CIB1 contributes to oncogenic signalling by Ras via modulating the subcellular localisation of sphingosine kinase 1.

CIB1 (calcium and integrin binding protein 1) is a small intracellular protein with numerous interacting partners, and hence has been implicated in various cellular functions. Recent studies have revealed emerging roles of CIB1 in regulating cancer cell survival and angiogenesis, although the mechanisms involved have remained largely undefined. In investigating the oncogenic function of CIB1, we initially found that CIB1 is widely up-regulated across a diverse range of cancers, with this upregulation frequently correlating with oncogenic mutations of KRas. Consistent with this, we found that ectopic expression of oncogenic KRas and HRas in cells resulted in elevated CIB1 expression. We previously described the Ca2+-myristoyl switch function of CIB1, and its ability to facilitate agonist-induced plasma membrane localisation of sphingosine kinase 1 (SK1), a location where SK1 is known to elicit oncogenic signalling. Thus, we examined the role this may play in oncogenesis. Consistent with these findings, we demonstrated here that over-expression of CIB1 by itself is sufficient to drive localisation of SK1 to the plasma membrane and enhance the membrane-associated enzymatic activity of SK1, as well as its oncogenic signalling. We subsequently demonstrated that elevated levels of CIB1 resulted in full neoplastic transformation, in a manner dependent on SK1. In agreement with our previous findings that SK1 is a downstream mediator of oncogenic signalling by Ras, we found that targeting CIB1 also inhibited neoplastic growth of cells induced by oncogenic Ras, suggesting an important pro-tumorigenic role for CIB1. Thus, we have demonstrated for the first time a role for CIB1 in neoplastic transformation, and revealed a novel mechanism facilitating oncogenic signalling by Ras and SK1.

Molecular targets of FTY720 (fingolimod).

FTY720 is a recently approved first line therapy for relapsing forms of multiple sclerosis. In this context, FTY720 is a pro-drug, with its anti-multiple sclerosis, immunosuppressive effects largely elicited following its phosphorylation by sphingosine kinase 2 and subsequent modulation of G protein-coupled sphingosine 1-phosphate (S1P) receptor 1 that induces lymphopenia by altering lymphocyte trafficking. A number of other biological effects of FTY720 have, however, been described, including considerable evidence that this drug also has anti-cancer properties. These other effects of FTY720 are independent of S1P receptors, and appear facilitated by modulation of a range of other recently described protein targets by nonphosphorylated FTY720. Here, we review the direct targets of FTY720 that contribute to its anti-cancer properties. We also discuss other recently described protein effectors that, in combination with S1P receptors, appear to contribute to its immunosuppressive effects.

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.

New approaches in the treatment of asthma.

Asthma is a common and complex inflammatory disease of the airways that remains incurable. Current forms of therapy are long term and may exhibit associated side-effect problems. Major participants in the development of an asthma phenotype include the triggering stimuli such as the allergens themselves, cells such as T cells, epithelial cells and mast cells that produce a variety of cytokines including IL-5, GM-CSF, IL-3, IL-4 and IL-13 and chemokines such as eotaxin. Significantly, the eosinophil, a specialized blood cell type, is invariably associated with this disease. The eosinophil has long been incriminated in the pathology of asthma due to its ability to release preformed and unique toxic substances as well as newly formed pro-inflammatory mediators. The regulation of eosinophil production and function is carried out by soluble peptides or factors. Of these IL-5, GM-CSF and IL-3 are of paramount importance as they control eosinophil functional activity and are the only known eosinophilopoietic factors. In addition they regulate the eosinophil life span by inhibiting apoptosis. While one therapeutic approach in asthma is directed at inhibiting single eosinophil products such as leukotrienes or single eosinophil regulators such as IL-5, we believe that the simultaneous inhibition of more than one component is preferable. This may be particularly important with eosinophil regulators in that not only IL-5, but also GM-CSF has been repeatedly implicated in clinical studies of asthma. The fact that GM-CSF is produced by many cells in the body and in copious amounts by lung epithelial cells highlights this need further. Our approach takes advantage of the fact that the IL-5 and GM-CSF receptors (as well as IL-3 receptors) utilize a shared subunit to bind, with high affinity, to these cytokines and the same common subunit mediates signal transduction culminating in all the biological activities mentioned. By generating the monoclonal antibody BION-1 to the cytokine binding region of the common subunit (betac) we have shown that the approach of inhibiting IL-5, GM-CSF and IL-3 binding and the resulting stimulation of eosinophil production and function with a single agent is feasible. Furthermore we have used BION-1 as a tool to crystallize and define the structure of the cytokine binding domain of betac. This knowledge and this approach may lead to the generation of novel therapeutics for the treatment of asthma.

GM-CSF binding to its receptor induces oligomerisation of the common beta-subunit.

The stoichiometry of the granulocyte-macrophage colony-stimulating factor (GM-CSF) receptor complex is still unresolved. We have utilised a sensitive, functional assay for receptor homodimerisation to show that GM-CSF induces dimerisation of the common signalling subunit, hbeta(c). We generated a chimeric cytokine receptor in which the extracellular and transmembrane domains of hbeta(c)are fused to the cytoplasmic domain of erythropoietin receptor (EPO-R). Given that to induce EPO-R activation and mitogenic signalling there is a requirement for formation of a specific homodimeric complex, we reasoned that the cytoplasmic domain of EPO-R could be utilised as a highly sensitive reporter for functional homodimer formation. We show that, in the presence of a cytoplasmically truncated GM-CSF alpha-subunit, the hbetac-EPO receptor chimera transduces a mitogenic signal in BaF-B03 in response to GM-CSF. This is consistent with formation of a hbeta(c)homodimer following GM-CSF binding and implies that ligand stimulation induces formation of a higher order complex that contains the hbeta(c)homodimer.

Characterization of IL-4 receptor components expressed on monocytes and monocyte-derived macrophages: variation associated with differential signaling by IL-4.

The anti-inflammatory effects of IL-4 on activated monocytes differ from those on monocyte-derived macrophages (MDMac). While IL-4 suppresses LPS-induced IL-1beta , IL-12, IL-10 and TNFalpha production by monocytes, IL-4 suppresses only IL-1beta and IL-12 production by MDMac. The U937 and Mono Mac 6 cell lines have similar cytokine responses to IL-4 as monocytes and MDMac, respectively. The IL-4Ralpha and IL-2Rgamma (gammac) chains are well-characterized components of the IL-4 receptor. Cross-linking studies with 125I-IL-4 revealed that for monocytes and U937 cells, the binding of IL-4 to the receptor components was approximately 1:1 for IL-4Ralpha:gammac. In contrast, for MDMac and Mono Mac 6 cells that have a relative reduction in gammac surface expression, the binding of IL-4 to IL-4Ralpha:gammac was approximately 3:1. Furthermore, IL-4 induced IL-4Ralpha chain phosphorylation more rapidly in MDMac and Mono Mac 6 cells than in monocytes and U937 cells. This study identifies a correlation between altered 125I-IL-4 cross-linking to IL-4Ralpha:gammac, IL-4-induced signaling and regulation of pro-inflammatory cytokine production by IL-4.

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

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

The role of disulfide-linked dimerization in interleukin-3 receptor signaling and biological activity.

Cysteine residues 86 and 91 of the beta subunit of the human interleukin (hIL)-3 receptor (hbetac) participate in disulfide-linked receptor subunit heterodimerization. This linkage is essential for receptor tyrosine phosphorylation, since the Cys-86 --> Ala (Mc4) and Cys-91 --> Ala (Mc5) mutations abolished both events. Here, we used these mutants to examine whether disulfide-linked receptor dimerization affects the biological and biochemical activities of the IL-3 receptor. Murine T cells expressing hIL-3Ralpha and Mc4 or Mc5 did not proliferate in hIL-3, whereas cells expressing wild-type hbetac exhibited rapid proliferation. However, a small subpopulation of cells expressing each mutant could be selected for growth in IL-3, and these proliferated similarly to cells expressing wild-type hbetac, despite failing to undergo IL-3-stimulated hbetac tyrosine phosphorylation. The Mc4 and Mc5 mutations substantially reduced, but did not abrogate, IL-3-mediated anti-apoptotic activity in the unselected populations. Moreover, the mutations abolished IL-3-induced JAK2, STAT, and AKT activation in the unselected cells, whereas activation of these molecules in IL-3-selected cells was normal. In contrast, Mc4 and Mc5 showed a limited effect on activation of Erk1 and -2 in unselected cells. These data suggest that whereas disulfide-mediated cross-linking and hbetac tyrosine phosphorylation are normally important for receptor activation, alternative mechanisms can bypass these requirements.

Structure of the activation domain of the GM-CSF/IL-3/IL-5 receptor common beta-chain bound to an antagonist.

Heterodimeric cytokine receptors generally consist of a major cytokine-binding subunit and a signaling subunit. The latter can transduce signals by more than 1 cytokine, as exemplified by the granulocyte-macrophage colony-stimulating factor (GM-CSF), interleukin-2 (IL-2), and IL-6 receptor systems. However, often the signaling subunits in isolation are unable to bind cytokines, a fact that has made it more difficult to obtain structural definition of their ligand-binding sites. This report details the crystal structure of the ligand-binding domain of the GM-CSF/IL-3/IL-5 receptor beta-chain (beta(c)) signaling subunit in complex with the Fab fragment of the antagonistic monoclonal antibody, BION-1. This is the first single antagonist of all 3 known eosinophil-producing cytokines, and it is therefore capable of regulating eosinophil-related diseases such as asthma. The structure reveals a fibronectin type III domain, and the antagonist-binding site involves major contributions from the loop between the B and C strands and overlaps the cytokine-binding site. Furthermore, tyrosine(421) (Tyr(421)), a key residue involved in receptor activation, lies in the neighboring loop between the F and G strands, although it is not immediately adjacent to the cytokine-binding residues in the B-C loop. Interestingly, functional experiments using receptors mutated across these loops demonstrate that they are cooperatively involved in full receptor activation. The experiments, however, reveal subtle differences between the B-C loop and Tyr(421), which is suggestive of distinct functional roles. The elucidation of the structure of the ligand-binding domain of beta(c) also suggests how different cytokines recognize a single receptor subunit, which may have implications for homologous receptor systems. (Blood. 2000;95:2491-2498)

The functional basis of granulocyte-macrophage colony stimulating factor, interleukin-3 and interleukin-5 receptor activation, basic and clinical implications.

The cytokines granulocyte-macrophage colony stimulating factor, interleukin-3 and interleukin-5 have overlapping activities on cells expressing their receptors. This is explained by their sharing a receptor signal transduction subunit, beta c. This communal signaling subunit is also required for high affinity binding of all three cytokines. Therapeutic approaches attempting to interfere or modulate haemopoietic cells using cytokines or their analogues can in some instances be limited due to functional redundancy amongst cytokines using shared receptor signaling subunits. Therefore, a better approach would be to develop therapeutics against the shared subunit. Studies examining the GM-CSF, IL-3 and IL-5 receptors have identified the key events leading to functional receptor activation. With this knowledge, it is now possible to identify new targets for the development of a new class of antagonist that blocks the biological activity of all the cytokines utilizing beta c. This approach may be extended to other receptor systems such as IL-4 and IL-13 where receptor activation is dependent on a common signaling and binding subunit.

Simultaneous antagonism of interleukin-5, granulocyte-macrophage colony-stimulating factor, and interleukin-3 stimulation of human eosinophils by targetting the common cytokine binding site of their receptors.

Human interleukin-5 (IL-5), granulocyte-macrophage colony-stimulating factor (GM-CSF), and IL-3 are eosinophilopoietic cytokines implicated in allergy in general and in the inflammation of the airways specifically as seen in asthma. All 3 cytokines function through cell surface receptors that comprise a ligand-specific alpha chain and a shared subunit (beta(c)). Although binding of IL-5, GM-CSF, and IL-3 to their respective receptor alpha chains is the first step in receptor activation, it is the recruitment of beta(c) that allows high-affinity binding and signal transduction to proceed. Thus, beta(c) is a valid yet untested target for antiasthma drugs with the added advantage of potentially allowing antagonism of all 3 eosinophil-acting cytokines with a single compound. We show here the first development of such an agent in the form of a monoclonal antibody (MoAb), BION-1, raised against the isolated membrane proximal domain of beta(c). BION-1 blocked eosinophil production, survival, and activation stimulated by IL-5 as well as by GM-CSF and IL-3. Studies of the mechanism of this antagonism showed that BION-1 prevented the high-affinity binding of (125)I-IL-5, (125)I-GM-CSF, and (125)I-IL-3 to purified human eosinophils and that it bound to the major cytokine binding site of beta(c). Interestingly, epitope analysis using several beta(c) mutants showed that BION-1 interacted with residues different from those used by IL-5, GM-CSF, and IL-3. Furthermore, coimmunoprecipitation experiments showed that BION-1 prevented ligand-induced receptor dimerization and phosphorylation of beta(c), suggesting that ligand contact with beta(c) is a prerequisite for recruitment of beta(c), receptor dimerization, and consequent activation. These results demonstrate the feasibility of simultaneously inhibiting IL-5, GM-CSF, and IL-3 function with a single agent and that BION-1 represents a new tool and lead compound with which to identify and generate further agents for the treatment of eosinophil-dependent diseases such as asthma.

Mechanism of activation of the GM-CSF, IL-3, and IL-5 family of receptors.

The process of ligand binding leading to receptor activation is an ordered and sequential one. High-affinity binding of GM-CSF, interleukin 3 (IL-3), and IL-5 to their receptors induces a number of key events at the cell surface and within the cytoplasm that are necessary for receptor activation. These include receptor oligomerization, activation of tyrosine kinase activity, phosphorylation of the receptor, and the recruitment of SH2 (src-homology) and PTB (phosphotyrosine binding) domain proteins to the receptor. Such a sequence of events represents a recurrent theme among cytokine, growth factor, and hormone receptors; however, a number of very recent and interesting findings have identified unique features in this receptor system in terms of: A) how GM-CSF/IL-3/IL-5 bind, oligomerize, and activate their cognate receptors; B) how multiple biological responses such as proliferation, survival, and differentiation can be transduced from activated GM-CSF, IL-3, or IL-5 receptors, and C) how the presence of novel phosphotyrosine-independent signaling motifs within a specific cytoplasmic domain of betaC may be important for mediating survival and differentiation by these cytokines. This review does not attempt to be all-encompassing but rather to focus on the most recent and significant discoveries that distinguish the GM-CSF/IL-3/IL-5 receptor subfamily from other cytokine receptors.

Identification of a Cys motif in the common beta chain of the interleukin 3, granulocyte-macrophage colony-stimulating factor, and interleukin 5 receptors essential for disulfide-linked receptor heterodimerization and activation of all three receptors.

The human interleukin 3 (IL-3) and granulocyte-macrophage colony-stimulating factor (GM-CSF) receptors undergo covalent dimerization of the respective specific alpha chains with the common beta subunit (betac) in the presence of the cognate ligand. We have now performed alanine substitutions of individual Cys residues in betac to identify the Cys residues involved and their contribution to activation of the IL-3, GM-CSF, and IL-5 receptors. We found that substitution of Cys-86, Cys-91, and Cys-96 in betac but not of Cys-100 or Cys-234 abrogated disulfide-linked IL-3 receptor dimerization. However, although Cys-86 and Cys-91 betac mutants retained their ability to form non-disulfide-linked dimers with IL-3Ralpha, substitution of Cys-96 eliminated this interaction. Binding studies demonstrated that all betac mutants with the exception of C96A supported high affinity binding of IL-3 and GM-CSF. In receptor activation experiments, we found that betac mutants C86A, C91A, and C96A but not C100A or C234A abolished phosphorylation of betac in response to IL-3, GM-CSF, or IL-5. These data show that although Cys-96 is important for the structural integrity of betac, Cys-86 and Cys-91 participate in disulfide-linked receptor heterodimerization and that this linkage is essential for tyrosine phosphorylation of betac. Sequence alignment of betac with other cytokine receptor signaling subunits in light of these data shows that Cys-86 and Cys-91 represent a motif restricted to human and mouse beta chains, suggesting a unique mechanism of activation utilized by the IL-3, GM-CSF, and IL-5 receptors.

The human granulocyte-macrophage colony-stimulating factor (GM-CSF) receptor exists as a preformed receptor complex that can be activated by GM-CSF, interleukin-3, or interleukin-5.

The granulocyte-macrophage colony-stimulating factor (GM-CSF) receptor is expressed on normal and malignant hematopoietic cells as well as on cells from other organs in which it transduces a variety of functions. Despite the widespread expression and pleiotropic nature of the GM-CSF receptor, little is known about its assembly and activation mechanism. Using a combination of biochemical and functional approaches, we have found that the human GM-CSF receptor exists as an inducible complex, analogous to the interleukin-3 (IL-3) receptor, and also as a preformed complex, unlike the IL-3 receptor or indeed other members of the cytokine receptor superfamily. We found that monoclonal antibodies to the GM-CSF receptor alpha chain (GMR alpha) and to the common beta chain of the GM-CSF, IL-3, and IL-5 receptors (beta(c)) immunoprecipitated both GMR alpha and beta(c) from the surface of primary myeloid cells, myeloid cell lines, and transfected cells in the absence of GM-CSF. Further association of the two chains could be induced by the addition of GM-CSF. The preformed complex required only the extracellular regions of GMR alpha and beta(c), as shown by the ability of soluble beta(c) to associate with membrane-anchored GMR alpha or soluble GMR alpha. Kinetic experiments on eosinophils and monocytes with radiolabeled GM-CSF, IL-3, and IL-5 showed association characteristics unique to GM-CSF. Significantly, receptor phosphorylation experiments showed that not only GM-CSF but also IL-3 and IL-5 stimulated the phosphorylation of GMR alpha-associated beta(c). These results indicate a pattern of assembly of the heterodimeric GM-CSF receptor that is unique among receptors of the cytokine receptor superfamily. These results also suggest that the preformed GM-CSF receptor complex mediates the instantaneous binding of GM-CSF and is a target of phosphorylation by IL-3 and IL-5, raising the possibility that some of the biologic activities of IL-3 and IL-5 are mediated through the GM-CSF receptor complex.

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

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

The in-vitro activity of faropenem, a novel oral penem.

The in-vitro activity of faropenem, a novel oral penem, was studied in comparison with other beta-lactam antimicrobials against 711 recent clinical isolates including Gram-negative, Gram-positive and anaerobic bacteria. MIC data showed that faropenem was active against most members of the Enterobacteriaceae (MICs < or = 4 mg/L), with reduced activity against Serratia spp. (MIC90 = 32 mg/L). In common with its comparators, faropenem had weak activity against Pseudomonas aeruginosa and Stenotrophomonas maltophilia (MIC > 128 mg/L). Faropenem was active against staphylococci, although for MRSA MICs were raised (MIC90 = 2 mg/L) compared with those for MSSA (MIC90 = 0.12 mg/L). Faropenem was also found to be active against streptococci, Neisseria spp., Enterococcus faecalis and beta-lactamase-producing and non-producing strains of Haemophilus influenzae and Moraxella catarrhalis. Of the anaerobic bacteria studied, faropenem was most active against peptostreptococci and Clostridium perfringens (MIC90 < or = 1 mg/L) and Bacteroides fragilis (MIC90 = 4 mg/L). An increase in inoculum from 10(4) to 10(6) cfu raised faropenem MICs for Morganella morganii from 0.06-1 mg/L to 2-4 mg/L and for MRSA from 0.25-2 mg/L to 8 mg/L (a similar increase was not observed for MSSA). The MICs of faropenem were not affected by the presence of either 20% or 70% (v/v) serum. MICs for faropenem to 11 well characterized beta-lactamase producers were similar to those of non-producers. In hydrolysis studies, faropenem was shown to be highly stable to a number of beta-lactamases, including TEM-1, SHV-1, the extended spectrum beta-lactamases, TEM-3 and TEM-9, and the beta-lactamase produced by Staphylococcus aureus (NCTC 11561).

In vitro activity of BAY 12-8039, a new fluoroquinolone.

The in vitro activity of BAY 12-8039, a new fluoroquinolone, was studied in comparison with those of ciprofloxacin, trovafloxacin (CP 99,219), cefpodoxime, and amoxicillin-clavulanate against gram-negative, gram-positive, and anaerobic bacteria. Its activity against mycobacteria and chlamydia was also investigated. BAY 12-8039 was active against members of the family Enterobacteriaceae (MIC at which 90% of strains tested were inhibited [MIC90S] < or = 1 microgram/ml, except for Serratia spp. MIC90 2 microgram/ml), Neisseria spp. (MIC90S, 0.015 microgram/ml), Haemophilus influenzae (MIC90, 0.03 microgram/ml), and Moraxella catarrhalis (MIC90, 0.12 micrgram/ml), and these results were comparable to those obtained for ciprofloxacin and trovafloxacin. Against Pseudomonas aeruginosa, the quinolones were more active than the beta-lactam agents but BAY 12-8039 was less active than ciprofloxacin. Strains of Stenotrophomonas maltophilia were fourfold more susceptible to BAY 12-8039 and trovafloxacin (MIC90S, 2 micrograms/ml) than to ciprofloxacin. BAY 12-8039 was as active as trovafloxacin but more active than ciprofloxacin against Streptococcus pneumoniae (MIC90, 0.25 microgram/ml) and methicillin-susceptible Staphylococcus auerus (MIC90S, 0.12 micrograms/ml). The activity of BAY 12-8039 against methicillin-resistant S. aureus (MIC90, 2 micrograms/ml) was lower than that against methicillin-susceptible strains. BAY 12-8039 was active against anaerobes (MIC90S < or = 2 micrograms/ml), being three- to fourfold more active against Bacteroides fragilis, Prevotella spp., and Clostridium difficile than was ciprofloxacin. Against Mycobacterium tuberculosis, BAY 12-8039 exhibited activity comparable to that of rifampin (MICs < or = 0.5 micrograms/ml). Against Chlamydia trachomatis and Chlamydia pneumoniae BAY 12-8039 was more active (MICs < or = 0.12 microgram/ml) than either ciprofloxacin or erythromycin and exhibited a greater lethal effect than either to these two agents. The protein binding of BAY 12-8039 was determined at 1 and 5 micrograms/ml as 30 and 26.4%, respectively. The presence of human serum (at 20 or 70%) had no marked effect on the in vitro activity of BAY 12-8039.

Receptors of the cytokine superfamily: mechanisms of activation and involvement in disease.

Cytokine receptors are members of a diverse family of proteins that serve the dual function of recognizing their cognate ligands among a plethora of other factors and of initiating a series of cellular signals that ultimately lead to multiple cellular functions. Although cytokine receptors are only activated by their specific cytokines, some functional overlap occurs as a result of receptor subunit promiscuity, kinase recruitment and the activation of coincident signalling pathways. Knock-out experiments are extremely useful in helping to elucidate functionally relevant interactions between cytokine receptor activation, signalling molecules and cellular function. Defects in cytokine receptors or activation, signalling molecules continue to be identified as the underlying cause of clinical conditions. We discuss newly recognized clinical syndromes and recent research into the molecular basis of cytokine receptor activation that provides new insights into the role of cytokine receptors in normal physiology and disease.

A single tyrosine residue in the membrane-proximal domain of the granulocyte-macrophage colony-stimulating factor, interleukin (IL)-3, and IL-5 receptor common beta-chain is necessary and sufficient for high affinity binding and signaling by all three ligands.

The beta-chain of the granulocyte-macrophage colony-stimulating factor (GM-CSF), interleukin-3 (IL-3), and interleukin-5 (IL-5) receptors functions as a communal receptor subunit and is often referred to as beta common (betac). Analogous to other shared receptor subunits including gp130 and the IL-2R gamma chain, betac mediates high affinity binding and signal transduction of all of its ligands. It is not clear, however, how these common receptor subunits can recognize several ligands and indeed whether they exhibit a common binding pocket to accomplish this. We have performed molecular modeling of betac based on the known structures of the growth hormone and prolactin receptors and targeted the putative F'-G' loop for mutagenesis. Substitution of this whole predicted loop region with alanines completely abrogated high affinity binding of GM-CSF, IL-3, and IL-5. Individual alanine substitutions across the loop revealed that a single residue, Tyr421, is critical for high affinity binding of GM-CSF, IL-3, and IL-5, whereas alanine substitution of adjacent residues has little or no effect on high affinity binding. Significantly, reintroducing Tyr421 into the polyalanine-substituted mutant restored high affinity ligand binding of GM-CSF, IL-3, and IL-5, indicating that within this region the tyrosine residue alone is sufficient for high affinity ligand interaction. Functional studies measuring STAT5 activation revealed that alanine substitution of Tyr421 severely impaired the ability of betac to signal. These results show for the first time that a single residue in a shared receptor subunit acts as a binding determinant for different ligands and may have implications for other receptor systems where communal receptor subunits exhibit hydrophobic residues in their putative F'-G' loops. These results also raise the possibility that a single compound targeted to this region may simultaneously inhibit the binding and function of multiple cytokines.