Growth signal transduction by the human interleukin-2 receptor requires cytoplasmic tyrosines of the beta chain and non-tyrosine residues of the gamma c chain

IL-2 Signaling Couples the MAPK and mTORC1 Axes to Promote T Cell Proliferation and Differentiation in Teleosts

IL-2 is a pleiotropic cytokine that is critical for T cell immunity. Although the IL-2-mediated regulation of T cell immunity in mammals is relatively well understood, it remains largely unknown whether and how IL-2 regulates T cell immunity in lower vertebrates. To address this knowledge gap, we investigated the role played by IL-2 in the regulation of T cell response, as well as the associated underlying mechanisms in a teleost fish, large yellow croaker (Larimichthys crocea). We found that large yellow croaker (L. crocea) IL-2 (LcIL-2) significantly promoted T cell proliferation both in vivo and in vitro; significantly induced the differentiation of Th1, Th2, regulatory T, and cytotoxic T cells while inhibiting Th17 differentiation; and participated in the elimination of invading pathogenic bacteria. Mechanistically, the binding of LcIL-2 to its heterotrimer receptor complex (LcIL-15Rα/LcIL-2Rβ/Lcγc) triggered the conserved JAK-STAT5 pathway, which in turn regulated the expression of genes involved in T cell expansion, differentiation, and biological function. The MAPK and mammalian target of rapamycin complex 1 (mTORC1) axes, which are involved in TCR-mediated signaling, were also required for LcIL-2-mediated T cell response. Collectively, our results demonstrated that fish IL-2 plays a comprehensive regulatory role in T cell response and highlighted the complex and delicate network regulating T cell-driven immune response. We propose that T cell immunity is regulated by the interplay between TCR signaling and cytokine signaling, and that this basic strategy evolved before the emergence of the tetrapod lineage. Our findings provide valuable insights into the regulatory mechanisms underlying T cell response in teleosts.

Biology and regulation of IL-2: from molecular mechanisms to human therapy

IL-2 was first identified as a growth factor capable of driving the expansion of activated human T cell populations. In the more than 40 years since its discovery, a tremendous amount has been learned regarding the mechanisms that regulate the expression of both IL-2 and its cell surface receptor, its mechanisms of signalling and its range of biological actions. More recently, the mechanisms by which IL-2 regulates CD4⁺ T cell differentiation and function have been elucidated. IL-2 also regulates the effector and memory responses of CD8⁺ T cells, and the loss of IL-2 or responsiveness to IL-2 at least in part explains the exhausted phenotype that occurs during chronic viral infections and in tumour responses. These basic mechanistic studies have led to the therapeutic ability to manipulate the action of IL-2 on regulatory T (T_reg) cells for the treatment of autoimmune disease and on CD8⁺ T cells for immunotherapy of cancer. IL-2 can have either positive or deleterious effects, and we discuss here recent ideas and approaches for manipulating the actions and overall net effects of IL-2 in disease settings, including cancer.

The common γ-chain cytokine receptor: tricks-and-treats for T cells

Originally identified as the third subunit of the high-affinity IL-2 receptor complex, the common γ-chain (γc) also acts as a non-redundant receptor subunit for a series of other cytokines, collectively known as γc family cytokines. γc plays essential roles in T cell development and differentiation, so that understanding the molecular basis of its signaling and regulation is a critical issue in T cell immunology. Unlike most other cytokine receptors, γc is thought to be constitutively expressed and limited in its function to the assembly of high-affinity cytokine receptors. Surprisingly, recent studies reported a series of findings that unseat γc as a simple housekeeping gene, and unveiled γc as a new regulatory molecule in T cell activation and differentiation. Cytokine-independent binding of γc to other cytokine receptor subunits suggested a pre-association model of γc with proprietary cytokine receptors. Also, identification of a γc splice isoform revealed expression of soluble γc proteins (sγc). sγc directly interacted with surface IL-2Rβ to suppress IL-2 signaling and to promote pro-inflammatory Th17 cell differentiation. As a result, endogenously produced sγc exacerbated autoimmune inflammatory disease, while the removal of endogenous sγc significantly ameliorated disease outcome. These data provide new insights into the role of both membrane and soluble γc in cytokine signaling, and open new venues to interfere and modulate γc signaling during immune activation. These unexpected discoveries further underscore the perspective that γc biology remains largely uncharted territory that invites further exploration.

Stability of the recombinant anti‑erbB2 scFv‑Fc‑interleukin‑2 fusion protein and its inhibition of HER2‑overexpressing tumor cells

The anti‑erbB2 scFv‑Fc‑IL‑2 fusion protein (HFI) is the basis for development of a novel targeted anticancer drug, in particular for the treatment of HER2‑positive cancer patients. HFI was fused with the anti‑erbB2 antibody and human IL‑2 by genetic engineering technology and by antibody targeting characteristics of HFI. IL‑2 was recruited to target cells to block HER2 signaling, inhibit or kill tumor cells, improve the immune capacity, reduce the dose of antibody and IL‑2 synergy. In order to analyse HFI drug ability, HFI plasmid stability was verified by HFI expression of the trend of volume changes. Additionally, HFI could easily precipitate and had progressive characteristics and thus, the buffer system of the additive phosphate‑citric acid buffer, arginine, Triton X‑100 or Tween‑80, the establishment of a microfiltration, ion exchange, affinity chromatography and gel filtration chromatography‑based purification process were explored. HFI samples were obtained according to the requirements of purity, activity and homogeneity. In vivo, HFI significantly delayed HER2 overexpression of non‑small cell lung cancer (Calu‑3) in human non‑small cell lung cancer xenografts in nude mice, and the inhibition rate was more than 60% (P<0.05) in the group treated with 1 mg/kg the HFI dose; HFI significantly inhibited HER2 expression of breast cancer (FVB/neu) transgenic mouse tumor growth in 1 mg/kg of the HFI dose group, and in the following treatment the 400 mm3 tumors disappeared completely. Combined with other HFI test data analysis, HFI not only has good prospects, but also laid the foundation for the development of antibody‑cytokine fusion protein‑like drugs.

The extracellular and transmembrane domains of the γC and interleukin (IL)-13 receptor α1 chains, not their cytoplasmic domains, dictate the nature of signaling responses to IL-4 and IL-13

Previously, we demonstrated that the γC subunit of type I IL-4 receptor was required for robust tyrosine phosphorylation of the downstream adapter protein, IRS-2, correlating with the expression of genes (ArgI, Retnla, and Chi3l3) characteristic of alternatively activated macrophages. We located an I4R-like motif (IRS-2 docking sequence) in the γC cytoplasmic domain but not in the IL-13Rα1. Thus, we predicted that the γC tail directed enhanced IRS-2 phosphorylation. To test this, IL-4 signaling responses were examined in a mutant of the key I4R motif tyrosine residue (Y325F) and different γC truncation mutants (γ285, γ308, γ318, γ323, and γFULL LENGTH (FL)) co-expressed in L-cells or CHO cells with wild-type (WT) IL-4Rα. Surprisingly, IRS-1 phosphorylation was not diminished in Y325F L-cell mutants suggesting Tyr-325 was not required for the robust insulin receptor substrate response. IRS-2, STAT6, and JAK3 phosphorylation was observed in CHO cells expressing γ323 and γFL but not in γ318 and γ285 mutants. In addition, when CHO cells expressed γ318, γ323, or γFL with IL-2Rβ, IL-2 induced phospho-STAT5 only in the γ323 and γFL clones. Our data suggest that a smaller (5 amino acid) interval than previously determined is necessary for JAK3 activation/γC-mediated signaling in response to IL-4 and IL-2. Chimeric receptor chains of the γC tail fused to the IL-13Rα1 extracellular and transmembrane domain did not elicit robust IRS-2 phosphorylation in response to IL-13 suggesting that the extracellular/transmembrane domains of the IL-4/IL-13 receptor, not the cytoplasmic domains, control signaling efficiency. Understanding this pathway fully will lead to rational drug design for allergic disease.

Expression of mutant p53 and of the multidrug resistant proteins P-glycoprotein and glutathione S-transferase-pi correlated in colorectal adenocarcinoma

OBJECTIVE

Accumulating studies suggest that multidrug resistance is related to expression of p53, P-glycoprotein (Pgp) and Glutathione S-Transferase-pi (GST-pi). This study was to estimate mutant p53 expression and its correlation with drug resistance related proteins Pgp and GST-pi expression in colorectal adenocarcinoma patients.

MATERIAL AND METHODS

Immunohistochemical ABC technique was used to detect the expression of mutant p53 protein, Pgp and GST-pi in 404 cases with colorectal adenocarcinoma.

RESULTS

A low frequency of mutant p53 accumulation was observed, consistent with findings in colorectal cancers (CRCs) from other Asian populations. Accumulation of mutant p53 was related to AJCC staging (p < 0.05). Pgp expression was significantly correlated with tumor location (p = 0.039) and gender (p = 0.043). The positive percentage of Pgp and GST-pi expression was all significantly higher in mutant p53 protein positive group than mutant p53 protein negative cases (r = 0.634, p < 0.001 and r = 0.680, p < 0.001, respectively).

CONCLUSIONS

These findings demonstrate association of three biomarkers with clinicopathologic parameters of colorectal carcinoma, and overexpression of Pgp and GST-pi was closely correlated with mutant p53.

The Interleukin (IL)‐2 Family Cytokines: Survival and Proliferation Signaling Pathways in T Lymphocytes

Lymphocyte populations in the immune system are maintained by a well-organized balance between cellular proliferation, cellular survival and programmed cell death (apoptosis). One of the primary functions of many cytokines is to coordinate these processes. In particular, the interleukin (IL)-2 family of cytokines, which consists of six cytokines (IL-2, IL-4, IL-7, IL-9, IL-15 and IL-21) that all share a common receptor subunit (gammac), plays a major role in promoting and maintaining T lymphocyte populations. The details of the molecular signaling pathways mediated by these cytokines have not been fully elucidated. However, the three major pathways clearly involved include the JAK/STAT, MAPK and phosphatidylinositol 3-kinase (P13K) pathways. The details of these pathways as they apply to the IL-2 family of cytokines is discussed, with a focus on their roles in proliferation and survival signaling.

Differential regulation of the IL-17 receptor by gammac cytokines: inhibitory signaling by the phosphatidylinositol 3-kinase pathway

The gammac-family cytokine IL-2 activates signaling events that contribute to cell survival and proliferation, the best-studied of which are the STAT-5 and phosphatidylinositol 3-kinase (PI3K) pathways. The starting point of this study was to define genes regulated by the IL-2R-mediated PI3K pathway in T cells. Accordingly, we used an erythropoietin (EPO) receptor chimeric receptor system in which IL-2-dependent HT-2 T cells expressed a mutant EPO-IL-2Rbeta construct where Tyr-338 is mutated to Phe. Cells expressing this mutant IL-2Rbeta chain fail to induce phosphorylation of PI3K-p85alpha/beta or activate Akt, but mediate normal IL-2-dependent proliferation and activation of JAK1 and STAT-5A/B. Microarray analyses revealed differential regulation of numerous genes compared with cells expressing a wild-type IL-2Rbeta, including up-regulation of the IL-17 receptor subunit IL-17RA. Blockade of the PI3K pathway but not p70S6K led to up-regulation of IL-17RA, and constitutive Akt activation was associated with suppressed IL-17RA expression. Moreover, similar to the mutant EPO-IL-2Rbeta chimera, IL-15 and IL-21 induced IL-17RA preferentially compared with IL-2, and IL-2 but not IL-15 or IL-21 mediated prolonged activation of the PI3K p85 regulatory subunit. Thus, there are intrinsic signaling differences between IL-2 and IL-15 that can be attributed to differences in activation of the PI3K pathway.

The molecular basis of IL-21-mediated proliferation

Interleukin-21 (IL-21) is a type I cytokine that modulates functions of T, B, natural killer (NK), and myeloid cells. The IL-21 receptor (IL-21R) is closely related to the IL-2 receptor beta chain and is capable of transducing signals through its dimerization with the common cytokine receptor gamma chain (gamma(c)), the protein whose expression is defective in humans with X-linked severe combined immunodeficiency. To clarify the molecular basis of IL-21 actions, we investigated the role of tyrosine residues in the IL-21R cytoplasmic domain. Simultaneous mutation of all 6 tyrosines greatly diminished IL-21-mediated proliferation, whereas retention of tyrosine 510 (Y510) allowed full proliferation. Y510 efficiently mediated IL-21-induced phosphorylation of Stat1 and Stat3, but not of Stat5, and CD8(+) T cells from Stat1/Stat3 double knock-out mice exhibited decreased proliferation in response to IL-21 + IL-15. In addition, IL-21 weakly induced phosphorylation of Shc and Akt, and consistent with this, specific inhibitors of the MAPK and PI3K pathways inhibited IL-21-mediated proliferation. Collectively, these data indicate the involvement of the Jak-STAT, MAPK, and PI3K pathways in IL-21 signaling.

The roles for cytokines in the generation and maintenance of regulatory T cells

As an essential mechanism for self-tolerance, immune suppression has attracted much attention since the discovery of suppressor T cells, now called regulatory T cells (Tregs), in the 1990s. Different types of Tregs have been described based on distinct expression patterns of surface markers and cytokines. Cytokines are not only essential for function but also important for the generation of Tregs. Interleukin-2 (IL-2), transforming growth factor-beta, IL-10, and other immunoregulatory molecules have been shown to control the generation of Tregs. The presence of other types of cells, in particular antigen-presenting cells (APCs), is critical for the generation of Tregs. Cytokines can serve as either initiators or intermediates for the interactions between APCs and Tregs. This review discusses our current knowledge of how cytokines regulate the generation and maintenance of Tregs.

Therapeutic resistance in lung cancer

Despite considerable progress over the last 25 years in the systemic therapy of lung cancer, intrinsic and acquired resistance to chemotherapeutic agents and radiation remains a vexing problem. The number of mechanisms of therapeutic resistance in lung cancer has expanded considerably over the past three decades, and the crucial role of stress resistance pathways is increasingly recognised as a cause of intrinsic and acquired chemo- and radiotherapy resistance. This paper reviews recent evidence for stress defence proteins, particularly RALBP1/RLIP76, in mediating intrinsic and acquired chemotherapy and radiation resistance in human lung cancer.

Cytokines and immunodeficiency diseases: critical roles of the gamma(c)-dependent cytokines interleukins 2, 4, 7, 9, 15, and 21, and their signaling pathways

In this review, we discuss the role of cytokines and their signaling pathways in immunodeficiency. We focus primarily on severe combined immunodeficiency (SCID) diseases as the most severe forms of primary immunodeficiencies, reviewing the different genetic causes of these diseases. We focus in particular on the range of forms of SCID that result from defects in cytokine-signaling pathways. The most common form of SCID, X-linked SCID, results from mutations in the common cytokine receptor gamma-chain, which is shared by the receptors for interleukin (IL)-2, IL-4, IL-7, IL-9, IL-15, and IL-21, underscoring that X-linked SCID is indeed a disease of defective cytokine signaling. We also review the signaling pathways used by these cytokines and the phenotypes in humans and mice with defects in the cytokines or signaling pathways. We also briefly discuss other cytokines, such as interferon-gamma and IL-12, where mutations in the ligand or receptor or signaling components also cause clinical disease in humans.

Geldanamycin, a heat shock protein 90-binding agent, disrupts Stat5 activation in IL-2-stimulated cells

The 90-kDa heat shock protein (Hsp90) is the most abundant molecular chaperone in eukaryotic cells. Hsp90 plays a critical role in regulating signal transduction pathways that control cell proliferation since its chaperone function is restricted to a subset of proteins including some signal molecules. Improper function of these proteins can be induced by an anti-tumor agent geldanamycin (GA) which is the specific inhibitor of Hsp90. In this study, it was demonstrated that GA interferes with IL-2-stimulated proliferation of murine CTLL-2 cells. As to the signaling mechanisms underlying this inhibitory effect, we discovered GA disrupts the IL-2-stimulated activation and phosphorylation of the transcription factor Stat5, indicating the proper function of Hsp90 is indispensable for Stat5 activation. This conclusion is validated by the observation that Hsp90 interacts with Stat5 in the immunoprecipitation assay and GA interrupts their interaction. Furthermore, by constructing deletion mutants, we identified the c-terminal half of Stat5 coiled-coil region is responsible for binding with Hsp90.

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.

IL-2 receptor signaling through the Shb adapter protein in T and NK cells

Interleukin-2 induces heterodimerization of the IL-2 receptor beta and gamma subunits. This study addresses a role of the Shb adapter protein in IL-2 receptor signaling in T and NK cells. The IL-2Rbeta and gamma chains were found to co-immunoprecipitate with Shb, when each alone was co-expressed with Shb in COS cells. Using fusion proteins, the Shb SH2 domain was found to associate in a phosphotyrosine-dependent manner with the IL-2 receptor beta and gamma subunits upon IL-2 stimulation in primary T cells and the NK cell line NK-92. The main binding site of the Shb SH2 domain was phosphorylated Tyr-510 in the IL-2Rbeta chain. Shb was also phosphorylated upon IL-2 stimulation when overexpressed together with IL-2Rbeta (in pre-B cells, which express the gamma chain constitutively). These cells were also less apoptotic in the presence of IL-2 than cells overexpressing a mutant Shb (with a defect SH2 domain) or cells expressing a mutant IL-2Rbeta, with the Shb binding sites mutated to phenylalanine (Y392F, Y510F). JAK1 and JAK3 were also found to associate with Shb, but in contrast to the Shb-IL-2 receptor association, JAK1 and 3 appear to associate with the proline-rich regions of Shb. In conclusion, Shb links the IL-2 receptor to other signaling proteins and mediates the regulation of apoptosis in the presence of IL-2.

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.

Critical sites for the interaction between IL-2Rgamma and JAK3 and the following signaling

JAK3 is the only known protein tyrosine kinase associating with IL-2Rgamma. This interaction is supposed to be very important to IL-2 signaling. In order to identify the critical residues for these two molecular interactions and the following signal events, various mutants of gammac and JAK3 were constructed on the basis of computer analysis. The direct interaction was determined via the yeast two-hybrid system, while the signaling was analyzed with reporter genes under the control of the c-fos, c-myc, or tnf-beta promoters, respectively. Results showed that there are two key sites on gammac involved in this interaction and the following signal transduction: the critical one is E327 via electrostatic interaction, the other is L293 via hydrophobic interaction. As to JAK3, the data indicated that Y100 is important for the interaction with gammac. These results also document that the requirement for interaction between gammac and JAK3 is different to activate different signaling pathways mediated by gammac, such as c-fos, c-myc, and JAK-STAT.

Cloning and expression of a putative common cytokine receptor gamma chain (gammaC) gene in rainbow trout (Oncorhynchus mykiss)

A full length cDNA of a putative common cytokine receptor gamma chain (gammaC) gene of rainbow trout (Oncorhynchus mykiss) has been cloned and sequenced. The contiguous cDNA contained 2291 nucleotides, consisting of an ORF of 1029 bp, with a 72 bp 5' UTR and a 1190 bp 3' UTR. The coding region showed 44-46% identity to mammalian gammaC genes. The ORF translated into a 343 amino acid protein, with some 28-30% amino acid identity to the coding region of mammalian sequences. A predicted signal peptide and transmembrane domain were identified, giving a 206 amino acid extracellular domain and a 98 amino acid intracellular domain in the trout molecule. Five potential glycosylation sites were present in the extracellular domain, as were six conserved cysteine residues and the W-S-X-W-S motif typical of haemopoietin receptors. One of the most interesting differences between the trout and mammalian sequences was the lack of tyrosines in the trout intracellular domain. RT-PCR studies revealed a wide tissue distribution of gammaC expression, with detectable transcript in blood, spleen, gill, kidney, brain and liver. Low levels of gammaC transcript were detectable in unstimulated macrophage cultures and expression was increased by stimulation of the cells with recombinant trout interleukin-1beta (IL-1beta) or LPS. Similarly, in the RTG cell line which exhibited even lower level constitutive expression, stimulation with IL-1beta increased gammaC transcript levels but LPS had no effect.

Surface chimeric receptors as tools in study of lymphocyte activation

In this chapter we have described a powerful technology that has allowed the functional dissection of individual subunits from oligomeric receptors. We have focused primarily on chimeras derived from antigen receptors or their downstream signaling components to illustrate the wide utility of the approach; however, the technology has been applied to numerous multimeric receptors of the immune system including cytokine receptors, Fc receptors, and natural killer (NK) cell inhibitory receptors. Although the significance of the structural complexity of oligomeric receptors is by no means understood, it is certain that valuable benefits must be derived from the integrated function of their subunits. In the case of antigen receptors, the multiplicity of ITAMs likely allows the cell to distinguish subtle variations in ligand affinities with exquisite sensitivity. Clearly, an isolated subunit that is ligated with antibodies cannot confer such complex function. For instance, it cannot reveal the subtle changes in signal transduction that likely occur on stimulation with altered antigenic peptide ligands or during a complex cell-cell interaction. However, before the intricacies of integrated receptor function can be appreciated, the potential or unique functional properties contributed by each individual receptor component must first be understood. Providing a tool to acquire this kind of understanding has been the greatest asset of this technology. Acknowledging its limitations, the use of surface chimeric receptors remains an invaluable approach toward our understanding the complex function of oligomeric receptors.

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).

The IL-2 receptor promotes lymphocyte proliferation and induction of the c-myc, bcl-2, and bcl-x genes through the trans-activation domain of Stat5

Studies assessing the role of Stat5 in the IL-2 proliferative signal have produced contradictory, and thus inconclusive, results. One factor confounding many of these studies is the ability of IL-2R to deliver redundant mitogenic signals from different cytoplasmic tyrosines on the IL-2R beta-chain (IL-2Rbeta). Therefore, to assess the role of Stat5 in mitogenic signaling independent of any redundant signals, all cytoplasmic tyrosines were deleted from IL-2Rbeta except for Tyr510, the most potent Stat5-activating site. This deletion mutant retained the ability to induce Stat5 activation and proliferation in the T cell line CTLL-2 and the pro-B cell line BA/F3. A set of point mutations at or near Tyr510 that variably compromised Stat5 activation also compromised the proliferative signal and revealed a quantitative correlation between the magnitude of Stat5 activation and proliferation. Proliferative signaling by a receptor mutant with a weak Stat5 activating site could be rescued by overexpression of wt Stat5a or b. Additionally, the ability of this receptor mutant to induce c-myc, bcl-x, and bcl-2 was enhanced by overexpression of wt Stat5. By contrast, overexpression of a version of Stat5a lacking the C-terminal trans-activation domain inhibited the induction of these genes and cell proliferation. Thus, Stat5 is a critical component of the proliferative signal from Tyr510 of the IL-2R and regulates expression of both mitogenic and survival genes through its trans-activation domain.

Constitutive Activation of STATs Upon In Vivo Human Immunodeficiency Virus Infection

Infection by the human immunodeficiency virus (HIV) either upregulates or downregulates the expression of several cytokines and interferons (IFNs) that use the Janus kinase/signal transducer and activator of transcription (JAK/STAT) pathway for signal transduction. However, very little is known on the state of activation of the JAK/STAT pathway after HIV infection either in vivo or in vitro. In this regard, we report here that a constitutive activation of a C-terminal truncated STAT5 (STAT5▵) and of STAT1 occurs in the majority (∼75%) of individuals with progressive HIV disease. We have further demonstrated that, among peripheral blood mononuclear cells (PBMCs), STAT5▵ is activated preferentially in CD4+ T cells. In contrast to a published report, expression of STATs from PBMCs of infected individuals was comparable with that of seronegative donors. In addition, in vitro infection of mitogen-activated PBMCs with a panel of laboratory-adapted and primary HIV strains characterized by differential usage of chemokine coreceptors did not affect STAT protein levels. However, enhanced activation of STAT was observed after in vitro infection of resting PBMCs and nonadherent PBMCs by different viral strains. Thus, constitutive STAT activation in CD4+T lymphocytes represents a novel finding of interest also as a potential new marker of immunological reconstitution of HIV-infected individuals.

Constitutive Activation of STATs Upon In Vivo Human Immunodeficiency Virus Infection

Infection by the human immunodeficiency virus (HIV) either upregulates or downregulates the expression of several cytokines and interferons (IFNs) that use the Janus kinase/signal transducer and activator of transcription (JAK/STAT) pathway for signal transduction. However, very little is known on the state of activation of the JAK/STAT pathway after HIV infection either in vivo or in vitro. In this regard, we report here that a constitutive activation of a C-terminal truncated STAT5 (STAT5▵) and of STAT1 occurs in the majority (∼75%) of individuals with progressive HIV disease. We have further demonstrated that, among peripheral blood mononuclear cells (PBMCs), STAT5▵ is activated preferentially in CD4+ T cells. In contrast to a published report, expression of STATs from PBMCs of infected individuals was comparable with that of seronegative donors. In addition, in vitro infection of mitogen-activated PBMCs with a panel of laboratory-adapted and primary HIV strains characterized by differential usage of chemokine coreceptors did not affect STAT protein levels. However, enhanced activation of STAT was observed after in vitro infection of resting PBMCs and nonadherent PBMCs by different viral strains. Thus, constitutive STAT activation in CD4+T lymphocytes represents a novel finding of interest also as a potential new marker of immunological reconstitution of HIV-infected individuals.

Genetic Evidence for an Additional Factor Required for Erythropoietin-Induced Signal Transduction

Erythropoietin (EPO) and its receptor (EPOR) are required for the development of mature erythrocytes. After binding of ligand, the EPOR activates a variety of signaling pathways that ultimately control cellular proliferation, survival, and specific gene expression. Although erythroid progenitors appear to be the principal EPO-responsive cell type in vivo due to the restricted expression of the EPOR, many growth factor–dependent cell lines expressing the EPOR can respond to EPO by activating many or all of these pathways. In the present study, we have identified a cellular context (the interleukin-2 [IL-2]–dependent HT-2 line) in which the EPO stimulation of the EPOR fails to support cellular proliferation, STAT-5 induction, or MAPK activation, despite efficient phosphorylation of the EPOR and JAK2 and inhibition of apoptosis after withdrawal of IL-2. Interestingly, when we fused HT-2 cells expressing the EPOR with Ba/F3 cells in a complementation assay, the resulting hybridomas proliferated and potently activated STAT-5 and MAPK in response to EPO. These data indicate that an unidentified cellular factor is needed to mediate signaling by the EPOR. Moreover, Ba/F3 cells apparently express this factor(s) and somatic fusions can, therefore, confer EPO-responsiveness to HT-2 cells that lack this factor.

Janus kinases and their role in growth and disease

Janus kinases (JAK) play a crucial role in the initial steps of cytokine signaling. Each of the four members (JAK1, JAK2, JAK3, TYK2) of this non-receptor tyrosine kinase family is indispensable for the effects of distinct cytokines. Moreover, recent reports have added to our knowledge on their highly specific functions: JAK3 knockout mice and JAK3 deficient patients cannot signal through the interleukin-2,4,7,9, or 15 receptors and suffer from severe combined immunodeficiency (SCID). JAK1 and JAK2 knockout mice do not survive, their cells again showing distinct patterns of cytokine signaling deficits. At the other end of the spectrum, JAK fusion proteins have been shown to play a role in leukemias. In addition, a new class of JAK-specific inhibitors was described by several groups, the CIS/SOCS/Jab family. This review on the rapidly growing field focuses on JAK function and regulation, and on their emerging role in development and human disease.

Biochemical analysis of interleukin-2 receptor beta chain phosphorylation by p56(lck)

Tyrosine phosphorylation of multiple proteins, including the receptor itself, is an initial event in IL-2 signaling and leads to recruitment of SH2 or PTB domain-containing proteins to the receptor. In this study, we have used subdomains of the IL-2 receptor beta chain (IL-2Rbeta) expressed in Escherichia coli as GST fusion proteins to identify the tyrosine residues that could be phosphorylated by p56(lck), one of the critical tyrosine kinases activated by IL-2. We report that recombinant p56(lck) phosphorylates in vitro tyrosine residues within the IL-2Rbeta chain but not those within the IL-2Rgamma chain. p56(lck) phosphorylates tyrosine residues 355, 358 and 361 but not 338 of the IL-2Rbeta chain acidic subdomain. Interestingly, phosphorylation of Tyr-358 appears to require the presence of either Tyr-355 or Tyr-361. p56(lck) also phosphorylates very efficiently the two tyrosines present in the IL-2Rbeta chain C-terminal region, Tyr-392 and Tyr-510. We also investigated the association of p56(lck) with the IL-2Rbeta chain which was found to depend on a short stretch of the IL-2Rbeta chain acidic subdomain, and to be independent of the presence of its tyrosine residues.

IL-2-Mediated Cell Cycle Progression and Inhibition of Apoptosis Does Not Require NF-κB or Activating Protein-1 Activation in Primary Human T Cells

The IL-2 growth hormone is the major growth factor of activated T lymphocytes during a developing immune response. IL-2 is required not only for cell cycle progression but also to protect Ag-activated T cells from programmed cell death. In several cell types, activation of NF-κB and/or activating protein-1 (AP-1) has been demonstrated to be extremely important in blocking apoptosis. To determine whether either or both of these transcription factors are involved in cell survival or cell cycle progression in response to IL-2, primary human T cells responsive to the growth factor were analyzed for NF-κB and AP-1 activation. The current study clearly demonstrates that IL-2 does not induce IκBα degradation or NF-κB activation in primary human T cells that respond to IL-2 by entering the cell cycle and avoiding apoptosis. Similarly, IL-2 neither activates JNK nor increases AP-1 binding activity to a consensus o-tetradecanoylphorbol 13-acetate (TPA) response element. On the other hand, the growth factor does induce the activation of STAT3 and STAT5 in these cells, as has been previously demonstrated. These data show that neither NF-κB nor AP-1 activation is required for IL-2-mediated survival or cell cycle progression in activated primary human T cells.

Autosomal SCID caused by a point mutation in the N-terminus of Jak3: mapping of the Jak3-receptor interaction domain

Signaling through the hematopoietic receptors requires activation of receptor-associated Janus (Jak) kinases. For example, Jak1 and Jak3 bind specifically to the IL-2 receptor beta (IL-2Rbeta) and common gamma (gammac) chains, respectively, and initiate biochemical signals critical in controlling immune responses. The region of Jak responsible for receptor interactions, however, is not well characterized. Here we describe a naturally occurring Jak3 mutation from a patient with autosomal severe combined immunodeficiency (SCID), where a single amino acid substitution, Y100C, in Janus homology domain 7 (JH7) prevents kinase-receptor interaction. This mutation also results in a loss of IL-2-induced signaling in a B-cell line derived from this patient. Using mutational analysis we have identified a region of Jak3, including portions of JH6 and JH7, that is sufficient for kinase-receptor contact and show that this segment interacts with the proline-rich Box1 region of the receptor. Furthermore, a Jak3-Jak1 chimera containing only the JH6 and JH7 domains of Jak3 interacts with gammac and can reconstitute IL-2-dependent responses, including receptor phosphorylation and activation of signal transducer and activator of transcription (STAT) 5b. Our results suggest that the N-terminus of Jak kinases is critical for receptor binding, and is therefore likely to determine specificity of Jak kinase-receptor interactions.

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.

Interleukin-7: physiological roles and mechanisms of action

Interleukin-7 (IL-7), a product of stromal cells, provides critical signals to lymphoid cells at early stages in their development. Two types of cellular responses to IL-7 have been identified in lymphoid progenitors: (1) a trophic effect and (2) an effect supporting V(D)J recombination. The IL-7 receptor is comprised of two chains, IL-7R alpha and gamma(c). Following receptor crosslinking, rapid activation of several classes of kinases occurs, including members of the Janus and Src families and PI3-kinase. A number of transcription factors are subsequently activated including STATs, c-myc, NFAT and AP-1. However, it remains to be determined which, if any, previously identified pathway leads to the trophic or V(D)J endpoints. The trophic response to IL-7 involves protecting lymphoid progenitors from a death process that resembles apoptosis. This protection is partly mediated by IL-7 induction of Bcl-2, however other IL-7-induced events are probably also involved in the trophic response. The V(D)J response to IL-7 is partly mediated through increased production of Rag proteins (which cleave the target locus) and partly by increasing the accessibility of a target locus to cleavage through chromatin remodeling.

Oligomerization and scaffolding functions of the erythropoietin receptor cytoplasmic tail

Signal transduction by the erythropoietin receptor (EPOR) is activated by ligand-mediated receptor homodimerization. However, the relationship between extracellular and intracellular domain oligomerization remains poorly understood. To assess the requirements for dimerization of receptor cytoplasmic sequences for signaling, we overexpressed mutant EPORs in combination with wild-type (WT) EPOR to drive formation of heterodimeric (i.e. WT-mutant) receptor complexes. Dimerization of the membrane-proximal portion of the EPOR cytoplasmic region was found to be critical for the initiation of mitogenic signaling. However, dimerization of the entire EPOR cytoplasmic region was not required. To examine this process more closely, we generated chimeras between the intracellular and transmembrane portions of the EPOR and the extracellular domains of the interleukin-2 receptor beta and gammac chains. These chimeras allowed us to assess more precisely the signaling role of each receptor chain because only heterodimers of WT and mutant receptor chimeras form in the presence of interleukin-2. Coexpression studies demonstrated that a functional receptor complex requires the membrane-proximal region of each receptor subunit in the oligomer to permit activation of JAK2 but only one membrane-distal tail to activate STAT5 and to support cell proliferation. Thus, this study defines key relationships involved in the assembly and activation of the EPOR signal transduction complex which may be applicable to other homodimeric cytokine receptors.

Functional dissection of the cytoplasmic subregions of the IL-2 receptor betac chain in primary lymphocyte populations

The interleukin 2 (IL-2) receptor betac chain (IL-2Rbetac) is known to regulate the development and function of distinct lymphocyte populations. Thus far, the functions of the IL-2Rbetac cytoplasmic subregions have been studied extensively by using cultured cell lines; however, this approach has limitations with respect to their functions in distinct primary lymphocyte populations. In the present study, we generated mice each expressing a mutant form of an IL-2Rbetac transgene, lacking the cytoplasmic A- or H-region, on an IL-2Rbetac null background. We show that lack of the H-region, which mediates activation of the Stat5/Stat3 transcription factors, selectively affects the development of natural killer cells and T cells bearing the gamma delta T cell receptor. This region is also required for the IL-2-induced proliferation of T cells in vitro, by upregulating IL-2Ralpha expression. In contrast, the A-region, which mediates activation of the Src family protein tyrosine kinase (PTK) members, contributes to downregulation of the T cell proliferation function. The IL-2Rbetac null mutant mice develop severe autoimmune symptoms; these are all suppressed following the expression of either of the mutants, suggesting that neither the Stats nor the Src PTK members are required. Thus, our present approach offers new insights into the functions of these cytoplasmic subregions of the IL-2Rbetac chain.

The IL-2 Receptor Promotes Proliferation, bcl-2 and bcl-x Induction, But Not Cell Viability Through the Adapter Molecule Shc

IL-2, the principal mitogenic factor for activated T cells, delivers a proliferative signal through ligation of the heterotrimeric IL-2R. This proliferative signal is critically dependent upon cytoplasmic tyrosines on the β-chain of this receptor (IL-2Rβ) becoming phosphorylated in response to ligand. We found that at least one of these tyrosines (Y338) also mediates cell survival and induction of bcl-2, bcl-x, and c-myc in the murine T cell line CTLL-2. Since the adapter molecule Shc binds to phosphorylated Y338, the specific contribution of Shc to these events was evaluated. An IL-2Rβ/Shc fusion protein, in which Shc was covalently tethered to a truncated version of IL-2Rβ lacking all cytoplasmic tyrosines, revealed a robust proliferative signal mediated through Shc. This Shc-mediated signal induced expression of c-myc as well as the antiapoptotic genes bcl-2 and bcl-x with normal magnitude and kinetics. Nonetheless, signals from this fusion protein failed to sustain the long-term viability of CTLL-2 cells. Thus, induction of bcl family genes and delivery of a competent proliferative signal are not sufficient to promote cell survival and mediate the antiapoptotic effects associated with a complete IL-2 signal.

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.

Absence of cytokine receptor-dependent specificity in red blood cell differentiation in vivo

Erythropoietin (EPO) is required for red blood cell development, but whether EPO-specific signals directly instruct erythroid differentiation is unknown. We used a dominant system in which constitutively active variants of the EPO receptor were introduced into erythroid progenitors in mice. Chimeric receptors were constructed by replacing the cytoplasmic tail of constitutively active variants of the EPO receptor with tails of diverse cytokine receptors. Receptors linked to granulocyte or platelet production supported complete erythroid development in vitro and in vivo, as did the growth hormone receptor, a nonhematopoietic receptor. Therefore, EPOR-specific signals are not required for terminal differentiation of erythrocytes. Furthermore, we found that cellular context can influence cytokine receptor signaling.

Jaks and STATs: biological implications

Cytokines and interferons are molecules that play central roles in the regulation of a wide array of cellular functions in the lympho-hematopoietic system. These factors stimulate proliferation, differentiation, and survival signals, as well as specialized functions in host resistance to pathogens. Although cytokines are known to activate multiple signaling pathways that together mediate these important functions, one of these pathways, the Jak-STAT pathway, is the focus of this chapter. This pathway is triggered by both cytokines and interferons, and it very rapidly allows the transduction of an extracellular signal into the nucleus. The pathway uses a novel mechanism in which cytosolic latent transcription factors, known as signal transducers and activators of transcription (STATs), are tyrosine phosphorylated by Janus family tyrosine kinases (Jaks), allowing STAT protein dimerization and nuclear translocation. STATs then can modulate the expression of target genes. The basic biology of this system, including the range of known Jaks and STATs, is discussed, as are the defects in animals and humans lacking some of these signaling molecules.

Heteromerization of the gammac chain with the interleukin-9 receptor alpha subunit leads to STAT activation and prevention of apoptosis

Interleukin-9 (IL-9) is a cytokine with pleiotropic effects on mast cell and T cell lines. It exerts its effects through the IL-9R complex consisting of IL-9Ralpha and the common gammac subunit. Here we report functional evidence for receptor heteromerization for efficient signal transduction, and we define minimal requirements in the two receptor subunits for IL-9R function. Tyrosine 336 of the IL-9Ralpha and the membrane-proximal segment of gammac are both crucial for signaling. The activated IL-9R complex employs the Janus kinases JAK1 and JAK3 for subsequent activation of the signal transducer and activator transcription (STAT) factors STAT-1, STAT-3, and STAT-5. This process is independent of Tyk2. We demonstrate further that the activated STAT complexes consist of STAT-1 and STAT-5 homodimers and STAT-1-STAT-3 heterodimers. Finally, we show that IL-9R signaling in a T cell line does not result in detectable mitogen-activated protein kinase activation and leads to unsustained proliferation. Nonetheless, these T cells are efficiently protected from dexamethasone-induced apoptosis. These results further define the molecular architecture of the IL-9R and its specific connections to various biologic responses.

Regulation of Cell Growth by IL-2: Role of STAT5 in Protection from Apoptosis But Not in Cell Cycle Progression

Cytokines play an essential role in the regulation of lymphocyte survival and growth. We have analyzed the pathways activated by IL-2 that lead to protection from apoptosis and cell proliferation. IL-2 can act as a long-term growth factor in 32D cells expressing the wild-type human (hu)IL-2Rβ. By contrast, cells expressing a truncated form of the huIL-2Rβ, which is able to induce Bcl-2 and c-myc expression but not STAT5 activation, were not protected from apoptosis by IL-2; consequently, they could not be grown long term in the presence of IL-2. However, IL-2 promoted cell cycle progression in cells bearing the truncated huIL-2Rβ with percentages of viable cells in the G0/G1, S, and G2/M phases similar to cells expressing the wild-type huIL-2Rβ. Transplantation of a region from the erythropoietin receptor, which contains a docking site for STAT5 (Y343) to the truncated huIL-2Rβ, restored the ability of IL-2 to signal both activation of STAT5 and protection from apoptosis. By contrast, transplantation of a region from the huIL-4Rα containing STAT6 docking sites did not confer protection from apoptosis. These results indicate that the IL-2-induced cell cycle progression can be clearly distinguished from protection from apoptosis and that STAT5 participates in the regulation of apoptosis.

Recruitment of SH2-containing protein tyrosine phosphatase SHP-1 to the interleukin 2 receptor; loss of SHP-1 expression in human T-lymphotropic virus type I-transformed T cells

Interleukin 2 (IL-2) rapidly induces tyrosine phosphorylation of intracellular substrates, including the IL-2 receptor beta chain (IL-2Rbeta), Janus kinase 1 (Jak1), Jak3, signal transducer/activator of transcription proteins, and Shc, but the mechanism underlying dephosphorylation of these proteins is not known. The src homology 2 (SH2) containing tyrosine phosphatase 1 (SHP-1) is recruited by several hematopoietic surface receptors indicating that this phosphatase plays an important role as a regulator of signaling. We have found that IL-2 induces association of SHP-1 with the IL-2 receptor complex, and that once SHP-1 is recruited to the activated receptor it is able to decrease tyrosine phosphorylation of IL-2Rbeta and the associated tyrosine kinases Jak1 and Jak3. This dephosphorylation is specific as expression of a catalytically inactive form of SHP-1, or expression of the related phosphatase SHP-2 did not result in dephosphorylation of the IL-2 receptor components. Furthermore, we have found that SHP-1 expression is greatly decreased or undetectable in a number of IL-2 independent HTLV-I transformed T cell lines that exhibit constitutive Jak/signal transducer/activator of transcription activation. In HTLV-I infected T cells, down-regulation of SHP-1 expression was also found to correlate with the acquisition of IL-2 independence. These observations suggest that SHP-1 normally functions to antagonize the IL-2 signal transduction pathway and that HTLV-I infection and oncogenic transformation can lead to loss of SHP-1 expression resulting in constitutive activation of IL-2 regulated T cell responses.

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.

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.

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.

An indirect effect of Stat5a in IL-2-induced proliferation: a critical role for Stat5a in IL-2-mediated IL-2 receptor alpha chain induction

Stat5a was identified as a prolactin-induced transcription factor but also is activated by other cytokines, including interleukin-2 (IL-2) and IL-7. We have now analyzed the immune system of Stat5a-deficient mice. Stat5a-/- splenocytes exhibited defective IL-2-induced expression of the IL-2 receptor alpha chain (IL-2R alpha), a protein that together with IL-2R beta and the common cytokine receptor gamma chain (gamma(c)) mediates high-affinity IL-2 binding. Correspondingly, Stat5a-/- splenocytes exhibited markedly decreased proliferation to IL-2, although maximal proliferation was still achieved at IL-2 concentrations high enough to titrate intermediate-affinity IL-2R beta/gamma(c) receptors. Thus, defective Stat5a expression results in diminished proliferation by an indirect mechanism, resulting from defective receptor expression. Correspondingly, we show that Stat5a is essential for maximal responsiveness to antigenic stimuli in vivo, underscoring the physiological importance of IL-2-induced IL-2R alpha expression.

An interleukin (IL)-13 receptor lacking the cytoplasmic domain fails to transduce IL-13-induced signals and inhibits responses to IL-4

Interleukin (IL)-13 is a pleiotropic immunoregulatory cytokine that shares many, although not all, of the biological activities of IL-4. The overlapping biological properties of IL-4 and IL-13 appear to be due to the existence of shared components of the receptors, and we and others showed that the IL-4 receptor-alpha is involved in signal transduction paths activated by both. We show here that expression of the IL-13 receptor-alpha in two factor-dependent cell lines, the premyeloid FD5 and the T lymphoid CT4.S, conferred the ability to grow continuously in response to IL-13; to respond to IL-13 with tyrosine phosphorylation of JAK1, Tyk2, IL-4Ralpha, IRS-2, and STAT6; and to respond to IL-4 with tyrosine phosphorylation of Tyk2 in addition to those induced in parental cell lines. Expression of a truncated IL-13 receptor-alpha that lacked the cytoplasmic domain demonstrated that this domain was essential for IL-13-dependent growth and phosphorylation of the above substrates. Expression of this truncated IL-13 receptor also resulted in an inhibition of biochemical and biological responses to IL-4 that was exacerbated by the presence of IL-13. These dominant inhibitory effects indicate that the extracellular domain of the truncated IL-13 receptor competes with gammac for complexes of IL-4 and the IL-4 receptor-alpha, or, when itself bound to IL-13, competes with IL-4 for the IL-4 receptor-alpha.

The amino terminus of JAK3 is necessary and sufficient for binding to the common gamma chain and confers the ability to transmit interleukin 2-mediated signals

JAK3 is a protein tyrosine kinase that specifically associates with the common gamma chain (gammac), a shared subunit of receptors for interleukin (IL) 2, 4, 7, 9, and 15. Patients deficient in either JAK3 or gammac presented with virtually identical forms of severe combined immunodeficiency (SCID), underscoring the importance of the JAK3-gammac interaction. Despite the key roles of JAK3 and gammac in lymphocytic development and function, the molecular basis of this interaction remains poorly understood. In this study, we have characterized the regions of JAK3 involved in gammac association. By developing a number of chimeric JAK3-JAK2 constructs, we show that the binding specificity to gammac can be conferred to JAK2 by transferring the N-terminal domains of JAK3. Moreover, those JAK3-JAK2 chimeras capable of binding gammac were also capable of reconstituting IL-2 signaling as measured by inducible phosphorylation of the chimeric JAK3-JAK2 protein, JAK1, the IL-2 receptor beta chain, and signal transducer and activator of transcription 5A. Subsequent deletion analyses of JAK3 have identified the N-terminal JH7-6 domains as a minimal region sufficient for gammac association. Furthermore, expression of the mutant containing only the JH7-6 domains effectively competed with full-length JAK3 for binding to gammac. We conclude that the JH7-6 domains of JAK3 are necessary and sufficient for gammac association. These studies offer clues toward a broader understanding of JAK-mediated cytokine signaling and may provide a target for the development of novel therapeutic modalities in immunologically mediated diseases.

Requirement for an initial signal from the membrane-proximal region of the interleukin 2 receptor gamma(c) chain for Janus kinase activation leading to T cell proliferation

The interleukin 2 receptor (IL-2R) generates proliferative signals in T lymphocytes by ligand-induced heterodimerization of two chains, IL-2Rbeta and gamma(c), which associate with the tyrosine kinases Jak1 and Jak3, respectively. Genetic and molecular studies have demonstrated that Jak3 is essential for mitogenic signaling by the gamma(c) chain; because it is also the only molecule known to associate with gamma(c), we speculated that Jak3 might be sufficient for signaling by this chain. Therefore, fusion proteins were constructed in which all or part of the cytoplasmic domain of gamma(c) was replaced by Jak3. Signaling was evaluated in the IL-2-dependent T cell line CTLL-2 using chimeric IL-2Rbeta and gamma(c) chains that bind and are activated by the cytokine granulocyte-macrophage colony-stimulating factor. Chimeric gamma(c) chains containing only Jak3 in the cytoplasmic domain failed to mediate proliferation of CTLL-2 cells, but addition of a conserved membrane-proximal (PROX) domain of gamma(c) in tandem with Jak3 fully reconstituted gamma(c) function. The requirement for the PROX domain reflected an essential role in the activation of Jak3 in vivo. Despite lacking defined catalytic motifs, PROX induced an early Jak-independent signal, including tyrosine phosphorylation of IL-2Rbeta and the tyrosine phosphatase SHP-2. The results define the minimal signaling components of gamma(c) and suggest a new mechanism by which the IL-2R initiates signaling in response to ligand.