Inhibition of an erythroid differentiation switch by the helix-loop-helix protein Id1

Transcription factor 3 promotes migration and invasion potential and maintains cancer stemness by activating ID1 expression in esophageal squamous cell carcinoma

Transcription factor 3 (TCF3) is a member of the basic Helix - Loop - Helix (bHLH) transcription factor (TF) family and is encoded by the TCF3 gene (also known as E2A). It has been shown that TCF3 functions as a key transcription factor in the pathogenesis of several human cancers and plays an important role in stem cell maintenance and carcinogenesis. However, the effect of TCF3 in the progression of esophageal squamous cell carcinoma (ESCC) is poorly known. In our study, TCF3 was found to express highly and correlated with cancer stage and prognosis. TCF3 was shown to promote ESCC invasion, migration, and drug resistance both from the results of in vivo and in vitro assays. Moreover, further studies suggested that TCF3 played these roles through transcriptionally regulating Inhibitor of DNA binding 1(ID1). Notably, we also found that TCF3 or ID1 was associated with ESCC stemness. Furthermore, TCF3 was correlated with the expression of cancer stemness markers CD44 and CD133. Therefore, maintaining cancer stemness might be the underlying mechanism that TCF3 transcriptionally regulated ID1 and further promoted ESCC progression and drug resistance.

ID1 marks the tumorigenesis of pancreatic ductal adenocarcinoma in mouse and human

Pancreatic Ductal Adenocarcinoma (PDAC) is a deadly disease that has an increasing death rate but no effective treatment to now. Although biological and immunological hallmarks of PDAC have been frequently reported recently, early detection and the particularly aggressive biological features are the major challenges remaining unclear. In the current study, we retrieved multiple scRNA-seq datasets and illustrated the genetic programs of PDAC development in genetically modified mouse models. Notably, the transcription levels of Id1 were elevated specifically along with the PDAC development. Pseudotime trajectory analysis revealed that Id1 was closely correlated with the malignancy of PDAC. The gene expression patterns of human PDAC cells were determined by the comparative analysis of the scRNA-seq data on human PDAC and normal pancreas tissues. ID1 levels in human PDAC cancer cells were dramatically increased compared to normal epithelial cells. ID1 deficiency in vitro significantly blunt the invasive tumor-formation related phenotypes. IPA analysis on the differentially expressed genes suggested that EIF2 signaling was the core pathway regulating the development of PDAC. Blocking EFI2 signaling remarkably decreased the expression of ID1 and attenuated the tumor-formation related phenotypes. These observations confirmed that ID1 was regulated by EIF2 signaling and was the critical determinator of PDAC development and progression. This study suggests that ID1 is a potential malignant biomarker of PDAC in both mouse models and human and detecting and targeting ID1 may be a promising strategy to treat or even rescue PDAC.

Inhibitor of DNA binding proteins revealed as orchestrators of steady state, stress and malignant hematopoiesis

Adult mammalian hematopoiesis is a dynamic cellular process that provides a continuous supply of myeloid, lymphoid, erythroid/megakaryocyte cells for host survival. This process is sustained by regulating hematopoietic stem cells (HSCs) quiescence, proliferation and activation under homeostasis and stress, and regulating the proliferation and differentiation of downstream multipotent progenitor (MPP) and more committed progenitor cells. Inhibitor of DNA binding (ID) proteins are small helix-loop-helix (HLH) proteins that lack a basic (b) DNA binding domain present in other family members, and function as dominant-negative regulators of other bHLH proteins (E proteins) by inhibiting their transcriptional activity. ID proteins are required for normal T cell, B cell, NK and innate lymphoid cells, dendritic cell, and myeloid cell differentiation and development. However, recent evidence suggests that ID proteins are important regulators of normal and leukemic hematopoietic stem and progenitor cells (HSPCs). This chapter will review our current understanding of the function of ID proteins in HSPC development and highlight future areas of scientific investigation.

Inhibitor of Differentiation 1 (Id1) in Cancer and Cancer Therapy

The inhibitor of DNA binding (Id) proteins are regulators of cell cycle and cell differentiation. Of all Id family proteins, Id1 is mostly linked to tumorigenesis, cellular senescence as well as cell proliferation and survival. Id1 is a stem cell-like gene more than a classical oncogene. Id1 is overexpressed in numerous types of cancers and exerts its promotion effect to these tumors through different pathways. Briefly, Id1 was found significantly correlated with EMT-related proteins, K-Ras signaling, EGFR signaling, BMP signaling, PI3K/Akt signaling, WNT and SHH signaling, c-Myc signaling, STAT3 signaling, RK1/2 MAPK/Egr1 pathway and TGF-β pathway, etc. Id1 has potent effect on facilitating tumorous angiogenesis and metastasis. Moreover, high expression of Id1 plays a facilitating role in the development of drug resistance, including chemoresistance, radiation resistance and resistance to drugs targeting angiogenesis. However, controversial results were also obtained. Overall, Id1 represent a promising target of anti-tumor therapeutics based on its potent promotion effect to cancer. Numerous drugs were found exerting their anti-tumor function through Id1-related signaling pathways, such as fucoidan, berberine, tetramethylpyrazine, crizotinib, cannabidiol and vinblastine.

Yap1 promotes proliferation of transiently amplifying stress erythroid progenitors during erythroid regeneration

In contrast to steady-state erythropoiesis, which generates new erythrocytes at a constant rate, stress erythropoiesis rapidly produces a large bolus of new erythrocytes in response to anemic stress. In this study, we illustrate that Yes-associated protein (Yap1) promotes the rapid expansion of a transit-amplifying population of stress erythroid progenitors in vivo and in vitro. Yap1-mutated erythroid progenitors failed to proliferate in the spleen after transplantation into lethally irradiated recipient mice. Additionally, loss of Yap1 impaired the growth of actively proliferating erythroid progenitors in vitro. This role in proliferation is supported by gene expression profiles showing that transiently amplifying stress erythroid progenitors express high levels of genes associated with Yap1 activity and genes induced by Yap1. Furthermore, Yap1 promotes the proliferation of stress erythroid progenitors in part by regulating the expression of key glutamine-metabolizing enzymes. Thus, Yap1 acts as an erythroid regulator that coordinates the metabolic status with the proliferation of erythroid progenitors to promote stress erythropoiesis.

Expansion and maintenance of primary corneal epithelial stem/progenitor cells by inhibition of TGFβ receptor I-mediated signaling

Transforming growth factor β (TGFβ) signaling is one of the most important signaling pathways regulating cell behavior in ocular tissues. Its functions are mainly linked to tissue fibrosis and inflammatory responses in ophthalmology. In epithelial cells, however, the growth inhibitory activity of TGFβ was reported in both non-ocular and ocular tissues. Since TGFβ is a bifunctional regulator that either inhibits or stimulates cell proliferation according to the specific context, we examined the effect of inhibition of TGFβ receptor (TβR) I-mediated signaling on primary corneal epithelial cells (CECs) in serum- and feeder-free conditions. The mouse CECs were isolated from the eyeballs of 6-8 weeks old female C57BL/6 mice using dispase and trypsin separately, cultivated in defined Keratinocyte serum-free medium (KSFM) with supplements (the complete medium) without feeder layer. Cells were divided into three groups, those cultured in complete medium additionally supplemented with 10 μM SB-431542, a specific inhibitor of TβR-I, were SB-CECs; those cultured in complete medium additionally supplemented with 10 ng/ml SRI-011381, a TGF-beta signaling agonist, were SRI-CECs; those cultured in complete medium without SB-431542 or SRI-011381 were control CECs. The growth rate and morphology were analyzed by light microscopy. The identity and stemness of cells was investigated through marker staining of p63, inhibitor of differentiation 1 (ID1), cytokeratin 12 (K12), cytokeratin 14 (K14), PAX6, pSmad3, alpha smooth muscle Actin (αSMA) and E-cadherin (E-cad); Real-time quantitative (RT-PCR) analysis of p63; Western blot analysis of ID1; as well as colony forming assay, sphere forming assay, healing wound in vitro assay and air-lifting interface assay. The results showed SB-CECs subcultured steadily, achieved sustained expansion, and expanded almost thrice faster than control CECs. Expanded SB-CECs exhibited smaller and more compact morphology, up-regulated p63 and ID1, as well as better performed colony-forming capacity, sphere-forming capacity, in vitro wound healing capacity, and the capacity to stratify and differentiate on air-lifting interface. Preliminary tests on human limbal epithelial cells (HLECs) showed the same results as mouse CECs. Interestingly, the ID1 expression pattern was almost identical to p63, the typical marker for corneal epithelial stem/progenitor cell (CESC/CEPC), in cultured CECs and normal corneal sections. Since ID1 has been proven to be regulated negatively by TGFβ signaling in epithelial cells and plays a role in blocking cell differentiation, its derepression by TβR-I inhibitor could be, at least in part, the underlying cause of CESC/CEPC expansion and the synchronously up-regulated expression of p63 in SB-CECs. In conclusion, inhibition of TβR-I-mediated signaling, CESCs/CEPCs achieved efficient long-term expansion in a feeder- and serum-free condition in vitro. And derepression of ID1 could be the underlying cause. Meanwhile, ID1 could serve as a marker for CESC/CEPC. These results may advance the basic and clinical CESC/CEPC research.

Combined Id1 and Id3 Deletion Leads to Severe Erythropoietic Disturbances

The Inhibitor of DNA Binding (Id) proteins play a crucial role in regulating hematopoiesis and are known to interact with E proteins and the bHLH family of transcription factors. Current efforts seek to elucidate the individual roles of Id members in regulating hematopoietic development and specification. However, the nature of their functional redundancies remains elusive since ablation of multiple Id genes is embryonically lethal. We developed a model to test this compensation in the adult. We report that global Id3 ablation with Tie2Cre-mediated conditional ablation of Id1 in both hematopoietic and endothelial cells (Id cDKO) extends viability to 1 year but leads to multi-lineage hematopoietic defects including the emergence of anemia associated with defective erythroid development, a novel phenotype unreported in prior single Id knockout studies. We observe decreased cell counts in the bone marrow and splenomegaly to dimensions beyond what is seen in single Id knockout models. Transcriptional dysregulation of hematopoietic regulators observed in bone marrow cells is also magnified in the spleen. E47 protein levels were elevated in Id cDKO bone marrow cell isolates, but decreased in the erythroid lineage. Chromatin immunoprecipitation (ChIP) studies reveal increased occupancy of E47 and GATA1 at the promoter regions of β-globin and E2A. Bone marrow transplantation studies highlight the importance of intrinsic Id signals in maintaining hematopoietic homeostasis while revealing a strong extrinsic influence in the development of anemia. Together, these findings demonstrate that loss of Id compensation leads to dysregulation of the hematopoietic transcriptional network and multiple defects in erythropoietic development in adult mice.

Genomic and transcriptome analysis revealing an oncogenic functional module in meningiomas

OBJECT

Meningiomas are among the most common primary adult brain tumors. Although typically benign, roughly 2%-5% display malignant pathological features. The key molecular pathways involved in malignant transformation remain to be determined.

METHODS

Illumina expression microarrays were used to assess gene expression levels, and Illumina single-nucleotide polymorphism arrays were used to identify copy number variants in benign, atypical, and malignant meningiomas (19 tumors, including 4 malignant ones). The authors also reanalyzed 2 expression data sets generated on Affymetrix microarrays (n = 68, including 6 malignant ones; n = 56, including 3 malignant ones). A weighted gene coexpression network approach was used to identify coexpression modules associated with malignancy.

RESULTS

At the genomic level, malignant meningiomas had more chromosomal losses than atypical and benign meningiomas, with average length of 528, 203, and 34 megabases, respectively. Monosomic loss of chromosome 22 was confirmed to be one of the primary chromosomal level abnormalities in all subtypes of meningiomas. At the transcriptome level, the authors identified 23 coexpression modules from the weighted gene coexpression network. Gene functional enrichment analysis highlighted a module with 356 genes that was highly related to tumorigenesis. Four intramodular hubs within the module (GAB2, KLF2, ID1, and CTF1) were oncogenic in other cancers such as leukemia. A putative meningioma tumor suppressor MN1 was also identified in this module with differential expression between malignant and benign meningiomas.

CONCLUSIONS

The authors' genomic and transcriptome analysis of meningiomas provides novel insights into the molecular pathways involved in malignant transformation of meningiomas, with implications for molecular heterogeneity of the disease.

Corepressor Rcor1 is essential for murine erythropoiesis

The corepressor Rcor1 has been linked biochemically to hematopoiesis, but its function in vivo remains unknown. We show that mice deleted for Rcor1 are profoundly anemic and die in late gestation. Definitive erythroid cells from mutant mice arrest at the transition from proerythroblast to basophilic erythroblast. Remarkably, Rcor1 null erythroid progenitors cultured in vitro form myeloid colonies instead of erythroid colonies. The mutant proerythroblasts also aberrantly express genes of the myeloid lineage as well as genes typical of hematopoietic stem cells (HSCs) and/or progenitor cells. The colony-stimulating factor 2 receptor β subunit (Csf2rb), which codes for a receptor implicated in myeloid cytokine signaling, is a direct target for both Rcor1 and the transcription repressor Gfi1b in erythroid cells. In the absence of Rcor1, the Csf2rb gene is highly induced, and Rcor1(-/-) progenitors exhibit CSF2-dependent phospho-Stat5 hypersensitivity. Blocking this pathway can partially reduce myeloid colony formation by Rcor1-deficient erythroid progenitors. Thus, Rcor1 promotes erythropoiesis by repressing HSC and/or progenitor genes, as well as the genes and signaling pathways that lead to myeloid cell fate.

ID2 and ID3 protein expression mirrors granulopoietic maturation and discriminates between acute leukemia subtypes

The inhibitors of DNA binding (ID) inhibit basic helix-loop-helix transcription factors and thereby guide cellular differentiation and proliferation. To elucidate the involvement of IDs in hematopoiesis and acute leukemias (AL), we analyzed ID2 and ID3 expression in hematopoiesis and leukemic blasts in bone marrow biopsies (BMB). BMB of healthy stem cell donors (n = 19) and BMB of patients with acute myeloid leukemia (AML) with myelodysplasia-related changes (AML-MD; n = 19), de novo AML (n = 20), B-acute lymphoblastic leukemia (B-ALL) (n = 23), T-ALL (n = 19), were immunohistochemically stained for ID2 and ID3 expression. The expression patterns were evaluated and quantified for each hematopoietic lineage and each leukemia subtype. In normal BMB, immature granulopoiesis showed weak ID2 and strong ID3 expression, which was lost during maturation (p < 0.001). Erythropoiesis remained negative for ID2/3 (p < 0.001). ID2/3 expression differed between immature granulopoiesis and leukemic blasts (p < 0.001). Moreover, differential ID2/3 expression was seen between AL subgroups: AML, especially AML-MD, had more ID2- (p < 0.001) and ID3-positive (p < 0.001) blasts than ALL. We show a comprehensive in situ picture of ID2/3 expression in hematopoiesis and AL. Morphologically, ID2/3 proteins seem to be involved in the granulopoietic maturation. Importantly, the distinct ID2/3 expression patterns in AL indicate a specific deregulation of ID2/3 in the various types of AL and may support subtyping of AL.

Bone morphogenetic protein 2 inhibits neurite outgrowth of motor neuron-like NSC-34 cells and up-regulates its type II receptor

Bone morphogenetic proteins (BMPs) regulate several aspects of neuronal behavior. For instance, BMP-2 has the ability to modulate, either positively or negatively, the outgrowth of neuronal processes in diverse cell types. In Drosophila motor neurons, the BMP type II receptor (BMPRII) homolog wishful thinking plays crucial roles on neuromuscular synaptogenesis signaling through Smad-dependent and Smad-independent pathways. However, a role for BMP signaling at the vertebrate neuromuscular junction has not been addressed. Herein, we have analyzed the expression of BMPRII and the effect of BMP-2 during the morphological differentiation of motor neuron-like NSC-34 cells. Our data indicate that BMPRII is up-regulated and becomes accumulated in somas and growth cones upon motor neuronal differentiation. BMP-2 inhibits the differentiation of NSC-34 cells, an effect that correlates with activation of a Smad-dependent pathway, induction of the inhibitory Id1 transcription factor, and down-regulation of the neurogenic factor Mash1. BMP-2 also activates effectors of Smad-independent pathways. Remarkably, BMP-2 treatment significantly increases the expression of BMPRII. Our findings provide the first evidence to suggest a role for BMP pathways on the differentiation of motor neurons leading to successful assembly and/or regeneration of the vertebrate neuromuscular synapse.

Induction of Id-1 by FGF-2 involves activity of EGR-1 and sensitizes neuroblastoma cells to cell death

Inhibitor of differentiation-1 (Id-1) is a member of helix-loop-helix (HLH) family of proteins that regulate gene transcription through their inhibitory binding to basic-HLH transcription factors. Similarly to other members of this family, Id-1 is involved in the repression of cell differentiation and activation of cell growth. The dual function of Id-1, inhibition of differentiation, and stimulation of cell proliferation, might be interdependent, as cell differentiation is generally coupled with the exit from the cell cycle. Fibroblast growth factor-2 (FGF-2) has been reported to play multiple roles in different biological processes during development of the central nervous system (CNS). In addition, FGF-2 has been described to induce "neuronal-like" differentiation and trigger apoptosis in neuroblastoma SK-N-MC cells. Although regulation of Id-1 protein by several mitogenic factors is well-established, little is known about the role of FGF-2 in the regulation of Id-1. Using human neuroblastoma cell line, SK-N-MC, we found that treatment of these cells with FGF-2 resulted in early induction of both Id-1 mRNA and protein. The induction occurs within 1 h from FGF-2 treatment and is mediated by ERK1/2 pathway, which in turn stimulates expression of the early growth response-1 (Egr-1) transcription factor. We also demonstrate direct interaction of Egr-1 with Id-1 promoter in vitro and in cell culture. Finally, inhibition of Id-1 expression results in G(2) /M accumulation of FGF-2-treated cells and delayed cell death.

Id1 promotes lung cancer cell proliferation and tumor growth through Akt-related pathway

Overexpression of Id family proteins inhibits cell differentiation and enhances cell proliferation and invasiveness. Although Id1 is the Id family member mostly linked to tumorigenesis, its role in lung cancer is unclear. An elevated Id1 expression was observed in lung cancer cell lines as well as lung cancer tissues. Id1 overexpression increased cell proliferation while Id1 knockdown decreased cell proliferation, mostly through Akt-related pathway. Nude mice study further confirmed an increased tumor growth in Id1-overexpressing cells and a decreased tumor growth in Id1-knockdowned cells. In conclusion, inactivation of Id1 may provide a novel strategy for treatment of lung cancer patients.

ID1 and ID3 represent conserved negative regulators of human embryonic and induced pluripotent stem cell hematopoiesis

Mechanisms that govern hematopoietic lineage specification, as opposed to the expansion of committed hematopoietic progenitors, from human pluripotent stem cells (hPSCs) have yet to be fully defined. Here, we show that within the family of genes called inhibitors of differentiation (ID), ID1 and ID3 negatively regulate the transition from lineage-specified hemogenic cells to committed hematopoietic progenitors during hematopoiesis of both human embryonic stem cells (hESCs) and human induced pluripotent stem cell (hiPSCs). Upon hematopoietic induction of hPSCs, levels of ID1 and ID3 transcripts rapidly increase, peaking at the stage of hemogenic precursor emergence, and then exclusively decrease during subsequent hematopoietic commitment. Suppression of ID1 and ID3 expression in hemogenic precursors using specific small interfering RNAs augments differentiation into committed hematopoietic progenitors, with dual suppression of ID1 and ID3 further increasing hematopoietic induction compared with upon knockdown of each gene alone. This inhibitory role of ID1 and ID3 directly affects hemogenic precursors and is not dependent on non-hemogenic cells of other lineages within developing human embryoid bodies from hESCs or hiPSCs. Our study uniquely identifies ID1 and ID3 as negative regulators of the hPSC-hematopoietic transition from a hemogenic to a committed hematopoietic fate, and demonstrates that this is conserved between hESCs and hiPSCs.

Id1 represses osteoclast-dependent transcription and affects bone formation and hematopoiesis

BACKGROUND

The bone-bone marrow interface is an area of the bone marrow microenvironment in which both bone remodeling cells, osteoblasts and osteoclasts, and hematopoietic cells are anatomically juxtaposed. The close proximity of these cells naturally suggests that they interact with one another, but these interactions are just beginning to be characterized.

METHODOLOGY/PRINCIPAL FINDINGS

An Id1(-/-) mouse model was used to assess the role of Id1 in the bone marrow microenvironment. Micro-computed tomography and fracture tests showed that Id1(-/-) mice have reduced bone mass and increased bone fragility, consistent with an osteoporotic phenotype. Osteoclastogenesis and pit formation assays revealed that loss of Id1 increased osteoclast differentiation and resorption activity, both in vivo and in vitro, suggesting a cell autonomous role for Id1 as a negative regulator of osteoclast differentiation. Examination by flow cytometry of the hematopoietic compartment of Id1(-/-) mice showed an increase in myeloid differentiation. Additionally, we found increased expression of osteoclast genes, TRAP, Oscar, and CTSK in the Id1(-/-) bone marrow microenvironment. Lastly, transplantation of wild-type bone marrow into Id1(-/-) mice repressed TRAP, Oscar, and CTSK expression and activity and rescued the hematopoietic and bone phenotype in these mice.

CONCLUSIONS/SIGNIFICANCE

In conclusion, we demonstrate an osteoporotic phenotype in Id1(-/-) mice and a mechanism for Id1 transcriptional control of osteoclast-associated genes. Our results identify Id1 as a principal player responsible for the dynamic cross-talk between bone and bone marrow hematopoietic cells.

ID1 promotes expansion and survival of primary erythroid cells and is a target of JAK2V617F-STAT5 signaling

The discovery of JAK2V617F as an acquired mutation in the majority of patients with myeloproliferative disorders (MPDs) and the key role of the JAK2-STAT5 signaling cascade in normal hematopoiesis has focused attention on the downstream transcriptional targets of STAT5. Despite evidence of its vital role in normal erythropoiesis and its ability to recapitulate many of the features of myeloid malignancies, including the MPDs, few functionally validated targets of STAT5 have been described. Here we used a combination of comparative genomics and chromatin immunoprecipitation assays to identify ID1 as a novel target of the JAK2-STAT5 signaling axis in erythroid cells. STAT5 binds and transactivates a downstream enhancer of ID1, and ID1 expression levels correlate with the JAK2V617F mutation in both retrovirally transfected fetal liver cells and polycythemia vera patients. Knockdown and overexpression studies in a well-characterized erythroid differentiation assay from primary murine fetal liver cells demonstrated a survival-promoting action of ID1. This hitherto unrecognized function implicates ID1 in the expansion of erythroblasts during terminal differentiation and suggests that ID1 plays an important role in the pathogenesis of polycythemia vera. Furthermore, our findings contribute to an increasing body of evidence implicating ID proteins in a wider range of cellular functions than initially appreciated.

Cell-nonautonomous function of Id1 in the hematopoietic progenitor cell niche

Development of hematopoietic stem cells (HSCs) and their immediate progeny is maintained by the interaction with cells in the microenvironment. We found that hematopoiesis was dysregulated in Id1(-/-) mice. Although the frequency of HSCs in Id1(-/-) bone marrow was increased, their total numbers remained unchanged as the result of decreased bone marrow cellularity. In addition, the ability of Id1(-/-) HSCs to self-renew was normal, suggesting Id1 does not affect HSC function. Id1(-/-) progenitors showed increased cycling in vivo but not in vitro, suggesting cell nonautonomous mechanisms for the increased cycling. Id1(-/-) HSCs developed normally when transplanted into Id1(+/+) mice, whereas the development of Id1(+/+) HSCs was impaired in Id1(-/-) recipients undergoing transplantation and reproduced the hematologic features of Id1(-/-) mice, indicating that the Id1(-/-) microenvironment cannot support normal hematopoietic development. Id1(-/-) stromal cells showed altered production of cytokines in vitro, and cytokine levels were deregulated in vivo, which could account for the Id1(-/-) hematopoietic phenotypes. Thus, Id1 is required for regulating the hematopoietic progenitor cell niche but is dispensable for maintaining HSCs.

Id-1 expression in androgen-dependent prostate cancer is negatively regulated by androgen through androgen receptor

This study discovered that Id-1 expression in androgen-dependent prostate cancer decreased immediately after androgen deprivation but increased after longer androgen deprivation both in vivo and in vitro. Id-1 expression in androgen-independent LNCaP cells was about 6 fold as that in their parental cells. As was the case with LNCaP cells, when androgen receptor (AR) was introduced into AR-negative PC-3 cells, dihydrotestosterone inhibited while flutamide increased Id-1 expression. Thus, Id-1 expression in androgen-dependent prostate cancer was negatively regulated by androgen in a receptor-dependent way. The re-increased Id-1 might partially contribute to the emergence of androgen-independent prostate cancer after longer androgen deprivation therapy.

Id1 is a common downstream target of oncogenic tyrosine kinases in leukemic cells

Oncogenic tyrosine kinases, such as BCR-ABL, TEL-ABL, TEL-PDGFbetaR, and FLT3-ITD, play a major role in the development of hematopoietic malignancy. They activate many of the same signal transduction pathways. To identify the critical target genes required for transformation in hematopoietic cells, we used a comparative gene expression strategy in which selective small molecules were applied to 32Dcl3 cells that had been transformed to factor-independent growth by these respective oncogenic alleles. We identified inhibitor of DNA binding 1 (Id1), a gene involved in development, cell cycle, and tumorigenesis, as a common target of these oncogenic kinases. These findings were prospectively confirmed in cell lines and primary bone marrow cells engineered to express the respective tyrosine kinase alleles and were also confirmed in vivo in murine models of disease. Moreover, human AML cell lines Molm-14 and K562, which express the FLT3-ITD and BCR-ABL tyrosine kinases, respectively, showed high levels of Id1 expression. Antisense and siRNA based knockdown of Id1-inhibited growth of these cells associated with increased p27(Kip1) expression and increased sensitivity to Trail-induced apoptosis. These findings indicate that Id1 is an important target of constitutively activated tyrosine kinases and may be a therapeutic target for leukemias associated with oncogenic tyrosine kinases.

Id2 intrinsically regulates lymphoid and erythroid development via interaction with different target proteins

Inhibitors of DNA binding (Id) family members are key regulators of cellular differentiation and proliferation. These activities are related to the ability of Id proteins to antagonize E proteins and other transcription factors. As negative regulators of E proteins, Id proteins have been implicated in lymphocyte development. Overexpression of Id1, Id2, or Id3 has similar effects on lymphocyte development. However, which Id protein plays a physiologic role during lymphocyte development is not clear. By analyzing Id2 knock-out mice and retroviral transduced hematopoietic progenitors, we demonstrated that Id2 is an intrinsic negative regulator of B-cell development. Hematopoietic progenitor cells overexpressing Id2 did not reconstitute B-cell development in vivo, which resembled the phenotype of E2A null mice. The B-cell population in bone marrow was significantly expanded in Id2 knock-out mice compared with their wild-type littermates. Knock-down of Id2 by shRNA in hematopoietic progenitor cells promoted B-cell differentiation and induced the expression of B-cell lineage-specific genes. These data identified Id2 as a physiologically relevant regulator of E2A during B lymphopoiesis. Furthermore, we identified a novel Id2 function in erythroid development. Overexpression of Id2 enhanced erythroid development, and decreased level of Id2 impaired normal erythroid development. Id2 regulation of erythroid development is mediated via interacting with transcription factor PU.1 and modulating PU.1 and GATA-1 activities. We conclude that Id2 regulates lymphoid and erythroid development via interaction with different target proteins.

A critical role for the co-repressor N-CoR in erythroid differentiation and heme synthesis

Co-repressor N-CoR (nuclear receptor co-repressor) has important roles in different biological processes, including proliferation, differentiation and development. Mutant mice lacking N-CoR are embryonically lethal and appear to die from anemia owing to defects in definitive erythropoiesis. However, the underlying molecular mechanisms of N-CoR-mediated erythroid differentiation are largely unknown. Using the human erythroleukemic K562 cell line, which can be chemically induced to differentiate into either erythroid or megakaryocytic lineages depending on the inducers used, we have investigated the role of N-CoR in erythroid differentiation. We show that knockdown of N-CoR either transiently (siRNA) or permanently (shRNA) impairs the cytosine arabinoside (Ara-C)- but not hemin-induced erythroid differentiation of K562 cells. RT-PCR analysis reveals that N-CoR is required for induction by Ara-C of 5-aminolevulinate synthase (ALA-S2), a key enzyme involved in heme biosynthesis. Furthermore, the amount of N-CoR proteins increases significantly during Ara-C-induced K562 differentiation, apparently through a post-transcriptional mechanism. Consistent with the data from N-CoR-null mice, N-CoR is not required for the differentiation of K562 cells into megakaryocytic lineages, induced by phorbol 12-myristate 13-acetate. Thus, our in vitro study confirms a role for N-CoR in erythroid differentiation and reveals for the first time that N-CoR is required for the induction of a key enzyme involved in heme synthesis.

Insulin receptor substrate (IRS)-1 regulates murine embryonic stem (mES) cells self-renewal

Mouse embryonic stem (mES) cells are pluripotent cells that can be propagated in vitro with leukemia inhibitory factor (LIF) and serum. Intracellular signaling by LIF is principally mediated by activation of STAT-3, although additional pathways for self-renewal have been described. Here, we identified a novel role for Insulin receptor substrate-1 (IRS-1) as a critical factor in mES cells self-renewal and differentiation. IRS-1 is expressed and tyrosyl phosphorylated during mES cells self-renewal. Differentiation of mES cells, by LIF withdrawal, is associated with a marked reduction in IRS-1 expression. Targeting of IRS-1 by si-IRS-1 results in a severe reduction of Oct-4 protein expression and alkaline phosphatase activity, markers of undifferentiated mES cells. IRS-1 targeting does not interfere with LIF-induced STAT-3 phosphorylation, but negatively affects protein kinase B (PKB/AKT) and glycogen synthase kinase-3 (GSK-3beta) phosphorylation, which are downstream effectors of the LIF-mediated PI3K signaling cascade. Targeting of IRS-1 also results in a marked down regulation of Id-1 and Id-2 proteins expression, which are important components for self-renewal of ES cells. Conversely, over expression of IRS-1 inhibits mES cell differentiation. Taken together, these results suggest that expression and activity of IRS-1 are critical to the maintenance of the self-renewal program in mES cells.

Lyl1 interacts with CREB1 and alters expression of CREB1 target genes

The basic helix-loop-helix (bHLH) transcription factor family contains key regulators of cellular proliferation and differentiation as well as the suspected oncoproteins Tal1 and Lyl1. Tal1 and Lyl1 are aberrantly over-expressed in leukemia as a result of chromosomal translocations, or other genetic or epigenetic events. Protein-protein and protein-DNA interactions described so far are mediated by their highly homologous bHLH domains, while little is known about the function of other protein domains. Hetero-dimers of Tal1 and Lyl1 with E2A or HEB, decrease the rate of E2A or HEB homo-dimer formation and are poor activators of transcription. In vitro, these hetero-dimers also recognize different binding sites from homo-dimer complexes, which may also lead to inappropriate activation or repression of promoters in vivo. Both mechanisms are thought to contribute to the oncogenic potential of Tal1 and Lyl1. Despite their bHLH structural similarity, accumulating evidence suggests that Tal1 and Lyl1 target different genes. This raises the possibility that domains flanking the bHLH region, which are distinct in the two proteins, may participate in target recognition. Here we report that CREB1, a widely-expressed transcription factor and a suspected oncogene in acute myelogenous leukemia (AML) was identified as a binding partner for Lyl1 but not for Tal1. The interaction between Lyl1 and CREB1 involves the N terminal domain of Lyl1 and the Q2 and KID domains of CREB1. The histone acetyl-transferases p300 and CBP are recruited to these complexes in the absence of CREB1 Ser 133 phosphorylation. In the Id1 promoter, Lyl1 complexes direct transcriptional activation. We also found that in addition to Id1, over-expressed Lyl1 can activate other CREB1 target promoters such as Id3, cyclin D3, Brca1, Btg2 and Egr1. Moreover, approximately 50% of all gene promoters identified by ChIP-chip experiments were jointly occupied by CREB1 and Lyl1, further strengthening the association of Lyl1 with Cre binding sites. Given the newly recognized importance of CREB1 in AML, the ability of Lyl1 to modulate promoter responses to CREB1 suggests that it plays a role in the malignant phenotype by occupying different promoters than Tal1.

Interleukin-3 and erythropoietin cooperate in the regulation of the expression of erythroid-specific transcription factors during erythroid differentiation

OBJECTIVE

To characterize how interleukin-3 and erythropoietin regulate cell fate by modulating the expression of lineage-specific transcription factors.

METHODS

This study analyzed mRNA and protein levels, gene transcription rates, and mRNA and protein stabilities of erythroid-specific transcription factors in lineage-restricted cells derived from the 32D cell line cultured either in interleukin-3 or erythropoietin.

RESULTS

Erythroid 32D subclones expressed levels of Idl, Gata-2, and Scl comparable and levels of Eklf and Gata-1 higher than those expressed by myeloid subclones. While maintained in interleukin-3, erythroid cells remained immature despite their high expression of Gata-1, Gata-2, Scl, Eklf, and Idl. Switching the erythroid cells to erythropoietin induced cell maturation (within 48 hours) and reduced expression of Gata-2 and Idl (in 24 hours) but did not alter the expression of Gata-1. The effects of interleukin-3 were mostly mediated by increases in transcription rates (Scl and Gata-2), and that of erythropoietin was apparently due to increased mRNA and protein (Gata-1, Scl, and Eklf) stability. In particular, erythropoietin increased the stability of the processed and transcriptionally more active form of GATA-1 protein.

CONCLUSIONS

These results suggest that interleukin-3 and erythropoietin cooperate to establish the lineage-specific transcription factor milieu of erythroid cells: interleukin-3 regulates mainly gene transcription and erythropoietin consistently increases mRNA and protein stability.

Role of the basic helix-loop-helix protein ITF2 in the hormonal regulation of Sertoli cell differentiation

Sertoli cells are a post-mitotic terminally differentiated cell population that forms the seminiferous tubules in the adult testis and provides the microenvironment and structural support for developing germ cells. During pubertal development, Sertoli cells are responsive to follicle-stimulating hormone (FSH) to promote the expression of differentiated gene products. The basic helix-loop-helix (bHLH) and inhibitors of differentiation (Id) transcription factors are involved in the differentiation of a variety of cell lineages during development. Both bHLH and Id transcription factors have been identified in Sertoli cells. A yeast two-hybrid screen was conducted using a rat Sertoli cell cDNA library to identify bHLH dimerization partners for the Id1 transcription factor. The ubiquitous bHLH protein ITF2 (i.e., E2-2) was identified as one of the interacting partners. The current study investigates the expression and function of ITF2 in Sertoli cells. ITF2 was found to be ubiquitously expressed in all testicular cell types including germ cells, peritubular myoid cells, and Sertoli cells. Stimulation of cultured Sertoli cells with FSH or dibutryl cAMP resulted in a transient decrease in expression of ITF2 mRNA levels followed by a rise in expression with FSH treatment. ITF2 expression was at its highest in mid-pubertal 20-day-old rat Sertoli cells. ITF2 was found to directly bind to negative acting Id HLH proteins and positive acting bHLH proteins such as scleraxis. Transient overexpression of ITF2 protein in cultured Sertoli cells stimulated transferrin promoter activity, which is a marker of Sertoli cell differentiation. Co-transfections of ITF2 and Id proteins sequestered the inhibitory effects of the Id family of proteins. Observations suggest ITF2 can enhance FSH actions through suppressing the inhibitory actions of the Id family of proteins and increasing the actions of stimulatory bHLH proteins (i.e., scleraxis) in Sertoli cells.

Id1 restrains myeloid commitment, maintaining the self-renewal capacity of hematopoietic stem cells

Appropriate hematopoietic stem cell (HSC) self-renewal reflects the tight regulation of cell cycle entry and lineage commitment. Here, we show that Id1, a dominant-negative regulator of E protein transcription factors, maintains HSC self-renewal by preserving the undifferentiated state. Id1-deficient HSCs show increased cell cycling, by BrdU incorporation in vivo, but fail to efficiently self-renew, leading to low steady-state HSC numbers and premature exhaustion in serial bone marrow transplant assays. The increased cycling reflects the perturbed differentiation process, because Id1 null HSCs more readily commit to myeloid differentiation, with inappropriate expression of myeloerythroid-specific genes. Thus, Id1 appears to regulate the fate of HSCs by acting as a true inhibitor of differentiation.

Non-redundant inhibitor of differentiation (Id) gene expression and function in human prostate epithelial cells

BACKGROUND

The four Id (inhibitor of differentiation) proteins (Id1, Id2, Id3, and Id4) dimerize and neutralize the transcriptional activity of basic helix-loop-helix (bHLH) proteins. The Id proteins negatively regulate differentiation and promote proliferation hence the expression of specific subsets of Id proteins is high in many different types of cancers. However, the expression of all the Id isoforms and their potential function in specific cancer cell types is not known. In this study, the expression and function of all four Id isoforms in prostate cancer cell lines was investigated to gain a better understanding of the role of each Id isoform in normal prostate epithelial and prostate cancer cells.

METHODS

Id gene and protein expression was evaluated in the context of androgen response. The cellular function of Id isoforms was evaluated by targeted loss of function of Id genes.

RESULTS

The four Id isoforms are differentially expressed and regulated in normal human prostate epithelial cells versus prostate cancer cell lines DU145 and LNCaP. Id4 is present only in AR positive cells (normal and LNCaP) and its expression regulated by androgens. Loss of Id1 and Id3 expression by siRNA results in loss of proliferation. Loss of Id2 had no effect on proliferation but increased apoptosis.

CONCLUSIONS

A complex equilibrium between Id isoforms determines the cell fate. Id1 and Id3 target cellular proliferation, Id2 targets apoptosis, and Id4 may act as a potential tumor suppressor in prostate epithelial cells.

C/EBPalpha determines hematopoietic cell fate in multipotential progenitor cells by inhibiting erythroid differentiation and inducing myeloid differentiation

C/EBPalpha is an essential transcription factor required for myeloid differentiation. While C/EBPalpha can act as a cell fate switch to promote granulocyte differentiation in bipotential granulocyte-macrophage progenitors (GMPs), its role in regulating cell fate decisions in more primitive progenitors is not known. We found increased numbers of erythroid progenitors and erythroid cells in C/EBPalpha(-/-) fetal liver (FL). Also, enforced expression of C/EBPalpha in hematopoietic stem cells resulted in a loss of erythroid progenitors and an increase in myeloid cells by inhibition of erythroid development and inducing myeloid differentiation. Conditional expression of C/EBPalpha in murine erythroleukemia (MEL) cells induced myeloid-specific genes, while inhibiting erythroid-specific gene expression including erythropoietin receptor (EpoR), which suggests a novel mechanism to determine hematopoietic cell fate. Thus, C/EBPalpha functions in hematopoietic cell fate decisions by the dual actions of inhibiting erythroid and inducing myeloid gene expression in multipotential progenitors.

Absence of the transcription factor CCAAT enhancer binding protein alpha results in loss of myeloid identity in bcr/abl-induced malignancy

The lineage-determining transcription factor CCAAT enhancer binding protein alpha (C/EBPalpha) is required for myeloid differentiation. Decreased function or expression of C/EBPalpha is often found in human acute myeloid leukemia. However, the precise impact of C/EBPalpha deficiency on the maturation arrest in leukemogenesis is not well understood. To address this question, we used a murine transplantation model of a bcr/abl-induced myeloproliferative disease. The expression of bcr/abl in C/EBPalphapos fetal liver cells led to a chronic myeloid leukemia-like disease. Surprisingly, bcr/abl-expressing C/EBPalpha-/- fetal liver cells failed to induce a myeloid disease in transplanted mice, but caused a fatal, transplantable erythroleukemia instead. Accordingly, increased expression of the transcription factors SCL and GATA-1 in hematopoietic precursor cells of C/EBPalpha-/-R01-EY-11298 ) fetal livers was found. The mechanism for the lineage shift from myeloid to erythroid leukemia was studied in a bcr/abl-positive cell line. Consistent with findings of the transplant model, expression of C/EBPalpha and GATA-1 was inversely correlated. Id1, an inhibitor of erythroid differentiation, was identified as a critical direct target of C/EBPalpha. Down-regulation of Id1 by RNA interference impaired C/EBPalpha-induced granulocytic differentiation. Taken together, our study provides evidence that myeloid lineage identity of malignant hematopoietic progenitor cells requires the residual expression of C/EBPalpha.

Inhibitors of differentiation and DNA binding (Ids) regulate Math1 and hair cell formation during the development of the organ of Corti

The basic helix-loop-helix (bHLH) transcription factor Math1 (also called Atoh1) is both necessary and sufficient for hair cell development in the mammalian cochlea (Bermingham et al., 1999; Zheng and Gao, 2000). Previous studies have demonstrated that a dynamic pattern of Math1 expression plays a key role in regulating the number and position of mechanosensory hair cells. However, the factors that regulate the temporal and spatial expression of Math1 within the cochlea are unknown. The bHLH-related inhibitors of differentiation and DNA binding (Id) proteins are known to negatively regulate many bHLH transcription factors, including Math1, in a number of different systems. Therefore, Id proteins are good candidates for regulating Math1 in the cochlea. Results from PCR and in situ hybridization indicate that Id1, Id2, and Id3 are expressed within the cochlear duct in a pattern that is consistent with a role in regulation of hair cell development. In particular, expression of Ids and Math1 overlapped in cochlear progenitor cells before cellular differentiation, but a specific downregulation of Id expression was observed in individual cells that differentiated as hair cells. In addition, progenitor cells in which the expression of Ids was maintained during the time period for hair cell differentiation were inhibited from developing as hair cells. These results indicate a key role for Ids in the regulation of expression of Math1 and hair cell differentiation in the developing cochlea.

Proteasome mediated degradation of Id-1 is associated with TNFalpha-induced apoptosis in prostate cancer cells

Overexpression of the helix-loop-helix protein Id-1 has been reported in over 20 types of cancer. While a number of factors have been demonstrated to regulate Id-1 gene transcription, little is known about the mechanisms responsible for its degradation. In this study, we have demonstrated that Id-1 protein stability was regulated by TNFalpha in prostate cancer cells. We found that exposure of prostate cancer cell lines, DU145 and PC-3, to TNFalpha resulted in a rapid and significant downregulation of the Id-1 protein level. The fact that neither the Id-1 promoter activity nor the Id-1 mRNA level was affected by the TNFalpha treatment suggested that the decrease in Id-1 protein was not due to the suppression of gene transcription. In addition, the half-life of the Id-1 protein was decreased in both cell lines in the presence of TNFalpha, and the addition of an ubiquitin/proteasome inhibitor (MG-132) prior to the TNFalpha treatment completely blocked the effect of the TNFalpha-induced Id-1 protein degradation. Furthermore, introduction of a Flag-tag sequence into the N-terminus region of the Id-1 protein, which has been shown to stabilize the protein, was able to protect the Id-1 protein from TNFalpha-induced degradation. These results suggest that TNFalpha downregulated Id-1 through activation of the ubiquitin/proteasome degradation pathway in prostate cancer cells. Interestingly, in both DU145 and PC-3 cells, the decrease of Id-1 protein was associated with the activation of apoptotic pathway, as evidenced by the increased expression of cleaved PARP and caspase 3. In addition, TNFalpha failed to downregulate Id-1 in a sub-line of LNCaP cells that was resistant to TNFalpha-induced apoptosis. These results further suggest that the downregulation of Id-1 may facilitate TNFalpha-induced apoptosis in prostate cancer cells. In conclusion, our findings indicate that Id-1 protein may be regulated by TNFalpha through the ubiquitin/proteasome degradation pathway and the stability of the Id-1 protein appears to correlate with the sensitivity of TNFalpha-induced apoptosis.

TEL/ETV6 accelerates erythroid differentiation and inhibits megakaryocytic maturation in a human leukemia cell line UT-7/GM

TEL/ETV6 accelerates erythroid differentiation in the murine erythroleukemia cell line. To clarify the effects of TEL on megakaryocytic maturation as well as erythroid differentiation, we chose the human leukemia cell line UT-7/GM that differentiates into the erythroid and megakaryocytic lineages by treatment with erythropoietin and thrombopoietin, respectively. Upon erythropoietin exposure, overexpressed TEL stimulated hemoglobin synthesis and accumulation of the erythroid differentiation-specific transcripts such as gamma-globin, delta-aminolevulinic acid synthase-erythroid, and erythropoietin receptor. Moreover, the glycophorin A(+)/glycoprotein IIb(-) fraction appeared more rapidly in the TEL-overexpressing cells. Interestingly, overexpression of TEL was associated with lower levels of the megakaryocytic maturation-specific glycoprotein IIb and platelet factor 4 transcripts under the treatment with thrombopoietin. Consistently, the glycophorin A(-)/glycoprotein IIb(+) fraction increased more slowly in the TEL-overexpressing cells. Finally, expression of endogenous TEL proteins in UT-7/GM cells was down-regulated following erythropoietin and thrombopoietin exposure. All these data suggest that TEL may decide the fate of human erythrocyte/megakaryocyte common progenitors to differentiate towards the erythroid lineage and against the megakaryocytic lineage.

High-dimensional switches and the modelling of cellular differentiation

Many genes have been identified as driving cellular differentiation, but because of their complex interactions, the understanding of their collective behaviour requires mathematical modelling. Intriguingly, it has been observed in numerous developmental contexts, and particularly haematopoiesis, that genes regulating differentiation are initially co-expressed in progenitors despite their antagonism, before one is upregulated and others downregulated. We characterise conditions under which three classes of generic "master regulatory networks", modelled at the molecular level after experimentally observed interactions (including bHLH protein dimerisation), and including an arbitrary number of antagonistic components, can behave as a "multi-switch", directing differentiation in an all-or-none fashion to a specific cell-type chosen among more than two possible outcomes. bHLH dimerisation networks can readily display coexistence of many antagonistic factors when competition is low (a simple characterisation is derived). Decision-making can be forced by a transient increase in competition, which could correspond to some unexplained experimental observations related to Id proteins; the speed of response varies with the initial conditions the network is subjected to, which could explain some aspects of cell behaviour upon reprogramming. The coexistence of antagonistic factors at low levels, early in the differentiation process or in pluripotent stem cells, could be an intrinsic property of the interaction between those factors, not requiring a specific regulatory system.

Comparison of basal gene expression profiles and effects of hepatocarcinogens on gene expression in cultured primary human hepatocytes and HepG2 cells

Toxicogenomics is a relatively new discipline of toxicology. Microarrays and bioinformatics tools are being used successfully to understand the effects of toxicants on in vivo and in vitro model systems, and to gain a better understanding of the relevance of in vitro models commonly used in toxicological studies. In this study, cDNA filter arrays were used to determine the basal expression patterns of human cultured primary hepatocytes from different male donors; compare the gene expression profile of HepG2 to that of primary hepatocytes; and analyze the effects of three genotoxic hepatocarcinogens; aflatoxin B(1) (AFB(1)), 2-acetylaminofluorene (2AAF), and dimethylnitrosamine (DMN), as well as one non-gentoxic hepatotoxin, acetaminophen (APAP) on gene expression in both in vitro systems. Real-time PCR was used to verify differential gene expression for selected genes. Of the approximately 31,000 genes screened, 3-6% were expressed in primary hepatocytes cultured on matrigel for 16 h. Of these genes, 867 were expressed in cultured hepatocytes from all donors. HepG2 cells expressed about 98% of the genes detectable in cultured primary hepatocytes, however, 31% of the HepG2 transcriptome was unique to the cell line. A number of these genes are expressed in human liver but expression is apparently lost during culture. There was considerable variability in the response to chemical carcinogen exposure in primary hepatocytes from different donors. The transcription factors, E2F1 and ID1 mRNA were increased three-fold and six-fold (P < 0.05, P < 0.01), respectively, in AFB(1) treated primary human hepatocytes but were not altered in HepG2. ID1 expression was also increased by dimethylnitrosamine, acetylaminofluorene and acetaminophen in both primary hepatocytes and HepG2. Identification of genes that are expressed in primary hepatocytes from most donors, as well as those genes with variable expression, will aid in understanding the variability in human reactions to drugs and chemicals. This study suggests that identification of biomarkers of exposure to some chemicals may be possible in the human through microarray analysis, despite the variability in responses.

E2A/HEB and Id3 proteins control the sensitivity to glucocorticoid-induced apoptosis in thymocytes by regulating the SRG3 expression

The E protein family transcription factors encoded by the E2A and HEB genes are known to play critical roles in the coordinate regulation of lymphocyte development. Previous studies have shown that T cell receptor (TCR) signals rapidly induce Id3, a dominant negative antagonist of E2A activity and allow thymocytes to survive selection events in the thymus. Here we show that SRG3 acts as a novel downstream target of E2A/HeLa E box-binding (HEB) complex and modulates glucocorticoid (GC) susceptibility in thymocytes in response to TCR signals. We have identified a putative E box element in the SRG3 promoter that is required for optimal promoter activity. The transcription factors E2A and HEB specifically associate with the E box element. Moreover, E2A-HEB heterodimers cooperated to activate SRG3 transcription, which was inhibited by the expression of Id proteins. TCR-mediated signals rapidly induced Id3 via MEK/ERK activation and thereby kept the E2A/HEB complex from binding to the E box element in the SRG3 promoter. Retroviral transduction of Id3 also repressed the SRG3 expression by inhibiting the E box binding activity of the E2A/HEB complex. Intriguingly, enforced Id3 expression conferred thymocyte resistance to GCs, which could be overcome by the overexpression of SRG3. Taken together, these results suggest that Id3 may enhance the viability of immature thymocytes by at least rendering them resistant to GCs through SRG3 down-regulation.

Id4 regulates mammary epithelial cell growth and differentiation and is overexpressed in rat mammary gland carcinomas

Id4 belongs to a family of helix-loop-helix (HLH) proteins that impact cellular growth and differentiation via regulation of basic HLH transcription factors. Herein the rat Id4 gene was cloned (GenBank Accession No. AF468681). The expression of rat Id4 was examined in rat mammary gland tumors induced by 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP), a carcinogen found in the human diet. By real-time polymerase chain reaction analysis, relative expression of Id4 mRNA in carcinomas, adenomas, and normal tissue was 27, 6, and 1, respectively. Immunohistochemical analysis indicated statistically elevated nuclear expression for Id4 protein in carcinomas in comparison to adenomas and normal mammary gland. In carcinomas, Id4 nuclear expression was positively correlated with proliferation, invasiveness, and tumor weight (Fisher Exact Test or Spearman Correlation, P < 0.05). The consequence of enforced expression of Id4 on mammary epithelial cell proliferation, differentiation, and growth in soft agar was examined in HC11 cells, a well-characterized model for studying various aspects of mammary epithelial cell biology. After transient and stable transfection of HC11 cells, Id4 overexpression increased cell proliferation and inhibited lactogenic hormone-mediated differentiation as revealed by inhibition of beta-casein promoter activity and beta-casein expression. In addition, enforced expression of Id4 in HC11 cells induced a statistically significant increase in colony growth in soft agar. The results implicate Id4 in rat mammary gland carcinogenesis and suggest that Id4 may contribute to carcinogenesis by inhibiting mammary epithelial cell differentiation and stimulating mammary epithelial cell growth.

Id proteins in development, cell cycle and cancer

Id proteins are important parts of signaling pathways involved in development, cell cycle and tumorigenesis. They were first shown to act as dominant negative antagonists of the basic helix-loop-helix family of transcription factors, which positively regulate differentiation in many cell lineages. The Id proteins do this by associating with the ubiquitous E proteins and preventing them from binding DNA or other transcription factors. Id proteins also associate with Ets transcription factors and the Rb family of tumor suppressor proteins, and are downstream targets of transforming growth factor beta and bone morphogenic protein signaling. Thus, the Id proteins have become important molecules for understanding basic biological processes as well as targets for potential therapeutic intervention in human disease.

Id proteins are overexpressed in human oral squamous cell carcinomas

BACKGROUND

The helix-loop-helix (HLH) proteins Id-1, Id-2 and Id-3 have been demonstrated to inhibit the activity of transcription factors and play an important role in regulating cell growth and tissue-specific differentiation.

METHODS

To elucidate the involvement of Id in human oral squamous cell carcinoma (OSCC), we examined 83 surgical specimens and eight normal gingival mucosae for the expression of Id proteins by immunohistochemistry; in addition, some specimens of the OSCC and the normal gingivae were also examined for the expression of Id-1 mRNA by in situ hybridization (ISH), while Western blots were performed on six of the tumours and on cell lysates of five OSCC cell lines. We also explored the correlation between Id expressions and cellular proliferation indicating Ki-67 or clinical parameters.

RESULTS

We discovered a higher and more frequent expression of Id-1 protein (27.7%) in human OSCC compared to that of Id-2 and Id-3 proteins (6.0 and 7.2%, respectively). Western blot analysis detected Id-1 protein in four of six tumour samples and in all cell lines. ISH demonstrated strong cytoplasmic localization of Id-1 mRNA in tumour samples at significantly higher levels compared to those of normal gingival mucosae.

CONCLUSION

Id-1 protein expression significantly correlates with lymph node status, indicating that increased Id expression may relate to the metastatic behaviour in human OSCC.

Induction of C/EBPalpha activity alters gene expression and differentiation of human CD34+ cells

The CCAAT/enhancer binding protein alpha (C/EBPalpha) belongs to a family of transcription factors that are involved in the differentiation process of numerous tissues, including the liver and hematopoietic cells. C/EBPalpha(-/-) mice show a block in hematopoietic differentiation, with an accumulation of myeloblasts and an absence of mature granulocytes, whereas expression of C/EBPalpha in leukemia cell lines leads to granulocytic differentiation. Recently, dominant-negative mutations in the C/EBPalpha gene and down-regulation of C/EBPalpha by AML1-ETO, an AML associated fusion protein, have been identified in acute myelogenous leukemia (AML). To better understand the role of C/EBPalpha in the lineage commitment and differentiation of hematopoietic progenitors, we transduced primary human CD34(+) cells with a retroviral construct that expresses the C/EBPalpha cDNA fused in-frame with the estrogen receptor ligand-binding domain. Induction of C/EBPalpha function in primary human CD34(+) cells, by the addition of beta-estradiol, leads to granulocytic differentiation and inhibits erythrocyte differentiation. Using Affymetrix (Santa Clara, CA) oligonucleotide arrays we have identified C/EBPalpha target genes in primary human hematopoietic cells, including granulocyte-specific genes that are involved in hematopoietic differentiation and inhibitor of differentiation 1 (Id1), a transcriptional repressor known to interfere with erythrocyte differentiation. Given the known differences in murine and human promoter regulatory sequences, this inducible system allows the identification of transcription factor target genes in a physiologic, human hematopoietic progenitor cell background.

Regulation of Id1 protein expression in mouse embryo fibroblasts by the type 1 insulin-like growth factor receptor

The activated type 1 insulin-like growth factor (IGF-IR) increases the expression of Id1 proteins in mouse embryo fibroblasts (MEF). Up-regulation depends on a functional receptor and on multiple pathways originating from different domains of the receptor. In MEF, Id1 protein expression is also up-regulated by serum and certain oncogenes. Signaling through Stat3 plays an important, but not exclusive, role in the up-regulation of Id1 protein levels. In all instances, the increase in Id1 protein expression is paralleled by a corresponding increase in Id1 promoter activity, as measured with a reporter gene.

Role of Id family proteins in growth control

Id proteins (inhibitors of DNA binding/differentiation) are negative regulators of basic helix-loop-helix (bHLH) type transcription factors, which promote the differentiation of various cell types. In addition to their "classical" ability to inhibit cell differentiation, they are able to stimulate cell cycle progression. These facts suggest that Id proteins play a role in keeping precursor cells immature and in expanding the cell population size during development. In vitro as well as in vivo analyses in the last several years have shown that Id proteins have more complex activities; they induce apoptosis or function as survival factors, depending on the cell context. Furthermore, dysregulated expression of Id proteins has been reported in several human tumors and seems to be related to the malignant character of tumors. Here, we summarize and discuss the biological activities of Id proteins from the standpoint of cell growth control.

Id proteins in cell cycle control and cellular senescence

The Id family of helix-loop-helix (HLH) proteins are thought to affect the balance between cell growth and differentiation by negatively regulating the function of basic-helix-loop-helix (bHLH) transcription factors. Although it has been suggested for some time that Id is involved in cell cycle regulation, little is known about the molecular mechanism of this control. Recent studies, however, have revealed that Id binds to important cell cycle regulatory proteins other than bHLH proteins. Two such proteins, pRB (retinoblastoma tumour suppressor protein) family proteins and Ets-family transcription factors are known to play key roles in cell cycle regulation, transformation and tumour suppression. Through the characterization of these pathways we will begin to understand the mechanisms by which Id controls normal and abnormal cell cycle progression.

Id-1, ITF-2, and Id-2 comprise a network of helix-loop-helix proteins that regulate mammary epithelial cell proliferation, differentiation, and apoptosis

Mammary epithelial cells proliferate, invade the stroma, differentiate, and die in adult mammals by mechanisms that are poorly understood. We found that Id-1, an inhibitor of basic helix-loop-helix transcription factors, regulates mammary epithelial cell growth, differentiation, and invasion in culture. Here, we show that Id-1 is expressed highly during mammary development in virgin mice and during early pregnancy, when proliferation and invasion are high. During mid-pregnancy, Id-1 expression declined to undetectable levels as the epithelium differentiated fully. Surprisingly, Id-1 increased during involution, when the epithelium undergoes extensive apoptosis. To determine whether Id-1 regulates both proliferation and apoptosis, we constitutively expressed Id-1 in mammary epithelial cell cultures. Id-1 stimulated proliferation in sparse cultures but induced apoptosis in dense cultures, which reflect epithelial cell density during early pregnancy and involution, respectively. To understand how Id-1 acts, we screened a yeast two-hybrid library from differentiating mammary epithelial cells and identified ITF-2, a basic helix-loop-helix transcription factor, as an Id-1-interacting protein. Overexpression of ITF-2 significantly reduced Id-1-stimulated proliferation and apoptosis. We show further that, in contrast to Id-1, Id-2 was expressed highly in differentiated mammary epithelial cells in vivo and in culture. In culture, Id-2 antisense transcripts blocked differentiation. Our results suggest that Id-1, ITF-2, and Id-2 comprise a network of interacting molecular switches that govern mammary epithelial cell phenotypes.

Regulation of Id gene expression by type I insulin-like growth factor: roles of Stat3 and the tyrosine 950 residue of the receptor

Id proteins are known to play important roles in the proliferation and differentiation of many cell types. The type 1 insulin-like growth factor receptor (IGF-IR), activated by its ligand, induces the differentiation of 32D IGF-IR cells, a murine hematopoietic cell line, expressing a human IGF-IR. Expression in 32D IGF-IR cells of a dominant negative mutant of Stat3 (DNStat3) inhibits IGF-I-mediated differentiation. DNStat3 causes a dramatic increase in Id2 gene expression. This increase, however, is IGF-I dependent and is abrogated by a mutation at tyrosine 950 of the IGF-IR. These results indicate that in 32D cells, the IGF-IR regulates the expression of the Id2 gene and that this regulation is modulated by both positive and negative signals. Our results also suggest that in this model, Id2 proteins influence the differentiation program of cells but are not sufficient for the full stimulation of their proliferation program.

The Rb pathway in neurogenesis

Cell division during embryogenesis plays a crucial role in the formation of the nervous system. During this developmental process, proliferating neural precursor cells commit to a neuronal fate and, as a consequence, undergo terminal mitosis and adopt a neuronal phenotype. A key cell cycle regulator, the tumor suppressor protein, retinoblastoma (Rb), is involved in both terminal mitosis and neuronal differentiation. Neural development is a complex process involving cell proliferation, cell fate determination and differentiation, as well as programmed cell death. In this review, we will examine each of these processes in turn, focussing on the role of the Rb family proteins to examine their many influences on these events.

Regulation of Id2 gene expression by the insulin-like growth factor I receptor requires signaling by phosphatidylinositol 3-kinase

The Id proteins play an important role in proliferation, differentiation, and tumor development. We report here that Id gene expression can be regulated by the insulin-like growth factor I receptor (IGF-IR), a receptor that also participates in the regulation of cellular proliferation and differentiation. Specifically, we found that the IGF-IR activated by its ligand was a strong inducer of Id2 gene expression in 32D murine hemopoietic cells. This activation was not simply the result of cellular proliferation, as Id2 gene expression was higher in 32D cells stimulated by IGF-I than in cells exponentially growing in interleukin-3. The up-regulation of Id2 gene expression was largely dependent on the presence of insulin receptor substrate-1, a major substrate of the IGF-IR and a potent activator of the phosphatidylinositol 3-kinase (PI3K) pathway. The role of PI3K activity in the up-regulation of Id2 gene expression by the IGF-IR was confirmed by different methods and in different cell types. In 32D cells, the up-regulation of Id2 gene expression by the PI3K pathway correlated with interleukin-3 independence and inhibition of differentiation.