The cytotoxic effect and glucose uptake modulation of Baeckea frutescens on breast cancer cells

Background Baeckea frutescens (B. frutescens) of the family Myrtaceae is a plant that has been used in traditional medicine. It is known to have antibacterial, antipyretic and cytoprotective properties. The objective of this study is to explore the mechanism of B. frutescens leaves extracts in eliminating breast cancer cells. Method B. frutescens leaves extracts were prepared using Soxhlet apparatus with solvents of different polarity. The selective cytotoxicity of these extracts at various concentrations (20 to 160 μg/ml) were tested using cell viability assay after 24, 48 and 72 h of treatment. The IC50 value in human breast cancer (MCF-7 and MDA-MB-231) and mammary breast (MCF10A) cell lines were determined. Apoptotic study using AO/PI double staining was performed using fluorescent microscope. The glucose uptake was measured using 2-NBDG, a fluorescent glucose analogue. The phytochemical screening was performed for alkaloids, flavonoids, tannins, triterpenoids, and phenols. Results B. frutescens leaves extracts showed IC50 value ranging from 10 -127μg/ml in MCF-7 cells after 72 h of treatment. Hexane extract had the lowest IC50 value (10μg/ml), indicating its potent selective cytotoxic activity. Morphology of MCF-7 cells after treatment with B. frutescens extracts exhibited evidence of apoptosis that included membrane blebbing and chromatin condensation. In the glucose uptake assay, B. frutescens extracts suppressed glucose uptake in cancer cells as early as 24 h upon treatment. The inhibition was significantly lower compared to the positive control WZB117 at their respective IC50 value after 72 h incubation. It was also shown that the glucose inhibition is selective towards cancer cells compared to normal cells. The phytochemical analysis of the extract using hexane as the solvent in particular gave similar quantities of tannin, triterpenoids, flavonoid and phenols. Presumably, these metabolites have a synergistic effect in the in vitro testing, producing the potent IC50 value and subsequently cell death. Conclusion This study reports the potent selective cytotoxic effect of B. frutescens leaves hexane extract against MCF-7 cancer cells. B. frutescens extracts selectively suppressed cancer cells glucose uptake and subsequently induced cancer cell death. These findings suggest a new role of B. frutescens in cancer cell metabolism. Electronic supplementary material The online version of this article (10.1186/s12906-019-2628-z) contains supplementary material, which is available to authorized users.


Background
Survival of patients with metastatic breast cancer has not improved significantly despite the development in early diagnosis and treatments of breast cancer [1]. Multidrug resistance remains the principal obstacle in treating metastatic breast cancer [1].
The inability of cells to respond to stress and repair damage underlies many forms of cancer. A fundamental characteristic of cancer cells is their ability to sustain indefinite cycles of proliferation. One of the mechanism which contributes to the inhibition of tumour growth is by impairing cancer cell metabolism. Reprogramming of core metabolism in tumours confers a selective growth advantage such as the ability to evade apoptosis and/or enhance cell proliferation, promotes tumour growth and progression [2]. The high proliferation rate in cancer cells requires increased energy, and most cancer cells rely on glycolysis as their primary energy source. Pharmacological inhibition of glucose uptake or glycolytic enzymes activity serves as a potential target for cancer therapeutic [3]. To date, natural product plays a dominant role in the discovery of leads for the development of drugs [4].
Enhance glycolytic flux in cancer cells contributes to tumour development. One of the mechanism which contributes to the inhibition of tumour growth is by impairing cancer cell metabolism. Thus, it is interesting to explore the potential role of B. frutescens in regulating cellular metabolism as an approach in inducing cell death and preventing tumour growth and progression.

Extracts preparation
B. frutescens or Cucur Atap was collected from Forest Research Institute Malaysia (FRIM) Research Station in Setiu, Terengganu and its voucher specimen was deposited at Institute of Bioscience, Universiti Putra Malaysia (voucher number: KLU 47909). The leaves of B. frutescens were air-dried under shade for 7 days and were pulverized into coarse powder [24]. The powder was grounded and filtered using 0.9 mm filter membrane by vacuum pump. The coarse powder was extracted using hexane, ethanol and water. B.frutescens leaves ethanol extracts were prepared according to Ahmad et al. [6] at 90% (L90), 70% (L70) and 50% (L50). 111 g, 142 g and 200 g of coarse powder were weighed and extracted in 5 l of ethanol to obtain L90, L70 and L50 respectively. All extraction was prepared by Soxhlet. A stock solution of each crude extract was prepared by suspending 100 mg of extract in 1 mL of pure dimethylsulphoxide (DMSO) and mixed by sonication for 30 min. The volume was adjusted to 1000 mL with culture media to provide assay solutions as required.

Cell viability assay
The cytotoxicity effect of B. frutescens leaves extracts was determined by measuring the IC 50 using cell viability assay as previously described by Mosmann [25]. Briefly, cells at a density of 5 × 10 3 cells/well were plated in 96-well microplates in triplicates and were treated for 24, 48 and 72 h with extracts concentration ranging from 0 to 1000 μg/ml. Etoposide served as a positive control whilst DMSO was the vehicle control (control). MTT (3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide) solution was added into each well and incubated for 4 h in dark. The formazan grains were dissolved in DMSO and the colour intensity was measured at 570 nm (wavelength range: 550 -600 nm) using an ELISA plate reader with the reference wavelength of higher than 650 nm.

Acridine Orange and Propidium iodide staining
Cell death was detected using propidium iodide (PI) (Sigma-Aldrich, USA) and acridine-orange (AO) (Sigma-Aldrich, USA) double staining and examined under fluorescence microscope as previously described by [27]. Briefly, 1 × 10 6 MCF-7 cells/ml were plated in 6-well plate. The IC 50 concentration were used for all five extracts and DMSO served as the vehicle control (control). Cells were treated for 24, 48 and 72 h. The treated cells were trypsinized and centrifuged at 1000×g for 10 min. After rinsing with PBS, the supernatant was discarded and 10 μl fluorescent dyes, AO (10 μg/ml) and PI (10 μg/ml), were added into the cellular pellet at equal volumes. Stained cells were transferred onto a glass slide and observed under ultraviolet (UV)-fluorescence microscope (Olympus, Japan) within 30 min.

Total tannins content
The intensity of blue colour formation after adding five drops of 5% ferric chloride (Sigma Aldrich) to the extract were measured at 560 nm. 0.01 g/ml tannic acid (Sigma Aldrich) was used as the positive control.

Total triterpenoids content
The leaves extracts in chloroform were added with concentrated sulfuric acid and the colour intensity was measured at 635 nm. 0.01 g/ml cholesterol (Sigma Aldrich) was used as the positive control.

Total flavonoids content
10% of lead acetate were added to the leaves extracts until yellow precipitate was formed and the colour intensity was measured at 590 nm. 0.01 mg/ml quercetin (Sigma Aldrich) was used as the positive control.

Total phenols content
Presence of phenols was measured by adding 10% ferric chloride (Sigma Aldrich) solution to the leaves extracts. The colour intensity was measured at 490 nm. 0.01 g/ml quercetin was used as the positive control.

Total alkaloids content
Brown precipitate that was formed when Dragendorff's reagent (Sigma Aldrich) were added to the extract indicates the presence of alkaloid. The colour intensity was measured at 435 nm. .01 g/ml quinine sulfate (Sigma Aldrich) was used as the positive control.

Statistical analysis
All experiments were conducted at least three times unless otherwise indicated and the data were expressed as mean ± SEM (Standard Error of the Mean). Statistical analyses were performed using SPSS (version 20) statistical software. The treatment effects were analysed using one way analysis of variance (ANOVA), followed by Bonferroni post-hoc test. p-value less than 0.01 (p < 0.01) was considered to be statistically significant.

Selective cytotoxic effect of B. frutescens on breast cancer cells
The cytotoxic effect of B. frutescens against MCF-7 were determined at 24, 48 and 72 h of incubation. After 24 h of incubation, no cytotoxic effect against MCF-7 cells was observed (Fig. 1a). Interestingly, two extracts (HX and L50) showed IC 50 values of 22 ± 0.04 μg/ml and 160 ± 0.04 μg/ml, respectively after 48 h of incubation (Figs. 1a and 2). When the incubation time was increased to 72 h, all extracts showed IC 50 values ranging from 10 to 124 μg/ml (Figs. 1a and 2). Of note, the extracts were also tested against MDA-MB-231 cancer cells at three incubation time; 24, 48 and 72 h. The IC 50 value of all extracts against MDA-MB-231 cells after 24 and 48 h of incubation cannot be calculated (Fig. 1a). Only hexane extract displayed IC 50 value of 80 ± 0.32 μg/ml after 72 h of incubation (Fig. 1a). This shows that B. frutescens extracts has low cytotoxic effect against MDA-MB-231 compared to MCF-7 cells.
The in vitro cytotoxicity of B. frutescens extracts were evaluated in normal mammary epithelial cells, MCF10A. MCF10A cells were incubated in three different concentrations of B. frutescens extracts; a concentration lesser than the IC 50 value obtained from MCF-7 cells, the IC 50 value and a concentration greater than the IC 50 value obtained from MCF-7 cells. All extracts except L50 showed 80% of cell viability at their respective IC 50 values after 72 h of incubation with B. frutescens extracts (Fig. 1b).

Apoptosis induction in breast cancer cells after B. frutescens treatment
Morphological changes in MCF-7 cells were observed at 24, 48 and 72 h of incubation with five B.frutescens extracts at their respective IC 50 values. At 24 h, the cells were stained green with intact nucleus (Fig. 3b).
Interestingly, after 48 h of incubation with HX and L50, chromatin condensation and membrane blebbing were noted (Fig. 3b). As shown in Fig. 3b, cells displayed orange coloured nuclei indicating cell death induction after 72 h incubation with B.frutescens extracts. To investigate the effect of B. frutescens extracts on glucose uptake in MCF-7 cells, the cells were incubated in the presence of five extracts at three different concentrations for 24, 48 and 72 h. All extracts except L70 and water extracts, significantly inhibited glucose uptake compared to control at all time points and concentration (Fig. 4).
At 24 h, all extracts except for L70 and water, significantly inhibited glucose uptake compared to the positive control, WZB117 and control at their IC 50 value. However, only L50 showed significant decrease in glucose uptake compared to WZB117 and control after 48 h of incubation for all three concentrations (Fig. 4d). Interestingly, all extracts at IC 50 value showed significant decrease in glucose uptake after 72 h of incubation compared to cells treated with WZB117 and control (Fig. 4). Of note, no glucose uptake inhibition was observed in MCF10A treated cells.

Secondary metabolites identified in leaves and branches of B. frutescens
Hexane, ethanol and water extracts showed the presence of various quantities of flavonoids, alkaloids, phenols, triterpenoids and tannins as indicated in Table 1. Alkaloid was undetected in all leaves extracts except very low amount in L90 and L70. Tannin was detected in all five leaves extracts with value of more than 100 (Additional file 1: Table S1). Interestingly, in the qualitative estimation of the secondary metabolites in B. frutescens showed that hexane extract has equal ratio of tannin, triterpenoids, flavonoid and phenols (Table 1).

Discussion
Tumours are collection of diverse cells with distinct molecular and morphological signatures [29]. Of note, established biomarkers in breast cancer such as oestrogen receptor, progesterone receptor and HER2 are used in clinical decision-making [30]. In this study, B. frutescens extracts were tested in two distinct human breast cancer cell lines: a human breast cancer cell line positive for oestrogen and progesterone receptors and negative for HER-2 receptor and a triple negative cell line, MCF-7 and MDA-MB-231, respectively. MCF-7 cells were found to be more sensitive to B. frutescens extracts effect in inducing cell death compared to MDA-MB-231 cells (Fig. 2a). Hence, the subsequent experiments to determine the role of B. frutescens in eliminating breast cancer cells were focused on MCF-7 cells.
Cell viability assay and AO/PI staining were used to investigate the cytotoxicity properties of B. frutescens leaves extracts on MCF-7 cells. This assay is a non-radioactive measurement of cell viability. Our results indicate hexane extract showed potent selective cytotoxicity against MCF-7 cells at 48 h and 72 h with IC 50 value less than 20 μg/ml (Figs. 1a and 2). According to the protocol from the American National Cancer Institute (NCI) recommends a crude extract is considered to possess significant cytotoxic activity with IC 50 value ≤20 μg/ml, whilst this value was deemed at ≤4 μg/ ml for a pure compound [31].
The apoptotic effect of these extracts were further established using AO/PI staining where cells predominantly displayed changes that are known to be associated to apoptotic features namely membrane blebbing and chromatin condensation (Fig. 3). Apoptosis is accompanied by series of dramatic cellular morphological changes which includes cell contraction, dynamic membrane blebbing and nuclear disintegration that ultimately lead cells to fragment into apoptotic bodies.
The altered pattern in glucose metabolism in cancer cells was initially described by Otto Warburg as aerobic glycolysis. Hypoxic cancer cells rely heavily on glucose transporters for their survival through the uptake of glucose and subsequent glycolysis. Many cancer cells overexpress GLUT1 [32]. Previous studies on compounds and extracts from B. frutescens were tested on cancer cells that exhibit upregulated glucose transporters, mainly GLUT1. Thus, in this study, effect of B. frutescens extracts on GLUT1 was investigated.
Glucose is an essential metabolic substrate in cells. GLUT1 is widely distributed in normal tissue and overexpressed in many tumours, including hepatic, pancreatic, breast, oesophageal, brain, renal, lung, cutaneous, colorectal, endometrial, ovarian, and cervical cancers [33]. One major concern about glucose transport inhibitors is the ability to selectively inhibit GLUT1 in tumour but not in normal cells. To address this concern, glucose inhibition assay was performed in the breast cancer cell line MCF-7 and normal mammary cell line MCF10A. This study demonstrated that B. frutescens extracts inhibited glucose uptake significantly in cancer cell lines and not in their non-cancerous counterparts (Fig. 4).
To date, knowledge on the mechanism of B. frutescens in eliminating cancer cells is limited. This study described a novel role of B. frutescens extracts in metabolic reprogramming through their ability in suppressing glucose uptake in cancer cells (Fig. 4). However, the inhibitory effect on GLUT1-mediated glucose uptake may not be the main mechanism in B. frutescens in inducing cell death as the inhibition does not correlate with the potency of the extracts which was observed in the MTT assay (Figs. 1, 2 and Table 1). There are several pathways that affect glucose regulation in cancer cells. Disrupting PI3K signalling and selective blocking of GLUT1 transporter lead to decreased glucose uptake by tumours  24,48 and 72 h at three different concentrations; lower than IC 50 value (low), IC 50 value (IC 50 ) and higher than IC 50 value (high). WZB117 served as the positive control. Data is expressed as mean ± standard error mean based on four independent experiments with triplicate wells for each concentration. *p < 0.01, compared with control, # p < 0.01 compared with the positive control (WZB117) [3,34]. Ritonavir, fasentine, genistein, STF13 and WZB117 are anticancer drugs designed to target glucose transporter GLUT1 and exert antitumour effect by inhibiting glucose uptake, thus leading to cell death through glucose deprivation [3,35]. Natural compounds such as cryptocaryone [36,37], methylxanthines [38] and resveratrol [39] have been recently identified to inhibit glucose uptake either via direct binding to GLUT1 or indirectly by influencing glucose metabolism.
The curative properties of medicinal plants are attributed to the presence of various secondary metabolites such as alkaloids, flavonoids, glycosides, natural phenols and terpenoids. In the continuous effort to discover phytochemical compounds from B.frutescens, five secondary metabolites, namely tannins, triterpenoids, flavonoids, alkaloids and phenol were tested in the preliminary screening. Hexane is able to extract the most non-polar compounds of all the secondary metabolite. Based on the phytochemical analysis, the active secondary metabolites are of very non-polar in nature, with almost equal amounts of the tannin, flavonoid and phenols. However, the absences of alkaloids is indicative of this nitrogen containing compound is not the active metabolite (Table 1 and Additional file 1: Table S1). Generally, tannin, flavanoid and phenol have been reported for their anticancer properties [50][51][52]. Potent anticancer activities have been observed in tannins with multiple mechanisms, such as apoptosis, cell cycle arrest, and inhibition of invasion and metastases [51]. This potentially indicates the overall activity of hexane extract is a result of the synergistic combination of the secondary metabolites especially tannin, flavonoid and phenol As the polarity of the solvent is increased from hexane to absolute ethanol, the total amount of triterpenoids, flavonoids and alkaloids increased, as these compounds usually have polar heteroatoms such as nitrogen, oxygen and sulphur. The most polar compounds in the leaves eventually were extracted when water was used as the extracting solvent.
In summary, from the five B. frutescens leaves extracts tested, hexane extract showed the most selectively potent cytotoxic activity against breast cancer cell lines, mainly MCF-7 cells. This finding corroborated with the morphological changes observed upon the extract treatment. The hexane extract also selectively inhibited glucose uptake in MCF-7 cells as early as 24 h after treatment.
Cancer is a systemic disease which involves multiple biological network, pathways, genes and proteins. Identification of single compound has shown great success; nonetheless, the effect of single compound is not always ideal [53]. The multiple constituents in B. frutescens leaves extracts suggest these extracts may have multiple targets and the efficacy can be achieved by the synergistic and dynamic interaction of multiple constituents. The present study reports the potent cytotoxic effect of B. frutescens leaves extracts against breast cancer cells and its novel role in cancer cell metabolism. However, additional studies on synergism and isolation of B.frutescens targeting the fractions showing the highest biological activity would be of great interest.

Conclusions
This study highlights the selective anti-cancer properties of B. frutescens in breast cancer cells. MCF-7 cells were Table 1 Qualitative analysis of B. frutescens leaves phytochemical constituents more susceptible to B. frutescens-mediated cytotoxic effect compared to MDA-MB2-31 cancer cells. The hexane extract showed the most potent effect in inducing cell death with IC 50 value of less than 20 μg/ml at 48 and 72 h of incubation. Limited publication is available regarding the property of hexane extract isolated from B. frutescens. Hence, correlating the potent cytotoxic effect of the hexane extract and its chemical constituents will be pertinent in determining therapeutically active constituent present in B. frutescens. Moreover, this study reports the novel role of B. frutescens in glucose-uptake inhibition in breast cancer cells. The mechanism of B. frutescens extracts in modulating glucose metabolism warrants for further investigation.

Additional file
Additional file 1: Table S1.