Introduction
Osteosarcoma (OS), primarily affecting adolescents and young adults, is among the most frequently occurring primary bone tumors (1). It arises from primitive transformed cells that exhibit osteoblastic differentiation and produce malignant osteoid tissue (2). Genetic changes as well as dysfunction of oncogenes or tumor suppressors have been demonstrated to be tightly associated with the development and progression (3,4). Therefore, understanding the molecular mechanisms in OS would benefit for the development of novel therapeutic targets or candidates for OS.
MicroRNAs (miRs), a class of 18–25 nucleotides in length non-coding RNAs, can suppress gene expression via directly binding to the 3′-untranslational region (UTR) of their target mRNAs, thus leading to mRNA degradation and translation repression (5). Through negative mediation of their target genes, miRs play a key role in a variety of cellular biological processes, including cell survival, proliferation, differentiation, apoptosis, autophagy, metabolism, and motility (6,7). Moreover, as many oncogenes or tumor suppressors are also targets of miRs, various miRs have been implicated in tumorigenesis and malignant progression of human cancers including OS (8–10). For instance, miR-143 inhibits OS metastasis by targeting matrix metalloprotease-13 expression (11). miR-199a-3p is downregulated in human OS and has suppressive effects on OS cell proliferation and migration (12). miR-205 generally acts as a tumor suppressor in a variety of human cancers. It is downregulated in prostate carcinoma and inhibits key oncogenic pathways including mitogen-activated protein kinase (MAPK) and androgen receptor (AR) signaling pathways (13). miR-205 is downregulated in renal cell carcinoma, and inhibits proliferation, migration, and invasion, and induces apoptosis of renal cell carcinoma cells (14). However, the expression profile and regulatory mechanism of miR-205 in OS still remains to be fully uncovered.
Runt-related transcription factor 2 (RUNX2), a member of the RUNX family of transcription factors, encodes a nuclear protein with a Runt DNA-binding domain (15). RUNX2 can bind DNA both as a monomer or as a subunit of a heterodimeric complex, and act as a scaffold for nucleic acids and regulatory factors involved in skeletal gene expression, and thus is essential for osteoblastic differentiation and skeletal morphogenesis (16–18). Moreover, RUNX2 has been suggested to play a promoting role in OS, knockdown of RUNX2 could inhibit the malignant phenotypes of OS cells (19,20). However, the regulatory mechanism of RUNX2 in OS is largely unclear.
The present study aimed to investigate the expression pattern as well as the regulatory mechanism of miR-205 in OS, involving the relationship between miR-205 and RUNX2.
Materials and methods
Clinical tissues
Our study was approved by the Ethics Committee of Xiangya Hospital of Central South University (Changsha, China). A total of 34 OS specimens and their matched adjacent normal tissues were collected from Xiangya Hospital from January 2012 to January 2014. A written informed consent was obtained from each patient, and none of the patients had received radiation therapy or chemotherapy prior to surgery. Tissue samples for use were stored in liquid nitrogen.
Cells culture and transfection
HEK293 cells, human OS cell lines, Saos-2, U2OS, SW1353, and MG63, and a human osteoblast cell line hFOB1.19 were obtained from the American Type Culture Collection (ATCC; Manassas, VA, USA). Cells were cultured in RPMI-1640 medium added with 10% fetal bovine serum (FBS) (both from Life Technologies, Carlsbad, CA, USA) at 37°C in a humidified atmosphere with 5% CO2. For cell transfection, OS cells were grown to 70% confluence, and transfected with pcDNA3.1 vector, pcDNA3.1-RUNX2 plasmid (Amspring, Changsha, China), miR-205 mimic or inhibitor (both from Thermo Fisher, Carlsbad, CA, USA) using Lipofectamine 2000, according to the manufacturer's recommendation.
Real-time RT-PCR assay
Total RNA was extracted using TRIzol reagent (Life Technologies), according to the manufacture's instruction. The mRNA expression was determined by using the standard SYBR-Green RT-PCR kit (Takara, Otsu, Japan), in accordance with the manufacturer's instructions. The reaction conditions were 95°C for 3 min, and 40 cycles of denaturation at 95°C for 15 sec and annealing/elongation at 60°C for 30 sec. The specific primers were as follows: RUNX2 forward, 5′-AAGTGAGGTTAGGGCGAAATG-3′ and reverse, 5′-AAGGTAGTTGATTGCCAACGAA-3′; and GAPDH forward, 5′-CTGGGCTACACTGAGCACC-3′ and reverse, 5′-AAGTGGTCGTTGAGGGCAATG-3′. GAPDH was used as the internal control. PrimeScript® miRNA RT-PCR kit (Life Technologies) was used to examine the miR expression, according to the manufacturer's instructions. U6 small nuclear RNA was used as an internal reference. The relative expression was analyzed by the 2−ΔΔCt method.
Western blotting
Western blotting was used to examine the protein expression. Briefly, cells were solubilized in cold RIPA lysis buffer. Proteins were separated with 10% SDS-PAGE, and transferred onto PVDF membrane, which was blocked by 5% skim milk for 1 h, and then incubated overnight at 4°C with primary antibodies, including rabbit anti-mouse RUNX2 monoclonal antibody (1:200) and GAPDH monoclonal antibody (1:500) (both from Abcam, Cambridge, MA, USA), and then with the mouse anti-rabbit secondary antibody (1:20,000; Abcam) for 40 min. An ECL kit (Pierce, Rockford, IL, USA) was used to visualize the protein bands.
Bioinformatics predication and luciferase assays
Bioinformatic analysis was used to analyze the putative targets of miR-205 using TargetScan (http://www.targetscan.org/). The wild-type (WT) of RUNX2 3′-UTR was amplified by PCR and cloned in pMIR-REPORT miRNA expression reporter (Thermo Fisher) to generate a reporter construct with firefly luciferase, named WT-RUNX2 vector. The mutant type (MUT) of RUNX2 3′-UTR was constructed by using Easy Mutagenesis system kit (Promega, Madison, WI, USA), in accordance with the manufacturer's protocol, and also cloned in pMIR-REPORT miRNA expression reporter to generate a reporter construct, named MUT-RUNX2 vector. HEK293 cells were co-transfected with scramble miR mimic or miR-205 mimic, and WT or MUT of RUNX2 3′-UTR luciferase reporter vector, together with Renilla plasmid (Promega, Beijing, China), respectively, using Lipofectamine 2000 according to the manufacturer's protocol. The firefly luciferase activity and Renilla luciferase activity were determined after transfection for 48 h using the Firefly and Renilla Luciferase Assay kit (Promega), according to the manufacturer's protocols. The firefly luciferase activity was normalized to that of Renilla luciferase.
Cell proliferation assay
Cells (5×103) in each group were suspended in 100 µl fresh serum-free RPMI-1640 medium, and seeded to a 96-well plate. After incubation at 37°C for 24, 48, 72, or 96 h, 0.5 g/l MTT (Sigma, USA) was added into the medium. After incubation at 37°C for 4 h, the medium was removed by aspiration. Then, 50 µl of dimethyl sulfoxide (DMSO; Sigma) was added, and incubated at 37°C for 20 min. The optical density (OD) at 570 nm was measured using the ELx800 Absorbance microplate reader (BioTek, USA).
Wound-healing assay
Cells in each group were seeded to a 24-well plate and cultured to full confluence. Wounds of approximately 1 mm width were created with a plastic scriber. Cells were washed and then cultured in Dulbecco's modified Eagle's medium (DMEM) containing 10% FBS for 48 h. Then, cells were observed and photographed under a microscope.
Cell invasion assay
Transwell chamber with a Matrigel-coated filter (BD Biosciences, Franklin Lakes, NJ, USA) was used to perform cell invasion assay. A total of 200 µl of cell suspension (5×103 cells) in serum-free RPMI-1640 medium was added to the upper chamber, and 500 µl of RPMI-1640 medium containing 10% FBS was added to the lower chamber. After incubation for 24 h, cells on the upper side of the filter were removed using cotton swabs. The invasive cells on the lower side were fixed, stained with 0.1% crystal violet solution (Sigma), and counted under a microscope.
Statistical analysis
All data were expressed as mean ± standard deviation (SD). Difference was analyzed by using Student's t-test or one-way analysis of variance (ANOVA). SPSS 17.0 software was used to perform statistical analysis. P<0.05 were considered statistically significant.
Results
miR-205 is significantly downregulated in OS
The exact role as well as the regulatory mechanism of miR-205 in OS remains largely unclear. In our study, we performed real-time RT-PCR to examine the miR-205 levels in 34 cases of OS tissues and paired adjacent non-tumor bone tissues. Our data showed that the expression of miR-205 was frequently and significantly decreased in OS tissues compared to matched adjacent normal tissues (Fig. 1A and B). Additionally we examined the miR-205 levels in OS cell lines, Saos-2, U2OS, SW1353, and MG63, as well as in human osteoblast cell line hFOB1.19. Real-time RT-PCR data indicated that miR-205 was also significantly downregulated in Saos-2, U2OS, SW1353, and MG63 cells, when compared with that in hFOB1.19 cells (Fig. 1C). These findings indicate that miR-205 is downregulated in OS.
miR-205 inhibits the malignant phenotypes of MG63 cells
As MG63 and U2OS cells showed the most significant decrease in the miR-205 level (Fig. 1C), we used these two cell lines in the following experiments in vitro. miR-205 mimic or miR-NC mimic was used to transfect these cells. After transfection, the miR-205 level was remarkably increased compared to the control group (Fig. 2A and B). However, transfection with miR-NC mimic did not affect the miR-205 level in MG63 and U2OS cells (Fig. 2A and B). MTT assay was then conducted to examine the cell proliferation. Our data showed that transfection with miR-205 mimic led to a significant decrease in the proliferation of MG63 and U2OS cells, indicating that miR-205 has an inhibitory effect on OS cell proliferation (Fig. 2C and D). Furthermore, wound-healing assay and Transwell assay were used to examine the cell migration and invasion. As indicated in Fig. 3, transfection with miR-205 mimic significantly decreased the migration and invasion of MG63 and U2OS cells, suggesting that miR-205 plays a suppressive role in OS metastasis.
RUNX2, significantly upregulated in OS, is a direct target gene of miR-205
We further performed bioinformatics analysis to predicate the putative targets of miR-205 by using TargetScan. RUNX2 was predicated to be a direct target gene of miR-205 (Fig. 4A), and this targeting relationship was evolutionally conserved (Fig. 4B). To verify whether miR-205 could directly bind to the RUNX2 3′-UTR, we generated WT-RUNX2 and MUT-RUNX2 reporter vectors containing the WT and MUT binding sequences of miR-205 within the 3′-UTR of RUNX2 mRNA, respectively (Fig. 4C and D). Luciferase reporter assay was then performed in HEK293 cells. As demonstrated in Fig. 4E, the luciferase activity was only decreased in HEK293 cells co-transfected with WT-RUNX2 reporter vector and miR-205 mimic, and showed no difference in cells co-transfected with MUT-RUNX2 reporter vector and miR-205 mimic, when compared to the control group. These data indicate that RUNX2 is indeed a target gene of miR-205.
Moreover, we examined the effect of miR-205 on the mRNA and protein expression of RUNX2 in OS cells. MG63 and U2OS cells were transfected with miR-205 inhibitor or negative control (NC) inhibitor, respectively. Our data showed that transfection with miR-205 inhibitor decreased the miR-205 level in MG63 and U2OS cells compared to the control group (Fig. 5A and B). Real-time RT-PCR and western blot assay were then conducted to examine the mRNA and protein level of RUNX2 in each group. As indicated in Fig. 5C–F, overexpression of miR-205 caused a decrease in both mRNA and protein expression of RUNX2, while downregulation of miR-205 resulted in increased mRNA and protein level of RUNX2 in MG63 and U2OS cells. Accordingly, miR-205 negatively mediates RUNX2 expression at both transcriptional and post-transcriptional levels in OS cells.
RUNX2 is involved in miR-205-mediated proliferation, migration, and invasion of MG63 and U2OS cells
We further investigated whether RUNX2 was involved in the miR-205-mediated malignant phenotypes of OS cells. MG63 and U2OS cells were transfected with miR-205 mimic, or co-transfected with miR-205 mimic and RUNX2 ORF plasmid, respectively. Western blotting data indicated that the protein level of RUNX2 was significantly higher in miR-205 + RUNX2 group compared to the miR-205 group (Fig. 6A and B), indicating that transfection with RUNX2 plasmid reversed the inhibitory effect of miR-205 overexpression on RUNX2 expression in OS cells. MTT assay, wound-healing assay and Transwell assay were conducted to examine the proliferation, migration and invasion of OS cells in each group. Our data showed that the proliferation of OS cells was markedly increased in the miR-205 + RUNX2 group compared to the miR-205 group (Fig. 6C and D). Moreover, the migration and invasion of OS cells were also higher in the miR-205 + RUNX2 group, when compared to the miR-205 group, respectively (Fig. 7). Taken together, we suggest that miR-205 may have suppressive effects on OS growth and metastasis via directly targeting RUNX2.
RUNX2 is upregulated in OS, and reversely correlated with miR-205 level
Finally, we performed real-time RT-PCR to examine the mRNA expression of RUNX2 in 34 cases of OS tissues and paired adjacent non-tumor bone tissues. Our data indicated that RUNX2 was frequently and significantly upregulated in OS tissues compared to their matched adjacent normal tissues (Fig. 8A and B). Besides, the mRNA and protein expression of RUNX2 was also increased in OS cell lines compared hFOB1.19 cells (Fig. 8C and D). Moreover, we observed a reverse correlation with the miR-205 level in OS tissues (Fig. 8E). Based on the above data, we suggest that the increased expression of RUNX2 may be due to the downregulation of miR-205 in OS.
Discussion
Accumulating evidence has revealed that some specific miRs act as oncogenes or tumor suppressors in OS. However, the molecular mechanisms by which miR-205 regulate the malignant phenotypes of OS cells still remains to be fully investigated. Here we found that miR-205 was frequently and significantly downregulated in OS tissues and cell lines, and had suppressive effects on the proliferation, migration and invasion of OS cells. RUNX2 was then identified as a direct target of miR-205, and was mediated by miR-205 in OS cells at both transcriptional and post-transcriptional levels. Moreover, overexpression of RUNX2 effectively reversed the suppressive effects of miR-205 on the proliferation, migration, and invasion of OS cells. Finally, we showed that RUNX2 was frequently and significantly upregulated in OS tissues and cell lines, and its expression was reversely correlated to the miR-205 level in OS tissues. These data expand the understanding of disease-associated mechanisms of miRs in OS.
miR-205, located in the second intron of the LOC642587 locus in chromosome 1, has been demonstrated to play different roles in different cancer types (13,14,21,22). Generally, it is frequently downregulated and acts as a tumor suppressor in a variety of human cancers (23,24). For instance, the expression of miR-205 was significantly reduced in melanoma tissues, and lower miR-205 level was associated with worse clinical outcome of melanoma patients (25,26). Moreover, overexpression of miR-205 inhibits the growth of melanoma cells in vitro and in vivo by targeting E2F1 and VEGF (27). On the contrary, miR-205 is upregulated and plays a promoting role in some other cancer types. For instance, miR-205 enhances the proliferation, migration, invasion and EMT of endometrial cancer cells by activation of AKT and downregulation of glycogen synthase kinase 3β (28). Niu et al reported that the miR-205 level was increased in ovarian cancer tissues, and its expression was significantly associated with high pathological grade and advanced clinical stage. They further showed that miR-205 enhanced the motility of ovarian cancer cells by directly targeting ZEB1 (29). These dual roles of miR-205 may depend on the different functions of miR-205 targets in different tumor microenvironments. In the present study, we found that miR-205 was frequently and significantly downregulated in OS tissues and cell lines (Saos-2, U2OS, SW1353, and MG63), and restoration of miR-205 expression led to a significant decrease of proliferation, migration and invasion of MG-63 and U2OS cells, consistent with a recent study by Wang et al that miR-205 was consistently suppressed in OS cell lines (HOS, SaOS-2, U2OS, and MG-63) compared to normal human osteoblast NHOst cells, and restored expression of miR-205 significantly inhibited the proliferation, migration, and invasion of MG-63 cells (23). Compared to their study, we examined the miR-205 expression in OS tissues, and used more than one OS cell line (U2OS) to investigate the miR-205 function in vitro (23). Thus, our findings further confirmed the suppressive role of miR-205 in OS.
As miRs function through mediating their target genes, we further investigated the potential targets of miR-205 by using bioinformatics analysis. Among the putative genes, we focused on RUNX2, as it has been implicated in osteoblastic differentiation, skeletal morphogenesis as well as OS development (30). RUNX2 has been suggested to convert (pre)-osteoblasts to OS cells by affecting the cell cycle control (30). Moreover, RUNX2 was also found to be associated with OS growth and metastasis, as well as bone metastasis in target cancers such as prostate and breast cancers (20,30–32). PI3K/AKT signaling pathway, one of the critical axes controlling cancer growth and metastasis, was demonstrated to be affected by RUNX2 (33). In our study, luciferase reporter assay data identified RUNX2 as a direct target gene of miR-205, and the mRNA and protein expression of RUNX2 was negatively mediated by miR-205 in OS cells, which further suggests that RUNX2 may be involved in the miR-205-mediated malignant phenotypes of OS cells. Previously, Zhang et al reported that miR-205 could directly target RUNX2 and affect osteoblast maturation through controlling the osteogenic activity of RUNX2 (34). Besides, Hu et al also reported that miR-205 could regulate SATB2 and RUNX2, and overexpression of SATB2 activated RUNX2 and reversed the negative effects of miR-205 on osteoblastic differentiation (35). However, the relationship between miR-205 and RUNX2 in OS has not been reported. Here we found that overexpression of RUNX2 effectively reversed the suppressive effects of miR-205 on the proliferation, migration, and invasion of OS cells, which further supports that the suppressive role of miR-205 is through targeting RUNX2.
Taken together, our findings suggest that miR-205 acts as a tumor suppressor in OS growth and metastasis via direct inhibition of RUNX2 expression. Therefore, the miR-205/RUNX2 may become a potential therapeutic target for OS, which should be further clarified in future studies.