Biomedical subjects

G Siu

Publications and source records attributed to G Siu.

At least 19 recordsLinked to original sources

PubMed2001

Negative regulation of CD4 gene expression by a HES-1-c-Myb complex.

Expression of the CD4 gene is tightly controlled throughout thymopoiesis. The downregulation of CD4 gene expression in CD4(-) CD8(-) and CD4(-) CD8(+) T lymphocytes is controlled by a transcriptional silencer located in the first intron of the CD4 locus. Here, we determine that the c-Myb transcription factor binds to a functional site in the CD4 silencer. As c-Myb is also required for CD4 promoter function, these data indicate that depending on the context, c-Myb plays both positive and negative roles in the control of CD4 gene expression. Interestingly, a second CD4 silencer-binding factor, HES-1, binds to c-Myb in vivo and induces it to become a transcriptional repressor. We propose that the recruitment of HES-1 and c-Myb to the silencer leads to the formation of a multifactor complex that induces silencer function and repression of CD4 gene expression.

Basic Helix-Loop-Helix Proteins↗

PubMed2001

Linking CD4 gene expression and T cell development.

The control of CD4 gene expression is essential for proper T lymphocyte development. Signals transmitted from the T-cell antigen receptor (TCR) during the selection process are believed to be linked to the regulation of CD4 gene expression during specific stages of T cell development. Thus, the control CD4 gene expression is an ideal model system for studying the molecular mechanisms that drive T cell development. Here, I discuss the characterization of transcriptional control elements in the CD4 locus and the factors that mediate their function. The study of these elements has led to significant insights into the mechanisms in which the T lymphocyte develops it mature functional characteristics.

Animals↗

PubMed2000

Precise arrangement of factor-binding sites is required for murine CD4 promoter function.

The control of CD4 expression is linked to the signaling events that mediate T-cell development and is directly dependent on the CD4 promoter. The CD4 promoter does not contain functionally redundant sites: all four factor-binding sites must be intact to achieve wild-type activity. Here we demonstrate that the precise position of three factor-binding sites relative to each other is essential for promoter activity, indicating that they function together as an inseparable cassette for assembly of the transcription initiation complex. Small changes in either phasing or distance between any two sites in this cassette leads to complete abrogation of promoter function. In addition, we demonstrate that one of the factors that bind the promoter cassette is not present in CD8 SP T(C) cells. Thus, this factor is a candidate for mediating the relative subclass specificity of CD4 promoter function in activated CD4 SP T(H) cells.

Animals↗

PubMed1999

Subclass-specific nuclear localization of a novel CD4 silencer binding factor.

The control of CD4 expression is essential for proper T lymphocyte development. We have previously described a cis-acting silencer element required for repressing transcription of the CD4 gene. Here we report the cloning and characterization of a novel factor that binds to a critical functional site in the CD4 silencer. This factor, referred to as silencer-associated factor (SAF), is a member of the helix-turn-helix factor family and shares sequence similarity with the homeodomain class of transcriptional regulators. Introduction of a specific mutation into the SAF binding site in the CD4 silencer abrogates silencer activity in transgenic mice, supporting the hypothesis that SAF is important in mediating silencer function. Although SAF is expressed in all lymphocytes, immunofluorescence studies indicate that SAF is present primarily in the cytoplasm in T cells in which the endogenous silencer is nonfunctional, whereas it is present primarily in the nucleus in T cells in which the silencer is functional. We thus hypothesize that the subclass-specific subcellular compartmentalization of SAF plays an important role in mediating the specificity of function of the CD4 silencer during T cell development.

Amino Acid Sequence↗

PubMed1999

A potential role for Elf-1 in CD4 promoter function.

The control of CD4 gene expression is believed to be linked directly to the signaling events that mediate T cell development and is directly dependent on the CD4 promoter. We have previously determined that this promoter contains four factor-binding sites important for its function. One of these sites, referred to as the P4 site, contains an Ets consensus recognition sequence. Using functional and biochemical analyses, we determine that Elf-1 binds to this site and specifically activates the CD4 promoter, indicating that Elf-1 is playing an important role in CD4 promoter function. In addition, a second nuclear factor binds to this region. Although there are consensus recognition sites for other factors, we demonstrate that none of these factors binds to the P4 site, nor do other known members of the Ets family. Thus, a novel transcription factor may bind to the CD4 promoter and help mediate its function.

Animals↗

PubMed1999

c-Myb is essential for early T cell development.

The c-Myb transcription factor is important for fetal hematopoiesis and has been proposed to mediate later stages of lymphocyte development. Using homozygous null c-Myb/Rag1 chimeric mice, we have determined that c-Myb plays an important role in the differentiation of macrophages and lymphocytes from precursor stem cells. We also determine that deletion of c-Myb leads to a complete block in early T cell development just before the oligopotent thymocyte matures into the definitive T cell precursor. These data indicate that c-Myb plays an important role at multiple stages of hematopoiesis and is required at an early stage of T cell development.

Animals↗

PubMed1999

Overexpression of the Helix-Loop-Helix protein Id2 blocks T cell development at multiple stages.

The Id proteins are inhibitors of basic-Helix-Loop-Helix transcription factor function that have been implicated in the control of cell differentiation and proliferation. To study the role of Id proteins in the control of T cell development, we generated transgenic mice that overexpress the Id2 protein in thymocytes. We detect a significant expansion of the early CD4(-)CD8(+)TCR(-) thymocyte stage and a depletion of the thymocytes of the subsequent developmental stages. These data indicate that the overexpression of Id2 leads to a stage-specific developmental block early in thymopoiesis. In addition, progeny mice from five of the six Id2 transgenic founder lines succumb to aggressive T cell hyperproliferation that resembles lymphoma. Thus, overexpression of the Id2 protein has profound effects on T cell development and oncogenesis, consistent with the hypothesis that the bHLH proteins play critical roles in these processes.

Animals↗

PubMed1999

Control of CD4 gene expression: connecting signals to outcomes in T cell development.

The control of CD4 gene expression is essential for proper T lymphocyte development. Signals transmitted from the T-cell antigen receptor (TCR) during the thymic selection processes are believed to be linked to the regulation of CD4 gene expression during specific stages of T cell development. Thus, a study of the factors that control CD4 gene expression may lead to further insight into the molecular mechanisms that drive thymic selection. In this review, we discuss the work conducted to date to identify and characterize the cis-acting transcriptional control elements in the CD4 locus and the DNA-binding factors that mediate their function. From these studies, it is becoming clear that the molecular mechanisms controlling CD4 gene expression are very complex and differ at each stage of development. Thus, the control of CD4 expression is subject to many different influences as the thymocyte develops.

Animals↗

PubMed1998

Transcription of a minimal promoter from the NF-IL6 gene is regulated by CREB/ATF and SP1 proteins in U937 promonocytic cells.

NF-IL6 is an important transcriptional regulator of genes induced in activated monocytes/macrophages, and NF-IL6 is the only CCAAT/enhancer-binding protein (C/EBP) family member whose steady-state mRNA levels increase upon activation of monocytes (1). We show that increased transcription of the NF-IL6 gene is responsible, at least in part, for induction of NF-IL6 mRNA following activation of U937 promonocytic cells. We have identified a 104-bp minimal promoter region of the NF-IL6 gene that is sufficient for basal and activation-dependent induction of transcription in U937 cells. This region contains binding sites for the cAMP response element-binding protein/activation transcription factor (CREB/ATF) and Sp1 families of transcription factors. Each site is functionally important and contributes independently to transcription of the NF-IL6 gene in U937 cells.

Activating Transcription Factors↗

PubMed1998

The notch pathway intermediate HES-1 silences CD4 gene expression.

We have previously identified a transcriptional silencer that is critical for proper expression of the CD4 gene during T-cell development. Here we report that the Hairy/Enhancer of Split homologue HES-1, a transcription factor in the lin12/Notch signaling pathway, binds to an important functional site in the CD4 silencer. Overexpression of HES-1 leads to the silencer site-dependent repression of CD4 promoter and enhancer function as well as the downregulation of endogenous CD4 expression in CD4(+) CD8(-) TH cells. Interestingly, overexpression of an activated form of Notch1 (NotchIC) leads to the repression of CD4 promoter and enhancer function both in the presence and absence of the silencer. NotchIC-mediated CD4 silencer function is not affected by the deletion of the HES-1-binding site, indicating that multiple factors binding to CD4 transcriptional control elements are responsive to signaling from this pathway, including other silencer-binding factors. Taken together, these data are consistent with the hypothesis that the lin12/Notch signaling pathway is important in thymic development and provide a molecular mechanism via the control of CD4 gene expression in which the lin12/Notch pathway affects T-cell developmental fate.

Animals↗

PubMed1997

Positive selection induces CD4 promoter and enhancer function.

Developmental expression of the CD4 gene in mature T cells is controlled by at least four transcriptional control elements: a promoter, two enhancers and a silencer. In this report we use a transgenic approach to study the mechanisms in which these elements interact to convey appropriate tissue- and cell-specific expression at all stages of T cell development. Our data indicate that the control of CD4 gene expression requires the interaction of multiple elements functioning in different combinations at different stages of T cell development. Expression of the CD4 gene in immature CD4+CD8+ thymocytes requires a third enhancer element located in the 3' flanking region of the CD4 gene. Interestingly, the CD4 promoter and proximal/distal enhancers first begin to function at the HSAlo CD69lo H-2Khi CD4 single-positive stage; cells of this phenotype are believed to have survived positive selection. These data indicate that the CD4 promoter and late enhancer elements are induced by positive selection; thus, the final maturation process is an active event that requires the initiation of a novel program of gene expression.

Animals↗

PubMed1996

Asymmetric redundancy in CD4 silencer function.

We and others have defined a transcriptional silencer critical for the proper expression of the CD4 gene at all stages of T cell development. In this report, we use biochemical techniques to identify three different factor-binding sites within the CD4 silencer, denoted sites I, II, and III. Using transgenic analyses, we determine that although all three factor-binding sites are important for silencer activity, there is significant redundancy in that the presence of either site II alone, or the combination of sites I and III permits silencer function. Thus, our data indicate that the mechanism of function of the CD4 silencer is extremely complex. Further biochemical analyses indicate that the factor binding to site II has the same sequence specificity as a factor binding to an E box site in the CD4 enhancer; thus, a member of the bHLH factor family may be important in mediating silencer function.

Animals↗

PubMed1995

A Myc-associated zinc finger protein binding site is one of four important functional regions in the CD4 promoter.

The CD4 promoter plays an important role in the developmental control of CD4 transcription. In this report, we show that the minimal CD4 promoter has four factor binding sites, each of which is required for full function. Using biochemical and mutagenesis analyses, we determined that multiple nuclear factors bind to these independent sites. We determined that an initiator-like sequence present at the cap site and an Ets consensus sequence are required for full promoter function. We also demonstrate that the Myc-associated zinc finger protein (MAZ) appears to be the predominant factor binding to one of these sites. This last site closely resembles the ME1a1 G3AG4AG3 motif previously shown to be a critical element in the P2 promoter of the c-myc gene. We therefore believe that the MAZ transcription factor is also likely to play an important role in the control of developmental expression of the CD4 gene.

Base Sequence↗

PubMed1994

A transcriptional silencer controls the developmental expression of the CD4 gene.

The appropriate expression of the CD4 glycoprotein is required for T-cell function and development. Here we define the transcriptional control elements in the CD4 locus that convey CD(4+)-specific expression of a marker gene in transgenic mice. Using nuclear run-on experiments, we have determined that the major mechanism for CD4 expression control during development is transcriptional. We have identified a developmental stage- and tissue-specific negative regulatory element in the first intron of the murine CD4 gene that has the characteristics of a transcriptional silencer. The CD4 silencer functions to inhibit marker gene expression at two different stages of T-cell development, as well as in non-T hematopoietic cells, and thus is the critical controlling element responsible for T-cell-specific, as well as developmental- and subclass-specific, expression.

Animals↗

PubMed1994

Elf-1 binds to a critical element in a second CD4 enhancer.

The coordinated expression of CD4 and CD8 during T-cell development is tightly coupled with the maturation state of the T cell. Additionally, the mutually exclusive expression of these receptors in mature T cells is representative of the functional T-cell subclasses (CD4+ helper T cells versus CD8+ cytotoxic T cells). We have studied the regulation CD4 gene transcription during T-cell development in an attempt to gain an understanding of the molecular mechanisms involved in T-cell development and differentiation. Here we present the identification of a second transcriptional enhancer in the murine CD4 locus 24 kb upstream of the CD4 promoter. This enhancer is active in mature T cells and is especially active in CD4+ helper T cells. A number of nuclear proteins bind to elements in the minimal CD4 enhancer that includes consensus sites for AP-1, Sp1, Gata, and Ets transcription factor families. We find that the Ets consensus site is crucial for enhancer activity and that the recently identified Ets factor, Elf-1, which is expressed at high levels in T cells and involved in the regulation of several other T-cell-specific genes, is a dominant protein in T-cell nuclear extracts that binds to this site.

Animals↗

PubMed1992

Expression of the CD4 gene requires a Myb transcription factor.

We have analyzed the control of developmental expression of the CD4 gene, which encodes an important recognition molecule and differentiation antigen on T cells. We have determined that the CD4 promoter alone functions at high levels in the CD4+ CD8- mature T cell but not at the early CD4+ CD8+ stage of T-cell development. In addition, the CD4 promoter functions only in T lymphocytes; thus, the stage and tissue specificity of the CD4 gene is mediated in part by its promoter. We have determined that a Myb transcription factor binds to the CD4 promoter and is critical for full promoter function. Thus, Myb plays an important role in the expression of T-cell-specific developmentally regulated genes.

Animals↗

PubMed1990

The biology of the T-cell antigen receptor and its role in the skin immune system.

T lymphocytes play an important role in the generation, maintenance, and specificity of the skin immune response. T cells are the predominant class of lymphocytes found in the skin and moderate many of the initial immune responses, such as allergic contact and delayed-type hypersensitivity. In addition, the primary class of cutaneous lymphomas is believed to be of T-cell lineage. All of the antigen and MHC-restriction capabilities are manifested by the T-cell antigen receptor (TCR), the study of which has been the primary focus of immunologists for many years. Proper recognition of antigen and MHC-restriction by the TCR is necessary for the activation of the T cell. The analysis of the TCR has proved to be a useful tool for the diagnosis of lymphomas and the study of the normal skin immune system. Recently, TCR subset populations were found to be expressed specifically within the epidermis and have been hypothesized to be important in the maintenance of immunity in the skin immune system. In this article, we discuss the relationship of T cells to the immune system and the importance of the TCR to its function and homeostasis.

Animals↗

PubMed1989

Isolation of the murine intercellular adhesion molecule 1 (ICAM-1) gene. ICAM-1 enhances antigen-specific T cell activation.

Cell adhesion molecules in the immune system are believed to play an important role in lymphocyte-target cell conjugate formation. One such molecule, intercellular adhesion molecule 1 (ICAM-1), is important in the function, aggregation, and adherence of leukocytes. Here, we report the isolation and characterization of the murine ICAM-1 gene. We report that the murine ICAM-1 gene is a member of the Ig gene superfamily, has limited homology to its human counterpart, and is expressed in cells of lymphocytic and myeloid lineages. Transfection of the ICAM-1 cDNA into MHC class II-transfected fibroblasts leads to enhancement of the Ag-specific T cell response when the transfectants are used as APC.

Amino Acid Sequence↗