Aspirin inhibits growth of ovarian cancer by upregulating caspase-3 and downregulating bcl-2.

The aim of the present study was to investigate the effect and mechanism of different concentrations of aspirin in inhibiting the ovarian cancer of p53N236S gene knock-in mice. In total, 28 male p53S mice, with an age range of 4-6 weeks and weight of 20-25 g were selected. The animals were transplanted with SKOV3 cells to establish subdermal human ovarian cancer. The mice were randomly divided into different groups according to the aspirin concentrations (mmol/l) used, i.e., 0, 1, 2 and 3. Subsequently, intraperitoneal injection was performed once every two days for 3 weeks. The tumor volume, lifetime, tumor cell proliferation inhibition rates, caspase-3 protein and bcl-2 protein expression of the four groups were analyzed and compared. Following aspirin treatment for 1, 2 and 3 weeks, the tumor volume of the 3 mmol/l aspirin group was significantly smaller than the other groups (P<0.05). The higher concentration of aspirin led to a smaller tumor size (P<0.05). The cell proliferation inhibition rate of the 3 mmol/l aspirin group was significantly larger than that of other groups (P<0.05). The relative expression level of caspase-3, bcl-2 protein of the 3 mmol/l aspirin group was significantly improved and reduced, respectively. In conclusion, aspirin can inhibit the growth of ovarian cancer of p53S rats due to its upregulation of the expression of caspase-3 protein and downregulation of the expression of bcl-2 protein.

Introduction

Aspirin is the most traditional antipyretic, analgesic, and anti-platelet drug. Aspirin is effective for obstetrical and gynecological diseases, such as pregnancy-induced hypertension, fetal growth restriction, anti-phospholipid syndrome, assisted reproductive technology, endometriosis, as well as ovarian cancer, endometrial and cervical cancer (1). However, the specific pharmacological mechanism, safety and adverse reactions remain to be investigated. The p53 gene is an important tumor suppressor, although the tumor-promoting effect of p53 knockout mice is not considered optimal (2).

Loss of regulation of tumor cells led to tumor occurrence and the monitoring of the mechanisms involved in DNA damage, such as cell senescence and apoptosis (3). Thus, tumor cells had to reactivate the telomere to prolong the mechanism. Approximately 85–90% tumors occurred by reactivating the telomerase. Telomerase reactivation may escape the inhibition of senescence, promoting the formation of tumor cells whose proliferation was not regulated by the body (4). A further 10–15% tumor maintained the length and function of telomere via a telomere lengthening substitution mechanism (5). Mutant p53 protein (p53N236S) lost the function of inducing cell cycle inhibition and apoptosis, thereby obtaining the functions of promoting tumor formation and metastasis (6). Caspase-3, also known as cysteine protease 32, is a type of apoptosis-related gene that was closely associated with tumor occurrence and development. It constitutes the core position of the apoptotic cascade reaction pathway and is also known as death protease (7). The bcl-2 protein is the encoding product of bcl-2 proto-oncogene. It is located in the mitochondria, endoplasmic reticulum, and continuous perinuclear membrane and may prevent the release of cytochrome c from the mitochondrion to cytoplasm, thereby further inhibiting cell apoptosis (8).

In the present study, p53 N236S (p53S) gene knockout mice were examined and tumor formation effects were shown to be relatively stable (9). By establishing ovarian cancer models, aspirin of different concentrations was used for intervention. Subsequently, the intervention results of different groups were compared and possible molecular mechanisms were determined to provide a new treatment modality for ovarian cancer.

Materials and methods

Animals and reagents

In total, 28 male mice with stable p53S, an age range of 4–6 weeks and weight of 20–25 g, were purchased from the Kunming University of Science and Technology (Yunnan, China). The animals were fed at a room temperature of 22–24°C, with humidity of 50–55%, and a 12 h light/dark cycle was followed. The animals were allowed to adapt to the new environment for 1–2 weeks. Operations were carried out in a laminar airflow clean room. The human SKOV3 ovarian cancer cell strain was donated by the key laboratory of Dongfang Hospital (Beijing, China) and cultured in Dulbecco's modified Eagle's medium (DMEM), as well as high glucose medium containing 10% fetal bovine serum, and 100 U/ml penicillin and 100 U/ml streptomycin. Cell morphology and growth was observed under an inverted microscope (Olympus, Tokyo, Japan), after which the cells were passaged and the medium was changed every 2–3 days.

Aspirin (Sigma-Aldrich, St. Louis, MO, USA) was dissolved into 10 mol/l sodium hydroxide solution with pH 7.0. Aspirin stock solution (50 mmol/l) was prepared in phosphate-buffered saline (PBS) and filter sterilized by 0.22 µmol/l prior to storing at 4°C. Before use, the solution was diluted to different concentrations, i.e., 1, 2 and 3 mmol/l by serum-free medium. PBS was purchased from Solarbio (Beijing, China), dimethyl sulfoxide (DMSO) was purchased from Sigma-Aldrich, and the Cell Counting Kit-8 (CCK-8) was purchased from Dojindo Molecular Technologies, Inc. (Kumamoto, Japan). The two-step general immunohistochemical detection kit was purchased from Dako (Shanghai, China), and 3,3′-diaminobenzidine (DAB) developing solution and 0.01 M citrate buffer solution were purchased from Gene Tech Biotechnology Co., Ltd. (Shanghai, China). Forma Series II CO2 incubator was the product of Thermo Fisher Scientific (Waltham, MA, USA); The XL-90 over-speed low temperature centrifuge was purchased from Beijing BJ-Genetool Co. Ltd. (Beijing, China); the DNM-9602 microplate reader was purchased from Beijing Perlong Medical Instrument Ltd. (Beijing, China); the adjustable pipette was purchased from Eppendorf AG (Hamburg, Germany); RM2125 type tissue sectioner was purchased from Leica (Mannheim, Germany); and 8122214 type optical microscope was purchased from Olympus (Tokyo, Japan).

Establishment of subdermal human SKOV3 ovarian cancer cell transplantation model

Skin from the back of mice left forelimb was selected and 4% sodium sulfide solution was injected. SKOV3 cells growing in the logarithmic phase were selected and washed twice with PBS to adjust cell concentration to 10×107/ml. The tumor cells were injected in the animals. The aspirin solution of different concentration was used with 1 ml syringe, for intraperitoneal injection once every two days for period of 3 weeks.

Grouping method and observation index

The mice were randomly divided into different concentration groups (mmol/l), i.e., 0, 1, 2 and 3, and were fed on normal diet and drinking water. Two mice from each group were sacrificed by cervical dislocation after 1, 2 and 3 weeks. The tumor tissue was dissected for histological observation, as described elsewhere (10). The tumor volume and maximum diameter (a) and minimum diameter (b) of the tumor body were measured by vernier caliper. The tumor volume was calculated using the formula V=1/2 × a × b2, and the proliferation inhibition rate was determined by the CCK-8 method. Caspase-3 and bcl-2 protein expression was determined using the immunohistochemical two-step method.

Procedures of CCK-8 detection

The cells were collected and centrifuged at 2,000 × g for 3 min. The pellet was washed with PBS, and 50 µl of cell suspension was taken to estimate cell concentration. The cell concentration was adjusted to 1×105/ml, and the cell suspension was inoculated into 96-well plates, with each well containing 200 µl. Each group was cultured at 37°C for 48 h, then 20 µl of 10% CCK-8 solution was added into each pore plate by pipette. The culture was continued for 2 h in the incubator, the supernatant was removed and 200 µl of DMSO was suspended. The culture was fully oscillated and dissolved to measure the absorbance (A) at 490 nm using a microplate reader (Shanghai Shenggong Technology Co. Ltd., Shanghai, China) within 30 min. Subsequently, the cell proliferation inhibition rate (IR) was calculated as: IR = (1-A of experiment group / A of control group) × 100%.

Immunohistochemical two-step detection

After dewaxing and hydration, the sections were washed with PBS three times for 3 min each. The sections were then placed in 0.01 M citrate buffer, to retrieve the antigen for 2.5 min and then cooled at 37°C. The sections were washed again three times with PBS for 3 min. Subsequently, 50 µl of 3% hydrogen dioxide solution was added to each section and incubated at room temperature for 10 min, followed by washing with PBS as mentioned earlier. Subsequently, 50 µl of 3% primary antibody (1:100 rabbit anti-human caspase-3 antibody and 1:150 bcl-2 antibody) was added to each section and incubated at 37°C for 1 h, followed by PBS washing. Fresh DAB solution (50 µl) was added to observe the slides for 5 min under a microscope (Olympus). The solution turned the sections brown, after which the sections were counterstained with hematoxylin dye. The sections were then washed with distilled water sealed with neutral gum, and incubated in xylene (Fangzheng Chemical Co., Ltd., Chengdu, China). For interpretation of results, sepia, claybank or flavescent granule in cytoplasm indicated positive staining. Five visual fields were selected under a ×400 magnified visual field, to calculate the positive rate and staining intensity. The positive rate of staining scoring was: 0 point, 0–5%; 1 point, 6–25%; 2 points, 26–50%; and 3 points, 51–100%; and scoring of staining intensity: 0 point, uncolored; 1 point, partially weak coloring; 2 points, weak to medium coloring; and 3 points, strong coloring.

The score value was calculated as: (sum of product of positive rate scoring and staining intensity scoring of each visual field)/5; range, 0–9 points.

Statistical processing

SPSS 19.0 software (IBM, Armonk, NY, USA) was applied to for statistical analysis. Quantitative data were presented as mean ± standard deviation. The variance analysis and repeated measurement data test were used in group comparisons, while enumeration data were presented by percentage (%). The χ2 test was applied for group comparisons, and P<0.05 was considered to be statistically significant.

Results

Comparison of tumor volume

After 1, 2 and 3 weeks of treatment with aspirin, the tumor volume of the 3 mmol/l aspirin group was significantly smaller than that of the other groups (P<0.05). The higher concentration of aspirin significantly reduced the tumor size (P<0.05) (Table I).

Table I.

Comparison of tumor volume (cm3).

Aspirin group	0 week	1 week	2 weeks	3 weeks
0 mmol/l	3.2±0.3	3.4±0.5	3.5±0.6	3.6±0.5
1	3.3±0.4	3.2±0.6	3.2±0.5	3.1±0.6
2	3.2±0.2	3.1±0.5	3.0±0.6	3.0±0.7
3	3.3±0.3	2.7±0.6	2.5±0.6	2.3±0.8
F	0.624	3.564	3.928	4.326
P-value	0.732	0.042	0.039	0.036

Comparison of proliferation inhibition rate

After 1, 2 and 3 weeks of aspirin treatment, the cell proliferation inhibition rate of the 3 mmol/l aspirin group was significantly larger than that of the other groups (P<0.05). The higher concentrations of aspirin significantly enhanced the cell proliferation inhibition rate (P<0.05) (Table II).

Table II.

Comparison of proliferation inhibition rate (%).

Aspirin group	0 week	1 week	2 weeks	3 weeks
0 mmol/l	0.22±0.03	0.23±0.04	0.24±0.03	0.23±0.05
1	0.24±0.03	9.25±1.25	9.63±1.36	9.55±1.53
2	0.28±0.02	18.62±4.28	19.27±4.32	20.33±5.16
3	0.25±0.04	27.73±5.36	28.54±5.93	30.32±4.78
F	0.854	4.128	4.329	4.502
P-value	0.329	0.034	0.027	0.025

Comparison of caspase-3 and bcl-2 protein expression

After 1, 2 and 3 weeks of treatment with aspirin, the relative expression of caspase-3 protein with the 3 mmol/l aspirin group was significantly improved. The expression level of bcl-2 protein was significantly reduced, and higher aspirin concentrations significantly enhanced the caspase-3 protein expression and reduced the bcl-2 protein expression (P<0.05) (Table III).

Table III.

Comparison of caspase-3 and bcl-2 protein expression.

	0 week		1 week		2 weeks		3 weeks
Aspirin group	Caspase-3	bcl-2	Caspase-3	bcl-2	Caspase-3	bcl-2	Caspase-3	bcl-2
0 mmol/l	0.32±0.04	3.45±0.23	0.33±0.05	3.37±0.35	0.32±0.03	3.33±0.34	0.31±0.04	3.51±0.32
1	0.31±0.03	3.34±0.33	0.67±0.12	3.25±0.41	0.72±0.15	3.15±0.25	0.83±0.26	3.13±0.53
2	3.33±0.02	3.38±0.42	0.92±0.23	2.93±0.39	1.13±0.24	2.72±0.28	1.27±0.32	2.66±0.46
3	3.32±0.04	3.35±0.34	1.27±0.42	2.54±0.36	1.62±0.53	2.13±0.36	1.84±0.26	1.97±0.78
F	0.238	0.924	4.632	4.867	5.123	5.148	5.237	5.439
P-value	0.621	0.736	0.023	0.021	0.019	0.017	0.014	0.012

Discussion

Schildkraut et al after comparing and analyzing 585 ovarian cancer patients and 627 healthy individuals under 5-year aspirin treatment, came to the conclusion that aspirin was able to reduce the risks of ovarian cancer (11). Findings of a previous study have showed that aspirin inhibits the proliferation and induces the apoptosis of ovarian cancer cells, and suppresses the expression of OVCAR-3 ovarian cancer cells by obstructing HER-2/neu receptor (12). Thus, inhibition of the metastasis of ovarian cancer by affecting metastasis-related genes, and its nitro derivative NCX-4016 may inhibit the proliferation of cisplatin-resistant ovarian cancer cells through obvious cytotoxic effects, thereby treating with recurrent ovarian cancer (13). In the recovery phase after epithelial ovarian cancer surgery, there is a possible risk of arterial thrombus. To improve the patient prognosis, Salman and Ayhan suggested treatment of the patients by conventional dose (75–100 mg/day) of aspirin until the end of chemotherapy (14). The results of their study showed that conventional dose (75–100 mg/day) of aspirin treatment after surgery significantly prevented the formation of thrombus, enhancing the cytotoxic effects of chemotherapeutics on microlesions, and improving the patient prognosis.

Although an increasing number of studies have suggested that aspirin inhibits the recurrence and metastasis of malignant tumors, such as colorectal, liver cancer, and ovarian cancer, and increases the sensitivity of chemotherapy (15), in clinic, there remains a lack of consensus regarding the specific mechanism of action and the optimal concentration (16). In the present study, ovarian cancer models were established, and aspirin of different concentrations was used for intervention. The intervention results of the different groups were compared and a possible molecular mechanism was determined. The results showed that 1, 2 and 3 weeks after aspirin treatment, the tumor volume with treatment of 3 mmol/l aspirin was significantly smaller than that of the other groups. The higher concentrations of aspirin significantly reduced the tumor size, but enhanced the cell proliferation inhibition rate compared to the other groups. The relative expression level of caspase-3 protein with 3 mmol/l aspirin was significantly improved. The expression of bcl-2 was significantly reduced, and higher concentrations of aspirin led to significant inhibition of cell proliferation. Additionally, caspase-3 expression was increased whereas that of bcl-2 was reduced. Although data from this study are promising, the small sample size included in the present study was a limitation of this study.

In conclusion, aspirin can inhibit the growth of ovarian cancer of p53S rats, possibly as it was able to upregulate the expression of caspase-3 protein and downregulate the expression of Bcl-2 protein.