There is no doubt that the development of multidrug resistance in cancer therapy, their serious side effects, as well as their high costs have stimulated new recent cancer therapy approaches to find out new, more efficacious and less toxic treatments.
Essential oils (EOs) are one of the most important natural products derived from plants, due to their various biological properties and their medicinal and nutritional uses [1
]. Many plant EOs have been used as medicine for centuries and have demonstrated several health benefits, including effects on infectious, chronic, and acute diseases [2
]. Numerous EOs have shown antimicrobial activity against several plant and human pathogens [3
Moreover, some EOs have acquired a great potential interest, being able to successfully treat different types of cancer cells. EOs from Melissa officinalis
L., Melaleuca alternifolia
(Maiden & Betche) Cheel, Salvia officinalis
L., Thymus vulgaris
L., and Origanum vulgare
L. are examined for their cytotoxic and antiproliferative actions in tumor cell lines to evaluate their efficiency as possible alternatives for cancer treatments [5
The genus Origanum
) includes 39 species widely distributed in the Mediterranean region [7
]. O. vulgare
is a perennial herbaceous plant, known commonly as “oregano”, with four subspecies in Italian flora.
Numerous studies have focused recently on the active secondary metabolites of this plant [9
]. Oregano EO displayed antimicrobial and antiviral activities [10
]. Mancini et al. [14
] studied the chemical composition of oregano EO from different regions in Italy and reported the presence of several constituents with promising antifungal activity against Monilinia laxa
(Aderh. & Ruhland) Honey, M. fructigena
Honey, and M. fructicola
(G.Winter) Honey. Moreover, oregano EO and its components significantly inhibited some phytopathogenic fungi such as Botrytis cinerea
Pers., Penicillium italicum
Wehmer, P. expansum
Link, Phytophthora citrophthora
(R.E. Sm. & E.H. Sm.) Leonian and Rhizopus stolonifer
(Ehrenb.: Fr.) Vuill. [15
] and Aspergillus niger
Tiegh., A. flavus
Link, A. ochraceus
K. Wilh., Fusarium oxysporum
W.C. Snyder & H.N. Hansen, F. solani
(Mart.) Sacc., Penicillium
sp., Pseudomonas aeruginosa
J.Schrt. ATCC 2730, Staphylococus aureus
Rosenbach ATCC 6538, Clavibacter michiganensis
S., P. infestans
Mont., Sclerotinia sclerotiorum
Lib., and Xanthomonas vesicatoria
Several studies stated that O. vulgare
EO is mainly composed of phenols, p
-cymene, and γ-terpinene [17
]. Mockute et al. [18
] mentioned that linalyl acetate, β
-caryophyllene and sabinene are the principal constituents of O. vulgare
EO, whereas, D’Antuono et al. [19
] and Perez et al. [20
] reported that linalool is the main volatile component of oregano EO. Recently, the chemical composition of the EO extracted from O. vulgare
ssp. was reported by Mancini et al. [14
] and the results explicated that the phenolic compounds thymol and carvacrol were the major constituents of oregano EO, followed by linalool and trans
El Babili et al. [21
] reported the cytotoxicity of ethyl acetate and ethanol extracts from leaves of O. compactum
against human breast cancer cells (MCF7). On the other hand, Bakkali et al. [22
] and Mezzoug et al. [23
] reported that carvacrol, the major component of oregano EO, showed antimutagenic activity, which seems to be mainly linked to the induction of mitochondrial dysfunction.
The main objectives of this study are to assess (i) the cytotoxic effects of oregano EO and its main constituents against hepatocarcinoma cell line HepG2. Healthy human renal cells HEK293 were used as a control to test the putative selectivity of EO and chosen compounds; (ii) the antibacterial activity of the above EO and its main constituents against Bacillus megaterium de Bary ITM100 gram positive (G+ve), B. megaterium Planch. (Act.) (G+ve) and Escherichia coli Migula gram negative (G−ve); iii) the phytotoxic effect of the studied EO against two common sensitive plant species (Raphanus sativus L. cv. “Saxa” and Lepidium sativum L.) and two weed species (Sinapis arvensis L. and Phalaris canariensis L.).
The composition of EO agrees partially with our previous studies, in which the EO of the same species presented the same chemical pattern [14
] except for the presence of limonene and E-citral. The EO belongs to the carvacrol/thymol chemotype. In fact, Origanum
taxa can be divided in three chemotypes: (i) linalool, terpinen-4-ol, and sabinene hydrate group; (ii) carvacrol and/or thymol group); and (iii) sesquiterpene group [24
]. Other authors proposed the existence of four chemotypes (thymol, carvacrol, thymol/carvacrol and carvacrol/thymol) of O. vulgare
grown in Calabria (Southern Italy) [17
]. The EO and its main components (carvacrol, thymol, limonene and citral) have been evaluated for their possible antiproliferative activity.
The results demonstrated the selective action of carvacrol and thymol against tumor cells rather than healthy renal cells.
The traditional chemotherapy methods for treating the cancer cells rely mostly on the characteristics of drugs to reduce the ability of cancer cells to continuously grow, multiply, and finally to induce the cell damage and death. For these reasons, some interesting natural alternatives have acquired a great potential interest, such as EOs, which are able to successfully treat different types of cancer cells.
Generally, the possible mechanisms of the neoplastic transformation of normal healthy cells could be explained by remaining the signaling of proliferation, inactivating the growth suppressors, and finally by facing the cell death [25
]. The potential efficacy of tested oregano EO is thought to depend on the specific toxicity of their single main active constituents and/or by their synergic effect [26
], Grbović et al. [28
] concluded that oregano is a significant source of biologically active substances that have cytotoxic activity against colon (HCT-116) and breast cancer (MDA-MB-231). In addition, Sivropoulou et al. [29
] reported that oregano EO exhibited high cytotoxicity against four permanent animal cell lines including two lines derived from human cancers. In particular, oregano EO caused complete cell death in all cell lines tested with dilutions of up to 1/10,000 and its ID50
on Vero cells was 1/36,000 [29
The possible expected mechanism of the cytotoxicity effect for either oregano EO or its main single constituents, especially citral, carvacrol, and thymol, could be due to the induction of the activity of Glutathione S-Transferase (GST) in various tissues. GST is thought to play an important role in detoxifying chemical carcinogens [26
]. Several studies have also examined the microscopic morphological changes for a variety of cells treated with different substances by using inverted a phase contrast microscope [30
] to understand the possible mechanism of the cytotoxicity effect. The obtained results of the microscopic morphological observations from this study underlined that the possible mechanism of the cytotoxicity of oregano EO and its main single constituents could be due to the induction of cell death by apoptosis and/or necrosis, which leads to increasing the cell permeability and hence losing several cytoplasmic organelles. Furthermore, the lipophilic nature and low molecular weights of the single constituents of studied oregano EO may ascribe their ability to immigrate easily through the cell membrane and alter its phospholipid layers which leads to increasing the fluidity of the cell membrane and finally destroying the cytoplasmic content [25
On the other hand, the IC50
values of both citral and limonene against renal health cells were estimated by 32 and 120 mg/L, as compared to 35 and 294 mg/L in the case of hepatocarcinoma cells. These results determine that these two single substances are not highly recommended as potential anticancer therapeutic agents. These observations agree with De Martino et al. [34
], who concluded that citral is considered as an apoptotic inductor both in leukocytes of healthy subjects and chronic myeloid leukemia patients [36
]. In addition, citral and some of its related compounds showed significant in vitro toxicity against pancreatic cell tumor lines (MIA PaCa2 cells) and human B-lymphoma as reported by Di Mola et al. [37
The antibacterial activity can be explained by the chemical structure of oregano EO and its single substances. In particular, carvacrol and thymol are phenolic compounds with similar structures (carvacrol: 5-isopropyl-2-methylphenol; thymol: 2-isopropyl-5-methylphenol), and the antimicrobial activity of phenolic compounds has been investigated against several phyto- and human pathogens [14
]. Generally, the single substances of oregano EO have a lipophilic nature and are able to interfere with membrane-catalyzed enzymes and with enzymes responsible for energy and protein production causing the cell death [13
The bioherbicidal effects of O. vulgare
EO could be also due to its composition of phenolic compounds such as thymol and carvacrol [38
]. Furthermore, oregano EO was able to inhibit significantly the seeds germination and the roots and shoots growth of Avena sterilis
L., Datura stramonium
L., Cucumis sativus
L. and Lactuca sativa
]. Further research demonstrated that several plant EOs and their phenolic compounds, carvacrol, and thymol, have herbicidal effects on weed germination and seedling growth of various plant species. In particular, the bibliographic research stated that O. vulgare
ssp. hirtum is an allelopathic plant and had showed a notable effect against some biosensor plants such as Avena sativa
and Lemna minor
. Hence, the ecological role of volatile terpenes in multiple ecological functions and in the phenomenon of allelopathy in case of oregano EO was confirmed [39
4. Materials and Methods
Dulbecco’s Modified Eagle Medium (DMEM) with 4.5 g/L glucose, dimethyl sulfoxide (DMSO), 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) were purchased from Sigma Aldrich (Milan, Italy). Fetal Bovine Serum (FBS), glutamine, penicillin-streptomycin, and Phosphate Buffered Saline (PBS) were purchased from Euroclone (Milan, Italy). All solvents and chemicals were of an analytical grade.
4.2. Plant Material
The aerial parts of O. vulgare were collected from populations growing wild in Marconia di Pisticci (Basilicata Region, Southern Italy). The plant was identified by Prof. Vincenzo De Feo and a voucher specimen, DFE203/2012, was deposited at the Herbarium in the Medical Botany Chair of the Salerno University. The representative homogeneous sample of the plant was collected during “balsamic time” corresponding to the flowering stage.
4.3. Isolation of the Volatile Oil
One hundred grams of aerial parts of O. vulgare
were ground in a Waring blender and then subjected to hydrodistillation for 3 h according to the standard procedure described in the European Pharmacopoeia [40
]. The yellow EO was solubilized in n
-hexane, filtered over anhydrous sodium sulphate and stored under N2
at +4 °C in the dark, until tested and analyzed.
4.4. GC-FID Analysis
Analytical gas chromatography was carried out on a Perkin-Elmer Sigma-115 gas chromatograph (Pelkin-Elmer, Waltham, MA, USA) equipped with a FID and a data handling processor. The separation was achieved using a HP-5MS fused-silica capillary column (30 m × 0.25 mm i.d., 0.25 μm film thickness). Column temperature: 40 °C, with a 5 min initial hold, and then to 270 °C at 2 °C/min, 270 °C (20 min); injection mode splitless (1 μL of a 1:1000 n-hexane solution). Injector and detector temperatures were 250 °C and 290 °C, respectively. Analysis was also run by using a fused silica HP Innowax polyethylenglycol capillary column (50 m × 0.20 mm i.d., 0.25 µm film thickness). In both cases, helium was used as carrier gas (1.0 mL/min).
4.5. GC/MS Analysis
Analysis was performed on an Agilent 6850 Ser. II apparatus (Agilent Technologies, Inc., Santa Clara, CA, USA), fitted with a fused silica DB-5 capillary column (30 m × 0.25 mm i.d., 0.33 μm film thickness), coupled to an Agilent Mass Selective Detector MSD 5973; ionization energy voltage 70 eV; electron multiplier voltage energy 2000 V. Mass spectra were scanned in the range 40–500 amu, scan time 5 scans/s. Gas chromatographic conditions were as reported in the previous paragraph; transfer line temperature, 295 °C.
4.6. Identification of the Essential Oil Components
Most constituents were identified by GC by comparison of their Kovats
retention indices (Ri) [determined relative to the tR
)], with either those of the literature [41
] and mass spectra on both columns with those of authentic compounds available in our laboratories by means NIST 02 and Wiley 275 libraries [45
] The components’ relative concentrations were obtained by peak area normalization. No response factors were calculated.
4.7. Cytotoxicity Test
4.7.1. Cell Cultures and Treatments
The human hepatocellular carcinoma cell line HepG2 and the human embryonic kidney HEK293 cells were cultured in DMEM (supplemented with 10% fetal bovine serum, 2 mM glutamine, 100 U/mL penicillin, and 100 μg/mL streptomycin) and maintained at 37 °C in a humidified atmosphere containing 5% CO2. The final DMSO concentrations in the cell cultures were no greater than 0.8%: these concentrations do not affect cell viability. DMSO treated cells were used as a control in all the experiments.
4.7.2. Cell Viability Assay (MTT Test)
In order to investigate the putative cytotoxic effects of the oregano EO, the 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl tetrazolium bromide (MTT) assay was carried out on HepG2 and HEK293 cells [45
]. The MTT assay is based on the capability of living cells to convert this yellow water-soluble tetrazolium salt into insoluble purple formazan crystals by cleavage of the tetrazolium ring by active dehydrogenase enzymes. This water insoluble product can be solubilized by using organic solvents and the resulting colored solutions are spectrophotometrically measured, getting an absorbance directly proportional to the percentage of viable cells. Briefly, HepG2 and HEK293 cells were plated in triplicate in 96-well plates at a density of 1.5 × 103
cells/well, and allowed to adhere for 24 h. The cultured cells were then treated with different concentrations of studied oregano EO (25, 50, 100, 200, 400, and 800 μg/μL) and of the single constituents (0.01, 0.02, 0.03, 0.06, 0.12, and 0.25 μg/μL for carvacrol and citral and 0.06, 0.11, 0.22, 0.45, and 0.90 μg/μL for limonene and thymol), and were incubated for 24 h at 37 °C. Control cells were incubated with a comparable amount of the vehicle (DMSO in DMEM medium). After discarding the medium from the wells, MTT solution (0.5 mg/mL in DMEM) was added to the cells, incubating for 4 h at 37 °C.
The formazan crystals were finally dissolved in DMSO:isopropanol (1:1) and 1% Triton solution. The absorbance was measured at a wavelength of 570 nm, with background subtraction at 630–690 nm, using a GLOMAX Multi detection System (Thermo scientific, Waltham, MA, USA). The untreated cells were considered 100% viable. The concentration that reduces the cell vitality to half is observed as IC50
. i.e., ″half maximal inhibitory concentration″ is a measure of the effectiveness of a substance in inhibiting cell multiplication.
4.7.3. Observation of Morphological Changes
The observation of morphological changes occurred in the tested cell lines were performed by using inverted phase contrast microscopy (Nikon Eclipse TS100, Nikon, Tokyo, Japan). HepG2 and HEK293 cell cultures were seeded in 12-well plates at a density of 2 × 105 cells/well, and then were treated for 24 h with different concentrations of oregano EO and of its single main constituents. The calculated values of IC50 for hepatoma cells have been selected for treatment of both studied cell lines (HepG2 and HEK293) in the case of a morphology-observation test.
4.8. Antibacterial Activity Test
4.8.1. Tested Bacterial Strains
Two G+ve bacterial strains were tested: B. megaterium ITM100 from the collection of “Istituto Tossine e Micotossine, Bari, Italy”, B. megaterium isolated from Actinidia chinensis (Act.). One G−ve bacterial strain E. coli. All tested bacterial strains were conserved in the collection of the School of Agricultural, Forestry, Food and Environmental Sciences of Basilicata University, Potenza, Italy.
4.8.2. Bactericidal Assay
The antibacterial test was carried out by using the disc diffusion method of Bhunia et al. [47
] on King B nutrient media [48
]. A suspension of each tested strain was prepared in sterile distilled water adjusted at 108
CFU/mL (OD ≈ 0.2 nm). Then, an aliquot of soft agar (0.7%) and prepared suspension (9:1; v
) was prepared and 4 mL were added over each plate (diameter 90 mm) containing 10 mL KB media. Blank Discs (6 mm)-OXOID were placed on the plate surfaces. Fifteen µL of EO and each single substance at concentrations 25% and 50% (v
) diluted in water and Tween 20 (0.2%) was carefully applied over discs. The bactericidal activity was estimated by measuring the diameter of inhibition zone (mm) formed around each treated point as compared to the control.
4.9. Phytotoxic Activity Test
A bioassay based on seeds germination and root/shoot elongation was carried out to evaluate the phytotoxic effect of the studied oregano EO on seeds R. sativus (commonly known as radish), L. sativum (commonly known as garden cress) and on seeds of two weed species S. arvensis (wild mustard) and P. canariensis (canary grass).
The seeds of radish and garden cress were purchased from Blumen srl (Piacenza, Italy), while wild mustard and canary grass was collected from wild plants. The seed-surface were initially sterilized by using 70% ethanol for 30 s and ten were immersed in distilled water for 15 min, and then 10 seeds were placed on each Petri dish (90 mm diameter) containing three filter paper layers (Sigma-Aldrich, Saint Louis, MO, USA) pre-treated with 7 mL of distilled water (-ve control) or 7 mL from each prepared concentration of oregano EO. The studied oil was dissolved in a mixture of water/acetone (99.5:0.5) and 0.2% soln. of Tween 20®, and then was prepared at the following doses (2.5, 1.25, 0.62, 0.25, 0.12, and 0.06 μg/mL).
The germination conditions were 23 ± 2 °C for 48 h with artificial photoperiod. Controls were performed with a water/acetone mixture. Seed germination was observed directly in petri dishes after 48 h from the incubation time, whereas the radical elongation was measured in centimeter after 120 h of incubation time. The whole phytotoxic assay was repeated 3 times with 3 replicates per each. The germination index was calculated using the formula:
G.I. % = [(S.G.t × R.E.t)/(S.G.c × R.E.c)] × 100
GI: germination index; S.G.t: average number of germinated treated seeds; R.E.t: average radical elongation for treated seeds; S.G.c: Average number of germinated seeds for control; R.E.c: average radical elongation for control.
Data are expressed as the mean ± SDs for germination, radicle elongation, and germination index. Data were analyzed using SPSS statistical program with Tukey test.
From the available literature, oregano EO and its main bioactive constituents seem to have a great potential as anticancer therapeutic agents especially thymol and carvacrol. In particular, the EO and its two constituents, thymol and carvacrol, explicated promising results against hepatocarcinoma cells (IC50), estimated to be 236, 289, and 48 mg/L, respectively. Conversely, they showed higher values of IC50 regarding the inhibition of health renal cells, which were estimated to be 310, 940, and 90 mg/L, respectively. Citral and limonene showed a cytotoxic effect (IC50) against renal health cells estimated to be 32 and 120 mg/L, as compared to 35 and 294 mg/L in the case of hepatocarcinoma cells. Therefore, these two substances are not recommended as potential anticancer therapeutic agents. Further studies will be carried out to explain the specific action mechanism(s) of the investigated single constituents. On the other hand, the potential toxicity and the exact side effects of the studied substance were rarely taken into consideration in the majority of related research. Therefore, future studies should be directed to obtain high-purified natural bioactive compounds from oregano and other aromatic plants and to study their possible potential toxicity on health human cells. On the other hand, mixtures of different compounds could show a higher efficiency than the single components, so the next studies should take into account this possibility with deep experiments using different percentages of main components of EOs to estimate the synergistic effect. Finally, the obtained results demonstrated that oregano EO and its main constituents have a significant antibacterial activity and a moderate phytotoxic effect.