circITGA7 Functions as an Oncogene by Sponging miR-198 and Upregulating FGFR1 Expression in Thyroid Cancer.

BACKGROUND: Emerging evidence has indicated that circular RNAs (circRNAs), recognized as functional noncoding transcripts in eukaryotic cells, may be involved in regulating many physiological or pathological processes. However, the regulation and function of circular RNA circITGA7 in thyroid cancer (TC) remains unknown.
METHODS: In this study, we found that circITGA7 is upregulated in TC cell lines. We then performed functional analyses in the cell lines to support clinical findings. Mechanistically, we demonstrated that circITGA7 can directly bind to miR-198 and reduce the inhibition effect of miR-198 on target FGFR1 expression.
RESULTS: We reported an upregulation of circITGA7 in patients with TC. Silencing of circITGA7 inhibits metastasis and proliferation of TC cell lines in vitro. In addition, in the TC cell lines, the knockdown of circITGA7 or overexpression of miR-198 significantly suppressed FGFR1 levels. Mechanistically, we found that circITGA7 acts as miR-198 competitive endogenous RNA (ceRNA) to regulate FGFR1 expression.
CONCLUSIONS: In summary, circRNA circITGA7 may play a regulatory role in TC and may be a potential marker for TC diagnosis or progression.

1. Introduction

Noncommunicable diseases had been a major cause of human death worldwide [1, 2]. With the development of economic and medical standards, cancer has been the leading cause of death in many countries, especially in developing countries [3, 4]. Thyroid cancer (TC) is one of the most common endocrine malignancies in females [5]. Until now, the mechanisms regulating TC remained to be unclear, although multiple evidences showed that abnormal iodine intake, BRAF mutations, obesity, smoking, hormone exposure, ionizing radiation, and certain environmental pollutants contribute to the development of this disease [6].

circRNA is a highly conserved covalently closed circular RNA that was first discovered 40 years ago [7]. The majority of circRNAs are generated by reverse splicing of exons or introns of protein-coding or noncoding genes [8]. circRNA is not easily degraded by conventional mechanisms within the cell and therefore has a longer half-life; due to that, circRNA has no free 3′ or 5′ end [9]. Studies have shown that some circRNAs are differently expressed in cancer cells, indicating that circRNAs played a regulatory role in cancer progression [10]. For example, studies have shown that upregulation of circABCB10 in breast cancer promotes cell proliferation [11]. Similarly, studies have detected a significant upregulation of circHIPK3 in liver cancer [12]. Functionally, the downregulation of circHIPK3 leads to the decrease of HCC cell proliferation [13]. Interestingly, many studies have shown circRNAs have different effects, even the opposite effect in different cancer types through different mechanisms [12]. For example, circHIPK3 was also revealed to have tumor-suppressive effect in several types of cancers, such as bladder cancer. Overexpression of circHIPK3 suppressed the invasiveness and proliferative properties in bladder cancer [14]. Exploring the roles and mechanisms of circRNAs could provide novel biomarkers for the diagnosis and treatment of cancers.

circITGA7 is derived from the fourth exon of the protein-coding gene integrin Fa7F with 256 bp in length. It has been reported to be a tumor suppressor in prostate cancer and melanoma [15]. The present study showed that circITGA7 can bind to miR-198 to upregulate the protein level of FGFR1 and can promote cell growth and metastasis of TC.

2. Material and Methods

2.1. Cell Culture, Cell Proliferation, Migration, and Invasion Assay

The human thyroid normal cell line Nthy-ori 3-1 and the TC cell lines CAL-62, TPC1, and K1 were purchased from ATCC (Manassas, VA, USA). The transfection was performed using Lipofectamine 2000. The cell proliferation assay was performed using CCK-8. The migration or invasion ability of cells was detected using the Transwell chamber coated without or with Matrigel mix (BD Biosciences, San Jose, CA, USA). All these assays were performed according to the manufacturer's protocol.

2.2. Dual-Luciferase Reporter Assay

The circITGA7 and FGFR1 which contain the binding site for miR-198 were inserted into the pmiR-GLO vector to obtain a wild type. Mutant circITGA7 and FGFR1 were constructed using a site-directed mutagenesis kit (Takara). The luciferase activity was measured with the Dual-Luciferase Reporter Assay System (Promega) according to the manufacturer's protocols.

2.3. qRT-PCR Assay

RNA was extracted using TRIzol reagent. The reverse transcription was conducted using EvoScript Universal cDNA Master (Roche, Basel, Switzerland). qRT-PCR was conducted using SYBR Green Mix (Roche). Gene expression was quantified using the 2− method. All these assays were performed according to the manufacturer's protocol.

2.4. Bioinformatics Analysis

The putative binding sites of miR-198 in the circITGA7 or FGFR1 sequence were predicted using miRBase (http://www.mirbase.org/) and TargetScan (http://www.targetscan.org).

2.5. Western Blot

Western blot assay was conducted according to the previous reports. The antibody used in this study included FGFR1 (1 : 2,000, CST, 2586) or GAPDH (1 : 1,000, CST, 2118).

2.6. Statistical Analysis

All data were represented as mean ± standard deviation (SD) from at least three independent experiments. The results were analyzed using SPSS 19.0 software (SPSS Inc., Chicago, IL, USA). The differences between two groups were analyzed using Student's t-test or one-way ANOVA. P value less than 0.05 was considered to be significant.

3. Results

3.1. circITGA7 Was Upregulated in TC Samples

We first determined circITGA7 expression in TC cells using qRT-PCR assay. We found that circITGA7 levels in TPC1, CAL-62, and K1 were remarkably higher than those in Nthy-ori 3-1 (Figure 1). Furthermore, we detected circITGA7 expression levels in 8 pairs of TC tissues. Our results demonstrated that circITGA7 expression was significantly upregulated in TC tissues compared to matched normal samples (Figure 2).

Figure 1

circITGA7 was significantly upregulated in the TC cell lines compared with that in the Nthy-ori 3-1.

Figure 2

The expression of circITGA7 in TC tissues was significantly higher than that in adjacent tissues by qRT-PCR.

3.2. Knockdown of circITGA7 Inhibited TC Cell Proliferation

To determine the biological functions of circITGA7 in TC, we first detected the subcellular location of the cytoplasm-located GAPDH, nucleus-located U6, and circITGA7 in the TPC1 and CAL-62 cells (Figure 3). As expected, U6 showed a nucleus-located expression pattern (Figure 3(a)), GAPDH showed a cytoplasm-located expression pattern (Figure 3(b)), and circITGA7 was mainly located in the cytoplasm of TC cells (Figure 3(c)).

Figure 3

circITGA7 was mainly located in the cytoplasm of the TC cells.

To investigate the function of circITGA7 knockdown on TC cell proliferation, CCK-8 analysis was performed. Our results showed that the proliferation rate of si-circITGA7-transfected cells was significantly downregulated in the CAL-62 (Figure 4(b)) and TPC1 cells (Figure 4(d)) compared to the control group. The knockdown efficacy of transfection in CAL-62 and TPC1 is also shown in Figure 4.

Figure 4

Knockdown of circITGA7 inhibited TC cell proliferation. (a, c) si-circITGA7 reduced the circITGA7 expression level in TC cells. (b, d) TC cells with si-circITGA7 had a lower rate of proliferation.

3.3. Knockdown of circITGA7 Inhibited TC Cell Migration and Invasion In Vitro

Furthermore, we then tested whether circITGA7 affected the metastatic abilities of TC cells. The results of Transwell assays indicated that knockdown of circITGA7 remarkably reduced the invasive (Figure 5) and migratory ability (Figure 6) of the CAL-62 and TPC1 cells.

Figure 5

Knockdown of circITGA7 inhibited invasion ability of the CAL-62 and TPC1 cells in vitro.

Figure 6

Knockdown of circITGA7 inhibited migration ability of the CAL-62 and TPC1 cells in vitro.

3.4. circITGA7 Served as a Sponge of miR-198 to Promote FGFR1

By using the RegRNA 2.0 online database (http://regrna2.mbc.nctu.edu.tw/index.html), we predicted miR-198 was a direct target of circITGA7. Dual-luciferase assays indicated that miR-198 could directly bind to circITGA7. After cotransfecting circITGA7 wild-type or mutant luciferase reporter plasmids with miR-198 in TC cells, we observed that the Renilla/firefly ratio of circITGA7 wild-type group was significantly reduced after overexpressing miR-198 (Figures 7(c) and 7(d)). However, miR-198 did not affect the luciferase activity of the circITGA7 mutant group (Figures 7(e) and 7(f)). Furthermore, our results showed that overexpression of miR-198 significantly suppressed circITGA7 levels (Figures 7(a) and 7(b)).

Figure 7

circITGA7 served as a sponge of miR-198 in TC cells. (a, b) Overexpression of miR-198 suppressed circITGA7 expression. (c–f) Dual-luciferase reporter assay showed that cotransfection with miR-198 mimic significantly inhibits the luciferase activity of the circITGA7 wild-type group.

FGFR1 was reported to be a direct target of miR-198 in multiple types of human cancers. Of note, FGFR1 was also found to regulate TC progression. Therefore, we detected whether circITGA7 could affect FGFR1 expression levels through miR-198. Dual-luciferase assays showed that miR-198 could inhibit the luciferase activity of wild-type FGFR1, but not mutant FGFR1 (Figures 8(a)–8(d)). Overexpression of miR-198 remarkably reduced the expression levels of FGFR1 in TC cells (Figures 8(e)–8(g)). Finally, our results revealed that knockdown of circITGA7 significantly suppressed the levels of FGFR1 in the CAL-62 and TPC1 cells (Figures 8(h)–8(j)).

Figure 8

FGFR1 was a target of miR-198. (a–d) Dual-luciferase reporter assay of wild-type and mutant FGFR1 3′UTR with miR-198. (e–i) TC cells transfected with miR-198 or si-circITGA7 had lower expression of FGFR1. (g, j) Western blot analysis detects the effect of miR-198 overexpression and circITGA7 knockdown on the expression of FGFR1 and GAPDH.

4. Discussion

Cancer metastasis is still the primary cause of mortality in patients with TC [16]. However, the mechanisms underlying TC metastasis remained largely unclear. This study for the first time demonstrated that circITGA7 acted as a metastasis promoter in TC. Knockdown of circITGA7 inhibits the miRNA-198/FGFR1 signaling pathway, which affects the proliferation and metastasis of TC cell lines. Interestingly, we also found that circITGA7 is highly expressed in thyroid tumors compared to adjacent tissues and is also highly expressed in thyroid tumor cell lines.

So far, a large number of people have discovered seven important roles of circRNA in tumors. For example, hsa_circ_0020397 enhances the proliferation, metastasis, and invasiveness of CRC cells by mutagenizing miR-138 [17]. hsa_circ_0014717 may inhibit the proliferation and metastasis of CRC by upregulating P16 expression [18]. In addition, hsa_circ_0008309 affects cell proliferation of various cancers by modulating the ATXN1 through miR-136-5p and miR-382-5p [19]. Consistently, our study demonstrates that downregulation of circITGA7 plays an important inhibitory role in TC cells.

circITGA7 is a novel circRNA involved in regulating cancer tumorigenesis and development. For example, Li et al. reported that circITGA7 promoted NF1 translation via sponging miR-370-3p [15]. In CRC, circITGA7 suppressed cancer progression through RAS signaling [15]. Our study suggests that circITGA7 plays a carcinogenic role in TC, suggesting that circITGA7 may be a potential therapeutic target. In addition, we prove that knockdown of circITGA7 significantly inhibited the expression of FGFR1. Overexpression of miR-198 significantly suppressed the expression levels of FGFR1 and circITGA7. Dual-luciferase assay showed that both FGFR1 and circITGA7 were the direct targets of miR-198.

miR-198 was reported to be a regulator of multiple cancers, such as liver, gastric, and breast cancers [20-22]. miR-198 was downregulated in gastric cancer, thus being a potential biomarker for this cancer [23]. The present study demonstrated that miR-198 is reduced in TC cells. FGFR1 acted as an oncogene in human cancers by affecting cell proliferation, apoptosis, and metastasis [24]. FGFR1 was involved in the regulation of the RAS/ERK1/2 pathway and the Wnt pathway [25]. The present study identified FGFR1 was a direct target of miR-198. Of note, we did not explore whether miR-198 is involved in the regulation of these pathways in this study. This is a limitation of the present study.

In conclusion, we demonstrated that circITGA7 enhanced TC progression via sponging miR-198 to upregulate FGFR1 expression. These findings suggested that circITGA7 could be a promising therapeutic strategy for TC.