Regulation of the pro-B-cell-specific enhancer of the Id1 gene involves the C/EBP family of proteins

ID1 and CEBPA coordinate epidermal progenitor cell differentiation

The regulatory circuits that coordinate epidermal differentiation during development are still not fully understood. Here, we report that the transcriptional regulator ID1 is enriched in mouse basal epidermal progenitor cells and find ID1 expression to be diminished upon differentiation. In utero silencing of Id1 impairs progenitor cell proliferation, leads to precocious delamination of targeted progenitor cells and enables differentiated keratinocytes to retain progenitor markers and characteristics. Transcriptional profiling suggests that ID1 acts by mediating adhesion to the basement membrane while inhibiting spinous layer differentiation. Co-immunoprecipitation reveals ID1 binding to transcriptional regulators of the class I bHLH family. We localize bHLH Tcf3, Tcf4 and Tcf12 to epidermal progenitor cells during epidermal stratification and establish TCF3 as a downstream effector of ID1-mediated epidermal proliferation. Finally, we identify crosstalk between CEBPA, a known mediator of epidermal differentiation, and Id1, and demonstrate that CEBPA antagonizes BMP-induced activation of Id1. Our work establishes ID1 as a key coordinator of epidermal development, acting to balance progenitor proliferation with differentiation and unveils how functional crosstalk between CEBPA and Id1 orchestrates epidermal lineage progression.

Id proteins: emerging roles in CNS disease and targets for modifying neural stemcell behavior

Neural stem/progenitor cells (NSPCs) are found in the adult brain and spinal cord, and endogenous or transplanted NSPCs contribute to repair processes and regulate immune responses in the CNS. However, the molecular mechanisms of NSPC survival and integration as well as their fate determination and functionality are still poorly understood. Inhibitor of DNA binding (Id) proteins are increasingly recognized as key determinants of NSPC fate specification. Id proteins act by antagonizing the DNA-binding activity of basic helix-loop-helix (bHLH) transcription factors, and the balance of Id and bHLH proteins determines cell fate decisions in numerous cell types and developmental stages. Id proteins are central in responses to environmental changes, as they occur in CNS injury and disease, and cellular responses in adult NSPCs implicate Id proteins as prime candidates for manipulating stemcell behavior. Here, we outline recent advances in understanding Id protein pleiotropic functions in CNS diseases and propose an integrated view of Id proteins and their promise as potential targets in modifying stemcell behavior to ameliorate CNS disease.

CCAAT/Enhancer Binding Protein β inhibits myogenic differentiation via ID3

Myogenesis is regulated by the coordinated expression of muscle regulatory factors, a family of transcription factors that includes MYOD, MYF5, myogenin and MRF4. Muscle regulatory factors are basic helix-loop-helix transcription factors that heterodimerize with E proteins to bind the regulatory regions of target genes. Their activity can be inhibited by members of the Inhibitor of DNA binding and differentiation (ID) family, which bind E-proteins with high affinity, thereby preventing muscle regulatory factor-dependent transcriptional responses. CCAAT/Enhancer Binding protein beta (C/EBPβ) is a transcription factor expressed in myogenic precursor cells that acts to inhibit myogenic differentiation, though the mechanism remains poorly understood. We identify Id3 as a novel C/EBPβ target gene that inhibits myogenic differentiation. Overexpression of C/EBPβ stimulates Id3 mRNA and protein expression, and is required for C/EBPβ-mediated inhibition of myogenic differentiation. Misexpression of C/EBPβ in myogenic precursors, such as in models of cancer cachexia, prevents the differentiation of myogenic precursors and we show that loss of Id3 rescues differentiation under these conditions, suggesting that the stimulation of Id3 expression by C/EBPβ is an important mechanism by which C/EBPβ inhibits myogenic differentiation.

Id proteins: small molecules, mighty regulators

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

Id1 has a physiological role in regulating early B lymphopoiesis

Basic helix-loop-helix E proteins play critical roles in B-cell development by stimulating B cell-specific gene expression and immunoglobulin gene rearrangement. The function of E proteins can be effectively suppressed by their naturally occurring inhibitors, Id1 to 4. Ectopic expression of Id1 has been shown to block B-cell development at the early pro-B cell stage. However, whether Id1 plays a physiological role in controlling B lymphopoiesis was not known. Although Id1-deficient mice do not exhibit significant abnormalities in steady-state B lymphopoiesis, we detected more robust B-cell engraftment in transplant recipients of Id1-deficient bone marrow compared to those of wild-type donor cells. In culture, Id1 ablation dramatically enhances B-lineage cell production without any marked effects on myeloid differentiation. Consistently, Id1 expression was found in pro-B but not pre-B cells as measured by enhanced green fluorescent protein (EGFP) fluorescence and by quantitative reverse transcription-PCR. Although loss of Id1 did not alter the number of B-cell colonies generated from whole bone marrow or the proliferation rate of developing B cells, B-cell colonies were detectable at a much earlier time point and the size of the colonies were larger. Therefore, we infer that Id1-deficient progenitors possess higher potential to differentiate to the pre-B cell stage when a proliferative burst occurs. Taken together, we present evidence to suggest that Id1 plays a physiological role in restraining the developmental progression, which may be important for proper B-cell differentiation in the bone marrow.

Interaction between HMGA1 and retinoblastoma protein is required for adipocyte differentiation

It is generally accepted that the regulation of adipogenesis prevents obesity. However, the mechanisms controlling adipogenesis have not been completely defined. We have previously demonstrated that HMGA1 proteins play a critical role in adipogenesis. In fact, suppression of HMGA1 protein synthesis by antisense technology dramatically increased growth rate and impaired adipocyte differentiation in 3T3-L1 cells. Furthermore, we showed that HMGA1 strongly potentiates the capacity of the CCAAT/enhancer-binding protein beta (C/EBPbeta) transcriptional factor to transactivate the leptin promoter, an adipocytic-specific promoter. In this study we demonstrate that HMGA1 physically interacts with retinoblastoma protein (RB), which is also required in adipocyte differentiation. Moreover, we show that RB, C/EBPbeta, and HMGA1 proteins all cooperate in controlling both Id1 and leptin gene transcriptions, which are down- and up-regulated during adipocyte differentiation, respectively. We also demonstrate that HMGA1/RB interaction regulates CDC25A and CDC6 promoter activities, which are induced by E2F-1 protein during early adipocyte differentiation, by displacing HDAC1 from the RB-E2F1 complex. Furthermore, by using Hmga1(-/-) embryonic stem cells, which failed to undergo adipocyte differentiation, we show the crucial role of HMGA1 proteins in adipocyte differentiation due to its pivotal involvement in the formation of the RB-C/EBPbeta complex. Altogether these data demonstrate a key role of the interaction between HMGA1 and RB in adipocyte differentiation.

Interleukin-6 aborts lymphopoiesis and elevates production of myeloid cells in systemic lupus erythematosus-prone B6.Sle1.Yaa animals

We previously reported the inhibitory action of interleukin-6 (IL-6) on B lymphopoiesis with SHIP(-/-) mice and showed that IL-6 biases lineage commitment toward myeloid cell fates in vitro and in vivo. Because elevated IL-6 is a feature of chronic inflammatory diseases, we applied an animal model of systemic lupus erythematosus (SLE) to determine whether IL-6 has similar effects on hematopoiesis. We found that IL-6 levels were elevated in the B6.Sle1.Yaa mice, and the increase was accompanied by losses of CD19(+) B cells and more primitive B-lymphoid progenitors in bone marrow. Both the CD19(+) B-cell population and their progenitors recovered in an IL-6(-/-) background. The uncommitted progenitors, containing precursors for both lymphoid and myeloid fates, expressed IL-6 receptor-alpha chain and responded to IL-6 by phosphorylation of STAT3. IL-6 stimulation caused uncommitted progenitors to express the Id1 transcription factor, which is known to inhibit lymphopoiesis and elevate myelopoiesis, and its expression was MAPK dependent. We conclude that chronic inflammatory conditions accompanied by increased IL-6 production bias uncommitted progenitors to a myeloid fate by inducing Id1 expression.

Linkage studies for T2D in Chop and C/EBPbeta chromosomal regions in Italians

The genes causing type 2 diabetes (T2D), a complex heterogeneous disorder, differ and/or overlap in various populations. Among others there are two loci in linkage to T2D, the chromosomes 20q12-13.1 and 12q15. These two regions harbor two genes, C/EBPbeta and CHOP, which are excellent candidate genes for T2D. In fact, C/EBPbeta protein cooperates with HNF4alpha (MODY1, monogenic form of diabetes) and 1alpha (MODY3, monogenic form of diabetes). C/EBPbeta mediates suppression of insulin gene transcription in hyperglycemia and may contribute to insulin-resistance. It interacts in a complex pathway with the CHOP protein. CHOP may play a role in altered beta-cell glucose metabolism, in beta-cell apoptosis, and in lack of beta-cell replication. Thus, both C/EBPbeta and CHOP genes may independently and interactively contribute to T2D. The chromosomal regions targeting C/EBPbeta and CHOP genes have never been previously explored in T2D. We planned to identify their potential contribution to T2D in Italians. We have genotyped a group of affected siblings/families with both late- and early-onset T2D around the C/EBPbeta and the CHOP genes. We have performed non-parametric linkage analysis in the total T2D group, in the late-onset and the early-onset group, separately. We have identified a suggestive linkage to T2D in the CHOP gene locus in the early-onset T2D group (P = 0.04). We identified the linkage to T2D in the chromosome 12q15 region in the early-onset T2D families and specifically target the CHOP gene. Our next step will be the identification of CHOP gene variants, which may contribute to the linkage to T2D in Italians.

The C/EBP family of transcription factors: a paradigm for interaction between gene expression and proliferation control

In recent years, a link between the transcriptional regulators of lineage-specific gene expression and progenitor proliferation control has emerged. A main exponent of this phenomenon is the CCAAT/enhancer binding protein (C/EBP) family of basic region-leucine zipper proteins. These transcription factors control the differentiation of a range of cell types, and have key roles in regulating cellular proliferation through interaction with cell cycle proteins. More recently, their position at the crossroads between proliferation and differentiation has made them strong candidate regulators of tumorigenesis, and C/EBPs have been described as both tumor promoters and tumor suppressors.

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.

BMP-6 inhibits human bone marrow B lymphopoiesis—Upregulation of Id1 and Id3

OBJECTIVE

In mammals, factors produced by bone marrow (BM) stromal cells are instrumental in orchestrating the developmental process of B lymphocytes. Bone morphogenetic proteins (BMPs) are multifunctional cytokines previously found to regulate hematopoietic stem cells. In the present study, we have explored the role of BMP-6 in human B progenitor cells.

MATERIALS AND METHODS

In vitro B lymphopoiesis of CD10(+) B progenitor cells from human BM was evaluated in the presence or absence of BMP-6 in short- or long-term coculture on MS-5 stromal cells, by tracking CFSE-labeled CD10(+) B progenitor cells or by quantification of CD19(+) cells. DNA synthesis in the pre-B cell line Nalm-6 was measured by (3)H-thymidine incorporation. BMP-6-induced phosphorylation of Smad1/5/8 was determined by Western blot analysis, whereas elevation of Id1-Id4 mRNA levels and basal BMP-6 mRNA levels were measured by real-time and conventional RT-PCR, respectively.

RESULTS

By in vitro coculture of CD10(+) B progenitor cells or monoculture of Nalm-6 cells, we found that BMP-6 inhibited B lymphopoiesis by impeding cell proliferation. Furthermore, in CD10(+) B progenitors as well as in Nalm-6 cells, BMP-6 rapidly induced phosphorylation of Smad1/5/8, followed by an upregulation of Id1 and Id3 mRNA levels. Finally, we demonstrated that human bone marrow stromal cells express BMP-6 mRNA whereas B progenitor cells did not.

CONCLUSIONS

We suggest that BMP-6, produced by the BM, may participate to fine-tune the balance between proliferation, apoptosis, and differentiation in human B progenitor cells during BM B lymphopoiesis.

Gene regulatory networks orchestrating B cell fate specification, commitment, and differentiation

The B cell developmental pathway represents a leading system for the analysis of regulatory circuits that orchestrate cell fate specification, commitment, and differentiation. We review the progress that has been achieved in the identification and characterization of regulatory components of such circuits, including transcription factors, chromatin modifying proteins, and signaling molecules. A comprehensive developmental model is proposed that invokes sequentially acting regulatory networks which dictate the generation of B cells from multipotential hematopoietic progenitors.

BMP-6 inhibits growth of mature human B cells; induction of Smad phosphorylation and upregulation of Id1

Background

Bone morphogenetic proteins (BMPs) belong to the TGF-β superfamily and are secreted proteins with pleiotropic roles in many different cell types. A potential role of BMP-6 in the immune system has been implied by various studies of malignant and rheumatoid diseases. In the present study, we explored the role of BMP-6 in normal human peripheral blood B cells.

Results

The B cells were found to express BMP type I and type II receptors and BMP-6 rapidly induced phosphorylation of Smad1/5/8. Furthermore, Smad-phosphorylation was followed by upregulation of Id1 mRNA and Id1 protein, whereas Id2 and Id3 expression was not affected. Furthermore, we found that BMP-6 had an antiproliferative effect both in naïve (CD19⁺CD27^-) and memory B cells (CD19⁺CD27⁺) stimulated with anti-IgM alone or the combined action of anti-IgM and CD40L. Additionally, BMP-6 induced cell death in activated memory B cells. Importantly, the antiproliferative effect of BMP-6 in B-cells was completely neutralized by the natural antagonist, noggin. Furthermore, B cells were demonstrated to upregulate BMP-6 mRNA upon stimulation with anti-IgM.

Conclusion

In mature human B cells, BMP-6 inhibited cell growth, and rapidly induced phosphorylation of Smad1/5/8 followed by an upregulation of Id1.

Genetic networks that regulate B lymphopoiesis

PURPOSE OF REVIEW

The B cell developmental pathway represents a leading model within the hematopoietic system for the analysis of genetic networks, which orchestrate cell fate specification and commitment. Considerable progress is being achieved in the characterization of regulatory components that comprise such networks and examining their connectivity. These components include the cytokine receptors Flk2 and IL-7R as well as the transcription factors PU.1, Ikaros, Bcl11a, E2A, EBF, and Pax-5. Based on new experimental evidence, a comprehensive model is proposed that invokes sequentially acting and inter-dependent regulatory modules that instruct the generation of B cell precursors from multipotential hematopoietic progenitors.

RECENT FINDINGS

The transcription factor PU.1 regulates the generation of lymphoid progenitors that express Flk2 and IL-7R. IL-7R receptor signaling appears to function in specification of the B cell fate. The transcription factor EBF can bypass the requirement for PU.1 and E2A in early B cell development. Pax-5 expression and function are contingent on EBF.

SUMMARY

Assembly of gene regulatory networks involved in cell fate specification may facilitate the efficient and directed generation of lineage-specific hematopoietic progenitors from embryonic stem cells for therapeutic purposes.

Regulation of Id2 expression by CCAAT/enhancer binding protein beta

Mice deficient for Id2, a negative regulator of basic helix–loop–helix (bHLH) transcription factors, exhibit a defect in lactation due to impaired lobuloalveolar development during pregnancy, similar to the mice lacking the CCAAT enhancer binding protein (C/EBP) β. Here, we show that Id2 is a direct target of C/EBPβ. Translocation of C/EBPβ into the nucleus, which was achieved by using a system utilizing the fusion protein between C/EBPβ and the ligand-binding domain of the human estrogen receptor (C/EBPβ-ERT), demonstrated the rapid induction of endogenous Id2 expression. In reporter assays, transactivation of the Id2 promoter by C/EBPβ was observed and, among three potential C/EBPβ binding sites found in the 2.3 kb Id2 promoter region, the most proximal element was responsible for the transactivation. Electrophoretic mobility shift assay (EMSA) identified this element as a core sequence to which C/EBPβ binds. Chromatin immunoprecipitation (ChIP) furthermore confirmed the presence of C/EBPβ in the Id2 promoter region. Northern blotting showed that Id2 expression in C/EBPβ-deficient mammary glands was reduced at 10 days post coitus (d.p.c.), compared with that in wild-type mammary glands. Thus, our data demonstrate that Id2 is a direct target of C/EBPβ and provide insight into molecular mechanisms underlying mammary gland development during pregnancy.

Contingent gene regulatory networks and B cell fate specification

The B cell developmental pathway represents a leading system for the analysis of regulatory circuits that orchestrate cell fate specification and commitment. Considerable progress has been achieved within the past decade in the identification and genetic analysis of various regulatory components. These components include the transcription factors PU.1, Ikaros, Bcl11a, E2A, EBF, and Pax-5, as well as the cytokine receptors Flk2 and IL-7R. Experimental evidence of connectivity among the regulatory components is used to assemble sequentially acting and contingent gene regulatory networks. Transient signaling inputs, self-sustaining positive feedback loops, and cross-antagonism among alternate cell fate determinants are key features of the proposed networks that instruct the development of B lymphocyte precursors from hematopoietic stem cells.

Characterization of the proximal enhancer element and transcriptional regulatory factors for murine recombination activating gene-2

Recombination-activating gene (RAG)-1 and RAG-2 are essential for V(D)J recombination and are expressed specifically in lymphoid cells. We previously identified two putative enhancer elements, the proximal and distal enhancers, located at -2.6 and -8 kb, respectively, 5' upstream of mouse RAG-2, and characterized the distal enhancer element in detail. In this study, to characterize the proximal enhancer in vitro as well as in vivo, we first defined a 170-bp core enhancer element within the proximal enhancer (Ep) and determined its activity in various cells. Ep conferred enhancer activity only in B-lymphoid cell lines, but not in T- or non-lymphoid cell lines. Analysis of the transgenic mice carrying an EGFP reporter gene linked with Ep revealed that Ep activated the transcription of the reporter gene in bone marrow and spleen, but not in thymus or non-lymphoid tissues. Ep was active in both B220+IgM- and B220+IgM+ subpopulations in the bone marrow and in the B220+ subpopulation in the spleen. Using electrophoretic mobility shift assays and mutational assays, we found that Ikaros and CCAAT/enhancer binding protein cooperatively bind Ep and function as the transcription factors responsible for B cell-specific enhancer activity. These results demonstrate the role of Ep as a cis-regulatory enhancer element for RAG-2-specific expression in B-lymphoid lineages.

Transcriptional control of B cell development and function

The generation, development, maturation and selection of mammalian B lymphocytes is a complex process that is initiated in the embryo and proceeds throughout life to provide the organism an essential part of the immune system it requires to cope with pathogens. Transcriptional regulation of this highly complex series of events is a major control mechanism, although control is also exerted on all other layers, including splicing, translation and protein stability. This review summarizes our current understanding of transcriptional control of the well-studied murine B cell development, which bears strong similarity to its human counterpart. Animal and cell models with loss of function (gene "knock outs") or gain of function (often transgenes) have significantly contributed to our knowledge about the role of specific transcription factors during B lymphopoiesis. In particular, a large number of different transcriptional regulators have been linked to distinct stages of the life of B lymphocytes such as: differentiation in the bone marrow, migration to the peripheral organs and antigen-induced activation.

STAT5-induced Id-1 transcription involves recruitment of HDAC1 and deacetylation of C/EBPbeta

Transcriptional activation is associated commonly with recruitment of histone acetylases, while repression involves histone deacetylases (HDACs). Here, we provide evidence to suggest that STAT5 activates gene expression by recruiting HDAC. The interleukin-3 (IL-3)-dependent expression of the Id-1 gene, encoding a helix-loop-helix (HLH) transcriptional inhibitor, is activated by both C/EBPbeta and STAT5 transcription factors bound to its pro-B-cell enhancer (PBE), but is inhibited by HDAC inhibitors in Ba/F3 cells. STAT5 interacts with HDAC1 in the PBE region, resulting in deacetylation of histones, as well as C/EBPbeta, whose acetylation diminishes its DNA-binding activity. Consistently, expression of an acetylation-resistant mutant of C/EBPbeta results in IL-3-independent expression of the Id-1 gene. Thus, we propose a novel mechanism by which STAT5 mediates the deacetylation of C/EBPbeta, allowing transcriptional activation.

Early B-cell factor, E2A, and Pax-5 cooperate to activate the early B cell-specific mb-1 promoter

Previous studies have suggested that the early-B-cell-specific mb-1(Igalpha) promoter is regulated by EBF and Pax-5. Here, we used in vivo footprinting assays to detect occupation of binding sites in endogenous mb-1 promoters at various stages of B-cell differentiation. In addition to EBF and Pax-5 binding sites, we detected occupancy of a consensus binding site for E2A proteins (E box) in pre-B cells. EBF and E box sites are crucial for promoter function in transfected pre-B cells, and EBF and E2A proteins synergistically activated the promoter in transfected HeLa cells. Other data suggest that EBF and E box sites are less important for promoter function at later stages of differentiation, whereas binding sites for Pax-5 (and its Ets ternary complex partners) are required for promoter function in all mb-1-expressing cells. Using DNA microarrays, we found that expression of endogenous mb-1 transcripts correlates most closely with EBF expression and negatively with Id1, an inhibitor of E2A protein function, further linking regulation of the mb-1 gene with EBF and E2A. Together, our studies demonstrate the complexity of factors regulating tissue-specific transcription and support the concept that EBF, E2A, and Pax-5 cooperate to activate target genes in early B-cell development.

Characterization of chromatin structure and enhancer elements for murine recombination activating gene-2

Recombination-activating genes (RAGs) play a critical role in V(D)J recombination machinery and their expression is specifically regulated during lymphocyte ontogeny. To elucidate the molecular mechanisms regulating murine RAG-2 expression, we examined a chromatin structure of 25-kb DNA segment adjacent to murine RAG-2 by analyzing DNase I hypersensitive (HS) sites. In a RAG-2-expressing murine pre-B cell line, three lymphoid cell-specific HS sites (HS1, HS2, and HS3) were identified. Among these HS sites, one HS site (HS3) that locates in the RAG-2 promoter was associated only with RAG-2-expressing cell lines. Using the transient enhanced green fluorescence protein reporter gene assays, we identified two enhancer elements in the 5'-upstream region of RAG-2 that corresponded to HS1 and HS2. One of the enhancer elements (D3) exhibited enhancer activity only in the lymphoid cell lines. Analysis of the transgenic mice carrying the enhanced green fluorescence protein-reporter gene linked with D3 revealed that D3 activated the reporter gene-expression in the primary lymphoid tissues, but not in the secondary lymphoid tissues or nonlymphoid tissues. D3 was active in CD4(-)CD8(-), but not in CD4(+)CD8(+) or CD4(+)CD8(-) thymocytes in the thymus, and also active in B220(+)IgM(-), but not in B220(+)IgM(+), cells in the bone marrow. Finally, our data suggested that C/EBP may bind to the D3 enhancer and function as one of the transcription factor(s) responsible for the enhancer activity. These results show that the tissue- and stage-specific expression of murine RAG-2 is regulated by alteration of the chromatin structure as well as cis-regulatory enhancer elements.

Critical role of the HMGI(Y) proteins in adipocytic cell growth and differentiation

The high-mobility group I (HMGI) nonhistone chromosomal proteins HMGI(Y) and HMGI-C have been implicated in defining chromatin structure and in regulating the transcription of several genes. These proteins have been implicated in adipocyte homeostasis: a severe deficiency of fat tissue is found in mice with targeted disruption of the HMGI-C locus, and lipomagenesis in humans is frequently associated with somatic mutations of HMGI genes. The aim of this study was to examine the role of HMGI(Y) proteins in adipocytic cell growth and differentiation. First, we found that differentiation of the preadipocytic 3T3-L1 cell line caused early induction of HMGI(Y) gene expression. Suppression of HMGI(Y) expression by antisense technology dramatically increased the growth rate and impaired adipocytic differentiation in these cells. The process of adipogenic differentiation involves the interplay of several transcription factors, among which is the CCAAT/enhancer-binding protein (C/EBP) family of proteins. These factors are required for the transcriptional activation of adipocyte-specific genes. We also tested the hypothesis that HMGI(Y) might participate in transcriptional control of adipocyte-specific promoters. We found that HMGI(Y) proteins bind C/EBPbeta in vivo and in vitro. Furthermore, we show that HMGI(Y) strongly potentiates the capacity of C/EBPbeta to transactivate the leptin promoter, an adipose-specific promoter. Taken together, these results indicate that the HMGI(Y) proteins play a critical role in adipocytic cell growth and differentiation.

Novel control motif cluster in the IgH delta-gamma 3 interval exhibits B cell-specific enhancer function in early development

The majority of the human Ig heavy chain (IgH) constant (C) region locus has been cloned and mapped. An exception is the region between C delta and C gamma 3, which is unstable and may be a recombination hot spot. We isolated a pBAC clone (pHuIgH3'delta-gamma 3) that established a 52-kb distance between C delta and C gamma 3. Sequence analysis identified a high number of repeat elements, explaining the instability of the region, and an unusually large accumulation of transcription factor-binding motifs, for both lymphocyte-specific and ubiquitous transcription activators (IKAROS, E47, Oct-1, USF, Myc/Max), and for factors that may repress transcription (Delta EF1, Gfi-1, E4BP4, C/EBP beta). Functional analysis in reporter gene assays revealed the importance of the C delta-C gamma 3 interval in lymphocyte differentiation and identified independent regions capable of either enhancement or silencing of reporter gene expression and interaction with the IgH intron enhancer E mu. In transgenic mice, carrying a construct that links the beta-globin reporter to the novel delta-gamma 3 intron enhancer (E delta-gamma 3), transgene transcription is exclusively found in bone marrow B cells from the early stage when IgH rearrangement is initiated up to the successful completion of H and L locus recombination, resulting in Ab expression. These findings suggest that the C delta-C gamma 3 interval exerts regulatory control on Ig gene activation and expression during early lymphoid development.

Regulated Expression and Functional Role of the Transcription Factor CHOP (GADD153) in Erythroid Growth and Differentiation

The hematopoietic growth factor erythropoietin (Epo) triggers changes in the expression of genes that encode important regulators of erythroid cell growth and differentiation. We now report that Epo markedly upregulates chop (gadd153) expression and that this transcription factor plays a role in erythropoiesis. Using a differential hybridization assay, we isolated a full-length cDNA ofchop as an Epo upregulated gene in Rauscher murine erythroleukemia cells. RNase protection assays demonstrated that Epo or dimethyl sulfoxide induction increased steady-state mRNA levels 10- to 20-fold after 24 to 48 hours. Western blot analysis confirmed a marked increase in CHOP protein. Among the other c/ebp family members, only c/ebp β was also upregulated during erythroid differentiation. Among normal hematopoietic cells examined, steady-state mRNA levels were highest in erythroid cells, with levels peaking during terminal differentiation. Transient overexpression ofchop in Rauscher cells resulted in a significant increase in Epo- or dimethyl sulfoxide (DMSO)-induced hemoglobinization, further linking chop upregulation to erythroid differentiation. Artificial downregulation of chop in normal murine bone marrow cells with antisense oligodeoxynucleotides inhibited colony-forming unit-erythroid (CFU-E)–derived colony growth in a concentration-dependent manner. Burst-forming unit-erythroid (BFU-E)–derived colony growth was not affected. Using a Far Western type of analysis, we detected several potential CHOP binding partners among the nuclear proteins of Rauscher cells. Importantly, the number and relative abundance of these proteins changed with differentiation. The results strongly suggest that CHOP plays a role in erythropoiesis, possibly through interactions with both C/EBP and non-C/EBP family members.

Transcriptional regulation during B cell development

Information is increasingly available concerning the molecular events that occur during primary and antigen-dependent stages of B cell development. In this review the roles of transcription factors and coactivators are discussed with respect to changes in expression patterns of various genes during B cell development. Transcriptional regulation is also discussed in the context of developmentally regulated immunoglobulin gene V(D)J recombination, somatic hypermutation, and isotype switch recombination.

Tumor necrosis factor alpha gene regulation: enhancement of C/EBPbeta-induced activation by c-Jun

Tumor necrosis factor alpha (TNF alpha) is a key regulatory cytokine whose expression is controlled by a complex set of stimuli in a variety of cell types. Previously, we found that the monocyte/macrophage-enriched nuclear transcription factor C/EBPbeta played an important role in the regulation of the TNF alpha gene in myelomonocytic cells. Abundant evidence suggests that other transcription factors participate as well. Here we have analyzed interactions between C/EBPbeta and c-Jun, a component of the ubiquitously expressed AP-1 complex. In phorbol myristate acetate (PMA)-treated Jurkat T cells, which did not possess endogenous C/EBPbeta, expression of c-Jun by itself had relatively little effect on TNF alpha promoter activity. However, the combination of C/EBPbeta and c-Jun was synergistic, resulting in greater than 130-fold activation. This effect required both the leucine zipper and DNA binding domains, but not the transactivation domain, of c-Jun, plus the AP-1 binding site centered 102/103 bp upstream of the transcription start site in the TNF alpha promoter. To determine if C/EBPbeta and c-Jun might cooperate to regulate the cellular TNF alpha gene in myelomonocytic cells, U937 cells that possess endogenous C/EBPbeta and were stably transfected with either wild-type c-Jun or the transactivation domain deletion mutant (TAM-67) were examined. U937 cells expressing ectopic wild-type c-Jun or TAM-67 secreted over threefold more TNF alpha than the control line in response to PMA plus lipopolysaccharide. Transient transfection of the U937 cells expressing TAM-67 suggested that TAM-67 binding to the -106/-99-bp AP-1 binding site cooperated with endogenous C/EBPbeta in the activation of the -120 TNF alpha promoter-reporter. DNA binding assays using oligonucleotides derived from the TNF alpha promoter suggested that C/EBPbeta and c-Jun interact in vitro and that the interaction may be DNA dependent. Our data demonstrate that the TNF alpha gene is regulated by the interaction of the ubiquitous AP-1 complex protein c-Jun and the monocyte/macrophage-enriched transcription factor C/EBPbeta and that this interaction contributes to the expression of the cellular TNF alpha gene in myelomonocytic cells. This interaction was unique in that it did not require the c-Jun transactivation domain, providing new insight into the cell-type-specific regulation of the TNF alpha gene.

Transcription factors C/EBP alpha, C/EBP beta, and CHOP (Gadd153) expressed during the differentiation program of keratinocytes in vitro and in vivo

CCAAT-enhancer binding proteins (C/EBP) are basic region/leucine zipper (bZIP) transcription factors selectively expressed during the differentiation of liver, adipose tissue, blood cells, and the endocrine pancreas. Here we show that C/EBP isoforms are differentially expressed in the skin. BALB/MK keratinocytes incubated in 0.12 mM calcium medium undergo a differentiation program featuring growth-arrest at 24-48 h, keratin K10 gene expression beginning at 24 h, and apoptosis commencing at 48 h. Within this framework, western immunoblot analysis and immunohistochemistry reveal that C/EBP alpha increases 5-fold at 1-2 d and remains elevated, C/EBP beta rises 2-fold at 2-4 d and gradually falls, and CHOP rises 9-fold in the first 24 h then returns rapidly to baseline. Several products of alternative translation are observed in BALB/MK cells, i.e., 42 kDa and 30 kDa forms of C/EBP alpha, and 32 kDa and 20 kDa forms of C/EBP beta. By immunohistologic examination of human, rat, and mouse skin, all three transcription factors are highly expressed within epithelial compartments in a spatially restricted distribution. C/EBP alpha is concentrated in the upper epidermis in a predominantly cytoplasmic location within cells, whereas the highest levels of C/EBP beta and CHOP are seen in the mid-epidermis, mainly within nuclei. High levels of C/EBP beta and CHOP (but not C/EBP alpha) are also observed in hair follicles and sebaceous glands. The identity of these factors in the epidermis is confirmed by western immunoblot analyses. In summary, C/EBP are expressed in a differentiation-associated manner in the skin, and may play an important role in regulating one or more aspects of the epidermal differentiation program.