On the mechanism of action of triptolide, beyond the NF-kB pathway

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Triptolide is a diterpene triepoxide extracted from Trypterygium wilfordii, a shrub commonly used in Chinese traditional medicine. Triptolide demonstrates potent antitumor activity in solid and circulating tumors implanted in nude mice. For this reason, over the past five years continuous efforts have been made to identify the molecular and cellular circuitry activated by triptolide in cancer cells. p53 and DNA damages induced by triptolide were investigated, as well as the effect of the compound against most of the conventional anticancer targets. Our results suggest that none of the targets tested, including topoisomerases, proteasome functions, microtubules organisation, and RNA/DNA synthesis, were affected by triptolide. The antiproliferative activity of triptolide on a panel of 12 different cell lines revealed a massive antiproliferative activity, with IC50 values in the 3-70 nM range. Interestingly, similar responses were observed regardless of the organ of origin, the metastatic, p53 or ER status, or doubling time of the cells. The antiproliferative properties of triptolide was correlated with apoptosis induction, and its effects were compared to those obtained with 5 established anticancer drugs. Triptolide triggers early apoptosis of human Namalwa Burkitt lymphoma cells more potently than camptothecin. Using propidium iodide staining to probe cell cycle alteration, we identified that triptolide delays asynchroneous population of A549 cell in S phase, following a 24 hours treatment. Other phenotypical tests were performed to determine the impact of the drug on cell morphology, mitochondrial, ER and Golgi integrity using A549 lung cancer cells. The NF-kB pathway was also investigated. Using a CEM/NF-kB transfected reporter cell line, we observed that a massive dose (100xIC50 of proliferation) of triptolide is required to fully inhibit activation of NF-kB induced by TNFalpha. The NCI-60 profiles of triptolide and the NF-kB inhibitor parthenolide are markedly distinct, suggesting therefore that these two compounds may have an different mode of action. Altogether, our results indicate that NF-kB may not be the unique altered pathway leading to cell death upon triptolide treatment. This study shed light on the mechanism of action of the diterpene triptolide, a potent anticancer agent likely acting via an original target.