Review
The hinge region in androgen receptor control

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Abstract

The region between the DNA-binding domain and the ligand-binding domain of nuclear receptors is termed the hinge region. Although this flexible linker is poorly conserved, diverse functions have been ascribed to it. For the androgen receptor (AR), the hinge region and in particular the 629RKLKKL634 motif, plays a central role in controlling AR activity, not only because it acts as the main part of the nuclear translocation signal, but also because it regulates the transactivation potential and intranuclear mobility of the receptor. It is also a target site for acetylation, ubiquitylation and methylation. The interplay between these different modifications as well as the phosphorylation at serine 650 will be discussed here. The hinge also has an important function in AR binding to classical versus selective androgen response elements. In addition, the number of coactivators/corepressors that might act via interaction with the hinge region is still growing. The importance of the hinge region is further illustrated by the different somatic mutations described in patients with androgen insensitivity syndrome and prostate cancer. In conclusion, the hinge region serves as an integrator for signals coming from different pathways that provide feedback to the control of AR activity.

Highlights

► The hinge region is more than a connection between DNA and ligand binding domain. ► The AR hinge is important for differentiating between clAREs and selAREs. ► AR transactivation potential and intranuclear mobility are regulated by the AR hinge. ► The AR hinge is a target site for posttranslational modifications. ► The AR hinge serves as an integrator for signals coming from different pathways.

Introduction

The human androgen receptor (AR) gene is located on chromosome X at position q11–12 and contains eight canonical exons, encoding a protein of 919 amino acids. The AR is a member of the nuclear receptor (NR) super family of ligand inducible transcription factors, which are defined by four functional domains (Robinson-Rechavi et al., 2003) (Fig. 1). The aminoterminal transactivation domain (NTD) is highly variable in length, amino acid sequence and mechanism of action. Importantly, this region contains an activation function 1 (AF-1), which is the major activation function for the AR (Jenster et al., 1995). Furthermore, the NTD contains a 23FQNLF27 motif which is important for the intramolecular N/C interaction with the ligand-binding domain (LBD) and for interaction with coactivators (Dubbink et al., 2004, He et al., 2004). This intramolecular N/C interaction is important for ligand dependent transcription activation of the AR (van Royen et al., 2007).

Next to the NTD lies the DNA-binding domain (DBD), which is the most conserved, and is the signature domain of the NR super family. This DBD consists of two zinc finger modules that are responsible for binding to the hormone response elements (Helsen et al., 2012, Umesono and Evans, 1989).

The main control of the activity of the AR happens via binding of androgens to the LBD that is situated at the carboxyterminal end of the DBD. This LBD contains a ligand-binding pocket composed of 11 α-helices. Upon androgen binding, helix 12 is repositioned to cover the helix binding pocket, forming the activation function 2 (AF-2) surface with which coactivators can interact (Heery et al., 1997, Moras and Gronemeyer, 1998). However, unlike AF-2 of the other NRs, AF-2 of the AR preferentially interacts with the 23FQNLF27 motif in the NTD to form the N/C interaction (van de Wijngaart et al., 2011).

The region between the DBD and the LBD is very sensitive to protease cleavage, is poorly conserved and most likely flexible in nature. This explains why initially it was called a hinge region. The flexibility of the hinge region of the NRs has been proposed to explain the fact that the retinoid X receptor (RXR) heterodimers can bind different types of response elements (Mangelsdorf and Evans, 1995). Later on, it was recognized that this hinge plays more crucial and partly receptor-specific roles in NR functioning, i.e. by influencing nuclear translocation, DNA binding, transactivation and receptor mobility. Here, we will describe the experimental evidence which has led to the postulation of the different hinge region functions of the AR, and discuss the role of its posttranslational modifications.

Section snippets

Nuclear translocation

In the absence of ligand, the AR resides mainly in the cytoplasm, but upon hormone binding, it translocates to the nucleus. The AR contains a bipartite nuclear localization signal (NLS) (617RKCYEAGMTLGARKLKKL634) consisting of two clusters of basic motifs (underlined) that span the carboxyterminal end of the second zinc finger (italics) and the aminoterminal part of the hinge region (bold) (Zhou et al., 1994). Nuclear import of the AR is mediated through the importin-α–importin-β complex.

Posttranslational modifications

An overview of possible posttranslational modifications of the AR hinge and the proposed interferences are summarized in Fig. 2.

Hinge interacting proteins

Several proteins affecting AR activity exert their effects via interaction with the hinge region. For example, SNURF (small nuclear RING finger protein) interacts with the AR DBD but also with residues 624 to 644 of the AR hinge region (Moilanen et al., 1998). This interaction is believed to facilitate nuclear translocation and retard nuclear export of the AR after hormone withdrawal (Poukka et al., 2000). Similar to SNURF, BAF57 (BRG1-associated factor 57) interacts with the AR via the DBD and

AR splice variants

The human AR gene, located on chromosome Xq11–12, contains eight canonical exons encoding the 110 kDa prototype AR, which consists of the four functional domains (NTD, DBD, hinge and LBD). The hinge region is encoded by part of the third and fourth exons. Recently, novel exons that map within intron 2 (exon 2b) and 3 (exon 3b–e) as well as an additional exon in the 3′ untranslated region (exon 9) have been described. Alternative splicing results in the inclusion of these cryptic exons in the

General conclusions

The human AR consists of four functional domains: NTD, DBD, hinge and LBD. The functions (and structure) of the DBD and LBD are well defined, but for the aminoterminal domain and the hinge region many questions still remain (Claessens et al., 2008).

For a long time, the hinge region of NRs has been considered to be nothing more than a polypeptide linker between the DBD and the LBD. For steroid receptors, it was shown to contain part of a bipartite NLS. Gradually it is becoming clear that the

Acknowledgements

This work was funded by a Ph.D. grant of the Agency for Innovation by Science and Technology (IWT) and by FWO grants G.0369.02 and G.0858.11. We thank the members of the Molecular Endocrinology Laboratory for interesting discussions.

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