Apoptosis induction through proteasome inhibitory activity of cucurbitacin D in human T-cell leukemia.

BACKGROUND: Human T-cell leukemia is an aggressive malignancy of T lymphocytes. T-cell leukemia has a very poor prognosis, even with intensive chemotherapy, indicating the need for development of new drugs to treat the disease. Triterpenoid cucurbitacins have been shown to have antitumor activity, but the mechanism of this activity is not fully understood. METHODS: The effects of cucurbitacin D on the proliferation and apoptotic induction of T-cell leukemia cells using the Cell viability assay and Annexin V staining were evaluated. To investigate the mechanisms of apoptosis, antiapoptotic protein, NF-kappa B, and the proteasome activity of leukemia cells treated with cucurbitacin D were evaluated by Western blotting both in vitro and in vivo. RESULTS: In this study, cucurbitacin D was found to inhibit proliferation and to induce apoptosis of T-cell leukemia cells. Constitutively activated NF-kappa B was inhibited by cucurbitacin D in the nucleus, which resulted in accumulation of NF-kappa B in the cytoplasm, leading to down-regulation of the expression of antiapoptotic proteins Bcl-xL and Bcl-2. Furthermore, cucurbitacin D induced the accumulation of inhibitor of NF-kappa B (I kappa B)alpha by inhibition of proteasome activity. Low doses of cucurbitacin D synergistically potentiated the antiproliferative effects of the histone deacetylase inhibitor VPA. Finally, the proapoptotic and proteasome inhibitory activities of cucurbitacin D also were demonstrated using SCID mice in an in vivo study. CONCLUSIONS: Cucurbitacin D induced apoptosis through suppression of proteasome activity both in vitro and in vivo, making cucurbitacin D a promising candidate for clinical applications in the treatment of T-cell leukemia. Cancer 2011; 117: 2735-46. (C) 2010 American Cancer Society.