Spatial and temporal regulation of protein kinase D (PKD)

Protein kinase D (PKD; also known as PKCmicro) is a serine/threonine kinase activated by diacylglycerol signalling pathways in a variety of cells. PKD has been described previously as Golgi-localized, but herein we show that it is present within the cytosol of quiescent B cells and mast cells and moves rapidly to the plasma membrane after antigen receptor triggering. The membrane redistribution of PKD requires the diacylglycerol-binding domain of the enzyme, but is independent of its catalytic activity and does not require the integrity of the pleckstrin homology domain. Antigen receptor signalling initiates in glycosphingolipid-enriched microdomains, but membrane-associated PKD does not co-localize with these specialized structures. Membrane targeting of PKD is transient, the enzyme returns to the cytosol within 10 min of antigen receptor engagement. Strikingly, the membrane-recycled PKD remains active in the cytosol for several hours. The present work thus characterizes a sustained antigen receptor-induced signal transduction pathway and establishes PKD as a serine kinase that temporally and spatially disseminates antigen receptor signals away from the plasma membrane into the cytosol.

Characterization of serine 916 as an in vivo autophosphorylation site for protein kinase D/Protein kinase Cmu

Activation of the serine kinase protein kinase D (PKD)/PKCmicro is controlled by the phosphorylation of two serine residues within its activation loop via a PKC-dependent signaling cascade. In this study we have identified the C-terminal serine 916 residue as an in vivo phosphorylation site within active PKD/PKCmu. An antibody that recognized PKD/PKCmu proteins specifically phosphorylated on the serine 916 residue was generated and used to show that phosphorylation of Ser-916 is induced by phorbol ester treatment of cells. Thus, the pS916 antibody is a useful tool to study the regulation of PKD/PKCmu activity in vivo. Antigen receptor ligation of T and B lymphocytes also induced phosphorylation of the serine 916 residue of PKD/PKCmu. Furthermore the regulatory FcgammaRIIB receptor, which mediates vital negative feedback signals to the B cell antigen receptor complex, inhibited the antigen receptor-induced activation and serine 916 phosphorylation of PKD/PKCmu. The degree of serine 916 phosphorylation during lymphocyte activation and inhibition exactly correlated with the activation status of PKD/PKCmu. Moreover, using different mutants of PKD/PKCmu, we show that serine 916 is not trans-phosphorylated by an upstream kinase but is rather an autophosphorylation event that occurs following activation of PKD/PKCmu.

Dynamic re-distribution of protein kinase D (PKD) as revealed by a GFP-PKD fusion protein: dissociation from PKD activation

Protein kinase D (PKD)/protein kinase Cmicro (PKCmicro, a serine/threonine protein kinase with distinct structural and enzymological properties, is rapidly activated in intact cells via PKC. The amino-terminal region of PKD contains a cysteine-rich domain (CRD) that directly binds phorbol esters with a high affinity. Here, we show that treatment of transfected RBL 2H3 cells with phorbol 12,13-dibutyrate (PDB) induces a striking CRD-dependent translocation of PKD from the cytosol to the plasma membrane, as shown by real time visualization of a functional green fluorescent protein (GFP)-PKD fusion protein. A single amino acid substitution in the second cysteine-rich motif of PKD (P287G) prevented PDB-induced membrane translocation but did not affect PKD activation. Our results indicate that PKD translocation and activation are distinct processes that operate in parallel to regulate the activity and localization of this enzyme in intact cells.

p70(s6k) integrates phosphatidylinositol 3-kinase and rapamycin-regulated signals for E2F regulation in T lymphocytes

In T lymphocytes, the hematopoietic cytokine interleukin-2 (IL-2) uses phosphatidylinositol 3-kinase (PI 3-kinase)-induced signaling pathways to regulate E2F transcriptional activity, a critical cell cycle checkpoint. PI 3-kinase also regulates the activity of p70(s6k), the 40S ribosomal protein S6 kinase, a response that is abrogated by the macrolide rapamycin. This immunosuppressive drug is known to prevent T-cell proliferation, but the precise point at which rapamycin regulates T-cell cycle progression has yet to be elucidated. Moreover, the effects of rapamycin on, and the role of p70(s6k) in, IL-2 and PI 3-kinase activation of E2Fs have not been characterized. Our present results show that IL-2- and PI 3-kinase-induced pathways for the regulation of E2F transcriptional activity include both rapamycin-resistant and rapamycin-sensitive components. Expression of a rapamycin-resistant mutant of p70(s6k) in T cells could restore rapamycin-suppressed E2F responses. Thus, the rapamycin-controlled processes involved in E2F regulation appear to be mediated by p70(s6k). However, the rapamycin-resistant p70(s6k) could not rescue rapamycin inhibition of T-cell cycle entry, consistent with the involvement of additional, rapamycin-sensitive pathways in the control of T-cell cycle progression. The present results thus show that p70(s6k) is able to regulate E2F transcriptional activity and provide direct evidence for the first time for a link between IL-2 receptors, PI 3-kinase, and p70(s6k) that regulates a crucial G1 checkpoint in T lymphocytes.

The dynamics of protein kinase B regulation during B cell antigen receptor engagement

This study has used biochemistry and real time confocal imaging of green fluorescent protein (GFP)-tagged molecules in live cells to explore the dynamics of protein kinase B (PKB) regulation during B lymphocyte activation. The data show that triggering of the B cell antigen receptor (BCR) induces a transient membrane localization of PKB but a sustained activation of the enzyme; active PKB is found in the cytosol and nuclei of activated B cells. Hence, PKB has three potential sites of action in B lymphocytes; transiently after BCR triggering PKB can phosphorylate plasma membrane localized targets, whereas during the sustained B cell response to antigen, PKB acts in the nucleus and the cytosol. Membrane translocation of PKB and subsequent PKB activation are dependent on BCR activation of phosphatidylinositol 3-kinase (PI3K). Moreover, PI3K signals are both necessary and sufficient for sustained activation of PKB in B lymphocytes. However, under conditions of continuous PI3K activation or BCR triggering there is only transient recruitment of PKB to the plasma membrane, indicating that there must be a molecular mechanism to dissociate PKB from sites of PI3K activity in B cells. The inhibitory Fc receptor, the FcgammaRIIB, mediates vital homeostatic control of B cell function by recruiting an inositol 5 phosphatase SHIP into the BCR complex. Herein we show that coligation of the BCR with the inhibitory FcgammaRIIB prevents membrane targeting of PKB. The FcgammaRIIB can thus antagonize BCR signals for PKB localization and prevent BCR stimulation of PKB activity which demonstrates the mechanism for the inhibitory action of the FcgammaRIIB on the BCR/PKB response.

Inhibition of Rho at different stages of thymocyte development gives different perspectives on Rho function

Development of thymocytes can be staged according to the levels of expression of the cell-surface markers CD4, CD8, CD44, CD25 and CD2. Thymocyte development is regulated by a complex signalling network [1], one component of which is the GTPase Rho. The bacterial enzyme C3 transferase from Clostridium botulinum selectively ADP-ribosylates Rho in its effector-binding domain and thereby abolishes its biological function [2,3]. To explore the function of Rho in thymocyte development, we previously used the proximal promoter of the gene encoding the Src-family kinase p56lck to make transgenic mice that selectively express C3 transferase in the thymus [4,6]. In these mice, which lack Rho function from the earliest thymocyte stages, thymocyte numbers are reduced by approximately 50- to 100-fold. Here, we describe transgenic mice that express C3 transferase under the control of the locus control region (LCR) of the CD2 gene; this regulatory element drives expression at a later stage of thymocyte development than the lck proximal promoter [7]. In these mice, thymocyte numbers were also reduced by 50- to 100-fold, but unlike the lck-C3 mice, in which the reduction predominantly results from defects in cell survival of CD25(+) thymocyte progenitors, the CD2-C3 transgenic mice had a pre-T-cell differentiation block at the CD25(+) stage after rearrangement of the T-cell receptor (TCR) beta chains. Analysis of CD2-C3 mice demonstrated that Rho acts as an intracellular switch for TCR beta selection, the critical thymic-differentiation checkpoint. These results show that Rho-mediated survival signals for CD25(+) pre-T cells are generated by the extracellular signals that act on earlier thymocyte precursors and also that temporal cell-type-specific elimination of Rho can reveal different functions of this GTPase in vivo.

p21ras initiates Rac-1 but not phosphatidyl inositol 3 kinase/PKB, mediated signaling pathways in T lymphocytes

p21ras is activated by the T cell antigen receptor (TCR) and then co-ordinates important signaling pathways for T lymphocyte activation. Effector pathways for this guanine nucleotide binding protein in T cells are mediated by the serine/threonine kinase Raf-1 and the Ras-related GTPase Rac-1. In fibroblasts, an important effector for the Ras oncogene is Phosphatidylinositol 3-kinase (PtdIns 3-kinase). Activation of this lipid kinase is able to induce critical Rac-1 signaling pathways and can couple p21ras to cell survival mechanisms via the serine/threonine kinase Akt/PKB. The role of PtdIns 3-kinase in Ras signaling in T cells has not been explored. In the present study, we examined the ability of PtdIns 3-kinase to initiate the Rac-1 signaling pathways important for T cell activation. We also examined the possibility that Akt/PKB is regulated by Ras signaling pathways in T lymphocytes. The results show that Ras can initiate a Rac-1 mediated pathway that regulates the transcriptional function of AP-1 complexes. PtdIns 3-kinase signals cannot mimic p21ras and induce the Rac mediated responses of AP-1 transcriptional activation. Moreover, neither TCR or Ras activation of AP-1 is dependent on PtdIns 3-kinase. PKB is activated in response to triggering of the T cell antigen receptor; PtdIns 3-kinase activity is both required and sufficient for this TCR response. In contrast, p21ras signals are unable to induce Akt/PKB activity in T cell nor is Ras function required for Akt/PKB activation in response to the TCR. The present data thus highlight that PtdIns 3-kinase and Akt/PKB are not universal Ras effector molecules. Ras can initiate Rac-1 regulated signaling pathways in the context of T cell antigen receptor function independently of PtdIns 3-kinase activity.

Lymphocyte signalling: a coordinating role for Vav?

Vav1 is a guanine nucleotide exchange factor that selectively activates the Rac1 GTPase and is expressed specifically in haematopoietic cells. Recent work has revealed how Vav1 integrates signals from lymphocyte antigen receptors and costimulatory receptors to control immune function.

Involvement of phosphoinositide 3-kinase and Rac in membrane ruffling induced by IL-2 in T cells

IL-2 is known to play a critical role in regulating T lymphocyte proliferation. We show here that IL-2 also provokes an instantaneous and sustained membrane ruffling in cloned human or murine T cells as well as in lectin-activated peripheral blood lymphocytes. In the IL-2-induced lamellipodia, tubulin is depolymerized whereas actin is strongly polymerized, forming caps. IL-2-induced membrane ruffling is protein kinase C (PKC) independent, as judged by the absence of effects of bisindolylmaleimide, an efficient inhibitor of all PKC isoforms. The formation of lamellipodia by IL-2 is blocked by wortmannin and LY294002, two inhibitors of phosphoinositide 3-kinase (PI3-kinase). Moreover, expression in murine T cells of an inactive form of P13-kinase inhibits IL-2-induced membrane ruffling, whereas expression of a constitutively active p110 increases the basal membrane ruffling. Rac is also involved in IL-2-induced membrane ruffling since an inactive form of Rac (N17rac) blocks the IL-2-induced lamellipodia, whereas the constitutive form of Rac (Val12rac) can also lead to membrane ruffling. In the signaling cascade, Rac is downstream of PI3-kinase since constitutive membrane ruffling in Val12rac cells is insensitive to wortmannin. Thus, through a signaling cascade involving PI3-kinase and Rac, IL-2 can induce profound alterations of the T cell cytoskeleton, a phenomenon which might be of importance for T cell physiology.

The real LAT steps forward

Antigen receptors initiate T-cell activation and determine the specificity of the immune response by activating membrane-localized protein tyrosine kinases. Signalling pathways initiated by these kinases control expression of the genes that mediate T-cell effector function. A major challenge in immunology is to work out the route taken by membrane-generated signals as they transit to the nucleus. Substrates for the ZAP70/Syk tyrosine kinases are important, but 'missing', links in this process. There has finally been some progress in characterizing one of these important linkers: LAT, an integral membrane protein that acts as an adaptor to couple antigen receptors to intracellular signalling cascades.

STAT3 is a serine kinase target in T lymphocytes. Interleukin 2 and T cell antigen receptor signals converge upon serine 727

Interleukin 2 (IL-2) induces tyrosine phosphorylation of STATs 3 and 5 (signal transducer and activator of transcription). We now show that IL-2 regulation of STAT3 proteins in T cells is a complex response involving activation of two forms of STAT3: 90-kDa STAT3alpha and an 83-kDa carboxyl-terminal truncated STAT3beta. The phosphorylation of STAT proteins on serine residues is also required for competent STAT transcription. A critical serine phosphorylation site in STAT3alpha is at position 727. In this study we have produced an antisera specific for STAT3alpha proteins phosphorylated on serine 727 and used this to monitor the phosphorylation of this residue during T lymphocyte activation. Our results show that phosphorylation of STAT3alpha on serine 727 is not constitutive in quiescent T cells but can be induced by the cytokine IL-2. Interestingly, triggering of the T cell antigen receptor complex or activation of protein kinase C with phorbol esters also induces phosphorylation of serine 727 but without simultaneously inducing STAT3 tyrosine phosphorylation or DNA binding. Hence, the present results show that STAT3 serine phosphorylation can be regulated independently of the tyrosine phosphorylation of this molecule. IL-2 and T cell antigen receptor complex induction of STAT3alpha serine 727 phosphorylation is dependent on the activity of the MEK/ERK pathway. Previous studies have identified H-7-sensitive kinase pathways that regulate STAT3 DNA binding. We show that H-7-sensitive pathways regulate STAT3 DNA binding in T cells. Nevertheless, we show that H-7-sensitive kinases do not regulate STAT3 tyrosine phosphorylation or phosphorylation of serine 727. These results thus show that STAT3 proteins are targets for multiple kinase pathways in T cells and can integrate signals from both cytokine receptors and antigen receptors.

Gabapentin monotherapy: II. A 26-week, double-blind, dose-controlled, multicenter study of conversion from polytherapy in outpatients with refractory complex partial or secondarily generalized seizures. The US Gabapentin Study Group 82/83

This study evaluated gabapentin monotherapy in 275 patients with medically refractory complex partial or secondarily generalized seizures who were taking one or two antiepileptic drugs (AEDs). Following an 8-week baseline, patients received randomized dosages of gabapentin (600, 1,200, or 2,400 mg/d) during a 26-week double-blind phase comprising 2 weeks gabapentin add-on therapy, an 8-week AED taper, and a 16-week gabapentin monotherapy period. Patients exited the study if they experienced a protocol-defined exit event. Results of outcome measures, including time to exit, completion rate, and mean time on monotherapy, showed no significant differences among dosage groups. Possible reasons for this lack of a dose-response relationship include withdrawal seizures and the limited range of gabapentin dosages studied. Overall, 20% of patients completed the study. Completion rates were higher among patients who had discontinued one AED (23%) than two AEDs (14%), and higher among patients who were not withdrawn from carbamazepine (27%) than among those who were (16%).

A negative role for phosphoinositide 3-kinase in T-cell antigen receptor function

BACKGROUND

A delicate balance between positive and negative regulatory mechanisms during T-cell activation determines the specificity and magnitude of an immune response. Phosphoinositide 3-kinase (PI 3-kinase) is activated by a diverse set of receptors that determine T-cell function, including the T-cell antigen receptor (TCR), the costimulatory receptor CD28, and negative regulators of T-cell activation such as CTLA-4. PI 3-kinase is also regulated by the haematopoietic cytokines that determine T-cell differentiation and lymphocyte proliferation. PI 3-kinase can thus dynamically influence the outcome of the immune reactions at various stages. In this study, we investigated the importance of PI 3-kinase in TCR-directed T-cell activation using activated or inhibitory versions of PI 3-kinase.

RESULTS

Certain aspects of TCR responses such as the induction of transcriptional activity of AP1 and serum response factor were not affected by expression of the mutant forms of PI 3-kinase. We found, however, that PI 3-kinase profoundly influenced the transactivation capacity of 'nuclear factor of activated T cells' (NF-AT) elicited by the TCR: expression of an activated form of PI 3-kinase inhibited TCR-mediated NF-AT responses, whereas expression of a dominant negative mutant of PI 3-kinase potently enhanced TCR-controlled NF-AT induction. These effects of PI 3-kinase were not mediated by previously identified PI 3-kinase effectors, such as protein kinase B, a positive regulator of PI 3-kinase, or the GTPase Rac, and are therefore likely to involve a novel, as yet unknown, effector molecule.

CONCLUSIONS

Our results establish that PI 3-kinase can both positively and negatively regulate T-cell function, and uncover a previously unrecognized function for PI 3-kinase in T cells as a selective negative regulator of TCR-signalling events and therefore as a determinant of T-cell homeostasis.

Multiple p21ras effector pathways regulate nuclear factor of activated T cells

The transcription factor, Nuclear Factor of Activated T cells (NFAT) is a major target for p21ras and calcium signalling pathways in the IL-2 gene and is induced by p21ras signals acting in synergy with calcium/calcineurin signals. One p21ras effector pathway involves the MAP kinase ERK-2, and we have examined its role in NFAT regulation. Expression of dominant negative MAPKK-1 prevents NFAT induction. Constitutively active MAPKK-1 fully activates ERK-2 and the transcription factor Elk-1, but does not substitute for activated p21ras and synergize with calcium/calcineurin signals to induce NFAT. Expression of dominant negative N17Rac also prevents TCR and p21ras activation of NFAT, but without interfering with the ERK-2 pathway. The transcriptional activity of the NFAT binding site is mediated by a complex comprising a member of the NFAT group and AP-1 family proteins. The induction of AP-1 by p21ras also requires Rac-1 function. Activated Rac-1 could mimic activated p21ras to induce AP-1 but not to induce NFAT. Moreover, the combination of activated MAPKK-1 and Rac-1 could not substitute for activated p21ras and synergize with calcium signals to induce NFAT. Thus, p21ras regulation of NFAT in T cells requires the activity of multiple effector pathways including those regulated by MAPKK-1/ERK-2 and Rac-1.

T cell antigen receptor signal transduction pathways

The T cell antigen receptor (TCR) regulates the activation and growth of T lymphocytes. The initial membrane proximal event triggered by the TCR is activation of protein tyrosine kinases with the resultant phosphorylation of cellular proteins. This biochemical response couples the TCR to a divergent array of signal transduction molecules including enzymes that regulate lipid metabolism, GTP binding proteins, serine/threonine kinases, and adapter molecules. The ultimate aim of studies of intracellular signaling mechanisms is to understand the functional consequences of a particular biochemical event for receptor function. The control of cytokine gene expression is one of the mechanism that allows the TCR to control immune responses. Accordingly, one object of the present review is to discuss the role of the different TCR signal transduction pathways in linking the TCR to nuclear targets: the transcription factors that control the expression of cytokine genes.

Interleukin-7 can induce the activation of Jak 1, Jak 3 and STAT 5 proteins in murine T cells

The activation of Janus protein tyrosine kinases (Jak) and STAT (signal transducer and activator of transcription) proteins has recently been linked to the signal transduction mechanism of several cytokines. IL-7 was observed to induce a rapid and dose-dependent tyrosine phosphorylation of Jak 1 and Jak 3 and concomitantly, the tyrosine phosphorylation and DNA binding activity of multiple STAT proteins. The STAT proteins utilized by IL-7 were identical to those induced by IL-2 and could be identified as various STAT 5 isoforms. Moreover, the induction of both Jak 1 and 3, and STAT 5 activity strongly correlated with the growth-promoting effects of IL-7, suggesting that this signal transduction mechanism may play a key role in IL-7-induced proliferation.

The role of tyrosine phosphorylation in the interaction of cellular tyrosine kinases with the T cell receptor zeta chain tyrosine-based activation motif

Immunoglobulin receptor family tyrosine-based activation motifs (ITAM) define a conserved signaling sequence, EX2YX2L/IX7YX2L/I, that mediates coupling of the T cell antigen receptor (TCR) to protein tyrosine kinases (PTK). In the present study, we explored the role of phosphorylation of the two ITAM tyrosine residues in the interactions of the motif with the PTK ZAP-70 and p59fyn. The data show that the phosphorylation of a single tyrosine within the motif enables binding of p59fyn, whereas phosphorylation of both tyrosines within the motif is required for maximal binding of the PTK ZAP-70. Quantitative binding experiments show that nanomolar concentrations of the doubly phosphorylated zeta 1-ITAM are sufficient for ZAP-70 recruitment, whereas micromolar levels of singly phosphorylated ITAM are necessary for p59fyn binding. ZAP-70 binds with low efficiency to a singly phosphorylated ITAM, but shows preferential binding to the C-terminal phosphotyrosine in the ITAM, whereas p59fyn binds selectively to the N-terminal phosphotyrosine. The present data thus show that there is the potential for a singly phosphorylated ITAM to couple to cellular PTK. Moreover, the data suggest a mechanism for heterogeneity in signal transduction responses by the TCR, since ITAM could differentially couple the TCR to downstream signaling events depending on their phosphorylation state.

A comparison of the interaction of Shc and the tyrosine kinase ZAP-70 with the T cell antigen receptor zeta chain tyrosine-based activation motif

Tyrosine-based activation motifs (TAMs) define a conserved signaling sequence, EX2YX2L/IX7YX2L/I, that couples the T cell antigen receptor to protein tyrosine kinases and adapter molecules. The present study shows that phosphorylation of both tyrosines within the motif is required for high affinity binding of the tyrosine kinase ZAP-70 whereas phosphorylation of the single COOH-terminal tyrosine within the motif is optimal for the binding of the adapter Shc. There were also quantitative differences in the ZAP-70 and Shc association with the zeta 1-TAM since nM concentrations of the doubly phosphorylated zeta 1-TAM are sufficient for ZAP-70 recruitment whereas micromolar levels of singly phosphorylated TAMs are necessary for Shc binding. Shc is tyrosine phosphorylated in antigen receptor-activated T cells and can potentially form a complex with the adapter molecule Grb2 and could thus recruit the Ras guanine nucleotide exchange protein Sos into the antigen receptor complex. The present data show that Grb2 can bind to the phosphorylated TAM, but this binding is independent of Shc and there is no formation of zeta 1-TAM.Shc.Grb2.Sos complexes in antigen receptor-activated cells. Accordingly, Shc function should not be considered in the context of Grb2/Sos recruitment to the T cell antigen receptor complex.

The regulation and function of p21ras during T-cell activation and growth

The delivery of signals that control the growth of T cells is a key event for effective co-ordination of T-cell-dependent immune responses. It is now recognized that guanine nucleotide binding proteins play an important role in signal transduction by the T-cell receptor (TCR) and cytokine receptors. Here, Manolo Izquierdo Pastor, Karin Reif and Doreen Cantrell review the numerous recent advances in understanding how the p21ras guanine nucleotide binding protein couples the TCR to the T-cell signalling cascade.

Regulation and function of p21ras in T lymphocytes

T cell activation is triggered by antigen stimulation and is characterized by the production of a wide range of cytokines and other immunomodulators crucial for the growth and development of other haemopoietic cells. Activation also induces the T cells to express, on their cell surface, receptors that enable the T cell to respond to the various cytokines generated during an immune response. One well characterized event that occurs when mature T cells are activated is the production of the cytokine IL2 and the acquisition by the T cell of IL2 receptors. Interaction between IL2 and its cellular receptor then directs T cell growth. Expression of the IL2 gene in T cells is regulated by signalling pathways that originate from the T cell antigen receptor complex (TCR). This review discusses the role of p21ras in these events. The TCR regulates the activity of p21ras, and a range of experiments have shown that p21ras couples the TCR to an intracellular kinase cascade involving the serine/threonine kinase Raf-1 and the MAP kinase ERK2. Analysis of more distal receptor signals shows that p21ras controls a signalling pathway that cooperates with a calcium/calcineurin controlled signalling system to stimulate the transcriptional factor NFAT and hence the IL2 gene. These studies identify p21ras as a critical signalling molecule in immune cells.

The role of protein kinase C in the regulation of extracellular signal-regulated kinase by the T cell antigen receptor

The aim of this study was to explore the role of protein kinase C (PKC) in the activation of mitogen-activated protein kinases (MAPK) in T lymphocytes. The MAPK extracellular signal-regulated kinase-2 (ERK2) is activated in response to phorbol esters which stimulate PKC, by transient expression of a constitutively active ras mutant by cell activation via the G protein-coupled type 1 muscarinic acetylcholine receptor (HM1R) or in response to triggering of the T cell antigen receptor (TCR). The relative contribution of PKC to TCR and HM1R regulation of ERK2 was explored by examining the effects of a PKC inhibitor (Ro 31-8425) on ERK2 activation. The data demonstrate that phorbol ester and HM1R regulation of ERK2 was prevented by the PKC inhibitor, but that the inhibitor had no effect on ERK2 activation induced by expression of a constitutively active ras mutant p21v-Ha-ras. Furthermore, the TCR stimulates both PKC and p21ras but TCR regulation of ERK2 was only weakly suppressed by the PKC inhibitor. These data indicate that PKC has a potential but not a predominant role in TCR regulation of ERK2.

The role of Raf-1 in the regulation of extracellular signal-regulated kinase 2 by the T cell antigen receptor

Triggering of the T cell antigen receptor (TCR) complex activates the serine/threonine kinase Raf-1 whose function is necessary for TCR induction of the interleukin 2 gene. Raf-1 has been identified as a candidate mitogen-activated protein (MAP) kinase kinase kinase (MKKK) and thus has the potential to couple the TCR to the activation of the MAP kinases such as ERK2. In the present study, the role of Raf-1 in ERK2 regulation of ERK2 in T cells has been explored. A constitutively active Raf-1 kinase, v-raf, or a dominant inhibitory Raf-1 mutant were expressed transiently from the pEF BOS vector in Jurkat cells and the effects of these Raf-1 mutants on a coexpressed ERK2 reporter was assessed. The action of the constitutively active Raf-1 was to stimulate the ERK2 kinase, whereas the dominant negative version of Raf-1 inhibited the ERK2 activation induced by triggering of the TCR. These data indicate a role for Raf-1 in the regulation of ERK2 in T cells.

G proteins in lymphocyte signalling

The guanine nucleotide binding proteins p21ras are activated by the T-cell antigen receptor and play a critical role in transducing signals from the T-cell receptor to the cell nucleus. The mechanism that couples the T-cell receptor to p21ras is the focus of many current studies, but is as yet unidentified.