Targets of p56(lck) activity in immature thymoblasts: stimulation of the Ras/Raf/MAPK pathway

Expression and mechanism of microRNA-181A on incidence and survival in late liver metastases of colorectal cancer

The emerging role of microRNA-181A (miR-181A) in CRC patients with late liver metastases was studied. In the present study we investigated the association between expression and mechanism of miRNA-181A, liver metastasis and prognosis of colorectal cancer (CRC). The expression of miR-181A and PTEN in CRC patients with late liver metastases was higher than that of the normal (control) group, whereas the expression of miR-181A and PTEN was lower in all pathological groups (TNM I-TNM IV). Overall survival (OS) of lower expression miR-181A CRC patients with late liver metastases was higher than that of higher expression miR-181A CRC patients with late liver metastases. The expression of miR-181A and PTEN in the colon cell line NCM460 was lower than that of the colon cancer SW620 cell line. Upregulation of miR-181A promoted cell viability and inhibited apoptosis of SW620 cells, suppressed PTEN expression and activated phosphorylation of AKT (p-AKT) in SW620 cells. Additionally, downregulation of miR-181A inhibited cell viability of SW620 cells through promotion of PTEN and inhibition of p-AKT. Together, our results indicate that miR-181A expression is associated with CRC patients with late liver metastases through PTEN/AKT signaling.

The differential regulation of Lck kinase phosphorylation sites by CD45 is critical for T cell receptor signaling responses

The molecular mechanisms whereby the CD45 tyrosine phosphatase (PTPase) regulates T cell receptor (TCR) signaling responses remain to be elucidated. To investigate this question, we have reconstituted CD45 (encoded by Ptprc)-deficient mice, which display severe defects in thymic development, with five different expression levels of transgenic CD45RO, or with mutant PTPase null or PTPase-low CD45R0. Whereas CD45 PTPase activity was absolutely required for the reconstitution of thymic development, only 3% of wild-type CD45 activity restored T cell numbers and normal cytotoxic T cell responses. Lowering the CD45 expression increased CD4 lineage commitment. Peripheral T cells with very low activity of CD45 phosphatase displayed reduced TCR signaling, whereas intermediate activity caused hyperactivation of CD4+ and CD8+ T cells. These results are explained by a rheostat mechanism whereby CD45 differentially regulates the negatively acting pTyr-505 and positively acting pTyr-394 p56(lck) tyrosine kinase phosphorylation sites. We propose that high wild-type CD45 expression is necessary to dephosphorylate p56(lck) pTyr-394, suppressing CD4 T+ cell lineage commitment and hyperactivity.

Constitutive Activation of Extracellular Signal-Regulated Kinase Predisposes Diffuse Large B-Cell Lymphoma Cell Lines to CD40-Mediated Cell Death

CD40 promotes survival, proliferation, and differentiation of normal B cells but can cause activation-induced cell death in malignant B lymphocytes. CD40 ligand and anti-CD40 antibodies have been used successfully to induce apoptosis in lymphoma lines both in vitro and in xenograft tumor models. Although this makes CD40 an attractive target for antitumor therapies, the response of malignant B cells to CD40 signaling is variable, and CD40 stimulation can enhance proliferation and can increase chemoresistance in some cell lines. It would therefore be useful to identify markers that predict whether a specific cell line or tumor will undergo apoptosis when stimulated with CD40 and to identify targets downstream of CD40 that affect only the apoptotic arm of CD40 signaling. We have analyzed gene expression patterns in CD40-sensitive and CD40-resistant diffuse large B-cell lymphoma (DLBCL) cell lines to identify signaling pathways that are involved in CD40-mediated apoptosis. CD40-resistant lines expressed pre-B-cell markers, including RAG and VPREB, whereas CD40-sensitive cells resembled mature B cells and expressed higher levels of transcripts encoding several members of the CD40 signaling pathway, including LCK and VAV. In addition, CD40-sensitive DLBCL cell lines also displayed constitutive activation of extracellular signal-regulated kinase (ERK) and failed to undergo apoptosis when ERK phosphorylation was inhibited. In contrast, CD40-resistant lines showed no constitutive activation of ERK and no increase in ERK activity in response to CD40 stimulation. Our results suggest that constitutive activation of ERK may be required for death signaling by CD40.

Transgenic Mice as an In Vivo Model of Lymphomagenesis

This review covers multiple data obtained on genetically modified mice that help to elucidate various intricate molecular mechanisms of lymphomagenesis in humans. We are in a "golden age" of mouse genetics. The mouse is by far the most accessible mammalian system physiologically similar to humans. Transgenic mouse models have illuminated how different genes contribute to human lymphomagenesis. Multiple experiments with transgenic mice have not only confirmed the data obtained for human lymphomas but also gave additional evidence for the role of some genes and cooperative participation of their products in the development of human lymphomas. Genes and gene networks detected on transgenic mice can successfully serve as molecular targets for tumor therapy. This review demonstrates the extraordinary possibilities of transgenic technology, which is presently one of the readily available, efficient, and accurate tools to solve the problem of cancer.

Requirement for cyclin D3 in lymphocyte development and T cell leukemias

The D-type cyclins (cyclins D1, D2, and D3) are components of the core cell cycle machinery in mammalian cells. Cyclin D3 gene is rearranged and the protein is overexpressed in several human lymphoid malignancies. In order to determine the function of cyclin D3 in development and oncogenesis, we generated and analyzed cyclin D3-deficient mice. We found that cyclin D3(-/-) animals fail to undergo normal expansion of immature T lymphocytes and show greatly reduced susceptibility to T cell malignancies triggered by specific oncogenic pathways. The requirement for cyclin D3 also operates in human malignancies, as knock-down of cyclin D3 inhibited proliferation of acute lymphoblastic leukemias deriving from immature T lymphocytes. These studies point to cyclin D3 as a potential target for therapeutic intervention in specific human malignancies.

Inhibition of MEK induces fas expression and apoptosis in lymphomas overexpressing Ras

Published results implicate PI3 kinase as a target of oncogenic Ras activity leading to the suppression of Fas but whether other Ras targets (e.g. Raf-1) are also involved is unclear. Here we report that thymic lymphomas overexpressing Ras and Raf-1 exhibit low expression of Fas. We show that expression of Fas in these lymphomas can be increased not only in the presence of a specific inhibitor (LY294002) of P13 kinase, but also in the presence of specific inhibitor (PD98059) of MEK, downstream target of Raf-1. Both treatments result in accumulation of ERK in cytosol of lymphoma cells suggesting cross-talk between these two pathways regulating Fas expression. Treatment with PD98059 also results in apoptosis of the lymphoma cells but not of normal thymocytes expressing low Raf-1 levels. These observations provide evidence for involvement of Raf-1/MEK/ERK pathway in Ras-mediated inhibition of Fas expression and in selective promotion of survival of lymphoma cells.

HA1004, an inhibitor of serine/threonine protein kinases, restores the sensitivity of thymic lymphomas to Ca2+-mediated apoptosis through a protein kinase A-independent mechanism

Our previous reports showed that thymic lymphomas arising in TCR transgenic mice are resistant to Ca2+-mediated apoptosis. Here we show that induction of apoptosis in thymic lymphomas involves a process that is cAMP-mediated and which depends on the activation of protein kinase A (PKA) despite the lower level of PKA type I in these lymphomas compared to thymocytes. Further, we show that treatment of the lymphomas with HA1004, a serine/threonine protein kinase inhibitor, restores their susceptibility to ionomycin-induced apoptosis. Results indicate that HA1004-induced restoration of sensitivity to ionomycin proceeds through a PKA-independent mechanism. Moreover, activation of PKA instead of its inhibition induces apoptosis of lymphoma cells.

The Raf signal transduction cascade as a target for chemotherapeutic intervention in growth factor-responsive tumors

This review focuses on the Ras-Raf-mitogen-activated protein kinase kinase (MEK)-extracellular signal-regulated kinase (ERK) signal transduction pathway and the consequences of its unregulation in the development of cancer. The roles of some of the cell membrane receptors involved in the activation of this pathway, the G-protein Ras, the Raf, MEK and ERK kinases, the phosphatases that regulate these kinases, as well as the downstream transcription factors that become activated, are discussed. The roles of the Ras-Raf-MEK-ERK pathway in the regulation of apoptosis and cell cycle progression are also analyzed. In addition, potential targets for pharmacological intervention in growth factor-responsive cells are evaluated.

CD28 and the tyrosine kinase lck stimulate mitogen-activated protein kinase activity in T cells via inhibition of the small G protein Rap1

Proliferation of T cells via activation of the T-cell receptor (TCR) requires concurrent engagement of accessory costimulatory molecules to achieve full activation. The best-studied costimulatory molecule, CD28, achieves these effects, in part, by augmenting signals from the TCR to the mitogen-activated protein (MAP) kinase cascade. We show here that TCR-mediated stimulation of MAP kinase extracellular-signal-regulated kinases (ERKs) is limited by activation of the Ras antagonist Rap1. CD28 increases ERK signaling by blocking Rap1 action. CD28 inhibits Rap1 activation because it selectively stimulates an extrinsic Rap1 GTPase activity. The ability of CD28 to stimulate Rap1 GTPase activity was dependent on the tyrosine kinase Lck. Our results suggest that CD28-mediated Rap1 GTPase-activating protein activation can help explain the augmentation of ERKs during CD28 costimulation.

TCR-independent T cell development mediated by gain-of-oncogene function or loss-of-tumor-suppressor gene function

The mechanisms that govern differentiation of T cell precursors during intrathymic development bridge an interdisciplinary research field of immunology, oncology and developmental biology. Critical checkpoints controlling early thymic T cell development and homeostasis are set by the proper signaling function of the IL-7 receptor, c-Kit receptor, and the pre-T cell antigen receptor (pre-TCR). Given the intimate link between cell cycle control and differentiation in T cell development, proto-oncogenes and tumor suppressors participate as physiological effectors downstream of these receptors not only to influence the cell cycle but also to determine differentiation and survival. Gain- or loss-of-function mutations of these downstream effectors uncouples partially or completely T cell precursors from these checkpoints, providing a selective advantage and enabling aberrant development. These effectors can be identified by provirus tagging in normal mice and more readily by complementation tagging in mice with a predefined block in T cell differentiation.

Development of T-leukaemias in CD45 tyrosine phosphatase-deficient mutant lck mice

The CD45 tyrosine phosphatase lowers T-cell antigen receptor signalling thresholds by its positive actions on p56(lck) tyrosine kinase function. We now show that mice expressing active lck(F505) at non-oncogenic levels develop aggressive thymic lymphomas on a CD45(-/-) background. CD45 suppresses the tumorigenic potential of the kinase by dephosphorylation of the Tyr394 autophosphorylation site. In CD45(-/-) thymocytes the kinase is switched to a hyperactive oncogenic state, resulting in increased resistance to apoptosis. Transformation occurs in early CD4(-)CD8(-) thymocytes during the process of TCR-beta chain rearrangement by a recombinase-independent mechanism. Our findings represent the first example in which a tyrosine phosphatase in situ prevents the oncogenic actions of a SRC: family tyrosine kinase.

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.

Overexpression of Ras, Raf and L-myc but not Bcl-2 family proteins is linked with resistance to TCR-mediated apoptosis and tumorigenesis in thymic lymphomas from TCR transgenic mice

Mice with transgenic TCR anti H-Y/Db develop spontaneous thymic tumors with a high frequency (up to 50%). Oncogenicity of TCR transgenes could depend on the deregulated expression of oncoproteins engaged in transduction pathways leading to proliferation or apoptosis. In agreement with this possibility we have found that cells of thymic lymphomas from TCR transgenic mice were largely resistant to TCR-dependent Ca++-mediated apoptosis but not to TCR-independent, p53-mediated (etoposide) apoptosis. Here we show raised expression of Bcl-2 protein in some but not in all thymic lymphoma cell lines. It suggests that the antiapoptotic function of Bcl-2 is not necessary for the process of tumorigenesis and the resistance of these lymphomas to Ca++-mediated apoptosis. On the other hand we show that all thymic lymphomas overexpressed Ras/Raf and L-myc proteins. Stimulation of the Ras/Raf pathway was reported to be required to maintain cell viability by preventing programmed cell death in thymic tumors derived from lck transgenic mice. Similarly, in TCR transgenic lymphomas overexpression of Ras, Raf and L-myc but not Bcl-2 family proteins may be responsible for the resistance of these lymphomas to TCR-mediated apoptosis but not affect p53-mediated apoptosis.

Commitment to the CD4 Lineage Mediated by Extracellular Signal-Related Kinase Mitogen-Activated Protein Kinase and Lck Signaling

The development of T cells results in a concordance between the specificity of the TCR for MHC class I and class II molecules and the expression of CD8 and CD4 coreceptors. Based on analogy to simple metazoan models of organ development and lineage commitment, we sought to determine whether extracellular signal-related kinase (Erk) mitogen-activated protein (MAP) kinase pathway signaling acts as an inductive signal for the CD4 lineage. Here, we show that, by altering the intracellular signaling involving the Erk/MAP kinase pathway, T cells with specificity for MHC class I can be diverted to express CD4, and, conversely, T cells with specificity for MHC class II can be diverted to express CD8. Furthermore, we find that activation of the src-family tyrosine kinase, p56lck is an upstream mediator of lineage commitment. These results suggest a simple mechanism for lineage commitment in T cell development.

Dysregulation of the protein tyrosine kinase LCK in lymphoproliferative disorders and in other neoplasias

Initially identified as a T-cell specific member of the Src family of protein tyrosine kinases, Lck has become the object of intensive investigations which have revealed a key role for this kinase in the central processes controlling T-cell development, activation, proliferation and survival. Experimental evidence of the oncogenic potential of Lck, together with the identification of defects in the regulation of Lck expression or activity in T-cell leukemias, suggests that dysregulation of Lck might play a role in neoplastic transformation. Here we review the data documenting a potential role for this kinase in the initiation and maintenance of the transformed state in human cancers.

Interferon-alpha induction of STATs1, -3 DNA binding and growth arrest is independent of Lck and active mitogen-activated kinase in T cells

Type I interferons (IFNs) are a family of cytokines that have antiviral and antiproliferative effects. Data regarding the processes by which these cytokines transduce signals from the cell membrane to the nucleus are becoming increasingly complex. The most characterized pathway is via JAK-STAT signaling. Previous studies established a potential role for the Src-family kinase Lck in JAK-STAT signaling. Therefore, this study was designed to analyze the role of Lck in IFN-alpha signaling by using the Jurkat, JCam (an Lck-defective cell line derived from Jurkat), and JCam/Lck (JCam cells with Lck restored). The results show that IFN-alpha can induce MAPK activity, but only in cells containing Lck. Furthermore, STATs1 and -3 are effectively phosphorylated and activated to bind DNA in the absence of Lck expression in IFN-alpha-treated cells. Finally, the results demonstrate that IFN-alpha exerts an antiproliferative effect in all three cell lines. These data indicate that Lck and active MAPK do not affect IFN-alpha-induced growth arrest or induction of STAT1s1 and -3 DNA binding ability.

Roles of Lck, Syk and ZAP-70 tyrosine kinases in TCR-mediated phosphorylation of the adapter protein Shc

The adapter protein Shc has been implicated in mitogenic signaling via growth factor receptors, antigen receptors and cytokine receptors. Recent studies have suggested that tyrosine phosphorylation of Shc may play a key role in T lymphocyte proliferation via interaction of phosphorylated Shc with downstream molecules involved in activation of Ras and Myc proteins. However, the sites on Shc that are tyrosine phosphorylated in response to TCR engagement and the ability of different T cell tyrosine kinases to phosphorylate Shc have not been defined. In this report, we show that during TCR signaling, the tyrosines Y239, Y240 and Y317 of Shc are the primary sites of tyrosine phosphorylation. Mutation of all three tyrosines completely abolished tyrosine phosphorylation of Shc following TCR stimulation. Our data also suggest that multiple T cell tyrosine kinases contribute to tyrosine phosphorylation on Shc. In T cells, CD4/Lck-dependent tyrosine phosphorylation on Shc was markedly diminished when Y317 was mutated, suggesting a preference of Lck for the Y317 site. The syk-family kinases (Syk and ZAP-70) were able to phosphorylate the Y239 and Y240 sites, and less efficiently the Y317 site. Moreover, co-expression of Syk or ZAP-70 with Lck resulted in enhanced phosphorylation of Shc on all three sites, suggesting a synergy between the syk-family and scr-family kinases. Of the two potential Grb2 binding sites (Y239 and Y317), Y239 appears to play a greater role in recruiting Sos through Grb2. These studies have implications for Ras activation and mitogenic signaling during T cell activation.

T cell activation through the CD43 molecule leads to Vav tyrosine phosphorylation and mitogen-activated protein kinase pathway activation

CD43, the most abundant membrane protein of T lymphocytes, is able to initiate signals that lead to Ca2+ mobilization and interleukin-2 production, yet the molecular events involved in signal transduction pathway of the CD43 molecule are only beginning to be understood. We have shown recently that cross-linking CD43 on the cell surface of human T lymphocytes with the anti-CD43 monoclonal antibody L10 leads to CD43-Fyn kinase interactions and to Fyn phosphorylation on tyrosine residues. This interaction seems to be mediated by the SH3 domain of Fyn and a proline-rich sequence located in the cytoplasmic domain of CD43. Here we show that CD43-specific activation of human T lymphocytes induced tyrosine phosphorylation of the adaptor protein Shc and of the guanine exchange factor Vav, as well as the formation of a macromolecular complex that comprises Shc, GRB2, and Vav. CD43 ligation resulted in enhanced formation of Vav.SLP-76 complexes and in the activation and nuclear translocation of ERK2. Cross-linking of the CD43 molecule in 3T3-CD43(+) cells induced luciferase activity from a construct under the control of the Fos serum responsive element. Altogether, these data suggest that the mitogen-activated protein kinase pathway is involved in CD43-dependent interleukin-2 gene expression.

Coxsackie B virus and its interaction with permissive host cells

BACKGROUND

Observations in humans and the results of experiments on laboratory animals have provided evidence that coxsackieviruses of group B (CVB) are major etiologic agents of acute and chronic enterovirus myocarditis and various other virus-induced diseases.

OBJECTIVE

This minireview briefly summarizes the investigations to elucidate various molecular mechanisms for the induction and maintenance of persistent CVB infections. With regard to the recent findings that CVB may use several different receptor proteins, this article focuses on virus-host cell interactions and the potential impact of these interactions for enteroviral replication.

STUDY DESIGN

The interaction of CVB with specific cell surface proteins was analyzed in cultured cell lines and murine tissues at the level of virus attachment and virus internalization. As example for the interaction of CVB with intracellular proteins, the state of p21rasGTPase-activating protein (RasGAP) was investigated in mock-infected and CVB3-infected HeLa cells.

RESULTS AND CONCLUSIONS

The experiments to elucidate the virus receptor interactions revealed the necessity to differentiate between CVB attachment proteins and proteins involved in virus internalization. Since more than one protein may be required to initiate the uptake of CVB into permissive host cells, a model of the putative interaction of these proteins within a multimeric receptor complex is proposed. It is further tempting to speculate that the presence of multiple attachment proteins may influence the tissue tropism of CVB as well as pathogenicity.