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. 2015 Apr 15;4:671. doi: 10.1038/bonekey.2015.38

Bone marrow stroma-derived miRNAs as regulators, biomarkers and therapeutic targets of bone metastasis

Maša Alečković 1, Yibin Kang 1,a
PMCID: PMC4398005  PMID: 25908970

Abstract

MicroRNAs (miRNAs) are short, endogenous RNA molecules that have essential roles in regulating gene expression. They control numerous physiological and cellular processes, including normal bone organogenesis and homeostasis, by enhancing or inhibiting bone marrow cell growth, differentiation, functional activity and crosstalk of the multiple cell types within the bone. Hence, elucidating miRNA targets in bone marrow stromal cells has revealed novel regulations during bone development and maintenance. Moreover, recent studies have detailed the capacity for bone stromal miRNAs to influence bone metastasis from a number of primary carcinomas by interfering with bone homeostasis or by directly influencing metastatic tumor cells. Owing to the current lack of good diagnostic biomarkers of bone metastases, such changes in bone stromal miRNA expression in the presence of metastatic lesions may become useful biomarkers, and may even serve as therapeutic targets. In particular, cell-free and exosomal miRNAs shed from bone stromal cells into circulation may be developed into novel biomarkers that can be routinely measured in easily accessible samples. Taken together, these findings reveal the significant role of bone marrow stroma-derived miRNAs in the regulation of bone homeostasis and bone metastasis.

Biogenesis and function of miRNAs

MicroRNAs (miRNAs) are a large family of noncoding ∼22-nucleotide-long RNA molecules that negatively regulate gene expression.1 It is estimated that miRNAs regulate about 50% of all protein-coding genes.2 They repress gene expression through complementary binding to sites in the 3′-untranslated region (UTR) of target mRNAs.3,4 miRNA-mediated inhibition occurs either by mRNA degradation or translational silencing. Cleavage of target mRNAs occurs when the miRNA and the target mRNA exhibit perfect complementarity.3 Conversely, miRNA–mRNA pairings with imperfect complementarity result in translational inhibition in the absence of target cleavage.3,5 Thus, even in the absence of perfect binding, the association between an miRNA and a target 3′-UTR still results in the suppression of a target protein. Owing to the relatively short length of the seed sequence that binds to the 3′-UTR (5–7 nucleotides), each miRNA can potentially recognize hundreds of mRNAs and is thus a powerful molecular manager that can control several gene regulatory networks simultaneously.

miRNA genes are typically transcribed into stem-loop structures from intra- or intergenic regions by RNA polymerase II and undergo sequential cleavage steps to produce mature miRNAs.2 In the nucleus, newly transcribed pri-miRNAs (primary miRNAs) are cleaved by a complex formed by the RNase III enzyme Drosha and the double-stranded RNA-binding protein Pasha (or DGCR8) to produce precursor miRNAs. These precursor miRNAs are about 70–100 nucleotides long.6 They are exported from the nucleus by Exportin 5 and the Ran-GTP cofactor,7 where they undergo a second cleavage by the endoribonuclease Dicer to produce a double-stranded, ∼18–25-nucleotide-long mature miRNA. One miRNA strand then combines with Argonaute (AGO2) proteins to form the RNA-induced silencing complex, allowing for directed pairing with target mRNAs and mediating rapid fine-tuning of gene expression.4

miRNA regulation within bone stromal cells

Bone formation and turnover are complex processes that involve differentiation and crosstalk of multiple cell types for the generation and remodeling of the skeleton.8,9,10 Inhibition of mRNA translation by miRNAs has emerged as an important regulator of developmental osteogenic signaling pathways, osteoblast growth and differentiation, osteoclast-mediated bone resorption activity and general bone homeostasis. Hence, it is not surprising that cell-type-specific disruption of miRNA biogenesis by deletion of Dicer would have a significant impact on physiological bone formation and remodeling (Table 1). For example, in vivo ablation of Dicer in osteoprogenitors, osteoblasts and chondrocytes by Col1a1-Cre resulted in severe skeletal deformities in fetal mice as it inhibited their maturation into cells that lay down bone mineral.11 The mice had a disproportione cartilage skeleton and a reduction in mineralized tissue at E14.5. Excision of Dicer in mature osteoblasts by Osteocalcin-Cre produced a viable mice, which exhibited delayed bone development that corresponded with reduced osteoblast numbers.11 Similarly, Dicer deletion in chondrocytes, using Col2a1-Cre mice, modulated the proliferation and differentiation of cells into postmitotic hypertrophic chondrocytes leading to significant skeletal defects.12 Osteoclast-specific Dicer knockout, using CD11b-Cre or CTSK-Cre transgenic mice, disrupted osteoclastogenesis and reduced the number and activity of multinuclear osteoclasts leading to an increase in bone mass.13,14,15 Cultured bone marrow from these transgenic mice was also incapable of producing osteoclasts ex vivo, further confirming the defect in osteoclast differentiation.14,15 Importantly, this defect was not confined to Dicer; ablation of DGCR8 and AGO2 using small interfering RNAs (siRNAs) similarly resulted in decreased osteoclast maturation and bone resorption, and osteoclast-specific DGCR8 knockout mice display impaired bone development (Table 1).13,16 Taken together, these results reveal the necessity for proper miRNA regulation during the differentiation and maintenance of multiple essential bone marrow cell types required for bone formation and homeostasis.

Table 1. Key miRNA regulators of bone organogenesis and homeostasis.

MiRNA biogenesis Regulatory function Activity Strategy for reduced expression Reference
Ago2 Essential for development Knockout inhibits osteoclast development siRNA (in vitro) 13
DGCR8 Essential for development Knockout inhibits osteoclast development CTSK-Cre, siRNA (in vitro) 13,16
Dicer Essential for development Knockout inhibits osteoblast, osteoclast, chondrocyte development Col1a1-Cre, osteocalcin-Cre, CD11b-Cre, CTSK-Cre, Col2a1-Cre 11,15
         
  Regulatory function Activity Targets Reference
Osteoclast        
 MiR-21 Activates Promotes differentiation PDCD4 14
 MiR-29b Activates and/or inhibits Activates and/or inhibits differentiation Cdc42, Srgap2, Fos, Mmp2 26,27
 MiR-31 Essential for development Essential for differentiation Rhoa 28
 MiR-34a Activates Promotes differentiation Tgif2 33
 MiR-125a Inhibits Inhibits differentiation TRAF6 30
 MiR-146a Inhibits Inhibits differentiation, protects joint destruction during collagen-induced arthritis   64
 MiR-148a Activates Promotes differentiation MAFB 65
 MiR-155 Inhibits Inhibits differentiation SCOS1, MITF 31,32
 MiR-223 Activates and/or inhibits Positive and negative regulator of differentiation   13,25
 MiR-503 Inhibits Inhibits differentiation RANK 29
 
Osteoblast
 MiR-15b Activates Promotes differentiation Smurf1 66
 MiR-17∼92 Activates Promotes proliferation and differentiation   67
 MiR-17-5p, miR-106 Inhibits Inhibits differentiation BMP2 68
 MiR-20a Activates Promotes differentiation PPARγ, Bambi, Crim1 69
 MiR-23a/27a/24-2 Inhibits Inhibits apoptosis FAK, Runx2, Satb2 21,22
 MiR-29a Activates Promotes differentiation Osteonectin, Dkk1, Kremen2, sFRP2 70,71,72
 MiR-30c Inhibits Inhibits differentiation Smad1, Runx2 21
 MiR-34c Inhibits Inhibits proliferation and differentiation SATB2, Runx2, Notch pathway 21,34,35
 MiR-93 Inhibits Inhibits mineralization Sp7/Osx 73
 MiR-100 Inhibits Inhibits differentiation BMPR2 74
 MiR-125b Inhibits Inhibits differentiation ErbB2, Sp7/Osx 75,76
 MiR-133a Inhibits Inhibits differentiation Runx2 21
 MiR-135a Inhibits Inhibits differentiation Runx2 21
 MiR-137 Inhibits Inhibits differentiation Runx2 21
 MiR-138 Inhibits Inhibits differentiation FAK, indirectly Sp7/Osx 24
 MiR-141, miR-200a Inhibits Inhibits differentiation Dlx5 77
 MiR-143 Inhibits Inhibits differentiation Sp7/Osx 78
 MiR-155 Inhibits Regulates TNFα inhibition of osteoblast differentiation SOCS1, MITF 31
 MiR-181a Activates Promotes differentiation Tgfbr1, Tgfbi 79
 MiR-182 Inhibits Inhibits proliferation and differentiation Foxo1 80
 MiR-204/211 Inhibits Inhibits differentiation Runx2, Sost 21
 MiR-205 Inhibits Inhibits differentiation Runx2 21
 MiR-206 Inhibits Inhibits differentiation Cx43 23
 MiR-208 Inhibits Inhibits differentiation Ets1 81
 MiR-214 Inhibits Inhibits activity and bone formation ATF4, Sp7/Osx 82,83
 MiR-217 Inhibits Inhibits differentiation Runx2 21
 MiR-218 Inhibits Inhibits differentiation Runx2, Dkk2, Sfrp2 21
 MiR-322 Activates Promotes differentiation Tob2, indirectly Sp7/Osx 84
 MiR-335-5p Activates Promotes differentiation DKK1 85
 MiR-338 Inhibits Inhibits differentiation Runx2 21
 MiR-542-3p Inhibits Inhibits differentiation BMP7 86
 MiR-637 Inhibits Inhibits differentiation Sp7/Osx, Col4a1, Bmp2k 87,88
 MiR-764-5p Inhibits Inhibits differentiation CHIP/STUB1 89
         
  Regulatory function Activity Targets Reference
Chondrocytes        
 Let-7 Activates Promotes proliferation Cdc34, E2f5 90
 MiR-17–92 Essential for development Loss induces microcephaly and other skeletal defects in patients   91
 MiR-34a Inhibits Inhibits differentiation Apoptosis 92,93
 MiR-140 Essential for activity Essential for proper activity Sp1, HDAC4, Pdgfra, Dnpep,Smad3, Rala 17,18
 MiR-145 Inhibits Inhibits differentiation Sox9 20
 MiR-199a* Inhibits Inhibits differentiation Smad1 19
 MiR-221 Inhibits Inhibits proliferation Mdm2 94
 MiR-335 Activates Promotes differentiation Rock1, Daam1 95
 MiR-337 Inhibits Inhibits differentiation Tgfbr2 96
 MiR-449 Inhibits Inhibits differentiation Lef1 97
 MiR-1247 Inhibits Inhibits differentiation Sox9 98
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Abbreviations: Ago2, argonaute; DGCR8, double-stranded RNA-binding protein Pasha; miR, microRNA; siRNA, small interfering RNA; TNFα, tumor necrosis factor-α.

miRNA regulation in bone homeostasis

Similar to studies investigating the effects of disruption of the general miRNA biogenesis pathway, several recent reports have revealed the capacity for individual miRNAs to direct the differentiation and activity of cells residing in the bone microenvironment. In this context, miRNAs can serve as both negative and positive regulators during differentiation and functional activation by disrupting the temporally organized coordination of transcription factors and regulatory proteins (Table 1). Characterization of miRNAs that operate through tissue-specific transcription factors as well as intricate feed-forward and reverse loops has provided novel insights into the supervision of signaling pathways and regulatory networks controlling normal bone development and turnover.

Chondrocytes are intricately regulated by a series of miRNAs (Figure 1 and Table 1), including miR-140, miR-199a* and miR-145. MiR-140 positively regulates chondrocyte differentiation by inhibiting HDAC4 and Dnpep, and contributes to craniofacial development and endochondral bone formation.17,18 Consistent with this, mice lacking miR-140 display accelerated bone formation at the embryonic and neonatal stages of development.18 Conversely, miR-199a* and miR-145 negatively regulate chondrogenesis; miR-199a* downregulates the expression of Smad1,19 whereas miR-145 targets Sox9, which encodes an essential transcription factor for chondrocyte differentiation.20

Figure 1.

Figure 1

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MicroRNA (miRNA) involvement in bone homeostasis. MiRNA-mediated regulation of osteoclast, osteoblast, and chondrocyte differentiation and activity is essential for proper bone development and homeostasis. Known positive and negative miRNA regulators and several of their key targets are indicated.

miRNA-based regulation was observed during osteoblast differentiation and activation (Figure 1 and Table 1). Runx2, a member of the Runt transcription factor family, is essential for osteogenesis and is subject to regulation by a number of miRNAs, including miR-23a, miR-30c, miR-34c, miR-133a and miR-204.21 The miR-23a/-27a/24-2 cluster is tied to osteogenesis through an intricate Runx2 regulatory loop. Runx2 transcriptionally represses the expression of the miR-23a/-27a/24-2 cluster by binding to its promoter region. On the other hand, miR-23a is capable of directly repressing Runx2 expression.22 Thus, Runx2-mediated inhibition of the miR-23a/-27a/24-2 cluster initiates a positive feedback loop. Moreover, inhibition of this miRNA cluster also derepresses another activator, SATB2, allowing for progression of the cell through osteogenesis.22 MiR-206 was also shown to inhibit osteoblast development by targeting Cx43, whereas miR-206 knockdown promoted differentiation.23 MiR-138 inhibited osteogenesis by directly targeting signaling through focal adhesion kinase leading to attenuated bone formation in vivo.24 Importantly, treatment of human mesenchymal stem cells (MSCs) with anti-miR-138 oligonucleotides increased osteogenic capacity and bone formation.24

Several miRNAs were shown to exhibit dichotomous roles during osteoclastogenesis by simultaneously enhancing and suppressing differentiation (Figure 1 and Table 1), including miR-223 (Sugatani and Hruska13,25) and the miR-29 family.26,27 Although one study reported that ectopic miR-29b expression inhibited osteoclast activity,26 a more recent report found miR-29 to be able to promote osteoclastogenesis by directly targeting Cdc42 and Srgap2, and knockdown of miR-29 in preosteoclasts to inhibit differentiation.27 Additional studies uncovered miRNAs with more easily defined roles during osteoclast differentiation (Figure 1 and Table 1). MiR-31, which is highly upregulated during osteoclast development, has been shown to be essential for proper differentiation,28 whereas miR-21 positively regulated osteoclastogenesis by targeting the c-Fos inhibitor PDCD4.14 Conversely, RANK targeting by miR-503 inhibited osteoclastogenesis, and silencing miR-503 enhanced in vivo bone resorption.29 MiR-125a was shown to inhibit differentiation by suppressing TRAF6 expression,30 and miR-155 regulated cell-fate commitment within macrophages through the downregulation of SOCS1 and MITF, thereby effectively inhibiting differentiation through an osteoclast lineage.31,32 Similarly, miR-34a-overexpressing transgenic mice exhibited lower bone resorption and higher bone mass, whereas miR-34a knockout and heterozygous mice displayed elevated bone resorption and reduced bone mass.33

As regulation of bone homeostasis depends on a carefully orchestrated crosstalk among bone-residing cells, miRNAs that are capable of regulating differentiation and/or activity of any bone stromal cells could markedly influence overall bone deposition and degradation. One such miRNA, miR-34c, is an essential regulator of Notch signaling in osteoblasts by directly targeting Notch1, Notch2 and Jagged1, as well as Satb2 and Runx2.34 Interestingly, miR-34c has been shown to function with both cell- and non-cell-autonomous activities. Upregulation of miR-34c inhibits osteogenesis by decreasing Satb2 and Runx2 in osteoblasts, whereas inhibition of Notch signaling may regulate the RANKL/OPG ratio, leading to increased osteoclast differentiation.34,35

Given the profound effect that regulated miRNA expression has on bone marrow cell differentiation and activity, it is perhaps not surprising that misregulation of miRNAs has also been linked to a number of bone-related pathologies, including osteoporosis36,37,38 and osteosarcoma.39,40,41,42,43 Importantly, misregulated miRNAs have been proposed as potential biomarkers of the specific bone-related diseases as they may accurately reflect the disease state of the bone. Because osteoporosis is caused by an imbalance between bone formation and resorption, misregulated bone marrow miRNAs represent promising indicators of assessment of bone fragility and provide potential therapeutic options to restore homeostasis. To this end, several circulating miRNAs (miR-21, miR-23a, miR-24, miR-25, miR-100, miR-125b and miR-133a), most of which have previously been linked to the regulation of bone metastasis (Table 1), have emerged as novel biomarkers of bone turnover in osteoporosis,44,45 demonstrating the feasibility of using miRNAs as biomarkers of bone disease.

Bone-derived miRNAs in bone metastasis

In addition to mediating bone development and homeostasis under physiological conditions, miRNAs can influence essential tumor–stroma interactions that mediate metastasis. Many bone metastatic cancers dysregulate tumor-intrinsic miRNA expression to establish these metastasis-promoting tumor–stroma interactions. For instance, miR-33a is downregulated in lung cancer cells because it functions as a bone metastasis suppressor by targeting PTHrP, a known activator of osteolytic bone resorption, and reducing the ability of cancer cells to induce osteoclast differentiation and activity.46 However, it is also possible for aberrant expression of stromal miRNAs to enhance bone metastatic progression. Thus, any of the miRNAs involved in bone formation and homeostasis described above may potentially participate in osteolytic or osteoblastic bone metastasis if their expression was altered by bone metastatic cancer cells. Such dysregulated miRNAs could potentially be used as biomarkers of bone metastatic progression or as therapeutic targets. However, the levels and function of the majority of these miRNAs during bone metastasis has first to be elucidated before their clinical usefulness can be assessed. Here, we review the known miRNAs expressed in bone stromal cells that are tumor-induced and/or influence bone metastasis (Figure 2 and Table 2).

Figure 2.

Figure 2

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Bone stromal cell microRNA (miRNA) involvement in bone metastasis. MiRNAs regulate bone metastasis activity, including the tumor–stroma crosstalk that mediates osteolysis. Bone metastatic tumor cells promote osteoclastogenesis by repressing a number of negative (red) regulators of osteoclast differentiation. In turn, bone marrow stromal cells use miRNA-mediated regulation to decrease proliferation and mobility and induce dormancy in bone metastatic cells. The modified bone microenvironment releases cell-free and exosomal miRNAs that can be detected in the serum of cancer patients.

Table 2. MiRNAs in bone metastasis.

miRNA Cell type Activity Species Cancer type Reference
Biomarkers of bone metastasis
 MiR-16 Osteoclasts Increased in presence of bone metastases Human and mouse Breast 47
 MiR-326 Osteoclasts Increased in presence of bone metastases Mouse Lung 57
 MiR-378 Osteoclasts Increased in presence of bone metastases Human and mouse Breast 47
miRNA Cell type Activity Targets Cancer type Reference
Regulators of bone metastasis
 Let-7 BM-MSC Decreases mobility of cancer cells IL-6 Prostate 58
 MiR-23b BM-MSC Decreases proliferation and mobility of cancer cells MARCKS Breast 60
 MiR-33a Osteoclasts Decreases osteoclastogenesis PTHrP Breast and lung 41,47
 MiR-34a Osteoclasts Decreases bone metastasis in transgenic mouse model Tgif2 Breast and skin 33
 MiR-127 BM stromal cells Decreases proliferation of cancer cells, induced dormancy CXCL12 Breast 59
 MiR-133a Osteoclasts Decreases osteoclastogenesis Mitf, Mmp14 Breast 47
 MiR-141 Osteoclasts Decreases osteoclastogenesis, inhibits bone metastasis in vivo Mitf, Calcr Breast 47
 MiR-190 Osteoclasts Decreases osteoclastogenesis Calcr Breast 47
 MiR-197 BM stromal cells Decreases proliferation of cancer cells, induced dormancy CXCL12 Breast 59
 MiR-219 Osteoclasts Decreases osteoclastogenesis, inhibits bone metastasis in vivo Mitf, Traf6 Breast 47
 MiR-222 BM stromal cells Decreases proliferation of cancer cells, induced dormancy CXCL12 Breast 59
 MiR-223 BM stromal cells Decreases proliferation of cancer cells, induced dormancy CXCL12 Breast 59
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Abbreviations: BM, bone marrow; miR, microRNA; MSC, mesenchymal stem cell.

During osteolytic metastasis, tumor cells co-opt the regulatory signaling pathways within bone marrow cells to recruit and activate osteoclasts, which in turn orchestrate bone degradation. Any miRNAs that functionally promote or inhibit osteoclastogenesis during this process could represent a good biomarker for osteolytic bone metastasis or serve as a therapeutic target to diminish bone metastasis. One such miRNA, miR-34a, has been shown to decrease osteoclastogenesis and bone resorption under physiological conditions and increase bone mass in transgenic mice, which specifically overexpress miR-34a in osteoclasts.33 MiR-34a, which is downregulated during osteoclast differentiation, could diminish osteoporosis as well as bone metastasis of breast and skin cancers in the same osteoclastic miR-34a transgenic mice model by directly inhibiting the expression of Tgif2.33 Hence, a key osteoclast suppressor miRNA involved in normal bone turnover could potentially become a therapeutic target to confer skeletal protection and ameliorate bone metastasis of cancers.

Recently, an miRNA signature induced by highly metastatic tumor cells that stimulates differentiation of osteoclasts and recruits preosteoclasts to the site of the tumor–bone interface has been described.47 Several miRNAs within the signature have been shown to directly influence osteoclast activity during bone metastasis, including miR-33a, miR-133a, miR-141, miR-190 and miR-219.47 Interestingly, similar to miR-34a, in vitro osteoclastogenesis assays revealed a necessity for these miRNAs to be downregulated. Their ectopic expression, on the other hand, inhibited differentiation through the functional targeting of essential osteoclast genes acting at different stages of osteoclast differentiation. Specifically, miR133a, miR-141 and miR-219 were all found to repress expression of Mitf. In addition, miR-133a was found to repress Mmp14, miR-141 and miR-190 repressed Calcr and miR-219 was able to repress Traf6.47 Importantly, systemic inoculation of pre-miRNA oligonucleotides for miR-141 or miR-219 was sufficient to inhibit bone metastasis from human breast cancer cells.47 Thus, it is possible for bone marrow cell miRNAs to be used as biomarkers of tumor-induced bone microenvironmental changes and osteolytic bone metastasis, as well as serve as potential therapeutic targets.

In addition to the biological significance of intracellular miRNAs, recent studies revealed the functional importance of secreted miRNAs.48,49,50 miRNAs can be released from cells as cell-free miRNAs either bound to protein, such as AGO2, or lipid carriers, such as HDL or LDL, or packaged into microvesicles such as exosomes (30–100 nm) or larger microparticles (100–1000, nm).51 The presence of their protein- or lipid-based carriers or their encapsulation into vesicles confer circulating miRNAs remarkable stability under a variety of harsh environmental conditions.50,51 As such, secreted miRNAs represent important potential biomarkers for diagnosis and prognosis that can be detected by noninvasive means in a reliable and reproducible manner. In fact, several studies have reported the use of circulating miRNAs as biomarkers to predict tumor progression and the presence of bone metastases.51,52,53,54 Furthermore, highly expressed circulating miRNAs from cancer patients have been reported to return to a normal level after tumor resection,55,56 suggesting that the level of circulating miRNAs reflects the disease state of the patient to some extent and could be used as a pharmacodynamic marker. Moreover, cell-free and exosomal miRNAs are of particular interest owing to their pleiotropic roles in modulating many physiological and pathological processes, including cancer metastasis.48

Serum miRNAs shown to be upregulated during metastasis-induced osteoclast differentiation could thus be used as an indicator of osseous metastases or as a potential therapeutic strategy. Two such osteoclastic miRNAs, miR-16 and miR-378, were found significantly elevated in the serum from mice bearing highly metastatic breast cancer cells, and in bone lesions and serum from patients with breast cancer metastatic to bone as compared with healthy female donors.47 In fact, miR-16 and miR-378 possessed comparable sensitivity relative to the bone turnover marker N-terminal telopeptide (NTX), indicating the potential for using these miRNAs as biomarkers for bone metastasis progression. In a separate study, serum levels of miR-326 strongly associated with tumor burden and a marker of bone metastatic burden, PINI (procollagen I amino-terminal propeptide), in a mouse model of lung cancer bone metastasis.57 These results indicate that miRNAs such as miR-16, miR-326 and miR-378 could potentially serve as novel biochemical markers for monitoring osteolytic bone metastatic progression.

Bone stromal miRNAs are also capable of directly influencing metastatic cells. Let-7c inhibited IL-6 within bone marrow MSCs (BM-MSCs), whereas ectopic expression of Let-7c was capable of repressing the migration- and invasion-promoting potential of tumor-associated BM-MSCs.58 A separate study found that several miRNAs were transferred from bone marrow stromal cells to nearby breast cancer cells through gap junctions and to a smaller extent via exosomes upon seeding to the bone.59 MiR-127, miR-197, miR-222 and miR-223, all direct inhibitors of CXCL12, inhibited proliferation and arrested the metastatic cells in a dormant state in which the latter could survive high-dose chemotherapy, making these miRNAs interesting targets for combination treatment with conventional therapy in which dormancy is an obstacle. Moreover, exosomal miR-23b has also recently been shown to induce dormancy in breast cancer cells after being released by BM-MSCs.60 Furthermore, transfer of stromal exosomes containing miR-23b was shown to inhibit cell cycling and mobility by targeting MARCKS in cancer cells. Interestingly, metastatic breast cancer cells in patient bone marrow had increased miR-23b and decreased MARCKS expression. Taken together, some bone stromal miRNAs may be useful biomarkers for specific metastasis-related processes such as increased chemoresistance or quiescence, which could direct the therapeutic regimen.

miRNAs as markers and therapeutic targets

It is currently difficult to accurately diagnose patients with emerging bone metastasis, and standard biomarkers, such as NTX, show only modest specificity as diagnostic markers. miRNAs expressed by the bone stroma could lead to improvements in diagnosis, treatment and prevention of bone metastases and elucidate the unique aspect of the bone microenvironment that supports tumor growth in the bone. More specifically, characterization of expression and secretion of bone marrow-derived miRNAs could enable the selection of patients at increased risk of bone metastasis for appropriate targeted therapies. Such patient selection may be achieved by simple quantitative diagnostic tests, such as quantitative reverse transcription-PCR, for specific miRNAs whose levels correlate with bone metastatic progression. Such diagnostic tests would probably analyze miRNA levels of a clinical ‘bone metastatic signature, which is likely to be made up of miRNAs specifically expressed by metastatic or bone metastatic cancer cells, as well as miRNAs whose expression is altered owing to tumor-induced changes in the bone microenvironment. Moreover, such miRNA-based diagnostic tests will likely be combined with other diagnostic and prognostic biomarkers of cancer progression or bone metastasis (such as serum proteins) to increase sensitivity and specificity of detection. This diagnostic approach would identify cancer patients at high risk for bone metastasis, therapeutic resistance or dormant cancer cells in the bone, who would greatly benefit from a preventive treatment or initiate therapies at an earlier state of metastatic lesion. Moreover, owing to their potential use as pharmacodynamic biomarkers whose levels depict the disease state of the patient, miRNAs possess the potential to guide treatment regimens during cancer therapy.

Furthermore, miRNAs with functional roles in promoting bone metastatic progression could become important therapeutic targets. Potential therapies for such metastasis-promoting miRNAs include siRNA or antisense oligonucleotides, miRNA sponges, miRNA masking and small-molecule inhibitors. Furthermore, endogenous or synthetic exosomes could be used to efficiently deliver these or other inhibitors in a tissue-specific manner.61,62 On the other hand, for metastasis-inhibiting miRNAs usually downregulated during cancer progression, administration of synthetic miRNAs or forced expression of endogenous miRNAs may be a useful therapeutic option to restore expression. Using systemic inoculation of pre-miRNA oligonucleotides for osteoclastic miRNAs miR-141 or miR-219, Ell et al.47 successfully inhibited bone metastasis from human breast cancer cells. As intravenously injected exosomes are detectable in the bone marrow of mice,63 miRNAs could also be delivered using exosomes. However, the development of miRNA-based therapeutics will require a more complete understanding of miRNA–mRNA targeting in diverse tissue types to minimize potential side effects owing to the multitarget nature of miRNAs in gene regulation. Moreover, identifying the specific targets through which these miRNAs exert their function in bone metastasis also opens possibilities to identify potential downstream targets of therapeutic interest.

Conclusions

miRNA activity represents an essential level of gene regulation during skeletal development and homeostasis. Ablation of miRNA activity, through the inhibition of miRNA processing pathway components such as Dicer or AGO2, has been shown to inhibit the development of multiple bone marrow stromal cells, resulting in devastating skeletal defects. In addition, dysregulation of individual miRNAs inhibited the differentiation and activity of multiple cell types and precipitated pathological events. Importantly, miRNA regulation has also been shown to have pivotal roles in regulating the development and progression of bone metastasis. In particular, the role of bone stromal miRNAs, such as those expressed in osteoclasts, as mediators or inhibitors of bone metastasis and the changes in their expression levels during disease progression have only recently started to be uncovered. Such knowledge, however, will be vital in further deepening our understanding of bone metastatic progression and transferring the knowledge from the bench into the clinical setting.

Bone marrow stroma-derived miRNAs constitute a novel class of dysregulated molecules that could provide new avenues for diagnosis and treatment of bone metastases. In particular, circulating miRNAs derived from tumor-induced changes in the bone microenvironment represent a novel class of biomarkers with large diagnostic and prognostic potential. This potential is based on their noninvasive detection in body fluids and their high resistance and stability under various conditions that could degrade the majority of RNAs. In addition, several methodologies are available for establishing miRNA expression levels and signatures in a high-throughput and robust manner. Although the recent data are exclusively preclinical evidence, the application of bone marrow stroma-derived miRNAs in metastasis therapy as adjuvant tools or targets appears very promising. A better understanding of the complex network of genes and cellular signaling transduction pathways regulated by miRNAs would enrich our knowledge of metastatic progression and hence improve the therapeutic outcome of cancer patients.

Acknowledgments

Research in our laboratory was supported by grants from the Komen for the Cure (KG110464), Breast Cancer Research Foundation, Department of Defense (BC123187), Brewster Foundation and the National Institutes of Health (R01CA134519 and R01CA141062) (to YK).

Footnotes

The authors declare no conflict of interest.

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