Interferons (IFNs) are known to possess potent antitumor properties. Previous studies have indicated that IFNs are capable of modulating the expression of various tumor suppressor genes and oncogenes. In this study, we looked at the effect of IFN-gamma on the neu/HER-2 proto-oncogene in the DU145, LNCaP, and PC-3 prostate cancer cell lines. IFN-gamma inhibited cell proliferation in both DU145 and PC-3 cells in a dose-dependent manner, whereas no inhibition of proliferation was seen in LNCaP cells. Correspondingly, IFN-gamma treatment of DU145 and PC-3 cells resulted in an increased production of the cyclin-dependent kinase inhibitor p21(WAF1), whereas no increase in p21(WAF1) was seen in LNCaP cells. In addition, IFN-gamma induced phosphorylation of signal transducer and activator of transcription (STAT) 1 in DU145 and PC-3 cells, but not in LNCaP cells. Consistent with these findings, we found that IFN-gamma treatment of DU145 and PC-3 cells caused a reduction in neu/HER-2 expression, with no change seen in the LNCaP cell line. Transfection and overexpression of the transcriptional coactivator p300 in PC-3 cells suppressed the reduction in neu/HER-2 expression after IFN-gamma treatment, suggesting a role for p300 in neu/HER-2 expression. The antiproliferative activity and p21(WAF1) production of these cells after IFN-gamma treatment were found to be reduced as well. We propose that the down-regulation of neu/HER-2 by IFN-gamma occurs via the interaction of phosphorylated STAT1 with p300 because IFN-gamma activities requiring phosphorylated STAT1 are reduced in cells overexpressing p300. These findings suggest that neu/HER-2 may play a role in the growth of some prostate cancers and that IFN-gamma may suppress such cancers by down-regulation of neu/HER-2.