Identification of a variable region within the cytoplasmic tail of the IL-2 receptor beta chain that is required for growth signal transduction

STAT5A and STAT5B-Twins with Different Personalities in Hematopoiesis and Leukemia

The transcription factors STAT5A and STAT5B have essential roles in survival and proliferation of hematopoietic cells-which have been considered largely redundant. Mutations of upstream kinases, copy number gains, or activating mutations in STAT5A, or more frequently in STAT5B, cause altered hematopoiesis and cancer. Interfering with their activity by pharmacological intervention is an up-and-coming therapeutic avenue. Precision medicine requests detailed knowledge of STAT5A's and STAT5B's individual functions. Recent evidence highlights the privileged role for STAT5B over STAT5A in normal and malignant hematopoiesis. Here, we provide an overview on their individual functions within the hematopoietic system.

Interleukin-2 receptor signaling in regulatory T cell development and homeostasis

Interleukin-2 (IL2) was initially identified from supernatants of activated lymphocytes over 30 years ago. In the ensuing 15 years, the cDNAs for both IL2 and the three chains of the interleukin-2 receptor (IL2R) were cloned. Subsequently, many of the downstream biochemical pathways activated by the IL2 receptor complex were identified and the structure of IL2 bound to this tripartite receptor complex was solved. Thus, we now have a very good understanding of how each chain contributes to high affinity IL2 binding and signal transduction. In contrast, over the past 30 years the role that IL2 plays in regulating lymphocyte function has involved many surprising twists and turns. For example, IL2 has been shown, paradoxically, to regulate both lymphocyte proliferation and lymphocyte death. In this review, we briefly outline the original findings suggesting a role for IL2 as a T cell growth factor, as well as subsequent studies pointing to its function as an initiator of activation-induced cell death, but then focus on the newly appreciated role for IL2 and IL2R signaling in the development and homeostasis of regulatory T cells.

Identification of a novel G245R polymorphism in the IL-2 receptor beta membrane proximal domain associated with human visceral leishmaniasis

Binding of the interleukin-2 (IL-2) to the IL-2 receptor (IL-2R) triggers a series of intracellular events culminating in lymphocyte proliferation and differentiation. We report here the identification of a novel G245R polymorphism in the membrane proximal domain of the IL-2 receptor beta chain (IL-2Rbeta). Present at a frequency of 7.2%, the IL-2-Rbeta G245R was identified in a population of Eastern Sudan exposed to a severe outbreak of visceral leishmaniasis (VL), a disease associated with a marked depression of T-cell antigen-specific responses. The location of the G245R polymorphism next to the box1/box2 proximal cytokine receptor homology segment and suggestive genetic association with the development of disease (P=0.043), suggest that it may affect Janus kinase (JAK) association and impair growth signal transduction. However, additional genetic association with a synonymous single nucleotide polymorphism (IL2RB+8777T) suggests that other variations of IL2RB or nearby genes participate in the highly significant linkage with VL at 22q12 previously reported for this population.

Compensatory energetic mechanisms mediating the assembly of signaling complexes between interleukin-2 and its alpha, beta, and gamma(c) receptors

Interleukin-2 is a key immuno-regulatory cytokine whose actions are mediated by three different cell surface receptors: the alpha, beta and the "common gamma" (gamma(c)) chains. We have undertaken a complete thermodynamic characterization of the stepwise assembly cycle for multiple possible combinations of the receptor-ligand, and receptor-receptor interactions that are necessary for formation of the high-affinity IL-2/alphabetagamma(c) signaling complex. We find an entropically favorable high affinity interaction between IL-2 and its alpha receptor, a moderately entropically favorable low affinity interaction between IL-2 and its beta receptor, and no interaction between IL-2 and the shared receptor, gamma(c). Formation of the stable intermediate trimolecular complexes of IL-2 with alpha and beta receptors, as well as IL-2 with beta and gamma(c) receptors proceeds through enthalpy-entropy compensation mechanisms. Surprisingly, we see a moderate affinity interaction between the unliganded receptor alpha and beta chains, suggesting that a preformed alphabeta complex may serve as the initial interaction complex for IL-2. Reconstitution of the IL-2/Ralphabetagamma(c) high-affinity quaternary signaling complex shows it to be assembled through cooperative energetics to form a 1:1:1:1 assembly. Collectively, the favorable entropy of the bimolecular interactions appears to be offset by the loss in rigid body entropy of the receptor components in the higher-order complexes, but overcome by the formation of increasingly enthalpically favorable composite interfaces. This enthalpic mechanism utilized by gamma(c) contrasts with the favorable entropic mechanism utilized by gp130 for degenerate cytokine interaction. In conclusion, we find that several energetically redundant pathways exist for formation of IL-2 receptor signaling complexes, suggesting a more complex equilibrium on the cell surface than has been previously appreciated.

Anti-apoptotic signaling by the interleukin-2 receptor reveals a function for cytoplasmic tyrosine residues within the common gamma (gamma c) receptor subunit

The interleukin-2 receptor (IL-2R) is composed of one affinity-modulating subunit (IL-2Ralpha) and two essential signaling subunits (IL-2Rbeta and gammac). Although most known signaling events are mediated through tyrosine residues located within IL-2Rbeta, no functions have yet been ascribed to gammac tyrosine residues. In this study, we describe a role for gammac tyrosines in anti-apoptotic signal transduction. We have shown previously that a tyrosine-deficient IL-2Rbeta chain paired with wild type gammac stimulated enhancement of bcl-2 mRNA in IL-2-dependent T cells, but it was not determined which region of the IL-2R or which pathway was activated to direct this signaling response. Here we show that up-regulation of Bcl-2 by an IL-2R lacking IL-2Rbeta tyrosine residues leads to increased cell survival after cytokine deprivation; strikingly, this survival signal does not occur in the absence of gammac tyrosine residues. These gammac-dependent signals are revealed only in the absence of IL-2Rbeta tyrosines, indicating that the IL-2R engages at least two distinct signaling pathways to regulate apoptosis and Bcl-2 expression. Mechanistically, the gammac-dependent signal requires activation of Janus kinases 1 and 3 and is sensitive to wortmannin, implicating phosphatidylinositol 3-kinase. Consistent with involvement of phosphatidylinositol 3-kinase, Akt can be activated via tyrosine residues on gammac. Thus, gammac mediates an anti-apoptotic signaling pathway through Akt which cooperates with signals from its partner chain, IL-2Rbeta.

Alternate signalling pathways from the interleukin-2 receptor

Interleukin-2 (IL-2) plays a major role in the proliferation of cell populations during an immune reaction. The beta(c) and gamma(c) subunits of the IL-2 receptor (IL-2R) are sufficient and necessary for signal transduction. Despite lacking known catalytic domains, receptor engagement leads to the activation of a diverse array protein tyrosine kinases (PTKs). In resting or anergised T cells, Jak3 is not activated. Signals arising from the PROX domain of the gamma(c) subunit activate p56(lck) (lck) leading to the induction of anti-apoptotic mechanisms. When Jak3 is activated, in primed T cells, other PTKs predominantly mediate the induction of anti-apoptotic mechanisms and drive cellular proliferation. This review intends to suggest a role for these differences within the context of the immune system.

Signaling domains of the interleukin 2 receptor

Interleukin (IL-)2 and its receptor (IL-2R) constitute one of the most extensively studied cytokine receptor systems. IL-2 is produced primarily by activated T cells and is involved in early T cell activation as well as in maintaining homeostatic immune responses that prevent autoimmunity. This review focuses on molecular signaling pathways triggered by the IL-2/IL-2R complex, with an emphasis on how the IL-2R physically translates its interaction with IL-2 into a coherent biological outcome. The IL-2R is composed of three subunits, IL-2Ralpha, IL-2Rbeta and gammac. Although IL-2Ralpha is an important affinity modulator that is essential for proper responses in vivo, it does not contribute to signaling due a short cytoplasmic tail. In contrast, IL-2Rbeta and gammac together are necessary and sufficient for effective signal transduction, and they serve physically to connect the receptor complex to cytoplasmic signaling intermediates. Despite an absolute requirement for gammac in signaling, the majority of known pathways physically link to the receptor via IL-2Rbeta, generally through phosphorylated cytoplasmic tyrosine residues. This review highlights work performed both in cultured cells and in vivo that defines the functional contributions of specific receptor subdomains-and, by inference, the specific signaling pathways that they activate-to IL-2-dependent biological activities.

A single amino acid substitution (Trp(666)-->Ala) in the interbox1/2 region of the interleukin-6 signal transducer gp130 abrogates binding of JAK1, and dominantly impairs signal transduction

gp130 is the common signal-transducing receptor chain of interleukin (IL)-6-type cytokines. Here we describe, for the first time, a single amino acid substitution (Trp(666)-->Ala) in the membrane-proximal interbox1/2 region that abrogates activation of STAT (signal transducer and activator of transcription) transcription factors and the proliferative response of pro-B-cell transfectants. Moreover, association of the Janus kinase JAK1 is prevented. No signalling of heterodimeric IL-5 receptor (IL-5R)/gp130 chimaeras occurs in COS-7 cells, even when only a single cytoplasmic chain of a gp130 dimer contains the Trp(666)Ala mutation, indicating that it acts dominantly.

Activation of STAT5 during EPO-directed suppression of apoptosis

The ligand-dependent activation of the JAK/STAT (Januskinase/Signal Transducer and Activator of Transcription) pathway has been implicated in the explanation of cytokine-specific regulation of gene expression. Previous studies have reported conflicting results on the role of the transcription factor STAT5 in erythropoietin (EPO)-induced cellular responses. In this study we focused on the functional importance of STAT5 docking sites in the intracellular EPO receptor (EPOR) domain for the mediation of antiapoptotic activities. We demonstrate that EPO-dependent survival of erythroleukemic cell lines is accompanied by sustained STAT5 DNA-binding activity. The role of single tyrosine residues was dissected by the analysis of myeloid FDCP-1 cells stably expressing mutant EPOR proteins. The data show that receptors having a high potential to mediate antiapoptotic signals also effectively activate STAT5, whereas receptors lacking STAT5 docking sites are diminished in both activities. We conclude that the transcription factor STAT5 is functionally implicated in the EPO-dependent survival of cells.

IL-4 Selectively Inhibits IL-2-Triggered Stat5 Activation, But Not Proliferation, in Human T Cells

IL-2 activates several distinct signaling pathways that are important for T cell activation, proliferation, and differentiation into both Th1 and Th2 phenotypes. IL-4, the major cytokine that promotes differentiation of Th2 cells, has been shown to block signaling of the Th1-promoting cytokine IL-12. As IL-2 synergizes with IL-12 in promoting Th1 differentiation, the effects of IL-4 on IL-2 signal transduction were investigated. IL-4 suppressed activation of DNA binding and tyrosine phosphorylation of the transcription factor Stat5 by IL-2, and suppressed the expression of the IL-2-inducible genes CD25, CIS, the PGE2 receptor, and cytokine responsive (CR) genes CR1 and CR8. Activation of Stat5 by cytokines that share a common γ receptor subunit, IL-2, IL-7, and IL-15, was suppressed by preculture in IL-4. Activation of the Jak1 and Jak3 kinases that are proximal to Stat5 in the IL-2-Jak-STAT signaling pathway was suppressed, and this correlated with inhibition of IL-2Rβ subunit expression. In contrast to suppression of Stat5, proliferative responses to IL-2 were augmented in IL-4-cultured cells, and activation of proliferative pathways leading to activation of mitogen activated protein kinases, induction of expression of Myc, Fos, Pim-1, and cyclin D3, and decreased levels of the cyclin-dependent kinase inhibitor p27 were intact. These results identify molecular mechanisms underlying interactions between IL-4 and IL-2 in T cells and demonstrate that one mechanism of regulation of IL-2 activity is selective and differential modulation of signaling pathways.

Functional and physical association of a cell surface phospholipid and interleukin-2 receptor p55(alpha) subunits

A phosphatidylcholine-like phospholipid expressed in the outer leaflet of the cell membrane shortly after mitogenic activation of T-cells is described, based on the binding of monoclonal antibody 90. 60.3. Expression of the 90.60.3 phospholipid antigen in T-cells is activation-dependent. Once expressed, the 90.60.3 phospholipid is in direct physical association with the interleukin-2 (IL-2) binding domain of IL-2 receptor alpha subunits, but does not affect IL-2 binding. The association is specific, because the 90.60.3 phospholipid is not found in association with other domains of IL-2 receptor alpha subunits, or near IL-2 receptor beta or gamma subunits. Culturing cytokine-dependent cell lines in the presence of monoclonal antibody 90.60.3 potentiates IL-2-dependent cell survival and proliferation in a dose-dependent manner. In contrast, IL-4-dependent responses are not potentiated. Taken together, the data suggest that specific plasma membrane phospholipids expressed in the outer leaflet after T-cell activation associate with the IL-2 binding domain of IL-2 receptor alpha subunits (and perhaps other cytokine receptors), and may play a role in regulating receptor mobility or signal transduction.

Scratching the (cell) surface: cytokine engineering for improved ligand/receptor trafficking dynamics

Cytokines can be engineered for greater potency in stimulating cellular functions. An obvious test criterion for an improved cytokine is receptor-binding affinity, but this does not always correlate with improved biological response. By combining protein-engineering techniques with studies of receptor trafficking and signaling, it might be possible to identify the ligand receptor-binding properties that should be sought.

JAK/STAT signaling by cytokine receptors

The JAK/STAT pathway is recognized as one of the major mechanisms by which cytokine receptors transduce intracellular signals. This system is regulated at multiple levels, including JAK activation, nuclear trafficking of STAT factors, and negative feedback loops. Gene deletion studies have implicated selected STAT factors as predominant mediators for a limited number of lymphokines. This signaling pathway influences normal cell survival and growth mechanisms and may contribute to oncogenic transformation.

Stoichiometric structure-function analysis of the prolactin receptor signaling domain by receptor chimeras

The intracellular domain of the prolactin (PRL) receptor (PRLr) is required for PRL-induced signaling and proliferation. To identify and test the functional stoichiometry of those PRLr motifs required for transduction and growth, chimeras consisting of the extracellular domain of either the alpha or beta subunit of human granulocyte-macrophage colony-stimulating factor (GM-CSF) receptor (GM-CSFr) and the intracellular domain of the rat PRLr were synthesized. Because the high-affinity binding of GM-CSF results from the specific pairing of one alpha- and one beta-GM-CSFr, use of GM-CSFr/PRLr chimera enabled targeted dimerization of the PRLr intracellular domain. To that end, the extracellular domains of the alpha- and beta-GM-CSFr were conjugated to one of the following mutations: (i) PRLr C-terminal truncations, termed alpha278, alpha294, alpha300, alpha322, or beta322; (ii) PRLr tyrosine replacements, termed Y309F, Y382F, or Y309+382F; or, (iii) PRLr wild-type short, intermediate, or long isoforms. These chimeras were cotransfected into the cytokine-responsive Ba/F3 line, and their expression was confirmed by ligand binding and Northern and Western blot analyses. Data from these studies revealed that heterodimeric complexes of the wild type with C-terminal truncation mutants of the PRLr intracellular domain were incapable of ligand-induced signaling or proliferation. Replacement of any single tyrosine residue (Y309F or Y382F) in the dimerized PRLr complex resulted in a moderate reduction of receptor-associated Jak2 activation and proliferation. In contrast, trans replacement of these residues (i.e., alphaY309F and betaY382F) markedly reduced ligand-driven Jak2 activation and proliferation, while cis replacement of both tyrosine residues in a single intracellular domain (i.e., alphaY309+382F) produced an inactive signaling complex. Analysis of these GM-CSFr-PRLr complexes revealed equivalent levels of Jak2 in association with the mutant receptor chains, suggesting that the tyrosine residues at 309 and 382 do not contribute to Jak association, but instead to its activation. Heterodimeric pairings of the intracellular domains from the known PRLr receptor isoforms (short-intermediate, short-long, and intermediate-long) also yielded inactive receptor complexes. These data demonstrate that the tyrosine residues at 309 and 382, as well as additional residues within the C terminus of the dimerized PRLr complex, contribute to PRL-driven signaling and proliferation. Furthermore, these findings indicate a functional requirement for the pairing of Y309 and Y382 in trans within the dimerized receptor complex.

Janus kinases in interleukin-2-mediated signaling: JAK1 and JAK3 are differentially regulated by tyrosine phosphorylation

BACKGROUND

Cytokines mediate a variety of effector cell functions, including cellular proliferation, differentiation, and modulation of the immune response. Many cytokines activate receptor-associated Janus kinases (JAKs) that promote tyrosine phosphorylation of signal transducers and activators of transcription (STAT) factors. Although JAK activation has been correlated with phosphorylation, the role of this tyrosine phosphorylation in the regulation of JAK1 and JAK3 remains unclear. Furthermore, the relative roles of JAK1 and JAK3 in the activation of STAT5 by interleukin-2 (IL-2) remain poorly understood.

RESULTS

We targeted two conserved tyrosine residues within the activation loop of the JAK1 and JAK3 kinase domains for substitution with phenylalanines. In an overexpression system, the catalytic function of JAK1 strictly required the presence of the first of these tyrosines, Y1033. In contrast, JAK3 retained catalytic activity when either or both of these activation-loop tyrosines were mutated. Analysis of JAK1/3 chimeras demonstrated that JAK activity was also controlled by intramolecular interactions involving the amino-terminal domain of the JAK as well as by the inherent signaling properties of the kinase domain. Finally, we have reconstituted IL-2-dependent STAT5 induction in a cell line that lacks detectable expression of JAK1 and JAK3. Catalytically active versions of both JAK1 and JAK3 must be present for effective induction of STAT5.

CONCLUSIONS

JAK1 and JAK3 are differentially regulated by specific tyrosines within their respective activation loops. Additionally, the amino-terminal domain of JAK3 appears to contain regulatory sequences that modify the function of the kinase domain. Finally, both JAK1 and JAK3 must retain catalytic function for IL-2-induced STAT5 activation.

Interleukin-2 (IL-2)-mediated induction of the IL-2 receptor alpha chain gene. Critical role of two functionally redundant tyrosine residues in the IL-2 receptor beta chain cytoplasmic domain and suggestion that these residues mediate more than Stat5 activation

The interleukin-2 receptor alpha chain (IL-2Ralpha) is potently induced by antigens, mitogens, and certain cytokines that include IL-2 itself. This induction leads to the formation of high affinity IL-2 receptors when IL-2Ralpha is co-expressed with the beta (IL-2Rbeta) and gamma (gammac) chains of this receptor. We investigated the signaling pathways mediating IL-2-induced IL-2Ralpha mRNA expression using 32D myeloid progenitor cells stably transfected with either wild type IL-2Rbeta or mutants of IL-2Rbeta containing tyrosine to phenylalanine substitutions. Of the six cytoplasmic tyrosines in IL-2Rbeta, we have found that only the two tyrosines that mediate Stat5 activation (Tyr-392 and Tyr-510) contribute to IL-2-induced IL-2Ralpha gene expression and that either tyrosine alone is sufficient for this process. Interestingly, the IL-7 receptor contains a tyrosine (Tyr-429)-based sequence resembling the motifs encompassing Tyr-392 and Tyr-510 of IL-2Rbeta. Further paralleling the IL-2 system, IL-7 could activate Stat5 and drive expression of IL-2Ralpha mRNA in 32D cells transfected with the human IL-7R. However, IL-3 could not induce IL-2Ralpha mRNA in 32D cells, despite its ability to activate Stat5 via the endogenous IL-3 receptor. Moreover, the combination of IL-3 and IL-2 could not "rescue" IL-2Ralpha mRNA expression in cells containing an IL-2Rbeta mutant with phenylalanine substitutions at Tyr-392 and Tyr-510. These data suggest that Tyr-392 and Tyr-510 couple to an additional signaling pathway beyond STAT protein activation in IL-2-mediated induction of the IL-2Ralpha gene.

Deletion of the carboxyl-terminal transactivation domain of MGF-Stat5 results in sustained DNA binding and a dominant negative phenotype

The Stat (signal transducer and activator of transcription) factors transmit cytokine, growth factor, and hormone responses. Seven members of the Stat gene family are known. MGF-Stat5a has been discovered as a mediator of the prolactin response in mammary epithelial cells. Two closely related variants of Stat5, Stat5a and Stat5b, are encoded by distinct genes. We examined the functional properties of the carboxyl termini of these molecules. Wild-type Stat5a (794 amino acids) and the carboxyl-terminal deletion mutant Stat5a delta 772 supported prolactin-induced transcription of a beta-casein promoter-reporter construct in COS7 cells; Stat5a delta 750 did not. Upon prolactin activation, tyrosine phosphorylation and the specificity of DNA binding were indistinguishable among the three Stat5a variants. Tyrosine dephosphorylation and the downregulation of the DNA-binding activity were delayed in the Stat5a delta 750 mutant. The carboxyl-terminal transactivation domain of Stat5a, amino acids 722 to 794, can be conferred to the DNA-binding domain of the yeast transcription factor GAL4. Coexpression of Stat5a or Stat5b and of the carboxyl-terminal deletion mutants resulted in the suppression of transcriptional induction in COS or Ba/F3 cells. We propose that Stat5a delta 750 and Stat5b delta 754 are lacking functional transactivation domains and exert their dominant negative effects by blocking the DNA-binding site in Stat5-responsive gene promoters.