Cucurbitacin E (CuE) or alpha-elaterin is a natural compound previously shown to be an antifeedant as well as a potent chemopreventive agent against several types of cancer. The present study investigated the anticancer effects of CuE on colorectal cancer (CRC) using primary cell lines isolated from five CRC patients in Taiwan, Specifically, we explored the anti-proliferation and cell cycle G2/M arrest induced by CuE in CRC cells. MPM-2 flow cytometry tests show that CuE-treated cells accumulated in metaphase (CuE 2.5-7.5 mu M). Results further indicate that CuE produced G(2)/M arrest as well as the downregulation of CDC2 and cyclin B1 expression and dissociation. Both effects increased proportionally with the dose of CuE; however, the inhibition of proliferation, arrest of mitosis, production of reactive oxygen species (ROS), and loss of mitochondrial membrane potential (Delta Psi m) were found to be dependent on the quantity of CuE used to treat the cancer cells. In addition, cell cycle arrest in treated cells coincided with the activation of the gene GADD45(alpha, beta, gamma). Incubation with CuE resulted in the binding of GADD45 gamma to CDC2, which suggests that the delay in CuE-induced mitosis is regulated by the overexpression of GADD45 gamma. Our findings suggest that, in addition to the known effects on cancer prevention, CuE may have antitumor activities in established CRC.