Structure-function analysis of the C-terminal segment of human interleukin-6

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 New IL6 Isoform in Chinese Soft-Shelled Turtle (Pelodiscus sinesis) Discovered: Its Regulation during Cold Stress and Infection

The Chinese soft-shelled turtle (Pelodiscus sinesis) is a widely cultured commercial species in East and Southeast Asian countries. The turtles frequently suffer from acute cold stress during farming in China. Stress-induced factor such as Interleukin-6 (IL6) is a multifunctional molecule that plays important roles in innate and adaptive immune response. In the present study, we found that the turtle possessed two IL6 transcripts, where one IL6 transcript contained a signal peptide sequence (psIL6), while the other IL6 transcript (psIL6ns) possessed no such signal peptide gene. To test any differential expression of the two isoforms during temperature and microbial stress, turtles were adapted to optimal environmental water temperature (25 °C), stressed by acute cooling for 24 h, followed with the challenge of Aeromonas hydrophila (1.8 × 10⁸ CFU) or Staphylococcus aureus (5.8 × 10⁸ CFU). Gene characterization revealed that psIL6ns, a splicer without codons encoding a signal peptide and identical to the one predicted from genomic sequence, and psIL6, a splicer with codons encoding a signal peptide, were both present. Inducible expression was documented in primary spleen cells stimulated with ConA and poly I: C. The splenic and intestinal expression of psIL6ns and psIL6 was increased in response to temperature stress and bacterial infection.

Interaction of Human Herpesvirus 8 Viral Interleukin-6 with Human Interleukin-6 Receptor Using In Silico Approach: The Potential Role in HHV-8 Pathogenesis

Introduction:

Human Herpesvirus 8 (HHV-8) causes classical, endemic (African), and Acquired Immunodeficiency Syndrome (AIDS)-related Kaposi’s Sarcoma (KS), Body Cavity-Based Primary Effusion Lymphomas (BCBL), HHV-8-associated peritoneal Primary Effusion Lymphoma (PEL), and Multicentric Castleman’s Disease (MCD). HHV8 genome encodes several structural and non-structural proteins, among which vIL6 is a functional homologue of Interleukin-6 (IL-6). It has been established that vIL6 plays a vital role in HHV8 infections; also, it has been suggested that its function was mediated through gp130, rather than the gp80 (IL-6 receptor [IL-6R]). This study aimed to investigate the physicochemical and structural properties as well as the immunological features, and finally the interaction between vIL6 and IL6 receptor (IL6R) by using several bioinformatics tools which could provide both valuable insight into vIL6 protein and advantageous data for further studies on HHV8 inhibitors and new vaccines.

Material and Methods:

vIL6, human IL6 (hIL6), and IL6R were obtained from NCBI GenBank and Uniport, which were aligned by The CLC Genomics Workbench. "Signal-BLAST" and “predisi" were employed to define signal peptide; also, “Expasy’sProtParam” was used to predict physicochemical properties as well as "DiANNA", and "SCRATCH" predicted the disulfide bonds. “NetPhosK”, “DISPHOS”, “NetPhos”, ”NetNGlyc”, and ”GlycoEP” were involved to determine post-modification sites. To define immunoinformatics analysis, “BcePred”, “ABCpred”, “Bepipred”, “AlgPred”, and "VaxiJen" were used. “SOPMA”, “I-TASSER”, “GalaxyRefine”, and “3D-Refine” predicted and refined the secondary and tertiary structures. TM-align server was used to align 3D structures. In addition, docking analysis was done by “Hex 5.0.”, and finally the results were illustrated by “Discovery Studio”.

Results:

A signal peptide (1-22) was defined in the vIL6 sequences and analysis has shown that vIL6 is an acidic protein which is significantly stable in all organisms. Three Disulfide bonds were predicted and immunoinformatics analysis showed 5 distinct B-cell epitopes. vIL6 is predicted as a non-allergen protein and the majority of its structure consists of Alpha helix. TM-align pointed the significant similarity between vIL6 and hIL6 in protein folding. The high energy value between vIL6 protein and IL6R was calculated and further analysis illustrated 5 conserved regions as well as 4 conserved amino acids which had a significant role in vIL6 and IL6R interaction.

Discussion:

An in silico study by numerous software determined the possible interaction between vIL6 and IL6R and the possible role of this interaction in HHV8 pathogenesis and the progress of infection. These have been overlooked by previous studies and will be beneficial to gain a more comprehensive understanding of vIL6 function during HHV8 lifecycle and infections. Structural analysis showed the significant similarity between vIL6 and hIL6 folding which can describe the similarity of the functions or interactions of both proteins. Furthermore, several conserved regions in the interaction site which interestingly were highly conserved among all vIL6 sequences can be used as new target for vIL6 inhibitors. Moreover, our results could predict immunological properties of vIL6 which suggested the ability of this protein in induction of the humoral immune response. Such a protein may be used for further studies on therapeutic vaccine fields.

Generation of a transgenic mouse line for conditional expression of human IL-6

IL-6 is a cytokine that is involved in various physiological and pathological conditions, and approaches using gain-of-function transgenic animals have contributed in elucidating IL-6 function. However, studies of the multiple functions of IL-6 in vivo are very time consuming because they require the generation of transgenic mice that harbor the gene encoding IL-6 under the control of specific promoters to mimic different pathologies. Here, we report the establishment of a conditional human IL-6 transgenic mouse, LGL-IL6, which conditionally expresses human IL-6 by taking advantage of the well-characterized Cre recombinase drivers.

G-CSF receptor-binding cyclic peptides designed with artificial amino-acid linkers

Designing small molecules that mimic the receptor-binding local surface structure of large proteins such as cytokines or growth factors is fascinating and challenging. In this study, we designed cyclic peptides that reproduce the receptor-binding loop structures of G-CSF. We found it is important to select a suitable linker to join two or more discontinuous sequences and both termini of the peptide corresponding to the receptor-binding loop. Structural simulations based on the crystallographic structure of KW-2228, a stable and potent analog of human G-CSF, led us to choose 4-aminobenzoic acid (Abz) as a part of the linker. A combination of 4-Abz with beta-alanine or glycine, and disulfide bridges between cysteins or homocysteins, gave a structure suitable for receptor binding. In this structure, the side-chains of several amino acids important for the interactions with the receptor are protruding from one side of the peptide ring. This artificial peptide showed G-CSF antagonistic activity in a cell proliferation assay.

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.

Function and molecular modeling of the interaction between human interleukin 6 and its HNK-1 oligosaccharide ligands

Interleukin 6 (IL-6) is endowed with a lectin activity for oligosaccharide ligands possessing the HNK-1 epitope (3-sulfated glucuronic acid) found on some mammalian glycoprotein N-glycans (Cebo, C., Dambrouck, T., Maes, E., Laden, C., Strecker, G., Michalski, J. C., and Zanetta, J. P. (2001) J. Biol. Chem. 276, 5685-5691). Using high affinity oligosaccharide ligands, it is demonstrated that this lectin activity is responsible for the early dephosphorylation of tyrosine residues found on specific proteins induced by interleukin 6 in human resting lymphocytes. The gp130 glycoprotein, the signal-transducing molecule of the IL-6 pathway, is itself a molecule possessing the HNK-1 epitope. This indicates that IL-6 is a bi-functional molecule able to extracellularly associate its alpha-receptor with the gp130 surface complex. Computational modeling indicates that the lower energy conformers of the high affinity ligands of IL-6 have a common structure. Docking experiments of these conformers suggest that the carbohydrate recognition domain of IL-6 is localized in the domain previously identified as site 3 of IL-6 (Somers, W., Stahl, M., and Seehra, J. S. (1997) EMBO J. 16, 989-997), already known to be involved in interactions with gp130.

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).

Kaposi's sarcoma-associated herpesvirus-encoded interleukin-6

Since the discovery of the virus in 1994, the rapid pace with which Karposi's sarcoma-associated herpesvirus (KSHV) research has progressed has quickly led to a broad understanding of the structure of the virus and its biology and pathology in humans. Molecular piracy of potentially useful cellular genes has emerged as a characteristic feature of this virus. The viral homolog of human IL-6, vIL-6 is an example in kind. Studies in vitro and in vivo have shown that vIL-6 can stimulate the growth of KSHV-infected primary infusion lymphoma (PEL) cells, can promote hematopoiesis, and act as an angiogenic factor through the induction of vascular endothelial growth factor (VEGF). It is not difficult to envision how vIL-6, through these properties and perhaps others yet to be identified, can contribute to KSHV survival and spread in the human population.

Human herpesvirus 8 interleukin-6 (vIL-6) signals through gp130 but has structural and receptor-binding properties distinct from those of human IL-6

Human herpesvirus 8 (HHV-8) has been associated with classical, endemic (African), and AIDS-related Kaposi's sarcoma (KS), body cavity-based primary effusion lymphomas, and multicentric Castleman's disease (MCD). HHV-8 encodes a functional homologue of interleukin-6 (IL-6), a cytokine that promotes the growth of KS and myeloma cells and is found at elevated levels in MCD lesions and patient sera. We have previously reported that the viral IL-6 (vIL-6) gene product can support the growth of the IL-6-dependent murine hybridoma cell line, B9, and that the gp80 (IL-6 receptor [IL-6R]) component of the IL-6 receptor-signal transducer (gp180) complex plays a role in mediating this activity. However, it has been shown by others that vIL-6 can function in human cells independently of IL-6R. Here we have extended our functional studies of vIL-6 by identifying transcription factors and pathways used in human Hep3B cells, investigating the utilization of gp130 and IL-6R by vIL-6, and undertaking mutational analyses of vIL-6 and gp130. The data presented here establish that vIL-6, in common with its endogenous counterparts, can mediate signal transduction through gp130 and activate multiple transcription factors, map residues within the vIL-6 protein that are and are not important for vIL-6 signalling, and identify a gp130 mutant that is nonfunctional with respect to vIL-6 signalling in the absence of IL-6R but that retains the ability to mediate vIL-6 and human IL-6 (hIL-6) signal transduction when IL-6R is coexpressed. The data presented demonstrate functional and mechanistic similarities between vIL-6 and endogenous IL-6 proteins but also highlight differences in the structural and receptor-binding properties of vIL-6 relative to its human counterpart.

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.

Interferons alpha/beta and their receptors: place in the hierarchy of cytokines

Interferons alpha/beta (IFNs-alpha/beta) are the first cytokines to be produced by recombinant DNA technology. They regulate growth and differentiation, affecting cellular communication, signal transduction pathways and immunological control. This review focuses on the relationships between the structure and biological activities of IFNs-alpha/beta induced as a result of specific interactions with different types of polypeptide receptors as well as on the role of glycolipids in the modulation of these activities. The discovery of the primary structure homology of HuIFNs-alpha and thymus hormone-thymosin alpha 1 (TM alpha 1), the experimental finding of the competition between IFN-alpha and TM alpha 1 for common receptors and the reproduction by reHuIFN-alpha 2 of TM alpha 1 immunomodulating activities create the basis of reHuIFN-alpha therapeutics instead of TM alpha 1, and potentiation of vaccines by reHuIFN-alpha. The first successful attempt at grafting of the HuIFN-alpha 2s TM alpha 1-like immunomodulating site to the designed de novo protein albeferon is described. This article also aims at reviewing recent data concerning the structure of other cytokines and their receptors. Their reciprocal structure-function taxonomy is proposed. The place of IFNs-alpha/beta and their receptors in the hierarchy of cytokines is determined.

Activating mechanism of CNTF and related cytokines

Ciliary neurotrophic factor (CNTF) shares structural and functional properties with members of the hematopoietic cytokine family. It is composed of a four-helix bundle structure and shares the transmembrane signal transducing proteins, glycoprotein-130 (gp130) and leukemia inhibitory factor receptor (LIF-R). Structure-function analysis showed that the gp130-interactive proteins bind in a similar manner to that of growth hormone (site I and II). In addition, gp130-interactive proteins and granulocyte colony-stimulating factor (G-CSF) utilize another binding site (site III) at the boundary between CD loop and helix D. CNTF triggers the association of receptor components, resulting in activation of a signal transduction cascade mediated by specific intracellular protein tyrosine kinases. The Janus kinase (JAK)/signal transducer and activator of transcription (STAT) and Ras/mitogen-activated protein kinase (MAPK) signaling pathways have been characterized in terms of gp130-interactive protein, and there should be other pathways and some crosstalk between them to enhance, prolong, or specify the signals.

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.

Structure-function analysis of the p35 subunit of mouse interleukin 12

Mouse IL-12 acts on both mouse and human cells; human IL-12 acts only on human cells. This species specificity is determined by the p35 subunit of the IL-12 heterodimer. Since mouse and human p35 sequences are 60% identical, the determinants for the species specificity most likely residue in the nonhomologous sequences of mouse p35. To identify the regions on the p35 subunit interacting with the mouse IL-12 receptor, we constructed a series of chimeric mouse-human p35 molecules by replacing mouse sequences with the nonhomologous human counterparts. An IL-12 heterodimer containing a mouse-human p35 chimera with five residues changed in three discontinuous sites had drastically reduced (750-3000-fold) bioactivities on mouse cells. However, the competitive binding activity of the same mutant IL-12 heterodimer on mouse cells was only reduced 30-fold relative to wild-type IL-12. These findings therefore suggest that 1) the mouse p35 subunit participates in both receptor binding and signaling, 2) the mutations introduced into p35 affect signaling to a much greater extent than receptor binding, and 3) the five residues identified on p35 are required for interacting with the mouse, but not with the human IL-12 receptor and as such contribute extensively to the observed species specificity of IL-12.

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.

Structure-function analysis of human IL-6: identification of two distinct regions that are important for receptor binding

Interleukin-6 (IL-6) is a multifunctional cytokine that plays an important role in host defense. It has been predicted that IL-6 may fold as a 4 alpha-helix bundle structure with up-up-down-down topology. Despite a high degree of sequence similarity (42%) the human and mouse IL-6 polypeptides display distinct species-specific activities. Although human IL-6 (hIL-6) is active in both human and mouse cell assays, mouse IL-6 (mIL-6) is not active on human cells. Previously, we demonstrated that the 5 C-terminal residues of mIL-6 are important for activity, conformation, and stability (Ward LD et al., 1993, Protein Sci 2:1472-1481). To further probe the structure-function relationship of this cytokine, we have constructed several human/mouse IL-6 hybrid molecules. Restriction endonuclease sites were introduced and used to ligate the human and mouse sequences at junction points situated at Leu-62 (Lys-65 in mIL-6) in the putative connecting loop AB between helices A and B, at Arg-113 (Val-117 in mIL-6) at the N-terminal end of helix C, at Lys-150 (Asp-152 in mIL-6) in the connecting loop CD between helices C and D, and at Leu-178 (Thr-180 in mIL-6) in helix D. Hybrid molecules consisting of various combinations of these fragments were constructed, expressed, and purified to homogeneity. The conformational integrity of the IL-6 hybrids was assessed by far-UV CD. Analysis of their biological activity in a human bioassay (using the HepG2 cell line), a mouse bioassay (using the 7TD1 cell line), and receptor binding properties indicates that at least 2 regions of hIL-6, residues 178-184 in helix D and residues 63-113 in the region incorporating part of the putative connecting loop AB through to the beginning of helix C, are critical for efficient binding to the human IL-6 receptor. For human IL-6, it would appear that interactions between residues Ala-180, Leu-181, and Met-184 and residues in the N-terminal region may be critical for maintaining the structure of the molecule; replacement of these residues with the corresponding 3 residues in mouse IL-6 correlated with a significant loss of alpha-helical content and a 200-fold reduction in activity in the mouse bioassay. A homology model of mIL-6 based on the X-ray structure of human granulocyte colony-stimulating factor is presented.

Mutagenesis of the human interleukin-6 fourth predicted alpha-helix: involvement of the Arg168 in the binding site

Random substitutions of amino acid 161-184 of human interleukin-6 (hIL-6) have been generated at the cDNA level using oligonucleotide-directed mutagenesis. Among the majority of the mutant proteins showing a reduced biological activity on murine hybridoma cells, only those having a substitution of Met161, Arg168, Arg179 or Met184, retained a tertiary structure similar to the IL-6 folding. These residues are thus probably involved in the interaction with the IL-6 receptor. However, the contacts established by Arg168 and Arg179 seem far more important for the biological activity. According to Bazan's model of cytokine folding and the receptor binding site on the fourth alpha-helix, based on growth hormone similarity, we propose that Arg168 and Arg179 are located on the exposed surface of this presumed helix.