Structure-function analysis of human interleukin-6. Evidence for the involvement of the carboxy-terminus in function

Unveiling novel insights into human IL-6 - IL-6R interaction sites through 3D computer-guided docking and systematic site mutagenesis

The cytokine interleukin-6 (IL-6) plays a crucial role in autoimmune and inflammatory diseases. Understanding the precise mechanism of IL-6 interaction at the amino acid level is essential to develop IL-6-inhibiting compounds. In this study, we employed computer-guided drug design tools to predict the key residues that are involved in the interaction between IL-6 and its receptor IL-6R. Subsequently, we generated IL-6 mutants and evaluated their binding affinity to IL-6R and the IL-6R - gp130 complex, as well as monitoring their biological activities. Our findings revealed that the R167A mutant exhibited increased affinity for IL-6R, leading to enhanced binding to IL-6R - gp130 complex and subsequently elevated intracellular phosphorylation of STAT3 in effector cells. On the other hand, although E171A reduced its affinity for IL-6R, it displayed stronger binding to the IL-6R - gp130 complex, thereby enhancing its biological activity. Furthermore, we identified the importance of R178 and R181 for the precise recognition of IL-6 by IL-6R. Mutants R181A/V failed to bind to IL-6R, while maintaining an affinity for the IL-6 - gp130 complex. Additionally, deletion of the D helix resulted in complete loss of IL-6 binding affinity for IL-6R. Overall, this study provides valuable insights into the binding mechanism of IL-6 and establishes a solid foundation for future design of novel IL-6 inhibitors.

A Computational–Experimental Investigation of the Molecular Mechanism of Interleukin-6-Piperine Interaction

Herein, we elucidate the biophysical aspects of the interaction of an important protein, Interleukin-6 (IL6), which is involved in cytokine storm syndrome, with a natural product with anti-inflammatory activity, piperine. Despite the role of piperine in the inhibition of the transcriptional protein NF-κB pathway responsible for activation of IL6 gene expression, there are no studies to the best of our knowledge regarding the characterisation of the molecular interaction of the IL6-piperine complex. In this context, the characterisation was performed with spectroscopic experiments aided by molecular modelling. Fluorescence spectroscopy alongside van’t Hoff analyses showed that the complexation event is a spontaneous process driven by non-specific interactions. Circular dichroism aided by molecular dynamics revealed that piperine caused local α-helix reduction. Molecular docking and molecular dynamics disclosed the microenvironment of interaction as non-polar amino acid residues. Although piperine has three available hydrogen bond acceptors, only one hydrogen-bond was formed during our simulation experiments, reinforcing the major role of non-specific interactions that we observed experimentally. Root mean square deviation (RMSD) and hydrodynamic radii revealed that the IL6-piperine complex was stable during 800 ns of simulation. Taken together, these results can support ongoing IL6 drug discovery efforts.

Molecular dynamics analysis of the binding of human interleukin-6 with interleukin-6 α-receptor

Human interleukin-6 (hIL-6) is a multifunctional cytokine that regulates immune and inflammatory responses in addition to metabolic and regenerative processes and cancer. hIL-6 binding to the IL-6 receptor (IL-6Rα) induces homodimerization and recruitment of the glycoprotein (gp130) to form a hexameric signaling complex. Anti-IL-6 and IL-6R antibodies are clinically approved inhibitors of IL-6 signaling pathway for treating rheumatoid arthritis and Castleman's disease, respectively. There is a potential to develop novel small molecule IL-6 antagonists derived from understanding the structural basis for IL-6/IL-6Rα interactions. Here, we combine homology modeling with extensive molecular dynamics (MD) simulations to examine the association of hIL-6 with IL-6Rα. A comparison with MD of apo hIL-6 reveals that the binding of hIL-6 to IL-6Rα induces structural and dynamic rearrangements in the AB loop region of hIL-6, disrupting intraprotein contacts and increasing the flexibility of residues 48 to 58 of the AB loop. In contrast, due to the involvement of residues 59 to 78 in forming contacts with the receptor, these residues of the AB loop are observed to rigidify in the presence of the receptor. The binary complex is primarily stabilized by two pairs of salt bridges, Arg181 (hIL-6)- Glu182 (IL-6Rα) and Arg184 (hIL-6)- Glu183 (IL-6Rα) as well as hydrophobic and aromatic stacking interactions mediated essentially by Phe residues in both proteins. An interplay of electrostatic, hydrophobic, hydrogen bonding, and aromatic stacking interactions facilitates the formation of the hIL-6/IL-6Rα complex.

Semisynthesis of biologically active glycoforms of the human cytokine interleukin 6

Human interleukin 6 (IL-6) is a potent cytokine with immunomodulatory properties. As the influence of N-glycosylation on the in vivo activities of IL-6 could not be elucidated so far, a semisynthesis of homogeneous glycoforms of IL-6 was established by sequential native chemical ligation. The four cysteines of IL-6 are convenient for ligations and require only the short synthetic glycopeptide 43-48. The Cys-peptide 49-183 could be obtained recombinantly by cleavage of a SUMO tag. The fragment 1-42 was accessible by the simultaneous cleavage of two inteins, leading to the 1-42 thioester with the native N-terminus. Ligation and refolding studies showed that the inherently labile Asp-Pro bond 139-140 was detrimental for the sequential C- to N-terminal ligation. A reversed ligation sequence using glycopeptide hydrazides gave full-length IL-6 glycoproteins, which showed full bioactivity after efficient refolding and purification.

Meprin metalloproteases inactivate interleukin 6

Meprins have been implicated in the pathogenesis of several inflammatory diseases, including inflammatory bowel disease, in which the cytokine IL-6 is a prominent effector molecule. Because IL-6 levels are elevated markedly in meprin α and α/β knockout mice in an experimental model of inflammatory bowel disease, the interaction between meprins and IL-6 was studied. The results demonstrate that rodent and human meprin A and B cleave IL-6 to a smaller product and, subsequently, are capable of extensive degradation of the cytokine. Analysis of the limited degradation product formed by meprin A indicated that three to five amino acids are removed from the C terminus of the cytokine. Meprin A and meprin B cleaved IL-6 with micromolar affinities (Km of 4.7 and 12.0 μM, respectively) and with high efficiencies (kcat/Km of 0.2 and 2.5 (M(-1)/s(-1)) × 10(6), respectively). These efficiency constants are among the highest for known meprin substrates. Madin-Darby canine kidney cells transiently transfected with meprin α or meprin β constructs also cleave exogenous IL-6. Both human and murine IL-6 cleaved by meprin A or B are inactivated, as demonstrated by their decreased capability to stimulate proliferation of B9 cells. These results are consistent with the proposition that one function of meprin metalloproteases is to modulate inflammation by inactivating IL-6.

Upd3--an ancestor of the four-helix bundle cytokines

The unpaired-like protein 3 (Upd3) is one of the three cytokines of Drosophila melanogaster supposed to activate the JAK/STAT signaling pathway (Janus tyrosine kinases/signal transducer and activator of transcription). This activation occurs via the type-I cytokine receptor domeless, an orthologue of gp130, the common signal transducer of all four-helix bundle interleukin-6 (IL-6) type cytokines. Both receptors are known to exist as preformed dimers in the plasma membrane and initiate the acute-phase response. These facts indicate an evolutionary relation between vertebrate IL-6 and the Drosophila protein Upd3. Here we presented data which strengthen this notion. Upd3 was recombinantly expressed, a renaturation and purification protocol was established which allows to obtain high amounts of biological active protein. This protein is, like human IL-6, a monomeric-α helical cytokine, implicating that Upd3 is an "ancestor" of the four-helix bundle cytokines.

Role of cytokines in endometrium and at the fetomaternal interface

Cytokines were first recognized as factors involved in the immune responses. However, emerging evidence suggests that cytokines serve as the unanimous language through which cells in organ systems as diverse as hypothalamus to endometrium communicate. Evidence for the participation of these factors in the normal function of the endometrium and uteroplacental unit and development of the embryo is being recognized. This review will highlight the significant insight gained over the past several years for the role of cytokines in the field of reproductive biology. Major emphasis will be placed on interleukins, tumour necrosis factor-α, transforming growth factors, colony stimulating factors and interferon-gamma.

Interleukin-6 N-terminal peptides modulate the expression of junB protooncogene and the production of fibrinogen in HepG2 cells

Interleukin-6 (IL-6) is a helical cytokine exerting pleiotropic activities including the regulation of hematopoiesis, B cell activation and acute-phase reaction. The structure-function relationship of the molecule is the subject of intensive investigation using point and deletion mutants. Our objective was to analyse the role of the N-terminal 18-46 region in IL-6-mediated expression of junB protooncogene and fibrinogen production, reflecting the acute phase response, with synthetic overlapping peptides. mRNA expression of junB was monitored by competitive RT-PCR, while sandwich ELISA was used for the detection of fibrinogen in the supernatant of HepG2 human hepatoma cells. We found that even short synthetic octapeptides can be stimulatory (in the absence of IL-6) or inhibitory (in the presence of IL-6) in both assays. To establish the molecular mechanism by which synthetic peptides exert their biological effects electromobility shift assay was carried out using HepG2 nuclear extracts. Peptides inducing junB expression initiate gel shifts of STAT3/DNA complexes, which may indicate the involvement of this signal transduction pathway. Circular dicroism spectroscopy data suggest that 8-11-mer peptides representing different parts of the 18-46 region have a marked tendency to adopt ordered conformations in a water/trifluoroethanol (1:1 v/v) mixture. Competition studies with rhIL-6 and selected fluorophore-labelled peptides indicate the presence of more than one binding site on soluble IL-6 receptor. Considering the possible multiple etiologic role of IL-6 in the pathogenesis of various diseases, these peptides could be useful for dissection of IL-6 related biological effects.

C-terminal peptides of interleukin-6 modulate the expression of junB protooncogene and the production of fibrinogen by HepG2 cells

Interleukin-6 (IL-6) is a 185 amino acid residue helical cytokine with various biological activities (e. g. B cell development, acute phase reaction). We have investigated the role of the 168-185 C-terminal region of IL-6 in the induction of fibrinogen synthesis and expression of junB mRNA using synthetic peptides corresponding to this region. Circular dichroism spectroscopy data suggest that even truncated peptides have a strong tendency to adopt an ordered conformation. Peptides were tested alone or in combination with recombinant hIL-6 on an IL-6 responsive human hepatoma HepG2 cell line. The expression of the protooncogene junB monitored by competitive RT-PCR represents an early, while the fibrinogen production detected by sandwich ELISA a late, marker of IL-6 initiated events. We found that peptides--depending on their structure--modulate spontaneous as well as IL-6 induced fibrinogen production and/or mRNA expression of junB by exhibiting inhibition (in the presence of IL-6) or stimulation (in the absence of IL-6).

Molecular analysis of hemolysin-mediated secretion of a human interleukin-6 fusion protein in Salmonella typhimurium

Previously, we reported a plasmid-bearing Salmonella typhimurium strain capable of secreting human interleukin-6 (hIL-6) when genetically fused to the Escherichia coli hemolysin transport signal (HlyA(S)). Stationary phase culture supernatants of this strain revealed three major forms of hIL-6-HlyA(S) fusion protein (apparent molecular masses 32.4, 30.3, 27.0 kDa), at which the largest protein presumably represented full-length hIL-6-HlyA(S). The biological activity of the hIL-6-HlyA(S) protein mixture was similar to that of mature hIL-6. Accumulation of hIL-6-HlyA(S) in the culture supernatant occurred only during the initial growth phase, whereas in stationary phase and under in vitro conditions successive cleavage into the two truncated forms was observed. On the other hand, in whole cell lysates only full-length hIL-6-HlyA(S) could be detected, accounting for more than 50% of the totally synthesized protein. Upon cell fractionation, cellular hIL-6-HlyA(S) was exclusively found in the membrane fraction. These results suggest, that in S. typhimurium production and secretion of hIL-6-HlyA(S) is restricted to growing cells. A specific processing by a Salmonella-derived protease did not affect the biological activity of the fusion protein.

Receptor recognition sites of cytokines are organized as exchangeable modules. Transfer of the leukemia inhibitory factor receptor-binding site from ciliary neurotrophic factor to interleukin-6

Interleukin-6 (IL-6) and ciliary neurotrophic factor (CNTF) are "4-helical bundle" cytokines of the IL-6 type family of neuropoietic and hematopoietic cytokines. IL-6 signals by induction of a gp130 homodimer (e.g. IL-6), whereas CNTF and leukemia inhibitory factor (LIF) signal via a heterodimer of gp130 and LIF receptor (LIFR). Despite binding to the same receptor component (gp130) and a similar protein structure, IL-6 and CNTF share only 6% sequence identity. Using molecular modeling we defined a putative LIFR binding epitope on CNTF that consists of three distinct regions (C-terminal A-helix/N-terminal AB loop, BC loop, C-terminal CD-loop/N-terminal D-helix). A corresponding gp130-binding site on IL-6 was exchanged with this epitope. The resulting IL-6/CNTF chimera lost the capacity to signal via gp130 on cells without LIFR, but acquired the ability to signal via the gp130/LIFR heterodimer and STAT3 on responsive cells. Besides identifying a specific LIFR binding epitope on CNTF, our results suggest that receptor recognition sites of cytokines are organized as modules that are exchangeable even between cytokines with limited sequence homology.

The membrane proximal cytokine receptor domain of the human interleukin-6 receptor is sufficient for ligand binding but not for gp130 association

Interleukin-6 (IL-6) belongs to the family of the "four-helix bundle" cytokines. The extracellular parts of their receptors consist of several Ig- and fibronectin type III-like domains. Characteristic of these receptors is a cytokine-binding module consisting of two such fibronectin domains defined by a set of four conserved cysteines and a tryptophan-serine-X-tryptophan-serine (WSXWS) sequence motif. On target cells, IL-6 binds to a specific IL-6 receptor (IL-6R), and the complex of IL-6.IL-6R associates with the signal transducing protein gp130. The IL-6R consists of three extracellular domains. The NH2-terminal Ig-like domain is not needed for ligand binding and signal initiation. Here we have investigated the properties and functional role of the third membrane proximal domain. The protein can be efficiently expressed in bacteria, and the refolded domain is shown to be sufficient for IL-6 binding. When complexed with IL-6, however, it fails to associate with the gp130 protein. Since the second and the third domain together with IL-6 can bind to gp130 and induce signaling, our data demonstrate the ligand binding function of the third domain and point to an important role of the second domain in complex formation with gp130 and signaling.

Interleukin-6: structure-function relationships

Interleukin-6 (IL-6) is a multifunctional cytokine that plays a central role in host defense due to its wide range of immune and hematopoietic activities and its potent ability to induce the acute phase response. Overexpression of IL-6 has been implicated in the pathology of a number of diseases including multiple myeloma, rheumatoid arthritis, Castleman's disease, psoriasis, and post-menopausal osteoporosis. Hence, selective antagonists of IL-6 action may offer therapeutic benefits. IL-6 is a member of the family of cytokines that includes interleukin-11, leukemia inhibitory factor, oncostatin M, cardiotrophin-1, and ciliary neurotrophic factor. Like the other members of this family, IL-6 induces growth or differentiation via a receptor-system that involves a specific receptor and the use of a shared signaling subunit, gp130. Identification of the regions of IL-6 that are involved in the interactions with the IL-6 receptor, and gp130 is an important first step in the rational manipulation of the effects of this cytokine for therapeutic benefit. In this review, we focus on the sites on IL-6 which interact with its low-affinity specific receptor, the IL-6 receptor, and the high-affinity converter gp130. A tentative model for the IL-6 hexameric receptor ligand complex is presented and discussed with respect to the mechanism of action of the other members of the IL-6 family of cytokines.

Harlequin granulocyte-colony stimulating factor interleukin 6 molecules with bifunctional and antagonistic activities

BACKGROUND

The granulocyte-colony stimulating factor (G-CSF) and the interleukin 6 (IL-6) are part of a large family of cytokines that regulate the production and the functional activity of hemopoietic cells. Recent studies have shown that IL-6 and G-CSF share structure homology and have partially overlapping functions.

OBJECTIVES

Our research gives some information for planning the production of new artificial cytokines in order to establish whether we could obtain molecules that had two functions or an inhibitory function.

STUDY DESIGN

Fourteen different chimeric molecules (called Harlequin molecules) of human IL-6 and G-CSF have been produced exchanging the sequence encoded by the 2nd to the 5th exon of the genes of the two cytokines.

RESULTS

In order to test their biological activity we performed different assays: proliferation of murine B9-cells and immunoglobulin production from human Epstein-Barr virus (EBV)-transformed B-cell lines for IL-6; induction of granulocytic differentiation of the murine 32DC13(G) cell line and normal bone marrow progenitor cells for G-CSF. Some chimeric molecules maintain the activity of either IL-6 and G-CSF and at least one (Harlequin 11) has both biological activities. One chimeric protein has no biological activity but competes, presumably at the receptor level, for the activity of the intact cytokine.

CONCLUSION

These studies can provide important information on the structure/function of the two cytokines. The bifunctional Harlequin molecule 11 could be a potential candidate as therapeutic agent.

I. A bioactive designer cytokine for human hematopoietic progenitor cell expansion

Efficient expansion of hematopoietic progenitor cells requires, at least, the simultaneous stimulation of the receptors c-kit and gp130. While c-kit is activated by SCF; gp130, in cells which do not express sufficient amounts of IL-6R, can be activated by the complex of soluble IL-6R (sIL-6R) and IL-6. The therapeutic use of IL-6/sIL-6R, however, has been hampered by the high concentrations of the sIL-6R protein required. We have designed a fusion protein of sIL-6R and IL-6, linked by a flexible peptide chain, that was expressed to high levels. On gp130 expressing cells the fusion protein turned out to be fully active at 100 to 1,000-fold lower concentration than the combination of unlinked IL-6 and IL-6R. The fusion protein was used to effectively expand human hematopoietic progenitor cells ex vivo in a dose dependent fashion.

Identification of single amino acid residues of human IL-6 involved in receptor binding and signal initiation

The pleiotropic cytokine interleukin-6 (IL-6) has been predicted to be a protein with four antiparallel alpha-helices. On target cells, IL-6 interacts with a specific ligand binding receptor subunit (IL-6R), and this complex associates with the signal-transducing subunit gp130. Human IL-6 acts on human and murine cells, whereas murine IL-6 is only active on murine cells. The construction of chimeric human/murine IL-6 proteins has allowed us to define a region (residues 77-95, region 2c) within the human IL-6 protein that is important for IL-6R binding and a region (residues 50-55, region 2a2) that is important for IL-6R dependent gp130 interaction. Guided by sequence alignment and molecular modeling, we have constructed several IL-6 variants with point mutations in these regions and have tested them for receptor binding and signal initiation. Within region 2c, phenylalanine 78 was involved in receptor binding, whereas lysine 54 within region 2a2 participated in gp130 activation. Furthermore, some IL-6 variants with lysine 54 replacements could be used to construct muteins that retained receptor binding but failed to activate gp130. Such IL-6 muteins were efficient IL-6 receptor antagonists.

The carboxy-terminal region of human interleukin-5 is essential for maintenance of tertiary structure but not for dimerization

The C-terminal region of interleukin-5 has previously been suggested to be important for biological activity [Mackenzie et al., (1991), Mol. Immunol. 28, 155-158; Kodama et al. (1991), Biochem. Biophys. Res. Commun. 178, 514-519]. We have investigated this region by making a series of truncation mutants. The proteins were expressed in Escherichia coli, purified from inclusion bodies, and were able to refold with the disulfide homodimeric topology typical of interleukin-5. Analysis of the truncated carboxy-terminal proteins in an interleukin-5-dependent proliferation assay on TF-1 cells showed a rapid loss of activity as the C-terminal was shortened by more than two amino acids. This loss of biological activity correlated with a drop in binding affinity to both the alpha chain of the receptor and the high-affinity complex consisting of the alpha and beta subunits. Analysis of the proteins by 1H-NMR showed that the truncated mutants have higher exchange rates with solvent, indicating a less rigid structure. The carboxy-terminal region is therefore necessary to maintain the stability of the four-helix bundle and to orient correctly the important residues of the fourth helix. Inspection of the structure determined by X-ray crystallography shows that Trp-110 acts as the major residue in anchoring the fourth helix.

Isomerization of the Xaa-Pro peptide bond induced by ionic interactions of arginine

Inclusion of Arg or Pro residues in proteins and peptides has been proved to play an essential role in biochemical functions through ionic interactions, conformational transitions, and formation of turns as well. In this study we present the conformational properties of the Ac-Arg-Ala-Pro (1), Ac-Arg-Ala-Pro-NH2 (2), Ac-Arg-Pro-Asp-NH2 (3), and Ac-Arg-Pro-Asp (4) tripeptides, using 1H-nmr spectroscopy and molecular dynamics. These peptides were modeled with the aim of studying the role of the Arg-guanidinium to carboxylate ionic interactions on the Xaa-Pro peptide bond isomerization. It was found with 1 and 4 that arginine preferentially interacts with the C-terminal carboxylate group, even though the beta-carboxylate is also accessible. This tendency of the Arg moiety was found to induce the cis disposition of the Ala-Pro peptide bond in 1. It was also confirmed that the Arg...Asp side chain-side chain ionic interaction in 3 plays a key role in backbone folding and structural stabilization through a type I beta-turn. The nmr pattern for 3 showed a remarkable similarity with that for various Arg-Tyr-Asp containing peptides, a sequence that is crucial for the adhesion properties of the Leishmania gp63 glycoprotein.

Use of immobilized synthetic peptides for the identification of contact sites between human interleukin-6 and its receptor

Synthetic peptides immobilized on cellulose membranes proved to be a powerful tool for the identification of sites in the cytokine IL-6 involved in receptor binding. Similarly, a region in the extracellular part of the IL-6 receptor which is important for interaction with its ligand was identified.

Preparation and a structure-function analysis of human ciliary neurotrophic factor

Ciliary neurotrophic factor (CNTF) is a trophic protein that promotes survival and/or differentiation of a variety of neuronal cell types including sensory, sympathetic, and motor neurons. CNTF, leukemia inhibitory factor (LIF), interleukin-6 (IL-6) and oncostatin M (OSM) share a predicted common helical framework and partially identical receptor components. In this study, we present the preparation and structure--functional analysis of recombinant human CNTF. The human CNTF gene was expressed under the control of the PL promoter in Escherichia coli, and the mutants were constructed by insertion, deletion and site-directed mutagenesis. The recombinant proteins were purified from bacteria via DEAE A-50 and Sephacryl S-200 chromatography, and their survival promoting activities were determined using cultures of embryonic chick dorsal root ganglion (DRG) neurons. Insertion at position 23 with APGL, or at position 79 with PRGA, or substitution of 162L163Q for PIDG resulted in proteins with no neurotrophic activity. However, insertion at position 186 with PRGI did not alter human CNTF activity. Deletion of the carboxy-terminal amino acid 186-200 did not reduce the biological activity, but elimination of the amino acid 162-186 abolished the activity. The mutant substituting of 17 Cys for Ser was found to display a biological activity equivalent to that of the wild type. Our data provided experimental confirmation for the structural prediction of CNTF.

Human ciliary neurotrophic factor: a structure-function analysis

Ciliary neurotrophic factor (CNTF) promotes survival in vitro and in vivo of several neuronal cell types including sensory and motor neurons. The primary structure of CNTF suggests it to be a cytosolic protein with strong similarity to the alpha-helical cytokine family which is characterized by a bundle of four anti-parallel helices. CNTF exerts its activity via complexation with CNTF receptor (CNTF-R). This complex consists of a CNTF-binding protein (CNTF-R) and two proteins important for signal transduction [gp130 and leukaemia inhibitory factor receptor (LIF-R)]. We have shortened the cDNA coding for CNTF at both the 5' and the 3' end and expressed the truncated proteins in bacteria. Biological activities of the protein preparations were determined by their ability to induce proliferation of BAF/3 cells that were stably transfected with CNTF-R, gp130 and LIF-R cDNAs. CNTF proteins with 14 amino acid residues removed from the N-terminus were biologically active whereas the removal of 23 amino acids resulted in an inactive protein. In addition, 18 amino acid residues could be removed from the C-terminus of the CNTF protein without apparent loss of bioactivity, but further truncation at the C-terminus yielded biologically inactive proteins. The introduction of two point mutations into the CNTF protein at a site that presumably interacts with one of the two signal-transducing proteins resulted in a CNTF mutant with no measurable bioactivity. In addition, a model of the three-dimensional structure of human CNTF was constructed using the recently established structural co-ordinates of the related cytokine, granulocyte colony-stimulating factor. CD spectra of CNTF together with our mutational analysis and our three-dimensional model fully support the view that CNTF belongs to the family of alpha-helical cytokines. It is expected that our results will facilitate the rational design of CNTF mutants with agonistic or antagonistic properties.

NMR studies of the solution properties of recombinant murine interleukin-6

The effects of solvent, pH and temperature on the 1H-NMR spectra of recombinant murine interleukin-6 (IL-6) are described. Assignments made from two-dimensional homonuclear spectra are presented for resonances of the fifteen aromatic amino-acid side chains. A time-dependent loss of intensity was observed for all resonances in the spectrum of IL-6, probably as a result of aggregation. This aggregation is markedly temperature-dependent. The pKa values of the four histidine residues in murine IL-6 has been measured; one has a value of 5.5, approx. one pH unit less than the value exhibited by the other three. Analysis of the NOESY spectra has allowed a preliminary characterisation of the nature of interactions among the aromatic side chains within the protein fold. 1H and 15N resonances of residues Thr-4 to Val-21 are assigned from three-dimensional 1H-15N correlated spectroscopy, and evidence is presented for these residues comprising a mobile N-terminal tail with little ordered structure. An N-terminal mutant lacking the first 22 residues of the murine IL-6 sequence and known to possess full biological activity was also examined and shown to have essentially retained the tertiary fold of the native molecule.

Site-directed mutagenesis of human CNTF: functional analysis of recombinant variants

Ciliary neurotrophic factor (CNTF), interleukin-6 (IL-6), leukemia inhibitory factor (LIF), and oncostatin M (OSM) share functional properties, a predicted common helical framework, and partially identical receptor components. CNTF is a survival promoting factor for various types of neurons in vitro and in vivo. In the present study, structural features essential for the biological function of human CNTF were investigated. Several recombinant CNTF variants were constructed by PCR and expressed in E. coli. Their survival promoting activities were determined using cultures of embryonic chick and newborn rat dorsal root ganglion cells. Deletion of 14 N-terminal and 18 C-terminal amino acids significantly increased bioactivity compared to wild-type (wt) CNTF. Further truncation of the CNTF molecule at the N- or C-terminus resulted in a significant reduction or complete loss of activity. Substitution of two amino acids (Lys154Glu and Trp157Pro) abolished the survival promoting effect. Recently described analogous substitutions in IL-6 had resulted in a partial IL-6 receptor antagonist. However, the double substitution variant had no significant inhibitory effect on wtCNTF activity in assays with both wt and mutant factor. The CNTF variants constructed had almost identical effects on both chick and rat neurons indicating a close similarity of the avian and the mammalian CNTF receptor complex. The present results also demonstrate that a core segment of the CNTF molecule is indispensable for biological function. Analogous segments important for activity have already been identified in the related molecules IL-6, LIF, and OSM. Thus, our data confirm the close structural relationship of CNTF to these "neuropoietic" cytokines. In addition, they demonstrate that site-directed mutagenesis of recombinant human CNTF can yield molecules which show increased survival promoting activity on mammalian neurons.

Combining two mutations of human interleukin-6 that affect gp130 activation results in a potent interleukin-6 receptor antagonist on human myeloma cells

The pleiotropic cytokine interleukin-6 (IL-6) interacts with the specific ligand binding subunit (IL-6R alpha) of the IL-6 receptor, and this complex associates with the signal-transducing subunit gp130 (IL-6R beta). Human IL-6 acts on human and murine cells, whereas murine IL-6 is only active on murine cells. The construction of a set of chimeric human/murine IL-6 proteins has recently allowed us to define a region (residues 43-55) within the human IL-6 protein, which is important for the interaction with gp130. Subdividing this region shows that mainly residues 50-55 of the human IL-6 are necessary for this interaction. Recently, another human IL-6 double mutant (Q159E and T162P) showed reduced affinity to gp130 but residual activity on the human myeloma cell line XG-1. Into this IL-6 mutant we introduced the murine residues 43-49 or 50-55 together with two point mutations, F170L and S176A, which had been reported to increase the affinity of IL-6 to the IL-6R alpha. The resulting IL-6 molecule, which contained the murine residues 50-55, was inactive on human myeloma cells and in addition completely inhibited wild type IL-6 activity on these cells. Such an antagonist may be used as a specific inhibitor of IL-6 activity in vivo.

Structure, stability and biological properties of a N-terminally truncated form of recombinant human interleukin-6 containing a single disulfide bond

A mutant species of the 185-residue chain of human interleukin-6 lacking 22-residues at its N-terminus and with a Cys-->Ser substitution at positions 45 and 51 was produced in Escherichia coli. The 163-residue protein des-(A1-S22)-[C45S, C51S]interleukin-6, containing a single disulfide bridge, formed inclusion bodies. Mutant interleukin-6 was solubilized in 6 M guanidine hydrochloride, subjected to oxidative refolding and purified to homogeneity by ammonium sulfate precipitation and hydrophobic chromatography. The purity of the mutant species was established by electrophoresis, isoelectrofocusing and reverse-phase HPLC and its structural identity was checked by N-terminal sequencing of both the intact protein and several of its proteolytic fragments. Electrospray mass spectrometry analysis of mutant interleukin-6 gave a molecular mass of 18,695 +/- 2 Da in excellent agreement with the calculated value. Circular dichroic, fluorescence emission and second-derivative ultraviolet absorption spectra indicated that mutant interleukin-6 maintains the overall secondary and tertiary structure, as well as stability characteristics, of the recombinant wild-type human interleukin-6. The urea-induced unfolding of mutant interleukin-6, monitored by circular dichroic measurements in the far-ultraviolet region, occurs as a highly cooperative process with a midpoint of denaturation at 5.5 M urea. The data of the reversible unfolding of mutant interleukin-6 mediated by urea were used to calculate a value of 20.9 +/- 0.4 kJ.mol-1 for the thermodynamic stability of the protein at 25 degrees C in the absence of denaturant. The biological activity of mutant interleukin-6 was evaluated in vitro by the hybridoma proliferation assay, and in vivo by measuring thrombopoiesis in monkeys. Dose/response effects of the mutant were comparable or even higher than those of the wild-type protein. Overall the results of this study show that mutant interleukin-6 is a biologically active cytokine, which could find practical use as a therapeutic agent.

Ciliary neurotrophic factor

Ciliary neurotrophic factor (CNTF) was first identified and partially purified from embryonic chick eye tissues. Subsequently, it was shown that CNTF is also present in large amounts in sciatic nerves of adult rats and rabbits, which led to its final purification and cloning. CNTF is not secreted by the classical secretory pathway involving the endoplasmatic reticulum and Golgi complex, but can be detected in high quantities within the cytoplasm of myelinating Schwann cells and astrocytes using immunohistochemistry. CNTF supports survival and/or differentiation of a variety of neuronal cell types including sensory, sympathetic, and motoneurons. Also, nonneuronal cells, such as oligodendrocytes, microglial cells, liver cells, and skeletal muscle cells, respond to exogenously administered CNTF, both in vitro and in vivo. During development, expression of CNTF is very low, if indeed it is expressed at all, and the phenotype of mice lacking endogenous CNTF after inactivation of the CNTF gene by homologous recombination suggests that CNTF does not play a crucial role for responsive cells during embryonic development. However, motoneurons are lost postnatally in mice lacking endogenous CNTF, suggesting that CNTF acts physiologically on the maintenance of these cells. The ability of exogenous CNTF to protect against motoneuron loss following lesion or in other animal models indicates that CNTF might be useful in the treatment of human motoneuron disorders, provided appropriate means of administration can be found.

Roles of disulfide bonds in recombinant human interleukin 6 conformation

Human IL-6 has two disulfide bonds linking Cys45 to Cys51 and Cys74 to Cys84, respectively. Previous site-directed mutagenesis studies have demonstrated that the Cys74-Cys84 bond is essential for full biological and receptor binding activities. To address the structural importance of these disulfide bonds in the formation and stabilization of IL-6 secondary and tertiary structures, we have generated a panel of disulfide bond-deficient rIL-6 analogs both by chemical reduction and alkylation as well as by site-directed mutagenesis. Conformational changes affecting these rIL-6 analogs were probed by circular dichroism spectroscopy, as well as reactivity with monoclonal antibodies, and correlated with changes in biological activities. We have shown that the first disulfide bridge (Cys45-Cys51) is highly sensitive to reduction and, therefore, more solvent-exposed or less thermodynamically stable. Contrary to previous reports, this bridge contributes, although minimally, to the full biological activity of the cytokine. However, no significant changes in secondary or tertiary structures were observed upon removal of this bond. In marked contrast, analogs lacking the disulfide bridge between Cys74 and Cys84 exhibited as little as 0.5% and 0.05% wild-type biological and receptor binding activities, respectively. These dramatic changes correlated with a slight reduction in alpha-helical content and a decreased reactivity with the neutralizing monoclonal antibody mAb8 which recognizes a conformational epitope associated with the active site. Our results suggest that the second disulfide bridge plays a critical role in maintaining the spatial relationship between the putative IL-6 A and D helices.

Production and structural characterization of amino terminally histidine tagged human oncostatin M in E. coli

Oncostatin M is a cytokine that acts as a growth regulator on a wide variety of cells and has diverse biological activities including acute phase protein induction, LDL receptor up-regulation and cell-specific gene expression. In order to gather information about the Onc M structure, we established a protocol for large scale production and single step purification of this functional cytokine from bacterial cells. The cDNA of human Onc M was cloned by RT-PCR from total RNA of PMA induced U937 cells. After the addition of a six histidine tag at the N-terminus, the coding region of mature Onc M was cloned in the pT7.7 expression vector. Histidine tagged Onc M was overexpressed in bacterial cells and purified to homogeneity in one step on a metal chelating column. We found that recombinant 6xHis-OncM remains fully active in a growth inhibition assay. Structural characterization of the purified protein was performed by electrospray mass spectrometry, automated Edman degradation and peptide mapping by high-pressure liquid chromatography/fast-atom-bombardment mass spectrometry. Thermal and pH stability dependence of Onc M was assessed by circular dichroism spectroscopy; the helical content is about 50%, in agreement with the four helix bundle fold postulated for cytokines that bind haematopoietic receptors of type I.

Structure-function studies of human ciliary neurotrophic factor

Ciliary neurotrophic factor (CNTF) is a polypeptide that promotes the survival and/or differentiation of a number of neural cell types. Here we present a structural and functional analysis of the human CNTF molecule. Variant proteins were synthesized by Escherichia coli transformed with mutant cDNA constructs, and purified by SDS-polyacrylamide gel electrophoresis and reverse phase high pressure liquid chromatography. Most variant CNTF proteins lacked neurotrophic activity, but two N- and C-terminal deletions (delta 2-14 and delta 173-200, respectively) actually displayed a several-fold increase in specific activity. Loss of biological activity was accompanied by changes in the alpha-helical nature of CNTF as measured by circular dichroism. These data strengthen the proposed similarity between CNTF and the family of hematopoietic cytokines.

Solution structure of synthetic peptides corresponding to the C-terminal helix of interleukin-6

Two synthetic peptides corresponding to the C-terminal 19 residues of human and murine interleukin-6, respectively, have been synthesized and their structures in solution investigated using high-resolution 1H-NMR spectroscopy. Both peptides show a marked dependence of chemical-shift dispersion on pH, with a greater degree of structure apparent above pH 4.5, where their glutamate carboxyl groups are ionised. In purely aqueous solution, neither peptide adopts a well-defined structure, although the murine peptide has characteristics of a nascent helix. Titration of the murine peptide with trifluoroethanol produced a significant increase in structure, which was then investigated using two-dimensional NMR. In 50% (by vol.) trifluoroethanol the murine peptide consists of a well-defined central helix of 12 residues with unstructured N-terminal and C-terminal regions. These observations lend experimental support to the current model of the interleukin-6 structure, which proposes a four-helical bundle with the last helix encompassing the C-terminal 20-30 residues. Furthermore, the fact that synthetic peptides corresponding to part of the putative receptor-binding surface of interleukin-6 are able to adopt a similar conformation in solution to that proposed for the intact protein suggests that such peptide analogues should be useful starting points in the design of peptide agonists and antagonists of interleukin-6.

Specific covalent modification of the tryptophan residues in murine interleukin-6. Effect on biological activity and conformational stability

Modification of recombinant murine interleukin-6 (mIL-6) with the tryptophan-specific reagent 2-nitrophenylsulfenyl chloride under mild acidic conditions, 0.1 M sodium acetate, pH 3.5, yielded a derivative containing 2.02 mol 2-nitrophenylsulfenyl tryptophan/mol protein. The sites of modification were identified as Trp36 and Trp160. No detectable side reactions occurred on other amino acids in the molecule, as indicated by the combination of endoproteinase Asp-N peptide mapping, Edman degradation and electrospray mass spectrometry. Sulfenylation of the two tryptophan residues in mIL-6 caused a 50% reduction in both the biological activity in the murine-hybridoma-growth-factor assay using 7TD1 cells and receptor-binding affinity to mIL-6 receptors. Sulfenylation of mIL-6 did not significantly affect the overall conformation of the protein as measured by farultraviolet circular dichroism and binding to the neutralizing anti-mIL-6 mAb 6B4. The sulfenylated protein was, however, significantly less stable [delta delta G(H2O) = 3.98 kJ/mol] than unmodified mIL-6 as measured by urea-gradient gel electrophoresis.

Role of the C-terminus in the activity, conformation, and stability of interleukin-6

Two murine interleukin-6 (mIL-6) variants were constructed using the polymerase chain reaction (PCR), one lacking the last five residues (183-187) at the C-terminus (pMC5) and another with the last five residues of mIL-6 substituted by the corresponding residues of human IL-6 (pMC5H). The growth stimulatory activity of pMC5 on the mouse hybridoma cell line 7TD1 was < 0.05% of mIL-6, whereas pMC5H and mIL-6 were equipotent. The loss of biological activity of pMC5 correlated with its negligible receptor binding affinity on 7TD1 cells, while the binding of pMC5H was comparable to that of mIL-6. Both pMC5 and pMC5H, like mIL-6, failed to interact with recombinant soluble human IL-6 receptor when assayed by surface plasmon resonance-based biosensor analysis. These studies suggest that the C-terminal seven amino acids of human IL-6, alone, do not define species specificity for receptor binding. A variety of biophysical techniques, as well as the binding of a conformational-specific monoclonal antibody, indicated that the global fold of the mIL-6 variants was similar to that of mIL-6, although small changes in the NMR spectra, particularly for pMC5, were observed. Some of these changes involved residues widely separated in the primary structure. For instance, interactions involving Tyr-22 were influenced by the C-terminal amino acids suggesting that the N- and C-termini of mIL-6 are in close proximity. Equilibrium unfolding experiments indicated that pMC5 was 0.8 kcal/mol less stable than mIL-6, whereas pMC5H was 1.4 kcal/mol more stable. These studies emphasize the structural importance of the C-terminal amino acids of IL-6 and suggest that truncation or mutation of this region could lead to small but significant alterations in other regions of the molecule.

Effect of pH and denaturants on the folding and stability of murine interleukin-6

The conformation and stability of a recombinant mouse interleukin-6 (mIL-6) has been investigated by analytical ultracentrifugation, fluorescence spectroscopy, urea-gradient gel electrophoresis, and near- and far-ultraviolet circular dichroism. On decreasing the pH from 8.0 to 4.0, the tryptophan fluorescence of mIL-6 was quenched 40%, the midpoint of the transition occurring at pH 6.9. The change in fluorescence quantum yield was not due to unfolding of the molecule because the conformation of mIL-6, as judged by both urea-gradient gel electrophoresis and CD spectroscopy, was stable over the pH range 2.0-10.0. Sedimentation equilibrium experiments indicated that mIL-6 was monomeric, with a molecular mass of 22,500 Da over the pH range used in these physicochemical studies. Quenching of tryptophan fluorescence (20%) also occurred in the presence of 6 M guanidine hydrochloride upon going from pH 7.4 to 4.0 suggesting that an amino acid residue vicinal in the primary structure to one or both of the two tryptophan residues, Trp-36 and Trp-160, may be partially involved in the quenching of endogenous fluorescence. In this regard, similar results were obtained for a 17-residue synthetic peptide, peptide H1, which corresponds to an N-terminal region of mIL-6 (residues Val-27-Lys-43). The pH-dependent acid quenching of endogenous tryptophan fluorescence of peptide H1 was 30% in the random coil conformation and 60% in the presence of alpha-helix-promoting solvents. Replacement of His-33 with Ala-33 in peptide H1 alleviated a significant portion of the pH-dependent quenching of fluorescence suggesting that the interaction of the imidazole ring of His-33 with the indole ring of Trp-36 is a major determinant responsible for the quenching of the endogenous protein fluorescence of mIL-6.

Recombinant human IL-6 expressed in E. coli undergoes selective N-terminal degradation: evidence that the protein consists of a stable core and a nonessential flexible N-terminal

A synthetic gene for human interleukin-6 has been expressed in E. coli. The protein has been purified and renatured and has the same activity as natural human IL-6 using the 7TD1 cell proliferation assay. The protein undergoes specific cleavage by a thiol protease, yielding two new N-termini at Arg-9 and His-15. The truncated proteins retain full biological activity. The degradation results in the loss of sharp amide resonances in the 1H-NMR spectrum, and little change to the ultraviolet CD spectrum. Several amino acid type assignments could be made for these sharp amides using a DQF-COSY 2D-NMR experiment. The N-terminal 15 amino acids exist as a flexible, random coil, attached to a central structure.

Molecular cloning and expression of an intracellular serpin: an elastase inhibitor from horse leucocytes

Horse blood leucocytes contain an elastase inhibitor (HLEI) belonging to the serpin family. Poly(A)+RNA isolated from these cells was used to construct a cDNA library in lambda gt10, which was first screened with a synthetic degenerate oligonucleotide probe corresponding to the amino acid sequence of the reactive centre of the inhibitor. Three clones were obtained covering the entire coding region of the protein. Sequencing of these clones showed identity with the amino acid sequence obtained from Edman degradation of the elastase inhibitor. The coding sequence of the HLEI cDNA was cloned into the bacterial expression vector pKK233-2 and expressed in Escherichia coli cells. Transformed bacteria expressed significant amounts of the protein, which was immunoprecipitated with a specific anti-HLEI antiserum. Furthermore, HLEI expressed in bacteria inhibited the activity of elastase but not trypsin.

Involvement of the Arg179 in the active site of human IL-6

Three internal-amino acid deletions of amino acids 171-179 of human interleukin 6 (IL-6) were introduced at the cDNA level. While all deletion proteins were biologically inactive, immunoprecipitations with a set of conformation-specific anti-(IL-6) monoclonal antibodies showed that only mutant delta 177-179 does not present major alterations in folding. This finding, together with the observation that delta 177-179 is not able to compete with IL-6 for binding to the soluble human IL-6 receptor, suggested that some or all of these three residues participate to the composition of the receptor-binding site of human IL-6. A large number of single-amino-acid-substitution mutants were generated in residues 177, 178 and 179. Their detailed analysis revealed that Arg179 is crucial for activity in mouse cells, because all amino acid substitutions in this position cause a dramatic drop of biological activity on murine hybridoma cells without affecting the overall protein folding. The only substitution which preserved some residual activity was the conservative Arg to Lys change. This demonstrates the absolute requirement for a positive charge in position 179 for the interaction of human IL-6 with its receptor.

Cells transfected with human interleukin 6 cDNA acquire binding sites for the hepatitis B virus envelope protein

Earlier studies revealed that human interleukin 6 (IL-6) contains recognition sites for the hepatitis B virus (HBV) envelope (env) protein, and that IL-6 and anti-IL-6 antibodies, respectively, inhibited the interaction of cells expressing a receptor for HBV with the preS(21-47) segment of the HBV env protein, encompassing the complementary attachment site for IL-6. This suggested that IL-6 mediates HBV-cell interactions. We report that: (a) Chinese hamster ovary cells transfected with human IL-6 cDNA and Spodoptera frugiperda ovarian insect cells infected with recombinant baculovirus carrying human IL-6 cDNA expressed receptors for the preS(21-47) region of the HBV env protein, indicating that expression of IL-6 on the surface of cells is sufficient to endow them with receptors for HBV. (b) Among peptides covering the entire sequence of human IL-6 and the corresponding antipeptide antibodies, the peptide IL-6[35-66] and anti-IL-6[35-66] most effectively inhibited the interaction between human hepatoma HepG2 cells and the preS(21-47) ligand, suggesting that this region of the human IL-6 sequence encompasses a binding site for the HBV env protein. (c) Studies with replacement set peptides from the preS(21-47) sequence indicated that residues 21-25, 28, 31, 33-35, 39, and 43-45 can be replaced by alanine (serine) residues, while all the other residues are essential for maintaining the cell receptor/IL-6 binding activity. Further delineation of complementary sites on IL-6 and on the HBV env protein may contribute to the design of compounds inhibiting HBV replication.

Analysis of human/mouse interleukin-6 hybrid proteins: both amino and carboxy termini of human interleukin-6 are required for in vitro receptor binding

The multifunctional cytokine interleukin-6 (IL-6) is a single polypeptide chain consisting of 184 amino acids in man and 187 amino acids in mouse. Despite the relatively high degree of sequence similarity of these two molecules (about 57%), the biological activity in mouse and human IL-6 shows species specificity. Starting with this observation, we constructed interspecies hybrids with the goal of defining which segments of the human IL-6 molecule are involved in human receptor binding. In this manner we generated multiple amino acid substitution mutants which do not contain insertions or deletions as compared with the parental proteins, and which, therefore, should not show dramatic changes in folding. Using two biological assays on cells of human and mouse origin and a recently developed in vitro binding assay to recombinant soluble human IL-6 receptor, we obtained results which indicate that both the amino and carboxy termini are necessary and sufficient for efficient binding, but that the carboxy terminus plays the dominant role in receptor recognition.

Purification and characterization of a recombinant murine interleukin-6. Isolation of N- and C-terminally truncated forms

Murine interleukin-6 (IL-6), when expressed in Escherichia coli using the pUC9 vector, accumulated as insoluble aggregates or 'inclusion bodies'. After selective urea washing of the inclusion bodies, to remove extraneous proteins, murine IL-6 was solubilized with 8 M guanidine hydrochloride and then rapidly purified to homogeneity by gel-permeation chromatography followed by reversed-phase HPLC. It was demonstrated that complete disulfide bond formation in murine IL-6 occurred during the early urea washing/guanidine hydrochloride extraction steps, so no refolding step was required. When fully reduced murine IL-6 was dissolved in 8 M guanidine hydrochloride and allowed to air-oxidize, complete disulfide bond formation, monitored by analytical reversed-phase HPLC, was shown to occur within 13 h at 6 degrees C. About 25 mg pure protein was obtained from 37 g wet cells. This recombinant murine IL-6 had a specific activity in the hybridoma growth factor assay of 2 x 10(8) U/mg, which is equivalent to that of native murine IL-6. During the purification procedure, a number of variant forms of murine IL-6 were isolated and partially characterized. Two of these forms, T1 and T3, were C-terminal deletants of murine IL-6 lacking about 60 and 20 amino acids from the C-terminus, respectively, while the other form, T2, was an N-terminal deletant lacking 37 amino acids from the N-terminus. None of these variant forms of murine IL-6 bound to the murine IL-6 receptor and, consequently, all were inactive in the hybridoma growth factor assay.

Sequence and structural relationships in the cytokine family

The sequences of nine different cytokines, growth hormone, and prolactin have been aligned and their secondary structure predicted. The alignment reveals that each exon has a characteristic sequence pattern shared by all cytokines. The most striking sequence similarity is observed in exon 4, where the residue pair Phe-Leu is conserved in many cytokines. In addition, there are discreet homologous regions between two specific growth factors, including a high degree of homology between granulocyte-macrophage colony-stimulating factor (GM-CSF) and interleukin 3 (IL-3). The secondary structure analysis predicts that exon 3 of all cytokines has an antiparallel helix-turn-helix motif, which is likely to form the central helical segments of a four alpha-helical bundle-type structure. Based on the secondary structure and the disulfide-bonding pattern, the topological connectivity for a number of cytokines has been predicted.

Interleukin 6: presence and future

This review article deals with the basic biological characteristics of the multifunctional cytokine interleukin-6 (IL-6) in man. Three central issues will be addressed more closely: the pathophysiological role of unbalanced IL-6 production in various disease states, the diagnostic usefulness of measurements of IL-6 in biological fluids, and the possible role of IL-6, IL-6 antagonists, and IL-6 derivatives as therapeutic tools in clinical medicine.