Diverse roles of androgen receptor (AR) domains in AR-mediated signaling

Androgens control male sexual development and maintenance of the adult male phenotype. They have very divergent effects on their target organs like the reproductive organs, muscle, bone, brain and skin. This is explained in part by the fact that different cell types respond differently to androgen stimulus, even when all these responses are mediated by the same intracellular androgen receptor. To understand these tissue- and cell-specific readouts of androgens, we have to learn the many different steps in the transcription activation mechanisms of the androgen receptor (NR3C4). Like all nuclear receptors, the steroid receptors have a central DNA-binding domain connected to a ligand-binding domain by a hinge region. In addition, all steroid receptors have a relatively large amino-terminal domain. Despite the overall structural homology with other nuclear receptors, the androgen receptor has several specific characteristics which will be discussed here. This receptor can bind two types of androgen response elements (AREs): one type being similar to the classical GRE/PRE-type elements, the other type being the more divergent and more selective AREs. The hormone-binding domain has low intrinsic transactivation properties, a feature that correlates with the low affinity of this domain for the canonical LxxLL-bearing coactivators. For the androgen receptor, transcriptional activation involves the alternative recruitment of coactivators to different regions in the amino-terminal domain, as well as the hinge region. Finally, a very strong ligand-induced interaction between the amino-terminal domain and the ligand-binding domain of the androgen receptor seems to be involved in many aspects of its function as a transcription factor. This review describes the current knowledge on the structure-function relationships within the domains of the androgen receptor and tries to integrate the involvement of different domains, subdomains and motifs in the functioning of this receptor as a transcription factor with tissue- and cell-specific readouts

Coactivator Interactions at the Transcription Activation Units of the Androgen Receptor

The androgen receptor (AR) belongs to the family of nuclear receptors (NRs). These NRs are ligand-inducible transcription factors that are directed to their target genes through an interaction of their DNA-binding domain with specific sequences in the DNA. A large number of coactivators is sequentiallyrecruited by the NRs via a coactivator-binding groove at the surface of their ligand-binding domain (LBD). Although the LBD of the AR is structurally very similar to that of other NRs, its activation function (called AF-2) is very weak, a characteristic that correlates with a strong interaction between theamino-terminal domain and the coactivator-binding groove. While the DNA-binding and the ligand-binding domains are strongly conserved within the complete family of nuclear receptors, the amino-terminal domain (NTD) is very divergent both in length and in sequence. The amino-terminal domains of the steroid receptors in particular are known to cover strong activation functions, but the mechanisms behind the activation remain much less clear compared to that of the ligand-binding domain. Indeed,the coactivator mechanisms of this domain do not seem to be conserved even between different steroid receptors. For the AR, the NTD is about 550 amino acids long and contains two transcription activation units, Tau-1 and Tau-5, which can work independently of each other as activation domains. For Tau-5, we and others described an interaction with p160 coactivators, not via their LxxLL motifs, but via aglutamine rich (Qr) region (Callewaert et al. 2006). In the first part of this study we analyzed the interactions between Tau-5 and the p160s. The boundaries and key characteristics of both the Tau-5 and the p160 Qr fragment were determined to delineate the specific interaction surface. Since both domains separately were difficult to express inprokaryotes, we aimed to purify the complex of Tau-5 with SRC1-Qr. Furtheroptimization is needed before the protein quantities will be sufficient forfurther structural analyses. In the second part of this work, we studied Tau-1, and focused on identifying its coactivator(s). The copurification of Tau-1 interacting proteins revealed interactions of HSP70-1a. We are convinced that this reflects the hydrophobic nature of the isolated AR-fragment that was used in the copurification assay, although a role of HSP70 in the action mechanisms of Tau-1 cannot be excluded.We also present the Transcription Intermediary Factor 1 ß (TIF1ß) as a Tau-1 coactivator. This factor can act both as a positive and a negative regulator for different transcription factors. We can show an interaction between TIF1ß and the AR and demonstrate that its coactivation depends on thepresence of Tau-1. Strong physiological evidence comes from the observation that a mutation in the Tau-1 domain found in an AIS patient destroys the coactivation by TIF1ß almost completely, indicating that the interaction also plays an important role in vivo. Compared to Tau-5, Tau-1 is less conserved, especially in Danio rerio, but we think this is linked with the fact that TIF1ß is lost in the Clupeocephala, where Danio rerio belongs to. A structure-function analysis of TIF1ß shows that the domains involved in its action as a corepressor are different from those involved in coactivation of the AR. In conclusion, we describe TIF1ß as a new coactivator for Tau-1. Since we demonstrate its expression in prostate cells, this AR-TIF1ß axis could become a a new target for prostate cancer therapy.status: publishe

The transcription intermediary factor 1β coactivates the androgen receptor

The androgen receptor (AR) is a ligand-inducible transcription factor. Its transcription activation domain consists of the two transcription activation units called Tau-1 and Tau-5. Tau-5 interacts with p160 coactivators like the transcription intermediary factor 2 (TIF2), which in their turn recruit histone modifiers and chromatin-remodelling complexes. The mechanism of action of Tau-1, however, remains elusive. Here, we demonstrate that transcription intermediary factor 1 beta (TIF1 beta) can induce the activity of the AR up to five fold when tested in vitro. Although there is no evidence for direct interactions between TIF1 beta and AR, mutation studies show that the activity of TIF1 beta depends on the integrity of Tau- 1 in AR on the one hand, and the so- called tripartite motif domain in TIF1 beta on the other. Surprisingly, the coactivation by TIF1 beta via Tau- 1 seems additive rather than cooperative with the AR coactivation by TIF2. Some mutations naturally occurring in androgen-insensitivity syndrome patients that reside in Tau-1 seem to impair the TIF1 beta coactivation of the AR, indicating that TIF1 beta could also be relevant for the in vivo androgen response in humans. Moreover, since TIF1 beta is well expressed in prostate cancer cells, its functional interaction with androgen signalling could in the long run be a therapeutic target for this disease. (J. Endocrinol. Invest. 36: 699- 706, 2013) (C) 2013, Editrice Kurti

A (RKLKK633)-R-629 motif in the hinge region controls the androgen receptor at multiple levels

The androgen receptor protein has specific domains involved in DNA binding, ligand binding, and transactivation, whose activities need to be integrated during transcription activation. The hinge region, more particular a (RKLKK633)-R-629 motif, seems to play a crucial role in this process. Indeed, although the motif is not part of the DNA-binding domain, its positive residues are involved in optimal DNA binding and nuclear translocation as shown by mutation analysis. When the mutated ARs are forced into the nucleus, however, the residues seem to play different roles in transactivation. Moreover, we show by FRAP analysis that during activation, the AR is distributed in the nucleus in a mobile and two immobile fractions, and that mutations in the (RKLKK633)-R-629 motif affect the distribution of the AR over these three intranuclear fractions. Taken together, the (RKLKK633)-R-629 motif is a multifunctional motif that integrates nuclear localization, receptor stability, DNA binding, transactivation potential and intranuclear mobility