Estrogen-Mediated Upregulation of Noxa Is Associated with Cell Cycle Progression in Estrogen Receptor-Positive Breast Cancer Cells

Noxa is a Bcl-2-homology domain (BH3)-only protein reported to be a proapoptotic member of the Bcl-2 family. Estrogen has been well documented to stimulate cell growth and inhibit apoptosis in estrogen receptor (ER)-positive breast cancer cells. Intriguingly, recent reports have shown that 17 beta-estradiol (E2) induces Noxa expression, although the mechanisms underlying E2-mediated induction of Noxa and its functional significance are unknown. Using MCF7 human breast cancer cells as an experimental model, we show that Noxa is upregulated by E2 via p53-independent processes that involve c-Myc and ER alpha. Experiments using small interfering ribonucleic acids (siRNA) to specifically knock down p53, c-Myc, and ER alpha demonstrated that c-Myc and ER alpha, but not p53, are involved in the transcriptional upregulation of Noxa following E2 treatment. Furthermore, while E2 promoted the recruitment of c-Myc and ER alpha to the NOXA promoter in chromatin immunoprecipitation (ChIP) assays, E2 did not induce p53 recruitment. Interestingly, E2-mediated upregulation of Noxa was not associated with apoptosis. However, siRNA-mediated knockdown of Noxa resulted in cell cycle arrest in G(0)/G(1)-phase and significantly delayed the G(1)-to-S-phase transition following E2 treatment, indicating that Noxa expression is required for cell cycle progression in ER-positive breast cancer cells.