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Basic research

Antiproliferative and apoptotic activity of glycyrrhizinic acid in MCF-7 human breast cancer cells and evaluation of its effect on cell cycle, cell migration and m-TOR/PI3K/Akt signalling pathway

Zhen Zhang
,
Yun Feng
,
Zhen-Yu Li
,
Xiao-Zhong Cao

Arch Med Sci 2019; 15, 1: 174–182
Online publish date: 2018/11/08
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Introduction

Cancer, which accounts for around 7.5 million deaths and 12.6 million cases of the disease annually, is characterized by abnormal cell growth and uncontrolled cell division. The development of cancer takes place over years and encompasses multiple genetic and phenotypic modifications. Cancer remains the second most common cause of mortality after cardiovascular diseases [1]. Among all cancers, breast cancer is the second most common cancer globally after lung cancer. In the USA, breast cancer is responsible for 25% of all cancers found in women, making it one of the fatal diseases [2]. Currently, conventional breast cancer treatment involves therapeutic regimens such as surgical resection, chemotherapy, radiotherapy or a combination of some of these. The 5-year survival rate has not shown any improvement with the currently used treatment stratefies [3]. Further, due to serious side-effects, multidrug resistance, non-selectivity as well as quick removal of the drugs from the systemic circulation, new, more effective and safer anticancer drugs are needed.

Natural products have always played crucial roles in the discovery of anticancer drugs and many of these naturally occurring anticancer drugs are currently used clinically [4]. Chemoprevention involves the use of naturally occurring or synthetically/semi-synthetically prepared compounds to intervene in the carcinogenesis process during the initiation or advanced stages of cancer. These compounds can actually reverse, inhibit or stop the process of carcinogenesis [5, 6]. Plant-based compounds such as terpenoids, flavonoids, phenolic compounds, alkaloids, xanthones, coumarins, anthocyanins, carotenoids, etc, have been reported to exhibit potent anticancer activity against a range of cancer cells in various in vitro and in vivo experimental models. These compounds have been shown to exert their anticancer effects via a variety of mechanisms including cell cycle arrest, apoptosis induction, inhibition of cell proliferation and angiogenesis, modulating protein expression of various cell signalling pathways including the PI3K/Akt/m-TOR pathway, etc [711]. Glycyrrhiza glabra is an important medicinal plant with tremendous pharmacological activities which include neuroprotection, antimicrobial and anticancer activities. Though several molecules from this plant have been evaluated pharmacologically, one of the active constituents, glycyrrhizinic acid, has not been evaluated against breast cancer [12]. Keeping in view the role played by naturally occurring compounds and tremendous potential of Glycyrrhiza glabra in anticancer drug discovery, the primary objective of the current research work was to study the anticancer effects of glycyrrhizinic acid in MCF-7 human breast cancer cells along with demonstrating its effects on cell cycle phase distribution, cancer cell migration and modulation of the m-TOR/PI3K/Akt signalling pathway.

Material and methods

Chemicals, cell line and culture conditions

In the current study, the following drugs and chemical reagents were used. Glycyrrhizinic acid (98% purity as certified by HPLC), Annexin V-FITC and propidium iodide were procured from Sigma-Aldrich, St. Louis, MO, USA. An MTT kit was purchased from Roche (USA). RPMI 1640 and Dulbecco’s modified Eagle’s medium (DMEM) were obtained from Gibco BRL, Carlsbad, CA, USA. All the antibodies for AKT, p-AKT, mTOR, p-mTOR and GAPDH were purchased from Cell Signaling Technology, USA. MCF-7, human breast cancer cell line was supplied by Institute of Cell Biology, Chinese Academy of Science, Shanghai, China. The cells were well maintained in RPMI 1640 medium containing 10% FBS and antibiotics (100 U/ml penicillin G and 100 µg/ml streptomycin).

MTT assay for cell proliferation

The cytotoxic efficacy of glycyrrhizinic acid was evaluated by MTT assay [13], which is a colorimetric assay based on the reduction of yellow coloured MTT by succinate dehydrogenase which is present in mitochondria. When MTT moves into the living cells, it gets reduced to insoluble formazan complex. MCF-7 cells at a density of 2 × 105 cells/well were seeded in a 96-well plate, incubated for 24 h and then treated with different doses (0, 5, 10, 25, 50, 100, 200 µM) of glycyrrhizinic acid for different time periods. The untreated cells were kept as a control group. After incubation, the cells were washed with PBS twice and then 100 µl of MTT solution was added and the whole cell culture was again incubated for 50 min. Finally the absorbance was measured at 490 nm using an ELISA plate reader (ELX 800; Bio-Tek Instruments, USA).

Colony formation assay

For this assay, MCF-7 cells were harvested and then counted using a haemocytometer. The cells were seeded at 200 cells/well, then incubated for 24 h, and the cells were then allowed to attach to form a complete monolayer of cells. Various doses (0, 10, 50 and 100 µM) of the drug (glycyrrhizinic acid) were added to the cell culture, following which the cells were incubated for 72 h, then washed with PBS and the colonies thus formed were fixed using methanol. The cells were stained with crystal violet for 20 min and then counted using a light microscope.

Apoptosis quantification using Annexin V-FITC assay

Induction of apoptosis was determined by Annexin V-FITC assay as described previously [14]. MCF-7 human breast cancer cells were seeded in 6-well plates at a cell density of 2 × 106 cells per ml, incubated for 12 h and then treated with varying doses (0, 10, 50 and 100 µM) of glycyrrhizinic acid for 48 h. The cells were then harvested via trypsinization and then washed with PBS twice, resuspended and 250 µl of binding buffer comprising 20 µl each of Annexin V-FITC and propidium iodide was added to the cells. The cells were then incubated for 30 min in the dark and finally the samples were observed by flow cytometry (BD Biosciences).

Cell cycle analysis using flow cytometry

MCF-7 human breast cancer cells were seeded at a density of 2 × 105 cells/ml and then incubated overnight as described previously [15]. The medium was substituted by fresh DMEM containing several concentrations (0, 10, 50 and 100 µM) of glycyrrhizinic acid and then further incubated for 48 h. The treated and untreated cells were trypsinized, washed with cold PBS twice and then fixed using 70% methanol for 30 min. The cells were again washed with ice-cold PBS, and then stained with 20 µg/ml propidium iodide and then 10 µg/ml RNase A was added for 30 min. Finally, the cells were analysed using a FACSCalibur flow cytometer (FACSCalibur; BD Biosciences), and the data were processed by cell cycle analysis software (Modifit 2.0).

Cell invasion assay

A 24-well plate invasion assay was performed to evaluate the effect of glycyrrhizinic acid on the invasion tendency of MCF-7 cells. A Matrigel coating was applied on a polyvinyl-pyrrolidone-free polycarbonate filter which had a pore size of 6 mm. Medium containing 10% FBS was added to the lower chamber, while the upper chamber along with the coated filter was placed onto the lower chamber. Various doses (0, 10, 50 and 100 µM) of glycyrrhizinic acid were added to the MCF-7 cell culture, which had a density of 2 × 106 cells per well. The cells were incubated for 40 min. The cell culture containing the drug was then seeded onto the upper chamber and incubated for another 40 min. The cells were then fixed and stained with 0.5% crystal violet for 20 min and then analysed under a light microscope.

Western blot assay

Glycyrrhizinic acid-treated cells were washed twice in ice-cold PBS and then extracted with a RIPA buffer composed of a mixture of 5% each of phosphate inhibitor and protease inhibitor. The cell lysates were centrifuged at 15,000 × g for 15 min and protein concentrations were estimated using a Bio-Rad protein analyser. The proteins were separated by sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE) and then transferred to nitrocellulose membranes (Millipore Corp-oration, MA, USA). The membranes were then probed with specific antibodies at 4°C overnight, then washed with buffered saline and incubated with the appropriate secondary antibody for 1 h. The bands were visualized using an ECL chemiluminescent detection kit (Perkin Elmer Cetus, Foster City, CA, USA).

Statistical analysis

The data are shown as mean ± SD. Student’s t-test was performed to compare the untreated control and glycyrrhizinic acid-treated groups. Differences were considered statistically significant at a p-value ≤ 0.05.

Results

Glycyrrhizinic acid induced potent cytotoxic effects and inhibited colony forming tendency

MTT assay as well as clonogenic assays revealed that glycyrrhizinic acid (Figure 1) could induce significant cytotoxic effects as well as inhibitory effects against colony formation. The results, which are depicted in Figure 2, showed that glycyrrhizinic acid led to concentration-dependent as well as time-dependent growth inhibitory effects in MCF-7 human breast cancer cells. Clonogenic assay showed that different doses of glycyrrhizinic acid had a tendency to inhibit or suppress the colony formation tendency of these MCF-7 breast cancer cells. It was also observed that this tendency of this naturally occurring compound was enhanced with increase of the dosage to which these cells were exposed (Figures 3 A, B). Thus glycyrrhizinic acid has a dual effect – one on cell viability and another on colony formation.

Figure 1

Chemical structure of glycyrrhizinic acid, which is also known as glycyrrhizin

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Figure 2

Cytotoxic effects of glycyrrhizinic acid in human breast cancer cells (MCF-7) using MTT cell viability assay

Data are shown as the mean ± SD of three independent experiments. *P < 0.05, **P < 0.01, vs. 0 μM (control).

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Figure 3

A – Glycyrrhizinic acid led to reduction in the fraction of cancer cell colonies after the MCF-7 human breast cancer cells were treated with: (a) 0 μM, (b) 10 μM, (c) 50 μM and (d) 100 μM of the drug for 48 h. B – Bar graph showing the percentage of colonies formed at the indicated concentrations of the drug

Data are shown as the mean ± SD of three independent experiments. *P < 0.05, **p < 0.01, vs. 0 μM (control).

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Glycyrrhizinic acid induced significant apoptotic effects in MCF-7 human breast cancer cells

In order to evaluate the mode of action behind the cytotoxic action of glycyrrhizinic acid in MCF-7 cancer cells, further experiments using flow cytometry in combination with Annexin V-FITC and propidium iodide were used. This assay is used to demonstrate whether the drug induces apoptosis or not and, if it does, what the extent of apoptosis is. The results, which are shown in Figure 4, reveal that glycyrrhizinic acid has a tendency to induce both early and apoptotic features in these cancer cells. As compared to the control group, glycyrrhizinic acid-treated cells showed a high percentage of apoptotic cells. As compared to the control which showed only 4.5% of cells in the apoptotic stage, however, after treatment with a 10, 50 and 100 µM dose of glycyrrhizinic acid, the percentage of apoptotic cells increased to 24.8%, 58.3% and 74.1% respectively.

Figure 4

Glycyrrhizinic acid led to apoptosis induction in MCF-7 human breast cancer cells. The cells were treated with 0 (A), 10 (B), 50 (C) and 100 (D) µM doses of glycyrrhizinic acid for 48 h. The cells were stained with Annexin V-FITC (20 µg/ml) and PI (20 µg/ml) in the dark. Finally cells were analysed by a FACSCalibur flow cytometer

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Glycyrrhizinic acid induced sub-G1 cell cycle arrest

The effect of glycyrrhizinic acid on the cell cycle phase distribution was assessed by flow cytometry using propidium iodide as a probe. The results are shown in Figures 5 A–D and reveal that glycyrrhizinic acid has a tendency to significantly alter the distribution of various cell cycle phases. As compared to the untreated control cells, glycyrrhizinic acid-treated cells with a 10, 50 and 100 µM dose led to a 24.3%, 41.5% and 82.1% increase in the sub-G1 phase (apoptotic cells) cells. Sub-G1 cells are actually apoptotic cells with the lowest DNA content, and as such these results indicate that glycyrrhizinic acid results in induction of apoptosis to a significant extent. In addition, the percentage of G2/M cells also increased as the dosage of glycyrrhizinic acid increased.

Figure 5

Sub-G1 cell cycle arrest induced by glycyrrhizinic acid in MCF-7 human breast cancer cells. The cells were treated with 0 (A), 10 (B), 50 (C) and 100 (D) µM doses of glycyrrhizinic acid for 48 h, stained with PI and then examined by flow cytometry

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Glycyrrhizinic acid inhibited MCF-7 cell invasion

One of the characteristic features of breast cancer is its ability to undergo metastasis and highly invasive behaviour. Therefore, a further experiment using Matrigel assay was carried out to demonstrate whether glycyrrhizinic acid could suppress the cell invasion of MCF-7 human breast cancer cells. The results showed that the compound in fact led to dose-dependent inhibition of cell invasion in these cells. As compared to the untreated control cells, glycyrrhizinic acid-treated cells showed significant suppression of cell invasion. The results are shown in Figures 6 A and B. The percentage of cell invasion decreased from 97.2% in control cells to 79.1%, 37.3% and 12.3% in 10, 50 and 100 µM glycyrrhizinic acid-treated cells respectively.

Figure 6

A – Glycyrrhizinic acid led to inhibition of cell invasion in MCF-7 human breast cancer cells. The cells were treated with 0 (a), 10 (b), 50 (c) and 100 µM (d) doses of glycyrrhizinic acid for 48 h. B – Bar graph showing the percentage of the invasive cells at the indicated concentrations of the drug

Data are shown as the mean ± SD of three independent experiments. *P < 0.05, **p < 0.01, vs. 0 μM (control).

/f/fulltexts/AOMS/34070/AMS-15-34070-g006_min.jpg

Glycyrrhizinic acid targets m-TOR/PI3K/Akt signalling pathway

The fact that glycyrrhizinic acid could modulate the protein expression of the m-TOR/PI3K/Akt signalling pathway was evaluated using Western blot assay. The findings are shown in Figure 7 and indicate an interesting outcome. As compared to the untreated control cells, glycyrrhizinic acid-treated cells showed concentration-dependent downregulation of m-TOR and pm-TOR proteins. It also showed downregulation of PI3K/Akt protein expression. Thus it may be concluded that glycyrrhizinic acid induced anticancer and apoptotic effects via the m-TOR/PI3K/Akt signalling pathway.

Figure 7

Glycyrrhizinic acid targets m-TOR, PI3K/Akt pathways in human breast cancer cells. The cells were treated with 0, 10, 50 and 100 µM doses of glycyrrhizinic acid and then the changes in protein expression were evaluated by Western blot assay. GAPDH served as a control

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Discussion

Due to the lack of effective treatments, cancer is a fatal disease and is rated as the second most common cause of death. The major issues limiting the available chemotherapies are severe side effects and chemo-resistance [1]. Natural products due to fewer side effects and higher pharmacological potential are considered important targets for development of drugs. Glycyrrhiza glabra, an important medicinal plant, is considered of tremendous potential [12]. Glycyrrhizinic acid, also known as glycyrrhizin, is a triterpene saponin mostly isolated from the root of Glycyrrhiza glabra. Glycyrrhizinic acid has been reported to possess various pharmacological effects [16]. It has been shown to inhibit liver cell injury and is given to patients suffering from chronic or viral hepatitis and liver cirrhosis. The compound has also been shown to be effective in the treatment of autoimmune hepatitis [1618]. Glycyrrhizinic acid has also been reported to exhibit anti-inflammatory effects and antiviral effects against a range of viruses including SARS coronavirus, HIV, hepatitis A virus, hepatitis B virus, and hepatitis C virus [1922]. However, the anticancer activity of glycyrrhizinic acid against MCF-7 human breast cancer cells as well as its effects on cell apoptosis, cell cycle arrest, cell invasion and the m-TOR/PI3K/Akt signalling pathway have not been reported so far to the best of our knowledge. Cell cycle arrest and apoptosis induction are regarded as effective strategies to stop the uncontrolled cell growth by eliminating cancerous cells. Apoptosis especially acts as a protective mechanism that abolishes harmful or damaged cells prior to the appearance of malignancy [2022]. Against this backdrop the present study aimed to evaluate the antitumor effects of glycyrrhizinic acid – a naturally occurring triterpene saponin compound – on MCF-7 human breast cancer cells. We also studied the effects of this compound on colony formation, cell cycle phase distribution, cell invasion and the m-TOR/PI3K/Akt pathway. Initially, the MTT assay and clonogenic assays were used to evaluate the effects of glycyrrhizinic acid on cell viability and colony formation tendency, which showed that this compound induced potent cytotoxic effects along with inhibiting colony formation efficacy of these cells. Our results are in agreement with previous studies. Several plant-derived molecules such as chrysophanol and caffeic acid have been reported to exert their antitumor effects via inhibition of colony formation [23]. Furthermore, using flow cytometry, effects of this drug on apoptosis induction were studied, and indicated that glycyrrhizinic acid induced both early and late apoptosis in these cells. After treatment with a 10, 50 and 100 µM dose of glycyrrhizinic acid, the percentage of apoptotic cells increased to 24.8%, 58.3% and 74.1% respectively. Cell cycle arrest is a common cause of growth inhibition. To determine whether the effect of extracts involves alterations in cell cycle progression, cell cycle distribution analysis flow cytometry using propidium iodide as a probe was used to study the effects of this compound on cell cycle progression. The binding of water soluble, DNA intercalating propidium iodide correlates with the amount of DNA within a given cell, and the relative content of DNA indicates the distribution of a population of cells throughout the cell cycle [24]. Glycyrrhizinic acid induced sub-G1 cell cycle arrest and led to a significant increase of sub-G1 cells in a dose-dependent manner. Further, it was shown that glycyrrhizinic acid could inhibit MCF-7 cancer cell invasion in a concentration-dependent manner. These findings are promising since it is well established that breast cancer is one of the most invasive cancers and glycyrrhizinic acid could inhibit this behaviour. Finally, the effects of glycyrrhizinic acid on the expression levels of various proteins including m-TOR, pm-TOR, PI3K, p-PI3K and Akt were studied using western blot assay. The results revealed that glycyrrhizinic acid-treated cells exhibited concentration-dependent downregulation of m-TOR and pm-TOR proteins. It also showed downregulation of PI3K/Akt protein expression.

In conclusion, glycyrrhizinic acid inhibited MCF-7 human breast cancer cell growth by inhibiting colony formation, inducing apoptosis and sub-G1 cell cycle arrest, inhibiting cell invasion and targeting the m-TOR/PI3K/Akt signalling pathway. Several compounds that target the m-TOR/PI3K/Akt pathway are promising anticancer agents [25, 26]. We strongly believe that glycyrrhizinic acid may prove an important lead molecule. However, further in-depth mechanistic studies are required.

Conflict of interest

The authors declare no conflict of interest.

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