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Introduction

Renal cell carcinoma (RCC) is a common human urologic cancer that accounts for approximately 3% of all malignancies (1). The incidence and death rates of RCC have been steadily increased in recently years (2). RCC can be histologically classified into four subtypes, namely, clear cell RCC (ccRCC), papillary RCC, chromophobe RCC and oncocytoma (3). ccRCC, the most common subtype of RCC, accounts for 70–80% of RCC cases (4). Several risk factors, including dietary habit, physical activity and occupational exposure to specific carcinogens, have been identified to play important roles in ccRCC pathogenesis and progression (5). Despite considerable advancements in cancer diagnosis and therapy, the curative effects on patients at advanced stages remain unsatisfactory with a 5-year survival rate of only 5–10% (6). Surgical resection offers no therapeutic benefit for patients diagnosed with advanced stages of ccRCC (7). In addition, ccRCC is resistant to standard chemotherapy and radiotherapy (8). Therefore, the mechanisms underlying the formation and progression of ccRCC should be elucidated, and novel therapeutic methods should be developed for the clinical management of patients with this malignancy.

microRNAs (miRNAs) are a large group of highly conserved, short and non-coding RNAs with lengths of 19–23 nucleotides (9). miRNAs have emerged as novel gene regulators that control gene expression by specifically binding to the 3′-untranslated regions (3′-UTRs) of their target genes in a base-pairing manner, thereby repressing translation and/or inducing mRNA degradation (10,11). It is estimated that at least one thirds of human genes are regulated by miRNAs (12). MiRNAs have been acknowledged to be aberrantly expressed in various types of human cancer, including ccRCC (13). The dysregulation of miRNAs in ccRCC is associated with clinicopathological characteristics and prognosis. For example, miR-181a is overexpressed in ccRCC, and this upregulation is strongly associated with tumour size, tumour/node/metastasis (TNM) staging and Fuhrman grade (14). Furthermore, abnormally expressed miRNAs contribute to the tumourigenesis and tumour development of ccRCC by controlling numerous crucial cellular processes (15–17). Therefore, miRNAs may be potential targets of therapeutic intervention for patients with ccRCC.

miR-599 plays critical roles in different types of human cancers, such as breast cancer (18), hepatocellular carcinoma (19) and glioma (20). However, the expression pattern, biological function and molecular mechanism of miR-599 in ccRCC remain unknown. Thus, this study aimed to detect the expression level of miR-599 in ccRCC, examine its effect on ccRCC progression and further explore the possible mechanisms underlying the tumour suppressive roles of miR-599 in ccRCC.

Materials and methods

Clinical samples

Twenty-one paired ccRCC tissues and normal adjacent tissues (NATs) were obtained from patients (17 males, 4 females; age range, 42–75 years old; mean age, 59 years old) who underwent nephrectomy in Yidu Central Hospital of Weifang between August 2014 and March 2016. None of these patients were treated with chemotherapy or radiotherapy prior to nephrectomy. All of the tissue specimens were immediately frozen and stored in liquid nitrogen until further RNA isolation. This study was approved by the Ethics Committee of Yidu Central Hospital of Weifang, and written informed consent was collected from all patients before they participated in this research.

Cell culture and transfection

Two human ccRCC cell lines, A498 (21–23) and 786-O and one normal renal cell line (HK-2) were purchased from American Type Culture Collection (ATCC, Manassas, VA, USA). All ccRCC cell lines were cultured in Dulbecco's Modified Eagle's Medium (DMEM) supplemented with 10% fetal bovine serum (FBS), and 1% penicillin/streptomycin (all from Gibco; Thermo Fisher Scientific, Inc., Waltham, MA, USA). HK-2 cells were grown at keratinocyte-SFM (Invitrogen; Thermo Fisher Scientific, Inc.) supplemented with bovine pituitary extract and human recombinant epidermal growth factor (all from Gibco; Thermo Fisher Scientific, Inc.). All these cell lines were maintained at 37°C in a humidified incubator containing 5% carbon dioxide (CO2).

miR-599 mimics, negative control miRNA mimics (miR-NC), small interfering RNA (siRNA) against the expression of HMGA2 (si-HMGA2) and negative control siRNA (si-NC) were designed and synthesised by Shanghai GenePharma Co., Ltd. (Shanghai, China). HMGA2 overexpression plasmid (pCMV-HMGA2) and empty pCMV plasmid were acquired from OriGene Technologies, Inc., (Beijing, China). Cells were inoculated into six-well plates and transfected with miRNA mimics, siRNA or plasmid by using Lipofectamine 2000 (Invitrogen; Thermo Fisher Scientific, Inc.) in accordance with the manufacturer's protocol. Reverse transcription-quantitative polymerase chain reaction (RT-qPCR) and western blot analysis were performed to evaluate the transfection efficacy.

RT-qPCR

Total RNA was isolated from tissue samples or cells by using TRIzol reagent (Invitrogen; Thermo Fisher Scientific, Inc.) in accordance with the manufacturer's protocol. The concentration and purity of total RNA were determined using a NanoDrop2000 spectrophotometer (Thermo Fisher Scientific, Inc.). The cDNA of miR-599 was synthesised by using a TaqMan MicroRNA reverse transcription kit (Applied Biosystems; Thermo Fisher Scientific, Inc.). Quantitative PCR (qPCR) was conducted to detect miR-599 expression by using a TaqMan MicroRNA PCR kit (Applied Biosystems; Thermo Fisher Scientific, Inc.) with U6 snRNA as an internal control. To quantify the mRNA expression of HMGA2, we conducted reverse transcription with a PrimeScript RT reagent kit (Takara Bio, Inc., Otsu, Japan) followed by qPCR with a SYBR Premix Ex Taq™ kit (Takara Bio, Inc.). GAPDH was employed as an internal control for the mRNA level of HMGA2. Relative gene expression was analysed through the 2−∆∆Ct method (24).

Cell Counting Kit-8 (CCK-8) assay

CCK-8 assay (Dojindo Molecular Technologies, Inc., Kumamoto, Japan) was applied to evaluate cell proliferation of ccRCC. Transfected cells were harvested at 24 h post-transfection. In each well of a 96-well plate, 3,000 transfected cells were plated and incubated at 37°C in a humidified incubator with 5% CO2 for 0, 24, 48 and 72 h. At every time point, CCK-8 assay was performed in accordance with the manufacturer's protocol. Briefly, 10 µl CCK-8 reagent was added into each well, and then incubated at 37°C for another 2 h. The absorbance value of each well was detected at a wavelength of 450 nm with the ELISA microplate reader (Bio-Rad Laboratories, Inc., Hercules, CA, USA).

Transwell invasion assay

Cellular invasion ability was examined by using a Boyden chamber containing 24-well Transwell plates (Corning Costar, Corning, NY, USA) with 8 µm pore membranes. After transfection for 48 h, cells were collected, suspended in FBS-free DMEM and seeded in the upper chamber of the insert, which was pre-coated with Matrigel (BD Biosciences, Franklin Lakes, NJ, USA). At 24 h post-incubation, the remaining cells at the upper side of the 8 µm pore membranes were wiped away with cotton swabs. The invasive cells that had invaded the bottom of the inserts were fixed in 4% cold paraformaldehyde and stained with 0.5% crystal violet. The stained cells were photographed and counted under an inverted microscope (Olympus Corporation, Tokyo, Japan) at ×200 magnification.

miR-599 target prediction and luciferase reporter assay

TargetScan (http://www.targetscan.org/) and microRNA (www.microrna.org) algorithms were used to predict the putative targets of miR-599. HMGA2 was predicted as a potential target of miR-599. Luciferase reporter assay was performed to further determine whether miR-599 could directly bind to the 3′-UTR of HMGA2. A total of 6×104 cells were seeded in triplicates in 24-well plates. Luciferase reporter plasmids, namely, pGL3-HMGA2-3′-UTR wild-type (Wt) and pGL3-HMGA2-3′-UTR mutant (Mut), were designed and synthesised by Shanghai GenePharma Co., Ltd. When the cell density reached 70–80% confluence, miR-599 mimics or miR-NC was transfected into cells with pGL3-HMGA2-3′-UTR Wt or pGL3-HMGA2-3′-UTR Mut by using Lipofectamine 2000. After 48 h of incubation, a dual luciferase reporter assay system (Promega Corp., Madison, Wisconsin, USA) was adopted to measure the luciferase activity in accordance with the manufacturer's procedure. The firefly luciferase activity was normalised to Renilla luciferase activity.

Western blot analysis

Total protein was extracted from tissue samples or cells in an ice bath by using RIPA lysis buffer (Santa Cruz Biotechnology, Inc., Dallas, TX, USA). The protein concentration was detected by using a BCA protein assay kit (Bio-Rad Laboratories, Inc.). Equal amounts of proteins were subjected to 10% sodium dodecyl sulphate polyacrylamide gel electrophoresis and transferred to a polyvinylidene difluoride membrane (EMD Millipore, Billerica, MA, USA), which was then blocked with 5% skimmed milk dissolved in TBS containing 0.1% Tween-20 (TBST). Afterwards, the membranes were incubated with primary antibodies against HMGA2 (1:1,000 dilution, ab184616; Abcam, Cambridge, UK) or GAPDH (1:1,000 dilution, sc-47724; Santa Cruz Biotechnology, Inc.) overnight at 4°C, rinsed with TBST, further incubated with the corresponding horseradish peroxidase-conjugated secondary antibody (1:5,000 dilution, sc-2005; Santa Cruz Biotechnology, Inc.) at room temperature for 2 h and washed with TBST. The protein signals were then visualised with an ECL detection kit (GE Healthcare Life Sciences, Chalfont, UK) and analysed with Image J 1.41 (National Institutes of Health, Bethesda, MD, USA). GAPDH was used as an internal control.

Statistical analysis

Data were expressed as mean ± standard deviation and analysed using SPSS v21.0 (SPSS Inc., Chicago, IL, USA). Each assay was repeated at least thrice. Differences between two groups were compared using t test, whereas differences between multiple groups were compared through one-way ANOVA. Student-Newman-Keuls test was used as a post hoc test. P<0.05 was considered to indicate a statistically significant difference.

Results

miR-599 is downregulated in ccRCC tissues and cell lines

miR-599 expression levels were detected in 21 paired ccRCC tissues and NATs. The results of RT-qPCR revealed that miR-599 was underexpressed in ccRCC tissues compared with that in NATs (P<0.05; Fig. 1A). The expression level of miR-599 was further examined in two ccRCC cell lines (A498 and 786-O) and one normal renal cell line (HK-2) by conducting RT-qPCR. As shown in Fig. 1B, miR-599 expression level decreased in the A498 and 786-O cell lines compared with HK-2 (P<0.05). These results suggest that the downregulation of miR-599 may be correlated with ccRCC progression.

Figure 1. miR-599 is downregulated in ccRCC tissues and cell lines. (A) RT-qPCR analysis was used to determine miR-599 expression in 21 paired ccRCC tissues and NATs. *P<0.05 vs. NATs. (B) miR-599 expression level in two ccRCC cell lines (A498 and 786-O) and one normal renal cell line (HK-2) was examined by RT-qPCR. *P<0.05 vs. HK-2. RT-qPCR, reverse transcription-quantitative polymerase chain reaction; miR, microRNA; ccRCC, clear cell renal cell carcinoma; NAT, normal adjacent tissue.

miR-599 attenuates the proliferation and invasion of ccRCC cells

To elucidate the effects of miR-599 in ccRCC development, we overexpressed miR-599 in A498 and 786-O cells. RT-qPCR analysis confirmed that miR-599 was markedly upregulated in A498 and 786-O cells transfected with miR-599 mimics (P<0.05; Fig. 2A). Then, CCK-8 assays were performed to determine the effects of miR-599 overexpression on the proliferation of A498 and 786-O cells. Ectopic expression of miR-599 significantly restricted the proliferation of A498 and 786-O cells compared with that in the cells transfected with miR-NC (P<0.05; Fig. 2B). Additionally, Transwell invasion assays were conducted to investigate the invasion abilities of A498 and 786-O cells transfected with miR-599 mimics or miR-NC. Upregulation of miR-599 resulted in the reduced invasion capabilities of A498 and 786-O cells (P<0.05; Fig. 2C). These data suggest that miR-599 may play tumour suppressive roles in ccRCC.

Figure 2. miR-599 re-expression attenuates cell proliferation and invasion of clear cell renal cell carcinoma. (A) miR-599 mimics were used to overexpress miR-599 in A498 and 786-O cells. The transfection of miR-599 mimics significantly increased the miR-599 expression in A498 and 786-O cells, as confirmed by reverse transcription-quantitative polymerase chain reaction. (B) A CCK-8 assay was performed to explore the effect of miR-599 overexpression on A498 and 786-O cells. Upregulation of miR-599 prohibited the proliferation of A498 and 786-O cells at 48 and 72 h. (C) A Transwell invasion assay was adopted to detect the invasion abilities of A498 and 786-O cells transfected with miR-599 mimics or miR-NC. Magnification, ×200. *P<0.05 vs. miR-NC. miR, microRNA; NC, negative control.

HMGA2 is a direct target of miR-599 in ccRCC

Bioinformatics analysis was performed to predict the putative targets of miR-599 and to investigate the molecular mechanism by which miR-599 affected the aggressive phenotype of ccRCC cells. Among the candidates, HMGA2 was selected for further confirmation (Fig. 3A) because it is aberrantly highly expressed in ccRCC and significantly associated with ccRCC progression (25–28). To identify whether HMGA2 is a direct target of miR-599, we subjected A498 and 786-O cells cotransfected with miR-599 mimics or miR-NC and pGL3-HMGA2-3′-UTR Wt or pGL3-HMGA2-3′-UTR Mut to luciferase reporter assays. As shown in Fig. 3B, enforced expression of miR-599 significantly decreased the luciferase activities of the wild-type 3′-UTR reporter plasmid in A498 and 786-O cells (P<0.05). However, this suppressive effect was abrogated in the mutant 3′-UTR reporter plasmid, in which the binding sequences mutated. RT-qPCR and western blot analysis were performed to detect the mRNA and protein expression of HMGA2 in A498 and 786-O cells transfected with miR-599 mimics or miR-NC and to evaluate the association between miR-599 and HMGA2 in ccRCC. The results indicated that restored miR-599 expression in the two ccRCC cell lines resulted in significantly reduced HMGA2 expression at both mRNA (Fig. 3C, P<0.05) and protein (P<0.05; Fig. 3D) levels. Therefore, HMGA2 is a direct target gene of miR-599 in ccRCC.

Figure 3. HMGA2 is a direct target of miR-599 in clear cell renal cell carcinoma. (A) Predicted wild-type and mutant of miR-599 binding sites in the 3′-UTR of HMGA2. (B) A498 and 786-O cells were transfected with miR-599 mimics or miR-NC and pGL3-HMGA2-3′-UTR wild type or pGL3-HMGA2-3′-UTR mutant. Luciferase activities were determined at 48 h post-transfection. (C) Reverse transcription-quantitative polymerase chain reaction and (D) western blot analysis of HMGA2 mRNA and protein levels in A498 and 786-O cells transfected with miR-599 mimics or miR-NC. *P<0.05 vs. miR-NC. UTR, untranslated region; miR, microRNA; NC, negative control; HMGA2, high-mobility group AT-hook 2.

HMGA2 knockdown inhibits cell proliferation and invasion in ccRCC

HMGA2 was validated as a direct target of miR-599 in ccRCC. As such, we hypothesised that the tumour suppressive roles of miR-599 in ccRCC might be exhibited by HMGA2 knockdown. To confirm this hypothesis, si-HMGA2 was transfected into A498 and 786-O cells to knock down endogenous HMGA2 expression. Western blot analysis revealed that HMGA2 was obviously downregulated in A498 and 786-O cells following transfection with si-HMGA2 (P<0.05; Fig. 4A). Subsequent CCK-8 and Transwell invasion assays demonstrated that the inhibition of HMGA2 prohibited the proliferation (P<0.05; Fig. 4B) and invasion (P<0.05; Fig. 4C) of A498 and 786-O cells. This phenomenon resembled the inhibitory effects of miR-599 overexpression on ccRCC cells, further suggesting that HMGA2 is a direct downstream target of miR-599 in ccRCC.

Figure 4. Effects of HMGA2 knockdown on the proliferation and invasion of clear cell renal cell carcinoma. (A) si-HMGA2 was introduced into A498 and 786-O cells to knock down the endogenous HMGA2 expression. After transfection for 72 h, the efficiency of siRNA transfection in A498 and 786-O cells was evaluated through western blot analysis. (B) A498 and 786-O cells were transfected with si-NC or si-HMGA2. A CCK-8 assay was performed to detect the indicated cell proliferation. (C) Invasive capabilities of A498 and 786-O cells transfected with si-HMGA2 or si-NC was assessed with Transwell invasion assays. Magnification, ×200. *P<0.05 vs. NC siRNA. si, short interfering; HMGA2, high-mobility group AT-hook 2; NC, negative control.

Restored expression of HMGA2 reverses the suppressive effects induced by miR-599 overexpression in ccRCC cells

A rescue experiment was performed to evaluate whether HMGA2 is responsible for the inhibitory effects of miR-599 on ccRCC cells. A498 and 786-O cells were cotransfected with miR-599 mimics and pCMV-HMGA2 or empty pCMV plasmid. Western blot analysis was conducted to examine the transfection efficiency, and the results confirmed that the downregulation of HMGA2 caused by miR-599 overexpression was recovered after the A498 and 786-O cells were cotransfected with pCMV-HMGA2 (P<0.05; Fig. 5A). In addition, CCK-8 and Transwell invasion assays revealed that recovered HMGA2 expression eliminated the inhibitory effects on cell proliferation (Fig. 5B, P<0.05) and invasion (Fig. 5C, P<0.05) induced by miR-599 overexpression in A498 and 786-O cells. Therefore, miR-599 inhibits cell proliferation and invasion of ccRCC partly by inhibiting the HMGA2 expression.

Figure 5. Restored HMGA2 expression reverses the suppressive effects on clear cell renal cell carcinoma proliferation and invasion induced by miR-599 mimics. miR-599 mimics, along with pCMV-HMGA2 or pCMV were introduced into A498 and 786-O cells. (A) HMGA2 protein expression in the indicated cells was assessed through western blot analysis. (B) CCK-8 and (C) Transwell invasion assays were performed to detect cell proliferative and invasive capabilities of the indicated cells. Magnification, ×200. *P<0.05 vs. miR-NC. #P<0.05 vs. miR-599 mimics+pCMV-HMGA2. miR, microRNA; HMGA2, high-mobility group AT-hook 2; NC, negative control.

Discussion

Dysregulation of miRNAs is involved in the occurrence and development of ccRCC through their participation in many important biological processes, such as cell proliferation, cycle, apoptosis, angiogenesis, migration, invasion and metastasis (29–31). Thus, an in-depth investigation on miRNAs and their biological roles in ccRCC may provide useful insights into the identification of novel therapeutic methods for patients with this disease. In this study, miR-599 expression was significantly reduced in ccRCC tissues and cell lines. Ectopic expression of miR-599 suppressed the proliferation and invasion of ccRCC cells. HMGA2 was identified as a direct target gene of miR-599 in ccRCC, and HMGA2 knockdown could mimic the suppressive effects of miR-599 overexpression on ccRCC cells. Moreover, restored HMGA2 expression rescued the inhibitory properties of ccRCC cells caused by miR-599 overexpression. These results suggest that miR-599 might be a novel therapeutic agent for ccRCC.

miR-599 has been reported to be aberrantly expressed in many types of human malignancies. For example, miR-599 expression is reduced in gastric cancer tissues and cell lines. The downregulation of miR-599 was associated with poor prognostic features, including lymph node metastasis and advanced TNM stage. The 5-year overall survival of patients with gastric cancer and a low miR-599 expression is shorter than that of patients with high miR-599 levels (32). miR-599 is also observed to be underexpressed in breast cancer (18), hepatocellular carcinoma (19) and glioma (20). Nevertheless, the miR-599 expression is relatively higher in non-small cell lung cancer tissues than in normal lung tissues (33). These conflicting studies suggest that the expression pattern of miR-599 exhibits tissue specificity and possibly represents a biomarker for the diagnosis of specific cancer types.

The dysregulation of miR-599 has been implicated in the carcinogenesis and progression of multiple types of human cancer. For instance, miR-599 overexpression suppresses cell metastasis and epithelial-mesenchymal transition in gastric cancer (32). Wang et al (18) reported that restored expression of miR-599 reduces cell proliferation, colony formation and metastasis in vitro and decreases tumour growth in vivo. Tian et al (19) revealed that enforced expression of miR-599 restricts cell growth and motility of hepatocellular carcinoma in vitro. Zhang et al (20) demonstrated that ectopic expression of miR-599 prohibits the migration and invasion of glioma cells. However, miR-599 serves as an oncogene in non-small cell lung cancer by promoting cell migration and invasion (33). These findings suggested that miR-599 may be a promising therapeutic target for patients with these human cancers.

Several miR-599 targets, including EIF5A2 (32) in gastric cancer, BRD4 (18) in breast cancer, MYC (19) in hepatocellular carcinoma, periostin (20) in glioma and STAB2 (33) in non-small cell lung cancer, have been validated. HMGA2, a membrane of high-mobility group A proteins, was identified as a novel target of miR-599 in ccRCC. Aberrantly overexpressed HMGA2 has been reported in various types of human cancer, such as breast cancer (34), thyroid cancer (35), colorectal cancer (36) and glioblastoma (37). HMGA2 is highly expressed in ccRCC at mRNA and protein levels (25). The upregulation of HMGA2 is strongly correlated with tumour size, lymph node metastasis and Fuhrman grade. In addition, the prognosis of patients with ccRCC with high HMGA2 levels is poorer than that of patients with low HMGA2 levels. Furthermore, HMGA2 expression level is an independent prognostic factor for patients with ccRCC (26). Besides, HMGA2 deregulation affected the onset and development of ccRCC by regulating cell proliferation, invasion and epithelial-mesenchymal transition (26,28). These findings suggested that targeting HMGA2 may show potential for the treatment of ccRCC in the future.

In conclusion, miR-599 was significantly downregulated in ccRCC tissues and cell lines. In vitro functional experiments demonstrated that miR-599 inhibited the proliferation and invasion of ccRCC cells. Mechanistically, HMGA2 was identified as a direct target gene of miR-599 in ccRCC. Therefore, the miR-599/HMGA2 pathway may provide a novel therapeutic target for the treatment of patients with ccRCC.

Acknowledgements

Not applicable.

Funding

No funding was received.

Availability of data and materials

The datasets used and/or analyzed during the present study are available from the corresponding author on reasonable request.

Authors' contributions

XL designed this research. HaZ and HuZ performed the experiments. XX analyzed the data. All authors have read and approved the final manuscript.

Ethics approval and consent to participate

This study was approved by the Ethics Committee of Yidu Central Hospital of Weifang and was performed in accordance with the Declaration of Helsinki. Written informed consent was collected from all patients prior to their participation.

Consent for publication

Written informed consent was obtained from all participants for the publication of their data.

Competing interests

The authors declare that they have no competing interests.

References