The role of androgen receptor phosphorylation in hormone refractory prostate cancer growth

Maintenance of androgen receptor (AR) pathway activity is crucial for hormone refractory prostate cancer growth. AR is activated by various kinase signaling pathways, but the mechanism by which this occurs is unclear. Here we use mass spectrometry and combinatorial mutagenesis to identify phosphorylation sites that are important for AR activity and hormone refractory disease. Treatment of LNCaP prostate cancer cells with the synthetic androgen R1881 elevates phosphorylation of serines 81, 94, 256, 308, and 650 on AR. Mutagenesis from serine to alanine at any of these sites on an individual basis does not affect AR activity. However, a pan-phosphonull mutant AR with serine to alanine mutations at each of the 5 identified residues shows a decrease in AR transcriptional activity of both an artificial reporter and the endogenous prostate specific antigen (PSA) gene. This decrease is partially attributed to an observed impairment in the ligand-induced, intramolecular AR amino-carboxy interaction. Furthermore, the pan-phosphonull mutant AR is impaired in its ability to cause progression to hormone refractory growth in a mouse prostate cancer xenograft model. Taken together these findings indicate direct phosphorylation at serine residues regulates AR transcriptional activity and is important for hormone refractory growth in vivo.