Differentiation-responsive elements in the 5' region of the mouse tissue plasminogen activator gene confer two-stage regulation by retinoic acid and cyclic AMP in teratocarcinoma cells

Regulation of E2F/cyclin A-containing complex upon retinoic acid-induced differentiation of teratocarcinoma cells

Retinoic acid-induced differentiation of mouse P19 teratocarcinoma cells is accompanied by alterations in the level of E2F transcription factor. P19 stem cells contain free, uncomplexed E2F and an E2F complex termed E2F/stem. This stem cell complex is a heterotrimeric protein aggregate consisting of E2F transcription factor, E2F-binding protein (E2F/bp1), and cyclin A. Retinoic acid treatment converts P19 stem cells into differentiated neurons, glial cells, and fibroblasts. The presented experiments clearly show that the level of uncomplexed E2F gradually decreases upon differentiation, and fully differentiated cells do not contain free E2F. In addition, the stem cell-specific E2F aggregate is converted into a smaller complex, termed E2F/diff. This smaller complex, which is specific for differentiated cells, does not contain cyclin A and consists of E2F transcription factor associated with E2F/bp1. Finally, the role of E2F complexes in the cessation of cell proliferation, which accompanies P19 cell differentiation, is discussed.

Retinoids regulate human amniotic tissue-type plasminogen activator gene by a two-step mechanism

The collagenolytic effects of the tissue-type plasminogen activator (t-PA) leading to extracellular matrix degradation are clearly involved in the physiopathology of human foetal membranes rupture. Nevertheless, the regulation of t-PA gene expression in extraembryonic developmental contexts remains unknown. The aim of our study is to propose the retinoic acids (RAs) as molecular regulators of t-PA expression in foetal membranes. RA induced t-PA mRNA and proteins in a time-dependent manner in amniotic membrane explants and Wistar Institute Susan Hayflick (WISH) cells. Furthermore, the use of cycloheximide revealed a two-step regulation of t-PA gene. Gene reporter assays confirmed that the RA-induced t-PA gene expression occurred through interactions of retinoid receptors (RARs and RXRs) with a DR5 response element located at –7 kb from the transcription site. Site-directed mutagenesis of this region of the t-PA promoter showed that SP1 factor was also retinoid-mediated induction, and immunoprecipitation assays revealed that SP1 and RAR/RXR interacted physically. Chromatin immunoprecipitation demonstrated that interactions between RARs, RXRs and t-PA promoter were time dependent: RAR-α/RXR-α bound DR5 motif before and up to 12 hrs of RA exposure, and RAR-β/RXR-α bound DR5 response element after 12 hrs of RA treatment. Finally, experiments using shRNA and RAR-β-specific antagonist revealed that reducing RAR-β induction decreased t-PA induction. Altogether, our results established that the RA-mediated regulation of t-PA in human foetal membranes occurred through two steps, with a major role played by RAR-β.

In vivo and in vitro analysis of the human tissue-type plasminogen activator gene promoter in neuroblastomal cell lines: evidence for a functional upstream kappaB element

Besides its well-established role in wound healing and fibrinolysis, tissue-type plasminogen activator (t-PA) has been shown to contribute to cognitive processes and memory formation within the central nervous system, and to promote glutamate receptor-mediated excitotoxicity. The t-PA gene is expressed and regulated in neuronal cells but the regulatory transcriptional processes directing this expression are still poorly characterized. We have used DNase I-hypersensitivity mapping and in vivo foot printing to identify putative regulatory elements and transcription factor binding sites in two human neuroblastomal (KELLY and SK-N-SH) and one human glioblastomal (SNB-19) cell lines. Hypersensitive sites were found in the proximal promoter region of all cell lines, and within the first exon for KELLY and SNB-19 cells. Mapping of methylation-protected residues in vivo detected a cluster of protected residues corresponding to a cAMP response element (CRE) and Sp1 sites in the proximal promoter previously shown to be essential for basal expression in other cell types. Protected residues were also found at other sites, notably a kappaB element at position bp -3081 to -3072 that was partly protected in KELLY and SNB-19 cells. Analysis of transfected reporter constructs in KELLY and SNB-19 cells confirmed that this particular element is functionally significant in the transactivation of the t-PA promoter in both cell types. This study defines, by in vivo and in vitro methods, a previously undescribed kappaB site in the t-PA gene promoter that influences t-PA expression in neuronal cells.

NF1 Regulatory Element Functions as a Repressor of Tissue Plasminogen Activator Expression

OBJECTIVE

Analysis of the distribution of endothelial cell tissue plasminogen activator (tPA) in the vasculature of rodents and primates demonstrated that tPA is constitutively expressed predominantly in small artery endothelial cells of brain and lung. The regulatory elements responsible for the highly selective expression of arterial endothelial cell tissue plasminogen activator were sought.

METHODS AND RESULTS

Transcription factor binding sites were defined by electrophoretic mobility-shift assay (EMSA) analysis using rat lung and brain nuclear extracts and the tPA promoter sequence from -609 to +37 bp. Protein binding to the promoter was found to be mediated by an NF1 site between -158 and -145 bp upstream from the transcriptional start site. Specific binding was confirmed through mutational analysis and competition binding studies. Infection of endothelial cells with a tPA promoter-green fluorescent protein (GFP) (-609 to +37 bp) reporter construct resulted in expression of the GFP, whereas no expression was found in smooth muscle cells. Mutation of the NF1 site increased the GFP expression indicating that the element acts as a repressor.

CONCLUSIONS

These results suggest that the 600 bp of the tPA promoter upstream of the transcription start site conveys cell specificity to tPA expression and that an NF1 site within this region acts as a repressor.

Absence of tissue plasminogen activator gene or activity impairs mouse cerebellar motor learning

Motor learning is thought to involve modulation of synaptic inputs in the cerebellar cortex, including granule neuron/Purkinje neuron contacts. During a complex motor task requiring mice to walk across irregularly spaced pegs, cerebellar granule neurons show a rapid and transient induction of mRNA for the extracellular protease tissue plasminogen activator (tPA). This induction of tPA mRNA is cerebellar specific, is not seen in the cerebella of exercised or stressed animals, and is distinct from simple performance phenomena. Knock-out mice lacking the tPA gene show a significant reduction in both rate and extent of learning. Furthermore, blocking tPA activity during training dramatically impaired motor learning. Thus, tPA plays an important role in motor learning, in which tPA may facilitate remodeling of the active synaptic zone.

Induction of tissue plasminogen activator in differentiated NG108-15 cells

Plasminogen activators may facilitate neurite outgrowth and neuronal migration in the developing nervous system. The expression of tissue plasminogen activator by NG108-15 neuroblastoma grown under a variety of conditions has been explored. High tissue plasminogen mRNA expression correlates with growth conditions which induce morphological differentiation and neurite outgrowth; however, NG108-15 cells grown in suspension with dibutyryl-cAMP also show a high level of tissue plasminogen activator expression.

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

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

The vimentin promoter as a tool to analyze the early events of retinoic acid-induced differentiation of cultured embryonal carcinoma cells

The vimentin gene encodes an intermediate filament protein expressed in the parietal endoderm, mesodermal, and early neural cells in vivo but by most in vitro-cultured cells regardless of their embryonic origin. Here we show that the vimentin gene promoter is very active in F9 embryonal carcinoma cells and increases in activity during differentiation. Using a series of 5'-deletion mutants, we provide evidence that the regions of the promoter involved in F9 cell activity are different from those previously demonstrated to be active in differentiated cell lines. Furthermore, we show that in differentiating F9 cells the activities of two different regions of the promoter are significantly enhanced. A distal region (-1710/-957) appears to contain functional binding sites for the murine Hox-A5 homeoprotein as demonstrated by band shift and footprinting experiments. A proximal region (-140/-78) contains a 30-bp repetitive sequence found in other genes activated during differentiation of F9 cells. Using band shift assays and methylation interference, we present evidence that a sequence-specific single-stranded DNA-binding protein(s) specifically interacts with the minus strand of the 30-bp sequence.

Identification of a retinoic acid response domain involved in the activation of the beta 1-adrenergic receptor gene by retinoic acid in F9 teratocarcinoma cells

The density of beta 1-adrenergic receptors (beta 1-AR) is up-regulated upon differentiation of embryonic F9 teratocarcinoma cells by retinoic acid (RA) to the primitive endodermal phenotype. To identify the domains involved in RA-mediated activation of beta 1-AR gene transcription, three kb of 5'-flanking sequence of the beta 1-AR gene were ligated to a luciferase reporter gene and transiently transfected into F9 cells that were pre-exposed to 100 nM RA for 2 days. By generating deletions in the beta 1-AR promoter, a region between -125 and -100 was found to mediate a 3-fold induction in cells exposed to RA for an additional 2 days. Through site-directed mutagenesis of this region, it was determined that the RA responsive element (RARE) was organized as a direct repeat separated by 5 nucleotides in which the 5'-most AGGTCG half-site was between nucleotides -106 and -101 and the 3'-most AGGTCA half-site was between nucleotides -117 and -112. The RA receptor alpha (RAR alpha) isoform bound to the oligomer representing the sequences between -125 and -100 as a heterodimer complex with the retinoid X receptor alpha (RXR alpha). In a separate study, it was determined that the nucleotides between -125 and -100 are involved in thyroid hormone-mediated activation of the beta 1-AR gene in ventricular myocytes. Therefore, transcriptional activation of the beta 1-AR gene by thyroid hormone or RA involves a single binding site in the promoter.

Characterisation of the rat tissue-type plasminogen activator gene promoter -- identification of a TAAT-containing promoter element

Tissue-type plasminogen activator (tPA) activates plasminogen to the active protease plasmin and is implicated in many biological processes that require extracellular proteolysis. In rat ovarian cells, gonadotropins induce the tPA gene by a cAMP-dependent pathway and this induction correlates with the time of follicular rupture. We have previously identified several promoter elements within the first 621 bp of the rat tPA promoter that are important for constitutive and cAMP-induced expression of the gene, including a cAMP responsive element (CRE), a nuclear factor 1 (NF1) element, a SP1-binding site and a G+C-rich box. In this report we have extended our study by analysing promoter constructs, ranging in size from 7.7 kb to 135 bp fused to the luciferase reporter gene. Transient transfection analysis of rat granulosa cells and human 293 cells, reveal that the proximal 268 bp of the promoter is enough to confer high basal and cAMP-induced expression of the gene. At position -162 to -172, between the previously identified CRE and NF1 sites, a novel TAAT-containing promoter element was identified. Mutational inactivation of the TAAT motif indicates that this element is important for both constitutive and cAMP-induced expression of the gene, and for the binding of a presumably novel nuclear factor that we have termed tPA promoter factor-1 (tPF-1).

Regulation of ribosomal RNA gene transcription during retinoic acid-induced differentiation of mouse teratocarcinoma cells

We have examined the mechanism of regulation of rRNA synthesis in mouse F9 teratocarcinoma cells that were induced to differentiate by retinoic acid and dibutyryl cAMP. Ribosomal RNA (rRNA) synthesis was significantly reduced during differentiation of F9 cells into parietal endoderm cells. Nuclear run-on assay revealed that the rRNA gene transcription rates were reduced in differentiated cells, and this phenomenon could be mimicked by in vitro transcription assay using nuclear extracts prepared from F9 stem and F9 parietal endoderm cells. Analysis of the DNA-binding activities of two RNA polymerase I (pol I) transcription factors E1BF/Ku and UBF revealed decreased affinity for their cognate recognition sequences. Immunoblot analysis showed a marked reduction in the amounts of E1BF/Ku and UBF in the differentiated cells. Analysis of the steady-state RNA levels for the smaller subunit of E1BF/Ku and for UBF in differentiating F9 cells revealed decreased mRNA synthesis and increase in message level for the differentiation-specific marker laminin B1 with progression of the differentiated status of the cells. This study has demonstrated that differentiation of mouse F9 teratocarcinoma cells into parietal endoderm cells leads to diminished rRNA synthesis, which may be mediated by reduced DNA-binding activities and amounts of at least two pol I transcription factors.

Glial and muscle embryonal carcinoma cell-specific independent regulation of expression of human JC virus early promoter by cyclic AMP response elements and adjacent nuclear factor 1 binding sites

The human polyoma JC virus (JCV) is a glial cell-specific virus and is the etiological agent for the terminal AIDS-associated brain disease, progressive multifocal leukoencephalopathy (PML). JCV contains several binding sites for transcriptional factors that are important for activity in glial cells, including cyclic AMP (cAMP) response elements (CREs) which are four nucleotides from nuclear factor 1 (NF1) sites within the two 98 bp repeat regions. We studied the combined role of cAMP and NF1 in regulating the expression of the JCV early promoter-enhancer (JCVE) in differentiating glial and muscle P19 embryonal carcinoma cells. JCVE expression remained several-fold higher in the presence of cAMP in glial cells, irrespective of whether the relatively strong activity of JCVE was greatly reduced by NF1 site mutations. In contrast, cAMP had no effect in muscle cells, independent of whether the modest activity of JCVE was two-fold higher due to NF1 site mutations. The in vivo effects were confirmed with in vitro transcription assays using glial cell extracts, competitors of CRE, and the NF1 site, and single repeat JCVE region with mutations in the NF1 II/ III binding sites as templates. The in vitro results also indicated that the effects were due to the CREs of JCV, rather than to the indirect effects of cAMP. Overall, the results indicated that NF1 and cAMP have independent, different, tissue-specific, and direct effects in the regulation of JCVE. These effects may contribute the neurotropic PML-inducing pattern of expression of JCVE.

Transcriptional regulation of alpha1,3-galactosyltransferase in embryonal carcinoma cells by retinoic acid. Masking of Lewis X antigens by alpha-galactosylation

Treatment of mouse teratocarcinoma F9 cells with all-trans-retinoic acid (RA) causes a 9-fold increase in steady-state levels of mRNA for UDP-Gal:beta-D-Gal alpha1,3-galactosyltransferase (alpha1,3GT) beginning at 36 h. Enzyme activity rises in a similar fashion, which also parallels the induction of laminin and type IV collagen. Nuclear run-on assays indicate that this increase in alpha1,3GT in RA-treated F9 cells, like that of type IV collagen, is transcriptionally regulated. Differentiation also results in increased secretion of soluble alpha1,3GT activity into the growth media. The major alpha-galactosylated glycoprotein present in the media of RA-treated F9 cells, but not of untreated cells, was identified as laminin. Differentiation of F9 cells is accompanied by an increase in alpha-galactosylation of membrane glycoproteins and a decrease in expression of the stage-specific embryonic antigen, SSEA-1 (also known as the Lewis X antigen or LeX), which has the structure Galbeta1-4(Fucalpha1-3)GlcNAcbeta1-R. However, flow cytometric analyses with specific antibodies and lectins, following treatment of cells with alpha-galactosidase, demonstrate that differentiated cells contain LeX antigens that are masked by alpha-galactosylation. Thus, RA induces alpha1,3GT at the transcriptional level, resulting in major alterations in the surface phenotype of the cells and masking of LeX antigens.

The species-specific differences in the cAMP regulation of the tissue-type plasminogen activator gene between rat, mouse and human is caused by a one-nucleotide substitution in the cAMP-responsive element of the promoters

In rat ovarian cells tissue-type plasminogen activator (tPA) is induced by gonadotropins, by a cAMP-dependent pathway and the induction correlates with the time of follicle rupture in vivo. However, in mice, gonadotropins induce the related but distinct protease urokinase-type plasminogen activator (uPA). Comparison of rat, mouse and human tPA genes reveal that there is a species-specific difference in the promoter that could explain the difference in regulation of the tPA gene between these species. At the position where the rat promoter contains a consensus cAMP-responsive element (CRE), the mouse and human counterparts contains a CRE variant with a one-nucleotide substitution. Transient transfection experiments of rat glial and granulosa cells demonstrated that reporter constructs driven by rat but not mouse or human tPA promoters were efficiently induced by the cAMP-inducing agents forskolin or follicle-stimulating hormone. Following the conversion of the mouse and human CRE-like sequences to rat consensus CRE these promoters became cAMP responsive. In contrast the rat promoter, following conversion of the consensus CRE to the corresponding mouse and human CRE-like sequence, lost the ability to efficiently respond to cAMP. Deoxyribonuclease I footprinting analysis and electrophoretic mobility shift assays were used to examine interactions of nuclear factors with the consensus and variant CRE. Compared to rat CRE, the mouse and human CRE-like sequences had a drastically reduced binding affinity for a nuclear factor identified as the cAMP-responsive element binding protein. Thus the inability of the mouse and human tPA promoters to respond efficiently to forskolin and follicle-stimulation hormone seem to be due to the inability of these CRE-like sequences to efficiently bind transcription factor CRE binding protein.

The kinetics of induction of Hox1.6 and C-jun mRNA during three different ways of inducing differentiation in teratocarcinoma F9 cells

Changes in Hox1.6 and c-jun gene expression were examined upon F9 cell differentiation that was induced by three independent methods: a drug treatment with retinoic acid (RA), that with sodium butyrate (NaB), and a genetic approach using the ts mutant. To obtain further information on the mechanism of teratocarcinoma cell differentiation we have examined the kinetics of the induction of Hox1.6 and c-jun mRNA whose gene products have been demonstrated to have specific roles in gene regulation. Expression of Hox1.6 mRNA was induced more rapidly than c-jun mRNA by all the above three inducing methods. Furthermore, protein synthesis was not required for the induction of Hox1.6 mRNA as well as of c-jun mRNA synthesis in all three methods. The data suggested that the transcriptional increase in the Hox1.6 mRNA was a primary response and could play an important role in F9 cell differentiation.

The mouse tissue plasminogen activator gene 5′ flanking region directs appropriate expression in development and a seizure-enhanced response in the CNS

Tissue plasminogen activator (t-PA) is a secreted serine protease implicated in multiple aspects of development. In the adult rat brain, transcription of t-PA is an immediate-early response in the hippocampus following treatments that induce neuronal plasticity. To study the sequence elements that govern transcription of this gene, in situ analysis was used to define t-PA's temporal and spatial expression pattern in midgestation embryos. Transgenic mice were then generated carrying t-PA 5′ flanking sequences linked to the E. coli lacZ gene. Constructs containing 4 kb of the flanking sequences (4.0TAMGAL) confer beta-galactosidase activity mostly to the same tissues that exhibit high levels of t-PA mRNA by in situ analysis. In 4.0TAMGAL embryos from embryonic day 8.5 (E8.5) to 13.5 (E13.5), the majority of expression observed is localized to neural ectoderm-derived tissues. beta-galactosidase activity is first detected in restricted neuromeres in the midbrain and diencephalon, at E8.5 and E9.5 respectively. At E10.5, transgene expression is observed in neural crest-derived cranial nerves and dorsal root ganglia, but not placode-derived cranial nerves. From E10.5 to E13.5, beta-galactosidase activity is observed in postmitotic neurons of the midbrain, spinal cord, neural retina and the developing olfactory system. beta-galactosidase activity is also detected in areas undergoing tissue remodeling such as the pinna of the ear, whisker follicles and the limbs. In adult mice, lacZ is expressed in the hippocampus and this expression was found to be enhanced upon seizure in the giant pyramidal neurons of CA3. These results reinforce the concept that t-PA plays a role in neurogenesis and morphogenesis, and identifies the promoter region that directs its transcriptional regulation both in development and in the CNS.

Down-regulation of retinoic acid receptor activity associated with decreased alpha and gamma isoforms expression in F9 embryonal carcinoma cells differentiated by retinoic acid

F9 embryonal carcinoma cells differentiate in response to retinoic acid (RA). To investigate the regulation of RA receptors (RARs) expression during this process, cDNA probes specific for the major RAR isoforms were used. In contrast to the level of RAR beta 2 mRNA which was high in cells treated 5 days with RA and below detection in untreated cells, as previously described, the steady state levels of RAR alpha 1, alpha 2, gamma 1, and gamma 2 mRNAs were markedly decreased in the RA-differentiated cells as compared to untreated cells. The down-regulation of the RA-responsive system in differentiated cells was also evident in gel shift assays as a marked decrease in binding capacity to a retinoid acid response element (beta 2RARE), as well as in chloramphenicol acetyltransferase (CAT) assays as a sixfold decrease in RA-mediated transacting activity via this element. The down-regulation of RAR DNA-binding and transacting activity coincided with the burst in tissue plasminogen activator secretion and thus, occurred at the hinge between early and late differentiation. The down-regulation of RA responsiveness may constitute an important event in the transition between early and late differentiation stage in F9 cells.

Binding at an NFI site is modulated by cyclic AMP-dependent activation of myelin basic protein gene expression

Using stable cell lines containing a series of deletions of the myelin basic protein (MBP) promoter directing the bacterial chloramphenicol acetyltransferase gene in a peripheral neurinoma cell line, we have studied the sequences in the MBP promoter needed for induction by cyclic AMP. Stimulation of expression from the MBP promoter by cyclic AMP is not a rapid response. Expression begins after 24 h and reaches a maximum at approximately 72 h. The results from the stable transformants indicate at least one region that appears to be essential to the induction of transcription directed by the MBP promoter. The region that is necessary for induction does not contain a consensus cyclic AMP response element. A specific binding site involved in the induction by cyclic AMP was localized to an NFI binding site.

Identification of a retinoic acid responsive enhancer 3' of the murine homeobox gene Hox-1.6

The putative vertebrate morphogen retinoic acid (RA) has been shown to induce expression of many mammalian homeobox genes in cell lines, suggesting expression of this gene family in developing vertebrate embryos may be controlled in part by RA. Using the teratocarcinoma cell line F9 as a model system, we have studied the RA-response of the murine homeobox gene Hox-1.6. RA treatment of F9 cells causes the appearance of a DNAse I hypersensitive site 3' of Hox-1.6, approximately 5 kb downstream of the Hox-1.6 promoter, and this site has been shown to reflect the presence of an RA-responsive enhancer 3' of the gene. The RA-responsiveness of the enhancer is controlled by a retinoic acid responsive element (RARE) identical to the RARE of the retinoic acid receptor (RAR) beta gene; however, other sequences also influence the activity of the enhancer, suggesting the presence of binding sites for novel proteins which regulate Hox-1.6 expression. Experiments with Hox-1.6 minigenes in which lacZ expression is controlled by the Hox-1.6 promoter and enhancer demonstrate that it is the 3' enhancer which confers RA responsiveness on the endogenous promoter, as constructs which lack the enhancer, or the RARE alone, do not respond to RA. Our results support the idea that RA is an endogenous vertebrate morphogen; identification of the RA-responsive enhancer downstream of Hox-1.6 demonstrates that RA directly controls the transcription of at least one member of a gene family that determines tissue identity in the vertebrate embryo.

TATA box-independent transcription of the human tissue plasminogen activator gene initiates within a sequence conserved in related genes

Transcription of the human tissue-type plasminogen activator (tPA) gene has been reported to initiate from a single site proximal to a TATA box motif [1985, J. Biol. Chem. 260, 11223-11230]. In this study, we utilized primer extension analysis to evaluate the tPA mRNA start site in phorbol-12-myristate 13-acetate (PMA) induced WI-38 human lung fibroblast cells. Whilst some tPA mRNA initiated from the predicted TATA-proximal location (+1), a 10-fold greater proportion of tPA mRNA transcripts initiated 110 bases downstream from a sequence conserved and utilized as the TATA-independent transcription start site in the rodent tPA genes. Moreover, the transfection and expression in different cell types of a cosmid containing the entire human tPA gene resulted in utilization of the same downstream (+110) start site. We propose that this, rather than the previously published position, is the major transcriptional initiation point for the human tPA gene. A core sequence (5'-CAGAGCTG-3') was identified which is common to the TATA-independent mRNA start sites of the human, mouse and rat tPA genes, and which demonstrates only partial similarity to sequences found at the initiation point of other TATA-independent genes.

Stimulation of tissue plasminogen activator production by retinoic acid: synergistic effect on protein kinase C-mediated activation

Trans retinoic acid (t-RA) stimulated the production of tissue plasminogen activator (tPA) in HeLa-S3 and human umbilical vein endothelial cells (huvecs) in a dose-dependent manner with maximal release (four to five times control) at 40 nmol/L and 40 mumol/L, respectively. In endothelial cells, the stimulation of tPA production by phorbol 12-myristate 13-acetate (PMA) was potentiated 1.9-fold by 10 mumol/L t-RA, or 1.8 times the additive effect. In HeLa cells, total tPA secretion with 10 nmol/L PMA was increased from 43 ng/mL to 96 ng/mL by 40 nmol/L t-RA, which was two times the additive effect. Higher concentrations of t-RA (400 nmol/L) depressed tPA secretion by itself and also suppressed PMA-induced tPA production by 50%. Histamine and thrombin also synergized with t-RA. t-RA (40 nmol/L) and 10 micrograms/mL histamine or 10 U/mL thrombin combined to induce tPA production 3.4 and 1.3 times the additive effect in HeLa cells. Cyclic adenosine monophosphate (cAMP) levels were not significantly affected by 10 nmol/L to 10 mumol/L t-RA. Nor did 10 nmol/L PMA and 40 nmol/L t-RA together affect cAMP levels, suggesting that t-RA-mediated potentiation of PMA-induced tPA production occurred via a mechanism that was independent of cAMP levels. Downregulation of protein kinase C (PKC) by pretreatment of huvecs with 100 nmol/L PMA completely blocked a secondary response to PMA, but did not have a significant effect on t-RA induction. Pretreatment with 10 mumol/L t-RA, on the other hand, did not significantly affect a secondary stimulus by 100 nmol/L PMA, but completely suppressed a secondary stimulation by 10 mumol/L t-RA alone. These studies suggest that the mechanism mediating t-RA stimulation of tPA production interacts with the PKC pathway, resulting in synergism.

Heparin inhibits collagenase gene expression mediated by phorbol ester-responsive element in primate arterial smooth muscle cells

Heparin is a potent inhibitor of arterial smooth muscle cell (SMC) migration and proliferation in vivo and in vitro. We propose that heparin affects these SMC functions by interfering with either the expression or the activity of secreted proteases required for cell movement. We have reported that heparin selectively inhibits the expression of tissue-type plasminogen activator in SMCs during mitogenesis. In this study we show that the gene expression of another kind of protease, interstitial collagenase, is induced by fetal bovine serum and is also suppressed by heparin. The inhibitory effect on the induced collagenase mRNA is specific to heparin-like molecules and does not depend on the anticoagulant activity of heparin. The induction of the collagenase gene depends on the protein kinase C pathway, since it can be induced by phorbol esters such as phorbol 12-myristate 13-acetate and blocked by inhibitors such as H-7 and staurosporine. In transient transfection assays with chloramphenicol acetyltransferase constructs containing the phorbol ester-responsive element introduced into baboon SMCs, heparin inhibits transcription induced by serum or phorbol 12-myristate 13-acetate. These results support the conclusion that, in primate SMCs, interstitial collagenase gene transcription mediated by the phorbol ester-responsive element is blocked by heparin.

Transcriptional regulation by cAMP

Cyclic AMP regulates a myriad of cellular processes through the cAMP-dependent protein kinase A. In many cases, activation of protein kinase A leads to altered patterns of gene transcription. Identification of the cAMP-response-element-binding-protein as a major nuclear substrate for protein kinase A accounts for many recent advances in understanding the signal transduction pathway. This review will focus on the role of cAMP-response-element-binding protein in coupling protein kinase A to the transcriptional machinery and the mechanisms that control cAMP signalling during mammalian development.

Structural organization of the murine gene encoding NAD-dependent methylenetetrahydrofolate dehydrogenase-methenyltetrahydrofolate cyclohydrolase

The structural organization of the entire nuclear gene (NMDMC) encoding the mitochondrial (mt) NAD-dependent methylenetetrahydrofolate dehydrogenase-methenyltetrahydrofolate cyclohydrolase enzyme (NMDMC) was determined by analysis of clones obtained from a lambda EMBL3 murine genomic DNA library. The gene is approx. 13 kb in length and contains eight exons and seven introns. All exon/intron splice junctions follow the GT/AG rule. The amino acid presequence, which is essential for transport of the NMDMC enzyme precursor into mt, is encoded almost entirely in the first exon. Two major transcriptional start points (tsp), located 33 and 75 nucleotides upstream from the AUG start codon, were revealed by S1 nuclease mapping and RNase protection analyses. The immediate 5'-flanking region of the first exon contains one CAAT box, a TATA-like box and three sites homologous to the consensus sequence for the binding of transcription factor Sp1.

Molecular mechanisms of cAMP-regulated gene expression

The ability of many genes to be induced by cAMP is dependent on the presence of enhancers located in the regions of DNA upstream of the start sites to the genes. The two best characterized enhancers are the CRE (5'-TGACGTCA-3') and the AP-2 site (5'-CCCCAGGC-3'). The activity of the CRE is modulated by sequences adjacent to the consensus sequence as well as by promoter context and cell type. The complex control of the CRE is reflected in the large number of cloned CRE binding proteins that arise both from unique genes and from splice variants. These factors are leucine zipper proteins that must dimerize before binding to DNA. Although all of the factors isolated can form active homodimers, many are also able to form heterodimers. The amino termini of these proteins contain consensus phosphorylation sites through which these factors trans-activate their cognate promoters. The diversity of the trans-acting factors and their cis-acting sequences reflects the precise control that cells require in the modulation of gene expression by cAMP.

Synergistic effect of glucocorticoids and androgens on the hormonal induction of tissue plasminogen activator activity and messenger ribonucleic acid levels in granulosa cells

Tissue-type plasminogen activator (tPA) is secreted by rat granulosa cells in response to treatment with activators of protein kinase A (follitropin, FSH), protein kinase C (gonadotropin-releasing hormone, GnRH) and tyrosine kinase (epidermal growth factor, EGF). Because steroid hormones have been shown to enhance the gonadotropin stimulation of ovarian differentiation, we investigated the effects of steroid hormones, alone or together with various kinase activators, on tPA activities and mRNA levels in cultured rat granulosa cells. Treatment of cells with dexamethasone (DEX; a glucocorticoid agonist) or R1881 (an androgen agonist) caused an increase in tPA secretion and mRNA levels. In addition, the stimulation of tPA activity and mRNA levels by FSH (50 ng/ml) was synergistically enhanced by cotreatment with DEX or R1881 in a time-dependent manner with 2.8- and 1.6-fold increase at 9 h after incubation as compared to cells treated with FSH alone. In contrast, treatment with diethylstilbestrol had no effect on tPA levels. Furthermore, tPA activity and mRNA levels induced by GnRH and EGF were also increased by cotreatment with DEX or R1881 as compared with cells treated with GnRH or EGF alone. Likewise, the stimulation of tPA mRNA levels by dibutyryl cAMP, a protein kinase A activator, and phorbol myristate acetate (PMA), a protein kinase C activator, was enhanced by cotreatment with DEX or R1881. These results demonstrate that glucocorticoid and androgen enhance tPA secretion and mRNA levels stimulated by FSH, GnRH and EGF in granulosa cells. The rat granulosa cells provide a useful model for studying the mechanism of regulation of tPA gene expression by steroid hormones.

Cyclic AMP stimulates somatostatin gene transcription by phosphorylation of CREB at serine 133

In this paper, we demonstrate that phosphorylation of CREB at Ser-133 is induced 6-fold in vivo, following treatment of PC12 cells with forskolin. By contrast, no such induction was observed in the kinase A-deficient PC12 line A126-1B2 (A126). Using F9 teratocarcinoma cells, which are unresponsive to cAMP, we initiated a series of transient expression experiments to establish a causal link between phosphorylation of CREB and trans-activation of cAMP-responsive genes. Inactivating the kinase A phosphorylation site by in vitro mutagenesis of the cloned CREB cDNA at Ser-133 completely abolished CREB transcriptional activity. As CREB mutants containing acidic residues in place of the Ser-133 phosphoacceptor were also transcriptionally inactive, these results suggest that phosphorylation of CREB may stimulate transcription by a mechanism other than by simply providing negative charge.