Protein inhibitor of activated STAT3 (PIAS3) is a specific inhibitor of signal transducer and activator of transcription 3 (STAT3). PIAS3 binds to STAT3 and inhibits its DNA-binding activity, and thereby STAT3-mediated gene activation. PIAS1, another member of the PIAS family, was recently shown to interact with the androgen receptor (AR), a nuclear hormone receptor that has an important role in both physiological and pathological processes, and acts as a cofactor for AR. Here we demonstrate that PIAS3 is expressed in prostate cancer cells and its expression is induced in response to dihydrotestosterone (DHT) treatment. Ectopic overexpression of PIAS3 suppressed AR-mediated gene activation induced by DHT-stimulation in LNCaP cells. We provide evidence that these activities were due to direct physical interactions between PIAS3 and AR. These results indicate that PIAS3 acts as a coregulator of AR signaling pathway in prostate cancer cells, (C) 2000 Academic Press.