The induction of G2/M cell-cycle arrest and apoptosis by cucurbitacin E is associated with increased phosphorylation of eIF2α in leukemia cells

The antiproliferative and apoptotic effects of cucurbitacin E, a natural product isolated from Cucurbitaceae, were determined in human leukemia HL-60 cells. Cucurbitacin E at low concentrations (3-50 nmol/l) inhibited the growth of HL-60 cells, which was associated with G(2)/M cell-cycle arrest, decrease in the levels of cyclin-dependent kinase1, and increase in the levels of p21(waf1). Cucurbitacin E at high concentrations (1-10 mu mol/l) induced apoptosis of HL-60 cells and activation of caspase-3, caspase-8, and caspase-9. Jurkat leukemia cells with or without caspase-8 expression were nearly equally sensitive to cucurbitacin E-induced apoptosis. Cucurbitacin E did not increase the levels of reactive oxygen species and antioxidants, N-acetylcysteine and catalase, did not block cucurbitacin E-induced apoptosis. Cucurbitacin E decreased the levels of the antiapoptotic proteins XIAP, survivin, and Mcl-1, but increased the level of the proapoptotic protein, Bax. The levels of phosphorylated eukaryotic translation initiation factor 2 subunit alpha (eIF2 alpha) were induced in cells undergoing both apoptosis and cell-cycle arrest. As phosphorylated eIF2 alpha is an inhibitor of protein translation initiation, our data suggest that cucurbitacin E induces cell growth arrest and apoptosis through the induction of eIF2 alpha phosphorylation, which leads to the inhibition of cyclin-dependent kinase 1, Mcl-1, survivin, and/or XIAP protein synthesis and that cucurbitacin E induces apoptosis mainly through a mitochondrial-mediated pathway. Anti-Cancer Drugs 21: 389-400 (C) 2010 Wolters Kluwer Health vertical bar Lippincott Williams & Wilkins.