Elevated PRC1 in gastric carcinoma exerts oncogenic function and is targeted by piperlongumine in a p53‐dependent manner

Abstract Gastric carcinoma is one of the most common malignancies worldwide and the second most frequent cause of cancer‐related death in China. Protein regulator of cytokinesis 1 (PRC1) is involved in cytokinesis and plays key roles in microtubule organization in eukaryotes. This study was aimed to analyse the expression and to investigate the functional role of PRC1 in gastric tumorigenesis. The expression of PRC1 was evaluated by qRT‐PCR, Western blot and immunohistochemistry. The biological function of PRC1 was determined by CCK‐8 proliferation assays, monolayer colony formation, xenografted nude mice and cell invasion assays by shRNA‐mediated knockdown in AGS and HGC27 cells. The regulation of PRC1 expression by piperlongumine was also investigated using dual‐luciferase reporter assay and ChIP‐qPCR analysis. PRC1 was up‐regulated in primary gastric cancers. Overexpression of PRC1 in gastric cancers was associated with poor disease‐specific survival and overall survival. PRC1 knockdown in AGS and HGC27 cell lines suppressed proliferation, reduced monolayer colony formation, inhibited cell invasion and migration ability and induced cell‐cycle arrest and apoptosis. Inhibition of PRC1 also suppressed tumour growth in vivo. We finally confirmed that PRC1 is a novel downstream target of piperlongumine in gastric cancer. Our findings supported the oncogenic role of PRC1 in gastric carcinogenesis. PRC1 might serve as a prognostic biomarker and potential therapeutic target for gastric carcinoma.

obviously a multi-step process with numerous genetic and/or epigenetic alterations. These alterations affect the downstream signal transduction pathways involved in cell proliferation and/or survival, whereas the underlying molecular mechanism is largely unknown [4]. Similar to other malignancies, gastric carcinoma is a very heterogeneous cancer type and has traditionally been classified into intestinal and diffuse types according to the Lauren classification [5]. Recently, The Cancer Genome Atlas (TCGA) network, classified gastric cancer into four subtypes: Epstein-Barr virus (EBV)-positive tumours, microsatellite instable (MSI) tumours, genomically stable (GS) tumours and tumours with chromosomal instability (CIN) [6]. Trastuzumab and ramucirumab (targeting HER2 and VEGFR, respectively) are currently the only gastric cancer-approved targeted therapies. Given that the use of targeted therapies in gastric tumours is much less common than in other cancers, the newly defined molecular classification of gastric cancer may represent a critical starting point to design more appropriate clinical trials based on the principles of precision medicine.
A hallmark of cancers is unlimited cellular proliferation; proper control of entry into and progression through the cell growth and division cycle is essential for cell proliferation and the maintenance of genome stability [7]. To do so, cells must precisely duplicate their chromosomal DNA during S phase, segregate the sister chromatids to opposite poles of the mitotic spindle during mitosis, assemble two nuclei and then divide their organelles, cytoplasm, plasma membrane and other cell contents into two daughter cells in the process known as cytokinesis, which is the final stage of the cell cycle [8]. The PRC1, also known as Ase1(yeast)/MAP65 (plant) gene was originally identified as a CDK substrate in an in vitro phosphorylation screen and was subsequently found to be a mid-zone-associated protein required for cytokinesis [9]. PRC1 is phosphorylated by CDK1 (Cdc2/cyclin B) in early mitosis and turns into an inactive and monomeric state [10]. During the metaphase-anaphase transition, it is dephosphorylated and interacts with KIF4, a kinesin motor that translocates PRC1 along mitotic spindles towards the plus end of antiparallel interdigitating microtubules. The dephosphorylated PRC1 protein bundles the antiparallel interdigitating microtubules to establish the mid-zone that is necessary for cytokinesis [11].
In addition to its fundamental role in cytokinesis, accumulating evidence also suggests that PRC1 appears to be linked with human carcinogenesis. PRC1 is overexpressed in a variety of cancers, including breast cancer [12], bladder cancer [13], hepatocellular carcinoma [14,15] and pancreatic cancer [16]. Knockdown of PRC1 using siRNA significantly suppresses the growth of breast and bladder cancer cells, indicating its crucial role in proliferation of cancer cells, and also suggesting PRC1 is a promising molecular target for human cancer treatment [12,13]. To date, however, the impact of PRC1 expression on gastric carcinoma patient survival and its potential oncogenic role and molecular mechanisms in gastric carcinoma has not been elucidated. In this study, we studied PRC1 expression status and its clinical significance in gastric carcinoma. Both in vitro and in vivo functional assays were performed to characterize the biological effects of PRC1 in gastric carcinoma. More importantly, we demonstrate, for the first time, that PRC1 can be targeted by piperlongumine (PL), an agent that has been previously proved to suppress gastric cancer cells by our group [17], via a p53-dependent mechanism. Our findings shown in this study suggest that PRC1 might play critical roles in tumour cell growth and be a promising target for the development of novel anticancer drugs to gastric carcinoma.

Gastric cancer cell lines and clinical samples
Human gastric cancer cell lines AGS and HGC27 were purchased from American Type Culture Collection of the Chinese Academy of Sciences (Shanghai, China) and were cultured in RPMI 1640 (Wisent Biotec, Co. Ltd. Montreal, QC, Canada) containing 10% foetal bovine serum (Wisent Biotec, Co. Ltd) in a humidified 5% CO 2 atmosphere at 37°C. A total of 17 primary gastric carcinomas and their paired non-cancerous gastric mucosal tissues were obtained from patients who underwent curative surgery in 2013 at the Department of Gastrointestinal Surgery (Nanjing Drum Tower Hospital, China) after obtaining written informed consent. All specimens were immediately snapped-frozen in liquid nitrogen and stored at À80°C until processing. Archival tissue blocks from 133 patients with gastric adenocarcinoma were retrieved from the Department of Anatomical and Cellular Pathology, Prince of Wales Hospital, the Chinese University of Hong Kong and arranged in tissue array blocks and have been described elsewhere [18][19][20]. All experiments were conducted and approved in accordance with the guidelines of ethics committees of Nanjing University and the Chinese University of Hong Kong.

Lentivirus production and transduction
Lentivirus expressing shPRC1#1, shPRC1#2 or shControl was produced as described previously [23]. Cells were transduced with appropriated amount of lentivirus for 24 hrs in the presence of polybrene (5 lg/ml). Then, the supernant was removed and replace with fresh medium. The cells were cultured for 4-6 days and then harvested for the experiments.

Cell proliferation assay
Cell proliferation in response to PRC1-silencing was measured by Cell Counting Kit-8, according to manufacturer's instructions (Dojindo, Kumamoto, Japan). Cells were seeded in 96-well plates after transduction of lentivirus in 6-well plates for 48 hrs. The result was expressed as the maximum absorbance at 450 nm, based on three independent experiments.

Colony formation assay
Cells were seeded into new 6-well plates (1000 cells per well) after transduction of lentivirus in 6-well plates for 2 days and incubated with medium containing 5 lg/ml of puromycin for 14 days. Colonies were fixed with methanol for 15 min. and stained with 0.5% crystal violet for 20 min. at room temperature sequentially. The number of colonies with more than 50 cells was counted manually under a microscope. Experiments were performed in triplicate.

RNA extraction and Quantitative real-time PCR
Total RNA in tissue specimens or cells was isolated using TRIzol reagent, and reverse transcription was carried out with 500 ng RNA in a total 10 ll reaction volume using PrimeScript TM RT Master Mix (TaKaRa, Shuzo Co., Ltd., Kyoto, Japan) according to the manufacturer's instructions. Quantitative real-time PCR experiments were carried out with the Step-One PCR System (Applied Biosystems, Foster City, CA). The sequences of the amplification primers are listed in Table S1. Real-time RT-PCR data analysis was conducted according to the 2 ÀDDCt methods using the threshold cycle (Ct) values for target genes and b-actin, as an endogenous control gene. All assays were performed in triplicate.

Western blot
Western blotting was performed as previously described [24,25]. Proteins were resolved by SDS-polyacrylamide electrophoresis and were transferred to a nitrocellulose membrane and immunoblotted with antibodies as indicated, followed by appropriate HRP-conjugated secondary antibodies. Signals generated by enhanced chemiluminescence (Millipore, Billerica, MA, USA) were recorded with a CCD camera (CLINX, Shanghai, China). Data are representative of at least three independent experiments.

Migration and invasion assays
Cell migratory and invasive abilities were assessed by way of transwell (Corning Life Sciences, Bedford, MA, USA) and Matrigel invasion (BD Biosciences, San Jose, CA, USA), respectively, which has been described previously [26]. Briefly, cells after 3 days of transduction were resuspended and cultured in medium containing only 1% FBS in the upper compartment of the chamber, whereas regular medium containing 10% FBS was added into the lower compartment of chambers. For transwell migration assay, 5 9 10 4 cells/well were seeded, whereas 8 9 10 4 cells/well were seeded for the invasion assay. After 24 hrs, the cells on the topside of the membrane were scraped using cotton swab, and the cells that migrated to the bottom were fixed with methanol and stained with 0.5% crystal violet. The migrated or invaded cells were counted in five sequential visual fields under 400-fold magnification. Each experiment was performed in replicate inserts, and mean value was calculated from three independent experiments.

Cell-cycle and apoptosis analysis
For cell-cycle analysis, cells were synchronized with serum starvation for 24 hrs and then transduced with lentivirus on 6-well plates for 4 days. Cell-cycle analysis was conducted using BD Cycle test TM plus DNA Reagent Kit (340242; BD Biosciences). Cell-cycle distribution was determined using a flow cytometer (BD FACSAria TM II, San Jose, CA, USA). Apoptotic cell death was measured by Annexin V fluorescein isothiocyanate (FITC) Apoptosis Detection Kit I (556547; BD Biosciences) according to the manufacturer's instructions. AGS and HGC27 cells transduced with indicated shRNAs were collected and washed with cold PBS twice and then were resuspended in 100 ll of Annexin V binding buffer and incubated with 5 ll of FITC-conjugated Annexin V and 5 ll of propidium iodide for 15 min. in the dark. Annexin V binding buffer (400 ll) was then added. Finally, the cells were analysed by BD FACSCanto II flow cytometer (BD Biosciences). The results were obtained from at least three independent experiments with triplicated wells each time.

Phalloidin and PKH67 staining
Phalloidin staining was conducted as previously described [26]. Cells transduced with lentivirus were replated on glass coverslips. Cells were first fixed in 4% ice-cold paraformaldehyde for 15 min. on ice. Subsequently, fixed cells were blocked with 1% bovine serum albumin (BSA)/ PBS for 1 hrs. FITC-Phalloidin (5 lg/ml, Sigma-Aldrich) was then used to stain filamentous actins for 2 hrs at room temperature. Fluorescence was observed using an inverted fluorescence microscope from Carl Zeiss (Axio Observer A1; Zeiss, Germany). PKH67 dye was used for cell membrane labelling. According to the manufacturer's instructions, the cells were rinsed with serum-free medium once. Dilution C was used to resuspend the cells, with PKH67 dye (2 lg/ml) added, and the cells were subcultured for three generations. Residual fluorescence was observed using an inverted fluorescence microscope from Carl Zeiss as mentioned above.

DNA transfection and luciferase reporter assay
DNA transfection and luciferase reporter assay were performed as previously described [26]. Cells were plated to 24-well plates 24 hrs prior to transfection experiments. Transfection experiment was carried out by adding 380 ng of reporter plasmid along with a pRL-TK reporter plasmid (20 ng) as a control for transfection efficiency. All assays were performed using a Dual Luciferase Reporter Assay System (Promega).

Chromatin immunoprecipitation
Chromatin immunoprecipitation (ChIP) assay was conducted as previously described [25]. Primers used in ChIP-qPCR experiments are shown in Table S2. Occupancy of p53 protein was normalized to input and expressed as a percentage of the input DNA (% Input). Data were obtained from at least three independent experiments.

Tumour xenograft model in nude mice
All animal procedures and care were approved by the Institutional Animal Care and Use Committee of Nanjing Drum Tower Hospital, Medical School of Nanjing University. HGC-27 cells (~3 9 10 6 cells/mice) transduced with indicated lentiviral shRNAs were resuspended in 100 ll serum-free medium and gently mixed with the same volume of ice-cold Matrigel and were injected subcutaneously into the right flank of 4-to 6-week-old female BALB/c nu/nu nude mice (n = 5). Tumour size was measured and calculated as previously described [27]. Tumour volume and animal weight were measured every 3 days. Mice were killed when control group tumours reached~1000 mm 3 . Tumours were then excised and weighed.

Statistical analysis
Data were analysed with GRAPHPAD PRISM 6.00 (GraphPad Software, La Jolla, CA, USA). Data were expressed as the means AE S.E.M. of at least three independent experiments. Statistically significant differences between the experimental and control groups were identified by Student's t-test, ordinary one-way ANOVA, and two-way ANOVA followed by Dunnett's multiple comparisons test. Differences were considered statistically significant when P < 0.05.

PRC1 is aberrantly expressed in gastric carcinoma and correlates with patients' survival
We first determined and compared PRC1 mRNA expression of 17 pairs of samples from patients who underwent resection of primary gastric carcinoma using qRT-PCR. As shown in Figure 1A, mRNA level of PRC1 gene is markedly increased in gastric carcinomas relative to the paired adjacent non-tumour tissues. To generalize our finding, we recapitulated PRC1 gene expression from the large cohorts of gastric cancer patients that are available from the GEO database (accession numbers GSE63089, GSE27342 and GSE65801) and data are given as scatterplots. Consistently, PRC1 gene expression was significantly up-regulated in all three different gastric cancer cohorts (Fig. 1B). We then compared the PRC1 expression level between different subtypes of gastric cancer by analysing the TGGA cohort. With regard to the Lauren classification, PRC1 mRNA level is significantly higher in intestinal type when compared with diffuse type (Fig. S1A). In addition, when dividing the cohort into four molecular subtypes, PRC1 mRNA is abundantly expressed in MSI groups, then CIN and EBV groups. It is relatively lowly expressed in GS group (Fig. S1B). Further analysis revealed that MSI gastric cancers are enriched for the intestinal histological variant (Fig. S1C); thus, further study is warranted to investigate why PRC1 mRNA level is elevated in intestinal type of gastric cancer. Next, computational analysis using the KM plotter integrative bioinformatic interface [28] revealed a significant correlation between high PRC1 mRNA expression and poor overall and disease-free survival (Fig. 1C). Western blotting showed that upregulation of PRC1 protein was detected in 7 of 13 (53.8%) randomly selected gastric carcinomas. Statistical analysis revealed that gastric cancer tissues expressed a significantly higher level of PRC1 protein than adjacent non-tumoural tissues ( Fig. 1D and Fig. S1D). Immunohistochemistry was then performed to assess the PRC1 protein expression in tissue microarrays containing 133 formalin-fixed, paraffin-embedded gastric carcinomas. The cancer cells often exhibited cytoplasmic expression of PRC1 (Fig. 1E). Negative staining of PRC1 was observed in 7 % (10/133) of samples, weak staining was noted in 38% (50/133), and strong immunoreactivity was seen in 55% (73/133) of the samples, respectively. PRC1 expression in immunohistochemistry did not show association with any clinicopathologic parameter (Table S3). More intriguingly, overexpression of PRC1 was correlated significantly with shorter overall survival (Fig. S1E) and shorter diseasespecific survival of patients (Fig. 1F). These results indicated that PRC1 expression was significantly increased in gastric cancer tissues and the up-regulation may represent a novel marker for patients' prognosis.  We next examined whether PRC1 is required for the tumorigenic phenotypes of gastric cancer cells by silencing PRC1 expression with short hairpin RNA. The introduction of shRNA into the gastric cancer cell lines AGS and HGC27 dramatically decreased the expression level of PRC1 relative to control cells expressing scrambled control shRNA ( Fig. 2A and B). Functional assays revealed that PRC1 depletion significantly decreased the cell growth as determined by CCK-8 assay (Fig. 2C), and colony formation efficiencies as determined by colony formation assay (Fig. 2D) compared with control cells. To further investigate whether PRC1 knockdown suppresses tumour growth in vivo, we generated stable PRC1 knockdown HGC27 cell population. When the stable cells were injected into mice subcutaneously, the PRC1-depleted tumours grew much more slowly than the scramble control tumours did ( Fig. 2E and F). The mean tumour weight from PRC1-depleted cells was less than one-third of those from scramble control cells after 21-day growth in vivo (Fig. 2G). Collectively, these data indicate that PRC1 is essential for gastric cancer cell proliferation both in vitro and in vivo.

Knockdown of PRC1 promotes cell-cycle arrest and apoptosis
To further test whether PRC1 depletion regulates the cell cycle, we knocked down PRC1 in AGS and HGC27 cells and then examined the cell cycle using fluorescence-activated cell sorting (FACS) analysis. As expected, PRC1 depletion in AGS cells significantly increased the percentage of cells in the G2 phase and decreased the percentage of cells in the G1 phase (Fig. 3A, upper). In contrast, PRC1 depletion in HGC27 cells significantly increased the percentage of cells in the G1 phase and decreased the percentage of cells in the G2 phase (Fig. 3A,  lower). Thus, it is likely that PRC1 depletion block cell-cycle progression at different phases. To validate these results and further investigate why PRC1 reduction affects cell-cycle distribution differently between the two cell lines, we examined the expression of cell-cycle regulators operative in G1 and G2 phases, including CDK1, CDK2, CDK6, cyclin B1, cyclin E1, p27 and GADD45a. As shown in Figure 3B, when PRC1 was depleted in AGS cells, expression of CDK1 and cyclin B1 was obviously inhibited, whereas when PRC1 was silenced in HGC27 cells, expression of cyclin E1 and CDK6 was obviously reduced. However, no significant changes were observed in the expression of other regulators, including cyclin A and CDK2 (Fig. 3B and data not shown). Furthermore, p27 and GADD45a, which inhibits cell-cycle progression from G2 to M phase in gastric cancer cells [24,29], were remarkably increased in AGS cells but not HGC27 cells when PRC1 was depleted (Fig. 3B). These data suggest the discrepancy of cell-cycle distribution may be due to the selective induction of p27 and GADD45a in gastric cancers following PRC1 depletion. We next investigated the potential apoptotic effect of PRC1 knockdown using the FACS assay. As shown in Figure 3C and D, sh1-and sh2transduced cells underwent apoptosis much more frequently than the negative control shRNA-transduced cells. Consistent with this observation, transduction with sh1 and sh2 in cells induced the cleavage of caspase-9, caspase-3 and PARP compared with the control shRNAtransduced cells (Fig. 3E). These results indicate that PRC1 gene silencing inhibits gastric cancer cell proliferation at least partially through induction of cell-cycle arrest and cellular apoptosis.

Knockdown of PRC1 impedes gastric cancer cell migration and invasion
Next, we investigated the effect of PRC1 depletion on migratory capacity of gastric cancer cells. Transwell migration assay revealed significantly reduced migrated cells in PRC1-depleted cells compared with that in control (Fig. 4A). Concordantly, knockdown of PRC1 gene also impaired cell invasion through Matrigel (Fig. 4B). Together, these data suggested a role of PRC1 in the motility and invasiveness of gastric cancer cells.

Knockdown of PRC1 selectively impairs cytokinesis in AGS but not HGC27 cells
Flow cytometry demonstrated that AGS cells transduced with PRC1 shRNAs contained a significantly larger number of tetraploid cells (Fig. S2), indicating that PRC1 genetic silencing impaired the elaborate co-ordination of nuclear division and cytokinesis. Consistently, fluorescence assay revealed that AGS cells expressing PRC1 shRNAs exhibited increased nuclear abnormality characterized by multi-nuclei formation, whereas HGC27 cells expressing PRC1 shRNAs exhibited no obvious nuclear abnormality compared with the control cells (Fig. 5A). To further identify whether PRC1 depletion blocked cytokinesis in AGS but not HGC27 cells, cell membrane was stained using the fluorescent dye PKH67. After continuous subculture for two generations, fluorescent microscopy found that the AGS cells but not HGC27 cells transduced with PRC1 shRNAs contained significantly more residual PKH67 dye than the control group (Fig. 5B), indicating that PRC1 depletion selectively postponed cytokinesis in AGS cells. We next sought to determine the underlying mechanism accounting for the differential role of PRC1 on cytokinesis. Our data further showed that PRC1 depletion probably disrupted mitotic checkpoint signalling because genes involved in mitotic arrest, including KIF4A, BUB1B, AURKB, NEK2, NUSAP1, MELK and TOP2A, were commonly repressed on PRC1 knockdown in AGS cells instead of HGC27 cells (Fig. 5C).

Piperlongumine inhibits PRC1 expression via a p53-dependent mechanism
Our recent work has identified that PL induces gastric cancer cell apoptosis and G2/M cell-cycle arrest both in vitro and in vivo. [17] Notably, we have demonstrated that PL induces p53 expression in the p53 wild-type AGS cells but not in the p53-mutated HGC27 cells [17]. Previously it was reported that p53 directly suppresses PRC1 gene transcription via interaction with PRC1 gene promoter in a variety of cancer cells [21]. Consistently, our data revealed that ectopic expression of p53 also suppresses PRC1 expression in gastric cancer cells ( Fig. 6A and B), indicating PRC1 is a common downstream target of p53. Furthermore, PL selectively suppresses PRC1 expression in AGS cells at both mRNA and protein levels where p53 was induced concordantly. However, it failed changing PRC1 expression level in HGC27 cells which revealed no p53 induction (Fig. 6C and D). We next determined whether PL suppresses PRC1 gene expression in a p53-dependent manner by knocking down p53 using siRNA targeting p53 mRNA. As expected, p53 depletion partly attenuated PRC1 inhibition led by PL treatment (10 lM) in AGS cells (Fig. 6E). To further validate the p53-dependent regulatory role of PL on PRC1, AGS cells were first transfected with different PRC1 promoter-driven luciferase reporter vectors (as shown in Fig. S3) and TK-Renilla reporter vector, followed by PL treatment for 24 hrs, and then, the PRC1 promoter activities in transfected cells were determined. Shortening of the PRC1 promoter progressively reduced the luciferase activity of the reporter vectors. Further shortening to 159 bp resulted in a drastic drop in activity which was even barely detected. In addition, PL led to further reduction in the luciferase activity of pGL3-1744, pGL3-1504, pGL3-904, pGL3-289 and pGL3-232, respectively (Fig. 6F). Similar results were obtained in AGS and HGC27 cells when transfected with p53-expressing plasmids or empty vector (Fig. S4A). These data suggested that the major PL-related functional promoter may located within the region between À232 and À159 bp of the PRC1 promoter, which coincides with the p53-responsive region identified by Li et al. [21] To further elucidate whether PL-induced p53 interacts with the functional PRC1 promoter in vivo, ChIP analysis was conducted in AGS cells treated or untreated with PL using anti-p53 antibodies followed by quantitative PCR amplification of amplicons that spread across a 4-kb region of the gene. As shown in Figure S4B, although the signal of p53 at all loci in control cells was very low, there was a remarkable enhancement of p53 signal within the region from À281 to +76 of the PRC1 gene that contained the p53-responsive region when cells were treated with PL. Concordantly, an amplicon spanning the core promoter of p21 gene was also enriched by the p53 antibody, suggesting the validity of the ChIP assay (Fig. S4B). Collectively, these data suggested that PL targets PRC1 via a p53dependent manner in gastric cancers.

Discussion
Gastric cancer is a lethal disease with dismal prognosis, and it can be initiated and driven by a multitude of intrinsic and extrinsic factors [30]. Despite the increasing knowledge on risk factors, the molecular mechanisms underlying gastric carcinogenesis have yet been  completely deciphered. In this study, we verified that PRC1 is significantly overexpressed in gastric cancers and that its overexpression is strongly associated with poor prognosis of patients. We further demonstrated that PRC1 is essential for the survival of gastric cancer cells. In addition, PRC1 was discovered to be a novel target of PL, which functions as a novel therapeutic agent targeting various types of human malignancies including gastric cancers [17,[31][32][33][34].
PRC1 was originally identified as a CDK substrate in an in vitro phosphorylation screen and was subsequently shown to be a mid-zone-associated protein required for cytokinesis [9]. PRC1 forms oligomers in vivo and has microtubule-binding and microtubule-bundling activities [9,10]. Perturbing the function of PRC1 or PRC1-related orthologs in various species inhibits the formation of mid-zone interdigitating microtubule bundles, resulting in two disarrayed half spindles [10,[35][36][37]. PRC1 is negatively regulated by p53 and overexpressed in p53-defective cells [21], suggesting that the gene is tightly regulated in a cancer-specific manner. However, studies on the functional role of PRC1 in malignancies have been quite limited. Shimo et al. first reported that significant overexpression of PRC1 in breast cancer and demonstrated its critical roles in tumour cell growth [12]. PRC1 was also found to be up-regulated in HCC samples and cooperated with Wnt signalling in a positive feedback loop to promote early recurrence [14,15]. Insofar as we know, PRC1 has not been previously implicated in gastric carcinogenesis. To the best of our knowledge, our current study is the first report describing an oncogenic role of PRC1 in regulating the growth and mobility of gastric cancer. This was supported by the infringed proliferation and motility ability by PRC1 inhibition. Gene silencing of PRC1 impaired cytokinesis and led to formation of multi-nucleated cells in AGS cells. However, depletion of PRC1 gene in HGC27 cells failed in leading to appearance of multi-nucleated cells. Our data indicated that PRC1 depletion down-regulated the mitotic-related genes including KIF4A, BUB1B, AURKB, NEK2, NUSAP1, MELK and TOP2A in AGS instead of HGC27 cells. Therefore, it is likely that PRC1 gene knockdown could contribute to cancer repression via mechanisms independent of its cytokinesis-related function. The precise molecular mechanisms by which PRC1 preferentially modulates multi-nucleation in AGS cells are under investigation in our laboratory and remain to be elucidated.
Another intriguing finding is the identification of PRC1 as a novel target of PL. PL is a biologically active component from long pepper and has been reported to kill multiple types of cancer cells by targeting the response to reactive oxygen species (ROS) and exerts antitumour activities in a variety of animal models [32,34,38,39]. We previously reported that PL significantly suppressed gastric cancer both in vitro and in vivo by increasing ROS generation and GADD45a expression and decreased the expression of telomerase reverse transcriptase (TERT) gene [17]. We now further demonstrated PL also targets PRC1 gene transcriptionally in gastric cancer cells in a functional p53-dependent manner. Recent genome study has confirmed the high frequency of p53 gene mutations in cancer [40]. In gastric carcinomas, p53 gene is mutated in nearly half of the cases [6,30], and clinical studies have shown that mutant p53-carrying tumours respond less well to conventional chemotherapeutic drugs and have worse prognosis than wild-type p53-carrying tumours [30]. Given the high frequency of p53 gene mutations in a wide range of human tumours, the development of efficient mutant p53-reactivating anticancer drugs has emerged as a promising strategy for improved cancer therapy. Thus far, several small molecules (i.e. APR-246 and ReACp53) which restore wild-type activity of mutant p53 have been identified using various approaches [41][42][43][44]. Further prospective studies are anticipated to identify whether these mutant p53-reactivating anticancer drugs may function with PL in a synergistic manner to block gastric carcinogenesis.
To summarize, we for the first time report the essential role of microtubule-associated gene PRC1 in gastric carcinogenesis. Our data should contribute to a better understanding of the molecular mechanism of gastric carcinogenesis and indicate that PRC1 is a promising molecular target for gastric cancer treatment.

Disclosure statement
The authors have no conflict of interest.

Supporting information
Additional Supporting Information may be found online in the supporting information tab for this article: Table S1 qRT-PCR primers.      Figure S4 (A) Normalized luciferase activity of various PRC1 promoter reporters in AGS and HGC27 cells transfected with p53-expressing plasmid (black) or empty vector (white). Results are the mean AE S.D. of triplicate measurements. Data shown are results of a representative from 3 independent experiments. *P < 0.01. (B) ChIP-qPCR analysis of the abundance of p53 at PRC1 gene in AGS cells treated with piperlongumine at 10 lM or with vehicle control for 24 hrs. Signal of p53 at the p21 gene promoter was used as a positive control. As a negative control, p53 antibody was replaced by IgG (not shown). Values are expressed as % of input. Results represent means AE S.D. from at least three independent experiments *P < 0.05 by Student's t-test.