Interleukin 2 activates extracellular signal-regulated protein kinase 2

Interleukin-2 as immunotherapeutic in the autoimmune diseases

Interleukins, also called cytokines are secretory proteins that bind to specific receptors and play a critical role in the intercellular communication between cells of the immune system. Cytokines are mainly produced by T lymphocytes, macrophages and eosinophils. Among its functions are the activation and suppression of immune system responses, induction of cell division and regulation of memory cells. Interleukin 2 (IL-2) is a secretory monomeric glycoprotein composed of 149 amino acids containing a signal peptide of 20 amino acids. It is classified as a member of the type I cytokines family. IL-2 binds to its receptor (IL-2R receptor) with high affinity and its signaling function promotes the activation of various subtypes of lymphocytes during the process of cell differentiation to generate an immune or homeostatic response. The specificity of IL-2 depends on its binding to low, medium or high-affinity receptors. Interleukin 2 acts as a regulator of the proliferation of CD4+ and CD8+ T cells. There is a relationship between IL-2 and autoimmune diseases due to its influence in the differentiation of T helper cells, which in turn directly influence immunological response processes. Therefore, IL-2 is a key element in the control and treatment of those diseases. In recent years, many therapeutic agents based on biomolecules and recombinant chimeric proteins have been developed to treat different autoimmune diseases. In this review, we focus on the use of interleukin 2 as a versatile therapeutic agent, alone or associated with other molecules to increase the efficiency of autoimmune disease treatment.

The challenges and molecular approaches surrounding interleukin-2-based therapeutics in cancer

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IL2-based cancer therapies are limited by their toxicity and pleiotropy.

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Current engineering approaches target IL2 half-life and cell/receptor specificity.

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IL2 may enhance the efficacy of checkpoint inhibitors and CAR-T-based therapies.

Interleukin-2 has had a long history as a promising cancer therapeutic, being capable of eliciting complete and durable remissions in patients with metastatic renal cell carcinoma and metastatic melanoma. Despite high toxicity and efficacy limited to only certain patient subpopulations and cancer types, the prospective use of novel, engineered IL2 formats in combination with the presently expanding repertoire of immuno-oncological targets remains very encouraging. This is possible due to the significant research efforts in the IL2 field that have yielded critical structural and biological insights that have made IL2 more effective and more broadly applicable in the clinic. In this review, we discuss some of the molecular approaches that have been used to further improve IL2 therapy for cancer.

IL-2 triggers specific signaling pathways in human NKT cells leading to the production of pro- and anti-inflammatory cytokines

NKT cells belong to a conserved T lymphocyte subgroup that has been implicated in the regulation of various immune responses, including responses to viruses, bacteria, and parasites. They express a semi-invariant TCR that recognizes glycolipids presented by the nonpolymorphic MHC class I-like molecule CD1d, and upon activation, they produce various pro- and anti-inflammatory cytokines. Recent studies have shed light on the nature of glycolipids and the environmental signals that may influence the production of cytokines by NKT cells and thus, modulate the immune response. To better understand the regulation mechanisms of NKT cells, we explored their behavior following activation by IL-2 and investigated the signaling pathways and biological responses triggered. We demonstrated that IL-2 activates not only STAT3 and -5 and the PI-3K and ERK-2 pathways as in all IL-2 responder cells but also STAT4 as in NK cells and the p38 MAPK pathway as in alphabeta T cells. We also showed that STAT6 is activated by IL-2 in NKT cells. Moreover, IL-2 induces the production of IFN-gamma and IL-4. The ability of IL-2 to induce pro- and anti-inflammatory cytokine production, in addition to proliferation, could open new therapeutic approaches for use in combination with molecules that activate NKT cells through TCR activation.

The Bax N terminus is required for negative regulation by the mitogen-activated protein kinase kinase and Akt signaling pathways in T cells

The Bcl-2 family proapoptotic protein, Bax, redistributes to the mitochondrion in response to varied stimuli, triggering loss of mitochondrial integrity and apoptosis. Suppression of MAPK kinase (MEK1) by the reagent UO126 in activated T cells maintained in the cytokine IL-2 disrupts cytoplasmic localization of Bax and cell survival. UO126 triggers mitochondrial translocation of ectopically expressed Bax-GFP, and both UO126 and dominant negative MEK-1 (DN-MEK1) trigger increased apoptosis in Bax-GFP-expressing T cell lines. Because inhibition of PI3K or its target Akt also triggers mitochondrial translocation of Bax in T cells and apoptosis in Bax-transfected cell lines, we generated Bax deletion mutants to identify the region(s) that confers sensitivity to regulation by MEK1 and Akt. A deletion mutant (Bax(1-171)) without the C terminus mitochondrial targeting sequence or an Akt target site (Ser(184)) localizes to the cytoplasm and triggers low level apoptosis that is enhanced by DN-Akt or DN-MEK1. A construct that lacks the first 29 aa (Bax-delta29) largely localizes to mitochondria, is highly apoptogenic, and is not inhibited by Akt or MEK1. Furthermore, Bax-delta29 overcomes IL-2-dependent survival in a T cell line, whereas Bax triggers comparatively low levels of apoptosis in these cells. Cytoplasmic localization and regulation by MEK1 and Akt are restored in a mutant deleted of the first 13 aa (Bax-delta13). Taken together, our results identify a region in the Bax N terminus that determines cellular localization regulated by MEK- and Akt-dependent signaling in T cells.

Specific Signaling Pathways Triggered by IL-2 in Human Vγ9Vδ2 T Cells: An Amalgamation of NK and αβ T Cell Signaling

The global immune response can be simplified into two components: the innate and the acquired systems. The innate immune response comprises primarily macrophages and NK cells, while B and T cells orchestrate the acquired response. Human Vgamma9Vdelta2 T cells represent a minor T cell subpopulation in blood (1-5%) that is activated via the TCR by small nonpeptidic molecules. Their percentage dramatically increases during the early phase of infection by intracellular pathogens, and they display many characteristics of NK cells, which places them at a unique position within the immune system. Our aim was to explore the behavior of these cells when they are activated by a receptor that is common to NK and alphabeta T cells, and to determine signaling pathways and biological responses induced in these cells through this receptor. Thus, we investigated whether Vgamma9Vdelta2 T cells behave as NK cells or as alphabeta T cells. We demonstrated that IL-2 activates not only STAT3, STAT5, the phosphatidylinositol 3-kinase pathway, and extracellular signal-regulated kinase-2 pathway, but also STAT4 as in NK cells, and the p38 mitogen-activated protein kinase pathway as in alphabeta T cells. Moreover, IL-2 induces the production of IFN-gamma in Vgamma9Vdelta2 T cells as observed in NK cells. Due to their double profiles, Vgamma9Vdelta2 T cells are at the interface of the innate and the acquired immune response and may therefore not only modulate the activity of innate cells, but also influence Th1/Th2 differentiation.

IL-4 synergistically enhances both IL-2- and IL-12-induced IFN-gamma expression in murine NK cells

Interleukin-4 (IL-4) is thought to influence T and natural killer (NK) cells by down-regulating T helper 1 (Th1)-type cytokines like interferon-gamma (IFN-gamma). While investigating IL-4 regulation of IFN-gamma expression, we found that IL-4 synergized with IL-2 or IL-12 to enhance IFN-gamma production and mRNA expression in spleen-derived, IL-2-cultured NK cells, as well as negatively sorted fresh DX5+/CD3- NK cells albeit at lower levels. The positive effect of IL-4 on IL-2-induced IFN-gamma production was dependent upon signal transducer and activator of transcription 6 (Stat6) because this response was virtually abrogated in Stat6-/- mice. Notably, though, IL-12 plus IL-4 synergy on IFN-gamma expression was intact in Stat6-/- mice. In exploring possible molecular mechanisms to account for the synergistic effects of IL-4 on murine NK cells, we found that IL-2 plus IL-4 stimulation resulted in a modest increase in tyrosine phosphorylation of Stat5, while IL-12 plus IL-4 treatment resulted in a more substantial increase in tyrosine-phosphorylated Stat4. Finally, to identify regions of the IFN-gamma promoter that may be involved, NK cells from human IFN-gamma promoter/luciferase transgenic mice were treated with cytokines. NK cells from proximal (-110 to +64) promoter region mice did not respond to cytokine stimulation; however, the intact -565 to +64 IFN-gamma promoter responded synergistically to IL-2 plus IL-4 and to IL-12 plus IL-4 in NK cells. These data demonstrate a role for IL-4 in enhancing IFN-gamma expression in murine NK cells that is partially dependent on Stat6 in IL-2 costimulation and completely independent of Stat6 in IL-12 costimulations.

IL-2 activation of a PI3K-dependent STAT3 serine phosphorylation pathway in primary human T cells

Interleukin-2 (IL-2) is the major growth factor of activated T lymphocytes. By inducing cell cycle progression and protection from apoptosis in these cells, IL-2 is involved in the successful execution of an immune response. Upon binding its receptor, IL-2 activates a variety of signal transduction pathways, including the Ras/mitogen-activated protein kinase kinase (MEK)/extracellular signal-regulated kinase (ERK) and Janus kinase (JAK)/STAT cascades. In addition, activation of phosphatidylinositol 3-kinase (PI3K) and several of its downstream targets has also been shown. However, the coupling of STAT3 serine phosphorylation to PI3K in response to IL-2 has yet to be shown in either T cell lines or primary human T cells. This report shows that the PI3K inhibitors LY294002 and wortmannin block activation of MEK and ERK by IL-2 in primary human T cells. Moreover, these inhibitors significantly reduce IL-2-triggered STAT3 serine phosphorylation without affecting STAT5 serine phosphorylation. Analysis of the effects of these inhibitors on cell cycle progression and apoptosis strongly suggests that PI3K-mediated events, which includes STAT3 activation, are involved in IL-2-mediated cell proliferation but not cell survival. Finally, results presented illustrate that in primary human T cells, activation of Akt is insufficient for IL-2-induced anti-apoptosis. Thus, these results demonstrate that IL-2 stimulates PI3K-dependent events that correlate with cell cycle progression, but not anti-apoptosis, in activated primary human T cells.

IL-4 regulation of p38 MAPK signalling is dependent on cell type

p38 MAPK was originally characterized as a stress-induced kinase, along with JNK. Subsequently, p38 MAPK was found to be activated by stimuli other than cellular stress, such as growth factors and mitogens, like interleukin (IL)-2, IL-7 and IL-3. A notable exception was IL-4, as studies in mast cells showed no activation of p38 MAPK by this cytokine. In this study we show that the regulation of p38 MAPK is cell type dependent. Like other cytokines that signal through the gamma (gamma)(c), IL-4 can activate p38 MAPK in the CT6 T-cell line and BA/F3 pro-B-cells. However, IL-4 was unable to activate p38 MAPK in the murine macrophage cell line, RAW 264.7 and, indeed, prolonged exposure of cells to IL-4 results in suppression of LPS-induced MAPK activation. This result correlates with the well defined inhibitory effect of IL-4 on tumour necrosis factor alpha (TNFalpha) production. In contrast, studies in primary human monocytes showed that prolonged exposure to IL-4 resulted in enhanced activation of LPS-stimulated p38 MAPK; this correlated with an enhanced TNFalpha production. These data highlight the complexity of IL-4 signalling mechanisms, the diversity that can exist in the regulation of a given signalling pathway by a given cytokine and, furthermore, indicate the problems that can arise from extrapolation between different cell systems.

Cytokine-driven proliferation and differentiation of human naive, central memory, and effector memory CD4(+) T cells

Memory T lymphocytes proliferate in vivo in the absence of antigen maintaining a pool of central memory T cells (T(CM)) and effector memory T cells (T(EM)) with distinct effector function and homing capacity. We compared human CD4(+) naive T, T(CM), and T(EM) cells for their capacity to proliferate in response to cytokines, that have been implicated in T cell homeostasis. Interleukin (IL)-7 and IL-15 expanded with very high efficiency T(EM), while T(CM) were less responsive and naive T cells failed to respond. Dendritic cells (DCs) and DC-derived cytokines allowed naive T cells to proliferate selectively in response to IL-4, and potently boosted the response of T(CM) to IL-7 and IL-15 by increasing the expression of the IL-2/IL-15Rbeta and the common gamma chain (gamma(c)). The extracellular signal regulated kinase and the p38 mitogen-activated protein (MAP) kinases were selectively required for TCR and cytokine-driven proliferation, respectively. Importantly, in cytokine-driven cultures, some of the proliferating T(CM) differentiated to T(EM)-like cells acquiring effector function and switching chemokine receptor expression from CCR7 to CCR5. The sustained antigen-independent generation of T(EM) from a pool of T(CM) cells provides a plausible mechanism for the maintenance of a polyclonal and functionally diverse repertoire of human CD4(+) memory T cells.

IL-2 induces the association of IL-2Rbeta, lyn, and MAP kinase ERK-1 in human neutrophils

IL-2, first identified as a T cell growth factor, has been proven to activate many cell types including polymorphonuclear neutrophils (PMN3). However, the mechanisms involved in PMN activation, especially the signaling pathways used by the IL-2R, are currently unknown. Here we demonstrate that IL-2 has the ability to induce protein tyrosine kinases in human PMN, and we provide the first evidence that lyn kinase is activated and physically associated with MAP kinase/ERK1. Co-immunoprecipitation experiments with anti-IL-2Rbeta and Western blotting with anti-p53/56lym revealed that lyn protein was present in IL-2R precipitates and that the association of lyn with IL-2Rbeta was markedly elevated by IL-2 stimulation. Furthermore the activity of lyn kinase, evaluated by an in vitro kinase assay with enolase as a substrate, increased following IL-2 stimulation. Another important finding was that, upon IL-2 activation, MAPK/ERK1 was also phosphorylated in PMN. A direct association between lyn and ERK1 was initially demonstrated by co-immunoprecipitation/Western blotting and then definitively proven by the use of a GST-ERK1 fusion protein. We showed that ERK1 binds lyn only in IL-2 stimulated PMN, but not in unstimulated PMN. These results suggest that IL-2 can promote the association of lyn protein tyrosine kinase with IL-2Rbeta as well as the direct binding of MAPK/ERK1 to lyn. The signaling pathway utilized by human PMN in response to IL-2 may thus involve the association of lyn with IL-2Rbeta and the activation process also triggers the recruitment and activation of a specific ERK.

Differential effects of N-acetyl-l-cysteine on IL-2- vs IL-12-driven proliferation of a T cell clone: implications for distinct signalling pathways

Using a T cell clone (2D6) capable of responding to IL-2 and IL-12, we compared the effects of NAC on IL-2 and IL-12-driven T cell proliferation. Addition of N-acetylcysteine (NAC) to 2D6 cultures did not affect IL-2 stimulated proliferation, but strikingly inhibited IL-12 stimulated proliferation. These differential NAC effects did not correlate with the patterns of the mitogen-activated protein kinase (MAPK) activation following cytokine stimulation and its regulation by NAC. Although a p38 MAPK inhibitor downregulated both IL-2- and IL-12-induced proliferation, this effect was seen at drug concentrations one order higher than those reportedly used to specifically inhibit p38 MAPK. The results suggest the existence of distinct signalling pathways and a common, indispensable signalling molecule in IL-2- and IL-12 driven T cell proliferation.

IL-2 activation of NK cells: involvement of MKK1/2/ERK but not p38 kinase pathway

IL-2 stimulates extracellular signal-regulated protein kinase (ERK) and p38 mitogen-activated protein kinase (MAPK) in various immune cell populations. The functional roles that these kinases play are still unclear. In this study, we examined whether MAPK kinase (MKK)/ERK and p38 MAPK pathways are necessary for IL-2 to activate NK cells. Using freshly isolated human NK cells, we established that an intact MKK/ERK pathway is necessary for IL-2 to activate NK cells to express at least four known biological responses: LAK generation, IFN-gamma secretion, and CD25 and CD69 expression. IL-2 induced ERK activation within 5 min. Treatment of NK cells with a specific inhibitor of MKK1/2, PD98059, during the IL-2 stimulation blocked in a dose-dependent manner each of four sequelae, with inhibition of lymphokine-activated killing induction being least sensitive to MKK/ERK pathway blockade. Activation of p38 MAPK by IL-2 was not detected in NK cells. In contrast to what was observed by others in T lymphocytes, SB203850, a specific inhibitor of p38 MAPK, did not inhibit IL-2-activated NK functions. This data indicate that p38 MAPK activation was not required for IL-2 to activate NK cells for the four functions examined. These results reveal selective signaling differences between NK cells and T lymphocytes; in NK cells, the MKK/ERK pathway and not p38 MAPK plays a critical positive regulatory role during activation by IL-2.

The Functional Synergy Between IL-12 and IL-2 Involves p38 Mitogen-Activated Protein Kinase and Is Associated with the Augmentation of STAT Serine Phosphorylation

IL-12 and IL-2 can stimulate mitogen- or CD3-activated T cells to proliferate, produce IFN-γ, and kill tumor cells. The magnitude of these functional responses is greatly augmented when T cells are activated by the combination of IL-12 and IL-2. Although peripheral blood T cells are largely unresponsive to these cytokines without prior activation, a small subset of CD8+ T cells (CD8+CD18bright) is strongly activated by the combination of IL-12 and IL-2. In this report we show that the functional synergy between IL-12 and IL-2 in CD8+CD18bright T cells correlates with the activation of the stress kinases, p38 mitogen-activated protein (MAP) kinase and stress-activated protein kinase (SAPK)/Jun N-terminal kinase, but not with the activation of the extracellular signal-regulated kinases. The functional synergy between IL-2 and IL-12 is also associated with a prominent increase in STAT1 and STAT3 serine phosphorylation over that observed with IL-12 or IL-2 alone. By contrast, STAT tyrosine phosphorylation is not augmented over that seen with either cytokine alone. A specific inhibitor of p38 MAP kinase completely inhibits the serine phosphorylation of STAT1 and STAT3 induced by IL-12 and IL-2 and abrogates the functional synergy between IL-12 and IL-2 without affecting STAT tyrosine phosphorylation. This suggests that p38 MAP kinase may play an important role in regulating STAT serine phosphorylation in response to the combination of IL-12 and IL-2. Furthermore, these findings indicate that the optimal activation of T cells by IL-12 and IL-2 may depend on an interaction between the p38 MAP kinase and Janus kinase/STAT signaling pathways.

Regulation of human immunodeficiency virus type 1 infectivity by the ERK mitogen-activated protein kinase signaling pathway

ERK1 and ERK2 mitogen-activated protein kinases (MAPK) play a critical role in regulation of cell proliferation and differentiation in response to mitogens and other extracellular stimuli. Mitogens and cytokines that activate MAPK in T cells have been shown to activate human immunodeficiency virus type 1 (HIV-1) replication. Little is known about the signal transduction pathways that activate HIV-1 replication in T cells upon activation by extracellular stimulation. Here, we report that activation of MAPK through the Ras/Raf/MEK signaling pathway enhances the infectivity of HIV-1 virions. Virus infectivity was enhanced by treatment of cells with MAPK stimulators, such as serum and phorbol myristate acetate, as well as by coexpression of constitutively activated Ras, Raf, or MEK (MAPK kinase) in the absence of extracellular stimulation. Treatment of cells with PD 098059, a specific inhibitor of MAPK activation, or with a MAPK antisense oligonucleotide reduced the infectivity of HIV-1 virions without significantly affecting virus production or the levels of virion-associated Gag and Env proteins. MAPK has been shown to regulate HIV-1 infectivity by phosphorylating Vif (X. Yang and D. Gabuzda, J. Biol. Chem. 273:29879-29887, 1998). However, MAPK activation enhanced virus infectivity in some cells lines that do not require Vif function. The HIV-1 Rev, Tat, p17(Gag), and Nef proteins were directly phosphorylated by MAPK in vitro, suggesting that other HIV-1 proteins are potential substrates for MAPK phosphorylation. These results suggest that activation of the ERK MAPK pathway plays a role in HIV-1 replication by enhancing the infectivity of HIV-1 virions through Vif-dependent as well as Vif-independent mechanisms. MAPK activation in producer cells may contribute to the activation of HIV-1 replication when T cells are activated by mitogens and other extracellular stimuli.

Biology of the interleukin-2 receptor

Studies of the biology of the IL-2 receptor have played a major part in establishing several of the fundamental principles that govern our current understanding of immunology. Chief among these is the contribution made by lymphokines to regulation of the interactions among vast numbers of lymphocytes, comprising a number of functionally distinct lineages. These soluble mediators likely act locally, within the context of the microanatomic organization of the primary and secondary lymphoid organs, where, in combination with signals generated by direct membrane-membrane interactions, a wide spectrum of cell fate decisions is influenced. The properties of IL-2 as a T-cell growth factor spawned the view that IL-2 worked in vivo to promote clonal T-cell expansion during immune responses. Over time, this singular view has suffered from increasing appreciation that the biologic effects of IL-2R signals are much more complex than simply mediating T-cell growth: depending on the set of conditions, IL-2R signals may also promote cell survival, effector function, and apoptosis. These sometimes contradictory effects underscore the fact that a diversity of intracellular signaling pathways are potentially activated by IL-2R. Furthermore, cell fate decisions are based on the integration of multiple signals received by a lymphocyte from the environment; IL-2R signals can thus be regarded as one input to this integration process. In part because IL-2 was first identified as a T-cell growth factor, the major focus of investigation in IL-R2 signaling has been on the mechanism of mitogenic effects in cultured cell lines. Three critical events have been identified in the generation of the IL-2R signal for cell cycle progression, including heterodimerization of the cytoplasmic domains of the IL-2R beta and gamma(c) chains, activation of the tyrosine kinase Jak3, and phosphorylation of tyrosine residues on the IL-2R beta chain. These proximal events led to the creation of an activated receptor complex, to which various cytoplasmic signaling molecules are recruited and become substrates for regulatory enzymes (especially tyrosine kinases) that are associated with the receptor. One intriguing outcome of the IL-2R signaling studies performed in cell lines is the apparent functional redundancy of the A and H regions of IL-2R beta, and their corresponding downstream pathways, with respect to the proliferative response. Why should the receptor complex induce cell proliferation through more than one mechanism or pathway? One possibility is that this redundancy is an unusual property of cultured cell lines and that primary lymphocytes require signals from both the A and the H regions of IL-2R beta for optimal proliferative responses in vivo. An alternative possibility is that the A and H regions of IL-2R beta are only redundant with respect to proliferation and that each region plays a unique and essential role in regulating other aspects of lymphocyte physiology. As examples, the A or H region could prove to be important for regulating the sensitivity of lymphocytes to AICD or for promoting the development of NK cells. These issues may be resolved by reconstituting IL-2R beta-/-mice with A-and H-deleted forms of the receptor chain and analyzing the effect on lymphocyte development and function in vivo. In addition to the redundant nature of the A and H regions, there remains a large number of biochemical activities mediated by the IL-2R for which no clear physiological role has been identified. Therefore, the circumstances are ripe for discovering new connections between molecular signaling events activated by the IL-2R and the regulation of immune physiology. Translating biochemical studies of Il-2R function into an understanding of how these signals regulate the immune system has been facilitated by the identification of natural mutations in IL-2R components in humans with immunodeficiency and by the generation of mice with targeted mutations in these gen

Recent advances in the understanding of interleukin-2 signal transduction

Interleukin-2 is one of the critical cytokines that control the proliferation and differentiation of cells of the immune system. The present article briefly reviews the current and recently established knowledge on the intracellular signaling events that convert the initial interaction of IL-2 with its receptor into pathways leading to the various biological functions. A first step in IL-2 signaling is the activation of several protein tyrosine kinases that phosphorylate a large array of intracellular substrates including the receptor complex. Phosphorylated tyrosine residues within the receptor then serve as docking sites for multimolecular signaling complexes that initiate three major pathways: the Jak-STAT pathway controlling gene transcription, the Ras-MAPK pathway leading to cell proliferation and gene transcription as well, and the PI3-kinase pathway involved in antiapoptotic signaling and organization of the cytoskeleton. Finally, other recently identified and presumably important tyrosine kinase substrates, whose significance is not yet fully understood, are described.

Transcription Factor E2F Controls the Reversible γδ T Cell Growth Arrest Mediated Through WC1

IL-2-stimulated expansion of T cells requires continued and sequential passage of the dividing cells through a major cell cycle check point in the G1 phase. We have previously shown that a γδ T cell-specific surface receptor, WC1, induces G0/G1 growth arrest, reversible with Con A, in proliferating IL-2-dependent γδ T cells. We now show that this reversible WC1-induced cell cycle arrest is correlated with induction of the cyclin kinase inhibitor p27kip1 and an associated down-regulation in cyclins A, D2, and D3 expression, along with dephosphorylation of pocket proteins p107, p130, and pRb. Together with diminished pocket protein phosphorylation, p107 expression levels are significantly down-regulated in response to WC1 stimulation. This coordinated sequence of signaling events is focused on E2F regulation so that, downstream of the pocket proteins, WC1 stimulation results in a diminished DNA binding activity for free E2F as a consequence of reduced E2F1 expression, whereas E2F4 expression is unaffected. Consistent with this interpretation, overexpression of E2F1 overcomes the growth-arresting effects induced by WC1 stimulation. Finally, in accordance with our previous observations at both the cellular and molecular level, subsequent mitogen stimulation can reverse all the above changes induced by WC1. These results, focused on E2F regulation, therefore provide a first insight into the effects of both positive (mitogen) and negative (anti-WC1) stimuli on cell cycle control in IL-2-dependent γδ T cells.

Interleukin-2 stimulates a late increase in phosphatidic acid production in the absence of phospholipase D activation

The signal transduction pathways involving phospholipid metabolism during T-cell proliferation remain partly undefined. Herein we show that interleukin-2 caused a late (> 12 h) rise in the intracellular phosphatidic acid content of CTLL-2 cells which was a consequence of the activation of the enzyme diacylglycerol kinase. No activation of phospholipase D was observed at similar times. Incubation of the cells with a recognized diacylglycerol kinase a isoform inhibitor, R59499, prior to interleukin-2 stimulation was able to block cell cycle entry, diacyglycerol kinase activation and phosphatidic acid accumulation. In contrast, when R59499 was added 3 h after interleukin-2, few or no observable effects on the above three parameters were noticed. These results suggest that the early signaling employed by IL-2 involving the alpha isoform of diacylglycerol kinase is sufficient to control the late increase in phosphatidic acid and that phosphatidic acid is a mitogenic agent in T-cells.

FRIP, a hematopoietic cell-specific rasGAP-interacting protein phosphorylated in response to cytokine stimulation

The human IL-4 receptor contains a sequence (the 14R motif) centered on Y497 that, when phosphorylated, interacts with phosphotyrosine-binding (PTB) domain proteins. Here, we describe a PTB domain protein, FRIP, that is phosphorylated in response to cytokine stimulation. FRIP is related to the rasGAP-associated protein p62dok and is bound by the N-terminal SH2 domain of rasGAP. The frip gene maps to the hairless (hr) locus on mouse chromosome 14. hr/hr mice exhibit lymphadenopathy, and their lymph node T cells proliferate more vigorously to anti-CD3 with IL-4 or IL-2 stimulation than +/hr T cells. FRIP expression is significantly reduced in T cells from hr/hr mice. FRIP may negatively regulate proliferation by acting as an adapter molecule between rasGAP and receptor complexes.

Signaling from the IL-2 receptor to the nucleus

Interleukin-2 has pleiotropic actions on the immune system and plays a vital role in the modulation of immune responses. Our current understanding of IL-2 signaling has resulted from in vitro studies that have identified the signaling pathways activated by IL-2, including the Jak-STAT pathways, and from in vivo studies that have analyzed mice in which IL-2, each chain of the receptor, as well a number of signaling molecules have been individually targeted by homologous recombination. Moreover, mutations in IL-2Ralpha, gamma(c) and Jak3 have been found in patients with severe combined immunodeficiency. In addition, with the discovery that two components of the receptor, IL-2Rbeta and gamma(c), are shared by other cytokine receptors, we have an enhanced appreciation of the contributions of these molecules towards cytokine specificity, pleiotropy and redundancy.

T cell proliferation in response to interleukins 2 and 7 requires p38MAP kinase activation

Interleukin-2 (IL-2) is a potent T cell mitogen. However, the signaling pathways by which IL-2 mediates its mitogenic effect are not fully understood. One of the members of the mitogen-activated protein kinase (MAPK) family, p42/44MAPK (ERK2/1), is known to be activated by IL-2. We have now investigated the response to IL-2 of two other members of the MAP kinase family, p54MAP kinase (stress-activated protein kinase (SAPK)/Jun-N-terminal kinase (JNK)) and p38MAP kinase (p38/Mpk2/CSBP/RK), which respond primarily to stressful and inflammatory stimuli (e.g. tumor necrosis factor-alpha, IL-1, and lipopolysaccharide). Here we show that IL-2, and another T cell growth factor, IL-7, activate both SAPK/JNK and p38MAP kinase. Furthermore, inhibition of p38MAP kinase activity with a specific pyrinidyl imidazole inhibitor SB203580 that prevents activation of its downstream effector, MAPK-activating protein kinase-2, correlated with suppression of IL-2- and IL-7-driven T cell proliferation. These data indicate that in T cells p38MAP kinase has a role in transducing the mitogenic signal.

Growth arrest of gammadelta T cells induced by monoclonal antibody against WC1 correlates with activation of multiple tyrosine phosphatases and dephosphorylation of MAP kinase erk2

WC1 is a 215-kDa type 1 transmembrane glycoprotein, the expression of which is restricted to gammadelta T lymphocytes. The binding of an anti-WC1 monoclonal antibody (mAb) (SC-29) induces reversible growth arrest in proliferating interleukin (IL)-2-dependent gammadalta T lymphocytes and this study has examined the relevant biochemical mechanisms. WC1 binding activates multiple protein tyrosine phosphatases causing specific tyrosine dephosphorylation in the absence of calcium mobilization. One of the dephosphorylated proteins was identified as the MAP kinase erk2. Another phosphotyrosine protein of 70 kDa, found to coprecipitate with p85 phosphoinositol (PI)3-kinase was either dephosphorylated or uncoupled from the p85 PI 3-kinase immunoprecipitate after WC1 receptor binding by mAb SC-29. The anti-WC1-induced tyrosine dephosphorylation was reversed by stimulation of gammadelta T cells with concanavalin A or anti-CD3 mAb, demonstrating that at the biochemical level, mitogen stimulation is dominant to the growth-arresting effects of anti-WC1. It is therefore proposed that the activation of tyrosine phosphatases by WC1 binding and the resultant dephosphorylation of certain key signaling protein such as erk2 correlates with and may cause the induction of growth arrest in IL-2-dependent gammadelta T cells, without affecting the cells ability to respond to antigen. Possible mechanisms, which include the inhibition of IL-2 signal transduction pathways, are discussed.

The signal transduction pathway of erythropoietin involves three forms of mitogen-activated protein (MAP) kinase in UT7 erythroleukemia cells

The survival and proliferation of the UT-7 human leukemic cell line is strictly dependent on the presence of either interleukin 3, granulocyte-macrophage colony-stimulating factor or erythropoietin. In these cells, erythropoietin stimulation led to the rapid phosphorylation of several proteins including the erythropoietin receptor and proteins with molecular masses around 45 kDa which could be mitogen-activated protein (MAP) kinases. Separation of cytosol from resting or erythropoietin-stimulated UT-7 cells by anion-exchange chromatography revealed two peaks of myelin basic protein kinase activity. The kinase activity of the first peak was independent of erythropoietin treatment of the cells and corresponded to an unidentified 50-kDa kinase, whereas the second peak was only present in erythropoietin-stimulated cells and corresponded to three forms of MAP kinases with molecular masses of 45, 44 and 42 kDa. The three forms were separated by hydrophobic chromatography and were shown to be activated in erythropoietin-stimulated cells. The 44-kDa and 42-kDa forms corresponded to extracellular signal-regulated kinase (ERK)-1 and ERK-2, respectively. Evidence was obtained showing that the 45-kDa form is not a shifted form of ERK-1 but corresponded to a less well defined form of MAP kinase which may be the previously described ERK-4. MAP kinase activation was detected after 1 min erythropoietin stimulation and remained detectable after more than 1 hour. A role for MAP kinase activation in erythropoietin-stimulated cell proliferation was suggested by the simultaneous inhibition of erythropoietin-induced MAP kinase stimulation and cell proliferation. The potential activator of MAP kinase, RAF-1, was hyperphosphorylated in erythropoietin-stimulated cells and its autophosphorylation activity was strongly increased. The protein adaptor Shc was heavily phosphorylated in UT-7 erythropoietin-stimulated cells and associated strongly with a unidentified 145-kDa protein. However, Shc bound poorly to the activated erythropoietin receptor and most Shc proteins were cytosolic in both unstimulated and erythropoietin-stimulated cells. In contrast, Grb2 associated efficiently with the activated erythropoietin receptor and a significant part of Grb2 was associated to a particulate subcellular fraction upon erythropoietin stimulation.

Activation of extracellular signal-regulated protein kinase (ERK/MAP kinase) following CD28 cross-linking: activation in cells lacking p56lck

T lymphocytes require two signals for activation. Recognition of antigen/MHC complexes by the T cell receptor delivers the first signal, while a second signal, delivered by the cell surface receptors CD80 and/or CD86 binding to the T cell surface molecule CD28, has been shown to be effective for the initiation of effective T cell responses. While some of the cytoplasmic effector molecules involved in T cell receptor signaling is known, little is known regarding those involved in the co-stimulation of T cells by CD28. Using the T cell leukemic cell line Jurkat as a model for T cell activation, we demonstrate that cross-linking CD28 using monoclonal antibodies causes tyrosine phosphorylation and activation of MAP kinase/ERK. This activation was rapid, peaking at approximately 5 minutes post CD28 cross-linking, and transient. Activation of MAP kinase/ERK occurred 3 fold less efficiently in a Jurkat line lacking functional p56lck (JCAM.1), and was almost undetectable in a line lacking CD45 (J45.01). These results suggest that CD28 cross-linking can activate intracellular signaling pathways via several different tyrosine kinases. Thus CD28 signaling can activate src family kinases lck and fyn, as well as the Tec family kinase emt/itk. Activation of any one or a combination of these tyrosine kinases may be sufficient for the activation of MAPK following CD28 cross-linking. Activation of MAPK has been shown to cause activation of AP-1 and other transcription factors via serine and/or threonine phosphorylation.(ABSTRACT TRUNCATED AT 250 WORDS)

Apoptosis overview emphasizing the role of oxidative stress, DNA damage and signal-transduction pathways

Apoptosis (programmed cell death) is a central protective response to excess oxidative damage (especially DNA damage), and is also essential to embryogenesis, morphogenesis and normal immune function. An understanding of the cellular events leading to apoptosis is important for the design of new chemotherapeutic agents directed against the types of leukemias and lymphomas that are resistant to currently used chemotherapeutic protocols. We present here a review of the characteristic features of apoptosis, the cell types and situations in which it occurs, the types of oxidative stress that induce apoptosis, the signal-transduction pathways that either induce or prevent apoptosis, the biologic significance of apoptosis, the role of apoptosis in cancer, and an evaluation of the methodologies used to identify apoptotic cells. Two accompanying articles, demonstrating classic apoptosis and non-classic apoptosis in the same Epstein-Barr virus-transformed lymphoid cell line, are used to illustrate the value of employing multiple criteria to determine the type of cell death occurring in a given experimental system. Aspects of apoptosis and programmed cell death that are not covered in this review include histochemistry, details of cell deletion processes in the sculpting of tissues and organs in embryogenesis and morphogenesis, and the specific pathways leading to apoptosis in specific cell types. The readers should refer to the excellent books and reviews on the morphology, biochemistry and molecular biology of apoptosis already published on these topics. Emphasis is placed, in this review, on a proposed common pathway of apoptosis that may be relevant to all cell types.

IL-2 signaling: recruitment and activation of multiple protein tyrosine kinases by the components of the IL-2 receptor

Cytokine receptors transduce signals to the cell interior upon binding of their cognate ligands, eventually leading to cellular responses such as cellular proliferation, differentiation and other effector functions. Most of the cytokine receptors, including the interleukin (IL)-2 receptor, consist of two or more distinct subunits, yet none possess any known catalytic activity such as protein tyrosine kinase activity. Significant advances have recently been made in identifying the multiple signaling molecules, including protein tyrosine kinases, that couple with the cytoplasmic regions of the IL-2 receptor, although their exact roles in cytokine signaling are still not fully understood. Another important development in the understanding of IL-2 signaling is the identification of the target genes, including nuclear proto-oncogenes. Furthermore, structure-function analyses of the components of the IL-2 receptor have enabled the dissection of multiple intracellular signaling pathways that lead to the induction of the respective target genes.

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

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

Regulation of GATA-2 phosphorylation by mitogen-activated protein kinase and interleukin-3

GATA-2 is a member of a family of transcription factors which bind a common DNA sequence motif (WGA-TAR) through an evolutionarily conserved zinc finger domain. An essential role for GATA-2 in the development of hematopoietic stem cells has recently been shown in gene targeting experiments in mice. Here we show that GATA-2 exists in hematopoietic progenitor cells as a phosphoprotein. Stimulation of progenitors with interleukin-3 (IL-3) results in enhanced phosphorylation of GATA-2 which occurs within 5 min. IL-3 is known to signal in part through mitogen-activated protein (MAP) kinase, and evidence for MAP kinase signaling in the control of GATA-2 phosphorylation was obtained by genetically manipulating the MAP kinase pathway in COS cells using either constitutively activating or interfering mutants of MAP kinase kinase. Furthermore, using an interfering mutant of MAP kinase kinase, we directly demonstrated a critical role for the MAP kinase pathway in the IL-3-dependent phosphorylation of GATA-2 in hematopoietic progenitor cells. Finally, in vitro phosphorylation experiments using recombinant GATA-2 raise the possibility that MAP kinase itself may phosphorylate GATA-2. Our results provide evidence for phosphorylation via the MAP kinase pathway constituting a cytoplasmic link between GATA-2 and growth factor receptors and are consistent with the hypothesis that GATA-2 is involved in the growth factor responsiveness and proliferation control of hematopoietic progenitor cells.

Raf-1 and ERK2 kinases are required for phorbol 12,13-dibutyrate-stimulated proliferation of rat lymphoblasts. ERK2 activation precedes Raf-1 hyperphosphorylation

Rat lymphoblasts are arrested in the G1 phase of the cell cycle and can be promoted to proceed up to the S phase, when they are stimulated by phorbol ester. In this work, we have studied some details of the phorbol 12,13-dibutyrate (PBu2)-stimulated proliferation. We show that in response to PBu2 at least four different protein kinase C (PKC) isoforms translocate to the membrane. A specific PKC zeta antibody recognizes two bands of 75 and 82 kDa. These two activities are separated using a Mono Q chromatography and we show that p75 is the classical PKC zeta isoform, while p82 might be a related isoform which is PBu2 sensitive. Our data show that there is a correlation between the ability of PBu2 to promote mitogenesis and to activate ERK2 kinase, suggesting that ERK2 kinase might be the limiting step of the process. We also show that ERK kinase activation precedes Raf-1 kinase hyperphosphorylation, suggesting that Raf-1 kinase activation is not required for ERK kinase activation. This idea was checked using a Raf-1 kinase antisense (AS) oligonucleotide. The results obtained with the Raf-1 AS oligonucleotide indicate that this serine/threonine kinase is dispensable for ERK kinase activation, but needed for the PBu2 mitogenic signaling even as late as 7 h after the delivery of the signal.

Signal transduction of interleukin 2 in human natural killer cells: involvement of the p56lck tyrosine kinase

Despite numerous reports, the role of the protein tyrosine kinase p56lck in IL-2 signal transduction has remained controversial. We show here, using IL-2-dependent human natural killer cell lines, that p56lck is regulated by IL-2 in two different ways: (1) IL-2 induces a rapid increase of p56lck kinase activity as assessed in vitro; and (2) following IL-2 stimulation, p56lck undergoes phosphorylation on serine residues that is reflected by a modification of its electrophoretic mobility in SDS-PAGE. Furthermore, dose response experiments, and blocking studies performed with anti-IL-2R alpha antibodies, indicated that binding of IL-2 to the IL-2R beta chain was sufficient to produce these modifications of p56lck. In contrast, activation of the CD2 pathway stimulated the kinase activity of p56lck, but did not induce a significant shift in NK cells, as opposed to T lymphocytes. Western blot analyses, and immunoprecipitations of cell lysates from 32P-preloaded NK cells demonstrated that seven major proteins are tyrosine phosphorylated in response to IL-2. These phosphoproteins, with apparent molecular weights of 190, 150, 120, 110, 85, 65 and 56, which may not all be p56lck substrates, undergo phosphorylation and dephosphorylation with different kinetics. Furthermore, pp120 was identified as rasGAP, by Western blot and immunoprecipitation experiments. rasGAP and some of its co-precipitating molecules become phosphorylated in response to IL-2, presumably by p56lck, which would thus provide a link between IL-2R and downstream events critical for NK cell proliferation and function.

Growth factors as survival factors: regulation of apoptosis

Apoptosis is now widely recognized as a common form of cell death and represents a mechanism of cell clearance in many physiological situations where deletion of cells is required. Peptide growth factors, initially characterised as stimulators of cell proliferation, have now been shown to inhibit death in many cell types. Deprivation of growth factors leads to the induction of apoptosis, i.e. condensation of chromatin and degradation in oligonucleosome-sized fragments, formation of plasma and nuclear membrane blebs and cell fragmentation into apoptotic bodies which can be taken up by neighbouring cells. Here we discuss the mechanism(s) by which growth factors may inhibit apoptosis.