Triptolide-Mediated Apoptosis by Suppression of Focal Adhesion Kinase through Extrinsic and Intrinsic Pathways in Human Melanoma Cells

Triptolide (TPL) has been shown to inhibit cell proliferation and induce apoptosis in various human cancer cells; however, the precise mechanism of apoptosis induced by TPL in human melanoma cells has not yet been elucidated. In this study, we investigated the precise mechanism underlying cytocidal effects of TPL on human melanoma cells. Treatment of human melanoma cells with TPL significantly inhibited cell growth and induced apoptosis, as evidenced by flow cytometry and annexin V-fluorescein isothiocyanate analyses. TPL increased the levels of Fas and Fas-associated death domain (FADD) and induced cleavage of Bid by activation of caspase-8 and cytochrome c release from mitochondria to the cytosol, which resulted in activation of caspase-9 and caspase-3. Moreover, TPL-induced apoptosis in SK-MEL-2 cells was mediated through dephosphorylation of focal adhesion kinase (FAK) and its cleavage by caspase-8-mediated caspase-3 activation via upregulation of Fas expression. We also found that TPL mediated the dissociation of receptor-interacting protein (RIP) from FAK and enhanced the formation of RIP/Fas complex formation initiating cell death. In conclusion, our data firstly demonstrated that TPL induces apoptosis by both extrinsic and intrinsic apoptosis pathways in human melanoma cells and identified that RIP shuttles between Fas and FAK to mediate apoptosis.