Interleukin 2- and polyomavirus middle T antigen-induced modification of phosphatidylinositol 3-kinase activity in activated T lymphocytes

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.

Molecular mechanisms of the diabetogenic effects of arsenic: inhibition of insulin signaling by arsenite and methylarsonous acid

BACKGROUND

Increased prevalences of diabetes mellitus have been reported among individuals chronically exposed to inorganic arsenic (iAs). However, the mechanisms underlying the diabetogenic effects of iAs have not been characterized. We have previously shown that trivalent metabolites of iAs, arsenite (iAs(III)) and methylarsonous acid (MAs(III)) inhibit insulin-stimulated glucose uptake (ISGU) in 3T3-L1 adipocytes by suppressing the insulin-dependent phosphorylation of protein kinase B (PKB/Akt).

OBJECTIVES

Our goal was to identify the molecular mechanisms responsible for the suppression of PKB/Akt phosphorylation by iAs(III) and MAs(III).

METHODS

The effects of iAs(III) and MAs(III) on components of the insulin-activated signal transduction pathway that regulate PKB/Akt phosphorylation were examined in 3T3-L1 adipocytes.

RESULTS

Subtoxic concentrations of iAs(III) or MAs(III) had little or no effect on the activity of phosphatidylinositol 3-kinase (PI-3K), which synthesizes phosphatidylinositol-3,4,5-triphosphate (PIP(3)), or on phosphorylation of PTEN (phosphatase and tensin homolog deleted on chromosome ten), a PIP(3) phosphatase. Neither iAs(III) nor MAs(III) interfered with the phosphorylation of 3-phosphoinositide-dependent kinase-1 (PDK-1) located downstream from PI-3K. However, PDK-1 activity was inhibited by both iAs(III) and MAs(III). Consistent with these findings, PDK-1-catalyzed phosphorylation of PKB/Akt(Thr308) and PKB/Akt activity were suppressed in exposed cells. In addition, PKB/Akt(Ser473) phosphorylation, which is catalyzed by a putative PDK-2, was also suppressed. Notably, expression of constitutively active PKB/Akt restored the normal ISGU pattern in adipocytes treated with either iAs(III) or MAs(III).

CONCLUSIONS

These results suggest that inhibition of the PDK-1/PKB/Akt-mediated transduction step is the key mechanism for the inhibition of ISGU in adipocytes exposed to iAs(III) or MAs(III), and possibly for impaired glucose tolerance associated with human exposures to iAs.

CD28 regulates the translation of Bcl-xL via the phosphatidylinositol 3-kinase/mammalian target of rapamycin pathway

In concert with the TCR, CD28 promotes T cell survival by regulating the expression of the antiapoptotic protein Bcl-x(L). The mechanism by which CD28 mediates the induction of Bcl-x(L) remains unknown. We show that although signaling through the TCR is sufficient to stimulate transcription of Bcl-x(L) mRNA, CD28, by activating PI3K and mammalian target of rapamycin, provides a critical signal that regulates the translation of Bcl-x(L) transcripts. We observe that CD28 induced 4E-binding protein-1 phosphorylation, an inhibitor of the translational machinery, and that CD28 costimulation directly augmented the translation of a Bcl-x(L) 5'-untranslated region reporter construct. Lastly, costimulation by CD28 shifted the distribution of Bcl-x(L) mRNA transcripts from the pretranslation complex to the translationally active polyribosomes. These results demonstrate that CD28 relieves the translational inhibition of Bcl-x(L) in a PI3K/mammalian target of rapamycin-dependent manner.

Impaired TCR-mediated induction of Ki67 by naive CD4+ T cells is only occasionally corrected by exogenous IL-2 in HIV-1 infection

Perturbations in naive T cell homeostasis and function may play a major role in the immunodeficiency that accompanies HIV infection. By examining naive CD4(+) T cell function on a single cell basis, we provide evidence that these cells have significant qualitative defects in HIV disease. Ki67, a molecule expressed during cell cycle progression, is induced less efficiently among naive CD4(+) T cells from HIV-infected individuals following activation with anti-TCR Ab. The impairment in Ki67 expression is evident even when a separate function, CD62L down-modulation, is within normal ranges. Moreover, the defects in Ki67 induction are only sometimes corrected by the addition of rIL-2 to cell cultures. An initial assessment of IL-2 unresponsiveness in cells from selected HIV-infected individuals suggests that the defect is not a consequence of impaired IL-2R expression or IL-2R signaling capability. Qualitative defects in naive T cells that cannot be routinely corrected by IL-2 have significant implications for disease pathogenesis and for strategies using IL-2 as a vaccine adjuvant in HIV disease.

The p38 MAPK pathway is involved in the IL-2 induction of TNF-beta gene via the EBS element

As an important cytokine of the immune system, interleukin-2 (IL-2) can induce the expression of various genes, one of which is the tumor necrosis factor-beta (TNF-beta). However, the induction mechanism of TNF-beta remains to be fully explored. We have previously shown JAK-STAT pathway mediates TNF-beta gene induction upon IL-2 stimulation through an upstream -200GAS element. In this study, we further demonstrated that there is another essential -130EBS element in TNF-beta gene promoter region. Using IL-2-dependent cell line BAF/BO3beta, we found that this -130EBS element can form a specific complex with nuclear protein, which contained a novel ETS transcription factor. Furthermore, using kinase inhibitors, we revealed that p38 MAP kinase is involved in the formation of -130EBS-protein complex and the subsequent transcriptional activation of TNF-beta gene in response to IL-2 stimulation. Taken together, our results suggested that the complicated IL-2 induction of TNF-beta gene expression requires not only the activation of JAK-STAT pathway on the -200GAS element, but also the cooperation of another signal pathway on the -130EBS element.

Deletion of the acidic-rich domain of the IL-2Rbeta chain increases receptor-associated PI3K activity

Interleukin-2 (IL-2) regulates the proliferation and homeostasis of lymphocytes through the coordinated activation of distinct signaling pathways. Deletion of the acidic-rich domain of the IL-2 receptor beta chain (IL-2Rbeta) prevents association of Src tyrosine kinases to the receptor, as well as IL-2-induced Akt activation. Cells bearing this deletion (BafbetaDeltaA) maintain full proliferation in response to IL-2 both in vivo and in vitro, suggesting that those pathways are dispensable for this important function of IL-2. In this study, we re-examined phosphatidylinositol-3 kinase (PI3K) activation in BafbetaDeltaA cells and found that, in BaF/3 IL-2RbetaDeltaA cells, deletion of the acidic domain induced constitutive activation of the receptor-associated PI3K activity. This, in turn, was responsible for the higher basal Akt activity observed in cells expressing this deletion. Based on these data, and since pharmacological abrogation of PI3K activity prevented IL-2-driven cell proliferation of BafbetaDeltaA cells, we conclude that the PI3K/Akt pathway is still functionally relevant in cells bearing this mutation. Moreover, we show that the PI3K-induced signals are, at least in part, responsible for c-myc expression. In conclusion, we have used this model to better identify those signals that are integral components of the molecular mechanisms responsible for IL-2-regulated cell proliferation.

p59fyn (Fyn) promotes the survival of anergic CD4-CD8- alpha beta TCR+ cells but negatively regulates their proliferative response to antigen stimulation

T cell anergy is characterized by alterations in TCR signaling that may play a role in controlling the unresponsiveness of the anergic cell. We have addressed questions regarding the importance of the Src kinase p59(fyn) (Fyn) in this process by using Fyn null mice. We demonstrate that a mature population of CD4(-)CD8(-) alphabeta TCR(+) anergic T cells lacking Fyn have a substantial recovery of their proliferation defect in response to Ag stimulation. This recovery cannot be explained by ameliorated production of IL-2, and the improved proliferation correlates with an enhanced ability of the Fyn(-/-) anergic T cells to up-regulate the high affinity IL-2 receptor. We also observe that anergic CD4(-)CD8(-) alphabeta TCR(+) T cells have a heightened survival ability that is partially dependent on the elevated levels of Fyn and IL-2 receptor beta-chain expressed by these cells. The enhanced survival correlates with an increased capacity of the anergic cells to respond to IL-15. We conclude that Fyn plays an important role in aspects of T cell anergy pertaining to TCR signaling and to cell survival.

Changes in the balance between mitogenic and antimitogenic lipid second messengers during proliferation, cell arrest, and apoptosis in T-lymphocytes

Control of lymphocyte cell survival and proliferation is critical for both the immune response and for the prevention of autoimmune and infectious diseases. The actions of interleukin-2, the major T-cell regulatory cytokine, are mediated by the complex network of divergent signalling pathways controlled by its high-affinity receptor. Various studies have indicated that the generation of certain lipid second messengers is an important mechanism in the control of proliferation and cell death. We have examined the relationship between diacylglycerol and ceramide and the levels of the lipids phosphatidylcholine and sphingomyelin, their potential precursors, in the human T-cell line Kit 225 cultured in three distinct conditions to favor apoptosis, cell arrest, and proliferation. Our data show that, in proliferating cells, the ratios of diacylglycerol/ceramide and phosphatidylcholine/sphingomyelin are higher than those found in arrested cells and increase with time in culture. These ratios are rapidly reversed in apoptotic cells. Further experiments reveal that de novo synthesis of both diacylglycerol and phosphatidylcholine is greatest in proliferating cells, whereas sphingomyelin synthase activity is increased in cells undergoing apoptosis. In summary, our results demonstrate for the first time that the ratio of mitogenic/antimitogenic lipids changes dramatically during T-cell proliferation and cell death. These results indicate that lipid second messengers and the enzymes that are responsible for their generation may provide targets for novel therapeutic interventions in the clinical management of immunosuppression and autoimmune disease.

The role of Stat5a and Stat5b in signaling by IL-2 family cytokines

The activation of Stat5 proteins (Stat5a and Stat5b) is one of the earliest signaling events mediated by IL-2 family cytokines, allowing the rapid delivery of signals from the membrane to the nucleus. Among STAT family proteins, Stat5a and Stat5b are the two most closely related STAT proteins. Together with other transcription factors and co-factors, they regulate the expression of the target genes in a cytokine-specific fashion. In addition to their activation by cytokines, activities of Stat5a and Stat5b, as well as other STAT proteins, are negatively controlled by CIS/SOCS/SSI family proteins. The outcome of Stat5 activation in regulating expression of target genes varies, depending upon the complexity of the promoter region of target genes and the other signaling pathways that are activated by each cytokine as well. Here, we mainly focus on the IL2-/IL-2 receptor system, as it is one of the best-studied systems that depend on Stat5-mediated signals. We will summarize what we have learned about the molecular mechanisms of how Stat5 is activated by IL-2 family cytokines from in vitro biochemical studies as well as the role that is played by Stat5 in each of the cytokine signaling pathways from in vivo gene-targeting analyses. Oncogene (2000).

Cytokine receptor signaling pathways

Cytokines represent a diverse group of molecules that transmit intercellular signals. These signals may either be autocrine (where the same cell both produces the cytokine and responds to it) or paracrine (where the cytokine is made by one cell and acts on another). Both these situations can occur simultaneously. Cytokines use multiple signaling pathways. This review will focus on signaling by type I cytokines and in particular on signaling by the IL-2 family of cytokines, as an illustrative example. The major signaling pathway that will be discussed is the Jak-STAT pathway, although other pathways will also be reviewed. The Jak-STAT pathway is a very rapid cytosol-to-nuclear signaling pathway that underscores how quickly extracellular signals can be transmitted to the nucleus. Aspects related to cytokine redundancy, pleiotropy, and specificity will be discussed.

The identification of phosphatidylinositol 3,5-bisphosphate in T-lymphocytes and its regulation by interleukin-2

In recent times 3-phosphoinositides have emerged as important regulators of cell metabolism, survival, and proliferation. During the last year, the phospholipid phosphatidylinositol 3, 5-bisphosphate (PtdIns3,5P2) was identified in yeast, fibroblasts, SV40-transformed kidney (COS-7) cells, and platelets. The discovery of this novel phospholipid has increased the complexity of the metabolism relating to the generation of biologically active inositol-containing lipids. We describe here the identification of PtdIns3,5P2 in the CTLL-2 mouse T-lymphocyte cell line using two in vivo radiolabeling protocols. Treatment of the cells with UV radiation led to an increase in the cellular content of PtdIns3,5P2. In contrast, preincubation of the cells with wortmannin or treatment with hypertonic medium (high concentration sorbitol) led to the opposite effect. Herein we demonstrate that interleukin-2 (IL-2), the growth factor required for CTLL-2 cell proliferation, was able to increase the level of PtdIns3,5P2 with similar kinetics to that of the formation of phosphatidylinositol 3,4-bisphosphate (PtdIns3, 4P2). An increase in this novel 3-phosphorylated lipid in response to IL-2 seems to be a general property of this cytokine because a similar result was obtained when the pre-B cell line BaF/3 expressing the high affinity IL-2 receptor was used. Using a constitutively active regulatory subunit of type I phosphatidylinositol 3-kinase and cells expressing a deletion of the serine-rich domain of the IL-2 receptor beta chain, which is required for IL-2-stimulated type I phosphatidylinositol 3-kinase activation, we demonstrate that IL-2-induced generation of PtdIns3, 5P2 is related to the activation of this enzyme. The results show for the first time the identification of PtdIns3,5P2 in both T- and B-lymphocytes and indicate its positive regulation by the mitogen IL-2.

Critical role of the membrane-proximal, proline-rich motif of the interleukin-2 receptor gammac chain in the Jak3-independent signal transduction

BACKGROUND

The interleukin-2 receptor (IL-2R) consists of three subunits, the IL-2Ralpha, IL-2Rbetac, and IL-2Rgammac chains. The essential role of the IL-2Rgammac cytoplasmic domain, consisting of 86 amino acids, in signal transmission has been well documented. Particularly, the carboxyl ter-minal region containing 48 amino acids. is essential for the association with and activation of the Jak3 protein tyrosine kinase. On the other hand, little is known about the role of the rest of the IL-2Rgammac cytoplasmic region consisting of the membrane-proximal 38 amino acids.

RESULTS

We show that a truncated mutant form of IL-2Rgammac which lacks the membrane-distal 48 amino acids is still capable of inducing the activation of Jak1 and Stat3/Stat5 in the absence of Jak3 activation. This membrane-proximal region can also mediate the IL-2-induced tyrosine phosphorylation of the p85 subunit of phosphatidylinositol-3-kinase (PI3K). Furthermore, these signalling events are completely abrogated when mutations are introduced into the proline-rich motif in this region.

CONCLUSIONS

In this study, we identified a Jak3-independent signalling pathway(s) from the membrane-proximal region of IL-2Rgammac. Our results indicate that the proline-rich motif in this region plays a critical role in this signalling pathway(s). The present study may provide further insight into the mechanism of cellular responses mediated by IL-2 and other cytokines which utilize the IL-2Rgammac for their signal transmission.

An IL-2 receptor beta subdomain that controls Bcl-X(L) expression and cell survival

IL-2 binding to its high-affinity receptor regulates signaling events that control both lymphocyte cell survival and cell cycle progression. Although many studies have examined the mechanisms by which IL-2 regulates cell growth, few studies have dissected the pathways involved in promoting cell survival or the coupling of these pathways to the receptor. In the present study, using the pre-B cell line Baf-B03 transfected with a truncated form of the IL-2 receptor (IL-2R) beta chain, we demonstrate that IL-2-dependent cell survival requires only the N-terminal 350 amino acids of the IL-2Rbeta chain. IL-2-dependent survival of cells expressing the truncated receptor correlates with increases in receptor-associated phosphatidylinositol 3-kinase (PI3K) activity and expression of Bcl-X(L), but not with changes in c-Myc expression or proliferation. Inhibition of the PI3K pathway in these cells, but not in cells expressing the wild-type receptor, has a marked effect on the capacity of IL-2 to prevent cell death and diminishes the Bcl-X(L) response. The requirement for IL-2-induced PI3K activity in suppressing the onset of apoptotic cell death is discussed.

Role of interleukin (IL)-2 receptor beta-chain subdomains and Shc in p38 mitogen-activated protein (MAP) kinase and p54 MAP kinase (stress-activated protein Kinase/c-Jun N-terminal kinase) activation. IL-2-driven proliferation is independent of p38 and p54 MAP kinase activation

We have shown recently that interleukin (IL)-2 activates the mitogen-activated protein (MAP) kinase family members p38 (HOG1/stress-activated protein kinase II) and p54 (c-Jun N-terminal kinase/stress-activated protein kinase I). Furthermore, the p38 MAP kinase inhibitor SB203580 inhibited IL-2-driven T cell proliferation, suggesting that p38 MAP kinase might be involved in mediating proliferative signals. In this study, using transfected BA/F3 cell lines, it is shown that both the acidic domain and the membrane-proximal serine-rich region of the IL-2Rbeta chain are required for p38 and p54 MAP kinase activation and that, as for p42/44 MAP kinase, this activation requires the Tyr338 residue of the acidic domain, the binding site for Shc. It is well established that the acidic domain of the IL-2Rbeta chain is dispensable for IL-2-driven proliferation, and thus our observations suggest that neither p38 nor p54 MAP kinase activation is required for IL-2-driven proliferation of BA/F3 cells. In addition, the tetravalent guanylhydrazone inhibitor of proinflammatory cytokine production, CNI-1493, can block the activation of p54 and p38 MAP kinases by IL-2 but has no effect on IL-2-driven proliferation of BA/F3 cells, activated primary T cells, or a cytotoxic T cell line. Furthermore, our observations provide evidence for the existence of an additional, unknown target of the p38 MAP kinase inhibitor SB203580, the activation of which is essential for mitogenic signaling by IL-2.

Molecular mechanism of basic calcium phosphate crystal-induced activation of human fibroblasts. Role of nuclear factor kappab, activator protein 1, and protein kinase c

Synovial fluid basic calcium phosphate (BCP) crystals are markers of severe joint degeneration in osteoarthritis. BCP crystals cause mitogenesis of articular cells and stimulate matrix metalloprotease production, thus promoting degradation of articular tissues. Previous work suggested that BCP crystal-induced cell activation required intracellular crystal dissolution, induction of proto-oncogene expression, and activation of signal transduction pathways involving protein kinase C and mitogen-activated protein kinases. Here we further elucidate the mechanisms of BCP crystal-induced cell activation as BCP crystals activate transcription factors nuclear factor kappaB and activator protein 1 in human fibroblasts. We confirm the role of protein kinase C in BCP crystal-induced mitogenesis in human fibroblasts. In contrast, we demonstrate that BCP crystals do not activate signal transduction pathways involving protein tyrosine kinases or phosphatidylinositol 3-kinase. These data further define the mechanism of cell activation by BCP crystals and confirm its selectivity, an observation that may have therapeutic implications.

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

A new tyrosine-phosphorylated 97-kDa adaptor protein mediates interleukin-2-induced association of SHP-2 with p85-phosphatidylinositol 3-kinase in human T lymphocytes

Interleukin (IL)-2 is a major cytokine that controls differentiation and proliferation of T lymphocytes. In this report we characterize an as yet unidentified 97-kDa protein that is a major tyrosine kinase substrate in IL-2-stimulated cells. pp97 was found to associate with the p85.p110 phosphatidylinositol 3-kinase complex, the Src homology 2 (SH2) domain-containing tyrosine phosphatase SHP-2, and the adaptor molecules CrkL and Grb2. We demonstrate that these interactions are directly mediated through the SH2 domains of CrkL, p85, and SHP-2 and through the SH3 domains of Grb2. pp97 was found to mediate the IL-2-induced interaction between p85 and both a phosphorylated and a non-phosphorylated form of SHP-2. In this study we show that pp97 behaves as a docking protein and associates with at least CrkL, p85, and SHP-2 in the same multimolecular complex. We thus characterized pp97 as a new tyrosine kinase substrate in human T lymphocytes which might play a central role in the regulation of several pathways activated by IL-2.

Polyomavirus middle T antigen as a probe for T cell antigen receptor-coupled signaling pathways

Stimulation of the T cell antigen receptor (TCR) triggers a complex series of signaling events that culminate in T cell activation and proliferation. The complex structure of the TCR has hindered efforts to link specific signaling events induced by TCR cross-linkage to downstream activation responses, such as interleukin-2 (IL-2) gene transcription. Previous studies have shown that the polyomavirus-derived oncoprotein, middle T antigen (mT), transforms rodent fibroblasts by interacting with and activating several cytoplasmic signaling proteins (Src kinases, phospholipase C (PLC)-gamma1, Shc, and phosphoinositide 3-kinase (PI3-K) implicated in cell growth control. In this study, we demonstrate that expression of mT activates Jurkat T cells, as measured by increases in IL-2 promoter- and NFAT (nuclear factor of activated T cells)-dependent reporter gene transcription. The transcriptional response provoked by mT was blocked by the immunosuppressive drug FK506, a potent inhibitor of TCR-mediated IL-2 gene expression. Mutations that disrupted the binding of mT to Src kinases or PLC-gamma1 abrogated the ability of mT to deliver the signals needed for IL-2 promoter activation. In contrast, a mT mutant that failed to bind PI3-K induced a markedly elevated transcriptional response in Jurkat cells, whereas mutation of the Shc binding site in mT had little effect on the transactivating potential of this viral oncoprotein. Additional studies demonstrated that the association of mT with PLC-gamma1 was necessary and sufficient to activate both Ca2+- and Ras-dependent signaling cascades in Jurkat cells. These results indicate that PLC-gamma1 activation plays pivotal and pleiotropic roles in the stimulation of IL-2 gene expression, whereas activation of PI3-K negatively modulates this response in Jurkat T cells.

Interleukin-2 stimulation induces tyrosine phosphorylation of p120-Cbl and CrkL and formation of multimolecular signaling complexes in T lymphocytes and natural killer cells

Interleukin (IL)-2, a major growth and differentiation factor for T lymphocytes, was found to induce tyrosine phosphorylation of the proto-oncogene products p120-Cbl and CrkL in IL-2-dependent cell lines. We established that, in unstimulated lymphocytes, the Src homology 2 (SH2) and SH3 domain-containing protein Grb2 and the p85 subunit of phosphatidylinositol 3-kinase, associate constitutively with Cbl via their SH3 domains. Furthermore, IL-2 stimulation increased the level of interaction of phosphorylated Cbl with the p85 SH2 domains, and we provide evidence that the preformed Cbl-Grb2 complex recruits the phosphorylated p52 Shc adaptor protein. In addition, we demonstrate that the SH2-SH3-SH3 adaptor protein CrkL is tyrosine-phosphorylated in an IL-2-dependent manner and, via its SH2 domain, associates with a large proportion of phosphorylated Cbl. We also show that p85 is preassociated with the CrkL SH3 domain. Furthermore, the association of CrkL and p85 is increased after IL-2 treatment by a mechanism involving intermediary tyrosine-phosphorylated proteins that remain to be identified. Our results show that CrkL associates independently with Cbl or p85 and suggest that it also participates in larger complexes containing Cbl and p85. Although the precise roles of Cbl and CrkL remain to be elucidated, their tyrosine phosphorylation, in addition to the multiple protein interactions described here, strongly suggest that Cbl and CrkL may play pivotal roles in the early steps of IL-2 signal transduction.

The p85 and p110 Subunits of Phosphatidylinositol 3-Kinase-α Are Substrates, In Vitro, for a Constitutively Associated Protein Tyrosine Kinase in Platelets

Phosphatidylinositol 3-kinase (PI3K) is a heterodimer lipid kinase consisting of an 85-kD subunit bound to a 110-kD catalytic subunit that also possesses intrinsic, Mn2+-dependent protein serine kinase activity capable of phosphorylating the 85-kD subunit. Here, we examine the Mn2+-dependent protein kinase activity of PI3Kα immunoprecipitated from normal resting or thrombin-stimulated platelets, and characterize p85/p110 phosphorylation, in vitro. Phosphoamino acid analysis of phosphorylated PI3Kα showed p85 and p110 were phosphorylated on serine, but in contrast to previous results, were also phosphorylated on threonine and tyrosine. Wortmannin and LY294002 inhibited p85 phosphorylation; however, p110 phosphorylation was also inhibited suggesting p110 autophosphorylation on serine/threonine. The protein tyrosine kinase inhibitor, erbstatin analog, partially inhibited p85 and p110 phosphorylation but did not appear to affect PI3K lipid kinase activity. The in vitro phosphorylation of p85α or p110α derived from thrombin-stimulated platelets was no different than that of resting platelets, but we confirm that in thrombin receptor-stimulated platelets enhanced levels of p85α and PI3K lipid kinase activity were recovered in antiphosphotyrosine antibody immunoprecipitates. These results suggest PI3Kα can autophosphorylate on serine and threonine, and both p85α and p110α are substrates for a constitutively-associated protein tyrosine kinase in platelets.

The p85 and p110 Subunits of Phosphatidylinositol 3-Kinase-α Are Substrates, In Vitro, for a Constitutively Associated Protein Tyrosine Kinase in Platelets

Phosphatidylinositol 3-kinase (PI3K) is a heterodimer lipid kinase consisting of an 85-kD subunit bound to a 110-kD catalytic subunit that also possesses intrinsic, Mn2+-dependent protein serine kinase activity capable of phosphorylating the 85-kD subunit. Here, we examine the Mn2+-dependent protein kinase activity of PI3Kα immunoprecipitated from normal resting or thrombin-stimulated platelets, and characterize p85/p110 phosphorylation, in vitro. Phosphoamino acid analysis of phosphorylated PI3Kα showed p85 and p110 were phosphorylated on serine, but in contrast to previous results, were also phosphorylated on threonine and tyrosine. Wortmannin and LY294002 inhibited p85 phosphorylation; however, p110 phosphorylation was also inhibited suggesting p110 autophosphorylation on serine/threonine. The protein tyrosine kinase inhibitor, erbstatin analog, partially inhibited p85 and p110 phosphorylation but did not appear to affect PI3K lipid kinase activity. The in vitro phosphorylation of p85α or p110α derived from thrombin-stimulated platelets was no different than that of resting platelets, but we confirm that in thrombin receptor-stimulated platelets enhanced levels of p85α and PI3K lipid kinase activity were recovered in antiphosphotyrosine antibody immunoprecipitates. These results suggest PI3Kα can autophosphorylate on serine and threonine, and both p85α and p110α are substrates for a constitutively-associated protein tyrosine kinase in platelets.

Cellular functions regulated by Src family kinases

Src family protein tyrosine kinases are activated following engagement of many different classes of cellular receptors and participate in signaling pathways that control a diverse spectrum of receptor-induced biological activities. While several of these kinases have evolved to play distinct roles in specific receptor pathways, there is considerable redundancy in the functions of these kinases, both with respect to the receptor pathways that activate these kinases and the downstream effectors that mediate their biological activities. This chapter reviews the evidence implicating Src family kinases in specific receptor pathways and describes the mechanisms leading to their activation, the targets that interact with these kinases, and the biological events that they regulate.

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.

STAT3 is a serine kinase target in T lymphocytes. Interleukin 2 and T cell antigen receptor signals converge upon serine 727

Interleukin 2 (IL-2) induces tyrosine phosphorylation of STATs 3 and 5 (signal transducer and activator of transcription). We now show that IL-2 regulation of STAT3 proteins in T cells is a complex response involving activation of two forms of STAT3: 90-kDa STAT3alpha and an 83-kDa carboxyl-terminal truncated STAT3beta. The phosphorylation of STAT proteins on serine residues is also required for competent STAT transcription. A critical serine phosphorylation site in STAT3alpha is at position 727. In this study we have produced an antisera specific for STAT3alpha proteins phosphorylated on serine 727 and used this to monitor the phosphorylation of this residue during T lymphocyte activation. Our results show that phosphorylation of STAT3alpha on serine 727 is not constitutive in quiescent T cells but can be induced by the cytokine IL-2. Interestingly, triggering of the T cell antigen receptor complex or activation of protein kinase C with phorbol esters also induces phosphorylation of serine 727 but without simultaneously inducing STAT3 tyrosine phosphorylation or DNA binding. Hence, the present results show that STAT3 serine phosphorylation can be regulated independently of the tyrosine phosphorylation of this molecule. IL-2 and T cell antigen receptor complex induction of STAT3alpha serine 727 phosphorylation is dependent on the activity of the MEK/ERK pathway. Previous studies have identified H-7-sensitive kinase pathways that regulate STAT3 DNA binding. We show that H-7-sensitive pathways regulate STAT3 DNA binding in T cells. Nevertheless, we show that H-7-sensitive kinases do not regulate STAT3 tyrosine phosphorylation or phosphorylation of serine 727. These results thus show that STAT3 proteins are targets for multiple kinase pathways in T cells and can integrate signals from both cytokine receptors and antigen receptors.

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.

Intermediate affinity interleukin-2 receptor mediates survival via a phosphatidylinositol 3-kinase-dependent pathway

Peripheral blood T lymphocytes require two signals to enter and progress along the cell cycle from their natural quiescent state. The first activation signal is provided by the stimulation through the T cell receptor, which induces the synthesis of cyclins and the expression of the high affinity interleukin-2 receptor. The second signal, required to enter the S phase, is generated upon binding of interleukin-2 to the high affinity alphabetagamma interleukin-2 receptor. However, resting T cells already express intermediate affinity betagamma interleukin-2 receptors. As shown here, T cell stimulation through intermediate affinity receptors is capable of inducing cell rescue from the apoptosis suffered in the absence of stimulation. Characterization of the signaling pathways utilized by betagamma interleukin-2 receptors in resting T cells, indicated that pp56(lck), but not Jak1 or Jak3, is activated upon receptor triggering. Compelling evidence is presented indicating that phosphatidylinositol 3-kinase associates with the intermediate affinity interleukin-2 receptor and is activated upon interleukin-2 addition. Bcl-xL gene was also found to be induced upon betagamma interleukin-2 receptor stimulation. Finally, pharmacological inhibition of phosphatidylinositol 3-kinase blocked both interleukin-2-mediated bcl-xL induction and cell survival. We conclude that betagamma interleukin-2 receptor mediates T-cell survival via a phosphatidylinositol 3-kinase-dependent pathway, possibly involving pp56(lck) and bcl-xL as upstream and downstream effectors, respectively.

A novel phosphatidylinositol-3,4,5-trisphosphate 5-phosphatase associates with the interleukin-3 receptor

To gain insight into the intracellular signaling cascades that are activated by the binding of interleukin-3 (IL-3) to its target cells, we have embarked on the identification of proteins that are associated with the IL-3 receptor (IL-3R). In a previous study we reported that a 110-kDa serine/threonine protein kinase is constitutively associated with the IL-3R and activated following IL-3 stimulation. We now report that a phosphatidylinositol-3,4, 5-trisphosphate (PtdIns-3,4,5-P3) 5-phosphatase (5-ptase) is also constitutively associated with the IL-3R. This 5-ptase is magnesium-dependent and removes the 5-position phosphate from PtdIns-3,4,5-P3 but does not metabolize PtdIns-4,5-P2, inositol (Ins)-1,3,4,5-P4, or Ins-1,4,5-P3. This substrate specificity distinguishes it from any previously characterized 5-ptase. Interestingly, it may be bound indirectly via phosphatidylinositol 3-kinase (PI 3-kinase), another enzyme that is constitutively bound to the IL-3R. However, unlike PI 3-kinase which becomes activated following IL-3 stimulation, this receptor-associated 5-ptase activity does not increase following IL-3 stimulation, and its primary function may be to keep the principal in vivo product of PI 3-kinase, PtdIns-3,4,5-P3, at low levels in unstimulated cells, to terminate the PI 3-kinase signal following IL-3 stimulation or to metabolize PtdIns-3,4,5-P3 to a metabolically active second messenger, i.e. PtdIns-3,4-P2.

Association of phosphatidylinositol 3 kinase to protein kinase C zeta during interleukin-2 stimulation

Interleukin-2 induces a serine-phosphorylated phosphatidylinositol 3 kinase activity in the mouse T cell line TS1 alpha beta. Moreover, protein kinase C (PKC) zeta directly or indirectly associates with the phosphatidylinositol 3 kinase and the association appears to be necessary for the serine-phosphorylated phosphatidylinositol 3 kinase activity, since release of zeta PKC by competition of binding with peptides spanning the p110 sequence from amino acids 907 to 925 abolishes the serine-phosphorylated phosphatidylinositol 3 kinase activity. This kinase activity is also blocked when zeta PKC expression is inhibited by antisense oligonucleotide. Inhibition of phosphatidylinositol 3 kinase activity by wortmannin does not abolish zeta PKC association.

Interleukin-10 stimulation of phosphatidylinositol 3-kinase and p70 S6 kinase is required for the proliferative but not the antiinflammatory effects of the cytokine

Interleukin-10 (IL-10) is a powerful suppressor of the proinflammatory monokine production by lipopolysaccharide-stimulated monocytes as well as a T- and B-cell growth cofactor. The signal transduction cascades initiated by IL-10 ligation to its cognate receptor remain to be elucidated. Here, we demonstrate that in both primary monocytes and the D36 cell line, IL-10 rapidly and transiently stimulated phosphatidylinositol 3-kinase activity associated with the p85 subunit of the enzyme. IL-10 also activated p70 S6 kinase in both cell types. The activation of both of these kinases was sensitive to wortmannin, an inhibitor of phosphatidylinositol 3-kinase. The activation of p70 S6 kinase was also inhibited by the immunosuppressive drug rapamycin. Both rapamycin and wortmannin inhibited the IL-10-induced proliferation of D36 cells but in contrast had no effect on the antiinflammatory effects of the cytokine on lipopolysaccharide-stimulated monocytes. Similar results on D36 proliferation and lipopolysaccharide-stimulated monocyte inhibition by IL-10 were obtained with another phosphatidylinositol 3-kinase inhibitor, LY294002. This suggests that the activation of phosphatidylinositol 3-kinase and p70 S6 kinase is involved in the proliferative functions of IL-10 and that other as yet uncharacterized pathways affect the suppressive effects on monocytes, indicating that multiple and distinct signaling pathways mediate the various pleiotropic activities of IL-10. Furthermore, these findings suggest that it may be possible in the future to modulate the antiinflammatory effects of IL-10 for therapeutic benefit without disrupting other functions of the cytokine.

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.

Interleukin-2 activation of STAT5 requires the convergent action of tyrosine kinases and a serine/threonine kinase pathway distinct from the Raf1/ERK2 MAP kinase pathway

Interleukin-2 (IL-2) induces DNA binding of STAT5, a member of the family of cytokine-regulated transcription factors termed 'signal transducers and activators of transcription'. IL-2-stimulated STAT5-DNA complexes include two tyrosine phosphoproteins which exhibit distinct mobilities in SDS-PAGE gels. Our studies have shown that IL-2 rapidly induces both tyrosine phosphorylation and serine phosphorylation of STAT5 and that the two STAT5 tyrosine phosphoproteins detected in IL-2-activated cells differ in their levels of phosphorylation on serine residues. The two different phosphoforms of STAT5 have identical in vitro DNA binding specificity and reactivity with tyrosine phosphopeptides, but differ in their cellular localization. As well, the present data indicate that the transcriptional activity of STAT5 is regulated by serine kinases in T lymphocytes. Two previously characterized serine kinases activated by IL-2, MAP kinase/ERK2 and p70 S6 kinase, do not appear to be involved in STAT5 regulation by this cytokine. Accordingly, STAT5 activation in T cells requires the convergent action of tyrosine kinases and a distinct serine/threonine kinase which has not previously been implicated in IL-2 signalling.

Effects of growth factors on phosphatidylinositol-3 kinase in astroglial cells

Growth factors differently regulate astroglial cell differentiation and proliferation. In an effort to understand the early intracellular events promoted by growth factors in astroglial cells, we have determined the effects of insulin-like growth factor I (IGF1), insulin, platelet-derived growth factor (PDGF), epidermal growth factor (EGF) and fibroblast growth factors (FGFs) on phosphatidylinositol-3 kinase (PI(3)-kinase). In astroglial cells cultured in serum-free medium, IGF1, PDGF, and EGF, which stimulate cell proliferation, increased PI(3)-kinase activity immunoprecipitated with anti-phosphotyrosine antibodies as shown by thin layer chromatography and high performance liquid chromatography. FGFa and FGFb, which strongly stimulate proliferation, glutamine synthetase, and deiodinase activities and modify cell morphology, have no effect on PI(3)-kinase activity. Addition of 1 nM PDGF, 10 nM IGF1, or 100 nM EGF to the culture medium rapidly stimulated PI(3)-kinase activity which declined slowly after 2 min. The stimulation of PI(3)-kinase increased with growth factor concentration. The maximum increase in PI(3)-kinase activity occurred with 50 nM IGF1, 1 nM PDGF, or 100 nM EGF. Since insulin was active only at high concentration (1 microM), its effect was probably mediated through IGF1 receptors and not through insulin receptors. IGF1 and PDGF, to a lesser degree, also increased the PI(3)-kinase activity associated with pp60c-src protein. Immunoblots performed with an antibody directed against the p85-subunit of the PI(3)-kinase confirmed that IGF1 increased the number of PI(3)-kinase molecules associated with phosphotyrosine-containing proteins or with c-src protein. Each growth factor affects in a different manner the association of PI(3)-kinase with phosphotyrosine-containing proteins and with pp60c-src and thus probably modulates intracellular signals downstream of PI(3)-kinase in astroglial cells.

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.

Antibody ligation of CD7 leads to association with phosphoinositide 3-kinase and phosphatidylinositol 3,4,5-trisphosphate formation in T lymphocytes

The CD7 40-kDa glycoprotein is present on a major subset of human T cells and in the presence of phorbol esters mediates an accessory pathway of T cell activation. Hitherto, the intracellular events elicited by CD7 have been ill-defined. This report demonstrates that cross-linking of CD7 results in the formation of phosphatidic acid in the absence of phosphatidylinositol-4,5-bisphosphate metabolism and also the formation of D-3 phosphoinositides lipids which have been postulated to act as intracellular regulatory molecules. The magnitude of D-3 phosphoinositide formation was similar to that induced by CD3. Both the CD7- and CD3-induced elevation of phosphatidylinositol 3,4,5-trisphosphate approximately 5-10 fold less than that elicited by ligation of the costimulatory molecule CD28 by its counter receptor CD80. The formation of D-3 phosphoinositides following ligation of CD7 coincided with the co-association of CD7 with phosphoinositide 3-kinase, the enzyme which mediates the formation of D-3 phosphoinositide lipids. In contrast, ligation of another reported T cell accessory molecule CD5, failed to elicit formation of D-3 phosphoinositides, implying that phosphoinositide 3-kinase is not coupled to all T cell molecules with accessory functions. Since D-3 phosphoinositides have been suggested to play a pivotal role in T cell costimulatory signals induced by CD28, the results presented in this study suggest that CD7 may also influence T cell activation via this pathway.

Contribution of p56lck to the upregulation of cytokine production and T cell proliferation by IL-2 in human CD3-stimulated T cell clones

Interaction of the interleukin 2 receptor (IL-2R) beta chain with the lymphocyte-specific protein tyrosine kinase (PTK), p56lck, has led to the speculation that p56lck participates in growth signal transduction. Although activation of T cells with interleukin 2 (IL-2) results in the activation of p56lck, accumulating data support the notion that Lck does not play an essential role in mitogenic signal delivery from the IL-2R. Since this src-related PTK has been shown to enhance TCR/CD3-mediated T cell responsiveness, here we investigated whether activation of Lck by IL-2 could contribute to enhance TCR/CD3-mediated T cell functions. This was achieved by using human CD4(+)-cloned T cells and comparing the effects of IL-2 on p56lck kinase activation and cytokine production. Results show that p56lck kinase activity increased as early as 1 min, reached a maximum within 5 min and decreased within 60 min after IL-2 stimulation. Such treatment with IL-2 also resulted in enhancing T cell responsiveness to CD3+PMA stimulation, as assessed by IL-2 and IFN-gamma secretion and by T cell proliferation. This increase of T cell functions was correlated with IL-2-induced p56lck activation in both dose-response and time-course experiments. Taken together these results strongly suggest that activation of Lck by IL-2 may play a role in regulating CD3-mediated T cell functions.

Protein tyrosine kinase Syk is associated with and activated by the IL-2 receptor: possible link with the c-myc induction pathway

The IL-2 receptor (IL-2R) consists of three subunits, the IL-2R alpha, IL-2R beta, and IL-2R gamma chains. The IL-2-induced proliferative signals emanate from the cytoplasmic domains of IL-2R beta and IL-2R gamma, but the nature and function of the signaling molecules that transmit these signals are not fully understood. Here, we report that Syk protein tyrosine kinase (PTK) is physically associated with IL-2R in peripheral blood lymphocytes. cDNA expression studies further revealed that this association is critical for the IL-2-induced activation of Syk PTK, which occurs primarily via the serine-rich region of the IL-2R beta chain, which is essential for proliferative signal transmission. Furthermore, we provide evidence that in the hematopoietic cell line, BAF-B03, the activation of Syk PTK results in the induction of the c-myc gene, an event critical for the cell proliferation. Thus, Syk PTK may be a critical integral member of the signaling molecules engaged by the IL-2R.

Activation of 70-kDa S6 kinase, induced by the cytokines interleukin-3 and erythropoietin and inhibited by rapamycin, is not an absolute requirement for cell proliferation

The cytokines interleukin (IL)-3 and erythropoietin (EPO) are critical regulators of the proliferation and differentiation of cells of the hematopoietic system, but their intracellular mechanisms of action are not fully understood. Binding of IL-3 to the IL-3 receptor (IL-3R) and binding of EPO to the EPOR both induce changes in intracellular tyrosine and serine/threonine phosphorylation; the phosphorylation of a number of polypeptides appears to be a shared response upon cytokine stimulation. We have previously shown that binding of IL-2 to the IL-2R activates the 70-kDa (p70) S6 kinase, a serine/threonine kinase whose activity is regulated by serine/threonine phosphorylation; the immunosuppressant rapamycin inhibits IL-2-dependent proliferation and IL-2-triggered activation of p70 S6 kinase. We, therefore, sought to examine whether induction of p70 S6 kinase activity is a conserved response upon cytokine triggering, and whether this activity is essential for cell proliferation. Proliferation of the IL-3-dependent pro-B cell line Ba/F3 transfected with the EPOR (Ba/F3-EPOR) can be supported by either IL-3 or EPO. In this cell line, both IL-3 and EPO induced p70 S6 kinase activity; rapamycin inhibited both the IL-3 and EPO-induced activation of the 70-kDa S6 kinase as well as cellular proliferation. Thus, p70 S6 kinase activation appears to be a common intermediate triggered by the stimulation of IL-3, EPO, and IL-2 receptors. The Friend spleen focus-forming virus gp55 renders the EPOR constitutively active, and confers growth factor independence on cells expressing EPOR. Ba/F3-EPOR cotransfected with gp55 (Ba/F3-EpoRgp55) and the erythroleukemia cell line MEL, which also expresses both the EPOR and gp55, were analyzed. Rapamycin inhibited the activation of p70 S6 kinase in both cell lines. However, rapamycin inhibited proliferation of Ba/F3-EpoRgp55 but not of MEL cells despite inhibition of p70 S6 kinase activity in both cells. Thus, p70 S6 kinase activation is not an absolute requirement for cell proliferation. These results are discussed in relation to the role of the activation of the 70-kDa S6 kinase activation pathway in the regulation of cell cycle progression.

Gene transfer of the interleukin (IL)-2 receptor beta chain into an IL-7-dependent pre-B cell line permits IL-2-driven proliferation: tyrosine phosphorylation of Shc is induced by IL-2 but not IL-7

Expression of the interleukin (IL)-2 receptor beta chain in the IL-7-dependent pre-B cell line IxN/2B permitted growth in presence of either IL-2 or IL-7, allowing for a direct comparison of intracellular signaling events. Protein tyrosine phosphorylation was essential for IL-2 and IL-7-induced signal transduction since the tyrosine kinase inhibitor herbimycin A blocked proliferation in response to both factors. Western blot analysis of tyrosine-phosphorylated proteins revealed that both IL-2 and IL-7 stimulation led to enhanced phosphorylation of proteins of 170-, 145-, 115- and 99-kDa, as well as induction of phosphorylation of a 96-kDa protein. However, a 55- and a 155-kDa protein were only phosphorylated after IL-2 stimulation. The 55-kDa protein specifically phosphorylated by IL-2 could be identified as p52shc which has recently been shown to be critically involved in Ras activation. Shc tyrosine phosphorylation as a result of IL-2 stimulation was consistently found in CTLL-2 cells and human T lymphoblasts. Taken together our results indicate that the IL-2- and IL-7-stimulated intracellular pathways are partially different and that Shc is a target of IL-2-, but not IL-7-, stimulated tyrosine phosphorylation.

Signaling pathways activated by protein tyrosine phosphorylation in lymphocytes

Antigen and cytokine receptors induce rapid tyrosine phosphorylation of receptor subunits, other membrane proteins, and signaling components. Each receptor induces phosphorylation of a number of proteins. Although there is often overlap between targets of different receptors, any given receptor only induces phosphorylation of a subset of possible targets. How this choice of targets is achieved for these receptors is not yet understood. The cellular events downstream of some signaling components are beginning to come into view. Recent progress in these areas is discussed.

Probing T-cell signal transduction pathways with the immunosuppressive drugs, FK-506 and rapamycin

In addition to their clinical utility in tissue transplantation the immunosuppressive agents FK-506 (Prograf) and rapamycin, have proven to be valuable tools for gaining insight into the biochemistry of T-cell activation. The findings that the protein phosphatase calcineurin and cell cycle control are key elements in T-cell activation and proliferation are the direct result of investigations into the mechanism of action of FK-506 and rapamycin and provide potentially novel therapeutic targets.