Phosphorylation at threonine-235 by a ras-dependent mitogen-activated protein kinase cascade is essential for transcription factor NF-IL6

NF-M (chicken C/EBP beta) induces eosinophilic differentiation and apoptosis in a hematopoietic progenitor cell line

CAAT/enhancer binding proteins (C/EBPs) are transcriptional activators implicated in the differentiation processes of various cell lineages. We have shown earlier that NF-M, the chicken homolog of C/EBP beta, is specifically expressed in myelomonocytic and eosinophilic cells of the hematopoietic system. To investigate the role of NF-M in hematopoietic cell lineage commitment, we constructed a conditional form of the protein by fusing it to the hormone binding domain of the human estrogen receptor. This construct was stably expressed in a multipotent progenitor cell line transformed by the Myb-Ets oncoprotein. We report here that both NF-M-dependent promoter constructs and resident genes could be activated by addition of beta-estradiol to the NF-M-estrogen receptor expressing progenitors. At the same time, we observed a down-regulation of progenitor-specific surface markers and the up-regulation of differentiation markers restricted to the eosinophil and myeloid lineages. In addition to the onset of differentiation, cell death was induced with typical apoptotic features. Our results suggest that NF-M plays an important role in commitment along the eosinophil lineage and in the induction of apoptosis.

Cytokine receptor signalling

Many cell functions are regulated by members of the cytokine receptor superfamily. Signalling by these receptors depends upon their association with Janus kinases (JAKs), which couple ligand binding to tyrosine phosphorylation of signalling proteins recruited to the receptor complex. Among these are the signal transducers and activators of transcription (STATs), a family of transcription factors that contribute to the diversity of cytokine responses.

Yeast RLM1 encodes a serum response factor-like protein that may function downstream of the Mpk1 (Slt2) mitogen-activated protein kinase pathway

The MPK1 (SLT2) gene of Saccharomyces cerevisiae encodes a mitogen-activated protein kinase that is regulated by a kinase cascade whose known elements are Pkc1 (a homolog of protein kinase C), Bck1 (Slk1) (a homolog of MEK kinase), and the functionally redundant Mpk1 activators Mkk1 and Mkk2 (homologs of MEK). An activated mutation of MKK1, MKK1P386, inhibits growth when overexpressed. This growth-inhibitory effect was suppressed by the mpk1 delta mutation, suggesting that hyperactivation of the Mpk1 pathway is toxic to cells. To search for genes that interact with the Mpk1 pathway, we isolated both chromosomal mutations and dosage suppressor genes that ameliorate the growth-inhibitory effect of overexpressed Mkk1P386. One of the genes identified by the analysis of chromosomal mutations is RLM1 (resistance to lethality of MKK1P386 overexpression), which encodes a protein homologous to a conserved domain of the MADS (Mcm1, Agamous, Deficiens, and serum response factor) box family of transcription factors. Although rlm1 delta cells grow normally at any temperature, they display a caffeine-sensitive phenotype similar to that observed in mutants defective in BCK1, MKK1/MKK2, or MPK1. A gene fusion that provides Rlm1 with a transcriptional activation domain of Gal4 suppresses bck1 delta and mpk1 delta. A screening for dosage suppressors yielded the MSG5 genes, which encode a dual-specificity protein phosphatase. Our results suggest that Rlm1 functions as a transcription factor downstream of Mpk1 that is subject to activation by the Mpk1 mitogen-activated protein kinase pathway.

cAMP activation of CAAT enhancer-binding protein-beta gene expression and promoter I of acetyl-CoA carboxylase

The acetyl-CoA carboxylase (ACC) gene contains two distinct promoters, denoted PI and PII. PI is responsible for the generation of class I ACC mRNAs which are induced in a tissue-specific manner under lipogenic conditions. PII generates class II ACC mRNAs which are expressed constitutively. During 30A5 preadipocyte differentiation, both promoters are activated; the preadipocytes must be pretreated with cAMP for this activation to occur. In this report, we present evidence that CAAT enhancer-binding protein-beta (C/EBP-beta) is induced and involved in the PI activation by cAMP. Expression of the reporter gene under the control of the PI promoter is activated within 3 h after treatment of 30A5 cells with a cyclic AMP analogue, 8-(4-chlorophenylthio)-adenosine 3',5'-cyclic monophosphate, and 3-isobutyl-1-methylxanthine, in association with the accumulation of C/EBP-beta mRNA and protein. These accumulations were inhibited in the presence of H8, a protein kinase inhibitor; H8 also inhibited activation of PI by cAMP. However, the induction of reporter gene expression and the increase of C/EBP-beta mRNA by cAMP were not affected by treatment with tumor necrosis factor alpha, which completely inhibited the accumulation of C/EBP-alpha mRNA. Overexpression of C/EBP-beta by transfection with the C/EBP-beta gene led to increased binding of C/EBP-beta to DNA and partial PI activation. cAMP did not affect the amount of C/EBP-beta binding to the DNA but did promote phosphorylation of C/EBP-beta and PI activation. As in the case of C/EBP-alpha, C/EBP-beta bound to the CCAAT box of the PI promoter. These results indicate that cAMP not only induces, but also activates, bound C/EBP-beta through phosphorylation for PI activation. Our studies also indicate that cAMP induces C/EBP-alpha. C/EBP-beta induction, however, precedes that of C/EBP-alpha.

Transcription stimulation of the Fas-encoding gene by nuclear factor for interleukin-6 expression upon influenza virus infection

We previously found that the level of Fas, a cell surface receptor for an apoptosis signal, increases at the mRNA level in influenza virus-infected HeLa cells prior to their death by apoptosis. Here we investigated the mechanism of activation of the Fas-encoding gene expression upon influenza virus infection. Nucleotide sequences for the binding of nuclear factor for interleukin-6 expression (NF-IL6), also known as CCAAT/enhancer-binding protein beta, were repeated 8 times in the 5'-end region of the human FAS gene, spanning from -1360 to +320. This region directed the expression of a downstream marker gene when introduced into HeLa cells and the activity of the FAS gene promoter was stimulated about 2-fold upon influenza virus infection. Gene expression driven by the FAS promoter was activated when human NF-IL6 was overproduced in a DNA when human NF-IL6 was overproduced in a DNA co-transfection study. Moreover, the DNA-binding activity of NF-IL6 increased after infection with the virus, whereas the amount of NF-IL6 seemed unchanged. The results suggest that NF-IL6 is activated upon influenza virus infection through post-translational modification and that the modified factor stimulates the transcription of the human FAS gene.

CCAAT/enhancer-binding protein isoforms beta and delta are expressed in mammary epithelial cells and bind to multiple sites in the beta-casein gene promoter

Lactogenic hormone-dependent expression of the rat beta-casein gene in mammary epithelial cells is controlled via a complex regulatory region in the promoter. The sequence between -176 and -82 is the minimal region to confer the response to glucocorticoid hormone and prolactin on a heterologous promoter. The response is further enhanced by the region between -282 and -176. DNase I footprinting experiments and electromobility shift assays revealed the presence of four binding sites for CCAAT/enhancer-binding protein (C/EBP) isoforms in the hormone response region between -220 and -132. In nuclear extracts from mammary epithelial cells, the prevalent C/EBP isoform binding to these sites is beta (C/EBP-beta). C/EBP-delta is also present in mammary epithelial cells, whereas C/EBP-alpha is not detectable. The C/EBP sites are located in close proximity to the previously characterized binding sites for the prolactin-inducible mammary gland factor/signal transducer and activator of transcription-5, the nuclear factor YY1, and the glucocorticoid receptor. The importance of the two proximal C/EBP binding sites at the 5' border of the minimal region was tested by mutational analysis. Mutations of each site were found to inhibit strongly both the basal and the lactogenic hormone-induced transcription of a beta-casein gene promoter chloramphenicol acetyltransferase construct. The results implicate C/EBPs as important regulators of beta-casein gene expression in the mammary epithelium.

Identification of a transcriptional regulatory factor for human aromatase cytochrome P450 gene expression as nuclear factor interleukin-6 (NF-IL6), a member of the CCAAT/enhancer-binding protein family

Human aromatase cytochrome P450 catalyzes the ultimate reaction in the estrogen biosynthetic pathway by coupling with another enzyme, NADPH-cytochrome P450 reductase, in the endoplasmic reticulum. The expression of the gene encoding the enzyme (CYP19) is regulated, in part, by tissue-specific promoters through the use of alternative-splicing mechanisms. Recently, we have localized a transcriptional activating element at positions -2141 to -2115 relative to the major cap site of the gene, by transient expression analyses in human BeWo choriocarcinoma cells using the bacterial chloramphenicol acetytransferase reporter gene ligated with CYP19 promoter sequences which regulate expression in this tissue. Here, we report the isolation of a cDNA encoding a DNA-binding protein which binds specifically to the regulatory element. The deduced amino-acid sequence of the insert is identical to that corresponding to the DNA-binding domain and the dimerization domain of a transcription factor, nuclear factor interleukin-6 (NF-IL6), a member of the CCAAT/enhancer-binding protein (C/EBP) family. Studies using specific antibodies against members of the C/EBP family demonstrate that NF-IL6 is the major nuclear factor binding to the regulatory element in BeWo cells; nevertheless. C/EBP alpha also seems to be involved. Disruption of the NF-IL6-binding site within the regulatory element resulted in the disappearance of the transcriptional enhancing activity of the element, indicating that NF-IL6 is at least one of the nuclear factor(s) which enhances transcription through binding to the cis-acting element. These results indicate the intrinsic importance of NF-IL6 in the transcriptional regulation of CYP19 expression.

The C/EBP family of transcription factors

The C/EBP proteins form a family of transcription factors with at least seven members. These proteins consist of three structural components which include a C-terminal leucine-zipper, a basic DNA-binding region and a N-terminal transactivating region. Dimerization through the leucine-zipper leads to formation of homo- and heterodimers which then bind with their two basic regions to often non-symmetric DNA-sequences in the promoter/enhancer regions of a variety of genes. Expression of C/EBP is prominent in adipocytes, hepatocytes and monocytes/macrophages, and here these proteins are involved in tissue-specific gene expression. Target genes for C/EBP include those for acute phase response genes in liver cells and for cytokine genes in monocytes/macrophages. Therefore, intervention at the level of C/EBP transcription factors may prove effective in controlling immune response.

Activation of Raf-1 and mitogen-activated protein kinase in murine macrophages partially mimics lipopolysaccharide-induced signaling events

Lipopolysaccharide (LPS), a highly conserved component of the outer membrane of gram-negative bacteria, stimulates macrophages to release various cytokine and eicosanoid mediators of the immune response. The mechanism by which LPS stimulates these cells is poorly characterized. One of the most rapid LPS-stimulated events is the phosphorylation and activation of the p42 and p44 isoforms of mitogen-activated protein (MAP) kinase. We wished to examine the role of MAP kinase in LPS-induced signaling in murine macrophages by activating MAP kinase independently of LPS. An expression vector encoding a Raf-1:estrogen receptor (ER) chimeric protein was transfected into the murine macrophage cell line RAW 264.7. Activation of this chimeric protein (delta Raf-1:ER) by estradiol resulted in rapid and prolonged activation of MAP kinase, as expected from previous results implicating Raf-1 as an upstream activator of this signaling cascade. LPS stimulation induced accumulation of MAP kinase phosphatase 1 messenger RNA, whereas delta Raf-1:ER activation did not, perhaps accounting for the more prolonged activation of MAP kinase seen in response to delta Raf-1:ER activation. Similarly, activation of DNA binding by the transcription factor, nuclear factor (NF) kappa B, as assessed by electrophoretic mobility shift assay, occurred in response to LPS stimulation but not in response to delta Raf-1:ER activation or phorbol myristate acetate (PMA) stimulation. Using an enzyme-linked immunosorbent assay for murine tumor necrosis factor alpha (TNF-alpha), we found that LPS and PMA stimulation and delta Raf-1:ER activation induced secretion of TNF-alpha, although the amount of TNF-alpha secreted in response to delta Raf-1:ER activation and PMA stimulation was approximately 20-fold less than that secreted in response to LPS. Correspondingly, accumulation of TNF-alpha messenger RNA was weakly induced by delta Raf-1:ER activation or PMA stimulation, whereas strong induction was noted in response to LPS. These results suggest that Raf-1 or PMA activation of MAP kinase in murine macrophages is sufficient for a small amount of TNF-alpha production and secretion in the absence of NF-kappa B activation, but LPS stimulation involves additional signaling events, such as NF-kappa B activation, that augment the response seen with activation of MAP kinase alone.

A novel cytokine-inducible gene CIS encodes an SH2-containing protein that binds to tyrosine-phosphorylated interleukin 3 and erythropoietin receptors

Cytokines manifest their function through alteration of gene expression. However, target genes for signals from cytokine receptors are largely unknown. We therefore searched for immediate-early cytokine-responsive genes and isolated a novel gene, CIS (cytokine inducible SH2-containing protein) which is induced in hematopoietic cells by a subset of cytokines including interleukin 2 (IL2), IL3, granulocyte-macrophage colony-stimulating factor (GM-CSF) and erythropoietin (EPO), but not by stem cell factor, granulocyte colony-stimulating factor and IL6. The CIS message encodes a polypeptide of 257 amino acids that contains an SH2 domain of 96 amino acids in the middle. To clarify the function of CIS in cytokine signal transduction, we expressed CIS in IL3-dependent hematopoietic cell lines under the control of a steroid-inducible promoter. The CIS product stably associated with the tyrosine-phosphorylated beta chain of the IL3 receptor as well as the tyrosine-phosphorylated EPO receptor. Forced expression of CIS by steroid reduced the growth rate of these transformants, suggesting a negative role of CIS in signal transduction. CIS induction requires the membrane-proximal region of the cytoplasmic domain of the EPO receptor as well as that of the common beta chain of the IL3, IL5 and GM-CSF receptor, whereas CIS binds to the receptor that is tyrosine phosphorylated by cytokine stimulation. Thus CIS appears to be a unique regulatory molecule for cytokine signal transduction.

Phosphorylation of tristetraprolin, a potential zinc finger transcription factor, by mitogen stimulation in intact cells and by mitogen-activated protein kinase in vitro

Tristetraprolin (TTP) is a potential transcription factor that contains three PPPPG repeats and two putative CCCH zinc fingers. TTP is encoded by the early response gene Zfp-36, which is highly expressed in response to growth factors and in several hematopoietic cell lines. In the present studies, we investigated the possibility that TTP is phosphorylated in intact cells. In NIH/3T3 cells that were made to overexpress TTP constitutively, we found that the protein was phosphorylated on serine residues, and that this phosphorylation was rapidly (within 10 min) stimulated by several mitogens. In cell-free assays, recombinant mouse TTP was a substrate for the mitogen-activated protein (MAP) kinase. By a combination of protease digestion experiments and site-directed mutagenesis strategies, we found that serine 220 was phosphorylated by p42 MAP kinase in vitro. Expression of mutant TTP in fibroblasts confirmed that serine 220 was one of the major, mitogen-stimulated phosphorylation sites on the protein in intact cells. These results suggest that TTP may be phosphorylated by MAP kinases in vivo and that this phosphorylation may regulate its function.

Autoregulated induction of the acute-phase response transcription factor gene, agp/ebp

AGP/EBP (C/EBP beta) is a key transcription factor responsible for transcriptional induction of many acute-phase protein genes. To characterize the regulation of this gene, we have isolated the mouse genomic DNA and sequenced the 5'-regulatory region. Using nuclear extracts from lipopolysaccharide (LPS)-stimulated or unstimulated mouse liver, three protein factor-binding motifs, UF1(-376 to -352), UF2(-254 to -223), and UF3(-220 to -190), as well as two Sp1 motifs (-309 to -277, and -264 to -241) were identified by DNase I footprinting assays. Biochemical analysis has shown that the AGP/EBP protein can bind to UF1 and UF2 sites, whereas an ubiquitous factor of unknown identity can bind to the UF3 site. Functional characterizations indicate that all of these factors play a major role in AGP/EBP induction. Thus, the agp/ebp gene is autoregulated during the acute-phase response.

Concerted participation of NF-kappa B and C/EBP heteromer in lipopolysaccharide induction of serum amyloid A gene expression in liver

The promoter region of the rabbit serum amyloid A (SAA) gene contains two adjacent C/EBP and one NF-kappa B binding element. Involvement of these elements in SAA gene induction, following lipopolysaccharide (LPS) stimulation of the liver, has been studied by investigating LPS-activated transcription factors and their interaction with the promoter elements of the SAA gene. Appearance of complexes in the electrophoretic mobility shift assay has indicated that DNA-binding proteins that interact with the NF-kappa B element of the SAA promoter are induced in the LPS-treated rabbit liver. Presence of RelA (p65 subunit of NF-kappa B) in these complexes was demonstrated by the ability of RelA-specific antisera to supershift the DNA-protein complexes. LPS also induced several members of the C/EBP family of transcription factors, which interacted with the C/EBP motifs of the SAA promoter. Activated C/EBP and RelA form a RelA-C/EBP heteromeric complex that associates with varying affinity to NF-kappa B and C/EBP elements of the SAA gene. Transfection assays using both transcription factor genes have demonstrated that the heteromeric complex of NF-kappa B and C/EBP is a much more potent transactivator of SAA expression than each transcription factor alone. The heteromeric complex efficiently promotes transcription from both NF-kappa B and C/EBP sites.

The MEK kinase activity of the catalytic domain of RAF-1 is regulated independently of Ras binding in T cells

Deletion of the amino-terminal domain of Raf-1, which contains the Ras-binding region, results in the constitutive activation of the liberated Raf-1 catalytic domain in fibroblast cell lines. We demonstrate that the MEK kinase activity of the isolated Raf-1 catalytic domain, Raf-BXB, is not constitutively active, but is regulated in Jurkat T cells. Raf-BXB is activated by engaging the antigen receptor-CD3 complex, or treating cells with phorbol myristate acetate or okadaic acid. Increasing intracellular cAMP inhibits Raf-1 activation stimulated by phorbol myristate acetate, but not the activation of Raf-BXB. Serine 621, but not serine 499, is essential for Raf-BXB MEK kinase activity. Because Raf-BXB does not bind Ras, the data establishes a Ras-independent signal in directly regulating the activity of the Raf-1 catalytic domain.

Hepatic nuclear factor 3- and hormone-regulated expression of the phosphoenolpyruvate carboxykinase and insulin-like growth factor-binding protein 1 genes

The rate of transcription of the hepatic phosphoenolpyruvate carboxykinase (PEPCK) and insulin-like growth factor-binding protein 1 (IGFBP-1) genes is stimulated by glucocorticoids and inhibited by insulin. In both cases, the effect of insulin is dominant, since it suppresses both basal and glucocorticoid-stimulated PEPCK or IGFBP-1 gene transcription. Analyses of both promoters by transfection of PEPCK or IGFBP-1-chloramphenicol acetyltransferase fusion genes into rat hepatoma cells has led to the identification of insulin response sequences (IRSs) in both genes. The core IRS, T(G/A)TTTTG, is the same in both genes, but the PEPCK promoter has a single copy of this element whereas the IGFBP-1 promoter has two copies arranged as an inverted palindrome. The IGFBP-1 IRS and PEPCK IRS both bind the alpha and beta forms of hepatic nuclear factor 3 (HNF-3), although the latter does so with a sixfold-lower relative affinity. Both the PEPCK and the IGFBP-1 IRSs also function as accessory factor binding sites required for the full induction of gene transcription by glucocorticoids. A combination of transient transfection and DNA binding studies suggests that HNF-3 is the accessory factor that supports glucocorticoid-induced gene transcription. In both genes, the HNF-3 binding site overlaps the IRS core motif(s). A model in which insulin is postulated to mediate its negative effect on glucocorticoid-induced PEPCK and IGFBP-1 gene transcription indirectly by inhibiting HNF-3 action is proposed.

Does mitogen-activated-protein kinase have a role in insulin action? The cases for and against

The discovery of the mitogen-activated protein (MAP) kinase family of protein kinases has sparked off an intensive effort to elucidate their role in the regulation of many cellular processes. These protein kinases were originally identified based on their rapid activation by insulin. In this review we concentrate on examining the evidence for and against a role for the MAP kinases Erk-1 and Erk-2 in mediating the effects of insulin. While there is good evidence in favour of a direct role for MAP kinase in the growth-promoting effects of insulin and the regulation of Glut-1 and c-fos expression, and AP-1 transcriptional complex activity, this is by no means conclusive. MAP kinase may also play a role in the control of mRNA translation by insulin. On the other hand, the evidence suggests that MAP kinase is not sufficient for the acute regulation of glucose transport (Glut-4 translocation), glycogen synthesis, acetyl-CoA carboxylase or pyruvate dehydrogenase activity. The findings suggest that insulin may utilise at least three distinct signalling pathways which do not involve MAP kinase.

Two distinct signalling pathways are involved in the control of the biphasic junB transcription induced by interleukin-6 in the B cell hybridoma 7TD1

We have measured the level of junB mRNA in the B hybridoma cell line 7TD1, under interleukin-6 (IL-6) stimulation. IL-6 increases junB mRNA in a biphasic fashion. The first early-induced peak was transient and likely corresponds to the well documented typical junB mRNA, stimulated in response to numerous growth factors, including IL-6. At variance, the second peak which has never been reported previously, lasted several hours. As a consequence of its effect on junB mRNA, IL-6 stimulated, in a biphasic fashion, the nuclear accumulation of the JunB protein. In this study, we demonstrated that IL-6 regulation occurred exclusively at the transcriptional level and that the bimodal increase of junB mRNA and JunB protein can be accounted for by a biphasic stimulation of junB transcription. Furthermore, our data point to two major differences between the mechanism of control of the early and the late IL-6-induced junB transcription waves. First, cycloheximide strongly potentiated the transcription of the second wave, whereas it failed to affect the early-induced burst. Second, tyrphostin, a tyrosine kinase inhibitor, impaired the expression of the first but not the second junB mRNA peak. Conversely, genistein, another tyrosine kinase inhibitor, totally abolished the expression of the second peak of junB mRNA whereas it did not affect the expression of the first peak. Altogether these data indicate that, in 7TD1 cells, IL-6 controls junB transcription in a biphasic fashion by means of two separate transduction pathways.

A novel cis-acting element required for lipopolysaccharide-induced transcription of the murine interleukin-1 beta gene

Regulatory elements important for transcription of the murine interleukin-1 beta (IL-1 beta) gene lie within a DNase I-hypersensitive region located > 2,000 bp upstream from the transcription start site. We have identified within this region a novel positive regulatory element that is required for activation of an IL-1 beta promoter-chloramphenicol acetyltransferase (CAT) fusion gene in the murine macrophage line RAW264.7. Electrophoretic mobility shift analysis of the 3' portion (-2315 to -2106) of the hypersensitive region revealed at least two nuclear factor binding sites, one of which is located between positions -2285 and -2256. Competitive inhibition studies localized the binding site to a 15-bp sequence between -2285 and -2271. Nuclear factor binding was lost by mutation of the 6-bp sequence from -2280 to -2275. The specific retarded complex formed with RAW264.7 nuclear extract was not detected under similar conditions with nuclear extracts from RLM-11, a murine T-cell line which does not express IL-1 beta RNA. Mutation of the 6-bp sequence (-2280 to -2275) in the chimeric IL-1 beta promoter -4093 +I CAT plasmid virtually eliminated the activation of this reporter gene by lipopolysaccharide (LPS) in transfected RAW264.7 cells. Multimerization of the 15-bp sequence containing the core wild-type 6-bp sequence 5' of minimal homologous or heterologous promoters in CAT reporter plasmids resulted in significant enhancement of CAT expression compared with parallel constructs containing the mutant 6-bp core sequence. This element was LPS independent and position and orientation dependent. The multimerized 15-bp sequence did not enhance expression in RLM-11 cells. Methylation interference revealed contact residues from -2281 to -2271, CCAAAAAGGAA. Because a search of the NIH TFD data bank with the 11-bp binding site sequence found no homology to known nuclear factor binding sites, we have designated this sequence the IL1 beta -upstream nuclear factor 1 (IL1 beta -UNF1) target. UV cross-linking and sodium dodecyl sulfate-polyacrylamide electrophoresis identified an IL1 beta -UNF1-specific binding factor approximately 85 to 90 kDa in size.

Transcription factors NF-IL6 and CREB recognize a common essential site in the human prointerleukin 1 beta gene

A site located between -2782 and -2729 of the human prointerleukin-1 beta (IL1B) gene functions as a strong lipopolysaccharide (LPS)-responsive enhancer independent of the previously identified enhancer located between -2896 and -2846 (F. Shirakawa, K. Saito, C.A. Bonagura, D.L. Galson, M. J. Fenton, A. C. Webb, and P. E. Auron, Mol. Cell. Biol. 13:1332-1344, 1993). Although these two enhancers appear to function cooperatively in the native sequence context, they function independently as LPS-responsive elements upon removal of an interposed silencer sequence. The new enhancer is not induced by dibutyryl cyclic AMP (dbcAMP) alone but is superinduced by costimulation with LPS-dbcAMP. This pattern of induction depends upon the nature of the sequence, a composite NF-IL6-cAMP response element (CRE) binding site. This pseudosymmetrical sequence is shown to contrast with a classical symmetric CRE which responds to dbcAMP but not LPS. DNA binding studies using in vivo nuclear extract, recombinant proteins, and specific antibodies show that LPS induces the formation of two different complexes at the enhancer: (i) an NF-IL6-CREB heterodimer and (ii) a heterodimer consisting of NF-IL6 and a non-CREB, CRE-binding protein. Cotransfection studies using NF-IL6 and CREB expression vectors show that NF-IL6 transactivates the enhancer in the presence of LPS, whereas CREB acts either positively or negatively, depending upon its cAMP-regulated phosphorylation state. Our data demonstrate that the newly identified enhancer is a specialized LPS-responsive sequence which can be modulated by cAMP as a result of the involvement of NF-IL6-CRE-binding protein heterodimers.

Novel mechanism of C/EBP beta (NF-M) transcriptional control: activation through derepression

Phosphorylation of transcription factors is regarded as a major mechanism to control their activity in regulation of gene expression. C/EBP beta is a transcription factor that becomes activated after phosphorylation to induce genes involved in inflammation, acute-phase response, cytokine expression, cell growth, and differentiation. The chicken homolog NF-M collaborates with Myb and various kinase oncogenes in normal myeloid differentiation as well as in the leukemic transformation of myelomonocytic cells. Here, we examined the structure of NF-M and its mechanism of activation. We show that NF-M is a repressed transcription factor with concealed activation potential. Derepressed NF-M exhibits enhanced transcriptional efficacy in reporter assays. More importantly, NF-M activates resident chromatin-embedded, myelomonocyte-specific target genes, even in heterologous cell types such as fibroblasts or erythroblasts. We identified two regions within NF-M that act to repress trans-activation. Repression is abolished by deletion of these regions, activation of signal transduction kinases including v-erbB, polyoma middle T, ras and mil/raf, or point mutation of a critical phosphorylation site for MAP kinases. We provide evidence that phosphorylation plays a unique role to derepress rather than to enhance the trans-activation domain as a novel mechanism to regulate gene expression by NF-M/C/EBP beta.

Transcription factors NF-IL6 and CREB recognize a common essential site in the human prointerleukin 1 beta gene

A site located between -2782 and -2729 of the human prointerleukin-1 beta (IL1B) gene functions as a strong lipopolysaccharide (LPS)-responsive enhancer independent of the previously identified enhancer located between -2896 and -2846 (F. Shirakawa, K. Saito, C.A. Bonagura, D.L. Galson, M. J. Fenton, A. C. Webb, and P. E. Auron, Mol. Cell. Biol. 13:1332-1344, 1993). Although these two enhancers appear to function cooperatively in the native sequence context, they function independently as LPS-responsive elements upon removal of an interposed silencer sequence. The new enhancer is not induced by dibutyryl cyclic AMP (dbcAMP) alone but is superinduced by costimulation with LPS-dbcAMP. This pattern of induction depends upon the nature of the sequence, a composite NF-IL6-cAMP response element (CRE) binding site. This pseudosymmetrical sequence is shown to contrast with a classical symmetric CRE which responds to dbcAMP but not LPS. DNA binding studies using in vivo nuclear extract, recombinant proteins, and specific antibodies show that LPS induces the formation of two different complexes at the enhancer: (i) an NF-IL6-CREB heterodimer and (ii) a heterodimer consisting of NF-IL6 and a non-CREB, CRE-binding protein. Cotransfection studies using NF-IL6 and CREB expression vectors show that NF-IL6 transactivates the enhancer in the presence of LPS, whereas CREB acts either positively or negatively, depending upon its cAMP-regulated phosphorylation state. Our data demonstrate that the newly identified enhancer is a specialized LPS-responsive sequence which can be modulated by cAMP as a result of the involvement of NF-IL6-CRE-binding protein heterodimers.

NF-IL6 and AP-1 cooperatively modulate the activation of the TSG-6 gene by tumor necrosis factor alpha and interleukin-1

Tumor necrosis factor alpha (TNF-alpha) and interleukin-1 (IL-1) activate transcription of the TSG-6 gene in normal human fibroblasts through a promoter region (-165 to -58) that encompasses an AP-1 and a NF-IL6 site. We show by deletion analysis and substitution mutagenesis that both sites are necessary for activation by TNF-alpha. Activation by IL-1 requires the NF-IL6 site and is enhanced by the AP-1 site. These results suggest that the NF-IL6 and AP-1 family transcription factors functionally cooperate to mediate TNF-alpha and IL-1 signals. Consistent with this possibility, IL-1 and TNF-alpha markedly increase the binding of Fos and Jun to the AP-1 site, and NF-IL6 activates the native TSG-6 promoter. Activation by NF-IL6 requires an intact NF-IL6 site and is modulated by the ratio of activator to inhibitor NF-IL6 isoforms that are translated from different in-frame AUGs. However, the inhibitor isoform can also bind to the AP-1 site and repress AP-1 site-mediated transcription. The finding that the inhibitor isoform antagonizes activation of the native TSG-6 promoter by IL-1 and TNF-alpha suggests that NF-IL6 has a physiologic role in these cytokine responses. Thus, the functionally distinct NF-IL6 isoforms cooperate with Fos and Jun to positively and negatively regulate the native TSG-6 promoter by TNF-alpha and IL-1.

NF-IL6 and AP-1 cooperatively modulate the activation of the TSG-6 gene by tumor necrosis factor alpha and interleukin-1

Tumor necrosis factor alpha (TNF-alpha) and interleukin-1 (IL-1) activate transcription of the TSG-6 gene in normal human fibroblasts through a promoter region (-165 to -58) that encompasses an AP-1 and a NF-IL6 site. We show by deletion analysis and substitution mutagenesis that both sites are necessary for activation by TNF-alpha. Activation by IL-1 requires the NF-IL6 site and is enhanced by the AP-1 site. These results suggest that the NF-IL6 and AP-1 family transcription factors functionally cooperate to mediate TNF-alpha and IL-1 signals. Consistent with this possibility, IL-1 and TNF-alpha markedly increase the binding of Fos and Jun to the AP-1 site, and NF-IL6 activates the native TSG-6 promoter. Activation by NF-IL6 requires an intact NF-IL6 site and is modulated by the ratio of activator to inhibitor NF-IL6 isoforms that are translated from different in-frame AUGs. However, the inhibitor isoform can also bind to the AP-1 site and repress AP-1 site-mediated transcription. The finding that the inhibitor isoform antagonizes activation of the native TSG-6 promoter by IL-1 and TNF-alpha suggests that NF-IL6 has a physiologic role in these cytokine responses. Thus, the functionally distinct NF-IL6 isoforms cooperate with Fos and Jun to positively and negatively regulate the native TSG-6 promoter by TNF-alpha and IL-1.

Transformation of mammalian cells by constitutively active MAP kinase kinase

Mitogen-activated protein (MAP) kinase kinase (MAPKK) activates MAP kinase in a signal transduction pathway that mediates cellular responses to growth and differentiation factors. Oncogenes such as ras, src, raf, and mos have been proposed to transform cells by prolonging the activated state of MAPKK and of components downstream in the signaling pathway. To test this hypothesis, constitutively active MAPKK mutants were designed that had basal activities up to 400 times greater than that of the unphosphorylated wild-type kinase. Expression of these mutants in mammalian cells activated AP-1-regulated transcription. The cells formed transformed foci, grew efficiently in soft agar, and were highly tumorigenic in nude mice. These findings indicate that constitutive activation of MAPKK is sufficient to promote cell transformation.

Multiple forms of C/EBP beta bind the EFII enhancer sequence in the Rous sarcoma virus long terminal repeat

In this report we demonstrate that C/EBP beta is a major component of three EFII DNA binding complexes, EFIIa, EFIIb, and EFIIc, which we have previously shown to specifically recognize a C/EBP consensus binding site found in the EFII enhancer sequence from the Rous sarcoma virus long terminal repeat (R. C. Sears and L. Sealy, J. Virol. 66:6338-6352, 1992). Three different forms of C/EBP beta, p42, p35, and p20, can bind the EFII DNA sequence as homodimers, and dimerization experiments show that EFIIa is a homodimer of p20 C/EBP beta, EFIIb is primarily composed of a p20/p35 heterodimer with minor amounts of p20/p42 heterodimer and p35 homodimer, and EFIIc is composed of p20 and/or p35 heterodimerized with a novel 60-kDa protein. p20 C/EBP beta is likely equivalent to the internally initiated translation product of C/EBP beta, LIP (liver inhibitor protein), described by P. Descombes and U. Schibler (Cell 67:569-579, 1991). In contrast to the low level of LIP expressed in liver, postulated to occur because of leaky ribosome scanning, we found high levels of expression of p20 C/EBP beta in fibroblasts and lymphocytes. In murine fibroblasts, p20 C/EBP beta has an extended half-life, four times longer than those of p42 and p35 C/EBP beta, which could contribute to its abundant accumulation in this cell type, even though its synthesis by leaky ribosome scanning might be inefficient. Interestingly, overexpression of either the long or short form of C/EBP beta represses EFII-mediated transcription, suggesting that another unidentified EFII transactivator(s) exists, which may be dominantly inhibited by C/EBP beta proteins, and/or that transactivation by C/EBP beta proteins requires posttranslational modifications that were lacking in the transient overexpression experiments.

Multiple forms of C/EBP beta bind the EFII enhancer sequence in the Rous sarcoma virus long terminal repeat

In this report we demonstrate that C/EBP beta is a major component of three EFII DNA binding complexes, EFIIa, EFIIb, and EFIIc, which we have previously shown to specifically recognize a C/EBP consensus binding site found in the EFII enhancer sequence from the Rous sarcoma virus long terminal repeat (R. C. Sears and L. Sealy, J. Virol. 66:6338-6352, 1992). Three different forms of C/EBP beta, p42, p35, and p20, can bind the EFII DNA sequence as homodimers, and dimerization experiments show that EFIIa is a homodimer of p20 C/EBP beta, EFIIb is primarily composed of a p20/p35 heterodimer with minor amounts of p20/p42 heterodimer and p35 homodimer, and EFIIc is composed of p20 and/or p35 heterodimerized with a novel 60-kDa protein. p20 C/EBP beta is likely equivalent to the internally initiated translation product of C/EBP beta, LIP (liver inhibitor protein), described by P. Descombes and U. Schibler (Cell 67:569-579, 1991). In contrast to the low level of LIP expressed in liver, postulated to occur because of leaky ribosome scanning, we found high levels of expression of p20 C/EBP beta in fibroblasts and lymphocytes. In murine fibroblasts, p20 C/EBP beta has an extended half-life, four times longer than those of p42 and p35 C/EBP beta, which could contribute to its abundant accumulation in this cell type, even though its synthesis by leaky ribosome scanning might be inefficient. Interestingly, overexpression of either the long or short form of C/EBP beta represses EFII-mediated transcription, suggesting that another unidentified EFII transactivator(s) exists, which may be dominantly inhibited by C/EBP beta proteins, and/or that transactivation by C/EBP beta proteins requires posttranslational modifications that were lacking in the transient overexpression experiments.

Serum amyloid A gene expression under acute-phase conditions involves participation of inducible C/EBP-beta and C/EBP-delta and their activation by phosphorylation

Serum amyloid A (SAA) is a plasma protein whose synthesis is markedly increased in the liver during the inflammatory process. Previous analysis of SAA promoter function implicated the involvement of the CCAAT/enhancer-binding protein (C/EBP) in controlling this process. In this study, using antibodies against three C/EBP isoforms in DNA-binding and Western blot (immunoblot) assays, we found that in response to inflammatory signals, both C/EBP-delta and C/EBP-beta are induced and that their interactions with the SAA promoter element are necessary for the increased SAA gene expression. Cotransfections of liver cells with an SAA promoter-linked reporter chloramphenicol acetyltransferase gene and murine sarcoma virus-expressed C/EBP-delta or C/EBP-beta confirm such phenomena. The increased transactivating ability in the presence of the cellular phosphatase inhibitors okadaic acid and sodium orthovanadate, coupled with the observation that dephosphorylation severely inhibits the DNA-binding ability in vitro, implicates a role of phosphorylation in the regulation of the activities of the C/EBP-delta isoform. Consistent with these findings, we have detected higher levels of DNA-binding activity of C/EBP-delta prepared from cells treated with phosphatase inhibitors. We also present evidence that C/EBP-delta is a phosphoprotein. These results suggest that C/EBP-delta is regulated by phosphorylation and, in conjunction with C/EBP-beta, is one of the major proteins responsible for the increased transcription of the SAA gene in response to inflammatory stimuli.

Expression of the gene encoding alpha 1-acid glycoprotein in rabbit liver under acute-phase conditions involves induction and activation of beta and delta CCAAT-enhancer-binding proteins

Transcription of the gene encoding alpha 1-acid glycoprotein is highly induced during acute inflammation which has been previously shown to be mediated by some inducible members of the CCAAT-enhancer-binding (C/EBP) transcription-factor family. In this study, we demonstrate that the involved inducible C/EBP isoforms are C/EBP-beta and C/EBP-delta, and together they control the high-level induction of the alpha 1-acid glycoprotein gene in response to inflammatory signals. We observed that dephosphorylation severely inhibits the DNA-binding ability of C/EBP-delta and its transactivating potential increases in the presence of cellular phosphatase inhibitors, such as okadaic acid and sodium orthovanadate. These results suggest that C/EBP-delta is regulated by phosphorylation. Transient transfections using expression vectors of C/EBP-alpha, C/EBP-beta and C/EBP-delta have shown that while individually all three isoforms can transactivate the alpha 1-acid glycoprotein-chloramphenicol-acetyltransferase gene transcription, co-expression of C/EBP-alpha and C/EBP-beta isoforms results in lower levels of reporter gene expression than the levels predicted from their additive transactivation level. In vitro DNA-binding studies have shown that C/EBP-alpha and C/EBP-beta isoforms both interact and form complexes with the alpha 1-acid glycoprotein gene C/EBP-binding element under normal noninduced conditions during which alpha 1-acid glycoprotein is expressed at a very low level. Higher than additive levels of reporter gene expression are observed when combinations of C/EBP-delta and C/EBP-beta or C/EBP-delta and C/EBP-alpha are used. Together, these data demonstrate that C/EBP-beta and C/EBP-delta are the major proteins responsible for the acute-phase induction of alpha 1-acid-glycoprotein gene expression and they require phosphorylation for transactivation potential.

Serum amyloid A gene expression under acute-phase conditions involves participation of inducible C/EBP-beta and C/EBP-delta and their activation by phosphorylation

Serum amyloid A (SAA) is a plasma protein whose synthesis is markedly increased in the liver during the inflammatory process. Previous analysis of SAA promoter function implicated the involvement of the CCAAT/enhancer-binding protein (C/EBP) in controlling this process. In this study, using antibodies against three C/EBP isoforms in DNA-binding and Western blot (immunoblot) assays, we found that in response to inflammatory signals, both C/EBP-delta and C/EBP-beta are induced and that their interactions with the SAA promoter element are necessary for the increased SAA gene expression. Cotransfections of liver cells with an SAA promoter-linked reporter chloramphenicol acetyltransferase gene and murine sarcoma virus-expressed C/EBP-delta or C/EBP-beta confirm such phenomena. The increased transactivating ability in the presence of the cellular phosphatase inhibitors okadaic acid and sodium orthovanadate, coupled with the observation that dephosphorylation severely inhibits the DNA-binding ability in vitro, implicates a role of phosphorylation in the regulation of the activities of the C/EBP-delta isoform. Consistent with these findings, we have detected higher levels of DNA-binding activity of C/EBP-delta prepared from cells treated with phosphatase inhibitors. We also present evidence that C/EBP-delta is a phosphoprotein. These results suggest that C/EBP-delta is regulated by phosphorylation and, in conjunction with C/EBP-beta, is one of the major proteins responsible for the increased transcription of the SAA gene in response to inflammatory stimuli.

An NFIL-6 sequence near the transcriptional initiation site is necessary for the lipopolysaccharide induction of murine interleukin-1 beta

We have analyzed the interleukin-1 beta (IL-1 beta) promoter region near the cap site. Specific DNA sequences required for lipopolysaccharide (LPS) induction within this region were identified using transfection of reporter plasmids that contained portions of the proximal IL-1 beta 5'-flanking sequence. An LPS-responsive activation area was localized between nucleotides -50 to -100, and down-regulating sequences were present between nucleotides -100 and -2,111. A NFIL-6 site between -92 and -84 was identified in the functionally active region. Base substitutions within this single NFIL-6 site in the context of a 4.1-kb IL-1 beta promoter segment resulted in dramatic reduction of LPS-induced gene transcription. Introduction of multimers of this NFIL-6 sequence immediately 5' to minimal homologous or heterologous promoters conferred LPS inducibility in each case. Anti-C/EBP beta (NFIL-6) and anti-C/EBP delta (NFIL-6 beta) antibodies identified both of these proteins in complexes formed between the NFIL-6 site and mononuclear cell nuclear extracts. These data show that the proximal NFIL-6 site is required for the activation of murine IL-1 beta gene expression by endotoxin.

Cloning and characterization of ERF-1, a human member of the Tis11 family of early-response genes

Members of the Tis11 family of early-response genes are characterized by a high degree of sequence similarity around a putative zinc finger motif. They are induced by a variety of cell agonists and polypeptide mitogens, including 12-O-tetradecanoylphorbol-13-acetate (TPA) and epidermal growth factor (EGF). We describe the cloning and sequencing of a human member of this gene family, EGF-response factor 1 (ERF-1), the homolog of the mouse Tis11b/rat cMG1 genes. The human and rodent genes are similar, with 5' UTR, coding sequence, and 3' UTR highly conserved. The promoter/enhancer region and intron sequences contain multiple putative transcription factor binding motifs characteristic of early-response genes. Amino acid sequence comparison of the seven members of the Tis11 family cloned so far identifies a repeated consensus motif of (x+)YKTELC(x+)x5GxCxYGx(x+)CxFxH involving the potential zinc finger. Toward the carboxyterminal end is a region with a high percentage of prolines (15/73) and, partially overlapping, a serine-rich domain (20/54). These may be important as trans-activation and phosphorylation sites. The 3' untranslated region is unusually long, extending over 1,860 bp. The sequence immediately downstream from the translational stop codon has extensive secondary structure potential. The 3' UTR is 60% AT rich, but contains two GC rich (> 70%) regions. In addition there are multiple reiterations of a destabilization sequence, as well as a single UUAUUUAU motif characteristic of mRNAs specifying proteins involved in the inflammatory response. The mRNA contains a consensus polyadenylation signal.

Listeria monocytogenes, an invasive bacterium, stimulates MAP kinase upon attachment to epithelial cells

Protein tyrosine phosphorylation is an important regulatory mechanism for many cellular processes in eucaryotic cells. During the invasion of the gram-positive pathogen, Listeria monocytogenes, into host epithelial cells, two host proteins become tyrosine phosphorylated. We have identified these major tyrosine phosphorylated species to be two isoforms of mitogen-activated protein (MAP) kinase, the 42 and 44 kDa MAP kinases. This activation begins within 5 to 15 min of bacterial infection. The tyrosine kinase inhibitor, genistein, blocks invasion as well as the tyrosine phosphorylation of these MAP kinases. Using cytochalasin D to block bacterial internalization but not adhesion, we showed that bacterial adherence rather than uptake is required for MAP kinase activation. Internalin mutants, which are unable to adhere efficiently to host cells, do not trigger MAP kinase activation. Other invasive bacteria, including enteropathogenic Escherichia coli (EPEC), and E. coli expressing Yersinia enterocolitica invasion, were not observed to activate MAP kinase during invasion into cultured epithelial cells. These results suggest that L. monocytogenes activates MAP kinase during invasion and a MAP kinase signal transduction pathway may be involved in mediating bacterial uptake.