A Bacterial Metabolite, Compound K, Induces Programmed Necrosis in MCF-7 Cells via GSK3β

Abstract

Ginsenosides, the major active component of ginseng, are traditionally used to treat various diseases, including cancer, inflammation, and obesity. Among these, compound K (CK), an intestinal bacterial metabolite of the ginsenosides Rb₁, Rb₂, and Rc from Bacteroides JY-6, is reported to inhibit cancer cell growth by inducing cell-cycle arrest or cell death, including apoptosis and necrosis. However, the precise effect of CK on breast cancer cells remains unclear. MCF-7 cells were treated with CK ($0-70{\mu}M$) for 24 or 48 h. Cell proliferation and death were evaluated by 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) and flow cytometry assays, respectively. Changes in downstream signaling molecules involved in cell death, including glycogen synthase kinase $3\beta$ ($GSK3\beta$), $GSK3\beta$, $\beta$-catenin, and cyclin D1, were analyzed by western blot assay. To block $GSK3\beta$ signaling, MCF-7 cells were pretreated with $GSK3\beta$ inhibitors 1 h prior to CK treatment. Cell death and the expression of $\beta$-catenin and cyclin D1 were then examined. CK dose- and time-dependently inhibited MCF-7 cell proliferation. Interestingly, CK induced programmed necrosis, but not apoptosis, via the $GSK3\beta$ signaling pathway in MCF-7 cells. CK inhibited $GSK3\beta$ phosphorylation, thereby suppressing the expression of $\beta$-catenin and cyclin D1. Our results suggest that CK induces programmed necrosis in MCF-7 breast cancer cells via the $GSK3\beta$ signaling pathway.

Introduction

Ginsenosides, active components of ginseng, have various biological effects including antidiabetic, antimutagenic, anti-inflammatory, antioxidant, and anticancer activities [5, 12]. Moreover, ginsenosides regulate the immune activity via modulation of cell survival or differentiation [24, 34]. Several researchers have shown that some ginsenosides cannot be absorbed in their native form through the intestinal barrier, due to their hydrophilicity. Instead, protopanaxadiol (PD) ginsenosides, including Rb1, Rb2, and Rc, are metabolized into an absorbable form in the intestinal tract [1, 3]. Compound K (CK), also known as IH-901 or M1, is the final bacterial metabolite of PD ginsenosides in the intestine [23, 30]. Moreover, ginsenosides are transformed to 20-O-β-D-glucopyranosyl-20(S)-protopanaxadiol (compound K, CK) by intestinal bacteria, namely Bacteroides sp., Eubacterium sp., and Bifidobacterium sp. [3] after the oral administration of which is absorbed into the blood [1]. CK is produced as a metabolic by-product after the sequential bioconverting of Rb1, Rd, and F2 in each step by β-glucosidase secreted by Leuconostoc citreum LH1 [36]. Interest in CK has increased owing to its anticancer effects against various cancer cell lines, including hepatoma, gastric carcinoma, and lung carcinoma cells [14, 27]. However, the mechanisms of CK-induced cancer cell death remain unclear.

There are three types of cell death: apoptosis, autophagy, and necrosis [7]. Apoptosis is characterized by DNA fragmentation, cellular shrinkage, and chromatin condensation [13]. In contrast, autophagy is characterized by activation of class III phosphatidylinositol 3-phosphate kinase (PI3K), generation of reactive oxygen species (ROS), and the formation of autophagosomes [8, 17]. Necrosis is generally considered to be an accidental event that occurs without the activation of signal transduction. Intriguingly, recent data suggest that necrosis could occur through programmed signaling, termed programmed necrosis [28, 29].

Glycogen synthase kinase 3β (GSK3β) is associated with programmed cell death. GSK3β promotes intrinsic apoptosis mediated by mitochondria, and inhibits extrinsic apoptosis induced by death receptors [4]. In contrast, suppression of GSK3β induces necrosis without Bax activation, which is primarily involved in apoptosis [38]. Recently, data showed that GSK3β may be associated with both autophagy [35] and programmed necrosis [16]. Together, these reports suggest that GSK3β could interact with different signaling molecules, depending on the type of cell death [26].

In the present study, we investigated the anticancer effects of CK in MCF-7 breast cancer cells by evaluating cell death and the related signaling pathways that are involved. We found that CK induces programed necrosis via GSK3βmediated signaling.

 

Materials and Methods

Materials

Ginsenoside CK was purified as previously described [18]. One hundred micromole of CK was dissolved in dimethyl sulfoxide (DMSO). 3-(4,5-Dimethylthiazol-2-yl)-2,5-diphenyl tetrazolium bromide (MTT) and 3-MA were purchased from Sigma-Aldrich (St. Louis, MO, USA). AnnexinV-FITC was purchased from BD Biosciences (San Jose, CA, USA) and GSK3β inhibitor VIII was purchased from Calbiochem (San Diego, CA, USA). Antibodies against phospho-GSK3β (Upstate Biotechnology, Lake Placid, NY, USA), GSK3β, β-catenin, cyclin D1 (Cell Signaling Technology, Danvers, MA, USA), and β-actin (Santa Cruz Biotechnology, Santa Cruz, CA, USA) were used.

Cell Proliferation Assay

MCF-7 cells were purchased from the American Type Culture Collection and maintained in DMEM with 10% fetal bovine serum and 1% antibiotics (Invitrogen, Grand Island, NY, USA) at 37℃ in a humidified incubator with 5% CO2. The cells were seeded in a 96-well plate at a density of 5 × 103 cells/well and incubated for 24 h. Then, the cells were treated with CK (0, 10, 30, 50, and 70 µM) for the indicated time period. In order to block autophagy, 3-MA (1 mM) was added for 1 h prior to CK (70 µM) treatment. Then, MTT solution (5 mg/ml) was added to each well at a final concentration of 0.5 mg/ml for 3 h at 37℃ in a humidified incubator with 5% CO2. The supernatant was removed and MTT formazan was dissolved with DMSO. The optical density of each well was measured at 570 nm by a microplate reader (Molecular Device, Sunnyvale, CA, USA).

Cell Cycle Analysis

For the detection of cell cycle, MCF-7 cells were seeded in a 6-well plate at a density of 3 × 105 cells/well and incubated for 24 h in a humidified incubator with 5% CO2 at 37℃. The cells were treated with CK (0, 10, 30, 50, and 70 µM) for the indicated time period. Then, the cells were trypsinized, washed twice in phosphate-buffered saline (PBS), and fixed with 70% ethanol at 4℃ overnight. After centrifugation (800 ×g, for 10 min at 4℃), the supernatant was discarded. The cells were stained with PI solution (0.05% Triton-X 100, 10 µg/ml RNase, and 10 µg/ml PI in PBS). Then, the DNA content was measured by flow cytometry (FACSCalibur) with Cell Quest software (BD Biosciences, Sparks, MD, USA). All flow cytometric data were analyzed by Flowjo software (Tree Star, San Carlos, CA, USA).

Annexin V/PI Staining Assay

MCF-7 was seeded in a 6-well plate at a density of 3 × 105 cells/well and incubated for 24 or 48 h at 37℃ in a humidified incubator with 5% CO2. The cells were treated with CK (0, 10, 30, 50, and 70 µM) for the indicated time periods. For blocking programmd necrosis, GSK3β inhibitor VIII (GSK3βi, 20 µM) was pretreated for 1 h before CK (70 µM) treatment. Floating and attached cells were collected, washed twice with PBS, and stained with Annexin VFITC and PI as described by the manufacturer’s instructions (BD Biosciences, Sparks, MD, USA). The samples were detected by flow cytometry with Cell Quest software (BD Biosciences, Sparks, MD, USA). All flow cytometric data were analyzed by Flowjo software (Tree Star).

Western Blot Analysis

MCF-7 cells were seeded in a 35 mm dish at a density of 3 × 105cells/dish in a humidified incubator at 5% CO2 and 37℃. The cells were treated with CK (0, 10, 30, 50, and 70 µM) for the indicated time periods. The cells were washed twice with cold PBS. Then, the cells were lysed with cold RIPA lysis buffer containing 150 mM NaCl, 1% NP-40, 0.5% deoxycholate, 0.1% SDS, 50 mM Tris-HCl (pH 7.4), and protease inhibitor cocktail (Roche, Mannheim, Germany) and incubated on ice for 30 min. After centrifugation (12,000 ×g for 10 min at 4℃), soluble proteins were obtained and the concentration of protein was determined by Bradford assay (Bio-Rad Laboratories, Hercules, CA, USA). Protein samples (20 µg) were separated by 10~12% SDS-PAGE and transferred to a polyvinylidene difluoride transfer membrane (Amersham Bioscience, Piscataway, NY, USA). The membrane was blocked with 5% skim milk in TBST (0.1 M Tris, 0.9% NaCl, and 0.1% Tween 20) at room temperature for 1 h. The membrane was washed twice with PBS and incubated with the appropriate primary antibody at 4℃ overnight. After washing with PBS three times, the membrane was incubated for 1 h with a horseradish peroxidase (HRP)-conjugated goat anti-mouse IgG or HRP-conjugated goat anti-rabbit IgG secondary antibody (SantaCruz Biotechnology) for 1 h at room temperature. Expected protein bands were visualized with the enhanced chemiluminescence system (GE Healthcare, Buckinghamshire, UK).

Statistical Analysis

All data were expressed as means ± standard deviation (SD). Statistical significance was analyzed by Student’s t-test or oneway ANOVA using Prism ver. 4.0 (GraphPad Software, Inc., San Diego, CA, USA). The significance of differences was considered statistically significant at p < 0.05.

 

Results

CK Inhibits the Proliferation of MCF-7 in a Time- and Dose-Dependent Manner

To examine the anticancer effects of CK, MCF-7 cells were treated with CK (0, 10, 30, 50, and 70 µM) and cell proliferation was assessed by MTT assay. CK significantly (p < 0.05-0.01) inhibited the proliferation at 50 and 70 µM for 24 and 48 h (Figs. 1A and 1B). CK at 50 and 70 µM resulted in 21% and 59% inhibition at 24 h, and 35% and 88% at 48 h, respectively, compared with the control group. These results suggest that CK has an inhibitory effect on the proliferation in time- and dose-dependent manner.

Fig. 1.Inhibitory effect of CK on the proliferation of MCF-7 cells. MCF-7 cells were seeded in a 96-well plate at a density of 5 × 103 cells/well and treated with CK (0, 10, 30, 50, and 70 µM). The proliferation was measured by MTT assay after (A) 24 h and (B) 48 h. The data are expressed as the mean ± SD from triplicate experiments. * p < 0.05, ** p < 0.01. Experimental results are the representative of three independent experiments.

Apoptosis and Autophagy Are Not Induced by CK in MCF-7 Cells

To examine whether CK induces apoptosis of MCF-7 cells, we measured the peak of sub G1 using flow cytometry after PI staining. The cells treated with CK showed a similar percentage of the sub G1 compared with that of the control (Figs. 2A and 2B). To further validate this result, we examined the expression pattern of PARP-1, which is known to be cleaved in apoptotic cells [33]. As a result, PARP-1 was not cleaved in cells treated with CK, suggesting that CK did not induce apoptosis in MCF-7 cells. It was further noted that neither sub G1 nor PARP-1 expression was different at 24 and 48 h compared with the control (data not shown).

Fig. 2.CK induced neither apoptosis nor autophagy in MCF-7 cells. (A) MCF-7 cells were treated with CK (0, 10, 30, 50, and 70 µM) for 24 h, and then stained with PI, and apoptosis was detected by using flow cytometry. (B) The cells were treated with 70 µM CK for 8 or 16 h. Then, the expression of PARP-1 level was examined by western blot assay. (C) MCF-7 cells were treated with 1 mM 3-MA, an autophagy inhibitor, 1 h prior to the treatment of CK (70 µM) for 24 h. The proliferation was measured by MTT assay. The data, expressed as the mean ± SD of three separate experiments, were analyzed by one-way ANOVA. Means with different superscripts are significantly different at p < 0.05.

Autophagy occurs with degradation and recycling of cellular components and it is often associated with cellular stress and eventual cell death. To investigate autophagy, the cells were treated with CK (70 µM) alone or together with 3-MA (an autophagy inhibitor). The cells treated with CK alone showed 39% of proliferation (i.e., 61% inhibition) at 24 h, and the cells pretreated with 1 mM of 3-MA at 1 h prior to the administration of CK showed 34% of proliferation (i.e., 66% inhibition) when compared with those of control (Fig. 2C). Taken together, these results suggest that CK induced neither apoptosis nor autophagy in MCF-7 cells.

Inhibition of GSK3β Suppressed the Programmed Necrosis in MCF-7 Cells Treated with CK

Next, in order to further investigate the type of cell death responsible for the inhibition of MCF-7 proliferation, the cells treated with CK were examined by using Annexin V and PI. Interestingly, necrosis (Annexin V- /PI+ region) was increased in CK-treated cells when compared with the control (Fig. 3A), suggesting that the reduced cell proliferation in the cells treated with CK is attributed to necrosis.

Fig. 3.Reduction of CK-induced necrosis in MCF-7 cells by GSK3β inhibitor VIII. (A) MCF-7 cells were treated with CK (0, 10, 30, 50, and 70 µM) for 24 or 48 h. The cells were stained with Annexin V and PI. Apoptosis and necrosis were measured in the cells by flow cytometry. (B) MCF-7 cells were pretreated with GSK3βi at 1 h before CK treatment (70 µM) for 24 or 48 h. Then, the cells were stained with Annexin V and PI and measured by flow cytometry. The consistency of all results were confirmed more than three times.

There is some evidence suggesting that GSK3β is involved in cell fate, including its survival, necrosis, and programmed necrosis [15, 16]. To examine whether CKinduced cell death was mediated via GSK3β, the cells were pretreated with GSK3β inhibitor VIII (GSK3βi) followed by CK treatment, and necrosis was examined by using flow cytometry. The results showed that necrosis in the cells pretreated with GSK3βi and then treated with CK were decreased when compared with the cells treated with CK alone (Fig. 3B). These results strongly suggested that GSK3β is involved in the CK-mediated cell death of MCF-7 cells.

CK Induces Programmed Necrosis via GSK3β in MCF-7 Cells

To further investigate the exact cause of CK-induced cell death in MCF-7, the cells were treated with CK and the expression of phospho-GSK3β, β-catenin, and cyclin D1 was examined by western blot assay. Phosphorylation of GSK3β and the expression of β-catenin and cyclin D1 were all decreased by CK treatment (Fig. 4A). As expected, the expression of β-catenin and cyclin D1 in the cells pretreated with GSK3βi was higher than that of cells treated with CK alone (Fig. 4B). Taken together, CK induced programmed necrosis of MCF-7 cells via GSK3β.

Fig. 4.Induction of programmed necrosis in MCF-7 cells treated with CK via GSK3β. (A) MCF-7 cells were treated with CK (0, 10, 30, 50, and 70 µM) for 16 h, and the expression levels of phospho-GSK3β, β-catenin, and cyclin D1 were measured by western blot assay. (B) MCF-7 cells were pretreated with GSK3βi before CK treatment (70 µM) for 6 h in β-catenin or 12 h in cyclin D1, respectively. The expression of β-catenin and cyclin D1 was measured by western blot assay. β-Actin was used as a loading control. The consistency of all results was confirmed more than three times.

 

Discussion

CK is known to exert anticancer effects [14]. However, its mechanism of action remains unclear. The aim of the study was to evaluate the anticancer effects of CK in the human breast cancer cell line MCF-7, focusing particularly on programmed necrosis and its associated signaling pathways. We investigated whether CK (i) has anticancer activity in MCF-7 cells and (ii) induces cell death via GSK3β. Our results showed that CK dose- and time-dependently inhibited MCF-7 cell proliferation and reduced GSK3β phosphorylation.

CK has been suggested to induce apoptosis and autophagy in human cancer cells [20, 21]. Nevertheless, our study revealed that CK did not suppress MCF-7 cell proliferation via these pathways. It is important to note that the previous report suggesting apoptotic MCF-7 cell death following CK treatment used DNA fragmentation and cell viability assays, which are limited in defining the type of cell death.

Moreover, CK has shown inhibition activity against various cancers, not only breast cancer. CK induced the apoptosis of colorectal cancer through the inhibition of histone deacetylase activity [19] and the down-regulation of cdc2 and cdc25A that arrested the G1 phase cell cycle [41]. For lung cancer, CK was effective for apoptosis by improving p53 expression [25]. Nasopharyngeal carcinoma was inhibited by CK through apoptosis-inducing factor activation [22].

In the steady state, PARP-1 plays a homeostatic protective and regulatory role, as it is associated with the DNA repair process [11, 32]. During apoptosis, PARP-1 is cleaved by caspases, which leads to the inactivation of poly(ADPribosyl)ation [33]. However, PARP-1 levels are increased in programmed necrosis [37]. Indeed, cleavage of PARP-1 was not observed, whereas its expression was increased in the present study. It has also been reported that PARP-1-dependent programmed necrosis relies on AIF [2]. Our results indicated that CK induces high AIF expression levels (data not shown), suggesting that CK induces programmed necrosis.

GSK3β, a serine threonine kinase, phosphorylates a number of proteins with various cellular functions, thereby regulating metabolism, structure, transcription, and gene expression [10]. GSK3β has numerous substrates, including metabolic and signaling proteins, structural proteins, and transcription factors [10]. Among them, β-catenin, cyclin D1, and c-myc are the oncogenic proteins. β-Catenin is phosphorylated by GSK3β in the Wnt signaling pathway [40]. Furthermore, Wnt/β-catenin signaling is involved in the development of cancer [39] by enhancing proliferation [31]. A number of oncogenic proteins are degraded via GSK3β-mediated phosphorylation [6, 9, 40], indicating that GSK3β also acts as a tumor suppressor. Moreover, GSK3β is involved in regulating various functions, including cell survival and death [15]. In our study, CK treatment induced GSK3β dephosphorylation, and reduced the expression of β-catenin and cyclin D1, suggesting that CK is associated with GSK3β-mediated programmed necrosis.

The present study demonstrated that CK induced programmed necrosis via the dephosphorylation and activation of GSK3β, which coincided with the decreased expression of β-catenin and cyclin D1. Understanding the involvement and the precise role of GSK3β signaling could become important for the development of therapeutic strategies to improve the efficacy of anticancer agents. Therefore, the present study provides a potential target for future investigation in breast cancer therapy.