Analyzing the Androgen Receptor Interactome in Prostate Cancer: Implications for Therapeutic Intervention

Abstract

The androgen receptor (AR) is a member of the ligand-activated nuclear receptor family of transcription factors. AR's transactivation activity is turned on by the binding of androgens, the male sex steroid hormones. AR is critical for the development and maintenance of the male phenotype but has been recognized to also play an important role in human diseases. Most notably, AR is a major driver of prostate cancer (CaP) progression, which remains the second leading cause of cancer deaths in American men. Androgen deprivation therapies (ADTs) that interfere with interactions between AR and its activating androgen ligands have been the mainstay for treatment of metastatic CaP. Although ADTs are effective and induce remissions, eventually they fail, while the growth of the majority of ADT-resistant CaPs remains under AR's control. Alternative approaches to inhibit AR activity and bypass resistance to ADT are being sought, such as preventing the interaction between AR and its cofactors and coregulators that is needed to execute AR-dependent transcription. For such strategies to be efficient, the 3D conformation of AR complexes needs to be well-understood and AR-regulator interaction sites resolved. Here, we review current insights into these 3D structures and the protein interaction sites in AR transcriptional complexes. We focus on methods and technological approaches used to identify AR interactors and discuss challenges and limitations that need to be overcome for efficient therapeutic AR complex disruption.

Keywords: androgen deprivation therapy; coactivators; corepressors; hormonal therapy; proteomics; transcription.

Figure 1

AR domain and motif organization. AR consist of an N-terminal domain (NTD) which contains a ligand-independent transcriptional function (AF) AF-1, a central DNA binding domain (DBD) and a C-terminal ligand-binding domain (LBD) which harbors the ligand-activated AF-2. DBD and LBD are connected by a hinge (H) region. FxxLF, WxxLF and LxxLF motifs represent short amino acid sequences relevant for AR domain interactions, AR transcriptional activity and AR-coregulator interaction, as discussed in the text. Numbers represent amino acids in AR protein, for which numbering is based on NM_000044.2. * and ** indicates a short 14 amino acid stretch and a proline rich region that has been used as bait in 2 hybrid screening or phage display assays in references He et al., 2004 and Blessing et al., 2015 respectively.

Figure 2

Basic mechanism of AR transcriptional activation. Testosterone or precursor androgens enter CaP cells and are converted into the most bioactive androgen dihydrotestosterone (DHT). DHT binding activates AR, which results in cytoplasmic to nuclear AR re-localization and AR dimerization. AR binds as a dimer to Androgen Response Elements (AREs) in AR target genes, where it associates with 3 classes of transcriptional regulators to mediate AR target gene transcription. These regulators consist of pioneer transcription factors (PFs), coregulators, and secondary transcription factors (TFs). TFBS, transcription factor binding site.

Figure 3

Contribution of AR domain interactions to AR dimerization, AR DNA binding and coregulator recruitment. (A) Current working model supports that AR dimerization occurs via intermolecular NTD-NTD, DBD-DBD and LBD-LBD interactions. Inter-and intramolecular NTD-LBD interactions further contribute to active AR conformation and dimerization. (B) Schematic representation of 3D AR domain interactions and interactions between AR domains and the AR-associated coregulators p300 and SRC-3. Black arrows, strongest interactions; dashed grey arrows, weak interactions. Data adapted from information provided in reference [11].