Dissecting the Relation between a nuclear receptor and GATA: binding affinity studies of thyroid hormone receptor and GATA2 on TSHβ promoter

Nuclear receptor: Structure and function

Ligand-dependent transcription factors are nuclear receptors (NRs) that regulate various critical cellular processes such as reproduction, metabolism, development, etc. NRs are classified into (subgroup 0 to subgroup 6) seven superfamilies based on ligand-binding characteristics. All NRs share a general domain structure (A/B, C, D, and E) with distinct essential functions. NRs as monomers, homodimers, or heterodimers bind to consensus DNA sequences known as Hormone Response Elements (HREs). Furthermore, nuclear receptor-binding efficiency depends on minor differences in the sequences of HREs, spacing between the two half-sites, and the flanking sequence of the response elements. NRs can trans-activate and repress their target genes. In positively regulated genes, ligand-bound NRs recruit coactivators to activate the target gene expression, and unliganded NRs cause transcriptional repression. On the other hand, NRs repress gene expression by different mechanisms: (i) ligand-dependent transcriptional repression, (ii) ligand-independent transcriptional repression. This chapter will briefly explain NR superfamilies, their structures, molecular mechanism of action and their role in pathophysiological conditions, etc. That could enable the discovery of new receptors and their ligands and may elucidate their roles in various physiological processes. In addition, therapeutic agonists and antagonists would be developed to control the dysregulation of nuclear receptor signaling.

Thyroid hormone receptor β sumoylation is required for thyrotropin regulation and thyroid hormone production

Thyroid hormone receptor β (THRB) is posttranslationally modified by small ubiquitin-like modifier (SUMO). We generated a mouse model with a mutation that disrupted sumoylation at lysine 146 (K146Q) and resulted in desumoylated THRB as the predominant form in tissues. The THRB K146Q mutant mice had normal serum thyroxine (T4), markedly elevated serum thyrotropin-stimulating hormone (TSH; 81-fold above control), and enlargement of both the pituitary and the thyroid gland. The marked elevation in TSH, despite a normal serum T4, indicated blunted feedback regulation of TSH. The THRB K146Q mutation altered the recruitment of transcription factors to the TSHβ gene promoter, compared with WT, in hyperthyroidism and hypothyroidism. Thyroid hormone content (T4, T3, and rT3) in the thyroid gland of the THRB K146Q mice was 10-fold lower (per gram tissue) than control, despite normal TSH bioactivity. The expression of thyroglobulin and dual oxidase 2 genes in the thyroid was reduced and associated with modifications of cAMP response element-binding protein DNA binding and cofactor interactions in the presence of the desumoylated THRB. Therefore, thyroid hormone production had both TSH-dependent and TSH-independent components. We conclude that THRB sumoylation at K146 was required for normal TSH feedback regulation and TH synthesis in the thyroid gland, by a TSH-independent pathway.

Thioredoxin-1 Negatively Modulates ADAM17 Activity Through Direct Binding and Indirect Reductive Activity

AIMS

A disintegrin and metalloprotease 17 (ADAM17) modulates signaling events by releasing surface protein ectodomains such as TNFa and the EGFR-ligands. We have previously characterized cytoplasmic thioredoxin-1 (Trx-1) as a partner of ADAM17 cytoplasmic domain. Still, the mechanism of ADAM17 regulation by Trx-1 is unknown, and it has become of paramount importance to assess the degree of influence that Trx-1 has on metalloproteinase ADAM17.

RESULTS

Combining discovery and targeted proteomic approaches, we uncovered that Trx-1 negatively regulates ADAM17 by direct and indirect effect. We performed cell-based assays with synthetic peptides and site-directed mutagenesis, and we demonstrated that the interaction interface of Trx-1 and ADAM17 is important for the negative regulation of ADAM17 activity. However, both Trx-1^K72A and catalytic site mutant Trx-1^C32/35S rescued ADAM17 activity, although the interaction with Trx-1^C32/35S was unaffected, suggesting an indirect effect of Trx-1. We confirmed that the Trx-1^C32/35S mutant showed diminished reductive capacity, explaining this indirect effect on increasing ADAM17 activity through oxidant levels. Interestingly, Trx-1^K72A mutant showed similar oxidant levels to Trx-1^C32/35S, even though its catalytic site was preserved. We further demonstrated that the general reactive oxygen species inhibitor, Nacetylcysteine (NAC), maintained the regulation of ADAM17 dependent of Trx-1 reductase activity levels; whereas the electron transport chain modulator, rotenone, abolished Trx-1 effect on ADAM17 activity.

INNOVATION

We show for the first time that the mechanism of ADAM17 regulation, Trx-1 dependent, can be by direct interaction and indirect effect, bringing new insights into the cross-talk between isomerases and mammalian metalloproteinases.

CONCLUSION

This unexpected Trx-1^K72A behavior was due to more dimer formation and, consequently, the reduction of its Trx-1 reductase activity, evaluated through dimer verification, by gel filtration and mass spectrometry analysis. Antioxid. Redox Signal. 29, 717-734.

Protein Disulfide Isomerase Modulates the Activation of Thyroid Hormone Receptors

Thyroid hormone receptors (TRs) are responsible for mediating thyroid hormone (T3 and T4) actions at a cellular level. They belong to the nuclear receptor (NR) superfamily and execute their main functions inside the cell nuclei as hormone-regulated transcription factors. These receptors also exhibit so-called "non-classic" actions, for which other cellular proteins, apart from coregulators inside nuclei, regulate their activity. Aiming to find alternative pathways of TR modulation, we searched for interacting proteins and found that PDIA1 interacts with TRβ in a yeast two-hybrid screening assay. The functional implications of PDIA1-TR interactions are still unclear; however, our co-immunoprecipitation (co-IP) and fluorescence assay results showed that PDI was able to bind both TR isoforms in vitro. Moreover, T3 appears to have no important role in these interactions in cellular assays, where PDIA1 was able to regulate transcription of TRα and TRβ-mediated genes in different ways depending on the promoter region and on the TR isoform involved. Although PDIA1 appears to act as a coregulator, it binds to a TR surface that does not interfere with coactivator binding. However, the TR:PDIA1 complex affinity and activation are different depending on the TR isoform. Such differences may reflect the structural organization of the PDIA1:TR complex, as shown by models depicting an interaction interface with exposed cysteines from both proteins, suggesting that PDIA1 might modulate TR by its thiol reductase/isomerase activity.

RXR agonist modulates TR: corepressor dissociation upon 9-cis retinoic acid treatment

Transcriptional regulation controlled by thyroid hormone receptor (TR) drives events such as development, differentiation, and metabolism. TRs may act either as homodimers or as heterodimers with retinoid X receptor (RXR). Thyroid hormone T3 preferentially binds TR-RXR heterodimers, which activate transcription through coactivator recruitment. However, it is unclear whether TR-RXR heterodimers may also be responsive to the canonical RXR agonist 9-cis retinoic acid (9C) in the context of physiological gene regulation. New structural studies suggest that 9C promotes the displacement of bound coactivators from the heterodimer, modifying TR-RXR activity. To shed light on the molecular mechanisms that control TR-RXR function, we used biophysical approaches to characterize coregulator recruitment to TR-TR or to TR-RXR in the presence of T3 and/or 9C as well as cell-based assays to establish the functional significance of biophysical findings. Using cell-based and fluorescence assays with mutant and wild-type TR, we show that 9C does indeed have a function in the TR-RXR heterodimer context, in which it induces the release of corepressors. Furthermore, we show that 9C does not promote detectable conformational changes in the structure of the TR-RXR heterodimer and does not affect coactivator recruitment. Finally, our data support the view that DNA binding domain and Hinge regions are important to set up NR-coactivator binding interfaces. In summary, we showed that the RXR agonist 9C can regulate TR function through its modulation of corepressor dissociation.

Genome-wide binding patterns of thyroid hormone receptor beta

Thyroid hormone (TH) receptors (TRs) play central roles in metabolism and are major targets for pharmaceutical intervention. Presently, however, there is limited information about genome wide localizations of TR binding sites. Thus, complexities of TR genomic distribution and links between TRβ binding events and gene regulation are not fully appreciated. Here, we employ a BioChIP approach to capture TR genome-wide binding events in a liver cell line (HepG2). Like other NRs, TRβ appears widely distributed throughout the genome. Nevertheless, there is striking enrichment of TRβ binding sites immediately 5′ and 3′ of transcribed genes and TRβ can be detected near 50% of T3 induced genes. In contrast, no significant enrichment of TRβ is seen at negatively regulated genes or genes that respond to unliganded TRs in this system. Canonical TRE half-sites are present in more than 90% of TRβ peaks and classical TREs are also greatly enriched, but individual TRE organization appears highly variable with diverse half-site orientation and spacing. There is also significant enrichment of binding sites for TR associated transcription factors, including AP-1 and CTCF, near TR peaks. We conclude that T3-dependent gene induction commonly involves proximal TRβ binding events but that far-distant binding events are needed for T3 induction of some genes and that distinct, indirect, mechanisms are often at play in negative regulation and unliganded TR actions. Better understanding of genomic context of TR binding sites will help us determine why TR regulates genes in different ways and determine possibilities for selective modulation of TR action.

Negative regulation by nuclear receptors: a plethora of mechanisms

Nuclear receptors are arguably the best understood transcriptional regulators. We know a great deal about the mechanisms through which they activate transcription in response to ligand binding and about the mechanisms through which they repress transcription in the absence of ligand. However, endocrine regulation often requires that ligand-bound receptors repress transcription of a subset of genes. An understanding of the mechanism for ligand-induced repression and how this differs from activation has proven elusive. A number of recent studies have directly or indirectly addressed this problem. Yet it seems the more evidence that accumulates, the more complex the mystery becomes.