Long noncoding RNA lncTCF7, induced by IL-6/STAT3 transactivation, promotes hepatocellular carcinoma aggressiveness through epithelial-mesenchymal transition.

BACKGROUND: Accumulating evidence suggests the pro-inflammatory cytokine interleukin-6 (IL-6) in tumor microenvironment may promote the development of hepatocellular carcinoma (HCC). However, the underlying mechanism remains largely unknown.
METHODS: The expression and promoter activity of lncTCF7 were measured by quantitative real-time polymerase chain reaction (qRT-PCR) and luciferase reporter assay. The function of the STAT3 binding site in the lncTCF7 promoter region was tested by luciferase reporter assay with nucleotide substitutions. The binding of STAT3 to the lncTCF7 promoter was confirmed by chromatin immunoprecipitation assay (CHIP) in vivo. The effects of decreasing STAT3 with small interference RNA and inhibiting STAT3 activation by small molecular inhibitor on lncTCF7 expression were also determined.
RESULTS: We demonstrate that IL-6 could induce lncTCF7 expression in a time- and dose-dependent manner, and we showed that IL-6 transcriptionally activated the expression of lncTCF7 in HCC cells by activating STAT3, a transcription activator which binds to promoter regions of lncTCF7. Furthermore, knocking-down STAT3 and inhibiting STAT3 activation reduced lncTCF7 expression. Importantly, RNA interference-based attenuation of lncTCF7 prevented IL-6-induced EMT and cell invasion.
CONCLUSION: Thus, these data provides evidence to the existence of an aberrant IL-6/STAT3/ lncTCF7 signaling axis that leads to HCC aggressiveness through EMT induction, which could be novel therapeutic targets in malignancies.

Introduction

Liver cancer is the fifth-most common solid tumor worldwide and the second most frequent cause of cancer-related death in China [1, 2]. Hepatocellular carcinoma (HCC) represents the major histological subtype and accounts for about 78 % cases of primary liver cancer [2]. In spite of the recent advances in the management of HCC, the 5-year survival rate of HCC remains poor, at approximately 50 % (range, 17–69) [3]. The high incidence of cancer recurrence and metastasis is still the main obstacle in the treatment of HCC [1, 2]. Although remarkable progress have been made in the knowledge of HCC tumorigenesis, the precise details of the molecular mechanisms underlying HCC carcinogenesis remain to be elucidated [4, 5].

Increasing evidence has established a linked between chronic inflammation and liver cancer risk in epidemiological studies [6-8], and several pro-inflammatory cytokines released from infiltrating inflammatory cells and other cells in the microenvironment have been suggested to regulate HCC carcinogenesis [9-11]. In particular, interleukin-6 (IL-6) and its intracellular signaling molecule signal transducer and activator of transcription 3 (STAT3) seem to play a vital role in bridging chronic inflammation to HCC progression [9, 12]. Furthermore, IL-6 levels in cancer tissues and serum are elevated in HCC patients, and are correlated with tumor metastasis and reduced patient survival [13, 14].

Long noncoding RNAs (lncRNAs) are a class of novel noncoding RNAs, defined as transcripts longer than 200 nucleotides (nt) with limited protein-coding potential [15]. Although once regarded as “transcriptional noise”, lncRNAs have been demonstrated to take a part in a variety of cellular processes, including carcinogenesis [16-19]. Alterations in a number of lncRNAs have been shown to exhibit tumor-suppressive or pro-oncogenic activities in HCC, including Linc00974 [20], HOTAIR [21], H19 [22], DANCR [23], lncTCF7 [16], Dreh [24], lncRNA MVIH [25], lncRNA-HEIH [26], HULC [27], LET [28], lncRNA-ATB [11], and PVT-1 [29]. However, the effect of IL-6 on lncRNAs remains largely unknown.

The epithelial-mesenchymal-transition (EMT) is a well-coordinated process that take places during embryonic development and a pathological feature in tumorigenesis [30, 31]. During this process, the epithelial phenotype cells lose expression of membranous epithelial marker E-cadherin and other components of cell to cell junctions and adopt a mesenchymal phenotype [32]. The EMT process has been demonstrated to play an important role in cancer invasion, metastasis, therapeutic resistance and expansion of the population of CSCs [32]. As lncTCF7 is necessary for liver CSC self-renewal and tumor propagation, we hereby propose a novel IL-6/STAT3/lncTCF7 signaling axis that may contribute to the EMT program and self-renewal of CSCs.

In this study, we sought to investigate the effects of IL-6 on the expression and function of HCC-specific lncRNAs. Our data provides solid evidence to verify that lncTCF7 is significantly upregulated in response to IL-6 stimulation and plays a central role in IL-6-induced epithelial-mesenchymal transition process of HCC cells. We demonstrated that IL-6 transcriptionally activated the expression of lncTCF7 in HCC cells by activating STAT3, a transcription activator which binds to promoter regions of lncTCF7. Our study propose the existence of an aberrant IL-6/STAT3/ lncTCF7 signaling axis leading to HCC aggressiveness through EMT induction, which could be novel therapeutic targets in malignancies.

Materials and methods

Cell culture

Two HCC cell lines (SK-Hep-1 and BEL-7402) were obtained from the Institute of Biochemistry and Cell Biology of the Chinese Academy of Sciences (Shanghai, China). The cell lines were grown in DMEM (Gibco BRL, Grand Island, NY, USA) supplemented with 10 % fetal bovine serum (FBS, HyClone, Camarillo, CA, USA) as well as 100 U/ml penicillin and 100 μg/ml streptomycin (Invitrogen, Carlsbad, CA, USA). Cells were maintained in a humidified incubator at 37 °C in the presence of 5 % CO2. All cell lines have been passaged for fewer than 6 months and tested routinely by Hoechst DNA staining to ensure that there is no mycoplasma contamination.

Antibodies and reagants

Recombinant IL-6 and negative control Ab were purchased from R&D Systems. All other reagents used were of analytical grade or the highest grade available. Antibodies against E-cadherin, vimentin, STAT3, p-STAT3 (Y705) and β-Actin were obtained from Abcam.

RNA extraction and quantitative real-time PCR

Total RNA was extracted from HCC cells utilizing Trizol reagent (Invitrogen, Carlsbad, CA, USA) according to the manufacturer’s instructions. cDNA was transcribed using AMV reverse transcriptase (Promega, Madison, Wisconsin, USA). The quantitative real-time polymerase chain reaction (qRT-PCR) was achieved on ABI 7500 system (Applied Biosystems, CA, USA). GAPDH was uesd as an internal control, and mRNA values of target gene was normalized to GAPDH. The relative expression fold change of mRNAs was calculated with the 2-ΔΔCt method. The results are representative of at least three independent experiments. qRT-PCR results were expressed and analyzed relative to CT (threshold cycle) values, and then converted to fold changes. The sequences of the qRT-PCR primers used in this study were listed in Table 1.

Table 1

Primer and siRNA list

mRNA/gene promoter	sequence (5′-3′)	Experimental use
linc00974	5′-TCTAACGTGCCTGGGACCTA-3′(forward)	Real-time PCR
	5′-AAATGCCTACCGCCAGTTCA-3′(reverse)
HOTAIR	5′-CAGTGGGGAACTCTGACTCG-3′(forward)	Real-time PCR
	5′-GTGCCTGGTGCTCTCTTACC-3′(reverse)
H19	5′-ACTCAGGAATCGGCTCTGGAA-3′(forward)	Real-time PCR
	5′-CTGCTGTTCCGATGGTGTCTT-3′(reverse)
DANCR	5′-GCGCCACTATGTAGCGGGTT-3′(forward)	Real-time PCR
	5′-TCAATGGCTTGTGCCTGTAGTT-3′(reverse)
lncTCF7	5′-AGGAGTCCTTGGACCTGAGC-3′(forward)	Real-time PCR
	5′-AGTGGCTGGCATATAACCAACA-3′(reverse)
Dreh	5′-CCTGTATGACGATGGAGCCT-3′(forward)	Real-time PCR
	5′-TGACACATTTGCGATGGGTAT-3′(reverse)
lncRNA MVIH	5′-GAGACAGGATTTAGCCGTGTTG-3′(forward)	Real-time PCR
	5′-AGCACTTTGGAAGGCTTAGACA-3′(reverse)
lncRNA-HEIH	5′-CCTCTTGTGCCCCTTTCTT-3′(forward)	Real-time PCR
	5′-ATGGCTTCTCGCATCCTAT-3′(reverse)
HULC	5′-CCATCCAATCGGTAGTAGCG-3′(forward)	Real-time PCR
	5′-TCCAGAAAGAGGGAGTTG-3′(reverse)
LET	5′-CCTTCCTGACAGCCAGTGTG-3′(forward)	Real-time PCR
	5′-CAGAATGGAAATACTGGAGCAAG-3′ (reverse)
lncRNA-ATB	5′-TCTGGCTGAGGCTGGTTGAC-3′(forward)	Real-time PCR
	5′-ATCTCTGGGTGCTGGTGAAGG-3′(reverse)
PVT-1	5′-GCTGCAAGGTCAAGATGGTT-3′(forward)	Real-time PCR
	5′-GCTGGGTGGCGTTCTATC-3′(reverse)
β-actin	5′-TCCCTGGAGAAGAGCTACGA-3′(forward)	Real-time PCR
	5′-AGCACTGTGTTGGCGTACAG-3′(reverse)
GADPH	5′-GCATCCTGGGCTACACTG-3′(forward)	Real-time PCR
	5′-TGGTCGTTGAGGGCAAT-3′(reverse)
lncTCF7 promoter	5′-AGCCAGACAGAAGAGTGGA-3′ (forward)	ChIP-PCR
	5′-TGGGATGGGGATGTCAGAAC-3′ (reverse)
lncTCF7 promoter SIE3	5′-ACTGGTACCTAAGCGGAGAGAGTCCCACACAGG-3′ (forward)	Site-directed mutagenesis
	5′-ACTAAGCTTGAGTCAGAGTTCCCCAC-3′ (reverse)
lncTCF7 promoter SIE4	5′-ACTGGTACCTAAGCGGAGAGAGTCCCACACAGG-3′(forward)	Site-directed mutagenesis
	5′-ACTAAGCTTGAGTCAGAGTTCCCCAC-3′ (reverse)
lncTCF7 siRNA-1	5′-AGCCAACATTGTTGGTTAT-3′,	RNA interference
lncTCF7 siRNA-2	5′-CACCTAGGTGCTCACTGAA-3′	RNA interference
STAT3 siRNA	5′-AAAUCCAGAACCCUCUGACAUUUGC-3′	RNA interference
siRNA control	5′-UUCUCCGAACGUGUCACGUTT-3′	RNA interference

Western blot analysis

The harvested HCC cells were centrifuged at 2000 rpm for 4 min. The total cellular proteins were lysed in RIPA buffer (Cell Signaling Technology) supplemented with protease inhibitors. The lysates were collected and subjected to ultrasonication and centrifugation. The supernatants were collected, and the protein concentrations were determined using BCA Protein Assay Kit (Pierce). Equal amounts (30–40 μg) of proteins were separated by 8–12 % SDS-polyacrylamide gel electrophoresis and transferred to a PVDF Immobilon-P membrane (Millipore). The membrane was blocked with 5 % nonfat milk in TBST and then probed with indicated primary antibodies at 4 °C overnight with gentle shaking. The membranes were washed with TBST (3 × 5 min), incubated in secondary antibodies for 1 h at room temperature. Antibody-bound proteins were detected by BeyoECL Plus kit. Intensity of the bands was quantified by densitometry (Image J 1.47 software) and normalized to the corresponding β-Actin bands.

The primary antibodies used in these experiments include rabbit polyclonal anti-human STAT3 (1:1000, Abcam), p-STAT3 (Y705) (1:1000, Abcam), rabbit monoclonal anti-human E-cadherin (1:500, Epitomics, Burlingame, CA, USA), rabbit monoclonal anti-human Vimentin (1:500, Epitomics), and rabbit polyclonal anti-human Actin (1:4,000, Abcam). HRP-conjugated goat anti-rabbit IgG antibody (Abcam) was used as the secondary antibody.

Cell transfection

LncTCF7 siRNA, STAT3 siRNA and Control siRNA were purchased from Qiagen, Hilden, Germany. Cells were transfected using Lipofectamine 2000 (Invitrogen) according to the manufacturer’s instructions. Forty-eight hours after transfection, cells were harvested and subjected to qRT-PCR or western blot analyses.

Immunofluorescence analysis

IL-6 treated cell layers on glass coverslips. For membrane staining (E-cadherin), cells were fixed with 100 % methanol for 15 min. For intracellular staining (Vimentin), cells were fixed for 15 min with 4 % (wt/vol) paraformaldehyde in PBS, permeabilized with 0.5 % Triton X-100 in PBS for 2 min, incubated with 5 % bovine serum albumin in PBS for 20 min at room temperature, and then probed with primary antibody at 4 °C overnight. After washing in PBS, the cells were incubated with FITC-conjugated secondary antibodies (Jackson ImmunoResearch, West Grove, PA) for 1 h at room temperature. The nuclei were stained with 4,6-diamidino-2-phenylindole (DAPI). The slides were mounted and visualized using a fluorescence microscope (AX70, Olympus, Tokyo, Japan). Antibody dilutions of 1:300 were used for E-cadherin (Abcam), and Vimentin (Cell Signaling Technology).

Chromatin immunoprecipitation

HCC cells were serum-starved overnight and treated with 50 ng/ml IL-6 for 1 h. Chromatin was cross-linked with 1 % formaldehyde for 10 min. After cell lysis, the chromatin was sonicated to obtain a DNA smear with an average size of 500 bp. After centrifugation, the supernatants were subjected to immunoprecipitation overnight with antibodies against STAT3 at 4 °C, or with isotype rabbit IgG at 4 °C overnight. Chromatin-antibody complexes were isolated using Protein A/G PLUS Agarose (Santa Cruz). The crosslinks for the enriched and the input DNA were reversed and the DNA was cleaned by RNase A (0.2 mg/mL) and proteinase K (2 mg/mL) before phenol/chloroform-purification. PCR was employed to analyzed the specific sequences from immunoprecipitated and input DNA utilizing the following primer sequences for lncTCF7 promoter region: 5′-AGCCAGACAGAAGAGTGGA-3′ (forward) and 5′-TGGGATGGGGATGTCAGAAC 3′ (reverse). The results are representative of at least three independent experiments.

Luciferase reporter assay

The genomic regions neighboring the promoter region of human lncTCF7 gene were amplified by PCR and then inserted into the pGL3 vector. The reporter constructs were generated by subsequent PCR-based cloning and they contained various lengths of lncTCF7 promoter or mutated STAT3 binding sites. The luciferase reporter assay was performed by transfecting the indicated cell lines with the reporter construct containing wild-type (wt) or mutant (mut) STAT3 expression vectors. Each sample was cotransfected with the pRL-SV40 vector as an internal control for transfection efficiency. At 24 h post-transfection, cells were incubated with IL-6 50 ng/ml for 6 h and the luciferase activities were measured utilizing the Dual-Luciferase Reporter Assay System (Promega, Madison, WI) and the luminometer (LB 9507, Berthold, Bad Wildbad, Germany). The relative luciferase expression equals the expression of Renilla luciferase divided by the expression of firefly luciferase. and all the experiments were carried out in triplicate.

Cell invasion assay

The cancer cell invasion assay was performed in 24-well transwell plates (Costar, Cambridge, MA) with 8 μm-pore inserts precoated with Matrigel (40 μl, BD Biosciences, San Jose, CA). Briefly, cells (1 × 105) in serum-free medium were added into a culture insert, whereas complete medium (supplemented with 10 % FBS) with or without 50 ng/ml IL-6 was applied to the lower compartment. After incubation for 48 h, cells on the upper surface of the filter were scraped and washed away, whereas the undersurface adherent cells were fixed in 4 % formaldehyde and stained with 0.05 % crystal violet for 2 h. The air-dried filter membrane was viewed under a microscope and three random fields were selected for cell counting.

Statistical analysis

All statistical analyses were performed using SPSS 17.0 (SPSS, Chicago, USA). The data are representative of three independent experiments and are presented as mean value ± standard deviation. Statistical evaluations were analyzed using unpaired student’s t-test. A two-sided p value of less than 0.05 was considered to be statistically significant.

Results

lncRNAs are elevated in HCC cells exposed to IL-6

To determine the effect of IL-6 on the expression of HCC specific lncRNAs, qRT-PCR analysis was performed in HCC cell line SK-Hep1 exposed to IL-6 50 ng/ml for 24 h. As demonstrated in Fig. 1a, several HCC specific lncRNAs were significantly dysregulated in response to IL-6 stimulation. Among them, lncTCF7 was most strongly dysregulated. Furthermore, IL-6 can increase the expression of lncTCF7 in a dose- and time-dependent manner in both SK-Hep1 and BEL-7402 cells (Fig. 1b, c) confirming that its expression can be stimulated by IL-6.

Fig. 1

IL-6 increases the level of HCC specific lncRNA mRNA in HCC cells. a Relative mRNA levels of HCC specific lncRNAs in SK-Hep1 cells treated with IL-6 50 ng/ml for 24 h (normalized to GAPDH). b Treatment of SK-Hep-1 cells with IL-6 induced the expression of lncTCF7 in a time- and dose-dependent manner. qRT-PCR analysis of lncTCF7 in SK-Hep-1 cells incubated with gradient concentrations of IL-6 for 24 h. qRT-PCR analysis of lncTCF7 in SK-Hep-1 cells incubated with IL-6 50 ng/ml for different periods of time. c Similar to effects seen in SK-Hep-1 cells, IL-6 also induced the expression of lncTCF7 in BEL-7402 cells. Data represent the mean ± S.D. from three independent experiments.*p < 0.05.*p < 0.05

Effect on IL-6 on the activation of STAT3 in HCC cells

IL-6 has been reported to be a pro-inflammatory cytokine characterized as a potent activator of STAT3 [9]. Accordingly, we examined the activation status of STAT3 in response to IL-6 treatment in HCC cells. As illustrated in Fig. 2a, IL-6 induced dose-dependent STAT3 phosphorylation (Y705). Expression of pSTAT3 (Y705) was elevated in the cells treated with IL-6 for 4 h and peaked in cells treated with 50 ng/ml IL-6. In contrast, IL-6 did not affect the level of unphosphorylated STAT3. In a time-course experiment, STAT3 (Y705) phosphorylation peaked after 2 h of IL-6 treatment (Fig. 2b). The level of STAT3 (Y705) phosphorylation declined after the cells were treated with IL-6 for 4 h. We confirmed that IL-6 induced phosphorylation of STAT3 in a dose- and time-dependent manner in SK-Hep1 cells; moreover, the activation of STAT3 was abrogated by pretreatment with LLL12, an inhibitor of STAT3 (Fig. 2c). Simultaneously, the upregulation of lncTCF7 induced by IL-6 was obviously attenuated by pretreatment with the inhibitor (Fig. 2d). In addition, siRNA-mediated STAT3 knockdown significantly ameliorated the IL-6 induced lncTCF7 expression in HCC cells (Fig. 2e, f). These data suggest that IL-6 induced STAT3 activation is vital in the upregulation of lncTCF7.

Fig. 2

IL-6 induces STAT3 (Y705) phosphorylation. a IL-6 induces STAT3 (Y705) phosphorylation in a dose-dependent manner. SK-Hep-1 cells were treated with the indicated doses of IL-6 for 4 h and analyzed by western blotting using the indicated antibodies. pSTAT3, Y705-phosphorylated STAT3. b Time-course studies of IL-6-induced STAT3 (Y705) phosphorylation. SK-Hep-1 cells were treated with 50 ng/ml IL-6 for the indicated times. Total cellular proteins were prepared for western blotting at the end of each treatment. c STAT3 inhibition blocked IL-6-induced STAT3 activation. SK-Hep-1 cells were cultured in the presence or absence of 5 μM LLL12, a STAT3 inhibitor for 2 h and then with 50 ng/ml IL-6 for an additional 12 h. Total cellular proteins were prepared for western blotting analysis. d STAT3 inhibition blocked IL-6-induced lncTCF7 upregulation. qRT-PCR analysis of lncTCF7 in SK-Hep-1 cells cultured in the presence or absence of 5 μM LLL12, a STAT3 inhibitor for 2 h and then with 50 ng/ml IL-6 for an additional 12 h. e SK-Hep-1 cells were transfected with a negative control, 50 nM control siRNA (siCon) or 50 nM STAT3 siRNA (siSTAT3) as described in the Materials and methods. Forty-eight hours after transfection, the cells were treated with 50 ng/ml IL-6 for 12 h. At the end of the cell culture period, cell lysates were prepared for determination STAT3 activation by western blotting. Data are representative of at least three independent experiments. f The cells were treated as described in section E. At the end of the cell culture period, cells were prepared for qRT-PCR analysis of lncTCF7 (n = 3)

STAT3 binds directly to the promoter region of lncTCF7 upon IL-6 stimulation

With the help of online bioinformatical software programs MatInspector (www.genomatix.de/online_help/help_matinspector/matinspector_help.html) and TFSEARCH (www.brc.jp/research/db/TFSEARCH.html) in the promoter region of lncTCF7, we predicted that the promoter sequence of the lncTCF7 gene harbored 7 potential STAT-inducible elements (SIEs). Serial truncations of the lncTCF7 gene promoter were generated created based on the location of the STAT3-binding sites to identify the transcriptional regulatory region responsive to IL-6/STAT3 signaling (Fig. 3a). Luciferase reporter assay demonstrated that the region between −720 and −476 on the lncTCF7 promoter was responsible for STAT3-mediated activation (Fig. 3b). This region contained two potential SIEs (SIE3 and SIE4) on the lncTCF7 promoter. Mutation in SIE3 or SIE4 markedly reduced the reporter activity induced by IL-6 (Fig. 3c). To confirm this finding, a ChIP assay was employed to confirm the interaction of STAT3 with the promoter regions of lncTCF7 in SK-Hep1 cells exposed to IL-6. ChIP-PCR results demonstrated that STAT3 directly bounds to the lncTCF7 promoter region upon IL-6 stimulation (Fig. 3d). Our data suggest that IL-6-activated STAT3, which transcriptionally activates lncTCF7 by binding directly to the promoter.

Fig. 3

lncTCF7 is transcriptionally regulated by STAT3 in response to IL-6 stimulation. a Schematic representation of lncTCF7 promoter with seven potential SIEs and the primer pair used in ChIP-PCR assays. The reporter construct lncTCF7-Luc and its truncated and mutated derivatives are also shown. b Transcription activity in response to IL-6 treatmemt for 6 h measured by luciferase assay in SK-Hep-1 cells with a series of deletion mutants of lncTCF7-luc (internal control, pRL-TK). *p < 0.05. c Relative luciferase activity 6 h after IL-6 incubation in SK-Hep-1 cells transfected with the wild-type or SIE mutated lncTCF7 promoter reporter construct. *p < 0.05. d Chromatin prepared from SK-Hep-1 cells stimulated with IL-6 for 1 h was immunoprecipitated with the indicated antibodies; PCR was performed on immunoprecipitated DNAs or soluble chromatin using specific primer pair for the lncTCF7 promoter

lncTCF7 is involved in IL-6 induced epithelial-mesenchymal transition, invasion and mobility of HCC cells

As previous studies have demonstrated that IL-6 efficiently induced epithelial-mesenchymal transition (EMT) in cancer [12, 33], we firstly explored the effect of IL-6 on EMT in HCC cells. After exogenous IL-6 treatment, SK-Hep1 cells became more spindle-shaped and came up with a mesenchymal phenotype. With the immunofluorescence analysis, IL-6 exposure resulted in an obvious downregulation in the expression of membranous epithelial marker E-cadherin and a great upregulation in the expression of cytoplasmic mesenchymal marker Vimentin (Fig. 4a). The western blot analysis confirmed the data from the immunofluorescence analysis (Fig. 4b). It suggests that IL-6 can induce EMT in HCC cells.

Fig. 4

Effect of IL-6 on the EMT progress in HCC. a Phase-contrast images (left) and immunofluorescence images (right) of SK-Hep-1 cells stained using antibodies against E-cadherin or Vimentin after treated with or without 50 ng/ml IL-6 for 72 h. b Western-blot analysis of phenotypic markers after treated with or without 50 ng/ml IL-6 for 72 h in SK-Hep-1 cells

We would like to explore whether lncTCF7 is involved in IL-6 induced EMT in HCC cells. We modulated the expression level of lncTCF7 through RNA interference experiments. 48 h after transfection, qRT-PCR analysis of lncTCF7 expression levels was performed. As demonstrated in Fig. 5a, lncTCF7 was significantly knocked down by silncTCF7-2, the most effective siRNA. The mesenchymal-like morphological conversion stimulated by IL-6 was substantially attenuated after transfection of lncTCF7 siRNA (Fig. 5b). The expression changes in EMT markers were blunted with lncTCF7 knockdown in SK-Hep1 cells both at the transcript and protein levels (Fig. 5c, d).

Fig. 5

lncTCF7 plays a critical role in IL-6 induced EMT. a qRT-PCR analysis of lncTCF7 expression following treatment of SK-Hep-1 cells with two individual siRNAs targeting lncTCF7. b Morphology of SK-Hep-1 cells with siRNAs against lncTCF7 or scrambled control and stimulated with 50 ng/mL of IL-6 for 72 h. qRT-PCR analysis (c) and western blots (d) of 72 h IL-6 treated SK-Hep-1 cells receiving the indicated siRNAs of EMT specific marker E-cadherin and Vimentin. Data represent the mean ± S.D. from three independent experiments.*p < 0.05.*p < 0.05

EMT process has been consistently reported to be associated with increased cancer cell invasion [9, 12]. Thus, we hypothesize that IL-6 can enhance cancer cell invasiveness at least partially via upregulating lncTCF7. The acquisition of invasive capacity in response to IL-6 stimulation was ameliorated by lncTCF7 silencing (Fig. 6). The above results indicate that lncTCF7 transactivated by STAT3 is essential for IL-6 induced EMT process and more malignant phenotypes in HCC cells.

Fig. 6

lncTCF7 was involved in the IL-6-induced invasiveness of cancer cells. lncTCF7 knockdown inhibited the invasion of SK-Hep-1 cells and further decreased the IL-6-induced promotion of SK-Hep-1 cell invasiveness. The results suggested that lncTCF7 was also involved in the IL-6-induced promotion of SK-Hep-1 cell invasiveness. The data represent the mean ± SD of three independent experiments

Discussion

There has been an established link between chronic inflammation and cancer risk, and various pro-inflammatory cytokines contribute to the transformation of cancer cells into more aggressive phenotypes through the regulation of oncogenes, which enhance the development and progression of cancer [6, 34]. In particular, IL-6/STAT3 signaling axis seem to play a vital role in bridging chronic inflammation to HCC progression [9, 12]. However, whether lncRNAs are also involved in this process remains largely unknown. In the present study, we identified lncTCF7 as an IL-6-inducible lncRNA that is vital for the IL-6 mediated malignant phenotype.

lncTCF7, a lncRNA initially identified in HCC, was reported to be highly expressed in HCC tumors and liver cancer stem cells (CSCs) [16]. LncTCF7 is necessary for liver CSC self-renewal and tumor propagation. Mechanistically, lncTCF7 recruits the SWI/SNF complex to the promoter region of TCF7 to regulate its expression, leading to activation of Wnt signaling [16]. Yet, up until now, few studies have concentrated on the transcriptional factors that contributes to its upregulation. In this study, we proposed a link between IL-6/STAT3 signaling and lncTCF7, which are two well-known driver of malignancy.

We first explored the effect of IL-6 on the expression of previously identified HCC specific lncRNAs. We revealed that lncTCF7 was most strongly upregulated in response to IL-6 stimulation. STAT3, a mediator of IL-6/STAT3 signaling, is considered to be a potent oncogene as it is often constitutively activated in most solid and hematological tumors and exerts multiple pro-tumorigenic activities, including promotion of tumor cell proliferation, invasion, metastasis, survival and angiogenesis [35-37]. We found that STAT3 is phosphorylated after IL-6 exposure, and acted a potent transcriptional factor that directly binds to the promoter region of human lncTCF7 gene. STAT3 knockdown or inhibiting STAT3 activation abrogated the IL-6-depenendent transcriptional activation. Functionally, lncTCF7 silencing attenuated IL-6 induced epithelial-mesenchymal transition and invasion of HCC cells.

Except for lncTCF7, other studies have identified that lncRNA HOTAIR as IL-6-inducible lncRNA and may be involved in IL-6-induced signal transduction in cancer [38]. These studies, together with ours, suggest that lncRNAs, which are a group of largely uncharacterized molecules, may play an important role in the regulation of IL-6/STAT3 signaling. miRNAs (19–25 nt), another class of noncoding RNAs, play vital regulatory roles in cancer mainly at the posttranscriptional level [39-41]. Previous studies have characterized a number of IL-6-regulated miRNAs in cancer, such as miR-17/19A [39], miR-21 [40] and miR-34a [41]. Our study expanded the downstream effectors of IL-6/STAT3 signaling.

Based on the data, we propose a model that depicts a role of lncTCF7 in the regulation of IL-6-mediated aggressive phenotype. This is the first study which shows that lncTCF7 is an IL-6-inducible lncRNA and is involved in IL-6 induced epithelial-mesenchymal transition and invasion of HCC cells. The present study may also cast light on the prophylactic treatment for the primary HCC.