Du145, C4-2 and PC3 cells were purchased from the American Type Culture Collection (ATCC; Manassas, VA, USA). Du145, C4-2 and PC3 cells were cultured in Dulbecco's modified Eagle's medium (DMEM), RPMI-1640 and DMEM/F12 medium respectively, with 10% (v/v) fetal bovine serum (FBS; Gibco/Life Technologies; Thermo Fisher Scientific, Inc., Grand Island, NY, USA) and 1% (v/v) penicillin and streptomycin at 37°C in a humidified incubator supplemented with 5% CO₂. Docetaxel was obtained from Selleck Chemicals (Houston, TX, USA). PC3 monoclonal cells stably expressing pGPU6-GFP-Neo-PARP1-siRNA-1706 or pGPU6-GFP-Neo-PARP1-siRNA-NC were generated and cultured in the presence of G418 (0.5 mg/ml).

siRNA that targeted the human PARP1 gene was synthesized by RiboBio Co., Ltd. (Guangzhou, China). The synthesized siRNA sequences were the following: siRNA-NC, 5′-UUCUCCGAACGUGUCACGUTT-3′; siRNA-1706, 5′-GAGGAAGGUAUCAACAAAUTT-3′; siRNA-2003, 5′-GAGCACUUCAUGAAAUUAUTT-3′; and siRNA-2907, 5′-GAGACCCAAUAGGUUAAUTT-3′.

Transfection of synthesized siRNAs was carried out using Lipofectamine 2000 (Invitrogen; Thermo Fisher Scientific, Inc., Carlsbad, CA, USA) according to the manufacturer's instructions.

Du145 cells were plated in 96-well plates. After 24 h, the cells were transfected with PARP1-siRNA-NC or PARP1-siRNA-1 using a riboFECT™ CP Transfection Kit from RiboBio Co., Ltd. (Guangzhou, China). Approximately 72 h later, the cells were replaced with DMEM without FBS and incubated for another 24 h. 3-(4,5-Dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) was added to cells for a 3 to 4-h reaction, and dimethyl sulfoxide (DMSO) was applied to dissolve the sediment. The optical density of the MTT results was assessed at a wavelength of 490 nm and the cell survival rate was normalized.

Cells were transfected with the indicated siRNA and cultured to confluence. Then, an artificial scratch wound was generated using a 200-µl pipette tip, and cells were washed to remove the detached cells and debris. Cell migration was captured using an inverted microscope and evaluated by measuring the difference in the wound width and counting the migrated number of cells.

Forty-eight hours after transfection, cells were washed with phosphate-buffered saline (PBS), trypsinized, and reseeded into 6-pore dishes at a density of 500 cells/well. Under the same culture conditions, the cells were cultured for another 9–12 days, and then fixed and dyed with 4% crystal violet in ethanol. The number of cell colonies was calculated.

Cell invasion was evaluated using a Transwell^® permeable support invasion chamber (Corning Incorporated, Corning, NY, USA) according to the manufacturer's protocol. Forty-eight hours after transfection, the cells were cultured in the insert for 48 h and stained with Mayer's hematoxylin. Eight fields in each sample were imaged, and invasion was evaluated by counting the invaded number of cells.

All animal experimental procedures were carried out in accordance with a mouse protocol, which was approved by the Animal Research Committee of Guangzhou Medical University. Male nude mice (aged 8 weeks; weighing 18–20 g) were fed a standard diet and water during the study. Fifteen mice were randomly assigned to 3 experimental groups evenly (5 mice in each group): PC3-siRNA-1706, PC3-siRNA-NC and PC3 C (control). PC3 monoclonal cells stably expressing pGPU6-GFP-Neo-PARP1-siRNA-1706 or pGPU6-GFP-Neo-PARP1-siRNA-NC were inoculated subcutaneously in the flank regions of the mice to establish the tumor model. Following subcutaneous inoculation, the mice were examined daily for tumor formation and growth. The sizes of the subcutaneous tumors were monitored twice weekly by assessing the length and width. The tumor volumes were calculated using the formula: Volume (mm³) = length (mm) × width² (mm²)/2. At 40 days after inoculation, all experimental mice were euthanized for further study.

For protein extraction, cells or homogenized tissue were rinsed with PBS and treated in ice-cold lysis buffer. After scrape, collection and ultra-sonication, the lysate solutions were centrifuged at 12,000 RCF for 30 min at 4°C. The Bradford protein assay method was used to assess the protein concentration. Equal amounts of protein were separated on a 10% SDS polyacrylamide gel and transferred to a NC membrane (Millipore, Billerica, MA, USA). The membranes were blocked in 5% non-fat dry milk in PBS-T for 2 h at room temperature and incubated with primary antibodies appropriately diluted in PBS-T overnight at 4°C. Primary antibodies against PARP1 (1:200; mouse, monoclonal; cat. no. sc-8007), vimentin (1:200; mouse, monoclonal; cat. no. sc-6260) and GAPDH (1:1,000; mouse, monoclonal; cat. no. sc-47724) were obtained from Santa Cruz Biotechnology (Santa Cruz, CA, USA). EGFR (1:2,000; rabbit, monoclonal; cat. no. 1902-1) was obtained from Epitomics (Cambridge, UK). Anti-tubulin (1:2,000; rabbit, ployclonal; cat. no. 2148), GSK3-β (1:1,000; rabbit, monoclonal; cat. no. 12456), pGSK3-β (ser9) (1:1,000; rabbit, monoclonal; cat. no. 5558), E-cadherin (1:1,000; mouse, monoclonal; cat. no. 14472), Cx43 (1:1,000; rabbit, ployclonal; cat. no. 3512) and cleaved-caspase3 (1:1,000; rabbit, monoclonal; cat. no. 9664) were purchased from Cell Signaling Technology, Inc. (Danvers, MA, USA). The membranes were incubated with secondary antibodies appropriately diluted in PBS-T for 2 h at room temperature. The secondary antibodies were anti-mouse or anti-rabbit IgG conjugated to HRP (Santa Cruz Biotechnology). The membranes were developed for chemiluminescence detection using an ECL detection kit (Thermo Fisher Scientific, Inc., Waltham, MA, USA). The band intensities were quantified using ImageJ software, and all western blot analyses were repeated 3 times.

Total RNA was extracted with a MiniBEST Universal RNA Extraction kit (Takara, Otsu, Japan). An equal amount of RNA was reverse transcribed to cDNA and amplified by PCR according to the manufacturer's protocol (Takara). Quantitative real-time PCR was performed using the SYBR-Green PCR Master mix (Takara) according to the manufacturer's protocol. The primers were as follows: PARP1 forward, 5′-AAGGCGAATGCCAGCGTTAC-3′ and reverse, 5′-GGCACTCTTGGAGACCATGTCA-3′; E-cadherin forward, 5′-TCCTCCCAATACATCTCCCTTCA-3′ and reverse, 5′-TCTCCGCCTCCTTCTTCATCATA-3′; vimentin forward 5′-TTCGCCAACTACATCGACAAGG-3′and reverse, 5′-TTCAAGGTCAAGACGTGCCAG-3′; GAPDH forward, 5′-GCACCGTCAAGGCTGAGAAC-3′and reverse, 5′-AUAAUUUCAUGAAGUGC-UCTT-3′. The real-time PCR reaction was maintained at 95°C for 5 min, followed by 40 cycles at 95°C for 15 sec, 57°C for 30 sec and 72°C for 20 sec. All samples were processed in triplicate, and all values were normalized to the expression levels of GAPDH. The relative quantification was determined by the comparative Ct method.

The tumor tissues were washed with PBS, fixed with 4% paraformaldehyde, paraffin-embedded, and sliced to a 3-µm thickness to generate serial sections. Slices were dewaxed in xylene for 10 min and placed into a mixture of alcohol and xylene (1:1) for ~5 min. After the slices were sequentially immersed in 100, 95, 85 and 70% alcohol for 2 min per step, they were placed in distilled water before staining. Then, the slices were stained with hematoxylin for ~10 min. Subsequently, the slices were immersed into water for 1 sec, and then 0.5–1% hydrochloric acid alcohol was used for a short time to clear the excess hematoxylin. The time was controlled by microscopy until the nuclei and nuclear chromatin were clear. After rinsing with water for 15–30 min, the nuclei became blue, and the slices were rinsed again in distilled water (short time). Thereafter, 0.5% eosin was applied to stain the slices for 1 min, and the slices were sequentially immersed in 70, 85, 95, and 100% alcohol for dehydration for 2–3 min per step. The slices were then cleared with xylene two times (5 min/time). Excess xylene in the sections was wiped away, and the specimens were not dried. A small amount of neutral gum was quickly added, and the glass was covered to seal the specimens.

All sections were formalin-fixed, paraffin-embedded and stained according to the standard immunohistochemistry protocol. Following incubation with primary antibodies for PARP1, E-cadherin and vimentin, the sections were incubated with biotinylated secondary antibodies (Maixin Biotech Co., Ltd., Fuzhou, China). Subsequently, the avidin-biotin horseradish peroxidase complex and diaminobenzidine chromogen (Maixin Biotech Co., Ltd.) were used as the detection system. Each stained slide was assessed by an experienced laboratory technician independently in a blinded fashion without any information regarding the group.

Statistical analysis was performed using SPSS 16.0 software (SPSS, Inc., Chicago, IL, USA). All data are expressed as the mean ± SD. The mean differences (P<0.05) were compared using a one-way ANOVA (>2 groups) or Student's t-test (2 groups). Western blot data was analyzed with ImageJ software. In the figures, an asterisk represents P<0.05 compared to the corresponding group and indicates a statistically significant result.

Two small interfering RNAs (1706 and 2003) used to knock down the expression of PARP1 in PC3 cells in our previous study (10), were synthesized and transfected into PC3 cells. Real-time PCR and western blot analyses revealed that both PARP1 siRNAs, 1706 and 2003, significantly decreased PARP1 mRNA and protein expression compared to the negative control in PC3 cells (Fig. 1A and B). Then, we investigated the effect of PARP1-siRNA on the migration and invasion capacity of PC3 cells. The results of the wound healing assays revealed that both siRNA-1706 and siRNA-2003 significantly suppressed the migration capacities of PC3 cells in vitro (Fig. 1C). The results of the Transwell invasion assays revealed that both siRNA-1706 and siRNA-2003 significantly suppressed the invasion capacities of PC3 cells in vitro (Fig. 1D).

Since siRNA-1706 was more effective than siRNA-2003 in suppressing the expression of PARP1, siRNA-1706 was further used in the following experiment. Colony formation assays were analyzed, and the results revealed that the siRNA-1706 group grew significantly more slowly than the negative control group (Fig. 1E). In addition, our previous study confirmed that PARP1 siRNAs could inhibit the EGFR/Akt/FoxO pathway. In the present study, we found that PARP1-siRNA could inhibit p-GSK-3β which is also downstream of AKT (Fig. 1F). Therefore, PARP1 knockdown may suppress PCa progression by inhibiting EGFR/Akt and its downstream including the FoxO and GSK-3β signaling pathway. To further investigate whether PARP1 knockdown reduced the tumorigenesis and metastasis capacity of PCa cells in vivo, we constructed a stable PARP1 knockout model in PC3 cells by recombinant plasmid. The PCR and western blot results revealed that stable transfection of the plasmid could effectively inhibit the expression of PARP1 (Fig. 1G).

After a stable PARP1 knockout model was constructed and confirmed, we investigated whether PARP1 knockdown could reduce the tumor growth and tumorigenesis capacity of PCa cells in vivo. The results revealed that in comparison with the PC3 control (C) group and the PC3-siRNA-negative control group (NC), the tumor growth in the PC3-siRNA-1706 group was markedly reduced (Fig. 2A and B). The mean volumes and wet weights of the tumors from the PC3-siRNA-1706 group were significantly decreased in comparison with the PC3 control group and the PC3-siRNA-NC group (Fig. 2C and D). The H&E staining results revealed that the control group and no-load group promoted a larger cell volume, rich in cytoplasm, in which the nucleus was larger and deeply stained; the nuclei were round and had prominent nucleoli. In this group, mitotic activity was more common, the nucleus/cytoplasm ratio was increased, cells were rich in cytoplasm, and there was a higher number of interstitial vessels. The PC3-siRNA-1706 group exhibited smaller PC3 cells in the nude mice with smaller cytoplasms, in which the nuclei were small, round and lighter in color; the nucleoli were obvious, the nucleus/cytoplasm ratio was smaller, and there were fewer interstitial blood vessels (Fig. 2E).

Since EMT is a key step for cancer cells to acquire invasion and metastasis abilities (11), we analyzed the expression of some EMT-associated markers in PC3 cells by PARP1 knockdown. Real-time PCR analysis revealed that PARP1 knockdown significantly upregulated the mRNA level of E-cadherin, which was accompanied with significant downregulation of vimentin mRNA (Fig. 3A). The western blot analysis results further validated the increased E-cadherin expression and decreased vimentin expression in PC3 cells after knockdown of PARP1 (Fig. 3B). To further explore whether PARP1 knockdown suppressed EMT in PCa cells in vivo, we detected the expression of EMT-associated markers in xenograft tumors. Consistent with the in vitro results, real-time PCR and western blot analyses revealed the same results in vivo (Fig. 3C and D). Immunohistochemical analysis revealed that, compared with the PC3 control group and the PC3-siRNA-NC group, the PC3-siRNA-1706 group had a higher level of the E-cadherin protein and a lower level of the vimentin protein (Fig. 3E).

In order to further confirm the function of PARP1 siRNAs in other PCa cells, siRNA-1706, siRNA-2003, siRNA-2907 and non-specific siRNA [negative control (NC)] were used to knock down the expression of PARP1 in C4-2 cells. The results revealed that siRNA-1706 was the most efficient and was further used in the following experiment (Fig. 4A). After PARP1 knockdown by PARP1 siRNA-1706, EGFR was decreased in C4-2 cells (Fig. 4B). After PARP1 was knocked down by PARP1 siRNA-1706 in C4-2 cells, docetaxel was used to induce apoptosis and the expression of cleaved-caspase3 was detected via western blotting (Fig. 4C). Connexin 43 (Cx43) may be associated with apoptosis since it is a component of gap junctions, therefore its expression was also detected (Fig. 4C). Compared with the NC group, EGFR expression in the PARP1 siRNA-1706 group was significantly decreased in Du145 cells (Fig. 4D). Similar to the C4-2 cells, cleaved-caspase3 and Cx43 expression in the siRNA-1706 group was significantly increased with docetaxel (100 nM) treatment in Du145 cells (Fig. 4E). After PARP1 was knocked down by PARP1 siRNA-1706 in Du145 cells, its proliferation was decreased (Fig. 4F). Collectively, these results indicated that PARP1 knockdown reduced the progression and metastasis capacities of PCa cells both in vitro and in vivo.