Antiproliferative Activity of Flavonoids from Croton sphaerogynus Baill. (Euphorbiaceae)

Croton sphaerogynus is a shrub from the Atlantic Rain Forest in southeastern Brazil. A lyophilized crude EtOH extract from leaves of C. sphaerogynus, obtained by maceration at room temperature (seven days), was suspended in methanol and partitioned with hexane. The purified MeOH phase was fractionated over Sephadex LH-20 yielding five fractions (F1–F5) containing flavonoids, as characterized by HPLC-DAD and HPLC-MS analyses. The antiproliferative activity of the crude EtOH extract, MeOH and hexane phases, and fractions F1–F5 was evaluated on in vitro cell lines NCI-H460 (nonsmall cell lung), MCF-7 (breast cancer), and U251 (glioma). The MeOH phase showed activity (mean log GI50 0.54) higher than the hexane phase and EtOH extract (mean log GI50 1.13 and 1.19, resp.). F1 exhibited activity against NCI-H460 (nonsmall cell lung) (GI50 1.2 μg/mL), which could be accounted for the presence of flavonoids and/or diterpenes. F4 showed moderate activity (mean log GI50 1.05), while F5 showed weak activity (mean log GI50 1.36). It is suggested that the antiproliferative activity of the crude EtOH extract and MeOH phase is accounted for a synergistic combination of flavonoids and diterpenes.


Introduction
Croton L. (Euphorbiaceae) has approximately 1300 species of herbaceous, shrubs, trees, and lianas forms. The genus is widely distributed in tropical and subtropical regions around the world, including Brazil, a country with 316 species, among which 253 are endemic [1,2].
Medicinal and toxic properties of Croton species have been ascribed to a wide variety of chemical compounds, such as terpenoids and steroids, alkaloids, and phenolic compounds, the latter including predominantly flavonoids, lignans, and proanthocyanidins [3][4][5].
According to the International Agency for Research on Cancer (IARC), the world impact of cancer has more than duplicated in the last 30 years [6]. The 2014 and 2015 annual estimative regarding Brazil foresees the emergence of approximately 576,000 new cancer cases, including nonmelanoma skin cancer. This latter cancer type is expected to become the most frequent among the Brazilian population (32% new cases), followed by prostate tumors (12%), female breast (10%), colon and rectum (6%), lung (5%), stomach (3.5%), and cervical (3%) [7].
Among drugs currently used in cancer treatment, over 60% are products directly or indirectly derived from plants [8]; most of them related to alkaloids and terpenoids [9].
The main goal of this study was to characterize the major polar constituents of the EtOH extract from leaves of C. sphaerogynus and evaluate their in vitro antiproliferative activities against tumor cell lines. This study differs from that of Motta et al. [20], which focused on the antiproliferative activity of diterpenes of nonpolar extracts from C. sphaerogynus and thus did not deal with its flavonoid constituents and their antiproliferative activity.
MeOH phase (MP) was also analyzed using a Bruker Daltonics equipment Esquire 3000 Plus HPLC with a Zorbax-C18 (250 × 4.6 mm, 3.5 m, Agilent, USA) column at 40 ∘ C, using the same conditions cited above. Solvent flow rates was .90 L/min, voltage 4000 V, nebulizer 27 psi, drying gas at 320 ∘ C, and flow of 7 L/min. Constituents were identified by comparing the corresponding UV-Vis and ESI/MS-MS spectra with MS data from the literature.
A purified compound from F5 was analyzed by 1 H NMR at 500 MHz, using a Bruker DRX-500 spectrometer. DMSOd 6 (Aldrich) was used as solvent and the residual peak of the nondeuterated solvent as internal standard.

Antiproliferative Assay.
Cancer cell lines used were kindly provided by the National Cancer Institute (NCI) at Frederick MA-USA and included NCI-H460 (nonsmall cell lung), MCF-7 (breast cancer), and U251 (glioma). Stock cell cultures were grown in medium containing RPMI 1640, supplemented with 5% of fetal bovine serum. Experimental cultures were supplemented also with penicilin : streptomicin (10 g/mL : 10 UI/mL).
Cells (100 L cells/well, inoculation density from 3-6 × 10 4 cell/mL) in 96-well plates were exposed to various sample concentrations (0.25 to 250 g/mL, 100 L/well) in DMSO/ RPMI 1640/FBS 5% at 37 ∘ C, 5% of CO 2 in air for 48 h. Final DMSO concentration did not affect cell viability. Cells were then fixed with 50% trichloroacetic acid and cell proliferation was determined by spectrophotometric quantification of cellular protein content at 540 nm, using the sulforhodamine B assay. Doxorubicin (DOX; 0.025-25 g/mL) was used as positive control. Three measurements were obtained at the beginning of incubation (time zero, 0 ) and 48 h after incubation for compound-free ( ) and tested ( ) cells. Cell proliferation was determined according to the equation 100× [( − 0 )/ − 0 ], for 0 < ≤ , and 100 × [( − 0 )/ 0 ], for ≤ 0 and a concentration-response curve for each cell line was plotted using software ORIGIN 7.5 (OriginLab Corporation) [21].

Data Analysis.
Using the concentration-response curve for each cell line, GI 50 (concentration causing 50% growth inhibition) [22] was determined by means of nonlinear regression analysis, using software ORIGIN 7.5 (OriginLab Corporation). The average activity (mean of log GI 50 ) of the extracts tested was also determined using MS Excel software. Extracts were regarded as inactive (mean > 1.5), weakly (1.1 < mean < 1.5), moderately (0 < mean < 1.1), or potently (mean < 0) active on basis of the NCI criteria for the mean of log GI 50 [23].

Results and Discussion
Retention times and UV and MS data analysis of the constituents from the crude EtOH extract (EE), MeOH (MP) and hexane (HP) partition phases, and fractions F1-F5 are given in Table 1 compound 17).
C. sphaerogynus was previously described as a major producer of diterpenes. Using serial extraction with hexane, CH 2 Cl 2 , and MeOH, Motta et al. [20] identified a great diversity of diterpenes, especially in the CH 2 Cl 2 extract. In the present study, maceration with EtOH at room temperature yielded an extract with a diterpene profile similar to that described by Motta et al. [20]. The lyophilized crude ethanol extract was resuspended in MeOH and partitioned with hexane. Partition did not eliminate the diterpenes from the polar fraction MP. Column chromatography using Sephadex and MeOH gave five fractions (F1-F5), the first of which (F1) contained diterpenes and flavonoids, while the further fractions (F2-F5) contained flavonoids exclusively. Crude EtOH extract(EE), hexane phase (HP), F1, and F2 showed weak antiproliferative activity ( Table 2). Methanol phase (MP) and F4 showed moderate activity, mainly against cell lines NCI-H460 (nonsmall cell lung) (mean log GI 50 0.54 and 1.05, resp.). On the other hand, F3 was inactive, while F5, containing virtually only quercetin-3-O-methyl ether ( Table 1, compound 17), also showed weak antiproliferative activity ( Table 2).
According to Motta et al. [20] the antiproliferative activity of C. sphaerogynus extract was a result of the massive presence of abietane and/or podocarpane diterpenes in nonpolar extracts. The present study tested two different sets of samples: extract and phases composed by different proportions of diterpenes and flavonoids (EE, HP, MP,and fraction F1) and fractions composed exclusively by flavonoids (F2-F5). Table 3 compares data obtained by Motta et al. [20] and the diterpene mixture obtained in the present work. According to Motta et al. [20] the CH 2 Cl 2 extract showed higher activity (mean log GI 50 0.86), compared with hexane and MeOH extracts (mean log GI 50 1.26 and 1.49, resp.). Regarding the diterpene profile, the latter extract was the most similar to MP and HP phases, although some qualitative and quantitative differences are evident. No crotonin derivative was detected in the present study, which may explain the moderate antiproliferative activity of the CH 2 CL 2 extract (mean 0.86 log GI 50 ) reported by Motta et al. [20] and the weak antiproliferative activity exerted by HP. The CH 2 Cl 2 extract from C. macrobothrys, which contains a crotonin derivative, showed moderate antiproliferative activity (mean log GI 50 0.89) [11]. Grynberg et al. [31] tested trans-dehydrocrotonin and trans-crotonin isolated from C. cajucara Benth. on the survival of mice bearing Sarcoma 180 and Ehrlich carcinoma and observed a significant antitumor activity when mice were treated with trans-dehydrocrotonin.
MP phase was the most active sample, showing moderate antiproliferative activity. The relative proportion of diterpenes and flavonoids (Table 4) might be important to enhance the antiproliferative activity. Either extracts with high contents of diterpenes or fractions with high contents of flavonoids presented weak or no antiproliferative activity. F2, F3, and F4 (fractions lacking diterpenes) were shown to be inactive. F1, though still containing diterpenes, showed flavonols in smaller amount than MP; F5, composed virtually by quercetin-3-O-methyl ether, also showed weak antiproliferative activity. The hexane phase (HP) contained no detectable flavonoids. These results suggest that Croton species lacking crotonin derivatives might have moderate antiproliferative activity if they have a combination of other diterpenes and flavonols.
Extracts from Croton species are frequently reported as exerting antiproliferative activity. The essential oil from the stem bark of C. lechleri showed mutagen-protective efficacy [32] and crude extracts from stems of C. cajucara [19], containing clerodane diterpenes, exert antitumor activity against the K562 leukemic cell line. The CH 2 Cl 2 extract of C. macrobothrys leaves, also containing clerodane diterpenes, exhibited moderate antiproliferative activity against several cell lines, in particular NCI-H460 (nonsmall cell lung) and K562 (leukemia) [11].
On the other hand, flavonoids such as apigenin dihexoside and tiliroside (kaempferol-p-coumaroyl glucoside) detected in leaf extract of C. dichrous, C. myrianthus, and C. erythroxyloides showed weak or no growth cell inhibition. However, extracts or fractions with substantial amounts of these compounds showed weak activity [30]. However, MeOH extract from C. erythroxyloides obtained under reflux   [30]. Angst et al. [33] observed that the flavonol quercetin inhibits the growth of pancreatic cancer cell lines by inducing apoptosis. The association of gemcitabine (a standard chemotherapeutic drug administered to patients with pancreatic cancer) and quercetin had no additional effect when compared with quercetin administered alone. The authors also observed a significant apoptotic effect and reduced tumor cell proliferation in in vivo assay using quercetin.
Besides synergism, the chemical structure of the flavonoids seems to be directly related to their antiproliferative activity. Burmistrova et al. [34] showed that synthetic flavonols with a hydroxyl group at the C3 position are 7-fold more potent than flavonols with a methoxyl group at the same position. This result suggests that a C3 methoxyl group at C3 is a reducing factor of cytotoxicity. In the present study, quercetin-3-O-methyl ether alone showed weak antiproliferative activity (mean of log GI 50 1.36). However, according to Seito et al. [35] flavonoids with methoxyl groups at positions other than C3 seem to have inhibitory effect on cell growth.

Conclusion
The antiproliferative activity of Croton sphaerogynus seems to be related to the presence of diterpenes and flavonoids. MeOH phase (MP) presented the highest antiproliferative activity among all samples tested and is showed to be composed by diterpenes and a high amount of flavonoids, in comparison with the crude EtOH extract (EE) and F1. Fractions containing no diterpenes showed weak antiproliferative activity. Samples containing small proportions of flavonoids also showed weak antiproliferative activity. The relative proportions of representatives of these two metabolite classes (flavonoids and diterpenes) in C. sphaerogynus extracts seem to be crucial to determine their antiproliferative activity.