TR3 enhances AR Variant Production and Transactivation, Increasing Androgen Independency of AR Signaling in Prostate Cancer Cells

Background Increased expression of constitutively active androgen receptor variants (AR-Vs) is associated with the development of advanced castration-resistant prostate cancer. The pro-oncogenic function of TR3, an orphan nuclear receptor, has been reported in various cancers including prostate cancer. However, the roles of TR3 in androgen receptor (AR) expression and signaling in prostate cancer cells are poorly understood. Methods Western blotting and quantitative RT-PCR were used to evaluate AR and AR-V expression levels affected by TR3 expression level. RNA-seq analysis, coimmunoprecipitation, cross-linked RNA-immunoprecipitation, and single-strand RNA protection and pull-down assays were conducted to elucidate the molecular mechanisms by which TR3 affected AR-V production. Results Database analysis revealed that TR3 expression level is elevated in prostate tumors, and is positively correlated with that of AR. TR3 overexpression increased the production of AR splice variants in addition to general upregulation of AR expression. TR3 interacted with some spliceosomal complex components and AR precursor mRNA, altering the splice junction rates between exons. TR3 also enhanced androgen-independent AR function. Furthermore, TR3 overexpression increased cell proliferation and mobility of AR-positive prostate cancer cells and stimulated tumorigenesis of androgen-independent prostate cancer cells in mouse xenograft models. This is the first study to report that TR3 is a multifunctional regulator of AR signaling in prostate cancer cells. Conclusions TR3 alters AR expression, splicing process, and activity in prostate cancer cells, increasing the androgen independency of AR signaling. Therefore, TR3 may play a crucial role in the progression of prostate cancer to advanced castration-resistant form.