Differences in DNA Binding Characteristics of the Androgen and Glucocorticoid Receptors Can Determine Hormone-specific Responses

Abstract

The basis for specificity of gene regulation by steroid hormone receptors remains an important problem in the study of steroid hormone action. One possible mechanism for steroid specificity is the difference in DNA binding characteristics of the receptors, although they share a high homology in their DNA-binding domains. Indeed, the androgen-specific expression of, for example, the probasin (PB) gene can be explained by the presence of an androgen response element (ARE) in its promoter (PB-ARE-2), specifically recognized by the androgen and not by the glucocorticoid receptor. Three residues in the DNA-binding domain of the AR were identified as main determinants for its high affinity for the PB-ARE-2. In addition, the direct repeat nature of this ARE seems to prohibit high affinity binding by the glucocorticoid receptor. This is confirmed by the fact that several imperfect direct repeats of the 5-TGT-TCT-3 core recognition sequence are recognized by the androgen receptor and not by the glucocorticoid receptor. Up to now, only differences between the androgen and glucocorticoid receptor in the transcription activation functions were invoked to explain the specificity of their genomic actions. In the present study, we describe the influence of the DNA-binding domain on the specificity of androgen action. The novelty of our working hypothesis resides in the demonstration of the capacity of the AR-DNA-binding domain to recognize elements with a direct repeat structure.Steroid hormones are important endocrine messengers that activate their receptors, which translocate to the cell nucleus and regulate gene expression mainly after interaction with DNA sequences, called response elements (1, 2). The steroid receptors are a subfamily of the nuclear receptor superfamily, a large group of structurally homologous transcription factors. A problem with the explanation of the specificity of these hormone responses arose when several studies pointed out that the class I receptors (androgen receptor (AR), 1 glucocorticoid receptor (GR), progesterone receptor, and mineralocorticoid receptor) have identical consensus response elements (3, 4) and that their DNA-binding domains were highly conserved (5). This contrasts with the fact that the in vivo expression of several genes is specifically controlled by only one steroid hormone (6). Several possible mechanisms have been described to explain the steroid specificity of transcriptional control, e.g. steroid metabolism, tissue-specific receptor presence (7), influence of coactivator complexes (8), and chromatin structure (9, 10). In addition, more recent reports indicate that the AR on the one hand and the GR, progesterone receptor, and mineralocorticoid receptor on the other exhibit different DNA binding characteristics (11-15). One AR-specific response element was found in the promoter of the rat probasin gene (PB-ARE-2) (12,16,17). Probasin is an androgen-regulated protein exclusively expressed in the dorsolateral epithelium of the prostate (18). Two cis-acting androgen response e...

Structural basis of androgen receptor binding to selective androgen response elements

“…Because cooperative dimerization greatly increases the affinity of receptors for their bipartite response elements, these two changes should also increase the relative affinity of the AR for a given response element compared with GR. In support of this hypothesis, GR DBD mutants containing a serine in place of Gly-478 [39] in the D box or a threonine in place of GR Ile-483 [44] show increased affinity for both palindromic and direct-repeat response elements compared with wild type (34), confirming the importance of these interactions for dimer stability.…”

“…All the steroid receptors (MR, PR, GR, and AR) show a 5-to 10-fold lower affinity for the naturally occurring PB-ARE-2 DR3-type element than the C3 (1) IR3-type element (34). However, the AR DBD binds 3-to 10-fold better to both elements relative to the other steroid receptors.…”

“…Because the concentration of individual steroid receptors in the cell is approximately nanomolar, differences in binding constants of this order are likely to be significant. AR substitutions in the GR dimerization interface, including Gly483Ser and Ile483Thr, show higher affinity binding to both DR3 and IR3 response elements (34), thus mimicking the behavior of the AR. Together with the structural data, these observations suggest a model where, because of the increased strength of the AR dimer interface, AR-selective gene activation arises from the ability of the AR to bind to IR3 response elements that have a greater deviation from the consensus half-site sequence.…”

Differential modulation of glucocorticoid and progesterone receptor transactivation

“…HRE flanking and spacer DNA can contribute to the affinity and specificity of receptor binding (Nelson et al, 1999). The DBD of androgen receptor (AR) has lower (76%) homology with the GR DBD (Chang et al, 1988), which seems to be sufficient for DNA binding specificity of AR vs. GR (Massaad et al, 2000;Schoenmakers et al, 2000). Thus subtle differences in HRE sequence may regulate the relative activities of GR and PR (Thackray et al, 1998).…”

Dual Function of an Amino-terminal Amphipatic Helix in Androgen Receptor-mediated Transactivation through Specific and Nonspecific Response Elements

“…Selective DNA binding by the AR could be one possible mechanism for hormone-specific gene regulation. It has been reported that the AR-DBD does not only recognize response elements organized as inverted repeats of 5Ј-TGTTCT-3Ј sequences with a threenucleotide spacer but can also bind to direct repeats of 5Ј-TGTTCT-3Ј-like sequences, which is proposed to contribute to the AR specificity of transcriptional responses (7)(8)(9)(10)(11)(12)(13)(14). The NTD of the AR is indispensable for AR transactivation and contains a strong transcription activation function (AF-1).…”