Prostate cancer is the most frequent non-skin cancer in men. Although the mechanisms involved in the progression of prostate cancer are not entirely understood, androgen receptor has been shown to play an important role. Androgen receptor is expressed in both early and late-stage prostate cancer. Also, androgen-regulated pathways are thought to be active as evidenced by elevated levels of prostate-specific antigen (PSA). In addition, several androgen receptor co-activators and cytokines are involved in prostate cancer progression. In this regard, we have shown previously that the coactivator p300 plays a major role in the androgen-independent activation of PSA by interleukin 6 (IL-6), a cytokine involved in late-stage prostate cancer. In this study, we investigated the role of p300 and its homologue CREB-binding protein in prostate cancer cells treated chronically with IL-6. We found that p300 but not CREB-binding protein induced activation of PSA in these cells and that the histone acetyltransferase activity of p300 was critical. This effect was independent of the presence of androgens or antiandrogens. Moreover, we found markedly reduced levels of androgen receptor in these cells and p300 transfection did not affect those levels, suggesting that the p300 effect on PSA could be bypassing the androgen receptor. Transfection with exogenous androgen receptor showed minimal response of PSA to androgens but higher response to p300. We found similar effects of p300 on the androgen response element III, which mediates the androgen receptor-dependent activation of PSA. Finally, we showed that p300 alone regulates expression of the endogenous PSA gene in the IL-6-treated cells. These findings reveal a new insight in the progression of prostate cancer, suggesting that coactivators, such as p300, play more important roles in late-stage prostate cancer, and could regulate androgen-dependent genes in the absence or with very low levels of androgen receptor.