Forsythia suspensa Leaves Triterpenoids Induce Breast Cancer Cell Apoptosis via the Mitochondrial Pathway

Forsythia suspensa is a traditional Chinese medicine. The leaves of F. suspensa, specifically the dried leaves of the plant, are commonly consumed as tea in China. In this study, F. suspensa leaves triterpenoids (FLT) were isolated and purified from the dried leaves of F. suspensa, and their in vitro antitumor activity as well as associated molecular mechanisms were systematically investigated. First, the primary components of FLT were determined using high-performance liquid chromatography (HPLC), and the results revealed ursolic acid (70.67%), oleanolic acid (16.23%), and betulinic acid (4.59%) as the major components. Next, the results of the 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay showed that FLT exhibited strong antiproliferative activity (p < 0.05) toward human breast cancer MCF-7 and MDA-MB-231 cells. Moreover, the flow cytometry detection of apoptosis using Annexin V-fluorescein isothiocyanate/propidium iodide (Annexin V-FITC/PI) revealed that FLT significantly induced apoptosis in MCF-7 and MDA-MB-231 cells in a dose-dependent manner (p < 0.001). Laser confocal microscopy showed that FLT was mainly located in the mitochondria and lysosome of cells. Meanwhile, after the FLT (15 μg/mL) treatment of MCF-7 and MDA-MB-231 cells, the mitochondrial membrane potential (MMP) (p < 0.001) and intracellular reactive oxygen species (ROS) level (p < 0.001) were significantly reduced. Finally, western blotting demonstrated that FLT (10 and 15 μg/mL) significantly reduced B-cell lymphoma-2 (Bcl-2) (p < 0.001) and cysteinyl aspartate specific proteinase-3 (caspase-3) protein levels (p < 0.001), but significantly increased Bcl-2 homologous antagonist/killer (Bak) protein (p < 0.05 or p < 0.001), cleaved-caspase-3 protein (p < 0.001), dynamic-related protein 1 (DRP1), and mitochondrial fission 1 protein (FIS1) levels (p < 0.01 or p < 0.001). Taken together, the results indicated that FLT promoted the apoptosis of MCF-7 and MDA-MB-231 cells by activating the mitochondrial pathway. This effect may be attributed to FLT affecting the expression of the Bcl-2 family in mitochondria by promoting DRP1 and FIS1-mediated mitochondrial division, thereby activating the cleavage of caspase-3 and ultimately leading to cell apoptosis.