Trichosanthes kirilowii Ethanol Extract and Cucurbitacin D Inhibit Cell Growth and Induce Apoptosis through Inhibition of STAT3 Activity in Breast Cancer Cells.

Trichosanthes kirilowii tuber is a traditional medicine which exhibits various medicinal effects including antidiabetic and anticancer activities in several cancer cells. Recently, it was reported that Cucurbitacin D (CuD) isolated from Trichosanthes kirilowii also induces apoptosis in several cancer cells. Constitutive signal transducer and activator of transcription 3 (STAT3), which is an oncogenic transcription factor, is often observed in many human malignant tumor, including breast cancer. In the present study, we tested whether Trichosanthes kirilowii ethanol extract (TKE) or CuD suppresses cell growth and induces apoptosis through inhibition of STAT3 activity in breast cancer cells. We found that both TKE and CuD suppressed proliferation and induced apoptosis and G2/M cell cycle arrest in MDA-MB-231 breast cancer cells by inhibiting STAT3 phosphorylation. In addition, both TKE and CuD inhibited nuclear translocation and transcriptional activity of STAT3. Taken together, our results indicate that TKE and its derived compound, CuD, could be potent therapeutic agents for breast cancer, blocking tumor cell proliferation and inducing apoptosis through suppression of STAT3 activity.

1. Introduction

Breast cancer is the most common disease in women and the leading cause of cancer death. The cause of breast cancer involves a genetic disorder, life style, or dietary factors. There are effects in breast cancer therapy that have improved the survival period and quality of life. However, breast cancer patients continue to die, and thus new therapeutic strategies of breast cancer are required. Signal transducer and activator of transcription3 (STAT3) is an oncogenic transcription factor which is constitutively activated in more than 50% of primary breast tumor and tumor-derived cell lines [1, 2].

Apoptosis plays a crucial role during embryonic development and tissue homeostasis [3]. It is related to the activation of the family of central components of the apoptotic machinery known as Caspases [4, 5]. To date, research indicates that there are two main apoptotic pathways: the extrinsic or death receptor pathway and the intrinsic or mitochondrial pathway. The extrinsic and intrinsic pathways converge on the same terminal execution pathway. This pathway is initiated by the cleavage of caspase-3 and results in DNA fragmentation, degradation of cytoskeletal and nuclear proteins, cross-linking of proteins, formation of apoptotic bodies, expression of ligands for phagocytic cell receptors, and finally uptake by phagocytic cells. Apoptotic cell death is controlled by diverse signaling pathways, which may activate either extrinsic inducers (caspase-8) or intrinsic inducers (caspase-9). The extrinsic signaling pathways that initiate apoptosis involve transmembrane receptor-mediated interactions. The intrinsic signaling pathways that initiate apoptosis involve a diverse array of non-receptor-mediated stimuli that produce intracellular signals that act directly on targets within the cell and are mitochondrial-initiated events. Bcl-2 family members (Bcl-2, Bcl-XL, BAX, Bak, Bad, and Bid) are associated with mitochondrial death pathway and cell cycle progression [6].

Trichosanthes kirilowii tuber extract (TKE) is a traditional medicine used in East of Asia for patients with diabetes symptoms. Trichosanthin (TCS), which is also known as type 1 ribosome-inactivating protein, is a chemical component derived from TKE [7]. TCS acts as a potent inhibitor of HIV-1 replication and is known to have various antitumor functions [8, 9]. Recently, it was reported that Cucurbitacin D, isolated from TKE, induces apoptosis in human hepatocellular carcinoma cells [10].

Cucurbitacins refer to a group of tetracyclic triterpenoids initially identified in the plant family of Cucurbitaceae. In traditional medicine, cucurbitacin-containing plants have been known for their anti-inflammatory, antimicrobial, and antitumor activities [11, 12]. There are 17 main molecules from cucurbitacin A to cucurbitacin T and hundreds of derivatives from them. Among those, cucurbitacin B, D, E, I, and their derivatives have been studied extensively for their strong anticancer activities. Also, cucurbitacin F, O, P, Q, and their derivatives are known to have modest anticancer activities [11]. JAK-STAT pathway, AKT-PKB pathway, and MAPK pathway are significantly associated with cancer and action of cucurbitacin family [13, 14].

In this study, we hypothesized that TKE and its derived compound, CuD, may modulate the STAT3 signaling pathway and sensitize breast cancer cells to apoptosis. To confirm this hypothesis, we treated MDA-MB-231 cells with TKE or CuD and performed experiments to observe the changes in STAT3 expression and its transcriptional activity.

2. Material and Method

2.1. Reagents

4′,6-diamidino-2-phenylindole (DAPI), (3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT), and dimethyl sulfoxide (DMSO) were purchased from Sigma-Aldrich (St. Louis, MO, USA). Propidium Iodide (PI) and Alexa Flour 488 goat anti-rabbit antibody were purchased from Invitrogen (Carlsbad, CA, USA). Cucurbitacin D was purchased from Extrasynthese (Genay Cedex, France). Antibodies against phospho-STAT3 (Tyr 705) and total STAT3 were obtained from Cell signaling (Danvers, MA, USA). Antibodies against pro-caspase-9, cleaved-caspase-9, cleaved-caspase-8, pro-caspase-3, cleaved-caspase-3, and PARP/p85 were obtained from Santa Cruz Biotechnology (Santa Cruz, CA, USA).

2.2. Preparation of Trichosanthes kirilowii Alcohol Extract (TKE)

Trichosanthes kirilowii was purchased from Omniherb (Yeongcheon, Republic of Korea). A ground powder (a mass of 100 g) was extracted twice with 80% (V/V) ethanol by using an ultrasonicator (Branson Ultrasonics, Danbury, CT, USA) for 30 min at room temperature. The alcohol extract was evaporated at 40°C (Evaporator, Eyela, Japan) and then freeze-dried for 72 h (Freezedryer, Matsushita, Japan). The powder from the extract was dissolved in distilled water to make a concentration of 200 mg/mL and centrifuged at 13,000 rpm at 4°C for 5 min. The supernatant TKE was stored in aliquots at −80°C until further analysis.

2.3. Cell Culture

MCF7, SK-BR3, and MDA-MB-231 breast cancer cells obtained from American Type Culture Collection (ATCC) were maintained in RPMI1640 supplemented with 10% heat-inactivated fetal bovine serum (Invitrogen, Carlsbad, CA, USA) and 100 U/mL antibiotic-antimycotic (Invitrogen). Cells were maintained at 37°C in a humidified incubator with 5% CO2.

2.4. Cell Viability Assay

Cell viability was measured using MTT assay. Cells were plated in 96-well flat bottom tissue culture plates at a density of 5 × 103 cells/well and incubated for 24 h. Cells were cultured for additional 24 h with TKE (10–200 μg/mL) or CuD (0.1–10 μg/mL). After incubation, MTT reagents (0.5 mg/mL) were added to each well, and the plates were incubated in the dark at 37°C for another 2 h. Medium was removed, formazan was dissolved in DMSO, and optical density was measured at 570 nm using an ELISA plate reader.

2.5. Flow Cytometric Analysis

Flow cytometry was used to analyze cell cycle distribution. Cells (3 × 105) were seeded in 60 mm dishes. After 24 h, cells were cultured for additional 6 or 24 h in the absence (control) or presence of 60 μg/mL TKE. Trypsinized cells were washed with phosphate-buffer saline (PBS), fixed in 95% ethanol containing 0.5% Tween-20 overnight at −20°C. After washing with PBS, cells were then incubated with 1 U/mL of RNase A and 10 μg/mL of PI for 30 min at room temperature in the dark. DNA content in each cell nucleus was determined by a FACScalibur flow cytometer (Becton-Dickinson, San Jose, CA, USA), and cell cycle was analyzed using a ModFit LT V2.0 software.

2.6. Western Blot Analysis

Cells were harvested, and cell pellets were incubated in one volume of lysis buffer [50 mM Tris-Cl pH 7.4, 1% NP-40, 0.25% sodium deoxycholate, 0.1% SDS, 150 mM NaCl, 1 mM EDTA, and protease inhibitor] for 20 min and centrifuged at 13,000 rpm at 4°C for 20 min. Aliquots containing 20 μg of protein were separated by SDS-polyacrylamide gel electrophoresis using 8–12% gels and transferred to nitrocellulose membranes (Protran nitrocellulose membrane, Whatman, UK). Membranes were blocked with 5% nonfat milk and probed with specific primary antibodies. Membranes were then incubated with horseradish peroxidase-conjugated secondary IgG antibody (Calbiochem, San Diego, CA, USA) and visualized using the enhanced chemiluminescence detection system (Amersham ECL kit, Amersham Pharmacia Biotech Inc. Piscataway, NJ, USA).

2.7. Immunocytofluorescence

Cells were plated in 8-well chamber slide at a density of 2 × 104 cells/well and incubated for 24 h. Cells were cultured for additional 24 h with 20 μg/mL of TKE or culture medium alone (control). After incubation, cells were fixed with 4% paraformaldehyde for 30 min, and the slides were incubated with 5% BSA for 1 h. After overnight incubation with rabbit polyclonal anti-human STAT3 antibody (dilution, 1 : 500), the slides were washed with PBS and then incubated with FITC-conjugated secondary antibody (dilution, 1 : 500) at room temperature for 1 h. After a brief washing in PBS, the slides were incubated with DAPI solution at room temperature for 5 min for counter staining. The slides were then washed with H2O and mounted by antifade mounting solution. The immunofluorescence staining of the cells was observed with confocal microscope (Zeiss, Germany).

2.8. Terminal Nucleotidyl Transferase-Mediated Nick End Labeling (TUNEL) Assay

Cells were plated in 8-well chamber slide at a density of 2 × 104 cells/well and incubated for 24 h. Cells were cultured for additional 24 h with 20 μg/mL of TKE or culture medium alone (control). After incubation, cells were fixed with 4% paraformaldehyde and incubated with 1% Triton X-100 in PBS for 2 min on ice for permeabilization. TUNEL staining was performed using TUNEL-label, enzyme kit (Roche) according to the manufacturer's instructions. The immunofluorescence staining of the cells was observed with confocal microscope.

2.9. Luciferase Assay

For the assay, MDA-MB-231 cells were plated and allowed to attach by overnight incubation. Next day, cells were transfected with control siRNA (Qiagen, Venlo, Netherlands) and STAT3 siRNA (SanTa Cruz, CA, USA) in the presence of STAT3-luciferase reporter using lipofectamine 2000 (Invitrogen, Carlsbad, CA, USA). Cells were then treated with TKE or CuD for 24 h. Luciferase assays were performed using a dual-luciferase assay kit (Promega, Madison, WI, USA) according to the manufacturer's instructions. Briefly, cells were lysed using a passive lysis buffer. Cell lysates were then centrifuged, and the supernatant was saved for analysis. Finally, luciferase activities were determined using a luminometer (BMG Labtech, Ortenberg, Germany).

2.10. Liquid Chromatography-Mass Spectrometry Analysis

An Agilent 1100 series liquid chromatography-mass spectroscopy (LC-MS) with an atmospheric pressure chemical ionization interface was used in negative and positive ionization modes. Data were collected using Chemstation software version A.09.03. A Shiseido capcell-pak UG120 column (4.6 mm × 250 mm, 5 μm) was used with an injection volume of 10 μL for the HPLC separation. The mobile phases consisted of (A) water and (B) methanol at a flow rate of 0.7 mL/min. The gradient of the mobile phases (A : B) for separation was 0–60 min (95 : 5 to 10 : 90). CuD was used as standard. Mass spectrometry was operated with an electrospray ionization source and positive mode.

2.11. Statistical Analysis

All experiments were expressed as the mean ± standard deviation (SD) of at least three separate tests. Student's t-test was used for single variable comparisons, and a P value < 0.05 was considered statistically significant.

3. Results

3.1. TKE and CuD Inhibits Proliferation of Human Breast Cancer Cell Lines

Figure 1(a) shows the chemical structure of CuD. We investigated whether TKE and CuD exhibit antiproliferative activity in MCF7, SK-BR3, and MDA-MB-231 cells. For that purpose, cells were treated with various concentrations of TKE (10, 30, 60, 100, 200 μg/mL) or CuD (0.1, 0.5, 1, 5 μg/mL), and cell growth rate was determined using MTT assay at 24 h. We found that both TKE and CuD significantly inhibited growth of all breast cancer cell lines (Figures 1(b) and 1(c)).

Figure 1

TKE and CuD inhibits cell proliferation in MCF7, SK-BR3, and MDA-MB-231 breast cancer cells. (a) Chemical structure of CuD. ((b), (c)) Different breast cancer cells were seeded into 96-well plates and treated with different concentrations of TKE (0, 10, 30, 60, 100, 200 μg/mL) or CuD (0, 0.1, 0.5, 1, 5 μg/mL) for 24 h. Cell viability measured by MTT assay. Data was reported as the percentage change in comparison to the 0 μg/mL concentration group, which were assigned 100% viability. The error bars represent the mean ± standard deviation (SD).

3.2. TKE and CuD Cause Apoptosis and G2/M Cell Cycle Arrest in MDA-MB-231 Cells

To further characterize the inhibitory effect of TKE and CuD on cell proliferation, we monitored cell cycle progression using flow cytometry. Both TKE and CuD induced the increase of sub G1 apoptotic cell fractions in MDA-MB-231 cells suggesting that antiproliferative activity of TKE and CuD resulted from apoptosis (Figures 2(a) and 2(b)). Moreover, exposure to TKE and CuD resulted in an increase in G2/M phase cells, accompanied by a decrease in G1 phase cells in MDA-MB-231 cells (Figures 2(c) and 2(d)).

Figure 2

TKE and CuD cause apoptosis and G2/M cell cycle arrest in MDA-MB-231 cells. (a) MDA-MB-231 cells were treated with 60 μg/mL of TKE or 1 μg/mL of CuD for 24 h. Cell cycle distribution was analyzed using a FACStar flow cytometer. (b) Graphs shows FACS-based subG1 phase distribution of MDA-MB-231 cells incubated with 60 μg/mL TKE for 24 h. (c) Bar graphs show FACS-based cell cycle distribution of MDA-MB-231 cells incubated with 60 μg/mL TKE for 24 h. (d) Bar graphs show FACS-based cell cycle distribution of MCF7 cells and MDA-MB-231 cells incubated with 60 μg/mL TKE or 1 μg/mL of CuD for 24 h. The error bars represent the mean ± standard deviation (SD).

3.3. TKE and CuD Regulate Apoptotic Molecules in MDA-MB-231 Cells

To examine the effect of TKE and CuD on the apoptotic signaling pathway, the levels of apoptosis-related molecules were measured in MDA-MB-231 cells by western blot using specific antibodies. We found that TKE and CuD upregulated levels of cleaved caspase-8, cleaved caspase-9, and cleaved caspase-3 and induced the cleavage of PARP in MDA-MB-231 cells (Figures 3(a) and 3(b)). These results indicate that TKE and CuD induce apoptosis in MDA-MB-231 cells producing the cleavage of caspases and PARP. On the other hand, TUNEL assay demonstrated that both TKE and CuD induced apoptotic cell death increasing many apoptotic green and yellow nuclei (Figures 3(c) and 3(d)).

Figure 3

Effect of TKE and CuD on the expression of apoptosis-related molecules. ((a), (b)) MDA-MB-231 cells were treated with 60 μg/mL TKE or 1 μg/mL CuD for the indicated time periods, and the cell lysates were subjected to western blot analysis using specific antibodies. ((c), (d)) MDA-MB-231 cells were incubated in the absence or presence of TKE or CuD and submitted to TUNEL assay. For the TUNEL assay, cells were fixed, permeabilized, and visualized for DNA degradation using dUTP-labeling. Red fluorescence: nuclei stained with PI. Green or yellow (resulting from merged red and green) fluorescence: apoptotic nuclei containing fragmented DNA.

3.4. TKE and CuD Inhibit Constitutive STAT3 Phosphorylation

High expression of STAT3 has been found to be involved in cancer progression. Knowing that STAT3 is activated in MDA-MB-231 cells, we investigated whether TKE and CuD suppress STAT3 phosphorylation. Figure 4(a) demonstrates that both TKE and CuD suppress constitutive activation of STAT3 within 6 to 12 h.

Figure 4

TKE and CuD suppress STAT3 phosphorylation and inhibit the translocation of STAT3 to the nucleus. (a) Cells were treated with 60 μg/mL TKE or 1 μg/mL CuD for different time periods (0–24 h). Total cell lysates were subjected to western blot analysis using specific antibodies (upper part) and the relative level of pSTAT3 as measured from the western blot analysis by densitometry (lower part). (b) MDA-MB-231 cells were plated in 8-well chamber slide. The next day, cells were incubated with 60 μg/mL TKE or 1 μg/mL CuD for 12 h and submitted to immunocytofluorescence assay. STAT3 distribution was exhibited by green fluorescence, and nuclei were counter-stained by DAPI.

3.5. TKE and CuD Suppress STAT3 Nuclear Translocation in MDA-MB-231 Cells

Normally, the active STAT3 homodimer translocates to the nucleus from the cytoplasm and induces specific target gene expression [15, 16]. Thus, we determined whether TKE and CuD suppress STAT3 nuclear translocation using immunofluorescence analysis. We found that TKE and CuD blocked the translocation of STAT3 into the nucleus in MDA-MB-231 cells (Figure 4(b)).

3.6. TKE and CuD Repress STAT3-Dependent Reporter Gene Expression

Although TKE and CuD has been shown to exhibit growth-inhibitory effect on MDA-MB-231 cells, it is not known whether it can modulate STAT3 signaling pathway. It should be noted that the constitutive activation of STAT3 is frequently detected in breast cancer tissues and cell lines. Therefore, we performed STAT3-dependent luciferase reporter gene assay to detect STAT3 transcriptional activity. We found that TKE and CuD significantly repressed STAT3-dependent reporter gene activity in MDA-MB-231 cells (Figures 5(a) and 5(b)).

Figure 5

TKE and CuD suppress the STAT3 transcription activity. MDA-MB-231 cells were transfected with the indicated siRNA or plasmid and then treated with each drug for 24 hours and submitted to dual-luciferase reporter assay.

3.7. CuD Is Extorted from TKE

We employed LC-MS analysis to confirm that CuD is extorted from TKE. Through analysis, chromatograms were acquired at 230 nm by UV detection (Figure 6(a)), and retention times of CuD were 25.9 min (Figure 6(b)).

Figure 6

LC-MS chromatogram. (a) Identification of CuD from TKE. (b) Mass spectrum peak at 25.9 min.

4. Discussion

The roots of Trichosanthes kirilowii have been used as a traditional herbal remedy and are an attractive medical resource. In addition, recent paper reported that CuD is an active compound of Trichosanthes kirilowii, and CuD induces apoptosis via activation of caspases and JNK in hepatocellular carcinoma cells [10]. Here, we found that TKE and its derived compound, CuD, inhibit cell proliferation and induce G2/M phase cell cycle arrest in MDA-MB-231 cells. In addition, we found that antiproliferative activity induced by TKE was associated with reduction of STAT3 phosphorylation.

STAT3 promotes inflammation, survival, immunity, proliferation, and angiogenesis of tumor cells [17-19]. It was reported that constitutive STAT3 activation was significantly and inversely related to overall 5-year survival in a cohort of breast cancer patients [2, 20]. Therefore, the inhibition of constitutive STAT3 activity is important for the prevention and treatment of breast cancer. The STAT3 phosphorylation at Tyr705 causes its nuclear translocation and DNA binding to specific DNA sequences in the promoter of target genes. In the present study, we found that TKE inhibited nuclear translocation of STAT3 as revealed by immunofluorescence assay resulting in the inhibition of expression of STAT3 target genes (data not shown).

Several earlier studies indicated that Cucurbitacin and their derivatives are triterpenoids found in TKE [21]. Recently, it was reported that Cucurbitacin induces apoptosis in human cancer cells by targeting JAK-STAT pathway [13, 14]. Many studies confirmed that Cucurbitacin I is a powerful JAK-STAT inhibitor by blocking the tyrosine phosphorylation of STAT3 and JAK2 in various human cancers [22-25]. Also, Cucurbitacin I chemical structure is similar to that of CuD [11]. In our study, both TKE and CuD inhibited phosphorylation and nuclear translocation of STAT3 in MDA-MB-231 cells. In addition, endogenously activated transcriptional activity of STAT3 was inhibited by TKE and CuD in MDA-MB-231 cells.

5. Conclusions

TKE and its derived compound, CuD, inhibit cell proliferation and induce apoptosis through inhibition of STAT3 activity in breast cancer cells. Therefore, our results suggest that TKE and CuD might be a potential anticancer agent which inhibits breast cancer associated with STAT3 activation.