Suppl 1: Role of MicroRNAs in Fibrosis

doi:10.2174/1874312901206010130

Open Rheumatol J. 2012; 6: 130–139.

Published online 2012 June 15. doi: 10.2174/1874312901206010130

PMCID: PMC3396185

Suppl 1

Role of MicroRNAs in Fibrosis

Serena Vettori,^1,² Steffen Gay,¹ and Oliver Distler^*¹

¹Center of Experimental Rheumatology, ZIHP, University Hospital Zurich, Gloriastrasse 25, CH-8091 Zurich, Switzerland

²Department of Clinical Medicine, Cardiovascular and Immunologic Sciences, University of Naples “Federico II”, via Pansini 5, 80131 I-Naples, Italy

^*Address correspondence to this author at the University Hospital Zurich, Gloriastrasse 25, 8091 Zurich, Switzerland; Tel: +41-44-255-2977; Fax: +41-44-255-4170; E-mail: oliver.distler/at/usz.ch

Received March 3, 2012; Revised March 27, 2012; Accepted April 4, 2012.

This is an open access article licensed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/3.0/) which permits unrestricted, non-commercial use, distribution and reproduction in any medium, provided the work is properly cited.

Abstract

Fibrosis is the leading cause of organ dysfunction in diseases such as systemic sclerosis, liver cirrhosis, cardiac fibrosis, progressive kidney disease, and idiopathic pulmonary fibrosis. The hallmark of fibrosis is tissue remodeling with excess deposition of extracellular matrix components, predominantly collagens. Different cell types, cytokines, growth factors, and enzymes interact in complex pathogenic networks with myofibroblasts playing a pivotal role. MicroRNAs are small non-coding RNAs acting as negative regulators of gene expression at the post-transcriptional level. MicroRNAs have been associated with many basic cellular processes as well as with a wide spectrum of diseases, most notably cancer. This review provides a comprehensive overview of microRNAs regulating profibrotic pathways and extracellular matrix synthesis. The potential of miRNA for targeted therapeutic approaches in fibrotic disorders is also discussed.

Keywords: Fibrosis, fibroblasts, microRNA (miRNA)-mediated gene regulation regulation, transforming growth factor-beta (TGF-β), connective tissue growth factor (CTGF), extracellular matrix (ECM), epithelial-to-mesenchymal transition (EMT), signaling pathways, antagomirs.

INTRODUCTION

Fibrosis is a leading cause of organ dysfunction in diseases, either as the outcome of an uncontrolled reaction to chronic tissue injury, or as the primary disease itself in predisposed individuals [1-4] (Table 11). Independently from the initiating trigger and the tissue-specific response, the hallmark of fibrosis is excess deposition of extracellular matrix (ECM) components, predominantly collagens and associated glycoproteins. Accumulating ECM replaces functional tissue, and disrupts organ architecture [1]. Different cell types, cytokines, growth factors, and enzymes are orchestrated together in complex pathogenic networks, converging on the activation and/or recruitment of fibroblasts and their differentiation into myofibroblasts, which are α-smooth muscle actin (α-SMA) enriched cells overproducing ECM [5-7]. It is currently believed that along with these cellular key players, a number of regulatory factors also have substantial effects on fibrosis and may be responsible for inter-organ variability of fibrotic manifestations [8].

Table 1.

Examples of Primary (Idiopathic) and Secondary Fibrotic Disorders with Multiple/Single Organ Presentation

In this scenario, microRNAs (miRNAs) may play the role of conductors in the pathogenesis of fibrosis [9]. miRNAs are small, non-coding RNAs with a strictly regulated biogenesis. This is combined with an extremely flexible and sophisticated regulatory function, allowing simultaneous targeting of multiple proteins involved in different crucial biological pathways of specific cell types and tissues [10]. Consequently, miRNAs have been associated with many basic cellular processes such as cell death, proliferation and differentiation. On the other hand, miRNA dysregulation has also been linked to a wide spectrum of diseases, most notably cancer [11].

A number of recent reviews have addressed the role of miRNA in fibrosis with a focus on organ-specific miRNA alteration [12-15]. Here we paid special attention to miRNA alterations common to different fibrotic disorders such as systemic sclerosis (SSc) [16], liver cirrhosis (LC) [17], cardiac fibrosis (CF) [18, 19], chronic kidney disease (CKD) [20], and idiopathic pulmonary fibrosis (IPF) [21]. miRNAs directly regulating ECM synthesis and miRNAs regulating TGF-β/CTGF signaling as a key profibrotic pathway were considered for this review.

MicroRNA BIOGENESIS AND FUNCTION

miRNAs are a class of small, evolutionarily conserved, non-coding RNAs, which repress the expression of protein coding genes at the post-transcriptional level via a partially complementary binding to the 3’ untranslated region (UTR) of target mRNAs ([22], for comprehensive review). To date, 1426 miRNA encoding sequences have been identified in the human genome and more than half of them have been experimentally validated (ftp://mirbase.org/pub/mirbase/CU RRENT/genomes/hsa.gff) [23]. Most miRNA genes are located in introns, exons and UTRs of protein coding genes. Approximately one quarter of miRNAs are encoded in intergenic regions, and display an independent transcription unit, whereas intronic and exonic miRNAs are usually co-transcribed and co-expressed with the host gene [22]. In addition, a half of known miRNAs are encoded by polycistronic units, generating multiple miRNAs, often related in their structure and function [22, 24].

miRNAs are transcribed as long polyadenylated RNAs, pri-miRNAs, which contain one or more mature miRNA sequences. Pri-miRNAs are cut by the endonuclease Drosha and its co-factor DiGeorge syndrome critical region 8 (DGCR8), originating intermediate 60-100 nucleotide hairpin structures called pre-miRNAs [22]. Pre-miRNAs are transported to the cytoplasm by exportin 5 and further processed by another endonuclease, Dicer, releasing functionally active, mature miRNAs with a length of ≈ 22 nucleotides. Each mature miRNA is then incorporated into the RNA-induced silencing complex (RISC), which allows the recognition of target mRNA and the formation of a miRNA:mRNA duplex [22]. The base pairing of the duplex is mediated by a distinct seed region at nucleotides 2-8 of the miRNA, and alters mRNA stability or affects protein translation [25]. Translational repression and also target mRNA destabilization predominate in humans, although in rare cases miRNAs can switch their negative regulatory function towards gene activation by promoting mRNA translation [26].

The “imperfect” miRNA:mRNA binding is the basis for an extremely flexible regulatory system, as one miRNA can regulate hundreds of genes, and one gene containing multiple partially complementary binding sites in its 3’UTR can be regulated by multiple miRNAs [24]. In addition, this flexibility in miRNA:mRNA interaction is further expanded by a number of variables: miRNA binding site accessibility, RNA secondary structure, position of the binding site in the 3’UTR and its distance from the stop codon, and reciprocal interference between different miRNAs and RNA binding proteins sharing the same target [27]. As a consequence, miRNA activity depicts a gray scale rather than a black-or-white regulatory scenario [27, 28]. This aspect is particularly relevant for the regulation of multistep biological processes such as signaling pathways: while the effect of a given miRNA on the overall activity of the pathway can be potent, this often results from the sum of subtle modulations on multiple constituents of the same pathway rather than from strong effects on a single molecule [28]. Comprehensive in silico analysis of miRNA targets is provided at the miRGen database (http://www.diana.pcbi.upenn.edu/miRGen.html), which merges the results of multiple miRNA prediction databases, such as DIANA microT, miRanda (microrna.org), miRanda (miRBase), PicTar (4-way and 5-way), and TargetScanS [29].

Despite this versatile regulatory activity, the expression of miRNA genes is tightly controlled, and specific cell types often display a certain miRNA profile [22]. Regulation of miRNA expression at both the transcriptional and post-transcriptional level is still poorly understood. However, multiple mechanisms may play a role. At the transcriptional level, miRNAs can control their own expression establishing either positive or negative regulatory loops via an indirect regulation of transcriptional activators or repressors [30]. Epigenetic modifications such as DNA and histone methylation and histone deacetylation also participate in the modulation of miRNA transcription [31, 32]. At the post-transcriptional level, factors influencing the activity of the endonucleases Drosha and Dicer affect selective miRNA cluster maturation [33, 34]. In addition, some pseudogene mRNAs can act as miRNA decoy through a 3’UTR which is highly homologous to the one of the related protein-coding mRNA, thus modulating cell levels of “free” miRNA [35]. Finally, exosome-mediated cell-cell exchange of RNA has also been described as a determinant of cellular miRNA profile [36].

Fig. (11) gives an overview of miRNA biogenesis and function.

Fig. (1)

Schematic miRNA biogenesis and regulation. The miRNA gene is transcribed (pri-miRNA) and cut in the nucleus to generate pre-miRNA. Pre-miRNA is transported to the cytoplasm and is further processed there. Mature miRNA is originated as a mature miRNA:miRNA* (more ...)

SSC: A MODEL FOR FIBROTIC DISEASES?

Despite fibrotic disorders include a large number of primary and secondary entities with distinct features, major extracellular mediators of fibrosis and related signaling pathways are nevertheless common to most of them. This includes cytokines, chemokines, growth factors, and other small secreted signaling proteins (Table 22) [2].

Table 2.

Major Molecular Mediators Elevated in Fibrotic Disorders

SSc is a systemic fibrotic disease of unknown origin characterized by the presence of three main pathophysiological pathways in each target organ/system: vascular injury, immune system activation, and interstitial fibrosis [37]. Thus, we propose SSc as a paradigm of fibrotic disorders, based on the following observations: i) fibrosis in SSc can virtually affect all organs/systems, although the disease displays apparent clinical involvement at preferential sites (skin, lungs, heart, kidneys, gastrointestinal tract, muscles); ii) initial factors triggering pro-fibrotic pathways in other fibrotic diseases are also discussed in SSc and SSc-like diseases including infections, autoimmune phenomena, and exposure to toxics or radiations [38-41]; iii) consistent evidence for the activation of major pro-fibrotic pathways common to most fibrotic diseases has been reported in SSc, including transforming growth factor-β (TGF-β) pathways, the platelet-derived growth factor (PDGF) pathway, and, most recently, the canonical Wnt-β-catenin pathway and the serotonin pathway [42-47].

OVERVIEW OF PRO- AND ANTI-FIBROTIC MicroRNAs

According to the current literature, ≈ 40 miRNAs have been linked to fibrosis in various organs and disease settings (Table 33). In general, the miRNA profile is specific for a given cell-type, and even more specific in relation to the developmental and functional organ status [22]. This might explain, why many miRNAs with pro- or anti-fibrotic properties were found to be dysregulated in specific organs and specific fibrotic diseases [48, 49, 52-54, 57, 62, 63, 67, 71-73, 78-83]. On the other hand, some miRNAs have been found to be abnormally expressed in more than one fibrotic disorder [47, 50, 51, 55, 56, 58-61, 64-66, 68-70, 74-77]. Predicted targets of approximately half of miRNAs involved in fibrosis have been validated by reporter gene assay [84]. Most of these miRNAs directly induce or protect from fibrosis by targeting TGF-β canonical and non-canonical pathways [53, 54, 68-71], connective tissue growth factor (CTGF) [51, 52, 60, 61], ECM structural proteins or enzymes involved in ECM remodeling [47, 74-77, 81, 83]. Another set of miRNAs indirectly regulates fibrogenesis by affecting epithelial-to-mesenchymal transition (EMT) [48, 50, 51, 58, 62, 63, 66, 82], or by inducing proliferation and resistance to apoptosis in myofibroblasts [51, 55-57, 73, 78, 80] (Table 33).

Table 3.

MicroRNAs Linked to Fibrosis

MicroRNA REGULATION OF TGF-B/CTGF SIGNALING

TGF-β is a central mediator of fibrosis [1] and it is ubiquitously expressed in tissues [85]. TGF-β also induces expression of CTGF, and CTGF can in turn enhance TGF-β signaling, along with a number of other pro-fibrotic factors including vascular endothelial growth factor (VEGF), endothelin-1 (ET), insulin-like growth factor-1 (IGF-1), integrins, and Wnts [86]. Therefore, miRNA regulation of TGF-β and CTGF signaling is of paramount interest. Up-to-date, 10 miRNAs showed consistent experimental evidence of being TGF-β/CTGF signaling regulators. Six of them were dysregulated in more than one fibrotic disorder (see Table 33).

miR-17-92 cluster. miR-17-92 is a proto-oncogenic cluster (also called oncomir-1) consisting of six miRNAs. Very recently, miR-18a and miR-19a/b from this cluster have been shown to regulate CTGF and thrombospondin-1 (TSP-1) in the context of liver and cardiac fibrosis [60, 61]. In a previous report, CTGF and TSP-1 were validated as direct miR-18a/miR-19a/b targets in cancer [87]. Kodama et al. [60] reported down-regulation of the miR-17-92 cluster in liver fibrosis in a p53-dependent manner and an associated up-regulation of CTGF. Similarly, van Almen et al. [61] showed down-regulation of the miR-17-92 cluster and spontaneous upregulation of CTGF and TSP-1 in a mouse model of age-related heart failure, which is characterized by widespread interstitial EMC accumulation. Down-regulation of the miR-17-92 cluster paralleled by CTGF and TSP-1 gene up-regulation was confirmed in human idiopathic cardiomyopathy specimens. Transfection experiments in aged neonatal rat cardiomyocytes were used to confirm CTGF and TSP-1 modulation by miR-18a and 19a/b. Down-regulation of the miR-17-92 cluster has also been reported in human IPF lung tissue [48].

miR-133, miR-30 and miR-590. Two other major regulators of CTGF expression in cardiac fibrosis are miR-133 and miR-30. miR-133 is a cardiac specific miRNA and its expression is limited to cardiomyocytes, while miR-30 is expressed in both cardiac fibroblasts and cardiomyocytes as well as in other tissues. Duisters et al. [52] reported miR-133- and miR-30c-mediated repression of CTGF in two animal models of overloaded heart disease: a rat renin-dependent hypertensive failing heart model (Ren2 rats) and a mouse mechanical stress-induced model (mice undergone transverse aortic constriction). They first showed down-regulation of the two miRNA families and up-regulation of CTGF in vivo, then confirmed the direct interaction of miR-133 and miR-30c with CTGF mRNA in vitro using cultured cardiomyocytes and cardiac fibroblasts, and finally proved the functional effects of this interaction on collagen synthesis. Interestingly, the authors also showed that miR-133 and miR-30c do not exert additive effects on CTGF regulation, which might be explained by the overlapping seed regions of these miRs in the 3’UTR of the CTGF gene. Similar down-regulation of miR-133 and miR-30c and increased levels of CTGF were observed in human hypertrophic heart.

The importance of miR-133 down-regulation in cardiac fibrosis is further supported by the findings of Shan and colleagues [53], who investigated the expression of miR-133 in the heart of smokers with chronic atrial fibrillation (AF), and in a canine model of nicotine-induced AF. Fibrotic structural remodeling was strongly induced by nicotine in both human and canine heart tissue. This was associated with an increase in TGF-β1 and the TGF-β pathway downstream targets CTGF and collagens both in vivo and in vitro. By in silico analysis, the authors selected miR-133 and miR-590 as potential regulators of TGF-β1 and TGF-βRII, respectively. miR-133 and miR-590 were strongly down-regulated in dog atrial fibroblasts treated with nicotine, and the direct interaction of the two miRNAs with their predicted targets could be confirmed by reporter gene assay. miR-30c down-regulation has also been reported in human IPF lung tissue [48].

miR-200. Most data link the five miR-200 family members to EMT [54, 66, 88], thus indicating an indirect role in fibrogenesis. Recent data from Wang et al. [54] showed in addition that the miR-200 family contributes directly to fibrosis by regulation of TGF-β2. Expression of miR-141 and miR-200a has been investigated in mouse models of early (ApoE knockout mice injected with streptozotocin) and advanced (C57BL6 mice treated with adenine) diabetic nephropathy, as well as in rat proximal tubular epithelial cells (NRK52E). Both miRNAs were down-regulated in mice, but only miR-200a was sensitive to TGF-β stimulation in NRK52E cells. At the same time, TGF-β stimulation induced transformation of NRK52E cells towards a mesenchymal phenotype as indicated by reduced E-cadherin expression and increased vimentin and α-SMA expression along with enhanced ECM synthesis. In turn, forced up-regulation of both miR-141 and miR-200a in NRK52E cells reduced TGF-β2, Smad3 activity and ECM proteins, and also prevented EMT, suggesting a miR-200a/TGF-β feedback loop in the pathogenesis of diabetic CKD. In addition, reporter gene assays confirmed TGF-β2 as a direct target of miR-141/miR-200a.

miR-21. Like the miR-17-92 cluster, miR-21 is traditionally regarded as an oncomir, and due to its ubiquitous expression it is one of the most widely investigated miRs [89]. Evidence for an important role of miR-21 in fibrosis comes from studies on heart, lung and kidney [68-70]. Thum and colleagues [69] described miR-21 up-regulation in three mouse models of heart failure: β₁-adrenergic receptor transgenic mice, C57BL/6 mice undergone transverse aortic constriction, and C57BL/6 mice treated with isopropanol. In different cell types isolated from failing hearts, miR-21 was predominantly up-regulated in cardiac fibroblasts, and mediated protection from apoptosis. Anti-miR-21 treatment and extracellular signal-regulated kinase/mitogen activated protein kinase (ERK/MAPK) blockade induced apoptosis to a similar extent, suggesting a possible regulation of ERK/MAPK signaling by miR-21. Accordingly, overexpression of miR-21 in both mouse and rat fibroblasts led to increased expression and activation of ERK/MAPK proteins. The authors identified sprouty1, a negative regulator of ERK/MAPK, as the direct target of miR-21 mediating the effects on this pathway. Nevertheless, more recently, Patrick et al. [90] demonstrated in mir21^-/- mice, which were exposed to different stimuli resulting in cardiac overload, that miR-21 contributes to, but is not essential for ERK/MAPK activation in stress-dependent cardiac remodeling.

Liu et al. [68] confirmed the pro-fibrotic activity of miR-21 in human IPF and murine bleomycin-induced lung fibrosis. Like in heart failure, miR-21 up-regulation was proportional to the severity of lung fibrosis in the animal model. TGF-β1 expression showed the same trend. Therefore, the authors elucidated the causal relation between miR-21 and TGF-β1, showing that miR-21 expression was reduced in TGFβRII dominant negative mice injected with bleomycin. Consistently, TGF-β1 treatment of cultured pulmonary fibroblasts induced miR-21 up-regulation and, in turn, miR-21 overexpression induced TGF-β1 expression, Smad2 activation, and ECM synthesis. These effects were mediated by direct repression of Smad7, the inhibitory Smad which counter-regulates Smad2/3 activation. Finally, administration of miR-21 antisense oligonucleotides in vivo attenuated bleomycin-induced lung fibrosis.

Studies on the role of miR-21 in renal fibrosis further elucidated the interaction of this miRNA with TGF-β/Smad signaling [70]. In these experiments, TGF-β-mediated upregulation of miR-21 in tubular epithelial cells was prevented by knockdown of Smad3, but not by knockdown of Smad2. Consistently, Smad3 deficient mice, but not Smad2 deficient mice, exposed to kidney injury by unilateral urethral obstruction were protected from miR-21 up-regulation and fibrosis.

miR-216, miR-217 and miR-192. miR-216 and miR-217 are two clustered miRNAs encoded in the second intron of the RP23 gene. They showed pro-fibrotic effects in murine mesangial cells (MCs) [71]. Kato et al. reported upregulation of miR-216/217 in mouse MCs following TGF-β treatment, with consequent amplification of TGF-β signaling. miR-216 and miR-217 directly target phosphatase and tensin homolog (PTEN), which inhibits the phosphoinositide-3-kinase pathway (PI3K), thereby preventing activation of Akt. Thus, down-regulation of PTEN de-represses Akt signaling in MCs leading to prolonged cell survival, hypertrophy and increased collagen synthesis. In addition, since the promoter region of the RP23 gene contains multiple Smad binding elements and E-boxes, the authors could demonstrate that the expression of miR-216 and miR-217 in MCs was induced by TGF-β and zinc finger E-box-binding homebox (Zeb) 1 and 2. In turn, Zeb 1 and 2 are regulated by miR-192 [62].

MicroRNA REGULATION OF ECM PROTEINS

miR-29. miR-29 is the best characterized direct regulator of ECM synthesis. Its involvement in fibrosis as an anti-fibrotic miRNA has been reported in SSc as well as in other major fibrotic disorders. The miR-29 family includes three members: miR-29a/b/c. We could show that all three miRNAs are down-regulated in human SSc dermal fibroblasts and SSc skin [47]. miR-29a showed the strongest down-regulation in tissues and cells, and the most consistent functional effects on the expression of collagen type I and type III. In addition, COL3A1 was validated as a direct target of miR-29a. We also demonstrated that PDGF-B and TGF-β down-regulate miR-29a, and that down-regulation of miR-29a leads to further upregulation of PDGF-B and TGF-β. This creates a positive feed-back loop among profibrotic mechanisms leading to the uncontrolled release of extracellular matrix proteins in SSc. Results were confirmed in a bleomycin-induced mouse model of skin fibrosis, in which the tyrosine-kinase inhibitor imatinib mesylate was able to restore miR-29a levels.

In murine and human hearts, down-regulation of the miR-29 family following myocardial infarction has been reported [74]. Van Rooji et al. showed that cardiac fibroblasts, but not cardiomyocytes, are responsible for the effects of miR-29 on ECM synthesis in myocardial infarction. They hypothesized that miR-29 down-regulation is mediated by TGF-β following cardiac injury, and that this axis is controlled by the parallel induction of brain natriuretic peptide (BNP), which causes repression of TGF-β. COL1A1, COL1A2, COL3A1, FBN1, and ELN1 genes were validated as direct miR-29 targets. In addition, miR-29b knockdown with cholesterol-conjugated antagomirs in C57BL/6 male mice induced COL1A1, COL1A2, and COL3A1 mRNA overexpression in cardiac tissue.

A clear anti-fibrotic function of the miR-29 family was also shown in hypertensive kidney disease [76]. Liu et al. found that miR-29b was overexpressed in a mouse model, which is protected against salt-induced hypertensive renal medullary fibrosis (SS-13^BN) after high-salt diet. Silencing of miR-29b in the kidney of these animals caused up-regulation of a large number of ECM and ECM-modulating genes, subsequently validated as direct targets of miR-29b by reporter gene assay (COL1A1, COL3A1, COL4A1, COL5A1, COL5A2, COL5A3, COL7A1, COL8A1, MMP2, and ITGB1). As reported by Du et al. [77], miR-29a down-regulation can also be induced in vitro in human proximal tubular epithelial cells (HK-2 cells) by high glucose and TGF-β stimulation, with similar effects on the expression of multiple collagen genes, suggesting a role for miR-29 also in diabetic nephropathy.

Regarding lung fibrosis, down-regulation of miR-29 has been reported by Cushing et al. [75] in bleomycin-treated mice, which was paralleled by an increase in COL3A1 and COL4A1 expression. In addition, the same authors investigated the effects of silencing of miR-29 family members in human fetal lung fibroblasts. They identified new direct targets, among them both structural ECM genes and enzymes involved in tissue remodeling, namely NID1, ITGA11, ADAMTS9, and ADAMTS12. In lungs as well as in the skin, heart and kidney, TGF-β was able to reduce miR-29 levels.

Lastly, miR-29 down-regulation (miR-29c) has been reported in a mouse model of non-alcoholic steatohepatitis [59], although functional investigations of miR-29 in the liver are still lacking.

Other miRNAs. Additional miRNAs have been shown to regulate ECM synthesis in the kidney: miR-377 and miR-449. Wang et al. [81] showed pro-fibrotic effects of miR-377 in human mesangial cells following high glucose and TGF-β administration and in animal models of type I diabetes. These effects were mediated by an indirect upregulation of fibronectin via targeting the fibronectin inhibitors p21-activated kinase and superoxide dismutase. Muth et al. [83] demonstrated that the expression of miR-449a/b was reduced in human fibroblasts cultured under hypoxic conditions and in human kidney transplants displaying chronic allograft nephropathy. miR-449 down-regulation induced Serpine-1 (PAI-1) expression. Serpine-1 inhibits tissue-type (tPA) and urokinase-type plasminogen activator (uPA), which mediate proteolysis of ECM proteins and activation of latent metalloproteinases (MMPs). Thus, the authors hypothesized that miR-449 down-regulation promotes ECM accumulation in hypoxic fibroblasts and chronic post-transplant allograft remodeling in the kidney.

CONCLUSIONS AND PERSPECTIVES

Our review outlines the potentially important role of miRNAs in fibrosis. With some exceptions, most of the miRNAs reported in this review are ubiquitously expressed, indicating that the majority of pro- or anti-fibrotic miRNAs might be common regulators of fibrosis in different organs and disease settings. In addition, several key pro-fibrotic molecules are regulated by multiple miRNAs [51-54, 60, 61, 68-71].

The partially redundant interaction between miRNAs and their targets might hamper the use of miRNA mimics or inhibitors as potential therapeutic tools. However, data on the use of antagomirs in animal models show promising results [68, 74, 90]. It is important for the development of targeted therapies that miRNAs exert rather mild to moderate effects on single molecules, but that these effects are additive when multiple molecules of a certain pathophysiological pathway are regulated. Therefore, miRNAs are of particular interest for targeted therapies in fibrosis, when they regulate multiple rate limiting molecules of profibrotic pathways. In addition, since differentially expressed miRNAs are often disease specific and do not play major roles under normal conditions in adult tissue or in quiescent cells, targeting of an aberrantly expressed miRNA is expected to be well tolerated [91]. Indeed, many miRNA knockout animal models do not display a specific phenotype unless they are challenged with a stimulus causing cellular stress in target organs [69]. However, the miRNA machinery in fibrosis is still far from being fully elucidated, although research in this field in the last years has been considerably fostered.

ACKNOWLEDGEMENT

Supported by a grant from the EULAR ODP program.

CONFLICT OF INTEREST

O. Distler had consultancy relationship and/or has received research funding from Actelion, Pfizer, Ergonex, BMS, Sanofi-Aventis, United BioSource Corporation, medac, Biovitrium, Boehringer Ingelheim Pharma, Novartis, 4 D Science, Bayer and Active Biotec in the area of potential treatments of scleroderma and its complications, he is holding a patent on miR-29 for the treatment of scleroderma.

REFERENCES

1. Wynn TA. Cellular and molecular mechanisms of fibrosis. J Pathol . 2007;214:199–210. [PMC free article] [PubMed]

2. Wynn TA. Common and unique mechanisms regulate fibrosis in various fibroproliferative diseases. J Clin Invest. 2007;117:524–9. [PMC free article] [PubMed]

3. Zimmer V, Lammert F. Genetics in liver disease: new concepts. Curr Opin Gastroenterol. 2011;27:231–9. [PubMed]

4. Romano E, Manetti M, Guiducci S, Ceccarelli C, Allanore Y, Matucci CM. The genetics of systemic sclerosis: an update. Clin Exp Rheumatol. 2011;29:S75–86. [PubMed]

5. Tomasek JJ, Gabbiani G, Hinz B, Chaponnier C, Brown RA. Myofibroblasts and mechano-regulation of connective tissue remodeling. Nat Rev Mol Cell Biol. 2002;3:349–63. [PubMed]

6. Quan TE, Cowper SE, Bucala R. The role of circulating fibrocytes in fibrosis. Curr Rheumatol Rep. 2006;8:145–50. [PubMed]

7. Kalluri R, Neilson EG. Epithelial-mesenchymal transition and its implications for fibrosis. J Clin Invest. 2003;112:1776–84. [PMC free article] [PubMed]

8. Mehal WZ, Iredale J, Friedman SL. Scraping fibrosis. Nat Med . 2011;17:552–3. [PMC free article] [PubMed]

9. Chau BN, Brenner DA. What goes up must come down: the emerging role of microRNA in fibrosis. Hepatology. 2011;53:4–6. [PubMed]

10. Liu N, Olson EN. MicroRNA regulatory networks in cardiovascular development. Dev Cell. 2010;18:510–25. [PMC free article] [PubMed]

11. Kloosterman WP, Plasterk RH. The diverse functions of microRNAs in animal development and disease. Dev Cell. 2006; 11:441–50. [PubMed]

12. van Rooji E, Olson EN. Searching for MIR-acles in cardiac fibrosis. Circ Res. 2009;104:138–40. [PMC free article] [PubMed]

13. Jiang X, Tsitsiou E, Herrick SE, Lindsay M. MicroRNAs and the regulation of fibrosis. FEBS J. 2010;277:2015–21. [PMC free article] [PubMed]

14. Pandit KV, Milosevic J, Kaminski N. MicroRNAs in idiopathic pulmonary fibrosis. Transl Res. 2011;157:191–9. [PubMed]

15. Lorenzen JM, Haller H, Thum T. MicroRNAs as mediators and therapeutic targets in chronic kidney disease. Nat Rev Nephrol . 2011;7:286–94. [PubMed]

16. Wei J, Bhattacharyya S, Tourtellotte WG, Varga J. Fibrosis in systemic sclerosis: emerging concepts and implications for targeted therapy. Autoimmun Rev. 2011;10:267–75. [PubMed]

17. Pinzani M, Rosselli M, Zuckermann M. Liver cirrhosis. Best Pract Res Clin Gastroenterol. 2011;25:281–90. [PubMed]

18. Krenning G, Zeisberg EM, Kalluri R. The origin of fibroblasts and mechanisms of cardiac fibrosis. J Cell Physiol. 2010;225:631–7. [PMC free article] [PubMed]

19. Creemers EE, Pinto YM. Molecular mechanisms that control interstitial fibrosis in the pressure-overloaded heart. Cardiovasc Res . 2011;89:265–82. [PubMed]

20. Dussaule JC, Guerrot D, Huby AC, et al. The role of cell plasticity in progression and reversal of renal fibrosis. Int J Exp Pathol. 2011; 92:151–7. [PubMed]

21. Wynn TA. Integrating mechanisms of pulmonary fibrosis. J Exp Med. 2011;208:1339–50. [PMC free article] [PubMed]

22. Bartel DP. MicroRNAs: genomics, biogenesis, mechanism, and function. Cell. 2004;116:281–97. [PubMed]

23. Kozomara A, Griffiths JS. miRBase: integrating microRNA annotation and deep-sequencing data. Nucleic Acids Res. 2011;39: D152–7. [PMC free article] [PubMed]

24. Ambros V. The functions of animal microRNAs. Nature. 2004;431: 350–5. [PubMed]

25. Filipowicz W, Bhattacharyya SN, Sonenberg N. Mechanisms of post-transcriptional regulation by microRNAs: are the answers in sight? Nat Rev Genet. 2008;9:102–14. [PubMed]

26. Vasudevan S, Tong Y, Steitz JA. Switching from repression to activation: microRNAs can up-regulate translation. Science. 2007; 318:1931–4. [PubMed]

27. Bartel DP. MicroRNAs: target recognition and regulatory functions. Cell. 2009;136:215–33. [PubMed]

28. Inui M, Martello G, Piccolo S. MicroRNA control of signal transduction. Nat Rev Mol Cell Biol. 2010;11:252–63. [PubMed]

29. Megraw M, Sethupathy P, Corda B, Hatzigeorgiou AG. miRGen: a database for the study of animal microRNA genomic organization and function. Nucleic Acids Res. 2007;35:D149–55. [PubMed]

30. Chen CY, Chen ST, Fuh CS, Juan HF, Huang HC. Coregulation of transcription factors and microRNAs in human transcriptional regulatory network. BMC Bioinformatics. 2011;12:S41. [PMC free article] [PubMed]

31. Vrba L, Garbe JC, Stampfer MR, Futscher BW. Epigenetic regulation of normal human mammary cell type specific miRNAs. Genome Res. 2011;21:2026–37. [PubMed]

32. Lee S, Jung JW, Park SB, et al. Histone deacetylase regulates high mobility group A2-targeting microRNAs in human cord blood-derived multipotent stem cell aging. Cell Mol Life Sci. 2011;68: 325–36. [PMC free article] [PubMed]

33. Davis BN, Hilyard AC, Nguyen PH, Lagna G, Hata A. Smad protein bind a conserved RNA sequence to promote microRNA maturation by Drosha. Mol Cell. 2010;39:373–84. [PMC free article] [PubMed]

34. Boominathan L. The tumor suppressors p53, p63, and p73 are regulators of microRNA processing complex. PLoS One. 2010;5: e10615. [PMC free article] [PubMed]

35. Poliseno L, Salmena L, Zhang J, Carver B, Haveman WJ, Pandolfi PP. A coding-independent function of gene and pseudogene mRNAs regulates tumor biology. Nature. 2010;465:1033–40. [PMC free article] [PubMed]

36. Valadi H, Ekström K, Bossios A, Sjöstrand M, Lee JJ, Lötval JO. Exosome-mediated transfer of mRNAs and microRNAs is a novel mechanism of genetic exchange between cells. Nat Cell Biol. 2007; 9:654–9. [PubMed]

37. Varga J, Denton CP. Scleroderma, inflammatory myopathies, and overlap syndromes. In: Firestein GS, Budd RC, Harris ED Jr, McInnes IB, Ruddy S, Sergent JS, editors. Kelley’s Textbook of Rheumatology. 8th. USA: Saunders Elsevier; 2009. pp. 1311–51.

38. Markiewicz M, Smith EA, Rubinchik S, Dong JY, Trojanowska M, LeRoy EC. The 72-kilodalton IE-1 protein of human cytomegalovirus (HCMV) is a potent inducer of connective tissue growth factor (CTGF) in human dermal fibroblasts. Clin Exp Rheumatol. 2004;22:S31–4. [PubMed]

39. Hasegawa M, Fujimoto M, Takehara K, Sato S. Pathogenesis of systemic sclerosis: altered B cell function is the key linking systemic autoimmunity and tissue fibrosis. J Dermatol Sci. 2005; 39:1–7. [PubMed]

40. Garabrant DH, Lacey JV, Jr, Laing TJ, et al. Scleroderma and solvent exposure among women. Am J Epidemiol. 2003;157:493–500. [PubMed]

41. Varga J, Haustein UF, Creech RH, Dwyer JP, Jimenez SA. Exaggerated radiation-induced fibrosis in patients with systemic sclerosis. JAMA. 1991;265:3292–5. [PubMed]

42. Ishida W, Mori Y, Lakos J, et al. Intracellular TGF-β blockade abrogates Smad-dependent fibroblast activation in vitro and in vivo. J Invest Dermatol. 2006;126:1733–44. [PubMed]

43. Pannu J, Gore HE, Yamanaka M, et al. An increased transforming growth factor β receptor type I: type II ratio contributes to elevated collagen protein synthesis that is resistant to inhibition via a kinase-deficient transforming growth factor β receptor type II in scleroderma. Arthritis Rheum. 2004;50:1566–77. [PubMed]

44. Svegliati S, Cancello R, Sambo P, et al. Platelet-derived growth factor and reactive oxygen species (ROS) regulate RAS protein levels in primary human fibroblasts via ERK1/2. Amplification of ROS and RAS in systemic sclerosis fibroblasts. J Biol Chem. 2005; 280:36474–82. [PubMed]

45. Bergmann C, Akhmetshina A, Dees C, et al. Inhibition of glycogen synthase kinase3{beta} induces dermal fibrosis by activation of the canonical Wnt pathway. Ann Rheum Dis. 2011;70:2191–8. [PubMed]

46. Dees C, Akhmetshina A, Zerr P, et al. Platelet-derived serotonin links vascular disease and tissue fibrosis. J Exp Med. 2011;208: 961–72. [PMC free article] [PubMed]

47. Maurer B, Stanczyck J, Jungel A, et al. MicroRNA-29, a key regulator of collagen expression in systemic sclerosis. Arthritis Rheum. 2010;62:1733–43. [PubMed]

48. Pandit KV, Corcoran D, Yousef H, et al. Inhibition and role of let-7d in idiopathic pulmonary fibrosis. Am J Respir Crit Care Med . 2010;182:220–9. [PMC free article] [PubMed]

49. Scian MJ, Maluf DG, David KG, et al. MicroRNA profiles in allograft tissues and paired urine associate with chronic allograft dysfunction with IF/TA. Am J Transplant. 2011;11:2110–22. [PMC free article] [PubMed]

50. Katare R, Riu F, Mitchell K, et al. Transplantation of human pericyte progenitor cells improves the repair of infarcted heart through activation of an angiogenic program involving micro-RNA-132. Circ Res. 2011;109:894–906. [PubMed]

51. Mann J, Chu DC, Maxwell A, et al. MeCP2 controls an epigenetic pathway that promotes myofibroblasts transdifferentiation and fibrosis. Gastroenterology. 2010;138:705–14. [PMC free article] [PubMed]

52. Duisters RF, Tijsen AJ, Schroen B, et al. miR-133 and miR-30 regulate connective tissue growth factor: implications for a role of microRNAs in myocardial matrix remodeling. Circ Res. 2009;104: 170–8. [PubMed]

53. Shan H, Zhang Y, Lu Y, et al. Down-regulation of miR-133 and miR-590 contributes to atrial remodelling in canines. Cardiovasc Res. 2009;83:465–72. [PubMed]

54. Wang B, Koh P, Winbanks C, et al. miR-200a prevents renal fibrogenesis through repression of TGF-β2 expression. Diabetes . 2011;60:280–7. [PMC free article] [PubMed]

55. Guo CJ, Pan Q, Li DG, Sun H, Liu BW. miR-15b and miR-16 are implicated in activation of the rat hepatic stellate cell: an essential role for apoptosis. J Hepatol. 2009;50:766–78. [PubMed]

56. Ezzie ME, Crawford M, Choo JH, et al. Gene expression network in COPD: microRNA and mRNA regulation. Thorax. 2012;67: 122–31. [PubMed]

57. Venugopal SK, Jiang J, Kim TH, et al. Liver fibrosis causes down-regulation of miR-150 and miR-194 in hepatic stellate cells, and their overexpression causes decreased stellate cell activation. Am J Physiol Gastrointest Liver Physiol. 2010;298:G101–6. [PubMed]

58. Pottier N, Maurin T, Chevalier B, et al. Indentification of keratinocyte growth factor as a target of miR-155 in lung fibroblasts: implication in epithelial-mesenchymal interactions. PLoS One. 2009;4:e6718. [PMC free article] [PubMed]

59. Pogribny IP, Starlard DA, Tryndyak VP, et al. Difference in expression of hepatic miR-29c, miR-34a, miR-155, and miR-200b is associated with strain-specific susceptibility to dietary nonalcoholic steatohepatitis in mice. Lab Invest. 2010;90:1437–46. [PubMed]

60. Kodama T, Takehara T, Hikita H, et al. Increases in p53 expression induce CTGF synthesis by mouse and human hepatocytes and result in liver fibrosis in mice. J Clin Invest. 2011;121:3343–56. [PMC free article] [PubMed]

61. van Almen GC, Verhesen W, van Leeuwen RE, et al. MicroRNA- 18 and microRNA-19 regulate CTGF and TSP-1 expression in age-related heart failure. Aging Cell. 2011;10:769–79. [PMC free article] [PubMed]

62. Kato M, Zhang J, Wang M, et al. MicroRNA-192 in diabetic kidney glomeruli and its function in TGF-beta-induced collagen expression via inhibition of E-box repressors. Proc Natl Acad Sci USA. 2007;104:3432–7. [PubMed]

63. Wang B, Herman EM, Koh P. E-cadherin expression is regulated by miR-192/215 by a mechanism that is independent of the profibrotic effects of transforming growth factor-beta. Diabetes . 2010;59:1794–802. [PMC free article] [PubMed]

64. da Costa Martins PA, Salic K, et al. MicroRNA-199b targets the nuclear kinase Dyrk1a in an auto amplification-loop promoting calcineurin-NFAT signaling. Nat Cell Biol. 2010;12:1220–7. [PubMed]

65. Murakami Y, Toyoda H, Tanaka M, et al. The progression of liver fibrosis is related to overexpression of miR-199 and miR-200 families. PLoS One. 2011;6:e16081. [PMC free article] [PubMed]

66. Oba S, Kumano S, Suzuki E, et al. miR-200b precursor can ameliorate renal tubulointerstitial fibrosis. PLoS One. 2010;5: e13614. [PMC free article] [PubMed]

67. van Rooji E, Sutherland LB, Qi X, Richardson JA, Hill J, Olson EN. Control of stress-dependent cardiac growth and gene expression by a microRNA. Science. 2007;316:575–9. [PubMed]

68. Liu G, Friggeri A, Yang Y, et al. miR-21 mediates fibrogenic activation of pulmonary fibroblasts and lung fibrosis. J Exp Med . 2010;207:1589–97. [PMC free article] [PubMed]

69. Thum T, Gross C, Fiedler J, et al. MIcroRNA-21 contributes to myocardial disease by stimulating MAP kinase signalling in fibroblasts. Nature. 2008;456:980–4. [PubMed]

70. Zhong X, Chung AC, Chen XY, Meng XM, Lan HY. Smad3-mediated up-regulation of miR-21 promotes renal fibrosis. J Am Soc Nephrol. 2011;22:1668–81. [PubMed]

71. Kato M, Putta S, Wang M, et al. TGF-beta activates Akt kinase through a microRNA-dependent amplifying circuit targeting PTEN. Nat Cell Biol. 2009;11:881–9. [PMC free article] [PubMed]

72. Cho JH, Gelinas R, Wang K, et al. Systems biology of interstitial lung disease: mRNA and microRNA expression changes. BMC Med Genomics. 2011;4:8. [PMC free article] [PubMed]

73. Ji J, Zhang J, Huang G, Qian J, Wang X, Mei S. Over-expressed miRNA-27a and 27b influence fat accumulation and cell proliferation during rat hepatic stellate cell activation. FEBS Lett . 2009;583:759–66. [PubMed]

74. van Rooji E, Sutherland LB, Thatcher JE, et al. Dysregulation of microRNAs after myocardial infarction reveals a role of mIR-29 in cardiac fibrosis. Proc Natl Acad Sci USA. 2008;105:13027–32. [PubMed]

75. Cushing L, Kuang PP, Qian J, et al. miR-29 is a major regulator of genes associated with pulmonary fibrosis. Am J Respir Cell Mol Biol. 2011;45:287–94. [PMC free article] [PubMed]

76. Liu Y, Taylor NE, Lu L, et al. Renal medullary microRNAs in Dahl salt-sensitive rats: miR-29b regulates several collagens and related genes. Hypertension. 2010;55:974–82. [PMC free article] [PubMed]

77. Du B. High glucose down-regulates miR-29a to increase collagen IV production in HK-2 cells. FEBS Lett. 2010;584:811–6. [PubMed]

78. Chen C, Wu CQ, Zhang ZQ, Yao DK, Zhu L. Loss of expression of miR-335 is implicated in hepatic stellate cell migration and activation. Exp Cell Res. 2011;317:1714–25. [PubMed]

79. Zhang H, Liu X, Chen S, et al. Tectorigenin inhibits the in vitro proliferation and enhances miR-338* expression of pulmonary fibroblasts in rats with idiopathic pulmonary fibrosis. J Ethnopharmacol. 2010;131:165–73. [PubMed]

80. Li WQ, Chen C, Xu MD, et al. The rno-miR-34 family is upregulated and targets ACSL1 in dimethylnitrosamine-induced hepatic fibrosis in rats. FEBS J. 2011;278:1522–32. [PubMed]

81. Wang Q, Wang Y, Minto AW, Wang J, Shi Q, Li X. MicroRNA-377 is up-regulated and can lead to increased fibronectin production in diabetic nephropathy. FASEB J. 2008;22:4126–35. [PubMed]

82. Kriegel AJ, Fang Y, Liu Y, et al. MicroRNA-target pairs in human renal epithelial cells treated with transforming growth factor β1: a novel role of miR-382. Nucleic Acids Res. 2010;38:8338–47. [PMC free article] [PubMed]

83. Muth M, Theophile K, Hussein K, Jacobi C, Kreipe H, Bock O. Hypoxia-induced down-regulation of microRNA-449a/b impairs control over targeted SERPINE-1 (PAI-1) mRNA – a mechanism involved in SERPINE-1 (PAI-1) overexpression. J Transl Med . 2010;8:33. [PMC free article] [PubMed]

84. Nicolas FE. Experimental validation of microRNA targets using a luciferase reporter system. Methods Mol Biol. 2011;732:139–52. [PubMed]

85. Santibanez JF, Quintanilla M, Bernabeu C. TGF-β/ TGF-β receptor system and its role in physiological and pathological conditions. Clin Sci. 2011;121:233–51. [PubMed]

86. Leask A, Abraham DJ. All in the CCN family: essential matricellular signaling modulators emerge from the bunker. J Cell Sci. 2006;119:4803–10. [PubMed]

87. Dews M, Homayouni A, Yu D, et al. Augmentation of tumor angiogenesis by a Myc-activated microRNA cluster. Nat Gen. 2006; 38:1060–5. [PMC free article] [PubMed]

88. Gregory PA, Bert AG, Paterson EL, et al. The miR-200 family and miR-205 regulate epithelial to mesenchymal transition by targeting ZEB1 and SIP1. Nat Cell Biol. 2008;10:593–601. [PubMed]

89. Kumarswamy R, Volkmann I, Thum T. Regulation and function of miR-21 in health and disease. RNA Biol. 2011;8:1–8. [PMC free article] [PubMed]

90. Patrick DM, Montgomery RL, Qi X, Obad S, Kauppinen S, Hill JA. Stress-dependent cardiac remodeling occurs in the absence of microRNA-21 in mice. J Clin Invest. 2010;120:3912–6. [PMC free article] [PubMed]

91. Broderick JA, Zamore D. MicroRNA therapeutics. Gene Ther. 2011;18:1104–10. [PMC free article] [PubMed]

Articles from The Open Rheumatology Journal are provided here courtesy of
Bentham Science Publishers