Domains of ERRgamma that mediate homodimerization and interaction with factors stimulating DNA binding.

The estrogen receptor-related receptor gamma (ERRgamma/ERR3/NR3B3) is an orphan member of the nuclear receptor superfamily closely related to the estrogen receptors. To explore the DNA binding characteristics, the protein-DNA interaction was studied in electrophoretic mobility shift assays (EMSAs). In vitro translated ERRgamma binds as a homodimer to direct repeats (DR) without spacing of the nuclear receptor half-site 5'-AGGTCA-3' (DR-0), to extended half-sites, and to the inverted estrogen response element. Using ERRgamma deletion constructs, binding was found to be dependent on the presence of sequences in the ligand binding domain (LBD). A far-Western analysis revealed that ERRgamma forms dimers even in the absence of DNA. Two elements, located in the hinge region and in the LBD, respectively, are necessary for DNA-independent dimerization. DNA binding of bacterial expressed ERRgamma requires additional factors present in the serum and in cellular extracts. Fusion proteins of the germ cell nuclear factor (GCNF/NR6A1) with ERRgamma showed that the characteristic feature to be stimulated by additional factors can be transferred to a heterologous protein. The stimulating activity was further characterized and its target sequence narrowed down to a small element in the hinge region.     

Development of a coactivator displacement assay for the orphan receptor estrogen-related receptor-gamma using time-resolved fluorescence resonance energy transfer

The estrogen-related receptor-gamma (ERRgamma) is a constitutively active orphan receptor that belongs to the nuclear receptor superfamily and is most closely related to the estrogen receptors. Although its physiological ligand is unknown, ERRgamma has been shown to interact with synthetic estrogenic compounds such as 4-hydroxytamoxifen (4-OHT), tamoxifen, and diethylstilbestrol (DES). To assess how coregulator proteins interact with ERRgamma in response to ligand, an in vitro interaction methodology using time-resolved fluorescence resonance energy transfer (TR-FRET) was developed using glutathione S-transferase (GST)-tagged ERRgamma ligand-binding domain (LBD), a terbium-labeled anti-GST antibody, a fluorescein-labeled peptide containing sequences derived from coregulator proteins, and various ligands. An initial screen of these coregulator peptides bearing the coactivator LXXLL motif, the corepressor LXXI/HIXXXI/L motif, or other interaction motifs from natural coactivator sequences or random phage display peptides indicated that the peptides PGC1alpha, D22, and SRC1-4, known as class III coregulators, interacted most strongly with ERRgamma in the absence of ligand. Given its assay window and biological relevance in energy metabolism and obesity, further studies were conducted with PGC1alpha. Fluorescein-labeled PGC1alpha peptide was displaced from the ERRgamma LBD in the presence of increasing concentrations of 4-OHT and tamoxifen, but DES was less effective in PGC1alpha displacement. The statistical parameter Z' factor that measures the robustness of the assay was greater than 0.8 for displacement of PGC1alpha from ERRgamma LBD in the presence of saturating 4-OHT over an assay incubation time of 1-6 h, indicating an excellent assay. These findings also suggest that binding of 4-OHT, tamoxifen, or DES to ERRgamma results in differential affinity of coregulators for ERRgamma due to unique ligand-induced conformations.     

Estrogen-related receptors-stimulated monoamine oxidase B promoter activity is down-regulated by estrogen receptors

Although there are studies published about the neuroprotective effect of estrogen, little is known about the mechanisms and cellular targets of the hormone. Recent reports demonstrate that estrogen down-regulates the expression of monoamine oxidase A and B (MAO-A and MAO-B) in the hypothalamus of the Macaques monkey, both of which are key isoenzymes in the neurotransmitter degradation pathway. Additionally, estrogen-related receptor alpha (ERRalpha) up-regulates MAO-B gene expression in breast cancer cells. ERRalpha recognizes a variety of estrogen response elements and shares many target genes and coactivators with estrogen receptor alpha (ERalpha). In this study, we investigate the interplay of ERs and ERRs in the regulation of MAO-B promoter activity. We demonstrate that ERRalpha and ERRgamma up-regulate MAO-B gene activity, whereas ERalpha and ERbeta decrease stimulation in both a ligand-dependent and -independent manner. Ectopically expressed ERRalpha and ERRgamma stimulate the expression of MAO-B mRNA and protein as well as increase the MAO-B enzymatic activity in ER-negative HeLa cells. The ability of ERRs to stimulate MAO-B promoter activity was reduced in ER-positive MCF-7 and T47D cells. Several AGGTCA motifs of the MAO-B promoter are responsible for up-regulation by ERRs. Interestingly, ERalpha or ERbeta alone have no effect on MAO-B promoter activity but can down-regulate the activation function of ERRs, whereas glucocorticoid receptor does not. By using chromatin immunoprecipitation assay, we demonstrate that ERs compete with ERRs for binding to the MAO-B promoter at selective AGGTCA motifs, thereby changing the chromatin status and cofactor recruitment to a repressed state. These studies provide new insight into the relationship between ERalpha, ERbeta, ERRalpha, and ERRgamma in modulation of MAO-B gene activity.     

Structure-guided synthesis of tamoxifen analogs with improved selectivity for the orphan ERRgamma

The design and synthesis of 4-hydroxytamoxifen (4-OHT) derivatives are described. The binding affinities of these compounds toward the orphan estrogen-related receptor gamma and the classical estrogen receptor alpha demonstrate that analogs bearing hydroxyalkyl groups display improved binding selectivity profiles compared with that of 4-OHT. An X-ray crystal structure of one of the designed compounds bound to ERRgamma LBD confirms the molecular basis of the selectivity.     

X-ray crystal structures of the estrogen-related receptor-gamma ligand binding domain in three functional states reveal the molecular basis of small molecule regulation

X-ray crystal structures of the ligand binding domain (LBD) of the estrogen-related receptor-gamma (ERRgamma) were determined that describe this receptor in three distinct states: unliganded, inverse agonist bound, and agonist bound. Two structures were solved for the unliganded state, the ERRgamma LBD alone, and in complex with a coregulator peptide representing a portion of receptor interacting protein 140 (RIP140). No significant differences were seen between these structures that both exhibited the conformation of ERRgamma seen in studies with other coactivators. Two structures were obtained describing the inverse agonist-bound state, the ERRgamma LBD with 4-hydroxytamoxifen (4-OHT), and the ERRgamma LBD with 4-OHT and a peptide representing a portion of the silencing mediator of retinoid and thyroid hormone action protein (SMRT). The 4-OHT structure was similar to other reported inverse agonist bound structures, showing reorientation of phenylalanine 435 and a displacement of the AF-2 helix relative to the unliganded structures with little other rearrangement occurring. No significant changes to the LBD appear to be induced by peptide binding with the addition of the SMRT peptide to the ERRgamma plus 4-OHT complex. The observed agonist-bound state contains the ERRgamma LBD, a ligand (GSK4716), and the RIP140 peptide and reveals an unexpected rearrangement of the phenol-binding residues. Thermal stability studies show that agonist binding leads to global stabilization of the ligand binding domain. In contrast to the conventional mechanism of nuclear receptor ligand activation, activation of ERRgamma by GSK4716 does not appear to involve a major rearrangement or significant stabilization of the C-terminal helix.     

Crystal structure of human estrogen-related receptor alpha in complex with a synthetic inverse agonist reveals its novel molecular mechanism

Inverse agonists of the constitutively active human estrogen-related receptor alpha (ERRalpha, NR3B1) are of potential interest for several disease indications (e.g. breast cancer, metabolic diseases, or osteoporosis). ERRalpha is constitutively active, because its ligand binding pocket (LBP) is practically filled with side chains (in particular with Phe(328), which is replaced by Ala in ERRbeta and ERRgamma). We present here the crystal structure of the ligand binding domain of ERRalpha (containing the mutation C325S) in complex with the inverse agonist cyclohexylmethyl-(1-p-tolyl-1H-indol-3-ylmethyl)-amine (compound 1a), to a resolution of 2.3A(.) The structure reveals the dramatic multiple conformational changes in the LBP, which create the necessary space for the ligand. As a consequence of the new side chain conformation of Phe(328) (on helix H3), Phe(510)(H12) has to move away, and thus the activation helix H12 is displaced from its agonist position. This is a novel mechanism of H12 inactivation, different from ERRgamma, estrogen receptor (ER) alpha, and ERbeta. H12 binds (with a surprising binding mode) in the coactivator groove of its ligand binding domain, at a similar place as a coactivator peptide. This is in contrast to ERRgamma but resembles the situation for ERalpha (raloxifene or 4-hydroxytamoxifen complexes). Our results explain the novel molecular mechanism of an inverse agonist for ERRalpha and provide the basis for rational drug design to obtain isotype-specific inverse agonists of this potential new drug target. Despite a practically filled LBP, the finding that a suitable ligand can induce an opening of the cavity also has broad implications for other orphan nuclear hormone receptors (e.g. the NGFI-B subfamily).     

Modulation of adipogenesis-related gene expression by estrogen-related receptor gamma during adipocytic differentiation

Estrogen-related receptor gamma (ERRgamma) is an orphan nuclear receptor that regulates cellular energy metabolism by modulating gene expression involved in oxidative metabolism and mitochondrial biogenesis in brown adipose tissue and heart. However, the physiological role of ERRgamma in adipogenesis and the development of white adipose tissue has not been well studied. Here we show that ERRgamma was up-regulated in murine mesenchyme-derived cells, especially in ST2 and C3H10T1/2 cells, at mRNA levels under the adipogenic differentiation condition including the inducer of cAMP, glucocorticoid, and insulin. The up-regulation of ERRgamma mRNA was also observed in inguinal white adipose and brown adipose tissues of mice fed a high-fat diet. Gene knockdown by ERRgamma-specific siRNA results in mRNA down-regulation of adipogenic marker genes including fatty acid binding protein 4, PPARgamma, and PGC-1beta in a preadipocyte cell line 3T3-L1 preadipocytes and mesenchymal ST2 and C3H10T1/2 cells in the adipogenesis medium. In contrast, stable expression of ERRgamma in 3T3-L1 cells resulted in up-regulation of these adipogenic marker genes under the adipogenic condition. These results suggest that ERRgamma positively regulate the adipocyte differentiation with modulating the expression of various adipogenesis-related genes.