PKC-theta is required for TCR-induced NF-kappaB activation in mature but not immature T lymphocytes

Human Notch-1 inhibits NF-kappa B activity in the nucleus through a direct interaction involving a novel domain

Notch participates in diverse cell fate decisions throughout embryonic development and postnatal life. Members of the NF-kappaB/Rel family of transcription factors are involved in the regulation of a variety of genes important for immune function. The biological activity of the NF-kappaB transcription factors is controlled by IkappaB proteins. Our previous work demonstrated that an intracellular, constitutively active form of human Notch-1/translocation-associated Notch homologue-1 (Notch(IC)) functions as an IkappaB molecule with specificity for the NF-kappaB p50 subunit and physically interacts with NF-kappaB in T cells. In the current study, we investigated the roles of different domains of Notch(IC) in the regulation of NF-kappaB-directed gene expression and NF-kappaB DNA binding activity. We found that Notch(IC) localizes to the nucleus and that a region in the N-terminal portion of Notch(IC), not the six ankyrin repeats, is responsible for the inhibitory effects of Notch on NF-kappaB-directed gene expression and NF-kappaB DNA binding activity. The N-terminal portion of Notch(IC) inhibited p50 DNA binding and interacted specifically with p50 subunit, not p65 of NF-kappaB. The interaction between Notch and NF-kappaB indicates that in addition to its role in the development of the immune system, Notch-1 may also have critical functions in the immune response, inflammation, viral infection, and apoptosis through control of NF-kappaB-mediated gene expression.

Antigen-induced translocation of PKC-theta to membrane rafts is required for T cell activation

Protein kinase C-theta (PKC-theta) is essential for mature T cell activation; however, the mechanism by which it is recruited to the TCR signaling machinery is unknown. Here we show that T cell stimulation by antibodies or peptide-major histocompatibility complex (MHC) induces translocation of PKC-theta to membrane lipid rafts, which localize to the immunological synapse. Raft translocation was mediated by the PKC-theta regulatory domain and required Lck but not ZAP-70. In addition, PKC-theta was associated with Lck in the rafts. An isolated PKC-straight theta catalytic fragment did not partition into rafts or activate the transcription factor NF-kappa B, although addition of a Lck-derived raft-localization sequence restored these functions. Thus, physiological T cell activation translocates PKC-theta to rafts, which localize to the T cell synapse; this PKC-theta translocation is important for its function.

Cyclic AMP-independent activation of protein kinase A by vasoactive peptides

Protein kinase A (PKA) is an important effector enzyme commonly activated by cAMP. The present study focuses on our finding that the vasoactive peptide endothelin-1 (ET1), whose signaling is not coupled to cAMP production, stimulates PKA in two independent cellular models. Using an in vivo assay for PKA activity, we found that ET1 stimulated PKA in HeLa cells overexpressing ET1 receptors and in aortic smooth muscle cells expressing endogenous levels of ET1 receptors. In these cell models, ET1 did not stimulate cAMP production, indicating a novel mechanism for PKA activation. The ET1-induced activation of PKA was found to be dependent on the degradation of inhibitor of kappaB, which was previously reported to bind and inhibit PKA. ET1 potently stimulated the nuclear factor-kappaB pathway, and this effect was inhibited by overexpression of the inhibitor of kappaB dominant negative mutant (IkappaBalpham) and by treatment with the proteasome inhibitor MG-132. Importantly, IkappaBalpham and MG-132 had similar inhibitory effects on ET1-induced activation of PKA without affecting G(s)-mediated activation of PKA or ET1-induced phosphorylation of mitogen-activated protein kinase. Finally, another vasoactive peptide, angiotensin II, also stimulated PKA in a cAMP-independent manner in aortic smooth muscle cells. These findings suggest that cAMP-independent activation of PKA might be a general response to vasoactive peptides.

NF-kappa B activation in tumor necrosis factor alpha-stimulated neutrophils is mediated by protein kinase Cdelta. Correlation to nuclear Ikappa Balpha

The transcription factor NF-kappaB is critical for the expression of multiple genes involved in inflammatory responses and apoptosis. However, the signal transduction pathways regulating NF-kappaB activation in human neutrophils in response to stimulation with tumor necrosis factor-alpha (TNFalpha) are undefined. Since recent studies implicated activation of NF-kappaB as well as protein kinase C-delta (PKCdelta) in neutrophil apoptosis, we investigated involvement of PKCdelta in the activation of NF-kappaB in TNFalpha-stimulated neutrophils. Specific inhibition of PKCdelta by rottlerin prevented IkappaBalpha degradation and NF-kappaB activation in TNFalpha-stimulated neutrophils. This regulation of NF-kappaB activation by PKCdelta was specific only for TNFalpha signaling, since lipopolysaccharide- or interleukin-1beta-induced NF-kappaB activation and IkappaBalpha degradation were not inhibited by rottlerin. In addition, we show that in human neutrophils, but not monocytes, IkappaBalpha localizes in significant amounts in the nucleus of unstimulated cells, and the amount of IkappaBalpha in the nucleus, as well as in the cytoplasm, correlates with the NF-kappaB DNA binding. These results suggest that in human neutrophils, the presence of IkappaBalpha in the nucleus may function as a safeguard against initiation of NF-kappaB dependent transcription of pro-inflammatory and anti-apoptotic genes, and represents a distinct and novel mechanism of NF-kappaB regulation.

Phosphoinositide 3-kinases in T lymphocyte activation

Biochemical experiments have established that the metabolism of inositol phospholipids by phosphoinositide 3-kinases (PI3Ks) and lipid-phosphatases is triggered by many receptors that control T lymphocyte function, including antigen-receptors, costimulatory molecules, cytokines and chemokines. Novel effectors of PI3K have been identified in the immune system and shown to be important in the control of lymphocyte activation. Moreover, key lipid-phosphatases have been identified that act to terminate or modulate PI3K signalling in cells of the immune system.

Protein kinase C-theta mediates a selective T cell survival signal via phosphorylation of BAD

Protein kinase C (PKC)-activating phorbol esters protect T cells from Fas-induced apoptosis. However, the mechanism of this protective effect and the identity of the relevant PKC isoform(s) are poorly understood. Here, we show that PKCtheta plays a selective and important role in this protection. Fas triggering led to a selective caspase-3-dependent cleavage of the enzyme and proteasome-mediated degradation and inactivation of its catalytic fragment. These events preceded the onset of apoptosis. Pharmacological inhibition of PKCtheta promoted Fas-mediated apoptosis in three different types of T cells. Conversely, constitutively active PKCtheta (and, to a lesser degree, PKCepsilon) selectively protected T cells from Fas-induced apoptosis. We provide evidence that the distant Bcl-2 family member, BAD, is a PKCtheta substrate, is phosphorylated by TCR stimulation, and can mediate at least in part the anti-apoptotic effect of PKCtheta.

Vav-Rac1-mediated activation of the c-Jun N-terminal kinase/c-Jun/AP-1 pathway plays a major role in stimulation of the distal NFAT site in the interleukin-2 gene promoter

Vav, a hematopoiesis-specific signaling protein, plays an important role in T-cell development and activation. Vav upregulates the expression of the interleukin-2 (IL-2) gene, primarily via activation of the distal NFAT site in the IL-2 gene promoter (NFAT-IL-2). However, since this site cooperatively binds NFAT and AP-1, the relative contribution of Vav to NFAT versus AP-1 activation has not been determined. Here, we studied the respective roles of the AP-1 and NFAT pathways in the T-cell receptor (TCR)-mediated, Vav-dependent activation of NFAT-IL-2. Although Vav stimulated the transcriptional activity of an NFAT-IL-2 reporter gene, it failed to stimulate the transcriptional or DNA-binding activities of an AP-1-independent NFAT site derived from the human gamma interferon gene promoter. Vav also did not stimulate detectable Ca(2+) mobilization and nuclear translocation of NFATc or NFATp. On the other hand, Vav induced the activation of Rac1 or Cdc42 and c-Jun N-terminal kinase (JNK), enhanced the transcriptional and DNA-binding activities of AP-1, and induced increased phosphorylation of c-Jun. Dominant-negative Vav and/or Rac1 mutants blocked the TCR-mediated stimulation of these events, demonstrating the physiological relevance of these effects. Vav also associated with Rac1 or Cdc42 in T cells, and anti-CD3 antibody stimulation enhanced this association. These findings indicate that a Rac1-dependent JNK/c-Jun/AP-1 pathway, rather than the Ca(2+)/NFAT pathway, plays the predominant role in NFAT-IL-2 activation by Vav.

ZAP-70 and SLP-76 regulate protein kinase C-theta and NF-kappa B activation in response to engagement of CD3 and CD28

The transcription factor NF-kappaB is a critical regulator of T cell function that becomes strongly activated in response to coengagement of TCR and CD28. Although events immediately proximal to NF-kappaB activation are well understood, uncertainty remains over which upstream signaling pathways engaged by TCR and CD28 lead to NF-kappaB activation. By using Jurkat T cell lines that are deficient or replete for either the protein tyrosine kinase ZAP-70 or the cytosolic adapter molecule SLP-76, the role of these proteins in modulating NF-kappaB activation was examined. NF-kappaB was not activated in response to coengagement of TCR and CD28 in either the ZAP-70- or SLP-76-negative cells, whereas stimuli that bypass these receptors (PMA plus A23187, or TNF-alpha) activated NF-kappaB normally. Protein kinase C (PKC) theta activation, which is required for NF-kappaB activation, also was defective in these cells. Reexpression of ZAP-70 restored PKCtheta and NF-kappaB activation in response to TCR and CD28 coengagement. p95(vav) (Vav)-1 tyrosine phosphorylation was largely unperturbed in the ZAP-70-negative cells; however, receptor-stimulated SLP-76/Vav-1 coassociation was greatly reduced. Wild-type SLP-76 fully restored PKCtheta and NF-kappaB activation in the SLP-76-negative cells, whereas 3YF-SLP-76, which lacks the sites of tyrosine phosphorylation required for Vav-1 binding, only partially rescued signaling. These data illustrate the importance of the ZAP-70/SLP-76 signaling pathway in CD3/CD28-stimulated activation of PKC theta and NF-kappaB, and suggest that Vav-1 association with SLP-76 may be important in this pathway.

Partners in transcription: NFAT and AP-1

Combinatorial regulation is a powerful mechanism that enables tight control of gene expression, via integration of multiple signaling pathways that induce different transcription factors required for enhanceosome assembly. The four calcium-regulated transcription factors of the NFAT family act synergistically with AP-1 (Fos/Jun) proteins on composite DNA elements which contain adjacent NFAT and AP-1 binding sites, where they form highly stable ternary complexes to regulate the expression of diverse inducible genes. Concomitant induction of NFAT and AP-1 requires concerted activation of two different signaling pathways: calcium/calcineurin, which promotes NFAT dephosphorylation, nuclear translocation and activation; and protein kinase C (PKC)/Ras, which promotes the synthesis, phosphorylation and activation of members of the Fos and Jun families of transcription factors. A fifth member of the NFAT family, NFAT5, controls the cellular response to osmotic stress, by a mechanism that requires dimer formation and is independent of calcineurin or of interaction with AP-1. Pharmacological interference with theNFAT:AP-1 interaction may be useful in selective manipulation of the immune response. Balanced activation of NFAT and AP-1 is known to be required for productive immune responses, but the role of NFAT:AP-1 interactions in other cell types and biological processes remains to be understood.

Transcriptional regulation in lymphocytes

Lymphocytes have been used to investigate many cellular processes, including lineage commitment, differentiation, proliferation and apoptosis. The transcription factors that mediate these processes are often expressed broadly in many cell types. The emerging theme is one of cell-type-specific regulation, affecting not only the functional activation of transcription factors but also their access to appropriate regions of DNA.

Molecular determinants of TCR expression and selection

The process of T cell development in the thymus is tightly regulated, being dependent on the integration of signals required for thymocyte maturation and survival. Rearrangements, expression and signaling of TCR genes play an indispensable role in this developmental program. Recent advances have provided insights into the molecular mechanisms that regulate TCR repertoire formation at the level of alphabeta versus gammadelta T cell fate and CD4(+) versus CD8(+) lineage determination.

Membrane lipid microdomains and the role of PKCtheta in T cell activation

Productive T cell activation depends on the assembly of a highly ordered and compartmentalized immunological synapse or supramolecular activation complex (SMAC). Reorganization of the actin cytoskeleton and clustering of specialized membrane microdomains, or lipid rafts, occur early following TCR/CD3 and costimulatory receptor ligation. Many key signaling molecules localize in lipid raft patches during T cell activation. Lipid raft reorganization is required for T cell activation, where it plays an apparently important role in stabilizing the T cell synapse. Here we review recent evidence supporting the role of lipid rafts in T cell activation. Particular emphasis is placed on the coupling of protein kinase C-theta(PKCtheta), which is selectively expressed in T cells and is known to function as an essential signal for T cell activation, and lipid rafts.

IKKbeta is essential for protecting T cells from TNFalpha-induced apoptosis

Transcription factor NF-kappaB, whose activation depends on the IKKbeta catalytic subunit of the IkappaB kinase, was assigned with both anti- and proapoptotic functions in T lymphocytes. To critically evaluate these functions, we transferred Ikkbeta-/- or wild-type (wt) fetal liver (FL) stem cells into lethally irradiated mice. Ikkbeta-/- radiation chimeras show thymic rudiments, aberrant lymphoid organs, and absence of T cells. T lymphopoiesis is rescued when Ikkbeta-/- stem cells are cotransferred with wt bone marrow, suggesting that IKKbeta may mediate its lymphopoietic function via extrinsic factors. However, almost normal development of Ikkbeta-/- T cells is observed upon removal of type 1 TNFalpha receptor, indicating that TNFalpha signaling accounts for the absence of Ikkbeta-/- T cells. Indeed, Ikkbeta-/- radiation chimeras exibit elevated circulating TNFalpha, and Ikkbeta-/- thymocytes display increased TNFalpha sensitivity.

Allelic exclusion and differentiation by protein kinase C-mediated signals in immature thymocytes

Pre-T cell receptor (preTCR)-derived signals mediate the transition of thymocytes from the CD4(-) CD8(-) double-negative (DN) to CD4(+) CD8(+) double-positive stage of T lymphocyte development. This progression, termed beta-selection, is limited to thymocytes that have generated a functional TCR-beta chain able to associate with pTalpha to form the preTCR complex. Formation of the preTCR complex not only induces differentiation, survival, and proliferation of DN thymocytes; it also inhibits further TCR-beta gene rearrangement through an ill-defined process known as allelic exclusion. The signaling pathways controlling this critical developmental checkpoint have not been characterized. Here we demonstrate that formation of the preTCR complex leads to the activation of protein kinase C (PKC), and that activation of PKC is necessary for the differentiation and expansion of DN thymocytes. Importantly, we also show that allelic exclusion at the TCR-beta gene loci is enforced by PKC-mediated signals. These results define PKC as a central mediator of both differentiation and allelic exclusion during thymocyte development.

Allelic exclusion and differentiation by protein kinase C-mediated signals in immature thymocytes

Pre-T cell receptor (preTCR)-derived signals mediate the transition of thymocytes from the CD4(-) CD8(-) double-negative (DN) to CD4(+) CD8(+) double-positive stage of T lymphocyte development. This progression, termed beta-selection, is limited to thymocytes that have generated a functional TCR-beta chain able to associate with pTalpha to form the preTCR complex. Formation of the preTCR complex not only induces differentiation, survival, and proliferation of DN thymocytes; it also inhibits further TCR-beta gene rearrangement through an ill-defined process known as allelic exclusion. The signaling pathways controlling this critical developmental checkpoint have not been characterized. Here we demonstrate that formation of the preTCR complex leads to the activation of protein kinase C (PKC), and that activation of PKC is necessary for the differentiation and expansion of DN thymocytes. Importantly, we also show that allelic exclusion at the TCR-beta gene loci is enforced by PKC-mediated signals. These results define PKC as a central mediator of both differentiation and allelic exclusion during thymocyte development.

Bcl10 is a positive regulator of antigen receptor-induced activation of NF-kappaB and neural tube closure

Bcl10, a CARD-containing protein identified from the t(1;14)(p22;q32) breakpoint in MALT lymphomas, has been shown to induce apoptosis and activate NF-kappaB in vitro. We show that one-third of bcl10-/- embryos developed exencephaly, leading to embryonic lethality. Surprisingly, bcl10-/- cells retained susceptibility to various apoptotic stimuli in vivo and in vitro. However, surviving bcl10-/- mice were severely immunodeficient and bcl10-/- lymphocytes are defective in antigen receptor or PMA/Ionomycin-induced activation. Early tyrosine phosphorylation, MAPK and AP-1 activation, and Ca2+ signaling were normal in mutant lymphocytes, but antigen receptor-induced NF-kappaB activation was absent. Thus, Bcl10 functions as a positive regulator of lymphocyte proliferation that specifically connects antigen receptor signaling in B and T cells to NF-kappaB activation.

p21-activated kinase 1 activates the nuclear factor kappa B (NF-kappa B)-inducing kinase-Ikappa B kinases NF-kappa B pathway and proinflammatory cytokines in Helicobacter pylori infection

Helicobacter pylori, the causative agent of several human gastric diseases, induces activation of the immediate early response transcription factor nuclear factor kappaB (NF-kappaB), which subsequently triggers release of proinflammatory cytokines in colonized epithelial cells. Here we report that in H. pylori infection p21-activated kinase 1 (PAK1) activates NF-kappaB. Activated PAK1 associates with NF-kappaB-inducing kinase, which upon activation directs the activity of IkappaB kinases to IkappaBalpha. Our results indicate that in epithelial cells PAK1 participates in a unique pathway that links H. pylori-dependent effector molecules to the activation of NF-kappaB and the induction of the innate immune response.

The physical association of protein kinase C theta with a lipid raft-associated inhibitor of kappa B factor kinase (IKK) complex plays a role in the activation of the NF-kappa B cascade by TCR and CD28

We investigated the role of protein kinase C theta (PKCtheta) in the activation of the NF-kappaB cascade in primary human CD4(+) lymphocytes. Among six or so PKC isoforms expressed in T cells, only PKCtheta participates in the assembly of the supramolecular activation clusters at the contact site of the TCR with Ag. Signaling via both the TCR and CD28 is required for optimal activation of the multisubunit IkappaB kinase (IKK) complex in primary human T lymphocytes; this activation could be inhibited by a Ca(2+)-independent PKC isoform inhibitor, rottlerin. Moreover, endogenous PKCtheta physically associates with activated IKK complexes in CD3/CD28-costimulated primary CD4(+) T cells. The same set of stimuli also induced relocation of endogenous PKCtheta and IKKs to a GM1 ganglioside-enriched, detergent-insoluble membrane compartment in primary T cells. IKKs recruited to these lipid rafts were capable of phosphorylating a recombinant IkappaBalpha sustrate. Confocal microscopy further demonstrated that exogenously expressed PKCtheta and IKKss colocalize in the membrane of CD3/CD28-costimulated Jurkat T cells. Constitutively active but not kinase-inactive PKCtheta activated IKKbeta in Jurkat T cells. Expression of dominant-active PKCtheta also had stimulatory effects on the CD28 response element of the IL-2 promoter. Taken together, these data show that the activation of PKCtheta by the TCR and CD28 plays an important role in the assembly and activation of IKK complexes in the T cell membrane.

Evidence for protein kinase C-dependent and -independent activation of mitogen-activated protein kinase in T cells: potential role of additional diacylglycerol binding proteins

Activation of mitogen-activated protein kinases (MAPK) is a critical signal transduction event for CTL activation, but the signaling mechanisms responsible are not fully characterized. Protein kinase C (PKC) is thought to contribute to MAPK activation following TCR stimulation. We have found that dependence on PKC varies with the method used to stimulate the T cells. Extracellular signal-regulated kinase (ERK) activation in CTL stimulated with soluble cross-linked anti-CD3 is completely inhibited by the PKC inhibitor bisindolylmaleimide (BIM). In contrast, only the later time points in the course of ERK activation are sensitive to BIM when CTL are stimulated with immobilized anti-CD3, a condition that stimulates CTL degranulation. Surprisingly, MAPK activation in response to immobilized anti-CD3 is strongly inhibited at all time points by the diacylglycerol (DAG)-binding domain inhibitor calphostin C implicating the contribution of a DAG-dependent but PKC-independent pathway in the activation of ERK in CTL clones. Chronic exposure to phorbol ester down-regulates the expression of DAG-responsive PKC isoforms; however, this treatment of CTL clones does not inhibit anti-CD3-induced activation of MAPK. Phorbol ester-treated cells have reduced expression of several isoforms of PKC but still express the recently described DAG-binding Ras guanylnucleotide-releasing protein. These results indicate that the late phase of MAPK activation in CTL clones in response to immobilized anti-CD3 stimulation requires PKC while the early phase requires a DAG-dependent, BIM-resistant component.

Branching out to gain control: how the pre-TCR is linked to multiple functions

How is signaling specificity achieved by the pre-TCR during selection of T-cell fate? Like the TCR, this receptor controls many functions, and recent studies define which pathways couple the pre-TCR to the molecular events controlling survival, proliferation, allelic exclusion at the TCRbeta locus, and further differentiation.

T cell expressed PKCtheta demonstrates cell-type selective function

T lymphocyte stimulation leading to interleukin-2 (IL-2) expression requires activation of protein kinase C (PKC); however, the relevant PKC isoform(s) have not yet been systematically defined. Here we examine seven major T cell expressed PKC isoforms (PKCalpha, delta, epsilon, zeta, nu, theta and iota) and identify PKCtheta to be essential for IL-2 expression (via the critical NF-AT and NF-kappaB enhancer) in Jurkat T cells. Employing a conditionally activated PKCtheta estrogen-receptor fusion mutant, a de novo synthesis-independent transactivation of JNK2 was established. Based on mRNA in situ hybridization to mouse whole body sections, PKCtheta was found to be highly expressed in lymphoid organs but also skeletal muscle and the nervous system. PKCtheta function appears to be cell-type specific, since its isoenzyme-selective function was not observed in ectopic expression studies, employing COS-1 or NIH3T3 cells. These results confirm PKCtheta to be the prime target for the activating effect of phorbol ester in T cell signaling and suggest that gene expression as well as gene function of PKCtheta is strictly controlled by the cell type.

A critical role for p59(fyn) in CD2-based signal transduction

TCR- but not CD2-triggered IL-2 production is p56(lck) dependent. To test the hypothesis that p59(fyn), a second src-family protein tyrosine kinase (PTK) expressed in T lymphocytes, might be an essential upstream component of the CD2 signaling pathway, we generated human (h) CD2 transgenic (tg) fyn(+/+) and fyn(-/-) mice. Clustering of hCD2 molecules on resting peripheral T lymphocytes results in Ca(2+) mobilization, activation of MAPK and cellular proliferation. In contrast, in the absence of p59(fyn), these CD2-initiated activities are markedly reduced, while TCR-triggered proliferation is unaffected. Several CD2 pathway components regulated by p59(fyn) have been identified including phospholipase C-gamma1 (PLC-gamma1), Vav, protein kinase C-theta isoform (PKC-theta), docking protein (Dok), focal adhesion kinase (FAK) and Pyk2. Decreased inducible PKC-theta catalytic activity and Vav phosphorylation likely account for diminished p38 and JNK activation in hCD2tg fyn(-/-) mice. Moreover, deficiency in fyn-dependent PLC-gamma1 catalytic activity may contribute to reduced PKC-alpha-dependent ERK activation. Of note, CD2-dependent Dok but not linker from activated T cells (LAT) tyrosine phosphorylation requires p59(fyn). Furthermore, that FAK and Pyk2 are target substrates implies that p59(fyn) may be an important regulator of T cell adhesion as well. Collectively, these data identify p59(fyn) as a key PTK in CD2-mediated activation of mature T lymphocytes.

The lack of NF-kappa B transactivation and PKC epsilon expression in CD4(+)CD8(+) thymocytes correlates with negative selection

Deletion of autoreactive thymocytes at the DP stage is the basis for tolerance to thymus-expressed self antigens. In this study we investigated whether distinct signalling pathways are induced in DP thymocytes as compared to mature T cells upon stimulation with antigen. Using triple transgenic mice expressing a TCR transgene, dominant negative ras/Mek proteins and a reporter gene construct with AP-1 or NF-kappa B binding sites, we showed a complete lack of transcriptional activity of NF-kappa B but not AP-1 in DP thymocytes, whereas both were transcriptionally active in mature T cells after antigenic stimulation. Lack of NF-kappa B induction correlated with increased death in response to antigen. AP-1 induction was dependent on the integrity of the ras/Mek pathway indicating that this pathway was activated in the DP thymocytes. In contrast, we found a complete lack of constitutive expression of the epsilon isoform of Protein Kinase C (PKC) in DP thymocytes, although it was present in mature thymocytes and peripheral T cells. Taken together the results suggest that the lack of PKC epsilon in DP thymocytes could lead to the absence of NF-kappa B activity after antigenic stimulation contributing to negative selection. Cell Death and Differentiation (2000) 7, 1253 - 1262.

Up-regulation of the IKCa1 potassium channel during T-cell activation. Molecular mechanism and functional consequences

We used whole cell recording to evaluate functional expression of the intermediate conductance Ca(2+)-activated K(+) channel, IKCa1, in response to various mitogenic stimuli. One to two days following engagement of T-cell receptors to trigger both PKC- and Ca(2+)-dependent events, IKCa1 expression increased from an average of 8 to 300-800 channels/cell. Selective stimulation of the PKC pathway resulted in equivalent up-regulation, whereas a calcium ionophore was relatively ineffective. Enhancement in IKCa1 mRNA levels paralleled the increased channel number. The genomic organization of IKCa1, SKCa2, and SKCa3 were defined, and IK(Ca) and SK(Ca) genes were found to have a remarkably similar intron-exon structure. Mitogens enhanced IKCa1 promoter activity proportional to the increase in IKCa1 mRNA, suggesting that transcriptional mechanisms underlie channel up-regulation. Mutation of motifs for AP1 and Ikaros-2 in the promoter abolished this induction. Selective Kv1.3 inhibitors ShK-Dap(22), margatoxin, and correolide suppressed mitogenesis of resting T-cells but not preactivated T-cells with up-regulated IKCa1 channel expression. Selectively blocking IKCa1 channels with clotrimazole or TRAM-34 suppressed mitogenesis of preactivated lymphocytes, whereas resting T-cells were less sensitive. Thus, Kv1.3 channels are essential for activation of quiescent cells, but signaling through the PKC pathway enhances expression of IKCa1 channels that are required for continued proliferation.

NF-kappa B is required for the positive selection of CD8+ thymocytes

To examine the role of NF-kappaB in T cell development, we analyzed thymocyte ontogeny in transgenic (mutant I-kappaBalpha (mI-kappaBalpha)) mice that express a superinhibitory form of the NF-kappaB inhibitory protein, I-kappaBalpha (I-kappaBalpha(A32/36)), under the control of the T cell-specific CD2 promoter and enhancer. Thymi from mI-kappaBalpha mice contained increased numbers of double-positive (DP) and decreased numbers of both CD4(+) and CD8(+) single-positive cells, consistent with a block in DP thymocyte maturation. In addition, expression of CD69, a marker of positive selection, was decreased on DP thymocytes from the mI-kappaBalpha mice. To test directly whether NF-kappaB was required for positive or negative selection, we generated mI-kappaBalpha mice expressing the H-Y or 2C alphabeta TCR transgenes. Expression of the I-kappaBalpha(A32/36) transgene caused a block in the positive selection of CD8(+) single-positive cells in both strains of TCR transgenic animals. In contrast, negative selection was unaffected by expression of the I-kappaBalpha(A32/36) transgene. Taken together, these results identified a NF-kappaB-dependent transcriptional pathway that is selectively required for the positive selection of CD8(+) thymocytes.

The calcium-independent protein kinase C participates in an early process of CD3/CD28-mediated induction of thymocyte apoptosis

Thymocyte negative selection eliminates self-reactive clones and involves both a T-cell receptor (TCR)/CD3-mediated signal and a costimulatory signal, which can be delivered via CD28. Anti-CD3/anti-CD28-triggered apoptosis in isolated CD4+CD8+ thymocytes in vitro provides a basic model for negative selection. Effects of isoform-selective and non-isoform-selective inhibitors of protein kinase C (PKC) on this apoptotic process suggest that activation of Ca2+-independent PKC isoforms during the first 2-3 hr of culture is essential for inducing apoptosis, and that Ca2+-dependent PKC isoforms may be influential, but not essential, for apoptosis. To assess the CD3/CD28-mediated activation of PKC in the apoptotic process, we prepared CD4+CD8+ thymocytes (without contamination with cells that had received negative or positive selection signals in vivo) by establishing TCR-transgenic mice with RAG-2-deficient and non-selecting major histocompatibility complex (MHC) backgrounds, in addition to a CD4+CD8+ thymocyte-enriched population from normal mice. Translocation of Ca2+-independent PKC from the cytosolic fraction to the particulate fraction of CD4+CD8+ thymocytes was induced by CD3/CD28-mediated stimulation, but not by CD3- or CD28-mediated stimulation alone, and peaked 2 hr after the start of culture. The kinase activity of the translocated Ca2+-independent PKC was dependent on cofactors in vitro, indicating that novel (n)PKC, but not atypical (a)PKC or a proteolytic PKC fragment, was responsible for the activity. Immunoblotting analysis indicated that the nPKC-theta isoform was the major contributor among nPKC isoforms, and that the classical (c)PKC-alpha isoform was the major contributor among cPKC isoforms. These results suggest that activation of nPKC (especially the theta isoform) in CD4+CD8+ thymocytes is involved in a pathway for negative selection.

Protein kinase Ctheta: a new essential superstar on the T-cell stage

Recent studies have identified protein kinase Cθ (PKCtheta), a member of the Ca(2+)-independent PKC family, as an essential component of the T-cell synapse that cooperates with calcineurin to activate the interleukin-2 (IL-2) gene. Several selective functions of PKCtheta involved in the activation and survival of T cells are reviewed herein. Among these, the nuclear factor-kappaB (NF-kappaB) signaling cascade appears to be the most critical target of PKCtheta in the T-cell receptor/CD28 costimulatory pathway that leads to T-cell activation.

Calcineurin and beyond: cardiac hypertrophic signaling

In response to increased ventricular wall tension or neurohumoral stimuli, the myocardium undergoes an adaptive hypertrophy response that temporarily augments pump function. Although initially beneficial, sustained cardiac hypertrophy can lead to decompensation and cardiomyopathy. Recent studies have focused on characterizing the molecular mechanisms that underlie cardiac hypertrophy. An increasing number of signal transduction pathways have been identified as important regulators of the hypertrophic response, including the low-molecular weight GTPases (Ras, RhoA, and Rac), mitogen-activated protein kinases, protein kinase C, and calcineurin. This review will discuss an emerging body of evidence that implicates the calcium-calmodulin-activated protein phosphatase calcineurin as a physiological regulator of the cardiac hypertrophic response. Although the sufficiency of calcineurin to promote cardiomyocyte hypertrophy in vivo and in vitro is established, its overall necessity as a hypertrophic mediator is currently an area of ongoing debate. The use of the calcineurin-inhibitory agents cyclosporine A and FK506 have suggested a necessary role for calcineurin in many, but not all, animal models of hypertrophy or cardiomyopathy. The evidence implicating a role for calcineurin signaling in the heart will be weighed against a growing body of literature suggesting necessary roles for a diverse array of intracellular signaling pathways, highlighting the multifactorial nature of the hypertrophic program.

Functional complementation of BLNK by SLP-76 and LAT linker proteins

Recent studies have demonstrated a requirement for the SLP-76 (SH2 domain-containing leukocyte protein of 76 kDa) and LAT (linker for activation of T cells) adaptor/linker proteins in T cell antigen receptor activation and T cell development as well as the BLNK (B cell linker) linker protein in B cell antigen receptor (BCR) signal transduction and B cell development. Whereas the SLP-76 and LAT adaptor proteins are expressed in T, natural killer, and myeloid cells and platelets, BLNK is preferentially expressed in B cells and monocytes. Although BLNK is structurally homologous to SLP-76, BLNK interacts with a variety of downstream signaling proteins that interact directly with both SLP-76 and LAT. Here, we demonstrate that neither SLP-76 nor LAT alone is sufficient to restore the signaling deficits observed in BLNK-deficient B cells. Conversely, the coexpression of SLP-76 and LAT together restored BCR-inducible calcium responses as well as activation of all three families of mitogen-activated protein kinases. Together, these data suggest functional complementation of SLP-76 and LAT in T cell antigen receptor function with BLNK in BCR function.

TCR-independent T cell development mediated by gain-of-oncogene function or loss-of-tumor-suppressor gene function

The mechanisms that govern differentiation of T cell precursors during intrathymic development bridge an interdisciplinary research field of immunology, oncology and developmental biology. Critical checkpoints controlling early thymic T cell development and homeostasis are set by the proper signaling function of the IL-7 receptor, c-Kit receptor, and the pre-T cell antigen receptor (pre-TCR). Given the intimate link between cell cycle control and differentiation in T cell development, proto-oncogenes and tumor suppressors participate as physiological effectors downstream of these receptors not only to influence the cell cycle but also to determine differentiation and survival. Gain- or loss-of-function mutations of these downstream effectors uncouples partially or completely T cell precursors from these checkpoints, providing a selective advantage and enabling aberrant development. These effectors can be identified by provirus tagging in normal mice and more readily by complementation tagging in mice with a predefined block in T cell differentiation.

Protein kinase C isozymes and the regulation of diverse cell responses

Individual protein kinase C (PKC) isozymes have been implicated in many cellular responses important in lung health and disease, including permeability, contraction, migration, hypertrophy, proliferation, apoptosis, and secretion. New ideas on mechanisms that regulate PKC activity, including the identification of a novel PKC kinase, 3-phosphoinositide-dependent kinase-1 (PDK-1), that regulates phosphorylation of PKC, have been advanced. The importance of targeted translocation of PKC and isozyme-specific binding proteins (like receptors for activated C-kinase and caveolins) is well established. Phosphorylation state and localization are now thought to be key determinants of isozyme activity and specificity. New concepts on the role of individual PKC isozymes in proliferation and apoptosis are emerging. Opposing roles for selected isozymes in the same cell system have been defined. Coupling to the Wnt signaling pathway has been described. Phenotypes for PKC knockout mice have recently been reported. More specific approaches for studying PKC isozymes and their role in cell responses have been developed. Strengths and weaknesses of different experimental strategies are reviewed. Future directions for investigation are identified.

The immunological synapse and the actin cytoskeleton: molecular hardware for T cell signaling

The actin cytoskeleton seems to play two critical roles in the activation of T cells. One of these roles is T cell shape development and movement, including formation of the immunological synapse. The other is the formation of a scaffold for signaling components. This review focuses on the recent convergence of cell biology and immunology studies to explain the role of the actin cytoskeleton in creating the molecular basis for immunological synapse formation and T cell signaling.