p53 directly represses Id2 to inhibit the proliferation of neural progenitor cells

Structural variants shape driver combinations and outcomes in pediatric high-grade glioma

We analyzed the contributions of structural variants (SVs) to gliomagenesis across 179 pediatric high-grade gliomas (pHGGs). The most recurrent SVs targeted MYC isoforms and receptor tyrosine kinases (RTKs), including an SV amplifying a MYC enhancer in 12% of diffuse midline gliomas (DMG), indicating an underappreciated role for MYC in pHGG. SV signature analysis revealed that tumors with simple signatures were TP53 wild type (TP53WT) but showed alterations in TP53 pathway members PPM1D and MDM4. Complex signatures were associated with direct aberrations in TP53, CDKN2A and RB1 early in tumor evolution and with later-occurring extrachromosomal amplicons. All pHGGs exhibited at least one simple-SV signature, but complex-SV signatures were primarily restricted to subsets of H3.3K27M DMGs and hemispheric pHGGs. Importantly, DMGs with complex-SV signatures were associated with shorter overall survival independent of histone mutation and TP53 status. These data provide insight into the impact of SVs on gliomagenesis and the mechanisms that shape them.

Lineage tracing reveals the dynamic contribution of Id2+ progenitor cells to branching morphogenesis

Branching morphogenesis is an important process in shaping the arborized structures of several organs. However, the driving force that directs this process from progenitor pools remains incompletely understood. In this lineage tracing study, we investigated the role of Id2+ embryonic progenitor cells in branching organs such as the pancreas, kidney, mammary gland, thyroid gland, and salivary gland. We found that a subset of Id2+ distal progenitor cells in the embryonic pancreas and kidney can give rise to multiple lineages of progeny cells during branching morphogenesis. Id2-labelled cells also supported the postnatal development of the mammary glands. However, Id2+ cells did not contribute to the development of the salivary and thyroid glands. We found the Id2+ cells located in the tip progenitor pools of pancreas and kidney have self-renewal potential and contribute descendents to multiple epithelial cell lineages. Our findings enrich the current model of distal progenitor pools driving branching morphogenesis and provide a new marker to investigate the regularity of branching in these organs.

Emerging Roles of Inhibitor of Differentiation-1 in Alzheimer's Disease: Cell Cycle Reentry and Beyond

Inhibitor of DNA-binding/differentiation (Id) proteins, a family of helix-loop-helix (HLH) proteins that includes four members of Id1 to Id4 in mammalian cells, are critical for regulating cell growth, differentiation, senescence, cell cycle progression, and increasing angiogenesis and vasculogenesis, as well as accelerating the ability of cell migration. Alzheimer’s disease (AD), the most common neurodegenerative disease in the adult population, manifests the signs of cognitive decline, behavioral changes, and functional impairment. The underlying mechanisms for AD are not well-clarified yet, but the aggregation of amyloid-beta peptides (Aβs), the major components in the senile plaques observed in AD brains, contributes significantly to the disease progression. Emerging evidence reveals that aberrant cell cycle reentry may play a central role in Aβ-induced neuronal demise. Recently, we have shown that several signaling mediators, including Id1, hypoxia-inducible factor-1 (HIF-1), cyclin-dependent kinases-5 (CDK5), and sonic hedgehog (Shh), may contribute to Aβ-induced cell cycle reentry in postmitotic neurons; furthermore, Id1 and CDK5/p25 mutually antagonize the expression/activity of each other. Therefore, Id proteins may potentially have clinical applications in AD. In this review article, we introduce the underlying mechanisms for cell cycle dysregulation in AD and present some examples, including our own studies, to show different aspects of Id1 in terms of cell cycle reentry and other signaling that may be crucial to alter the neuronal fates in this devastating neurodegenerative disease. A thorough understanding of the underlying mechanisms may provide a rationale to make an earlier intervention before the occurrence of cell cycle reentry and subsequent apoptosis in the fully differentiated neurons during the progression of AD or other neurodegenerative diseases.

MicroRNA-934 is a novel primate-specific small non-coding RNA with neurogenic function during early development

Integrating differential RNA and miRNA expression during neuronal lineage induction of human embryonic stem cells we identified miR-934, a primate-specific miRNA that displays a stage-specific expression pattern during progenitor expansion and early neuron generation. We demonstrate the biological relevance of this finding by comparison with data from early to mid-gestation human cortical tissue. Further we find that miR-934 directly controls progenitor to neuroblast transition and impacts on neurite growth of newborn neurons. In agreement, miR-934 targets are involved in progenitor proliferation and neuronal differentiation whilst miR-934 inhibition results in profound global transcriptome changes associated with neurogenesis, axonogenesis, neuronal migration and neurotransmission. Interestingly, miR-934 inhibition affects the expression of genes associated with the subplate zone, a transient compartment most prominent in primates that emerges during early corticogenesis. Our data suggest that mir-934 is a novel regulator of early human neurogenesis with potential implications for a species-specific evolutionary role in brain function.

Mitogenic and progenitor gene programmes in single pilocytic astrocytoma cells

Pilocytic astrocytoma (PA), the most common childhood brain tumor, is a low-grade glioma with a single driver BRAF rearrangement. Here, we perform scRNAseq in six PAs using methods that enabled detection of the rearrangement. When compared to higher-grade gliomas, a strikingly higher proportion of the PA cancer cells exhibit a differentiated, astrocyte-like phenotype. A smaller proportion of cells exhibit a progenitor-like phenotype with evidence of proliferation. These express a mitogen-activated protein kinase (MAPK) programme that was absent from higher-grade gliomas. Immune cells, especially microglia, comprise 40% of all cells in the PAs and account for differences in bulk expression profiles between tumor locations and subtypes. These data indicate that MAPK signaling is restricted to relatively undifferentiated cancer cells in PA, with implications for investigational therapies directed at this pathway.

Pilocytic astrocytoma is a low-grade pediatric glioma, characterized by a single BRAF rearrangement. Here, Reitman and colleagues use single-cell RNA sequencing to reveal molecular hallmarks of the disease that might be targeted therapeutically.

Interaction Between Sympk and Oct4 Promotes Mouse Embryonic Stem Cell Proliferation

The scaffold protein Symplekin (Sympk) is involved in cytoplasmic RNA polyadenylation, transcriptional modulation, and the regulation of epithelial differentiation and proliferation via tight junctions. It is highly expressed in embryonic stem cells (ESCs), in which its role remains unknown. In this study, we found Sympk overexpression in mouse ESCs significantly increased colony formation, and Sympk deletion via CRISPR/Cas9 decreased colony formation. Sympk promoted ESC growth and its overexpression sustained ESC pluripotency, as assessed by teratoma and chimeric mouse formation. Genomic stability was preserved in these cells after long-term passage. The domain of unknown function 3453 (DUF3453) in Sympk was required for its interaction with the key pluripotent factor Oct4, and its depletion led to impaired colony formation. Sympk activated proliferation-related genes and suppressed differentiation-related genes. Our results indicate that Sympk interacts with Oct4 to promote self-renewal and pluripotency in ESCs and preserves genome integrity; accordingly, it has potential value for stem cell therapies. Stem Cells 2019;37:743-753.

Similarities Between Embryo Development and Cancer Process Suggest New Strategies for Research and Therapy of Tumors: A New Point of View

Here, I propose that cancer stem cells (CSCs) would be equivalent to para-embryonic stem cells (p-ESCs), derived from adult cells de-re-programmed to a ground state. p-ESCs would differ from ESCs by the absence of genomic homeostasis. A p-ESC would constitute the cancer cell of origin (i-CSC or CSC0), capable of generating an initial tumor, corresponding to a pre-implantation blastocyst. In a niche with proper signals, it would engraft as a primary tumor, corresponding to a post-implantation blastocyst. i-CSC progeny would form primary pluripotent and slow self-renewing CSCs (CSC1s), blocked in an undifferentiated state, corresponding to epiblast cells; CSC1s would be tumor-initiating cells (TICs). CSC1s would generate secondary CSCs (CSC2s), corresponding to hypoblast cells; CSC2s would be tumor growth cells (TGCs). CSC1s/CSC2s would generate tertiary CSCs (CSC3s), with a mesenchymal phenotype; CSC3s would be tumor migrating cells (TMCs), corresponding to mesodermal precursors at primitive streak. CSC3s with more favorable conditions (normoxia), by asymmetrical division, would differentiate into cancer progenitor cells (CPCs), and these into cancer differentiated cells (CDCs), thus generating a defined cell hierarchy and tumor progression, mimicking somito-histo-organogenesis. CSC3s with less favorable conditions (hypoxia) would delaminate and migrate as quiescent circulating micro-metastases, mimicking mesenchymal cells in gastrula morphogenetic movements. In metastatic niches, these CSC3s would install and remain dormant in the presence of epithelial/mesenchymal transition (EMT) signals and hypoxia. But, in the presence of mesenchymal/epithelial transition (MET) signals and normoxia, they would revert to self-renewing CSC1s, reproducing the same cell hierarchy of the primary tumor as macro-metastases. Further similarities between ontogenesis and oncogenesis involving crucial factors, such as ID, HSP70, HLA-G, CD44, LIF, and STAT3, are strongly evident at molecular, physiological and immunological levels. Much experimental data about these factors led to considering the cancer process as ectopic rudimentary ontogenesis, where CSCs have privileged immunological conditions. These would consent to CSC development in an adverse environment, just like an embryo, which is tolerated, accepted and favored by the maternal organism in spite of its paternal semi-allogeneicity. From all these considerations, novel research directions, potential innovative tumor therapy and prophylaxis strategies might, theoretically, result.

Inhibitor of binding/differentiation 2 (Id2) is regulated by CCAAT/enhancer-binding protein-α (C/EBPα) and promotes the proliferation of hepatocellular carcinoma

Inhibitor of DNA binding/differentiation (Id2) is an important regulator involved in the initiation and progression of cancer. However, the function and mechanism of the regulation of Id2 in hepatocellular carcinoma (HCC) was unclear. In the present study, we found that the overexpression of Id2 increased HCC cell proliferation in vitro and in vivo. Knockdown of Id2 inhibited HCC cell proliferation in vitro and in vivo. Furthermore, knockdown of Id2 enhanced sorafenib-induced apoptosis in HCC. Conversely, overexpression of Id2 weakened sorafenib-induced apoptosis in HCC. In addition, the transcription factor CCAAT/enhancer-binding protein alpha (C/EBPα) bound to the Id2 promoter and decreased its expression in HCC cells. Therefore, all results suggest that Id2 promotes the proliferation of HCC cells by inhibiting cell apoptosis. Id2 may serve as a potential target in HCC therapy.

Delivery of a TNF-α-derived peptide by nanoparticles enhances its antitumor activity by inducing cell-cycle arrest and caspase-dependent apoptosis

Prostate cancer is the second-most common malignancy of the male genitourinary system. TNF-α has attracted intense attention as a potential therapeutic agent against various cancers. However, its therapeutic application is restricted by short half life and severe toxic side-effects. In this study, we constructed a stable nanodrug, called TNF-α-derived polypeptide (P16)-conjugated, chitosan (CTS)-modified selenium nanoparticle (SC; SCP), which is composed of SC as a slow-release carrier conjugated to P16. SCP had significant inhibitory effects on multiple types of tumor cells, especially DU145 prostate cancer cells, but not on RWPE-1 normal human prostate epithelial cells. SCP could induce G0/G1 cell-cycle arrest and apoptosis in DU145 cells more effectively than could P16 and TNF-α. In DU145 xenograft tumor models, SCP exerted much stronger antitumor effects than P16 or estramustine (the clinical drug for prostate cancer) but caused fewer toxic side-effects. In addition, SCP significantly inhibited proliferation and accelerated apoptosis in DU145 xenograft tumors. Further mechanistic studies revealed that SCP exerted antitumor effects via activation of the p38 MAPK/JNK pathway, thus inducing G0/G1 cell-cycle arrest and caspase-dependent apoptosis. These findings suggest that SCP may represent a potential long-lasting therapeutic agent for human prostate cancer with fewer side effects.-Yan, Q., Chen, X., Gong, H., Qiu, P., Xiao, X., Dang, S., Hong, A., Ma, Y. Delivery of a TNF-α-derived peptide by nanoparticles enhances its antitumor activity by inducing cell-cycle arrest and caspase-dependent apoptosis.

Alternative RNA splicing of the MEAF6 gene facilitates neuroendocrine prostate cancer progression

Although potent androgen receptor pathway inhibitors (ARPI) improve overall survival of metastatic prostate cancer patients, treatment-induced neuroendocrine prostate cancer (t-NEPC) as a consequence of the selection pressures of ARPI is becoming a more common clinical issue. Improved understanding of the molecular biology of t-NEPC is essential for the development of new effective management approaches for t-NEPC. In this study, we identify a splice variant of the MYST/Esa1-associated factor 6 (MEAF6) gene, MEAF6-1, that is highly expressed in both t-NEPC tumor biopsies and neuroendocrine cell lines of prostate and lung cancers. We show that MEAF6-1 splicing is stimulated by neuronal RNA splicing factor SRRM4. Rather than inducing neuroendocrine trans-differentiation of cells in prostate adenocarcinoma, MEAF6-1 upregulation stimulates cell proliferation, anchorage-independent cell growth, invasion and xenograft tumor growth. Gene microarray identifies that these MEAF6-1 actions are in part mediated by the ID1 and ID3 genes. These findings suggest that the MEAF6-1 variant does not induce neuroendocrine differentiation of prostate cancer cells, but rather facilitates t-NEPC progression by increasing the proliferation rate of cells that have acquired neuroendocrine phenotypes.

Census and evaluation of p53 target genes

The tumor suppressor p53 functions primarily as a transcription factor. Mutation of the TP53 gene alters its response pathway, and is central to the development of many cancers. The discovery of a large number of p53 target genes, which confer p53's tumor suppressor function, has led to increasingly complex models of p53 function. Recent meta-analysis approaches, however, are simplifying our understanding of how p53 functions as a transcription factor. In the survey presented here, a total set of 3661 direct p53 target genes is identified that comprise 3509 potential targets from 13 high-throughput studies, and 346 target genes from individual gene analyses. Comparison of the p53 target genes reported in individual studies with those identified in 13 high-throughput studies reveals limited consistency. Here, p53 target genes have been evaluated based on the meta-analysis data, and the results show that high-confidence p53 target genes are involved in multiple cellular responses, including cell cycle arrest, DNA repair, apoptosis, metabolism, autophagy, mRNA translation and feedback mechanisms. However, many p53 target genes are identified only in a small number of studies and have a higher likelihood of being false positives. While numerous mechanisms have been proposed for mediating gene regulation in response to p53, recent advances in our understanding of p53 function show that p53 itself is solely an activator of transcription, and gene downregulation by p53 is indirect and requires p21. Taking into account the function of p53 as an activator of transcription, recent results point to an unsophisticated means of regulation.

siRNA-mediated knockdown of ID1 disrupts Nanog- and Oct-4-mediated cancer stem cell-likeness and resistance to chemotherapy in gastric cancer cells

DNA-binding protein inhibitor ID-1 (ID1) serves an essential role in tumor progression, and the self-renewal and pluripotency of embryonic stem cells. However, the effect of ID1 on the stemness and cancer stem cell (CSC)-like properties of gastric adenocarcinoma cells remains to be elucidated. In the present study, effective ID1 knockdown was achieved in gastric cancer (GC) cells using small interfering RNA, and the self-renewal ability and cisplatin (DDP) sensitivity of GC cells was subsequently examined. ID1 knockdown in the MKN-28 and MGC-803 cell lines was demonstrated to significantly suppress colony formation (P=0.005 in MKN-28 and P=0.001 in MGC-803), tumor spheroid formation (P=0.021 in MKN-28 and P=0.037 in MGC-803), cell proliferation (P=0.028 in MKN-28 and P=0.001 in MGC-803) and migration (P=0.002 in MKN-28 and P=0.015 in MGC-803). To the best of our knowledge, the present study revealed for the first time that ID1 knockdown suppresses the expression of the key CSC-associated factors Nanog and octamer-binding protein 4 (Oct-4). It was further demonstrated that ID1 knockdown sensitized GC cells to DDP. In conclusion, knockdown of ID1 attenuates the stem cell like-properties of self-renewal in normal GC cells, potentially through the targeting of Nanog and Oct-4, and subsequently decreases cell proliferation and resistance to DDP. The results of the present study suggest that ID1 functions as an oncogene in GC and regulates the stem cell like-properties of gastric cancer cells by targeting Nanog and Oct-4.

ID2 promotes survival of glioblastoma cells during metabolic stress by regulating mitochondrial function

Tumor cells proliferate in cellular environments characterized by a lack of optimal tissue organization resulting oftentimes in compromised cellular metabolism affecting nutrition, respiration, and energetics. The response of tumor cells to adverse environmental conditions is a key feature affecting their pathogenicity. We found that inhibitor of DNA binding 2 (ID2) expression levels significantly correlate with the ability of glioblastoma (GBM)-derived cell lines to survive glucose deprivation. ID2 suppressed mitochondrial oxidative respiration and mitochondrial ATP production by regulating the function of mitochondrial electron transport chain (mETC) complexes, resulting in reduced superoxide and reactive oxygen species (ROS) production from mitochondria. ID2 suppression of ROS production reduced mitochondrial damage and enhanced tumor cell survival during glucose deprivation. Bioinformatics analysis of GBM gene expression data from The Cancer Genome Atlas (TCGA) database revealed that expression of ID2 mRNA is unique among ID gene family members in correlating with the expression of nuclear genes involved in mitochondrial energy metabolism and assembly of mETC. Our data indicate that the expression level of ID2 in GBM cells can predict the sensitivity of GBM-derived tumor cells to decreased glucose levels. Low levels of ID2 expression in human GBM tissues may identify a clinical group in which metabolic targeting of glycolytic pathways can be expected to have the greatest therapeutic efficacy.

The Id-protein family in developmental and cancer-associated pathways

Inhibitors of DNA binding and cell differentiation (Id) proteins are members of the large family of the helix-loop-helix (HLH) transcription factors, but they lack any DNA-binding motif. During development, the Id proteins play a key role in the regulation of cell-cycle progression and cell differentiation by modulating different cell-cycle regulators both by direct and indirect mechanisms. Several Id-protein interacting partners have been identified thus far, which belong to structurally and functionally unrelated families, including, among others, the class I and II bHLH transcription factors, the retinoblastoma protein and related pocket proteins, the paired-box transcription factors, and the S5a subunit of the 26 S proteasome. Although the HLH domain of the Id proteins is involved in most of their protein-protein interaction events, additional motifs located in their N-terminal and C-terminal regions are required for the recognition of diverse protein partners. The ability of the Id proteins to interact with structurally different proteins is likely to arise from their conformational flexibility: indeed, these proteins contain intrinsically disordered regions that, in the case of the HLH region, undergo folding upon self- or heteroassociation. Besides their crucial role for cell-fate determination and cell-cycle progression during development, other important cellular events have been related to the Id-protein expression in a number of pathologies. Dysregulated Id-protein expression has been associated with tumor growth, vascularization, invasiveness, metastasis, chemoresistance and stemness, as well as with various developmental defects and diseases. Herein we provide an overview on the structural properties, mode of action, biological function and therapeutic potential of these regulatory proteins.

The Id-protein family in developmental and cancer-associated pathways

Inhibitors of DNA binding and cell differentiation (Id) proteins are members of the large family of the helix-loop-helix (HLH) transcription factors, but they lack any DNA-binding motif. During development, the Id proteins play a key role in the regulation of cell-cycle progression and cell differentiation by modulating different cell-cycle regulators both by direct and indirect mechanisms. Several Id-protein interacting partners have been identified thus far, which belong to structurally and functionally unrelated families, including, among others, the class I and II bHLH transcription factors, the retinoblastoma protein and related pocket proteins, the paired-box transcription factors, and the S5a subunit of the 26 S proteasome. Although the HLH domain of the Id proteins is involved in most of their protein-protein interaction events, additional motifs located in their N-terminal and C-terminal regions are required for the recognition of diverse protein partners. The ability of the Id proteins to interact with structurally different proteins is likely to arise from their conformational flexibility: indeed, these proteins contain intrinsically disordered regions that, in the case of the HLH region, undergo folding upon self- or heteroassociation. Besides their crucial role for cell-fate determination and cell-cycle progression during development, other important cellular events have been related to the Id-protein expression in a number of pathologies. Dysregulated Id-protein expression has been associated with tumor growth, vascularization, invasiveness, metastasis, chemoresistance and stemness, as well as with various developmental defects and diseases. Herein we provide an overview on the structural properties, mode of action, biological function and therapeutic potential of these regulatory proteins.

E Proteins and ID Proteins: Helix-Loop-Helix Partners in Development and Disease

The basic Helix-Loop-Helix (bHLH) proteins represent a well-known class of transcriptional regulators. Many bHLH proteins act as heterodimers with members of a class of ubiquitous partners, the E proteins. A widely expressed class of inhibitory heterodimer partners-the Inhibitor of DNA-binding (ID) proteins-also exists. Genetic and molecular analyses in humans and in knockout mice implicate E proteins and ID proteins in a wide variety of diseases, belying the notion that they are non-specific partner proteins. Here, we explore relationships of E proteins and ID proteins to a variety of disease processes and highlight gaps in knowledge of disease mechanisms.

Phosphorylation Regulates Id2 Degradation and Mediates the Proliferation of Neural Precursor Cells

Inhibitor of DNA binding proteins (Id1-Id4) function to inhibit differentiation and promote proliferation of many different cell types. Among the Id family members, Id2 has been most extensively studied in the central nervous system (CNS). Id2 contributes to cultured neural precursor cell (NPC) proliferation as well as to the proliferation of CNS tumors such as glioblastoma that are likely to arise from NPC-like cells. We identified three phosphorylation sites near the N-terminus of Id2 in NPCs. To interrogate the importance of Id2 phosphorylation, Id2(-/-) NPCs were modified to express wild type (WT) Id2 or an Id2 mutant protein that could not be phosphorylated at the identified sites. We observed that NPCs expressing this mutant lacking phosphorylation near the N-terminus had higher steady-state levels of Id2 when compared to NPCs expressing WT Id2. This elevated level was the result of a longer half-life and reduced proteasome-mediated degradation. Moreover, NPCs expressing constitutively de-phosphorylated Id2 proliferated more rapidly than NPCs expressing WT Id2, a finding consistent with the well-characterized function of Id2 in driving proliferation. Observing that phosphorylation of Id2 modulates the degradation of this important cell-cycle regulator, we sought to identify a phosphatase that would stabilize Id2 enhancing its activity in NPCs and extended our analysis to include human glioblastoma-derived stem cells (GSCs). We found that expression of the phosphatase PP2A altered Id2 levels. Our findings suggest that inhibition of PP2A may be a novel strategy to regulate the proliferation of normal NPCs and malignant GSCs by decreasing Id2 levels. Stem Cells 2016;34:1321-1331.

The transcription factor p53: not a repressor, solely an activator

The predominant function of the tumor suppressor p53 is transcriptional regulation. It is generally accepted that p53-dependent transcriptional activation occurs by binding to a specific recognition site in promoters of target genes. Additionally, several models for p53-dependent transcriptional repression have been postulated. Here, we evaluate these models based on a computational meta-analysis of genome-wide data. Surprisingly, several major models of p53-dependent gene regulation are implausible. Meta-analysis of large-scale data is unable to confirm reports on directly repressed p53 target genes and falsifies models of direct repression. This notion is supported by experimental re-analysis of representative genes reported as directly repressed by p53. Therefore, p53 is not a direct repressor of transcription, but solely activates its target genes. Moreover, models based on interference of p53 with activating transcription factors as well as models based on the function of ncRNAs are also not supported by the meta-analysis. As an alternative to models of direct repression, the meta-analysis leads to the conclusion that p53 represses transcription indirectly by activation of the p53-p21-DREAM/RB pathway.

Differential expression of id genes and their potential regulator znf238 in zebrafish adult neural progenitor cells and neurons suggests distinct functions in adult neurogenesis

Teleost fish display a remarkable ability to generate new neurons and to repair brain lesions during adulthood. They are, therefore, a very popular model to investigate the molecular mechanisms of constitutive and induced neurogenesis in adult vertebrates. In this study, we investigated the expression patterns of inhibitor of DNA binding (id) genes and of their potential transcriptional repressor, znf238, in the whole brain of adult zebrafish. We show that while id1 is exclusively expressed in ventricular cells in the whole brain, id2a, id3 and id4 genes are expressed in broader areas. Interestingly, znf238 was also detected in these regions, its expression overlapping with id2a, id3 and id4 expression. Further detailed characterization of the id-expressing cells demonstrated that (a) id1 is expressed in type 1 and type 2 neural progenitors as previously published, (b) id2a in type 1, 2 and 3 neural progenitors, (c) id3 in type 3 neural progenitors and (d) id4 in postmitotic neurons. Our data provide a detailed map of id and znf238 expression in the brain of adult zebrafish, supplying a framework for studies of id genes function during adult neurogenesis and brain regeneration in the zebrafish.

p53 and ΔNp63α Coregulate the Transcriptional and Cellular Response to TGFβ and BMP Signals

The TGFβ superfamily regulates a broad range of cellular processes, including proliferation, cell-fate specification, differentiation, and migration. Molecular mechanisms underlying this high degree of pleiotropy and cell-type specificity are not well understood. The TGFβ family is composed of two branches: (i) TGFβs, activins, and nodals, which signal through SMAD2/3, and (ii) bone morphogenetic proteins (BMP), which signal through SMAD1/5/8. SMADs have weak DNA-binding affinity and rely on coactivators and corepressors to specify their transcriptional outputs. This report reveals that p53 and ΔNp63α act as transcriptional partners for SMAD proteins and thereby influence cellular responses to TGFβ and BMPs. Suppression of p53 or overexpression of ΔNp63α synergistically enhance BMP-induced transcription. Mechanistically, p53 and ΔNp63α physically interact with SMAD1/5/8 proteins and co-occupy the promoter region of inhibitor of differentiation (ID2), a prosurvival BMP target gene. Demonstrating further convergence of these pathways, TGFβ-induced canonical BMP regulated transcription in a ΔNp63α- and p53-dependent manner. Furthermore, bioinformatic analyses revealed that SMAD2/3 and ΔNp63α coregulate a significant number of transcripts involved in the regulation of epithelial-to-mesenchymal transition. Thus, p53 and ΔNp63α are transcriptional partners for a subset of TGFβ- and BMP-regulated SMAD target genes in the mammary epithelium. Collectively, these results establish an integrated gene network of SMADs, p53, and ΔNp63α that contribute to EMT and metastasis.

IMPLICATIONS

This study identifies aberrant BMP activation as a result of p53 mutation or ΔNp63α expression.

SHIP1 regulates MSC numbers and their osteolineage commitment by limiting induction of the PI3K/Akt/β-catenin/Id2 axis

Here, we show that Src homology 2-domain-containing inositol 5'-phosphatase 1 (SHIP1) is required for the efficient development of osteoblasts from mesenchymal stem cells (MSCs) such that bone growth and density are reduced in mice that lack SHIP1 expression in MSCs. We find that SHIP1 promotes the osteogenic output of MSCs by limiting activation of the PI3K/Akt/β-catenin pathway required for induction of the MSC stemness factor Id2. In parallel, we demonstrate that mice with myeloid-restricted ablation of SHIP1, including osteoclasts (OCs), show no reduction in bone mass or density. Hence, diminished bone mass and density in the SHIP1-deficient mice results from SHIP deficiency in MSC and osteolineage progenitors. Intriguingly, mice with a SHIP-deficient MSC compartment also exhibit decreased OC numbers. In agreement with our genetic findings we also show that treatment of mice with an SHIP1 inhibitor (SHIPi) significantly reduces bone mass. These findings demonstrate a novel role for SHIP1 in MSC fate determination and bone growth. Further, SHIPi may represent a novel therapeutic approach to limit bone development in osteopetrotic and sclerotic bone diseases.

Taking advantage of neural development to treat glioblastoma

Glioblastoma (GBM) is by far the most common and most malignant primary adult brain tumor (World Health Organization grade IV), containing a fraction of stem-like cells that are highly tumorigenic and multipotent. Recent research has revealed that GBM stem-like cells play important roles in GBM pathogenesis. GBM is thought to arise from genetic anomalies in glial development. Over the past decade, a wide range of studies have shown that several signaling pathways involved in neural development, including basic helix-loop-helix, Wnt-β-catenin, bone morphogenetic proteins-Smads, epidermal growth factor-epidermal growth factor receptor, and Notch, play important roles in GBM pathogenesis. In this review, we highlight the significance of these pathways in the context of developing treatments for GBM. Extrapolating knowledge and concepts from neural development will have significant implications for designing better strategies with which to treat GBM.

ID proteins regulate diverse aspects of cancer progression and provide novel therapeutic opportunities

The inhibitor of differentiation (ID) proteins are helix-loop-helix transcriptional repressors with established roles in stem cell self-renewal, lineage commitment, and niche interactions. While deregulated expression of ID proteins in cancer was identified more than a decade ago, emerging evidence has revealed a central role for ID proteins in neoplastic progression of multiple tumor types that often mirrors their function in physiological stem and progenitor cells. ID proteins are required for the maintenance of cancer stem cells, self-renewal, and proliferation in a range of malignancies. Furthermore, ID proteins promote metastatic dissemination through their role in remodeling the tumor microenvironment and by promoting tumor-associated endothelial progenitor cell proliferation and mobilization. Here, we discuss the latest findings in this area and the clinical opportunities that they provide.

Elevated Id2 expression results in precocious neural stem cell depletion and abnormal brain development

Id2 is a helix-loop-helix transcription factor essential for normal development, and its expression is dysregulated in many human neurological conditions. Although it is speculated that elevated Id2 levels contribute to the pathogenesis of these disorders, it is unknown whether dysregulated Id2 expression is sufficient to perturb normal brain development or function. Here, we show that mice with elevated Id2 expression during embryonic stages develop microcephaly, and that females in particular are prone to generalized tonic-clonic seizures. Analyses of Id2 transgenic brains indicate that Id2 activity is highly cell context specific: elevated Id2 expression in naive neural stem cells (NSCs) in early neuroepithelium induces apoptosis and loss of NSCs and intermediate progenitors. Activation of Id2 in maturing neuroepithelium results in less severe phenotypes and is accompanied by elevation of G1 cyclin expression and p53 target gene expression. In contrast, activation of Id2 in committed intermediate progenitors has no significant phenotype. Functional analysis with Id2-overexpressing and Id2-null NSCs shows that Id2 negatively regulates NSC self-renewal in vivo, in contrast to previous cell culture experiments. Deletion of p53 function from Id2-transgenic brains rescues apoptosis and results in increased incidence of brain tumors. Furthermore, Id2 overexpression normalizes the increased self-renewal of p53-null NSCs, suggesting that Id2 activates and modulates the p53 pathway in NSCs. Together, these data suggest that elevated Id2 expression in embryonic brains can cause deregulated NSC self-renewal, differentiation, and survival that manifest in multiple neurological outcomes in mature brains, including microcephaly, seizures, and brain tumors.

Transient axonal glycoprotein-1 induces apoptosis-related gene expression without triggering apoptosis in U251 glioma cells

Previous studies show that transient axonal glycoprotein-1, a ligand of amyloid precursor protein, increases the secretion of amyloid precursor protein intracellular domain and is involved in apoptosis in Alzheimer's disease. In this study, we examined the effects of transient axonal glycoprotein-1 on U251 glioma cells. 3-(4,5-Dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide assay showed that transient axonal glycoprotein-1 did not inhibit the proliferation of U251 cells, but promoted cell viability. The terminal deoxynucleotidyl transferase dUTP nick end labeling assay showed that transient axonal glycoprotein-1 did not induce U251 cell apoptosis. Real-time PCR revealed that transient axonal glycoprotein-1 substantially upregulated levels of amyloid precursor protein intracellular C-terminal domain, and p53 and epidermal growth factor receptor mRNA expression. Thus, transient axonal glycoprotein-1 increased apoptosis-related gene expression in U251 cells without inducing apoptosis. Instead, transient axonal glycoprotein-1 promoted the proliferation of these glioma cells.

The ID proteins: master regulators of cancer stem cells and tumour aggressiveness

Inhibitor of DNA binding (ID) proteins are transcriptional regulators that control the timing of cell fate determination and differentiation in stem and progenitor cells during normal development and adult life. ID genes are frequently deregulated in many types of human neoplasms, and they endow cancer cells with biological features that are hijacked from normal stem cells. The ability of ID proteins to function as central 'hubs' for the coordination of multiple cancer hallmarks has established these transcriptional regulators as therapeutic targets and biomarkers in specific types of human tumours.

Id2 mediates oligodendrocyte precursor cell maturation arrest and is tumorigenic in a PDGF-rich microenvironment

Maturation defects occurring in adult tissue progenitor cells have the potential to contribute to tumor development; however, there is little experimental evidence implicating this cellular mechanism in the pathogenesis of solid tumors. Inhibitor of DNA-binding 2 (Id2) is a transcription factor known to regulate the proliferation and differentiation of primitive stem and progenitor cells. Id2 is derepressed in adult tissue neural stem cells (NSC) lacking the tumor suppressor Tp53 and modulates their proliferation. Constitutive expression of Id2 in differentiating NSCs resulted in maturation-resistant oligodendroglial precursor cells (OPC), a cell population implicated in the initiation of glioma. Mechanistically, Id2 overexpression was associated with inhibition of the Notch effector Hey1, a bHLH transcription factor that we here characterize as a direct transcriptional repressor of the oligodendroglial lineage determinant Olig2. Orthotopic inoculation of NSCs with enhanced Id2 expression into brains of mice engineered to express platelet-derived growth factor in the central nervous system resulted in glioma. These data implicate a mechanism of altered NSC differentiation in glioma development and characterize a novel mouse model that reflects key characteristics of the recently described proneural subtype of glioblastoma multiforme. Such findings support the emerging concept that the cellular and molecular characteristics of tumor cells are linked to the transformation of distinct subsets of adult tissue progenitors.

Ovarian cancer stem cell-specific gene expression profiling and targeted drug prescreening

Cancer stem cells, with unlimited self-renewal potential and other stem cell characteristics, occur in several types of cancer, including ovarian cancer (OvC). Although CSCs can cause tumor initiation, malignant proliferation, relapse and multi-drug resistance, ways to eliminate them remain unknown. In the present study, we compared ovarian cancer stem cell (OVCSC) expression profiles in normal ovarian surface epithelium and ovarian cells from patients with advanced disease to identify key pathways and specific molecular signatures involved in OVC progression and to prescreen candidate small-molecule compounds with anti-OVCSC activity. Comparison of genome-wide expression profiles of OvC stemness groups with non-stemness controls revealed 6495, 1347 and 509 differentially expressed genes in SDC, SP1 and SP2 groups, respectively, with a cut-off of fold-change set at >1.5 and P<0.05. NAB1 and NPIPL1 were commonly upregulated whereas PROS1, GREB1, KLF9 and MTUS1 were commonly downregulated in all 3 groups. Most differentially expressed genes consistently clustered with molecular functions such as protein receptor binding, kinase activity and chemo-repellent activity. These genes regulate cellular components such as centrosome, plasma membrane receptors, and basal lamina, and may participate in biological processes such as cell cycle regulation, chemoresistance and stemness induction. Key Kyoto Encyclopedia of Genes and Genomes (KEGG) pathways such as ECM receptor, ErbB signaling, endocytosis and adherens junction pathways were enriched. Gene co-expression extrapolation screening by the Connectivity Map revealed several small-molecule compounds (such as SC-560, disulfiram, thapsigargin, esculetin and cinchonine) with potential anti-OVCSC properties targeting OVCSC signature genes. We identified several key CSC features and specific regulation networks in OVCSCs and predicted several small molecules with potential anti-OVCSC pharmacological properties, which may aid the development of OVCSC-specific drugs.

Id proteins: small molecules, mighty regulators

The family of inhibitor of differentiation (Id) proteins is a group of evolutionarily conserved molecules, which play important regulatory roles in organisms ranging from Drosophila to humans. Id proteins are small polypeptides harboring a helix-loop-helix (HLH) motif, which are best known to mediate dimerization with other basic HLH proteins, primarily E proteins. Because Id proteins do not possess the basic amino acids adjacent to the HLH motif necessary for DNA binding, Id proteins inhibit the function of E protein homodimers, as well as heterodimers between E proteins and tissue-specific bHLH proteins. However, Id proteins have also been shown to have E protein-independent functions. The Id genes are broadly but differentially expressed in a variety of cell types. Transcription of the Id genes is controlled by transcription factors such as C/EBPβ and Egr as well as by signaling pathways triggered by different stimuli, which include bone morphogenic proteins, cytokines, and ligands of T cell receptors. In general, Id proteins are capable of inhibiting the differentiation of progenitors of different cell types, promoting cell-cycle progression, delaying cellular senescence, and facilitating cell migration. These properties of Id proteins enable them to play significant roles in stem cell maintenance, vasculogenesis, tumorigenesis and metastasis, the development of the immune system, and energy metabolism. In this review, we intend to highlight the current understanding of the function of Id proteins and discuss gaps in our knowledge about the mechanisms whereby Id proteins exert their diverse effects in multiple cellular processes.

Upregulated H19 contributes to bladder cancer cell proliferation by regulating ID2 expression

Long noncoding RNAs have been shown to have important regulatory roles in cancer biology, and long noncoding RNA 19 (H19) is essential for human tumor growth. However, little is known about how abnormal expression of H19 contributes to bladder cancer cell proliferation. In this study, we first evaluated the expression of H19 in bladder cancer tissues by real-time PCR, and defined the biological functions. We found that H19 expression levels were remarkably increased in bladder cancer tissues as compared with adjacent normal control tissue, and forced expression of H19 promoted bladder cancer cell proliferation in vitro. Inhibitor of DNA binding/differentiation 2 (ID2) expression levels were upregulated in bladder cancer tissues and in bladder cancer cells. A significant positive correlation was observed between H19 levels and ID2 levels in vivo. We further demonstrated that overexpression of H19 resulted in a significant increase in the expression of ID2, whereas H19 knockdown decreased ID2 expression in vitro. Gain-of-function and loss-of-function studies demonstrated that upregulated H19 increased bladder cancer cell proliferation by increasing ID2 expression. In conclusion, upregulated H19 increases bladder cancer growth by regulating ID2 expression, and thus may be helpful in the development of effective treatment strategies for bladder cancer.

p53 regulates neural stem cell proliferation and differentiation via BMP-Smad1 signaling and Id1

Neural stem cells (NSCs) play essential roles in nervous system development and postnatal neuroregeneration and their deregulation underlies the development of neurodegenerative disorders. Yet how NSC proliferation and differentiation are controlled is not fully understood. Here we present evidence that tumor suppressor p53 regulates NSC proliferation and differentiation via the bone morphogenetic proteins (BMP)-Smad1 pathway and its target gene inhibitor of DNA binding 1 (Id1). p53 deficiency led to increased neurogenesis in vivo, and biased neuronal differentiation and augmented NSC proliferation of ex vivo NSCs. This is accompanied by elevated Smad1 expression/activation in the brain and NSC, which contributes to accelerated neuronal differentiation of p53(-/-) NSCs. p53 deficiency also leads to upregulation of Id1, whose expression is repressed by p53 in BMP-Smad1-dependent and -independent manners. Elevated Id1 expression contributes to augmented proliferation and, unexpectedly, accelerated neuronal differentiation of p53(-/-) NSCs as well. This study reveals a molecular mechanism by which tumor suppressor p53 controls NSC proliferation and differentiation and establishes a connection between p53 and Id1.

Therapeutic targeting of the p53 pathway in cancer stem cells

INTRODUCTION

Cancer stem cells (CSCs) are a high profile drug target for cancer therapeutics due to their indispensable role in cancer progression, maintenance and therapeutic resistance. Restoring wild-type (WT) p53 function is an attractive new therapeutic approach for the treatment of cancer due to the well-described powerful tumor suppressor function of p53. As emerging evidence intimately links p53 and stem cell biology, this approach also provides an opportunity to target CSCs.

AREAS COVERED

This review covers the therapeutic approaches to restore the function of WT p53, cancer and normal stem cell biology in relation to p53 and the downstream effects of p53 on CSCs.

EXPERT OPINION

The restoration of WT p53 function by targeting p53 directly, its interacting proteins or its family members holds promise as a new class of cancer therapies. This review examines the impact that such therapies may have on normal and CSCs based on the current evidence linking p53 signaling with these populations.

A new module in neural differentiation control: two microRNAs upregulated by retinoic acid, miR-9 and -103, target the differentiation inhibitor ID2

The transcription factor ID2 is an important repressor of neural differentiation strongly implicated in nervous system cancers. MicroRNAs (miRNAs) are increasingly involved in differentiation control and cancer development. Here we show that two miRNAs upregulated on differentiation of neuroblastoma cells – miR-9 and miR-103 – restrain ID2 expression by directly targeting the coding sequence and 3′ untranslated region of the ID2 encoding messenger RNA, respectively. Notably, the two miRNAs show an inverse correlation with ID2 during neuroblastoma cell differentiation induced by retinoic acid. Overexpression of miR-9 and miR-103 in neuroblastoma cells reduces proliferation and promotes differentiation, as it was shown to occur upon ID2 inhibition. Conversely, an ID2 mutant that cannot be targeted by either miRNA prevents retinoic acid-induced differentiation more efficient than wild-type ID2. These findings reveal a new regulatory module involving two microRNAs upregulated during neural differentiation that directly target expression of the key differentiation inhibitor ID2, suggesting that its alteration may be involved in neural cancer development.

Didymin induces apoptosis by inhibiting N-Myc and upregulating RKIP in neuroblastoma

Neuroblastomas arise from the neural crest cells and represent the most common solid tumors outside the nervous system in children. The amplification of N-Myc plays a primary role in the pathogenesis of neuroblastomas, whereas acquired mutations of p53 lead to refractory and relapsed cases of neuroblastomas. In this regard, dietary compounds which can target N-Myc and exert anticancer effects independent of p53 status acquire significance in the management of neuroblastomas. Hence, we investigated the anticancer properties of the flavonoid didymin in neuroblastomas. Didymin effectively inhibited proliferation and induced apoptosis irrespective of p53 status in neuroblastomas. Didymin downregulated phosphoinositide 3-kinase, pAkt, Akt, vimentin, and upregulated RKIP levels. Didymin induced G(2)/M arrest along with decreasing the levels of cyclin D1, CDK4, and cyclin B1. Importantly, didymin inhibited N-Myc as confirmed at protein, mRNA, and transcriptional level by promoter-reporter assays. High-performance liquid chromatography analysis of didymin-treated (2 mg/kg b.w.) mice serum revealed effective oral absorption with free didymin concentration of 2.1 μmol/L. Further in vivo mice xenograft studies revealed that didymin-treated (2 mg/kg b.w.) animals had significant reductions in tumors size compared with controls. Didymin strongly inhibited the proliferation (Ki67) and angiogenesis (CD31) markers, as well as N-Myc expression, as revealed by the histopathologic examination of paraffin-embedded section of resected tumors. Collectively, our in vitro and in vivo studies elucidated the anticancer properties and mechanisms of action of a novel, orally active, and palatable flavonoid didymin, which makes it a potential new approach for neuroblastoma therapy (NANT) to target pediatric neuroblastomas.