Objectives  To investigate the effect of a Chinese traditional medicine, gambogic acid (GA), on human malignant melanoma (MM) A375 cells and to study the mechanism of apoptosis induced by GA.

Methods  A375 cells were treated with GA at different doses and for different times, and their proliferation and viability were detected by 3-[4,5-dimethylthiazol-2-yl]-2,5-diphenyltetrazolium bromide (MTT) assay. Apoptosis induced by GA in A375 cells was observed by annexin-V/propidium iodide doubling staining flow cytometry assay and Hoechst staining. To further determine the molecular mechanism of apoptosis induced by GA, the changes in expression of Bcl-2 and Bax were detected by real-time reverse transcriptase-polymerase chain reaction (RT-PCR) and Western blot, and caspase-3 activity was measured by fluorescence resonance energy transfer (FRET) probe.

Results  After incubation with GA, A375 cell proliferation was dramatically inhibited in a dose-dependent manner. After these cells had been exposed to GA for 24, 36 and 48 h, the IC50 values were 1.57 ± 0.05, 1.31 ± 0.20, and 1.12 ± 0.19 µg/mL, respectively. Treatment of A375 cells with GA (2.5–7.5 µg/mL) for 36 h resulted in an increased number of early apoptotic cells, which ranged from 27.6% to 41.9%, in a dose-dependent manner, compared with only 3.5% apoptotic cells in the non-GA-treated group. An increase in Bax and decrease in Bcl-2 expression were found by real-time RT-PCR and Western blot. Caspase-3 activity was increased in a dose-dependent manner, observed by FRET probe.

Conclusion  GA can inhibit the proliferation of A375 cells and induce their apoptosis, which may be related to the up-regulation of the Bax/Bcl-2 ratio and caspase-3 activity.