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The Journal of Immunology, 2008, 180, 5805 -5815
Copyright © 2008 by The American Association of Immunologists, Inc.
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Lck-Dependent Tyrosine Phosphorylation of Diacylglycerol Kinase {alpha} Regulates Its Membrane Association in T Cells1

Ernesto Merino*, Antonia Ávila-Flores*, Yasuhito Shirai{dagger}, Ignacio Moraga*, Naoaki Saito{dagger} and Isabel Mérida2,*

* Department of Immunology and Oncology, Centro Nacional de Biotecnología/Consejo Superior de Investigaciones Cientificas, Madrid, Spain; and {dagger} Biosignal Research Center, Kobe, Japan


   Abstract
Top
 Abstract
Introduction
 Materials and Methods
 Results
 Discussion
 Disclosures
 References
 
TCR engagement triggers phospholipase C{gamma}1 activation through the Lck-ZAP70-linker of activated T cell adaptor protein pathway. This leads to generation of diacylglycerol (DAG) and mobilization of intracellular Ca2+, both essential for TCR-dependent transcriptional responses. TCR ligation also elicits transient recruitment of DAG kinase {alpha} (DGK{alpha}) to the lymphocyte plasma membrane to phosphorylate DAG, facilitating termination of DAG-regulated signals. The precise mechanisms governing dynamic recruitment of DGK{alpha} to the membrane have not been fully elucidated, although Ca2+ influx and tyrosine kinase activation were proposed to be required. We show that DGK{alpha} is tyrosine phosphorylated, and identify tyrosine 335 (Y335), at the hinge between the atypical C1 domains and the catalytic region, as essential for membrane localization. Generation of an Ab that recognizes phosphorylated Y335 demonstrates Lck-dependent phosphorylation of endogenous DGK{alpha} during TCR activation and shows that pY335DGK{alpha} is a minor pool located exclusively at the plasma membrane. Our results identify Y335 as a residue critical for DGK{alpha} function and suggest a mechanism by which Lck-dependent phosphorylation and Ca2+ elevation regulate DGK{alpha} membrane localization. The concerted action of these two signals results in transient, receptor-regulated DGK{alpha} relocalization to the site at which it exerts its function as a negative modulator of DAG-dependent signals.


   Introduction
Top
 Abstract
 Introduction
Materials and Methods
 Results
 Discussion
 Disclosures
 References
 
Activation of T lymphocytes in response to foreign Ags is essential for an adequate immune response to tumors and infection (1). The mechanisms governing T cell activation require fine-tuning, because T cell activation defects induce immunosuppression and anergy, whereas hyperactivation can lead to immunoproliferative disorders and autoimmunity. Peripheral T cells are activated following TCR triggering in the presence of costimulatory signals (2). This initiates the concerted activation of several protein tyrosine kinases (PTK)3 that, in turn, promote assembly of signaling complexes, leading to second messenger generation and cytoskeletal reorganization (3, 4).

The Src kinase family members p56lck (Lck) and p59fyn (Fyn) are the major PTK activated during TCR triggering. These kinases phosphorylate the ITAM motifs in CD3 and TCR{zeta}, enabling binding of ZAP70 ({zeta}-chain TCR-associated protein kinase 70 kDa), which subsequently phosphorylates linker of activated T cell adaptor protein (LAT). Phosphorylated LAT provides a docking site for an array of proteins involved in TCR signaling (5, 6). Although Fyn and Lck are closely related, each is predicted to exert discrete, unique functions (7). In fact, Lck (but not Fyn) is primarily responsible for initiating phospholipase C{gamma}1 (PLC{gamma}) phosphorylation in peripheral T cells (8).

The activation of PLC{gamma} must be accurate, because this enzyme is responsible for generating inositol 1,4,5-triphosphate (IP3) and diacylglycerol (DAG), two essential mediators in the initiation and maintenance of the T cell activation program (9, 10). IP3 mediates an increase in intracellular Ca2+ ([Ca2+]i) levels and ensures accurate activation of NF-AT-modulated genes (11, 12), whereas DAG generation at the membrane regulates localization and activation of several signaling molecules, including protein kinase C (PKC){theta}, Ras guanyl nucleotide-releasing protein 1 (Ras-GRP1), and protein kinase D (3, 13). DAG- and [Ca2+]i-regulated signals are both necessary for an appropriate immune response; in addition, an adequate balance between these two messengers guarantees correct initiation and termination of the T cell activation program. Accordingly, Ca2+ flux generation in the absence of sufficient DAG is proposed to lead to anergy (14, 15), whereas defects in DAG signal termination are linked to lymphoproliferative disease and/or autoimmunity (16, 17). The requirement for mechanisms to oversee DAG consumption thus emerges as an important aspect in T cell response control.

The DAG kinases (DGK) are a family of signaling proteins that modulate DAG levels by catalyzing its conversion into phosphatidic acid (PA) (18, 19, 20, 21). Mammalian cells express five DGK subtypes, characterized by the presence of a common catalytic domain and at least two protein kinase C-like type 1 (C1) domains. In addition, each DGK subtype has distinct regulatory motifs, suggesting the existence of diverse regulatory mechanisms and/or participation in different signaling complexes. DGK{alpha} is a type I DGK characterized by two EF hand Ca2+-binding domains (22) and a recoverin-like domain in the N-terminal region (23) that is abundantly expressed in the thymus and mature T cells (24, 25). Early studies in T cells demonstrated that, during T cell activation, DGK{alpha} located at the cytosol in resting T cells translocates to the membrane (24). DGK{alpha} membrane localization and activation act as a switch-off signal for Ras activation, mediated by localization to the membrane of Ras-GRP1 (24, 26). The recent generation of DGK{alpha}-deficient mice confirmed these results, showing that stimulation of DGK{alpha}-null T cells elicits increased RasGTP levels and MAPK activation (17). Together, these studies indicate that DGK{alpha} controls the magnitude of the TCR response, acting as a brake at the initial steps of TCR signaling.

Like most DGK, DGK{alpha} must translocate to the membrane to exert its regulatory function. Studies using GFP-DGK{alpha} fusion proteins and the Jurkat T cell model have allowed a detailed analysis of the signals required for receptor-dependent translocation (27). The DGK{alpha} N-terminal domain acts as a negative regulator of enzyme localization, maintaining the enzyme in a cytosolic/inactive conformation unless modified by receptor-derived [Ca2+]i to an active/membrane-bound conformation. Nonetheless, in T lymphocytes, Ca2+ mobilization is necessary, but not sufficient, to induce DGK{alpha} localization to the plasma membrane unless PTK are also activated. This suggested a more complex mechanism by which PTK-dependent signaling is required to regulate DGK{alpha} membrane localization (27). Additional studies in lymphoid and nonlymphoid cells point to PTK-dependent regulation of this isoform. IL-2 was shown to activate DGK{alpha} by a Ca2+-independent mechanism that requires PTK-mediated PI3K activation (28, 29). Studies in nonlymphoid cells suggest that Src kinase-dependent phosphorylation is necessary to promote DGK{alpha} activation (30). There are nonetheless no reports of direct tyrosine phosphorylation of endogenous DGK{alpha} in response to physiological stimulation.

In this study, we investigated DGK{alpha} regulation by tyrosine phosphorylation during T cell activation. We found that TCR triggering induces tyrosine phosphorylation of endogenous DGK{alpha} by a mechanism dependent on Lck. Because previous studies identified Y335 as a residue phosphorylated in a Src-dependent manner in nonlymphoid cells, we examined phosphorylation of this residue using a phospho-Y335-specific Ab. We found that endogenous DGK{alpha} is phosphorylated at Y335 and that this phosphorylation is not observed in cells lacking Lck. TCR triggering induces rapid, transient elevation of Y355 phosphorylation, and fractionation analysis showed that phosphorylated DGK{alpha} localized specifically at the membrane. These results suggest that phosphorylation at Y335 stabilizes DGK{alpha} membrane localization. Accordingly, the use of a nonphosphorylatable mutant showed that this tyrosine is essential for DGK{alpha} translocation to the membrane, where it exerts its function. This is the first description of tyrosine phosphorylation of endogenous DGK{alpha} in T lymphocytes as a mechanism to modulate membrane localization of the enzyme and, thus, to attenuate DAG-dependent signals.


   Materials and Methods
Top
 Abstract
 Introduction
 Materials and Methods
Results
 Discussion
 Disclosures
 References
 
Cell culture

The human leukemia Jurkat T cell line, the Lck-defective Jurkat variant JCaM1 (31), and the kidney epithelial cell line HEK293 were maintained in DMEM (Invitrogen) supplemented with 10% heat-inactivated FBS (Invitrogen) and 2 mM L-glutamine (37°C, 5% CO2). BaF/3 cells and the human leukemia Molt4 cell line were maintained in RPMI 1640 (Invitrogen) supplemented as above; BaF/3 cell medium also contained 5 x 10–5 M 2-ME and 5% WEHI-3B supernatant as an IL-3 source. Human PBL were prepared from buffy coats using a Ficoll density gradient. T lymphocyte purity was >90%, as analyzed by flow cytometry using the T3b anti-CD3 mAb.

Abs and reagents

Polyclonal rabbit anti-Lck, anti-PLC{gamma}, and anti-phosphotyrosine (clone AG10) mAbs were from Upstate Biologicals; anti-Lck, anti-CD3, and anti-CD28 mAb were from BD Pharmingen. Mouse anti-MAPK (ERK1 plus ERK2) was from Zymed Laboratories; anti-pMAPK (ERK1 plus ERK2), anti-pI{kappa}B{alpha}, anti-I{kappa}B{alpha}, anti-pPLC{gamma} (Tyr783), and anti-p(serine) PKC substrate were from Cell Signaling Technology; anti-GFP mAb was from BD Clontech; anti-actin (clone AC-15), anti-vimentin, and anti-{alpha}-tubulin were from Sigma-Aldrich; HRP-coupled polyclonal goat anti-mouse and anti-rabbit Ig were from DakoCytomation; rabbit anti-mouse IgG was from Jackson ImmunoResearch Laboratories; and anti-hemagglutinin (HA) Ab was from Covance. The DGK{alpha}-specific mouse mAb mixture was a gift from W. van Blitterswijk (Netherlands Cancer Institute, Amsterdam, The Netherlands). Mouse mAb against the Myc tag and rabbit anti-NF-AT Ab were provided by P. Hawkins (Babraham Institute, Cambridge, U.K.) and J. Redondo (Centro Nacional de Investigaciónes Cardiovasculares Carlos III, Madrid, Spain). PMA, leupeptin, and aprotinin were from Sigma-Aldrich. Ionomycin, DGK inhibitor R59949, PI3K inhibitor LY294002, and the Src PTK family inhibitor PP2 were from Calbiochem. {gamma}-binding Sepharose was from GE Healthcare.

Plasmids and transfections

Plasmid encoding human DGK{alpha} fused to Myc (Myc-DGK{alpha}) was a gift from A. Graziani (Department of Medical Sciences, University Amadeo Avogadro, Piamonte Orientale, Novara, Italy) and has been previously described (30); plasmids encoding Lck and a constitutive active version bearing Y505 to F mutation (Lck505) were a gift from A. Carrera (Centro Nacional de Biotecnología/CSIC, Madrid, Spain) (29); plasmids encoding DGK{alpha} or a catalytically inactive mutant fused to GFP (GFP-DGK{alpha}, GFP-DGK{alpha}KD) or to HA (HA-DGK{alpha}) were previously described (27, 29). DGK{alpha} and DGK{alpha}KD mutants Y335F were generated using the Quickchange site-directed mutagenesis kit (Stratagene) and the following primers: 5'-CCTCCATCTTCCATCTTTCCCAGTGTCCTGGCC-3' and 3'- GGAGGTAGAAGGTAGAAAGGGTCACAGGACCGG-5'.

For transfection, lymphocytes in logarithmic growth were electroporated with 25 µg of plasmid DNA. HEK293 cells were transfected with LipofectAmine (Invitrogen). All experiments were performed 24 h after transfection.

Cell stimulation, lysis, and Western blot

Jurkat T cells were washed once with DMEM, then starved (1 h) or resuspended immediately in complete medium (5 x 106 cells/ml). Cells were stimulated with anti-CD3 alone or with anti-CD28 mAb (1 µg/ml each). For TCR cross-linking, 107 cells were resuspended in 250 µl of complete medium and incubated (10 min, on ice), after which 2.5 µg each of anti-CD3 and anti-CD28 was added and the mixture incubated (45 min, on ice). Cells were washed twice with cold medium and resuspended in 250 µl of cold medium containing 7.5 µg of rabbit anti-mouse IgG Ab. Cells were incubated for the indicated times at 37°C, pelleted, and submitted to subcellular fractionation. Where indicated, cells were incubated (37°C, 5% CO2, 1 h) with PI3K inhibitor LY294002 (10–40 µM), DGK inhibitor R59949 (30 µM), or the Src PTK family inhibitor PP2 (1–20 µM). In some cases, cells were treated with PMA (200 nM) and/or ionomycin (1 µM) plus 2 mM CaCl2 as a Ca2+ source. PBL were stimulated with anti-CD3 mAb or anti-CD3/CD28 mAb (1 µg/ml each) for the times indicated.

After treatment, cells were lysed immediately in ice-cold lysis buffer (10 mM HEPES (pH 7.5), 15 mM KCl, 1 mM EDTA, 1 mM EGTA, 10% glycerol, 0.2% Nonidet P-40, 1 mM DTT, 50 mM NaF, 10 µg/ml each leupeptin and aprotinin, 1 mM PMSF, 1 mM sodium orthovanadate, and 20 mM glycerol phosphate) by incubation with gentle rocking (20 min, 4°C). Cell lysates were centrifuged (15,000 x g, 15 min, 4°C), and supernatant proteins were resolved by SDS-PAGE. Gels were blotted onto nitrocellulose filters, which were incubated with the indicated Abs diluted in TBST containing either 5% milk or 5% BSA (4°C, overnight). After incubation with secondary Ab (room temperature, 1 h), blots were visualized using ECL (Amersham Biosciences). Where indicated, pY335DGK{alpha} bands were quantified by analysis of films using the Image J Program, and values were normalized against the corresponding total DGK{alpha} protein band.

Immunoprecipitation

Cells were collected and lysed in ice-cold Nonidet P-40 lysis buffer (1% Nonidet P-40, 50 mM Tris-HCl (pH 7.5), 150 mM NaCl, 10 mM NaF, 10 mM Na4P2O7, 1 mM Na3VO4, 1 mM PMSF, 10 ng/µl aprotinin, and 10 ng/µl leupeptin). Cell lysates were centrifuged (15,000 x g, 15 min, 4°C) and subsequently quantified.

DGK{alpha} and the pY proteins were immunoprecipitated by incubating cell lysates with the appropriate Ab (2 h, 4°C). Complexes were then precipitated by addition of {gamma}-binding Sepharose beads (1 h, 4°C). Bead-bound complexes were washed three times in lysis buffer, once with 0.5 M LiCl, and twice with 150 mM Tris (pH 7.5). Finally, beads were resuspended in Laemmli buffer and analyzed by SDS-PAGE and immunoblot.

For in vitro kinase assay, purified GFP-DGK{alpha} was incubated with or without 0.25 µg of rLck (MBL) in 50 µl of reaction buffer (50 mM HEPES (pH 7.5), 10 mM MnCl2, 0.01% Triton X-100, and 2.5 mM DTT) for 5 min at 30°C. In vitro phosphorylation reaction was inititated by ATP (500 µM) addition; after 5 min at 30°C, the reaction was stopped by adding loading buffer. Samples were analyzed by SDS-PAGE.

DGK assay

DGK activity was determined by measuring radioactive phosphate incorporation into PA, using C8-DAG as substrate, as described (28). The reaction was conducted for 10 min at room temperature, followed by lipid extraction using CHCl3/MeOH/2 N HCl (20:10:5, v/v/v). Dried radioactive-labeled lipid products were dissolved in 40 µl of CHCl3/MeOH (1:1, v/v) and applied to a silica gel thin-layer chromatography plate, with unlabeled C8-PA as a migration standard. Plates were developed in a chloroform/methanol/4 M ammonia solvent system (9:7:2, v/v/v) and autoradiographed.

Subcellular fractionation

Fractionation was performed, as described (32), with some modifications. Briefly, cells were harvested and resuspended in cold lysis buffer 1 (5 mM Tris-HCl (pH 7.5), 10 mM NaCl, 0.5 mM MgCl2, 1 mM EGTA, 1 mM DTT, and 40 µg/ml digitonin) supplemented with a mixture of protein inhibitors and lysed (15 min, on ice). After centrifugation (4500 x g, 4 min, 4°C), supernatants (cytosolic fraction, C) were collected and pellets were resuspended in cold lysis buffer 2 (as for buffer 1, with 0.2% Nonidet P-40 instead of digitonin) and lysed (10 min, on ice). After centrifugation (15,000 x g, 15 min, 4°C), supernatants (membrane fraction 1, M1) were collected. The pellet was further extracted with lysis buffer 3 (as for lysis buffer 2, with 1% Nonidet P-40). The supernatant contained membrane-associated proteins and was designated as M2. The pellet was solubilized in Laemmli buffer and corresponds to the cytoskeleton proteins. The different fractions were resolved in SDS-PAGE and analyzed by immunoblot.

Immunofluorescence microscopy

Jurkat cells were harvested 24 h after transfection, washed once, and resuspended in HEPES balanced solution. The cell suspension was transferred to chambered coverslips coated with anti-CD3/CD28 mAb (final concentration 5 µg/ml, 4°C, overnight). Cells were imaged with a laser-scanning confocal microscope (TSC-NT; Leica Microsystems).

Analysis of cell surface CD69 expression

CD69 expression on the cell surface of the GFP-positive population was analyzed 24 h after transfection using a PE-conjugated anti-human CD69 mAb. Cells were stimulated with anti-CD3/CD28 mAb (0.2 µg/ml) for 2.5 h, and immunofluorescence intensity of the cells was determined by flow cytometry (Excalibur; BD Biosciences). Flow cytometry data are presented both as histograms and as plots of mean fluorescence intensity (MFI) of CD69, normalized to that of the GFP-negative population within each sample, against the mean fluorescence intensity of GFP.

Generation of the pY335 Ab

A synthetic peptide corresponding to the porcine DGK{alpha} sequence (NH2-CPPSSI(phospho-Y)PSVLA-COOH) was conjugated to keyhole limpet hemocyanin, and 300 µg of keyhole limpet hemocyanin-conjugated Ag emulsified with CFA was injected into a 10-wk-old female rabbit (Japanese White). Booster injections (150 µg of Ag with IFA) were given at 2-wk intervals. Three days after the sixth boost, the rabbit was bled and antiserum was collected. Ab specificity for phosphotyrosine was confirmed by dot and Western blotting, using nonphosphorylated peptide (NH2-CPPSSIYPSVLA-COOH) and the Y334F mutant of porcine DGK{alpha} as negative controls.


   Results
Top
 Abstract
 Introduction
 Materials and Methods
 Results
Discussion
 Disclosures
 References
 
Endogenous DGK{alpha} is tyrosine phosphorylated in response to TCR stimulation

TCR triggering in T lymphocytes is rapidly followed by activation of cytosolic PTK (33), which in turn phosphorylate several signaling proteins and adaptors (10). Our previous results demonstrated that DGK{alpha} translocation to the plasma membrane requires [Ca2+]i and the action of PTK (27). To determine whether endogenous DGK{alpha} is present in tyrosine-phosphorylated complexes, we mimicked TCR triggering by stimulating Jurkat T cells with anti-CD3 mAb (34) for different time periods; proteins phosphorylated in tyrosine residues (pY) were then purified from cell lysates by immunoprecipitation with anti-pY mAb. Endogenous DGK{alpha} was readily detected in the anti-pY complex from 5 min poststimulation until the latest time tested (30 min; Fig. 1A).


Figure 1
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  		FIGURE 1. DGK{alpha} is tyrosine phosphorylated in response to TCR stimulation. A, Western blot analysis of DGK{alpha} in anti-PY immunopellets obtained from Jurkat cells stimulated at times indicated with anti-CD3. B, Same as in A, except than DGK{alpha} was immunoprecipitated using anti-DGK{alpha} and the blot was probed with anti-pY and then with anti-DGK{alpha}. C, Lck-dependent phosphorylation of DGK{alpha}. BaF/3 cells were cotransfected with a plasmid encoding HA-DGK{alpha} and an empty plasmid or a plasmid encoding an active p56Lck mutant (p56Lck 505). At 24 h, cells were untreated or incubated (1 h) with R59949 (5 µM) or orthovanadate (1 mM). HA-DGK{alpha} was immunoprecipitated using anti-HA. Immunoprecipitated proteins and 20 µg of total cell lysates were resolved by SDS-PAGE, blotted, and probed with the indicated Abs. Tyrosine phosphorylation in total cell lysates (bottom panel) was determined as a control of p56Lck expression/activity. D, BaF/3 cells were cotransfected as above, then left untreated or incubated with the PI3K inhibitor LY294002 (2 h) at the concentrations indicated. GFP-DGK{alpha} was immunoprecipitated with anti-GFP. Blots were probed with anti-pY mAb and reprobed with anti-DGK{alpha} mAb to determine total expression levels. Tyrosine phosphorylation was determined in total cell lysates (bottom panel) as an indicator of p56Lck expression/activity. E, GFP-DGK{alpha} was immunoprecipitated from transfected HEK293 cells and subjected to an in vitro phosphorylation reaction without further addition or in the presence of rLck. Proteins were resolved in SDS-PAGE and probed with anti-pY Ab. All experiments are representative of three or four different experiments performed with similar results.

 
Detection of DGK{alpha} in anti-pY pellets suggested tyrosine phosphorylation of the protein, although it could also reflect DGK{alpha} association to tyrosine-phosphorylated proteins. To analyze the effect of TCR triggering on DGK{alpha} tyrosine phosphorylation, we purified endogenous protein by immunoprecipitation and assessed phosphorylation by immunoblot using anti-pY mAb. We observed transient phosphorylation of DGK{alpha} on tyrosine residues, with a maximum from 5 to 15 min poststimulation, followed by a decrease (Fig. 1B).

DGK{alpha} is tyrosine phosphorylated by Lck

Previous experiments in endothelial cells suggested Src-dependent phosphorylation of DGK{alpha} (30). In T lymphocytes, the Src kinase family members Lck and Fyn initiate signaling events downstream of TCR stimulation (31). In contrast to Lck, Fyn expression was reported to be very low in Jurkat cell lines (35); we therefore focused on the action of Lck. To study the role of this PTK on DGK{alpha} phosphorylation, we analyzed phosphorylation of ectopically expressed DGK{alpha} in BaF/3 cells. This proB cell line lacks endogenous Lck, but expresses other Src family PTK, such as Fyn and Lyn (36). In this cell line, we previously showed that constitutive active Lck induces elevated DGK{alpha} activity (29). We observed tyrosine phosphorylation of ectopically expressed DGK{alpha} only when BaF/3 cells were cotransfected with a plasmid encoding a constitutive active Lck form (Fig. 1C). Cell pretreatment with the pharmacological DGK{alpha} inhibitor R59949 did not alter enzyme phosphorylation on tyrosine, suggesting that Lck-dependent phosphorylation of DGK{alpha} was independent of enzyme activity. Orthovanadate pretreatment of the cells further increased the level of protein phosphorylation on tyrosine. Our earlier studies suggested that whereas TCR triggering promotes DGK{alpha} localization to the plasma membrane (27), IL-2 induces perinuclear localization of this protein and its activation by a PI3K-dependent mechanism (28, 29). Accordingly, inhibition of PI3K activity did not alter Lck-dependent phosphorylation of DGK{alpha} (Fig. 1D), suggesting that these two pathways represent independent DGK{alpha} regulatory mechanisms. Finally, an in vitro kinase assay using purified Lck confirmed direct phosphorylation of DGK{alpha} (Fig. 1E).

Identification of Y335 as a DGK{alpha} phosphorylation site

Amino acid sequence alignment of human, porcine, murine, and rat DGK{alpha} orthologues shows a high degree of conservation of Y335 (numbering based on the human sequence), located at the hinge between the second C1 domain and the catalytic domain (Fig. 2A). This residue, which is not present in the other two type I isoforms (DGKβ and DGK{gamma}), was recently proposed to be phosphorylated by Src in nonhematopoietic cells in response to hepatocyte growth factor (37) or {alpha}-D-tocopherol (38). We therefore studied the role of this residue in Lck-dependent DGK{alpha} phosphorylation in hematopoietic cells. BaF/3 cells were transiently cotransfected with a plasmid that coded for Lck and a plasmid encoding either wild-type (wt) GFP-DGK{alpha} or a mutant in which Y335 was replaced by F (DGK{alpha}Y335F). Analysis of pY in immunoprecipitated proteins showed that DGK{alpha} Y335F was phosphorylated to a much lower extent than the wt protein, suggesting that Y335 is a target for Lck-dependent phosphorylation (Fig. 2B), albeit not the only one. In contrast to results for the wt DGK{alpha} (29), enzymatic activity of the DGK{alpha}Y335F mutant was not increased by cotransfection with Lck (Fig. 2C).


Figure 2
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  		FIGURE 2. A, Sequence comparison of the type I DGK family. Top, The cartoon represents the modular organization of DGK{alpha} and the localization of Y335. Bottom, An alignment of different DGK{alpha} orthologues indicates the conservation of Y335 (arrowhead). A proline region probably important as a hinge for DGK{alpha} folding is also depicted (dot line). B, Lck phosphorylates DGK{alpha} at Y335. BaF/3 cells were cotransfected with a plasmid encoding GFP-DGK{alpha} wt or GFP-DGK{alpha} Y335F mutant, together with an empty plasmid or a plasmid encoding Lck, as indicated. GFP-DGK{alpha} was immunoprecipitated using anti-GFP, proteins were resolved by SDS-PAGE, and blots were probed with the indicated Ab. Tyrosine phosphorylation was determined in total cell lysates as a control of Lck expression/activation. C, Lack of Y335 phosphorylation impairs Lck-dependent DGK{alpha} activation. BaF/3 cells were transfected with plasmids encoding GFP-DGK{alpha} Y335F and Lck where indicated. DGK{alpha} activity was determined in the immunopellets by P32 incorporation into C8-DAG, as described in Materials and Methods. An autoradiogram of the TLC shows the radioactivity comigrating with the C8-PA standard. One-third of the immunopellet was analyzed by SDS-PAGE and immunoblot with anti-DGK{alpha} mAb to determine DGK{alpha} expression. Tyrosine phosphorylation was determined in total cell lysates as a control of Lck expression/activation.

 
Y335 determines DGK{alpha} function

These results prompted us to analyze whether impairment of DGK{alpha} Y335 phosphorylation affected enzyme translocation and/or function. Our previous studies established that DGK{alpha} membrane translocation is a rapid, transient event. Membrane-bound DGK{alpha} can be visualized in conditions that strongly inhibit enzyme relocalization, i.e., in the presence of DGK or tyrosine phosphatase inhibitors (27). We examined whether differences between TCR-triggered membrane localization of wt DGK{alpha} and the Y335F mutant could be detected by confocal analysis of live T cells. Jurkat cells were transfected with GFP-fused wt or Y335 mutant DGK{alpha}, and cells were plated on anti-CD3/CD28 mAb-coated plates, alone or with orthovanadate. In the case of the wt enzyme, membrane localization was observed (Fig. 3A, left); this was more pronounced when cells were pretreated with orthovanadate (data not shown). On the contrary, Y335F mutant-transfected cells did not show this membrane pattern (Fig. 3A, right), even in the presence of orthovanadate (data not shown). These results suggest that DGK{alpha} phosphorylation at Y335 is a decisive event for membrane stabilization of the enzyme.


Figure 3
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  		FIGURE 3. DGK{alpha} Y335 phosphorylation is essential for DGK{alpha} function. A, DGK{alpha} Y335F mutant does not translocate to plasma membrane following TCR stimulation. Jurkat cells were transfected with either GFP-DGK{alpha} or GFP-DGK{alpha} Y335F mutant. Twenty-four hours after transfection, cells were plated onto anti-CD3/anti-CD28-coated plates, and the subcellular localization of the GFP-fused proteins was determined by confocal microscopy. B, PMA-dependent ERK phosphorylation in HEK293 cells. HEK293 cells were left untreated or stimulated with PMA (1 h). Cells were lysed, and ERK1/2 phosphorylation was determined by analysis of total cell lysates with anti-pERK1/2-specific Ab. C, The DGK{alpha} Y335F mutant does not attenuate ERK phosphorylation. HEK293 cells were transfected with a plasmid encoding HA-DGK{alpha}, the HA-DGK{alpha} Y335F mutant, or empty vector. ERK1/2 phosphorylation was determined as in B. The same membrane was blotted with anti-HA Ab as a control of protein transfection.

 
Jurkat T cells express a large amount of endogenous DGK{alpha}, and enzyme overexpression does not affect TCR-dependent functions (27). We thus examined wt and mutated DGK{alpha} function in the HEK293 cell line, which expresses very low DGK{alpha} levels. PMA addition to HEK293 cells induced strong ERK phosphorylation (Fig. 3B), confirming that DAG-dependent signals regulate this mechanism (39). Expression of wt DGK{alpha} markedly reduced ERK phosphorylation, whereas the effect of expressing the nonphosphorylatable Y335F mutant was much less pronounced (Fig. 3C). This result confirms the role of DGK{alpha} as a negative modulator of DAG-dependent functions and demonstrates that phosphorylation of DGK{alpha} at Y335 is essential for enzyme function.

We have previously shown that lack of enzyme activity alters DGK{alpha} translocation kinetics, conferring dominant-negative properties on this kinase-dead DGK{alpha} mutant (DGK{alpha} KD). Expression of this mutant in Jurkat cells enhanced CD69 expression, as a consequence of promoted Ras/MAPK signaling (27). Because lack of activity does not prevent tyrosine phosphorylation of the enzyme (see Fig. 1C), we compared membrane translocation of DGK{alpha} KD with that of a construct bearing the Y335F mutation (DGK{alpha} KDY335F). Like its active counterpart, DGK{alpha} KDY335F failed to relocate to the membrane after stimulation (Fig. 4A). Accordingly, CD69 expression analysis indicates that Y335F mutation fully impaired the effect of the DGK{alpha} KD mutant (Fig. 4B). These results further support that Y335 phosphorylation is absolutely necessary for DGK{alpha} function at the membrane.


Figure 4
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  		FIGURE 4. Y335 phosphorylation is required for the dominant-negative properties of DGK{alpha} KD. Jurkat cells were transfected with either DGK{alpha} KD or DGK{alpha} KD Y335F mutant fused to GFP. A, The subcellular localization of the GFP-fused proteins was determined as in Fig. 3. B, CD69 expression was determined by flow cytometry after 2.5-h stimulation with CD3/CD28. At the top, analysis of CD69 expression in the GFP-positive population is shown. At the bottom, CD69 MFI was plotted vs GFP MFI. The results shown are from a single experiment representative of three performed with similar results.

 
Generation of a phosphorylated Y335-specific Ab

To improve our analysis of DGK{alpha} phosphorylation at Y335, we generated a polyclonal phospho-specific Ab (pY335) and tested its specificity in in vitro studies using the wt and mutant versions of DGK{alpha}. We cotransfected HEK293 cells with a plasmid that coded for Lck and with increasing amounts of a plasmid encoding Myc-fused human DGK{alpha}. In these conditions, the pY335 Ab recognized human DGK{alpha}. Recognition was linear (Fig. 5A), and was further enriched following immunoprecipitation of DGK{alpha} (Fig. 5B). The pY335 Ab also recognized HA-tagged murine DGK{alpha} when overexpressed with Lck in HEK293 cells (Fig. 5C). Although the Ab clearly recognized DGK{alpha} when the cells coexpressed Lck, a weak signal was also observed in the absence of Lck (Fig. 5C, longer exposure). Because HEK293 cells express other PTK of the Src family, these kinases might phosphorylate the overexpressed DGK{alpha}.


Figure 5
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  		FIGURE 5. pY335 DGK{alpha} Ab recognizes DGK{alpha} tyrosine phosphorylation at Y335. A, HEK293 cells were transfected with different amounts of plasmid encoding myc-tagged human DGK{alpha} or empty vector (PMT2), alone or with a plasmid encoding Lck, as indicated. After 24 h, cells were collected and lysed, and pDGK{alpha}Y335, DGK{alpha}, Lck, and actin levels were evaluated using the indicated Ab. B, myc-DGK{alpha} was immunoprecipitated, and blots were probed with anti-pDGK Y335. C, Same as in A, except that murine HA-tagged DGK{alpha} was used. D, HEK293 cells were transfected with a plasmid encoding HA-DGK{alpha} or HA-DGK{alpha} Y335F mutant, alone or with Lck. At 24 h posttransfection, cells were lysed, and proteins were resolved by SDS-PAGE and analyzed by Western blot with the indicated Ab.

 
To confirm the specificity of the pY335 Ab, we transiently transfected HEK293 cells with wt HA-tagged murine DGK{alpha} or DGK{alpha}Y335F alone or together with a plasmid encoding Lck. Analysis of total cell lysates showed strong pY335 Ab reactivity with wt DGK{alpha} when cotransfected with Lck. The pY335 Ab did not recognize the DGK{alpha}Y335F mutant, although wt and mutant proteins were expressed at similar levels (Fig. 5D).

To test endogenous DGK{alpha} phosphorylation on Y335 in Jurkat T cells, we analyzed total cell lysates in immunoblot using the pY335 Ab, which revealed a clear band corresponding to the Mr of DGK{alpha} (Fig. 6A). We used the Src family inhibitor PP2 to corroborate our hypothesis that DGK{alpha} phosphorylation was Src family kinase activity dependent. Analysis showed that DGK{alpha} phosphorylation at Y335 decreased at the same inhibitor dose that reduced tyrosine phosphorylation in total cell lysates (Fig. 6A). To examine the role of Lck in Y335 phosphorylation, we used JCaM, a Jurkat cell variant that lacks Lck (31). Although endogenous DGK{alpha} expression is similar in both cell lines, the pY335 Ab reacted strongly with endogenous DGK{alpha} in Jurkat, but not in JCaM cells (Fig. 6B). We also observed marked reactivity in the Molt4 leukemia T cell line, which also expresses Lck. These experiments confirm phosphorylation of endogenous DGK{alpha} at Y335 and suggest that, in T lymphocytes, this phosphorylation is Lck dependent.


Figure 6
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  		FIGURE 6. A, DGK{alpha} phosphorylation depends on a Src kinase activity. Jurkat cells were left untreated or incubated with the Src inhibitor PP2 at the indicated concentrations (1 h). Cells were collected and lysed, and proteins were resolved by SDS-PAGE and immunoblot using pY335 Ab to detect DGK{alpha} phosphorylation and anti-DGK{alpha} Ab to determine DGK{alpha} expression. B, DGK{alpha} phosphorylation depends on Lck expression. Total cell lysates from HEK293, Jurkat, JcaM, and Molt4 cells were analyzed by SDS-PAGE. Blots were probed with pY335 Ab to detect DGK{alpha} phosphorylation. DGK{alpha} expression and Lck expression were determined using specific DGK{alpha} and Lck Abs.

 
TCR triggering induces DGK{alpha} phosphorylation at Y335

We showed above that DGK{alpha} tyrosine phosphorylation is Lck dependent. TCR triggering activates Lck and also generates DAG and an increase in [Ca2+]i. We next determined DGK{alpha} Y335 phosphorylation kinetics following TCR triggering. The basal Y335 phosphorylation observed in Jurkat T cells increased rapidly following TCR triggering, to then decrease at longer times (Fig. 7A). These rapid, transient kinetics mirrored those of PLC{gamma}, suggesting a correlation between the mechanisms that govern DAG generation and consumption (Fig. 7B).


Figure 7
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  		FIGURE 7. TCR stimulation increases DGK{alpha} Y335 phosphorylation. A, Jurkat cells were stimulated with anti-CD3 and anti-CD28 for the times indicated. Cells were lysed and proteins were analyzed by SDS-PAGE and immunoblot. Nitrocellulose membranes were blotted with the indicated Ab. B, As in A, but membranes were blotted with anti-pDGK{alpha}-Y335 and anti-pPLC{gamma} Y783 and reprobed with anti-DGK{alpha}, anti-PLC{gamma}, and anti-actin to detect total proteins.

 
Effect of costimulation on DGK{alpha} Y335 phosphorylation

DGK{alpha} tyrosine phosphorylation is observed in the absence of the CD28 costimulatory signal (Fig. 1, A and B). To evaluate the importance of costimulation in DGK{alpha} Y335 phosphorylation kinetics, we stimulated Jurkat cells with anti-CD3, alone or with anti-CD28 Ab. We observed phosphorylation even after serum deprivation, when the total tyrosine phosphorylation activity level is greatly decreased. Anti-CD3 stimulation was sufficient to induce robust DGK{alpha} Y335 phosphorylation (Fig. 8A); after costimulation, phosphorylation was also observed, albeit with more transient kinetics than following anti-CD3 stimulation (Fig. 8A).


Figure 8
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  		FIGURE 8. DGK{alpha} Y335 phosphorylation kinetics depends on costimulatory signals. A, Jurkat cells were stimulated with anti-CD3 alone or anti-CD3/CD28 for the times indicated and lysed, and proteins were resolved by SDS-PAGE and analyzed by immunoblot with anti-pDGK{alpha} Y335. Membranes were then reblotted with anti-DGK{alpha} to detect total protein. Immunoblots of duplicate samples were probed with anti-Lck, anti-pERK1/2, and anti-pI{kappa}B as a control of stimulation. DGK{alpha} phosphorylation was quantified with Image J software. B, Human PBLs were purified, as indicated in Materials and Methods, and treated as in A.

 
We next determined whether DGK{alpha} was also phosphorylated in response to TCR ligation in human primary T cells. We detected DGK{alpha} phosphorylation in unstimulated PBL, which increased further following TCR triggering. As also found for Jurkat T cells, phosphorylation decreased more rapidly when cells were costimulated (Fig. 8B).

DGK{alpha} phosphorylated at Y335 represents a membrane-bound DGK{alpha} pool

In Jurkat T cells, we previously showed rapid, transient translocation to membranes of both GFP-DGK{alpha} fusion proteins and the endogenous enzyme in response to receptor stimulation (24, 27) (Fig. 3A). Analysis of endogenous DGK{alpha} by subcellular fractionation of murine lymph node T cells shortly after in vivo engagement of the TCR confirmed that this enzyme, which is cytosolic in resting T cells, translocated to the membrane in response to receptor triggering (24). To analyze the correlation between DGK{alpha} Y335 phosphorylation and membrane location, we determined the location of phosphorylated DGK{alpha} in subcellular fractions of Jurkat T cells. We found that DGK{alpha} is present mainly in the cytosolic fraction, with a very small amount in the membrane fraction, as previously observed (24). The pY335 Ab did not recognize DGK{alpha} in cytosol, but only in membrane fractions, where Lck is also located (Fig. 9A). This demonstrates that phosphorylation at Y335 is only detected when DGK{alpha} is located at the membrane, and suggests that the pY335 Ab represents an excellent tool for detection of the membrane-associated/active DGK{alpha} fraction.


Figure 9
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  		FIGURE 9. A, DGK{alpha} pY335 DGK{alpha} subcellular distribution. A, Jurkat subcellular fractions were isolated by fractionation, as described in Materials and Methods. The proteins obtained in the sequential extractions were resolved by SDS-PAGE, and Western blot membranes were blotted with anti-Lck, anti-I{kappa}B{alpha}, and anti-vimentin Abs as membrane, cytosol, and cytoskeleton isolation controls, respectively. Anti-pDGK{alpha}-Y335 and anti-DGK{alpha} were used to analyze phosphorylated DGK{alpha} and DGK{alpha} cellular distribution. B, Jurkat cells were stimulated by TCR cross-linking with anti-CD3/CD28, and DGK{alpha} phosphorylation was assessed in the cytosol and membrane fractions, as in A. C, DGK{alpha} associates to Lck in response to TCR stimulation. Lck was immunoprecipitated from lysates of Jurkat cells stimulated for the indicated times with anti-CD3/CD28. Total cell lysates and immunoprecipitated pellets were analyzed by SDS-PAGE and immunoblot, and membranes were blotted with the indicated Ab.

 
Because phosphorylated DGK{alpha} appears to represent the membrane-bound pool, we tested phosphorylation kinetics in membranes isolated after stimulation. We identified changes in phosphorylation almost exclusively in the membrane fraction, whereas phosphorylation of the cytosolic protein was barely detected (Fig. 9B).

The finding that DGK{alpha} Y335 phosphorylation was Lck activity dependent and that, like Lck, DGK{alpha} pY335 was found exclusively in the membrane fraction, suggested that DGK{alpha} translocation to the membrane is required for Lck-dependent phosphorylation. We thus analyzed the possible association between these two proteins. We stimulated Jurkat cells with anti-CD3/CD28 mAb, followed by Lck immunoprecipitation and DGK{alpha} detection in the complexes. Following stimulation, we detected DGK{alpha} in anti-Lck pellets (Fig. 9C). The pY335 Ab nonetheless indicated that the Lck-associated DGK{alpha} fraction was not phosphorylated at this residue. These data suggest that, whereas Lck associates DGK{alpha}, tyrosine phosphorylation at residue 335 induces DGK{alpha} dissociation from the complex while maintaining the enzyme at the membrane. This indicates that TCR-dependent Lck activation facilitates interaction with DGK{alpha}, allowing phosphorylation at Y335, which in turn promotes DGK{alpha} stabilization at the membrane, while causing its dissociation from Lck.

Ca2+ flux enhances DGK{alpha} phosphorylation at Y335

DGK{alpha} has two EF hand domains, characteristic of the Ca2+-binding proteins, and is activated by Ca2+ in vitro (23). Deletion of the EF hand domains induces constitutive membrane localization, suggesting that a [Ca2+]i-dependent conformational change is necessary to allow membrane localization of the enzyme (27). We therefore evaluated the individual effects of [Ca2+]i and DAG on DGK{alpha} phosphorylation at Y335. We raised [Ca2+]i levels using the calcium ionophore ionomycin; to mimic DAG generation, we used PMA, which is often used as a costimulatory signal in TCR stimulation (40, 41). Ionomycin treatment promoted a considerable increase in DGK{alpha} Y335 phosphorylation, as well as NF-AT translocation to the nucleus (Fig. 10). Although PMA addition strongly induced ERK phosphorylation, it did not affect Y335 phosphorylation. Finally, maximum Y335 phosphorylation was observed using PMA and ionomycin together (Fig. 10A). These results demonstrate that the concerted activation of DAG and Ca2+-dependent signals induces maximum DGK{alpha} phosphorylation at Y335. Ca2+ mobilization alone, probably through direct binding to the EF hand motifs, appears to induce a conformational change that facilitates enzyme phosphorylation.


Figure 10
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  		FIGURE 10. Ca2+ flux increases DGK phosphorylation at Tyr335. Jurkat cells were left untreated or stimulated with Ca2+ ionophore (ionomycin), PMA, or both (5 min). Cells were lysed, and proteins were separated by SDS-PAGE. A, Nitrocellulose membranes were blotted with anti-pY335DGK{alpha} and anti-pERK1/2 Ab as a control of PMA stimulation, and anti-DGK{alpha}, anti-ERK1/2, and anti-Lck Ab as protein controls. Phosphorylated proteins were detected with anti-pY mAb. B, Duplicate samples were probed with anti-NF-AT Ab to visualize NF-AT dephosphorylation (top) and translocation to the insoluble fraction (bottom).

 

   Discussion
Top
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
Disclosures
 References
 
The exquisite tuning of the TCR is possible because this receptor is coupled to signaling networks controlled by down-regulator proteins and interconnected negative feedback loops. TCR activation initiates a cascade of tyrosine phosphorylation that leads to DAG production and Ca2+ release. Tyrosine phosphorylation is controlled and terminated by the action of different tyrosine phosphatases (33), such as Src homology (SH) region 2 domain-containing phosphatase-1 (SHP-1), whereas DAG levels are strictly controlled by DGK action (16, 17, 24).

The Src kinase Lck is one of the main PTK activated during TCR triggering. In addition to its enzymatic function, this kinase serves as an adaptor protein through its SH domains, SH2 and SH3 (7). Recent studies using naive T cells from Lck-deficient mice pointed out that through Y136 LAT phosphorylation, Lck is the main trigger for PLC{gamma} activation. Lck is thus the main Src kinase controlling subsequent DAG production and activation of DAG-responsive molecules, such as Ras-GRP1 and PKC{theta} (8). Lck also exerts a suppressive role on the TCR, either through TCR internalization (42) or by turning on a negative feedback loop through SHP-1 (SH2-containing phosphotyrosine phosphatase type I) activation. Lck phosphorylates SHP-1 on residues Y536 and Y566, leading to an increase in SHP-1 phosphatase activity. Lck down-regulation by small interfering RNA in Jurkat cells thus suppresses proximal TCR signaling and also increases the downstream response, evaluated as ERK phosphorylation and IL-2 production (43).

In this study, we demonstrate that Lck also phosphorylates another negative regulatory molecule of the TCR response, DGK{alpha}. Consequently, Lck controls not only the switching off of tyrosine phosphorylation, but also the termination of DAG-derived signals. Phosphorylation of DGK{alpha} by a PTK activated during TCR triggering concurs with our previous results establishing that in addition to Ca2+ flux, DGK{alpha} membrane translocation requires tyrosine phosphorylation (27).

We identified Y335, located at the hinge between the C1 and the catalytic domains, as a Lck-dependent DGK{alpha} phosphorylation site. Generation of a specific Y335DGK{alpha} Ab demonstrated that, both in T cell lines and primary human lymphocytes, DGK{alpha} is phosphorylated at this residue. Phosphorylation of endogenous DGK{alpha} is inhibited by Src-family PTK inhibitors. Experiments comparing Jurkat T cells and the Lck-deficient variant indicate that, at least in T lymphocytes, Lck is the PTK responsible for DGK{alpha} phosphorylation.

Other Src family PTK (Src, Fyn, Lyn, Blk, Yes, Fgr, Hck) may be able to phosphorylate DGK{alpha} at this residue in nonlymphoid cells. In HEK293 cell overexpression experiments, we detected weak Y335 phosphorylation of DGK{alpha}, even in the absence of Lck. HEK293 cells do not express Lck, although they express other Src kinases that might phosphorylate the ectopically expressed DGK{alpha}. Accordingly, Src kinases are reported to be essential for DGK{alpha} modulation in various adherent cell lines in response to receptors such as vascular endothelial growth factor, hepatocyte growth factor, or {alpha}-D-tocopherol (30, 37, 38, 44, 45).

Experiments with overexpressed enzyme suggest that DGK{alpha} Y335 phosphorylation targets DGK{alpha} to the plasma membrane (37, 38). Analysis of endogenous protein in Jurkat T lymphocytes demonstrates that tyrosine-phosphorylated DGK{alpha} represents a membrane-associated fraction, which probably corresponds to the active pool of the enzyme. This is consistent with the fact that the small amount of phosphorylated DGK{alpha}, derived from DGK overexpression in HEK293 cells, reduces ERK phosphorylation. In addition, impairment of Y335 phosphorylation in DGK{alpha} KD dramatically inhibits the dominant-negative properties of this mutant, further suggesting a direct correlation between Y335 phosphorylation and DGK{alpha} function.

During TCR activation, tyrosine phosphorylation of DGK{alpha} (as detected with total pY or pY335DGK{alpha} Ab) increases to a maximum level, which then decreases sharply. This pattern resembles the tyrosine phosphorylation kinetics of PLC{gamma}, suggesting that Lck activation turns on a coordinated mechanism that controls DAG production and consumption in response to TCR (Figs. 7 and 11). There may be similar coordination in the modulation of PLCβ-generated DAG, because DGK{alpha} modulates DAG produced by PLCβ following activation of the carbachol receptor (27). DGK{alpha} and Lck participation in the signaling by other G protein-coupled receptor, such as the chemokine receptors, remains to be determined.


Figure 11
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  		FIGURE 11. Model of TCR-mediated DGK{alpha} activation. Following TCR engagement, Lck is activated and triggers a signaling cascade that leads to PLC{gamma} activation, which generates DAG and IP3. This last messenger provokes a Ca2+ flux from endoplasmic reticulum that promotes a conformational change in the DGK{alpha}. In its open conformation, DGK{alpha} is able to interact with Lck at the plasma membrane. This interaction leads to DGK{alpha} phosphorylation at Y335, which induces its stabilization at the membrane and its rapid dissociation from Lck. The pY335 DGK{alpha} is active and, because it is membrane bound, can easily metabolize the TCR-generated DAG. DGK{alpha} activity at the membrane down-regulates DAG-dependent signals and at the same time promotes its own inactivation, most likely through a PA-sensitive tyrosine phosphatase.

 
Because DGK{alpha} requires both tyrosine phosphorylation and Ca2+ for full activation, it is reasonable to assume that DGK{alpha} is phosphorylated only after a [Ca2+]i increase; this idea is also supported by our experiments using PMA and the Ca2+ ionophore ionomycin. Although Ca2+ influx increases DGK{alpha} Y335 phosphorylation, PMA alone does not. This shows that Ca2+ enhances Lck-dependent DGK{alpha} phosphorylation, and further supports a model in which Ca2+ influx induces a closed-to-open conformation transition of DGK{alpha}, allowing its interaction with membrane (27).

Lck-dependent DGK{alpha} phosphorylation suggests a direct interaction between these two proteins (45). Our fractionation experiments indicate that DGK{alpha} pY335 is located at the membrane, where Lck is also concentrated, and TCR stimulation induces an increase in the phosphorylation of the protein at the membrane. Rapid association of DGK{alpha} and Lck is observed after TCR triggering, concurring with earlier studies reporting an association between DGK{alpha} and Src kinase activity (30, 44, 46). It is striking that DGK{alpha} pY335 is not observed in the Lck immunoprecipitates. Although we cannot discard resolution limitations of the pY335 Ab, this would appear to indicate that phosphorylation of DGK{alpha} induces its rapid dissociation from Lck, simultaneously allowing DGK{alpha} association with the plasma membrane. The physiological significance of this rapid loss of association remains to be determined, but it may favor DGK{alpha} association with other partners. A recent publication showed that phosphorylated Y335 might interact with certain SH2 domains in vitro (37). The rapid dissociation of DGK{alpha} pY335 and Lck might also control and/or restrict an inhibitory DGK{alpha} effect on Lck activity. ERK phosphorylates Lck, converting the p56Lck isoform to the phosphorylated p59Lck form. This phosphorylation affects the SH2 tyrosine-binding properties of Lck (47), primarily its binding and subsequent inactivation by SHP-1 (48). Transitory activation of DGK{alpha}, through its effects on Ras/ERK pathway, may indirectly influence the temporality of Lck activity.

Because DAG metabolism is essential during TCR-mediated responses, it is important to understand the mechanisms that control DGK{alpha} activation. Our results demonstrating DGK{alpha} tyrosine phosphorylation at Y335 in response to TCR ligation in cultured cell lines and in primary human PBLs reveal a new aspect of DGK{alpha} regulation. It is interesting that CD28-mediated costimulation causes a more transient phosphorylation of Y335, suggesting that costimulation results in a more rapid termination of DGK{alpha}-mediated signals. Although the mechanism by which CD28 orchestrates the temporality of DGK{alpha} activation remains unknown, DGK{alpha} inactivation by costimulatory signals correlates with the proposed role of this DGK isoform as a negative regulator of T cell functions and of an anergy-induced gene (14). We previously showed that DGK{alpha} overexpression prevents Ras-GRP1 membrane localization (24), thereby blocking the Ras/ERK pathway. Recent studies of DGK{alpha} overexpression in an in vivo model of T lymphocyte activation confirm these results, and demonstrate that DGK{alpha} contributes to establishment of anergy (49). This function concurs with studies using DGK{alpha}-deficient mice, in which the absence of DGK{alpha} contributes to an anergy-resistant state, and indicates that DAG metabolism is essential to anergy development (17).

TCR engagement with its ligand leads to a cellular response in which different factors are integrated, to give rise to either activation or tolerance. In the periphery, low-affinity ligands such as self-ligands lead to an inefficient response and thus to tolerance; in contrast, pathogenic ligands elicit strong, productive activation that promotes effective host defense. Our finding suggests that those stimuli that provoke tolerance or anergy could induce disproportionate DGK{alpha} activation, either through Ca2+ flux or initial Lck activity. In such cases, loss of temporality of the DGK{alpha} response would block the Ras/ERK pathway and, through SHP-1, turn off Lck, leading to incomplete, nonproductive T cell activation. Moreover, tolerized T and B lymphocytes show a basal increase in [Ca2+]i levels (14, 50). It is not known whether such control mechanisms are applicable to other DGK isoforms. At difference from DGK{alpha}, DGK{zeta} activity does not appear to be regulated directly by Ca2+ flux, although this isoform is reported to associate with a Src kinase activity in gonadotrophic cells (51).

In summary, our data suggest a model in which TCR-dependent activation of Lck regulates DAG generation and removal through the concerted activation of PLC{gamma} and DGK{alpha} (Fig. 11). Tyrosine phosphorylation of DGK{alpha} is facilitated by the PLC{gamma}-generated Ca2+ flux. This Ca2+-dependent priming of DGK{alpha} provides a unique mechanism that guarantees the correct timing of PLC{gamma} and DGK{alpha} activation.


   Acknowledgments
 
We are grateful to colleagues from I. Mérida’s group for stimulating discussion, and Catherine Mark for excellent editorial assistance.


   Disclosures
Top
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
 Disclosures
References
 
The authors have no financial conflict of interest.


   Footnotes
 
The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked advertisement in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

1 This work was supported in part by Grants G03/79 from the Instituto de Salud Carlos III (Spanish Ministry of Health), BFU2004-01756 (Spanish Ministry of Education), and S-SAL-0311 from Comunidad de Madrid. E.M. is a Spanish Ministry of Science and Technology predoctoral fellow; A.Á.-F. is supported by the Juan de la Cierva programme from the Spanish Ministry of Science and Technology. The Department of Immunology and Oncology was founded and is supported by the Spanish National Research Council (Consejo Superior de Investigaciones Cientificas) and by Pfizer. Back

2 Address correspondence and reprint requests to Dr. Isabel Mérida, Department of Immunology and Oncology, Centro Nacional de Biotecnología/Consejo Superior de Investigaciones Cientificas, C/Darwin, 3, Universidad Autónoma de Madrid Campus de Cantoblanco, E-28049 Madrid, Spain. E-mail address: imerida{at}cnb.uam.es Back

3 Abbreviations used in this paper: PTK, protein tyrosine kinase; [Ca2+]i, intracellular Ca2+; DAG, diacylglycerol; DGK, DAG kinase; HA, hemagglutinin; IP3, inositol 1,4,5-triphosphate; KD, kinase dead; LAT, linker of activated T cell adaptor protein; MFI, mean fluorescence intensity; PA, phosphatidic acid; PKC, protein kinase C; PLC{gamma}, phospholipase C{gamma}1; pY, phosphotyrosine; SH, Src homology; SHP-1, SH region 2 domain-containing phosphatase-1; wt, wild type; Ras-GRP1, Ras guanyl nucleotide-releasing protein 1. Back

Received for publication October 3, 2007. Accepted for publication February 19, 2008.


   References
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 Introduction
 Materials and Methods
 Results
 Discussion
 Disclosures
 References
 

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