Identification of a 14-3-3 Binding Sequence in the Common β 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

14-3-3ζ suppresses RANKL signaling by destabilizing TRAF6

Macrophages are essential regulators of inflammation and bone loss. Receptor activator of nuclear factor-κβ ligand (RANKL), a pro-inflammatory cytokine, is responsible for macrophage differentiation to osteoclasts and bone loss. We recently showed that 14-3-3ζ-knockout (YwhazKO) rats exhibit increased bone loss in the inflammatory arthritis model. 14-3-3ζ is a cytosolic adaptor protein that actively participates in many signaling transductions. However, the role of 14-3-3ζ in RANKL signaling or bone remodeling is unknown. We investigated how 14-3-3ζ affects osteoclast activity by evaluating its role in RANKL signaling. We utilized 14-3-3ζ-deficient primary bone marrow-derived macrophages obtained from wildtype and YwhazKO animals and RAW264.7 cells generated using CRISPR-Cas9. Our results showed that 14-3-3ζ-deficient macrophages, upon RANKL stimulation, have bigger and stronger tartrate-resistant acid phosphatase-positive multinucleated cells and increased bone resorption activity. The presence of 14-3-3ζ suppressed RANKL-induced MAPK and AKT phosphorylation, transcription factors (NFATC1 and p65) nuclear translocation, and subsequently, gene induction (Rank, Acp5, and Ctsk). Mechanistically, 14-3-3ζ interacts with TRAF6, an essential component of the RANKL receptor complex. Upon RANKL stimulation, 14-3-3ζ-TRAF6 interaction was increased, while RANK-TRAF6 interaction was decreased. Importantly, 14-3-3ζ supported TRAF6 ubiquitination and degradation by the proteasomal pathway, thus dampening the downstream RANKL signaling. Together, we show that 14-3-3ζ regulates TRAF6 levels to suppress inflammatory RANKL signaling and osteoclast activity. To the best of our knowledge, this is the first report on 14-3-3ζ regulation of RANKL signaling and osteoclast activation.

HDAC6 regulates human erythroid differentiation through modulation of JAK2 signalling

Among histone deacetylases, HDAC6 is unusual in its cytoplasmic localization. Its inhibition leads to hyperacetylation of non-histone proteins, inhibiting cell cycle, proliferation and apoptosis. Ricolinostat (ACY-1215) is a selective inhibitor of the histone deacetylase HDAC6 with proven efficacy in the treatment of malignant diseases, but anaemia is one of the most frequent side effects. We investigated here the underlying mechanisms of this erythroid toxicity. We first confirmed that HDAC6 was strongly expressed at both RNA and protein levels in CD34⁺ -cells-derived erythroid progenitors. ACY-1215 exposure on CD34⁺ -cells driven in vitro towards the erythroid lineage led to a decreased cell count, an increased apoptotic rate and a delayed erythroid differentiation with accumulation of weakly hemoglobinized immature erythroblasts. This was accompanied by drastic changes in the transcriptomic profile of primary cells as shown by RNAseq. In erythroid cells, ACY-1215 and shRNA-mediated HDAC6 knockdown inhibited the EPO-dependent JAK2 phosphorylation. Using acetylome, we identified 14-3-3ζ, known to interact directly with the JAK2 negative regulator LNK, as a potential HDAC6 target in erythroid cells. We confirmed that 14-3-3ζ was hyperacetylated after ACY-1215 exposure, which decreased the 14-3-3ζ/LNK interaction while increased LNK ability to interact with JAK2. Thus, in addition to its previously described role in the enucleation of mouse fetal liver erythroblasts, we identified here a new mechanism of HDAC6-dependent control of erythropoiesis through 14-3-3ζ acetylation level, LNK availability and finally JAK2 activation in response to EPO, which is crucial downstream of EPO-R activation for human erythroid cell survival, proliferation and differentiation.

14-3-3ζ-TRAF5 axis governs interleukin-17A signaling

IL-17A is a therapeutic target in many autoimmune diseases. Most nonhematopoietic cells express IL-17A receptors and respond to extracellular IL-17A by inducing proinflammatory cytokines. The IL-17A signal transduction triggers two broad, TRAF6- and TRAF5-dependent, intracellular signaling pathways to produce representative cytokines (IL-6) and chemokines (CXCL-1), respectively. Our limited understanding of the cross-talk between these two branches has generated a crucial gap of knowledge, leading to therapeutics indiscriminately blocking IL-17A and global inhibition of its target genes. In previous work, we discovered an elevated expression of 14-3-3 proteins in inflammatory aortic disease, a rare human autoimmune disorder with increased levels of IL-17A. Here we report that 14-3-3ζ is essential for IL-17 signaling by differentially regulating the signal-induced IL-6 and CXCL-1. Using genetically manipulated human and mouse cells, and ex vivo and in vivo rat models, we uncovered a function of 14-3-3ζ. As a part of the molecular mechanism, we show that 14-3-3ζ interacts with several TRAF proteins; in particular, its interaction with TRAF5 and TRAF6 is increased in the presence of IL-17A. In contrast to TRAF6, we found TRAF5 to be an endogenous suppressor of IL-17A-induced IL-6 production, an effect countered by 14-3-3ζ. Furthermore, we observed that 14-3-3ζ interaction with TRAF proteins is required for the IL-17A-induced IL-6 levels. Together, our results show that 14-3-3ζ is an essential component of IL-17A signaling and IL-6 production, an effect that is suppressed by TRAF5. To the best of our knowledge, this report of the 14-3-3ζ-TRAF5 axis, which differentially regulates IL-17A-induced IL-6 and CXCL-1 production, is unique.

The role of IL-3 in bone

In the recent past, there has been a burgeoning interest in targeting cytokines such as IL-3 for specific disease conditions of bone such as rheumatoid arthritis and multiple myeloma. Unlike other cytokines, IL-3 is a cytokine with a multilineage potential and broad spectrum of target cells and it plays a vital role in hematopoiesis. Due to its common receptor subunit, the action of IL-3 shows functional redundancy with other cytokines such as the granulocyte-macrophage colony-stimulating factor and IL-5. IL-3 has been successfully used in ameliorating radiation-induced bone marrow aplasia and similar conditions. However, the role of IL-3 in bone cells has not been fully unraveled yet; therefore, the aim of this overview is to present the effects of IL-3 in bone with a special emphasis on osteoclasts and osteoblasts in a concise manner.

14-3-3 Proteins in Brain Development: Neurogenesis, Neuronal Migration and Neuromorphogenesis

The 14-3-3 proteins are a family of highly conserved, multifunctional proteins that are highly expressed in the brain during development. Cumulatively, the seven 14-3-3 isoforms make up approximately 1% of total soluble brain protein. Over the last decade, evidence has accumulated implicating the importance of the 14-3-3 protein family in the development of the nervous system, in particular cortical development, and have more recently been recognized as key regulators in a number of neurodevelopmental processes. In this review we will discuss the known roles of each 14-3-3 isoform in the development of the cortex, their relation to human neurodevelopmental disorders, as well as the challenges and questions that are left to be answered. In particular, we focus on the 14-3-3 isoforms and their involvement in the three key stages of cortical development; neurogenesis and differentiation, neuronal migration and neuromorphogenesis and synaptogenesis.

Interactome analysis of myeloid-derived suppressor cells in murine models of colon and breast cancer

In solid cancers, myeloid derived suppressor cells (MDSC) infiltrate (peri)tumoral tissues to induce immune tolerance and hence to establish a microenvironment permissive to tumor growth. Importantly, the mechanisms that facilitate such infiltration or a subsequent immune suppression are not fully understood. Hence, in this study, we aimed to delineate disparate molecular pathways which MDSC utilize in murine models of colon or breast cancer. Using pathways enrichment analysis, we completed interactome maps of multiple signaling pathways in CD11b+/Gr1(high/low) MDSC from spleens and tumor infiltrates of mice with c26GM colon cancer and tumor infiltrates of MDSC in 4T1 breast cancer. In both cancer models, infiltrating MDSC, but not CD11b+ splenic cells, have been found to be enriched in multiple signaling molecules suggestive of their enhanced proliferative and invasive phenotypes. The interactome data has been subsequently used to reconstruct a previously unexplored regulation of MDSC cell cycle by the c-myc transcription factor which was predicted by the analysis. Thus, this study represents a first interactome mapping of distinct multiple molecular pathways whereby MDSC sustain cancer progression.

TRAF6 is required for the GM-CSF-induced JNK, p38 and Akt activation

JNK, p38 and Akt signalings have been shown to be activated by granulocyte-macrophage colony-stimulating factor (GM-CSF) and are pivotal for GM-CSF-mediated survival, proliferation and differentiation of macrophages and their progenitors. However, the detailed mechanism of how these signalings is activated by GM-CSF is not fully elucidated. We report here that E3 ligase TRAF6 is required for the GM-CSF-induced activation of JNK, p38 and Akt. GM-CSF triggers autoubiquitination of TRAF6 and TRAF6 knocked down results in impaired activation of JNK and p38 signaling. TRAF6 is also required for GM-CSF-induced ubiquitination and activation of Akt. These findings reveal novel roles of TRAF6 in GM-CSF signaling.

The GM-CSF receptor family: mechanism of activation and implications for disease

Granulocyte-macrophage colony-stimulating factor (GM-CSF) is a pluripotent cytokine produced by many cells in the body, which regulates normal and malignant hemopoiesis as well as innate and adaptive immunity. GM-CSF assembles and activates its heterodimeric receptor complex on the surface of myeloid cells, initiating multiple signaling pathways that control key functions such as cell survival, cell proliferation, and functional activation. Understanding the molecular composition of these pathways, the interaction of the various components as well as the kinetics and dose-dependent mechanics of receptor activation provides valuable insights into the function of GM-CSF as well as the related cytokines, interleukin-3 and interleukin-5. This knowledge provides opportunities for the development of new therapies to block the action of these cytokines in hematological malignancy and chronic inflammation.

Neurodevelopmental and neuropsychiatric behaviour defects arise from 14-3-3ζ deficiency

Complex neuropsychiatric disorders are believed to arise from multiple synergistic deficiencies within connected biological networks controlling neuronal migration, axonal pathfinding and synapse formation. Here, we show that deletion of 14-3-3ζ causes neurodevelopmental anomalies similar to those seen in neuropsychiatric disorders such as schizophrenia, autism spectrum disorder and bipolar disorder. 14-3-3ζ-deficient mice displayed striking behavioural and cognitive deficiencies including a reduced capacity to learn and remember, hyperactivity and disrupted sensorimotor gating. These deficits are accompanied by subtle developmental abnormalities of the hippocampus that are underpinned by aberrant neuronal migration. Significantly, 14-3-3ζ-deficient mice exhibited abnormal mossy fibre navigation and glutamatergic synapse formation. The molecular basis of these defects involves the schizophrenia risk factor, DISC1, which interacts isoform specifically with 14-3-3ζ. Our data provide the first evidence of a direct role for 14-3-3ζ deficiency in the aetiology of neurodevelopmental disorders and identifies 14-3-3ζ as a central risk factor in the schizophrenia protein interaction network.

Interleukins, from 1 to 37, and interferon-γ: receptors, functions, and roles in diseases

Advancing our understanding of mechanisms of immune regulation in allergy, asthma, autoimmune diseases, tumor development, organ transplantation, and chronic infections could lead to effective and targeted therapies. Subsets of immune and inflammatory cells interact via ILs and IFNs; reciprocal regulation and counter balance among T(h) and regulatory T cells, as well as subsets of B cells, offer opportunities for immune interventions. Here, we review current knowledge about ILs 1 to 37 and IFN-γ. Our understanding of the effects of ILs has greatly increased since the discoveries of monocyte IL (called IL-1) and lymphocyte IL (called IL-2); more than 40 cytokines are now designated as ILs. Studies of transgenic or knockout mice with altered expression of these cytokines or their receptors and analyses of mutations and polymorphisms in human genes that encode these products have provided important information about IL and IFN functions. We discuss their signaling pathways, cellular sources, targets, roles in immune regulation and cellular networks, roles in allergy and asthma, and roles in defense against infections.

miR-451 protects against erythroid oxidant stress by repressing 14-3-3zeta

The bicistronic microRNA (miRNA) locus miR-144/451 is highly expressed during erythrocyte development, although its physiological roles are poorly understood. We show that miR-144/451 ablation in mice causes mild erythrocyte instability and increased susceptibility to damage after exposure to oxidant drugs. This phenotype is deeply conserved, as miR-451 depletion synergizes with oxidant stress to cause profound anemia in zebrafish embryos. At least some protective activities of miR-451 stem from its ability to directly suppress production of 14-3-3zeta, a phospho-serine/threonine-binding protein that inhibits nuclear accumulation of transcription factor FoxO3, a positive regulator of erythroid anti-oxidant genes. Thus, in miR-144/451(-/-) erythroblasts, 14-3-3zeta accumulates, causing partial relocalization of FoxO3 from nucleus to cytoplasm with dampening of its transcriptional program, including anti-oxidant-encoding genes Cat and Gpx1. Supporting this mechanism, overexpression of 14-3-3zeta in erythroid cells and fibroblasts inhibits nuclear localization and activity of FoxO3. Moreover, shRNA suppression of 14-3-3zeta protects miR-144/451(-/-) erythrocytes against peroxide-induced destruction, and restores catalase activity. Our findings define a novel miRNA-regulated pathway that protects erythrocytes against oxidant stress, and, more generally, illustrate how a miRNA can influence gene expression by altering the activity of a key transcription factor.

Defective erythroid differentiation in miR-451 mutant mice mediated by 14-3-3zeta

Erythrocyte formation occurs throughout life in response to cytokine signaling. We show that microRNA-451 (miR-451) regulates erythropoiesis in vivo. Mice lacking miR-451 display a reduction in hematrocrit, an erythroid differentiation defect, and ineffective erythropoiesis in response to oxidative stress. 14-3-3zeta, an intracellular regulator of cytokine signaling that is repressed by miR-451, is up-regulated in miR-451(-/-) erythroblasts, and inhibition of 14-3-3zeta rescues their differentiation defect. These findings reveal an essential role of 14-3-3zeta as a mediator of the proerythroid differentiation actions of miR-451, and highlight the therapeutic potential of miR-451 inhibitors.

14-3-3 protein signaling in development and growth factor responses

Tyrosine and serine phosphorylation are central to cellular signaling in growth and development. 14-3-3 proteins function as dimeric phosphoserine-binding proteins with documented interactions throughout the eukaryotic proteome and are highly conserved in both the animal and plant kingdoms. Binding of 14-3-3 to a client protein can have a range of context-dependent effects, including conformational change, enzyme inhibition, a shielding effect, re-localization, and bridging between two molecules. Proteome-based strategies utilizing mass spectrometry have revealed an unprecedented central stage for 14-3-3 in signal transduction with interacting partners composing at least 0.6% of the cellular proteome. 14-3-3 has been shown to bind to the human GM-CSF, IL-3, and IL-5 receptors and is required for the transmission of cell survival. 14-3-3 is involved in survival-specific signals, acting not only at the receptor level but also at critical steps downstream of the receptor. This phosphoserine-mediated pathway works independently of tyrosine kinases, highlighting an alternative mechanism of signaling for this receptor family. Other growth factor receptors and their adaptors are also being shown to associate with 14-3-3 and/or have putative 14-3-3 interaction sequences, such as the prolactin receptor, IGF-1 receptor, and some G-protein coupled receptors. 14-3-3 proteins are remarkably conserved through eukaryotic organisms and in Drosophila are required for photoreceptor development, learning, timing of cell cycles, and maintenance of cellular polarity. These findings are elevating our initial description of biochemical interactions to a better understanding of 14-3-3 function at the level of the whole organism. Further study should explore the integration of phosphoserine and phosphotyrosine signaling by 14-3-3 proteins and the role of isoform-specific functions in higher organisms. The prevalence of functional 14-3-3 binding sites throughout the proteome, and especially among growth factor receptors and signaling molecules, reflects a global role for 14-3-3 in multiple cellular decision making.

14-3-3:Shc scaffolds integrate phosphoserine and phosphotyrosine signaling to regulate phosphatidylinositol 3-kinase activation and cell survival

Integrated cascades of protein tyrosine and serine/threonine phosphorylation play essential roles in transducing signals in response to growth factors and cytokines. How adaptor or scaffold proteins assemble signaling complexes through both phosphotyrosine and phosphoserine/threonine residues to regulate specific signaling pathways and biological responses is unclear. We show in multiple cell types that endogenous 14-3-3zeta is phosphorylated on Tyr(179) in response to granulocyte macrophage colony-stimulating factor. Importantly, 14-3-3zeta can function as an intermolecular bridge that couples to phosphoserine residues and also directly binds the SH2 domain of Shc via Tyr(179). The assembly of these 14-3-3:Shc scaffolds is specifically required for the recruitment of a phosphatidylinositol 3-kinase signaling complex and the regulation of CTL-EN cell survival in response to cytokine. The biological significance of these findings was further demonstrated using primary bone marrow-derived mast cells from 14-3-3zeta(-/-) mice. We show that cytokine was able to promote Akt phosphorylation and viability of primary mast cells derived from 14-3-3zeta(-/-) mice when reconstituted with wild type 14-3-3zeta, but the Akt phosphorylation and survival response was reduced in cells reconstituted with the Y179F mutant. Together, these results show that 14-3-3:Shc scaffolds can act as multivalent signaling nodes for the integration of both phosphoserine/threonine and phosphotyrosine pathways to regulate specific cellular responses.

Fibroblast growth factor receptor 2 phosphorylation on serine 779 couples to 14-3-3 and regulates cell survival and proliferation

The fibroblast growth factors (FGFs) exert their diverse (or pleiotropic) biological responses through the binding and activation of specific cell surface receptors (FGFRs). While FGFRs are known to initiate intracellular signaling through receptor tyrosine phosphorylation, the precise mechanisms by which the FGFRs regulate pleiotropic biological responses remain unclear. We now identify a new mechanism by which FGFR2 is able to regulate intracellular signaling and cellular responses. We show that FGFR2 is phosphorylated on serine 779 (S779) in response to FGF2. S779, which lies adjacent to the phospholipase Cgamma binding site at Y766, provides a docking site for the 14-3-3 phosphoserine-binding proteins and is essential for the full activation of the phosphatidylinositol 3-kinase and Ras/mitogen-activated protein kinase pathways. Furthermore, S779 signaling is essential for promoting cell survival and proliferation in both Ba/F3 cells and BALB/c 3T3 fibroblasts. This new mode of FGFR2 phosphoserine signaling via the 14-3-3 proteins may provide an increased repertoire of signaling outputs to allow the regulation of pleiotropic biological responses. In this regard, we have identified conserved putative phosphotyrosine/phosphoserine motifs in the cytoplasmic domains of diverse cell surface receptors, suggesting that they may perform important functional roles beyond the FGFRs.

A functional 14-3-3zeta-independent association of PI3-kinase with glycoprotein Ib alpha, the major ligand-binding subunit of the platelet glycoprotein Ib-IX-V complex

Engagement of the adhesion receptor glycoprotein (GP) Ib-IX-V by von Willebrand factor (VWF) mediates platelet adhesion to damaged vessels and triggers platelet activation and thrombus formation in heart attack and stroke. GPIb-IX-V contains distinct 14-3-3zeta-binding sites at the GPIb alpha C-terminus involving phosphorylation of Ser609, an upstream site involving phosphorylated Ser587/Ser590, and a protein kinase A (PKA)-dependent site on GPIb beta involving Ser166. 14-3-3zeta regulates the VWF-binding affinity of GPIb-IX-V and inhibiting 14-3-3zeta association blocks receptor signaling, suggesting a key functional role for 14-3-3zeta. We used deletion mutants of GPIb alpha expressed in Chinese hamster ovary (CHO) cells to define the relationship of 14-3-3zeta binding to another GPIb-IX-V-associated signaling protein, phosphoinositide 3-kinase (PI3-kinase). Pull-down experiments involving glutathione S-transferase (GST)-PI3-kinase/p85-subunit and GST-14-3-3zeta indicated that both proteins interacted with contiguous GPIb alpha sequences 580 to 590/591 to 610. Deleting these, but not upstream sequences of GPIb alpha expressed in CHO cells, inhibited VWF/ristocetin-dependent Akt phosphorylation, relative to wild-type receptor, confirming this region encompassed a functional PI3-kinase-binding site. Pull-down experiments with GST-p85 truncates indicated the GPIb alpha-binding region involved the p85 breakpoint cluster region (BCR) domain, containing RSXSXP. However, pull-down of GPIb-IX was unaltered by mutation/deletion/phosphorylation of this potential 14-3-3zeta-binding sequence in mutant constructs of GST-p85, suggesting PI3-kinase bound GPIb alpha independently of 14-3-3zeta; 14-3-3zeta inhibitor peptide R18 also blocked pull-down of receptor by GST-14-3-3zeta but not GST-p85, and GST-p85 pull-downs were unaffected by excess 14-3-3zeta. Together, these data suggest the GPIb alpha C-terminus regulates signaling through independent association of 14-3-3zeta and PI3-kinase.

14-3-3 proteins: a historic overview

This chapter includes a historic overview of 14-3-3 proteins with an emphasis on the differences between potentially cancer-relevant isoforms on the genomic, protein and functional level. The focus will therefore be on mammalian 14-3-3s although many important developments in the field have involved Drosophila 14-3-3 proteins for example and the cross-fertilisation from parallel studies on plant 14-3-3 should not be underestimated. In the major part of this review I will attempt to focus on some novel data and aspects of 14-3-3 structure and function, in particular regulation of 14-3-3 isoforms by oncogene-related protein kinase phosphorylation and aspects of 14-3-3 research with which newcomers to the field may be less familiar.

Growth factor pleiotropy is controlled by a receptor Tyr/Ser motif that acts as a binary switch

Pleiotropism is a hallmark of cytokines and growth factors; yet, the underlying mechanisms are not clearly understood. We have identified a motif in the granulocyte macrophage-colony-stimulating factor receptor composed of a tyrosine and a serine residue that functions as a binary switch for the independent regulation of multiple biological activities. Signalling occurs either through Ser585 at lower cytokine concentrations, leading to cell survival only, or through Tyr577 at higher cytokine concentrations, leading to cell survival as well as proliferation, differentiation or functional activation. The phosphorylation of Ser585 and Tyr577 is mutually exclusive and occurs via a unidirectional mechanism that involves protein kinase A and tyrosine kinases, respectively, and is deregulated in at least some leukemias. We have identified similar Tyr/Ser motifs in other cell surface receptors, suggesting that such signalling switches may play important roles in generating specificity and pleiotropy in other biological systems.

Systematic discovery of new recognition peptides mediating protein interaction networks

Many aspects of cell signalling, trafficking, and targeting are governed by interactions between globular protein domains and short peptide segments. These domains often bind multiple peptides that share a common sequence pattern, or “linear motif” (e.g., SH3 binding to PxxP). Many domains are known, though comparatively few linear motifs have been discovered. Their short length (three to eight residues), and the fact that they often reside in disordered regions in proteins makes them difficult to detect through sequence comparison or experiment. Nevertheless, each new motif provides critical molecular details of how interaction networks are constructed, and can explain how one protein is able to bind to very different partners. Here we show that binding motifs can be detected using data from genome-scale interaction studies, and thus avoid the normally slow discovery process. Our approach based on motif over-representation in non-homologous sequences, rediscovers known motifs and predicts dozens of others. Direct binding experiments reveal that two predicted motifs are indeed protein-binding modules: a DxxDxxxD protein phosphatase 1 binding motif with a K_D of 22 μM and a VxxxRxYS motif that binds Translin with a K_D of 43 μM. We estimate that there are dozens or even hundreds of linear motifs yet to be discovered that will give molecular insight into protein networks and greatly illuminate cellular processes.

Many protein interactions are mediated by short amino acid motifs. The authors describe a new approach to identify these interaction motifs and experimentally validate some of their binding predictions.

Inhibition of granulocyte-macrophage colony-stimulating factor signaling and microglial proliferation by anti-CD45RO: role of Hck tyrosine kinase and phosphatidylinositol 3-kinase/Akt

Increasing evidence suggests that CD45, a transmembrane protein tyrosine phosphatase, is an important modulator of macrophage activation. Microglia, resident brain macrophages, express CD45 and proliferate under pathologic conditions. In this study, we examined the role of CD45 in modulating GM-CSF-induced proliferation and signal transduction in primary human microglial cultures. Soluble, but not immobilized anti-CD45RO induced tyrosine phosphatase activity and inhibited GM-CSF-induced microglial proliferation. Microglial proliferation was also inhibited by PP2 (Src inhibitor), LY294002 (PI3K inhibitor), and U0126 (MEK inhibitor). GM-CSF induced phosphorylation of Jak2, Stat5, Hck (the myeloid-restricted Src kinase), Akt, Stat3, and Erk MAPKs in microglia. Of these, anti-CD45RO inhibited phosphorylation of Hck and Akt, and PP2 inhibited phosphorylation of Hck and Akt. In a macrophage cell line stably overexpressing wild-type or kinase-inactive Hck, GM-CSF increased proliferation of the control (empty vector) and wild-type but not kinase-inactive cells, and this was inhibited by anti-CD45RO. Together, these results demonstrate that, in macrophages, Hck tyrosine kinase is activated by GM-CSF, and that Hck plays a pivotal role in cell proliferation and survival by activating the PI3K/Akt pathway. Ab-mediated activation of macrophage and microglial CD45 tyrosine phosphatase may have therapeutic implications for CNS inflammatory diseases.

Regulation of haematopoiesis by growth factors - emerging insights and therapies

Haematopoiesis is regulated by a wide variety of glycoprotein hormones, including stem cell factor, granulocyte-macrophage colony-stimulating factor, thrombopoietin and IL-3. These haematopoietic growth factors (HGFs) share a number of properties, including redundancy, pleiotropy, autocrine and paracrine effects, receptor subunit oligomerisation and similar signal transduction mechanisms, yet each one has a unique spectrum of haematopoietic activity. Ongoing studies with knockout mice have discovered previously unrecognised physiological roles for HGFs, linking haematopoiesis to innate immunity, pulmonary physiology and bone metabolism. The regulation of stem cells by HGFs within niches of the bone marrow microenvironment is now well recognised and similar mechanisms appear to exist in the regulation of other stem cell compartments. Alternative signalling strategies, other than tyrosine kinase activation and phosphotyrosine cascades, may account for some of the more subtle differences between HGFs. Accumulating evidence suggests that some, but not all, HGF receptors can transduce a genuine lineage-determining signal at certain points in haematopoiesis. Further studies, primarily at the receptor level, are needed to determine the mechanisms of instructive signalling, which may include phosphoserine cascades. Novel haematopoietic regulators, as well as the development of biological therapies, including growth factor antagonists and peptide mimetics, are also discussed.

The phosphoserine-585-dependent pathway of the GM-CSF/IL-3/IL-5 receptors mediates hematopoietic cell survival through activation of NF-kappaB and induction of bcl-2

We have recently identified a novel mechanism of hematopoietic cell survival that involves site-specific serine phosphorylation of the common beta subunit (beta(c)) of the granulocyte-macrophage colony-stimulating factor (GM-CSF), interleukin-3 (IL-3), and IL-5 receptors. However, the downstream components of this pathway are not known, nor is its relationship to survival signals triggered by tyrosine phosphorylation of the receptor clear. We have now found that phosphorylation of Ser585 of beta(c) in response to GM-CSF recruited 14-3-3 and phosphatidyl inositol 3-OH kinase (PI 3-kinase) to the receptor, while phosphorylation of the neighboring Tyr577 within this "viability domain" promoted the activation of both Src homology and collagen (Shc) and Ras. These are independent processes as demonstrated by the intact reactivity of phosphospecific anti-Ser585 and anti-Tyr577 antibodies on the cytotoxic T-lymphocyte-ecotrophic retroviral receptor neomycin (CTL-EN) mutants beta(c)Tyr577Phe and beta(c)Ser585Gly, respectively. Importantly, while mutants in which either Ser585 (beta(c)Ser585Gly) or all tyrosines (beta(c)F8) were substituted showed a defect in Akt phosphorylation, nuclear factor kappaB (NF-kappaB) activation, bcl-2 induction, and cell survival, the mutant beta(c)Tyr577Phe was defective in Shc, Ras, and extracellular signal-related kinase (ERK) activation, but supported CTL-EN cell survival in response to GM-CSF. These results demonstrate that both serine and tyrosine phosphorylation pathways play a role in hematopoietic cell survival, are initially independent of each other, and converge on NF-kappaB to promote bcl-2 expression.

Constitutive mutants of the GM-CSF receptor reveal multiple pathways leading to myeloid cell survival, proliferation, and granulocyte-macrophage differentiation

Activation of the granulocyte-macrophage colony-stimulating factor (GM-CSF) family of receptors promotes the survival, proliferation, and differentiation of cells of the myeloid compartment. Several signaling pathways are activated downstream of the receptor, however it is not clear how these induce specific biologic outcomes. We have previously identified 2 classes of constitutively active mutants of the shared signaling subunit, human (h) betac, of the human GM-CSF/interleukin-3 (IL-3)/IL-5 receptors that exhibit different modes of signaling. In a factor-dependent bipotential myeloid cell line, FDB1, an activated mutant containing a substitution in the transmembrane domain (V449E) induces factor-independent proliferation and survival, while mutants in the extracellular domain induce factor-independent granulocyte-macrophage differentiation. Here we have used further mutational analysis to demonstrate that there are nonredundant functions for several regions of the cytoplasmic domain with regard to mediating proliferation, viability, and differentiation, which have not been revealed by previous studies with the wild-type GM-CSF receptor. This unique lack of redundancy has revealed an association of a conserved membrane-proximal region with viability signaling and a critical but distinct role for tyrosine 577 in the activities of each class of mutant.

The dimeric versus monomeric status of 14-3-3zeta is controlled by phosphorylation of Ser58 at the dimer interface

The 14-3-3 proteins play a central role in the regulation of cell growth, cycling, and apoptosis by modulating the functional activities of key signaling proteins. Through binding to a phosphoserine motif, 14-3-3 alters target proteins activities by sequestering them, relocalizing them, conformationally altering their functional activity, or by promoting interaction with other proteins. These functions of 14-3-3 are facilitated by, if not dependent on, its dimeric structure. We now show that the dimeric status of 14-3-3 is regulated by site-specific serine phosphorylation. We found that a sphingosine-dependent kinase phosphorylates 14-3-3 in vitro and in vivo on a serine residue (Ser58) located within the dimer interface. Furthermore, by developing an antibody that specifically recognizes 14-3-3zeta phosphorylated on Ser58 and employing native-PAGE and cross-linking techniques, we found that 14-3-3 phosphorylated on Ser58 is monomeric both in vitro and in vivo. Phosphorylated 14-3-3 was detected solely as a monomer, indicating that phosphorylation of a single monomer within a dimer is sufficient to disrupt the dimeric structure. Significantly, phosphorylation-induced monomerization did not prevent 14-3-3 binding to a phosphopeptide target. We propose that this regulated monomerization of 14-3-3 controls its ability to modulate the activity of target proteins and thus may have significant implications for 14-3-3 function and the regulation of many cellular processes.

Threonine 391 phosphorylation of the human prolactin receptor mediates a novel interaction with 14-3-3 proteins

The prolactin receptor (PrlR) is a member of the cytokine receptor superfamily that lacks an intrinsic kinase domain and relies on the cytoplasmic Jak tyrosine kinases to transduce signals. Prolactin-induced Jak2 activation and consequent tyrosine phosphorylation of the receptor and downstream signaling molecules have been studied, but phosphorylation of the PrlR on serine or threonine residues has not been reported. Here we describe a novel interaction between the PrlR and the phosphoserine/phosphothreonine-binding 14-3-3 proteins. This association is mediated by the KCST391WP motif, which occurs in the major functional isoform of the human receptor and is conserved among a wide variety of species. Mutagenesis of threonine 391 to alanine significantly impaired 14-3-3 binding to the PrlR in both glutathione S-transferase pulldown and coimmunoprecipitation assays. In breast carcinoma and mouse mammary epithelial cell lines, the endogenous receptor was found to associate with glutathione S-transferase-14-3-3 proteins independent of prolactin stimulation. A phospho-specific peptide antibody was generated and used to demonstrate phosphorylation of Thr391 in vivo. Phosphorylation of this site was found to be sensitive to okadaic acid, a specific inhibitor of serine/threonine protein phosphatases. Interestingly, the T391A PrlR mutant exhibited increased basal and prolactin-induced tyrosine phosphorylation compared with the wild-type receptor. This was accompanied by a ligand-induced increase in protein kinase B and Erk activation but not that of Stat5a. Phosphorylation of the receptor on Thr391 may therefore provide a new mechanism by which prolactin signaling is attenuated.

GMCSF activates NF-kappaB via direct interaction of the GMCSF receptor with IkappaB kinase beta

Granulocyte-macrophage colony-stimulating factor (GMCSF) has a central role in proliferation and differentiation of hematopoetic cells. Furthermore, it influences the proliferation and migration of endothelial cells. GMCSF elicits these functions by activating a receptor consisting of a ligand-specific alpha-chain and a beta-chain, which is common for GMCSF, interleukin-3 (IL-3), and IL-5. It is known that various signaling molecules such as Janus kinase 2 or transcription factors of the signal transducer and activator of transcription (STAT) family bind to the common beta-chain and initiate signaling cascades. However, alpha-chain-specific signal transduction adapters have to be postulated given that IL-3, IL-5, and GMCSF induce partly distinct biologic responses. Using a yeast 2-hybrid system, we identified the alpha-chain of the GMCSF receptor (GMRalpha) as putative interaction partner of IkappaB kinase beta, one of the central signaling kinases activating the transcription factor nuclear factor-kappaB (NF-kappaB). Using endogenous protein levels of endothelial cell extracts, we could verify the interaction by coimmunoprecipitation experiments. Fluorescence resonance energy transfer (FRET) microscopy confirmed the direct interaction of CFP-IKKbeta and YFPGMRalpha in living cells. Functional studies demonstrated GMCSF-dependent activation of IkappaB kinase activity in endothelial cells, degradation of IkappaB, and activation of NF-kappaB. Further biologic studies using GMCSF-dependent TF-1 cells indicated that GMCSF-triggered activation of NF-kappaB is important for cell survival and proliferation.

Self-association of the spindle pole body-related intermediate filament protein Fin1p and its phosphorylation-dependent interaction with 14-3-3 proteins in yeast

The Fin1 protein of the yeast Saccharomyces cerevisiae forms filaments between the spindle pole bodies of dividing cells. In the two-hybrid system it binds to 14-3-3 proteins, which are highly conserved proteins involved in many cellular processes and which are capable of binding to more than 120 different proteins. Here, we describe the interaction of the Fin1 protein with the 14-3-3 proteins Bmh1p and Bmh2p in more detail. Purified Fin1p interacts with recombinant yeast 14-3-3 proteins. This interaction is strongly reduced after dephosphorylation of Fin1p. Surface plasmon resonance analysis showed that Fin1p has a higher affinity for Bmh2p than for Bmh1p (K(D) 289 versus 585 nm). Sequences in both the central and C-terminal part of Fin1p are required for the interaction with Bmh2p in the two-hybrid system. In yeast strains lacking 14-3-3 proteins Fin1 filament formation was observed, indicating that the 14-3-3 proteins are not required for this process. Fin1 also interacts with itself in the two-hybrid system. For this interaction sequences at the C terminus, containing one of two putative coiled-coil regions, are sufficient. Fin1p-Fin1p interactions were demonstrated in vivo by fluorescent resonance energy transfer between cyan fluorescent protein-labeled Fin1p and yellow fluorescent protein-labeled Fin1p.

Identification of primary structural features that define the differential actions of IL-3 and GM-CSF receptors

Activation of human interleukin 3 (IL-3) and granulocyte-macrophage colony-stimulating factor (GM-CSF) receptors, ectopically expressed in FDCP-mix multipotent cells, stimulates self-renewal or myeloid differentiation, respectively. These receptors are composed of unique alpha subunits that interact with common beta(c) subunits. A chimeric receptor (hGM/beta(c)), comprising the extracellular domain of the hGM-CSF receptor alpha subunit (hGM Ralpha) fused to the intracellular domain of hbeta(c), was generated to determine whether hbeta(c) activation is alone sufficient to promote differentiation. hGM-CSF activation of hGM/beta(c), expressed in the presence and absence of the hbeta(c) subunit, promoted maintenance of primitive phenotype. This indicates that the cytosolic domain of the hGM Ralpha chain is required for differentiation mediated by activation of the hGM Ralpha, beta(c) receptor complex. We have previously demonstrated that the alpha cytosolic domain confers signal specificity for IL-3 and GM-CSF receptors. Bioinformatic analysis of the IL-3 Ralpha and GM Ralpha subunits identified a tripeptide sequence, adjacent to the conserved proline-rich domain, which was potentially a key difference between them. Cross-exchange of the equivalent tripeptides between the alpha subunits altered receptor function compared to the wild-type receptors. Both the mutant and the corresponding wild-type receptors promoted survival and proliferation in the short-term but had distinct effects on developmental outcome. The mutated hGM Ralpha promoted long-term proliferation and maintenance of primitive cell morphology, whereas cytokine activation of the corresponding hIL-3 Ralpha mutant promoted myeloid differentiation. We have thus identified a region of the alpha cytosolic domain that is of critical importance for defining receptor specificity.

A novel MPL point mutation resulting in thrombopoietin-independent activation

Thrombopoietin (TPO) and its receptor (MPL) are important regulators of megakaryopoiesis. MPL belongs to a cytokine receptor superfamily. To date, all constitutively active MPL mutants have been artificially constructed with amino acid substitutions in the transmembrane domain or extracellular domain of the protein, and they activate signal transduction pathways in Ba/F3 cells that can also be activated by the normal MPL. In this paper, we report a novel spontaneously occurring mutation of MPL, with an amino acid substitution of Trp(508) to Ser(508) in the intracellular domain of MPL, that induces the factor-independent growth of Ba/F3 cells. Examination of intracellular signaling pathways demonstrated that the mutant MPL protein constitutively activates three distinct signaling pathways, SHC-Ras-Raf-MAPK/JNK, JAK-STAT, and PI3K-Akt-Bad.

Specificity of interleukin-2 receptor gamma chain superfamily cytokines is mediated by insulin receptor substrate-dependent pathway

Interleukins 9 (IL-9) and 4 are cytokines within the IL-2 receptor gamma chain (IL-2R gamma) superfamily that possess similar and unique biological functions. The signaling mechanisms, which may determine cytokine specificity and redundancy, are not well understood. IRS proteins are tyrosine-phosphorylated following IL-9 and IL-4 stimulation, a process in part mediated by JAK tyrosine kinases (Yin, T. G., Keller, S. R., Quelle, F. W., Witthuhn, B. A., Tsang, M. L., Lienhard, G. E., Ihle, J. N., and Yang, Y. C. (1995) J. Biol. Chem. 270, 20497--20502). In the present study, we used 32D cells stably transfected with insulin receptor (32D(IR)), which do not express any IRS proteins, as a model system to study the requirement of different structural domains of IRS proteins in IL-9- and IL-4-mediated functions. Overexpression of IRS-1 and IRS-2, but not IRS-4, induced proliferation of 32D(IR) cells in response to IL-9. The pleckstrin homology (PH) domain of IRS proteins is required for IRS-mediated proliferation stimulated by IL-9. The phosphotyrosine binding and Shc and IRS-1 NPXY binding domains are interchangeable for IRS to transduce the proliferative effect of IL-4. Therefore, the PH domain plays different roles in coupling IRS proteins to activated IL-9 and IL-4 receptors. The role of IRS proteins in determining cytokine specificity was corroborated by their ability to interact with different downstream signaling molecules. Although phosphatidylinositol 3' -kinase (PI3K) and Grb-2 interact with tyrosine-phosphorylated IRS proteins, Shp-2 only binds to IRS proteins following IL-4, but not IL-9, stimulation. Although PI3K activity is necessary for the IRS-1/2-mediated proliferative effect of IL-9 and IL-4, Akt activation is only required for cell proliferation induced by IL-4, but not IL-9. These data suggest that IRS-dependent signaling pathways work by recruiting different signaling molecules to determine specificity of IL-2R gamma superfamily cytokines.

Structural and functional hot spots in cytokine receptors

The activation of cytokine receptors is a stepwise process that depends on their specific interaction with cognate cytokines, the formation of oligomeric receptor complexes, and the initiation of cytoplasmic phosphorylation events. The recent determination of the structure of extracellular domains of several cytokine receptors allows comparison of their cytokine-binding surfaces. This comparison reveals a common structural framework that supports considerable diversity and adaptability of the binding surfaces that determine both the specificity and the orientation of subunits in the active receptor complex. These regions of the cytokine receptors have been targeted for the development of specific agonists and antagonists. The physical coupling of signaling intermediates to the intracellular domains of their receptors plays a major role in determining biological responses to cytokines. In this review, we focus principally on the receptors for cytokines of the granulocyte-macrophage colony-stimulating factor (GM-CSF) family and, where appropriate, compare them with related cytokine receptors. Several paradigms are beginning to emerge that focus on the ability of the extracellular portion of the cytokine receptor to recognize the appropriate cytokine and on a phosphorylated motif in the intracellular region of the GM-CSF receptor that couples to a specific signaling pathway.

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.

A structure-function analysis of serine/threonine phosphorylation of the thrombopoietin receptor, c-Mpl

Thrombopoietin (TPO), the critical regulator of platelet production, acts by binding to its cell surface receptor, c-Mpl. Numerous studies have shown that TPO binding leads to JAK2 kinase activation and Tyr phosphorylation of c-Mpl and several intracellular signaling intermediates, events vital for the biological activity of the hormone. In contrast, virtually nothing is known of the role of Ser or Thr phosphorylation of c-Mpl. By using phosphoamino acid analysis we found that Ser residues of c-Mpl were constitutively phosphorylated in receptor-bearing cells, levels that were increased following exposure of cells to TPO. To identify which residues were modified, and to determine the functional consequences of their phosphorylation, we generated a series of Ser to Ala mutations of a truncated c-Mpl receptor (T69) capable of supporting TPO-induced cell growth. Of the eight Ser within T69 we found that at least four are phosphorylated in TPO-stimulated cells. The mutation of each of these residues alone had minimal effects on TPO-induced proliferation, but substitution of all of the phosphoserine residues with Ala reduced the capacity of the receptor to support cell growth by over 50%. Additionally, the Ser at cytoplasmic position 18 is not detectably phosphorylated. However, the mutation of Ser-18 to Ala nearly abrogates TPO-induced proliferation and co-precipitation of JAK2 with Mpl. This study provides the first systematic analysis of the role of Ser residues in c-Mpl signaling.

The solution structure of the cytokine-binding domain of the common beta-chain of the receptors for granulocyte-macrophage colony-stimulating factor, interleukin-3 and interleukin-5

The haemopoietic cytokines, granulocyte-macrophage colony-stimulating factor, interleukin-3 and interleukin-5 bind to cell-surface receptors comprising ligand-specific alpha-chains and a shared beta-chain. The beta-chain is the critical signalling subunit of the receptor and its fourth domain not only plays a critical role in interactions with ligands, hence in receptor activation, but also contains residues whose mutation can lead to ligand-independent activation of the receptor. We have determined the NMR solution structure of the isolated human fourth domain of the beta-chain. The protein has a fibronectin type III fold with a well-defined hydrophobic core and is stabilised by an extensive network of pi-cation interactions involving Trp and Arg side-chains, including two Trp residues outside the highly conserved Trp-Ser-Xaa-Trp-Ser motif (where Xaa is any amino acid) that is found in many cytokine receptors. Most of the residues implicated in factor-independent mutants localise to the rigid core of the domain or the pi-cation stack. The loops between the B and C, and the F and G strands, that contain residues important for interactions with cytokines, lie adjacent at the membrane-distal end of the domain, consistent with their being involved cooperatively in binding cytokines. The elucidation of the structure of the cytokine-binding domain of the beta-chain provides insight into the cytokine-dependent and factor-independent activation of the receptor.