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# Role of microRNAs in the immune system, inflammation and cancer

Jennifer Raisch, Arlette Darfeuille-Michaud, - Hang Thi Thu Nguyen

► To cite this version:

Jennifer Raisch, Arlette Darfeuille-Michaud, - Hang Thi Thu Nguyen. Role of microRNAs in the immune system, inflammation and cancer. World Journal of Gastroenterology, 2013, 19 (20), pp.2985 - 2996. 10.3748/wjg.v19.i20.2985. hal-02650775

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REVIEW

# Role of microRNAs in the immune system, inflammation and cancer

Jennifer Raisch, Arlette Darfeuille-Michaud, Hang Thi Thu Nguyen

Jennifer Raisch, Arlette Darfeuille-Michaud, Hang Thi Thu Nguyen, M2iSH, UMR 1071 Inserm, University of Auvergne, 63000 Clermont-Ferrand, France

Jennifer Raisch, Arlette Darfeuille-Michaud, Hang Thi Thu Nguyen, INRA USC 2018, 63000 Clermont-Ferrand, France
Arlette Darfeuille-Michaud, Centre Hospitalier Universitaire, 63000 Clermont-Ferrand, France

**Author contributions:** Raisch J and Nguyen HTT wrote the paper; Darfeuille-Michaud A supervised the manuscript.

Supported by The Ministère de la Recherche et de la Technologie (JE2526, France), Inserm and University of Auvergne (UMR1071), INRA (USC-2018); Grants from the Association François Aupetit (AFA to Arlette Darfeuille-Michaud), and the European Union FP7 People Marie Curie International Incoming Fellowship (IIF to Hang Nguyen)

Correspondence to: Hang Thi Thu Nguyen, PhD, Assistant Professor, M2iSH, UMR 1071 Inserm, University of Auvergne, 28 place Henri Dunant, 63000 Clermont-Ferrand, France. hang.nguyen@udamail.fr

Telephone: +33-4-73178372 Fax: +33-4-73178371

Received: January 24, 2013 Revised: March 29, 2013

Accepted: April 10, 2013

Published online: May 28, 2013

## Abstract

MicroRNAs, a key class of gene expression regulators, have emerged as crucial players in various biological processes such as cellular proliferation and differentiation, development and apoptosis. In addition, microRNAs are coming to light as crucial regulators of innate and adaptive immune responses, and their abnormal expression and/or function in the immune system have been linked to multiple human diseases including inflammatory disorders, such as inflammatory bowel disease, and cancers. In this review, we discuss our current understanding of microRNAs with a focus on their role and mode of action in regulating the immune system during inflammation and carcinogenesis.

© 2013 Baishideng. All rights reserved.

**Key words:** MicroRNAs; Immune response; Inflammation; Inflammatory bowel disease; Colorectal cancer

**Core tip:** MicroRNAs (miRNAs), a key class of gene expression regulators, have emerged as crucial players in various biological processes such as cellular proliferation and differentiation, development and apoptosis. A better understanding of the function of miRNAs is providing new insights into the molecular basis of human pathologies, and new biomarkers for disease diagnosis and therapy.

Raisch J, Darfeuille-Michaud A, Nguyen HTT. Role of microRNAs in the immune system, inflammation and cancer. *World J Gastroenterol* 2013; 19(20): 2985-2996 Available from: URL: http://www.wjgnet.com/1007-9327/full/v19/i20/2985.htm DOI: http://dx.doi.org/10.3748/wjg.v19.i20.2985

## INTRODUCTION

MicroRNAs (miRNAs, miR) are small (approximately 20-22 nucleotides), non-coding RNAs that post-transcriptionally regulate gene expression by binding to the 3'-untranslated region of target mRNAs, leading to mRNA degradation or translational inhibition[1].

Since the identification of the first miRNA, *lin-4*, in *Caenorabditis elegans* in 1993[2,3], thousands of miRNA genes have been identified in animal and plant genomes[4]. As a class, miRNAs account for about 1%-2% of genes in worms, flies, and mammals[5]. Each miRNA can target hundreds of mRNAs within a given cell type, and a single mRNA is often the target of multiple miRNAs, and thus over half of the human transcriptome is predicted to be under miRNA regulation, embedding this post-transcriptional control pathway within nearly every biological process[5].

Given its fundamental biological roles, it is not sur-
prising that miRNA expression is tightly controlled and
that its deregulation can lead to various diseases. In this
review, we summarize our current knowledge about the
physiological role of miRNAs in mammalian biology and
the manner in which miRNA activities contribute to dis-
eases including inflammatory disorders and cancer.

MIRNA BIOSYNTHESIS AND
REGULATION

Biosynthesis

Our knowledge of miRNA biogenesis and regulation has
been greatly expanded in recent years[1]. The canonical
miRNA biogenesis takes place in a multi-step process
and involves two RNAse III endonucleases, Dicer and
Drosha. MiRNAs are encoded by genomic DNA and
are most commonly transcribed by RNA polymerase II,
which generates a primary miRNA (pri-miRNA) tran-
script. Within the primary transcripts, miRNAs form
stem-loop structures, which contain the mature miRNA
as part of an imperfectly paired double-stranded stem
connected by a short terminal loop. Pri-miRNAs are then
processed by a microprocessor complex, a multiprotein
complex with the two core components, Drosha and Di
George Syndrome critical region 8 (DGCR8)[6-8]. This re-
sults in the formation of a hairpin-shaped RNA molecule
of 70-100 bp called miRNA precursor or pre-miRNA,
which is then exported into the cytoplasm in a process
involving the nucleocytoplasmic shuttle Exportin-5 and
in a Ran-GTP-dependent manner[9-11]. In cytoplasm,
the pre-miRNA hairpin is cleaved by the endonuclease
DICER into an imperfect miRNA:miRNA* duplex of
21-23 nucleotides in length[12]. After separation of the
two strands of the duplex, one of the strands (the mature
miRNA) is transferred into an Argonote (Ago) protein
located in the RNA-induced silencing complex (RISC
or miRISC), which is involved in the repression of gene
expression by leading miRNAs to specific target mRNAs,
whereas the other strand (the star-strand) is degraded. It
has been shown that strand selection and RISC assembly
in mammals are accomplished by a complex that contains
Dicer, Ago and the double-stranded RNA binding pro-
tein TRBP[13-15]. MiRNAs target mRNAs by interacting
with sites of imperfect complementarity. Short “seed”
sequences at the 5’-ends of miRNAs (nucleotides 2-8) are
critical, and in some cases fully sufficient, for target selec-
tion[16,17].

Regulation

Although there have been recent advances in our knowl-
edge of the biogenesis of the miRNA pathway, relatively
little is known about the mechanisms regulating the activ-
ity of the pathway's components. Several recent studies
indicate that the regulation of miRNA expression and
function occurs at three levels: transcription, processing
and subcellular localization[¹⁷,¹⁸].

The first, and one of the most important, mechanisms
controlling miRNA abundance is the regulation of pri-

miRNA transcription, which could be positively or nega-
tively regulated by different factors such as transcription
factors, enhancers, silencers and epigenetic modification
in miRNA promoters[¹⁶]. For example, the oncogene
c-myc can bind to the promoter of the miR-17-5p cluster,
thereby up-regulating expression of the miRNAs encoded
by the cluster[¹⁹,²⁰]. Similarly, the tumour suppressor p53
has been shown to upregulate the transcription of miR-34
family members, inhibiting important factors of cell pro-
liferation and survival, such as Bcl2 and Cdk4 and 6[²¹⁻²⁴].
A region of miRNA genes is located within CpG islands
involving the epigenetic control of miRNA transcription.
It is estimated from recent works that 5%-10% of mam-
malian miRNAs are epigenetically regulated[¹⁹,²⁵⁻²⁷].

Several post-transcriptional regulatory mechanisms
that affect miRNA processing at different stages, from
the pri-miRNA transcripts to the delivery of mature miR-
NAs to their target mRNAs, have recently been investi-
gated[¹⁸]. For example, p53 can form a complex with Dro-
sha, which increases the processing of pri-miRNAs to
pre-miRNAs[²⁸]. Histone deacetylase I can enhance pri-
miRNA processing by deacetylating the protein DGCR8
of the microprocessor complex[²⁹]. Cytokines such as
interferons have been shown to inhibit Dicer expression,
decreasing the processing of pre-miRNAs[³⁰].

MICRORNAS AND IMMUNE SYSTEM

The immune system has evolved to maintain self-tolerance and to recognize efficiently specific pathogens. The innate immune system acts as a first protector providing an immediate response to pathogens, and propagation of the innate response activates the adaptive immune system. Both innate and adaptive immune responses are highly regulated, and recent studies have shed light on the role of miRNAs in this intricate system[³¹,³²]. The role of miRNAs in immune responses will be discussed in this section.

MicroRNAs and innate immune response

The innate immune system is activated *via* recognition of pathogen-associated molecular patterns by toll-like receptors (TLRs)[³³], which will recruit adaptor proteins to the receptor, followed by activation of downstream signalling pathways such as the nuclear factor kappa-light-chain-enhancer of activated B cells (NF-κB) pathway[³⁴]. This signal transduction ultimately leads to induction of immune gene expression.

The first study examining the effect of lipopolysac-
charide (LPS)-mediated activation of TLR signalling on
miRNA production identified miR-155, miR-146a and
miR-132, which are induced in human macrophages
by LPS[³⁵]. Further analysis showed that miR-155 is in-
duced by LPS, cytokine IFN-β and various TLR ligands
in murine macrophages[³⁶,³⁷]. MiR-155, once induced, is
involved in the activation of tumor necrosis factor-α
(TNF-α) and interleukin-6 (IL-6), enzyme linked immu-
nosorbent assay pathway *via* targeting the Fas-associated
death domain protein, I B kinase ε, and receptor (TNF receptor superfamily)-interacting serine-threonine kinase 1[37]. MiR-155 plays a role in the innate immune response by regulating suppressor of cytokine signalling (SOCS)-1, a negative regulator of dendritic cell antigen-presenting capacity[37-39]. Likewise, miR-155-deficient dendritic cells exhibit impaired antigen presentation and therefore are unable to activate T cells to promote inflammation[39]. One study demonstrated that in human myeloid-derived DCs, knockdown of miR-155 expression significantly increased protein expression of the pro-inflammatory cytokine IL-1[40]. The same study also showed that miR-155 directly inhibited expression of the pro-inflammatory signalling protein TAK1-binding protein 2 (TAB2, also known as MAP3K7IP2), which could be a mechanism underlying its anti-inflammatory property[40]. In contrast, other studies have shown that miR-155 can enhance inflammatory responses. Overexpression of miR-155 in mouse bone marrow leads to a myeloproliferative phenotype that is similar to that observed transiently after LPS stimulation[41]. MiR-155 can negatively regulate SHIP1, an important negative regulator of phosphoinositide 3-kinase (PI3K) and the downstream AKT pathway[42,43]. SHIP1, which is similar to SOCS1, is a negative regulator of TLR4 signaling[44], and hence repression of SHIP1 by miR-155 may counter this negative regulation and increase downstream AKT signalling.

Like miR-155, miR-146a is induced by LPS, TNF-α and IL-1β in a NF-κB-dependent manner. MiR-146a in turn inhibits expression of two components of the TLR4 signaling pathway, IL-1 receptor associated kinase and TNF receptor-associated factor-6[35]. Thus, miR-146a functions as a negative feedback regulator of the TLR/ NF-κB pathway. MiR-155 and miR-146 expression is increased in macrophages in response to LPS stimulation, while miR-125b expression is decreased. MiR-125b can target TNF-α mRNA, and a decrease in its expression leads to elevated TNF-α production and consequently increased inflammatory response[37].

Macrophage inflammatory response to infection involves the upregulation of several miRNAs, such as miR-21, miR-9 and miR-147[45-47]. These miRNAs can also be induced by TLR signaling, and can negatively regulate activation of inflammatory pathways in myeloid cells. MiR-9 represses NF-κB subunit 1 (NFKB1/p50 unit) and helps to maintain a constant level of NF-κB1 protein expression during TLR4-mediated activation of monocytes and neutrophils[46]. MiR-147 has been shown to attenuate TLR2, TLR3 and TLR4-mediated production of inflammatory proteins such as TNF-α and IL-6[47]. Induction of miR-21 inhibits PDCD4, an IL-10 inhibitor, thereby derepressing IL-10. IL-10 in turn inhibits miR-155, allowing SHIP1 to be derepressed and inhibit TLR signaling[45,48]. Hence, immune responses are highly regulated by TLRs-mediated upregulation of different miRNAs.

In addition to miRNA induction by TLR signaling, recent studies have also reported inflammatory repres-

sion, such as miR-155 repression, in response to anti-inflammatory cytokine IL-10[49].

### MicroRNAs and adaptive immune response

In addition to their role in regulating the innate immune system, miRNAs have been implicated in adaptive immunity by controlling the development and activation of T and B cells.

### T cells

Specific miRNA expression profiles have been reported in different T cell subsets and stages of development[50-52], suggesting that miRNA-mediated regulation of signaling networks in T cells, and probably other immune cells, is dynamic and highly regulated. Interestingly, miRNA profiling in naive, effector and memory CD8+ T cells has revealed that a few highly expressed miRNAs are dynamically regulated during antigen-specific T-cell differentiation[52]. Mice exhibiting T-cell specific deletion of Dicer had lower numbers of mature T cells with abnormally developed T-cell subsets than wild-type mice, indicating that miRNAs are required for T cell development[53,54]. Two specific miRNAs have been implicated in T cell development, and probably account for some of the phenotype of Dicer-deficient T cells. The miR-17-92 cluster suppresses expression of pro-apoptotic proteins, including BCL-2-interacting mediators of cell death (BIM or BCL2L11) and phosphatase and tensin homologue. This miRNA cluster is thought to increase T cell survival during development and is expressed during the double negative 2 stage of thymopoiesis[55].

The role of miRNAs in the differentiation of T cells into distinct effector T helper cell subsets has been recently reported. It was demonstrated that miR-326 regulates differentiation of T cell subpopulations both *in vitro* and *in vivo*[56]. MiR-155 is implicated in regulatory T (Treg) cell formation and function, since forkhead box P3 (FOXP3), a transcription factor that is required for the development and function of Treg cells, may directly regulate the expression of this miRNA[57]. Furthermore, miRNA-155-deficient mice are immunodeficient, indicating the implication of miR-155 in homeostasis and the immune system[39]. Similarly, using genetic deletion and transgenic approaches, Thai and colleagues showed the important role of miR-155 in the mammalian immune system, specifically in regulating T helper cell differentiation and the germinal center reaction to produce an optimal T cell-dependent antibody response[58]. Certain miRNAs, such as the miR-17-92 cluster, might be involved in the development and function of T follicular helper cells (specialized T cells that provide selective signals to supporting germinal center B cells), which are essential for long-lived antibody responses[59,60]. In addition, miR-181a, which is increased during early T cell development and down-regulated in mature CD4 T cells such as Th1 and Th2 effector cells, can enhance TCR signaling strength by inhibiting multiple phosphatases that negatively regulate the TCR signaling cascade[61]. Finally, conditional deletion
of *Dicer* or *Drosha* in *T*reg cells led to lethal autoimmune inflammatory disease, accompanied by impaired development or function of *T*reg cells, indicating the role of miRNAs in *T*reg cell biology[62-64].

## B cells

Distinct miRNA profiles in naive, germinal central and post-germinal central B cells have been reported[65-67], suggesting the implication of miRNAs in B cell development and maturation. A pioneer study showed that miR-181 is highly expressed in B cells of mouse bone marrow, and its ectopic expression in hematopoietic stem and progenitor cells resulted in an increase in the percentage of B-lineage cells but not in T cells or myeloid cells[68], indicating the role of lineage-specific miRNAs in regulating lymphocyte development. Conditional deletion of *Dicer* in B cells completely arrested B cell development in mice, which is thought to be due to dysregulated expression of the pro-apoptotic protein BIM, probably during the selection of effective antigen receptors[69]. Notably, B cells lacking miR-17-92 family and *Dicer*-deficient B cells exhibited similar gene expression profiles[70], suggesting that this miRNA cluster could play a determining role in the regulation of B cell development.

Recent studies have explored the role of miR-150, a miRNA specifically expressed by mature lymphocytes, in B cell differentiation[51,71,72]. MiR-150 expression increases during B-cell maturation in bone marrow, and its constitutive expression blocked B cell development at the transition from the pro-B-cell to pre-B-cell developmental stage, leading to severe defects in the production of mature B cells[71]. MiR-150-deficient mice exhibited a 2-fold increase in splenic B-1 cell numbers, with a relative decrease in those of B-2 cells, but had no apparent defect in the development of other lymphoid-derived T- and B-cell types[72]. Mice expressing a miR-150 transgene early in life also had dramatically impaired B cell development with normal T cell levels. These defects in miR-150 gain- and loss-of-function were further shown to be due to dysregulation of c-Myb, a target of miR-150 and a transcription factor that controls multiple steps of lymphocyte development[72]. MiR-155-deficient B cells showed defects in antibody class switching and differentiation into plasma cells, resulting in an impaired humoral response to T cell-dependent antigenic stimulation[39,58,73]. The constitutive expression of miR-34a blocked B cell development at the pro-B to pre-B cell transition, leading to a reduction in mature B cells[74]. This block appeared to be mediated by miR-34a-inhibited expression of the transcription factor Foxp1[74], which is an essential regulator of B cell development[75]. Together, these studies show the important role of miRNAs in normal B cell development.

## MICRORNAS AND INFLAMMATORY BOWEL DISEASE

As miRNAs play a critical role in the regulation of the immune system, failure of miRNA regulation is associ-

<table><caption>Table 1 MicroRNAs dysregulated in ulcerative colitis and/or Crohn's disease</caption><thead><tr><th></th><th>Up-regulated</th><th>Down-regulated</th><th>Source, reference</th></tr></thead><tbody><tr><td rowspan="10">UC vs healthy</td><td>miR-16, miR-21, miR-23a, miR-24, miR-29a, miR-126, miR-195, let-7f, miR-21, miR-155</td><td>miR-192, miR-375, miR-422b</td><td>Sigmoid colon[77]</td></tr><tr><td>miR-7, miR-31, miR-135b, miR-223, miR-29a, miR-29b, miR-126*, miR-127-3p, miR324-3p</td><td>miR-188-5p, miR-25, miR-320a, miR-346</td><td>Colon[78]<br>Colonic mucosa[79]</td></tr><tr><td>(miR-196a, miR-29a, miR-29b, miR-126*, miR-127-3p, miR324-3p)<sup>1</sup></td><td>(miR-188-5p, miR-25, miR-320a, miR-346)<sup>2</sup></td><td></td></tr><tr><td>miR-28-5p, miR-151-5p, miR-199a-5p, miR-340*, miRplus-E1271, miR-103-2*, miR-362-3p, miR-532-3p</td><td>miR-505*</td><td>Peripheral blood[80]</td></tr><tr><td>miR-9, miR-126, miR-130a, miR-181c, miR-375, miR-26a, miR-29b, miR-30b, miR-34c-5p, miR-126*, miR127-3p, miR-133b, miR-155, miR-196a, miR324-3p, miR-21, miR-22, miR-29c, miR-31, miR-106a,</td><td></td><td>Colonic mucosa[79]</td></tr><tr><td>CD vs healthy</td><td>miR-146a, miR146b-3p, miR-150 (miR-9*, miR-30a*, miR-30c, miR-223, miR-25a, miR-29b, miR-30b, miR-34c-5p, miR-126*, miR127-3p, miR-133b, miR-155, miR-196a, miR324-3p, miR-21, miR-22, miR-29c, miR-31, miR-106a, miR-146a, miR146b-3p, miR-150)<sup>1</sup></td><td>miR-149*, miRplus-F1065</td><td>Peripheral blood[80]</td></tr><tr><td>UC vs CD</td><td>miR-199p-5a, miR-362-3p, miR-340*, miR-532-3p, miRplus-E1271</td><td>(miR-150, miR-196b, miR-199a-3p, miR-199b-5p, miR-223, miR-320a)<sup>2</sup></td><td>Colonic mucosa[79]</td></tr><tr><td></td><td>miR-28-5p, miR-103-2*, miR-149*, miR-151-5p, miR-340*, miR-532-3p, miRplus-E1153</td><td>miR-505*</td><td>Peripheral blood[80]</td></tr></tbody></table>

ated with several human disorders such as inflammatory bowel disease (IBD) (Table 1), which is a chronic inflammatory gastrointestinal disorder. Although the etiology of IBD remains largely unknown, extensive studies in the last decades have suggested that it involves environmental and genetic factors that lead to dysfunction of the epithelial barrier with consequent deregulation of the mucosal immune system and responses to gut microbiota[76].

Distinguished miRNA expression profiles have been recently described in tissues of patients with active and inactive UC, CD, irritable bowel syndrome (IBS), infectious colitis (IC), and microscopic colitis (MC)[77]. Wu and colleagues demonstrated that active UC was associated

<footnote n="1">¹miRNAs upregulated specifically in non-inflamed colonic mucosa; ²miRNAs downregulated specifically in non-inflamed tissue colonic mucosa. UC: Ulcerative colitis; CD: Crohn's disease.</footnote>
with the differential expression of 11 miRNAs (3 signifi-
cantly decreased and 8 significantly increased in UC tis-
sues). MiR-192, the expression of which is decreased in
active UC, was predominantly localized to colonic epithe-
lial cells, and targeted macrophage inflammatory peptide
(MIP)-2α, a chemokine expressed by epithelial cells[77].
In colonic epithelial cells, TNF-α-induced MIP-2α ex-
pression was inhibited by a miR-192 mimic. In contrast,
miR-21 is significantly increased in patients with active
UC compared to healthy subjects. In inactive UC patients,
miR-375 and miR-422 expression was increased, while
that of miR-192 was unaltered compared to healthy sub-
jects[77]. Inactive UC showed similar expression levels of
miR-375, miR-422b, and miR-23a to IBS and IC tissues.
The miRNAs differently expressed in active UC were not
dysregulated in MC and CD. This study highlights the
specific miRNA expression patterns in active and inactive
IBD tissues, and suggests that miRNAs could regulate
expression of proteins implicated in the pathogenesis.

Another study showed the upregulated expression of
several miRNAs in active UC compared to healthy co-
lonic biopsies, suggesting that upregulation of miRNAs
may be responsible for the development of intestinal
inflammation in UC[78]. MiR-21 was found among the up-
regulated miRNAs, which is consistent with the findings
of Takagi et al[78].

Of interest, Fasseu and colleagues identified restricted
subsets of miRNAs abnormally expressed in inactive
colonic mucosa of IBD patients[79]. This elegant study
identified 14 (in UC) and 23 (in CD) miRNAs with
significantly altered expression (> 5-fold increase or <
0.05-fold decrease) in quiescent colonic mucosa com-
pared to healthy control tissues. Eight of the miRNAs
(miR-26a, -29a, -29b, -30c, -126*, -127-3p, -196a, -324-3p)
were commonly dysregulated in non-inflamed UC and
CD. Six miRNAs (miR-196b, -199a-3p, -199b-5p, -320a,
-150, -223) displayed significantly distinct dysregulation
of expression between non-inflamed UC and CD colonic
biopsies. Interestingly, several miRNA genes with dys-
regulated expression mapped within acknowledged IBD-
susceptibility loci. In addition, significant dysregulated
expression of four and five miRNAs specific to inflamed
UC or CD tissues, respectively, compared to healthy
controls was observed[79]. This study sheds light on the
role of miRNAs as contributors to IBD susceptibility, in
particular their implication in the onset and/or relapse of
inflammation from quiescent mucosa of IBD patients.

There have been recent reports of differential miR-
NA expression profiles in the peripheral blood of IBD
patients[80]. Four miRNAs (miR-199a5p, -362-3p, -532-3p
and miRplus-E1271) were upregulated and one miRNA
(miRplus-F1065) was downregulated in the peripheral
blood of patients with active CD, but not inactive CD,
compared to healthy controls[80]. Both active and inactive
CD patients had increased expression of miR-340 and
decreased expression of miR-149 in the blood. Expres-
sion of three miRNAs (miR-103-2, 262-3p, 532-3p) was
increased in the blood of both active and inactive UC

patients. In addition, a subset of 11 miRNAs can distin-
guish active CD from active UC[80]. This study important-
ly supports the evidence that distinct peripheral blood
miRNA profiles in different circulating immune cell types
are associated with IBD.

Efforts have been made to understand the mecha-
nisms underlying the implication of miRNAs in the
pathogenesis of IBD. The potential association between
single nucleotide polymorphisms (SNPs) in pre-miRNA
coding regions and IBD susceptibility has been analyzed.
A study in a Japanese cohort of 170 UC patients and 403
healthy controls revealed the association of three SNPs
(rs11614913, rs2910164, and rs3746444) in coding regions
of pre-miR-196a2, pre-miR-146a and pre-miR-499[81].
Of particular interest, the CD-associated SNP C313T in
immunity-related GTPase family, M (IRGM) gene caused
a loss in binding of miR-196[82] (Figure 1). IRGM plays
an important role in the immune system by its involve-
ment in the autophagy process. In addition, miR-196 is
overexpressed in the inflamed epithelium of CD patients
and downregulates the IRGM protective variant (c.313C)
but not the risk-associated allele (c.313T)[82]. Loss of tight
regulation of IRGM expression by miR-196 resulted in
defects in autophagy-mediated control of intracellular
replication of adherent-invasive Escherichia coli (AIEC),
leading to abnormal persistence of AIEC in host cells
(Figure 1). This suggests that the association of IRGM
with CD could arise from abnormal miRNA-mediated
IRGM regulation, which affects the efficacy of autopha-
gy, thereby contributing a synonymous polymorphism as
a likely causal variant.

Intestinal microbiota is increasingly recognized as a
risk for, and a causal factor of, IBD. Our recent studies
showed that miRNAs are involved in the regulation of
host gene expression by gut microbiota[83]. In another
study, we showed that miRNAs play a role in determining
the unique physiological characteristics of intestinal epi-
thelial cells, such as their differentiation during migration
along the crypt/villus axis[84]. In particular, expression of
CD98, a transmembrane glycoprotein that regulates inte-
grin signalling, cellular homeostasis and innate immune
response in the gut[85], and its function are directly under
the control of miRNAs during the differentiation of in-
testinal epithelial cells[86]. MiRNAs could also be involved
in the upregulation of CD98 during intestinal inflamma-
tion and IBD[86]. The biological importance of miRNAs
in the pathogenesis of IBD is becoming clearer, and
targeting miRNAs in the gastrointestinal tract may be a
promising approach for future therapeutic opportunities.

MICRORNAS AND COLORECTAL CANCERS

The transformation of a normal epithelium into a can-
cerous state involves modifications in several genes that
are involved in different stages of carcinogenesis such
as apoptosis, proliferation, limitless replicative potential
of tumor cells, angiogenesis, migration and invasion[87].
<figure><img src="image_4.png" /><figcaption>**Figure 1** Hypothetical model for the involvement of miR-196 in the pathogenesis of Crohn's disease. MicroRNAs (miRNAs, miR) 196 normally targets immunity-related GTPase family, M (IRGM) mRNA within RNA-induced silencing complex (RISC) for a negative regulation (left panel). The IRGM risk allele (IRGM*) mRNA lacks the binding site for miR-196 and therefore is not regulated by this miRNA (right panel). During Crohn's disease, loss of tight regulation of IRGM* expression by miR-196 may lead to defects in autophagy with most intracellular bacteria replication occurring in dysfunctional vacuoles[82] (dotted cycle). This consequently results in abnormal persistence of pathogens in host cells, which could further worsen disease status[82].</figcaption></figure>

Colorectal cancer (CRC) is one of the most common
cancers worldwide. Its incidence is greater in industrial
countries than in developing countries[88]. MiRNAs have
been shown to play an important role in oncogenesis by
regulating the expression of genes involved in cancer
initiation, promotion and development[89]. Hundreds of
miRNAs mapped to the human genome regions that
are known to be altered in cancer, and a similar number
of miRNAs are aberrantly expressed in cancerous tis-
sues[90,91]. By analyzing miRNA expression profile (miR-
Nome) of prostate, stomach, pancreas, lung, breast and
colon tumors, Volinia and colleagues identified a solid
cancer miRNA signature including those with well-char-
acterized cancer association, such as miR-17-5p, miR-20a,
miR-21, miR-92, miR-106a, and miR-155[92]. In particular,
21 miRNAs are up-regulated and 1 is down-regulated in
colon tumors compared to normal tissue[92]. MiRNA pro-
files can identify different tissue and tumor types better
than mRNA expression patterns, making them attractive
targets for development as cancer biomarkers[93]. Distin-
guished miRNA profiles can even be found in the serum
of patients with cancers. The functions of such circulat-
ing miRNAs have not been identified, but profiling of
serum miRNAs might be a powerful approach for early
cancer diagnosis. The cancer-associated miRNAs may
function as oncogenes or tumor suppressors depending
on their role in carcinogenesis. Some of the best ex-
amples of such miRNAs will be discussed in this section

(Figure 2).

Oncogenic miRNAs

MiR-21 is one of the most up-regulated miRNAs in vari-
ous cancers, including CRC[92,94], and was identified as an
independent predictor of overall survival in the valida-
tion set containing tumor samples from 113 patients with
CRC[95]. It has been shown that miR-21 is involved in
invasion, intravasation and metastasis processes by target-
ing the tumor suppressor PDCD4[96], and in CRC tissues
expression of miR-21 is inversely correlated with that of
PDCD4 compared to normal tissue[97]. Shibuya and col-
leagues suggested that miR-21 expression may predict
poor prognosis in CRC[98]. Likewise, these authors also
examined the prognostic value of miR-155 in CRC since
its expression is up-regulated in tumor tissues compared
to normal adjacent tissues from CRC patients[98]. MiR-155
was previously shown to target the tumor protein 53-in-
duced nuclear protein 1 (TP53INP1), a pro-apoptotic
stress-induced p53 target, and significant reduction or
loss of TP53INP1 expression was detected during ad-
enocarcinoma progression[99].

MiR-17-92 and miR-106b-25 clusters are known to
be up-regulated in CRC stromal tissues compared with
normal stroma[100]. They include, respectively, multiple
mature miRNAs, miR-17, miR-18a, miR-19a, miR-20a,
miR-19b1, miR-92-1[101], and miR-106b, miR-93 and
miR-25[102]. These miRNA clusters play an important role
<figure><img src="image_5.png" /><figcaption>**Figure 2** Overview of "oncogenic" and "tumor suppressor" microRNAs related to colorectal cancer described in this review, their targets and different carcinogenesis pathways in which they have been implicated.</figcaption></figure>

during carcinogenesis[92,100,103,104]. An anti-apoptotic ef-
fect of miR-17-92 appears to be one of the mechanisms
underlying its procarcinogenic role in CRC develop-
ment and progression[105]. Abrogation of miR-92a leads
to cell apoptosis, and there is a correlation between the
miR-17-92 overexpression in tumors of CRC patients
and the downregulated expression of BIM, a member of
the Bcl-2 family that promotes apoptosis[105]. Some works
have reported that there is an interconnection between
the expression of miR-17-92 cluster and angiogenesis,
which occurs later in tumor development and is one of
the most important stages in carcinogenesis. Dews and
colleagues demonstrated that the anti-angiogenic fac-
tors thrombospondin-1 (tsp-1) and connective tissue
growth factor (CTGC) are down-regulated by this cluster
in intestinal epithelial cells expressing constitutively the
oncogene c-myc[106], which was shown to be involved in
regulation of miR-17-92 expression[20]. MiR-18 targets
tsp-1 and miR-19 modulates the expression of CTGF[107].

Other miRNAs have also been identified as causal
factors in colon carcinogenesis. For example, miR-196a
had higher expression level in CRC tissues than in normal
epithelial tissues[108]. MiR-196a exerts a pro-oncogenic in-
fluence in CRC as a high level of its expression promotes
the oncogenic phenotype of colorectal cancer cells such
as increased cell detachment, migration and invasion[109].
MiR-31 is often up-regulated in CRC and its high expres-
sion associated with advanced tumor stage but the clinical
significance is unclear[110]. MiR-181b-1, miR-135a, miR-
135b, miR-675 are also known to be up-regulated in CRC
tumors[111]. MiR-135a is able to promote the growth and
invasion of CRC cells by targeting the metastasis sup-
pressor 1[112].

Tumor suppressor miRNAs

Mir-143 and miR-145 are among the best examples of
tumor suppressor miRNAs. The expression of these
miRNAs is down-regulated in CRC tumors, and in other

cancers such as breast, prostate, cervical and lymphoid
cancer[113-115]. Many studies have reported that down-
regulation of miR-143 and miR-145 correlates with poor
prognosis[110,115,116]. The expression and post-transcrip-
tional maturation of these miRNAs were recently shown
to be enhanced by the tumor suppressor p53 in response
to DNA damages in CRC cell lines[28,117]. In particular,
miR-143 is involved in inhibition of oncogene KRAS ex-
pression[118]. MiR-145 is reported to inhibit tumor growth
and angiogenesis by directly targeting p70S6K1[117],
which is activated by mTOR, and its overexpression in cancer
cells induces tumor angiogenesis[119-121]. Another study
reported that this miRNA is able to inhibit tumor growth
and angiogenesis in breast cancer by targeting N-RAS
and VEGF-A, which are key players in carcinogenesis[122].

It was recently demonstrated that miR-34a is down-
regulated in colon tumors and also in circulating
blood[123]. Furthermore, ectopic expression of miR-34a
in CRC cell line reduces cell proliferation, demonstrating
that this miRNA has a tumor suppressive function in co-
lon carcinogenesis[124]. Several studies conducted in 2007
revealed that miR-34a can target p53, leading to apoptosis
and cell cycle arrest[21-24,125]. MiR-203 is identified as an-
other tumor suppressor miRNA. Its low expression was
found *in vitro* in CRC cell lines and was correlated with
tumor size in CRC. MiR-203 can inhibit proliferation of
cancer cell lines[126]. Li *et al*[127] showed that miR-203 over-
expression significantly decreased cell proliferation and
survival and induced cell apoptosis in the p53-mutated
CRC cells. The tumor suppressive role of miR-203 was
mediated by negatively regulating Akt2 expression *via*
mRNA degradation. In addition, overexpression of
miR-203 decreased expression of the anti-apoptotic gene
Bcl-xL, leading to a resistance to apoptosis[127]. MiR-126
is specifically expressed in endothelial cells and is known
to be down-regulated in CRC compared to normal tissue
*via* an unknown mechanism[128]. *In vitro* studies suggested
that a loss in negative regulation of p85 subunit of PI3K
by miR-126 could lead to a selective growth advantage
during colon carcinogenesis[129].

CONCLUSION

MiRNAs are a class of gene regulators that have recently
emerged as key players in the innate and adaptive immune
system. Changes in miRNA expression are observed in
many human diseases such as inflammatory bowel disease
and cancers. Dysregulated miRNA expression profiles in
IBD have been reported and could be used as diagnostic
biomarkers but further studies are needed to examine
the mechanism of their action in the etiopathogenesis of
this disease and their clinical utility. Emerging evidence
suggests that miRNAs play important roles in the patho-
genesis of a limited range of human cancers. Some miR-
NAs may be directly involved in cancer development by
controlling cell differentiation and apoptosis, while others
may be involved in cancers by targeting cancer oncogenes
and/or tumor suppressors. Given the critical role of
miRNAs, current studies are focusing on their association
with CRC incidence and prognosis and on the possibility
of using circulating miRNAs or fecal miRNA expression
as noninvasive early detection biomarkers. These data
suggest that miRNAs may be potential molecular classi-
fiers, early detection biomarkers, and therapeutic targets
for CRC. Finally, miRNA-based cancer therapy has been
limited to targeting a single miRNA[130,131]. However, it has
been recently shown that the small molecule enoxacin,
a fluoroquinolone used as an antibacterial compound,
enhances the miRNA-processing machinery by binding
to TRBP[132]. Thus, if most cancers are characterized by
a dysregulation of global mature miRNA expression,
restoration of the global miRNome may be an attractive
approach in cancer therapy. In conclusion, a better un-
derstanding of the function of miRNAs is providing new
insights into the molecular basis of human pathologies,
and new biomarkers for disease diagnosis and therapy.

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P- Reviewers Chi SG, Monticelli S S- Editor Gou SX

L- Editor A E- Editor Zhang DN

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