Homeobox protein Nkx-3.1
nuclear receptor activity
Androgen regulation of the prostatic tumour suppressor NKX3.1 is mediated by its 3' untranslated region.
The homeodomain transcription factor NKX3.1 is a prostate-specific tumour suppressor, expression of which is reduced or undetectable in the majority of metastatic prostate tumours. In the normal prostate and in prostate cancer cells, NKX3.1 expression is under tight androgenic control that we have shown to be mediated by its ~2.5 kb 3'UTR (3' untranslated region). Reporter deletion analysis of the NKX3.1 3'UTR identified three regions that were transactivated by DHT (5alpha-dihydrotestosterone) in the AR (androgen receptor)-expressing prostate cancer cell line LNCaP. Reversal of DHT effects by the anti-androgen bicalutamide supported an AR-mediated mechanism, and bioinformatic analysis of the NKX3.1 3'UTR identified canonical AREs (androgen-response elements) in each of the androgen-responsive regions. EMSAs (electrophoretic mobility-shift assays) indicated binding of the AR DNA-binding domain to two of the AREs, a proximal ARE at +2378-2392 from the transcription start site, and a more distal ARE at +3098-3112. ChIP (chromatin immunoprecipitation) analysis provided further evidence of ligand-dependent recruitment of endogenous AR to sequence encompassing each of the two elements, and site-directed mutagenesis and deletion analysis confirmed the contribution of each of the AREs in reporter assays. The present studies have therefore demonstrated that the NKX3.1 3'UTR functions as an androgen-responsive enhancer, with the proximal ARE contributing the majority and the distal ARE providing a smaller, but significant, proportion of the androgen responsiveness of the NKX3.1 3'UTR. Characterization of androgen-responsive regions of the NKX3.1 gene will assist in the identification of transcriptional regulatory mechanisms that lead to the deregulation of NKX3.1 expression in advanced prostate cancers.