Abstract

Beclin 1 is an essential mediator of autophagy and a regulator of cell growth and cell death. We examined the effect of Beclin 1 overexpression on the action of estradiol (E 2 ) and two antiestrogens, raloxifene and 4-hydroxytamoxifen, in estrogen receptor (ER )-positive MCF-7 breast cancer cells. 3 H-thymidine incorporation studies showed that Beclin 1–overexpressing cells (MCF-7.beclin) had a lower proliferative response to E 2 compared with cells transfected with vector control (MCF-7.control). There was only a 35% increase in 3 H-thymidine incorporation, after 24 hours of E 2 treatment of MCF-7.beclin cells compared with untreated cells, whereas this increase was 2-fold for MCF-7.control cells. E 2 -induced changes in the expression of early-response genes were examined by real-time quantitiative PCR. There were significant differences in the pattern of expression of E 2 -induced genes c- myc , c- fos, Erg -1, and Nur 77 between MCF-7.beclin and MCF-7.control cells two hours after treatment. Although E 2 -induced growth of MCF-7.control cells was completely inhibited by 500 nmol/L raloxifene or 500 nmol/L 4-hydroxytamoxifen, these concentrations of antiestrogens had no significant effect on the growth of MCF-7.beclin cells. Confocal microscopic and coimmunoprecipitation studies showed evidence for colocalization and association of Beclin 1 and ER . In addition, E 2 caused a decrease in Akt phosphorylation in MCF-7.beclin cells, compared with a 3-fold increase in MCF-7 cells, five minutes after treatment. These results indicate that Beclin 1 can down-regulate estrogenic signaling and growth response, and contribute to the development of antiestrogen resistance. This observation might be useful to define and overcome antiestrogen resistance of breast cancer. Cancer Res 2008;68(19):7855–63