Cytochrome c release and apoptosis induced by mitochondrial targeting of nuclear orphan receptor TR3

Adenovirus-mediated expression of orphan nuclear receptor NR4A2 targeting hepatic stellate cell attenuates liver fibrosis in rats

Liver fibrosis is a wound-healing response characterized with the accumulation of extracellular matrix (ECM). And hepatic stellate cells (HSCs) are the principal cell source of ECM. NR4A2 (Nurr1) is a member of orphan nuclear receptor NR4A family and acts as transcription factor. It participates in regulating cell differentiation, proliferation and apoptosis. We previously demonstrated that NR4A2 expression in fibrotic liver reduced significantly compared with normal liver and NR4A2 knockout in HSCs promoted ECM production. In the present study we explored the role of NR4A2 on liver fibrosis. Studies in cultured HSCs demonstrated that NR4A2 over-expression suppressed the activation of HSCs, such as ECM production and invasion ability. Moreover cell cycle was arrested, cell apoptosis was promoted and cell signaling pathway was influenced. Adenovirus-mediated delivery of NR4A2 in rats ameliorated significantly dimethylnitrosamine (DMN) induced liver fibrosis. The In vivo experiments produced results consistent with in vitro experiments. Taken together these results demonstrate NR4A2 enhancement attenuates liver fibrosis via suppressing the activation of HSCs and NR4A2 may be an ideal target for anti-fibrotic therapy.

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.

RARγ Downregulation Contributes to Colorectal Tumorigenesis and Metastasis by Derepressing the Hippo-Yap Pathway

The Hippo-Yap pathway conveys oncogenic signals, but its regulation during cancer development is not well understood. Here, we identify the nuclear receptor RARγ as a regulator of the Hippo-Yap pathway in colorectal tumorigenesis and metastasis. RARγ is downregulated in human colorectal cancer tissues, where its expression correlates inversely with tumor size, TNM stage, and distant metastasis. Functional studies established that silencing of RARγ drove colorectal cancer cell growth, invasion, and metastatic properties both in vitro and in vivo Mechanistically, RARγ controlled Hippo-Yap signaling to inhibit colorectal cancer development, acting to promote phosphorylation and binding of Lats1 to its transcriptional coactivator Yap and thereby inactivating Yap target gene expression. In clinical specimens, RARγ expression correlated with overall survival outcomes and expression of critical Hippo-Yap pathway effector molecules in colorectal cancer patients. Collectively, our results defined RARγ as tumor suppressor in colorectal cancer that acts by restricting oncogenic signaling by the Hippo-Yap pathway, with potential implications for new approaches to colorectal cancer therapy. Cancer Res; 76(13); 3813-25. ©2016 AACR.

Roles of d-Amino Acids on the Bioactivity of Host Defense Peptides

Host defense peptides (HDPs) are positively-charged and amphipathic components of the innate immune system that have demonstrated great potential to become the next generation of broad spectrum therapeutic agents effective against a vast array of pathogens and tumor. As such, many approaches have been taken to improve the therapeutic efficacy of HDPs. Amongst these methods, the incorporation of d-amino acids (d-AA) is an approach that has demonstrated consistent success in improving HDPs. Although, virtually all HDP review articles briefly mentioned about the role of d-AA, however it is rather surprising that no systematic review specifically dedicated to this topic exists. Given the impact that d-AA incorporation has on HDPs, this review aims to fill that void with a systematic discussion of the impact of d-AA on HDPs.

High expression of orphan nuclear receptor NR4A1 in a subset of ovarian tumors with worse outcome

OBJECTIVE

Nuclear receptors (NRs) play a vital role in the development and progression of several cancers including breast and prostate. Using TCGA data, we sought to identify critical nuclear receptors in high grade serous ovarian cancers (HGSOC) and to confirm these findings using in vitro approaches.

METHODS

In silico analysis of TCGA data was performed to identify relevant NRs in HGSOC. Ovarian cancer cell lines were screened for NR expression and functional studies were performed to determine the significance of these NRs in ovarian cancers. NR expression was analyzed in ovarian cancer tissue samples using immunohistochemistry to identify correlations with histology and stage of disease.

RESULTS

The NR4A family of NRs was identified as a potential driver of ovarian cancer pathogenesis. Overexpression of NR4A1 in particular correlated with worse progression free survival. Endogenous expression of NR4A1 in normal ovarian samples was relatively high compared to that of other tissue types, suggesting a unique role for this orphan receptor in the ovary. Expression of NR4A1 in HGSOC cell lines as well as in patient samples was variable. NR4A1 primarily localized to the nucleus in normal ovarian tissue while co-localization within the cytoplasm and nucleus was noted in ovarian cancer cell lines and patient tissues.

CONCLUSIONS

NR4A1 is highly expressed in a subset of HGSOC samples from patients that have a worse progression free survival. Studies to target NR4A1 for therapeutic intervention should include HGSOC.

Pronounced peptide selectivity for melanoma through tryptophan end-tagging

Effects of oligotryptophan end-tagging on the uptake of arginine-rich peptides into melanoma cells was investigated under various conditions and compared to that into non-malignant keratinocytes, fibroblasts, and erythrocytes, also monitoring resulting cell toxicity. In parallel, biophysical studies on peptide binding to, and destabilization of, model lipid membranes provided mechanistic insight into the origin of the selectivity between melanoma and non-malignant cells. Collectively, the results demonstrate that W-tagging represents a powerful way to increase selective peptide internalization in melanoma cells, resulting in toxicity against these, but not against the non-malignant cells. These effects were shown to be due to increased peptide adsorption to the outer membrane in melanoma cells, caused by the presence of anionic lipids such as phosphatidylserine and ganglioside GM1, and to peptide effects on mitochondria membranes and resulting apoptosis. In addition, the possibility of using W-tagged peptides for targeted uptake of nanoparticles/drug carriers in melanoma was demonstrated, as was the possibility to open up the outer membrane of melanoma cells in order to facilitate uptake of low Mw anticancer drugs, here demonstrated for doxorubicin.

Orphan Nuclear Receptor NR4A1 Binds a Novel Protein Interaction Site on Anti-apoptotic B Cell Lymphoma Gene 2 Family Proteins

B cell lymphoma gene 2 (Bcl-2) family proteins are key regulators of programmed cell death and important targets for drug discovery. Pro-apoptotic and anti-apoptotic Bcl-2 family proteins reciprocally modulate their activities in large part through protein interactions involving a motif known as BH3 (Bcl-2 homology 3). Nur77 is an orphan member of the nuclear receptor family that lacks a BH3 domain but nevertheless binds certain anti-apoptotic Bcl-2 family proteins (Bcl-2, Bfl-1, and Bcl-B), modulating their effects on apoptosis and autophagy. We used a combination of NMR spectroscopy-based methods, mutagenesis, and functional studies to define the interaction site of a Nur77 peptide on anti-apoptotic Bcl-2 family proteins and reveal a novel interaction surface. Nur77 binds adjacent to the BH3 peptide-binding crevice, suggesting the possibility of cross-talk between these discrete binding sites. Mutagenesis of residues lining the identified interaction site on Bcl-B negated the interaction with Nur77 protein in cells and prevented Nur77-mediated modulation of apoptosis and autophagy. The findings establish a new protein interaction site with the potential to modulate the apoptosis and autophagy mechanisms governed by Bcl-2 family proteins.

Toxoplasma gondii ROP18: potential to manipulate host cell mitochondrial apoptosis

Toxoplasma gondii is an obligate intracellular parasite that may manipulate host cell mitochondrial apoptosis pathways. In our experiment, 293T cells were transfected with the p3×FLAG-CMV-Myc-ROP18 vector and expressed the ROP18-Myc fusion protein. Cell apoptosis was induced by 0.5 μg/mL actinomycin D (ActD) and was detected by Annexin V-FITC/PI assay. The cell mitochondrial membrane potential was determined by JC-1. Cytochrome c (Cyto-c) from mitochondria and the cytoplasm was measured by Western blot. The Bcl-2 and Bax coding gene expression levels were detected by real-time PCR. We found, in vitro, that T. gondii ROP18 significantly suppressed 293T cell apoptosis induced by ActD and maintained mitochondrial membrane potential and integrity, thereby preventing the release of Cyto-c from mitochondria into the cytoplasm. The ratio of Bcl-2/Bax in ROP18-overexpressing cells was significantly higher than that of the negative control. Therefore, we speculate that ROP18 could suppress host cell apoptosis via the mitochondrial apoptosis pathway in vitro.

Nuclear receptor 4A (NR4A) family - orphans no more

The orphan nuclear receptors NR4A1, NR4A2 and NR4A3 are immediate early genes induced by multiple stressors, and the NR4A receptors play an important role in maintaining cellular homeostasis and disease. There is increasing evidence for the role of these receptors in metabolic, cardiovascular and neurological functions and also in inflammation and inflammatory diseases and in immune functions and cancer. Despite the similarities of NR4A1, NR4A2 and NR4A3 and their interactions with common cis-genomic elements, they exhibit unique activities and cell-/tissue-specific functions. Although endogenous ligands for NR4A receptors have not been identified, there is increasing evidence that structurally-diverse synthetic molecules can directly interact with the ligand binding domain of NR4A1 and act as agonists or antagonists, and ligands for NR4A2 and NR4A3 have also been identified. Since NR4A receptors are key factors in multiple diseases, there are opportunities for the future development of NR4A ligands for clinical applications in treating multiple health problems including metabolic, neurologic and cardiovascular diseases, other inflammatory conditions, and cancer.

Targeting truncated RXRα for cancer therapy

Retinoid X receptor-alpha (RXRα), a unique member of the nuclear receptor superfamily, is a well-established drug target, representing one of the most important targets for pharmacologic interventions and therapeutic applications for cancer. However, how RXRα regulates cancer cell growth and how RXRα modulators suppress tumorigenesis are poorly understood. Altered expression and aberrant function of RXRα are implicated in the development of cancer. Previously, several studies had demonstrated the presence of N-terminally truncated RXRα (tRXRα) proteins resulted from limited proteolysis of RXRα in tumor cells. Recently, we discovered that overexpression of tRXRα can promote tumor growth by interacting with tumor necrosis factor-alpha-induced phosphoinositide 3-kinase and NF-κB signal transduction pathways. We also identified nonsteroidal anti-inflammatory drug Sulindac and analogs as effective inhibitors of tRXRα activities via a unique binding mechanism. This review discusses the emerging roles of tRXRα and modulators in the regulation of cancer cell survival and death as well as inflammation and our recent understanding of tRXRα regulation by targeting the alternate binding sites on its surface.

TR3 is preferentially expressed by bulge epithelial stem cells in human hair follicles

TR3 is an orphan member of the steroid/thyroid/retinoid nuclear receptor superfamily of transcription factors and it plays a pivotal role in regulating cell growth and apoptosis. The expression and function of TR3 in skin have not been well investigated. Using a cDNA expression assay, we discover that TR3 is significantly enriched in human telogen bulge compared with anagen bulb. Immunohistochemical staining confirms that TR3 is highly expressed in the bulge region of human hair follicles and it colocalizes with cytokeratin 15 (K15), an epithelial stem cell marker. To study the function of TR3 in the effect of androgens in keratinocytes, we treat HaCaT keratinocytes and primary human keratinocytes with dihydrotestosterone (DHT) and testosterone (T). The treated keratinocytes show a dose-dependent growth reduction to DHT and T. DHT increases the expression of TR3 in keratinocytes, associated with a concomitant increase of BAD and decrease of Bcl-2 expression. Knockdown TR3 expression by siRNA blocks the inhibitory effect of DHT on keratinocyte proliferation. Our results demonstrate that TR3 is localized to the stem cell compartment in the human hair follicles. Androgen increases TR3 expression in cultured keratinocytes. Our data suggest that TR3 mediates at least part of the inhibitory effect of androgens on keratinocytes.

Sinoporphyrin sodium mediated photodynamic therapy inhibits the migration associated with collapse of F-actin filaments cytoskeleton in MDA-MB-231 cells

OBJECTIVE

We previously demonstrated that the photosensitizer sinoporphyrin sodium (DVDMS) mediated photodynamic therapy (PDT) had potential advantages in inhibiting tumor growth and metastasis. However, details regarding the mechanism of cell migration inhibition remain unclear. Therefore, in this study, we aimed to investigate the effects of DVDMS-PDT on F-actin filaments, cell migration, apoptotic response and the possible interactions between them in human breast cancer MDA-MB-231 cells.

MATERIALS AND METHODS

The cell viability was evaluated by MTT and Guava ViaCount assays. The subcellular localization of DVDMS and reactive oxygen species (ROS) generation were analyzed by fluorescence microscope and flow cytometry. FITC-phalloidin was used to evaluate the changes of F-actin filaments. Cell migration was analyzed by scratch assay and Transwell assay. Cell apoptosis was determined by nuclear TUNEL staining and Annexin V-PE/7-AAD assay. Jasplakinolide, an F-actin stabilizer, was applied to dissect the influences of F-actin filaments disruption on cell migration and apoptosis.

RESULTS

DVDMS-PDT significantly suppressed cell proliferation, promoted early apoptotic response, triggered collapse of F-actin filaments and inhibited cell migration in MDA-MB-231 cells. Cell migration significantly increased when cells were pretreated with F-actin stabilizer jasplakinolide after PDT, while cell apoptosis was not obviously affected. Moreover, ROS was a key factor in causing collapse of F-actin filaments.

CONCLUSION

We demonstrated that DVDMS-PDT triggered cell apoptosis and collapse of F-actin filaments through the induction of ROS in MDA-MB-231 cells. F-actin filaments contributed to cell migration but produced no obvious effect on cell apoptosis.

NUR77 exerts a protective effect against inflammatory bowel disease by negatively regulating the TRAF6/TLR-IL-1R signalling axis

Nur77, an immediate-early response gene, participates in a wide range of biological functions. Its human homologue, NUR77, is known by several names and has the HGNC-approved gene symbol NR4A1. However, the role of Nur77 in inflammatory bowel disease (IBD) and its underlying mechanisms remain elusive. Here, using public data from the International Inflammatory Bowel Disease Genetics Consortium (IIBDGC) on the most recent genome-wide association studies (GWAS) for ulcerative colitis (UC) and Crohn's disease (CD), we found that genetic variants of the NUR77 gene are associated with increased risk for both UC and CD. Accordingly, Nur77 expression was significantly reduced in colon tissues from patients with UC or CD and mice treated with DSS. Nur77 deficiency increased the susceptibility of mice to DSS-induced experimental colitis and prevented intestinal recovery, whereas treatment with cytosporone B (Csn-B), an agonist for Nur77, significantly attenuated excessive inflammatory response in the DSS-induced colitis mouse model. Mechanistically, NUR77 acts as a negative regulator of TLR-IL-1R signalling by interacting with TRAF6. This interaction prevented auto-ubiquitination and oligomerization of TRAF6 and subsequently inhibited NF-κB activation and pro-inflammatory cytokine production. Taken together, our GWAS-based analysis and in vitro and in vivo studies have demonstrated that Nur77 is an important regulator of TRAF6/TLR-IL-1R-initiated inflammatory signalling, and loss of Nur77 may contribute to the development of IBD, suggesting Nur77 as a potential target for the prevention and treatment of IBD.

Honokiol sensitizes breast cancer cells to TNF-α induction of apoptosis by inhibiting Nur77 expression

BACKGROUND AND PURPOSE

The orphan nuclear receptor Nur77 is implicated in the survival and apoptosis of cancer cells. The purpose of this study was to determine whether and how Nur77 serves to mediate the effect of the inflammatory cytokine TNF-α in cancer cells and to identify and characterize new agents targeting Nur77 for cancer therapy.

EXPERIMENTAL APPROACH

The effects of TNF-α on the expression and function of Nur77 were studied using in vitro and in vivo models. Nur77 expression was evaluated in tumour tissues from breast cancer patients. The anticancer effects of honokiol and its mechanism of action were assessed by in vitro, cell-based and animal studies.

KEY RESULTS

TNF-α rapidly and potently induced the expression of Nur77 in breast cancer cells through activation of IκB kinase and JNK. Knocking down Nur77 resulted in TNF-α-dependent apoptosis, while ectopic Nur77 expression in MCF-7 cells promoted their growth in animals. Levels of Nur77 were higher in tumour tissues than the corresponding tissues surrounding the tumour in about 50% breast cancer patients studied. Our in vitro and animal studies also identified honokiol as an effective sensitizer of TNF-α-induced apoptosis by inhibiting TNF-α-induced Nur77 mRNA expression, which could be attributed to its interference of TNFR1's interaction with receptor-interacting protein 1 (RIPK1).

CONCLUSIONS AND IMPLICATIONS

TNF-α-induced Nur77 serves as a survival factor to attenuate the death effect of TNF-α in cancer cells. With its proven human safety profile, honokiol represents a promising agent that warrants further clinical development.

Role of the nucleus in apoptosis: signaling and execution

Since their establishment in the early 1970s, the nuclear changes upon apoptosis induction, such as the condensation of chromatin, disassembly of nuclear scaffold proteins and degradation of DNA, were, and still are, considered as the essential steps and hallmarks of apoptosis. These are the characteristics of the execution phase of apoptotic cell death. In addition, accumulating data clearly show that some nuclear events can lead to the induction of apoptosis. In particular, if DNA lesions resulting from deregulation during the cell cycle or DNA damage induced by chemotherapeutic drugs or viral infection cannot be efficiently eliminated, apoptotic mechanisms, which enable cellular transformation to be avoided, are activated in the nucleus. The functional heterogeneity of the nuclear organization allows the tight regulation of these signaling events that involve the movement of various nuclear proteins to other intracellular compartments (and vice versa) to initiate and govern apoptosis. Here, we discuss how these events are coordinated to execute apoptotic cell death.

Endogenous IGFBP-3 Mediates Intrinsic Apoptosis Through Modulation of Nur77 Phosphorylation and Nuclear Export

In nontransformed bovine mammary epithelial cells, the intrinsic apoptosis inducer anisomycin (ANS) induces IGFBP-3 expression and nuclear localization and knockdown of IGFBP-3 attenuates ANS-induced apoptosis. Others have shown in prostate cancer cells that exogenous IGFBP-3 induces apoptosis by facilitating nuclear export of the orphan nuclear receptor Nur77 and its binding partner, retinoid X receptor-α (RXRα). The goal of the present work was to determine whether endogenous IGFBP-3 plays a role in ANS-induced apoptosis by facilitating nuclear transport of Nur77 and/or RXRα in nontransformed cells. Knockdown of Nur77 with siRNA decreased ANS-induced cleavage of caspase-3 and -7 and their downstream target, PARP, indicating a role for Nur77 in ANS-induced apoptosis. In cells transfected with IGFBP-3, IGFBP-3 associated with RXRα but not Nur77 under basal conditions, however, IGFBP-3 co-precipitated with phosphorylated forms of both proteins in ANS-treated cells. Indirect immunofluorescence and cell fractionation techniques showed that ANS induced phosphorylation and transport of Nur77 from the nucleus to the cytoplasm and these effects were attenuated by knockdown of IGFBP-3. These data suggest that endogenous IGFBP-3 plays a role in intrinsic apoptosis by facilitating phosphorylation and nuclear export of Nur77 to the cytoplasm where it exerts its apoptotic effect. Whether this mechanism involves a physical association between endogenous IGFBP-3 and Nur77 or RXRα remains to be determined.

Nuclear receptor 4A1 as a drug target for breast cancer chemotherapy

The orphan nuclear receptor 4A1 (NR4A1) is overexpressed in mammary tumors and breast cancer cell lines. The functional activity of this receptor was investigated by RNA interference with oligonucleotides targeted to NR4A1 (siNR4A1) and by treatment with NR4A1 antagonists. Breast cancer cells were treated with NR4A1 antagonists or transfected with siNR4A. Effects on cell proliferation and apoptosis as well as specific genes associated with these responses were investigated in MCF-7, SKBR3, and MDA-MB-231 cells, and in athymic nude mice bearing MDA-MB-231 cells as xenografts. Transfection of MCF-7, MDA-MB-231, and SKBR3 breast cancer cells with siNR4A1 decreased cell proliferation and induced apoptosis in these cell lines. Transfection of breast cancer cells with siNR4A1 also decreased expression of Sp-regulated genes including survivin, bcl-2, and epidermal growth factor receptor, inhibited mTOR signaling in MCF-7 cells that express WT p53, and activated oxidative and endoplasmic reticulum stress through downregulation of thioredoxin domain-containing 5 and isocitrate dehydrogenase 1. 1,1-Bis(3'-indolyl)-1-(p-substituted phenyl)methanes (C-DIMs) are NR4A1 ligands that act as NR4A1 antagonists. Treatment with selected analogs also inhibited breast cancer cell and tumor growth and induced apoptosis. The effects of C-DIM/NR4A1 antagonists were comparable to those observed after NR4A1 knockdown. Results with siNR4A1 or C-DIMs/NR4A1 antagonists in breast cancer cells and tumors were similar to those previously reported in pancreatic, lung, and colon cancer cells. They demonstrate the potential clinical applications of NR4A1 antagonists in patients with tumors that overexpress this receptor.

The orphan nuclear receptor NR4A1 specifies a distinct subpopulation of quiescent myeloid-biased long-term HSCs

Hematopoiesis is maintained throughout life by self-renewing hematopoietic stem cells (HSCs) that differentiate to produce both myeloid and lymphoid cells. The NR4A family of orphan nuclear receptors, which regulates cell fate in many tissues, appears to play a key role in HSC proliferation and differentiation. Using a NR4A1(GFP) BAC transgenic reporter mouse we have investigated NR4A1 expression and its regulation in early hematopoiesis. We show that NR4A1 is most highly expressed in a subset of Lin(-) Sca-1(+) c-Kit(+) CD48(-) CD150(+) long-term (LT) HSCs, and its expression is tightly associated with HSC quiescence. We also show that NR4A1 expression in HSCs is induced by PGE2, a known enhancer of stem cell engraftment potential. Finally, we find that both NR4A1(GFP+) and NR4A1(GFP-) HSCs successfully engraft primary and secondary irradiated hosts; however, NR4A1(GFP+) HSCs are distinctly myeloid-biased. These results show that NR4A1 expression identifies a highly quiescent and distinct population of myeloid-biased LT-HSCs.

Induction of Autophagic Death in Cancer Cells by Agonizing TR3 and Attenuating Akt2 Activity

Apoptotic resistance is becoming a significant obstacle for cancer therapy as the majority of treatment takes the route of apoptotic induction. It is of great importance to develop an alternative strategy to induce cancer cell death. We previously reported that autophagic cell death mediated by nuclear receptor TR3 and driven by a chemical agonist, 1-(3,4,5-trihydroxyphenyl)nonan-1-one (THPN), is highly effective in the therapy of melanoma but not any other cancer types. Here, we discovered that the insensitivity of cancer cells to THPN originated from a high cellular Akt2 activity. Akt2 phosphorylation interferes with TR3 export to cytoplasm and targeting to mitochondria, which lead to the autophagic induction. Therefore, the TR3-mediated autophagy could be effectively induced in the otherwise insensitive cells by downregulating Akt2 activity. Highly effective antineoplastic compounds are developed through optimizing the structure of THPN. This study implicates a general strategy for cancer therapy by the induction of autophagic cell death.

NSC-640358 acts as RXRα ligand to promote TNFα-mediated apoptosis of cancer cell

Retinoid X receptor α (RXRα) and its N-terminally truncated version tRXRα play important roles in tumorigenesis, while some RXRα ligands possess potent anti-cancer activities by targeting and modulating the tumorigenic effects of RXRα and tRXRα. Here we describe NSC-640358 (N-6), a thiazolyl-pyrazole derived compound, acts as a selective RXRα ligand to promote TNFα-mediated apoptosis of cancer cell. N-6 binds to RXRα and inhibits the transactivation of RXRα homodimer and RXRα/TR3 heterodimer. Using mutational analysis and computational study, we determine that Arg316 in RXRα, essential for 9-cis-retinoic acid binding and activating RXRα transactivation, is not required for antagonist effects of N-6, whereas Trp305 and Phe313 are crucial for N-6 binding to RXRα by forming extra π–π stacking interactions with N-6, indicating a distinct RXRα binding mode of N-6. N-6 inhibits TR3-stimulated transactivation of Gal4-DBD-RXRα-LBD by binding to the ligand binding pocket of RXRα-LBD, suggesting a strategy to regulate TR3 activity indirectly by using small molecules to target its interacting partner RXRα. For its physiological activities, we show that N-6 strongly inhibits tumor necrosis factor α (TNFα)-induced AKT activation and stimulates TNFα-mediated apoptosis in cancer cells in an RXRα/tRXRα dependent manner. The inhibition of TNFα-induced tRXRα/p85α complex formation by N-6 implies that N-6 targets tRXRα to inhibit TNFα-induced AKT activation and to induce cancer cell apoptosis. Together, our data illustrate a new RXRα ligand with a unique RXRα binding mode and the abilities to regulate TR3 activity indirectly and to induce TNFα-mediated cancer cell apoptosis by targeting RXRα/tRXRα.

The Orphan Nuclear Receptor NR4A1 Protects Pancreatic β-Cells from Endoplasmic Reticulum (ER) Stress-mediated Apoptosis

The role of NR4A1 in apoptosis is controversial. Pancreatic β-cells often face endoplasmic reticulum (ER) stress under adverse conditions such as high free fatty acid (FFA) concentrations and sustained hyperglycemia. Severe ER stress results in β-cell apoptosis. The aim of this study was to analyze the role of NR4A1 in ER stress-mediated β-cell apoptosis and to characterize the related mechanisms. We confirmed that upon treatment with the ER stress inducers thapsigargin (TG) or palmitic acid (PA), the mRNA and protein levels of NR4A1 rapidly increased in both MIN6 cells and mouse islets. NR4A1 overexpression in MIN6 cells conferred resistance to cell loss induced by TG or PA, as assessed by MTT (3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide) assay, and TUNEL assays indicated that NR4A1 overexpression also protected against ER stress-induced apoptosis. This conclusion was further confirmed by experiments exploiting siRNA to knockdown NR4A1 expression in MIN6 cells or exploiting NR4A1 knock-out mice. NR4A1 overexpression in MIN6 cells reduced C/EBP homologous protein (CHOP) expression and Caspase3 activation induced by TG or PA. NR4A1 overexpression in MIN6 cells or mouse islets resulted in Survivin up-regulation. A critical regulatory element was identified in Survivin promoter (-1872 bp to -1866 bp) with a putative NR4A1 binding site; ChIP assays demonstrated that NR4A1 physically associates with the Survivin promoter. In conclusion, NR4A1 protects pancreatic β-cells against ER stress-mediated apoptosis by up-regulating Survivin expression and down-regulating CHOP expression, which we termed as "positive and negative regulation."

Nuclear Receptor 4A1 (NR4A1) as a Drug Target for Renal Cell Adenocarcinoma

The orphan nuclear receptor NR4A1 exhibits pro-oncogenic activity in cancer cell lines. NR4A1 activates mTOR signaling, regulates genes such as thioredoxin domain containing 5 and isocitrate dehydrogenase 1 that maintain low oxidative stress, and coactivates specificity protein 1 (Sp1)-regulated pro-survival and growth promoting genes. Transfection of renal cell carcinoma (RCC) ACHN and 786-O cells with oligonucleotides that target NR4A1 results in a 40-60% decrease in cell proliferation and induction of apoptosis. Moreover, knockdown of NR4A1 in RCC cells decreased bcl-2, survivin and epidermal growth factor receptor expression, inhibited of mTOR signaling, induced oxidative and endoplasmic reticulum stress, and decreased TXNDC5 and IDH1. We have recently demonstrated that selected 1,1-bis(3'-indolyl)-1-(p-substituted phenyl)methane (C-DIM) compounds including the p-hydroxyphenyl (DIM-C-pPhOH) and p-carboxymethyl (DIM-C-pPhCO2Me) analogs bind NR4A1 and act as antagonists. Both DIM-C-pPhOH and DIM-C-pPhCO2Me inhibited growth and induced apoptosis in ACHN and 786-O cells, and the functional and genomic effects of the NR4A1 antagonists were comparable to those observed after NR4A1 knockdown. These results indicate that NR4A1 antagonists target multiple growth promoting and pro-survival pathways in RCC cells and in tumors (xenograft) and represent a novel chemotherapy for treating RCC.

Orphan nuclear receptors as drug targets for the treatment of prostate and breast cancers

Nuclear receptors (NRs), a family of 48 transcriptional factors, have been studied intensively for their roles in cancer development and progression. The presence of distinctive ligand binding sites capable of interacting with small molecules has made NRs attractive targets for developing cancer therapeutics. In particular, a number of drugs have been developed over the years to target human androgen- and estrogen receptors for the treatment of prostate cancer and breast cancer. In contrast, orphan nuclear receptors (ONRs), which in many cases lack known biological functions or ligands, are still largely under investigated. This review is a summary on ONRs that have been implicated in prostate and breast cancers, specifically retinoic acid-receptor-related orphan receptors (RORs), liver X receptors (LXRs), chicken ovalbumin upstream promoter transcription factors (COUP-TFs), estrogen related receptors (ERRs), nerve growth factor 1B-like receptors, and ‘‘dosage-sensitive sex reversal, adrenal hypoplasia critical region, on chromosome X, gene 1’’ (DAX1). Discovery and development of small molecules that can bind at various functional sites on these ONRs will help determine their biological functions. In addition, these molecules have the potential to act as prototypes for future drug development. Ultimately, the therapeutic value of targeting the ONRs may go well beyond prostate and breast cancers.

Nuclear receptors of the NR4a family are not required for the development and function of follicular T helper cells

Follicular T helper (Tfh) cells promote germinal center (GC) reaction and high-affinity antibody production. The molecular mechanisms that regulate development and function of Tfh cells are not fully understood. Here we report that ligand-independent nuclear receptors of the Nr4a family are highly expressed in Tfh cells. In a well-established adoptive transfer model, enforced expression of Nr4a receptors reduces helper T cell expansion but apparently increased the T cell capacity to promote the GC response. On the other hand, deletion of all Nr4a receptors in T cells did not significantly affect expansion or differentiation of Tfh cells or the development of GC reaction. These findings suggest that Nr4a receptors may promote but are not necessary for Tfh development or function in vivo.

Apoptosis Signal-regulating Kinase 1 (ASK1)-p38 Pathway-dependent Cytoplasmic Translocation of the Orphan Nuclear Receptor NR4A2 Is Required for Oxidative Stress-induced Necrosis

p38 mitogen-activated protein kinases (MAPKs) play important roles in various cellular stress responses, including cell death, which is roughly categorized into apoptosis and necrosis. Although p38 signaling has been extensively studied, the molecular mechanisms of p38-mediated cell death are unclear. ASK1 is a stress-responsive MAP3K that acts as an upstream kinase of p38 and is activated by various stresses, such as oxidative stress. Here, we show that NR4A2, a member of the NR4A nuclear receptor family, acts as a necrosis promoter downstream of ASK1-p38 pathway during oxidative stress. Although NR4A2 is well known as a nucleus-localized transcription factor, we found that it is translocated into the cytosol after phosphorylation by p38. Because the phosphorylation site mutants of NR4A2 cannot rescue the cell death-promoting activity, ASK1-p38 pathway-dependent phosphorylation and subsequent cytoplasmic translocation of NR4A2 may be required for oxidative stress-induced cell death. In addition, NR4A2-mediated cell death does not depend on caspases and receptor-interacting protein 1 (RIP1)-RIP3 complex, suggesting that NR4A2 promotes an RIP kinase-independent necrotic type of cell death. Our findings may enable a more precise understanding of molecular mechanisms that regulate oxidative stress-induced and p38-mediated necrosis.

Non-genomic effects of the NR4A1/Nur77/TR3/NGFIB orphan nuclear receptor

The orphan nuclear receptor NR4A1/Nur77/TR3/NGFIB acts primarily as a transcription factor to regulate the expression of multiple genes. However, increasing research attention has recently been given to non-genomic activities of NR4A1. The first description of a non-genomic action of NR4A1 referred to the conversion of anti-apoptotic Bcl-2 into a pro-apoptotic protein by direct interaction with NR4A1. In response to certain apoptotic stimuli, NR4A1 translocates from the nucleus to the mitochondrial outer membrane (MOM) where it associates with Bcl-2 and thereby causes apoptosis. Afterwards, it appeared that NR4A1 could also bind and convert other anti-apoptotic Bcl-2 family members. The latest studies indicate a significant role of NR4A1 in the process of autophagy. For example, a new NR4A1-mediated pathway specific for melanoma cells has been described where NR4A1 interacts with the adenine nucleotide translocase 1 (ANT1) on the mitochondrial inner membrane (MIM) leading to induction of the autophagy pathway. Moreover, NR4A1 interaction with cytoplasmic p53 may also contribute to the induction of autophagy. In addition to mitochondria, NR4A1 could be translocated to the outer membrane of the endoplasmic reticulum (ER) and associate with Bcl-2 or translocon-associated protein subunit γ (TRAPγ) causing ER stress-induced apoptosis. NR4A1 also contributes to the proteasomal degradation of β-catenin in colon cancer cells in vitro and in vivo, as well as to the stabilization of hypoxia-inducible factor-1α (HIF-1α) under non-hypoxic conditions. This review summarizes research findings on non-genomic effects of NR4A1 in normal and cancer cells.

High cytoplasmic expression of the orphan nuclear receptor NR4A2 predicts poor survival in nasopharyngeal carcinoma

OBJECTIVE

This study aimed at investigating whether the orphan nuclear receptor NR4A2 is significantly associated with clinicopathologic features and overall survival of patients with nasopharyngeal carcinoma (NPC).

METHODS

Immunohistochemistry was performed to determine NR4A2 protein expression in 84 NPC tissues and 20 non-cancerous nasopharyngeal (NP) tissues. The prognostic significance of NR4A2 protein expression was evaluated using Cox proportional hazards regression models and Kaplan-Meier survival analysis.

RESULTS

We did not find a significant association between total NR4A2 expression and clinicopathological variables in 84 patients with NPC. However, we observed that high cytoplasmic expression of NR4A2 was significantly associated with tumor size (T classification) (P = 0.006), lymph node metastasis (N classification) (P = 0.002) and clinical stage (P = 0.017). Patients with higher cytoplasmic NR4A2 expression had a significantly lower survival rate than those with lower cytoplasmic NR4A2 expression (P = 0.004). Multivariate Cox regression analysis analysis suggested that the level of cytoplasmic NR4A2 expression was an independent prognostic indicator for overall survival of patients with NPC (P = 0.033).

CONCLUSIONS

High cytoplasmic expression of NR4A2 is a potential unfavorable prognostic factor for patients with NPC.

EBNA2 and Its Coactivator EBNA-LP

While all herpesviruses can switch between lytic and latent life cycle, which are both driven by specific transcription programs, a unique feature of latent EBV infection is the expression of several distinct and well-defined viral latent transcription programs called latency I, II, and III. Growth transformation of B-cells by EBV in vitro is based on the concerted action of Epstein-Barr virus nuclear antigens (EBNAs) and latent membrane proteins(LMPs). EBV growth-transformed B-cells express a viral transcriptional program, termed latency III, which is characterized by the coexpression of EBNA2 and EBNA-LP with EBNA1, EBNA3A, -3B, and -3C as well as LMP1, LMP2A, and LMP2B. The focus of this review will be to discuss the current understanding of how two of these proteins, EBNA2 and EBNA-LP, contribute to EBV-mediated B-cell growth transformation.

Regulation of the nongenomic actions of retinoid X receptor-α by targeting the coregulator-binding sites

Retinoid X receptor-α (RXRα), a unique member of the nuclear receptor superfamily, represents an intriguing and unusual target for pharmacologic interventions and therapeutic applications in cancer, metabolic disorders and neurodegenerative diseases. Despite the fact that the RXR-based drug Targretin (bexarotene) is currently used for treating human cutaneous T-cell lymphoma and the fact that RXRα ligands (rexinoids) show beneficial effects in the treatment of cancer and diseases, the therapeutic potential of RXRα remains unexplored. In addition to its conventional transcription regulation activity in the nucleus, RXRα can act in the cytoplasm to modulate important biological processes, such as mitochondria-dependent apoptosis, inflammation, and phosphatidylinositol 3-kinase (PI3K)/AKT-mediated cell survival. Recently, new small-molecule-binding sites on the surface of RXRα have been identified, which mediate the regulation of the nongenomic actions of RXRα by a class of small molecules derived from the nonsteroidal anti-inflammatory drug (NSAID) Sulindac. This review discusses the emerging roles of the nongenomic actions of RXRα in the RXRα signaling network, and their possible implications in cancer, metabolic and neurodegenerative disorders, as well as our current understanding of RXRα regulation by targeting alternate binding sites on its surface.

The interplay of NR4A receptors and the oncogene-tumor suppressor networks in cancer

Nuclear receptor (NR) subfamily 4 group A (NR4A) is a family of three highly homologous orphan nuclear receptors that have multiple physiological and pathological roles, including some in cancer. These NRs are reportedly dysregulated in multiple cancer types, with many studies demonstrating pro-oncogenic roles for NR4A1 (Nur77) and NR4A2 (Nurr1). Additionally, NR4A1 and NR4A3 (Nor-1) are described as tumor suppressors in leukemia. The dysregulation and functions of the NR4A members are due to many factors, including transcriptional regulation, protein-protein interactions, and post-translational modifications. These various levels of intracellular regulation result from the signaling cross-talk of the NR4A members with various signaling pathways, many of which are relevant to cancer and likely explain the family members' functions in oncogenesis and tumor suppression. In this review, we discuss the multiple functions of the NR4A receptors in cancer and summarize a growing body of scientific literature that describes the interconnectedness of the NR4A receptors with various oncogene and tumor suppressor pathways.

Diindolylmethane analogs bind NR4A1 and are NR4A1 antagonists in colon cancer cells

1,1-Bis(3'-indolyl)-1-(p-substituted phenyl)methane (C-DIM) compounds exhibit antineoplastic activity in multiple cancer cell lines and the p-hydroxyphenyl analog (DIM-C-pPhOH) inactivates nuclear receptor 4A1 (NR4A1) in lung and pancreatic cancer cell lines. Using a series of 14 different p-substituted phenyl C-DIMs, we show that several compounds including DIM-C-pPhOH directly interacted with the ligand binding domain of NR4A1. Computational-based molecular modeling studies showed high-affinity interactions of DIM-C-pPhOH and related compounds within the ligand binding pocket of NR4A1, and these same compounds decreased NR4A1-dependent transactivation in colon cancer cells transfected with a construct containing 3 tandem Nur77 binding response elements linked to a luciferase reporter gene. Moreover, we also show that knockdown of NR4A1 by RNA interference (small interfering NR4A1) or treatment with DIM-C-pPhOH and related compounds decreased colon cancer cell growth, induced apoptosis, decreased expression of survivin and other Sp-regulated genes, and inhibited mammalian target of rapamycin signaling. Thus, C-DIMs such as DIM-C-pPhOH directly bind NR4A1 and are NR4A1 antagonists in colon cancer cells, and their antineoplastic activity is due, in part, to their interactions with nuclear NR4A1.

Induction of Nur77-dependent apoptotic pathway by a coumarin derivative through activation of JNK and p38 MAPK

Coumarins are plant-derived natural products with a broad range of known pharmacological activities including anticancer effects. However, the molecular mechanisms by which this class of promising compounds exerts their anticancer effects remain largely unknown. We report here that a furanocoumarin named apaensin could effectively induce apoptosis of cancer cells through its activation of Jun N-terminal kinase (JNK) and p38 mitogen-activated protein kinase (MAPK). Apoptosis induction by apaensin in cancer cells was suppressed by chemical inhibitors of JNK and p38 MAPK. Inhibition of the expression of orphan nuclear receptor Nur77 by small interfering RNA (siRNA) approach also abrogated the death effect of apaensin. Molecular analysis demonstrated that JNK activation was required for the nuclear export of Nur77, a known apoptotic event in cancer cells. Although p38 MAPK activation was not involved in Nur77 nuclear export, it was essential for Nur77 mitochondrial targeting through induction of Nur77 interaction with Bcl-2, which is also known to convert Bcl-2 from an antiapoptotic to a proapoptotic molecule. Together, our results identify a new natural product that targets orphan nuclear receptor Nur77 through its unique activation of JNK and p38 MAPK and provide insight into the complex regulation of the Nur77-Bcl-2 apoptotic pathway.

Nuclear receptor nur77 promotes cerebral cell apoptosis and induces early brain injury after experimental subarachnoid hemorrhage in rats

Nur77 is a potent proapoptotic member of the nuclear receptor superfamily that is expressed predominantly in brain tissue. It has been demonstrated that Nur77 mediates apoptosis in multiple organs. Nur77-mediated early brain injury (EBI) involves a conformational change in BCL-2 and triggers cytochrome C (cytoC) release resulting in cellular apoptosis. This study investigates whether Nur77 can promote cerebral cell apoptosis after experimentally induced subarachnoid hemorrhage (SAH) in rats. Sprague Dawley rats were randomly assigned to three groups: 1) untreated group, 2) treatment control group, and 3) SAH group. The experimental SAH group was divided into four subgroups, corresponding to 12 hr, 24 hr, 48 hr, and 72 hr after experimentally induced SAH. It remains unclear whether Nur77 can play an important role during EBI after SAH as a proapoptotic protein in cerebral cells. Cytosporone B (Csn-B) was used to demonstrate that Nur77 could be enriched and used to aggravate EBI after SAH. Rats treated with Csn-B were given an intraperitoneal injection (13 mg/kg) 30 min after experimentally induced SAH. We found that Nur77 promotes cerebral cell apoptosis by mediating EBI and triggering a conformational change in BCL-2, resulting in cytoC release. Nur77 activity, along with cerebral cell apoptosis, peaked at 24 hr after SAH onset. After induction of SAH, an injection of Csn-B, an agonist for Nur77, enhanced the expression and function of Nur77. In summary, we have demonstrated the proapoptotic effect of Nur77 within cerebral cells, an effect that can be further exacerbated with Csn-B stimulation.

Apoptins: selective anticancer agents

Therapies that selectively target cancer cells for death have been the center of intense research recently. One potential therapy may involve apoptin proteins, which are able to induce apoptosis in cancer cells leaving normal cells unharmed. Apoptin was originally discovered in the Chicken anemia virus (CAV); however, human gyroviruses (HGyV) have recently been found that also harbor apoptin-like proteins. Although the cancer cell specific activity of these apoptins appears to be well conserved, the precise functions and mechanisms of action are yet to be fully elucidated. Strategies for both delivering apoptin to treat tumors and disseminating the protein inside the tumor body are now being developed, and have shown promise in preclinical animal studies.

Sinoporphyrin sodium, a novel sensitizer, triggers mitochondrial-dependent apoptosis in ECA-109 cells via production of reactive oxygen species

Background

Sonodynamic therapy (SDT) is a promising method that uses ultrasound to activate certain chemical sensitizers for the treatment of cancer. The purpose of this study was to investigate the sonoactivity of a novel sensitizer, sinoporphyrin sodium (DVDMS), and its sonotoxicity in an esophageal cancer (ECA-109) cell line.

Methods

The fluorescence intensity of DVDMS, hematoporphyrin, protoporphyrin IX, and Photofrin II was detected by fluorescence microscopy and flow cytometry. Generation of singlet oxygen was measured using a 1, 3-diphenylisobenzofuran experiment. A 3-(4, 5-dimethylthiazol-2-yl)-2, 5-diphenyl tetrazolium bromide assay was used to examine cell viability. Production of reactive oxygen species (ROS) and destabilization of the mitochondrial membrane potential were assessed by flow cytometry. Apoptosis was analyzed using Annexin-PE/7-amino-actinomycin D staining. Confocal microscopy was performed to assess mitochondrial damage and identify release of cytochrome C after treatment. Western blots were used to determine expression of oxidative stress-related and apoptosis-associated protein. Ultrastructural changes in the cell were studied by scanning electron microscopy.

Results

DVDMS showed higher autofluorescence intensity and singlet oxygen production efficiency compared with other photosensitizers in both cancerous and normal cells. Compared with hematoporphyrin, DVDMS-mediated SDT was more cytotoxic in ECA-109 cells. Abundant intracellular ROS was found in the SDT groups, and the cytotoxicity induced by SDT was effectively remitted by ROS scavengers. DVDMS located mainly to the mitochondria of ECA-109 cells, which were seriously damaged after exposure to SDT. Release of cytochrome C, an increased rate of apoptosis, and activated apoptosis protein were detected in the SDT group. In addition, relatively severe cell damage was observed on scanning electron microscopy after treatment with DVDMS and SDT.

Conclusion

These results suggest that DVDMS could be activated by ultrasound, and that DVDMS mediates SDT-induced mitochondrial-dependent apoptosis in ECA-109 cells via production of ROS.

Inhibition of c-Jun N-terminal kinase ameliorates early brain injury after subarachnoid hemorrhage through inhibition of a Nur77 dependent apoptosis pathway

Nur77 is a potent pro-apoptotic member of the orphan nuclear receptor superfamily. Our previous study revealed Nur77-mediated apoptotic also involved in early brain injury (EBI) after experimental subarachnoid hemorrhage (SAH). Previous researches show that c-Jun N-terminal kinase (JNK) positively regulates Nur77 nuclear export and apoptosis by phosphorylating Nur77. To determine whether activation of JNK is directly associated with Nur77 dependent apoptosis pathway. We hypothesized that SP600125, a chemical inhibitor of JNK, may effectively ameliorate EBI by inhibiting Nur77 phosphorylation and its transcriptional activity. Hence, in this study was designed to explore the neuroprotective effects of SP600125 in EBI after SAH. Adult male SD rats were randomly assigned to four groups: control; SAH + DMSO; SAH + SP10 and SAH + SP30, a dose of 10 and 30 mg/kg SP600125 was directly administered intraperitoneally 30 min before and 2 h after SAH induction. SP600125 markedly decreased expressions of p-JNK, p-Nur77, Bcl-2, cyto C, caspase-3 and inhibited apoptosis. Improvement of neurological deficit, alleviation of brain edema and amelioration of EBI were obtained after treatment of SP600125. Transferase-mediated dUTP nick end labeling-positive cells were reduced markedly in brain cortex by SP600125. Our studies indicate JNK plays important roles in Nur77 activation. These findings strongly support the hypothesis that SP600125 treatment can ameliorate EBI after experimentally induced SAH by inhibiting a Nur77-dependent apoptotic pathway.

Twenty-Four Genes are Upregulated in Patients with Hypospadias

Hypospadias is a congenital hypoplasia of the penis, with displacement of the urethral opening along the ventral surface, and has been reported to be one of the most common congenital anomalies, occurring in approximately 1:250 to 1:300 live births. As hypospadias is reported to be an easily diagnosed malformation at the crossroads of genetics and environment, it is important to study the genetic component in order to elucidate its etiology. In this study, the gene expression profiles both in human hypospadias tissues and normal penile tissues were studied by Human Gene Expression Array. Twenty-four genes were found to be upregulated. Among these, ATF3 and CYR61 have been reported previously. Other genes that have not been previously reported were also found to be upregulated: BTG2, CD69, CD9, DUSP1, EGR1, EIF4A1, FOS, FOSB, HBEGF, HNRNPUL1, IER2, JUN, JUNB, KLF2, NR4A1, NR4A2, PTGS2, RGS1, RTN4, SLC25A25, SOCS3 and ZFP36 (p <0.05). Further studies including genome-wide association studies (GWAS) with expression studies in a large patient group will help us for identifiying the candidate gene(s) in the etiology of hypospadias.

The Nuclear Orphan Receptor NR4A1 is Involved in the Apoptotic Pathway Induced by LPS and Simvastatin in RAW 264.7 Macrophages

Macrophage death plays a role in several physiological and inflammatory pathologies such as sepsis and arthritis. In our previous work, we showed that simvastatin triggers cell death in LPS-activated RAW 264.7 mouse macrophage cells through both caspase-dependent and independent apoptotic pathways. Here, we show that the nuclear orphan receptor NR4A1 is involved in a caspase-independent apoptotic process induced by LPS and simvastatin. Simvastatin-induced NR4A1 expression in RAW 264.7 macrophages and ectopic expression of a dominant-negative mutant form of NR4A1 effectively suppressed both DNA fragmentation and the disruption of mitochondrial membrane potential (MMP) during LPS- and simvastatin-induced apoptosis. Furthermore, apoptosis was accompanied by Bcl-2-associated X protein (Bax) translocation to the mitochondria. Our findings suggest that NR4A1 expression and mitochondrial translocation of Bax are related to simvastatin-induced apoptosis in LPS-activated RAW 264.7 macrophages.

Mitochondrial translocation of Nur77 induced by ROS contributed to cardiomyocyte apoptosis in metabolic syndrome

Metabolic syndrome is a major risk factor for cardiovascular diseases, and increased cardiomyocyte apoptosis which contributes to cardiac dysfunction after myocardial ischemia/reperfusion (MI/R) injury. Nur77, a nuclear orphan receptor, is involved in such various cellular events as apoptosis, proliferation, and glucose and lipid metabolism in several cell types. Apoptosis is positively correlated with mitochondrial translocation of Nur77 in the cancer cells. However, the roles of Nur77 on cardiac myocytes in patients with metabolic syndrome remain unclear. The objective of this study was to determine whether Nur77 may contribute to cardiac apoptosis in patients with metabolic syndrome after I/R injury, and, if so, to identify the underlying molecular mechanisms responsible. We used leptin-deficient (ob/ob) mice to make metabolic syndrome models. In this report, we observed that, accompanied by the substantial decline in apoptosis inducer Nur77, MI/R induced cardiac dysfunction was manifested as cardiomyopathy and increased ROS. Using the neonatal rat cardiac myocytes cultured in a high-glucose and high-fat medium, we found that excessive H2O2 led to the significant alteration in mitochondrial membrane potential and translocation of Nur77 from the nucleus to the mitochondria. However, inhibition of the relocation of Nur77 to mitochondria via Cyclosporin A reversed the changes in membrane potential mediated by H2O2 and reduced myocardial cell injury. Therefore, these data provide a potential underlying mechanism for cardiac dysfunction in metabolic syndrome and the suppression of Nur77 translocation may provide an effective approach to reduce cardiac injury in the process.

Elevation of NR4A3 expression and its possible role in modulating insulin expression in the pancreatic beta cell

Background

NR4A3/NOR-1 is a member of the NR4A orphan nuclear receptor subfamily, which contains early response genes that sense and respond to a variety of stimuli in the cellular environment. The role of NR4A3 in insulin expression in pancreatic beta cells remains unknown.

Methods

Dynamic changes in NR4A3 were examined in a pancreatic beta-cell line, MIN6, treated with thapsigargin (TG), palmitate (PA), tunicamycin (TM), and dithiothreitol (DTT), chemicals that produce cell stress and even apoptosis. We exploited virus infection techniques to induce expression of NR4A3 or three deletion mutants, and determined expression of insulin and insulin regulatory genes in MIN6 cells.

Results

TG and PA, two endoplasmic reticulum (ER) stress inducers, were able to induce unfolded protein response (UPR) activation and elevation of NR4A3 expression in MIN6 cells, whereas TM and DTT, two other ER stress inducers, were able to induce UPR activation but not NR4A3 elevation. MIN6 cells over-expressing NR4A3 protein after adenoviral infection exhibited reduced transcription of the insulin genes Ins1 and Ins2, and reduced insulin protein secretion, which were negatively correlated with NR4A3 expression levels. Functional analysis of different deletion mutants of NR4A3 showed that deleting the activation domain AF1 or the DNA-binding domain abolished the down-regulation of insulin transcription by NR4A3 in MIN6 cells, indicating that this down-regulative role was closely related to the NR4A3 trans-activation activity. Over-expression of NR4A3 in MIN6 cells resulted in reduced mRNA transcription of the insulin positive-regulation genes, Pdx1 and NeuroD1.

Conclusion

Some ER stress inducers, such as TG or PA, are able to elevate NR4A3 expression in MIN6 cells, while others, such as TM or DTT, are not. Over-expression of NR4A3 in MIN6 cells results in down-regulation of insulin gene transcription and insulin secretion. NR4A3 reduces insulin gene expression by modulating the expression of Pdx1 and NeuroD1.

Minireview: role of orphan nuclear receptors in cancer and potential as drug targets

The nuclear orphan receptors for which endogenous ligands have not been identified include nuclear receptor (NR)0B1 (adrenal hypoplasia congenita critical region on chromosome X gene), NR0B2 (small heterodimer partner), NR1D1/2 (Rev-Erbα/β), NR2C1 (testicular receptor 2), NR2C2 (testicular receptor 4), NR2E1 (tailless), NR2E3 (photoreceptor-specific NR [PNR]), NR2F1 chicken ovalbumin upstream promoter transcription factor 1 (COUP-TFI), NR2F2 (COUP-TFII), NR2F6 (v-erbA-related protein), NR4A1 (Nur77), NR4A2 (Nurr1), NR4A3 (Nor1), and NR6A1 (GCNF). These receptors play essential roles in development, cellular homeostasis, and disease including cancer where over- or underexpression of some receptors has prognostic significance for patient survival. Results of receptor knockdown or overexpression in vivo and in cancer cell lines demonstrate that orphan receptors exhibit tumor-specific pro-oncogenic or tumor suppressor-like activity. For example, COUP-TFII expression is both a positive (ovarian) and negative (prostate and breast) prognostic factor for cancer patients; in contrast, the prognostic activity of adrenal hypoplasia congenita critical region on chromosome X gene for the same tumors is the inverse of COUP-TFII. Functional studies show that Nur77 is tumor suppressor like in acute leukemia, whereas silencing Nur77 in pancreatic, colon, lung, lymphoma, melanoma, cervical, ovarian, gastric, and some breast cancer cell lines induces one or more of several responses including growth inhibition and decreased survival, migration, and invasion. Although endogenous ligands for the orphan receptors have not been identified, there is increasing evidence that different structural classes of compounds activate, inactivate, and directly bind several orphan receptors. Thus, the screening and development of selective orphan receptor modulators will have important clinical applications as novel mechanism-based agents for treating cancer patients overexpressing one or more orphan receptors and also for combined drug therapies.

The aryl hydrocarbon receptor mediates raloxifene-induced apoptosis in estrogen receptor-negative hepatoma and breast cancer cells

Identification of new molecular targets for the treatment of breast cancer is an important clinical goal, especially for triple-negative breast cancer, which is refractory to existing targeted treatments. The aryl hydrocarbon receptor (AhR) is a ligand-activated transcription factor known primarily as the mediator of dioxin toxicity. However, the AhR can also inhibit cellular proliferation in a ligand-dependent manner and act as a tumor suppressor in mice, and thus may be a potential anticancer target. To investigate the AhR as an anticancer target, we conducted a small molecule screen to discover novel AhR ligands with anticancer properties. We identified raloxifene, a selective estrogen receptor (ER) modulator currently used in the clinic for prevention of ER-positive breast cancer and osteoporosis in post-menopausal women, as an AhR activator. Raloxifene directly bound the AhR and induced apoptosis in ER-negative mouse and human hepatoma cells in an AhR-dependent manner, indicating that the AhR is a molecular target of raloxifene and mediates raloxifene-induced apoptosis in the absence of ER. Raloxifene selectively induced apoptosis of triple-negative MDA-MB-231 breast cancer cells compared with non-transformed mammary epithelial cells via the AhR. Combined with recent data showing that raloxifene inhibits triple-negative breast cancer xenografts in vivo (Int J Oncol. 43(3):785-92, 2013), our results support the possibility of repurposing of raloxifene as an AhR-targeted therapeutic for triple-negative breast cancer patients. To this end, we also evaluated the role of AhR expression on survival of patients diagnosed with breast cancer. We found that higher expression of the AhR is significantly associated with increased overall survival and distant metastasis-free survival in both hormone-dependent (ER-positive) and hormone-independent (ER and progesterone receptor (PR)-negative) breast cancers. Together, our data strongly support the possibility of using the AhR as a molecular target for the treatment of hormone-independent breast cancers.