Green tea polyphenol EGCG antagonizes androgen function: Novel insight into the mechanism of chemopreventive effects of green tea on prostate cancer

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Oral consumption of green tea polyphenols containing epigallocatechin-3-gallate (EGCG) to TRAMP and other mouse models affords substantial prostate cancer (PCa) chemo-preventive effects. Epidemiological and clinical studies indicate that these animal data are also applicable to human PCa patients. Androgens play a central role in prostate development and PCa proliferation. Androgen receptor (AR) manifests the biological actions of androgens. AR activity is critical for PCa progression and its transactivation is known to enhance PCa cell survival. AR is known to act as a homodimer on the regulatory elements of the target genes to modulate their expression and the interdomain interaction (N-C terminus interaction) of AR is required for the optimal/maximal activation of the receptor. Since the growth of prostate cancer cells is regulated by androgens, we hypothesized that EGCG will blunt AR function in PCa. We first asked whether EGCG can act as a ligand for AR. EGCG was therefore screened in AR competitive binding assay employing time-resolved fluorescence resonance energy transfer (TR-FRET) format where ligands for AR are identified by their ability to compete with and displace fluorescent dihydrotestosterone (DHT) from the receptor. EGCG was found to compete and displace fluorescent DHT and disrupted FRET, resulting in a lower TR-FRET ratio with an effective IC₅₀ of 0.4 µM. To evaluate the functional consequence of this interaction on transcription activation function of AR, a reporter assay was performed to determine how EGCG modulates hormone-dependent AR action. We observed that in LNCaP cells, EGCG led to a dose-dependent decrease in AR transcription activation function on a cognate reporter element MMTV containing AR binding sites. This indicates that physical interaction of EGCG with AR leads to attenuation of its transactivation function implicating EGCG to be a novel antagonist of AR function. To further explain decrease in AR transactivation, an interdomain interaction assay was performed which clearly indicated a decrease in AR amino-carboxy termini interaction by EGCG. Since AR transactivation is known to positively stimulate growth of PCa cells and EGCG led to a potent inhibition of this function, next we tested the effect of EGCG on the growth of hormone R1881 stimulated PCa LNCaP cells employing MTT assay. R1881 treatment enhanced cell growth and this enhancement was blunted by EGCG. Taken together, we propose EGCG as a novel candidate antagonist of AR signaling in PCa that may function as a PCa chemotherapeutic agent alone or in combination with currently employed chemotherapeutic regimens.