An activated epidermal growth factor receptor/Lck chimera restores early T cell receptor-mediated calcium response in a CD45-deficient T cell line

Ca2+ Signaling and Src Functions in Tumor Cells

Signaling by calcium ion (Ca2+) plays a prominent role in cell physiology, and these mechanisms are frequently altered in tumor cells. In this review, we consider the interplay of Ca2+ signaling and the functions of the proto-oncogene non-receptor tyrosine kinase c-Src in tumor cells, and the viral oncogenic variant v-Src in transformed cells. Also, other members of the Src-family kinases are considered in this context. The role of Ca2+ in the cell is frequently mediated by Ca2+-binding proteins, where the Ca2+-sensor protein calmodulin (CaM) plays a prominent, essential role in many cellular signaling pathways. Thus, we cover the available information on the role and direct interaction of CaM with c-Src and v-Src in cancerous cells, the phosphorylation of CaM by v-Src/c-Src, and the actions of different CaM-regulated Ser/Thr-protein kinases and the CaM-dependent phosphatase calcineurin on v-Src/c-Src. Finally, we mention some clinical implications of these systems to identify mechanisms that could be targeted for the therapeutic treatment of human cancers.

Src-family tyrosine kinases and the Ca2+ signal

In this review, we shall describe the rich crosstalk between non-receptor Src-family kinases (SFKs) and the Ca²⁺ transient generated in activated cells by a variety of extracellular and intracellular stimuli, resulting in diverse signaling events. The exchange of information between SFKs and Ca²⁺ is reciprocal, as it flows in both directions. These kinases are main actors in pathways leading to the generation of the Ca²⁺ signal, and reciprocally, the Ca²⁺ signal modulates SFKs activity and functions. We will cover how SFKs participate in the generation of the cytosolic Ca²⁺ rise upon activation of a series of receptors and the mechanism of clearance of this Ca²⁺ signal. The role of SFKs modulating Ca²⁺-translocating channels participating in these events will be amply discussed. Finally, the role of the Ca²⁺ sensor protein calmodulin on the activity of c-Src, and potentially on other SFKs, will be outlined as well. This article is part of a Special Issue entitled: ECS Meeting edited by Claus Heizmann, Joachim Krebs and Jacques Haiech.

Selective regulation of TCR signaling pathways by the CD45 protein tyrosine phosphatase during thymocyte development

In CD45-deficient animals, there is a severe defect in thymocyte-positive selection, resulting in an absence of mature T cells and the accumulation of thymocytes at the DP stage of development. However, the signaling defect(s) responsible for the block in development of mature single-positive T cells is not well characterized. Previous studies have found that early signal transduction events in CD45-deficient cell lines and thymocytes are markedly diminished following stimulation with anti-CD3. Nevertheless, there are also situations in which T cell activation and TCR signaling events can be induced in the absence of CD45. For example, CD45-independent TCR signaling can be recovered upon simultaneous Ab cross-linking of CD3 and CD4 compared with cross-linking of CD3 alone. These data suggest that CD45 may differentially regulate TCR signaling events depending on the nature of the signal and/or on the differentiation state of the cell. In the current study, we have assessed the role of CD45 in regulating primary thymocyte activation following physiologic stimulation with peptide. Unlike CD3-mediated stimulation, peptide stimulation of CD45-deficient thymocytes induces diminished, but readily detectable TCR-mediated signaling events, such as phosphorylation of TCR-associated zeta, ZAP70, linker for activation of T cells, and Akt, and increased intracellular calcium concentration. In contrast, phosphorylation of ERK, which is essential for positive selection, is more severely affected in the absence of CD45. These data suggest that CD45 has a selective role in regulating different aspects of T cell activation.

Involvement of CD45 in DNA fragmentation in apoptosis induced by mitochondrial perturbing agents

CD45 is a type I transmembrane molecule with phosphatase activity which comprises up to 10% of the cell surface area in nucleated haematopoietic cells. We have previously demonstrated the absence of nuclear apoptosis in CD45-negative T cells after chemical-induced apoptosis. The aim of this study was to characterize the role of CD45 in nuclear apoptosis. In contrast to wild type CD45-positive T cells, the CD45-deficient T cell lines are resistant to the induction of DNA fragmentation and chromatin condensation following tributyltin (TBT) or H2O2 exposure, but not to cycloheximide-induced apoptosis. CD45 transfection in deficient cell lines led to the restoration of chromatin condensation and DNA fragmentation following TBT exposure. In both CD45-positive and negative T cell lines, TBT exposure mediates intracellular calcium mobilization, caspase-3 activation and DFF45 cleavage. Moreover, DNA fragmentation was also induced by TBT in cells deficient in expression of p56lck, ZAP-70 and SHP-1. Subcellular partitioning showed a decrease in nuclear localisation of caspase-3 and DFF40. Together, these results demonstrate for the first time, that CD45 expression plays a key role in internucleosomal DNA fragmentation and chromatin condensation processes during apoptosis. CD45 activity or its substrates' activity, appears to be located downstream of caspase-3 activation and plays a role in retention of DFF40 in the nucleus.

Subdomain Switching Reveals Regions That Harbor Substrate Specificity and Regulatory Properties of Protein Tyrosine Kinases

Csk and Src are two protein tyrosine kinases that share a similar overall multidomain structural organization and a high degree of sequence homology but have different substrate specificities and regulatory properties. In this study, we generated chimeric kinases of Csk and Src by switching the C-terminal lobes of their catalytic domains, and we characterized their substrate specificity and regulatory properties. First, both Csk and Src phosphorylate Src as a common substrate, but on different Tyr residues. The C-terminal lobes of the kinase catalytic domain determined the site of phosphorylation on Src. Furthermore, toward several physiological substrates of Src, the substrate specificity was also determined by the C-terminal lobe of the catalytic domain regardless of the regulatory domains and the N-terminal lobe of the catalytic domain. Second, Csk and Src represent two general regulatory strategies for protein tyrosine kinases. Csk catalytic domain is inactive and is positively regulated by the regulatory domains, while Src catalytic domain is active and suppressed by its interactions with the regulatory domains. The regulatory properties of the chimeric kinases were more complicated. The regulatory domains and the N-lobe did not fully determine the response to a regulatory ligand, suggesting that the C-lobe also contributes to such responses. On the other hand, the intrinsic kinase activity of the catalytic domain correlates with the identity of the N-lobe. These results demonstrate that the chimeric strategy is useful for detailed dissection of the mechanistic basis of substrate specificity and regulation of protein tyrosine kinases.

Changes in the Role of the CD45 Protein Tyrosine Phosphatase in Regulating Lck Tyrosine Phosphorylation during Thymic Development

CD45-dependent dephosphorylation of the negative regulatory C-terminal tyrosine of the Src family kinase Lck, promotes efficient TCR signal transduction. However, despite the role of CD45 in positively regulating Lck activity, the distinct phenotypes of CD45 and Lck/Fyn-deficient mice suggest that the role of CD45 in promoting Lck activity may be differentially regulated during thymocyte development. In this study, we have found that the C-terminal tyrosine of Lck (Y505) is markedly hyperphosphorylated in total thymocytes from CD45-deficient mice compared with control animals. In contrast, regulation of the Lck Y505 phosphorylation in purified, double-negative thymocytes is relatively unaffected in CD45-deficient cells. These changes in the role of CD45 in regulating Lck phosphorylation during thymocyte development correlate with changes in coreceptor expression and the presence of coreceptor-associated Lck. Biochemical analysis of coreceptor-associated and nonassociated Lck in thymocytes, and in cell lines varying in CD4 and CD45 expression, indicate that CD45-dependent regulation of Lck Y505 phosphorylation is most evident within the fraction of Lck that is coreceptor associated. In contrast, Lck Y505 phosphorylation that is not coreceptor associated is less affected by the absence of CD45. These data define distinct pools of Lck that are differentially regulated by CD45 during T cell development.

Expression of CD45 Lacking the Catalytic Protein Tyrosine Phosphatase Domain Modulates Lck Phosphorylation and T Cell Activation

The function of the second protein tyrosine phosphatase domain (D2) in two-domain protein tyrosine phosphatases (PTP) is not well understood. In CD45, D2 can interact with the catalytic domain (D1) and stabilize its activity. Although D2 itself has no detectable catalytic activity, it can bind substrate and may influence the substrate specificity of CD45. To further explore the function of D2 in T cells, a full-length construct of CD45 lacking the D1 catalytic domain (CD45RABC-D2) was expressed in CD45+ and CD45- Jurkat T cells. In CD45- Jurkat T cells, CD45RABC-D2 associated with Lck but, unlike its active counterpart CD45RABC, did not restore the induction of tyrosine phosphorylation or CD69 expression upon T cell receptor (TCR) stimulation. Expression of CD45RABC-D2 in CD45+ Jurkat T cells resulted in its association with Lck, increased the phosphorylation state of Lck, and reduced T cell activation. TCR-induced tyrosine phosphorylation was delayed, and although MAPK phosphorylation and CD69 expression were not significantly affected, the calcium signal and IL2 production were severely reduced. This indicates that the non-catalytic domains of CD45 can interact with Lck in T cells. CD45RABC-D2 acts as a dominant negative resulting in an increase in Lck phosphorylation and a preferential loss of the calcium signaling pathway, but not the MAPK pathway, upon TCR signaling. This finding suggests that, in addition to their established roles in the initiation of TCR signaling, CD45 and Lck may also influence the type of TCR signal generated.

HIV-1 infection is facilitated in T cells by decreasing p56lck protein tyrosine kinase activity

Several studies have suggested an important role for the protein tyrosine kinase p56lck (Lck) in HIV infection; however, the exact nature of this role remains unclear. Using a series of well characterized Jurkat-derived cell lines having a wide range of Lck kinase activity, our results showed that, while the entry of HIV-1 into these cell lines was similar, the kinetics of virus production by these cells were very different. Cells expressing a kinase-inactive Lck showed accelerated viral replication, whereas, cells expressing Lck with normal or elevated enzymatic activity showed a delay in virus replication that was proportional to the initial level of endogenous Lck activity. The cell line having the highest initial Lck kinase activity showed the slowest rate of productive HIV-1 infection. Analysis of 2-LTR circles revealed that this inhibitory effect of Lck was not due to inhibition of reverse transcription of HIV-1 genome or migration of the proviral DNA into the nuclei. This affect of Lck was confirmed in additional studies that used either the S1T cell line lacking completely Lck or where the Lck activity was altered in Jurkat cells prior to infection. S1T cells showed a 3- to 12-fold increase in the level of infection compared to Jurkat cells despite similar CD4 and chemokine coreceptor expression and cell doubling times. Pretreatment of Jurkat with an antisense lck oligodeoxynucleotide inhibited the synthesis of functional Lck and facilitated the viral replication by the cells as did expressing a dominant-negative mutant Lck which increased the productive infection>3-fold. Conversely, whereas IL-16 had no affect on productive infection in S1T cells that lack Lck, IL-16 pretreatment of Jurkat cells resulted in an immediate (within 5 min) and sustained and gradual (over 5 h) increase in Lck activity that resulted in a reduction of HIV-1 replication that paralleled the increasing Lck kinase activity. These results show that the enzymatic activity of Lck kinase can affect viral replication, that a lack of, or decreased Lck activity facilitates viral replication. Conversely, Lck can mediate a delay in HIV-1 infection that is proportional to the initial endogenous Lck enzyme activity.

Receptor/ligand avidity determines the capacity of Ly49 inhibitory receptors to interfere with T-cell receptor-mediated activation

The specificity and the relative affinity of many Ly49 receptors for major histocompatibility complex class I ligands have been studied in detail in various adhesion and binding assays. However, how the level of cell surface expression of a given Ly49 receptor and its ligand affinity influence the strength of the inhibition signal is not well documented. To address this issue, we developed a series of human Jurkat T-cell transfectants expressing the whole range of Ly49A and Ly49C levels found in vivo on natural killer and T cells and evaluated their capacity to alter superantigen-induced NF-AT activation and interleukin-2 production. We show that the strength of the inhibition induced by Ly49A/H-2Dd interaction correlates with Ly49A density up to a certain level after which increasing expression does not further inhibit significantly the T-cell receptor-induced activation. This system also represents a valuable tool for the determination of the relative strength of the inhibitory signals of Ly49 receptors following their interactions with different ligands. Even at high levels of expression there was no evidence that engagement of Ly49A with H-2b class I molecules provided an inhibitory signal. Moreover, we showed that functional inhibitory interactions of Ly49C with H-2b class I molecules were only the result of H-2Kb and that H-2d represent lower affinity ligands for Ly49C than H-2b. Therefore, depending on the relative affinity of Ly49 receptors for their ligands, the modulation of their expression level will be determinant for the functional outcome of activated T cells.

Evidence that Llck-mediated phosphorylation of p56dok and p62dok may play a role in CD2 signaling

The Lck tyrosine kinase is involved in signaling by T cell surface receptors such as TCR/CD3, CD2, and CD28. As other downstream protein-tyrosine kinases are activated upon stimulation of these receptors, it is difficult to assign which tyrosine-phosphorylated proteins represent bona fide Lck substrates and which are phosphorylated by other tyrosine kinases. We have developed a system in which Lck can be activated independently of TCR/CD3. We have shown that activation of an epidermal growth factor receptor/Lck chimera leads to the specific phosphorylation of Ras GTPase-activating protein (RasGAP) and two RasGAP-associated proteins, p56(dok) and p62(dok). Activation of the chimeric protein correlates with an increase in cellular Ca(2+) in the absence of ZAP-70 and phospholipase Cgamma1 phosphorylation. Furthermore, we have found that p62(dok) co-immunoprecipitates with the activated epidermal growth factor receptor/LckF505 and that phosphorylated Dok proteins bind to the Src homology 2 domain of Lck in vitro. In addition, we have shown that activation via the CD2 but not the TCR/CD3 receptor leads to the phosphorylation of p56(dok) and p62(dok). Using JCaM1.6 cells, we have demonstrated that Lck is required for CD2-mediated phosphorylation of Dok proteins. We propose that phosphorylation and Src homology 2-mediated association of p56(dok) and p62(dok) with Lck play a selective function in accessory receptor signal transduction mechanisms.

Biochemical association of CD45 with the T cell receptor complex: regulation by CD45 isoform and during T cell activation

CD45 is the predominant transmembrane tyrosine phosphatase in lymphocytes and is required for the efficient induction of T cell receptor signaling and activation. However, the regulation of CD45 activity and substrate specificity are poorly understood. In the present study, we demonstrate a basal biochemical association of CD45 with the T cell receptor complex that is regulated in part by CD45 isoform expression. Further, maintenance of CD45/TCR association is differentially regulated following TCR ligation with peptide: a partial agonist peptide induces CD45/TCR dissociation while an agonist peptide promotes sustained association in a CD4-dependent manner. These data suggest that T cell receptor signaling pathways may be modulated by altering access of CD45 to TCR-associated substrates involved in T cell activation.

Tyrosine kinase chimeras for antigen-selective T-body therapy

Protein tyrosine kinases (PTKs) transmit activation signals in almost every cell type, including immune effector cells. The aberrant or constitutive activation of PTKs can often cause neoplastic transformation. The use of chimeric receptors based on PTKs may enable us to elucidate the signaling pathways of normal immune cells and other cell types, and the abnormal events that can lead to malignant transformation. In this review, we focus on antigen specific chimeric PTKs in which antibody-derived scFv are joined to the Syk family of PTKs. These chimeric receptors yielded reagents that can selectively redirect immune effector cells and specifically activate them to produce cytokines or lyse their target. The advantages of using such PTK-based chimeras to redirect lymphocytes to tumor targets and their potential as an immunotherapeutic approach to malignant disease is discussed.

Role of phosphatases in lymphocyte activation

Many lymphocyte signaling pathways are regulated by protein tyrosyl phosphorylation, which is controlled by protein tyrosine kinases (PTKs) and protein tyrosine phosphatases (PTPs). Substantial progress has been made in defining the functions of lymphocyte PTPs. Individual PTPs can enhance or diminish cell signaling levels. The transmembrane PTP CD45 is a key positive element in multiple lymphocyte signaling pathways in vivo. New insights into the function of individual CD45 isoforms have emerged. Anti-CD45 antibodies with potent immunosuppressant activity have been identified, suggesting that CD45 may be a propitious target for drug design. Progress has also been made in elucidating the function and targets of specific nontransmembrane PTPs, particularly those with Src homology 2 domains.