Biomedical subjects

H C Liou

Publications and source records attributed to H C Liou.

44 records · Page 3Linked to original sources

PubMed1990

A cDNA for a human cyclic AMP response element-binding protein which is distinct from CREB and expressed preferentially in brain.

The cyclic AMP response element (CRE) is found in many cellular genes regulated by cyclic AMP, and similar elements are present in the early genes of adenovirus that are activated by E1A. The transcription factor CREB has previously been shown to bind this site, and cDNAs for CREB have recently been characterized. We report here the isolation of a cDNA encoding a human DNA-binding protein that also recognizes this motif in cellular and viral promoters. This protein, HB16, displays structural similarity to CREB and to c-Jun and c-Fos, which bind the related 12-O-tetradecanoylphorbol-13-acetate response element (TRE). HB16 contains a highly basic, putative DNA-binding domain and a leucine zipper structure thought to be involved in dimerization. Deletional analysis of HB16 demonstrated that the leucine zipper is required for its interaction with DNA. In addition, HB16 could form a complex with c-Jun but not with c-Fos. Despite its structural similarity to c-Jun and c-Fos and its interaction with c-Jun, HB16 had approximately a 10-fold-lower affinity for the TRE sequence than for the CRE sequence. Although HB16 and CREB both recognized the CRE motif, an extensive binding analysis of HB16 revealed differences in the fine specificity of binding of the two proteins. HB16 mRNA was found at various levels in many human tissues but was most abundant in brain, where its expression was widespread. The existence of more than one CRE-binding protein suggests that the CRE motif could serve multiple regulatory functions.

Activating Transcription Factor 2↗

PubMed1990

mXBP/CRE-BP2 and c-Jun form a complex which binds to the cyclic AMP, but not to the 12-O-tetradecanoylphorbol-13-acetate, response element.

Proto-oncogene products c-Fos and c-Jun form a complex which binds with high affinity to the 12-O-tetradecanoylphorbol-13-acetate (TPA) response DNA element and which stimulates transcription of phorbol ester- inducible genes. We have previously identified, by screening a lambda gt11 expression library, murine protein mXBP, which binds to a sequence which overlaps the 3' end of the murine class II major histocompatibility complex A alpha gene X box, a conserved transcription element found upstream of all class II genes. Here, we demonstrate that the target sequence for mXBP is a consensus cyclic AMP response element (CRE). mXBP is a member of the leucine zipper family of DNA-binding proteins and has significant homology to oncoproteins c-Fos and c-Jun. The inferred amino acid sequence of mXBP shows near identity to human CRE-BP1, except it does not contain an internal proline-rich domain. Immunoprecipitation and glutaraldehyde cross-linking studies show that mXBP/CRE-BP2 can form a complex with c-Jun. Complex formation is dependent on intact leucine zipper domains in both proteins. mXBP-c-Jun complexes can coexist with c-Fos-c-Jun complexes and can bind with high affinity to CRE, but not to TPA response DNA element, sequences. These results suggest that changes in the expression of mXBP/CRE-BP2, c-Fos, and c-Jun, which alter the ratio of mXBP-c-Jun to c-Fos-c-Jun complexes, would affect the relative expression of cyclic AMP and phorbol ester-responsive genes. This provides support for a combinatorial model of gene regulation, whereby protein-protein interactions which alter the DNA binding specificity of protein complexes can expand the flexibility of cellular transcriptional responses.

Activating Transcription Factor 2↗

PubMed1989

Differences in DNA sequence specificity among MHC class II X box binding proteins.

The class II (Ia) MHC Ag are integral membrane proteins whose expression is limited to specific cell types. A pair of consensus sequences, X and Y, is found upstream from all class II genes and deletion of each of these sequences eliminates expression of transfected genes. Cells that express Ia demonstrate a coordinate response to lymphokines and other stimuli. These conserved sequences might, therefore, play a role in tissue specificity or lymphokine inducibility of Ia gene expression. The X box sequence of the murine class II A alpha gene diverges much more substantially from the X consensus than does the Y box motif of this gene. We demonstrate that this X box motif is nonetheless recognized by sequence-specific DNA-binding proteins, as is the more closely conserved Y box. Gel retardation assays and DNase I footprints were compared for a panel of Ia+ and Ia- cells as well as for cells stimulated with the Ia-inducing lymphokines IL-4 and IFN-gamma. The level, retardation pattern and region of DNA contact were comparable in all instances. Thus the availability of active DNA-binding X and Y box factors cannot alone account for the regulation of A alpha expression. To test whether the same set of proteins binds all class II MHC conserved motifs, oligonucleotide probe binding and cross-competition experiments with X box sequences from A alpha, E alpha, and E beta genes were performed. These studies demonstrated A alpha, E alpha, and E beta DNA-protein complexes with unique mobilities and specificities. In addition, all three X box oligonucleotide probes generated one faint complex with an affinity profile of E beta greater than E alpha much greater than A alpha. These three complexes comigrated and thus may represent a communal binding protein. The data are most consistent with the conclusion that multiple proteins bind class II MHC X boxes. For A alpha, the predominant complexes represent different specificities from the predominant E alpha and E beta X box binding proteins.

Animals↗

PubMed1988

A DNA binding protein regulated by IL-4 and by differentiation in B cells.

The class II (Ia) major histocompatibility complex antigens are a family of integral membrane proteins whose expression is limited to certain cell types, predominantly B lymphocytes, macrophages, and thymic epithelial cells. In B cells, Ia expression is both developmentally regulated and responsive to external stimuli. The differentiation of early B stem cells to mature B lymphocytes is accompanied by the appearance of cell surface Ia antigens; the transition to plasma cells results in loss of class II gene expression. In Ia-expressing B cells, the T cell-derived lymphokine interleukin-4 (IL-4) increases such expression by an as yet undefined mechanism. Chloramphenicol acetyltransferase gene expression was cis-activated by a region of the Ia A alpha k gene in a B lymphoma line, but not in a myeloma line. A nuclear protein that bound to two sites within this region, upstream from previously described transcription elements, was found in normal spleen cells. This binding activity was also found in spleen extracts from athymic mice, which lack T lymphocytes, and in Ia-positive B lymphocyte tumor cell lines, demonstrating that it is a B cell protein. Further analysis showed the activity to be undetectable in an Ia-negative pre-B cell line and in three plasmacytoma cell lines that are Ia negative. IL-4 treatment of normal and athymic mouse spleen cells greatly increased the binding of this nuclear protein to these two sites, concomitant with increased MHC class II gene transcription. Thus, B cells contain a sequence-specific DNA-binding activity whose level is influenced both by IL-4 and by differentiation signals.

B-Lymphocytes↗

PubMed1988

Distinct cloned class II MHC DNA binding proteins recognize the X box transcription element.

The class II (Ia) major histocompatibility complex (MHC) antigens are a family of integral membrane proteins whose expression is limited to certain cell types. A pair of consensus sequences, X and Y, is found upstream of all class II genes, and deletion of each of these sequences eliminates expression of transfected genes. Furthermore, the absence of a specific X box binding protein in patients with severe combined immunodeficiency disease whose cells lack class II suggests an important role for these proteins in class II regulation. Here, the cloning of two lambda gt11 complementary DNAs encoding DNA binding proteins (murine X box binding proteins lambda mXBP and lambda mXBP-2) is reported. Both phage-encoded fusion proteins bind specifically to the X box of the A alpha, but not to E alpha or E beta class II genes. These two independent isolates do not cross-hybridize. The lambda mXBP complementary DNA hybridizes to two RNA species, 6.2 and 3.0 kilobases in mouse, that are expressed in both Ia positive and Ia negative cells. By means of DNA blot analysis with the lambda mXBP complementary DNA insert and probes generated from each end of this complementary DNA insert, lambda mXBP was found to arise from a multigene family. These data illustrate the high degree of complexity in the transcriptional control of this coordinately regulated gene family.

Animals↗

PubMed1988

A tissue-specific DNase I-hypersensitive site in a class II A alpha gene is under trans-regulatory control.

Class II major histocompatibility complex molecules are integral membrane glycoproteins whose distribution is limited to certain tissues. To identify the molecular basis for such specificity, the chromatin configuration of the class II A alpha gene was examined in intact nuclei from various cell types. We show that there are three DNase I-hypersensitive sites in the A alpha gene. One of these sites, located near the promoter region, is specific to cells that normally express class II molecules at some stage of differentiation. Furthermore, this tissue-specific site appears to be under trans-regulatory control.

Animals↗

PubMed1988

Differential induction of class II gene expression in murine pre-B-cell lines by B-cell stimulatory factor-1 and by antibodies to B-cell surface antigens.

We have previously reported that BSF-1 and an alloantibody to the B-cell differentiation antigen Lyb2 induce class II gene expression in two Ia negative pre-B-cell lines. Two questions were asked in these studies. The first question is whether the different stimuli which we and others have shown to induce class II expression in B-cells act via the same signal transduction mechanisms. The second question is whether the traditionally accepted pathway of B-cell differentiation, as defined by immunoglobulin (Ig) gene rearrangement, is applicable to other events that occur during B-cell differentiation. In this report, we have therefore examined a large panel of pre-B-cell lines at different stages of Ig gene rearrangement in an attempt to 1) identify the stage in B-cell development where class II gene expression occurs and where it becomes inducible by BSF-1 or anti-Lyb2, and 2) compare the signal transduction mechanisms used by these ligands. The majority of pre-B-cell lines tested did not express BSF-1 receptors and were consequently noninducible for class II by BSF-1; such cell lines were, however, inducible for class II expression by anti-Lyb2 and, in addition, by antibodies to the B220 membrane glycoprotein. The induction of class II molecules by BSF-1 and by anti-Lyb2 and anti-B220 differed in several respects: 1) Induction by anti-Lyb2 and anti-B220 did not require the presence of BSF-1 receptors; 2) BSF-1 selectively induced class II antigen expression while anti-Lyb2 and anti-B220 induced the expression of other surface markers as well; and 3) PGE2 inhibited BSF-1 but not antibody-mediated class II induction. Finally, the presence of receptors for BSF-1 and the baseline expression of cell surface Ia was shown to be unlinked to Ig gene rearrangement and expression in this series of pre-B-cell lines. The independent regulation of Ia and Ig genes observed here may reflect a branching rather than a linear pathway for B-cell differentiation. The differentiation of pre-B-cells to mature Ig-secreting cells should probably not be defined solely by rearrangement of Ig genes, since this is likely to represent an oversimplified view of B-cell differentiation.

Animals↗

PubMed1986

Molecular cloning, germ-line transformation, and transcriptional analysis of the zeste locus of Drosophila melanogaster.

Approximately 170 kilobase pairs (kb) of contiguous DNA sequences derived from bands 3A3,4 of the Drosophila melanogaster X chromosome have been isolated by molecular cloning. Sequences required for the wild-type expression of the zeste locus are located within a 6-kb fragment of this chromosomal region, as shown by phenotypic rescue of zeste mutants in P element-mediated germ-line transformation. Expression of zeste is correlated with a 2.2-kb poly(A)+ RNA species transcribed at all postzygotic stages of Drosophila development. Many zeste alleles, including several producing neomorphic phenotypes, are not associated with detectable rearrangements of DNA.

Animals↗