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

The reversion of highly tumorigenic cell lines to non-tumorigenic phenotype is associated with c-jun down-expression

Using model spontaneously reverting cell lines, c-jun, junB, junD and c-fos oncogene expression was investigated. c-jun, but not junB, junD or c-fos, was overexpressed in highly tumorigenic clones. The reversion of cells to the non-tumorigenic phenotype resulted in a dramatic decrease in c-jun expression. CAT assays revealed that c-jun overexpression in tumorigenic cells was associated with higher transcription activity. No correlation between c-jun oncogene expression and AP-1 transcription factor activity in tumorigenic and non-tumorigenic clones was found.

Analysis of transcriptional activation of a cyclic AMP response element by 2,6,10,14-tetramethylpentadecane (pristane) in JB6 mouse epidermal cells

Pristane is a naturally occurring isoprenoid that is believed to be derived from the phytyl moiety of chlorophyll. Thus, it is not surprising that pristane is present in many common fruits and vegetables. Furthermore, pristane can be detected in the tissues of fish and mammals. In animal models using rodents, pristane can function as a potent tumor promoter. At the molecular level, pristane can induce changes in the plasma membrane, alter the conformation of chromatin, and selectively activate gene expression. Addition of pristane to a mouse epidermal cell line (JB6 P+) allows these cells to grow in an anchorage-independent manner. In contrast, JB6 P-cells are not transformed by pristane. Our study was undertaken to correlate transformation of P+ cells with changes induced by pristane. Transcriptional activation of a cyclic AMP response element (CRE) was induced by pristane in P+ and P-cells. Point mutations in the CRE abolished activation by pristane, thus indicating that an intact CRE was necessary for pristane activation. In P+ cells, pristane repressed phosphodiesterase activity. However, protein kinase A was activated by pristane in P+ and P-cells. Taken together, these results indicated pristane induced novel changes in P+ cells that in turn may facilitate neoplastic transformation.

Inhibition of the DNA-binding activity of Drosophila suppressor of hairless and of its human homolog, KBF2/RBP-J kappa, by direct protein-protein interaction with Drosophila hairless

We have purified the sequence-specific DNA-binding protein KBF2 and cloned the corresponding cDNA, which is derived from the previously described RBP-J kappa gene, the human homolog of the Drosophila Suppressor of Hairless [Su(H)] gene. Deletion studies of the RBP-J kappa and Su(H) proteins allowed us to define a DNA-binding domain conserved during evolution. Because Su(H) mutant alleles exhibit dose-sensitive interactions with Hairless (H) loss-of-function mutations, we have investigated whether the RBP-J kappa or Su(H) proteins directly interact with the H protein in vitro. We show here that H can inhibit the DNA binding of both Su(H) and RBP-J kappa through direct protein-protein interactions. Consistent with this in vitro inhibitory effect, transcriptional activation driven by Su(H) in transfected Drosophila S2 cells is inhibited by H. These results support a model in which H acts, at least in part, as a negative regulator of Su(H) activity. This model offers a molecular view to the antagonistic activities encoded by the H and Su(H) genes for the control of sensory organ cell fates in Drosophila. We further propose that a similar mechanism might occur in mammals.

Inhibition of activation of transcription factor AP-1 by CD28 signalling in human T-cells

Co-stimulation of T-lymphocytes by T-cell receptor (TcR) occupancy and activation of the CD28 surface molecule results in enhanced proliferation and interleukin 2 (IL-2) production. The increase in IL-2 gene expression triggered by CD28 involves a kappa B-like sequence in the 5'-regulatory region of the IL-2 promoter, called CD28-responsive element. Stimulation of T-cells by agonistic anti-CD28 antibodies in conjunction with phorbol 12-myristate 13-acetate (PMA)- or TcR-derived signals induces the enhanced activation of the transcription factor NF-kappa B. Here we report that CD28 engagement, however, exerts opposite effects on the transcription factor AP-1. Whereas anti-CD28 together with PMA increased the DNA binding and trans-activation activity of NF-kappa B, PMA-induced activation of AP-1 was significantly suppressed. The inhibitory effect exerted by anti-CD28 was observed at the level of DNA binding as well as in functional reporter-gene assays. These results suggest that the two transcription factors are independently regulated and may perform different functions during T-cell activation.

Multiple regulatory elements in the interleukin-6 gene mediate induction by prostaglandins, cyclic AMP, and lipopolysaccharide

Induction of interleukin-6 (IL-6) gene expression is mediated by numerous agents involving all major signal transduction pathways. We have compared the effects of prostaglandins and their second messenger cyclic AMP (cAMP) with the effect of lipopolysaccharide (LPS) on IL-6 gene expression. We demonstrate that secretion of IL-6 is induced by cAMP in murine monocytic PU5-1.8 cells, even though to a lesser extent than by LPS. Nevertheless, cAMP and prostaglandins of the E series in the presence of theophylline induce transcription of the IL-6 promoter more strongly than LPS, suggesting distinctive effects of cAMP and LPS on posttranscriptional events. Mutations within four regulatory elements, namely, the multiple response element (MRE), AP-1, NF-IL6, and NF-kappa B sites, significantly reduce, but do not completely abrogate, inducibility by cAMP and prostaglandin E1, whereas alterations of four additional sites have no effects. LPS-induced promoter activity, however, is almost completely abolished by mutations in the NF-kappa B site, suggesting that a single regulatory element is crucial for inducibility by LPS. Stimulation by cAMP is correlated with the binding of inducible factors to the AP-1, NF-IL6, and NF-kappa B elements, whereas factors binding to the MRE are constitutively expressed. Recombinant cAMP response element-binding protein binds to the MRE, indicating a potential role for this factor in the cAMP response. Our results suggest that cAMP and prostaglandins act through multiple, partially redundant regulatory elements to induce IL-6 expression in monocytic cells. Nuclear events that overlap partially with the LPS response but also exhibit distinctive features are involved.

Multiple regulatory elements in the interleukin-6 gene mediate induction by prostaglandins, cyclic AMP, and lipopolysaccharide

Induction of interleukin-6 (IL-6) gene expression is mediated by numerous agents involving all major signal transduction pathways. We have compared the effects of prostaglandins and their second messenger cyclic AMP (cAMP) with the effect of lipopolysaccharide (LPS) on IL-6 gene expression. We demonstrate that secretion of IL-6 is induced by cAMP in murine monocytic PU5-1.8 cells, even though to a lesser extent than by LPS. Nevertheless, cAMP and prostaglandins of the E series in the presence of theophylline induce transcription of the IL-6 promoter more strongly than LPS, suggesting distinctive effects of cAMP and LPS on posttranscriptional events. Mutations within four regulatory elements, namely, the multiple response element (MRE), AP-1, NF-IL6, and NF-kappa B sites, significantly reduce, but do not completely abrogate, inducibility by cAMP and prostaglandin E1, whereas alterations of four additional sites have no effects. LPS-induced promoter activity, however, is almost completely abolished by mutations in the NF-kappa B site, suggesting that a single regulatory element is crucial for inducibility by LPS. Stimulation by cAMP is correlated with the binding of inducible factors to the AP-1, NF-IL6, and NF-kappa B elements, whereas factors binding to the MRE are constitutively expressed. Recombinant cAMP response element-binding protein binds to the MRE, indicating a potential role for this factor in the cAMP response. Our results suggest that cAMP and prostaglandins act through multiple, partially redundant regulatory elements to induce IL-6 expression in monocytic cells. Nuclear events that overlap partially with the LPS response but also exhibit distinctive features are involved.

Activation of c-Jun transcription factor by substitution of a charged residue in its N-terminal domain

C-Jun is a cellular transcription factor that can control gene expression in response to treatment of cells with phorbol esters, growth factors, and expression of some oncogenes. The ability of c-Jun to catalyze the transcription of certain genes is controlled, in part, by changes in the phosphorylation state of specific amino acids in c-Jun. One of the major sites that is phosphorylated during signal response is Ser73. Here we show that substitution of a negatively charged aspartic acid residue at 73 constitutively increased transcriptional activity of c-Jun. The Asp73 substitution also enhanced its availability to bind to DNA in a whole cell extract without altering its intrinsic DNA binding activity since the intrinsic activity was unaltered for the c-Jun mutant proteins expressed in a bacterial system. The negatively charged Asp substitution may mimic the negative charge of a phosphorylated serine at 73. The substitution of an uncharged alanine at 73 resulted in lowered activities. The N-terminal end of c-Jun containing these substitutions was fused to the DNA-binding region of the bovine papilloma virus E2 protein, and was able to confer the same activation properties to the fusion protein at the heterologous E2 DNA-binding site. Ser73 lies in a region of c-Jun previously proposed to bind an uncharacterized inhibitor, perhaps related to a protein of approximately 17.5 kD that coprecipitates along with our c-Jun or the JunE2 fusion products.

Follicle-stimulating hormone regulation of AP-1: inhibition of c-jun and stimulation of jun-B gene transcription in the rat Sertoli cell

The proto-oncogenes c-fos and c-jun and the related jun-B encode the components of transcription factor, AP-1, a heterodimeric DNA-binding protein that mediates hormone and growth factor-regulated gene expression. In the rat Sertoli cell, FSH rapidly inhibited c-jun gene expression while it stimulated c-fos and jun-B as well as the expression of the more slowly responding, tissue plasminogen activator (tPA) and inhibin alpha-subunit. These early effects of FSH were not inhibited by cycloheximide. Nuclear run-off analyses demonstrated that the FSH-dependent decline in c-jun and increases in c-fos, jun-B, tPA and inhibin alpha-subunit mRNAs were regulated at the transcriptional level. The rates of degradation of c-fos, c-jun and jun-B mRNAs were unaffected by FSH while tPA and inhibin alpha-subunit mRNAs were stabilized. After 8 h of FSH treatment, the transcription of all five genes returned to basal rates. These data demonstrate immediate-early regulation by FSH of the expression of genes encoding components of the transcription factor, AP-1.

Induction of the DNA-binding activity of c-jun/c-fos heterodimers by the hepatitis B virus transactivator pX

The hepatitis B virus (HBV) X protein (pX) is capable of activating transcription regulated by viral and cellular promoters containing binding sites for different transcription factors, including AP1. In this study we have analyzed the mechanisms of AP1 induction by pX. The hepatitis B virus transactivator was able to activate TRE (12-O-tetradecanoylphorbol-13-acetate response element)-directed transcription in different cell lines, including HepG2, HeLa, CV1, and PLC/PRF/5 cells. pX-induced AP1 activation in HepG2 cells was associated with an increase in the DNA-binding activity of c-Jun/c-Fos heterodimers, which was not dependent either on an increase in the overall amount of c-Fos and c-Jun proteins in the cells or on formation of dimers between pX and the two proteins, thus suggesting the involvement of posttranslational modifications of the transcription factor. The observation that the overexpression of c-Jun and c-Fos in the cells results in a strong augmentation of the effect of pX on TRE-directed transcription is additional evidence indicating the involvement of posttranscriptional modifications of c-Jun/c-Fos heterodimers. The increased AP1 binding observed in the presence of pX was unaffected by the protein kinase C inhibitors calphostin C and sphingosine and by the protein kinase A inhibitor HA1004, while it was almost completely blocked by staurosporine, a potent and nonspecific protein kinase inhibitor, suggesting that protein kinase C- and A-independent phosphorylation events might play a role in the phenomenon. The ability of pX also to increase TRE-directed transcription in cell lines in which AP1-binding activity is not increased (i.e., HeLa, CV1, and PLC/PRF/5 cells) suggests that pX can activate canonical TRE sites by different mechanisms as well.

Induction of the DNA-binding activity of c-jun/c-fos heterodimers by the hepatitis B virus transactivator pX

The hepatitis B virus (HBV) X protein (pX) is capable of activating transcription regulated by viral and cellular promoters containing binding sites for different transcription factors, including AP1. In this study we have analyzed the mechanisms of AP1 induction by pX. The hepatitis B virus transactivator was able to activate TRE (12-O-tetradecanoylphorbol-13-acetate response element)-directed transcription in different cell lines, including HepG2, HeLa, CV1, and PLC/PRF/5 cells. pX-induced AP1 activation in HepG2 cells was associated with an increase in the DNA-binding activity of c-Jun/c-Fos heterodimers, which was not dependent either on an increase in the overall amount of c-Fos and c-Jun proteins in the cells or on formation of dimers between pX and the two proteins, thus suggesting the involvement of posttranslational modifications of the transcription factor. The observation that the overexpression of c-Jun and c-Fos in the cells results in a strong augmentation of the effect of pX on TRE-directed transcription is additional evidence indicating the involvement of posttranscriptional modifications of c-Jun/c-Fos heterodimers. The increased AP1 binding observed in the presence of pX was unaffected by the protein kinase C inhibitors calphostin C and sphingosine and by the protein kinase A inhibitor HA1004, while it was almost completely blocked by staurosporine, a potent and nonspecific protein kinase inhibitor, suggesting that protein kinase C- and A-independent phosphorylation events might play a role in the phenomenon. The ability of pX also to increase TRE-directed transcription in cell lines in which AP1-binding activity is not increased (i.e., HeLa, CV1, and PLC/PRF/5 cells) suggests that pX can activate canonical TRE sites by different mechanisms as well.

Transcriptional regulation of gene expression: mechanisms and pathophysiology

Transcription factors are key mediators of the genetic programs that underlie human development and physiology. Mutations in genes that encode transcription factors or in DNA sequences to which these factors bind may adversely affect gene expression and result in disease. Mutations in genes encoding transcription factors often have pleiotropic effects because each transcription factor is involved in the regulation of multiple genes. For several transcription factors, germline mutations have been shown to result in malformation syndromes whereas somatic mutations in the same genes contribute to the multistep process of tumorigenesis. The study of transcription factors and their involvement in human disease thus provides insight into the molecular mechanisms underlying human development, physiology, dysmorphology, and oncology.

EBV-immortalized isogenic human B-cell clones exhibit differences in DNA-protein complex formation on the BZLF1 and BRLF1 promoter regions among latent, lytic and TPA-activated cell lines

The reactivation of Epstein-Barr virus from latency requires the transcriptional induction of the viral encoded lytic cycle initiator gene, BZLF1, and a concomitant switch from OriP to OriLyt directed viral DNA replication. To investigate the role of host cell factors in these events, a series of EBV-immortalized clonal lymphoblastoid cell lines (LCL) were derived from the spontaneous outgrowth of peripheral blood lymphocytes from a single EBV-seropositive individual. We show that the state of virus activation among this family of isogenic clonal LCL differs, suggesting that each B-cell clone expresses a different complement of cellular factors that influence the state of viral activation. As a first step in the identification of factors involved in EBV reactivation, nuclear extracts were prepared from tightly latent, spontaneously replicating and latent LCL treated with 12-O-tetradecanoyl-phorbol-13-acetate (TPA) and sodium butyrate. The extracts were used in gel mobility shift analyses to compare DNA-protein complex formation among a series of target DNA sequences, including OriLyt and promoter sequences from BZLF1 and BRLF1. The DNA-protein complex patterns were reproducible and indistinguishable among extracts obtained from the latent LCL, but differed from those observed in extracts obtained from the spontaneously replicating LCL, particularly in regard to the binding of a CREB protein to the BZLF1 promoter. Moreover, extracts prepared from LCL treated with TPA to induce virus reactivation resulted in the formation of complexes that differed from those prepared from the spontaneously replicating LCL. Taken together, these data suggest that B-cell factors govern the state of viral activation and that EBV may be reactivated by more than one pathway.

UV-responsive element (TGACAACA) from rat fibroblasts to human melanomas

When taken together, several lines of evidence suggest that the URE-bound proteins are associated with DNA replication. (1) A trans-acting factor of 60 kDa (which may include the 68-kDa URE-binding proteins) was found to be induced by UV and to mediate polyoma DNA replication; (2) the URE was able to compete for the binding of factors that promote polyoma replication in rodent cells; (3) URE-bound proteins are expressed to a higher extent at the S phase of the cell cycle; and (4) they are induced following treatment with aphidicholin. These observations may suggest that the URE may play a role in growth "release" (as opposed to growth "arrest") which would assist in restoring normal growth following DNA damage. It is clear that URE-bound proteins consist of multiple transcription factors, some of which are well characterized (i.e., jun, CREB, and fos families); however, it is likely that the growth release phenomenon we relate to is also mediated by (1) other members of these transcription factor families which have not been identified as yet and (2) a specific combination of known transcription factors which bind to this response element under certain circumstances. This hypothesis is outlined in the enclosed model (Fig. 3).

Transcriptional regulation of multidrug resistance in breast cancer

The development of cross-resistance to many natural product anticancer drugs, termed multidrug resistance (MDR), is one of the major reasons why cancer chemotherapy ultimately fails. This type of MDR is often associated with over-expression of the MDR1 gene product, P-glycoprotein (Pgp), a multifunctional drug transporter. The expression of MDR in breast tumors is related to their origination from a tissue that constitutively expresses Pgp as well as to the development of resistance during successive courses of chemotherapy. Therefore, understanding the mechanisms that regulate the transcriptional activation of MDR1 may afford a means of reducing or eliminating MDR. We have found that MDR1 expression can be modulated by type I cAMP-dependent protein kinase (PKA), opening up the possibility of modulating MDR by selectively down-regulating the activity of PKA-dependent transcription factors which upregulate MDR1 expression. High levels of type I PKA occurs in primary breast carcinomas and patients exhibiting this phenotype show decreased survival. The selective type I cAMP-dependent protein kinase (PKA) inhibitors, 8-Cl-cAMP and Rp8-Cl-cAMP[S] may be particularly useful for downregulating PKA-dependent MDR-associated transcription factors, and we have found these compounds to downregulate transient expression of a reporter gene under the control of several MDR1 promoter elements. Thus, investigations of this nature should not only lead to a greater understanding of the mechanisms governing the expression of MDR, but also provide a focus for pharmacologic intervention by a new class of inhibitors.

Regulatory implications of translational frameshifting in cellular gene expression

The genetic code, once thought to be rigid, has been found to be quite flexible, permitting several different reading alternatives. One of these is translational frameshifting, a process programmed in the mRNA sequence and which enables a +1 or -1 shift from the reading frame of the initiation codon. So far, the involvement of translational frameshifting in gene expression has been described mainly in viruses (particularly retroviruses), retrotransposons, and bacterial insertion elements. In this MicroReview, we present a survey of the cellular genes, mostly in Escherichia coli, which have been found to be expressed through a translational frameshifting process, as well as a discussion of the regulatory implications of this process.

Trypanosoma cruzi infection suppresses nuclear factors that bind to specific sites on the interleukin-2 enhancer

Interleukin-2 (IL-2) gene expression, a critical early event during T lymphocyte activation, is severely suppressed in mice infected with the protozoan parasite Trypanosoma cruzi, the causative agent of human Chagas' disease. Our previous observation that reduction of IL-2 mRNA in T cells from T. cruzi-infected mice is not due to an increased degradation of the mRNA suggests a repression of the IL-2 gene at the transcriptional level. In this study, we have measured the level of nuclear factors that bind to specific sites on the IL-2 enhancer. Splenocytes and splenic T cells from acutely infected mice show a marked decrease in the level of AP-1, and a modest decrease in the level of NF-kappa B and nuclear factor of activated T cells (NF-AT). DNA-binding activity of Oct-1 was least affected in T cells from infected mice. Although the basal level of AP-1 activity is comparable in cells from uninfected and infected mice, mitogen-induced AP-1 activation is absent in the cells from T. cruzi-infected mice. Sodium deoxycholate treatment slightly enhances NF-kappa B-binding activity in splenocyte nuclear and whole-cell extracts from infected mice, suggesting that a blockage of the activation of NF-kappa B is only partially responsible for the decrease in the level of NF-kappa B in T cells from T. cruzi-infected mice. These data identify the molecular basis of IL-2 gene regulation in T. cruzi infection and suggest that T cells are anergized as a result of the infection.

The ubiquitous subunit of erythroid transcription factor NF-E2 is a small basic-leucine zipper protein related to the v-maf oncogene

Erythroid transcription factor NF-E2 is a tissue-restricted heterodimeric protein which recognizes an extended AP-1 motif [(T/C)TGCTGA(C/G)TCA(T/C)] found in the upstream locus control regions of the alpha- and beta-globin gene clusters. A cDNA clone encoding a cell-type-specific subunit of NF-E2, designated p45 NF-E2, has previously been characterized and shown to encode a basic-leucine zipper DNA-binding protein. Here we describe protein purification and cloning of cDNA that encodes the second basic-leucine zipper subunit of the native NF-E2 heterodimer. This polypeptide, designated p18, is widely expressed. It displays extensive homology to the v-maf oncogene product and a human retinal-specific protein, NRL. Unusual features in the basic region shared by v-Maf, NRL, and p18 place them in a distinct subfamily of AP-1-like proteins.

Compositional changes of AP-1 DNA-binding proteins are regulated by light in a mammalian circadian clock

Recent reports have shown that the nuclear phosphoprotein Fos is induced by light in a mammalian circadian clock, the suprachiasmatic nucleus. To learn how light and circadian phase affect the binding of Fos to DNA, we analyzed the photic and temporal regulation of immunoreactive Jun protein expression and AP-1 DNA-binding activity in the rat suprachiasmatic nucleus. Immunohistochemistry and gel mobility shift assays suggest that AP-1 activity during the night and after a light pulse consists of constant, as well as variable, protein components; JunD could be identified as a constituent of both dark- and light-activated binding complexes, whereas binding by JunB and Fos could be implicated only after photic stimulation. Since JunD or JunB could be colocalized with Fos in individual suprachiasmatic nucleus cell nuclei, light may be acting in at least some suprachiasmatic nucleus cells by altering AP-1 protein composition rather than binding site occupancy.

A fibroblast protein binds the 3'-untranslated region of pro-alpha 1(I) collagen mRNA

Post-transcriptional regulation of the expression of the pro alpha 1(I) chain of type I collagen (COL1A1) was studied by analysing cytoplasmic RNA-binding proteins and by transient transfections with collagen minigene plasmids. In this paper we present evidence for a factor from NIH 3T3 cells and human skin fibroblasts that interacts with the conserved 3'-untranslated region (UTR) of the shorter 4.8 kb mRNA species of the COL1A1 gene. The specificity of the interaction was confirmed by using (i) unlabelled specific and non-specific competitor RNAs and (ii) oligodeoxyribonucleotides annealed to the probe or used as single-stranded competitors. An antisense oligonucleotide annealed to the RNA probe near its 3'-terminus [20-42 nucleotides upstream of the first polyadenylation signal of the alpha 1(I) collagen mRNA] inhibited the binding, whereas other sense or antisense oligonucleotides had no effect on the interaction. The binding was sensitive to alkylation of free SH groups but not to phosphatase treatment of the extracts. In u.v. cross-linking analysis this factor migrated as a single polypeptide chain of about 67 kDa, and was named alpha 1-RBF67 (type I collagen alpha 1 chain RNA-binding factor). Dexamethasone treatment of fibroblasts, which is known to accelerate the turnover of COL1A1 mRNA, decreased the alpha 1-RBF67 activity markedly as evaluated by gel-retardation and u.v. cross-linking assays. Transient transfections with plasmids carrying the alpha 1(I) collagen promoter and 3'-UTR sequences demonstrated that the 3'-UTR participates in the response to dexamethasone. Thus the loss of alpha 1-RBF67 activity might be associated with decreased alpha 1(I) collagen mRNA levels after dexamethasone treatment.

Transformation by the fos or jun oncogene does not increase AP-1 DNA-binding activity

The c-fos and c-jun proto-oncogenes encode components of the transcription factor AP-1. To determine whether transformation by the v-fos or v-jun oncogene results in alterations in the level or regulation of this factor, we have characterized AP-1 DNA-binding activity in nuclear extracts prepared from v-fos- and c-fos-transformed rat fibroblast cell lines and v-jun-transformed chicken embryo fibroblasts under various growth conditions. During proliferation, the level of AP-1 DNA-binding activity does not differ among the v-fos, c-fos, or v-jun-transformed cells and their normal progenitors, despite constitutive overexpression of the corresponding oncoproteins. Therefore, although necessary, it is not likely that an increase in DNA binding is sufficient for fos or jun transformation. Normal rat and chicken fibroblasts demonstrate very low levels of AP-1 DNA-binding activity when quiescent, and upon serum stimulation a biphasic increase is observed. A similar cyclical pattern is seen in v-fos-transformed cells, but in v-jun-transformed cells AP-1 DNA-binding activity does not fluctuate in response to serum stimulation, which suggests that this level of control may be exerted through the Jun component of the AP-1 complex.

Identification of c-fos-responsive elements downstream of TAR in the long terminal repeat of human immunodeficiency virus type-1

Activation of HIV-1 requires the binding of host cell transcription factors to cis elements in the proviral long terminal repeat (LTR). This study identifies c-fos-responsive sequence motifs in the U5 transcribed noncoding leader sequences downstream of the viral transactivator responsive (TAR) element. These DNA sequence motifs are the most downstream regulatory elements described thus far in the HIV-1 LTR. Functional studies, using human colon epithelial cell lines, demonstrate that the downstream elements are transactivated by expression of the c-fos protooncogene and can transmit PMA and TNF alpha activation signals to the viral LTR. Moreover, the c-fos-responsive elements mediate HIV-1 LTR transcription independent of Tat and the NF kappa B-binding enhancer element. Nuclear extracts of colon epithelial cells form distinct gel mobility shift complexes with the c-fos-responsive elements. These complexes comigrate with a gel shift complex formed on a classical CRE oligonucleotide and are competed by CRE oligonucleotides. These data indicate that the HIV-1 LTR contains previously unrecognized functional DNA cis-regulatory elements downstream of TAR in the transcribed noncoding 5' leader sequence and suggest that early response genes such as c-fos play a role in the activation of HIV-1 gene expression.

Nuclear factor binding to an AP-1 site is associated with the activation of pro-alpha 1(I)-collagen gene in dedifferentiating chondrocytes

Isolated chondrocytes grown on plastic gradually lose their differentiated phenotype upon subculturing. This dedifferentiation is manifested by an altered production of extracellular-matrix molecules (ECM): e.g., the cartilage specific type II collagen is replaced by types I and III. We have studied the regulation of ECM gene expression in dedifferentiating human and murine fetal chondrocytes. Nuclear extracts from dedifferentiated cells, human fetal fibroblasts and 3T3 cells contained a protein that bound in an electrophoretic mobility shift assay to an AP-1 site in the first intron of the human alpha 1(I) collagen gene. This binding activity was not present in freshly isolated human or murine chondrocytes, which produced type II, but not type I, collagen mRNA in culture. Thus the binding activity was induced simultaneously with alpha 1(I)-collagen-gene expression during dedifferentiation. The specific interaction was sensitive to dephosphorylation of the nuclear extract and to chemical modification of reduced cysteine residues. The AP-1 site we studied had previously been shown to be a positive transcriptional contributor in the first intron to the expression of the alpha 1(I) collagen gene. In transient transfections into dedifferentiating chondrocytes, an alpha 1(I) collagen expression plasmid carrying a mutated AP-1 site in the first intron resulted in three-times-lower reporter gene RNA levels than a plasmid carrying the respective functional AP-1 site. These data suggest that the AP-1 sequence and its respective trans-acting factors may play a role in the transcriptional regulation of the alpha 1(I) collagen gene during dedifferentiation of chondrocytes.

A Fos-Jun element in the first intron of an alpha 2u-globulin gene

The hepatic expression of the alpha-2u-globulin gene family is controlled by a variety of hormones including steroids, growth hormone and insulin. The mechanisms by which these hormones affect alpha 2u-globulin expression are only partially understood. Recently we isolated and characterized clone RAP 01, an alpha 2u-globulin gene expressed in the liver. In preliminary experiments we noted that partial hepatectomy, a procedure which results in a sharp rise in the level of the oncoproteins c-Fos and c-Jun, also causes a transient induction of the messenger RNA corresponding to clone RAP 01. Using the DNAseI footprinting technique we were able to show that this clone contains a TPA (phorbol 12-myristate 13-acetate)-responsive element (TRE) in its first intron. This element (denoted as element X) is identical to the consensus AP-1 binding site (TGACTCAG) and is protected by rat liver nuclear extracts as well as by purified c-Jun. Gel retardation experiments show that an oligonucleotide containing the TRE consensus sequence competes for binding of liver nuclear proteins to element X and that antibodies directed against the M2 peptide of the mouse Fos protein or the PEP-2 peptide of Jun prevent the formation of specific complexes with the same element. Moreover, element X functions as a TRE in transfected BWTG3 hepatoma cells treated with TPA. Co-transfection with fos and jun expression vectors mimics the effects of TPA suggesting that AP-1 is in fact the mediator of the observed response. It is concluded that the first intron of RAP 01 contains a functional Fos-Jun element.

Developmental extinction of liver lipoprotein lipase mRNA expression might be regulated by an NF-1-like site

The molecular mechanism underlying the extinction of lipoprotein lipase (LPL) expression in rat liver during development was investigated. A mouse (BWTG3) and a rat (7777) hepatoma, both of which exhibit characteristics of fetal hepatocytes, were found to contain LPL mRNA, whereas the more differentiated human (Hep G2 and Hep 3B) or rat (Fa32) hepatoma cell lines did not. Somatic cell hybrids between LPL-producing hepatoma cells and non-LPL-producing cells, such as adult rat hepatocytes or fibroblasts, exhibited extinction of LPL gene expression. Assay of expression of nested deletions in the 5' regulatory sequences of the LPL gene in the Hep G2 cell line and in BWTG3 cells localized sequences involved in the suppression of LPL production to a region between -591 and -288 relative to the transcription initiation site. A site with sequence homology to a glucocorticoid responsive element (GRE) was shown not to play an important role in the extinction process. A novel transcription factor, termed RF-1-LPL, was shown to bind to an NF-1-like site in this region. In contrast to neonatal animals, in adult animals an additional protein complex (RF-2-LPL), was formed on the NF-1-like site, suggesting that this sequence might recruit a trans-acting factor involved in the extinction of LPL gene expression in adult rat liver.

Translational introns: an additional regulatory element in gene expression

The linear expression of a gene can be interrupted by the well-known RNA introns and the recently discovered protein introns. In both cases, splicing mechanisms physically excise the unexpressed segments. In this article we describe a third category of introns that we call 'translational introns'. These functional introns are not excised through a splicing mechanism; instead, the translational machinery bypasses a segment of the coding sequence of an mRNA. We suggest that 'translational introns' are part of a regulatory mechanism that may sense changes in the rate of translation and thereby control the ratio of alternative gene products.

A Jun-binding protein related to a putative tumor suppressor

A lambda gt11 cDNA library of chicken embryo fibroblasts was screened with biotinylated Jun protein to identify Jun-binding clones. Eight such clones were isolated; one contains a gene referred to as jif-1 that is homologous to the putative tumor suppressor gene QM. jif-1 codes for a protein of 25 kDa that binds to the leucine zipper of viral and cellular Jun. The Jif-1 protein also binds to itself. Jif-1 does not contain a leucine zipper, and it does not bind to the 12-O-tetradecanoylphorbol 13-acetate response element DNA sequence. Complex formation of Jif-1 with Jun inhibits DNA binding and reduces transactivation by Jun. Addition of Fos protein to Jun-Jif-1 complexes restores DNA-binding activity. These observations suggest that Jif-1 is a negative regulator of Jun.

Resolution of multiple AP-1 complexes in HL-60 cells induced to differentiate by 1,25-dihydroxyvitamin D3

Activator protein-1 (AP-1) complex plays a central role in the regulation of both growth and differentiation in many cell types. Monocytic differentiation of HL-60 cells by TPA (12-0-tetradecanoyl phorbol-13-acetate) has been reported to be paralleled by increased AP-1 binding to DNA and by elevated c-jun expression, suggesting transcriptional level of control. We show that two forms of AP-1 complex, designated AP-1/1 and AP-1/2, can be demonstrated in logarithmically growing HL-60 cells, that the exposure of these cells to 10(-8) M 1,25-dihydroxyvitamin D3 (1,25(OH)2D3) results in increased binding of these complexes to the AP-1 DNA element, and that the AP-1 complex can be resolved into at least three forms in differentiated cells. Binding to, or competition with, a mutated form of the AP-1 binding site shows that the most slowly migrating complex (AP-1/3) binds to DNA with greater specificity than do complexes AP-1/1 and AP-1/2, while antibody inhibition and binding studies performed at 37 degrees C indicate that jun proteins predominate in AP-1/2 complexes. Exposure of extracts from differentiated, but not untreated, HL-60 cells to 2 mM ATP increases the prominence of AP-1/3 complexes, and reduces the DNA binding of AP-1/1 complexes. Treatment of the extracts with phosphatases abolishes the binding of AP-1/2 and AP-1/3 to DNA, and increases the binding intensity of AP-1/1. When extracts from differentiated cells are mixed with extracts from undifferentiated cells the AP-1/3 complexes become less prominent, suggesting than an inhibitory activity in undifferentiated cells prevents the formation of AP-1/3 complexes. These studies show the association of multiple forms of AP-1 complex with the mature monocytic phenotype, and suggest several levels of control of monocytic differentiation.

Molecular regulation of the IL-2 gene: rheostatic control of the immune system

The delivery of costimulation and the effects of the anergic state impinge on IL-2 production via different molecular mechanisms. The strongest experimental support at this stage suggests that CD28 signaling effects mRNA stability of several lymphokine genes including IL-2. While there may also be transcriptional effects of CD28 signals in human cells, controversy surrounding relevant TCR mimics must be addressed. In the case of clonal anergy, however, transcriptional non-responsiveness is evident when anergic cells are restimulated with TCR and costimulatory signals. This repression affects predominantly AP-1 activity. So far, the nature of the repression has not been identified.

Recognition of distinct HLA-DQA1 promoter elements by a single nuclear factor containing Jun and Fos or antigenically related proteins

The activity of MHC class II promoters depends upon conserved regulatory signals one of which, the extended X-box, contains in its X2 subregion a sequence related to the cAMP response element, CRE and to the TPA response element, TRE. Accordingly, X2 is recognized by the AP-1 factor and by other c-Jun or c-Fos containing heterodimers. We report that the X-box dependent promoter activity of the HLA-DQA1 gene is down-modulated by an array of DNA elements each of which represented twice either in an invertedly or directly repeated orientation. In this frame, we describe a nuclear binding factor, namely DBF, promiscuously interacting with two of these additional signals, delta and sigma, and with a portion of the X-box, namely the X-core, devoid of X2. The presence of a single factor recognizing divergent DNA sequences was indicated by the finding that these activities were co-eluted from a heparin-Sepharose column and from DNA affinity columns carrying different DNA binding sites as ligands. Competition experiments made with oligonucleotides representing wild type and mutant DNA elements showed that each DNA element specifically inhibited the binding of the others, supporting the contention that DBF is involved in recognition of different targets. Furthermore, we found that DBF also exhibits CRE/TRE binding activity and that this activity can be competed out by addition of an excess of sigma, delta and X-core oligonucleotides. Anti-Jun peptide and anti-Fos peptide antibodies blocked not only the binding activity of DBF, but also its X-core and sigma binding; this blockade was removed by the addition of the Jun or Fos peptides against which the antibodies had been raised. In vitro synthesized Jun/Fos was able to bind to all these boxes, albeit with seemingly different affinities. The cooperativity of DBF interactions may explain the modulation of the X-box dependent promoter activity mediated by the accessory DNA elements described here.

Synergism between protein-kinase C and cAMP-dependent pathways in the expression of the interleukin-1 beta gene is mediated via the activator-protein-1 (AP-1) enhancer activity

In many different cell types treatment with phorbol esters (e.g. 4 beta-phorbol 12-myristate 13-acetate, PMA) leads to the activation of protein-kinase C (PKC) and subsequently to the activation of the activator-protein-1(AP-1)-responsive gene expression. We have previously reported that a structural analog of cAMP (dibutyryl cAMP, Bt2cAMP) or agents elevating the endogenous cAMP levels strongly enhanced the PMA-induced interleukin-1 beta(IL-1 beta)-gene expression in human myeloid leukemia cells (THP-1, HL-60). We have now examined the role of AP-1 in the regulation of the IL-1 beta gene expression by PKC and cAMP in THP-1 cells. AP-1 is a complex composed of products of the jun and fos gene families. Our studies show that Bt2cAMP enhances the PMA-induced c-fos and jun-B expression, but inhibits c-jun expression. Electrophoretic mobility-shift assay revealed that Bt2cAMP also increased the PMA-induced AP-1 DNA-binding activity. The functional role of the increased AP-1 DNA-binding activity was studied by transfecting THP-1 cells with reporter constructs containing AP-1 sites [Col-TREx5/TK-CAT and IL-1 beta-X-CAT, which contains the putative 12-O-tetradecanoyl-phorbol-13-acetate(TPA)-responsive element of the IL-1 beta gene]. Transient transfection assay demonstrated that Bt2cAMP similarly increased the PMA-induced transcription from both of these reporter constructs. Taken together, these results suggest that cAMP increases the PMA-induced AP-1 activity which then leads to increased IL-1 beta expression.

Techniques for cloning cDNAs encoding interactive transcriptional regulatory proteins

Several approaches aimed at detecting and cloning interactive transcriptional regulatory proteins have been presented. All of the techniques can effectively identify specific interactions between two transcription proteins. However, interaction cloning and the two hybrid system have the added advantage of yielding a cDNA expression clone directly. The other methods, EMSA-mediated cloning, co-immunoprecipitation, oligonucleotide/PCR-facilitated cloning, Southwestern, and Farwestern, require additional manipulations to obtain a cDNA clone. Clearly, the interactive cloning system of choice will depend on the proteins under investigation.

The mechanism by which the human apolipoprotein B gene reducer operates involves blocking of transcriptional activation by hepatocyte nuclear factor 3

Previously, we showed that when a DNA fragment extending from -3067 to -2734 of the human apolipoprotein B (apo-B) gene is inserted immediately upstream of an apo-B promoter segment (-139 to +121), transcription from this promoter is reduced by about 10-fold in cultured colon carcinoma cells (CaCo-2) but not in cultured hepatoma cells (HepG2). We postulated that this reducer operates by a mechanism involving active repression of a transcriptional activator that binds to the segment from -111 to -88 of the apo-B promoter (B. Paulweber and B. Levy-Wilson, J. Biol. Chem. 266:24161-24168 1991). In the current study, the reducer element has been localized to a 24-bp sequence from -2801 to -2778 of the apo-B gene that contains a binding site for the negative regulatory protein ARP-1. Furthermore, we have demonstrated that the transcription factor hepatocyte nuclear factor 3 alpha (HNF-3 alpha) binds to the sequence 5'-TGTTTGCTTTTC-3' from -95 to -106 of the apo-B promoter, to stimulate transcription. Transcriptional activation by HNF-3 is repressed when the reducer sequence is inserted immediately upstream of the HNF-3 binding site, suggesting a mechanism by which the reducer-bound protein blocks the activation promoted by HNF-3. Data from cotransfection experiments in which ARP-1 is overexpressed in the absence of its binding site suggest that ARP-1 interacts either directly or via a mediator protein with proteins recognizing the HNF-3 site and that this interaction is sufficient to repress transcriptional activation by HNF-3. Because transcriptional activation by Sp1 is not affected by the reducer, it is unlikely that the reducer interacts directly with basic components of the transcriptional machinery.

Expression of c-jun/AP-1 during myogenic differentiation in mouse C2C12 myoblasts

Mitogen withdrawal triggers myogenic differentiation in skeletal myoblasts in culture. We have examined the expression of the proto-oncogene c-jun during this process in mouse C2C12 myoblasts. c-jun belongs to a family of immediate early genes whose expression is activated in cultured cells in response to the addition of serum growth factors. Interestingly, expression of c-jun was maintained in mouse C2C12 and rat L6 myoblasts undergoing myogenic differentiation under low-serum conditions. Previously it has been reported that expression of c-jun is downregulated during differentiation of C2 cells. However, our results using C2C12 cells, a subclone of the C2 line, show that c-jun mRNA, protein and the activator-protein 1 (AP-1) DNA-binding activity were easily detected in proliferating myoblasts and differentiated myotubes. Although overexpression of c-jun has been shown to block myogenic differentiation in C2 cells, results presented here suggest that expression of c-jun at physiological levels may not interfere with skeletal myogenesis.

The mechanism by which the human apolipoprotein B gene reducer operates involves blocking of transcriptional activation by hepatocyte nuclear factor 3

Previously, we showed that when a DNA fragment extending from -3067 to -2734 of the human apolipoprotein B (apo-B) gene is inserted immediately upstream of an apo-B promoter segment (-139 to +121), transcription from this promoter is reduced by about 10-fold in cultured colon carcinoma cells (CaCo-2) but not in cultured hepatoma cells (HepG2). We postulated that this reducer operates by a mechanism involving active repression of a transcriptional activator that binds to the segment from -111 to -88 of the apo-B promoter (B. Paulweber and B. Levy-Wilson, J. Biol. Chem. 266:24161-24168 1991). In the current study, the reducer element has been localized to a 24-bp sequence from -2801 to -2778 of the apo-B gene that contains a binding site for the negative regulatory protein ARP-1. Furthermore, we have demonstrated that the transcription factor hepatocyte nuclear factor 3 alpha (HNF-3 alpha) binds to the sequence 5'-TGTTTGCTTTTC-3' from -95 to -106 of the apo-B promoter, to stimulate transcription. Transcriptional activation by HNF-3 is repressed when the reducer sequence is inserted immediately upstream of the HNF-3 binding site, suggesting a mechanism by which the reducer-bound protein blocks the activation promoted by HNF-3. Data from cotransfection experiments in which ARP-1 is overexpressed in the absence of its binding site suggest that ARP-1 interacts either directly or via a mediator protein with proteins recognizing the HNF-3 site and that this interaction is sufficient to repress transcriptional activation by HNF-3. Because transcriptional activation by Sp1 is not affected by the reducer, it is unlikely that the reducer interacts directly with basic components of the transcriptional machinery.

Current concepts in DRA gene regulation

The purpose of this review is to provide an interpretative view of work from our laboratory on the DRA gene, and incorporate it with work from other laboratories. Specially, we will deal with: (a) the functional roles of transcription factors in DRA gene regulation; (b) the mechanisms of DRA induction by cytokines; (c) the analysis of DRA gene control in primary untransformed cells, and (d) interactions among transcription factors critical for DRA gene expression.

Reversion of a transcriptionally defective MHC class II-negative human B-cell mutant

RJ2.2.5, a mutant derived from the human B-lymphoma cell, Raji, is unable to express the MHC class II genes because of a recessive transcriptional defect attributed to the lack of an activator function. We report the isolation of a RJ2.2.5 revertant, namely AR, in which the expression of the mRNAs encoded by these genes is restored. Comparison of the binding of nuclear extracts or of partially purified nuclear preparations from the wild-type, the mutant and the revertant cells to a conserved MHC class II promoter element, the X-box, showed no alteration in the mobility of the complexes thus formed. However, in extracts from RJ2.2.5, and other MHC class II negative cell lines, such as HeLa, the amount of complex observed was significantly higher than in wild-type Raji cells. Furthermore, the binding activity exhibited by the AR revertant was lower than that of the RJ2.2.5 and higher than that of Raji. The use of specific monoclonal antibodies indicated that in all cases c-Jun and c-Fos or antigenically related proteins were required for binding. An inverse correlation between the level of DNA-protein complex formed and the level of MHC class II gene mRNA expressed in the three cell lines was apparent, suggesting that overexpression of a DNA binding factor forming complexes with class II promoter elements may cause repression of MHC class II transcription. A model which reconciles the previously ascertained recessivity of the phenotype of the mutation carried by RJ2.2.5 with the findings reported here is discussed.

Cross-coupling of signal transduction pathways: the dioxin receptor mediates induction of cytochrome P-450IA1 expression via a protein kinase C-dependent mechanism

Signal transduction by dioxin (2,3,7,8-tetrachlorodibenzo-p-dioxin) is mediated by the intracellular dioxin receptor which, in its dioxin-activated state, regulates transcription of target genes encoding drug-metabolizing enzymes, such as cytochrome P-450IA1 and glutathione S-transferase Ya. Exposure of the dioxin receptor to dioxin leads to an apparent translocation of the receptor to the nucleus in vivo and to a rapid conversion of the receptor from a latent, non-DNA-binding form to a species that binds to dioxin-responsive positive control elements in vitro. This DNA-binding form of receptor appears to be a heterodimeric complex with the helix-loop-helix factor Arnt. In this study, we show that activation of the cytochrome P-450IA1 gene and minimal dioxin-responsive reporter constructs by the dioxin receptor was inhibited following prolonged treatment of human keratinocytes with the phorbol ester 12-O-tetradecanoylphorbol-13-acetate. Inhibition of the receptor-mediated activation response was also achieved by treatment of the cells with a number of protein kinase inhibitors, one of which, calphostin C, shows selectivity for protein kinase C. Taken together, these data suggest that protein kinase C-dependent phosphorylation may play an essential role in the dioxin signaling pathway. This hypothesis is supported by the observation that pretreatment of the cells with 12-O-tetradecanoylphorbol-13-acetate inhibited the DNA-binding activity of the dioxin receptor in vivo. In vivo, the dioxin receptor was found to be a phosphoprotein. In vitro, dephosphorylation of the ligand-activated, heteromeric dioxin receptor form or dephosphorylation of the individual ligand-binding and Arnt receptor subunits inhibited the xenobiotic response element-binding activity. Moreover, dephosphorylation experiments with the individual receptor subunits prior to assembly of the xenobiotic response element-binding receptor form indicated that phosphorylation seemed to be important for the DNA-binding activity per se of the receptor, whereas Arnt appeared to require phosphorylation to interact with the receptor. Finally, a protein kinase C inhibitor-sensitive cytosolic catalytic activity that could restore the DNA-binding activity of the dephosphorylated dioxin receptor form was identified.

Endothelins stimulate c-fos and nerve growth factor expression in astrocytes and astrocytoma

Endothelin receptors have been identified on astrocytes and astrocytoma, but their physiological significance has remained elusive. It is shown here that endothelins induce c-fos in primary cultures of mouse embryo astrocytes, as well as in two subclones of rat astrocytoma C6 cells, although with different kinetics. In addition, nerve growth factor expression is stimulated, as seen by mRNA accumulation and protein secretion, in primary astrocytes and one of the two C6 subclones, with an apparent correlation with the transience of c-fos induction. The activation of protein kinase C appears as an obligatory step during these processes, because (a) inhibition of protein kinase C by staurosporine blocks the induction by endothelin or phorbol esters of both c-fos and nerve growth factor, and (b) phorbol ester-evoked down-regulation of protein kinase C completely abolishes the c-fos induction by endothelin, but not that by the beta-adrenergic agonist isoproterenol, a known activator of the cyclic AMP-dependent pathway. Our results support the hypothesis that c-fos product might be implicated in nerve growth factor expression by astrocytes, and also suggest that endothelins may participate in vivo in the modulation of the glial neurotrophic activity during brain development or wound healing.