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.

Retinoid X receptor-alpha (RXRα) binds to DNA either as homodimers or heterodimers, but it also forms homotetramers whose function is poorly defined. We previously discovered that an N-terminally-cleaved form of RXRα (tRXRα), produced in tumour cells, activates phosphoinositide 3-kinase (PI3K) signalling by binding to the p85α subunit of PI3K and that K-80003, an anti-cancer agent, inhibits this process. Here, we report through crystallographic and biochemical studies that K-80003 binds to and stabilizes tRXRα tetramers via a ‘three-pronged’ combination of canonical and non-canonical mechanisms. K-80003 binding has no effect on tetramerization of RXRα, owing to the head–tail interaction that is absent in tRXRα. We also identify an LxxLL motif in p85α, which binds to the coactivator-binding groove on tRXRα and dissociates from tRXRα upon tRXRα tetramerization. These results identify conformational selection as the mechanism for inhibiting the nongenomic action of tRXRα and provide molecular insights into the development of RXRα cancer therapeutics.

Nur77 (also called TR3 or NGFI-B), an orphan member of the nuclear receptor superfamily, induces apoptosis by translocating to mitochondria where it interacts with Bcl-2 to convert Bcl-2 from an antiapoptotic to a pro-apoptotic molecule. Nur77 posttranslational modification such as phosphorylation has been shown to induce Nur77 translocation from the nucleus to mitochondria. However, small molecules that can bind directly to Nur77 to trigger its mitochondrial localization and Bcl-2 interaction remain to be explored. Here, we report our identification and characterization of DIM-C-pPhCF 3 þ MeSO 3 À (BI1071), an oxidized product derived from indole-3-carbinol metabolite, as a modulator of the Nur77-Bcl-2 apoptotic pathway. BI1071 binds Nur77 with high affinity, promotes Nur77 mitochondrial targeting and interaction with Bcl-2, and effectively induces apoptosis of cancer cells in a Nur77-and Bcl-2-dependent manner. Studies with animal model showed that BI1071 potently inhibited the growth of tumor cells in animals through its induction of apoptosis. Our results identify BI1071 as a novel Nur77binding modulator of the Nur77-Bcl-2 apoptotic pathway, which may serve as a promising lead for treating cancers with overexpression of Bcl-2.

BACKGROUND AND PURPOSEThe 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 APPROACHThe 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 RESULTSTNF-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 IMPLICATIONSTNF-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.

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