Mapping the intracytoplasmic regions of the alpha granulocyte-macrophage colony-stimulating factor receptor necessary for cell growth regulation

Messing with βc: A unique receptor with many goals

Our understanding of the biological role of the βc family of cytokines has evolved enormously since their initial identification as bone marrow colony stimulating factors in the 1960's. It has become abundantly clear over the intervening decades that this family of cytokines has truly astonishing pleiotropic capacity, capable of regulating not only hematopoiesis but also many other normal and pathological processes such as development, inflammation, allergy and cancer. As noted in the current pandemic, βc cytokines contribute to the cytokine storm seen in acutely ill COVID-19 patients. Ongoing studies to discover how these cytokines activate their receptor are revealing insights into the fundamental mechanisms that give rise to cytokine pleiotropy and are providing tantalizing glimpses of how discrete signaling pathways may be dissected for activation with novel ligands for therapeutic benefit.

STAT3-activated GM-CSFRα translocates to the nucleus and protects CLL cells from apoptosis

Here, it was determined that chronic lymphocytic leukemia (CLL) cells express the α subunit, but not the β subunit, of the granulocyte-macrophage colony-stimulating factor receptor (GM-CSFR/CSF2R). GM-CSFRα was detected on the surface, in the cytosol, and in the nucleus of CLL cells via confocal microscopy, cell fractionation, and GM-CSFRα antibody epitope mapping. Because STAT3 is frequently activated in CLL and the GM-CSFRα promoter harbors putative STAT3 consensus binding sites, MM1 cells were transfected with truncated forms of the GM-CSFRα promoter, then stimulated with IL6 to activate STAT3 and to identify STAT3-binding sites. Chromatin immunoprecipitation (ChIP) and an electoromobility shift assay (EMSA) confirmed STAT3 occupancy to those promoter regions in both IL6-stimulated MM1 and CLL cells. Transfection of MM1 cells with STAT3-siRNA or CLL cells with STAT3-shRNA significantly downregulated GM-CSFRα mRNA and protein levels. RNA transcripts, involved in regulating cell survival pathways, and the proteins KAP1 (TRIM28) and ISG15 coimmunoprecipitated with GM-CSFRα. GM-CSFRα-bound KAP1 enhanced the transcriptional activity of STAT3, whereas GM-CSFRα-bound ISG15 inhibited the NF-κB pathway. Nevertheless, overexpression of GM-CSFRα protected MM1 cells from dexamethasone-induced apoptosis, and GM-CSFRα knockdown induced apoptosis in CLL cells, suggesting that GM-CSFRα provides a ligand-independent survival advantage.

IMPLICATIONS

Constitutively, activation of STAT3 induces the expression of GM-CSFRα that protects CLL cells from apoptosis, suggesting that inhibition of STAT3 or GM-CSFRα may benefit patients with CLL.

Crossing paths: interactions between the cell death machinery and growth factor survival signals

Cytokines and growth factors play a crucial role in the maintenance of haematopoietic homeostasis. They transduce signals that regulate the competing commitments of haematopoietic stem cells, quiescence or proliferation, retention of stem cell pluripotency or differentiation, and survival or demise. When the balance between these commitments and the requirements of the organisms is disturbed, particularly when it favours survival and proliferation, cancer may result. Cell death provoked by loss of growth factor signalling is regulated by the Bcl-2 family of apoptosis regulators, and thus survival messages transduced by growth factors must regulate the activity of these proteins. Many aspects of direct interactions between cytokine signalling and regulation of apoptosis remain elusive. In this review, we explore the mechanisms by which cytokines, in particular Interleukin-3 and granulocyte-macrophage colony-stimulating factor, promote cell survival and suppress apoptosis as models of how cytokine signalling and apoptotic pathways intersect.

Alternative modes of GM-CSF receptor activation revealed using activated mutants of the common beta-subunit

Granulocyte/macrophage colony-stimulating factor promotes growth, survival, differentiation, and activation of normal myeloid cells and plays an important role in myeloid leukemias. The GM-CSF receptor (GMR) shares a signaling subunit, beta(c), with interleukin-3 and interleukin-5 receptors and has recently been shown to induce activation of Janus kinase 2 (JAK2) and downstream signaling via formation of a unique dodecameric receptor complex. In this study we use 2 activated beta(c) mutants that display distinct signaling capacity and have differential requirements for the GMR alpha-subunit (GMR-alpha) to dissect the signaling pathways associated with the GM-CSF response. The V449E transmembrane mutant selectively activates JAK2/signal transducer and activator of transcription 5 and extracellular signal-regulated kinase (ERK) pathways, resulting in a high level of sensitivity to JAK and ERK inhibitors, whereas the extracellular mutant (FIDelta) selectively activates the phosphoinositide 3-kinase/Akt and IkappaKbeta/nuclear factorkappaB pathways. We also demonstrate a novel and direct interaction between the SH3 domains of Lyn and Src with a conserved proline-rich motif in GMR-alpha and show a selective requirement for Src family kinases by the FIDelta mutant. We relate the nonoverlapping nature of signaling by the activated mutants to the structure of the unique GMR complex and propose alternative modes of receptor activation acting synergistically in the mature liganded receptor complex.

The alpha subunit of the granulocyte-macrophage colony-stimulating factor receptor interacts with c-Kit and inhibits c-Kit signaling

The cytokine granulocyte-macrophage colony-stimulating factor (GM-CSF) regulates hematopoiesis and the function of mature host defense cells through the GM-CSF receptor (GMR), which is composed of alpha (alphaGMR) and beta (betaGMR) subunits. Stem cell factor is another important hematopoietic cytokine that signals through c-Kit, a receptor tyrosine kinase, and regulates hematopoietic stem cell maintenance and erythroid development. Like other cytokine receptors, GMR and c-Kit are generally deemed as independent adaptor molecules capable of transducing cytokine-specific signals. We found that the alphaGMR directly interacts with c-Kit and that the interaction is mediated by the cytoplasmic domains. Furthermore, alphaGMR inhibited c-Kit auto-phosphorylation induced by the ligand stem cell factor. Consistent with the inhibitory effect, the expression of alphaGMR was suppressed in cells whose viability was dependent on c-Kit signaling. In contrast, the alternatively spliced alpha2 isoform of the alphaGMR could not inhibit c-Kit signaling, providing a rationale for the existence of the alpha2 isoform. Our results suggest that in addition to having the commonly appreciated roles in cytokine signal transduction, the receptors alphaGMR and c-Kit could interact to coordinate their signal initiation.

The laminin receptor modulates granulocyte-macrophage colony-stimulating factor receptor complex formation and modulates its signaling

Basement membrane matrix proteins are known to up-regulate granulocyte-macrophage colony-stimulating factor (GM-CSF) signaling in neutrophils and mononuclear phagocytes, but the mechanisms involved are poorly understood. We used the intracellular portion of the alpha subunit of the GM-CSF receptor (alphaGMR) to search for interacting proteins and identified the 67-kDa laminin receptor (LR), a nonintegrin matrix protein receptor expressed in several types of host defense cells and certain tumors, as a binding partner. LR was found to interact with the beta subunit of the GMR (betaGMR) as well. Whereas GM-CSF functions by engaging the alphaGMR and betaGMR into receptor complexes, LR inhibited GM-CSF-induced receptor complex formation. Laminin and fibronectin binding to LR was found to prevent the binding of betaGMR to LR and relieved the LR inhibition of GMR. These findings provide a mechanistic basis for enhancing host defense cell responsiveness to GM-CSF at transendothelial migration sites while suppressing it in circulation.

Functional identification of secondary mutations inducing autonomous growth in synergy with a truncated interleukin-3 receptor Implications for multi-step oncogenesis

OBJECTIVE

A truncated common beta chain (Deltabeta(C)) of the interleukin-3 (IL-3) receptor complex was previously identified as a key factor in inducing autonomous growth of IL-3-independent mutants. Expression of Deltabeta(C) in IL-3-dependent hematopoietic cells does not result in immediate factor-independent growth, but increases the frequency of obtaining autonomous mutants by three to four orders of magnitude. This study was designed to delineate the mechanisms by which Deltabeta(C) increases the frequency to autonomous growth.

DESIGN AND METHODS

Retroviral vectors were used to express Deltabeta(C) into IL-3-dependent myeloid cells, which were then tested for factor-independent growth. To determine if secondary genetic events were required for conversion to autonomous growth, elements of the Cre-loxP recombinant system were used to excise Deltabeta(C) in factor-independent clones.

RESULTS

Excision of Deltabeta(C) in factor-independent clones revealed two types of phenotypes: reversion to factor-dependent growth (1/8) or continued IL-3-dependent growth (7/8). Analysis of cells that remained factor independent revealed constitutive activation of STAT5, not observed in factor-dependent revertants. Analysis of revertant cells demonstrated the presence of interacting secondary mutations that synergize with Deltabeta(C)-induced proliferation. A cysteine residue within the truncated extracellular domain of Deltabeta(C) was also found to be required for its oncogenic potential, supporting a model of dimerization for receptor activation.

CONCLUSIONS

The high incidence of obtaining factor-independent mutants from cells expressing Deltabeta(C) results from the selection of mutations that either complement Deltabeta(C) expression to promote proliferation or that singly or in synergy with other secondary mutations negate the requirement of Deltabeta(C) expression for proliferation.

Distinct domains of the human granulocyte-macrophage colony-stimulating factor receptor alpha subunit mediate activation of Jak/Stat signaling and differentiation

The alpha subunit of the human granulocyte-macrophage colony-stimulating factor (GM-CSF) receptor has several isoforms that result from alternative splicing events. Two forms, alpha-1 and alpha-2, have intracytoplasmic sequences that are identical within a membrane-proximal domain but differ completely distally. Variant and mutated GM-CSF receptor alpha subunits, along with the beta subunit (beta(c) protein) were expressed in M1 murine leukemia cells. and the ability of the receptors to signal for differentiation events and to activate Jak/Stat signaling pathways was examined. All cell lines expressing both alpha and beta(c) proteins exhibited high-affinity binding of radiolabeled human GM-CSF. Receptor alpha subunits with intact membrane-proximal intracellular domains could induce expression of the macrophage antigen F4/80 and down-regulate the expression of CD11b. Addition of recombinant human GM-CSF to cells expressing alpha-1 subunits induced the expression of CD86 and tyrosine phosphorylation of Jak-2 and its putative substrates SHPTP-2, Stat-5, and the GM-CSF receptor beta(c) subunit. Cells containing alpha subunits that lacked a distal domain (term-3) or had the alternatively spliced alpha-2 distal domain showed markedly decreased ability to support tyrosine phosphorylation of Jak-2 and its substrates or to up-regulate CD86. Ligand binding induced stable association of the alpha-1 subunit and beta(c) protein. In contrast, the alpha-2 subunit did not stably associate with the beta(c) subunit. These data identify potential molecular mechanisms for differential signaling of the alpha-1 and alpha-2 proteins. The association of unique signaling events with the 2 active GM-CSF alpha subunit isoforms offers a model for variable response phenotypes to the same ligand.

Specificity in cytokine signal transduction: lessons learned from the IL-3/IL-5/GM-CSF receptor family

Cytokines mediate the transduction of proliferative, differentiation and survival signals in the hematopoietic system. Although the cytokine family is large and diverse, many different cytokines display broadly overlapping functions. This can be explained by the fact that cytokine receptors often share multiple subunits. Specificity in signal transduction can however be achieved through several mechanisms. This review focuses on how signal specificity can be achieved within the IL-3, IL-5 and GM-CSF receptor family. This is discussed in terms of receptor expression, recent advances in our understanding of intracellular signalling components, and analysis of null mutant knock-out mice.

Capture of cytokine-responsive genes (NACA and RBM3) using a gene trap approach

We have developed a gene trap approach to select specific cytokine receptor/ligand responsive genes in the cell line TF-1. This cell line exhibits a dependency on granulocyte-macrophage colony-stimulating factor (GM-CSF) or interleukin-3 (IL-3) and responds to interleukin-5 (IL-5). In an attempt to detect genes modulated by one of these factors, cells were infected with the Rosaβgeo retrovirus in the presence of GM-CSF, IL-3, or IL-5 and clones were selected for retroviral integration on the basis of G418 resistance. Housekeeping and cytokine-regulated trapped genes were then differentiated on the basis of G418 resistance versus sensitivity in the presence of the different cytokines. To determine the reliability of this screen, DNA sequences upstream of the proviral integration site were identified by 5′ rapid amplification of DNA ends polymerase chain reaction (RACE PCR) from selected GM-CSF–treated and –infected clones. Comparison of the sequences with those in the Genbank database revealed that 2 sequences correspond to known genes: NACA and RBM3. NACAwas recently defined as a coactivator of c-jun–mediated transcription factors in osteoblasts, and RBM3 as a protein from the heterogeneous nuclear ribonucleoprotein family. Data from transcriptional analysis of these 2 genes in TF-1 cells showed a specific up-regulation by GM-CSF. Both transcripts were also found to be up-regulated in purified CD34+ cells, suggesting their involvement in proliferative processes during hematopoiesis. Interestingly, down-regulation was observed during monocytic differentiation of TF-1 cells, suggesting their extinction could contribute to monocytic lineage development. This study demonstrates that this gene trap approach is a useful method for identifying novel, specific cytokine-responsive genes that are involved in the regulation of hematopoiesis.

Capture of cytokine-responsive genes (NACA and RBM3) using a gene trap approach

We have developed a gene trap approach to select specific cytokine receptor/ligand responsive genes in the cell line TF-1. This cell line exhibits a dependency on granulocyte-macrophage colony-stimulating factor (GM-CSF) or interleukin-3 (IL-3) and responds to interleukin-5 (IL-5). In an attempt to detect genes modulated by one of these factors, cells were infected with the Rosaβgeo retrovirus in the presence of GM-CSF, IL-3, or IL-5 and clones were selected for retroviral integration on the basis of G418 resistance. Housekeeping and cytokine-regulated trapped genes were then differentiated on the basis of G418 resistance versus sensitivity in the presence of the different cytokines. To determine the reliability of this screen, DNA sequences upstream of the proviral integration site were identified by 5′ rapid amplification of DNA ends polymerase chain reaction (RACE PCR) from selected GM-CSF–treated and –infected clones. Comparison of the sequences with those in the Genbank database revealed that 2 sequences correspond to known genes: NACA and RBM3. NACAwas recently defined as a coactivator of c-jun–mediated transcription factors in osteoblasts, and RBM3 as a protein from the heterogeneous nuclear ribonucleoprotein family. Data from transcriptional analysis of these 2 genes in TF-1 cells showed a specific up-regulation by GM-CSF. Both transcripts were also found to be up-regulated in purified CD34+ cells, suggesting their involvement in proliferative processes during hematopoiesis. Interestingly, down-regulation was observed during monocytic differentiation of TF-1 cells, suggesting their extinction could contribute to monocytic lineage development. This study demonstrates that this gene trap approach is a useful method for identifying novel, specific cytokine-responsive genes that are involved in the regulation of hematopoiesis.

A model for assembly and activation of the GM-CSF, IL-3 and IL-5 receptors: insights from activated mutants of the common beta subunit

Granulocyte-macrophage colony stimulating factor (GM-CSF), Interleukin-3 (IL-3) and Interleukin-5 (IL-5) have overlapping, pleiotropic effects on hematopoietic cells, including neutrophils, eosinophils, monocytes and early progenitor cells. The high-affinity receptors for human GM-CSF, IL-3, and IL-5 share a common beta-subunit (hbeta(c)), which is essential for signalling and plays a major role in recruiting intracellular signalling molecules. While activation of the cytoplasmic tyrosine kinase JAK2 appears to be the initiating event for signalling, the immediate events that trigger this are still unclear. We have isolated a number of activated mutants of hbeta(c), which can be grouped into classes defined by their state of receptor phosphorylation, their requirement for alpha subunit as a cofactor, and their activities in primary cells and cell lines. We discuss these findings with regard to the stoichiometry, activation, and signalling of the normal GM-CSF/IL-3/IL-5 receptor complexes. Specifically, this work has implications for the role of the ligand-specific alpha-subunits in initiating the signalling through the beta-subunit, the role of beta subunit dimerization as a receptor trigger, and the function of receptor tyrosine phosphorylation in generating growth and survival signals. Based on the properties of the activated mutants and the recent structures of erythropoietin receptor (Epo-R) complexes, we propose a model in which (1) activation of hbeta(c) can occur via alternative states that differ with respect to stoichiometry and subunit assembly, but which all mediate proliferative responses, and (2) each of the different classes of activated mutants mimics one of these alternative states.

Molecular determinants of the granulocyte-macrophage colony-stimulating factor receptor complex assembly

The granulocyte-macrophage colony-stimulating factor (GM-CSF) receptor (GMR) is composed of two chains that belong to the superfamily of cytokine receptors typified by the growth hormone receptor. A common structural element found in cytokine receptors is a module of two fibronectin-like domains, each characterized by seven beta-strands denoted A-G and A'-G', respectively. The alpha-chain (GMRalpha) confers low affinity GM-CSF binding (K(d) = 1-5 nM), whereas the beta-chain (beta(c)) does not bind GM-CSF by itself but confers high affinity binding when associated with alpha (K(d) = 40-100 pM). In the present study, we define the molecular determinants required for ligand recognition and for stabilization of the complex through a convergence of several approaches, including the construction of chimeric receptors, the molecular dynamics of our three-dimensional model of the GM.GMR complex, and site-directed mutagenesis. The functional importance of individual residues was then investigated through ligand binding studies at equilibrium and through determination of the kinetic constants of the GM.GMR complex. Critical to this tripartite complex is the establishment of four noncovalent bonds, three that determine the nature of the ligand recognition process involving residues Arg(280) and Tyr(226) of the alpha-chain and residue Tyr(365) of the beta-chain, since mutations of either one of these residues resulted in a significant decrease in the association rate. Finally, residue Tyr(365) of beta(c) serves a dual function in that it cooperates with another residue of beta(c), Tyr(421) to stabilize the complex since mutation of Tyr(365) and Tyr(421) result in a drastic increase in the dissociation rate (Koff). Interestingly, these four residues are located at the B'-C' and F'-G' loops of GMRalpha and of beta(c), thus establishing a functional symmetry within an apparently asymmetrical heterodimeric structure.

Heterodimerization of the  and β Chains of the Interleukin-3 (IL-3) Receptor Is Necessary and Sufficient for IL-3–Induced Mitogenesis

The high-affinity receptor for interleukin-3 (IL-3) is a complex of the IL-3–binding subunit (IL-3) and a larger β chain—βc, or, in the mouse, βc or its close relative βIL-3. There is evidence that the critical event that initiates signaling is not the approximation of the cytoplasmic domains of IL-3 and βIL-3, but is, rather, the formation of a β-β homodimer. Many of these studies involved the analyses of receptor chimeras where the cytoplasmic domains were derived from IL-3, βc or βIL-3, and the extracellular domains were derived from other cytokine receptors, such as the erythropoietin receptor (EpoR). However, evidence that the EpoR may also associate with other receptors clouds the interpretation of these experiments. Therefore, we reevaluated the structure of the functional IL-3R using chimeric receptors with extracellular domains derived not from members of the cytokine-receptor family, but from CD8 or CD16. We show, by expression of these chimeras in Ba/F3 or CTLL-2 cells, that mitogenic signals were only generated by heterodimerization of the cytoplasmic domains of IL-3 and βIL-3. Homodimers of either IL-3 or βIL-3, alone or in combination, were nonfunctional. Furthermore, the ability of heterodimers to stimulate mitogenesis correlated with their ability to induce tyrosine phosphorylation of JAK-2. These data suggest that the physiological activation of the IL-3R involves the generation of simple heterodimers of IL-3 and βIL-3.

Heterodimerization of the  and β Chains of the Interleukin-3 (IL-3) Receptor Is Necessary and Sufficient for IL-3–Induced Mitogenesis

The high-affinity receptor for interleukin-3 (IL-3) is a complex of the IL-3–binding subunit (IL-3) and a larger β chain—βc, or, in the mouse, βc or its close relative βIL-3. There is evidence that the critical event that initiates signaling is not the approximation of the cytoplasmic domains of IL-3 and βIL-3, but is, rather, the formation of a β-β homodimer. Many of these studies involved the analyses of receptor chimeras where the cytoplasmic domains were derived from IL-3, βc or βIL-3, and the extracellular domains were derived from other cytokine receptors, such as the erythropoietin receptor (EpoR). However, evidence that the EpoR may also associate with other receptors clouds the interpretation of these experiments. Therefore, we reevaluated the structure of the functional IL-3R using chimeric receptors with extracellular domains derived not from members of the cytokine-receptor family, but from CD8 or CD16. We show, by expression of these chimeras in Ba/F3 or CTLL-2 cells, that mitogenic signals were only generated by heterodimerization of the cytoplasmic domains of IL-3 and βIL-3. Homodimers of either IL-3 or βIL-3, alone or in combination, were nonfunctional. Furthermore, the ability of heterodimers to stimulate mitogenesis correlated with their ability to induce tyrosine phosphorylation of JAK-2. These data suggest that the physiological activation of the IL-3R involves the generation of simple heterodimers of IL-3 and βIL-3.

Regulation and function of protein kinase B and MAP kinase activation by the IL-5/GM-CSF/IL-3 receptor

Interleukin (IL)-3, IL-5 and granulocyte-macrophage colony-stimulating factor (GM-CSF) regulate proliferation, differentiation and apoptosis of target cells. Receptors for these cytokines consist of a cytokine-specific alpha subunit and a common shared beta c subunit. Tyrosine phosphorylation of the beta c is thought to play a critical role in mediating signal transduction events. We have examined the effect of mutation of beta c tyrosines on the activation of multiple signal transduction pathways. Activation of protein kinase B (PKB) required JAK2 and was inhibited by dominant-negative phosphatidylinositol 3-kinase (P13K). Overexpression of JAK2 was sufficient to activate both protein kinase B (PKB) and extracellular regulated kinase-1 (ERK1). Tyrosine 577 and 612 were found to be critical for the activation of PKB and ERK1, but not activation of STAT transcription factors. Activation of both PKB and ERK have been implicated in the regulation of proliferation and apoptosis. We generated GM-CSFR stable cell lines expressing receptor mutants to evaluate their effect on these processes. Activation of both PKB and ERK was perturbed, while STAT activation remained unaffected. Tyrosines 577 and 612 were necessary for optimal proliferation, however, mutation of these tyrosine residues did not affect GM-CSF mediated rescue from apoptosis. These data demonstrate that while phosphorylation of beta c tyrosine residues 577 and 612 are important for optimal cell proliferation, rescue from apoptosis can be mediated by alternative signalling routes apparently independent of PKB or ERK activation.

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

Regulation of proliferation, differentiation and survival by the IL-3/IL-5/GM-CSF receptor family

The receptors for the I1-3/IL-5/GM-CSF cytokine family are composed of a heterodimeric complex of a cytokine-specific alpha chain and a common beta chain (betac). Binding of IL-3/IL-5/GM-CSF to their respective receptors rapidly induces activation of multiple intracellular signalling pathways, including the Ras-Raf-ERK, the JAK/STAT, the phosphatidylinositol 3-kinase PKB, and the JNK/SAPK and p38 signalling pathways. This review focuses on recent advancements in understanding how these different signalling pathways are activated by IL-3/IL-5/GM-CSF receptors, and how the individual pathways contribute to the pleiotropic effects of IL-3/IL-5/GM-CSF on their target cells, including proliferation, differentiation, survival, and effector functions.

Characterization of the Role of the Human Granulocyte-Macrophage Colony-Stimulating Factor Receptor α Subunit in the Activation of JAK2 and STAT5

The high-affinity human granulocyte-macrophage colony-stimulating factor (GM-CSF) receptor (GMR) consists of an alpha (GMRα) and a common beta (βc) subunit. The intracellular domain of βc has been extensively characterized and has been shown to be critical for the activation of both the JAK/STAT and MAP kinase pathways. The function of the intracellular domain of GMRα, however, is not as well characterized. To determine the role of this domain in GMR signaling, an extensive structure-function analysis was performed. Truncation mutants α362, α371, and α375 were generated, as well as the site-directed mutants αVQVQ and αVVVV. Although α375β, αVQNQβ, and αVVVVβ stimulated proliferation in response to human GM-CSF, the truncation mutants α362β and α371β were incapable of transducing a proliferative signal. In addition, both α371 and αVVVV were expressed at markedly reduced levels, indicating the importance of residues 372 to 374 for proper protein expression. More importantly, we show that GMRα plays a direct role in the activation of the JAK/STAT pathway, and electrophoretic mobility shift assays (EMSA) indicate that both GMRα and βc play a role in determining the STAT5 DNA binding complex activated by the GMR. Thus, the intracellular domain of the human GMRα is important for activation of the JAK/STAT pathway and protein stabilization.

© 1998 by The American Society of Hematology.

Characterization of the Role of the Human Granulocyte-Macrophage Colony-Stimulating Factor Receptor α Subunit in the Activation of JAK2 and STAT5

The high-affinity human granulocyte-macrophage colony-stimulating factor (GM-CSF) receptor (GMR) consists of an alpha (GMRα) and a common beta (βc) subunit. The intracellular domain of βc has been extensively characterized and has been shown to be critical for the activation of both the JAK/STAT and MAP kinase pathways. The function of the intracellular domain of GMRα, however, is not as well characterized. To determine the role of this domain in GMR signaling, an extensive structure-function analysis was performed. Truncation mutants α362, α371, and α375 were generated, as well as the site-directed mutants αVQVQ and αVVVV. Although α375β, αVQNQβ, and αVVVVβ stimulated proliferation in response to human GM-CSF, the truncation mutants α362β and α371β were incapable of transducing a proliferative signal. In addition, both α371 and αVVVV were expressed at markedly reduced levels, indicating the importance of residues 372 to 374 for proper protein expression. More importantly, we show that GMRα plays a direct role in the activation of the JAK/STAT pathway, and electrophoretic mobility shift assays (EMSA) indicate that both GMRα and βc play a role in determining the STAT5 DNA binding complex activated by the GMR. Thus, the intracellular domain of the human GMRα is important for activation of the JAK/STAT pathway and protein stabilization.

© 1998 by The American Society of Hematology.

Homodimerization of IL-2 receptor beta chain is necessary and sufficient to activate Jak2 and downstream signaling pathways

Cytokine receptor signaling involves the Jak/Stat pathways. Heterotrimeric IL-2R (alpha, beta, gamma[c] chains) activates Jak1 and Jak3, whereas homodimeric PRLR activates Jak2. The requirements directing such specificity of Jak activation are unknown. We show that chimeric receptors containing the intracellular domain of IL-2Rbeta chain fused to the extracellular domain of either EPOR or Kit, a non-cytokine receptor, activate Jak2. This observation provides evidence that IL-2Rbeta intrinsically possesses the ability to activate Jak2, but that this property is only displayed in homodimerized complexes. Our data suggest a role for the stoichiometry of cytokine receptors in selective activation of Janus kinases.

The cytoplasmic domain of granulocyte-macrophage colony-stimulating factor (GM-CSF) receptor alpha subunit is essential for both GM-CSF-mediated growth and differentiation

Granulocyte-macrophage colony-stimulating factor (GM-CSF) regulates differentiation, survival, and proliferation of colony-forming unit-granulocyte-macrophage progenitor cells. The biologic actions of GM-CSF are mediated by binding to a specific receptor consisting of two chains designated as alpha and beta subunits. We have demonstrated that the murine FDC-P1-derived cell line WT-19 transfected with the human GM-CSF receptor alpha and beta subunits (GM-CSFRalpha and beta) can be induced to differentiate by the addition of human GM-CSF (hGM-CSF). By expressing a series of GM-CSFRalpha mutants in WT19 cells, we have determined the amino acid domains of the GM-CSFRalpha cytoplasmic domain that regulate cell differentiation, proliferation, and survival. We found that the membrane proximal proline-rich domain and adjacent 16 residues are essential for both hGM-CSF-dependent cell proliferation and differentiation. In contrast, the C-terminal region of the GM-CSFRalpha cytoplasmic domain was not necessary for cell differentiation mediated by hGM-CSF, but the removal of this region severely impaired the ability of hGM-CSF to support cell survival. While the activation of JAK2, Shc, Erk, and STAT5 proteins correlated with hGM-CSF-mediated cell growth, cellular differentiation occurred in the absence of activation of these signal transduction pathways.

The kinase domain of Jak2 mediates induction of bcl-2 and delays cell death in hematopoietic cells

Granulocyte-macrophage colony-stimulating factor (GM-CSF), interleukin (IL)-3, and IL-5 stimulate DNA synthesis and proliferation and inhibit apoptosis in hematopoietic cells. Multiple signal pathways are activated by binding of these ligands to their receptors, which share a common beta subunit. Janus protein kinase 2 (Jak2) binds to the membrane proximal domain of the beta chain and is phosphorylated on receptor ligation. To explore the role of Jak2 in the regulation of specific signal transduction pathways, we constructed fusion proteins with a CD16 external domain, a CD7 transmembrane region, and a Jak2 cytoplasmic domain. This cytoplasmic domain consisted either of wild type Jak2 (CD16/Jak2-W) or Jak2 mutations with deletions of (a) the amino terminus (CD16/Jak2-N), (b) kinase-like domain (CD16/Jak2-B), (c) kinase domain (CD16/Jak2-C), or (d) amino-terminal and kinase-like domains, leaving the kinase domain (CD16/Jak-K) intact. In contrast to the CD16/Jak2-W fusion protein, which requires cross-linking for activation, CD16/Jak2-N, CD16/Jak2-B, and CD16/Jak2-K were constitutively phosphorylated, and they stimulated Shc phosphorylation and increased binding of STAT to DNA in Ba/F3 cells. Cell lines derived from IL-3-dependent Ba/F3 cells stably transfected with CD16/Jak2-W, CD16/Jak2-N, or CD16/Jak2-B mammalian expression vectors died at a rate similar to that of the parental cells on IL-3 deprivation. In contrast, CD16/Jak2-K cell lines exhibited increased expression of bcl-2 and pim-1 mRNA and maintained their viability when compared with control cell lines. Thus, activation of tyrosine phosphorylation by creating a CD16/Jak2-K fusion is sufficient to activate pathways that prevent cell death.

The apoptosis-inducing granulocyte-macrophage colony-stimulating factor (GM-CSF) analog E21R functions through specific regions of the heterodimeric GM-CSF receptor and requires interleukin-1beta-converting enzyme-like proteases

The granulocyte-macrophage colony-stimulating factor (GM-CSF) analog E21R induces apoptosis of hemopoietic cells. We examined the GM-CSF receptor subunit requirements and the signaling molecules involved. Using Jurkat T cells transfected with the GM-CSF receptor we found that both receptor subunits were necessary for E21R-induced apoptosis. Specifically, the 16 membrane-proximal residues of the alpha subunit were sufficient for apoptosis. This sequence could be replaced by the corresponding sequence from the interleukin-2 receptor common gamma subunit, identifying this as a conserved cytokine motif necessary for E21R-induced apoptosis. Cells expressing the alpha subunit and truncated betac mutants showed that the 96 membrane-proximal residues of betac were sufficient for apoptosis. E21R, in contrast to GM-CSF, did not alter tyrosine phosphorylation of betac, suggesting that receptor-associated tyrosine kinases were not activated. Consistent with this, E21R decreased the mitogen-activated protein kinase ERK (extracellular signal-regulated kinase). E21R-induced apoptosis was independent of Fas/APO-1 (CD95) and required interleukin-1beta-converting enzyme (ICE)-like proteases. In contrast, Bcl-2, which protects cells from growth factor deprivation-induced cell death, did not prevent this apoptosis. These findings demonstrate the GM-CSF receptor and ICE-like protease requirements for E21R-induced apoptosis.

Cloning and chromosomal location of a novel member of the myotonic dystrophy family of protein kinases

We have cloned a novel serine/threonine protein kinase (PK428) which is highly related (65%) within the kinase domain to the myotonic dystrophy protein kinase (DM-PK), as well as the cyclic AMP-dependent protein kinase (33%). Northern blots demonstrate that PK428 mRNA is distributed widely among tissues and is expressed at the highest levels in pancreas, heart, and skeletal muscle, with lower levels in liver and lung. Two PK428 mRNAs 10 and 3.8 kilobase pairs in size are seen in a number of cell lines, including hematopoietic and breast cancer cells. An antibody generated to a glutathione S-transferase-PK428 fusion protein detects a 65-kDa protein in these cell lines, and a similarly sized protein when the cloned cDNA is transiently expressed in Cos 7 cells. Immunoprecipitation of the transiently expressed PK428 protein and incubation with [gamma-32P]ATP demonstrate that it is capable of autophosphorylation. In addition, immunoprecipitates of the PK428 protein kinase also phosphorylated histone H1 and a peptide encoding a cyclic AMP-dependent protein kinase substrate. The gene corresponding to the 3.8-kb PK428 mRNA, and its corresponding 65-kDa protein, was isolated by polymerase chain reaction screening of a P1 phage human genomic library. Using this P1 phage clone as a probe, the PK428 gene was located on 1q41-42, a possible location for a human senescence gene, a gene associated with Rippling muscle disease, as well as a region associated with genetically acquired mental retardation.

Functional replacement of cytokine receptor extracellular domains by leucine zippers

Granulocyte-macrophage colony-stimulating factor receptor signals by a complex which includes the ligand and two different receptor subunits: a low affinity alpha receptor binding chain (granulocyte-macrophage colony-stimulating factor receptor alpha subunit (GM-Ralpha)) and a signal-transducing beta chain (GM-Rbeta). To investigate two unresolved issues in the initiation of signaling, the role of receptor extracellular domains and receptor stoichiometry, we replaced the mouse GM-Ralpha and GM-Rbeta extracellular domains with the leucine zipper domain of either the Fos or Jun molecule. We co-transfected combinations of chimeric receptors into Ba/F3 cells and found that both simple heterodimers of the GM-Ralpha and GM-Rbeta intracellular domains and homodimers of the GM-Rbeta intracellular domain signaled for proliferation. Surprisingly, homodimers of the GM-Ralpha intracellular domain also signaled for prevention of apoptosis in transfected cells. We similarly engineered dimers of the intracellular domain of the human interferon gamma receptor beta subunit and found that homodimers of the intracellular domain signaled for proliferation. When Fos peptide was added to Ba/F3 cells expressing the human interferon gamma receptor beta subunit construct, thereby preventing homodimer formation, the cells no longer proliferated in the absence of mouse interleukin 3.

A dominant negative granulocyte-macrophage colony-stimulating factor receptor alpha chain reveals the multimeric structure of the receptor complex

The receptor for the hemopoietic growth factor granulocyte-macrophage colony-stimulating factor (GM-CSF) is composed of two chains, both of which belong to the superfamily of cytokine receptors. The alpha chain confers low affinity binding only, whereas the beta chain (betac) confers high affinity binding when associated with alpha. Ectopic expression of both chains of the receptor in murine NIH-3T3 fibroblasts results in signal transduction, mitogenesis, and morphologic transformation. The cytoplasmic domain of the GM-CSF receptor alpha subunit (GMR-alpha) comprises 54 amino acids that have been shown to be important for signal transduction through the beta chain. The present study was designed to address the possibility of receptor oligomerization and its functional implication. Cross-linking studies with 125I-GM-CSF on NIH-3T3 transfectants is consistent with the presence of alpha and betac dimers and of receptor oligomers. We have, therefore, generated an inert alpha chain through polymerase chain reaction-mediated truncation of 47 amino acids of the COOH-terminal domain of alpha (alphat), and coexpressed alphat, alpha, and betac in NIH-3T3. In cells in which alphat and alpha are present in stoichiometric proportion within the GM-CSF-binding complex, we provide evidence that alphat is dominant negative over wild type alpha on the basis of two different functional assays: cell proliferation and foci formation. Hence, our results suggest the requirement for at least two functional alpha chains for signal transduction. Together with the cross-linking studies, our data indicate that the functional GMR is an oligomer that contains at least two alpha chains.

Interleukin-3 (IL-3) inhibits erythropoietin-induced differentiation in Ba/F3 cells via the IL-3 receptor alpha subunit

Introduction of erythropoietin receptors (EpoRs) into the interleukin-3 (IL-3)-dependent murine hemopoietic cell line, Ba/F3, enables these cells to not only proliferate, after an initial lag in G1, but also to increase beta-globin mRNA levels in response to erythropoietin (Epo). With IL-3 and Epo costimulation, IL-3-induced signaling appears to be dominant since no increase in beta-globin mRNA occurs. Differentiation and proliferation signals may be uncoupled since EpoRs lacking all eight intracellular tyrosines were compromised in proliferative signaling but retained erythroid differentiation ability. Intriguingly, a chimeric receptor of the extracellular domain of the EpoR and the transmembrane and intracellular domains of IL-3RbetaIL-3 chain (EpoR/IL-3RbetaIL-3) was capable of Epo-induced proliferative and differentiating signaling, suggesting either the existence of a second EpoR subunit responsible for differentiation or that the alpha subunit of the IL-3 receptor (IL-3R) prevents it. Arguing against the former, a truncated EpoR lacking an intracellular domain was incapable of promoting proliferation or differentiation. An EpoR/IL-3Ralpha chimera, in contrast, was capable of transmitting a weak Epo-induced proliferative signal but failed to stimulate accumulation of beta-globin mRNA. Most significantly, coexpression of the EpoR/IL-3Ralpha chimera with either EpoR/IL-3Rbeta or wild-type EpoRs suppressed Epo-induced beta-globin mRNA accumulation. Taken together, these results suggest an active role for the IL-3Ralpha subunit in inhibiting EpoR-specific differentiating signals.

Roles of the cytoplasmic domains of the alpha and beta subunits of human granulocyte-macrophage colony-stimulating factor receptor

The high-affinity and functional granulocyte-macrophage colony-stimulating factor receptor (GMR) is composed of two distinct subunits, alpha and beta; and the cytoplasmic domain of the beta subunit is essential to transduce growth-promoting signals. In contrast to the beta subunit, the role of the alpha subunit is not well characterized. We examined the requirement of the cytoplasmic domain of the alpha subunit and its functional region by deletion analyses. We demonstrated that the cytoplasmic domain of the alpha subunit, especially 29 amino acids residues near the transmembrane domain, was absolutely required for various signaling events including activation of immediate early genes, induction of tyrosine phosphorylation of cellular proteins, and cell growth. We further analyzed the role of the cytoplasmic domain of each subunit by constructing chimeric subunits, designated alpha/beta and beta/alpha, by exchanging cytoplasmic domains of the alpha and beta subunits of human (h) GMR. Reconstituted high-affinity chimeric hGMRs, hGMR(alpha/beta,beta/alpha) and hGMR(alpha/beta,beta), transduced signals at levels similar to the wild type hGMR(alpha,beta) in Ba/F3 cells and in NIH3T3 cells. These observations indicate that the original configuration between the extracellular and the cytoplasmic domains of the hGMR(alpha,beta) subunits is not required and that hGMR(alpha/beta,beta) transduced signals through the cytoplasmic domain of the beta subunit in an oligomeric form, without involvement of the cytoplasmic domain of the alpha subunit. Therefore human granulocyte-macrophage colony-stimulating factor signals are mainly transduced through the beta subunit, and the cytoplasmic domain of the alpha subunit is likely to activate the beta subunit in the normal hGMR.

The amino-terminal portion of the JAK2 protein kinase is necessary for binding and phosphorylation of the granulocyte-macrophage colony-stimulating factor receptor beta c chain

The binding of granulocyte-macrophage colony stimulating factor (GM-CSF) to its receptor stimulates JAK2 protein kinase activation, protein phosphorylation, and JAK2 association with the beta c chain of the GM-CSF receptor. To better understand how different domains of the JAK2 function to regulate association and phosphorylation of the beta c receptor, the minimal portion of the beta c receptor necessary for JAK2 binding has been determined. Using glutathione S-transferase (GST) fusion proteins expressing different portions of the membrane-proximal domain of the beta c chain, we demonstrate that JAK2 binds to amino acids 458-495, but showed little binding to fusion proteins containing amino acids 483-559, 483-530, or 458-484. The GST-beta c 458-495 bound equally well to the wild type (WT) JAK2, a carboxyl-terminal deletion of JAK2 removing the protein kinase domain (amino acids 1000-1129), and a deletion of the kinase-like domain (amino acids 523-746). However, an amino-terminal JAK2 deletion (amino acids 2-239) markedly reduced binding to this GST-beta c. Far Western blotting demonstrated that a GST fusion protein containing amino acids 1-294 of JAK2, but not fusion proteins containing amino acids 295-522, 523-746, or 747-1127, bound GST-beta c 458-559. When the JAK2 WT and deletions were transiently expressed along with the alpha and beta c subunits of the GM-CSF receptor and the cells were treated with GM-CSF, the following results were obtained: 1) WT JAK2 phosphorylated the beta c subunit in a GM-CSF-dependent manner, 2) the kinase-like domain deletion phosphorylated the beta c subunit, and 3) both the kinase domain deletion and the amino-terminal deletion failed to stimulate phosphorylation of the beta c subunit. Therefore, phosphorylation of the beta c subunit requires the binding of JAK2 through its amino terminus.

Signaling mechanisms through cytokine receptors that share signal transducing receptor components

Most of the receptors for soluble factors functioning in immune and hematopoietic systems belong to the cytokine receptor family. These receptors often share common signal transducing receptor components with other members of the same family. Such receptors and signal transducers possess no intrinsic tyrosine kinase domain but have recently been found to be associated with members of a JAK family of cytoplasmic tyrosine kinases. The JAK kinases become activated after ligand-induced dimerization of the receptor components. This activation appears to link the cell surface receptors to the nuclear genes through tyrosine phosphorylation and activation of latent cytoplasmic transcription factors called signal transducers and activators of transcription (STATs).