Estrogen receptor beta modulates prostate carcinogenesis
Prostate cancer (PC) is characterised by dependence upon androgen receptor (AR) as its driving oncogene. When organ-confined, radical treatment can be curative, however there is no cure for advanced, castration-resistant prostate cancer (CRPC). There is therefore a need to better understand the biology of PC, and how influencing AR can modify disease progression.
Estrogen is essential for prostate carcinogenesis with evidence from epidemiological, in vitro, human tissue and animal studies. Most suggests that estrogen receptor beta (ERβ) is tumour-suppressive, but trials of ERβ-selective agents have not improved clinical outcomes. ERβ has also been implicated as an oncogene, therefore its role remains unclear. Additional evidence suggests interplay between ERβ and AR, the mechanisms of which are uncertain. The study hypothesis ‘ERβ is an important modulator of prostate carcinogenesis’ was developed to establish whether targeting ERβ could affect PC progression.
Much of the confusion around ERβ stems from use of inadequately validated antibodies and cell line models. The first phase of this work was to test ERβ antibodies using an ERβ-inducible cell system. Eight ERβ antibodies were assessed by multiple techniques, showing that commonly used antibodies are either non-specific or only specific in one modality. Two reliable antibodies were identified. Next, cell lines previously used to study ERβ were assessed using validated antibodies and independent approaches. No ERβ expression was detected; an important finding that casts doubt on previously published ERβ biology. Subsequently, a PC cell line with inducible ERβ expression (LNCaP-ERβ) was developed and validated to enable controlled experiments on the effects of ERβ on proliferation, gene expression and ERβ/AR genomic cross-talk.
Phase three of this work focused on ERβ biology in PC and its relationship to AR. Interrogation of clinical datasets showed that greater ERβ expression associated with favourable prognosis. Gene expression data from men treated with androgen deprivation therapy revealed that AR represses ERβ. This was confirmed in vitro. The LNCaP-ERβ cell line was treated with androgen and/or ERβ-selective estrogen. Activated ERβ in the presence of androgen-stimulated AR inhibited cell proliferation and down-regulated androgen-dependent genes. Genome-wide mapping of ERβ binding sites reveals that ERβ antagonises AR through competition for shared DNA binding sites.
In conclusion, ERβ expression is down-regulated by AR during malignant transformation of prostate epithelium. We reveal an antagonistic relationship between ERβ and AR whereby sustaining or replacing ERβ may inhibit tumour growth through down-regulation of AR-target genes. In future, an ERβ-selective compound may be used to slow or abrogate PC progression.