A Feedback Loop between the Androgen Receptor and a NEDD4-binding Protein, PMEPA1, in Prostate Cancer Cells

PMEPA1 was identified originally as a highly androgen-inducible gene with prostate-abundant expression that was restricted to prostatic epithelial cells. PMEPA1 protein is a NEDD4 (ubiquitin-protein isopeptide ligase)-binding protein, which negatively regulates prostate cancer cell growth. In this study we establish that PMEPA1 is a direct transcriptional target of the androgen receptor (AR). We also demonstrate that PMEPA1 negatively regulates AR protein levels in different cell culture models. Transient expression of PMEPA1 down-regulates AR protein levels and AR transcriptional targets in prostate cancer cells. Conversely, knockdown of PMEPA1 leads to elevated levels of AR protein, AR transcriptional targets (prostate-specific antigen), and increased cell cycle S phase. We define that the PMEPA1-dependent down-regulation of AR is because of AR ubiquitination and proteasome-mediated degradation. The mutant PMEPA1 (PY1/2 motif mutation) that is impaired in NEDD4 recruitment shows attenuated AR ubiquitination and AR protein down-regulation. These data support the hypothesis that PMEPA1 negatively regulates the stability of AR protein by enhancing AR ubiquitination and proteasome-mediated degradation through NEDD4. The effect of PMEPA1 on AR ubiquitination and degradation appears to be MDM2-independent. Thus, the PMEPA1-AR degradation pathway may represent a new androgen-dependent mechanism for regulating AR levels in prostate epithelial cells. These findings underscore that the decreased PMEPA1 expression frequently noted in prostate cancers may lead to increased AR functions and strengthen the biological role of PMEPA1 in prostate cancers.