Background: Previously, stingless bee (Trigona spp.) products from East Kalimantan, Indonesia, were successfully screened for in vitro antiproliferative activity against human cancer derived cell lines. It was established that propolis from T. incisa presented the highest in vitro cytotoxicity against the SW620 colon cancer cell line (6% cell survival in 20 μg/mL).
Materials and methods: Propolis from T. incisa was extracted with methanol and further partitioned with n-hexane, ethyl acetate and methanol. The in vitro cytotoxicity of the extracts was assessed by the MTT assay against human colon (SW620), liver (Hep-G2), gastric (KATO-III), lung (Chago) and breast (BT474) cancer derived cell lines. The active fractions were further enriched by silica gel quick column, absorption and size exclusion chromatography. The purity of each fraction was checked by thin layer chromatography. Cytotoxicity in BT-474 cells induced by cardanol compared to doxorubicin were evaluated by MTT assay, induction of cell cycle arrest and cell death by flow cytometric analysis of propidium iodide and annexin-V stained cells.
Results: A cardol isomer was found to be the major compound in one active fraction (F45) of T. incisa propolis, with a cytotoxicity against the SW620 (IC50 of 4.51±0.76 μg/mL), KATO-III (IC50 of 6.06±0.39 μg/mL), Hep-G2 (IC50 of 0.71±0.22 μg/mL), Chago I (IC50 of 0.81±0.18 μg/mL) and BT474 (IC50 of 4.28±0.14 μg/mL) cell lines. Early apoptosis (programmed cell death) of SW620 cells was induced by the cardol containing F45 fraction at the IC50 and IC80 concentrations, respectively, within 2-6 h of incubation. In addition, the F45 fraction induced cell cycle arrest at the G1 subphase.
Conclusions: Indonesian stingless bee (T. incisa) propolis had moderately potent in vitro anticancer activity on human cancer derived cell lines. Cardol or 5-pentadecyl resorcinol was identified as a major active compound and induced apoptosis in SW620 cells in an early period (≤6 h) and cell cycle arrest at the G1 subphase. Thus, cardol is a potential candidate for cancer chemotherapy.