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

Myeloid-derived growth factor alleviates non-alcoholic fatty liver disease alleviates in a manner involving IKKβ/NF-κB signaling

Whether bone marrow modulates systemic metabolism remains unknown. Our recent study suggested that myeloid-derived growth factor (MYDGF) improves insulin resistance. Here, we found that myeloid cell-specific MYDGF deficiency aggravated hepatic inflammation, lipogenesis, and steatosis, and show that myeloid cell-derived MYDGF restoration alleviated hepatic inflammation, lipogenesis, and steatosis. Additionally, recombinant MYDGF attenuated inflammation, lipogenesis, and fat deposition in primary mouse hepatocytes (PMHs). Importantly, inhibitor kappa B kinase beta/nuclear factor-kappa B (IKKβ/NF-κB) signaling is involved in protection of MYDGF on non-alcoholic fatty liver disease (NAFLD). These data revealed that myeloid cell-derived MYDGF alleviates NAFLD and inflammation in a manner involving IKKβ/NF-κB signaling, and serves as a factor involved in the crosstalk between the liver and bone marrow that regulates liver fat metabolism. Bone marrow functions as an endocrine organ and serves as a potential therapeutic target for metabolic disorders.

Back on the scene: advances and challenges in CD3-related drugs in tumor therapy

CD3 molecules are mainly distributed on the membrane of mature T cells. They are involved in T cell antigen recognition, signal transduction, and regulation of T cell development. CD3-related monoclonal antibodies (mAbs) are mainly used in the treatment of autoimmune diseases. Nearly half of all bispecific antibodies developed are used in tumor therapy, one of which is CD3 antigen. In this review, we discuss the importance of biological function and the crucial role of CD3 in tumor therapy. We highlight the research status of antibodies and small molecules targeting CD3 to provide guidance for future drug research.

A sensitive S-Trap-based approach to the analysis of T cell lipid raft proteome

The analysis of T cell lipid raft proteome is challenging due to the highly dynamic nature of rafts and the hydrophobic character of raft-resident proteins. We explored an innovative strategy for bottom-up lipid raftomics based on suspension-trapping (S-Trap) sample preparation. Mouse T cells were prepared from splenocytes by negative immunoselection, and rafts were isolated by a detergent-free method and OptiPrep gradient ultracentrifugation. Microdomains enriched in flotillin-1, LAT, and cholesterol were subjected to proteomic analysis through an optimized protocol based on S-Trap and high pH fractionation, followed by nano-LC-MS/MS. Using this method, we identified 2,680 proteins in the raft-rich fraction and established a database of 894 T cell raft proteins. We then performed a differential analysis on the raft-rich fraction from nonstimulated versus anti-CD3/CD28 T cell receptor (TCR)-stimulated T cells. Our results revealed 42 proteins present in one condition and absent in the other. For the first time, we performed a proteomic analysis on rafts from ex vivo T cells obtained from individual mice, before and after TCR activation. This work demonstrates that the proposed method utilizing an S-Trap-based approach for sample preparation increases the specificity and sensitivity of lipid raftomics.

Exercise and Muscle Lipid Content, Composition, and Localization: Influence on Muscle Insulin Sensitivity

Accumulation of lipid in skeletal muscle is thought to be related to the development of insulin resistance and type 2 diabetes. Initial work in this area focused on accumulation of intramuscular triglyceride; however, bioactive lipids such as diacylglycerols and sphingolipids are now thought to play an important role. Specific species of these lipids appear to be more negative toward insulin sensitivity than others. Adding another layer of complexity, localization of lipids within the cell appears to influence the relationship between these lipids and insulin sensitivity. This article summarizes how accumulation of total lipids, specific lipid species, and localization of lipids influence insulin sensitivity in humans. We then focus on how these aspects of muscle lipids are impacted by acute and chronic aerobic and resistance exercise training. By understanding how exercise alters specific species and localization of lipids, it may be possible to uncover specific lipids that most heavily impact insulin sensitivity.

Enhanced interaction between SEC2 mutant and TCR Vβ induces MHC II-independent activation of T cells via PKCθ/NF-κB and IL-2R/STAT5 signaling pathways

SEC2, a major histocompatibility complex class II (MHC II)-dependent T-cell mitogen, binds MHC II and T-cell receptor (TCR) Vβs to induce effective co-stimulating signals for clonal T-cell expansion. We previously characterized a SEC2 mutant with increased recognition of TCR Vβs, ST-4, which could intensify NF-κB signaling transduction, leading to IL-2 production and T-cell activation. In this study, we found that in contrast to SEC2, ST-4 could induce murine CD4⁺ T-cell proliferation in a Vβ8.2- and Vβ8.3-specific manner in the absence of MHC II⁺ antigen-presenting cells (APCs). Furthermore, although IL-2 secretion in response to either SEC2 or ST-4 stimulation was accompanied by up-regulation of protein kinase Cθ (PKCθ), inhibitor of κB (IκB), α and β IκB kinase (IKKα/β), IκBα, and NF-κB in mouse splenocytes, only ST-4 could activate CD4⁺ T cells in the absence of MHC II⁺ APCs through the PKCθ/NF-κB signaling pathway. The PKCθ inhibitor AEB071 significantly suppressed SEC2/ST-4-induced T-cell proliferation, CD69 and CD25 expression, and IL-2 secretion with or without MHC II⁺ APCs. Further, SEC2/ST-4-induced changes in PKCθ/NF-κB signaling were significantly relieved by AEB071 in a dose-dependent manner. Using Lck siRNA, we found that Lck controlled SEC2/ST-4-induced phosphorylation of PKCθ. We also demonstrated that the IL-2R/STAT5 pathway is essential for SEC2/ST-4-induced T-cell activation. Collectively, our data demonstrate that an enhanced ST-4-TCR interaction can compensate for lack of MHC II and stimulate MHC II-free CD4⁺ T-cell proliferation via PKCθ/NF-κB and IL-2R/STAT5 signaling pathways. Compared with SEC2, intensified PKCθ/NF-κB and IL-2R/STAT5 signals induced by ST-4 lead to enhanced T-cell activation. The results of this study will facilitate better understanding of TCR-based immunotherapies for cancer.

Role of IκB kinase β in regulating the remodeling of the CARMA1-Bcl10-MALT1 complex

The current work investigates the notion that inducible clustering of signaling mediators of the IKK pathway is important for platelet activation. Thus, while the CARMA1, Bcl10, and MALT1 (CBM) complex is essential for triggering IKK/NF-κB activation upon platelet stimulation, the signals that elicit its formation and downstream effector activation remain elusive. We demonstrate herein that IKKβ is involved in membrane fusion, and serves as a critical protein kinase required for initial formation and the regulation of the CARMA1/MALT1/Bcl10/CBM complex in platelets. We also show that IKKβ regulates these processes via modulation of phosphorylation of Bcl10 and IKKγ polyubiquitination. Collectively, our data demonstrate that IKKβ regulates membrane fusion and the remodeling of the CBM complex formation.

CADM1 is essential for KSHV-encoded vGPCR-and vFLIP-mediated chronic NF-κB activation

Approximately 12% of all human cancers worldwide are caused by infections with oncogenic viruses. Kaposi's sarcoma herpesvirus/human herpesvirus 8 (KSHV/HHV8) is one of the oncogenic viruses responsible for human cancers, including Kaposi's sarcoma (KS), Primary Effusion Lymphoma (PEL), and the lymphoproliferative disorder multicentric Castleman's disease (MCD). Chronic inflammation mediated by KSHV infection plays a decisive role in the development and survival of these cancers. NF-κB, a family of transcription factors regulating inflammation, cell survival, and proliferation, is persistently activated in KSHV-infected cells. The KSHV latent and lytic expressing oncogenes involved in NF-κB activation are vFLIP/K13 and vGPCR, respectively. However, the mechanisms by which NF-κB is activated by vFLIP and vGPCR are poorly understood. In this study, we have found that a host molecule, Cell Adhesion Molecule 1 (CADM1), is robustly upregulated in KSHV-infected PBMCs and KSHV-associated PEL cells. Further investigation determined that both vFLIP and vGPCR interacted with CADM1. The PDZ binding motif localized at the carboxyl terminus of CADM1 is essential for both vGPCR and vFLIP to maintain chronic NF-κB activation. Membrane lipid raft associated CADM1 interaction with vFLIP is critical for the initiation of IKK kinase complex and NF-κB activation in the PEL cells. In addition, CADM1 played essential roles in the survival of KSHV-associated PEL cells. These data indicate that CADM1 plays key roles in the activation of NF-κB pathways during latent and lytic phases of the KSHV life cycle and the survival of KSHV-infected cells.

Teuvincenone F Suppresses LPS-Induced Inflammation and NLRP3 Inflammasome Activation by Attenuating NEMO Ubiquitination

Inflammation causes many diseases that are serious threats to human health. However, the molecular mechanisms underlying regulation of inflammation and inflammasome activation are not fully understood which has delayed the discovery of new anti-inflammatory drugs of urgent clinic need. Here, we found that the natural compound Teuvincenone F, which was isolated and purified from the stems and leaves of Premna szemaoensis, could significantly inhibit lipopolysaccharide (LPS)-induced pro-inflammatory cytokines production and NLRP3 inflammasome activation. Our results showed that Teuvincenone F attenuated K63-linked ubiquitination of NF-κB-essential modulator (NEMO, also known as IKKγ) to suppress LPS-induced phosphorylation of NF-κB, and inhibited mRNA expression of IL-1β, IL-6, TNF-α, and NLRP3. In addition, we found that decreased NLRP3 expression by Teuvincenone F suppressed NLRP3 inflammasome activation and IL-1β/IL-18 maturation. In vivo, we revealed that Teuvincenone F treatment relieved LPS-induced inflammation. In conclusion, Teuvincenone F is a highly effective natural compound to suppress LPS-induced inflammation by attenuating K63-linked ubiquitination of NEMO, highlighting that Teuvincenone F may be a potential new anti-inflammatory drug for the treatment of inflammatory and NLRP3 inflammasome-driven diseases.

The Novel PKCθ from Benchtop to Clinic

The protein kinases C (PKCs) are a family of serine/threonine kinases involved in regulating multiple essential cellular processes such as survival, proliferation, and differentiation. Of particular interest is the novel, calcium-independent PKCθ which plays a central role in immune responses. PKCθ shares structural similarities with other PKC family members, mainly consisting of an N-terminal regulatory domain and a C-terminal catalytic domain tethered by a hinge region. This isozyme, however, is unique in that it translocates to the immunological synapse between a T cell and an antigen-presenting cell (APC) upon T cell receptor-peptide MHC recognition. Thereafter, PKCθ interacts physically and functionally with downstream effectors to mediate T cell activation and differentiation, subsequently leading to inflammation. PKCθ-specific perturbations have been identified in several diseases, most notably autoimmune disorders, and hence the modulation of its activity presents an attractive therapeutic intervention. To that end, many inhibitors of PKCs and PKCθ have been developed and tested in preclinical and clinical studies. And although selectivity remains a challenge, results are promising for the future development of effective PKCθ inhibitors that would greatly advance the treatment of several T-cell mediated diseases.

Kurarinone regulates immune responses through regulation of the JAK/STAT and TCR-mediated signaling pathways

Sophora flavescens is a medicinal herb that contains flavonoids and quinolizidine alkaloids and has a wide range of biological activities due to its anti-inflammatory, anti-bacterial and anti-cancer properties. We isolated a series of flavonoids from the roots of Sophora flavescens and examined their ability to inhibit immune responses. Among the flavonoids, kurarinone exhibited the strongest inhibitory effect on immune responses. Kurarinone suppressed the differentiation of CD4(+) T cells by inhibiting the expression and production of T-cell lineage-specific master regulators and cytokines. Our results also demonstrated that kurarinone directly suppressed the cytokine-induced Janus kinase/signal transducer and activator of transcription (JAK/STAT) signaling and T-cell receptor (TCR) pathways. In two established animal models of chronic inflammatory skin disease, one in which psoriasis-like skin disease was induced by an interleukin 23 (IL-23) injection into mouse ears and another in which 2,4,6-trinitrochlorobenzene (TNCB) application on the abdomens of mice was used to induce contact dermatitis, kurarinone repressed disease development by inhibiting the expression of pro-inflammatory mediators, including cytokines, chemokines and enzyme in murine ear skin. This study provides new evidence that kurarinone may ameliorate chronic inflammatory skin diseases through the suppression of pathogenic CD4(+) T-cell differentiation and the overall immune response.

CD80 and CD86 differentially regulate mechanical interactions of T-cells with antigen-presenting dendritic cells and B-cells

Functional T-cell responses are initiated by physical interactions between T-cells and antigen-presenting cells (APCs), including dendritic cells (DCs) and B-cells. T-cells are activated more effectively by DCs than by B-cells, but little is known about the key molecular mechanisms that underpin the particular potency of DC in triggering T-cell responses. To better understand the influence of physical intercellular interactions on APC efficacy in activating T-cells, we used single cell force spectroscopy to characterize and compare the mechanical forces of interactions between DC:T-cells and B:T-cells. Following antigen stimulation, intercellular interactions of DC:T-cell conjugates were stronger than B:T-cell interactions. DCs induced higher levels of T-cell calcium mobilization and production of IL-2 and IFNγ than were elicited by B-cells, thus suggesting that tight intercellular contacts are important in providing mechanically stable environment to initiate T-cell activation. Blocking antibodies targeting surface co-stimulatory molecules CD80 or CD86 weakened intercellular interactions and dampen T-cell activation, highlighting the amplificatory roles of CD80/86 in regulating APC:T-cell interactions and T-cell functional activation. The variable strength of mechanical forces between DC:T-cells and B:T-cell interactions were not solely dependent on differential APC expression of CD80/86, since DCs were superior to B-cells in promoting strong interactions with T-cells even when CD80 and CD86 were inhibited. These data provide mechanical insights into the effects of co-stimulatory molecules in regulating APC:T-cell interactions.

PKC-theta-mediated signal delivery from the TCR/CD28 surface receptors

Protein kinase C-theta (PKCθ) is a key enzyme in T lymphocytes, where it plays an important role in signal transduction downstream of the activated T cell antigen receptor (TCR) and the CD28 costimulatory receptor. Interest in PKCθ as a potential drug target has increased following recent findings that PKCθ is essential for harmful inflammatory responses mediated by Th2 (allergies) and Th17 (autoimmunity) cells as well as for graft-versus-host disease (GvHD) and allograft rejection, but is dispensable for beneficial responses such as antiviral immunity and graft-versus-leukemia (GvL) response. TCR/CD28 engagement triggers the translocation of the cytosolic PKCθ to the plasma membrane (PM), where it localizes at the center of the immunological synapse (IS), which forms at the contact site between an antigen-specific T cell and antigen-presenting cells (APC). However, the molecular basis for this unique localization, and whether it is required for its proper function have remained unresolved issues until recently. Our recent study resolved these questions by demonstrating that the unique V3 (hinge) domain of PKCθ and, more specifically, a proline-rich motif within this domain, is essential and sufficient for its localization at the IS, where it is anchored to the cytoplasmic tail of CD28 via an indirect mechanism involving Lck protein tyrosine kinase (PTK) as an intermediate. Importantly, the association of PKCθ with CD28 is essential not only for IS localization, but also for PKCθ-mediated activation of downstream signaling pathways, including the transcription factors NF-κB and NF-AT, which are essential for productive T cell activation. Hence, interference with formation of the PKCθ-Lck-CD28 complex provides a promising basis for the design of novel, clinically useful allosteric PKCθ inhibitors. An additional recent study demonstrated that TCR triggering activates the germinal center kinase (GSK)-like kinase (GLK) and induces its association with the SLP-76 adaptor at the IS, where GLK phosphorylates the activation loop of PKCθ, converting it into an active enzyme. This recent progress, coupled with the need to study the biology of PKCθ in human T cells, is likely to facilitate the development of PKCθ-based therapeutic modalities for T cell-mediated diseases.

NEMO differentially regulates TCR and TNF-α induced NF-κB pathways and has an inhibitory role in TCR-induced NF-κB activation

NF-κB essential modulator (NEMO), the regulatory subunit of the IκB kinase (IKK) complex, is an essential adaptor both for inflammation stimuli and TCR-induced NF-κB activation. However, the exact mechanism of its function has not been fully understood. Here, we report that knockdown of NEMO by RNA interference in Jurkat E6.1 cells enhanced TCR-induced NF-κB report gene activity and IL-2 production by promotion of IκBα degradation and p65 nuclear translocation, whereas inhibited TNF-α and LPS-induced IκBα degradation without influencing the phosphorylation of MAPKs. In human primary T and Jurkat E6.1 cells, both CD3/CD28 and PMA/Ionomycin induced NF-κB activation showed a para-curve correlation with the dosage of small interfering RNA targeting NEMO (siNEMO): the NF-κB report gene activity was increased along with ascending doses of transfected siNEMO and reached the highest activity when knockdown about 70% of NEMO, then turned to decline and gradually be blocked once almost thoroughly knockdown of NEMO. Meanwhile, TNF-α induced NF-κB was always inhibited no matter how much NEMO was knockdown. Subcellular fractionation results suggested that upon CD3/CD28 costimulation, NEMO and IKKβ may not cotranslocate to cytoskeleton fraction as a conventional NEMO/IKK complex with a static stoichiometric ratio, instead the ratio of NEMO: IKKβ continuously shift from high to low. Depletion of NEMO accelerated TCR-induced cytoskeleton translocation of IKKβ. Altogether, this study suggests that NEMO may function as a rheostat exerting a negative action on TCR-induced NF-κB activation and differentially regulates TNF-α and TCR-induced NF-κB pathways.

Palmitate-activated macrophages confer insulin resistance to muscle cells by a mechanism involving protein kinase C θ and ε

Background

Macrophage-derived factors contribute to whole-body insulin resistance, partly by impinging on metabolically active tissues. As proof of principle for this interaction, conditioned medium from macrophages treated with palmitate (CM-PA) reduces insulin action and glucose uptake in muscle cells. However, the mechanism whereby CM-PA confers this negative response onto muscle cells remains unknown.

Methodology/Principal Findings

L6-GLUT4myc myoblasts were exposed for 24 h to palmitate-free conditioned medium from RAW 264.7 macrophages pre-treated with 0.5 mM palmitate for 6 h. This palmitate-free CM-PA, containing selective cytokines and chemokines, inhibited myoblast insulin-stimulated insulin receptor substrate 1 (IRS1) tyrosine phosphorylation, AS160 phosphorylation, GLUT4 translocation and glucose uptake. These effects were accompanied by a rise in c-Jun N-terminal kinase (JNK) activation, degradation of Inhibitor of κBα (IκBα), and elevated expression of proinflammatory cytokines in myoblasts. Notably, CM-PA caused IRS1 phosphorylation on Ser1101, and phosphorylation of novel PKCθ and ε. Co-incubation of myoblasts with CM-PA and the novel and conventional PKC inhibitor Gö6983 (but not with the conventional PKC inhibitor Gö6976) prevented PKCθ and ε activation, JNK phosphorylation, restored IκBα mass and reduced proinflammatory cytokine production. Gö6983 also restored insulin signalling and glucose uptake in myoblasts. Moreover, co-silencing both novel PKC θ and ε isoforms in myoblasts by RNA interference, but not their individual silencing, prevented the inflammatory response and restored insulin sensitivity to CM-PA-treated myoblasts.

Conclusions/Clinical Significance

The results suggest that the block in muscle insulin action caused by CM-PA is mediated by novel PKCθ and PKCε. This study re-establishes the participation of macrophages as a relay in the action of fatty acids on muscle cells, and further identifies PKCθ and PKCε as key elements in the inflammatory and insulin resistance responses of muscle cells to macrophage products. Furthermore, it portrays these PKC isoforms as potential targets for the treatment of fatty acid-induced, inflammation-linked insulin resistance.

NF-κB as a target for oncogenic viruses

NF-κB is a pivotal transcription factor that controls cell survival and proliferation in diverse physiological processes. The activity of NF-κB is tightly controlled through its cytoplasmic sequestration by specific inhibitors, IκBs. Various cellular stimuli induce the activation of an IκB kinase, which phosphorylates IκBs and triggers their proteasomal degradation, causing nuclear translocation of activated NF-κB. Under normal conditions, the activation of NF-κB occurs transiently, thus ensuring rapid but temporary induction of target genes. Deregulated NF-κB activation contributes to the development of various diseases, including cancers and immunological disorders. Accumulated studies demonstrate that the NF-κB signaling pathway is a target of several human oncogenic viruses, including the human T cell leukemia virus type 1, the Kaposi sarcoma-associated herpesvirus, and the Epstein-Bar virus. These viruses encode specific oncoproteins that target different signaling components of the NF-κB pathway, leading to persistent activation of NF-κB. This chapter will discuss the molecular mechanisms by which NF-κB is activated by the viral oncoproteins.

Lipid raft assembly and Lck recruitment in TRAIL costimulation mediates NF-κB activation and T cell proliferation

The TNF-related apoptosis-inducing ligand was shown to provide a costimulatory signal that cooperates with the TCR/CD3 complex to induce T cell proliferation and cytokine production. Although a number of signaling pathways were linked to the TCR/CD3 complex, it is not known how these two receptors cooperate to induce T cell activation. In this study, we show that TRAIL-induced costimulation of T cells depends on activation of the NF-κB pathway. TRAIL induced the NF-κB pathway by phosphorylation of inhibitor of κB factor kinase and protein kinase C in conjunction with anti-CD3. Furthermore, we demonstrated that TRAIL costimulation induced phosphorylation of the upstream TCR-proximal tyrosine kinases, Lck and ZAP70. Ligation of the TRAIL by its soluble receptor, DR4-Fc, alone was able to induce the phosphorylation of Lck and ZAP70 and to activate the NF-κB pathway; however, it was insufficient to fully activate T cells to support T cell proliferation. In contrast, TRAIL engagement in conjunction with anti-CD3, but not TRAIL ligation alone, induced lipid raft assembly and recruitment of Lck and PKC. These results demonstrate that TRAIL costimulation mediates NF-κB activation and T cell proliferation by lipid raft assembly and recruitment of Lck. Our results suggest that in TRAIL costimulation, lipid raft recruitment of Lck integrates mitogenic NF-κB-dependent signals from the TCR and TRAIL in T lymphocytes.

c-Rel phenocopies PKCtheta but not Bcl-10 in regulating CD8+ T-cell activation versus tolerance

Elucidating the signaling events that promote T-cell tolerance versus activation provides important insights for manipulating immunity in vivo. Previous studies have suggested that the absence of PKCtheta results in the induction of anergy and that the balance between the induction of the transcription factors NFAT, AP1 and NF-kappaB plays a key role in determining whether T-cell anergy or activation is induced. Here, we examine whether Bcl-10 and specific family members of NF-kappaB act downstream of PKCtheta to alter CD8(+) T-cell activation and/or anergy. We showed that T cells from mice deficient in c-Rel but not NF-kappaB1 (p50) have increased susceptibility to the induction of anergy, similar to T cells from PKCtheta-deficient mice. Surprisingly T cells from Bcl-10-deficient mice showed a strikingly different phenotype to the PKCtheta-deficient T cells, with a severe block in TCR-mediated activation. Furthermore, we have also shown that survival signals downstream of NF-kappaB, are uncoupled from signals that mediate T-cell anergy. These results suggest that c-Rel plays a critical role downstream of PKCtheta in controlling CD8(+) T-cell anergy induction.

NF-kappaB activation in T cells requires discrete control of IkappaB kinase alpha/beta (IKKalpha/beta) phosphorylation and IKKgamma ubiquitination by the ADAP adapter protein

NF-kappaB activation following engagement of the antigen-specific T cell receptor involves protein kinase C-theta-dependent assembly of the CARMA1-BCL10-MALT1 (CBM) signalosome, which coordinates downstream activation of IkappaB kinase (IKK). We previously identified a novel role for the adhesion- and degranulation-promoting adapter protein (ADAP) in regulating the assembly of the CBM complex via an interaction of ADAP with CARMA1. In this study, we identify a novel site in ADAP that is critical for association with the TAK1 kinase. ADAP is critical for recruitment of TAK1 and the CBM complex, but not IKK, to protein kinase C-theta. ADAP is not required for TAK1 activation. Although both the TAK1 and the CARMA1 binding sites in ADAP are essential for IkappaB alpha phosphorylation and degradation and NF-kappaB nuclear translocation, only the TAK1 binding site in ADAP is necessary for IKK phosphorylation. In contrast, only the CARMA1 binding site in ADAP is required for ubiquitination of IKKgamma. Thus, distinct sites within ADAP control two key activation responses that are required for NF-kappaB activation in T cells.

Generation of tumor-specific T lymphocytes using dendritic cell/tumor fusions and anti-CD3/CD28

Adoptive immunotherapy with tumor-specific T cells represents a promising treatment strategy for patients with malignancy. However, the efficacy of T-cell therapy has been limited by the ability to expand tumor-reactive cells with an activated phenotype that effectively target malignant cells. We have developed an anticancer vaccine in which patient-derived tumor cells are fused with autologous dendritic cells (DCs), such that a wide array of tumor antigens are presented in the context of DC-mediated costimulation. In this study, we demonstrate that DC/tumor fusions induce T cells that react with tumor and are dramatically expanded by subsequent ligation of the CD3/CD28 costimulatory complex. These T cells exhibit a predominantly activated phenotype as manifested by an increase in the percentage of cells expressing CD69 and interferon gamma. In addition, the T cells upregulate granzyme B expression and are highly effective in lysing autologous tumor targets. Targeting of tumor-specific antigen was demonstrated by the expansion of T cells with specificity for the MUC1 tetramer. Stimulation with anti-CD3/CD28 followed by DC/tumor fusions or either agent alone failed to result in a similar expansion of tumor-reactive T cells. Consistent with these findings, spectratyping analysis demonstrates selective expansion of T-cell clones as manifested by considerable skewing of the Vbeta repertoire following sequential stimulation with DC/tumor fusions and anti-CD3/CD28. Gene expression analysis was notable for the upregulation of inflammatory pathways. These findings indicate that stimulation with DC/tumor fusions provides a unique platform for subsequent expansion with anti-CD3/CD28 in adoptive T-cell therapy of cancer.

Dynamic regulation of T-cell costimulation through TCR-CD28 microclusters

SUMMARY

T-cell activation requires contact between T cells and antigen-presenting cells (APCs) to bring T-cell receptors (TCRs) and major histocompatibility complex peptide (MHCp) together to the same complex. These complexes rearrange to form a concentric circular structure, the immunological synapse (IS). After the discovery of the IS, dynamic imaging technologies have revealed the details of the IS and provided important insights for T-cell activation. We have redefined a minimal unit of T-cell activation, the 'TCR microcluster', which recognizes MHCp, triggers an assembly of assorted molecules downstream of the TCR, and induces effective signaling from TCRs. The relationship between TCR signaling and costimulatory signaling was analyzed in terms of the TCR microcluster. CD28, the most valuable costimulatory receptor, forms TCR-CD28 microclusters in cooperation with TCRs, associates with protein kinase C theta, and effectively induces initial T-cell activation. After mature IS formation, CD28 microclusters accumulate at a particular subregion of the IS, where they continuously assemble with the kinases and not TCRs, and generate sustained T-cell signaling. We propose here a 'TCR-CD28 microcluster' model in which TCR and costimulatory microclusters are spatiotemporally formed at the IS and exhibit fine-tuning of T-cell responses by assembling with specific players downstream of the TCR and CD28.

HTLV-1 Tax is a critical lipid raft modulator that hijacks IkappaB kinases to the microdomains for persistent activation of NF-kappaB

Upon T cell activation, IkappaB kinases (IKKs) are transiently recruited to the plasma membrane-associated lipid raft microdomains for activation of NF-kappaB in promoting T cell proliferation. Retroviral Tax proteins from human T cell leukemia virus type 1 and type 2 (HTLV-1 and -2) are capable of activating IKK, yet only HTLV-1 infection causes T cell leukemia, which correlates with persistent activation of NF-kappaB induced by Tax1. Here, we show that the Tax proteins exhibit differential modes of IKK activation. The subunits of IKK are constitutively present in lipid rafts in activated forms in HTLV-1-infected T cells that express Tax. Disruption of lipid rafts impairs IkappaB kinase activation by Tax1. We also show that the cytoplasmic Tax1 protein persistently resides in the Golgi-associated lipid raft microdomains. Tax1 directs lipid raft translocation of IKK through selective interaction with IKKgamma and accordingly, depletion of IKKgamma impairs Tax1-directed lipid raft recruitment of IKKalpha and IKKbeta. In contrast, Tax2 activates NF-kappaB in a manner independent of lipid raft recruitment of IKK. These findings indicate that Tax1 actively recruits IKK to the lipid raft microdomains for persistent activation of NF-kappaB, thereby contributing to HTLV-1 oncogenesis.

Mechanosensing machinery for cells under low substratum rigidity

Mechanical stimuli are essential during development and tumorigenesis. However, how cells sense their physical environment under low rigidity is still unknown. Here we show that low rigidity of collagen gel downregulates beta(1)-integrin activation, clustering, and focal adhesion kinase (FAK) Y397 phosphorylation, which is mediated by delayed raft formation. Moreover, overexpression of autoclustered beta(1)-integrin (V737N), but not constitutively active beta(1)-integrin (G429N), rescues FAKY397 phosphorylation level suppressed by low substratum rigidity. Using fluorescence resonance energy transfer to assess beta(1)-integrin clustering, we have found that substratum rigidity between 58 and 386 Pa triggers beta(1)-integrin clustering in a dose-dependent manner, which is highly dependent on actin filaments but not microtubules. Furthermore, augmentation of beta(1)-integrin clustering enhances the interaction between beta(1)-integrin, FAK, and talin. Our results indicate that contact with collagen fibrils is not sufficient for integrin activation. However, substratum rigidity is required for integrin clustering and activation. Together, our findings provide new insight into the mechanosensing machinery and the mode of action for epithelial cells in response to their physical environment under low rigidity.

Cell type-specific regulation of ITAM-mediated NF-kappaB activation by the adaptors, CARMA1 and CARD9

Activating NK cell receptors transduce signals through ITAM-containing adaptors, including FcRgamma and DAP12. Although the caspase recruitment domain (CARD)9-Bcl10 complex is essential for FcRgamma/DAP12-mediated NF-kappaB activation in myeloid cells, its involvement in NK cell receptor signaling is unknown. Herein we show that the deficiency of CARMA1 or Bcl10, but not CARD9, resulted in severe impairment of cytokine/chemokine production mediated by activating NK cell receptors due to a selective defect in NF-kappaB activation, whereas cytotoxicity mediated by the same receptors did not require CARMA1-Bcl10-mediated signaling. IkappaB kinase (IKK) activation by direct protein kinase C (PKC) stimulation with PMA plus ionomycin (P/I) was abrogated in CARMA1-deficient NK cells, similar to T and B lymphocytes, whereas CARD9-deficient dendritic cells (DCs) exhibited normal P/I-induced IKK activation. Surprisingly, CARMA1 deficiency also abrogated P/I-induced IKK activation in DCs, indicating that CARMA1 is essential for PKC-mediated NF-kappaB activation in all cell types, although the PKC-CARMA1 axis is not used downstream of myeloid ITAM receptors. Consistently, PKC inhibition abrogated ITAM receptor-mediated activation only in NK cells but not in DCs, suggesting PKC-CARMA1-independent, CARD9-dependent ITAM receptor signaling in myeloid cells. Conversely, the overexpression of CARD9 in CARMA1-deficient cells failed to restore the PKC-mediated NF-kappaB activation. Thus, NF-kappaB activation signaling through ITAM receptors is regulated by a cell type-specific mechanism depending on the usage of adaptors CARMA1 and CARD9, which determines the PKC dependence of the signaling.

Regulation of programmed cell death by NF-kappaB and its role in tumorigenesis and therapy

The Rel/NF-kappaB transcription factors are key regulators of programmed cell death (PCD). Their activity has significant physiological relevance for normal development and homeostasis in various tissues and important pathological consequences are associated with aberrant NF-kappaB activity, including hepatocyte apoptosis, neurodegeneration, and cancer. While NF-kappaB is best characterized for its protective activity in response to proapoptotic stimuli, its role in suppressing programmed necrosis has come to light more recently. NF-kappaB most commonly antagonizes PCD by activating the expression of antiapoptotic proteins and antioxidant molecules, but it can also promote PCD under certain conditions and in certain cell types. It is therefore important to understand the pathways that control NF-kappaB activation in different settings and the mechanisms that regulate its anti- vs pro-death activities. Here, we review the role of NF-kappaB in apoptotic and necrotic PCD, the mechanisms involved, and how its activity in the cell death response impacts cancer development, progression, and therapy. Given the role that NF-kappaB plays both in tumor cells and in the tumor microenvironment, recent findings underscore the NF-kappaB signaling pathway as a promising target for cancer prevention and treatment.

Identification of flotillin-2, a major protein on lipid rafts, as a novel target of p53 family members

p73 and p63 are members of the p53 gene family and have been shown to play an important role in development and homeostasis mainly by regulating the transcription of a variety of genes. A subset of these genes encodes secreted proteins and receptors that may be involved in the communication between adjacent cells. We report here that flotillin-2, a major hydrophobic protein on biomembrane microdomain lipid rafts, is a direct transcriptional target of the p53 family member genes. It has been suggested that such rafts could play an important role in many cellular processes including signal transduction, membrane trafficking, cytoskeletal organization, and pathogen entry. We found that the expression of flotillin-2 was specifically up-regulated by either TAp73beta or TAp63gamma, but not significantly by p53. In addition, flotillin-2 transcription is activated in response to cisplatin in a manner dependent on endogenous p73. By using small interference RNA designed to target p73, we showed that silencing endogenous p73 abolishes the induction of flotillin-2 transcription following cisplatin treatment. Furthermore, we identified a p73/p63-binding site located upstream of the flotillin-2 gene that is responsive to the p53 family members. This response element is highly conserved between humans and rodents. We also found that ectopic expression of TAp73 as well as TAp63 enhances signal transduction by assessing the interleukin-6-mediated phosphorylation of signal transducers and activators of transcription 3. Thus, in addition to direct transactivation, p53 family member genes enhance a set of cellular processes via lipid rafts.

T-cell receptor-induced NF-kappaB activation is negatively regulated by E3 ubiquitin ligase Cbl-b

It has previously been shown that E3 ubiquitin ligase Casitas B-lineage lymphoma-b (Cbl-b) negatively regulates T-cell activation, but the molecular mechanism(s) underlying this inhibition is not completely defined. In this study, we report that the loss of Cbl-b selectively results in aberrant activation of NF-kappaB upon T-cell antigen receptor (TCR) ligation, which is mediated by phosphatidylinositol 3-kinase (PI3-K)/Akt and protein kinase C-theta (PKC-theta). TCR-induced hyperactivation of Akt in the absence of Cbl-b may potentiate the formation of caspase recruitment domain-containing membrane-associated guanylate kinase protein 1 (CARMA1)-B-cell lymphoma/leukemia 10 (Bcl10)-mucosa-associated lymphatic tissue 1(MALT1) (CBM) complex, which appears to be independent of PKC-theta. Cbl-b associates with PKC-theta upon TCR stimulation and regulates TCR-induced PKC-theta activation via Vav-1, which couples PKC-theta to PI3-K and allows it to be phosphorylated. PKC-theta then couples IkappaB kinases (IKKs) to the CBM complex, resulting in the activation of the IKK complex. Therefore, our data provide the first evidence to demonstrate that the down-regulation of TCR-induced NF-kappaB activation by Cbl-b is mediated coordinately by both Akt-dependent and PKC-theta-dependent signaling pathways in primary T cells.

Stromal cell-derived factor 1-alpha (SDF)-induced human T cell chemotaxis becomes phosphoinositide 3-kinase (PI3K)-independent: role of PKC-theta

Stromal cell-derived factor 1alpha (SDF-1alpha) is the exclusive ligand for the chemokine receptor CXCR4. This receptor plays a pivotal role in immune responses, the pathogenesis of infection such as HIV, and cellular trafficking. However, the signaling mechanisms regulating SDF-driven T cell migration are not well defined. In this study, we determined the role of PI3K and protein kinase C- theta (PKC-theta) in SDF-induced human T cell migration in fresh versus cultured T cells. Purified human T cells (fresh vs. 48 h in media, unstimulated or activated by anti-CD3+anti-CD28) were used. Western blots showed that SDF induced phospho-(p)-Akt [threonine (Thr)308 and serine 473], a proxy for PI3K activity, in fresh cells and p-PKC-theta in 48 h unstimulated cells. LY294002 (PI3K inhibitor) reduced SDF-induced chemotaxis in fresh cells by 51%, whereas it minimally affected chemotaxis in 48 h unstimulated or activated cells. However, a specific PKC-theta inhibitor, pseudosubstrate for PKC-theta, reduced chemotaxis in 48 h unstimulated and stimulated T cells by 72% and 87%, respectively. Thus, chemotaxis becomes independent of PI3K signaling in human T cells cultured for 48 h. Under these conditions, PKC-theta is phosphorylated (Thr538) by SDF, and chemotaxis becomes largely PKC-theta-dependent.

PKCtheta cooperates with atypical PKCzeta and PKCiota in NF-kappaB transactivation of T lymphocytes

Using yeast two-hybrid, we isolated atypical PKCzeta as a PKCtheta-interacting kinase and demonstrated that it selectively interacted with, and was phosphorylated by, PKCtheta. Importantly, however, both atypical PKCzeta and PKCiota were functionally required in TCR/CD28-mediated activation of NF-kappaB downstream of PKCtheta in Jurkat T cells albeit, activation responses of PKCzeta-deficient CD3+ T cells were comparable with wildtype controls. This normal activation thresholds of PKCzeta-/- T cells suggested that PKCiota, the closest structural relative, might play a compensatory role in TCR/CD28-induced signalling. Consistently, both PKCzeta and PKCiota resided in the plasma membrane lipid raft microdomains of Jurkat as well as primary mouse CD3+ T cells. Thus, PKCtheta, the established constituent of the immunological synapse, physically and functionally interacted with PKCzeta and PKCiota. Together, these data demonstrate that atypical PKCzeta/iota isotypes serve as direct downstream targets of PKCtheta in the signalling pathway leading to NF-kappaB activation in T lymphocytes.

Regulation of peripheral T cell tolerance by the E3 ubiquitin ligase Cbl-b

The family of the Casitas B-lineage Lymphoma (Cbl) proteins, c-Cbl, Cbl-b, and Cbl-3, function as E3 ubiquitin ligases and molecular adaptors. In particular, Cbl-b acts as a gatekeeper in T cell activation that controls activation thresholds and the requirement for co-stimulation. Loss of Cbl-b expression renders animals susceptible to antigen-triggered autoimmunity suggesting that Cbl-b is a key autoimmunity gene. In addition, Cbl-b plays a critical role in T cell anergy and escape from regulatory T cells (Treg) suppression. Modulation of Cbl-b might provide us with a unique opportunity for future immune treatment of human disorders such as autoimmunity, immunodeficiency, or cancer.

Redistribution of P-selectin glycoprotein ligand-1 (PSGL-1) in chemokine-treated neutrophils: a role of lipid microdomains

P-selectin glycoprotein ligand-1 (PSGL-1) is a mucin-like cell adhesion molecule expressed on leukocyte plasma membranes and involved in platelet-leukocyte and endothelium-leukocyte interactions. The treatment of neutrophils with a low concentration of IL-8 induced the redistribution of PSGL-1 to one end of the cell to form a cap-like structure. We investigated the role of lipid microdomains in the redistribution of PSGL-1 and its effect on the adhesive characteristics of IL-8-treated neutrophils. The redistribution of PSGL-1 induced by IL-8 was inhibited by cholesterol-perturbing agents such as methyl-beta-cyclodextrin and filipin. Sucrose density gradient centrifugation analysis revealed that PSGL-1 was enriched in a low-density fraction together with the GM1 ganglioside after solubilization of the cell membranes with a nonionic detergent, Brij 58. However, when Triton X-100 was used for the solubilization, PSGL-1 was no longer recovered in the low-density fraction, although GM1 ganglioside remained in the low-density fraction. Furthermore, immunofluorescence microscopic observation demonstrated that the localization of PSGL-1 differed from that of GM1 ganglioside, suggesting that PSGL-1 is associated with a microdomain distinct from that containing the GM1 ganglioside. Treatment of neutrophils with IL-8 increased the formation of microaggregates composed of neutrophils and activated platelets, and this treatment also enhanced reactive oxygen species production in neutrophils induced by the cross-linking of PSGL-1 with antibodies. These results suggest that the association of PSGL-1 with lipid microdomains is essential for its redistribution induced by IL-8 stimulation and that the redistribution modulates neutrophil functions mediated by interactions with P-selectin.

Antigen-receptor signaling to nuclear factor kappa B

Signal transduction events leading to the survival, differentiation, or apoptosis of cells of the innate or adaptive immune system must be properly coordinated to ensure the normal mounting and termination of immune responses. One of the key transcription factors in immune responses is nuclear factor kappaB (NF-kappaB), which has been the focus of intense investigation over the past two decades. With the identification of the CARMA1-BCL10-MALT1 complex and ongoing progress in understanding the molecular mechanisms connecting T cell and B cell receptor proximal signals to the IkappaB kinase (IKK) complex, a cohesive model of antigen receptor (AgR)-dependent signaling to NF-kappaB activation is beginning to emerge. In this review, we provide an overview of the current state of research into the mechanisms that regulate AgR-mediated NF-kappaB transcriptional activity, with particular focus on the events leading to activation of the IKK complex.

NF-kappaB and the immune response

One of the primary physiological roles of nuclear factor-kappa B (NF-kappaB) is in the immune system. In particular, NF-kappaB family members control the transcription of cytokines and antimicrobial effectors as well as genes that regulate cellular differentiation, survival and proliferation, thereby regulating various aspects of innate and adaptive immune responses. In addition, NF-kappaB also contributes to the development and survival of the cells and tissues that carry out immune responses in mammals. This review, therefore, describes the role of the NF-kappaB pathway in the development and functioning of the immune system.

Spatial compartmentalization of tumor necrosis factor (TNF) receptor 1-dependent signaling pathways in human airway smooth muscle cells. Lipid rafts are essential for TNF-alpha-mediated activation of RhoA but dispensable for the activation of the NF-kappaB and MAPK pathways

Tumor necrosis factor (TNF)-alpha-induced activation of RhoA, mediated by TNF receptor 1 (TNFR1), is a prerequisite step in a pathway that leads to increased 20-kDa light chain of myosin (MLC20) phosphorylation and airway smooth muscle contraction. In this study, we have investigated the proximal events in TNF-alpha-induced RhoA activation. TNFR1 is localized to both lipid raft and nonraft regions of the plasma membrane in primary human airway smooth muscle cells. TNF-alpha engagement of TNFR1 recruited the adaptor proteins TRADD, TRAF-2, and RIP into lipid rafts and activated RhoA, NF-kappaB, and MAPK pathways. Depletion of cholesterol from rafts with methyl-beta-cyclodextrin caused a redistribution of TNFR1 to nonraft plasma membrane and prevented ligand-induced RhoA activation. By contrast, TNF-alpha-induced activation of NF-kappaB and MAPKs was unaffected by methyl-beta-cyclodextrin indicating that, in airway smooth muscle cells, activation of these pathways occurred independently of lipid rafts. Targeted knockdown of caveolin-1 completely abrogated TNF-alpha-induced RhoA activation, identifying this raft-resident protein as a positive regulator of the activation process. The signaling adaptors TRADD and RIP were also found to be necessary for ligand-induced RhoA activation. Taken together, our results suggest that in airway smooth muscle cells, spatial compartmentalization of TNFR1 provides a mechanism for generating distinct signaling outcomes in response to ligand engagement and define a mechanistic role for lipid rafts and caveolin-1 in TNF-alpha-induced activation of RhoA.

Rottlerin inhibits human T cell responses

Rottlerin is a pharmacological inhibitor of protein kinase C (PKC) theta, a novel PKC selectively expressed in T lymphocytes. PKC theta is known to regulate T cell receptor (TCR)/CD28 signalling pathways in T lymphocytes, but the impact of PKC theta inhibition on human T cell responses remains undefined. In this work, we describe the effects of rottlerin on the responses of CD4+ and CD8+ human T lymphocytes upon polyclonal activation. We observed a dose-dependent inhibition of CD4+ and CD8+ T cell proliferation in response to anti-CD3/anti-CD28 antibodies stimulation in the presence of rottlerin. This inhibition was associated with impaired CD25 expression and decreased interleukin (IL)-2 production in activated T cells. In contrast, rottlerin did not alter IL-2-induced T cell proliferation. Furthermore, we demonstrated that rottlerin blocked interferon (IFN) gamma, IL-10 and IL-13 mRNA expression in TCR/CD28 activated CD4+ T cells. These findings place rottlerin as a potent immunosuppressive agent for the development of novel therapies in T cell mediated immune disorders.

Coincident enrichment of phosphorylated IkappaBalpha, activated IKK, and phosphorylated p65 in the axon initial segment of neurons

Phosphorylation of the inhibitory protein IkappaBalpha by the activated IkappaB kinase (IKK) is a crucial step in the activation of the transcription factor NF-kappaB. In neurons of the mammalian central nervous system, constitutive activation of NF-kappaB has been previously documented. The cellular compartments involved in this activation have not yet been fully identified. Here we document a striking enrichment of several molecules involved in NF-kappaB activation in the axon initial segment (AIS) of neurons: Phosphorylated-IkappaBalpha (pIkappaBalpha), activated IKK, and p65 phosphorylated at serine 536 were found to be enriched in the AIS in vivo as well as in vitro. Both, pIkappaBalpha and activated IKK, were associated with cytoskeletal components of the AIS. Activated IKK was associated with the membrane cytoskeleton, whereas pIkappaBalpha was sequestered to microtubules of the AIS. Colchicine-induced depolymerization of microtubules resulted in the loss of pIkappaBalpha in the AIS, demonstrating that the integrity of the axonal cytoskeleton is essential for the clustering of this NF-kappaB pathway component. These data provide the first evidence for a compartmentalized clustering of NF-kappaB pathway components in the AIS and implicate this neuronal compartment in the activation of NF-kappaB.

Lipid raft adhesion receptors and Syk regulate selectin-dependent rolling under flow conditions

Selectins and their ligand P-selectin glycoprotein ligand-1 (PSGL-1) mediate leukocyte rolling along inflamed vessels. Cell rolling is modulated by selectin interactions with their ligands and by topographic requirements including L-selectin and PSGL-1 clustering on tips of leukocyte microvilli. Lipid rafts are cell membrane microdomains reported to function as signaling platforms. Here, we show that disruption of leukocyte lipid rafts with cholesterol chelating agents depleted raft-associated PSGL-1 and L-selectin and strongly reduced L-, P-, and E-selectin-dependent rolling. Cholesterol repletion reversed inhibition of cell rolling. Importantly, leukocyte rolling on P-selectin induced the recruitment of spleen tyrosine kinase (Syk), a tyrosine kinase associated to lipid raft PSGL-1. Furthermore, inhibition of Syk activity or expression, with pharmacologic inhibitors or by RNA interference, strongly reduced leukocyte rolling on P-selectin, but not on E-selectin or PSGL-1. These observations identify novel regulatory mechanisms of leukocyte rolling on selectins with a strong dependency on lipid raft integrity and Syk activity.

CARMA1 is critical for the development of allergic airway inflammation in a murine model of asthma

CARMA1 has been shown to be important for Ag-stimulated activation of NF-kappaB in lymphocytes in vitro and thus could be a novel therapeutic target in inflammatory diseases such as asthma. In the present study, we demonstrate that mice with deletion in the CARMA1 gene (CARMA1(-/-)) do not develop inflammation in a murine model of asthma. Compared with wild-type controls, CARMA1(-/-) mice did not develop airway eosinophilia, had no significant T cell recruitment into the airways, and had no evidence for T cell activation in the lung or draining lymph nodes. In addition, the CARMA1(-/-) mice had significantly decreased levels of IL-4, IL-5, and IL-13, did not produce IgE, and did not develop airway hyperresponsiveness or mucus cell hypertrophy. However, adoptive transfer of wild-type Th2 cells into CARMA1(-/-) mice restored eosinophilic airway inflammation, cytokine production, airway hyperresponsiveness, and mucus production. This is the first demonstration of an in vivo role for CARMA1 in a disease process. Furthermore, the data clearly show that CARMA1 is essential for the development of allergic airway inflammation through its role in T lymphocytes, and may provide a novel means to inhibit NF-kappaB for therapy in asthma.

Phosphorylation of the CARMA1 linker controls NF-kappaB activation

PKC isoforms and CARMA1 play crucial roles in immunoreceptor-dependent NF-kappaB activation. We tested whether PKC-dependent phosphorylation of CARMA1 directly regulates this signaling cascade. B cell antigen receptor (BCR) engagement led to the progressive recruitment of CARMA1 into lipid rafts and to the association of CARMA1 with, and phosphorylation by, PKCbeta. Furthermore, PKCbeta interacted with the serine-rich CARMA1 linker, and both PKCbeta and PKCtheta phosphorylated identical serine residues (S564, S649, and S657) within this linker. Mutation of two of these sites ablated the functional activity of CARMA1. In contrast, deletion of the linker resulted in constitutive, receptor- and PKC-independent NF-kappaB activation. Together, our data support a model whereby CARMA1 phosphorylation controls NF-kappaB activation by triggering a shift from an inactive to an active CARMA1 conformer. This PKC-dependent switch regulates accessibility of the CARD and CC domains and controls assembly and full activation of the membrane-associated IkappaB kinase (IKK) signalosome.

Deciphering the pathway from the TCR to NF-κB

A major regulator of lymphocyte survival and activation is the transcription factor nuclear factor-kappaB (NF-kappaB). Controlled activation of NF-kappaB is essential for the immune and inflammatory response as well as for cell proliferation and protection against apoptosis. The NEMO/IkappaB kinase (IKK) complex is the central integrator of most stimuli leading to NF-kappaB activation, but a detailed knowledge of the upstream events is available only for a limited number of stimuli. In particular, although most players have probably been identified, relatively little is known about the detailed molecular mechanisms involved in the cascade leading to NF-kappaB activation following engagement of the T-cell receptor by a foreign antigen. In this review, we discuss recent insights into this specific signal transduction cascade, and the way it is controlled both spatially and temporally.

Activation or suppression of NFkappaB by HPK1 determines sensitivity to activation-induced cell death

Restimulation of the T-cell receptor (TCR) in activated T cells induces CD95 (Fas/Apo-1)-mediated activation-induced cell death (AICD). The TCR-proximal mechanisms leading to AICD are elusive. Here we characterize hematopoietic progenitor kinase 1 (HPK1) as a differentially regulated TCR-proximal signaling protein involved in AICD of primary T cells. We show that HPK1 is a functional component of the endogenous IkappaB kinase (IKK) complex and is crucial for TCR-mediated NFkappaB activation. While full-length HPK1 enhances IKKbeta phosphorylation, siRNA-mediated knockdown of HPK1 blunts TCR-mediated NFkappaB activation and increases cell death. We also demonstrate proteolytic processing of HPK1 into HPK1-C, specifically in AICD-sensitive primary T cells. The cleavage product HPK1-C sequesters the inactive IKK complex and suppresses NFkappaB upon TCR restimulation by binding to IKKalpha and IKKbeta. T cells of HPK1-C transgenic mice are sensitized towards TCR-mediated AICD. Consequently, preventing HPK1-C generation in primary T cells by siRNA-mediated knockdown results in decreased AICD. Thus, these results show a novel mechanism of sensitization of T lymphocytes towards AICD by suppression of NFkappaB, and propose that HPK1 is a life/death switch in T lymphocytes.

Activation of Nuclear Factor κB by Somatostatin Type 2 Receptor in Pancreatic Acinar AR42J Cells Involves Gα14 and Multiple Signaling Components

Medications targeting the somatostatin type 2 receptor (SSTR2) have been employed for pancreatic inflammations and cancers, possibly via the regulation of the transcription factor nuclear factor kappaB (NFkappaB). Here we demonstrate that in tumoral pancreatic acinar AR42J cells, activation of SSTR2 leads to stimulation of the inhibitor kappaB kinase (IKK)/NFkappaB signaling cascade via pertussis toxin-insensitive G proteins in a time- and dose-dependent manner. The inability of G(q/11) and G(12/13) proteins to activate IKK/NFkappaB by SSTR2 in transfected human embryonic kidney 293 cells and the lack of Galpha(16) in AR42J cells suggested a possible role of Galpha(14) in mediating SSTR2-induced responses. This regulatory role of Galpha(14) was further confirmed by the activation of IKK and NFkappaB in human embryonic kidney 293 cells expressing SSTR2 and Galpha(14) upon induction. The stimulatory effect of Gbeta(1)gamma(2) and the abrogation by overexpressing transducin confirmed the participation of Gbetagamma in SSTR2-mediated IKK/NFkappaB activation. By the application of specific inhibitors and dominant negative mutants, phospholipase Cbeta, protein kinase C, and calmodulin-dependent kinase II were shown to be involved in SSTR2-induced responses. Inhibition of c-Src and numerous intermediates, including Ras, Raf-1 kinase, MEK1/2, along with the extracellular signal-regulated kinase cascade attenuated somatostatin-mediated IKK/NFkappaB activation. Although c-Jun N-terminal kinase and p38 mitogen-activated protein kinase (MAPK) were also stimulated by SSTR2, suppression of these two MAPKs was ineffective in altering the somatostatin-mediated responses. Similar results were also obtained using AR42J cells. These data suggest that activation of the IKK/NFkappaB signaling cascade by SSTR2 requires a complicated network consisting of Galpha(14) and multiple intermediates.

Anti-apoptotic action of API2-MALT1 fusion protein involved in t(11;18)(q21;q21) MALT lymphoma

At least three distinct chromosomal translocations, t(11;18)(q21;q21), t(1;14)(p22;q32) and t(14;18)(q32;q21) involving the API2 (also known as c-IAP2)-MALT1 fusion protein, BCL10, and MALT1, respectively, have been implicated in the molecular pathogenesis of mucosa associated lymphoid tissue (MALT) lymphoma. Our findings showed that several variants of the API2-MALT1 fusion protein can occur in patients with t(11;18)(q21;q21), and that API2-MALT1 can potently enfance activation of nuclear factor (NF)-kappaB signaling, which may be relevant to the pathogenesis of MALT lymphomas. We also found that MALT1 is rapidly degraded via the ubiquitin-proteasome pathway, as is the case with API2, but upon the synthesis of fusion, API2-MALT1 becomes stable against this pathway. This stability of API2-MALT1 may thus result in inappropriate nuclear factor (NF)-kappaB activation, thereby contributing to the pathogenesis of MALT lymphoma. Recent biochemical and genetic studies have clearly shown that BCL10 and MALT1 form a physical and functional complex and are both required for NF-kappaB activation by antigen receptor stimulation in T and B lymphocytes. It has also been shown that CARMA1, a newly discovered member of the membrane-associated guanylate kinase (MAGUK) families, is critical for antigen receptor-stimulated NF-kappaB activation. It can be assumed that API2-MALT1 can bypass this normal BCL10/MALT1 cellular signaling pathway linked to NF-kappaB activation, thereby inducing antigen receptor-independent proliferation of lymphocytes. Furthermore, BCL10/MALT1- and API2-MALT1-induced NF-kappaB activation may contribute to anti-apoptotic action probably through NF-kappaB-mediated upregulation of apoptotic inhibitor genes. We recently provided direct evidence that API2-MALT1 indeed exerts anti-apoptotic action, in part, through its direct interaction with apoptotic regulators including Smac. Taken together, these findings prompt us to hypothesize that the anti-apoptotic action of API2-MALT1 may be mediated partly by the direct interaction with apoptotic regulators as well as partly by upregulation of apoptotic inhibitor genes. Further studies can be expected to stimulate research into the development of therapeutic drugs that specifically inhibit the antigen receptor signaling-stimulated NF-kappaB activation pathway: such molecule targeting drugs should be useful for interfering with inappropriate proliferation of lymphocytes associated with inflammatory and neoplastic disorders.

Phosphatidylinositol-3-kinase regulates PKCθ activity in cytotoxic T cells

Protein kinase C (PKC) theta plays a crucial role in T cell activation. We, therefore, examined the regulation of PKCtheta activity in cytotoxic T lymphocytes (CTL). We demonstrated that PMA did not stimulate PKCtheta activation and phospholipase C inhibition did not block anti-CD3-stimulated PKCtheta activation in a CTL clone. This suggests that diacylglycerol is neither sufficient nor required for PKCtheta activation. Furthermore, PKCtheta was only activated in a CTL clone stimulated with plate-bound anti-CD3 but not soluble anti-CD3. However, PMA or cross-linked anti-CD3 stimulated phosphorylation of PKCtheta as measured by a migratory shift, suggesting that phosphorylation was not sufficient for activity. Phosphatidylinositol 3-kinase activity was required for anti-CD3, but not PMA, stimulated phosphorylation and for immobilized anti-CD3-triggered PKCtheta activity. A substantial fraction of PKCtheta was constitutively membrane associated and PMA or CD3 stimulation did not significantly increase membrane association. Our data indicate that phosphorylation of PKCtheta is not a suitable surrogate measurement for PKCtheta activity and that additional, yet to be defined steps, are required for the regulation of PKCtheta enzymatic activity in CTL.

Novel Mechanism of Inhibition of Nuclear Factor-κB DNA-Binding Activity by Diterpenoids Isolated from Isodon rubescens

The development of specific inhibitors that can block nuclear factor-kappaB (NF-kappaB) activation is an approach for the treatment of cancer, autoimmune, and inflammatory diseases. Several diterpenoids, oridonin, ponicidin, xindongnin A, and xindongnin B were isolated from the herb Isodon rubescens. These compounds were found to be potent inhibitors of NF-kappaB transcription activity and the expression of its downstream targets, cyclooxygenase-2 and inducible nitric-oxide synthase. The mechanisms of action of the diterpenoids against NF-kappaB are similar, but significant differences were also identified. All of the diterpenoids directly interfere with the DNA-binding activity of NF-kappaB to its response DNA sequence. Oridonin and ponicidin have an additional impact on the translocation of NF-kappaB from the cytoplasm to nuclei without affecting IkappaB-alpha phosphorylation and degradation. The effect of these compounds on the interaction of NF-kappaB with consensus DNA sequences is unique. Different inhibitory effects were observed when NF-kappaB bound to various DNA sequences. Both p65/p65 and p50/p50 homodimers, as well as p65/p50 heterodimer association with their responsive DNA, were inhibited. Kinetic studies on NF-kappaB-DNA interaction indicate that the diterpenoids decrease the B(max app) but have no effect on K(d app). This suggests that this class of compounds interacts with both p65 and p50 subunits at a site other than the DNA binding site and subsequently modulates the binding affinity of the transcription factor toward DNA with different NF-kappaB binding sequences. The diterpenoid structure could therefore serve as a scaffold for the development of more potent and selective NF-kappaB inhibitors that target regulated gene transcription.