RPN2 promotes metastasis of hepatocellular carcinoma cell and inhibits autophagy via STAT3 and NF-κB pathways

RPN2 level is increased in HCC cells and tissue specimens of HCC patients

Expression of RPN2 is relevant for the prognosis and diagnosis of different cancers. In the current research, when RPN2 gene expression was determined in HCC specimens vs normal healthy hepatic tissue, it was found to be significantly upregulated in the HCC specimens, compared to the normal healthy tissue (Figure 1A). Then, the mRNA levels of RPN2 in the human HCC cell lines, Huh-7 and HepG2 were compared with normal human hepatic cells (NHC) by qPCR. As shown in Figure 1B and 1C, mRNA levels of RPN2 were upregulated in HCC cells compared to the healthy cells. WB, performed to confirm the protein expression of RPN2 in the HCC cell lines and healthy cells, showed that the expression of RPN2 protein was considerably elevated in HCC cell lines, in comparison to the healthy hepatic cells (Figure 1D and 1E). In IFA imaging, more RPN2 staining was observed in Huh-7 and HepG2 than that in normal hepatic cells (Figure 1F and 1G).

RPN2 mediates HCC cell proliferation

To confirm that RPN2 regulates the proliferation rate of HCC cells, the overexpression of RPN2 in Huh-7 and HepG2 cells was demonstrated with WB and qPCR (Figure 2A–2D). Further, MTT assay (Figure 2E and 2F) revealed that multiplication of Huh-7 and HepG2 cells, 12–72 h post-transfection, was greatly increased when they were transfected with RPN2-expressing adenovirus (AD-RPN2). The RPN2 overexpression caused a noticeable increase in colony numbers, evaluated by the soft agar colony formation assay, while transfection with the control (AD-NC) did not affect colony numbers of HepG2 cells (Figure 2G and 2H). To determine whether RPN2 overexpression promotes tumor phenotypes in normal hepatocytes, we performed RPN2 overexpression in normal hepatocytes (NHCs). However, there was no significant different in cell proliferation between RPN-overexpressing NHCs and control NHCs, indicating that RPN2 only exert its function in malignant cells (Figure 2I and 2J).

Previous research had reported that invasion and migration of HCC cells is a major cause of mortality during HCC development and progression [9]. To determine whether RPN2 influences the invasion and migration of HCC cells, transwell migration and wound-healing assays were carried out after transfection of HepG2 and Huh-7 cells with the RPN2-expressing adenovirus (AD-RPN2) and control (AD-NC). In the wound healing assay, overexpression of RPN2 promoted migration of Huh-7 and HepG2 cells towards the gap created by scratching of the cell monolayer (Figure 3A and 3B). Overexpression of RPN2 clearly increased migration of HCC cells (Figure 3C and 3D), especially in HepG2 cells, which is consistent with data from the wound healing assay. Moreover, we examined the effect of RPN2 overexpression on EMT; the ectopic expression RPN2 led to a decrease in E-cadherin and an increase in N-cadherin expression in both the cell lines, as determined by WB (Figure 3E and 3F). These data suggested that RPN2 overexpression facilitates the metastatic and invasive attribute of HCC cells in vitro.

Next, the effect of RPN2 silencing in HepG2 and Huh-7 cells was determined by transfecting the cell lines with vector containing shRNA-RPN2 and control (vector containing shRNA), and then the gene and protein expression of RPN2 were determined by qPCR and WB (Figure 4A–4D). Cell proliferation was determined by MTT assay, and we found that RPN2 silencing caused a significant reduction in the number of HepG2 and Huh7 cells (Figure 4E and 4F). Additionally, colony formation assay showed a decrease in the number of colonies, compared to the control (Figure 4G and 4H).

The effect of RPN2 silencing on the migration and invasion of HepG2 and Huh-7 cells was also determined by transwell migration and wound healing assays. The wound healing assay displayed that silencing of RPN2 inhibited the invasive capability of SMMC-7721 and HepG2 cells, compared to the control (Figure 5A and 5B). Further, RPN2 silencing remarkably reduced the number of migrated Huh-7 and HepG2 cells (Figure 5C and 5D), especially for HepG2 cells, which is consistent with wound healing data. Additionally, we examined the EMT-related protein expression upon RPN2 silencing. E-cadherin was significantly increased while N-cadherin was decreased due to RPN2 silencing (Figure 5E and 5F).

RPN2 regulates autophagy of HepG2 cells

It had been reported that RPN2 depletion induced autophagy via hypoxia signaling [27]. Therefore, our study looked at the effect of RPN2 overexpression and silencing on induction of autophagy in HCC cell lines. RPN2 overexpression did not significantly alter the autophagy rate of HepG2 cells, which was proved by the LC3B dynamic and processing (reduced LC3B I and II levels), but showed p62 upregulation (LC3B and p62, 2 main indicators of autophagic process [28]). On the contrary, RPN2 silencing induced a strong autophagy reaction: elevated LC3B IIlevel and (Figure 6A and 6B), and p62 degradation (Figure 6C and 6D). Quantitative PCR was also carried out to determine the p62 expression at mRNA level, and the results showed that its expression positively correlated with RPN2 expression (FIGURE 6E and 6F). Immuno-fluorescent assay was carried out to trace the location of LC3B protein upon RPN2 silencing. We observed that LC3B (green) gathered due to RPN2 silencing (Figure 6G). Electron microscopy was also performed to detect the autophagic vacuoles of cells with treatment to validate the autophagy state change. RPN2 silencing caused more autophagic vacuoles, when compared to the NCi groups (Figure 6H). These results demonstrated that RPN2 inhibited autophagy of HepG2 cells, which is partially responsible for its facilitative effect on proliferation of HCC cells.

RPN2 induced MMP-9 expression through STAT3 and NF-κB pathways

It has been well-recognized that the invasive and metastatic nature of carcinoma cells were promoted, although partially, via generation of factors like MMP-9 [29]. Thus, we examined the expression of MMP-9 at both gene and protein level under varying RPN2 expression. We found that ectopic expression of RPN2 led to a drastic increasing of MMP-9 production (Figure 7A), while RPN2 silencing caused depletion of MMP-9 in HepG2 cell (Figure 7B). The qPCR data further confirmed that MMP-9 expression is positively correlated with RPN2 expression (Figure 7C and 7D). These data suggested that RPN2 played an essential role in MMP-9 expression in HepG2 cells.

A previous study had shown that MMP-9 expression was modulated through STAT3 phosphorylation [30]; therefore, we next examined the STAT3 activation level with respect to RPN2 overexpression and silencing. As expected, the overexpression of RPN2 increased the phosphorylation level of STAT3 (Figure 8A) without affecting its expression, while RPN2 silencing remarkably reduced the phosphorylation level of STAT3 (Figure 8B). Furthermore, to confirm the effect of STAT3 in RPN2-induced MMP-9 upregulation, AG490 (an inhibitor binding to the upstream region of Janus kinase, the activating agent of STAT3) was utilized to block the signal transduction of STAT3 pathway. HepG2 cells stably expressing RPN2 were processed with 20 μM AG490 for 24 h. In response to AG490 treatment, STAT3 phosphorylation was clearly inhibited while its expression was not impaired. Meanwhile, MMP-9 expression was significantly reduced due to AG490 (Figure 8C). These evidences strongly indicated that STAT3 pathway was involved in the RPN2-regulated MMP-9 expression.

We then determined the function of NF-κB p65 in RPN2-induced MMP-9 production of HepG2 cells, as it is reported that the expression of NF-κB is related to the MMP-9 in different cell lines and diseases [31–33]. Therefore, we examined the p65 expression in HepG2 cells expressing RPN2. The expression of p65 and MMP-9 was significantly upregulated due to RPN2 overexpression (Figure 9A). In contrast, RPN2 silencing reduced p65 and MMP-9 level at protein level (Figure 9B). Moreover, qPCR data confirmed that p65 level is correlated with RPN2 expression (Figure 9C and 9D). In order to show the role of NF-κB p65 in RPN2-induced MMP-9 expression, NF-κB p65 expression was silenced with shRNA. The shRNA-p65 was transfected in HepG2 cell stably expressing RPN2, and the potency of transfection was further verified by WB. The p65 protein expression decreased in RPN2+shRNA-p65 group, due to which the MMP-9 expression was impaired, partially showing that NF-κB p65 regulated RPN2-induced expression of MMP-9 in HepG2 cells (Figure 9E).

Antibodies and chemicals

Antibodies for RPN2, STAT3, pSTAT3, p65, E-cadherin, N-cadherin, LC3B, p62, MMP-9, and Tubulin were purchased from Cell Signaling Technology (Danver, USA) and Santa Cruz Biotechnology (Dallas, USA), and Anti-HMGB1 antibody from GeneTex (Irvine, USA). The chemical AG490 was procured from Selleck (Beijing, China).

Cells and transfection

Human cells Huh-7 and HepG2 were obtained from the Cell Bank of Academia Sinica (Shanghai, China), and cultivated in Dulbecco's modified Eagle's medium (DMEM; Gibco) supplemented with 10% fetal bovine serum, 2 mM L-glutamine, 100 μg/mL streptomycin and 100 U/mL penicillin. They were incubated at 37°C in a humidified atmosphere with 5% CO₂. 5 × 10⁶ cells were seeded onto 6 and 12-well plates as per requirement and incubated overnight until 80–90% confluence.

Clinical samples

The study group consisted of ten cases of HCC, with the age of the subjects ranging from 30–60 years. Clinicopathological data were obtained from the patients’ medical records. We harvested the para-neoplastic tissues at least one centimeter from the neoplastic tissues. All cases were hepatectomy candidates in the First Affiliated Hospital, Xi’an Jiaotong University. The study was approved by the Ethical Committee of the First Affiliated Hospital, Xi’an Jiaotong University. All study participants had given their written informed consent before participating in the study. Three pathologists reviewed and diagnosed the samples independently.

Adenovirus generation and transfection

The adenovirus was produced using the RAPAd^® CMV Adenoviral Bicistronic Expression System (Cell Biolabs, San Diego, USA) according to the manufacturer’s instructions. Human RPN2 cDNA was cloned into pacAd5 CMV-IRES linearized vector to form AD-RPN2. The purified linear DNAs were then co-transfected in 293T cells along with Lipofectamine™ 2000 reagent. Five days after the transfection, the adenoviruses were released from the 293T cells by performing the freezing-thawing procedure twice; the cell lysate was centrifuged to pellet the debris and the adenoviral lysate was stored at −80°C. For transfection of Huh-7 and HepG2 cells, 20 μL of AD-RPN2 from the adenoviral stock was used, and the cells were cultured for a period of 8 to 12 h. One adenovirus that expressed Green Fluorescent Protein (GFP) without RPN2 (AD-NC) served as the control.

RNA interference

To reduce the expression of RPN2 and p65, short hairpin RNA (shRNA) against RPN2 and p65 were used. RPN2 and p65-targeted shRNA was designed and manufactured by Genomics Co., Beijing. The shRNA was added to 5 μL of RNase free water at a concentration of 100 pmol/μL, mixed with 10 μL transfection reagent, and kept for 15 min at 25 °C.

For the transfection assay, 5 × 10⁶ HCC cells Huh-7 and HEPG2 were seeded onto 6-well plates and incubated overnight at 37°C in 5% CO₂ until 80-90% confluence, and then they were transfected with shRNA.

MTT assessment

The MTT assay was performed to evaluate cell survival. Briefly, 20 μL of MTT (0.5 mg/mL) was added to the cells and incubated for 4 h at 37°C in an atmosphere of 5% CO₂. The supernatant was discarded, 150 μL DMSO was added to each well, and rotated every 10 min to dissolve the purple formazan crystals. The absorbance was measured at 490 nm with an Infinite M200 microplate reader (Tecan, Männedorf, Switzerland).

3-Methyladenine (3-MA) treatment

For autophagy analysis, the cells have been pre-treated with 10 nM of 3-Methyladenine (3-MA), autophagy inhibitor. 3-M) is an inhibitor of phosphatidylinositol 3-kinases (PI3K). PI3K plays an important role in many biological processes, including controlling the activation of mTOR, a key regulator of autophagy. 3-MA inhibits autophagy by blocking autophagosome formation via the inhibition of class III PI3K [47].

WB

For the extraction of proteins, the cells were washed with PBS and then processed in RIPA Lysis Buffer (Cell Signaling Inc). Same volumes of total cellular proteins were separated with the SDS-PAGE, transferred onto the PVDF membrane, and immunoblotted with the commercial antibodies, as per requirement. Finally, the samples were immunoblotted with antibodies and treated as the control. For autophagy analysis, the cells have been pre-treated with 10 nM of 3-MA, autophagy inhibitor.

Immuno-fluorescence assay (IFA)

For the LC3B puncta assay, the human prostate cancer cell line PC-3 was transfected with GFP-LC3B expression constructs (Addgene). Images were recorded under the fluorescent microscope as mentioned earlier [48]. The GFP puncta were calculated from 10 different microscopic fields. For autophagy analysis, the cells have been pre-treated with 10 nM of 3-MA, autophagy inhibitor.

RNA extraction and qPCR

Trizol reagent was used to isolate total RNA from the cells. For, qPCR, the reaction mixture consisting of cDNA, forward and reverse primers and SYBR Green PCR Master Mix in a total volume of 20 μL was subjected to amplification in the Light-Cycler 480 Real Time PCR system (Roche, Basel, Switzerland) using the following program: 95°C for 10 min followed by 40 cycles at 95°C for 15 sec, 60°C for 30 s, and 72°C for 30 s. GAPDH was used as the internal reference. Quantification was performed according to 2^-ΔΔCT method through normalization to GAPDH. For autophagy analysis, the cells have been pre-treated with 10 nM of 3-Methyladenine (3-MA), autophagy inhibitor.

Transwell migration assays

After 24 h transfection, the HCC cells were trypsinized, followed by washing with D-Hanks solution. Matrigel inserts (8 μm pore size) were used to separate the wells of a 24-well culture plate into top and bottom compartments. F-12 medium (400 μL) containing HGF (20 ng/mL) and FBS (10%) was added into the bottom chamber and the cells were seeded in the upper chamber. Following incubation, crystal violet was added to stain the cells that had migrated through the pore, and observed under a microscope (Zeiss).

Wound healing assay

After 48 h of treatment, Cells (1 × 10⁶ cells/mL) were seeded in 24-well culture plate and grown until they reached 70–80% confluence. Using a sterile pipette tip, a scratch wound was created by scraping the monolayer of cells. The wells were washed twice with PBS to remove the detached cells, fresh medium was added and the cells were incubated for 0 (control) and 48 h. The cell images were obtained with an inverted microscope (Nikon).

Statistical analysis

Results are represented as mean ± standard deviation (SD). Differences were analyzed using one-way ANOVA or Student’s t-tests. The threshold for significance was set at P < 0.05.