Vitamin E succinate inhibits proliferation of BT-20 human breast cancer cells: increased binding of cyclin A negatively regulates E2F transactivation activity.
Vitamin E succinate (VES) inhibited the proliferation of the estrogen receptor-negative human breast cancer cell line, BT-20, in the G1 phase of the cell cycle. The E2F proteins are integral transcriptional components in the regulation of cell growth. Overexpression of E2F-1 blocked the ability of VES to inhibit BT-20 cell growth, suggesting that VES regulation of E2F-1 activity leads to growth arrest of BT-20 cells. VES, although having little effect on E2F-1 steady-state protein levels, decreased E2F-1 phosphorylation and transactivation activity and increased cyclin A binding to E2F-1. GAL4-E2F-1 deletion mutant studies indicated that cyclin A negatively regulates E2F function. In VES-treated BT-20 cells, the cyclin A protein exhibited reduced kinase activity, which correlated with decreased steady-state levels and binding of cyclin-dependent kinase-2 to cyclin A and increased steady-state levels and binding of p21cip1 to cyclin A and cyclin-dependent kinase-2. The functional consequence of the negative regulatory effect of VES on E2F-1 function was shown by the ability of VES to inhibit the transcriptional activation of an E2F-1 responsive gene, c-myc. These studies show that VES induces growth inhibition of BT-20 cells through a mechanism that involves cyclin A-negative regulation of E2F-mediated transcription.