Estrogen receptor β upregulates FOXO3a and causes induction of apoptosis through PUMA in prostate cancer

Estrogen receptor beta (ER beta) is emerging as a critical factor in understanding prostate cancer biology. Although reduced in prostate cancer above Gleason grade 3, ER beta is a potential drug target at the initial stage of the disease. In human prostate cancer cells, we found that ERb causes apoptosis by increasing the expression of pro-apoptotic factor p53-upregulated modulator of apoptosis (PUMA), independent of p53, but dependent on the forkhead transcription factor class-O family member, FOXO3a. FOXO3a has previously been shown to induce PUMA after growth factor withdrawal and inhibition of the Akt pathway. Surprisingly, the phosphorylation of FOXO3a remained unchanged, while the mRNA and total protein levels of FOXO3a were increased in response to ER beta expression or treatment of PC3, 22Rv1 and LNCaP cells with the ER beta-specific ligands 3 beta-Adiol (5 alpha-androstane-3 beta, 17 beta-diol), DPN (diarylpropionitrile) or 8 beta-VE2 (8-vinylestra-1,3,5 (10)-triene-3,17 beta-diol). Knockdown of FOXO3a or ER beta expression abolished the increase of PUMA in response to 3 beta-Adiol in LNCaP and PC3 cells, suggesting that FOXO3a mediates the apoptotic effect of 3 beta-Adiol-activated ER beta. Moreover, the ventral prostate of ER beta-/- mice had decreased expression of FOXO3a and PUMA compared with the ERb-/- mice, indicating a relationship between ER beta and FOXO3a expression. The regulation of FOXO3a by ER beta in normal basal epithelial cells indicates a function of ER beta in cell differentiation and maintenance of cells in a quiescent state. In addition, the expression of ER beta, FOXO3a and PUMA is comparable and higher in benign prostatic hyperplasia than in prostate cancer Gleason grade 4 or higher, where there is substantial loss of ER beta, FOXO3a and PUMA. We conclude that ER beta induces apoptosis of prostate cancer cells by increasing transcription of FOXO3a, leading to an increase of PUMA and subsequent triggering of apoptosis via the intrinsic pathway involving caspase-9. Furthermore, we conclude that ligands specifically activating ER beta could be useful pharmaceuticals in the treatment of prostate cancer.