The tyrosine 974 within the LIF-R-chain of the gp130/LIF-R heteromeric receptor complex mediates negative regulation of LIF signalling

Roles of leukemia inhibitory factor receptor in cancer

Leukemia inhibitory factor receptor (LIFR), in complex with glycoprotein 130 (gp130) as the receptor for leukemia inhibitory factor (LIF), can bind to a variety of cytokines and subsequently activate a variety of signaling pathways, including Janus kinase/signal transducer and activator of transcription 3. LIF, the most multifunctional cytokines of the interleukin-6 family acts as both a growth factor and a growth inhibitor in different types of tumors. LIF/LIFR signaling regulates a broad array of tumor-related processes including proliferation, apoptosis, migration, invasion. However, due to the activation of different signaling pathways, opposite regulatory effects are observed in certain tumor cells. Therefore, the role of LIFR in human cancers varies across different tumor and tissue, despite their recognized value in tumor treatment and prognosis observation is affirmed. Given its aberrant expression in numerous tumor cells and crucial regulatory function in tumorigenesis and progression, LIFR is considered as a promising targeted therapeutic agent. This review provides an overview of LIFR's initiating signaling pathway function as a cytokine receptor and summarize the current literature on the role of LIFR in cancer and its possible use in therapy.

LIF and the heart: just another brick in the wall?

Multiple studies have shown that the cytokine leukemia inhibitory factor (LIF) is protective of the myocardium in the acute stress of ischemia-reperfusion. All three major intracellular signaling pathways that are activated by LIF in cardiac myocytes have been linked to actions that protect against oxidative stress and cell death, either at the level of the mitochondrion or via nuclear transcription. In addition, LIF has been shown to contribute to post-myocardial infarction cardiac repair and regeneration, by stimulating the homing of bone marrow-derived cardiac progenitors to the injured myocardium, the differentiation of resident cardiac stem cells into endothelial cells, and neovascularization. Whether LIF offers protection to the heart under chronic stress such as hypertension-induced cardiac remodeling and heart failure is not known. However, mice with cardiac myocyte restricted knockout of STAT3, a principal transcription factor activated by LIF, develop heart failure with age, and cardiac STAT3 levels are reported to be decreased in heart failure patients. In addition, endogenously produced LIF has been implicated in the cholinergic transdiffrentiation that may serve to attenuate sympathetic overdrive in heart failure and in the peri-infarct region of the heart after myocardial infarction. Surprisingly, therapeutic strategies to exploit the beneficial actions of LIF on the injured myocardium have received scant attention. Nor is it established whether the purported so-called adverse effects of LIF observed in isolated cardiac myocytes have physiological relevance in vivo. Here we present an overview of the actions of LIF in the heart with the goal of stimulating further research into the translational potential of this pleiotropic cytokine.

Differential STAT3 signaling in the heart: Impact of concurrent signals and oxidative stress

Multiple lines of evidence suggest that the transcription factor STAT3 is linked to a protective and reparative response in the heart. Thus, increasing duration or intensity of STAT3 activation ought to minimize damage and improve heart function under conditions of stress. Two recent studies using genetic mouse models, however, report findings that appear to refute this proposition. Unfortunately, studies often approach the question of the role of STAT3 in the heart from the perspective that all STAT3 signaling is equivalent, particularly when it comes to signaling by IL-6 type cytokines, which share the gp130 signaling protein. Moreover, STAT3 activation is typically equated with phosphorylation of a critical tyrosine residue. Yet, STAT3 transcriptional behavior is subject to modulation by serine phosphorylation, acetylation, and redox status of the cell. Unphosphorylated STAT3 is implicated in gene induction as well. Thus, how STAT3 is activated and also what other signaling events are occurring at the same time is likely to impact on the outcome ultimately linked to STAT3. Notably STAT3 may serve as a scaffold protein allowing it to interact with other singling pathways. In this context, canonical gp130 cytokine signaling may function to integrate STAT3 signaling with a protective PI3K/AKT signaling network via mutual involvement of JAK tyrosine kinases. Differences in the extent of integration may occur between those cytokines that signal through gp130 homodimers and those through heterodimers of gp130 with a receptor α chain. Signal integration may have importance not only for deciding the particular gene profile linked to STAT3, but for the newly described mitochondrial stabilization role of STAT3 as well. In addition, disruption of integrated gp130-related STAT3 signaling may occur under conditions of oxidative stress, which negatively impacts on JAK catalytic activity. For these reasons, understanding the importance of STAT3 signaling to heart function requires a greater appreciation of the plasticity of this transcription factor in the context in which it is investigated.

Protein tyrosine phosphatase SHP2/PTPN11 mistargeting as a consequence of SH2-domain point mutations associated with Noonan Syndrome and leukemia

SHP2/PTPN11 is a key regulator of cytokine, growth factor and integrin signaling. SHP2 influences cell survival, proliferation and differentiation by regulating major signaling pathways. Mutations in PTPN11 cause severe diseases like Noonan, LEOPARD syndrome or leukemia. Whereas several of these mutations result in altered enzymatic activity due to impaired auto-inhibition, not all disease patterns can be explained by this mechanism. In this study we analyzed altered binding properties of disease-related SHP2-mutants bearing point mutations within the SH2-domain (T42A, E139D, and R138Q). Mutants were chosen according to SPR assays, which revealed different binding properties of mutated SH2 towards phosphorylated receptor peptides. To analyze global changes in mutant binding properties we applied quantitative mass spectrometry (SILAC). Using an in vitro approach we identified overall more than 1000 protein candidates, which specifically bind to the SH2-domain of SHP2. We discovered that mutations in the SH2-domain selectively affected protein enrichment by altering the binding capacity of the SH2-domain. Mutation-dependent, enhanced or reduced exposure of SHP2 to its binding partners could have an impact on the dynamics of signaling networks. Thus, disease-associated mutants of SHP2 should not only be discussed in the context of deregulated auto-inhibition but also with respect to deregulated protein targeting of the SHP2 mutants.

BIOLOGICAL SIGNIFICANCE

Using quantitative mass spectrometry based proteomics we provided evidence that disease related mutations in SHP2 domains of SHP2 are able to influence SHP2 recruitment to its targets in mutation dependent manner. We discovered that mutations in the SH2-domain selectively affected protein enrichment ratios suggesting altered binding properties of the SH2-domain. We demonstrated that mutations within SHP2, which had been attributed to affect the enzymatic activity (i.e. affect the open/close status of SHP2), also differ in respect to binding properties. Our study indicates that SHP2 mutations need to be discussed not only in terms of deregulated auto-inhibition but also with respect to deregulated protein targeting properties of the SHP2 mutants. Discovery of the new binding partners for disease-related SHP2 mutants might provide a fruitful foundation for developing strategies targeting Noonan-associated leukemia.

Characterization of the rat oncostatin M receptor complex which resembles the human, but differs from the murine cytokine receptor

Evaluation of a pathophysiological role of the interleukin-6-type cytokine oncostatin M (OSM) for human diseases has been complicated by the fact that mouse models of diseases targeting either OSM or the OSM receptor (OSMR) complex cannot fully reflect the human situation. This is due to earlier findings that human OSM utilizes two receptor complexes, glycoprotein 130 (gp130)/leukemia inhibitory factor receptor (LIFR) (type I) and gp130/OSMR (type II), both with wide expression profiles. Murine OSM on the other hand only binds to the gp130/OSMR (type II) receptor complex with high affinity. Here, we characterize the receptor usage for rat OSM. Using different experimental approaches (knock-down of the OSMR expression by RNA interference, blocking of the LIFR by LIF-05, an antagonistic LIF variant and stably transfected Ba/F3 cells) we can clearly show that rat OSM surprisingly utilizes both, the type I and type II receptor complex, therefore mimicking the human situation. Furthermore, it displays cross-species activities and stimulates cells of human as well as murine origin. Its signaling capacities closely mimic those of human OSM in cell types of different origin in the way that strong activation of the Jak/STAT, the MAP kinase as well as the PI3K/Akt pathways can be observed. Therefore, rat disease models would allow evaluation of the relevance of OSM for human biology.

The biology and mechanism of action of suppressor of cytokine signaling 3

Suppressors of cytokine signaling 3 (SOCS3) has been shown to be an important and non-redundant feedback inhibitor of several cytokines including leukemia inhibitory factor, IL-6, IL-11, Ciliary neurotrophic factor (CNTF), leptin, and granulocyte colony-stimulating factor (G-CSF). Loss of SOCS3 in vivo has profound effects on placental development, inflammation, fat-induced weight gain, and insulin sensitivity. SOCS3 expression is induced by Janus kinase (JAK)/signal transducers and activators of transcription (STAT) signaling and it then binds to specific cytokine receptors (including gp130, G-CSF, and leptin receptors). SOCS3 then inhibits JAK/STAT signaling in two distinct ways. First, SOCS3 is able to directly inhibit the catalytic activity of JAK1, JAK2, or TYK2 while remaining bound to the cytokine receptor. Second, SOCS3 recruits elongins B/C and Cullin5 to generate an E3 ligase that ubiquitinates both JAK and cytokine receptor targeting them for proteasomal degradation. Detailed in vivo studies have revealed that SOCS3 action not only limits the duration of cytokine signaling to prevent overactivity but it is also important in maintaining the specificity of cytokine signaling.

Leukemia inhibitory factor: a paracrine mediator of bone metabolism

Leukemia inhibitory factor (LIF) is a soluble interleukin-6 family cytokine that regulates a number of physiologic functions, including normal skeletal remodeling. LIF signals through the cytokine co-receptor glycoprotein-130 in complex with its cytokine-specific receptor [LIF receptor (LIFR)] to activate signaling cascades in cells of the skeletal system, including stromal cells, chondrocytes, osteoblasts, osteocytes, adipocytes, and synovial fibroblasts. LIF action on skeletal cells is cell-type specific, and frequently dependent on the state of cell differentiation. This review describes the expression patterns of LIF and LIFR in bone, their regulation by physiological and inflammatory agents, as well as cell-specific influences of LIF on osteoblast, osteoclast, chondrocyte, and adipocyte differentiation. The actions of LIF in normal skeletal growth and maintenance, in pathological states (e.g. autocrine tumor cell signaling and growth in bone) and inflammatory conditions (e.g. arthritis) will be discussed, as well as the signaling pathways activated by LIF and their importance in bone formation and resorption.

LIF in the regulation of T-cell fate and as a potential therapeutic

At the heart of lineage commitment within the adaptive immune response is the intrinsic genetic plasticity of the naive peripheral T lymphocyte (T cell). Primary activation by presentation of cognate antigen is coupled to rapid T-cell cycling and progressive epigenetic changes that guide the cell down distinct T-cell lineages, either effector (Th1, Th2, Th17) or tolerogenic (Treg). Fate choice is influenced both by strength of the priming activation signal and by cues from the micro-environment that are integrated with lineage-specific gene expression profiles, eventually becoming hard-wired in the fully differentiated cell. The micro-environmental cues include cytokines, and the discovery that leukaemia inhibitory factor (LIF) and interleukin (IL)-6 counter-regulate development of the Treg and Th17 lineages places LIF within the core regulatory circuitry of T cells. I first summarise current understanding of LIF and the LIF receptor in the context of T cells. Next, the central relevance of the LIF/IL-6 axis in immune-mediated disease is set in the context of (i) a new nano-therapeutic approach for targeted delivery of LIF and (ii) MARCH-7, a novel E3-ligase discovered to have a central mechanistic role in LIF-mediated T-cell biology, functioning as a rheostat-type regulator of endogenous LIF-signalling.

Sensitivity analysis of intracellular signaling pathway kinetics predicts targets for stem cell fate control

Directing stem cell fate requires knowledge of how signaling networks integrate temporally and spatially segregated stimuli. We developed and validated a computational model of signal transducer and activator of transcription-3 (Stat3) pathway kinetics, a signaling network involved in embryonic stem cell (ESC) self-renewal. Our analysis identified novel pathway responses; for example, overexpression of the receptor glycoprotein-130 results in reduced pathway activation and increased ESC differentiation. We used a systematic in silico screen to identify novel targets and protein interactions involved in Stat3 activation. Our analysis demonstrates that signaling activation and desensitization (the inability to respond to ligand restimulation) is regulated by balancing the activation state of a distributed set of parameters including nuclear export of Stat3, nuclear phosphatase activity, inhibition by suppressor of cytokine signaling, and receptor trafficking. This knowledge was used to devise a temporally modulated ligand delivery strategy that maximizes signaling activation and leads to enhanced ESC self-renewal.

SHPS-1/SIRP1alpha contributes to interleukin-6 signalling

The transmembrane glycoprotein signal regulatory protein/SHP2-substrate (SIRP1alpha/SHPS-1) has been implicated in growth factor- and cell adhesion-induced signalling. Here we report on the contribution of SIRP1alpha to IL-6 type cytokine signalling. SIRP1alpha binds the protein tyrosine phosphatase SHP2 upon treatment with interleukin-6 in a stimulation-dependent manner. Mouse embryonic fibroblasts expressing a SIRP1alpha protein which lacks the intracellular part show enhanced SHP2 phosphorylation and ERK1/2 activation in response to IL-6, suggesting that SIRP1alpha affects IL-6-signalling through SHP2. Whereas SHP2 phosphorylation is enhanced in SIRP1alpha-deficient cells STAT3 activation is delayed and STAT3-dependent gene induction is reduced which correlates with reduced STAT3 serine phosphorylation. Our results indicate that SIRP1alpha contributes to IL-6 signalling by counteracting SHP2 phosphorylation which consequently affects ERK-activation and STAT3-dependent transactivation as well as target gene expression. Our observations will help to understand the tight balance of MAPK- and STAT3-activation in response to IL-6 which was found to be misbalanced in many autoimmune diseases, inflammatory proliferative diseases and cancer.

Can the protective actions of JAK-STAT in the heart be exploited therapeutically? Parsing the regulation of interleukin-6-type cytokine signaling

Activation of the transcription factor signal transducers and activators of transcription (STAT) 3 is a defining feature of the interleukin (IL)-6 family of cytokines, which include IL-6, leukemia inhibitory factor, and cardiotrophin-1. These cytokines, as well as STAT3 activation, have been shown to be protective for cardiac myocytes and necessary for ischemia preconditioning. However, the mechanisms that regulate IL-6-type cytokine signaling in cardiac myocytes are largely unexplored. We propose that the protective character of IL-6-type cytokine signaling in cardiac myocytes is determined principally by three mechanisms: redox status of the nonreceptor tyrosine kinase Janus kinase 1 (JAK) 1 that activates STAT3, phosphorylation of STAT3 within the transcriptional activation domain on serine 727, and STAT3-mediated induction of suppressor of cytokine signaling (SOCS) 3 that terminates IL-6-type cytokine signaling. Moreover, we hypothesize that hyperactivation of the JAK kinases, particularly JAK2, mismatched STAT3 serine-tyrosine phosphorylation or heightened STAT3 transcriptional activity, and SOCS3 induction may ultimately prove detrimental. Here we summarize recent evidence that supports this hypothesis, as well as additional possible mechanisms of JAK-STAT regulation. Understanding how IL-6-type cytokine signaling is regulated in cardiac myocytes has great significance for exploiting the therapeutic potential of these cytokines and the phenomenon of preconditioning.

Sequence specificity of SHP-1 and SHP-2 Src homology 2 domains. Critical roles of residues beyond the pY+3 position

A combinatorial phosphotyrosyl (pY) peptide library was screened to determine the amino acid preferences at the pY+4 to pY+6 positions for the four SH2 domains of protein-tyrosine phosphatases SHP-1 and SHP-2. Individual binding sequences selected from the library were resynthesized and their binding affinities and specificities to various SH2 domains were further evaluated by SPR studies, stimulation of SHP-1 and SHP-2 phosphatase activity, and in vitro pulldown assays. These studies reveal that binding of a pY peptide to the N-SH2 domain of SHP-2 is greatly enhanced by a large hydrophobic residue (Trp, Tyr, Met, or Phe) at the pY+4 and/or pY+5 positions, whereas binding to SHP-1 N-SH2 domain is enhanced by either hydrophobic or positively charged residues (Arg, Lys, or His) at these positions. Similar residues at the pY+4 to pY+6 positions are also preferred by SHP-1 and SHP-2 C-SH2 domains, although their influence on the overall binding affinities is much smaller compared with the N-SH2 domains. A structural model was generated to qualitatively interpret the contribution of the pY+4 and pY+5 residues to the overall binding affinity. Examination of pY motifs from known SHP-1 and SHP-2-binding proteins shows that many of the pY motifs contain a hydrophobic or positively charged residue(s) at the pY+4 and pY+5 positions.

SOCS3 negatively regulates LIF signaling in neural precursor cells

Cytokines that signal through the LIFRbeta/gp130 receptor complex, including LIF and CNTF, promote the self-renewal of embryonic and adult neural precursor cells (NPCs). In non-CNS tissues, the protein suppressor of cytokine signaling-3 (SOCS3) negatively regulates signaling through gp130. Here, we analyze the role of SOCS3 in inhibiting LIF signaling in NPCs in vitro. SOCS3 is rapidly expressed by NPCs in response to LIF stimulation, with this expression largely dependent on recruitment of STAT proteins to the activated gp130 receptor. Proliferating NPC cultures can be generated from SOCS3 knockout (SOCS3KO/KO) embryos and display prolonged STAT3 phosphorylation and induction of the GFAP gene in response to LIF. In comparison with SOCS3 wild-type (SOCS3WT/WT) NPCs, SOCS3KO/KO cultures display enhanced self-renewal capacity. However, the clonal potential of SOCS3WT/WT but not SOCS3KO/KO NPCs is enhanced by exogenous LIF. Thus, SOCS3 acts as a negative regulator of LIF signaling in NPCs.

Oncostatin M receptor-mediated signal transduction is negatively regulated by SOCS3 through a receptor tyrosine-independent mechanism

Down-regulation of interleukin (IL)-6-type cytokine signaling has been shown to occur, among other mechanisms, via induction of the feedback inhibitor SOCS3 (suppressor of cytokine signaling 3). Binding of SOCS3 to the phosphorylated Tyr(759) in the cytoplasmic region of gp130, the common signal transducing receptor chain of all IL-6-type cytokines, is necessary for inhibition of Janus kinase-mediated signaling. In the present study, we analyzed the effect of SOCS3 on signal transduction by the proinflammatory cytokine oncostatin M (OSM), which signals through a receptor complex of gp130 and the OSM receptor (OSMR). OSM leads to a much stronger and prolonged induction of SOCS3 in HepG2 hepatoma cells and murine embryonal fibroblasts (MEF) compared with IL-6. A negative effect of SOCS3 on OSM signaling was confirmed using MEF cells lacking SOCS3. We can show that the OSMR-mediated signaling is inhibited by SOCS3 to a similar extent as previously described for gp130. However, the inhibition occurs independent of tyrosine motifs within the OSMR. Instead, SOCS3 interacts directly with JAK1 in a stimulation-dependent manner, a mechanism so far only known for SOCS1.