Oncogene jun encodes a sequence-specific trans-activator similar to AP-1

The effects of glucocorticoid hormone on the expression of c-jun

The effects of glucocorticoid hormone on the expression of c-jun in the fibroblasts were studied. The expression of c-jun was repressed by dexamethasone in the NIH3T3 cells, but not in the transformed B104-1 or EJ-Ras cells. The repression was not relieved by the addition of cycloheximide.

IP-1: a dominant inhibitor of Fos/Jun whose activity is modulated by phosphorylation

Transcription factor AP-1 is inducible by phorbol esters and thus could be considered to be one final target of the protein kinase C signal transduction pathway. AP-1 consists of the products of the fos and jun oncogenes, which associate as dimers to bind TPA-responsive promoter elements (TRE) efficiently. We show that AP-1 activity is modulated by an inhibitory protein (IP-1), present both in the nucleus and cytoplasm of several cell types. IP-1 specifically blocks DNA binding of AP-1 from nuclear extracts and of in vitro synthesized Fos/Jun proteins. It is a labile protein of 30-40 kd, which exerts its activity only in the nonphosphorylated form. Block of IP-1 function is obtained by PKA-mediated phosphorylation, possibly suggesting a cross talk mechanism at transcriptional level. Competition experiments with synthetic peptides suggest that IP-1 could interact with Fos and/or Jun leucine zippers. We speculate that IP-1 might act as a transcriptional antioncogene.

Myristylation alters DNA-binding activity and transactivation of FBR (gag-fos) protein

FBR murine sarcoma virus (gag-fos) protein, a virally transduced Fos protein, exhibits decreased gene transactivation in comparison with the cellular Fos protein. Biochemical analysis suggests that myristylation of the virally encoded N-terminal gag region results in decreased DNA binding and transcriptional activation without affecting heterodimerization with Jun protein. These findings demonstrate that protein myristylation can modulate gene regulation by a DNA-binding protein.

Induction of the HTLV-I LTR by Jun occurs through the Tax-responsive 21-bp elements

The HTLV-I LTR is known to be induced by a variety of cellular signals. Tax protein is one potent viral trans-activator of LTR-directed transcription. We demonstrate here that Jun is another transcription factor that can strongly modulate the activity of this LTR. Using deletion and competition studies, the minimal portion of the LTR for Jun activation was found to coincide with the Tax-responsive 21-bp elements. In binding experiments, nuclear factors that bound to the HTLV-I 21-bp sequence were competed by an excess of AP-1 motif oligonucleotide. Although the Tax-responsive elements do not contain a strictly conserved AP-1 motif, these findings suggest that they function as AP-1 sites. We found, however, that in cells depleted for AP-1 activity (F9 teratocarcinoma), Tax activation of the HTLV-I LTR was maintained. Thus while Jun/AP-1 may be involved in the basal expression of the HTLV-I LTR, it may not be essential for Tax-mediated activation.

Interleukin-6 signals activating junB and TIS11 gene transcription in a B-cell hybridoma

The events in interleukin-6 (IL-6) signal transduction leading to primary response gene activation were analyzed in murine B-cell hybridoma and plasmacytoma cells which require IL-6 for growth. IL-6 stimulation of IL-6-deprived cells resulted in the rapid and transient tyrosine phosphorylation of a 160-kDa cellular protein (p160). This was followed by the highly selective induction of two primary response genes, junB/AP-1 transcription factor and TIS11. junB and TIS11 inductions were unaffected by cycloheximide, suggesting that posttranslational modifications accounted for their activation. Activation of junB and TIS11 transcription required rapid tyrosine kinase activity as well as a different protein kinase activity sensitive to the potent kinase inhibitor, H7, and activated following p160 tyrosine phosphorylation. This H7-sensitive kinase appears to be distinct from any well-characterized protein kinase-second messenger system. On the basis of these findings, we propose that IL-6-induced signal transduction proceeds through a novel protein kinase cascade which activates junB and TIS11 gene transcription.

Repression by Jun of the Polyoma-virus enhancer overrides activation in a cell specific manner

The activities of promoters and enhancers are generated by the combinatorial effects of the factors which interact with them. The Polyoma virus (Py) enhancer contains sequences that are positively regulated by the proto-oncogene Jun. Surprisingly, Jun has an additional and overriding repressing effect on enhancer activity, which is cell specific. Thus overall enhancer activity cannot be simply deduced from the properties of individual elements. We present evidence that repression is indirect.

Activation of junB by PKC and PKA signal transduction through a novel cis-acting element

The product of the junB gene, a gene homologous to the proto-oncogene c-jun, is a component of transcription factor AP-1. JunB expression is modulated by a wide variety of extracellular stimuli, such as serum, growth factors, phorbol esters (TPA) and activators of protein kinase A (PKA). In order to study the molecular basis of this complex regulation, we have cloned the mouse junB gene from a genomic testis library, and characterized the junB promoter. Here we show that the junB promoter is activated by serum, TPA, and activated PKA. Sequences located between -91 and -44 are necessary for induction. These sequences contain a CAAT box, a G-C rich region and a previously undescribed inverted repeat (IR). The IR element can mediate induction by TPA and PKA when coupled to a heterologous promoter, and specifically binds a protein of 110 kD.

E1A dependent up-regulation of c-jun/AP-1 activity

E1A, the early region 1A transcription unit of human adenovirus, exhibits multiple functions that regulate the expression of some cellular genes and promote cell growth and division. We found that E1A stimulated c-jun gene expression at least fifty-fold in rat 3Y1 cells in a serum-independent manner, concomitantly with E1A down-regulation of jun B expression. The E1A-dependent induction of c-jun transcription resulted in increase amount of cJun/AP1. This induction was mediated by the enhancement of the binding activity of the transcription factor cJun/AP1 to an AP1 binding site in the c-jun promoter. Additionally, this induction can be repressed by introducing junB into the cells. Taken collectively, these results suggest that the differential expression of two closely related proteins greatly expands their cellular regulation. Induction of c-jun expression by E1A as well as c-jun autoregulation may amplify the action of E1A during adenovirus infection. Therefore, some of the biological effects of E1A may include mediating the constitutive activation of c-jun, which is important in transcriptional regulation and oncogenic transformation.

Activation of protein kinase C decreases phosphorylation of c-Jun at sites that negatively regulate its DNA-binding activity

In resting human epithelial and fibroblastic cells, c-Jun is phosphorylated on serine and threonine at five sites, three of which are phosphorylated in vitro by glycogen synthase kinase 3 (GSK-3). These three sites are nested within a single tryptic peptide located just upstream of the basic region of the c-Jun DNA-binding domain (residues 227-252). Activation of protein kinase C results in rapid, site-specific dephosphorylation of c-Jun at one or more of these three sites and is coincident with increased AP-1-binding activity. Phosphorylation of recombinant human c-Jun proteins in vitro by GSK-3 decreases their DNA-binding activity. Mutation of serine 243 to phenylalanine blocks phosphorylation of all three sites in vivo and increases the inherent trans-activation ability of c-Jun at least 10-fold. We propose that c-Jun is present in resting cells in a latent, phosphorylated form that can be activated by site-specific dephosphorylation in response to protein kinase C activation.

Rapid induction of the c-jun protooncogene in the avian oviduct by the antiestrogen tamoxifen

This report describes a rapid regulation of the expression of the c-jun protooncogene by the antiestrogen tamoxifen (Tam). The c-jun protooncogene codes for an important component of the AP-1 transcription factor complex, which regulates the expression of many unlinked genes. Repeated experiments have shown that Tam rapidly increases the steady-state c-jun mRNA levels in the avian oviduct but decreases the levels in the liver. The Tam effects are time- and dose-dependent. These results are supported by other studies that have demonstrated that 17 beta-estradiol decreases steady-state levels of c-jun protooncogene mRNA in oviducts of animals fully withdrawn from estradiol. The effect of Tam in the avian oviduct is in contrast to the reported effects of Tam on the expression of practically all other genes in the avian oviduct and other animal tissues. Transcription analyses using nuclear runoff experiments with oviduct nuclei demonstrate a decrease in the c-jun gene transcription within minutes after Tam treatment with a return to 75% of control values by 4 hr. The fact that Tam transiently decreases the transcription of the c-jun gene but increases the steady-state c-jun mRNA levels suggests that Tam must alter both transcriptional and post-transcriptional events. The results support a role of the c-jun protooncogene as a regulatory gene in the cascade model for steroid action whereby steroids rapidly regulate the regulatory genes, which in turn regulate many other structural genes.

Myristylation alters DNA-binding activity and transactivation of FBR (gag-fos) protein

FBR murine sarcoma virus (gag-fos) protein, a virally transduced Fos protein, exhibits decreased gene transactivation in comparison with the cellular Fos protein. Biochemical analysis suggests that myristylation of the virally encoded N-terminal gag region results in decreased DNA binding and transcriptional activation without affecting heterodimerization with Jun protein. These findings demonstrate that protein myristylation can modulate gene regulation by a DNA-binding protein.

An arginine to lysine substitution in the bZIP domain of an opaque-2 mutant in maize abolishes specific DNA binding

The opaque-2 (o2) locus in maize encodes a transcription factor involved in the regulation of zein storage proteins. We have shown previously that the O2 protein contains a leucine zipper domain that binds to promoters of 22-kD zein genes. In this paper we characterize an EMS-induced o2 allele, o2-676, that causes a 50% reduction in zein. We have found that the o2-676 mutant protein does not show specific recognition of zein promoter fragments because of the substitution of a lysine residue for an arginine residue within the bZIP domain of o2-676. This particular arginine is conserved within the bZIP domains of all mammalian, fungal, and plant DNA binding proteins of this class. The correlation between this mutation in o2 and the altered pattern of zein expression strongly suggests that O2 regulates transcription of certain members of the zein multigene family through direct interaction with the zein promoters and not through the transcriptional activation of some other regulator of zein gene expression.

Stage and tissue-specific expression of fosB during mouse development

The product of the fos-related fosB gene shares many properties with c-Fos such as inducibility by growth factors, complex formation with members of the Jun family and cooperative binding with Jun to the TPA response element (TRE). To investigate whether in contrast to these functional similarities, the two genes might be differentially regulated, we have analysed the expression of fosB during mouse development by in situ hybridization. A spatially restricted accumulation of fosB mRNA in the visceral yolk sac and the nervous system was observed during late gestation. The highest levels of fosB mRNA were found in the cortex and the dorsal columns of the spinal cord. Moreover, stage-specific expression was seen in sensory organs such as retina and vibrissae, where the levels of fosB RNA either increased (retina) or decreased (vibrissae) between days 15 and 18. Our results suggest that fosB may have a specific function in the development of ectoderm-derived tissues. Expression of fosB during prenatal development differs markedly from the known expression pattern of c-fos, pointing to different tissue-specific functions for c-fos and fosB.

The human leukemia cell line, THP-1: a multifacetted model for the study of monocyte-macrophage differentiation

THP-1 is a human monocytic leukemia cell line. After treatment with phorbol esters, THP-1 cells differentiate into macrophage-like cells which mimic native monocyte-derived macrophages in several respects. Compared to other human myeloid cell lines, such as HL-60, U937, KG-1, or HEL cell lines, differentiated THP-1 cells behave more like native monocyte-derived macrophages. Because of these characteristics, the THP-1 cell line provides a valuable model for studying the mechanisms involved in macrophage differentiation, and for exploring the regulation of macrophage-specific genes as they relate to physiological functions displayed by these cells.

Expression of c-jun, jun B and jun D proto-oncogenes in human peripheral-blood granulocytes

We have found that purified human peripheral-blood granulocytes express constitutively significant levels of proto-oncogenes c-jun, jun B and jun D mRNA. Upon functional activation of granulocytes by 4 beta-phorbol 12-myristate 13-acetate (PMA), the levels of c-jun, jun B and jun D transcripts were increased. The three jun genes showed a similar time course in their induction by PMA, maximal mRNA levels being reached after 60 min of induction. These results suggest that expression of c-jun, jun B and jun D genes might be involved in terminal granulocyte differentiation or in regulating granulocyte functionality.

N-methyl-D-aspartate receptor activation in the neostriatum increases c-fos and fos-related antigens selectively in medium-sized neurons

In the neostriatum a selective loss of neurons occurs following exposure to N-methyl-D-aspartate receptor agonists. One hypothesis emerging from this observation is that an excitotoxic process via N-methyl-D-aspartate receptors may contribute to the pathogenesis of Huntington's disease, which is characterized by the loss of medium-sized neurons. However, whether there is a selective distribution of N-methyl-D-aspartate receptors in specific populations of neostriatal neurons is unknown. In this study the expression of c-fos mRNA and protein was used to examine the response of neostriatal cells to N-methyl-D-aspartate receptor stimulation in the rat. After intrastriatal injection of the N-methyl-D-aspartate receptor agonist, quinolinic acid, an increase in c-fos mRNA concentrations was detected using in situ hybridization and Northern blot analysis. Western blot analysis showed that not only the c-Fos mRNA protein product but also other Fos-related antigens capable of binding to DNA were increased in response to N-methyl-D-aspartate receptor activation. The selectivity of the neuronal response to N-methyl-D-aspartate receptor activation was examined immunohistochemically at the light and ultrastructural levels. Our results indicate that N-methyl-D-aspartate receptor activation by quinolinic acid stimulates medium spiny neurons to increase c-Fos expression; to a lesser extent, medium aspiny interneurons and glial cells also respond. In contrast, negligible change in c-Fos expression is observed in large neurons. These results are consistent with other evidence that medium-sized spiny neurons are preferentially vulnerable to the toxic effects of excitatory amino acids acting at N-methyl-D-aspartate receptors. An additional implication of these findings is that activation of the N-methyl-D-aspartate receptor in medium spiny neurons leads to increased expression of candidate AP-1 transcription factors, thereby coupling the N-methyl-D-aspartate receptor and regulation of gene expression in signal transduction processes of the neostriatal medium spiny neuron.

Fission yeast genes that confer resistance to staurosporine encode an AP-1-like transcription factor and a protein kinase related to the mammalian ERK1/MAP2 and budding yeast FUS3 and KSS1 kinases

Staurosporine, a potent inhibitor of protein kinase C, arrests fission yeast cell elongation specifically at a stage immediately after cell division. We isolated two genes, which, when carried on multicopy plasmids, confer drug resistance in fission yeast. One, spk1+, encodes a protein kinase highly similar (54% identity) to those encoded by the mammalian ERK1/MAP2 kinase and the budding yeast KSS1 and FUS3 genes. It is not essential for vegetative growth of Schizosaccharomyces pombe cells but is required for conjugation. The spk1+ gene product is a 45-kD protein enriched in the nucleus, and its level increases 10-fold after addition of staurosporine. The other gene pap1+ encodes an AP-1-like transcription factor that contains a region rich in basic amino acids followed by a "leucine zipper" motif. The pap1+ gene is required for spk1(+)-conferred staurosporine resistance. These two genes appear to function as a part of the fission yeast growth control pathway.

Neurokinin A induces c-fos, c-jun, and c-myc expression in L6 rat myoblasts

Neurokinin A (NKA), a neuropeptide belonging to the tachykinin family, induced c-fos proto-oncogene mRNA expression in serum-deprived L6J1 rat skeletal myoblasts in vitro. The marked increase reached maximal levels after 15 to 30 min. In contrast to this, c-jun and c-myc proto-oncogene expression were only slightly induced, with peak levels after 30 min. NKA did not stimulate DNA synthesis or cell proliferation in serum-deprived L6J1 myoblasts. We demonstrate a relationship between NKA treatment and induction of c-fos, c-jun and c-myc mRNA expression in serum-deprived L6J1 rat myoblasts. The results on DNA synthesis and cell proliferation indicate that the induced proto-oncogene expression alone is not enough to induce a cellular response to NKA. Possible mechanisms of action are discussed.

Deprivation of a single amino acid induces protein synthesis-dependent increases in c-jun, c-myc, and ornithine decarboxylase mRNAs in Chinese hamster ovary cells

Genes of higher eucaryotic cells are considered to show only a limited response to nutritional stress. Here we show, however, that omission of a single essential amino acid from the medium caused a marked rise in the mRNA levels of c-myc, c-jun, junB and c-fos oncogenes and ornithine decarboxylase (ODC) in CHO cells. There was no general accumulation of mRNAs in amino acid-starved cells, since the gamma-actin, beta-tubulin, protein kinase C, RNA polymerase II, and glyceraldehyde-3-phosphate dehydrogenase mRNAs and the total poly(A)+ mRNA were not increased. The levels of c-myc, ODC, and c-jun mRNAs were elevated more by amino acid starvation than by inhibition of protein synthesis with cycloheximide, which is known to increase the levels of these mRNAs. Importantly, however, cycloheximide present during amino acid starvation reduced the rise in the levels of the mRNAs down to the level obtained with cycloheximide alone. This implies that protein synthesis is required for the accumulation of c-myc, ODC, and c-jun mRNAs in amino acid-deprived cells. The junB and c-fos mRNAs, instead, were increased to the same extent or less by amino acid starvation than by cycloheximide treatment. The accumulation of the c-myc mRNA in amino acid-starved cells was due to both stabilization of the mRNA and increase of its transcription. The rise in the c-jun mRNA level seemed to be caused merely by stabilization of the mRNA. Further, despite the inhibition of general protein synthesis, amino acid starvation led to an increase in the synthesis of c-myc polypeptide. The results suggest that mammalian cells have a specific mechanism for registering shortages of amino acids in order to make adjustments compatible with cellular growth.

Human immunodeficiency virus type 1 infection of U937 cells promotes cell differentiation and a new pathway of viral assembly

The differentiation of U937 monoblastoid cells after human immunodeficiency virus type 1 (HIV-1) infection was studied using the following approaches: reverse transcriptase activity measurement, immunofluorescence labeling, and electron microscopy. For comparison, uninfected U937 cells were induced to differentiate from monocyte to macrophage by phorbol 12-myristate 13-acetate (PMA) or retinoic acid (RA) treatment. Both infected and drug-treated cells showed important and similar ultrastructural cell modifications, with a phenotype that decreased in monocyte specificity and increased in that of macrophages. When U937 cells were induced to differentiate upon HIV-1 infection, a very different pathway of viral production was observed. Production and accumulation of the virus in a vacuolar compartment of intracytoplasmic origin and escape to the antiviral lysosomal activity could explain virus persistence. This makes the cell system a good model with which to study the relationship between HIV-1 production and cell differentiation.

Control of c-Jun activity by interaction of a cell-specific inhibitor with regulatory domain delta: differences between v- and c-Jun

Analysis of transcriptional activation properties of c-Jun chimeras in different cell lines suggests that it contains an activator domain (A1) that is negatively regulated by a cell type-specific inhibitor. A regulatory domain of c-Jun, delta, previously identified by in vitro experiments, also regulates transcriptional activation by c-Jun in vivo. The delta domain facilitates or stabilizes the interaction of the cellular inhibitor with A1. v-Jun, which lacks delta, is a stronger transcriptional activator than c-Jun, since its activity is not efficiently repressed by the cellular inhibitor. In vitro transcription with chimeric Jun proteins and extracts from different cell types confirms that the A1 and delta domains are repressed in a cell type-specific manner. These findings implicate a specific cellular factor in the negative regulation of c-Jun activity and suggest a molecular basis for the observed difference in transcriptional properties between v-Jun and c-Jun.

Induction of "General Control" and thermotolerance in cdc mutants of Saccharomyces cerevisiae

In Saccharomyces cerevisiae starvation for a single amino acid activates the transcription of a set of genes belonging to different amino acid biosynthetic pathways (General Control, GC). We show that mutants affected in GC regulation are also affected in their response to thermal stress. Moreover, growth conditions that are known to induce heat shock proteins induce the GC response. However, unlike heat shock proteins, the transcriptional activator of GC, GCN4, is not induced after a short exposure to heat, and in gcn mutant strains induction of heat resistance is normal.

Deprivation of a single amino acid induces protein synthesis-dependent increases in c-jun, c-myc, and ornithine decarboxylase mRNAs in Chinese hamster ovary cells

Genes of higher eucaryotic cells are considered to show only a limited response to nutritional stress. Here we show, however, that omission of a single essential amino acid from the medium caused a marked rise in the mRNA levels of c-myc, c-jun, junB and c-fos oncogenes and ornithine decarboxylase (ODC) in CHO cells. There was no general accumulation of mRNAs in amino acid-starved cells, since the gamma-actin, beta-tubulin, protein kinase C, RNA polymerase II, and glyceraldehyde-3-phosphate dehydrogenase mRNAs and the total poly(A)+ mRNA were not increased. The levels of c-myc, ODC, and c-jun mRNAs were elevated more by amino acid starvation than by inhibition of protein synthesis with cycloheximide, which is known to increase the levels of these mRNAs. Importantly, however, cycloheximide present during amino acid starvation reduced the rise in the levels of the mRNAs down to the level obtained with cycloheximide alone. This implies that protein synthesis is required for the accumulation of c-myc, ODC, and c-jun mRNAs in amino acid-deprived cells. The junB and c-fos mRNAs, instead, were increased to the same extent or less by amino acid starvation than by cycloheximide treatment. The accumulation of the c-myc mRNA in amino acid-starved cells was due to both stabilization of the mRNA and increase of its transcription. The rise in the c-jun mRNA level seemed to be caused merely by stabilization of the mRNA. Further, despite the inhibition of general protein synthesis, amino acid starvation led to an increase in the synthesis of c-myc polypeptide. The results suggest that mammalian cells have a specific mechanism for registering shortages of amino acids in order to make adjustments compatible with cellular growth.

Transcriptional regulation of c-jun gene expression by arabinofuranosylcytosine in human myeloid leukemia cells

Previous studies have demonstrated that 1-beta-D-arabinofuranosylcytosine (ara-C) induces terminal differentiation of human myeloid leukemia cells. Other studies have shown that the c-jun protooncogene is expressed during phorbol ester-induced myeloid differentiation. This work examines the effects of ara-C on c-jun gene expression in human KG-1 myeloid leukemia cells. The results demonstrate that c-jun transcripts are undetectable in uninduced KG-1 cells and that ara-C induces expression of this gene in a concentration- and time-dependent manner. Ara-C treatment was also associated with increases in c-jun transcripts in U-937, THP-1, and HL-60 myeloid leukemia cells. Furthermore, transcriptional run-on analysis has demonstrated that exposure to ara-C increases the rate of c-jun gene transcription. The results also demonstrate that while inhibition of protein synthesis superinduces c-jun mRNA levels in phorbol ester-treated KG-1 cells, cycloheximide had no effect on the induction of c-jun transcripts during ara-C treatment. Moreover, the half-life of c-jun transcripts in ara-C-treated KG-1 cells was 42 min. These findings suggest that the increase in c-jun mRNA observed during ara-C treatment is regulated by a transcriptional mechanism, and that c-jun may be involved in the induction of differentiation and regulation of gene expression by ara-C.

The molecular basis for inhibition of adipose conversion of murine 3T3-L1 cells by retinoic acid

The effect of retinoic acid (RA) on the adipose conversion of 3T3 cells has been studied. Differentiation of 3T3-L1 cells was initiated by addition of 0.5 mM methylisobutylxanthine, 0.3 microM dexamethasone and 10 micrograms/ml insulin (MDI) to confluent monolayers of preadipocytes for 48 h. During this time, the cells underwent DNA replication and cell division prior to the expression of adipose specific genes. RA administration had no apparent effect on the rate or extent of cell growth, cell division, or DNA replication. However, RA treatment concomitant with MDI addition inhibited triacylglycerol accumulation (I0.5 = 6 nM) and the accumulation of the differentiation-dependent mRNAs encoding the adipocyte lipid-binding protein (ALBP) and stearoyl-CoA desaturase 1 (SCD1). No inhibition occurred with RA addition either prior to or after MDI treatment. Runoff transcription revealed that the inhibitory effects of RA occurred at the level of transcription and were persistent. Cells treated with RA during the MDI regimen did not appreciably transcribe ALBP or SCD1 mRNAs several days following RA withdrawal. The effects of RA were specific for differentiation-dependent transcripts: 10(-6) M RA did not inhibit expression of the mRNAs encoding beta-tubulin or glutamine synthase. Examination of immediate-early transcription factor expression during the MDI regimen revealed that RA mediated an elevated, prolonged expression of c-Jun mRNA accompanied by diminished expression of c-Fos and Jun-B mRNAs. Given the previously demonstrated role of transcription factor AP-1 in ALBP gene expression, our results suggest that the initiation of expression of this and other adipocyte-specific genes during adipose conversion is regulated by the relative composition of transcription factor AP-1.

Evidence for DNA binding activity of numatrin (B23), a cell cycle-regulated nuclear matrix protein

Stimulation of various cell types with growth factors is associated with a rapid induction in the synthesis of a nuclear matrix protein, termed 'numatrin' which was shown to be identical to the nucleolar protein B23. The abundance of numatrin was shown to be correlated with entry and progression through the S-phase. Thus, experiments were undertaken to examine whether numatrin also has DNA binding activity. Using whole nuclear extract, we showed that numatrin binds to both double-stranded (DS) DNA and to single-stranded (SS) DNA cellulose columns. Purified numatrin, which was extracted either under native conditions (in oligomeric form) or under urea conditions (in monomeric form), demonstrated significant binding to either [3H]DS-DNA or [3H]DS-DNA as shown by nitrocellulose filter binding assay. However, numatrin binding to DS-DNA was qualitatively and quantitatively different from its binding to SS-DNA. Thus, the binding of numatrin was several fold higher to DS-DNA as compared to SS-DNA. The binding to DS-DNA was reduced by 77% in the presence of 0.5 M NaCl, while the binding to SS-DNA was not affected under this condition. Furthermore, treatment of the native numatrin under conditions which caused monomerization of the protein resulted in a significant enhancement of numatrin binding to SS-DNA but did not affect the binding to DS-DNA. Following heparin-Sepharose chromatography purification (under native conditions), numatrin at picomole amounts showed significant binding to both DS-DNA and SS-DNA. Finally, numatrin was found to copurify with the complex of DNA polymerase alpha primase together with other proteins required for SV-40 in vitro replication activity. These results demonstrate that numatrin has DNA binding activity, and imply a possible role for numatrin/B23 in DNA-associated processes.

Induction of the proto-oncogene c-jun by angiotensin II

Angiotensin (Ang) II causes hypertrophy of rat aortic smooth muscle cells in culture and results in the rapid activation of c-fos. This study demonstrated that Ang II also activated c-jun and, in addition, could activate the AP-1 enhancer element. These data add support for a role of Ang II as an important mediator of vascular smooth muscle cell growth.

The leucine zipper of c-Myc is required for full inhibition of erythroleukemia differentiation

The leucine zipper motif has been observed in a number of proteins thought to function as eucaryotic transcription factors. Mutation of the leucine zipper interferes with protein dimerization and DNA binding. We examined the effect of point mutations in the leucine zipper of c-Myc on its ability to dimerize in vitro and to inhibit Friend murine erythroleukemia (F-MEL) differentiation. Glutaraldehyde cross-linking studies failed to provide evidence for homodimerization of in vitro-synthesized c-Myc protein, although it was readily demonstrated for c-Jun. Nevertheless, whereas transfected wild-type c-myc sequences strongly inhibited F-MEL differentiation, those with single or multiple mutations in the leucine zipper were only partially effective in this regard. Since the leucine zipper domain of c-Myc is essential for its cooperative effect in ras oncogene-mediated transformation, this study emphasizes the close relationship that exists between transformation and hematopoietic commitment and differentiation. c-Myc may produce its effects on F-MEL differentiation through leucine zipper-mediated heterodimeric associations rather than homodimeric ones.

The interaction of a c-Jun/Fos related protein factor with the U3 sequences of the mouse mammary tumor virus LTR

Using polyacrylamide gel mobility shift assay we have detected a nuclear factor (NF-S) in a mouse mammary tumor cell line (GR) that binds to an upstream sequence domain (-766 to -737) near the 5'-end of the mouse mammary tumor virus (MMTV) long terminal repeat (LTR). Antibodies to the products of the Jun and Fos oncogenes interfered with the binding potential of this factor, indicating that the factor shares antigenic determinants with c-Jun/AP-1. In vitro translated c-Jun and c-Fos were also found to bind to the NF-S binding domain consisting of the sequence TGA(A/G)TCA that are known to be recognized by c-Jun/AP-1. Our results raise the possibility that the MMTV-LTR sequence element-766 to -737 by interacting with a Jun/Fos related protein play a role in MMTV transcription and/or the activation of int protooncogenes that are associated with murine mammary tumorigenesis.

Cooperation of v-jun and v-erbB oncogenes in embryo fibroblast transformation in vitro and in vivo

Retroviral vectors carrying either the v-jun and v-erbB sequences or the v-jun gene linked to the neomycin resistance gene were constructed on the basis of the structural genome organization of avian erythroblastosis virus (AEV). These viruses, called JB and JN, respectively, were rescued as Rous-associated virus-1 pseudotypes, and they were shown to successfully transform chicken embryo fibroblasts in vitro. However, in agar, colonies developed from JB-infected fibroblasts were three to five times larger than those obtained after infection with JN or with AEV Pst124 carrying only a functional v-erbB gene. In vivo, on chorioallantoic membrane (CAM) assays, JB produced fibrosarcomas that were more rapidly growing and much larger than those induced by JN or AEV Pst124. Moreover, in chickens infected in ovo with JB, multiple fibrosarcomas arose in different organs a few days after birth, whereas no tumor could be detected in parallel experiments in either JN- or AEV Pst124-infected animals. These results demonstrate that in embryo fibroblast cells, v-jun and v-erbB can act synergistically to enhance the transformation potential of either oncogene alone both in vitro and in vivo.

Efficient transformation of chicken embryo fibroblasts by c-Jun requires structural modification in coding and noncoding sequences

To assess the transforming capability of the c-Jun protein, we introduced the chicken c-jun proto-oncogene into a replication competent avian retroviral expression vector (RCAS). Viral Jun efficiently transformed chicken embryo fibroblasts (CEFs) when expressed from this vector. Overexpression of c-Jun leads to transformation of CEFs with an efficiency that is 15- to 25-fold less than that seen for v-Jun, suggesting that v-Jun contains structural features that increase its oncogenic potential relative to c-Jun. There are four structural differences between v-Jun and c-Jun. To determine the relative contribution that each of these structural differences between v-Jun and c-Jun has on oncogenic activity, several deletion and substitution mutants were constructed. Each of these mutants was expressed in CEF and assayed for transformation by focus formation. Analysis of the results reveals that deletion of a region of 27 amino acids near the amino terminus of c-Jun and deletion of 3'-untranslated sequences are critical in activating the full oncogenic potential of Jun.

Induction of the proto-oncogene c-jun by angiotensin II

Angiotensin (Ang) II causes hypertrophy of rat aortic smooth muscle cells in culture and results in the rapid activation of c-fos. This study demonstrated that Ang II also activated c-jun and, in addition, could activate the AP-1 enhancer element. These data add support for a role of Ang II as an important mediator of vascular smooth muscle cell growth.

The leucine zipper of c-Myc is required for full inhibition of erythroleukemia differentiation

The leucine zipper motif has been observed in a number of proteins thought to function as eucaryotic transcription factors. Mutation of the leucine zipper interferes with protein dimerization and DNA binding. We examined the effect of point mutations in the leucine zipper of c-Myc on its ability to dimerize in vitro and to inhibit Friend murine erythroleukemia (F-MEL) differentiation. Glutaraldehyde cross-linking studies failed to provide evidence for homodimerization of in vitro-synthesized c-Myc protein, although it was readily demonstrated for c-Jun. Nevertheless, whereas transfected wild-type c-myc sequences strongly inhibited F-MEL differentiation, those with single or multiple mutations in the leucine zipper were only partially effective in this regard. Since the leucine zipper domain of c-Myc is essential for its cooperative effect in ras oncogene-mediated transformation, this study emphasizes the close relationship that exists between transformation and hematopoietic commitment and differentiation. c-Myc may produce its effects on F-MEL differentiation through leucine zipper-mediated heterodimeric associations rather than homodimeric ones.

Purification of a mouse nuclear factor that binds to both the A and B cores of the polyomavirus enhancer

We have previously identified a protein factor, PEBP2 (polyomavirus enhancer-binding protein), in the nuclear extract from mouse NIH 3T3 cells which binds to the sequence motif, PEA2, located within the polyomavirus enhancer A element. Upon cellular transformation with activated oncogene c-Ha-ras, this factor frequently undergoes drastic molecular modifications into an altered form having a considerably reduced molecular size. In this study, the altered form, PEBP3, was purified to near homogeneity. The purified PEBP3 comprised two sets of families of polypeptides, alpha-1 to alpha-4 and beta-1 to beta-2, which were 30 to 35 kilodaltons and 20 to 25 kilodaltons in size, respectively. Both kinds of polypeptides possessed DNA-binding activities with exactly the same sequence specificity. Individual alpha or beta polypeptides complexed with DNA showed faster gel mobilities than did PEBP3. However, the original gel retardation pattern was restored when alpha and beta polypeptides were mixed together in any arbitrary pair. These observation along with the results of UV- and chemical-cross-linking studies led us to conclude that PEBP3 is a heterodimer of alpha and beta subunits, potentially having a divalent DNA-binding activity. Furthermore, PEBP3 was found to bind a second, hitherto-unnoticed site of the polyomavirus enhancer that is located within the B element and coincides with the sequence previously known as the simian virus 40 enhancer core homology. From comparison of this and the original binding sites, the consensus sequence for PEBP3 was defined to be PuACCPuCA. These findings provided new insights into the biological significance of PEBP3 and PEBP2.

Domain swapping reveals the modular nature of Fos, Jun, and CREB proteins

The products of the Jun and Fos proto-oncogenes form a heterodimer that binds to and activates transcription from 12-O-tetradecanoylphorbol-13-acetate-responsive promoter elements (TGACTCA) and AP-1-binding sites (TGACATCA). These two proteins belong to a family of related transcription factors which contain similar domains required for protein dimerization and DNA binding but display different protein and DNA binding specificities. The basic region, required for DNA binding, is followed by a leucine zipper structure, a domain that mediates protein-protein interactions. To assess the role of these two domains in three related proteins, Fos, Jun, and CREB, we carried out extensive domain-swapping analysis. We found that (i) dimers formed by two Jun leucine zipper-containing proteins were unable to bind DNA as efficiently as a Fos-Jun combination, regardless of the source of the basic region; (ii) the Fos leucine zipper was unable to form either homo- or heterodimers with a chimeric protein containing a Fos leucine zipper; (iii) the Fos basic region was capable of binding to an AP-1 site; (iv) replacement of the Jun amino terminus with that of CREB had little effect on dimerization, whereas replacement with the amino terminus of Fos disrupted both protein-protein and protein-DNA interactions; (v) changes in relative affinities of the Fos and Jun basic regions for the AP-1 element were dependent on the secondary contributions of amino-terminal residues; and (vi) the Fos-Jun chimeric constructs cooperated in transcriptional transactivation of the Jun promoter in NIH 3T3 cells.

Transcriptional interference between c-Jun and the glucocorticoid receptor: mutual inhibition of DNA binding due to direct protein-protein interaction

Glucocorticoids are potent inhibitors of collagenase induction by phorbol esters and inflammatory mediators. The target for this negative effect is the AP-1 site within the collagenase promoter, which also mediates its induction. Negative regulation is due to repression of AP-1 activity by the glucocorticoid receptor (GCR). While the GCR is a potent inhibitor of AP-1 activity (Jun/Fos), both c-Jun and c-Fos are potent repressors of GCR activity. In vitro experiments using purified GCR and c-Jun proteins suggest that mutual repression is due to direct interaction between the two. Direct interaction between GCR and either c-Jun or c-Fos is demonstrated by cross-linking and coimmunoprecipitation. These findings reveal a cross talk between two major signal transduction systems used to control gene transcription in response to extracellular stimuli, and a novel mechanism for transcriptional repression.

Antitumor promotion and antiinflammation: down-modulation of AP-1 (Fos/Jun) activity by glucocorticoid hormone

Glucocorticoid hormones counteract inflammation and phorbol ester tumor promotion and drastically decrease the expression of several extracellular proteases, including collagenase I. Glucocorticoid hormone inhibits basal and induced transcription of collagenase by interfering with AP-1, the major enhancer factor of the collagenase promoter. The mechanism of interference is novel in that it does not require protein synthesis, it depends on the hormone receptor but not its binding to DNA, it occurs at hormone doses one order of magnitude below those required for gene activation, and it involves down-modulation of the trans-activating function of preexisting unbound and DNA-bound AP-1. Coprecipitation experiments suggest direct AP-1-hormone receptor interaction, which also possibly explains the reverse experiment: overexpression of Fos or Jun inhibits the expression of hormone-dependent genes.

Functional antagonism between oncoprotein c-Jun and the glucocorticoid receptor

We present evidence that the glucocorticoid receptor (GR) and transcription factor Jun/AP-1 can reciprocally repress one another's transcriptional activation by a novel mechanism that is independent of DNA binding. Overexpression of c-Jun prevents the glucocorticoid-induced activation of genes carrying a functional glucocorticoid response element (GRE). Conversely, GR is able to repress AP-1-mediated transcriptional activation. Mutant analysis reveals that the ligand binding and DNA binding domains of GR and the region including the leucine zipper of c-Jun are required for repression. Gel retardation analysis demonstrates that bacterially expressed c-Jun disrupts GR-GRE complexes. These data indicate that members of two distinct classes of transcription factors can oppose one another's activity through a mechanism likely involving protein-protein interactions.

Increased expression of c-jun gene during spontaneous hepatocarcinogenesis in LEC rats

We have studied the expressions of nine proto-oncogenes (c-myc, N-myc, c-fos, C-jun, p53, H-ras, N-ras, c-raf, hst) and two other genes (PCNA, GST-P) during the spontaneous development of hepatocellular carcinomas (HCCs) in LEC rats. Expression of c-myc, H-ras, N-ras, C-raf, p53 and PCNA genes was detected, but this did not significantly change during the development of HCCs in LEC rats. Expression of N-myc and hst genes was not detectable. Expression of c-fos gene was detected in one HCC case out of four. Significantly increased expression of c-jun gene was observed in the liver tissues of LEC rats aged 8 months. This high expression was decreased with the development of HCCs. On the other hand, the expression of GST-P gene increased in parallel with the clinical course of the development of HCCs in LEC rats. The pattern of c-jun mRNA augmentation was different from that of GST-P mRNA. These observations suggest that c-jun gene may play a role in the spontaneous development of HCCs in LEC rats.

Hepadnavirus enhancer and its binding proteins

In the hepadnavirus enhancer region, a 33 bp DNA sequence is strongly conserved among mammalian hepadnavirus genomes. To elucidate the role of the sequence, we tested enhancer activities and capability to form DNA-protein complex of several synthetic DNAs. Not only two tandem copies of a 46 bp DNA covering the sequence but also two tandem copies of a 23 bp in the sequence exhibit enhancer activity. Also the activity was augmented by treatment of a tumor promoter, TPA. DNA binding proteins complexes with the 23 bp DNA were augmented in extracts of HepG2 or HeLa cells stimulated with TPA. These results imply that the conserved sequence of hepadnavirus enhancer is a TPA-inducible enhancer which is transactivated by ubiquitous DNA-binding proteins. We presented results showing that DNA-protein complexes with a 23 bp DNA are similar to but distinct from those with a TPA-responsive element DNA, the recognition site for c-jun/fos products. We also presented results suggesting that hepadnavirus X protein may not directly or indirectly affect DNA-protein complex formation with the conserved sequence in the hepadnavirus enhancer.

Domain swapping reveals the modular nature of Fos, Jun, and CREB proteins

The products of the Jun and Fos proto-oncogenes form a heterodimer that binds to and activates transcription from 12-O-tetradecanoylphorbol-13-acetate-responsive promoter elements (TGACTCA) and AP-1-binding sites (TGACATCA). These two proteins belong to a family of related transcription factors which contain similar domains required for protein dimerization and DNA binding but display different protein and DNA binding specificities. The basic region, required for DNA binding, is followed by a leucine zipper structure, a domain that mediates protein-protein interactions. To assess the role of these two domains in three related proteins, Fos, Jun, and CREB, we carried out extensive domain-swapping analysis. We found that (i) dimers formed by two Jun leucine zipper-containing proteins were unable to bind DNA as efficiently as a Fos-Jun combination, regardless of the source of the basic region; (ii) the Fos leucine zipper was unable to form either homo- or heterodimers with a chimeric protein containing a Fos leucine zipper; (iii) the Fos basic region was capable of binding to an AP-1 site; (iv) replacement of the Jun amino terminus with that of CREB had little effect on dimerization, whereas replacement with the amino terminus of Fos disrupted both protein-protein and protein-DNA interactions; (v) changes in relative affinities of the Fos and Jun basic regions for the AP-1 element were dependent on the secondary contributions of amino-terminal residues; and (vi) the Fos-Jun chimeric constructs cooperated in transcriptional transactivation of the Jun promoter in NIH 3T3 cells.

CREB regulation of cellular cyclic AMP-responsive and adenovirus early promoters

The cyclic AMP (cAMP) response element-binding protein (CREB) has been demonstrated to be a key mediator of cellular promoter response to cAMP. The binding site for this protein in many cellular cAMP inducible promoters (CRE) contains the palindrome sequence TGACGTCA, which contains two half-sites for CREB binding. A related promoter element, with the core sequence TGACG, has significant homology to an AP1-binding site and contains only one half-site for CREB binding. A group of factors known as activating transcription factors (ATF) have been found to bind to the latter and related sequences found upstream of early adenovirus promoters induced by E1A, and these factors are highly homologous to the CREB protein. We wished to characterize CREB, c-jun, and c-fos binding to these sites in the somatostatin gene (CRE) and in the adenovirus early region 3 promoter (E3/ATF). Oligonucleotides complementary to each of these sites were used in gel retardation assays with in vitro-translated CREB protein. These studies indicated that CREB bound primarily as a dimer to both a single and two half-sites, though there was increased affinity to the double compared with the single half-site. The c-jun and c-fos proteins also bound to both the somatostatin CRE- and E3/ATF-binding sites, but CREB did not bind to AP1 recognition sites nor was it capable of forming heterodimers with either c-jun or c-fos. Truncations of the CREB protein, which eliminated regions of the protein containing consensus sites for phosphorylation by protein kinase A, protein kinase C, and casein kinase II, bound to both the CRE and ATF sites, indicating that these consensus sites were not essential for DNA binding or dimer formation. Transfection of CREB and protein kinase A expression constructs into F9 cells with promoters containing either a single or two half-sites for CREB binding indicated that CREB was capable of similar levels of activation of these constructs. However, the fold activation by CREB was higher for constructs containing a single half-site compared with those containing two half-sites. These results demonstrate that multiple mechanisms may regulate CREB binding, including variations in the sequences in the promoter-binding site and the presence of related DNA-binding proteins.