Toxoplasma gondii tachyzoites inhibit proinflammatory cytokine induction in infected macrophages by preventing nuclear translocation of the transcription factor NF-kappa B

Control of microbial infection requires regulated induction of NF-kappaB-dependent proinflammatory cytokines such as IL-12 and TNF-alpha. Activation of this important transcription factor is driven by phosphorylation-dependent degradation of the inhibitory IkappaB molecule, an event which enables NF-kappaB translocation from the cytoplasm to the nucleus. In this study, we show that intracellular infection of macrophages with the protozoan parasite Toxoplasma gondii induces rapid IkappaB phosphorylation and degradation. Nevertheless, NF-kappaB failed to translocate to the nucleus, enabling the parasite to invade cells without triggering proinflammatory cytokine induction. Infected cells subsequently subjected to LPS triggering were severely crippled in IL-12 and TNF-alpha production, a result of tachyzoite-induced blockade of NF-kappaB nuclear translocation. Our results are the first to demonstrate the ability of an intracellular protozoan to actively interfere with the NF-kappaB activation pathway in macrophages, an activity that may enable parasite survival within the host.

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.

Insulin suppresses transactivation by CAAT/enhancer-binding proteins beta (C/EBPbeta). Signaling to p300/CREB-binding protein by protein kinase B disrupts interaction with the major activation domain of C/EBPbeta

CAAT/enhancer-binding proteins (C/EBPs) play an important role in the regulation of gene expression in insulin-responsive tissues. We have found that a complex containing C/EBPbeta interacts with an insulin response sequence in the insulin-like growth factor-binding protein-1 (IGFBP-1) gene and that a C/EBP-binding site can mediate effects of insulin on promoter activity. Here, we examined mechanisms mediating this effect of insulin. The ability of insulin to suppress promoter activity via a C/EBP-binding site is blocked by LY294002, a phosphatidylinositol 3-kinase inhibitor, but not by rapamycin, which blocks activation of p70(S6 kinase). Dominant negative phosphatidylinositol 3-kinase and protein kinase B (PKB) block the effect of insulin, while activated PKB suppresses promoter function via a C/EBP-binding site, mimicking the effect of insulin. Coexpression studies indicate that insulin and PKB suppress transactivation by C/EBPbeta, but not C/EBPalpha, and that N-terminal transactivation domains in C/EBPbeta are required. Studies with Gal4 fusion proteins reveal that insulin and PKB suppress transactivation by the major activation domain in C/EBPbeta (AD II), located between amino acids 31 and 83. Studies with E1A protein indicate that interaction with p300/CBP is required for transactivation by AD II and the effect of insulin and PKB. Based on a consensus sequence, we identified a PKB phosphorylation site (Ser(1834)) within the region of p300/CBP known to bind C/EBPbeta. Mammalian two-hybrid studies indicate that insulin and PKB disrupt interactions between this region of p300 and AD II and that Ser(1834) is critical for this effect. Signaling by PKB and phosphorylation of Ser(1834) may play an important role in modulating interactions between p300/CBP and transcription factors and mediate effects of insulin and related growth factors on gene expression.

C/EBPbeta, when expressed from the C/ebpalpha gene locus, can functionally replace C/EBPalpha in liver but not in adipose tissue

Knockout of C/EBPalpha causes a severe loss of liver function and, subsequently, neonatal lethality in mice. By using a gene replacement approach, we generated a new C/EBPalpha-null mouse strain in which C/EBPbeta, in addition to its own expression, substituted for C/EBPalpha expression in tissues. The homozygous mutant mice C/ebpalpha(beta/beta) are viable and fertile and show none of the overt liver abnormalities found in the previous C/EBPalpha-null mouse line. Levels of hepatic PEPCK mRNA are not different between C/ebpalpha(beta/beta) and wild-type mice. However, despite their normal growth rate, C/ebpalpha(beta/beta) mice have markedly reduced fat storage in their white adipose tissue (WAT). Expression of two adipocyte-specific factors, adipsin and leptin, is significantly reduced in the WAT of C/ebpalpha(beta/beta) mice. In addition, expression of the non-adipocyte-specific genes for transferrin and cysteine dioxygenase is reduced in WAT but not in liver. Our study demonstrates that when expressed from the C/ebpalpha gene locus, C/EBPbeta can act for C/EBPalpha to maintain liver functions during development. Moreover, our studies with the C/ebpalpha(beta/beta) mice provide new insights into the nonredundant functions of C/EBPalpha and C/EBPbeta on gene regulation in WAT.

Generation of truncated C/EBPbeta isoforms by in vitro proteolysis

Multiple forms of the transcriptional regulator CCAAT/enhancer-binding protein beta (C/EBPbeta) with molecular masses of approximately 38, 34, 20, and 14 kDa have been observed in cell extracts. It has been proposed that these proteins arise by alternative initiation at in-frame AUG codons. The truncated C/EBPbeta isoforms (p14 and p20/LIP) lack transactivation domains but retain DNA-binding and dimerization sequences and are therefore assumed to function as competitive inhibitors of C/EBP-mediated transcription in vivo. By comparing various extraction procedures to analyze endogenous and overexpressed C/EBPbeta proteins, we determined that p20-C/EBPbeta is generated predominantly by in vitro proteolytic cleavage during isolation from cells and that p14-C/EBPbeta is produced exclusively by this mechanism. In transfected cells, the full-length (p34 and p38) isoforms but not the truncated proteins were detectable in the cytoplasm, indicating that the latter are not primary translation products. In addition, the C/EBPbeta leucine zipper dimerization domain was essential for the appearance of the truncated species, demonstrating that protein folding or dimerization are critical determinants of proteolytic sensitivity. Our findings suggest that the presence of truncated C/EBPbeta proteins in cell extracts must be interpreted with caution and that assumptions about the in vivo relevance of these isoforms should be re-evaluated.

The C/EBP bZIP domain can mediate lipopolysaccharide induction of the proinflammatory cytokines interleukin-6 and monocyte chemoattractant protein-1

C/EBPalpha, beta, and delta are all expressed by bone marrow-derived macrophages. Ectopic expression of any of these transcription factors is sufficient to confer lipopolysaccharide (LPS)-inducible expression of interleukin-6 (IL-6) and monocyte chemoattractant protein-1 (MCP-1) to a B lymphoblast cell line, which normally lacks C/EBP factors and does not display LPS induction of proinflammatory cytokines. Thus, the activities of C/EBPalpha, beta, and delta are redundant in regard to expression of IL-6 and MCP-1. Surprisingly, the bZIP region of C/EBPbeta, which lacks any previously described activation domains, can also confer LPS-inducible expression of IL-6 and MCP-1 in stable transfectants. Transient transfections reveal that the bZIP regions of C/EBPbeta, C/EBPdelta, and, to a lesser extent, C/EBPalpha can activate the IL-6 promoter and augment its induction by LPS. Furthermore, the transdominant inhibitor, LIP, can activate expression from the IL-6 promoter. The ability of the C/EBPbeta bZIP region to activate the IL-6 promoter in transient transfections is completely dependent upon an intact NF-kappaB-binding site, supporting a model where the bZIP protein primarily functions to augment the activity of NF-kappaB. Replacement of the leucine zipper of C/EBPbeta with that of GCN4 yields a chimeric protein that can dimerize and specifically bind to a C/EBP consensus sequence, but shows a markedly reduced ability to activate IL-6 and MCP-1 expression. These results implicate the leucine zipper domain in some function other than dimerization with known C/EBP family members, and suggest that C/EBP redundancy in regulating cytokine expression may result from their highly related bZIP regions.

C/EBPbeta modulates the early events of keratinocyte differentiation involving growth arrest and keratin 1 and keratin 10 expression

The epidermis is a stratified squamous epithelium composed primarily of keratinocytes that become postmitotic and undergo sequential changes in gene expression during terminal differentiation. The expression of the transcription factor CCAAT/enhancer binding protein beta (C/EBPbeta) within mouse epidermis and primary keratinocytes has recently been described; however, the function of C/EBPbeta within the epidermal keratinocyte is unknown. We report here that transient transfection of mouse primary keratinocytes with a C/EBP-responsive promoter-reporter construct resulted in a sevenfold increase in luciferase activity when keratinocytes were switched to culture conditions that induce growth arrest and differentiation. Forced expression of C/EBPbeta in BALB/MK2 keratinocytes inhibited growth, induced morphological changes consistent with a more differentiated phenotype, and upregulated two early markers of differentiation, keratin 1 (K1) and keratin 10 (K10) but had a minimal effect on the expression of late-stage markers, loricrin and involucrin. Analysis of the epidermis of C/EBPbeta-deficient mice revealed a mild epidermal hyperplasia and decreased expression of K1 and K10 but not of involucrin and loricrin. C/EBPbeta-deficient primary keratinocytes were partially resistant to calcium-induced growth arrest. Analysis of terminally differentiated spontaneously detached keratinocytes or those induced to differentiate by suspension culture revealed that C/EBPbeta-deficient keratinocytes displayed striking decreases in K1 and K10, while expression of later-stage markers was only minimally altered. Our results demonstrate that C/EBPbeta plays an important role in the early events of stratified squamous differentiation in keratinocytes involving growth arrest and K1 and K10 expression.

CCAAT/enhancer-binding proteins regulate expression of the human steroidogenic acute regulatory protein (StAR) gene

Two putative CCAAT/enhancer-binding protein (C/EBP) response elements were identified in the proximal promoter of the human steroidogenic acute regulatory protein (StAR) gene, which encodes a key protein-regulating steroid hormone synthesis. Expression of C/EBPalpha and -beta increased StAR promoter activity in COS-1 and HepG2 cells. Cotransfection of C/EBPalpha or -beta and steroidogenic factor 1, a transcription factor required for cAMP regulation of StAR expression, into COS-1 augmented 8-bromoadenosine 3':5'-cyclic monophosphate (8-Br-cAMP)-stimulated promoter activity. When the putative C/EBP response elements were mutated, individually or together, a pronounced decline in basal StAR promoter activity in human granulosa-lutein cells resulted, but the fold stimulation of promoter activity by 8-Br-cAMP was unaffected. Recombinant C/EBPalpha and -beta bound to the two identified sequences but not the mutated elements. Human granulosa-lutein cell nuclear extracts also bound these elements but not the mutated sequences. An antibody to C/EBPbeta, but not C/EBPalpha, supershifted the nuclear protein complex associated with the more distal element. The complex formed by nuclear extracts with the proximal element was not supershifted by either antibody. Western blot analysis revealed the presence of C/EBPalpha and C/EBPbeta in human granulosa-lutein cell nuclear extracts. C/EBPbeta levels were up-regulated 3-fold by 8-Br-cAMP treatment. Our studies demonstrate a role for C/EBPbeta as well as yet to be identified proteins, which can bind to C/EBP response elements, in the regulation of StAR gene expression and suggest a mechanism by which C/EBPbeta participates in the cAMP regulation of StAR gene transcription.

Autocrine signals control CCAAT/enhancer binding protein beta expression, localization, and activity in macrophages

The transcription factor CCAAT/enhancer binding protein beta (C/EBPbeta, or NF-IL6) is expressed in macrophages, where it participates in lipopolysaccharide (LPS)-mediated induction of proinflammatory cytokine genes such as interleukin-6 (IL-6) and IL-1beta. We have identified activities in conditioned medium from a macrophage tumor cell line that regulates the expression, localization, and transcriptional activity of C/EBPbeta. One factor was shown to be tumor necrosis factor- (TNF-), which increased C/EBPbeta expression by a posttranscriptional mechanism. A second activity, designated autocrine macrophage factor (AMF), elicited a change in C/EBPbeta localization from a punctate nuclear staining pattern to diffuse nuclear distribution. The punctate form of C/EBPbeta correlated with increased susceptibility of this protein to cleavage by an endogenous protease during nuclear extract preparation. Conditioned medium stimulated the ability of C/EBPbeta to transactivate a reporter gene and activated the expression of two cytokine genes that are putative targets of C/EBPbeta. These observations suggest that diffuse distribution of C/EBPbeta in the nucleus corresponds to an activated form of this protein. AMF activity could not be mimicked by an extensive set of recombinant cytokines and growth factors and therefore may represent a novel extracellular factor.

Tumor necrosis factor alpha transcription in macrophages is attenuated by an autocrine factor that preferentially induces NF-kappaB p50

Macrophages are a major source of proinflammatory cytokines such as tumor necrosis factor alpha (TNF-alpha), which are expressed during conditions of inflammation, infection, or injury. We identified an activity secreted by a macrophage tumor cell line that negatively regulates bacterial lipopolysaccharide (LPS)-induced expression of TNF-alpha. This activity, termed TNF-alpha-inhibiting factor (TIF), suppressed the induction of TNF-alpha expression in macrophages, whereas induction of three other proinflammatory cytokines (interleukin-1beta [IL-1beta], IL-6, and monocyte chemoattractant protein 1) was accelerated or enhanced. A similar or identical inhibitory activity was secreted by IC-21 macrophages following LPS stimulation. Inhibition of TNF-alpha expression by macrophage conditioned medium was associated with selective induction of the NF-kappaB p50 subunit. Hyperinduction of p50 occurred with delayed kinetics in LPS-stimulated macrophages but not in fibroblasts. Overexpression of p50 blocked LPS-induced transcription from a TNF-alpha promoter reporter construct, showing that this transcription factor is an inhibitor of the TNF-alpha gene. Repression of the TNF-alpha promoter by TIF required a distal region that includes three NF-kappaB binding sites with preferential affinity for p50 homodimers. Thus, the selective repression of the TNF-alpha promoter by TIF may be explained by the specific binding of inhibitory p50 homodimers. We propose that TIF serves as a negative autocrine signal to attenuate TNF-alpha expression in activated macrophages. TIF is distinct from the known TNF-alpha-inhibiting factors IL-4, IL-10, and transforming growth factor beta and may represent a novel cytokine.

Selectively enhanced contextual fear conditioning in mice lacking the transcriptional regulator CCAAT/enhancer binding protein delta

CCAAT/enhancer binding protein delta (C/EBPdelta) is a transcriptional regulator implicated in the hepatic acute phase response and in adipogenic and myeloid cell differentiation. We found that C/EBPdelta is widely expressed in the peripheral and central nervous systems, including neurons of the hippocampal formation, indicating a role in neural functions. To examine the role of C/EBPdelta in vivo, we generated mice with a targeted deletion of the C/EBPdelta gene. This mutation does not interfere with normal embryonic and postnatal development. Performance in a battery of behavioral tests indicates that basic neurological functions are normal. Furthermore, performance in a Morris water maze task suggests that C/EBPdelta mutant mice have normal spatial learning. However, in the contextual and auditory-cue-conditioned fear task, C/EBPdelta null mice displayed significantly more conditioned freezing to the test context than did wild-type controls, but equivalent conditioning to the auditory cue. These data demonstrate a selectively enhanced contextual fear response in mice carrying a targeted genomic mutation and implicate C/EBPdelta in the regulation of a specific type of learning and memory.

beta-adrenergic receptor-induced activation of nerve growth factor gene transcription in rat cerebral cortex involves CCAAT/enhancer-binding protein delta

Stimulation of beta-adrenergic receptors (BAR) by clenbuterol (CLE) increases nerve growth factor (NGF) biosynthesis in the rat cerebral cortex but not in other regions of the brain. We have explored the transcription mechanisms that may account for the cortex-specific activation of the NGF gene. Although the NGF promoter contains an AP-1 element, AP-1-binding activity in the cerebral cortex was not induced by CLE, suggesting that other transcription factors govern the brain area-specific induction of NGF. Because BAR activation increases cAMP levels, we examined the role of CCAAT/enhancer-binding proteins (C/EBP), some of which are known to be cAMP-inducible. In C6-2B glioma cells, whose NGF expression is induced by BAR agonists, (i) CLE increased C/EBPdelta-binding activity, (ii) NGF mRNA levels were increased by overexpressing C/EBPdelta, and (iii) C/EBPdelta increased the activity of an NGF promoter-reporter construct. Moreover, DNase footprinting and deletion analyses identified a C/EBPdelta site in the proximal region of the NGF promoter. C/EBPdelta appears to be responsible for the BAR-mediated activation of the NGF gene in vivo, since CLE elicited a time-dependent increase in C/EBPdelta-binding activity in the cerebral cortex only. Our data suggest that, while AP-1 may regulate basal levels of NGF expression, C/EBPdelta is a critical component determining the area-specific expression of NGF in response to BAR stimulation.

The C/EBPbeta transcription factor regulates epithelial cell proliferation and differentiation in the mammary gland

Studies of C/EBPbeta-deficient mice have demonstrated a pivotal role for this transcription factor in hematopoiesis, adipogenesis, and ovarian function. Here we show that C/EBPbeta is also essential for normal development and function of the mammary gland. Ductal morphogenesis in virgin C/EBPbeta-deficient mice was disrupted, with ducts displaying reduced growth and branching. To distinguish whether the effect of C/EBPbeta deficiency on mammary epithelium is indirect or cell autonomous, we performed ovarian and mammary gland transplants. Transplants of wild-type ovaries into mutant females partially restored ductal morphogenesis during puberty but failed to support mammopoiesis during pregnancy. At term, mutant mice harboring wild-type ovaries exhibited reduced alveolar proliferation and impaired epithelial cell differentiation, including a complete absence of milk protein expression. Mammary gland transplant experiments demonstrated that development of C/EBPbeta-deficient epithelium was defective within a wild-type stroma and host background. Cell proliferation during pregnancy was reduced and differentiation, as measured by the activity of milk protein genes, was inhibited. However, wild-type epithelium developed in a C/EBPbeta-deficient stroma. Thus, C/EBPbeta plays an essential, cell autonomous role in the proliferation and differentiation of mammary secretory epithelial cells and is required for the activation of milk protein genes.

CCAAT/enhancer binding protein beta is a neuronal transcriptional regulator activated by nerve growth factor receptor signaling

CCAAT/enhancer binding protein beta (C/EBPbeta) is a transcriptional regulator of the basic leucine zipper family. By in situ hybridization analysis, we found that C/EBPbeta is widely expressed in the CNS of adult mice, including cells of the hippocampus and dentate gyrus and cerebellar Purkinje and granule cells. Expression of C/EBPbeta had also been reported in the PC12 cell line, which undergoes differentiation to neuron-like cells in response to nerve growth factor (NGF). We show that C/EBPbeta mRNA expression increases while protein levels decrease during differentiation of PC12 cells. In transactivation assays, C/EBPbeta activity was stimulated by NGF receptor signaling. Mutations of a phosphorylation site for mitogen-activated protein kinase in C/EBPbeta affected its capacity to transactivate in a promoter-specific manner. Our data identify the C/EBPbeta protein and gene as direct downstream targets of the NGF receptor and suggest a role for C/EBPbeta in neurotrophin signaling in the brain.

C/EBPepsilon is a myeloid-specific activator of cytokine, chemokine, and macrophage-colony-stimulating factor receptor genes

C/EBPepsilon is a member of the CCAAT/enhancer binding protein family of basic region/leucine zipper transcriptional activators. The C/EBPepsilon protein is highly conserved between rodents and humans, and its domain structure is very similar to C/EBPalpha. In mice C/EBPepsilon mRNA is only detected in hematopoietic tissues, including embryonic liver and adult bone marrow and spleen. Within the hematopoietic system, C/EBPepsilon is expressed primarily in myeloid cells, including promyelocytes, myelomonocytes, and their differentiated progeny. To identify potential functions of C/EBPepsilon, cell lines over-expressing the C/EBPepsilon protein were generated in the P388 lymphoblastic cell line. In contrast to the parental cell line, C/EBPepsilon-expressing cell lines displayed lipopolysaccharide-inducible expression of the interleukin-6 and monocyte chemoattractant protein 1 (MCP-1) genes as well as elevated basal expression of the MIP-1alpha and MIP-1beta chemokine genes. In the EML-C1 hematopoietic stem cell line, C/EBPepsilon mRNA levels increased as the cells progressed along the myeloid lineage, just preceding activation of the gene encoding the receptor for macrophage-colony-stimulating factor (M-CSFR). M-CSFR expression was stimulated in C/EBPepsilon-expressing P388 cell lines, when compared with either the parental P388 cells or P388 cell lines expressing either C/EBPalpha or C/EBPbeta. These results suggest that C/EBPepsilon may be an important regulator of differentiation of a subset of myeloid cell types and may also participate in the regulation of cytokine gene expression in mature cells.

Inhibition of CCAAT/enhancer-binding protein alpha and beta translation by upstream open reading frames

CCAAT/enhancer-binding protein (C/EBP) alpha is a bZIP transcription factor whose expression is restricted to specific cell types. Analysis of C/EBPalpha mRNA and protein levels in various mammalian cells indicates that expression of this gene is controlled both transcriptionally and post-transcriptionally. We report here that C/EBPalpha translation is repressed in several cell lines by an evolutionarily conserved upstream open reading frame (uORF), which acts in cis to inhibit C/EBPalpha translation. Mutations that disrupt the uORF completely abolished translational repression of C/EBPalpha. The related c/ebpbeta gene also contains an uORF that suppresses translation. The length of the spacer sequence between the uORF terminator and the ORF initiator codon (7 bases in all c/ebpalpha genes and 4 bases in c/ebpbeta homologs) is precisely conserved. The effects of insertions, deletions, and base substitutions in the C/EBPalpha spacer showed that both the length and nucleotide sequence of the spacer are important for efficient translational repression. Our data indicate that the uORFs regulate translation of full-length C/EBPalpha and C/EBPbeta and do not play a role in generating truncated forms of these proteins, as has been suggested by start site multiplicity models.

Interleukin-6-specific activation of the C/EBPdelta gene in hepatocytes is mediated by Stat3 and Sp1

C/EBPdelta (CCAAT/enhancer binding protein delta) has been implicated as a regulator of acute-phase response (APR) genes in hepatocytes. Its expression increases dramatically in liver during the APR and can be induced in hepatic cell lines by interleukin-6 (IL-6), an acute-phase mediator that activates transcription of many APR genes. Here we have investigated the mechanism by which C/EBPdelta expression is regulated by IL-6 in hepatoma cells. C/EBPdelta promoter sequences to -125 bp are sufficient for IL-6 inducibility of a reporter gene and include an APR element (APRE) that is essential for IL-6 responsiveness. DNA binding experiments and transactivation assays demonstrate that Stat3, but not Stat1, interacts with this APRE. Two Sp1 sites, one of which is adjacent to the APRE, are required for IL-6 induction and transactivation by Stat3. Thus, Stat3 and Sp1 function cooperatively to activate the C/EBPdelta promoter. Replacement of the APRE with Stat binding elements (SBEs) from the ICAM-1 or C/EBPbeta promoter, both of which recognize both Stat1 and Stat3, confers responsiveness to gamma interferon, a cytokine that selectively activates Stat1. Sequence comparisons suggest that the distinct Stat binding specificities of the C/EBPdelta and C/EBPbeta SBEs are determined primarily by a single base pair difference. Our findings indicate that the cytokine specificity of C/EBPdelta gene expression is governed by the APRE sequence.

Redundancy of C/EBP alpha, -beta, and -delta in supporting the lipopolysaccharide-induced transcription of IL-6 and monocyte chemoattractant protein-1

C/EBP alpha, -beta, and -delta are members of the CCAAT/enhancer binding protein family of transcriptional regulators. All three of these factors are expressed by bone marrow-derived macrophages, with the DNA binding activity of C/EBP beta and -delta increased by treatment with LPS while that of C/EBP alpha is decreased. We have ectopically expressed each C/EBP protein in P388 lymphoblasts. The expression of any of these transcription factors is sufficient to confer the LPS-inducible expression of IL-6 and monocyte chemoattractant protein-1 to lymphoblasts, which normally lack C/EBP factors and do not display LPS induction of proinflammatory cytokines. Thus, the activities of C/EBP alpha, -beta, and -delta are redundant in regard to the expression of IL-6 and monocyte chemoattractant protein-1. Since C/EBP beta-deficient mice have been reported to be largely normal in their expression of proinflammatory cytokines, it is likely that the lack of C/EBP beta is compensated for by the induction of C/EBP delta upon LPS treatment.

Disruption of the c/ebp alpha gene in adult mouse liver

The liver-enriched transcription factor C/EBP alpha has been implicated in the regulation of numerous liver-specific genes. It was previously reported that mice carrying a homozygous null mutation at the c/ebp alpha locus died as neonates due to the absence of hepatic glycogen and the resulting hypoglycemia. However, the lethal phenotype precluded further analysis of the role of C/EBP alpha in hepatic gene regulation in adult mice. To circumvent this problem, we constructed a conditional knockout allele of c/ebp alpha by using the Cre/loxP recombination system. Homozygous c/ebp-loxP mice, (c/ebp alpha(fl/fl);fl, flanked by loxP sites) were found to be indistinguishable from their wild-type counterparts. However, when Cre recombinase was delivered to hepatocytes of adult c/ebp alpha(fl/fl) mice by infusion of a recombinant adenovirus carrying the cre gene, more than 80% of the c/ebp alpha(fl/fl) genes were deleted specifically in liver and C/EBP alpha expression was reduced by 90%. This condition resulted in a reduced level of bilirubin UDP-glucuronosyltransferase expression in the liver. After several days, the knockout mice developed severe jaundice due to an increase in unconjugated serum bilirubin. The expression of genes encoding phosphoenolpyruvate carboxykinase, glycogen synthase, and factor IX was also strongly reduced in adult conditional-knockout animals, while the expression of transferrin, apolipoprotein B, and insulin-like growth factor I genes was not affected. These results establish C/EBP alpha as an essential transcriptional regulator of genes encoding enzymes involved in bilirubin detoxification and gluconeogenesis in adult mouse liver.

An essential role for C/EBPbeta in female reproduction

A large number of intercellular signaling molecules have been identified that orchestrate female reproductive physiology. However, with the exception of steroid hormone receptors, little information exists about the transcriptional regulators that mediate cellular responses to these signals. The transcription factor C/EBP beta (CCAAT/enhancer-binding protein beta) is expressed in ovaries and testes, as well as many other tissues of adult mice. Here we show that mice carrying a targeted deletion of the C/EBP beta gene exhibit reproductive defects. Although these animals develop normally and males are fertile, adult females are sterile. Transplantation of normal ovaries into mutant females restored fertility, thus localizing the primary reproductive defect to the ovary proper. In normal ovaries, C/EBP beta mRNA is specifically induced by luteinizing hormone (LH/hCG) in the granulosa layer of preovulatory antral follicles. C/EBP beta-deficient ovaries lack corpora lutea and fail to down-regulate expression of the prostaglandin endoperoxidase synthase 2 and P450 aromatase genes in response to gonadotropins. These findings demonstrate that C/EBP beta is essential for periovulatory granulosa cell differentiation in response to LH. C/EBP beta is thus established as a critical downstream target of G-protein-coupled LH receptor signaling and one of the first transcription factors, other than steroid hormone receptors, known to be required for ovarian follicle development in vivo.

The ability of C/EBP beta but not C/EBP alpha to synergize with an Sp1 protein is specified by the leucine zipper and activation domain

The rat CYP2D5 P-450 gene is activated in the liver during postnatal development. We previously showed that liver-specific transcription of the CYP2D5 gene is dictated by a proximal promoter element, termed 2D5, that is composed of a binding site for Sp1 or a related factor, and an adjacent cryptic C/EBP (CCAAT/enhancer-binding protein) site. Despite the fact that both C/EBP alpha and C/EBP beta are expressed abundantly in liver, only C/EBP beta is capable of stimulating the 2D5 promoter in HepG2 hepatocarcinoma cells. In addition, activation of the 2D5 promoter by C/EBP beta is completely dependent on the presence of the Sp1 site. Domain switch experiments reveal that C/EBP beta proteins containing either the leucine zipper or the activation domain of C/EBP alpha are unable to stimulate the 2D5 promoter yet are fully capable of transactivating an artificial promoter bearing a high-affinity C/EBP site. Thus, the leucine zipper and the activation domain of C/EBP beta are absolutely required to support transactivation of the 2D5 promoter. Using Drosophila cells that lack endogenous Sp1 activity, we show that the serine/threonine- and glutamine-rich activation domains A and B of Sp1 are required for efficient cooperatively with C/EBP beta. Furthermore, analysis of c/ebp beta-deficient mice shows that mutant animals are defective in expression of a murine CYP2D5 homolog in hepatic cells, confirming the selective ability of C/EBP beta to activate this liver-specific P-450 gene in vivo. Our findings illustrate that two members of a transcription factor family can achieve distinct target gene specificities through differential interactions with a cooperating Sp1 protein.

Interleukin-6 induces expression of peripherin and cooperates with Trk receptor signaling to promote neuronal differentiation in PC12 cells

In contrast to the intensively studied nerve growth factor (NGF)-related family of cytokines, relatively little is known about the mechanisms of neurotrophic activity elicited by the cytokine interleukin-6 (IL-6). We have examined the mechanisms of IL-6-induced neuronal differentiation of the pheochromocytoma cell line PC12. IL-6 independently induced the expression of peripherin, identifying this gene as the first neuronal-specific target of IL-6. However, IL-6 alone failed to elicit neurite outgrowth in PC12 cells and instead required low levels of Trk/NGF receptor tyrosine kinase activity to induce neuronal differentiation. The cooperating Trk signal could be provided by either overexpression of Trk or exposure to low concentrations of NGF. IL-6 also functioned cooperatively with basic fibroblast growth factor to promote PC12 differentiation. IL-6 and Trk/NGF synergized in enhancing tyrosine phosphorylation of the Erk-1 mitogen-activated protein kinase and in activating expression of certain NGF target genes. NGF also induced expression of the gp80 subunit of the IL-6 receptor, providing another potential mechanism of cooperativity between NGF and IL-6 signaling. We propose that IL-6 functions as an enhancer of NGF signaling rather than as an autonomous neuronal differentiation signal. Moreover, our results demonstrate that a Trk receptor-specific cellular response can be achieved in the absence of NGF through amplification of its basal signaling activity by the IL-6 receptor system.

Design of a C/EBP-specific, dominant-negative bZIP protein with both inhibitory and gain-of-function properties

We have developed a bZIP protein, GBF-F, with both dominant-negative (DN) and gain-of-function properties. GBF-F is a chimera consisting of two components: the DNA binding (basic) region from the plant bZIP protein GBF-1 (GBF) and a leucine zipper (F) designed to preferentially heterodimerize with the C/EBP alpha leucine zipper. Biochemical studies show that GBF-F preferentially forms heterodimers with C/EBP alpha and thus binds a chimeric DNA sequence composed of the half-sites recognized by the C/EBP and GBF basic regions. Transient transfections in HepG2 hepatoma cells show that both components of GBF-F are necessary for inhibition of C/EBP alpha transactivation. When the C/EBP alpha leucine zipper is replaced with that of either GCN4 or VBP, the resulting protein can transactivate a C/EBP cis-element but is not inhibited by GBF-F, indicating that the specificity of dominant-negative action is determined by the leucine zipper. All known members of the C/EBP family contain similar leucine zipper regions and are inhibited by GBF-F. GBF-F also exhibits gain-of-function properties, since, with the essential cooperation of a C/EBP family member, it can transactivate a promoter containing the chimeric C/EBP/GBF site. This protein therefore has potential utility both as a dominant-negative inhibitor of C/EBP function and as an activator protein with novel DNA sequence specificity.

Role of the transcription factor C/EBP beta in expression of a rat pregnancy-specific glycoprotein gene

Pregnancy-specific glycoproteins (PSGs), which are the major placental proteins, and the carcinoembryonic antigens comprise a subfamily within the immunoglobulin superfamily. To understand the molecular mechanisms underlying the control of PSG expression, we characterized the promoter elements of a rodent PSG gene, rnCGM3, and showed that DNA elements at nucleotides -326 to -185 (PI) relative to the translation start site of rnCGM3 function as a promoter. The rnCGM3 PI promoter contains two placental factor binding sites, PISI and PISII. Both are transcription activation elements. In the present report, we screened a placental expression cDNA library with a rnCGM3-PISII probe (nucleotides -263 to -233) encompassing two overlapping palindromes (TGTTGCTCAACATGTTG) and demonstrated that the PISII-binding factor is C/EBP beta, a leucine zipper family of transcription factor. Gel mobility-shift and transient expression analyses showed that C/EBP beta and C/EBP isoforms, C/EBP alpha and C/EBP delta, bind to the PISII element and trans-activate rnCGM3 gene expression. Deletion of PISII from the rnCGM3 PI promoter greatly reduced the basal as well as the C/EBP-activated rnCGM3 expression. Gel supershift assays demonstrated that C/EBP beta is the placental isoform that binds to the PISII site rnCGM3. Moreover, C/EBP beta is expressed in high levels in the placenta, ovary, liver, lung, heart, and spleen, in contrast to C/EBP alpha, which is expressed primarily in the liver and only low levels in the placenta. Our results demonstrate that C/EBP beta is one of the transcription factors that positively regulate rnCGM3 expression during pregnancy.

Mouse chromosomal location of the CCAAT/enhancer binding proteins C/EBP beta (Cebpb), C/EBP delta (Cebpd), and CRP1 (Cebpe)

There are four known members of the C/EBP family of basic region-leucine zipper transcription factors: Cebpa, Cebpb, Cebpd, and Cebpe. Cebpa has previously been mapped to mouse chromosome 7. Here, we show that Cebpb maps to mouse chromosome 2, Cebpd to chromosome 16, and Cebpe to chromosome 14. The assignment of Cebpd to chromosome 16 identifies a new region of homology between mouse chromosome 16 and human chromosome 8.

CRP2 (C/EBP beta) contains a bipartite regulatory domain that controls transcriptional activation, DNA binding and cell specificity

Two members of the C/EBP family of basic region-leucine zipper proteins enriched in the liver, C/EBP (C/EBP alpha) and CRP2 (C/EBP beta), were previously shown to transactivate the albumin promoter in a cell type-dependent manner. These proteins function efficiently in HepG2 hepatoma cells, but inefficiently in HeLa (epithelial) and L (fibroblastic) cells. Here we have investigated the mechanism for cell-specific control of CRP2 activity. We show that CRP2 contains a negative regulatory region composed of two elements, RD1 and RD2. Deletions of RD2 relieve the inhibition of CRP2 activity in L cells, but do not affect CRP2 function in HepG2 cells. These deletions also increase the DNA binding activity of CRP2 approximately 3-fold, suggesting that RD2-mediated repression of DNA binding activity is responsible for CRP2 inhibition in L cells. The adjacent RD1 element functions independently of RD2 and modulates the CRP2 activation domain, which we show to be composed of three subdomains that are conserved within the C/EBP protein family. RD1 does not affect cell type specificity, but inhibits the transactivation potential of GAL4-CRP2 hybrid proteins by 50-fold. These findings suggest that CRP2 assumes a tightly folded conformation in which the DNA binding and activation domains are masked by interactions with the regulatory domain and that to function efficiently in HepG2 cells the protein must undergo an activation step. We propose that relief of inhibition conferred by the regulatory domains also accounts for CRP2 activation in response to extracellular signals.

Cytokine regulation of the liver transcription factor hepatocyte nuclear factor-3 beta is mediated by the C/EBP family and interferon regulatory factor 1

Three distinct hepatocyte nuclear factor-3 (HNF-3) proteins (alpha, beta, and gamma) regulate the transcription of numerous liver-enriched genes. The HNF-3 proteins bind DNA via a homologous winged helix motif common to a number of proteins known to be critical for determination events in embryogenesis. We have demonstrated previously that two binding sites in the -184 HNF-3 beta promoter are recognized by widely distributed factors and that there is also a critical autoregulatory site, we identified a binding site for a cell-specific factor, LF-H3 beta, that may function in restricting HNF-3 beta gene expression to hepatocytes. Our present study demonstrates that members of the C/EBP and proline and acidic amino acid-rich subfamilies of basic region leucine zipper transcription factors bind the LF-H3 beta site, and cotransfection of HepG2 cells shows that these factors are able to activate an HNF-3 beta promoter reporter construct. The LF-H3 beta-C/EBP binding sequence also confers HNF-3 beta promoter stimulation in response to interleukin (IL)-1 and IL-6. Upstream of this HNF-3 beta proximal promoter region, an IFN-stimulated response element core sequence (-231 to -210) was found that mediates transcriptional induction by IFN-gamma but not IFN-alpha. Gel mobility supershift assay demonstrates that an IFN-gamma-induced protein-DNA complex is disrupted by an antibody specific for interferon regulatory factor-1/interferon-stimulated gene factor-2. Consistent with this finding, we observed that IFN-gamma induction requires ongoing protein synthesis. Surprisingly, the effect of the three cytokines (IL-1, IL-6, and IFN-gamma) in combination as assayed by the same model is not synergistic. HNF-3beta joins the C/EBP family on the list of liver-enriched transcription factors, the expression of which is modulated by cytokines.

DNA-binding specificity of the PAR basic leucine zipper protein VBP partially overlaps those of the C/EBP and CREB/ATF families and is influenced by domains that flank the core basic region

The PAR subfamily of basic leucine zipper (bZIP) factors comprises three proteins (VBP/TEF, DBP, and HLF) that have conserved basic regions flanked by proline- and acidic-amino-acid-rich (PAR) domains and functionally compatible leucine zipper dimerization domains. We show that VBP preferentially binds to sequences that consist of abutted GTAAY half-sites (which we refer to as PAR sites) as well as to sequences that contain either a C/EBP half-site (GCAAT) or a CREB/ATF half-site (GTCAT) in place of one of the PAR half-sites. Since the sequences that we describe as PAR sites and PAR-CREB/ATF chimeric sites, respectively, were both previously described as high-affinity binding sites for the E4BP4 transcriptional repressor, we infer that these sequences may be targets for positive and negative regulation. Similarly, since the sequences that we describe as PAR-C/EBP and PAR-CREB/ATF chimeric sites are known to be high-affinity binding sites for C/EBP and CREB/ATF factors, respectively, we infer that these sites may each be targets for multiple subfamilies of bZIP factors. To gain insights regarding the molecular basis for the binding-site specificity of PAR factors, we also carried out an extensive mutational analysis of VBP. By substituting five amino acid residues that differ between the Drosophila giant bZIP factor and the vertebrate PAR bZIP factors, we show that the fork region, which bridges the basic and leucine zipper domains, contributes to half-site sequence specificity. In addition, we report that at least two domains amino terminal to the core basic region are required for VBP to bind to the full spectrum of PAR target sites. Thus, whereas direct base contacts may be restricted to basic-region residues (as indicated by GCN4-DNA crystal structures), several other domains also influence the DNA-binding specificity of PAR bZIP proteins.

C/EBP-related protein 2 confers lipopolysaccharide-inducible expression of interleukin 6 and monocyte chemoattractant protein 1 to a lymphoblastic cell line

C/EBP-related proteins 2 and 3 (CRP2 and CRP3) are differentially expressed by P388 lymphoblasts and their derivative P388D1(IL1) macrophages. We have ectopically expressed CRP2, the predominant CRP in macrophages, in P388 lymphoblasts. The expression of CRP2 is sufficient to confer the lipopolysaccharide (LPS)-inducible expression of interleukin 6 and monocyte chemoattractant protein 1 to lymphoblasts, which normally do not display LPS induction of inflammatory cytokines. Consistent with these findings, the expression of CRP2 antisense RNA blocks the LPS induction of IL-6 expression in P388D1(IL1) macrophages. This work clearly establishes the essential role of CRP2 in the induction of cytokine genes by LPS. Additionally, these data add MCP-1 to the list of cytokines showing an involvement of CRP2 in their expression.

A revised sequence of the rat c/ebp gene

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A novel cis-acting element controlling the rat CYP2D5 gene and requiring cooperativity between C/EBP beta and an Sp1 factor

The rat CYP2D5 gene encodes a cytochrome P450 and is expressed in liver cells. Its expression commences a few days after birth, and maximal mRNA levels are achieved when animals reach puberty. Transfection and DNA binding studies were performed to investigate the mechanism controlling developmentally programmed, liver-specific expression of CYP2D5. Transfection studies using a series of CYP2D5 upstream DNA chloramphenicol acetyltransferase gene fusion constructs identified a segment of DNA between nucleotides -55 and -156 that conferred transcriptional activity in HepG2 cells. Activity was markedly increased by cotransfection with a vector expressing C/EBP beta but was unaffected by vectors producing other liver-enriched transcription factors (C/EBP alpha, HNF-1 alpha, and DBP). DNase I footprinting revealed a region protected by both HepG2 and liver cell nuclear extracts between nucleotides -83 and -112. This region displayed some sequence similarity to the Sp1 consensus sequence and was able to bind the Sp1 protein, as assessed by a gel mobility shift assay. The role of Sp1 in CYP2D5 transcription was confirmed by trans activation of the 2D5-CAT construct in Drosophila melanogaster cells by using an Sp1 expression vector. C/EBP beta alone was unable to directly bind the -83 to -112 region of the promoter but was able to produce a ternary complex when combined with HepG2 nuclear extracts or recombinant human Sp1. C/EBP alpha was unable to substitute for C/EBP beta in forming this ternary complex. A poor C/EBP binding site is present adjacent to the Sp1 site, and mutagenesis of this site abolished formation of the ternary complex with the CYP2D5 regulatory region. These result establish that two transcription factors can work in conjunction, possibly by protein-protein interaction, to activate the CYP2D5 gene.

Identification of C/EBP basic region residues involved in DNA sequence recognition and half-site spacing preference

C/EBP and GCN4 are basic region-leucine zipper (bZIP) DNA-binding proteins that recognize the dyad-symmetric sequences ATTGCGCAAT and ATGAGTCAT, respectively. The sequence specificities of these and other bZIP proteins are determined by their alpha-helical basic regions, which are related at the primary sequence level. To identify amino acids that are responsible for the different DNA sequence specificities of C/EBP and GCN4, two kinds of hybrid proteins were constructed: GCN4-C/EBP chimeras fused at various positions in the basic region and substitution mutants in which GCN4 basic region amino acids were replaced by the corresponding residues from C/EBP. On the basis of the DNA-binding characteristics of these hybrid proteins, three residues that contribute significantly to the differences in C/EBP and GCN4 binding specificity were defined. These residues are clustered along one face of the basic region alpha helix. Two of these specificity residues were not identified as DNA-contacting amino acids in a recently reported crystal structure of a GCN4-DNA complex, suggesting that the residues used by C/EBP and GCN4 to make base contacts are not identical. A random binding site selection procedure also was used to define the optimal recognition sequences for three of the GCN4-C/EBP fusion proteins. These experiments identify an element spanning the hinge region between the basic region and leucine zipper domains that dictates optimal half-site spacing (either directly abutted for C/EBP or overlapping by one base pair for GCN4) in high-affinity binding sites for these two proteins.

Racial norms: esthetic and prosthodontic implications

A review of the orthodontic literature reveals that different races have different cephalometric norms. This has great implications in the placement and arrangement of teeth and in the evaluation of esthetics in people of dissimilar ethnic origins. In the diverse American society, prosthodontists and restorative dentists need to be concerned with the phylogenetic characteristics of their patients. The resultant esthetic differences can have significant importance in the function and design of prostheses.

A family of C/EBP-related proteins capable of forming covalently linked leucine zipper dimers in vitro

Mouse and rat genomic DNA libraries were screened by reduced stringency hybridization with the DNA-binding domain of the c/ebp gene as a probe. Three genes were isolated that encode bZIP DNA-binding proteins (designated CRP1, CRP2, and CRP3) with strong amino acid sequence similarities to the C/EBP-binding domain. CRP2 is identical to the protein described recently by other groups as NF-IL6, LAP, IL-6DBP, and AGP/EBP, whereas CRP1 and CRP3 represent novel proteins. Several lines of evidence indicate that these three proteins, along with C/EBP, comprise a functional family. Each bacterially expressed polypeptide binds to DNA as a dimer with recognition properties that are virtually identical to those of C/EBP. Every member also bears a conserved cysteine residue at or near the carboxyl terminus, immediately following the leucine zipper, that at least in vitro allows efficient disulfide cross-linking between paired zipper helices. We developed a gel assay for covalent dimers to assess leucine zipper specificities among the family members. The results demonstrate that all pairwise combinations of dimer interactions are possible. To the extent that we have examined them, the same heterodimeric complexes can be detected intracellularly following cotransfection of the appropriate pair of genes into recipient cells. All members are also capable of activating in vivo transcription from promoters that contain a C/EBP-binding site. Our findings indicate that a set of potentially interacting C/EBP-like proteins exists, whose complexity is comparable to that of other bZIP protein subfamilies such as Jun, Fos, and ATF/CREB.

Cognate DNA binding specificity retained after leucine zipper exchange between GCN4 and C/EBP

Both C/EBP and GCN4 are sequence-specific DNA binding proteins that control gene expression. Recent evidence implicates C/EBP as a transcriptional regulator of genes involved in lipid and carbohydrate metabolism. The C/EBP protein binds avidly to the dyad symmetric sequence 5'-ATTGCGCAAT-3'; GCN4 regulates the transcription of genes that control amino acid biosynthesis in yeast, and binds avidly to the dyad symmetric sequence 5'-ATGA(G/C)TCAT-3'. Both C/EBP and GCN4 bind DNA via the same structural motif. This motif has been predicted to be bipartite, consisting of a dimerization interface termed the "leucine zipper" and a DNA contact surface termed the "basic region." Specificity of DNA binding has been predicted to be imparted by the basic region. As a test of this hypothesis, recombinant proteins were created wherein the basic regions and leucine zippers of GCN4 and C/EBP were reciprocally exchanged. In both of the recombinant polypeptides, DNA binding specificity is shown to track with the basic region.

The DNA binding domain of the rat liver nuclear protein C/EBP is bipartite

C/EBP is a rat liver nuclear protein capable of sequence-specific interaction with DNA. The DNA sequences to which C/EBP binds in vitro have been implicated in the control of messenger RNA synthesis. It has therefore been predicted that C/EBP will play a role in regulating gene expression in mammalian cells. The region of the C/EBP polypeptide required for direct interaction with DNA has been identified and shown to bear amino acid sequence relatedness with the product of the myc, fos, and jun proto-oncogenes. The arrangement of these related amino acid sequences led to the prediction of a new structural motif, termed the "leucine zipper," that plays a role in facilitating sequence-specific interaction between protein and DNA. Experimental tests now provide support for the leucine zipper hypothesis.

In situ detection of sequence-specific DNA binding activity specified by a recombinant bacteriophage

We have used a recombinant bacteriophage that expresses the DNA-binding domain of C/EBP to optimize conditions for a screening technique that may facilitate the cloning of genes that encode sequence-specific DNA-binding proteins. The method relies on the expression of cDNA inserts in bacteriophage lambda gt11. Fusion protein adsorbed onto nitrocellulose filters is probed with radioactive, double-stranded DNA as a ligand. Two procedures greatly increase the level of binding between ligand and recombinant fusion protein. First, nitrocellulose filters are processed through a denaturation/renaturation regimen using 6 M guanidine hydrochloride. Second, synthetic DNA corresponding to the specific binding site is catenated extensively using DNA ligase. The combination of these procedures leads to remarkably strong detection signals. Specific DNA-binding signals can be detected on duplicate filters, and filters can be washed and reused by repeating the cycle of denaturation/renaturation.

Isolation of a recombinant copy of the gene encoding C/EBP

In two previous studies we described the properties of a heat-stable DNA-binding protein present in rat liver nuclei. This protein, hereafter termed C/EBP, is capable of selective binding to the CCAAT homology of several viral promoters (Graves et al. 1986), as well as the core homology common to many viral enhancers (Johnson et al. 1987). We now report the isolation of a recombinant clone of the gene that encodes C/EBP. Expression of the clone in bacterial cells yields a protein that binds in vitro to both the CCAAT homology and the enhancer core homology, providing conclusive evidence that a single gene product accounts for both binding activities. By examining the properties of protease-derived fragments of C/EBP, we have localized its DNA-binding domain to a 14-kD fragment. A 60-amino-acid segment located within the DNA-binding domain of C/EBP bears sequence similarity to the products of the myc and fos oncogenes.