Deregulated NFATc1 activity transforms murine fibroblasts via an autocrine growth factor-mediated Stat3-dependent pathway

The nuclear factor of activated T cells (NFAT) family of transcription factors has recently been implicated with a role in tumorigenesis. Forced expression of a constitutively active NFATc1 mutant (caNFATc1) has been shown to transform immortalized murine fibroblasts in vitro, while constitutive activation of the NFAT-signaling pathway has been found in a number of human cancers, where it has been shown to contribute towards various aspects of the tumor phenotype. Here we have investigated the molecular mechanisms underlying the oncogenic potential of deregulated NFAT activity. We now show that ectopic expression of caNFATc1 in murine 3T3-L1 fibroblasts induces the secretion of an autocrine factor(s) that is sufficient to promote the transformed phenotype. We further demonstrate that this NFATc1-induced autocrine factor(s) specifically induces the tyrosine phosphorylation of the Stat3 transcription factor via a JAK kinase-dependent pathway. Interestingly, this effect of sustained NFAT signaling on the autocrine growth factor-mediated activation of Stat3 is not restricted to murine fibroblasts, but is also observed in the PANC-1 and MCF10A human cell lines. Most importantly, we find that the shRNA-mediated depletion of endogenous Stat3 significantly attenuates the ability of caNFATc1 to transform 3T3-L1 fibroblasts. Taken together, our results afford significant new insights into the molecular mechanisms underlying the oncogenic potential of deregulated NFATc1 activity by demonstrating that constitutive NFATc1 activity transforms cells via an autocrine factor-mediated pathway that is critically dependent upon the activity of the Stat3 transcription factor.

Prostaglandin F2alpha induces the normoxic activation of the hypoxia-inducible factor-1 transcription factor in differentiating 3T3-L1 preadipocytes: Potential role in the regulation of adipogenesis

Prostaglandin F2alpha (PGF2alpha) is a potent paracrine inhibitor of adipocyte differentiation. Here we show that treatment of differentiating 3T3-L1 preadipocytes with PGF2alpha induces the expression of DEC1, a transcriptional repressor that has previously been implicated in the inhibition of adipogenesis in response to hypoxia as a downstream effector of the hypoxia-inducible factor-1 (HIF-1) transcription factor. Surprisingly, despite performing our experiments under normal ambient oxygen conditions, we find that treatment of differentiating 3T3-L1 preadipocytes with PGF2alpha also results in the marked activation of HIF-1, as measured by an increase in the accumulation of the HIF-1alpha regulatory subunit. However, unlike the effects of hypoxia, this PGF2alpha-induced normoxic increase in HIF-1alpha is not mediated by an increase in the stability of the HIF-1alpha polypeptide, rather we find that PGF2alpha selectively increases the expression of the alternatively spliced HIF-1alpha I.1 mRNA isoform. Significantly, we demonstrate that the shRNA-mediated knockdown of endogenous HIF-1alpha expression attenuates the PGF2alpha-induced expression of DEC1, overcomes the inhibitory effects of PGF2alpha on the expression of proadipogenic transcription factors C/EBPalpha and PPARgamma and partially rescues the PGF2alpha-induced inhibition of adipogenesis. Taken together, these results indicate that PGF2alpha promotes the activation of the HIF-1 transcription factor pathway under normal oxygen conditions, and highlight a potential role for the normoxic activation of the HIF-1/DEC1-pathway in mediating the inhibitory effects of PGF2alpha on adipocyte differentiation.

Modulation of NFAT-dependent gene expression by the RhoA signaling pathway in T cells

We have reported previously that p115Rho guanine nucleotide exchange factor, its upstream activator Galpha13, and its effector RhoA are able to inhibit HIV-1 replication. Here, we show that RhoA is able to inhibit HIV-1 gene expression through the NFAT-binding site in the HIV long-terminal repeat. Constitutively active NFAT counteracts the inhibitory activity of RhoA, and inhibition of NFAT activation also inhibits HIV-1 gene expression. We have shown further that RhoA inhibits NFAT-dependent transcription and IL-2 production in human T cells. RhoA does not inhibit nuclear localization of NFAT but rather, inhibits its transcriptional activity. In addition, RhoA decreases the level of acetylated histone H3, but not NFAT occupancy, at the IL-2 promoter. These data suggest that activation of RhoA can modulate IL-2 gene expression by inhibiting the transcriptional activity of NFAT and chromatin structure at the IL-2 promoter during T cell activation.

Prostaglandin F2alpha inhibits adipocyte differentiation via a G alpha q-calcium-calcineurin-dependent signaling pathway

Prostaglandin F2alpha (PGF2alpha) is a potent physiological inhibitor of adipocyte differentiation, however the specific signaling pathways and molecular mechanisms involved in mediating its anti-adipogenic effects are not well understood. In the current study, we now provide evidence that PGF2alpha inhibits adipocyte differentiation via a signaling pathway that requires heterotrimeric G-protein G alpha q subunits, the elevation of the intracellular calcium concentration ([Ca2+]i), and the activation of the Ca2+/calmodulin-regulated serine/threonine phosphatase calcineurin. We show that while this pathway acts to inhibit an early step in the adipogenic cascade, it does not interfere with the initial mitotic clonal expansion phase of adipogenesis, nor does it affect either the expression, DNA binding activity or differentiation-induced phosphorylation of the early transcription factor C/EBPbeta. Instead, we find that PGF2alpha inhibits adipocyte differentiation via a calcineurin-dependent mechanism that acts to prevent the expression of the critical pro-adipogenic transcription factors PPARgamma and C/EBPalpha. Furthermore, we demonstrate that the inhibitory effects of PGF2alpha on both the expression of PPARgamma and C/EBPalpha and subsequent adipogenesis can be attenuated by treatment of preadipocytes with the histone deacetylase (HDAC) inhibitor trichostatin A. Taken together, these results indicate that PGF2alpha inhibits adipocyte differentiation via a G alpha q-Ca2+-calcineurin-dependent signaling pathway that acts to block expression of PPARgamma and C/EBPalpha by a mechanism that appears to involves an HDAC-sensitive step.

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.

Induction of FucT-VII by the Ras/MAP kinase cascade in Jurkat T cells

Induction of the alpha1,3-fucosyltransferase FucT-VII in T lymphocytes is crucial for selectin ligand formation, but the signaling and transcriptional pathways that govern FucT-VII expression are unknown. Here, using a novel, highly phorbol myristate acetate (PMA)-responsive variant of the Jurkat T-cell line, we identify Ras and downstream mitogen-activated protein (MAP) kinase pathways as essential mediators of FucT-VII gene expression. PMA induced FucT-VII in only a subset of treated cells, similar to expression of FucT-VII in normal activated CD4 T cells. Introduction of constitutively active Ras or Raf by recombinant retroviruses induced FucT-VII expression only in that subset of cells expressing the highest levels of Ras, suggesting that induction of FucT-VII required a critical threshhold of Ras signaling. Both PMA treatment and introduction of active Ras led to rolling on E-selectin. Pharmacologic inhibition studies confirmed the involvement of the classic Ras-Raf-MEK-extracellular signal-regulated kinase (Ras-Raf-MEK-ERK) pathway in FucT-VII induction by PMA, Ras, and Raf. These studies also revealed a second, Ras-induced, Raf-1-independent pathway that participated in induction of FucT-VII. Strong activation of Ras represents a major pathway for induction of FucT-VII gene expression in T cells.

Infection with Theiler's murine encephalomyelitis virus directly induces proinflammatory cytokines in primary astrocytes via NF-kappaB activation: potential role for the initiation of demyelinating disease

Theiler's virus infection in the central nervous system (CNS) induces a demyelinating disease very similar to human multiple sclerosis. We have assessed cytokine gene activation upon Theiler's murine encephalomyelitis virus (TMEV) infection and potential mechanisms in order to delineate the early events in viral infection that lead to immune-mediated demyelinating disease. Infection of SJL/J primary astrocyte cultures induces selective proinflammatory cytokine genes (interleukin-12p40 [IL-12p40], IL-1, IL-6, tumor necrosis factor alpha, and beta interferon [IFN-beta]) important in the innate immune response to infection. We find that TMEV-induced cytokine gene expression is mediated by the NF-kappaB pathway based on the early nuclear NF-kappaB translocation and suppression of cytokine activation in the presence of specific inhibitors of the NF-kappaB pathway. Further studies show this to be partly independent of dsRNA-dependent protein kinase (PKR) and IFN-alpha/beta pathways. Altogether, these results demonstrate that infection of astrocytes and other CNS-resident cells by TMEV provides the early NF-kappaB-mediated signals that directly activate various proinflammatory cytokine genes involved in the initiation and amplification of inflammatory responses in the CNS known to be critical for the development of immune-mediated demyelination.

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.

Calcineurin mediates the calcium-dependent inhibition of adipocyte differentiation in 3T3-L1 cells

Recent studies have revealed that the calcium-dependent serine/threonine phosphatase calcineurin mediates the effects of intracellular calcium in many different cell types. In this study we investigated the role of calcineurin in the regulation of adipocyte differentiation. We found that the specific calcineurin inhibitors cyclosporin A and FK506 overcame the antiadipogenic effect of calcium ionophore on the differentiation of 3T3-L1 preadipocytes. This finding suggests that calcineurin is responsible for mediating the previously documented Ca(2+)-dependent inhibition of adipogenesis. We further demonstrate that the expression of a constitutively active calcineurin mutant potently inhibits the ability of 3T3-L1 cells to undergo adipocyte differentiation by preventing expression of the proadipogenic transcription factors peroxisome proliferator-activated receptor gamma (PPARgamma) and CCAAT/enhancer-binding protein alpha (C/EBPalpha). This calcineurin-mediated block in adipocyte differentiation is rescued by ectopic expression of PPARgamma1. Finally, we demonstrate that inhibition of endogenous calcineurin activity with either FK506 or a specific calcineurin inhibitory peptide enhances differentiation of 3T3-L1 cells in response to suboptimal adipogenic stimuli, suggesting that endogenous calcineurin activity normally sets a signaling threshold that antagonizes efficient adipocyte differentiation. Collectively, these data indicate that calcineurin acts as a Ca(2+)-dependent molecular switch that negatively regulates commitment to adipocyte differentiation by preventing the expression of critical proadipogenic transcription factors.

Sustained NFAT signaling promotes a Th1-like pattern of gene expression in primary murine CD4+ T cells

T cell activation is known to be critically regulated by the extent and duration of TCR-induced signaling pathways. The NFAT family of transcription factors is believed to play an important role in coupling these quantitative differences in TCR-induced signaling events into changes in gene expression. In this study we have specifically investigated the effects of sustained NFAT signaling on T cell activation by introducing a constitutively active mutant version of NFATc1 (caNFATc1) into primary murine CD4(+) T cells and examining its effects on gene expression. We now report that ectopic expression of caNFATc1 partially mimics TCR signaling, resulting in enhanced expression of CD25 and CD40 ligand and down-regulation of CD62L. More importantly, we find that expression of caNFATc1 in T cells maintained under either nonpolarizing or Th1-skewing conditions leads to a marked selective increase in the number of cells expressing the prototypical Th1 cytokine, IFN-gamma. Furthermore, when expressed in Th2-skewed cells, caNFATc1 appears to attenuate Th2 differentiation by decreasing production of IL-4 and promoting the expression of IFN-gamma. Finally, we find that caNFATc1 enhances expression of functional P-selectin glycoprotein ligand-1, up-regulates Fas ligand expression, and increases susceptibility to activation-induced cell death, cellular traits that are preferentially associated with Th1 effector cells. Taken together, these results suggest that sustained NFAT signaling, mediated by ectopic expression of caNFATc1, acts to promote a Th1-like pattern of gene expression and thereby serves to highlight the important relationship between the degree of NFAT signaling and the qualitative pattern of gene expression induced during T cell activation.

Differential abilities of central nervous system resident endothelial cells and astrocytes to serve as inducible antigen-presenting cells

Microglial cells and astrocytes are capable of processing and presenting antigens for efficient activation of T cells. However, the antigen-presenting function and role of cerebrovascular endothelial cells (CVEs) in central nervous system inflammatory responses remain controversial. We compared the expression of necessary accessory molecules and the functional antigen-presenting capacity of cloned SJL/J CVEs and primary astrocytes in response to the pro-inflammatory cytokines interferon-gamma (IFN-gamma) and tumor necrosis factor-alpha (TNF-alpha). Astrocytes and CVEs up-regulated major histocompatibility complex (MHC) class II, and primarily B7-1 as opposed to B7-2, in response to IFN-gamma. TNF-alpha inhibited the IFN-gamma-induced up-regulation of MHC class II on CVEs correlating to a decrease in the mRNA for the class II transactivator (CIITA), whereas CIITA expression in astrocytes was unaffected. Unlike astrocytes, CVEs did not elicit significant MHC class II-restricted T-cell responses. Furthermore, we have found that CVE monolayers are altered following T-cell contact, implicating CVE/T-cell contact in the breakdown of the blood-brain barrier during neuro-inflammatory responses.

T cell activation signals upregulate CBP-dependent transcriptional activity

The transcriptional coactivator CREB-binding protein (CBP) is known to play an important role in coupling signal transduction pathways to changes in gene expression. In many cases, this is achieved by the stimulus-specific recruitment of CBP to promoter-bound transcription factors. However, a number of recent studies have suggested that signal transduction pathways can also directly influence CBP-mediated transcriptional activity. Here we show that in Jurkat cells the activity of the CBP C-terminal transactivation domain is strongly upregulated in response to either T cell receptor stimulation or the combination of ionomycin and phorbol ester. We further show that maximal stimulation of CBP-mediated transcription requires the synergistic activation of both the calcineurin and Ras-MAPK signaling pathways. These results indicate that CBP can function as a T cell activation-inducible transcriptional coactivator and is therefore likely to play an important role in T cell activation-induced gene expression.

Glycogen synthase kinase-3 inhibits the DNA binding activity of NFATc

The NFAT family of transcription factors is required for the expression of numerous immunologically important genes and plays a pivotal role in both the initiation and coordination of the immune response. NFAT family members appear to be regulated primarily at the level of their subcellular localization. Here we show that NFATc is additionally regulated at the level of its DNA binding activity. Using gel mobility shift assays, we demonstrate that the intrinsic DNA binding activity of NFATc is negatively regulated by phosphorylation. We found that activation of calcineurin activity in cells and dephosphorylation of NFATc in vitro enhanced NFATc DNA binding activity, whereas phosphorylation of NFATc in vitro inhibited its ability to bind DNA. Through the analysis of NFATc mutants, we identified the conserved Ser-Pro repeat motifs as critical quantitative determinants of NFATc DNA binding activity. In addition, we provide several lines of evidence to suggest that the phosphorylation of the Ser-Pro repeats by glycogen synthase kinase-3 inhibits the ability of NFATc to bind DNA. Taken together, these studies afford new insights into the regulation of NFATc and underscore the potential role of glycogen synthase kinase-3 in the regulation of NFAT-dependent gene expression.

Identification of amino acid residues and protein kinases involved in the regulation of NFATc subcellular localization

The subcellular localization of the transcription factor NFATc is tightly regulated by the calcium-regulated phosphatase calcineurin, which acts to directly dephosphorylate NFATc, causing its rapid translocation from the cytoplasm to the nucleus. The calcineurin-mediated nuclear localization of NFATc is opposed by poorly defined protein kinases that act either to directly antagonize nuclear import or, alternatively, to promote nuclear export. Here, we provide evidence that the cellular protein kinases JNK, ERK, p38, and CK2 (formerly casein kinase II) are involved in the regulation of NFATc subcellular localization. We show that JNK, ERK, and p38 physically associate with the NFATc N-terminal regulatory domain and can directly phosphorylate functionally important residues involved in regulating NFATc subcellular localization, namely Ser(172) and the conserved NFATc Ser-Pro repeats. Moreover, we found that overexpression of JNK, ERK, or p38 is able to block ionomycin-induced NFATc nuclear translocation, whereas treatment of cells with both PD98059 and SB202190, which inhibit MAPK/SAPK signaling pathways, is sufficient to trigger NFATc nuclear localization. Finally, we show that CK2 also binds the N terminus of NFATc and phosphorylates functionally important amino acid residues, including a conserved amino acid motif located downstream of each of the NFATc Ser-Pro repeats that appears to be important for regulating NFATc nuclear export. Collectively, these studies identify functionally important amino acid residues and protein kinases involved in the regulation of NFATc subcellular localization.

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.

Rapid shuttling of NF-AT in discrimination of Ca2+ signals and immunosuppression

Cells need to distinguish between transient Ca2+ signals that induce events such as muscle contraction, secretion, adhesion and synaptic transmission, and sustained Ca2+ signals that are involved in cell proliferation and differentiation. The latter class of events is blocked in lymphocytes by the immunosuppressive drugs cyclosporin A and FK506, which inhibit calcineurin, a Ca2+-activated serine/threonine phosphatase necessary for the nuclear import of NF-AT transcription factors. Here we report that sustained high concentrations of Ca2+, but not transient pulses, are required to maintain NF-AT transcription factors in the nucleus, where they participate in Ca2+-dependent induction of genes required for lymphocyte activation and proliferation. Furthermore, overexpression and constitutive nuclear localization of NF-AT, but not Jun, Fos, NF-kappaB, Oct or Ets family members, renders the interleukin-2 enhancer in Jurkat T lymphocytes resistant to FK506 and cyclosporin A. Thus a primary effect of these immunosuppressive reagents is to control the subcellular localization of the NF-AT family of transcription factors.

Isolation of mutant T lymphocytes with defects in capacitative calcium entry

Calcium and calcium-binding proteins play important roles in the signaling cascade leading from the initial engagement of TCRs on T cells to the fully activated state. To undertake a molecular dissection of this cascade, we first isolated a Jurkat T cell line derivative containing the NF-AT promoter element driving transcription of the diphtheria toxin A chain gene (dipA), resulting in rapid cell death. Selecting viable cells that fail to activate NF-AT-dependent transcription, we isolated two independent cell lines possessing defects in capacitative Ca2+ entry. NF-AT-dependent transcription can be restored in these cells by expression of a constitutively active calcineurin, but not overexpression of the Ca2+ regulatory protein CAML, which can normally replace the Ca2+ signal. The defect in these cell lines probably lies between CAML and calcineurin in the T cell activation cascade.

Molecular analysis of the interaction of calcineurin with drug-immunophilin complexes

The calcium/calmodulin-regulated phosphatase calcineurin (CN) is the site of action of the immunosuppressive drugs cyclosporin A (CsA) and FK506. CN has recently been established as a key signaling enzyme in the T cell signal transduction cascade and an important regulator of transcription factors such as NF-AT and OAP/Oct-1, which are involved in the expression of a number of important T cell early genes. CsA and FK506 act by forming complexes with their respective intracellular receptors cyclophilin and FKBP (immunophilins), which can then bind to CN, inhibiting its enzymatic activity and thereby preventing early gene expression. CN is comprised of two subunits: a 59-kDa catalytic subunit (CNA), which contains a calmodulin binding domain and autoinhibitory region, and a 19-kDa intrinsic calcium binding regulatory subunit (CNB). In this study, we have utilized a series of deletion mutants of the CNA subunit to investigate the subunit and molecular requirements that govern the interaction of CN with drug-immunophilin complexes. The calmodulin binding and autoinhibitory domains of the CNA subunit were found to be dispensable for the binding of CN to drug-immunophilin complexes. In contrast, we found that the regulatory CNB subunit appears to play an obligatory role in this interaction and have defined an amino acid sequence of the CNA subunit which forms the binding site for CNB. Although necessary, the CNB subunit per se is not sufficient to mediate an interaction with drug-immunophilin complexes; amino acid residues of the CNA subunit, specifically a region located within the putative catalytic domain, are also required for the interaction of CN with both FKBP-FK506 and cyclophilin A-CsA.

p21ras and calcineurin synergize to regulate the nuclear factor of activated T cells

In T lymphocytes, triggering of the T cell receptor (TCR) induces several signaling cascades which ultimately synergize to induce the activity of the nuclear factor of activated T cells (NFAT), a DNA binding complex critical to the inducibility and T cell specificity of the T cell growth factor interleukin 2. One immediate consequence of T cell activation via the TCR is an increase in cytosolic calcium. Calcium signals are important for NFAT induction, and recent studies have identified calcineurin, a calcium-calmodulin dependent serine-threonine phosphatase, as a prominent component of the calcium signaling pathway in T cells. A second important molecule in TCR signal transduction is the guanine nucleotide binding protein, p21ras, which is coupled to the TCR by a protein tyrosine kinase dependent mechanism. The experiments presented here show that expression by transfection of mutationally activated calcineurin or activated p21ras alone is insufficient for NFAT transactivation. However, coexpression of the activated calcineurin with activated p21ras could mimic TCR signals in NFAT induction. These data identify calcineurin and p21ras as cooperative partners in T cell activation.

Identification of calcineurin as a key signalling enzyme in T-lymphocyte activation

The immunosuppressive drugs cyclosporin A (CsA) and FK506 both interfere with a Ca(2+)-sensitive T-cell signal transduction pathway, thereby preventing the activation of specific transcription factors (such as NF-AT and NF-IL2A) involved in lymphokine gene expression. CsA and FK506 seem to act by interaction with their cognate intracellular receptors, cyclophilin and FKBP, respectively (see ref. 11 for review). The Ca2+/calmodulin-regulated phosphatase calcineurin is a major target of drug-isomerase complexes in vitro. We have therefore tested the hypothesis that this interaction is responsible for the in vivo effects of CsA/FK506. We report here that overexpression of calcineurin in Jurkat cells renders them more resistant to the effects of CsA and FK506 and augments both NFAT- and NFIL2A-dependent transcription. These results identify calcineurin as a key enzyme in the T-cell signal transduction cascade and provide biological evidence to support the notion that the interaction of drug-isomerase complexes with calcineurin underlies the molecular basis of CsA/FK506-mediated immunosuppression.

Signaling requirements for the expression of the transactivating factor NF-AT in human T lymphocytes

A protein called nuclear factor of activated T cells (NF-AT) binds to DNA sequences within the enhancer region of the interleukin 2 (IL 2) gene and appears necessary for both the inducibility and T cell specificity of IL 2 expression. IL 2 production is regulated by multiple signals including those generated via activation of the T cell antigen receptor complex (TcR/CD3), CD2 antigen, protein kinase C (PKC) or elevation of intracellular free calcium concentration ([Ca2+]i). We have, therefore, explored the role of these different stimuli in regulating the nuclear expression of NF-AT in human peripheral blood-derived lymphocytes. Results presented herein indicate that maximal expression of NF-AT in T cells requires at least two signals: PKC activation and TcR/CD3 or CD2 triggering, [Ca2+]i increases and TcR/CD3 or CD2 triggering. Data are presented that indicate that either the [Ca2+]i or PKC signal generated via the TcR/CD3 complex would not alone induce NF-AT expression, and that the TcR/CD3 complex probably regulates NF-AT expression because of its ability to regulate multiple intracellular signals in T cells, and not via any single biochemical event. The combination of CD2 mAb GT2/OKT11 used in the present study to trigger the CD2 antigen is able to act in synergy with phorbol 12,13-dibutyrate or ionomycin to induce NF-AT expression. However, these CD2 mAb do not elevate [Ca2+]i or activate PKC, suggesting that signals other than [Ca2+]i or PKC can regulate NF-AT expression in peripheral blood-derived T cells.

Stable expression of the cDNA encoding the human T lymphocyte-specific CD2 antigen in murine L cells

The human CD2 antigen is a T lymphocyte cell surface glycoprotein that has been implicated in both T lymphocyte adhesion and activation. The requirement for a T lymphocyte-specific factor(s) in the mechanism of signal transduction initiated via the CD2 antigen has been explored using murine L cells transfectants that stably express the human CD2 antigen at the cell surface. Such transfectants express the three CD2 epitopes previously defined on human T lymphocytes. However, combinations of CD2 monoclonal antibodies, that stimulated an increase in the cytoplasmic calcium concentration of the human T cell line Jurkat, failed to induce a similar signal in the transfectants. These results indicate that the transfected CD2 antigen cannot alone transduce intracellular signals in response to stimulatory combinations of CD2 monoclonal antibodies, and suggest that the functional CD2 antigen expressed on human T lymphocytes requires the association of another, as yet undefined factor(s).