Fibroblast growth factor and cyclic AMP (cAMP) synergistically activate gene expression at a cAMP response element

Investigation of axonal regeneration of Triturus ivanbureschi by using physiological and proteomic strategies

Peripheral nerve injuries are frequently observed and successful treatment depends mainly on the injury type, location of the damage, and the elapsed time prior to treatment. The regenerative capacity is limited only to the embryonic period in many mammalian tissues, but urodele amphibians do not lose this feature during adulthood. The main purpose of this study is to define the recovery period after serious sciatic nerve damage of a urodele amphibian, Triturus ivanbureschi. Experimental transection damage was performed on the sciatic nerves of T. ivanbureschi specimens. The recovery period of sciatic nerves were investigated by walking track analysis, electrophysiological recordings, and bottom-up proteomic strategies at different time points during a 35-day period. A total of 34 proteins were identified related to the nerve regeneration process. This study showed that the expression levels of certain proteins differ between distal and proximal nerve endings during the regeneration period. In distal nerve stumps, transport proteins, growth factors, signal, and regulatory molecules are highly expressed, whereas in proximal nerve stumps, neurite elongation proteins, and cytoskeletal proteins are highly expressed.

Driving GDNF expression: the green and the red traffic lights

Glial cell line-derived neurotrophic factor (GDNF) is widely recognized as a potent survival factor for dopaminergic neurons of the nigrostriatal pathway that degenerate in Parkinson's disease (PD). In animal models of PD, GDNF delivery to the striatum or the substantia nigra protects dopaminergic neurons against subsequent toxin-induced injury and rescues previously damaged neurons, promoting recovery of the motor function. Thus, GDNF was proposed as a potential therapy to PD aimed at slowing down, halting or reversing neurodegeneration, an issue addressed in previous reviews. However, the use of GDNF as a therapeutic agent for PD is hampered by the difficulty in delivering it to the brain. Another potential strategy is to stimulate the endogenous expression of GDNF, but in order to do that we need to understand how GDNF expression is regulated. The aim of this review is to do a comprehensive analysis of the state of the art on the control of endogenous GDNF expression in the nervous system, focusing mainly on the nigrostriatal pathway. We address the control of GDNF expression during development, in the adult brain and after injury, and how damaged neurons signal glial cells to up-regulate GDNF. Pharmacological agents or natural molecules that increase GDNF expression and show neuroprotective activity in animal models of PD are reviewed. We also provide an integrated overview of the signalling pathways linking receptors for these molecules to the induction of GDNF gene, which might also become targets for neuroprotective therapies in PD.

Fibroblast growth factor 2 and cyclic AMP synergistically regulate bone sialoprotein gene expression

Bone sialoprotein (BSP) is a noncollagenous protein of the mineralized bone extracellular matrix. We here report that FGF2 and cAMP act synergistically to stimulate BSP gene expression. Treatment of ROS 17/2.8 cells with either 10 ng/ml FGF2 or 1 microM FSK for 6 h resulted in 5.4- and 8.2-fold increases, respectively, in the levels of BSP mRNA. However, in the presence of both FGF2 and forskolin (FGF/FSK), BSP mRNA levels were increased synergistically by 20.4-fold. Using a luciferase reporter construct, encompassing BSP promoter nucleotides -116 to +60, transcription was also increased synergistically by 15.0-fold with FGF/FSK, compared to stimulations of 2.6- and 5.3-fold, respectively, for FGF2 and FSK alone. Transcriptional stimulation by FGF/FSK abrogated in constructs included 2 bp mutations in the inverted CCAAT, CRE, FRE and Pit-1 elements. Whereas the FRE-protein complex was increased by FGF2 and FGF/FSK, the Pit-1-protein complex was decreased by FSK and FGF/FSK. Notably, transcriptional activity induced by FGF/FSK was blocked by protein kinase A, tyrosine kinase and MEK inhibitors. These studies indicate that the combinatorial effects of FGF and FSK act through PKA, tyrosine kinase and MAP-kinase-dependent pathways, which target the inverted CCAAT, CRE, FRE and Pit-1 elements in the BSP gene to synergistically increase BSP expression.

Cooperation of protein kinase A and Ras/ERK signaling pathways is required for AP-1-mediated activation of fibroblast growth factor-inducible response element (FiRE)

Recent studies suggest a crucial role for protein kinase A (PKA) in the regulation of growth factor signaling. However, the effect of PKA on the transcription of growth factor-responsive genes has drawn far less attention. Here we have investigated the signaling mechanisms involved in the activation of an activator protein-1 (AP-1)-driven, growth factor-specific enhancer element, fibroblast growth factor-inducible response element (FiRE). The activation was found to be mediated by three phorbol 12-O-tetradecanoate-13-acetate-response element-related DNA elements of FiRE, including motif 4 and two distinct elements of motif 5 (referred to as M5-1 and M5-2). All three elements were required for full FiRE activity. Stimulation of cells with fibroblast growth factor-2 (FGF-2) induced the binding of AP-1 to motif 4 and M5-2, whereas M5-1 did not show detectable binding. The FGF-2-induced FiRE activation appeared to require cooperational function of the Ras/ERK and PKA pathways. Inhibition of either of the pathways abolished the binding of AP-1 complexes to motif 4 and motif 5 and the subsequent FiRE activation. By contrast, costimulation of cells with FGF-2 and the PKA activator 8-bromo-cyclic AMP increased the binding of AP-1 to FiRE and potentiated the level of transcriptional activity. The cooperational function of these two pathways was confirmed by experiments with cell lines stably expressing 4-hydroxytamoxifen-inducible oncogenic Raf-1 (DeltaRaf-1:ER[DD]). Noticeably, the induction systems showed variations with respect to regulation of AP-1-driven activation of FiRE. These differences were likely to originate from the ability of these two systems to induce the differential activation pattern of the Ras/ERK pathway.

Intrinsic GABA neurons inhibit proenkephalin gene expression in slice cultures of rat neostriatum

In the neostriatum, the proenkephalin gene is expressed in medium spiny GABA neurons, which project to the globus pallidus. The expression is activated by glutamatergic projections from the neocortex via NMDA receptors. In these experiments we have used slice cultures of rat neostriatum to study the role of GABA in proenkephalin gene expression. Our results show that GABA is released from neostriatal neurons and negatively regulates the proenkephalin gene expression induced by NMDA receptor stimulation. The GABAA receptors involved seem to be colocalized with NMDA receptors on the projection neurons, which express the proenkephalin gene. In further experiments, we have found that the proenkephalin gene expression is not only activated by neocortical projection neurons but also by intrinsic striatal neurons as well as by projections from the thalamus. All these glutamatergic afferents enhance the proenkephalin gene expression via NMDA receptors. Their efficacy is regulated by endogenous GABA.

Identification of a novel response element in the rat bone sialoprotein (BSP) gene promoter that mediates constitutive and fibroblast growth factor 2-induced expression of BSP

Bone sialoprotein (BSP) is a sulfated and phosphorylated glycoprotein, found almost exclusively in mineralized connective tissues, that may function in the nucleation of hydroxyapatite crystals. We have found that expression of BSP in osteoblastic ROS 17/2.8 cells is stimulated by fibroblast growth factor 2 (FGF2), a potent mitogen for mesenchymal cells. Stimulation of BSP mRNA with 10 ng/ml FGF2 was first evident at 3 h ( approximately 2.6-fold) and reached maximal levels at 6 h ( approximately 4-fold). From transient transfection assays, a FGF response element (FRE) was identified (nucleotides -92 to -85, "GGTGAGAA") as a target of transcriptional activation by FGF2. Ligation of two copies of the FRE 5' to an SV40 promoter was sufficient to confer FGF-responsive transcription. A sequence-specific protein-DNA complex, formed with a double-stranded oligonucleotide encompassing the FRE and nuclear extracts from ROS 17/2.8 cells, but not from fibroblasts, was increased following FGF2 stimulation. Several point mutations within the critical FRE sequence abrogated the formation of this complex and suppressed both basal and FGF2-mediated promoter activity. These studies, therefore, have identified a novel FRE in the proximal promoter of the BSP gene that mediates both constitutive and FGF2-induced BSP transcription.

Transcriptional autorepression of the stress-inducible gene ATF3

Previously, we demonstrated that ATF3 (activating transcription factor-3) is a stress-inducible gene, and the protein it encodes is a transcriptional repressor. In this report, we present evidence suggesting that ATF3 represses the transcription of its own gene. Interestingly, efficient repression requires a consensus ATF/cAMP-responsive element site in the promoter and a previously unidentified ATF3-binding site immediately downstream from the TATA box. Although this new site resembles the known ATF/cAMP-responsive element sequences at the flanking sequence, it differs from them at the center key residues. These observations indicate that ATF3 can tolerate variations in the center of the binding sites if the flanking sequences are favorable. The repression of the ATF3 promoter by its own gene product provides a mechanistic explanation, at least in part, for the transient expression pattern of the ATF3 gene upon stress induction.

Evidence for cell specific regulation by PACAP38 of the proenkephalin gene expression in neocortical cells

During the first postnatal week, glial cell production for the neocortex continues in the neocortical subventricular zone. During this time, the proenkephalin gene (PEnk) is expressed in numerous cells of the subventricular zone and of the adjacent neocortex. When neocortical astroglial cells are brought into dissociation culture, they also produce PEnk mRNA. We have investigated the effect of pituitary adenylate cyclase activating peptide-38 (PACAP38) on PEnk gene expression in dissociation cultures as well as in slice cultures, which contained the subventricular zone and the adjacent neocortex. PACAP38 enhanced the levels of PEnk mRNA in both culture systems. In dissociated astroglial cells, inhibition of protein kinase A, of p44,42 mitogen-activated protein kinase as well as of the EGF-receptor tyrosine kinase by H89, PD98059 and AG1478, respectively, reduced the PACAP38-induced expression in a synergistic manner. In the neocortical part of the slice cultures, the effect of PACAP38 on PEnk gene expression was inhibited only by H89 and PD98059. Here, protein kinase A and p44,42 MAP kinases shared a mechanism which increased the gene expression. Surprisingly, the expression of the PEnk gene in the glial progenitors of the subventricular zone as induced by PACAP38 was not affected by any of the three protein kinase inhibitors, but was blocked by the unspecific kinase inhibitor H7. It is concluded that PACAP38 induced the PEnk gene expression in both culture systems in a cell-type specific manner.

Basic fibroblast growth factor decreases elastin gene transcription through an AP1/cAMP-response element hybrid site in the distal promoter

Previous studies demonstrated that basic fibroblast growth factor (bFGF) decreases elastin gene transcription in pulmonary fibroblasts. In this study we pursue the identification of the element and the trans-acting factors responsible. Gel shift analyses show that bFGF increases protein binding to a sequence located at -564 to -558 base pairs (bp), which possesses homology to both AP1 and cAMP-response consensus elements yet displays a unique affinity for heterodimer binding. Site-directed mutation of the -564- to -558-bp sequence results in an increase in promoter activity and abrogates the effect of bFGF. Western blot analysis shows that bFGF induces a sustained increase in the steady-state levels of Fra 1, and co-transfection of a Fra 1 expression vector with an elastin promoter reporter construct results in an inhibition of elastin promoter activity. Overall the results suggest that bFGF represses elastin gene transcription by increasing the amount of the Fra 1 that subsequently binds to the -564- to -558-bp as a heterodimer with c-Jun to form an inhibitory complex. We propose that the identified bFGF response element can serve to down-regulate elastin transcription in elastogenic cells and, conversely, can serve to up-regulate elastogenesis in cells where endogenous bFGF signaling is attenuated or altered.

A unique bFGF-responsive transcriptional element

The mitogen-regulated protein/proliferin (mrp/plf) genes encode closely related proteins that stimulate cell proliferation and angiogenesis. Basic fibroblast growth factor (bFGF) increases mrp/plf mRNA and protein production by 3T3 cells. Although the three cloned mrp/plf gene promoters are over 97% identical, only mrp3 is transcriptionally activated by bFGF. A series of truncated mrp3 promoter sequences were tested to determine the minimal promoter sequence necessary for bFGF-responsive transcription. Within the minimal bFGF-responsive mrp3 promoter fragment, a putative FGF-regulatory element (FRE) was identified. Nuclear factors that bind the FRE are present in 3T3 cells. When present upstream of a thymidine kinase basal promoter, the FRE exhibits high transcriptional activity and responds to bFGF. Thus, the FRE is a strong transcriptional element that is regulated by bFGF and that may participate in regulating the mrp3 gene and perhaps other FGF-regulated genes.

Circadian regulation of cAMP response element-mediated gene expression in the suprachiasmatic nuclei

A program of stringently-regulated gene expression is thought to be a fundamental component of the circadian clock. Although recent work has implicated a role for E-box-dependent transcription in circadian rhythmicity, the contribution of other enhancer elements has yet to be assessed. Here, we report that cells of the suprachiasmatic nuclei (SCN) exhibit a prominent circadian oscillation in cAMP response element (CRE)-mediated gene expression. Maximal reporter gene expression occurred from late-subjective night to mid-subjective day. Cycling of CRE-dependent transcription was not observed in other brain regions, including the supraoptic nucleus and piriform cortex. Levels of the phospho-active form of the transcription factor CREB (P-CREB) varied as a function of circadian time. Peak P-CREB levels occurred during the mid- to late-subjective night. Furthermore, photic stimulation during the subjective night, but not during the subjective day, triggered a marked increase in CRE-mediated gene expression in the SCN. Reporter gene experiments showed that activation of the p44/42 mitogen-activated protein kinase signaling cascade is required for Ca2+-dependent stimulation of CRE-mediated transcription in the SCN. These findings reveal the CREB/CRE transcriptional pathway to be circadian-regulated within the SCN, and raise the possibility that this pathway provides signaling information essential for normal clock function.

Fibroblast Growth Factor-1 (FGF-1) Enhances IL-2 Production and Nuclear Translocation of NF-κB in FGF Receptor-Bearing Jurkat T Cells

Fibroblast growth factors (FGFs) are heparin-binding proteins crucial to embryogenesis, angiogenesis, and wound healing. FGF-1 is abundantly expressed in the synovium in rheumatoid arthritis and in rejecting allografts, sites of chronic immune-mediated inflammation. The frequency of FGF-1-responsive T cells is increased in the peripheral blood of these disorders, and a high percentage of infiltrating T cells in rheumatoid arthritis synovium express receptors for FGF-1. To understand the action of FGF-1 in T cells, studies were initiated in Jurkat T cells that express the signaling isoform of FGF receptor-1. These experiments show that FGF-1 stimulation of Jurkat T cells provides a second signal that augments TCR-mediated IL-2 production. Analogous to costimulation via CD28, this activity is mediated through activation of Rel/κB, a family of transcription factors known to regulate IL-2 and other activation-inducible proteins. FGF-1 alone induces modest nuclear translocation of κB-binding proteins, and this translocation is enhanced by the combination of anti-CD3 and FGF-1. This NF-κB binding complex is composed of transcriptionally active p65(RelA)/p50 heterodimers and results primarily from the targeted degradation of IκB-α, an inhibitor that sequesters Rel/κB in the cytoplasm. These data are the first to show a connection between FGF-1 signaling and NF-κB activation outside of embryonic development. The signaling events that link FGF receptor-1 engagement and NF-κB activation in Jurkat are probably distinct from the CD28 costimulation pathway, since FGF-1-induced Rel/κB binding proteins do not contain significant levels of c-Rel and are not identical with the CD28 response complex.

Differential regulation of glial cell line-derived neurotrophic factor (GDNF) expression in human neuroblastoma and glioblastoma cell lines

Human SK-N-AS neuroblastoma and U-87MG glioblastoma cell lines were found to secrete relatively high levels of glial cell line-derived neurotrophic factor (GDNF). In response to growth factors, cytokines, and pharmacophores, the two cell lines differentially regulated GDNF release. A 24-hr exposure to tumor necrosis factor-alpha (TNFalpha; 10 ng/ml) or interleukin-1beta (IL-1,; 10 ng/ml) induced GDNF release in U-87MG cells, but repressed GDNF release from SK-N-AS cells. Fibroblast growth factors (FGF)-1, -2, and -9 (50 ng/ml), the prostaglandins PGA2, PGE2, and PGI2 (10 microM), phorbol 12,13-didecanoate (PDD; 10 nM), okadaic acid (10 nM), dexamethasone (1 microM), and vitamin D3 (1 microm) also differentially effected GDNF release from U-87MG and SK-N-AS cells. A result shared by both cell lines, was a two- to threefold increase in GDNF release by db-cAMP (1 mM), or forskolin (10 microM). In general, analysis of steady-state GDNF mRNA levels correlated with changes in extracellular GDNF levels in U-87MG cells but remained static in SK-N-AS cells. The data suggest that human GDNF synthesis/release can be regulated by numerous factors, signaling through multiple and diverse secondary messenger systems. Furthermore, we provide evidence of differential regulation of human GDNF synthesis/release in cells of glial (U-87MG) and neuronal (SK-N-AS) origin.

ATF3 and stress responses

The purpose of this review is to discuss ATF3, a member of the ATF/CREB family of transcription factors, and its roles in stress responses. In the introduction, we briefly describe the ATF/CREB family, which contains more than 10 proteins with the basic region-leucine zipper (bZip) DNA binding domain. We summarize their DNA binding and heterodimer formation with other bZip proteins, and discuss the nomenclature of these proteins. Over the years, identical or homologous cDNA clones have been isolated by different laboratories and given different names. We group these proteins into subgroups according to their amino acid similarity; we also list the alternative names for each member, and clarify some potential confusion in the nomenclature of this family of proteins. We then focus on ATF3 and its potential roles in stress responses. We review the evidence that the mRNA level of ATF3 greatly increases when the cells are exposed to stress signals. In animal experiments, the signals include ischemia, ischemia coupled with reperfusion, wounding, axotomy, toxicity, and seizure; in cultured cells, the signals include serum factors, cytokines, genotoxic agents, cell death-inducing agents, and the adenoviral protein E1A. Despite the overwhelming evidence for its induction by stress signals, not much else is known about ATF3. Preliminary results suggest that the JNK/SAPK pathway is involved in the induction of ATF3 by stress signals; in addition, IL-6 and p53 have been demonstrated to be required for the induction of ATF3 under certain conditions. The consequences of inducing ATF3 during stress responses are not clear. Transient transfection and in vitro transcription assays indicate that ATF3 represses transcription as a homodimer; however, ATF3 can activate transcription when coexpressed with its heterodimeric partners or other proteins. Therefore, it is possible that, when induced during stress responses, ATF3 activates some target genes but represses others, depending on the promoter context and cellular context. Even less is understood about the physiological significance of inducing ATF3. We will discuss our preliminary results and some reports by other investigators in this regard.

Synergistic increase in c-fos expression by simultaneous activation of the ras/raf/map kinase- and protein kinase A signaling pathways is mediated by the c-fos AP-1 and SRE sites

Expression of the c-fos proto-oncogene is induced by numerous stimuli some of which are transmitted through the Ras/Raf/MAP kinase or the cAMP-dependent protein kinase (PKA) pathways. The effect of cell-specific interactions between these pathways on c-fos expression was investigated by exposing quiescent NIH3T3 cells to serum, forskolin, or a combination. Co-stimulation with serum and forskolin resulted in a more than additive increase in c-fos transcription. Synergistic increase in c-fos promoter activity was also observed in transient transfection studies after co-stimulation with serum plus forskolin or co-transfection with c-Raf and PKA expression plasmids. Analysis of the cAMP signaling pathway revealed that the synergy was neither due to an increase in PKA activity nor to Ser-133 phosphorylation/activation of CREB. The activation status of the MAP kinases ERK1 and ERK2 in co-treated cells was comparable to that in serum-treated cells. Co-stimulation with forskolin did not alter the phosphorylation state of Elk-1 compared to serum-induced phosphorylation of Elk-1. Deletion of c-fos promoter elements previously shown to be important for regulation of c-fos expression in response to mitogens indicates a role for SRE and FAP-1 elements.

Switching of DNA secondary structure in proenkephalin transcriptional regulation

Proper transcriptional regulation of the proenkephalin gene requires a switch between distinct factor binding sites that cannot exist at the same time. Each of the sites is formed from a nearly palindromic region that contains two functionally defined cAMP response elements. The region can switch between cruciform and linear duplex. Formation of the cruciform creates an alternative binding site for mediators of second messenger-directed transcription and abolishes the site present in the native duplex form. Use of the cruciform site has been shown to correlate with activated transcription. Analysis of DNA structure, protein binding, and gene expression from plasmids with mutant enhancers shows, however, that both sites are required for regulation of transcription. The two distinct structures form within the same enhancer. Shifting the balance between the two alters transcriptional response.

Functional components of fibroblast growth factor (FGF) signal transduction in pituitary cells. Identification of FGF response elements in the prolactin gene

Fibroblast growth factors (FGFs) have been implicated in pituitary lactotroph tumorigenesis; however, little is known about the molecular mechanisms of FGF signal transduction. We used a transient transfection approach, in GH4 cells, to identify components of the FGF signaling pathway leading to activation of the rat prolactin (rPRL) promoter. Using dominant-negative constructs of p21(Ras), Raf-1 kinase, and mitogen-activated protein (MAP) kinase, we show that FGF activation of the rPRL promoter is independent of Ras and Raf-1 but requires MAP kinase. Furthermore, MAP kinase but not Raf-1 kinase catalytic activity is stimulated by FGFs. The rPRL promoter FGF response maps to two Ets binding sites, centered at -212 (FRE1) and -96 (FRE2), and co-transfection of dominant-negative Ets inhibits FGF activation. FRE1 co-localizes with a composite, Ets/GHF-1, Ras response element. However, overexpression of Ets-1 and GHF-1, which potentiate the Ras response, inhibits FGF stimulation of the rPRL promoter, implying that Ras and FGF signaling pathways target distinct factors to elicit their effects. These data suggest that Ets factors serve to sort and integrate MAP kinase-dependent growth factor signals, allowing highly specific transcriptional responses to be mediated via the interaction of distinct Ets proteins and cofactors at common response elements.

A new dexamethasone-induced gene of the leucine zipper family protects T lymphocytes from TCR/CD3-activated cell death

By comparing mRNA species expressed in dexamethasone (DEX)-treated and untreated murine thymocytes, we have identified a gene, glucocorticoid-induced leucine zipper (GILZ), encoding a new member of the leucine zipper family. GILZ was found expressed in normal lymphocytes from thymus, spleen, and lymph nodes, whereas low or no expression was detected in other nonlymphoid tissues, including brain, kidney, and liver. In thymocytes and peripheral T cells, GILZ gene expression is induced by DEX. Furthermore, GILZ expression selectively protects T cells from apoptosis induced by treatment with anti-CD3 monoclonal antibody but not by treatment with other apoptotic stimuli. This antiapoptotic effect correlates with inhibition of Fas and Fas ligand expression. Thus, GILZ is a candidate transcription factor involved in the regulation of apoptosis of T cells.

Requirement for cAMP/calcium response element but not AP-1 site in fibroblast growth factor-2-elicited activation of tyrosine hydroxylase gene expression in PC12 cells

Basic fibroblast growth factor (FGF-2) mediates numerous important physiological processes, including differentiation and survival of dopaminergic neurons. FGF-2 was found to trigger elevation of tyrosine hydroxylase (TH) gene expression in PC12 cells that was sustained for 1-8 days. FGF-2 induced chloramphenicol acetyltransferase (CAT) reporter activity under control of the TH promoter, indicating that the induction is transcriptionally mediated. The transcriptional activation of TH by FGF-2 was examined using various deletions and point mutations of the 5' flanking region controlling CAT reporter activity. In contrast to the reported mechanisms of transcriptional regulation of TH expression by NGF and phorbol esters, the AP-1 site at -205/-199 was not required for the activation by FGF-2. A construct containing only 60 nucleotides of the promoter was still inducible by FGF-2. However, a construct with a point mutation in the CRE/CaRE was not responsive to induction by FGF-2. These findings indicate that the CRE/CaRE, but not the AP-1, element is required for induction by FGF-2 and point to differences between NGF and FGF-2 in the regulation of TH gene expression.

Fibroblast growth factor receptor signaling activates the human interstitial collagenase promoter via the bipartite Ets-AP1 element

Interstitial collagenases participate in the remodeling of skeletal matrix and are regulated by fibroblast growth factor (FGF). A 0.2-kb fragment of the proximal human interstitial collagenase [matrix metalloproteinase (MMP1)] promoter conveys 4- to 8-fold induction of a luciferase reporter in response to FGF2 in MC3T3-E1 osteoblasts. By 5'-deletion, this response maps to nucleotides -100 to -50 relative to the transcription initiation site. The 63- bp MMP1 promoter fragment -123 to -61 confers this FGF2 response on the rous sarcoma virus minimal promoter. Intact Ets and AP1 cognates in this element are both required for responsiveness. The AP1 site supports basal and FGF-inducible promoter activity. The intact Ets cognate represses basal transcriptional activity in both heterologous and native promoter contexts and is also required for FGF activation. FGF2 up-regulates a DNA-binding activity that recognizes the MMP1 AP1 cognate and contains immunoreactive Fra1 and c-Jun. Both constitutive and FGF-inducible DNA-binding activities are present in MC3T3-E1 cells that recognize the MMP1 Ets cognate; prototypic Ets transcriptional activators are not present in these complexes. Inhibitors of protein kinase C, phosphatidyl inositol 3-OH kinase, and calmodulin-dependent protein kinase do not attenuate MMP1 promoter activation. FGF2 activates ERK1/ERK2 signaling in osteoblasts; however, 25 microM MAPK-ERK kinase (MEK) inhibitor PD98059 (inhibits by > 85% the phosphorylation of ERK1/ERK2) has no effect on MMP1 promoter activation by FGF2. Ligand-activated and constitutively active FGF receptors initiate MMP1 induction. Dominant negative Ras abrogates MMP1 induction by constitutively active FGFR2-ROS, but dominant negative Rho and Rac do not inhibit induction. The mitogen-activated protein kinase (MAPK) phosphatase MKP2 [inactivates extracellular regulated kinase (ERK) = Jun N-terminal kinase (JNK) > p38 MAPK] completely abrogates MMP1 activation, whereas PAC1 (inactivates ERK = p38 > JNK) attenuates but does not completely prevent induction. Thus, a Ras- and MKP2-regulated MAPK pathway, independent of ERK1/ERK2 MAPK activity, mediates FGF2 transcriptional activation of MMP1 in MC3T3-E1 osteoblasts, converging upon the bipartite Ets-AP1 element. The DNA-protein interactions and signal cascades mediating FGF induction of the MMP1 promoter are distinct from two other recently described FGF response elements: the MMP1 promoter (-123 to -61) represents a third FGF-activated transcriptional unit.

The rat galanin-gene promoter: response to members of the nuclear hormone receptor family, phorbol ester and forskolin

We have cloned a rat genomic DNA fragment of approximately 12.5 kb. Nine kb of the cloned fragment lie in the 5'-flanking region of the gene and contain the promoter elements, while the remaining 3.5 kb contain the first four complete exons, the first three introns, and part of the fourth intron of the rat galanin gene. We have partially analysed some of the elements within the proximal sequence of this promoter which may influence the transcriptional regulation of the rat galanin gene. The rat galanin-gene promoter contains many regions which share homology with both the human and the bovine galanin genes and certain cis-elements appear to be conserved among the three species. In an attempt to test whether some of these elements are functional in the rat gene, transient transfection studies were carried out in selected cell lines. Estrogen, thyroid hormone and retinoic acid all showed a minimal degree of promoter stimulation when the rat galanin-gene promoter was co-transfected with the appropriate hormone receptors in Neuro 2A cells, while co-transfection of the nuclear orphan receptor ELP1 was able to stimulate transcription of a galanin promoter-driven reporter-gene construct (-374 bp) by 35-fold. The galanin promoter mediated a 3-4-fold induction in response to forskolin or TPA. Deletion of a 5-bp element at -50 bp from the start of transcription was able to greatly reduce the forskolin response but not the TPA response. These results point to several elements that may be targets of transcription factors linked to extracellular stimuli.

Differential regulation of insulin-like growth factor-I (IGF-I) receptor gene expression by IGF-I and basic fibroblastic growth factor

Insulin-like growth factor-I receptor (IGF-IR) gene expression is regulated by various stimuli, including hormones, growth factors, and nutritional status. We have investigated the molecular mechanism by which two growth factors, insulin-like growth factor-I (IGF-I) and basic fibroblast growth factor (bFGF) regulate IGF-IR gene expression. bFGF increases the endogenous IGF-IR mRNA levels and IGF-IR promoter activity. This effect is mediated by a region of the IGF-IR promoter located between nucleotides -476 and -188 in the 5'-flanking region. In contrast, IGF-I decreases the IGF-IR mRNA levels. IGF-I down-regulates IGF-IR transcriptional activity as deduced from experiments in which the levels of pre-mRNA and mRNA were measured. IGF-I reduced pre-mRNA and mRNA levels in parallel, while the mRNA stability was found to be unchanged by IGF-I treatment. While these results strongly suggest an effect of IGF-I on IGF-IR transcriptional activity, no specific IGF-I response element was demonstrated in the 5'-untranslated region or 5'-flanking region studied. Thus, bFGF and IGF-I have differential effects on IGF-IR gene transcription, with the IGF-I response region as yet unidentified.

FGF and stress regulate CREB and ATF-1 via a pathway involving p38 MAP kinase and MAPKAP kinase-2

Fibroblast growth factor (FGF) activates a protein kinase cascade in SK-N-MC cells that regulates gene expression at a cyclic-AMP response element (CRE) by stimulating the transcriptional activity of CREB. The activation of CREB is prevented by a dominant negative mutant of Ras and triggered via the same site (Ser133) that becomes phosphorylated in response to cyclic AMP and Ca2+. However, the effect of FGF is not mediated by cyclic AMP-dependent protein kinase, TPA-sensitive isoforms of protein kinase-C, p70S6K or p90rsk (all of which phosphorylate CREB at Ser133 in vitro). Instead, we identify the FGF-stimulated CREB kinase as MAP kinase-activated protein (MAPKAP) kinase-2, an enzyme that lies immediately downstream of p38 MAP kinase, in a pathway that is also stimulated by cellular stresses. We show that MAPKAP kinase-2 phosphorylates CREB at Ser133 in vitro, that the FGF- or stress-induced activation of MAPKAP kinase-2 and phosphorylation of CREB and ATF-1 are prevented by similar concentrations of the specific p38 MAP kinase inhibitor SB 203580, and that MAPKAP kinase-2 is the only detectable SB 203580-sensitive CREB kinase in SK-N-MC cell extracts. We also show that transfection of RK/p38 MAP kinase in SK-N-MC cells, but not transfection of p44 MAP kinase, activates Gal4-CREB-dependent transcription via Ser133. These findings identify a new growth factor and stress-activated signaling pathway that regulates gene expression at the CRE.

Synergistic induction of osteocalcin gene expression: identification of a bipartite element conferring fibroblast growth factor 2 and cyclic AMP responsiveness in the rat osteocalcin promoter

Fibroblast growth factors (FGFs) are important regulators of calvarial osteoblast growth and differentiation. We have studied the regulation of the osteoblast-specific gene osteocalcin (OC) by FGF2 in phenotypically immature MC3T3-E1 calvarial osteoblastic cells. FGF2 markedly induces OC mRNA accumulation in MC3T3-E1 cells in the presence of forskolin (FSK). Similarly, OC promoter activity (luciferase reporter) is up-regulated 6-10-fold by FGF2/FSK or by FGF2/8-bromo cyclic AMP. Half-maximal induction of OC promoter activity occurs at 1 nM FGF2. By 5' deletion analysis and dinucleotide point mutations, we map one component of this FGF2/FSK response to a GCAGTCA motif in the region -144 to -138 relative to the OC transcription initiation site. The OC promoter region -154 to -90 confers FGF2/FSK responsiveness on the Rous sarcoma virus minimal promoter. By 3' and internal deletion analyses, the region between -90 to -99 is also found to be necessary for FGF2/FSK synergy (encodes a PuGGTCA motif previously identified as a component of FSK induction). A DNA binding activity that recognizes the region -148 to -125 of the rat OC promoter is induced in crude nuclear extracts from MC3T3-E1 cells treated with FGF2 or FGF2/FSK. This binding activity is sequence-specific and does not recognize the TCAGTCA DNA cognate of AP1. Members of the ATF, Fos, and Jun family are not immunologically detected in this inducible DNA binding activity. However, transient co-expression of ATF3 but not ATF2 selectively attenuates the FGF2 component of induction. Thus, a novel FGF2-regulated DNA-protein interaction in the OC promoter participates in the transcriptional control of OC expression by FGF and cyclic AMP in MC3T3-E1 calvarial osteoblasts.

Neuron-like phenotypic changes in pancreatic β-cells induced by NGF, FGF, and dbcAMP

We studied the effects of nerve growth factor (NGF), fibroblast growth factor (FGF), and dibutyryl-cAMP (dbcAMP) on rat pancreatic β-cell morphology and of NGF and dbcAMP on insulin secretion. After 2 wk in culture, nearly 3% of β-cells extended neurite-like processes spontaneously; when cells were treated with NGF, almost 30% of them extended processes. In the presence of dbcAMP, almost all β-cells flattened, and the extension of neurite-like processes was more pronounced in fetal than in adult cells. The most prominent effect, regardless of age, was observed in cells treated with NGF and dbcAMP together, since the percentage of neurite-like bearing β-cells increased to 50%. β-cells cultured under these conditions maintained their immunoreactivity to insulin and nearly all β-cells and their neurite-like processes were also positive to GABA, tubulin, tau protein, and N-CAM. FGF increased the percentage of adult β-cells bearing neurite-like processes to 13%, and FGF and dbcAMP applied together to 40%. β-cells treated with NGF and dbcAMP for 5 to 7 d preserved their capability to secrete the hormone in response to different extracellular glucose concentrations. Insulin secretion of dbcAMP-treated β-cells was 2.5-fold higher than in control cells. NGF-treated cells were able to discriminate between different glucose concentrations, a property lost in control cells with time in culture.

DNA structure determines protein binding and transcriptional efficiency of the proenkephalin cAMP-responsive enhancer

Two precisely arranged proenkephalin cAMP response elements (CREs) behave as a single protein binding site. The experiments described support a model in which a secondary structural change creates a new binding site, which is made up of sequences from both of the CREs. The CRE-binding protein (CREB) binds CRE-1, but binding there is entirely dependent on the presence of CRE-2. Electron spectroscopic images show that a CREB dimer occupies twice as much DNA in the proenkephalin gene as in the prodynorphin gene. The enhancer region is sensitive to P1 nuclease in a CREB concentration-dependent manner, and sensitivity is strand-specific, indicating protein-stabilized structural change. DNase I analysis shows that in the native proenkephalin gene, CREB binds both CRE-1 and CRE-2. In vivo, both CREs are occupied in the transcriptionally active proenkephalin gene, while neither is in the silent gene. Whereas CREB can bind CRE-2, mutation or elimination of either proenkephalin CRE alters response to second messengers and transcription factors. Thus, binding to CRE-2 alone is not sufficient. Specific and efficient transcription of the proenkephalin gene requires the presence of both CREs, precisely arranged to allow them to form a single protein binding site.

Activated Ha-Ras but not TPA induces transcription through binding sites for activating transcription factor 3/Jun and a novel nuclear factor

We report the identification of a 20-base pair sequence mediating induced transcription in response to an activated Ha-ras gene and epidermal growth factor (EGF) but not 12-O-tetradecanoylphorbol-13-acetate stimulation. This signal-specific nuclear target is present in the long terminal repeat of a mouse VL30 retrotransposon expressed in epidermis. Functional studies and in vitro binding analyses using cultured keratinocytes (Balb/MK) reveal that the response element is composed of two cooperating sequence motifs in juxtaposed position, both of which are targets for induced binding activity 1-2 h after EGF stimulation. Of many different activating transcription factor/cAMP-responsive element binding protein/activating protein 1 factors tested, one part of the sequence selectively binds endogenous proteins immunologically related to activating transcription factor 3 (ATF3) and Jun isotypes. The other sequence is a target for a nuclear factor showing binding specificity unrelated to factors known to mediate EGF- or ras-induced transcription as determined by its sequence specificity and by antibody experiments. This component has been characterized and partially purified by gel filtration chromatography and velocity centrifugation revealing a Stokes radius of 43.6 A and a sedimentation coefficient of 9.7 S in solution. Based on these parameters, a molecular mass of 178,000 Da was calculated. The results indicate that the specific binding of ATF3/Jun and a previously uncharacterized factor account for signal-specific transcription in response to EGF or an activated Ha-ras gene in a cell type in which the cooperative action of an activated Ha-ras gene and 12-O-tetradecanoylphorbol-13-acetate cause tumor growth.