Involvement of mitochondrial pathway in triptolide-induced cytotoxicity in human normal liver L-02 cells

Triptolide, a purified diterpenoid triepoxide compound derived from a traditional Chinese medicine, Tripterygium wilfordii HOOK. f (TWHf), has been used in the treatment of autoimmune and inflammatory diseases. However, the toxicity of triptolide limits its application to a great extent. In the present study, we treated human normal liver L-02 cells (L-02 cells) with triptolide in vitro and investigated its toxic effects. The cytotoxicity was measured by 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay for cellular viability and by flow cytometry and Hoechst 33258 staining for apoptosis. The mitochondrial membrane potential (Delta Psi m) was evaluated by flow cytometry with JC-1 as probe. After treatment with triptolide, a decrease in the viability of L-02 cells and increase in apoptosis were observed. Triptolide-induced apoptosis was accompanied by loss of mitochondrial membrane potential and release of cytochrome c (cyt-c) from the mitochondria to the cytosol and down-regulation of anti-apoptotic protein Bcl-2 levels with concurrent up-regulation in pro-apoptotic protein Bax levels and tumor suppressor protein p53 levels. Triptolide-increased activity of caspase 9 and caspase 3 was also observed. These results indicate that triptolide induced cytotoxicity in L-02 cells by apoptosis, which is mediated through mitochondrial pathway.