Induction of mitochondria-involved apoptosis in estrogen receptor-negative cells by a novel tamoxifen derivative, ridaifen-B

Tamoxifen is an antagonist of estrogen receptor, which is used widely as an estrogen receptor-positive breast cancer drug that blocks growth signals and provokes apoptosis. However, recent studies have revealed that tamoxifen induces apoptosis even in estrogen receptor-negative cells. In the present study, we synthesized several tamoxifen derivatives to augment the apoptosis-inducing effect of tamoxifen and evaluated the apoptosis-inducing pathway. The estrogen receptor-positive human leukemia cell line HL-60 and estrogen receptor-negative human leukemia cell line Jurkat were treated with tamoxifen and synthesized tamoxifen derivatives, and thereafter subjected to cell viability-detection assays. Tamoxifen derivatives, as well as the lead compound tamoxifen, decreased the cell viability despite the expression of estrogen receptor. Among all of the synthesized tamoxifen derivatives, ridaifen-B had more potent cancer cell-damaging activity than tamoxifen. Ridaifen-B fragmented Jurkat cell DNA and activated caspases, suggesting that the ridaifen-B-induced apoptosis pathway is estrogen receptor independent. Moreover, mitochondrial involvement during ridaifen-B-induced apoptosis was estimated. Ridaifen-B significantly reduced mitochondrial membrane potential, and overexpression of Bcl-2 inhibited ridaifen-B-induced apoptosis. These results suggest that the induction of apoptosis by ridaifen-B, a novel tamoxifen derivative, is dependent on mitochondrial perturbation without estrogen receptor involvement. (Cancer Sci 2008; 99: 608–614)