Abstract 864: Androgen receptor stability in prostate cancer is regulated by the cochaperone Bag-1L
Authors: Cato Laura , Neeb Antje , Sharp Adam , Ficarro Scott , Buzon Victor , Salvatella Xavier , Marto Jarrod A. , de Bono Johann S. , Cato Andrew C. , Brown Myles 
Published: July 2016

The androgen receptor (AR) is an important determinant of normal and malignant prostate growth and its function is usually regulated by circulating levels of androgens, which act by binding to the ligand-binding domain (LBD) of the receptor. Currently available therapeutic intervention in prostate cancer concentrates on reducing the androgen-mediated activation of the receptor by either blocking the production of androgens or by competing with endogenous androgens for the ligand-binding pocket. However, these treatments are often palliative as almost all patients eventually develop castration-resistant prostate cancer (CRPC). Therefore a great need exists to discover alternative modes of AR inhibition, outside of targeting the LBD. Factors that control AR stability and receptor turnover may constitute new regulatory targets for inhibiting AR action.

Bag-1L is a nuclear-resident cochaperone with the ability to control AR transactivation function by directly interacting with the receptor. Here we show that a conserved domain (BAG) within the C-terminus of Bag-1L induces significant structural changes within the intrinsically disordered N-terminal domain of the AR upon binding the receptor. A consequence of this may be increased accessibility of either domain for additional protein interactions. Using a mass spectrometry approach involving the combination of tandem affinity purification (TAP) and rapid immunoprecipitation mass spectrometry of endogenous proteins (RIME) we could show that Bag-1L, besides its ability to interact with AR, forms a complex involving the E3 ubiquitin-protein ligase CHIP and its known interaction partner Hsp70/Hsc70. Both proteins are involved in the folding and proteolytic turnover of AR, implying a role of Bag-1L in both processes. To further test this, we investigated the function of Bag-1L in human patient samples and in prostate cancer cell lines. We observed that nuclear Bag-1 protein levels are increased from hormone naïve to CRPC status in prostate cancer patients and this correlates with an increase in AR protein level. TALEN-mediated loss of Bag-1L in a CRPC cell line model leads to a stabilization of the AR protein, yet a drastic reduction in genome-wide AR binding and about a 50% decrease in AR-regulated target genes critical for cell proliferation and migration. Correspondingly, we observed a significant reduction in prostate cancer growth, which could be rescued when Bag-1L was re-expressed. Combined these results demonstrate the importance of Bag-1L for AR stability and function, and the potential of this protein for the therapeutic intervention in prostate cancer.

Citation Format: Laura Cato, Antje Neeb, Adam Sharp, Scott Ficarro, Victor Buzon, Xavier Salvatella, Jarrod A. Marto, Johann S. de Bono, Andrew C. Cato, Myles Brown. Androgen receptor stability in prostate cancer is regulated by the cochaperone Bag-1L. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 864.