Discovery of novel ID2 antagonists from pharmacophore-based virtual screening as potential therapeutics for glioma.

Glioma, especially the most aggressive type glioblastoma multiforme, is a malignant cancer of the central nervous system with a poor prognosis. Traditional treatments are mainly surgery combined with radiotherapy and chemotherapy, which is still far from satisfactory. Therefore, it is of great clinical significance to find new therapeutic agents. Serving as an inhibitor of differentiation, protein ID2 (inhibitor of DNA binding 2) plays an important role in neurogenesis, neovascularization and malignant development of gliomas. It has been shown that ID2 affects the malignant progression of gliomas through different mechanisms. In this study, a pharmacophore-based virtual screening was carried out and 16 hit compounds were purchased for pharmacological evaluations on their ID2 inhibitory activities. Based on the cytotoxicity of these small-molecule compounds, two compounds were shown to effectively inhibit the viability of glioma cells in the micromolar range. Among them, AK-778-XXMU was chosen for further study due to its better solubility in water. A SPR (Surface Plasma Resonance) assay proved the high affinity between AK-778-XXMU and ID2 protein with the KD value as 129 nM. The plausible binding mode of ID2 was studied by molecular docking and it was found to match AGX51 very well in the same binding site. Subsequently, the cancer-suppressing potency of the compound was characterized both in vitro and in vivo. The data demonstrated that compound AK-778-XXMU is a potent ID2 antagonist which has the potential to be developed as a therapeutic agent against glioma.

Id2a is required for hepatic outgrowth during liver development in zebrafish.

During development, inhibitor of DNA binding (Id) proteins, a subclass of the helix-loop-helix family of proteins, regulate cellular proliferation, differentiation, and apoptosis in various organs. However, a functional role of Id2a in liver development has not yet been reported. Here, using zebrafish as a model organism, we provide in vivo evidence that Id2a regulates hepatoblast proliferation and cell death during liver development. Initially, in the liver, id2a is expressed in hepatoblasts and after their differentiation, id2a expression is restricted to biliary epithelial cells. id2a knockdown in zebrafish embryos had no effect on hepatoblast specification or hepatocyte differentiation. However, liver size was greatly reduced in id2a morpholino-injected embryos, indicative of a hepatic outgrowth defect attributable to the significant decrease in proliferating hepatoblasts concomitant with the significant increase in hepatoblast cell death. Altogether, these data support the role of Id2a as an important regulator of hepatic outgrowth via modulation of hepatoblast proliferation and survival during liver development in zebrafish.

Inhibitor of differentiation 4 (ID4) acts as an inhibitor of ID-1, -2 and -3 and promotes basic helix loop helix (bHLH) E47 DNA binding and transcriptional activity.

The four known ID proteins (ID1-4, Inhibitor of Differentiation) share a homologous helix loop helix (HLH) domain and act as dominant negative regulators of basic-HLH transcription factors. ID proteins also interact with many non-bHLH proteins in complex networks. The expression of ID proteins is increasingly observed in many cancers. Whereas ID-1, ID-2 and ID-3, are generally considered as tumor promoters, ID4 on the contrary has emerged as a tumor suppressor. In this study we demonstrate that ID4 heterodimerizes with ID-1, -2 and -3 and promote bHLH DNA binding, essentially acting as an inhibitor of inhibitors of differentiation proteins. Interaction of ID4 was observed with ID1, ID2 and ID3 that was dependent on intact HLH domain of ID4. Interaction with bHLH protein E47 required almost 3 fold higher concentration of ID4 as compared to ID1. Furthermore, inhibition of E47 DNA binding by ID1 was restored by ID4 in an EMSA binding assay. ID4 and ID1 were also colocalized in prostate cancer cell line LNCaP. The alpha helix forming alanine stretch N-terminal, unique to HLH ID4 domain was required for optimum interaction. Ectopic expression of ID4 in DU145 prostate cancer line promoted E47 dependent expression of CDKNI p21. Thus counteracting the biological activities of ID-1, -2 and -3 by forming inactive heterodimers appears to be a novel mechanism of action of ID4. These results could have far reaching consequences in developing strategies to target ID proteins for cancer therapy and understanding biologically relevant ID-interactions.

Regulation of ACVR1 and ID2 by cell-secreted exosomes during follicle maturation in the mare.

BACKGROUND: Ovarian follicle growth and maturation requires extensive communication between follicular somatic cells and oocytes. Recently, intercellular cell communication was described involving cell-secreted vesicles called exosomes (50-150 nm), which contain miRNAs and protein, and have been identified in ovarian follicular fluid. The goal of this study was to identify a possible role of exosomes in follicle maturation. METHODS: Follicle contents were collected from mares at mid-estrous (~35 mm, before induction of follicular maturation) and pre-ovulatory follicles (30-34 h after induction of follicular maturation). A real time PCR screen was conducted to reveal significant differences in the presence of exosomal miRNAs isolated from mid-estrous and pre-ovulatory follicles, and according to bioinformatics analysis these exosomal miRNAs are predicted to target members belonging to the TGFB superfamily, including ACVR1 and ID2. Granulosa cells from pre-ovulatory follicles were cultured and treated with exosomes isolated from follicular fluid. Changes in mRNA and protein were measured by real time PCR and Western blot. RESULTS: ACVR1 mRNA and protein was detected in granulosa cells at mid-estrous and pre-ovulatory stages, and real time PCR analysis revealed significantly lower levels of ID2 (an ACVR1 target gene) in granulosa cells from pre-ovulatory follicles. Exposure to exosomes from follicular fluid of mid-estrous follicles decreased ID2 levels in granulosa cells. Moreover, exosomes isolated from mid-estrous and pre-ovulatory follicles contain ACVR1 and miR-27b, miR-372, and miR-382 (predicted regulators of ACVR1 and ID2) were capable of altering ID2 levels in pre-ovulatory granulosa cells. CONCLUSIONS: These data indicate that exosomes isolated from follicular fluid can regulate members of the TGFB/BMP signaling pathway in granulosa cells, and possibly play a role in regulating follicle maturation.

Smad mediated regulation of inhibitor of DNA binding 2 and its role in phenotypic maintenance of human renal proximal tubule epithelial cells.

The basic-Helix-Loop-Helix family (bHLH) of transcriptional factors plays a major role in regulating cellular proliferation, differentiation and phenotype maintenance. The downregulation of one of the members of bHLH family protein, inhibitor of DNA binding 2 (Id2) has been shown to induce de-differentiation of epithelial cells. Opposing regulators of epithelial/mesenchymal phenotype in renal proximal tubule epithelial cells (PTEC), TGFß1 and BMP7 also have counter-regulatory effects in models of renal fibrosis. We investigated the regulation of Id2 by these growth factors in human PTECs and its implication in the expression of markers of epithelial versus myofibroblastic phenotype. Cellular Id2 levels were reduced by TGFß1 treatment; this was prevented by co-incubation with BMP7. BMP7 alone increased cellular levels of Id2. TGFß1 and BMP7 regulated Id2 through Smad2/3 and Smad1/5 dependent mechanisms respectively. TGFß1 mediated Id2 suppression was essential for α-SMA induction in PTECs. Although Id2 over-expression prevented α-SMA induction, it did not prevent E-cadherin loss under the influence of TGFß1. This suggests that the loss of gate keeper function of E-cadherin alone may not necessarily result in complete EMT and further transcriptional re-programming is essential to attain mesenchymal phenotype. Although BMP7 abolished TGFß1 mediated α-SMA expression by restoring Id2 levels, the loss of Id2 was not sufficient to induce α-SMA expression even in the context of reduced E-cadherin expression. Hence, a reduction in Id2 is critical for TGFß1-induced α-SMA expression in this model of human PTECs but is not sufficient in it self to induce α-SMA even in the context of reduced E-cadherin.

BMP signaling is responsible for serum-induced Id2 expression.

Ids function as negative regulators of basic helix-loop-helix transcription factors and their expression is rapidly induced by serum stimulation in various cell types. In this study, we investigated the molecular basis of serum-induced expression of the mouse Id2 gene in NIH3T3 cells. A small-molecule inhibitor of bone morphogenetic protein (BMP) type I receptor kinases blocked the serum induction of Id2 mRNA. The chemical compound and several inhibitory proteins specific for BMP signaling suppressed the serum-induced activation of the luciferase construct with the mouse Id2 4.6-kb promoter region. Importantly, serum stimulation evoked rapid phosphorylation of Smad1/5/8 and significant activation of the reporter plasmid containing the recently identified BMP-responsive element (BRE) of the mouse Id2. Mutation analysis demonstrated that the binding sites for Smad proteins in the Id2 BRE were critical for serum response of the 4.6-kb whole construct. Gel shift and chromatin immunoprecipitation (ChIP) assays confirmed the serum-inducible binding of Smad1/5/8 and Smad4 to the Id2 BRE in vitro and in vivo. Finally, a knockdown experiment revealed the functional importance of Smad1 in the serum induction of Id2 expression. Thus, we concluded that BMP signaling is primarily responsible for the serum-induced Id2 expression. Our results also suggest that some of the cellular effects caused by serum are mediated through BMP signaling.

Id gene regulation and function in the prosensory domains of the chicken inner ear: a link between Bmp signaling and Atoh1.

Bone morphogenetic proteins (Bmps) regulate the expression of the proneural gene Atoh1 and the generation of hair cells in the developing inner ear. The present work explored the role of Inhibitor of Differentiation genes (Id1-3) in this process. The results show that Id genes are expressed in the prosensory domains of the otic vesicle, along with Bmp4 and Bmp7. Those domains exhibit high levels of the phosphorylated form of Bmp-responding R-Smads (P-Smad1,5,8), and of Bmp-dependent Smad transcriptional activity as shown by the BRE-tk-EGFP reporter. Increased Bmp signaling induces the expression of Id1-3 along with the inhibition of Atoh1. Conversely, the Bmp antagonist Noggin or the Bmp-receptor inhibitor Dorsomorphin elicit opposite effects, indicating that Bmp signaling is necessary for Id expression and Atoh1 regulation in the otocyst. The forced expression of Id3 is sufficient to reduce Atoh1 expression and to prevent the expression of hair cell differentiation markers. Together, these results suggest that Ids are part of the machinery that mediates the regulation of hair cell differentiation exerted by Bmps. In agreement with that, during hair cell differentiation Bmp4 expression, P-Smad1,5,8 levels and Id expression are downregulated from hair cells. However, Ids are also downregulated from the supporting cells which contrarily to hair cells exhibit high levels of Bmp4 expression, P-Smad1,5,8, and BRE-tk-EGFP activity, suggesting that in these cells Ids escape from Bmp/Smad signaling. The differential regulation of Ids in time and space may underlie the multiple functions of Bmp signaling during sensory organ development.

Repression of Id2 expression by Gfi-1 is required for B-cell and myeloid development.

The development of mature blood cells from hematopoietic stem cells requires coordinated activities of transcriptional networks. Transcriptional repressor growth factor independence 1 (Gfi-1) is required for the development of B cells, T cells, neutrophils, and for the maintenance of hematopoietic stem cell function. However, the mechanisms by which Gfi-1 regulates hematopoiesis and how Gfi-1 integrates into transcriptional networks remain unclear. Here, we provide evidence that Id2 is a transcriptional target of Gfi-1, and repression of Id2 by Gfi-1 is required for B-cell and myeloid development. Gfi-1 binds to 3 conserved regions in the Id2 promoter and represses Id2 promoter activity in transient reporter assays. Increased Id2 expression was observed in multipotent progenitors, myeloid progenitors, T-cell progenitors, and B-cell progenitors in Gfi-1(-/-) mice. Knockdown of Id2 expression or heterozygosity at the Id2 locus partially rescues the B-cell and myeloid development but not the T-cell development in Gfi-1(-/-) mice. These studies demonstrate a role of Id2 in mediating Gfi-1 functions in B-cell and myeloid development and provide a direct link between Gfi-1 and the B-cell transcriptional network by its ability to repress Id2 expression.

(-)-Epigallocatechin-3-gallate induces Du145 prostate cancer cell death via downregulation of inhibitor of DNA binding 2, a dominant negative helix-loop-helix protein.

(-)-Epigallocatechin-3-gallate (EGCG) is one of the major polyphenol components in green tea. It effectively induces apoptosis in prostate cancer cells. The anticancer effect of this reagent is appealing because it is a natural component of a popular daily beverage that has proven harmless for thousands of years, making it a good candidate chemopreventive agent. EGCG suppresses cell growth and causes cell death, but the mechanisms are not well characterized, especially in androgen-independent prostate cancer cells. In the present study, using Affymetrix genechip Hu133 2.0, we analyzed the gene expression patterns of the androgen-independent prostate cancer cell line Du145, treated with or without EGCG, and found 40 genes whose expression levels were altered (>twofold, either upregulated or downregulated, P < 0.01) upon treatment with EGCG. These gene products are involved in the functions of transcription, RNA processing, protein folding, phosphorylation, protein degradation, cell motility, and ion transport. Among them, inhibitor of DNA binding 2 (ID2), known as a dominant anti-retinoblastoma (Rb) helix-loop-helix protein, was found to be downregulated fourfold by EGCG treatment. Forced expression of ID2 in Du145 cells reduced apoptosis and increased cell survival in the presence of EGCG, and knockdown ID2 expression in Du145 cells using a morpholino oligonucleotide specific for ID2 mimicked the apoptosis effect generated by EGCG treatment, although it was milder. To our knowledge, this is the first report indicating that ID2 is one of the critical factors in the signaling pathway of Du145 cell death induced by EGCG.

FHL2 interacts with and acts as a functional repressor of Id2 in human neuroblastoma cells.

Inhibitor of differentiation 2 (Id2) is a natural inhibitor of the basic helix-loop-helix transcription factors. Although Id2 is well known to prevent differentiation and promote cell-cycle progression and tumorigenesis, the molecular events that regulate Id2 activity remain to be investigated. Here, we identified that Four-and-a-half LIM-only protein 2 (FHL2) is a novel functional repressor of Id2. Moreover, we demonstrated that FHL2 can directly interact with all members of the Id family (Id1-4) via an N-terminal loop-helix structure conserved in Id proteins. FHL2 antagonizes the inhibitory effect of Id proteins on basic helix-loop-helix protein E47-mediated transcription, which was abrogated by the deletion mutation of Ids that disrupted their interaction with FHL2. We also showed a competitive nature between FHL2 and E47 for binding Id2, whereby FHL2 prevents the formation of the Id2-E47 heterodimer, thus releasing E47 to DNA and restoring its transcriptional activity. FHL2 expression was remarkably up-regulated during retinoic acid-induced differentiation of neuroblastoma cells, during which the expression of Id2 was opposite to that. Ectopic FHL2 expression in neuroblastoma cells markedly reduces the transcriptional and cell-cycle promoting functions of Id2. Altogether, these results indicate that FHL2 is an important repressor of the oncogenic activity of Id2 in neuroblastoma cells.

Modulation of retinal capillary endothelial cells by Müller glial cell-derived factors.

PURPOSE: The inner blood-retinal barrier (BRB) is a gliovascular unit in which macroglial cells surround capillary endothelial cells and regulate retinal capillaries by paracrine interactions. The purpose of the present study was to identify genes of retinal capillary endothelial cells whose expression is modulated by Müller glial cell-derived factors. METHODS: Conditionally immortalized rat retinal capillary endothelial (TR-iBRB2) and Müller (TR-MUL5) cell lines were chosen as an in vitro model. TR-iBRB2 cells were incubated with conditioned medium of TR-MUL5 (MUL-CM) for 24 h and subjected to microarray and quantitative real-time PCR analysis. RESULTS: TR-MUL5 cell-derived factors increased alkaline phosphatase activity in TR-iBRB2 cells, indicating that paracrine interactions occurred between TR-iBRB2 and TR-MUL5 cells. Microarray analysis demonstrated that MUL-CM treatment leads to a modulation of several genes including an induction of plasminogen activator inhibitor 1 (PAI-1) and a suppression of an inhibitor of DNA binding 2 (Id2) in TR-iBRB2 cells. Treatment with TGF-beta1, which is incorporated in MUL-CM, also resulted in an induction of PAI-1 and a suppression of Id2 in TR-iBRB2 cells. CONCLUSIONS: In vitro inner BRB model study revealed that Müller glial cell-derived factors modulate endothelial cell functions including the induction of anti-angiogenic PAI-1 and the suppression of pro-angiogenic Id2. Therefore, Müller cells appear to be one of the modulators of retinal angiogenesis.

Suppression of inhibitor of differentiation 2, a target of mutant p53, is required for gain-of-function mutations.

Overexpression of mutant p53 is a common theme in human tumors, suggesting a tumor-promoting gain-of-function for mutant p53. To elucidate whether and how mutant p53 acquires its gain-of-function, mutant p53 is inducibly knocked down in the SW480 colon cancer cell line, which contains mutant p53(R273H/P309S), and the MIA PaCa-2 pancreatic cancer cell line, which contains mutant p53(R248W). We found that knockdown of mutant p53 markedly inhibits cell proliferation. In addition, knockdown of mutant p53 sensitizes tumor cells to growth suppression by various chemotherapeutic drugs. To determine whether a gene involved in cell growth and survival is regulated by mutant p53, gene expression profiling analysis was performed and showed that the expression level of Id2, a member of the inhibitor of differentiation (Id) family, was markedly increased upon knockdown of mutant p53. To confirm this, Northern blot analysis was performed and showed that the expression level of Id2 was regulated by various mutant p53s in multiple cell lines. In addition, we found that the Id2 promoter is responsive to mutant but not wild-type p53, and mutant p53 binds to the Id2 promoter. Consistent with these observations, expression of endogenous Id2 was found to be inhibited by exogenous mutant p53 in p53-null HCT116 cells. Finally, we showed that knockdown of Id2 can restore the proliferative potential of tumor cells inhibited by withdrawal of mutant p53. Together, these findings suggest that one mechanism by which mutant p53 acquires its gain-of-function is through the inhibition of Id2 expression.

Inhibitor of DNA binding 2 is a small molecule-inducible modulator of peroxisome proliferator-activated receptor-gamma expression and adipocyte differentiation.

We previously identified the small molecule harmine as a regulator of peroxisome proliferator activated-receptor gamma (PPARgamma) and adipocyte differentiation. In an effort to identify signaling pathways mediating harmine's effects, we performed transcriptional profiling of 3T3-F442A preadipocytes. Inhibitor of DNA biding 2 (Id2) was identified as a gene rapidly induced by harmine but not by PPARgamma agonists. Id2 is also induced in 3T3-L1 preadipocytes treated with dexamethasone, 3-isobutyl-1-methylxanthine, and insulin, suggesting that Id2 regulation is a common feature of the adipogenic program. Stable overexpression of Id2 in preadipocytes promotes expression of PPARgamma and enhances morphological differentiation and lipid accumulation. Conversely, small interfering RNA-mediated knockdown of Id2 antagonizes adipocyte differentiation. Mice lacking Id2 expression display reduced adiposity, and embryonic fibroblasts derived from these mice exhibit reduced PPARgamma expression and a diminished capacity for adipocyte differentiation. Finally, Id2 expression is elevated in adipose tissues of obese mice and humans. These results outline a role for Id2 in the modulation of PPARgamma expression and adipogenesis and underscore the utility of adipogenic small molecules as tools to dissect adipocyte biology.

A molecular pathway involved in the generation of microtubule-associated protein 2-positive cells from microglia.

We have recently demonstrated that microglia as multipotential stem cells give rise to microtubule-associated protein 2 (MAP2)-positive and glial fibrillary acidic protein (GFAP)-positive cells and that microglia-derived MAP2-positive cells possess properties of functional neurons. In this study, we investigated the molecular pathways involved in the generation of microglia-derived MAP2-positive and GFAP-positive cells. Western blot analyses demonstrated that expression levels of Id2 protein, an inhibitory basic helix-loop-helix transcription factor of the inhibitor of differentiation and DNA binding family, and Smad proteins were upregulated under differentiation conditions. Immunocytochemical analyses demonstrated that the generation of MAP2-positive and GFAP-positive cells from microglia was promoted by bone morphogenetic proteins (BMPs) and was inhibited by noggin which is a BMP antagonist, Smad4 siRNA and Id2 siRNA. These results indicate that activation of BMP signaling through Smad and Id2 proteins is one of the molecular pathways involved in the generation of microglia-derived MAP2-positive and GFAP-positive cells.

Id2 gene-targeted crosstalk between Wnt and retinoid signaling regulates proliferation in human keratinocytes.

We investigated the effect of all-trans-retinoic acid (atRA) on proliferation in several human skin cell lines and found that antiproliferative potency of atRA correlated with the endogenous activity of canonical Wnt signaling. In HaCaT keratinocytes, we found that atRA significantly suppressed the expression of Id2, a member of the inhibitor of differentiation family of transcription factors that regulate cell growth and differentiation. However, no apparent change in the expression of other Wnt targets, like c-Myc or cyclin D1, was observed. Retinoid-induced Id2 gene suppression was associated with decreased levels of histone H3 and H4 acetylation and histone H3 Lys-4 methylation, and with recruitment of the LSD1 demethylase at the Wnt-response element (WRE) (TCF/LEF-binding site), in the Id2 gene promoter. None of such changes was detected at the WRE of c-Myc and cyclin D1 gene promoters. Inhibition of Id2 by short interfering RNA (siRNA) had a similar effect on the proliferation of HaCaT cells as exposure to atRA, whereas anti-beta-catenin siRNA significantly inhibited its antiproliferative effect. These data suggest that downregulation of Id2 gene expression through transcriptional convergence between Wnt and retinoid signaling pathways underlies the antiproliferative effect of retinoids in keratinocytes, and provide evidence of gene-targeted crosstalk between signaling pathways.