Biological role of the CCAAT/enhancer-binding protein family of transcription factors

Molecular cloning, genetic mapping, and expression analysis of four zebrafish c/ebp genes

The CCAAT/enhancer binding protein family (C/EBP) are transcription factors that play integral roles in the development and function of many organ systems, including hematopoietic cells, adipose tissues, and liver. We have identified and characterized putative zebrafish orthologs of mammalian C/EBP alpha, beta, gamma, and delta using low-stringency hybridization screening and computer searches of the GenBank EST database. c/ebpa and g were mapped within 1 cM of each other on linkage group (LG) 7, syntenic with human CEBPA and G genes on chromosome 19. c/ebpb was mapped to LG8, and c/ebpd was mapped to LG24, on the same LG as a recently identified unique c/ebp in zebrafish, c/ebp1. The mapping of these genes established new syntenic relationships between LG8 and human chromosome 20, extended existing synteny between LG7 and human chromosome 19, and confirmed the synteny between LG24 and human chromosome 8. In addition, these syntenies between zebrafish and human chromosomes are also conserved in the mouse genome. To characterize the expression of these genes, RNA in situ hybridization in embryos of wild type and a hematopoietic mutant, cloche, was performed. The results showed that zebrafish c/ebpa, b, g, and d were expressed in many embryonic tissues. c/ebpa and b were expressed in a subset of hematopoietic cells in a region consistent with myeloid expression. In addition, there was expression of c/ebpa and b in the liver and c/ebpa, b, and d in regions of the gastrointestinal tract. The expression of the c/ebps may serve as important markers for analysis of myelopoiesis, hepatic development, and other developmental processes in the future.

Expression of CCAAT/enhancer binding protein family genes in monolayer and sandwich culture of hepatocytes: induction of stress-inducible GADD153

Removal of hepatocytes from their physiological environment for experimentation in vitro activates loss of liver-specific phenotype. Hepatocytes cultured in a sandwich configuration reportedly maintain greater expression of certain liver-specific genes than hepatocytes in monolayer cultures. We show that sandwich culture of rat hepatocytes improves retention of expression of a liver-enriched transcription factor, C/EBPalpha (CCAAT/enhancer binding protein alpha), which regulates many liver-specific genes. However, we also demonstrate increased expression of a stress-responsive C/EBP homologue, GADD153 (growth arrest and DNA damage gene 153), during monolayer culture, which may promote dedifferentiation. Induction of GADD153 was not prevented in sandwich cultured hepatocytes. Activation of a homologue of the mouse GADD153 target gene, doc1, was observed in monolayer and sandwich culture, suggesting that GADD153 was transcriptionally active. We suggest that the capability of sandwich cultures to maintain hepatocyte phenotype may be limited by the altered profile of transcription factor activity.

The role of C/EBP in nutrient and hormonal regulation of gene expression

C/EBPs are a family of transcription factors that play important roles in energy metabolism. Although initially thought to be constitutive regulators of transcription, an increasing amount of evidence indicates that their transactivating capacity within the cell can be modulated by nutrients and hormones. There are several mechanisms whereby this occurs. First, hormones/nutrients are known to directly alter the expression of C/EBPs. Second, hormones/nutrients may cause an alteration in the phosphorylation state of C/EBPs, which can affect their DNA-binding activity or transactivating capacity. Third, C/EBPs can function as accessory factors on gene promoters within a hormone response unit, interacting with other transcription factors to enhance the degree of responsiveness to specific hormones. Given their role in regulating genes involved in a wide variety of metabolic events, advancing our understanding of the molecular mechanism of action of C/EBPs will undoubtedly further our appreciation for the role these transcription factors play in both health and disease.

p38 MAPK regulates group IIa phospholipase A2 expression in interleukin-1beta -stimulated rat neonatal cardiomyocytes

Group IIa phospholipase A(2) (GIIa PLA(2)) is released by some cells in response to interleukin-1beta. The purpose of this study was to determine whether interleukin-1beta would stimulate the synthesis and release of GIIa PLA(2) from cardiomyocytes, and to define the role of p38 MAPK and cytosolic PLA(2) in the regulation of this process. Whereas GIIa PLA(2) mRNA was not identified in untreated cells, exposure to interleukin-1beta resulted in the sustained expression of GIIa PLA(2) mRNA. Interleukin-1beta also stimulated a progressive increase in cellular and extracellular GIIa PLA(2) protein levels and increased extracellular PLA(2) activity 70-fold. In addition, interleukin-1beta stimulated the p38 MAPK-dependent activation of the downstream MAPK-activated protein kinase, MAPKAP-K2. Treatment with the p38 MAPK inhibitor, SB202190, decreased interleukin-1beta stimulated MAPKAP-K2 activity, GIIa PLA(2) mRNA expression, GIIa PLA(2) protein synthesis, and the release of extracellular PLA(2) activity. Infection with an adenovirus encoding a constitutively active form of MKK6, MKK6(Glu), which selectively phosphorylates p38 MAPK, induced cellular GIIa PLA(2) protein synthesis and the release of GIIa PLA(2) and increased extracellular PLA(2) activity 3-fold. In contrast, infection with an adenovirus encoding a phosphorylation-resistant MKK6, MKK6(A), did not result in GIIa PLA(2) protein synthesis or release by unstimulated cardiomyocytes. In addition, infection with an adenovirus encoding MKK6(A) abrogated GIIa PLA(2) protein synthesis and release by interleukin-1beta-stimulated cells. These results provide direct evidence that p38 MAPK activation was necessary for interleukin-1beta-induced synthesis and release of GIIa PLA(2) by cardiomyocytes.

Role of specific CCAAT/enhancer-binding protein isoforms in intestinal epithelial cells

Intestinal epithelial cells participate in the acute phase response in response to inflammation. We have shown that acute phase protein genes are induced during intestinal acute phase response, and that the CCAAT/enhancer binding protein family of transcription factors are involved. To address the role of specific C/EBP isoforms, we generated IEC-6 rat intestinal epithelial cell lines expressing different C/EBP isoforms, by retroviral infection. Overexpression of C/EBPalpha p30 and C/EBPdelta led to increases in C/EBPbeta LAP and C/EBPbeta LIP endogenous protein levels, as determined by electrophoretic mobility shift assays and Western blot. Inhibition of C/EBP activity with dominant negative C/EBPs (C/EBPbeta LIP, 3hF, 4hF) decreased glucocorticoid-, cAMP- and IL-1 responsiveness of the endogenous haptoglobin gene, while overexpression of each C/EBP isoform increased the responsiveness to these regulators. In contrast, dominant negative C/EBPs or C/EBP isoforms did not alter the expression of alpha-acid glycoprotein in response to dexamethasone and of C/EBPbeta and C/EBPdelta in response to various regulators as assessed by Northern blot. These data show that the three C/EBP isoforms are involved in the regulation of haptoglobin and that C/EBPbeta, C/EBPdelta, and alpha-acid glycoprotein expression are not induced by C/EBP isoforms in contrast to other cell types. C/EBPbeta LAP-expressing cells showed an inhibition of cell growth characterized by a delay in p27(Kip1) decrease in response to serum and a decrease in cyclin D isoforms and cyclin E protein levels. Finally, C/EBP isoforms interact with the E2F4 transcription factor. Thus, specific C/EBP isoforms are involved in the differential expression of acute phase protein genes in response to hormones and cytokines. Furthermore, C/EBP isoforms may play a role in the control of cell cycle progression.

Involvement of CCAAT/enhancer-binding protein in regulation of the rat serine:pyruvate/alanine:glyoxylate aminotransferase gene expression

In the rat liver, transcription of the serine:pyruvate/alanine:glyoxylate aminotransferase (SPT/AGT) gene occurs from two sites, +1 and +66, in exon 1, resulting in the formation of two mRNAs, one for a precursor of mitochondrial SPT/AGT and the other for peroxisomal SPT/AGT, respectively. In this study, we attempted to characterize the downstream promoter responsible for generation of peroxisomal SPT/AGT. The minimal downstream promoter was confined to the +21-+90 region. We demonstrated that C/EBPalpha and C/EBPbeta bound around the downstream start site (+66) contribute to the promoter activity. The downstream promoter activity is also regulated positively by a short inverted repeat, located 20-30 bp upstream of the downstream start site, through a protein factor(s) bound to this region. On the other hand, the sequence just downstream of the start site may negatively regulate the promoter activity.

DNase I hypersensitivity analysis of the human CCAAT enhancer binding protein epsilon (C/EBPepsilon) gene

Human C/EBPepsilon is a recently cloned member of the C/EBP family of transcriptional factors. Previous studies demonstrated that the expression of this gene is tightly regulated in a tissue-specific manner; it is expressed almost exclusively in myeloid cells. To understand the mechanism by which the expression of C/EBPepsilon gene is controlled, we cloned a large genomic region surrounding the C/EBPepsilon gene and performed a DNase I hypersensitivity analysis of this locus. These sites probably represent areas of binding of proteins modulating gene transcription. Hypersensitive (HS) regions in 30 kb of DNA surrounding the C/EBPepsilon gene were examined in C/EBPepsilon high-expressing (NB4, HL-60), low-expressing (Jurkat), very-low-expressing (KG-1), and non-expressing (K562) hematopoietic cells as well as in non-hematopoietic-non-expressing cells (MCF-7, DU 145, PC-3). Three HS sites were detected near the first exon of C/EBPepsilon gene. They were found only in hematopoietic cells and were especially prominent in C/EBPepsilon expressing cells, suggesting that these sites play an important role in transcribing the gene. These hypersensitive bands did not change when the cells were cultured with retinoids. Gel-shift assays using 200 bp of nucleotide sequences that encompassed the hypersensitive sites and nuclear extracts from NB4 and Jurkat cells (C/EBPepsilon expressing) as well as K562 and MCF-7 cells (non-expressing) showed different retarded bands on gel electrophoresis. A fourth HS site, located about 11 kb upstream of exon 1, was found only in cells highly expressing C/EBPepsilon. Two sites, one about 4.5 kb upstream of exon 1 and another about 8.5 kb downstream of exon 2, were positive only in non-expressing cell lines, suggesting that repressors may bind in these areas. Taken together, we have found six specific DNase I hypersensitive sites in the region of C/EBPepsilon that may be involved in regulating transcription of this gene.

Acholeplasma laidlawii up-regulates granulysin gene expression via transcription factor activator protein-1 in a human monocytic cell line, THP-1

An antimicrobial protein granulysin is constitutively expressed in cytotoxic T lymphocytes (CTL) and natural killer (NK) cells. However, little is known about the precise regulatory mechanisms underlying granulysin gene expression. In this study, we examined the regulatory mechanisms underlying granulysin gene expression using a human monocytic cell line, THP-1, treated with Acholeplasma laidlawii. The level of granulysin mRNA expression in THP-1 cells was significantly augmented in response to stimulation with A. laidlawii. The transfection of reporter gene constructs into THP-1 cells indicated that DNA sequences between residues -329 and -239, relative to the transcriptional start site of the granulysin gene, are responsible for mediating gene induction. In addition, mutagenesis of a putative activator protein-1 (AP-1)-binding site between residues -277 and -271 in the granulysin promoter resulted in the reduction of granulysin promoter activity. Electrophoretic mobility shift assays (EMSA) demonstrated that nuclear extract prepared from A. laidlawii-treated THP-1 cells can generate specific binding to DNA oligonucleotides encompassing the AP-1-binding site, whereas unstimulated nuclear extract from the cells failed to do so. Furthermore, competition and supershift assays confirmed that A. laidlawii can induce the activation of AP-1. These results indicate that AP-1 dominantly participates in the regulation of inducible granulysin gene expression in THP-1 cells. Therefore, the finding of inducible granulysin gene expression by A. laidlawii suggests that inducible granulysin in macrophages may function as a protective weapon when microbial invasion occurs.

CCAAT/enhancer binding protein beta regulates expression of the cystatin-related epididymal spermatogenic (Cres) gene

The CRES protein is a member of the cystatin superfamily of cysteine protease inhibitors with restricted expression in stage-specific germ cells, proximal caput epididymidis, and anterior pituitary gonadotroph cells. To elucidate the molecular mechanisms regulating the highly restricted expression of the cres gene, we have sequenced 1.6 kilobases of mouse cres 5' flanking sequence and performed studies to examine the cres gene promoter. Two putative CCAAT/enhancer binding protein (C/EBP) transcription factor binding motifs exist within the first 135 base pairs of cres promoter. Furthermore, our studies demonstrate that cres mRNA levels are dramatically reduced in the epididymides of C/EBP beta-deficient mice. These data suggest that the C/EBP family of transcription factors, in particular C/EBP beta, plays a role in the regulation of cres gene expression. In support of this finding, Northern blot analysis showed that C/EBP beta is the predominant C/EBP family member expressed in the L beta T2 gonadotroph cell line and the proximal caput epididymidis. Also, gel shift and supershift assays demonstrated that C/EBP beta protein in nuclear extracts from L beta T2 gonadotroph cells and epididymal cells bound to the two C/EBP sites in the cres promoter. Finally, to test the in vivo function of the C/EBP sites in cres gene expression, transfection studies were performed in L beta T2 gonadotroph cells and two heterologous cell systems. These experiments showed a significant reduction of cres transactivation when either C/EBP sites were mutated, and no transC/EBP activation of the cres promoter when both C/EBP sites were mutated. Taken together, these studies demonstrate that the C/EBP beta transcription factor is necessary for high levels of cres gene expression in the proximal caput epididymidis and anterior pituitary gonadotroph cells.

The multifaceted roles of glycogen synthase kinase 3beta in cellular signaling

Glycogen synthase kinase-3beta (GSK3beta) is a fascinating enzyme with an astoundingly diverse number of actions in intracellular signaling systems. GSK3beta activity is regulated by serine (inhibitory) and tyrosine (stimulatory) phosphorylation, by protein complex formation, and by its intracellular localization. GSK3beta phosphorylates and thereby regulates the functions of many metabolic, signaling, and structural proteins. Notable among the signaling proteins regulated by GSK3beta are the many transcription factors, including activator protein-1, cyclic AMP response element binding protein, heat shock factor-1, nuclear factor of activated T cells, Myc, beta-catenin, CCAAT/enhancer binding protein, and NFkappaB. Lithium, the primary therapeutic agent for bipolar mood disorder, is a selective inhibitor of GSK3beta. This raises the possibility that dysregulation of GSK3beta and its inhibition by lithium may contribute to the disorder and its treatment, respectively. GSK3beta has been linked to all of the primary abnormalities associated with Alzheimer's disease. These include interactions between GSK3beta and components of the plaque-producing amyloid system, the participation of GSK3beta in phosphorylating the microtubule-binding protein tau that may contribute to the formation of neurofibrillary tangles, and interactions of GSK3beta with presenilin and other Alzheimer's disease-associated proteins. GSK3beta also regulates cell survival, as it facilitates a variety of apoptotic mechanisms, and lithium provides protection from many insults. Thus, GSK3beta has a central role regulating neuronal plasticity, gene expression, and cell survival, and may be a key component of certain psychiatric and neurodegenerative diseases.

Expression and function of CCAAT/enhancer binding proteinbeta (C/EBPbeta) LAP and LIP isoforms in mouse mammary gland, tumors and cultured mammary epithelial cells

CCAAT/Enhancer binding proteins (C/EBPs) play important roles in the regulation of cell growth and differentiation. This study investigated the expression and function of C/EBPbeta isoforms in the mouse mammary gland, mammary tumors, and a nontransformed mouse mammary epithelial cell line (HC11). C/EBPbeta mRNA levels are 2-5-fold higher in mouse mammary tumors derived from MMTV/c-neu transgenic mice compared with lactating and involuting mouse mammary gland. The "full-length" 38 kd C/EBPbeta LAP ("Liver-enriched Activator Protein") isoform is the predominant C/EBPbeta protein isoform in mammary tumor whole cell lysates, however, the truncated 20 kd C/EBPbeta LIP ("Liver-enriched Inhibitory Protein") isoform is also present at detectable levels (mean LAP:LIP ratio 5.3:1). The mammary tumor C/EBPbeta LAP:LIP ratio decreases 70% (from 5.3:1 to 1.6:1) when lysate preparation is switched from a rapid whole cell lysis protocol to a multistep nuclear/cytoplasmic fractionation protocol. In contrast to mammary tumors, only the C/EBPbeta LAP isoform is detectable in the mammary gland whole cell and nuclear lysates; the truncated "LIP" isoform is undetectable regardless of isolation protocol. Ectopic over expression of C/EBPbeta LIP or C/EBPbeta LAP did not alter HC11 growth rates. However, C/EBPbeta LIP over expressing HC11 cells (LAP:LIP ratio of approximately 1:1) exhibited a consistent 2-4 h delay in G(0)/S phase transition. C/EBPbeta LIP overexpressing HC11 cells did not express beta-casein mRNA (mammary epithelial cell differentiation marker) in response to lactogenic hormones. This defect in beta-casein expression was not corrected by carrying out the differentiation protocol in the presence of an artificial extracellular matrix. These results demonstrate that the "full-length" C/EBPbeta LAP isoform is the predominant C/EBPbeta protein isoform expressed in mouse mammary gland in vivo and mouse mammary epithelial cell cultures in vitro. C/EBPbeta LIP detected in mammary tumor lysates may result from in vivo production or ex vivo isolation-induced proteolysis of C/EBPbeta LAP. Ectopic overexpression of C/EBPbeta LIP (LAP:LIP ratio of approximately 1:1) inhibits mammary epithelial cell differentiation (beta-casein expression).

Estrogen-dependent E2a/Pbx1 myeloid cell lines exhibit conditional differentiation that can be arrested by other leukemic oncoproteins

The molecular pathways of normal myeloid differentiation, as well as the mechanisms by which oncogenes disrupt this process, remain poorly understood. A major limitation in approaching this problem has been the lack of suitable cell lines that exhibit normal, terminal, and synchronous differentiation in the absence of endogenous oncoproteins and in response to physiologic cytokines, and whose differentiation can be arrested by ectopically expressed human oncoproteins. This report describes clonal, granulocyte-macrophage colony-stimulating factor-dependent myeloid cell lines that exhibit these properties. The cell lines were established by conditional immortalization of primary murine marrow progenitors with an estrogen-regulated E2a/Pbx1-estrogen receptor fusion protein. Clones were identified that proliferated as immortalized blasts in the presence of estrogen, and that exhibited granulocytic, monocytic, or bipotential (granulocytic and monocytic) differentiation on estrogen withdrawal. Differentiation was normal and terminal as evidenced by morphology, cell surface markers, gene expression, and functional assays. The differentiation of the cells could be arrested by heterologous oncoproteins including AML1/ETO, PML/RARalpha, PLZF/RARalpha, Nup98/HoxA9, and other Hox proteins. Furthermore, the study examined the effects of cooperating oncoproteins such as Ras or Bcr/Abl, which allowed for both factor-independent proliferation and differentiation, or Bcl-2, which permitted factor-independent survival but not proliferation. These myeloid cell lines provide tools for examining the biochemical and genetic pathways that accompany normal differentiation as well as a system in which to dissect how other leukemic oncoproteins interfere with these pathways.

Characterization of C/EBPbeta isoforms in normal versus neoplastic mammary epithelial cells

A member of the CCAAT Enhancer Binding Proteins (C/EBPs) family of transcription factors, C/EBPbeta, has recently proven to be an important player in both growth and differentiation of the epithelial cells in the mammary gland. When the gene for C/EBPbeta is disrupted in mice, these mice fail to either develop normal mammary ducts during puberty or pregnancy, or to lactate upon parturition. C/EBPbeta can be present in cells in three isoforms: C/EBPbeta-1, -2, and -3. These isoforms have the same carboxy terminus but different N-termini due to alternative translational initiation at three different initiator codons within the C/EBPbeta mRNA. Using a commercially available antibody specific to the C-terminus of C/EBPbeta and a novel antibody specific to the N-terminus of C/EBPbeta-1, we have uncovered a striking difference in the forms of C/EBPbeta present in normal mammary epithelial cells versus breast cancer cell lines. C/EBPbeta- 1 is found exclusively in normal mammary epithelial cells, whereas C/EBPbeta- 2 is found only in dividing cells, both normal and neoplastic. Our preliminary data suggest that the prevalent form of C/EBPbeta in cancer cells, C/EBPbeta- 2, can activate genes which push the cell to divide, such as cyclin D1.

Gene structure of the extracellular glutathione S-transferase from Onchocerca volvulus and its overexpression and promoter analysis in transgenic Caenorhabditis elegans

Two highly similar genes encoding unique extracellular, glycosylated glutathione S-transferases (GSTs) of the human-pathogenic nematode, Onchocerca volvulus (Ov-GST1a and Ov-GST1b), have been isolated and characterised. The genes are approximately 3 kb in length and consist of seven exons interrupted by introns of approximately 100 bp in length, with the exception of intron II, which is approximately 1.6 kb in length. Interestingly, exon I and II encode a signal peptide and an N-terminal extension before sequence homology to other GSTs begins. The 5' flanking region was sequenced and analysed for transcription factor binding sites. Consistent with the lack of a TATA box, analysis of the mRNAs by primer extension showed multiple transcription start sites spread over a 60 bp region. To examine the activity and specificity of the Ov-GST1a gene promoter, we have exploited Caenorhabditis elegans as a heterologous transformation system. To analyse whether transgenic C. elegans are able to carry out processing and post-transcriptional modifications of the Ov-GST1a correctly, the protein was ectopically overexpressed in C. elegans. The parasite-derived Ov-GST1a gene product was correctly processed in transgenic C. elegans and posttranslational modifications, such as signal peptide cleavage and N-glycosylation, were performed successfully. This further demonstrates the potential of C. elegans as a host for expression of candidate vaccine antigens from O. volvulus and affirms the role of C. elegans as a model for parasitic nematodes.

Ectopic expression of CCAAT/enhancer binding protein beta (C/EBPbeta) in long-term bone marrow cultures induces granulopoiesis and alters stromal cell function

CCAAT/enhancer binding proteins (C/EBP) have been demonstrated to impact directly the development of multiple hematopoietic lineages. However, the role of C/EBPbeta in the differentiation of various hematopoietic lineages has not been thoroughly examined. We used primary bone marrow cultures to assess directly the ability of C/EBPbeta to influence myelopoiesis. Retroviral expression vectors were used to express C/EBPbeta ectopically in murine primary long-term bone marrow cultures. The differentiation potential of these cells was determined using hematopoietic colony assays and differential staining of cells within the cultures. Bone marrow cultures that overexpressed C/EBPbeta had fewer myeloid progenitors and a significant increase in the number of granulocytes. The ability of C/EBPbeta to alter hematopoiesis in vitro was dependent on the presence of the transcriptional activation domain because LIP, which lacks this functional domain, did not decrease the ability of bone marrow cultures to support myeloid progenitors. These data also show that C/EBPbeta influences hematopoiesis by altering stromal cell function rather than the intrinsic developmental potential of myeloid progenitor cells.

Specific localization of transcription factors in the chromatin of mouse mature spermatozoa

We previously characterized a nuclease-hypersensitive fraction of mouse sperm chromatin, which is organized in a typical nucleosomal structure. A partial genomic library was constructed with the DNA from the nuclease-hypersensitive chromatin, which revealed a high content in retroposon/retroviral DNA sequences. Here we report that the cloned nuclease-hypersensitive DNA also contains clusters of potential sites for transcription factors: among those, binding sites for Oct-1, Oct-4, TBP, Ets-1, and C/EBP are most abundant. This observation prompted us to ask whether mature spermatozoa contain the corresponding protein factors. Indirect immunofluorescence experiments show that all analyzed factors are indeed present in the sperm heads. Moreover, transcription factors are associated with the nuclease-hypersensitive chromatin of spermatozoa, as endogenous nucleases that degrade the hypersensitive fraction also cause the concomitant release of transcription factors from sperm cells into the medium. Band-shift assays with proteins extracted from the supernatant, and immunofluorescence analysis of sperm pellets, indicate that transcription factors are largely recovered in the supernatant while being absent or poorly retained in spermatozoa. The possible involvement of these factors in early embryogenesis is discussed.

Contribution of cyclooxygenase 2 to liver regeneration after partial hepatectomy

Partial hepatectomy (PH) triggers a rapid regenerative response in the remaining tissue to reinstate the organ function and the cell numbers. Among the molecules that change in the course of regeneration is an accumulation of prostaglandin E2 in the sera of rats with PH. Analysis of the cyclooxygenase (COX) isoenzymes in the remnant liver showed the preferential expression of COX-2 in hepatocytes. Cultured regenerating hepatocytes expressed significant levels of COX-2, a process that was not observed in the sham counterparts. Maximal expression of COX-2 was detected 16 h after PH with increased levels present even at 96 h. Pharmacological inhibition of COX-2 activity with NS398 shunted the up-regulation of cell proliferation after PH, which suggests a positive interaction of prostaglandins with the progression of the cell cycle. Similar results were obtained after PH of mice lacking the COX-2 gene. The expression of COX-2 in regenerating liver was concomitant with a decrease in CCAAT-enhancer binding protein (C/EBP-a) level and an increase in the expression of C/EBP-b and C/EBP-d. These results suggest a contribution of the enhanced synthesis of prostaglandins to liver regeneration observed after PH.

Retinoblastoma protein complexes with C/EBP proteins and activates C/EBP-mediated transcription

The retinoblastoma protein (RB) recruits histone deacetylase (HDAC) to repress E2F-mediated transactivation that plays a critical role in cell cycle regulation. RB is also involved in activation of expression of a number of tissue specific- and differentiation-related genes. In this study, we examined the mechanism by which RB stimulated the expression of a differentiation-related gene, the surfactant protein D (SP-D), which plays important roles in innate host defense and the regulation of surfactant homeostasis. We demonstrated that RB specifically stimulated the activity of human SP-D gene promoter. The RB family member, p107 but not p130, also increased SP-D promoter activity. Activation by RB was mediated through a NF-IL6 (C/EBP beta) binding motif in the human SP-D promoter, and this sequence specifically bound to C/EBP alpha, C/EBP beta, and C/EBP delta. RB formed stable complexes with all three C/EBP family members. RB small pocket (amino acid residues 379-792), but not the C-pocket (amino acid residues 792-928), was necessary and sufficient for its interaction with C/EBP proteins. Furthermore, we demonstrated that the complexes containing RB and C/EBP proteins directly interacted with C-EBP binding site on DNA. These findings indicate that RB plays a positive, selective, and direct role in the C/EBP-dependent transcriptional regulation of human SP-D expression.

Dichotomy of AML1-ETO functions: growth arrest versus block of differentiation

The fusion gene AML1-ETO is the product of t(8;21)(q22;q22), one of the most common chromosomal translocations associated with acute myeloid leukemia. To investigate the impact of AML1-ETO on hematopoiesis, tetracycline-inducible AML1-ETO-expressing cell lines were generated using myeloid cells. AML1-ETO is tightly and strongly induced upon tetracycline withdrawal. The proliferation of AML1-ETO(+) cells was markedly reduced, and most of the cells eventually underwent apoptosis. RNase protection assays revealed that the amount of Bcl-2 mRNA was decreased after AML1-ETO induction. Enforced expression of Bcl-2 was able to significantly delay, but not completely overcome, AML1-ETO-induced apoptosis. Prior to the onset of apoptosis, we also studied the ability of AML1-ETO to modulate differentiation. AML1-ETO expression altered granulocytic differentiation of U937T-A/E cells. More significantly, this change of differentiation was associated with the down-regulation of CCAAT/enhancer binding protein alpha (C/EBPalpha), a key regulator of granulocytic differentiation. These observations suggest a dichotomy in the functions of AML1-ETO: (i) reduction of granulocytic differentiation correlated with decreased expression of C/EBPalpha and (ii) growth arrest leading to apoptosis with decreased expression of CDK4, c-myc, and Bcl-2. We predict that the preleukemic AML1-ETO(+) cells must overcome AML1-ETO-induced growth arrest and apoptosis prior to fulfilling their leukemogenic potential.

Molecular characterization of the Xenopus CCAAT-enhancer binding protein beta gene promoter

Transcription factors belonging to the CCAAT-enhancer binding protein (C/EBP) family play key roles in the regulation of genes implicated in the control of growth, differentiation, metabolism, and inflammation. The recent limited studies on the promoter regions of C/EBP genes, particularly C/EBPalpha, have indicated the potential existence of species-specific regulatory mechanisms. It is therefore essential that the promoter regions of different C/EBP genes from a wide range of species are investigated in detail. As an important step toward this goal, we report here the characterization of the Xenopus laevis C/EBPbeta gene promoter. Sequence analysis showed that the 1.6-kb promoter region contained putative binding sites for several transcription factors that have previously been implicated in the regulation of the C/EBPs, including C/EBP, CREB, Myb, STAT, and USF. The -288/+91 promoter region was capable of directing high levels of expression in the hepatoma Hep3B cell line. In addition, this minimal promoter could be autoregulated by both C/EBPalpha and C/EBPbeta and activated by lipopolysaccharide, interleukin-6 and CREB. These results therefore demonstrate that several aspects of C/EBPbeta regulation in mammals have been highly conserved in amphibians. However, a comparison of C/EBPbeta gene promoters characterized to date does indicate the existence of species-specific differences in autoregulation.

Renal tubule-specific transcription and chromosomal localization of rat thiazide-sensitive Na-Cl cotransporter gene

The molecular mechanism underlying the renal expression localization of the thiazide-sensitive Na-Cl cotransporter (TSC) gene was studied. The TSC gene was localized to chromosome 19p12-14. In cultured cells, tissue-specific transcription activity of the 5'-flanking region of the rat rTSC gene (5'FL/rTSC) was demonstrated, and the major promoter region was located between position -580 and -141. To further examine the tissue-specific transcription, transgenic rats harboring the 5'FL/rTSC fused upstream of the LacZ gene were generated. Immunohistochemical analysis clearly showed that LacZ gene expression was co-localized to distal convoluted tubules (DCT) with TSC, indicating that the 5'FL/rTSC regulates the renal tubule-specific TSC expression. Because a transcription factor, HFH-3 (hepatocyte nuclear factor-3/folk head homologue-3), had also been localized to DCT, a possible role of the putative cis-acting element (HFH-3/rTSC, -400/-387 position) for HFH-3 binding in the tissue-specific transcription was examined. Deletion and mutation analyses suggested that transcription of the HFH-3/rTSC was actually responsive to HFH-3, and electrophoretic mobility shift assay showed a direct binding of in vitro synthesized HFH-3 to the HFH-3/rTSC. In conclusion, the rTSC gene is localized to rat chromosome 19p12--24. The transcription regulatory region of the TSC gene confers DCT-specific gene expression. DCT-specific transcription factor HFH-3 may be involved in the renal tubule-specific transcription of TSC gene.

Elk-1, C/EBPalpha, and Pit-1 confer an insulin-responsive phenotype on prolactin promoter expression in Chinese hamster ovary cells and define the factors required for insulin-increased transcription

The transcription factor(s) that mediate insulin-increased gene transcription are not well defined. These studies use phenotypic conversion of Rat2 and Chinese hamster ovary (CHO) cells with transcription factors to identify components required for regulation of prolactin promoter activity and its control by insulin. The pituitary-derived GH4 cells contain all of the transcription factors required for insulin-increased prolactin-chloramphenicol acetyltransferase (CAT) expression while HeLa cells require only Pit-1, a pituitary-specific factor. However, Rat2 and CHO cells require additional factors. We had determined previously that the transcription factor that mediates insulin-increased prolactin gene expression was likely an Ets-related protein. Elk-1 and Sap-1 were the only Ets-related transcription factors tested as chimeras with LexA DNA-binding domain that were able to mediate insulin-increased expression of a LexA-CAT reporter plasmid. Elk-1 and Sap-1 are expressed in GH4 and HeLa cells but Rat2 and CHO cells express Sap-1, but not Elk-1. Expression of Elk-1 made Rat2 cells (but not CHO cells) insulin responsive. C/EBPalpha also binds to the prolactin promoter at a sequence overlapping the binding site for Elk-1. Expression of both C/EBPalpha and Pit-1 in CHO cells is required for high basal transcription of prolactin-CAT. Expression of Elk-1 converts CHO cells into a phenotype in which prolactin gene expression is increased by insulin treatment. Finally, antisense mediated reduction of Elk-1 in GH4 cells decreased insulin-increased prolactin gene expression and confirmed the requirement for Elk-1 for insulin-increased prolactin gene expression. Thus, both C/EBPalpha and Pit-1 were required for high basal transcription while insulin sensitivity required Elk-1.

Characterization of domains in C/EBPalpha that mediate its constitutive and cAMP-inducible activities

Structure/function analysis of CCAAT/enhancer binding proteins (C/EBP) alpha and beta have shown that they possess both constitutive and cAMP inducible activities. Three regions conserved between C/EBPalpha and beta were identified which lie within the cAMP inducible domains of each protein. Deletion analysis of these conserved regions within C/EBPalpha show that conserved region 2 plays a particularly critical role in mediating the PKA inducible activity of the protein, however, the constitutive activity of conserved region 2 depends on promoter context. This data supports previous findings that constitutive and cAMP responsiveness are mediated by domains of the protein that do not directly overlap, suggesting that they occur through distinct mechanisms.

C/EBP epsilon mediates myeloid differentiation and is regulated by the CCAAT displacement protein (CDP/cut)

Neutrophils from CCAAT enhancer binding protein epsilon (C/EBP epsilon) knockout mice have morphological and biochemical features similar to those observed in patients with an extremely rare congenital disorder called neutrophil-specific secondary granule deficiency (SGD). SGD is characterized by frequent bacterial infections attributed, in part, to the lack of neutrophil secondary granule proteins (SGP). A mutation that results in loss of functional C/EBP epsilon activity has recently been described in an SGD patient, and has been postulated to be the cause of the disease in this patient. We have previously demonstrated that overexpression of CCAAT displacement protein (CDP/cut), a highly conserved transcriptional repressor of developmentally regulated genes, suppresses expression of SGP genes in 32Dcl3 cells. This phenotype resembles that observed in both C/EBP epsilon(-/-) mice and in SGD patients. Based on these observations we investigated potential interactions between C/EBP epsilon and CDP/cut during neutrophil maturation. In this study, we demonstrate that inducible expression of C/EBP epsilon in 32Dcl3/tet cells results in granulocytic differentiation. Furthermore, Northern blot analysis of G-CSF-induced CDP/cut overexpressing 32Dcl3 cells revealed absence of C/EBP epsilon mRNA. We therefore hypothesize that C/EBP epsilon positively regulates SGP gene expression, and that C/EBP epsilon is itself negatively regulated by CDP/cut during neutrophil maturation. We further demonstrate that the C/EBP epsilon promoter is regulated by CDP/cut during myeloid differentiation.

Overexpression of and RNA interference with the CCAAT enhancer-binding protein on long-term facilitation of Aplysia sensory to motor synapses

In the marine mollusk Aplysia, the CCAAT/enhancer-binding protein, ApC/EBP, serves as an immediate early gene in the consolidation of long-term facilitation in the synaptic connection between the sensory and motor neurons of the gill-withdrawal reflex. To further examine the role of ApC/EBP as a molecular switch of a stable form of long-term memory, we cloned the full-length coding regions of two alternatively spliced forms, the short and long form of ApC/EBP. Overexpression of each isoform by DNA microinjection resulted in a l6-fold increase in the expression of the coinjected luciferase reporter gene driven by an ERE promoter. In addition, when we overexpressed ApC/EBP in Aplysia sensory neurons, we found that the application of a single pulse of 5-HT that normally induced only short-term facilitation now induced long-term facilitation. Conversely, when we attempted to block the synthesis of native ApC/EBP by microinjecting double-strand RNA or antisense RNA, we blocked long-term facilitation in a sequence-specific manner. These data support the idea that ApC/EBP is both necessary and sufficient to consolidate short-term memory into long-term memory. Furthermore, our results suggest that this double-strand RNA interference provides a powerful tool in the study of the genes functioning in learning and memory in Aplysia by specifically inhibiting both the constitutive and induced expression of the genes.

The role of C/EBP(epsilon) in the terminal stages of granulocyte differentiation

As a consequence of its characterization using both in vitro and knockout mouse models, the myeloid-specific transcription factor, CCAAT/enhancer binding protein (C/EBP)epsilon, has been identified as a critical regulator of terminal granulopoiesis and one of the causative mutations in the human disease, neutrophil-specific granule deficiency. C/EBPs are a family of transcription factors sharing numerous structural and functional features and to date include C/EBPalpha, -beta, -gamma, -delta, -epsilon, and -zeta. C/EBPalpha was the first family member isolated and characterized, its essential role in hepatocyte and adipocyte differentiation demonstrated in knockout mouse models. Subsequent analysis of the hematopoietic elements in fetal mouse liver revealed its critical role in myelopoiesis. Understanding the role of C/EBPepsilon in terminal granulopoiesis in the context of other known transcription factors is ongoing with analysis of deficient and conditionally expressing cell lines and knockout models. Mouse models with targeted gene disruptions have contributed greatly to our understanding of the transcriptional regulation of granulopoiesis. Further manipulation of these models and other conditional expression systems have bypassed some of the limitations of knockout models and helped delineate the interactions of different transcription factors in affecting granulocyte development. Phenotypic expression of the loss of C/EBPepsilon in mice is extreme, resembling absolute neutropenia with systemic infection with P. aeruginosa. Future work will need to explore the regulation of C/EBPepsilon expression, its functional interactions with other transcriptional regulators such as PU.1, and its role in monocyte differentiation and function in the mouse.

CCAAT/enhancer-binding protein beta mediates interferon-gamma-induced p48 (ISGF3-gamma ) gene transcription in human monocytic cells

Previous studies have identified a novel interferon-stimulated response element-like element, termed gamma-interferon-activating transcription element, within the interferon-stimulating gene factor-3gamma (p48) promoter region that is bound by novel transcription factors in response to stimulation with interferons (IFNs) (Weihua, X., Kolla, V., and Kalvakolanu, D. V. (1997) Proc. Natl. Acad. Sci. U. S. A. 94, 103-108). In the present study, we have identified CCAAT/enhancer-binding protein beta (C/EBP-beta) as one of the gamma-interferon-activating transcription element cognate transcription factors by screening a human monophage-derived cDNA library in a yeast one-hybrid system. Electrophoretic mobility shift assay studies suggest that C/EBP-beta dynamically regulates p48 gene expression upon IFN-gamma stimulation by undergoing changes in its heterodimerization partners. Transient transfection studies demonstrate that overexpression of C/EBP-beta strongly enhanced IFN-gamma-induced transcription from the p48 promoter. However, deletion mutants of C/EBP-beta that lack the N-terminal transactivation domain were unable to stimulate the p48 promoter. Western blotting revealed that C/EBP-beta is induced by IFN-gamma stimulation in THP-1-derived macrophages. Collectively, these results suggest that C/EBP-beta plays an important role in the human IFN-gamma signaling pathway by transcriptional regulation of p48 gene expression, an essential component in the IFN signaling pathway.

Epimorphin mediates mammary luminal morphogenesis through control of C/EBPbeta

We have shown previously that epimorphin (EPM), a protein expressed on the surface of myoepithelial and fibroblast cells of the mammary gland, acts as a multifunctional morphogen of mammary epithelial cells. Here, we present the molecular mechanism by which EPM mediates luminal morphogenesis. Treatment of cells with EPM to induce lumen formation greatly increases the overall expression of transcription factor CCAAT/enhancer binding protein (C/EBP)β and alters the relative expression of its two principal isoforms, LIP and LAP. These alterations were shown to be essential for the morphogenetic activities, since constitutive expression of LIP was sufficient to produce lumen formation, whereas constitutive expression of LAP blocked EPM-mediated luminal morphogenesis. Furthermore, in a transgenic mouse model in which EPM expression was expressed in an apolar fashion on the surface of mammary epithelial cells, we found increased expression of C/EBPβ, increased relative expression of LIP to LAP, and enlarged ductal lumina. Together, our studies demonstrate a role for EPM in luminal morphogenesis through control of C/EBPβ expression.

Autoregulation enables different pathways to control CCAAT/enhancer binding protein beta (C/EBP beta) transcription

CCAAT/enhancer binding protein beta (C/EBP beta) also named liver-enriched transcriptional activating protein (LAP) is a member of the C/EBP family of transcription factors and is involved in hepatocyte-specific gene expression and in the process of tissue differentiation. The activity of LAP/C/EBP beta can be regulated at the transcriptional and posttranslational level or by protein-protein interaction with other transcription factors. In this study we show that LAP/C/EBP beta can stimulate its own transcription. Deletion analysis of the rat LAP/C/EBP beta promoter in luciferase reporter gene experiments demonstrated that the region located between nucleotide -121 to -71, comprising two recently characterized cAMP responsive element (CRE)-like elements, is important for autoregulation. Gel shift experiments using oligonucleotides with overlapping point mutations identified the sequence GCAATGA (beta-site) adjacent to and partially overlapping the first CRE-like site as core motif for LAP/C/EBP beta binding. Analysis of a mutated beta-site in reporter gene experiments showed the functional relevance of this site for autoregulation. The composite C/EBP beta-CRE-element in the promoter enables synergistic activation of transcription by LAP/C/EBP beta and the protein kinase A (PKA)/cAMP responsive element binding protein (CREB) pathway in a cell-type specific manner. In hepatoma cells nuclear factor kappa B (NF-kappa B) increased autoregulation and therefore could mediate enhanced activation during inflammatory responses. In summary, our results demonstrated that the assembly of the three binding sites in the promoter and thus the interaction between LAP/C/EBP beta and members of the CREB or NF-kappa B family allows the control of LAP/C/EBP beta gene transcription as a response to different stimuli in a tissue specific manner.

PEA3 is up-regulated in response to Wnt1 and activates the expression of cyclooxygenase-2

The inducible prostaglandin synthase cyclooxygenase-2 (COX-2) is aberrantly expressed in intestinal tumors resulting from APC mutation, and is also transcriptionally up-regulated in mouse mammary epithelial cells in response to Wnt1 expression. beta-Catenin stabilization is a consequence of both APC mutation and Wnt signaling. We have previously observed coordinate regulation of the matrilysin promoter by beta-catenin and Ets family transcription factors of the PEA3 subfamily. Here we show that while beta-catenin only weakly activates the COX-2 promoter, PEA3 family transcription factors are potent activators of COX-2 transcription. Consistent with this, PEA3 is up-regulated in Wnt1-expressing mouse mammary epithelial cells, and PEA3 factors are highly expressed in tumors from Wnt1 transgenic mice, in which Cox-2 is also up-regulated. Promoter mapping experiments suggest that the NF-IL6 site in the COX-2 promoter is important for mediating PEA3 responsiveness. The NF-IL6 site is also important for COX-2 transcription in some colorectal cancer lines (Shao, J., Sheng, H., Inoue, H., Morrow, J. D., and DuBois, R. N. (2000) J. Biol. Chem. 275, 33951-33956), and PEA3 factors are highly expressed in colorectal cancer cell lines. Therefore, we speculate that PEA3 factors may contribute to the up-regulation of COX-2 expression resulting from both APC mutation and Wnt1 expression.

Dysregulated cytokine production in human cystic fibrosis bronchial epithelial cells

Although pulmonary inflammation is an important pathologic event in cystic fibrosis (CF), the relationship between expression of the CF gene and the inflammatory response is unclear. We studied tumor necrosis factor (TNF) alpha and IL-1beta stimulated production of IL-6 and IL-8 by CF, corrected CF, and normal human bronchial epithelial cells in culture. During the first 24 hours of TNFalpha stimulation, CF cells produced significantly more IL-8 than normal or corrected CF cells. In the second 24 hours of TNFalpha stimulation, IL-6 and IL-8 generation ceased in normal and corrected CF cells but accelerated in CF cells, resulting in marked IL-6 and IL-8 accumulation in CF cells. Similar results were found when cells were stimulated with IL-1beta. Finally, when CF cells were grown at 27 degrees C (a culture condition which results in transport of CF transmembrane conductance regulator, CFTR, to the cell membrane and normalization of chloride conductance) TNFalpha-stimulated production of IL-6 and IL-8 reverted to normal. We conclude that dysregulation of cytokine generation by CF bronchial epithelial cells is directly related to expression of mutant CFTR and these observations provide a potential mechanism for persistence of airway inflammation in CF.

c-Myc is a critical target for c/EBPalpha in granulopoiesis

CCAAT/enhancer binding protein alpha (C/EBPalpha) is an integral factor in the granulocytic developmental pathway, as myeloblasts from C/EBPalpha-null mice exhibit an early block in differentiation. Since mice deficient for known C/EBPalpha target genes do not exhibit the same block in granulocyte maturation, we sought to identify additional C/EBPalpha target genes essential for myeloid cell development. To identify such genes, we used both representational difference analysis and oligonucleotide array analysis with RNA derived from a C/EBPalpha-inducible myeloid cell line. From each of these independent screens, we identified c-Myc as a C/EBPalpha negatively regulated gene. We mapped an E2F binding site in the c-Myc promoter as the cis-acting element critical for C/EBPalpha negative regulation. The identification of c-Myc as a C/EBPalpha target gene is intriguing, as it has been previously shown that down-regulation of c-Myc can induce myeloid differentiation. Here we show that stable expression of c-Myc from an exogenous promoter not responsive to C/EBPalpha-mediated down-regulation forces myeloblasts to remain in an undifferentiated state. Therefore, C/EBPalpha negative regulation of c-Myc is critical for allowing early myeloid precursors to enter a differentiation pathway. This is the first report to demonstrate that C/EBPalpha directly affects the level of c-Myc expression and, thus, the decision of myeloid blasts to enter into the granulocytic differentiation pathway.

Elements involved in the regulation of the StAR gene

The steroidogenic acute regulatory protein (StAR) mediates the transfer of cholesterol from the outer to the inner mitochondrial membrane, the regulated step in steroidogenesis. A most interesting facet of this protein is the manner in which its expression is acutely regulated. In this regard, a number of studies have concentrated on the search for consensus cis regulatory elements within its promoter, and, more importantly, on whether these elements are involved in its expression. This short review will summarize some of the findings that have been reported concerning the nature of how the expression of this gene is regulated.

A novel myeloid-restricted zebrafish CCAAT/enhancer-binding protein with a potent transcriptional activation domain

The CCAAT/enhancer-binding protein (C/EBP) family consists of transcription factors essential for hematopoiesis. The defining feature of the C/EBPs is a highly conserved carboxy-terminal bZIP domain that is necessary and sufficient for dimerization and DNA binding, whereas their amino-terminal domains are unique. This study reports a novel c/ebp gene (c/ebp1) from zebrafish that encodes a protein homologous to mammalian C/EBPs within the bZIP domain, but with an amino terminus lacking homology to any C/EBP or to any known sequence. In zebrafish embryos, c/ebp1 expression was initially observed in cells within the yolk sac circulation valley at approximately the 16-to 18-somite stage, and at 24 hours postfertilization (hpf), also in circulating cells. Most c/ebp1(+) cells also expressed a known early macrophage marker, leukocyte-specific plastin (l-plastin). Expression of both markers was lost in cloche, a mutant affecting hematopoiesis at the level of the hemangioblast. Expression of both markers was retained in m683 and spadetail, mutants affecting erythropoiesis, but not myelopoiesis. Further, c/ebp1 expression was lost in a mutant with defective myelopoiesis, but intact erythropoiesis. These data suggest that c/ebp1 is expressed exclusively in myeloid cells. In electrophoretic mobility shift assays, c/ebp1 was able to bind a C/EBP consensus DNA site. Further, a chimeric protein containing the amino-terminal domain of c/ebp1 fused to the DNA-binding domain of GAL4 induced a GAL4 reporter 4000-fold in NIH3T3 cells. These results suggest that c/ebp1 is a novel member of the C/EBP family that may function as a potent transcriptional activator in myeloid cells.

Interleukin-6-induced tethering of STAT3 to the LAP/C/EBPbeta promoter suggests a new mechanism of transcriptional regulation by STAT3

LAP/C/EBPbeta is a member of the C/EBP family of transcription factors and contributes to the regulation of the acute phase response in hepatocytes. Here we show that IL-6 controls LAP/C/EBPbeta gene transcription and identify an IL-6 responsive element in the LAP/C/EBPbeta promoter, which contains no STAT3 DNA binding motif. However, luciferase reporter gene assays showed that STAT3 activation through the gp130 signal transducer molecule is involved in mediating IL-6-dependent LAP/C/EBPbeta transcription. Southwestern analysis indicated that IL-6 induces binding of a 68-kDa protein to the recently characterized CRE-like elements in the LAP/C/EBPbeta promoter. Transfection experiments using promoter constructs with mutated CRE-like elements revealed that these sites confer IL-6 responsiveness. Further analysis using STAT1/STAT3 chimeras identified specific domains of the protein that are required for the IL-6-dependent increase in LAP/C/EBPbeta gene transcription. Overexpression of the amino-terminal domain of STAT3 blocked the IL-6-mediated response, suggesting that the STAT3 amino terminus has an important function in IL-6-mediated transcription of the LAP/C/EBPbeta gene. These data lead to a model of how tethering STAT3 to a DNA-bound complex contributes to IL-6-dependent LAP/C/EBPbeta gene transcription. Our analysis describes a new mechanism by which STAT3 controls gene transcription and which has direct implication for the acute phase response in liver cells.

TNF-alpha inhibits UCP-1 expression in brown adipocytes via ERKs. Opposite effect of p38MAPK

Tumor necrosis factor-alpha (TNF-alpha) activates extracellular-regulated kinases (ERKs) and p38 mitogen-activated protein kinase (p38MAPK), and inhibits the expression of uncoupling protein-1 (UCP-1) and adipocyte-specific genes in rat fetal brown adipocytes. MEK inhibition with PD98059 abolished the inhibitory effect of TNF-alpha on UCP-1, but not on adipogenic genes. In contrast, inhibition of p38MAPK with SB203580 potentiated the negative effect of TNF-alpha on UCP-1 and adipogenic genes. The inhibitory action of TNF-alpha was partially correlated with changes in C/EBPalpha and beta protein levels and in their DNA binding activity, suggesting a role for these transcription factors. However, other transcription factors might explain the different regulation of UCP-1 and adipogenic genes by ERKs.

Molecular regulation of adipogenesis

Adipogenesis, or the development of fat cells from preadipocytes, has been one of the most intensely studied models of cellular differentiation. In part this has been because of the availability of in vitro models that faithfully recapitulate most of the critical aspects of fat cell formation in vivo. More recently, studies of adipogenesis have proceeded with the hope that manipulation of this process in humans might one day lead to a reduction in the burden of obesity and diabetes. This review explores some of the highlights of a large and burgeoning literature devoted to understanding adipogenesis at the molecular level. The hormonal and transcriptional control of adipogenesis is reviewed, as well as studies on a less well known type of fat cell, the brown adipocyte. Emphasis is placed, where possible, on in vivo studies with the hope that the results discussed may one day shed light on basic questions of cellular growth and differentiation in addition to possible benefits in human health.

Human RNA helicase II/Gu gene: genomic organization and promoter analysis

Human RNA helicase II/Gu (hRH II/Gu) protein unwinds double-stranded RNA, folds single-stranded RNA, and may play important roles in ribosomal RNA biogenesis, RNA editing, RNA transport, and general transcription. The genomic organization and the regulation of expression of the hRH II/Gu gene were analyzed. The hRH II/Gu gene is over 29 kb in length, and includes 15 exons. All intron-exon junctions contain consensus GT-AG sequences. An 85 bp basic promoter was identified which lacks a functional TATA box. In LNCaP cells, the hRH II/Gu promoter was up to 50 times as active as pGL2-control vector which has an SV 40 early promoter. Mutation of a putative c-Myc/USF binding site in the region between -110 and -85 caused a decrease in the promoter activity by 2- to 3-fold. A negative regulatory region was also found between -324 and -264. Studies of transcriptional regulation of hRH II/Gu under different physiological and pathophysiological conditions may provide clues to its biological function.

Regulation of the Clara cell secretory protein/uteroglobin promoter in lung

Clara cell secretory protein/uteroglobin (CCSP/UG) is specifically expressed in the conducting airway epithelium of the lung in a differentiation-dependent manner. The proximal promoter region of the rodent CCSP/UG gene directs Clara cell specificity. Previously, it was shown that the forkhead transcription factors HNF-3 alpha and beta and the homeodomain factor TTF-1 are important transcription factors acting through this region, suggesting that they contribute to cell specificity of the CCSP/UG gene. Members of the C/EBP family of transcription factors can also interact with elements of the proximal rat and mouse CCSP/UG promoters. The onset of C/EBP alpha expression in Clara cells correlates with the strong increase of CCSP/UG expression. Thus, C/EBP alpha may play a crucial role for differentiation-dependent CCSP/UG expression. Transfection studies demonstrate that C/EBP alpha and TTF-1 can synergistically activate the murine CCSP/UG promoter. Altogether, these results suggest that C/EBP alpha, TTF-1, and HNF-3 determine the Clara cell-specific, differentiation-dependent expression of the CCSP/UG gene in murine lung. The relative importance of these three transcription factors, however, differs in rabbits and humans.

Links among growth factors, hormones, and nuclear factors with essential roles in bone formation

Research performed during the last several years implicates important roles for a variety of growth factors that affect osteoblasts or their precursors during bone development, remodeling, or repair. Of these, three families of growth factors in particular-the transforming growth factor betas (TGF-betas), insulin-like growth factors (IGFs), and bone morphogenetic proteins (BMPs)-are considered to be principal local regulators of osteogenesis, although none is specific for cells of the osteoblast lineage. Therefore, mechanisms to induce skeletal tissue specificity might occur through interactions among these growth factors, with circulating hormones, or through specific intracellular mediators. In the latter case, even more recent studies point to two nuclear transcription factors, termed Core Binding Factor a1 (CBFa1) and CCAAT/Enhancer Binding Protein delta (C/EBPdelta), as significant regulators of the expression or activity of specific bone growth factors or their receptors. Perhaps more importantly, events that link these growth factors to nuclear proteins occur in response to glucocorticoids, sex steroids, parathyroid hormone (PTH), or prostaglandin E2 (PGE2), which themselves have well-known effects on bone biology. In this review, we discuss the situations and processes that initially suggested growth-factor- and hormone-specific interactions on cells within the osteoblast lineage, and present evidence for roles that CBFa1 and C/EBPdelta have on osteoblast function. Finally, we offer examples for how these factors integrate events that are associated with various aspects of bone formation.

Molecular and in silico characterization of a promoter module and C/EBP element that mediate LPS-induced RANTES/CCL5 expression in monocytic cells

The chemokine RANTES/CCL5 is a proinflammatory agent produced by a variety of tissues in response to specific stimuli. In human monocytes, RANTES/CCL5 transcription is up-regulated rapidly and transiently in response to LPS. We describe here two regions that help control LPS-driven transcription from the human RANTES/CCL5 promoter in monocytic cells. These sites were analyzed by using DNase I footprinting, transient transfection assays, site-directed mutagenesis, and EMSA. RANTES site E (R(E), -125/-99) constitutively binds C/EBP proteins in monocytic Mono Mac 6 cells. Mutation of region R(E) led to a significant (40%-50%) reduction in LPS-induced promoter reporter activity. Region R(AB) is composed of tandem kB-like elements R(A) and R(B) (-73/-34). These sites working in concert act as an LPS-responsive promoter module. R(A) constitutively binds Sp1, and Rel p50/p65 following LPS stimulation. Either factor can mediate transcriptional effects at R(A). Induced Rel p50/p50 binding to site R(B) is required for LPS regulation of RANTES/CCL5 transcription. A series of computer models based on the RANTES/CCL5 promoter were generated to represent the organization of these functional elements. The models could identify LPS-regulated promoters in human, other vertebrate, and viral sequences in various databases.

Transcriptional control of adipogenesis

The major transcriptional factors involved in the adipogenic process include proteins belonging to the CCAAT/enhancer binding protein family, peroxisome proliferator-activated receptor gamma, and adipocyte determination and differentiation dependent factor 1, also known as sterol regulatory element-binding protein 1. This process has been characterized with the aid of cell lines that represent various stages in the path of adipocyte commitment, ranging from pluripotent mesodermal fibroblasts to preadipocytes. Molecular analyses have led to a cascade model for adipogenesis based on timed expression of CCAAT/enhancer-binding proteins and peroxisome proliferator-activated receptor gamma. Gene targeting and transgenic-mouse technologies, which allow the manipulation of endogenous genes for these transcription factors, have also contributed to the understanding of adipogenesis. This review aims to integrate this information to gain an understanding of the transcriptional regulation of fat cell formation.

Leptin receptor 5'untranslated regions in the rat: relative abundance, genomic organization and relation to putative response elements

Hypothalamic sensitivity to leptin has been suggested to be important for regulation of body fat mass. Mice heterozygous for a mutation in the leptin receptor (leptin-R) have an increased body fat mass suggesting that the abundance of leptin-R may be an important determinator of leptin sensitivity. Leptin-R cDNAs from several species contain alternative 5'untranslated regions (5'UTRs), suggesting that several distinct regulatory regions may exist. To investigate possible mechanisms by which leptin-R expression may be regulated, we searched for possible alternative 5'UTRs of the leptin-R in the rat and determined their location in relation to putative response elements. Four leptin-R 5'UTRs (exons 1A-1D), which diverged 23 bp upstream of the start codon, were identified by 5'Rapid Amplification of cDNA Ends (5'RACE) and sequencing. Exons 1B and 1C were present in 31 and 61%, respectively, of all leptin-R transcripts in the hypothalamus as determined by a ribonuclease protection assay. Analysis of the 5' flanking genomic sequences revealed an imperfect estrogen response element (ERE), two Spl-sites, three CCAAT-boxes and one octamer. Exons 1A and 1D corresponded to a putative second gene, encoding the OB-Receptor Gene Related Protein (OB-RGRP), which is transcribed from a promoter shared with the leptin-R. DNA sequencing revealed that the rat OB-RGRP had 98 and 97% homology with the mouse and human sequence, respectively. We report here that transcription of the rat leptin-R gene may generate transcripts with four alternative 5'UTRs. The presence of a putative ERE, close to the most frequently used transcriptional start sites of the leptin-R gene in the hypothalamus, provides a possible mechanism by which estrogen may exert its effects on food intake.

The chum salmon insulin-like growth factor II promoter requires Sp1 for its activation by C/EBPbeta

The chum salmon insulin-like growth factor II (IGF-II) gene is highly expressed in liver tissue. In this study we demonstrate that two transcription factors, Sp1 and C/EBPbeta, are involved in the enhanced expression of the salmon IGF-II gene. The presence of the fish homolog for C/EBPbeta in salmon liver RNA was confirmed by Northern blotting. The sIGF-II promoter was activated up to 20-fold by co-transfection with C/EBPbeta. The functional importance of four out of the five putative C/EBPbeta binding sites was demonstrated with mutational analysis in transient transfection assays. The transcription factor Sp1 binds to two sites within the salmon IGF-II promoter. Interestingly, mutation of the Sp1 binding sites decreases not only the basal IGF-II promoter activity but also the C/EBPbeta-induced transactivation. These results demonstrate that liver-enriched C/EBPbeta and ubiquitously expressed Sp1 each activate the sIGF-II promoter and that Sp1 is required for full transactivation of the sIGF-II gene by C/EBPbeta. This suggests that C/EBPbeta and Sp1 act in synergy.

Transcription factors that regulate growth and differentiation of myeloid cells

Recently much progress has been made in our understanding of how myeloid progenitor cells undergo commitment and become mature granulocytes or monocytes/macrophages. Studies of normal and leukemic myeloid cells as well as those of cells derived from mice with targeted disruption showed that a series of transcription factors play a major role in both commitment and maturation of myeloid cells. This is primarily because these transcription factors direct an ordered pattern of gene expression according to a well-defined developmental program. PU.1, an Ets family member, is one of the master transcription factors identified to regulate development of both granulocytes and monocytes/macrophages. Further, C/EBPalpha and C/EBPvarepsilon of the bZip family have important roles in directing granulocytic maturation. A number of additional transcription factors such as AML1, RARalpha, MZF-1, Hox and STAT families of transcription factors, Egr-1 and c-myb etc are shown to play roles in myeloid cell differentiation. Our laboratory has recently obtained evidence that ICSBP, a member of the IRF family, is involved in lineage commitment during myeloid cell differentiation and stimulates maturation of functional macrophages. Future elucidation of pathways and networks through which these transcription factors act in various stages of development would provide a more definitive picture of myeloid cell commitment and maturation.

Genomic characterization, localization, and functional expression of FGL2, the human gene encoding fibroleukin: a novel human procoagulant

For diseases in which thrombosis plays a pivotal role, such as virus-induced fulminant hepatitis, fetal loss syndrome, and xenograft rejection, the major procoagulant has remained elusive. Here we describe the isolation and functional expression of a distinct human prothrombinase, termed FGL2. The murine fgl2 gene product has been implicated in the pathophysiology of murine fulminant hepatitis. The predicted ORF corresponds to a 439-amino-acid type II integral membrane protein that contains a carboxy-terminal Fibrinogen-related domain. Functional analysis showed that FGL2-encoded protein is indeed a prothrombinase. This enzyme is a serine protease and directly cleaves prothrombin to thrombin. The FGL2 gene is a single-copy gene in the haploid human genome and has two exons separated by a 2195-bp intron expressing two mRNA transcripts of 1.5 and 5.0 kb. The 5'-flanking region contains putative cis-elements including a TATA box, an AP1 site, CEBP sites, Sp1 site, and Ets binding domains. By both radiation hybrid analyses and fluorescence in situ hybridization, human FGL2 was localized to 7q11.23.

Coiled coils: a highly versatile protein folding motif

The alpha-helical coiled coil is one of the principal subunit oligomerization motifs in proteins. Its most characteristic feature is a heptad repeat pattern of primarily apolar residues that constitute the oligomer interface. Despite its simplicity, it is a highly versatile folding motif: coiled-coil-containing proteins exhibit a broad range of different functions related to the specific 'design' of their coiled-coil domains. The architecture of a particular coiled-coil domain determines its oligomerization state, rigidity and ability to function as a molecular recognition system. Much progress has been made towards understanding the factors that determine coiled-coil formation and stability. Here we discuss this highly versatile protein folding and oligomerization motif with regard to its structural architecture and how this is related to its biological functions.

Analysis of the Xenopus laevis CCAAT-enhancer binding protein alpha gene promoter demonstrates species-specific differences in the mechanisms for both auto-activation and regulation by Sp1

Transcription factors belonging to the CCAAT-enhancer binding protein (C/EBP) family have been implicated in the regulation of gene expression during differentiation, development and disease. Autoregulation is relatively common in the modulation of C/EBP gene expression and the murine and human C/EBPalpha genes have been shown to be auto-activated by different mechanisms. In the light of this finding, it is essential that autoregulation of C/EBPalpha genes from a wider range of different species be investigated in order to gauge the degree of commonality, or otherwise, that may exist. We report here studies that investigate the regulation of the Xenopus laevis C/EBPalpha gene (xC/EBPalpha). The -1131/+41 promoter region was capable of directing high levels of expression in both the human hepatoma Hep3B and the Xenopus kidney epithelial A6 cell lines, and was auto-activated by expression vectors specifying for xC/EBPalpha or xC/EBPss. Deletion analysis showed that the -321/+41 sequence was sufficient for both the constitutive promoter activity and auto-activation and electrophoretic mobility shift assays identified the interaction of C/EBPs and Sp1 to this region. Although deletion of either the C/EBP or the Sp1 site drastically reduced the xC/EBPalpha promoter activity, multimers of only the C/EBP site could confer autoregulation to a heterologous SV40 promoter. These results indicate that, in contrast to the human promoter and in common with the murine gene, the xC/EBPalpha promoter was subject to direct autoregulation. In addition, we demonstrate a novel species-specific action of Sp1 in the regulation of C/EBPalpha expression, with the factor able to repress the murine promoter but activate the Xenopus gene.