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FoxD2-AS1 promotes glioma progression by regulating miR-185-5P/HMGA2 axis and PI3K/AKT signaling pathway

Background/Aims: The present study was aimed at exploring the role of long noncoding RNA (lncRNA) FOXD2-AS1 in the development and progression of glioma and the underlying mechanism of FOXD2-AS1/miR-185-5p/HMGA2 network in glioma via regulation of PI3K/Akt signaling pathway. Methods: Microarray analysis was used for preliminary screening for candidate lncRNAs and mRNAs in glioma tissues. qRT-PCR and Western blot were used to determine the expression of FOXD2-AS1. The potential effects of FOXD2-AS1 on the viability, mobility and apoptosis of glioma cells were evaluated using MTT assay, Transwell assays and flow cytometry. The xenograft tumor model was performed to examine the influence of the lncRNA FOXD2-AS1/miR-185-5p/HMGA2 network on the biological functions of glioma cells. Luciferase assay and immunoprecipitation assay were examined to dissect molecular mechanisms. Results: LncRNA FOXD2-AS1 was overexpressed in human glioma, and upregulated FOXD2-AS11 expression indicated higher WHO grade (p < 0.05). MiR-185-5p was downregulated, whereas HMGA2 was upregulated in glioma tissues in comparison with para-carcinoma tissues. FOXD2-AS1 could regulate the expression of HMGA2 via miR-185-5p. Knockdown of FOXD2-AS1 significantly inhibited proliferation and metastatic potential of glioma cells, whereas endogenous expression FOXD2-AS1 inhibited the glioma cell activity through targeting HMGA2. Conclusions: lncRNA FOXD2-AS1 acted as a sponge of miR-185-5p and influenced the PI3K/Akt signaling pathway through regulating HMGA2. LncRNA FOXD2-AS1 modulated HMGA2 and PI3K/Akt downstream signaling through sponging miR-185-5p, thereby promoting tumorigenesis and progression of glioma.


INTRODUCTION
Glioma is one of the most common and invasive carcinomas in the central nervous system [1,2]. According to the WHO classification criterion, gliomas can be classified as grade I-IV based upon the pathological characteristics of malignant tumors. In spite of recent progress in cancer treatment, clinical prognosis and survival rate of glioma patients are still extremely low [3,4]. The malignant growth and high invasiveness of AGING glioma cells severely constrain the therapeutic effect and cause a high recurrence rate [5]. Therefore, a better understanding of the key molecular mechanisms that mediate the development and progression of glioma will contribute to exploring novel and effective interventions.
Longnon-coding RNA (lncRNA) is constantly defined as an RNA that does not encode a protein, and the transcription length exceeds 200 nucleotides and lacks of the ability to encode a protein [6][7][8]. LncRNA HOXD-AS1 has been proven to be a carcinogen in human cancers [9][10][11]. At present, lncRNAs have been found to be abnormally regulated in various cancers [12]. Certain carcinogenic lncRNAs, such as H19, CAS C2 and HOTAIR have been identified to be poorly regulated [13]. It has been reported that lncRNA FOXD2-AS1 plays a pivotal role in tumor progression. For instance, Su et al. have found that FOXD2-AS1 promotes the progression and recurrence of bladder cancer through positive feedback loops of AKT and E2F1 [14]. Yang et al. have demonstrated that FOXD2-AS1 acts as a tumor promoter in colorectal cancer by regulating the EMT and Notch signaling pathways [15]. Chen et al. have proposed FOXD2-AS1 can promote the incidence of nasopharyngeal carcinoma by regulating the miR-363-5p/S100a1 signaling pathway [16]. However, the role and potential mechanism of FOXD2-AS1 in glioma remain elusive.
Consequently, the purpose of this study is to explore the exact role of lncRNA FOXD2-AS1 in pathogenesis of glioma and the potential mechanism underlying lncRNA FOXD2-AS1/miR-185-5P/HMGA2 signaling pathway in glioma.

Overexpression of lncRNA FOXD2-AS1 in glioma tissues and cells
The most significantly differentially-expressed lncRNA or mRNA were screened from the glioma and normal brain tissue samples according to the criteria of multiple change greater than 2 and P<0.05. In the glioma tissues, 285 lncRNAs were up-regulated and 483 lncRNAs were down-regulated. In the overexpression of lncRNA, the expression level of FOXD2-AS1 was up-regulated by 3.17 times on average. In addition, the expression levels of four lncRNAs were quantitatively detected by qRT-PCR and the results demonstrated that the expression levels of these lncRNAs in the glioma were significantly up-regulated than those in the adjacent tissues ( Figure 1A, P<0.05). Among them, the expression level of FOXD2-AS1 was significantly up-regulated in low-/high-grade gliomas. HMGA2 plays a critical role in glioma progression [17]. Consistently, qRT-PCR revealed that HMGA2 was overexpressed in low-/highgrade glioma tissues (P<0.05, Figure 1B). Moreover, the expression levels of FOXD2-AS1 and HMGA2 were also significantly up-regulated in the U87 and U251 glioma cell lines compared with HEB cells (P<0.01, Figure 1C-D).

MiR-185-5p targets HMGA2
A binding site of miR-185-5p on HMGA2 3'UTR was observed by two bioinformatics programs, including miranda and TargetScan ( Figure 4A). Luciferase assay demonstrated that the fluorescence intensity in the HMGA2-wt and miR-185-5p co-transfection groups was significantly decreased compared with that in the control group (both P<0.01), whereas no statistical significance was observed in the HMGA2-mut and miR-185-5p co-transfection groups (both P>0.05, Figure 4B), indicating a regulatory relationship exists between HMGA2 and miR-185-5p. Indeed, transfection with miR-185-5p mimic significantly reduced the protein and mRNA expression of HMGA2 ( Figure 4C, D). In contrast, inhibition of miR-185-5p enhanced HMGA2 expression ( Figure 4E-H). Moreover, treatment with miR-185-5p inhibitor remarkably abrogated the inhibitory effect of FOXD2-AS1 on HMGA2 protein expression (both P<0.01, Figure 4E, G). Similar results were also observed in the expression of HMGA2 mRNA as demonstrated by qRT-PCR ( Figure 4F, H). Together, these results suggest that HMGA2 is a direct target of miR-185-5p.

FOXD2-AS1 regulates the proliferation and migration of glioma cells via miR-185-5p/HMGA2 axis
Next, we investigated whether the miR-185-5p/HMGA2 axis was critical for the function of FOXD2-AS1 in glioma cells. MTT assay showed that overexpression of miR-185-5p significantly inhibited the viability of U251 and U87 cells and this inhibitory effect was reversed by co-overexpression of HMGA2 (P<0.01, Figure. 5A-B). Transfection of HMGA2 alone markedly increased the growth of U251 and U87 cells (P<0.01, Figure. 5A-B). Transwell migration assays also demonstrated that cotransfection of miR-185-5p mimic and HMGA2 abolished the migration inhibition induced by miR-185-5p overxpression in U251 and U87 cells (P<0.01, Figure 5C, D). Overexpression of HMGA2 alone remarkably increased the growth of U251 and U87 cells (P<0.01, Figure 5C, D). Moreover, overexpression of HMGA2 restored the cell viability and migration ability of U251 cells inhibited by FOXD2-AS1 knockdown (P<0.01, Figure 5E, F). These results indicate that FOXD2-AS1 regulates the proliferation and migration of glioma cells through regulation of miR-185-5p/HMGA2 cascade.

FOXD2-AS1 regulates PI3K/AKT signaling
qRT-PCR and Western blot results demonstrated that the mRNA and protein expression levels of PI3K and p-AKT were significantly down-regulated in the si-FOXD2-AS1 and miR-185-5p mimics groups, but dramatically up-regulated in the miR-185-5p inhibitor and p-HMGA2 groups ( Figure 5G-H). Moreover, cotransfection with si-FOXD2-AS1 and miR-185-5p inhibitor reversed the down-regulation of PI3K expression and AKT phosphorylation induced by FOXD2-AS1 knockdown. On the other hand, coexpression of miR-185-5p and HMGA2 abrogated the inhibitory effects of miR-185-5p on levels of PI3K and p-AKT ( Figure 5G-H). Furthermore, treatment with an activator of AKT, SC79 effectively blocked the AGING suppressive effects of FOXD2-AS1 knockdown on cell viability and migration capability of U251 cells. Together, these results suggest that FOXD2-AS1 silencing inactivates PI3K/AKT signaling through the miR-185-5p/HMGA2 axis and thus suppresses the cell growth and migration of glioma cells.

Knockdown of FOXD2-AS1 inhibits tumor growth in xenografts
To further investigate the effect of FOXD2-AS1 in vivo, a xenograft assay was performed. The results showed that the tumor volume and tumor weight in the AGING si-FOXD2-AS1 group were significantly decreased compared with those in the control and NC groups (both P<0.01, Figure 6A-D). Moreover, qRT-PCR showed that the expression of miR-185-5p was significantly upregulated in glioma tissues after FOXD2-AS1 silencing (P<0.01, Figure 6 E-F), which was consistent with the results found in U251 and U87 cells ( Figure 2E, F).

DISCUSSION
Previous results have demonstrated that the pathogenesis of glioma requires the changes in multiple genetic pathways, and each primary and secondary glioma possesses a unique combination of these genetic changes [18]. Primary glioma is constantly characterized with loss of heterozygosity at 10q (LOH, 70%), EGFR amplification (36%), deletion ofp16INK4a (31%) and mutation of PTEN (25%) [17]. In our study, we focused on lncRNA FOXD2-AS1, which has been found to be aberrantly expressed in various cancers. In this experiment, lncRNA FOXD2-AS1 was highly expressed in human glioma tissues and cell lines. Next, we investigated the downstream molecular mechanism of FOXD2-AS1 in glioma. In this study, we found a regulatory relationship between lncRNA FOXD2-AS1 and miR-185-5p, and between miR-185-5p and HMGA2. Compared with adjacent normal tissues, the expression of miR-185-5p was down-regulated, whereas the expression of HMGA2 was up-regulated in glioma tissues. In addition, HMGA2 overexpression had the potential to promote the proliferation and migration of glioma cells, whereas miR-185-5p mimics inhibited the biological function of glioma cells by targeting HMGA2. Finally, lncRNA FOXD2-AS1 was proven to act as the sponge of miR-185-5p and affects the PI3K/AKT signal transduction pathway via regulating HMGA2.  [20]. Besides, lncRNA can serve as a competitive endogenous RNA sponge for miRNA to regulate the degradation of miRNA target, thereby affecting the post-transcriptional regulation in bladder cancer [21]. In the present investigation, we first screened out the significantly differentially-expressed lncRNAs based on microarray assay and lncRNA FOXD2-AS1 was highly expressed in glioma.  AGING Subsequently, bioinformatics analysis, luciferase reporter gene assay and RIP test were conducted to confirm the regulatory relationship between FOXD2-AS1 and miR-185-5p. The findings of this study further validate that the transcriptional activation of FOXD2-AS1 contributes to tumorigenesis, whereas FOXD2-AS1 knockdown impairs the carcinogenic function of miR-185-5p in glioma cells.
It has been reported that HMGA2 plays a role in carcinogenesis and up-regulation of HMGA2 triggers the migration of cancer cell population via activation of PI3K/AKT downstream signal transduction [22]. In addition, previous studies have demonstrated that the expression of PI3K/AKT is up-regulated in glioma. In most circumstances, the loss of PI3K inhibitor PTEN (phosphatase and tens in homologue) or the functional acquisition mutation of PI3KCA5 is a potential cause.
In the current study, we explicitly elaborated the regulatory mechanism of FOXD2-AS1/miR-185-5p pathway upon the HMGA2 and PI3K/AKT signaling pathways, and eventually demonstrated that FOXD2-AS1 promoted HMGA2 by suppressing miR-185-5p, thereby regulating the proteins of PI3K and p-AKT. A diagram summarizing the main findings of this study was shown in Figure 7. In this study, the expression levels of PI3K and p-AKT were significantly downregulated in the si-FOXD2-AS1 and miR-185-5p mimics groups, whereas considerably up-regulated in the miR-185-5p inhibitor and p-HMGA2 groups, indicating that FOXD2-AS1 regulates the PI3K/AKT signal pathway through the miR-185-5p/HMGA2 axis.

CONCLUSION
Taken together, lncRNA FOXD2-AS1 and HMGA2 are overexpressed in glioma tissues and cell lines. A regulatory relationship exists between miR-185-5p and lncRNA FOXD2-AS1 or HMGA2. Moreover, lncRNA FOXD2-AS1 acts as a sponge of miR-185-5p to regulate HMGA2 expression and PI3K/AKT signaling pathway and subsequently promote the occurrence and progression of glioma. The findings in this study not only contribute to in-depth exploration of the exact mechanism of lncRNA FOXD2-AS1, but also provide a promising therapeutic target for the treatment of glioma.

Patients and samples
Human glioma samples were collected from 48 glioma patients (31 males and 17 females) who underwent neurosurgery in the Department of Neurosurgery of the Third Affiliated Hospital of Kunming Medical University from January 2013 to October 2015. Normal brain tissues were obtained from 24 patients with brain injury who were randomly selected from 2014 to 2016 and underwent brain tissue resection. All samples were restored in liquid nitrogen, and all participants did not receive radiotherapy before the experiment. Clinical parameters of glioma patients were shown in Supplementary Table 1. Written informed consents were obtained from all participants. The study procedures were approved by the Ethics Committee of the Third Affiliated Hospital of Kunming Medical University.

Microarray analysis
The Gene Expression Omnibus (GEO) database was utilized to screen the differentially expressed lncRNAs and mRNAs from tumor and normal tissues. The Blimma tool kit in the R script was adopted to identify differentially expressed genes under experimental conditions. The microarray data were subjected to logarithmic transformation, Benjamini-Hochberg adjust method [23] for Bayesian test. The differentially expressed genes were screened using multiple changes (P<0.05, |log (multiple changes) |>2).

qRT-PCR
Total RNA was extracted from frozen tissues or cultured cell lines using TRIZOL reagent (Life Technologies, Carlsbad, CA). The total RNA was reversetranscribed into cDNA using the ProtoScript® First

RNA immunoprecipitation
The RNA immunoprecipitation (RIP) test was conducted using the Magna RIP RNA-binding protein immunoprecipitation kit (Millibo, Billyka, MA, USA) according to the manufacturers' protocols. The U87 and U251 cells were lysed using a complete RIP lysis buffer, and 100 μl of the whole cell extract was incubated with RIPA buffer containing human anti-Argonaute 2 (AgO2) antibody (Millipore)-conjugated magnetic beads (#AK6979) at 4 for 6-8 h. Normal mouse IgG (Millipore) was used as negative control.

MTT assay
Cells were seeded into a 96-well plate and incubated at a density of 5×10 3 at 37 . After cell incubation for 0, 24, 48 and 72 h, MTT was supplemented into each well. The mixture was incubated at 37 and 5% CO 2 for 4 h, supplemented with 150 mL DMSO and dissolved at room temperature for 20 min. The absorbance value of each well was measured at a wavelength of 490 nm by an ultraviolet spectrophotometer.

Apoptosis analysis
The cells were seeded in a 6-well plate for 24 h. The cell apoptosis was detected by using Annexin V-FITC/PI apoptosis detection kit according to the manufacturers' instructions (BD Biosciences). The mixture was then shaken thoroughly in a dark room at room temperature for 15 min and supplemented with 2.5 μL PI. Cytomics FC 500 flow cytometer was adopted to analyze cell apoptosis.

Tumor xenograft transplantation
Four-week-old specific pathogen-free (SPF) BALB/c nude mice were selected and divided into 6 groups. After 1-week adaptation, approximately 5×10 6 U251 or U87 cells transfected with NC, shNC, sh-FOXD2-AS1, NC, mimics-NC or miR-185-5p mimics were subcutaneously inoculated into the right side of the mice in each group (n=8). The volume was measured every week and calculated using the following formula: Volume (cm 3 ) = (length × width 2 )/2. After 4 weeks, the mice were sacrificed. The tumor tissues were extracted and prepared for subsequent experiment. This study was approved by the Ethics Committee of Animal Experiment Center of the Third Affiliated Hospital of Kunming Medical University.

Statistical analysis
All data were presented as mean ± SD. All statistical analyses were performed by SPSS statistical software 17.0 (SPSS Inc., Chicago, IL, USA) and GraphPad Prism software (version 5.0). Group comparison was conducted by using Student's t-test or one-way ANOVA. The relationship between FOXD2-AS1 level and clinicopathological features was determined by χ2 test. P-values of statistical significance are represented as *P<0.05, **P<0.01 and ***P<0.001.