Promotion of cell cycle progression by basic helix-loop-helix E2A

Musculin does not modulate the disease course of Experimental Autoimmune Encephalomyelitis and DSS colitis

Previous evidences show that Musculin (Msc), a repressor member of basic helix-loop-helix transcription factors, is responsible in vitro for the low responsiveness of human Th17 cells to the growth factor IL-2, providing an explanation for Th17 cells rarity in inflammatory tissue. However, how and to what extent Musculin gene can regulate the immune response in vivo in an inflammatory context is still unknown. Here, exploiting two animal models of inflammatory diseases, the Experimental Autoimmune Encephalomyelitis (EAE) and the dextran sodium sulfate (DSS)-induced colitis, we evaluated the effect of Musculin gene knock-out on clinical course, performing also a deep immune phenotypical analysis on T cells compartment and an extended microbiota analysis in colitis-sick mice. We found that, at least during the early phase, Musculin gene has a very marginal role in modulating both the diseases. Indeed, the clinical course and the histological analysis showed no differences between wild type and Msc knock-out mice, whereas immune system appeared to give rise to a regulatory milieu in lymph nodes of EAE mice and in the spleen of DSS colitis-sick mice. Moreover, in the microbiota analysis, we found irrelevant differences between wild type and Musculin knock-out colitis-sick mice, with a similar bacterial strains' frequency and diversity after the DSS treatment. This work strengthened the idea of a negligible Msc gene involvement in these models.

Complex interactomes and post-translational modifications of the regulatory proteins HABP4 and SERBP1 suggest pleiotropic cellular functions

The 57 kDa antigen recognized by the Ki-1 antibody, is also known as intracellular hyaluronic acid binding protein 4 and shares 40.7% identity and 67.4% similarity with serpin mRNA binding protein 1, which is also named CGI-55, or plasminogen activator inhibitor type-1-RNA binding protein-1, indicating that they might be paralog proteins, possibly with similar or redundant functions in human cells. Through the identification of their protein interactomes, both regulatory proteins have been functionally implicated in transcriptional regulation, mRNA metabolism, specifically RNA splicing, the regulation of mRNA stability, especially, in the context of the progesterone hormone response, and the DNA damage response. Both proteins also show a complex pattern of post-translational modifications, involving Ser/Thr phosphorylation, mainly through protein kinase C, arginine methylation and SUMOylation, suggesting that their functions and locations are highly regulated. Furthermore, they show a highly dynamic cellular localization pattern with localizations in both the cytoplasm and nucleus as well as punctuated localizations in both granular cytoplasmic protein bodies, upon stress, and nuclear splicing speckles. Several reports in the literature show altered expressions of both regulatory proteins in a series of cancers as well as mutations in their genes that may contribute to tumorigenesis. This review highlights important aspects of the structure, interactome, post-translational modifications, sub-cellular localization and function of both regulatory proteins and further discusses their possible functions and their potential as tumor markers in different cancer settings.

Up-regulation of Transcription Factor 3 Is Correlated With Poor Prognosis in Cervical Carcinoma

OBJECTIVES

Transcription factor 3 (TCF3, or E2A) is a multifunctional bHLH (basic helix loop helix) transcription factor. The role of TCF3 expression in cancer and the multiple cell signaling pathways that regulate or are influenced by TCF3 are unclear. Therefore, the expression level of TCF3 in patients with cervical squamous cell carcinoma (CSCC) is discussed in this study.

METHODS

Total RNA was extracted using real-time quantitative reverse transcription-polymerase chain reaction. Western blotting was applied to confirm the results. Immunohistochemistry was used to characterize the expression patterns of TCF3 in CSCC specimens. The close relationship between the expression levels of TCF3 and the 5-year overall survival time was described by survival curves. The association between TCF3 expression and clinicopathological characteristics of 119 CSCC patients was analyzed by Chi-square, Fisher exact test, and Cox regression analysis. TCF3 was overexpressed or inhibited by plasmid transfection, and the proliferation, invasion, and migration of cells were detected using 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT), wound healing, and Transwell assays.

RESULTS

The expression of TCF3 was higher in CSCC tissues than in nonmalignant cervical tissues. Messenger RNA (mRNA) and protein in patient tissues were increased compared with nonmalignant cervical tissues. Moreover, the level of expression in early-stage disease was higher than in the advanced stage. From FIGO (International Federation of Gynecology and Obstetrics) stages I to IV, immunohistochemistry staining intensity gradually increased. A high level of expression was closely related to clinical stages. The expression of TCF3 was negatively correlated with overall survival time. TCF3 can promote HeLa cell growth, invasion, and migration in vitro.

CONCLUSIONS

Based on our results, TCF3 is clearly associated with the progression of CSCC. This is the first time that it has been reported that TCF3 can act as a tumor promoter in cervical cancer and thus might be of great significance in the prognosis of CSCC.

New insights into transcriptional and leukemogenic mechanisms of AML1-ETO and E2A fusion proteins

BACKGROUND

Nearly 15% of acute myeloid leukemia (AML) cases are caused by aberrant expression of AML1-ETO, a fusion protein generated by the t(8;21) chromosomal translocation. Since its discovery, AML1-ETO has served as a prototype to understand how leukemia fusion proteins deregulate transcription to promote leukemogenesis. Another leukemia fusion protein, E2A-Pbx1, generated by the t(1;19) translocation, is involved in acute lymphoblastic leukemias (ALLs). While AML1-ETO and E2A-Pbx1 are structurally unrelated fusion proteins, we have recently shown that a common axis, the ETO/E-protein interaction, is involved in the regulation of both fusion proteins, underscoring the importance of studying protein-protein interactions in elucidating the mechanisms of leukemia fusion proteins.

OBJECTIVE

In this review, we aim to summarize these new developments while also providing a historic overview of the related early studies.

METHODS

A total of 218 publications were reviewed in this article, a majority of which were published after 2004.We also downloaded 3D structures of AML1-ETO domains from Protein Data Bank and provided a systematic summary of their structures.

RESULTS

By reviewing the literature, we summarized early and recent findings on AML1-ETO, including its protein-protein interactions, transcriptional and leukemogenic mechanisms, as well as the recently reported involvement of ETO family corepressors in regulating the function of E2A-Pbx1.

CONCLUSION

While the recent development in genomic and structural studies has clearly demonstrated that the fusion proteins function by directly regulating transcription, a further understanding of the underlying mechanisms, including crosstalk with other transcription factors and cofactors, and the protein-protein interactions in the context of native proteins, may be necessary for the development of highly targeted drugs for leukemia therapy.

Basic helix loop helix (bHLH) transcription factor 3 (TCF3, E2A) is regulated by androgens in prostate cancer cells

TCF3 (E2A) is a multifunctional basic helix loop helix (bHLH) transcription factor that is over-expressed in prostate cancer (PCa) as compared to normal prostate and that it acts as a tumor promoter in PCa. Given the diverse biological pathways regulated/influenced by TCF3, little is known about the mechanisms that regulate its expression. TCF3 expression in androgen sensitive LNCaP and insensitive C81 PCa cell lines was determined following treatments with androgen receptor (AR) agonist R1881 and antagonist Casodex. In silico analysis was used to discover putative Androgen Response Elements (ARE) in the TCF3 promoter/intron region. Chromatin Immunoprecipitation (ChIP) with AR antibody and luciferase reporter assays on the above mentioned cell lines was used to confirm AR biding and AR dependent transcriptional activity respectively. The results were confirmed by demonstrating TCF3 expression in LNCaP PCa xenograft models. The results suggested that TCF3 transcript increased in response to R1881 in LNCaP cells but was constitutively expressed in C-81 cell lines. The promoter/Intron region of the TCF3 gene was predicted to contain two putative ARE sites ARE1 and ARE2. ChIP after treatment of LNCaP and C81 cells with R1881 and Casodex showed that the ARE1 and ARE2 were bound by AR in LNCaP cells only in the presence of R1881, whereas C81 cells showed constitutive AR binding. Similar results were observed in luciferase reporter assays indicating that TCF3 is activated by AR in LNCaP cell lines whereas it is independent of androgens in C81 cell line. Luciferase reporter assays also confirmed that ARE1 alone drives androgen dependent transcription. TCF3 expression was only observed in castration resistant LNCaP xenografts in castrated mice. In conclusion, we demonstrate that in PCa androgen receptor regulates the expression of TCF3 which is mediated in part via a consensus androgen response element. The shift in TCF3 expression from androgen regulated to androgen independent during prostate cancer progression, together with lack of expression in normal prostate may provide mechanistic basis underlying the transition of androgen receptor from a tumor suppressor to an oncogene in prostate cancer.

Enforced expression of E47 has differential effects on Lmo2-induced T-cell leukemias

LIM domain only-2 (LMO2) overexpression in T cells induces leukemia but the molecular mechanism remains to be elucidated. In hematopoietic stem and progenitor cells, Lmo2 is part of a protein complex comprised of class II basic helix loop helix proteins, Tal1and Lyl1. The latter transcription factors heterodimerize with E2A proteins like E47 and Heb to bind E boxes. LMO2 and TAL1 or LYL1 cooperate to induce T-ALL in mouse models, and are concordantly expressed in human T-ALL. Furthermore, LMO2 cooperates with the loss of E2A suggesting that LMO2 functions by creating a deficiency of E2A. In this study, we tested this hypothesis in Lmo2-induced T-ALL cell lines. We transduced these lines with an E47/estrogen receptor fusion construct that could be forced to homodimerize with 4-hydroxytamoxifen. We discovered that forced homodimerization induced growth arrest in 2 of the 4 lines tested. The lines sensitive to E47 homodimerization accumulated in G1 and had reduced S phase entry. We analyzed the transcriptome of a resistant and a sensitive line to discern the E47 targets responsible for the cellular effects. Our results suggest that E47 has diverse effects in T-ALL but that functional deficiency of E47 is not a universal feature of Lmo2-induced T-ALL.

Ki-1/57 and CGI-55 ectopic expression impact cellular pathways involved in proliferation and stress response regulation

Ki-1/57 (HABP4) and CGI-55 (SERBP1) are regulatory proteins and paralogs with 40.7% amino acid sequence identity and 67.4% similarity. Functionally, they have been implicated in the regulation of gene expression on both the transcriptional and mRNA metabolism levels. A link with tumorigenesis is suggested, since both paralogs show altered expression levels in tumor cells and the Ki-1/57 gene is found in a region of chromosome 9q that represents a haplotype for familiar colon cancer. However, the target genes regulated by Ki-1/57 and CGI-55 are unknown. Here, we analyzed the alterations of the global transcriptome profile after Ki-1/57 or CGI-55 overexpression in HEK293T cells by DNA microchip technology. We were able to identify 363 or 190 down-regulated and 50 or 27 up-regulated genes for Ki-1/57 and CGI-55, respectively, of which 20 were shared between both proteins. Expression levels of selected genes were confirmed by qRT-PCR both after protein overexpression and siRNA knockdown. The majority of the genes with altered expression were associated to proliferation, apoptosis and cell cycle control processes, prompting us to further explore these contexts experimentally. We observed that overexpression of Ki-1/57 or CGI-55 results in reduced cell proliferation, mainly due to a G1 phase arrest, whereas siRNA knockdown of CGI-55 caused an increase in proliferation. In the case of Ki-1/57 overexpression, we found protection from apoptosis after treatment with the ER-stress inducer thapsigargin. Together, our data give important new insights that may help to explain these proteins putative involvement in tumorigenic events.

E2A predicts prognosis of colorectal cancer patients and regulates cancer cell growth by targeting miR-320a

BACKGROUND AND OBJECTIVE

Transcriptional factor E2A is crucial for the normal development and differentiation of B and T lymphocytes. Dysregulation of E2A leads to leukemia and tumorigenesis of some solid tumors. The expression and clinical significance of E2A as well as its role in colorectal cancer (CRC) are still unknown. This study aims to assess E2A expression in CRC tissues, evaluate its prognosis value, and investigate its role in colon cancer cell growth.

METHODS

E2A expression in CRC tissues and normal mucosa was detected by immunohistochemical staining; Kaplan-Meier survival curve and Cox regression model were used to evaluate the prognostic value of E2A. Lentivirus was used to construct E2A stably knocked-down cells. MTT assay was employed to detect cell proliferation change; cell cycle was analyzed by flow cytometry; and chromatin immunoprecipitation (ChIP) assay was used to validate the predicted binding target of E2A.

RESULTS

Expression of E2A was lower in CRC tissues than normal mucosa; low E2A expression correlated with advanced TNM stage and larger tumor size, and predicted poor prognosis of CRC patients. E2A knockdown resulted in increased cell proliferation rate and cell cycle acceleration. ChIP assay showed miR-320a was a direct target of E2A and upregulation of miR-320a in E2A downregulated cells could reverse cell proliferation and cell cycle changes caused by E2A deficiency.

CONCLUSIONS

E2A is an independent prognostic factor for CRC patients and targets miR-320a to regulate cell proliferation of colon cancer cells.

E proteins in lymphocyte development and lymphoid diseases

As members of the basic helix-loop-helix (bHLH) family of transcription factors, E proteins function in the immune system by directing and maintaining a vast transcriptional network that regulates cell survival, proliferation, differentiation, and function. Proper activity of this network is essential to the functionality of the immune system. Aberrations in E protein expression or function can cause numerous defects, ranging from impaired lymphocyte development and immunodeficiency to aberrant function, cancer, and autoimmunity. Additionally, disruption of inhibitor of DNA-binding (Id) proteins, natural inhibitors of E proteins, can induce additional defects in development and function. Although E proteins have been investigated for several decades, their study continues to yield novel and exciting insights into the workings of the immune system. The goal of this chapter is to discuss the various classical roles of E proteins in lymphocyte development and highlight new and ongoing research into how these roles, if compromised, can lead to disease.

Increased expression of bHLH transcription factor E2A (TCF3) in prostate cancer promotes proliferation and confers resistance to doxorubicin induced apoptosis

E2A (TCF3) is a multifunctional basic helix loop helix (bHLH), transcription factor. E2A regulates transcription of target genes by homo- or heterodimerization with cell specific bHLH proteins. In general, E2A promotes cell differentiation, acts as a negative regulator of cell proliferation in normal cells and cancer cell lines and is required for normal B-cell development. Given the diverse biological pathways regulated/influenced by E2A little is known about its expression in cancer. In this study we investigated the expression of E2A in prostate cancer. Unexpectedly, E2A immuno-histochemistry demonstrated increased E2A expression in prostate cancer as compared to normal prostate. Silencing of E2A in prostate cancer cells DU145 and PC3 led to a significant reduction in proliferation due to G1 arrest that was in part mediated by increased CDKN1A(p21) and decreased Id1, Id3 and c-myc. E2A silencing in prostate cancer cell lines also resulted in increased apoptosis due to increased mitochondrial permeability and caspase 3/7 activation. Moreover, silencing of E2A increased sensitivity to doxorubicin induced apoptosis. Based on our results, we propose that E2A could be an upstream regulator of Id1 and c-Myc which are highly expressed in prostate cancer. These results for the first time demonstrate that E2A could in fact acts as a tumor promoter at least in prostate cancer.

Forced expression of cyclin-dependent kinase 6 confers resistance of pro-B acute lymphocytic leukemia to Gleevec treatment

The gene encoding c-ABL, a nonreceptor protein tyrosine kinase, is involved in a chromosomal translocation resulting in expression of a BCR-Abl fusion protein that causes most chronic myelogenous and some acute lymphocytic leukemias (CML and ALL) in humans. The Abelson murine leukemia virus (A-MuLV) expresses an alternative form of c-Abl, v-Abl, that transforms murine pro-B cells, resulting in acute leukemia and providing an experimental model for human disease. Gleevec (STI571) inhibits the Abl kinase and has shown great utility against CML and ALL in humans, although its usefulness is limited by acquired resistance. Since STI571 is active against A-MuLV-transformed cells in vitro, we performed a retroviral cDNA library screen for genes that confer resistance to apoptosis induced by STI571. We found that forced expression of Cdk6 promotes continued cell division and decreased apoptosis of leukemic cells. We then determined that the transcription factor E2A negatively regulates Cdk6 transcription in leukemic pro-B cells and that the v-Abl kinase stimulates Cdk6 expression via an extracellular signal-regulated kinase 1-dependent pathway. Finally, we show that the cyclin-dependent kinase 4 and 6 (CDK4/6) inhibitor PD0332991 can act synergistically with STI571 to enhance leukemic cell death, suggesting a potential role for CDK6 inhibitors in the treatment of STI571-resistant CML or ALL.

A tamoxifen inducible knock-in allele for investigation of E2A function

Background

E-proteins are transcription factors important for the development of a variety of cell types, including neural, muscle and lymphocytes of the immune system. E2A, the best characterized E-protein family member in mammals, has been shown to have stage specific roles in cell differentiation, lineage commitment, proliferation, and survival. However, due to the complexity of E2A function, it is often difficult to separate these roles using conventional genetic approaches. Here, we have developed a new genetic model for reversible control of E2A protein activity at physiological levels. This system was created by inserting a tamoxifen-responsive region of the estrogen receptor (ER) at the carboxyl end of the tcfe2a gene to generate E2AER fusion proteins. We have characterized and analyzed the efficiency and kinetics of this inducible E2A^ER system in the context of B cell development.

Results

B cell development has been shown previously to be blocked at an early stage in E2A deficient animals. Our E2A^ER/ER mice demonstrated this predicted block in B cell development, and E2AER DNA binding activity was not detected in the absence of ligand. In vitro studies verified rapid induction of E2AER DNA binding activity upon tamoxifen treatment. While tamoxifen treatment of E2A^ER/ER mice showed inefficient rescue of B cell development in live animals, direct exposure of bone marrow cells to tamoxifen in an ex vivo culture was sufficient to rescue and support early B cell development from the pre-proB cell stage.

Conclusion

The E2A^ER system provides inducible and reversible regulation of E2A function at the protein level. Many previous studies have utilized over-expression systems to induce E2A function, which are complicated by the toxicity often resulting from high levels of E2A. The E2A^ER model instead restores E2A activity at an endogenous level and in addition, allows for tight regulation of the timing of induction. These features make our E2A^ER ex vivo culture system attractive to study both immediate and gradual downstream E2A-mediated events.

E47 controls the developmental integrity and cell cycle quiescence of multipotential hematopoietic progenitors

Little is known about the transcriptional regulators that control the proliferation of multipotent bone marrow progenitors. Understanding the mechanisms that restrict proliferation is of significant interest since the loss of cell cycle integrity can be associated with hematopoietic exhaustion, bone marrow failure, or even oncogenic transformation. Herein, we show that multipotent LSKs (lineage(-)Sca(high)c-kit(+)) from E47-deficient mice exhibit a striking hyperproliferation associated with a loss of cell cycle quiescence and increased susceptibility to in vivo challenge with a mitotoxic drug. Total LSKs contain long-term self-renewing hematopoietic stem cells and downstream multipotential progenitors (MPPs) that possess very limited or no self-renewal ability. Within total LSKs, we found specific developmental and functional deficits in the MPP subset. E47 knockout mice have grossly normal numbers of self-renewing hematopoietic stem cells but a 50-70% reduction in nonrenewing MPPs and downstream lineage-restricted populations. The residual MPPs in E47 knockout mice fail to fully up-regulate flk2 or initiate V(D)J recombination, hallmarks of normal lymphoid lineage progression. Consistent with the loss of normal cell cycle restraints, we show that E47-deficient LSKs have a 50% decrease in p21, a cell cycle inhibitor and known regulator of LSK proliferation. Moreover, enforced expression studies identify p21 as an E47 target gene in primary bone marrow LSKs. Thus, E47 appears to regulate the developmental and functional integrity of early hematopoietic subsets in part through effects on p21-mediated cell cycle quiescence.

The MAPK signaling cascade is a central hub in the regulation of cell cycle, apoptosis and cytoskeleton remodeling by tripeptidyl-peptidase II

Tripeptidyl-peptidase II (TPPII) is a serine peptidase highly expressed in malignant Burkitt's lymphoma cells (BL). We have previously shown that overexpression of TPPII correlates with chromosomal instability, centrosomal and mitotic spindle abnormalities and resistance to apoptosis induced by spindle poisons. Furthermore, TPPII knockdown by RNAi was associated with endoreplication and the accumulation of polynucleated cells that failed to complete cell division, indicating a role of TPPII in the cell cycle. Here we have applied a global approach of gene expression analysis to gain insights on the mechanism by which TPPII regulates this phenotype. mRNA profiling of control and TPPII knockdown BL cells identified one hundred and eighty five differentially expressed genes. Functional categorization of these genes highlighted major physiological functions such as apoptosis, cell cycle progression, cytoskeleton remodeling, proteolysis, and signal transduction. Pathways and protein interactome analysis revealed a significant enrichment in components of MAP kinases signaling. These findings suggest that TPPII influences a wide network of signaling pathways that are regulated by MAPKs and exerts thereby a pleiotropic effect on biological processes associated with cell survival, proliferation and genomic instability.

Suppression of E-protein activity interferes with the development of BCR-ABL-mediated myeloproliferative disease

E-proteins are a class of helix-loop-helix (HLH) proteins, which play multiple roles throughout lymphoid development. The DNA binding activities of the E-proteins are regulated by a distinct class of antagonistic HLH proteins, named Id1-4. Here we demonstrate that Id2 deficient mice in a C57BL/6 genetic background exhibit increased cellularity in the granulocyte/myeloid progenitor compartment and show significantly higher numbers of maturing neutrophils. Within 6 months of age, Id2 deficient mice succumbed from overwhelming granulocytosis. The disease closely mimicked the distinctive features of human chronic myeloid leukemia: leukocytosis with maturing neutrophils, splenomegaly, hepatomegaly, and myeloid infiltration into peripheral tissues, including spleen, liver, and lungs. Strikingly, forced Id2 expression in murine bone marrow cells substantially delayed the onset of myeloproliferative disease (MPD). Collectively, these studies show that suppression of E-protein activity interferes with the development of BCR-ABL-mediated MPD.

ID proteins as targets in cancer and tools in neurobiology

In eukaryotic organisms, ID proteins are key regulators of development when they function to preserve the stem cell state and prevent lineage determination. By fueling several key features of tumor progression (deregulated proliferation, invasiveness, angiogenesis and metastasis), ID proteins contribute to multiple steps of tumorigenesis. Through oncogenic processes that lead to their aberrant activation in tumors, ID proteins transfer the phenotypic traits of embryonic stem cells to cancer cells. However, ID proteins have recently emerged as highly specialized factors in post-mitotic neurons. The elevated expression of ID proteins arrests neurons in the axon growth mode and prevents cessation of axonal elongation. Here, we discuss how unique properties of ID proteins in cancer cells and neurons pave the way to unexpected therapeutic opportunities.

Expression of the B cell-associated transcription factors PAX5, OCT-2, and BOB.1 in acute myeloid leukemia: associations with B-cell antigen expression and myelomonocytic maturation

The aberrant expression of the B-cell transcription factor PAX5 has been described in a subset of acute myeloid leukemia (AML) with t(8;21)(q22;q22) in association with B-cell antigen expression. However, the expression of other B cell-associated transcription factors, particularly OCT-2 and its B cell-specific coactivator BOB.1, has not been described in AML. In this study, expression of PAX5, OCT-2 and BOB.1 was evaluated by immunohistochemical staining of bone marrow samples from 83 cases of AML. The expression patterns were correlated with t(8;21)(q22;q22), B cell-associated antigen expression, and AML subtype. We confirmed the expression of PAX5 in AML with t(8;21)(q22;q22), but also demonstrated its expression in cases that express B-cell antigens but lack this translocation. Although OCT-2 and BOB.1 were not associated with PAX5 expression, we report expression of OCT-2 in AML with myelomonocytic/monocytic maturation and BOB.1 in normal hematopoietic elements.

The basic helix-loop-helix transcription factor HEBAlt is expressed in pro-T cells and enhances the generation of T cell precursors

The basic helix-loop-helix (bHLH) transcription factors HEB and E2A are critical mediators of gene regulation during lymphocyte development. We have cloned a new transcription factor, called HEBAlt, from a pro-T cell cDNA library. HEBAlt is generated by alternative transcriptional initiation and splicing from the HEB gene locus, which also encodes the previously characterized E box protein HEBCan. HEBAlt contains a unique N-terminal coding exon (the Alt domain) that replaces the first transactivation domain of HEBCan. Downstream of the Alt domain, HEBAlt is identical to HEBCan, including the DNA binding domain. HEBAlt is induced in early thymocyte precursors and down-regulated permanently at the double negative to double positive (DP) transition, whereas HEBCan mRNA expression peaks at the DP stage of thymocyte development. HEBAlt mRNA is up-regulated synergistically by a combination of HEBCan activity and Delta-Notch signaling. Retroviral transduction of HEBAlt or HEBCan into hemopoietic stem cells followed by OP9-DL1 coculture revealed that HEBAlt-transduced precursors generated more early T lineage precursors and more DP pre-T cells than control transduced cells. By contrast, HEBCan-transduced cells that maintained high level expression of the HEBCan transgene were inhibited in expansion and progression through T cell development. HEB(-/-) fetal liver precursors transduced with HEBAlt were rescued from delayed T cell specification, but HEBCan-transduced HEB(-/-) precursors were not. Therefore, HEBAlt and HEBCan are functionally distinct transcription factors, and HEBAlt is specifically required for the efficient generation of early T cell precursors.

E Proteins and Id2 converge on p57Kip2 to regulate cell cycle in neural cells

A precise balance between proliferation and differentiation must be maintained during neural development to obtain the correct proportion of differentiated cell types in the adult nervous system. The basic helix-loop-helix (bHLH) transcription factors known as E proteins and their natural inhibitors, the Id proteins, control the timing of differentiation and terminal exit from the cell cycle. Here we show that progression into S phase of human neuroblastoma cells is prevented by E proteins and promoted by Id2. Cyclin-dependent kinase inhibitors (CKI) have been identified as key effectors of cell cycle arrest in differentiating cells. However, p57Kip2 is the only CKI that is absolutely required for normal development. Through the use of global gene expression analysis in neuroblastoma cells engineered to acutely express the E protein E47 and Id2, we find that p57Kip2 is a target of E47. Consistent with the role of Id proteins, Id2 prevents activation of p57Kip2 expression, and the retinoblastoma tumor suppressor protein, a known Id2 inhibitor, counters this activity. The strong E47-mediated inhibition of entry into S phase is entirely reversed in cells in which expression of p57Kip2 is silenced by RNA interference. During brain development, expression of p57Kip2 is opposite that of Id2. Our findings identify p57Kip2 as a functionally relevant target recruited by bHLH transcription factors to induce cell cycle arrest in developing neuroblasts and suggest that deregulated expression of Id proteins may be an epigenetic mechanism to silence expression of this CKI in neural tumors.

A unique function for cyclin D3 in early B cell development

During hematopoiesis, stem cell proliferation is dependent on expression of the D-type cyclins. However, little is known about how each cyclin D contributes to the development of specific hematopoietic lineages. Here, analysis of Ccnd1(-/-), Ccnd2(-/-), Ccnd3(-/-) and Ccnd2(-/-)Ccnd3(-/-) mice showed that cyclin D3 was uniquely required for the development of pre-B cells. Transcription of Ccnd3 was dependent on expression of the common gamma-chain. In contrast, expression of the pre-B cell receptor and activation of 'downstream' signaling pathways prevented proteasome-mediated degradation of cyclin D3. Cyclin D3 has a key function in B cell development by integrating cytokine and pre-B cell receptor-dependent signals to expand the pool of pre-B cells that have successfully rearranged immunoglobulin heavy chain.

At the crossroads: diverse roles of early thymocyte transcriptional regulators

Transcriptional regulation of T-cell development involves successive interactions between complexes of transcriptional regulators and their binding sites within the regulatory regions of each gene. The regulatory modules that control expression of T-lineage genes frequently include binding sites for a core set of regulators that set the T-cell-specific background for signal-dependent control, including GATA-3, Notch/CSL, c-myb, TCF-1, Ikaros, HEB/E2A, Ets, and Runx factors. Additional regulators in early thymocytes include PU.1, Id-2, SCL, Spi-B, Erg, Gfi-1, and Gli. Many of these factors are involved in simultaneous regulation of non-T-lineage genes, T-lineage genes, and genes involved in cell cycle control, apoptosis, or survival. Potential and known interactions between early thymic transcription factors such as GATA-3, SCL, PU.1, Erg, and Spi-B are explored. Regulatory modules involved in the expression of several critical T-lineage genes are described, and models are presented for shifting occupancy of the DNA-binding sites in the regulatory modules of pre-Talpha, T-cell receptor beta (TCRbeta), recombinase activating genes 1 and 2 (Rag-1/2), and CD4 during T-cell development. Finally, evidence is presented that c-kit, Erg, Hes-1, and HEBAlt are expressed differently in Rag-2(-/-) thymocytes versus normal early thymocytes, which provide insight into potential regulatory interactions that occur during normal T-cell development.

Helix-loop-helix proteins and lymphocyte development

Helix-loop-helix (HLH) proteins are transcriptional regulators that control a wide variety of developmental pathways in both invertebrate and vertebrate organisms. Results obtained in the past decade have shown that HLH proteins also contribute to the development of lymphoid lineages. A subset of HLH proteins, the 'E proteins', seems to be particularly important for proper lymphoid development. Members of the E protein family include E12, E47, E2-2 and HEB. The E proteins contribute to B lineage- and T lineage-specific gene expression programs, regulate lymphocyte survival and cellular proliferation, activate the rearrangement of antigen receptor genes and control progression through critical developmental checkpoints. This review discusses HLH proteins in lymphocyte development and homeostasis.

Constitutively active Notch1 induces growth arrest of HPV-positive cervical cancer cells via separate signaling pathways

Notch signaling plays a key role in cell-fate determination and differentiation in different organisms and cell types. Several reports suggest that Notch signaling may be involved in neoplastic transformation. However, in primary keratinocytes, Notch1 can function as a tumor suppressor. Similarly, in HPV-positive cervical cancer cells, constitutively active Notch1 signaling was found to cause growth suppression. Activated Notch1 in these cells represses viral E6/E7 expression through AP-1 down-modulation, resulting in increased p53 expression and a block of pRb hyperphosphorylation. Here we show that in cervical cancer cell lines in which Notch1 ability to repress AP-1 activity is impaired, Notch1-enforced expression elicits an alternative pathway leading to growth arrest. Indeed, activated Notch1 signaling suppresses activity of the helix-loop-helix transcription factor E47, via ERK1/2 activation, resulting in inhibition of cell cycle progression. Moreover, we found that RBP-Jkappa-dependent Notch signaling is specifically repressed in cervical cancer cells and this repression could provide one such mechanism that needs to be activated for cervical carcinogenesis. Finally, we show that inhibition of endogenous Notch1 signaling, although results in a proliferative advantage, sensitizes cervical cancer cell lines to drug-induced apoptosis. Together, our results provide novel molecular insights into Notch1-dependent growth inhibitory effects, counteracting the transforming potential of HPV.

ID1 and ID2 are retinoic acid responsive genes and induce a G0/G1 accumulation in acute promyelocytic leukemia cells

Acute promyelocytic leukemia (APL) is uniquely sensitive to treatment with all-trans retinoic acid (ATRA), which results in the expression of genes that induce the terminal granulocytic differentiation of the leukemic blasts. Here we report the identification of two ATRA responsive genes in APL cells, ID1 and ID2. These proteins act as antagonists of basic helix-loop-helix (bHLH) transcription factors. ATRA induced a rapid increase in ID1 and ID2, both in the APL cell line NB4 as well as in primary patient cells. In addition, a strong downregulation of E2A was observed. E2A acts as a general heterodimerization partner for many bHLH proteins that are involved in differentiation control in various tissues. The simultaneous upregulation of ID1 and ID2, and the downregulation of E2A suggest a role for bHLH proteins in the induction of differentiation of APL cells following ATRA treatment. To test the relevance of this upregulation, ID1 and ID2 were overexpressed in NB4 cells. Overexpression inhibited proliferation and induced a G0/G1 accumulation. These results indicate that ID1 and ID2 are important retinoic acid responsive genes in APL, and suggest that the inhibition of specific bHLH transcription factor complexes may play a role in the therapeutic effect of ATRA in APL.

Engagement of membrane immunoglobulin enhances Id3 promoter activity in WEHI-231 B lymphoma cells

AIM

We have recently shown that engagement of membrane immunoglobulin (mIg) induced upregulation of inhibitor of differentiation 3 (Id3) mRNA, resulting in growth arrest at G1 phase in WEHI-231 cells. In the present study, we examined whether engagement of mIg will affect promoter activity of the Id3 gene in WEHI-231 cells.

METHODS

DNA fragments corresponding to the 5'-flanking region of mId3 gene were amplified by polymerase chain reaction (PCR) using genomic DNA as the template. Three DNA fragments upstream of the transcription start site (+1) of the mId3 gene were subcloned into the luciferase reporter vector PGV-B2. The recombinant constructs were transiently transfected into WEHI-231 cells by an electroporation method. After incubation for 24 h, WEHI-231 cells were stimulated with 10 mg/L anti-IgM or irradiated CD40L-expressing NIH3T3 cells or control NIH3T3 cells for further 24 h, followed by assay for luciferase activity.

RESULTS

The luciferase analysis demonstrated that basal promoter activity of the Id3 gene was found in the region between -200 and +54. The Id3 promoter activity was increased 2-fold following stimulation with anti-IgM, but not CD40L, compared with medium alone.

CONCLUSION

The mIg-mediated upregulation of Id3 expression is controlled, at least in part, through transcriptional regulation, as assessed by luciferase assay.

Expression of the bHLH transcription factor Tcf12 (ME1) gene is linked to the expansion of precursor cell populations during neurogenesis

In this study, we focused on the potential function of the murine gene Tcf12 (also known as ME1 or HEB) encoding the bHLH E-protein ME1 during brain development. An exencephaly phenotype of low penetrance has consistently been observed in both Tcf12 null mice and Tcf12(dm) homozygous mice. Thus, to address the possible underlying mechanism of the Tcf12 gene during the early steps of brain development, we performed a detailed analysis of its spatio-temporal expression pattern at distinct steps of gastrulation and neurogenesis. We found that Tcf12 transcripts are detected in the embryonic ectoderm prior to neural induction during gastrulation. During neurulation, Tcf12 transcripts are evident at high levels in the proliferating neuroepithelium of the neural folds and the cephalic mesenchyme. Thus, Tcf12 gene expression coincides with the massive proliferation occurring in the forming neuroepithelium and cephalic mesenchyme during neural tube formation, which is consistent with the exencephaly phenotype of Tcf12 null mice. In the developing cortex and spinal cord, Tcf12 expression is restricted to the proliferative ventricular zones, indicating that Tcf12 expression is down regulated when these neuronal cells undergo their final differentiation. Interestingly, we found that the postnatal Tcf12 expression parallels the ongoing adult neurogenesis in the mitotically active subventricular zone. Thus, the timing and location of Tcf12 expression combined with this severe neurulation defect support our hypothesis that the Tcf12 gene may be involved in the control of proliferating neural stem cells and progenitor cells and that it may be critical to sustain their undifferentiated state during embryonic and adult neurogenesis.

Identification of cyclin D3 as a direct target of E2A using DamID

The transcription factor E2A can promote precursor B cell expansion, promote G(1) cell cycle progression, and induce the expressions of multiple G(1)-phase cyclins. To better understand the mechanism by which E2A induces these cyclins, we characterized the relationship between E2A and the cyclin D3 gene promoter. E2A transactivated the 1-kb promoter of cyclin D3, which contains two E boxes. However, deletion of the E boxes did not disrupt the transactivation by E2A, raising the possibility of indirect activation via another transcription factor or binding of E2A to non-E-box DNA elements. To distinguish between these two possibilities, promoter occupancy was examined using the DamID approach. A fusion construct composed of E2A and the Escherichia coli DNA adenosine methyltransferase (E47Dam) was subcloned in lentivirus vectors and used to transduce precursor B-cell and myeloid progenitor cell lines. In both cell types, specific adenosine methylation was identified at the cyclin D3 promoter. Chromatin immunoprecipitation analysis confirmed the DamID findings and localized the binding to within 1 kb of the two E boxes. The methylation by E47Dam was not disrupted by mutations in the E2A portion that block DNA binding. We conclude that E2A can be recruited to the cyclin D3 promoter independently of E boxes or E2A DNA binding activity.

Id family of transcription factors and vascular lesion formation

Vascular smooth muscle cell (VSMC) modulation to a de-differentiated phenotype and proliferation are key components of vascular lesion formation. Understanding how these processes are regulated is essential to understanding the progression of vascular diseases such as atherosclerosis and in-stent restenosis. The Id family of helix-loop-helix (HLH) transcription factors has emerged as important regulators of cellular growth and differentiation. Recent published findings have implicated the Id proteins as important regulators of growth and phenotypic modulation in VSMC and in the vascular response to injury. In this review, we summarize what is known regarding how the Id proteins function to control cellular growth and differentiation and their role in vascular lesion formation.

Differential functions for the transcription factor E2A in positive and negative gene regulation in pre-B lymphocytes

The transcription factors encoded by the E2A gene have been shown to play essential roles in the initiation and progression of lymphocyte development. However, there is still a lack of comprehensive understanding of E2A downstream genes in B-cell development. We previously developed a gene tagging-based chromatin immunoprecipitation (ChIP) system to directly evaluate E2A target genes in B-cell development. Here, we have improved this ChIP strategy and used it in conjunction with microarray analysis on E2A-deficient pre-B-cell lines to determine E2A target genes in lymphocyte development. Both microarray data and ChIP studies confirmed that E2A directly controls IgH gene expression. The microarray assay also revealed genes that were significantly up-regulated after E2A disruption. ChIP analysis showed that E2A was most likely to be directly involved in repression of some of these target genes such as Nfil3 and FGFR2. An inducible E2A reconstitution system further demonstrated that E2A-mediated repression of Nfil3 and FGFR2 was reversible. Collectively, these findings indicate that E2A is a positive regulator for one set of genes and a negative regulator for another set of genes in developing B lymphocytes.

CD40 Ligand Rescues Inhibitor of Differentiation 3-Mediated G1 Arrest Induced by Anti-IgM in WEHI-231 B Lymphoma Cells

The engagement of membrane-bound Igs (mIgs) results in growth arrest, accompanied by apoptosis, in the WEHI-231 murine B lymphoma cells, a cell line model representative of primary immature B cells. Inhibitor of differentiation (Id) proteins, members of the helix-loop-helix protein family, functions in proliferation, differentiation, and apoptosis in a variety of cell types. In this study, we analyzed the involvement of Id protein in mIg-induced growth arrest and apoptosis in WEHI-231 cells. Following stimulation with anti-IgM, expression of Id3 was up-regulated at both the mRNA and protein levels; this up-regulation could be reversed by CD40L treatment. Retrovirus-mediated transduction of the Id3 gene into WEHI-231 cells resulted in an accumulation of the cells in G(1) phase, but did not induce apoptosis. E box-binding activity decreased in response to anti-IgM administration, but increased after stimulation with either CD40L alone or anti-IgM plus CD40L, suggesting that E box-binding activity correlates with cell cycle progression. WEHI-231 cells overexpressing Id3 accumulated in G(1) phase, which was accompanied by reduced levels of cyclin D2, cyclin E, and cyclin A, and a reciprocal up-regulation of p27(Kip1). Both the helix-loop-helix and the C-terminal regions of Id3 were required for growth-suppressive activity. These data suggest that Id3 mimics mIg-mediated G(1) arrest in WEHI-231 cells.

Altered T-dependent antigen responses and development of autoimmune symptoms in mice lacking E2A in T lymphocytes

E2A has been shown to be an important transcription factor downstream of the T-cell receptor (TCR) signal during T-cell development. The TCR signal is known to elicit different cellular responses at different stages of T-cell development. Whether E2A is still required for normal TCR signalling in mature T cells is unknown. Here we examined T-cell function after disruption of the E2A gene exclusively in the T-cell lineage. The conditional E2A-deficient mice show enhanced humoral immunity to a T-dependent antigen. We further show that E2A is involved in regulating TCR-induced T-cell proliferation events. However, E2A seems to play opposite roles in naïve and effector T cells. In the absence of E2A, TCR-induced proliferation is increased in naïve T cells and decreased in effector T cells. At older ages, these mice frequently develop antinuclear antibodies and proteinuria. Our studies suggest that E2A regulates T-cell function and the loss of E2A may promote age-dependent autoimmune diseases.

Orphan nuclear receptor small heterodimer partner, a novel corepressor for a basic helix-loop-helix transcription factor BETA2/neuroD

Small heterodimer partner (SHP; NR0B2) is an atypical orphan nuclear receptor that lacks a conventional DNA binding domain (DBD) and represses the transcriptional activity of various nuclear receptors. In this study, we examined the novel cross talk between SHP and BETA2/NeuroD, a basic helix-loop-helix transcription factor. In vitro and in vivo protein interaction studies showed that SHP physically interacts with BETA2/NeuroD, but not its heterodimer partner E47. Moreover, confocal microscopic study and immunostaining results demonstrated that SHP colocalized with BETA2 in islets of mouse pancreas. SHP inhibited BETA2/NeuroD-dependent transactivation of an E-box reporter, whereas SHP was unable to repress the E47-mediated transactivation and the E-box mutant reporter activity. In addition, SHP repressed the BETA2-dependent activity of glucokinase and cyclin-dependent kinase inhibitor p21 gene promoters. Gel shift and in vitro protein competition assays indicated that SHP inhibits neither dimerization nor DNA binding of BETA2 and E47. Rather, SHP directly repressed BETA2 transcriptional activity and p300-enhanced BETA2/NeuroD transcriptional activity by inhibiting interaction between BETA2 and coactivator p300. We also showed that C-terminal repression domain within SHP is also required for BETA2 repression. However, inhibition of BETA2 activity was not observed by naturally occurring human SHP mutants that cannot interact with BETA2/NeuroD. Taken together, these results suggest that SHP acts as a novel corepressor for basic helix-loop-helix transcription factor BETA2/NeuroD by competing with coactivator p300 for binding to BETA2/NeuroD and by its direct transcriptional repression function.

E2A proteins enforce a proliferation checkpoint in developing thymocytes

E2A proteins regulate multiple stages of thymocyte development and suppress T-cell lymphoma. The activity of E2A proteins throughout thymocyte development is modulated by signals emanating from the pre-TCR and TCR. Here we demonstrate that E2A is required for the complete arrest in both differentiation and proliferation observed in thymocytes with defects in proteins that mediate pre-TCR signaling, including LAT, Lck and Fyn. We show that E2A proteins are required to prevent the accumulation of TCRbeta negative cells beyond the pre-TCR checkpoint. E2A-deficient thymocytes also exhibit abnormal cell-cycle progression prior to pre-TCR expression. Furthermore, we demonstrate that E47 can act in concert with Bcl-2 to induce cell-cycle arrest in vitro. These observations indicate that E2A proteins function during early thymocyte development to block cell-cycle progression prior to the expression of TCRbeta. In addition, these data provide further insight into how deficiencies in E2A lead to T lymphoma.

E-proteins directly regulate expression of activation-induced deaminase in mature B cells

Activated mature B cells in which the DNA-binding activity of E-proteins has been disrupted fail to undergo class switch recombination. Here we show that activated B cells overexpressing the antagonist helix-loop-helix protein Id3 do not induce expression of the murine Aicda gene encoding activation-induced deaminase (AID). A highly conserved intronic regulatory element in Aicda binds E-proteins both in vitro and in vivo. The transcriptional activity of this element is regulated by E-proteins. We show that the enforced expression of AID in cells overexpressing Id3 partially restores class switch recombination. Taken together, our observations link helix-loop-helix activity and Aicda gene expression in a common pathway, in which E-protein activity is required for the efficient induction of Aicda transcription.

Inhibition of p300/CBP by early B-cell factor

Early B-cell factor (EBF) is a DNA binding protein required for early B-cell development. It activates transcription of several B-cell-specific genes, including the lambda5 gene, which encodes a protein necessary for signaling by the pre-B-cell receptor. In an effort to understand the mechanism by which EBF activates transcription, we examined its interaction with the coactivator protein p300/CBP. We found that two domains of EBF each bind the histone acetyltransferase (HAT)/CH3 domain of p300/CBP both in vitro and in vivo. Surprisingly, transcriptional activation by EBF was not sensitive to E1A, a potent p300/CBP inhibitor. In fact, overexpressed EBF mimicked E1A by severely repressing the activity of several other transcription factors, including E47, a protein that acts cooperatively with EBF to promote transcription of the lambda5 gene. This broad inhibitory profile correlated with EBF's ability to repress the HAT activity of p300/CBP in vivo and in vitro. However, such a repressed complex is not likely to form at the lambda5 promoter in vivo since (i) EBF could not bind p300/CBP and DNA simultaneously and (ii) the cooperativity imparted by E47 was sensitive to E1A. Our data reveal an intriguing inhibitory property of EBF-a property shared only by E1A, Twist, Pu.1, and the Hox family of homeodomain proteins-and suggest that E47 and EBF play distinct roles during lambda5 promoter activation.

Effects of aging on proliferation and E47 transcription factor activity induced by different stimuli in murine splenic B cells

In the present paper, we have investigated the effects of aging on the expression and function of the E2A-encoded transcription factor E47 in splenic B lymphocytes, unactivated or activated with different stimuli (LPS, anti-CD40, anti-IgM, alone or in combination with IL-4). Results indicate that unstimulated splenic B cells show very low E47 protein levels as well as E47 DNA-binding activity and that, upon B cell activation, E47 expression and DNA-binding activity are strongly induced in young and, to a significantly lesser extent, in old mice. The level of E47 protein expression in stimulated splenic B cells was found significantly higher in young than in old mice, suggesting that DNA-binding activity correlates with protein expression. These results altogether suggest that the reduced expression of the transcriptional regulator E47 could help explain the reduced B cell functions in aging mice.

Decreased E12 and/or E47 transcription factor activity in the bone marrow as well as in the spleen of aged mice

The E2A-encoded transcription factors E12 and E47 are key regulators of B cell functions. They bind to the E-box site, found in regulatory regions of B cell-specific genes; promote cell survival of early pre-B cells; help to initiate Ig rearrangements; and are also involved in class switch in mature B cells in the periphery. We have investigated the expression and function of E47 and E12 in IL-7-expanded pro-B/pre-B cell precursors and in unstimulated or LPS-activated splenic B cells from young and old BALB/c mice. Results show that B cell precursors from the bone marrow of old mice exhibit a reduced expression of E2A proteins and a reduced ability to bind DNA, as compared with young mice. In the spleen, E2A protein expression and DNA binding are present in unstimulated B cells from young mice and, to a significantly lesser extent, from old mice. These are both strongly induced by activation in splenic B cells from young mice but only moderately induced in old mice, indicating that aging affects the expression and activity of E2A-encoded genes and also that DNA binding correlates with the amount of protein expression. The levels of E2A DNA binding in the spleen correlate with those in the bone marrow for individual mice. In splenic mature B cells, only E47/E47 complexes bind DNA; whereas in bone marrow B cell precursors, E47/E12 complexes participate in DNA binding. Only nuclear extracts of splenic mature B cells, but both nuclear and cytoplasmic extracts of bone marrow B cell precursors, exhibit DNA binding.

Regulation of early lymphocyte development by E2A family proteins

Lymphocytes develop from hematopoietic stem cells through a series of highly regulated differentiation events in the bone marrow and thymus. A number of transcription factors are known to collaborate in controlling the timing and specificity of gene expression required for these developmental processes to occur. The basic helix-loop-helix (bHLH) proteins encoded by the E2A gene have been shown to play particularly important roles in the initiation and progression of lymphocyte differentiation. Gene targeting experiments in mice have demonstrated a requirement for E2A proteins at the onset of B lymphocyte development. More recent studies have broadened our view on the function of E2A proteins at multiple stages of lymphopoiesis and in the regulation of lymphoid-specific gene expression. Here we review the mammalian E2A proteins and the accumulated evidence demonstrating central roles for E2A throughout early B and T lymphocyte development. We also speculate on the direction of future research on the mechanisms underlying the lineage and stage-specific functions of E2A in lymphopoiesis.

Levels of E2A protein expression in B cell precursors are stage-dependent and inhibited by stem cell factor (c-kit ligand)

OBJECTIVE

The E2A-encoded proteins E47 and E12 are crucial to the development of pro-B and pre-B cells. The expression of E2A protein and mRNA during early B lymphopoiesis was determined and effects of stem cell factor (SCF; Steel factor; c-kit ligand) on E2A expression were evaluated.

MATERIALS AND METHODS

Ex vivo murine pro-B cells and pre-B cells were isolated and in vitro B cell precursors were derived after culture of bone marrow with rmIL-7. Levels of E2A proteins were determined by Western analysis and mRNA by RT-PCR. E2A expression in vitro was also assessed in cultures supplemented with IL-7 +/- recombinant murine SCF (rmSCF), insulin-like growth factor-1 (rhIGF-1), or Flt3-ligand (rhFlt3-L). Turnover of E2A proteins was assessed following cycloheximide treatment.

RESULTS

Ex vivo, pro-B cells had lower E47 protein levels than did pre-B cells but had comparable E2A mRNA levels. E2A protein, but not mRNA, levels were reduced in pro-B cells upon culture in vitro with IL-7 + rmSCF. This was associated with increased turnover of E2A proteins. In contrast, culture with IL-7 + IGF-1 or Flt3-L had minimal effects on E2A protein levels.

CONCLUSION

Pre-B cells expressed higher levels of E2A protein than did pro-B cells and this mainly resulted from posttranscriptional regulation. Exogenous SCF inhibited E2A protein, but not mRNA, expression in cultured B cell precursors, possibly by increasing E2A protein turnover. The capacity to respond to SCF may influence the levels of E2A during B-cell development.

Proviral activation of the tumor suppressor E2a contributes to T cell lymphomagenesis in EμMyc transgenic mice

The basic helix-loop-helix factor E2A plays an important role in the development of B and T lymphocytes. In addition, E2a has been implicated as a gene with tumor suppressor activity, since mice deficient for E2a succumb to T cell lymphomas. We have performed retroviral tagging in EmuMyc transgenic mice to identify genes that contribute to lymphomagenesis. The EmuMyc transgenic mouse is a well-established model of a common translocation in human B cell lymphomas. Analyses of the proviral insertion sites in the MuLV-induced lymphomas revealed that a number of T cell lymphomas carried proviral insertions in the promoter region of E2a. These proviral insertions yield hybrid viral-E2a mRNAs resulting in a marked rise in E2A protein levels. The proviral insertions in E2a were predominantly of clonal origin indicating that E2a insertions are early events in these T cell lymphomas. The primary oncogenic effect of E2A is likely to be associated with enhancement of transcription of the c-Myc transgene via binding to the regulatory immunoglobulin enhancers. The results herein thus provide the first evidence that in a specific setting E2A overexpression can contribute to T-lymphomagenesis. This implies that E2a contains oncogenic features in addition to the previously described tumor suppressive properties.

daughterlesscoordinates somatic cell proliferation, differentiation and germline cyst survival during follicle formation inDrosophila

During Drosophila oogenesis two distinct stem cell populations produce either germline cysts or the somatic cells that surround each cyst and separate each formed follicle. From analyzing daughterless (da) loss-of-function, overexpression and genetic interaction phenotypes, we have identified several specific requirements for da+ in somatic cells during follicle formation. First, da is a critical regulator of somatic cell proliferation. Also, da is required for the complete differentiation of polar and stalk cells, and elevated da levels can even drive the convergence and extension that is characteristic of interfollicular stalks. Finally, da is a genetic regulator of an early checkpoint for germline cyst progression: Loss of da function inhibits normally occurring apoptosis of germline cysts at the region 2a/2b boundary of the germarium, while da overexpression leads to postmitotic cyst degradation. Collectively, these da functions govern the abundance and diversity of somatic cells as they coordinate with germline cysts to form functional follicles.

Gradient of E2A activity in B-cell development

The E2A locus is a frequent target of chromosomal translocations in B-cell acute lymphoblastic leukemia (B-ALL). E2A encodes two products, E12 and E47, that are part of the basic helix-loop-helix (bHLH) family of transcription factors and are central in B lineage differentiation. E2A haplo-insufficiency hinders progression through three major checkpoints in B-cell development: commitment into the B lineage, at the pro-B to pre-B transition, and in the induction of immunoglobulin M (IgM) expression required for a functional BCR. These observations underscore the importance of E2A gene dosage in B-cell development. Here we show that a higher proportion of pro-B cells in E2A(+/-) mice is in the cell cycle compared to that in wild-type littermates. This increase correlates with lower p21(waf/cip1) levels, indicating that E2A has an antiproliferative function in B-cell progenitors. Ectopic expression in the B lineage of SCL/Tal1, a tissue-specific bHLH factor that inhibits E2A function, blocks commitment into the B lineage without affecting progression through later stages of differentiation. Furthermore, ectopic SCL expression exacerbates E2A haplo-insufficiency in B-cell differentiation, indicating that SCL genetically interacts with E2A. Taken together, these observations provide evidence for a gradient of E2A activity that increases from the pre-pro-B to the pre-B stage and suggest a model in which low levels of E2A (as in pro-B cells) are sufficient to control cell growth, while high levels (in pre-B cells) are required for cell differentiation. The antiproliferative function of E2A further suggests that in B-ALL associated with t(1;19) and t(17;19), the disruption of one E2A allele contributes to leukemogenesis, in addition to other anomalies induced by E2A fusion proteins.

CD40 Ligand Rescues Inhibitor of Differentiation 3-Mediated G1Arrest Induced by Anti-IgM in WEHI-231 B Lymphoma Cells

The engagement of membrane-bound Igs (mIgs) results in growth arrest, accompanied by apoptosis, in the WEHI-231 murine B lymphoma cells, a cell line model representative of primary immature B cells. Inhibitor of differentiation (Id) proteins, members of the helix-loop-helix protein family, functions in proliferation, differentiation, and apoptosis in a variety of cell types. In this study, we analyzed the involvement of Id protein in mIg-induced growth arrest and apoptosis in WEHI-231 cells. Following stimulation with anti-IgM, expression of Id3 was up-regulated at both the mRNA and protein levels; this up-regulation could be reversed by CD40L treatment. Retrovirus-mediated transduction of the Id3 gene into WEHI-231 cells resulted in an accumulation of the cells in G1phase, but did not induce apoptosis. E box-binding activity decreased in response to anti-IgM administration, but increased after stimulation with either CD40L alone or anti-IgM plus CD40L, suggesting that E box-binding activity correlates with cell cycle progression. WEHI-231 cells overexpressing Id3 accumulated in G1phase, which was accompanied by reduced levels of cyclin D2, cyclin E, and cyclin A, and a reciprocal up-regulation of p27Kip1. Both the helix-loop-helix and the C-terminal regions of Id3 were required for growth-suppressive activity. These data suggest that Id3 mimics mIg-mediated G1arrest in WEHI-231 cells.