Heterogeneity of protein kinase C expression and regulation in T lymphocytes

The Phosphorylation of Kv1.3: A Modulatory Mechanism for a Multifunctional Ion Channel

The voltage-gated potassium channel Kv1.3 plays a pivotal role in a myriad of biological processes, including cell proliferation, differentiation, and apoptosis. Kv1.3 undergoes fine-tuned regulation, and its altered expression or function correlates with tumorigenesis and cancer progression. Moreover, posttranslational modifications (PTMs), such as phosphorylation, have evolved as rapid switch-like moieties that tightly modulate channel activity. In addition, kinases are promising targets in anticancer therapies. The diverse serine/threonine and tyrosine kinases function on Kv1.3 and the effects of its phosphorylation vary depending on multiple factors. For instance, Kv1.3 regulatory subunits (KCNE4 and Kvβ) can be phosphorylated, increasing the complexity of channel modulation. Scaffold proteins allow the Kv1.3 channelosome and kinase to form protein complexes, thereby favoring the attachment of phosphate groups. This review compiles the network triggers and signaling pathways that culminate in Kv1.3 phosphorylation. Alterations to Kv1.3 expression and its phosphorylation are detailed, emphasizing the importance of this channel as an anticancer target. Overall, further research on Kv1.3 kinase-dependent effects should be addressed to develop effective antineoplastic drugs while minimizing side effects. This promising field encourages basic cancer research while inspiring new therapy development.

Effects of PKI55 protein, an endogenous protein kinase C modulator, on specific PKC isoforms activity and on human T cells proliferation

PKI55 protein, coded by the recently identified KI55 gene [R. Selvatici, E. Melloni, M. Ferrati, C. Piubello, F.C. Marincola, E. Gandini, J. Mol. Evol. 57 (2003) 131-139] is synthesized following protein kinase C (PKC) activation and acts as a PKC modulator, establishing a feedback loop of inhibition. In this work, PKI55 was found to inhibit recombinant alpha, beta(1), beta(2), gamma, delta, zeta and eta PKC isoforms; the effect on conventional PKC was lost in the absence of calcium. Confocal immunofluorescence experiments showed that PKI55 can penetrate into peripheral blood mononuclear cells (PBMC), following a coordinated movement of calcium ions. The addition of PKI55 protein down-regulated the PKC enzyme activity in phytohaemagglutinin-activated PBMC, decreasing the activity of alpha, beta(1) and beta(2) PKC isoforms. Moreover, inhibition in PBMC proliferation was observed. Similar effects were detected in Jurkat T cells transfected with a plasmid containing the coding sequence of PKI55. The PKI55 protein functional role could be to control the pathological over-expression of specific PKC isoforms and to regulate proliferation.

Inhibition of Fas-mediated apoptotic cell death of murine T lymphocytes in a mouse model of immunosenescence in linkage to deterioration in cell membrane raft function

We previously developed a transgenic mouse line into which a rabbit protein kinase Calpha (PKCalpha) gene fused to a human CD2 promoter/enhancer was introduced, and we found that immunosenescence was facilitated in these transgenic mice. In this study, we found that along with age-dependent increase in the level of protein expression of PKCalpha and its translocation to the membrane, activated T cells became less sensitive to apoptosis-inducing anti-Fas antibody. The capacity of T cells to express Fas antigen on their surfaces in response to anti-CD3 and interleukin-2 was impaired in PKCalpha-transgenic mice of relatively advanced age, although background Fas expression levels on T cells from those mice were high. We then found that out of proportion to a high level of cell surface Fas expression the density of cholera toxin B (CTx)-binding raft elements decreased in PKCalpha-transgenic mice of relatively advanced age and to a lesser extent in normal mice of advanced age. Correspondingly, the expression level of raft-associating Lck was decreased in these mice. These findings suggest for the first time that immunosenescence of T cells involves a decrease in density of cell surface CTx-binding raft elements, which might underlie a deterioration in T-cell signal pathway for either cell death or cell activation.

Characterization of expression of protein kinase C isozymes in human B-cell lymphoma: Relationship between its expression and prognosis

Protein kinase C (PKC) enzymes play a major role in signal transduction and contribute to the regulation of cellular differentiation and proliferation. However, little is known about subtype-specific intracellular expression of PKC in human malignant lymphoma. To characterize the relationship between expression of PKC and B-cell lymphomas based on the different subspecies, we investigated the expression of four subspecies (alpha, beta II, gamma and delta) in five cases of reactive lymphoid tissues, 77 cases of human B-cell lymphoma and 17 human lymphoma cell lines. In the reactive lymphoid tissues, PKC beta II-positive cells were found in the mantle zones and marginal zones, and centroblasts and centrocytes in the germinal centers showed cytoplasmic staining with strong intensity against PKC delta. The present study is the first report to examine the expression of PKC delta in reactive lymphoid tissues. In interfollicular areas, a small number of T-cells were positive for PKC alpha. Protein kinase C gamma-positive cells were not found in these lymphoid tissues. Eight cases of Burkitt lymphoma (BL) (8/10; 80%) showed the overexpression of PKC alpha (P < 0.01), but other B-cell lymphoma cases except three cases of diffuse large B-cell lymphoma did not express PKC alpha. In addition, six and eight out of nine BL cell lines expressed the protein and mRNA of PKC alpha, respectively. These results indicate that PKC alpha was predominantly expressed on BL in comparison with other types of lymphoma. The expression of PKC gamma was observed in only five cases of BL. The overall survival of PKC gamma-positive BL was significantly better than that of PKC gamma-negative BL (P < 0.05). The expression of PKC gamma seems to be associated with a better prognosis in the limited number of BL cases in the present study.

Metabolic labelling of membrane microdomains/rafts in Jurkat cells indicates the presence of glycerophospholipids implicated in signal transduction by the CD3 T-cell receptor

Cell membranes contain sphingolipids and cholesterol, which cluster together in distinct domains called rafts. The outer-membrane leaflet of these peculiar membrane domains contains glycosylphosphatidylinositol-anchored proteins, while the inner leaflet contains proteins implicated in signalling, such as the acylated protein kinase p56(lck) and the palmitoylated adaptator LAT (linker for activation of T-cells). We present here an approach to study the lipid composition of rafts and its change upon T-cell activation. Our method is based on metabolic labelling of Jurkat T-cells with different precursors of glycerophospholipid synthesis, including glycerol and fatty acids with different lengths and degrees of saturation as well as phospholipid polar head groups. The results obtained indicate that lipid rafts isolated by the use of sucrose density-gradient centrifugation after Triton X-100 extraction in the cold, besides sphingolipids and cholesterol, contain unambiguously all classes of glycerophospholipids: phosphatidylserine, phosphatidylinositol, phosphatidylethanolamine and phosphatidylcholine. Fatty acid labelling shows that lipid rafts are labelled preferentially with saturated fatty acids while the rest of the plasma membrane incorporates mostly long-chained polyunsaturated fatty acids. To see whether the raft composition as measured by metabolic labelling of phospholipids is involved in T-cell activation, we investigated the production of sn-1,2-diacylglycerol (DAG) in CD3-activated cells. DAG production occurs within rafts, confirming previous demonstration of protein kinase C translocation into membrane microdomains. Our data demonstrate that raft disorganization by methyl-beta-cyclodextrin impairs both CD3-induced DAG production and changes in cytosolic Ca(2+) concentration. These lines of evidence support the conclusion that the major events in T-cell activation occur within or due to lipid rafts.

Protein kinase C and calcineurin synergize to activate IkappaB kinase and NF-kappaB in T lymphocytes

The nuclear factor of kappaB (NF-kappaB) is a ubiquitous transcription factor that is key in the regulation of the immune response and inflammation. T cell receptor (TCR) cross-linking is in part required for activation of NF-kappaB, which is dependent on the phosphorylation and degradation of IkappaBalpha. By using Jurkat and primary human T lymphocytes, we demonstrate that the simultaneous activation of two second messengers of the TCR-initiated signal transduction, protein kinase C (PKC) and calcineurin, results in the synergistic activation of the IkappaBalpha kinase (IKK) complex but not of another putative IkappaBalpha kinase, p90(rsk). We also demonstrate that the IKK complex, but not p90(rsk), is responsible for the in vivo phosphorylation of IkappaBalpha mediated by the co-activation of PKC and calcineurin. Each second messenger is necessary, as inhibition of either one reverses the activation of the IKK complex and IkappaBalpha phosphorylation in vivo. Overexpression of dominant negative forms of IKKalpha and -beta demonstrates that only IKKbeta is the target for PKC and calcineurin. These results indicate that within the TCR/CD3 signal transduction pathway both PKC and calcineurin are required for the effective activation of the IKK complex and NF-kappaB in T lymphocytes.

Phorbol ester-induced disruption of the CD4-Lck complex occurs within a detergent-resistant microdomain of the plasma membrane. Involvement of the translocation of activated protein kinase C isoforms

Recent studies have highlighted the existence of discrete microdomains at the cell surface that are distinct from caveolae. The function of these microdomains remains unknown. However, recent evidence suggests that they may participate in a subset of transmembrane signaling events. In hematopoietic cells, these low density Triton-insoluble (LDTI) microdomains (also called caveolae-related domains) are dramatically enriched in signaling molecules, such as cell surface receptors (CD4 and CD55), Src family tyrosine kinases (Lyn, Lck, Hck, and Fyn), heterotrimeric G proteins, and gangliosides (GM1 and GM3). Human T lymphocytes have become a well established model system for studying the process of phorbol ester-induced down-regulation of CD4. Here, we present evidence that phorbol 12-myristate 13-acetate (PMA)-induced down-regulation of the cell surface pool of CD4 occurs within the LDTI microdomains of T cells. Localization of CD4 in LDTI microdomains was confirmed by immunoelectron microscopy. PMA-induced disruption of the CD4-Lck complex was rapid (within 5 min), and this disruption occurred within LDTI microdomains. Because PMA is an activator of protein kinase C (PKC), we next evaluated the possible roles of different PKC isoforms in this process. Our results indicate that PMA induced the rapid translocation of cytosolic PKCs to LDTI microdomains. We identified PKCalpha as the major isoform involved in this translocation event. Taken together, our results support the hypothesis that LDTI microdomains represent a functionally important plasma membrane compartment in T cells.

Inhibition of protein kinase C alpha expression by antisense RNA in transfected Jurkat cells

Taking the antisense approach to inhibit the expression of specific protein kinase C (PKC) isoforms, we investigated the function of PKC alpha in T cell activation by transfecting Jurkat cells with an episomal vector (pREP3) containing a copy of the corresponding gene in the antisense orientation. Transfected Jurkat cells were selected with hygromycin and cloned by limiting dilution. Two (as1/as2) stably transfected antisense PKC alpha-pREP3 clones (as PKC alpha-pREP3) exhibited consistently reductions (76% and 85%, respectively) of PKC alpha levels when analyzed by immunoblotting and immunoprecipitation and also of PKC alpha mRNA (75%, as determined by Northern blotting) when compared to control clones (C1/C2) containing the pREP3 vector alone. The ability of the as-PKC alpha-pREP3 construct to specifically reduce PKC alpha levels in both clones was demonstrated by Western blots probed with antibodies against the PKC beta isozyme (the form structurally more similar to PKC alpha) and other representative isoenzymes expressed in Jurkat cells (PKC delta, epsilon, theta, and mu). Stimulation of transfected Jurkat clones with phorbol-12-myristate-13 alone or in the presence of ionomycin resulted in significant reduction of IL-2R alpha expression, TNF-alpha production, and the induction of transcriptional activity of a pIL-2/Luc construct in both as PKC alpha-reduced clones. The magnitude of these decrements paralleled the reductions of PKC alpha expression. The loss of the effects in clone as1 after a high number of passages correlated with the recovery of normal levels of PKC alpha protein, suggesting a link between these processes. Thus, the findings of this study demonstrate the essential role that PKC alpha plays in major events of the T lymphocyte activation process.

Differential regulation of discrete apoptotic pathways by Ras

The products of the ras genes are known to regulate cell proliferation and differentiation; recently, they have been found to play a role in apoptosis. The expression of oncogenic p21(ras) in a number of cell types, including Jurkat (a human T lymphoblastoid cell line) and murine fibroblasts, makes the cells susceptible to apoptosis following suppression of protein kinase C (PKC) activity (PKC/Ras-mediated apoptosis). Engagement of Fas antigen, a potent effector of apoptosis, activates cellular p21(ras), which may be required for completion of the cell death program. To further investigate the role of p21(ras) in the regulation of apoptosis, the cellular mechanisms employed in these two apoptotic processes in which Ras activity is involved (PKC/Ras-related and Fas-triggered apoptosis), was explored. Increasing p21(ras) activity by expressing v-ras or by treatment with an antisense oligonucleotide to the GTPase-activating protein was found to accelerate the Fas-mediated apoptotic process in Jurkat and mouse LF cells. PKC/Ras-related apoptosis was associated with, and required, cell cycle progression, accompanied by the expression of the G1/S cyclins. In contrast, Fas engagement, although inducing a vigorous and PKC-independent activation of endogenous p21(ras), did not alter cell cycle progression, nor did it require such progression for apoptosis. Both the protein synthesis inhibitor cycloheximide and cyclin E antisense oligonucleotides partially abolished PKC/Ras-mediated apoptosis but had only a moderate effect on Fas-induced apoptosis. In contrast, the CED-3/interleukin-1beta-converting enzyme (ICE) protease inhibitor Z-VADfmk efficiently suppressed Fas-induced apoptosis and only marginally inhibited PKC/Ras-mediated apoptosis. Induction of both pathways resulted in activation of the Jun NH2-terminal kinase/JUN signaling system. These results suggest that different cell death programs, such as PKC/Ras-mediated and Fas-mediated apoptosis, may be interconnected via p21(ras) and perhaps Jun NH2-terminal kinase/JUN. In response to various death stimuli, p21(ras) may act as a common intermediate regulator in the transduction of apoptotic signals.

Linkage of protein kinase C-beta activation and intracellular interleukin-2 accumulation in human naive CD4 T cells

A critical role for protein kinase C (PKC) in signal transduction events has been well established. Moreover, studies of regulation in PKC levels suggest participation in mediating long-term cellular functions. Protein kinase C-beta (PKC-beta) has been reported to be involved in interleukin-2 (IL-2) synthesis in T lymphocytes. In this study, the role of PKC-beta in intracellular accumulation of IL-2 was investigated using specific inhibitors. Preincubation with two different PKC inhibitors, one specific for classical isotypes (alpha and beta I) Go6976, and one which inhibits both classical and non-classical isotypes, GF109203X, caused a complete block in cytoplasmic IL-2 accumulation when naive CD4 T cells were stimulated in the presence of CD2+CD28+phorbol myristate acetate (PMA). In contrast, preincubation with up to 1000 ng/ml of cyclosporin A (CsA) resulted in a reduction in the intracellular IL-2 detected, as observed by a decrease in the proportion of positive cells as well as a fall in the mean fluorescence intensity (MFI). CsA did not influence PKC-beta translocation. Flow cytometric assessments of PKC-beta and its isoforms beta I and beta II correlated with Western blotting analysis and these results were further supported by the use of PKC-beta-positive (HUT 78) and -negative (BW5147) T-cell lines. Using the specific inhibitors, Go6976 and GF109203X, the findings in this study suggest that activation and translocation of PKC-beta is critical for accumulation of intracellular IL-2. The influence of CsA in reducing but not blocking IL-2 synthesis is discussed. PMA-induced down-regulation of the CD4 antigen was observed in the presence of Go6976 and but not GF109203X, suggesting regulation by non-classical PKC isoforms.

A role for protein kinase CbetaI in the regulation of Ca2+ entry in Jurkat T cells

T cell activation leading to cytokine production and cellular proliferation involves a regulated increase and subsequent decrease in the intracellular concentration of Ca2+ ([Ca2+]i). While much is understood about agonist-induced increases in [Ca2+]i, less is known about down-regulation of this pathway. Understanding the mechanism of this down-regulation is critical to the prevention of cell death that can be the consequence of a sustained elevation in [Ca2+]i. Protein kinase C (PKC), activated by the diacylglycerol produced as a consequence of T cell receptor engagement, has long been presumed to be involved in this down-regulation, although the precise mechanism is not wholly clear. In this report we demonstrate that activation of PKC by phorbol esters slightly decreases the rate of Ca2+ efflux from the cytosol of Jurkat T cells following stimulation through the T cell receptor or stimulation in a receptor-independent manner by thapsigargin. On the other hand, phorbol ester treatment dramatically reduces the rate of Ca2+ influx following stimulation. Phorbol ester treatment is without an effect on Ca2+ influx in a different T cell line, HSB. Down-regulation of PKCbetaI expression by 18-h phorbol ester treatment is associated with a loss of the response to acute phorbol ester treatment in Jurkat cells, suggesting that PKCbetaI may be the isozyme responsible for the effects on Ca2+ influx. Electroporation of an anti-PKCbetaI antibody, but not antibodies against PKCalpha or PKCgamma, led to an increase in the rate of Ca2+ influx following stimulation. Taken together, these data suggest that PKCbetaI may be a component of the down-regulation of increases in [Ca2+]i associated with Jurkat T cell activation.

Calcium-dependent immediate-early gene induction in lymphocytes is negatively regulated by p21Ha-ras

The induction of immediate-early (IE) response genes, such as egr-1, c-fos, and c-jun, occurs rapidly after the activation of T lymphocytes. The process of activation involves calcium mobilization, activation of protein kinase C (PKC), and phosphorylation of tyrosine kinases. p21(ras), a guanine nucleotide binding factor, mediates T-cell signal transduction through PKC-dependent and PKC-independent pathways. The involvement of p21(ras) in the regulation of calcium-dependent signals has been suggested through analysis of its role in the activation of NF-AT. We have investigated the inductions of the IE genes in response to calcium signals in Jurkat cells (in the presence of activated p21(ras)) and their correlated consequences. The expression of activated p21(ras) negatively regulated the induction of IE genes by calcium ionophore. This inhibition of calcium-activated IE gene induction was reversed by treatment with cyclosporin A, suggesting the involvement of calcineurin in this regulation. A later result of inhibition of this activation pathway by p21(ras) was down-regulation of the activity of the transcription factor AP-1 and subsequent coordinate reductions in IL-2 gene expression and protein production. These results suggest that p2l(ras) is an essential mediator in generating not only positive but also negative modulatory mechanisms controlling the competence of T cells in response to inductive stimulations.

Protein kinase C isotype expression and regulation of lymphoid cell motility

Lymphocyte migration into inflammatory sites involves a change from a spherical, non-motile phenotype to an irregular, constantly shape-changing, motile phenotype. We have previously shown that lymphocytes are maintained in the non-motile state by the constitutive activity of protein kinase C (PKC). In this paper we have attempted to identify the PKC isotype which regulates these morphological changes by three different approaches. (a) Motile and non-motile T-cell lines were compared for expression of the alpha, beta I, beta II, gamma, delta, epsilon, eta, zeta and theta isotypes by Western blotting. There was no obvious correlation of isotype expression with motility. (b) Two different PKC inhibitors, one specific for classical isotypes, Go6976 and the other GF109203X, which inhibits both classical and non-classical isotypes were compared for induction of motility in non-motile lymphocytes. Only GF109203X induced motility implying that a non-classical isotype is involved. (c) Non-motile lymphocytes were chronically treated with the PKC activator bryostatin and the time courses of induction of motility and downregulation of PKC isotypes were compared. Induction of motility correlated better with downregulation of epsilon, eta and theta than with alpha or beta. It is concluded that the data fit best with the involvement of a non-classical PKC isotype in regulating lymphocyte motility although no association with a particular isotype was found.

Two signaling pathways can lead to Fas ligand expression in CD8+ cytotoxic T lymphocyte clones

As shown previously, a given cytotoxic T lymphocyte (CTL) clone (KB5.C20) could be induced to express the Fas ligand (FasL) by either T cell receptor (TCR) engagement or phorbol 12-myristate 13-acetate (PMA)/ionomycin stimulation. In contrast, another CTL clone (BM3.3) has now been found to exert Fas-based cytotoxicity only after TCR engagement, but not after PMA/ionomycin stimulation. This suggested the existence of a PMA-insensitive, antigen-induced pathway leading to FasL expression. The inability of PMA to promote Fas-based cytotoxicity in BM3.3 cells was correlated with a defect in expression of the classical protein kinase C (PKC) isoforms alpha and beta I. In KB5.C20 cells depleted of PMA-sensitive PKC isoforms and thus no longer responsive to PMA, Fas-based cytotoxicity could still be induced via the TCR/CD3 pathway. On the other hand, a requirement for phosphatidylinositol-3 kinase (PI3K) selectively in this TCR/CD3-induced pathway was demonstrated by specific inhibition with wortmannin. These results suggest that FasL expression when induced via the TCR/CD3 involves PI3K, and when induced by PMA/ionomycin requires the expression of PMA-sensitive PKC isoforms absent in clone BM3.3. Additional data suggest that in neither case was NF-kappa B activation implicated in FasL expression.

Protein kinase C modulates cytosolic free calcium by stimulating calcium pump activity in Jurkat T cells

Although protein kinase C (PKC) activation has been shown to inhibit Ca2+ influx in T lymphocytes, the role of PKC on Ca2+ sequestration or extrusion processes has not been fully explored. We examined the effect of CD3 stimulation and PKC activators on cytosolic Ca2+ (Ca2+i) extrusion and 45Ca2+ efflux in human leukemic Jurkat T cells. Treatment of Fura-2 loaded cells with phorbol 12-myristate 13-acetate (PMA) or thymeleatoxin (THYM) resulted in a decrease in Ca2+i both in the presence and absence of extracellular Ca2+, whereas inactive phorbol esters had no effect. PKC activators added at the peak of a Ca2+i transient induced by anti-CD3 mAb, ionomycin or thapsigargin (TG) stimulated the rate and extent of return of Ca2+i to basal levels by 17-53%. PKC stimulation of the Ca2+i decline was not enhanced by the presence of Na+, indicating that PKC activators increase Ca2+ pump activity rather than a Na+/Ca2+ exchange mechanism. As CD3 receptor activation enhanced the Ca2+i decline in TG-treated cells, antigen-mediated activation of phospholipase C (PLC) signaling includes enhanced Ca2+ extrusion at the plasma membrane. The effect of PKC activators on parameters of Ca2+i extrusion were further explored. PMA significantly increased the rate of Ca2+ extrusion in TG-treated cells from 0.28 +/- 0.02 to 0.35 +/- 0.03 s-1 (mean +/- SEM) and stimulated the initial rate of 45Ca2+ efflux by 69% compared to inactive phorbol ester treated cells. The effects of PKC activation on the Ca2+i decline were eliminated by PKC inhibitors, PKC down regulation (24 h PMA pretreatment), ATP-depletion and conditions that inhibited the Ca2+ pump. In contrast, pretreatment of cells with okadaic acid enhanced the PMA-stimulated response. We suggest that Jurkat T cells contain a PKC-sensitive Ca2+ extrusion mechanism likely to be the Ca2+ pump. In lymphocytes, receptor/PLC-linked PKC activation modulates Ca2+i not only by inhibiting Ca2+ influx but also by stimulating plasma membrane Ca2+i extrusion.

Elucidation of the protein kinase C-dependent apoptosis pathway in distinct subsets of T lymphocytes in MRL-lpr/lpr mice

MRL-lpr mice are severely impaired in the Fas pathway of apoptosis induction. We here evaluate another pathway of apoptosis induction in MRL-lpr mice which is protein kinase C (PKC) dependent. Despite the defect of the Fas pathway, apoptosis developed during culture in vitro in splenic T lymphocytes from MRL-lpr mice more extensively than in T lymphocytes from MRL-(+/+) mice. Apoptosis induction in the former cells was then found to be greatly promoted by PKC inhibitor H-7, and partially prevented by PKC activator phorbol 12-myristate 13-acetate (PMA). High sensitivity to H-7, but not to PKA inhibitor HA 1004, of these cells for apoptosis induction was confirmed by detailed time course and dose-dependency experiments of the drug effect. Population analysis showed that both CD4+ T lymphocytes and CD8+ T lymphocytes from MRL-lpr mice were highly sensitive to H-7, whereas CD8+ T lymphocytes, but not CD4+ T lymphocytes, from MRL-(+/+) mice were susceptible to the reagent. Interestingly, B220+ Thy-1+ CD4-CD8- T lymphocytes from MRL-lpr mice were most sensitive to H-7 for apoptosis induction. Correspondingly, the membrane-translocated activated PKC-alpha level in splenic T lymphocytes from MRL-lpr was more extensively up-regulated by PMA than in splenic T lymphocytes from MRL-(+/+). These results suggest that some signal consistently activates PKC in MRL-lpr T lymphocytes, and this event is needed for survival of these cells. On the other hand, CD4+ CD8+ thymocytes were deleted by apoptosis in culture with PMA, whether these thymocytes were from MRL-lpr mice or MRL-(+/+) mice. This finding suggested that the apoptosis induction pathway linked to PKC activation is intact in CD4+ CD8+ thymocytes from the Fas-defective MRL-lpr mice. We conclude from these results that the PKC-dependent signal pathways for either cell death or cell activation are intact or even accelerated in lpr mice, which could both compensate for the loss of the Fas pathway and promote the generation of autoreactive T lymphocytes.

Anti-CD3-induced changes in protein kinase C isozymes expression in human CD4+ and CD8+ T lymphocytes

In order to determine whether there is a differential expression and activation of PKC isozymes between CD4+ and CD8+ T cells, peripheral blood mononuclear cells were stimulated with anti-CD3 monoclonal antibody (moAb) for various time intervals and the expression of calcium-dependent PKC isozymes (alpha, beta, gamma) and calcium-independent PKC isozymes (delta, epsilon, zeta) was analyzed with dual color flow cytometry, using anti-PKC isozyme antibodies and anti-CD4 or anti-CD8 antibodies. The basal fluorescence intensity of all PKC isozymes was comparable between CD4+ T cells and CD8+ T cells. Following activation with anti-CD3 moAb a marked increase in the fluorescence intensity of all PKC isozymes in both CD4+ and CD8+ T cells, albeit to a different extent and with different kinetics was observed. Among all PKC isozymes studied, the least striking changes were observed in PKC zeta isozyme and the most striking changes were observed in PKC-epsilon isozyme. Laser-based confocal microscopic studies confirmed that the increase in fluorescence intensity of PKC isozymes following anti-CD3 moAb stimulation, as measured by flow cytometry was accompanied by the translocation of PKC isozymes from cytosol to the plasma membrane. This study demonstrates a differential effect of anti-CD3 moAb on the expression of PKC isozymes between CD4+ and CD8+ T cells and suggests that flow cytometry can be used to study the translocation of PKC isozymes from cytosol to the plasma membrane.

Regulation of non-classical protein kinase C isoenzymes in a human T cell line

We have examined the expression and responses to activation, of novel/atypical protein kinase C (PKC) isoforms epsilon, zeta, and delta in the T cell lymphoma cell line K-4. The effects of 1-h phorbol 12-myristate 13-acetate (PMA) and OKT3 activation of K-4 cells on PKC isoform distribution were examined. In addition, the effects of PMA-mediated down-regulation on the expression of PKC epsilon and zeta were determined using high concentrations of PMA over 24- and 48-h time periods in these cells. PKC zeta expression was not altered by incubation of K-4 cells with up to 200 ng/ml PMA over a 24- or 48-h period. PKC epsilon was down-regulated in a concentration-dependent manner by PMA after both 24- and 48-h of activation. Expression of PKC epsilon was not completely depressed, however, even at the highest concentration of the phorbol ester after 48-h incubation with PMA. The presence of PKC epsilon, zeta, and delta was confirmed by immunohistochemistry with distinct patterns of expression observed. PMA-induced PKC activation for a 1-h period resulted in a translocation of PKC delta from resting cytoplasmic/nuclear staining to a cytoplasmic aggregate. Following 1-h activation through the T cell receptor-associated complex CD3, PKC delta translocated from a peri-nuclear/cytoplasmic compartment to a putative cytoskeletal location in K-4 cells. This translocation was time dependent and redistributed to a cytoplasmic aggregate prior to the cytoskeleton. Similarly, following 1-h activation through the T cell receptor, PKC zeta redistributed directly to what is possibly a cytoskeletal cell compartment. The cytoplasmic distribution of PKC zeta was unaltered following activation with PMA over a 1-h time period. There was no apparent redistribution of PKC epsilon cytoplasmic staining pattern following a 1-h direct or indirect activation. These results underline the differences in individual PKC isoform distribution, and responses to different stimuli, thereby providing additional evidence for the use of discrete PKC isoform signaling pathways in T cells. Furthermore, this data underlines the differences in PMA-mediated PKC activation and activation through the T cell receptor.

Raf-1 and ERK2 kinases are required for phorbol 12,13-dibutyrate-stimulated proliferation of rat lymphoblasts. ERK2 activation precedes Raf-1 hyperphosphorylation

Rat lymphoblasts are arrested in the G1 phase of the cell cycle and can be promoted to proceed up to the S phase, when they are stimulated by phorbol ester. In this work, we have studied some details of the phorbol 12,13-dibutyrate (PBu2)-stimulated proliferation. We show that in response to PBu2 at least four different protein kinase C (PKC) isoforms translocate to the membrane. A specific PKC zeta antibody recognizes two bands of 75 and 82 kDa. These two activities are separated using a Mono Q chromatography and we show that p75 is the classical PKC zeta isoform, while p82 might be a related isoform which is PBu2 sensitive. Our data show that there is a correlation between the ability of PBu2 to promote mitogenesis and to activate ERK2 kinase, suggesting that ERK2 kinase might be the limiting step of the process. We also show that ERK kinase activation precedes Raf-1 kinase hyperphosphorylation, suggesting that Raf-1 kinase activation is not required for ERK kinase activation. This idea was checked using a Raf-1 kinase antisense (AS) oligonucleotide. The results obtained with the Raf-1 AS oligonucleotide indicate that this serine/threonine kinase is dispensable for ERK kinase activation, but needed for the PBu2 mitogenic signaling even as late as 7 h after the delivery of the signal.

12-Deoxyphorbol-13-O-phenylacetate 20 acetate [an agonist of protein kinase C beta 1 (PKC beta 1)] induces DNA synthesis, interleukin-2 (IL-2) production, IL-2 receptor alpha-chain (CD25) and beta-chain (CD122) expression, and translocation of PKC beta isozyme in human peripheral blood lymphocytes: evidence for a role of PKC beta 1 in human T cell activation

To determine a role of protein kinase C (PKC) isozymes in lymphocyte activation, human peripheral blood mononuclear cells were activated with 12-deoxyphorbol-13-O-phenylacetate (dPP; an agonist of both calcium-dependent and calcium-independent PKC isozymes), thymeleatoxin (TX; an activator of calcium-dependent PKC alpha, beta, and gamma), and 12-deoxyphorbol-13-O-phenylacetate 20 acetate (dPPA; an activator of PKC beta 1 isozyme) and examined for DNA synthesis, lymphocyte proliferation, interleukin-2 (IL-2) production, expression of IL-2 receptor alpha and beta chains on CD3+, CD4+, and CD8+ T lymphocytes and CD20+ B lymphocytes, and translocation of PKC beta isozyme from cytosol to membrane fraction. The results show that dPPA activates lymphocytes by inducing the above changes in a manner analogous to that of dPP, TX, and phorbol myristate acetate. These data suggest that PKC beta 1 is involved in the activation of human peripheral blood T and B lymphocytes.

Differential expression of PKC alpha and PKC beta isozymes in CD4+, CD8+ and CD4+/CD8+ double positive human T cells

Using specific monoclonal and polyclonal antibodies, we have analyzed protein kinase C alpha and beta isozyme expression in human T cells from peripheral blood (PB) and from thymus. While the PKC beta isozyme was present in all T cell sub-types isolated from both PB and thymus, the alpha isozyme was found only in single positive CD4+ thymocytes, in PB-CD4+ lymphocytes and in PB-CD8+ T cells from several donors. It was absent from both, CD8+ and double positive CD4+/CD8+ thymocytes. These results show that PKC alpha and -beta are differentially regulated during intra-thymic development and suggest that PKC alpha plays a specific role in helper T cell function.

Kinetics of calcium transport across the lymphocyte plasma membrane

We have investigated plasma membrane Ca2+ transport by monitoring the fluorescence of human peripheral T-lymphocytes loaded with fura 2. Thapsigargin (TG) was utilized the block the Ca(2+)-ATPase of the endoplasmic reticulum and elevate the cytosolic Ca2+ (Ca2+i). Ca2+ influx was inhibited by chelating extracellular Ca2+ with ethylene glycol-bis(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid (EGTA). The rate of decline in the Ca2+i signal of TG-treated lymphocytes after exposure to EGTA was used to assess Ca2+ extrusion across the plasma membrane. Initial rates of Ca2+i decline were examined in cells suspended in Na(+)-containing and Na(+)-free solutions; initial rates were linearly related to the [Ca2+]i at the onset of the Ca2+i decline and were unaffected by varying the extracellular Ca2+. Extracellular Na+ increased the rate of Ca2+ extrusion and decreased the threshold [Ca2+]i for extrusion, indicating a substantial role for the Na(+)-Ca2+ exchange in Ca2+i homeostasis. Both decreased temperature and calmodulin inhibition significantly slowed the Ca2+i decline in Na(+)-free HEPES-buffered solution, suggesting Ca2+ extrusion under these conditions was mediated by the Ca2+ pump. Protein kinase C (PKC) activation or inhibition did not affect the Ca2+i decline parameters. However, Ca2+ accumulation and Mn2+ (a Ca2+ surrogate) uptake were significantly and Mn2+ (a Ca2+ surrogate) uptake were significantly inhibited by activators of PKC. Cyclic nucleotides altered neither the parameters of the Ca2+i decline nor Mn2+ uptake. Thus human T-lymphocytes exhibit Na(+)- and Ca(2+)-dependent transporters characterized as the Na(+)-Ca2+ exchanger and Ca2+ pump. The main effect of PKC in these cells is the modulation of Ca2+ entry across the lymphocyte plasma membrane.

Protein kinase C isoenzymes: divergence in signal transduction?

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CD3 antigen-mediated calcium signals and protein kinase C activation are higher in CD45R0+ than in CD45RA+ human T lymphocyte subsets

T lymphocytes may be separated into subsets according to their expression of CD45 isoforms. The CD45R0+ T cell subset has been reported to proliferate in response to recall antigen and to mitogenic mAb to a much greater extent than the CD45RA+ subset. This difference could be due to more efficient coupling of the T cell antigen receptor complex to mitogenic signaling pathways. To investigate this possibility, CD3 antigen-induced calcium signals, diacylglycerol (DAG) production and protein kinase C (PKC) activation levels were compared in CD45RA+ and CD45R0+ human T lymphocyte subsets derived from peripheral blood. The mean CD3-induced rise in intracellular calcium was 80% greater in CD45R0+ than in CD45RA+ cells. Basal DAG levels in CD45R0+ cells were found to be, on average, 60% higher than in CD45RA+ cells (p = 0.002), but the CD3-induced production of DAG over background was not different in the two subsets (p = 0.4). Basal PKC activity, and CD3-induced PKC activation levels over background, were found to be 50% and 140% higher, respectively, in CD45R0+ cells than in CD45RA+ cells (p = 0.015 and 0.023). The CD45R0+ subset contained a higher proportion of cells expressing activation markers, such as CD25, CD71 and major histocompatibility complex class II, when compared to the CD45RA+ subset. Our results suggest that the elevated basal DAG levels observed in the CD45R0+ subset may reflect the recent activation of these cells. Both the higher basal DAG and CD3-induced elevation in intracellular calcium observed in the CD45R0+ cells may contribute to the greater PKC activation signals triggered by CD3 mAb in this subset. These findings elucidate the greater response of CD45R0+ T cells to mitogenic stimuli compared to CD45RA+ cells.

Role of protein kinase C in T-cell antigen receptor regulation of p21ras: evidence that two p21ras regulatory pathways coexist in T cells

T-lymphocyte activation via the antigen receptor complex (TCR) results in accumulation of p21ras in the active GTP-bound state. Stimulation of protein kinase C (PKC) can also activate p21ras, and it has been proposed that the TCR effect on p21ras occurs as a consequence of TCR regulation of PKC. To test the role of PKC in TCR regulation of p21ras, a permeabilized cell system was used to examine TCR regulation of p21ras under conditions in which TCR activation of PKC was blocked, first by using a PKC pseudosubstrate peptide inhibitor and second by using ionic conditions that prevent phosphatidyl inositol hydrolysis and hence diacylglycerol production and PKC stimulation. The data show that TCR-induced p21ras activation is not mediated exclusively by PKC. Thus, in the absence of PKC stimulation, the TCR was still able to induce accumulation of p21ras-GTP complexes, and this stimulation correlated with an inactivation of p21ras GTPase-activating proteins. The protein tyrosine kinase inhibitor herbimycin could prevent the non-PKC-mediated, TCR-induced stimulation of p21ras. These data indicate that two mechanisms for p21ras regulation coexist in T cells: one PKC mediated and one not. The TCR can apparently couple to p21ras via a non-PKC-controlled route that may involve tyrosine kinases.

Immunomodulatory effects of therapeutic gold compounds. Gold sodium thiomalate inhibits the activity of T cell protein kinase C

Previous studies have shown that the gold compounds, gold sodium thiomalate (GST) and auranofin (AUR), which are effective in the treatment of rheumatoid arthritis, inhibit functional activities of a variety of cells, but the biochemical basis of their effect is unknown. In the current studies, human T cell proliferation and interleukin 2 production by Jurkat cells were inhibited by GST or AUR at pharmacologically relevant concentrations. Because it has been documented that protein kinase C (PKC) is involved in T cell activation, the capacity of gold compounds to inhibit PKC partially purified from Jurkat cells was assayed in vitro. GST was found to inhibit PKC in a dose-dependent manner, but AUR caused no significant inhibition of PKC at pharmacologically relevant concentrations. The inhibitory effect of GST on PKC was abolished by 2-mercaptoethanol. To investigate the effect of GST on the regulation of PKC in vivo, the levels of PKC activity in Jurkat cells were examined. Cytosolic PKC activity decreased slowly in a concentration- and time-dependent manner as a result of incubation of Jurkat cells with GST. To ascertain whether GST inhibited PKC translocation and down-regulation, PKC activities associated with the membrane and cystosolic fractions were evaluated after phorbol myristate acetate (PMA) stimulation of GST incubated Jurkat cells. Translocation of PKC was markedly inhibited by pretreatment of Jurkat cells with GST for 3 d, but the capacity of PMA to down-regulate PKC activity in Jurkat cells was not altered by GST preincubation. The functional impact of GST-mediated downregulation of PKC in Jurkat cells was examined by analyzing PMA-stimulated phosphorylation of CD3. Although GST preincubated Jurkat cells exhibited an increased density of CD3, PMA-stimulated phosphorylation of the gamma chain of CD3 was markedly inhibited. Specificity for the inhibitory effect of GST on PKC was suggested by the finding that GST did not alter the mitogen-induced increases in inositol trisphosphate levels in Jurkat cells. Finally, the mechanism of the GST-induced inhibition of PKC was examined in detail, using purified PKC subspecies from rat brain. GST inhibited type II PKC more effectively than type III PKC, and also inhibited the enzymatic activity of the isolated catalytic fragment of PKC. The inhibitory effect of GST on PKC activity could not be explained by competition with phospholipid or nonspecific interference with the substrate. These data suggest that the immunomodulatory effects of GST may result from its capacity to inhibit PKC activity.

Role of protein kinase C in T-cell antigen receptor regulation of p21ras: evidence that two p21ras regulatory pathways coexist in T cells

T-lymphocyte activation via the antigen receptor complex (TCR) results in accumulation of p21ras in the active GTP-bound state. Stimulation of protein kinase C (PKC) can also activate p21ras, and it has been proposed that the TCR effect on p21ras occurs as a consequence of TCR regulation of PKC. To test the role of PKC in TCR regulation of p21ras, a permeabilized cell system was used to examine TCR regulation of p21ras under conditions in which TCR activation of PKC was blocked, first by using a PKC pseudosubstrate peptide inhibitor and second by using ionic conditions that prevent phosphatidyl inositol hydrolysis and hence diacylglycerol production and PKC stimulation. The data show that TCR-induced p21ras activation is not mediated exclusively by PKC. Thus, in the absence of PKC stimulation, the TCR was still able to induce accumulation of p21ras-GTP complexes, and this stimulation correlated with an inactivation of p21ras GTPase-activating proteins. The protein tyrosine kinase inhibitor herbimycin could prevent the non-PKC-mediated, TCR-induced stimulation of p21ras. These data indicate that two mechanisms for p21ras regulation coexist in T cells: one PKC mediated and one not. The TCR can apparently couple to p21ras via a non-PKC-controlled route that may involve tyrosine kinases.

Evidence for clonal heterogeneity of the expression of six protein kinase C isoforms in murine B and T lymphocytes

Protein kinase C (PKC), which plays a pivotal role in lymphocyte activation, represents a homologous family of at least nine proteins. Seven genes that encode PKC proteins have been identified. Since the regulatory properties and substrate specificities of the isoforms are not identical in vitro, it is possible that each isoform plays a unique role in cell activation. Toward an understanding of the role of PKC isoforms in lymphocyte activation we have studied the expression of mRNA encoding six of the isoforms (alpha, beta, gamma, delta, epsilon, and zeta) in T cell clones and B cell lines. PKC isoform phenotyping was done by MAPPing using isoform-specific primers and slot-blot analyses of mRNA were performed using specific probes. T cell clones and B cell lines were determined to express levels of the delta, epsilon, and zeta isoforms of PKC that were detectable by MAPPing. Plasmacytomas did not express PKC-beta message detectable by MAPPing. Slot blot analyses and Western blot analyses with peptide-specific antibody confirmed that B cell plasmacytomas did not express PKC-beta mRNA or protein. T cell clones and B cell lines were similar in that none expressed PKC-gamma. In cells that expressed PKC isoforms that were detectable by the MAPPing protocol, there was heterogeneity in the relative abundance of isoform mRNA (PKC-delta and -beta) and protein (PKC-beta and -epsilon). Such diversity of isoform expression could be responsible for the differential responsiveness of lymphocyte clones to activating stimuli.

Stathmin phosphorylation patterns discriminate between distinct transduction pathways of human T lymphocyte activation through CD2 triggering

CD2 triggering of human T lymphocyte activation has been associated with the activation of different interacting protein kinases, including protein kinase C (PKC). However the precise roles of its phosphorylated substrates are still unknown. We show here that PKC-dependent and -independent pathways are responsible for the CD2-induced phosphorylation of stathmin, a ubiquitous soluble phosphoprotein, most likely acting as a general intracellular relay integrating various second messenger pathways. The phosphorylated variants of stathmin provide a fingerprint reflecting the second messenger pathway(s) stimulated. The respective roles of both PKC and stathmin in the regulation of T lymphocyte proliferation are discussed.

T cell receptor/CD3 complex internalization following activation of a cytolytic T cell clone: evidence for a protein kinase C-independent staurosporine-sensitive step

The fate of the T cell receptor (TcR)/CD3 complex was examined on a cytotoxic T lymphocyte (CTL) clone (KB5.C20) activated either via binding of an anti-TcR monoclonal antibody (mAb) or by a Ca2+ ionophore and phorbol 12-myristate 13-acetate (PMA). After binding of the anti-TcR mAb, electron microscopy revealed internalization through coated vesicles followed by slow degradation of the antibody as shown by use of radiolabeled mAb. The influence of activation on TcR/CD3 internalization was analyzed. The Ca2+ ionophore alone had no effect on internalization, whereas PMA induced an accelerated internalization of anti-TcR mAb. PMA-induced internalization was dependent on protein kinase C (PKC) as shown by its absence in PKC-depleted cells or in the presence of the PKC inhibitor staurosporine. Anti-TcR mAb-induced internalization was maintained in PKC-depleted cells, but unexpectedly remained sensitive to inhibition by staurosporine. The monovalent anti-TcR mAb Fab fragment is non-stimulatory for the CTL. It was poorly internalized but its internalization was induced by PMA. Surprisingly, on PKC-depleted cells, the Fab was internalized more readily than in untreated cells and this internalization was sensitive to inhibition by staurosporine. Inhibition of PMA-induced phosphorylation of gamma and epsilon subunits of CD3 was demonstrated after depletion of PKC or in the presence of staurosporine, confirming that PKC function was inhibited in those conditions. Cross-linking of the TcR via plastic-coated anti-TcR mAb led to phosphorylation of CD3 gamma and epsilon and also of zeta, known to be phosphorylated on tyrosines. All of these phosphorylation events were inhibited by treatment with staurosporine. Our results indicate that staurosporine inhibits the receptor internalization induced by anti-TcR mAb by means other than inhibition of PKC, suggesting that other kinases may control a step of this internalization process.

Translocation of the alpha- and beta-isoforms of protein kinase C following activation of human T-lymphocytes

We have analyzed how activation of human Jurkat T-cells by the mitogenic lectin, concanavalin A (Con A), may affect the cellular distribution of the alpha- and beta-isoforms of protein kinase C (PKC) in T-cells. In non-stimulated cells almost all of the alpha- and beta-PKC was localized to the cytoplasmic compartment. Stimulation with Con A caused a transient translocation of both alpha- and beta-PKC from the cytoplasm to the cell membrane. The alpha-isoform appeared to be translocated to a somewhat greater extent and for a longer period of time than the beta-form. Translocation was maximal between 1 and 5 min for both of the isoforms. 30 min after stimulation, beta-PKC had returned to basal levels, whereas a substantial amount of alpha-PKC remained associated with the particulate fraction. We conclude that activation of human T-cells causes the translocation of at least two different isoforms of PKC, alpha-PKC and beta-PKC.

Protein kinase C in transmembrane signalling

Protein kinase C functions as the transducer of a second messenger, diacylglycerol, and is the major receptor for tumour-promoting phorbol esters. The enzyme is a family of proteins with closely but distinct structures and individual enzymological properties. Members of the family are differently distributed in particular cell types and limited intracellular locations from lower organisms to mammalian tissues. The enzyme appears to interact with many signalling pathways, and display functions in the processing and modulation of cellular responses to external stimuli. Presumably, each member of the family plays discrete roles in the control of a variety of membrane functions and activation of gene transcription.