Site-specific serine phosphorylation of the IL-3 receptor is required for hemopoietic cell survival.

In the hemopoietic compartment, IL-3, GM-CSF, and IL-5 receptors are major transducers of survival signals; however, the receptor-proximal events that determine this vital function have not been defined. We have found that IL-3 stimulation induces phosphorylation of Ser-585 of beta(c). This promotes the association of phospho-Ser-585 of beta(c) with 14-3-3 and the p85 subunit of PI 3-K. Mutation of Ser-585 specifically impairs the PI 3-K signaling pathway and reduces cell survival in response to IL-3. These results define a distinct IL-3 receptor-mediated survival pathway regulated by site-specific receptor serine phosphorylation and 14-3-3 binding and suggest that this novel mode of signaling may be utilized by disparate transmembrane receptors that have as a common theme the transduction of survival signals.

Identification of a 14-3-3 binding sequence in the common beta chain of the granulocyte-macrophage colony-stimulating factor (GM-CSF), interleukin-3 (IL-3), and IL-5 receptors that is serine-phosphorylated by GM-CSF.

The common beta chain (beta(c)) of the granulocyte-macrophage colony-stimulating factor (GM-CSF), interleukin-3 (IL-3), and IL-5 receptors is the major signaling subunit of these receptors coupling ligand binding to multiple biological activities. It is thought that these multiple functions arise as a consequence of the recruitment of specific signaling molecules to tyrosine-phosphorylated residues in the cytoplasmic domain of beta(c). However, the contribution of serine phosphorylation in beta(c) to the recruitment of signaling molecules is not known. We show here the identification of a phosphoserine motif in the cytoplasmic domain of beta(c) that interacts with the adaptor protein 14-3-3zeta. Coimmunoprecipitation and pull-down experiments with a glutathione S-transferase (GST):14-3-3zeta fusion protein showed that 14-3-3 directly associates with beta(c) but not the GM-CSF receptor alpha chain. C-terminal truncation mutants of beta(c) further showed that a region between amino acids 544 and 626 in beta(c) was required for its association with 14-3-3zeta. This region contains the sequence (582)HSRSLP(587), which closely resembles the RSXSXP (where S is phosphorylated) consensus 14-3-3 binding site identified in a number of signaling molecules, including Raf-1. Significantly, substitution of (582)HSRSLP(587) for EFAAAA completely abolished interaction of beta(c) with GST-14-3-3zeta. Furthermore, the interaction of beta(c) with GST-14-3-3 was greatly reduced in the presence of a peptide containing the 14-3-3 binding site, but only when (585)Ser was phosphorylated. Direct binding experiments showed that the peptide containing phosphorylated (585)Ser bound 14-3-3zeta with an affinity of 150 nmol/L. To study the regulation of (585)S phosphorylation in vivo, we raised antibodies that specifically recognized (585)Ser-phosphorylated beta(c). Using these antibodies, we showed that GM-CSF stimulation strongly upregulated (585)Ser phosphorylation in M1 myeloid leukemic cells. The proximity of the SHC-binding site ((577)Tyr) to the 14-3-3-binding site ((582)HSRSLP(587)) and their conservation between mouse, rat, and human beta(c) but not in other cytokine receptors suggest that they form a distinct motif that may subserve specialized functions associated with the GM-CSF, IL-3, and IL-5 receptors.

Interferon-gamma upregulates interleukin-3 (IL-3) receptor expression in human endothelial cells and synergizes with IL-3 in stimulating major histocompatibility complex class II expression and cytokine production.

The human interleukin-3 (IL-3) receptor is constitutively expressed on certain hematopoietic cells where it mediates proliferation and differentiation, or functional activation. We have recently found that human umbilical vein endothelial cells (HUVECs) also express IL-3 receptors and that the expression is enhanced by stimulation with the monokine tumor necrosis factor alpha. In this report we show that the lymphokine interferon gamma (IFN gamma) is a powerful stimulator of the IL-3 receptor of HUVECs and that the combination of IL-3 and IFN gamma has a synergistic effect on major histocompatibility complex (MHC) class II expression and on the production of the early-acting hematopoietic cytokines IL-6 and granulocyte colony-stimulating factor (G-CSF). IFN gamma caused a time- and dose-dependent up-regulation of mRNA for both the alpha and beta chains of the IL-3 receptor, with maximal effects occurring 12 to 24 hours after stimulation with IFN gamma at 100 U/mL. Induction of mRNA correlated with protein expression on the cell surface, as judged by monoclonal antibody staining of both receptor chains and by the ability of HUVEC to specifically bind 125I-labeled IL-3 (125I-IL-3). Scatchard analysis of HUVECs stimulated with IFN gamma at 100 U/mL for 24 hours showed approximately 6,300 IL-3 receptors per cell that were of a high affinity class (dissociation constant [kd] = 500 pmol/L) only. The addition of IL-3 to IFN gamma-treated HUVECs strongly enhanced the expression of MHC class II antigen. Importantly, IFN gamma and IL-3 also exhibited a synergistic effect in the induction of the mRNA for G-CSF and IL-6. This was reflected in increased amounts of G-CSF and IL-6 protein in HUVEC supernatants. In contrast, IFN gamma and IL-3 did not stimulate granulocyte-macrophage colony-stimulating factor (GM-CSF) or IL-8 production in HUVECs. These results show that IFN gamma is a strong stimulator of IL-3 receptor expression in HUVECs and suggest that in vivo T-cell activation, causing the concomitant production of IFN gamma and IL-3, may lead to enhanced endothelial MHC class II expression and to the selective production of early-acting hematopoietic cytokines. Thus, IL-3 could influence immunity and hematopoiesis by acting not only on hematopoietic cells, but also on vascular endothelium.

Specific human granulocyte-macrophage colony-stimulating factor antagonists.

Human granulocyte-macrophage colony-stimulating factor (GM-CSF) is a pleiotropic hemopoietic growth factor and activator of mature myeloid cell function. We have previously shown that residue 21 in the first helix of GM-CSF plays a critical role in both biological activity and high-affinity receptor binding. We have now generated analogues of GM-CSF mutated at residue 21, expressed them in Escherichia coli, and examined them for binding, agonistic, and antagonistic activities. Binding experiments showed that GM E21A, E21Q, E21F, E21H, E21R, and E21K bound to the GM-CSF receptor alpha chain with a similar affinity to wild-type GM-CSF and had lost high-affinity binding to the GM-CSF receptor alpha-chain-common beta-chain complex. From these mutants, only the charge reversal mutants E21R and E21K were completely devoid of agonistic activity. Significantly we found that E21R and E21K antagonized the proliferative effect of GM-CSF on the erythroleukemic cell line TF-1 and primary acute myeloid leukemias, as well as GM-CSF-mediated stimulation of neutrophil superoxide production. This antagonism was specific for GM-CSF in that no antagonism of interleukin 3-mediated TF-1 cell proliferation or tumor necrosis factor alpha-mediated stimulation of neutrophil superoxide production was observed. E. coli-derived GM E21R and E21K were effective antagonists of both nonglycosylated and glycosylated wild-type GM-CSF. These results show that low-affinity GM-CSF binding can be dissociated from receptor activation and have potential clinical significance for the management of inflammatory diseases and certain leukemias where GM-CSF plays a pathogenic role.

Identification of residues in the first and fourth helices of human granulocyte-macrophage colony-stimulating factor involved in biologic activity and in binding to the alpha- and beta-chains of its receptor.

Residues within the first and fourth helices of human granulocyte-macrophage colony-stimulating factor (hGM-CSF) were analyzed for their role in biologic activity and interaction with the alpha- and beta-chains of the hGM-CSF receptor. Within the first helix substitution of the surface residues Glu14, Asn17, Gln20, Arg23, Arg24, and Asn27 or the buried residues Ala18, Leu25, and Leu28 did not significantly impair bioactivity or receptor binding. Substitutions at the buried residues Ala22 and Leu26 had intermediate bioactivity. However, substitutions of the surface residue Glu21 or the buried residue Ile19 reduced the relative bioactivity of the analogues to as little as 0.45% and 0.3%, respectively. Substitution of the charged surface residues of the fourth helix showed that substitution at Glu104, Lys107, and Lys111 had no significant effect on bioactivity, but substitution at Glu108 and Asp112 reduced the potency of the analogues to 34% and 7%, respectively. Receptor binding studies showed that, whereas Glu21 is the critical residue for binding to the hGM-CSF-receptor beta-chain, Asp112 is likely to be involved in binding to the GM-CSF-receptor alpha-chain. These results establish the relative contribution of residues in the first and fourth helices for GM-CSF bioactivity and receptor binding, and support a model where the fourth helix of GM-CSF interacts with the alpha-chain, and the first helix with the beta-chain of the GM-CSF receptor.

Two contiguous residues in human interleukin-3, Asp21 and Glu22, selectively interact with the alpha- and beta-chains of its receptor and participate in function.

We have previously reported that the predicted first helix of human interleukin (IL)-3 contains a hydrophilic region encompassing residues Asp21, Glu22, and Thr25 that is crucial for biological activity and IL-3 receptor binding. Using single amino acid substitution mutagenesis, we have now determined that Asp21 and Glu22, but not Thr25, were crucial for full IL-3 activity. Mutant D21R was 30-fold less potent than wild type IL-3 in the stimulation of biological activity. It also exhibited a similar reduction in its ability to bind to the cloned high affinity IL-3 receptor complex (alpha- and beta-chains) or to the receptor alpha-chain alone, indicating that residue 21 is involved in contacts with the alpha-chain. Mutant E22R was approximately 20,000-fold less potent than wild type IL-3 in the stimulation of biological activity and in binding to the IL-3 receptor high affinity complex. However, the binding of E22R to the IL-3 receptor alpha-chain alone was similar to that of wild type IL-3, suggesting that this mutant was defective in interactions with the receptor beta-chain. These results show that two contiguous residues in the N-terminal region of IL-3 mediate binding to the two different chains of the IL-3 receptor and emphasize the functional significance of the conserved Glu in the first helix of the IL-3, granulocyte-macrophage colony-stimulating factor, and IL-5 cytokine subfamily.

The receptor for interleukin 3 is selectively induced in human endothelial cells by tumor necrosis factor alpha and potentiates interleukin 8 secretion and neutrophil transmigration.

Interleukin (IL)-3 stimulates hemopoiesis in vitro. However, IL-3 is not normally found in bone marrow, raising doubts as to the in vivo role of IL-3. We have found that human umbilical vein endothelial cells (HUVEC) express functional high-affinity receptors for IL-3 after stimulation with tumor necrosis factor alpha (TNF-alpha), IL-1 beta, or lipopolysaccharide, and that this receptor is involved in inflammatory phenomena. TNF-alpha caused time- and dose-dependent up-regulation of mRNA for the IL-3 receptor alpha and beta chains, with maximal effects occurring 16-36 h after stimulation with TNF-alpha at 100 units/ml. Induction of mRNA correlated with protein expression on the cell surface as judged by monoclonal antibody staining and by the ability of HUVEC to specifically bind 125I-labeled IL-3. Scatchard analysis under optimal conditions of TNF-alpha stimulation revealed approximately 1500 IL-3 receptors per cell, which were of a high-affinity class (Kd = 500 pM) only. In contrast to a previous report, receptors for granulocyte-macrophage colony-stimulating factor could not be detected. IL-3 binding to TNF-alpha-activated HUVEC enhanced IL-8 production, E-selection expression, and neutrophil transmigration. The selective induction of a functional IL-3 receptor on endothelial cells suggests that, beyond hemopoiesis, IL-3 may have an important role in chronic inflammation and in allergic diseases.

A human interleukin 3 analog with increased biological and binding activities.

Human interleukin 3 (IL-3) variants generated by site-directed mutagenesis were analyzed in multiple biological and binding assays to identify residues critical for IL-3 activity. Two mutants carrying substitutions in the predicted hydrophilic region within the first alpha-helix, [Ala21,Leu22]IL-3 and [Ala21,Leu22,Ala25]IL-3 showed loss of biological activity and high-affinity binding. Mutants in a second predicted hydrophilic region, [Ala44,Leu45,Ala46]IL-3 and [Ala44,Ala46]IL-3, however, showed similar biological and binding activities to wild-type IL-3. Mutations in a C-terminal hydrophilic region that overlaps the fourth predicted alpha-helix led to either loss or gain of function. IL-3 analogs [Glu104,Asp105]-, [Leu108]-, [Asn108]-, [Thr108]-, and [Ala101,Leu108]IL-3 were less active than wild-type IL-3, whereas [Ala101]IL-3 and [Val116]IL-3 were 2- to 3-fold more potent. Significantly, the double mutant [Ala101,Val116]IL-3 exhibited a 15-fold greater potency than native IL-3. Receptor binding studies showed that [Ala101,Val116]IL-3 exhibited increased binding to the high- and low-affinity receptors of monocytes. These results show the generation of an IL-3 analog with increased biological and binding activities and support a model where the C terminus of IL-3 interacts with the alpha chain of the IL-3 receptor, making this region a useful focus for the development of more potent IL-3 agonists or antagonists.

Residue 21 of human granulocyte-macrophage colony-stimulating factor is critical for biological activity and for high but not low affinity binding.

The functional role of the predicted first alpha-helix of human granulocyte-macrophage colony-stimulating factor (GM-CSF) was analysed by site-directed mutagenesis and multiple biological and receptor binding assays. Initial deletion mutagenesis pointed to residues 20 and 21 being critical. Substitution mutagenesis showed that by altering Gln20 to Ala full GM-CSF activity was retained but that by altering Glu21 for Ala GM-CSF activity and high affinity receptor binding were decreased. Substitution of different amino acids for Glu21 showed that there was a hierarchy in the ability to stimulate the various biological activities of GM-CSF with the order of potency being Asp21 greater than Ser21 greater than Ala21 greater than Gln21 greater than Lys21 = Arg21. To distinguish whether position 21 was important for GM-CSF binding to high or low affinity receptors, GM-CSF (Arg21) was used as a competitor for [125I]GM-CSF binding to monocytes that express both types of receptor. GM-CSF (Arg21) exhibited a greatly reduced capacity to compete for binding to high affinity receptors, however, it competed fully for [125I]GM-CSF binding to low affinity receptors. Furthermore, GM-CSF (Arg21) was equipotent with wild-type GM-CSF in binding to the cloned low affinity alpha-chain of the GM-CSF receptor. These results show that (i) this position is critical for high affinity but not for low affinity GM-CSF receptor binding thus defining two functional parts of the GM-CSF molecule; (ii) position 21 of GM-CSF is critical for multiple functions of GM-CSF; and (iii) stimulation of proliferation and mature cell function by GM-CSF are mediated through high affinity receptors.

Differential binding of IL-3 and GM-CSF to human monocytes.

Human monocytes respond to IL-3 and GM-CSF with a similar range of functional activities, and at similar cytokine concentrations. We have recently shown, however, that the rate of monocyte activation is greater in response to GM-CSF than to IL-3. In order to understand the basis of this phenomenon we investigated the interaction of IL-3 and GM-CSF with their surface receptors by means of kinetic binding experiments. 125I-GM-CSF showed very rapid association to monocytes at 37 degrees C, with a half-time of only 40 sec. The pattern of binding with this ligand was complex, with a decline in overall cell-associated radioactivity after 2 min of incubation. In contrast, 125I-IL-3 showed slower association, with a half-time at 37 degrees C of 2.5 min. The different rates of association correlated well with the different rates of cell activation induced by the two cytokines. On the other hand, rates of internalisation were similar for the two cytokines, with half-times of 14-15 min. Competition binding experiments performed under high affinity conditions showed that IL-3 and GM-CSF cross-competed for binding on the surface of monocytes. In contrast, under low affinity conditions IL-3 did not compete for 125I-GM-CSF binding while GM-CSF was a strong competitor of 125I-IL-3 binding. In quantitative inhibition experiments GM-CSF showed inhibitory effects on low affinity 125I-IL-3 binding at lower concentrations than those needed with unlabelled IL-3. It is suggested that current models of IL-3/GM-CSF receptor interactions need to be revised in order to accommodate the unique pattern of competition on human monocytes presented here.