Autoregulation of fos: the dyad symmetry element as the major target of repression

Activation of c-fos expression by transforming Ha-ras in HC11 mouse mammary epithelial cells is PKC-dependent and mediated by the serum response element

The mechanism by which transforming Ha-ras induces c-fos expression in HC11 mouse mammary epithelial cells was investigated with regard to controversial data concerning the role of protein kinase C (PKC) and the required promoter elements of the fos gene. HC11 cells carrying a glucocorticoid-inducible Ha-ras (val12) construct were transfected with a chloramphenicol acetyltransferase (CAT) reporter gene under the control of a human fos promoter which includes the serum response element (SRE), the adjacent c-fos AP-1 site (FAP) and the cAMP response element (CRE). Induction of the Ha-ras gene by dexamethasone lead to a transactivation of expression of the transfected fos promoter construct which was inhibited by the PKC inhibitor BM41440 and abrogated in PKC-'depleted' cells. A similar transactivation was observed when the fos promoter construct was co-transfected with a constitutively active ras expression vector. Again, this effect was depressed by the PKC inhibitor and abolished in PKC-'depleted' cells. 'PKC-depletion' was achieved by long-term exposure to 12-O-tetradecanoylphorbol-13-acetate. This procedure was shown to deplete cells of PKC alpha and to reduce significantly PKC epsilon. Long-term exposure to bryostatin 1 selectively depletes PKC alpha. Depletion of PKC alpha by bryostatin 1 does not reduce the transcriptional activation of the SRE-FAP-TK-CAT (TK: thymidine kinase) construct by Ha-ras. In order to delineate the promoter elements mediating the transcriptional activation, constructs which lack the FAP and the CRE sites but contain an intact SRE were co-transfected with the ras construct. Elimination of the FAP and CRE sequences did not affect the transcriptional activation by Ha-ras (val12). It is concluded that in HC11 cells, transforming Ha-ras activates c-fos expression in a PKC-dependent manner, presumably implying PKC epsilon, and that the SRE is sufficient to mediate transcriptional activation.

Role of protein kinase C in ras-mediated fos-expression

HC-11 mouse mammary epithelial cells stably transfected with a glucocorticoid-inducible Ha-ras construct encoding a transforming (val12) p21Ha-ras were cotransfected with a c-fos-CAT construct containing the human c-fos promoter up to position -711 and the CAT reporter gene. Expression of Ha-ras by dexamethasone leads to a transcriptional activation of the fos-CAT construct which was found to be sensitive to the PKC inhibitor ilmofosine (BM41440) and abrogated by PKC depletion following long-term exposure to TPA. The responsiveness to Ha-ras is retained if only the portion of the fos promoter covering the serum response element (SRE) and the adjacent fos AP-1 (FAP) site are put in front of a CAT gene linked to a thymidine kinase (TK) promoter. Further depletion of the FAP-site does not affect the inducibility by Ha-ras. Transcriptional activation of the SRE-FAP-TK-CAT as well as the SRE-TK-CAT constructs by Ha-ras is sensitive to the PKC-inhibitor ilmofosine (BM41440) and blocked by long-term exposure to TPA. Long-term exposure to TPA depletes cells of PKC alpha and significantly reduces the PKC epsilon levels. Long-term exposure in bryostatin 1 selectively depletes PKC alpha. Depletion of PKC alpha by bryostatin 1 does not reduce the transcriptional activation of the SRE-FAP-TK-CAT-construct by Ha-ras. It is concluded that (i) transforming Ha-ras induces c-fos in HC-11 cells via PKC (presumably epsilon), (ii) the signal is mediated to the serum response element (SRE) of the fos promoter and (iii) the fos AP-1 (FAP) site is not required for this mechanism.

The mammalian UV response: mechanism of DNA damage induced gene expression

DNA damage inducing treatment of cultured mammalian cells triggers the activation of transcription factors and the prolongation of the half life of p53. As the earliest event detectable in the nucleus (5 min), AP-1 (c-Jun/c-Fos) is post-translationally modified. Triggering this early event and triggering subsequent transcription factor dependent processes requires extra-nuclear components of signal transduction such as Src, Ras, Raf-1 and MAP-2 kinase. Recent efforts have concentrated on examining whether DNA damage or other secondary effects of the damaging agent generate the signal then passed on to transcription factors. Further, it has been studied whether a pathway of reverse signalling exists that originates in the nucleus and reaches the cell surface. At the cell surface the UV induced signalling chain can be interrupted experimentally. Beyond this step DNA damage and signal transduction induced by phorbol esters and growth factors merge and reach the nuclear proteins through common components.

Negative regulation of transcription in eukaryotes

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An AP-1 binding site in the upstream region of Deb-A is part of a developmentally regulated negative element

The Deb-A gene from Drosophila melanogaster encodes a small membrane-associated protein, regulated during development, with peak abundance at 12-15 h of embryogenesis. The cis-acting regulatory elements that control expression of Deb-A during embryogenesis were localized using a somatic transformation assay. The Adh gene of D. melanogaster was used as a 'reporter' gene. The promoterless ADH coding sequence was fused to the 5'-upstream control region of Deb-A. Deletions were introduced into the 5'-region using various restriction sites and Bal31 deletion mutagenesis. A negative regulatory element, or silencer, was localized to a segment 47 base pairs long, between -395 and -442. It is responsible for 80% of the repression of gene expression during late development and reduces levels of Deb-A RNA nearly 5-fold. This negative element is temporally functional. It becomes active after 15 h of embryogenesis and it has no effect on gene expression prior to that. Within this negative element of 47 base pairs, two footprint regions were protected from DNase I digestion by embryonic nuclear extracts: one region contains an AP-1 binding site, but the other footprint is due to unknown element(s). High molecular weight DNA-protein complexes on an oligonucleotide probe spanning the AP-1 binding site were identified in gel retardation assays using partially purified bacterially expressed Djun protein or nuclear extracts from Drosophila embryos. These data suggest that the AP-1 site may be partly responsible for decreasing Deb-A expression during the late embryonic developmental stages of D. melanogaster.

Differential expression of immediate early genes in the superior cervical ganglion after nicotine treatment

We examined the effects of single and multiple systemic injections of nicotine on the expression of five immediate early genes in the rat superior cervical ganglion by in situ hybridization histochemistry. A single nicotine injection resulted in a rapid and transient activation phase of nerve growth factor I-A, c-fos and jun-B at 20 min, and a later and less prominent activation of c-jun, which stayed high from 20 to 60 min. there was a parallel slow and long-lasting activation of jun-D, which remained high 4 h after nicotine treatment. Tyrosine hydroxylase mRNA, but not neuropeptide Y mRNA, was also induced by nicotine. Denervation of the ganglion did not prevent the induction of immediate early genes, but the nicotine antagonists hexamethonium and mecamylamine completely blocked the induction of immediate early genes, indicating that nicotine acted directly on receptors present on ganglion cells. When repeated nicotine injections were given, there was a refractory period of 1-2 h for c-fos, nerve growth factor I-A and jun-B induction. Repeated nicotine injections at 1-h intervals prevented about 80% of c-fos, nerve growth factor I-A and jun-B mRNA induction seen after a single injection. Because nicotine is known to induce immediate early genes in the adrenal glands as well, we examined whether similar kinetics of the gene induction could be seen in the adrenal medulla. However, no refractory period for repeated nicotine treatment or down regulation of the induction of the immediate early genes could be demonstrated in the adrenal medulla. The results show that sympathetic neurons respond to nicotine with altered expression of immediate early genes. Nicotine-induced expression of immediate early genes may be mediated and regulated by different factors in neuronal and endocrine noradrenergic cells.

Immediate early gene expression in experimental epilepsy

Neuronal excitation by experimentally induced seizures elicits the rapid induction of a set of genes called immediate early genes (IEGs). The gene products of fos, jun and Krox, multimember gene families that belong to the class of IEGs, participate in a fundamental biological control mechanism, the regulation of gene transcription. IEG encoded proteins act as third messengers in an intracellular signal transduction cascade between neural cell surface receptors, cytoplasmic second messenger systems and specific target genes in the nucleus, a process for which the term 'stimulus transcription coupling' has been given. Almost all types of seizures cause dynamic alterations of IEG expression in neurons of the limbic system, but also in non-limbic areas, such as the cortex, striatum and thalamus. IEG encoded transcription factors are thought to up- or down-regulate effector genes with preferential expression in the central nervous system, including genes for neurotransmitters, growth factors, receptors, synaptic and axonal proteins. If the concept holds true that IEGs act as molecular switches converting epileptic short-term excitation of neurons into alterations of the molecular phenotype, future research may help to explain hitherto unexplained phenomena in epileptogenesis including changes of synaptic efficacy, kindling and sprouting.

C-terminal phosphorylation of the serum-response factor

The serum-response factor (SRF) is essential for the induction and repression of the protooncogene c-fos. Phosphorylation of SRF has been implicated to be involved in these processes and five phosphorylation sites have already been mapped within the N-terminal region. Here we show that in vivo additional phosphorylation of SRF does occur. This modification is located primarily within amino acids 206-289, which probably contain more than one phosphorylation site. Microsequencing allowed the identification of one phosphorylation site at Ser253, which is a potential target of casein kinase II. Mutational analysis revealed that, in contrast to N-terminal phosphorylation, Ser253 phosphorylation does not affect DNA-binding properties. In addition, phosphorylation at Ser253 does not seem to change transactivation activity of SRF but rather influences its contribution to transcriptional repression. Thus, C-terminal phosphorylation of SRF may modulate c-fos basal repression.

Hepatitis B virus transactivator HBx uses a tumour promoter signalling pathway

The hepatitis B virus (HBV) transactivator protein HBx is enigmatic in that it stimulates a striking variety of promoters which do not share a common cis-regulatory element. As it does not bind to DNA, it has been speculated that HBx acts indirectly through cellular pathways. Under certain conditions HBx can have an oncogenic potential, which may be relevant for HBV-associated liver carcinogenesis, but until now the mechanism for transactivation and cell transformation by HBx was unclear. We report here that HBx uses a complex signal transduction pathway for transactivation. An increase in the endogenous protein kinase C (PKC) activator sn-1,2-diacylglycerol and the subsequent activation of PKC give rise to activation of the transcription factor AP-1 (Jun-Fos). As a result, HBx transactivates through binding sites for AP-1 and other PKC-dependent transcription factors (AP-2, NF-kappa B), thereby explaining the as-yet incomprehensible variety of HBx-inducible genes. As the PKC signal cascade also mediates cell transformation by tumour-promoting agents, the mechanism presented here might account for the oncogenic potential of HBx.

c-fos gene expression in cell revertants from a transformed to a pseudonormal phenotype

c-fos gene expression in two types of mouse sarcoma cells of spontaneous origin and in revertants to pseudonormal phenotype has been investigated. In the latter cells the content of c-fos mRNA is similar to that in normal fibroblasts. Activity of transcription factors interacting with the regulatory elements, SRE, DSE and TRE, in the c-fos promoter do not correlate with the c-fos mRNA concentration. However, experiments with cells transformed with the indicator plasmid, fos-CAT, showed that the 600 bp c-fos promoter region provides the chloramphenicol acetyltransferase activity correlating with c-fos mRNA expression in cell revertants to a pseudonormal phenotype.

Oxidant carcinogenesis and antioxidant defense

Growth promotion by oxidants is observed with cultured human and mouse fibroblasts as well as epidermal cells. It is expected to play a role in inflammation, fibrosis, and tumorigenesis. Indeed, oxidants trigger (patho)physiological reactions that resemble those induced by growth and differentiation factors. For example, active oxygen activates protein kinases, causes DNA breakage, and induces the growth competence-related protooncogenes c-fos and c-myc. The cellular antioxidant defenses affect the consequences of oxidant exposure. Transfectants of mouse epidermal cells that overproduce Cu,Zn-superoxide dismutase (SOD) were sensitized to the toxic effects of an extracellular burst of O2-. plus H2O2, whereas overproducers of catalase (CAT) were protected. Transfection of SOD overproducers with CAT corrected their hypersensitivity. Inducibility of the protooncogene c-fos by oxidants was diminished in SOD and CAT overproducers, albeit probably for different reasons. It is concluded that a fine balance of the multiple components of the antioxidant defense determines the growth response of cells to oxidative stress. In studies of the mechanism of the transcriptional induction of c-fos by oxidants, we identified the joint DSE-AP1 elements (dyad symmetry element, DSE) as major enhancer motifs in the 5'-upstream regulatory sequences of c-fos. Oxidants also increased the de novo synthesis of protein factors that bind to the fos-AP1 enhancer motif. Protein kinase and ADPR transferase inhibitors suppressed the transcriptional induction of c-fos as well as the increase in factor binding to fos-AP1. We conclude that protein phosphorylation and protein polyADP-ribosylation are required for the transcriptional induction of c-fos and the synthesis of protein factors that bind to fos-AP1. It is likely that the FOS and JUN proteins are among these factors and that they participate in the regulation of c-fos expression by oxidants.

Antisense promoter mapping. Inhibitory methods of transcriptional analysis

We have employed antisense methods to study the transcriptional functions of c-fos protein (Fos). Clones expressing inducible anti-fos RNA have been employed to inhibit c-fos expression, resulting in activation of c-fos transcription by inhibiting its normal repressor function. The sites of negative regulation by Fos have been mapped using this antisense mapping method which demonstrates that the serum response element represents the major site of repression by endogenous c-fos protein. A similar strategy (antisense cloning) has been employed to clone four target genes that are Fos dependent. These cDNAs encode mRNAs that are rapidly induced by serum (although this induction is blocked by cycloheximide) but are blocked by induction of anti-fos RNA. These inhibitory methods of studying transcription factor function are extremely useful for transcription factors (like Fos) that require cooperation with other factors to modulate gene transcription.

Yeast GCN4 as a probe for oncogenesis by AP-1 transcription factors: transcriptional activation through AP-1 sites is not sufficient for cellular transformation

The Jun and Fos oncoproteins belong to the AP-1 family of transcriptional activators and are believed to induce cellular transformation by inappropriately activating genes involved in cell replication. To determine whether transcriptional activation through AP-1 sites is sufficient for transforming activity, we examined the properties of an autonomous and heterologous AP-1 protein, yeast GCN4, in rat embryo fibroblasts. GCN4 induces transcriptional activation through AP-1 sites but, unlike Jun and Fos, fails to induce cellular transformation, in cooperation with Ha-ras. Jun-GCN4 and Fos-GCN4 homodimers independently induce cellular transformation indicating that the amino-terminal regions of Jun and Fos each contain regulatory functions that are required for oncogenesis but are distinct from generic transcriptional activation domains. In addition, these observations have implications for the nature of the oncogenically relevant target genes that respond to Jun and Fos.

Interference between pathway-specific transcription factors: glucocorticoids antagonize phorbol ester-induced AP-1 activity without altering AP-1 site occupation in vivo

Phorbol esters stimulate and glucocorticoid hormones down-regulate a variety of promoters such as that of the collagenase gene through the transcription factor AP-1 (Fos/Jun). We now show by genomic footprinting of the collagenase promoter that phorbol ester treatment of cells results in the binding of AP-1 to its cognate DNA binding site in vivo. The DNA-protein contacts obtained in living cells are also found in vitro using cloned DNA and purified AP-1. Although in vitro synthesized glucocorticoid receptor can disturb the DNA binding of Jun homodimers, it does not interfere with the binding of Fos-Jun heterodimers or of purified AP-1 in vitro. Consistently, fully inhibitory doses of glucocorticoid hormone cause no change in apparent occupation of the AP-1 binding site in vivo. The hormone receptor acts without itself binding to DNA.

The serum response element and an AP-1/ATF sequence immediately downstream co-operate in the regulation of c-fos transcription

Transcription of the c-fos gene is activated in response to a wide variety of extracellular stimuli and several cis-acting transcriptional control elements have been characterized. One of these elements is called the serum response element (SRE) and here we investigate an interaction between this element and an AP-1/ATF-like sequence immediately downstream from the SRE. In growing cells these sequences activate transcription in an additive fashion whereas in quiescent cells they co-operate to repress transcription. This co-operation is disrupted upon separation of the elements which also alters the response of the elements to serum or 12-O-tetradecanoyl-phorbol-13-acetate (TPA) stimulation of quiescent cells. This separation also results in an increase of transcription in growing cells. A consensus AP-1 DNA-binding site can substitute for the AP-1/ATF-like sequence present in the c-fos promoter to activate transcription in an additive fashion with the SRE in growing cells, and co-operate in repression in quiescent cells. These observations show that any interaction that may be occurring between proteins binding to these elements results in a different pattern of transcriptional control in growing and quiescent cells. Alternatively, different proteins (or modified proteins) may complex with these sequences in the two different states of cell growth.

Fos and Jun repress transcriptional activation by myogenin and MyoD: the amino terminus of Jun can mediate repression

Myogenin and MyoD belong to a family of muscle-specific helix-loop-helix (HLH) proteins that have the potential to activate muscle-specific genes in nonmyogenic cells. Peptide growth factors can block the ability of myogenin and MyoD to activate their target genes. Here, we show that the growth factor-inducible proto-oncogenes c-fos, c-jun, and junB mimic the effects of exogenous growth factors and suppress trans-activation of the muscle creatine kinase (MCK) enhancer by myogenin and MyoD. In contrast, JunD, which shares DNA-binding specificity with JunB and c-Jun but is expressed constitutively in muscle cells, is an inefficient inhibitor of the trans-activating capacity of myogenin and MyoD. Transcriptional repression by Fos and Jun is specific to myogenic HLH proteins and is not observed with the widely expressed HLH protein E47, which recognizes the same DNA sequence. Repression of the MCK enhancer by Fos and Jun is targeted at the myogenin and MyoD DNA recognition sequence and can be mediated by the amino terminus of c-Jun. Comparison of several myogenin mutants for their responsiveness to Fos and Jun shows that repression is directed at the basic-HLH region. These results indicate that members of the Jun family can be distinguished on the basis of their effects on muscle-specific transcription and suggest there is cross talk between transcription factors that control myogenesis and those involved in cell proliferation.

Overexpression of the glucocorticoid receptor represses transcription from hormone responsive and non-responsive promoters

The glucocorticoid receptor (GR) is a regulable transcription factor which can bind to DNA response elements in the vicinity of inducible gene promoters and enhance the rate of transcription initiation. The concentration of endogenously expressed GR has been shown to limit the magnitude of the transcriptional induction response in cultured cells. We have investigated the consequence of increased GR expression on the transcriptional activity of a hormone responsive promoter, the MMTV LTR, and on three non-responsive promoters, the RSV LTR, the SV40 early promoter and the c-fos promoter in transiently transfected cells. Low receptor concentrations allow a slight hormonal induction of the MMTV LTR while maximal inducibility can be observed at intermediate GR concentrations. High GR expression reduces the hormonal induction on the MMTV LTR and also adversely effects transcription from the RSV LTR, the SV40 and c-fos promoters. This repression effect is dependent on GR activation. These experiments suggest that the GR interacts with a nuclear factor that is required for the activation of all four promoters. It is probable that "squelching", i.e. protein-protein complex formation in the nucleus leads to the sequestration of interacting proteins(s) essential for the transcription machinery, causing a limitation for the initiation events.

Ultraviolet-radiation induced c-jun gene transcription: two AP-1 like binding sites mediate the response

In HeLa cells transcription of the c-jun gene is activated strongly and rapidly by ultraviolet (UV) irradiation and, to a somewhat lesser extent, by treatment with phorbol ester tumor promoters. In the same cells UV and phorbol esters only marginally enhance the abundance of RNA transcribed from the jun D gene and from the gene coding for the serum response factor (which in turn acts on the UV and phorbol ester response element of the c-fos gene). In contrast to c-jun, jun B transcription is induced more efficiently by phorbol ester than by UV irradiation, suggesting that the members of the jun family are differently regulated. The promoter of c-jun carries two enhancer elements resembling AP-1 binding sites: the jun1 UV response element (URE-71 TGACATCA -64) and the jun2 URE (-190 TTACCTCA-183). These elements act independently in the UV induced expression of c-jun. In the context of the complete c-jun promoter they seem not to be required for c-jun induction by phorbol esters. When fused to the Herpes simplex thymidine kinase promoter, however, the isolated elements mediate induction by both UV and phorbol esters. UV and phorbol ester treatment of cells increases the binding of transcription factors to both elements. Both elements bind factors different in modification or/and constitution from AP-1, the heterodimeric transcription factor composed of c-Fos and c-Jun that controls the activity of the UV and phorbol ester response element (-72 TGAGTCA-66) of the human collagenase gene.

Differential activation of NGF receptor and early response genes in neural crest-derived cells

Nerve growth factor (NGF) binds to a specific cell surface receptor (NGFR) that exists in high affinity (now called trk) and low affinity (now called p75NGFR) forms. NGF-responsive neurons express both forms of the receptor, while Schwann cells, during early development and after nerve injury, express only low affinity p75NGFR. In an attempt to determine whether NGF alters patterns of gene expression in p75NGFR-bearing Schwann cells, we examined the regulation of three early response genes (NGFI-A, NGFI-B, and c-fos) in JS1 rat schwannoma cells. Although these genes are markedly activated by NGF in PC12 (rat pheochromocytoma) cells, NGF has no effect on their transcription in JS1 cells. In contrast to PC12 cells, NGFI-A and NGFI-B are constitutively expressed in JS1 cells, whereas the c-fos gene is not expressed. Treating JS1 cells with cycloheximide (CHX), an inhibitor of protein synthesis that commonly potentiates induction of early response genes by presumably inhibiting synthesis of transcriptional repressors, markedly induces the transcription of NGFI-A and c-fos as well as p75NGFR genes. These data suggest that transcriptional repression plays a major role in the regulation of these genes and that the markedly different regulation of NGFI-A, NGFI-B, and c-fos, all of which encode transcriptional regulators, may be important in guiding the differentiation of these cell types.

Differential c-jun expression in response to tumor promoters in JB6 cells sensitive or resistant to neoplastic transformation

The activity of AP-1, a trans-acting transcription factor, is stimulated by 12-O-tetradecanoylphorbol-13-acetate (TPA) and epidermal growth factor (EGF) in promotion-sensitive (P+) but not in promotion-resistant (P-) JB6 mouse epidermal cell lines. TPA and EGF also promote neoplastic transformation only in P+ cells. Thus, it has been proposed that AP-1-dependent gene expression is involved in determining sensitivity to tumor promotion. This paper explores the possible basis for the differential inducibility of AP-1 activity in P+ and P- JB6 cells, focusing in particular on the regulation of expression of the components of the AP-1 complex at the mRNA level. The expression of jun and fos gene family members, which make up the AP-1 complex, can be stimulated by serum and a number of growth factors, including EGF, and by TPA. Therefore, the possibility that differential expression of one or more forms of jun or fos contributes to the differential AP-1 activity was considered. The data presented here demonstrate both similarities and differences in the basal and TPA- or EGF-induced levels of fos and jun family members between P+ and P- cells. The most striking observation was that the overall TPA- and EGF-induced levels of jun but not fos expression were higher in P+ cells. This suggests that tumor promoter-regulated c-jun expression may contribute to the differential AP-1 activation observed in these cells and may be important in determining sensitivity to promotion of neoplastic transformation.

Characteristics of c-fos and jun B gene expression in A5 cells after beta-adrenoreceptor stimulation and during the cell cycle

The beta-adrenoreceptor agonist isoproterenol elevates cAMP concentrations in the A5 rat salivary epithelial cell line and rapidly and transiently induces the expression of c-fos and jun B at 30 and 60 min following continuous stimulation of these cells. The induction of both genes is mediated by cAMP. We show here that the inducibility of these genes by isoproterenol or 8-BrcAMP is transcriptionally regulated and short (5 min) incubations of A5 cells with either agent is sufficient to trigger the induction of c-fos and jun B. We also have investigated the expression and inducibility of these genes during the A5 cell cycle. Both c-fos and jun B mRNA are elevated at the early phase of the cell cycle and are detectable throughout the cycle. At different stages of the cell cycle in synchronous A5 cells, both genes are as highly induced by isoproterenol or 8-BrcAMP as in asynchronous A5 cells. These studies provide the first evidence for the transcriptional regulation of c-fos and jun B by beta-adrenergic receptor stimulation or cAMP in an epithelial cell line (A5) and demonstrate the coordinate expression and inducibility of these genes at the different stages of the A5 cell cycle.

The AP-1 site and the cAMP- and serum response elements of the c-fos gene are constitutively occupied in vivo

The c-fos proto-oncogene is inducible by cAMP, phorbol esters, serum, and growth factors. The induction by cAMP is mediated by the conserved cAMP response element (CRE), while induction by phorbol esters, serum, and growth factors requires a distal element called the serum response element (SRE). In addition to these elements, a consensus AP-1 transcription factor binding site is located next to SRE. Upstream regions of the mouse and human c-fos genes were footprinted in vivo by the ligation-mediated polymerase chain (PCR). Our results show that all three elements are constitutively protected in mouse liver and lung and in cultured human A431 cells. No major change in the protection profile was detected in A431 cells following stimulation with epidermal growth factor or in mice at birth, when c-fos is known to be induced. These results suggest that the inducible cis elements of the c-fos gene are poised, ready to respond immediately to external signals.

Alternative splicing of fosB transcripts results in differentially expressed mRNAs encoding functionally antagonistic proteins

We show that serum-stimulated fibroblasts transiently express two different forms of fosB mRNA, which are generated by alternative splicing of the transcript from a single gene. In addition to the known long form (fosB-L), encoding a protein of 338 amino acids (FosB-L), a second shorter form (fosB-S) with a deletion of 140 bp was detected. This deletion creates a stop codon 3' to the leucine repeat, giving rise to a protein of 237 amino acids (FosB-S) lacking the carboxyl terminus of FosB-L. Only the long FosB form efficiently induces transformation in mouse and rat fibroblast cell lines and trans-represses the c-fos promoter. Both of these functions are suppressed by coexpressed FosB-S. Upon serum stimulation, maximum expression of the oncogenic fosB-L form precedes the expression of the antagonistic fosB-S form, indicating a new mechanisms regulating the action of members of the Fos family. However, FosB-L and FosB-S do not differ in all trans-regulatory properties: Trans-activation of a 5x TRE-CAT reporter construct in HeLa and NIH-3T3 cells was found with both FosB forms. These observations suggest a correlation between fosB-induced transformation and trans-repression, thus pointing to different mechanisms involved in transformation by fosB and c-fos/v-fos.

Differences in induction of c-fos transcription by cholera toxin-derived cyclic AMP and Ca2+ signals in astrocytes and 3T3 fibroblasts

The B subunit of cholera toxin, a protein which binds specifically to membrane ganglioside GM1, is known to affect cell growth and differentiation. To investigate the mechanism of these cellular responses at the nuclear level, we used the induction of c-fos in astrocytes and 3T3 fibroblasts as a model. Northern blot analysis showed that treatment with B subunit provokes a rapid and transient expression of c-fos mRNA, independent of a measurable increase in cyclic AMP. The B subunit signal, which is mediated by Ca2+, was compared to cholera toxin and other agents which increase intracellular cyclic AMP levels. In transient transfection assays of astrocytes and fibroblasts, functional analysis of c-fos promoter deletions was used to identify the elements involved in transcriptional activation by B subunit. In astrocytes, the DNA region including the serum response element and the cyclic AMP response element (CRE) are equally required, whereas 3T3 cells require only the CRE for maximal induction. A synergistic effect of signal transduction was mediated by calcium and cyclic AMP on the CRE, being positive in 3T3 cells and negative in astrocytes. Diverse regulatory elements may be thus involved in responses of different cell types to the same extracellular signal. Furthermore, a single regulatory element (CRE) can integrate both calcium and cyclic AMP signals in the control of gene expression.

Gene regulation and genetic susceptibility to neoplastic transformation: AP-1 and p80 expression in JB6 cells

The mouse epidermal JB6 cell system consists of clonal genetic variants that are sensitive (P+) or resistant (P-) to the promotion of neoplastic transformation by phorbol esters and other tumor-promoting agents. P+ cells display AP-1-dependent phorbol-ester-inducible transactivation of gene expression, whereas P- cells have a defect in transactivation. Transfection of promotion sensitivity gene pro-1 into P- cells reconstituted both P+ phenotype and AP-1-dependent phorbol-ester-inducible transactivation. P- and P+ cells exhibited induction of c-jun and c-fos messenger RNA levels by phorbol ester, but P- cells had significantly lower basal and induced levels of jun mRNA than P+ cells. Basal and induced levels of c-jun protein were significantly lower in P- cells as well. Differences in levels the 80-kDa pI 4.5 protein p80 were also observed in JB6 cells as a function of preneoplastic progression; high levels of p80 protein and mRNA were observed in P- cells, intermediate levels in P+ cells, and negligible levels were observed in transformed derivatives of JB6 cells. Phorbol ester treatment induced phosphorylation but not synthesis of p80. These data are consistent with the hypotheses that AP-1 is required in the signal transduction pathway for promotion of neoplastic transformation by tumor promoter, that pro genes may control AP-1 activity, that threshold levels of Jun mRNA and protein may play a role in transactivation and promotion sensitivity, and that the p80 protein in JB6 cells may behave in vivo as a suppressor of cellular transformation.

Myeloperoxidase is a primary response gene in HL60 cells, directly regulated during hematopoietic differentiation

The expression of myeloperoxidase was studied in three human myeloid leukemic cell lines. The myeloperoxidase transcript was strongly expressed in promyelocytic HL 60 cells, whereas much lower levels were detected in immature monocytic U 937 cells. Phorbol-12-myristate-13-acetate induction resulted in inhibition of myeloperoxidase expression within 24 hrs. This regulatory event could not be blocked by cycloheximide. Furthermore, cycloheximide did not superinduce myeloperoxidase mRNA levels in KG 1, HL 60 and U 937 cells, arguing against the existence of a negative gene regulator for myeloperoxidase. Therefore, the myeloperoxidase gene can be classified as a primary response gene.

A naturally occurring truncated form of FosB that inhibits Fos/Jun transcriptional activity

Fos and Jun transcription factors are induced by a variety of extracellular signaling agents. We describe here an unusual member of the Fos family that is also induced, namely, a truncated form of FosB (delta FosB) missing the C-terminal 101 amino acids of FosB. delta FosB retains the dimerization and DNA-binding activities of FosB but has lost the ability in transfection assays to activate a promoter with an AP-1 site and to repress the c-fos promoter. Rather, delta FosB inhibits gene activation by Jun or Jun + Fos and inhibits repression of the c-fos promoter by FosB or c-Fos, presumably by competing with full-length Fos proteins at the steps of dimerization with Jun and binding to DNA. In stimulated cells delta FosB may act to limit the transcriptional effects of Fos and Jun proteins.

Cutting the chain of command: specific inhibitors of transcription

Cell growth and differentiation are regulated (at least in part) by changes in gene transcription. The cloning and characterization of transcription factors has revealed that these factors coordinately regulate the transcription of specific genetic programs; for example, a number of phorbol ester-induced genes are activated by binding of the transcription factors Fos and Jun to specific DNA sequences. Clearly, inhibition of either the production or function of specific transcription factors would alter complete genetic programs, changing the expression of a great number of genes (analogous to cutting the chain of military command and affecting an entire brigade or division). Our laboratory and others have employed genetic methods to specifically inhibit transcription by two distinct methods: (1) antisense inhibition of the production of transcription factors; and (2) introduction of target DNA sequences to "soak up"or quench transcription factors. In this report, we present data showing that serum-stimulated induction of the c-fos gene may be reduced more than 90% by introduction of target DNA sequences containing the serum response element (SRE); identical amounts of mutant SRE sequences have no effect on gene induction. These studies demonstrate that specific inhibitors of transcription can have significant effects on cellular gene expression. The challenge is to modulate transcriptional programs without deleterious effects on normal cells.

Antisense mapping of the c-fos promoter: role of the serum response element

Using an antisense RNA approach to eliminate endogenous expression of the c-fos protein, we have verified by nuclear run-on and transient expression assays that the Fos protein is a negative regulator of its own transcription in vivo. The negative autoregulation of the c-fos promoter by Fos was further confirmed by overexpression of an antisense-resistant c-fos expressing vector. Antisense mapping of the c-fos promoter demonstrated that the serum responsive element (SRE) represents the major site for c-fos suppression only during the first hour, but that additional adjacent DNA sequences are required for suppression at later times. We propose that antisense inhibition of transcriptional repressors provides a useful method for analyzing the significance and mechanism of transcriptional repression in vivo.

The SIF binding element confers sis/PDGF inducibility onto the c-fos promoter

The c-fos proto-oncogene is rapidly and transiently induced by a variety of extracellular stimuli. We have previously shown that conditioned media from v-sis transformed NRK cells rapidly induces a DNA binding protein which binds to a conserved sequence upstream of the human c-fos gene. We now show that purified recombinant c-sis/PDGF can induce this binding activity which we have termed SIF, for sis-inducible factor. Oligonucleotides which bind to the SIF protein will confer sis/PDGF inducibility onto a truncated, unresponsive c-fos promoter. However, sequences lying between -100 and -57 of the c-fos gene are required for this induction. The sis-responsive element functions independently of a region of dyad symmetry previously identified as the serum responsive element (SRE). The time course of c-fos expression driven by the sis-responsive element is similar to that mediated by the SRE. Unlike the SRE, which can respond to signals generated by sis/PDGF, serum or phorbol esters, the SIF binding element mediates c-fos induction only in response to sis/PDGF. The SRE and SIF elements function in an additive manner to stimulate the transcription of the c-fos gene in response to sis/PDGF.

Murine ferritin heavy chain: isolation and characterization of a functional gene

A mouse liver genomic library screened with a full-length cDNA encoding murine ferritin heavy chain (mFHC) [Torti et al., J. Biol. Chem. 263 (1988) 12638-12644] yielded a functional genomic clone mFHC. The genomic clone isolated included a region of approximately 3 kb containing four exons and three introns. Sequence comparisons of the mouse genomic clone with other genomic clones from rat, human and chicken showed a high degree of similarity among species in the coding regions. Introns and flanking sequences were less conserved. However, comparison of mFHC promoter elements with FHC genes from other species revealed common elements. Analysis of the genomic structure of FHC suggested the presence of pseudogenes. S1 nuclease analysis, however, confirmed that this mouse clone, when transfected into human MRC-5 fibroblasts, was transcribed, indicating that this clone contains an FHC functional gene.

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.

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.