Evaluation of the antioxidant and cytotoxic potency of Euphorbia rigida and Arbutus andrachne methanol extracts in human hepatocellular carcinoma cell lines in vitro

Ethnobotanical studies on plants and their active compounds take a great interest in traditional medicine. After pharmacological and toxicological studies, there will be a possibility to be used in therapy. This study aimed to examine the in vitro antioxidant and cytotoxic activity of the methanol extracts of Arbutus andrachne L. and Euphorbia rigida M.Bieb. 10, 25, 50, 75, 100 and 150 µg mL−1 concentrations of A. andrachne and E. rigida were tested for antioxidant activity by using DPPH radical scavenging assays, total antioxidant capacity (phosphomolybdate assay) and and metal ion chelating activity. In addition, in vitro cytotoxic effects of this plants methanol extracts on Hep3B and HepG2 human hepatocellular carcinoma cell lines were evaluated at 24, 48 and 72 h. The cytotoxicity test was carried using the MTT [3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl tetrazolium bromide] assay. Methanol extract obtained from both plants showed increased antioxidant activity depending on the increase in concentration. When A. andrachne and E. rigida methanol extracts were compared in free DPPH scavenging activity, total antioxidant capacity and metal ion chelating activity, A. andrachne methanol extract was found more effective than E. rigida. Results from MTT assay revealed that except for 72 h treatment of HepG2 cells with 400 and 500 µgmL−1 extract concentrations, A. andrachne methanol extract did not show significant cytotoxic effects on either Hep3B or HepG2 cells at any concentration and treatment time. On the contrary, it significantly increased proliferation in Hep3B cells from 48 h and at a concentration of 100 µg mL−1. E. rigida methanol extract exhibited statistically significant cytotoxic activity on HepG2 cells after 48 and 72 h treatment. However, the treatment concentrations of E. rigida methanol extract were not as effective on Hep3B cells as on HepG2 cells. According to our findings, it was determined that A. andrachne methanol extract did not have cytotoxic activity on neither Hep3B nor HepG2 cells, while E. rigida methanol extract had cytotoxic activity especially on HepG2 hepatocellular carcinoma cells. Further research is needed to identify and purify the active ingredients in E. rigida extracts.

WHO estimates that 80% of the population around the world use medicinal plants for primary health care. Usually, plant seeds, berries, roots, leaves, bark, or flowers are used for medicinal purposes [1]. There are very many reports on the use of herbal medicines for the treatment of individual health conditions or multiple health conditions. Several plant parts used for herbal medicines have been extensively studied.
Cancer, an abnormal malignant growth of body tissue or cell, is main health problem in both developed and developing countries. Hepatocellular carcinoma is the sixth most widespread cancer and the third most common cause of death from cancer [2]. Today, the active substances of many drugs used in cancer treatment are obtained from medicinal plants [3]. Medicinal plants have to be screened for anticancer activity for more of use. Herbal drugs show their anticancer effects by mechanisms such as carcinogen inactivation, antiproliferation, cell cycle suspension, induction of apoptosis and differentiation, suppression of angiogenesis, antioxidation and reduction of multiple drug resistance [4]. The growing cost for conventional treatments of cancer has prompted people to depend more on traditional medicine [5,6]. As a therapeutic alternative and a safe choice, herbal medicine might even increase the success rate of most cancer treatments by having a lower systemic toxicity indicated in chemotherapy [7]. Despite the afore-mentioned advantages, little is known about the possible medicinal application of medicinal plants or their cytotoxicity [8].
Arbutus andrachne is belonging to the Ericaceae family and distributed in the Eastern Mediterranean, mainly in Greece and Turkey [9]. The local name in Turkey is "sandalağacı" [10]. Fruits of this plant contain tannin, anthocyanin and carotenoids, and fruits are used widely in food products such as marmalade, fruit jelly and jam, and also in alcoholic beverages like wine and liquor [11]. Whole plant is used for various ethno medicinal purposes in the Mediterranean countries [12]. The antioxidant activities of methanol extracts from roots, leaves and fruits of the A. andrachne were investigated. Also, the effects of the extracts on the cardiodynamics of isolated perfused rabbit hearts. It was reported that methanol extract of the roots possesses high antioxidant activity and antihypertensive effect [13].
The genus Euphorbia is the largest in the plant family Euphorbiaceae, comprising about 2000.
known species and ranging from annuals to trees. 91. Euphorbia species are growing in Turkey [14]. All contain latex and have unique flower structures Euphorbia genus is known to contain a wide variety of terpenoids, ranging from mono-, sesqui-, and diterpenes to triterpenoids and steroids. Many of these compounds have been investigated for their toxicity or their potential therapeutic activity, promising anticancer activity and some have been used as medicines since ancient times [15]. This Euphorbia species have been used in folk medicine to treat skin diseases, migraines, inflammatory infections, respirational infections, body pain, intestinal parasites and warts [12,16,17]. The biological activities of the genus, including antitumor, antiviral, cytotoxic properties [18][19][20][21][22][23][24][25][26]. In this study, we investigated the antioxidant activities of the methanol extracts from Arbutus andrachne and Euphorbia rigida by measuring the free radical scavenging, total antioxidant capacity and metal chelating activity. Also, we evaluated the in vitro cytotoxic effects of the extracts on Hep3B and HepG2 human hepatocellular carcinoma cells by MTT assay. To the knowledge, there is no comparative study has been published to evaluate the cytotoxic properties of A. andrachne stem (wood and bark) methanol extract. Also, there are studies on the biological effects of various Euphorbia and Arbutus species but, we did not find any studies on the cytotoxic effects of plants in the literature review.

Plant material and methanol extraction
The branches (anatomically stem that consists of wood and bark) of A. andrachne and aerial parts of the E. rigida were used in this study. Collections of the branches and identifications were done by Assoc. Prof. Dr. Yelda Güzel in 2018 from the natural habitats from Hatay, TURKEY. Voucher specimens were deposited in the Herbarium of the Biology Department of Hatay Mustafa Kemal University. Voucher numbers are; Y. Güzel-1104 for A. andrachne and Y. Güzel-1105 for E. rigida. Plant samples air dried. Then, stems of A. andrachne and aerial parts of E. rigida (air-dried) samples were chopped and grounded into powder using an electric blender (HR2118 Philips, Netherlands). These plant powders (20 g) were extracted with 500 mL methanol at room temperature for 24-48 h. After filtration, the extract was evaporated at 40 °C under reduced pressure (Laborota 4002, Heidolph). The crude extracts were kept at +4 °C until the experimental studies.

1-diphenyl-2-picrylhydrazyl (DPPH) radical scavenging assay
The DPPH free radical scavenging potential assay is based on the stable DPPH scavenging activity. Quantitative measurements of radical scavenging assay were carried out according to the method described by Brand-Williams et al. [27]. One milliliter of 0, 1 mM DPPH methanol solution was added to 3 mL of different concentrations (10,25,50,75, 100 and 150 µg mL -1 ) of extracts in methanol. The mixture was vigorously shaken, then left at room temperature to stand. Using a microplate reader (Elisa Reader, Biotek Co, USA) the absorbance of the mixture was measured at λ = 517 nm after 30 min. 25 and 50 µg mL −1 ascorbic acid, the commercial known antioxidant was used as a positive control. All experiments were performed in triplicate. The percentage of the DPPH free radical was calculated using the following equation: where A 0 was the absorbance of the control and A 1 was the absorbance in the presence of the A. andrachne and E. rigida methanol extracts. The actual decrease in absorption induced by the test was compared with the positive controls.

Phosphomolybdate assay (total antioxidant capacity)
The total antioxidant capacity (TAC) assay of samples was carried out by the phosphomolybdenum method [28]. A 0.1-mL aliquot of the extract (10, 25, 50, 75, 100 and 150 µg mL −1 ) solution was shaken with 1 mL of reagent solution (0.6 M sulfuric acid, 28 mM sodium phosphate and 4 mM ammonium molybdate). The test tubes were covered with aluminum foil and incubated in a water bath at 95 °C for 90 min. After the samples were cooled, the absorbances were measured at 765 nm. Ascorbic acid was used as standard (25 and 50 µg mL −1 ) and the results were expressed as µg mL −1 of ascorbic acid equivalents. All experiments were performed in triplicate. The total antioxidant capacity (TAC) of the extracts was estimated using the following formula: Abs: absorbance.

Metal ion (Iron II) chelating ability
The chelation of ferrous ions by extracts was estimated by method of Dinis et al. [29]. Briefly, 50 µL of 2 mM FeCl 2 was added to 1 mL of different concentrations of the methanol extracts 10, 25, 50, 75, 100 and 150 µg mL −1 ). The reaction was initiated by the addition of 0.2 mL of 5 mM ferrozine solution. The mixture was vigorously shaken and left to stand at room temperature for 10 min. All experiments were performed in triplicate. The absorbance of the solution was thereafter measured at 562 nm. The percentage inhibition of ferrozine -Fe 2+ complex formation was calculated as: where A 0 was the absorbance of the control, and As was the absorbance of the methanol extracts/standard. Na 2 EDTA was used as positive control (standard). [(A 0 − As)/As] × 100

Cell culture and cytotoxicity assay
In cytotoxicity assays, Hep3B (ATCC HB-8016) and HepG2 (ATCC HB-8065) human hepatocellular carcinoma cells were used, because these cell lines are widely used hepatocellular carcinoma cells in studies about hepatotoxicity and drug metabolism. These cell lines possess well-known similarities each other. However, there are significant differences between these cell lines such as ethnic origins, intrinsic and drug-dependent gene expressions, cell growth inhibition, and signaling pathways associated with differential drug responses [30]. Hep3B and HepG2 cells were cultured in DMEM (Dulbecco's Modified Eagle's Medium) supplemented with 10% FBS, 1% penicilin-streptomycin and Amphotericin B under humidified atmosphere of 5% CO 2 at 37 °C until confluent. The cells were trypsinized and cytotoxicity assays were carried out in 96 well-plates.

Statistical analysis
Each experiment was performed in three replicates. Results were expressed as means ± SD and analyzed by One Way ANOVA (SPSS 20.00 software package program). Statistically significant difference was considered at the level of p < 0.05.

DPPH assay
The antioxidant potential of methanol extracts of A. andrachne and E. rigida was evaluated on the basis of their ability to scavenge stable free DPPH radicals. This test is based on change in color of DPPH solution from purple to yellow, due to scavenging of stable free DPPH radicals, which from purple to yellow measured at 517 nm [32]. A stronger yellow color indicates a greater ability of the extract to scavenge free DPPH radicals and stronger antioxidant potential. In the present study, the antioxidant potential of the methanol extracts from A. andrachne and E. rigida was explored in a dose-dependent (10-150 µg mL -1 ) manner as shown in Table 1 and Fig. 1. An increase in DPPH scavenging ability was observed with increase in concentration of extracts. The DPPH scavenging ability of A. andrachne methanol extract was higher than that of E. rigida methanol extract at each concentration tested. E. rigida methanol extract, on the other hand, showed high DPPH scavenging activity from the concentration of 50 µg mL −1 .  The DPPH scavenging activity of A. andrachne stem methanol extract was even higher than the standard antioxidant ascorbic acid from a concentration of 50 µg mL -1 .

Total antioxidant capacity assay (TAC)
The results in Table 1 and Fig. 1 show that A. andrachne stem methanol extract possess high TAC starting from 75 µg mL -1 , while the highest TAC was at 150 µg mL -1 (81.30 ± 0.235%). These results were even higher than the TAC value of ascorbic acid (76.50 ± 0.059). Although E. rigida methanol extract also showed total antioxidant activity, the total antioxidant capacity of E. rigida was considerably lower than both ascorbic acid and A. andrachne stem methanol extract.

Metal chelating activity
Phosphomolybdate assay is based on the reduction of phosphomolybdate ion in the presence of an antioxidant resulting in the formation of a green phosphate/MoV complex which is measured spectrophotometrically [33,34]. Metal chelating ability assay is based on the measurement of iron-ferrozine absorbance at 562 nm in presence of an antioxidant compound [29].

Cell proliferation assay
To investigate the cytotoxic potential of the A. andrachne and E. rigida methanol extracts on Hep3B and HepG2 cancer cells, the MTT colorimetric assay was performed. The cytotoxic activity of A. andrachne stem methanol extract against two cancer cell lines (Hep3B and HepG2) was evaluated ( Table 2  The cytotoxic effect of A. andrachne stem methanol extract on the proliferation and viability of HepG2 cells is presented in Table 2 and Fig. 3. A. andrachne methanol extracts at 50, 100, 150, and 200 40 µg mL -1 increased the viability of HepG2 cells somewhat at 24 h of treatment, but slightly decreased at the other three concentrations (25, 400, and 500 40 µg mL −1 ) (Table 2; Fig. 3). This effect was not statistically significant when compared with solvent control and farmorubicin ((40 µg mL -1 ). As a result of 48 h of treatment, cell viability started to decrease due to the increase in concentration with the application of 200, 400 and 500 µg mL -1 concentrations in the viability and proliferation of HepG2 cells. The most significant decrease in cell viability occurred after 72 h of treatment with 500 µg mL −1 extract concentration (67.05 ± 0.031%). Farmorubicin used as control produced a very high cytotoxic effect on HepG2 cells at all three treatment times and killed almost all cells.
Effect of E. rigida methanol extract on the proliferation and viability of Hep3B cells are present in Table 3 and Fig. 4. After 24 h of treatment of E. rigida methanol extract, all concentrations except 500 µg mL −1 increased proliferation in Hep3B cells. This effect was found to be significantly high in 25, 50 and 100 µg mL −1 (128.40 ± 0.015%, 125 ± 0.033%, 123 ± 0.098%, respectively) extract treatments compared to the control and solvent control (DMSO) group (p < 0.05). After 48 h of treatment, no significant effect of methanol extract was Effect of E. rigida methanol extract on the proliferation and viability of HepG2 cells are also present in Table 3 and Fig. 5. 400 and 500 µg mL −1 concentrations of E. rigida methanol extract reduced the viability of HepG2 cells at 24 h of treatment (p < 0.05), and all concentrations except for 25 and 50 µg mL −1 at 48 h of treatment resulted in a significant decrease in cell viability. After 48 h of treatment with E. rigida methanol extract, the concentration that most reduced HepG2 cell viability and proliferation was 500 µg mL −1 (58.92 ± 0.087%). The 72 h treatment of E. rigida methanol extract caused a concentration-dependently and significantly decrease in the Table 3 The effect of E. rigida methanol extract on the viability of Hep3B and HepG2 cells Data are the mean ± SD of three separate determinations. Means with an asterisk in the same line indicate a significant (p < 0.05) difference between the concentrations tested. Means with asterisks in the same column indicate the significant (p < 0.05) difference between the solvent used Concentrations (µg mL −1 ) Cell viability % ± SD  HepG2 cell viability decreased to 45.91% at 500 µg mL −1 concentration treatment (Table 3; Fig. 5). In HepG2 cells treated with E. rigida methanol extract decreased of the cell viability and proliferation was observed at most after 72 h of treatment, and this decrease was parallel to the increase in concentration. Farmorubicin killed almost all of the cells at all treatment time.

Discussion
Cancer is one of the most dangerous diseases with fast progression and high mortality around the world. Today, about 60-75% of anticancer agents are derived from natural sources [33][34][35]. The chemical constituents of the plants or crude extracts are known to be biologically active ingredients. Plant polyphenols have been highlighted as potential anticancer agents as well as being chemical inhibitors [36] because of their high antioxidant activity, targeting signaling molecules, and preventing or protecting cells from further damage and/or transformation into cancer cells [36,37]. In our literature review, we found antioxidant, anti-inflammatory and cell protective effects of Arbutus spp. but, we could not find any data regarding the cytotoxic effect of Arbutus species including A. andrachne stem (wood and bark). Furthermore, it has been shown that Euphorbia spp. is promising plants antitumor activity [38,39]. Also, there are reports about antioxidant activities of different Euphorbia species [25,26]. In this regard, the antioxidant activities of methanol extracts from A. andrachne and E. rigida were investigated using DPPH assay, phosphomolybdate assay and metal chelating assay. Also, cytotoxic effects of the extracts on two human hepatocellular carcinoma (Hep3B and HepG2) cell lines were investigated using MTT assay. When A. andrachne and E. rigida methanol extracts were compared in terms of antioxidant activities (DPPH free radical scavenging activity, total antioxidant capacity and metal chelating activity), A. andrachne methanol extract possess higher antioxidant activity. Differences between the antioxidant activities of the extracts may be attributed to different phytochemical contents of A. andrachne and E. rigida plants. The transition metal ion Fe 2+ have the ability to maintain the formation of free radicals by gain or loss of electrons. Therefore, the decrease of reactive oxygen species genesis can be carried out by the chelation of metal ions with chelating agents. Considering the cellular damages of reactive oxygen species and their relation with various diseases, it is important that plant extracts possess metal chelating activity besides free radical scavenging activity [40,41]. In our previous studies, we reported that diethyl ether, ethyl acetate and aqueous extracts from fruits of Arbutus unedo possess high total phenolic content and antioxidant activity [42]. Recently, it was suggested also, Arbutus unedo leaf aqueous extract has antioxidant activity, in vitro anti-inflammatory effect, and heat protective effect on the membrane integrity of human red blood cells against heat [43]. Furthermore, some studies reported vasorelaxant and antiaggregant effects of Arbutus unedo leaf extract on human platelets [44,45] and antihaemolytic and radical scavenging activities [46,47]. Hmaidosh et al. [48] reported that A. andrachne flowers, leaves and barks contain high amounts of total phenolic compounds. They reported also that these parts of plant possess antioxidant activity (FRAP), which was similar to the total phenolic contents. We reported also ethyl acetate, methanol and water extracts of Euphorbia platyphyllos possess high DPPH radical scavenging activity, previously [24]. Gapuz and Besagas suggested that leaf methanol extracts from three Euphorbia species (E. milii, E. trigona, and E. antiquorum) possess high total phenolic content and high DPPH radical scavenging activity [49]. They reported also that the extracts contain alkaloids, flavonoids, carbohydrates, saponin, and tannins. In this study, it was investigated also whether A. andrachne and E. rigida methanol extracts possess cytotoxic effect on Hep3B and HepG2 cells. A. andrachne methanol extract showed no cytotoxic effect on Hep3B cells and increased cell viability and proliferation. Also, A. andrachne methanol extract showed no significant cytotoxic effect on HepG2 cells at 24 and 48 h treatments. Only 72 h treatment with 400 and 500 µg mL −1 extract concentrations significantly reduced the viability and proliferation of HepG2 cells (Table 2; Fig. 3). This may be due to different responses of the cells to the components of the extract. When the results were examined, it was understood that methanol extract had no significant cytotoxic effect on both cells. Moreover, methanol extract appears to increase cell proliferation in Hep3B cells depending on time and concentration (Table 2; Fig. 2).
In previous studies conducted with cancer cells, it was found that extracts of Arbutus species don't possess cytotoxic and/or antiproliferative effect. Fortalezas et al. reported that hydroethanolic extract of A. unedo fruits caused no effect on viability of SK-N-MC human neuroblastoma cells [50]. Also, it was suggested that A. andrachne did not exhibit significant antiproliferative effect on breast, colorectal and skin cancer cells [4]. In addition, it was reported that arbutin, a main compound in Arbutus species, has not significant effect on cell viability, even at high concentrations on TCCSUP human bladder cancer cell line, HepG2 human hepatocellular carcinoma cells, and U937 human lymphoma cells [51][52][53]. In our study, very insignificant dose-dependent results were obtained for Arbutus extract. Because of the high antioxidant activity of the A. andrachne stem methanol extract, free radicals forming in the cell may be eradicated and cytotoxic effect is neutralized. In addition, it has been reported that the polyphenol compounds of plants to be anticancer agents [54][55][56].
Methanol extract of E. rigida exhibited statistically significant cytotoxic activity on HepG2 cells after 48 and 72 h treatment (Table 3; Fig. 5). However, the tried concentrations of E. rigida methanol extract were not as effective on Hep3B cells as on HepG2 cells (Table 3; Figs. 4 and 5). This result could be explained by different sensitivity of tumor cells to the chemical content of the extract. Previous studies suggested that triterpenoids, alkaloids, diterpene esters, organic acids, acetophenone derivatives, and flavonoids are the main chemical ingredients of Euphorbiaceae plants [57,58]. Therefore, the cytotoxic effect of E. rigida methanol extract on HepG2 cells may be considered to be due to the presence of these chemical ingredients. It was reported that Euphorbia plants possess a various biological activities such as antioxidative, antiproliferative, cytotoxic, anti-anaphylactic, antimicrobial, and anti-arthritic activities [59,60]. Javidnia et al. [61] reported that Euphorbia hebecarpa extracts possess cytotoxic effect on K562 and U937 cells but, they had no effects on KB cells. Sadeghi-Aliabadi et al. [62] showed that ethyl acetate extracts of E. macroclada were cytotoxic against MDA-MB-468 cells. Metin and Bürün reported that the aqueous extract of E. rigida aerial parts possess negative impacts on mitosis and, cytotoxic and genotoxic effects on Allium cepa root meristematic cells at concentrations over 50 ppm [63].

Conclusions
Turkey has a great history of folk medicine, but this knowledge has not been documented extensively so far. In the recent years researchers have carried out many studies about traditional medicine in Turkey. Now we have important information, documents and great opportunities to study on medicinal plants. According to our findings it could be conclude that constituents extracted by methanol from E. rigida possess cytotoxic activity on HepG2 hepatocellular carcinoma cells and it required further investigations for the identification and purification of the active components. However, A. andrachne stem methanol extract possess cytotoxic activity neither on Hep3B nor on HepG2 cells. For this reason, different studies can be carried out on this plant as well as other potential effects other than anticancer effects in the future.