UPF1 Participates in the Progression of Endometrial Cancer by Inhibiting the Expression of lncRNA PVT1.

BACKGROUND: Endometrial carcinoma (EC) is the primary cause of death associated with cancer globally. Thus, the possible molecular mechanism of EC needs further exploration. Up-frameshift protein 1 (UPF1) is an ATPase depending on RNA/DNA and RNA helicase depending on ATP. Long noncoding RNA (lncRNA) plasmacytoma variant translocation 1 (PVT1) was dysregulated in diverse diseases.
METHODS: qRT-PCR and Western blot were applied to detect UPF1 and PVT1 in EC. CCK-8, colony formation, and Transwell assays were used to test the effects of UPF1/PVT1 on cell proliferation and migration. Cells were cultured with actinomycin D to observe mRNA stability, and RNA immunoprecipitation assay was applied to verified the relationship between UPF1 and PVT1. Glucose consumption and lactate generation were measured when cells were transfected with siRNA.
RESULTS: Results demonstrated that the expression of UPF1 exhibited a remarkable decrement in EC tissues relative to that in non-tumor tissues. Subsequent functional experiments suggested that UPF1 decrement stimulated EC cells to grow and migrate. Moreover, UPF1 was discovered to be linked to PVT1 and had an inverse correlation with PVT1. Besides, PVT1 expression affected EC growth and migration, and PVT1 decrement alleviated the influence of UPF1 decrement on EC growth and migration and strengthened glycolysis in EC.
CONCLUSION: In this study, we found that UPF1 was down-regulated in EC tissues, and UPF1 might exert its role by regulating the expression of PVT1.

Background

Endometrial carcinoma (EC) is one of the three malignant tumors arising in the reproductive tract of women. It is an endometrial epithelial malignant tumor with secluded onset, and it is prone to metastasis and migration.1,2 The onset of EC is generally believed to increase with age, and prognosis is poor.3

Up-frameshift protein 1 (UPF1) is an ATPase depending on RNA/DNA and an RNA helicase depending on ATP; it is an evolutionarily conserved phosphorylated protein with extensive expression.5 UPF1 exerts a crucial effect on nonsense-mediated decay (NMD) and non-NMD RNA decay.6 In the NMD process, UPF1 is related to a translation termination codon based on the translation termination complex. Additionally, UPF1 stimulates cells to progress to G1/S, increasing the likelihood that NMD facilitates the decay of mRNAs encoding repressive proteins, which prevent evolution in this cell cycle stage.7 A recent study discovered that UPF1 modulates tumor formation.8 However, reports about UPF1 in EC remain limited.

An increasing number of lncRNAs have been ascertained to modulate the expression of genes correlated with tumors at the transcriptional, post-transcriptional, chromatin, and genomic levels.4 Thus, an investigation on the functions of pivotal lncRNAs in EC growth may contribute to the prediction of prognosis, elevation of early diagnosis rate, and increase in the survival rate in patients with EC.

In this study, we aimed to reveal the roles of UPF1 in the occurrence and progression of EC. Given that UPF1 can exert its roles by affecting downstream genes in many diseases, we also intended to explore the potential mechanism by which UPF1 exerts its function and provide insight into the study of EC.

Materials and Methods

Patients

Twenty-four fresh EC tissues and paired adjacent non-cancerous tissue samples were obtained from patients who underwent surgical treatment at the affiliated hospital of Jiamusi University. None of the patients received anti-cancer treatment before surgery, including radiotherapy and chemotherapy. EC diagnosis was confirmed through pathology by three pathologists. This research gained the approval from the Institutional Review Board of the first affiliated hospital of Jiamusi University, and all subjects signed informed consent.

Cell Culture

EC cell lines (AN3CA, KLE, RL-95, HEC1A, and Ishikawa) and endometrial epithelial cells (hEECs) were acquired from ATCC Cell Lines (USA). All cells were cultured in DMEM with 10% fetal bovine serum (FBS) purchased from Thermo Fisher Scientific and McCoy’s 5a medium. The culture environment was 37 °C and 5% CO2.

Quantitative Real-Time Polymerase Chain Reaction (qRT-PCR)

TRIzol reagent provided by Invitrogen (USA) was utilized to harvest total RNA from clinical specimens, followed by reverse transcription via a PrimeScriptRT reagent Kit from Promega (USA) following the manufacturer’s protocol. The ABI7900HT RT-PCR system acquired from Applied Biosystems (USA) and SYBR Green Master Mix provided by Thermo Fisher Scientific (USA) were adopted for qRT-PCR, with GAPDH as internal control. The applied primers are shown below: UPF1 (F:5ʹ-ACCGACTTTACTCTTCCTAGCC-3ʹ; R:5ʹ-AGGTCCTTCGTGTAATAGGTGTC-3ʹ),

PVT1 (F:5ʹ-GTCTTGGTGCTCTGTGTTC-3ʹ;

R:5ʹ-CCCGTTATTCTGTCCTTCT-3ʹ)

GAPDH (F:5ʹ-CCATGTTCGTCATGGGTGTGAACCA-3ʹ;

R:5ʹ-GCCAGTAGAGGCAGGGATGATGTTG-3ʹ).

The 2−ΔΔct method was employed to calculate the relative expression level of each gene.

siRNA Synthesis and Cell Transfection

The PVT1- or UPF1-specific siRNAs, pcDNA3.1-UPF1, negative control siRNA (siR-NC), and pcDNA3.1 were provided by Riobobio (China). Lipofectamine 2000 transfection reagent from Thermo Fisher Scientific (USA) was utilized to treat cells in accordance with the manufacturer’s instructions. After transfection for about 24–48 h, cells from every group were collected and applied for subsequent research. In addition, plasmid treatment was carried out using the same method as above.

Immunohistochemistry

Immunohistochemical detection was carried out with general approaches. Anti‐UPF1 (Abcam, Cambridge, UK) was used as the primary antibody. Image Pro Plus 6.0 (Media Cybernetics, Inc., Rockville, MD, USA) software was applied to calculate the mean optical density. For each tissue section, at least five non‐overlapping cortical fields were analyzed.

CCK-8 Test

Following inoculation into a 96-well plate (2×103 cells/well), the cells were cultured for 24, 48, and 72 h. Thereafter, the wells were added with CCK-8 from Dojindo Laboratories (Japan) for 4 h of incubation. A Varioskan Flash Spectral Scanning Multimode Reader provided by Thermo Fisher Scientific (USA) was employed to examine the absorbance at 450 nm. Each experiment was repeated three times independently.

Western Blot

On the basis of the manufacturer’s guidelines, a standard BCA test was executed to determine the protein concentration in cell lysate. After isolation via SDS-PAGE (10%) electrophoresis, the proteins were transferred to PVDF membranes at 4 °C and sealed with skim milk (5%) in TBST for 1 h. These membranes were incubated with anti-GAPDH or anti-UPF1 antibody (Cell Signaling, USA) overnight at 4 °C. The membranes were washed three times with TBST and incubated with secondary antibody at indoor temperature for 1 h. A Phototope-horseradish peroxidase Western blot detection kit (Cell Signaling Technology, Danvers, MA, USA) was applied to detect the expression of proteins. The UPF1 protein expression levels were normalized to that of GAPDH by calculating the relative expression levels.

Colony Formation Experiment

A number of 1×103 Ishikawa or HEC1A cells were put into agar (1.5 mL) on the top that was then added onto agar on the bottom in each well. Complete medium (2 mL) was replenished twice a week. After 3 weeks, colonies were dyed with 0.1% crystal violet (0.5 mL) for 1 h, and a. TE2000-U dissection microscope acquired from Nikon (Japan) was used to quantify colonies ≥0.5 mm. Each experiment was repeated three times independently.

Transwell Experiment

The ability of the cells to migrate was assessed by Corning Transwell insert chambers (Corning). Approximately 1×104 (migration assay) of transfected cells in 200 μL of serum-free medium was seeded in the upper well; the chambers were then incubated with medium plus 20% fetal bovine serum for 48 h at 37 °C to allow the cells to migrate to the lower well. The cells that had migrated through the membrane were fixed in methanol and stained with crystal violet (Invitrogen). Finally, the migrated cells were imaged and counted using a microscope.

Detection of RNA Stability

HEC1A or Ishikawa cells undergoing treatment with siRNA specific to PVT1/UPF1 or siR-NC were incubated using 5 μg/mL Actinomycin D (Sigma-Aldrich, USA) in the medium. Subsequently, total RNA was obtained at the denotative time, and the mRNA expression level was evaluated by qRT-PCR. Finally, the half-life period of mRNAs was examined before and after Actinomycin D addition. Each experiment was repeated three times independently.

RNA Immunoprecipitation Assay

RNA immunoprecipitation (RIP) experiments were performed using a Magna RIP kit (Millipore, Bedford, MA) following the manufacturer’s instructions. In summary, a mixed buffer was utilized to obtain cells on ice for 20 min. Ten nuclei were subjected to 15 min of centrifugation at 2500 g through pelleting, and resuspension was carried out to obtain nuclear pellets in RIP buffer. Centrifugation was then implemented again to pellet nuclear debris and membrane. The supernatant was added with protein G beads and rabbit UPF1 or IgG antibody from Cell Signaling (MA) for incubation overnight at 4°C. Finally, RNAs undergoing co-precipitation were separated, and PVT1 was subjected to qRT-PCR. Each experiment was repeated three times independently.

Glucose Consumption and Lactate Generation Experiment

In accordance with the manufacturer’s protocol, a glucose and lactate assay kit (BioVision, Milpitas, CA, USA) was employed to examine the harvested cell supernatants and assess lactate and glucose. Each experiment was repeated three times independently.

Statistical Analysis

Statistical processing was executed with the use of SPSS.20 software (IBM, USA). Assays in this research were carried out three times, and the mean ± SD was applied to the present data. Student’s t-test and one-way ANOVA were conducted to analyze the results. Statistical significance was set at p<0.05.

Results

UPF1 Expression Declined in EC

First, UPF1 expression in 24 EC tissues and 24 non-tumor tissues was tested via RT-PCR, and the mRNA expression of UPF1 was down-regulated in EC tissues (Figure 1A). The protein expression of UPF1 was detected by Western blot, and it also declined in EC (Figure 1B). We also applied immunohistochemistry to detect the expression of UPF1 in EC tissues; UPF1 was up-regulated in EC tissues (Figure 1C). Finally, we detected the expression of UPF1 in EC cell lines, and UPF1 was down-regulated obviously in both HEC1A and Ishikawa cell lines relative to the other cell lines (Figure 1D). Therefore, we chose these two cell lines for the subsequent experiments. The above results revealed that UPF1 might play a part in EC and predominantly influenced tumor evolution.

Figure 1

UPF1 expression in human EC tissues. (A) RT-PCR is implemented to test UPF1 expression in 24 pairs of EC and no-tumor tissue specimens. UPF1 expression is lowered in EC tissues. (B) UPF1 expression in EC tissues relative to matched non-tumor tissues. Western blot is executed to test UPF1 expression. (C) Immunohistochemistry also shows that the expression of UPF1 in EC tissues is down-regulated. (D) The expression of UPF1 in EC cell lines. (*P<0.05).

UPF1 Silencing Facilitated EC Cells to Grow and Migrate

To determine UPF1’s function in EC, HEC1A cells were treated with siRNAs specific to UPF1 to repress UPF1 expression (Figure 2A). Among three siRNAs, number #2 and #3 were highly effective, so the two were applied for later assays. CCK-8 and colony formation experiments showed that the HEC1A cells’ proliferation and growth abilities were inhibited when UPF1 expression was reduced (Figure 2B and C). On the basis of the obtained findings (Figure 2D), the number of migrated cells increased due to UPF1 decrement. By contrast, UPF1 overexpression showed the opposite results (Figure 2E–H). Similar trends and results were also observed in the Ishikawa cell line when the expression of UPF1 was up- or down-regulated (Figure 3A–H). In conclusion, UPF1 repression boosted EC cells to grow and migrate.

Figure 2

UPF1 repression boosts HEC1A cells to migrate and grow. (A) UPF1 decrement efficiency in HEC1A. (B) CCK-8 assay is used to test cell proliferation in HEC1A. (C) UPF1’s impact on cell growth is examined via colony formation experiment. (D) UPF1 decrement predominantly aggrandizes the number of migrated cells. (E) UPF1 overexpression efficiency in HEC1A is confirmed by Western blot. (F–H) Cell growth and migration are tested when UPF1 is up-regulated. (*P<0.05).

Figure 3

UPF1 repression boosts Ishikawa cells to migrate and grow. (A) UPF1 decrement efficiency in Ishikawa cells. (B) Cell proliferation in Ishikawa. (C) UPF1’s impact on Ishikawa cell growth is examined via colony formation experiment. (D) UPF1 decrement predominantly promotes Ishikawa cell migration. (E) UPF1 overexpression efficiency in Ishikawa. (F–H) Ishikawa cell growth and migration are tested when UPF1 is up-regulated. (*P<0.05).

UPF1 Linked lncRNA PVT1

A recent study reported that numerous lncRNAs play a part in molecular regulatory pathways by interacting with proteins.9 LncRNAs that possibly link UPF1 were verified via bioinformatics analysis, and PVT1 might be related to UPF1. First, PVT1 expression level in EC was analyzed by RT-PCR, and the findings revealed that it was higher in EC tissues than in non-tumor tissues (Figure 4A). To continuously verify the association between PVT1 and UPF1 in EC, the relationship between their expression levels in EC tissues was tested. As shown in Figure 4B, they had an inverse relationship as indicated by RT-PCR. The stability of PVT1 mRNA was then examined in EC cells with UPF1 decrement. The results ascertained that PVT1 decay rate increased in HEC1A after UPF1 decrement (Figure 4C). The linage between UPF1 and PVT1 was tested by RIP, and the results demonstrated that UPF1 was specifically linked PVT1 (Figure 4D). Moreover, HEC1A cells were treated with UPF1 expression plasmids, and UPF1 overexpression was discovered to lower PVT1 expression (Figure 4E), which was reversed by UPF1 decrement (Figure 4F). In conclusion, UPF1 linked PVT1 and was likely to participate in EC evolution.

Figure 4

LncRNA PVT1 links UPF1. (A) PVT1 expression is examined by RT-PCR, with GAPDH expression as normalization. (B) Association between expression levels of UPF1 and PVT1 RNAs in 24 EC tissues. (C) Actinomycin D is employed for cell treatment for the denoted time, and RT-PCR is implemented for the assessment of PVT1 RNA level. (D) RIP test denotes the linkage between UPF1 and PVT1. (E) UPF1 overexpression reduces PVT1 expression. HEC1A cells are treated with UPF1 plasmid. (F) UPF1 decrement in HEC1A cells raises PVT1 expression. (*P<0.05).

Effects of PVT1 Expression on EC Growth and Migration

The expression of PVT1 in HEC1A cells was lowered to determined PVT1’s function, and #2 and #3 siRNAs were more efficient PVT1 targets for further assays than the other tested siRNAs (Figure 5A). The CCK-8 assay was executed to test the functions of UPF1 in EC growth. In Figure 5B, PVT1 decrement reduced the evolution of EC cells, and this phenomenon was reversed by PVT1 overexpression. Furthermore, colony formation experiment demonstrated that PVT1 decrement repressed the growth of HEC1A cells (Figure 5C). Subsequently, PVT1’s functions in cell migration were assessed through a Transwell experiment. The decrement in PVT1 expression weakened the migration ability (Figure 5D), but PVT1 overexpression enhanced cell growth and migration (Figure 5E–H). In Ishikawa cells, these results could also be observed (Figure 6A–H). The abovementioned data denoted that PVT1 may participate in EC migration and growth.

Figure 5

PVT1 expression’s functions in HEC1A migration and growth. (A) PVT1 expression in HEC1A cells is repressed. (B) CCK-8 is utilized for assessment of HEC1A cell proliferation. (C) Colony formation experiment validates that PVT1 decrement slows down HEC1A cell growth. (D) PVT1’s impacts on cell migration in HEC1A cells is also assessed by Transwell assay. (E–H) Up-regulation of PVT1 accelerates the proliferation, growth, and migration of HEC1A cells. (*P<0.05).

Figure 6

PVT1 expression’s functions in Ishikawa migration and growth. (A) si-PVT1s’ efficiency in Ishikawa cells. (B–D) Ishikawa cell proliferation, growth, and migration are detected when PVT1 is down-regulated. (E–H) Up-regulation of PVT1 accelerates the proliferation, growth, and migration of Ishikawa cells. (*P<0.05).

PVT1 Decrement Alleviated the Function of UPF1 Decrement in EC Migration and Growth

To explore the functional association between PVT1 and UPF1, EC cells were treated with PVT1 siRNA following UPF1 decrement. First, RT-PCR showed that PVT1 reduction repressed PVT1 expression after UPF1 decrement raised PVT1 (Figure 7A). Second, CCK-8 assay proved that UPF1 decrement facilitated the proliferation of EC cells, whereas PVT1 decrement weakened the cell proliferation capacity (Figure 7B). Third, cell growth was continuously researched via colony formation experiment, and the results indicated that cell growth was also blocked following PVT1 decrement (Figure 7C). Finally, UPF1 decrement weakened cell migration ability (Figure 7D). Thus, PVT1 decrement alleviated the influences on EC cell migration and growth exerted by UPF1 decrement.

Figure 7

PVT1 decrement alleviates the influence of UPF1 decrement on EC migration and growth. (A) Exploration of PVT1 expression in HEC1A cells. (B) MTT is used to analyze cell proliferation. (C) Colony formation experiment. (D) Transwell assay is implemented for cell migration analysis. (*P<0.05).

UPF1 Decrement Strengthened Glycolysis in EC

In general, normal cells display a lower glucose metabolism rate than tumor cells,10 and PVT1 is reported to participate in the glycolysis of tumor cells.11 In this research, glycolysis changes in EC cells with UPF1 decrement were examined, and the results verified that UPF1 decrement elevated the glucose consumption rate in EC cells (Figure 8A–D).

Figure 8

UPF1 decrement strengthens glycolysis in EC cells. (A) Analysis of glucose consumption in EC in HEAC1 cells. (B) Examination of lactate generation in EC in HEAC1 cells. (C) Analysis of glucose consumption in EC in Ishikawa cells. (D) Examination of lactate generation in EC in Ishikawa cells. (*P<0.05).

Discussion

The UPF complex helps degrade abnormal mRNAs.12,13 UPF1 has been considered to be a mainstay factor for NMD,14,15 and it plays a remarkable part in embryonic survival and growth.16,17 In addition, UPF1 represses cell growth but triggers apoptosis in Drosophila melanogaster,18 and it functions in cancer evolution. Moreover, UPF1 possibly modulates MALAT1, and the UPF1/MALAT1 pathway may be a target in gastric cancer therapy.19 UPF1 exhibits higher expression level in normal lung tissues relative to human lung adenocarcinoma tissues, implying that NMD decrement contributes to the formation of lung adenocarcinoma.20 UPF1 has also been reported as a tumor repressor, which is consistent with the findings of this research.

In the current research, UPF1’s association with EC was explored. RT-PCR revealed a reduction of UPF1 expression in EC, and UPF1 may affect tumor evolution. Additionally, UPF1’s functions in EC cells were confirmed using loss-of-function tests. The obtained data distinctly ascertained that UPF1 decrement boosted EC cells to migrate and grow.

LncRNAs, with >200 nucleotides (nt), originated from the genome “noisy region.” They are novel biomarkers for the relapse and evolution of disease.21 Increasing attention has been paid to the impacts of lncRNAs on cell biology and tumor growth.22–24 Plasmacytoma variant translocation 1 (PVT1), a lncRNA, is abnormally expressed in numerous cancer cells and tissues.25,26 In particular, PVT1 acts as an oncogene in tumor metastasis and growth.27–29 Nevertheless, the molecular mechanism of PVT1 in cancer evolution remains unclear. In this research, we found that UPF1 was capable of linking PVT1, and they had an inverse correlation in EC. PVT1 decrement in EC cells impeded cells to migrate and grow. Notably, we discovered that UPF1 might perform its effects on cell growth and migration by binding to PVT1.

The current research unfolded a new mechanism mediated by UPF1 of cell growth and migration by targeting lncRNA PVT1 in EC cells. The findings revealed that PVT1/UPF1 influenced EC formation and functions as a speculated target for diagnosing and treating EC.