SMAD3 represses androgen receptor-mediated transcription

The androgen-signaling pathway is important in the growth and progression o f prostate cancer. Androgen ablation therapy, which may result in programme d cell death, is often used to treat advanced prostate cancer. The growth-p romoting effects of androgen are mediated mostly through the androgen recep tor (AR), Transforming growth factor beta (TGF-beta) plays critical roles i n controlling prostate cell proliferation, differentiation, and apoptosis, Normal transcripts and proteins of TGF-beta receptors are frequently lost i n prostate cancer cells, especially in advanced stages of the disease. Howe ver, the mechanisms by which TGF-beta inhibits proliferation and induces ap optosis in prostate cancer cells is not clear. We investigated the molecula r mechanism by which TGF-beta inhibits transcriptional activation mediated by AR. Using transient transfection systems, we demonstrated that Smad3 spe cifically represses transcriptional activation mediated by AR on two natura l androgen-responsive promoters. This repression is transmitted through TGF -beta signaling and can be regulated by other Smad proteins, A protein-prot ein interaction between AR and Smad3 was identified in vitro and in vivo, a nd the transcription activation domain of AR and the MH2 of Smad3 were iden tified as being responsible for binding. Additional functional experiments showed that the repression of AR by Smad3 is mediated solely through the MH 2 domain. These results provide fresh insight for understanding the mechani sm by which TGF-beta regulates the androgen-signaling pathway in prostate c ancer cells.