Nuclear export of NF-ATc enhanced by glycogen synthase kinase-3

The role of GSK3beta in regulating neuronal differentiation in Xenopus laevis

The serine threonine protein kinase encoded by the shaggy locus has been implicated in neurogenesis in Drosophila. In vertebrates, the shaggy homolog, GSK3beta, is involved in early pattern formation, specifically in setting up the dorsal ventral axis. In the present study we have cloned the Xenopus homolog of the shaggy kinase and show (1) that GSK3beta is expressed in the right time and place to play a role in primary neurogenesis in Xenopus; (2) that overexpression of wild-type GSK3beta leads to a decrease in the number of primary neurons; (3) that inhibition of endogenous GSK3beta activity with overexpression of a dominant negative GSK3beta construct leads to an increase in the number of primary neurons; and (4) that GSK3beta inhibits the ability of neurogenin and NeuroD to produce ectopic tubulin expression, but does not inhibit the ability of neurogenin to produce ectopic NeuroD. On the basis of these data we propose that GSK3beta inhibits the function of NeuroD and therefore prevents neuronal differentiation at a relatively late stage in the developmental pathway.

Phosphoinositide 3-kinase regulation of T cell receptor-mediated interleukin-2 gene expression in normal T cells

Phosphoinositide (PI) 3-kinase has been implicated in T cell receptor (TCR) signaling, either as a positive or a negative regulatory molecule. Here, we show that for normal mouse lymph node T cells, PI 3-kinase activity is required for interleukin-2 (IL-2) production following TCR-mediated activation. Furthermore, in normal T cells, inhibition of PI 3-kinase prevented activation of enzymes in the extracellular signal-regulated protein kinase (ERK) signaling pathway (MEK-1 and ERK-2). Overexpression of a dominant-negative mutant of PI 3-kinase and pharmacological inhibitors of PI 3-kinase prevented transcriptional activation of AP-1 and NF-AT, transcription factors regulated by ERK-2 and pivotal for IL-2 gene expression. Although a constitutively active form of Akt kinase, a downstream mediator of PI 3-kinase function, enhanced TCR-induced IL-2 gene transcription, it could not bypass the requirement for PI 3-kinase activity. Therefore, PI 3-kinase is likely to be involved in signaling for IL-2 production in at least two steps in the TCR-initiated signaling pathway.

Survival factors and apoptosis

This chapter will explore the role of survival factors in suppression of apoptosis, and illustrate how survival signals play a critical role in the survival of both normal and tumor cells. Survival factors necessary for the development and maintenance of the nervous system and hemopoietic system will be surveyed. This will be followed by a detailed discussion of the role of insulin-like growth factor I (IGF-I) and its receptor in suppression of apoptosis. The importance of survival signals from the IGF-IR for development and tumorigenesis will be discussed, and results of a mutational analysis of the receptor to assign domains necessary for suppression of apoptosis will be summarized. Finally, a discussion of the signal transduction pathways involved in survival factor-signaling will review the roles played by PI-3 kinase and AKT and speculate on how activation of these kinases by survival factors might regulate the apoptotic pathway.

Signalling into the T-cell nucleus: NFAT regulation

The nuclear factor of activated T cells (NFAT) plays an important role in T-cell biology. Activation of T cells results in the rapid calcineurin-dependent translocation of NFAT transcription factors from the cytoplasm to the nucleus. This translocation process coupled to the subsequent active maintenance of NFAT in the nucleus compartment is critical for the induction of expression of several genes encoding cytokines and membrane proteins that modulate immune responses. The molecular cloning of the NFAT family of transcription factors has facilitated rapid progress in the understanding of the signalling mechanisms that control the activity of NFAT.

The transcription factor NFAT4 is involved in the generation and survival of T cells

Nuclear factor of activated T cells (NFAT) is a family of four related transcription factors implicated in cytokine and early response gene expression in activated lymphocytes. Here we report that NFAT4, in contrast to NFATp and NFATc, is preferentially expressed in DP thymocytes. Mice lacking NFAT4 have impaired development of CD4 and CD8 SP thymocytes and peripheral T cells as well as hyperactivation of peripheral T cells. The thymic defect is characterized by increased apoptosis of DP thymocytes. The increased apoptosis and hyperactivation may reflect heightened sensitivity to TcR-mediated signaling. Further, mice lacking NFAT4 have impaired production of Bcl-2 mRNA and protein. NFAT4 thus plays an important role in the successful generation and survival of T cells.

Reconstitution of HIV-1 rev nuclear export: independent requirements for nuclear import and export

The Rev protein of HIV-1 actively shuttles between nucleus and cytoplasm and mediates the export of unspliced retroviral RNAs. The localization of shuttling proteins such as Rev is controlled by the relative rates of nuclear import and export. To study nuclear export in isolation, we generated cell lines expressing a green fluorescent protein-labeled chimeric protein consisting of HIV-1 Rev and a hormone-inducible nuclear localization sequence. Steroid removal switches off import thus allowing direct visualization of the Rev export pathway in living cells. After digitonin permeabilization of these cells, we found that a functional nuclear export sequence (NES), ATP, and fractionated cytosol were sufficient for nuclear export in vitro. Nuclear pore-specific lectins and leptomycin B were potent export inhibitors. Nuclear export was not inhibited by antagonists of calcium metabolism that block nuclear import. These data further suggest that nuclear pores do not functionally close when luminal calcium stores are depleted. The distinct requirements for nuclear import and export argue that these competing processes may be regulated independently. This system should have wide applicability for the analysis of nuclear import and export.

Rac-1 regulates nuclear factor of activated T cells (NFAT) C1 nuclear translocation in response to Fcepsilon receptor type 1 stimulation of mast cells

Transcription factors of the nuclear factor of activated T cells (NFAT) family play a key role in antigen receptor-mediated responses in lymphocytes by controlling induction of a wide variety of cytokine genes. The GTPases Ras and Rac-1 have essential functions in regulation of NFAT transcriptional activity in the mast cell system, where Fcepsilon receptor type 1 (FcepsilonR1) ligation results in induction of multiple NFAT target genes. This report examines the precise biochemical basis for the Rac-1 dependency of FcepsilonR1 activation of NFAT in mast cells. We are able to place Rac-1 in two positions in the signaling network that regulates the assembly and activation of NFAT transcriptional complexes in lymphocytes. First, we show that activity of Rac-1 is required for FcepsilonR1-mediated NFATC1 dephosphorylation and nuclear import. Regulation of NFAT localization by the FcepsilonR1 is a Rac-dependent but Ras-independent process. This novel signaling role for Rac-1 is distinct from its established regulation of the actin cytoskeleton. Our data also reveal a second GTPase signaling pathway regulating NFAT transcriptional activity, in which Rac-1 mediates a Ras signal. These data illustrate that the GTPase Rac-1 should now be considered as a component of the therapeutically important pathways controlling NFATC1 subcellular localization. They also reveal that GTPases may serve multiple functions in cellular responses to antigen receptor ligation.

Calcineurin and the biological effect of cyclosporine and tacrolimus

The mechanism of the immunosuppressive effect of CyA and FK 506 can be monitored in vivo in humans. The picture emerging is of a close relationship between drug concentrations and CN inhibition. But many puzzles of the drugs remain. What is the role of CyA in the activation of transforming growth factor-beta (TGF-beta), particularly in relationship to nephrotoxicity and fibrogenesis? How important are the anti-inflammatory (non T) effects of CyA, and which cells do they operate in? Are there effects of CyA and FK 506 all attributable to CN inhibition, and how much of them are mediated through the NFATC family of transcription factors? Finally, it would be useful to know what the inhibitory effects of CyA are on tolerance and negative regulatory events.

The cyclosporin A-sensitive nuclear factor of activated T cells (NFAT) proteins are expressed in vascular smooth muscle cells. Differential localization of NFAT isoforms and induction of NFAT-mediated transcription by phospholipase C-coupled cell surface receptors

Expression of the antigen-regulated, cyclosporin A-sensitive nuclear factor of activated T cells (NFAT) is not restricted to lymphoid cells, as thought initially, but the physiological inducers of NFAT-mediated transcription in non-lymphoid cells are unknown. Here, cultured vascular smooth muscle cells (VSMC) are shown to express two isoforms of the NFAT family endogenously, which are localized differentially in cells under resting conditions. Using a retroviral NFAT-specific luciferase reporter, we show that VSMC support previously unrecognized complexities in NFAT-mediated transcription, including evidence for negative regulation by Ca2+ signaling and positive regulation through co-activation of adenylyl cyclase and Ca2+ mobilization. The VSMC mitogen platelet derived growth factor-BB (PDGF-BB) induces NFAT-mediated transcription in VSMC. Thrombin and angiotensin II, which activate Galphaq-coupled receptors, are significantly weaker inducers of NFAT-mediated luciferase expression than is PDGF-BB. However, co-stimulation studies show that Galphaq receptor agonists augment the NFAT-mediated transcriptional response to PDGF-BB. This synergy can be explained in part by augmented intracellular Ca2+ transients elicited by multiple agonist challenges. These data indicate that agonists for phospholipase C-coupled receptors stimulate NFAT-mediated transcription in VSMC differentially, and that NFAT can function to integrate co-activating signals in the extracellular environment.

V7 (CD101) Ligation Inhibits TCR/CD3-Induced IL-2 Production by Blocking Ca2+ Flux and Nuclear Factor of Activated T Cell Nuclear Translocation

Ligation of the V7 (CD101) molecule on T cells with anti-V7 mAb blocks TCR/CD3-induced proliferation by inhibiting IL-2 transcription. To explore the basis for this observation, we analyzed the effects of V7 ligation on CD3/TCR-induced changes in intracellular free Ca2+ and Ca2+-dependent nuclear factor of activated T cells (NF-AT) translocation to the nucleus, which is required for IL-2 transcription. T cells exposed to anti-V7 mAb fluxed Ca2+ transiently, but did not flux Ca2+ in response to subsequent treatment with anti-CD3; however, they recovered the capacity to flux Ca2+ after treatment with pervanadate, indicating that tyrosine dephosphorylation of a critical V7-related substrate is required in the desensitization process. One such substrate, phospholipase C (PLC)-γ1, becomes tyrosine phosphorylated on CD3/TCR activation and mediates inositol triphosphate-dependent Ca2+ flux. Co-cross-linking of T cells with anti-CD3 and anti-V7 resulted in selective inhibition of PLC-γ1 tyrosine phosphorylation, which may explain V7-mediated blockade of anti-CD3-induced Ca2+ flux. Moreover, anti-CD3-induced binding of transcription factors to a consensus NF-AT-binding oligonucleotide, which is dependent on Ca2+, was blocked completely by treatment of the cells with anti-V7, whereas binding to a consensus-activating protein-1 oligonucleotide was unaffected. Western blot analysis of cytoplasmic and nuclear extracts confirmed that anti-V7 prevented nuclear translocation of NF-ATc induced by anti-CD3. We conclude that V7 ligation interferes with T cell activation and IL-2 secretion through a Ca2+ and tyrosine kinase-dependent pathway that inhibits PLC-γ1 phosphorylation and prevents NF-AT translocation to the nucleus.

Signals mediating nuclear targeting and their regulation: application in drug delivery

The recent progress with respect to understanding the signals mediating the transport of proteins in both directions through the NPC, and cellular proteins interacting with these signals to effect the transport process has made possible a number of advances in terms of the use of this information in a clinical setting. In particular, our knowledge of the mechanism of regulation of the process, and of how we may exploit the cellular transport machinery itself in a therapeutic situation, especially where there may be transport pathways specific to particular viruses, has advanced considerably. In this context, this review expounds current understanding of the signals conferring targeting to the nucleus, and their practical and potential use in delivering molecules of interest to the nucleus in a clinical context. It also deals with targeting signals conferring nuclear protein export/ shuttling between nuclear and cytoplasmic compartments as well as with those conferring nuclear or cytoplasmic retention, and with the specific mechanisms regulating the activity of these signals, and in particular those regulating signal-dependent nuclear protein import. Detailed understanding of the processes of signal-mediated nuclear protein import/export and its regulation enables the considered application and optimization of approaches to target molecules of interest, such as plasmid DNA or toxic molecules, efficiently to the nucleus according to need in a clinical or research context, and enhance the expression or efficiency of their action, respectively. The use of nuclear targeting signals in this context is reviewed, and future possibilities in terms of the application of our growing understanding of nuclear transport and its regulation are discussed.

Nuclear import and export of viruses and virus genomes

Many viruses replicate in the nucleus of their animal and plant host cells. Nuclear import, export, and nucleo-cytoplasmic shuttling play a central role in their replication cycle. Although the trafficking of individual virus proteins into and out of the nucleus has been well studied for some virus systems, the nuclear transport of larger entities such as viral genomes and capsids has only recently become a subject of molecular analysis. In this review, the general concepts emerging are discussed and a survey is provided of current information on both plant and animal viruses. Summarizing the main findings in this emerging field, it is evident that most viruses that enter or exit the nucleus take advantage of the cell's nuclear import and export machinery. With a few exceptions, viruses seem to cross the nuclear envelope through the nuclear pore complexes, making use of cellular nuclear import and export signals, receptors, and transport factors. In many cases, they capitalize on subtle control systems such as phosphorylation that regulate traffic of cellular components into and out of the nucleus. The large size of viral capsids and their composition (they contain large RNA and DNA molecules for which there are few precedents in normal nuclear transport) make the processes unique and complicated. Prior capsid disassembly (or deformation) is required before entry of viral genomes and accessory proteins can occur through nuclear pores. Capsids of different virus families display diverse uncoating programs which culminate in genome transfer through the nuclear pores.

GBP, an inhibitor of GSK-3, is implicated in Xenopus development and oncogenesis

Dorsal accumulation of beta-catenin in early Xenopus embryos is required for body axis formation. Recent evidence indicates that beta-catenin is dorsally stabilized by the localized inhibition of the kinase Xgsk-3, utilizing a novel Wnt ligand-independent mechanism. Using a two-hybrid screen, we identified GBP, a maternal Xgsk-3-binding protein that is homologous to a T cell protooncogene in three well-conserved domains. GBP inhibits in vivo phosphorylation by Xgsk-3, and ectopic GBP expression induces an axis by stabilizing beta-catenin within Xenopus embryos. Importantly, antisense oligonucleotide depletion of the maternal GBP mRNA demonstrates that GBP is required for the establishment of the dorsal-ventral axis in Xenopus embryos. Our results define a family of GSK-3-binding proteins with roles in development and cell proliferation.

Phosphorylation and inhibition of rat glucocorticoid receptor transcriptional activation by glycogen synthase kinase-3 (GSK-3). Species-specific differences between human and rat glucocorticoid receptor signaling as revealed through GSK-3 phosphorylation

Transcriptional activation by the glucocorticoid receptor (GR) is regulated by both glucocorticoid binding and phosphorylation. The rat GR N-terminal transcriptional regulatory domain contains four major phosphorylation sites: threonine 171 (Thr171), serine 224 (Ser224), serine 232 (Ser232), and serine 246 (Ser246). We have previously demonstrated that Ser224 and Ser232 are phosphorylated by cyclin-dependent kinases, while Ser246 is phosphorylated by the c-Jun N-terminal kinase. We report here that the remaining GR phosphorylation site, Thr171, is a target for glycogen synthase kinase-3 (GSK-3) in vitro and in cultured mammalian cells. Increasing GSK-3 activity through its overexpression in cultured cells inhibits GR transcriptional enhancement, an effect dependent upon Thr171. Correspondingly, overexpression of a constitutively active form of the GSK-3 inhibitor, protein kinase B/Akt, increases GR transcriptional enhancement. Overexpression of GSK-3 had no effect on GR-mediated transcriptional repression of AP1-dependent gene expression. Importantly, transcriptional activation by the human GR (hGR), which contains an alanine (Ala150) at the position equivalent to Thr171 in rat GR, is not affected by GSK-3 overexpression. Introduction of a threonine residue at this position (A150T) establishes GSK-3-mediated inhibition of hGR transcriptional activation. These findings demonstrate species-specific differences in GR signaling, as revealed through GSK-3 phosphorylation, which suggests that GR function in rodents may not fully recapitulate receptor action in humans and that hGR is capable of adopting the GSK-3 signaling pathway through a somatic mutation.

Intramolecular masking of nuclear import signal on NF-AT4 by casein kinase I and MEKK1

T cell activation requires the import of NF-AT transcription factors to the nucleus, a process promoted by calcineurin-dependent dephosphorylation and inhibited by poorly understood protein kinases. Here, we report the identification of two protein kinases that oppose NF-AT4 nuclear import. Casein kinase Ialpha directly binds and phosphorylates NF-AT4, resulting in the inhibiton of NF-AT4 nuclear translocation. MEKK1 indirectly suppresses NF-AT4 nuclear import by stabilizing the interaction between NF-AT4 and CKIalpha. CKIalpha thus acts to establish an intramolecular masking of the nuclear location signal on NF-AT4, while MEKK1 augments this mechanism, and may further provide a link to signal transduction pathways regulating NF-AT4.

Nucleocytoplasmic shuttling factors including Ran and CRM1 mediate nuclear export of NFAT In vitro

We have developed a permeabilized cell assay to study the nuclear export of the shuttling transcription factor NFAT, which contains a leucine-rich export signal. The assay uses HeLa cells that are stably transfected with NFAT fused to the green fluorescent protein (GFP). Nuclear export of GFP-NFAT in digitonin-permeabilized cells occurs in a temperature- and ATP-dependent manner and can be quantified by flow cytometry. In vitro NFAT export requires the GTPase Ran, which is released from cells during the digitonin permeabilization. At least one additional rate-limiting export factor is depleted from permeabilized cells by a preincubation at 30 degrees C in the absence of cytosol. This activity can be provided by cytosolic or nucleoplasmic extracts in a subsequent export step. Using this assay, we have purified a second major export activity from cytosol. We found that it corresponds to CRM1, a protein recently reported to be a receptor for certain leucine-rich export sequences. CRM1 appears to be imported into the nucleus by a Ran-dependent mechanism that is distinct from conventional signaling pathways. Considered together, our studies directly demonstrate by fractionation and reconstitution that nuclear export of NFAT is mediated by multiple nucleocytoplasmic shuttling factors, including Ran and CRM1.

Quantitative aspects of T-cell recognition: from within the antigen-presenting cell to within the T cell

T lymphocytes circulate continually throughout the peripheral lymphoid organs, where they scrutinize the surface of cells to detect the presence of nonself protein fragments. During the last years, many facets of T-cell function have been unravelled. After being bound by major histocompatibility complex (MHC) molecules, peptides derived from nonself as well as from self proteins are delivered to the cell surface. A few copies of a nonself peptide "presented" at the cell surface in the context of an MHC molecule can be detected by specific T cells, and suffice to trigger T-cell activation. This paper reviews the requirements imposed on T cells to fulfill this exquisite sensitivity.

Role of transcriptional repressor ICER in cyclic AMP-mediated attenuation of cytokine gene expression in human thymocytes

Proliferating human medullary thymocytes can exhibit characteristic T helper cell type 1 cytokine responses exemplified by the immediate early expression of interleukin-2, interferon-gamma, tumor necrosis factor-alpha, and lymphotoxin-beta. Here we report that cAMP-mediated attenuation of the transcription of T helper-1-specific cytokine genes in human medullary thymocytes correlates with the induction of the cAMP-mediated transcriptional repressor ICER (inducible cAMP early repressor). We show that ICER binds specifically to several NFAT/AP-1 (nuclear factor of activated T cells/activating protein-1) composite DNA sites essential for the activation of the interleukin (IL)-2 promoter as well as to a homologous DNA motif present in the proximal segment of the interferon-gamma promoter. In the presence of the minimal NFAT DNA-binding domain, which is sufficient for both DNA binding and AP-1 complex formation, ICER and NFAT form NFAT/ICER ternary complexes on several NFAT/AP-1 DNA composite sites previously identified as essential for the expression of the immunoregulatory cytokines such as IL-2, IL-4, granulocyte-macrophage colony-stimulating factor, and tumor necrosis factor-alpha. In extracts prepared from human medullary thymocytes treated with forskolin and ionomycin, these composite sites bind endogenously expressed ICER either singly or in complexes. Moreover, in Jurkat cells, ectopically expressed ICER represses transcription from NFAT-mediated, phorbol ester/ionophore-activated IL-2, granulocyte-macrophage colony-stimulating factor, and tumor necrosis factor-alpha promoters. We present evidence that ICER interactions with NFAT/AP-1 composite DNA sites correlate with its ability to repress transcription. These findings provide further insight into the mechanisms involved in cAMP-mediated transcriptional attenuation of cytokine expression.

The Tpl-2 protooncoprotein activates the nuclear factor of activated T cells and induces interleukin 2 expression in T cell lines

Tpl-2 expression is induced within 30-60 min after ConA stimulation of rat splenocytes, suggesting that it may contribute to the induction of IL-2 during T cell activation. Herein we show that wild-type and carboxyl-terminally truncated (activated) Tpl-2 activate the nuclear factor of activated T cells (NFAT) and induce interleukin 2 (IL-2) expression in EL4 cells. In Jurkat cells the truncated Tpl-2 activates NFAT and induces IL-2, whereas wild-type Tpl-2 activates NFAT only when cotransfected with NFAT expression constructs, suggesting that Tpl-2 may induce NFAT activation signals. Experiments in NIH 3T3 cells revealed that the NFATp isoform, but not the NFATc or NFATx isoform, undergoes nuclear translocation when coexpressed with wild-type Tpl-2 and confirmed this hypothesis. Activation of NFAT by anti-CD3 stimulation but not by phorbol 12-myristate 13-acetate and ionomycin in Jurkat cells was inhibited by the kinase-dead Tpl-2K167M, suggesting that Tpl-2 contributes to the transduction of NFAT activation signals originating in the T cell receptor. The Tpl-2-mediated induction of IL-2 was not observed in T cell lymphoma lines other than EL4 and Jurkat, as well as in normal T cells. NFAT activation by Tpl-2, however, was observed in several cell lines including some of nonhematopoietic origin. The activation of NFAT by Tpl-2 in different cell types defines a molecular mechanism that may contribute to its oncogenic potential.

Protein kinase B/Akt mediates effects of insulin on hepatic insulin-like growth factor-binding protein-1 gene expression through a conserved insulin response sequence

Insulin regulates the expression of multiple hepatic genes through a conserved insulin response sequence (IRS) (CAAAAC/TAA) by an as yet undetermined mechanism. Protein kinase B/Akt (PKB/Akt), a member of the PKA/PKC serine/threonine kinase family, functions downstream from phosphatidylinositol 3'-kinase (PI3K) in mediating effects of insulin on glucose transport and glycogen synthesis. We asked whether PKB/Akt mediates sequence-specific effects of insulin on hepatic gene expression using the model of the insulin-like growth factor binding protein-1 (IGFBP-1) promoter. Insulin lowers IGFBP-1 mRNA levels, inhibits IGFBP-1 promoter activity, and activates PKB/Akt in HepG2 hepatoma cells through a PI3K-dependent, rapamycin-insensitive mechanism. Constitutively active PI3K and PKB/Akt are each sufficient to mediate effects of insulin on the IGFBP-1 promoter in a nonadditive fashion. Dominant negative K179 PKB/Akt disrupts the ability of insulin and PI3K to activate PKB/Akt and to inhibit promoter activity. The IGFBP-1 promoter contains two IRSs each of which is sufficient to mediate sequence-specific effects of insulin, PI3K, and PKB/Akt on promoter activity. Highly related IRSs from the phosphoenolpyruvate carboxykinase and apolipoprotein CIII genes also are effective in this setting. These results indicate that PKB/Akt functions downstream from PI3K in mediating sequence-specific effects of insulin on the expression of IGFBP-1 and perhaps multiple hepatic genes through a conserved IRS.

Role of the NF-ATc transcription factor in morphogenesis of cardiac valves and septum

In lymphocytes, the expression of early immune response genes is regulated by NF-AT transcription factors which translocate to the nucleus after dephosphorylation by the Ca2+-dependent phosphatase, calcineurin. We report here that mice bearing a disruption in the NF-ATc gene fail to develop normal cardiac valves and septa and die of circulatory failure before day 14.5 of development. NF-ATc is first expressed in the heart at day 7.5, and is restricted to the endocardium, a specialized endothelium that gives rise to the valves and septum. Within the endocardium, specific inductive events appear to activate NF-ATc: it is localized to the nucleus only in endocardial cells that are adjacent to the interface with the cardiac jelly and myocardium, which are thought to give the inductive stimulus to the valve primordia. Treatment of wild-type embryos with FK506, a specific calcineurin inhibitor, prevents nuclear localization of NF-ATc. These data indicate that the Ca2+/calcineurin/NF-ATc signalling pathway is essential for normal cardiac valve and septum morphogenesis; hence, NF-ATc and its regulatory pathways are candidates for genetic defects underlying congenital human heart disease.

Nuclear localization of the C2H2 zinc finger protein Msn2p is regulated by stress and protein kinase A activity

Msn2p and the partially redundant factor Msn4p are key regulators of stress-responsive gene expression in Saccharomyces cerevisiae. They are required for the transcription of a number of genes coding for proteins with stress-protective functions. Both Msn2p and Msn4p are Cys2His2 zinc finger proteins and bind to the stress response element (STRE). In vivo footprinting studies show that the occupation of STREs is enhanced in stressed cells and dependent on the presence of Msn2p and Msn4p. Both factors accumulate in the nucleus under stress conditions, such as heat shock, osmotic stress, carbon-source starvation, and in the presence of ethanol or sorbate. Stress-induced nuclear localization was found to be rapid, reversible, and independent of protein synthesis. Nuclear localization of Msn2p and Msn4p was shown to be correlated inversely to cAMP levels and protein kinase A (PKA) activity. A region with significant homologies shared between Msn2p and Msn4p is sufficient to confer stress-regulated localization to a SV40-NLS-GFP fusion protein. Serine to alanine or aspartate substitutions in a conserved PKA consensus site abolished cAMP-driven nuclear export and cytoplasmic localization in unstressed cells. We propose stress and cAMP-regulated intracellular localization of Msn2p to be a key step in STRE-dependent transcription and in the general stress response.

The role of calcium in activity-dependent neuronal gene regulation

Synaptic transmission is a key signaling event, whereby an action potential-induced release of chemical neurotransmitters again generates a positive or negative electrical activity via opening of postsynaptic channels. Thereafter, information spreads through space, from the postsynaptic membranes to the dendrites, to the soma, to the nucleus, to the presynaptic terminals and, in some cases, back to the originally stimulated synapses. Furthermore, information is also often converted in time, either by shifting the phase of electrical activity during the integration of EPSPs and IPSPs into the generation of an action potential, or by triggering a long-lasting cascade of enzymatic or protein-protein interaction-mediated events in the cytoplasm and in the nucleus. Recent studies of the signaling from the synapse to the nucleus now allow us to consider how various patterns of synaptic activity could couple with activation of specific nuclear transcription factors and thus regulate neuronal gene expression. The critical importance of Ca(2+)-dependent signaling processes in such regulatory events will be discussed below.

Signal transduction by the Wnt family of ligands

The Wnt genes encode a large family of secreted polypeptides that mediate cell-cell communication in diverse developmental processes. The loss or inappropriate activation of Wnt expression has been shown to alter cell fate, morphogenesis and mitogenesis. Recent progress has identified Wnt receptors and components of an intracellular signalling pathway that mediate Wnt-dependent transcription. This review will highlight this 'core' Wnt signal-transduction pathway, but also aims to reveal the potential diversity of Wnt signalling targets. Particular attention will be paid to the overlap between developmental biology and oncogenesis, since recent progress shows Wnt signalling forms a paradigm for an interdisciplinary approach.

Protection of CD95-mediated apoptosis by activation of phosphatidylinositide 3-kinase and protein kinase B

Apoptosis may be triggered, in a variety of tissues, by interaction of the cell surface molecule CD95 with its specific ligand, CD95L. CD95 plays a physiological role in the regulation of the immune response; furthermore, alterations in CD95/CD95L function may contribute to the pathogenesis of a number of human diseases, including cancer, autoimmune diseases and viral infections. Many cells that express CD95, however, are not susceptible to CD95-mediated apoptosis. It is therefore important to identify the mechanisms that counteract the CD95 apoptotic process that are still poorly understood. Growth factors and lymphokines such as interleukin (IL)-4 that counteract CD95-mediated apoptosis may activate phosphatidylinositide 3-kinase (PI 3-kinase). We therefore used two different approaches to investigate the role of PI 3-kinase on CD95-mediated apoptosis. First we tested the effect of two pharmacological PI 3-kinase inhibitors, wortmannin and LY294002, on CD95 agonistic antibody-induced apoptosis in three different cell lines. Second, we co-expressed in COS7 cells CD95 with constitutively active PI 3-kinase. Results of both approaches indicate that active PI 3-kinase effectively protects against CD95-mediated apoptosis. Furthermore we extended our studies on the CD95 downstream mediator, FADD, and on the PI 3-kinase downstream mediator, the serine/threonine protein kinase PKB, using the co-expression approach in COS7 cells. We provide evidence that apoptosis induced by triggering the CD95 cell death receptor is counteracted by PI 3-kinase activation; moreover, PKB but not p70S6K represents the relevant downstream target of PI 3-kinase signaling.

LEF-1/TCF proteins mediate wnt-inducible transcription from the Xenopus nodal-related 3 promoter

The Xenopus nodal-related 3 gene (Xnr3) is expressed in the Spemann organizer of the embryo and encodes a member of the transforming growth factor beta family that mediates some activities of the organizer. Xnr3 is transcriptionally activated by wnt signaling during gastrulation in the Xenopus embryo. Here we show that a small region of the Xnr3 promoter is sufficient to confer wnt-inducible transcription. By mutational analysis of the promoter, we have identified two distinct sequence elements required for the response to wnt signals. One regulatory sequence interacts with a factor which accumulates in Xenopus gastrulae independent of wnt signaling. The other functionally important site can bind mammalian LEF-1 protein, a member of the LEF-1/TCF family of transcription factors. In addition, misexpression of LEF-1 in embryo explants induces transcription of the endogenous Xnr3 gene. Taken together, these data provide further evidence for a role of LEF-1/TCF proteins in wnt signaling and identify the Spemann organizer-specific gene Xnr3 as a direct target of these transcription factors in vertebrates.

Nuclear accumulation of NFAT4 opposed by the JNK signal transduction pathway

The nuclear factor of activated T cells (NFAT) group of transcription factors is retained in the cytoplasm of quiescent cells. NFAT activation is mediated in part by induced nuclear import. This process requires calcium-dependent dephosphorylation of NFAT caused by the phosphatase calcineurin. The c-Jun amino-terminal kinase (JNK) phosphorylates NFAT4 on two sites. Mutational removal of the JNK phosphorylation sites caused constitutive nuclear localization of NFAT4. In contrast, JNK activation in calcineurin-stimulated cells caused nuclear exclusion of NFAT4. These findings show that the nuclear accumulation of NFAT4 promoted by calcineurin is opposed by the JNK signal transduction pathway.

Molecular cloning and functional characterization of murine cDNA encoding transcription factor NFATc

Transcription factors of the NFAT (nuclear factor of activated T cells) family play important roles in immune and inflammatory responses by regulating the expression of genes encoding cytokines and immunoregulatory proteins. Here we describe cloning and characterization of full-length cDNA encoding murine (m) NFATc which predicts that the protein has all the conserved structural motifs of NFAT family members, including the rel homology domain, the NFAT homology domain and the nuclear translocation signals. mNFATc complexed with AP-1 bound specifically to the murine IL-2 NFAT recognition sequence and activated transcription from the co-transfected IL-2 promoter in COS-7 cells. Northern blot analysis showed that the cDNA probe hybridized with a 4.5 kb transcript which is highly inducible in murine T cells. By Northern and in situ hybridization, mNFATc transcript was detected from the early stage of development. In the mouse embryo, mNFATc transcript was strongly expressed in thymus, lung and submandibular gland and weakly in skeletal muscle and heart suggesting that mNFATc may have a role both in embryogenesis and in mature T cells.

Blockade of T-cell activation by dithiocarbamates involves novel mechanisms of inhibition of nuclear factor of activated T cells

Dithiocarbamates (DTCs) have recently been reported as powerful inhibitors of NF-kappaB activation in a number of cell types. Given the role of this transcription factor in the regulation of gene expression in the inflammatory response, NF-kappaB inhibitors have been suggested as potential therapeutic drugs for inflammatory diseases. We show here that DTCs inhibited both interleukin 2 (IL-2) synthesis and membrane expression of antigens which are induced during T-cell activation. This inhibition, which occurred with a parallel activation of c-Jun transactivating functions and expression, was reflected by transfection experiments at the IL-2 promoter level, and involved not only the inhibition of NF-kappaB-driven reporter activation but also that of nuclear factor of activated T cells (NFAT). Accordingly, electrophoretic mobility shift assays (EMSAs) indicated that pyrrolidine DTC (PDTC) prevented NF-kappaB, and NFAT DNA-binding activity in T cells stimulated with either phorbol myristate acetate plus ionophore or antibodies against the CD3-T-cell receptor complex and simultaneously activated the binding of AP-1. Furthermore, PDTC differentially targeted both NFATp and NFATc family members, inhibiting the transactivation functions of NFATp and mRNA induction of NFATc. Strikingly, Western blotting and immunocytochemical experiments indicated that PDTC promoted a transient and rapid shuttling of NFATp and NFATc, leading to their accelerated export from the nucleus of activated T cells. We propose that the activation of an NFAT kinase by PDTC could be responsible for the rapid shuttling of the NFAT, therefore transiently converting the sustained transactivation of this transcription factor that occurs during lymphocyte activation, and show that c-Jun NH2-terminal kinase (JNK) can act by directly phosphorylating NFATp. In addition, the combined inhibitory effects on NFAT and NF-KB support a potential use of DTCs as immunosuppressants.

Rapid targeting of nuclear proteins to the cytoplasm

BACKGROUND

The transcription factor NF-ATc plays a key role in the activation of many early immune response genes and is regulated by subcellular localization. NF-ATc translocates from the cytoplasm to the nucleus in response to a rise in intracellular calcium, and immediately returns to the cytoplasm when intracellular calcium levels fall. The rapid nuclear exit of NF-ATc is thought to be one mechanism by which cells distinguish between sustained and transient calcium signals.

RESULTS

To study the nuclear export of NF-ATc, we have developed a general, non-invasive assay for the identification and study of nuclear export signals (NESs). The NES is defined by its ability to translocate a protein from the nucleus to the cytoplasm when the two are tethered by a membrane-permeable ligand. This procedure has allowed us to identify a NES within NF-ATc that functions in concert with a glycogen synthase kinase-regulated process to direct the rapid nuclear exit of NF-ATc.

CONCLUSIONS

The rapid nuclear export of NF-ATc via its NES and a glycogen synthase kinase-regulated event may be an important mechanism for insulating cells from transient spikes in intracellular calcium which might otherwise lead to inappropriate activation. The assay we have developed allows the rapid identification of NESs and can be used as a general method for the inducible cytoplasmic export of nuclear proteins.

Nuclear localization of NF-ATc by a calcineurin-dependent, cyclosporin-sensitive intramolecular interaction

The NF-AT family of transcription factors participates in the regulation of early immune response genes such as IL-2, IL-4, CD40 ligand, and Fas ligand in response to Ca2+/calcineurin signals initiated at the antigen receptor. Calcineurin activation leads to the rapid translocation of NF-AT family members from cytoplasm to nucleus, an event that is blocked by the immunosuppressive drugs cyclosporin A and FK506. We show that translocation requires two redundant nuclear localization sequences and that one sequence is in an intramolecular association with phosphorserines in a conserved motif located at the amino terminus of each NF-AT protein. Mutation of serines in this motif in NF-ATc both disrupts this intramolecular interaction and leads to nuclear localization, suggesting a model of NF-AT nuclear import in which dephosphorylation by calcineurin causes exposure of two nuclear localization sequences.