Cyclic AMP response element-binding protein and the catalytic subunit of protein kinase A are present in F9 embryonal carcinoma cells but are unable to activate the somatostatin promoter

Modulation of Early Host Innate Immune Response by an Avipox Vaccine Virus' Lateral Body Protein

The avian pathogen fowlpox virus (FWPV) has been successfully used as a vaccine vector in poultry and humans, but relatively little is known about its ability to modulate host antiviral immune responses in these hosts, which are replication-permissive and nonpermissive, respectively. FWPV is highly resistant to avian type I interferon (IFN) and able to completely block the host IFN-response. Microarray screening of host IFN-regulated gene expression in cells infected with 59 different, nonessential FWPV gene knockout mutants revealed that FPV184 confers immunomodulatory capacity. We report that the FPV184-knockout virus (FWPVΔ184) induces the cellular IFN response as early as 2 h postinfection. The wild-type, uninduced phenotype can be rescued by transient expression of FPV184 in FWPVΔ184-infected cells. Ectopic expression of FPV184 inhibited polyI:C activation of the chicken IFN-β promoter and IFN-α activation of the chicken Mx1 promoter. Confocal and correlative super-resolution light and electron microscopy demonstrated that FPV184 has a functional nuclear localisation signal domain and is packaged in the lateral bodies of the virions. Taken together, these results provide a paradigm for a late poxvirus structural protein packaged in the lateral bodies, capable of suppressing IFN induction early during the next round of infection.

Effects of mutation of the CREB binding site of the somatostatin promoter on cyclic AMP responsiveness in CV-1 cells

The transcription factors CREB (cAMP response element binding protein) and ATF (activating transcription factor) recognize DNA containing the consensus sequence TGACGTCA. We compared the neuropeptide somatostatin promoter, which binds CREB and is activated by cAMP, to the adenovirus E2A promoter, which binds ATF but is not activated by cAMP, to determine which specific nucleotides within a CREB/ATF recognition sequence confer cAMP responsiveness. Several mutant somatostatin promoters were generated containing part of all of the E2A ATF binding site. Some of the hybrid CREB/ATF binding sites competed for factor binding to a wild-type somatostatin promoter probe. However, only the wild-type CREB binding site promoter could confer cAMP activation on a linked CAT plasmid. Furthermore, this wild-type CREB binding site could confer cAMP activation on the CAT plasmid only if it was adjacent to a wild-type somatostatin TATA box and cap site. These results suggest that slight deviation from a wild-type CREB recognition sequence might be tolerated by factor(s) binding to cAMP response element-like sequences. However, transcription activation may require a particular CREB recognition sequence, as well as additional promoter elements that bind proteins that interact with CREB.

Control of the induction of type I interferon by Peste des petits ruminants virus

Peste des petits ruminants virus (PPRV) is a morbillivirus that produces clinical disease in goats and sheep. We have studied the induction of interferon-β (IFN-β) following infection of cultured cells with wild-type and vaccine strains of PPRV, and the effects of such infection with PPRV on the induction of IFN-β through both MDA-5 and RIG-I mediated pathways. Using both reporter assays and direct measurement of IFN-β mRNA, we have found that PPRV infection induces IFN-β only weakly and transiently, and the virus can actively block the induction of IFN-β. We have also generated mutant PPRV that lack expression of either of the viral accessory proteins (V&C) to characterize the role of these proteins in IFN-β induction during virus infection. Both PPRV_ΔV and PPRV_ΔC were defective in growth in cell culture, although in different ways. While the PPRV V protein bound to MDA-5 and, to a lesser extent, RIG-I, and over-expression of the V protein inhibited both IFN-β induction pathways, PPRV lacking V protein expression can still block IFN-β induction. In contrast, PPRV C bound to neither MDA-5 nor RIG-I, but PPRV lacking C protein expression lost the ability to block both MDA-5 and RIG-I mediated activation of IFN-β. These results shed new light on the inhibition of the induction of IFN-β by PPRV.

LGP2 plays a critical role in sensitizing mda-5 to activation by double-stranded RNA

The DExD/H box RNA helicases retinoic acid-inducible gene-I (RIG-I) and melanoma differentiation associated gene-5 (mda-5) sense viral RNA in the cytoplasm of infected cells and activate signal transduction pathways that trigger the production of type I interferons (IFNs). Laboratory of genetics and physiology 2 (LGP2) is thought to influence IFN production by regulating the activity of RIG-I and mda-5, although its mechanism of action is not known and its function is controversial. Here we show that expression of LGP2 potentiates IFN induction by polyinosinic-polycytidylic acid [poly(I:C)], commonly used as a synthetic mimic of viral dsRNA, and that this is particularly significant at limited levels of the inducer. The observed enhancement is mediated through co-operation with mda-5, which depends upon LGP2 for maximal activation in response to poly(I:C). This co-operation is dependent upon dsRNA binding by LGP2, and the presence of helicase domain IV, both of which are required for LGP2 to interact with mda-5. In contrast, although RIG-I can also be activated by poly(I:C), LGP2 does not have the ability to enhance IFN induction by RIG-I, and instead acts as an inhibitor of RIG-I-dependent poly(I:C) signaling. Thus the level of LGP2 expression is a critical factor in determining the cellular sensitivity to induction by dsRNA, and this may be important for rapid activation of the IFN response at early times post-infection when the levels of inducer are low.

A transcription assay for EWS oncoproteins in Xenopus oocytes

Aberrant chromosomal fusion of the Ewing's sarcoma oncogene (EWS) to several different cellular partners produces the Ewing's family of oncoproteins (EWS-fusion-proteins, EFPs) and associated tumors (EFTs). EFPs are potent transcriptional activators, dependent on the N-terminal region of EWS (the EWS-activation-domain, EAD) and this function is thought to be central to EFT oncogenesis and maintenance. Thus EFPs are promising therapeutic targets, but detailed molecular studies will be pivotal for exploring this potential. Such studies have so far largely been restricted to intact mammalian cells while recent evidence has indicated that a mammalian cell-free transcription system may not support bona fide EAD function. Therefore, the lack of manipulatable assays for the EAD presents a significant barrier to progress. Using Xenopus laevis oocytes we describe a plasmid-based micro-injection assay that supports efficient, bona fide EAD transcriptional activity and hence provides a new vehicle for molecular dissection of the EAD.

Identification and characterization of TLR8 and MyD88 homologs in Atlantic salmon (Salmo salar)

Toll-like receptor 8 (TLR 8) belongs to a subgroup of the TLR family that recognizes nucleic acids and that is involved in the protection against viruses. In mammals, TLR7 and 8 have been characterized as receptors for viral and synthetic single-stranded RNA. Here we describe the cloning of a TLR8 homolog in Atlantic salmon (Salmo salar) and its proximal adaptor protein MyD88. The mRNA expression of SsTLR8 was tissue-restricted and its highest level was detected in the spleen while SsMyD88 was expressed in all of the tested organs. SsTLR8 and SsMyD88 mRNAs were up-regulated in TO cells treated with recombinant IFN alpha1 and IFN gamma. In vivo, the expression of SsTLR8 was not significantly affected following challenge with salmon alphavirus subtype 3 (SAV3). By contrast, infection with SAV3 up-regulated SsMyD88 transcripts on day 7 post-challenge and the expression remained elevated at day 28. The SsMyD88 expression in vivo paralleled type I IFN expression. In vitro stimulation of salmon head kidney leukocytes with CpG ODNs and IFN gamma also up-regulated SsMyD88 mRNA. Furthermore, ectopic expression of SsMyD88 in HEK cells was able to activate a NF-kappaB reporter construct indicating that the cloned salmon molecule was a functional MyD88 homologue.

The rinderpest virus non-structural C protein blocks the induction of type 1 interferon

The innate immune response, in particular the production of type 1 interferons, is an essential part of the mammalian host response to viral infection. We have previously shown that rinderpest virus, a morbillivirus closely related to the human pathogen measles virus, blocks the actions of type 1 and type 2 interferons. We show here that this virus can also block the induction of type 1 interferon. The viral non-structural C protein appears to be the active agent, since expressing this protein in cells makes them resistant to activation of the interferon-beta promoter while recombinant virus that does not express the C protein activates this promoter much more than virus expressing the C protein. In addition, differences in activation of the interferon-beta promoter by different strains of rinderpest virus are reflected in differing abilities of their respective C proteins to block activation of the promoter by dsRNA. The C protein blocks the activation of this promoter induced by either cytoplasmic dsRNA or by Newcastle disease virus (NDV) infection, as well as activation induced by overexpression of several elements of the signalling pathway, including mda-5, RIG-I and IRF-3. The RPV C protein also blocks transcription from promoters responsive individually to the three transcription factors that make up the interferon-beta promoter enhanceosome, although it does not appear to block the activation of IRF-3.

Mapuera virus, a rubulavirus that inhibits interferon signalling in a wide variety of mammalian cells without degrading STATs

Mapuera virus (MPRV) is a paramyxovirus that was originally isolated from bats, but its host range remains unknown. It was classified as a member of the genus Rubulavirus on the basis of structural and genetic features. Like other rubulaviruses it encodes a V protein (MPRV/V) that functions as an interferon (IFN) antagonist. Here we show that MPRV/V differs from the IFN antagonists of other rubulaviruses in that it does not induce the proteasomal degradation of STAT proteins, key factors in the IFN signalling cascade. Rather, MPRV/V prevents the nuclear translocation of STATs in response to IFN stimulation and inhibits the formation of the transcription factor complex ISGF3. We also show that MPRV/V blocks IFN signalling in cells from diverse mammalian species and discuss the IFN response as a barrier to cross-species infections.

Improved growth of enteric adenovirus type 40 in a modified cell line that can no longer respond to interferon stimulation

Human enteric adenoviruses propagate poorly in conventional human cell lines used to grow other adenovirus serotypes. As human enteric adenoviruses have a defect in counteracting the cellular interferon (IFN) response in cell culture, to aid in growth of the virus, a 293-based cell line defective in its ability to respond to IFN was constructed. This cell line (293-SV5/V) constitutively expresses V-protein of the paramyxovirus Simian virus 5, which degrades the signal transducer and activator of transcription 1 (STAT1) and thereby prevents the STAT1-mediated IFN response. Analysis of human enteric adenovirus type 40 (HAdV-40)-infected 293-SV5/V cells compared with parental 293 cells shows that the recombinant line allows more rapid production of virus and results in higher titres. These results suggest that the defect in HAdV-40 in counteracting the IFN response can be overcome at least partially through the use of 293-SV5/V cell lines.

Characterization of three distinct CpG oligonucleotide classes which differ in ability to induce IFN alpha/beta activity and cell proliferation in Atlantic salmon (Salmo salar L.) leukocytes

Oligodeoxynucleotides (ODNs) containing unmethylated CpG dinucleotides within specific sequence contexts (CpG motifs) are in humans divided into three distinct classes (A, B and C). The CpG ODNs, like baterial DNA, are recognized by the vertebrate immune system and are known to stimulate immune responses. The characterization of the different classes is based on their structure and biological activity. In this study, we report the effects of these classes of CpG ODNs as they are defined in humans on IFN alpha/beta production and cell proliferation in Atlantic salmon spleen, head kidney and blood leukocytes. These studies revealed that CpG A together with CpG C are strong inducers of IFN alpha/beta in spleen and head kidney leukocytes, whilst CpG B and CpG C had the highest capacity to stimulate cell proliferation in all three cell populations. These findings are the first to establish that the effects of the different CpG classes are comparable between fish and man.

mda-5, but not RIG-I, is a common target for paramyxovirus V proteins

The induction of IFN-beta by the paramyxovirus PIV5 (formerly known as SV5) is limited by the action of the viral V protein that targets the cellular RNA helicase mda-5. Here we show that 12 other paramyxoviruses also target mda-5 by a direct interaction between the conserved cysteine-rich C-terminus of their V proteins and the helicase domain of mda-5. The inhibition of IFN-beta induction is not species-restricted, being observed in a range of mammalian cells as well as in avian cells, and we show that the inhibition of mda-5 function is also not restricted to mammalian cells. In contrast, the V proteins do not bind to the related RNA helicase RIG-I and do not inhibit its activity. The relative contributions of mda-5 and RIG-I to IFN-beta induction are discussed.

The UL144 gene product of human cytomegalovirus activates NFkappaB via a TRAF6-dependent mechanism

Molecular mimicry of cytokines and cytokine receptors is a strategy used by poxviruses and herpesviruses to modulate host immunity. The human cytomegalovirus (HCMV) UL144 gene, situated in the UL/b' region of the viral genome, has amino-acid sequence similarity to members of the tumour necrosis factor receptor superfamily. We report that UL144 is a potent activator of NFkappaB-induced transcription in a TRAF6-dependent manner. This NFkappaB activation enhances expression of the chemokine CCL22 through the NFkappaB responsive elements found in its promoter. In contrast to the clinical HCMV isolates, extensively passaged laboratory strains lack the UL/b' region and hence do not encode UL144. Consistent with this, infection with viruses that carry UL/b' causes NFkappaB activation and CCL22 expression, a phenotype that is not observed after infections with strains lacking the UL/b' region. Moreover, knockdown of UL144, TRAF6 or NFkappaB by specific siRNA in infections with UL144-encoding HCMV prevents the activation of CCL22 expression normally observed after infection with UL/b' positive HCMV. Upregulation of CCL22, which attracts Th2 and regulatory T cells, may help HCMV evade immune surveillance.

Stimulation of type I IFN activity in Atlantic salmon (Salmo salar L.) leukocytes: synergistic effects of cationic proteins and CpG ODN

Unmethylated CpG motifs in DNA are recognised by vertebrate immune cells as pathogen signatures. Consequently, oligodeoxynucleotides containing CpG motifs (CpG ODNs) are able to enhance and direct immune responses. Recent studies have demonstrated that CpG ODNs activate antiviral immune responses in Atlantic salmon (Salmo salar L.) leukocytes, and are therefore promising agents as vaccine adjuvants or immunostimulants in fish. In this work, we report synergy of CpG ODN and cationic proteins in the stimulation of type I IFN activity in Atlantic salmon leukocytes. Different fractions of cationic histone proteins derived from cod milt or poly-l-arginine and poly-l-lysine were screened for their ability to enhance CpG ODN induced type I IFN activity in Atlantic salmon leukocytes. Optimal ratio of histones to CpG ODN was identified, and effects on transcription of type I IFN and antiviral Mx genes were studied. Delivery of CpG ODN with cationic proteins enhanced the production of type I IFN and succeeding Mx transcripts after two and five days of stimulation at substimulatory concentrations of CpG ODN. These results indicate that co-delivery of CpG ODN and cationic proteins enhance antiviral mechanisms in Atlantic salmon leukocytes as compared to CpG ODN alone.

Activation of Interferon-Stimulated Response Element in Huh-7 Cells Replicating Hepatitis C Virus Subgenomic RNA

Interferon-alpha (IFN(alpha)) binds to receptors on the cell surface, which initiate a cascade of signal transduction pathways that leads to transcription of selected genes. This transduction pathway involves binding of transcription factors to a common cis-acting DNA sequence called IFN-stimulated response element (ISRE). To test whether these signaling pathways are functional in hepatitis C virus (HCV)-replicating cells, we studied the regulation of ISRE-mediated transcription of firefly luciferase gene in stable replicon cell lines. A plasmid construct was prepared (pISRELuc) which contains four tandem repeats of 9-27 ISRE sequences positioned directly upstream of the herpes virus 1 thymidine kinase promoter TATA box that drives the expression of firefly luciferase. Regulation of ISRE-mediated expression of firefly luciferase by IFN(alpha) was studied by transfecting this clone into Huh-7 cells replicating HCV subgenomic HCV RNA. The significance of ISRE-mediated transcriptional activation was studied in a replicon cell line by pretreatment of cells with actinomycin D, which inhibits cellular DNA-dependent RNA transcription. IFN treatment activates ISRE-mediated expression of luciferase, indicating that this pathway is functional in Huh-7 cells. Activation of ISRE-mediated transcription of luciferase is relatively high in two Huh-7 stable cell lines replicating HCV subgenomic RNA. Inhibition of ISRE-mediated transcription of luciferase by actinomycin D also makes HCV replication totally resistant to IFN(alpha). These in vitro studies suggest that activation of IFN-inducible genes is important in mounting a successful antiviral response against HCV.

Quantification of Atlantic salmon type-I interferon using an Mx1 promoter reporter gene assay

We here describe an assay for the detection of interferon-like activity in Atlantic salmon based on the transient transfection of chinook salmon embryo cells (CHSE-214 cells) with a rainbow trout Mx1 promoter linked to a luciferase reporter. A beta-galactosidase gene under the control of a constitutively expressed beta-actin promoter was used as a transfection standard, and luciferase and beta gal expression were measured by a commercially available kit. Interferon containing supernatants from poly I:C- or CpG-stimulated leucocytes added to transfected CHSE-cells induced high luciferase expression (>60-fold induction compared to supernatants from non-stimulated cells). There was no response to supernatants from LPS- and ConA/PMA-stimulated leucocytes, demonstrating the specificity for type I interferon-like activity. Duplicate samples analysed using a cell protection assay for detection of antiviral activity correlated well with levels obtained by the Mx1 promoter reporter gene assay (R2=0.97), confirming the reporter assay as a reliable substitute for the standard antiviral assay. The Mx reporter gene assay also has advantages in terms of sensitivity, high dynamic range and reliability over the conventional cell protection assay.

Transactivation of the fucosyltransferase VII gene by human T-cell leukemia virus type 1 Tax through a variant cAMP-responsive element

Human T-cell leukemic virus type 1 (HTLV-1)-infected T cells express the fucosyltransferase (Fuc-T) VII gene involved in the biosynthesis of the leukocyte sialyl Lewis X, which may be related to tissue infiltration in patients with malignant adult T-cell leukemia. HTLV-1 induces Fuc-T VII transcription through the viral transactivator Tax, although the underlying molecular mechanism remains unknown. In the present study, we analyzed the role of the cis-activating element in Tax activation using reporter constructs bearing the 5'-regulatory region of Fuc-T VII in Jurkat T cells. A sequence (GGCTGTGGGGGCGTCATATTGCCCTGG) covering a half-palindromic cyclic adenosine monophosphate (cAMP)-responsive element (CRE) was found to be required for Tax activation of the Fuc-T VII promoter. We further demonstrated that transcription factors of the CRE-binding protein (CREB)/activating transcription factor (ATF) family bind to this CRE-like sequence and that Tax binds in association with CREB and the coactivator CREB-binding protein (CBP) in Jurkat T cells. This element, containing the G+C-rich flanking sequences, is homologous to the Tax-responsive viral CREs in the HTLV-1 long terminal repeat (LTR)-promoter. Furthermore, CREM alpha, an isoform of CREB deficient in the glutamine-rich domains, was found to activate the Fuc-T VII promoter in a phosphorylation-independent manner, similar to the viral CRE in HTLV-1 LTR but in contrast to the phosphorylation-dependent activation of the cellular CREs by Tax. These findings indicate that the Fuc-T VII promoter is transactivated by Tax in concert with CBP through a CRE-like sequence in a manner similar to that of viral CRE in HTLV-1 LTR.

The V proteins of simian virus 5 and other paramyxoviruses inhibit induction of interferon-beta

In this article we show that the paramyxovirus SV5 is a poor inducer of interferon-beta (IFN-beta). This inefficient induction is a consequence of the expression of an intact viral V protein. In the absence of the viral V protein cysteine-rich C-terminal domain, IFN-beta mRNA is strongly induced and the transcription factors NF-kappaB and IRF-3 are activated significantly. The V protein can work in isolation from SV5 to block intracellular dsRNA signaling. The mechanism of block to dsRNA signaling is distinct from that previously observed for blocking IFN signaling in that proteolysis of candidate factors cannot be detected, and furthermore, the respective blocks require distinct protein domains. Blocking of the induction of IFN-beta by dsRNA requires the C-terminal cysteine-rich domain, a feature that is highly conserved among paramyxoviruses. We demonstrate that the V proteins from other paramyxoviruses have equivalent functions and speculate that limiting the yield of IFN-beta during infection may be a general property of paramyxoviruses.

Retinoic acid induces parietal endoderm but not primitive endoderm and visceral endoderm differentiation in F9 teratocarcinoma stem cells with a targeted deletion of the Rex-1 (Zfp-42) gene

Cultured murine F9 teratocarcinoma stem cells resemble pluripotent stem cells of the inner cell mass of the mouse blastocyst and, depending upon their treatment, can be induced to differentiate along the primitive endoderm, the parietal endoderm (PE), or the visceral endoderm (VE) pathway. The Rex-1 gene encodes a zinc finger family transcription factor which is expressed at high levels in undifferentiated F9 stem cells, embryonic stem cells, and in other types of stem cells. To examine whether the Rex-1 protein plays a role in F9 cell differentiation, homologous recombination was employed to generate F9 cell lines which lack both alleles of Rex-1. F9 wild type cells in monolayer culture require both retinoic acid and cyclic AMP analogs to differentiate into PE, whereas the F9 Rex-1(-/-) cells differentiate into PE, as assessed by several molecular markers, including thrombomodulin and laminin B1, in the presence of RA alone. The F9 Rex-1(-/-) cells do not completely differentiate into VE after RA treatment in aggregate culture; they do not express alpha-fetoprotein, a definitive marker of VE differentiation. These results indicate that the Rex-1 transcription factor regulates the differentiation of F9 stem cells along several distinct cell lineages found in the early embryo.

In vivo effects of the Epstein-Barr virus small RNA EBER-1 on protein synthesis and cell growth regulation

Recent studies have suggested a role for the Epstein-Barr virus-encoded RNA EBER-1 in malignant transformation. EBER-1 inhibits the activity of the protein kinase PKR, an inhibitor of protein synthesis with tumour suppressor properties. In human 293 cells and murine embryonic fibroblasts, transient expression of EBER-1 promoted total protein synthesis and enhanced the expression of cotransfected reporter genes. However reporter gene expression was stimulated equally well in cells from control and PKR knockout mice. NIH 3T3 cells stably expressing EBER-1 exhibited a greatly increased frequency of colony formation in soft agar, and protein synthesis in these cells was relatively resistant to inhibition by the calcium ionophore A23187. Nevertheless clones containing a high concentration of EBER-1 were not invariably tumourigenic. We conclude that EBER-1 can enhance protein synthesis by a PKR-independent mechanism and that, although this RNA may contribute to the oncogenic potential of Epstein-Barr virus, its expression is not always sufficient for malignant transformation.

Differences in interferon sensitivity and biological properties of two related isolates of simian virus 5: a model for virus persistence

CPI(+) and CPI(-) are two canine isolates of simian virus 5 (SV5). CPI(+) was originally isolated from the cerebrospinal fluid of a dog with temporary posterior paralysis and CPI(-) was recovered at 12 days p.i. from the brain tissue of a dog experimentally infected with CPI(+). We have previously shown that the V protein of SV5 blocks interferon (IFN) signalling by targeting STAT1 for degradation. Here we report that whilst CPI(+) targets STAT1 for degradation, CPI(-) fails to and as a consequence, CPI(+) blocks IFN signalling but CPI(-) does not. Three amino acid differences in the P/V N-terminal common domain of the V protein are responsible for the observed difference in the abilities of CPI(+) and CPI(-) to block IFN signalling. In cells persistently infected with CPI(-) the virus may become repressed in response to IFN, under which circumstances virus glycoproteins are lost from the surface of infected cells and virus nucleocapsid proteins accumulate in cytoplasmic inclusion bodies. We suggest that in vivo cells infected with IFN-resistant viruses (in which there would be continuous virus protein synthesis) may be more susceptible to killing by cytotoxic T cells than cells infected with IFN-sensitive viruses (in which virus protein synthesis was repressed), and a model of virus persistence is put forward in which there is alternating selection of IFN-resistant and IFN-sensitive viruses depending upon the state of the adaptive immune response.

Two cysteine residues in the DNA-binding domain of CREB control binding to CRE and CREB-mediated gene expression

The cAMP-responsive element-binding protein (CREB) has been implicated in the regulation of numerous physiological functions including those of several hypoxia-responding genes. All CREB transcription-regulated genes harbor the eight base-pair cAMP-responsive element (CRE) or the seven base-pair AP-1 sequence. Utilizing mutational analysis and biochemical assays, we found that reduction of two cysteine residues located in the DNA-binding basic domain of CREB, enhances the binding efficiency of CREB to DNA and regulates CRE-mediated gene expression. Substitution of these residues to serine renders insensitivity to reduction, hypoxia and to the sulfhydryl-specific modifying agent, N-ethylmaleimide. These substitutions enhance the binding of CREB to its cognate DNA sites under oxidative conditions, and of the CREB-dependent gene expression during normoxia. These findings are supported by results of molecular modeling of the CREB-CRE interactions. We also found that HTLV-1 Tax enhancement of CREB binding to the cellular and the viral DNA sites and activation of the CRE-dependent gene expression are independent of CREB activation exerted by redox conditions. The genetic biochemical and molecular modeling presented in this work indicate that the two cysteine residues in the bZIP domain of CREB regulate the binding efficiency of CREB to its cognate DNA sites and as a consequence the activation of CREB-mediated gene expression.

Transcriptional Regulation of Expression of Carbohydrate Ligands for Cell Adhesion Molecules in the Selectin Family

Cell adhesion mediated by selectins and their carbohydrate ligands is involved in the adhesion of cancer cells to endothelial cells during the course of hematogenous metastasis of cancer. In patients with leukemia, this adhesion is involved in the extravascular infiltration of leukemic cells. Extravasation and tissue infiltration of malignant cells in patients with adult T-cell leukemia is mediated by the interaction of selectins and their carbohydrate ligand sialyl Lewis X, which is strongly and constitutively expressed on the leukemic cells. Constitutive expression of Lewis X in these cells is due to the transcriptional activation of Fuc-T VII, the rate-limiting enzyme in the sialyl Lewis X synthesis, induced by the Tax protein encoded by the human T-cell leukemia virus-1, the etiological virus for this leukemia. This transactivation is in clear contrast to the regulation of typical CRE-element found in various cellular genes in that it is independent of phosphorylation-dependent regulation. This must be the reason for the strong and constitutive expression of sialyl Lewis X, which exacerbates the tissue infiltration of leukemic cells. This is a good example corroborating the proposition that the abnormal expression of carbohydrate determinant at the surface of malignant cells is intimately associated with the genetic mechanism of malignant transformation of cells.

Paramyxoviridae use distinct virus-specific mechanisms to circumvent the interferon response

STAT1 and STAT2 are cellular transcription factors involved in interferon (IFN) signaling and are thus critical for the IFN-induced antiviral state. We have previously shown that the paramyxovirus Simian Virus 5 (SV5) blocks both type I and type II interferon (IFN) signaling by targeting STAT1 for proteasome-mediated degradation. To determine whether this is a feature common to all Paramyxoviridae, we examined the abilities of SV5, Sendai virus (SeV), human respiratory syncytial virus (RSV), and human parainfluenza viruses types 2 and 3 (hPIV2 and hPIV3, respectively) to block interferon signaling. The results showed that in reporter assays SV5, SeV, and hPIV3 blocked both type I and type II IFN-signaling; hPIV2 blocked type I but not type II IFN-signaling; and RSV failed to block either type I or type II IFN-signaling. In agreement with these results, SV5 and SeV inhibited the formation of the ISGF3 and GAF transcription complexes (essential for type I and type II signaling, respectively). Surprisingly, although hPIV3 inhibited IFN-induction of the ISGF3 complex, GAF complexes were detected in hPIV3-infected cells. hPIV2 also blocked the formation of the ISGF3 complex but not the GAF complex, whereas RSV failed to block the induction of either complex. SV5 was the only virus that caused the degradation of STAT1. Indeed, in SeV- and hPIV3-infected cells STAT1 was phosphorylated on tyrosine 701 (Y701), a characteristic of IFN receptor activation. However, consistent with these viruses blocking IFN signaling downstream of receptor activation, there was a specific reduction in the levels of serine 727 (S727)-phosphorylated forms of STAT1alpha in SeV- and hPIV3-infected cells. In contrast both (Y701)- and (S727)-phosphorylated forms of STAT1 were detected in hPIV2-infected cells but there was a specific loss of STAT2. Both STAT1 (including Y701- and S727-phosphorylated forms) and STAT2 could readily be detected in RSV-infected cells. Despite not being able to block type I or type II IFN signaling, RSV was able to replicate in human cells that produce and respond to IFN, suggesting that RSV must have an alternative method(s) for circumventing the IFN response. These results demonstrate that, although interference with IFN signaling is a common strategy among Paramyxovirinae, distinct virus-specific mechanisms are used to achieve this end.

Transcriptional regulation of the transforming growth factor-beta2 promoter by cAMP-responsive element-binding protein (CREB) and activating transcription factor-1 (ATF-1) is modulated by protein kinases and the coactivators p300 and CREB-binding protein

Transcription of the transforming growth factor-beta2 (TGF-beta2) gene is dependent on a cAMP-response element/activating transcription factor (CRE/ATF) site that is bound by CREB and ATF-1 as well as an E-box motif that is bound by upstream stimulatory factors 1 and 2 (USF1 and USF2). To identify additional factors involved in the expression of the TGF-beta2 gene, we employed F9 embryonal carcinoma (EC) cells, which express TGF-beta2 only after the cells differentiate. We show that overexpression of the transcription factors, CREB, ATF-1, USF1, and USF2 dramatically increases TGF-beta2 promoter activity in F9-differentiated cells. We further show that the coactivators p300 and CBP up-regulate the TGF-beta2 promoter when CREB and ATF-1 are expressed in conjunction with protein kinases that phosphorylate CREB on serine 133 and ATF-1 on serine 63. Importantly, we identify the presence of serine 133-phosphorylated CREB in the nucleus of F9-differentiated cells but not in the nucleus of F9 EC cells. This phosphorylated form is present in whole cell extracts of both the parental and differentiated cells, suggesting that nuclear accumulation of serine 133-phosphorylated CREB is regulated during differentiation of F9 EC cells and is likely to play an important role in the activation of the TGF-beta2 gene.

CREB regulates MHC class II expression in a CIITA-dependent manner

The X2 box of MHC class II promoters is homologous to TRE/CRE elements and is required for expression of MHC class II genes. The X2 box-specific DNA binding activity, X2BP, was purified to homogeneity, sequenced, and identified as CREB. Transient transactivation experiments showed that CREB can cooperate with CIITA to enhance activation of transcription from MHC class II promoters in a dose-dependent manner. Binding of CREB to the class II promoter in vivo was demonstrated by a chromatin immunoprecipitation assay. Additionally, ICER, a dominant inhibitor of CREB function, was found to repress class II expression. These results demonstrate that CREB binds to the X2 box in vivo and cooperates with CIITA to direct MHC class II expression.

The mouse elongation factor-2 gene: isolation and characterization of the promoter

Elongation factor 2 (EF-2) is a protein involved in peptide chain elongation in eukaryotes. We isolated the mouse EF-2 gene and characterized its promoter. We showed that the majority of enhancer elements were located within 500 bp of the flanking sequence and identified a factor binding site sequence (CGTCACGTGACGC) located between nucleotides -58 and -47 containing two CGTCA motifs separated by two nucleotides. The motif represents a half-site for binding of the cAMP response element (CRE) binding protein (CREB). Mutation analysis indicated that the presence of one CGTCA site alone conferred cAMP inducibility, but the presence of one or two CGTCA sites and spacing nucleotides elicited cAMP-independent, constitutive expression. UV cross-linking and DNA affinity chromatography revealed that three 40-, 43-, and 65-kD proteins bound to the CRE-like element. Of these, the 65-kD protein was unique to the CRE-like element. The 40-kD protein was ATF1 and the 43-kD protein with the molecular size of CREB was not CREB, on the basis of reactivity to their respective antibodies. Because ATF1 responds poorly to cAMP induction, it is likely the contributor to the constitutive expression rather than inductive expression of the CRE-like element, and, thus, the EF-2 gene.

Differential activation of viral and cellular promoters by human T-cell lymphotropic virus-1 tax and cAMP-responsive element modulator isoforms

We have previously proposed that cAMP-responsive element-binding protein (CREB) activity is stimulated by human T-cell lymphotropic virus-1 (HTLV-1) Tax through two mechanisms that are differentially dependent upon CREB phosphorylation. We have tested this model by examining how Tax affects transcriptional activation mediated by the cAMP-responsive element (CRE) modulator (CREM). The CREM proteins are highly homologous to CREB, particularly in their DNA-binding domains and the kinase-inducible domain (KID), a region that interacts with the coactivator CREB-binding protein (CBP) in a phosphorylation-dependent manner. Despite this similarity, most CREM isoforms are transcriptional repressors. CREMalpha lacks the glutamine-rich domains found in CREB that are essential for transcriptional activation. We show that the normally repressive CREMalpha activates the HTLV-1 and cellular CREs in the presence of Tax; activation of the viral element is phosphorylation-independent, and activation of the cellular CRE is phosphorylation-dependent. CREMDelta(C-G) lacks both the KID and the glutamine-rich regions. This isoform activates the HTLV-1 long terminal repeat in a phosphorylation-independent manner, but does not activate the cellular CRE. This study suggests that Tax, interacting with the basic/zipper region of CREM, recruits CBP to the viral promoter. Tax activation of the cellular CRE depends on the KID and its ability to interact with CBP in a phosphorylation-dependent manner.

Interaction of nuclear factors with the cAMP response elements of the human β(3)-adrenoceptor gene

Four potential cyclic adenosine 3',5'-monophosphate (cAMP) response elements (CREs), each having at most two mismatches from the classical canonical sequence, have been identified in the 5'UTR of the human β(3)-adrenoceptor gene by Liggett and Schwinn (1991). Recently, three of these CREs were shown to confer responsiveness to cAMP when cloned into a CAT reporter vector (Thomas et al., 1992). In this study, in vitro gel-retardation assays have shown that recombinant human CRE binding protein-1 (CREB-1) or activating transcription factor-1 (ATF-1) can interact specifically with these four putative CREs (termed β(3)CRE2), although with different affinities. Nuclear extracts from human brown or white adipose tissue contain proteins interacting with β(3)CRE3 and β(3)CRE2. These adipose nuclear factors were shown by competition assays and the use of antibodies to differ from CREB-1 or ATF-1. The nuclear factor(s) interacting with β(3)CRE2 was found in human and rat brown and white adipose tissues, but not in the other nonadipose tissues examined, i.e., rat lung, liver, kidney, and heart, suggesting an adipose tissue-specific DNA binding or expression pattern. β(3)CRE2 is found to constitute a hexameric element that is highly homologous to the binding site for members of the nuclear hormone receptor superfamily, and a competition assay using this site has provided evidence that an adipose tissuespecific orphan member of this superfamily may bind to β(3)CRE2. Reporter gene assays have indicated that β(3)CRE2 and β(3)CRE3 slightly repress the basal level of transcription and that β(3)CRE2 confers cAMP responsiveness, whereas β(3)CRE3 does not.

CCAAT box enhancer binding protein alpha (C/EBP-alpha) stimulates kappaB element-mediated transcription in transfected cells

A construct comprising three tandemly repeated copies of the kappaB element from the interleukin-8 gene linked to chloramphenicol acetyltransferase (CAT) (3xNF-kappaBCAT) was transcriptionally activated in normal human FS-4 fibroblasts by co-transfection with expression vectors for NF-kappaB p50, p65, or p52. Unexpectedly, a significant activation of 3xNF-kappaBCAT was also seen upon its co-transfection with the expression vector for CCAAT box enhancer binding protein alpha (C/EBP-alpha) (but not C/EBP-beta or C/EBP-delta). Stimulation by C/EBP-alpha required some other factor(s) present in FS-4 cells because no transcriptional activation of 3xNF-kappaBCAT was seen after co-transfection with C/EBP-alpha in F9 mouse embryonic carcinoma cells, known to be deficient in several transcription factors. To determine whether transcriptional activation was the result of interaction with one of the major NF-kappaB proteins, we co-transfected C/EBP-alpha with NF-kappaB p50, p65, p50 + p65, or p52 into F9 or FS-4 cells. No cooperative interaction was seen; in fact, C/EBP- alpha reduced p65-stimulated transcription, especially in F9 cells. Electrophoretic mobility shift assay with a kappaB probe revealed that the addition of recombinant C/EBP-alpha protein to nuclear extracts from untreated FS-4 cells resulted in the appearance of four bands. Only one of these bands was supershifted by antibody to p50, whereas antibodies to p65 or other NF-kappaB proteins had no effect. Our findings show that C/EBP-alpha may cause activation of some kappaB element-containing genes lacking C/EBP binding sites.

Identification of a functional cAMP response element in the secretogranin II gene

Secretogranin II is an acidic secretory protein with a widespread distribution in secretory granules of neuronal and endocrine cells. The secretogranin II gene contains, like other members of the granin family, a cAMP response element (CRE) in its upstream region. To investigate the functional significance of this motif, intracellular cAMP levels were increased in a neuronal cell line derived from the septal region of the brain and the level of secretogranin II gene expression was analysed. It was found that increased cAMP levels did, in fact, induce secretogranin II gene expression. To analyse the cis-acting sequence responsible for this induction, a hybrid gene containing the upstream region of the mouse secretogranin II gene fused to beta-globin as a reporter was constructed. Transfection analysis revealed that cAMP-induced transcription of the secretogranin II promoter/beta-globin gene in septal and insulinoma cells. DNA-protein binding assays showed that recombinant CRE-binding protein (CREB), produced in bacteria or human cells, bound in a sequence-specific manner to the secretogranin II promoter CRE. Moreover, deletion mutagenesis revealed that the CRE motif is a bifunctional genetic regulatory element in that it mediates basal as well as cAMP-stimulated transcription. Interestingly, cAMP had no effect upon secretogranin II gene transcription in PC12 and neuroblastoma cells. An increase in the intracellular cAMP concentration activated a GAL4-CREB fusion protein upon transcription in neuroblastoma cells indicating the integrity of the cAMP signaling pathway to the nucleus. Basal as well as cAMP-stimulated transcription, directed from the secretogranin II promoter was, however, impaired in insulinoma cells by overexpression of CREB-2, a negative-acting CRE-binding protein. These results indicate that competitive effects are likely to occur between CRE-bound transcriptional activators and repressors. We conclude that cAMP-stimulated induction of secretogranin II gene transcription is mediated by the CRE motif in a cell-type-specific manner, and is likely to depend on the balance between positive and negative CRE-binding proteins in a particular cell type.

Role of LFB3 in cell-specific cAMP induction of the urokinase-type plasminogen activator gene

In previous work we suggested that a kidney-specific transcription factor LFB3 cooperates with cAMP-response element (CRE)-binding proteins within a cAMP regulatory unit comprised of three protein-binding domains and located 3.4 kilobase pairs upstream of the urokinase-type plasminogen activator (uPA) gene in LLC-PK1 cells (Menoud, P.-A., Matthies, R., Hofsteenge, J., and Nagamine, Y. (1993) Nucleic Acids Res. 21, 1845-1852). The two domains contain a CRE-like sequence, and the third domain is recognized by LFB3. The absolute requirement of LFB3 as well as the cooperation among the three domains for cAMP regulation were confirmed by transient transfection assays in F9 teratocarcinoma cells, in which the level of LFB3 was negligible. Suspecting a possible feedback regulation of LFB3 mRNA expression during cAMP-dependent uPA gene induction in LLC-PK1 cells, we measured LFB3 mRNA levels after cAMP treatment and found a strong reduction. This reduction was not due to a change in template activity of the LFB3 gene because run-on transcription showed no significant change in LFB3 gene transcription. RNA synthesis inhibitor-chase experiments indicated that the down-regulation was post-transcriptional. Interestingly, when the inhibitor was added at the same time as cAMP, the cAMP-induced decrease in LFB3 mRNA levels was abrogated, suggesting that ongoing RNA synthesis is required for the decrease. Similar effects on LFB3 mRNA metabolism were observed with all agents that induce uPA mRNA in LLC-PK1 cells, including 12-O-tetradecanoylphorbol-13-acetate, okadaic acid, colchicine, and cytochalasin. We discuss the significance of this regulation in uPA gene expression.

The CRE consensus sequence in the synapsin I gene promoter region confers constitutive activation but no regulation by cAMP in neuroblastoma cells

Synapsin I is implicated in the modulation of neurotransmitter release and in synaptogenesis and is regulated by phosphorylation. The rat and human synapsin I genes both carry CRE and TRE consensus sequences in their promoter regions. This suggested that protein kinase-mediated signal pathways might also regulate synapsin I activity at the level of gene expression and thus contribute, on a slower time scale, to synaptic plasticity. We have therefore investigated, in neuroblastoma cell lines, the effects of agents that activate protein kinases on synapsin I gene expression. Unexpectedly, treatment with forskolin/IBMX was not found to enhance synapsin I mRNA levels. Rather, it causes a decrease to approximately 50% within 1 day although several CRE-dependent control genes are strongly induced. The calcium ionophore, A23187, lowers synapsin I mRNA to approximately 75%, and the phorbol ester, TPA, is without effect. Transient expression of a CAT fusion gene under the control of the synapsin I promoter region is also inhibited by forskolin/IBMX, as well as by protein kinase A (PKA) overexpression, suggesting that the decrease of synapsin I mRNA in response to forskolin/IBMX is due to the inhibition of transcription. Mutation of the CRE consensus does not affect the response to PKA, but it reduces the constitutive activity of synapsin I promoter constructs down to 30-50%. Nuclease footprinting experiments demonstrate sequence-specific binding proteins from brain, liver and NS20Y cell nuclear extracts to the CRE consensus sequence of the rat synapsin I promoter.

Regulation of the transforming growth factor-beta 2 gene promoter in embryonal carcinoma cells and their differentiated cells: differential utilization of transcription factors

Previous studies demonstrated that differentiation of embryonal carcinoma (EC) cells increases the expression of the TGF-beta 2 gene and identified a CRE/ATF-like motif in the TGF-beta 2 promoter that is necessary for its activity. This suggested that differentiation may increase the transcription of this gene by differential binding of transcription factors to the CRE/ATF-like motif. To test this possibility, we performed gel mobility shift analysis using double-stranded oligodeoxynucleotides containing the TGF-beta 2 CRE/ATF-like motif and nuclear extracts prepared from F9 EC cells and F9-differentiated cells. We determined that the DNA/protein complexes formed by the EC nuclear extracts, but not the complexes formed by differentiated cell nuclear extracts, are recognized and supershifted by an ATF-1 specific antibody. This observation is consistent with our Western immunoblot analysis that detects AFT-1 in the EC cells, but not in their differentiated counterparts. In addition, we provide evidence that protein phosphorylation influences the formation of complexes between F9 nuclear proteins and the CRE/ATF-like motif. Together, our studies identify a likely role for the CRE/ATF-like motif in the regulation of TGF-beta 2 and suggest that this site binds one set of nuclear proteins in EC cells, where the gene is not expressed, and a different set of nuclear proteins in the differentiated cells, where the gene is expressed.

Endogenous and transfected mouse alpha-fetoprotein genes in undifferentiated F9 cells are activated in transient heterokaryons

Mouse F9 teratocarcinoma cells provide a system to study developmentally regulated alpha-fetoprotein (AFP) gene expression. AFP is not expressed in undifferentiated F9 cells but is induced when cells differentiate as cell aggregates in the presence of retinoic acid. Previous studies have led to the suggestion that undifferentiated F9 cells contain negative regulators of AFP expression. To test this, we have used transient heterokaryons to ask whether inactive AFP genes in undifferentiated F9 cells are responsive to positively acting trans-acting factors. Our results indicate that silent endogenous and transfected AFP genes are activated when undifferentiated F9 cells are fused to human hepatoma HepG2 cells. This suggests that the lack of AFP expression in undifferentiated F9 cells is due to the absence or insufficient level of positive-acting transcription factors, rather than the presence of dominant negative regulators. We also demonstrate that stably transfected AFP genes, although unmethylated, are properly regulated in F9 cells.

Gene regulation of cyclic AMP-dependent protein kinase subunits (C alpha, beta; RI alpha, beta and RII alpha, beta) in rat facial motoneurons after nerve transection

In this study we investigated the changes in gene expressions of catalytic (C alpha, beta) and regulatory (RI alpha, beta and RII alpha, beta) subunits of cAMP-dependent protein kinase (PKA) in axotomized facial motoneurons of the rat. Nerve transection induced changes in the expression of C subunit messenger RNAs and RII subunit messenger RNAs. Control facial motoneurons had a high expression of both C alpha and C beta subunit messenger RNAs, but their expression declined after axotomy. The decrease was most pronounced at postoperative week 2 and returned to basal level within postoperative week 4. In contrast, the expression of both RII alpha and beta subunit messenger RNAs, which were low in control facial motoneurons, was increased after axotomy. Enhancement of RII subunits messenger RNAs was apparent during postoperative weeks 1 and 3, and then returned to the basal level. RI alpha, beta subunits messenger RNAs were strongly expressed in normal facial motoneurons, but were not clearly influenced by axotomy. These results indicate an attenuation of total PKA activity in axotomized facial motoneurons. Furthermore, such gene regulation may imply a change of the targets for PKA in facial motoneurons during the process of neurite regeneration.

DNA-binding proteins that interact with the 19-base pair (CRE-like) element from the HCMV major immediate early promoter in differentiating human embryonal carcinoma cells

The pluripotent human embryonal carcinoma (EC) cell line NTERA-2 provides a useful tool for investigating cell differentiation in a way that is pertinent to the development of the early human embryo. The major immediate early (MIE) gene of human cytomegalovirus (HCMV), which is not transcribed in undifferentiated NTERA-2 EC cells but is transcribed in their differentiated derivatives, offers a model with which to study the developmental regulation of gene activity during the differentiation of these cells. We have investigated the regulatory activity of the cAMP response elements (CRE) and the activation protein (AP1) site found within several repeated 19-base-pair (bp) elements from the HCMV MIE promoter, and the developmental regulation of nuclear DNA-binding factors that interact with these sites. The 19-bp CRE but not the AP1 site is responsive to cAMP in undifferentiated NTERA-2 EC and its activity is enhanced upon differentiation. Nuclear proteins of the CREB, Fos, and Jun families bind to these sites, but, surprisingly, their levels only show limited regulation during NTERA-2 differentiation. This contrasts with results obtained with murine EC cells. However, additional and apparently novel proteins with molecular weights between 80,000 and 90,000, and binding specificities for both CRE and AP1 sites, were detected in undifferentiated EC cells. The activity of these proteins decreased markedly after differentiation, indicating their involvement in negative regulation of the CRE/AP1-like site in undifferentiated EC cells. This suggests novel members able to interact via leucine zippers with other members of the Jun-Fos-CREB family of DNA binding proteins that are also involved in this regulation.