Differential interaction of steroid hormone receptors with LXXLL motifs in SRC-1a depends on residues flanking the motif

Electrostatic modulation in steroid receptor recruitment of LXXLL and FXXLF motifs

Coactivator recruitment by activation function 2 (AF2) in the steroid receptor ligand binding domain takes place through binding of an LXXLL amphipathic alpha-helical motif at the AF2 hydrophobic surface. The androgen receptor (AR) and certain AR coregulators are distinguished by an FXXLF motif that interacts selectively with the AR AF2 site. Here we show that LXXLL and FXXLF motif interactions with steroid receptors are modulated by oppositely charged residues flanking the motifs and charge clusters bordering AF2 in the ligand binding domain. An increased number of charged residues flanking AF2 in the ligand binding domain complement the two previously characterized charge clamp residues in coactivator recruitment. The data suggest a model whereby coactivator recruitment to the receptor AF2 surface is initiated by complementary charge interactions that reflect a reversal of the acidic activation domain-coactivator interaction model.

Ligands differentially modulate the protein interactions of the human estrogen receptors alpha and beta

The interactions of human estrogen receptor subtypes ERalpha and ERbeta with DNA and a 210 amino acid residue fragment of the coactivator protein SRC-1 bearing three nuclear receptor interaction motifs were investigated quantitatively using fluorescence anisotropy in the presence of agonist and antagonist ligands. ERalpha and ERbeta were found to bind in a similar manner to DNA, and both salt and temperature affected the affinity and/or stoichiometry of these interactions. The agonist ligands estradiol, estrone and estriol did not modify the binding of ERalpha to the fluorescein-labeled target estrogen response element. However, in the case of ERbeta, these ligands led to the formation of some higher-order protein-DNA complexes and a small decrease in affinity. The partial agonist 4-hydroxytamoxifen had little effect on either ER subtype, whereas the pure antagonist ICI 182,780 led to the cooperative formation of protein-DNA complexes of higher order than dimer, as further demonstrated by competition experiments and gel mobility-shift assays. In addition to DNA binding, the interaction of both ER subtypes with the Alexa488-labeled SRC-1 coactivator fragment was investigated by fluorescence anisotropy. The agonist ligands estrone, estradiol, estriol, genistein and ethynyl estradiol exhibited distinct capacities for inducing the recruitment of SRC-1 that were not correlated with their affinity for the receptor. Moreover, estrone and genistein exhibited subtype specificity in that they induced SRC-1 recruitment to ERbeta with much higher efficiency than in the case of ERalpha. The differential coactivator recruitment capacities of the ER agonists and their receptor subtype coactivator recruitment specificity may be linked to the molecular structure of the agonists with respect to their interactions with a specific histidine residue located at the back of the ligand-binding pocket. Altogether, these quantitative in vitro studies of ER interactions reveal the complex energetic and stoichiometric consequences of changes in the chemical structures of these proteins and their ligands.

Modulation of induction properties of glucocorticoid receptor-agonist and -antagonist complexes by coactivators involves binding to receptors but is independent of ability of coactivators to augment transactivation

Coactivators such as TIF2 and SRC-1 modulate the positioning of the dose-response curve for agonist-bound glucocorticoid receptors (GRs) and the partial agonist activity of antiglucocorticoid complexes. These properties of coactivators differ from their initially defined activities of binding to, and increasing the total levels of transactivation by, agonist-bound steroid receptors. We now report that constructs of TIF2 and SRC-1 lacking the two activation domains (AD1 and AD2) have significantly less ability to increase transactivation but retain most of the activity for modulating the dose-response curve and partial agonist activity. Mammalian two-hybrid experiments show that the minimum TIF2 segment with modulatory activity (TIF2.4) does not interact with p300, CREB-binding protein, or PCAF, which also modulates GR activities. DRIP150 and DRIP205 have been implicated in coactivator actions but are unable to modulate GR activities. The absence of synergism by PCAF or DRIP150 with SRC-1 or TIF2, respectively, further suggests that these other factors are not involved. The ability of a TIF2.4 fragment (i.e. TIF2.37), which is not known to interact with proteins, to block the actions of TIF2.4 suggests that an unidentified binder mediates the modulatory activity of TIF2. Pull-down experiments with GST/TIF2.4 demonstrate a direct interaction of TIF2 with GR in a hormone-dependent fashion that requires the receptor interaction domains of TIF2 and is equally robust with agonists and most antiglucocorticoids. These observations, which are confirmed in mammalian two-hybrid assays, suggest that the capacity of coactivators such as TIF2 to modulate the partial agonist activity of antisteroids is mediated by the binding of coactivators to GR-antagonist complexes. In conclusion, the modulatory activity of coactivators with GR-agonist and -antagonist complexes is mechanistically distinct from the ability of coactivators to augment the total levels of transactivation and appears to involve the binding to both GR-steroid complexes and an unidentified TIF2-associated factor(s).

A GAL4-based yeast three-hybrid system for the identification of small molecule-target protein interactions

We report the development of a yeast strain designed for assaying compound-protein interactions through activation of reporter gene expression. Activation of lacZ expression, driven by the GAL4 promoter, has been demonstrated for precedented compound-protein interactions between FK506 and FK506 binding protein 12 (FKBP12) and also between methotrexate (MTX) and dihydrofolate reductase (DHFR). Reporter gene expression was completely abrogated in a competitive manner by the presence of excess FK506 or MTX, respectively. In addition, a strain expressing a mutated DHFR clone with decreased binding affinity for MTX was not capable of activating reporter gene expression. While strain sensitivity is compound-dependent, the minimum compound concentration necessary to drive reporter gene expression was 20 nM for the FK506-FKBP12 interaction. The utility of this strain as a tool for identifying unknown compound-binding proteins has been demonstrated by screening a mouse cDNA library for clones that encode proteins capable of binding MTX. Four library clones of mouse DHFR were identified after screening 5 x 10(6) clones. The screen background was low and false positives were easily identified, making this yeast system particularly amenable for use in a screening context for novel compound-protein interactions.

Androgen receptor (AR) coregulators: an overview

The biological action of androgens is mediated through the androgen receptor (AR). Androgen-bound AR functions as a transcription factor to regulate genes involved in an array of physiological processes, most notably male sexual differentiation and maturation, and the maintenance of spermatogenesis. The transcriptional activity of AR is affected by coregulators that influence a number of functional properties of AR, including ligand selectivity and DNA binding capacity. As the promoter of target genes, coregulators participate in DNA modification, either directly through modification of histones or indirectly by the recruitment of chromatin-modifying complexes, as well as functioning in the recruitment of the basal transcriptional machinery. Aberrant coregulator activity due to mutation or altered expression levels may be a contributing factor in the progression of diseases related to AR activity, such as prostate cancer. AR demonstrates distinct differences in its interaction with coregulators from other steroid receptors due to differences in the functional interaction between AR domains, possibly resulting in alterations in the dynamic interactions between coregulator complexes.

The FXXLF motif mediates androgen receptor-specific interactions with coregulators

The androgen receptor (AR) activation function 2 region of the ligand binding domain binds the LXXLL motifs of p160 coactivators weakly, engaging instead in an androgen-dependent, interdomain interaction with an FXXLF motif in the AR NH(2) terminus. Here we show that FXXLF motifs are present in previously reported AR coactivators ARA70/RFG, ARA55/Hic-5, and ARA54, which account for their selection in yeast two-hybrid screens. Mammalian two-hybrid assays, ligand dissociation rate studies, and glutathione S-transferase adsorption assays indicate androgen-dependent selective interactions of these FXXLF motifs with the AR ligand binding domain. Mutagenesis of residues within activation function 2 indicates distinct but overlapping binding sites where specificity depends on sequences within and flanking the FXXLF motif. Mutagenesis of the FXXLF motifs eliminated interaction with the ligand binding domain but only modestly reduced AR coactivation in transcription assays. The studies indicate that the FXXLF binding motif is specific for the AR and mediates interactions both within the AR and with coregulatory proteins.

Characterization of the interaction between androgen receptor and a new transcriptional inhibitor, SHP

SHP (short heterodimer partner) is an orphan nuclear receptor, first described for its interaction with nuclear receptors. This study explores a new way of inhibiting the androgen-signaling pathway. We demonstrated that SHP inhibited up to 97% of AR-induced activity. Characterization of AR/SHP interaction provided evidence of a clear ligand dependency. We also showed that the LXXI/LL motifs previously found on SHP mediated the interaction with the AR ligand-binding domain (AR-LBD), the motif responsible for the interaction being slightly different from that found with ER. The AR N-terminal domain (AR-NTD), in contrast to that of other nuclear receptors, accounts for most of the entire receptor transactivation potential. SHP also interacted with AR-NTD, thus stabilizing the interaction with AR. We demonstrated that SHP inhibited both AR-LBD and NTD-dependent transactivation, which evidenced for the first time a protein capable of inhibiting a steroid receptor amino-terminal-dependent transactivation. We further characterized the SHP mechanism of action by showing that SHP reversed AR coactivator-mediated activation. Conversely, FHL2 and TIF2 counteracted SHP-mediated inhibition of AR. SHP evidences a new way of inhibiting AR activity by competing with AR coactivators. This new type of inhibitor could dictate the activity of nuclear receptors, depending on the equilibrium between activators and inhibitors.

Multiplexed molecular interactions of nuclear receptors using fluorescent microspheres

BACKGROUND

We describe a novel microsphere-based system to identify and characterize multiplexed interactions of nuclear receptors with peptides that represent the LXXLL binding region of coactivator proteins.

METHODS

In this system, individual microsphere populations with unique red and orange fluorescent profiles are coupled to specific coactivator peptides. The coactivator peptide-coupled microsphere populations are combined and incubated with a nuclear receptor that has been coupled to a green fluorochrome. Flow cytometric analysis of the microspheres simultaneously decodes each population and detects the binding of receptor to respective coactivator peptides by the acquisition of green fluorescence.

RESULTS

We have used this system to determine the binding affinities of human estrogen receptor beta ligand binding domain (ERbeta LBD) and human peroxisome proliferator activated receptor gamma ligand binding domain (PPARgamma LBD) to a set of 34 coactivator peptides. Binding of ERbeta LBD to a coactivator peptide sequence containing the second LXXLL motif of steroid receptor coactivator-1 (SRC-1(2) (676-700) is shown to be specific and saturable. Analysis of receptor binding to a multiplexed set of coactivator peptides shows PPARgamma LBD binds with high affinity to cAMP response element binding protein (CBP) peptides and to the related P300 peptide while ERbeta LBD exibits little binding to these peptides. Using the microsphere-based assay we demonstrate that ERbeta LBD and PPARgamma LBD binding affinities for the coactivator peptides are increased in the presence of agonist (estradiol or GW1929, respectively) and that ERbeta LBD binding is decreased in the presence of antagonist (raloxifene or tamoxifen).

CONCLUSIONS

This unique microsphere-based system is a sensitive and efficient method to simultaneously evaluate many receptor-coactivator interactions in a single assay volume. In addition, the system offers a powerful approach to study small molecule modulation of nuclear receptor binding.

Ligands Specify Coactivator Nuclear Receptor (NR) Box Affinity for Estrogen Receptor Subtypes

Nuclear receptors (NRs) require coactivators to efficiently activate transcription of their target genes. Many coactivators including the p160 proteins utilize a short NR box motif to recognize the ligand-binding domain of the NR when it is activated by ligand. To investigate the ability of various ligands to specify the affinity of NR boxes for a ligand-bound NR, we compared the capacity of p160 NR boxes to be recruited to estrogen receptor (ERα) and ERβ in the presence of 17β-estradiol, diethylstilbestrol, and genestein. A time-resolved fluorescence-based binding assay was used to determine the dissociation constants for the 10 NR boxes derived from the three p160 coactivators for both ER subtypes in the presence of the each of the agonists. While the affinity of some NR boxes for ER was independent of the agonist, we identified several NR boxes that had significantly different affinities for ER depending on which agonist was bound to the receptor. Therefore, an agonist may specify the affinity of an NR for various NR boxes and thus regulate the coactivator selectivity of the receptor.

Core LXXLL motif sequences in CREB-binding protein, SRC1, and RIP140 define affinity and selectivity for steroid and retinoid receptors

An alpha-helical motif containing the sequence LXXLL is required for the ligand-dependent binding of transcriptional co-activators to nuclear receptors. By using a peptide inhibition assay, we have defined the minimal "core" LXXLL motif as an 8-amino acid sequence spanning positions -2 to +6 relative to the primary conserved leucine residue. In yeast two-hybrid assays, core LXXLL motif sequences derived from steroid receptor co-activator (SRC1), the 140-kDa receptor interacting protein (RIP140), and CREB-binding protein (CBP) displayed differences in selectivity and affinity for nuclear receptor ligand binding domains. Although core LXXLL motifs from SRC1 and RIP140 mediated strong interactions with steroid and retinoid receptors, three LXXLL motifs present in the global co-activator CBP were found to have very weak affinity for these proteins. Core motifs with high affinity for steroid and retinoid receptors were generally found to contain a hydrophobic residue at position -1 relative to the first conserved leucine and a nonhydrophobic residue at position +2. Our results indicate that variant residues in LXXLL core motifs influence the affinity and selectivity of co-activators for nuclear receptors.