Histone deacetylase inhibitor activates the WAF1/Cip1 gene promoter through the Sp1 sites

Treatment of cultured cells with trichostatin A (TSA), a specific histone deacetylase inhibitor, induces the histone hyperacetylation and modulates expression of some mammalian genes. We examined the effects of TSA on cell growth arrest, and its relation to expression of the WAF1/Cip1 gene, a potent inhibitor of cyclin-dependent kinases, in a p53-mutated human osteosarcoma cell line MG63. TSA at 500 ng/ml induced growth arrest at both G1 and G2/M phases, and the expressions of the WAF1/Cip1 mRNA and protein. We also examined the changes of acetylated isoforms of histone H4. Dose-response and kinetic analysis suggest a close correlation between the level of histone acetylation and the induction of the WAF1/Cip1 expressions. Using several mutant WAF1/Cip1 promoter fragments, we found that the TSA responsive elements are two Sp1 sites at -82 and -69 relative to the transcription start site. These findings indicate that TSA induces the WAF1/Cip1 promoter through the typical Sp1 sites, in a p53-independent fashion. Furthermore, the Sp1-luc plasmid, containing SV40 promoter-derived three consensus Sp1 binding sites, was markedly activated by TSA, compared to the mutant Sp1-luc plasmid. These results demonstrate that transcriptional activation through the Sp1 sites of the WAF1/Cip1 promoter by TSA coincides with induced hyperacetylation of histone H4.

The homeodomain protein Pbx1 is involved in cAMP-dependent transcription of human CYP17

Pbx1 is a homeodomain transcription factor involved in cAMP-dependent transcriptional regulation of the bovine CYP17 gene. In this study, we have investigated the involvement of Pbx1 in the transcriptional regulation of the human CYP17 gene. Although a sequence identical to previously determined Pbx-binding sites is not present in the promoter region of the human CYP17 gene, three putative Pbx-binding sites are identified by sequence similarity analysis. Coexpression of Pbx1 and a catalytic subunit of protein kinase A (PKA) greatly enhances reporter gene transcription via the 5'-flanking region of the human CYP17 gene. Upon gel shift analysis utilizing nuclear extracts from human adrenal H295R cells, one of the three putative Pbx1-binding sites, -250/-241 bp, shows the typical intense doublet observed with other Pbx-binding sites. 5'-Deletion analyses of the reporter construct containing this Pbx-binding site showed approximately sixfold induction by coexpression of Pbx1 and PKA compared to the basal transcription, suggesting that Pbx1 binds the -250/-241 bp sequence and participates in cAMP-dependent regulation of the human CYP17 gene.

Posttranscriptional regulation of protein expression in human epithelial carcinoma cells by adenine-uridine-rich elements in the 3'-untranslated region of tumor necrosis factor-alpha messenger RNA

Eukaryotic mRNAs contain 3'-untranslated regions (UTR) that are involved in posttranscriptional control of gene expression. AU-rich octanucleotide repeats, UUAUUUAU, present in the 3'-UTR of mature lymphokine and other cytokine transcripts, have been implicated in the regulation of mRNA stability and translational efficiency. For example, previous evidence suggests that the AU-rich element (ARE) present in the 3'-UTR of murine tumor necrosis factor-alpha (TNF-alpha) can affect the posttranscriptional regulation of murine TNF-alpha gene expression in hematopoietic cells. Although cytokines are produced in epithelial cells, little is known about the regulation of TNF-alpha and other cytokine gene expression by 3'-UTR elements in human malignant epithelial cells. To better understand the function of the 3'-UTR of the human TNF-alpha gene in the regulation of TNF-alpha protein production in human epithelial cancer cells, a series of luciferase reporter constructs with portions of the 3'-UTR of human TNF-alpha was transfected into human breast carcinoma cell lines ZR-75-1 and ZR-75-1R (which overexpresses TNF-alpha). The 3'-UTR of TNF-alpha markedly suppressed luciferase activity in both cell lines, and the suppression of activity was reversed by deletion of the AU-rich sequences. This suppression was quantitative, with six repeats causing more inhibition than two repeats. Increased levels of luciferase activity were observed 3 h after TNF-alpha stimulation in ZR-75-1 cells transfected by constructs containing AU-rich repeats. In addition, cytoplasmic extracts from both cell lines were assayed for factors that bind to the 3'-UTR of human TNF-alpha mRNA. RNA-protein binding activities were found in both cell lines. Competition studies showed that these proteins specifically bound to AU-rich repeats present in the 3'-UTR of TNF-alpha. No binding activity was observed when the AU-rich repeats were deleted. TNF-alpha exposure markedly increased activity of several RNA-binding proteins, especially a novel Mr 50,000-55,000 RNA-binding protein. The binding activity in untreated ZR-75-1R was higher than that in untreated ZR-75-1 cells, suggesting that the level of RNA-protein binding correlates with the expression level of TNF-alpha in human epithelial cancer cells and that the RNA-binding proteins may control expression of TNF-alpha in ZR-75-1 cells. We conclude that the AU-rich repeats in the 3'-UTR of human TNF-alpha mRNA may regulate gene expression in human epithelial cancer cells by binding to AU sequence-specific proteins, including a previously undescribed Mr 50,000-55,000 protein not observed in hematopoietic cells.

Activation of the human transglutaminase 1 promoter in transgenic mice: terminal differentiation-specific expression of the TGM1-lacZ transgene in keratinized stratified squamous epithelia

Transglutaminase 1 (TGase 1) is a tissue-specific enzyme which is expressed in the keratinized stratified squamous epithelia and which catalyzes straightepsilon-(gamma-glutamyl) lysine cross-links of proteins to form the cell envelope at the periphery of cornified cells. A transient expression assay using a luciferase reporter gene linked to the 2.5 kb 5' upstream region of the human TGase 1 gene (TGM1) showed phorbol ester-responsive promoter activity in cultured normal human keratinocytes. To assess its promoter activity in vivo, we generated transgenic mice expressing the Escherichia coli beta-galactosidase gene (lacZ) directed by the 5' upstream region. beta-Galactosidase histochemistry revealed that the TGM1-lacZ transgene was expressed in terminally differentiating keratinocytes in upper layers of stratified squamous epithelia in embryonic, neonatal and adult transgenic mice. The expression pattern was similar to that of endogenous TGase 1 mRNA detected by in situ hybridization. Furthermore, topical application of a phorbol ester to adult tail skin enhanced expression of the transgene as well as TGase 1 mRNA in the epidermis. Thus, the 2.5 kb 5' upstream sequence of TGM1 includes elements regulating tissue- and terminal differentiation-specific gene expression in stratified squamous epithelia.

Activation of the human histamine H2 receptor is linked to cell proliferation and c-fos gene transcription

We examined whether histamine could regulate cell proliferation and expression of the early response gene c-fos in HEK-293 cells stably transfected with the human H2 receptor (HEK-H2). Histamine stimulated [3H]thymidine incorporation [50% effective concentration (EC50) = 3.6 x 10(-6) M] in HEK-H2 cells in a cimetidine-sensitive manner and increased c-fos mRNA in a time-dependent fashion, reaching maximal induction after 30 min. Histamine induced luciferase activity in HEK-H2 cells transiently transfected with a construct containing the luciferase reporter gene (Luc) coupled to the serum response element (SRE) of the c-fos gene promoter (EC50 = 1.5 x 10(-6) M) or a plasmid containing the SRE core fragment (bases -320 to -298). The protein kinase C (PKC) inhibitor staurosporine and long-term pretreatment of HEK cells with phorbol ester inhibited the effect of histamine on PKC activation, SRE-Luc activity, and [3H]thymidine incorporation. We have demonstrated that activation of the human H2 receptor can lead to induction of c-fos gene transcription and cell proliferation through a PKC-dependent mechanism.

Overexpression of manganese superoxide dismutase selectively modulates the activity of Jun-associated transcription factors in fibrosarcoma cells

Manganese superoxide dismutase (MnSOD) is reduced in a variety of tumor cells and has been proposed to be a new type of tumor suppressor gene. The mechanism(s) by which MnSOD suppresses cancer development is currently unknown. However, expression of this antioxidant might play a significant role in maintaining cellular redox status. The relationship between MnSOD expression and modulation of DNA-binding activity and transcriptional activation of redox-sensitive oncoproteins and tumor suppressor proteins was studied in a murine fibrosarcoma cell line (FSa-II). Electrophoretic mobility shift assay and transcriptional activation studies revealed an inverse correlation between MnSOD expression and activity of c-jun-associated transcription factors, activator protein 1 and cyclic AMP-responsive element binding protein. Furthermore, expression of an activator protein 1 target gene, bcl-xL, was decreased in MnSOD-transfected cell lines. The results suggest that overexpression of MnSOD may exert its tumor suppressor activity, in part, by modulation of specific oncogenes.

Characterization of density-dependent regulation of the tyrosinase gene promoter: role of protein kinase C

The rate-limiting step in melanogenesis is catalyzed by tyrosinase, a multifunctional enzyme encoded by the albino locus. We have previously reported that depletion of protein kinase C by long-term treatment of B16 mouse melanoma cells with phorbol dibutyrate (PDBu) prevented cell density-dependent melanogenesis. This was accompanied by a lack of induction of tyrosinase protein and mRNA. We report here the effect of PDBu on the functional activity of the mouse tyrosinase promoter by reporter gene assay and its effect on the binding of nuclear proteins from B16 cells to the "M-box" region of the mouse tyrosinase promoter. Short-term PDBu treatment of B16 cells transfected with a mouse tyrosinase promoter-luciferase construct resulted in increased reporter gene activity, while long-term PDBu treatment inhibited reporter gene activity. Using an oligonucleotide containing the M-box and its flanking residues in electrophoretic mobility shift assays, we found a density-dependent change in the pattern of DNA-protein complexes. One complex was found to be negatively regulated by long-term PDBu treatment. Competition experiments with various mutated oligonucleotides demonstrated that both the M-box and flanking residues are important for nuclear protein binding. The complex whose formation was inhibited by long-term PDBu treatment was shown to contain the basic helix-loop-helix leucine zipper protein microphthalmia-associated transcription factor (MITF). These results suggest that chronic PDBu treatment might inhibit tyrosinase expression (and subsequent melanogenesis) by affecting the amount or function of MITF.

Identification of cDNAs encoding two G protein-coupled receptors for lysosphingolipids

The structural similarity of lysosphingolipids to lysophosphatidic acid (LPA) prompted a sequence-based search for lysosphingolipid receptors using cDNA sequence of the Edg2 human LPA receptor. Two closely related G protein-coupled receptors, rat H218 and human Edg3, are highly similar to Edg2. When overexpressed in Jurkat cells, H218 and Edg3 activated serum response element-driven transcriptional reporter gene in response to sphingosine 1-phosphate (S1P), dihydro-S1P and sphingosylphosphorylcholine, but not to LPA. H218 and Edg3 expressed in Xenopus oocytes conferred responsiveness to S1P and dihydro-S1P in agonist-triggered 45Ca2+ efflux. Therefore, H218 and Edg3 are functional receptors for S1P and perhaps other closely related lysosphingolipids.

Estrogens stimulate tamoxifen-induced neuronal cell apoptosis in vitro: a possible nongenomic action

Estrogens are implicated in the regulation of neuronal cell death and survival in the nervous system. However, the molecular mechanisms are largely unknown. Here, we investigated effects of estrogens and an anti-estrogen compound, tamoxifen (TMX), on the death/survival of GT1-7 hypothalamic neuronal cells. Endogenous nuclear estrogen receptors (ERs) in these cells were found to be inactive on the basis of luciferase assay. Treatment of cells with TMX stimulated cell death, which was associated with DNA ladder formation characteristic of apoptosis. Both 17-beta estradiol, which stimulates ER-mediated transcription, and 17-alpha estradiol, which does not, had little effect on cell survival. Both estradiols, however, significantly potentiated TMX-induced cell death. Similar effects were obtained by estriol, but more remarkable effects were observed by quinestrol, an ethinyl estradiol derivative, which has an ether-modification at the C3 position. Furthermore, either TPA or forskolin, a potent stimulator of protein kinase C or A, respectively, also stimulated TMX-induced cell death. Taken together, these results may suggest that genomic activity through ERs is not prerequisite for estrogen stimulation of TMX-induced apoptosis, but that the cell death pathway of TMX could be modulated at the cytoplasmic level by estrogens, whose activity is dependent upon their molecular structure.

Free luciferase may acquire a more favorable conformation than ribosome-associated luciferase for its activity expression

A variant of firefly luciferase in which the C-terminal end was extended with 44 amino acid residues served as a model protein in this study. After transcription and translation in vitro, the enzyme activity was measured when still attached to the ribosome and when released from the ribosome by incubation with RNase A or puromycin. It was found that the C-terminally extended luciferase already had activity when linked to the ribosome, but its activity was greatly increased when released from the ribosome. These results indicate that the luciferase is folded during synthesis on the ribosome; however, some conformational adjustments occur after its release from the ribosome which are required for the full expression of its enzymatic activity.

Suppression of GnRH gene expression in GT1-1 hypothalamic neuronal cells: action of protein kinase C

We attempted to elucidate molecular mechanisms of gonadotropin-releasing hormone (GnRH) gene regulation by the protein kinase C (PKC) pathway in GT1-1 cells. Activation of PKC with 12-tetra-decanoylphorbol-13-acetate (TPA) or inhibition with staurosporine or calphostin C down-regulated GnRH mRNA levels. A serial deletion mutant analysis revealed that this suppression was mediated by the proximal region (-187/-69) of the mouse GnRH promoter. TPA transiently induced c-fos mRNA, whereas staurosporine or calphostin C failed to do so. However, PKC inhibitors blocked the TPA-evoked c-fos induction. Over-expression of PKC alpha down-regulated GnRH promoter activity, indicating that PKC activation was sufficient to inhibit GnRH gene expression. These results suggest that both activation and inhibition of PKC decrease the GnRH gene expression in the GT1-1 cells probably through different signal cascade mechanisms.

Basic amino acid residues at the carboxy-terminal eleven amino acid region of the phosphoprotein (P) are required for transcription but not for replication of vesicular stomatitis virus genome RNA

The phosphoprotein (P) of vesicular stomatitis virus (VSV) serotypes New Jersey [P(NJ)] and Indiana [P(I)] contains a highly conserved carboxy-terminal domain which is required for binding to the cognate N-RNA template as well as to form a soluble complex with the nucleocapsid protein N in vivo. We have shown that the deletion of 11 amino acids from the C terminal end of the P(I) protein abolishes both the template binding and the complex forming activity with the N protein. Within this region, there are conserved basic amino acid residues (R260 and K262) that are potential candidates for such interactions. We have generated mutant P proteins by substitution of these basic amino acid residues with alanine and studied their role in both transcription and replication. We have found that the R260A mutant failed to bind to the N-RNA template, whereas the K262A mutant bound efficiently as the wild-type protein. The R260A mutant, as expected, was unable to support mRNA synthesis in vitro in a transcription reconstitution reaction as well as transcription in vivo of a minigenome using a reverse genetic approach. However, the K262A mutant supported low level of transcription (12%) both in vitro and in vivo, suggesting that direct template binding of P protein through the C-terminal domain is necessary but not sufficient for optimal transcription. Using a two-hybrid system we have also shown that both R260A and K262A mutants interact inefficiently with the L protein, suggesting further that the two point mutants display differential phenotype with respect to binding to the template. In addition, both R260A and K262A mutants were shown to interact efficiently with the N protein in vivo, indicating that these mutants form N-P complexes which are presumably required for replication. This contention is further supported by the demonstration that these mutants support efficient replication of a DI RNA in vivo. Since the transcription defective P mutants can support efficient replication, we propose that the transcriptase and the replicase are composed of two distinct complexes containing (L-P2-3) and L-(N-P), respectively.

AtLTP1 luciferase expression during carrot somatic embryogenesis

The carrot (Daucus carota L.) EP2 gene encodes a Lipid Transfer Protein (LTP) which is expressed during protoderm formation in developing embryos. To develop a vital reporter system for gene expression during somatic embryo development a 1.1 kB fragment of the Arabidopsis thaliana LTP1 promoter was fused to the firefly luciferase (LUC) coding sequence. The AtLTP1 luciferase expression pattern in transformed carrot suspension cultures was identical to the expression pattern of the endogenous carrot EP2 gene. Cell tracking experiments revealed that all somatic embryos were derived from AtLTP1 luciferase expressing cell clusters. However, not all cell clusters that expressed the AtLTP1 luciferase reporter gene developed into a somatic embryo, suggesting that initiation of an embryogenic pathway in tissue culture does not always lead to development of a somatic embryo.

Ethanol down-regulates the transcription of microsomal triglyceride transfer protein gene

Microsomal triglyceride transfer protein (MTP) plays a central role in the assembly and secretion of apoB-containing lipoproteins. In this study, we investigated the effect of ethanol on the expression of the large subunit of MTP in a human liver hepatoma cell line, the HepG2 cells. Exposure of HepG2 cells to low concentrations of ethanol reduced MTP mRNA levels in a concentration- and time-dependent manner. The level of MTP mRNA decreased significantly (P<0.05, -26% relative to pretreatment control) when the concentration of ethanol in the culture medium was 50 ppm (0.005%, v/v). Maximal suppression (-50%) was observed at 100 ppm ethanol; the MTP mRNA levels remained at 50% of control when the ethanol concentration was raised to 10,000 ppm. Furthermore, a 10-day ethanol treatment caused a significant 50% decrease in the MTP activity and apoB secretion rate in HepG2 cells. To investigate the molecular mechanisms underlying this phenomenon, we examined the effect of ethanol on the promoter activity of the MTP gene. Transient transfection analysis of human MTP promoter-driven luciferase gene expression showed that ethanol down-regulates MTP promoter activity in a manner parallel to that observed for mRNA levels. Deletion analysis suggested that the MTP promoter sequence contains a negative ethanol response element -612 to -142 bp upstream of the transcription start site. To evaluate the in vivo relevance of the effect of ethanol on MTP mRNA levels, rats were given a single oral dose of ethanol, with hepatic and intestinal MTP mRNA measured 3 h after dosing. Rats receiving 1 or 3 g/kg of ethanol exhibited substantially lower hepatic and intestinal MTP mRNA levels. Taken together, these results strongly suggest that ethanol can modulate the secretion of apoB-containing lipoproteins by down-regulating the expression of MTP large subunit, primarily through inhibiting the transcription of the MTP gene.

Role of nitric oxide in poly(I-C)-induced endothelial cell expression of leukocyte adhesion molecules

Polyinosinic-polycytidylic acid [poly(I-C)] is a synthetic double-stranded RNA (dsRNA) that simulates a viral-infected state in cells. It has been shown that viral infection, as well as poly(I-C), stimulates leukocyte adhesion to endothelial cell (EC) monolayers and that this is mediated through the surface expression of the adhesion molecules E-selectin, vascular cell adhesion molecule 1 (VCAM-1), and intercellular adhesion molecule 1. We have tested the involvement of nitric oxide (NO) in poly(I-C)-induced monocytic cell adhesion to human vascular EC. Using primary cultured EC for these studies, we confirmed the results from previous reports that these cells have higher basal levels of NO production than passaged cells. Poly(I-C)-induced monocytic cell adhesion to primary EC was concentration-dependently inhibited by 40-74% by the nitric oxide synthase (NOS) inhibitor NG-methyl-L-arginine (L-NMA), as well as three other NOS inhibitors, without significantly affecting interleukin-1 beta-induced adhesion. L-NMA inhibited poly(I-C)-induced surface expression of E-selectin and VCAM-1 by 25 and 45%, respectively, and mRNA levels of E-selectin and VCAM-1 by 62 and 74%, respectively. Primary EC transiently transfected with a plasmid containing an E-selectin promoter-driven luciferase reporter gene showed that L-NMA treatment reduced poly(I-C)-induced E-selectin promoter activity to basal levels. Electrophoretic mobility shift analysis indicated that poly(I-C)-induced nuclear factor-kappa B (NF-kappa B) binding to a radiolabeled oligonucleotide corresponding to the consensus NF-kappa B binding domain of the E-selectin promoter was decreased by L-NMA pretreatment. Hence, NO appears to augment E-selectin gene expression in response to poly(I-C) at the transcriptional level in vascular EC. Collectively, these data support the hypothesis that NO augments poly(I-C)-induced EC activation. These data suggest a novel role for NO as a response mediator in dsRNA-induced leukocyte adhesion to EC.

An Arabidopsis mutant showing reduced feedback inhibition of photosynthesis

Many plant genes are responsive to sugars but the mechanisms used by plants to sense sugars are unknown. A genetic approach has been used in Arabidopsis to identify genes involved in perception and transduction of sugar signals. For this purpose, an in vivo reporter system was established consisting of the light- and sugar-regulated plastocyanin promoter, fused to the luciferase coding sequence (PC-LUC construct). At the seedling stage, expression of the PC-LUC gene is repressed by sucrose, and a number of sucrose-uncoupled (sun) mutants were selected in which sucrose is unable to repress the activity of the PC promoter. Three mutants have been characterized in more detail. The sugar analog 2-deoxy-D-glucose (2DG) was used to repress whole plant photosynthesis, PC-LUC gene expression and total ribulose-1,5-bisphosphate activity. It was found that the sun6 mutation makes plants unresponsive to these 2DG-induced effects. Moreover, unlike wild-type plants, sun6 mutants are insensitive to elevated levels of glucose in the growth medium. These findings suggest that the SUN6 gene is active in a hexose-activated signal transduction pathway.

Expression of cholera toxin B subunit oligomers in transgenic potato plants

A gene encoding the cholera toxin B subunit protein (CTB), fused to an endoplasmic reticulum (ER) retention signal (SEKDEL) was inserted adjacent to the bi-directional mannopine synthase P2 promoter in a plant expression vector containing a bacterial luciferase AB fusion gene (luxF) linked to the P1 promoter. Potato leaf explants were transformed by Agrobacterium tumefaciens carrying the vector and kanamycin-resistant plants were regenerated. The CTB-SEKDEL fusion gene was identified in the genomic DNA of bioluminescent plants by polymerase chain reaction amplification. Immunoblot analysis indicated that plant-derived CTB protein was antigenically indistinguishable from bacterial CTB protein, and that oligomeric CTB molecules (M(r) approximately 50 kDa) were the dominant molecular species isolated from transgenic potato leaf and tuber tissues. Similar to bacterial CTB, plant-synthesized CTB dissociated into monomers (M(r) approximately 15 kDa) during heat or acid treatment. The maximum amount of CTB protein detected in auxin-induced transgenic potato leaf and tuber tissues was approximately 0.3% of total soluble plant protein. Enzyme-linked immunosorbent assay methods indicated that plant-synthesized CTB protein bound specifically to GM1-ganglioside, the natural membrane receptor of cholera toxin. In the presence of the SEKDEL signal, CTB protein accumulates in potato tissues and is assembled into an oligomeric form that retains native biochemical and immunological properties. The expression of oligomeric CTB protein with immunological and biochemical properties identical to native CTB protein in edible plants opens the way for preparation of inexpensive food plant-based oral vaccines for protection against cholera and other pathogens in endemic areas throughout the world.

A novel dicistronic AAV vector using a short IRES segment derived from hepatitis C virus genome

Adeno-associated virus (AAV) vectors have a limited capacity for packaging DNA. To insert both a therapeutic gene and a selectable marker gene in the same AAV vector efficiently, we developed a novel dicistronic AAV vector containing a 230 base pairs (bp) internal ribosome entry site (IRES) element derived from hepatitis C virus (HCV) genome and a 420 bp blasticidin S-resistance gene (bsr) as a small selectable marker in the second cistron. The 650 bp HCV IRES-bsr construct was placed downstream of the 3' end of the luciferase gene (Luc) under the control of the human cytomegalovirus (CMV) promoter. This dicistronic gene conferred blasticidin S-resistance to 293 cells besides luciferase activity, when examined not only by transfection but also by transduction using AAV vectors. The dicistronic AAV vector harbouring HCV IRES-bsr is capable of expressing a therapeutic gene of up to 3.6 kilobases (kb) (including promoter/enhancer elements) as well as a selectable marker gene. If a selectable marker gene is not necessary, this vector is able to incorporate two different kinds of therapeutic genes more easily than that containing EMCV IRES. The dicistronic AAV vector described here is useful for expressing many kinds of cDNA besides a selectable marker.

Gastrin and phorbol 12-myristate 13-acetate regulate the human histidine decarboxylase promoter through Raf-dependent activation of extracellular signal-regulated kinase-related signaling pathways in gastric cancer cells

Gastrin stimulates transcription of the human histidine decarboxylase (HDC) gene through binding to the G-protein-coupled cholecystokinin-B/gastrin receptor. We have explored the possibility that mitogen-activated protein kinase cascades play a role in mediating the effects of gastrin on transcription in a gastric cancer (AGS-B) cell line. Gastrin and phorbol 12-myristate 13-acetate (PMA) treatment of AGS-B cells was found to increase the phosphorylation of tyrosine residues of extracellular signal-regulated kinases (ERKs) 1 and 2 and increase ERK activity as determined by the in vitro phosphorylation of myelin basic protein. Reporter gene assays also demonstrated that gastrin and PMA stimulated Elk-1- and c-Myc-dependent transactivation, consistent with gastrin- and PMA-induced activation of ERKs. Overexpression of wild type ERK-1 and ERK-2 or activation of endogenous ERKs using activated MEK-1 (mitogen-activated protein kinase kinase or ERK kinase) overexpression stimulated HDC promoter activity in a dose-dependent fashion. Interruption of the ERK-related pathway using expression vectors for kinase-deficient ERKs or an ERK-specific phosphatase (PAC-1) blocked gastrin- and PMA-stimulated HDC promoter activity. In contrast, inhibition of the Jun kinase pathway using an interfering dominant negative SEK-1 (stress-activated protein kinase/ERK-1) mutant did not inhibit HDC promoter activity. Furthermore, whereas gastrin stimulated phosphorylation of Shc proteins and association with Grb2, activation of the HDC promoter was not influenced by expression of dominant negative Ras (N15 or N17) proteins. However, gastrin stimulated Raf-1 kinase activity, and activation of the HDC promoter was blocked by coexpression of a dominant negative Raf-1 construct. Overall, these data demonstrate that gastrin regulates HDC transcription in a Rafdependent, Ras-independent fashion predominantly through activation of the ERK-related pathway.

Heat shock factor 1 represses Ras-induced transcriptional activation of the c-fos gene

Heat shock factor 1, the critical molecular regulator of the stress response is conserved throughout eukaryotic organisms and activates the transcription of heat shock genes. We now show that heat shock factor 1 inhibits the expression of c-fos, an immediate early gene that controls responses to extracellular stimuli for growth and differentiation. Heat shock factor 1 inhibits the transcription of the c-fos gene and antagonizes the activating effects of the signal transducing protein Ras on the c-fos promoter and on the promoter of another Ras responsive gene uPA. This property was specific for heat shock factor 1; c-fos repression was not seen with the structurally related protein heat shock factor 2. Repression involved different molecular mechanisms compared with those involved in transcriptional activation by heat shock factor 1 and specifically did not require binding to the c-fos promoter. Thus, in addition to its known role as a transcriptional activator of the cellular heat shock response, heat shock factor 1 also antagonizes the expression of Fos, a key component of the ubiquitous AP-1 transcription factor complex and as such could influence multiple aspects of cell regulation.

Activation of the HIV-1 long terminal repeat by nerve growth factor

The brain is an important target for the human immunodeficiency virus type 1 (HIV-1). We show here that nerve growth factor (NGF), which induces neuronal differentiation and survival, causes a strong activation of the HIV-1 long terminal repeat by a Ras/Raf-dependent mechanism in PC12 cells. Mutation of the kappaB sequences contained whithin the long terminal repeat reduces NGF-mediated stimulation. NGF does not activate NF-kappaB in PC12 cells, but rather increases binding of other nuclear factors to the kappaB sequences. Furthermore, a nuclear receptor response element contributes to the stimulatory effect of NGF. The retinoids receptors have been identified as components of the nuclear binding to the nuclear receptor response element in NGF-treated PC12 cells. These results reveal the importance of neurotrophins and nuclear receptor signaling pathways as specific activators of HIV-1 gene expression in neural cells.