Triptolide cooperates with chemotherapy to induce apoptosis in acute myeloid leukemia cells.

OBJECTIVE: Triptolide has shown antitumor activity in a broad range of solid tumors and on leukemic cells in vitro. MATERIALS AND METHODS: The THP1 cell line and primary acute myeloid leukemia (AML) cells were cultured with triptolide alone or in association with AraC or idarubicin in increasing concentrations. Apoptosis was measured by flow cytometry using DiOC6(3) for the cell line and fluorescein isothiocyanateAnnexin-V and CD45 labeling for fresh blast cells. Protein expression was measured by Western blot. Cell cycle distribution of apoptotic cells was measured by flow cytometry. RESULTS: A synergistic effect was observed when triptolide was added to idarubicin or to AraC to induce apoptosis of THP-1 leukemic cells. The triptolide/AraC association was also investigated in vitro on primary blast cells from 25 AML patients. This combination induced significantly higher percentages of apoptosis vs treatment with each drug separately (p<0.005). The IkappaB and X-linked inhibitor of apoptosis protein contents, which were altered by triptolide in idarubicin-treated cells, were not modified in AraC-treated cells. The association of AraC with triptolide increased the number of cells blocked in the S phase and most underwent apoptosis. CONCLUSION: These results suggest that, by modifying the cell cycle kinetics, AraC sensitizes AML cells to apoptosis induced by low concentration triptolide. The in vitro proapoptotic effect of triptolide associated with the antiproliferative activity of AraC warrants further clinical investigation for treatment of AML patients, especially elderly patients for whom low-dose AraC treatment could be improved by the addition of triptolide.

Proteasome inhibition specifically sensitizes leukemic cells to anthracyclin-induced apoptosis through the accumulation of Bim and Bax pro-apoptotic proteins.

Proteasome inhibitors are a novel class of compounds that might increase sensitivity to chemotherapy for acute myeloid leukemia (AML). We quantified apoptosis in THP-1 cells incubated with idarubicin (IDA) alone or together with a low concentration of MG132 or bortezomib. The combination of both drugs yielded a percentage of apoptotic cells that was significantly higher than the additive effect of both drugs administered separately (p < 0.01). Isobologram analysis showed that both MG132 and bortezomib interacted synergistically with IDA to induce apoptosis of THP1 cells. Western blot analysis of Bax and Bim show an acumulation of these pro-apoptotic proteins in THP1 treated cells. This increase in Bim preceded the induction of apoptosis and participated in idarubicin-induced apoptosis. Proteasome inhibition also potentiated IDA-induced apoptosis in primary blast cells from 22 AML patients while no such effect was found on normal lymphocytes, PHA-stimulated lymphocytes, normal cord blood CD34+ cells or bone marrow normal myeloid cells. These data show that MG132 and bortezomib specifically sensitize leukemic cells to IDA through an increase in BIM and Bax pro-apoptotic Bcl-2 family proteins.