Abstract A4: Androgen receptor (AR) degradation is controlled by the co-operation of PMEPA1 and the E3 ubiquitin ligase NEDD4-1

Abstract

Abstract Introduction and Objective: Androgen receptor (AR) plays critical roles in prostate growth and differentiation and prostate tumorigenesis. Our recent investigations have discovered a novel mechanism of AR degradation in CaP cells involving PMEPA1 and NEDD4-1 which is independent of MDM2. Previously we have shown that PMEPA1 is a highly androgen-inducible gene in CaP cells that recruits the NEDD4-1, an E3 ubiquitin ligase, to degrade AR. In an independent study NEDD4-1 has been identified as an E3 ligase controlling the levels of PTEN. We hypothesize that decreased or loss of PMPEA1 expression that is frequently observed in human prostate tumor cells may lead to enhanced AR functions. Elevated AR levels may increase NEDD4-1 and in turn destabilize PTEN in prostate cancer cells. Here, we further investigate the co-operation between PMEPA1 and NEDD4-1 to alter AR and PTEN protein levels. Methods: PMEPA1 and NEDD4-1 were ectopically expressed in androgen responsive LNCaP and VCaP cells by pCMV-XL4 and pcDNA3.1 vectors, respectively. AR, PMEPA1, NEDD4-1, MDM2 and PTEN were silenced by using specific siRNAs. Cell growth was monitored in response modulation of PMEPA1, NEDD4 and AR. Protein levels of AR, PMEPA1, NEDD4-1, PTEN and PSA were assessed by immunoblot assays. Results: Decreases in AR protein levels, inhibited AR signaling and cell growth in response to heterologous expression of NEDD4-1. Consistent with this observation, inhibition of NEDD4-1 by siRNA resulted in elevated AR protein levels, AR signaling and increased cell growth. Reduced cell growth by AR knock-down or by heterologous expression of NEDD4-, were rescued by the knockdown of PMEPA1. It was also shown that AR up-regulates the expression of NEDD4-1 in hormone responsive CaP cells. In VCaP cells, overexpression of NEDD4-1 resulted in decreased PTEN protein levels. Remarkably, PMEPA1 knockdown resulted in elevated AR and NEDD4-1 levels and the downregulation of PTEN protein. Conclusions: NEDD4-1 mediated AR degradation in hormone responsive CaP cells requires PMEPA1. The frequently observed decreased expression of PMEPA1 results in elevated AR protein levels and may facilitate NEDD4-1 mediated degradation of PTEN in CaP. Our findings revealed a complex interplay of the AR, PMEPA1, NEDD4 and PTEN underscoring the impact of the loss of PMEPA1 to increased AR and decreased PTEN, both proteins critical in prostate tumorigenesis. Citation Format: Hua Li, Albert Dobi, Shiv Srivastava. Androgen receptor (AR)degradation is controlled by the cooperation of PMEPA1 and the E3 ubiquitin ligase NEDD4-1 [abstract]. In: Proceedings of the AACR Special Conference on Advances in Prostate Cancer Research; 2012 Feb 6-9; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2012;72(4 Suppl):Abstract nr A4.