Activation of caspase-3 and c-Jun NH2-terminal kinase-1 signaling pathwaysin tamoxifen-induced apoptosis of human breast cancer cells

Tamoxifen (TAM) is widely used in the treatment of breast cancer. The cytos tatic effects of TAM have been attributed to the antagonism of estrogen rec eptor (ER) and inhibition of estrogen-dependent proliferative events. Howev er, the mechanism by which TAM is also effective against certain ER-negativ e breast tumors remains to be elucidated. Here we report that TAM induced t he activity of caspase-3-like proteases in ER-negative breast cancer cell l ines MDA-MB-231 and BT-20, as evidenced by the cleavage of fluorogenic tetr apeptide substrate and of poly(ADP-ribose) polymerase. The activation of ca spase-1-like proteases preceded TAM-induced chromatin condensation and nucl ear fragmentation, the typical apoptotic morphologies. Pretreatment of cell s with a specific inhibitor of caspase-3, acetyl-Asp-Glu-Val-Asp-aldehyde, or with a general inhibitor of caspases, benzyloxycarbonyl-Val-Ala-Asp-fluo romethyl ketone, prevented TAM-induced apoptosis. TAM also stimulated c-Jun NH2-terminal kinase (JNK) 1 activity, and interfering with the JNK pathway by overexpressing a DN JNK1 mutant attenuated TAM-induced apoptosis. In ad dition, treatment of cells with a lipid-soluble antioxidant vitamin E block ed TAM-induced caspase-3 and JNK1 activation as well as apoptosis, whereas water-soluble antioxidants N-acetyl L-cysteine and glutathione had little e ffect. Thus, this study demonstrates that TAM induces apoptosis in ER-negat ive breast cancer cells through caspase-3 and JNK1 pathways, which are prob ably initiated at the cell membrane by an oxidative mechanism.