Estrogen and Antiestrogen Actions on Human Prostate Cancer: A Dissertation

Abstract

Prostate cancer increases its incidence with age after men in their fifth decade as the ratio of estrogen to androgen rises. Epidemiological studies indicated that high levels of estrogens are associated with the high-risk ethnic groups for prostate cancer. Therefore, estrogens may be involved in prostatic carcinogenesis. It is widely believed that the actions of estrogens are mediated by estrogen receptors. However, expression of estrogen receptor in normal prostate and lesions of the gland was controversial. With the recent discovery of second estrogen receptor (ER-β), this issue became more complicated and it needs to be readdressed. In addition, the biological involvement of ER-β in human prostate remains to be investigated. In this study, we demonstrated that human normal prostate epithelial cells express ER-β but not ER-α, suggesting that estrogens act directly on these epithelial cells via ER-β. Using RT-PCR analysis, the transcripts of ER-β were detected in our primary human prostatic epithelial cell cultures that were derived from the ultrasound-guided peripheral zone biopsies and the cells express two estrogen-regulated genes such as progesterone receptor (PR) and pS2. Moreover, we had developed an ER-β antibody with fully characterizations and used it for immunohistochemistry. Results indicated that ER-p protein is expressed in the basal compartment of prostatic epithelium of the gland. Our findings lead to a new hypothesis that estrogens directly act on human prostatic epithelial cells to modulate its biological functions. To investigate expression of ERs in prostate cancer, RT-PCR analysis was used. We found that all three human prostate metastatic cancer cell lines, DU145, PC-3 and LNCaP, express ER-β transcripts while ER-α mRNA expression only in PC-3 cells. Expressions of PR and pS2 in these cell lines are various. LNCaP cells express both PR and pS2 mRNAs but DU145 cells with only PR and PC-3 cells with only pS2. Our immunohistochemical results on prostatic lesions revealed down-regulation of ER-β expression in high-grade of dysplasia and carcinoma of peripheral zone of the prostate compared to their low-grade lesions. This down-regulation in high-grade carcinoma was verified in transcriptional level by RT-PCR analysis on micro dissected normal epithelium and lesion samples of the gland. In the metastasis, ER-β was found to be reactivated as we observed ER-β mRNA expression in prostate cancer cell lines. Recent evidence suggests that ER-β may be antiproliferative factor for a protective effect against the mitogenic activity of estrogens in breast and androgens in prostate. Activation of the receptor may exhibit cell growth inhibition. We demonstrated that antiestrogens [ICI-182,780 (ICI) and 4-hydroxytamoxifen], raloxifene and phytoestrogen (resveratrol), but not estrogens (17β-estradiol and diethylstilbestrol), inhibit growth of DU145 cells which express only ER-β while PC-3 cells with both ERs showed growth inhibition in response to estrogen and antiestrogen treatments. In DU145 cells, the ICI-induced cell growth inhibition was prevented by blockade of ER-β expression using antisense oligonucleotide. It indicated that the inhibition is mediated via ER-p associated pathway. Using flow cytometry, we found that ICI-treatment could induce accumulation of cells at GO-G1 phase of cell cycle. Similarly, this GO-G1 cell accumulation was also induced by raloxifene in DU145 cells. For resveratrol, the treatment exhibited dual effects on cell cycle distribution in DU145 cells. In the early treatment, resveratrol induced cell cycle arrests at GO-G1phase. The prolonged treatment leads to S-phase cell cycle arrest. To study the molecular mechanism of this ER-p associated cell growth inhibition, real-time RT-PCR analysis was used to semi-quantitate the transcript levels of tentative ER-β regulated genes such as telomerase reverse transcriptase (TERT), survivin and thymidylate synthase (TS) in the treated cells compared to those in control. Results demonstrated that the treatment of ICI could down-regulate TERT and survivin mRNA expressions with dose-dependent fashion. As the ICI-treatment, resveratrol downregulated expression levels of TERT, survivin and TS in DU145 cells. Down-regulation of TS may be related to the S-phase cell cycle arrest observed in the prolonged treatment of resveratrol. Taken together, our findings support the concept that ER-β participates in cell cycle regulation in normal and malignant prostatic epithelial cells. Presence of ER-β in basal cells of the prostate acini indicates that the direct actions of estrogens may be involved in the normal physiology of the gland. Loss of this receptor in primary prostate cancer and its re-expression in metastasis suggests the roles of ER-β in the cancer progression. Activation of the receptor by antiestrogen and phytoestrogen induced cell growth inhibition in prostate cancer cells. The mechanism may be mediated by reduction of cell survival factors and eventually decrease in cell viability and induction of cell cycle arrests.