Cucurbitacin E’s Anti-Cancer Effects on HCT116 Human Colon Cancer Cells by Controlling Expression and Phosphorylation levels of Caspase-9, eIF-2α, and ATF-4

Abstract

Cucurbitacin E is a pivotal member of the cucurbitacin family and has been shown to have anti-cancer effects. However, until now, the anti-cancer effect and mode of action of cucurbitacin E in human colon cancer cells remain unclear. In this study, we investigated whether cucurbitacin E inhibits the growth of HCT116 human colorectal cancer cells. Treatment of cucurbitacin E at 1 µM markedly reduced the survival of HCT116 cells. Moreover, treatment of cucurbitacin E at 1 µM caused nuclear DNA fragmentation in HCT116 cells, pointing out its apoptosis-inducing effect. Treatment of cucurbitacin E at 1 µM also led to the activation of caspase-9 and poly(ADP-ribose) polymerase (PARP) cleavage without affecting expression of death receptor (DR)-4/5 in HCT116 cells. Furthermore, while treatment of cucurbitacin E at 1 µM had no effect on expression of Mcl-1, it largely increased expression and phosphorylation of eukaryotic translation initiation factor-2α (eIF-2α) and activating transcription factor-4 (ATF-4) in HCT116 cells. Treatment of cucurbitacin E at 1 µM further up-regulated phosphorylation of extracellular signal-regulated kinase-1/2 (ERK-1/2), but not c-Jun N-terminal kinase1/2 (JNK-1/2), in HCT116 cells. However, treatment with PD98059, an inhibitor of ERK-1/2, that strongly blocked activation of ERK-1/2 had no effect on reduction of survival of HCT116 cells treated with cucurbitacin E at 1 µM. Taken together, these findings demonstrate that cucurbitacin E at 1 µM has strong anti-survival and pro-apoptotic effects on HCT116 cells, which are mediated through control of the expression and phosphorylation levels of caspase-9, PARP, eIF-2α, and ATF-4.