Ha-Ras augments c-Jun activity and stimulates phosphorylation of its activation domain

Ha-Ras augments c-Jun-mediated transactivation by potentiating the activity of the c-Jun activation domain. Ha-Ras also causes a corresponding increase in phosphorylation of specific sites in that part of the c-Jun protein. A Ha-Ras-induced protein kinase cascade resulting in hyperphosphorylation of the c-Jun activation domain could explain how these oncoproteins cooperate to transform rat embryo fibroblasts.

Rapid and preferential activation of the c-jun gene during the mammalian UV response

Exposure of mammalian cells to DNA-damaging agents leads to activation of a genetic response known as the UV response. Because several previously identified UV-inducible genes contain AP-1 binding sites within their promoters, we investigated the induction of AP-1 activity by DNA-damaging agents. We found that expression of both c-jun and c-fos, which encode proteins that participate in formation of the AP-1 complex, is rapidly induced by two different DNA-damaging agents: UV and H2O2. Interestingly, the c-jun gene is far more responsive to UV than any other immediate-early gene that was examined, including c-fos. Other jun and fos genes were only marginally affected by UV or H2O2. Furthermore, UV is a much more efficient inducer of c-jun than phorbol esters, the standard inducers of c-jun expression. This preferential response of the c-jun gene is mediated by its 5' control region and requires the TPA response element, suggesting that this element also serves as an early target for the signal transduction pathway elicited by DNA damage. Both UV and H2O2 lead to a long-lasting increase in AP-1 binding activity, suggesting that AP-1 may mediate the induction of other damage-inducible genes such as human collagenase.

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.

A simplified method for the preparation of transcriptionally active liver nuclear extracts

We have developed a simplified method for the preparation of liver nuclear extracts to study gene regulation and protein-DNA interactions. This protocol uses conventional laboratory equipment and standard reagents. The liver tissue is homogenized in a low-salt solution at physiological molarity with subsequent adjustment of the molarity and purification of nuclei by density sedimentation. The nuclear extracts are transcriptionally active in a validated cell-free transcription assay and contain functional DNA-binding proteins. This protocol results in the rapid preparation of highly reproducible and active liver nuclear extracts.

Human metallothionein genes: structure of the functional locus at 16q13

The functional human metallothionein (MT) genes are located on chromosome 16q13. We have physically mapped the functional human MT locus by isolation and restriction digest mapping of cloned DNA. The mapped region contains all sequences on chromosome 16 that hybridize to metallothionein gene probes and comprises 14 tightly linked MT genes, 6 of which have not been previously described. This analysis defines the genetic limits of metallothionein functional diversity in the human genome.

Multiple doses of diacylglycerol and calcium ionophore are necessary to activate AP-1 enhancer activity and induce markers of macrophage differentiation

In contrast to phorbol esters, multiple doses of diacylgycerols are needed to differentiate U937 human monoblastic leukemic cells to a macrophage-like phenotype. Although both of these agents similarly activate protein kinase C in vitro, it is not known why these agents appear to have differing biologic effects. One possibility is that they regulate gene transcription in slightly different ways. Regulation of gene transcription by phorbol esters is complex and involves the stimulation of the transactivating proteins Jun and Fos which form dimers and bind to the AP-1 enhancer elements (5'-TGAGTCA-3'). To understand whether diacylglycerols regulate gene transcription similarly to phorbol esters and to examine whether activation of AP-1 enhancer activity is correlated with differentiation, we have treated U937 human monoblastic leukemic cells with these agents and examined activation of transcription from AP-1 enhancer elements. We find that, although a single dose of diacylglycerol, like phorbol esters, is sufficient to elevate mRNA levels of both the c-jun and c-fos protooncogenes, in contrast to phorbol esters there is no increase in either Jun protein or activation of AP-1 enhancer activity. However, multiple doses of this agent given over 24 h stimulate repeated elevations in c-jun and c-fos mRNA, increases in Jun protein, and enhancer activation. Treatment of U937 cells with ionomycin, a calcium ionophore, also stimulates an increase in c-jun mRNA, but neither activates AP-1 enhancer activity nor stimulates differentiation of these cells. However ionomycin functions to enhance the effects of diacylglycerols both on transcriptional activation and U937 differentiation. These results suggest a complex regulation of AP-1 enhancer activity in U937 cells by diacylglycerols involving both transcriptional and post-transcriptional regulatory mechanisms. Maximal activation of AP-1 enhancer elements, and not changes in jun and fos mRNA, is correlated with increases in markers of U937 differentiation. These changes may be important in the early events leading to differentiation of hematopoietic 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.

A single amino acid change in CUP2 alters its mode of DNA binding

CUP2 is a copper-dependent transcriptional activator of the yeast CUP1 metallothionein gene. In the presence of Cu+ and Ag+) ions its DNA-binding domain is thought to fold as a cysteine-coordinated Cu cluster which recognizes the palindromic CUP1 upstream activation sequence (UASc). Using mobility shift, methylation interference, and DNase I and hydroxyl radical footprinting assays, we examined the interaction of wild-type and variant CUP2 proteins produced in Escherichia coli with the UASc. Our results suggest that CUP2 has a complex Cu-coordinated DNA-binding domain containing different parts that function as DNA-binding elements recognizing distinct sequence motifs embedded within the UASc. A single-amino-acid substitution of cysteine 11 with a tyrosine results in decreased Cu binding, apparent inactivation of one of the DNA-binding elements and a dramatic change in the recognition properties of CUP2. This variant protein interacts with only one part of the wild-type site and prefers to bind to a different half-site from the wild-type protein. Although the variant has about 10% of wild-type DNA-binding activity, it appears to be completely incapable of activating transcription.

Interleukin-1 stimulates and all-trans-retinoic acid inhibits collagenase gene expression through its 5' activator protein-1-binding site

Collagenase production by synovial fibroblast-like cells (synoviocytes) plays a major role in cartilage and bone destruction in rheumatoid arthritis. Interleukin-1 (IL-1) increases collagenase secretion by elevating the steady state levels of collagenase mRNA in cultured rheumatoid synoviocytes, while all-trans-retinoic acid (RA) has the opposite effect. We have studied the regulation of collagenase gene transcription by IL-1 and RA in synoviocytes by transient transfection of plasmid constructs containing deletion mutants of the 5'-flanking region of the collagenase gene or the isolated phorbol ester-responsive element ligated to a chloramphenicol acetyltransferase reporter gene. We show that the phorbol ester-responsive element of the collagenase gene mediates both positive and negative regulatory effects, respectively, of IL-1 and RA on transcription. In addition, we show that IL-1 and 12-O-tetradecanoyl-phorbol-13-acetate transiently induce c-jun and c-fos expression and that retinoic acid inhibits IL-1 and 12-O-tetradecanoyl-phorbol-13-acetate induction of c-fos, but not c-jun. These results suggest that RA inhibits collagenase transcription at least in part through inhibition of c-fos.

Regulation of the pituitary-specific homeobox gene GHF1 by cell-autonomous and environmental cues

Homeodomain proteins function in determination of mating type in yeast, segmentation in fruit flies and cell-type specific gene expression in mammals. In Drosophila, expression of homeobox genes is controlled by cell-autonomous interactions between regulatory proteins and environmental clues. Similar controls may operate during mammalian limb development and frog embryogenesis. But, the exact way in which expression of homeodomain proteins is regulated in these systems is not clear and requires biochemical analysis of homeobox gene transcription. We now describe such an analysis of the GHF1 gene, which encodes a mammalian homeodomain protein specifying expression of the growth hormone (GH) gene in anterior pituitary somatotrophs. GHF1 is transcribed in a highly restricted manner and the presence of GHF1 protein is correlated both temporally and spatially with activation of the GH gene during pituitary development. Analysis of the GHF1 promoter indicates that transcription is also controlled by cell-autonomous interactions involving positive autoregulation by GHF1, and environmental cues that modulate the intracellular level of cyclic AMP and thereby the activity of cAMP response element binding protein (CREB), a ubiquitous transactivator that binds to the GHF1 promoter.

A novel T-cell trans-activator that recognizes a phorbol ester-inducible element of the interleukin-2 promoter

The interleukin 2 (IL-2) gene promoter is recognized by several cell-type-specific and ubiquitous transcriptional regulators that integrate information transmitted by various signaling systems leading to IL-2 production and T-cell activation. Using a combination of transfection, protein-DNA binding, and in vitro transcription methods, we have discovered the novel T-cell-specific transcriptional activator TCF-1 (for T-Cell Factor-1), which recognizes a T-cell-specific response element (TCE) located within the IL-2 promoter. Although the TCE is similar in sequence to a consensus NF kappa B site, several criteria indicate that TCF-1 is distinct from NF kappa B. However, like NF kappa B, TCF-1 activity is induced by phorbol esters and other T-cell activators.

DNA-binding activity of Jun is increased through its interaction with Fos

Transcription factor AP-1 mediates induction of a set of genes in response to the phorbol ester tumor promoter TPA. Recently, AP-1 preparations from HeLa cells were shown to contain a product of the c-JUN protooncogene (Jun/AP-1) which forms a tight complex with the Fos protein. In this paper, we examine the role of the Fos protein in the DNA-binding activity of the AP-1 complex. We show that the DNA-binding activity of bacterially expressed trpE-Jun fusion proteins is increased many-fold upon their interaction with Fos (or a Fos-related antigen) expressed from a baculovirus vector. The site of Fos interaction is within the DNA-binding domain of Jun/AP-1, and anti-Fos antibodies interfere with the binding of affinity purified AP-1 to DNA. These results suggest that, by associating with Jun/AP-1, Fos is responsible for the formation of a multimeric protein complex that has greater affinity for the target sequence than does Jun/AP-1 alone.

Autoinduction of transforming growth factor beta 1 is mediated by the AP-1 complex

The multifunctional actions of transforming growth factor beta 1 (TGF-beta 1) indicate that it has a pivotal control function in many physiological and pathological processes. An important property of TGF-beta 1 is its ability to activate its own mRNA expression and thereby increase its own secretion. Two distinct regions of the promoter of the TGF-beta 1 gene are responsive to autoregulation: one 5' to the upstream transcriptional start site and another located between the two major start sites. In both promoter regions, autoinduction is mediated by binding of the AP-1 (Jun-Fos) complex. An important contribution to this positive regulation is the autoactivation of c-jun transcription by AP-1. Cotransfection of antisense c-jun or antisense c-fos expression vectors prevents TGF-beta 1 autoinduction. These results demonstrate that both components of the AP-1 complex are required for TGF-beta 1 autoinduction. Induction of jun expression by TGF-beta 1, as well as jun autoinduction, may amplify the action of TGF-beta 1 during normal development and oncogenesis.

Elevation of AP1 activity during F9 cell differentiation is due to increased c-jun transcription

The regulation of jun family genes and AP1 activity during the course of differentiation of F9 embryonal carcinoma stem cells was investigated. The induction of differentiation by retinoic acid (RA) leads to an accumulation of c-jun mRNA caused by increased c-jun transcription. This induction is an indirect response to RA and requires a functional AP1 binding site within the c-jun promoter. Expression of jun-B mRNA, however, is transiently induced but at a later time point is repressed by RA. The third member of the family, jun-D, is already active in undifferentiated cells and is only slightly induced after differentiation. Differentiation also converts c-jun from being refractory to phorbol esters to a highly inducible state. The development of this response is correlated with increased AP1 activity in RA-treated cells. By contrast, the induction of c-fos by phorbol esters or cAMP is greatly diminished after RA treatment. Transfection experiments indicate that, in the absence of c-Fos, only c-Jun is an effective transactivator. Hence, the major increase in AP1 activity is due to elevated c-jun expression and probably involves positive autoregulation by the c-Jun protein. Furthermore, these results demonstrate that AP1 activity can be stimulated by phorbol ester without concomitant c-fos induction. Forced expression of c-Jun and v-Jun results in activation of at least two differentiation marker genes, EndoB and tissue plasminogen activator, whose regulatory regions contain AP1 binding sites. Thus, the induction of c-jun transcription by RA, although indirect, can have an important role in the differentiation process.

Overexpressed human metallothionein IIA gene protects Chinese hamster ovary cells from killing by alkylating agents

Experiments were designed to detect survival advantages that cells gain by overexpressing metallothionein (MT). Chinese hamster ovary K1-2 cells and an x-ray-sensitive derivative were transfected with a bovine papillomavirus (BPV)-linked construct carrying the human metallothionein IIA (hMT-IIA) gene. Transfectants survived 40-fold higher levels of cadmium chloride, harbored at least 30 copies of hMT-IIA, and contained 25- to 166-fold more MT than the parent cells. Even under conditions of reduced glutathione synthesis, the transfectants were not more resistant to the lethal effects of ionizing radiation and bleomycin than the parent cells. Thus free radicals generated by these agents cannot be scavenged efficiently by MT in vivo. The hMT-IIA transfectants, however, but not control transfectants harboring a BPV-MT promoter-neo construct, tolerated significantly higher doses of the alkylating agents N-methyl-N-nitrosourea and N-methyl-N'-nitro-N-nitrosoguanidine. Resistance and MT overexpression occurred irrespective of selection and cultivation in cadmium and zinc. There was no increase in resistance to methyl methanesulfonate and N-hydroxyethyl-N-chloroethylnitrosourea. MT did not affect the degree of overall DNA methylation after N-methyl-N-nitrosourea treatment nor the level of O6-methylguanine-DNA methyltransferase. The results suggest that MT participates as a cofactor or regulatory element in repair or tolerance of toxic alkylation lesions.

Expression of GHF-1 protein in mouse pituitaries correlates both temporally and spatially with the onset of growth hormone gene activity

The relationship between expression of the pituitary-specific transcription factor, GHF-1, and activation of the growth hormone and prolactin genes during mouse anterior pituitary development was investigated. While GHF-1 transcripts were detected within 24 hr of the first observable events in anterior pituitary differentiation, no GHF-1 protein could be detected until about 3 days later. The appearance of GHF-1 protein showed good temporal and spatial correlation with activation of the growth hormone gene. Prolactin gene expression, on the other hand, was observed transiently during embryonic day 16 in two different populations of cells, of which the major one does not contain GHF-1 or growth hormone. These results suggest that expression of GHF-1 is controlled both transcriptionally and posttranscriptionally. The spatial and temporal correlation between the appearance of GHF-1 protein and growth hormone gene activation suggests that GHF-1 is responsible for this very last step in the specialization of somatotrophic cells.

Growth hormone gene regulation: a paradigm for cell-type-specific gene activation

The growth hormone (GH) gene is specifically expressed within specialized cells of the anterior pituitary. The central role in GH gene activation is played by GHF-1, a homeodomain protein that is itself specifically expressed in the anterior pituitary.

Regulation of collagenase gene expression by okadaic acid, an inhibitor of protein phosphatases

Human collagenase gene expression is regulated transcriptionally and is inducible by various mitogens in many cell types. To investigate the molecular mechanisms of this response, we examined the effects on collagenase gene expression of okadaic acid, a non-12-O-tetradecanoyl-phorbol-13-acetate (TPA)-type tumor promoter, which induces apparent "activation" of protein kinases by inhibition of protein phosphatases. Steady state levels of collagenase mRNA were markedly increased by okadaic acid treatment. We show that the AP-1 consensus sequence in the collagenase promoter is required for the induction of collagenase gene expression by okadaic acid, even though sequences upstream of the AP-1 consensus site have an additive effect. We also examined the regulation by okadaic acid of expression of the components of the AP-1 complex, c-fos and c-jun. c-fos expression is dramatically stimulated by okadaic acid, whereas c-jun expression is stimulated to a lesser extent. Induction of c-fos gene mRNA occurs through a region known to contain multiple regulatory elements. These results suggest that phosphorylation regulates collagenase gene expression mediated by an AP-1 binding site.

Too many transcription factors: positive and negative interactions

Eukaryotic transcription factors can be classified into several families on the basis of conserved sequences among their DNA-binding domains. Because of such structural conservation, several different trans-acting factors can often interact with a common binding site. Recent findings reviewed herein indicate that the interaction of different factors with a common target site does not necessarily result in equivalent transcriptional responses. While some factors activate transcription, others that bind to the same site repress this process. The examples described here illustrate an emerging new mode of cellular regulation mediated by closely related but functionally distinct transcription factors that appear to compete for common binding sites.

Jun-B differs in its biological properties from, and is a negative regulator of, c-Jun

c-Jun, Jun-B, and Jun-D proteins bind to the TPA response element (TRE) either as homodimers or as Jun-Fos heterodimers. We demonstrate that c-Jun and Jun-B nevertheless differ markedly in their ability to activate AP-1 responsive genes. c-Jun is an efficient activator of the c-jun and collagenase promoters, which contain a single TRE; Jun-B is not. Furthermore, Jun-B inhibits activation of these promoters by c-Jun. On the other hand, like c-Jun, Jun-B is an efficient activator of constructs containing multimeric TREs. Using chimeric proteins, we show that the distinct behavior of c-Jun and Jun-B is due to differences in their activation domains. Trans-activation by Jun-B depends on cooperative interactions between adjacently bound factors, while activation by c-Jun does not require such interactions. This differential behavior greatly expands the regulatory potential of the Jun family.

Dissection of functional domains of the pituitary-specific transcription factor GHF-1

The specific expression of growth hormone (GH) in the somatotrophic cells of the anterior pituitary is largely attributable to a short promoter in the 5' flanking region of the GH gene. This promoter contains two binding sites for the transcription factor GHF-1, the expression of which is also specific to cells of the somatotrophic lineage and correlates with activation of the GH gene in the developing mouse pituitary. Various studies indicate that GHF-1 is the main determinant of cell type-specific expression of the GH gene. GHF-1 is a member of the POU-domain class of proteins that each contain two highly conserved sequence motifs, the homoeodomain and the POU-specific domain. Here we report that the GHF-1 homoeodomain is sufficient for sequence-specific DNA binding, although its activity is stimulated by the POU-specific domain, which does not interact directly with the DNA. Transcriptional activation is mediated by a separate domain rich in hydroxylated amino-acid residues. Similar sequences are present in other cell type-specific transcription factors.

Different requirements for formation of Jun: Jun and Jun: Fos complexes

The cFos proto-oncoprotein associates with cJun to form a heterodimer with increased DNA binding and transcriptional activities. It has been suggested that dimerization of these proteins is mediated by the interdigitation of an orderly repeat of leucine residues forming a leucine zipper. In agreement with this model, we find that binding to the AP-1 site requires dimerization of these proteins. Although cFos, itself, does not seem to dimerize and bind to the AP-1 site, Jun: Fos heterodimers have higher stability than Jun homodimers, which accounts for their increased DNA binding activity. Mutational analysis indicates that at least three of the repeated leucines of cJun are important for homodimer formation. However, these residues can be mutated without affecting formation of Jun: Fos heterodimers. In addition, several other residues present between the leucines are also important for both homo- and heterodimerization. These findings provide support for the recent proposal that these proteins dimerize via formation of a coiled coil and suggest that residues other than leucines provide specificity for this interaction. Assuming that dimerization is required for proper alignment of the DNA recognition sites, we generated a cJun mutant containing a small insertion between the dimerization and the DNA recognition domains. This mutant fails to bind DNA, but it acts as a trans-dominant inhibitor of cJun and cFos because it still dimerizes with the wild-type proteins.

Interleukin-1 costimulatory activity on the interleukin-2 promoter via AP-1

Interleukin-1 (IL-1) is a major regulator of inflammation and immunity. IL-1 induces T lymphocyte growth by acting as a second signal (together with antigen) in enhancing the production of interleukin-2 (IL-2). An IL-1-responsive element in the promoter region of the human IL-2 gene was similar to the binding site for the transcription factor AP-1. IL-1 enhanced expression of c-jun messenger RNA, whereas the antigenic signal enhanced messenger RNA expression of c-fos. Thus, the two components of the AP-1 factor are independently regulated and the AP-1 factor may serve as a nuclear mediator for the many actions of IL-1 on cells.

The CUP2 gene product, regulator of yeast metallothionein expression, is a copper-activated DNA-binding protein

CUP2 is a regulatory gene controlling expression of CUP1, which encodes the Cu-binding yeast metallothionein. CUP2, which is identical to the ACE1 gene, encodes a Cu-regulated DNA-binding protein. The CUP2 protein contains a cysteine-rich DNA-binding domain dependent on Cu+ and Ag+ ions which bind the cysteine residues and direct the refolding of the metal-free apoprotein. CUP2 mutant alleles from Cu-sensitive yeast strains have point mutations affecting the DNA-binding activity. These results establish CUP2 as the primary sensor of intracellular Cu+ in the yeast Saccharomyces cerevisiae, functioning as a Cu+-regulated transcriptional activator.

Induction of human growth hormone promoter activity by the adenosine 3',5'-monophosphate pathway involves a novel responsive element

Regulation of GH gene expression by GRF involves cAMP as a second messenger. We have demonstrated that a 500-basepair fragment of the human GH (hGH) gene 5' flanking region can confer cAMP inducibility upon the chloramphenicol acetyltransferase transcription unit in transient transfections of rat pituitary tumor cells treated with forskolin, an activator of adenyl cyclase. The same hGH construct is not induced by forskolin in nonpituitary-derived cells. Experiments with hGH deletion constructs reveal that binding sites for transcription factor AP-2 and the pituitary-specific factor GHF-1 are not required for forskolin stimulation, but that GHF-1 may potentiate the effect. RNA analyses reveal that forskolin also stimulates accumulation of transcripts initiated at the hGH promoter. Other agents that elevate cAMP levels also stimulate hGH expression. Since the hGH 5' flanking region contains no sequences homologous to the cAMP-responsive element of the somatostatin gene, and the AP-2 sites do not appear to be required for the forskolin response, these results suggest that a novel cAMP-responsive element exists within 82 basepairs upstream from the transcriptional start of the hGH gene and that hGH regulation by GRF may involve interaction between a tissue-specific element and a cAMP-inducible element.

The Fos and Jun/AP-1 proteins are involved in the downregulation of Fos transcription

The low basal expression of Fos and the rapid and effective turn-off of serum induced Fos transcription is due to autoregulation. Fos and Jun/AP-1 protein cooperate in the repression mechanism. Overexpressions of Fos and Jun decrease basal and induced transcription from Fos-CAT constructs and from the endogenous gene in NIH3T3 cells. The introduction into cells of either antisense Fos or antisense Jun sequences leads to elevated basal Fos promoter activity. Gel retardation experiments with synthetic oligonucleotides define two target sequences in the Fos promoter which bind Fos-Jun/AP-1 (centering at about -296 and -60). In vivo competition with these oligonucleotides relieves repression.

Transcription factors AP-3 and AP-2 interact with the SV40 enhancer in a mutually exclusive manner

The 'core' sequence is critical for efficient transcriptional activity of the SV40 enhancer. Moreover, the core was shown to be involved in a signal transduction pathway elicited by treatment of cells with phorbol ester tumor promoters. We report here the identification and characterization of activator protein-3 (AP-3), which recognizes the core element. AP-3 was purified to near homogeneity and identified as a 48K polypeptide. The purified protein is an efficient transcriptional activator in vitro. In addition, we show that AP-3 and a second factor that recognizes the SV40 enhancer, AP-2, interact in a mutually exclusive manner. These studies should facilitate understanding of the mechanism by which the SV40 enhancer achieves its characteristic broad cell-type specificity.

Prolonged activation of jun and collagenase genes by tumour necrosis factor-alpha

Tumour necrosis factor-alpha (TNF-alpha) is secreted by macrophages in response to inflammation, infection and cancer. Sublethal doses of recombinant TNF-alpha to rats causes cachexia, anaemia and inflammation. TNF-alpha plays a major part in tissue inflammation and remodelling by stimulating production of collagenase. Cellular responses to TNF-alpha are initiated by binding to high-affinity cell surface receptors. TNF-alpha then profoundly affects gene regulation, stimulating the fos, myc, interleukin-1 and interleukin-6 genes and inhibiting the type I collagen gene. Here we demonstrate that TNF-alpha also stimulates collagenase gene transcription; this stimulation is mediated by an element of the gene that is responsive to the transcription factor AP-1, the major component of which (jun/AP-1) is encoded by the jun gene; and that TNF-alpha stimulates prolonged activation of jun gene expression. This prolonged induction of jun contrasts with its transient activation by the phorbol ester TPA and provides a physiological example of the ability of jun/AP-1 to stimulate its own transcription. This may be a key mechanism for mediating at least some of the biological effects of TNF-alpha.

Purification of growth hormone-specific transcription factor GHF-1 containing homeobox

Pituitary-specific expression of the growth hormone (GH) gene is governed by a transcription factor, GHF-1, that binds to two sites within its promoter. Recently, GHF-1 was shown to be a member of the homeobox family of DNA-binding proteins. An important question is whether GHF-1 controls the expression of other pituitary specific genes, such as prolactin (Prl), expressed in closely related cell types. To this end, GHF-1 was purified from extracts of GH- and Prl-expressing pituitary tumor cells and identified as a 33-kilodalton polypeptide. Although GHF-1 bound to and activated the GH promoter, it did not recognize the Prl promoter. However, at least one other factor in the same extracts, which was easily separated from GHF-1, bound to several sites within the Prl but not the GH promoter. Antibodies to GHF-1 did not react with the Prl binding activity. These results imply that the pituitary-specific expression of GH and Prl is governed by two distinct trans-acting factors.

Ubiquitous and lymphocyte-specific factors are involved in the induction of the mouse interleukin 2 gene in T lymphocytes

The immediate upstream region of the mouse interleukin 2 (Il-2) gene harbors a strong transcriptional enhancer. This enhancer contains most, if not all of the sequence elements necessary for the T cell specific induction of the Il-2 gene by the phorbol ester TPA and the plant lectin Concanavalin A. DNase I footprinting studies with fractionated extracts obtained from induced and uninduced E14 T cells revealed numerous recognition sites for potential trans-acting factors. Five of these sites are also recognized by the TPA-activated HeLa cell factors AP-1 and AP-3. Other sites including two TATA-boxes, two purine-rich sequence motifs and two copies of the GGGPuTTTCAA motif are recognized by lymphocyte specific factors. The latter motif is highly conserved between several lymphokine genes and is therefore designated as a T cell element (TCE). In E14 T cells, pentamers of the distal TCEd confer an activity similar to that of the entire Il-2 enhancer, whereas in B and HeLa cells, the TCEd-pentamer is inactive as is the Il-2 enhancer. These data indicate the involvement of the TCEd and its recognition factor(s) in the cell type specific induction of the Il-2 gene during T cell activation.

The CUP2 gene product regulates the expression of the CUP1 gene, coding for yeast metallothionein

The yeast CUP1 gene codes for a copper-binding protein similar to metallothionein. Copper sensitive cup1s strains contain a single copy of the CUP1 locus. Resistant strains (CUP1r) carry 12 or more multiple tandem copies. We isolated 12 ethyl methane sulfonate-induced copper sensitive mutants in a wild-type CUP1r parental strain, X2180-1A. Most mutants reduce the copper resistance phenotype only slightly. However, the mutant cup2 lowers resistance by nearly two orders of magnitude. We cloned CUP2 by molecular complementation. The smallest subcloned fragment conferring function was approximately 2.1 kb. We show that CUP2, which is on chromosome VII, codes for or controls the synthesis or activity of a protein which binds the upstream control region of the CUP1 gene on chromosome VIII. Mutant cup2 cells produced extremely low levels of CUP1-specific mRNA, with or without added copper ions and lacked a factor which binds to the CUP1 promoter. Integrated at the cup2 site, the CUP2 plasmid restored the basal level and inducibility of CUP1 expression and led to reappearance of the CUP1-promoter binding factor. Taken collectively, our data establish CUP2 as a regulatory gene for expression of the CUP1 metallothionein gene product.

Sequence-specific and general transcriptional activation by the bovine papillomavirus-1 E2 trans-activator require an N-terminal amphipathic helix-containing E2 domain

The sequence-specific trans-activator protein of bovine papillomavirus (BPV)-1, E2, strongly increases transcription at promoters containing papillomaviral ACCG(N)4CGGT (E2P) cis motifs, but can also activate a wide range of co-transfected promoters without E2P cores to a lower extent. Analysis of multiple E2 mutants in transfected cells revealed that the C-terminal DNA binding E2 domain binds to the E2P cis sequences in the form of pre-existing nuclear dimers. The DNA binding function of E2 was required for specific trans-activation of the E2P elements, as well as for the function of the previously described C-terminal 'short E2' transrepressor. In addition to the C terminus, specific trans-activation also required an intact N-terminal half of the E2 protein. When expressed alone, the N-terminal E2 domain was found to activate heterologous promoters without E2P elements to an extent comparable to wild-type E2, and therefore represents the functional transcription activation domain of the E2 factor. In contrast to other DNA-binding activator proteins described to date, the transcriptional activation by the E2 factor can occur without specific DNA binding. Its mechanism may thus involve protein--protein interactions between common transcription factors and the N-terminal E2 domain which contains amphipathic helix motifs.

The jun proto-oncogene is positively autoregulated by its product, Jun/AP-1

Binding of the human transcription factor Jun/AP-1 to a conserved 8 bp nucleotide sequence (TRE) is responsible for increased transcription of different cellular genes in response to tumor promoters, such as TPA, and serum factors. Enhanced Jun/AP-1 activity in TPA-stimulated cells is regulated by two different mechanisms: a posttranslational event acting on pre-existing Jun/AP-1 molecules, and transcriptional activation of jun gene expression leading to an increase in the total amount of Jun/AP-1. Induction of jun transcription in response to TPA is mediated by binding of Jun/AP-1 to a high-affinity AP-1 binding site in the jun promoter region. Site-specific mutagenesis of this binding site prevents TPA induction and trans-activation by Jun/AP-1. These results clearly demonstrate that jun transcription is directly stimulated by its own gene product. This positive regulatory loop is likely to be responsible for prolonging the transient signals generated by activation of protein kinase C.

Structure and chromosomal localization of the functional intronless human JUN protooncogene

The JUN protooncogene encodes a protein that is functionally and biochemically identical to the transcription factor AP-1 (activator protein 1). To understand the structure and regulation of this important gene, a genomic clone of human JUN was isolated and its primary structure and transcription pattern were determined. Most surprisingly, the sequence of the genomic clone was found to be contiguous with the sequence of the JUN cDNA, suggesting that it lacks introns. RNase protection experiments confirm that JUN is an intronless gene that yields several transcripts due to 5' and 3' heterogeneities. Transfection experiments show that the cloned gene is functional, as it encodes a trans-acting factor that stimulates transcription of AP-1-dependent reporter gene. In situ hybridization was used to map JUN to chromosomal region 1p31-32. Interestingly, this region is frequently deleted in neuroblastomas, suggesting that elimination of AP-1 may play an important role in the pathogenesis of this disease.

The pituitary-specific transcription factor GHF-1 is a homeobox-containing protein

Growth hormone factor 1 (GHF-1) is a pituitary-specific transcription factor that plays a critical role in cell type-specific expression of the growth hormone (GH) gene. Here, we describe the isolation of bovine and rat GHF-1 cDNA clones. These cDNAs encode proteins whose molecular mass, 33K, is identical to purified GHF-1 and whose sequence agrees with a partial GHF-1 peptide sequence. The predicted GHF-1 sequence contains a region, near its C-terminus, that exhibits considerable similarity to a homeobox consensus sequence. DNAase I footprinting with bacterially expressed fusion protein containing a fragment of GHF-1 encompassing the homeobox indicates that this region of the protein functions as its DNA binding domain. Expression of GHF-1 is restricted to cells of the somatotropic lineage in the pituitary. This remarkable specificity of GHF-1 expression correlates with the selective transcription of its target, the GH gene. Other mammalian homeobox-containing proteins may function similarly as transcription factors controlling cell type-specific expression in other locations.

Extinction of growth hormone expression in somatic cell hybrids involves repression of the specific trans-activator GHF-1

Growth hormone (GH) expression in pituitary-derived cells has been attributed to the presence of a positive trans-activator, GHF-1, which binds to two sites on the GH promoter. Somatic cell hybridization of non-GH-expressing L cells with pituitary-derived GH3 cells usually results in extinction of GH production. While previous studies showed that extinction occurs at the level of GH transcription, the exact mechanism remained elusive. We therefore characterized two parental cell lines and three hybrids, two of which extinguish GH expression and one in which GH is reexpressed after loss of mouse chromosomal material. Using in vivo transfections, in vitro transcription, DNAase I footprints, and immunoblotting experiments, no evidence for a direct repressor of GH transcription was found. Rather, extinction of GH expression in fibroblast x pituitary hybrids was accompanied by loss of GHF-1 protein and mRNA expression, suggesting that extinction occurs by repression of this trans-activator.

A family of immunologically related transcription factors that includes multiple forms of ATF and AP-1

ATF is a cellular transcription factor involved in the regulation of multiple adenovirus E1A- and cellular cAMP-inducible promoters. Using DNA affinity chromatography, we have purified ATF and found that a series of polypeptides copurify in a sequence-specific manner. We demonstrate that these polypeptides represent a family of proteins that are related by DNA-binding specificity and by immunological cross-reactivity. This family includes the transcription factor AP-1, whose recognition sequence, GTGAGTCAA, differs from the ATF consensus, GTGACGTCAA, by the absence of a cytosine residue. Our results further indicate that there are multiple forms of both ATF and AP-1. The immunological cross-reactivity and related DNA-binding specificities suggest that ATF and AP-1 contain similar amino acid sequences and may have originated from a common gene.

Activation of the c-fos gene by UV and phorbol ester: different signal transduction pathways converge to the same enhancer element

In NIH3T3 cells stably transfected with the human c-fos gene, serum, platelet derived growth factor (PDGF), phorbol ester (12-O-tetradecanoyl-phorbol-13-acetate, TPA), ultraviolet irradiation (UV) and 3'-5'-cyclic adenosine monophosphate (cAMP) cause a transient and rapid activation of both the endogenous and the transfected c-fos genes. While serum, TPA, UV and PDGF dependent activation of the gene is severely impaired, when the serum responsive element from position -319 to -300 (SRE, Treisman, 1985) is destroyed, a full response to cAMP is retained. Insertion of a synthetic oligonucleotide corresponding to the SRE element upstream of position -96 restores the responses to TPA and serum, and large parts of the responses to UV and PDGF. The signal transduction chains elicited by UV and TPA are blocked by an inhibitor of protein kinase. Only TPA, however, causes the translocation of protein kinase C to the membrane. UV and TPA treated cells become refractory to a second stimulation by the same agent at 3 or 24 hours after the first treatment. Alternating the agents, however, leads to full responses. In addition, saturating doses of UV and TPA are at least additive. Ca-ionophores severely reduce only UV induced c-fos expression. These data indicate, that different signal transduction pathways elicited by growth promoting agents and by UV induced stress converge onto the same enhancer element.

The c-Fos protein interacts with c-Jun/AP-1 to stimulate transcription of AP-1 responsive genes

Cell lines stably transfected with metal inducible, MT-fos chimeric genes were used to study the ability of the c-fos gene product, Fos, to act as a transcriptional trans-activator. In 3T3MTfos cells, induction of Fos expression led to specific trans-activation of an AP-1 responsive reporter gene. Induction of Fos expression in F9MTfos cells, however, did not lead to trans-activation. Since, unlike NIH3T3 cells, F9 cells do not contain detectable levels of AP-1, we examined whether a c-Jun/AP-1 expression vector can restore the trans-activating effect of Fos in F9MTfos cells. Transfection with a functional c-Jun/AP-1 vector restored the specific trans-activating effect of Fos on AP-1 responsive constructs. When incubated with nondenatured cell extracts, anti-cFos antisera precipitated a protein complex composed of Fos and several Fos associated proteins (FAP). One of these, FAP p39, is structurally identical to c-Jun/AP-1. These results suggest that Fos is a trans-acting factor that is capable of stimulating gene expression not by direct binding to DNA but by interaction with the sequence-specific transcription factor AP-1. Therefore recognition of specific cis-elements by AP-1 is a prerequisite for Fos-mediated stimulation of gene expression.

Progesterone induction of metallothionein-IIA gene expression

Expression of the metallothionein gene is known to be induced by glucocorticoids in a variety of cells. Here we show that in human cell lines containing functional progesterone receptors, the endogenous metallothionein-IIA (hMTIIA) gene is inducible by the synthetic progestins R5020 and medroxy-progesterone acetate. That this effect reflects a direct interaction with the metallothionein gene is supported by our finding that the partially purified progesterone receptor binds to the promoter region of the gene in vitro. The limits of the DNase I footprint and the guanine residues protected in methylation studies with the progesterone receptor are similar to those previously described for the glucocorticoid receptor. Thus, the hormone regulatory element of the human metallothionein-IIA gene can mediate regulation by both glucocorticoids and progestins, as does the hormone regulatory element of mouse mammary tumor virus.

Cytosine methylation does not affect binding of transcription factor Sp1

DNA methylation may be a component of a multilevel control mechanism that regulates eukaryotic gene expression. We used synthetic oligonucleotides to investigate the effect of cytosine methylation on the binding of the transcription factor Sp1 to its target sequence (a G + C-rich sequence known as a "GC box"). Concatemers of double-stranded 14-mers containing a GC box successfully competed with the human metallothionein IIA promoter for binding to Sp1 in DNase I protection experiments. The presence of 5-methylcytosine in the CpG sequence of the GC box did not influence Sp1 binding. The result was confirmed using double-stranded 20-mers containing 16 base pairs of complementary sequence. Electrophoretic gel retardation analysis of annealed 28-mers containing a GC box incubated with an Sp1-containing HeLa cell nuclear extract demonstrated the formation of DNA-protein complexes; formation of these complexes was not inhibited when an oligomer without a GC box was used as a competitor. Once again, the presence of a 5-methylcytosine residue in the GC box did not influence the binding of the protein to DNA. The results therefore preclude a direct effect of cytosine methylation on Sp1-DNA interactions.

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

Proto-oncogenes encode proteins with three main sites of action: the cell-surface membrane, the cytoplasm and the nucleus. Although the exact biochemical function of most proto-oncogene products is not understood, several of them are known to be involved in signal transduction. A role in gene regulation through DNA binding has been suggested for a recently isolated member of the group of oncogenes acting at the nucleus, v-jun. The C-terminus of the putative v-jun-encoded protein is similar in sequence to the C-terminus of the yeast transcriptional activator GCN4 (refs 8, 9), which forms its minimal DNA-binding domain. GCN4 binds to specific sites whose consensus sequence is highly similar to the recognition sequence of the mammalian transcriptional activator AP-1 (refs 12, 13). Like GCN4, AP-1 binds to promoter elements of specific genes and activates their transcription. Because of the similarity between the recognition sites for GCN4 and AP-1, we examined the possibility that AP-1 could be the product of the c-jun proto-oncogene. The experimental results reported here indicate that the JUN oncoprotein is a sequence-specific transcriptional activator similar to AP-1.

In vitro activation of the HIV-1 enhancer in extracts from cells treated with a phorbol ester tumor promoter

The transition from persistent to lytic infection by the human immunodeficiency virus, HIV, is marked by a burst of viral replication and gene expression that occurs when infected cells are stimulated by physiological inducers or tumor promoters like 12-O-tetradecanoyl phorbol acetate (TPA). We report here that the HIV enhancer is activated specifically by TPA in several non-lymphoid cell types, and that this transcriptional regulation can be reproduced in a cell-free system. In vitro transcription experiments revealed a 6-fold activation of the HIV promoter in nuclear extracts prepared from TPA-induced HeLa tk- cells, whereas a control (human alpha-globin) promoter was transcribed with equal efficiency in either induced or uninduced cell extracts. A corresponding increase in the activity of a cellular DNA-binding protein that interacts with the HIV enhancer was detected in TPA-treated cells with DNase I footprint experiments. This increase occurred in the absence of de novo protein synthesis, suggesting a post-transcriptional activation mechanism. Analysis of HIV deletion mutants suggests that the enhancer is the target for the TPA effect both in vitro and in vivo. The cell-free system described here should facilitate studies on the mechanism of phorbol ester induction of gene-specific transcription factors.

Transcription factor AP-2 mediates induction by two different signal-transduction pathways: protein kinase C and cAMP

We have purified and characterized the 50 kd activator protein 2 (AP-2), another enhancer-binding protein interacting with the human metallothionein IIA (hMT-IIA) gene control region. Purified AP-2 activates transcription in vitro from a hybrid promoter containing hMT-IIA upstream sequences. AP-2 also recognizes control elements of the human growth hormone, c-myc, and H-2Kb genes, and the SV40 and bovine papilloma virus enhancers. Multiple synthetic copies of the hMT-IIA high-affinity AP-2 binding site can act as efficient, cell-type-specific enhancer elements; their activity increases after treatment of cells with phorbol ester or cAMP-elevating agents. In contrast, a synthetic enhancer recognized by factor AP-1 is activated only by phorbol ester. AP-2 appears to mediate transcriptional activation in response to two different signal-transduction pathways, one involving the phorbol-ester- and diacylglycerol-activated protein kinase C, the other involving cAMP-dependent protein kinase A.