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BPTF promotes hepatocellular carcinoma growth by modulating hTERT signaling and cancer stem cell traits

Bromodomain PHD finger transcription factor (BPTF), a core subunit of nucleosome-remodeling factor (NURF) complex, plays an important role in chromatin remodeling. However, its precise function and molecular mechanism involved in hepatocellular carcinoma (HCC) growth are still poorly defined. Here, we demonstrated the tumor-promoting role of BPTF in HCC progression. BPTF was highly expressed in HCC cells and tumor tissues of HCC patients compared with normal liver cells and tissues. Knockdown of BPTF inhibited cell proliferation, colony formation and stem cell-like traits in HCC cells. In addition, BPTF knockdown effectively sensitized the anti-tumor effect of chemotherapeutic drugs and induced more apoptosis in HCC cells. Consistently, knockdown of BPTF in a xenograft mouse model also suppressed tumor growth and metastasis accompanied by the suppression of cancer stem cells (CSC)-related protein markers. Moreover, the mechanism study showed that the tumor-promoting role of BPTF in HCC was realized by transcriptionally regulating the expression of human telomerase reverse transcriptase (hTERT). Furthermore, we found that HCC patients with high BPTF expression displayed high hTERT expression, and high BPTF or hTERT expression level was positively correlated with advanced malignancy and poor prognosis in HCC patients. Collectively, our results demonstrate that BPTF promotes HCC growth by targeting hTERT and suggest that the BPTF-hTERT axis maybe a novel and potential therapeutic target in HCC.


Introduction
Hepatocellular carcinoma (HCC) is one of the most common malignancies, and ranks as the fifth and eighth leading cause of cancerrelated death in men and women respectively [1]. The preferred and most effective therapy option for HCC remains surgical resection, and the other treatments include transplantation, chemotherapy, radiotherapy and biotherapy [2][3][4]. However, the high recurrence rate and metastatic rate still forms severe challenges in HCC treatment. Cancer stem-like cells (CSC) are considered as a cell population which [5] sustain cell immortalization, promote cell self-renewal, and are responsible for the tumor metastasis, relapse and chemoresistance [6][7][8].
Therefore, the increased understanding of the molecular mechanisms that regulate cellular unlimited proliferation and stemness maintenance in HCC progression and the discovery and identification of new candidate targets responsible for HCC initiation and development might open up the new avenues to accelerate the development of novel diagnostic and therapeutic strategies to improve the prognosis for patients with HCC.
Nucleosome remodeling factor (NURF) complex exists in nearly all eukaryotic species by displaying its effect via catalyzing ATP-dependent nucleosome sliding and facilitating transcription mediated by chromatin [9,10]. As the largest subunit of the NURF chromatin remodeling complex, bromodomain PHD finger transcription factor (BPTF) is critical for epigenetically regulating DNA accessibility and gene expression [9]. It is also reported to be essential in embryonic development [11,12]. BPTF has recently been found to affect tumor progression, especially melanoma survival, by activating oncogenic signaling directly or by synergizing with other key protein factors, such as c-MYC [13][14][15]. Its prognostic significance associated with EMT marker in colorectal carcinoma and HCC was also mentioned [16,17]. Nevertheless, to date, its precise function and molecular mechanisms in HCC development, especially in the maintenance of HCC cancer stem celllike straits, are still poorly understood.
As the catalytic subunit of the telomerase holoenzyme complex, human telomerase reverse transcriptase (hTERT), uses its own RNA as template to synthesize telomeres, and then add telomeres to the ends of chromosomes to extend the shortened telomeres and to protect chromosomes [18,19]. Therefore, hTERT plays an important role in cell senescence and tumorigenesis. It has been reported to be involved in cell signaling transductions governing cell proliferation, apoptosis, migration and stem cell function [20,21]. In HCC, the serum and tissue expression of hTERT was significantly increased than that in patients with liver cirrhosis [22], and TERT mRNA was reported to serve as a tumor marker for early detection of hepatocellular carcinoma [23]. Sustained activation of telomerase is even thought to be essential for the growth and progression of HCC, and targeting therapy against hTERT holds great promises in HCC treatment [24]. The expression and function of hTERT gene are known to be regulated at various molecular levels, and among all the regulatory events, transcription modulation is one of the most important. Therefore, the study on how hTERT is transcriptionally regulated, focusing primarily on the discovery and verification of the unknown trans-acting regulators, including transcription factors and epigenetic modifiers, will provide new and potential therapeutic strategies for HCC treatment.
In the study, we investigated the functional roles of BPTF in HCC and its effect on the stemness of HCC cancer stem cells in vitro and in vivo. We also assessed the response of chemotherapeutic drugs to BPTF knockdown in HCC cells, and evaluated the transcriptional regulation of hTERT by BPTF. Our data show that BPTF promotes stemness maintenance, tumor growth and metastasis by transcriptionally activating hTERT expression in HCC cells. The simultaneous high expression of BPTF and hTERT was positively correlated with the advanced malignancy and predicted poor prognosis for patients with HCC.

Cell lines and cell culture
Human hepatoma cells Hep3B, HepG2, SNU-449, and human normal liver cell L-O2 were purchased from the American Type Culture Collection (ATCC, Manassas, VA). L-O2 were cultured in Dulbecco's modified Eagle's medium supplemented with 10% fetal bovine serum. Hep3B and HepG2 were cultured in Eagle's Minimum Essential Medium (MAC GENE) supplemented with 10% fetal bovine serum. SNU-449 were maintained in RPMI-1640 Medium containing 10% fetal bovine serum. Bel7402 cell line was obtained from Peking University Hepatology Institute (China), and cultured in Dulbecco's modified Eagle's medium supplemented with 10% fetal bovine serum. All referred cell lines were cultured in a humidified atmosphere containing 5% CO 2 at 37 ℃.

MTT assay
Cells plated in 96-well plates (2000 cells/well) were transfected with control or specific shRNAs. 48 h later, 10ul MTT reagent (5 mg/ ml) was added in single well and treated for 3.5 h. Then MTT was discarded and 150ul Dimethyl Sulphoxide (DMSO) was added in single well. Finally, the OD value at 490 nm was measured.

Colony formation assay
Cells transfected by control or specific shRNAs were trypsinized into single cell and seeded in 6-well plates (1000 cells/well) with continuous culture for 8 days. The cells were washed with PBS twice and fixed with the mixture (methanol: glacial:acetic 1:1:8) for 10 min, and then were stained with 0.1% crystal violet for 10 min. The plates were washed with PBS and the colonies were photographed and counted.

Wound scratch assay
Cells were plated in a 6-well plate and grown to nearly 70-80% confluence. Then the cells were treated with plasmids, or siRNA, or inhibitor for 24 h and scraped in a straight line to create a "scratch". The images of the cells at the beginning and at regular intervals during cell migration to close the scratch were captured and compared through quantifying the migration rate of the cells.

Transwell invasion assay
Cell invasion ability was detected using 24-well chemotaxis chambers (Corning, CA, USA, Cat: 3422). The cells were washed twice with PBS, resuspended in 100 μl serum-free medium and added into the upper chambers. The lower chambers were filled with 500 μl medium containing 20% fetal bovine serum (FBS). The cells were incubated for 48 h in the upper chamber coated with a mixture of serum-free medium and Matrigel (4:1; BD Biosciences, Cat: 356234). The membrane were fixed in methyl alcohol for 10 min at room temperature, stained with crystal violet for 10 min, washed 3 times with PBS and dried off. The crystal violet was dissolved with 500 μl 33% acetic acid and the OD570 value was measured.

Tumorsphere culture
Cells transfected by control or specific shRNAs were digested into single cell with trypsin-EDTA and were respectively seeded in 35 mm non-treated cell culture dishes (BIOFIL, 2000 cells/dish) with continuous culture in DMEM/F12 medium (HyClone) containing B27 supplement (gibco), N2 supplement (gibco), bFGF (20 ng/ml), and EGF (20 ng/ml) for two weeks. Then the picture of the formed tumorspheres were taken by inverted microscope(Leica)and spheres with diameters larger than 50 µm were counted.

Plasmids and lentiviral infections
BPTF-targeting shRNAs (shBPTF-1, shBPTF-2, shBPTF-3) and control non-targeting shRNA plasmids purchased from GeneCopoeia were used for RNA interference. The infection was mediated by lentivirus and conducted according to the manufacturer's protocol (Lenti-Pac HIV expression packaging kit, GeneCopoeia). Briefly, one 10-cm culture dish containing 1.3-1.5 × 10 6 293 T cells were transfected with 2.5 μg shRNA expression plasmids and 5.0 μl lenti-Pac HIV mix using Lipofectamine 3000 transfection reagent (Invitrogen). Supernatants were collected after 48 h, centrifuged and filtered, and the viral titre was determined by serial dilutions. Then the virus was used to infect target cells to obtain the stably transduced cells after puromycin selection.

Acridine orange/ethidium bromide (AO/EB) fluorescence staining
The HepG2 cells and Bel7402 cells seeded in 6-well plates and transfected with shRNA for 24 h were then treated with cis-Dichlorodiamineplatinum (DDP, 26.7uM/well) for another 24 h. Cells were collected and washed with PBS to remove detached cells. AO (Solarbio, China) and EB (Solarbio, China) were respectively diluted 100 times in PBS and mixed according to 1:1. 250ul mixed liquor was added into each well and discarded after shaking several seconds. The staining results were observed under the inverted fluorescence microscope(Leica)using blue light as excitation light.

Apoptosis assay
Detection of cell apoptosis was based on FACS analysis by FITC-AV/ PI staining. Cells were plated in 6-well plates and transfected with the negative control shRNA plasmids or specific shRNA plasmids for 24 h followed by cis-Dichlorodiamineplatinum (DDP, 26.7uM/well) treatment for another 24 h. Then the cells were harvested, washed twice in ice-cold PBS with 2% BSA and centrifuged at 300 ×g for 3 min. The cells were resuspended in 500 μl binding buffer and stained with 5ul Annexin V-FITC (AV), 5ul propidiumiodide (PI) using an Annexin V-FITC/PI staining kit (KeyGene BioTech). The status of cell apoptosis was analyzed by flow cytometry (BD ACCURI C6).

Flow cytometry assay of CD24 and CD44
Expression of stemness-associated marker, CD24 and CD44, was detected by flow cytometer. Cells were plated in 6-well plates and transfected with siRNA for 10 h. After continuous culture of 48 h, cells were digested with trypsin-EDTA and washed twice in ice-cold PBS containing 2% BSA and centrifuged at 300 ×g for 3 min. Cells were divided into two groups and resuspended in 100 μl PBS with 2% BSA on ice. Then the antibody APC-IgG, PE-IgG and APC-CD44, PE-CD44 (BD Pharmingen) were respectively added into single tube of each group on ice to incubate for 30 min. The fluorescence value was detected finally by flow cytometer.

Lysate preparation from tissues
The experimental materials used for lysate preparation include lung tissue, xenografts of HCC cells in mice and hepatic carcinoma tumors, adjacent normal tissues obtained from patients who underwent surgery therapy at The First Affiliated Hospital Of Dalian Medical University between 2015 and 2016 with the consent of the patients. These tissues were washed with PBS to remove blood, and transferred to liquid nitrogen immediately. Tissues were grinded by TGrinder (TIANGEN) into 500ul RIPA buffer with protease inhibitor for 5 min and sonicated for 24 s on ice. Then the lysate were centrifuged at 12,000 ×g for 10 min at 4 ℃, and the supernatants were transferred to new tubes for the following determinations.

ChIP assay
ChIP assay was performed using conventional protocol. Hep3B and HepG2 cells with stable knockdown of BPTF were used to perform ChIP assay. First of all, 1 × 10 7 cells were fixed with 1% formaldehyde for 10 min at RT. Next, 10% 1.25 M glycine was added in the mixture for 5 min to end the excessive crosslink. The mixture was abandoned, the cells were washed three times with cold PBS and then were scraped and harvested in PBS buffer containing protease inhibitors and centrifuged at 240 g for 4 min at 4 ℃. The cell pellets were resuspended twice with PBS buffer containing protease inhibitors and centrifuged at 600 g for 4 min at 4 ℃. The finally collected cells were sonicated five times in IP buffer (SDS buffer: Triton buffer=2:1) for 5 s each time, centrifuged at 14,000 g for 20 min at 4 ℃ and supernatants were transferred to a new tube. 25ul protein A/G agarose beads (Santa Cruz Biotechnology) were mixed with the above supernatant containing 1 mg total proteins and rotated for 30 min at 4 ℃. After centrifugation for 15 min at full speed, the chromatin in the supernatant was immunoprecipitated overnight with 2 μg antibodies against BPTF (Santa Cruz Biotechnology) or IgG (Santa Cruz Biotechnology). Then 25ul protein A/G agarose beads were added into the mixture and rotated for 12 h at 4 ℃. Beads were then washed sequentially for 5 min with the following buffers: 1 ml mixed wash buffer, buffer 500, LiCl/detergent wash buffer twice, and TE buffer one time. The beads were reversely cross-linked by heating at 65 ℃ for 12 h in buffer containing 1% SDS, 0.1 M NaHCO 3 . After microcentrifuge briefly, the supernatant was treated with 1 ml RNaseA at 37 ℃ for 15 min and digested with 2 ml proteinase K solution at 37 ℃ for 2 h. DNA was finally obtained by LiCl, phenol/chloroform and ethanol extractions and used as DNA templates to amplify the specific hTERT promoter region. The primers used for PCR reaction was as following: forward primer, 5′-TTTCCCACCCTTTCTCGACG-3′; reverse primer, 5′-CAGCGGAGAGAGGTCGAATC −3′

Animal studies
All animal maintenance and operational procedures were carried in accordance with the animal licence protocol approval by Animal Care and Ethics Committee of Dalian Medical University. Male nude mice (Balb/c) aged 4-6 weeks were purchased from Beijing Vital River Laboratory Animal Technology Co., Ltd. Mice were randomized for xenograft tumor growth and experimental metastasis assays. For xenograft tumor formation, the mice were randomly divided into three groups, sh-control, sh-BPTF-2, sh-BPTF-3. 4 × 10 6 Hep3B cells with stable knockdown of BPTF gene or control vector expressing mCherry protein were respectively suspended in 100ul PBS and injected subcutaneously into the right flank of each mouse. The tumor volume was measured after two weeks of injection. The tumor volume was calculated as V= (width 2 ×length)/2 and the data was recorded every two days within two weeks. Mice were sacrificed and tumors were taken from mice for weighting and photographing. For the metastasis assays, the mice were randomly divided into four groups. PBS(No Hep3B group) or 4 × 10 6 Hep3B cells with stable knockdown of BPTF gene (Hep3B-sh2 or Hep3B-sh3 group) or control vector expressing mCherry protein (Hep3B-control group) were respectively injected into the tail vein of nude mice. 45 days after injection, the distribution of tumor cells in mice body was detected by in vivo imaging system (CRI maestro). Parameter setting: Excitation: 523 nm, Emission: 560-660 nm, Filter: 560 nm, Exposure time: 1.000 s. Then the mice were sacrificed and lung tissues were taken. Partial tissues were transferred to liquid nitrogen immediately and lysed for western blot analysis, and partial were fixed in formalin for IHC assay.

Immunohistochemistry staining assay
The examined tissues were fixed in 4% paraformaldehyde, washed with PBS three times, transferred to 70% ethanol, cut into small pieces and then embedded in paraffin in accordance with standard procedures. After being dewaxed and antigen retrieval, the tissue sections were stained using SP kit (SP-9000, ZSGB-BIO, China) according to its instructions. Briefly, the sections were blocked with blocking reagents, and then were exposed overnight at 4 ℃ to antibodies against BPTF, hTERT, EpCAM, or CD44. The slides were washed with PBS and incubated with anti-mouse/rabbit biotin antibodies for 1 h. After washing, the slides were added with HRP-conjugated streptavidin, developed with HRP substrate(DAB), and counterstained with hematoxylin. In the end, the paraffin sections were dehydrated by using Gradient alcohol, sealed by means of neutral balsam (Solarbio) and photographed with microscope.

IHC scoring system
The percentage of positive cells was divided into 5 levels. 0 point for no positive cells, 1 point for positive cells≤ 25%, 2 point for 26-50%, 3 point for 51-75%, and 4 point for > 75%. The coloring strength was divided into 4 levels. 0 point for colorlessness and 1point for pale yellow，brown is 2 point and tan is 3 point. The final score of immunohistochemistry is summed up by these two parts: the color intensity integral and the quantity integral: 0 point are (-), 2-3 point are ( ± ), 4-5 point are (+), 6-7 point are (++). In the final evaluation, (+) (++) was judged to be positive.

RNA extraction from frozen tissues and PCR
Total RNA was extracted from xenografts of HCC cells in mice and hepatic carcinoma tumors and adjacent normal tissues from patients using Trizol (Invitrogen Life Technologies), following the manufacturer's protocol. Briefly, 50-100 mg of tissue sections were added to 1 ml Trizol, centrifuge at 12000 rpm for 10 min to remove insoluble matter. After chloroform extraction and precipitation with isopropanol, RNA was washed twice with 75% ethanol, and finally the RNA pellet was dissolved in 15 μl of RNase-free water.

Statistical analysis
All results were presented as the mean ± SE. A Student t-test was performed to compare the two independent groups of data. The associations between BPTF or hTERT expression and categorical variables were compared by Pearson Chi-squared test. Survival curves were calculated using the Kaplan Meier method. The log-rank test was used to analyze overall survival (OS) time between different clinicopathological factors in hepatocellular carcinoma. Univariate and multivariate analysis were performed using the Cox regression model. Data was analyzed by the SPSS 20 software (Inc, Chicago, IL). P < 0.05 was considered to be significant.

BPTF was highly expressed in HCC cells and tumor tissues and positively correlated with patients' advanced stages
We firstly detected the expression of BPTF in human HCC cells and tumor tissues. Western blot analysis indicated that BPTF was highly expressed in five different HCC cell lines and one immortalized liver cell line (L-O2) (Fig. 1A), as well as tumor tissues from 9 cases of patients with HCC, by comparison to the levels of BPTF in the corresponding adjacent non-cancer tissues (Fig. 1B). These results were further confirmed by the obtained data from Oncomine database. The BPTF mRNA level of clinical tissue samples collected in Wurmbach Liver showed that the expression of BPTF was higher in cirrhosis, hepatocellular carcinoma, liver cell dysplasia than in normal tissues, and was the highest in HCC samples (Fig. 1C). Consistently, TCGA data also showed that the BPTF DNA copy number in HCC samples were greatly more than that in normal tissues (Fig. 1D). Furthermore, we performed immunohistochemistry (IHC) assay of BPTF expression based on tissue microarray of 81 HCC patients. The representative IHC staining from 6 different patients were shown in Fig. 1E. BPTF had a high expression level in HCC in most cases (high in 53 cases, and low in 28 cases) (Fig. 1F). In addition, the TNM-staging results indicated that patients displaying high BPTF expression were usually categorized in advanced malignancy stages (mostly at Ⅱ, Ⅲ and Ⅳ stage), while patients displaying low BPTF expression were categorized in preliminary stages of malignancy (mostly atⅠ,Ⅱ, and Ⅲ stage) (Fig. 1G). All these results together demonstrate that BPTF was overexpressed in HCC cells.

BPTF knockdown inhibited HCC cell proliferation and invasion
To study the functional significance of BPTF in HCC, we knocked down its expression using its specific shRNAs in three different HCC cell lines (Bel7402, Hep3B2.1-7, HepG2). Among the designed three different shRNAs targeting BPTF, two of them (sh2 and sh3) caused more effective silencing ( Fig. 2A). Therefore, these two shRNAs were used in the following studies. MTT assay showed that BPTF knockdown led to decreased proliferative capacity in HCC cells (Fig. 2B). Consistent with this, the inhibition of proliferation was also reflected by the decreased colony formation under BPTF silencing in HCC cells (Fig. 2C). We then examined whether knockdown of BPTF could weaken the cell migration and invasion viability in Bel7402 and HepG2 cells. Wound scratch assay revealed that BPTF knockdown reduced the cells migaration capacity (Fig. 2D). Similarly, cell invasion capacity was suppressed by BPTF knockdown (Fig. 2E).

BPTF regulated stemness phenotypes of HCC cells
Combined with the previous reports that BPTF is required for the establishment of the anterior-posterior axis of the mouse embryo during the earliest stages of development [11] and mediates pro-oncogenic role in cancer progression, including melanoma and lung cancer [14,15,25],we deduced BPTF might play a critical role in the stemness maintenance of HCC. Initially, we observed the effect of BPTF knockdown on the ability of tumorsphere formation. HCC cells with stable knockdown of BPTF by lentivirus-mediated shRNA transfection were cultured for two weeks in serum-free medium specific for CSCs, and the number and size of the formed tumorspheres were observed. Compared to the control group, BPTF silencing significantly suppressed the tumorsphere formation ability in HCC cells (Fig. 3A). The three-dimentional images of the formed tumorspheres and the corresponding fluorescent expression of mCherry gene fused with shRNA targeting BPTF under inverted fluorescence microscope also indicated such attenuated trend of tumorsphere formation caused by BPTF knockdown (Fig. S1).
CD44/CD24 has been reported to drive CSC progression in different cancer types [26][27][28]. We detected the influence of BPTF on their expression level in HCC cells. We knocked down BPTF expression using its specific shRNAs in Bel7402, Hep3B, and HepG2 cells and determined the CD44 or CD24 level by flowcytometry. Compared with the control group, BPTF-specific shRNAs-treated cells inhibited the levels of CD44 and CD24 (Fig. 3B, C). Only CD24 expression change was shown in Hep3B and HepG2 cells as the change for CD44 was subtle (Fig. 3C). Besides CD44 and CD24, the expression of other stemness marker in case of silencing BPTF, such as CD133 and c-Kit, was also significantly down-regulated in Bel7402 and Hep3B cells (Fig. 3D). All these findings collectively demonstrate the crucial role of BPTF in maintaining the stemness of HCC stem cells.    (caption on next page)

BPTF knockdown sensitized HCC cells to chemotherapeutic drugs
The acquisition of chemoresistance seems to be closely related to the intrinsic or required properties of CSCs [29]. Thus, CSC-focused therapy is destined to form the core of any effective anticancer strategy. Given the above findings of the stemness-promoting role for BPTF in HCC cells, we hypothesized that BPTF expression might modulate the sensitivity of HCC cells to chemotherapeutic drugs. In accordance to our hypothesis, BPTF shRNA-expressing HepG2 and Bel7402 cells were more sensitive to cisplatin (DDP) or 5-Fluorouracil (5-FU) treatment (Fig. 4A), displaying a significantly reduced IC50 of these two drugs, compared to the control shRNA-expressing cells (Fig. 4B), suggesting the improved anti-tumor function for chemotherapeutic treatment upon BPTF silencing.
In order to further clarify the potential molecular mechanisms of BPTF in its regulation of sensitivity to chemotherapy drugs in HCC cells, we then examined the apoptosis induction by chemotherapeutic drugs with BPTF knockdown. The AO/EB staining was initially carried out to study such induction. The upregulation of apoptosis in cells induced by DDP treatment plus BPTF shRNA expression was observed (Fig. 4C). Consistently, BPTF silencing in HepG2 and Bel7402 cells significantly increased apoptosis induction under DDP treatment under PI/Annexin V staining (Fig. 4D) and was accompanied by the down-regulated antiapoptotic protein Bcl-2 and the up-regulated pro-apoptotic protein cleaved caspase-7, cleaved caspase-9, and cleaved PARP (Fig. 4E). Thus, BPTF knockdown sensitized HCC cells to chemotherapy at least partially via inducing more apoptosis.

hTERT was required for the BPTF-mediated proliferative promotion and stemness maintenance in HCC cells
Given the role of BPTF in mediating chromatin remodeling and gene expression, we hypothesized one or more potential target genes transcriptionally regulated by BPTF might contribute to its function in HCC progression. Considering hTERT functionalizes as a pivotal factor in HCC tumorigenesis and stemness promotion [30,31], we then investigated the expression regulation of hTERT by BPTF in HCC to explore the molecular mechanism accounting for BPTF's tumor-promoting effect and stemness maintenance in HCC development. Firstly, we analyzed hTERT expression in tumor tissues from 9 cases of HCC patients, the samples of which were the same as the ones used in Fig. 1 for BPTF expression determination. hTERT similarly displayed high expression at mRNA and protein level in 5 cases, and its expression trend was positively correlated with BPTF expression (Fig. S3A, B). Consistently, in HepG2 and Hep3B cells, BPTF knockdown significantly down-regulated hTERT expression at the protein and mRNA levels, and hTERT promoter-driven luciferase expression when the promoter sequence corresponds to −2063-+40, −1605-+ 40 and −902-+ 40, but not to other regions (Fig. 5A, B, C), suggesting the transcriptional regulation of hTERT by BPTF. These results also showed that the possible regulatory site was located at hTERT promoter region −902-−321. ChIP assay further verified the binding of BPTF at this fragment of hTERT promoter (Fig. 5D). We further analyzed the binding of methylated histones at hTERT promoters upon BPTF silencing. As shown in Fig. 5D, BPTF knockdown caused nearly no change of their bindings at hTERT promoters.
In order to clarify whether the elevated proliferative capacity and stemness traits mediated by BPTF depend on its regulation on hTERT, we first investigated the effect of hTERT itself on stemness maintenance in HCC cells. As shown in Fig. S2, when hTERT expression was downregulated, the expression of BPTF was nearly unchanged, while the expression of hallmarks related to stemness maintenance (EpCAM, CD44 and c-kit) were inhibited. We next overexpressed hTERT in HCC cells following BPTF knockdown. Colony formation and tumorsphere formation analysis indicated that hTERT overexpression reversed BPTF silencing-caused inhibition of tumor cell growth and tumorsphereforming ability (Fig. 5E, F). The down-regulated expression of hallmarks related to stemness maintenance mediated by BPTF silencing, including EpCAM, CD44 and c-kit, was also reversed under hTERT overexpression (Fig. 5G). Thus, the suppression of tumor cell growth and CSC-related features with BPTF silencing in HCC is at least partially mediated by the down-regulated hTERT level.

BPTF knockdown suppressed tumor growth and CSC-associated characteristics in a HCC xenograft mouse model
The effect of BPTF on HCC development was further validated in vivo based in a HCC xenograft mouse model. Hep3B cells with or without stably knocked down expression of BPTF were subcutaneously injected into the right flank of nude mice respectively. 14 days later, the sizes of the formed tumors were monitored once every two days for nearly two weeks. Then the mice were sacrificed and the tumor weight was measured. Compared with the control group, the tumor size, weight and volume were significantly diminished in BPTF knockdown group (Fig. 6A-D). Of note, the expression regulation of hTERT by BPTF was confirmed again in vivo. Consistent with the effect in vitro, RT-PCR, western blot and immunohistochemistry staining analysis indicated BPTF knockdown considerably suppressed the expression level of hTERT (Fig. 6E-G). In addition, the level of stemness-related hallmarks, CD44 and EpCAM in xenografts of HCC, was also impeded with BPTF knockdown (Fig. 6F, G). These findings demonstrated again that BPTF expression is required to maintain tumor cell growth in HCC progression by activating hTERT expression.

HCC with BPTF knockdown exhibits low metastatic potential
The maintenance of stemness is required for tumor cells to migrate and recolonize at new sites [32,33]. Given the regulation of CSC-related traits by BPTF in HCC in vitro, we then assessed the metastatic potential of HCC driven by BPTF using an in vivo lung metastasis mouse model. 3 × 10 6 Hep3B cells stably expressing BPTF shRNA fused with mCherry fluorescent protein were injected into the tail vein of nude mice. 7 weeks after injection, compared to the animals injected with cells expressing control shRNA, considerably fewer lung tumor nodules and wrinkles were observed in animals injected with cells expressing BPTF shRNA (Fig. 7A, B). Western blot and IHC analyses showed that these tumors in lung tissues formed with BPTF knockdown cells exhibited low expression of hTERT, EpCAM, CD44 and c-kit (Fig. 7C, D). These results reveal that BPTF promote the metastatic potential of HCC and its inhibition impedes survival and regrowth of HCC at a distant site.  . (A-B) The knockdown of BPTF down-regulated hTERT expression at the protein and mRNA levels in Hep3B and HepG2 cells. (C) Detection of the luciferase activity driven by different fragments of hTERT promoter in Hep3B and HepG2 cells when BPTF was stably knocked down. (D) The binding of BPTF, H3K4me3 and H3K27me3 at hTERT promoter(−902 to −321)was validated by chromatin immunoprecipitation assay. (E) Colony formation assay in Hep3B and HepG2 cells upon stable silencing of BPTF followed by hTERT overexpression. Number of colonies was presented on the right. (F) Tumorsphere formation assay in Hep3B and HepG2 cells upon stable silencing of BPTF followed by hTERT overexpression. The number of spheres bigger than 50 µm was quantified and shown. Scale bars, 100 µm. (G) The expression of stemness-associated markers, including EpCAM, CD44, and c-Kit, was shown by western blot in Hep3B and HepG2 cells upon stable silencing of BPTF followed by hTERT overexpression.

BPTF expression was positively correlated with hTERT expression and poor prognosis in patients with HCC
We next evaluated the clinical significance and correlation of BPTF with hTERT in HCC. IHC staining was used to detect the expression of BPTF and hTERT in tumor tissue samples of HCC patients based on tissue microarray from 81 HCC cases. The level of gene expression was divided into 2 classes (high and low). The relationship between BPTF or hTERT expression and the clinicopathologic characteristics of patients was analyzed firstly. As shown in Fig. 8A, BPTF and hTERT expression was tightly relevant to the TNM stage (P = 0.025, P = 0.007 respectively), but not to sex, age and differentiation. Among all these 81 patients tested, 27 cases (about 35%) showed high expression of BPTF and hTERT and 18(22%) showed their low expression simultaneously (Fig. 8B). Furthermore, among patients with high expression of BPTF, nearly half of them displayed high expression of hTERT, and among patients with low expression of BPTF, nearly 64% showed low expression of hTERT (Fig. 8C), suggesting a potential correlation between BPTF and hTERT level in HCC. In addition, Kaplan Meier analysis showed that BPTF and hTERT expression was remarkably associated with 5-year survival of patients (Fig. 8D). Consistent with the tumorpromoting effect of BPTF in vitro and in vivo, the overall survival analysis indicated patients with high expression of BPTF or hTERT owned significantly lower survival rate (P = 0.025 or P = 0.007) compared to the ones with low expression (Fig. 8E). Also, the multivariate Cox proportional hazards model analysis indicated BPTF expression was an independent prognostic risk factor in HCC (HR=0.1947, 95%CI: 1.009-3.756, P = 0.047) (Fig. 8F). Thus, consistent with the results from in vitro and animal study, BPTF expression is positively correlated with hTERT expression clinically, and BPTF expression is predictive of the short survival and is an independent predictor of survival in HCC patients.

Discussion
Although BPTF has been previously reported to be implicated in chromatin remodeling, embryonic development and thymocyte maturation [34], little is known about its effect in tumorigenesis and development. Here, integrating all the experimental data in our study, we reported for the first time the functional role of BPTF and the associated molecular mechanisms in HCC progression. The silencing of BPTF inhibited HCC cell proliferation and colony formation, weakened the CSC characteristics and sensitized chemotherapeutic drugs in vitro. Similarly, BPTF knockdown led to the inhibited tumor growth and the decreased metastatic potential in vivo, confirming the tumor-promoting role of BPTF in HCC again. Moreover, the further mechanistic studies revealed hTERT functioned as the pivotal downstream factor regulated by BPTF to mediate its oncogenic function, including stemness maintenance.
Considering the basic role of BPTF in the stemness maintenance of embryonic stem cells and embryonic development [11], we supposed and verified its involvement in the stemness maintenance of CSCs, as evidenced by the remarkably down-regulated tumorsphere-forming ability applied in studying cell self-renewal [35,36]. The levels of CSCrelated protein markers, including EpCAM, CD133, CD44 and c-Kit, were down-regulated when BPTF was silenced in vitro and in vivo. Of note, BPTF silencing obviously enhanced the anti-cancer effects of chemotherapeutic drugs in HCC cells, resulting in more tumor cell apoptosis. Given the contribution of CSC in the acquirement of therapeutic resistance [37][38][39], the fact that chemosensitivity was regulated by BPTF confirmed again its effect on CSCs in HCC development.
From current reports, hTERT has been treated as a critical factor that promotes cell immortalization [40,41]. Our experimental evidence demonstrated knockdown of BPTF expression produced pronounced HCC cell growth inhibition and led to markedly decreased expression of hTERT. The overexpression of hTERT gene partially reversed the reduced capabilities of colony formation and tumorsphere formation and suppressed expression of CSC-related markers caused by BPTF shRNA treatment, suggesting the key role of hTERT in BPTF-regulated HCC progression. More importantly, we have identified the binding of BPTF at hTERT promoter and its direct regulation on hTERT at transcriptional level. Considering BPTF was defined as nuclear transcription factor to achieve its biological function by regulating downstream genes, most likely, hTERT functions as its direct downstream effector to mediate its protumorigenic function. Conceivably, there must be downstream targets other than hTERT, such as Myc and its downstream signaling molecules, regulated by BPTF directly or indirectly to mediate its function. What are these targets? Whether they play the same role as hTERT in BPTF's carcinogenic function? All these points deserve to be better explored in our future study.
Analysis of the tissue microarray revealed the remarkable consistence in the aspect of BPTF and hTERT expression and their clinical significance. Based on the clinical statistics in a cohort of 81 HCC cases, the expression of BPTF and hTERT was found to be significantly associated with the TNM stage and overall survival of patients. Moreover, patients showing high BPTF expression similarly displayed more percentage of high hTERT expression, implying again the potential regulation of hTERT by BPTF in HCC progression. Combined with our experimental data in vivo, which demonstrated the suppressed tumor growth and metastasis in mouse model and the significantly downregulated hTERT expression in tumor tissues upon BPTF silencing, the clinical analysis not only confirmed the key role of BPTF in promoting HCC development but also suggested the synergy and indispensability of hTERT in such an oncogenic role. Of course, it is better to confirm the diagnostic and prognostic significance of BPTF and its relationship with hTERT in a larger patient cohort. However, based on our current determination, targeting BPTF and hTERT simultaneously might provide an important strategy in HCC treatment.
In summary, our data provide functional evidence and mechanistic insights that BPTF drives HCC progression, including promoting tumor cell proliferation, sustaining CSC traits, and increasing tumor metastasis, by transcriptionally activating the expression of hTERT. Our data also demonstrate the contribution of BPTF in regulating the sensitivity of HCC cells to chemotherapeutic drugs, as evidenced by the elevated sensitization to DDP and 5-FU and increased apoptosis induction upon BPTF knockdown. All the results suggest that the activation of BPTF might serve as a biomarker to drive tumor progression and predict poor prognosis and as a promising therapeutic target for patients suffering HCC.