Functional analysis of a growth factor-responsive transcription factor complex

Oxidative stress and superoxide dismutase in development, aging and gene regulation

Free radicals and other reactive oxygen species are produced in the metabolic pathways of aerobic cells and affect a number of biological processes. Oxidation reactions have been postulated to play a role in aging, a number of degenerative diseases, differentiation and development as well as serving as subcellular messengers in gene regulatory and signal transduction pathways. The discovery of the activity of superoxide dismutase is a seminal work in free radical biology, because it established that free radicals were generated by cells and because it made removal of a specific free radical substance possible for the first time, which greatly accelerated research in this area. In this review, the role of reactive oxygen in aging, amyotrophic lateral sclerosis (a neurodegenerative disease), development, differentiation, and signal transduction are discussed. Emphasis is also given to the role of superoxide dismutases in these phenomena.

Oncogenic Ras modulates epidermal growth factor responsiveness in endometrial carcinomas

Since the majority of endometrial carcinomas do not contain any detectable ras mutations, the precise contribution of aberrant Ras function, if any, to endometrial carcinoma development remains to be determined. Since there is considerable evidence that Ras transformation is associated with a decreased requirement for growth factors, we compared the growth response of endometrial carcinoma cells harbouring wild-type (Ishikawa cells) or mutated (HHUA cells) K-ras to epidermal growth factor (EGF). K-ras mutation did not significantly affect the level of the EGF receptor (EGFR) expressed in these carcinoma cells. EGF could stimulate the growth of Ishikawa, but not HHUA cells. Furthermore, EGF caused elevation of Ras-GTP levels in Ishikawa, but not HHUA cells. However, the introduction of mutated, but not normal, K-ras into Ishikawa cells rendered them non-responsive to EGF growth stimulation. Thus, the presence of mutated K-ras alone modulated the growth response of endometrial carcinoma cells to EGF. An inhibitor of the EGFR tyrosine kinase activity could prevent soft agar colony formation of Ishikawa cells, but not HHUA or mutant K-ras(12V)-transfected Ishikawa cells. Taken together, these results suggest that mutated K-ras causes a loss of responsiveness to EGF stimulation and that EGFR function is dispensable for the growth of mutant Ras-positive endometrial carcinoma cells.

A composite DNA element in the promoter of the polymeric immunoglobulin receptor regulates its constitutive expression

The polymeric immunoglobulin receptor (pIgR), which is constitutively expressed on the basolateral surface of secretory epithelial cells, mediates external translocation of polymeric IgA and pentameric IgM (collectively called pIg) to exocrine secretions. A high level of synthesis must be maintained because the receptor is continuously cleaved to release bound secretory component (SC) in secretory IgA and secretory IgM, as well as free SC from unoccupied receptor. We have isolated the promoter of the pIgR gene and identified a short activating region that is required for the expression of pIgR promoter-driven reporter genes. This region contained an E-box and an inverted repeat sequence (IRS). Gel electrophoresis mobility shift assays with nuclear extracts from different pIgR-expressing epithelial cell lines demonstrated proteins that bind independently to both the E-box and the IRS sequence of the pIgR promoter. In addition, a DNA probe that contained both the E-box and the IRS gave rise to a larger complex that could not be competed by either element on its own. Binding was confirmed by DNase I footprinting of the E-box and IRS sequences with nuclear extracts, and by dimethyl sulfide footprinting in living HT-29 epithelial cells. Finally, a mutation in the pIgR promoter that inhibited protein binding to the E-box and the formation of the larger complex, abolished activated transcription from the reporter gene.

Regulation of glucose transport and c-fos and egr-1 expression in cells with mutated or endogenous growth hormone receptors

To identify mechanisms by which GH receptors (GHR) mediate downstream events representative of growth and metabolic responses to GH, stimulation by GH of c-fos and egr-1 expression and glucose transport activity were examined in Chinese hamster ovary (CHO) cells expressing mutated GHR. In CHO cells expressing wild-type GHR(GHR(1-638)), GH stimulated the expression of c-fos and egr-1, and stimulated 2-deoxyglucose uptake, responses also mediated by endogenous GHR in 3T3-F442A cells. Deletion of the proline-rich box 1 of GHR (GHR(deltaP)) abrogated all of these responses to GH, indicating that box 1, a site of association of GHR with the tyrosine kinase JAK2, is crucial for these GH-stimulated responses. As the C-terminal half of the cytoplasmic domain of GHR is required for GH-stimulated calcium flux and for stimulation of spi-2.1 transcription, GHR lacking this sequence (GHR(1-454)) were examined. Not only did GHR(1-454) mediate stimulation of c-fos and egr-1 expression and 2-deoxyglucose uptake, but they also mediated GH-stimulated transcriptional activation via Elk-1, a transcription factor associated with the c-fos Serum Response Element. Thus, the C-terminal half of the cytoplasmic domain of GHR is not required for GH-stimulated c-fos transcription, suggesting that increased calcium is not required for GH-stimulated c-fos expression. In CHO cells lacking all but five N-terminal residues of the cytoplasmic domain (GHR(1-294)), GH did not induce c-fos or egr-1 expression or stimulate 2-deoxyglucose uptake. Further, in 3T3-F442A fibroblasts with endogenous GHR, GH-stimulated c-fos expression and 2-deoxyglucose uptake were reduced by the tyrosine kinase inhibitors herbimycin A, staurosporine, and P11. Herbimycin A and staurosporine inhibit JAK2 and tyrosyl phosphorylation of all proteins stimulated by GH, whereas P11 inhibits the GH-dependent tyrosyl phosphorylation of only some proteins, including extracellular signal regulated kinases ERK1 and -2, but not JAK2. Taken together, these results implicate association of GHR with JAK2 and GH-stimulated tyrosyl phosphorylation of an additional cellular protein in GH-stimulated glucose transport and c-fos and egr-1 expression. These studies also indicate that, in contrast to spi-2.1, the N-terminal half of the cytoplasmic domain of GHR is sufficient to mediate stimulation of c-fos and egr-1 expression and Elk-1 activation, supporting multiple mechanisms for GH signaling to the nucleus.

Studies of perinuclear and nuclear translocation of the Raf-1 protein in rodent fibroblasts

Raf-1, A-Raf and B-Raf comprise a small family of highly conserved serine/threonine protein kinases, whose activities play a fundamental role in the control of proliferation and differentiation. The best studied family member, Raf-1, is expressed ubiquitously and constitutively, and its activity is regulated by post-translational mechanisms. Raf-1 can be activated by many signals that include growth factors, tumor promoters, inflammatory cytokines, calcium mobilization, DNA damaging agents, and oxygen radicals. Ras-mediated translocation of Raf-1 to the plasma membrane is a crucial step in its activation process, and is thought to facilitate phosphorylation by membrane-bound kinases. Raf-1 has also been reported to undergo intracellular redistribution following its activation: to the perinuclear space in murine NIH3T3 cells and rat hepatic Ito cells, and into the nucleus in gerbil hippocampal pyramidal cells and human MO7 leukemia cells. In contrast to the translocation to the plasma membrane, the perinuclear and/or nuclear translocation of Raf-1 has not been investigated in detail. In this paper, we report an examination of the subcellular localization of endogenous Raf-1 in a fibroblastic cell line (Rat-1) commonly used in transformation assays. Using the methods of cellular fractionation as well as in situ immunofluorescence, we show that no detectable movement of Raf-1 to the perinuclear or nuclear space can be observed. Tethering of activated Raf to the plasma membrane does not interfere with its transforming activity.

Thrombin aivation of the 9E3/CEF4 chemokine involves tyrosine kinases including c-src and the epidermal growth factor receptor

The 9E3/CEF4 gene codes for a chemokine that is highly homologous to human interleukin-8 and melanoma growth-stimulating activity/groalpha. These chemokines belong to a family of molecular mediators that are importantly involved in inflammation, wound healing, tumor development, and viral entry into cells. On the chorioallantoic membrane the 9E3 protein is chemotactic for monocyte/macrophages and lymphocytes and is angiogenic. In cultured chicken embryo fibroblasts, which have many of the properties of wound fibroblasts, the gene is stimulated by a variety of agents including oncogenes, growth factors, phorbol esters, and thrombin. The strong stimulation of 9E3 by thrombin in culture correlates well with the observation that in young chicks this gene is stimulated to very high levels in fibroblasts upon wounding and remains high throughout wound repair. Activation of 9E3 by thrombin: (i) occurs very rapidly, one minute exposure to thrombin is sufficient to initiate the signals necessary for gene activation; (ii) is independent of mitogenesis; (iii) operates through the proteolytically activated receptor for thrombin; (iv) is mediated by tyrosine kinases, including c-src and the epidermal growth factor (EGF) receptor, rather than Ser/Thr kinases such as protein kinase C and protein kinase A. Inhibition of either c-src or the EGF receptor tyrosine kinase inhibits the stimulation of 9E3 by thrombin. We show here for the first time that activation of the EGF receptor through a cell-surface receptor that does not have tyrosine kinase activity can lead to expression of an immediate early response gene which encodes for a secreted protein, a chemokine. This rapidly activated tyrosine kinase pathway may be a general stress response by which in vivo a localized cell population reacts to emergency situations such as viral infection, wounding, or tumor growth.

Receptor-like protein-tyrosine phosphatase alpha specifically inhibits insulin-increased prolactin gene expression

A physiologically relevant response to insulin, stimulation of prolactin promoter activity in GH4 pituitary cells, was used as an assay to study the specificity of protein-tyrosine phosphatase function. Receptor-like protein-tyrosine phosphatase alpha (RPTPalpha) blocks the effect of insulin to increase prolactin gene expression but potentiates the effects of epidermal growth factor and cAMP on prolactin promoter activity. RPTPalpha was the only protein-tyrosine phosphatase tested that did this. Thus, the effect of RPTPalpha on prolactin-chloramphenicol acetyltransferase (CAT) promoter activity is specific by two criteria. A number of potential RPTPalpha targets were ruled out by finding (a) that they are not affected or (b) that they are not on the pathway to insulin-increased prolactin-CAT activity. The negative effect of RPTPalpha on insulin activation of the prolactin promoter is not due to reduced phosphorylation or kinase activity of the insulin receptor or to reduced phosphorylation of insulin receptor substrate-1 or Shc. Inhibitor studies suggest that insulin-increased prolactin gene expression is mediated by a Ras-like GTPase but is not mitogen-activated protein kinase dependent. Experiments with inhibitors of phosphatidylinositol 3-kinase suggest that insulin-increased prolactin-CAT expression is phosphatidylinositol 3-kinase-independent. These results suggest that RPTPalpha may be a physiological regulator of insulin action.

Shear stress activates p60src-Ras-MAPK signaling pathways in vascular endothelial cells

The aim of this study was to elucidate the upstream signaling mechanism that mediates the fluid shear stress activation of mitogen-activated protein kinases (MAPKs), including c-Jun NH2-terminal kinase (JNK) and extracellular signal-regulated kinases (ERKs), in vascular endothelial cells (ECs). Our results indicate that p60src is rapidly activated by fluid shear stress in bovine aortic endothelial cells (BAECs). Shear stress induction of the hemagglutinin (HA) epitope-tagged HA-JNK1 and the Myc epitope-tagged Myc-ERK2 was significantly attenuated by v-src(K295R) and c-src(K295R), the kinase-defective mutants ofv-src and c-src, respectively. HA-JNK1 and Myc-ERK2 were activated by c-src(F527), a constitutively activated form of p60src, and the activation was abolished by RasN17, a dominant-negative mutant of p2lras. In contrast, although HA-JNK1 and Myc-ERK2 were also activated by RasL61, an activated form of p21ras, the activation was not affected by v-src(K295R). These results indicate that p60src is upstream to the Ras-JNK and Ras-ERK pathways in response to shear stress. The shear stress inductions of the promoters of monocyte chemotactic protein-1 (MCP-1) and c-fos, driven by TPA-responsive element (TRE) and serum-responsive element (SRE), respectively, were attenuated by v-src(K295R). This attenuation is associated with decreased transcriptional activities of c-Jun and Elk-1, the transcription factors targeting TRE and SRE, respectively. Thus, p60src plays a critical role in the shear stress activation of MAPK pathways and induction of Activating Protein-1 (AP- 1)/TRE and Elk-1/SRE-mediated transcription in ECs.

The developmentally regulated expression of serum response factor plays a key role in the control of smooth muscle-specific genes

Serum response factor (SRF) is a MADS box transcription factor that has been shown to be important in the regulation of a variety of muscle-specific genes. We have previously shown SRF to be a major component of multiple cis/trans interactions found along the smooth muscle gamma-actin (SMGA) promoter. In the studies reported here, we have further characterized the role of SRF in the regulation of the SMGA gene in the developing gizzard. EMSA analyses, using nuclear extracts derived from gizzards at various stages in development, showed that the SRF-containing complexes were not present early in gizzard smooth muscle development, but appeared as development progressed. We observed an increase in SRF protein and mRNA levels during gizzard development by Western and Northern blot analyses, with a large increase just preceding an increase in SMGA expression. Thus, changes in SRF DNA-binding activity were paralleled with increased SRF gene expression. Immunohistochemical analyses demonstrated a correspondence of SRF and SMGA expression in differentiating visceral smooth muscle cells (SMCs) during gizzard tissue development. This correspondence of SRF and SMGA expression was also observed in cultured smooth muscle mesenchyme induced to express differentiated gene products in vitro. In gene transfer experiments with SMGA promoter-luciferase reporter gene constructs we observed four- to fivefold stronger SMGA promoter activity in differentiated SMCs relative to replicating visceral smooth muscle cells. Further, we demonstrate the ability of a dominant negative SRF mutant protein to specifically inhibit transcription of the SMGA promoter in visceral smooth muscle, directly linking SRF with the control of SMGA gene expression. Taken together, these data suggest that SRF plays a prominent role in the developmental regulation of the SMGA gene.

In vivo expression and regulation of Elk-1, a target of the extracellular-regulated kinase signaling pathway, in the adult rat brain

The transcription factor Elk-1, a nuclear target of extracellular-regulated kinases (ERKs), plays a pivotal role in immediate early gene induction by external stimuli. Notably, the degree of phosphorylation of Elk-1 is tightly correlated with the level of activation of transcription of c-fos by proliferative signals. No data yet indicate the role of Elk-1 in the adult brain in vivo. To address this question, we have analyzed in the present work (1) Elk-1 mRNA and protein expression in the adult rat brain, and (2) the regulation of Elk-1 (i.e., its phosphorylation state) in an in vivo model of immediate early gene (IEG) induction: an electrical stimulation of the cerebral cortex leading to c-fos and zif268 mRNA induction in the striatum. Using in situ hybridization, we show that Elk-1 mRNA is expressed in various brain structures of adult rat, and that this expression is exclusively neuronal. We demonstrate by immunocytochemistry using various specific Elk-1 antisera that the protein is not only nuclear (as shown previously in transiently transfected cell lines) but is also present in soma, dendrites, and axon terminals. On electrical stimulation of the glutamatergic corticostriatal pathway, we show a strict spatiotemporal correspondence among ERK activation, Elk-1 phosphorylation, and IEG mRNA induction. Furthermore, both activated proteins, analyzed by immunocytochemistry, are found in cytosolic and nuclear comparments of neuronal cells in the activated area. Our data suggest that the ERK signaling pathway plays an important role in regulating genes controlled by serum response element sites via phosphorylation of Elk-1 in vivo.

Reductions in the activation of ERK and JNK are associated with decreased IL-2 production in T cells from elderly humans stimulated by the TCR/CD3 complex and costimulatory signals

T cells from elderly humans often display impaired IL-2 production, but the mechanisms are unknown. Because the activities of extracellular signal-regulated kinases (ERK) and c-Jun NH2-terminal kinases (JNK) are important for IL-2 production, the current study evaluated if aberrancies in the expression and activation of ERK2 or JNK might underlie decreased IL-2 production by human T cells during aging. The present results show that diminished ERK2 and JNK catalytic activities were commonly detected in T cells from elderly humans stimulated with anti-CD3 mAb OKT3 plus PMA. These reductions did not represent temporal shifts in activation or altered expression of ERK2 or JNK. In addition, the reductions of ERK2 activation in stimulated T cells from elderly individuals were accompanied by decreased Raf-1 kinase activation and could be observed without coexisting impairments in JNK activation. Stimulation of ERK2 activation in elderly T cells correlated with IL-2 production and decreased ERK2 activation was consistently associated with reduced IL-2 production. Although the age-related decreases in JNK activation were accompanied by reduced IL-2 production, substantial impairments of JNK activation were observed with diminished ERK2 activation. Moreover, anti-CD3/PMA-stimulated T cells from elderly individuals that displayed normal JNK activation and impaired ERK2 activation continued to demonstrate reduced IL-2 production. These findings show that impairments in the activation of ERK2 and JNK can accompany decreased IL-2 production by T cells from elderly humans and further suggest that aberrancies in TCR/CD3-dependent activation of the Raf-1/MEK/ERK2 cascade may be rate-limiting for the full induction of IL-2.

The CREB-binding protein (CBP) cooperates with the serum response factor for transactivation of the c-fos serum response element

The serum response element is one of the major promoter elements of the immediate early response to extracellular signals. The serum response element includes two main binding sites for proteins: the Ets box, which binds p62(TCF), and the CArG box, which binds p67(SRF). These two proteins are direct targets for signal transduction pathways; p62(TCF) is a nuclear end point of the Ras/mitogen-activated protein kinase pathway, and p67(SRF) is targeted by the Rho/Rac small G-proteins. The mechanism by which the signal is further transduced from the transcription factors to the basal transcriptional machinery is poorly understood. Recent data have suggested that the cAMP-responsive element-binding protein (CREB)-binding protein, a transcriptional adaptor involved in the transactivation through a wide variety of enhancer elements, participates in p62(TCF) activity. We here show that the CREB-binding protein also cooperates in the process of transactivation by p67(SRF). Cotransfections of expression vectors for the CREB-binding protein increased the expression, in response to serum, of reporters under the control of the c-fos serum response element. Interestingly, the C-terminal moiety of the CREB-binding protein was not necessary to observe this effect. The cooperation did not require the Ets box in the serum response element, and the CArG box was sufficient, indicating that the CREB-binding protein is able to cooperate with p67(SRF) in the absence of an Ets protein. Co-immunoprecipitation experiments using cell extracts showed that p67(SRF) could be retained with antibodies directed against the CREB-binding protein, suggesting that the two proteins form a multimolecular complex in live cells. The physical interaction between p67(SRF) and the CREB-binding protein was further confirmed by two-hybrid assays in mammalian cells. Our results indicate that the CREB-binding protein cooperates with p67(SRF) and, thus, suggest that the serum response element is regulated by a multimolecular complex, which includes the CREB-binding protein, p67(SRF), and p62(TCF), with multiple interactions between the components of the complex.

Signal transduction pathways and their relevance in human astrocytomas

Aberrations in a number of signal transduction pathways have been identified as playing a key role in the molecular pathogenesis of astrocytomas and their progression to high grade glioblastoma multiforme (GBM). GBMs are characterized by overexpression of the Platelet Derived Growth Factor Receptors (PDGFR) and their ligands (PDGF), as well as the Epidermal Growth Factor Receptor (EGF-R). These receptors activate the Ras pathway, a key cellular signal transduction pathway, culminating in the activation of a wide range of Ras-dependent cellular events. GBMs have also been found to either overexpression or lose expression of various Protein Kinase C (PKC) isoforms. Major strides are being made in developing pharmacological agents which specifically inhibit these growth factor receptors and intracellular signal transduction pathways. Elucidating the role of these pathways in GBMs is thus of major clinical importance, as these novel molecularly-targeted agents may prove of use in the clinical management of GBMs in the future.

Enhanced serum response element binding activity correlates with down-regulation of c-fos mRNA expression in the rat brain following repeated cortical lesions

Repeated lesions of rat cerebral cortex result in transient peaks in the level of the c-fos transcript, but after the second lesion, this peak is substantially diminished. Using this lesion paradigm, we have analyzed the participation of the c-fos promoter elements SRE and DSE in the regulation of c-fos transcription. Following a single lesion, SRE/DSE binding activity peaked at 2 h, subsequent to the maximal levels of c-fos mRNA and parallel to the peak of c-Fos protein. After a second lesion (reinduction), 4 h following the initial lesion, SRE/DSE binding activity peaked after only 30 min and was significantly higher than following the first lesion. Once again, this peak occurred after the peak of c-fos mRNA expression and parallel with the second peak of c-Fos protein expression. These results suggested that the SRE and DSE promoter elements participated in the induction and down-regulation of c-fos transcription in vivo and suggested the possible involvement of Fos protein in its own regulation. The ability of Fos/Fra proteins to participate in a transcriptional complex was confirmed in gel-shift experiments with an AP-1 element, and the biphasic trend of binding activity was observed. Supershift experiments were performed to directly determine whether Fos protein was participating in SRE and/or DSE transcriptional complexes. No alterations in the position or intensity of the shifted band were observed using Fos/Fra antiserum suggesting that Fos/Fra proteins could be involved in c-fos down-regulation through mechanisms other than direct participation in the SRE/DSE transcription complex.

Using altered specificity Oct-1 and Oct-2 mutants to analyze the regulation of immunoglobulin gene transcription

Oct-1 and Oct-2 represent the prototypical example of related transcription factors with identical DNA recognition properties. They bind functionally critical octamer elements found in diverse regulatory sequences. It has not been possible to determine directly if these factors are functionally redundant or selective when interacting with different regulatory sequences in the appropriate cell type. An equivalent pair of altered DNA-binding specificity mutants of Oct-1 and Oct-2 are used to examine their function from varied regulatory contexts in B cells. These factors function as redundant activators of immunoglobulin (Ig) gene promoters (Vkappa and VH) and a histone H2B promoter. The structural basis of redundancy resides in the highly conserved DNA-binding POU domain, because this domain of either protein can activate transcription from both Ig and H2B promoters. We find that the nature of a distal enhancer dictates the relative potency of Oct-1 versus Oct-2 bound to a promoter. Oct-1 preferentially stimulates transcription from a VH or Vkappa promoter in combination with enhancers from the IgH or Igkappa locus, respectively. In this context, the more potent action of Oct-1 is dependent on a region external to the POU domain. These results suggest that Oct-1 may be the critical regulator of Ig gene transcription during B cell development and provide an explanation for selective Ig isotype expression defects in Oct-2 and OCA-B null mice.

Activated RhoA stimulates c-fos gene expression in myocardial cells

Rho regulates various cell functions, including cell morphology and motility. However, the functional role of Rho on the signaling pathway in myocardial cells (MCs) is unknown. In the present study, we attempted to explore the mode of Rho action for c-fos gene expression in MCs. Expression of the c-fos promoter/enhancer linked to the luciferase reporter gene (c-fos luciferase) was stimulated by the wild type of RhoA and the point-mutated active form of RhoA (RhoA Val14) but not the biologically inactive effector domain mutant of RhoA. Rho GDP dissociation inhibitor inhibited the action of RhoA on c-fos luciferase expression. The deletion analysis revealed that the c-fos serum response element (SRE) and the 12-O-tetradecanoylphorbol-13-acetate response element (TRE) mainly account for c-fos luciferase expression by RhoA Val14. The c-fos SRE mutant, which contains an intact binding site for the serum response factor but lacks the ternary complex factor binding site, was activated by RhoA Val14. The action of RhoA Val14 on c-fos luciferase expression was not inhibited by downregulation of protein kinase C, protein kinase C inhibitors, or tyrosine kinase inhibitors. These results indicate that activated RhoA stimulates c-fos gene expression through the c-fos SRE and TRE and that the signaling pathway from activated RhoA to the c-fos promoter/enhancer is independent of these inhibitor-sensitive pathways in MCs.

Activation of mitogen-activated protein kinases (ERK/JNK) and AP-1 transcription factor in rat carotid arteries after balloon injury

Smooth muscle cell proliferation is a key event in neointimal formation after balloon angioplasty. The molecular signals that mediate this process have yet to be identified. Mitogen-activated protein (MAP) kinases are thought to play a pivotal role in transmitting transmembrane signals required for cell proliferation in vitro. The present studies were designed to investigate whether the signal transduction pathways of MAP kinases were involved in the development of restenosis in the injured arteries. Rat carotid arteries were isolated at various time points after balloon injury, and activities of MAP kinases, including extracellular signal-regulated kinases (ERK), and stress activated protein kinases (SAPK)/c-Jun N-terminal protein kinases (JNK), were determined in protein extracts of the vasculature using protein kinase assay and Western blot analysis. After balloon angioplasty, ERK2 and JNK1 activities in the vessel wall increased rapidly, reached a high level in 5 minutes and maintained for 1 hour. A sustained increase in ERK2 kinase activity was observed over the next 7 days in the arterial wall and 14 days in neointima after injury. In contrast, opposite and uninjured arteries did not show significant changes in these kinase activities. Concomitantly, Western blot analysis confirmed that the ERK2 kinase in the injured vessels was indeed activated or phosphorylated, showing a slowly migrating species of a 42-kDa protein containing phosphorylated tyrosine. Kinase activation is followed by an increase in c-fos and c-jun gene expression and enhanced activator protein 1 (AP-1) DNA-binding activity. Thus, balloon injury rapidly activates the MAP kinases in rat carotid arteries. These kinase activations may be crucial in mediating smooth muscle cell proliferation in response to vascular angioplasty.

Protein and DNA contact surfaces that mediate the selective action of the Phox1 homeodomain at the c-fos serum response element

The human homeodomain protein Phox1 can impart serum-responsive transcriptional activity to the c-fos serum response element (SRE) by interacting with serum response factor (SRF). This activity is shared with other Paired class homeodomains but not with more distantly related homeodomains. To understand the mechanism of action of Phox1 at the SRE and the basis for the selective activity of Paired class homeodomains in this context, we performed a detailed mutagenesis of the Phox1 homeodomain. We found that amino acid residues that contact the major groove of the DNA are required for SRE activation in vivo, suggesting an in vivo requirement for major-groove DNA contact by the homeodomain. In contrast, substitution of a lysine residue in the N-terminal arm of the Phox1 homeodomain appeared to abolish DNA binding without affecting activity in vivo. Certain substitutions on the exposed surfaces of helices 1 and 2, not required for DNA binding, abolished activity in vivo, suggesting that these surfaces contact an accessory protein(s) required for this activity. We also found that transfer of a single amino acid residue from the surface of Phox1 helix 1 to the corresponding position in the distantly related Deformed (Dfd) homeodomain imparts to Dfd the ability to activate the SRE in vivo. We propose that Phox1 interacts with one or more factors at the SRE, in addition to SRF, and that the specificity of this interaction is determined by residues on the surfaces of helices 1 and 2.

Growth hormone regulates ternary complex factors and serum response factor associated with the c-fos serum response element

For insight into the mechanisms of gene regulation by growth hormone (GH), the regulation of transcription factors associated with the serum response element (SRE) located upstream of c-fos was examined. The SRE can mediate induction of reporter expression in response to GH. For insight into the mechanism by which GH regulates transcription factors, regulation of SRE-associated proteins by GH was examined. In nuclear extracts from 3T3-F442A fibroblasts, several SRE-binding complexes were identified by electrophoretic mobility shift assay. GH treatment for 2-10 min transiently increased binding of two complexes; binding returned to control values within 30 min. The two GH-stimulated complexes were supershifted by antibodies against the serum response factor (SRF), indicating that they contained SRF or an antigenically related protein. One of the GH-stimulated complexes was supershifted by antibody against Elk-1, suggesting that it contains a ternary complex factor (TCF) such as Elk-1 in addition to SRF. Induction of binding by GH was lost when the SRF binding site in the SRE was mutated, and mutation of either the SRF or TCF binding site altered the pattern of protein binding to the SRE. Mutation of the SRF or TCF binding site in SRE-luciferase plasmids inhibited the ability of GH to stimulate reporter expression, supporting a role for both SRF and TCF in GH-induced transcription of c-fos via the SRE. The TCF family member Elk-1 is capable of mediating GH-stimulated transcription, since GH-stimulated reporter expression was mediated by the transcriptional activation domain of Elk-1. Consistent with this stimulation, GH rapidly and transiently stimulated the serine phosphorylation of Elk-1. The increase was evident within 10 min and subsided after 30 min. Taken together, these data indicate that SRF and TCF contribute to GH-promoted transcription of c-fos via the SRE and are consistent with GH-promoted phosphorylation of Elk-1 contributing to GH-promoted transcriptional activation via the SRE.

Growth-inhibitory effects of N,N-diethyl-2-[4-(phenylmethyl)phenoxy]-ethanamine-HCl combined with cisplatin on human ovarian cancer cells inoculated into nude mice

In 5-day incubation of an estrogen receptor-negative human ovarian cancer cell line (KF) with N,N-diethyl-2-[4-(phenylmethyl)phenoxy]ethanamine-HCl (DPPE), the concentration of DPPE required for 50% inhibition of KF cell proliferation (IC50) was 1.7 microM. The IC50 of DPPE for inhibition of protein kinase C (PKC) activity was 3.0 microM, a similar value to those of other antiestrogens such as tamoxifen and clomiphene. DPPE also inhibited phosphorylation of mitogen-activated protein kinase in KF cells. When treatment with DPPE was started 7 days after inoculation of KF cells into nude mice, 50 mg/kg DPPE alone resulted in a significant growth retardation in the early stage of tumor growth. Although 25 mg/kg DPPE showed a similar effect to 2 mg/kg cisplatin (CDDP), the combination had the most marked tumor growth-inhibitory effect. Nude mice treated with combinations of CDDP and DPPE survived significantly longer than not only untreated, but also CDDP-alone-treated mice, while 50 mg/kg but not 25 mg/kg DPPE alone had an effect comparable to that of 2 mg/kg CDDP alone. If treatment with DPPE was begun from the day after tumor inoculation, the inhibitory effect of DPPE was further enhanced, especially when combined with CDDP. If treatment with DPPE was started in nude mice with a lower tumor burden, 25 mg/kg as well as 50 mg/kg DPPE had a similar effect to 2 mg/kg CDDP, in terms of survival. When DPPE was combined with CDDP, the effect was significantly enhanced, compared to that of either alone. These treatments could be done without any adverse side effect. Thus, we conclude that DPPE has an antiestrogen action and its tumor growth-inhibiting activity is enhanced on administration in combination with CDDP.

Angiotensin II-induced stimulation of smooth muscle alpha-actin expression by serum response factor and the homeodomain transcription factor MHox

The objective of the present study was to examine the molecular mechanisms whereby angiotensin II (Ang II) stimulates smooth muscle (SM) alpha-actin expression in rat aortic smooth muscle cells (SMCs). Nuclear run-on analysis and transfection studies indicated that the effects of Ang II on SM alpha-actin were mediated at least in part at the transcriptional level. Transfection of various rat SM alpha-actin promoter/chloramphenicol acetyltransferase (CAT) constructs into SMCs demonstrated that the first 155 bp of the SM alpha-actin promoter was sufficient to confer maximal Ang II responsiveness, conferring an approximately 4-fold increase in reporter activities in these SMCs compared with vehicle-treated SMCs. Mutation of either of two highly conserved CArG elements, designated A (-62) and B (-112), completely abolished Ang II-induced increases in reporter activity, whereas mutation of a homeodomain-like binding sequence at -145 (ATTA) reduced reporter activity by half. Results of EMSAs showed that nuclear extracts from Ang II-treated SMCs exhibited enhanced binding activity of serum response factor (SRF) to the CArG elements and of a homeodomain factor, MHox, to the ATTA element. Northern analyses showed that Ang II also stimulated marked increases in MHox mRNA levels. Western analyses demonstrated that Ang II-induced increases in SRF binding were not due to increased SRF protein expression. Recombinant MHox markedly enhanced binding activity of SRF in EMSAs. Finally, MHox overexpression transactivated a SM alpha-actin promoter/CAT reporter construct by approximately 3.5-fold in transient cotransfection studies. These results provide evidence for involvement of a homeodomain transcription factor, MHox, in Ang II-mediated stimulation of SM alpha-actin via a CArG/SRF-dependent mechanism.

Hypoxia induces c-fos transcription via a mitogen-activated protein kinase-dependent pathway

Hypoxia is a pathophysiological condition that occurs during injury, ischemia, and stroke. It is characterized by a decrease of reactive oxygen intermediates and a change of the intracellular redox level. In tumors hypoxia is regarded as a trigger for enhanced growth and metastasis. Here we report that in HeLa cells, hypoxic conditions induce the transcriptional activation of c-fos transcription via the serum response element. Mutations in the binding site for the ternary complex factor Elk-1 and the serum response factor abolished this induction, indicating that a ternary complex at the serum response element is necessary for the induction of the c-fos gene under hypoxia. The transcription factor Elk-1 was covalently modified by phosphorylation in response to hypoxia. Furthermore this hyperphosphorylation of Elk-1, the activation of mitogen-activated protein kinase (MAPK), and the induction of c-fos transcripts were blocked by PD98059, a specific inhibitor of mitogen-activated protein kinase kinase/extracellular signal-regulated protein kinase kinase 1. An in vitro kinase assay with Elk-1 as substrate showed that MAPK is activated under hypoxia. The activation of MAPK corresponds temporally with the phosphorylation and activation of Elk-1. Thus, a decrease of the intracellular reactive oxygen intermediate level by hypoxia induces c-fos via the MAPK pathway. These results suggest that the intracellular redox levels may be directly coupled to tumor growth, invasion, and metastasis via Elk-1-dependent induction of c-Fos controlled genes.

Raf-induced proliferation or cell cycle arrest is determined by the level of Raf activity with arrest mediated by p21Cip1

The Raf family of protein kinases display differences in their abilities to promote the entry of quiescent NIH 3T3 cells into the S phase of the cell cycle. Although conditional activation of deltaA-Raf:ER promoted cell cycle progression, activation of deltaRaf-1:ER and deltaB-Raf:ER elicited a G1 arrest that was not overcome by exogenously added growth factors. Activation of all three deltaRaf:ER kinases led to elevated expression of cyclin D1 and cyclin E and reduced expression of p27Kip1. However, activation of deltaB-Raf:ER and deltaRaf-1:ER induced the expression of p21Cip1, whereas activation of deltaA-Raf:ER did not. A catalytically potentiated form of deltaA-Raf:ER, generated by point mutation, strongly induced p21Cip1 expression and elicited cell cycle arrest similarly to deltaB-Raf:ER and deltaRaf-1:ER. These data suggested that the strength and duration of signaling by Raf kinases might influence the biological outcome of activation of this pathway. By titration of deltaB-Raf:ER activity we demonstrated that low levels of Raf activity led to activation of cyclin D1-cdk4 and cyclin E-cdk2 complexes and to cell cycle progression whereas higher Raf activity elicited cell cycle arrest correlating with p21Cip1 induction and inhibition of cyclin-cdk activity. Using green fluorescent protein-tagged forms of deltaRaf-1:ER in primary mouse embryo fibroblasts (MEFs) we demonstrated that p21Cip1 was induced by Raf in a p53-independent manner, leading to cell cycle arrest. By contrast, activation of Raf in p21Cip1(-/-) MEFs led to a robust mitogenic response that was similar to that observed in response to platelet-derived growth factor. These data indicate that, depending on the level of kinase activity, Raf can elicit either cell cycle progression or cell cycle arrest in mouse fibroblasts. The ability of Raf to elicit cell cycle arrest is strongly associated with its ability to induce the expression of the cyclin-dependent kinase inhibitor p21Cip1 in a manner that bears analogy to alpha-factor arrest in Saccharomyces cerevisiae. These data are consistent with a role for Raf kinases in both proliferation and differentiation of mammalian cells.

Interaction of ATF6 and serum response factor

Serum response factor (SRF) is a transcription factor which binds to the serum response element (SRE) in the c-fos promoter. It is required for regulated expression of the c-fos gene as well as other immediate-early genes and some tissue-specific genes. To better understand the regulation of SRF, we used a yeast interaction assay to screen a human HeLa cell cDNA library for SRF-interacting proteins. ATF6, a basic-leucine zipper protein, was isolated by binding to SRF and in particular to its transcriptional activation domain. The binding of ATF6 to SRF was also detected in vitro. An ATF6-VP16 chimera activated expression of an SRE reporter gene in HeLa cells, suggesting that ATF6 can interact with endogenous SRF. More strikingly, an antisense ATF6 construct reduced serum induction of a c-fos reporter gene, suggesting that ATF6 is involved in activation of transcription by SRF. ATF6 was previously partially cloned as a member of the ATF family. The complete cDNA of ATF6 was isolated, and its expression pattern was described.

Calcium controls gene expression via three distinct pathways that can function independently of the Ras/mitogen-activated protein kinases (ERKs) signaling cascade

Calcium ions are the principal second messenger in the control of gene expression by electrical activation of neurons. However, the full complexity of calcium-signaling pathways leading to transcriptional activation and the cellular machinery involved are not known. Using the c-fos gene as a model system, we show here that the activity of its complex promoter is controlled by three independently operating signaling mechanisms and that their functional significance is cell type-dependent. The serum response element (SRE), which is composed of a ternary complex factor (TCF) and a serum response factor (SRF) binding site, integrates two calcium-signaling pathways. In PC12 cells, calcium-regulated transcription mediated by the SRE requires the TCF site and is not inhibited by expression of the dominant-negative Ras mutant, RasN17, nor by the MAP kinase kinase 1 inhibitor PD 98059. In contrast, TCF-dependent transcriptional regulation by nerve growth factor or epidermal growth factor is mediated by a Ras/MAP kinases (ERKs) pathway targeting the TCF Elk-1. In AtT20 cells and hippocampal neurons, calcium signals can stimulate transcription via a TCF-independent mechanism that requires the SRF binding site. The cyclic AMP response element (CRE), which cooperates with the TCF site in growth factor-regulated transcription, is a target of a third calcium-regulated pathway that is little affected by the expression of RasN17 or by PD 98059. Thus, calcium can stimulate gene expression via a TCF-, SRF-, and CRE-linked pathway that can operate independently of the Ras/MAP kinases (ERKs) signaling cascade in a cell type-dependent manner.

Three-dimensional structure of the Ras-interacting domain of RalGDS

The Ras-interacting domains of the the protein-kinase Raf and the Ral guanine nucleotide dissociation stimulator, RalGDS, lack extensive sequence similarity, but their overall three-dimensional structures are very similar to each other. Mutational analysis indicated that three residues in the RalGDS domain are critical for its interaction with Ras.

Organization and myogenic restricted expression of the murine serum response factor gene. A role for autoregulation

Serum response factor (SRF), a member of an ancient family of DNA-binding proteins, is generally assumed to be a ubiquitous transcription factor involved in regulating growth factor-responsive genes. However, avian SRF was recently shown (Croissant, J. D., Kim, J.-H., Eichele, G., Goering, L., Lough, J., Prywes, R., and Schwartz, R. J. (1996) Dev. Biol. 177, 250-264) to be preferentially expressed in myogenic lineages and is required for regulating post-replicative muscle gene expression. Given the central importance of SRF for the muscle tissue-restricted expression of the striated alpha-actin gene family, we wanted to determine how SRF might contribute to this muscle-restricted expression. Here we have characterized the murine SRF genomic locus, which has seven exons interrupted by six introns, with the entire locus spanning 11 kilobases. Murine SRF transcripts were processed to two 3'-untranslated region polyadenylation signals, yielding 4.5- and 2.5-kilobase mRNA species. Murine SRF mRNA levels were the highest in adult skeletal and cardiac muscle, but barely detected in liver, lung, and spleen tissues. During early mouse development, in situ hybridization analysis revealed enrichment of SRF transcripts in the myotomal portion of somites, the myocardium of the heart, and the smooth muscle media of vessels of mouse embryos. Likewise, murine SRF promoter activity was tissue-restricted, being 80-fold greater in primary skeletal myoblasts than in liver-derived HepG2 cells. In addition, SRF promoter activity increased 6-fold when myoblasts withdrew from the cell cycle and fused into differentiated myotubes. A 310-base pair promoter fragment depended upon multiple intact serum response elements in combination with Sp1 sites for maximal myogenic restricted activity. Furthermore, cotransfected SRF expression vector stimulated SRF promoter transcription, whereas dominant-negative SRF mutants blocked SRF promoter activity, demonstrating a positive role for an SRF-dependent autoregulatory loop.

Evidence for serum response factor-mediated regulatory networks governing SM22alpha transcription in smooth, skeletal, and cardiac muscle cells

SM22alpha is an adult smooth muscle-specific protein that is expressed in the smooth, cardiac, and skeletal muscle lineages during early embryogenesis before becoming restricted specifically to all vascular and visceral smooth muscle cells (SMC) in late fetal development and adulthood. We have used the SM22alpha gene as a marker to define the regulatory mechanisms that control muscle-specific gene expression in SMCs. Previously, we reported that the 445-base-pair promoter of SM22alpha was sufficient to direct transcription of a lacZ reporter gene in early cardiac and skeletal muscle cell lineages and in a subset of arterial SMCs, but not in venous nor visceral SMCs in transgenic mice. Here we describe two evolutionarily conserved CArG (CC(A/T)6GG) boxes in the SM22alpha promoter, both of which are essential for full promoter activity in cultured SMCs. In contrast, only the promoter-proximal CArG box is essential for specific expression in developing smooth, skeletal, and cardiac muscle lineages in transgenic mice. Both CArG boxes bind serum response factor (SRF), but SRF binding is not sufficient for SM22alpha promoter activity, since overexpression of SRF in the embryonal teratocarcinoma cell line F9, which normally expresses low levels of SRF, fails to activate the promoter. However, a chimeric protein in which SRF was fused to the transcription activation domain of the viral coactivator VP16 is able to activate the SM22alpha promoter in F9 cells. These results demonstrate the SM22alpha promoter-proximal CArG box is a target for the regulatory programs that confer smooth, skeletal, and cardiac muscle specificity to the SM22alpha promoter and they suggest that SRF activates SM22alpha transcription in conjunction with additional regulatory factors that are cell type-restricted.

Genomic structure and chromosomal localization of the novel ETS factor, PE-2 (ERF)

The members of the ETS family of transcription factors are grouped because they share a highly conserved DNA binding domain. These factors are involved in growth factor pathways and regulate both proliferation and differentiation. To identify ETS factors that may be involved in early hematopoietic progenitor regulation, we isolated a novel member of the ETS family by reverse transcriptase-PCR of the conserved DNA binding domain using degenerate oligonucleotides. This gene directs the synthesis of a 2704-nucleotide transcript whose largest open reading frame encodes a 548-amino-acid protein. Northern blot analysis reveals ubiquitous expression in all human tissues and cell lines tested, with highest levels in the testis, ovary, pancreas, and heart. Comparison of this gene with the available databases reveals very significant homology to the ETS factor PE-1 and probable near-identity with the recently cloned factor ERF. The PE-2 gene is composed of four exons spanning over 9 kb of genomic DNA. Sequence analysis of the promoter region reveals a GC-rich sequence without a TATA motif and with putative binding motifs for CREB, c-myb, and AP-1 factors. Using mouse-human somatic hybrids and FISH analysis, the PE-2 gene is localized to human chromosome 19q13.2, a region involved in translocations and deletions in leukemias and several solid tumors, suggesting that this novel ETS factor may play a role in carcinogenesis.

Molecular characterization of the B-box protein-protein interaction motif of the ETS-domain transcription factor Elk-1

The ternary complex factor (TCF) subfamily of ETS-domain transcription factors form ternary complexes with the serum response factor (SRF) and the c-fos SRE. Extracellular signals are relayed via MAP kinase signal transduction pathways through the TCF component of the ternary complex. Protein-protein interactions between TCFs and SRF play an essential role in formation of this ternary complex. A 30 amino acid sequence encompassing the TCF B-box is sufficient to mediate interactions with SRF. In this study we have identified amino acids which are critical for this interaction and derived a molecular model of the SRF binding interface. Alanine scanning of the Elk-1 B-box reveals five predominantly hydrophobic residues which are essential for binding to SRF and for ternary complex formation in vitro and in vivo. These amino acids are predicted to lie on one face of an alpha-helix. Peptides encompassing the B-box retain biological activity and have helix-forming propensity. alpha-Helix and ternary complex formation is disrupted by the introduction of helix-breaking proline residues. Our results are consistent with a model in which the Elk-1 B-box forms an inducible alpha-helix which presents a hydrophobic face for interaction with SRF. We discuss the wider applicability of our results to similar short protein-protein interaction motifs found in other transcription factors.

Transcriptional activation and transformation by FosB protein require phosphorylation of the carboxyl-terminal activation domain

The transcription factor AP-1, composed of Fos-Jun dimers, mediates some aspects of the cellular response to growth factors. Transcriptional activation and neoplastic transformation by FosB, a member of the Fos family of proteins, require the presence of a potent C-terminal activation domain. Here we show by mutational analysis that the FosB C-terminal domain has a proline-based motif that is essential for both of these functions. Phosphopeptide mapping experiments show that the C terminus of FosB is phosphorylated within a cluster of functionally redundant serine residues that is adjacent to this proline-based motif. Mutation of these serine residues to alanine severely reduces the ability of this region to function as an activation domain and inhibits the ability of FosB protein to function as a transforming protein. Several observations suggest that the kinase responsible for phosphorylation of these sites is distinct from the mitogen-activation protein kinases and stress-activated protein kinases. Our results show that transcriptional activation and neoplastic transformation by the FosB protein are dependent on phosphorylation within the C terminus. This form of control may provide a potential mechanism of signal integration at the level of a single transcription factor.

Rapid phosphorylation of Ets-2 accompanies mitogen-activated protein kinase activation and the induction of heparin-binding epidermal growth factor gene expression by oncogenic Raf-1

Heparin-binding epidermal growth factor (HB-EGF) gene transcription is rapidly activated in NIH 3T3 cells transformed by oncogenic Ras and Raf and mediates the autocrine activation of the c-Jun N-terminal kinases (JNKs) observed in these cells. A 1.7-kb fragment of the promoter of the murine HB-EGF gene linked to a luciferase reporter was strongly induced following activation of deltaRaf-1:ER, a conditionally active form of oncogenic human Raf-1. Promoter activation by deltaRaf-1:ER required a composite AP-1/Ets transcription factor binding site located between bp -974 and -988 upstream of the translation initiation site. In vivo genomic footprinting indicated that the basal level of occupancy of this composite AP-1/Ets element increased following deltaRaf-1:ER activation. Cotransfection of Ets-2 and p44 mitogen-activated protein (MAP) kinase expression vectors strongly potentiated HB-EGF promoter activation in response to deltaRaf-1:ER. Potentiated activation required both p44 MAP kinase catalytic activity and threonine 72 in the Pointed domain of Ets-2. Biochemical assays demonstrated the ability of the p42 and p44 MAP kinases to phosphorylate Ets-2 on threonine 72. Importantly, in intact cells, the kinetics of phosphorylation of Ets-2 on this residue closely mirror the activation of the p42 and p44 MAP kinases and the observed onset of HB-EGF gene transcription following deltaRaf-1:ER activation. These data firmly establish Ets-2 as a direct target of the Raf-MEK-MAP kinase signaling pathway and strongly implicate Ets-2 in the regulation of HB-EGF gene expression.

A negative role for phosphoinositide 3-kinase in T-cell antigen receptor function

BACKGROUND

A delicate balance between positive and negative regulatory mechanisms during T-cell activation determines the specificity and magnitude of an immune response. Phosphoinositide 3-kinase (PI 3-kinase) is activated by a diverse set of receptors that determine T-cell function, including the T-cell antigen receptor (TCR), the costimulatory receptor CD28, and negative regulators of T-cell activation such as CTLA-4. PI 3-kinase is also regulated by the haematopoietic cytokines that determine T-cell differentiation and lymphocyte proliferation. PI 3-kinase can thus dynamically influence the outcome of the immune reactions at various stages. In this study, we investigated the importance of PI 3-kinase in TCR-directed T-cell activation using activated or inhibitory versions of PI 3-kinase.

RESULTS

Certain aspects of TCR responses such as the induction of transcriptional activity of AP1 and serum response factor were not affected by expression of the mutant forms of PI 3-kinase. We found, however, that PI 3-kinase profoundly influenced the transactivation capacity of 'nuclear factor of activated T cells' (NF-AT) elicited by the TCR: expression of an activated form of PI 3-kinase inhibited TCR-mediated NF-AT responses, whereas expression of a dominant negative mutant of PI 3-kinase potently enhanced TCR-controlled NF-AT induction. These effects of PI 3-kinase were not mediated by previously identified PI 3-kinase effectors, such as protein kinase B, a positive regulator of PI 3-kinase, or the GTPase Rac, and are therefore likely to involve a novel, as yet unknown, effector molecule.

CONCLUSIONS

Our results establish that PI 3-kinase can both positively and negatively regulate T-cell function, and uncover a previously unrecognized function for PI 3-kinase in T cells as a selective negative regulator of TCR-signalling events and therefore as a determinant of T-cell homeostasis.

Identification and characterization of a novel Ets-2-related nuclear complex implicated in the activation of the human interleukin-12 p40 gene promoter

Interleukin-12 (IL-12) is a proinflammatory cytokine produced by antigen-presenting cells in response to many microbial infections. IL-12 plays an important role in the generation of T helper type-1 cells, which favor cell-mediated immune response. IL-12 is composed of two different subunits, p40 and p35, whose expression can be regulated concomitantly or differentially. Monocytic cells, the major producers of IL-12, can be primed by interferon-gamma (IFN-gamma) to produce optimal amounts of IL-12 in response to LPS stimulation as a consequence of bacterial infection. The priming effect is exerted primarily at the transcriptional level on the p40 promoter in conjunction with the effects of LPS, possibly by inducing specific transcription factors, which individually have no direct effect but which cooperatively can activate the promoter. We examined in detail one of these DNA-protein interactions observed around an Ets-2 element situated at -211/-207 of the p40 promoter, which is known to be a functionally critical site. This region interacts with a nuclear complex termed F1 that appears to be highly inducible by either IFN-gamma treatment for 16 h or lipopolysaccharide stimulation for 8 h. F1 binding to the Ets-2 site requires a considerable amount of spacing around the Ets-2 site, as revealed by gel mobility shift and in vitro methylation assays. Supershift experiments and DNA affinity purification indicated that both Ets-2 and a novel, antigenically related protein with an approximate molecular mass of 109 kDa are part of the F1 complex, together with additional components including IRF-1 and c-Rel. This novel protein is designated GLp109 for its inducibility by IFN-gamma or lipopolysaccharide. Its possible role in the activation of the IL-12 p40 promoter is discussed.

Convergence of MAP kinase pathways on the ternary complex factor Sap-1a

The serum response element (SRE), which is pivotal for transcriptional up-regulation of the c-fos protooncogene, is constitutively occupied by a protein complex comprising the serum response factor and a ternary complex factor (TCF). Phosphorylation of the TCFs Elk-1 and Sap-1a by the ERK and JNK subclasses of MAP kinases triggers c-fos transcription. We demonstrate here that Elk-1 is barely activated by a third subclass of MAP kinases (p38), most likely because the critical residues Ser383 and Ser389 are poorly phosphorylated by p38 MAP kinase. In contrast, the TCF Sap-1a is efficiently phosphorylated by p38 MAP kinase in vitro and in vivo on the homologous residues Ser381 and Ser387. Mutation of these sites to alanine severely reduces c-fos SRE-dependent transcription mediated by Sap-1a and p38 MAP kinase. Thus, Sap-1a may be an important target for mitogens, stress and apoptotic signals to elicit a nuclear response. However, signaling from p38 MAP kinase to Sap-1a or from Sap-1a to the basal transcription machinery does not occur in all cell types nor at promoters other than the c-fos SRE, which may ensure the specificity of signaling.

DNA-binding specificity of Mcm1: operator mutations that alter DNA-bending and transcriptional activities by a MADS box protein

The yeast Mcm1 protein is a member of the MADS box family of transcriptional regulatory factors, a class of DNA-binding proteins found in such diverse organisms as yeast, plants, flies, and humans. To explore the protein-DNA interactions of Mcm1 in vivo and in vitro, we have introduced an extensive series of base pair substitutions into an Mcm1 operator site and examined their effects on Mcm1-mediated transcriptional regulation and DNA-binding affinity. Our results show that Mcm1 uses a mechanism to contact the DNA that has some significant differences from the one used by the human serum response factor (SRF), a closely related MADS box protein in which the three-dimensional structure has been determined. One major difference is that 5-bromouracil-mediated photo-cross-linking experiments indicate that Mcm1 is in close proximity to functional groups in the major groove at the center of the recognition site whereas the SRF protein did not exhibit this characteristic. A more significant difference is that mutations at a position outside of the conserved CC(A/T)6GG site significantly reduce Mcm1-dependent DNA bending, while these substitutions have no effect on DNA bending by SRF. This result shows that the DNA bending by Mcm1 is sequence dependent and that the base-specific requirements for bending differ between Mcm1 and SRF. Interestingly, although these substitutions have a large effect on DNA bending and transcriptional activation by Mcm1, they have a relatively small effect on the DNA-binding affinity of the protein. This result suggests that the degree of DNA bending is important for transcriptional activation by Mcm1.

Regulation of the cfos serum response element by C/EBPbeta

Serum response element binding protein (SRE BP) is a novel binding factor present in nuclear extracts of avian and NIH 3T3 fibroblasts which specifically binds to the cfos SRE within a region overlapping and immediately 3' to the CArG box. Site-directed mutagenesis combined with transfection experiments in NIH 3T3 cells showed that binding of both serum response factor (SRF) and SRE BP is necessary for maximal serum induction of the SRE. In this study, we have combined size fractionation of the SRE BP DNA binding activity with C/EBPbeta antibodies to demonstrate that homodimers and heterodimers of p35C/EBPbeta (a transactivator) and p20C/EBPbeta (a repressor) contribute to the SRE BP complex in NIH 3T3 cells. Transactivation of the SRE by p35C/EBPbeta is dependent on SRF binding but not ternary complex factor (TCF) formation. Both p35C/EBPbeta and p20C/EBPbeta bind to SRF in vitro via a carboxy-terminal domain that probably does not include the leucine zipper. Moreover, SRE mutants which retain responsiveness to the TCF-independent signaling pathway bind SRE BP in vitro with affinities that are nearly identical to that of the wild-type SRE, whereas mutant SRE.M, which is not responsive to the TCF-independent pathway, has a nearly 10-fold lower affinity for SRE BP. We propose that C/EBPbeta may play a role in conjunction with SRF in the TCF-independent signaling pathway for SRE activation.

Serum response factor and protein-mediated DNA bending contribute to transcription of the dystrophin muscle-specific promoter

The minimal muscle-specific dystrophin promoter contains the consensus sequence CC(A/T)6GG, or the CArG element, which can be found in serum-inducible or muscle-specific promoters. The serum response factor (SRF), which mediates the transcriptional activation of the c-fos gene in response to serum stimulation, can bind to different CArG box elements, suggesting that it could be involved in muscle-constitutive transcription. Here we show that SRF binds to the dystrophin promoter and regulates its muscle-specific transcription. In transient transfections, an altered-binding-specificity SRF mutant restores the muscle-constitutive transcription of a dystrophin promoter with a mutation in its CArG box element. The muscle-constitutive transcription of the dystrophin promoter also requires the sequence GAAACC immediately downstream of the CArG box. This sequence is recognized by a novel DNA bending factor which was named dystrophin promoter-bending factor (DPBF). Mutations of the CArG flanking sequence abolish both DPBF binding and the promoter activity in muscle cells. Its replacement with a p62/ternary complex factor binding site changes the promoter specificity from muscle constitutive to serum responsive. These results show that, on the dystrophin promoter, the transcriptional activation induced by SRF requires the DNA bending induced by DPBF. The bending, next to the CArG box, could promote interactions between SRF and other proteins in the transcriptional complex.

Ras-GTPase activating protein (GAP): a putative effector for Ras

One attractive candidate for a Ras effector protein, other than the Raf kinases, is Ras-GAP. Indeed, recent literature suggests that besides the Raf/MAP kinase cascade, additional pathways must be stimulated to elicit a full biological response to Ras. Ras binds the COOH terminal domain of Ras-GAP, while the NH2 terminal domain appears to be essential for triggering downstream signals. Since Ras-GAP itself has no obvious enzymatic function that might explain a role in processes associated with proliferation, differentiation or apoptosis, candidates for downstream Ras-GAP effectors that fulfill this role remain to be identified. The newly found GAP-SH3 domain Binding Protein (G3BP) may be one of these. This review will briefly overview the candidates Ras effectors and discuss the results that position Ras-GAP as a critical effector downstream of Ras.

Effect of growth factors on the expression of 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase in Rat-1 fibroblasts

The activation of glycolytic flux is a biochemical characteristic of growing cells. Several reports have demonstrated the role of fructose 2,6-bisphosphate in this process. In this paper we show that the levels of 6-phosphofructo-2-kinase/fructose-2, 6-bisphosphatase (6PF2K/Fru-2,6-P2ase) mRNA are modulated in response to serum and growth factors and this effect is due to regulation of its transcription rate. The modulation of the expression of this enzyme by growth factors differs according their mitogenic effect; both lysophosphatidic acid and epidermal growth factor, when added alone, increased the mRNA levels, but endothelin had no effect. Furthermore, cAMP, which acts as an antimitogenic signal in Rat-1 fibroblasts, produced a decrease in 6PF2K/Fru-2, 6-P2ase mRNA and inhibited the effects of lysophosphatidic acid and epidermal growth factor on 6PF2K/Fru-2,6-P2ase expression. PD 098059, a specific inhibitor of the activation of the mitogen-activated protein kinase, was able to prevent the effect of EGF on 6PF2K/Fru-2, 6-P2ase gene expression. These results imply that activation of mitogen-activated protein kinase is required for the stimulation of the transcription of 6PF2K/Fru-2,6-P2ase by EGF.

What does Mos do in oocytes and somatic cells?

Mos, a protein kinase, is specifically expressed and functions during meiotic maturation (or G2/M progression) of vertebrate oocytes. When expressed ectopically, however, it can also readily induce oncogenic transformation (or uncontrolled G1/S transitions) in somatic cells. In both of these cell types, Mos activates mitogen-activated protein kinase (MAPK), which seems largely to mediate its different functions in both oocyte maturation and cellular transformation. In oocyte maturation, the Mos-MAPK pathway probably serves to activate and stabilize M-phase promoting factor (MPF) (possibly by inhibiting some negative regulator(s) of this factor), while in cellular transformation, it seems to stabilize and activate the nuclear oncoprotein c-Fos as well as to induce transcription of its gene. Thus, the different functions of Mos in oocytes and somatic cells may arise chiefly from its different MAPK-mediated targets in the respective cell types. This review discusses the cellular basis that may enable Mos to act differently in oocytes and somatic cells.

Calcium influx via the NMDA receptor induces immediate early gene transcription by a MAP kinase/ERK-dependent mechanism

The regulation of gene expression by neurotransmitters is likely to play a key role in neuroplasticity both during development and in the adult animal. Therefore, it is important to determine the mechanisms of neuronal gene regulation to understand fully the mechanisms of learning, memory, and other long-term adaptive changes in neurons. The neurotransmitter glutamate stimulates rapid and transient induction of many genes, including the c-fos proto-oncogene. The c-fos promoter contains several critical regulatory elements, including the serum response element (SRE), that mediate glutamate-induced transcription in neurons; however, the mechanism by which the SRE functions in neurons has not been defined. In this study, we sought to identify transcription factors that mediate glutamate induction of transcription through the SRE in cortical neurons and to elucidate the mechanism(s) of transcriptional activation by these factors. To facilitate this analysis, we developed an improved calcium phosphate coprecipitation procedure to transiently introduce DNA into primary neurons, both efficiently and consistently. Using this protocol, we demonstrate that the transcription factors serum response factor (SRF) and Elk-1 can mediate glutamate induction of transcription through the SRE in cortical neurons. There are at least two distinct pathways by which glutamate signals through the SRE: an SRF-dependent pathway that can operate in the absence of Elk and an Elk-dependent pathway. Activation of the Elk-dependent pathway of transcription seems to require phosphorylation of Elk-1 by extracellular signal-regulated kinases (ERKs), providing evidence for a physiological function of ERKs in glutamate signaling in neurons. Taken together, these findings suggest that SRF, Elk, and ERKs may have important roles in neuroplasticity.

Mammalian mitogen-activated protein kinase pathways are regulated through formation of specific kinase-activator complexes

Mammalian cells contain at least three signaling systems which are structurally related to the mitogen-activated protein kinase (MAPK) pathway. Growth factors acting through Ras primarily stimulate the Raf/MEK/MAPK cascade of protein kinases. In contrast, many stress-related signals such as heat shock, inflammatory cytokines, and hyperosmolarity induce the MEKK/SEK(MKK4)/SAPK(JNK) and/or the MKK3 or MKK6/p38(hog) pathways. Physiological agonists of these pathway types are either qualitatively or quantitatively distinct, suggesting few common proximal signaling elements, although past studies performed in vitro, or in cells using transient over-expression, reveal interaction between the components of all three pathways. These studies suggest a high degree of cross-talk apparently not seen in vivo. We have examined the possible molecular basis of the differing agonist profiles of these three MAPK pathways. We report preferential association between MAP kinases and their activators in eukaryotic cells. Furthermore, using the yeast 2-hybrid system, we show that association between these components can occur independent of additional eukaryotic proteins. We show that SAPK(JNK) or p38(hog) activation is specifically impaired by co-expression of cognate dominant negative MAP kinase kinase mutants, demonstrating functional specificity at this level. Further divergence and insulation of the stress pathways occurs proximal to the MAPK kinases since activation of the MAPK kinase kinase MEKK results in SAPK(JNK) activation but does not cause p38(hog) phosphorylation. Therefore, in intact cells, the three MAPK pathways may be independently regulated and their components show specificity in their interaction with cognate cascade members. The degree of intermolecular specificity suggests that mammalian MAPK signaling pathways may remain distinct without the need for specific scaffolding proteins to sequester components of individual pathways.

Phosphatidylinositol 3-kinase signals activate a selective subset of Rac/Rho-dependent effector pathways

BACKGROUND

Phosphatidylinositol 3'-hydroxyl kinase (PI 3-kinase) is activated by many growth factor receptors and is thought to exert its cellular functions through the elevation of phosphatidylinositol (3,4,5)-triphosphate levels in the cell. PI 3-kinase is required for growth-factor induced changes of the actin cytoskeleton which are mediated by the GTPases Rac and Rho. Recently, a role for Rac and Rho in regulating gene transcription has become evident.

RESULTS

Here, we show that membrane targeting of the p110 catalytic subunit, but not the p85 regulatory subunit, of PI 3-kinase generates a constitutively active enzyme that allows us to assess the relative contribution of PI 3-kinase activation to a particular cellular response. Expression of this active PI 3-kinase induced actin reorganization in the form of Rac-mediated lamellipodia and focal complexes, and Rho-mediated stress fibres and focal adhesions. However, expression of active PI 3-kinase did not induce the Ras/Rac/Rho signalling pathways that regulate gene transcription controlled by the c-fos promoter, the c-fos serum response element or the transcription factors Elk-1 and AP-1.

CONCLUSIONS

Our results demonstrate that PI 3-kinase induces a selective subset of cellular responses, but is not sufficient to stimulate the full repertoire of Rac- or Rho-mediated responses.

Neither the homeodomain nor the activation domain of Bicoid is specifically required for its down-regulation by the Torso receptor tyrosine kinase cascade

Bicoid (Bcd) is a maternal morphogen responsible for patterning the head and thorax of the Drosophila embryo. Correct specification of head structure, however, requires the activity of the Torso receptor tyrosine kinase cascade, which also represses expression of Bcd targets at the most anterior tip of the embryo. Here, we investigate the role of both the homeodomain (HD) and the activation domain of Bcd in the anterior repression of its targets. When a Bcd mutant protein whose HD has been replaced by the Gal4 DNA-binding domain is expressed in early embryos, a reporter gene driven by Gal4 DNA-binding sites is first activated in an anterior domain and then repressed from the anterior pole. The down-regulation of Bcd-Gal4 activity requires torso function but does not depend on endogenous bcd activity, indicating that the Bcd protein alone and none of its targets is required to mediate the effect of torso. Functional analysis of a chimeric protein, whose activation domain has been replaced by a generic activation domain, indicates that the activation domain of Bcd is also not specifically required for its down-regulation by Torso. We propose that Torso does not affect the ability of Bcd to bind DNA, but instead directs modification of Bcd or of a potential Bcd co-factor, which renders the Bcd protein unable to activate transcription.