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The benzofuran lignans egonol and homoegonol are found in all species of the genus Styrax. Since natural products are important sources of new anticancer drugs, this study evaluated the cytotoxic activity of a hydroalcoholic extract of the stems of S. camporum (SCHE) and their chemical markers, egonol (EG) and homoegonol (HE), against different tumor cell lines (B16F10, MCF-7, HeLa, HepG2, and MO59J). A normal human cell line (GM07492A) was included. Cytotoxic activity was evaluated at different treatment times (24, 48 and 72 h) using the XTT assay. More effective results were observed after 72 h of treatment. The lowest IC50 values were found for the HepG2 cell line, ranging from 11.2 to 55.0 µg/mL. The combination of EG and HE exerted higher cytotoxic activity than SCHE or treatment with either lignan alone, with the lowest IC50 (13.31 µg/mL) being observed for the MCF-7 line. Furthermore, treatment with these lignans was significantly more cytotoxic for some tumor cell lines compared to the normal cell line, GM07492A, indicating selectivity. These results suggest that these lignans may be used to treat cancer without affecting normal cells.
The cancer remains a major public health issue as more than one million people are diagnosed with cancer each year. Natural products constitute the major sources of chemical diversity, in purified or structurally identified form, and many drugs used for therapeutic applications are complex natural products or their derivatives (Salvador et al. 2013).
The plant species Styrax camporum Pohl (Styracaceae), popularly known as “estoraque do campo” or “cuia de brejo”, is found in the states of São Paulo and Minas Gerais, Brazil, and is used in folk medicine to treat ulcers (Lorenzi 1982). Previous phytochemical studies have isolated egonol (EG; Fig. 1a) and homoegonol (HE; Fig. 1b), benzofuran neolignans used as phytochemical markers for the quality control of extracts of the genus Styrax (Moraes et al. 2011). Important biological activities are described in the literature on hydroalcoholic extract of S. camporum stems such as antiulcer (Bacchi and Sertié 1994; Bacchi et al. 1995) and antiparasitic activities (Braguini et al. 2012) as well as chemical genus markers egonol and homoegonol such as antifungal, antibacterial (Pauletti et al. 2000), anti-complement (Min et al. 2004) and cytotoxic activities (Li et al. 2005).
The development of chemoresistance, toxicity and side effects requires the identification of relatively non-toxic drugs or natural products to wage a more humane war against cancer (Pan and Ho 2008). However, the success of cancer chemotherapy depends on the development of drugs that selectively destroy tumor cells, or at least limit their proliferation without causing severe side effects (Nussbaumer et al. 2011).
Considering the need for new selective molecules for cancer therapy, the present study investigated the cytotoxic activity of a hydroalcoholic extract of S. camporum (SCHE) and its chemical markers, EG and HE, against different tumor cell lines.
Styrax camporum Pohl and the fruits of S. ferrugineus were collected in May and October 2012 in the Santa Cecilia garden (20°46′12″S and 47°14′24″W), Patrocínio Paulista, São Paulo, Brazil. The specimens were identified by Prof. Dr. Alba Regina Barbosa Araújo, University of Franca, São Paulo, Brazil. Voucher specimens (SPFR 13754 and SPFR 12169, respectively) were deposited in the Herbarium of the Department of Biology, Faculty of Philosophy, Sciences and Letters of Ribeirão Preto, University of São Paulo.
About 790 g of S. camporum Pohl stems were dried in a hot air oven at 40 °C. The material was ground in a blender and the powder was immersed in a solution of EtOH–H2O (7:3 v/v) for 24 h at room temperature and then filtered three times through filter paper. This solution was concentrated at a temperature of less than 40 °C under vacuum to remove the solvent, yielding 95 g of a crude extract.
The fruits of S. ferrugineus (94.7 g) were submitted to extraction with MeOH, yielding 8.6 g of a crude extract. Approximately 3.5 g of this extract was subjected to column chromatography on silica gel (70–230 mesh American Society for Testing and Materials (ASTM), 60 Å, São Paulo, Brazil, Sigma-Aldrich). The mobile phase consisting of n-hexane:EtOAc was used in increasing order of polarity: n-hexane (500 mL), n-hexane:EtOAc (9:1, 500 mL; 8:2, 500 mL; 7:3, 500 mL; 6:4, 500 mL; 5:5, 500 mL; 4:6, 500 mL; 2:8, 500 mL; v/v), EtOAc (500 mL), and EtOAc:MeOH (9:1, v/v; 500 mL). Sixty-six subfractions were thus obtained. Egonol was isolated from subfractions 32 to 36 and HE from subfractions 19 to 30. These subfractions were purified by Preparative thin-layer chromatography (TLC preparative) using dichloromethane (DCM)-MeOH (97:3, v/v) as the mobile phase, analyzed by HPLC–UV in comparison to authentic samples, and also identified by nuclear magnetic resonance spectroscopy (1H-NMR).
The following cancer cell lines were used in the present study after the 4th passage: murine melanoma (B16F10) kindly provided by the Department of Biochemistry (Faculty of Medicine, University of São Paulo), human breast adenocarcinoma (MCF-7), human hepatocellular liver carcinoma (HepG2) (courtesy of Mutagenesis Laboratory, Department of Biological Sciences, Universidade Estadual Paulista), human cervical adenocarcinoma (HeLa), and human glioblastoma (MO59J) obtained from the Cell Bank of the Federal University of Rio de Janeiro. A normal human cell line (lung fibroblasts, GM07492A) (courtesy of Mutagenesis Laboratory of the University of São Paulo) was included to evaluate the possible selective activity of the natural products tested. The different cell lines were maintained as monolayers in plastic culture flasks (25 cm2) containing HAM-F10 plus DMEM (1:1; Sigma-Aldrich) for B16F10, MCF-7, HeLa, MO59J and GM07492A or only DMEM for HepG2, both supplemented with 10 % fetal bovine serum (Nutricell, Campinas, Brazil), 2.38 mg/mL Hepes (Sigma-Aldrich) and antibiotics (0.01 mg/mL streptomycin and 0.005 mg/mL penicillin; Sigma-Aldrich). The cells were incubated at 37 °C in a humidified 5 % CO2 atmosphere.
The cytotoxic effects of the crude extract and isolated compounds was determined by monitoring the growth of untreated and treated cells using the Cell Proliferation Kit (an XTT-based colorimetric assay, Roche, Mannheim, Germany) after 24, 48 and 72 h of incubation. SCHE was directly diluted in complete culture medium at concentrations of 2.44–5000 µg/mL, while EG and HE were dissolved in dimethylsulfoxide (0.02 µg/mL; Sigma-Aldrich) and complete medium to obtain concentrations ranging from 0.78 to 1250 µg/mL. For the purpose of comparison, the chemotherapeutic agents doxorubicin (Zodiac, São Paulo, Brazil), (S)-(+)-camptothecin (Sigma-Aldrich) and etoposide (Sigma-Aldrich) were examined under the same experimental conditions. For the experiments, 104 cells were plated onto 96-well microplates. Each well received 100 µL HAM-F10/DMEM or DMEM medium containing the different concentrations of SCHE, EG and HE and the cells were cultured in a 5 % CO2 atmosphere for 24, 48 and 72 h at 37 °C. After incubation, the culture medium was removed and the cells were washed with 100 µL phosphate-buffered saline and exposed to 100 µL HAM-F10 culture medium without phenol red. At designated time points, the tetrazolium salt (sodium 3'-[1-phenylaminocarbonyl)-3,4-tetrazolium]-bis(4-methoxy-6-nitro)benzene sulfonic acid hydrate (XTT, Roche Life Science, Indianapolis, IN, USA) was added to each well (25 µL) and the microplates were incubated for 17 h at 37 °C to allow the formation of an orange formazan dye product by metabolically active cells. Absorbance was read spectrophotometrically in an ELISA reader (Asys UVM 340/Microwin 2000 (Biochrom, Holliston, MA, USA)) at a wavelength of 450 nm and at a reference wavelength of 620 nm. The experiments were performed in triplicate.
The selectivity index was calculated by dividing the IC50 value of the isolated compound obtained for GM07492A cells by the IC50 value obtained for the cancer cell line.
Cytotoxicity was assessed using the IC50 value (50 % cell growth inhibition) as a response parameter, which was calculated with the GraphPad Prism program by plotting cell survival against the respective concentrations of the natural products tested. One-way ANOVA was used for the comparison of means (P < 0.05).
The results obtained are summarized in Table 1. As can be seen in the table, the natural products exerted a time-dependent cytotoxic effect against most of the cell lines tested. For all cell lines, the lowest IC50 values were observed at 72 h of treatment with SCHE, EG and/or HE.
The lowest IC50 values were observed when the cells were treated simultaneously with EG and HE for most of the cell lines tested. The lower IC50 values were obtained for MCF-7 cells (40.9, 23.3 and 13.3 µg/mL at 24, 48 and 72 h of treatment, respectively). The value observed for these cells at 72 h of treatment is similar to that obtained for the doxorubicin (13.4 µg/mL at 72 h of treatment).
Considering the selectivity, most of the tumor cell lines treated with HE exhibited lower IC50 values than those observed for the normal cell line, wherein the selectivity index ranged from 1.2 (MCF-7 cells, 24 h of treatment) to 2.8 (HeLa cells, 48 h of treatment).
Although low IC50 values were observed for cells treated with EG plus HE, the lowest IC50 value was obtained for HepG2 cells treated with EG (11.2 µg/mL at 72 h of treatment), with a selectivity index of 6.3.
According to Suffness and Pezzuto (1990), a selectivity index of 2.0 or higher is interesting since this value indicates that the compound is two-fold more cytotoxic against the tumor cell line compared to the normal cell line.
According to the U.S. National Cancer Institute, only extracts with IC50 values of <30 µg/mL against experimental tumor cell lines are promising agents for anticancer drug development (Suffness and Pezzuto 1990). The present results showed that EG and HE were more cytotoxic than SCHE against all tumor cell lines tested. The combination of EG and HE showed IC50 values of less than 30 µg/mL for MCF-7 and EG for HepG2. Furthermore, the natural products exhibited high selective indices in some cases. These findings suggest the combination of EG and HE as well as EG alone to be a promising alternative for the development of anticancer drugs.
Literature data have demonstrated the cytotoxic activity of these benzofuran lignans, especially EG. Teles et al. (2005), evaluating the two benzofuran lignans, EG and HE, in cultures of Hep-2 (larynx epidermoid carcinoma), HeLa and C6 (rat glioma) cells, showed a dose-dependent cytotoxic effect of the two lignans against all cell lines tested. The authors observed significant activities (IC50 < 10 µg/mL) of EG against C6 and Hep-2 cells and of HE against C6 and HeLa cells when compared to the positive control streptonigrin after 24 h of treatment. Reiter et al. (2014), evaluating the cytotoxicity of EG and derivatives against a wild-type human T cell lymphoblast-like cell line (CCRF-CEM) and a human leukemia cell line (CEM/ADR5000), observed considerable cytotoxicity of EG (IC50 = 1.88 ± 0.41 µM).
Li et al. (2005) evaluated the cytotoxicity of fractions of an EtOH extract of S. perkinsiae and isolated compounds in two breast cancer cell lines (MCF-7 and MDA-MB-231) after 48 h of treatment using a colorimetric chemosensitivity assay with sulforhodamine B and demonstrated the cytotoxic activity of EG at high concentrations (IC50 > 100 µg/mL). These authors believe that most benzofuran neolignans and nor-lignans with phenolic hydroxy groups possess cytotoxic activity. It has been demonstrated that the phenolic hydroxy groups largely contribute to this biological activity and significant differences in activity have been related to the lignan structure (Hayakawa et al. 2004).
Further biological and phytochemical studies should be conducted to identify the mechanism of action underlying the cytotoxicity of lignans EG and HE.
This work was supported by the São Paulo Research Foundation (FAPESP; Brazil; Grant # 2013/13903-9). P.F. Oliveira was the recipient of a Doctoral fellowship from FAPESP (Grant # 2011/201310-2). The authors are grateful to Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) for the fellowships granted.
The authors declared that they have no conflicts of interest.