Recombinant soluble human interleukin-6 receptor. Expression in Escherichia coli, renaturation and purification

Navigating IL-6: From molecular mechanisms to therapeutic breakthroughs

This concise review navigates the intricate realm of Interleukin-6 (IL-6), an important member of the cytokine family. Beginning with an introduction to cytokines, this narrative review unfolds with the historical journey of IL-6, illuminating its evolving significance. A crucial section unravels the three distinct signaling modes employed by IL-6, providing a foundational understanding of its versatile interactions within cellular landscapes. Moving deeper, the review meticulously dissects IL-6's signaling mechanisms, unraveling the complexities of its pleiotropic effects in both physiological responses and pathological conditions. A significant focus is dedicated to the essential role IL-6 plays in inflammatory diseases, offering insights into its associations and implications for various health conditions. The review also takes a therapeutic turn by exploring the emergence of anti-IL-6 monoclonal inhibitors, marking a profound stride in treatment modalities. Diving into the molecular realm, the review explores small molecules as agents for IL-6 inhibition, providing a nuanced perspective on diverse intervention strategies. As the review embarks on the final chapters, it contemplates future aspects, offering glimpses into potential research trajectories and the evolving landscape of IL-6-related studies.

Siltuximab as a primary treatment for cytokine release syndrome in a patient receiving a bispecific antibody in a clinical trial setting

Introduction

Cytokine release syndrome (CRS) and immune effector cell-associated neurotoxicity syndrome (ICANS) are common toxicities associated with immunotherapies, including T cell redirecting bispecific antibodies. Although cooperative group guidelines recommend the use of tocilizumab or other IL-6/IL-6R inhibitors for the management of CRS and ICANS, reports on the use of siltuximab, an IL-6 inhibitor, for the treatment of CRS are limited.

Case report

We present the case of a 77-year-old male who received T cell redirecting bispecific antibody therapy with talquetamab for relapsed/refractory multiple myeloma (RRMM) and developed CRS with concurrent ICANS after receiving a second dose of talquetamab.

Management and Outcome

The patient received an infusion of siltuximab. The patient recovered from CRS within 1 h of siltuximab administration and ICANS within 7 h of siltuximab administration. Patient tolerated the subsequent dose of talquetamab with no evidence of CRS and continued on study.

Discussion

This case describes the successful use of siltuximab for the management of CRS in a patient treated with a T cell redirecting bispecific antibody for RRMM.

Anti-EGFR VHH Antibody under Thermal Stress Is Better Solubilized with a Lysine than with an Arginine SEP Tag

In this study, we assessed the potential of arginine and lysine solubility-enhancing peptide (SEP) tags to control the solubility of a model protein, anti-EGFR V_HH-7D12, in a thermally denatured state at a high temperature. We produced V_HH-7D12 antibodies attached with a C-terminal SEP tag made of either five or nine arginines or lysines (7D12-C5R, 7D12-C9R, 7D12-C5K and 7D12-C9K, respectively). The 5-arginine and 5-lysine SEP tags increased the E. coli expression of V_HH-7D12 by over 80%. Biophysical and biochemical analysis confirmed the native-like secondary and tertiary structural properties and the monomeric nature of all V_HH-7D12 variants. Moreover, all V_HH-7D12 variants retained a full binding activity to the EGFR extracellular domain. Finally, thermal stress with 45-minute incubation at 60 and 75 °C, where V_HH-7D12 variants are unfolded, showed that the untagged V_HH-7D12 formed aggregates in all of the four buffers, and the supernatant protein concentration was reduced by up to 35%. 7D12-C5R and 7D12-C9R did not aggregate in Na-acetate (pH 4.7) and Tris-HCl (pH 8.5) but formed aggregates in phosphate buffer (PB, pH 7.4) and phosphate buffer saline (PBS, pH 7.4). The lysine tags (either C5K or C9K) had the strongest solubilization effect, and both 7D12-C5K and 7D12-C9K remained in the supernatant. Altogether, our results indicate that, under a thermal stress condition, the lysine SEP tags solubilization effect is more potent than that of an arginine SEP tags, and the SEP tags did not affect the structural and functional properties of the protein.

Insights into the Sensitivity of Arginine Concentration to Preserve the Folded Form of Insulin Monomer under Thermal Stress

Arginine, although popularly known as aggregation suppressor additive, has been found to quench proteins' structure and function by destabilizing their conformations. Driven by such controversial evidence, in this work we performed a series of atomistic molecular dynamics simulations of insulin monomer, a biologically active hormone protein, in arginine solution of varying concentrations (0.5, 1, and 2 M) at ambient and elevated temperature (400 K) to explore the arginine concentration driven structure-based stability of the protein. Our study reveals that the flexibility of the protein's structure is dependent on the arginine concentration, and among all the used solutions, 2 M arginine, a "neutral crowder" that mimics the cellular environment, can preserve the native folded form of the protein at ambient temperature in an excellent manner. Further, while the protein unfolds at 400 K in pure water, this solution worked satisfactorily to preserve the protein's folded conformation more firmly than the other solutions. The replica-exchange MD of insulin in 2 M arginine solution further supports the fact. In this aspect an important issue in molecular pharmacology is to identify and recognize the physical origin of the stability of a protein, i.e, in this case, how arginine directs the conformational flexibility of the protein and preserves its native folded form. We identified that the exclusion of arginine from the protein surface increases the local structuration of water around the protein, thereby preserving its "biological water" layer, and makes the protein more hydrated at 2 M concentration as compared to the other arginine solutions. Additionally, our microscopic investigation on the interactions of the protein-solvation layer revealed that the structural heterogeneity of the protein surface, arising from the differential physicochemical nature of the amino acid residues, controls the favorable formation of sluggish water-arginine mixed solvation layer at higher arginine concentration that helps the protein to maintain its structural rigidity. Importantly, apart from the protein-solvent hydrogen-bonding interactions, the anion-pi interactions, established between the carboxyl group of arginine and the aromatic amino acid residues of insulin, were recognized to facilitate the protein to maintain its native folded form at the experimental temperatures.

Interleukin-6 signaling requires only few IL-6 molecules: Relation to physiological concentrations of extracellular IL-6

Introduction

The aim of this study was to give quantitative insight into the number of cytokine molecules needed to activate a target cell and relate this to the physiological consequences of the amounts of cytokines typically detectable in humans. As a model system blood interleukin‐6 (IL‐6) was chosen since this cytokine is one of the most studied and clinically monitored cytokines, and because of the tools for the present investigations such as fully bioactive iodinated recombinant IL‐6, cellular cytokine binding assays, and bioassays have been thoroughly validated.

Methods

The key intermediates of the basic equilibrium principles that govern cytokine binding and exchange were deduced and applied on concrete estimations of cellular and extracellular IL‐6 binding in the bloodstream based on experimental binding data and data from the literature. In parallel, in vitro cellular IL‐6 binding data was substantiated by paired measurements of IL‐6 bioactivity on IL‐6 sensitive B9 hybridoma cells.

Results

Blood leucocytes and B9 cells expressed 50 to 300, 10 to 20 picomolar affinity, IL‐6 binding sites per cell and at physiological concentrations of IL‐6 less than 10 IL‐6 molecules seemed to be bound to blood cells. Nonetheless, binding off as few as four IL‐6 molecules per cell seemed to result in statistically significant bioactivity, whereas binding of 16 IL‐6 molecules triggered extensive cellular responses.

Conclusion

Together, the estimations and the measurements support the notion that target cells with more than 100 bioactive cytokine receptors per cell, such as T cells and hepatocytes, are likely to be under steady and substantial cytokine‐induced endocrine activation.

Interleukin-6 signal transduction and its role in hepatic lipid metabolic disorders

Hepatic lipid dysregulation can lead to spectrum of metabolic disease conditions including metabolic syndrome (MS), fatty liver and diabetes. Liver lipids are regulated by a complex set of extra-hepatic and intra-hepatic factors including cellular cross-talk with variety of cells, inducing various cytokines. Interleukin 6(IL-6) is a pleiotropic cytokine that exerts both pro-inflammatory and anti-inflammatory effects on hepatic system through either JNK/STAT or ERK/MAPK signaling. Although, IL-6 has shown to protect the liver from fat storage in both rodent and human models and various IL-6(-/-) studies have supported this notion yet a question remains over its deleterious pro-inflammatory effects on hepatocytes. IL-6 ability to produce reactive oxygen species (ROS) and subsequently disturb the hepatic lipid balance has created a conundrum. Furthermore, IL-6 has shown to behave differently under different disease states within hepatocytes and hence, modulating the hepatic lipids accordingly. This review deals with the role of IL-6 on hepatic lipid metabolism and analyzes various data presented on this topic.

Expression of recombinant human IL-4 in Pichia pastoris and relationship between its glycosylation and biological activity

Secretory human interleukin 4 (hIL4) is an N-glycosylated pleiotropic cytokine. It is unknown if these N-linked glycans are required and essential for hIL4 protein stability, expression, secretion, and activity in vivo, and hIL4 expressed from Pichia pastoris yeast has not been tested to date. In this study, we successfully expressed human hIL4 in P. pastoris, the methylotrophic yeast, with a yield of 15.0mg/L. Using the site-directed mutagenesis technique, we made two mutant hIL4 cDNA clones (N38A and N105L) and subsequently expressed them in P. pastoris to analyze the relevant function of each N-glycosylation site on hIL4. Our results demonstrate that the glycosylation only occurs at position Asn38, but not Asn105. The glycosylated form of hIL4 unexpectedly has lower biological activity and lower stability when compared to its non-glycosylated form. The implications of this are discussed.

Arginine inhibits adsorption of proteins on polystyrene surface

Nonspecific adsorption of protein on solid surfaces causes a reduction of concentration as well as enzyme inactivation during purification and storage. However, there are no versatile inhibitors of the adsorption between proteins and solid surfaces at low concentrations. Therefore, we examined additives for the prevention of protein adsorption on polystyrene particles (PS particles) as a commonly-used material for vessels such as disposable test tubes and microtubes. A protein solution was mixed with PS particles, and then adsorption of protein was monitored by the concentration and activity of protein in the supernatant after centrifugation. Five different proteins bound to PS particles through electrostatic, hydrophobic, and aromatic interactions, causing a decrease in protein concentration and loss of enzyme activity in the supernatant. Among the additives, including arginine hydrochloride (Arg), lysine hydrochloride, guanidine hydrochloride, NaCl, glycine, and glucose, Arg was most effective in preventing the binding of proteins to PS particles as well as activity loss. Moreover, even after the mixing of protein and PS particles, the addition of Arg caused desorption of the bound protein from PS particles. This study demonstrated a new function of Arg, which expands the potential for application of Arg to proteins.

The DEAD-box helicase DDX3 supports the assembly of functional 80S ribosomes

The DEAD-box helicase DDX3 has suggested functions in innate immunity, mRNA translocation and translation, and it participates in the propagation of assorted viruses. Exploring initially the role of DDX3 in the life cycle of hepatitis C virus, we observed the protein to be involved in translation directed by different viral internal ribosomal entry sites. Extension of these studies revealed a general supportive role of DDX3 in translation initiation. DDX3 was found to interact in an RNA-independent manner with defined components of the translational pre-initiation complex and to specifically associate with newly assembling 80S ribosomes. DDX3 knock down and in vitro reconstitution experiments revealed a significant function of the protein in the formation of 80S translation initiation complexes. Our study implies that DDX3 assists the 60S subunit joining process to assemble functional 80S ribosomes.

IL-6/IL-6 receptor system and its role in physiological and pathological conditions

IL (interleukin)-6, which was originally identified as a B-cell differentiation factor, is a multifunctional cytokine that regulates the immune response, haemopoiesis, the acute phase response and inflammation. IL-6 is produced by various types of cell and influences various cell types, and has multiple biological activities through its unique receptor system. IL-6 exerts its biological activities through two molecules: IL-6R (IL-6 receptor) and gp130. When IL-6 binds to mIL-6R (membrane-bound form of IL-6R), homodimerization of gp130 is induced and a high-affinity functional receptor complex of IL-6, IL-6R and gp130 is formed. Interestingly, sIL-6R (soluble form of IL-6R) also binds with IL-6, and the IL-6-sIL-6R complex can then form a complex with gp130. The homodimerization of receptor complex activates JAKs (Janus kinases) that then phosphorylate tyrosine residues in the cytoplasmic domain of gp130. The gp130-mediated JAK activation by IL-6 triggers two main signalling pathways: the gp130 Tyr759-derived SHP-2 (Src homology 2 domain-containing protein tyrosine phosphatase-2)/ERK (extracellular-signal-regulated kinase) MAPK (mitogen-activated protein kinase) pathway and the gp130 YXXQ-mediated JAK/STAT (signal transducer and activator of transcription) pathway. Increased IL-6 levels are observed in several human inflammatory diseases, such as rheumatoid arthritis, Castleman's disease and systemic juvenile idiopathic arthritis. IL-6 is also critically involved in experimentally induced autoimmune diseases. All clinical findings and animal models suggest that IL-6 plays a number of critical roles in the pathogenesis of autoimmune diseases. In the present review, we first summarize the IL-6/IL-6R system and IL-6 signal transduction, and then go on to discuss the physiological and pathological roles of IL-6.

Ternary system of solution additives with arginine and salt for refolding of beta-galactosidase

(L)-Arginine hydrochloride (Arg HCl) has been used for protein refolding as a universal aggregation suppressor for monomeric proteins. This paper presents an investigation of the refolding of tetrameric beta-galactosidase (beta-gal) using Arg HCl and other salts. In a binary system using only Arg HCl, the refolding yield of beta-gal increased with increasing concentration up to 0.2 M. However, the refolding yield sharply decreased above this concentration, reaching the level below the control yield of 5% at 0.5 M and near zero above 0.75 M, an observation unexpected from monomeric proteins. In a ternary system using both 0.2 M Arg HCl and another salt, the refolding yield increased up to 1.5-fold higher than that in the binary system. These data indicate that aggregation suppressive effects of protein increase with Arg HCl concentration, but also are deleterious to self-association of the protein. This dual nature of Arg HCl effects may have to be taken into account in its application for refolding of oligomeric proteins.

Contribution of leptin receptor N-linked glycans to leptin binding

The extracellular domain of the human leptin receptor (Ob-R) contains 20 potential N-glycosylation sites whose role in leptin binding remains to be elucidated. We found that a mammalian cell-expressed sOb-R (soluble Ob-R) fragment (residues 22-839 of the extracellular domain) bound leptin with a dissociation constant of 1.8 nM. This binding was inhibited by Con A (concanavalin A) or wheatgerm agglutinin. Treatment of sOb-R with peptide N-glycosidase F reduced leptin binding by approximately 80% concurrently with N-linked glycan removal. The human megakaryoblastic cell line, MEG-01, expresses two forms of the Ob-R, of approx. 170 and 130 kDa molecular mass. Endo H (endoglycosidase H) treatment and cell culture with alpha-glucosidase inhibitors demonstrated that N-linked glycans are of the complex mature type in the 170 kDa form and of the high-mannose type in the 130 kDa form. Both isoforms bound leptin, but not after peptide N-glycosidase F treatment. An insect-cell-expressed sOb-R fragment, consisting of the Ig (immunoglobulin), CRH2 (second cytokine receptor homology) and FNIII (fibronectin type III) domains, bound leptin with affinity similar to that of the entire extracellular domain, but this function was abolished after N-linked glycan removal. The same treatment had no effect on the leptin-binding activity of the isolated CRH2 domain. Our findings show that N-linked glycans within Ig and/or FNIII domains regulate Ob-R function, but are not involved in essential interactions with the ligand.

l-Argininamide improves the refolding more effectively than l-arginine

L-arginine (Arg) is a widely used additive for suppressing protein aggregation during refolding. Systematic screening of Arg analogs provides superior additives that enhance the refolding yield more effectively than Arg. The refolding yield of hen egg lysozyme in the presence of 500 mM L-argininamide (ArgAd) increases 1.7-fold higher than Arg. Thermal unfolding experiments indicate that ArgAd has a greater denaturing effect than Arg. The refolding yield positively relates to the net charge of Arg analogs. Moreover ArgAd was also effective for the refolding of bovine carbonic anhydrase. High potency to increase the refolding yield of ArgAd compared to Arg results from high positive net charge and the denaturing property.

Tocilizumab inhibits signal transduction mediated by both mIL-6R and sIL-6R, but not by the receptors of other members of IL-6 cytokine family

To characterize the biological activity of tocilizumab, a humanized anti-human interleukin-6 receptor (IL-6R) monoclonal antibody, we examined its binding activity to both soluble IL-6R (sIL-6R) and membrane bound IL-6R (mIL-6R) and its neutralizing activity to other IL-6 family cytokines. ELISA assay demonstrated that tocilizumab bound to sIL-6R and inhibited IL-6 binding to sIL-6R in a dose-dependent manner. The dissociation constant (Kd value) for IL-6R was determined to be 2.54+/-0.12 nmol/L by Scatchard analysis. In addition, tocilizumab had the ability to dissociate IL-6 and sIL-6R from their preformed complex. The immune complex of tocilizumab and sIL-6R did not transmit signaling. Moreover, tocilizumab suppressed the IL-6/sIL-6R complex-induced proliferation of human gp130-transfected cell, BAF-h130. In addition, tocilizumab had the ability to bind to human IL-6R expressing COS-7 cells and to suppress the growth of the IL-6-dependent myeloma cell line, KPMM2. Finally, to analyze the specificity of this antibody, the effects on signal transduction of IL-6 family cytokines such as interleukin-11 (IL-11), oncostatin M (OSM), leukemia inhibitory factor (LIF), and ciliary neurotrophic factor (CNTF) were examined using murine transfectant cell lines (BaF/IL-6R, BaF/IL-11R, BaF/OSMR, BaF/LIFR and BaF/CNTFR) that proliferate depending on IL-6, IL-11, OSM, LIF and human CNTF, respectively. Tocilizumab inhibited the proliferation of BaF/IL-6R induced by IL-6, but did not inhibit the proliferation of BaF/IL-11R, BaF/OSMR, BaF/LIFR and BaF/CNTFR cells induced by their corresponding cytokines. These lines of evidence indicate that tocilizumab is able to bind to both sIL-6R and mIL-6R and to inhibit IL-6 binding to its receptors, leading to the blockade of the IL-6 signaling through both sIL-6R and mIL-6R, but not block the signaling of other IL-6 family cytokines.

Optimization of the primary recovery of human interferon alpha2b from Escherichia coli inclusion bodies

The human interferon alpha2b (hu-IFNalpha2b) gene was cloned in Escherichia coli JM109(DE3) and the recombinant protein was expressed as cytoplasmic inclusion bodies (IB). The present work discusses the recovery of hu-IFNalpha2b IB from the E. coli cells. An optimized protocol is proposed based on the sequential evaluation of recovery steps and parameters: (i) cell disruption, (ii) IB recovery and separation from cell debris, (iii) IB washing, and (iv) IB solubilization. Parameters such as hu-IFNalpha2b purity and recovery yield were measured after each step. The optimized recovery protocol yielded 60% of hu-IFNalpha2b with a purity of up to 80%. The protein was renatured at high concentration after recovery and it was found to display biological activity.

Structure of the extracellular domains of the human interleukin-6 receptor alpha -chain

Dysregulated production of IL-6 and its receptor (IL-6R) are implicated in the pathogenesis of multiple myeloma, autoimmune diseases and prostate cancer. The IL-6R complex comprises two molecules each of IL-6, IL-6R, and the signaling molecule, gp130. Here, we report the x-ray structure (2.4 A) of the IL-6R ectodomains. The N-terminal strand of the Ig-like domain (D(1)) is disulfide-bonded to domain D(2), and domains D(2) and D(3), the cytokine-binding domain, are structurally similar to known cytokine-binding domains. The head-to-tail packing of two closely associated IL-6R molecules observed in the crystal may be representative of the configuration of the physiological dimer of IL-6R and provides new insight into the architecture of the IL-6R complex.

Renaturation and stabilization of the telomere-binding activity of Saccharomyces Cdc13(451-693)p by L-arginine

Production of recombinant proteins can be valuable in studying their biological functions. However, recombinant proteins expressed in Escherichia coli sometimes form undesirable insoluble aggregates. Solubilization and renaturation of these aggregates becomes a problem that one needs to solve. Here we used recombinant Cdc13(451-693)p as example to show the presence of l-arginine during renaturation greatly enhanced the renaturation efficiency. Cdc13p is the single-stranded telomere-binding protein of yeast Saccharomyces cerevisiae. The telomere-binding domain has been mapped within amino acids 451-693 of Cdc13p, Cdc13(451-693)p. Recombinant Cdc13(451-693)p was expressed in E. coli as insoluble protein aggregates. Purification of insoluble Cdc13(451-693)p was achieved by denaturing the protein with 6 M guanidine-HCl and followed by Ni-nitrilotriacetic acid agarose column chromatography. Renaturation of Cdc13(451-693)p to the active form was achieved by dialyzing denatured protein in the presence of l-arginine. Moreover, the presence of l-arginine was also helped in maintaining the telomere-binding activity of Cdc13(451-693)p. Taking together, l-arginine might have a general application in renaturation of insoluble aggregates.

The adipocyte: a model for integration of endocrine and metabolic signaling in energy metabolism regulation

The ability to ensure continuous availability of energy despite highly variable supplies in the environment is a major determinant of the survival of all species. In higher organisms, including mammals, the capacity to efficiently store excess energy as triglycerides in adipocytes, from which stored energy could be rapidly released for use at other sites, was developed. To orchestrate the processes of energy storage and release, highly integrated systems operating on several physiological levels have evolved. The adipocyte is no longer considered a passive bystander, because fat cells actively secrete many members of the cytokine family, such as leptin, tumor necrosis factor-alpha, and interleukin-6, among other cytokine signals, which influence peripheral fuel storage, mobilization, and combustion, as well as energy homeostasis. The existence of a network of adipose tissue signaling pathways, arranged in a hierarchical fashion, constitutes a metabolic repertoire that enables the organism to adapt to a wide range of different metabolic challenges, such as starvation, stress, infection, and short periods of gross energy excess.

Shedding of interleukin-6 receptor and tumor necrosis factor alpha. Contribution of the stalk sequence to the cleavage pattern of transmembrane proteins

A functionally and structurally diverse group of transmembrane proteins including transmembrane forms of mediators or receptors can be proteolytically cleaved to form soluble growth factors or receptors. Recently, the proteolytic activity responsible for pro-tumor necrosis factor alpha (proTNFalpha) processing has been identified and named TACE (TNFalpha converting enzyme). In experiments with TACE deficient (TACE-/-) fibroblasts we found that 4beta-phorbol 12-myristate 13-acetate (PMA)-induced shedding of the interleukin-6 receptor (IL-6R) is strongly reduced. A basal hydroxamate sensitive release of IL-6R, however, could still be detected. This result demonstrates that TACE plays a role in IL-6R processing and that additional metalloproteases might be involved. PMA-induced shedding of IL-6R in TACE deficient mouse fibroblasts could be restored by stable transfection of a TACE cDNA. To characterize differences between shedding of IL-6R and proTNFalpha we generated chimeric IL-6R and proTNFalpha proteins wherein the endogenous cleavage sites (CS) had been replaced by the corresponding region of proTNFalpha and IL-6R, respectively. Interestingly, proTNFalpha chimeric proteins showed only minimal shedding. In contrast, IL-6R chimeras containing the proTNFalpha CS were shed spontaneously, processing was not further induced by PMA. Thus, the cleavage pattern transferred by the introduction of the proTNFalpha CS is similar to that of proTNFalpha itself. We conclude that the amino-acid sequence at the proteolytic CS contributes to the cleavage characteristics of a protein. However, this information alone is not sufficient to transfer cleavability as seen with proTNFalpha chimeras containing the IL-6R CS and which were resistant to shedding.

Neural activities of IL-6-type cytokines often depend on soluble cytokine receptors

Cytokines of the interleukin-6 (IL-6) family participate in regulatory and inflammatory processes within the nervous system. IL-6, ciliary neurotrophic factor (CNTF) and IL-11 act via specific membrane receptors which, together with their ligands, associate with signal-transducing receptor subunits thereby initiating cytoplasmic signalling. Cells which only express signal-transducing receptor subunits but no ligand binding subunits for IL-6, CNTF and IL-11 are refractory to these cytokines. An unusual feature of the IL-6 cytokine family is that the soluble forms of the ligand binding receptor subunits generated by one cell type in complex with their ligands can directly stimulate the signal-transducing receptor subunits on different cell types which lack ligand binding receptor subunits. This process has been named transsignalling. This article focuses on the importance of transsignalling events in neuronal differentiation and survival responses.

A role for the immunoglobulin-like domain of the human IL-6 receptor. Intracellular protein transport and shedding

Interleukin (IL)-6, IL-11 and cililary neurotrophic factor (CNTF) belong to the same family of hematopoietic and neurotrophic cytokines. Their receptor complexes contain a cytokine-binding alpha receptor and the common glycoprotein (gp)130 subunit for signal transduction. The extracellular parts of the alpha-receptor subunits consist of a membrane-proximal cytokine-binding domain and an N-terminal immunoglobulin (Ig)-like domain with unknown function. We examined the role of the Ig-like domain of IL-6R by constructing deletion mutants lacking the Ig domain (IL-6RDeltaIg and soluble IL-6RDeltaIg). IL-6RDeltaIg was shed as effectively as wild-type IL-6R from transfected COS-7 cells upon 4beta-phorbol 12-myristate 13-acetate (PMA) treatment, whereas nonstimulated shedding of IL-6RDeltaIg was not observed. The shed sIL-6RDeltaIg from PMA-treated cells, as well as the transmembrane IL-6RDeltaIg, had the same biological activity as wild-type sIL-6R, as measured by the induction of haptoglobin secretion in HepG2-IL-6 cells and IL-6-dependent proliferation of IL-6RDeltaIg transfected BAF/gp130 cells. In COS-7 cells transfected with IL-6RDeltaIg or soluble IL-6RDeltaIg cDNA, transport of the deletion mutants through the secretory pathway appeared to be delayed because a sizeable proportion of the mutants was detected as an endo-beta-N-acetylglucosaminidase-sensitive intermediate, suggesting that transport and processing of the DeltaIg mutants on the secretory pathway were impaired. These experiments suggest that the Ig-like domain of the IL-6R is important for intracellular transport of IL-6R through the secretory pathway. Furthermore, the Ig-like domain is necessary for noninduced shedding of the IL-6R, whereas it has no function in PKC-dependent shedding of the IL-6R.

Hydrostatic pressure rescues native protein from aggregates

Misfolding and misassembly of proteins are major problems in the biotechnology industry, in biochemical research, and in human disease. Here we describe a novel approach for reversing aggregation and increasing refolding by application of hydrostatic pressure. Using P22 tailspike protein as a model system, intermediates along the aggregation pathway were identified and quantitated by size-exclusion high-performance liquid chromatography (HPLC). Tailspike aggregates were subjected to hydrostatic pressures of 2.4 kbar (35,000 psi). This treatment dissociated the tailspike aggregates and resulted in increased formation of native trimers once pressure was released. Tailspike trimers refolded at these pressures were fully active for formation of infectious viral particles. This technique can facilitate conversion of aggregates to native proteins without addition of chaotropic agents, changes in buffer, or large-scale dilution of reagents required for traditional refolding methods. Our results also indicate that one or more intermediates at the junction between the folding and aggregation pathways is pressure sensitive. This finding supports the hypothesis that specific determinants of recognition exist for protein aggregation, and that these determinants are similar to those involved in folding to the native state. An increased understanding of this specificity should lead to improved refolding methods.

Disulfide bond structure and N-glycosylation sites of the extracellular domain of the human interleukin-6 receptor

The high affinity interleukin-6 (IL-6) receptor is a hexameric complex consisting of two molecules each of IL-6, IL-6 receptor (IL-6R), and the high affinity converter and signaling molecule, gp130. The extracellular "soluble" part of the IL-6R (sIL-6R) consists of three domains: an amino-terminal Ig-like domain and two fibronectin-type III (FN III) domains. The two FN III domains comprise the cytokine-binding domain defined by a set of 4 conserved cysteine residues and a WSXWS sequence motif. Here, we have determined the disulfide structure of the human sIL-6R by peptide mapping in the absence and presence of reducing agent. Mass spectrometric analysis of these peptides revealed four disulfide bonds and two free cysteines. The disulfides Cys102-Cys113 and Cys146-Cys157 are consistent with known cytokine-binding domain motifs, and Cys28-Cys77 with known Ig superfamily domains. An unusual cysteine connectivity between Cys6-Cys174, which links the Ig-like and NH2-terminal FN III domains causing them to fold back onto each other, has not previously been observed among cytokine receptors. The two free cysteines (Cys192 and Cys258) were detected as cysteinyl-cysteines, although a small proportion of Cys258 was reactive with the alkylating agent 4-vinylpyridine. Of the four potential N-glycosylation sites, carbohydrate moieties were identified on Asn36, Asn74, and Asn202, but not on Asn226.

Immunoadhesins of interleukin-6 and the IL-6/soluble IL-6R fusion protein hyper-IL-6

Signal transduction in response to interleukin-6 (IL-6) results from homodimerization of gp130. This dimerization occurs after binding of IL-6 to its surface receptor (IL-6R) and can also be triggered by the complex of soluble IL-6R and IL-6. We fused IL-6 to the constant region of a human IgG1 heavy chain (Fc). IL-6Fc was expressed in COS-7 cells and purified via Protein A Sepharose. Using three different assays we found that the biological activity of this dimeric IL-6 protein is comparable with monomeric IL-6. Recently, we described the designer cytokine Hyper-IL-6 (H-IL-6) in which soluble IL-6R and IL-6 are connected via a flexible peptide linker. This molecule turned out to be 100-1000 times more effective than unlinked IL-6 and soluble IL-6R. Hyper-IL-6 acts on cells only expressing gp130 and is a potent stimulator of in vitro expansion of early hematopoietic precursors. Here we show that a Fc fusion protein of H-IL-6 (H-IL-6Fc) has the same biological activity on BAF/gp130 cells as H-IL-6. Furthermore, both H-IL-6 forms have a similar ability to induce the synthesis of acute phase proteins in human hepatoma cells HepG2 and in mice in vivo. The introduction of a thrombin cleavage site between H-IL-6 and the Fc portion of H-IL-6Fc made it possible to specifically recover biologically active monomeric H-IL-6 by limited proteolysis of the fusion protein. A more general use of cleavable immunoadhesins expressed in mammalian cells is discussed.

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.

Isolation and expression of murine carbonic anhydrase IV

A 1193-bp cDNA containing the complete murine carbonic anhydrase IV coding sequence was isolated from a Balb/c kidney cDNA library. The entire coding sequence plus shorter segments was used in an Escherichia coli T7 expression vector system to produce four forms of murine CA IV, including (1) a protein representing the full-length coding sequence, (2) an amino-truncated protein lacking the 18 N-terminal amino acid plasma membrane targeting sequence, (3) a protein which lacked the plasma membrane targeting sequence and 26 C-terminal amino acids, and (4) a protein which lacked both 36 N-terminal residues (the plasma membrane targeting sequence plus 18 additional amino acids which included the first two cysteines) and 26 C-terminal residues. All four proteins were expressed as catalytically inactive inclusion bodies. After rapid dilution of washed, guanidine hydrochloride-denatured inclusion bodies into a glutathione-, l-arginine-containing renaturation buffer, an active carbonic anhydrase IV at yields of 3-4 mg/liter was easily purified from cultures expressing the form lacking the N-terminal targeting sequence and 26 C-terminal residues. The longest and shortest forms of carbonic anhydrase IV failed to refold into active enzyme under these conditions. The activity of purified recombinant carbonic anhydrase IV was highly resistant to sodium dodecyl sulfate, as is the native enzyme. This resistance presumably results from intramolecular disulfide bonds maintaining a functional active site configuration even in the presence of denaturing agents.

Recombinant human single chain Fv antibodies recognizing human interleukin-6. Specific targeting of cytokine-secreting cells

A human antibody library was displayed on the surface of filamentous bacteriophage and screened for binding to human interleukin-6 (IL-6). Two antibody-bearing phages were selected that bound IL-6. The complementary-determining region 3 loops of the variable heavy chains of these two antibodies differed in length and sequence and recognized two distinct epitopes. One of the single chain Fv fragments isolated (H1) was found to bind human (but not murine) IL-6 with an affinity comparable to that of the human IL-6 receptor. H1 also recognized newly synthesized human IL-6 intracellularly, as shown by indirect immunofluorescence. H1 did not neutralize human IL-6, and the H1 epitope was mapped to a region of IL-6 not involved in interactions with IL-6, IL-6 receptor, or the signal-transducing protein gp130. To target IL-6-secreting cells, we then constructed a bispecific antibody fragment (a diabody) comprising H1 and the antigen binding site of the T-cell activating monoclonal antibody OKT3. The diabody led to T-cell-mediated killing of cells secreting IL-6.

Beryllium-stimulated release of tumor necrosis factor-alpha, interleukin-6, and their soluble receptors in chronic beryllium disease

Chronic beryllium disease (CBD) provides a model system in which to evaluate the antigen-stimulated, cell-mediated, immune response that leads to granulomatous lung disease. We hypothesized that beryllium salts would stimulate bronchoalveolar lavage (BAL) cell release of tumor necrosis factor-alpha (TNF-alpha) and lnterleukin-6 (IL-6), and their soluble receptors, soluble TNF receptor I (sTNF RI), sTNF RII, and sIL-6R and that chronic exposure to antigen would increase production of soluble receptors in the serum and BAL fluid (BALF) of beryllium-sensitized and CBD patients. We have demonstrated (1) similar constitutive TNF-alpha, IL-6, and soluble receptor production by control subjects and CBD patients, (2) a BeSO4-stimulated increase in TNF-alpha and IL-6 production by CBD-derived BAL cells, and (3) a BeSO4-induced decrease in sTNF RII production by BAL cells from control subjects. We measured increased serum sTNF RI and serum and BALF sIL-6R in beryllium-sensitized subjects and increased sTNF RI and RII in serum and sIL-6R and sTNF RII and BALF in CBD patients. These changes correlated with pulmonary lymphocytosis and clinical measures of disease severity, indicating that soluble receptors may reflect disease status.

Major role of the soluble interleukin-6/interleukin-6 receptor complex for the proliferation of interleukin-6-dependent human myeloma cell lines

Interleukin-6 (IL-6) is a proinflammatory cytokine which possesses a central growth factor activity for certain tumor cells such as plasma cells in multiple myeloma (MM). Upon binding of IL-6, soluble IL-6 receptor (sIL-6R) has been shown to retain its affinity for IL-6 and to associate with the signal-transducing gp130 chain. Therefore, contrary to the majority of soluble cytokine receptors, it plays an agonist role in IL-6 signaling. In order to test its physiological importance as compared to that of its membrane counterpart, we studied cells from two myeloma cell lines which need exogenous IL-6 to proliferate and release sIL-6R into their culture supernatant. Using a new culture system where the supernatant recirculated permanently through an anti-IL-6R affinity column, all sIL-6R was removed from the culture medium throughout the culture period. Under these conditions IL-6-dependent cells were unable to grow in the presence of physiological concentrations of IL-6, showing the major role of the sIL-6R for sustaining the proliferation of these cell lines. Increasing IL-6 concentrations well over the physiological values allowed the cells to proliferate again. No effect was seen when sIL-6R was removed from the supernatant of an IL-6-independent myeloma cell line. These results show that the levels of circulating sIL-6R (and thus those of IL-6/sIL-6R complex) are worth looking at in pathologies involving IL-6 hyperactivity.

Interleukin 6 is autoregulated by transcriptional mechanisms in cultures of rat osteoblastic cells

Interleukin 6 (IL-6), a cytokine produced by skeletal cells, stimulates osteoclast recruitment. The IL-6 soluble receptor (sIL-6R) increases IL-6 activity, and IL-6 and sIL-6R levels are increased in conditions of increased bone resorption. We examined the production of IL-6 by primary rat osteoblasts (Ob cells) cultured in the presence of IL-6 and sIL-6R. IL-6 alone did not induce IL-6 transcripts, but IL-6 was stimulatory in the presence of sIL-6R. Furthermore, sIL-6R by itself increased IL-6 transcripts. Cycloheximide superinduced IL-6 transcripts and did not prevent the effect of IL-6 and sIL-6R. IL-6 in the presence of sIL-6R stimulated IL-6 rates of transcription and the activity of IL-6 promoter fragments in transiently transfected Ob cells. 5' deletions of the IL-6 promoter and targeted mutations of the multiple response element (MRE)/cAMP responsive element (CRE), the nuclear factor for IL-6 (NF-IL-6), and the nuclear factor-kappaB (NF-kappaB) binding sites indicated that NF-IL-6 and NF-kappaB, in combination with MRE/CRE, binding sites are required for the induction of the IL-6 promoter by IL-6. In conclusion, IL-6 induces its own synthesis in osteoblasts by transcriptional mechanisms. This positive feedback may be important in conditions of increased bone resorption.

Molecular modeling-guided mutagenesis of the extracellular part of gp130 leads to the identification of contact sites in the interleukin-6 (IL-6).IL-6 receptor.gp130 complex

The transmembrane protein gp130 is involved in many cytokine-mediated cellular responses and acts therein as the signal-transducing subunit. In the case of interleukin-6 (IL-6), the signal-transducing complex is composed of the ligand IL-6, the IL-6 receptor (IL-6R, gp80, CD126), and at least two gp130 (CD130) molecules. The extracellular part of the signal transducer gp130 consists of six fibronectin type III-like domains. It has recently been shown that the three membrane distal domains bind to the IL-6. IL-6R complex. A structural model of the IL-6.IL-6R.gp130 complex enabled us to propose amino acid residues in these domains of gp130 interacting with IL-6 bound to its receptor. The proposed amino acid residues located in the B'C' loop (Val252) and in the F'G' loop (Gly306, Lys307) of domain 3 and in the hinge region (Tyr218) connecting domains 2 and 3 of gp130 were mutated to disturb ternary complex formation. Binding of wild type and mutants of the extracellular region of gp130 was studied by use of a co-precipitation assay and Scatchard analysis. All mutants showed decreased binding to the IL-6.IL-6R complex. Biological function of the membrane-bound gp130 mutants was studied by STAT (signal transducer and activator of transcription) activation in COS-7 cells and by proliferation of stably transfected Ba/F3 cells. Reduced binding of the mutants was accompanied by decreased biological activity. The combined approach of molecular modeling and site-directed mutagenesis has led to the identification of amino acid residues in gp130 required for complex formation with IL-6 and its receptor.

Interleukin-6 and its soluble receptor cause a marked induction of collagenase 3 expression in rat osteoblast cultures

Interleukin-6 (IL-6), a cytokine produced by skeletal cells, increases bone resorption, but its effects on collagenase expression are unknown. We tested the effects of IL-6 and its soluble receptor on collagenase 3 expression in osteoblast-enriched cells from fetal rat calvariae (Ob cells). IL-6 caused a small increase in collagenase mRNA levels, but in the presence of IL-6-soluble receptor (IL-6sR), IL-6 caused a marked increase in collagenase transcripts after 2-24 h. In addition, IL-6sR increased collagenase mRNA when tested alone. IL-6 and IL-6sR increased immunoreactive collagenase levels. Cycloheximide and indomethacin did not prevent the effect of IL-6 and IL-6sR on collagenase mRNA levels. IL-6 and IL-6sR did not alter the decay of collagenase mRNA in transcriptionally arrested Ob cells and increased the levels of collagenase heterogeneous nuclear RNA and the rate of collagenase gene transcription in Ob cells. IL-6 and IL-6sR increased collagenase 3 mRNA in MC3T3 cells but only modestly in skin fibroblasts. IL-6 and IL-6sR enhanced the expression of tissue inhibitor of metalloproteinases 1. In conclusion, IL-6, in the presence of IL-6sR, increases collagenase 3 synthesis in osteoblasts by transcriptional mechanisms. This effect may contribute to the action of IL-6 on bone matrix degradation and bone resorption.

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.

The function of the soluble IL-6 receptor in vivo

Interleukin-6 (IL-6) is considered an important mediator of acute inflammatory responses. Moreover, IL-6 functions as a differentiation and growth factor of hematopoietic precursor cells, B-cells, T-cells, keratinocytes, neuronal cells, osteoclasts and endothelial cells. IL-6 exhibits its action via a receptor complex consisting of a specific IL-6 receptor (IL-6R) and a signal-transducing subunit (gp130). Soluble forms of both receptor components are generated by shedding and are found in patients with various diseases such as AIDS, rheumatoid arthritis and others. The function of the soluble IL-6R in vivo is unknown. To discriminate between the biologic function of hIL-6 alone and that of the hIL-6/hsIL-6R complex, mice transgenic for human IL-6, for the human soluble IL-6R and for both, human IL-6 and the human soluble IL-6R were analyzed and compared with nontransgenic littermates. While IL-6 transgenic mice exhibit elevated acute phase protein levels and develop plasmacytomas, hsIL-6R single transgenic mice are hypersensitized towards human IL-6, mounting an acute phase protein gene induction at significantly lower IL-6 dosages compared to control animals. Furthermore, in hsIL-6R transgenic mice, the acute phase response persists for a longer period of time and the IL-6 plasma half life was markedly prolonged. IL-6/sI1-6R mice, however, develop massive hepatosplenomegaly caused by extramedullary hematopoisis in these organs. In IL-6- and IL-6R-single transgenic mice, no such effects were observed. Our study discloses a novel biologic effect of the hIL-6/hsIL-6R complex, which is clearly distinct from that of hIL-6 alone. We provide evidence that the activation of the gp130 signal transducer represents a major stimulation of growth and differentiation of hematopoietic progenitor cells.

Yeast expression of the cytokine receptor domain of the soluble interleukin-6 receptor

The complex of the soluble interleukin-6 receptor (sIL-6R) and IL-6 (IL-6) is a potent agonist on cells expressing the signal transducing protein gp 130. In contrast, IL-6 alone only stimulates cells which express a membrane bound form of the IL-6R and gp 130. The natural occurring sIL-6R is generated by shedding of the membrane receptor and to a lesser extend by alternative splicing. We have inserted the coding sequence of the 323 amino acid residues of the human sIL-6R into an expression/secretion vector suitable for the methylotrophic yeast Pichia pastoris. We obtained, however, no detectable expression and secretion of the recombinant protein. When we used only the coding sequence of the cytokine receptor domain of the sIL-6R for the construction of an expression plasmid, this truncated version of the sIL-6R accumulated in the supernatant to 1-5 mg/l. The protein was purified by a single affinity chromatography step using a monoclonal antibody directed against the human IL-6R. Following the same approach, we expressed a truncated splice variant of the sIL-6R. Both, the secreted truncated sIL-6R and the splice variant showed full agonistic biological activity on human hepatoma cells. The described expression strategy will be useful for large scale production of biologically active sIL-6R and might be adapted for the expression of other members of the hematopoietic cytokine receptor family.

Characterization and biologic activities of recombinant rat soluble interleukin-6 receptor

The soluble interleukin-6 receptor (sIL-6R) consists of the extracellular domain of the membrane-bound IL-6 receptor (gp80) found on many types of cells. Contrary to most other soluble cytokine receptors, it possesses in vitro agonistic properties, yet its physiologic role remains unknown. We have generated a cDNA encoding the rat sIL-6R and have expressed and purified the protein using Escherichia coli and baculovirus systems. Analysis of purified protein by electrophoresis and silver staining showed a single band migrating at 35 kDa for E. coli (nonglycosylated) and at 47 kDa for baculovirus-derived material. The purified protein is biologically active, as determined by the ability to convert human hepatoma cells (HepG2) from nonresponsive to responsive to rat IL-6 and induce acute-phase protein synthesis. Most important, we show that rat sIL-6R directly induces proliferation of the IL-6-dependent murine hybridoma cell line (B9) in an IL-6-like manner, with 50% proliferation induced by 100 ng/ml of baculovirus-derived receptor protein. Physiologic concentrations of sIL-6R dramatically enhance the sensitivity of B9 cells to IL-6, indicating that the bioassay for IL-6 is susceptible to modulation by the presence of sIL-6R in rodent serum samples. This sIL-6R-dependent B9 cell proliferation is fully abrogated by antibodies directed against rodent IL-6 and indicates autocrine production of low amounts of IL-6 by the B9 cell line.

Novel pathogenic mechanism of microbial metalloproteinases: liberation of membrane-anchored molecules in biologically active form exemplified by studies with the human interleukin-6 receptor

Certain membrane-anchored proteins, including several cytokines and cytokine receptors, can be released into cell supernatants through the action of endogenous membrane-bound metalloproteinases. The shed molecules are then able to fulfill various biological functions; for example, soluble interleukin-6 receptor (sIL-6R) can bind to bystander cells, rendering these cells sensitive to the action of IL-6. Using IL-6R as a model substrate, we report that the metalloproteinase from Serratia marcescens mimics the action of the endogenous shedding proteinase. Treatment of human monocytes with the bacterial protease led to a rapid release of sIL-6R into the supernatant. This effect was inhibitable with TAPI [N-(D,L-[2-(hydroxyaminocarbonyl)methyl]-4-methylpentanoyl) L-3-(2' naphthyl)-alanyl-L-alanine, 2-aminoethyl amide], a specific inhibitor of the membrane-bound intrinsic metalloproteinase, but not with other conventional proteinase inhibitors. sIL-6R-liberating activity was also detected in culture supernatants of Staphylococcus aureus, Pseudomonas aeruginosa, and Listeria monocytogenes, organisms that are known to produce metalloproteinases. sIL-6R released through the action of S. marcescens metalloproteinase retained biological activity and rendered IL-6-unresponsive human hepatoma cells sensitive to stimulation with IL-6. This was shown by Northern (RNA) blot detection of haptoglobin mRNA and by quantitative measurements of de novo-synthesized haptoglobin in cell supernatants. Analysis of immunoprecipitated, radiolabeled sIL-6R revealed that the bacterial protease cleaved IL-6R at a site distinct from that utilized by the endogenous protease. These studies show that membrane-anchored proteins can be released in active form through cleavage at multiple sites, and they uncover a novel mechanism via which microbial proteases possibly provoke long-range biological effects in the host organism.

Influence of interleukin-6 (IL-6) dimerization on formation of the high affinity hexameric IL-6.receptor complex

The high affinity interleukin-6 (IL-6) signaling complex consists of IL-6 and two membrane-associated receptor components: a low affinity but specific IL-6 receptor and the affinity converter/signal transducing protein gp130. Monomeric (IL-6M) and dimeric (IL-6D) forms of Escherichia coli-derived human IL-6 and the extracellular ("soluble") portions of the IL-6 receptor (sIL-6R) and gp130 have been purified in order to investigate the effect of IL-6 dimerization on binding to the receptor complex. Although IL-6D has a higher binding affinity for immobilized sIL-6R, as determined by biosensor analysis employing surface plasmon resonance detection, IL-6M is more potent than IL-6D in a STAT3 phosphorylation assay. The difference in potency is significantly less pronounced when measured in the murine 7TD1 hybridoma growth factor assay and the human hepatoma HepG2 bioassay due to time-dependent dissociation at 37 degrees C of IL-6 dimers into active monomers. The increased binding affinity of IL-6D appears to be due to its ability to cross-link two sIL-6R molecules on the biosensor surface. Studies of the IL-6 ternary complex formation demonstrated that the reduced biological potency of IL-6D resulted from a decreased ability of the IL-6D (sIL-6R)2 complex to couple with the soluble portion of gp130. These data imply that IL-6-induced dimerization of sIL-6R is not the driving force in promoting formation of the hexameric (IL-6 IL-6R gp130)2 complex. A model is presented whereby the trimeric complex of IL-6R, gp130, and IL-6M forms before the functional hexamer. Due to its increased affinity for the IL-6R but its decreased ability to couple with gp130, we suggest that a stable IL-6 dimer may be an efficient IL-6 antagonist.

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.

Pore-forming toxins trigger shedding of receptors for interleukin 6 and lipopolysaccharide

Cleavage of membrane-associated proteins with the release of biologically active macromolecules is an emerging theme in biology. However, little is known about the nature and regulation of the involved proteases or about the physiological inducers of the shedding process. We here report that rapid and massive shedding of the interleukin 6 receptor (IL-6R) and the lipopolysaccharide receptor (CD14) occurs from primary and transfected cells attacked by two prototypes of pore-forming bacterial toxins, streptolysin O and Escherichia coli hemolysin. Shedding is not induced by an streptolysin O toxin mutant which retains cell binding capacity but lacks pore-forming activity. The toxin-dependent cleavage site of the IL-6R was mapped to a position close to, but distinct from, that observed after stimulation with phorbol myristate acetate. Soluble IL-6R that was shed from toxin-treated cells bound its ligand and induced an IL-6-specific signal in cells that primarily lacked the IL-6R. Transsignaling by soluble IL-6R and soluble CD14 is known to dramatically broaden the spectrum of host cells for IL-6 and lipopolysaccharide, and is thus an important mechanism underlying their systemic inflammatory effects. Our findings uncover a novel mechanism that can help to explain the long-range detrimental action of pore-forming toxins in the host organism.

Participation of two Ser-Ser-Phe-Tyr repeats in interleukin-6 (IL-6)-binding sites of the human IL-6 receptor

The alpha-subunit of interleukin-6 (IL-6) receptor is a member of the hematopoietin receptor family. The alignment of its amino acid sequence with those of other members of this family (human somatotropin receptor/murine IL-3 receptor beta and human IL-2 receptor beta) has suggested that amino acids included in two SSFY repeats found in each of its hematopoietin receptor domains, contribute to the binding of the ligand. The involvement of these amino acids in IL-6 binding and signal transduction was studied by site-directed mutagenesis and molecular modelling. We present a computer-derived three-dimensional model of the IL-6/IL-6 receptor complex based on the structure of the human somatotropin/human somatotropin receptor complex. This model allowed the location of distinct regions important for IL-6 and gp130 binding. We show that some of the residues included in the SSFY repeats located in our IL-6 receptor model in the loops between beta-strands E and F of domain-I and B' and C', of domain-II, participate in the formation of a major IL-6-binding site. These residues are necessary for IL-6 and gp130 binding and for signal transduction. Using our IL-6 receptor mutants we mapped the epitopes of our anti-(IL-6 receptor) neutralising monoclonal antibodies to these residues. Our results demonstrate that a generic hematopoietin receptor family structural module can be used for the study of both alpha and beta receptor subunits belonging to this family.

Diverse cell surface protein ectodomains are shed by a system sensitive to metalloprotease inhibitors

The extracellular domains of a diverse group of membrane proteins are shed in response to protein kinase C activators such as phorbol 12-myristate 13-acetate (PMA). The lack of sequence similarity in the cleavage sites suggests the involvement of many proteases of diverse specificity in this process. However, a mutant Chinese hamster ovary cell line recently isolated for being defective in PMA-activated shedding of the membrane-anchored growth factor transforming growth factor alpha precursor (proTGF-alpha) is concomitantly defective in the shedding of many other unrelated membrane proteins. Here we show that independent mutagenesis and selection experiments yield shedding mutants having the same recessive phenotype and belonging to the same genetic complementation group. Furthermore, two structurally distinct agents, TAPI-2 and 1,10-phenanthroline, which are known to inhibit metalloproteases, block PMA-activated shedding of proTGF-alpha, cell adhesion receptor L-selectin, interleukin 6 receptor alpha subunit, beta-amyloid precursor protein, and an entire set of anonymous Chinese hamster ovary cell surface proteins. Certain serine protease inhibitors prevent release of these proteins by interfering with their maturation and transport to the cell surface but do not inhibit ectodomain shedding from the cell surface. The results suggest the existence of a common system for membrane protein ectodomain shedding involving one or several proteolytic activities sensitive to metalloprotease inhibitors, whose ability to act can be disrupted by recessive mutations in a single gene.

The function of the soluble interleukin 6 (IL-6) receptor in vivo: sensitization of human soluble IL-6 receptor transgenic mice towards IL-6 and prolongation of the plasma half-life of IL-6

Interleukin 6 (IL-6) is considered an important mediator of acute inflammatory responses. Moreover, IL-6 functions as a differentiation and growth factor of hematopoietic precursor cells, B cells, T cells, keratinocytes, neuronal cells, osteoclasts, and endothelial cells. IL-6 exhibits its action via a receptor complex consisting of a specific IL-6 receptor (IL-6R) and a signal transducing subunit (gp130). Soluble forms of both receptor components are generated by shedding and are found in patients with various diseases such as acquired immune deficiency syndrome, rheumatoid arthritis, and others. The function of the soluble (s)IL-6R in vivo is unknown. Since human (h)IL-6 acts on human and murine target cells, but murine IL-6 on murine cells only, we constructed transgenic mice expressing the hsIL-6R. We report here that in the presence of hsIL-6R, mice are hypersensitized towards hIL-6, mounting an acute phase protein gene induction at significantly lower IL-6 dosages compared to control animals. Furthermore, in hsIL-6R transgenic mice, the detected acute phase response persists for a longer period of time. The IL-6/IL-6R complex prolongs markedly the Il-6 plasma half-life. Our results reinforce the role of the hsIL-6R as an agonistic protein, help to understand the function of the hsIL-6R in vivo, and highlight the significance of the receptor in the induction of the acute phase response.

Alteration of the cystic fibrosis transmembrane conductance regulator folding pathway

The cellular phenotype of the most common cystic fibrosis-causing mutation, deletion of phenylalanine 508 (deltaF508) in the amino-terminal nucleotide binding domain (NBD1) of the cystic fibrosis transmembrane conductance regulator (CFTR), is the inability of the mutant protein to fold and transit to the apical membrane of affected epithelial cells. Expressed NBD1s were purified and folded in vitro into soluble monomers capable of binding nucleotide. Here we report that the deltaF508 mutation has little effect on the thermodynamic stability of the folded NBD1. The deltaG(0)(D,GdnHCl) is 15.5 kJ/mol for the wild type NBD1 and 14.4 kJ/mol for NBD1deltaF. In contrast, the mutation significantly reduces the folding yield at a variety of temperatures, indicating that Phe-508 makes crucial contacts during the folding process, but plays little role in stabilization of the native state. Under conditions that approximate the efficiency of maturation in vivo, the rate off-pathway is significantly increased by the disease causing mutation. These results establish a molecular mechanism for most cases of cystic fibrosis and provide insight into the complex processes by which primary sequence encodes the three-dimensional structure.