Prostate cancer (PCa) is initially androgen-dependent. Consequently, standard therapy for non-organ confined PCa aims to block the production or action of androgens. Although most PCas initially regress following anti-androgen therapy, a majority of these tumors eventually start to regrow, giving rise to the so-called androgen depletion independent (ADI) state of the disease for which no efficient treatment exists. Interestingly, in ADI PCas the presence of a functional androgen receptor (AR), the transcription factor mediating the effects of androgens, is critical for cell proliferation. Among the mechanisms that have been proposed to explain activation of the AR in ADI PCa cells, aberrant expression of coregulatory proteins, required for formation of a productive AR transcriptional complex, has gained considerable support. Previously, we have shown that the transcriptional coactivator p300 is required for ADI activation of the AR. Moreover, we demonstrated that p300 is upregulated in PCa where its expression is associated with cell proliferation, predicts aggressive tumor features, and modulates nuclear morphology. The molecular mechanism responsible for the observed deregulated expression of p300 in the advanced stages of PCa however remains elusive. Here, we provide a first clue towards such a mechanism by demonstrating that expression of p300 protein in PCa cells is subject to androgen regulation. In several PCa model systems, addition of both synthetic (methyltrienolone and mibolerone) as well as natural (dihydrotestosterone) androgens led to decreased protein expression of p300. The effects of androgens were evident at concentrations of 1nM and were first seen 24 hours after stimulation. Experiments using AR-negative PCa cell lines, AR antagonists or siRNA targeting the AR revealed that downregulation of p300 expression depended entirely on the presence of a functional AR. Conversely, both short-term and long-term androgen deprivation of PCa cells resulted in marked upregulation of p300 protein expression. The increase in p300 expression induced by androgen deprivation was not affected by addition of cytokines (e.g. IL-6) or growth factors (e.g. EGF and IGF-1). Similarly, treatment of PCa cells with antiandrogens (biculatamide, hydroxyflutamide and cyproterone acetate), commonly used in PCa treatment, did not affect the increased p300 protein levels following androgen deprivation. Noteworthy, when evaluated by quantitative real-time PCR, androgen treatment did not affect p300 mRNA levels, suggesting p300 protein stability to be increased by androgen deprivation. Taken together, these data provide the first insight into a mechanism by which the expression of the transcriptional coactivator p300 is enhanced in PCa progression and suggest the existence of feedback loops in which androgens, through the AR, influence and regulate transactivation by the AR in PCa cells. Support : NIH, TJ Martell Foundation.
[Proc Amer Assoc Cancer Res, Volume 47, 2006]