Synthesis, SAR study, and bioactivity evaluation of a series of Quinoline-Indole-Schiff base derivatives: Compound 10E as a new Nur77 exporter and autophagic death inducer.

We previously reported 5-((8-methoxy-2-methylquinolin-4-yl)amino)-1H-indole- 2-carbohydrazide derivatives as new Nur77 modulators. In this study, we explored whether the 8-methoxy-2-methylquinoline moiety and bicyclic aromatic rings at the N'-methylene position were critical for their antitumor activity against hepatocellular carcinoma (HCC). For this purpose, a small library of 5-substituted 1H-indole-2-carbohydrazide derivatives was designed and synthesized. We found that the 8-methoxy-2-methylquinoline moiety was a fundamental structure for its biological function, while the introduction of the bicyclic aromatic ring into the N'-methylene greatly improved its anti-tumor effect. We found that the representative compound 10E had a high affinity to Nur77. The KD values were in the low micromolar (2.25-4.10 µM), which were coincident with its IC50 values against the tumor cell lines (IC50 < 3.78 µM). Compound 10E could induce autophagic cell death of liver cancer cells by targeting Nur77 to mitochondria while knocking down Nur77 greatly impaired anti-tumor effect. These findings provide an insight into the structure-activity relation of Quinoline-Indole-Schiff base derivatives and further demonstrate that antitumor agents targeting Nur77 may be considered as a promising strategy for HCC therapy.

Structural basis of NR4A1 bound to the human pituitary proopiomelanocortin gene promoter.

The nuclear receptor NR4A subfamily (NR4A1/NGFI-B, NR4A2/Nurr1 and NR4A3/NOR-1) can recognize different classes of DNA response elements either as a monomer, homodimer, or heterodimer. In this study, we determined the structure of the NR4A1 DNA-binding domain (NR4A1-DBD) bound to natural Nur-responsive elements (NurREs) in the promoter region of the pituitary proopiomelanocortin (POMC) gene (NurREPOMC) at 3.12 Å resolution. The NR4A1-DBD molecules bound independently to this element in our structure. The N-terminal helix H1 forms specific contacts with major groove, and C-terminal extension interact extensively with minor groove. Moreover our modelling structure of NR4A1 large fragment complexed with NurREPOMC indicated that ligand binding domain of NR4A might form dimer interactions to help recognize DNA. Overall, our analyses provide a molecular basis for DNA binding of NR4A proteins as a homodimer and novel insight into the molecular functions of NR4A modulation of gene expression.

Reporters of TCR signaling identify arthritogenic T cells in murine and human autoimmune arthritis.

How pathogenic cluster of differentiation 4 (CD4) T cells in rheumatoid arthritis (RA) develop remains poorly understood. We used Nur77-a marker of T cell antigen receptor (TCR) signaling-to identify antigen-activated CD4 T cells in the SKG mouse model of autoimmune arthritis and in patients with RA. Using a fluorescent reporter of Nur77 expression in SKG mice, we found that higher levels of Nur77-eGFP in SKG CD4 T cells marked their autoreactivity, arthritogenic potential, and ability to more readily differentiate into interleukin-17 (IL-17)-producing cells. The T cells with increased autoreactivity, nonetheless had diminished ex vivo inducible TCR signaling, perhaps reflective of adaptive inhibitory mechanisms induced by chronic autoantigen exposure in vivo. The enhanced autoreactivity was associated with up-regulation of IL-6 cytokine signaling machinery, which might be attributable, in part, to a reduced amount of expression of suppressor of cytokine signaling 3 (SOCS3)-a key negative regulator of IL-6 signaling. As a result, the more autoreactive GFPhi CD4 T cells from SKGNur mice were hyperresponsive to IL-6 receptor signaling. Consistent with findings from SKGNur mice, SOCS3 expression was similarly down-regulated in RA synovium. This suggests that despite impaired TCR signaling, autoreactive T cells exposed to chronic antigen stimulation exhibit heightened sensitivity to IL-6, which contributes to the arthritogenicity in SKG mice, and perhaps in patients with RA.

SAR study of celastrol analogs targeting Nur77-mediated inflammatory pathway.

Nur77, an orphan member of the nuclear receptor superfamily, plays an important role in the regulation of inflammatory processes. Our previous work found that celastrol, a pentacyclic triterpene, bound to Nur77 to inhibit inflammation in a Nur77-dependent manner. Celastrol binding to Nur77 promotes Nur77 translocation from nucleus to cytoplasm, resulting in clearance of inflamed mitochondria and then alleviation of inflammation. Here, we report the design, synthesis, SAR study and biological evaluation of a series of celastrol analogs. A total of 24 celastrol derivatives were made. Compound 3a with a Kd of 0.87 µM was found to be less toxic than celastrol and could be a hit molecule for further optimization.

NR4A Nuclear Receptors Target Poly-ADP-Ribosylated DNA-PKcs Protein to Promote DNA Repair.

Although poly-ADP-ribosylation (PARylation) of DNA repair factors had been well documented, its role in the repair of DNA double-strand breaks (DSBs) is poorly understood. NR4A nuclear orphan receptors were previously linked to DSB repair; however, their function in the process remains elusive. Classically, NR4As function as transcription factors using a specialized tandem zinc-finger DNA-binding domain (DBD) for target gene induction. Here, we show that NR4A DBD is bi-functional and can bind poly-ADP-ribose (PAR) through a pocket localized in the second zinc finger. Separation-of-function mutants demonstrate that NR4A PAR binding, while dispensable for transcriptional activity, facilitates repair of radiation-induced DNA double-strand breaks in G1. Moreover, we define DNA-PKcs protein as a prominent target of ionizing radiation-induced PARylation. Mechanistically, NR4As function by directly targeting poly-ADP-ribosylated DNA-PKcs to facilitate its autophosphorylation-promoting DNA-PK kinase assembly at DNA lesions. Selective targeting of the PAR-binding pocket of NR4A presents an opportunity for cancer therapy.

Metabolic gene NR4A1 as a potential therapeutic target for non-smoking female non-small cell lung cancer patients.

BACKGROUND: Although cigarette smoking is considered one of the key risk factors for lung cancer, 15% of male patients and 53% of female patients with lung cancer are non-smokers. Metabolic changes are critical features of cancer. Therapeutic target identification from a metabolic perspective in non-small cell lung cancer (NSCLC) tissue of female non-smokers has long been ignored. RESULTS: Based on microarray data retrieved from Affymetrix expression arrays E-GEOD-19804, we found that the downregulated genes in non-smoking female NSCLC patients tended to participate in protein/amino acid and lipid metabolism, while upregulated genes were more involved in protein/amino acid and carbohydrate metabolism. Combining nutrient metabolic co-expression, protein-protein interaction network construction and overall survival assessment, we identified NR4A1 and TIE1 as potential therapeutic targets for NSCLC in female non-smokers. To accelerate the drug development for non-smoking female NSCLC patients, we identified nilotinib as a potential agonist targeting NR4A1 encoded protein by molecular docking and molecular dynamic stimulation. We also show that nilotinib inhibited proliferation and induced senescence of cells in non-smoking female NSCLC patients in vitro. CONCLUSIONS: These results not only uncover nutrient metabolic characteristics in non-smoking female NSCLC patients, but also provide a new paradigm for identifying new targets and drugs for novel therapy for such patients.

Design and synthesis of biotinylated cardiac glycosides for probing Nur77 protein inducting pathway.

The orphan nuclear receptor Nur77 (also known as TR3 or nerve growth factor-induced clone B NGFI-B) functions as a nuclear transcription factor in the regulation of target gene expression and plays a critical role in the regulation of differentiation, proliferation, apoptosis, and survival of many different cell types. Recent studies demonstrate that Nur77 also involves many important physiological and pathological processes including cancer, inflammation and immunity, cardiovascular diseases, and bone diseases. Our previous studies showed that cardiac glycosides could induce the expression of Nur77 protein and its translocation from the nucleus to the cytoplasm and subsequent targeting to mitochondria, leading to apoptosis of cancer cells. In order to probe the Nur77 protein inducting pathway, we designed and synthesized a series of novel biotinylated cardiac glycosides from ß-Antiarin and α-Antiarin, two typical cardiac glycosides from the plant of Antiaris toxicaria. The induction of Nur77 protein expression of these biotinylated cardiac glycosides and their inhibitory effects on NIH-H460 cancer cell proliferation were evaluated. Results displayed that some biotinylated cardiac glycosides could significantly induce the expression of Nur77 protein comparable with their parent compounds ß-Antiarin and α-Antiarin. Also, their streptavidin binding activities were evaluated. Among them, biotinylated cardiac glycosides P4b and P5a exhibited significant effect on the induction of Nur77 expression along with high binding capacity with streptavidin, suggesting that they can be used as probes for probing Nur77 protein inducting pathway.

Characteristics of Nur77 and its ligands as potential anticancer compounds (Review).

Nuclear receptor subfamily 4 group A member 1 (NR4A1; also termed Nur77/TR3/NGFIB), a member of the nuclear receptor superfamily, is expressed as an early response gene to regulate the expression of multiple target genes. Nur77 has the typical structure of a nuclear receptor, including an N­terminal domain, a DNA binding domain, and a ligand­binding domain. The expression and localization of Nur77 are closely associated with its roles in cell proliferation and apoptosis. Nur77 was first identified as an orphan receptor, the endogenous ligand of which has not yet been identified; however, an increasing number of compounds targeting Nur77 have been reported to have beneficial effects in the treatment of cancer and other diseases. This review provides a brief overview of the identification, structure, expression and localization, transcriptional role and non­genomic function of Nur77, and summarizes the ligands that have been shown to interact with Nur77, including cytosporone B, cisplatin, TMPA, PDNPA, CCE9, THPN, Z­ligustilide, celastrol and bisindole methane compounds, which may potentially be used to treat cancer in humans.

The E3 ubiquitin ligase Trim13 regulates Nur77 stability via casein kinase 2α.

Nur77 is a member of the NR4A subfamily of nuclear receptors and has been shown to regulate various biological processes such as apoptosis and inflammation. Here, we show that Nur77 ubiquitination is mediated by the tripartite motif 13 (Trim13), a RING-type E3 ubiquitin ligase. The interaction between Nur77 and Trim13 was confirmed by co-immunoprecipitation. Moreover, we found that Lys539 in Nur77 ubiquitination is targeted for Trim13, which leads to Nur77 degradation. The Trim13-mediated ubiquitination of Nur77 was optimal in the presence of the E2 enzyme UbcH5. Importantly, in addition to Trim13-mediated ubiquitination, the stability of Nur77 was also regulated by casein kinase 2α (CK2α). Pharmacological inhibition of CK2 markedly increased Nur77 levels, whereas overexpression of CK2α, but not its inactive mutant, dramatically decreased Nur77 levels by promoting Nur77 ubiquitination. CK2α phosphorylated Ser154 in Nur77 and thereby regulated Nur77 protein levels by promoting its ubiquitin-mediated degradation. Importantly, we also show that degradation of Nur77 is involved in TNFα-mediated IL-6 production via CK2α and Trim13. Taken together, these results suggest that the sequential phosphorylation and ubiquitination of Nur77 controls its degradation, and provide a therapeutic approach for regulating Nur77 activity through the CK2α-Trim13 axis as a mechanism to control the inflammatory response.

Nur77 variants solely comprising the amino-terminal domain activate hypoxia-inducible factor-1α and affect bone marrow homeostasis in mice and humans.

Gene targeting via homologous recombination can occasionally result in incomplete disruption of the targeted gene. Here, we show that a widely used Nur77-deficient transgenic mouse model expresses a truncated protein encoding for part of the N-terminal domain of nuclear receptor Nur77. This truncated Nur77 protein is absent in a newly developed Nur77-deficient mouse strain generated using Cre-Lox recombination. Comparison of these two mouse strains using immunohistochemistry, flow cytometry, and colony-forming assays shows that homologous recombination-derived Nur77-deficient mice, but not WT or Cre-Lox-derived Nur77-deficient mice, suffer from liver immune cell infiltrates, loss of splenic architecture, and increased numbers of bone marrow hematopoietic stem cells and splenic colony-forming cells with age. Mechanistically, we demonstrate that the truncated Nur77 N-terminal domain protein maintains the stability and activity of hypoxia-inducible factor (HIF)-1, a transcription factor known to regulate bone marrow homeostasis. Additionally, a previously discovered, but uncharacterized, human Nur77 transcript variant that encodes solely for its N-terminal domain, designated TR3ß, can also stabilize and activate HIF-1α. Meta-analysis of publicly available microarray data sets shows that TR3ß is highly expressed in human bone marrow cells and acute myeloid leukemia samples. In conclusion, our study provides evidence that a transgenic mouse model commonly used to study the biological function of Nur77 has several major drawbacks, while simultaneously identifying the importance of nongenomic Nur77 activity in the regulation of bone marrow homeostasis.

Molecular dynamics study of TMPA mediated dissociation of Nur77-LKB1 complex.

LKB1 protein is involved in the regulation of cell polarity by phosphorylating the AMPK under energetic stress conditions. LKB1 protein is expressed in both cytoplasm and nucleus. In the nucleus, LKB1 interacts with orphan nuclear receptor protein Nur77. It is reported that the interaction of LKB1 with Nur77 is disrupted by the small molecular ligand TMPA (ethyl 2-[2,3,4-trimethoxy-6-(1-octanoyl)phenyl]acetate), such that the LKB1 is enabled to play its role in cytoplasm and further to regulate/reduce the blood glucose level. In the present study, atomistic molecular dynamics simulations are performed to understand the dissociation mechanism of Nur77-LKB1 complex. The present study reveals that TMPAs induce an open-close motion of Nur77 which further decrease the stability of Nur77-LKB1 complex. As a consequence, the interface region in LKB1-Nur77 complex is more exposed for solvation and further releases the interactions existing between Nur77 and LKB1. Altogether, this study explains the TMPAs mediated Nur77-LKB1 complex dissociation.

New Drug Candidate Targeting the 4A1 Orphan Nuclear Receptor for Medullary Thyroid Cancer Therapy.

Medullary thyroid cancer (MTC) is a relatively rare thyroid cancer responsible for a substantial fraction of thyroid cancer mortality. More effective therapeutic drugs with low toxicity for MTC are urgently needed. Orphan nuclear receptor 4A1 (NR4A1) plays a pivotal role in regulating the proliferation and apoptosis of a variety of tumor cells. Based on the NR4A1 protein structure, 2-imino-6-methoxy-2H-chromene-3-carbothioamide (IMCA) was identified from the Specs compounds database using the protein structure-guided virtual screening approach. Computationally-based molecular modeling studies suggested that IMCA has a high affinity for the ligand binding pocket of NR4A1. MTT [3-(4,5-dimethyl-2-thiazolyl)-2,5-diphenyl-2-H-tetrazolium bromide] and apoptosis assays demonstrated that IMCA resulted in significant thyroid cancer cell death. Immunofluorescence assays showed that IMCA induced NR4A1 translocation from the nucleus to the cytoplasm in thyroid cancer cell lines, which may be involved in the cell apoptotic process. In this study, the quantitative polymerase chain reaction results showed that the IMCA-induced upregulation of sestrin1 and sestrin2 was dose-dependent in thyroid cancer cell lines. Western blot showed that IMCA increased phosphorylation of adenosine 5'-monophosphate-activated protein kinase (AMPK) and decreased phosphorylation of ribosomal protein S6 kinase (p70S6K), which is the key enzyme in the mammalian target of rapamycin (mTOR) pathway. The experimental results suggest that IMCA is a drug candidate for MTC therapy and may work by increasing the nuclear export of NR4A1 to the cytoplasm and the tumor protein 53 (p53)-sestrins-AMPK-mTOR signaling pathway.

Simultaneous targeting of MyD88 and Nur77 as an effective approach for the treatment of inflammatory diseases.

Myeloid differentiation primary response protein 88 (MyD88) has long been considered a central player in the inflammatory pathway. Recent studies clearly suggest that it is an important therapeutic target in inflammation. On the other hand, a recent study on the interaction between the orphan nuclear receptor (Nur77) and p38α, leading to increased lipopolysaccharide-induced hyperinflammatory response, suggests this binary complex as a therapeutic target. In this study, we have designed inhibitors that can inhibit both MyD88 and Nur77 at the same time. Since both MyD88 and Nur77 are an integral part of the pathways involving lipopolysaccharide-induced activation of NF-κB-mediated inflammation, we tried to target both proteins with the same library in order to retrieve compounds having dual inhibitory properties. To perform this, we developed a homodimeric model of MyD88 and, along with the crystal structure of Nur77, screened a virtual library of compounds from the traditional Chinese medicine database containing ~61,000 compounds. We analyzed the resulting hits for their efficacy for dual binding and probed them for developing a common pharmacophore model that could be used as a prototype to screen compound libraries as well as to guide combinatorial library design to search for ideal dual-target inhibitors. Thus, our study explores the identification of novel leads having dual inhibiting effects due to binding to both MyD88 and Nur77 targets.

In Silico Adoption of an Orphan Nuclear Receptor NR4A1.

A 4.1 µs molecular dynamics simulation of the NR4A1 (hNur77) apo-protein has been undertaken and a previously undetected druggable pocket has become apparent that is located remotely from the 'traditional' nuclear receptor ligand-binding site. A NR4A1/bis-indole ligand complex at this novel site has been found to be stable over 1 µs of simulation and to result in an interesting conformational transmission to a remote loop that has the capacity to communicate with a NBRE within a RXR-α/NR4A1 heterodimer. Several features of the simulations undertaken indicate how NR4A1 can be affected by alternate-site modulators.

Selective ligand activity at Nur/retinoid X receptor complexes revealed by dimer-specific bioluminescence resonance energy transfer-based sensors.

Retinoid X receptors (RXRs) play a role as master regulators because of their capacity to form heterodimers with other nuclear receptors (NRs). Accordingly, retinoid signaling is involved in multiple biologic processes, including development, cell differentiation, metabolism, and cell death. However, the role and function of RXRs in different heterodimer complexes remain unidentified, mainly because most RXR drugs (called rexinoids) are not selective of specific heterodimer complexes. The lack of selectivity strongly limits the use of rexinoids for specific therapeutic approaches. To better characterize rexinoids at specific NR complexes, we have developed and optimized luciferase (Luc) protein complementation(PCA)-based bioluminescence resonance energy transfer (BRET) assays that can directly measure recruitment of a coactivator (CoA) motif fused to yellow fluorescent protein (YFP) by specific NR dimers. To validate the assays, we compared rexinoid modulation of CoA recruitment by the RXR homodimer and by the heterodimers Nur77/RXR and Nurr1/RXR. Results revealed that some rexinoids display selective CoA recruitment activities with homo- or heterodimer complexes. In particular, SR11237 (BMS649) has stronger potency for recruitment of CoA motif and transcriptional activity with the heterodimer Nur77/RXR than other complexes. This technology should be useful in identifying new compounds with specificity for individual dimeric species formed by NRs.

Requirement of novel amino acid fragments of orphan nuclear receptor TR3/Nur77 for its functions in angiogenesis.

Pathological angiogenesis is a hallmark of many diseases. We demonstrated that TR3/Nur77 is an excellent target for pro-angiogenesis and anti-angiogenesis therapies. Here, we report that TR3 transcriptionally regulates endothelial cell migration, permeability and the formation of actin stress fibers that is independent of RhoA GTPase. 1) Amino acid residues 344-GRR-346 and de-phosphorylation of amino acid residue serine 351 in the DNA binding domain, and 2) phosphorylation of amino acid residues in the 41-61 amino acid fragment of the transactivation domain, of TR3 are required for its induction of the formation of actin stress fibers, cell proliferation, migration and permeability. The 41-61 amino acid fragment contains one of the three potential protein interaction motifs in the transactivation domain of TR3, predicted by computational modeling and analysis. These studies further our understanding of the molecular mechanism, by which TR3 regulates angiogenesis, identify novel therapeutic targeted sites of TR3, and set the foundation for the development of high-throughput screening assays to identify compounds targeting TR3/Nur77 for pro-angiogenesis and anti-angiogenesis therapies.

[Expression and purification of DNA binding domain of NR4A1].

OBJECTIVE: To express and purify NR4A1-DNA binding domain (DBD) protein of nuclear receptors. METHODS: The fusion protein PET28a-NR4A1-DBD was constructed and purified with the nickel affinity chromatography, cation-exchange chromatography and gel filtration chromatography. RESULTS: The protein PET28a-NR4A1-DBD was mostly soluable at 24 °C. A total of 2-3 mg/L pure NR4A1 proteins were yielded in bacterial culture and the purity for final fractions of NR4A1-DBD protein were great than 95% by SDS-PAGE analysis. CONCLUSION: Nickel affinity chromatography is effective to purify protein. The protein purity can be further improved by the following methods including cation-exchange chromatography and gel filtration chromatography.

Self-consistent field convergence for proteins: a comparison of full and localized-molecular-orbital schemes.

Proteins in the gas phase present an extreme (and unrealistic) challenge for self-consistent-field iteration schemes because their ionized groups are very strong electron donors or acceptors, depending on their formal charge. This means that gas-phase proteins have a very small band gap but that their frontier orbitals are localized compared to "normal" conjugated semiconductors. The frontier orbitals are thus likely to be separated in space so that they are close to, but not quite, orthogonal during the SCF iterations. We report full SCF calculations using the massively parallel EMPIRE code and linear scaling localized-molecular-orbital (LMO) calculations using Mopac2009. The LMO procedure can lead to artificially over-polarized wavefunctions in gas-phase proteins. The full SCF iteration procedure can be very slow to converge because many cycles are needed to overcome the over-polarization by inductive charge shifts. Example molecules have been constructed to demonstrate this behavior. The two approaches give identical results if solvent effects are included.

The orphan receptor TR3 participates in angiotensin II-induced cardiac hypertrophy by controlling mTOR signalling.

Angiotensin II (AngII) induces cardiac hypertrophy and increases the expression of TR3. To determine whether TR3 is involved in the regulation of the pathological cardiac hypertrophy induced by AngII, we established mouse and rat hypertrophy models using chronic AngII administration. Our results reveal that a deficiency of TR3 in mice or the knockdown of TR3 in the left ventricle of rats attenuated AngII-induced cardiac hypertrophy compared with the respective controls. A mechanistic analysis demonstrates that the TR3-mediated activation of mTORC1 is associated with AngII-induced cardiac hypertrophy. TR3 was shown to form a trimer with the TSC1/TSC2 complex that specifically promoted TSC2 degradation via a proteasome/ubiquitination pathway. As a result, mTORC1, but not mTORC2, was activated; this was accompanied by increased protein synthesis, enhanced production of reactive oxygen species and enlarged cell size, thereby resulting in cardiac hypertrophy. This study demonstrates that TR3 positively regulates cardiac hypertrophy by influencing the effect of AngII on the mTOR pathway. The elimination or reduction of TR3 may reduce cardiac hypertrophy; therefore, TR3 is a potential target for clinical therapy.