MicroRNA-379 suppresses osteosarcoma progression by targeting PDK1.

Osteosarcoma is the most common primary bone tumour. Increasing evidence has demonstrated the pathogenic role of microRNA (miRNAs) dysregulation in tumour development. miR-379 was previously reported to function as an oncogenic or tumour-suppressing miRNA in a tissue-dependent manner. However, its function in osteosarcoma remains unknown. In this study, we found that the expression of miR-379 was downregulated in osteosarcoma tissues and cell lines. Further functional characterization revealed that miR-379 suppressed osteosarcoma cell proliferation and invasion in vitro and retarded the growth of osteosarcoma xenografts in vivo. Mechanistically, PDK1 was identified as the direct target of miR-379 in osteosarcoma, in which PDK1 expression was up-regulated and showed inverse correlation with miR-379. Enforced expression of PDK1 promoted osteosarcoma cell proliferation and rescued the anti-proliferative effect of miR-379. These data suggest that miR-379 could function as a tumour-suppressing miRNA via targeting PDK1 in osteosarcoma.

Introduction

Osteosarcoma is the most common primary tumour of bone with an incidence of 4–5 cases per million in young adults and adolescents 1, 2, 3, 4. Because of the advancement of its treatment, including surgery and multi‐agent chemotherapy, the 5‐year survival rate in patients with primary osteosarcoma has improved over the past several decades 5, 6, 7, 8. However, the survival rate for this tumour remains low 9, 10, 11, 12. Therefore, it is necessary to better understand the pathogenesis of osteosarcoma and identify novel markers to improve its treatment strategies.

MicroRNAs (miRNAs) are highly conserved, endogenous small noncoding RNAs (18–25 nucleotides in length) that repress gene expression by binding to the 3′‐untranslated region (3′UTR) of their target mRNAs, inducing their degradation or translational repression 13, 14, 15, 16. Several studies have demonstrated abnormal expression of miRNAs in different types of cancer, including gastric cancer, hepatocellular carcinoma, glioblastoma, ovarian carcinoma, breast cancer and laryngeal cancer 17, 18, 19, 20, 21, 22. Moreover, increasing evidence showed that miRNAs play important roles in many biological processes, including cell proliferation, apoptosis, metabolism, differentiation and invasion 23, 24, 25, 26. Some studies have also identified miRNAs as novel diagnostic and prognostic biomarkers for cancers 27, 28, 29, 30.

vPrevious studies showed that aberrant expression of miR‐379 contributes to cancer development 31, 32, 33. For instance, Khan et al. demonstrated that the expression of miR‐379 was significantly reduced in breast cancer, in which this miRNA significantly inhibited cell proliferation by repressing cyclin B1 expression 34. Haenisch et al. also showed that miR‐379 could be induced by rifampicin, thereby impeding the protein overexpression of ABCC2 (multidrug resistance associated protein 2) after treatment with this pregnane X receptor ligand in HepG2 cells 35. However, the oncogenic role of miR‐379 has also been reported. It has been demonstrated that miR‐379 expression was elevated in bone‐metastatic prostate cancer cell lines and tissues. The expression of miR‐379 was also correlated with shortened progression‐free survival of patients with prostate cancer 32. In addition, treatment with miR‐379 inhibitor decreased bone metastasis and increased survival of mice transplanted with bone metastatic ARCaPM prostate cancer cells 32. In osteosarcoma, the expression and function of miR‐379 remain unknown.

In the present study, we found that miR‐379 expression was downregulated in osteosarcoma tissues and cell lines. Further investigation demonstrated that miR‐379 repressed the in vitro and in vivo malignant phenotypes of osteosarcoma cells. Importantly, we identified PDK1 as the direct target of miR‐379 in osteosarcoma.

Materials and methods

Tissue specimens, cell lines and cell transfection

A total of 30 pairs of osteosarcoma and adjacent non‐cancer bone tissues (located >3 cm from the tumour) were obtained between 2012 and 2014 (see Table S1 for demographic and clinicopathological information). All samples were immediately placed in liquid nitrogen until use. All patients had provided consents and this study was approved by the institutional ethical board of Peking Union Medical College Hospital and complied with the Declaration of Helsinki. Four human osteosarcoma cell lines MG‐63 (14 years old, male), U2OS (15 years old, female), SOSP‐9607(17 years old, male), and SAOS‐2 (11 years old, female) and a non‐cancer osteoblastic cell line (hFOB) were purchased from the Cell Resource Center of Chinese Academy of Medical Sciences (Beijing, China). Cells were cultured in DMEM (Invitrogen, Carlsbad, CA, USA) and supplemented with 10% foetal bovine serum (Gibco, Grand Island, NY, USA). miR‐379 mimics and scrambled control (GenePharma, Shanghai, China) were transfected into cells at 40–60% confluence using Lipofectamine 2000 (Invitrogen, Carlsbad, CA, USA) according to the manufacturer's recommendations. For PDK1 overexpression, approximate 4 × 105 cells were seeded into each well of a 6‐well plate the night before followed by transfection with 2 μg pcDNA3.1‐PDK1 or empty plasmids DNA at 60–80% confluence using Lipofectamine 2000.

Quantitative reverse transcription‐PCR

Total RNA was isolated from cells or tissues by Trizol reagent (Invitrogen). The expression of miRNA and mRNA was quantified by quantitative reverse transcription (qRT)‐PCR on the iQ5 Real‐Time PCR System (Bio‐Rad, Hercules, CA, USA). U6 was used for miRNA normalization 36 and GAPDH was used as a control for mRNA (see Table S2 for primer sequences). Relative expression was calculated using the 2−ΔΔCt method.

Cell proliferation and invasion assays

Cell proliferation was determined using CCK‐8 assay (Dojindo, Kumamoto, Japan) following the manufacturer's instruction. The proliferation rate was measured at 0, 24, 48 and 72 hrs post‐transfection (1 × 105 cells). For invasion analysis, cells were seeded onto a Matrigel‐coated membrane. Foetal bovine serum was appended and non‐invading cells were removed. Cells on the lower surface of the Matrigel‐coated chamber were stained with 0.1% crystal violet (Sigma‐Aldrich, St. Louis, MO, USA). Cells on the entire membrane were counted.

Western blots

Total tissue or cellular protein was separated using 10% SDS‐PAGE and transferred to polyvinylidene difluoride membranes (Amersham, Buckinghamshire, UK). The membranes were blocked with 5% non‐fat milk and then incubated with primary antibodies (against PDK1, GAPDH or Ki‐67) for 2 hrs. After washing with Tris‐buffered saline with Tween 20, the membranes were incubated with horseradish peroxidase‐conju gated secondary antibody and developed with a chemiluminescence detection kit (Millipore, Boston, MA, USA).

Luciferase reporter assay

The mutant (MUT) or wild‐type (WT) 3′UTR of PDK1 was cloned into pGL3 luciferase promoter vector (Promega, Madison, WI, USA). Cells were cotransfected with miR‐379 mimics or scrambled control and the vectors carrying PDK1 MUT or WT 3′UTR using Lipofectamine 2000 (Invitrogen). Activities of firefly and Renilla luciferases were measured using a dual‐luciferase assay system (Promega).

In vivo tumourigenesis assay

MG‐63 cells (5 × 106 cells per mouse) were inoculated subcutaneously into the dorsal flanks of 6‐week‐old female nude mice. On day 10, when tumours reached ~50 mm3, miR‐379 mimics or scrambled control (100 nmol per mouse in 100 μl total volume containing 90 μl phosphate‐buffered saline and 10 μl Lipofectamine 2000; 5 mice per group) was injected directly into the tumours with 26‐gauge needles every 3 days. The tumour width, length and weight were measured every 3 days. Mice were killed by cervical dislocation on the 25th days after cancer cell inoculation. All experimental procedures were conducted in conformity with institutional guidelines for the care and use of laboratory animals, and protocols were approved by the institutional animal ethics committee of Peking Union Medical College.

Immunofluorescence

Cells were fixed with 3.5% formaldehyde, permeabilized with 0.1% Triton X‐100, blocked with 3% bovine serum albumin and 0.05% Tween 20 in PBS and then incubated with primary antibody(dilutions 1:1000, Ki‐67; Sigma‐Aldrich) overnight at 4°C. Fluorescence was measured by a confocal microscope (TCS SP2; Leica, Mannheim, Germany).

Statistical analysis

All data were shown as mean ± S.D. Differences between two groups and among three or more groups were measured by Student's t‐test and one‐way ANOVA, respectively. P < 0.05 was considered statistically significant.

Results

Downregulation of miR‐379 in osteosarcoma tissues and cell lines

The expression of miR‐379 was downregulated in osteosarcoma tissues compared with their adjacent non‐cancerous tissues (Fig. 1A). Among them, 24 cases (80%) showed significant reduction in miR‐379 in osteosarcoma tissues, whereas six cases (20%) showed up‐regulation. The overall expression of miR‐379 in osteosarcoma tissues was significantly lower than that in cancer‐adjacent tissues (Fig. 1B). Moreover, miR‐379 expression in 4 osteosarcoma cell lines (MG‐63, U2OS, SOSP‐9607 and SAOS‐2) was lower as compared with the non‐cancer osteoblastic cell line hFOB (Fig. 1C). Consistent with our findings, data from the public database S‐MED (Sarcoma microRNA Expression Database; http://www.oncomir.umn.edu/) indicated that the levels of miR‐379 in osteosarcoma tissues were much lower than that in normal bone tissues (Fig. 1D).

Figure 1

miR‐379 expression was downregulated in osteosarcoma tissues and cell lines. (A) The expression levels of miR‐379 were measured in 30 osteosarcoma samples by qRT‐qPCR. (B) The expression of miR‐379 in osteosarcoma tissues was significantly lower than that in cancer‐adjacent tissues. (C) miR‐379 expression was determined in four osteosarcoma cell lines (MG‐63, U2OS, SOSP‐9607, and SAOS‐2) and a non‐cancer osteoblastic cell line (hFOB) using qRT‐PCR. (D) Expression data of miR‐397 in normal bone tissues and osteosarcoma tissues were extracted from the public database S‐MED.

Inhibition of osteosarcoma cell proliferation by miR‐379

miR‐379 mimics or scrambled control was transfected into MG‐63 and U2OS. The efficiency of miR‐379 transfection was confirmed by qRT‐PCR (Fig. 2A). Restored expression of miR‐379 inhibited MG‐63 and U2OS cell proliferation (Fig. 2B). In addition, the mRNA and protein expression of Ki‐67, a proliferation marker, was decreased in these two cell lines upon transfection with miR‐379 mimics (Fig. 2C and D).

Figure 2

miR‐379 inhibited osteosarcoma cell proliferation. (A) The expression of miR‐379 in MG‐63 and U2OS cells was determined by qRT‐qPCR. (B) Transfection of miR‐379 mimics inhibited MG‐63 and U2OS cell proliferation. (C) miR‐379 suppressed the mRNA expression of Ki‐67 in MG‐63 and U2OS cells. (D) The protein expression of Ki‐67 in MG‐63 and U2OS cells was determined by Western blots. *P < 0.05; **P < 0.01 and ***P < 0.001.

Inhibition of osteosarcoma cell invasion by miR‐379

Cell invasion assay was performed to measure the effect of miR‐379 on the cell invasive ability in miR‐379‐transfected MG‐63 and U2OS cells. Data showed that restored expression of miR‐379 inhibited both MG‐63 and U2OS cell invasion (Fig. 3).

Figure 3

miR‐379 inhibited osteosarcoma cell invasion. Trans‐well invasion assays were conducted using MG‐63 and U2OS cells transfected with miR‐379 mimics or scrambled control. The relative ratio of invaded cells per field was shown, ***P < 0.001.

PDK1 as the direct target of miR‐379 in osteosarcoma

Using TargetScan, we identified PDK1 as the tentative target of miR‐379 (Fig. 4A). To confirm this possibility, luciferase reporter assay was performed. The relative luciferase activity of the reporter containing PDK1 WT‐3′UTR was significantly decreased by about 60% upon miR‐379 transfection as compared with the reporter containing PDK1 MUT‐3′UTR in both MG‐63 and U2OS cells (Fig. 4B). Restored expression of miR‐379 also repressed the mRNA and protein expression of PDK1 in both MG‐63 and U2OS cells (Fig. 4C and D).

Figure 4

PDK1 was the direct target of miR‐379 in osteosarcoma. (A) The 3′UTR of PDK1 mRNA contains the binding sequence of miR‐379. (B) Relative luciferase activity of the indicated PDK1 reporter construct in MG‐63 cells is shown. Firefly luciferase values were normalized to Renilla luciferase activity and plotted as relative luciferase activity. (C) Relative luciferase activity of the indicated PDK1 reporter construct in U2OS cells is shown. (D and E) The mRNA expression of PDK1 in (D) MG‐63 and (E) U2OS cells was measured by qRT‐PCR. (F and G) The protein expression of PDK1 in (F) MG‐63 and (G) U2OS cells was measured by Western blots, ***P < 0.001.

Up‐regulation of PDK1 in osteosarcoma and its inverse correlation with miR‐379

The expression of PDK1 was up‐regulated in osteosarcoma tissues compared with corresponding non‐cancerous adjacent tissues (Fig. 5A). Among them, 26 cases (87%) showed significantly higher levels of PDK1 in osteosarcoma samples, whereas four cases (13%) exhibited downregulation. The overall expression of PDK1 in osteosarcoma tissues was significant higher than that in cancer‐adjacent tissues (Fig. 5B). Moreover, the PDK1 levels were inversely correlated with miR‐379 expression (r 2 = −0.347; P < 0.001; Fig. 5C). PDK1 expression was also increased in the four osteosarcoma cell lines (MG‐63, U2OS, SOSP‐9607, and SAOS‐2) as compared with hFOB (Fig. 5D).

Figure 5

PDK1 expression was up‐regulated in osteosarcoma and was inversely correlated with miR‐379 expression. (A) The expression levels of PDK1 were measured in 30 osteosarcoma samples by qRT‐qPCR. (B) PDK1 expression was up‐regulated in osteosarcoma tissues compared with cancer‐adjacent tissues. (C) PDK1 levels were inversely correlated with miR‐379 expression (r 2 = −0.347; P < 0.001). (D) PDK1 expression was measured in 4 osteosarcoma cell lines (MG‐63, U2OS, SOSP‐9607, and SAOS‐2) and a non‐cancerous osteoblastic cell line (hFOB) using qRT‐PCR.

Tumour‐suppressing function of miR‐379 rescued by PDK1

The PDK1 expression vector was constructed to restore PDK1 expression. The protein level of PDK1 was up‐regulated in MG‐63 cells upon transfection with PDK1 vector (Fig. 6A). Enforced expression of PDK1 per se promoted MG‐63 and U2OS cell proliferation (Fig. 6B and C). Importantly, enforced expression of PDK1 rescued miR‐379‐mediated inhibition of cell proliferation in both cell lines (Fig. 6D and E). Ectopic expression of PDK1 promoted the U2OS cell invasion (Fig. 6F).

Figure 6

PDK1 downregulation was involved in the tumour‐suppressing function of miR‐379. (A) The protein expression of PDK1 was measured by Western blots. (B and C) Enforced expression of PDK1 promoted (B) MG‐63 and (C) U2OS cell proliferation. (D and E) Enforced expression of PDK1 rescued the miR‐379‐mediated inhibition of cell proliferation in (D) MG‐63 and (E) U2OS cells. (F) Ectopic expression of PDK1 promoted the U2OS cell invasion. The relative invasion cell was shown. *P < 0.05; **P < 0.01 and ***P < 0.001.

Growth inhibition of MG‐63 xenografts by miR‐379

MG‐63 cells were inoculated subcutaneously in posterior flanks of nude mice to study the therapeutic effect of miR‐379 in vivo. When tumours reached 50 mm3, miR‐379 mimics or scrambled control was injected directly into the tumours. After 4 weeks, we found that injection with miR‐379 mimics increased the intratumoural levels of miR‐379 (Fig. 7A) and repressed the growth of MG‐63 xenografts compared with scrambled oligonucleotides‐injected tumours (Fig. 7B). Consistent with the tumour growth curve, the weight of tumours injected with miR‐379 was significantly lower than scrambled control‐injected tumours (Fig. 7C). The protein expression of PDK1 was also lower in miR‐379‐injected tumours (Fig. 7D). Moreover, the mRNA expression of Ki‐67 was also reduced by miR‐379 injection (Fig. 7E), accompanied by less Ki‐67‐positive cells (Fig. 7F).

Figure 7

miR‐379 repressed the growth of MG‐63 xenografts in nude mice. (A) Injection with miR‐379 mimics increased the levels of miR‐379 in MG‐63 xenografts as determined by quantitative RT‐PCR. (B) Graph representing tumour volumes at the indicated days during the experiment for the two groups: scrambled control and miR‐379 mimics (n = 5 per group). (C) Average tumour weight of scrambled control and miR‐379 mimics‐injected mice at the end of the experiment (25 days post‐miRNA injection). (D) PDK1 protein expression was measured by Western blots. (E) The mRNA expression of Ki‐67 was measured by qRT‐PCR. (F) Ki‐67‐positive cells were counted in the two groups: scrambled control and miR‐379 mimics. *P < 0.05 and ***P < 0.001.

Discussion

In this study, we investigated the expression and function of miR‐379 in osteosarcoma. We found that the miR‐379 expression was downregulated in osteosarcoma tissues and cell lines. Further investigation demonstrated that miR‐379 repressed cell proliferation and invasion in two osteosarcoma cells lines (MG‐63 and U2OS) and inhibited in vivo tumourigenicity. Mechanistically, our data indicated that PDK1 was the direct target of miR‐379 in osteosarcoma, in which PDK1 was up‐regulated and showed inverse correlation with miR‐379. In addition, we demonstrated that enforced expression of PDK1 promoted osteosarcoma cell proliferation and PDK1 downregulation was involved in the tumour‐suppressing function of miR‐379. These data suggest that miR‐379 repressed the development of osteosarcoma.

In the present study, we identified PDK1 as the direct target of miR‐379 in osteosarcoma. We first used the TargetScan to identify PDK1 followed by validating the physical interaction of its 3′UTR with miR‐379 using the luciferase assay. Mutation of the predicted binding sequence abrogated such interactions. The downregulation of PDK1 at mRNA and protein levels upon miR‐379 transfection was also confirmed in MG‐63 and U2OS. Importantly, the inverse correlation between PDK1 and miR‐379 in clinical samples was demonstrated. PDK1 is a critical component of the oncogenic phosphoinositide 3‐kinase signalling and its overexpression has been documented in breast cancer 37, acute myeloid leukaemia 38 and multiple myeloma 39. Activation of large number of proteins, including Akt, some protein kinase C isoforms, S6K and SGK, by PDK1 has also been reported. In breast cancer, targeting PDK1 inhibits migration and experimental metastasis 40. Herein, we demonstrated for the first time the up‐regulation of PDK1 and its mitogenic action in osteosarcoma. Our data also suggested that derepression of PDK1 by miR‐379 downregulation may be an important oncogenic cascade in osteosarcoma.

One of the major limitations of the present study is the use of transient transfection of miR‐379 instead of stable expression in osteosarcoma cell lines. Furthermore, knockdown miR‐379 in other non‐cancerous osteoblastic cell lines should further substantiate its tumour‐suppressing role in osteosarcoma. In conclusion, we found that the expression of miR‐379 was downregulated in osteosarcoma tissues and cell lines, and overexpression of miR‐379 suppressed the osteosarcoma cell proliferation and invasion and tumour growth. The function of miR‐379 was mediated by the downregulation of PDK1. These data suggested that miR‐379 downregulation may play crucial roles in osteosarcoma development and that miR‐379 may be a potential therapeutic target for the treatment of osteosarcoma.

Conflict of interest

The authors declare no conflict of interest.

Table S1 Summary of clinicopathological parameters of patients with osteosarcoma.

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Table S2 Primer sequences.

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