A reciprocal feedback between the PDZ binding kinase and androgen receptor drives prostate cancer.

Elucidation of mechanisms underlying the increased androgen receptor (AR) activity and subsequent development of aggressive prostate cancer (PrCa) is pivotal in developing new therapies. Using a systems biology approach, we interrogated the AR-regulated proteome and identified PDZ binding kinase (PBK) as a novel AR-regulated protein that regulates full-length AR and AR variants (ARVs) activity in PrCa. PBK overexpression in aggressive PrCa is associated with early biochemical relapse and poor clinical outcome. In addition to its carboxy terminus ligand-binding domain, PBK directly interacts with the amino terminus transactivation domain of the AR to stabilise it thereby leading to increased AR protein expression observed in PrCa. Transcriptome sequencing revealed that PBK is a mediator of global AR signalling with key roles in regulating tumour invasion and metastasis. PBK inhibition decreased growth of PrCa cell lines and clinical specimen cultured ex vivo. We uncovered a novel interplay between AR and PBK that results in increased AR and ARVs expression that executes AR-mediated growth and progression of PrCa, with implications for the development of PBK inhibitors for the treatment of aggressive PrCa.

Tissue control of androgen action: The ups and downs of androgen receptor expression.

The hormone testosterone plays crucial roles during male development and puberty and throughout life, as an anabolic regulator of muscle and bone structure and function. The actions of testosterone are mediated, primarily, through the androgen receptor, a member of the nuclear receptor superfamily. The androgen receptor gene is located on the X-chromosome and receptor levels are tightly controlled both at the level of transcription of the gene and post-translationally at the protein level. Sp1 has emerged as the major driver of expression of the androgen receptor gene, while auto-regulation by androgens is associated with both positive and negative regulation in a possible cell-selective manner. Research into the networks of positive and negative regulators of the androgen receptor gene are vital in order to understand the temporal and spatial control of receptor levels and the consequences for healthy aging and disease. A clear understanding of the multiple transcription factors participating in regulation of the androgen receptor gene will likely aid in the development and application of hormone therapies to boast or curb receptor activity.

Promoter-dependent activity on androgen receptor N-terminal domain mutations in androgen insensitivity syndrome.

Androgen receptor (AR) mutations are associated with androgen insensitivity syndrome (AIS). Missense mutations identified in the AR-N-terminal domain (AR-NTD) are rare, and clinical phenotypes are typically mild. We investigated 7 missense mutations and 2 insertion/deletions located in the AR-NTD. This study aimed to elucidate the pathogenic role of AR-NTD mutants in AIS and to use this knowledge to further define AR-NTD function. AR-NTD mutations (Q120E, A159T, G216R, N235K, G248V, L272F, and P380R) were introduced into AR-expression plasmids. Stably expressing cell lines were established for del57L and ins58L. Transactivation was measured using luciferase reporter constructs under the control of GRE and Pem promoters. Intrinsic fluorescence spectroscopy and partial proteolysis studies were performed for mutations which showed reduced activities by using a purified AR-AF1 protein. Pem-luciferase reporter activation was reduced for A159T, N235K, and G248V but not the GRE-luciferase reporter. Protein structure analysis detected no significant change in the AR-AF1 region for these mutations. Reduced cellular expression and transactivation activity were observed for ins58L. The mutations Q120E, G216R, L272F, P380R, and del57L showed small or no detectable changes in function. Thus, clinical and experimental analyses have identified novel AR-signalling defects associated with mutations in the structurally disordered AR-NTD domain in patients with AIS.

Negative regulation of the androgen receptor gene through a primate-specific androgen response element present in the 5' UTR.

The androgen receptor (AR) is a widely expressed ligand-activated transcription factor which mediates androgen signalling by binding to androgen response elements (AREs) in normal tissue and prostate cancer (PCa). Within tumours, the amount of AR plays a crucial role in determining cell growth, resistance to therapy and progression to fatal castrate recurrent PCa in which prostate cells appear to become independent of androgenic steroids. Despite the pivotal role of the AR in male development and fertility and all stages of PCa development, the mechanisms governing AR expression remain poorly understood. In this work, we describe an active nonconsensus androgen response element (ARE) in the 5' UTR of the human AR gene. The ARE represses transcription upon binding of activated AR, and this downregulation is relieved by disruption of the regulatory element through mutation. Also, multiple species comparison of the genomic region reveals that this ARE is specific to primates, leading to the conclusion that care must be exercised when elucidating the operation of the human AR in PCa based upon rodent promoter studies.

An androgen receptor N-terminal domain antagonist for treating prostate cancer.

Hormone therapies for advanced prostate cancer target the androgen receptor (AR) ligand-binding domain (LBD), but these ultimately fail and the disease progresses to lethal castration-resistant prostate cancer (CRPC). The mechanisms that drive CRPC are incompletely understood, but may involve constitutively active AR splice variants that lack the LBD. The AR N-terminal domain (NTD) is essential for AR activity, but targeting this domain with small-molecule inhibitors is complicated by its intrinsic disorder. Here we investigated EPI-001, a small-molecule antagonist of AR NTD that inhibits protein-protein interactions necessary for AR transcriptional activity. We found that EPI analogs covalently bound the NTD to block transcriptional activity of AR and its splice variants and reduced the growth of CRPC xenografts. These findings suggest that the development of small-molecule inhibitors that bind covalently to intrinsically disordered proteins is a promising strategy for development of specific and effective anticancer agents.

Regulation of the androgen receptor by SET9-mediated methylation.

The androgen receptor (AR) is a member of the nuclear hormone receptor family of transcription factors that plays a critical role in regulating expression of genes involved in prostate development and transformation. Upon hormone binding, the AR associates with numerous co-regulator proteins that regulate the activation status of target genes via flux to the post-translational modification status of histones and the receptor. Here we show that the AR interacts with and is directly methylated by the histone methyltransferase enzyme SET9. Methylation of the AR on lysine 632 is necessary for enhancing transcriptional activity of the receptor by facilitating both inter-domain communication between the N- and C-termini and recruitment to androgen-target genes. We also show that SET9 is pro-proliferative and anti-apoptotic in prostate cancer cells and demonstrates up-regulated nuclear expression in prostate cancer tissue. In all, our date indicate a new mechanism of AR regulation that may be therapeutically exploitable for prostate cancer treatment.

Conformation of the mineralocorticoid receptor N-terminal domain: evidence for induced and stable structure.

The mineralocorticoid receptor (MR) binds the steroid hormones aldosterone and cortisol and has an important physiological role in the control of salt homeostasis. Regions of the protein important for gene regulation have been mapped to the amino-terminal domain (NTD) and termed activation function (AF)1a, AF1b, and middle domain (MD). In the present study, we used a combination of biophysical and biochemical techniques to investigate the folding and function of the MR-NTD transactivation functions. We demonstrate that MR-AF1a and MR-MD have relatively little stable secondary structure but have the propensity to form α-helical conformation. Induced folding of the MR-MD enhanced protein-protein binding with a number of coregulatory proteins, including the coactivator cAMP response element-binding protein-binding protein and the corepressors SMRT and RIP140. By contrast, the MR-AF1b domain appeared to have a more stable conformation consisting predominantly of ß-secondary structure. Furthermore, MR-AF1b specifically interacted with the TATA-binding protein, via an LxxLL-like motif, in the absence of induced folding. Together, these data suggest that the MR-NTD contains a complex transactivation system made up of distinct structural and functional domains. The results are discussed in the context of the induced folding paradigm for steroid receptor NTDs.

Regression of castrate-recurrent prostate cancer by a small-molecule inhibitor of the amino-terminus domain of the androgen receptor.

Castration-recurrent prostate cancer (CRPC) is suspected to depend on androgen receptor (AR). The AF-1 region in the amino-terminal domain (NTD) of AR contains most, if not all, of the transcriptional activity. Here we identify EPI-001, a small molecule that blocked transactivation of the NTD and was specific for inhibition of AR without attenuating transcriptional activities of related steroid receptors. EPI-001 interacted with the AF-1 region, inhibited protein-protein interactions with AR, and reduced AR interaction with androgen-response elements on target genes. Importantly, EPI-001 blocked androgen-induced proliferation and caused cytoreduction of CRPC in xenografts dependent on AR for growth and survival without causing toxicity.

Using intrinsic fluorescence emission spectroscopy to study steroid receptor and coactivator protein conformation dynamics.

X-ray crystallography and nuclear magnetic resonance (NMR) spectroscopy have proved powerful methods for studying the structure of the isolated ligand and DNA-binding domains of nuclear receptors. However, the N-terminal domain (NTD), which in some members of the superfamily is important for transcriptional regulation, and the full-length receptor proteins have proved more challenging. The NTD of different nuclear receptors show little sequence homology and can vary dramatically in length from a few to several hundred amino acids. Low resolution structural analysis using circular dichroism, NMR, steady-state fluorescence spectroscopy, and Fourier transformed infrared spectroscopy has provided valuable information on the conformation and folding of the structurally plastic NTD. In this chapter, we discuss protocols for measuring the intrinsic fluorescence emission spectrum for tryptophan residues under different experimental conditions of protein folding and unfolding.

Consequences of poly-glutamine repeat length for the conformation and folding of the androgen receptor amino-terminal domain.

Poly-amino acid repeats, especially long stretches of glutamine (Q), are common features of transcription factors and cell-signalling proteins and are prone to expansion, resulting in neurodegenerative diseases. The amino-terminal domain of the androgen receptor (AR-NTD) has a poly-Q repeat between 9 and 36 residues, which when it expands above 40 residues results in spinal bulbar muscular atrophy. We have used spectroscopy and biochemical analysis to investigate the structural consequences of an expanded repeat (Q45) or removal of the repeat (DeltaQ) on the folding of the AR-NTD. Circular dichroism spectroscopy revealed that in aqueous solution, the AR-NTD has a relatively limited amount of stable secondary structure. Expansion of the poly-Q repeat resulted in a modest increase in alpha-helix structure, while deletion of the repeat resulted in a small loss of alpha-helix structure. These effects were more pronounced in the presence of the structure-promoting solvent trifluoroethanol or the natural osmolyte trimethylamine N-oxide. Fluorescence spectroscopy showed that the microenvironments of four tryptophan residues were also altered after the deletion of the Q stretch. Other structural changes were observed for the AR-NTDQ45 polypeptide after limited proteolysis; in addition, this polypeptide not only showed enhanced binding of the hydrophobic probe 8-anilinonaphthalene-1-sulphonic acid but was more sensitive to urea-induced unfolding. Taken together, these findings support the view that the presence and length of the poly-Q repeat modulate the folding and structure of the AR-NTD.

Natural disordered sequences in the amino terminal domain of nuclear receptors: lessons from the androgen and glucocorticoid receptors.

Steroid hormones are a diverse class of structurally related molecules, derived from cholesterol, that include androgens, estrogens, progesterone and corticosteroids. They represent an important group of physiologically active signalling molecules that bind intracellular receptor proteins and regulate genes involved in developmental, reproductive and metabolic processes. The receptor proteins share structurally and functionally related ligand binding and DNA-binding domains, but possess distinct N-terminal domains (NTD) of unique length and amino acids sequence. The NTD contains sequences important for gene regulation, exhibit structure plasticity and are likely to contribute to the specificity of the steroid hormone/receptor response.

Induced alpha-helix structure in the aryl hydrocarbon receptor transactivation domain modulates protein-protein interactions.

The aryl hydrocarbon receptor (AhR) is an intracellular receptor protein that regulates gene transcription in response to both man-made and natural ligands. A modular transactivaton domain (TAD) has been mapped to the 304 C-terminal amino acids and consists of acidic, Q-rich, and P/S/T-rich subdomains. We have used steady-state intrinsic tryptophan fluorescence and circular dichroism spectroscopy to investigate the conformation of the acidic Q-rich region. The results reveal that this region of the protein is structurally flexible but adopts a more folded conformation in the presence of the natural osmolyte trimethylamine N-oxide (TMAO) and the solvent trifluoroethanol (TFE). In protein-protein interaction studies, the acidic Q-rich region bound to components of the general transcription machinery [TATA-binding protein (TBP), TAF4, and TAF6] as well as the coactivator proteins SRC-1a and TIF2. The binding site for TBP mapped to the acidic subdomain, while SRC-1a bound preferentially to the Q-rich sequence. Significantly, the binding of TBP was modulated by induced folding of the TAD with TMAO. The results indicate that the AhR TAD makes multiple interactions with the transcriptional machinery and protein conformation plays a critical role in receptor function. Taken together, these findings support a role for protein folding in AhR action and suggest possible mechanisms of receptor-dependent gene activation.

The androgen receptor interacts with multiple regions of the large subunit of general transcription factor TFIIF.

The androgen receptor (AR) is a ligand-activated transcription factor that regulates genes important for male development and reproductive function. The main determinants for the transactivation function lie within the structurally distinct amino-terminal domain. Previously we identified an interaction between the AR-transactivation domain (amino acids 142-485) and the general transcription factor TFIIF (McEwan, I. J., and Gustafsson, J.-A. (1997) Proc. Natl. Acad. Sci. U. S. A. 94, 8485-8490). We have now mapped the binding sites for the AR-transactivation domain within the RAP74 subunit of TFIIF. Both the amino-terminal 136 amino acids and the carboxyl-terminal 155 amino acids of RAP74 interacted with the AR-transactivation domain and were able to rescue basal transcription after squelching by the AR polypeptide. Competition experiments demonstrated that the AR could interact with the holo-TFIIF protein and that the carboxyl terminus of RAP74 represented the principal receptor-binding site. Point mutations within AR-transactivation domain distinguished the binding sites for RAP74 and the p160 coactivator SRC-1a and identified a single copy of a six amino acid repeat motif as being important for RAP74 binding. These data indicate that the AR-transactivation domain can potentially make multiple protein-protein interactions with coactivators and components of the general transcriptional machinery in order to regulate target gene expression.

Conformational analysis of the androgen receptor amino-terminal domain involved in transactivation. Influence of structure-stabilizing solutes and protein-protein interactions.

The androgen receptor (AR) is a member of the nuclear receptor superfamily. Sequences within the large amino-terminal domain of the receptor have been shown to be important for transactivation and protein-protein interactions; however, little is known about the structure and folding of this region. In the present study we show that a 344-amino acid polypeptide representing the main determinants for transactivation has the propensity to form alpha-helical structure and that mutations which disrupt putative helical regions alter conformation. Folding of the AR was observed in the presence of the helix-stabilizing solvent trifluoroethanol and the natural osmolyte trimethylamine N-oxide (TMAO). TMAO resulted in the movement of two tryptophan residues to a less solvent-exposed environment and the formation of secondary/tertiary structure resistant to protease cleavage. Critically, binding to the RAP74 subunit of the general transcription factor TFIIF resulted in extensive protease resistance, consistent with induced folding of the receptor transactivation domain. These data indicate that this region of the AR is structurally flexible and folds into a stable conformation upon interactions with a component of the general transcription machinery.