NFAT: ubiquitous regulator of cell differentiation and adaptation

Blockade of NFAT activation by the second calcineurin binding site

Activation of NFAT transcription factors requires their dephosphorylation by the phosphatase calcineurin (CN). NFATs contain two CN binding motifs: PxIxIT and CnBP-B/CNBR2 (which we call LxVP). Here we carry out a detailed comparative analysis of the CN binding activity displayed by the PxIxIT and LxVP sites from different NFATs. Dose-response CN binding experiments with GST fusion proteins of NFATc1 and NFATc2 showed that NFATc1 binds CN in vitro more efficiently than does NFATc2. This difference in binding appears to be caused by the different CN binding potencies of the corresponding LxVP sites; thus while the LxVPc2 peptide fused to GST did not bind CN, GST-LxVPc1 bound it more efficiently than did GST-PxIxITc1 or GST-PxIxITc2. Furthermore, an NFATc2 chimera protein containing the LxVP motif from NFATc1 interacted with CN much more potently than did wild-type NFATc2. Free peptides spanning the LxVP motifs from NFATc1, c3 or c4 displaced CN from GST-NFATc1 and GST-NFATc2 more efficiently than any PxIxIT peptide. PxIxITc2 and LxVPc1 peptides were each able to cross-compete GST-LxVPc1-CN and GST-PxIxITc2-CN binding. In contrast with PxIxITc2, the LxVP peptide not only blocked CN-NFAT binding but also inhibited CN phosphatase activity in vitro. Furthermore, exogenous LxVPc1 blocked NFATc2 phosphorylation and nuclear translocation in vivo. These results suggest a model in which the different CN binding characteristics of the PxIxIT and LxVP sites enable different NFAT members to influence each others activities in cells where they are co-expressed.

HCMV-encoded G-protein-coupled receptors as constitutively active modulators of cellular signaling networks

Several herpesviruses encode G-protein-coupled receptor (vGPCR) proteins that are homologous to human chemokine receptors. In contrast to chemokine receptors, many vGPCRs signal in a ligand-independent (constitutive) manner. Such constitutive signaling is of major significance because various pathologies are associated with activating GPCR mutations. Constitutive activity of the human herpesvirus 8-encoded GPCR (ORF74), for example, is essential for its oncogenic potential to cause angioproliferative Kaposi's sarcoma-like lesions. The human cytomegalovirus (HCMV) encodes four GPCRs, of which US28 and UL33 display constitutive activity in transfected, but also HCMV-infected, cells. In addition, US28 is activated by a broad spectrum of chemokines. Furthermore, both US28 and UL33 show promiscuous G-protein coupling, whereas chemokine receptors activate primarily G(i/o) proteins. Thus, these vGPCRs are versatile signaling devices, reprogramming cellular signaling networks to modulate cellular function after infection. By these means, these HCMV-encoded receptors might contribute to HCMV-related pathologies.

NFATc3 deficiency may contribute to the development of mammary gland adenocarcinoma in aging female mice

The nuclear transcription factor of activated T cells (NFAT) suggested to be a tumor suppressor. Here we report that two out of three NFATc3-/- and two in four NFATc3 +/- female mice developed aggressive mammary adenocarcinoma by 12.5 and 16 mo of age, respectively, with no occurrences in age-matched wild-type littermates (N-14). Thus, our data suggest that NFATc3 can suppress the development of mammary gland tumors in female mice.

Close encounters: regulation of vertebrate skeletal myogenesis by cell-cell contact

Cells of the vertebrate skeletal muscle lineage develop in a highly ordered process that includes specification, migration and differentiation into multinucleated myofibers. The changes in gene expression and cell morphology that occur during myogenic differentiation must be coordinated with each other in a spatiotemporal fashion; one way that this might occur is through regulation of these processes by cell-cell adhesion and resultant signaling. The past several years have witnessed the identification of molecules that are likely to be mediators of the promyogenic effects of cell-cell contact and some of the mechanisms by which they work. These include: the community factor, embryonic fibroblast growth factor (eFGF); classical cadherins, which mediate both adhesion and signaling; and cadherin-associated immunoglobulin superfamily members such as CDO, BOC and neogenin. Genetic evidence for the promyogenic roles of some of these factors is emerging. In other cases, potential compensatory or redundant functions necessitate future construction of double or triple mutants. Mechanistic studies in vitro indicate that specific cadherins and immunoglobulin superfamily proteins exert some of their effects in an interdependent fashion by signaling from a multiprotein complex found at sites of cell-cell contact.

Homer modulates NFAT-dependent signaling during muscle differentiation

While changes in intracellular calcium are well known to influence muscle contraction through excitation contraction coupling, little is understood of the calcium signaling events regulating gene expression through the calcineurin/NFAT pathway in muscle. Here, we demonstrate that Ca(+2) released via the inositol trisphosphate receptor (IP3R) increases nuclear entry of NFAT in undifferentiated skeletal myoblasts, but the IP3R Ca(+2) pool in differentiated myotubes promotes nuclear exit of NFAT despite a comparable quantitative change in [Ca(+2)]i. In contrast, Ca(+2) released via ryanodine receptors (RYR) increases NFAT nuclear entry in myotubes. The scaffolding protein Homer, known to interact with both IP3R and RYR, is expressed as part of the myogenic differentiation program and enhances NFAT-dependent signaling by increasing RYR Ca(+2) release. These results demonstrate that differentiated skeletal myotubes employ discrete pools of intracellular calcium to restrain (IP3R pool) or activate (RYR pool) NFAT-dependent signaling, in a manner distinct from undifferentiated myoblasts. The selective expression of Homer proteins contributes to these differentiation-dependent features of calcium signaling.

Genistein, a Dietary Isoflavone, Down-Regulates the MDM2 Oncogene at Both Transcriptional and Posttranslational Levels

Although genistein has chemopreventive effects in several human malignancies, including cancers of the breast, colon, and prostate, the mechanisms of action are not fully understood. Herein we report novel mechanisms whereby genistein down-regulates the MDM2 oncogene, perhaps explaining some of its anticancer activities. In a dose- and time-dependent manner, genistein reduced MDM2 protein and mRNA levels in human cell lines of breast, colon, and prostate cancer; primary fibroblasts; and breast epithelial cells. The inhibitory effects were found at both transcriptional and posttranslational levels and were independent of tyrosine kinase pathways. We found that the NFAT transcription site in the region between -132 and +33 in the MDM2 P2 promoter was responsive to genistein. At the posttranslational level, genistein induced ubiquitination of MDM2, which led to its degradation. Additionally, genistein induced apoptosis and G2 arrest and inhibited proliferation in a variety of human cancer cell lines, regardless of p53 status. We further showed that MDM2 overexpression abrogated genistein-induced apoptosis in vitro and that genistein inhibited MDM2 expression and tumor growth in PC3 xenografts. In conclusion, genistein directly down-regulates the MDM2 oncogene, representing a novel mechanism of its action that may have implications for its chemopreventive and chemotherapeutic effects.

The Sarcolemmal Calcium Pump Inhibits the Calcineurin/Nuclear Factor of Activated T-cell Pathway via Interaction with the Calcineurin A Catalytic Subunit

The calcineurin/nuclear factor of activated T-cell (NFAT) pathway represents a crucial transducer of cellular function. There is increasing evidence placing the sarcolemmal calcium pump, or plasma membrane calcium/calmodulin ATPase pump (PMCA), as a potential modulator of signal transduction pathways. We demonstrate a novel interaction between PMCA and the calcium/calmodulin-dependent phosphatase, calcineurin, in mammalian cells. The interaction domains were located to the catalytic domain of PMCA4b and the catalytic domain of the calcineurin A subunit. Endogenous calcineurin activity, assessed by measuring the transcriptional activity of its best characterized substrate, NFAT, was significantly inhibited by 60% in the presence of ectopic PMCA4b. This inhibition was notably reversed by the co-expression of the PMCA4b interaction domain, demonstrating the functional significance of this interaction. PMCA4b was, however, unable to confer its inhibitory effect in the presence of a calcium/calmodulin-independent constitutively active mutant calcineurin A suggesting a calcium/calmodulin-dependent mechanism. The modulatory function of PMCA4b is further supported by the observation that endogenous calcineurin moves from the cytoplasm to the plasma membrane when PMCA4b is overexpressed. We suggest recruitment by PMCA4b of calcineurin to a low calcium environment as a possible explanation for these findings. In summary, our results offer strong evidence for a novel functional interaction between PMCA and calcineurin, suggesting a role for PMCA as a negative modulator of calcineurin-mediated signaling pathways in mammalian cells. This study reinforces the emerging role of PMCA as a molecular organizer and regulator of signaling transduction pathways.

The p53-induced gene-6 (proline oxidase) mediates apoptosis through a calcineurin-dependent pathway

Proline oxidase is a p53-induced redox gene that can generate reactive oxygen species (ROS) and mediate apoptosis in tumor cells. We report that proline oxidase is a downstream effector in p53-mediated activation of the calcium/calmodulin-dependent phosphatase calcineurin in lung, renal, colon, and ovarian carcinoma cells. The activation of calcineurin by p53 and proline oxidase was detected by activation of the nuclear factor of activated T cells (NFAT), an established indicator of activated calcineurin. Both proline oxidase- and p53-induced activation of NFAT were sensitive to the calcineurin inhibitors cyclosporin A and FK-506, to scavengers of ROS, and to inhibitors of calcium mobilization. A proline oxidase antisense vector suppressed the ability of p53 to up-regulate proline oxidase, activate calcineurin, and induce apoptosis. Moreover, two renal carcinoma-derived mutant p53 proteins were deficient in inducing proline oxidase expression and in activating calcineurin. Inhibitors of calcineurin and calcium mobilization abolished proline oxidase-mediated apoptosis and reduced p53-induced apoptosis. Treatment of colon and ovarian carcinoma cells with the anticancer genotoxic agent etoposide up-regulated both p53 and proline oxidase, activated calcineurin, and induced apoptosis. The etoposide-mediated activation of calcineurin and induction of apoptosis was markedly suppressed by FK-506 calcineurin inhibitor. We propose that proline oxidase mediates apoptosis through the generation of proline-dependent ROS, which then mobilize calcium and activate calcineurin. The activation of calcineurin-regulated transcription factor pathways by proline oxidase might affect gene expression events important to p53 regulation of cell growth and apoptosis.

Activity-dependent and -independent nuclear fluxes of HDAC4 mediated by different kinases in adult skeletal muscle

Class II histone deacetylases (HDACs) may decrease slow muscle fiber gene expression by repressing myogenic transcription factor myocyte enhancer factor 2 (MEF2). Here, we show that repetitive slow fiber type electrical stimulation, but not fast fiber type stimulation, caused HDAC4-GFP, but not HDAC5-GFP, to translocate from the nucleus to the cytoplasm in cultured adult skeletal muscle fibers. HDAC4-GFP translocation was blocked by calmodulin-dependent protein kinase (CaMK) inhibitor KN-62. Slow fiber type stimulation increased MEF2 transcriptional activity, nuclear Ca²⁺ concentration, and nuclear levels of activated CaMKII, but not total nuclear CaMKII or CaM-YFP. Thus, calcium transients for slow, but not fast, fiber stimulation patterns appear to provide sufficient Ca²⁺-dependent activation of nuclear CaMKII to result in net nuclear efflux of HDAC4. Nucleocytoplasmic shuttling of HDAC4-GFP in unstimulated resting fibers was not altered by KN-62, but was blocked by staurosporine, indicating that different kinases underlie nuclear efflux of HDAC4 in resting and stimulated muscle fibers.

Regulation of p70S6k, GSK-3β, and calcineurin in rat striated muscle during aging

In this study we compared the content and phosphorylation levels of several molecules believed to regulate muscle hypertrophy and fiber type changes in the extensor digitorum longus (EDL), soleus, diaphragm, and heart of adult (6 months), aged (30 months), and very aged (36 months) Fischer 344 x Brown Norway rats. With aging, the mass of the EDL and soleus decreased significantly (approximately 38% and approximately 36%, respectively), the diaphragm's mass remained unchanged while the mass of the heart increased (approximately 35%). Western blotting demonstrated that calcineurin (CnA), the 70-kDa ribosomal S6 kinase (p70(S6k)), glycogen synthase kinase-3beta (GSK-3beta), and the phosphorylated forms of GSK-3beta and p70(S6k) (p-GSK-3beta(Ser9) and p-p70(S6kThr389)) were regulated differently with aging and between muscle types. Total p70(S6k), GSK-3beta, and p-GSK-3beta(Ser9) decreased in the aged-atrophic EDL and soleus while p-p70(S6kThr389) increased. Although total p70(S6k) content diminished in the continuously active diaphragm, phosphorylation of p70(S6k )remained unchanged. Conversely, the expression of GSK-3beta and p-GSK-3beta(Ser9) increased in the diaphragm. With aging, the amount of p-p70(S6kThr389) decreased approximately 56% in the heart while p-GSK-3beta( Ser9) increased approximately 193%. Interestingly, CnA content remained unchanged in the diaphragm, increased approximately 204% in the EDL, and decreased approximately 30% and approximately 65% with aging in the soleus and heart, respectively. These results indicate remarkable differences in the regulation of molecules thought to govern protein synthesis and changes in contractile protein expression.

Nuclear factor of activated T cells and serum response factor cooperatively regulate the activity of an alpha-actin intronic enhancer

Expression of alpha-actin in smooth muscle cells (SMCs) is regulated, in part, by an intronic serum response factor (SRF)-binding CArG element. We have identified a conserved nuclear factor of activated T cells (NFAT) binding site that overlaps this CArG box and tested the hypothesis that this site plays a previously unrecognized role in regulating alpha-actin expression. A reporter construct prepared using a 56-bp region of the mouse alpha-actin first intron containing SRF, NFAT, and AP-1 sites (SNAP) acted as an enhancer element in the context of a minimal thymidine kinase promoter. Basal reporter activity following expression in SMCs was robust and sensitive to the calcineurin-NFAT pathway inhibitors cyclosporin A and FK506. Mutating either the NFAT or SRF binding site essentially abolished reporter activity, suggesting that both NFAT and SRF binding are required. Basal activity in non-smooth muscle HEK293 cells was SRF-dependent but NFAT-independent and approximately 8-fold lower than that in SMCs. Activation of NFAT in HEK293 cells induced an approximately 4-fold increase in activity that was dependent on the integrity of both NFAT and SRF binding sites. NFATc3.SRF complex formation, demonstrated by co-immunoprecipitation, was facilitated by the presence of SNAP oligonucleotide. Inhibition of the calcineurin-NFAT pathway decreased alpha-actin expression in cultured SMCs, suggesting that the molecular interaction of NFAT and SRF at SNAP may be physiologically relevant. These data provide the first evidence that NFAT and SRF may interact to cooperatively regulate SMC-specific gene expression and support a role for NFAT in the phenotypic maintenance of smooth muscle.

DIM stimulates IFNgamma gene expression in human breast cancer cells via the specific activation of JNK and p38 pathways

3,3'-Diindolylmethane (DIM) is a promising anticancer agent derived from Brassica vegetables, but the mechanisms of DIM action are largely unknown. We have shown that DIM can upregulate the expression and stimulate the secretion of interferon-gamma (IFNgamma) in the human MCF-7 breast cancer cell line. This novel effect may provide important clues to explain the anticancer effects of DIM because it is well known that IFNgamma plays an important role in preventing the development of primary and transplanted tumors. Utilizing promoter deletions, we show here that the region between -108 and -36 bp in the IFNgamma promoter, which contains two conserved and essential regulatory elements, is required for DIM-induced IFNgamma expression. DIM activates both JNK and p38 pathways, induces the phosphorylation of c-Jun and ATF-2, and increases the binding of the homodimer or heterodimer of c-Jun/ATF-2 to the proximal AP-1.CREB-ATF-binding element. Moreover, studies with specific enzyme inhibitors showed that up-stream Ca2+-dependent kinase(s) is required for the inducing effects of DIM in MCF-7 cells. These results establish that DIM-induced IFNgamma expression in human breast tumor cells is mediated by activation of both JNK and p38 pathways, which is ultimately dependent on intracellular calcium signaling.

Functional Characterization of SDF-1 Proximal Promoter

Stromal-cell derived factor 1 (SDF1) is a CXC chemokine that binds and signals through the CXCR4 receptor, playing an essential role in embryonic B lymphopoiesis, myelopoiesis and organogenesis. The CXCR4/SDF1 pathway is associated with several pathologies. CXCR4 serves as a fusion cofactor for lymphotropic strains of human immunodeficiency virus type 1 and SDF1 inhibits viral entry. Moreover, recent works suggest an important role for SDF1 in metastasis progression and autoimmune diseases such as rheumatoid arthritis. To understand the molecular mechanisms that regulate SDF1 expression, we have cloned and functionally analysed its 5' flanking regulatory region. An SDF1-promoter luciferase construct showed high levels of reporter gene activity in transient transfection experiments. DNase I footprinting analysis revealed that the proximal promoter was occupied by six putative Sp1-binding motifs. Binding of Sp1 to the promoter was confirmed by electrophoretic mobility shift assay, and its importance in SDF1 gene expression verified by in vitro mutagenesis. Particularly, mutation of an Sp1 motif located between -57 and -39 upstream of the main transcription start-site resulted in a marked reduction in promoter activity. It has been shown that the SDF1 expression could be induced by mitogenic stimuli, X-ray radiation or treatment with IL1beta, depending on cell environment. We have analysed the effect of these stimuli on SDF1 promoter transactivation in three different cell lines. Phorbol myristated acetate plus ionomycin increased promoter activity in U373 and LC5 but repressed it in MS5 cells. On the contrary, gamma irradiation promoted SDF1 transcription in MS5 cells but not in the other cell lines. Interferon-gamma acted as a transcriptional repressor in U373 and LC5 but not in MS5 cells. Finally, IL1beta functions as mild activator only in U373 cells. The present study demonstrates that these stimuli mediate SDF1 production through promoter activation in a cell-specific manner.

Protection against melanoma by vaccination with Bacille Calmette-Guerin (BCG) and/or vaccinia: an epidemiology-based hypothesis on the nature of a melanoma risk factor and its immunological control

A multicentre case-control study conducted by the FEBrile Infections and Melanoma (FEBIM) group has demonstrated a reduced risk of melanoma associated with Bacille Calmette-Guerin (BCG) and/or vaccinia vaccination in early childhood and/or with infectious diseases later in life. This has led to the recognition of a new risk indicator of melanoma; namely 'not being vaccinated with either with BCG or vaccinia'. On the basis of these findings, we propose a hypothesis of immune surveillance for melanoma induced or enhanced by prior contacts with pathogens unexpectedly cross-reactive to a cellular 'marker of melanoma risk'. The reduced risk of melanoma due to BCG and vaccinia, as well as certain common causes of infectious disease, is shown to be associated with antigenic determinants exhibiting sequence homologies with the HERV-K-MEL-antigen. The latter is a product of a pseudo-gene that is closely associated with the env-gene of the endogenous human retrovirus K (HERV-K). A suppressive immune reaction appears to inhibit the expression of endogenous retroviral genes, such as the HERV-K env-gene, that could otherwise result in malignant transformation years or even decades later. The HERV-K env-protein has homologous amino acid sequences with the human nuclear factor Oxygen Responsive Element Binding Protein (OREBP) that controls the expression of glutathione peroxidase. The formation of this and other redox-enzymes, needed to maintain appropriate levels of the normal intracellular redox potential, seems to be suppressed by the OREBP-homologous protein. The present hypothesis is in accordance with the concept that immune dysregulation due to adverse environmental impacts is a risk factor not only for some autoimmune disorders, as previously described, but also for certain malignancies such as melanoma.

Nuclear localization and chromatin targets of p21-activated kinase 1

Pak1 (p21-activated kinase 1), a conserved, mammalian signaling kinase, is a downstream effector of small GTPases Rac1 and Cdc42 and of growth factor signaling. Until now, a major focus of study has been on the cytosolic functions of Pak1, where it is an important modulator of cytoskeletal reorganization, consequently playing a major role in cell survival, migration, and invasion. In this report, we demonstrate the nuclear localization of Pak1 upon stimulation by epidermal growth factor. Three nuclear localization signals (NLSs) were identified in the N-terminal domain of Pak1. With mutational analysis, the importance of each NLS was elucidated. Mutation of all three NLSs eliminated the nuclear localization of Pak1. Expression of Pak1 as a fusion protein with Gal4-DNA binding domain and Gal4-luciferase activity showed that Pak1 might increase transcription. To identify the potential targets of nuclear Pak1, we used a Pak1-specific chromatin immunoprecipitation-based screening assay and identified a series of Pak1-interacting target chromatins, including phosphofructokinase-muscle isoform (PFK-M) and nuclear factor of activated T-cell (NFAT1) genes. Pak1 associated with the upstream enhancer sequence and promoter of PFK-M and was involved in the stimulation of the PFK-M expression. It also associated with a portion of the NFAT1 gene and its upstream region, leading to the repression of NFAT1 expression. These investigations provide proof-of-principle evidence that Pak1 could influence the expression of its putative chromatin targets in both a positive and a negative manner. Together, for the first time, these findings defined the NLSs of the Pak1, its association with chromatin, and the resulting modulation of transcription, thus opening new avenues to further the search for nuclear Pak1 functions and identify putative Pak1-interacting nuclear proteins.

The NFATc1 transcription factor is widely expressed in white cells and translocates from the cytoplasm to the nucleus in a subset of human lymphomas

Stimulation of lymphoid cells via their surface receptors triggers signalling pathways that terminate in the nucleus, where they induce alterations in gene transcription. Nuclear factor of activated T cells (NFAT) transcription factors, involved in a major Ca2+-dependent signalling pathway, normally reside in the cytoplasm but re-locate to the nucleus when activation of the pathway (e.g. following ligation of antigen receptors) leads to their dephosphorylation. This study found that one member of the NFAT family (NFATc1/NFAT2) can be detected in routine biopsy samples, where it is seen in essentially all lymphoid cells, but is absent from the great majority of non-haematopoietic cells. An immunohistological evaluation of NFATc1 in almost 300 lymphomas showed that most neoplastic lymphoid cells also express NFATc1 as a cytoplasmic constituent, although it is absent in classical Hodgkin's disease and plasma cell proliferations. Of particular interest was the finding that NFATc1 was relocated to the nucleus in a minority of lymphoid neoplasms (usually diffuse large B-cell lymphomas or Burkitt lymphoma), presumably reflecting activation of the NFAT pathway. It would be of interest to correlate this feature with patterns of gene expression and also with prognosis, since it may identify a subset of human lymphoma that is distinct in its molecular and clinical features.

Expression and Function of the Nuclear Factor of Activated T Cells in Colon Carcinoma Cells

Increasing evidence shows a crucial role of the Ca2+/ calcineurin-mediated activation of the nuclear factor of activated T cells (NFAT) in the regulation of a variety of processes in nonimmune cells. Here we provide evidence that NFATc1 and NFATc2 are expressed in human colon carcinoma cell lines. These proteins are translocated from the cytoplasm to the nucleus upon treatment with a combination of phorbol 12-myristate 13-acetate plus the calcium ionophore A23187. Subsequent to translocation to the nucleus, NFATc1 and NFATc2 were able to bind to a NFAT response element in the DNA, regulating transcriptional activation of genes containing a NFAT-responsive element such as cyclooxygenase-2 (COX-2). COX-2 expression and prostaglandin E2 (PGE2) production were induced upon pharmacological stimuli leading to NFAT activation and blunted by inhibition of calcineurin phosphatase with cyclosporin A or tacrolimus (FK506). Expression of NFAT wild type protein or the active catalytic subunit of calcineurin transactivates COX-2 promoter activity, whereas a dominant negative mutant of NFAT inhibited COX-2 induction in colon carcinoma cell lines. Furthermore, mutation or deletion of NFAT binding sites in the human COX-2 promoter greatly diminished its induction by phorbol 12-myristate 13-acetate/calcium ionophore A23187. These findings demonstrate the presence and activation of NFAT in human colon carcinoma cells, with important implications in the regulation of genes involved in the transformed phenotype as COX-2.

The Transcription Factor NFAT3 Mediates Neuronal Survival

Neuronal apoptosis is critical for normal development of the mammalian nervous system and also contributes to the pathogenesis of ischemic and degenerative diseases of the brain. Apoptosis of neurons is tightly regulated by extrinsic signals including growth factors and neuronal activity, but the intracellular mechanisms by which these signals promote neuronal survival are incompletely understood. We report that the transcription factor NFAT3 plays a critical role in mediating survival of granule neurons of the developing cerebellum. NFAT3 accumulated in the nucleus of primary granule neurons under survival conditions of serum growth factors and neuronal activity that was elicited by depolarization with high K(+). In contrast, deprivation of serum and K(+), which leads to neuronal apoptosis, triggered NFAT3 nuclear export. Treatment of granule neurons with Li(+), an inhibitor of the NFAT export kinase GSK3, prevented the nuclear export of NFAT3 and increased granule cell survival even under pro-apoptotic conditions. Thus, the nuclear localization of NFAT3 correlated tightly with granule neuron survival. Consistent with a pro-survival function for NFAT3, genetic knockdown of NFAT3 by RNA interference in primary granule neurons led to increased apoptosis even in neurons cultured under survival conditions. Conversely, expression of a constitutively active form of NFAT protected neurons against apoptosis induced by serum withdrawal and low K(+). Taken together, these results reveal an essential function for NFAT3-mediated transcription in neuronal survival that may play important roles in the developing and mature brain.

US28 actions in HCMV infection: lessons from a versatile hijacker

Mimicking host proteins is a strategy adopted by several herpesviruses to exploit the host cell for their own benefit. In this respect the human cytomegalovirus (HCMV) chemokine receptor homologue US28, has been extensively studied. Molecular pirates such as US28 can teach us about crucial events in HCMV infection and may either offer a potential target for antiviral therapy or provide an alternative strategy to immune suppression. Despite elaborate research into the chemokine binding affinity, signalling properties, intracellular trafficking and expression kinetics of US28, a solid hypothesis about the role of US28 in HCMV infection has not yet been proposed. It appears that US28 may behave as a molecular pirate that employs smart strategies for cell entry, host gene regulation and immune evasion. This review will elaborate on these aspects of US28 biology and discuss possible implications for HCMV infection.

NFAT expression in human osteoclasts

Nuclear factor of activated T-cells cytoplasmic (NFATc) is a family of transcription factors originally identified in T-cells. The gene family is currently known to have four members (NFATc1 through NFATc4) which have roles both within and outside the immune system. We show that NFATc1 is the major induced NFAT in human osteoclasts, with expression greatly exceeding that of NFATc2 through NFATc4. In macrophage-like cells in culture, NFATc1 through NFATc4 are expressed at similar low levels. NFATc1 is comprised of five mRNA transcript variants known to encode three different protein isoforms. The mRNA encoding isoform C (mRNA variant 3) was the most expressed with 38 copies per nanogram followed by isoform B (mRNA variant 5) with 17 copies per nanogram of total RNA. Isoform A (mRNA variant 1) and mRNA variants 2 and 4 made up less than 1% of the total NFATc1 expressed. NFATc1 is activated by calcineurin after calcium-calmodulin signalling. The induction of NFATc1 in osteoclasts was not altered in the presence of cyclosporin A, an inhibitor of calcineurin, suggesting that NFATc1 does not participate in autoregulatory activation of its own promoter. The NFATc1 variants expressed by human osteoclasts are not those normally expressed by effector T-cells but are similar to those seen in naïve T-cells.

Sp3 proteins negatively regulate beta myosin heavy chain gene expression during skeletal muscle inactivity

In adult skeletal muscle, beta myosin heavy chain (betaMyHC) gene expression is primarily restricted to slow type I fibers; however, its expression is down-regulated in response to muscle inactivity. Little is known about the signaling pathways and transcription factors that mediate this important functional response. This study demonstrates that increased binding of Sp3 to GC-rich elements in the betaMyHC promoter is a critical event in down-regulation of betaMyHC gene expression under non-weight-bearing conditions. Conversely, binding of Sp3 to these elements decreased while Sp1 binding increased with nuclear extracts from plantaris muscle exposed to mechanical overload, a stimulus that increases betaMyHC gene expression. In addition, these experiments revealed the existence of an Sp4-DNA binding complex when using adult skeletal muscle nuclear extract was used but not when nuclear extracts from cultured myotubes were used. Sp3 proteins are competitive inhibitors of Sp1-mediated betaMyHC reporter gene transactivation in both Drosophila SL-2 and mouse C2C12 myotubes. Sp4 is a weak activator of betaMyHC gene expression in SL-2 cells, which lack endogenous Sp1 activity, but does not activate betaMyHC gene expression in C2C12 myotubes, which have high levels of Sp1. These results suggest that competitive binding of Sp family proteins regulate betaMyHC gene transcription in response to altered neuromuscular activity.

Cellular and molecular mechanisms in environmental and occupational inhalation toxicology

The central issue of this review are inflammatory changes that take place in the mucous membranes of the respiratory tract as a result of inhaled pollutants. Of particular relevance are dusts, SO(2), ozone, aldehydes und volatile organic compounds. Bioorganic pollutants, especially fragments of bacteria and fungi, occur predominantly in indoor dusts. They activate the toll-like/IL-1 receptor and lead to the activation of the transcription factor NF-κB for the release of numerous proinflammatory cytokines. Metals are predominant in ambient air dust particles. They induce the release of reactive oxygen species that cause damage to lipids, proteins and the DNA of the cell. As well as NF-κB, transcription factors that foster proliferation are activated via stress activated protein kinases. Organic compounds such as polycyclic aromatic hydrocarbons and nitroso-compounds of incomplete combustion processes activate additional via the cytosolic arylhydrocarbon receptor for detoxification enzymes. Sulphur dioxide leads to acid stress, and ozone to oxidative stress of the cell. This is accompanied by the release of proinflammatory cytokines via stress activated protein kinases. Aldehydes and volatile organic compounds activate the vanilloid receptor of trigeminal nerve fibres and induce a hyperreactivity of the mucous membrane via the release of nerve growth factors. The mechanisms described work synergistically and lead to a chronic inflammatory reaction of the mucous membranes of the upper respiratory tract that is regularly demonstrable in inhabitants of western industrial nations. It is unclear whether we are dealing here with a physiological inflammation or with an at least partially avoidable result of chronic pollutant exposure.

Optimised real-time quantitative PCR assays for RANKL regulated genes

Osteoclasts are multinucleated giant cells that differentiate from precursors of the monocyte-macrophage lineage. We used receptor activator of NF-kappa B ligand (RANKL) and macrophage colony stimulating factor (M-CSF) to differentiate authentic human osteoclasts from peripheral blood mononuclear cells (PBMCs). We previously described a series of genes that are strongly regulated by RANKL. Here, we provide a set of reliable quantitative real-time PCR based assays of RANKL regulated genes as reference genes that may prove useful in the study of human osteoclasts. The SYBR-green I assays are free of primer dimer and other artefacts, and are designed to amplify in parallel, thus permitting simultaneous analysis of 12 genes. Optimised primers for 18S rRNA provide a valid housekeeping reference gene. Standard curves have been constructed for all assays, thus allowing for absolute quantification of mRNA transcript copy number. As an example, the regulation of expression of the chemokine RANTES in osteoclasts is demonstrated. These gene assays have potential utility in a variety of cell types, tissues and organs, in addition to macrophages and osteoclasts.

RANK-L induces the expression of NFATc1, but not of NFκB subunits during osteoclast formation

We report the program of gene expression during osteoclast formation from RAW264.7 cell precursors in response to RANK-ligand (RANK-L) using a combination of quantitative real time PCR and Affymetrix gene chip assays. We found that genes obligatory to osteoclast formation and function, namely tartrate-resistant acid phosphatase, cathepsin K, beta3 integrin, and calcitonin receptors, were up-regulated by RANK-L markedly by up to approximately 2000-fold. In contrast, we found a cluster of genes that were significantly down-regulated: these included interleukin-18, insulin-like growth factor-1, interleukin-6 receptor, and cathepsins B, C, and L. These results from real time PCR were broadly concordant with those obtained from Affymetrix. We also explored the expression of the transcription factors of the NFAT and NFkappaB family at days 3 and 5 of culture. Whereas NFATc1 expression was increased significantly at days 3 and 5 following RANK-L exposure, there were no significant increases in the expression of NFkappaB subunits, namely p65, p50, c-Rel, IkappaBalpha, and IkappaBbeta. There were also no significant differences in transcription modulator expression between days 3 and 5, except for c-Rel and NFATc4, which were both decreased significantly at day 5. The studies suggest RANK-L regulates the expression only of NFATc1, while it signals through both NFATc1 and NFkappaB.

Discovery of Small-Molecule Inhibitors of the NFAT−Calcineurin Interaction by Competitive High-Throughput Fluorescence Polarization Screening

The direct protein-protein interaction between the phosphatase calcineurin and transcription factor NFAT plays important roles in a number of crucial mammalian cell signaling and regulatory events, such as activation of T cells and developmental genetic programs. In this paper, we report on the identification of small organic molecules for the targeted disruption of the NFAT-calcineurin interaction in vitro. In the preceding paper (21), we devise a theoretical and procedural framework for high-throughput fluorescence polarization screens to aid in this effort. The results presented here ground on this work and illustrate the stringency and successful general applicability of our approach. The identified compounds provide valuable molecular tools for probing calcineurin signaling and for the NFAT-specific inhibition of calcineurin in cells and organisms.

Calcium-calcineurin signaling in the regulation of cardiac hypertrophy

Cardiac hypertrophy is a leading predicator of progressive heart disease that often leads to heart failure and a loss of cardiac contractile performance associated with profound alterations in intracellular calcium handling. Recent investigation has centered on identifying the molecular signaling pathways that regulate cardiac myocyte hypertrophy, as well as the mechanisms whereby alterations in calcium handling are associated with progressive heart failure. One potential focal regulator of cardiomyocyte hypertrophy that also responds to altered calcium handling is the calmodulin-activated serine/threonine protein phosphatase calcineurin (PP2B). Once activated by increases in calcium, calcineurin mediates the hypertrophic response through its downstream transcriptional effector nuclear factor of activated T cells (NFAT), which is directly dephosphorylated by calcineurin resulting in nuclear translocation. While previous studies have convincingly demonstrated the sufficiency of calcineurin to mediate cardiac hypertrophy and progressive heart failure, its necessity remains an area of ongoing investigation. Here we weigh an increasing body of literature that suggests a causal link between calcineurin signaling and the cardiac hypertrophic response and heart failure through the use of pharmacologic inhibitors (cyclosporine A and FK506) and genetic approaches. We will also discuss the manner in which calcineurin-NFAT signaling is negatively regulated in the heart through a diverse array of kinases and inhibitory proteins. Finally, we will discuss emerging theories as to the mechanisms whereby alterations in intracellular calcium handling might stimulate calcineurin within the context of a contractile cell continually experiencing calcium flux.

Netrins and neogenin promote myotube formation

Differentiation of skeletal myoblasts into multinucleated myotubes is a multistep process orchestrated by several families of transcription factors, including myogenic bHLH and NFAT proteins. The activities of these factors and formation of myotubes are regulated by signal transduction pathways, but few extracellular factors that might initiate such signals have been identified. One exception is a cell surface complex containing promyogenic Ig superfamily members (CDO and BOC) and cadherins. Netrins and their receptors are established regulators of axon guidance, but little is known of their function outside the nervous system. We report here that myoblasts express the secreted factor netrin-3 and its receptor, neogenin. These proteins stimulate myotube formation and enhance myogenic bHLH- and NFAT-dependent transcription. Furthermore, neogenin binds to CDO in a cis fashion, and myoblasts lacking CDO are defective in responding to recombinant netrin. It is proposed that netrin-3 and neogenin may promote myogenic differentiation by an autocrine mechanism as components of a higher order complex of several promyogenic cell surface proteins.

Co-operative interactions between NFAT (nuclear factor of activated T cells) c1 and the zinc finger transcription factors Sp1/Sp3 and Egr-1 regulate MT1-MMP (membrane type 1 matrix metalloproteinase) transcription by glomerular mesangial cells

The transition of normally quiescent glomerular MCs (mesangial cells) to a highly proliferative phenotype with characteristics of myofibroblasts is a process commonly observed in inflammatory diseases affecting the renal glomerulus, the ultimate result of which is glomerulosclerosis. Generation of proteolytically active MMP (matrix metalloproteinase)-2 by the membrane-associated membrane type 1 (MT1)-MMP is responsible for the transition of mesangial cells to the myofibroblast phenotype [Turck, Pollock, Lee, Marti and Lovett (1996) J. Biol. Chem. 271, 15074-15083]. In the present study, we show that the expression of MT1-MMP within the context of MCs is mediated by three discrete cis -acting elements: a proximal non-canonical Sp1 site that preferentially binds Sp1; an overlapping Sp1/Egr-1-binding site that preferentially binds Egr-1; and a more distal binding site for the NFAT (nuclear factor of activated T cells) that binds the NFAT c1 isoform present in MC nuclear extracts. Transfection with an NFAT c1 expression plasmid, or activation of calcineurin with a calcium ionophore, yielded major increases in NFAT c1 nuclear DNA-binding activity, MT1-MMP transcription and protein synthesis, which were additive with the lower levels of transactivation provided by the proximal Sp1 and the overlapping Sp1/Egr-1 sites. Specific binding of NFAT c1 to the MT1-MMP promoter was confirmed by chromatin immunoprecipitation studies, while MT1-MMP expression was suppressed by treatment with the calcineurin inhibitor, cyclosporin A. These studies are the first demonstration that a specific NFAT isoform enhances transcription of an MMP (MT1-MMP) that plays a major role in the proteolytic events that are a dominant feature of acute glomerular inflammation. Suppression of MT1-MMP by commonly used calcineurin inhibitors may play a role in the development of renal fibrosis following renal transplantation.

Structural delineation of the calcineurin-NFAT interaction and its parallels to PP1 targeting interactions

Calcineurin is a phosphoprotein phosphatase that channels intracellular Ca signals into multiple biological pathways. Calcineurin is known to interact directly with its substrate nuclear factor of activated T cells (NFAT or NFATc), with other substrates, and with several targeting and scaffold proteins including AKAP79 and Cabin1/cain. The calcineurin-NFAT interaction depends on recognition of a PxIxIT sequence motif present in NFAT-family proteins and in certain other calcineurin-interacting proteins. Here, we define the structural basis for the interaction of calcineurin with NFAT and with other proteins possessing the PxIxIT motif. The calcineurin-PxIxIT contact has a direct parallel in the contact of protein phosphatase 1 with its regulatory proteins, suggesting that the evolution of these related phosphatases involved local remodelling of an ancestral docking site.

Gene array identification of osteoclast genes: differential inhibition of osteoclastogenesis by cyclosporin A and granulocyte macrophage colony stimulating factor

Treatment of adherent peripheral blood mononuclear cells (PBMCs) with macrophage colony stimulating factor (M-CSF) and receptor activator of NF-kappaB ligand (RANKL) stimulates the formation of multinucleate osteoclast-like cells. Treatment with M-CSF alone results in the formation of macrophage-like cells. Through the use of Atlas human cDNA expression arrays, genes regulated by RANKL were identified. Genes include numerous cytokines and cytokine receptors (RANTES and CSF2R proportional, variant ), transcription factors (nuclear factor of activated T-cells cytoplasmic 1 (NFATc1) and GA binding protein transcription factor alpha (GABPalpha)), and ribosomal proteins (60S L17 and 40S S20). Real-time PCR analysis showed significant correlation (R2 of 0.98 P < 0.01) with array data for all genes tested. Time courses showed differential activation patterns of transcription factors with early induction of FUSE binding protein 1 (FBP) and c-Jun, and later steady upregulation of NFATc1 and GABP by RANKL. Treatment with cyclosporin A, a known NFATc1 inhibitor, resulted in a blockade of osteoclast formation. The mononuclear cells resulting from high cyclosporin treatment (1,000 ng/ml) were cathepsin K (CTSK) and tartrate-resistant acid phosphatase (TRAP) positive but expression of calcitonin receptor (CTR) was downregulated by more than 30-fold. Constant exposure of M-CSF- and RANKL-treated cells to GM-CSF resulted in inhibition of osteoclast formation and the downregulation of CTSK and TRAP implicating the upregulation of CSF2R in a possible feedback inhibition of osteoclastogenesis.

Polycystin-1 activates the calcineurin/NFAT (nuclear factor of activated T-cells) signaling pathway

Regulation of intracellular Ca(2+) mobilization has been associated with the functions of polycystin-1 (PC1) and polycystin-2 (PC2), the protein products of the PKD1 and PKD2 genes. We have now demonstrated that PC1 can activate the calcineurin/NFAT (nuclear factor of activated T-cells) signaling pathway through Galpha(q) -mediated activation of phospholipase C (PLC). Transient transfection of HEK293T cells with an NFAT promoter-luciferase reporter demonstrated that membrane-targeted PC1 constructs containing the membrane proximal region of the C-terminal tail, which includes the heterotrimeric G protein binding and activation domain, can stimulate NFAT luciferase activity. Inhibition of glycogen synthase kinase-3beta by LiCl treatment further increased PC1-mediated NFAT activity. PC1-mediated activation of NFAT was completely inhibited by the calcineurin inhibitor, cyclosporin A. Cotransfection of a construct expressing the Galpha(q) subunit augmented PC1-mediated NFAT activity, whereas the inhibitors of PLC (U73122) and the inositol trisphosphate and ryanodine receptors (xestospongin and 2-aminophenylborate) and a nonspecific Ca(2+) channel blocker (gadolinium) diminished PC1-mediated NFAT activity. PC2 was not able to activate NFAT. An NFAT-green fluorescent protein nuclear localization assay demonstrated that PC1 constructs containing the C-tail only or the entire 11-transmembrane spanning region plus C-tail induced NFAT-green fluorescent protein nuclear translocation. NFAT expression was demonstrated in the M-1 mouse cortical collecting duct cell line and in embryonic and adult mouse kidneys by reverse transcriptase-PCR and immunolocalization. These data suggest a model in which PC1 signaling leads to a sustained elevation of intracellular Ca(2+) mediated by PC1 activation of Galpha(q) followed by PLC activation, release of Ca(2+) from intracellular stores, and activation of store-operated Ca(2+) entry, thus activating calcineurin and NFAT.

Glucocorticoid treatment alleviates dystrophic myofiber pathology by activation of the calcineurin/NF-AT pathway

Duchenne muscular dystrophy (DMD) is a progressive and ultimately fatal skeletal muscle disease. Currently, the most effective therapy is the administration of a subclass of glucocorticoids, most notably deflazacort. Although deflazacort treatment can attenuate DMD progression, extend ambulation, and maintain muscle strength, the mechanism of its action remains unknown. Prior observations have shown that activation of a JNK1-mediated signal transduction cascade contributes to the progression of the DMD phenotype, in part by phosphorylation and inhibition of a calcineurin sensitive NF-ATc1 transcription factor. Here, we observed that deflazacort treatment restored myocyte viability in muscle cells with constitutive activation of JNK1 and in dystrophic mdx mice. However, deflazacort treatment did not alter JNK1 activity itself, but rather led to an increase in the activity of the calcineurin phosphatase and an up-regulation of NF-ATc1-dependent gene expression. The prophylactic effect of deflazacort treatment was associated with increased expression of NF-ATc1 target genes such as the dystrophin homologue utrophin. Moreover, the muscle sparing effects of deflazacort were completely abolished when used in conjunction with the calcineurin inhibitor cyclosporine. Collectively, these results show that deflazacort attenuates loss of dystrophic myofiber integrity by up-regulating the activity of the phosphatase calcineurin, which in turn negates JNK1 inhibition of NF-ATc1-mediated phosphorylation and nuclear exclusion of NF-ATc1.

Forming a multinucleated cell: molecules that regulate myoblast fusion

In mammals, cell fusion occurs among a limited number of cell types: sperm and oocytes during fertilization, trophoblasts during placenta formation, macrophages during giant cell and osteoclast formation and myoblasts in the formation of myofibers and myotubes. The molecular mechanisms involved in these membrane fusion events largely are unknown. This review will focus on the known molecules that regulate myoblast fusion with an emphasis on a novel signaling pathway involving the calcium-regulated transcription factor NFATC2 in the regulation of myoblast fusion.

Nuclear factor of activated T cells balances angiogenesis activation and inhibition

It has been demonstrated that vascular endothelial cell growth factor (VEGF) induction of angiogenesis requires activation of the nuclear factor of activated T cells (NFAT). We show that NFATc2 is also activated by basic fibroblast growth factor and blocked by the inhibitor of angiogenesis pigment epithelial–derived factor (PEDF). This suggests a pivotal role for this transcription factor as a convergence point between stimulatory and inhibitory signals in the regulation of angiogenesis.

We identified c-Jun NH₂-terminal kinases (JNKs) as essential upstream regulators of NFAT activity in angiogenesis. We distinguished JNK-2 as responsible for NFATc2 cytoplasmic retention by PEDF and JNK-1 and JNK-2 as mediators of PEDF-driven NFAT nuclear export.

We identified a novel NFAT target, caspase-8 inhibitor cellular Fas-associated death domain–like interleukin 1β–converting enzyme inhibitory protein (c-FLIP), whose expression was coregulated by VEGF and PEDF. Chromatin immunoprecipitation showed VEGF-dependent increase of NFATc2 binding to the c-FLIP promoter in vivo, which was attenuated by PEDF. We propose that one possible mechanism of concerted angiogenesis regulation by activators and inhibitors may be modulation of the endothelial cell apoptosis via c-FLIP controlled by NFAT and its upstream regulator JNK.

Expression and regulation of NFAT (nuclear factors of activated T cells) in human CD34+cells: down-regulation upon myeloid differentiation

The calcineurin-dependent, cyclosporin A (CsA)-sensitive transcription factor nuclear factor of activated T cells (NFAT) represents a group of proteins, which is well-characterized as a central regulatory element of cytokine expression in activated T cells. In contrast, little is known about the expression or function of NFAT family members in myeloid cells; moreover, it is unclear whether they are expressed by hematopoietic stem/progenitor cells. Here, we show that NFATc2 (NFAT1) is expressed at high levels in CD34+ cells and megakaryocytes but not in cells committed to the neutrophilic, monocytic, or erythroid lineages. Cytokine-induced in vitro differentiation of CD34+ cells into neutrophil granulocytes results in the rapid suppression of NFATc2 RNA and protein. NFATc2 dephosphorylation/rephosphorylation as well as nuclear/cytoplasmic translocation in CD34+ cells follow the same calcineurin-dependent pattern as in T lymphocytes, suggesting that NFATc2 activation in these cells is equally sensitive to inhibition with CsA. Finally, in vitro proliferation, but not differentiation, of CD34+ cells cultured in the presence of fms-like tyrosine kinase 3 ligand (FLT3L), stem cell factor, granulocyte macrophage-colony stimulating factor (GM-CSF), interleukin-3, and G-CSF is profoundly inhibited by treatment with CsA in a dose-dependent manner. These results suggest a novel and unexpected role for members of the NFAT transcription factor family in the hematopoietic system.

Selective inhibition of calcineurin-NFAT signaling by blocking protein-protein interaction with small organic molecules

Transient or reversible protein-protein interactions are commonly used to ensure efficient targeting of signaling enzymes to their cellular substrates. These interactions include direct binding to substrate, interaction with an accessory or scaffold protein, and positioning at subcellular locations in proximity to substrates. The existence of specialized targeting mechanisms raises the possibility of designing inhibitors that do not block enzyme activity per se, but rather interfere with targeting of the enzyme to one or more of its substrates within the cell. Here, we identify small organic molecules that specifically block targeting of the protein phosphatase calcineurin to its substrate nuclear factor of activated T cells (NFAT, also termed NFATc) and show that they are effective inhibitors of calcineurin-NFAT signaling.

Calcineurin mediates pancreatic growth in protease inhibitor-treated mice

CCK acts on pancreatic acinar cells to increase intracellular Ca(2+) leading to secretion of digestive enzymes and, in the long term, pancreatic growth. Calcineurin (CN) is a serine/threonine-specific protein phosphatase activated by Ca(2+) and calmodulin that recently has been shown to participate in the growth regulation of cardiac and skeletal myocytes. We therefore tested the effect of two different CN inhibitors, cyclosporine A (CsA) and FK506, on mouse pancreatic growth induced by oral administration of the synthetic protease inhibitor camostat, a known stimulator of endogenous CCK release. Mice were fed a powdered diet with or without 0.1% camostat. Pancreatic wet weight, protein, and DNA were increased in response to camostat in a time-dependent manner over 10 days in ICR mice but not in CCK-deficient mice. Both CsA (15 mg/kg) and FK506 (3 mg/kg) given twice daily blocked the increase in pancreatic wet weight and protein and DNA content induced by camostat. The increase in plasma CCK induced by camostat was not blocked by CsA or FK506. Camostat feeding also increased the relative amount of CN protein, whereas levels of MAPKs, ERKs, and p38 were not altered. In summary, 1) CCK released by chronic camostat feeding induces pancreatic growth in mice; 2) this growth is blocked by treatment with both CsA and FK506, indicating a role for CN; 3) CCK stimulation also increases CN protein. In conclusion, activation and possibly upregulation of CN may participate in regulation of pancreatic growth by CCK in mice.

Reciprocal regulation of the nuclear factor of activated T cells and HIV-1

The human immunodeficiency virus type 1 (HIV-1) has evolved to coordinate its replication with the activation state of the host CD4T cell. To this end, it taps into major host cell signaling pathways and their associated transcription factors. Of these, T-cell activation and the transcription factor NF-kappaB, respectively, have become the best-studied examples. The past several years have revealed compelling evidence that another transcription factor family involved in T-cell activation, the nuclear factor of activated T cells (NFAT), plays an important role in the regulation of HIV-1. Major advances have been made in our understanding of the interaction of HIV-1 with this intriguing transcription factor. The duplicated NF-kappaB binding sites in the HIV-1 enhancer surprisingly also bind NFAT proteins and appear to be the most important targets for NFAT transactivation of the HIV-1 long terminal repeat. The crystal structure of NFAT1 bound to one of these duplicated sites was solved recently. Interestingly, it showed that NFAT1 binds to this site as a homodimer and occupies the core of the NF-kappaB site, suggesting mutually exclusive binding and alternate transactivation by these two factors. NFAT also regulates HIV-1 infection indirectly, as it can relieve a block to reverse transcription in quiescent T cells. In turn, HIV-1, and particularly its Tat and Nef gene products, can upregulate NFAT expression and activity. This reciprocal regulation between virus and transcription factor potentially creates a positive feedback loop, which may facilitate the establishment of early HIV-1 infection and, later, the transition from latent to productive infection. The immunosuppressive drug cyclosporin A (CsA) inhibits NFAT activity and thus represents a potential treatment for HIV-1 infection. Recent small-scale clinical trials have yielded optimistic results, suggesting roles for CsA after organ transplantation in HIV-1+ individuals and as adjunct treatment in stable early HIV-1 infection.

The calcineurin/nuclear factor of activated T cells signaling pathway regulates osteoclastogenesis in RAW264.7 cells

Although best known for its role in T lymphocyte activation, the calcineurin/nuclear factor of activated T cells (NFAT) signaling pathway is also known to be involved in a wide range of other biological responses in a variety of different cell types. Here we have investigated the role of the calcineurin/NFAT signaling pathway in the regulation of osteoclast differentiation. Osteoclasts are bone-resorbing multinucleated cells that are derived from the monocyte/macrophage cell lineage after stimulation with a member of the tumor necrosis factor family of ligands known as receptor activator of nuclear factor-kappaB ligand (RANKL). We now report that inhibition of calcineurin with either the immunosuppressant drugs cyclosporin A and FK506, or the retrovirally mediated ectopic expression of a specific calcineurin inhibitory peptide, all potently inhibit the RANKL-induced differentiation of the RAW264.7 monocyte/macrophage cell line into mature multinucleated osteoclasts. In addition, we find that NFAT family members are expressed in RAW264.7 cells and that their expression is up-regulated in response to RANKL stimulation. Most importantly, we find that ectopic expression of a constitutively active, calcineurin-independent NFATc1 mutant in RAW264.7 cells is sufficient to induce these cells to express an osteoclast-specific pattern of gene expression and differentiate into morphologically distinct, multinucleated osteoclasts capable of inducing the resorption of a physiological mineralized matrix substrate. Taken together, these data define calcineurin as an essential downstream effector of the RANKL-induced signal transduction pathway leading toward the induction of osteoclast differentiation and furthermore, indicate that the activation of the NFATc1 transcription factor is sufficient to initiate a genetic program that results in the specification of the mature functional osteoclast cell phenotype.

Changes in apoptosis-related pathways in acute myelocytic leukemia

Expression analysis of apoptotic genes was performed for 15 patients with acute myelocytic leukemia (AML) at the time of diagnosis to identify genes and signaling pathways involved in the regulation of cell survival and apoptosis during leukemogenesis. cDNA array analysis revealed 34 genes whose expression was significantly different compared to others. Tumor suppressor genes TP53 and CDKN2A were downregulated and protooncogenes JUN and GRB10 were upregulated. Furthermore, several cellular signaling pathways acting either in cell cycle regulation or in apoptosis were altered. Deregulation was found in pathways that contribute to genomic stability (by downregulation of either TP53 or CSE1L and by upregulation of GADD45A) and regulate cell cycle progression (by downregulation of CDKN2A and upregulation of RBBP4, CDC37, and NEDD5). Alterations at the transcriptional level were identified, namely, upregulation of JUN and E2F5. Abnormalities were observed in the regulation of the caspases through upregulation of CASP8 and by altered expression of BCL2-related pathway. Extrinsic apoptotic signals mediated by IGFs were deregulated and the glutathione detoxification pathway was downregulated. These findings provide insight into the regulation of balance between apoptosis and cell proliferation signals, and suggest that these genes and pathways may have an important role in the pathogenesis of AML.

Common Signaling Pathways Link Activation of Murine PAR-1, LPA, and S1P Receptors to Proliferation of Astrocytes

Receptors for the serine protease thrombin and for lysophospholipids are coupled to G proteins and control a wide range of cellular functions, including mitogenesis. Activators of these receptors are present in blood, and can enter the brain during central nervous system (CNS) injury. Reactive astrogliosis, a prominent component of CNS injury with potentially harmful consequences, may involve proliferation of astrocytes. In this study, we have examined the expression and activation of protease activated receptors (PARs), lysophosphatidic acid (LPA) receptors, and sphingosine-1-phosphate (S1P) receptors on murine astrocytes. We show that activation of these three receptor classes can lead to astrogliosis in vivo and proliferation of astrocytes in vitro. Cultured murine cortical astrocytes express mRNA for multiple receptor subtypes of PAR (PAR-1-4), LPA (LPA-1-3) and S1P (S1P-1, -3, -4, and -5) receptors. Comparison of the intracellular signaling pathways of glial PAR-1, LPA, and S1P receptors indicates that each receptor class activates multiple downstream signaling pathways, including Gq/11-directed inositol lipid/Ca2+ signaling, Gi/o activation of mitogen-activated protein kinases (MAPK) (extracellular signal-regulated kinase 1/2 and stress activated protein kinase/c-jun N-terminal kinase, but not p38), and activation of Rho pathways. Furthermore, activation of these different receptor classes can differentially regulate two transcription factor pathways, serum response element and nuclear factor of activated T cells. Blockade of Gi/o signaling with pertussis toxin, MAPK activation with 1,4-diamino-2,3-dicyano-1,4-bis(2-aminophynyltio)butadiene (U0126), or Rho kinase signaling with R-(+)-trans-N-(4-pyridyl)-4-(1-aminoethyl)-cyclohexane carboxamide (Y27632) can markedly reduce the proliferative response of glial cells to PAR-1, LPA, or S1P receptor activation, suggesting that each of these pathways is important in coupling of receptor activation to glial proliferation.

The threshold pattern of calcineurin-dependent gene expression is altered by loss of the endogenous inhibitor calcipressin

Calcineurin links calcium signaling to transcriptional responses in the immune, nervous and cardiovascular systems. To determine the function of the calcipressins, a family of putative calcineurin inhibitors, we assessed the calcineurin-dependent process of T cell activation in mice engineered to lack the gene encoding calcipressin 1 (Csp1). Csp1 regulated calcineurin in vivo, and genes triggered in an immune response had unique transactivation thresholds for T cell receptor stimulation. In the absence of Csp1, the apparent transactivation thresholds for all these genes were shifted because of enhanced calcineurin activity. This unbridled calcineurin activity drove Fas ligand expression, which normally requires high T cell receptor stimulation and results in the premature death of T helper type 1 cells. Thus, calcipressins modulate the pattern of calcineurin-dependent transcription, and may influence calcineurin activity beyond calcium to integrate a broad array of signals into the cellular response.

Polymorphisms of genes of the cardiac calcineurin pathway and cardiac hypertrophy

The study investigated the role of genetic polymorphisms in four genes of the calcineurin pathway on cardiac hypertrophy and dilated cardiomyopathy. The cardiac calcineurin pathway has been suggested to play a role in the development of cardiac hypertrophy in response to a number of physiological and pathological stimuli. Calcineurin, a heterodimeric protein composed of a catalytic and a regulatory subunit, activates the nuclear factor NFATC4 which after translocation to the nucleus associates with the transcription factor GATA4 to activate several cardiac genes involved in hypertrophic response. We have screened the genes encoding the four major components of the heart calcineurin pathway in 95 individuals and identified 27 polymorphisms. These polymorphisms were investigated in 400 selected subjects obtained from a population-based study (LOVE) in relation to echocardiographic parameters. A Gly/Ala substitution at position 160 of the NFATC4 protein (G160A) was associated with left ventricular mass and wall thickness (P=0.02 and 0.006, respectively, GA+AA vs GG), the minor allele (Ala) being associated with lower mean values of these parameters. The other polymorphisms identified by the gene screen were not associated with cardiac phenotypes. For the G160A polymorphism in NFATC4, genotype frequencies were compared between patients with dilated cardiomyopathy and controls obtained from the CARDIGENE Study. Allele A carriers were less frequent in the patient than in the control group (P=0.04). Although the strength of the associations was rather weak, these observations raise the hypothesis that the G160A polymorphism of the NFATC4 gene plays a role in the development of human cardiac hypertrophy.

Altered patterns of gene expression distinguishing ascending aortic aneurysms from abdominal aortic aneurysms: complementary DNA expression profiling in the molecular characterization of aortic disease

OBJECTIVES

The purpose of this study was to profile altered patterns of gene expression that characterize degenerative ascending thoracic aortic aneurysms and to compare these patterns with those observed for infrarenal abdominal aortic aneurysms.

METHODS

Full-thickness aortic wall tissues were obtained during surgical repair of degenerative thoracic aortic aneurysms and infrarenal abdominal aortic aneurysms (n = 4 each), with normal thoracic and abdominal aortas from organ transplant donors used as control preparations. Radiolabeled complementary DNA was prepared for each specimen and hybridized to complementary DNA microarrays, and differential levels of gene expression between aneurysmal and normal aortic tissues at each site were assessed by parametric statistics.

RESULTS

Of 1185 genes examined, 112 (9.5%) were differentially expressed (P <.05) between thoracic aortic aneurysms and normal thoracic aorta, with 105 increased and 7 decreased. There were 104 genes (8.8%) differentially expressed between infrarenal abdominal aortic aneurysms and normal abdominal aorta (65 increased and 39 decreased). Quantitative increases in expression for 97 genes were unique to thoracic aortic aneurysms, whereas increases for 61 genes were unique to infrarenal abdominal aortic aneurysms. Although 8 gene products were significantly altered in both thoracic and infrarenal abdominal aortic aneurysms, these changes were directionally concordant for only 4 (matrix metalloproteinase 9/gelatinase B, v-yes-1 oncogene, mitogen-activated protein kinase 9, and intercellular adhesion molecule 1/CD54). Results for 9 genes were independently confirmed by quantitative reverse transcriptase-polymerase chain reaction.

CONCLUSIONS

Thoracic aortic aneurysms and infrarenal abdominal aortic aneurysms exhibit distinct patterns of gene expression relative to normal aorta from the same sites, with most alterations being unique to each disease. Degenerative aneurysms arising in different locations are thus characterized by a high degree of molecular heterogeneity, reflecting different pathophysiologic mechanisms.

NFATc3 and NFATc4 are required for cardiac development and mitochondrial function

Activation of the nuclear factor of activated T-cell (NFAT) family of transcription factors is associated with changes in gene expression and myocyte function in adult cardiac and skeletal muscle. However, the role of NFATs in normal embryonic heart development is not well characterized. In this report, the function of NFATc3 and NFATc4 in embryonic heart development was examined in mice with targeted disruption of both nfatc3 and nfatc4 genes. The nfatc3-/-nfatc4-/- mice demonstrate embryonic lethality after embryonic day 10.5 and have thin ventricles, pericardial effusion, and a reduction in ventricular myocyte proliferation. Cardiac mitochondria are swollen with abnormal cristae, indicative of metabolic failure, but hallmarks of apoptosis are not evident. Furthermore, enzymatic activity of complex II and IV of the respiratory chain and mitochondrial oxidative activity are reduced in nfatc3-/-nfatc4-/- cardiomyocytes. Cardiac-specific expression of constitutively active NFATc4 in nfatc3-/-nfatc4-/- embryos prolongs embryonic viability to embryonic day 12 and preserves ventricular myocyte proliferation, compact zone density, and trabecular formation. The rescued embryos also maintain cardiac mitochondrial ultrastructure and complex II enzyme activity. Together, these data support the hypothesis that loss of NFAT activity in the heart results in a deficiency in mitochondrial energy metabolism required for cardiac morphogenesis and function.

Requirement of transcription factor NFAT in developing atrial myocardium

Nuclear factor of activated T cell (NFAT) is a ubiquitous regulator involved in multiple biological processes. Here, we demonstrate that NFAT is temporally required in the developing atrial myocardium between embryonic day 14 and P0 (birth). Inhibition of NFAT activity by conditional expression of dominant-negative NFAT causes thinning of the atrial myocardium. The thin myocardium exhibits severe sarcomere disorganization and reduced expression of cardiac troponin-I (cTnI) and cardiac troponin-T (cTnT). Promoter analysis indicates that NFAT binds to and regulates transcription of the cTnI and the cTnT genes. Thus, regulation of cytoskeletal protein gene expression by NFAT may be important for the structural architecture of the developing atrial myocardium.

Atypical mechanisms regulate the PMA-induced expression of IFN-gamma in a porcine trophectoderm cell line

Interferon-gamma (IFN-gamma) is a major effector cytokine of the immune system with an expression pattern strictly restricted to cells of the lymphoid lineage. Several years ago, we reported that, during early pregnancy, the trophectoderm of the pig blastocyst, which represents a monolayer of polarized epithelial cells secretes high amount of IFN-gamma in a transient and developmentally regulated manner. In an effort to study the molecular basis of this atypical IFN-gamma gene expression, a pig trophectoderm cell line, TBA B4-3, was established in our laboratory. These cells developed a polarized phenotype with high transepithelial electrical resistance (TER) when grown on a microporous membrane. We found that treatment of polarized TBA B4-3 cells with the strong PKC agonist PMA induced, 3-4 days later, a transient IFN-gamma mRNA expression and vectorial IFN-gamma protein secretion. In order to better understand IFN-gamma gene regulation in TBA B4-3 cells, we examined in this system the effect of several drugs and factors known to affect the inducibility of this cytokine in T lymphocytes, the main source of IFN-gamma in the immunocompetent animal. We found that cyclosporine A (CsA) treatment of TBA B4-3 cells induces a partial inhibition of IFN-gamma secretion, thus indicating a minor role for the calcineurin signaling pathway in IFN-gamma expression. In addition, we found that although PMA alone can induce IFN-gamma secretion, the calcium ionophore A23187 synergizes with PMA for induction. We also analyzed by Southern blot the methylation status of a CpG dinucleotide in the 5' flanking region of IFN-gamma promoter and found that it was unmethylated in TBA B4-3 cells and in several pig epithelial cell lines that do not express IFN-gamma thus indicating the absence of correlation between demethylation and the ability to express IFN-gamma. Taken together, these results indicate that the mechanisms involved in IFN-gamma induction in TBA B4-3 cells are atypical compared to those presently known to operate in the T cell lineage.

A constitutively active NFATc1 mutant induces a transformed phenotype in 3T3-L1 fibroblasts

The calcineurin/nuclear factor of activated T cells (NFAT) signaling pathway is best known for its role in T lymphocyte activation. However, it has become increasingly apparent that this signaling pathway is also involved in the regulation of cell growth and development in a wide variety of different tissues and cell types. Here we have investigated the effects of sustained NFATc1 signaling on the growth and differentiation of the murine 3T3-L1 preadipocyte cell line. Remarkably, we find that expression of a constitutively active NFATc1 mutant (caNFATc1) in these immortalized cells inhibits their differentiation into mature adipocytes and causes them to adopt a transformed cell phenotype, including loss of contact-mediated growth inhibition, reduced serum growth requirements, protection from growth factor withdrawal-induced apoptosis, and formation of colonies in semisolid media. Furthermore, we find that caNFATc1-expressing cells acquire growth factor autonomy and are able to proliferate even in the complete absence of serum. We provide evidence that this growth factor independence is caused by the NFATc1-dependent production of a soluble heat-labile autocrine factor that is capable of promoting the growth and survival of wild type 3T3-L1 cells as well as potently inhibiting their differentiation into mature adipocytes. Finally, we demonstrate that cells expressing caNFATc1 form tumors in nude mice. Taken together, these results indicate that deregulated NFATc1 activity is able to induce the immortalized 3T3-L1 preadipocyte cell line to acquire the well established hallmarks of cellular transformation and thereby provide direct evidence for the oncogenic potential of the NFATc1 transcription factor.

Prostaglandin F2(alpha) stimulates growth of skeletal muscle cells via an NFATC2-dependent pathway

Skeletal muscle growth requires multiple steps to form large multinucleated muscle cells. Molecules that stimulate muscle growth may be therapeutic for muscle loss associated with aging, injury, or disease. However, few factors are known to increase muscle cell size. We demonstrate that prostaglandin F2alpha (PGF2alpha) as well as two analogues augment muscle cell size in vitro. This increased myotube size is not due to PGF2alpha-enhancing cell fusion that initially forms myotubes, but rather to PGF2alpha recruiting the fusion of cells with preexisting multinucleated cells. This growth is mediated through the PGF2alpha receptor (FP receptor). As the FP receptor can increase levels of intracellular calcium, the involvement of the calcium-regulated transcription factor nuclear factor of activated T cells (NFAT) in mediating PGF2alpha-enhanced cell growth was examined. We show that NFAT is activated by PGF2alpha, and the isoform NFATC2 is required for PGF2alpha-induced muscle cell growth and nuclear accretion, demonstrating the first intersection between prostaglandin receptor activation and NFAT signaling. Given this novel role for PGF2alpha in skeletal muscle cell growth, these studies raise caution that extended use of drugs that inhibit PG production, such as nonsteroidal antiinflammatory drugs, may be deleterious for muscle growth.