The relevance of estrogen receptor-β expression to the antiproliferative effects observed with histone deacetylase inhibitors and phytoestrogens in prostate cancer treatment

In the prostate, estrogen receptor beta (ER beta), the preferred receptor for phytoestrogens, has features of a tumor suppressor. To investigate the mechanisms underlying the beneficial effects on prostate cancer of histone deacetylase inhibitor valproic acid (VPA) and phytoestrogen tectorigenin, we analyzed the expression of ER after tectorigenin or VPA treatment. For further functional analysis, we knocked down ER beta expression by RNA interference. LNCaP prostate cancer cells were treated with 5 mmol/L VPA or 100 mu mol/L tectorigenin and transfected with small interfering RNA (siRNA) against ER beta. Control transfections were done with luciferase (LUC) siRNA. Expression of ER beta was assessed by Western blot. mRNA expression was quantitated by real-time reverse transcription-PCR. Expression of ER beta mRNA and protein markedly increased after VPA or tectorigenin treatment. When ER beta was knocked down by siRNA, the expression of prostate-derived Ets factor, prostate-specific antigen, prostate cancer-specific indicator gene DD3(PCA3), insulin-like growth factor-1 receptor, the catalytic subunit of the telomerase, and ER beta was up-regulated and the tectorigenin effects were abrogated. ER beta levels were diminished in prostate cancer and loss of ER beta was associated with proliferation. Here, we show that siRNA-mediated knockdown of ER beta increases the expression of genes highly relevant to tumor cell proliferation. In addition, we show that one prominent result of treatment with VPA or tectorigenin is the up-regulation of ER beta resulting in antiproliferative effects. Thus, these drugs, by restoring the regulatory function of ER in tumor cells, could become useful in the intervention of prostate cancer.