Introduction
Osteosarcoma (OS) is a prevalent primary bone tumour mainly occurring in adolescents less than 20 years old [
1]. OS most frequently occurs in the long extremity bone metaphysis. The currently reported 5-year overall survival rate of localized OS is 60–70%[
2]. However, OS metastasis decreases the survival rate; the 5-year survival rate for individuals with lung metastases is 20–30% [
3,
4]. Therefore, there is an urgent need to clarify the pathological mechanisms underlying OS metastasis to improve the survival of patients with metastatic OS.
Barx homeobox 1 (BARX1), belonging to the Barx homeobox protein family, has been investigated in multiple developmental contexts, including cranium, face, stomach, and muscle development [
5‐
7]. BARX1 regulates the proliferation and metastasis of many tumours. Evidence has demonstrated that BARX1 promotes the proliferation of renal cell carcinoma and oesophageal adenocarcinoma [
8,
9]. However, enforced BARX1 expression in gastric cancer can inhibit tumour proliferation. In endometrial carcinoma, silencing of BARX1 suppresses ERK/MEK signalling pathway activity, which decreases migration and cell viability [
10]. BARX1 is considered a promising oncogenic transcription factor in non-small cell lung cancer (NSCLC) [
11]. Notably, BARX1 expression is decreased in hepatocellular carcinoma (HCC), and its absence causes enhancement of tumour invasion and metastasis by inducing MGAT5 and MMP9 expression [
12]. However, the function of BARX1 in OS is still unknown.
We aimed to clarify the role of BARX1 in OS. We detected BARX1 expression in tissues of OS patients. BARX1 was found to be significantly overexpressed in OS tumour tissue compared with adjacent normal tissue. In addition, our functional experiment demonstrated that silencing BARX1 in OS cells inhibited cell proliferation and invasion. We also identified the genes targeted by BARX1 via RNA sequencing (RNA-seq) and luciferase reporter assays. Above all, our study demonstrated that BARX1 plays an essential role in OS progression by regulating transcription and may be a therapeutic target.
Materials and methods
Human samples
Tumour tissues and normal adjacent tissues were obtained from OS patients from Shanghai Changzheng Hospital (Shanghai, China). All tissue samples were confirmed by histopathological biopsy. All participating patients were aware of the study and signed an informed consent form. Experiments were approved by the Naval Medical University Ethics Committee of Biomedicine and were performed following the Declaration of Helsinki.
Immunofluorescence (IF)
The tissue slices were treated with EDTA buffer for antigen repair and subsequently blocked with 3% BSA. Anti-BARX1 (ab220859, Abcam) and anti-HSPA6 (13616-1-AP, Proteintech) primary antibodies were applied dropwise to the sections, and they were then incubated at 4 °C overnight, followed by treatment with the matching secondary antibody and incubation for 1 h at normal temperature. Immunofluorescence was detected by a three-colour fluorescence kit (Shanghai Recordbio Biological Technology, Shanghai, China). Then, DAPI was used to stain the nucleus. The images were observed and collected under an inverted microscope. The nucleus is dyed blue by DAPI, HSPA6 is coloured orange, and BARX1 is coloured green.
Real-time PCR (RT-PCR)
RNA extraction from the tissue samples was performed by homogenization. Briefly, tissue samples were cut into fragments and transferred into 2 ml tubes, followed by the addition of TRIzol (1 ml) to each tube. The samples were then subjected to homogenization, and the supernatant was collected. The following steps were performed according to the instructions for TRIzol (Invitrogen Corporation, 15,596–018). We utilized Evo M-MLV RT Premix for qPCR and the SYBR Green Premix Pro Taq HS qPCR kit; reverse transcription and RT-PCR were performed using GAPDH as an internal control. All qPCRs were conducted on a 7900HT Fast Real-Time PCR System. Table
1 lists the primers used.
Table 1
RT-PCR primer sequences
DNAJB1 | AAGGCATGGACATTGATGACC | GGCCAAAGTTCACGTTGGT |
HSPB1 | ACGGTCAAGACCAAGGATGG | AGCGTGTATTTCCGCGTGA |
BARX1 | TTCCACGCCGGACAGAATAGA | AGTAAGCTGCTCGCTCGTTG |
HSPA6 | CAAGGTGCGCGTATGCTAC | GCTCATTGATGATCCGCAACAC |
SERPINH1 | TCAGTGAGCTTCGCTGATGAC | CATGGCGTTGACTAGCAGGG |
CLDN6 | TGTTCGGCTTGCTGGTCTAC | CGGGGATTAGCGTCAGGAC |
ARC | AGCGGGACCTGTACCAGAC | GCAGGAAACGCTTGAGCTTG |
GAPDH | CTGGGCTACACTGAGCACC | AAGTGGTCGTTGAGGGCAATG |
Western blotting (WB)
The protein content of the tissue samples was extracted using radioimmunoprecipitation assay buffer (RIPA) containing a phosphatase inhibitor, and the protein concentration was determined with a BCA kit. By using 10–12% polyacrylamide gels, the proteins were separated and then transferred to PVDF membranes, which were incubated with the primary antibodies BARX1 and HSPA6 at 4 °C for a whole night after being blocked with 5% skim milk, followed by adding the corresponding horseradish peroxidase-conjugated secondary antibodies.
Cell culture and transfection
The OS cell lines and bone marrow mesenchymal stem cells (BMSCs) were procured from the Shanghai Cell Bank Type Culture Collection Committee (Shanghai, China). BMSCs and MG63, U2OS, HOS, and Saos-2 cells were cultured in DMEM with 10% foetal bovine serum (FBS) and 1% penicillin–streptomycin supplements at 37 °C and 5% CO2 in a humidified atmosphere. Plasmids and siRNAs were purchased from Shanghai GeneChem (Shanghai, China). A density of 2 × 105 cells/well was used for culturing OS cells. Plasmid and siRNA cell transfections were performed following the Lipofectamine™ 2000 reagent protocols.
Cell counting Kit-8 (CCK-8) assay
After seeding cells in 96-well plates, plasmids or siRNA were transfected into the cells. After 72 h of culture, a CCK-8 assay was utilized to evaluate cell viability per the protocols.
Invasion assay
In a 24-well plate, the cell invasion assay was carried out. When the cell density reached 80–90%, the cells were resuspended and injected at a 1 × 105 cells/millilitre into the Matrigel-coated upper layer of the Transwell chamber (Scipu002874; Corning Inc.), while for the chamber bottom layer, serum-containing medium was added. The cells were fixed with 4% formaldehyde and stained with 0.1% crystal violet after a 24-h culture period (C0121; Beyotime).
Luciferase reporter assay
OS cells were transfected with HSPA6-luciferase reporter gene constructs for 48 h by Lipofectamine 2000 per the protocols, followed by OE-BARX1 treatment. Luciferase activity was measured by using a luciferase assay kit.
RNA-seq
TRIzol Reagent (Invitrogen, USA) was applied to extract total RNA from MG63 cells (transfected with pcDNA-BARX1 or vectors for 24 h), followed by Illumina mRNA deep sequencing. The sequence results were obtained for each transcript as fragments per kilobase of exon per million reads.
Statistics
All values are reported as the mean ± standard deviation (SD). Student’s t test was used to assess significant differences between group; the Kaplan‒Meier method and log-rank test were used to compare survival curves and determine statistical significance, respectively. Counting data were analysed with a χ2 test. p < 0.05 indicates a significant difference.
Discussion
OS, which is derived from primitive bone-forming mesenchymal cells, is a common primary bone malignancy [
14]. High-grade OS treatment has progressed remarkably since the advent of chemotherapy in the 1970s. Nevertheless, the survival rate remains unsatisfactory because of metastasis and relapse. The biology of OS is complex and not well understood [
15], and there are many clinical challenges related to OS, including wide histological heterogeneity, lack of biomarkers, high local aggressiveness, and rapid metastasis.
Increasing evidence shows that BARX1, as a transcription factor, is a two-way regulator of tumour diseases: it can not only promote but also inhibit tumour occurrence and progression [
12,
16]. Our study found that the increased expression of BARX1 was associated with a poor prognosis in OS patients and promoted OS cell proliferation and invasion, while downregulation of BARX1 suppressed OS cell proliferation and invasion, indicating that BARX1 acts as an oncogene in OS.
HSPA6 was first identified as an HSP70 family member by Leung et al. in 1990 [
17]. Heat shock proteins are overexpressed in different tumours, including breast, prostate, colorectal, and lung cancers, as well as OS [
18], where their overexpression is correlated with a poor prognosis and a high rate of chemotherapy resistance [
19‐
21]. High HSP70 levels in many cancers facilitate the survival of these cancer cells [
22]. Downregulation of HSP70 strongly reduced tumorigenicity in experimental models [
23]. HSP70 also participates in several biological processes that directly enhance tumorigenesis, including cell proliferation, apoptosis, angiogenesis, migration, and drug resistance. In this study, we performed bioinformatics analysis and found that HSPA6 may be the direct target gene of BARX1. HSPA6 can directly or indirectly inhibit or promote tumour occurrence and development. For example, bladder and cervical cancer tumour cell growth, migration, and invasion could be inhibited by increasing the expression of HSPA6 [
24]. Knocking down HSPA6 promoted triple-negative breast cancer cell growth, migration, and invasion [
25]. Our results revealed the coexpression of BARX1 and HSPA6 in the OS cell line; HSPA6 expression was also high in cells with BARX1 overexpression, and the expression of HSPA6 was not significant in cells with low expression of BARX1. Furthermore, through cell experiments, we found that knocking out HSPA6 while overexpressing BARX1 almost completely abolished the proliferation- and invasion-promoting effects of BARX1. The above results suggest that HSPA6 may be an effector molecule of the BARX1-mediated malignant behaviour of OS.
In summary, this research clinically confirmed the tumour-promoting effect of BARX1 in OS at the cellular and molecular levels and demonstrated that BARX1 enhances OS cell proliferation and invasion by regulating the downstream effector molecule HSPA6, providing an innovative basic theory for studying OS. However, there are still some shortcomings to this study, and an animal model needs to be used to further verify the experimental findings.
The obtained results are helpful for further understanding the biological function of BARX1 and its role in OS; these results provide an in-depth understanding of the OS pathogenesis and recurrence mechanism and provide ideas for OS prognosis evaluation and targeted treatment.
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