Binding of leukemia inhibitory factor (LIF) to mutants of its low affinity receptor, gp190, reveals a LIF binding site outside and interactions between the two cytokine binding domains

Therapeutic approaches targeting CD95L/CD95 signaling in cancer and autoimmune diseases

Cell death plays a pivotal role in the maintenance of tissue homeostasis. Key players in the controlled induction of cell death are the Death Receptors (DR). CD95 is a prototypic DR activated by its cognate ligand CD95L triggering programmed cell death. As a consequence, alterations in the CD95/CD95L pathway have been involved in several disease conditions ranging from autoimmune diseases to inflammation and cancer. CD95L-induced cell death has multiple roles in the immune response since it constitutes one of the mechanisms by which cytotoxic lymphocytes kill their targets, but it is also involved in the process of turning off the immune response. Furthermore, beyond the canonical pro-death signals, CD95L, which can be membrane-bound or soluble, also induces non-apoptotic signaling that contributes to its tumor-promoting and pro-inflammatory roles. The intent of this review is to describe the role of CD95/CD95L in the pathophysiology of cancers, autoimmune diseases and chronic inflammation and to discuss recently patented and emerging therapeutic strategies that exploit/block the CD95/CD95L system in these diseases.

Endocannabinoids as biomarkers of human reproduction

BACKGROUND

Infertility is a condition of the reproductive system that affects ∼10-15% of couples attempting to conceive a baby. More than half of all cases of infertility are a result of female conditions, while the remaining cases can be attributed to male factors, or to a combination of both. The search for suitable biomarkers of pregnancy outcome is a challenging issue in human reproduction, aimed at identifying molecules with predictive significance of the reproductive potential of male and female gametes. Among the various candidates, endocannabinoids (eCBs), and in particular anandamide (AEA), represent potential biomarkers of human fertility disturbances. Any perturbation of the balance between synthesis and degradation of eCBs will result in local changes of their tone in human female and male reproductive tracts, which in turn regulates various pathophysiological processes, oocyte and sperm maturation included.

METHODS

PubMed and Web of Science databases were searched for papers using relevant keywords like 'biomarker', 'endocannabinoid', 'infertility', 'pregnancy' and 'reproduction'.

RESULTS

In this review, we discuss different studies on the measurements of AEA and related eCBs in human reproductive cells, tissues and fluids, where the local contribution of these bioactive lipids could be critical in ensuring normal sperm fertilizing ability and pregnancy.

CONCLUSION

Based on the available data, we suggest that the AEA tone has the potential to be exploited as a novel diagnostic biomarker of infertility, to be used in association with assays of conventional hormones (e.g. progesterone, β-chorionic gonadotrophin) and semen analysis. However further quantitative research of its predictive capacity is required.

Functional characterization of a chimeric soluble Fas ligand polymer with in vivo anti-tumor activity

Binding of ligand FasL to its receptor Fas triggers apoptosis via the caspase cascade. FasL itself is homotrimeric, and a productive apoptotic signal requires that FasL be oligomerized beyond the homotrimeric state. We generated a series of FasL chimeras by fusing FasL to domains of the Leukemia Inhibitory Factor receptor gp190 which confer homotypic oligomerization, and analyzed the capacity of these soluble chimeras to trigger cell death. We observed that the most efficient FasL chimera, called pFasL, was also the most polymeric, as it reached the size of a dodecamer. Using a cellular model, we investigated the structure-function relationships of the FasL/Fas interactions for our chimeras, and we demonstrated that the Fas-mediated apoptotic signal did not solely rely on ligand-mediated receptor aggregation, but also required a conformational adaptation of the Fas receptor. When injected into mice, pFasL did not trigger liver injury at a dose which displayed anti-tumor activity in a model of human tumor transplanted to immunodeficient animals, suggesting a potential therapeutic use. Therefore, the optimization of the FasL conformation has to be considered for the development of efficient FasL-derived anti-cancer drugs targeting Fas.

Precise mapping of the CD95 pre-ligand assembly domain

Pre-association of CD95 at the plasma membrane is mandatory for efficient death receptor signaling. This homotrimerization occurs through self-association of an extracellular domain called the pre-ligand assembly domain (PLAD). Using novel molecular and cellular tools, we confirmed that CD95-PLAD is necessary to promote CD95 multimerization and plays a pivotal role in the transmission of apoptotic signals. However, while a human CD95 mutant deleted of the previously described PLAD domain (amino acids 1 to 66) fails to interact with its wild-type counterpart and trigger autonomous cell death, deletion of amino acids 1 to 42 does not prevent homo- or hetero (human/mouse)-oligomerization of CD95, and thus does not alter transmission of the apoptotic signal. Overall, these findings indicate that the region between amino acids 43 to 66 corresponds to the minimal motif involved in CD95 homotypic interaction and is necessary to convey an efficient apoptotic signal. Interfering with this PLAD may represent a new therapeutic strategy for altering CD95-induced apoptotic and non-apoptotic signals.

Direct anti-cancer effect of oncostatin M on chondrosarcoma

The cytokine Oncostatin M (OSM) is cytostatic, pro-apoptotic and induces differentiation of osteosarcoma cells into osteocytes, suggesting new adjuvant treatment for these bone-forming sarcomas. However, OSM systemic over-expression could lead to adverse side effects such as generalized inflammation, neoangiogenesis and osteolysis. We determine here the effect of OSM on chondrosarcoma, another primary bone sarcoma characterized by the production of cartilage matrix and altered bone remodelling. Chondrosarcomas are resistant to conventional chemotherapy and radiotherapy, and wide surgical excision remains the only available treatment. We found that OSM blocked the cell cycle in four of five chondrosarcoma cell lines, independently of p53 and presumably through the JAK3/STAT1 pathway. In two tested cell lines, OSM induced a hypertrophic chondrocyte differentiation, with an induced Cbfa1/SOX9 ratio and induced Coll10, matrix metalloproteinase 13 (MMP13) and RANKL expression. Adenoviral gene transfer of OSM (AdOSM) in the Swarm rat chondrosarcoma (SRC) model indicated that local intra-tumoral OSM over-expression reduces chondrosarcoma development not only with reduced tumor proliferation and enhanced apoptosis but also with enhanced RANKL expression, osteoclast formation and reduced bone volumes. Flu-like symptoms were induced by the AdOSM, but there was no effect on tumor angiogenesis. Therefore, OSM could be considered as a new adjuvant anti-cancer agent for chondrosarcomas. A local application of this cytokine is presumably needed to overcome the poor vascularization of these tumors and to limit the deleterious effect on other tissues. Its side effect on bone remodeling could be managed with anti-resorption agents, thus offering potential new lines of therapeutic interventions.

Interplay between endocannabinoids, steroids and cytokines in the control of human reproduction

The use of marijuana, which today is the most used recreational drug, has been demonstrated to affect adversely reproduction. Marijuana smokers, both men and women, show impaired fertility, owing to defective signalling pathways, aberrant hormonal regulation, or wrong timing during embryo implantation. Anandamide (N-arachidonoylethanolamine, AEA) and 2-arachidonoylglycerol (2-AG) mimic Delta(9)-tetrahydrocannabinol (THC), the psychoactive principle of Cannabis sativa, by binding to both the brain-type (CB(1)) and the spleen-type (CB(2)) cannabinoid receptors. These 'endocannabinoids' exert several actions either in the central nervous system or in peripheral tissues, and are metabolised by specific enzymes that synthesise or hydrolyse them. In this review, we shall describe the elements that constitute the endocannabinoid system (ECS), in order to put in a better perspective the role of this system in the control of human fertility, both in females and males. In addition, we shall discuss the interplay between ECS, sex hormones and cytokines, which generates an endocannabinoid-hormone-cytokine array critically involved in the control of human reproduction.

Cutting edge: SDS-stable Fas microaggregates: an early event of Fas activation occurring with agonistic anti-Fas antibody but not with Fas ligand

The 45 kDa Fas or CD95 receptor triggers apoptosis via the caspase cascade when stimulated by its ligand FasL or by agonistic Abs. Activated Fas receptors seem to oligomerize very early into SDS-stable and reducing agent-resistant microaggregates of 200-250 kDa on SDS-PAGE. However, these microaggregates have so far only been reported using agonistic anti-Fas Abs, and no results have been reported using FasL. Here, we demonstrate that the microaggregates do not form in response to FasL, while they always appear in response to the agonistic Ab, in four different cell lines and in normal lymphocytes from human blood. Therefore, the Fas microaggregates are not required for the induction of apoptosis via FasL. These results also suggest that subtle differences exist in the apoptotic pathways triggered by anti-Fas agonistic Abs and by FasL.

Leukemia inhibitory factor (LIF), cardiotrophin-1, and oncostatin M share structural binding determinants in the immunoglobulin-like domain of LIF receptor

Leukemia inhibitory factor (LIF), cardiotrophin-1 (CT-1), and oncostatin M (OSM) are four helix bundle cytokines acting through a common heterodimeric receptor composed of gp130 and LIF receptor (LIFR). Binding to LIFR occurs through a binding site characterized by an FXXK motif located at the N terminus of helix D (site III). The immunoglobulin (Ig)-like domain of LIFR was modeled, and the physico-chemical properties of its Connolly surface were analyzed. This analysis revealed an area displaying properties complementary to those of the LIF site III. Two residues of the Ig-like domain of LIFR, Asp214 and Phe284, formed a mirror image of the FXXK motif. Engineered LIFR mutants in which either or both of these two residues were mutated to alanine were transfected in Ba/F3 cells already containing gp130. The F284A mutation impaired the biological response induced by LIF and CT-1, whereas the response to OSM remained unchanged. The Asp214 mutation did not alter the functional responses. The D214A/F284A double mutation, however, totally impaired cellular proliferation to LIF and CT-1 and partially impaired OSM-induced proliferation with a 20-fold increase in EC50. These results were corroborated by the analysis of STAT3 phosphorylation and Scatchard analysis of cytokine binding to Ba/F3 cells. Molecular modeling of the complex of LIF with the Ig-like domain of LIFR provides a clue for the superadditivity of the D214A/F284A double mutation. Our results indicate that LIF, CT-1, and OSM share an overlapping binding site located in the Ig-like domain of LIFR. The different behaviors of LIF and CT-1, on one side, and of OSM, on the other side, can be related to the different affinity of their site III for LIFR.

Analysis of the leukemia inhibitory factor receptor functional domains by chimeric receptors and cytokines

In contrast to other hematopoietic cytokine receptors, the leukemia inhibitory factor receptor (LIFR) possesses two cytokine binding modules (CBMs). Previous studies suggested that the NH(2)-terminal CBM and the Ig-like domain of the LIFR are most important for LIF binding and activity. Using the recently engineered designer cytokine IC7, which induces an active heterodimer of the LIFR and gp130 after binding to the IL-6R, and several receptor chimeras of the LIFR and the interleukin-6 receptor (IL-6R) carrying the CBM of the IL-6R in place of the COOH-terminal LIFR CBM, we could assign individual receptor subdomains to individual binding sites of the ligand. The NH(2)-terminal CBM and the Ig-like domain of the LIFR bind to ligand site III, whereas the COOH-terminal CBM contacts site I. Furthermore, we show that LIFR mutants carrying the IL-6R CBM instead of the COOH-terminal CBM can replace the IL-6R by acting as an alpha-receptor for IL-6. However, in situations where a signaling competent receptor is bound at IL-6 site I, ligand binding to site III is an absolute requirement for participation of the receptor in a signaling heterodimer with gp130; i.e., a functional receptor complex of IL-6 type cytokines cannot be assembled solely via site I and II as in the growth hormone receptor complex.

Mutations in the immunoglobulin-like domain of gp190, the leukemia inhibitory factor (LIF) receptor, increase or decrease its affinity for LIF

The leukemia inhibitory factor (LIF) receptor comprises the low affinity binding chain gp190 and the high affinity converter gp130. The ectodomain of gp190 is among the most complex in the hematopoietin receptor family, because it contains two typical cytokine receptor homology domains separated by an immunoglobulin-like (Ig-like) domain. Human and murine gp190 proteins share 76% homology, but murine gp190 binds human LIF with a much higher affinity, a property attributed to the Ig-like domain. Using alanine-scanning mutagenesis of the Ig-like domain, we mapped a LIF binding site at its carboxyl terminus, mainly involving residue Phe-328. Mutation of selected residues into their orthologs in the murine receptor (Q251E and N321D) significantly increased the affinity for human LIF. Interestingly, these residues, although localized at both the amino and carboxyl terminus, make a spatially unique LIF binding site in a structural model of the Ig-like module. These results demonstrate definitively the role of the Ig-like domain in LIF binding and the potential to modulate receptor affinity in this family with very limited amino acid changes.

Separate functions for the two modules of the membrane-proximal cytokine binding domain of glycoprotein 190, the leukemia inhibitory factor low affinity receptor, in ligand binding and receptor activation

The receptor for the cytokine leukemia inhibitory factor (LIF) associates the low affinity binding component gp190 and the high affinity converter gp130. Both are members of the hematopoietic receptors family characterized by the cytokine receptor homology (CRH) domain, which consists of two barrel-like modules of around 100 amino acids each. The gp190 is among the very few members of this large family to contain two CRH domains. The membrane-distal one (herein called D1) is followed by an immunoglobulin-like domain, a membrane-proximal CRH domain called D2, and three type III fibronectin-like repeats. A minimal D1IgD2 fragment is required for binding LIF. By using transmembrane forms of deletion mutants in gp190 ectodomain, we demonstrated that removal of D1 led to spontaneous activation of the receptor and that this property was devoted to a peptidic sequence localized within the last 42 amino acids of the carboxyl-terminal module of D2. By using soluble forms of deletion mutants made by progressive truncations from the end of the D1IgD2 fragment, we demonstrated that the carboxyl-terminal module of D2 was dispensable for LIF binding and that the correct conformation of the D1Ig fragment required a full amino-terminal module of D2. Therefore, the two constitutive modules of the membrane-proximal CRH domain D2 of gp190 fulfill two distinct roles in gp190 function, i.e. in stabilizing the conformation of gp190 allowing LIF binding and in activating the receptor.

Membrane distal cytokine binding domain of LIFR interacts with soluble CNTFR in vitro

Ciliary neurotrophic factor (CNTF) is a member of the gp130 family of cytokines. The functional receptor complex of CNTF is composed of the CNTF receptor alpha (CNTFR), gp130 and the leukemia inhibitory factor receptor (LIFR). Three regions on CNTF have been identified as binding sites for its receptors. The ligand-receptor interactions are mediated through the cytokine binding domains (CBDs) and/or the immunoglobulin-like domains of the receptors. However, in the case of CNTF, the precise nature of the protein-protein contacts in the signaling complex has not yet been resolved. In this study, we provide the first demonstration that the membrane distal CBD (CBD1) of LIFR associates in vitro with soluble CNTFR in the absence of CNTF. Moreover, purified CBD1 partially blocks CNTF signaling, but not that of interleukin-6 or LIF, in human embryonal carcinoma cell line Ntera/D1 cells. These data raise the possibility that LIFR has the capability to form a ligand-free complex with CNTFR.

The upper cytokine-binding module and the Ig-like domain of the leukaemia inhibitory factor (LIF) receptor are sufficient for a functional LIF receptor complex

To elucidate the function of the two cytokine-binding modules (CBM) of the leukemia inhibitory factor receptor (LIFR), receptor chimeras of LIFR and the interleukin-6 receptor (IL-6R) were constructed. Either the NH(2)-terminal (chimera RILLIFdeltaI) or the COOH-terminal LIFR CBM (chimera RILLIFdeltaII) were replaced by the structurally related CBM of the IL-6R which does not bind LIF. Chimera RILLIFdeltaI is functionally inactive, whereas RILLIFdeltaII binds LIF and mediates signalling as efficiently as the wild-type LIFR. Deletion mutants of the LIFR revealed that both the NH(2)-terminal CBM and the Ig-like domain of the LIFR are involved in LIF binding, presumably via the LIF site III epitope. The main function of the COOH-terminal CBM of the LIFR is to position the NH(2)-terminal CBM and the Ig-like domain, so that these can bind to LIF. In analogy to a recently published model of the IL-6R complex, a model of the active LIFR complex is suggested which positions the COOH-terminal CBM at LIF site I and the NH(2)-terminal CBM and the Ig-like domain at site III. An additional contact is postulated between the Ig-like domain of gp130 and the NH(2)-terminal CBM of the LIFR.

Identification of agonistic and antagonistic antibodies against gp190, the leukemia inhibitory factor receptor, reveals distinct roles for its two cytokine-binding domains

The receptor for the cytokine leukemia inhibitory factor (LIF) associates the low affinity binding component gp190 and the high affinity converter gp130, both of which are members of the family of hematopoietic receptors characterized by the cytokine receptor homology (CRH) domain. The gp190 is among the very few members of this large family to contain two CRH domains. The membrane-distal one (herein called D1) is followed by an Ig-like domain, a membrane-proximal CRH domain called D2, and three type III fibronectin repeats. We raised a series of monoclonal antibodies specific for the human gp190. Among them was the blocking antibody 1C7, which was directed against the D1Ig region and which impaired the binding of LIF to gp190. Another blocking antibody, called 12D3, was directed against domain D2 and interfered with the reconstitution of the high affinity receptor complex, independently of the interaction between LIF and gp190. The blocking effect of these two antibodies concerned four cytokines known to use gp190, i.e. LIF, oncostatin M, ciliary neurotrophic factor, and cardiotrophin-1. Among 23 antibodies tested alone or in combination (two anti-D2 and 21 anti-D1Ig), only the mixture of the two anti-D2 antibodies displayed agonistic activity in the absence of the cytokine. Taken together, these results demonstrate that the two CRH domains of gp190 play different functions in ligand binding and receptor activation.

Two different epitopes of the signal transducer gp130 sequentially cooperate on IL-6-induced receptor activation

Cytokines are key mediators for the regulation of hemopoiesis and the coordination of immune responses. They exert their various functions through activation of specific cell surface receptors, thereby initiating intracellular signal transduction cascades which lead to defined cellular responses. As the common signal-transducing receptor subunit of at least seven different cytokines, gp130 is an important member of the family of hemopoietic cytokine receptors which are characterized by the presence of at least one cytokine-binding module. Mutants of gp130 that either lack the Ig-like domain D1 (DeltaD1) or contain a distinct mutation (F191E) within the cytokine-binding module have been shown to be severely impaired with respect to IL-6 induced signal transduction. After cotransfection of COS-7 cells with a combination of both inactive gp130 mutants, signal transduction in response to IL-6 is restored. Whereas cells transfected with DeltaD1 do not bind IL-6/sIL-6R complexes, cells transfected with the F191E mutant bind IL-6/sIL-6R with low affinity. Combination of DeltaD1 and F191E, however, leads to high-affinity ligand binding. These data suggest that two different gp130 epitopes, one on each receptor chain, sequentially cooperate in asymmetrical binding of IL-6/IL-6R in a tetrameric signaling complex. On the basis of our data, a model for the mechanism of IL-6-induced gp130 activation is proposed.

Receptor subunit-specific action of oncostatin M in hepatic cells and its modulation by leukemia inhibitory factor

The related cytokines, interleukin-6 (IL-6), oncostatin M (OSM), and leukemia inhibitory factor (LIF) direct the formation of specific heteromeric receptor complexes to achieve signaling. Each complex includes the common signal-transducing subunit gp130. OSM and LIF also recruit the signaling competent, but structurally distinct OSMRbeta and LIFRalpha subunits, respectively. To test the hypothesis that the particularly prominent cell regulation by OSM is due to signals contributed by OSMRbeta, we introduced stable expression of human or mouse OSMRbeta in rat hepatoma cells which have endogenous receptors for IL-6 and LIF, but not OSM. Both mouse and human OSM engaged gp130 with their respective OSMRbeta subunits, but only human OSM also acted through LIFR. Signaling by OSMRbeta-containing receptors was characterized by highest activation of STAT5 and ERK, recruitment of the insulin receptor substrate and Jun-N-terminal kinase pathways, and induction of a characteristic pattern of acute phase proteins. Since LIF together with LIFRalpha appear to form a more stable complex with gp130 than OSM with gp130 and OSMRbeta, co-activation of LIFR and OSMR resulted in a predominant LIF-like response. These results suggest that signaling by IL-6 cytokines is not identical, and that a hierarchical order of cytokine receptor action exists in which LIFR ranks as dominant member.

Crystal structure and functional dissection of the cytostatic cytokine oncostatin M

BACKGROUND

The cytokine oncostatin M (OSM) inhibits growth of certain tumour-derived cells, induces proliferation in other cell types (e.g. haemangioblasts) and is a mediator of inflammatory responses. Its mechanism of action is via specific binding to gp130 and either the leukaemia inhibitory factor receptor (LIFR) or oncostatin M receptor (OSMR) systems at the cell surface to form an active signalling complex.

RESULTS

We report here the crystal structure of human oncostatin M (hOSM) along with mutagenesis data which map the receptor-binding epitopes of the molecule. The structure was determined to a resolution of 2.2 A and conforms to the haematopoietin cytokine up-up-down-down four-helix bundle topology. The site 2 epitope, responsible for gp130 binding, is centred around Gly120 which forms a 'dimple' on the surface of the molecule located on helices A and C. The site 3 motif, responsible for LIFR and OSMR binding, consists of a protruding Phe160/Lys163 pair located at the start of helix D.

CONCLUSIONS

The data presented allow functional dissection of the receptor-binding interfaces to atomic resolution. Modelling suggests that the gp130 residue Phe169 packs into the site 2 dimple in an analogous fashion to structurally equivalent residues at the growth hormone-growth hormone receptor interface, implying that certain key features may underlie recognition across the whole cytokine/receptor superfamily. Conversely, detailed comparison of the available structures suggests that variations on a common theme dictate the specificity of receptor-ligand interactions within the gp130 family of cytokines.

Leukemia-inhibitory factor-neuroimmune modulator of endocrine function

Leukemia-inhibitory factor (LIF) is a pleiotropic cytokine expressed by multiple tissue types. The LIF receptor shares a common gp130 receptor subunit with the IL-6 cytokine superfamily. LIF signaling is mediated mainly by JAK-STAT (janus-kinase-signal transducer and activator of transcription) pathways and is abrogated by the SOCS (suppressor-of cytokine signaling) and PIAS (protein inhibitors of activated STAT) proteins. In addition to classic hematopoietic and neuronal actions, LIF plays a critical role in several endocrine functions including the utero-placental unit, the hypothalamo-pituitary-adrenal axis, bone cell metabolism, energy homeostasis, and hormonally responsive tumors. This paper reviews recent advances in our understanding of molecular mechanisms regulating LIF expression and action and also provides a systemic overview of LIF-mediated endocrine regulation. Local and systemic LIF serve to integrate multiple developmental and functional cell signals, culminating in maintaining appropriate hormonal and metabolic homeostasis. LIF thus functions as a critical molecular interface between the neuroimmune and endocrine systems.

Receptor recognition by gp130 cytokines

Cytokines of the gp130 family exert their diverse biological effects by formation of stable high affinity transmembrane receptor complexes that are characterized by the presence of the shared transmembrane signalling receptor gp130. Different gp130 ligands form signalling complexes that vary in both composition and stoichiometry. Analysis of the three-dimensional structure of selected ligands and receptor elements indicates that ligands display three topologically conserved receptor recognition epitopes that interact with complementary ligand recognition elements. The composition of the signalling complex and downstream biological responses is defined by the relative affinity of different receptor components for these epitopes. The detailed structure of receptor recognition epitopes indicates that the generation of small molecule cytokine mimetics may be a feasible objective.