cell lines
Just three studies, all concerning COPD, were published on cell lines. All the three took into account different miRNAs and corresponding antagomiRs, but the results were seen to be overall positive, except the above reported work by Hsu et al. [76], which highlighted the pivotal importance of combining antagomiRs with protein kinase R to increase antiviral stress granules formation, induction of p300 and IFN-β in COPD primary bronchial epithelial cells.

Pulmonary Hypertension
Six studies investigated pulmonary hypertension in the literature retrieved, mostly dealing with animal models. Overall, the various antagomiRs used (as previously, the heterogeneity in terms of miRNAs studied was high) demonstrated a good efficacy in challenging the corresponding miRNA and in improving the biological outcome.

Humans
Only one work, published by Potus and colleagues [77] investigated the use of antagomiRs in pulmonary hypertension in humans, through biopsy specimens, with the surprising result that AntagomiR-126 in healthy CD31+ cells mimicked the PAH phenotype.

Animals
Six works, including that by Potus and colleagues [77] above reported, have used animal models to this aim. All of them used either mice or rat models, with complete heterogeneity on the miRNAs studied. Overall, the treatment with antagomiRs improved the outcome with respect to pulmonary hypertension, except in the study by Gubrij et al. [78], where AntagomiR-223 was efficient in reducing levels of miR-223 in pulmonary artery and lungs of rats as compared to controls, but failed to attenuate pulmonary hypertension.

Cell lines
Three articles also took into account cell lines on this specific topic. Despite using different compounds, tailoring different miRNAs, all the three works highlighted the positive role of antagomiRs in improving the outcome related to pulmonary hypertension, overall confirming the results obtained in vivo.

Lung Injury
In this category, acute lung injury and pulmonary inflammation were included. Overall, eight articles were included, four each for the two conditions above mentioned. No studies on human subjects were retrieved, while works dealing with animal models and cell lines were relatively numerous. As in other sub-groups and possibly due to the slight heterogeneity of the clinical conditions included in this category, just one miRNA was studied in more than one article even if taken into account in two studies by the same research team (miR-374a, investigated in Adyshev et al., [80,81]). Summarizing the results, even in this group of conditions, antagomiRs were extremely useful in improving the outcome both in vivo and in vitro, with no report of lack of efficacy in any of the studies retrieved.

Animals
Six studies were retrieved on animal models. As in most of the cases previously reported, all of them were performed on mice models. Three of them investigated miRNAs in lung injury, two of them in lung inflammation and one [79] in both.
As stated, all the antagomiRs employed successfully reduced the pathological changes, starting from cellular level, in vivo, without evidence of inefficacy on the samples studied.

Cell lines
Six studies were found also on cell lines. Three of them took into account lung inflammation, two lung injury and the one by Xie and colleagues [77] reported above both the conditions.
The results obtained confirmed the good reliability and efficacy of antagomiRs in improving the biomarkers of diseases in vitro, independently from the miRNA (and, therefore, the mechanism) tailored, with just one antagomiR, antagomiR-374a, that was used in more than one study [80,81], on lung injury and lung inflammation, respectively.

Other Conditions
In this category, several conditions, not matched by the groups above explained, were included, each of them featuring a small number of studies, and displaying positive results, overall.

Humans
Four studies were performed on humans, one each about lung cell dysfunction, lung fibrosis, HIV infection and substance abuse, and tuberculosis. Given the different etiopathological processes of such conditions, also completely different mechanisms were investigated, and different miRNAs tailored. However, in all four cases, an improvement of the outcome was found in vivo, independently from the condition studied.

Animals
Sixteen studies on animals were found for this macro-category, with the condition more largely studied being influenza (four studies). All but two of these studies were carried out on mice or rats, whereas Asquith et al. [85], studied chronic ethanol consumption in non-human primates, and Zhou et al. [88] assessed the effect of miRNAs and antagomiRs in influenza in one-month-old beagles.
Three studies [31,72,85] on chronic ethanol consumption, pneumonia, and systemic lupus erythematosus, investigated the role of miR-155, two [88,89] respectively on tuberculosis and influenza, of miR-143 and the others showed complete heterogeneity in the miRNAs studied.
Overall, antagomiRs demonstrated their efficacy in tailoring such conditions in vivo also on animal models, as already seen on humans, despite a lack of complete efficacy noticed by Chiba et al. [83] for miR-133b and let-7a antagomiRs in abnormal bronchial smooth muscle contraction studied in BALB/c mice.

Cell Lines
Nineteen studies were included in this category, most of which were already mentioned in the two previous (humans and animals) groups, as performed both in vivo and in vitro. Three of them were performed on influenza [86,88,90] and three others on lung fibrosis [26,82,84]. Other conditions were more rarely studied.
Concerning the miRNAs studied, as above, a slight prevalence (three studies: Bhattacharyya et al. [29] on cystic fibrosis, Podsiad et al. [31] on pneumonia, Zhou et al. [86] on systemic lupus erythematosus) included miR-155, with the positive results about the use of the respective antagomiR seen in vivo that were confirmed in vitro. miR-143 was successfully tailored in two studies (Tamgue et al. [89] on tuberculosis, Zhou et al., [88] on influenza) confirming the positive retrievals already seen in vivo.
Overall, the other antagomiRs were also seen to be efficient in positively tailoring the activity of the respective miRNAs, thus improving the biological outcome of the various conditions they were applied to, also in vitro, despite minor evidence of lack of (or reduced) efficacy seen in Chiba et al. [83] on abnormal bronchial smooth muscle contraction and in Morales et al. [87] on SARS-CoV.

Discussion
It is well established that miRNAs are pleiotropic molecules involved in almost all major biological processes. This concept was particularly studied in the lung, where specific miRNAs demonstrated to cooperate with organ development and pulmonary diseases. During the last years, much data has been collected on this topic, with special regards to obstructive and restrictive lung diseases [91][92][93]. In fact, as Sessa and Hata [94] reported, a typical miRNAs expression profile was noticed and different miRNAs play an active role among different processes including hemostasis, viral infection, and inflammation. Lung-specific miRNAs can be used as novel biomarkers in lung disorders. To date, several pieces of research focused on specific lung disease miRNAs expression patterns. However, specific miRNAs expression profiles may be noticed also among different organs. Previously, Wang et al. [20] conducted a study on rats, demonstrating that two specific miRNAs (miR-195 and miR-200c) were peculiarly expressed in the lung, while eight miRNAs were co-expressed in the lung and heart and one miRNA was co-expressed in the lung and kidney. As interest increased on this topic, accessible databases as, for example, MiRmine were created in order to allow researchers to retrieve expression profiles of single or multiple miRNAs for a specific tissue or cell line, either normal or with disease information [95]. According to our results, miR-21 seems to be the most represented miRNA among lung conditions. MiR-21 is often up-regulated in lung carcinoma. This fact is believed to be a result of the capacity of miR-21 to inhibit tumor suppressor phosphatase and tensin-homolog [96]. Collison et al. [24] characterized miRNAs expression among house dust mite allergic mice. A group was treated with antagomiRs that inhibited the function of specific miRNAs in the lung, and the other group received standard steroid therapy with dexamethasone. Finally, inflammatory lesions and airway hyper-responsiveness were measured. Researchers found that, although miR-21 and let-7b were highly expressed during allergic inflammation, blockade of their function was ineffective at modulating the expression of disease. On the other hand, Kim et al. [27] conducted a study on BALB/c mice noticing that antagomiR-21 increased phosphatase and tensin homolog (PTEN) levels (p < 0.05). Treatment with Ant-21 reduced phosphoinositide 3-kinase (PI3K) activity and restored histone deacetylase (HDAC2) levels (p < 0.05), leading to suppression of airway hyper-responsiveness and to restore of steroid sensitivity to allergic airway disease. Lee et al. [60] also investigated allergic inflammation among mouse models, reporting that MiR-21 expression was down-regulated in mice lungs treated with anti-miR-21. In fact, specific antagomiR reduced both eosinophil count (p < 0.01) and Th2 cytokines levels, including IL-5 and IL-13 in mice BAL fluid (p < 0.05). MiRNA21 demonstrated positive effects also upon lung ischemia. Ischemia/reperfusion injury (IRI) is the primary cause of acute lung injury (ALI) and primary graft dysfunction (PGD) after lung transplantation [97]. Li et al. [28] conducted a study on murine lung ischemia/reperfusion (I/R) and in vitro hypoxia/reoxygenation (H/R) models demonstrating that pre-treatment of mesenchymal stromal cells with miR-21-5p antagomiR ameliorated IRI in the lung. Regarding PAH, Pullamsetti et al. [25] conducted a study both on animal models and cell lines demonstrating that Ant-17 and Ant-21 reduced right ventricular systolic pressure and pulmonary arterial muscularization. Moreover, Ant-17 decreased hypoxia-induced right ventricular hypertrophy and improved pulmonary artery acceleration time. In mice, Ant-17 therapy reduced right ventricular systolic pressure and total pulmonary vascular resistance index, stabilized cardiac output and reduced pulmonary vascular remodeling. In human pulmonary artery smooth muscle cells, Ant-17 increased the cyclin-dependent kinase inhibitor 1A (p21). MiRNA 21 demonstrated to have a role also upon lung fibrosis. In fact, Shentu et al. [26] demonstrated that human mesenchymal stem cell-derived extracellular vesicles (mEVs) do contain several specific miRNAs including 21-5p and 630. MEVs suppress TGFβ1-induced myofibroblastic differentiation of normal and idiopathic pulmonary fibrosis (IPF) lung fibroblasts, thus mitigating tissue fibrotic response. Investigating the role of miRNA regarding the pathogenesis and progression of lung fibrosis, Liu et al. [98] found that miR-21 was up-regulated both in the lungs of mice presenting with bleomycin-induced lung fibrosis and IPF patients. In this setting, miR-21 was highly expressed by myofibroblasts in the fibrotic lungs. Furthermore, researchers noticed that miR-21 reduced bleomycin-induced lung fibrosis in rats' lungs.
Overall, a simple explanation of the mechanisms involving antagomiR-21 in lung conditions is provided in Figure 2. and progression of lung fibrosis, Liu et al. [98] found that miR-21 was up-regulated both in the lungs of mice presenting with bleomycin-induced lung fibrosis and IPF patients. In this setting, miR-21 was highly expressed by myofibroblasts in the fibrotic lungs. Furthermore, researchers noticed that miR-21 reduced bleomycin-induced lung fibrosis in rats' lungs.
Overall, a simple explanation of the mechanisms involving antagomiR-21 in lung conditions is provided in Figure 2. Another miRNA molecule, which is widely expressed in several lung conditions, is miR-155. Several studies demonstrated that this molecule is upregulated in activated immune cells, such as T and B lymphocytes, macrophages, and dendritic cells (DCs). Indeed, miR-155 levels increase in response to inflammatory mediators. Moreover, miRNA-155 can regulate B-cell proliferation, malignancy, antibody production, and the differentiation and function of IL-17-producing helper T cells. Furthermore, miR-155 is induced by LPS, as well as other TLR ligands and proinflammatory Another miRNA molecule, which is widely expressed in several lung conditions, is miR-155. Several studies demonstrated that this molecule is upregulated in activated immune cells, such as T and B lymphocytes, macrophages, and dendritic cells (DCs). Indeed, miR-155 levels increase in response to inflammatory mediators. Moreover, miRNA-155 can regulate B-cell proliferation, malignancy, antibody production, and the differentiation and function of IL-17-producing helper T cells. Furthermore, miR-155 is induced by LPS, as well as other TLR ligands and proinflammatory cytokines [99][100][101]. As miR-155 is involved in several processes, it is feasible to understand its role upon many lung disorders. Basing on the hypothesis that miR-155 upregulation could inhibit IL-17 expression and therefore increase susceptibility to secondary bacterial pneumonia, Podsiad et [31] al. conducted a study on wild-type C57BL/6 mice and human lung macrophages in order to investigate the role of miR-155 and the respective antagoMiR upon viral and bacterial pneumonia. They concluded that miR-155 antagomiR ameliorated lung bacterial clearance compared with controls. MiR-155 plays a crucial role also upon ARDS. Triggering receptors expressed on myeloid cells (TREM) proteins are a family of immunoglobulin cell surface receptors expressed on myeloid cells and they are considered as amplifiers of Toll-like receptor (TLR)-induced inflammation. Experiments with antagomiR-155 confirmed that TREM-1-mediated changes were dependent on miR-155. Yuan et al. [32] conducted a study on wild-type C57BL/6J mice and bone marrow-derived macrophages demonstrating that TREM-1 boosted inflammatory response by inducing the expression of miR-155 in macrophages. Therefore, researchers inhibited TREM-1 using a nanomicellar strategy. Neutrophilic inflammation was reduced, thus suggesting that TREM-1 inhibition is a potential therapeutic target for neutrophilic lung inflammation and ARDS. Systemic lupus erythematosus (SLE) is a complex auto-immune disease which can involve several systems, including lungs. Diffuse alveolar hemorrhage (DAH) is a rare but severe complication of SLE and miR-155 showed to have a relevant role. In fact, Zhou et al. [86] found that miR-155 expression was up-regulated during the development of DAH, noticing that this molecule targets several pro-inflammatory mediators. The extent of lung inflammation was markedly reduced in miR-155-knockout (miR-1552/2) mice. Moreover, in vivo silencing of miR-155 using miR-155 antagomiR reduced the incidence of iatrogenic-induced DAH. MiR-155 cooperates with Th2 responses too. In fact, it is extensively expressed in the Th cell, DCs, and macrophages in the lung. MiR-155 was also found to be up-regulated in the nasal mucosa and airway smooth muscle cells of allergic asthmatic patients [27,100,101]. Recently, Plank et al. [30] conducted a study on murine asthmatic models noticing that MiR-155-5p is highly upregulated in mice. However, while targeting of miR-155-5p with a specific antagomiR resulted in specific inhibition in vivo, it was not able to alter the disease phenotype. Authors hypothesized that this could be due to the variation in antagomiR uptake, which demonstrated to be effective in myeloid cells and weak in lymphocytes. Cystic fibrosis (CF) is an autosomal recessive disease, due to the occurrence of cystic fibrosis transmembrane conductance regulator (CFTR) gene mutations and it is characterized by a variable cytokines pro-inflammatory milieu. Bhattacharyya et al. [29] demonstrated that antagomiR-155 down-regulates miR-155 expression suppressing IL-8 and other proinflammatory genes in CF cells.
The mechanisms involving antagomiR-155 in lung conditions are displayed in Figure 3.
development of DAH, noticing that this molecule targets several pro-inflammatory mediators. The extent of lung inflammation was markedly reduced in miR-155-knockout (miR-1552/2) mice. Moreover, in vivo silencing of miR-155 using miR-155 antagomiR reduced the incidence of iatrogenicinduced DAH. MiR-155 cooperates with Th2 responses too. In fact, it is extensively expressed in the Th cell, DCs, and macrophages in the lung. MiR-155 was also found to be up-regulated in the nasal mucosa and airway smooth muscle cells of allergic asthmatic patients [27,100,101]. Recently, Plank et al. [30] conducted a study on murine asthmatic models noticing that MiR-155-5p is highly upregulated in mice. However, while targeting of miR-155-5p with a specific antagomiR resulted in specific inhibition in vivo, it was not able to alter the disease phenotype. Authors hypothesized that this could be due to the variation in antagomiR uptake, which demonstrated to be effective in myeloid cells and weak in lymphocytes. Cystic fibrosis (CF) is an autosomal recessive disease, due to the occurrence of cystic fibrosis transmembrane conductance regulator (CFTR) gene mutations and it is characterized by a variable cytokines pro-inflammatory milieu. Bhattacharyya et al. [29] demonstrated that antagomiR-155 down-regulates miR-155 expression suppressing IL-8 and other proinflammatory genes in CF cells.
The mechanisms involving antagomiR-155 in lung conditions are displayed in Figure 3.

Conclusions
This systematic review provides numerous shreds of evidence regarding dysregulation in miRNAs expression in lung diseases. It remains to understand the sources of the various miRNAs, and whether they have mainly disease-or organ-specific effects. However, these findings may contribute to a better definition of the complex network of miRNAs involved in lung diseases. Thus, miRNAs have been proposed as diagnostic or prognostic biomarkers and therapeutic targets for future treatments. Notably, antagomiRs are chemically modified oligonucleotides that are used to silence microRNAs, having the property of bind specifically to particular microRNAs. These could represent a therapeutic opportunity to modulate miRNA-induced, post-transcriptional mRNA regulation. To our best knowledge, this is the first article describing evidence on the involvement of miRNAs and the efficacy of respective antagomiRs in different lung diseases, including studies conducted on humans, animals, and cells. In conclusion, these findings provide new knowledge on the network of miRNAs in lung diseases and suggest that antagomiRs may represent a target for a specific therapy for these diseases.

Key Points
• Strong evidence confirmed that miRNAs play a crucial role in several pathologic mechanisms, therefore they have been proposed as diagnostic or prognostic biomarkers and therapeutic targets for future treatments. • AntagomiRs are chemically modified oligonucleotides able of silencing microRNAs and are now emerging as novel therapeutic agents in several conditions.
• It has been widely demonstrated that miRNAs have a fundamental role among several lung conditions, moreover, some of these molecules proved to have a lung-specific tropism, thus suggesting the idea of lung-specific miRNAs patterns.

•
To our best knowledge, this is the first article describing the evidence on the involvement of miRNAs and the efficacy of respective antagomiRs in different lung diseases, including studies conducted on humans, animals, and cells.

•
Coherence between these groups has been demonstrated, thus suggesting the importance of developing new studies on these agents as target therapies.