Transcriptional regulation by extracellular signals: mechanisms and specificity

Nuclear activation and translocation of mitogen-activated protein kinases modulated by ethanol in embryonic liver cells

Activation and nuclear translocation of mitogen activated protein (MAP) kinases in ethanol-treated embryonic liver cells (BNLCL2) was investigated. The relative amount of MAPK proteins, MAP kinase activity and MAPK/LDH (lactate dehydrogenase) ratios were determined in nuclear and cytosolic fractions before and after serum stimulation. In ethanol-treated cells, serum-stimulated MAPK activation was potentiated in both cytosolic and nuclear fractions. Levels of both the p42 and p44 MAPK proteins increased in nuclear fractions from cells treated with ethanol alone for 24 h. Serum-stimulated nuclear translocation of both p42 and p44 MAPK was potentiated in ethanol-treated cells. Nuclear fractions from ethanol-treated cells had a modest increase in MAP kinase activity concurrent with the increased MAPK protein levels. The ratio of MAPK/LDH increased in nuclear fractions with increasing concentrations of ethanol and after serum stimulation. This further confirmed the nuclear translocation of MAPK and also demonstrated that it is not a non-specific effect of ethanol. These results demonstrate, for the first time, that in BNLCL2 liver cells ethanol treatment has dual effects. First, ethanol triggered nuclear translocation of MAPK without causing its activation. Second, it potentiated serum-stimulated activation and translocation of MAPK in the nucleus. These findings provide a novel mechanism through which ethanol may affect cellular and nuclear processes in liver cells.

Induction of smooth muscle alpha-actin in vascular smooth muscle cells by arginine vasopressin is mediated by c-Jun amino-terminal kinases and p38 mitogen-activated protein kinase

Exposure of vascular smooth muscle cells to arginine vasopressin (AVP) increases smooth muscle alpha-actin (SM-alpha-actin) expression through activation of the SM- alpha-actin promoter. The goal of this study was to determine the role of the mitogen-activated protein kinase (MAP kinase) family in regulation of SM-alpha-actin expression. AVP activated all three MAP kinase family members: ERKs, JNKs, and p38 MAP kinase. Inhibition of JNKs or p38 decreased AVP-stimulated SM-alpha-actin promoter activity, whereas inhibition of ERKs had no effect. A 150-base pair region of the promoter containing two CArG boxes was sufficient to mediate regulation by vasoconstrictors. Mutations in either CArG box decreased AVP-stimulated promoter activity. Electrophoretic mobility shift assays using oligonucleotides corresponding to either CArG box resulted in a complex of similar mobility whose intensity was increased by AVP. Antibodies against serum response factor (SRF) completely super-shifted this complex, indicating that SRF binds to both CArG boxes. Overexpression of SRF increased basal promoter activity, but activity was still stimulated by AVP. AVP stimulation rapidly increased SRF phosphorylation. These data indicate that both JNKs and p38 participate in regulation of SM- alpha-actin expression. SRF, which binds to two critical CArG boxes in the promoter, represents a potential target of these kinases.

Contortrostatin activates ERK2 and tyrosine phosphorylation events via distinct pathways

We report that cells adhering to contortrostatin show transient increases in activation of Extracellular signal Regulated Kinase 2 (ERK2). The kinetics and degree of activation are similar to cells adhering to fibronectin or vitronectin. We have recently shown that contortrostatin induces tyrosine phosphorylation in tumor cells. Contortrostatin is shown here to stimulate activation of ERK2 in suspended cells, but this activation follows a different dose-response pattern than contortrostatin-induced tyrosine phosphorylation. Since contortrostatin induces tyrosine phosphorylation via alphavbeta3, we explored the effects of an alphavbeta3-blocking antibody, 7E3, on contortrostatin-stimulated ERK2 activation. While 7E3 completely blocks the effect of contortrostatin on tyrosine phosphorylation, this antibody had no effect on activation of ERK2. In cells lacking expression of alphavbeta3, tyrosine phosphorylation was unaffected by contortrostatin treatment, but ERK2 was activated. This is strong evidence that contortrostatin is regulating tyrosine phosphorylation events and ERK2 activation via separate pathways and through different integrin receptors.

The transcription factor EGR-1 directly transactivates the fibronectin gene and enhances attachment of human glioblastoma cell line U251

EGR-1, a transcription factor with important functions in the regulation of growth and differentiation, is highly expressed in brain. Previous studies have shown that EGR-1 suppresses the transformed phenotype. However, the expression and role of EGR-1 in human glioblastoma cells are not yet determined. In this study, we found that the basal expression of the EGR-1 protein is undetectable, but is inducible in four human glioblastoma cell lines. To determine EGR-1 functions, we re-expressed EGR-1 in human glioblastoma U251 cells and found that the secretion of transforming growth factor-beta1 (TGF-beta1), plasminogen activator inhibitor-1 (PAI-1), and fibronectin (FN) was greatly enhanced. Addition of anti-TGF-beta antibodies completely inhibited the secretion of PAI-1, but had little effect on secretion of FN, indicating that PAI-1 is under the control of EGR-1-induced TGF-beta1. An examination of the promoter of the FN gene revealed two EGR-1-binding sites between positions -75 and -52 and positions -4 and +14 that specifically bound EGR-1 in gel mobility shift experiments. Utilizing wild-type and mutant FN promoter/luciferase reporter genes, we demonstrated that EGR-1 positively regulated the activity of the FN gene. In addition, cell adhesion and migration were greatly increased in the EGR-1-expressing cells, and adhesion was reversed by addition of RGD-containing peptides. These results suggest that EGR-1 may regulate cell interaction with the extracellular matrix by coordinated induction of TGF-beta1, FN, and PAI-1 in human glioblastoma cells.

Involvement of the Ras/MAPK signaling pathway in the modulation of urokinase production and cellular invasiveness by transforming growth factor-beta(1) in transformed keratinocytes

Transformed PDV keratinocytes respond to TGF-beta(1) by stimulating cell motility and invasiveness concomitantly to enhancement of the urokinase-type plasminogen activator (uPA) expression/secretion. Depletion of extracellular signal-regulated kinase (ERK1, 2) proteins by treatment of PDV cells with antisense oligonucleotides reduced basal uPA production and abolished stimulation of uPA secreted levels and cell motility by TGF-beta(1). PD098059, an inhibitor of mitogen-activated protein kinase (MAPK) kinase (MEK), decreased TGF-beta(1)-induced uPA mRNA expression, secreted activity in a dose-dependent manner, and abrogated TGF-beta(1)-stimulated cell motility and invasiveness. PDV-derived dominant-negative RasN17 cell transfectants secreted similar amounts of uPA and exhibited similar invasive abilities as the parental cells or control clones, but were unable to respond to TGF-beta(1) for stimulation of uPA-secreted levels and invasiveness. These results suggest that a Ras/MAPK transduction pathway is involved in the invasive response of transformed keratinocytes to TGF-beta(1).

A systemic administration of NMDA induces immediate early gene pip92 in the hippocampus

In the mammalian CNS, aspartate and glutamate are major excitatory amino acids, and their receptors are believed to mediate a wide range of physiological and pathological processes, including neurotransmission, plasticity, excitotoxicity, and various forms of neurodegeneration. The immediate early gene pip92 has been identified in serum-stimulated BALB/c 3T3 fibroblasts, activated T lymphocytes treated with cycloheximide, and fibroblast growth factor-stimulated hippocampal cells during neuronal differentiation. In this study we have demonstrated that pip92 is expressed in the mouse brain after a single intraperitoneal injection of NMDA. The distribution of pip92 mRNA levels in the NMDA-treated mouse brain was investigated using in situ RT-PCR. The region-specific activation of pip92 in the CNS was observed 3 h after NMDA injection, and high levels of pip92 mRNA were detected in the hippocampal dentate gyrus and piriform cortex regions. In addition, the activation of pip92 by NMDA was mediated by activation of mitogen-activated protein kinases (MAPKs), such as c-Jun N-terminal kinase (JNK) and p38 kinase, but not extracellular signal-regulated kinase (ERK) in the mouse hippocampus and immortalized rat hippocampal progenitor cells. This study suggests that pip92 is likely to play an important role in neuronal cell death induced by excitotoxic NMDA injury in the CNS.

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.

Molecular mechanisms of increased nitric oxide (NO) in asthma: evidence for transcriptional and post-translational regulation of NO synthesis

Evidence supporting increased nitric oxide (NO) in asthma is substantial, although the cellular and molecular mechanisms leading to increased NO are not known. Here, we provide a clear picture of the events regulating NO synthesis in the human asthmatic airway in vivo. We show that human airway epithelium has abundant expression of NO synthase II (NOSII) due to continuous transcriptional activation of the gene in vivo. Individuals with asthma have higher than normal NO concentrations and increased NOSII mRNA and protein due to transcriptional regulation through activation of Stat1. NOSII mRNA expression decreases in asthmatics receiving inhaled corticosteroid, treatment effective in reducing inflammation in asthmatic airways. In addition to transcriptional mechanisms, post-translational events contribute to increased NO synthesis. Specifically, high output production of NO is fueled by a previously unsuspected increase in the NOS substrate, l -arginine, in airway epithelial cells of asthmatic individuals. Finally, nitration of proteins in airway epithelium provide evidence of functional consequences of increased NO. In conclusion, these studies define multiple mechanisms that function coordinately to support high level NO synthesis in the asthmatic airway. These findings represent a crucial cornerstone for future therapeutic strategies aimed at regulating NO synthesis in asthma.

p38 and extracellular signal-regulated kinases regulate the myogenic program at multiple steps

The extracellular signals which regulate the myogenic program are transduced to the nucleus by mitogen-activated protein kinases (MAPKs). We have investigated the role of two MAPKs, p38 and extracellular signal-regulated kinase (ERK), whose activities undergo significant changes during muscle differentiation. p38 is rapidly activated in myocytes induced to differentiate. This activation differs from those triggered by stress and cytokines, because it is not linked to Jun-N-terminal kinase stimulation and is maintained during the whole process of myotube formation. Moreover, p38 activation is independent of a parallel promyogenic pathway stimulated by insulin-like growth factor 1. Inhibition of p38 prevents the differentiation program in myogenic cell lines and human primary myocytes. Conversely, deliberate activation of endogenous p38 stimulates muscle differentiation even in the presence of antimyogenic cues. Much evidence indicates that p38 is an activator of MyoD: (i) p38 kinase activity is required for the expression of MyoD-responsive genes, (ii) enforced induction of p38 stimulates the transcriptional activity of a Gal4-MyoD fusion protein and allows efficient activation of chromatin-integrated reporters by MyoD, and (iii) MyoD-dependent myogenic conversion is reduced in mouse embryonic fibroblasts derived from p38alpha(-/-) embryos. Activation of p38 also enhances the transcriptional activities of myocyte enhancer binding factor 2A (MEF2A) and MEF2C by direct phosphorylation. With MEF2C, selective phosphorylation of one residue (Thr293) is a tissue-specific activating signal in differentiating myocytes. Finally, ERK shows a biphasic activation profile, with peaks of activity in undifferentiated myoblasts and postmitotic myotubes. Importantly, activation of ERK is inhibitory toward myogenic transcription in myoblasts but contributes to the activation of myogenic transcription and regulates postmitotic responses (i.e., hypertrophic growth) in myotubes.

Inhibition of human gastric H(+)-K(+)-ATPase alpha-subunit gene expression by Helicobacter pylori

Clinical studies and in vitro data from isolated parietal cells suggest that acute Helicobacter pylori infection inhibits acid secretion. Gastric acidification is mediated by H(+)-K(+)-ATPase, an integral protein of parietal cell apical membranes. To test the hypothesis that H. pylori downregulates H(+)-K(+)-ATPase alpha-subunit (HKalpha) gene expression and to identify potential intracellular signaling pathways mediating such regulation, we transfected human gastric adenocarcinoma (AGS) cells with human and rat HKalpha 5'-flanking DNA fused to a luciferase reporter plasmid. Histamine caused dose-dependent, cimetidine-sensitive (10(-4) M) increases in cAMP, free intracellular Ca(2+), and HKalpha promoter activation in AGS cells. H. pylori infection of transfected AGS cells dose dependently inhibited basal and histamine-stimulated HKalpha promoter activity by 80% and 66%, respectively. H. pylori dose dependently inhibited phorbol myristate acetate-induced (10(-7) M) and staurosporine- (10(-7) M) and calphostin C-sensitive (5 x 10(-8) M) activation of HKalpha promoter. Also, H. pylori inhibited epidermal growth factor (EGF) (10(-8) M), genistein-sensitive (5 x 10(-5) M) activation of HKalpha promoter, reducing activity to 60% of basal level. These data suggest that H. pylori inhibits HKalpha gene expression via intracellular pathways involving protein kinases A and C and protein tyrosine kinase, AGS cells have functional histamine H(2) and EGF receptors, and transiently transfected AGS cells are a useful model for studying regulation of HKalpha gene expression.

Transcriptional responses to growth factor and G protein-coupled receptors in PC12 cells: comparison of alpha(1)-adrenergic receptor subtypes

Transcriptional responses to growth factor and G protein-coupled receptors were compared in PC12 cells using retroviral luciferase reporters. In cells stably expressing alpha(1A)-adrenergic receptors, norepinephrine activated all five reporters [AP1 (activator protein-1), SRE (serum response element), CRE (cyclic AMP response element), NFkappaB) (nuclear factor-kappaB), and NFAT (nuclear factor of activated T cells)], whereas nerve growth factor (NGF) and epidermal growth factor activated only AP1 and SRE. Activation of P2Y2 receptors by UTP did not activate any reporters. Protein kinase C inhibition blocked NFkappaB activation by norepinephrine, but potentiated CRE. Mitogen-activated protein kinase kinase inhibition blocked AP1 activation by norepinephrine, but also potentiated CRE. p38 mitogen-activated protein kinase inhibition reduced most norepinephrine responses, but not NGF responses. inhibition of Src eliminated SRE responses to norepinephrine and NGF, and reduced all responses except CRE. Phosphatidylinositol 3-kinase inhibitors markedly potentiated CRE activation by norepinephrine, with only small effects on the other responses. Comparison of the three human subtypes showed that the alpha(1A) activated all five reporters, the alpha(1B) showed smaller effects, and the alpha(1D) was ineffective. Cell differentiation caused by norepinephrine, but not NGF, was reduced by all inhibitors studied. These experiments suggest that alpha(1A)-adrenergic receptors activate a wider array of transcriptional responses than do growth factors in PC12 cells. These responses are not linearly related to second messenger production, and different subtypes show different patterns of activation.

Nucleolin and YB-1 are required for JNK-mediated interleukin-2 mRNA stabilization during T-cell activation

Regulated mRNA turnover is a highly important process, but its mechanism is poorly understood. Using interleukin-2 (IL-2) mRNA as a model, we described a role for the JNK-signaling pathway in stabilization of IL-2 mRNA during T-cell activation, acting via a JNK response element (JRE) in the 5′ untranslated region (UTR). We have now identified two major RNA-binding proteins, nucleolin and YB-1, that specifically bind to the JRE. Binding of both proteins is required for IL-2 mRNA stabilization induced by T-cell activation signals and for JNK-induced stabilization in a cell-free system that duplicates essential features of regulated mRNA decay. Nucleolin and YB-1 are required for formation of an IL-2 mRNP complex that responds to specific mRNA stabilizing signals.

Endothelial proliferation, migration, and differentiation are blunted by conditionally expressed protein kinase C pseudosubstrate peptides

Peptides based on the pseudosubstrate (PS) sequence of conventional protein kinase C isoenzymes (alpha, beta, gamma) specifically inhibit PKC activity in permeabilized cells, but whether PS can be used to study the role of PKC in the proliferation or migration of intact endothelial cells (EC) and angiogenesis is unknown. Peptides based on the PKCeta pseudosubstrate (etaPS) sequence were 3.5- to 8-fold more potent in inhibiting the PKCalpha, delta, epsilon, or eta kinase activity than was the peptide based on the PKCalpha pseudosubstrate (alphaPS) sequence. Thus, etaPS was conditionally overexpressed in intact EC and compared to alphaPS. Serum-induced growth of EC expressing etaPS was significantly slower than that of control EC. etaPS EC demonstrated slower rate of serum stimulated migration than that of either control or alphaPS EC. Expression of either etaPS or alphaPS produced slower rates of PMA induced EC migration, as compared to control EC. In an in vitro three-dimensional assay in which EC organize into capillary tubules, the EC that expressed etaPS formed fewer such tubules. This study shows that pseudosubstrate inhibitors derived from PKCeta are more potent both in vitro and in vivo than one based on the conventional isoenzyme PKCalpha. These data further support a role for PKC in proliferation and migration of intact EC, and angiogenesis.

Promoter-selective properties of the TBP-related factor TRF1

The TATA-binding protein (TBP)-related factor 1 (TRF1) is expressed in a tissue-restricted fashion during Drosophila embryogenesis and may serve as a promoter-specific recognition factor that can replace TBP in regulating transcription. However, bona fide target promoters that would preferentially respond to TRF1 have remained elusive. Polytene chromosome staining, chromatin immunoprecipitation, direct messenger RNA analysis, and transient cotransfection assays identified the Drosophila gene tudor as containing a TRF1-responsive promoter. Reconstituted in vitro transcription reactions and deoxyribonuclease I footprinting assays confirmed the ability of TRF1 to bind preferentially and direct transcription of the tudor gene from an alternate promoter. Thus, metazoans appear to have evolved gene-selective and tissue-specific components of the core transcription machinery to regulate gene expression.

Role of the cyclic AMP response element binding complex and activation of mitogen-activated protein kinases in synergistic activation of the glycoprotein hormone alpha subunit gene by epidermal growth factor and forskolin

The aim of these studies was to elucidate a role for epidermal growth factor (EGF) signaling in the transcriptional regulation of the glycoprotein hormone alpha subunit gene, a subunit of chorionic gonadotropin. Studies examined the effects of EGF and the adenylate cyclase activator forskolin on the expression of a transfected alpha subunit reporter gene in a human choriocarcinoma cell line (JEG3). At maximal doses, administration of EGF resulted in a 50% increase in a subunit reporter activity; forskolin administration induced a fivefold activation; the combined actions of EGF and forskolin resulted in synergistic activation (greater than eightfold) of the alpha subunit reporter. Mutagenesis studies revealed that the cyclic AMP response elements (CRE) were required and sufficient to mediate EGF-forskolin-induced synergistic activation. The combined actions of EGF and forskolin resulted in potentiated activation of extracellular signal-regulated kinase (ERK) enzyme activity compared with EGF alone. Specific blockade of ERK activation was sufficient to block EGF-forskolin-induced synergistic activation of the alpha subunit reporter. Pretreatment of JEG3 cells with a p38 mitogen-activated protein kinase inhibitor did not influence activation of the alpha reporter. However, overexpression of c-Jun N-terminal kinase (JNK)-interacting protein 1 as a dominant interfering molecule abolished the synergistic effects of EGF and forskolin on the alpha subunit reporter. CRE binding studies suggested that the CRE complex consisted of CRE binding protein and EGF-ERK-dependent recruitment of c-Jun-c-Fos (AP-1) to the CRE. A dominant negative form of c-Fos (A-Fos) that specifically disrupts c-Jun-c-Fos DNA binding inhibited synergistic activation of the alpha subunit. Thus, synergistic activation of the alpha subunit gene induced by EGF-forskolin requires the ERK and JNK cascades and the recruitment of AP-1 to the CRE binding complex.

Novel biochemical and functional insights into nuclear Ca(2+) transport through IP(3)Rs and RyRs in osteoblasts

We report the first biochemical and functional characterization of inositol trisphosphate receptors (IP(3)Rs) and ryanodine receptors (RyRs) in the nuclear membrane of bone-forming (MC3T3-E1) osteoblasts. Intact nuclei fluoresced intensely with anti-RyR (Ab(34)) and anti-IP(3)R (Ab(40)) antisera in a typically peripheral nuclear membrane pattern. Isolated nuclear membranes were next subjected to SDS-PAGE and blotted with isoform-specific anti-receptor antisera, notably Ab(40), anti-RyR-1, anti-RyR-2 (Ab(129)), and anti-RyR-3 (Ab(180)). Only anti-RyR-1 and Ab(40) showed bands corresponding, respectively, to full-length RyR-1 ( approximately 500 kDa) and IP(3)R-1 (approximately 250 kDa). Band intensity was reduced by just approximately 20% after brief tryptic proteolysis of intact nuclei; this confirmed that isolated nuclear membranes were mostly free of endoplasmic reticular contaminants. Finally, the nucleoplasmic Ca(2+) concentration ([Ca(2+)](np)) was measured in single nuclei by using fura-dextran. The nuclear envelope was initially loaded with Ca(2+) via Ca(2+)-ATPase activation (1 mM ATP and approximately 100 nM Ca(2+)). Adequate Ca(2+) loading was next confirmed by imaging the nuclear envelope (and nucleoplasm). Exposure of Ca(2+)-loaded nuclei to IP(3) or cADP ribose resulted in a rapid and sustained [Ca(2+)](np) elevation. Taken together, the results provide complementary evidence for nucleoplasmic Ca(2+) influx in osteoblasts through nuclear membrane-resident IP(3)Rs and RyRs. Our findings may conceivably explain the direct regulation of osteoblastic gene expression by hormones that use the IP(3)-Ca(2+) pathway.

Differential expression of egr-1 in osteoarthritic compared to normal adult human articular cartilage

OBJECTIVE

The purpose of this study was to identify genes that are differentially expressed in normal versus osteoarthritic human articular cartilage as either potential novel therapeutic targets or diagnostic markers of this disease.

DESIGN

mRNA was isolated from histologically normal and osteoarthritic adult human articular cartilage. The Differential Display technique was employed which identified differentially expressed genes in the normal and diseased tissue. Northern and reverse Northern hybridization were used to confirm the gene expression pattern. Immunohistochemistry and in-situ hybridization were used to localize expression of Egr-1 protein and mRNA respectively in cartilage.

RESULTS

A transcription factor, early growth response protein-1 (Egr-1) was found to be down-regulated more than six-fold in multiple human OA cartilage samples when compared to normal tissue. Immunohistochemistry indicated that Egr-1 was expressed throughout normal adult cartilage, in deep-, mid- and superficial-zones. In contrast, in OA cartilage there was expression of Egr-1 mRNA and protein only in the chondrocytes undergoing cloning.

CONCLUSIONS

Egr-1 is differentially expressed in OA versus normal cartilage and because of its role in transcriptional activation and repression and regulation of proliferation, differentiation and apoptosis, Egr-1 may play an important role in the pathogenesis of OA. Up-regulation of Egr-1 may therefore provide a novel therapeutic approach for either the prevention or treatment of OA.

The ERK-1/2 signaling pathway is involved in the stimulation of branching morphogenesis of fetal mouse submandibular glands by EGF

We have previously reported that epidermal growth factor (EGF) stimulates branching morphogenesis of the fetal mouse submandibular gland (SMG) (M. Kashimata and E. W. Gresik, 1997, Dev. Dyn. 208, 149-161) and that the EGF receptor (EGFR) is localized principally, if not exclusively, on the epithelial components of the fetal SMG (E. W. Gresik, M. Kashimata, Y. Kadoya, R. Mathews, N. Minami, and S. Yamashina, 1997, J. Histochem. Cytochem. 45, 1651-1657). The EGFR is a receptor tyrosine kinase, and after binding of its ligand, it triggers several intracellular signaling cascades, among them the one activating the mitogen-activated protein kinases (MAPK) ERK-1/2. Here we investigated whether EGF utilizes the ERK-1/2 signaling cascade to stimulate branching morphogenesis in the fetal mouse SMG. SMG rudiments were collected as matched pairs at E14, E16, and E18 (E0 = day of vaginal plug); placed into wells of defined medium (BGJb); and exposed to EGF for 5 or 30 min or to medium alone (controls). By Western blotting we found that EGF induced the appearance of multiple bands of phosphotyrosine-containing proteins, including bands at 170 kDa and 44 kDa/42 kDa, presumably corresponding to the phosphorylated forms of EGFR and ERK-1/2, respectively. Other blots showed the specific appearance of the phosphorylated EGFR and of phospho-ERK-1/2 in response to EGF. Immunohistochemical staining for phosphotyrosine increased at the plasma membrane after EGF stimulation for 5 or 30 min. Diffuse cytoplasmic staining for MEK-1/2 (the MAPK kinase that activates ERK-1/2) increased near the cell membrane after EGF stimulation. Phospho-ERK-1/2 was localized in the nuclei of a few epithelial cells after EGF for 5 min, but in the nuclei of many cells after EGF for 30 min. PD98059, an inhibitor of phosphorylation and activation of MEK-1/2, by itself inhibited branching morphogenesis and, furthermore, decreased the stimulatory effect of EGF on branching. Western blots confirmed that this inhibitor blocked phosphorylation of ERK-1/2 in fetal SMGs exposed to EGF. These results show that components of the ERK-1/2 signaling cascade are present in epithelial cells of the fetal SMG, that they are activated by EGF, and that inhibition of this cascade perturbs branching morphogenesis. However, EGF did not cause phosphorylation of two other MAPKs, SAPK/JNK or p38MAPK, in fetal SMGs. These results imply that the ERK-1/2 signaling is responsible, at least in part, for the stimulatory effect of EGF on branching morphogenesis of the fetal mouse SMG.

Activation of c-fos expression in hypothalamic nuclei by mu- and kappa-receptor agonists: correlation with catecholaminergic activity in the hypothalamic paraventricular nucleus

Administration of the preferential mu-opioid receptor agonist, morphine, and selective K-opioid receptor agonists elicits activation of the hypothalamus-pituitary-adrenocortical axis, although the site or the molecular mechanisms for these effects have not been determined. The expression ofFos, the protein product of the c-fos protooncogene, has been widely used as an anatomical marker of monitoring neuronal activity. In the present study we evaluated 1) the effects of the mu-opioid receptor agonist, morphine, and those of the selective K-opioid receptor agonist, trans-3,4-dichloro-N-methyl-N-[2-(1-pyrrolidinyl)cyclohexyl-]benzeneacet amide methane sulfonate (U-50,488H), administration on the expression of Fos in hypothalamic nuclei; and 2) the possible modification of the activity of noradrenergic neurons known to send afferent projections to the paraventricular nucleus (PVN), the site of CRF neurons involved in initiating ACTH secretion. Using immunohistochemical staining of Fos, the present results indicate that acute treatment with either morphine or U-50,488H induces marked Fos immunoreactivity within the hypothalamus, including the medial parvicellular PVN and supraoptic and suprachiasmatic nuclei. Pretreatment with naloxone attenuated the effect of morphine, whereas nor-binaltorphimine, a selective kappa-opioid receptor antagonist, abolished the effect of U-50,488H on Fos induction. Correspondingly, morphine and U-50,488H injection increased the production of the cerebral noradrenaline metabolite 3-methoxy-4-hydroxyphenylethylene glycol as well as noradrenaline turnover in the PVN. These effects were antagonized by naloxone and nor-bin-altorphimine, respectively. All of these findings are discussed in terms of specific events that couple opioid-induced activation of the hypothalamus-pituitary-adrenocortical axis and noradrenergic activity with changes in gene expression in selective hypothalamic nuclei.

Synergistic interaction of MEK kinase 2, c-Jun N-terminal kinase (JNK) kinase 2, and JNK1 results in efficient and specific JNK1 activation

Mitogen-activated protein kinases (MAPKs) are activated through cascades or modules consisting of a MAPK, a MAPK kinase (MAPKK), and a MAPKK kinase (MAPKKK). Investigating the molecular basis of activation of the c-Jun N-terminal kinase (JNK) subgroup of MAPK by the MAPKKK MEKK2, we found that strong and specific JNK1 activation by MEKK2 was mediated by the MAPKK JNK kinase 2 (JNKK2) rather than by JNKK1 through formation of a tripartite complex consisting of MEKK2, JNKK2, and JNK1. No scaffold protein was required for the MEKK2-JNKK2-JNK1 tripartite-complex formation. Expression of JNK1, JNKK2, and MEKK2 significantly augmented the coprecipitation of, respectively, MEKK2-JNKK2, MEKK2-JNK1, and JNKK2-JNK1, indicating that the interaction of MEKK2, JNKK2, and JNK1 is synergistic. Finally, the JNK1 was activated more efficiently in the MEKK2-JNKK2-JNK1 complex than was the JNK1 excluded from the complex. Thus, formation of a signaling complex through synergistic interaction of a MAPKKK, a MAPKK, and a MAPK molecule like MEKK2-JNKK2-JNK1 is likely to be responsible for the efficient, specific flow of information via MAPK cascades.

Lysophosphatidylcholine inhibits endothelial cell migration and proliferation via inhibition of the extracellular signal-regulated kinase pathway

Lysophosphatidylcholine (lysoPC), a major lipid component of oxidized low density lipoprotein, inhibits endothelial cell (EC) migration and proliferation, which are critical processes during angiogenesis and the repair of injured vessels. However, the mechanism(s) of lysoPC-induced inhibition of EC migration and proliferation has not been clarified. In this report, we demonstrate the critical role of extracellular signal-regulated kinase (ERK) in growth factor-stimulated EC migration and proliferation as well as their inhibition by lysoPC. EC migration and proliferation stimulated by basic fibroblast growth factor (FGF-2) were blocked by inhibition of ERK activity by both the specific mitogen-activated protein kinase kinase (MEK) 1 inhibitor PD98059 and the overexpression of a dominant-negative mutant of MEK1. Conversely, overexpression of a constitutively active mutant of MEK1 increased EC migration and proliferation, which were comparable to those of ECs stimulated with FGF-2. LysoPC inhibited FGF-2-induced ERK activation via prevention of Ras activation without inhibiting tyrosine phosphorylation of phospholipase C-gamma. Taken together, our data demonstrate that ERK activity is required for FGF-2-induced EC migration and proliferation and suggest that inhibition of the Ras/ERK pathway by lysoPC contributes to the reduced EC migration and proliferation.

EGF stimulates gastrin promoter through activation of Sp1 kinase activity

Epidermal growth factor (EGF) receptor activation stimulates gastrin gene expression through a GC-rich element called gastrin EGF response element (gERE). This element is bound by Sp1 family members and is a target of the ras-extracellular signal-regulated kinase (Erk) signal transduction cascade. This raised the possibility that Sp1 may be phosphorylated by kinases of this signaling pathway. Erk is capable of phosphorylating other mitogen-inducible transcription factors, e.g., Elk and Sap, suggesting that Erk may also mediate EGF-dependent phosphorylation of Sp1. This possibility was tested by studying Sp1-dependent kinase activity in extracts prepared from EGF-activated AGS cells by use of solid-phase kinase assays and immunoprecipitation of metabolically labeled Sp1. The results revealed that Sp1 kinase activity (like gastrin promoter activation) is inhibited by PD-98059 and, therefore, is dependent on mitogen-activated protein kinase kinase 1 (Mek 1). However, EGF-dependent activation of endogenous Erk did not account for most of the Sp1 kinase activity, since Erk and additional Sp1 kinase activity analyzed in a solid-phase kinase assay eluted from an ion-exchange column in different fractions. Phosphoamino acid analysis of in vivo radiolabeled Sp1 demonstrated that the kinase phosphorylates Sp1 on Ser and Thr in response to EGF. Therefore, most EGF-stimulated Sp1 kinase activity is Mek 1 dependent and distinct from Erk.

The dominant negative Ras mutant, N17Ras, can inhibit signaling independently of blocking Ras activation

Ras plays an important role in a variety of cellular functions, including growth, differentiation, and oncogenic transformation. For instance, Ras participates in the activation of Raf, which phosphorylates and activates mitogen-activated protein kinase kinase (MEK), which then phosphorylates and activates extracellular signal-regulated kinase (ERK), a mitogen-activated protein (MAP) kinase. Activation of MAP kinase appears to be essential for propagating a wide variety of extracellular signals from the plasma membrane to the nucleus. N17Ras, a GDP-bound dominant negative mutant, is used widely as an interfering mutant to assess Ras function in vivo. Surprisingly, we observed that expression of N17Ras inhibited the activity and phosphorylation of Elk-1, a physiological substrate of MAP kinases, in response to phorbol myristate acetate. The activity and phosphorylation of the MAP kinase hemagglutinin epitope (HA)-ERK1 were not affected by N17Ras in response to the same stimulus. Additionally, expression of N17Ras, but not L61S186Ras, a GTP-bound interfering mutant, inhibited MEK-induced Elk-1 phosphorylation, suggesting that inhibition of Elk-1 may be unique to GDP-bound Ras mutants. Finally, we observed that V12Ras-induced focus formation in NIH3T3 cells is inhibited by coexpression of GDP-bound Ras mutants, such as N17, A15, and N17N69. Therefore, N17Ras and V12 Ras may be codominant with respect to Elk-1 activation and cellular transformation. These results indicate that N17Ras appears to have at least two distinguishable functions: interference with endogenous Ras activation and inhibition of Elk-1 and transfomation. Furthermore, our data imply the possibility that GDP-bound Ras, like N17Ras, may have a direct role in signal transduction.

Neurotensin-mediated activation of MAPK pathways and AP-1 binding in the human pancreatic cancer cell line, MIA PaCa-2

Neurotensin (NT), a gastrointestinal (GI) hormone, binds its receptor (NTR) to stimulate proliferation of normal and neoplastic GI tissues; the molecular mechanisms remain largely undefined. Mitogen-activated protein kinases (MAPKs) are a family of intracellular kinases that transmit mitogenic signals by translocating to the nucleus and activating transcription factors. The purposes of this study were: (1) to identify whether the MAPKs (ERK1/2 and JNK) are activated by NT and (2) to determine the effect of NT on downstream transcription factors using the human pancreatic adenocarcinoma cell line, MIA PaCa-2, which possesses high-affinity NTR. Both ERK and JNK activity were stimulated within 3-6 min by treatment with NT (10 nM); steady-state levels of ERK and JNK protein were unchanged. Moreover, NT treatment resulted in increased AP-1 binding activity as determined by gel shift analysis. Delineating the signal transduction mechanisms regulating the cellular effects of NT will provide important insights into the molecular pathways responsible for NT-mediated effects on both normal and neoplastic cells.

Reconstitution of angiotensin receptor mRNA down-regulation in vascular smooth muscle. Post-transcriptional control by protein kinase a but not mitogenic signaling directed by the 5'-untranslated region

Cell surface receptor activation generally leads to changes in mRNA abundance, which may involve regulatory targets in processes working at the post-transcriptional level. Many types of agonists down-regulate vascular smooth muscle angiotensin receptor (AT(1)-R) gene expression, but it is unclear which of these activate post-transcriptional mechanisms. To reconstitute faithfully the normal AT(1)-R mRNA regulatory environment, tetracycline-suppressible promoters drive highly accurate recombinant AT(1)-R mRNA mimics in vascular smooth muscle cells that co-express an endogenous AT(1)-R mRNA. Down-regulation of the latter occurs shortly after stimulating mitogenic receptors or by using forskolin, but only cAMP signaling reduces expression of the recombinant AT(1)-R mRNA. Transcription of the recombinant mRNA is unaffected by cAMP signaling. Deletions of the AT(1)-R mRNA 3'-untranslated region do not impair cAMP-mediated down-regulation. Both loss of function and gain of function mutants show the response is mediated by the 5'-untranslated region. These observations provide the first direct functional evidence for modulation of vascular AT(1)-R gene expression by a mechanism involving a protein kinase A-regulated post-transcriptional process.

A population of in vivo anergized T cells with a lower activation threshold for the induction of CD25 exhibit differential requirements in mobilization of intracellular calcium and mitogen-activated protein kinase activation

Chronic exposure of mature T cells with specificity for self-Ags can lead to the induction of a nonfunctional state which is referred to as T cell anergy. It is unclear whether anergic T cells are destined for cell death and thereby harmless or whether they can contribute to the induction of autoimmunity and/or regulation of anti-self reactivity. We have begun to address this issue. In a recent study, we showed that a population of mature CD4-CD8- T cells that express a transgenic TCR specific for the Ld MHC class I molecule are rendered anergic in Ld-expressing mice. In this study, we show that this population of anergic T cells possess a lower activation threshold for the induction of CD25 and CD69 in response to stimulation by antigenic ligands. Furthermore, these anergic T cells undergo extensive proliferation when stimulated with a low-affinity ligand in the presence of an exogenous source of IL-2. Biochemical analysis of the early intracellular signaling events of these in vivo anergized T cells showed that they have a signaling defect at the level of ZAP-70 and linker for the activation of T cell (LAT) phosphorylation. They also exhibit a defect in mobilization of intracellular calcium in response to TCR signaling. However, these anergic T cells demonstrate no defect in SLP-76 phosphorylation and extracellular signal-regulated kinase 1/2 activation. These biochemical characteristics of the anergic T cells were associated with an elevated level of Fyn, but not Lck expression. The potential contributions of these anergic T cells in the induction and/or regulation of autoimmune responses are discussed.

Single amino acid (arginine) deprivation induces G1 arrest associated with inhibition of cdk4 expression in cultured human diploid fibroblasts

Withdrawal of a single amino acid (arginine) from freely cycling early passage primary human fibroblasts caused a halt to proliferation, characterized by an accumulation of cells in the G1 phase of the cell cycle. This arrest was accompanied by the suppression of cyclin D1- and cyclin E-associated kinase activities and the appearance of hypophosphorylated retinoblastoma protein. Arginine-deprived cells remained viable for in excess of 4 days and could be made to synchronously reenter the cell cycle by restoration of the amino acid, with kinetics characteristic of exit from a quiescent state. Stimulation of cells arrested by serum withdrawal did not result in S-phase entry when arginine was omitted from the culture medium. Although cyclin D1 accumulated on normal schedule, cdk4, which increased following restimulation in amino acid-replete medium, was not induced when arginine was absent. These results suggest that arginine deprivation-in common with other "suboptimal" conditions-inhibits the passage of normal human cells through the restriction point and implicate cdk4 as the key regulatory element in amino acid-sensitive cell cycle control.

p38 mitogen-activated protein kinase and c-jun-NH2-terminal kinase regulate RANTES production by influenza virus-infected human bronchial epithelial cells

Airway epithelial cells which are the initial site of influenza virus (IV) infection are suggested to participate in airway inflammatory response by expressing various cytokines including RANTES; however, the intracellular signal that regulates RANTES expression has not been determined. In the present study, we examined the role of p38 mitogen-activated protein (MAP) kinase, extracellular signal-regulated kinase (Erk), and c-Jun-NH2-terminal kinase (JNK) in RANTES production by IV-infected human bronchial epithelial cells. The results showed that IV infection induced increases in p38 MAP kinase, and Erk and JNK phosphorylation and activity. SB 203580, PD 98059, and CEP-1347 attenuated IV-infection induced p38 MAP kinase activity, Erk activity, and JNK activity, respectively. SB 203580 and CEP-1347 attenuated RANTES production by 45.3% and 45.2%, respectively, but a combination of these inhibitors additively attenuated by 69.1%. In contrast, PD 98059 did not attenuate. Anti-IL-1alpha mAb, anti-IL-1beta mAb, anti-TNF-alpha mAb, anti-IL-8 mAb, anti-IFN-beta mAb, anti-RANTES mAb, and a combination of these mAbs did not affect IV infection-induced increases in p38 MAP kinase, Erk, and JNK phosphorylation, indicating that each cytokine neutralized by corresponding Ab was not involved in IV infection-induced phosphorylation of MAP kinases. N-acetylcysteine (NAC) did not affect IV infection-induced increases in MAP kinase phosphorylation, whereas NAC attenuated RANTES production by 18.2%, indicating that reactive oxygen species may act as a second messenger leading to RANTES production via p38 MAP kinase- and JNK-independent pathway. These results indicate that p38 MAP kinase and JNK, at least in part, regulate RANTES production by bronchial epithelial cells.

Mechanistic basis for catalytic activation of mitogen-activated protein kinase phosphatase 3 by extracellular signal-regulated kinase

The dual specificity mitogen-activated protein kinase phosphatase MKP3 has been shown to down-regulate mitogenic signaling through dephosphorylation of extracellular signal-regulated kinase (ERK). Camps et al. (Camps, M., Nichols, A., Gillieron, C., Antonsson, B., Muda, M., Chabert, C., Boschert, U., and Arkinstall, S. (1998) Science 280, 1262-1265) had demonstrated that ERK binding to the noncatalytic amino-terminal domain of MKP3 can dramatically activate the phosphatase catalytic domain. The physical basis for this activation has not been established. Here, we provide detailed biochemical evidence that ERK activates MKP3 through the stabilization of the active phosphatase conformation, inducing closure of the catalytic "general acid" loop. In the closed conformation, this loop structure can participate efficiently in general acid/base catalysis, substrate binding, and transition-state stabilization. The pH activity profiles of ERK-activated MKP3 clearly indicated the involvement of general acid catalysis, a hallmark of protein-tyrosine phosphatase catalysis. In contrast, unactivated MKP3 did not display this enzymatic group as critical for the low activity form of the enzyme. Using a combination of Brönsted analyses, pre-steady-state and steady-state kinetics, we have isolated all catalytic steps in the reaction and have quantified the specific rate enhancement. Through protonation of the leaving group and transition-state stabilization, activated MKP3 catalyzes formation of the phosphoenzyme intermediate approximately 100-fold faster than unactivated enzyme. In addition, ERK-activated MKP3 catalyzes intermediate hydrolysis 5-6-fold more efficiently and binds ligands up to 19-fold more tightly. Consistent with ERK stabilizing the active conformation of MKP3, the chemical chaperone dimethyl sulfoxide was able to mimic this activation. A general protein-tyrosine phosphatase regulatory mechanism involving the flexible general acid loop is discussed.

Hyperoxia in cell culture. A non-apoptotic programmed cell death

Here we discuss the morphological features and our current understanding of the pathways involved in non-apoptotic cell death from O2 toxicity. Preliminary data on hyperoxic signaling indicate that NF-kappa B translocation (and presumptive activation) is not a result of the p42/p44 MAPK pathway, but a likely downstream consequence of activation of the JNK pathway. Our observations suggest the existence of multiple signal transduction pathways in hyperoxia-induced cell death: one involved in the stress response which appears to be NF-kappa B-dependent and another in cell death.

Rapid activation of NF-kappaB and AP-1 and target gene expression in postischemic rat intestine

BACKGROUND & AIMS

The molecular mechanisms underlying intestinal mucosal damage-repair processes induced by ischemia-reperfusion (IR) remain unknown. We determined nuclear factor-kappaB (NF-kappaB) and activator protein 1 (AP-1) activities and the expression of potential target genes relevant to damage-repair events.

METHODS

Rat jejunal segment was subjected to ischemia for 30 minutes followed by reperfusion for defined times. NF-kappaB and AP-1 activities; mucosal p105, p50, and inhibitor kappaB-alpha (IkappaB-alpha) levels; and c-fos, neurotensin, and ferritin H expression were determined by electrophoretic mobility shift assay and Western and Northern analyses, respectively.

RESULTS

NF-kappaB and AP-1 activities were significantly elevated from 1 to 12 hours after reperfusion. The activated NF-kappaB in the nuclear extract consisted of solely p50 homodimers. Activation of p50 was associated with a decrease of p105, generation of p50, and increased phosphorylation and degradation of IkappaB-alpha. The activated AP-1 contained c-fos but not c-jun, fosB, and Fra-1. Reperfusion induced a transient elevation of c-fos, prolonged increase of neurotensin, and early reduction followed by recovery of ferritin H messenger RNA.

CONCLUSIONS

The intestine shows organ-specific responses to IR, characterized by prolonged NF-kappaB and AP-1 activation involving NF-kappaB p50 dimers and excluding AP-1 c-jun protein. Degradation of the IkappaB-gamma component of p105 and partial reduction IkappaB-alpha selectively activate p50/p50 dimers. Temporal patterns of target gene expression reflect functional relevance to mucosal damage-repair processes after IR.

Induction of terminal differentiation by constitutive activation of p38 MAP kinase in human rhabdomyosarcoma cells

MyoD inhibits cell proliferation and promotes muscle differentiation. A paradoxical feature of rhabdomyosarcoma (RMS), a tumor arising from muscle precursors, is the block of the differentiation program and the deregulated proliferation despite MyoD expression. A deficiency in RMS of a factor required for MyoD activity has been implicated by previous studies. We report here that p38 MAP kinase (MAPK) activation, which is essential for muscle differentiation, is deficient in RMS cells. Enforced induction of p38 MAPK by an activated MAPK kinase 6 (MKK6EE) restored MyoD function and enhanced MEF2 activity in RMS deficient for p38 MAPK activation, leading to growth arrest and terminal differentiation. Stress and cytokines could activate the p38 MAPK in RMS cells, however, these stimuli did not promote differentiation, possibly because they activated p38 MAPK only transiently and they also activated JNK, which could antagonize differentiation. Thus, the selective and sustained p38 MAPK activation, which is distinct from the stress-activated response, is required for differentiation and can be disrupted in human tumors.

Adaptive responses of the endothelium to stress

It is now established that endothelial cells acquire several functional properties in response to a diverse array of extracellular stimuli. This expression of an altered phenotype is referred to as endothelial cell activation, and it includes several activities that promote inflammation and coagulation. While it is recognized that endothelial cell activation has a principal role in host defense, recent studies also demonstrate that endothelial cells are capable of complex molecular responses that protect the endothelium against various forms of stress including heat shock, hypoxia, oxidative stress, shock, ischemia-reperfusion injury, toxins, wounds, and mechanical stress. In this review, we examine endothelial cell genotypic and phenotypic responses to stress. Also, we highlight important cellular stress responses that, although not yet demonstrated directly in endothelial cells, likely exist as part of the repertoire of stress responses in endothelium. A detailed understanding of the molecular mechanisms mediating the adaptive responses of endothelial cells to stress should facilitate the development of novel therapeutics to aid in the management of diverse surgical diseases and their complications.

Heat-stress induced modulation of protein phosphorylation in virulent promastigotes of Leishmania donovani

In parasites such as Leishmania, the study of molecular events induced in response to heat stress is of immense interest since temperature increase is an integral part of the life cycle. Protein phosphorylation is known to control major steps of proliferation and differentiation in eukaryotic cells. Studies on intracellular signaling systems in protozoa are relatively recent. We have examined the effect of heat shock on the protein phosphorylation status in promastigotes of Leishmania donovani. The patterns of total protein phosphorylation and specific phosphorylation at tyrosine residues were examined using [32P]-orthophosphate labelling of the parasites and immunoblotting with a monoclonal anti-phosphotyrosine antibody. The major proteins of L. donovani that were phosphorylated at 24 degrees C had apparent molecular weights of 110, 105, 66-68, 55, 36-40 and 20 kDa. Heat shock (from 24 to 37 degrees C) led to a significant decrease in phosphorylation of the majority of phosphoproteins in the virulent promastigotes. On the other hand, the avirulent promastigotes did not show any decrease in protein phosphorylation on exposure to heat stress. Predominant phosphorylation at tyrosine residues was detectable in proteins of putative size 105-110 kDa in both virulent and avirulent parasites. Heat shock led to a reduction in the level of phosphotyrosine in both these proteins in the case of virulent parasites, while no such reduction was detectable in avirulent parasites. Significant modifications in the phosphorylation status of proteins in response to heat stress including that of tyrosine containing proteins, observed exclusively in virulent parasites, suggest that modulation of protein phosphorylation/dephosphorylation may play a role in signal transduction pathways in the parasite upon heat shock encountered on entering the mammalian host.

Role of Ras and Mapks in TGFbeta signaling

Normal signaling by TGFbeta, in the absence of serum or exogenous factors, involves a rapid activation of Ras, Erks, and Sapks in proliferating cultures of TGFbeta-sensitive untransformed epithelial cells and human carcinoma cells. Expression of either RasN17 or dominant-negative (DN) MKK4, or addition of the MEK1 inhibitor PD98059, can block the ability of TGFbeta to induce AP-1 complex formation at the TGFbeta(1) promoter and to autoinduce its own production. The primary components present in this TGFbeta-stimulated AP-1 complex are JunD and Fra-2, although c-Jun, and possibly Fos B, may also be present. While there are two potential Smad binding elements (SBE's) in the TGFbeta(1) promoter, supershift assays suggest that at least one of these does not bind Smad4, and the other is unable to bind factors activated by TGFbeta. In contrast, TGFbeta autoinduction is Smad3-dependent, as DN Smad3 inhibits the ability of TGFbeta to stimulate TGFbeta(1) promoter activity. Our results indicate that TGFbeta can activate both the MKK4/Sapk and MEK/Erk pathways, through Ras and TGFbeta R(I) and R(II), to induce TGFbeta(1) production; Smad4 does not appear to be involved, and Smad3 appears to function independently of this Smad4. We also demonstrate that activation of the Ras/Mapk pathway by TGFbeta positively modulates Smad1-signaling-pathway activation by TGFbeta. In addition, Smad1 could enhance TGFbeta activation of the SBE reporter SBE-luc and this effect could be blocked by co-expression of a DN TGFbeta R(I) receptor or by the MEK1 inhibitor PD98059. This cross-talk between the MEK/Erk and Smad1 pathways was mediated through the four Erk consensus phosphorylation sites in the linker region of Smad1. Mutation of these sites resulted in a loss of the ligand-dependence of both Smad1-Smad4 interactions and nuclear accumulation of Smad1, as well as a loss of the ability of Smad1 to enhance TGFbeta-mediated SBE activation. Our results provide evidence that Erk-mediated phosphorylation of Smad1 in response to TGFbeta is critical for regulating Smad1 subcellular localization; this may be a key determinant in maintaining TGFbeta-dependent transcriptional activation.

Diphosphorylation and involvement of extracellular signal-regulated kinases (ERK1/2) in glutamate-induced apoptotic-like death in cultured rat cortical neurons

Glutamate-induced excitotoxicity, with certain characteristics of apoptosis, has been implicated in a variety of neuronal degenerative disorders. In some physiological cases, extracellular signal-regulated kinases (ERK1/2) are activated by stimulation of glutamate receptors. In the present study, the activation (diphosphorylation) and role of ERK1/2 in glutamate-induced apoptotic-like death in cultured cortical neurons were investigated. Protein levels and activation (diphosphorylation) levels of ERK1/2 were examined by Western immunoblot, probed with anti-ERK1/2 and anti-active (diphosphorylated) ERK1/2 antibodies, respectively. Apoptotic-like death was determined by DAPI staining. Before a remarkable increase of apoptotic-like cell death was observed at 9-18 h after 15 min exposure to 50 microM glutamate, diphosphorylation levels of ERK1/2 were rapidly increased, peaked at 5-15 min of the exposure, and reverted to sham control level 3 h after the exposure, while the protein levels of ERK1/2 were unaffected. The glutamate concentration effective for inducing apoptotic-like cell death was correlated with that for inducing ERK1/2 diphosphorylation. Both ERK1/2 diphosphorylation and the apoptotic-like cell death were largely prevented by MK-801, a specific NMDA receptor (a subtype receptor of glutamate) antagonist, or the elimination of extracellular Ca(2+) with EGTA. PD98059, a specific inhibitor of ERK1/2 kinase, completely inhibited ERK1/2 diphosphorylation and partially inhibited the apoptotic-like cell death. These results suggest that largely via NMDA receptor-mediated influx of extracellular Ca(2+), ERK1/2 were rapidly and transiently activated and were involved in glutamate-induced apoptotic-like death in cultured rat cortical neurons.

Tyrosine kinase inhibitors in preclinical development

Due to the limited efficacy of cytotoxic chemotherapy in the treatment of advanced malignancy and its excessive toxicity precluding its use in chemoprevention, new therapeutic and preventive strategies have been sought. One of the most interesting of these new approaches is the manipulation of signal transduction pathways. Among the approaches being considered to eventuate such a strategy is the inhibition of autophosphorylation, a critical first step in the signal transduction pathways of many cell surface receptor tyrosine kinases, as well as of non-receptor tyrosine kinases. This article is intended to review those tyrosine kinase inhibitors that are currently in preclinical development, for which there are data to support consideration for their use in chemoprevention or cancer treatment. We will focus upon those agents that have received attention in the past several years.

The tetraspanin CD9 associates with transmembrane TGF-alpha and regulates TGF-alpha-induced EGF receptor activation and cell proliferation

Transforming growth factor-alpha (TGF-alpha) is a member of the EGF growth factor family. Both transmembrane TGF-alpha and the proteolytically released soluble TGF-alpha can bind to the EGF/TGF-alpha tyrosine kinase receptor (EGFR) and activate the EGFR-induced signaling pathways. We now demonstrate that transmembrane TGF-alpha physically interacts with CD9, a protein with four membrane spanning domains that is frequently coexpressed with TGF-alpha in carcinomas. This interaction was mediated through the extracellular domain of transmembrane TGF-alpha. CD9 expression strongly decreased the growth factor- and PMA- induced proteolytic conversions of transmembrane to soluble TGF-alpha and strongly enhanced the TGF- alpha-induced EGFR activation, presumably in conjunction with increased expression of transmembrane TGF-alpha. In juxtacrine assays, the CD9-induced EGFR hyperactivation by transmembrane TGF-alpha resulted in increased proliferation. In contrast, CD9 coexpression with transmembrane TGF-alpha decreased the autocrine growth stimulatory effect of TGF-alpha in epithelial cells. This decrease was associated with increased expression of the cdk inhibitor, p21(CIP1). These data reveal that the association of CD9 with transmembrane TGF-alpha regulates ligand-induced activation of the EGFR, and results in altered cell proliferation.

Prolactin-releasing peptide activation of the prolactin promoter is differentially mediated by extracellular signal-regulated protein kinase and c-Jun N-terminal protein kinase

Regulation of the mitogen-activated protein kinase (MAPK) family by prolactin-releasing peptide (PrRP) in both GH3 rat pituitary tumor cells and primary cultures of rat anterior pituitary cells was investigated. PrRP rapidly and transiently activated extracellular signal-regulated protein kinase (ERK) in both types of cells. Both pertussis toxin, which inactivates G(i)/G(o) proteins, and exogenous expression of a peptide derived from the carboxyl terminus of the beta-adrenergic receptor kinase I, which specifically blocks signaling mediated by the betagamma subunits of G proteins, completely blocked the PrRP-induced ERK activation, suggesting the involvement of G(i)/G(o) proteins in the PrRP-induced ERK activation. Down-regulation of cellular protein kinase C did not significantly inhibit the PrRP-induced ERK activation, suggesting that a protein kinase C-independent pathway is mainly involved. PrRP-induced ERK activation was not dependent on either extracellular Ca(2+) or intracellular Ca(2+). However, the ERK cascade was not the only route by which PrRP communicated with the nucleus. JNK was also shown to be significantly activated in response to PrRP. JNK activation in response to PrRP was slower than ERK activation. Moreover, to determine whether a MAPK family cascade regulates rat prolactin (rPRL) promoter activity, we transfected the intact rPRL promoter ligated to the firefly luciferase reporter gene into GH3 cells. PrRP activated the rPRL promoter activity in a time-dependent manner. Co-transfection with a catalytically inactive form of a MAPK construct or a dominant negative JNK, partially but significantly inhibited the induction of the rPRL promoter by PrRP. Furthermore, co-transfection with a dominant negative Ets completely abolished the response of the rPRL promoter to PrRP. These results suggest that PrRP differentially activates ERK and JNK, and both cascades are necessary to elicit rPRL promoter activity in an Ets-dependent mechanism.

The Pit-1 homeodomain and beta-domain interact with Ets-1 and modulate synergistic activation of the rat prolactin promoter

Pit-1/GHF-1 is a pituitary-specific, POU homeodomain transcription factor required for development of somatotroph, lactotroph, and thyrotroph cell lineages and regulation of the temporal and spatial expression of the growth hormone, prolactin (PRL), and thyrotropin-beta genes. Synergistic interaction of Pit-1 with a member of the Ets family of transcription factors, Ets-1, has been shown to be an important mechanism regulating basal and Ras-induced lactotroph-specific rat (r) PRL promoter activity. Pit-1beta/GHF-2, an alternatively spliced isoform containing a 26-amino acid insert (beta-domain) within its transcription-activation domain, physically interacts with Ets-1 but fails to synergize. By using a series of Pit-1 internal-deletion constructs in a transient transfection protocol to reconstitute rPRL promoter activity in HeLa cells, we have determined that the functional and physical interaction of Pit-1 and Ets-1 is mediated via the POU homeodomain, which is common to both Pit-1 and Pit-1beta. Although the Pit-1 homeodomain is both necessary and sufficient for direct binding to Ets-1 in a DNA-independent manner, an additional interaction surface was mapped to the beta-domain, specific to the Pit-1beta isoform. Thus, the unique transcriptional properties of Pit-1 and Pit-1beta on the rPRL promoter may be due to the formation of functionally distinct complexes of these two Pit-1 isoforms with Ets-1.

Hepatocyte growth factor/scatter factor-induced intracellular signalling

Hepatocyte growth factor (HGF) identical to scatter factor (SF) is a glycoprotein involved in the development of a number of cellular phenotypes, including proliferation, mitogenesis, formation of branching tubules and, in the case of tumour cells, invasion and metastasis. This fascinating cytokine transduces its activities via its receptor encoded by the c-met oncogene, coupled to a number of transducers integrating the HGF/SF signal to the cytosol and the nucleus. The downstream transducers coupled to HGF/MET, most of which participate in overlapping pathways, determine the development of the cell's phenotype, which in most cell types is dual.

Differential activation of mitogen-activated protein kinases in experimental mesangioproliferative glomerulonephritis

Multiple extracellular mitogens are involved in the pathogenesis of proliferative forms of glomerulonephritis (GN). In vitro studies demonstrate the pivotal role of mitogen-activated protein (MAP) kinases in the regulation of cellular proliferation. This study was conducted to examine whether these kinases, as a convergence point of mitogenic stimuli, are activated in mesangioproliferative GN in vivo. Therefore, anti-Thy1 GN was induced in rats using a monoclonal anti-Thy1.1 antibody (OX-7). Whole cortical tissue as well as isolated glomeruli were examined at different time points using kinase activity assays and Western blot analysis. A maximal increase in the number of glomerular mitotic figures (9.7-fold) was demonstrated 6 d after injection of the anti-Thy1.1 antibody. In parallel with this finding, a significant increase in cortical, and more dramatically glomerular, activity of extracellular signal-regulated kinase (ERK) was detected. Maximal activation of ERK was detectable on day 6. This activation of ERK was accompanied by an increase in the expression of MEK (MAP kinase/ERK kinase), the ERK-activating kinase. A marked induction of glomerular apoptosis at 2 h after injection of the anti-Thy1.1 antibody, which subsided subsequently, was demonstrated using the terminal deoxynucleotidyl transferase-mediated dUTP-biotin nick end labeling assay as well as staining for single-stranded DNA. However, no significant activation of stress-activated protein kinase or p38 MAP kinase, both MAP kinases that are suggested to induce apoptosis and to inhibit cellular growth, was detectable at this early time point. Rather, on day 6 a dramatic decrease in the activity of p38 MAP kinase, which might have contributed to the overshooting glomerular cellular proliferation, was observed. Treatment of rats with heparin blunted glomerular proliferation as well as ERK activation and restored p38 MAP kinase activity. These observations point to ERK and p38 MAP kinase as putative mediators of the proliferative response in mesangioproliferative GN and suggest that upregulation of MEK is involved in the long-term regulation of ERK in vivo.

Molecular biology of pancreatic cancer; oncogenes, tumour suppressor genes, growth factors, and their receptors from a clinical perspective

Pancreatic cancer represents the fourth leading cause of cancer death in men and the fifth in women. Prognosis remains dismal, mainly because the diagnosis is made late in the clinical course of the disease. The need to improve the diagnosis, detection, and treatment of pancreatic cancer is great. It is in this type of cancer, in which the mortality is so great and the clinical detection so difficult that the recent advances of molecular biology may have a significant impact. Genetic alterations can be detected at different levels. These alterations include oncogene mutations (most commonly, K-ras mutations, which occur in 75% to more than 95% of pancreatic cancer tissues), tumour suppressor genes alterations (mainly, p53, p16, DCC, etc.), overexpression of growth factors (such as EGF, TGF alpha, TGF beta 1-3, aFGF, bTGF, etc.) and their receptors (i.e., EGF receptor, TGF beta receptor I-III, etc.). Insights into the molecular genetics of pancreatic carcinogenesis are beginning to form a genetic model for pancreatic cancer and its precursors. These improvements in our understanding of the molecular biology of pancreatic cancer are not simply of research interest, but may have clinical implications, such as risk assessment, early diagnosis, treatment, and prognosis evaluation.

Effect of PRL on MAPK activation: negative regulatory role of the C-terminal part of the PRL receptor

Prolactin induces cell proliferation and cell differentiation through well-known MAPK Erk, and JAK2/STAT5 pathways depending on the cell line. The aim of the present study was to delineate the functional domains of the PRL receptor involved in PRL induced MAPK regulation. Using various PRL-R mutants of the cytoplasmic domain we found, that the membrane proximal domain is necessary for PRL induced MAPK activation and that the C-terminal part of the receptor exerts a negative regulatory role. A pharmacological approach, using different types of inhibitors, provided evidence that PRL induced MAPK activation requires both a MEK dependent pathway and a PI3K dependent pathway. The negative regulation induced by the carboxy-terminal part of the receptor involves a combination of tyrosine phosphatases and serine/threonine phosphatases as concluded from the actions of the phosphatase inhibitors: pervanadate, PAO and okadaic acid. The mechanism by which these phosphatases are recruited or are induced by the last 141 cytoplasmic residues of the receptor remains to be determined. Finally the negative regulatory role of the carboxy-terminal part of the receptor, first demonstrated in the present study, is discussed in terms of the regulation of different effects of PRL on growth and differentiation.

Janus kinase 2-dependent activation of p38 mitogen-activated protein kinase by growth hormone. Resultant transcriptional activation of ATF-2 and CHOP, cytoskeletal re-organization and mitogenesis

We demonstrate here that p38 mitogen-activated protein (MAP) kinase is activated in response to cellular stimulation by human GH (hGH) in Chinese hamster ovary cells stably transfected with GH receptor cDNA. This activation requires the proline-rich box 1 region of the GH receptor required for JAK2 association and is prevented by pretreatment of cells with the JAK2-specific inhibitor AG490. ATF-2 is both phosphorylated and transcriptionally activated by hGH, and its transcriptional activation also requires the proline-rich box 1 region of the GH receptor. Expression of wild type JAK2 can further enhance hGH-induced ATF-2-, CHOP-, and Elk-1-mediated transcriptional activation, whereas pretreatment with AG490 is inhibitory. Use of either specific pharmacological inhibitors or transient transfection of cells with p38alpha MAP kinase cDNA or a dominant negative variant demonstrated that hGH-stimulated transcriptional activation of ATF-2 and CHOP, but not Elk-1, is regulated by p38 MAP kinase. Both the p38 MAP kinase and p44/42 MAP kinase are critical for hGH-stimulated mitogenesis, whereas only p38 MAP kinase is required for hGH-induced actin cytoskeletal re-organization. p38 MAP kinase is therefore an important regulator in coordinating the pleiotropic effects of GH.

S phase-specific DNA-binding proteins interacting with the Hex and Oct motifs in type I element of the wheat histone H3 promoter

The type I element (CCACGTCANCGATCCGCG), consisting of the Hex motif (CCACGTCA) and the reverse-oriented Oct motif (GATCCGCG), is necessary and sufficient to confer the S phase-specific transcription of the wheat histone H3 (TH012) gene. The transcriptional regulation via the type I element is thought to occur through interactions between transcription factors which bind specifically to the Hex and Oct motifs. Here we report S phase-specific DNA-binding proteins interacting with the type I element in partially synchronized wheat cultured cells. Hex motif-binding proteins found here resembled HBP-1a, as reported previously in terms of DNA-binding specificity. DNA-binding activities of the HBP-1a-like proteins were modulated by phosphorylation/dephosphorylation. In the electrophoretic mobility shift assay of the wheat nuclear extract, we also found three Oct motif-specific binding proteins, named OBRF (octamer-binding regulatory factor)-1, -2 and -3. One of the HBP-1a-like proteins and OBRF-1 appeared predominantly at the S phase. Thus, it was supposed that these two factors play a crucial role in the S phase-specific regulation of wheat histone gene expression.

Relationship between Ras pathways and cell cycle control

The ordered execution of the two main events of cellular reproduction, duplication of the genome and cell division, characterize progression through the cell cycle. Cultured cells can be switched between cycling and non-cycling states by alteration of extracellular conditions and the notion that a critical cellular control mechanism presides on this decision, whose temporal location is known as the restriction point, has become the focus for the study of how extracellular mitogenic signalling impinges upon the cell cycle to influence proliferation. This review attempts to cover the disparate pathways of Ras-mediated mitogenic signal transduction that impact upon restriction point control.

Glutamine metabolism stimulates intestinal cell MAPKs by a cAMP-inhibitable, Raf-independent mechanism

BACKGROUND & AIMS

Infectious diarrhea caused by viruses plus enterotoxigenic bacteria is often more severe than diarrhea induced by either pathogen alone. We postulated that the increased cell adenosine 3',5'-cyclic monophosphate (cAMP) concentration observed during infection by enterotoxigenic organisms retards the intestinal repair process by blocking activation of mitogen-activated protein kinases (MAPKs) in proliferating intestinal cells.

METHODS

We evaluated the effects of glutamine on MAPK activity, thymidine incorporation, and cell number in glutamine-starved and -sufficient rat intestinal crypt cells (IEC-6).

RESULTS

In glutamine-starved cells, 10 mmol/L glutamine in the absence of serum stimulated [(3)H]thymidine incorporation 8-fold. This effect was inhibited by 60% with 8-(4-chlorophenylthio) (8-CPT)-cAMP (100 micromol/L) + isobutyl methylxanthine (100 micromol/L). In cells not starved of glutamine, glutamine stimulated thymidine incorporation by 3-fold, and 8-CPT-cAMP completely blocked the mitogenic effect. Inhibition of proliferation by cAMP persisted for at least 68 hours after cAMP removal. In vitro kinase assays showed that glutamine signaling requires an intact ERK (extracellular signal-related kinase) pathway in unstarved cells. In starved cells, at least one other pathway (JNK) was activated by glutamine, and the mitogenic inhibition by 8-CPT-cAMP was incomplete. Other intestinal fuels (glucose and acetate) were not mitogenic.

CONCLUSIONS

Increased levels of intracellular cAMP inhibit ERKs but only partially reduce glutamine-stimulated proliferation in enterocytes adapted to low glutamine.