Bone is increasingly viewed as an endocrine organ with key biological functions. The skeleton produces hormones and cytokines, such as FGF23 and osteocalcin, which regulate an extensive list of homoeostatic functions. Some of these functions include glucose metabolism, male fertility, blood cell production and calcium/phosphate metabolism. Many of the genes regulating these functions are specific to bone cells. Some of these genes can be wrongly expressed by other malfunctioning cells, driving the generation of disease. The miRNAs are a class of non-coding RNA molecules that are powerful regulators of gene expression by suppressing and fine-tuning target mRNAs. Expression of one such miRNA, miR-140, is ubiquitous in chondrocyte cells during embryonic bone development. Activity in cells found in the adult breast, colon and lung tissue can silence genes required for tumour suppression. The realization that the same miRNA can be both normal and detrimental, depending on the cell, tissue and time point, provides a captivating twist to the study of whole-organism functional genomics. With the recent interest in miRNAs in bone biology and RNA-based therapeutics on the horizon, we present a review on the role of miR-140 in the molecular events that govern bone formation in the embryo. Cellular pathways involving miR-140 may be reactivated or inhibited when treating skeletal injury or disorder in adulthood. These pathways may also provide a novel model system when studying cancer biology of other cells and tissues.
Bone and cartilage are specific to vertebrates. The first organisms to develop such an endoskeleton, Chondrichthyes, evolved around 422 million years ago. Two million years later and our first ancestor, Osteichthyes, contained an endoskeleton with all the same four types of mineralized tissue (bone, cartilage, enamel and dentin) as modern mammals . The tissue classification criteria were considered controversial due to fossils of primitive vertebrates containing other tissues difficult for palaeontologists to classify . Further controversy existed about whether the four tissue types developed early in vertebrate evolution were a result of phenotypic plasticity or that cartilage and bone developed analogous to each other. Put to rest in the 2000s using new technology in evolutionary genetics, skeletogenesis is now regarded as involving the four mineralized tissue types expressing specific secretory calcium-binding phosphoprotein genes and other core developmental gene networks essential to vertebrate phylogeny . The expression of runt-related transcription factors (RUNX1, RUNX2 and RUNX3) is vital for these criteria. RUNX proteins are critical in regulating skeletogenesis and cartilage development in all vertebrates. They are also important for differentiating the bone cell population from mesenchymal and haematopoietic stem cells .
In a human embryo it takes 8 weeks post-conception for the skeleton to begin to form, one of the last organs to develop before becoming a fetus. Within the three developmental lineages that generate the skeleton, the somites produce the axial skeleton (skull, spine and ribs), the lateral plate mesoderm generates the appendicular skeleton (limbs and hips), and the neural crest develops the pharyngeal arches, craniofacial bones and future cartilage [3,4]. With the exception of the neurocranium, parts of the jaw and the medial clavicle where bones are formed anew, embryonic bone development is regulated by endochondral ossi-fication [5,6]. Conserved among vertebrates, endochondral ossification governs the differentiation of mesenchymal progenitors into chondrocytes, where the entire skeleton is first laid out as an intermediate cartilage template before replacement with bone . Cell–cell communication is critical for fate determination and anteroposterior patterning of the developing skeleton.
Paracrine signalling acts on mesenchymal precursor cells to condense and commit to the chondrocyte cell lineage . Induction of chondrogenesis begins with production of the transcription factor paired box 1 (PAX1) . Cell adhesion molecules such as N-cadherin bind immature chondrocytes into compact nodules. This structure formation further induces specialization with the activation of requisite transcription factors for chondrogenesis–RUNX2, sex-determining region Y (SRY)-box 9 (SOX9) and collagen type II α1 (COL2A1) . These transcription factors are required for driving the proliferation of the chondrocyte population and determination of osteoblast cell fates, as well as overseeing other transcriptional regulation in chondrocyte differentiation. Molecular cues and signalling pathways derived from immature chondrocytes sustain the differentiation process. The wingless-type MMTV integration site (Wnt) signalling pathway is shut down, with reduced levels of pericellular Wnt ligands, intracellular β-catenin and expression of the transmembrane receptors, low-density lipoprotein receptor-related protein (LRP) 5 and 6 . The co-activation of transforming growth factor β (TGF-β) signalling pushes the propagation and patterning of the entire skeletal model as cartilage .
Once the model is complete, chondrocytes lying within the central regions of the nascent skeleton undergo a molecular switch. The small, round chondrocytes stop dividing and increase their cell size, by at least 20-fold, giving rise to hypertrophic chondrocytes [5,9]. The extracellular matrix produced by hypertrophic chondrocytes changes, with increased production of collagen type X α1 (COL10A1) and fibronectin 1 (FN1). This new matrix enables mineralization with hydroxyapatite, using calcium and phosphate from maternal blood. The formation of bone in this region is termed the ‘primary ossification centre’. As the generation of bone gradually replaces the cartilage model from the primary ossification centre, the assembly and maturation of chondrocytes are restricted to the epiphysis. Here, they turn over within their own secondary ossification centre [5,10] (Figure 1). Blood vessels invade the cartilage model with hypertrophic chondrocytes undergoing programmed cell death. The empty space left behind, caused by chondrocyte apoptosis, is replaced with bone marrow.
The skeleton is first formed as a cartilage model
The final phase of skeletogenesis is the differentiation of osteoblasts; RUNX2 interacts with other transcription factors such as bone morphogenetic protein 2 (BMP2) and zinc finger protein 521 (ZFP521) to encourage osteoblast differentiation of mesenchymal cells that have migrated and enveloped the cartilage model . As the antithesis to earlier in skeletal development, increased paracrine signalling of Wnt ligands and Indian hedgehog (IHH) supports the reduction of the chondrocyte population. These signalling pathways also act to increase osteoblast maturation and bone formation .
As greater Wnt and IHH signalling progresses throughout the developing skeleton, it is imperative that the chondrocytes are not replaced by osteoblasts in the primitive joints. Chondrocyte survival within the secondary ossification centre at the epiphysis relies on a reduced exposure to Wnt and IHH signalling. Now developing under hypoxia, chondrocytes express vascular endothelial growth factor (VEGF), hypoxia-inducible factor 1 (HIF1) and its oxygen-sensitive component HIF1α to mediate the survival response within the joints . These molecules are common to any cell where there is a reduction in oxygen availability.
At the end of adolescence, when bone modelling has ceased and bone remodelling takes over, immature chondrocytes in the joints secrete parathyroid hormone-related protein (PTHrP) in response to hypoxia and IHH signalling from hypertrophic chondrocytes at the bone–cartilage border [12,14] (Figure 2). The activation of parathyroid hormone receptor 1 (PTHR1) by PTHrP slows the rate of immature chondrocyte molecular switching to hypertrophic chondrocytes . Reports suggest that PTHrP regulates the movement of a transcriptional suppressor, histone deacetylase 4 (HDAC4), into the nucleus to negatively regulate the activity of downstream transcription factors by direct binding. These transcription factors include myocyte enhancer factor 2 (MEF2), ZFP521 and RUNX2 [15–18]. PTHrP is also endogenously present in other cartilaginous sites, e.g. in the perichondrium and chondrocyte population adjacent to hyaline cartilage.
Hypoxia drives the secretion of PTHrP from both periarticular and perichondral chondrocytes
Understanding the molecular mechanisms that oversee chondrogenesis, bone formation and cartilage homoeostasis allows researchers and physicians to reactivate or modify these pathways with treatments to promote skeletal tissue repair after injury or disease. Early bone development is a highly conserved arrangement across vertebrate species, from fish to humans. In animal models of embryonic development, a number of studies have identified cellular responses to chemical genetics that underlie human bone and cartilage disorders [19–22]. Manipulation of ‘transcription dynamics’–described as the cyclical nature of RNA polymerase assembly, remodelling of chromatin, binding of transcription factors, transcriptional elongation, RNA editing, RNA splicing and post-transcriptional regulation –is a hot topic in regenerative medicine.
BIOLOGY OF THE miRNAs
The miRNAs are a class of non-coding RNA molecules that are the most well-known components behind the cellular machinery of ‘RNA silencing’–a mechanism of RNA-mediated gene silencing first discovered in nematode worms . Oscillations of mRNA transcription and translation are buffered by co-expression of complementary miRNAs, with ensuing negative regulation of gene activity. In the canonical biogenesis pathway, miRNA genes are transcribed from the genome by RNA polymerase II to produce pri-miRNA transcripts . The transcripts are enzymatically cleaved within the nucleus by the microprocessor complex: a protein assembly composed of Drosha and DiGeorge's critical region 8 (DGCR8). The following approximately 70-nucleotide pre-miRNA forms a characteristic hairpin secondary structure and is transported to the cytoplasm by RanGTP-dependent, nuclear envelope-bound Exportin-5 [25,26]. Here, the multi-domain enzyme Dicer processes the pre-miRNA into an approximately 22-bp miR-5p and miR-3p duplex. Dicer achieves this through its possession of a dsRNA-binding domain for initial capture of the pre-miRNA, two-tandem RNase III nucleases for cleavage and an N-terminal ATPase/helicase domain for unwinding the 3p and 5p strands . One of the strands is immediately loaded into the Argonaute 2 (AGO2) effector structure of a ribonucleoprotein complex known as the RNA-induced silencing complex (RISC). The other strand, previously known as miRNA star, is usually degraded. RISC uses the mature single-stranded miRNA sequence as a guide for targeting complementary sequences in the mRNA 3′-UTR, also known as the miRNA recognition element (MRE) . Less commonly the 5′-UTR and mRNA-coding sequence also contain MREs [29,30]. The mRNA is silenced via translational suppression or mRNA decay (and very rarely through target cleavage when the complementarity between the miRNA and the MRE is near perfect). There are 2588 human mature miRNAs recorded in miRBase version 21 , collectively believed to regulate as much as 60 per cent of the coding transcriptome . The regulation of gene expression at this level is important in many fundamental biological processes, including cellular differentiation, proliferation, migration and extracellular communication.
Although the core model of miRNA biogenesis and regulation of gene expression is widely known, turnover of miRNAs is less clear. In a similar fashion to mRNA transcripts, miRNA production is differentially controlled through the association of regulatory factors. This can take place both within the biogenesis pathway and post-transcriptionally . The first miRNA to be discovered, lin-4 in Caenorhabditis elegans, was primarily described as a ‘small temporal RNA’ owing to its transient pattern of expression . When this finding was discovered to be conserved through to humans, it added weight to the concept that miRNAs could be produced in tissue-specific spatiotemporal patterns similar to those of coding genes . RNA polymerase II-mediated transcription provides a major regulator for the production of miRNAs . Mapping of human miRNA promoters through nucleosome positioning and chromatin immunoprecipitation suggests that the promoter structure of miRNA genes, including the frequencies of CpG sites, cis-regulatory elements and histone modifications, is indistinguishable between miRNA and mRNA loci [35,36]. Protein complexes, such as transcription factors, that facilitate mRNA expression are largely those that also modulate miRNA expression. The tumour-suppressor protein p53 was recently identified to up-regulate miRNA production through the generation of a complex with Drosha and an RNA helicase, DDX5, when there is damage to DNA . Similarly, proto-oncogenes such as c-Myc may modulate miRNA expression through binding to enhancer boxes in miRNA cluster promoters . As miRNAs are transcribed in an almost identical manner to mRNAs, the mechanisms of epigenetic control known for protein-coding genes are likely to apply to miRNA loci . Hypermethylation of tumour-suppressor genes is a common occurrence in cancer biology; the same is true for tumour-suppressor miRNAs such as miR-9-1, miR-193a, miR-137, miR-342, miR-203 and miR-34b/c, which are also found to be hypermethylated in various human cancers [39,40].
Not only are miRNA loci subject to transcriptional and epigenetic control, but mature miRNAs are also under post-transcriptional regulation. Circular RNAs (circRNAs) are highly expressed in the human and mouse brain where they act as a molecular sponge to soak up miRNA transcripts, inhibiting their activity . The circRNA counterpart for miR-7, or ciRS-7, contains more than 70 conserved miRNA target sites but is entirely resistant to miRNA-mediated destabilization . CiRS-7 strongly suppresses miR-7 activity, resulting in an increase in miR-7 target mRNAs. The testis-determining factor SRY is also a circRNA for miR-138 . This finding supports that the miRNA sponge effects achieved by circRNA formation are a general phenomenon. It is also the first time that natural RNA circles have been shown to regulate mature miRNAs. Expansion of this discovery will provide further insight into biological processes and pathophysiology where miRNAs have been characterized in the aetiology of disease.
THE miRNAs IN EMBRYONIC BONE DEVELOPMENT
Bone formation requires a multifaceted sequence of cellular events to drive progenitor differentiation and assembly of the bone cell population. Extracellular signalling provides the molecular signals to accomplish this process, which is ultimately achieved through targeted gene expression. A key element of this complexity is the tuning of signal-induced genes by miRNAs, as has been demonstrated in Dicer knock-out studies in mice. A global reduction of miRNA expression in chondrocytes and limb bud mesenchymal progenitors was previously shown to cause significant growth defects in skeletal development . On such a genome-wide scale these effects are typically expected. The biological function of individual miRNAs in skeletal development will provide the key to understanding normal development, while exposing a process that may be manipulated by medical treatments.
In mice, the miR-199a/214 cluster is expressed from the opposite strand of dyanamin 3 (Dnm3os) . Replacing this locus with the lacZ gene in mice caused growth retardation, craniofacial and vertebral hypoplasia, and osteopenia [42,43]. This strongly suggests the functional role of this miRNA cluster in developing chondrocytes. Mechanistic insight of other miRNAs promoting chondrogenesis includes miR-365, which performs by targeting HDAC4 . The miRNAs miR-30a, miR-30c and miR-125b enhance the differentiation of tracheal chondrocytes by suppressing a chondrogenesis inhibitor, snail zinc finger 1 (SNAIL1) . The miRNA miR-337 also promotes chondrogenesis in developing rat bone by down-regulating TGF-β receptor II (TGFBR2) .
Let-7 was recently shown to regulate chondrocyte proliferation specifically in the epiphyseal growth plate, providing further evidence for the biological role of individual miRNAs in chondrogenesis . Overexpression of a Let-7 inhibitor, lin-28 homologue A (LIN28A), caused growth impairment through the action of reduced chondrocyte proliferation . An up-regulation of predicted Let-7 target genes such as cell division cycle 34 (Cdc34) and E2F transcription factor 5 (E2f5) was also observed. The phenotype of Lin28a transgenic mice was mild; however, when combined with a reduction of another miRNA, miR-140, the observed phenotype of reduced growth was significantly amplified . The findings suggest that these miRNAs work in synergy to promote normal skeletal development, or miR-140 has a key role in chondrogenesis and the generation of cartilage.
miR-140 in embryonic bone development
Several miRNAs have been discovered and proposed as playing a crucial role in modulating oscillations of gene expression during embryonic development . One such miRNA, miR-140, was first reported in zebrafish. Its expression was later linked with mesenchymal stem cell frequency and chondrogenesis [49–52]. Among vertebrates miR-140 is evolutionarily conserved and is almost exclusively specific to chondrocytes, where it supports cellular homoeostasis and identity [53,54]. The pri-miR-140 gene is housed within an intron of WW domain containing E3 ubiquitin protein ligase 2 (WWP2)–known for its role in mediating the TGF-β signalling pathway through interaction with SMAD proteins. Expression of miR-140 has an overlapping pattern with WWP2, SOX9 and COL2A1, known regulators of chondrogenesis . Previous research in chondrocyte cell cultures has confirmed that SOX9 is a promoter for miR-140 expression . Studies in mice have confirmed the association between SOX9 and miR-140. These studies show that a proximal upstream region of the pri-miR-140 gene possesses a chondrocyte-specific promoter, which is directly regulated by SOX9 .
During the early stages of endochondral ossification, before the formation of articular cartilage at the epiphysis, miR-140 is highly expressed . Timing of expression is crucial because premature miR-140 activity may result in craniofacial defects through modulation of palatogenesis . Two independent studies in mice lacking miR-140 identified growth defects but with two different phenotypes [51,59]. In the first study, miR-140−/− mice presented with early onset, age-related osteoarthritis, supporting the concept that miR-140 has a role in the preservation and homoeostasis of articular cartilage. The second study showed proliferative growth defects of endochondral bones–resulting in dwarfism and craniofacial deformities. This resulted in the hypothesis that miR-140 may play a newly defined role in embryonic bone development.
Locked nucleic acid (LNA) probes in whole-embryo in situ hybridization were used to detect miR-140 at stages of development not tested in previous studies . Using the bones of the digits in chicken embryos as a model, the presence of miR-140 was detected as early as day 6 in embryonic development . At this stage, the limbs were shown to have developed a toe plate with a pointed contour and expression of miR-140 in two rudimentary digit bones. High magnification showed no distinct or well-shaped bones. These results indicated that miR-140 first appears at the early stage of bone development when the condensed mesenchyme is forming a hyaline cartilage template for future bone . In later stages of development, when the bones of the digits are distinctly formed, miR-140 expression is seen in the whole body of the bone, including the upper region where future articular cartilage is formed. The shift of miR-140 expression was also observed along the bones of the metatarsus and phalanges of the digits. The last bone of the phalanx showed a homogeneous distribution of miR-140 throughout the whole bone. The other three bones of the phalanx showed intermediate movement of miR-140 to the tops and centre of the bone, leaving two clear zones in between. The central zone where miR-140 appeared in bones three and four, and remained well defined in bone five, is the same location of primary ossification used during endochondral ossification . This demonstrates that miR-140 is present during the early stages of bone development when the hyaline cartilage template of the bone is formed. For a short period after, miR-140 is expressed in the primary ossification centre . Lack of miR-140 in mutant mice at these early stages, therefore lacking chondrocyte identity, could lead to premature chondrocyte hypertrophy which would explain the observed dwarfism in the mouse experiment discussed earlier .
miR-140 in chondrogenesis and adult cartilage homoeostasis
In the adult skeleton, chondrocytes are restricted to the ends of bones, or epiphyses, where they are the only cell type present in the cartilage. They are responsible for the synthesis and turnover of the extracellular matrix crucial to their function of bone protection at the joints. Increased miR-140 expression correlates with increased expression of SOX9 and COL2A1 during human chondrogenesis . Healthy chondrocytes steadily express miR-140 throughout their lifetime . Degenerative diseases such as osteoarthritis show significantly decreased expression of miR-140 as well as dysregulation of other miRNAs, such as miR-455 . Target genes of miR-140 provide an insight into the homoeostasis of chondrocyte maintenance. However, target prediction of miRNAs in animals is computationally difficult due to the low complementary annealing efficiency between the miRNA and target mRNA. In animals, perfect base-pairing is commonly restricted to the miRNA seed sequence (nucleotide positions 2–8). Overexpressing and antagonizing miR-140 in mouse fibroblast cells previously revealed a cross-section of 49 co-differentially expressed mRNAs. Therefore, correlating miRNA manipulation and mRNA expression provides a general experimental approach to target prediction . One of the potential targets–chemokine (C-X-C motif) ligand 12 (Cxcl12), known for its role in skeletogenesis, metastasis and tumour angiogenesis–was experimentally validated as a real miR-140 target by luciferase assay .
In an extension to the experimental identification of the miR-140 target study, the mRNA profiling datasets were used to show that Smad3 is a target at the protein level . SMAD3 is a key negative regulator of proliferation in the TGF-β signalling pathway. When Smad3 mRNA is silenced by miR-140, the cell cannot achieve G1 cell cycle arrest. As bone is one of the richest sources of TGF-β ligands in the human body, this pathway is attractive as a model for studying cancer development in which TGF-β signalling is a driver for both proliferation and tumour suppression. It could be hypothesized that miR-140 is a candidate molecule for investigating the development of malignant chondrocytes. Studies of chondrosarcoma have so far identified other miRNAs that are more likely contenders [63,64]. The favoured position of miR-140 at present is as a key regulator of chondrocyte homoeostasis, loss of which results in cartilage breakdown and progressive diseases such as osteoarthritis. Other experimentally validated miR-140 targets include ADAM metallopeptidase with thrombospondin type 1 motif 5 (ADAMTS5), aspartyl aminopeptidase (DNPEP) and V-ral simian leukaemia viral oncogene homologue A (ras related) (RALA) [49,59,65].
miR-140: A DUAL ROLE AS AN ONCOGENE AND TUMOUR SUPPRESSOR
MiR-140 in colorectal and hepatic cancer
The expression of miR-140 is a marker of chondrocyte identity. Dysregulation and inappropriate expression in other cells have been shown to drive tumorigenesis. In colorectal tumours, miRNA-network analysis has shown that miR-140 was among the most differentially expressed miRNAs . Although the target genes of miR-140 were not studied, the authors suggest that the down-regulation of miR-140 in colorectal cancer is tissue specific. Mechanistic insight of miR-140’s role could include analysis of cellular pathways known to be perturbed in colorectal cancer such as the Wnt signalling pathway. Loss of miR-140 has also been linked to hepatocellular carcinoma . DDX20 is an RNA helicase that regulates the function of miR-140 in the liver. Here, miR-140 has two confirmed targets–nuclear factor-κB (NF-κB) and DNA methyltransferase 1 (DNMT1). It was previously known that loss of DDX20 enhanced the activity of NF-κB in driving carcinogenesis of hepatocytes. Increased expression of DNMT1 in DDX20-deficient cells resulted in hypermethylation of metallothionein gene promoters. Decreased expression was linked with increased NF-κB activity . The action of miR-140 here is 2-fold: without upstream regulation it cannot prevent the downstream methylation and subsequent bioactivity of NF-κB.
MiR-140 in breast cancer stem cells
Cancer stem cells are the key drivers of tumorigenesis, malignancy, multiple drug resistance and treatment failure . Whether they are hyperactive stem cells or senescent cells that have recovered stem-like behaviours is still a topic of debate. They are, however, very similar to embryonic stem cells in their physiology and behaviour. They are capable of self-renewal and express totipotency-associated transcription factors such as SOX2 and octamer-binding transcription factor 4 (OCT4) . Oestrogen, a contributor to hormone-sensitive breast cancer, promotes the vigour of breast cancer stem cells . In studying the mechanistic driver of breast cancers overexpressing the oestrogen receptor, it was demonstrated that activation of the receptor inhibits transcription of miR-140 . As miR-140 has been reported as a tumour suppressor in breast cancer, down-regulation could initiate cancer stem cell activity. Computational prediction of miR-140 mRNA targets using TargetScan 6.0 revealed SOX2 as a potential target of miR-140. A luciferase reporter assay confirmed that the SOX2 3′-UTR is targeted by miR-140 . Loss of miR-140 through oestrogen receptor activation up-regulates SOX2, prompting self-renewal of cancer stem cells.
Duplication of the
miR-140 gene in a head and neck tumour
Copy number variations or gene duplications are common to most cancers. Consequently, gene dosage is significantly increased. If a proto-oncogene is duplicated, the increased gene expression can be key to cellular transformation and malignancy. A DNA microarray to detect not only genome-wide DNA copy number changes but also transcriptome alterations revealed a duplication of the miR-140 loci in human oral squamous cell carcinoma . An over-representation of cellular miR-140 transcripts resulted in the suppression of genes involved in normal cell cycling, connective tissue homoeostasis, cellular function and cell–cell signalling . The suppressed mRNAs included those for p53, NF-κB and HDAC1, consequently driving lymph node metastasis .
miR-140 and up-regulation of RAD51AP1 in ovarian cancer
Application of a novel method to detect conserved MRE-seed sequence matching and significant ‘anti-correlation’ of miRNA and mRNA expression revealed a down-regulation of miR-140 in ovarian serous cystadenocarcinoma . Using mRNA profiling data from the Cancer Genome Atlas, the up-regulated mRNA corresponding to a reduction of miR-140 was RAD51-associated protein 1 (RAD51AP1), which is involved in double-stranded break repair of DNA . Overexpression of RAD51AP1 is a tactic utilized by tumours to evade the impact of platinum-based chemotherapy agents, such as cisplatin, which specifically function by triggering DNA damage in cancer cells.
Chemoresistance of osteosarcoma, and colorectal and gastric cancers is influenced by
High-throughput miRNA expression analysis in osteosarcoma and colorectal cancer cells revealed that miR-140 expression was associated with chemosensitivity . Tumour cells in vitro were transfected with a pre-miR-140 mimic and exhibited resistance to methotrexate and 5-fluorouracil–two commonly used cancer chemotherapy agents. Overexpression of miR-140 inhibited cell proliferation and encouraged G1 and G2 cell cycle arrest in p53-positive cell lines, but less so in cell lines with a mutant p53 . This is an important finding because there is a p53-associated mutation in half of all human cancers. As first reported in embryonic development of bone , this study supported the contention that HDAC4 is a target of miR-140. The study also revealed endogenous miR-140 up-regulation in CD133 (+hi) CD44 (+hi) colorectal cancer stem cells exhibiting chemoresistance. Antagonizing endogenous miR-140 with LNA inhibitors partially sensitized the cancer cells to 5-fluorouracil treatment. The study summarized that miR-140 was involved in chemoresistance by reduced cell proliferation through G1 and G2 phase arrest, mediated in part by the suppression of HDAC4 . The reverse is true in stomach cancer where miR-140 was present within a list of miRNAs linked to increased chemosensitivity . Taken together, the results highlight the importance of recognizing that one miRNA may have very different roles contingent on the cell and tissue of origin. Therapeutics involving miRNA delivery, as demonstrated by the use of LNA inhibitors, must be organ targeted.
SUMMARY AND FUTURE DIRECTIONS
In the chondrocyte, miR-140 is a highly expressed non-coding RNA and could be used as a marker of chondrocyte differentiation. Genes known to interact with miR-140 are accepted markers of chondrocyte proliferation and homoeostasis–these include SOX9, COL2A1 and COL10A1. Key mRNA targets of miR-140 gene silencing include HDAC4, CXCL12, SMAD3, DNMT1 and NF-κB (Figure 3). Targets within the TGF-β signalling pathway are crucial to regulating cellular proliferation in skeletogenesis (Figure 4). They are central to endochondral ossification and cartilage homoeostasis.
TGF-β ligands bind to a membrane-bound type II receptor dimer, a serine/threonine kinase
Loss of miR-140 expression can lead to degenerative diseases such as osteoarthritis, although dysregulation is not thought to be involved in the tumorigenesis of chondrocytes. Conversely, in other cell types, differential expression of miR-140 can play a key role in malignant neoplasia. However, this is strictly determined by the tissue type. In some cancers, such as colorectal and oral, miR-140 plays the role of a tumour suppressor. Here, it silences proto-oncogene transcription where loss of function drives tumorigenesis. In other tumours such as liver, breast, skin and lung, miR-140 is an oncogene or ‘oncomiR’ where gain of function drives tumorigenesis.
The Wnt signalling pathway and
Expression of miR-140 promotes the proliferation of chondrocytes and sustains chondrogenesis in the developing embryo. Once the cartilage skeletal model is complete, a molecular switch rapidly escalates Wnt ligand production and its complement signalling molecules β-catenin, LRP5 and LRP6. Chondrogenesis is inhibited and restricted to the epiphysis, before osteoblast differentiation and proliferation become the primary developmental process. In adulthood, miR-140 regulates cartilage homoeostasis, with Wnt signalling regulating the homoeostasis of bone turnover. Activating mutations in the Wnt signalling pathway can cause high bone mass . In contrast, decline of Wnt signalling can cause bone density disorders such as osteoporosis .
Wnt signalling is also employed in the colon for intestinal barrier turnover. Inhibition of miR-140 with overexpression of Wnt signalling has been shown to drive the bulk of colorectal cancer cases . Although miR-140 has yet to be experimentally linked to the Wnt signalling pathway, there is a clear paradigm between this gene network and the colon and bone. If there is an interaction between Wnt signalling and miR-140, embryonic bone development may provide an exceptional model for studying colorectal cancer progression. One potential avenue of investigation is the newly discovered Wnt signalling antagonist molecule sclerostin, or SOST. If SOST mRNA is a real target of miR-140, LNA inhibitors for up-regulated miR-140 may rescue SOST in a novel treatment for colorectal cancer. Intriguingly, targeting SOST through immunotherapy has been shown to reverse osteoporosis .
Our work is supported by Big C [14–08R] and Arthritis Research UK .
ADAM metallopeptidase with thrombospondin type 1 motif 5
collagen type X α1
collagen type II α1
chemokine (C-X-C motif) ligand 12
DNA methyltransferase 1
histone deacetylase 4
hypoxia-inducible factor 1
locked nucleic acid
low-density lipoprotein receptor-related protein
myocyte enhancer factor 2
miRNA recognition element
parathyroid hormone receptor 1
parathyroid hormone-related protein
RAD51-associated protein 1
V-ral simian leukaemia viral oncogene homologue A (ras related)
RNA-induced silencing complex
runt-related transcription factor
sex-determining region Y
transforming growth factor β
TGF-β receptor II
wingless-type MMTV integration site
WW domain containing E3 ubiquitin protein ligase 2
zinc finger protein 521