Triptolide-mediated downregulation of FLIP in hepatoma cells occurs at the post-transcriptional level independently of proteasome-mediated pathways.

Cellular c-FLIP prevents apoptosis mediated by death receptor through inhibiting activation of caspase-8. Therefore, when c-FLIP is downregulated or eliminated, caspase-8 activation is promoted, and death receptor ligand-induced apoptosis is activated. It was reported that triptolide (TPL) sensitized tumor cells to TNF-α-induced apoptosis by blocking TNF-α-induced activation of NF-κB and transcription of c-IAP1 and c-IAP2. However, the effect of TPL on basal c-FLIP expression was not understood. In this study, we found that the combination of TNF-α and TPL accelerated apoptosis in human hepatocellular carcinoma cells and TNF-α-induced elevated as well as basal level of FLIP protein were downregulated by TPL. Additionally, we demonstrated that the basal level of FLIP in Huh7 cells was continuously downregulated following the incubation of TPL and downregulated more when dosage of TPL for treatment was increased. Subsequently, we showed that TPL reduced FLIP level in a transcription- and degradation-independent mechanism. Our findings suggest that TPL induces loss of FLIP at the post-transcriptional level independently of proteasome-mediated pathway, an additional mechanism of TPL sensitizing cancer cells to TNF-α-induced apoptosis.