INTRODUCTION
Cancer has emerged as one of the most alarming diseases in the last few decades throughout the world. The steep rise in the number of cancer cases may be attributed to the changes in food habits, use of tobacco and alcohol, chronic infections, exposure to harmful radiations and chemicals, or more widely due to change in lifestyle and environmental pollution.1 No extremely effective drug to treat most cancers is available in the market. There is a general call for new drugs that are highly effective, possess low toxicity and have minor environmental impact. Novel natural products offer opportunities for innovation in drug discover.2 Questions concerning the safety of synthetic agents have increased the interest in the use of natural compounds and have encouraged more detailed studies of plant resources, which are a rich source of bionutrients or bioactive phytochemicals.3
Essential oils from aromatic plants have been reported to possess anticancer properties as well as antioxidant activity. Most of essential oils have been first characterized for the treatment of inflammatory and oxidative diseases. It appeared that these essential oils could also have anticancer effects as there is a relationship between the production of reactive oxygen species to the origin of oxidation and inflammation that can lead to cancer.4,5
Origanum L. (Lamiaceae) is represented by 30 taxa in Turkey, 15 of them are endemic and 5 taxa are hybrid.6,7,8,9,10 Origanum species have traditionally been used as spices and herbal tea. They are traditionally used as a sedative, diuretic, degasifier, sweater and antiseptic, and also in the treatment of gastrointestinal diseases and constipation. This genus is rich in essential oils and bitter substances.11 Oregano essential oil has been found to be amongst the most effective antioxidant natural agents.12 The antioxidant effect of these essential oils is attributed to their major components, carvacrol and thymol, and it is the result of various possible mechanisms: free-radical scavenging activity, transition-metal-chelating activity, and/or singlet-oxygen quenching capacity.13
Origanum acutidens (Hand.-Mazz.) Ietswaart is an endemic, herbaceous, and perennial plant growing mainly in East Anatolia.8 Essential oil of this plant possesses a variety of biological activities, including antioxidant activity.14 The aim of this study was, i) to investigate the essential oil composition of O. acutidens by gas chromatography/mass spectrometry (GC-MS), ii) to evaluate the cytotoxic effect of the oil on the growth of human cervical carcinoma cell line (HeLa) and HT-29 cell lines by xCELLigence method, and iii) to determine its radical scavenging effect by 2,2-diphenyl-1-picrylhydrazyl (DPPH) assay.
MATERIALS AND METHODS
Chemicals and reagents
DPPH, butylated hydroxyanisole (BHA), 2,6-di-tert-butyl-4-methylphenol (BHT), dimethyl sulfoxide (DMSO), formic acid, Trypsin-ethylenediaminetetraacetic acid, fetal bovine serum (FBS), penicillin/streptomycin and Dulbecco’s modified Eagle’s medium (DMEM)-high glucose were purchased from Sigma-Aldrich GmbH (Taufkirchen, Germany).
Plant materials
Origanum acuditens materials were collected from Bingöl city to Aşağıköy village, 8-9. km, in Quercus L. woods (Bingöl, Turkey) in June 2011. The identification of plant materials was confirmed by taxonomist in the Department of Biology, Bingöl University, Turkey. A voucher specimen (L. Behçet 8046) was deposited at the Herbarium of the Biology Department, Bingöl University, Turkey.
Essential oil isolation procedure
Two kilograms of dried aerial parts of the plant were shredded and combined with liquid nitrogen in a clean metal container. Then, the plant materials were processed for approximately 3 h in a Clevenger hydro-distillation apparatus and the essential oil was obtained. Anhydrous sodium sulfate was added to eliminate residual water in the oil. Finally, filtration was performed using blue band filter paper. The essential oil was stored in amber glass bottles at 4°C.
Gas chromatography/mass spectrometry analysis
The essential oil of O. acutidens were analyzed with an Agilent Technology 7890A GC system coupled to a 5975C inert MSD with Triple-Axis Detector (Agilent Technologies) on a capillary column [Agilent Technologies HP5-MS (30 m x 0.25 mm I.D. x 0.25 µm film thickness)]. GC temperature program was as follow: From 60 to 150°C at a rate of 3°C/min and holding there for 10 min, from 150 to 200°C at a rate of 5°C/min and holding there for 3 min, from 200 to 250°C at a rate of 15°C/min. Inlet temperature was 250°C. Spectra were obtained for the range of 50-550 m/z. The GC temperature program was run with helium as carrier gas, at a flow rate of 1 mL/min and injections in split mode (1:50). The mass-spectrometer interface temperature was set to 250°C. The temperature of the ion source was 230°C, electron energy 70 eV and quadruples temperature 150°C. The injection volume was 1 µL.
xCELLigence real time cell analyzer assay
The cytotoxic effect of the essential oil against the HT-29 and HeLa cell lines was determined by using the xCELLigence real-time cell analyzer-single plate (RTCA) instrument (Roche Applied Science, Basel, Switzerland) according to method of Koldas et al.15 DMEM with 10% FBS and 2% penicillin-streptomycin was used as the cell culture medium during the assessments. First, 50 µL of medium was added to each well of E-plate 96 and the plate was left in the hood for 15 min and then in the incubator for 15 min to let both the E-plate’s golden electrode well bottoms and the medium reach a thermal equilibrium. Then, the E-plate was inserted into the RTCA station in the incubator and a background measurement was performed. After ejection of the Eplate from the station, 100 µL HT-29/HeLa cell suspensions were added to the wells to obtain a 2.5x104 cell/well concentration in each well except for three of the wells. The cell concentration (cells/well) was analysed by using fully automated Cedex Hires Analyzer system (Roche Diagnostic Ltd, Rotkreuz, Switzerland) based on the manual Trypan Blue Exclusion Method. Three wells were left without cells to check if there would be an increase in cell index (CI) originating from the medium. 100 µL of medium was added to these wells instead of the cell suspension. After leaving the E-plate in the hood for 30 min, it was inserted to the RTCA station and the second step measurement was initiated for 80 min. During this period, the cells adapted to the bottom of the wells and entered into a growth and division phase. After this step, the E-plate was ejected from the station and solutions of the essential oil that was prepared with DMSO (final concentration of DMSO was less than 1% in each of the wells) and medium were added to the wells to obtain final concentrations of 250, 100, 50, and 10 µg/mL. Then final volume of the wells was completed to 200 µL by adding medium. After this step, the E-plate was inserted into the station and the main measurement period of 48 hours was initiated.
2,2-diphenyl-1-picrylhydrazyl radical scavenging assay
The free radical scavenging activity of the essential oil was determined by the method of, Blois16 with some modifications.17 The solution of DPPH in methanol (0.004%) was prepared fresh daily and 1 mL of this solution in methanol was mixed with 1 mL of sample solution of varying concentrations. Each mixture was kept in the dark for 30 min at room temperature. The absorbance was measured at 517 nm against a blank on a ultraviolet (UV) visible light spectrophotometer (Rayleigh, UV-2601). BHT and BHA were used as positive controls. The activity was calculated using the following formula: Scavenging %= [(Acontrol-Asample)/Acontrol]x100.
Statistical analysis
All experiments were done in triplicate. The results were expressed as means ± standard deviations. Statistical analyses were performed using the SPSS 11.5 (SPSS, Chicago, IL). Differences among means were done by ANOVA, and averages were compared using the Tukey test. The level of statistical significance was taken at p<0.01.
RESULTS AND DISCUSSION
Essential oil composition
Hydro-distillated essential oil of O. acutidens was analyzed by GC/MS. Twenty eight components were identified representing 98.99% of the oil (Table 1).
According to the GC-MS analysis results, carvacrol was the most abundant component of the oil (61.69%). Other main components of the oil were found to be p-cymene (17.32%), γ-terpinene (4.05%), and borneol (3.96%). As seen in Table 1, our findings are in agreement with previously published studies determined carvacrol (64.58-72%) and p-cymene (7.5-13.99%) were the major components of the oil.14,18,19
Cytotoxic effect by using real time cell analyzer
The cytotoxic effect of the essential oil was examined on HT-29 cell line at the concentrations of 10, 50 and 100 µg/mL by using real time cell analyzer xCELLigence method. The system measures impedance differences in order to derive CI values at time points whose intervals can be set by the operator. These impedance differences and thus the CI values depend on the cell activity at the bottom of the wells.20 CI is a dimensionless parameter derived as a relative change in measured electrical impedance to represent cell status. If it is decreasing, it shows us that the cancer cells are dying. Our data showed that the essential oil displayed significant cytotoxic effect against HT-29 cell line (Figure 1). The essential oil was most active at the concentration of 50 and 100 µg/mL.
Cytotoxic effect of the essential oil was also examined on HeLa cell line at the concentrations of 50, 100 and 250 µg/mL. The essential oil exhibited excellent cytotoxic effect against HeLa cell line at the tested concentrations [inhibitory concentration (IC)50<10 µg/mL]. The Figure 2 belonging to the CI changing according to the cell numbers against to time was obtained.
Begnini et al.21 reported that Origanum vulgare essential oil was composed mostly of 4-terpineol and induced a high cytotoxic effect in HT-29. Sivropoulou et al.22 investigated cytotoxicity of three Origanum essential oils and the oils exhibited high levels of cytotoxicity against four permanent animal cell lines including HeLa. Hussain et al.23 reported the inhibitory effect of Origanum essential oils (O. majorana, O. vulgare) on the cell proliferation of two human cancer (MCF-7 and LNCaP) cell lines and the IC50 values indicating that the tested Origanum essential oils showed prominent cytotoxicity against both the cancer cell lines.
In the present study, carvacrol (61.69%) was found to be the main component of the essential oil. It may be an important component in the cytotoxic effect of this oil. Mehdi et al.24 demonstrated that carvacrol is an effective anticancer compound with an IC50 of 50 mg/L at 48 h inducing growth inhibition in human cervical cells by using 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide and lactate dehydrogenase assays. In our previous study, we reported the inhibitory effect of the essential oil and the isolated pure or mixture components from Satureja boissieri essential oil on HeLa. In that study, the components containing the high concentrations of p-cymene, thymol and carvacrol effectively inhibited the growth of HeLa cells.25 Akalin and Incesu26 demonstrated that the carvacrol has cytotoxic effect on H-ras transformed 5RP7 and N-ras transformed CO25 cell lines upon time- and concentrations. Therefore, in our present study, the high cytotoxic effect of the essential oil against HeLa and HT-29 cells may be attributed the presence of carvacrol.
2,2-diphenyl-1-picrylhydrazyl radical scavenging activity of the oil
The free radical scavenging activity of essential oil from O. acutidens was measured by DPPH assay. DPPH radical scavenging activity assay is a widely used method to evaluate antioxidant activities in a relatively short time compared with other methods. This assay measures the ability of a sample to donate hydrogen to DPPH radical. DPPH radical scavenging activities of the essential oil, BHT and BHA are given in Table 2. Lower IC50 values indicate higher free radical scavenging activity. The radical scavenging activity of the essential oil increased by increasing the oil concentration (Figure 3). The IC50 value of the oil was determined as 0.387±0.005 mg/mL.
The essential oil of O. acutidens had moderate radical scavenging activity compared to the synthetic antioxidants. Sokmen et al.14 was determined the radical scavenging effect of O. acutidens essential oil with a IC50 value of 133.7±0.5 µg/mL and the oil exhibited lower radical scavenging activity than BHT (19.8±0.5 µg/mL). Hussain et al.23 reported the radical scavenging activities of Origanum oils (O. majorana and O. vulgare) was less than that of the positive control.
Sharopov et al.27 investigated the radical scavenging activities of the eighteen different essential oil components and carvacrol showed the one of best radical scavenging activity in the DPPH and 2,2’-azino-bis-3-ethylbenzthiazoline-6-sulphonic acid assays. Lin et al.28 reported that thymol and carvacrol were the major components attributing the DPPH free radical scavenging activity in the thyme wild essential oil. In the study of Ali et al.29 carvacrol exhibited 33.9% radical scavenging activity at 5 mM concentration. Therefore, the observed radical scavenging activity of the essential oil is mainly attributed to its main constituent carvacrol.
CONCLUSION
In this study, we have identified the chemical composition of O. acutidens essential oil and evaluated the oil’s cytotoxic effect on HT-29 and HeLa cell line. Our results clearly show that this essential oil is active against both tumor cell lines tested. These cytotoxic properties could be explained, in part, by the presence of carvacrol which is a main component of the oil. According to the results, O. acutidens essential oil may be suggested as a promising natural agent for alleviating HeLa and HT-29 cell growth.
ACKNOWLEDGEMENTS
The authors would like to thank Prof. Dr. Lütfi Behçet for the collection and identification of the plant material, Dr. Muhammed Altun for his help in the xCELLigence experiment, and Mehmet Ali Demirci for his help in the GC/MS analysis.
Conflict of Interest: No conflict of interest was declared by the authors.