PKC theta cooperates with PKC alpha in alloimmune responses of T cells in vivo

Potential implications of protein kinase Cα in pathophysiological conditions and therapeutic interventions

PKCα is a molecule with many functions that play an important role in cell survival and death to maintain cellular homeostasis. Alteration in the normal functioning of PKCα is responsible for the complicated etiology of many pathologies, including cancer, cardiovascular diseases, kidney complications, neurodegenerative diseases, diabetics, and many others. Several studies have been carried out over the years on this kinase's function, and regulation in normal physiology and pathological conditions. A lot of data with antithetical results have therefore accumulated over time to create a complex framework of physiological implications connected to the PKCα function that needs comprehensive elucidation. In light of this information, we critically analyze the multiple roles played by PKCα in basic cellular processes and their molecular mechanism during various pathological conditions. This review further discusses the current approaches to manipulating PKCα signaling amplitude in the patient's favour and proposed PKCα as a therapeutic target to reverse pathological states.

Protein Kinase C at the Crossroad of Mutations, Cancer, Targeted Therapy and Immune Response

The frequent PKC dysregulations observed in many tumors have made these enzymes natural targets for anticancer applications. Nevertheless, this considerable interest in the development of PKC modulators has not led to the expected therapeutic benefits, likely due to the complex biological activities regulated by PKC isoenzymes, often playing ambiguous and protective functions, further driven by the occurrence of mutations. The structure, regulation and functions of PKCs have been extensively covered in other publications. Herein, we focused on PKC alterations mostly associated with complete functional loss. We also addressed the modest yet encouraging results obtained targeting PKC in selected malignancies and the more frequent negative clinical outcomes. The reported observations advocate the need for more selective molecules and a better understanding of the involved pathways. Furthermore, we underlined the most relevant immune mechanisms controlled by PKC isoforms potentially impacting the immune checkpoint inhibitor blockade-mediated immune recovery. We believe that a comprehensive examination of the molecular features of the tumor microenvironment might improve clinical outcomes by tailoring PKC modulation. This approach can be further supported by the identification of potential response biomarkers, which may indicate patients who may benefit from the manipulation of distinctive PKC isoforms.

T cell-intrinsic protein kinase D3 is dispensable for the cells' activation

Protein kinases D (PKDs) are implicated in T cell receptor (TCR) signaling. Of the two T cell-expressed isoforms PKD2 and PKD3, however, only the former one is rather well understood in this immune cell type. Recently, we have observed a putative hyper-phenotype of T cells from conventional PKD3-knockout mice, which we explained as a secondary effect due to a skewed T cell compartment from naïve towards effector/memory T cells already under steady state conditions. Nonetheless, to this end it is not clear whether these aberrations are mediated by a T cell-intrinsic or -extrinsic function of PKD3. To address this question, we have investigated mice lacking PKD3 specifically in the T cell compartment. We could show that T cells from CD4-Cre-driven conditional knockout mice did not phenocopy the ones from conventional PKD3-knockout mice. In brief, no skewing in the T cell compartment of peripheral lymphoid organs, no hyper-activation upon stimulation in vitro or in vivo as well as no aberrations in follicular helper T cells (T_FH) upon immunization were observed. Hence, although PKD3 is strongly regulated upon TCR stimulation, in T cells this kinase seems to be dispensable for their activation. The described skewing in the T cell compartment of conventional PKD3-deficient mice seems to be mediated by T cell-extrinsic mechanisms, thus once more emphasizing the importance of cell type-specific mouse models.

A MLR-Based Approach to Analyze Regulators of T Lymphocyte Activation In Vivo

Depending on the context, robust and durable T lymphocyte activation is either desirable, as in the case of anti-tumor responses, or unwanted, in cases of autoimmunity when chronic stimulation leads to self-tissue damage. Therefore, reliable in vivo models are of great importance to identify and validate regulatory pathways of T lymphocyte activation. Here, we describe an in vivo mixed-lymphocyte-reaction (MLR) approach, which is based on the so-called parent-into-F1 (P → F1) mouse model in combination with the congenic marker CD45.1/2 and cell proliferation dye-labeling. This setup allows us to track adoptively transferred allogenic CD4⁺ and CD8⁺ T lymphocytes and analyze their phenotype as well as the proliferation by flow cytometry in the blood and spleen. We could show hypo-reactive responses of T lymphocytes isolated from knockout mice with a known defect in T lymphocyte activation. Thus, this MLR-based in vivo model provides the opportunity to analyze positive regulators of T cell responses under physiological conditions of polyclonal T lymphocyte activation in vivo.

Addressing the role of PKD3 in the T cell compartment with knockout mice

Background

The Protein kinase D3 (PKD3) has been implicated in signal transduction downstream of the T cell receptor (TCR). However, its role for the activation of primary T lymphocytes has not been elucidated so far.

Methods

Expression of PKD isoforms in primary murine T cells was determined by RT-PCR and SDS-Page. A germline PKD3-knockout mouse line was analyzed for its immune response to OVA/alum intraperitoneal immunization. Phenotyping of the T cell compartment ex vivo as well as upon stimulation in vitro was performed by flow cytometry. Additionally, cytokine expression was assessed by flow cytometry, RT-PCR and Luminex technology.

Results

PKD expression in T cells is modulated by TCR stimulation, leading to a rapid down-regulation on mRNA and on protein level. PKD3-deficient mice respond to immunization with enhanced T follicular helper cell generation. Furthermore, peripheral PKD3-deficient CD4⁺ T cells express more interleukin-2 than wild type CD4⁺ T cells upon TCR stimulation ex vivo. However, purified naïve CD4⁺ T cells do not differ in their phenotype upon differentiation in vitro from wild type T cells. Moreover, we observed a shift towards an effector/memory phenotype of splenic T cells at steady state, which might explain the contradictory results obtained with pan-T cells ex vivo and naïve-sorted T cells.

Conclusion

While PKD3-deficiency in vivo in mice leads to a skewing of the T cell compartment towards a more activated phenotype, this kinase seems to be dispensable for naïve CD4⁺ T cell differentiation in vitro.

Evidence that PKCα inhibition in Dalton's Lymphoma cells augments cell cycle arrest and mitochondrial-dependent apoptosis

Protein kinase Cα (PKCα), belonging to ser/thr protein kinase, perform various biological functions. Overexpression of PKCα has been observed in multiple human malignancies including lymphoma. However, the molecular pathogenesis and involvement of PKCα in Non-Hodgkin lymphoma (NHL) are not clearly understood. Hence, deciphering the role of PKCα in NHL management may provide a better therapeutic option. In the present study, we used selective pharmacological inhibitors Gö6976 and Ro320432 that potentially inhibit PKCα-mediated signaling in DL cells, resulting in the inhibition of cell growth and mitochondrial-dependent apoptosis. PKCα inhibition by these inhibitors also displays cell cycle arrest at the G1 phase and causes growth retardation of DL cells. Our results extended the mechanism of PKCα in NHL, and provided potential implications for its therapy.

The complexities of PKCα signaling in cancer

Protein kinase C α (PKCα) is a ubiquitously expressed member of the PKC family of serine/threonine kinases with diverse functions in normal and neoplastic cells. Early studies identified anti-proliferative and differentiation-inducing functions for PKCα in some normal tissues (e.g., regenerating epithelia) and pro-proliferative effects in others (e.g., cells of the hematopoietic system, smooth muscle cells). Additional well documented roles of PKCα signaling in normal cells include regulation of the cytoskeleton, cell adhesion, and cell migration, and PKCα can function as a survival factor in many contexts. While a majority of tumors lose expression of PKCα, others display aberrant overexpression of the enzyme. Cancer-related mutations in PKCα are uncommon, but rare examples of driver mutations have been detected in certain cancer types (e. g., choroid gliomas). Here we review the role of PKCα in various cancers, describe mechanisms by which PKCα affects cancer-related cell functions, and discuss how the diverse functions of PKCα contribute to tumor suppressive and tumor promoting activities of the enzyme. We end the discussion by addressing mutations and expression of PKCα in tumors and the clinical relevance of these findings.

IDO inhibits a tryptophan sufficiency signal that stimulates mTOR: A novel IDO effector pathway targeted by D-1-methyl-tryptophan

Tryptophan catabolism by indoleamine 2,3-dioxygenase (IDO) alters inflammation and favors T-cell tolerance in cancer, but the underlying molecular mechanisms remain poorly understood. The integrated stress response kinase GCN2, a sensor of uncharged tRNA that is activated by amino acid deprivation, is recognized as an important effector of the IDO pathway. However, in a mouse model of inflammatory carcinogenesis, ablation of Gcn2 did not promote resistance against tumor development like the absence of IDO does, implying the existence of additional cancer-relevant pathways that operate downstream of IDO. Addressing this gap in knowledge, we report that the IDO-mediated catabolism of tryptophan also inhibits the immunoregulatory kinases mTOR and PKC-Θ, along with the induction of autophagy. These effects were relieved specifically by tryptophan but also by the experimental agent 1-methyl-D-tryptophan (D-1MT, also known as NLG8189), the latter of which reversed the inhibitory signals generated by IDO with higher potency. Taken together, our results implicate mTOR and PKC-Θ in IDO-mediated immunosuppressive signaling, and they provide timely insights into the unique mechanism of action of D-1MT as compared with traditional biochemical inhibitors of IDO. These findings are important translationally, because they suggest broader clinical uses for D-1MT against cancers that overexpress any tryptophan catabolic enzyme (IDO, IDO2 or TDO). Moreover, they define mTOR and PKC-Θ as candidate pharmacodynamic markers for D-1MT responses in patients recruited to ongoing phase IB/II cancer trials, addressing a current clinical need.

Protein kinase D2: a versatile player in cancer biology

Protein kinase D2 (PKD2) is a serine/threonine kinase that belongs to the PKD family of calcium-calmodulin kinases, which comprises three isoforms: PKD1, PKD2, and PKD3. PKD2 is activated by many stimuli including growth factors, phorbol esters, and G-protein-coupled receptor agonists. PKD2 participation to uncontrolled growth, survival, neovascularization, metastasis, and invasion has been documented in various tumor types including pancreatic, colorectal, gastric, hepatic, lung, prostate, and breast cancer, as well as glioma multiforme and leukemia. This review discusses the versatile functions of PKD2 from the perspective of cancer hallmarks as described by Hanahan and Weinberg. The PKD2 status, signaling pathways affected in different tumor types and the molecular mechanisms that lead to tumorigenesis and tumor progression are presented. The latest developments of small-molecule inhibitors selective for PKD/PKD2, as well as the need for further chemotherapies that prevent, slow down, or eliminate tumors are also discussed in this review.

Protein kinase C theta is dispensable for suppression mediated by CD25+CD4+ regulatory T cells

The activation of conventional T cells upon T cell receptor stimulation critically depends on protein kinase C theta (PKCθ). However, its role in regulatory T (Treg) cell function has yet to be fully elucidated. Using siRNA or the potent and PKC family-selective pharmacological inhibitor AEB071, we could show that murine Treg-mediated suppression in vitro is independent of PKCθ function. Likewise, Treg cells of PKCθ-deficient mice were fully functional, showing a similar suppressive activity as wild-type CD25⁺CD4⁺ T cells in an in vitro suppression assay. Furthermore, in vitro-differentiated wild-type and PKCθ-deficient iTreg cells showed comparable Foxp3 expression as well as suppressive activity. However, we observed a reduced percentage of Foxp3⁺CD25⁺ CD4⁺ T cells in the lymphatic organs of PKCθ-deficient mice. Taken together, our results suggest that while PKCθ is involved in Treg cell differentiation in vivo, it is dispensable for Treg-mediated suppression.

Intracellular Delivery of Anti-pPKCθ (Thr538) via Protein Transduction Domain Mimics for Immunomodulation

Targeting cellular proteins with antibodies, to better understand cellular signaling pathways in the context of disease modulation, is a fast-growing area of investigation. Humanized antibodies are increasingly gaining attention for their therapeutic potential, but the collection of cellular targets is limited to those secreted from cells or expressed on the cell surface. This approach leaves a wealth of intracellular proteins unexplored as putative targets for antibody binding. Protein kinase Cθ (PKCθ) is essential to T cell activation, proliferation, and differentiation, and its phosphorylation at specific residues is required for its activity. Here we report on the design, synthesis, and characterization of a protein transduction domain mimic capable of efficiently delivering an antibody against phosphorylated PKCθ (Thr538) into human peripheral mononuclear blood cells and altering expression of downstream indicators of T cell activation and differentiation. We used a humanized, lymphocyte transfer model of graft-versus-host disease, to evaluate the durability of protein transduction domain mimic:Anti-pPKCθ modulation, when delivered into human peripheral mononuclear blood cells ex vivo. We demonstrate that protein transduction domain mimic:Antibody complexes can be readily introduced with high efficacy into hard-to-transfect human peripheral mononuclear blood cells, eliciting a biological response sufficient to alter disease progression. Thus, protein transduction domain mimic:Antibody delivery may represent an efficient ex vivo approach to manipulating cellular responses by targeting intracellular proteins.

PKD3 deficiency causes alterations in microtubule dynamics during the cell cycle

Protein kinase D 3 (PKD3) is a member of the PKD family that has been linked to many intracellular signaling pathways. However, defined statements regarding isoform specificity and in vivo functions are rare. Here, we use mouse embryonic fibroblast cells that are genetically depleted of PKD3 to identify isoform-specific functions. We show that PKD3 is involved in the regulation of the cell cycle by modulating microtubule nucleation and dynamics. In addition we also show that PKD1 partially can compensate for PKD3 function. Taken together our data provide new insights of a specific PKD3 signaling pathway by identifying a new function, which has not been identified before.

Therapeutics for Graft-versus-Host Disease: From Conventional Therapies to Novel Virotherapeutic Strategies

Allogeneic hematopoietic stem cell transplantation (allo-HSCT) has a curative potential for many hematologic malignancies and blood diseases. However, the success of allo-HSCT is limited by graft-versus-host disease (GVHD), an immunological syndrome that involves inflammation and tissue damage mediated by donor lymphocytes. Despite immune suppression, GVHD is highly incident even after allo-HSCT using human leukocyte antigen (HLA)-matched donors. Therefore, alternative and more effective therapies are needed to prevent or control GVHD while preserving the beneficial graft-versus-cancer (GVC) effects against residual disease. Among novel therapeutics for GVHD, oncolytic viruses such as myxoma virus (MYXV) are receiving increased attention due to their dual role in controlling GVHD while preserving or augmenting GVC. This review focuses on the molecular basis of GVHD, as well as state-of-the-art advances in developing novel therapies to prevent or control GVHD while minimizing impact on GVC. Recent literature regarding conventional and the emerging therapies are summarized, with special emphasis on virotherapy to prevent GVHD. Recent advances using preclinical models with oncolytic viruses such as MYXV to ameliorate the deleterious consequences of GVHD, while maintaining or improving the anti-cancer benefits of GVC will be reviewed.

Metabolic reprogramming of alloantigen-activated T cells after hematopoietic cell transplantation

Alloreactive donor T cells are the driving force in the induction of graft-versus-host disease (GVHD), yet little is known about T cell metabolism in response to alloantigens after hematopoietic cell transplantation (HCT). Here, we have demonstrated that donor T cells undergo metabolic reprograming after allogeneic HCT. Specifically, we employed a murine allogeneic BM transplant model and determined that T cells switch from fatty acid β-oxidation (FAO) and pyruvate oxidation via the tricarboxylic (TCA) cycle to aerobic glycolysis, thereby increasing dependence upon glutaminolysis and the pentose phosphate pathway. Glycolysis was required for optimal function of alloantigen-activated T cells and induction of GVHD, as inhibition of glycolysis by targeting mTORC1 or 6-phosphofructo-2-kinase/fructose-2,6-biphosphatase 3 (PFKFB3) ameliorated GVHD mortality and morbidity. Together, our results indicate that donor T cells use glycolysis as the predominant metabolic process after allogeneic HCT and suggest that glycolysis has potential as a therapeutic target for the control of GVHD.

Protein kinase Cθ: the pleiotropic T-cell signalling intermediate

Activating as well as inhibitory circuits tightly regulate T-cell activation thresholds and effector differentiation processes enabling proper immune response outcomes. Recently, an additional molecular link between T-cell receptor signalling and CD4⁺ Th17 cell skewing has been reported, namely that protein kinase C (PKC) θ critically regulates Th17/Th1 phenotypic differentiation and plasticity in CD4⁺ T-cells by selectively acting as a 'reprogramming element' that suppresses Th1-typical genes during Th17-mediated immune activation in order to stabilize a Th17 cell phenotype.

From innate to adaptive immune response in muscular dystrophies and skeletal muscle regeneration: the role of lymphocytes

Skeletal muscle is able to restore contractile functionality after injury thanks to its ability to regenerate. Following muscle necrosis, debris is removed by macrophages, and muscle satellite cells (MuSCs), the muscle stem cells, are activated and subsequently proliferate, migrate, and form muscle fibers restoring muscle functionality. In most muscle dystrophies (MDs), MuSCs fail to properly proliferate, differentiate, or replenish the stem cell compartment, leading to fibrotic deposition. However, besides MuSCs, interstitial nonmyogenic cells and inflammatory cells also play a key role in orchestrating muscle repair. A complete understanding of the complexity of these mechanisms should allow the design of interventions to attenuate MDs pathology without disrupting regenerative processes. In this review we will focus on the contribution of immune cells in the onset and progression of MDs, with particular emphasis on Duchenne muscular dystrophy (DMD). We will briefly summarize the current knowledge and recent advances made in our understanding of the involvement of different innate immune cells in MDs and will move on to critically evaluate the possible role of cell populations within the acquired immune response. Revisiting previous observations in the light of recent evidence will likely change our current view of the onset and progression of the disease.

Protein kinase C inhibitors for immune disorders

Protein kinase C (PKC) proteins are a group of well-conserved, intracellular signaling enzymes expressed in all cells and tissues, including immune cells. Much of the molecular insight into PKC immunobiology has been gleaned from studies using PKC gene (Prkc) knockout mice and the analysis of different disease models in these animals. More-recent studies have revealed that PKCs also have crucial roles in the pathogenesis of human immune disorders. Therefore, strategies to modulate the functions of PKC enzymes could have a major impact on the treatment and therapies of autoimmune diseases and other immune disorders.

Protein kinase C θ regulates the phenotype of murine CD4+ Th17 cells

Protein kinase C θ (PKCθ) is involved in signaling downstream of the T cell antigen receptor (TCR) and is important for shaping effector T cell functions and inflammatory disease development. Acquisition of Th1-like effector features by Th17 cells has been linked to increased pathogenic potential. However, the molecular mechanisms underlying Th17/Th1 phenotypic instability remain largely unknown. In the current study, we address the role of PKCθ in differentiation and function of Th17 cells by using genetic knock-out mice. Implementing in vitro (polarizing T cell cultures) and in vivo (experimental autoimmune encephalomyelitis model, EAE) techniques, we demonstrated that PKCθ-deficient CD4⁺ T cells show normal Th17 marker gene expression (interleukin 17A/F, RORγt), accompanied by enhanced production of the Th1-typical markers such as interferon gamma (IFN-γ) and transcription factor T-bet. Mechanistically, this phenotype was linked to aberrantly elevated Stat4 mRNA levels in PKCθ^−/− CD4⁺ T cells during the priming phase of Th17 differentiation. In contrast, transcription of the Stat4 gene was suppressed in Th17-primed wild-type cells. This change in cellular effector phenotype was reflected in vivo by prolonged neurological impairment of PKCθ-deficient mice during the course of EAE. Taken together, our data provide genetic evidence that PKCθ is critical for stabilizing Th17 cell phenotype by selective suppression of the STAT4/IFN-γ/T-bet axis at the onset of differentiation.

Pharmacologic inhibition of PKCα and PKCθ prevents GVHD while preserving GVL activity in mice

Allogeneic hematopoietic cell transplantation (HCT) is the most effective therapy for hematopoietic malignancies through T-cell-mediated graft-vs-leukemia (GVL) effects but often leads to severe graft-vs-host disease (GVHD). Given that protein kinase Cθ (PKCθ), in cooperation with PKCα, is essential for T-cell signaling and function, we have evaluated PKCθ and PKCα as potential therapeutic targets in allogeneic HCT using genetic and pharmacologic approaches. We found that the ability of PKCα(-/-)/θ(-/-) donor T cells to induce GVHD was further reduced compared with PKCθ(-/-) T cells in relation with the relevance of both isoforms to allogeneic donor T-cell proliferation, cytokine production, and migration to GVHD target organs. Treatment with a specific inhibitor for both PKCθ and PKCα impaired donor T-cell proliferation, migration, and chemokine/cytokine production and significantly decreased GVHD in myeloablative preclinical murine models of allogeneic HCT. Moreover, pharmacologic inhibition of PKCθ and PKCα spared T-cell cytotoxic function and GVL effects. Our findings indicate that PKCα and θ contribute to T-cell activation with overlapping functions essential for GVHD induction while less critical to the GVL effect. Thus, targeting PKCα and PKCθ signaling with pharmacologic inhibitors presents a therapeutic option for GVHD prevention while largely preserving the GVL activity in patients receiving HCT.

Protein kinase C signaling and cell cycle regulation

A link between T cell proliferation and the protein kinase C (PKC) family of serine/threonine kinases has been recognized for about 30 years. However, despite the wealth of information on PKC-mediated control of, T cell activation, understanding of the effects of PKCs on the cell cycle machinery in this cell type remains limited. Studies in other systems have revealed important cell cycle-specific effects of PKC signaling that can either positively or negatively impact proliferation. The outcome of PKC activation is highly context-dependent, with the precise cell cycle target(s) and overall effects determined by the specific isozyme involved, the timing of PKC activation, the cell type, and the signaling environment. Although PKCs can regulate all stages of the cell cycle, they appear to predominantly affect G0/G1 and G2. PKCs can modulate multiple cell cycle regulatory molecules, including cyclins, cyclin-dependent kinases (cdks), cdk inhibitors and cdc25 phosphatases; however, evidence points to Cip/Kip cdk inhibitors and D-type cyclins as key mediators of PKC-regulated cell cycle-specific effects. Several PKC isozymes can target Cip/Kip proteins to control G0/G1 → S and/or G2 → M transit, while effects on D-type cyclins regulate entry into and progression through G1. Analysis of PKC signaling in T cells has largely focused on its roles in T cell activation; thus, observed cell cycle effects are mainly positive. A prominent role is emerging for PKCθ, with non-redundant functions of other isozymes also described. Additional evidence points to PKCδ as a negative regulator of the cell cycle in these cells. As in other cell types, context-dependent effects of individual isozymes have been noted in T cells, and Cip/Kip cdk inhibitors and D-type cyclins appear to be major PKC targets. Future studies are anticipated to take advantage of the similarities between these various systems to enhance understanding of PKC-mediated cell cycle regulation in T cells.

PKCθ/β and CYLD are antagonistic partners in the NFκB and NFAT transactivation pathways in primary mouse CD3+ T lymphocytes

In T cells PKCθ mediates the activation of critical signals downstream of TCR/CD28 stimulation. We investigated the molecular mechanisms by which PKCθ regulates NFκB transactivation by examining PKCθ/β single and double knockout mice and observed a redundant involvement of PKCθ and PKCβ in this signaling pathway. Mechanistically, we define a PKCθ-CYLD protein complex and an interaction between the positive PKCθ/β and the negative CYLD signaling pathways that both converge at the level of TAK1/IKK/I-κBα/NFκB and NFAT transactivation. In Jurkat leukemic T cells, CYLD is endoproteolytically processed in the initial minutes of stimulation by the paracaspase MALT1 in a PKC-dependent fashion, which is required for robust IL-2 transcription. However, in primary T cells, CYLD processing occurs with different kinetics and an altered dependence on PKC. The formation of a direct PKCθ/CYLD complex appears to regulate the short-term spatial distribution of CYLD, subsequently affecting NFκB and NFAT repressional activity of CYLD prior to its MALT1-dependent inactivation. Taken together, our study establishes CYLD as a new and critical PKCθ interactor in T cells and reveals that antagonistic PKCθ/β-CYLD crosstalk is crucial for the adjustment of immune thresholds in primary mouse CD3⁺ T cells.

The kinase PKCα selectively upregulates interleukin-17A during Th17 cell immune responses

Transforming growth-factor β (TGFβ) has been implicated in T helper 17 (Th17) cell biology and in triggering expression of interleukin-17A (IL-17A), which is a key Th17 cell cytokine. Deregulated TGFβ receptor (TGFβR) signaling has been implicated in Th17-cell-mediated autoimmune pathogenesis. Nevertheless, the full molecular mechanisms involved in the activation of the TGFβR pathway in driving IL-17A expression remain unknown. Here, we identified protein kinase C α (PKCα) as a signaling intermediate specific to the Th17 cell subset in the activation of TGFβRI. We have shown that PKCα physically interacts and functionally cooperates with TGFβRI to promote robust SMAD2-3 activation. Furthermore, PKCα-deficient (Prkca^−/−) cells demonstrated a defect in SMAD-dependent IL-2 suppression, as well as decreased STAT3 DNA binding within the Il17a promoter. Consistently, Prkca^−/− cells failed to mount appropriate IL-17A, but not IL-17F, responses in vitro and were resistant to induction of Th17-cell-dependent experimental autoimmune encephalomyelitis in vivo.

Transient PKCα shuttling to the immunological synapse is governed by (DGK)ζ and regulates L-selectin shedding

Considerable evidence indicates that diacylglycerol (DAG) generation at the immunological synapse (IS) determines T cell functions by regulating the duration and amplitude of Ras/ERK signals. The exact mechanism by which DAG regulates Ras/ERK activation downstream of the T cell receptor (TCR) nonetheless remains poorly understood. Here we characterize PKCα as a previously unrecognized component of the machinery that translates cell receptor occupancy into Ras/ERK-propagated signals. We show transient translocation of PKCα to the IS, mediated by DAG generation at the contact area. Diacylglycerol kinase (DGK)ζ negatively regulated PKCα translocation kinetics, whereas PKCα activity limited its own persistence at the IS. Coordinated activation of DGKζ and PKCα in response to antigen recognition regulated the amplitude and duration of Ras/ERK activation; this in turn mediated early processes of T cell surface proteolysis such as L-selectin shedding. Analysis of DGKζ-deficient mice further showed that increased DAG signaling is translated to downstream elements of this pathway, as reflected by enhanced PKCα-dependent L-selectin shedding. We propose that early activation of a DAG/PKCα axis contributes to the mechanisms by which antigen affinity translates into TCR biological responses.

The yin and yang of protein kinase C-theta (PKCθ): a novel drug target for selective immunosuppression

Protein kinase C-theta (PKCθ) is a protein kinase C (PKC) family member expressed predominantly in T lymphocytes, and extensive studies addressing its function have been conducted. PKCθ is the only T cell-expressed PKC that localizes selectively to the center of the immunological synapse (IS) following conventional T cell antigen stimulation, and this unique localization is essential for PKCθ-mediated downstream signaling. While playing a minor role in T cell development, early in vitro studies relying, among others, on the use of PKCθ-deficient (Prkcq(-/-)) T cells revealed that PKCθ is required for the activation and proliferation of mature T cells, reflecting its importance in activating the transcription factors nuclear factor kappa B, activator protein-1, and nuclear factor of activated T cells, as well as for the survival of activated T cells. Upon subsequent analysis of in vivo immune responses in Prkcq(-/-) mice, it became clear that PKCθ has a selective role in the immune system: it is required for experimental Th2- and Th17-mediated allergic and autoimmune diseases, respectively, and for alloimmune responses, but is dispensable for protective responses against pathogens and for graft-versus-leukemia responses. Surprisingly, PKCθ was recently found to be excluded from the IS of regulatory T cells and to negatively regulate their suppressive function. These attributes of PKCθ make it an attractive target for catalytic or allosteric inhibitors that are expected to selectively suppress harmful inflammatory and alloimmune responses without interfering with beneficial immunity to infections. Early progress in developing such drugs is being made, but additional studies on the role of PKCθ in the human immune system are urgently needed.

Effect of short-term hyperglycemia on protein kinase C alpha activation in human erythrocytes

BACKGROUND

Diabetes mellitus, characterized by chronic hyperglycemia, is known to have a deleterious effect on erythrocyte structure and hemodynamic characteristics, which eventually contribute to diabetes-associated vascular complications. Protein kinase C alpha (PKCα) is a major regulator of many metabolic processes and structural changes in erythrocytes, and may play a significant role in the development of hyperglycemia-mediated cellular abnormalities.

AIM

We hypothesized that acute hyperglycemic stress may affect erythrocyte structure and metabolic properties through its effect on PKCα membrane content and activity.

RESULTS

Erythrocytes, from healthy individuals acutely exposed to a glucose enriched media, showed a significant decrease in the membranous fraction of PKCα and its phosphorylation (p = 0.005 and p = 0.0004, respectively). These alterations correlated with decreased affinity of PKCα to its membrane substrates (4.1R and GLUT1) and reduced RBC deformability (p = 0.017). Pre-activation of erythrocytes with PKC activator, PMA, minimized the effect of glucose on the membrane PKCα fraction and RBC deformability (p > 0.05).

CONCLUSIONS

Acute glycemia-induced inhibition of PKCα membranous translocation and activation is associated with reduced erythrocyte membrane deformability.

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.

Involvement of distinct PKC gene products in T cell functions

It is well established that members of the protein kinase C (PKC) family seem to have important roles in T cells. Focusing on the physiological and non-redundant PKC functions established in primary mouse T cells via germline gene-targeting approaches, our current knowledge defines two particularly critical PKC gene products, PKCθ and PKCα, as the "flavor of PKC" in T cells that appear to have a positive role in signaling pathways that are necessary for full antigen receptor-mediated T cell activation ex vivo and T cell-mediated immunity in vivo. Consistently, in spite of the current dogma that PKCθ inhibition might be sufficient to achieve complete immunosuppressive effects, more recent results have indicated that the pharmacological inhibition of PKCθ, and additionally, at least PKCα, appears to be needed to provide a successful approach for the prevention of allograft rejection and treatment of autoimmune diseases.

Th1/Th2 Differentiation and B Cell Function by the Atypical PKCs and Their Regulators

The members of the atypical Protein Kinase Cs (aPKC) kinase subfamily, PKCζ and PKCλ/ɩ, as well as their adapters, p62 and Par-6, form part of the PB1-domain-containing group of signaling regulators. Both adapters serve to locate through heterotypic interactions the aPKCs into the NF-κB and cell polarity pathways, respectively. Both signaling cascades have been critically implicated in T cell function in vitro and in vivo. The analysis of gene-knockout (KO) mice deficient in the different PB1 molecules is providing more definitive information on the actual role that the aPKCs and other PB1-containing molecules play in B cell biology and T cell polarity, survival, and differentiation toward the different effector lineages in vivo and at the cellular ex vivo level. Here we discuss recent data generated from the analysis of KO mice linking the control of cell polarity by PKCλ/ɩ and PKCζ, their adapter p62, and the Par-4 inhibitor, in the control of B and T cell signaling and differentiation. Altogether, these genetic and biochemical evidences reveal the existence of a PB1-orchestrated signaling network that acts to control Th2 differentiation in vitro and in vivo, and the gene transcriptional programs that are essential during the B cell maturation and function and Th2 differentiation.

Shaping up the membrane: diacylglycerol coordinates spatial orientation of signaling

Diacylglycerol signals by binding and activating C1 domain-containing proteins expressed principally in neuronal and immune tissues. This restricted expression profile suggests that diacylglycerol-regulated signals are particularly relevant in cell-cell communication processes in which active endocytosis and exocytosis take place. Not surprisingly, various experimental approaches have demonstrated a crucial role for diacylglycerol effectors and metabolizing enzymes in the control of immune responses, neuron communication and phagocytosis. Current research delineates a scenario in which coordinated decoding of diacylglycerol signals is translated into complex biological responses such as neuronal plasticity, T cell development or cytolytic killing. Diacylglycerol functions reach maximal diversity in these highly specialized systems in which signal intensity directly regulates distinct biological outcomes. This review brings together the most recent studies, emphasizing the contribution of compartmentalized DAG metabolism to orientated signaling events.

PKC-θ function at the immunological synapse: prospects for therapeutic targeting

Protein kinase C (PKC)-θ regulates conventional effector T (Teff) cell function. Since this initial finding, it has become clear that the role of PKC-θ in T cells is complex. PKC-θ plays a central role in Teff cell activation and survival, and negatively regulates stability of the immunological synapse (IS). Recent studies demonstrated that PKC-θ is required for the development of natural CD4(+)Foxp3(+) regulatory T (Treg) cells, and mediates negative regulation of Treg cell function. Here, we examine the role of PKC-θ in the IS, evidence for its distinct localization in Treg cells and the therapeutic implications of inhibiting PKC-θ in Teff cells, to reduce effector function, and in Treg cells, to increase suppressor function, for the prevention and treatment of autoimmune and alloimmune disease states.

Unique functions for protein kinase D1 and protein kinase D2 in mammalian cells

Mammalian PKD (protein kinase D) isoforms have been implicated in the regulation of diverse biological processes in response to diacylglycerol and PKC (protein kinase C) signalling. To compare the functions of PKD1 and PKD2 in vivo, we generated mice deficient in either PKD1 or PKD2 enzymatic activity, via homozygous expression of PKD1^S744A/S748A or PKD2^S707A/S711A ‘knockin’ alleles. We also examined PKD2-deficient mice generated using ‘gene-trap’ technology. We demonstrate that, unlike PKD1, PKD2 catalytic activity is dispensable for normal embryogenesis. We also show that PKD2 is the major PKD isoform expressed in lymphoid tissues, but that PKD2 catalytic activity is not essential for the development of mature peripheral T- and B-lymphocytes. PKD2 catalytic activity is, however, required for efficient antigen receptor-induced cytokine production in T-lymphocytes and for optimal T-cell-dependent antibody responses in vivo. Our results reveal a key in vivo role for PKD2 in regulating the function of mature peripheral lymphocytes during adaptive immune responses. They also confirm the functional importance of PKC-mediated serine phosphorylation of the PKD catalytic domain for PKD activation and downstream signalling and reveal that different PKD family members have unique and non-redundant roles in vivo.

TCR-induced Akt serine 473 phosphorylation is regulated by protein kinase C-alpha

Akt signaling plays a central role in T cell functions, such as proliferation, apoptosis, and regulatory T cell development. Phosphorylation at Ser(473) in the hydrophobic motif, along with Thr(308) in its activation loop, is considered necessary for Akt function. It is widely accepted that phosphoinositide-dependent kinase 1 (PDK-1) phosphorylates Akt at Thr(308), but the kinase(s) responsible for phosphorylating Akt at Ser(473) (PDK-2) remains elusive. The existence of PDK-2 is considered to be specific to cell type and stimulus. PDK-2 in T cells in response to TCR stimulation has not been clearly defined. In this study, we found that conventional PKC positively regulated TCR-induced Akt Ser(473) phosphorylation. PKC-alpha purified from T cells can phosphorylate Akt at Ser(473) in vitro upon TCR stimulation. Knockdown of PKC-alpha in T-cell-line Jurkat cells reduced TCR-induced phosphorylation of Akt as well as its downstream targets. Thus our results suggest that PKC-alpha is a candidate for PDK-2 in T cells upon TCR stimulation.