Neoplastic transformation induced by an activated lymphocyte-specific protein tyrosine kinase (pp56lck)

The Activity and Stability of p56Lck and TCR Signaling Do Not Depend on the Co-Chaperone Cdc37

Lymphocyte-specific protein tyrosine kinase (Lck) is a pivotal tyrosine kinase involved in T cell receptor (TCR) signaling. Because of its importance, the activity of Lck is regulated at different levels including phosphorylation of tyrosine residues, protein-protein interactions, and localization. It has been proposed that the co-chaperone Cdc37, which assists the chaperone heat shock protein 90 (Hsp90) in the folding of client proteins, is also involved in the regulation of the activity/stability of Lck. Nevertheless, the available experimental data do not clearly support this conclusion. Thus, we assessed whether or not Cdc37 regulates Lck. We performed experiments in which the expression of Cdc37 was either augmented or suppressed in Jurkat T cells. The results of our experiments indicated that neither the overexpression nor the suppression of Cdc37 affected Lck stability and activity. Moreover, TCR signaling proceeded normally in T cells in which Cdc37 expression was either augmented or suppressed. Finally, we demonstrated that also under stress conditions Cdc37 was dispensable for the regulation of Lck activity/stability. In conclusion, our data do not support the idea that Lck is a Cdc37 client.

Stimulation strength controls the rate of initiation but not the molecular organisation of TCR-induced signalling

Millions of naïve T cells with different TCRs may interact with a peptide-MHC ligand, but very few will activate. Remarkably, this fine control is orchestrated using a limited set of intracellular machinery. It remains unclear whether changes in stimulation strength alter the programme of signalling events leading to T cell activation. Using mass cytometry to simultaneously measure multiple signalling pathways during activation of murine CD8+ T cells, we found a programme of distal signalling events that is shared, regardless of the strength of TCR stimulation. Moreover, the relationship between transcription of early response genes Nr4a1 and Irf8 and activation of the ribosomal protein S6 is also conserved across stimuli. Instead, we found that stimulation strength dictates the rate with which cells initiate signalling through this network. These data suggest that TCR-induced signalling results in a coordinated activation program, modulated in rate but not organization by stimulation strength.

Phosphoproteomic Analyses of Interleukin 2 Signaling Reveal Integrated JAK Kinase-Dependent and -Independent Networks in CD8(+) T Cells

Interleukin-2 (IL-2) is a fundamental cytokine that controls proliferation and differentiation of T cells. Here, we used high-resolution mass spectrometry to generate a comprehensive and detailed map of IL-2 protein phosphorylations in cytotoxic T cells (CTL). The data revealed that Janus kinases (JAKs) couple IL-2 receptors to the coordinated phosphorylation of transcription factors, regulators of chromatin, mRNA translation, GTPases, vesicle trafficking, and the actin and microtubule cytoskeleton. We identified an IL-2-JAK-independent SRC family Tyr-kinase-controlled signaling network that regulates ∼10% of the CTL phosphoproteome, the production of phosphatidylinositol (3,4,5)-trisphosphate (PIP3), and the activity of the serine/threonine kinase AKT. These data reveal a signaling framework wherein IL-2-JAK-controlled pathways coordinate with IL-2-independent networks of kinase activity and provide a resource toward the further understanding of the networks of protein phosphorylation that program CTL fate.

The phosphatase JKAP/DUSP22 inhibits T-cell receptor signalling and autoimmunity by inactivating Lck

JNK pathway-associated phosphatase (JKAP, also known as DUSP22 or JSP-1) is a JNK activator. The in vivo role of JKAP in immune regulation remains unclear. Here we report that JKAP directly inactivates Lck by dephosphorylating tyrosine-394 residue during T-cell receptor (TCR) signalling. JKAP-knockout T cells display enhanced cell proliferation and cytokine production. JKAP-knockout mice show enhanced T-cell-mediated immune responses and are more susceptible to experimental autoimmune encephalomyelitis (EAE). In addition, the recipient mice that are adoptively transferred with JKAP-knockout T cells show exacerbated EAE symptoms. Aged JKAP-knockout mice spontaneously develop inflammation and autoimmunity. Thus, our results indicate that JKAP is an important phosphatase that inactivates Lck in the TCR signalling turn-off stage, leading to suppression of T-cell-mediated immunity and autoimmunity.

Nuclear localization of lymphocyte-specific protein tyrosine kinase (Lck) and its role in regulating LIM domain only 2 (Lmo2) gene

LIM domain only protein 2 (Lmo2) is a transcription factor that plays a critical role in the development of T-acute lymphoblastic leukemia (T-ALL). A previous report established a link between Lmo2 expression and the nuclear presence of oncogenic Janus kinase 2 (JAK2), a non-receptor protein tyrosine kinase. The oncogenic JAK2 kinase phosphorylates histone H3 on Tyr 41 that leads to the relief of Lmo2 promoter repression and subsequent gene expression. Similar to JAK2, constitutive activation of lymphocyte-specific protein tyrosine kinase (Lck) has been implicated in lymphoid malignancies. However, it is not known whether oncogenic Lck regulates Lmo2 expression through a similar mechanism. We show here that Lmo2 expression is significantly elevated in T cell leukemia LSTRA overexpressing active Lck kinase and in HEK 293 cells expressing oncogenic Y505FLck kinase. Nuclear localization of active Lck kinase was confirmed in both Lck-transformed cells by subcellular fractionation and immunofluorescence microscopy. More importantly, in contrast to oncogenic JAK2, oncogenic Lck kinase does not result in significant increase in histone H3 phosphorylation on Tyr 41. Instead, chromatin immunoprecipitation experiment shows that oncogenic Y505FLck kinase binds to the Lmo2 promoter in vivo. This result raises the possibility that oncogenic Lck may activate Lmo2 promoter through direct interaction.

The human cytomegalovirus UL11 protein interacts with the receptor tyrosine phosphatase CD45, resulting in functional paralysis of T cells

Human cytomegalovirus (CMV) exerts diverse and complex effects on the immune system, not all of which have been attributed to viral genes. Acute CMV infection results in transient restrictions in T cell proliferative ability, which can impair the control of the virus and increase the risk of secondary infections in patients with weakened or immature immune systems. In a search for new immunomodulatory proteins, we investigated the UL11 protein, a member of the CMV RL11 family. This protein family is defined by the RL11 domain, which has homology to immunoglobulin domains and adenoviral immunomodulatory proteins. We show that pUL11 is expressed on the cell surface and induces intercellular interactions with leukocytes. This was demonstrated to be due to the interaction of pUL11 with the receptor tyrosine phosphatase CD45, identified by mass spectrometry analysis of pUL11-associated proteins. CD45 expression is sufficient to mediate the interaction with pUL11 and is required for pUL11 binding to T cells, indicating that pUL11 is a specific CD45 ligand. CD45 has a pivotal function regulating T cell signaling thresholds; in its absence, the Src family kinase Lck is inactive and signaling through the T cell receptor (TCR) is therefore shut off. In the presence of pUL11, several CD45-mediated functions were inhibited. The induction of tyrosine phosphorylation of multiple signaling proteins upon TCR stimulation was reduced and T cell proliferation was impaired. We therefore conclude that pUL11 has immunosuppressive properties, and that disruption of T cell function via inhibition of CD45 is a previously unknown immunomodulatory strategy of CMV.

The human cytomegalovirus (CMV) belongs to a class of viruses that interferes with the immune response of its host. Accordingly, infection with CMV is a severe risk for immunologically immature newborns and immunocompromised patients such as transplant recipients. The mechanisms by which CMV affects the immune system are not completely understood. Here we show that a CMV protein, pUL11, which is expressed on the surface of cells, binds to leukocytes by interacting with the receptor tyrosine phosphatase CD45. In T cells, CD45 is essential for transmission of activating signals received via the T cell receptor (TCR) to downstream effector molecules that ultimately lead to activation and proliferation of these immune cells. Binding of the CMV pUL11 protein to CD45 on T cells prevents signal transduction via the TCR and restricts T cell proliferation. Interestingly, the mechanism by which the activity of CD45 is regulated is a matter of debate and no specific cellular ligand of CD45 has yet been described. The identification of a first viral ligand for CD45 may provide the means to investigate CD45 regulatory mechanisms and also allow the development of therapies to interfere with CMV-mediated immunomodulation.

B-1 cell lymphoma in mice lacking the steroid and xenobiotic receptor, SXR

The steroid and xenobiotic receptor (SXR) is a broad-specificity nuclear hormone receptor that is highly expressed in the liver and intestine, where its primary function is to regulate drug and xenobiotic metabolism. SXR is expressed at lower levels in other tissues, where little is known about its physiological functions. We previously linked SXR with immunity and inflammation by showing that SXR antagonizes the activity of nuclear factor (NF)-κB in vitro and in vivo. SXR(-/-) mice demonstrate aberrantly high NF-κB activity and overexpression of NF-κB target genes. Here we show that SXR(-/-) mice develop B cell lymphoma in an age-dependent manner. SXR(-/-) mice develop multiple hyperplastic lymphoid foci composed of B-1a cells in the intestine, spleen, lymph nodes, peritoneal cavity, and blood. In all circumstances, these lymphocytes possess cell surface and molecular characteristics of either chronic lymphocytic leukemia or non-Hodgkin's lymphoma originating from B-1 lymphocytes. These results demonstrate a novel and unsuspected role for SXR signaling in the B-1 cell compartment, establish SXR as a tumor suppressor in B-1 cells, and may provide a link between metabolism of xenobiotic compounds and lymphomagenesis.

Lyn kinase and ZAP70 are substrates of PTPROt in B-cells: Lyn inactivation by PTPROt sensitizes leukemia cells to VEGF-R inhibitor pazopanib

We have recently shown that the gene encoding the truncated form of protein tyrosine phosphatase receptor-type O (PTPROt) expressed predominantly in hematopoietic cells is epigenetically silenced in human primary chronic lymphocytic leukemia (B-CLL). To determine whether increased phosphorylation of the PTPROt substrates following PTPROt suppression alters signal transduction pathway(s) that impart a growth advantage to the leukemic lymphocytes, it is critical to discern the key substrates of PTPROt. Here, we used substrate-trapping assay to identify two novel substrates of PTPROt, the tyrosine kinases Lyn and ZAP70. Both Lyn and ZAP70 were dephosphorylated by wild-type PTPROt, but not by its catalytic site (CS) mutant. A critical phosphorylation site in Lyn, Y397, essential for its activity was dephosphorylated by PTPROt. Consequently, the activity of Lyn kinase was compromised when co-expressed with PTPROt-WT compared to vector control or upon co-expression with PTPROt-CS. Ectopic expression of PTPROt in Raji cells reduced phosphorylation of Lyn in the absence of any change in its protein levels. These results have revealed the physiological importance of PTPROt in regulating B-cell receptor signaling at Lyn kinase. Further, ectopic expression of PTPROt also sensitized the cells to the VEGF-R inhibitor Pazopanib.

Plasma membrane subdomain partitioning of Lck in primary human T lymphocytes

Uncovering the plasma membrane distribution of tyrosine kinase Lck is crucial to understanding T lymphocyte triggering. Several studies of Lck species partitioning have given contradictory results. We decided to re-address this point by using phospho-specific antibodies to characterize active and inactive Lck partitioning in raft and non-raft membranes from primary human peripheral blood T lymphocytes. We show that most inactive Lck was localized in rafts and was associated with nearly all CD4 coreceptors and its negative regulator Csk in resting cells, while T cell receptor (TCR) engagement promoted a sustained dephosphorylation of inactive Lck. In contrast, active Lck had a more discrete distribution interacting with only a small number of CD4 coreceptors, and the kinase showed a rapid and short phosphorylation after TCR triggering. The differences in distribution and kinetics may be related to T lymphocyte signalling threshold modulation by Lck species and suggest how TCR triggering is first initiated. This study furthers our knowledge of the TCR activation model in primary human T lymphocytes.

T-cell receptor proximal signaling via the Src-family kinases, Lck and Fyn, influences T-cell activation, differentiation, and tolerance

T-cell development in the thymus and activation of mature T cells in secondary lymphoid organs requires the ability of cells to respond appropriately to environmental signals at multiple stages of their development. The process of thymocyte selection insures a functional T-cell repertoire, while activation of naive peripheral T cells induces proliferation, gain of effector function, and, ultimately, long-lived T-cell memory. The T-cell immune response is initiated upon engagement of the T-cell receptor (TCR) and coreceptor, CD4 or CD8, by cognate antigen/major histocompatibility complexes presented by antigen-presenting cells. TCR/coreceptor engagement induces the activation of biochemical signaling pathways that, in combination with signals from costimulator molecules and cytokine receptors, direct the outcome of the response. Activation of the src-family kinases p56(lck) (Lck) and p59(fyn) (Fyn) is central to the initiation of TCR signaling pathways. This review focuses on our current understanding of the mechanisms by which these two proteins orchestrate T-cell function.

Structurally distinct phosphatases CD45 and CD148 both regulate B cell and macrophage immunoreceptor signaling

The receptor-type protein tyrosine phosphatase (RPTP) CD148 is thought to have an inhibitory function in signaling and proliferation in nonhematopoietic cells. However, its role in the immune system has not been thoroughly studied. Our analysis of CD148 loss-of-function mice showed that CD148 has a positive regulatory function in B cells and macrophages, similar to the role of CD45 as a positive regulator of Src family kinases (SFKs). Analysis of CD148 and CD45 doubly deficient B cells and macrophages revealed hyperphosphorylation of the C-terminal inhibitory tyrosine of SFKs accompanied by substantial alterations in B and myeloid lineage development and defective immunoreceptor signaling. Because these findings suggest the C-terminal tyrosine of SFKs is a common substrate for both CD148 and CD45 phosphatases and imply a level of redundancy not previously appreciated, a reassessment of the function of CD45 in the B and myeloid lineages based on prior data from the CD45-deficient mouse is warranted.

Identification of Lck-derived peptides applicable to anti-cancer vaccine for patients with human leukocyte antigen-A3 supertype alleles

The identification of peptide vaccine candidates to date has been focused on human leukocyte antigen (HLA)-A2 and -A24 alleles. In this study, we attempted to identify cytotoxic T lymphocyte (CTL)-directed Lck-derived peptides applicable to HLA-A11⁺, -A31⁺, or -A33⁺ cancer patients, because these HLA-A alleles share binding motifs, designated HLA-A3 supertype alleles, and because the Lck is preferentially expressed in metastatic cancer. Twenty-one Lck-derived peptides were prepared based on the binding motif to the HLA-A3 supertype alleles. They were first screened for their recognisability by immunoglobulin G (IgG) in the plasma of prostate cancer patients, and the selected candidates were subsequently tested for their potential to induce peptide-specific CTLs from peripheral blood mononuclear cells of HLA-A3 supertype⁺ cancer patients. As a result, four Lck peptides were frequently recognised by IgGs, and three of them – Lck₉₀₋₉₉, Lck_449−458, and Lck_450−458 – efficiently induced peptide-specific and cancer-reactive CTLs. Their cytotoxicity towards cancer cells was mainly ascribed to HLA class I-restricted and peptide-specific CD8⁺ T cells. These results indicate that these three Lck peptides are applicable to HLA-A3 supertype⁺ cancer patients, especially those with metastasis. This information could facilitate the development of peptide-based anti-cancer vaccine for patients with alleles other than HLA-A2 and -A24.

T cell receptor induced intracellular redistribution of type I protein kinase A

The productive activation of CD4(+) T lymphocytes, leading to proliferation and cytokine secretion, requires precise temporal regulation of intracellular cyclic AMP concentrations. The major effector molecule activated by cyclic AMP in mammalian cells is the cyclic AMP-dependent protein kinase A (PKA). The type I PKA isozyme mediates the inhibitory effects of cyclic AMP on T-cell activation. Using laser scanning confocal microscopy, we demonstrated that the regulation of PKA type I activity involves spatial redistribution of PKA type I molecules following T-cell receptor (TCR) stimulation. In resting T cells, PKA type I was located in membrane proximal regions and distributed equally across the cell. Shortly after antigen engagement, T cells and antigen-presenting cells formed an area of intense contact, known as the immunological synapse. TCR concentrated at the synapse, whereas PKA type I molecules redistributed to the opposite cell pole within 10 min after T-cell stimulation. Type I PKA redistribution was solely dependent on TCR signalling, because we observed the same temporal and spatial distribution after antibody-mediated cross-linking of the TCR-associated CD3 complex. Segregation of TCR and PKA type I molecules was maintained for at least 20 min. Thirty minutes after stimulation, PKA type I partially colocalized with the TCR. After 60 min, PKA type I distribution again approached the resting state. Considering that initial TCR signals lead to increases in intracellular cyclic AMP, PKA type I molecules may be targeted towards localized cyclic AMP accumulations or transported away from these areas, depending on the requirements of the cellular response.

Evaluation of the anti-inflammatory and atopic dermatitis-mitigating effects of BSASM, a multicompound preparation

For effective management of atopic dermatitis (AD), it is important to introduce a therapeutic agent, which although having the fewest side effects, has the greatest anti-inflammatory effect. In the course of screening anti-inflammatory agents, we obtained BSASM, a mixture of several plant extracts. This study was designed to investigate the AD-mitigating effect of BSASM in patients, as well as its anti-inflammatory and immunomodulatory effects in an in vitro experiment. The anti-inflammatory effects of BSASM were evaluated by the level of production of proinflammatory cytokines. Clinical evaluation was also done using eczema area severity index (EASI) score in AD patients. BSASM inhibited LPS-induced activation of NF-kappaB promoter. In addition, LPS-induced an increase of IL-8, and the TNF-alpha production in THP-1 cells was also inhibited. These results suggest that BSASM has an anti-inflammatory activity. A clinical study in patients with AD showed that BSASM induced a reduction of EASI score, degree of pruritus, and TEWL on both the antecubital fossa and abdomen. Besides, BSASM had no irritative or allergic effects. Based on these results, we conclude that BSASM has anti-inflammatory and AD-mitigating effects.

Function of the Src-family kinases, Lck and Fyn, in T-cell development and activation

The function of the Src-family kinases (SFKs) Lck and Fyn in T cells has been intensively studied over the past 15 years. Animal models and cell line studies both indicate a critical role for Lck and Fyn in proximal T-cell antigen receptor (TCR) signal transduction. Recruited SFKs phosphorylate TCR ITAMs (immunoreceptor tyrosine-based activation motifs) in the CD3 and zeta chains, which then serve as docking sites for Syk-family kinases. SFKs then phosphorylate and activate the recruited Syk-family kinase. Lck and Fyn are spatially segregated in cell membranes due to differential lipid raft localization, and may undergo sequential activation. In addition to the CD4 and CD8 coreceptors, a recently described adaptor, Unc119, may link SFKs to the TCR. CD45 and Csk provide positive and negative regulatory control of SFK functions, respectively, and Csk is constitutively bound to the transmembrane adapter protein, PAG/Cbp. TCR-based signaling is required at several stages of T-cell development, including at least pre-TCR signaling, positive selection, peripheral maintenance of naive T cells, and lymphopenia-induced proliferation. SFKs are required for each of these TCR-based signals, and Lck seems to be the major contributor.

Tyrosine Kinase p56 Regulates Cell Motility and Nuclear Factor κB-mediated Secretion of Urokinase Type Plasminogen Activator through Tyrosine Phosphorylation of IκBα following Hypoxia/Reoxygenation

Nuclear factor kappaB (NFkappaB) plays major role in regulating cellular responses as a result of environmental injuries. The molecular mechanism(s) by which hypoxia/reoxygenation (H/R) regulates p56lck-dependent activation of NFkappaB through tyrosine phosphorylation of IkappaBalpha and modulates the expression of downstream genes that are involved in cell migration in human breast cancer cells are not well defined. In this paper, we investigated the involvement of protein-tyrosine kinase p56lck in the redox-regulated activation of NFkappaB following H/R in highly invasive (MDA-MB-231) and low invasive (MCF-7) breast cancer cells. We demonstrated that H/R induces tyrosine phosphorylation of p56lck, nuclear translocation of NFkappaB, NFkappaB-DNA binding, and transactivation of NFkappaB through tyrosine phosphorylation of IkappaBalpha. Transfection of these cells with wild type Lck but not with mutant Lck F394 followed by H/R induces the tyrosine phosphorylation of inhibitor of nuclear factor kappaB (IkappaBalpha) and transcriptional activation of NFkappaB, and these are inhibited by Lck inhibitors. In vitro kinase assay demonstrated that immunoprecipitated p56lck but not Lyn or Fyn directly phosphorylate IkappaBalpha in presence of H/R. Pervanadate, H2O2, and H/R induce the interaction between Lck and tyrosine-phosphorylated IkappaBalpha, and this interaction is inhibited by Src homology 2 domain inhibitory peptide, suggesting that tyrosine-phosphorylated IkappaBalpha interacts with Src homology 2 domain of Lck. Luciferase reporter gene assay indicated that Lck induces NFkappaB-dependent urokinase type plasminogen activator (uPA) promoter activity in presence of H/R. Furthermore, H/R stimulates the cell motility through secretion of uPA. To our knowledge, this is the first report that p56lck in presence of H/R regulates NFkappaB activation, uPA secretion, and cell motility through tyrosine phosphorylation of IkappaBalpha and further demonstrates an important redox-regulated pathway for NFkappaB activation following H/R injury that is independent of IkappaB kinase/IkappaBalpha-mediated signaling pathways.

Cutting edge: a chemical genetic system for the analysis of kinases regulating T cell development

To understand the regulatory activities of kinases in vivo requires their study across a biologically relevant window of activity. To this end, ATP analog-sensitive kinase alleles (ASKAs) specifically sensitive to a competitive inhibitor have been developed. This article tests whether ASKA technology can be applied to complex immunological systems, such as lymphoid development. The results show that when applied to reaggregate thymic organ culture, novel p56(Lck) ASKAs readily expose a dose-dependent correlation of thymocyte development with a range of p56(Lck) activity. By regulating kinase activity, rather than amounts of RNA or protein, ASKA technology offers a general means for assessing the quantitative contributions to immunology of numerous kinases emerging from genomics analyses. It can obviate the generation of multiple lines of mice expressing different levels of kinase transgenes and should permit specific biological effects to be associated with defined biochemical activities.

Identification of Lck-derived peptides capable of inducing HLA-A2-restricted and tumor-specific CTLs in cancer patients with distant metastases

The Lck protein (p56(lck)), a src family tyrosine kinase essential for T cell development and function, is aberrantly expressed in various types of cancers. We revealed recently that Lck can be a tumor antigen recognized by HLA-A24-restricted and tumor-specific cytotoxic T lymphocytes (CTLs) of cancer patients with metastases. In this study, we tried to identify Lck-derived epitopes capable of inducing HLA-A2-restricted and tumor-specific CTLs in cancer patients. The tumor-infiltrating lymphocytes (TILs) from 2 HLA-A2 cancer patients were found to respond to COS-7 cells when co-transfected with the lck gene and either HLA-A0201, -A0206, or A0207 cDNA. These TILs contained CTLs capable of recognizing either the Lck(61-69), the Lck(246-254), or the Lck(422-430) peptide among 24 different peptides, all of which were prepared based on the HLA-A2 binding motif. Importantly, in vitro sensitization with the latter 2 peptides induced tumor-specific CTLs in HLA-A2(+) cancer patients with metastases, but not in those without metastases. Overall, the Lck(246-254) and Lck(422-430) peptides could be useful for specific immunotherapy of HLA-A2(+) cancer patients, especially with distant metastases.

Activated p56lck directs maturation of both CD4 and CD8 single-positive thymocytes

p56(lck) is a protein tyrosine kinase expressed throughout T cell development. It associates noncovalently with the cytoplasmic domains of the CD4 and CD8 coreceptor molecules and has been implicated in TCR signaling in mature T cells. Its role in early thymocyte differentiation has been demonstrated in vivo, both by targeted gene disruption and by transgene expression. Previously, we showed that expression of a dominant-negative form of p56(lck) in double-positive thymocytes inhibits positive selection. We now demonstrate that expression of constitutively activated p56(lck) (p56(lck)F505) accelerates the transition from the double-positive to the single-positive stage. Importantly, p56(lck)F505 drives survival and lineage commitment of thymocytes in the absence of TCR engagement by appropriate MHC molecules. These results indicate that activation of p56(lck) constitutes an early step in conveying maturational signals after TCR ligation by a positively selecting ligand. Our study provides direct in vivo evidence for the role of p56(lck) in regulating TCR signaling.

Recognition of the Lck tyrosine kinase as a tumor antigen by cytotoxic T lymphocytes of cancer patients with distant metastases

The Lck protein (p56(lck)), a src family tyrosine kinase that is essential for T cell development and function, is aberrantly expressed in metastatic colon cancers. p56(lck) seems to facilitate the malignant transformation of epithelial cells through initiation of anchorage-independent proliferation. We demonstrate that the lck gene encodes antigenic epitopes recognized by the HLA class I-restricted and tumor-specific CTL of metastatic cancer patients. Lck peptides augmented CTL activity in peripheral blood mononuclear cells (PBMC) of colon and other epithelial cancer patients with distant metastases, but not those without distant metastases. CTL precursors recognizing the Lck peptide were identified in freshly prepared PBMC of patients with distant metastases, and their frequency was significantly augmented by stimulation with the peptide. Thus, Lck peptides could be useful in developing a specific immunotherapy for cancer patients with distant metastases.

Activation of the Lck tyrosine protein kinase by the Herpesvirus saimiri tip protein involves two binding interactions

The Tip protein of Herpesvirus saimiri strain 484C binds to and activates the Lck tyrosine protein kinase. Two sequences in the Tip protein were previously shown to be involved in binding to Lck. A proline-rich region, residues 132-141, binds to the SH3 domain of the Lck protein. We show here that the other Lck-binding domain, residues 104-113, binds to the carboxyl-terminal half of Lck and that this binding does not require the Lck SH3 domain. Mutated Tip containing only one functional Lck-binding domain can bind stably to Lck, although not as strongly as wild-type Tip. Interaction of Tip with Lck through either Lck-binding domain increases the activity of Lck in vivo. Simultaneous binding of both domains is required for maximal activation of Lck. The transient expression of Tip in T cells was found to stimulate both Stat3-dependent and NF-AT-dependent transcription. Mutant forms of Tip lacking one or the other of the two Lck-binding domains retained the ability to stimulate Stat3-dependent transcription. Tip lacking the proline-rich Lck-binding domain exhibited almost wild-type activity in this assay. In contrast, ablation of either Lck-binding domain abolished the ability of Tip to stimulate NF-AT-dependent transcription. Full biological activity of Tip, therefore, appears to require both Lck-binding domains.

The activation loop in Lck regulates oncogenic potential by inhibiting basal kinase activity and restricting substrate specificity

The activities of Src-family non-receptor tyrosine kinases are regulated by structural changes that alter the orientation of key residues within the catalytic domain. In this study, we investigate the effects of activation loop mutations on regulation of the lymphocyte-specific kinase Lck (p56lck). Substitution of 5 - 7 residues amino terminal to the conserved activation loop tyrosine (Y394) increases kinase activity and oncogenic potential regardless of regulatory C-terminal tail phosphorylation levels (Y505), while most mutations in the 13 residues carboxyl to Y394 decrease kinase activity. Phosphorylation of the C-terminal regulatory tail is carried out by the cytosolic tyrosine kinase Csk and we find that mutations upstream or downstream of Y394 or mutation of Y394 do not affect the level of Y505 phosphorylation. In addition, we report that mutations on either side of Y394 affect substrate specificity in vivo. We conclude that the high degree of conservation across the entire activation loop of Src-family kinases is critical for normal regulation of kinase activity and oncogenicity as well as substrate selection. Oncogene (2000) 19, 3961 - 3961.

Defective p56Lck activity in T cells from an adult patient with idiopathic CD4+ lymphocytopenia

Idiopathic CD4(+) lymphocytopenia (ICL) is defined by a stable loss of CD4(+) T cells in the absence of any known cause of immune deficiency. This syndrome is still of undetermined origin. It affects adult patients, some of them displaying opportunistic infections similar to HIV-infected subjects. The hypothesis that the cellular immune defect may be due to biochemical failures of the CD3-TCR pathway is investigated here in a patient associating a severe selective CD4(+) lymphocytopenia with an increased CD8(+) T cell count discovered in the course of a cryptococcal meningitidis. A 40% reduction of T cell proliferation to CD3-TCR stimulation is observed only in the CD4(+) subpopulation. The early CD3-induced protein tyrosine phosphorylations are conserved in both CD4(+) and CD8(+) subsets, and the levels of the T cell protein tyrosine kinases p56(Lck), p59(Fyn) and ZAP-70 are normal. However, we find a 50% reduction of p56(Lck) kinase activity in the patient's T cells compared to a healthy control donor. p59(Fyn) activity does not appear to be altered. Nevertheless, we do not find any genetic abnormality of p56(Lck). These results thus suggest that a defect of an unknown protein regulating p56(Lck) activity takes place in this patient's T cells. Taken together, these findings reveal p56(Lck) alteration in ICL and confirm the critical role of this kinase in the maintenance of the peripheral CD4(+) T cell subpopulation.

Lck domains differentially contribute to pre-T cell receptor (TCR)- and TCR-alpha/beta-regulated developmental transitions

Maturational changes at the CD4(-)CD8(-) double negative (DN) to CD4(+)CD8(+) double positive (DP) transition are dependent on signals generated via the pre-T cell receptor (TCR) and the nonreceptor protein tyrosine kinase p56(lck) (Lck). How Lck activities are stimulated or relayed after pre-TCR formation remains obscure. Our structure-function mapping of Lck thymopoietic properties reveals that the noncatalytic domains of Lck are specialized to signal efficient cellular expansion at DN to DP transition. Moreover, although substitution of the Lck catalytic domain with FynT sequences minimally impacts DP development, single positive thymocytes are most efficiently produced in the presence of kinases containing both the NH(2)-terminal and catalytic regions of Lck. These findings demonstrate that the Lck structure is uniquely adapted to mediate signals at both major transitions in thymopoiesis.

Activation of the lck tyrosine-protein kinase by the binding of the tip protein of herpesvirus saimiri in the absence of regulatory tyrosine phosphorylation

The Tip protein of herpesvirus saimiri 484 binds to the Lck tyrosine-protein kinase at two sites and activates it dramatically. Lck has been shown previously to be activated by either phosphorylation of Tyr394 or dephosphorylation of Tyr505. We examined here whether a change in the phosphorylation of either site was required for the activation of Lck by Tip. Remarkably, mutation of both regulatory sites of tyrosine phosphorylation did not prevent activation of Lck by Tip either in vivo or in a cell free in vitro system. Tip therefore appears to be able to activate Lck through an induced conformational change that does not necessarily involve altered phosphorylation of the kinase. Tip may represent the prototype of a novel type of regulator of tyrosine-protein kinases.

Interactions of CD45-associated protein with the antigen receptor signaling machinery in T-lymphocytes

CD45 is a transmembrane protein tyrosine phosphatase playing an essential role during T-cell activation. This function relates to the ability of CD45 to regulate p56(lck), a cytoplasmic protein tyrosine kinase necessary for T-cell antigen receptor (TCR) signaling. Previous studies have demonstrated that CD45 is constitutively associated in T-lymphocytes with a transmembrane molecule termed CD45-AP (or lymphocyte phosphatase-associated phosphoprotein). Even though the exact role of this polypeptide is unclear, recent analyses of mice lacking CD45-AP have indicated that its expression is also required for optimal T-cell activation. Herein, we wished to understand better the function of CD45-AP. The results of our studies showed that in T-cells, CD45-AP is part of a multimolecular complex that includes not only CD45, but also TCR, the CD4 and CD8 coreceptors, and p56(lck). The association of CD45-AP with TCR, CD4, and CD8 seemed to occur via the shared ability of these molecules to bind CD45. However, binding of CD45-AP to p56(lck) could take place in the absence of other lymphoid-specific components, suggesting that it can be direct. Structure-function analyses demonstrated that such an interaction was mediated by an acidic segment in the cytoplasmic region of CD45-AP and by the kinase domain of p56(lck). Interestingly, the ability of CD45-AP to interact with Lck in the absence of other lymphoid-specific molecules was proportional to the degree of catalytic activation of p56(lck). Together, these findings suggest that CD45-AP is an adaptor molecule involved in orchestrating interactions among components of the antigen receptor signaling machinery. Moreover, they raise the possibility that one of the functions of CD45-AP is to recognize activated Lck molecules and bring them into the vicinity of CD45.

Modular folding and evidence for phosphorylation-induced stabilization of an hsp90-dependent kinase

The de novo folding of the individual domains of the src family kinase p56(lck) was examined within the context of full-length p56(lck) molecules produced in rabbit reticulocyte lysate containing active chaperone machinery. The catalytic domain required geldanamycin-inhibitable heat shock protein 90 (hsp90) function to achieve its active protease-resistant conformation, but the src homology 2 (SH2) domain acquired phosphopeptide-binding competence independently of hsp90 function. The SH2 domain of hsp90-bound p56(lck) was folded and functional. In addition to the facilitation by hsp90 of kinase biogenesis, a conditional role in maintenance folding could be demonstrated; although wild type p56(lck) molecules with a negative-regulatory C-terminal tyrosine matured to a nearly hsp90-independent state, p56(lck) molecules with a mutated C-terminal tyrosine continued to require hsp90-mediated maintenance. De novo folding could be distinguished from maintenance folding on the basis of proteolytic fingerprints and the effects of different temperatures on folding behavior. Results indicate that during p56(lck) biogenesis, the SH2 domain rapidly folds independently of hsp90 function, followed by the slower hsp90-dependent folding of the catalytic domain and suggest the final stabilization of p56(lck) structure by phosphorylation-mediated interdomain interactions.

Inhibition of HIV-1 replication by a monoclonal antibody directed toward the complementarity determining region 3-like domain of CD4 in CD45 expressing and CD45-deficient cells

Monoclonal antibodies directed toward the complementarity determining region (CDR)3-like loop of the aminoterminal domain of CD4 have been shown to inhibit the replication of human immunodeficiency virus (HIV) in CD4 positive T cells. The mechanism of action of these antibodies is not yet elucidated, although several observations suggest that they inhibit viral transcription by signal transduction through the CD4 molecule, potentially implicating the activation of a protein tyrosine kinase (PTK) cascade. Since CD45 is the major protein tyrosine phosphatase associated to the plasma membrane in T cells, and has been shown to regulate the activity of several PTK, we postulated that CD45 may be necessary for the inhibitory action of the CDR3-like specific anti-CD4 antibodies. Therefore we tested the effect of one such anti-CD4 monoclonal antibody, 13B8.2, in repressing HIV replication in CD45 positive cell lines and CD45 deficient variants. Our data show that cells respond to 13B8.2 postinfection treatment regardless of CD45 expression, indicating that neither CD45 nor PTK regulated by CD45 are implicated in the mechanism of action of this antibody.

CD45 and Src-related protein tyrosine kinases regulate the T cell response to phorbol esters

Protein kinase C (PKC)-dependent activation of the Ras signal transduction cascade is essential for induction of the IL-2 promoter during stimulation of T lymphocytes via the T cell receptor (TCR). In this study, the effects of PKC-activating phorbol myristate acetate (PMA) on Ras-dependent activation of transcription from the ets/AP-1 Ras-responsive promoter element were examined in human T cells. Pretreatment of Jurkat cells with the Src-family PTK inhibitor herbimycin A resulted in a 50% inhibition of transactivation of the reporter following incubation with PMA. Evidence was also obtained to suggest the participation of the leukocyte-specific protein tyrosine phosphatase CD45, a regulator of Src-like PTKs, in the PMA-induced activation of the Ras/Raf pathway. First, PMA-induced transactivation of ets/AP-1 is diminished 75% in CD45-negative variants, compared with CD45-positive cells. Second, engagement of CD45 by monoclonal antibodies suppresses the PMA response from the reporter construct. Taken together, these data suggest that Src-related proteins mediate PKC-dependent activation of the Ras/Raf pathway and implicate CD45 in the TCR-independent activation of T lymphocytes induced by agents such as PMA.

Modification of phosphatidylinositol 3-kinase SH2 domain binding properties by Abl- or Lck-mediated tyrosine phosphorylation at Tyr-688

In cells expressing the oncogenic Bcr-Abl tyrosine kinase, the regulatory p85 subunit of phosphatidylinositol 3-kinase is phosphorylated on tyrosine residues. We report that this phosphorylation event is readily catalyzed by the Abl and Lck protein-tyrosine kinases in vitro, by Bcr-Abl or a catalytically activated Lck-Y505F in co-transfected COS cells, and by endogenous kinases in transfected Jurkat T cells upon triggering of their T cell antigen receptor. Using these systems, we have mapped a major phosphorylation site to Tyr-688 in the C-terminal SH2 domain of p85. Tyrosine phosphorylation of p85 in vitro or in vivo was not associated with detectable change in the enzymatic activity of the phosphatidylinositol 3-kinase heterodimer, but correlated with a strong reduction in the binding of some, but not all, phosphoproteins to the SH2 domains of p85. This provides an additional candidate to the list of SH2 domains regulated by tyrosine phosphorylation and may explain why association of phosphatidylinositol 3-kinase with some cellular ligands is transient or of lower stoichiometry than anticipated.

Mutation of C-terminal tyrosine residues Y497/Y504 of the Src-family member Bsk/Iyk decreases NIH3T3 cell proliferation

To elucidate the properties of the Src-family member Bsk/Iyk, NIH3T3 cells were transfected with wild-type Bsk/Iyk or Bsk/Iyk carrying Y497F, Y504F or Y497/504F mutations. These positions are putatively homologous to tyr-527 in Src. The Bsk/IykY497/504F cells displayed a decreased cell growth rate, parallelled by an augmentation of the fraction of cells in G1-phase. The Bsk/IykY497/504F double-mutation decreased the [3H]thymidine incorporation. No effects on NIH3T3 cell growth could be seen in cells expressing wild-type Bsk/Iyk or the other Bsk/Iyk mutants. In vitro kinase reactions performed on immunoprecipitates from NIH3T3 cells expressing wild-type or mutated Bsk/Iyk revealed increased relative [32P]-incorporation into Bsk/Iyk isoforms containing the Y504F and Y497/504F mutations compared with wild-type Bsk/Iyk. The Y497F and Y497/504F mutations elevated the proportion of [32P]-incorporation into a 57 kDa Bsk/Iyk product relative to that into the 60 kDa isoform. The Y497F Bsk/Iyk mutant not only increased the relative amount of p57 Bsk/Iyk but also transferred this isoform to the nuclear subcellular fraction. The results suggest that Bsk/Iyk has unique regulatory properties, and that this kinase might serve a role in inhibiting cell replication.

Activation of the Lck tyrosine kinase targets cell surface T cell antigen receptors for lysosomal degradation

The mechanism by which TCR expression is regulated was explored by expressing a constitutively active form of the tyrosine kinase Lck (Lck505F) in T cells. Expression of Lck505F down-regulated TCR levels, an effect that was even more pronounced in CD45- T cells, in which the activity of this tyrosine kinase is further enhanced. Cells expressing Lck505F synthesized all TCR subunits, but lysosomal degradation of assembled receptors was enhanced. TCRs were rapidly internalized and degraded after removal of a tyrosine kinase inhibitor that had permitted cell surface expression. Finally, TCR levels on thymocytes were increased by an Lck inhibitor, and activation- but not phorbol ester-induced internalization of TCRs on Jurkat cells was prevented by inhibition or loss of Lck. These studies identify a regulated nonreceptor tyrosine kinase-mediated pathway for targeting cell surface receptors for lysosomal degradation.

The activated form of the Lck tyrosine protein kinase in cells exposed to hydrogen peroxide is phosphorylated at both Tyr-394 and Tyr-505

Members of the Src family of non-receptor tyrosine protein kinases are known to be inhibited by the intramolecular association between a phosphorylated carboxyl-terminal tyrosine residue and the SH2 domain. We have previously shown that exposure of cells to H2O2 strongly activates Lck, a lymphocyte-specific Src family kinase, by inducing phosphorylation on Tyr-394, an absolutely conserved residue within the activation loop of the catalytic domain. Here we show that Lck that has been activated by H2O2 is simultaneously phosphorylated at both the carboxyl-terminal tyrosine (Tyr-505) and Tyr-394. Thus, dephosphorylation of Tyr-505 is not a prerequisite for either phosphorylation of Lck at Tyr-394 or catalytic activation of the kinase. These results indicate that activation of Lck by phosphorylation of Tyr-394 is dominant over any inhibition induced by phosphorylation of Tyr-505. We propose that these results may be extended to all Src family members.

Posttranslational regulation of Lck and a p36-38 protein by activators of protein kinase C: differential effects of the tumor promoter, PMA, and the non-tumor-promoter, bryostatin

T cell activation via the antigen receptor or by PKC-activating drugs results in phosphorylation of Lck and alteration of its electrophoretic mobility. Although tyrosine phosphorylation appears to regulate Lck enzymatic activity, the significance of phosphorylation of serine residues and its relevance to the cell proliferation process are yet unclear. We found that the PKC activator, bryostatin, like PMA, induced the conversion of p56lck to a slower migrating form with an apparent molecular mass of 60 kDa. The effect of PMA lasted over 48 hr but that of bryostatin was transient and correlated in time kinetics with that of the bryostatin-induced degradation of PKC. The effects of bryostatin were dominant over those of PMA. In addition, PKC was found to affect both serine and tyrosine phosphorylation of Lck but had no significant effect on the in vitro catalytic activity of Lck. To test whether serine phosphorylation of Lck may affect its ability to bind tyrosine phosphoproteins, we compared Lck immunoprecipitates from PMA- and bryostatin-treated T cells. We found that a 36- to 38-kDa tyrosine phosphoprotein co-immunoprecipitated with Lck from cells that were treated for 24 hr with PMA, but not bryostatin. A p36-38 from PMA- but not bryostatin-treated cells also interacted with an Lck-SH2 fusion protein, suggesting differential regulation of p36-38 by PMA and bryostatin. Furthermore, in vitro phosphorylation of p36-38 occurred in lysates of cells that were treated for 24 hr with PMA, but not in lysates of bryostatin-treated cells. The results show that tyrosine phosphorylation and the association of p36-38 with Lck are differentially affected by bryostatin and PMA and suggest that PKC regulates the interaction of potential signaling molecules with Lck, thereby regulating biochemical events that are relevant to T cell mitogenesis and/or transformation.

Restoration of thymopoiesis in pT alpha-/- mice by anti-CD3epsilon antibody treatment or with transgenes encoding activated Lck or tailless pT alpha

Mice deficient for the pre-TCR alpha (pT alpha) chain cannot form a pre-T cell receptor (TCR) and exhibit a severe defect in early T cell development, characterized by lack of "beta selection" and impaired generation of double-positive (DP) thymocytes. Here, we demonstrate that intraperitoneal injection of CD3epsilon-specific antibodies into pT alpha-/- x RAG-/- mice or introduction of an activated p56(lck) transgene in pT alpha-/- mice fully restores the number of DP thymocytes, and that expression of a transgenic pT alpha chain lacking its cytoplasmic portion can overcome all developmental defects associated with pT alpha deficiency. These results allow a better definition of the role of pT alpha in pre-TCR signal transduction and provide conclusive evidence that the cytoplasmic tail of pT alpha is not essential for pre-TCR signaling.

Reconstitution of interactions between protein-tyrosine phosphatase CD45 and tyrosine-protein kinase p56(lck) in nonlymphoid cells

To further understand the functional interactions between CD45 and p56(lck) in T-cells, we stably reconstituted their expression in a nonlymphoid system. The results of our analyses demonstrated that CD45 could dephosphorylate tyrosine 505 of p56(lck) in NIH 3T3 fibroblasts. As is the case for T-cells, removal of the unique domain of p56(lck) interfered with dephosphorylation of tyrosine 505 in fibroblasts, further stressing the importance of this region in the interactions between CD45 and p56(lck). The ability of CD45 to dephosphorylate tyrosine 505 in NIH 3T3 cells was also greatly influenced by the catalytic activity of p56(lck). Indeed, whereas CD45 provoked dephosphorylation of kinase-defective Lck molecules in this system, it failed to stably dephosphorylate kinase-active p56(lck) polypeptides. Finally, our studies showed that CD45 was also able to inhibit the oncogenic potential of a constitutively activated version of p56(lck) in NIH 3T3 cells. This effect did not require the Lck unique domain and apparently resulted from selective dephosphorylation of substrates of activated p56(lck) in fibroblasts. In addition to providing insights into the nature and regulation of the interactions between CD45 and p56(lck) in T-cells, these results indicated that CD45 clearly has the capacity to both positively and negatively regulate p56(lck)-mediated functions in vivo.

Regulation of the Lck SH2 domain by tyrosine phosphorylation

Src homology 2 (SH2) domains bind to phosphotyrosine (Tyr(P)) residues in specific sequence contexts in other proteins and thereby mediate tyrosine phosphorylationdependent protein-protein interactions. The SH2 domain of the Src family kinase Lck is phosphorylated at tyrosine 192 in T cells upon T cell antigen receptor triggering. We have studied the consequences of this phosphorylation on the properties of the SH2 domain and on the function of Lck in T cell activation. We report that phosphorylation at Tyr192 reduced the capacity of the isolated SH2 domain to bind a high affinity peptide ligand and Tyr(P)-containing cellular proteins. This effect was mimicked by mutation of Tyr192 to an acidic residue. In intact T cells, where Lck participates in T cell antigen receptor signal transduction in an SH2 domain-dependent manner, phosphorylation of Tyr192 correlated with reduced downstream signaling. Our results indicate that tyrosine phosphorylation of the SH2 domain of Lck terminates its high affinity binding to ligands, thereby negatively regulating its participation in T cell antigen receptor signaling. This represents a novel mechanism for the regulation of the function of SH2 domains.

T cell receptor and CD8-dependent tyrosine phosphorylation events in cytotoxic T lymphocytes: activation of p56lck by CD8 binding to class I protein

Tyrosine phosphorylation of proteins plays a central role in T cell activation. Mitogens or anti-receptor antibodies have been employed to study these signaling events, but the extent to which these mimic receptor interactions with native ligands is unclear. Cytotoxic T lymphocytes can be activated for functional responses using purified, native class I ligands presented on a surface. Previous work showed that stimulation with fluid-phase anti-T cell receptor (TCR) monoclonal antibody (mAb) activates CD8 to mediate adhesion to class I proteins and that activated CD8 generates a co-stimulatory signal upon binding to class I. Changes in tyrosine phosphorylation of substrates and activity of the p56lck kinase have now been examined in this two-step process. The observed changes are small in comparison to those found using more potent nonphysiological stimuli, but may more accurately reflect the events required for activation of functional responses. Fluid-phase anti-TCR mAb caused increased tyrosine phosphorylation of a discrete subset of cellular substrates. Increased phosphorylation of additional substrates occurred upon CD8 binding to class I, resulting in a phosphorylation pattern comparable to that found in cells stimulated with class I alloantigen. Anti-TCR mAb alone caused increased tyrosine phosphorylation of p56lck. When CD8 bound to class I, phosphorylation of p56lck decreased to below the basal level found in unstimulated cells, accompanied by a substantial increase in kinase activity. These results are consistent with the two-step model for TCR activation of CD8/class I interactions and directly demonstrate that CD8 binding to class I leads to up-regulation of p56lck activity.

Augmented expression of LCK message directed from the downstream promoter in human colorectal cancer specimens

Lck protein is expressed in some colon carcinoma cell lines but its expression in colon cancer cells in vivo has not been clarified. LCK transcription is regulated from two distinct promoters and initiated exclusively from the downstream promoter in colon carcinoma cell lines in contrast to peripheral lymphocytes. We investigated the expression of the downstream promoter-initiated LCK transcript in 18 colorectal primary cancer and normal mucosae, and two hepatic metastases, using a RNase protection assay with the EcoRI-BglII fragment of human LCK cDNA, YT16. In normal tissues, only traces of the LCK transcript were detected. The expression of the LCK transcript was augmented in 3/18 cancer specimens. The relative level of the LCK transcript in the cancer tissue compared to the average value of normal adjacent tissue was 10-60 in 3 cases, and 3-10 in 7 cases. One hepatic metastasis expressed more LCK message than the primary lesion. Our results indicate that the LCK message is strongly expressed in some colorectal cancers.

Regulation, substrates and functions of src

Src is the best understood member of a family of 9 tyrosine kinases that regulates cellular responses to extracellular stimuli. Activated mutants of Src are oncogenic. Using Src as an example, and referring to other Src family members where appropriate, this review describes the structure of Src, the functions of the individual domains, the regulation of Src kinase activity in the cell, the selection of substrates, and the biological functions of Src. The review concentrates on developments in the last 6-7 years, and cites data resulting from the isolation and characterization of Src mutants, crystallographic studies of the structures of SH2, SH3 and tyrosine kinase domains, biochemical studies of Src kinase activity and binding properties, and the biology of transgenic and knockout mouse strains.

Stimulation of phosphorylation of Tyr394 by hydrogen peroxide reactivates biologically inactive, non-membrane-bound forms of Lck

Lck, a lymphocyte-specific tyrosine protein kinase, is bound to cellular membranes as the result of myristoylation and palmitoylation of its amino terminus. Its activity is inhibited by phosphorylation of tyrosine 394. The Tyr-505 --> Phe mutant of Lck (F505Lck) exhibits elevated biological activity and constitutive phosphorylation of Tyr-394 in vivo. Mutations at sites of fatty acylation that prevent F505Lck from associating with cellular membranes abolish the biological activity as a protein kinase in vivo and in vitro, and eliminate the phosphorylation of Tyr-394. Here, we show that exposure of cells expressing cytoplasmic or nuclear forms of F505Lck to H2O2, a general inhibitor of tyrosine protein phosphatases, restores the catalytic activity of these mutant proteins through stimulation of phosphorylation of Tyr-394. H2O2 treatment induced the phosphorylation of Tyr-394 therefore need not occur by autophosphorylation. Thus, there appear to be two mechanisms through which the phosphorylation of Lck at Tyr-394 can occur. One is restricted to the plasma membrane and does not require the presence of oxidants. The other is operational in the nucleus as well as the cytosol and is responsive to oxidants.

Selective activation of T cell kinase p56lck by Herpesvirus saimiri protein tip

Infection with Herpesvirus saimiri, a T lymphotropic virus of non-human primates, immortalizes human T cells in vitro. The cells show a mature activated phenotype and retain their antigen specificity. We have previously shown that in H. saimiri transformed cells a viral gene product termed tyrosine kinase interacting protein (Tip) associates with the T cell-specific tyrosine kinase p56lck and becomes phosphorylated by the enzyme on tyrosine residues. Here we show that p56lck is activated by recombinant and native Tip in cell-free systems. A dramatic increase of Lck activity was also observed in T cell lines transfected with Tip. p60fyn and p53/56lyn, the other Src-related kinases expressed in H. saimiri transformed T cells, did not phosphorylate Tip, and they were not activated by the protein. The selective activation of p56lck by Tip could contribute to the transformed phenotype of H. saimiri infected cells, and it might explain the T cell selectivity of the transformation event.

Characterization of two activated mutants of human pp60c-src that escape c-Src kinase regulation by distinct mechanisms

Two activated transforming mutants of human pp60c-src were found to possess single point mutations within the regulatory carboxyl terminus (E527K in CY CST201) and the kinase domain (E381G in WO CST1), respectively, that do not directly interfere with either the regulatory c-Src kinase (CSK) phosphorylation site (Tyr530) or the SH2/3 domains. In vivo, both mutant proteins are hypophosphorylated on their carboxyl-terminal regulatory tyrosines and are hyperactive. In an in vitro Src kinase inactivation assay, both mutant Src proteins exhibited resistance to inactivation by CSK relative to wild-type Src. Under these in vitro conditions, E381G c-Src was found to be phosphorylated by CSK to wild-type levels, while E527K c-Src was not detectably phosphorylated. The ability of CSK to phosphorylate a carboxyl-terminal peptide modelled against E527K c-Src was also impaired, suggesting that CSK is unable to recognize E527K c-Src as an efficient substrate. In the case of E381G c-Src, examination of whether its SH2/3 domains were accessible to the carboxyl-terminal regulatory phosphotyrosine revealed a highly reduced ability of autophosphorylated E381G c-Src to bind to a synthetic phosphopeptide modelled from the SH2-binding region of polyoma middle-T antigen which binds to Src SH2 with high affinity. This suggests that the E381G c-Src mutation results in an altered or reduced accessibility of the SH2 domain of the autophosphorylated form of E381G c-Src and may represent a previously undescribed mode of Src activation. Further study of these and other Src mutants may offer additional new insights into the regulation of "Src family" kinases.

The kinase-dependent function of Lck in T-cell activation requires an intact site for tyrosine autophosphorylation

The cytoplasmic protein tyrosine kinase p56lck (Lck) has important signaling roles in T-cell development and activation. We have mutated the two known regulatory tyrosine residues of CD4-associated Lck and examined the effects on its kinase-dependent function in an antigen-specific CD4-dependent T-cell hybridoma. Substitution of phenylalanine for the negative regulatory tyrosine-505 within a CD4/Lck chimera resulted in a slightly increased response to antigen, whereas mutation of the major in vitro autophosphorylation site (tyrosine-394) completely abolished the kinase-dependent function of Lck. Even though its kinase activity was only slightly affected, the F394 mutant behaved similarly to a catalytically inactive chimeric protein. Cross-linking of the F505 mutant, but not of wild-type Lck or F394 mutants, resulted in tyrosine phosphorylation of multiple cellular proteins. Although the pattern of tyrosine phosphorylation resembled that observed upon T-cell receptor cross-linking, there was no induction of interleukin-2 synthesis upon cross-linking of the chimeric protein. These results suggest that the activity of the Lck kinase domain in vivo is controlled by dephosphorylation at the negative regulatory site and phosphorylation at the positive regulatory (autophosphorylation) site. Additionally, our data show that the specific kinase activity of Lck towards an artificial substrate need not correlate with its ability to phosphorylate cellular proteins or its biological function.

Molecular interactions in the activation of effector and precursor cytotoxic T lymphocytes

Cell-cell interactions are influenced by parameters that cannot readily be studied using either intact cells or soluble molecules. Replacing one of the pair of interacting cells with an artificial cell surface construct allows novel insights to be gained into some of these parameters. Application of this approach to the study of CTL has helped to clarify the contrasting roles of some of the various receptors that are involved in recognition, adhesion and activation. In addition, it has revealed features of these receptor ligand interactions that help to explain how CTL are able to carry out effective immune surveillance and elimination of virus-infected or tumor cells. Although not discussed in this review, artificial cell surface constructs have also been effectively employed to study the interaction of TH cells with class II bearing surfaces. Class I protein and peptide antigen can be sufficient to mediate adhesion and activate CTL effector function through the TCR and CD8. In addition, interactions of other co-receptors with their ligands can act along with TCR and CD8 in a cascade of activated adhesion and co-stimulatory signal generation to allow adhesion and response when antigen and/or class I surface densities are too low to be sufficient by themselves to initiate response. The relative contributions of the various receptor/ligand interactions to a given CTL/target encounter will depend upon the affinity of the TCR for antigen and on the densities and types of ligands, including antigen, displayed on the target cell surface. It appears that the CTL has the ability to accomplish its task in a variety of ways, providing it with considerable flexibility in recognizing and eliminating antigen-bearing target cells. Thus, downregulation of any one particular ligand on a virus-infected or tumor cell does not allow escape from CTL surveillance provided that at least a low level of class I antigen remains present. The CTL is able to employ several co-receptors specific for ligands common to many cell types without being diverted from effective immune surveillance, since these receptors only become activated to mediate high-avidity adhesion when antigen is detected by the TCR. Cloned effector CTL are most amenable to studies of the kind reviewed here, since large numbers of homogenous cells can be obtained, antigen-specific adhesion can be readily measured and response is rapid and easily quantitated.(ABSTRACT TRUNCATED AT 400 WORDS)

Three distinct IL-2 signaling pathways mediated by bcl-2, c-myc, and lck cooperate in hematopoietic cell proliferation

Two interleukin-2 receptor-dependent signaling pathways have thus far been identified: the c-fos/c-jun induction pathway mediated by src family protein-tyrosine kinases and the c-myc induction pathway. Here, we provide evidence for the existence of a third, rapamycin-sensitive pathway, which results in the induction of another proto-oncogene, bcl-2. In the hematopoietic cell line BAF-B03, the expression of any two of lckF505 (an active form of p56lck), Bcl-2, or c-Myc is sufficient to promote transit of the cell cycle, regardless of the activation state of the third pathway. We also provide evidence that epidermal growth factor receptor signaling may act through the same pathway that involves p56lck. These studies demonstrate a novel approach to dissecting signaling pathways regulating cellular proliferation.

p56lck plays a key role in transducing apoptotic signals in T cells

The CD4 receptor synergizes with the T-cell antigen receptor (TCR) in helper T-cell activation. However CD4 cross-linking in the absence of simultaneous TCR engagement leaves the cells primed to activation dependent apoptosis. To assess the role of the CD4 associated protein tyrosine kinase p56lck in CD4 priming to apoptosis we have constructed Jurkat T-cell lines stably transfected with a constitutively active form of p56lck. These cells were constitutively primed to undergo apoptosis upon TCR crosslinking with specific antibodies. In addition the Jurkat JCaM1 line, which is defective for p56lck expression, was resistant to TCR induced apoptosis. These data indicate that p56lck is required for T-cell apoptosis and that CD4 priming of T-cells for antigen dependent apoptosis is due to inappropriate or partial activation of the p56lck signal transduction pathway.

Interactions between the tyrosine kinases p56lck, p59fyn and p50csk in CD4 signaling in T cells

Interaction of the CD4 co-receptor with major histocompatibility complex (MHC) class II molecules during antigen presentation results in enhancement of antigen receptor signaling. The synergism between the two receptors is believed to result from the juxtaposition of the CD4-associated tyrosine kinase p56lck with the cytoplasmic domains of CD3 complex components. Here, we report that cross-linking of CD4 on the surface of Jurkat cells using monoclonal antibodies results in activation of the CD3-associated kinase p59fyn. Co-cross-linking of CD4 and CD3 results in synergistic activation of p59fyn. The p59fyn kinase is also hyperactive in a Jurkat cell line stably transfected with a constitutively active p56lck mutant, indicating that p56lck mediates CD4 activation of p59fyn. In support of this hypothesis, expression of a dominant inhibitory mutant of p59fyn blocks CD4 signals involved in gene activation. In addition, the p59fyn dominant inhibitor mutant blocks gene-activating signals induced by expression of a constitutively active mutant of p56lck. Overexpression of the regulatory kinase p50csk, which attenuates TcR signaling by inactivation of p59fyn, inhibits signaling from the constitutively active form of p56lck. Taken together, these data suggest that CD4/p56lck enhancement of TcR signaling is, at least in part, mediated by activation of p59fyn, and may be regulated by p50csk.