Natural plant extract tubeimoside I induces cytotoxicity via the mitochondrial pathway in human normal liver cells

Tubeimoside I (TBMS I) is a natural compound extracted from Bolbostemma paniculatum (Maxim.) Franquet (Cucurbitaceae), a traditional Chinese herbal medicine widely used for the treatment of inflammation. Recently, it has been suggested that TBMS I may be a potent anticancer agent for a variety of human cancers. However, TBMS I is known to distribute preferentially in the liver, and thus may harm normal liver cells if it is delivered systemically for cancer treatment. This safety concern warrants careful evaluation of the hepatotoxicity of TBMS I to normal liver cells, which to date has not been carried out. Here, we report the cytotoxic effects of TBMS I on one type of normal liver cells (L-02 cells), and the associated molecular events as underlying mechanisms. Cultured human normal liver L-02 cells were treated with TBMS I at concentrations of 0, 15 and 30 µM for 24, 48 and 72 h, respectively. Subsequently, the cell survival rate was evaluated by the MTT dye method, and several key molecular events associated with apoptosis were assayed, including mitochondrial depolarization, release of cytochrome c (cyt-c), activation of caspases, and the balance between Bax and Bcl-2 protein expression. Our results indicate that TBMS I inhibited the proliferation of L-02 cells in a dose- and time-dependent manner. The TBMS I-induced growth inhibition of L-02 cells was accompanied by the collapse of mitochondrial membrane potential, release of cyt-c from the mitochondria to the cytosol, activation of caspase-9 and -3, decrease of anti-apoptotic protein Bcl-2 levels and increase of the pro-apoptotic protein Bax levels, all indicative of apoptosis through the mitochondrial pathway. Taken together, these results confirm that TBMS I has a significant apoptotic effect on normal liver L-02 cells, which may be significant to its clinical applications.