The Antiproliferative Effects And The Mechanisms Of Action Of Cucurbitacin E In Human Leukemia And Hepatoma Cells

Cucurbitacin E is a natural product isolated from Cucurbitaceae which has been reported to have antitumor effects. We determined the antiproliferative effects and explored its mechanisms of action in leukemia and hepatoma cells. Cucurbitacin E at low concentrations inhibited growth of leukemia cells, HL-60 and K562, and hepatoma cells, Bel7402 and HepG2. Cell cycle analyses revealed that cucurbitacin E induced G2/M cell cycle arrest in all of the tested cell lines. Western blot analyses indicated that cucurbitacin E decreased the levels of Cdk1 and increased the levels of p21waf1 but did influence the levels of cyclin A, cyclin B1 and Cdc25C protein. These data suggest the G2/M cell cycle arrest may be caused by the decreased levels of Cdk1 and/or increased levels of p21waf1. Cucurbitacin E at high concentrations induced apoptosis of HL-60 cells with the activation of caspase-3, caspase-8, and caspase-9 as determined by Western blot analysis. Cucurbitacin E did not increase the levels of reactive oxygen species and that the antioxidants 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 (eIF2α) were induced in cells undergoing both apoptosis and cell cycle arrest. Since phosphorylated eIF2αis an inhibitor of protein translation initiation, our data suggest that cucurbitacin E induces cell growth arrest and apoptosis probably through induction of eIF2αphosphorylation which leads to inhibition of Cdk1, p21, Mcl-1, survivin and/or XIAP protein synthesis and that cucurbitacin E induces apoptosis mainly through a mitochondrial-mediated pathway in HL-60 cells. Senescence but not apoptosis was detected in K562 cells after treatment with cucurbitacin E at high concentrations. It seems that cucurbitacin E induces cell death in different cell lines through different pathways that need to be further investigated.