The Emerging Roles of Long Noncoding RNAs in Bone Homeostasis and Their Potential Application in Bone-Related Diseases

Abstract

Increasing evidence has announced the emerging roles of long noncoding RNAs (lncRNAs) in modulating bone homeostasis due to their potential regulating effects on bone-related cells' proliferation, migration, differentiation and apoptosis. Thus, lncRNAs have been considered as a promising gene tool to facilitate the bone regeneration process and then to predict and cure bone-related diseases such as osteosarcoma, osteoporosis, and osteoarthritis. In this review, we first enumerated several kinds of dysregulated lncRNAs and concisely summarized their regulating role in bone formation as well as resorption process. The related mechanisms were also discussed, respectively. Then, the positive or negative behavior of these lncRNAs in bone-related diseases was elucidated. This review provides an in-depth sight about the lncRNA's clinical values and limitations, which is conducive to explore new gene targets and further establish new therapeutic strategies for bone-related disease.

Introduction

Bone homeostasis (BH) is one kind of dynamic balance between bone resorption resulted from osteoclasts and bone formation by osteoblasts (Rodan, 1998; Nakashima et al., 2011). Maintenance of constant BH is profound to human body because this physiological process can effectively control the bone's structure and amount and ensure normal development of skeleton (Harada and Rodan, 2003). Once BH turns into a continuous maladjustment state, the risk of some severe bone metabolism diseases such as osteosarcoma (OS), osteoporosis (OP), and osteoarthritis (OA) will increase (Arantzazu et al., 2015; Huo et al., 2018; Zhao et al., 2019). Therefore, exploring the influence factors and mechanisms involved in BH is beneficial to preventing the occurrence of those diseases. Existing evidences revealed that multiple internal factors, including genes, cells, proteins, and external factors such as stress, drugs, and ultrasound, can bring significant impact to both osteogenesis and bone resorption process (Manolagas, 2000; Isaia et al., 2007; Schiavone et al., 2016). However, many preclinical trials have announced that these factors are not conducive to clinical use because it is difficult to obtain stable therapeutic effects under such uncontrollable conditions that resulted in an impossibly complicated therapeutic microenvironment (Rodan, 1998; Nakahama, 2010). Gene therapy is considered as a promising approach for these diseases due to its remarkable efficacy and safety, but the biggest challenge in gene therapy is how to find out the accurate gene target to achieve favorable therapeutic effect (Verma and Somia, 1997).

Long noncoding RNAs (lncRNAs) are a subset of noncoding RNAs (ncRNAs) with strand length exceeding 200 bp, which have been demonstrated to regulate various biological processes during gene transcription, cell proliferation, differentiation, and cancerization (Mercer et al., 2009; Ponting et al., 2009). Specifically, it is well established that the dysregulation of specific lncRNAs plays a key role in the pathogenesis and progression of bone diseases, which might provide insights into innovative treatment for alleviating or abating relevant diseases. According to some previous reports, the relationship between lncRNAs and osteoblasts has reached a definite conclusion, however, their roles and related mechanisms in bone resorption, or in other words, their connection with osteoclasts remains obscure (Hassan et al., 2015; Li et al., 2018c). Moreover, this deficit in understanding lncRNA-osteoclast interaction might be compounded by the microenvironmental diversity and osteoclast inherent properties.

Therefore, in the first section of this review, we take a deep insight into the emerging roles of several typical lncRNAs, specifically addressing their effects on the BH. In addition, we also analyze the regulating role of lncRNAs in the development and differentiation of bone and cartilage. Then, this review highlights the recent advances of the positive or negative behavior of those lncRNAs applied on some challengeable bone-related diseases. This review provides an in-depth understanding about the lncRNA's clinical values and limitations, which is conducive to explore new gene targets and further establish new therapeutic guidance for bone-related disease.

Typical Dysregulated lncRNAs in BH

LncRNAs are considered as essential regulators in BH because they can regulate the expression of key related enzymes and proteins during bone formation and bone resorption process. In this section, the actual functions and mechanisms of some typical dysregulated lncRNAs are addressed (Table 1) and the schematic is illustrated in Figure 1.

FIG. 1.

The functions and inner mechanisms of key lncRNAs, including (a) HOTAIR (b) DANCR (c) MEG3 (d) H19, and (e) GAS5 in bone homeostasis. DANCR, differentiation antagonizing nonprotein coding RNA; GAS5, growth arrest-specific transcript 5; lncRNA, long noncoding RNA; MEG3, maternally expressed gene 3.

Table. 1.

The Expression and Related Mechanisms of Long Non-Coding RNAs in Bone-Related Biological Processes (Listed in Alphabetical Order)

LncRNAs	Expression levels	Related biological processes	Mechanisms
AK028326	Up	Osteogenesis	Increases the expression of osteogenic genes by upregulating CXCL13 in hMSCs and MC3T3-E1 (Cao et al., 2016).
AK077216	Up	Osteoclastogenesis	Accelerates bone resorption by enhancing the abilities of osteoclasts through modulating NIP45/NFATc1 axis (Liu et al., 2019).
BDNF	Up	Osteogenesis	/
BDNF-AS	Down	Osteogenesis	/
CIR	Up	OA	Promotes ECM degradation under OA conditions, and affects OA progression by regulating autophagy (Wang et al., 2018a).
CRNDE	Up	PMOP	Promotes the proliferation of osteoclasts (Li et al., 2018b).
DANCR	Up/down	Osteogenesis, osteoclastogenesis, chondrogenesis, odontogenesis, OS, OA	Inhibits osteogenic differentiation of hBMSCs by suppressing the p38 MAPK signaling pathway (Zhang et al., 2018); knockdown of DANCR reduces FOXO1 degradation via blocking Skp2-mediated ubiquitination (Tang et al., 2018); suppresses osteoblast differentiation via decreasing the expression of Runx2 (Zhu and Xu, 2013).
			Activates osteoclasts via regulating Jagged1 expression in compression force-treated hPDLCs (Zhang et al., 2019b); increases IL-6 and TNF-α expression in blood mononuclear cells and enhances osteoclastic activity (Tong et al., 2015).
			Overexpression of DANCR promotes the proliferation and chondrogenesis of hSMSCs via modulating the expression of myc, Smad3, STAT3 mRNA (Zhang et al., 2017b), and by regulating miR-1305/Smad4 axis (Zhang et al., 2017a).
			Downregulated DANCR inhibits odontogenic differentiation of hDPCs via inhibiting the Wnt/β-catenin pathway (Chen et al., 2016).
			Overexpression of DANCR accelerates proliferation and metastasis of OS cells by sponging miR-1972 and miR-335-5p (Wang et al., 2018b).
			Promotes OA chondrocytes proliferation via DANCR/miR-577/SphK2 (Fan et al., 2018).
GAS5	Up/down	Osteogenesis, OS, OA	Upregulated GAS5 facilitates osteogenic differentiation of BMSCs via miR-135a-5p/FOXO1 axis (Wang et al., 2019).
GAS5	Up/down	Osteogenesis, OS, OA	The increased expression of GAS5 promotes OA development through sponging miR-21 (Song et al., 2014; Xing et al., 2014). GAS5 modulates the proliferation and apoptosis of growth plate chondrocytes through miR-21/FGF1 axis (Liu et al., 2018).
H19	Up	Osteogenesis, OS	Accelerates the BMSCs' osteogenic differentiation through Foxc2 (Zhou et al., 2019); promote the BMSCs' osteogenic differentiation by sponging miR-141 and miR-22 (Liang et al., 2016); regulates the osteogenic differentiation through the interaction between miR-675-5p and H19 (Zhang et al., 2019a); Modulates osteogenic differentiation and adipocyte differentiation of BMSCs via H19/miR-675/HDAC (Huang et al., 2016); promotes osteogenic differentiation of BMSCs by modulating miR-138/FAK axis (Wu et al., 2018); Inhibits osteogenesis via H19/DNMT1/ERK axis by mechanical unloading (Li et al., 2018a).
H19	Up	Osteogenesis, OS	Overexpression of H19 contributes to OS by binding to the miR-200 family; down-regulated H19 suppresses OS via NF-κB pathway (Li et al., 2016; Zhao and Ma, 2018). Hh signaling promotes OS development via Yap1 and H19 overexpression (Chan et al., 2014).
HOTAIR	Up/down	Osteogenesis, skeletal diseases, RA, OA, OS	HOTAIR inhibits miR-17-5p to regulate osteogenic differentiation and proliferation in non-traumatic osteonecrosis of femoral head (Wei et al., 2017).
			Knockdown of HOTAIR causes skeletal malformations via targeting at PRC2 and silencing the HOXD gene (Peng et al., 2018a).
			High expression of HOTAIR might lead to RA development (Song et al., 2015).
			In temporomandibular Joint OA rabbits, increased HOTAIR expression contributes to cartilage destruction by upregulating MMP-1, MMP-3 and MMP-9 (Zhang et al., 2016).
			High expression of OS cells regulates the apoptosis of tumor cells by regulating miR-126/DNMT1/CDKN2A axis (Li et al., 2017b). HOTAIR inhibits mineralization in osteoblastic OS cells through by HOTAIR/H3K4me/ALPL (Aya and Hideo, 2018).
HULC	Up	OS	Knockdown of HULC inhibits the viability and metastasis by upregulating miR-122 (Kong and Wang, 2018).
MALAT1	Up	Osteogenesis, OS	Regulates osteogenic differentiation of hBMSCs via miRNA-143/osterix aixs (Gao et al., 2018); promotes osteoblast differentiation of human aortic VICs via MALAT1/miR-204/Smad4 (Xiao et al., 2017).
MALAT1	Up	Osteogenesis, OS	MALAT1 has been reported to be highly expressed in human OS tissues. It might contribute to OS progression via activating the PI3K/Akt pathway (Dong et al., 2015).
MEG3	Up/Down	Osteogenesis, OS, MM, PMOP, OA	MEG3 has been significantly downregulated in hDFSCs. Downregulated MEG3 promotes osteogenic differentiation of hDFSCs by epigenetically regulating the Wnt/β-catenin signaling (Deng et al., 2018).
			MEG3 has been reported significantly decreased in OS tissues, which is correlated with poor prognosis (Tian et al., 2015). Knockdown of MEG3 promotes the growth and metastasis of OS cells by sponging miR-127 (Wang and Kong, 2018).
			Promotes osteocyte differentiation of hASCs by decreasing the expression of miR-140-5p (Li et al., 2017c).
			Downregulation of MEG3 significantly inhibits osteogenic differentiation and promotes adipogenic differentiation of hBMSCs from patients with MM through modulating SOX2/BMP-4 axis (Zhuang et al., 2015).
			Overexpression of MEG3 decreases osteogenic differentiation ability of hBMSCs by regulating miR-133a-3p expression in PMOP patients (Wang et al., 2017b).
			MEG3 was downregulated in OA patients (Su et al., 2015).
MIAT	Down	Osteogenesis	Knockdown of MIAT promotes osteogenesis of hASCs (Jin et al., 2017).
MIR31HG	Down	Osteogenesis	Affects the osteogenic action of hASCs under inflammatory microenvironment through MIR31HG-NF-κB regulatory loop (Jin et al., 2016).
MODR	Up	Osteogenesis	/
NTF3–5	Up	Osteogenesis	NTF3–5 promotes osteogenic differentiation of MSMSCs via sponging miR-93-3p (Peng et al., 2018b).
POIR	Up	Osteogenesis	LncRNA POIR promotes osteogenic differentiation of hPDLSCs through miR-182/FOXO1 axis (Wang et al., 2016).
TUG1	Up	Osteogenesis	Silencing TUG1 promotes osteogenic differentiation of VICs via modulating miR-204-5p expression (Yu et al., 2017).

BDNF-AS, brain-derived neurotrophic factor-antisense; BMP, bone morphogenetic protein; BMSC, bone mesenchymal stem cell; DANCR, differentiation antagonizing non-protein coding RNA; ECM, extracellular matrix; FAK, focal adhesion kinase; Foxc2, forkhead box C2; FOXO1, forkhead box protein O1; GAS5, growth arrest-specific transcript 5; hASCs, human adipose-derived stem cells; hBMSCs, human BMSCs; HDAC, histone deacetylase; hDFSCs, human dental follicle stem cells; hDPCs, human dental pulp cells; hPDLCs, human periodontal ligament cells; hPDLSCs, human periodontal ligament stem cells; hSMSCs, human synovium-derived stem cells; lncRNA, long non-coding RNA; MALAT1, metastasis-associated lung adenocarcinoma transcript 1; MEG3, maternally expressed gene 3; miRNA, microRNA; MM, multiple myeloma; MMP, matrix metalloproteinase; mRNA, messenger RNA; MSMSCs, maxillary sinus membrane stem cells; NF-κB, nuclear factor-kappa B; OA, osteoarthritis; OS, osteosarcoma; PMOP, postmenopausal osteoporosis; PRC2, polycomb repressive complex 2; RA, rheumatoid arthritis; Runx2, runt-related transcription factor 2; VICs, valve interstitial cells.

lncRNA H19 (H19)

H19, as one kind of the most abundant and conserved lncRNAs, is confirmed to have an upregulation of expression in bone mesenchymal stem cells (BMSCs) during osteogenesis (Li et al., 2016). Abundance of work have revealed that H19 can significantly accelerate the BMSCs' osteogenic differentiation with the help of forkhead box C2 (Foxc2), which is a key factor in regulating Wnt promoter expression in postmenopausal osteoporosis (PMOP) patients (Zhou et al., 2019). However, it is worth noting that the expression of H19 in human adipose-derived stem cells (hASCs) was lower than that in human BMSCs (hBMSCs), suggesting a more stable state of hASCs than hBMSCs because lower H19 expression contributes to genomic stability (Ravid et al., 2014).

Besides, H19 also has the ability to regulate the expression of microRNA (miRNA) and messenger RNA (mRNA) in bone development. According to Liang et al.'s (2016) research, two miRNAs named miR-141 and miR-22 can inhibit the osteogenesis by downregulating the expression of β-catenin. H19 can effectively sponge these two miRNAs and thereby accelerate the osteogenesis via activating the Wnt/β-catenin signaling pathway. Moreover, the interaction between miR-675-5p and its host gene H19 is also recognized as a crucial factor to influence the osteogenic differentiation. In brief, the expression of miR-675-5p can be elevated by overexpression of H19, but miR-675-5p downregulated H19 expression conversely during osteogenesis, suggesting the reduced osteogenic differentiation when miR-675-5p was overexpressed (Zhang et al., 2019a). Epigenetic modulation plays an important role in mediating the interaction between H19 and miR-675. The downregulation of H19 and miR-675 can increase adipocyte differentiation of BMSCs, but inhibit osteogenic differentiation by targeting histone deacetylase (HDAC), and thus lead to the progression of OP (Huang et al., 2016). Another study also showed that the interaction between H19 and miR-675 played a key role in regulating hBMSCs' differentiation via a novel signaling pathway H19/miR-675/TGF-β1/Smad3/HDAC (Huang et al., 2015). Similarly, based on Zhao's report, H19/miR-675/NOMO1 axis was also considered as one of the major reasons to elicit DLX3 gene mutation, which could further lead to serious tricho-dento-osseous (TDO) syndrome, which is a systemic genetic disorder characterized by joint abnormalities of hair, teeth, and bones (Zhao et al., 2017).

In addition, many researchers have proved that applying proper mechanical tension is another propelling way to enhance the BMSCs' osteogenic ability. The related mechanism can be briefly summarized that high-level expression of H19 can suppress the function of miR-138 and correspondingly upregulate its target focal adhesion kinase (FAK) (Wu et al., 2018). Therefore, it is likely to regulate the expression level of H19 by applying various mechanical stimuli under specific circumstances, which is promising to postpone the development of some hypokinesia-induced bone diseases such as disuse osteoporosis (DOP) (Li et al., 2017a). Conversely, DOP can elicit mechanical unloading to increase H19 methylation via DNMT1 as well as inhibit the ERK signaling, followed by impairment of osteogenesis (Li et al., 2018a).

It also should be noted that overexpression of dysregulated H19 may elicit several pathogenic processes. For example, H19 can promote OS metastasis by means of competitively binding to the miR-200 family and thereby increasing the expression of ZEB1 and ZEB2, which are two major proteins related to tumor metastasis and progression. Meanwhile, downregulating H19 expression would productively restrain development of OS via suppressing nuclear factor-kappa B (NF-κB) pathway (Li et al., 2016; Zhao and Ma, 2018). Previous report has explained that the aberrant Hedgehog signaling might act as an inducer for H19 overexpression as well as Yap1 upregulation in mature osteoblasts, which contributes to the development of osteoblastic OS (Chan et al., 2014).

Differentiation antagonizing nonprotein coding RNA

In recent decades, many researchers have illustrated the downregulating role of lncRNA differentiation antagonizing nonprotein coding RNA (DANCR) in hBMSCs' proliferation and differentiation. One possible mechanism is that DANCR inhibits osteogenic differentiation of hBMSCs via suppressing the p38 MAPK signaling pathway (Zhang et al., 2018). Further evidence has shown that knockdown of DANCR resulted in reduced degradation of forkhead box protein O1 (FOXO1) via blocking Skp2-mediated ubiquitination, thus led to elevated FOXO1 expression and improved bone formation (Tang et al., 2018). The study by Zhu and Xu (2013) has shown that DANCR had ability to decrease the expression of runt-related transcription factor 2 (Runx2) and suppressed osteoblast differentiation by interacting with enhancer of zeste homolog 2 (EZH2). Similarly, in the coculture system of hBMSCs and human amniotic mesenchymal stem cells (hAMSCs), DANCR significantly decreased, which elicit to an increasing expression level of Runx2 mRNA. Therefore, hAMSCs was regarded as potential regulator to osteogenic differentiation of hBMSCs via altering the DANCR's expression (Wang et al., 2017a).

It should be noted that DANCR also involves in osteoclastogenesis. As generally accepted, the maintenance of bone physiological state mainly relies on the balance between bone formation and resorption. Overactivated osteoclasts are responsible for reduced bone mineralization and subsequent bone diseases. During this process, DANCR can promote the osteoclast activation via positively regulating Jagged1 expression in compression force-treated human periodontal ligament cells (Zhang et al., 2019b). Meanwhile, DANCR can also increase the expression of IL-6 and TNF-α in blood mononuclear cells and accordingly enhance osteoclastic activity, which endows it with the possibility as the biomarkers for PMOP (Tong et al., 2015).

In addition, DANCR exerts an essential impact on chondrogenesis of human synovium-derived stem cells (hSMSCs). Overexpression of DANCR can stimulate the proliferation of hSMSCs via modulating the expression of myc, Smad3, and STAT3 mRNA and further triggering the chondrogenesis through regulating Smad3 and STAT3 (Zhang et al., 2017b). In addition, SRY-related HMG-box 4 (SOX4) as a crucial molecular determinant of hSMSC activity might result in the upregulation of DANCR through interacting with the related promoters (Zhang et al., 2015). It was also highlighted that miR-1305, a downstream target of DANCR, was greatly downregulated in DANCR-overexpressed hSMSCs, which can reduce the expression of Smad4 and finally activated hSMSC proliferation and chondrogenesis (Zhang et al., 2017a).

Recent research announced that DANCR was mainly downregulated in a time-dependent manner during odontogenic differentiation of human dental pulp cells in view of the inhibition of Wnt/β-catenin pathway (Chen et al., 2016). The same variation trend exhibited during the phases of proliferation and osteogenic differentiation in human periodontal ligament stem cells (hPDLSCs) (Jia et al., 2015)

DANCR is also considered as a crucial regulator in OS. Specifically, overexpression of DANCR can facilitate OS cell line's proliferation and metastasis. The reasons can be expounded that DANCR as one kind of competing endogenous RNA (ceRNA) can decoy both miR-1972 and miR-335-5p, further negatively modulate the expression of the OS's oncogene named rho-associated coiled-coil containing protein kinase 1 (ROCK1) (Wang et al., 2018b). As for cartilage disease, DANCR significantly increased in OA patients mainly because this special lncRNA can facilitate the proliferation of OA chondrocytes via sponging miR-577 and regulate the transcription of the downstream molecular SphK2 (Fan et al., 2018). In general, by means of complicated interaction with several molecules, DANCR is indispensable in either the process of osteogenesis or chondrogenesis, suggesting its boundless feasibility as a therapeutic target for bone- and cartilage-related diseases.

LncRNA HOTAIR

As generally accepted, HOTAIR is a negative factor to osteogenic differentiation mainly because HOTAIR can activate HOXC11 gene in hMSCs (the HOX-family are believed to regulate osteogenesis as well as the control of bone tissue organization) (Rinn et al., 2007). Recent evidence has confirmed that HOTAIR was harmful to normal osteogenesis and it participated in several bone diseases. The related mechanisms explained by Kalwa et al. (2016) have revealed that the purine motif triple helix formed within the HOTAIR sequence resulted in abnormal expression such as overexpression or knockdown of HOTAIR in MSCs. This would definitely affect the MSC's differentiation. The role of HOTAIR in bone development discussed by Peng et al. (2018a) has illustrated that knockdown of HOTAIR possibly resulted in homologous transformation and skeletal malformations by means of targeting at polycomb repressive complex 2 (PRC2) and silencing the HOXD gene.

HOTAIR has shown negative regulation of osteogenic differentiation by downregulating miR-17-5p expression and upregulating its downstream target SMAD7 in nontraumatic osteonecrosis of femoral head (Wei et al., 2017).

In addition, HOTAIR might play a part in the development of rheumatoid arthritis (RA). High expression of HOTAIR in blood monocytes and serum exosome of RA patients may cause macrophage migration and abnormal immune response. Meanwhile, matrix metalloproteinase (MMP)-2 and MMP-13 expression decreased in differentiated osteoclasts and rheumatoid synoviocytes when a lower level of HOTAIR was detected (Song et al., 2015). HOTAIR functioned in the progression of cartilage destruction as well. In IL-1β-induced temporomandibular joint OA, HOTAIR expression increased in primary rabbit condylar chondrocytes, resulting in the upregulation of MMP-1, MMP-3, and MMP-9, impairing the normal construction and function of joints (Zhang et al., 2016).

HOTAIR controlled the epithelial cell memory program and involved in tumor pathogenesis (Procino, 2016). In brief, OS cells exhibited a high level of HOTAIR, which could regulate the OS cell apoptosis. HOTAIR depletion contributed to the decrease of DNMT1 and DNA methylation level via regulating miR-126 and thus eliminated the inhibition of CDKN2A expression induced by DNA hypermethylation (Li et al., 2017b). Since HOTAIR contributes to ALPL (Tissue Nonspecific Alkaline Phosphatase) repression via modulating histone H3K4 methylation levels at ALPL promoter region, HOTAIR could attenuate mineralization in osteoblastic OS cells (Aya and Hideo, 2018).

Maternally expressed gene 3

Maternally expressed gene 3 (MEG3) is known as a tumor suppressor for its capacity of modulating cell growth and differentiation as well as inhibiting angiogenesis (Zhang et al., 2003). MEG3 expression was significantly decreased in OS tissues compared to adjacent nontumor tissues, and the downregulation was correlated with poor prognosis (Tian et al., 2015). In addition, knockdown of MEG3 prompted the growth and metastasis of OS cells by sponging miR-127 (Smolle and Pichler, 2018; Wang and Kong, 2018). Correspondingly, Sun et al. (2016) discovered that lncRNA Ewing sarcoma-associated transcript 1 (EWSAT1) was able to promote the proliferation, migration, and invasion of OS cells through suppressing MEG3.

The impact of MEG3 on hASCs also attracted many researchers' attention. Abundant experimental results have revealed that MEG3 might promote osteocyte differentiation of hASCs via decreasing the expression of miR-140-5p, which was considered as a key factor in facilitating adipogenic differentiation, while inhibit osteogenic differentiation of hASCs (Li et al., 2017c).

Similarly, Zhuang et al. also directly proved that downregulation of MEG3 significantly inhibited osteogenic differentiation and promoted adipogenic differentiation of hBMSCs from patients with multiple myeloma (MM). Knockdown of MEG3 reduced the expression of several key osteogenic markers, including Runx2, osterix, and osteocalcin. The possible mechanisms could be summarized as follows: MEG3 directly eliminated the inhibitory effect of SOX2, which was located at the bone morphogenetic protein (BMP)-4 promoter region, thereby enhancing the transcription of BMP-4 (Zhuang et al., 2015). However, another research lies in PMOP patients announced that overexpression of MEG3 would positively regulate the expression of miR-133a-3p, further led to the decreased osteogenic differentiation ability of hBMSCs (Wang et al., 2017b). Interestingly, downregulation of MEG3 was observed in OA patients compared to normal cartilage, suggesting that MEG3 may be involved in OA development (Su et al., 2015). Besides, MEG3 was significantly downregulated in human dental follicle stem cells (hDFSCs), which activated the Wnt/β-catenin signaling by epigenetic regulation, and thus promoted osteogenic differentiation of hDFSCs (Deng et al., 2018). Therefore, it is rational to infer that cell types and microenvironment are two major factors to determine the actual function of MEG3 under pathological conditions.

Growth arrest-specific transcript 5

Traditionally, growth arrest-specific transcript 5 (GAS5) was widely known as a growth suppressor. However, accumulative evidence has manifested its important role in affecting the cells' behavior under specific circumstances. Upregulated GAS5 facilitated osteogenic differentiation of BMSCs via miR-135a-5p/FOXO1 axis (Wang et al., 2019). The expression level of GAS5 was remarkably downregulated in OS cells. Furthermore, genetic variant rs145204276, located in the promoter region of GAS5, is concerned with the development of OS (Xu et al., 2018). Hence, it is possible to explore GAS5 as a promising gene target for OS treatment.

Another interesting fact is that the expression of GAS5 was higher in cartilage of patients with OA compared with normal cartilage due to the varied expression level of miR-21 according to Song's study (Song et al., 2014; Xing et al., 2014). The proliferation and apoptosis of growth plate chondrocytes could be modulated through GAS5/miR-21/FGF1 axis (Liu et al., 2018). Hence, GAS5 could promote OA development through inhibiting expression of miR-21 which would likely to induce chondrocyte apoptosis. Therefore, GAS5 also shows its huge potential as a novel therapeutic target to block the progression of OA.

Metastasis-associated lung adenocarcinoma transcript 1

Metastasis-associated lung adenocarcinoma transcript 1 (MALAT1) was reported to play an important role in bone metastasis in patients with nonsmall cell lung cancer (Liu et al., 2016). Gao et al. (2018) demonstrated that MALAT1 regulated osteogenic differentiation of hBMSCs via miRNA-143/osterix axis. Furthermore, MALAT1 acts as a ceRNA for miR-204 to increase Smad4 expression, thus promoting osteoblast differentiation of human aortic valve interstitial cells (VICs) (Xiao et al., 2017). In addition, MALAT1 was highly expressed in human OS tissues. It might contribute to OS progression via activating the PI3K/Akt pathway (Dong et al., 2015).

Other lncRNAs

Some other lncRNAs have also played important roles in regulating the BH. Their actual functions are briefly described as follows.

AK028326

According to recent reports, lncRNA AK028326 positively regulated the expression of osteogenic genes in hMSCs and MC3T3-E1 cells via increasing CXCL13 expression at both mRNA and protein level. Glucose as the negative factor can decrease the expression of AK028326 in hMSCs featured in a time-dependent manner, further downregulated the expression of osteogenesis-related genes (Cao et al., 2016).

AK077216

The expression of lncRNA AK077216 has been confirmed to have an obvious increasing trend in osteoclastogenesis, suggesting its optimal effect to accelerate the bone resorption process. Taking RAW264.7 cells from ovariectomized mice as example, AK077216 upregulated the expression of NFATc1, a crucial regulator in osteoclastogenesis, by inhibiting NIP45 in bone marrow and spleen tissues, finally resulted in enhanced activity of osteoclasts (Liu et al., 2019)

Brain-derived neurotrophic factor, brain-derived neurotrophic factor-antisense

Brain-derived neurotrophic factor-antisense (BDNF-AS) transcript was upregulated during osteogenic differentiation of hBMSCs. Conversely BDNF-AS was downregulated suggesting its adverse effect on osteogenesis (Feng et al., 2018). Furthermore, the expression of BDNF-AS gradually downregulated during the proliferation process of OS cells, which means that high expression of BDNF-AS may decrease the cells' proliferation (Huang et al., 2018).

LncRNA-CIR

A study by Wang et al. (2018a) confirmed that high level of LncRNA-CIR (a special cartilage injury-related lncRNA) in cartilage promoted the degradation of extracellular matrix under OA conditions. In addition, CIR contributed to the inhibition of OA progression by activating autophagy and aggravating cartilage degeneration.

CRNDE

The expression of lncRNA CRNDE upregulated dramatically in osteoclasts under OP conditions suggesting its optimal role in osteoclasts' proliferation. In addition, under the PMOP circumstance with estrogen deficiency, CRNDE also displayed close relationship with the osteoclasts' proliferation. This result strongly proved that CRNDE has the potential to serve as the therapeutic target for treating these above-mentioned diseases (Li et al., 2018b).

LncRNA HULC

LncRNA highly upregulated in liver cancer (HULC) exhibited high level expression in OS cell lines. Knockdown of HULC would suppress tumor cell viability and metastasis, correspondingly increase apoptosis rate by upregulating one kind of tumor suppressor named miR-122 (Kong and Wang, 2018).

MIAT

LncRNA MIAT, acting as a sponge molecule for miR-150-5p, was downregulated in hASCs during osteogenic differentiation. Knockdown of MIAT could facilitate osteogenesis of hASCs and reverse the inflammation-caused repression by affecting the inflammatory factors expression level (Jin et al., 2017).

MIR31HG

Based on previous researches, MIR31HG was downregulated during osteogenesis of hASCs in vitro. Thereby, it was feasible to promote final osteogenesis effect of hASCs by knockdown of MIR31HG. MIR31HG also plays an important role in reversing the inflammation-induced repression of bone formation. Accumulative evidence has identified that through targeting MIR31HG-NF-κB regulatory loop, the osteogenic action of hASCs under inflammatory microenvironment could be improved (Jin et al., 2016).

MODR

LncRNA MODR was gradually upregulated during osteogenesis of maxillary sinus membrane stem cells (MSMSCs) and then increased the expression of Runx2 (Weng et al., 2017).

NTF3-5

Many researchers have confirmed the important regulating role of lncRNA NTF3-5 in osteoblastogenesis. Through cotransfection of NTF3-5 and miR-93-3p inhibitor, the MSMSCs could be stimulated and promote the bone formation effect (Peng et al., 2018b).

POIR

LncRNA POIR facilitated osteogenic differentiation of hPDLSCs both in vitro and in vivo. Acting as a ceRNA for miR-182, POIR might lead to the depression of FOXO1, which prompted bone formation of hPDLSCs through inhibiting Wnt/β-catenin pathway. However, the expression of POIR could be reduced by upregulating miR-182 that resulted from NF-κB pathway. Hence, POIR/miR-182/FOXO1 axis may keep an imbalance state under the inflammatory microenvironment (Wang et al., 2016).

TUG1

Previous studies have confirmed that high expression of lncRNA TUG1 was detected in human primary VICs. Silencing TUG1 caused an increase in miR-204-5p expression, and correspondingly increased Runx2 expression, thereby promoting osteogenic differentiation (Yu et al., 2017).

LncRNAs in Bone-Related Diseases

Although so far there are only a few clinical cases to show the application potential of lncRNAs in bone-related diseases, researchers still consider lncRNAs as promising candidates for biomarkers or therapeutic targets due to their diversities and vital effects on BH. This section will concisely review the recent advances about lncRNAs potential applications in several typical bone-related diseases. Figure 2 schematically shows the relationship between the lncRNAs and related diseases.

FIG. 2.

The relationship between lncRNAs and bone-related diseases, including OS, OP, OA, RA, and MM. MM, multiple myeloma; OA, osteoarthritis; OP, osteoporosis; OS, osteosarcoma; RA, rheumatoid arthritis.

Osteosarcoma

OS is an aggressive malignant neoplasm, featured by osteogenic differentiation of tumor cells and formation of malignant osteoid (Kong and Wang, 2018). LncRNAs LUCAT1could serve as predictive biomarkers to identify MTX-sensitivity of patients with OS if it lies in a relatively high expression level. This phenomenon provides a new strategy for the OS diagnosis. Smolle and Pichler (2018) displayed that MALAT1 might be a promising therapeutic target for OS treatment mainly because of its outstanding property to inhibit tumor cell's proliferation, migration, and invasion. In addition, MEG3, HULC, and DANCR have been proved to have different impact on OS prognosis. High MEG3 expression possibly resulted in a good prognosis. On contrast, overexpression of HULC or DANCR caused a poor prognosis in OS patients (Sun et al., 2015; Tian et al., 2015). In general, lncRNAs have potential to be selected as predictive, therapeutic, and prognostic biomarkers for OS's prevention and cure.

Osteoporosis

OP is a common skeletal disorder in older people. Its noticeable characterization is excessive bone loss and/or increased risk of bone spontaneous fracture. LncRNAs have positive effects during osteoblastogenesis or osteoclastogenesis process and thereby be recognized as potential tools to OP treatment. For instance, Tong's work has announced that DANCR could act as a diagnostic biomarker and therapeutic target for POMP. First, the expression of DANCR in the blood mononuclear cells was significantly upregulated in patients with low bone mineral density. Meanwhile, the expression levels of some osteoclastogenesis proinflammatory factors such as IL6 and TNF-α, which were derived from blood mononuclear cells, were also positively induced by DANCR, which means that limited DANCR function could effectively decrease those factors expression and thus elicit reduced bone resorption. However, our ongoing research has revealed that a certain amount of proinflammatory factors is essential conditions for the ectopic osteogenesis, suggesting that DNACR has the potential to serve as the regulator to quantitatively control this process (Tong et al., 2015; Compston et al., 2017).

Other lncRNAs can also play important roles in PMOP's treatment. MEG3 can be used as a novel therapy target mainly because it participates in the pathogenesis of PMOP (Wang et al., 2017b). AK077216 and CRNDE are closely associated with the osteoclasts existing amount as well as activation process, which suggest a possible indicative effect of POMP (Li et al., 2018b; Liu et al., 2019).

Osteoarthritis

OA is a common type of bone disease characterized by the degradation of degenerative joint cartilage and peripheral bone matrix (Arden et al., 2014). Recent researches have summarized 152 kinds of lncRNAs had dysregulated expressions in OA cartilage (Liu et al., 2014). Among these lncRNAs, the expression level of HOTAIR and GAS5 were significantly increased in OA patients, which might result in the cartilage destruction. Moreover, CIR's high expression in OA pathogenesis can inhibit the disease's progression by activating autophagy and aggravating cartilage degeneration. Thus CIR can be selected as a potential target in OA treatment (Xing et al., 2014; Zhang et al., 2016).

Repair and regeneration of bone tissues

The regulating effect of some lncRNAs on stem cells' proliferation and osteogenic differentiation has been confirmed suggesting their potential applications on facilitating the regeneration of bone tissues. Taking oral tissues for example, periodontal ligament stem cells (PDLSCs), as one kind of cells with self-renewal and multilineage differentiation properties, are responsible for the regeneration of cementum and adjacent alveolar bone (Gu et al., 2017). LncRNAs like DANCR can affect the differentiation of PDLSCs via acting as ceRNAs to regulate protein-coding transcripts. In other words, combined application of lncRNAs and PDLSCs provides a possible way to promote the regeneration of oral tissues. In addition, NTF3-5 promoted osteogenic differentiation of MSMSCs, suggesting a promising strategy to improve bone regeneration of the posterior maxilla (Peng et al., 2018b).

Rheumatoid arthritis

RA is a disabling autoimmune disease which could result in joint pain and arthrosclerosis (Singh, et al., 2016). According to recent findings, lncRNAs played vital roles in RA via modulating MMP expression, NF-κB pathway, T cell response, and other essential factors in autoimmunity and inflammation (Li et al., 2018d). In addition, the circulating level of HOTAIR might have clinical values to diagnose RA (Song et al., 2015).

Multiple myeloma

It is well known that the feature of MM is existed neoplastic plasma cells in the bone marrow and damaged osteogenic differentiation capacity of MSCs. The expression of lncRNA serum PCAT-1 in MM patients was significantly upregulated (Shen et al., 2017). Meanwhile, overexpression of MEG3 enhanced osteogenic differentiation of MSCs in MM patients (Zhuang et al., 2015). The overexpressed MALAT1 may serve as a biomarker to predict MM progression (Cho et al., 2014).

Conclusion

LncRNAs as an important regulator in many physiological activities have attracted increasing attention in recent decades. Specifically, in orthopedics research realm, lncRNAs have already exhibited the application potential to serve as the biomarker or gene therapy target to diagnose and even cure the related diseases. However, lack of adequate understanding of lncRNA's inner functional mechanisms during osteogenesis and bone resorption stages still greatly limited their clinical application. In this review, the regulating function and underlying mechanisms of several kinds of typical lncRNAs in BH were summarized at first in detail. Then, their potential applications in diverse bone-related diseases have also been highlighted. This review provides some theoretical supports for researchers to understand the lncRNA's clinical values and limitations, which is conducive to explore new gene targets and further establish new therapeutic strategies for bone-related disease.

Footnotes

Disclosure Statement

No competing financial interests exist.

Funding Information

This review was financially supported by the grants from National Natural Science Foundation of China (grant numbers 81701027) and National Key Research and Development Program of China (2016YFC1102700).

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